# PHOTOCATALYSIS FOR ENVIRONMENTAL APPLICATIONS

EDITED BY : Fan Dong, Sen Zhang and Yuxin Zhang PUBLISHED IN : Frontiers in Chemistry

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The above represents a summary only. For the full conditions see the Conditions for Authors and the Conditions for Website Use. ISSN 1664-8714 ISBN 978-2-88945-902-5 DOI 10.3389/978-2-88945-902-5

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# PHOTOCATALYSIS FOR ENVIRONMENTAL APPLICATIONS

Topic Editors:

Fan Dong, Chongqing Technology and Business University, China; University of Electronic Science and Technology of China, China Sen Zhang, University of Virginia, United States Yuxin Zhang, Chongqing University, China

Citation: Dong, F., Zhang, S., Zhang, Y., eds. (2019). Photocatalysis for Environmental Applications. Lausanne: Frontiers Media. doi: 10.3389/978-2-88945-902-5

# Table of Contents


Shiying He, Pengfu Hou, Evangelos Petropoulos, Yanfang Feng, Yingliang Yu, Lihong Xue and Linzhang Yang

*91 Photocatalytic Oxidation of Acetone Over High Thermally Stable TiO2 Nanosheets With Exposed (001) Facets*

Ting Shi, Youyu Duan, Kangle Lv, Zhao Hu, Qin Li, Mei Li and Xiaofang Li

*101 Compressible and Recyclable Monolithic g-C3N4 /Melamine Sponge: A Facile Ultrasonic-Coating Approach and Enhanced Visible-Light Photocatalytic Activity*

Ye Yang, Qian Zhang, Ruiyang Zhang, Tao Ran, Wenchao Wan and Ying Zhou

*111 Insights Into Highly Improved Solar-Driven Photocatalytic Oxygen Evolution Over Integrated Ag3PO4 /MoS2 Heterosttructures* Xingkai Cui, Xiaofei Yang, Xiaozhai Xian, Lin Tian, Hua Tang and Qinqin Liu


Wei Guan, Gaoge Sun, Lei Yin, Zhenghua Zhang and Shichao Tian


Jin Han, Kejian Li, Hanyun Cheng and Liwu Zhang

# Editorial: Photocatalysis for Environmental Applications

#### Fan Dong1,2 \*, Yuxin Zhang<sup>3</sup> \* and Sen Zhang<sup>4</sup> \*

<sup>1</sup> Chongqing Key Laboratory of Catalysis and New Environmental Materials, Engineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education, College of Environment and Resources, Chongqing Technology and Business University, Chongqing, China, <sup>2</sup> Research Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, China, <sup>3</sup> College of Materials Science and Engineering, Chongqing University, Chongqing, China, <sup>4</sup> Department of Chemistry, University of Virginia, Charlottesville, VA, United States

Keywords: photocatalysis, environmental catalysis, air pollution, reaction mechanism, nanomaterials

#### **Editorial on the Research Topic**

#### **Photocatalysis for Environmental Applications**

#### Edited by:

Simelys Hernández, Polytechnic University of Turin, Italy

#### Reviewed by:

Fatwa Abdi, Helmholtz-Zentrum Berlin für Materialien und Energie, Helmholtz-Gemeinschaft Deutscher Forschungszentren (HZ), Germany

#### \*Correspondence:

Fan Dong dfctbu@126.com Yuxin Zhang zhangyuxin@cqu.edu.cn Sen Zhang sz3t@virginia.edu

#### Specialty section:

This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry

Received: 12 February 2019 Accepted: 16 April 2019 Published: 01 May 2019

#### Citation:

Dong F, Zhang Y and Zhang S (2019) Editorial: Photocatalysis for Environmental Applications. Front. Chem. 7:303. doi: 10.3389/fchem.2019.00303 Environmental pollution is one of the major challenges because of the rapid development of urbanization and industrialization. Considering this environmental challenge, providing a clean environment for human beings is very important for the sustainability. The nanostructured photocatalysts with intriguing physiochemical property have offered opportunities to solve the issue of environmental sustainability (Chen et al., 2019; Huo et al., 2019; Li J. et al., 2019). In recent years, significant advances have been witnessed on the synthesis and application of photocatalyst in environmental remediation (He et al., 2018a; Li et al., 2018c; Li X. et al., 2018, 2019; Wang et al., 2018c). These new photocatalysts have enabled wide applications in the air purification, wastewater treatment, and so on (Cui et al., 2018; He et al., 2018b; Li et al., 2018b; Xiong et al., 2018) The rapid development in catalysis science, nanoscience, and materials enable the significant advances in new strategies for the controlled preparation, photocatalysis reaction mechanism, and structure-activity relationship of photocatalysts (Dong et al., 2018a; Li et al., 2018a; Wang et al., 2018a,b). The structural features of photocatalysts can be further tuned to achieve enhanced photocatalytic performance in environmental applications (Dong et al., 2018b; Li X. et al., 2018; Wang et al., 2018d).

The rapid development in photocatalysis for environment has inspired this interesting Research Topic. We have invited scientists worldwide to contribute original research and review articles which could enhance our understanding of the key problems in environmental applications of nanostructured photocatalysts. The original articles describing the photocatalysts for environmental control, and for sustainable development have been accepted for publication after peered review. In this topic issue, the readers will find very interesting results covering the following aspects (1) design and synthesis of photocatalysts with new morphology and active catalytic sites; (2) photocatalysts for green synthesis; (3) photocatalysts for CO<sup>2</sup> conversion to solar fules; (4) photocatalysts for wastewater treatment and air purification; and (5) revealing the photocatalysis reaction mechanism as applied in environmental problems.

For the g-C3N<sup>4</sup> based photocatalysts, Guan et al. synthesized Ti4O7/g-C3N<sup>4</sup> composites by a low temperature method. The enhanced photocatalytic activity for Ti4O7/g-C3N<sup>4</sup> could be ascribed to the promoted charge separation and photoabsorption efficiency. Yang et al. fabricated a monolithic g-C3N4/melamine sponge by a cost-effective ultrasonic-coating method. The monolithic g-C3N4/melamine demonstrated high photocatalytic activity for NO removal and CO<sup>2</sup> reduction. Guan et al. prepared the Ti4O7/g-C3N<sup>4</sup> photocatalysts by a hydrolysis method. The Ti4O7/g-C3N<sup>4</sup> exhibited remarkably improved photocatalytic activity for hypophosphite oxidation, which can be ascribed to the heterojunction structure of Ti4O7/g-C3N<sup>4</sup> that enhanced charge carrier efficiency (Guan et al.).

Xu et al. prepared BiVO<sup>4</sup> by a facile method and conducted a trapping experiment to study the free radical transformation mechanisms. They identified •OH and h<sup>+</sup> as the main active radicals for oxidation. Han et al. developed a new photoelectrochemical (PEC) technology for simultaneous SO<sup>2</sup> removal and H<sup>2</sup> production. The enhanced H<sup>2</sup> production and SO<sup>2</sup> removal efficiency can be attributed to the improved charge carrier transfer after Mo doping (Han et al.). Regmi et al. reviewed recent advances on the microbial decontamination by photocatalysts and their possible mechanisms are highlighted.

Cui et al. fabricated the Ag3PO4/MoS<sup>2</sup> nanocomposites and revealed that the improved performance of Ag3PO4/MoS<sup>2</sup> can be ascribed to wide spectra response, efficient charge separation and enhanced oxidation capacity. He et al. developed a two-step ZnO-modified strategy to immobilize the catalyst on rGO sheets. The high ammonia degradation efficiency of ZnO/Cu/rGO can be attributed to the enhanced ROSs production efficiency and the activated interfacial catalytic sites. Shi et al. prepare high energy faceted TiO<sup>2</sup> nanosheets by calcination of TiOF<sup>2</sup> cubes. The 500◦ C-calcined sample exhibits the highest photocatalytic activity for removal of acetone owing to the high energy TiO2-NSs and the surface adsorbed fluorine.

Kim et al. synthesized the nitrogen doped TiO<sup>2</sup> by a novel plasma electrolysis method. The 0.4 at.% N doped TiO<sup>2</sup> catalyst showed the highest photocatalytic performance. Xu et al. developed a BiOCl/NaNbO<sup>3</sup> p-n heterojunction by an in-situ method. The BiOCl/NaNbO<sup>3</sup> composites exhibited much

#### REFERENCES


enhanced photocatalytic activity attributed to the formation of p-n junction between NaNbO<sup>3</sup> and BiOCl that facilitated the charge separation (Xu et al.). Ren et al. synthesized the AgBr@Ag modified titanium phosphate composites. The AgBr@Ag/titanium phosphate exhibited higher photocatalytic activity and the photocatalytic degradation mechanisms were proposed.

She et al. reported selective activation of saturated C–H bond to generate the high-value-added chemicals on Ni-doped CdS nanoparticles. The high photocatalytic performance can be attributed to the cubic and hexagonal phases, Ni-doping and the charge carriers separation. Li et al. synthesized Au/BiFeO<sup>3</sup> homojunctions via a simple method. The Au1.2-BFO showed efficient photocatalytic activity due to the hierarchical structure, SPR effect of Au particles, and the defects (Li et al.). Zhang and Liang fabricated the new 2D g-C3N4@BiOCl/Bi12O17Cl<sup>2</sup> by a facile approach, which showed enhanced visible light absorption and electron-hole separation efficiency and thus highly enhanced photocatalytic activity for NO removal.

At last, as the Guest Editors of this topic issue, we would like to appreciate all the authors for the contributed articles and thank for all the referees for their comments on the manuscripts. We hope that the readers will find the results in articles of this topic issue interesting and useful for their research. Finally, we appreciate the editorial staff of Frontiers in Chemistry for their work in publishing of this topic issue.

#### AUTHOR CONTRIBUTIONS

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Dong, Zhang and Zhang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Facile Synthesis of Ternary g-C3N4@BiOCl/Bi12O17Cl<sup>2</sup> Composites With Excellent Visible Light Photocatalytic Activity for NO Removal

#### Wendong Zhang\* and Yi Liang

Chongqing Key Laboratory of Inorganic Functional Materials, Department of Scientific Research Management, Chongqing Normal University, Chongqing, China

#### Edited by:

Nicolas Keller, l'Environnement et la Santé (ICPEES), France

#### Reviewed by:

Dong Guohui, Shaanxi University of Science and Technology, China Bitao Liu, Chongqing University of Arts and Sciences, China Liwu Zhang, Fudan University, China

> \*Correspondence: Wendong Zhang w5i1@163.com

#### Specialty section:

This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry

Received: 19 August 2018 Accepted: 25 March 2019 Published: 11 April 2019

#### Citation:

Zhang W and Liang Y (2019) Facile Synthesis of Ternary g-C3N4@BiOCl/Bi12O17Cl2 Composites With Excellent Visible Light Photocatalytic Activity for NO Removal. Front. Chem. 7:231. doi: 10.3389/fchem.2019.00231 In this study, novel two-dimensional (2D) g-C3N4@BiOCl/Bi12O17Cl<sup>2</sup> composites have been fabricated through a facile deposition-precipitation process. The as-prepared photocatalysts were characterized by XRD, SEM, TEM, XPS, UV-vis DRS, PL, Photocurrent, EIS, ESR, and N<sup>2</sup> adsorption-desorption. The photocatalytic activities were investigated through NO removal test in gas under visible light irradiation (λ > 420 nm). The g-C3N4@BiOCl/Bi12O17Cl<sup>2</sup> composites exhibit enhanced visible light absorption and photo-induced electron-hole separation efficiency, compared with pristine g-C3N<sup>4</sup> and BiOCl/Bi12O17Cl2. The intimated contact interfaces between g-C3N<sup>4</sup> and BiOCl/Bi12O17Cl<sup>2</sup> nanosheets are responsible for the more efficient photochemical interactions. The present work provides a new direction to develop a class of ternary g-C3N4-based visible-light-driven photocatalysts for environmental purification.

Keywords: facile synthesis, g-C3N4@BiOCl/Bi12O17Cl2 , visible light, photocatalytic activity, nitrogen oxide (NO) removal

## INTRODUCTION

In the past decades, with the rapid development of modern industrial society, a large amount of highly harmful and toxic contaminants have been discharged into environmental system, which have been the focus of world attention (Han et al., 2017; Li et al., 2018). It remains a great challenge to completely achieve the degradation of environmental contaminants through the conventional treatment process, especially for the low-level concentrations of contaminants (Jiang et al., 2017, 2018; Zhong et al., 2017).

Photocatalysis, as a novel technique, have potential application in degradation low-level concentrations of environmental contaminants under visible-light irradiation (Xiong et al., 2015; Zheng and Zhang, 2016; Guan et al., 2017; Jin et al., 2018). Up to now, although a large number of photocatalysts have been explored for environmental purification, most of them still suffer from the limited utilization of solar light, resulting in quite low visible light photocatalytic activity (Chibac et al., 2017; Zhang et al., 2017). Hence, it is desirable to develop highly efficient visible-light-driven photocatalysts for satisfying the requirements of practical applications.

**8**

Currently, graphitic carbon nitride (g-C3N4), an organic semiconductor photocatalyst, has attracted intensive research interest in energy conversion and environmental remediation fields, largely due to its typical physicochemical properties, such as suitable band gap, good chemical and thermal stability, environmental friendly, etc (Wang et al., 2011; Dong and Zhang, 2013; Dong et al., 2015). However, the photocatalytic efficiency of g-C3N<sup>4</sup> is far below the requirement of practical applications, mainly due to its fast recombination of photoexcited electron-hole pairs. So far, various strategies have been developed to improve the photocatalytic performance of g-C3N4, including electronic structure engineering, nanostructure optimization, and heterojunction construction (Cao et al., 2015; Zhao et al., 2015; Ong et al., 2016). Furthermore, it is well-known that the g-C3N<sup>4</sup> nanosheets can provide a good two-dimensional surface and interface platform for growth of other nanostructured semiconductors. In particular, the heterojunction with other semiconductor photocatalysts has been regarded as an attractive and effective solution in enabling the efficient separation of photo-excited electronhole pairs, which not only helps to prolong the life-time of photo-excited charge carriers, but also endows g-C3N4-based heterojunctions with more abundant active sites, leading to the significantly enhancement of photocatalytic performance. Successful examples include g-C3N4/TiO<sup>2</sup> (Wei et al., 2016), g-C3N4/MoS<sup>2</sup> (Hou et al., 2013), g-C3N4/BiOBr (Sun et al., 2014), g-C3N4/WO<sup>3</sup> (Huang et al., 2013), g-C3N4/CdS (Liu, 2015), g-C3N4/g-C3N<sup>4</sup> (Dong et al., 2013), g-C3N4/(BiO)2CO<sup>3</sup> (Zhang et al., 2014), g-C3N4/graphene (Kim et al., 2016), and g-C3N4/Ag (Olga et al., 2016), etc. However, little information about ternary g-C3N4@BiOCl/Bi12O17Cl<sup>2</sup> heterojunction for visible light photocatalytic removal of NO has been reported.

Here, we report a highly cost-effective method based on the in situ self-assembly of BiOCl/Bi12O17Cl<sup>2</sup> binary nanoplates onto the surface of g-C3N<sup>4</sup> nanosheets at room temperature. It takes into consideration advantages of well-matched band structures

among g-C3N4, BiOCl and Bi12O17Cl2, the as-prepared ternary g-C3N4@BiOCl/Bi12O17Cl<sup>2</sup> heterojunctions exhibit apparent characteristics including larger surface area, improved visible light absorption ability, and efficient separation of photo-induced charge carries, which are extremely favorable for improving the photocatalytic activity.

# Fabrication of g-C3N4@BiOCl/Bi12O17Cl<sup>2</sup> Composites

The pure g-C3N<sup>4</sup> nanosheets and TiO<sup>2</sup> powders were fabricated according to the previous reports (Dong et al., 2011; Zhang et al., 2014), respectively. It's a typical synthesize that 1.33 g of BiCl<sup>3</sup> and 0.25 g of as-obtained g-C3N<sup>4</sup> were added to 50 mL absolute ethyl alcohol and then were ultrasonicated for 30 min. Afterwards, dripping 12.6 mL of NaOH solution (2.0 mol/L) dropwise into BiCl<sup>3</sup> solution and then stirring vigorously for 4 h at room temperature. After that, filtering and washing the resulting precipitate for times with distilled water and ethanol. The final samples were obtained after drying under vacuum at 60◦C for 24 h. Five types of samples were prepared. The mass rations of BiOCl/Bi12O17Cl<sup>2</sup> to g-C3N<sup>4</sup> are 1:2, 1:4, 1:1, 2:1, and 4:1, respectively. Accordingly, the final samples were labeled as BOC-CN-1-2, BOC-CN-1-4, BOC-CN-1-1, BOC-CN-2-1, and BOC-CN-4-1, respectively. The pure BiOCl/Bi12O17Cl<sup>2</sup> samples were synthesized under the same conditions without adding g-C3N<sup>4</sup> nanosheets, and the g-C3N4@BiOCl/Bi12O17Cl<sup>2</sup> sample was labeled as BOC.

## Materials Characterization

In a typical analysis of the crystal phase of the as-obtained samples, X-ray diffraction with Cu Kα radiation (XRD: model D/max RA, Japan) was applied. Meanwhile scanning electron microscope (SEM, JEOL model JSM-6490, Japan) and transmission electron microscopy (TEM: JEM-2010, Japan) were utilized to characterize the morphology and structure. And the surface properties were examined by X-ray photoelectron spectroscopy with Al Kα X-rays (hν = 1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA). By using a Scan UV-vis spectrophotometer (UVvis DRS: UV-2450, Shimadzu, Japan) equipped with an integrating sphere assembly and BaSO<sup>4</sup> as reflectance sample, the UV-vis diffuse reflection spectra was gained. Nitrogen adsorption-desorption was conducted on a nitrogen adsorption apparatus (ASAP 2020, USA) to insure the specific surface areas and total pore volumes. Photoluminescence (PL: F-7000, HITACHI, Japan) was used to investigate the charge transfer properties. ESR spectrometer (FLsp920, England) was applied to detect the electron spin resonance (ESR) signals of •OH and •O − 2 , respectively. The photocurrent measurements (CHI 660B electrochemical system: Shanghai, China) and electrochemical impedance spectroscopy (EIS) were carried out to analyze the photo-generated charge separation properties. All the samples were degassed at 150◦C prior to measurements.

#### Appraisement of Photocatalytic Activity

The photocatalytic activity was investigated by removal of NO at ppb (1 × 10−<sup>9</sup> ) levels in a continuous flow reactor at ambient

FIGURE 4 | SEM (a,b) and TEM (c,d) images of BOC-CN. EDS elemental mapping (e–k) of the same region, indicating the spatial distribution of Bi (g), C (h), Cl (i), N (j), and O (k), respectively.

temperature. The volume of the rectangular reactor, made of stainless steel and covered with Saint-Glass, was 4.5 L (30 × 15 × 10 cm). A 100-W commercial tungsten halogen lamp (THL100, Beijing, China) was vertically placed outside the reactor, and the light spectra range (see **Supplementary Figure 1**) from 350 to 2,400 nm. A UV cut-off filter (Aike UBG-420, Shenzhen, China) was adopted to remove UV light in the light beam. Photocatalyst (0.2 g) was coated onto a dish with a diameter of 12.0 cm, and the irradiance received by the photocatalyst powder is about 0.66 W/cm<sup>2</sup> . The coated dish was then pretreated at 70◦C to remove water in the suspension. The catalyst adhesion on the dish was firm enough to avoid the erosion (or removal) of the catalyst during air flowing. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 × 10−<sup>6</sup> NO (N<sup>2</sup> balance, BOC gas). The initial concentration of NO was diluted to about 500 × 10−<sup>9</sup> by the air stream. The desired relative humidity (RH) level of the NO flow was controlled at 50% by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 2.4 L·min−<sup>1</sup> by a mass flow controller. After the adsorption-desorption equilibrium was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., 42i-TL), which monitors NO, NO2, and NO<sup>x</sup> (NO<sup>x</sup> represents NO + NO2) with a sampling rate of 1.0 L·min−<sup>1</sup> . The removal ratio (η) of NO was calculated by η (%) = (1-C/C0) ×100%, where C and C<sup>0</sup> are concentrations of NO in the outlet stream and the feeding stream, respectively.

# RESULTS AND DISCUSSION

The phase structures of the obtained samples were investigated by XRD patterns. As shown in **Figure 1**, the BOC sample exhibits typical XRD peaks of BiOCl and Bi12O17Cl<sup>2</sup> (He et al., 2016; Huang et al., 2016), thus suggesting the formation of BiOCl/Bi12O17Cl<sup>2</sup> heterojunctions, the result is good agreement with the previous reports (Zhang et al., 2017). For the pure CN sample, the typical diffraction peaks appeared at 27.4 and 13.1◦ are indexed to the g-C3N<sup>4</sup> (002) and (100) planes (Wang et al., 2011; Cao et al., 2015), respectively. Based on further observation, the peak intensity of the CN decreased with increasing BOC content, implying that interactions exist between the BOC and CN. Finally, two peaks of CN cannot be observed in the BOC-CN composites, which can be ascribed to a good dispersion of BOC onto the surface of CN.

The morphologies and microstructures of BOC, CN, and BOC-CN were characterized by SEM and TEM. For BOC samples (**Figure 2**), the layered and irregular microstructures consist of smooth nanosheets with different sizes. Moreover, there are two different fringes with the lattice spacing of 0.59 and 0.25 nm (**Figure 2d**), which can be indexed to the (006) crystal plane of Bi12O17Cl<sup>2</sup> and (003) crystal plane of BiOCl, respectively. For CN samples (**Figure 3**), it presents the lamellar morphology is composed of numerous nanosheets with a much looser pore structure. As shown in **Figures 4a–d**, the BOC-CN composites also consist of a large number of layered nanosheets with different shapes. Obviously, the BOC nanosheets were in situ growth on the surface of g-C3N4, resulting in the formation of closely interface in g-C3N4@BiOCl/Bi12O17Cl<sup>2</sup> composites, which is beneficial for the separation and transfer of the photo-induced electron-hole pairs. The surface element dispersion state of BOC-CN composites are studied by EDS mapping. As shown in **Figures 4e–k**, the Bi, C, Cl, N, and O elements are uniformly distributed in BOC-CN samples.

The XPS measurements were applied to verify the composition and chemical state of the elements. Two peaks at 159.1 and 164.3 eV are consistent with Bi4f7/<sup>2</sup> and Bi4f5/<sup>2</sup> (**Figure 5A**), respectively. The XPS spectrum for Cl shows two peaks at 197.8 and 199.4 eV attributed to Cl2p3/<sup>2</sup> and Cl2p1/<sup>2</sup> (**Figure 5B**), respectively. The peak centered at 530.3 eV that corresponds to the binding energy of O 1s (**Figure 5C**) (Bi et al., 2016; Zhang et al., 2017). The C peak at 284.8 eV can be ascribed to the adventitious carbon atom. **Figure 5D** shows one peak at 288.5 is identified as overlapped peaks of N-C=N and the O-C=O. Two different peaks are observed in **Figure 5E**, the N peak at 398.9 eV correspond to the C=N-C and the N

peak at 400.8 eV is attributed to the residual amino groups (Zhang et al., 2014).

The optical absorption property of the as-obtained samples was investigated by UV-vis DRS. As can be seen from **Figure 6A**, BOC and CN exhibit absorbance edge around 550 and 475 nm, respectively, displaying that BOC and CN possess good visible light absorption ability. Interestingly, BOC-CN-4-1 shows relatively stronger visible light absorption ability lies in the range of 550–800 nm, because of the synergetic effect between BOC and CN. **Figure 6B** shows the PL spectra for BOC, CN,

and BOC-CN-4-1 using the exciting light of 320 nm. Compared to CN, the PL intensity of BOC-CN-4-1 significantly decreases, demonstrating that the interface interaction between BOC and CN could inhibit the recombination rate of photo-generated electrons and holes.

The photocurrent and electrochemical impedance experiments were used to investigated the photo-generated charges separation and transfer property of BOC, CN, and BOC-CN-4-1 samples under visible light irradiation (Zhang et al., 2018a). Compared with the pure BOC and CN, it was interesting to find that BOC-CN-4-1 exhibits significantly enhanced photocurrent density (**Figure 7A**), suggesting that BOC-CN-4-1 possesses higher photo-generated charge separation property. As can be seen from the **Figure 7B**, the arc radius on the EIS Nyquist plot of BOC-CN-4-1 was smaller than that of the pure BOC and CN, demonstrating that BOC-CN-4-1 has much more efficient photo-generated charge separation and transfer property.

**Figure 8A** shows that the adsorption-desorption isotherms of all the samples are type IV according to the IUPAC classification, demonstrating that all the samples have mesopores (Dong et al., 2013; Zhang et al., 2014). However, the typical H3 hysteresis loop at low pressure can be ascribed to the aggregation of nanosheets with slit-like pores. **Figure 8B** further confirms the presence of mesopores in BOC, CN, and BOC-CN samples. In addition,

the specific surface area (SBET) and total pore volume (Vp) are 32.8 m<sup>2</sup> /g and 0.176 cm<sup>3</sup> /g for BOC-CN composites, which are higher than those of pure BOC (29.7 m<sup>2</sup> /g and 0.168 cm<sup>3</sup> /g) and CN (21.8 m<sup>2</sup> /g and 0.132 cm<sup>3</sup> /g), the enlarged SBET and V<sup>p</sup> of BOC-CN composites can be attributed to the stack of nanosheets layered by layered. The enlarged SBET and V<sup>p</sup> of BOC-CN composites can facilitate the reactants adsorb and transfer, and provide more active sites for the photocatalytic reaction.

The electron spin resonance (ESR) experiments were further used to confirm the active species during the photocatalytic reaction process under visible light irradiation. As shown in **Figures 9A,B**, the superoxide (•O − 2 ) radicals and hydroxyl (•OH) radicals have been successfully detected by the ESR technique, respectively. Moreover, the intensity of all peaks increase significantly with the irradiation time, demonstrating that •O − 2 and •OH are continuously generated during the reaction. The result shows that both •O − 2 and •OH are the main photocatalytic reaction active species. The formation of •O − 2 and •OH from the photochemical reaction shows in Equations 1–3 (Zhang et al., 2018a).

$$\text{e}^- + \text{O}\_2 \rightarrow \text{\textbullet O}\_2^- \tag{1}$$

$$\text{\textbullet O}\_2^- + 2\text{H}^+ + \text{e}^- \rightarrow \text{H}\_2\text{O}\_2 \tag{2}$$

$$\mathrm{H}\_{2}\mathrm{O}\_{2} + \mathrm{e}^{-} \rightarrow \bullet \mathrm{OH} + \mathrm{OH}^{-} \tag{3}$$

Furthermore, the possible mechanism for the photocatalytic reaction at the BOC-CN interface are presented in **Figure 10**. After the visible light irradiation, the Bi12O17Cl<sup>2</sup> and g-C3N<sup>4</sup> can be excited and then produce electron-hole pairs (Dong et al., 2013; Zhang et al., 2018b). On the one hand, the excited electrons in CB of g-C3N<sup>4</sup> can directly transfer to CB of BiOCl (Zhang et al., 2013). On the other hand, the excited holes in VB of Bi12O17Cl<sup>2</sup> can transfer to VB of g-C3N4, the excited electrons in CB of g-C3N<sup>4</sup> can transfer to CB of Bi12O17Cl2, and the electrons can further transfer to the CB of BiOCl from the CB of Bi12O17Cl2, the band structures of the three components are well-matched resulting in efficient separation and transfer of the photo-induced carriers. Hence, the suitable band structures clearly show that the efficient electron-hole pair separation plays critical role in improving the photochemical reaction (Zhang et al., 2013, 2014).

The visible-light-induced photocatalytic activities of the TiO2, BOC, CN, and BOC-CN samples toward NO were revealed in **Figure 11A**. The pure BOC and CN only removed 36.2 and 14.6% of NO after 30 min visible light irradiation due to its fast recombination of photo-induced carriers, respectively. However, the visible-light-induced photocatalytic activities of the TiO<sup>2</sup> sample can be neglected under the same conditions, indicating that the activity does not result from a UV-A light induced photocatalytic activity due to some trace of UV-A light after the cut-off filter. When the heterojunction was formed, the NO removal ratio over BOC-CN composites was increased to 46.8%. For ruling out and evidencing the NO degradation, the adsorption experiment of the optimized BOC-CN-4-1was carried out under dark condition, the result shows that adsorption property of NO over the BOC-CN-4-1-dark sample can also be ignored. The photocatalytic stability experiment of the BOC-CN-4-1 sample was evaluated by repeating the reaction for five runs under visible light irradiation. As shown in in **Figure 11B**, the photocatalytic performance shows slightly loss after five run, indicating that BOC-CN-4-1 photocatalyst possesses good photocatalytic stability. Interestingly, the BOC-CN composites exhibit even higher visible light photocatalytic activity than that of BiOBr/C3N<sup>4</sup> (removal rate of 32.7%) and p-doped g-C3N<sup>4</sup> (removal rate of 42.3%) (Sun et al., 2014; Zhang et al., 2016). The enhanced photocatalytic activity of BOC-CN can be attributed to the synergistic contribution of BOC and g-C3N<sup>4</sup> with respect to the suitable band structure, enlarged SBET and Vp, improved visible light absorption, and efficient photo-induced carrier separation at the interface of BOC and g-C3N<sup>4</sup> (Dong et al., 2013; Hou et al., 2013; Sun et al., 2014; Liu, 2015; Wei et al., 2016).

#### CONCLUSION

In summary, we have synthesized ternary BOC-CN heterojunctions with outstanding visible light photocatalytic

performance by self-assembly of BiOCl/Bi12O17Cl<sup>2</sup> nanosheets on the surface of g-C3N<sup>4</sup> nanosheets via a chemical depositionprecipitation method. The results reveal that g-C3N4, BiOCl, and Bi12O17Cl<sup>2</sup> possess well-matched band structures, which is helpful to the separation and transport of photo-induced carriers. This work provides a new perspective for the design and fabrication of high performance and stable BiOCl/Bi12O17Cl2-based photocatalysts via a facile method at room temperature.

#### AUTHOR CONTRIBUTIONS

WZ: experiment, data analysis, and paper writing. YL: paper writing.

#### REFERENCES


#### FUNDING

This research is financially supported by the National Natural Science Foundation of China (No. 51708078), Natural Science Foundation of Chongqing (CSTC, 2018jcyjA1040), and the Chongqing Postdoctoral Science Foundation funded project (No. Xm2016027).

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2019.00231/full#supplementary-material

Supplementary Figure 1 | The light spectra range of tungsten halogen lamp.


macroscopic visible-light photocatalyst. Chin. J. Catal. 2, 313–320. doi: 10.1016/S1872-2067(16)62568-8


at room temperature. Ind. Eng. Chem. Res. 52, 6740–6746. doi: 10.1021/ie 400615f


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Zhang and Liang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# In situ Synthesis of Au-Induced Hierarchical Nanofibers/Nanoflakes Structured BiFeO<sup>3</sup> Homojunction Photocatalyst With Enhanced Photocatalytic Activity

#### Yan'an Li <sup>1</sup> , Jiao Li <sup>1</sup> \*, Long Chen<sup>1</sup> \*, Haibin Sun<sup>1</sup> , Hua Zhang<sup>1</sup> , Hong Guo<sup>2</sup> and Liu Feng<sup>2</sup>

*<sup>1</sup> School of Materials Science and Engineering, Shandong University of Technology, Zibo, China, <sup>2</sup> Analysis & Testing Center, Shandong University of Technology, Zibo, China*

In order to further improve the photocatalytic performance of BiFeO<sup>3</sup> (BFO), novel Au-induced hierarchical nanofibers/nanoflakes structured BiFeO<sup>3</sup> homojunctions (Au*x*-BFO, *x* = 0, 0. 6, 1.2, 1.8, 2.4 wt%) were *in situ* synthesized through a simple reduction method with assist of sodium citrate under the analogous hydrothermal environment. The effect of loading amount of Au nanoparticles (NPs) on the physicochemical properties and photocatalytic activity was investigated in detail. The Au1.2-BFO NFs sample show the best photocatalytic activity (85.76%), much higher than that for pure BFO samples (49.49%), mainly due to the hierarchical nanofibers/nanoflakes structured homojunction, the surface plasmon resonance (SPR) effect of Au NPs, as well as the presence of defects (Fe2+/Fe3<sup>+</sup> pairs and oxygen vacancy). Furthermore, the possible formation mechanism of the unique homojunction and the enhanced photocatalytic mechanism for the degradation of methylene blue (MB) dye are proposed. It is proven that holes (h+) play the decisive role in the photocatalytic process. The present work provides a fascinating way to synthesize efficient homojunctions for the degradation of organic pollutes.

Keywords: bismuth ferrite, homojunction, SPR effect, defects, photodegradation

# INTRODUCTION

Nowadays, energy crisis and environmental deterioration issues are severely detrimental for economic development and human health (Wang et al., 2013). Semiconductor photocatalysis as an efficient and green technology can convert solar energy into chemical energy to dispose of these issues (Wang et al., 2013; Wang H. et al., 2017). In order to fully exploit the solar energy, it is necessary to explore visible light responsive photocatalysts with excellent photocatalytic activities (Zhang et al., 2017).

Among the variety of visible-light-driven semiconductor materials, BiFeO<sup>3</sup> (BFO) has attracted a great deal of attention due to its narrow band gap (2.2∼2.5 eV), good chemical stability and low cost (Wang et al., 2013; Niu et al., 2015). In addition, the room temperature multiferroic property makes it easily recovered from the treated water to avoid the secondary pollution (Zhang et al., 2017). However, the rapid recombination rate of photogenerated electron-hole pairs and low quantum yield limit its practical application (Huo et al., 2011; Srivastav and Gajbhiye, 2012; Niu et al., 2015). Therefore, much efforts have been made to improve its photocatalytic performance, such as element doping (Wang et al., 2012; Ahmada et al., 2017; Irfan et al., 2017; Yang et al., 2018),

#### Edited by:

*Yuxin Zhang, Chongqing University, China*

#### Reviewed by:

*Dong Guohui, Shaanxi University of Science and Technology, China Xiaoying Liu, Chongqing Technology and Business University, China*

#### \*Correspondence:

*Jiao Li haiyan9943@163.com Long Chen lchen830@sdut.edu.cn*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *03 September 2018* Accepted: *12 December 2018* Published: *04 January 2019*

#### Citation:

*Li Y, Li J, Chen L, Sun H, Zhang H, Guo H and Feng L (2019) In situ Synthesis of Au-Induced Hierarchical Nanofibers/Nanoflakes Structured BiFeO3 Homojunction Photocatalyst With Enhanced Photocatalytic Activity. Front. Chem. 6:649. doi: 10.3389/fchem.2018.00649*

**18**

morphology control (Mohan and Subramanian, 2013; Zhang Q. et al., 2016), noble metal deposition (Li S. et al., 2013; Zhang et al., 2015), and semiconductor coupling (Li Z. et al., 2013; Humayun et al., 2016).

Compared to that of bulk materials, the reduced radial dimension and the extremely large surface-to-volume ratio of one-dimension (1D) nanofibrous structure can promote the rapid transfer of photogenerated charge carriers to the surface, and thus enhance the separation efficiency (Li S. et al., 2013). Furthermore, the perpendicular transport direction of photogenerated electrons and holes can further effectively inhibit the recombination of photogenerated electron-hole pairs (Mohan and Subramanian, 2013). Therefore, the synthesis of BFO sample with nanofibrous structure is in favor of enhancing the photocatalytic activity.

Noble metal (e.g., Ag, Au, Pt, etc.) deposition is also an efficient way to enhance the photocatalytic activity (Li et al., 2015; Niu et al., 2015; Zhang et al., 2015). Among them, Au nanoparticles (NPs) exhibit excellent a chemical stability and a characteristic absorption peak in the visible wavelength range due to the strong surface plasmon resonance (SPR) effect (Li S. et al., 2013; Li et al., 2015). The deposited Au NPs can not only act as electron-trapping centers to reduce the recombination rate of photogenerated electron-hole pairs, but also can act as light harvesters enhancing the light absorbing ability and as catalytic sites for the photocatalytic reaction (Lin et al., 2017; Chiu et al., 2018). Significantly, the formed Schottky junctions between Au NPs and BFO nanofibers (NFs) can adjust the interfacial band structure and thus promote the separation and transfer of photogenerated charge carriers.

In comparison with heterojunction built with different semiconductors. Besides the band alignment of two semiconductors, the creation of heterojunctions is also dependent on other properties of semiconductor, such as electron affinity and work function (Ye et al., 2017). The homojunction is constructed by the same semiconductor materials with different crystal phases, exposing facets or semiconductor types, etc., which can avoid the disadvantages of heterojunction fabrication (Huang et al., 2017). The enhanced photocatalytic activity of heterojunctions or homojunctions is mainly dependent on the space charge accumulation or depletion at the interfaces of two phases (Ye et al., 2017).The homojunction structure can also introduce an internal field to effectively facilitate surface charge separation, retard the recombination of photogenerated carriers, and thus remarkably improve the photocatalytic performance. However, there are few reports on the fabrication of BFO homojunction, especially for the aspect of in situ synthesis.

In this work, an distinctive Au NPs deposited BFO homojunction, in which BFO nanoflakes are assembled on the surface of BFO NFs, was in situ synthesized through a simple reduction method under the analogous hydrothermal environment. The effect of loading amount of Au NPs on the physicochemical properties and photocatalytic activity was investigated in detail. Furthermore, it proposed a possible formation mechanism of the unique homojunction and the enhanced photocatalytic mechanism for the degradation of methylene blue (MB) dye over Aux-BFO NFs under simulated solar light irradiation.

# EXPERIMENTAL

# Preparation of Aux-BFO NFs

BFO NFs (Aux-BFO NFs, x = 0 wt%) were prepared via a sol-gel method combined with an electrospinning technique. All the chemical reagents used for the synthesis were analytical grade. Bismuth nitrate (Bi(NO3)3·5H2O) and ferric nitrate (Fe(NO3)3·9H2O) with the Bi: Fe molar ratio of 1.11:1 (to make up for the loss of Bi ions during the calcination process) were dispersed in methylglycol by ultrasonic to form a homogeneous solution. Then the viscosity and pH value of the solution were adjusted by 2.5 mL of glacial acetic acid, called solution A. g of poly(vinylpyrrolidone) (PVP, Mw = 1,300,000) was added to the mixed solvent of N'N-dimethylformamide (DMF) and ethanol with the volume ratio of 2:1, called solution B. Solution B was added dropwise to solution A with constantly stirring for 24 h to form the pre-spinning solution which has a pH of 2.0. The pre-spinning solution was loaded into a plastic syringe connected with a stainless needle and then fixed onto the electrospinning system. An optimized high voltage of 15 kV was applied to the needle with the flow rate of 0.4 mL/h. The distance was 20 cm from the needle to the rotating drum collector. The as-spun NFs were dried at 60◦C for 4 h, calcined at 350◦C for 30 min with a heating rate of 2◦C/min, and then calcined at 600◦C for 120 min with a heating rate of 5◦C/min in air.

Au NPs deposited BFO NFs (Aux-BFO NFs, x = 0.6, 1.2, 1.8, 2.4 wt%) were synthesized by a simple reduction method with sodium citrate. In a typical procedure, 57.38 mg of BFO NFs were first dispersed in 50 mL of deionized water, followed by adding a certain amount of HAuCl<sup>4</sup> (3 mmol/L) and sodium citrate (0.04 mol/L). Then the suspension was kept stirring at 150◦C for 30 min in oil bath. After cooled to room temperature naturally, the obtained products were centrifuged, washed with deionized water and absolute ethanol for several times, and then dried at 80◦C for 24 h. The loading amount of Au NPs on BFO NFs were 0.6, 1.2, 1.8, and 2.4 wt% (mass ratio of Au to BFO), which were defined as Au0.6-BFO NFs, Au1.2-BFO NFs, Au1.8-BFO NFs, and Au2.4-BFO NFs, respectively.

# Characterization

The phase structures were identified by X-ray diffraction (XRD; D8 Advance, Bruker AXS, German) with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 50 mA in a 2θ ranging from 20 to 70◦ . The morphologies were observed by field emission scanning electron microscope (FE-SEM; Sirion 200, FEI, USA). The transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were performed at a field emission transmission electron microscope (TEM; Tecnai G2 F20 S-TWIN, FEI, USA) with an accelerating voltage of 200 kV. UV-vis diffuse reflectance spectra (DRS) were measured at room temperature by UV-Vis spectrometer (UV-Vis; UV-3600plus, Shimadzu, Japan) with the wavelength range of 350–800 nm using BaSO<sup>4</sup> as a reflectance. The measurements of magnetic properties were carried out by a superconducting quantum interference device magnetometer (FM, MPMS XL-7, Qunatum Design, USA). The element chemical states were characterized by X-ray photoelectron spectrometer (XPS; PHI-5300, PHI, USA) with an Al Kα X-ray radiation. The photoluminescence (PL) spectra were measured on the fluorescence spectrophotometer (PL; Hitachi F-4600, Hitachi, Japan) with the excitation wavelength of 230 nm and a 150 W xenon lamp as the light source.

# Photocatalytic Activity Evaluation

Twenty five milligram of Aux-BFO NFs were added to 30 mL of 10 mg/L MB solution with ultrasound for 10 min. After stirring in the dark for 30 min to reach the adsorptiondesorption equilibrium, the suspension system was exposed to simulated solar light irradiation (CHF-XM-500W xenon lamp, Beijing Perfactlight Company, China). During the photocatalytic reaction process, the suspension was taken out at every 30 min interval and centrifuged to remove the photocatalyst. The MB concentration was measured by a UV-vis spectrophotometer at the wavelength of 666 nm.

# RESULTS AND DISCUSSION

# XRD Analysis

The XRD patterns of Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%) are shown in **Figure 1**. All the diffraction peaks of pure BFO NFs can be indexed to the rhombohedral perovskite structure with R3m space group of the BiFeO<sup>3</sup> phase (JCPDS card no. 86–1518) (Chen et al., 2017). The strong diffraction peaks also imply the good crystallinity. In addition, the crystalline structures of BFO NFs are not changed after depositing Au NPs.

# Microstructures

**Figure 2** shows the SEM images of Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%). It can be seen from **Figure 2A** that the pure BFO NFs exhibit an uniform nanofibrous structure with compactly packed and continuous nanoparticles. The diameter of BFO NFs is about 100–200 nm. As shown in **Figures 2B–E**, there is no

obvious change of the nanofibrous structure for BFO NFs with Au NPs deposition, but some thin nanoflakes are embedded in the surface of nanofibers, and the number of nanoflakes increases with the increase of the loading amount of Au NPs.

The EDS mapping is performed to clarify the chemical composition and element distribution in Au1.2-BFO NFs, as shown in **Figure 3**. It can be clearly seen that Bi, Fe, and O elements are uniformly distributed throughout the single nanofiber (**Figures 3A–D**). Au NPs are dispersed well on BFO NFs, instead of agglomerating together (**Figure 3E**). More importantly, The EDS result (**Figure 3F**) recorded at the "+" position in **Figure 3A** shows that the main elements of the nanoflake are Bi, Fe, Au, and O. It is worthy to note that the atomic ratio of Bi/Fe is almost 1:1, nearly corresponding to the composition of BFO. The trace amount of Au shows that Au NPs are also deposited on the nanoflake.

In order to further illustrate the morphology and microstructure of pure BFO NFs and Au1.2-BFO NFs, TEM technology is used. As shown in **Figure 4A**, the pure BFO sample is nanofibrous structure compactly packed with nanoparticles. The HRTEM image in **Figure 4D** is collected from the square area in **Figure 4A**. The interplanar spacing of 0.278 nm corresponds to the (110) lattice plane of the rhombohedral perovskite BFO structure. **Figure 4B** shows the TEM image

FIGURE 3 | (A) The SEM image of a single representative Au1.2-BFO NFs, the corresponding elemental mappings of (B) Bi, (C) Fe, (D) O, (E) Au elements, and (F) EDS pattern.

of Au1.2-BFO NFs. It can be seen that the morphology is still the nanofibous structure. However, the surface of the nanofiber becomes rougher and the composed nanoparticles appear to be more prominent after depositing the Au NPs on the surface of BFO NFs. Furthermore, new nanoflake-like structures are formed and embedded in the BFO NFs. In order to confirm the structure of nanoflakes, the HRTEM is used, and the result of which is shown in **Figure 4E**. The interplanar spacing of 0.397 nm is ascribed to the (012) lattice plane of the rhombohedral perovskite BFO structure, further implying that the new formed nanoflake-like structure is indeed BFO phase. Therefore, homojunctions will be formed between BFO NFs

and BFO nanoflakes. From **Figure 4B**, there is no obvious Au NPs deposited on the BFO surface. In order to clearly observe the deposited Au NPs, the magnified TEM image is shown in **Figure 4C**. It can be seen that Au NPs with the size of about 5 nm are indeed deposited uniformly on the BFO surface. The according HRTEM image is shown in **Figure 4F**. The interplanar spacing of 0.236 nm matches well with the (111) lattice plane of the cubic Au structure (Wang P. et al., 2017).

The possible formation mechanism of the Au NPs deposited hierarchical nanofibers/nanoflakes structured BFO homojunction is proposed in **Figure 5**. Firstly, the HAuCl<sup>4</sup> is decomposed to AuCl<sup>3</sup> and HCl in the aqueous solution. BiFeO3, being regarded as the product of a combination of Bi2O<sup>3</sup> and Fe2O3, is soluble in the weak acidic environment (Sakar et al.,

FIGURE 7 | *M–H* hysteresis loops of Au*x*-BFO NFs (*x* = 0, 0.6, 1.2, 1.8, 2.4 wt%) measured at room-temperature; the inset shows the partially magnified curves.

2013). Therefore, the generated HCl can decompose the BiFeO<sup>3</sup> to generate Bi3<sup>+</sup> and Fe3<sup>+</sup> ions in the solution (**Figure 5A**). In particular, the decomposition reaction prefers to occur at the defects on the surface of BFO NFs where the higher surface energy have. The defects may be generated during the oxidation decomposition of organic material (PVP) under the high calcination temperature. Secondly, the citrate present in the solution can act as a chelating agent to react with Bi3+, Fe3+, and Au3<sup>+</sup> ions to form stable metal-chelate complexes (**Figure 5B**). In addition, due to the presence of defects, the citrate can anchor on the surface of BFO NFs (Mudunkotuwa and Grassian, 2010). Finally, the uniform Au NPs and nanoflake-like BFO structure are formed on the surface of BFO NFs under the effect of the citric acid in the analogous hydrothermal environment (**Figure 5C**). With the increase of the HAuCl<sup>4</sup> content (the increase of the loading amount of Au NPs), more Bi3<sup>+</sup> and Fe3<sup>+</sup> ions pass from the defects on the surface of BFO NFs to the solution, and more nanoflake-like BFO structure are formed.

### XPS Analysis

The chemical state and elemental composition of Au1.2-BFO NFs are analyzed by XPS technology. According to the overall XPS survey (**Figure 6A**), only Bi, Fe, O, Au, and C elements can be detected in the sample. The peak of C 1s located at 284.5 eV is used for calibration (Dou et al., 2015; Chen et al., 2016). As shown in **Figure 6B**, the two peaks centered at around 158.11 eV and 163.42 eV can be assigned to the binding energy of Bi 4f7/<sup>2</sup> and Bi 4f5/2, respectively. It is confirmed that the bismuth species in Au1.2-BFO NFs is the oxidation state of +3. From the high resolution XPS spectra of Fe 2p in **Figure 6C**, the three peaks located at 710.79, 717.12, and 723.20 eV can be ascribed to the Fe 2p3/2, satellite, and Fe 2p1/<sup>2</sup> peaks, respectively. In addition, the Fe 2p3/<sup>2</sup> peak can be fitted into two peaks located at 711.05 and 709.11 eV, which are attributed to Fe3<sup>+</sup> and Fe2<sup>+</sup> species, respectively (Wu et al., 2016). The presence of Fe2<sup>+</sup> species may come from the reduction of Fe3<sup>+</sup> species by citric acid under high temperature environment. In order to meet

the requirement of charge equilibrium, oxygen vacancy should be formed accompanied by the Fe2<sup>+</sup> species. The O 1s curve in **Figure 6D** can be splitted into two peaks with Lorentzian-Gaussian function. The peaks at 531.51 and 528.96 eV can be ascribed to the surface adsorbed oxygen (Oads) and lattice oxygen (Olatt) species, respectively (Bharathkumar et al., 2016; Chen et al., 2016). The Oads species may originate from the –OH groups bonded to the metal cations in the oxygen defcient region (Guo et al., 2016; Sun et al., 2017), which can be converted to oxygenrelated free radicals to participate in the photocatalytic reaction. Moreover, the more Oads species usually mean the more oxygen vacancies it possesses, and also the better photocatalytic activity. From **Figure 6E**, the binding energies of 83.69 eV and 87.40 eV are ascribed to the Au 4f7/<sup>2</sup> and Au 4f5/<sup>2</sup> peaks, respectively, indicating that the gold is present in the form of metallic gold (Au<sup>0</sup> ) in the Au1.2-BFO NFs. This can be ascribed to the reduction by citric acid under high temperature environment. Therefore, the metallic gold have been successfully deposited on the surface of BFO NFs. Whereas, the binding energies of Au 4f in the Au1.2-BFO NFs are 0.3 eV below the standard binding energy of a metallic gold foil (Jovic et al., 2013), indicating the existence of electron transfer from BFO NFs to Au NPs for the different Fermi levels (Li S. et al., 2013). In addition, the loading amount of Au NPs is measured to be 1.14%, almost equal to the theoretical value of 1.2%.

#### Ferromagnetism Analysis

In order to investigate the effect of Au on the magnetic property of Aux-BFO NFs, the room-temperature magnetic hysteresis (M-H) loops are shown in **Figure 7**. It can be seen that all BFO NFs exhibit the weak-ferromagnetic (FM) characters, which is due to the suppressed cycloidal spin structure and the coexistence of Fe3<sup>+</sup> and Fe2+, thereby resulting in the FM spins in the system (Xu et al., 2014; Modak et al., 2016). The pure BFO NFs show the saturation magnetization (Ms) value of 0.32 emu/g. When Au NPs are deposited on the surface of BFO NFs, the Ms value is enhanced with the increase in the loading amount of Au NPs. The Ms values are 1.07, 1.19, 1.22, and 1.27 emu/g for BFO NFs with the loading amount of Au NPs of x = 0.6, 1.2, 1.8, and 2.4 wt%, respectively. The gradually enhanced magnetic properties of Aux-BFO NFs could be attributed to the new formed nanoflake-like structures, where the confined dimensions (15 nm) facilitate the enhancement of FM properties (Chauhan et al., 2012). In addition, the rather large magnetic moment of Au NPs explains the enhanced magnetic properties with increase in loading amount of Au NPs (Nealon et al., 2012). From the inset of **Figure 7**, an obvious off-center displacement in magnetic field axis is shown in the M–H curves. This can be ascribed to the exchange bias (EB) effect of antiferromagnet (AFM) - FM ordering in BFO NFs, where the AFM-core/FM-shelllike structure can be formed in the well-defined 1D structure (Sakar et al., 2016). Compared to that of the pure BFO NFs, the off-center displacement of Aux-BFO NFs is relatively less pronounced, which is ascribed to the enhanced FM spins caused by the emergence of nanoflake-like structures.

## Optical Properties

**Figure 8** shows the UV-vis DRS spectra of Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%) samples. It can be seen that the pure BFO NFs exhibit excellent absorption in both UV and visible light region. The absorption edge located at 575 nm is due to the bandgap transition. When the Au NPs are deposited on the surface of BFO NFs, the optical properties are obviously enhanced. In addition, the Aux-BFO NFs exhibit a weak absorption peak in the range of 600∼725 nm, which are attributed to the SPR effect of Au NPs (Villa et al., 2016; Wang P. et al., 2017). The enhanced absorption in visible light is in favor of boosting the photocatalytic activity. Moreover, the band gap energies can be calculated from the plots of the (αhν) 2 vs. the photon energy (hν) (Xu et al., 2015; Singh et al., 2018), the

results of which are shown in the inset of **Figure 8**. The band gap energies are calculated to be 2.11, 2.08, 2.02, 2.01, and 1.97 eV for the x = 0, 0.6, 1.2, 1.8, 2.4 wt%, respectively. It is obvious that the band gap energies decrease slightly with the increase in the loading amount of Au NPs. The possible reason is that the deposited Au NPs can produce some lower energy levels due to the formed Schottky junction at the interface between Au NPs and BFO NFs (Papadas et al., 2015).

# PL Analysis

Photoluminescence (PL) spectra are extensively used to study the transfer and separation of photogenerated electron-hole pairs in the semiconductor material. The peak intensity can reflect the capture, migration and transfer efficiency of photogenerated electron-hole pairs. The lower PL intensity usually means the lower recombination rate of photogenerated electron-hole pairs, and also the higher photocatalytic activity (Tang et al., 2018). It can be seen from **Figure 9** that the PL intensity decreases obviously once the Au NPs are loaded on the surface of BFO NFs. Significantly, the PL intensity of Au1.2-BFO NFs is the lowest. This indicates that the deposition of Au NPs on the surface of BFO NFs can efficiently inhibit the recombination of photogenerated charge carriers, which may be attributed to that the Schottky junction formed between Au NPs and BFO NFs can efficiently promote electron transfer from BFO NFs to Au NPs, and thus decreased the recombination rate of photogenerated charge carriers (Li S. et al., 2013). However, the PL intensity increases when the loading amount of Au NPs is too much. This can be ascribed to the fact that the excess amount of Au NPs can result in an increase of surface defects, which can act as recombination centers for photogenerated charge carriers, thus leading to the increase in the recombination rate. Therefore, it is necessary to deposit an appropriate amount of Au NPs on the surface of BFO NFs to promote the efficient separation of photogenerated electron-hole pairs, thus enhancing the photocatalytic activity.

#### Photocatalytic Activities

**Figure 10A** shows the photocatalytic degradation efficiencies of Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%) on the degradation of MB dye under simulated solar light irradiation. For contrast, the blank experiment without any photocatalyst is carried out under the same conditions. The result shows that the MB dye is hardly degraded under simulated solar light irradiation. The degradation efficiency is only 2.77% after irradiation of 3 h. When the pure BFO sample is added to the reaction system, the degradation efficiency reaches to be 49.49% after irradiation of 3 h. However, the photocatalytic activity is still relatively poor. Once Au NPs are deposited on the surface of BFO NFs, the photocatalytic activities of Aux-BFO NFs are obviously enhanced. The enhanced activities can be ascribed to the hierarchical nanofibers/nanoflakes structured homojunction, the SPR effect of Au NPs, as well as the presence of defects (Fe2+/Fe3<sup>+</sup> pairs and oxygen vacancy). In addition, the photocatalytic activity is increased and then suppressed with the increase in the loading amount of Au NPs. When the loading amount of Au NPs is 1.2%, the photocatalytic activity is the best with the photocatalytic efficiency of 85.76% after irradiation of 3 h.

To evaluate the reaction kinetics of the degradation of MB, the plot of ln(C0/C) vs. time is plotted in **Figure 10B**, which shows the kinetics of the degradation process by the five photocatalysts which follow Langmuir-Hinshelwood pseudo first-order kineticsequation, ln(C0/C) = kt, where t is the irradiation time and k is the rate constant (Niu et al., 2015). The observed calculated rate constants k in the presence of blank sample, Aux-BFO NFs (x = 0, 0.6, 1.2, 1.8, 2.4 wt%) are 8.28 × 10−<sup>3</sup> , 200.23 × 10−<sup>3</sup> , 411.46 × 10−<sup>3</sup> , 623.27 × 10−<sup>3</sup> , 552.85 × 10−<sup>3</sup> , and 472.69 × 10−<sup>3</sup> h −1 , respectively. which also indicates that the Au1.2-BFO NFs sample exhibits the strongest photocatalytic activity among the Aux-BFO NFs (x = 0, 0.6, 1.8, 2.4 wt%) samples.

In order to evaluate the stability of photocatalysts, the recycle experiment is carried out. After each recycle, the photocatalysts

are collected by simple filtration and washed with deionized water and absolute ethanol. As shown in **Figure 11**, there is no obvious loss in photocatalytic activity after five recycle test, indicating that Aux-BFO NFs photocatalysts have good stability during the photocatalytic degradation of MB dye under simulated solar light irradiation.

In order to explore the possible reaction pathways during the photocatalytic degradation of MB dye under simulated solar light irradiation, the trapping experiments are used to determine the main active species in this reaction system. Ethylenediaminetetraacetic acid (EDTA), tertbutyl alcohol (TBA) and benzoquinone (BQ) are used as scavengers for holes (h+), hydroxyl radicals (•OH), and superoxide radicals (•O − 2 ), respectively (Chen et al., 2016; Qiao et al., 2017; Ji et al., 2018). As can be seen from **Figure 12**, when the TBA or BQ is added, a slight reduction in MB degradation efficiency is occurred in comparison to the initial activity without addition of any scavengers, which means that •O − 2 and •OH radicals are not the main active species. However, the photocatalytic activity is severely suppressed when EDTA is added to the reaction system, implying that photogenerated holes are the main active species and play the decisive role in the photocatalytic activity.

## Photocatalytic Mechanism

On the basis of the aforementioned experimental and active species trapping results, the possible enhanced photocatalytic mechanism is illustrated in **Figure 13**. It can be ascribed to the following aspects: (1) Nanofibers/nanoflakes structured homojunction. It is well-known that nanoflakes with ultrathin structure possess higher valence band (VB) energy than that of nanofibers with larger particle size (Weng et al., 2014). After forming homojunctions between BFO NFs and BFO nanoflakes, the staggered band potentials would introduce a new internal electrical field to provide a driving force to reduce the charge transfer barrier. The formed homojunction can promote the transfer of photogenerated holes (h+) from the VB of BFO NFs to the VB of BFO nanoflakes, whereas photogenerated electrons (e−) from the conduction band (CB) of BFO nanoflakes to that of BFO NFs. (2) SPR effect of Au NPs. The enhanced local electric field in the near-surface region of BFO induced by the SPR effect of Au NPs results in the further separation of photogenerated electrons and holes. The photogenerated electrons are thermodynamically favorable to transfer from the CB of BFO to Au NPs due to the Fermi energy of Au NPs is more positive than the CB potential of BFO (Li S. et al., 2013). The formed Schottky junction at the interfaces can act as trapping centers for photogenerated electrons, contributing the accumulation of photogenerated electrons at the surface of Au NPs and preventing the recombination of photogenerated electron-hole pairs. (3) Defects. The presence of Fe2+/Fe3<sup>+</sup> pairs located below the CB of BFO can act as electron/hole trapping centers and thus reduce the recombination rate (Zhang et al., 2014; Verma and Kotnala, 2016). In addition, the oxygen vacancy can introduce the defect state energy level and effectively regulate the band structure, thus significantly improve the separation of photogenerated charge carriers (Zhang C. et al., 2016). Under the effect of these aspects, the photocatalytic activities are remarkably enhanced.

During the process of photocatalytic reaction, a few photogenerated electrons in the CB of BFO nanoflakes can

Li et al. Au-Induced BiFeO3 Homojunction Photocatalyst

react with oxygen molecules adsorbed on the surface to form superoxide radicals (•O − 2 ). The photogenerated electrons transferred to the Au NPs can react with Fe2+/Fe3<sup>+</sup> pairs and oxygen vacancy defects. Meanwhile, a little part of photogenerated holes in the VB of BFO NFs can oxidize the absorbed OH<sup>−</sup> and H2O to hydroxyl radicals (•OH) (Dong et al., 2016; Hu et al., 2017). Finally, the photogenerated holes transferred the VB of BFO nanoflakes as well as the formed •O − 2 and •OH species can oxidation decomposition of MB dye molecules.

# CONCLUSIONS

Novel Au-induced hierarchical nanofibers/nanoflakes structured BFO homojunction (Aux-BFO, x=0, 0.6, 1.2, 1.8, 2.4 wt%) were in situ synthesized by a simple reduction method under the analogous hydrothermal environment. The deposited Au NPs were distributed uniformly on the surface of BFO sample. The formed BFO nanoflakes were vertically assembled on BFO nanofibers to form a hierarchical architecture. The photocatalytic results showed that Au1.2-BFO NFs samples exhibited the best photocatalytic activity with the photocatalytic efficiency of 85.76% after irradiation of 3 h. The remarkable enhanced photocatalytic activity could be

# REFERENCES


mainly attributed to the hierarchical nanofibers/nanoflakes structured homojunction, the SPR effect of Au NPs, as well as the presence of defects (Fe2+/Fe3<sup>+</sup> pairs and oxygen vacancy). This work provided a novel method to design efficient homojunctions with multiple functions in solar energy utilization.

## AUTHOR CONTRIBUTIONS

YL synthesized Aux-BFO NFs samples and analyzed part of characterizations. JL is the supervisor of this research work. LC and HS helped writing. HZ analyzed Ferromagnetism measurements. HG and LF performed SEM, EDS, and TEM imaging and analyses. All authors have contributed to preparing the review article.

#### FUNDING

This work was supported by the National Natural Science Foundation of China (Grant No. 51702189), the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2017BEM033), and Science & Technology Plan of the Education Department of Shandong Province (grant No. J18KA002).


BiFeO<sup>3</sup> nanowires under visible-light. Chem. Commun. 49, 5856–5858. doi: 10.1039/c3cc40363g


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Li, Li, Chen, Sun, Zhang, Guo and Feng. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Titanium Phosphate Nanoplates Modified With AgBr@Ag Nanoparticles: A Novel Heterostructured Photocatalyst With Significantly Enhanced Visible Light Responsive Activity

#### Manli Ren, Jiaqiu Bao, Peifang Wang, Chao Wang and Yanhui Ao\*

*Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing, China*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Guohong Wang, Hubei Normal University, China Qizhao Wang, Northwest Normal University, China*

#### \*Correspondence:

*Yanhui Ao andyao@hhu.edu.cn*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *17 July 2018* Accepted: *25 September 2018* Published: *17 October 2018*

#### Citation:

*Ren M, Bao J, Wang P, Wang C and Ao Y (2018) Titanium Phosphate Nanoplates Modified With AgBr@Ag Nanoparticles: A Novel Heterostructured Photocatalyst With Significantly Enhanced Visible Light Responsive Activity. Front. Chem. 6:489. doi: 10.3389/fchem.2018.00489* AgBr@Ag modified titanium phosphate composites were fabricated through a two-step approach. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The optical properties of the composites were characterized by using UV-vis diffuse reflectance spectroscopy. The photocatalytic activities of the composites were investigated on the degradation of Rhodamine B and ciprofloxacin under visible light irradiation. AgBr@Ag/titanium phosphate was determined to exhibit considerably higher photocatalytic activity than the corresponding individual components. The mechanism on the enhancement of the photocatalytic activity was proposed based on the results of photoluminescence spectra and photocurrent measurements. Furthermore, the possible photocatalytic mechanisms of organic compounds degradation were also proposed.

Keywords: heterojunction, titanium phosphate, AgBr@Ag, photocatalysis, visible light

## INTRODUCTION

Environmental pollution has become a global problem due to the accelerated development of human society (Ye et al., 2012; Sun et al., 2017; Zhang et al., 2017; Li et al., 2018a; Lu et al., 2018). Therefore, it is urgent to solve this severe problem via green technology (Liu et al., 2012; Hao et al., 2016; Zou et al., 2016; Chen et al., 2018; Feng et al., 2018; Guo et al., 2018). Currently, photocatalysis technology has recieved intense attention as an efficient technology for solving the problem of environment (Fan et al., 2013; Ao et al., 2016; Wang et al., 2016; Dong et al., 2017; Hao et al., 2017). Furthermore, it is well known that composite photocatalysts always show higher activity than the corresponding single component (Ma et al., 2018; Wang et al., 2018; Zhang et al., 2018). The improved photocatalytic activity is due to the different electronic energy levels of different components, which create an internal electric field that can accelerate the separation rate of photogenerated electrons and holes, and reduce their recombination rate (Guo et al., 2017).

Recently, titanium phosphate has been widely studied for the treatment of organic compounds. Titanium phosphate [α-Ti(HPO4)2•H2O, hereafter TP] consists of layers of titanium atoms bonding on both plane sides of the monohydrogen phosphate groups. Water molecules located in the interlayer region to form a network of hydrogen-bonding with phosphate groups (Ekambaram et al., 2000; Guo et al., 2013; Guo and Han, 2014; Zhu et al., 2014). However, titanium phosphate can only absorb ultraviolet light (<5% of the sunlight). Furthermore, the high recombination rate of photoinduced electron-hole pairs is still exist, which would lead to lower

photocatalytic activity. Thus, it is urgent to find effective ways to improve the activity of the TP.

Recently, silver halide materials have received intense attention due to their excellent visible light responsive photocatalytic activity. Silver halide@silver nanoparticles (AgNPs) based heterojunctions have been found to be excellent photocatalysts (Chen et al., 2015; Bai et al., 2016; Li et al., 2018b; Xuan et al., 2018). It has been proved that plasmon AgNPs on the surface of AgX can enhance the light absorption of the composites and accelerate the separation rate of the photogenerated charges (Daupor and Wongnawa, 2014; Li et al., 2015a,b; Ding et al., 2018; Xiao et al., 2018). Moreover, it was also considered that AgNPs decrease the recombination rate of Ag<sup>+</sup> with photogenerated electrons, and allow the formation of Br<sup>0</sup> species (photogenerated holes react with Br<sup>−</sup> ions) that can degrade the organic compounds (Xia and Halas, 2005; Jiang et al., 2014; Zhang et al., 2015a). Therefore, it would be a good way to obtain higher visible light activated activity through the conbination of AgBr@Ag and TP.

In this work, we prepared visible light active AgBr@Ag/TP photocatalysts by a simple and feasible method. The morphology, crystal structure, optical properties, and composition of the photocatalysts were investigated. Moreover, the possible photocatalytic degradation mechanism of ciprofloxacin and the enhancement mechanism photocatalytic activity were proposed.

## EXPERIMENTAL

All experimental details are shown in **Supporting Information**.

Ag@AgBr/TP-3.

# RESULTS AND DISCUSSION

#### Characterization

The phase structure was characterized by XRD, and the obtained patterns are shown in **Figure 1**. The peaks at about 11.6, 21.0, 26.0, 35.4, and 35.8◦ are indexed to the (002), (200), (202), (006), and (020) directions, respectively. All peaks can be ascribed to TP (JCPDS Card No: 83-0109; Ortíz-Oliveros et al., 2014; Yada et al., 2014). Diffraction peaks are also detected at 26.73, 30.96, 44.35, 55.04, 64.49, and 73.26◦ , which can be indexed to the (111), (200), (220), (222), (400), and (420) directions of AgBr (JCPDS Card NO: 06-0438; Chen et al., 2015; Bai et al., 2016). Furthermore, with the increasing of AgBr mass, the intensity of the characteristic peaks of AgBr increased gradually. No diffraction peaks of Ag can be observed, possibly due to its low mass, high dispersion or small size (Wang et al., 2013, 2014).

The morphologies of the composites were investigated by TEM and SEM. The obtained SEM image of pure TP is displayed in **Figure 2a**. It can be seen that the TP sample is composed

of uniform plates with sub-micrometer width and 30∼40 nm thickness. The SEM image of AgBr@Ag/TP-3 taken from **Figure 2b** displays smaller AgBr@Ag nanoparticles decorating

FIGURE 5 | Photocatalytic degradation of CIP over the as-prepared samples under visible light irradiation.

the TP surface. The TEM image of pure TP is displayed in **Figure 2c**. It can also be seen that the titanium phosphate plate is hexagonal, which is in good agreement with the results of SEM. The TEM image of AgBr@Ag/TP is shown in **Figure 2d**. It can be seen that AgBr nanoparticles decorated the TP surface, indicating the formation of heterojunctions in the prepared samples (Cui et al., 2018). As shown in the inset of **Figure 2d**, the measured lattice spacings of 0.289 nm corresponds to the (200) lattice planes of AgBr.

The elemental composition and chemical states of the elements in the composites were analyzed by XPS. The peaks of Br 3d, P 2p, Ag 3d, C 1s, Ti 2p, and O1s can be observed in **Figures 3A,B** shows the electron binding energies of Ag 3d5/<sup>2</sup> and Ag 3d3/<sup>2</sup> orbitals, which can be further divided into 366.6/367.4 and 372.6/373.4 eV, respectively. The peaks at 366.6 and 372.6 eV indicate the presence of Ag+. Both the peak separation (6.0 eV) and peak positions (367.4 and 373.4 eV) indicate the presence of metallic form (Ag<sup>0</sup> ; Yang et al., 2015; Tang et al., 2017a,b). As shown in **Figure 3C**, two peaks at 68.0 and 67.0 eV corresponding to the binding energies of Br 3d3/<sup>2</sup> and Br 3d5/<sup>2</sup> can be found (Wang et al., 2009). All above results prove the presence of AgBr and Ag in the composite. In **Figure 3D**, the Ti peaks observed at 459.0 and

464.8 eV can be attributed to Ti 2p3/<sup>2</sup> and Ti 2p1/2. The peak separation of 5.8 eV for binding energy between the two peaks indicates that Ti was mainly Ti4<sup>+</sup> in the composite (Yada et al., 2014).

The optical properties of photocatalysts significantly affect their photocatalytic performance (He et al., 2015). Therefore, optical properties of the composites were investigated by DRS and the obtained results are shown in **Figure 4**. Pure TP exhibited no absorption in the visible-light region, whereas the Ag@AgBr/TP composites had better absorption performance, possibly due to the deposition of AgBr and formation of AgNPs on the surface of the composites. Moreover, Ag@AgBr/TP-3 showed the highest absorption in visible light region, indicating that high absorption capability is ascribed to an optimal loading. In other words, when the loading of AgBr@Ag exceeds a critical value, the AgBr@Ag nanoparticles would agglomerate and decrease the absorption capability.

#### Photocatalytic Activity

To investigate the photocatalytic performance of the composites, we conducted experiments on the degradation of colorless organic pollutant (CIP) under visible light irradiation. As shown in **Figure 5**, AgBr@Ag/TP-3 displayed the highest photocatalytic activity for CIP degradation among all samples. Approximately 51.4, 44.2, 61.3, 71.5, and 65.4% of CIP was degraded in 180 min by AgBr@Ag, AgBr@Ag/TP-1, AgBr@Ag/TP-2, AgBr@Ag/TP-3, and AgBr@Ag/TP-4, respectively. Furthermore, the TP showed a low photocatalytic efficiency for CIP degradation. The degradation curve of CIP by TP exhibits a fluctuating trend. There may be some CIP molecules desorbed from the surface of TP. These results show that the Ag@AgBr/TP photocatalysts are generally effective for the degradation of colorless organic compounds.

#### Intermediates Analysis

Prior to the complete mineralization of CIP, there are many degradation intermediates in the photocatalytic process. The sample solution was detected by HPLC/MS during the photocatalytic degradation. Some intermediates in the photocatalytic process were identified: A1 (m/z = 362.2), A2 (m/z = 334.2), A3 (m/z = 306.2), A4 (m/z = 291.2), and A5 (m/z = 263.2). These results show that A1∼A5 are the main intermediates during the degradation process of CIP. As observed in many other oxidation system (Sturini et al., 2012), desethylene ciprofloxacin (A3) was dominant intermediate of CIP, and was formed by a net loss of C2H<sup>2</sup> at the piperazinyl substituent of CIP. Prior to the formation of desethylene ciprofloxacin, two intermediates were detected. The hydroxyl keto-derivative of the piperazinyl substituent (A1) was detected to be the first oxidized intermediate on the way to the formation of A3. Apparently, the subsequent loss of CO from A1 results in the cleavage of the piperazinyl ring to form keto-derivatives A2 and A2', in which a carbonyl group remains on the N atom of aniline and alkylamine, respectively. A second loss of a CO molecule from A2 and A2' leads to the formation of desethylene ciprofloxacin (A3). The further oxidation of desethylene ciprofloxacin (A3) generated A4 with a keto group through the loss of a N atom. The decarbonylation of A4 forms aniline (A5), completely destroying the piperazinyl substituent of CIP. The probable pathways for CIP degradation are shown in **Figure 6**.

## Possible Photocatalytic Mechanism

Several experiments were performed to further understand the mechanism on the enhancement of photocatalytic performance for the AgBr@Ag/TP composites. The photoluminescence (PL) spectrum was measured to investigate the recombination rate of the photogenerated electron-hole pairs. The photoluminescence spectra of the prepared samples are shown in **Figure 7**. The intensity of PL is directly proportional to the electron-hole

irradiation.

recombination rate, and lower PL intensity means lower recombination rate (Zhang et al., 2015b; Shi et al., 2016). The main emission peak of pure TP centers at ∼470 nm, which was ascribed to the band gap recombination of electronhole pairs. The PL intensity decreases significantly after the deposition of AgBr@Ag. The decreasing emission intensity of the AgBr@Ag/TP suggests that the recombination of electronhole pairs can be suppressed after the deposition of AgBr@Ag (Xu et al., 2013). Furthermore, it can be seen that the PL intensity increase as the amount of Ag@AgBr increase to a certain value, and AgBr@Ag/TP-3 composite showed the lowest PL intensity, which means Ag@AgBr/TP-3 composite has the lowest recombination rate of photogenerated electrons-hole pairs. Therefore, it would lead to the highest photocatalytic performance. However, the PL intensity increase as the amount of Ag@AgBr increase further. It can be ascribed to the reason that superfluous Ag@AgBr nanoparticles would aggregate thus cannot form effective heterojunctions with TP. The above results demonstrate that the deposition of AgBr on TP and the formation of AgNPs on the surface of the composite can decrease the recombination efficiency and enhance the separation efficiency of the photogenerated electrons and holes.

It is well known that the efficiency of the electron-hole pairs separation significantly affects the photocatalytic performance (Jiang et al., 2011). The electrons in the valence band of photocatalysts can be activated and then transfer to the conduction band, leading to the formation of photocurrent. A higher photocurrent means higher electron-hole pairs separation rate, which induces to better photocatalytic performance (Xiang et al., 2011). **Figure 8** shows the photocurrents for different samples under 20 s intermittent visible-light irradiation with a bias of 0.3 V. The photocurrents of AgBr@Ag/TP composites are all much higher than that of TP. Moreover, AgBr@Ag/TP-3 exhibits the highest value. The results are in good agreement with the photocatalytic activity, indicating that the efficient separation of photo-induced electrons and holes results in the high photocatalytic activity.

It was demonstrated that the separation efficiency of photogenerated electrons and holes is important to the photocatalytic performance. Furthermore, the conduction band (CB) and valence band (VB) potential have a significantly effect on the separation of the photogenerated electron-hole pairs. The CB and VB potential of AgBr and TP can be obtained using Equations. (1) and (2) (Zhang et al., 2008):

$$\rm E\_{vb} = X - E\_0 + 0.5E\_{\rm g} \tag{1}$$

$$\mathbf{E\_{cb}} = \mathbf{E\_{vb}} - \mathbf{E\_{\mathcal{g}}} \tag{2}$$

**w**here Evb is the potential of VB, X is the absolute electronegativity of photocatalyst (X is 6.85 eV and 5.81 eV of TP and AgBr, respectively), E<sup>g</sup> is the band gap energy and E<sup>0</sup> is the energy of free electrons (∼4.5 eV). Based on these equations, the CB potential (Ecb) and VB potential (Evb) of TP are calculated as 0.43 and 4.28 eV, while Ecb and Evb of AgBr are calculated as 0.01 eV and 2.62 eV, respectively.

Therefore, a possible mechanism on the photocatalytic degradation of RhB by AgBr@Ag/TP under the irradiation of visible light is illustrated in **Figure 9**. The plasmon induced holes can react with Br<sup>−</sup> and form active Br<sup>0</sup> atoms. The Br<sup>0</sup> atoms exhibit an excellent oxidation capability to oxidize organic compounds. Meanwhile, plasmon induced electrons are formed on the surface of AgNPs, part of these electrons are trapped by O<sup>2</sup> to produce •O − 2 , mainly contributing to the organic compounds removal (Ye et al., 2012; Zhang and Yates, 2012; Cheng et al., 2015; Ding et al., 2018). Besides, the other electrons would transferto the conduction band of AgBr, and then further transfer from AgBr to the conduction band of TP. Meanwhile, the h<sup>+</sup>

stored in valence band of AgBr and TP was also involved in the organic compounds photocatalytic degradation, though they are not the dominant active species. The relevant equations are shown as following:

$$\text{Ag} + \text{visible} - \text{light} \rightarrow \text{e}^- + \text{h}^+ \tag{3}$$

$$\text{e}^- + \text{O}\_2 \rightarrow \text{\textbullet O}\_2^- \tag{4}$$

$$\text{Br}^+ + \text{Br}^- \rightarrow \text{Br}^0 \tag{5}$$

$$\text{CIP} + \bullet \text{O}\_2^- \text{(orBr}^0) \rightarrow \text{degradation products} \tag{6}$$

Based on above results, the possible mechanism on the enhancement of photocatalytic activity for AgBr@Ag/TP composites can be proposed. AgBr@Ag nanoparticles deposited on TP improve the light absorption capability, as confirmed by UV-vis DRS studies. Furthermore, the heterojunctions between AgNPs, AgBr, and TP are conducive to improving the transfer rate and separation efficiency of the electron-hole pairs, and hindering the recombination of the pairs, thus inducing to the enhancement of the photocatalytic activity.

#### CONCLUSIONS

AgBr@Ag/TP composites exhibited higher photocatalytic performance than pure TP and AgBr@Ag, and can degrade **a**

#### REFERENCES


**typical antibiotic (CIP)** under the irradiation of visible light. The excellent photocatalytic activity can be ascribed to their enhanced visible-light absorption, low recombination and high transferring rate of the photo-generated charges, as shown by DRS, PL, and photocurrent experiments.

#### AUTHOR CONTRIBUTIONS

YA and CW designed the project and guided the work. MR and JB conducted the experiments and characterizations of the samples. PW revised and polished the manuscript.

#### ACKNOWLEDGMENTS

We are grateful for grants from National Science Funds for Creative Research Groups of China (No. 51421006), Natural Science Foundation of China (51679063), the Fundamental Research Funds for The Central Universities (2018B14514), the Key Program of National Natural Science Foundation of China (No. 91647206), the National Key Plan for Research and Development of China (2016YFC0502203), and PAPD.

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00489/full#supplementary-material


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**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Ren, Bao, Wang, Wang and Ao. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Photocatalytic Activation of Saturated C–H Bond Over the CdS Mixed-Phase Under Visible Light Irradiation

#### Houde She<sup>1</sup> , Liangshan Li <sup>1</sup> , Hua Zhou<sup>1</sup> , Lei Wang<sup>1</sup> , Jingwei Huang<sup>1</sup> and Qizhao Wang1,2 \*

*<sup>1</sup> College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, China, <sup>2</sup> Gansu International Scientific and Technological Cooperation Base of Water-Retention Chemical Functional Materials, Lanzhou, China*

Selective activation of saturated C–H bond in hydrocarbons to produce high-value-added chemicals is of great significance for chemical synthesis and transformation. Herein, we present a facile procedure to achieve Ni-doped CdS nanoparticles with mixed (cubic and hexagonal) phases, as well as its application to the photocatalytic activation of saturated primary C–H bond of toluene and its derivatives. The photocatalytic oxidation rate of toluene into benzaldehyde of formation reached up to 216.7 µmolh−1g <sup>−</sup><sup>1</sup> under visible light irradiation. The excellent photocatalytic performance of Ni(II)-doped CdS [Ni(II)/CdS] can be attributed to its unique structural assembly with cubic and hexagonal phases and also the addition of Ni ions, together taking effect in promoting the separation of photogenerated charge carriers. The possible reaction mechanism for the photocatalytic selective oxidation is illustrated in this work. The band width of the as-prepared mixed phase CdS is reduced, which can effectively expand the response range and improve photocatalytic performance.

Keywords: toluene, benzaldehyde, ion modify, mixed-phase, C–H bond, selective oxidation

# INTRODUCTION

The highly selective oxidation of inactive C–H bond in hydrocarbons to form high value-added oxygenated products under mild conditions is one of the major challenges in the industrial field (Pal et al., 2014; Yang et al., 2017). For example, toluene is a simple aromatic compound and can be selectively oxidized into benzaldehyde, benzyl alcohol, benzoic acid and benzyl benzoate, all of which are crucial intermediate for the manufacture of fine chemicals, such as pharmaceuticals, dyes, preservatives and perfume. At present, benzaldehyde is primarily synthesized via traditional route of chlorination hydrolysis of toluene. The discharges of toxic gases and organic waste bring on serious environmental concerns. Therefore, it is essential to develop an environment-friendly technique to address such problem. Photocatalysis is a promising alternative strategy that has been widely applied in the activation of C–H bond under mild conditions (Zhang et al., 2012; Hao et al., 2018) owning to its clean and low energy consumption, etc. For example, Yuan et al. reported that BiOBr/TiO<sup>2</sup> can be used in oxidation of toluene into benzaldehyde under visible light irradiation (Yuan et al., 2013). He's group (He et al., 2018) presented a toluene selective oxidation that has been carried out by Cd3(C3N3S3)2/CdS porous composites under visible light irradiation. Cd3(C3N3S3)2/CdS exhibits excellent performance for the transformation of toluene into benzaldehyde.

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Zhu Gangqiang, Shaanxi Normal University, China Guidong Yang, Xi'an Jiaotong University, China Xiaole Weng, Zhejiang University, China*

#### \*Correspondence:

*Qizhao Wang wangqizhao@163.com; qizhaosjtu@gmail.com*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *23 August 2018* Accepted: *14 September 2018* Published: *09 October 2018*

#### Citation:

*She H, Li L, Zhou H, Wang L, Huang J and Wang Q (2018) Photocatalytic Activation of Saturated C–H Bond Over the CdS Mixed-Phase Under Visible Light Irradiation. Front. Chem. 6:466. doi: 10.3389/fchem.2018.00466*

It is well-known that the CdS is economical and has been extensively employed in photocatalysis research because of its adequate band gap (2.45 eV), which is unique among photocatalytic materials (Wang et al., 2014; Cheng and Xiang, 2016; Xiang et al., 2016). Although CdS-based photocatalysts are widely studied in the field of photocatalysis (Yan et al., 2009; He et al., 2016b; Li et al., 2018) some inherent defects like high charge carrier recombination rate and photo-corrosion, debase their photocatalytic activity and practical application (He et al., 2016a). To hold back electrons-holes recombination and improve photocatalytic activity, various strategies, such as the loading of co-catalysts, forming heterojunctions have been scrupulously developed (Wang et al., 2013; Wei et al., 2017, 2018). In the past decade, Pt (Luo et al., 2015b), Au (Majeed et al., 2016; Wang et al., 2018b), Pd (Luo et al., 2015a), Ag (Gupta et al., 2017), and Ru (Nosheen et al., 2017) have been used as co-catalysts in various photocatalyst systems to enhance charge separation and surface reactions. Despite the high photocatalytic performance achieved by these noble metal-based co-catalysts contained catalyst, their use in mass production are limited due to the scarcity and high cost of them. Therefore, it is desirable to investigate new catalyst system with non-noble metal as co-catalyst, which can meet the general industrial requirements, such as low cost, high efficiency, and good durability. In this regard, many non-precious metals [such as Fe (Yan et al., 2017; Deka and Kalita, 2018), Cu (Kumar et al., 2018; Wang et al., 2018a), Co (Dong et al., 2017), Ni (Yang et al., 2018), Mo (Zhang et al., 2017)] materials have been studied as a co-catalyst to improve performance of main catalyst. In particular, nickel-based materials [such as metallic nickel, nickel ion (Song et al., 2017), nickel oxides and hydroxides (Zhang et al., 2015)] exhibit excellent activity and attract much attention as promoters for enhancing photocatalytic activity. Murugesan et al. (Murugesan et al., 2017) prepared Ba and Ni doped CdS by a spray pyrolysis method. Doping Ba and Ni can significantly improve optical performance of CdS. Besides, it has also been suggested that Ni doped CdS was prepared via an impregnation method (Yu et al., 2016), which doped samples can efficiently promotes separation of holes and electrons and consequently enhances the photocatalytic hydrogen production.

However, in the above works, only modified single phase CdS can enhance the photocatalytic activity. But, as reported by some researchers, the CdS base can also form a homojunction for enhancing its photocatalytic activity (Dai et al., 2014). The mixed-phase CdS has been reported by Zhao's group (Zhao et al., 2018), which demonstrated that the photoresponse range can be broadened to ameliorate photocatalytic activity.

In this work, we report the synthesis of Ni(II) doped CdS (Ni(II)/CdS) with mixed-phase (cubic and hexagonal) via a simple impregnation method. The Ni(II)/CdS was further used as photocatalyst in the activation of saturated primary C–H bond of toluene and its derivatives at room temperature and under two barometric pressure (0.2 MPa). The yield of benzaldehyde can reach up to 216.70 µmolh−<sup>1</sup> g −1 , which is 7.7 times higher than that of mixed-phase CdS. Controlled experiments using different radical scavenger were also conducted to elucidate the possible reaction mechanism for the selective oxidation of C–H bond in toluene over the mix phase of Ni(II)/CdS.

## EXPERIMENTAL

#### Materials

All reagents were analytical grade and used without further purification. Benzotrifluoride (BTF) (>99%)was purchased from Aladdin Chemical Reagent Co., Ltd. Oleic acid (C18H34O2), Cadmium nitrate tetrahydrate [Cd(NO3)2·4H2O], 8% of ammonium sulfide [(NH4)2S], sodium borohydride (NaBH4), ethyl alcohol (C2H6O), toluene (>99.5%) and other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

# Preparation

#### Synthesis of Mixed-Phase CdS

Typically, Cd(NO3)2·4H2O (1.0 mmol) and oleic acid (0.0375 ml) were first dissolved in deionized water (50 mL) in a beaker placed on a magnetic stirrer for 15 min at room temperature to form solution A. NaBH<sup>4</sup> (4.0 mmol) was also dissolved in deionized water (250 mL) in a beaker placed on a magnetic stirrer at room temperature to form solution B. The two solutions were then mixed and quickly put into a microwave oven (M1-L213B, at a fixed frequency of 2,450 MHz, Guangdong midea kitchen appliances manufacturing Co., Ltd., China) followed by 20 s irradiation. The Cd nanoparticles (NPs) were collected and washed by ethyl alcohol three times, and dispersed in 20 mL ethyl alcohol to get sulfurized by adding certain amount of (NH4)2S (2.0 mmol) and followed by 1 h stirring at room temperature. Then the yellowish products were collected and washed by ethyl alcohol three times. Finally, the products were dried in drying oven at 60◦C for 4 h.

#### Preparation of Ni(II)-Mixed-Phase CdS Samples

The Ni(II) doped mixed phase CdS samples (Ni(II)/CdS) were synthesized by a simple impregnation method. Typically, CdS (200.0 mg) was dispersed into10.0 mL of NiCl2·6H2O solution and stirred at 80◦C for 2 h. The products was collected and washed by deionized water several times. The amount of Ni (the weight ratio of Ni(II) to CdS) was about 0.5, 1.0, 3.0, and 5.0 wt%, respectively. Finally, the samples dried in drying oven at 60◦C for 4 h.

#### Characterization

X-ray diffraction (XRD) patterns were acquired by a Rigaku D/MAX-2200/PCX-ray diffractometer in the angular range of 10–90◦ (2θ) with Cu Kα radiation (40 kV, 20 mA). UVvis diffuse reflectance spectra (DRS) were acquired from 230 to 800 nm by a UV-vis spectrophotometer (PuXin TU-1901). The morphologies and microstructures of the samples were obtained using field emission scanning electron microscope (FE-SEM, Ultra Plus, Carl Zeiss) and transmission electron microscope (TEM, F20, FEI). X-ray photoelectron spectroscopy (XPS) was used to study the chemical compositions and the valence states by a photoelectron spectrometer (PHI5702). The photoelectrochemical (PEC) performances of photoanodes were acquired by a three-electrode system (CHI-660D Co., Shanghai, China) under a LED lamp (λ > 420 nm, CEL-LED100) illumination. A Pt wire and Ag/AgCl were used as counter electrode and reference electrode, respectively. The working electrodes were made on the fluride-tin oxide (FTO) conductor glasses. The samples (10 mg) were homogeneously dispersed in anhydrous ethanol and ultrasound for 60 min, which were slowly dripped on FTO glasses. The electrolyte was 0.5 M Na2SO<sup>4</sup> (pH = 7.35) aqueous solution in a quartz ware 0.5 V of the bias voltage was used for photoelectrochemical testing. Illumination through the back-side of FTO was used with an illumination area of about 1.0 cm−<sup>2</sup> . Photoluminescence (PL) spectra of the samples were recorded by a fluorescence spectrophotometer (PE, LS-55) at room temperature, using a 390 nm excitation wavelength.

#### Evaluation of Photocatalytic Activity

The photocatalytic reaction was performed in a 25 mL glass bottle under visible light irradiation in this work. Typically, 0.5 mmol of substrate and 80 mg of catalyst were dispersed in the 5.0 mL of solvent (BTF) in a 25 mL glass bottle. Then the glass bottle was transferred into a closed reactor (**Figure S1**) and stirred for 30 min in dark to make the catalyst fully contact with the solution and achieve adsorption equilibrium. Meanwhile, the reactor was bubbled into oxygen molecules from a gas cylinder for 30 min at absolute pressure of 0.2 MPa. A 300 W Xe arc lamp (CEL-HXF 300, Beijing Perfectlight Co. Ltd.) with a ultraviolet cutoff filter (λ < 420 nm) was used in the following photocatalytic reaction. After 2 h of irradiation, the mixture was centrifuged to remove catalyst powders at 8,000 rmp for 5 min by a centrifuge (TG16-WS). The liquid supernatant was analyzed by a gas chromatograph (GC-9600). A series of controlled experiments were carried out similar to the photocatalytic oxidation process of toluene, except that the radical scavengers (0.1 mmol) were added to the reaction system. The conversion (Con.) rate of substrates, yield of aldehyde, and selectivity (Sel.) were defined with the following equations:

$$\text{Con.(\%)} = [(\text{C}\_0 - \text{C}\_1)/\text{C}\_0] \times 100\% \tag{1}$$

$$\text{Yield(\%)} = \text{(C}\_{\text{aldehydde}}/\text{C}\_{\text{0}}) \times 100\text{\%} \tag{2}$$

$$\text{Sel.(\%)} = [\text{C}\_{\text{aldehydde}} / (\text{C}\_0 - \text{C}\_1)] \times 100\text{\%} \tag{3}$$

Where C<sup>0</sup> is the initial concentration of substrates; C<sup>1</sup> and Caldehyde are the concentration of the remaining substrates and the corresponding aldehyde at a certain time after the photocatalytic reaction, respectively.

#### RESULTS AND DISCUSSION

## DRS, XRD and Photoluminescence (PL) Spectra Analysis

**Figure 1A** shows the UV–vis diffuse reflectance spectra of mixed-phase CdS, 0.5 wt%Ni(II)/CdS, 1.0 wt%Ni(II)/CdS, 3.0 wt%Ni(II)/CdS, 5.0 wt%Ni(II)/CdS samples. All samples of the spectra display absorption in the visible-light region. The mixedphase CdS NPs exhibit a absorption edges at 518 nm, while the

Ni(II)/CdS samples show red shift of absorption edges compared with prestine CdS sample. In particular, the absorption edges of 3.0 wt%Ni(II)/CdS NPs was observed at 555 nm. Hence, it is clear that the modification of Ni(II) co-catalyst can significantly affect the optical absorption feature of CdS NPs. The optical band gap value has been calculated via the equation: (αhv)<sup>2</sup> = A(hv — Eg) (Su et al., 2013; Kuang et al., 2015), where α, h, A and v correspond to absorption coefficient, Planck constant, proportionality and light frequency, respectively. As shown in **Figure 1B**, the band gaps estimated are 2.39, 2.26, 2.24, 2.23 and 2.26 eV for mixed-phase CdS, 0.5 wt%Ni(II)/CdS, 1.0 wt% Ni(II)/CdS, 3.0 wt%Ni(II)/CdS and 5.0 wt%Ni(II)/CdS, respectively. The above results indicate that the Ni(II)/CdS have changed the band gap of the mixed-phase CdS. Compared with the single-phase CdS of absorption edge, the absorption edge of the mixed phase CdS is red shift. Prior to this, the cubic CdS was synthesized by a solvothermal method. In addition, the UV–vis diffuse reflectance spectra and Tauc plot are shown in **Figure S2**.

The X-ray diffraction (XRD) pattern of as-prepared CdS sample is shown in **Figure 1C**. It is evident that the sample contain hexagonal phase CdS because the peaks at 24.8, 26.5, and 28.2◦ correspond to the (100), (002) and (101) crystal planes (hexagonal CdS, JCPDS No. 41–1049), respectively. Meanwhile, peaks at 26.5, 44.0 and 52.1◦ can be indexed to (111), (220), and (311) planes of cubic phase CdS (JCPDS No. 75-1546) or (002), (110), and (112) planes of hexagonal phase CdS. So it is clear that most of the strong diffraction peaks of JCPDS No. 75–1546 (cubic phase) overlap with the peaks of JCPDS No. 41–1049 (hexagonal phase) except for the peak at 30.7◦ , which corresponds to (200) plane of cubic phase CdS. But it is not safe to judge the existence of cubic phase CdS from the XRD patterns simply according to this distinctive peak because the intensity of (200) peak is only one fifth of that of (111) plane due to its weak and even indiscernible signal in the whole pattern. However, the nature of mixed phases of our samples can be alternatively judged from the intensity distribution of diffraction peaks, as suggested by the peak of 26.5◦ corresponds to the (111) crystal plane and overlaps with the (002) crystal plane of the hexagonal phase. Noting that peak of (002) plane is not the strongest line in JCPDS No. 41-1049, it is unreasonable to conclude that the strongest peak is given at 26.5◦ if our samples are hexagonal phase CdS, the only possible explanation is that our sample should be mixed-phase CdS so that the diffraction wave of (111) plane (cubic CdS) will stack up with that of (002) plane (hexagonal CdS) and presents the strongest peak at 26.5◦ . Another possibility for the presence of the strongest peak at 26.5◦ can be explained by anisotropic growth of hexagonal phase CdS. The dominant growth along (002) plane will give similar result even if our sample is not mixed-phase CdS. To rule out this possibility, SEM and TEM were performed on all the samples and the results (shown in **Figure 3**) do not support this supposition. So it is now safe to demonstrate that the CdS sample should be composed of cubic CdS and hexagonal CdS. According to the patterns shown in **Figure 1C**, the same diffraction peak intensity and width as mixed-phase CdS suggests that the appearance and crystallite size of CdS NPs are not affected by doping different amount of Ni(II). Photoluminescence (PL) spectroscopy is an effective method to study the electronic structure and optical properties of semiconductor materials. **Figure 1D** shows the PL spectra of all samples at the excitation wavelength of 390 nm in room temperature. It can be seen from **Figure 1D** that the catalyst samples have a strong signal peak at a wavelength of about 540 nm. In general, a high intensity of the photoluminescent signal indicates a high recombination probability of photogenerated electrons (e−) and holes (h+) as well as a low photocatalytic activity (Martha et al., 2013; Wu et al., 2014; Weng et al., 2016; Cui et al., 2017a; Nie et al., 2018; She et al., 2018). In this regard, it is seen from **Figure 1D** that the activity of the catalyst 3.0 wt%Ni(II)/CdS is significantly higher than that of the other samples, which is in a good agreement with the result of catalyst activity tests.

#### Morphology and Microstructures Analysis

**Figure 2** shows the SEM images of the mixed-phase CdS and 3.0 wt%Ni(II)/CdS. It is seen that the mixed-phase CdS sample consists of irregular particles with a size range of 20–100 nm in **Figures 2a,b**. As exhibited in the **Figures 2c,d**, 3.0 wt%Ni(II)/CdS sample is also constituted of irregular nanoparticle, and its microscopic structure is similar to that of mixed-phase CdS. But its surface is not so smooth as that of mixed-phase CdS due to the modification of Ni(II). The EDX analysis was also performed to confirm the existence of Ni element and the result is shown in **Figure 3f**. The SEM images of the 0.5 wt%Ni(II)/CdS, 1.0 wt% Ni(II)/CdS and 5.0 wt% Ni(II)/CdS are displayed in **Figure S3**, respectively. It can be clearly seen that all the samples are constituted of irregular nanoparticle with the same microscopic structure.

To study the microscopic structure and morphology of asprepared catalysts, TEM, HRTEM, SAED, and EDX analysis were performed and the results are shown in **Figure 3**. As shown in **Figure 3b**, the lattice spacing of 0.359 and 0.335 nm can be

FIGURE 2 | SEM images of the mixed-phase CdS (a,b), and 3.0 wt% Ni(II)/CdS (c,d).

ascribed to the (100) and (002) crystal face of hexagonal phase and the spacing of 0.290 nm can be ascribed to the (200) crystal face of cubic CdS. This image analysis further confirms that the as-prepared catalysts are composed of cubic and hexagonal phase CdS NPs and is consistent with the XRD results as discussed above. Compared with prestine CdS (**Figure 3a**), it is observed that the microscopic structure and morphology of 3.0 wt%Ni(II)/CdS (**Figure 3c**) is not significantly changed. The lattice spacing of Ni is not observed in **Figure 3d**, which could be attributed to the low dosage of Ni(II) ions. As shown in **Figure 3e**, the SAED pattern shows that the sample possesses a polycrystalline nature. In addition, the energy dispersive X-ray (EDX) analysis also suggests the coexistence of Cd, S and Ni elements in **Figure 3f**.

# XPS Analysis

The surface nature of the as-prepared CdS and 3.0 wt%Ni(II)/CdS samples were characterized by XPS measurements. As shown in **Figure 4A**, Cd and S elements are derived from the CdS phase, while the O element might come from H2O (Chen et al., 2015). We can see that the peaks at the binding energy of 404.9 and 411.7 eV correspond to Cd 3d5/<sup>2</sup> and Cd 3d3/<sup>2</sup> in **Figure 4B** (Jin et al., 2015). **Figure 4C** indicates the S 2p peak bifurcates as two peaks of S 2p3/<sup>2</sup> and S 2p1/<sup>2</sup> (Xue et al., 2018), corresponding to the binding energy of 161.4 and 162.7 eV (Li et al., 2015b), respectively. As shown in the **Figure 4D** (3.0 wt%Ni(II)/CdS), the peaks at about 855.5 and 873.1 eV are attributed to Ni 2p3/<sup>2</sup> and Ni 2p1/<sup>2</sup> of Ni(II) (Song et al., 2017), with a spin-energy separation of 17.6 eV. The peaks at 861.3 and 879.3 eV are ascribed to the satellite peaks of Ni 2p3/<sup>2</sup> and Ni 2p1/2, respectively (Oros-Ruiz et al., 2014, 2016). The above results also suggest that Ni(II) was successfully decorated on the surface of the CdS nanoparticles.

# Electrochemical Characterization

To study the efficiency of the photogenerated electron-hole, the transient photocurrent tests were carried out on all as-prepared samples (mixed-phase CdS, 0.5 wt%Ni(II)/CdS, 1.0 wt%Ni(II)/CdS, 3.0 wt%Ni(II)/CdS and 5.0 wt%Ni(II)/CdS). As shown in **Figure 5A**, it is clear that all Ni(II)-doped samples have a higher photocurrent than that of prestine CdS, indicating that the separation efficiency of photoinduced charge carriers is improved by doping Ni(II) ions into CdS lattice. The results are consistent with the PL test results (**Figure 1D**; Zou et al., 2016). **Figure 5B** shows the electrochemical impedance spectroscopy (EIS) curves of the mixed-phase CdS and Ni(II)/CdS. Comparing the Nyquist plots circle radius of the mixed-phase CdS with that of 3.0 wt% Ni(II)/CdS, it can be observed that the radius of arc curve of 3.0 wt%Ni(II)/CdS is the shortest among all tested samples, suggesting that Ni(II) ions in CdS lattice can facilitate electron-hole separation and interfacial migration (Cui et al., 2017b). It can be also inferred that the Ni(II) ions act as a carrier for transferring electrons. **Figures 5C**, **Figure S4** show the Mott-Schottky curves of the as-prepared samples. The slopes of the two Mott-Schottky curves are all positive in the range of −0.5 and 1.2 V, indicating that the samples are n-type semiconductor, which is consistent with the reported results. The flat band potentials of CdS and 3.0 wt%Ni (II)/CdS are −0.56 and −0.51 V, respectively. The flat band potential is approximately equal to the conduction band (CB) edge for n-type semiconductors. Therefore, the CB positions of CdS and 3.0 wt%Ni (II)/CdS are approximately −0.56 and −0.51 V, respectively. The valence band (VB) edge of samples can be

determined by the following equation: Eg = EVB – ECB (Jin et al., 2015; Wang et al., 2018c), where ECB is the CB edge potential, EVB is the VB edge potential, and Eg is the band gap of the semiconductors. The calculated VB positions of CdS and 3.0 wt%Ni (II)/CdS are approximately 1.83 and 1.72 V, respectively.

## Photocatalytic Performance Experiment

To study the effect of amount of doping Ni(II) on the conversion level, we have tested the photocatalytic performance of the mixed-phase CdS, 0.5 wt%Ni(II)/CdS, 1.0 wt%Ni(II)/CdS, 3.0 wt%Ni(II)/CdS, and 5.0 wt%Ni(II)/CdS for selective oxidation of toluene under visible light irradiation for 2 h. It can be seen from **Table 1** that the formation rate of benzaldehyde can be improved when the amount of Ni(II) increases from 0.5 to 3.0 wt%. The formation rate of benzaldehyde can reach to 216.70 µmolh−<sup>1</sup> g −1 using 3.0 wt%Ni(II)/CdS as catalyst after 2 h irradiation. The result suggest that 3.0 wt%Ni(II)/CdS is highly active visiblelight-driven photocatalyst for the selective oxidation of primary C–H bond in toluene. As we known, metal ions may become the capture center of photogenerated electrons, and effectively suppress the recombination of photogenerated electrons and holes, thereby improving the photocatalytic activity. As the doping amount, the trapping sites of the carriers increase, which increases the carrier lifetime and improves the separation of photogenerated electron-holes. When the doping amount is increases, the carrier capture sites are increased and the carrier lifetime is extended, resulting in a great improvement in photoelectron-hole separation. Therefore, this process enhances the photocatalytic activity. However, when the doping amount is increased to a certain extent, the distance between the trapping sites of the trapping carriers becomes small, the doping ions evolve into a recombination center of electrons and holes, and the activity of the catalyst is lowered. However, when the doping amount is increased to a certain amount, the distance between the trapping sites of the trapping carriers becomes small, resulting in the doping ions evolve into a recombination center of electrons and holes. So, the photocatalyst exhibits lower photocatalytic activity. When the doping amount of 3 wt% is an optimum value, the photocatalyst exhibits the best photocatalytic activity.

To demonstrate the general applicability of the as-prepared Ni(II)/CdS photocatalysts for selective oxidation of such primary C–H bond in alkyl aromatics, the visible light photoactivity of 3.0 wt%Ni(II)/CdS toward the selective oxidation of other substituted toluenes were further tested, and the results are listed in **Table 2**. It is clear to see that the 3.0 wt%Ni(II)/CdS is also active for the oxidation of primary C–H bond of substituted toluenes to the corresponding aldehydes. The above photoactivity results suggest that the as-prepared 3.0 wt%Ni(II)/CdS, that is

TABLE 1 | Oxidation of toluene over the as-prepared samples under visible light irradiation for 2 h.


achieved by such a very simple method at room temperature, can be used as an efficient and visible-light-driven photocatalyst toward the selective activation of primary C–H bond in a variety of substituted toluenes.

# Cycle and Possible Reaction Mechanism Experiment

The 4 times cycling test was performed over the 3.0 wt%Ni(II)/CdS to verify its stability under the same conditions. After irradiation, the reaction mixture was centrifuged to separate the catalyst, which was then washed three times with ethanol and deionized water and dried before next cycle test. In **Figure 6A**, it can be seen that the conversion of tolueneand the selectivity of benzaldehyde are almost unchanged. To further


TABLE 2 | Oxidation of toluene and substituted toluenes over the 3.0

wt%Ni(II)/CdS under visible light irradiation for 2 h.

confirm the stability of our sample as catalyst, the XRD test were performed to measure the fresh 3.0 wt%Ni(II)/CdS as well as the used 3.0 wt%Ni(II)/CdS. As shown in **Figure 6B**, the

crystal structure of 3.0 wt%Ni(II)/CdS sample does not change significantly after the photocatalytic reaction. Furthermore, considering the XRD test on stability of our sample together with the SEM images (shown in **Figure S5**) of used the 3.0 wt%Ni(II)/CdS after photocatalytic reaction, it can be inferred that the morphology and structure of the sample as photocatalyst remain unchanged during reaction, indicating that the 3.0 wt% Ni(II)/CdS composite is a stable photocatalyst for the selective oxidation of substituted toluenes to the corresponding aldehydes under the experimental conditions. Furthermore, the valence of Ni and its effect on the photocatalytic process through XPS detection of the 3.0 wt%Ni(II)/CdS after photocatalytic reaction had been analyzed. According to XPS (**Figure S6**), Cd and S present undetectable change between fresh and used 3.0 wt%Ni(II)/CdS. As shown in **Figure S6D**, two weak peaks at 850.6 and 868.9 eV can be respectively attributed to the 2p3/<sup>2</sup> and 2p1/<sup>2</sup> peaks of Ni metal (Li et al., 2015a), indicating the formation of the nickel metallic state of the catalyst during the photocatalytic reaction. It is considered that photogenerated electrons in the CdS CB tended to transfer to Ni(II) clusters and then effectively reduce a portion of Ni(II) to Ni<sup>0</sup> atoms (Weng et al., 2017). However, the combination of electrons and oxygen molecules quickly forms a process of the long-term transport of electrons. Therefore, more holes are combined with toluene to form an oxidation reaction process. This process facilitates efficient separation of carriers to ameliorate photocatalytic activity. As shown in **Figure 6C**, toluene oxidation over 3.0 wt%Ni(II)/CdS under visible light irradiation for 10 h can give a toluene conversion rate of 20.4%. It presents a great potential for the industrial synthesis for the fine chemicals. To study the role of photogenerated radical species involved in photocatalytic oxidation of toluene on 3.0 wt%Ni(II)/CdS sample under visible light irradiation and the reaction mechanism involved, a series of control experiments were carried out and the result is shown in **Figure 6D**. In the control experiments, all conditions were kept unchanged except that different scavengers (AO, BQ and AgNO3) were added to capture h+, •O − 2 and e−, respectively. As shown in **Figure 6D**, ammonium oxalate (AO) scavenger was added and the conversion of toluene was almost terminated under visible light irradiation. Adding benzoquinone (BQ) or AgNO<sup>3</sup> to the reaction system also significantly suppressed the conversion of toluene. Although electrons cannot directly participate in the oxidation of toluene, it can affect the activation of oxygen molecules. A series of control experiments result suggest that the h<sup>+</sup> play the important role in photocatalytic oxidation of toluene over 3.0 wt%Ni(II)/CdS. The O<sup>2</sup> is activated by e<sup>−</sup> to form the superoxide radicals, which acts as the main oxidant for the photocatalytic toluene process. The above results suggest that the major active species for the photocatalytic selective oxidation of toluene is h<sup>+</sup> rather than e−.

Thus, the possible mechanism is proposed, as given in **Figure 7**. Compared with the common CdS of band width (2.41 eV), the band width (2.39 eV) of the mixed phase CdS is reduced. Besides, doping Ni(II) to the photocatalyst is capable of greatly enhancing its electron transport and charge separation. The photogenerated electrons of CdS can be rapidly transferred to the Ni(II) promoter because of the positive potential of Ni2+/Ni<sup>0</sup> (−0.23 V vs. SHE, pH = 0) than the CB potential of the mixed-phase CdS (about −0.56 V, **Figure 5C**; Ran et al., 2011; Meng et al., 2018), thereby forming a long-term electron transfer process. The toluene is adsorbed on the surface of the 3.0 wt%Ni(II)/CdS and oxidized by the holes to the corresponding cationic radicals. Meanwhile, the electrons react with adsorbed O<sup>2</sup> to give activated oxygen species. The activated oxygen species then selectively oxidize the cationic radicals, finally leading to the formation of benzaldehyde.

According to the experimental and simulated results, the photocatalytic mechanism of oxidation of toluene coupling with was proposed as following scheme:

#### CONCLUSION

In summary, we have synthesized Ni(II)-doped cubic and hexagonal phases CdS semiconductor with nanosphere structure morphology by a simple method, which is able to be used as a well-photocatalyst to the activation of saturated primary C–H bond in toluene and substituted toluenes under mild conditions. The mixed phase CdS forms a homojunction, resulting in a reduction in its band width. It can effectively expand the response range and improve photocatalytic performance. Besides, doping Ni(II) to the photocatalyst is capable of greatly enhancing its electron transport and charge separation. The superior photocatalytic performance of Ni(II)-doped CdS is attributed to its unique structure assembly of specific morphology, which can be efficient transport and separation of photogenerated charge carriers under visible light irradiation. Mechanism research shows that Ni as a co-catalyst can improve the catalytic activity of the CdS. The as-prepared 3.0 wt%Ni(II)/CdS has highly active for the selective oxidation of inert primary C–H bond, which has great potential in photocatalytic selective activation of C–H bond to fine chemicals.

#### AUTHOR CONTRIBUTIONS

HS and LL designed and conducted the experiments. LL, HZ, and JH analyzed the data. HS, LL, LW, and QW wrote the paper.

#### FUNDING

This work was financially supported by the National Natural Science Foundation of China (21261021, 21663027, 51262028), the Science and Technology Support Project of Gansu Province (1504GKCA027), the Program for the Young Innovative Talents of Longyuan (NWNU-LKQN-15-2) and the Undergraduate Academic Innovative Research Team of Northwest Normal University.

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00466/full#supplementary-material

$$\mathbf{0\_2} + \mathbf{e}^\cdot \longrightarrow \longrightarrow \cdot \mathbf{0\_2}^\cdot \qquad (4)$$

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over V2O5@TiO<sup>2</sup> under simulated solar light irradiation. Green Chem. 19, 311–318. doi: 10.1039/C6GC02748B


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 She, Li, Zhou, Wang, Huang and Wang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# BiOCl Decorated NaNbO<sup>3</sup> Nanocubes: A Novel p-n Heterojunction Photocatalyst With Improved Activity for Ofloxacin Degradation

#### Jingjing Xu<sup>1</sup> \*, Bingbing Feng<sup>1</sup> , Ying Wang<sup>1</sup> , Yadi Qi <sup>1</sup> , Junfeng Niu<sup>2</sup> and Mindong Chen<sup>1</sup>

*<sup>1</sup> Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Engineering Technology Research Center of Environmental Cleaning Materials, Nanjing University of Information Science and Technology, Nanjing, China, <sup>2</sup> Research Center for Eco-Environmental Engineering, Dongguan University of Technology, Dongguan, China*

#### Edited by:

*Fan Dong, Chongqing Technology and Business*

#### *University, China* Reviewed by:

*Huogen Yu, Wuhan University of Technology, China Tianhua Zhou, Fujian Institute of Research on the Structure of Matter (CAS), China*

> \*Correspondence: *Jingjing Xu xujj@nuist.edu.cn*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *12 July 2018* Accepted: *14 August 2018* Published: *02 October 2018*

#### Citation:

*Xu J, Feng B, Wang Y, Qi Y, Niu J and Chen M (2018) BiOCl Decorated NaNbO3 Nanocubes: A Novel p-n Heterojunction Photocatalyst With Improved Activity for Ofloxacin Degradation Front. Chem. 6:393. doi: 10.3389/fchem.2018.00393* BiOCl/NaNbO<sup>3</sup> p-n heterojunction photocatalysts with significantly improved photocatalytic performance were fabricated by a facile *in-situ* growth method. The obtained BiOCl/NaNbO<sup>3</sup> samples were characterized by UV-vis absorption spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), photocurrent (PC) and photoluminescence spectroscopy (PL). The photocatalytic activity of the BiOCl/NaNbO<sup>3</sup> samples was investigated by the degradation of a typical antibiotic Ofloxacin (OFX). The experimental results showed that BiOCl/NaNbO<sup>3</sup> composites exhibited much higher photocatalytic activity for OFX degradation compared to pure NaNbO<sup>3</sup> and BiOCl. The degradation percent of OFX reached 90% within 60 min, and the apparent rate constant was about 8 times as that of pure NaNbO<sup>3</sup> and BiOCl. The improved activity can be attributed to the formation of p-n junction between NaNbO<sup>3</sup> and BiOCl. The formed p-n junction facilitated the separation of photogenerated holes and electrons, thereby enhancing photocatalytic activity. In addition, the composite photocatalyst showed satisfactory stability for the degradation of OFX. Due to the simple synthesis process, high photocatalytic activity, and the good recyclability of these composite photocatalysts, the results of this study would provide a good example for the rational design of other highly efficient heterojunction photocatalytic materials.

Keywords: photocatalysis, BiOCl, NaNbO3 , p–n junction, ofloxacin

## INTRODUCTION

The problem of water pollution has attracted increasing attention worldwide due to many toxic pollutants continue to enter into our water bodies (Ao et al., 2016b; Li et al., 2018; Yuan et al., 2018). As a green and sustainable technology, semiconductor photocatalysis can completely eliminate most pollutants in water and air (Zhang et al., 2016, 2017a,b; Chen et al., 2018; Guo et al., 2018; He et al., 2018; Yang et al., 2018). The contaminants are degraded by hydroxyl radical or other free radicals which are produced in the process after the absorption of light (Li et al., 2015; Cao et al., 2016; Xu et al., 2016; Yu et al., 2016; Zhang and Xiao, 2017; Han et al., 2018; Zhong et al., 2018). There are many semiconductors (such as oxides, sulfides, nitrates and silver based compounds etc.) can be served as photocatalysts to induce the degradation of pollutants.

In recent years, NaNbO<sup>3</sup> has attracted much attention due to its unique properties of nonlinear optics, ferroelectric, ionic conduction, photorefractive and photocatalytic properties (Katsumata et al., 2010; Lv et al., 2010; Li et al., 2012a, 2013; Fan et al., 2015; Qian et al., 2018). Because NaNbO<sup>3</sup> is a typical n type semiconductor with unique crystal [NbO6] angle eight-hedral crystal structure, it has the advantage of relative high transfer efficiency of electrons during the photocatalytic process (Shi et al., 2014b; Song et al., 2015). Recent studies have shown that NaNbO<sup>3</sup> can be used as an efficient photocatalyst for the evolution of H<sup>2</sup> and the reduction of CO<sup>2</sup> (Li et al., 2012b, 2014). In addition, NaNbO<sup>3</sup> also shows the ability for the decomposition of organic pollutants (Li et al., 2008; Li, 2010; Chen et al., 2014). However, because NaNbO<sup>3</sup> has a large bandgap, it cannot be used as a visible light responsive photocatalyst. Various experiments have shown that forming heterojunction with other semiconductor materials was a promising way to improve the photocatalytic activity of NaNbO3. For example, an effective composite photocatalyst NaNbO3/WO<sup>3</sup> has been proved to has stronger dye degradation activity compared with the corresponding single component (Shifu et al., 2013). Moreover, other NaNbO<sup>3</sup> based composite photocatalysts, such as AgO2/NaNbO<sup>3</sup> (Zhang et al., 2017), Pt/NaNbO<sup>3</sup> (Liu et al., 2016) and CdS/NaNbO<sup>3</sup> (Kumar et al., 2014), were also successfully prepared for photocatalytic degradation of pollutants.

On the other hand, although BiOCl (a typical p-type oxyhalide semiconductor) has been extensively investigated in the field of photocatalysis (Ye et al., 2014; Ao et al., 2016a; Hu et al., 2016; Mao et al., 2016; Ma Y. C. et al., 2016; Wang et al., 2018). However, the recombination rate of photogenerated charges in pure phase of BiOCl is high, resulting in its limited photocatalytic activity under UV light. It has recently been found that the photocatalytic activity of BiOCl can be promoted through the formation of p-n junctions with n type semiconductors (Rajeshwar et al., 2001; Wang et al., 2012; Fang et al., 2016). The enhanced activity can be ascribed to the fact that the internal electric field at the p-n junction interface improved the separation and transfer of photo-generated charges (Yan et al., 2017). However, as far as we know, the preparation and activity of BiOCl/NaNbO<sup>3</sup> p-n heterojunction photocatalyst have not been studied yet.

In the present work, we constructed a new type of p-n heterojunction photocatalyst (BiOCl/NaNbO3) by a simple in situ growth method. Compared with pure BiOCl and NaNbO3, the composite has better photocatalytic activity for the degradation of ofloxacin (OFX). The experimental results proved that the p-n junctions formed between BiOCl/NaNbO<sup>3</sup> composites promote the transfer and separation of photogenerated electron-hole pairs, thus enhancing the photocatalytic activity.

# EXPERIMENTAL

# Synthesis of NaNbO<sup>3</sup> Cubes

All the reagents and materials used in this experiment were analytical grades. All of them were purchased from China pharmaceutical chemical reagents Co., Ltd. NaNbO<sup>3</sup> was directly synthesized by hydrothermal method described as following. 2 g Nb2O<sup>5</sup> was added to the 120 mL NaOH solution (10 M) and stirred for 120 min before the suspension was transferred to the 200 ml Teflon-lined stainless steel autoclave. The reaction kettle was kept at 150◦C for 48 h. Subsequently, the autoclave was cooled to room temperature naturally after the reaction. Then remove the reactor and precipitate it naturally, pour the supernatant out of the precipitate and centrifugate, then wash it with deionized water and ethanol for 3–5 times. Finally, the product was dried for 12 h in a vacuum oven at 60◦C.

# Synthesis of BiOCl/NaNbO<sup>3</sup> p–n Junction Photocatalysts

BiOCl/NaNbO<sup>3</sup> heterostructures were prepared by a simple insitu growth method. A certain amount of Bi(NO3)<sup>3</sup> . 5H2O was added to 300 mL of ultrapure water and stirred for 1 h to form solution A. Meanwhile, NaNbO<sup>3</sup> powder was dispersed in 80 mL ultrapure water and sonicated for 30 min to form solution B. A certain amount of KCl was added to 60 mL ultrapure water to form solution C. Finally, B and C were slowly added to A under rapid stirring. Afterwards, the stirring was continued for 24 h. The product was then filtered and thoroughly washed with distilled water and ethanol before it was dried at 60◦C for 24 h. By Changing the amount of Bi(NO3)3·5H2O and KCl added, NaNbO3/BiOCl composites with different theoretical BiOCl mass ratios of 10, 25, 75, and 100% (expressed as BN-1, BN-2, BN-3, and BN-4) were produced. In order to facilitate comparison, pure BIOCl samples are also prepared in the same way, that is to say, no NaNbO<sup>3</sup> is added in the process of making composite samples.

# Characterization of Photocatalysts

The results of X ray diffraction (XRD) scanning of pure BiOCl, NaNbO<sup>3</sup> and BiOCl/NaNbO<sup>3</sup> were recorded by Shimadzu XRD 6100 X-ray diffractometer. The morphology and microstructure of the samples were characterized by scanning electron microscopy (SEM, Hitach S-4800). The band gap (Eg) of the sample was measured by UV-vis absorption spectroscopy at 200∼800 nm at room temperature, and the integrated ball attachment was installed on the ultraviolet visible spectrophotometer (UV-3600). Photocurrent (PC) and mott-Schottky (MS) were carried out on the traditional three electrode system (the electrochemical workstation of CHI-660D Chenhua chemical equipment company in Shanghai, China). The contrast electrode and the reference electrode are platinum electrodes and Ag/AgCl electrodes respectively.

# Photocatalytic Activity Experiments

The experimental vessel is a double-layer quartz beaker with cooling water circulation system for photocatalytic reaction. During the experiment, the temperature of the solution can be maintained at about 25 ◦C, so as to reduce the influence of temperature on the experiment. The light source used in the photocatalytic process is a 300 W xenon lamp(200 nm<λ<400 nm), and before irradiation, 25 mg photocatalyst was added to 50 mL OFX solution (5 mg/L). The suspension was stirred magnetically in the dark for 30 min to achieve adsorption/desorption equilibrium between photocatalysts and OFX. During the photocatalytic process, a certain suspension was sampled at the prescribed time intervals. The sampled suspension was centrifuged before the determination of OFX concentration.

# RESULTS AND DISCUSSION

### Characterizations

The XRD diagrams of pure BiOCl, NaNbO<sup>3</sup> and BiOCl/NaNbO<sup>3</sup> composites were shown in **Figure 1**. It can be seen that there are seven different diffraction peaks, which can be indexed to tetragonal BiOCl (JCPDS card number 01-0600) (Ma W. et al., 2016). Sharp and narrow diffraction peaks indicate good crystallinity for pure BiOCl. From the pattern of pure NaNbO3, several main diffraction peaks can be seen at 22.7, 32.5, 46.4, 52.7, 58.1, and 68.11◦ corresponding to (001), (110), (002), (021), (112), and (022) planes of NaNbO<sup>3</sup> orthorhombic phases (JCPDS card number 33-1270) (Saito and Kudo, 2013). The diffraction peaks of pure BiOCl and NaNbO<sup>3</sup> and the diffraction peaks of heterostructure BiOCl/NaNbO<sup>3</sup> composites can be clearly observed by XRD diagram. In addition, when the mass ratio of BiOCl to NaNbO<sup>3</sup> increases from 10 to 100%, the diffraction peak intensity of BiOCl increases and the corresponding NaNbO<sup>3</sup> peak intensity decreases. Furthermore, when BiOCl grows on NaNbO3, its diffraction peak is wider than that of pure BiOCl. This phenomenon indicates that BiOCl on the surface of NaNbO<sup>3</sup> cube has smaller particle size. The result indicates that the addition of NaNbO<sup>3</sup> can effectively inhibit the growth of BiOCl (Shang et al., 2009). XRD showed that the samples were

clean and had no other phase of impurity, so the composites were basically composed of BiOCl and NaNbO3.

The morphology and structure of BiOCl, NaNbO3, and BiOCl/NaNbO<sup>3</sup> composites can be clearly seen by SEM characterization. The SEM image of pure BiOCl is displayed in **Figure 2a**. As can be seen from the graph, pure BiOCl is composed of regular, square like nano plates. At the same time, we can see that the synthesized NaNbO<sup>3</sup> has irregular cubic shape and smooth surface in **Figure 2b**. A typical SEM image of the BN-3 composite is shown in **Figure 2c**. It can be seen that NaNbO<sup>3</sup> cubes are anchored by BiOCl nanosheets which are thiner and smaller than pure BiOCl. During the in-sith growth process, the NaNbO<sup>3</sup> particles played a role of heterogeneity and inhibited the

FIGURE 2 | SEM images of pure BiOCl (a), NaNbO3 (b) and BN-3 (c).

growth of BiOCl to lager ones. After analyzing the SEM image, the conclusion is in good agreement with the previous XRD analysis. BiOCl and NaNbO<sup>3</sup> form p-n junctions at the interface because of the close contact between BiOCl and NaNbO3. This improves the carrier transport rate and separation efficiency in photocatalysis and helps to improve the activity.

The optical properties of the samples have a great influence on the utilization of sunlight. Therefore, UV-DRS for pure BiOCl, NaNbO<sup>3</sup> and BiOCl/NaNbO<sup>3</sup> composites are investigated. As we can see from **Figure 3**, the absorbance threshold of NaNbO<sup>3</sup> and pure BiOCl are at about 400 and 380 nm, respectively. The band-gap of photocatalysts can be obtained from the following equation:

$$\text{Eg} = 1240/\lambda,$$

where Eg is the band-gap energy and λ is the cut off wavelength. Therefore, the band gaps of NaNbO<sup>3</sup> and BiOCl prepared were 3.10 and 3.26 eV, respectively, which are similar to those reported previously (Shi et al., 2014a; Ma W. et al., 2016). It can also be seen

from **Figure 3** that BiOCl/NaNbO<sup>3</sup> exhibits higher UV absoption performance compared to pure BiOCl. Furthermore, the band edge of BiOCl/NaNbO<sup>3</sup> exhibits a little red shift compared to pure BiOCl.

Through the band structure of the catalyst, we further explore the way of carrier transfer. The type and surface potential of NaNbO<sup>3</sup> and BiOCl semiconductors are determined by the Mott-Schottky plots. The positive tangent in the graph indicates that the sample is n type semiconductor, while the negative tangent in the graph indicates that the sample is p type semiconductor (Kuang et al., 2015). The results shown in **Figures 4A,B** show that BiOCl and NaNbO<sup>3</sup> are p-type and n-type semiconductors, respectively. In addition, the flat potential (relative to Ag/AgCl) of BiOCl and NaNbO<sup>3</sup> are 2.30 V and −0.70 V respectively through the intersection point between tangent and abscissa. According to the following equation, the potential (relative to Ag/AgCl) can be converted to a normal hydrogen electrode (NHE). (1) (Zhou et al., 2017; Liu et al., 2018):

$$\mathbf{E\_{NHE}} = \mathbf{E\_{Ag/A\&Cl}} + \mathbf{E\_{Ag/AgCl}}$$

where E<sup>0</sup> Ag/AgCl <sup>=</sup> 0.197 V. In particular, the maximum of the valence band (VB) is about 0.1 V lower than the flat potential of the p-type semiconductor; the minimum value of the conduction band (CB) is about 0.1 V higher than the flat potential of the ntype semiconductor (Liu et al., 2017b). Therefore, the VB position of BiOCl and the CB position of NaNbO<sup>3</sup> are 2.60 and −0.60 V (vs. NHE), respectively. The Eg of BiOCl and NaNbO<sup>3</sup> were 3.26 and 3.10, respectively. Therefore, according to Eg. (2) (Liu et al., 2017a):

$$\text{Eg} = \text{E}\_{\text{VB}} - \text{E}\_{\text{CB}}$$

the position of CB and VB for BiOCl and NaNbO<sup>3</sup> are −0.66 and 2.50 V, respectively.

#### Photocatalytic Activity

The photocatalytic activity of BiOCl/NaNbO<sup>3</sup> composites with p-n junctions is mainly studied on the degradation of OFX under UV irradiation. As shown in **Figure 5A**, In the presence of NaNbO<sup>3</sup> or BiOCl, about 20% OFX are decomposed within

60 min. It can also be seen that all BiOCl/NaNbO<sup>3</sup> composites exhibit significantly higher photocatalytic activity than that of pure BiOCl and NaNbO3. In addition, it was found that the mass ratio of BiOCl has a significant effect on the activity of BiOCl/NaNbO<sup>3</sup> composites. Specifically, the activity increase gradually when the BiOCl content is increased from 10 to 75%. When the content of BiOCl is further increased to 100%, the activity of the sample begin to decrease. Therefore, sample BN-3 with BiOCl mass ratio of 75% exhibites the highest activity (the degradation percent of OFX is about 90% after 60 min irradiation). The time evolution of the OFX solution spectra in the degradation process by BN-3 were measured and are shown in **Figure 5B**. The absorption peak of OFX decreases as the increase of irradiation time and reaches the minimum at 60 min.

It has also been found that the photocatalytic degradation of OFX follows pseudo-first order kinetic calculated from the following formula (as shown in **Figure 5C**):

$$-\ln(\mathcal{C}\_t/\mathcal{C}\_0) = \text{kt}$$

The obtained k of OFX removal rate of pure BiOCl, NaNbO<sup>3</sup> and BiOCl /NaNbO<sup>3</sup> composites can be calculated by this formula. In **Figure 5D**, it is more clear to see that the kinetic constants of BiOCl, NaNbO3, BN-1, BN-2, BN-3, and BN-4 are 0.0021, 0.002, 0.0071, 0.0094, 0.016, and 0.011 min−<sup>1</sup> , respectively. Therefore, compared with the single phase BiOCl and NaNbO3, the BiOCl/NaNbO<sup>3</sup> composite can promote the separation of the electron-hole pairs on the interface, thus promoting the enhancement of the photocatalytic activity.

The stability of one photocatalyst is very important to the practical application. Therefore, the recycle experiments are carried out for BN-3. As shown in **Figure 6**, the photocatalytic performance of the catalyst has only slight loss after four cycles of reutilization of the photocatalyst. This experimental phenomenon indicates that the photocatalyst is stable to the photodegradation of OFX. The experimental results show that the photocatalytic degradation of organic pollutants by the prepared BiOCl/NaNbO<sup>3</sup> composites is stable, and it is of great significance for the practical application of the catalyst.

#### Photocatalytic Mechanism

The interfacial charge transfer kinetics between BiOCl and NaNbO<sup>3</sup> can be further elucidated by photocurrent measurements. **Figure 7** shows the photocurrent-time curves of the BiOCl, NaNbO3, and BN-3 composites under an on/off cycle of intermittent UV irradiation. As shown in **Figure 7**, BN-3 composite exhibits much higher photocurrent than

FIGURE 7 | Transient photocurrent responses for NaNbO3, BiOCl, and BN-3 composite.

that of pure BiOCl and NaNbO3. Therefore, the BN-3 can transfer and separate photogenerated electron hole pairs more efficiently under ultraviolet light irradiation. In addition, the photoluminescence (PL) spectra of pure BiOCl, NaNbO<sup>3</sup> and BiOCl/NaNbO<sup>3</sup> composite samples were tested to determine the separation efficiency of photo-generated charges. The excitation wavelength used in the test is 285 nm. In **Figure 8**, it is found that pure NaNbO<sup>3</sup> is the strongest relative to other sample emission peaks within the range of 350–550 nm. For pure BiOCl samples, their emission peaks are relatively weak relative to pure NaNbO3. The PL luminescence strength of BN-3 composites is the lowest in the three samples. The results show that the combination of pure BiOCl and NaNbO<sup>3</sup> can effectively inhibit the recombination of electron hole pairs. The results are in good agreement with the measurement results of photocurrent.

In order to study the degradation mechanism, the main active species produced in the degradation process were determined. Therefore, three radical scavengers such as isopropanol (IPA, •OH scavenger) (Cao et al., 2012), ammonium oxalate (AO, h <sup>+</sup> scavenger) (Chang et al., 2013) and p-benzoquinone (BQ, •O2<sup>−</sup> scavenger) (Sun et al., 2017) have been added separately in the degradation systems. When BQ and AO were introduced, the degradation rate of OFX decreased significantly (see **Figure 9**). Moreover, the degradation rate of OFX was also decreased in the presence of IPA. These results indicate that h<sup>+</sup> and •O2<sup>−</sup> play an important role in the degradation process. •OH has a certain effect on the degradation of OFX. Therefore, we can conclude that the free radicals of •O2<sup>−</sup> , h<sup>+</sup> and •OH in active species play a certain role in the degradation of OFX.

The formation of heterojunction in the composite system has a positive effect on the separation of photogenerated electrons and holes. Based on the above experimental results, we propose a reasonable photocatalytic mechanism for BiOCl/NaNbO<sup>3</sup> (**Scheme 1**). The enhanced photocatalytic activity of BiOCl/NaNbO<sup>3</sup> heterojunction originates from the

degradation of OFX over BN-3 under UV light irradiation.

unique matching band location of these two semiconductors after forming heterojunction. Based on the experimental data, we know that the band gap of BiOCl is larger than that of NaNbO3. The Fermi level (E<sup>f</sup> ) of BiOCl is located near the valence band because it is a p type semiconductor, and the Fermi level of n type semiconductor NaNbO<sup>3</sup> approaches the conduction band. When the mixture of BiOCl and NaNbO<sup>3</sup> forms a p-n type heterojunction of BiOCl/NaNbO3, the Fermi level of BiOCl moves upwards, while the Fermi level of the NaNbO<sup>3</sup> moves downward until the Fermi level of BiOCl and NaNbO<sup>3</sup> is at the same level. Finally, the CB position of BiOCl becomes higher than NaNbO<sup>3</sup> after moving up. At the same time, BiOCl and NaNbO<sup>3</sup> form an internal electric field in the final equilibrium state. At the same time, the valence band (VB) of NaNbO<sup>3</sup> is also lower than that of BiOCl. Due to the special band structure of the BiOCl/NaNbO<sup>3</sup> composite, the photoelectrons are excited from the VB of the BiOCl to the CB, because the special ladder like structure of the BiOCl/NaNbO<sup>3</sup> heterostructure leads to the electronic transfer from CB of BiOCl to the CB of NaNbO3. Electrons stored in NaNbO3's CB can react with dissolved O<sup>2</sup> to form •O2<sup>−</sup> , while the adsorbed hydroxy or H2O molucules can also react with h<sup>+</sup> to form •OH or directly oxidize organic pollutants. Therefore, the formed p-n junction between BiOCl and NaNbO<sup>3</sup> provide an effective electron transfer path to reduce the recombination of electron-hole pairs, thus improving the activity of the as fabricated BiOCl/NaNbO<sup>3</sup> composites.

## REFERENCES


# CONCLUSIONS

In this work, BiOCl/NaNbO<sup>3</sup> composites with p-n heterojunctions were synthesized by simple hydrothermal method. Through experiments and characterization, it is confirmed that p-n junctions formed on the interface between BiOCl and NaNbO3. The results indicted that the separation efficiency of photo-generated carriers has been greatly improved because of the formation of p-n junctions. Recycle experiments showed that the composite photocatalyst exhibited good stability. The experimental results also show that pollutants are mainly degraded by h<sup>+</sup> and ·O2<sup>−</sup> radicals. Therefore, BiOCl/NaNbO<sup>3</sup> heterojunction nanocomposites can be used as a highly efficient and reusable photocatalyst, which has potential applications in the removal of organic pollutants.

# AUTHOR CONTRIBUTIONS

JX and MC designed the project, guided the study and polished the manuscript. BF, YW, and YQ conducted the experiments and characterized the samples. JN revised the manuscript.

# ACKNOWLEDGMENTS

We are grateful for grants from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Xu, Feng, Wang, Qi, Niu and Chen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# A Novel Synthetic Method for N Doped TiO<sup>2</sup> Nanoparticles Through Plasma-Assisted Electrolysis and Photocatalytic Activity in the Visible Region

Tae Hyung Kim<sup>1</sup> , Gwang-Myeong Go<sup>2</sup> , Hong-Baek Cho<sup>2</sup> , Yoseb Song<sup>2</sup> , Chan-Gi Lee<sup>1</sup> \* and Yong-Ho Choa<sup>2</sup> \*

*<sup>1</sup> Advanced Materials and Processing Center, Institute for Advanced Engineering (IAE), Yongin, South Korea, <sup>2</sup> Department of Fusion Chemical Engineering, Hanyang University, Ansan, South Korea*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Guohong Wang, Hubei Normal University, China Jingyu Wang, Huazhong University of Science and Technology, China*

#### \*Correspondence:

*Chan-Gi Lee cglee@iae.re.kr Yong-Ho Choa choa15@hanyang.ac.kr*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *06 July 2018* Accepted: *13 September 2018* Published: *02 October 2018*

#### Citation:

*Kim TH, Go G-M, Cho H-B, Song Y, Lee C-G and Choa Y-H (2018) A Novel Synthetic Method for N Doped TiO*2 *Nanoparticles Through Plasma-Assisted Electrolysis and Photocatalytic Activity in the Visible Region. Front. Chem. 6:458. doi: 10.3389/fchem.2018.00458* Nitrogen doped TiO<sup>2</sup> (N-TiO2) nanoparticles were synthesized via a novel plasma enhanced electrolysis method using bulk titanium (Ti) as a source material and nitric acid as the nitrogen dopant. This method possesses remarkable merits with regard to the direct-metal synthesis of nanoparticles with its one-step process, eco-friendliness, and its ability to be mass produced. The nanoparticles were synthesized from bulk Ti metal and dipped in 5–15 mmol of a nitric acid electrolyte under the application of AC 500 V, the minimum range of voltage to generate plasma. By controlling the electrolyte concentration, the nanoparticle size distribution could be tuned between 12.1 and 24.7 nm using repulsion forces via variations in pH. The prepared N-TiO<sup>2</sup> nanoparticles were calcined at between 100 and 300◦C to determine their photocatalytic efficiency within the visible-light region, which depended on their crystal structure and N doping content. Analysis showed that the temperature treatment yielded an anatase TiO<sup>2</sup> crystalline structure when the N doping content was varied from 0.4 to 0.54 at.%. In particular, the 0.4 at.% N doped TiO<sup>2</sup> catalyst exhibited the highest catalytic performance with quadruple efficiency compared to the P-25 standard TiO<sup>2</sup> nanoparticles, which featured a 91% degradation of methyl orange organic dye within 300 min. This solid-liquid reaction based on plasma enhanced electrolysis could open new pathways with regard to high purity mass producible ceramic nanoparticles with advanced properties.

Keywords: plasma enhanced electrolysis, amorphous N-doped TiO2 , visible region photocatalyst, metal-direct synthesis, nanoparticle synthesis

## INTRODUCTION

Intensive research has been performed to produce highly photocatalytic titanium dioxide (TiO2) nanoparticles (Ba-abbad et al., 2012; Mattioli et al., 2014; Vukoje et al., 2016) for a large application space, including water splitting (Galinska and Walendziewski, 2005; Ni et al., 2007; Tang et al., 2008) and industrial pollutant degradation (Liu et al., 2010; Lin et al., 2012; Dong et al., 2015). However, crystalline TiO<sup>2</sup> nanoparticles possess limited photocatalytic activity due to their wide band gap regardless of crystal structure from anatase to rutile (anatase ∼ 3.2 eV, rutile ∼ 3.0 eV);

**58**

the nanoparticles only absorb within the UV region, comprising a small portion of sunlight of <5% of the solar flux. Thus, to efficiently enhance the photocatalytic activity, many researchers have focused on utilizing the visible region, which contains nearly half of the energy from sunlight (∼45%) by doping metal cations (Ag, C, Zn, etc.) into the Ti lattice (Cao et al., 2008; Aware and Jadhav, 2015). However, cation dopants located at d-states within the band gap act as trap centers for excited electron and holes, which can additionally shift the conduction band below the redox potential (Wu et al., 2007). Thus, some drawbacks exist such as a low thermal stability and can act as recombination centers in the form of charge carriers. Alternatively, anion dopants such as N, S, and P have attracted considerable attention due to their intrinsic properties, which possess similar characteristics to oxygen. Among those anions, nitrogen appears to be the most plausible ingredient to be investigated for doping with the formation of a metastable center, low ionization energy state, and reduced atomic size (Dunnill et al., 2011). The light absorption range can thus be extended into the visible region (up to 380 nm) through existing p-states near the valance band (Scanlon et al., 2013).

The preparation of N-TiO<sup>2</sup> nanoparticles has been extensively studied with aqueous-based chemical processes such as sol-gel (Yu et al., 2007; Dunnill et al., 2011; Caratto et al., 2012), hydrothermal (Zhou et al., 2011), solvothermal (Li and Liu, 2009; Yang et al., 2010), and polymeric precursor methods (Soares et al., 2011; Yu et al., 2017) being the most frequently employed. These are general methods for the preparation of nanoparticles that can be used to control morphology, structure, and particle size. However, synthetic conditions such as the solution temperature, pH, reaction time, and precursor ratio must be sensitively controlled. Moreover, long reaction times and drying processes are essential in order to obtain highly pure and crystalline nanoparticles, making mass production difficult. To overcome these drawbacks, a noble synthetic strategy using plasma enhanced electrolysis was adopted herein. The process utilizes a small amount of electrolyte, characterizes a quick production time, and entails the direct synthesis of nanoparticles from bulk metal in a manner suitable for mass production. Furthermore, this technology is environmental friendly due to employing a reusable electrolyte and not requiring an additional nitrogen source such as ammonia (Gao et al., 2014).

Here we report a novel method for the preparation of interstitial N-TiO<sup>2</sup> nanoparticles by bulk-direct synthesis using an eco-friendly and mass producible method via plasma enhanced electrolysis, enhancing photocatalytic activity through the narrowing of band gap structures, which will widen the light absorption region into the visible spectrum. During the nanoparticle synthetic process, arc plasma was formed on the metal surface, creating gaseous anionic species that subsequently reacted with cationic Ti species to grow into N-TiO<sup>2</sup> through nucleation. The relationship between N doping content and nanoparticle crystallinity, which primarily affected the photocatalytic efficiency was analyzed and elucidated by comparing samples prepared through the variation of electrolyte concentrations and calcination temperatures.

# MATERIALS AND METHODS

# Synthesis of Amorphous N-TiO<sup>2</sup> Nanoparticles

**Figure 1** shows a brief schematic diagram for the synthesis of N-TiO<sup>2</sup> nanoparticles by the generation of plasma. A Ti metal plate (99.99% purity) was first washed twice with acetone and deionized water in order to remove other possible impurities. The organic-free Ti plate was hanged in 1 L of a nitric acid electrolyte in the middle of a double-jacket reactor. The Ti metal was connected to an anode and stainless steel mesh (SUS 304, 60 mesh) with a cylindrical shape that was connected to the cathode; the shape of the cathode was designed to radially impart an equal supply of electric voltage to the metal surface. The solution temperature was controlled using a bath circulator and stirred with a magnetic bar to exchange heat efficiently under the high temperature plasma reaction. After the initial generation of plasma on the metal surface, the voltage and current density of the power supply was maintained at certain conditions such as AC 500 V at 100 Hz for 10 min to synthesize nanoparticles continuously; the nitric acid concentration was varied from 5 to 15 mmol to observe the effect of the electrolyte concentration on the morphology of TiO2. After the reaction, the nanoparticle suspension was centrifuged to remove the remaining electrolyte and dried in a vacuum oven at 80◦C for 24 h. The as-synthesized powders were calcined at different temperatures: 100, 150, 200, 250, and 300◦C to transform amorphous TiO<sup>2</sup> into the crystalline anatase phase to investigate their respective photocatalytic activity.

# Sample Characterization

Scanning electron microscopy (SEM) images were obtained with a Hitachi S-4800/Horiba EX-250 to analyze the morphology of the synthesized N-TiO<sup>2</sup> nanoparticles. High-resolution transmission electron microscopy (HR-TEM) images were obtained with a JEOL, JEM-2100F to measure and calculate lattice parameters with selected area electron diffraction (SAED) patterns. The nanoparticle crystallinity and structures were analyzed with a RigaKu D/MAX-2500/PC X-ray diffractometer (XRD) using Cu Kα<sup>1</sup> radiation (λ = 1.54 nm 2θ range between 20 and 80◦ ). X-ray photoelectron spectroscopy (XPS) was performed with a Thermofisher Scientific K-Alpha+ MXP10. The photocatalytic activity of the N-TiO<sup>2</sup> nanoparticles was assessed by analyzing the visible region efficiency of the decomposing methyl orange (MO, Sigma-Aldrich Co.) solution in a dark chamber equipped with a UV cut-off filter (Edmund Optics, 400–50 mm Diameter, OD 2 long pass filter) and a 300 W Zenon lamp. While being exposed to visible light, degraded methyl orange was collected with nanoparticles at 2 h intervals and separated via centrifugation. After separation, a Dong-il Shimadzu Co. UV-2600 UV-vis spectrophotometer was employed to calculate the photocatalytic activity. To compare the recombination rates of the samples, photoluminescence (PL) measurements were performed using a 290 nm laser (Horiba, Nanolog) as the excitation source at room temperature.

# Photocatalytic Activity Analysis

The photocatalytic efficiency of the N-TiO<sup>2</sup> samples was compared after various degrees of heat treatment: 100, 150, 200, 250, and 300◦C for 4 h to transform the crystal structure and yield variations with regard to the degree of N doping with Degussa P-25 standard TiO<sup>2</sup> nanoparticles. After calcination, 100 mg of nanoparticles were added and mixed with 100 ml of a 1 × 10−<sup>5</sup> mol/L MO organic dye solution to measure the decomposition efficiency of each sample. The 100 mg/L powder mixed MO solution was sonicated for 10 min prior to the degradation step to enhance solution dispersity. The well-dispersed organic solution was placed in a dark chamber and stirred for an additional 30 min for stability. The degradation process was performed in a doublejacketed reactor to control the solution temperature and was irradiated with light using a 300 W Xe lamp with a UV cut-off filter. The reaction solution was collected every 2 h and analyzed via UV-vis spectroscopy to calculate the degradation efficiency for a 6 h long reaction.

# RESULTS AND DISCUSSION

# Synthesis of N-TiO<sup>2</sup> Nanoparticles

**Figures 2a–c** show the TiO<sup>2</sup> nanoparticle morphologies produced at different concentrations of nitric acid. The particle size distribution analyzed via SEM decreased between the range of 12.1–24.7 nm as the nitric acid concentration was increased (size distribution in **Figure 2d**). The mean particle size decreased to 24.7, 19.4, and 12.1 nm at concentrations of 5, 10, and 15 mmol, respectively. It could be seen that at lower concentrations, some rod-like nanoparticle shapes could be found. Irregular shaped nanoparticles indicated unbalanced nanoparticle synthesis between the nucleation and growth steps. At higher concentrations with a lower pH electrolyte, more uniform and non-agglomerated nanoparticles could be produced. This could be attributed to two factors; repulsion forces and balance between the nucleation and growth steps (Alqadi et al., 2014). When the electrolyte solution pH decreased to lower values, ions such as OH−, which reacted with metal ions, scarcely existed and facilitated a slower nucleation and growth process. This also increased repulsion forces between nanoparticles, thus producing much more uniform nanoparticles with a smaller size distribution than at higher pH values (Wu et al., 2011). At higher electrolyte concentrations, the solution conductivity increased due to a larger quantity of ionized ions such as H<sup>+</sup> and SO<sup>−</sup> 4 (when H2SO<sup>4</sup> was used as an electrolyte), yielding a higher current density that was related to the plasma intensity; this would produce transparent solutions without forming colloid nanoparticles, thus necessitating the use of additional reducing agents (Kim et al., 2016). As a result, 15 mmol was the optimum nitric acid concentration for nanoparticle synthesis.

**Figures 2d,e** show size distribution and XRD data for the synthesized amorphous N-TiO<sup>2</sup> nanoparticles. The size distribution showed the nanoparticles ranging from 12.1 to 24.7 nm (**Figure 2d**); the size deviation increased noticeably from ±4.7 to ±22.2 nm as the electrolyte concentration increased. A broad peak at the lower degree with no other specific peaks

in the XRD data (**Figure 2e**) indicated the pristine amorphous structure of N-TiO2. Moreover, no peaks for the Ti bulk metal starting material were detected, implying that Ti was completely transformed to TiO<sup>2</sup> nanoparticles by adopting the plasma enhanced electrolysis process. If detached from the metal surface, the particle peaks would exhibit XRD peaks for bulk Ti and several irregularly shaped particles.

After the synthesis of nanoparticles using a 15 mmol electrolyte concentration, amorphous N-TiO<sup>2</sup> nanoparticles were calcined at various temperatures to compare the XRD peaks between the treated nanoparticles in **Figure 3**. The as-prepared amorphous N-TiO<sup>2</sup> nanoparticles revealed that the amorphous phase was transformed to a metastable anatase structure after being calcined at 100, 150, 200, 250, and 300◦C for 4 h. In **Figure 3A**, analyzed peaks for the (101), (004), (200), and (105) lattice planes matched those for anatase TiO<sup>2</sup> (JCPDS card no. 21-1272) and no nitrogen source peaks were detected, which indicated that nitrogen was perfectly doped into the TiO<sup>2</sup> lattice (as confirmed in **Figure 6**). With an increase in calcination temperature, the XRD peak intensity also increased, indicating that the calcination temperature enhanced the crystallinity of the N-TiO<sup>2</sup> nanoparticles with a deformation of amorphous bonding between atoms during calcination. Additional evidence for nitrogen doping was observed by a shift in the enlarged peak of the (101) lattice in **Figure 3B**. Due to the larger atomic radius of nitrogen compared to oxygen (N3<sup>−</sup> = 171 Ã > O2<sup>−</sup> = 140 Ã), a large degree of nitrogen doping would lead to XRD peak broadening effects. A reduction in doping with an increase in the calcination temperature could shift the peak to much higher diffraction angles compared to non-doped TiO<sup>2</sup> (Sun et al., 2016). This result will be compared and identified via XPS and PL analysis in later sections.

To analyze a non-agglomerated single nanoparticle, a welldispersed colloid solution was loaded onto a Cu grid to perform TEM analysis. According to the TEM images of amorphous N-TiO<sup>2</sup> structures in **Figures 4a,b**, there were no signs of lattice fringes in the nanoparticles that matched the XRD analysis in **Figure 2**. The primary particle size distribution of amorphous N-TiO<sup>2</sup> in **Figure 4c** revealed a primary mean particle size of 6.57 nm, which was smaller than that in the SEM images mentioned previously (in **Figure 2**). After calcination at 200◦C, the polycrystalline structure with a ring SAED pattern identified the anatase phase of N-TiO<sup>2</sup> with a lattice plane of (101), (004), (200), and (105) in **Figures 4d–f**. The mean primary particle size was 8.37 nm, indicating a minor growth in morphology after calcination (**Figure 4g**).

## XPS Analysis of N-TiO<sup>2</sup> Nanoparticles From Different Calcination Temperatures

The N doping concentration in the TiO<sup>2</sup> lattice primarily influenced the electronic structure according to the doping position and state. Thus, synthesized N-TiO<sup>2</sup> nanoparticles at different calcination temperatures were analyzed via XPS to compare the dopant tendency, as can be seen in **Figure 5**. The 1s peak located at 284.8 eV was used to calculate the peak shift. The peak at 398.7 eV in **Figures 5A,B** was evidence of the replacement of N dopants instead of O sites with N-Ti-O bonds (Mohamed et al., 2015)**.** When the N 1s state at 399.6 eV, which was identical and compared to the interstitial doping state with N-Ti-O bonds (**Figure 5C**), it could be confirmed that the N concentration decreased dramatically from 0.54 to 0.4 at.% by increasing the calcination temperature and lowering the intensity of the N peak, as summarized in **Figure 5D**, which was consistent with that reported by Cheng et al. (2012). To discuss the interstitial

and substitutional N doping states in TiO<sup>2</sup> lattices, the energy conservation law equation was adapted:

$$E\_f = E\_{\text{TIO}\_2:N} - E\_{\text{TIO}\_2} - \frac{m}{2}E\_{N\_2} + \frac{n}{2}E\_{O\_2},\tag{1}$$

where ETiO<sup>2</sup> : <sup>N</sup> and ETiO<sup>2</sup> are the total energies for N-doped TiO<sup>2</sup> and pure TiO2, EN<sup>2</sup> and EO<sup>2</sup> indicate the gas energies of N<sup>2</sup> and O<sup>2</sup> molecules, and m and n are integers of doped N atoms and substituted O atoms. According to this theory, energies tended to increase when the N doping level increased by acting as a formed impurity, making it more difficult to achieve a higher doping condition. Also, doping with N atoms in the lattice instead of O atoms required greater bonding energies; interstitial doping was more generally preferable (Zhao and Liu, 2008). In this scenario, the plasma enhanced electrolysis reaction using 5–15 mmol nitric acid could only yield interstitial N doped TiO<sup>2</sup> nanoparticles.

# Band Gap Analysis Depending on N Content

**Figure 6** and **Figure S1** shows the diffuse reflectance spectra of Degussa P-25, amorphous, and various N-TiO<sup>2</sup> nanoparticle samples calcined at 100, 150, 200, 250, and 300◦C, which possess various N doping molar ratios. The peaks of each sample showed absorptions between 300 and 400 nm in **Figure 6A** and the enlarged peaks in **Figure 6B**, which exhibited a calculated band gap of 3.4, 3.0, 2.96, 2.94, 2.94, 2.97, and 2.98 eV, respectively. The

band gap values exhibited a tendency to red shift toward lower wavelengths with an increase in N doping content that strongly affected the absorption region. Thus, it could be anticipated that nitrogen sources from nitric acid reacted with Ti ions during plasma generation in a form of interstitial doping and modifying the energy state of the TiO<sup>2</sup> nanoparticle band gap, which could be reduced toward the visible region.

# Photocatalytic Activity Depending on N Doping Content and Crystallinity

To observe and compare photocatalytic activity depending on the calcination temperature and N doping content ratio, samples were mixed with a 1 × 10−<sup>5</sup> MO solution and the degradation efficiency was analyzed. In **Figures 7A–H**, the asprepared solution was irradiated with visible light (>400 nm) and the photocatalytic decomposition process was initiated. Degussa P-25 in **Figure 7A**, a standard TiO<sup>2</sup> nanopowder, exhibited a reduced degradation activity of the MO solution, which was only 21% under visible region irradiation. The calcined samples in **Figures 7B–F** at 100–300◦C of the anatase phase with various degrees of N doping and crystallinity exhibited an enhanced maximum efficiency activity of 91% that was 4 times greater than the degradation performance of P-25 within a reaction period of 6 h. The 200◦C calcined N-TiO<sup>2</sup> nanoparticles in **Figure 7D** yielded the greatest photodegradation results and recyclability was measured by cycling test in **Figure S2**. These results indicated that the anatase structure with 0.4 at.% N doping performed much better than the P-25 standard nanoparticles; the other calcination temperature samples yielded somewhat unpredictable phenomena, which should be discussed with regard to crystallinity and N doping content. The band gap of TiO<sup>2</sup> generally depended on the electron state of the doping species and content. Among those, the N doping effect was widely accepted; in the TiO<sup>2</sup> crystalline lattice, N incorporated and modified the electronic band structure, forming an N 2p band above the original O 2p valance band leading to the formation of a narrower band gap of minimum 2.5 eV and shifting the absorption region toward that of visible light. Undoped anatase TiO<sup>2</sup> structure has larger band gap of 3.2 eV, which cannot be excited with visible light. On the other hand, when the Ndoped TiO<sup>2</sup> was exposed in the visible region, valence band at N 2p state above original O 2p reduces band gap and generates electrons in the conduction band while remaining holes in the valence band. The generated electrons on conduction band captures oxygen molecules in solution which produces highly reactive superoxide anions and hydroxyl radicals on the surface while holes in the valence band produces highly oxidative hydroxyl radicals by reacting with OH<sup>−</sup> ions (Ansari

et al., 2016). Not only N doping but also the Ti3<sup>+</sup> donor contributed toward the absorption of light. Under visible light irradiation, nanoparticles excited and produced electron-hole pairs, which diffused into the particle surface. These electrons reduced Ti4<sup>+</sup> into Ti3<sup>+</sup> species that existed under the conduction band unless they were not trapped (Yang et al., 2010). However, prior studies proposed and certified that the N state above the valance band and Ti3<sup>+</sup> that was produced by Ti4<sup>+</sup> capturing electrons on the 1.2 eV conduction band that narrowed the band gap state could act as electron traps near the oxygen vacancy sites, reducing the electron-hole recombination duration time. Another parameter that affected the photodegradation efficiency was the crystallinity of the nanoparticles. It was already known that an amorphous-like layer of nanoparticles would occur due to its adsorption of water molecules on the particle surface. This would reduce the catalytic performance by masking interactions between particles and dye molecules. By removing the adsorbed species and increasing crystallinity, the photocatalytic properties could be enhanced (Yang et al., 2011). Thus, the conditions to obtain the highest photocatalytic properties within the visible light region should be optimized by adjusting the degree of crystallinity and N doping content (Jang et al., 2001; Zhoa et al., 2008; Wang et al., 2009). Following this approach, N-TiO<sup>2</sup> calcined at 100◦C with a doping content of 0.52 at.% could be applied as a trapping agent; the photodegradation efficiency was reduced compared to the 200◦C condition. Additionally, at lower calcination temperatures, physically embedded or adsorbed water molecules and applied electrolytes on the surface were crucial factors that limited active sites, motivating the reduction in degradation efficiency. At conditions >250◦C, negligible N doping content was observed due to an unstable state amid a highly crystalline structure; the poor degradation properties compared to P25 indicated that N doping was passivated by oxidation and returning the material to its original anatase

structure while widening the band gap, which would limit its contribution toward enhancement of the visible light region catalyst.

#### Photoluminescence Analysis From Various Calcination Temperatures

To evaluate the radiative recombination of excited electrons and holes, synthesized N-TiO<sup>2</sup> with 200◦C calcined samples were analyzed via PL spectra under an excitation wavelength of 290 nm and compared to an undoped P-25 TiO<sup>2</sup> powder reference in **Figure 8**. Excited peaks in the spectra were observed at 390, 424, 441, and 452 nm indicating different emission sources. The peak at 390 nm could be attributed to the conduction to valence band energy state transition, peaks at 424 and 452 nm were signals from free excitations at the band edge and the excitation peak at 441 nm was due to localized self-trapped excitons (Jagadale et al., 2008; Kho et al., 2010; Ablat et al., 2014). In addition to the PL analysis, the intensity virtually decreased for the 200◦C calcined sample, showing that the lifetimes of excited electrons and holes with regard to the N doped TiO<sup>2</sup> nanoparticles were greatly enhanced and contributed to decomposition reactions with organics until the electrons and holes recombined. These results coincided with the photocatalytic activity and XPS analysis showing that for temperatures >250◦C, the N doping content was eliminated and the band gap recovered to its original value. Thus, photocatalytic activity would only occur within the UV region.

# Growth Mechanism of N-TiO<sup>2</sup> Nanoparticles via Plasma Enhanced Electrolysis

To clarify the growth mechanism of the N-TiO<sup>2</sup> nanoparticles using plasma assisted electrolysis, a schematic diagram can be

seen in **Figure 9** with the following description: (1) During the initial stage, low voltages produced a porous thin TiO<sup>2</sup> oxide layer due to anodization during the pre-spark treatment period. (2) This increased the resistivity between the anode-electrolyte, forming an electric field strong enough to liberate and form as gas enveloped the anode surface and initially increased the applied voltage. (3) High voltages greater than the breakdown voltage at the electron avalanche birthplace excited the electrolyte near the anode via ionization and dissociation, transforming gases into ions such as NO2−, N2−, and O2−, which were subsequently localized onto the anode surface by applying considerable kinetic energy (Mikheev et al., 2015). (4) The ionized gaseous layer provided sufficient energy to initiate Joule heating, which entailed high temperatures and pressures, forming a plasma discharge within the gas envelope. Such arc plasma created Ti4<sup>+</sup> metal cations from the outer surface of the anode into the electrolyte solution while the plasma gradually penetrated the inner part of the anode. (5) The metal cations then reacted with the asprepared anionic gaseous species; the ionized cations initially reacted with N species from the ionization of nitric acid, a doping source of nitrogen, forming N-Ti-O bond structures within the Ti lattice through nucleation and growth mechanism; N-TiO<sup>2</sup> nanoparticles were subsequently produced and assisted by controlled temperature induced cooling processes (Cheng et al., 2012). Moreover, the scarcity of existing anions in solution could be attributed to the repression of particle formation; acid-based nanoparticle synthesis conditions would prevent agglomeration between particles.

#### CONCLUSIONS

Through the adoption of a novel synthetic method for amorphous N doped TiO<sup>2</sup> nanoparticles using plasma enhanced electrolysis, massive N-doped TiO<sup>2</sup> nanoparticles could be produced by merely supplying sufficient voltage to the anode, which was a metal source. This was because the synthetic phenomenon of the metal oxide nanoparticles occurred simultaneously via plasma generation with a rapid reaction.

Arc plasma formation was performed by applying a 500 V AC electric field to the nitric acid electrolyte. The particle morphology and size was verified from 12.1 to 24.7 nm relying on various nitric acid concentrations between the range of 5–15 mmol influenced by repulsive forces during nucleation. The N doping content and N-TiO<sup>2</sup> nanoparticle crystallinity could be controlled by varying the calcination conditions and completely transforming the amorphous structure to anatase, enabling the control of N doping content from 0.54 to 0.4 at.%. N-TiO<sup>2</sup> nanoparticles started to be synthesized just after

#### REFERENCES


applying voltage to the anode due to plasma generation, entailing a high temperature and pressure. The N-TiO<sup>2</sup> nanoparticles synthesized after calcination at 200◦C featured 0.4 at.% of N doping content and exhibited the greatest MO degradation performance of 91% in the visible region, which could be attributed to a narrow band gap structure due to N doping; PL analysis also revealed the most delayed recombination rate as well (see **Figures 6**, **7**). Thus, this one-step metal-direct synthetic method via plasma generation demonstrated great potential for the novel and high-quality synthesis of N-doped metal oxide nanoparticles and could be scaled-up for industrial applications.

# AUTHOR CONTRIBUTIONS

TK prepared in writing this paper work through experiment and data analysis. G-MG helped measuring photocatalytic activities of nanoparticles. H-BC helped and advised result of this paper and modified it as logical, YS helped searching for the related information about this paper. C-GL and Y-HC equally contributed to this manuscript and accepted responsibility for conduct of research and final approval.

#### ACKNOWLEDGMENTS

This study was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which is funded by the Ministry of Trade, Industry and Energy of the Republic of Korea (No. 20152510101950), by the Nano•Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2016M3A7B4900044) and the Future Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016M3D1A1027836).

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00458/full#supplementary-material


and its photocatalytic properties. Appl. Catal. B: Environ. 106, 287–294. doi: 10.1016/j.apcatb.2011.05.018


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Kim, Go, Cho, Song, Lee and Choa. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Ti4O7/g-C3N<sup>4</sup> for Visible Light Photocatalytic Oxidation of Hypophosphite: Effect of Mass Ratio of Ti4O7/g-C3N<sup>4</sup>

#### Wei Guan<sup>1</sup> , Zhenghua Zhang<sup>2</sup> \* † , Shichao Tian<sup>3</sup> \* † and Jianwei Du<sup>1</sup>

*<sup>1</sup> South China Institute of Environmental Sciences, The Ministry of Environment Protection of PRC, Guangzhou, China, <sup>2</sup> Graduate School at Shenzhen, Research Institute of Environmental Engineering and Nano-Technology, Tsinghua University, Shenzhen, China, <sup>3</sup> Shenzhen Environmental Science and New Energy Technology Engineering Laboratory, Tsinghua-Berkeley Shenzhen Institute, Shenzhen, China*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Guohong Wang, Hubei Normal University, China Aihua Xu, Wuhan Textile University, China Shaowen Cao, Wuhan University of Technology, China Hongwei Huang, China University of Geosciences, China*

#### \*Correspondence:

*Zhenghua Zhang zhenghua.zhang@sz.tsinghua.edu.cn Shichao Tian 18812672619@163.com*

> *†These authors have contributed equally to this work.*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

> Received: *07 May 2018* Accepted: *09 July 2018* Published: *24 July 2018*

#### Citation:

*Guan W, Zhang Z, Tian S and Du J (2018) Ti4O7/g-C3N4 for Visible Light Photocatalytic Oxidation of Hypophosphite: Effect of Mass Ratio of Ti4O7/g-C3N4 Front. Chem. 6:313. doi: 10.3389/fchem.2018.00313*

Hypophosphite wastewater treatment is still a critical issue in metallurgical processes and the oxidation of hypophosphite to phosphate followed by the precipitation of phosphate is an important strategy for hypophosphite wastewater treatment. Herein, Ti4O7/g-C3N<sup>4</sup> photocatalysts with various mass ratios (Ti4O<sup>7</sup> (m): g-C3N<sup>4</sup> (m) = 0.5, 0.2, 0.1, and 0.05) were synthesized by a hydrolysis method and the effect of the mass ratio of Ti4O<sup>7</sup> (m): g-C3N<sup>4</sup> (m) on Ti4O7/g-C3N<sup>4</sup> visible light photocatalytic oxidation of hypophosphite was evaluated. The as-prepared Ti4O7/g-C3N<sup>4</sup> were characterized and confirmed by SEM, XPS, XRD and FTIR. Moreover, the specific surface area and the distribution of pore size of Ti4O7/g-C3N<sup>4</sup> was also analyzed. Our results showed that Ti4O7/g-C3N<sup>4</sup> exhibited remarkably improved photocatalytic performance on hypophosphite oxidation compared with g-C3N<sup>4</sup> and meanwhile 1:2-Ti4O7/g-C3N<sup>4</sup> with a mass ratio of 0.5 showed the best photocatalytic performance with the highest oxidation rate constant (17.7-fold and 91.0-fold higher than that of pure g-C3N<sup>4</sup> and Ti4O7, respectively). The enhanced performance of photocatalytic oxidation of hypophosphite was ascribed to the heterojunction structure of Ti4O7/g-C3N<sup>4</sup> with broader light absorption and significantly enhanced efficiency of the charge carrier (e−-h+) generation and separation. Additionally, the generated ·OH and ·O − 2 radicals contributed to the hypophosphite oxidation during the photocatalytic system.

Keywords: photocatalysts, hypophosphite oxidation, graphitic carbon nitride, sub-stoichiometric titanium oxides, visible light irradiation

## INTRODUCTION

Hypophosphite wastewater is produced in metallurgical processes where hypophosphite is a widely used reducing reagent for chemical nickel deposition (Gan et al., 2007; Huang et al., 2009). The discharge of hypophosphite wastewater may result in eutrophication and therefore the further treatment is required (Piveteau et al., 2017; Tian et al., 2017). Coagulants such as Fe have been widely used for phosphorus removal (Shih et al., 2013), however, the hypophosphite precipitants are not stable due to the high solubility constant (Zhao et al., 2017). As such, the pre-oxidation of hypophosphite to phosphate is very important for hypophosphite wastewater treatment so as to facilitate the following precipitation of phosphate in the form of insoluble salts precipitates.

Photocatalysis is considered to be a useful technology for water treatment with advantages of energy-free by using solar energy and high oxidation efficiency of pollutants by hydroxyl radicals (·OH) and superoxide radicals (·O − 2 ) generated during the photocatalytic process (Hao et al., 2017). The most commonly used TiO<sup>2</sup> photocatalyst, however, is greatly limited in wide applications especially under visible light or sunlight due to its main drawback of wide band gap (3.2 eV) (Hao et al., 2016; Ma et al., 2018). Therefore, photocatalysts with wide range of response wavelength as well as good photogenerated charge separation properties are urgently required for photocatalytic applications.

Recently, graphite-like carbon nitride (g-C3N4) has attracted lots of attentions due to its advantages including small band gap of 2.73 eV, robust chemical stability over a wide pH range of 0–14 (Zhang et al., 2014; Li et al., 2016a,b). Nevertheless, limitations including low surface area and poor photogenerated charge separation properties still hinder the photocatalytic applications of g-C3N<sup>4</sup> (Shao et al., 2017). Preparation of g-C3N4-based heterojunction is an alternative pathway to facilitate the enhancement of charge separation and photocatalytic performance (Masih et al., 2017). For instance, photocatalysts with p–n junction exhibited excellent photocatalytic performance in environmental and energy applications (Huang et al., 2015a,b, 2016; Zhang et al., 2018).

Magnéli phase titanium suboxides (TinO2n−1) are substoichiometric titanium oxides, where n is an integer between 4 and 10 (i.e., 4, 5, 6, and 8) (Zaky and Chaplin, 2014). Among various compositions of TinO2n−1, Ti4O<sup>7</sup> has the properties of best conductivity (1,500 S cm−<sup>1</sup> ) and the robust resistance to aggressive chemical conditions (Ganiyu et al., 2016). Construction of g-C3N4/Ti4O<sup>7</sup> heterojunction significantly improved the photocatalytic performance as a result of the effective enhancement of charge separation was reported in our previous work (Guan et al., 2018). However, the effect of the mass ratio of Ti4O<sup>7</sup> (m): g-C3N<sup>4</sup> (m) of the g-C3N4/Ti4O<sup>7</sup> heterojunction on photocatalytic performance is still unknown. In this study, the effect of the mass ratio of Ti4O7/g-C3N<sup>4</sup> (Ti4O<sup>7</sup> (m): g-C3N<sup>4</sup> (m) = 0.5, 0.2, 0.1 and 0.05) on photocatalytic oxidation of hypophosphite was evaluated in view of the photocatalytic performance, optical and electrochemical properties as well as the contributions of ·OH and ·O − 2 radicals generated during the photocatalytic process. Our results indicated that Ti4O7/g-C3N<sup>4</sup> with a mass ratio of 0.5 showed the highest rate constant of photocatalytic oxidation of hypophosphite (17.7-fold and 91.0-fold higher than that of pure g-C3N<sup>4</sup> and Ti4O7, respectively).

## MATERIALS AND METHODS

#### Chemicals

The reagents used for the preparation and performance characterization of g-C3N4/Ti4O<sup>7</sup> were analytical grade and included sub-stoichiometric titanium oxide, melamine, urea, sodium hypophosphite, sodium sulfate, isopropanol, sodium hydroxide, sulfuric acid. All chemicals were bought from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). In addition, all solutions were prepared using freshly prepared Milli-Q water (Millipore, 18.2 M cm).

# Preparation of Ti4O7/g-C3N<sup>4</sup> Photocatalysts

Graphite-like carbon nitride was prepared first using a liquidbased growth method (Sun et al., 2018). Then, g-C3N<sup>4</sup> (2 g) and Ti4O<sup>7</sup> (1, 0.4, 0.2, 0.1 g) were well mixed in the 0.1 mol/L NaOH solution (100 mL) using ultrasonication following by the annealing procedure at 160◦C for 20 h. Subsequently, the asprepared g-C3N4/Ti4O<sup>7</sup> with various mass ratios (Ti4O<sup>7</sup> (m): g-C3N<sup>4</sup> (m) = 0.5, 0.2, 0.1, and 0.05) were dried at 60◦C for 12 h before usage and noted as 1:2-Ti4O7/g-C3N4, 1:5-Ti4O7/g-C3N4, 1:10-Ti4O7/g-C3N<sup>4</sup> and 1:20-Ti4O7/g-C3N4, respectively.

#### Analysis and Test Methods

The as-prepared g-C3N4/Ti4O<sup>7</sup> were characterized by scanning electron microscopy (SEM) (JEOL JSM-6701F), X-ray photoelectron spectroscopy (XPS) (Phi Quantern instrument with C 1s peak (284.8 eV) as the calibrated reference), X-ray diffraction (XRD) (model D/max RA, Rigaku Co., Japan), and fourier transforms infrared spectroscopy spectra (FTIR) (Bruker Tensor-27). The specific surface area and the pore size distribution of the as-prepared g-C3N4/Ti4O<sup>7</sup> were calculated by Brunauer-Emmett-Teller (BET) equation and Barrett-Joyner-Halenda (BJH) method according to the N<sup>2</sup> adsorption/desorption isotherms. The optical and electrochemical properties of the as-prepared g-C3N4/Ti4O<sup>7</sup> were evaluated by Ultraviolet-visible diffraction spectra (UV-vis DRS) (UV-2450, Shimadzu, Japan) and CHI 660B electrochemical system in view of photocurrent (PC), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The contributions of ·OH and ·O − 2 radicals generated during the visible light photocatalytic process were identified by comparing the efficiencies of hypophosphite oxidation in the absence and presence of isopropanol (IPA) and N<sup>2</sup> purging.

#### Analysis of Photocatalytic Performance

The photocatalytic performance of g-C3N4/Ti4O<sup>7</sup> was characterized by photocatalytic oxidation of hypophosphite with the concentration of hypophosphite measured by ion chromatography using a 732 IC detector. A metal-halide lamp (35 W, Philips) with the light strength of ∼5 mW cm−<sup>2</sup> was used as the light source and a UV-cutoff filter of 420 nm was used to provide the visible light with the wavelength over 420 nm. Before the photocatalytic experiments, g-C3N4/Ti4O<sup>7</sup> with various mass ratios (10 mg) dispersed in 100 mg L−<sup>1</sup> hypophosphite aqueous solution (100 mL) were continuously stirred for 30 min in the dark to achieve the adsorption–desorption equilibrium. After that, the visible light photocatalytic oxidation of hypophosphite was conducted with the above solutions exposing to the visible light irradiation and sampling conducted at 1 h intervals over

photocatalyst.

FIGURE 2 | Structure and morphology analysis of g-C3N4 and Ti4O7/g-C3N4 photocatalysts: (A) XRD patterns analysis; (B) FTIR spectrometer analysis.

6 h experimental period. More detailed information about the photocatalytic experiment can be found in our most recent paper (Guan et al., 2018). The oxidation efficiency of hypophosphite (η) was calculated using the following equation (Guan et al., 2017):

$$
\eta = \frac{\text{Co} - \text{C}\_{\text{t}}}{\text{C}\_{\text{0}}} \times 100\% \tag{1}
$$

where C<sup>0</sup> and C<sup>t</sup> represent the concentrations of hypophosphite at initial and given time, respectively.

# RESULTS AND DISCUSSION

#### Materials Characterization

The SEM images of pure g-C3N<sup>4</sup> and Ti4O7/g-C3N<sup>4</sup> photocatalysts were shown in **Figure 1** that g-C3N<sup>4</sup> had a sheet-like structure (**Figure 1A**) and spheroidal Ti4O<sup>7</sup> crystals were deposited on the surface of C3N<sup>4</sup> (**Figure 1B**). The corresponding XPS high resolution spectra of Ti4O7/g-C3N<sup>4</sup> were analyzed as shown in **Figure 1**. There were two components for the XPS spectra of C 1s core level (**Figure 1C**), the standard reference carbon (284.8 eV) and the sp<sup>2</sup> bonded C in N=C(–N)<sup>2</sup> (288.3 eV) (Jo and Natarajan, 2015). With regard to the N 1s

hypophosphite using different photocatalysts.

spectra, there were three peaks (**Figure 1D**), the main peak at 398.8 eV assigned to sp<sup>2</sup> nitrogen (C=N–C) involved in triazine rings, the peak at 400.0 eV originated from the tertiary nitrogen bonded to carbon atoms in the form of N–(C)<sup>3</sup> (Wu et al., 2013) and the peak at 401.3 eV ascribed to amino functions (C–N–H) (Gao et al., 2014). These assignments of C 1s and N 1s were agreed well with the XPS results of g-C3N<sup>4</sup> reported previously. Meanwhile, Ti4O<sup>7</sup> is a mixed-valence compound with two evenly occupied Ti4<sup>+</sup> (3d<sup>0</sup> ) and Ti3<sup>+</sup> (3d<sup>1</sup> ) configurations. Four peaks were observed for Ti 2p spectra (**Figure 1E**) that two broad peaks at 458.6 eV and 464.7 eV were respectively assigned to Ti 2p1/<sup>2</sup> and Ti 2p3/<sup>2</sup> peaks of Ti4+, and another two peaks at 457.9 eV and 463.8 eV were assigned to Ti3<sup>+</sup> (Zeng et al., 2017). In terms of the O 1s spectra, three peaks were observed (**Figure 1F**) that the peak at 533.5 eV was assigned to the C–O functional groups, and the peaks at 531.8 and 529.7 eV were ascribed to the OH–Ti and O–Ti bonds (Li et al., 2017). The XPS results confirmed the presence of Ti4O<sup>7</sup> on the surface of g-C3N<sup>4</sup> with covalent bonds.

The XRD phase structures of Ti4O7/g-C3N<sup>4</sup> photocatalysts with various mass ratios were shown in **Figure 2A**. Peaks at 13.10◦ and 27.40◦ were indexed as (1 0 0) plane of tri-s-triazine units and (0 0 2) plane of the conjugated aromatic system of g-C3N4, respectively (Liang and Zhu, 2016). Meanwhile, major peaks of Ti4O<sup>7</sup> including 20.7, 26.3, 29.5, 31.7, 34.0, 36.3, 40.5, 53.1, 55.0, 63.8, and 66.4◦ were also found (Guo et al., 2016). With increase of the mass ratio of Ti4O<sup>7</sup> (m): g-C3N<sup>4</sup> (m), the intensity of the Ti4O<sup>7</sup> peaks became stronger while that of the C3N<sup>4</sup> peaks became weaker. Furthermore, Ti4O7/g-C3N<sup>4</sup> especially 1:2-Ti4O7/g-C3N<sup>4</sup> (mass ratio of Ti4O7/g-C3N<sup>4</sup> of 0.5) matched well with the reference of pure Ti4O<sup>7</sup> and g-C3N4, indicating that the main structures of Ti4O<sup>7</sup> and g-C3N<sup>4</sup> were not destroyed during the synthesis process of Ti4O7/g-C3N4.

The as-prepared Ti4O7/g-C3N<sup>4</sup> photocatalysts with various mass ratios were further characterized by FTIR as shown in **Figure 2B**. Typical absorption peaks of 1,230–1,630 cm−<sup>1</sup> were attributed to tri-s-triazine ring moieties of g-C3N4. For example, the absorption peaks of 1,638 and 1,570 cm−<sup>1</sup> were related to C=N stretching, and the peaks of 1,474, 1,410, 1,322, 1,241 cm−<sup>1</sup> were attributed to C–N stretching. Meanwhile, the sharp peak of 810 cm−<sup>1</sup> was due to the bending vibration of heptazine rings, indicating that the heptazine units might exist in C3N<sup>4</sup> (Hatamie et al., 2018). With increase of the mass ratio of Ti4O7/g-C3N4, the absorption intensity related to the vibrational bands

of g-C3N<sup>4</sup> became weaker. The presence of Ti4O<sup>7</sup> in Ti4O7/g-C3N<sup>4</sup> restricted the evolution trend of g-C3N<sup>4</sup> and thus alleviated the severe stacking of aromatic units of g-C3N4. Additionally, the delocalized π-π conjugated electronic system of g-C3N<sup>4</sup> facilitated the transfer of photogenerated electron-hole pairs during the photocatalytic process (Jourshabani et al., 2018).

The nitrogen adsorption-desorption isotherms and pore size distribution of pure Ti4O7, g-C3N<sup>4</sup> and 1:2-Ti4O7/g-C3N<sup>4</sup> photocatalysts were shown in **Figure 3**. The specific surface area calculated according to **Figure 3A** was 116.0, 56.5, and 174.0 m<sup>2</sup> ·g −1 for Ti4O7, g-C3N<sup>4</sup> and 1:2-Ti4O7/g-C3N4, respectively. Generally, a catalyst with larger surface area could offer more active sites for adsorption and photodegradation toward organic pollutants, resulting in an enhanced photodecomposition activity (Dong et al., 2015). As such, compared with pure Ti4O<sup>7</sup> and g-C3N4, 1:2- Ti4O7/g-C3N<sup>4</sup> with larger specific surface area would facilitate photocatalytic oxidation of hypophosphite. Moreover, the pore size distribution of 1:2-Ti4O7/g-C3N<sup>4</sup> was between 5

and 15 nm and that of pure Ti4O<sup>7</sup> was mainly in the range of 10–25 nm, while no obvious mesopores were observed for g-C3N<sup>4</sup> (**Figure 3B**).

# Photocatalytic Performance Analysis

The photocatalytic performance of different photocatalysts was analyzed and compared as shown in **Figure 4A**. Ti4O7/g-C3N<sup>4</sup> exhibited the significantly improved efficiency in hypophosphite oxidation under visible light irradiation (λ > 420 nm) in comparison to the pure g-C3N<sup>4</sup> and Ti4O7. Additionally, the oxidation efficiency of hypophosphite was increased with increase of the mass ratio of Ti4O7/g-C3N<sup>4</sup> and 1:2-Ti4O7/g-C3N<sup>4</sup> illustrated the best photocatalytic performance with the highest oxidation efficiency of 83.6%. Furthermore, the photocatalytic oxidation reaction of hypophosphite was well fitted by pseudo first-order kinetic (R <sup>2</sup> > 0.95) as shown in **Figure 4B** and the rate constant of 1:2-Ti4O7/g-C3N<sup>4</sup> was 17.7-fold and 91.0-fold higher than that of pure g-C3N<sup>4</sup> and Ti4O7, respectively. The low efficiency of photocatalytic oxidation of hypophosphite for pure g-C3N<sup>4</sup> was possibly because of the rapid combination of electron-hole pairs (Yan et al., 2018). Similarly, the photocatalytic performance of pure Ti4O<sup>7</sup> was extremely limited herein, which was mainly due to the wide band gap of 2.9 eV of pure Ti4O<sup>7</sup> (Maragatha et al., 2017). Nevertheless, the efficiency of photocatalytic oxidation of hypophosphite was remarkably improved especially for 1:2-Ti4O7/g-C3N<sup>4</sup> possibly as a result of the Ti4O7/g-C3N<sup>4</sup> heterojunction with effectively increased recombination lifetime of electron-hole pairs and better charge transfer during the photocatalytic process, which was discussed in the following parts.

# Optical Properties Analysis

Generally, the band gap of a semiconductor is related to its photocatalytic performance, because it determines the absorption properties of the incident photon, the recombination lifetime of the electron-hole pairs and transfer of charge carriers. As shown in **Figure 5**, Ti4O7/g-C3N<sup>4</sup> showed a distinct red-shift in comparison to pure g-C3N<sup>4</sup> in the UV–vis diffuse reflectance spectra. Moreover, the absorption intensities ranging from 450 to 800 nm gradually strengthened with increase of the mass ratio of Ti4O<sup>7</sup> (m): g-C3N<sup>4</sup> (m), indicating that the Ti4O7/g-C3N<sup>4</sup> heterojunction with good interaction between Ti4O<sup>7</sup> and g-C3N<sup>4</sup> facilitated the enhancement of visible-light harvesting (Chang et al., 2018). The band gap of Ti4O7/g-C3N<sup>4</sup> was determined such that (Ai et al., 2018):

$$(\text{ahv})^2 = A(\text{hv} - \text{E}\_{\text{g}}) \tag{2}$$

where α is the optical absorption coefficient; h is the Plank's constant; ν is the photonic frequency; A is the proportionality constant; E<sup>g</sup> is the band gap.

The band gap followed the order: 1:2-Ti4O7/g-C3N<sup>4</sup> (2.07 eV) < 1:5-Ti4O7/g-C3N<sup>4</sup> (2.25 eV) < 1:10-Ti4O7/g-C3N<sup>4</sup> (2.33 eV) < 1:20-Ti4O7/g-C3N<sup>4</sup> (2.43 eV) < pure g-C3N<sup>4</sup> (2.70 eV). As such, the narrowed band gap of Ti4O7/g-C3N<sup>4</sup> would effectively enhance the photoabsorption efficiency, which thus contributed to the improved efficiency of photocatalytic oxidation of hypophosphite as shown in **Figure 4**.

#### Electrochemical Properties Analysis

CV and EIS results can indicate the combination efficiency of electron-hole pairs during the photocatalytic process. As shown in **Figure 6A**, the transient photoelectrochemical response current density of Ti4O7/g-C3N<sup>4</sup> increased with the increase of the mass ratio of Ti4O7/g-C3N4. 1:2-Ti4O7/g-C3N<sup>4</sup> had the highest photocurrent density of 0.30 µA cm−<sup>2</sup> , while the photocurrent density for pure g-C3N<sup>4</sup> was only 0.10 µA cm−<sup>2</sup> . The enlarged photocurrent density of Ti4O7/g-C3N<sup>4</sup> suggested the more efficient charge carrier (e−-h+) generation on the Ti4O7/g-C3N<sup>4</sup> surface (Liu et al., 2017). Herein, the electrons in the valence band of g-C3N<sup>4</sup> were excited upon visible light irradiation and then migrated to the conduction band of Ti4O<sup>7</sup> with effectively enhanced efficiency of the charge carrier (e− h <sup>+</sup>) generation and separation (Samanta and Srivastava, 2017), which improved the photocatalytic performance as indicated in **Figure 4**. In addition, the photoelectrochemical response current density of Ti4O7/g-C3N<sup>4</sup> switched reversibly and was unchanged after repetitive ON/OFF illumination cycles, indicating the good photoelectrochemical stability of Ti4O7/g-C3N4.

Charge transfer of photocatalysts is another very important factor determining the photocatalytic performance (Kang et al., 2016). EIS was applied to analyze the photogenerated electron transfer process of Ti4O7/g-C3N4. As shown in **Figure 6B**, the arc radius decreased gradually with increase of the mass ratio of Ti4O7/g-C3N<sup>4</sup> and the arc radius was lowest for 1:2-Ti4O7/g-C3N4. It is well known that the radius of Nyquist circle is related to the interfacial charge transfer that the smaller radius of Nyquist circle indicates a faster interfacial charge transfer and lower recombination rate of electron and hole (Guo et al., 2017). Herein, the decreased arc radius of Ti4O7/g-C3N<sup>4</sup> indicated that 1:2-Ti4O7/g-C3N<sup>4</sup> in comparison to pure g-C3N<sup>4</sup> would also facilitate the photocatalytic oxidation of hypophosphite as evident in **Figure 4**.

#### Proposed Mechanism

Reactive oxygen species (ROS) including ·O − 2 and ·OH would be generated during the photocatalytic system (Huang et al., 2017a,b). Herein, the contributions of ·OH and ·O − 2 to the photocatalytic oxidation of hypophosphite were analyzed by evaluation of the photocatalytic oxidation efficiency of hypophosphite in the presence of isopropanol (IPA) acting as the scavenger of ·OH (Tian et al., 2015) and N<sup>2</sup> purging applied to reduce the superoxide ·O − 2 radicals (Zhang et al., 2017). As shown in **Figure 7**, the oxidation efficiency of hypophosphite was decreased to 42 and 27% in the presence of IPA and N2, respectively. In contrast, that value was 83% without radical scavengers. These results indicated that ·OH and ·O − 2 radicals significantly contributed to the photocatalytic oxidation of hypophosphite and other radicals such as singlet oxygen (1O2) and peroxyl (RO2·) may also make a contribution as evident by the lowest efficiency of 27% rather than 0% (Huang et al., 2017a).

The photocatalytic mechanism of Ti4O7/g-C3N<sup>4</sup> was proposed as shown in **Figure 8**. Under visible light irradiation, electrons in the valence band of g-C3N<sup>4</sup> were excited and then the excited electrons migrated to the conduction band of Ti4O<sup>7</sup> through the heterojunction surface of Ti4O7/g-C3N<sup>4</sup> with the generation of holes in the valence band of g-C3N<sup>4</sup> (Zhu et al., 2018). The electrons on the surface of Ti4O<sup>7</sup> could easily react with O<sup>2</sup> adsorbed on the Ti4O7/g-C3N<sup>4</sup> surface to generate ·O − 2 radicals for hypophosphite oxidation. Meanwhile, the active ·OH radicals with high redox potential were generated through ·O − 2 radicals, which further facilitated the oxidation of hypophosphite (Nosaka and Nosaka, 2017; Guan et al., 2018). Therefore, it can be concluded that the oxidation of hypophosphite was mainly ascribed to ·OH and ·O − 2 radicals generated during the photocatalytic process as shown in **Figure 8**.

#### Analysis of Stability

The stability of photocatalyst is another vital consideration to evaluate the photocatalytic performance. As shown in **Figure 9**,

## REFERENCES


relatively robust reusability was exhibited by 1:2-Ti4O7/g-C3N<sup>4</sup> photocatalyst that the efficiency of photocatalytic oxidation of hypophosphite was almost stable in the range of 79–84% after four repetitive experiments. The slight decrease of oxidation efficiency was possibly due to the inevitable mass loss of photocatalyst during the recycling process.

# CONCLUSION

In this work, Ti4O7/g-C3N<sup>4</sup> with various mass ratios (Ti4O<sup>7</sup> (m): g-C3N<sup>4</sup> (m) = 0.5, 0.2, 0.1, and 0.05) were synthesized by a hydrolysis method and the effect of the mass ratio of Ti4O7/g-C3N<sup>4</sup> on Ti4O7/g-C3N<sup>4</sup> visible light photocatalytic oxidation of hypophosphite was evaluated. Ti4O7/g-C3N<sup>4</sup> exhibited remarkably improved photocatalytic performance on hypophosphite oxidation in comparison to pure g-C3N<sup>4</sup> and 1:2-Ti4O7/g-C3N<sup>4</sup> with a mass ratio of 0.5 showed the best photocatalytic performance with the highest oxidation rate constant of photocatalytic (17.7-fold and 91.0-fold higher than that of pure g-C3N<sup>4</sup> and Ti4O7, respectively). The heterojunction structure of Ti4O7/g-C3N<sup>4</sup> with broader light absorption significantly enhanced the efficiency of the charge carrier (e−-h+) generation and separation. ·OH and ·O − 2 radicals generated during the photocatalytic process were the main radicals contributing to the oxidation of hypophosphite.

## AUTHOR CONTRIBUTIONS

WG: experiment. ZZ: paper writing. ST: data analysis. JD: sample analysis.

# FUNDING

This work was supported by National Natural Science Foundation of China (No. 51608086); Science and Technology Planning Project of Guangdong Province (2017A050506032); Science and Technology Project from Chongqing (CSTC2015JCYJA20027); Chongqing Key Laboratory of Environmental Materials and Remediation Technology, Chongqing University of Arts and Sciences (CKE1407); Scientific Research Foundation of Chongqing University of Arts and Sciences (R2014CH08).


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Zhu, Z., Huo, P. W., Lu, Z. Y., Yan, Y. S., Liu, Z., Shi, W. D., et al. (2018). Fabrication of magnetically recoverable photocatalysts using g-C3N<sup>4</sup> for effective separation of charge carriers through like-Z-scheme mechanism with Fe3O<sup>4</sup> mediator. Chem. Eng. J. 331, 615–625. doi: 10.1016/j.cej.2017.08.131

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Guan, Zhang, Tian and Du. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# High Efficient Visible-Light Photocatalytic Performance of Cu/ZnO/rGO Nanocomposite for Decomposing of Aqueous Ammonia and Treatment of Domestic Wastewater

Shiying He<sup>1</sup> , Pengfu Hou<sup>1</sup> , Evangelos Petropoulos <sup>2</sup> , Yanfang Feng<sup>1</sup> , Yingliang Yu<sup>1</sup> , Lihong Xue<sup>1</sup> and Linzhang Yang<sup>1</sup> \*

*1 Institute of Agricultural Resources and Environment, Jiangsu Academy of Agricultural Sciences, Nanjing, China, <sup>2</sup> School of Engineering, Newcastle University, Newcastle upon Tyne, United Kingdom*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Zhihua Xu, Jianghan University, China Guohong Wang, Hubei Normal University, China*

> \*Correspondence: *Linzhang Yang lzyang@issas.ac.cn*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

> Received: *04 April 2018* Accepted: *28 May 2018* Published: *12 June 2018*

#### Citation:

*He S, Hou P, Petropoulos E, Feng Y, Yu Y, Xue L and Yang L (2018) High Efficient Visible-Light Photocatalytic Performance of Cu/ZnO/rGO Nanocomposite for Decomposing of Aqueous Ammonia and Treatment of Domestic Wastewater. Front. Chem. 6:219. doi: 10.3389/fchem.2018.00219* Photocatalytic removal of ammonium-nitrogen (NH<sup>+</sup> 4 -N) from water using solar energy is an approach of high interest and applicability due to the convenience in application. ZnO has a great potential in photocatalytic decomposition of NH<sup>+</sup> 4 -N and conversion of this nutrient to under visible light irradiations. However the applicability of pristine ZnO though is limited due to its reduced capacity to utilize light from natural light. Herein, we report a two-step ZnO-modified strategy (Cu-doped ZnO nanoparticles, immobilized on reduced graphene oxide (rGO) sheets) for the promotion of photocatalytic degradation of NH<sup>+</sup> 4 -N under visible light. UV-Vis spectra showed that the Cu/ZnO/rGO can be highly efficient in the utilization of photons from the visible region. Hence, Cu/ZnO/rGO managed to demonstrate adequate photocatalytic activity and effective NH<sup>+</sup> 4 -N removal from water under visible light compared to single ZnO. Specifically, up to 83.1% of NH<sup>+</sup> 4 -N (initial concentration 50 mg·L −1 , catalyst dosage 2 g·L −1 , pH 10) was removed within 2 h retention time under Xe lamp irradiation. From the catalysis, the major by-product was N2. The high ammonia degradation efficiency from the ZnO/Cu/rGO is attributed to the improvement of the reactive oxygen species (ROSs) production efficiency and the further activation of the interfacial catalytic sites. This study also demonstrated that such nanocomposite is a recyclable agent. Its NH<sup>+</sup> 4 -N removal capacity remained effective even after five batch cycles. In addition, Cu/ZnO/rGO was applied to treat real domestic wastewater, and it was found that chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) removal efficiencies can reach 84.3, 80.7, and 90.3%, respectively. Thus, Cu/ZnO/rGO in the presence of solar light can be a promising photocatalyst in the field of wastewater treatment.

Keywords: ZnO, graphene oxide, nanocomposite, photocatalysis, ammonia-nitrogen removal, water treatment

#### INTRODUCTION

Accumulation of ammonia nitrogen (NH<sup>+</sup> 4 -N) to aqueous ecosystems, as a result of the rapid agricultural and industrial development (Kirkpatrick et al., 2014; Li et al., 2014), severely impacts the aquatic ecosystems due to eutrophication. Specifically, increased presence of ammoniacal compounds in water leads to cyanobacteria blooms harmful to various aquatic species due to their competitive behavior for respiration (Hued et al., 2006). Furthermore, the ammoniacal oxidative forms (NO<sup>−</sup> 2 , NO<sup>−</sup> 3 ) are harmful to human health (Sun et al., 2012). Numerous techniques have been developed to treat ammoniarich wastewater including biological processing (Li et al., 2016), constructed wetlands (Wang et al., 2016), adsorption (Halim et al., 2010), chemical oxidation (Kurniawan et al., 2006), and photocatalytic processes (Meeroff et al., 2012). Among these, photocatalytic removal of aqueous NH<sup>+</sup> 4 -N has been extensively investigated due to its high efficiency, low cost, and the absence of secondary pollutants (Nemoto et al., 2007; Yuzawa et al., 2012).

Semiconductor photocatalysts such as TiO2, ZnO, g-C3N4, and Bi2O<sup>3</sup> have attracted enormous research interest in environmental pollution in contiguous years (Hao et al., 2017; Ma et al., 2018; Zhang et al., 2018). Among various semiconductor photocatalysts, ZnO is considered as one of the most promising photocatalysts for solar energy conversion and photodegradation of pollutants. Its advantages are the high oxidative capacity, low toxicity, high abundance at low cost, and optical stabilities (Daneshvar et al., 2007b; Kuriakose et al., 2015; Akir et al., 2017). When ZnO is illuminated with UV light an electron (e−) excites from the valence band (VB) to the conduction band (CB) leaving a hole (h+) in the VB, generating electron-hole pairs (Equation 1) (Lee et al., 2016). Holes are powerful oxidants that can be excited to split water molecules producing hydroxyl radicals (•OH) (Equation 2), whereas electrons can act as O<sup>2</sup> reductants for hydroxyl radicals (•OH) formation (Equations 3, 4) (Lee et al., 2016). Both •OH and holes could oxidize ammonia to nitrogen gas (Reaction Scheme I) and/or to nitrate (Reaction Scheme II) (Altomare et al., 2012). Reaction Scheme I is the ideal path for ammonia conversion due to the production of harmless N<sup>2</sup> and H<sup>2</sup> gases. The production of H<sup>2</sup> also highlights the capacity of this photocatalytic process for carbon neutral/positive "waste" water treatment and/or bio-refinement.

$$ZnO + h\nu \to ZnO\left(e\_{cb}{}^- + h\_{vb}{}^+\right) \tag{1}$$

$$h^{+} + H\_{2}O \rightarrow H^{+} + \cdot OH \tag{2}$$

$$\text{O}\_2 + e^- + H^+ \to H\_2\text{O}\_2\tag{3}$$

$$\cdot H\_2O\_2 + e^- \rightarrow \cdot OH + OH^- \tag{4}$$

Reaction Scheme I:

$$\cdot \text{NH}\_4 + \cdot \text{OH} \rightarrow \cdot \text{NH}\_2 + \text{H}\_2\text{O} \tag{5}$$

$$\cdot \cdot NH\_2 + \cdot OH \rightarrow NH + H\_2O \text{(or } H^+\text{)}\tag{6}$$

$$\cdot NH + \cdot OH \rightarrow N + H\_2O \text{(or } H^+\text{)}\tag{7}$$

$$\rm NH\_x + NH\_\mathcal{Y} \to \rm N\_2H\_{x+\mathcal{Y}} \text{ (x, } y = \ 0, 1, 2\text{)}\tag{8}$$

$$\left(N\_2H\_{\text{x+y}} + \left(\text{x+y}\right)h^+ \rightarrow N\_2 + (\text{x+y})H^2 \text{(N}\_2 \text{ formation)}\right) \tag{9}$$

Reaction Scheme II:

$$\mathrm{NH\_3 + OH^- \to NH\_2OH + H^+} \tag{10}$$

$$\text{NH}\_2\text{OH} + \text{OH}^- \rightarrow \text{NO}\_2^- \rightarrow \text{NO}\_3^- \tag{11}$$

The applicability of pristine ZnO for photocatalysis purposes is limited due to the reduced light utilization from natural light (∼4% of the solar spectrum) as a result of its wide band gap (3.37 eV). Moreover, the photocatalytic efficiency of ZnO is often hindered by the fast charge recombination of the photoinduced electron-hole pairs (h+/e−) and the lack of active sites on the catalyst surface (Daneshvar et al., 2007a; Akir et al., 2017). Thus, development of efficient procedures to overcome these bottlenecks and promote the light absorption from the visible-light region is essential. Numerous studies have tried to develop a low-cost, highly-active, and easily recycled photocatalysts. Various approaches were employed to achieve that including doping of cations and anions, creating oxygen vacancies, semiconductor coupling, and combining with carbon materials (Hao et al., 2016; Lee et al., 2016; Qi et al., 2017; Lu et al., 2018). Additions of Cu2<sup>+</sup> have been reported as a promising method to decrease the band gap (Kuriakose et al., 2015; Yildirim et al., 2016). Copper ions have the similar ionic radii (0.73 Å) to the ionic radii of Zn (0.74 Å), which can easily form extra impurities at the ZnO lattice and add a localized electronic energy level within the band gap. The dopant energy level is temporally located below the CB, which subsequently facilitates electron trapping and inhibits the recombination of the h+/e<sup>−</sup> pairs. It is observed that the photocatalytic efficiency and the absorption in visible-light enhances with Cu2<sup>+</sup> doping (Sharma et al., 2017).

Regardless the potential benefits of the Cu doped ZnO, its efficiency as a visible-light photocatalyst is yet limited. Reports have shown that addition of carbon materials (such as carbon nanotubes, graphene, amorphous carbon) could improve the stability and photocatalytic performance of ZnO, not only by enhancing charge separation, but also by extending its optical absorption to the visible light range. Graphene oxide (GO) [two-dimensional (2D) carbon nanostructure] is a promising additive as it possesses high electron mobility (exceeding 2,000 cm<sup>2</sup> /V), good optical transparency, high surface area (2,600 m2 g −1 ) (Bolotin et al., 2008; Jang et al., 2013) and contains series of reactive oxygen functional groups on their surface (e.g., carboxylic acid, hydroxyl, and epoxide groups) (Akir et al., 2017). All these properties make GO one of the most desirable materials for photocatalyst support (Sun et al., 2015). ZnO/GO composite has been previously fabricated using hydrothermal and electrochemical methods with improved photodegradation performance under visible light irradiation. The 2D GO acts both as an electron-acceptor as well as an electron-transport material that manipulates the charge transfer across the ZnO-graphene interface. The electrons transfer from the ZnO conduction band to the GO energy level can reside there for longer periods (Zhao et al., 2017). This phenomenon results in effective separation of the photo-generated carriers that often inhibit the h <sup>+</sup>/e<sup>−</sup> recombination and in their absence the photocatalytic activity of ZnO can be drastically enhanced (Hsieh and Ting, 2018).

Many studies demonstrated improved ZnO photocatalytic performance by either doping or via GO addition for the enhancement of light absorption and recombination reduction. Therefore, combining the advantage of Cu doping with those of graphene support, Cu/ZnO/rGO nanocomposite could in theory promote visible-light-driven photocatalysis. Thus far, our knowledge on applications of Cu/ZnO/rGO composite for catalysis is limited, especially in the case of nutrient treatment from water. To shed further light on this field we fabricated a Cu/ZnO/rGO nanocomposite to investigate the photocatalytic activity of this specific nanocomposite to ammonia decomposition. Its practical application in the treatment of real domestic wastewater was also evaluated.

# MATERIALS AND METHODS

#### Synthesis of Cu/ZnO/rGO Photocatalyst

Cu/ZnO/rGO nanocomposite was prepared via a twostep hydrothermal method. Zinc acetate hydrate (50 mL, 0.34 M), copper nitrate hydrate (50 mL, 0.015 M) and sodium oxalate (40 mL, 0.5 M) were mixed (magnetic stirrer) at room temperature until homogeneity was achieved. KOH (20 mL, 1 M) was then added drop wise. After stirring for 2 h a homogeneous white solution was obtained (200 mL), sealed and sterilized (autoclave, with a filling fraction of 0.8 and kept at 120◦C for 10 h). The white precipitate was formed, collected by filtration and washed with DI water and ethanol (in turn, three and two times respectively) to remove residues. The resultant Cu/ZnO nanostructure was dried in an oven at 80◦C and then left on air for 12 h. 25 mg of GO powder was added to 50 mL deionized (DI) water under ultrasonic dispersion for 30 min. 0.4 g Cu/ ZnO (as synthesized) and 30 mL DI water were added to the GO solution and sonicated for 30 min. The mixture was transferred to a 100 mL Teflon liner and kept at 160◦C for 12 h (sterilization). The white color of Cu/ZnO changed to dark gray. Meanwhile, GO were thermally reduced after the hydrothermal process, the gray precipitate was filtered and washed several times with DI water and ethanol separately. The final product (Cu/ZnO/rGO photocatalyst) (4 mol% of Cu and 6 mass% of rGO) was left in an oven at 70◦C for 10 h. Pure ZnO nanostructures were synthesized using the same procedure without addition of copper nitrate hydrate as controls.

# Characterization of Cu/ZnO/rGO Photocatalyst

The morphology of the nanostructured samples was characterized by field emission gun scanning electron microscopy (FESEM, JEOL, JSM-5610LV) and transmission electron microscopy (TEM, JEOL, JEM-200CX). The crystal structure of the as-synthesized Cu/ZnO/rGO was determined by X-ray diffractometer (XRD, Bruker, D8 ADVANCE) using high-intensity Cu Karadiation (k = 1.54051 Å) at an accelerating voltage of 40 kV and emission current of 30 mA. The spectrum was recorded from the region of 2θ (20–80◦ ). UV-visible spectroscopy was used to evaluate the optical absorption in the range 300–850 nm (UV-Vis spectrophotometer, Evolution 220). X-ray photoelectron spectroscopy (XPS, ThermoFisher Scientific ESCALAB250Xi) was used to analyze the surface chemical state.

#### Photocatalytic Removal of Aqueous NH<sup>+</sup> 4 -N

The photocatalytic removal of aqueous ammonia was performed in a cylindrical quartz batch photoreactor. A certain dose of photocatalyst was suspended in 100 mL NH4Cl aqueous solution. Prior to photocatalytic process, adsorption of ammonia in darkness for 30 min was performed to achieve adsorption equilibrium. The reaction solution was then irradiated under light illumination for 2 h under the xenon lamp (120 W, light with wavelengths 400–700 nm, visible light region). The highpressure Hg lamp (125 W, with a peak light intensity at 254 nm, UV light region) was used to compare the NH<sup>+</sup> 4 -N removal efficiency of photocatalysts under different light sources. The distance between light source and photoreactor was set as 10 cm. The reaction solution was supplied with oxygen via bubbling (150 mL/min flow rate) to develop a consistent amount of dissolve oxygen. For comparison, a blank control without photocatalyst was also evaluated to quantify NH<sup>+</sup> 4 -N volatilization.

In this study, the effect of operational parameters including initial ammonia concentration, initial pH and Cu/ZnO/rGO dosage on the photocatalytic degradation efficiency were investigated. For the evaluation of the initial pH, the photocatalytic oxidation of aqueous NH<sup>+</sup> 4 -N at different pH (4, 7, 9, 10, and 11) was performed by adding 2 g photocatalyst to 100 mL aqueous ammonia solution (50 mg/L). The pH was adjusted to desired values (4, 7, 9, 10, and 11) by an addition HCl or KOH solutions. To evaluate the impact of catalyst excess, different amounts of Cu/ZnO/rGO (0.2, 0.5, 1, 2, and 3 g/L) were dispersed in 100 mL aqueous ammonia solution (50 mg/L), the solution pH was adjusted to 10. The effect of the initial ammonia concentration (10, 30, 50, 70, and 100 mg/L) on the photocatalytic degradation of ammonia was investigated at 2 g/L of the photocatalyst and initial pH 10. All trials were performed at room temperature (25◦C) using constant magnetic stirring, the illumination time was up to 2 h. A volume of 1 mL of the treated solution was sampled every 30 min and then immediately centrifuged at 8,000 rpm for 10 min to remove photocatalyst-originated particles prior analysis. The concentrations of NH<sup>+</sup> 4 -N, NO<sup>−</sup> 3 -N, and NO<sup>−</sup> 2 -N in the supernatant were measured via flow injection analysis on a Skalar San++ System (Skalar Co., The Netherlands). For the catalyst stability test, leaching of Cu2<sup>+</sup> and Zn2<sup>+</sup> ions in the supernatant were analyzed by inductively coupled plasma mass spectroscopy (ICP-MS) (Thermo Scientific, Waltham, MA). To examine reusability of Cu/ZnO/rGO, the photocatalyst was recovered by centrifugation and washed with DI water prior re-application. The consecutive photocatalytic experiments were conducted for five runs. All trials were conducted in triplicate. And the data were expressed as the means with standard deviation (SD). The significance of the difference between the treatments means was assessed by Tukey's multiple range tests. Differences at P < 0.05 were considered statistically significant.

# Treatment of Real Domestic Wastewater by Cu/ZnO/rGO

The real wastewater used in the present study was collected from a manhole shaft in the residential area of Jiangsu Academy of Agricultural Sciences. The general characteristics of the wastewater were as follows: NH<sup>+</sup> 4 -N: 43.5 ± 0.76 mg/L; total nitrogen (TN): 47.6 ± 0.95 mg/L; total phosphorus (TP): 5.44 ± 0.36 mg/L and chemical oxygen demand (COD): 68.5 ± 1.2 mg/L. It is suggested that nitrogen mainly exists as NH<sup>+</sup> 4 -N. Cu/ZnO/rGO nanocomposite (2 g·L −1 ) was added to 100 mL wastewater in the photoreactor. Its performance in the simultaneous removal of P, COD, and N (NH<sup>+</sup> 4 -N and TN) was evaluated in this investigation. After visible light illumination for 2 h, aqueous samples were analyzed three times. COD was measured using a COD analyzer (DR1010 COD, HACH, China). N (NH<sup>+</sup> 4 -N and TN) and P were analyzed on a Skalar San++ System (Skalar Co., The Netherlands).

# RESULTS AND DISCUSSION

# Characterization of Photocatalysts

**Figures 1**, **2** show the morphology of the as-synthesized pure ZnO, Cu/ZnO, and Cu/ZnO/rGO nanostructures (SEM, TEM). It is observed that pure ZnO exhibits nanosheet-like structure with a rough surface and irregular shape (**Figures 1A**, **2A**). The images reveals that numerous ZnO nanosheets (about 50–100 nm) aggregated and assembled flower-like microsphere structures (with the size ranging from 500 nm to 1µm). The morphology and size considerably changed by the addition of Cu2+. Different morphologies of these Cu/ZnO nanostructures including nanospheres, nanorods and nanosheets were observed (**Figures 1C**, **2B**). Cu doping also reduced the agglomeration of ZnO. The average nanostructures size decreased (about 10– 60 nm) when Cu2<sup>+</sup> was incorporated to the ZnO matrix. These variations in morphology and size can be attributed to potential changes in the ZnO nucleation and growth process. The decrease in the average size of Cu/ZnO implies that the overall growth rate decreased in all dimensions for the treatment with Cu2<sup>+</sup> doping. Since the ionic radii of Cu2<sup>+</sup> ions is different from that of Zn2+, the presence of Cu2<sup>+</sup> ions in the precursor solution induces a thermodynamic barrier that hinders the ZnO nucleation rate and impedes Zn species from adsorption for further growth. Therefore, the further nanostructure growth at all ZnO dimensions decreases. It has been reported that doping ZnO with Ce and Cu ions can form Zn–Ce–Cu–O in the crystal lattice and decrease the size of the nanoparticles (Mary et al., 2015). The smaller particles can provide with increased reactive sites promoting photocatalysis. Smaller particles have the tendency to

interact with nanoparticles, resulting to irregular shapes (Mary et al., 2015).

With the incorporation of rGO, the close interfacial connection between ZnO and rGO became well-defined. SEM and TEM images showed the formation of Cu/ZnO/rGO nanocomposite ranging from sub-micrometers to several micrometers (**Figures 1D**, **2C,D**). The well-defined Cu/ZnO nanostructure anchors on the surface of rGO sheets, illustrating the excellent adhesion between rGO and Cu/ZnO nanostructures. It is evident that rGO effectively inhibits ZnO agglomeration. This close contact between rGO and ZnO is beneficial to the effective photo-generated charge transfer, effectively inhibiting the e−/h<sup>+</sup> pair recombination, which is favorable for photocatalytic reaction. It was also observed that the morphology and the size range of the anchored ZnO nanostructures are not affected by the addition of the hydrothermally produced rGO. rGO acts as an ideal support for semiconductors due to its unique two dimensional structure, high electron conductivity and mobility, high surface area, and excellent chemical stability. It is also reported that rGO has a series of oxygen functional moieties, including hydroxyl, and epoxy groups on its panels as well as carboxyl and carbonyl groups at its edges (Fu et al., 2011). The presence of the functional groups on the surface of rGO acts as anchoring sites for ZnO nanoparticles and prevents from nanopaticles agglomeration (Najafi et al., 2017).

The crystal structural parameters of the synthesized ZnO, Cu/ZnO and Cu/ZnO/rGO nanostructures (**Figure 3A**) showed that all samples have almost identical patterns. The diffraction peaks exhibit a single phase hexagonal wurtzite structure without any impurities indicating Cu2<sup>+</sup> and rGO do not change the crystalline structure of ZnO. The diffraction peaks (at 2θ = 31.8, 34.3, 36.1, 47.6, 56.6, 62.8, 68, and 69.1◦ Corresponding to the (100), (002), (101), (102), (110), (103), (200), and (112) planes) well match with the hexagonal wurtzite structure of crystalline ZnO (00-036-1415). The sharp and intense peaks in all samples indicate high single crystallinity. No additional peak from copper or its complex oxides was detected in the recorded patterns within the detection limits. This indicates that the doping does not alter the original structure by adding an additional phase. Decreased diffraction peak intensities were observed in Cu/ZnO and Cu/ZnO/rGO due to the substitution of Cu ion by ZnO crystal lattice at the Zn2<sup>+</sup> sites. This replacement can be attributed to the fact that ionic radii (0.73 Å) of Cu2<sup>+</sup> is

close to that of Zn2<sup>+</sup> (0.74 Å) (Meshram et al., 2016). It is difficult to see the rGO peaks in Cu/ZnO/rGO which may be due to the low content of rGO as well as the variation of the stacking mode of r caused by the incorporation of the ZnO nanostructures.

The internal changes of the synthesized nanocomposite samples are also reflected at their light absorption, based on UV–Vis diffuse reflectance spectra (**Figure 3B**). Pure ZnO shows a strong absorption peak at ≈360 nm (typical band-to-band transition characteristics of the wurtzite hexagonal ZnO) (Zhao et al., 2017). No apparent absorption was observed in the visible light region. It is known that ZnO consists of O 2p orbital of valence band (VB) and Zn 4s orbital of conduction band (CB). The distance between the VB (O 2p) and the CB (Zn 4s) of pure ZnO is ∼3.2 eV, hence, it can absorb a major part of UV light (Xia et al., 2014; Lee et al., 2016). When Cu2<sup>+</sup> ions are doped to the ZnO, an additional broad absorption peak in the visible light (range 500–800 nm) was observed due to a reduction in the band gap. The optical absorption measurement supports the Cu substitution in the ZnO lattice. As an impurity in the ZnO lattice, the localized d-electrons of Cu2<sup>+</sup> ions narrow the ZnO VB and CB gap by forming strong Cu (3d)-O (2p) and Cu (3d)-Zn (4s) bonds (Xia et al., 2014). Furthermore, with the addition of rGO, the adsorption in the visible light region is enhanced compared to pure ZnO and Cu/ZnO nanostructures. rGO is a zero-gap semiconductor, the interaction of ZnO with rGO can generate an electronic energy level below the CB of ZnO (Bolotin et al., 2008). Thus, the Cu/ZnO/rGO nanocomposite requires decreased band gap energy and an increased light absorption. Moreover, the increased absorption in the visible light is also ascribed to the black body properties of rGO sheets. The white color of ZnO changed to dark gray after rGO introduction, indicating that the addition of rGO leads to an enhancement of background absorption in the visible light region (for Cu/ZnO/rGO nanocomposite). The band gaps of the as-prepared photocatalysts were calculated using the following formula for near-edge optical absorption of semiconductors:

$$\left(\frac{\alpha h\nu}{A}\right)^2 = \left(h\nu - E\mathfrak{g}\right) \tag{12}$$

Where α is the absorption coefficient, hv is the photon energy, A is a constant. **Figure 3C** shows the plot of (αhν) 2 vs. hν. The calculated Eg values are 3.20, 3.14, and 2.87 eV for ZnO, Cu/ZnO and Cu/ZnO/rGO, respectively. It can be clearly seen that Cu/ZnO/rGO composite has the lowest band gap value (2.87 eV), which is beneficial for the absorption of the visible light absorption.

The full scan XPS spectrum and high-resolution XPS spectra of Zn 2p, Cu 2p, and O 1s were carried out to clarify the elemental composition and chemical state of the as-synthesized Cu/ZnO/rGO nanocomposite. It is obvious from the **Figure 4A** that the detected peaks were assigned to Zn, Cu, O, and C. As also shown in **Figure 4B**, the intense character peaks centered at 1,022.62 eV (Zn 2p3/2) and 1,048.42 eV (Zn 2p1/2). This clearly indicates the oxidation state of Zn atoms. The Cu 2p binding energies of the as-prepared samples were 932.15 eV (Cu 2p3/2 and 952.36 eV (Cu 2p1/2) (**Figure 4C**), respectively, which also proved the oxidation state of Cu atoms in the Cu/ZnO/rGO. From O 1s spectra (**Figure 4D**), it was observed that the two peaks belonging to Zn-O, and O-H bonding located at 530.69 and 532.51 eV, respectively.

Combining the results from XRD spectroscopy, SEM, TEM images, UV-Vis, and XPS analyses, the Cu/ZnO/rGO

nanocomposite possesses a smaller and better-dispersed ZnO on rGO with notably enhanced absorption in the visible light region. This suggests the theoretically, at least, improvement of the visible-light photocatalytic activity for aqueous ammonia removal in wastewater.

#### Photocatalytic Removal of NH<sup>+</sup> 4 -N With Cu/ZnO/rGO Under UV And Visible Light

The aim of this study is to develop a new photocatalyst for aqueous ammonia degradation under visible light. The photocatalytic activities of the as synthesized photocatalysts were compared under a xenon (400 nm cutoff filter, visible light region) and Hg lamp (UV light region). Controls without photocatalysts showed that the concentration of aqueous NH<sup>+</sup> 4 - N reduces by 10% (**Figure 5**), indicating the volatilization is not the major contributor in the NH<sup>+</sup> 4 -N removal process. All the photocatalysts (pure ZnO, Cu/ZnO, and Cu/ZnO/rGO) exhibited good photocatalytic activities for NH4-N treatment in the UV light region. As shown in **Figure 5**, the removal efficiency under UV light with ZnO, Cu/ZnO, and Cu/ZnO/rGO were 55.6, 64.5, and 79.2% respectively. Pure ZnO nanoparticles showed negligible photocatalytic activity on ammonia under visible-light, which is due to the large band gap of ZnO and the lack of absorption in the visible region. It is also observed that the

Cu doped ZnO nanocomposite exhibits higher photocatalytic activities than pure ZnO, especially in the visible light region. In the photocatalytic process, the photo-generated holes in the VB and the electrons from the CB could participate in

the oxidation of aqueous NH<sup>+</sup> 4 -N (Yuzawa et al., 2012). From literature, dopant ions incorporated in the ZnO matrix can modify the energy band of ZnO by creating new electron energy levels within the bandgap (Xia et al., 2014; Sharma et al., 2017). The photo-generated electrons could easily transfer from the CB of ZnO to the localized dopant energy levels and get trapped by the dopant ions. In the meantime, the photogenerated holes were left in the VB of ZnO and migrated to the catalyst surfaces. Therefore, the recombination of the electron and the hole could be inhibited. The excited electrons and holes subsequently could produce more reactive oxygen species (ROSs, mainly •OH and •O − 2 radicals) to degrade ammonia, resulting in the improved photocatalytic activity under visible light. The decrease in the particles size will increase the specific surface area and provide with larger active surface, which consequently would lead to a higher interfacial charge carrier transfer for photocatalysis. Cu ions doped with ZnO or TiO<sup>2</sup> have also been reported capable to enhance the methyl orange, phenol and humic substances degradation efficiency (Maleki et al., 2015; Zhang et al., 2015; Meshram et al., 2016).

The NH<sup>+</sup> 4 -N removal efficiency was substantially enhanced with rGO supporting. It is obvious that Cu/ZnO/rGO composite catalysts exhibited the highest photocatalytic degradation efficiency, with removal efficiency of 79.2% in UV light and 83.1% under visible light irradiation. The influence of rGO at the photocatalytic ammonia removal enhancement can be explained as follows: (i) In the Cu/ZnO/rGO composite, ZnO is one of the most efficient electron donors while rGO is a relatively effective electron acceptor. Irradiated with visible photon energy, photo-excited electrons and holes from ZnO were extracted by the rGO bridges through interfacial interactions. The high conductivity of rGO sheets enables quick charge transfer prolonging the charge carriers life-time, suppressing the h+/e<sup>−</sup> recombination (Zhao et al., 2017). These interactions can effectively enhance the generation of active radical species under photo irradiation which are eventually responsible for the photocatalytic decomposition of ammonia. (ii) From the obtained results, rGO nanosheets have the ability for ammonia adsorption due to their larger specific surface areas and functional oxygen groups. rGO could also prevent ZnO agglomeration enhancing the adsorption of ammonia molecules. The ammonia molecules adsorbed on photocatalyst are more favorable to interact with the ROSs than those in the solution form. (iii) Moreover, the high optical transparency of rGO facilitates light harvesting and subsequently enhances the power conversion efficiency. Furthermore, increase of the rGO concentration over 6% mass adversely affects the NH<sup>+</sup> 4 -N removal efficiency (Figure S1). The reduced efficiency is most likely caused by rGO overloading. The excessive rGO could act as a block restricting the light absorption of ZnO and encouraging the electron hole recombination. Similar behavior on the effect of rGO dosage has been reported previously, where rGO overloading caused a negative influence over the catalytic performance (Ranjith et al., 2017).

In conclusion, combined with Cu doping and rGO supporting, photocatalytic activity of ZnO under visiblelight irradiation is enhanced. A possible reaction mechanism and the photodegradation process are illustrated in **Figure 6**.

pH is an vital parameter in photocatalytic reactions (Shibuya et al., 2015). For the investigation of the photocatalytic NH<sup>+</sup> 4 - N removal with Cu/ZnO/rGO at various pH (4.0 to11.0), the

He et al. Cu/ZnO/rGO Photocatalyst

catalyst and the ammonia solution were kept in dark for 30 min then excited under light illumination for 2 h. The decrease of NH<sup>+</sup> 4 -N in the dark was less than 10%, and the reduction related to the physicochemical adsorption property of the photocalysts. From **Figure 7A**, Cu/ZnO/rGO demonstrated higher NH<sup>+</sup> 4 -N removal efficiency in alkaline conditions rather than in neutral or acidic ones. The concentration of NH<sup>+</sup> 4 -N remained unchanged at pH 4. The NH<sup>+</sup> 4 -N removal efficiency increased as per pH rising and reached the maximum at a pH >10. This is explained by the fact that pH influences not only the existing forms of the NH<sup>+</sup> 4 -N in solution but also the photocatalyst's surface charge properties (Shibuya et al., 2015). Firstly, ionized ammonium (NH<sup>+</sup> 4 ) and un-ionized ammonia (NH3) are the main types of ammonia nitrogen (at equilibrium in water with a pKa of 9.26). It has been reported that NH<sup>+</sup> 4 could not be oxidized by the VB holes (h<sup>+</sup> VB) or hydroxyl radicals (·OH). (Lee et al., 2002). When the solution pH is higher than 9.26, the dominant species of aqueous ammonia is NH3. The proportion of NH<sup>+</sup> 4 is reversely proportional with the solution pH when < 9.26 (Sun et al., 2015). Thus, ammonia degradation efficiency decreases in neutral or acidic solutions. When the solution pH dropped

TABLE 1 | Comparison of ammonia removal efficiency (%) using various photocatalysts.


to 7, almost all aqueous ammonia was in the form of NH<sup>+</sup> 4 , and impossible to be photocatalytically oxidized. Secondly, the number of hydroxide groups (OH−) increases with the pH increase, and more hydroxyl radical (·OH) can be generated by Cu/ZnO/rGO, resulting in the promotion of the NH<sup>+</sup> 4 -N removal efficiency. Thirdly, the surface of Cu/ZnO/rGO catalysts is maintained positively charged in acidic conditions, a condition unfavorable for the adsorption of ammonium molecules at the catalyst surface. Moreover, the space steric hindrance of NH<sup>3</sup> is smaller than the one of NH<sup>+</sup> 4 , which is more conducive to the reaction of NH<sup>3</sup> with ·OH (Luo et al., 2015). This finding is aligned with the theoretical relationship between pH and ammonia photocatalysis (Wang et al., 2014; Shibuya et al., 2015).

The optimum amount of catalysts is essential for effective photocatalytic removal of ammonia. **Figure 7B** shows the effect of Cu/ZnO/rGO dosage on photocatalytic performance under visible light. Increasing catalyst dosage from 0.2 to 2 g/L, ammonia degradation increases from 31.5 to 83.2% respectively. When the photocatalyst dosage is kept low, the production of ROSs is limited. Dosage increase could offer with more accessible active sites to be exposed to light irradiation and generate more ROSs leading to the enhanced photocatalytic activity. Further increase in the amount of Cu/ZnO/rGO (3 g/L) leads to a removal efficiency reduction. This is attributed to the excessive amount of photocatalysts that could reduce light transmittance and decrease the light penetration. Moreover, a large amount of catalysts could form larger agglomerates and consequently decrease the active catalyst surface. Therefore, a Cu/ZnO/rGO overdose limits photocatalytic performance. The optimal concentration of Cu/ZnO/rGO in the trial was found 2 g/L.

The initial ammonium concentration has a profound impact on the removal rate. As shown on **Figure 7C**, at high initial NH<sup>+</sup> 4 -N concentration a significant decrease (P < 0.05) in the rate of ammonium degradation was observed. This is due to the relationship of the rate of ammonia photo-degradation to the availability of catalytic sites on the catalyst surface that dictates the formation of ·OH. At this study, the amount of Cu/ZnO/rGO was kept constant, thus the high initial ammonia concentration could occupy a greater number of Cu/ZnO/rGO active sites available for degradation in the presence of excessive "influent" concentration. Thus, the formation of ·OH on the catalyst surface was inhibited. This results in decreased removal rates and subsequently to longer retention times to complete degradation. **Figure 7C** indicates that the percentage of degradation varies from 89.2 to 45.0% for 10– 100 mg/L respectively. The kinetic studies of the photocatalytic degradation of ammonia using Cu/ZnO/rGO at different initial concentrations were also investigated using the pseudo-firstorder kinetic reaction:

$$\ln\left\{\frac{C\_0}{C}\right\} = Kt\tag{13}$$

Where K (min−<sup>1</sup> ) is the first-order rate constant, which is calculated from the slope of ln (C0/C) vs. t plots, t is the irradiation time, <sup>C</sup><sup>0</sup> is the initial NH<sup>+</sup> 4 -N concentration at the beginning of the photocatalytic reaction, C is the NH<sup>+</sup> 4 -N concentration at different irradiation time. A linear relationship between the ln (C0/C) and t (**Figure 7D**) shows that the rate constant values K (min−<sup>1</sup> ) decreases with increased ammonia concentration. The values of rate constants for 10, 30, 50, 70, and 100 mg/L of NH<sup>+</sup> 4 - N are estimated to be 0.0238 min−<sup>1</sup> , 0.02073 min−<sup>1</sup> , 0.01819 min−<sup>1</sup> , 0.00947 min−<sup>1</sup> , and 0.00647 min−<sup>1</sup> , respectively.

As shown in Reaction Schemes I and II, nitrogen gas (N2), nitrite (NO<sup>−</sup> 2 -N), and nitrate (NO<sup>−</sup> 3 -N) are the plausible byproducts of the photocatalytic removal of aqueous NH<sup>+</sup> 4 -N. To further confirm the mechanism of the NH<sup>+</sup> 4 -N oxidation with Cu/ZnO/rGO the fate of the inorganic N atoms (including NH<sup>+</sup> 4 - N, NO<sup>−</sup> 2 -N, and NO<sup>−</sup> 3 -N) during the process were quantified. **Figure 8** shows that concentration of aqueous NH<sup>+</sup> 4 -N decreased from 50 mg/L to 8.5 mg/L after 2 h treatment. During the degradation process, only minor amounts of NO<sup>−</sup> 3 -N (0.43 mg/L) and NO<sup>−</sup> 2 -N (0.262 mg/L) were detected. Considering the weak volatilization of aqueous NH<sup>+</sup> 4 -N (about 4.8 mg/ L, control experiment without catalyst) it is proposed that ∼70% of NH<sup>+</sup> 4 - N is converted to N<sup>2</sup> according to the N-mass balance. The formation of nontoxic N<sup>2</sup> is an advantage of the specific process.

The stability of a photocatalyst is a crucial parameter for the evaluation of an agent as per its practical application. The stability of Cu/ZnO/rGO was evaluated by a successive cyclic experiment performed using 200 mg Cu/ZnO/rGO in 100 mL ammonia solution with an initial concentration of approximately 50 mg·L <sup>−</sup><sup>1</sup> under visible light irradiation. **Figure 9** shows that the photocatalyst did not exhibit any obvious loss of activity, and high NH<sup>+</sup> 4 -N efficiency of Cu/ZnO/rGO was maintained even after five successive trials. Specifically, the photocatalytic was reduced by up to ∼0.6%. Therefore, Cu/ZnO/rGO nanocomposite has a stable photocatalytic activity with a relatively satisfactory recyclability and reusability. The release of the Zn2<sup>+</sup> and Cu2<sup>+</sup> ions from the Cu/ZnO/rGO photocatalysts was also measured via ICP. The results pointed that Zn2<sup>+</sup> and Cu2<sup>+</sup> ions were not detected at any of the five-cycles. Moreover, the XRD analysis revealed that the structure of Cu/ZnO/rGO remained the same before and after the photocatalysis (Figure S2). The above indicate the good degree of stability of the Cu/ZnO/rGO underlying its high potential in practical applications.

In addition, we also compare our NH<sup>+</sup> 4 -N removal efficiency with some of the related literatures, it is shown that Cu/ZnO/rGO has similar NH<sup>+</sup> 4 -N removal efficiency with that of the reported catalysts (**Table 1**). The advantage of our investigation is that the removal efficiency was achieved within 2 h retention time. The short retention time dictates the size of the tank in water and wastewater treatment engineering. Subsequently the effective catalysts can reduce the capital cost of these applications paving the path toward sustainable treatment.

### The COD, N, and P Changes of Actual Domestic Wastewater

The applicability of Cu/ZnO/rGO nanocomposite in real domestic wastewater was also investigated. Our study clearly shows that Cu/ZnO/rGO exhibit highly efficient simultaneous removal of COD, N, and P (**Figure 10**). After treating wastewater with Cu/ZnO/rGO, the concentrations of COD were reduced by 57.7 mg/L (removal efficiency: 84.3%) by photocatalytic degradation. The removal efficiency of NH<sup>+</sup> 4 -N and TN were 78.5 and 80.7%, respectively. In addition, the concentration of TP decreased from 5.44 to 0.53 mg/L (removal efficiency: 90.3%). Based on these findings, it is clear that Cu/ZnO/rGO nanocomposite is an ideal agent for real wastewater treatment.

#### CONCLUSIONS

In summary, Cu-doped ZnO/rGO nanocomposite was sufficiently prepared via a two-step hydrothermal method. The incorporation of Cu ions and rGO with ZnO broadened the response of the photocatalyst to visible light. The fabricated Cu/ZnO/rGO showed higher ammonia removal efficiency (>80%) than the pure ZnO under visible-light irradiation, and the major oxidation product was N2. The facile modification of ZnO by Cu ions and rGO can improve the ROSs production efficiency and further activate the interfacial catalytic sites, which account for the higher degradation efficiency. This study demonstrates that Cu/ZnO/rGO composite is a promising alternative for NH<sup>+</sup> 4 -N removal and real domestic wastewater treatment with effective application of solar energy. Further work is now underway to combine the Cu/ZnO/rGO nanocomposite with a suitable reactor to degrade pollutants for the water treatment in practice.

#### AUTHOR CONTRIBUTIONS

SH, LX, and LY designed and conducted the experiments. PH, YF, and YY analyzed the data. SH, EP and LY wrote the paper.

#### ACKNOWLEDGMENTS

This work was supported by grants from the National Key Research and Development Program of China

#### REFERENCES


(2017YFD0800101), National Natural Science Foundation of China (grant numbers 41771295, 41601319, 41501320), and Jiangsu Provincial six top talent peak projects (NY-054).

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00219/full#supplementary-material


synthesized via a one-step hydrothermal method. J. Alloy. Compd. 722, 272–277. doi: 10.1016/j.jallcom.2017.06.001


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 He, Hou, Petropoulos, Feng, Yu, Xue and Yang. This is an openaccess article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Photocatalytic Oxidation of Acetone Over High Thermally Stable TiO<sup>2</sup> Nanosheets With Exposed (001) Facets

Ting Shi <sup>1</sup> , Youyu Duan<sup>1</sup> , Kangle Lv <sup>1</sup> \*, Zhao Hu<sup>1</sup> , Qin Li <sup>1</sup> , Mei Li <sup>1</sup> \* and Xiaofang Li 1,2

*<sup>1</sup> Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, College of Resources and Environmental Science, South-Central University for Nationalities, Wuhan, China, <sup>2</sup> College of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan, China*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Xin Li, South China Agricultural University, China Shiyong Sun, Southwest University of Science and Technology, China*

#### \*Correspondence:

*Kangle Lv lvkangle@mail.scuec.edu.cn Mei Li limei@mail.scuec.edu.cn*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

> Received: *07 April 2018* Accepted: *30 April 2018* Published: *18 May 2018*

#### Citation:

*Shi T, Duan Y, Lv K, Hu Z, Li Q, Li M and Li X (2018) Photocatalytic Oxidation of Acetone Over High Thermally Stable TiO2 Nanosheets With Exposed (001) Facets. Front. Chem. 6:175. doi: 10.3389/fchem.2018.00175* Anatase TiO<sup>2</sup> (A-TiO2) usually exhibits superior photocatalytic activity than rutile TiO<sup>2</sup> (R-TiO2). However, the phase transformation from A-TiO<sup>2</sup> to R-TiO<sup>2</sup> will inevitably happens when the calcination temperature is up to 600◦C, which hampers the practical applications of TiO<sup>2</sup> photocatalysis in hyperthermal situations. In this paper, high energy faceted TiO<sup>2</sup> nanosheets (TiO2-NSs) with super thermal stability was prepared by calcination of TiOF<sup>2</sup> cubes. With increase in the calcination temperature from 300 to 600◦C, TiOF<sup>2</sup> transforms into TiO<sup>2</sup> hollow nanoboxes (TiO2-HNBs) assembly from TiO2-NSs via Ostwald Rippening process. Almost all of the TiO2-HNBs are disassembled into discrete TiO2-NSs when calcination temperature is higher than 700◦C. Phase transformation from A-TiO<sup>2</sup> to R-TiO<sup>2</sup> begins at 1000◦C. Only when the calcination temperature is higher than 1200◦C can all the TiO2-NSs transforms into R-TiO2. The 500◦C-calcined sample (T500) exhibits the highest photoreactivity toward acetone oxidation possibly because of the production of high energy TiO2-NSs with exposed high energy (001) facets and the surface adsorbed fluorine. Surface oxygen vacancy, due to the heat-induced removal of surface adsorbed fluoride ions, is responsible for the high thermal stability of TiO2-NSs which are prepared by calcination of TiOF<sup>2</sup> cubes.

#### Keywords: TiO2 , photocatalytic degradation, acetone, fluorine, oxygen vacancy

# INTRODUCTION

Semiconductor photocatalysis has attracted much attention due to its potential applications such as water (Regmi et al., 2018; Xu et al., 2018) and air purification (Wen et al., 2015; Cui et al., 2017; Li et al., 2017a; Qi et al., 2017) and water splitting for clean H<sup>2</sup> energy (Cheng et al., 2018) due to its peculiar chemical and physical properties. As a typical semiconductor photocatalyst, anatase TiO<sup>2</sup> (A-TiO2) usually shows excellent photocatalytic activity. However, it usually transforms into poor photoreactive rutile TiO<sup>2</sup> (R-TiO2) when calcination temperature is higher than about 600◦C, which hampers the practical applications of A-TiO<sup>2</sup> in hyperthermal situations (Lv et al., 2011; Liang et al., 2017). For example, the temperature of some industrial waste gases after burning can be as high as 900–1000◦C, which makes the photocatalytic oxidation technology lose its power due to the phase transformation of A-TiO<sup>2</sup> at such high temperature. There are many photocatalytically active stable TiO2-coated ceramic materials which are used for the control of organic contaminants, including sanitary wares, bathroom tiles and self-cleaning glass. They require high processing temperatures and therefore need excellent stability at high temperature (Periyat et al., 2009). Thus, exploration of thermally stable TiO<sup>2</sup> photocatalyst is very important but this question remains unsolved.

As one of volatile organic compounds (VOCs), acetone is a widely used solvent especially in chemical plant. The emission of acetone can not only cause the problems to global environment, but also bring harms to human health (Zhu et al., 2016). When the concentration of acetone vapor is higher than 500 ppm, it can irritate eyes and discomfort respiratory system (Li et al., 2018). So, it is of great importance to explore an efficient method to decompose of acetone. Because semiconductor photocatalysis is considered as an environmentally benign way, it is no doubt very promising for VOCs removal under normal conditions.

It is generally accepted that the quantum efficiency of photocatalysis is highly related to the physical properties of TiO<sup>2</sup> such as crystal structure, relative crystallinity, size of the particle, and specific surface area (Xu et al., 2007; Lan et al., 2015; Wang et al., 2015, 2016; Wen et al., 2015; Li et al., 2016; Sajan et al., 2016; Liang et al., 2017; Xia et al., 2017; Lin et al., 2018a,b). Recently, growing interesting has been devoted to the synthesis of TiO<sup>2</sup> nanocrystals with exposed high energy {001} facets by surface fluorination of TiO<sup>2</sup> to reduce the surface energy (Yang et al., 2008, 2009; Chen et al., 2010; Yu et al., 2014; Sajan et al., 2016; Liang et al., 2017). On considering that TiOF<sup>2</sup> cubes are fluorine-containing materials, they prefer to transform into high energy TiO<sup>2</sup> nanocrystals during calcination. After the removal of surface adsorbed fluorine ions at high temperature, the formed oxygen vacancy (OV) on the surface of the photocatalyst will prevent the fusion of neighboring A-TiO<sup>2</sup> nanocrystals, which is believed to improve the thermal stability of TiO<sup>2</sup> nanocrystals. Herein, we systematically studied the dependence of the structure and photocatalytic activity of TiOF<sup>2</sup> cubes on the calcination temperature. Acetone was selected as the targeted VOCs to evaluate the photoreactivity of the as-prepared photocatalyst.

## EXPERIMENTAL SECTION

## Synthesis

Precursor TiOF<sup>2</sup> was synthesized through the solvothermal reaction of Tetrabutyl titanium (TBT), acetic acid (HAc) and hydrofluoric acid (HF) (Huang et al., 2013). Typically, 20.0 g of TBT was dropwise added into the mixed solution containing 6.4 ml of HF (47 wt%) and 40.0 ml of HAc under magnetic stirring. The resulted white suspensions were transferred to an autoclave with volume of 100 ml, which was then kept at 200◦C for 2 h. The white deposition was collected and washed with ethanol and distilled water until the filtrate is neutral (pH7). After oven dry at 80◦C, we obtained the precursor TiOF2.

Precursor TiOF<sup>2</sup> was then calcined at certain temperature (300-1200◦C) for 2 h by keeping the same heating rate (5◦C min−<sup>1</sup> ). The obtained photocatalyst is denoted as Tx (**Table 1**), where x is the calcination temperature. For example, T500 is that the photocatalyst which was prepared by calcination of the precursor TiOF<sup>2</sup> at 500◦C for 2 h.

TABLE 1 | Physical property of the photocatalyst.


*<sup>a</sup>A-TiO<sup>2</sup> and R-TiO<sup>2</sup> represent anatase TiO<sup>2</sup> and rutile TiO2, respectively.*

*<sup>b</sup>The BET surface area is determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range from 0.05 to 0.3.*

# Characterization

XRD patterns of the photocatalysts were performed on a X-ray diffractometer (D8- advance, Bruker Co., German), and the scan rate of Cu Ka radiation keeps 0.02◦ 2θ s −1 , using an accelerated voltage and applied current of 15 kV and 20 mA, respectively. We observe the morphology of the prepared photocatalyst by an FESEM with an acceleration voltage of 20 kV (Hitach, Japan) and a TEM (Tecnai G20, USA) using an acceleration voltage of 200 kV, respectively. The optical property of the photocatalyst was measured by a spectrophotometer (UV-2550, Shimadzu, Japan) from 200 to 800 nm using BaSO<sup>4</sup> as background. FTIR was obtained on a infrared spectrometer (NeXUS 470) using the KBr pellet technique. XPS was recorded using monochromatic Al-Ka radiation under vacuum at 2 × 10−<sup>6</sup> Pa on a photoelectron spectrometer (VG Multilab 2000). The C1s peak at 284.8 eV originated from the surface adventitious carbon is used to reference all the binding energies. EPR signal of the photocatalyst was recorded in an EPR spectrometer (JES-FA 200, JEOL) at room temperature (frequency of 100 kHz and microwave power of 0.99 mW). Nitrogen sorption isotherm was measured on an ASAP 2020 nitrogen adsorption equipment (Micromeritics, USA). Before investigating the surface areas of the photocatalysts, all samples were degassed firstly at 200◦C.

#### Photoelectrochemical Measurements

We use CHI760e as electrochemical workstation (Shanghai, China) to measure the transient photocurrent, EIS Nyquist plots and Mott-Schottky plots in a standard there-electrode system, where Pt wire was used as the counter electrode, and the prepared samples and Ag/AgCl in saturated KCl were used as the working and reference electrode, respectively. During the measurement, 0.4 M Na2SO<sup>4</sup> was used as electrolyte solution. In the Mott-Schottky measurement, direct current potential polarization was kept at a fixed frequency, and the working electrode was prepared on a glassy carbon electrode. Before the test of photocurrent, 50 mg photocatalysts and 30 µL Nafion were dispersed into 1 mL water/absolute ethanol mixed solvent (v/v = 1/1), and then the mixed aqueous solution was dispersed uniformly through ultrasound to form a homogeneous catalyst colloid. The ITO/TiO<sup>2</sup> electrode was prepared using the as-prepared photocatalyst colloid as precursor by a drip coating method. The light source is a 3W LED lamp (Shenzhen LAMPLIC, China) emitted mainly at 365 nm. The intensity for the lamp at working distance is measured to be 0.41 W/cm<sup>2</sup> .

### Evaluation of the Photocatalytic Activity

Photocatalytic oxidation of gasous acetone was used to evaluate the photocatalytic activity of the photocatalyst, which was performed in a 15 L reactor at ambient temperature under UV light irradiation. 0.3 g of the powder was firstly dispersed in 30 mL of double distilled water by sonicating treatment for 5 min. The obtained suspensions were then evenly transferred into three glass dishes in diameters of about 7.0 cm. After drying at 80◦C for about 2 h, the dishes that have been coated with a layer of the photocatalyst were placed in the reactor, and then 10 uL of acetone was injected into the reactor by a microsyringe. The vaporated acetone then begins to adsorb on the surface of the photocatalyst. Thirty minutes later, the adsorption–desorption equilibrium of acetone can be established. The photocatalytic oxidation of acetone begins after turning on the UV light, which is 5 cm above the dishes. The concentrations of acetone and the produced carbon dioxide in the reactor were determined online with a Photoacoustic IR Multigas Monitor (Model 1412, INNOVA). Before irradiation, the initial concentration of acetone after the adsorption equilibrium was about 300 ppm, which almost keeps unchanged for about 5 min before lighting a UV lamp (15W@365 nm).

# RESULTS AND DISCUSSION

#### Effect of Calcination Temperature on Phase Structure

Phase structure of the photocatalyst is important to the photoreactivity. Therefore, we used XRD to study the phase evolution of the photocatalyst during calcination. From **Figure 1**, a broad peak centering at 2θ = 23.4◦ , which corresponds to the (100) plane diffraction of TiOF2, was observed for the prepared precursor, and no any peak of TiO<sup>2</sup> phases (A-TiO<sup>2</sup> and R-TiO2) exists, indicating the successful synthesis of TiOF2(Huang et al., 2013). After calcination at 300◦C for 2 h, the phase struccture of the sample (T300) almost keeps unchanged. With increase in the calcination temperature to 400◦C, the peak intensity for TiOF<sup>2</sup> decreases. Simultaneously, a small peak at 2θ = 25.3◦ , which corresponds to the (101) plane diffraction of A-TiO2, can be observed for T400 sample, indicating that phase transformation from TiOF<sup>2</sup> to anatase begins. This phase transformation becomes obvious after calcination of TiOF<sup>2</sup> at 500◦C, and the prepared TiOF<sup>2</sup> totally transforms into anatase TiO<sup>2</sup> at 600◦C.

It is proposed that the reaction for the heat-induced phase transformation from TiOF<sup>2</sup> to TiO<sup>2</sup> is as follows (Equation 1) (Zhao et al., 2018).

$$2\text{TiOF}\_2 + \text{heat} \rightarrow \text{TiO}\_2 + \text{TiF}\_4\uparrow\tag{1}$$

Further increase in the calcination from 600 to 900◦C, the peak intensity of A-TiO<sup>2</sup> increases, indicating the enhanced crystallization. Meanwhile, the narrowing of the width for the (101) plane diffraction peak indicates the increase of A-TiO<sup>2</sup> crystallite size. The peak intensity for the (101) peak of anatase TiO<sup>2</sup> begins to decrease at calcination temperature of 1000◦C, which is a sign of anatase-to-rutile phase transfermation. The formation of rutile phase is confirmed from the XRD pattern of the T1100 sample due to the formation of a small peak at 2θ = 27.3◦ , which corresponds to the (110) plane diffraction of R-TiO2. Only when the calcination temperature reaches 1200◦C can all of the A-TiO<sup>2</sup> transform into R-TiO2.

Usually, A-TiO<sup>2</sup> nanocrystals begin transform into R-TiO<sup>2</sup> at about 600◦C. However, in the present study, the temperature for anatase-to-rutile transformation is as high as 1100◦C, which indicates that these samples are promising to be used in hyperthermal situations.

#### Evolution of the Morphology

The SEM images of the precursor TiOF<sup>2</sup> is shown in **Figure 2**. It can be clearly observed that these TiOF<sup>2</sup> nanoparticles are in cubic shape and relatively monodispersed (**Figure 2a**), which is consistent with the reported literatures (Chen et al., 2012; Wang

FIGURE 1 | XRD patterns of the photocatalysts calcined at different temperatures.

Frontiers in Chemistry | www.frontiersin.org

magnification (b).

et al., 2012). From the high resolution SEM image shown in **Figure 2b**, we can estimate that the sidelength of the TiOF<sup>2</sup> cube is about 250 nm.

After calcination of the precursor TiOF<sup>2</sup> at 300◦C, the morphology of the resulted sample (T300) almost keeps unchanged (SEM image not shown here). However, some nanoboxes with hollow interiors can be clearly seen from the SEM image of T400 sample (**Figure 3a**). These anatase TiO<sup>2</sup> hollow nanoboxes (TiO2-HNBs) assemble from TiO<sup>2</sup> nanosheets (TiO2-NSs) with exposed high energy (001) facets (Wen et al., 2011). The thickness of the TiO2-NSs is about 30 nm. It has been proven that high energy TiO<sup>2</sup> nanosheets with exposed (001) facets can be prepared by using fluoride ions as shapedirecting reagent (Yang et al., 2008; Lv et al., 2012). Then, it is not hard to understand the formation of high energy TiO2- NSs during calcination of TiOF2, a kind of fluorine-containing materials.

Since the sidelength of the obtained TiO2-HNBs is similar as these precursor TiOF<sup>2</sup> cube, it is proposed that the formation of TiO2-HNBs assembly from TiO2-NSs is through a Ostwald Rippening process (Lou et al., 2008; Huang et al., 2013).

After calcination at 500◦C, most of the TiOF<sup>2</sup> cubes transform into TiO2-HNBs (**Figure 3c**). **Figure 3d** shows an enlarged SEM image of a broken TiO<sup>2</sup> hollow nanobox, from which we can see that the thickness of the TiO<sup>2</sup> nanosheet increases to about 50 nm, much thicker than that of T400 sample.

The structure of TiO2-HNBs was further confirmed by the corresponding TEM image of T500 sample (**Figure 4a**), from which we can observe the presence of some erected TiO2- NSs and a TiOF<sup>2</sup> cube that has not totally transformed into anatase TiO2. This is consistent with the corresponding XRD characterization result (**Figure 1d**). From the side view HRTEM image of a discrete TiO2-NS (**Figure 4b**), we can clearly see the lattice spacing of 0.235 nm that parallels to the top and bottom facets, which corresponds to the (001) planes of A-TiO<sup>2</sup> (Han et al., 2009). This confirms that the obtained TiO2-HNBs are assembly from high energy TiO2-NSs with exposed (001) facets (Wang et al., 2010).

When the calcination temperature increases to 600◦C, we can see that almost all of the TiO2-HNBs decompose into discrete TiO2-NSs (**Figure 3e**). From the truncated biypramidal shape of a TiO<sup>2</sup> nanocrystals (**Figure 3f**), we can estimate that the thickness of the TiO2-NSs is about 100 nm (T600 sample), which increases to ca. 150 nm for T700 sample (**Figures 3g,h**). Even calcined at 1100◦C, some TiO<sup>2</sup> nanosheets still keep bipyramidal shape (decahedron) with exposed (001) and (101) facets (**Figures 3i,j**), further indicating the thermal stability of high energy TiO2-NSs.

The bipyramidal shapes of TiO<sup>2</sup> nanostructures disappear (**Figures 3k,l**) when calcination of TiOF<sup>2</sup> cubes at 1200◦C. This can be explained by the sintering of the sample due to phase transformation from A-TiO<sup>2</sup> to R-TiO<sup>2</sup> (**Figure 1**).

By comparing the morphologies of TiO2-NSs from **Figure 3**, we can clearly see that heat treatment of TiOF<sup>2</sup> cubes only results in the growth of TiO<sup>2</sup> nanosheet along (001) direction. **Scheme 1** illustrates the morphology evolution and phase transformation of TiOF<sup>2</sup> cube during calcination.

FIGURE 3 | Comparison of the SEM images for T400 (a,b), T500 (c,d), T600 (e,f), T700 (g,h), T1100 (i,j), and T1200 (k,l) samples.

# Uv-vis Absorption and FTIR Spectra

Light-harvesting ability plays a very important role on the photoreactivity of the photocatalyst (Li et al., 2017b). Therefore, we compared the UV-vis absorption spectra of the samples. It can be seen that from **Figure 5** that, when calcination temperature is below 1100◦C, all samples possess similar absorption spectra (**Figure 5**). The onset of the UV-vis absorption spectrum for T400 sample is at 389 nm, corresponding to a bangap of 3.19 eV. However, the absorption edge was obviously red-shifted for T1200 sample. The onset of the spectrum for T1200 sample begins at 424 nm, corresponding to a bandgap of 2.92 eV, which can be ascribed to the phase transformation (**Figure 1**).

**Figure 6** compares the FTIR spectra of the photocatalysts treated at different calcination temperature. From which, it can be seen that all samples exhibit strong absorption peaks cenerting at about 3,427, 1,628, 1,403, and 554 cm−<sup>1</sup> . The peaks of 3,427 and 1,628 cm−<sup>1</sup> originate from the -OH groups/H2O due to the adsorption of moisture from the air, while the peaks centering at about 1403 and 554 cm−<sup>1</sup> originate from the vibration of

600◦C, (e) 700◦C, (f) 1100◦C, and (g) 1200◦C.

Ti-O and O-Ti-O. Carefully view shows that there is a strong absorption peak at 978 cm−<sup>1</sup> , which decreases with increasing in the calcination temperature. This peak was attributed to the vibration of Ti-F bond of TiOF2. When calcination temperature

increases to 600◦C, the vibration of Ti-F disappears because of the complete phase transformation of TiOF2-to-anatase TiO<sup>2</sup> (**Figure 1**; Zhao et al., 2018).

#### Analysis of XPS Spectrum

**Figure 7**A compares the XPS survey spectra of photocatalysts for T400, T500, and T600 photocatalysts. From which, it can be observed that the XPS spectra are similar. All these samples contain titanium (Ti), oxygen (O), fluorine (F), and carbon (C) elements. The C element originates from the adventitious hydrocarbon from the XPS instrument itself. We can also be clearly see that the peak intensity of F element tends to decrease with increasing the calcination temperature from 400 to 600◦C. The atomic ratios of F:Ti were determined to be 1.19 for T400 sample, 0.50 for T500 sample, and 0.17 for T600 sample, respectively. The steady decrease in F content with increase in the calcination is because of the TiOF<sup>2</sup> to A-TiO<sup>2</sup> phase transformation (**Figure 1** and Equation 1), and the removal of adsorbed fluoride ions on the surface of A-TiO2.

**Figures 7B,C** show the high resolution XPS spectra in Ti 2p and O 1s regions, respectively. It can be seen that, both the binding energies for Ti 2p and O 1s of the samples steady decrease with increasing the calcination temperature. This is attributed to the TiOF2-to-TiO<sup>2</sup> phase transformation (**Figure 1** and Equation 1), and the formation of oxygen vacancy due to

the removal of surface adsorbed fluoride ions over TiO<sup>2</sup> upon calcination (Lv et al., 2011). It has been accepted that almost all of the fluoride ions adsorbed on the surface of high energy TiO<sup>2</sup> nanosheets can be removed after calcination at 500◦C.

The F 1s binding energy peak centering at about 684 eV (**Figure 7D**) is designated to the surface fluoride (Ti-F). No peak with binding energy of about 688.5 eV, corresponding to the lattice F<sup>−</sup> of TiO2, is found in all the photocatalysts, indicating that calcination of TiOF<sup>2</sup> cannot results in the doping of fluorine into anatase TiO<sup>2</sup> (Yu et al., 2002). We can also clearly see that the peak intensity of F 1s steady decreases with increasing the calcination temperature, this can be attributed to the heatinduced phase transformation from TiOF<sup>2</sup> to A-TiO<sup>2</sup> and the desorption of surface adsorbed fluoride ions (Yang et al., 2008).

#### Photocatalytic Oxidation of Acetone

Photocatalytic oxidation of acetone was used to monitor the photoreactivity of the prepared photocatalyst. The reaction for the photocatalytic oxidation of acetone is as follows (Equation 2).

$$\text{(CH}\_3\text{)}\_2\text{CO} + 4\text{O}\_2 = \text{ 3CO}\_2 + \text{3H}\_2\text{O} \tag{2}$$

**Figure 8A** compares the relative photoreactivity of the photocatalyst by calculation the decomposed acetone within 2 h. It was found that the photocatalytic activity of the precursor TiOF<sup>2</sup> can be neglected (only 4.0 ppm acetone was decomposed). The poor photocatalytic activity of TiOF<sup>2</sup> is possibly due to its easy recombination of photo-generated carriers.

The photocatalytic activity of the photocatalyst increases with increase in the calcination temperature from 300◦C (12.8 ppm) to 500◦C (102.5 ppm), which is due to the production of A-TiO2. When further increase in the heat temperature from 600◦C (94.6 ppm) to 1100◦C (25.4 ppm), the photocatalytic activity of the photocatalyst steady decreases, possibly due to the decreased specific surface area (**Table 1**). Only 5.9 ppm of acetone was found to decompose when T1200 sample is used as photocatalyst. This is because of the complete A-TiO<sup>2</sup> to R-TiO<sup>2</sup>

phase transformation (**Figure 1**) and smallest BET surface area (0.4 m<sup>2</sup> g −1 ).

Reusability of the photocatalyst is also important from the viewpoint of the practical applications (Liu et al., 2010). Therefore, we monitored the recycling use of T500 sample for 5 times in acetone oxidation (**Figure 8B**). It can be seen that no obvious reactivity decrease was observed for T500 sample even continual use for 5 times, indicating that T500 sample is potential to be used in practical applications.

#### Mechanism

The value of photocurrent is usually used to evaluate the ability to generate and transfer of charge carriers for illuminated semiconductor photocatalyst (Cheng et al., 2018; Huang et al., 2018). The photocurrent of the prepared photocatalyst was therefore tested for several on-off cycles (**Figure 9**). It can be

seen that, when irradiation of the TiO<sup>2</sup> film electrodes, obvious prompt photocurrent signals are produced, which exhibit good reproducibility. When the lamp is turned off, the photocurrent value for all the TiO<sup>2</sup> film electrodes are instantaneously close to zero. We can also see, with increasing the calcination temperature from 400 to 600◦C, the photocurrent of the photocatalyst increases first and then decreases. The photocurrent of T500 sample is the as high as 2.8 uAcm−<sup>2</sup> , which is much higher than that of T400 (1.1 uAcm−<sup>2</sup> ) and T600 (2.3 uAcm−<sup>2</sup> ) samples. The photoreactivity of semiconductor photocatalyst is closely related to the efficiency of the separation of the photo-generated electron-hole pairs (Fu et al., 2008). So it is safe to predict that the photoreactivity of T500 is higher than that of T400 and T600 samples, which keeps in line with the experimental results (**Figure 8**).

It has been accepted that fluoride ions show strong affinity to titanium (Equation 3), and the photoreactivity of TiO<sup>2</sup> nanocrystals can be obviously improved after the introduction of surface fluorination (Minero et al., 2000; Xu et al., 2007; Cheng et al., 2008). According to the study of Yu et al., the difference in surface energy makes the photo-generated electrons and holes migrate to (101) and (001) facets, respectively (Yu et al., 2014). The CB electrons aggregated in (101) facets are captured by surface adsorbed oxygen to produce super oxygen radicals (O·− 2 ), while the VB holes aggregated on the (001)facets are transfer into hydroxyl radicals (·OH). Both O·− 2 and ·OH are important reactive oxygen species (ROSs) for the degradation of organics. When compared with pristine TiO2, surface fluorination of TiO<sup>2</sup> changes the state of the formed hydroxyl radicals from surface bounded ·OH radicals (Equation 6) to mobile ·OH radicals (·OHfree). This is because the displacement of fluoride to surface OH<sup>−</sup> groups (Equation 3) induces the direct oxidization solvent water by holes (Equation 7).

$$\text{H} \equiv \text{Ti}-\text{OH} \text{ + F}^- \rightarrow \text{ } \equiv \text{Ti}-\text{F} + \text{OH}^- \tag{3}$$

$$\text{TiO}\_2 + \text{ hv} \rightarrow \text{TiO}\_2 \text{ (h}^+ \cdot \text{\textdegree e^-)}\tag{4}$$

$$\text{O}\_2 + \text{e}^- \rightarrow \text{O}\_2\text{\textdegree} \tag{5}$$

$$\text{H} \rightleftharpoons \text{Ti}-\text{OH} + \text{h}^+ \rightarrow \begin{array}{c} \text{Ti} \cdot \text{OH} \end{array} \tag{6}$$

$$\equiv \text{Ti}-\text{F} + \text{H}\_2\text{O} + \text{h}^+ \rightarrow \equiv \text{Ti}-\text{F} + \cdot\text{OH}\_{\text{free}} \quad \text{(7)}$$

As free ·OH radical are more active than surface bounded ·OH radicals, the oxidation of acetone into CO<sup>2</sup> and H2O is greatly enhanced due to the attacks of super oxygen radicals and free ·OH radicals (Equation 8). The proposed mechanism for the enhanced photoreactivity of surface fluorinated TiO<sup>2</sup> nanosheet toward acetone oxidation is shown in **Scheme 2**.

$$\text{C}\_3\text{H}\_6\text{O} + \text{O}\_2\text{\textdegree /}\cdot\text{OH}\_{\text{free}} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} \tag{8}$$

Recently, significantly growing interest has been devoted to studying of hierarchical nanostructures due to their unique properties and widespread potential applications (Lou et al., 2008). For example, when compared with solid spheres, TiO<sup>2</sup> hollow spheres usually shows much higher photocatalytic activity mainly because they possess better light-harvesting ability (Li et al., 2009). In the present study, the hierarchically structured

FIGURE 10 | EPR spectra of the photocatalysts.

T500 sample, that is TiO<sup>2</sup> hollow nanobox assembly from TiO<sup>2</sup> nanosheets should also benefit the use of light (**Figure 5**), enhancing the photoreactivity.

However, the photoreactivity of TiO2-NSs steady decrease with increase in the calcination temperature from 500 to 1100◦C due to the collapse of the hierarchical TiO<sup>2</sup> hollow nanobox and the removal of surface adsorbed fluorine, leaving surface oxygen vacancy (**Figure 8**; Cheng et al., 2018).

$$\equiv \text{Ti} - \text{F} \ + \text{ heat} \rightarrow \equiv \text{Ti} - \square \tag{9}$$

The formation of surface oxygen vacancy was proved by electron paramagnetic resonance (EPR) technique. It was found that the signal intensity for oxygen vacancy of the photocatalyst increases with increase in the calcination temperature from 400 to 600◦C (**Figure 10**) due to the heat-induced desorption of surface adsorbed fluoride ions (Equation 9).

$$\begin{aligned} \left( \begin{array}{c} \text{Ti} - \text{OH} \end{array} \right) \left( \begin{array}{c} \text{Ti} - \text{O} - \text{Ti} \end{array} \right) \left( \begin{array}{c} \text{Ti} - \text{O} - \text{Ti} \end{array} \right) \left( \begin{array}{c} \text{Ti} - \text{O} - \text{Ti} \end{array} \right) \left( \begin{array}{c} \text{Ti} - \text{O} - \text{Ti} \end{array} \right) \end{aligned} \tag{10}$$

$$\equiv \text{Ti} - \square \; + \; \Box - \text{Ti} \equiv \rightarrow \text{ no reaction} \tag{11}$$

It was believed that only when the crystalline size of the nanocrystal is larger than a critical size can phase transformation begin (Padmanabhan et al., 2007; Periyat et al., 2008, 2009). Therefore, the growth of TiO<sup>2</sup> nanocrystal is a prerequisite before the phase transformation of A-TiO<sup>2</sup> to R-TiO2. The growth of pristine TiO<sup>2</sup> nanocrystal is relatively easy by formation of =Ti-O-Ti= chain between two neighboring TiO<sup>2</sup> nanoparticles (Equation 10). However, the growth of the =Ti-O-Ti= chain is prevented due to the formation of surface oxygen vacancy (Equation 11). Only when the lattice oxygen is diffused from the bulk to the surface of TiO<sup>2</sup> nanosheet with oxygen vacancy at high temperaure can the fusion of neighboring TiO2-NSs become possible (Lv et al., 2011). Therefore, it is not hard to understand the super thermal stability of TiO2-NSs prepared by calcination of TiOF<sup>2</sup> cubes.

After the removal of surface adsorbed fluoride ions by calcination, these TiO2-NSs prefer to aggregate and grow along (001) direction to reduce the high surface energy (Lv et al., 2011). Then we can clearly observe the steady increase in the thickness of TiO2-NSs (**Figure 3**).

#### REFERENCES


#### CONCLUSIONS

TiO<sup>2</sup> nanosheets with high thermal stability were prepared by calcination of TiOF<sup>2</sup> cubes. The anatase-to-rutile phase transformation temperature reaches as high as 1100◦C. 500◦Ccalcined sample shows the highest photoreactivity toward acetone oxidation due to the surface fluorination. The high thermal stability of TiO<sup>2</sup> nanosheets is ascribed to the introduction of surface oxygen vacancy after removal of the surface adsorbed fluoride ions, which prevents the growth of TiO<sup>2</sup> nanosheets. The present study provide a novel way in design of thermally stable materials.

#### AUTHOR CONTRIBUTIONS

TS and YD performed the experiments. KL planned the project, designed the experiments, and wrote the manuscript. ZH, QL and XL assisted in the analysis and interpretation of the data. ML revised the manuscript.

#### ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (51672312, 21571192 and 21373275), the Science and Technology Program of Wuhan (2016010101010018) and Fundamental Research Funds for the Central Universities, South-Central University for Nationalities (CZT18016).

Bi2WO6@Bi2S<sup>3</sup> nanoplate via ion exchange. Chin. J. Catal. 39, 718–727. doi: 10.1016/S1872-2067(17)62913-9


arranged 3D hierarchical TiO<sup>2</sup> nanoboxes. CrystEngComm 14, 4578–4581. doi: 10.1039/C2CE25271F


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Shi, Duan, Lv, Hu, Li, Li and Li. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Compressible and Recyclable Monolithic g-C3N4/Melamine Sponge: A Facile Ultrasonic-Coating Approach and Enhanced Visible-Light Photocatalytic Activity

Ye Yang1,2, Qian Zhang<sup>1</sup> \*, Ruiyang Zhang<sup>1</sup> , Tao Ran<sup>1</sup> , Wenchao Wan<sup>1</sup> and Ying Zhou1,2 \*

*<sup>1</sup> The Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, China, <sup>2</sup> State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Liwu Zhang, Fudan University, China Hongwei Huang, China University of Geosciences, China*

#### \*Correspondence:

*Qian Zhang zhangqian@swpu.edu.cn Ying Zhou yzhou@swpu.edu.cn*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *02 March 2018* Accepted: *19 April 2018* Published: *18 May 2018*

#### Citation:

*Yang Y, Zhang Q, Zhang R, Ran T, Wan W and Zhou Y (2018) Compressible and Recyclable Monolithic g-C3N4/Melamine Sponge: A Facile Ultrasonic-Coating Approach and Enhanced Visible-Light Photocatalytic Activity. Front. Chem. 6:156. doi: 10.3389/fchem.2018.00156* Powdery photocatalysts seriously restrict their practical application due to the difficult recycle and low photocatalytic activity. In this work, a monolithic g-C3N4/melamine sponge (g-C3N4/MS) was successfully fabricated by a cost-effective ultrasonic-coating route, which is easy to achieve the uniform dispersion and firm loading of g-C3N<sup>4</sup> on MS skeleton. The monolithic g-C3N4/MS entirely inherits the porous structure of MS and results in a larger specific surface area (SSA) than its powdery counterpart. Benefit from this monolithic structure, g-C3N4/MS gains more exposed active sites, enhanced visible-light absorption and separation of photogenerated carriers, thus achieving noticeable photocatalytic activity on nitric oxide (NO) removal and CO<sup>2</sup> reduction. Specifically, NO removal ratio is as high as 78.6% which is 4.5 times higher than that of the powdery g-C3N4, and yield rate of CO and CH<sup>4</sup> attains 7.48 and 3.93 µmol g−<sup>1</sup> h −1 . Importantly, the features of low-density, high porosity, good elasticity, and firmness, not only endow g-C3N4/MS with flexibility in various environmental applications, but also make it easy to recycle and stable for long-time application. Our work provides a feasible approach to fabricate novel monolithic photocatalysts with large-scale production and application.

Keywords: g-C3N4 /melamine sponge, photocatalysis, NO removal, monolith, visible light

# INTRODUCTION

Semiconductor photocatalysis is one of the promising strategies for pollutants abatement (Maggos et al., 2007; Huang et al., 2013, 2016; Zhao et al., 2015) and has attracted intense investigation in the past decades. Up to now, hundreds of semiconductors have been explored and applied in the field of environmental remediation (Liu et al., 2008; Hossain and Mukherjee, 2013; Huang et al., 2015a, 2017a; Zhou et al., 2016). However, only a few of them have been considered as potential candidates for practical application in view of their nontoxic, suitable band gap, band edge energy, good stability and earth-abundant source. g-C3N<sup>4</sup> is one of those semiconductors, which possesses graphene-like structure and constituted mainly by carbon and nitrogen. Since g-C3N<sup>4</sup> was first reported to photocatalytic water splitting by Wang et.al (Wang et al., 2009), it quickly becomes a hot material in photocatalysis. Afterwards, g-C3N<sup>4</sup> has already been applied in various reactions such as CO<sup>2</sup> photoreduction, NO removal, and dye degradation (Yan et al., 2009; Dong et al., 2014a; Sun H. et al., 2017). However, extensive studies revealed that g-C3N<sup>4</sup> suffers from fast photo-generated carriers recombination, limited visible-light absorption, and low surface area. Various strategies have been carried out to overcome these intrinsic drawbacks of g-C3N4, such as elemental doping, composite with other materials and morphology control synthesis, etc. (Liu et al., 2010; Zhao et al., 2012; Hou et al., 2014; Cheng et al., 2015; Han Q. et al., 2015; Li et al., 2017; Yang et al., 2017). Besides above disadvantages, as a potential photocatalyst for practical application, g-C3N<sup>4</sup> is also hindered by difficult recycle originating from particle heavy loss during its complicated recovery process, inefficient utilization of active sites and light energy resulting from particle aggregation. Comparison with the intrinsic drawbacks of g-C3N4, these problems are vital to achieve successful application of photocatalyst on actual environmental issues, but they are rarely studied and generally beyond the aforementioned strategies to overcome.

Recently, monolithic or integrated photocatalysts are found to be a plausible way to solve the practical application problem of photocatalyst (Cheng et al., 2016; Wan et al., 2018). The socalled monolithic photocatalyst usually consists of two parts, one is a macroscopical support with porous three-dimensional (3D) skeleton, and the other is the loaded photocatalyst particles. After integrating powdery catalysts on its support, their recycling becomes easy to achieve by a tweezer (Liu W. J. et al., 2015; Tang et al., 2017). Meanwhile, the 3D porous structure of support gives the powdery catalysts a high dispersion and exposes more active sites by avoiding particles agglomeration. Moreover, this structure also benefits light energy harvest and transportation of liquid or gas pollutants. Until now, graphene aerogels are the most studied monolithic photocatalyst supports because of their inherent large surface area, high porosity and low density. We have fabricated monolithic C3N4/graphene oxide aerogel (GOA) in our previous work and found obvious activity enhancement (Wan et al., 2016), which is in line with the results from other monolithic photocatalyst/graphene aerogels (Fan et al., 2015; Cui et al., 2017; Wang et al., 2017). However, the intrinsic brittleness and weak firmness of aerogels make it easy to break into pieces during mechanical deformation, which seriously restrict its potential in practical application. Other firm supports, such as carbon foam and Al2O<sup>3</sup> ceramic foam with hard 3D framework, are also selected to fabricate monolithic photocatalyst (Dong et al., 2014b; Lin et al., 2016). Nevertheless, it is very difficult to achieve uniform loading by directly mixing photocatalyst with these hard supports. To overcome this problem, special strategies with high cost and energy consumption, like in-situ immobilizing approach and laser ablating deposition (Liang et al., 2015; Lin et al., 2016), are used to achieve the good catalyst dispersion. The sophisticated preparation method severely limits the hard supports to be extensively utilized in fabrication of monolithic photocatalyst. Based on the above considerations, the proper support remains an obstacle for the practical application of monolithic photocatalyst. Lately, melamine sponge (MS), a cheap commercial polymer foam which is widely used as kitchen and construction materials, is successfully used for oil-water separation by integrating with graphene (Liu T. T. et al., 2015; Zhao et al., 2016). The fabricated graphene/MS exhibits lowdensity, high porosity and high elasticity inherited from MS, which exactly match the support characteristics of the monolithic photocatalyst. Importantly, the good elasticity makes MS more ductile and avoids the drawbacks of brittle and hard materials. Therefore, MS is a potential alternative and selected as the support for monolithic photocatalyst fabrication.

Herein, we prepare a monolithic g-C3N4/MS by a facile ultrasonic-coating method at room temperature, which is very easy to achieve mass production. The monolithic structure endows g-C3N4/MS with enhanced light harvest and more exposed active sites, ensuring its good photocatalytic activity. Importantly, the as-prepared monolithic photocatalyst exhibits low-density, high porosity, high elasticity and good firmness, which not only make it flexible in various environmental applications including NO removal, and CO<sup>2</sup> photoreduction, but also make it easy to recycle and suitable for practical application. Overall, our results provide a novel strategy to develop monolithic photocatlyst for practical application with large-scale production.

# EXPERIMENTAL

# Synthesis of g-C3N<sup>4</sup>

The polymeric g-C3N<sup>4</sup> was prepared by pyrolysis of urea (Liu et al., 2011). In a typical process, 15 g urea was added into an alumina crucible with a cover and then heated to 550◦C in a muffle furnace for 1 h with a heating rate of about 55◦C min−<sup>1</sup> . After cool down to room temperature, the final yellow agglomerates were the pristine g-C3N<sup>4</sup> and subsequently ground into powder for further use.

# Preparation of Monolithic g-C3N4/MS

For the preparation of monolithic g-C3N4/MS, 2.5 g g-C3N<sup>4</sup> powder was dispersed in 500 mL water and sonicated for 1 h to form a g-C3N<sup>4</sup> suspension. MS was cut into suitable size and washed with deionized water and alcohol in order, then dried at room temperature. Next, the clean MS was immersed into g-C3N<sup>4</sup> suspension for 30 min, and then squeezed out the excess solution. After that, the sample was transferred into a culture dish and freeze-dried (−70◦C pre-freezing) for 48 h to obtain g-C3N4/MS. For comparison, powdery g-C3N<sup>4</sup> without MS was processed through the same procedures and denoted as sonicated g-C3N4. To obtain the best photocatalytic activity of monolithic g-C3N4/MS, we finely investigated the effect of g-C3N<sup>4</sup> suspension concentration (3 mg mL−<sup>1</sup> -40 mg mL−<sup>1</sup> ) and MS thickness (0.5–2.5 cm). The sizes of monolithic g-C3N4/MS varied with different experiments and were stated at their first appearance in the text.

## Characterization

Powder X-ray diffraction (PXRD) was performed on a PANalytical X'pert diffractometer with a Cu Ka radiation. Transmission electron microscopy (TEM) was performed on

a FEI tecnai G2 F30 microscope operated at 200 kV. The morphology of g-C3N4/MS was observed through scanning electron microscopy (SEM) on a ZEISS EVO MA15 microscopy. The Fourier transform infrared (FT-IR) spectra were measured using a Nicolet 6700 spectrometer on samples embedded in KBr pellets. UV-vis diffuse reflectance spectrum (DRS) data were recorded on a Shimadzu UV-2600 spectrophotometer. Photoluminescence spectra were recorded on F-7000 FL spectrofluorometer with an excitation wavelength at 320 nm. X-ray photoelectron spectroscopy (XPS) was performed by using a Thermo Scientific Escalab 250Xi spectrometer. The specific surface area (SSA) was determined via using methylene blue (MB) adsorption method on a UV-vis spectrophotometer (UV-5100, Anhui Wanyi; Tran et al., 2015), the SSA of g-C3N<sup>4</sup> and g-C3N4/MS were calculated by the following equation:

$$\text{SSA} = \frac{N\_A A\_{MB} (C\_0 - C\_e) V}{M\_{MB} m\_s}$$

Where N<sup>A</sup> represents Avogadro's constant (6.02×10<sup>23</sup> mol−<sup>1</sup> ), AMB represents the covered area of per MB molecule (typically assumed to be 1.35 nm<sup>2</sup> ), C<sup>o</sup> and C<sup>e</sup> are the initial and equilibrium concentrations of MB, V is the volume of MB solution, MMB is the relative molecular mass of MB, and m<sup>s</sup> is the mass of the sample.

#### Evaluation of Photocatalytic Activity

The photocatalytic activity of g-C3N4/MS was evaluated in both gaseous systems. Photocatalytic removal of NO at ppb level was previously reported in details (Zhang et al., 2014). Typically, A 150 W metal halide lamp with a visible light filter (>420 nm) was employed to operate the experiment. A piece of g-C3N4/MS was put into the reactor for photocatalytic activity test. The light intensity is 35.88 mW cm−<sup>2</sup> measured by a light intensity meter. The initial concentration of NO was diluted to 500 ppb by drying air. The flow rates of dry air and NO are set at 2 L min−<sup>1</sup> and 9.5 mL min−<sup>1</sup> , respectively. The formula of degradation rate of NO was counted by the following equation:

$$\omega\left(\%\right) = \frac{\left[C\left(NO\_X\right) - C\left(NO\right) - C\_0\{NO\_2\}\right]}{C\left(NO\_X\right)} \times 100\%$$

Where C (NOx) represents the concentration of total nitric oxide (NO<sup>2</sup> and NO), while C<sup>0</sup> is the initial concentration of NO<sup>2</sup> when reaching the adsorption-desorption equilibrium.

The photocatalytic reduction of CO<sup>2</sup> was performed in a 380 mL home-made reactor at ambient temperature and pressure. A 300 W Xe lamp was used as a light source and positioned 8 cm above the photocatalytic reactor. In a typical test, a plastic beaker with 20 mL deionized water was deposited at the bottom of the reactor, and a culture dish with 100 mg g-C3N<sup>4</sup> powder or g-C3N4/MS (38.5 cm<sup>2</sup> ) was placed on the plastic beaker. Before irradiation, the reactor was sealed and vacuumed by a pump, then removed air by blowing argon for 15 min. Subsequently, 1 mL CO<sup>2</sup> was injected into the reactor. After 4 h irradiation, 1 mL of gas was taken out from the reactor and analyzed by using a gas chromatograph (Techcomp GC7900) equipped with a flame ionized detector (FID) and a thermal conductivity detector (TCD). CO and CH<sup>4</sup> were analyzed by the FID, and H<sup>2</sup> was analyzed by TCD.

# RESULTS AND DISCUSSION

# Fabrication and Physical Properties of g-C3N4/MS

The general preparation approach of g-C3N4/MS was illustrated in **Figure 1A**. Direct mixture of powdery g-C3N<sup>4</sup> and MS is hard to gain a monolithic g-C3N4/MS with good particle dispersion and well contact between particle and MS skeleton. Therefore, the g-C3N<sup>4</sup> powder was firstly added into water and sonicated for 30 min to form uniform suspension, which make the particle diffusion easy and fast in the porous structure of MS. Then, the pretreated MS was immersed into this suspension through dipping and squeezing procedures until MS was fully covered by g-C3N<sup>4</sup> particles. Finally, followed by a conventional freezing-drying process, the monolithic g-C3N4/MS was obtained. Obviously, no large particles were observed on the cross-section photo in **Figure 1A**, indicating that the g-C3N<sup>4</sup> powder was uniformly coated on MS skeleton. For comparison, we also dipped MS in a saturated solution of urea, which is the g-C3N<sup>4</sup> precursor. After removing the water by freeze-drying, we attempted to fabricate the monolithic g-C3N4/MS by in-situ immobilizing approach at 550◦C. However, the obtained monolithic composite became very fragile due to the carbonization of MS skeleton. Moreover, the g-C3N<sup>4</sup> powder gathered on the composite surface with an inhomogeneous dispersion (Figure S1). The failed trial illustrated the mild ultrasonic-coating approach is superior to other methods for monolithic g-C3N4/MS fabrication. Importantly, the facile ultrasonic-coating approach developed here is also adapted to other porous supports, such as nickel foam, Al2O<sup>3</sup> ceramic foam, glass fiber and polyester fiber, and the corresponding monolithic products of g-C3N<sup>4</sup> are shown in Figure S2.

Physical properties of materials are pivotal to their practical application. As shown in **Figure 1B**, the monolithic g-C3N4/MS has an ultra-low density (11.5 mg cm−<sup>3</sup> ), which can stay on dog tail grass and remarkably lighter than that of reported BiOBr/reduced GOA (50 mg cm−<sup>3</sup> ) (Liu W. J. et al., 2015), TiO2/graphene aerogel (19 mg cm−<sup>3</sup> ) (Qiu et al., 2014), and MoS2/reduced GOA (56.1 mg cm−<sup>3</sup> ) (Zhang R. Y. et al., 2017). Moreover, SSA of g-C3N4/MS (7.6 m<sup>2</sup> g −1 ) is much more larger than that of the pristine g-C3N<sup>4</sup> (0.9 m<sup>2</sup> g −1 ), which not only offer more active sites but also have larger absorption capacity than its powdery counterpart. In addition, facile modification of shapes and sizes endows g-C3N4/MS with a good flexibility to handle different situations in practical application (**Figures 1C,D**). More importantly, the g-C3N4/MS presents excellent elasticity. As revealed in **Figure 1E**, the g-C3N4/MS can instantaneously recover and maintain its integrity after removing of the heavy loading (410 g counterweight), suggesting that g-C3N4/MS possesses enough mechanical strength to deal with intricate operation in environmental abatement. To further ensure the firmness of g-C3N<sup>4</sup> on MS skeletons, a test is carried

out by blowing the g-C3N4/MS with a strong airflow for 12 h, the detailed simulation device diagram is shown in Figure S3a. The dropping g-C3N<sup>4</sup> powder from g-C3N4/MS is collected and weighted up. The final weight loss is less than 7.60 mg, which account for 1.15% of g-C3N<sup>4</sup> loaded on MS skeletons (Figure S3b). The above result demonstrates that g-C3N<sup>4</sup> is firmly distributed on MS even under extreme work condition, which is pivotal to the recycle in practical application.

## Photocatalytic Activity

Benefit from the characteristics of low-density, high porosity and good elasticity, the monolithic g-C3N4/MS can be used to removal gaseous pollutants. Therefore, two different photocatalysis applications including NO removal, dye degradation, and CO<sup>2</sup> photoreduction, are selected to test the photocatalytic activity of the as-prepared monolithic g-C3N4/MS. NO, a typical air contaminants, mostly produced from the combustion of fossil fuels and the emission of vehicle exhaust, can cause a series of atmosphere pollution problems such as acid rain, photochemical smog and haze (Wang et al., 2016). Photo-oxidation technique is considered an alternative to remove NO at low concentration (Zhou et al., 2014). Therefore, the g-C3N4/MS samples are firstly investigated by NO removal at the indoor ppb level. **Figure 2A** shows the effect of g-C3N<sup>4</sup> concentration on NO photo-removal ratio occurring on g-C3N4/MS. Notably, the MS coated with 5 mg mL−<sup>1</sup> suspension achieves the highest removal ratio of 45% within 30 min. As the concentration of g-C3N<sup>4</sup> suspension less than 5 mg mL−<sup>1</sup> , the photocatalytic activity of g-C3N4/MS gradually enhanced with the increased concentration of g-C3N<sup>4</sup> suspension. With a concentration higher than 5 mg mL−<sup>1</sup> , the photocatalytic activity of the g-C3N4/MS slightly decreased, attributing to agglomeration of the excessive g-C3N<sup>4</sup> which not only hinder the NO transport by blocking pore channels in MS, but also cause a reduced light transmittance. Moreover, the thicknesses of MS have also been investigated, which is closely associated with the light utilization. Benefit from the good transparence of MS, the activity of g-C3N4/MS enhanced along with increased MS thickness in **Figure 2B**, but the corresponding best unit mass rate constant (0.868 min−<sup>1</sup> g −1 ) of NO removal is belong to g-C3N4/MS with 0.5 cm thickness (As shown in Figure S4). Based on above results, the optimal concentration and thickness were fixed at 5 mg mL−<sup>1</sup> and 0.5 cm, respectively.

irradiation.

The fabrication and utilization of monolithic photocatalysts with large area on macro-scale is significant to their practical application. Therefore, a monolithic g-C3N4/MS with area of 12.0 × 12.0 cm<sup>2</sup> is prepared for further test of NO removal under visible-light illumination (**Figure 3A**). For comparison, the NO removal on powdery g-C3N<sup>4</sup> and pristine MS are also performed under the same conditions, respectively. Surprisingly, monolithic g-C3N4/MS presents the highest NO removal ratio of 78.6% in initial 5 min which is about 4.5 times higher than that of powdery g-C3N<sup>4</sup> (17.6%), while no NO removal occurs on the pristine MS. After initial 5 min, NO removal ratio of g-C3N4/MS tends to be steady, which could be interpreted as partial active sites replaced by adsorptive NO<sup>−</sup> 3 or NO (Ai et al., 2009; Huang et al., 2010; Liu et al., 2017) and finally reached an adsorption and desorption equilibrium of these oxynitrides. Moreover, comparison with other photocatalysts in our previous work, large area g-C3N4/MS exhibits optimum activity (78.6%), which is approximate 4.25, 2.25 and 2.32 times higher than that of Bi2WO6/graphene (Zhou et al., 2014), C3N4/GOA (GOA) (Wan et al., 2016), and N-Bi2O2CO3/graphene quantum dots (Liu et al., 2017), respectively. Notably, the fraction of generated NO<sup>2</sup> is lower than 5.4% over all our samples (as shown in Figure S5a), indicating those samples selectively oxidize NO to NO<sup>−</sup> 3 rather than NO2. Based on above results, the g-C3N4/MS with large area shows obviously enhanced photocatalytic activity than its powdery counterpart, indicating the monolithic photocatalyst is an effective strategy to solve the problems encountered by powdery photocatalysts in large-scale application.

The stability and recyclability of photocatalysts are also important to their practical applications (Hu et al., 2017). Here, we carried out cycle and long-time tests to further evaluate the performance of the monolithic g-C3N4/MS. The result of cycle test is listed in **Figure 3B**. It can be seen that the NO removal ratio of g-C3N4/MS dropped quickly during the first two cycles. After that, the NO removal ratio approached to stabilization within third cycle and slightly decreased with incremental cycle-index, which attributed to temporary absorption equilibrium of oxynitrides (NO, NO<sup>−</sup> 3 ) in third cycle and their continuous accumulation on g-C3N4/MS. Fortunately, after UV lamp irradiation for 5 min, the adsorbed oxynitrides are desorbed and results in an immediate recovery of sample activity (**Figure 3B**). Furthermore, result of long-time test is displayed in Figure S5b. The g-C3N4/MS can achieve a stable catalytic performance after 1 h and keep a relatively high activity in the 2 h test interval. All above results confirm that the large area g-C3N4/MS has a superior recyclability and stability for NO removal, confirming its potential in practical application.

Apart from the photo-oxidation availability, the monolithic g-C3N4/MS also exhibits good ability of CO<sup>2</sup> photoreduction. CO<sup>2</sup> is the main greenhouse gas that generation from human activity and the combustion of fossil fuels which is responsible for global warming (Norby and Luo, 2004). As displayed in **Figure 3C**, the g-C3N4/MS shows higher photocatalytic activity (7.48 µmol g−<sup>1</sup> h <sup>−</sup><sup>1</sup> CO, 3.93 µmol g−<sup>1</sup> h <sup>−</sup><sup>1</sup> CH<sup>4</sup> and 0.26 µmol g−<sup>1</sup> h <sup>−</sup><sup>1</sup> H2) than that of powdery g-C3N<sup>4</sup> (6.27 µmol g <sup>−</sup><sup>1</sup> h <sup>−</sup><sup>1</sup> CO, 0.52 µmol g−<sup>1</sup> h <sup>−</sup><sup>1</sup> CH<sup>4</sup> and 0 µmol g−<sup>1</sup> h −1 H2). Notably, yield of CO is higher than that of CH<sup>4</sup> on both two samples, because the conversion from CO<sup>2</sup> to CO is 4 electron process, whereas that from CO<sup>2</sup> to CH<sup>4</sup> is 8-electrons process, obviously, the former is easier than the latter which can account for higher yield of CO. No H<sup>2</sup> generated on g-C3N<sup>4</sup> and traced H<sup>2</sup> appeared on g-C3N4/MS indicate that all samples have high selectivity for CO<sup>2</sup> reduction rather than H<sup>2</sup> reduction.

In view of above results of NO removal, and CO<sup>2</sup> photoreduction, the monolithic g-C3N4/MS does show practical potential in various applications and enhanced photocatalytic activity than its powdery counterpart. The large SAA must be responsible for this activity enhancement. However, to get a deep insight into the reasons of the improved performance, more investigations further carried out on the monolithic g-C3N4/MS.

#### Structure and Morphology

The PXRD patterns of MS, g-C3N4/MS, sonicated g-C3N<sup>4</sup> and g-C3N<sup>4</sup> are displayed in **Figure 4**. The peaks at ca. 13.1 and 27.3◦ can be assigned to (100) and (002) crystal planes of g-C3N4, respectively (Gholipour et al., 2016). Obviously, the PXRD patterns of g-C3N<sup>4</sup> are consistent before and after ultrasonic process, confirming the crystal structure of g-C3N<sup>4</sup> is very stable. Moreover, no difference is found in the PXRD patterns of g-C3N4/MS and MS, indicating the g-C3N<sup>4</sup> particles are uniformly dispersed on the porous framework of MS rather than aggregated on its surface.

To reveal the microstructure and morphology of samples, TEM and SEM were conducted as shown in **Figure 5**. The TEM images of g-C3N<sup>4</sup> and sonicated g-C3N<sup>4</sup> show similar thin nano-flake structure with some mesopores (**Figures 5A,B**), which derived from gases releasing such as NH<sup>3</sup> and CO<sup>2</sup> during the pyrolysis of urea (Mao et al., 2013). Combination with PXRD results, it is sure no noticeable change appeared on the structure and morphology of g-C3N<sup>4</sup> before and after the ultrasonic. In **Figure 5C**, the SEM image of MS reveals an interconnected 3D network structure with abundant opencell pores, which not only offer sufficient channels for reactant transport, but also offer enough locations for photocatalyst particle dispersion. It is notable that the g-C3N<sup>4</sup> was successfully coated on the smooth skeleton of MS according to the g-C3N4/MS image in **Figure 5D**. Based on the analysis of structure and morphology, the uniform dispersion of g-C3N<sup>4</sup> achieves in monolithic g-C3N4/MS, confirming the feasibility of our coating strategy.

**Figure 6** shows the FT-IR spectra of g-C3N4, g-C3N4/MS and MS. The spectrum of g-C3N<sup>4</sup> displays typical peaks at 3000-3600 cm−<sup>1</sup> , 1200-1700 cm−<sup>1</sup> , and 811 cm−<sup>1</sup> , which are ascribed to the vibrational absorption of N-H and O-H, CN heterocycles and triazine unit (Kang et al., 2015; Wei et al., 2016; Sun Z. X. et al., 2017). Moreover, in the spectrum of MS, the prominent

peaks located at 808, 1154, 1545, and 3422 cm−<sup>1</sup> , attributing to triazine ring bending, C-O stretching, C=N stretching and N-H stretching, while peaks centered at 988, 1329, and 1466 cm−<sup>1</sup> corresponding to C-H bending vibrations (Pham and Dickerson, 2014; Zhang W. B. et al., 2017). Particularly, a new peak and an intensive peak appeared at 1334 cm−<sup>1</sup> and 813 cm−<sup>1</sup> in g-C3N4/MS spectrum, indicating a weak chemical interaction exists between g-C3N<sup>4</sup> and MS skeletons. The above results reveal both van der waals and chemical interactions between g-C3N<sup>4</sup> and MS, which explain the good firmness of monolithic g-C3N4/MS.

# Band Structure and Photocatalytic Mechanism

The band gap of MS, g-C3N<sup>4</sup> and g-C3N4/MS were determined by the results of UV-vis DRS in **Figure 7A**. Obviously, g-C3N<sup>4</sup> absorption is located in visible region with a calculated band gap of 2.99 eV, while MS shows only UV light absorption with a wide band gap of 4.31 eV. Moreover, the DRS profile of the g-C3N4/MS exhibits a mechanical combination of the absorption features of the g-C3N<sup>4</sup> and MS alone. Importantly, visible-light absorption of g-C3N4/MS gets a slight enhancement (2.79 eV), which may be attributed to the light multistage

refraction and reflection on the MS framework (Dong et al., 2014b). In addition, the g-C3N4/MS not only enhances the light absorption, but also significantly suppresses the recombination of photo-generated carriers according to photoluminescence (PL) spectra in **Figure 7B**, which accounts for the enhancement of photocatalytic activity.

According to the above UV-vis DRS analysis (**Figure 7A**) and the XPS valence band spectrum (Figure S6), the band structure of g-C3N4/MS is proposed in **Figure 8** with VB edge and CB edge located at 2.01 and −0.98 eV, respectively. As shown in **Figure 8**, the potential of VB holes (h+) is slight positive than OH−/OH (1.99 eV), while the potential of CB electron (e−) is much negative than that of O2/O<sup>−</sup> 2 (−0.28 eV). Therefore, the photogenerated holes can directly oxidize OH<sup>−</sup> to OH, and the photogenerated electrons can reduce easily O<sup>2</sup> to O<sup>−</sup> 2 . It is well known that OH and O<sup>−</sup> 2 usually have strong oxidative ability and play the key role in photocatalytic oxidation reaction (Huang et al., 2015b,c, 2017b). Moreover, in consideration of the oxidation potentials of NO2/NO (1.03 eV), HNO2/NO (0.99 eV), HNO3/NO (0.94 eV) (Wan et al., 2016), all OH, O<sup>−</sup> 2 and hole generated on g-C3N4/MS are able to remove NO. In addition, the reduction potentials of E (CO2/CH4), E (CO2/CO) and E (H2O/H2) were located at −0.24, −0.52, and −0.41 eV, respectively (Yu et al., 2014; Han B. et al., 2015). Comparison with CB potential (−0.98 eV), the photogenerated electron is capable of reducing CO<sup>2</sup> on g-C3N4/MS. Based on above analyses and the results in photocatalytic activity part, the possible photocatalytic mechanism are simply illustrated in **Figure 8**. In short, there are three main pathways for NO removal, involving three different active oxidation species (OH, O<sup>−</sup> 2 and h+), while there is one pathway for CO<sup>2</sup> photoreduction, involving only one active species (e−).

# CONCLUSIONS

In summary, a novel monolithic g-C3N4/MS was fabricated by a facile ultrasonic-coating method. This monolithic g-C3N4/MS possesses a uniform dispersion of g-C3N<sup>4</sup> and large SSA, which not only facilitate exposing more active sites of g-C3N<sup>4</sup> but also enhancing the visible-light absorption. Consequently, monolithic g-C3N4/MS shows obviously improved visiblelight photocatalytic activity. The PL detection further reveals that enhanced separation of photogenerated carriers is also responsible for the activity enhancement. More importantly, the characteristics of low-density and high porosity allow the monolithic g-C3N4/MS to be applied in various environmental issues, while the high elasticity, good firmness, and mechanical strength give it noticeable recyclability and stability, confirming its feasibility for practical application. All in all, in this work, we fabricated a monolithic g-C3N4/MS with enhanced photocatalytic activity which can be easy to achieve large-scale production for practical photocatalysis application. Our results provide a low-cost and mild method for mass production of new monolithic photocatalysts.

## AUTHOR CONTRIBUTIONS

YY: Partly designed the experiments and wrote the manuscript; RZ, TR, and WW: Assisted in the analysis and interpretation of the data; YZ and QZ: Proposed the project, designed the experiment, and wrote the manuscript.

## ACKNOWLEDGMENTS

We gratefully acknowledge financial support from the Sichuan Provincial International Cooperation Project (2017HH0030), National Natural Science Foundation of China (21403172) and the Innovative Research Team of Sichuan Province (2016TD0011).

## SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2018.00156/full#supplementary-material

## REFERENCES


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enhanced photocatalytic hydrogen production. Appl. Catal. B Environ. 204, 335–345. doi: 10.1016/j.apcatb.2016.11.047


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Yang, Zhang, Zhang, Ran, Wan and Zhou. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Insights Into Highly Improved Solar-Driven Photocatalytic Oxygen Evolution Over Integrated Ag3PO4/MoS<sup>2</sup> Heterostructures

Xingkai Cui <sup>1</sup> , Xiaofei Yang1,2,3 \*, Xiaozhai Xian<sup>1</sup> , Lin Tian<sup>1</sup> , Hua Tang<sup>1</sup> and Qinqin Liu<sup>1</sup> \*

*<sup>1</sup> School of Materials Science and Engineering, Jiangsu University, Zhenjiang, China, <sup>2</sup> College of Science, Nanjing Forestry University, Nanjing, China, <sup>3</sup> State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, China*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Kangle Lv, South-Central University for Nationalities, China Yanhui Ao, Hohai University, China Shichao Tian, Research Institute of Tsinghua University in Shenzhen, China*

#### \*Correspondence:

*Xiaofei Yang xiaofei\_yang1980@163.com; xiaofei.yang@njfu.edu.cn Qinqin Liu liu\_qin\_qin@126.com*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *13 March 2018* Accepted: *03 April 2018* Published: *18 April 2018*

#### Citation:

*Cui X, Yang X, Xian X, Tian L, Tang H and Liu Q (2018) Insights Into Highly Improved Solar-Driven Photocatalytic Oxygen Evolution Over Integrated Ag3PO4/MoS2 Heterostructures Front. Chem. 6:123. doi: 10.3389/fchem.2018.00123* Oxygen evolution has been considered as the rate-determining step in photocatalytic water splitting due to its sluggish four-electron half-reaction rate, the development of oxygen-evolving photocatalysts with well-defined morphologies and superior interfacial contact is highly important for achieving high-performance solar water splitting. Herein, we report the fabrication of Ag3PO4/MoS<sup>2</sup> nanocomposites and, for the first time, their use in photocatalytic water splitting into oxygen under LED light illumination. Ag3PO<sup>4</sup> nanoparticles were found to be anchored evenly on the surface of MoS<sup>2</sup> nanosheets, confirming an efficient hybridization of two semiconductor materials. A maximum oxygen-generating rate of 201.6 µmol · L −1 · g −1 · h <sup>−</sup><sup>1</sup> was determined when 200 mg MoS<sup>2</sup> nanosheets were incorporated into Ag3PO<sup>4</sup> nanoparticles, which is around 5 times higher than that of bulk Ag3PO4. Obvious enhancements in light-harvesting property, as well as electron-hole separation and charge transportation are revealed by the combination of different characterizations. ESR analysis verified that more active oxygen-containing radicals generate over illuminated Ag3PO4/MoS<sup>2</sup> composite photocatalysts rather than irradiated Ag3PO4. The improvement in oxygen evolution performance of Ag3PO4/MoS<sup>2</sup> composite photocatalysts is ascribed to wide spectra response in the visible-light region, more efficient charge separation, and enhanced oxidation capacity in the valence band (VB). This study provides new insights into the design and development of novel composite photocatalytic materials for solar-to-fuel conversion.

Keywords: Ag3PO4 , MoS2 , composite photocatalyst, oxygen evolution, water splitting, Z-scheme

# INTRODUCTION

Inspired by natural photosynthesis, the construction of visible-light-responsive functional semiconducting materials for highly efficient photocatalytic water splitting and reduction of CO<sup>2</sup> has drawn considerable attention over the past few years (Maeda and Domen, 2010; Mikkelsen et al., 2010; Takanabe, 2017; Zheng et al., 2017). Especially, water splitting into hydrogen and oxygen has been intensively investigated due to the nature of clean and sustainable solar-to-fuel conversion. Compared with the hydrogen evolution reaction (HER), four-electron water oxidation process is more difficult to be fulfilled since a higher potential more than 1.23 eV is required Cui et al. Ag3PO4/MoS2 Composite Photocatalysts

(Kudo and Miseki, 2009), mostly an overpotential is also required. Thus, oxygen evolution is considered as the ratedetermining step in photocatalytic overall water splitting process. So far only few semiconductors such as WO3, BiVO4, have been employed as photocatalysts for oxygen production from water splitting (Xin et al., 2009; Wu et al., 2016; Zeng et al., 2017; He et al., 2018). Due to the limitations of band structures and light-harvesting properties in the visible light region, the utilization of sing-component semiconductors as catalysts for oxygen evolution has encountered serious difficulties, the design and development of novel composite materials for solar-driven photocatalytic water splitting are highly desirable.

Since the pioneer work of silver phosphate (Ag3PO4) semiconductor for photocatalytic applications in 2010 (Yi et al., 2010). Many efforts have been devoted to synthesize Ag3PO<sup>4</sup> photocatalysts with different nanostructures and a variety of Ag3PO4-based composite photocatalytic materials for energy and environmental applications (Bi et al., 2012; Wang et al., 2012; Hu et al., 2013; Cao et al., 2017). Nanostructure engineering of pristine Ag3PO<sup>4</sup> and hybridization of Ag3PO<sup>4</sup> with other semiconductors have been proven to offer superior advantages in light absorption, electronhole separation and charge transportation, resulting in the enhancement in the photocatalytic activity (Yang et al., 2015b; Lv et al., 2016; Wang et al., 2017; Zhou et al., 2018). The key to synthesizing highly efficient Ag3PO4-based composite photocatalysts lies in screening promising candidates with matched band structures and constructing heterojunctions with optimal morphologies and interfaces, where favorable visible light utilization and tandem charge transfer pathway should be taken into consideration. The past decades have witnessed the use of molybdenum disulfide (MoS2), a 2D lamellar material with excellent conductive property, as electrocatalysts and photocatalysts for applications in electrochemical and solar-tofuel conversion (Xiang et al., 2012; Dai et al., 2017; Yang et al., 2017). It was confirmed that the combination of MoS<sup>2</sup> with functional semiconductors enabled obvious enhancements in both photocatalytic and electrocatalytic hydrogen production activity (Sun et al., 2016; Zhang et al., 2016; Iqbal et al., 2017; Yuan et al., 2017). The hybridization of MoS<sup>2</sup> with Ag3PO<sup>4</sup> to produce MoS2/Ag3PO<sup>4</sup> composite materials has been primarily explored, however the application is restricted to the photodegradation of different kinds of organic pollutants (Wang L. et al., 2015; Wang P. F. et al., 2015; Gyawali and Lee, 2016; Li et al., 2016; Zhu et al., 2016; Wan et al., 2017), the employment of MoS2/Ag3PO<sup>4</sup> nanocomposites as photocatalysts for solar-lightdriven oxygen evolution from water splitting has not yet been explored.

Most recently, we reported the in-situ deposition of Ag3PO<sup>4</sup> on graphitic carbon nitride (g-C3N4) nanostructures for highly efficient Z-scheme oxygen evolution from water splitting (Yang et al., 2015a,c; Cui et al., 2018; Tian et al., 2018). In consideration of the matched band structures of bulk Ag3PO<sup>4</sup> and MoS<sup>2</sup> materials for redox reactions, in this work, we demonstrate the hybridization of oxygen-producing photocatalyst Ag3PO<sup>4</sup> with few-layered, two-dimensional MoS<sup>2</sup> nanosheets, and the use of Ag3PO4/MoS<sup>2</sup> nanocomposites for photocatalytic water oxidation under LED illumination. Asprepared hybrid materials exhibit well-organized nanostructures, in which sheet-like MoS<sup>2</sup> materials provide sufficient active sites for the deposition of Ag3PO<sup>4</sup> nanoparticles. It is for the first time that oxygen evolution performance over the obtained Ag3PO4/MoS<sup>2</sup> composite photocatalysts has been evaluated, moreover, the effects of highly conductive MoS<sup>2</sup> materials on visible light utilization, electron-hole separation and water oxidation efficiency are systematically revealed.

# EXPERIMENTAL

#### Preparation

MoS<sup>2</sup> nanosheets were fabricated by the ultrasonic stripping of commercially available MoS<sup>2</sup> materials. In a typical synthesis of Ag3PO4/MoS<sup>2</sup> nanocomposites, different amounts of sheetlike MoS<sup>2</sup> (50, 100, 200, 300 mg) were added into 90 mL H2O, respectively, followed by the ultrasonic treatment for 3 h. Next, 30 mL of AgNO<sup>3</sup> (18 mmol, 3.06 g) aqueous solution was added dropwise into the MoS<sup>2</sup> suspension, and stirred for further 12 h. And then 30 mL of Na3PO<sup>4</sup> (6 mmol, 2.28 g) aqueous solution was added slowly into the above mixture, followed by continuous stirring for 3 h. After high-speed centrifugation, solid products were washed with deionized water and ethanol repeatedly, and dried at 60◦C for 12 h. The final products were collected and are denoted as AM-50, AM-100, AM-200, and AM-300.

#### Characterizations

X-ray diffraction (XRD) was measured using Cu Kα radiation with the 2θ range from 5 to 80◦ at a scan rate of 5◦ min−<sup>1</sup> on D/MAX2500PC. Raman spectra were recorded by Thermo ScientificTM DXR spectrometer operating at 532 nm. X-ray photoelectron spectroscopy (XPS) was evaluated by Perkin-Elmer PHI 5000C. The field-emission scanning electron microscopy (FE-SEM) was performed on JSM-7001F. The Ultraviolet-visible diffuse reflectance spectrophotometer (UV-vis DRS) on UV2450 from 200 to 800 nm with BaSO<sup>4</sup> as reference standard. Photoluminescence (PL) emission measurements were carried out by a QuantaMasterTM 40 with an excitation wavelength of 420 nm. The electron spin resonance (ESR) measurements were recorded on a JES FA200 Spectrometer using the 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) as the radical capture reagent.

## Photocatalytic Measurements

The efficiency of photocatalytic oxygen evolution was monitored by in-situ oxygen sensor in a sealed system connected with a circulation system. Before the measurement, oxygen-free and air-saturated water were used to calibrate the oxygen probe with temperature compensation. For the measurement of oxygen evolution, 0.3 g of the photocatalyst powder was added into AgNO<sup>3</sup> aqueous solution (100 mL, 10 g/L), followed by an ultrasonic treatment for 5 min.

# RESULTS AND DISCUSSION

Photocatalytic oxygen evolution from water splitting over pure Ag3PO<sup>4</sup> and Ag3PO4/MoS<sup>2</sup> composites with different mass ratios were evaluated, and the results are presented in **Figure 1**. It can be observed (**Figure 1A**) that the amount of evolved oxygen increases gradually when 50 and 100 mg MoS<sup>2</sup> were employed, the highest concentration of produced oxygen was recorded when the content of MoS<sup>2</sup> was increased to 200 mg. Further increase in the MoS<sup>2</sup> content from 200 to 300 mg in the Ag3PO4/MoS<sup>2</sup> composite resulted in the deterioration in the oxygen-generating performance. Notably all the composites showed improved oxygen-evolving performance than bulk Ag3PO<sup>4</sup> material. The oxygen-evolving rates of as-prepared samples under LED illumination are further quantified and shown in **Figure 1B**. When 200 mg MoS<sup>2</sup> was introduced to hybridize with Ag3PO4, an optimal oxygen-generating rate of 201.6 µmol · L −1 · g −1 · h <sup>−</sup><sup>1</sup> was determined, which is about 4.5 times higher than that of pure Ag3PO4. It can be concluded from the oxygen evolution performance that a proper addition of MoS<sup>2</sup> may promote the water oxidation efficiency, while the use of more than 200 mg of MoS<sup>2</sup> is found to show negative effects on the oxygen evolution performance. For simplicity, only the composite AM-200 with the best oxygen-producing efficiency is chosen for comparisons with two starting materials in the following sections.

Following the evaluation of photocatalytic oxygen evolution performance, phase structures of the AM-200 composite were confirmed by XRD patterns (**Figure 2A**). Diffraction peaks (black line) appearing in 14.32, 32.62, 33.44, 35,82, 39.48, 44,1, 49.72, 58.24, 60.32, and 72.72◦ can be assigned to the (002), (100), (101), (102), (103), (006), (105), (110), (008), and (203) planes of hexagonal MoS<sup>2</sup> (JCPDS No. 37-1492). And the characteristic peaks (blue line) located at 20.96, 29.78, 33.38, 36.66, 47.86, 52.76, 55.1, 57.34, 61.72, and 73.94◦ correspond to planes (110), (200), (210), (211), (310), (222), (320), (321), (400), and (332) of bulk Ag3PO<sup>4</sup> (JCPDS No. 06-0505), respectively. No obvious difference can be observed in characteristic diffraction peaks of bulk Ag3PO<sup>4</sup> and Ag3PO4/MoS<sup>2</sup> composite (AM-200), presumably due to a relatively weaker diffraction intensity of MoS2. The molecular structures of pure MoS2, bulk Ag3PO<sup>4</sup> and the Ag3PO4/MoS<sup>2</sup> composite AM-200 were further characterized

FIGURE 3 | Low-magnification (A,C,E) and high-magnification (B,D,F) SEM images of pure Ag3PO4 (A,B), MoS2 (C,D) as well as the composite AM-200 (E,F); EDS element mapping images of AM-200 (G).

by Raman spectra (**Figure 2B**). In the spectrum of Ag3PO4, a sharp absorption peak at 908.5 cm−<sup>1</sup> can be attributed to the motion of terminal oxygen bond vibration in phosphate chains. The peak at 1002.4 cm−<sup>1</sup> is ascribed to the asymmetric stretching vibrations of O–P–O bonds in [PO4] clusters. The broad peak located at 554.4 cm−<sup>1</sup> arises from the asymmetric stretch of P–O–P bonds, while the peak centered at 406.1 cm−<sup>1</sup> corresponds to the symmetric bending vibration modes related to [PO4] clusters (Botelho et al., 2015). For pure MoS2, characteristic Raman shifts located at 375.8 and 402.1 cm−<sup>1</sup> are assigned to the E2g and A1g modes, while the peak appearing at 446.2 cm−<sup>1</sup> is suggested to come from the interaction of the longitudinal acoustic phonon and Raman inactive A2u modes (Koroteev et al., 2011; Lukowski et al., 2013). All characteristic peaks of both Ag3PO<sup>4</sup> and MoS<sup>2</sup> were detected in Raman spectrum of the Ag3PO4/MoS<sup>2</sup> composite AM-200, indicating a complete hybridization of Ag3PO<sup>4</sup> with MoS2.

The morphologies of as-prepared samples were recorded by SEM images (**Figure 3**). From **Figures 3A,B**, as-synthesized Ag3PO<sup>4</sup> products present an irregular polyhedron structure with the size of about 1–3µm, and a few of small particles are distributed around large particles. SEM images of ultrasonictreated MoS<sup>2</sup> material (**Figures 3C,D**) shows that stripped MoS<sup>2</sup> exhibit a thin sheet-like nanostructure. The stripped MoS<sup>2</sup> layer-like material has a certain accumulation and parts of MoS<sup>2</sup> materials have not been stripped into pieces. The Ag3PO4/MoS<sup>2</sup> composite AM-200 was synthesized by the in-situ deposition of Ag3PO<sup>4</sup> nanoparticles on the surface of MoS<sup>2</sup> nanosheets via electrostatically driven self-assembly. As displayed in **Figures 3E,F**, the Ag3PO<sup>4</sup> particles are uniformly distributed on the MoS<sup>2</sup> nanosheets without obvious agglomeration. It can

be observed that the particle size of Ag3PO<sup>4</sup> in AM-200 decreases slightly and is more uniform when a certain amount of MoS<sup>2</sup> were hybridized with Ag3PO4, suggesting that the addition of MoS<sup>2</sup> nanosheets have an effect on the particle size of Ag3PO4. The EDS element mapping images of AM-200 suggest that Mo, S, Ag, P, and O elements are homogeneously distributed, confirming the complete hybridization of Ag3PO<sup>4</sup> particles and MoS<sup>2</sup> nanosheets.

The surface chemical compositions and states of the Ag3PO4/MoS<sup>2</sup> composite AM-200 were investigated by XPS characterization, the results are shown in **Figure 4**. Ag, P, O, Mo, S, and C elements can be detected in the survey spectrum (**Figure 4A**) of as-prepared composite AM-200. The existence of C 1s peak is may due to the adventitious carbon on the surface of sample. In the high resolution spectrum of Ag 3d (**Figure 4B**), two peaks at 368.1 and 374.2 eV can be assigned to the Ag 3d5/<sup>2</sup> and Ag 3d3/2, respectively. The broad peak in the P 2p spectrum (**Figure 4C**) located at 133.0 eV originates from the P5<sup>+</sup> in the Ag3PO4. The high resolution spectrum of Mo 3d is displayed in **Figure 4D**, the peaks at 226.7, 229.7, 232.9, and 236.2 eV can be ascribed to the S 2s, Mo 3d5/2, Mo 3d3/2, and Mo-O binding, respectively. Particularly, the first three binding energies indicated that S and Mo elements in the MoS<sup>2</sup> are found in the form of S2<sup>−</sup> and Mo4+, respectively, and the last one might result from the exposed Mo atoms during the exfoliation process combining with the O of Ag3PO<sup>4</sup> (Wan et al., 2017). The S 2p XPS spectrum (**Figure 4E**) can be divided into two peaks centered at 162.5 and 163.6 eV, respectively, corresponding to the S 2p3/<sup>2</sup> and S 2p1/<sup>2</sup> in the MoS2. The spectrum of O 1s (**Figure 4F**) can be fitted into two peaks located at 530.8 and 532.5 eV, originating from the O2<sup>−</sup> in the Ag3PO<sup>4</sup> and the hydroxyl group, respectively.

It is well-known that the utilization of visible light is one of key factors affecting the activity of a photocatalyst, therefore, the light-harvesting properties of all samples were measured by UVvis DRS ranging from 200 to 800 nm and the absorption spectra are presented in **Figure 5A**. It can be seen that pure Ag3PO<sup>4</sup> has a clear absorption edge around 530 nm, and black MoS<sup>2</sup> material reveals full-spectrum absorption in the range of 200–800 nm. The light absorption intensity of Ag3PO4/MoS<sup>2</sup> composite AM-200 in the wavelength range of 500–800 nm was increased when a certain amount of MoS<sup>2</sup> (200 mg) were employed to hybridize with Ag3PO4, implying that the integration of MoS<sup>2</sup> with Ag3PO<sup>4</sup> favors a more efficient utilization of visible light. In addition to the light absorption, the separation of photoinduced electron-hole pairs is also believed to play a predominant role in determining the photocatalytic activity, thus the recombination of photogenerated charge carrier for the as-synthesized samples was analyzed by PL spectroscopy measurements. **Figure 5B** reveals Ag3PO<sup>4</sup> has a strong excitation peak around 630 nm, stemming from the recombination of electrons and holes. After the addition of MoS2, the Ag3PO4/MoS<sup>2</sup> composite AM-200 presented a similar position of excitation peak with Ag3PO4, and the PL emission intensity of AM-200 was weaker than those of pure MoS<sup>2</sup> and Ag3PO4, suggesting that the recombination efficiency of photoexcited charge carriers in the Ag3PO4/MoS<sup>2</sup> composite AM-200 has been effectively suppressed when the heterostructured composite was formed. A slower recombination rate of photogenerated electron-hole pairs boosts the enhancement in the photocatalytic performance.

To further determine the influence of the redox capacity of samples on the photocatalytic activity, as well as to investigate the mechanism behind the enhanced photocatalytic oxygen evolution from water splitting, the ESR measurement was carried out to confirm active radicals in-situ formed under light illumination. It can be observed in **Figure 6A** that no obvious peak was detected in dark. Under illumination, several strong peaks arising from DMPO-captured radicals can be detected in methanol dispersion for both pure Ag3PO<sup>4</sup> and the Ag3PO4/MoS<sup>2</sup> composite AM-200, typical peaks are assigned to the spin adducts (DMPO-O•− 2 ). Compared with signals derived from pure Ag3PO4, higher signal intensities of DMPO-captured superoxide radicals were observed in the methanol dispersion of AM-200. It is shown in **Figure 6B** that no radical signal was detected in dark. A typical intensity ratio of 1:2:2:1 was determined from the spin adducts in aqueous dispersions of both Ag3PO<sup>4</sup> and AM-200 under light irradiation, representing the generation of the spin adducts (DMPO-•OH). Similarly, the intensity of radical signal for AM-200 increased largely comparable to that of Ag3PO4.On the basis of the above results, it is concluded that higher intensities of both photo-induced O•− 2 and •OH were recorded when a proper amount of MoS<sup>2</sup> was employed.

Furthermore, the band edge positions of valence band (VB) and conduction band (CB) also have an important effect on the redox catalytic capability, which can be deduced by the following formula (Li et al., 2016):

$$\begin{aligned} \mathrm{E\_{VB}} &= \chi - \mathrm{E\_e} + 0.5\mathrm{E\_g} \\ \mathrm{E\_{CB}} &= \mathrm{E\_{VB}} - \mathrm{E\_g} \end{aligned}$$

Where ECB and EVB represent the CB and VB edge potentials, respectively; χ is the electro-negativity of the semiconductor, which is the geometric mean of the electro-negativities of the constituent atoms, and χ-values for Ag3PO<sup>4</sup> and MoS<sup>2</sup> are 5.96 and 5.32 eV (Wan et al., 2017), respectively. E<sup>e</sup> is about 4.5 eV, representing the free electron energy on the hydrogen scale. E<sup>g</sup> was the band gap energy of the semiconductor, and Egvalues for Ag3PO<sup>4</sup> and MoS<sup>2</sup> are about 2.45 and 1.9 eV (Yang et al., 2015c; Li et al., 2016), respectively. According to the calculation, the EVB-values of Ag3PO<sup>4</sup> and MoS<sup>2</sup> are about 2.69 and 1.77 eV, the top of which for both Ag3PO<sup>4</sup> and MoS<sup>2</sup> is more positive than the redox potential of O2/H2O (1.23 eV) (Xie et al., 2013), theoretically both two semiconductors are able to split water into oxygen. However, overpotential is generally required for practical water splitting, in this study, Ag3PO<sup>4</sup> acts as the oxygen-evolving catalyst for solar-driven water splitting due to its more positive potential higher than that for water oxidation. Subsequently ECB positions of Ag3PO<sup>4</sup> and MoS<sup>2</sup> are determined to be 0.24 and −0.13 eV, respectively. Therefore, the enhanced photocatalytic oxygen-generating performance over the Ag3PO4/MoS<sup>2</sup> composite photocatalyst could be explained as follows: first, the electrons in the VB of Ag3PO<sup>4</sup> could be initially excited into CB, and subsequently, may recombine with the holes in the VB of MoS<sup>2</sup> via a possible Z-scheme configuration. Thus, more efficient electron-hole separations and charge transporatation occur in the illuminated hybrid materials due to the existence of highly conductive MoS<sup>2</sup> sheets and possible Z-scheme pathway for electron transfer. The electronhole recombination on the surface of Ag3PO<sup>4</sup> can be suppressed, as a result, active holes left on the VB position of Ag3PO<sup>4</sup>

#### REFERENCES


may oxidize water into oxygen effectively, leading to highly efficient oxygen evolution performance over the Ag3PO4/MoS<sup>2</sup> composite photocatalysts.

#### CONCLUSIONS

In conclusion, effective Ag3PO4/MoS<sup>2</sup> composite photocatalysts were successfully fabricated by combining ion-exchange process and electrostatic assembly of Ag3PO<sup>4</sup> nanoparticles on the surface of MoS<sup>2</sup> nanosheets. The Ag3PO4/MoS<sup>2</sup> hybrid materials demonstrated superior interfacial contact and wide-spectrum light-harvesting property in the visible light region. When employed as the catalyst for photocatalytic water splitting, it exhibited highly improved oxygen evolution performance than bulk Ag3PO<sup>4</sup> under LED irradiation. The oxygen-evolving rate of the optimal Ag3PO4/MoS<sup>2</sup> composite (AM-200) is nearly five times faster than pure Ag3PO4. The combined characterizations and theoretical analysis on band structures suggest that the enhanced water oxidation efficiency is attributed to remarkable response to visible light, more efficient charge transportation and possibly specific Z-scheme pathway derived from matched band positions. The finding in this work offers a great opportunity in designing and synthesizing novel composite photocatalytic materials for applications in solar energy conversion, allowing us to develop an understanding of the fundamental mechanisms of Ag3PO4-based composite photocatalytic materials.

#### AUTHOR CONTRIBUTIONS

XY and QL: designed the project, guided the study, and polished the manuscript; XC, XX, and LT: conducted the experiments and characterized the samples; HT: revised the manuscript.

#### ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (51672113), Six Talent Peaks Project in Jiangsu Province (2015-XCL-026), Natural Science Foundation of Jiangsu Province (BK20171299), State Key Laboratory of Photocatalysis on Energy and Environment (SKLPEE-KF201705), Fuzhou University, Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJZZ16\_0192), and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (2016-KF-10), Wuhan University of Technology.


photoreduction activity under visible light irradiation. Appl. Surf. Sci. 403, 230–239. doi: 10.1016/j.apsusc.2017.01.171


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Cui, Yang, Xian, Tian, Tang and Liu. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Mechanisms for ·O<sup>−</sup> 2 and ·OH Production on Flowerlike BiVO<sup>4</sup> Photocatalysis Based on Electron Spin Resonance

Xuan Xu1,2, Yaofang Sun1,2, Zihong Fan<sup>3</sup> , Deqiang Zhao1,2, Shimin Xiong1,2 , Bingyao Zhang1,2, Shiyu Zhou1,2 and Guotao Liu1,2 \*

<sup>1</sup> Key Laboratory of Three Gorges Reservoir Region's Eco-Environment, Ministry of Education, Chongqing University, Chongqing, China, <sup>2</sup> National Centre for International Research of Low-Carbon and Green Buildings, Chongqing University, Chongqing, China, <sup>3</sup> College of Environmental and Resources, Chongqing Technology and Business University, Chongqing, China

#### Edited by:

Fan Dong, Chongqing Technology and Business University, China

#### Reviewed by:

Fatwa Abdi, Helmholtz-Zentrum Berlin für Materialien und Energie, Germany Alessandro Di Mauro, Istituto per la Microelettronica e Microsistemi (CNR), Italy Liqun Ye, Nanyang Normal University, China

> \*Correspondence: Guotao Liu liu-guotao@163.com

#### Specialty section:

This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry

Received: 19 October 2017 Accepted: 27 February 2018 Published: 26 March 2018

#### Citation:

Xu X, Sun Y, Fan Z, Zhao D, Xiong S, Zhang B, Zhou S and Liu G (2018) Mechanisms for ·O − 2 and ·OH Production on Flowerlike BiVO4 Photocatalysis Based on Electron Spin Resonance. Front. Chem. 6:64. doi: 10.3389/fchem.2018.00064 Many studies have focused on the use of BiVO<sup>4</sup> as a photocatalyst, but few have investigated the production of free radicals during the photocatalytic process. Following synthesis of flowerlike BiVO<sup>4</sup> and characterization by X-ray diffraction (XRD), Raman spectroscopy, Scanning electron microscopy (SEM) Scanning electron microscopy (EDX), UV-Vis and XPS, we successfully prepared BiVO4. Then we used electron spin resonance (ESR) to determine the production and degradation of individual active free radicals, including the superoxide radical (·O − 2 ) and the hydroxyl radical (·OH). In the first experiment, we used ESR to detect the signals of free radicals (·O − 2 and ·OH) under varying oxygen conditions. The results shown that in addition to production by ·O − 2 , ·OH could also be produced by oxidation of h<sup>+</sup> to OH−. In the next experiment, we detected ·OH under varying pH to identify the result of the first experiment, and found that signal intensities increased with increasing pH, indicating the mechanism for ·OH production. Finally, we conducted a trapping experiment to examine free radical degradation mechanisms. We identified ·OH and h<sup>+</sup> as the main active free radicals and showed the complete production about ·OH. These results improve current knowledge of free radical production mechanisms, which can be used to enhance the photocatalytic performance of BiVO4.

Keywords: electron spin resonance (ESR), degradation mechanism, bismuth vanadate (BiVO4 ), superoxide radical (·O<sup>−</sup> 2 ), hydroxyl radical (·OH)

# INTRODUCTION

The development of modern society and global economy has led to a series of environmental problems. The public's awareness of environmental issues has gradually increased with the growing realization that the environment requires continuous protection in order to sustain life (Zhang et al., 2010). Related to this area of thought is the utilization of solar energy. The development of photocatalytic semiconductors for organic pollutant degradation in wastewater has become a challenging research topic (Zhang et al., 2015). Many studies are now focusing on the photocatalytic decomposition of wastewater and degradation of organic pollutants under visible light irradiation

(Long et al., 2006). Since the discovery of photo-induced decomposition of water by TiO<sup>2</sup> electrodes (Inoue et al., 1979), semiconductor-based photocatalysis has attracted extensive interest and the properties of several photocatalytic materials, such as TiO<sup>2</sup> and ZnO, have been investigated. The development of visible-light-driven photocatalysts involves two strategies (Long et al., 2006): modification of TiO<sup>2</sup> and exploitation of novel semiconductor materials. Many studies have reported that TiO<sup>2</sup> is a promising material for Surface Enhanced Raman Scattering (SERS) owning to its high refractive index, versatility in terms of surface functionalization, and synergistic coupling with plasmonic nanoparticles (Xu et al., 2017). In addition, TiO<sup>2</sup> has many advantages that are beneficial to photocatalytic reactions, such as extraordinary chemical stability, low cost, and small environmental impact (Inoue et al., 1979). However, it also has some limitations, such as a low quantum efficiency and wide band-gap (3.2 eV); therefore, TiO2-based photocatalytic reactions can only occur under ultraviolet irradiation (Zhang et al., 1998; Ren et al., 2012; Wang et al., 2014). Bismuth vanadate (BiVO4), a promising photocatalyst, has received considerable attention because of its high absorption of visible light (VL), and VL accounts for over 50% of total sunlight (Xu et al., 2016). Due to its non-toxicity, environmentally friendly BiVO4-based paints have recently replaced paints containing toxic pigments, such as lead chromate (PbCrO4) and cadmium sulfide (CdS) (Nalbandian et al., 2015). As a new VL-active photocatalyst, BiVO<sup>4</sup> occurs in three main crystal forms: monoclinic scheelite, tetragonal zircon, and tetragonal scheelite structures. Monoclinic scheelite and tetragonal scheelite possess similar scheelite crystalline structures (Shan et al., 2014). Compared with the other two phases, the monoclinic scheelite phase of BiVO<sup>4</sup> exhibits much higher photocatalytic activity under visible light, as illustrated by extensive photocatalytic research (Xu et al., 2016). With the rise of BiVO<sup>4</sup> applications, various methods for the synthesis of BiVO<sup>4</sup> photocatalysts have been proposed (Zhao et al., 2016). In addition, many researchers have focused on modifying BiVO<sup>4</sup> to inhibit the recombination of electron carriers and improve its photocatalytic absorption efficiency (Kohtani et al., 2005; Xu et al., 2008; Wang et al., 2015; Zhang et al., 2016; Zhao et al., 2016).

Though many studies have focused on the modification of BiVO<sup>4</sup> and its photocatalytic properties, there are few published investigations on its photocatalytic mechanism. According to published literatures (Hashimoto et al., 2005; Lam et al., 2014), the main active free radicals that take part in the photocatalytic system include the hydroxyl radical (·OH), the superoxide radical (·O − 2 ), and the hole (h+). Among these active free radicals, ·OH is mainly generated by oxidation of h<sup>+</sup> to water or via a series of reactions involving ·O − 2 . The main activate free radicals that affecting the degradation of photocatalytic materials in different photocatalytic systems vary, although they typically include one of the three above-mentioned radicals. Previous research has focused on the direct effects that photocatalytic materials exert on pollutant degradation, and reaction mechanisms typically are based on trapping experiments (Xu et al., 2016). However, there is limited research on identification of the main active free radicals using various methods, and the mechanisms of ·OH and ·O − 2 generation are unclear (Ge et al., 2012b). Some studies on BiVO<sup>4</sup> have reported the reduction of oxygen by electrons as a mechanism for the generation of ·O − 2 , however no experiments have been conducted to confirm this conclusion. Furthermore, some studies have reported conflicting conclusions on free radical reaction mechanisms, indicating that ·OH can be generated by ·O − 2 but not by h<sup>+</sup> (Cheng et al., 2012). Thus, it is unclear whether ·OH can be generated by h+, and further research is necessary to clarify the mechanisms of free radical reactions.

Spin trapping in conjunction with electron spin resonance (ESR) has been widely used for the indirect detection of shortlived radicals (Ming et al., 2014). Spin trapping compounds are used to convert short-lived radicals into relatively longer-lived radical products, spin adducts, which can be easily detected by ESR (Castellanos et al., 2001; Kubo et al., 2012). This is a very intuitive method to detect the presence of unpaired electrons, such as free radicals, transition metals, electron holes, etc.

The aim of this study was to clarify the recent controversy regarding the formation mechanism of ·OH, particularly whether it can be generated by ·O − 2 or h+. In addition to the synthesis and characterization of BiVO4, we investigated its photocatalytic degradation mechanism with regard to the formation of ·OH. We used ESR to determine individual active free radicals, including ·O − 2 and ·OH. According to the results, oxygen plays multiple roles in this mechanism and primarily determines the pathway by which these two active free radicals are generated. We further investigated the main radicals participating in the photocatalytic process by changing the pH of the reaction system. Finally, a trapping experiment was conducted and the photocatalytic mechanism of BiVO<sup>4</sup> was discussed in detail.

## EXPERIMENTAL METHODS

#### Reagents, Preparation, and Characterization of Materials Reagents

Pure chemicals of analytical grade were used without further purification. Sodium hydroxide (NaOH), ammonium metavanadate (NH4VO3), and silver nitrate (AgNO3) were purchased from the Chongqing Chuandong Chemical Company. Bismuth nitrate (Bi (NO3)3·5H2O) and nitric acid (HNO3) were obtained from the Chengdu Area of the Industrial Development Zone (Xinde Mulan). Dimethyl pyridine N-oxide (DMPO), absolute ethyl alcohol (C2H5OH), and isopropanol (IPA) were acquired from the Industrial Development Zone, Mulan Town, Xindu District of Chengdu. Sodium oxalate (Na2C2O4), commercial Titania (P25) and L- cysteine were bought from the Sinopharm Chemical Reagent Co, Ltd, and p-benzoquin-one (PBQ) was obtained from the Chengdu Kelong Chemical Reagent Factory.

#### Synthesis of Flowerlike BiVO<sup>4</sup>

To synthesize flowerlike BiVO4, 0.072 g L-cysteine was dissolved in 4 ml of 4 mol/l HNO<sup>3</sup> and the mixture was stirred for 30 min, then 2 mmol of Bi (NO3)3·5H2O was dissolved in the mixture and was stirred for 30 min, resulting in solution A. Next, 2 mmol NH4VO<sup>3</sup> was dissolved in 4 ml of 2 mol·L <sup>−</sup><sup>1</sup> NaOH, resulting in solution B. Solutions A and B were mixed together and 50 ml distilled water was added in it, while regulating the pH (2.5), and the mixture was ultrasonically dispersed for 30 min. The mixture was then transferred to a reaction vessel and allowed to react at 180◦C for 16 h. Finally, the system was naturally cooled to room temperature and washed with absolute ethanol followed by distilled water about six times. The final product was dried under a vacuum overnight at 60◦C for 6 h.

#### Characterization

The crystalline structures of all samples were characterized by Xray diffraction (XRD) using a Rigaku D/Max-rB diffractometer with Cu Ka radiation. Scanning electron microscopy (SEM) images were acquired with a Zeiss AURIGA field emission microscope (electron high tension (EHT) = 5 kV, work distance (WD) = 8.8 nm; Zeiss, Oberkochen, Germany). The surface chemical environment was analyzed by XPS on a PHI5000 VersaProbe system with monochromatic Al Kα X-rays. Energy dispersive X-ray (EDX) images were acquired with an EDX-100A-4. UV-Vis DRS was performed with a Hitachi U-3010 UV-Vis spectrometer. ESR analyses were performed using a JES FA200 X-band ESR spectrometer operating in the X-band at 0.907 GHz and 0.998 mW.

#### Photocatalytic Degradation Experiment

The photocatalytic activity of BiVO<sup>4</sup> was assessed by evaluating the degree of photodegradation of Rhodamine B (RhB) solution under visible light at room temperature. In each experiment, 0.10 g of catalyst was added to an aqueous RhB solution (300 ml, 5 mg·L −1 ), and the mixture was magnetically stirred for 30 min in the dark to achieve high dispersion and adsorption–desorption equilibrium between the dye and catalyst. Then, the solution was placed in a 500 mL beaker and positioned 350 mm away from the visible light source. Samples were collected after every 2 h of irradiation, and then centrifuged at 12000 rpm to remove the catalyst powder. The concentration of the remaining dye was monitored by measuring the absorbance of the solution at 552 nm. For comparison, control experiments were performed using P25 as a catalyst under the same conditions. At the same time, we did the repeated test.

#### Mechanisms for ·O<sup>−</sup> 2 and ·OH Production ESR Experiment

The ESR experiment was primarily focused on the analysis of ·OH and ·O − 2 . In the case of ·OH, the whole test was performed in aqueous solution. First, 5 mg of BiVO<sup>4</sup> was weighed and dispersed in 5 mL water via ultrasound for 20 min; then, argon was bubbled through the solution for 20 min to strip oxygen from the solution; DMPO (100 mM) solution (also bubbled with argon to remove oxygen) was used for hybrid acquisition under visible light. Simultaneously, the same experiment was performed in the presence of oxygen as a control.

There were some differences in the O<sup>−</sup> 2 experiment. First, 5 mg of BiVO<sup>4</sup> was weighed and dispersed in methanol along with a capturing agent, DMPO (100 mM). The solution was then deoxygenated with argon as before, and the test was conducted under visible light. Again, the same experiment was conducted under aerobic conditions as a control. All tests were conducted under neutral conditions (pH = 7).

We also analyzed the DMPO spin-trapping ESR spectra of BiVO<sup>4</sup> in an aqueous dispersion for DMPO–·OH (1:2:2:1) (Zhou et al., 2012) and in a methanol dispersion for DMPO– ·O − 2 . The characteristic peaks of DMPO–·OH and DMPO–·O − 2 respectively indicated the production of ·OH and ·O − 2 species in the samples.

To identify whether h<sup>+</sup> can generate ·OH, we conducted the above ·OH experiment under varying oxygen levels, and then under varying pH (pH = 5, 9, 7, 11).

#### Trapping Experiment

The method of trapping experiment is the same to photocatalytic degradation experiment, what different is the addition of scavengers. In these experiments, we used PBQ (0.1 mmol), IPA (0.1 mol·L −1 ), AgNO<sup>3</sup> (0.1 mmol), and Na2C2O<sup>4</sup> (0.1 mmol) as ·O − 2 , ·OH, electron, and h<sup>+</sup> scavengers, respectively (Chen et al., 2013). In each experiment, 0.10 g of catalyst was added to an aqueous RhB solution (300 ml, 5 mg·L −1 ), and the mixture was magnetically stirred for 30 min in the dark to achieve high dispersion and adsorption–desorption equilibrium between the dye and catalyst. Then, the solution was placed in a 500 mL beaker and positioned 350 mm away from the visible light source. Samples were collected after every 2 h of irradiation, and then centrifuged at 12,000 rpm to remove the catalyst powder. The concentration of the remaining dye was monitored by measuring the absorbance of the solution at 552 nm. For comparison, control experiments were performed using BiVO<sup>4</sup> as a catalyst, or without any catalyst, under the same conditions.

## RESULTS AND DISCUSSION

#### Sample Composition and Morphology

X-ray diffraction enables the qualitative analysis and phase identification of a compound. Additionally this analytical technique facilitates determination of crystalline structures (regardless of symmetry measurements, etc.). From the XRD spectrum, the crystalline structure, lattice parameters, and defects (dislocations, etc.) can be determined (Zhang et al., 2006). As seen in **Figure 1A**, the diffraction peaks of the sample agree well with those for the standard monoclinic BiVO<sup>4</sup> (JCPDS No. 83–1697), indicating that the material consists of a single phase. Other authors have obtained similar XRD patterns of BiVO<sup>4</sup> (Zhang et al., 2010; Jiang et al., 2012), suggesting that highly monoclinic scheelite BiVO<sup>4</sup> was obtained.

Raman spectroscopy is commonly used to determine the structure of prepared BiVO<sup>4</sup> together with XRD, and we further examined the prepared BiVO<sup>4</sup> structure with this technique. Based on band component analysis of the Raman and IR spectra (Hardcastle et al., 1991; Gotic et al., ´ 2005; Zhang and Zhang, 2009), we identified Raman bands at 146, 210, 324, 698, and 853 cm−<sup>1</sup> (**Figure 1B**). The Raman bands of the prepared BiVO<sup>4</sup> are quite distinctive and sharp. The band with the highest intensity (853 cm−<sup>1</sup> ) was assigned to ν<sup>s</sup> (V–O) (Zhang and Zhang, 2009), and the weak shoulder at 698 cm−<sup>1</sup>

was assigned to νas (V–O) (Ye et al., 2012). Medium intensity bands were observed at 324 and 146 cm−<sup>1</sup> . External vibrational modes (rotation/translation) are visible at 210 cm−<sup>1</sup> (Jiang et al., 2012). These values correspond to the typical vibrations of monoclinic BiVO4, although there are minor differences in some parts of the spectrum due to the fact that Raman band positions are sensitive to short-range order, whereas band widths are more sensitive to the degree of crystallinity, defects, disorders, particle size, and/or aggregation of particles (Gotic´ et al., 2005).

The sizes and morphologies of the prepared samples were examined by SEM (**Figures 2a,b**). The shape of the material is similar to that of the flower, and each of the slices about the sample was about 2–5µm. Based on the EDX results shown in **Figures 2c,d**, it can be concluded that the components of the material mainly include V, Bi, O, and Si (Si signals come from the conductive adhesives used in the EDX measurement). These results show the exact composition of the material and are in good agreement with the XPS results (**Figure 3**).

XPS was used to examine the chemical states of elements in the photocatalytic material. **Figure 3A** shows the full spectra of BiVO<sup>4</sup> in the range of 0–800 eV. The survey spectrum shows that the composite contains Bi, O, V, and C. As is reported in some papers (Long et al., 2006; Xu et al., 2016), the form of the Bi is Bi3<sup>+</sup> in the compound BiVO<sup>4</sup> and the binding energies for Bi 4f7/2 and Bi4f5/2 are 158.8 and 163.4 eV, respectively (**Figure 3B**). **Figure 3C** shows the binding energy of C. The peaks at binding energies of 516.4 eV (V2p3/2), 522.0 eV (V2p1/2), and 524.2 eV (V2p1/2) are visible in **Figure 3D**. The peaks at binding energying of 516.4 eV (V2p3/2) and 524.2 eV (V2p1/2) indicate the V species in the composite is V5<sup>+</sup> (Long et al., 2006; Xu et al., 2016; Zhao et al., 2016). And the peak at binding energying of 522.0 eV also represent the element of V (V2p1/2), which means there are some impurities when preparation (Zhao et al., 2016). The three peaks (**Figure 3D**) all represent V2p, and you can find the same peaks of V in other paper (Jia et al., 2012; Ota et al., 2014). Finally, **Figure 3E** shows the binding energy of O. Together, these data indicate that the tested sample consists of BiVO<sup>4</sup> microspheres as the same as other paper although there contains some impurities when preparation (Zhou et al., 2011; Xu et al., 2016).

## Optical Properties and Degradation Properties

Optical absorption properties are key characteristics for a catalyst's photocatalytic activity (Hardcastle et al., 1991). The absorption of visible light by a photocatalyst is mainly due to band-gap transition. To investigate photocatalytic properties, the range of the photocatalyst's absorption wavelength within that of natural light must first be determined. **Figure 4A** shows the UV-Visible absorption spectra of the obtained BiVO<sup>4</sup> product, which exhibits strong absorption in both the visible and UV range, indicating its potential as a good photocatalyst for sunlightdriven applications.

As is known to us, energy band structures of a semiconductor continue to be important in determining its photocatalytic activity. And the relationship of absorbance and incident photon energy can be described by the following equation (Cheng et al., 2012):

$$\alpha \mathbf{h} \boldsymbol{\upsilon} = \left[ \mathbf{A} (\mathbf{h} \boldsymbol{\upsilon} - \mathbf{E} \mathbf{g})^{\frac{1}{2}} \right]$$

Where α, ν, and A are the absorption coefficient, light frequency, and proportionality constant, respectively. The band-gap energy (there means direct band-gap), Eg, of the obtained photocatalyst can be estimated from a plot depicting (αhν) 2 vs. hν (Cheng et al., 2012; Xu et al., 2016; Zhao et al., 2016). According to the data and corresponding theoretical calculations, the band-gap energies of

BiVO<sup>4</sup> were estimated to be about 2.45 eV as shown in **Figure 4B**. For a semiconductor, the CBM and VBM can be calculated according to the following empirical equation (Ye et al., 2012): ECBM = χ – Ee −0.5Eg, where ECBM is the CBM edge potential; χ is the electronegativity of the semiconductor (6.04 for BiVO4) (Long et al., 2006); Ee is the energy of free electrons (about 4.5 eV); and E<sup>g</sup> is the band-gap energy of the semiconductor (2.45 eV, as estimated previously). According to the equation EVBM = ECBM + Eg, the value of ECBM is 0.31 eV and EVBM is 2.76 eV. Similar results were also reported by Long et al (Long et al., 2006). What's more, the value of ECBM and EVBM are different owning to different test or different calculation method in different papers although the band-gap energy is very similar (Zhang et al., 1998; Long et al., 2006). A schematic illustration of the band-gap structure of BiVO<sup>4</sup> is shown in **Figure 4C**.

This paper also tested the photocatalytic efficiency of P25 and BiVO<sup>4</sup> and the result was shown in **Figure 5**. The **Figure 5A** experimental results showed that the efficiency of P25 was

9%, while the efficiency of BiVO<sup>4</sup> was 37%. The **Figure 5B** was the first order kinetic fitting of the degradation result, both of these results indicated a better efficiency of RhB degradation in the presence of BiVO<sup>4</sup> compared to the P25, and this exactly represented a good degradation property of BiVO4.

#### Mechanisms for ·O<sup>−</sup> 2 and ·OH Production ESR Analysis

Based on the ESR results as shown in **Figures 6A,B**, signals with intensities corresponding to the characteristic peaks of DMPO–·O − 2 adducts were observed when the reaction was performed under visible light irradiation, but not when the reaction was performed in the dark. The peak intensities further increased with increased irradiation time. Furthermore, DMPO–·O − 2 signals were observed under aerobic conditions (**Figure 6B**) but not under anaerobic conditions (**Figure 6A**). This is consistent with the production mechanism of ·O − 2 via O2, as previously reported (Ge et al., 2012b; Ye et al., 2012). Although there still were some studies mentioned no ·O − 2 contained in the degradation system of BiVO<sup>4</sup> (Ge et al., 2012a; Lopes et al., 2015). As shown in **Figures 6C,D**, DMPO–·OH was observed under both aerobic and anaerobic conditions, respectively, and the signal intensity was stronger in the aerobic case. These results demonstrate the validity of both aerobic and anaerobic pathways for the production of ·OH. In the first pathway, oxygen, which is an effective electron acceptor, absorbed on the catalyst surface is the main capturing agent targeting photogenerated electrons and can oxidize the hydroxylated products to produce ·OH. The h<sup>+</sup> produced by the photocatalyst can then oxidize OH<sup>−</sup> in water to produce ·OH, representing the second possible path. However there is no report for the identification of this phenomenon. The experimental results verified the validity of first path, and to ensure the possibility of the second path, we performed an experiment in which the pH was varied.

The ESR spectrum of BiVO<sup>4</sup> is shown in **Figures 6E,F** reveals a magnified section of the same spectrum to provide more detail. These confirm the separation of electron–hole pairs in the BiVO<sup>4</sup> and provide evidence for the existence of ·OH (Yang et al., 2015). The signal intensity of the 300.5 mT peak (g = 2.002) is evidently strong (**Figure 6E**) in comparison with the peak for the same sample in darkness, indicating the presence of h+. In order to further for the explanation about the recombination of electron–hole pairs, we did the photoluminescence (PL) test. PL spectra can reveal the movement, transfer and recombination of photoelectronhole pairs. It's an effective means to characterize the separation

efficiency of semiconductors. In general, the lower the intensity of the emission peak in the PL spectrum, the higher the efficiency of the electron and hole separation in the semiconductor and the higher the photocatalytic activity of the photocatalyst (Gao et al., 2014). From the PL result shown in **Figure 6G** shows the PL spectra of BiVO<sup>4</sup> under the excitation wavelength of 325 nm, we can find the strong peak under 367 nm, which means the existence about recombination of electron–hole pairs (Zhao et al., 2017).

According to the results of the pH experiment (**Figure 7**), the DMPO–·OH signal increased with increasing pH between 5 and 9, which could be found from the signal intensity under 5 min light; however, no obvious increase was observed when the pH increased to 11. This is because more ·OH radicals are produced under alkaline conditions than under acidic or neutral conditions. However, increased alkalinity may also alter the charge of the catalyst surface, thus affecting the production of hydroxyl radicals, which explains the lack of an increase at pH = 11 (Wang et al., 2011). These results confirm the validity of the second path, indicating that ·OH can be produced by the oxidation of h<sup>+</sup> for OH−. Together, the ESR results explain the complete production of ·OH. The trapping experiment was performed to determine the main pathway for ·OH generation.

#### Scavengers Study

The photocatalytic activities of the samples were evaluated by analyzing the degradation of RhB under visible light irradiation. The effect of BiVO<sup>4</sup> on RhB degradation was obvious when compared to the result without any catalyst (**Figure 8**). The experimental results indicate complete inhibition of RhB degradation in the presence of IPA, suggesting that ·OH is the main reactive species contributing to the photocatalytic degradation of RhB over BiVO4. ·O − 2 also influenced the photocatalytic degradation of RhB, but the effect was less obvious than that of ·OH. As is known to us, the h<sup>+</sup> and e<sup>−</sup> are the original active species in the degradation system. The presence of AgNO<sup>3</sup> captured e<sup>−</sup> and reduced the recombination of h<sup>+</sup> and e−, as a result it increased the effect of h+. At the same time, the h<sup>+</sup> could further generate ·OH, which promoted the degradation of RhB. Because the ·OH was also the production of hole, we concluded that h<sup>+</sup> also plays an important role, and similar results were also reported by Zhou et al (Xu et al., 2017). What's more, we found obviously decrease on the addition of Na2C2O4. The capture of h<sup>+</sup> meant there was no any hole and·OH in the degradation system, thus the degradation effect obviously decreased. As a result, we concluded the importance of hole. And its importance was further confirmed by the slight enhancement of RhB degradation after the addition of Na2C2O<sup>4</sup> to capture the photogenerated Xu et al. Mechanism Based on ESR

hole. Furthermore, the addition of AgNO<sup>3</sup> enhanced the degradation effect, which also demonstrates the strong influence of electron–hole recombination on the degradation process. These results demonstrate that h<sup>+</sup> can directly degrade RhB, and therefore it plays a significant role in the system, as indicted by the second pathway. In addition, ·OH is mainly generated by h+.

#### Elucidate the Mechanism

We proposed a reaction mechanism based on the results of the experiments described above (**Figure 9**). First, the adsorbed RhB is photo-activated by visible light, so that electrons can transfer from the singlet excited RhB (RhB<sup>∗</sup> ) to the CB of BiVO4, leaving h<sup>+</sup> in the VB. Then, the electrons in the CB react with O2, resulting in the production of ·O − 2 and ·OH radicals, as illustrated in process I (**Figure 9**). However, this degradation process is our speculation based on references and the ESR experiment. We cannot ignore the indirect effect of the ·O − 2 on the degradation of RhB owning to its little effect on the production of ·OH based on the trapping experiment result, and this process is not the main degradation mechanism. The O<sup>−</sup> 2 is perhaps only participating in a side reaction or maybe can further generate ·OH for the degradation. And the evidence for this process still requires further study. At the same time, h<sup>+</sup> in VB can oxidize the OH<sup>−</sup> produced by ionization of water to ·OH, which is the main active free radical for RhB degradation, as shown in process III (**Figure 9**). Finally, the ·O − 2 and ·OH degrade RhB<sup>+</sup> and RhB, indicating the major role of ·OH in the degradation system. At the same time, the h<sup>+</sup> can also degrade RhB<sup>+</sup> and RhB directly (which can be concluded from the trapping experiment result), as demonstrated in process II (**Figure 9**). However, the proportion of RhB degraded by this process is minor. So we conclude that there are three processes in the degradation system, the main mechanism is the process III, the process II occur very minor and the process I is the speculation based on our experiment result. Further study would be done for the process I. The entire sequence is summarized in the following reactions:

$$\text{BiVO}\_4 + \text{h}\nu \rightarrow \text{ h}^+ + \text{e}^- \tag{\text{R1}}$$

$$\text{O}\_2 + \text{e}^- + \text{H}^+ \rightarrow \text{H}\_2\text{O}\_2 \tag{\text{R2}}$$

$$\mathrm{H}\_{2}\mathrm{O}\_{2} + \mathrm{e}^{-} \rightarrow \cdot \mathrm{O}\_{2}^{-} + \mathrm{OH}^{-} \tag{\mathrm{R3}}$$

$$\cdot \mathrm{O}\_{2}^{-} + \mathrm{H}\_{2}\mathrm{O} \rightarrow \cdot \mathrm{OH} + \mathrm{OH}^{-} \tag{R4}$$

$$h^+ + \text{OH}^- \rightarrow \cdot \text{OH} \tag{\text{R5}}$$

$$\text{Rh}^+ + \cdot \text{O}\_2^- + \cdot \text{OH} + \text{RhB} \rightarrow \text{ Degation product} \quad \text{(R6)}$$

The mechanism and reactions for flowerlike BiVO<sup>4</sup> photocatalysis described in this study are the same as those from described in previous research (Liu et al., 2006; Li, 2012; Wei et al., 2013), which are generally accepted as true (Han et al., 2003; Cruz and Pérez, 2010; Tian et al., 2014; Aguilera-Ruiz et al., 2015; Gao et al., 2015). The difference between this study and others is that ESR identifies the production of ·O − 2 and provides a direct analysis of the pathways for the production of ·OH radicals, as shown in R4 and R5. These results indicate that h<sup>+</sup> can also produce ·OH, a result which has not been identified in previous research (Lam et al., 2014); thus, h<sup>+</sup> plays an important role in the degradation system by mainly producing ·OH. This result makes it a little more clearly about the production of ·OH in the complete photocatalytic system, of which is one of the most effective active free radicals.

### CONCLUSION

In summary, we successfully prepared flowerlike BiVO<sup>4</sup> and characterized the resulting samples using a variety of analytical methods to thoroughly investigate their morphologies and compositions. After the well synthesis of material, we compared the photocatalytic activity of BiVO<sup>4</sup> to P25 and the result shown a well degradation property of BiVO4. Based on the ESR experiment, we found the main free radicals in the reaction system and concluded the production of them by changing test conditions and trapping experiment. The mechanism for RhB degradation via BiVO<sup>4</sup> photocatalysis was successfully elucidated by these experiments mentioned earlier. The main active free radicals in the reaction system are h<sup>+</sup> and ·OH. For the production of ·OH, in addition to production by ·O − 2 , it can also be produced by oxidation of h <sup>+</sup> to OH−, which is the main pathway for ·OH generation. However, we cannot ignore the indirect effect of the ·O − 2 on the degradation of RhB owning to its little effect on the production of ·OH based on the trapping experiment result. Weather the ·O − 2 can further generate OH still require further future study .More ·OH will be generated if there is more OH<sup>−</sup> in the degradation system. At the same time, future research should further elucidate the degradation system by conducting experiments under varying pH. This new insight into radical production can be used in tandem with further experimental investigation to help improve photocatalyst performance.

#### AUTHOR CONTRIBUTIONS

XX, YS, and ZF: conceived of the study and designed the experiments; YS, SZ, and SX: performed the experiments; YS, DZ, and BZ: analyzed the data; GL: contributed reagents, materials, and analysis tools; YS wrote the paper.

#### ACKNOWLEDGMENTS

Financial support from the Science and Technology Innovation Special Projects of Social Undertakings and Livelihood Support, Chongqing (cstc2016shmszx20009), the Chongqing Research Program of Basic Research and Frontier Technology (cstc2015jcyjA20013, cstc2017jcyjBX0080), and the 111 Project (B13041) are gratefully acknowledged.

# REFERENCES


their high visible-light photocatalytic property[J]. J. Nanopart. Res. 19:124. doi: 10.1007/s11051-017-3818-6


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling Editor declared a shared affiliation, though no other collaboration, with one of the authors, FZ.

Copyright © 2018 Xu, Sun, Fan, Zhao, Xiong, Zhang, Zhou and Liu. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Ti4O7/g-C3N<sup>4</sup> Visible Light Photocatalytic Performance on Hypophosphite Oxidation: Effect of Annealing Temperature

#### Wei Guan<sup>1</sup> , Gaoge Sun<sup>2</sup> , Lei Yin<sup>3</sup> , Zhenghua Zhang<sup>4</sup> \* † and Shichao Tian<sup>5</sup> \* †

*<sup>1</sup> Chongqing Key Laboratory of Environmental Materials and Remediation Technologies, Chongqing University of Arts and Sciences, Chongqing, China, <sup>2</sup> Department of Chemistry, Tsinghua University, Beijing, China, <sup>3</sup> Heibei Yinfa Meifute Environmental Engineering Co., Ltd., Shijiazhuang, China, <sup>4</sup> Research Institute of Environmental Engineering and Nano-Technology, Graduate School at Shenzhen, Tsinghua University, Shenzhen, China, <sup>5</sup> Shenzhen Environmental Science and New Energy Technology Engineering Laboratory, Tsinghua-Berkeley Shenzhen Institute, Shenzhen, China*

# Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Hongwei Huang, China University of Geosciences, China Michael Nolan, University College Cork, Ireland*

#### \*Correspondence:

*Zhenghua Zhang zhenghua.zhang@sz.tsinghua.edu.cn Shichao Tian 18812672619@163.com*

> *† These authors have contributed equally to this work.*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *04 December 2017* Accepted: *14 February 2018* Published: *01 March 2018*

#### Citation:

*Guan W, Sun G, Yin L, Zhang Z and Tian S (2018) Ti*4*O*7*/g-C*3*N*4 *Visible Light Photocatalytic Performance on Hypophosphite Oxidation: Effect of Annealing Temperature. Front. Chem. 6:37. doi: 10.3389/fchem.2018.00037* The oxidation of hypophosphite to phosphate is the key to recover the phosphorus resource from the hypophosphite wastewater. In the present work, Ti4O7/g-C3N<sup>4</sup> composites were synthesized at two different temperatures (100 and 160◦C) and their performance on photocatalytic oxidation of hypophosphite under visible light irradiation and the corresponding mechanism were evaluated. A hydrolysis method using g-C3N<sup>4</sup> and Ti4O<sup>7</sup> was applied to synthesize the Ti4O7/g-C3N<sup>4</sup> composites with their hybrid structure and morphology confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectra (XPS). The annealing temperature significantly affected the photocatalytic performance of Ti4O7/g-C3N<sup>4</sup> that the 160-Ti4O7/g-C3N<sup>4</sup> composite (fabricated at 160◦C) showed the highest oxidation efficiency of hypophosphite of 81% and the highest photocatalytic oxidation rate of 0.467 h−<sup>1</sup> comparing with the 100-Ti4O7/g-C3N<sup>4</sup> composite (fabricated at 100◦C) and pure g-C3N4. The enhanced photocatalytic performance of 160-Ti4O7/g-C3N<sup>4</sup> could be ascribed to the effective charge separation and enhanced photoabsorption efficiency. Additionally, electron spin resonance (ESR) results showed that hydroxyl radicals and superoxide anion radicals were mainly responsible to the oxidation of hypophosphite with superoxide anion radicals accounting for a more significant contribution. Moreover, Ti4O7/g-C3N<sup>4</sup> photocatalysts showed the remarkable stability in the repetitive experiments.

Keywords: graphitic carbon nitride, sub-stoichiometric titanium oxides, hypophosphite, hydroxyl radicals, superoxide anion radicals

# INTRODUCTION

Hypophosphite is commonly used as a reducing agent in metallurgy industries especially in the processes of plating and surface finishing thereby generating large amounts of hypophosphite wastewater (Bulasara et al., 2011; Li et al., 2015). The hypophosphite contaminant should be further treated before being discharged into the river or lake, because it may lead to algae growth and cause eutrophication (Wang et al., 2016; Ge et al., 2017). In addition, phosphorus is a non-renewable resource mainly used as a nutrient in agricultural production (Montangero and Belevi, 2007). Due to an increase in the global demand for phosphorus resource, it will be depleted in the next 50–100 years (Takeda et al., 2010; Ye Y. Y. et al., 2017; Ye Z. L. et al., 2017). Therefore, the phosphorus recovery from wastewater is of considerable interest. However, a high solubility constant of the hypophosphite precipitants limits the transformation of hypophosphite into the precipitated products. In contrast, phosphate is easier to be recovered than hypophosphite by precipitation. As such, a highly efficient approach for the preoxidation of hypophosphite to phosphate becomes of great importance for phosphorus recovery. Photocatalysis, a nanoenabled technology, has been recognized for its promising applications with the generation of activated radicals such as hydroxyl radicals and superoxide anion radicals, which herein can be applied for hypophosphite oxidation.

Semiconductor photocatalysts have been recognized as a potential strategy to solve some serious challenges of the twentyfirst century, such as energy shortage, environmental pollution, and global warming (Lin et al., 2017). TiO<sup>2</sup> photocatalyst has attracted much attention due to its strong oxidizing power, low cost and high chemical stability. However, a large band gap (3.2 eV) of TiO<sup>2</sup> means that it can only absorb ultraviolet light (only about 3–5% of total sunlight), which greatly limits its performance in industrial applications (Teng et al., 2017; Noman et al., 2018). Therefore, it is urgent to develop novel semiconductor photocatalysts that respond to visible light.

Recently, graphitic carbon nitride (g-C3N4), constituted by numerous layers of two-dimensional (2D) counterparts, has attracted enormous attention given its advantages such as low cost and visible light driven semiconductor photocatalyst (Huang et al., 2017b; Liu et al., 2017a,b; Tian et al., 2017; Wang et al., 2017). The metal free g-C3N<sup>4</sup> can work as photocatalyst under visible light irradiation with a suitable band gap of 2.7 eV. In addition, g-C3N<sup>4</sup> possesses excellently structural stability, which is suitable for chemical modification. Nevertheless, the photocatalytic property of g-C3N<sup>4</sup> is still limited for its low surface area, low photoabsorption efficiency and high recombination rate of photo-induced electron-hole pairs (Jourshabani et al., 2017; Shao et al., 2017).

Decreasing the recombination of photo-induced carriers during the photocatalytic process would enhance the photocatalytic activity of the as-prepared g-C3N<sup>4</sup> photocatalyst (Li J. D. et al., 2017). Therefore, some methods, such as porosity engineering, doping with foreign elements, and compositing with other semiconductors to facilitate charge separation, were developed to enhance the photocatalytic performance of g-C3N4. For example, the horn-like hollow mesoporous ultrathin g-C3N<sup>4</sup> tubes were fabricated with high surface area, drastically boosted bulk charge separation, carrier density and surface charge transfer efficiency and showed the remarkably photocatalytic performance for H<sup>2</sup> evolution (Liu et al., 2017a). Meanwhile, the 3D mesoporous g-C3N<sup>4</sup> established by ultrathin self-doped nanosheets exhibited the superior photocatalytic performance on hydrogen evolution (Tian et al., 2017). Additionally, the porous and thin g-C3N<sup>4</sup> nanosheets, prepared via a novel thioureaassisted melamine-precursor hydrothermal pre-treatment followed by a traditional thermal polymerization method, profoundly enhanced visible-light photocatalytic performance on H<sup>2</sup> evolution and NO removal from the gaseous phase (Huang et al., 2017b). Moreover, the Cl intercalated mesoporous g-C3N<sup>4</sup> showed outstanding photocatalytic performance for water splitting into H2, CO<sup>2</sup> reduction, liquid and air contaminants removal (Liu et al., 2017b).

Substoichiometric titanium oxides, known as Magnéli phases (Sun et al., 2016), comprise a series of compounds with the generic formula TinO2n−<sup>1</sup> (4 ≤ n ≤ 10) (Kolbrecka and Przyluski, 1994; Guo et al., 2016). Among these compounds, Ti4O<sup>7</sup> possesses high electrical conductivity, thermal stability, and corrosion resistance in harsh conditions (Oturan et al., 2017). Therefore, Ti4O<sup>7</sup> is wildly used as catalyst coated material (Li et al., 2010), wastewater treatment material (You et al., 2016), support material in fuel cell (Chisaka et al., 2016), and additive to positive materials in batteries (Cao et al., 2017). However, it was also reported that pure Ti4O<sup>7</sup> as the photocatalyst had limited photocatalytic activity with the band gap of 2.9 eV (Maragatha et al., 2017). Herein, coupling Ti4O<sup>7</sup> and intrinsic g-C3N<sup>4</sup> to construct the well-matched Ti4O7/g-C3N<sup>4</sup> heterojunction would be an alternative novel pathway to address the intrinsic drawbacks of g-C3N<sup>4</sup> for photocatalytic applications.

In this study, novel Ti4O7/g-C3N<sup>4</sup> photocatalysts were synthesized at two different temperatures (100 and 160◦C) and their performance in photocatalytic oxidation of hypophosphite under visible light irradiation and the corresponding mechanism were compared and investigated. Ti4O7/g-C3N<sup>4</sup> photocatalysts exhibited an enhanced photocatalytic activity for hypophosphite oxidation under visible light irradiation and the annealing temperature significantly affected the photocatalytic performance. The separation mechanism of photogenerated electrons and holes of the photocatalysts was investigated by UV-Vis diffuse reflectance spectra, photoluminescence emission spectra, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The activated species generated in the photocatalytic process were measured by electron spin resonance (ESR). The enhanced photocatalytic performance could be ascribed to the efficient charge separation and transfer across the heterojunction interface and the enhanced photoabsorption efficiency. Our work demonstrated that the rational design and construction of isotype heterojunction was an effective strategy for the development of efficient photocatalysts under visible light irradiation.

## MATERIALS AND METHODS

#### Chemicals

Melamine (C3H6N6), urea [CO(NH2)2], sub-stoichiometric titanium oxide (Ti4O7), sodium hypophosphite (NaH2PO2), sodium sulfate (Na2SO4), isopropanol [IPA, (CH3)2CHOH], ethylenediaminetetraacetic acid disodium salt (C10H14N2Na2O8), sodium hydroxide (NaOH), and sulfuric acid (H2SO4) were provided by Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). 5,5-dimethylpyrroline-N-oxide (DMPO) was bought from Dojindo Co., Ltd. (Shanghai, China). The entire chemical reagents were analytical grade and all solutions were prepared using Milli-Q water (Millipore, 18.2 M cm).

# Synthesis of g-C3N<sup>4</sup> Materials

The g-C3N<sup>4</sup> materials were prepared using a liquid-based growth method (Sun et al., 2018). In a typical process, the mixture of melamine and urea (molar ratio = 1:1) was dissolved with 50 mL deionized water and then vigorously stirred for 1 h at room temperature. After that, the mixture suspension was centrifuged at 7,500 r/min for 15 min, and then dried at 60◦C for 24 h under the vacuum to obtain the white powder. After that, the prepared white powders were further grinded into smaller powders in a mortar and placed in a muffle furnace. The powders were then annealed at 550◦C in a muffle furnace for 4 h in static air at a ramp rate of 2.5◦C min−<sup>1</sup> . The resulting yellow products were collected for further usage.

# Preparation of Ti4O7/g-C3N<sup>4</sup> Photocatalysts

The preparation procedure of Ti4O7/g-C3N<sup>4</sup> photocatalysts was shown below: g-C3N<sup>4</sup> powder (2.0 g) and Ti4O<sup>7</sup> (1.0 g) were dispersed into 100 mL NaOH (0.1 mol/L) by ultrasonication for 0.5 h. Subsequently, the mixed liquor was transferred to reaction still and then annealed in different temperatures (100 and 160◦C) for 20 h. After that, the obtained precipitates were collected by centrifugation and washed with distilled water, and then dried at 60◦C for 12 h. Samples fabricated at different annealing temperatures were noted as 100-Ti4O7/g-C3N<sup>4</sup> (fabricated at 100◦C) and 160-Ti4O7/g-C3N<sup>4</sup> (fabricated at 160◦C), respectively.

# Samples Characterization

The concentration of NaH2PO<sup>2</sup> was measured by ion chromatography using a 732 IC detector (McDowell et al., 2004). The crystal phase composition and fineness of the samples were analyzed by X-ray diffraction (XRD) with Cu Kα radiation in the scanning range of 2θ = 5–80◦ (model D/max RA, Rigaku Co., Japan). The surface morphology of the as-developed samples was examined by scanning electron microscopy (SEM) (JEOL JSM-6701F). The valence state of the deposition was measured by X-ray photoelectron spectra (XPS) (PHI-5300/ESCA, ULVAC-PHI, INC). The UV–vis diffuse reflection spectra (UV–vis DRS) of the samples was obtained by a UV–vis spectrophotometer (UV-2450, Shimadzu, Japan). Electrochemical properties of the Ti4O7/g-C3N<sup>4</sup> and g-C3N<sup>4</sup> photocatalysts, including photocurrents (PC), CV, and EIS, were measured on a CHI 660B electrochemical system. Electron spin resonance (ESR) (ESRA-300, Bruker, Germany) signals were recorded by the probe molecular 5,5-dimethyl-1-pyrroline-Noxide (DMPO) to identify the radicals generated under visible light irradiation (λ > 420 nm) (Tian et al., 2015).

FIGURE 1 | Structure and morphology analyses. (a) XRD patterns of the g-C3N4, Ti4O7 and 160-Ti4O7/g-C3N4 photocatalysts; (b) SEM image of the 160-Ti4O7/g-C3N4 photocatalyst.

# Evaluation of Photocatalytic Activity

The photocatalytic activities of as-synthesized samples were evaluated by the oxidation of hypophosphite in aqueous solution under visible light irradiation. For hypophosphite oxidation, the light source was a 35 W metal-halide lamp (Philips) with a 420 nm UV-cutoff filter. The lamps were located 12 cm away from the surface of reaction solution (about 5 mW cm−<sup>2</sup> ). In each experiment, photocatalyst (10 mg) was dispersed in hypophosphite (100 ml, 100 mg L−<sup>1</sup> ) aqueous solution. Before irradiation, the solution was continuously stirred in the dark for 2 h to reach adsorption-desorption equilibrium between the hypophosphite and the photocatalyst. During the photocatalytic reaction, the solutions were kept magnetically stirring, and 4 ml mixture was collected at 1 h intervals followed by centrifugation (10,000 rpm, 5 min) to remove the photocatalyst.

# Radicals Quencher Experiment for Photocatalysis

In order to identify the contributions of the radicals generated in the photocatalytic oxidation process, IPA and N<sup>2</sup> purging were applied with IPA acting as the ·OH radicals quencher and N<sup>2</sup> purging reducing the superoxide ·O<sup>2</sup> <sup>−</sup> radicals (Yang et al., 2016). Adding different radical scavengers into reaction solutions would affect the photocatalytic performance. As such, the contributions of ·OH radicals and ·O<sup>2</sup> <sup>−</sup> radicals on photocatalytic oxidation of hypophosphite under visible light irradiation can be evaluated based on the change of photocatalytic oxidation efficiency of hypophosphite with and without IPA (1 mM) and N<sup>2</sup> purging (continuous purging).

# RESULTS AND DISCUSSION

## Structure and Morphology Analyses

The crystal structures of g-C3N4, Ti4O7, and 160-Ti4O7/g-C3N<sup>4</sup> photocatalysts were characterized by XRD. As shown in **Figure 1a**, two pronounced diffraction peaks in pure g-C3N<sup>4</sup>

were observed at 13.20◦ and 27.60◦ , respectively. The peak at 13.20◦ was corresponded to (1 0 0) plane of tri-striazine units (Zhang et al., 2012a). The peak at 27.60◦ indexed as (0 0 2) peak was due to the interlayer-stacking of aromatic systems as in graphite (Zhang et al., 2012b). The characteristic peaks of Ti4O<sup>7</sup> were matched well with the standard card (JCPDF 50-0787). The main diffraction peaks of 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst did not change obviously, indicating that the fabrication process did not destroy the main structure of both counterparts. The microstructure of the 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst was shown in **Figure 1b**. It was mainly composed of spheroidal crystals, and the shape of synthesized 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst was relatively uniform.

XPS measurements were carried out to investigate the compositions and elemental chemical states of the samples. As shown in **Figure 2**, the XPS spectra revealed that the elements of C, N, Ti and O existed on the surface of 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst. The corresponding high resolution spectra of C 1s, N 1s, Ti 2p, and O 1s were also analyzed. The XPS spectra of C 1s core level for 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst was shown

in **Figure 2A** that it could be divided into two components including the standard reference carbon (284.8 eV) and the sp<sup>2</sup> bonded C in N=C(–N)<sup>2</sup> (288.3 eV) (Jo and Natarajan, 2015). The N 1s spectra of Ti4O7/g-C3N<sup>4</sup> could be divided into four peaks as shown in **Figure 2B**. The main peak at 398.7 eV was assigned to sp<sup>2</sup> nitrogen (C=N–C) involved in triazine rings, and the peak at 399.8 eV originated from the tertiary nitrogen bonded to carbon atoms in the form of N–(C)<sup>3</sup> (Wu et al., 2013). The peak at 401.3 eV was ascribed to amino functions (C–N– H). Another peak centered at 404.4 eV was associated to charging effects or positive charge localization in heterocycles (Gao et al., 2014). These assignments of C 1s and N 1s were agreed well with the XPS results of g-C3N<sup>4</sup> reported previously. Ti4O<sup>7</sup> is a mixedvalence compound with two evenly occupied Ti4<sup>+</sup> (3d<sup>0</sup> ) and Ti3<sup>+</sup> (3d<sup>1</sup> ) configurations. As shown in **Figure 2C**, two broad peaks at about 458.6 and 464.7 eV were observed, corresponding to the

characteristic Ti 2p1/<sup>2</sup> and Ti 2p3/<sup>2</sup> peaks of Ti4+, respectively. Additionally, two peaks at 457.97 and 463.8 eV corresponding to Ti3<sup>+</sup> also appeared, as reported elsewhere (Zeng et al., 2017). The O 1 s spectra of Ti4O7/g-C3N<sup>4</sup> were shown in **Figure 2D**. The peak with binding energy of 533.5 eV was assigned to the C–O functional groups, and the peaks centered at the binding energies of 531.8 and 529.7 eV were ascribed to the OH–Ti and O–Ti bonds (Li Z. Q. et al., 2017). These results confirmed the presence of Ti4O<sup>7</sup> on the g-C3N<sup>4</sup> surface with covalent bonds.

#### Photocatalytic Activity Analysis

The photocatalytic oxidation of hypophosphite over various samples was analyzed. As shown in **Figure 3A**, the blank experiment indicated that the concentration of hypophosphite

was stable under visible light irradiation (λ > 420 nm) if there was no photocatalyst presented. Pure g-C3N<sup>4</sup> showed weak photocatalytic activity with the oxidation efficiency of only 10% possibly owing to the rapid recombination of photogenerated charge carriers and low charge transfer ability (Shi et al., 2017). The 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst had the highest photocatalytic activity with the oxidation efficiency of 81% compared with the pure g-C3N<sup>4</sup> and 100-Ti4O7/g-C3N<sup>4</sup> photocatalysts.

The photocatalytic oxidation kinetic of the prepared samples was fitted by a pseudo-first-order model, which was depicted by the following Equation (1) (Lu et al., 2018):

$$
\ln \text{ (C}\_0/\text{C}) = kt \tag{1}
$$

FIGURE 6 | EIS Nyquist plots of the g-C3N4, 100-Ti4O7/g-C3N4, and 160-Ti4O7/g-C3N4 photocatalysts under visible light irradiation.

Where C<sup>0</sup> and C are the hypophosphite concentrations in solution at times 0 and t, respectively, and k is the first-order rate constant. As shown in **Figure 3B**, the 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst showed the highest photocatalytic oxidation rate of 0.467 h−<sup>1</sup> , which was 2.5 and 26 times higher than that of 100-Ti4O7/g-C3N<sup>4</sup> and pure g-C3N4, respectively. Therefore, the results showed that the 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst

exhibited an excellent activity in photocatalytic oxidation of hypophosphite under visible light irritation.

The enhanced photocatalytic activity of Ti4O7/g-C3N<sup>4</sup> and the effect of annealing temperature on Ti4O7/g-C3N<sup>4</sup> photocatalytic activity were further investigated and explained from the perspectives of photoabsorption efficiency, band gap,

separation, transformation, and recombination processes of photogenerated carriers in the following sections.

# Optical Properties Analysis

The optical properties of Ti4O7/g-C3N<sup>4</sup> and pure g-C3N<sup>4</sup> photocatalysts were evaluated by UV–vis diffuse reflectance spectra. As shown in **Figure 4A**, the photoabsorption efficiency of Ti4O7/g-C3N<sup>4</sup> was remarkably enhanced compared with the pure g-C3N4. The pure g-C3N<sup>4</sup> held an absorption edge of around 430 nm while the Ti4O7/g-C3N<sup>4</sup> photocatalysts showed a distinct red-shift, indicating that the Ti4O7/g-C3N<sup>4</sup> photocatalysts were more efficient in light harvesting under visible light irradiation. The enhanced photoabsorption efficiency of Ti4O7/g-C3N<sup>4</sup> was due to the narrowed band gap. The band energy gap of the photocatalysts was determined from the formula αhν = A (hν – E<sup>g</sup> ) n/2 (Eg, α, h, ν, and A indicate the band gap, optical absorption coefficient, Plank's constant, photonic frequency and a proportionality constant, respectively) (Huang et al., 2017b; Liu et al., 2017a,b; Tian et al., 2017). The band gap calculated from the plot of absorption1/<sup>2</sup> vs. energy was 2.70, 2.32, and 2.13 eV for g-C3N4, 100-Ti4O7/g-C3N4, and 160-Ti4O7/g-C3N4, respectively.

Photoluminescence (PL) spectra was used to investigate the separation, transformation and recombination processes of photogenerated carriers. The band–band PL spectrum can directly reflect the separation performance of photo-induced charge carriers, viz. the stronger of the band-band PL signal, the higher of the recombination rate of photo-induced carriers. The PL spectroscopy of the photocatalysts was shown in **Figure 4B**. All photocatalysts exhibited a broad emission peak centered at around 460 nm, which was mainly caused by the recombination of photogenerated electrons and holes produced by g-C3N<sup>4</sup> (Shi et al., 2017). The PL emission intensity was highest for the pure g-C3N4, while the intensity was significantly lowered for Ti4O7/g-C3N4. This indicated that the charge carrier recombination was effectively suppressed for the Ti4O7/g-C3N<sup>4</sup> photocatalysts. It is well known that the noble metals, such as Ti4O7, are good conductors with excellent electric properties. After formation of noble metalssemiconductors heterostructures, the photogenerated electrons of semiconductors could transfer through these noble metals rapidly and the lifetime of these electrons and holes are prolonged (Cui et al., 2017). In addition, the PL intensity of 160-Ti4O7/g-C3N<sup>4</sup> was much lower than that of 100-Ti4O7/g-C3N4, indicating that 160-Ti4O7/g-C3N<sup>4</sup> had the effectively decreased charge carrier recombination compared with 100-Ti4O7/g-C3N4. The higher annealing temperature could etch and tailor g-C3N<sup>4</sup> with the possibly smaller and thinner nanosheet structure of 160-Ti4O7/g-C3N<sup>4</sup> compared with 100-Ti4O7/g-C3N<sup>4</sup> and thus shortened the distance between the photogenerated electrons and the heterostructure surface, which suppressed the recombination probability of photo-generated electron–hole pairs with the photogenerated electrons rapidly transferring through Ti4O<sup>7</sup> (Dong et al., 2015).

#### Electrochemical Properties Analysis

The PC responses of g-C3N4, 100-Ti4O7/g-C3N4, and 160- Ti4O7/g-C3N<sup>4</sup> photocatalysts under visible light irradiation were evaluated to further offer information about the separation and transformation efficiency of photogenerated electrons and holes. As shown in **Figure 5A**, the transient PC responses of all photocatalysts at light on and light off were reversible and stable, and the PC density of 160-Ti4O7/g-C3N<sup>4</sup> was much higher (0.30 µA cm−<sup>2</sup> ) than that of 100-Ti4O7/g-C3N<sup>4</sup> (0.23 µA cm−<sup>2</sup> ) and g-C3N<sup>4</sup> (0.10 µA cm−<sup>2</sup> ). This indicated that the Ti4O7/g-C3N<sup>4</sup> heterostructures promoted the separation of photogenerated charge carriers (Kang et al., 2016), in line with the PL spectra results as shown in **Figure 4B**. Additionally, this was also supported by the CV test. As shown in **Figure 5B**, a reduction peak at about −0.18 V was observed in g-C3N4, 100-Ti4O7/g-C3N4, and 160-Ti4O7/g-C3N4, but 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst possessed much higher reduction current, which indicated faster electron transfer in 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst (Samanta and Srivastava, 2017).

Electrochemical impedance spectroscopy was used to investigate the photogenerated charge separation process on the photocatalysts. The radius of the circular arc reflected the resistance of the interfacial charge transfer and separation efficiency of the electron-hole pairs (Leng et al., 2005; Liang and Zhu, 2016). As shown in **Figure 6**, the arc radius decreased gradually when Ti4O<sup>7</sup> was doped onto the g-C3N<sup>4</sup> photocatalyst. This meant that the photogenerated charge separation process occurred more easily on Ti4O7/g-C3N<sup>4</sup> compared with the pure g-C3N<sup>4</sup> because of the decreased energy barrier that the electrode reaction needed to overcome. Moreover, the arc radius of 160-Ti4O7/g-C3N<sup>4</sup> was smaller than that of 100-Ti4O7/g-C3N4, meaning that the separation of the photogenerated electron–hole pairs was more effective and the interfacial charge transfer of the electron donor/electron acceptor was faster on the 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst.

## Catalyst Stability Analysis

The stability was another vital consideration for an excellent photocatalyst. To evaluate the stability of the as-prepared 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst, the repetitive experiments of photocatalytic oxidation of hypophosphite were carried out. As shown in **Figure 7**, the oxidation efficiency of hypophosphite in the four-round continuous reaction tests using 160-Ti4O7/g-C3N<sup>4</sup> photocatalyst was 75, 81, 80, and 84%, respectively. The repetitive experiments results showed that the fabricated Ti4O7/g-C3N<sup>4</sup> photocatalysts had a stable structure possibly with the strong binding force. The N-H groups or conjugated structures in g-C3N<sup>4</sup> could tightly bond with Ti4<sup>+</sup> (3d<sup>0</sup> ) and Ti3<sup>+</sup> (3d<sup>1</sup> ) in Ti4O7, which effectively reduced the dissolution of bulk g-C3N<sup>4</sup> material during the photocatalytic process.

#### Proposed Mechanism

To clarify the reaction mechanism of photocatalytic oxidation of hypophosphite, the ROS generated under visible light irradiation were analyzed by ESR technique (with DMPO). As shown in **Figure 8A**, no ESR signals were observed in the dark while ·OH was observed under visible light irradiation with four peaks with intensities of 1:2:2:1 attributing to DMPO-·OH generated via a hole oxidative process on H2O and/or OH<sup>−</sup> (Tu et al., 2017). Moreover, ·O<sup>2</sup> <sup>−</sup> was also observed under visible light irradiation with a four-line spectrum with the relative intensities of 1:1:1:1 assigned to DMPO-·O<sup>2</sup> <sup>−</sup> adduct derived from O<sup>2</sup> reduction by electrons (Huang et al., 2015), however, no ESR signals were observed in the dark as shown in **Figure 8B**. Therefore, both ·OH and ·O<sup>2</sup> <sup>−</sup> would contribute to the photocatalytic oxidation of hypophosphite and their contributions were further investigated with and without radical scavengers. Isopropanol (IPA) and N<sup>2</sup> purging were applied with IPA acting as the ·OH radicals quencher and N<sup>2</sup> purging reducing the superoxide ·O<sup>2</sup> <sup>−</sup> radicals (Yang et al., 2016). As shown in **Figure 8C**, the photocatalytic oxidation efficiency of hypophosphite decreased from 83% (without radical scavengers) to 42% (with IPA) and even 27% (with N<sup>2</sup> purging). These results confirmed that both ·OH and O2· <sup>−</sup> radicals were the major active radical species for hypophosphite oxidation in the photocatalytic process with O2· <sup>−</sup> accounting for a more significant contribution. Note that the photocatalytic oxidation efficiency of hypophosphite did not decrease to zero with the lowest efficiency of 27% in the presence of N<sup>2</sup> purging, indicating that holes and some other radicals may also contribute to the photocatalytic oxidation process to some extent. It was reported that reactive oxygen species (ROS), such as superoxide (·O<sup>2</sup> <sup>−</sup>), hydroxyl radicals (·OH), singlet oxygen (1O2), peroxyl (RO2·), and alkoxyl (RO-) as well as hypochlorous acid (HOCl) are basically produced in the photocatalytic process (Huang et al., 2017a), which may also contribute to the photocatalytic oxidation of hypophosphite in this case.

According to the above results and those reported in the literature, the possible photocatalytic mechanism of Ti4O7/g-C3N<sup>4</sup> on hypophosphite oxidation was illustrated in **Figure 9**. The possible photocatalytic processes were as follows:

$$\text{-Ti}\_4\text{O}\_7/\text{g}\text{-C}\_3\text{N}\_4 + h\nu \rightarrow \text{ g-C}\_3\text{N}\_4\text{(e}^- + \text{h}^+)\tag{2}$$

$$\text{g-C}\_3\text{N}\_4\text{ (e}^-\text{)} + \text{Ti}\_4\text{O}\_7 \rightarrow \text{Ti}\_4\text{O}\_7\text{ (e}^-\text{)}\tag{3}$$

$$\text{Ti}\_4\text{O}\_7 + h\nu \rightarrow \text{Ti}\_4\text{O}\_7 \text{ (e}^-\text{)}\tag{4}$$

$$\cdot \text{Ti}\_4\text{O}\_7\text{ (e}^-\text{)} + \text{O}\_2 \rightarrow \cdot \text{O}\_2^-\tag{5}$$

$$\text{PO}\_2^- + \text{PO}\_2^{3-} \rightarrow \text{PO}\_4^{3-} \tag{6}$$

$$\text{g-C}\_3\text{N}\_4\text{ (h}^+\text{)} + \text{OH}^- \rightarrow \cdot \cdot \text{OH} \tag{7}$$

$$\cdot \text{OH} + \text{PO}\_2^{3-} \rightarrow \cdot \text{PO}\_4^{3-} \tag{8}$$

$$\rm{g-C\_3N\_4 (h^+) + PO\_2^{3-} \rightarrow \quad PO\_4^{3-}} \tag{9}$$

Firstly, the electrons (e−) in valence band of g-C3N<sup>4</sup> under visible light irradiation could be excited to the conduction band, leaving the holes (h+) in valence band of g-C3N<sup>4</sup> (Equation 2). Then the photogenerated electrons in the conduction band of g-C3N<sup>4</sup> would continually transfer to Ti4O<sup>7</sup> until the same Fermi levels reached (Equation 3). Thus, the photogenerated electrons and holes were located at Ti4O<sup>7</sup> and g-C3N4, respectively, leading to the effective separation of the photoinduced charge carriers. Furthermore, the electrons might be also generated from the Ti4O<sup>7</sup> particles (Equation 4) with the product easily reacting with the adsorbed oxygen molecules to produce ·O − 2 (Equation 5) followed by the oxidation of hypophosphite to phosphate (Equation 6). Meanwhile, the photogenerated holes as the strong oxidants could oxidize OH<sup>−</sup> to ·OH radicals, and then the hypophosphite was directly oxidized to phosphate (Equations 7 and 8). In addition, some photogenerated holes could directly oxidize hypophosphite to phosphate (Equation 9).

#### CONCLUSION

The enhancement of Ti4O7/g-C3N<sup>4</sup> visible light photocatalytic performance on hypophosphite oxidation and the effect of

#### REFERENCES


annealing temperature and the corresponding mechanism were investigated in this study. 160-Ti4O7/g-C3N<sup>4</sup> (fabricated at 160◦C) photocatalyst showed the highest oxidation efficiency of hypophosphite of 81% and the highest photocatalytic oxidation rate of 0.467 h−<sup>1</sup> comparing with 100-Ti4O7/g-C3N<sup>4</sup> (fabricated at 100◦C) and pure g-C3N4. The enhanced photocatalytic performance of 160-Ti4O7/g-C3N<sup>4</sup> could be ascribed to the effective charge separation and enhanced photoabsorption efficiency. Additionally, ESR results showed that hydroxyl radicals and superoxide anion radicals were mainly responsible to the oxidation of hypophosphite with O2· <sup>−</sup> accounting for a more significant contribution. Moreover, Ti4O7/g-C3N<sup>4</sup> photocatalysts showed the remarkable stability in the repetitive experiments. Our work demonstrated that the rational design and construction of isotype heterojunction was an effective strategy to develop the efficient photocatalysts under visible light irradiation.

#### AUTHOR CONTRIBUTIONS

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

#### FUNDING

This work was supported by National Natural Science Foundation of China (No. 51608086); Science and Technology Project from Chongqing (CSTC2015JCYJA20027); Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501103); Chongqing Key Laboratory of Environmental Materials and Remediation Technology, Chongqing University of Arts and Sciences (CKE1407); Natural Science Foundation of Yongchuan, Chongqing (Ycstc, 2014ac4001); Scientific Research Foundation of Chongqing University of Arts and Sciences (R2014CH08).


**Conflict of Interest Statement:** Author LY was employed by company of Heibei Yinfa Meifute Environmental Engineering Co., Ltd.

The other authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Guan, Sun, Yin, Zhang and Tian. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Understanding Mechanism of Photocatalytic Microbial Decontamination of Environmental Wastewater

Chhabilal Regmi 1†, Bhupendra Joshi 1†, Schindra K. Ray <sup>1</sup> , Gobinda Gyawali <sup>1</sup> \* and Ramesh P. Pandey 2,3 \*

*<sup>1</sup> Department of Environmental and Bio-chemical Engineering, Sun Moon University, Asan-si, South Korea, <sup>2</sup> Department of Life Science and Bio-chemical Engineering, Sun Moon University, Asan-si, South Korea, <sup>3</sup> Department of BT-Convergent Pharmaceutical Engineering, Sun Moon University, Asan-si, South Korea*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Leny Yuliati, Universitas Ma Chung, Indonesia Martin Reli, Technical University of Ostrava, Czechia Yuanguo Xu, Jiangsu University, China*

#### \*Correspondence:

*Gobinda Gyawali ggobinda@sunmoon.ac.kr Ramesh P. Pandey pandey@sunmoon.ac.kr*

*† These authors have contributed equally to this work.*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *16 November 2017* Accepted: *09 February 2018* Published: *28 February 2018*

#### Citation:

*Regmi C, Joshi B, Ray SK, Gyawali G and Pandey RP (2018) Understanding Mechanism of Photocatalytic Microbial Decontamination of Environmental Wastewater. Front. Chem. 6:33. doi: 10.3389/fchem.2018.00033* Several photocatalytic nanoparticles are synthesized and studied for potential application for the degradation of organic and biological wastes. Although these materials degrade organic compounds by advance oxidation process, the exact mechanisms of microbial decontamination remains partially known. Understanding the real mechanisms of these materials for microbial cell death and growth inhibition helps to fabricate more efficient semiconductor photocatalyst for large-scale decontamination of environmental wastewater or industries and hospitals/biomedical labs generating highly pathogenic bacteria and toxic molecules containing liquid waste by designing a reactor. Recent studies on microbial decontamination by photocatalytic nanoparticles and their possible mechanisms of action is highlighted with examples in this mini review.

Keywords: photocatalysis for environment, microbial inactivation, mechanisms of action, semiconductors, environmental pollutants

The natural ecosystem is being deteriorated continuously because of unrestrained flow of wastewater from various industries, hospital wastes, and laboratories due to rapid and poorly-planned urbanization in developing countries and development of mega cities. The urban population is rising unprecedentedly. The socio-economic disparities is exacerbating while creating unsanitary conditions that facilitates the outbreaks. The natural water bodies are contaminated with (i) pathogens originated from hospitals and biomedical laboratories, (ii) different hazardous materials such as organic dyes, chemicals, and toxins from industries. Meanwhile, fertile agricultural lands are also affected with those chemicals and excessively used hazardous pesticides, herbicides, and plastics while unduly used fertilizers are causing algal blooms in natural water bodies (Rosen et al., 2017; West et al., 2017). Ultimately, these harmful chemicals are entering to humans to cause deadly diseases via food chain from plants and animals. In some instances, the contaminated water and foods caused severe epidemics and pandemics due to super-bacteria resistant to almost all drugs. In another hand, the genetic disorders caused by chemical mutagens leaving thousands of people with deadly non-transmissible diseases such as cancers, reproductive disorders and adverse pregnancy outcomes, physical disorders, and disabilities (Frazier and Hage, 1998). All these factors are amplifying the global risks (www.weforum.org). Thus, the environment protection is always crucial to secure the human life and animals on the planet. Though governments of different nations issued standard guidelines to discharge treated effluents, sewerage, and other wastages to the environment, yet excessive discharge of waste is maltreating the environment and the ecosystem. Hence, there is an urgent need for a highly efficient system to treat waste for complete degradation of toxic molecules into simpler non-toxic forms while inactivating highly pathogenic multidrug-resistant superbugs to make water free of pathogens.

Many industries and laboratories treat chemical wastages using various approaches prior to discharge them. Commonly practiced approaches include- (1) physical methodssedimentation, floatation, filtration; (2) biological oxidation methods- includes trickling filters, rotating biological contractor, activated sludge, lagoons, and various aerobic and anaerobic methods; and (3) chemical treatment methods such as coagulation and flocculation by lime added with ferric chloride and lime added with aluminum sulfate. Hospitals and biomedical laboratories treat microbially contaminated wastes by either physical means such as autoclaving and irradiation, or chemical treatment methods using organic solvents and detergents to remove or inactivate pathogens prior to discharge. All these methods have their own merits and demerits. The major demerit lies in the incomplete removal of these wastage products as well as the formation of intermediates which are more toxic than the parent pollutants.

Recently, photocatalysis, a novel green technology which is based on advance oxidation processes (AOPs') and is capable of generating different reactive species such as reactive oxygen species (ROS), reactive nitrogen species (RNS), oxygen radicals (·O − 2 ), hydroxyl radicals (·OH) have attracted the great attention for their potential application for the degradation of organic and biological wastes (Cavassin et al., 2015; Ramazanzadeh et al., 2015; Waiskopf et al., 2016; Nosaka and Nosaka, 2017; Podporska-Carroll et al., 2017). In this method, the synthesized semiconductor NPs are activated by light of different wavelength (1) UV light, (2) visible light, and (3) NIR light depending on the energy band gap they possess (Sarina et al., 2013). The activated semiconductor degrade the organic and microbial cells by a simple photocatalytic redox mechanism. In recent year, extensive research has been carried out for the efficient removal of the toxic molecules and pathogenic microbes from wastewater using semiconductor photocatalytic NPs. Moreover, various modification on the photocatalytic nanoparticles such as formation of heterojunction, tuning the morphology, doping of metals/non-metals, and decorating of nanoparticles onto the photocatalytic surface have been carried out in order to make efficient inactivation of microorganism in waste water under visible light. Regmi et al. (2017a) have fabricated Fe doped BiVO<sup>4</sup> visible light active semiconductor and showed efficient removal of ibuprofen and Escherichia coli in water. Recently, the synthesis of Bi2MoO<sup>6</sup> on g-C3N<sup>4</sup> nanosheet is reported with enhanced photocatalytic hydrogen evolution and disinfection of E. coli under the visible light (Li et al., 2017). Meanwhile, Meng et al. (2017) used synthesized MoS<sup>2</sup> quantum dots interspersed Bi2WO<sup>6</sup> heterostructure for efficient detoxification and disinfection in wastewater. Doping of different size of metals such as Ag further improved the photocatalytic antibacterial activities (Esfandiari et al., 2014). Ning et al. (2017) synthesized dual couples Bi metal depositing and Ag@AgI islanding on BiOI 3D architectures showed synergistic bactericidal mechanism of E. coli under visible light. Similarly, Booshehri et al. (2014) studied the effect of depositing silver nanoparticles on BiVO<sup>4</sup> to enhance visible light photocatalytic inactivation of bacteria in water. Other illustrations of microbial inactivation using semiconductors include the heterogeneous photocatalytic inactivation of B. stearothermophilus endospores in aqueous suspensions under artificial and solar irradiation (Berberidou et al., 2012), visible light inactivation of bacteria and fungi by modified titanium dioxide (Mitoraj et al., 2007), and the efficient inactivation of E. coli by ZnO–Ag nanoparticles under solar radiation (Adhikari et al., 2015). Similarly, visible light driven morphology tuned α-NiMoO<sup>4</sup> photocatalyst were used to inactivate multidrug resistant Staphylococcus aureus (Ray et al., 2018). Diverse microbes such as pathogenic bacteria, viruses, fungi, protozoa, and other organisms are inactivated thus making the water devoid of microbes and safe to human, animal, and environment.

Another key importance of semiconductor NPs is its multiple applications in medicine, bio-imaging, drugs delivery, the coating of implantable devices, wound dressings, dental material, bone cement as well as personal care products (Smith and Nie, 2010; Pan et al., 2016; Wang et al., 2017). They can also prevent pathogenic microbes to restrict the biofilm formation. Biofilmforming bacteria such as S. aureus is one of the deadliest superbugs which is getting resistant to almost all kinds of drugs. NPs such as MgF2, Au-based NPs (Yu et al., 2016), AgBr-Ag-Bi2WO<sup>6</sup> (Zhang et al., 2010), Ag and Mg-based NPs (Lellouche et al., 2012; Markowska et al., 2013), ZnO, CuO, Fe3O4, TiO<sup>2</sup> and YF photocatalytic NPs are capable of inhibiting such microbes from developing biofilm (Wang et al., 2017).

Harmful algal bloom is another major problem in natural water bodies where the dissolved oxygen is extremely low, killing marine lives. Some of these algae produce toxins, algal toxins and metabolites such as domoic acid, brevotoxin, saxitoxin, microcystin, geosmin, anabena, nodularin etc. which are also detrimental to the health of the marine animals (Sivonen and Jones, 1999). As these harmful algae last for several months to years in the water bodies, these dead zones lost all forms of plants and marine animals while causing significant loss of environmental health, ecosystem, and economies. Thus, photocatalytic materials could play a potential role in decreasing such harmful algal blooms and preventing natural waters and different form of lives and ecosystem. The Ni-doped BiVO<sup>4</sup> photocatalyst exhibited the efficient algae inactivation under visible light irradiation while Co-BiVO4 degraded malachite green and inactivated harmful microorganisms in wastewater including green tides (Regmi et al., 2017b,c). Similarly, Ag-TiO<sup>2</sup> has shown efficient UV-photo elimination of harmful algal bloom (Rodríguez-González et al., 2010).

The multiple applications of photocatalytic NPs lead to discovering the modes of action of these particles in decomposing organic molecules to simpler form such as CO<sup>2</sup> and H2O and disabling actively growing pathogens from multiplying and reproducing. This potential of NPs has huge benefits to make the environment clean by eliminating toxic molecules and pathogens while keeping the entire ecosystem healthy.

The mechanisms of destruction of the microorganisms, algae, and various organic toxic molecules is different. Although several studies have been reported till now to explain the possible mechanism of degradation of organic compounds and killing of microbes, the antimicrobial mechanism of photocatalytic NPs has to be explored yet. The photocatalytic process which is based on AOPs' is considered as green technique for treatment of polluted water containing harmful microbes. In this process a semiconductor upon absorption of a photon of suitable energy (≥band gap energy) can act as a photocatalytic substrate by producing highly reactive radicals that can that can degrade indiscriminately micropollutants including harmful micro-organism present in wastewater (Pillai et al., 2017). But during the process majorities of the produced charges undergo recombination within the bulk/surface of photocatalyst, with the release of heat as a byproduct, before they are transferred to the adsorbate species creating the decrease in photocatalytic efficiency often greater than 90%. Thus, impurities substitution, lattice defects, and vacancies formation on the semiconductor can provide possible alternative pathways for the enhancement of photogenerated electrons and holes thus, prevent recombination.

The semiconductor photocatalyst on irradiation to light with energy equal or greater to its band gap energy produces an oxidative and reductive entity. In its first step, photo-generated holes and electrons are formed in the valence band (h<sup>+</sup> VB) and the conduction band (e<sup>−</sup> CB), respectively. Consequently, these photogenerated charge carriers then react with water or dissolved oxygen to produce reactive oxidizing species such as OH and O − 2 that decompose pollutants into smaller molecules as well as inactivate micro-organisms.

Photocatalyst + hν → e − CB + h + VB h + VB + H2O → ·OH + H + h + VB <sup>+</sup> OH<sup>−</sup> → ·OHad e − CB + O<sup>2</sup> → ·O − <sup>2</sup> <sup>→</sup>2H<sup>+</sup> <sup>H</sup>2O<sup>2</sup> <sup>→</sup>e<sup>−</sup> ·OH <sup>+</sup> OH<sup>−</sup> ·OH + pollutants/cellular constituent (micro − organism) + O<sup>2</sup> → simpler products (salts, CO2, H2O etc., ) ·OHad → ·OHfree + pollutants →simpler oxidation products

Photocatalytic materials when come in contact with microbes via different energy such as electrostatic attraction, hydrophobic interactions, van der Waals forces and receptor-ligand interactions, they exert their effect on the bacterial cell membrane and starts to influence the basic metabolism of the cells by various mechanisms. The cells could also experience different stresses such as oxidative stresses, membrane permeability imbalance, changes in cell shape, protein inhibition and alteration in metabolism and DNA damages. As shown in **Figure 1**, various reactive radicals generated by photocatalytic NPs on receiving the photon energy higher than the band gap energy enters to the cells and brings disorders to the cells' metabolism and gene expression (Wang et al., 2015). Single NP can also have multiple modes of actions (Hajipour et al., 2012). Thus, the exact mechanism of cell death caused by NPs is currently unknown. Currently understood mechanisms of destruction of microbes are discussed with examples henceforward. (i) Oxidative stress induction; in which the excess ROS, generated as a result of redox process, favors the oxidation process in the cells that leads to the peroxidation of lipid membrane and eventually attack proteins, depress the activity of certain periplasmic enzyme, and eventually interact with DNA and damage it. (ii) Metal ion release; where the metal ions released from metal oxide semiconductors are percolated through the cell membrane and directly interact with the –SH, -NH and –COOH group of nucleic acid and protein and finally damaging them. However, this method is less lethal than the other. Since detail studies of metal ion release on antimicrobial mechanism has not been carried out yet; it is not considered as the major cause of cell death (Hussein-Al-Ali et al., 2014). (iii) The non-oxidative mechanism involves the inactivation of microbes by decreasing the critical cellular metabolism such as protein, amino acid, nucleotide, energy, and carbohydrate metabolism without oxidative stress induction. The mechanism of non-oxidative stress cell death is poorly understood with very few MgO NP (Leung et al., 2014). Among these three possible mechanisms of antibacterial activities of NPs, the first mechanism has attracted the most attention of the researchers.

In general different types of ROS (·O − 2 , ·OH, H2O2,) are generated by NPs by reducing O<sup>2</sup> molecules (**Figure 1**). These ROS exert a different level of stress reactions to the cells to damage the cell components such as peptidoglycan layer, electron transport chain system, genomic materials (DNA, RNA), proteins, ribosomes etc. ROS alters the cell permeability by attacking the components of the cell membrane which changes cell integrity and release of cytoplasmic contents (Ansari et al., 2015; Huang et al., 2017; Ray et al., 2017). Moreover, ROS also depress the activities of various proteins essential for physiological processes in the cell (Padmavathy and Vijayaraghavan, 2011). H2O<sup>2</sup> is able to enter the cell membrane and damage cell components while increasing the expression pattern of oxidative stress-induced genes (Xu et al., 2013).

The usefulness of photocatalysis for the disinfection of water has been shown by the destruction effects it has on a wide range of micro-organisms i.e. bacteria, viruses, fungi and protozoa (Laxma Reddy et al., 2017; Ray et al., 2017). The disinfection efficiency of photocatalysis depends upon pH, chemical nature of bacterial suspension medium, photocatalyst type, size, surface morphology, specific surface area, structure, zeta potential, high surface energy, and atomic ligand deficiency, concentration, light intensity, and treatment time as well as the nature of the targeted organism. The accepted sequence of events which take place when micro-organisms undergo photocatalytic experience are thought to be an attack of the cell wall constituent by the hydroxyl radical produced during the catalysis process. These radicals being the strongest agent could break many organic covalent bonds, such as C-C, C-H, C-N, C-O, and H-O present in biomolecules such as carbohydrates, nucleic acid, proteins, amino acid, and even DNA (Wu et al., 2016; Laxma Reddy et al., 2017; Xu et al., 2018).

Even though several modes of cell death based on reactive radicals are proposed and elucidated in recent publications, the exact mechanism of the antibacterial mechanism of NPs is not understood yet and warrants further research. Since there

is variation in the type of NPs, strains, and environmental conditions used for antibacterial tests; it is very difficult to generalize the exact mechanism of killing of microbes by NPs. Different NPs exhibit different antibacterial activities over different strains. Thus, it is essential to bring such topics in the discussion forum for comprehensive analysis and elucidation of the exact mechanism of action of NPs. Several issues including entry of NPs to different cells having different structural integrity has to be elucidated. Moreover, the exact mechanism of NPs on protein degradation, DNA damage, and alteration of cells' basic metabolic pathways are hugely unknown.

green color arrow represents targets of Ag nanoparticle. Different targets in the microbial cells are labeled within the cell.

Understanding the real mechanism of inhibiting and destroying cells during photocatalytic disinfection helps to fabricate more efficient semiconductor photocatalyst by (1) tuning the morphology, (2) incorporating transition metal ions such as Ag, Au, Cu, Fe, or Ni and (3) designing heterojunction photocatalysts capable of utilizing efficient energy source such as sunlight (Guo et al., 2015; Podporska-Carroll et al., 2017). Upon getting insights into the mechanism of the photocatalytic bactericidal phenomenon of semiconductor NPs, the system could be employed for large-scale decontamination of environmental wastewater or industries and hospitals/biomedical labs generating highly pathogenic bacteria and toxic molecules containing liquid waste by designing a reactor. Moreover, designing of semiconductor NPs which can utilize the large fraction of the solar spectrum

#### the expense of energy. More importantly, if these semiconductor NPs are reusable, non-toxic, and easily synthesizable; the waste treatment system will be highly cost-effective and sustainable. Another important point is that microbes need significant alteration to overcome the bactericidal activity exerted by NPs. NPs act on wide target sites in the cell at a time, thus bacteria could not make such big changes in metabolism by making mutational changes in the genomes and bringing changes in the metabolism in short period of time. This eliminates the concern of pathogens getting resistance to such treatment systems. Not only wastewaters, such reactors can also be designed for different purposes such as disinfection of hospital medical utensils, detoxifying textile wastes, and inactivation of algal bloom in large water bodies.

(visible light) further expand the reactor's promiscuity without

#### AUTHOR CONTRIBUTIONS

CR, BJ, SR, GG, and RP wrote the manuscript. CR and BJ equally contributed to this article.

#### ACKNOWLEDGMENTS

This research was supported by grant from National Research Foundation of Korea to Ramesh Prasad Pandey (Grant No: 2017R1C1B5018056).

#### REFERENCES


against clinical isolates of multi-drug resistant Pseudomonas aeruginosa. World J. Microbiol. Biotechnol. 31, 153–164. doi: 10.1007/s11274-014-1 757-2

Berberidou, C., Paspaltsis, I., Pavlidou, E., Sklaviadis, T., and Poulios, I. (2012). Heterogenous photocatalytic inactivation of B. stearothermophilus endospores in aqueous suspensions under artificial and solar irradiation. Appl. Catal. B Environ. 125, 375–382. doi: 10.1016/j.apcatb.2012.06.005


in Advanced Oxidation Processes for Water Treatment: Fundamentals and Applications, ed. M. I. Stefan (London: IWA Publishing), 333–381.


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Regmi, Joshi, Ray, Gyawali and Pandey. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# A Green Desulfurization Technique: Utilization of Flue Gas SO<sup>2</sup> to Produce H<sup>2</sup> via a Photoelectrochemical Process Based on Mo-Doped BiVO<sup>4</sup>

Jin Han† , Kejian Li † , Hanyun Cheng and Liwu Zhang\*

*Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of Environmental Science and Engineering, Fudan University, Shanghai, China*

#### Edited by:

*Fan Dong, Chongqing Technology and Business University, China*

#### Reviewed by:

*Guangming Jiang, Chongqing Technology and Business University, China Hongwei Huang, China University of Geosciences, China Yanhui Ao, Hohai University, China*

> \*Correspondence: *Liwu Zhang zhanglw@fudan.edu.cn*

*† These authors have contributed equally to this work.*

#### Specialty section:

*This article was submitted to Catalysis and Photocatalysis, a section of the journal Frontiers in Chemistry*

Received: *27 October 2017* Accepted: *27 November 2017* Published: *12 December 2017*

#### Citation:

*Han J, Li K, Cheng H and Zhang L (2017) A Green Desulfurization Technique: Utilization of Flue Gas SO*2 *to Produce H*2 *via a Photoelectrochemical Process Based on Mo-Doped BiVO*4*. Front. Chem. 5:114. doi: 10.3389/fchem.2017.00114*

A green photoelectrochemical (PEC) process with simultaneous SO<sup>2</sup> removal and H<sup>2</sup> production has attracted an increasing attention. The proposed process uses flue gas SO<sup>2</sup> to improve H<sup>2</sup> production. The improvement of the efficiency of this process is necessary before it can become industrial viable. Herein, we reported a Mo modified BiVO<sup>4</sup> photocatalysts for a simultaneous SO<sup>2</sup> removal and H<sup>2</sup> production. And the PEC performance could be significantly improved with doping and flue gas removal. The evolution rate of H<sup>2</sup> and removal of SO<sup>2</sup> could be enhanced by almost three times after Mo doping as compared with pristine BiVO4. The enhanced H<sup>2</sup> production and SO<sup>2</sup> removal is attributed to the improved bulk charge carrier transportation after Mo doping, and greatly enhanced oxidation reaction kinetics on the photoanode due to the formation of SO2<sup>−</sup> 3 after SO<sup>2</sup> absorption by the electrolyte. Due to the utilization of SO<sup>2</sup> to improve the production of H2, the proposed PEC process may become a profitable desulfurization technique.

Keywords: hydrogen, sulfur dioxide, solar energy, Photoelectrochemical (PEC), Mo-doped BiVO4

## INTRODUCTION

Sulfur dioxide (SO2), as one of the acid gases, could transform into some atmospheric products (e.g., sulfate, sulfuric acid aerosol) through the chemical process in the atmosphere. It and its atmospheric products are detrimental to human health, and cause lots of environmental problems such as smog formation, acid deposition, and degradation of visibility. As is well-known, the SO<sup>2</sup> mainly enters the atmosphere through the anthropogenic processes, e.g., combustion and release of petroleum, fossil fuel, etc. (Lelieveld and Heintzenberg, 1992; McDonald-Buller et al., 2016). Till now, to remove SO<sup>2</sup> released from the burning of fossil fuels, several effective methods has been developed (Srinivasan and Grutzeck, 1999; Bashikova et al., 2001; Xia et al., 2011; Kaplan et al., 2013; Yang et al., 2013). Among these technologies, Wet Flue Gas Desulfurization (WFGD) has been one of the state-of-the-art technologies for SO<sup>2</sup> removal with high efficiency, simple equipment and obtaining multi-useful byproducts (Yang et al., 2012; Lu et al., 2013). Unfortunately, there are some inevitable drawbacks with the WFGD process: for example, the oxidation process energy of SO2<sup>−</sup> 3 to final SO2<sup>−</sup> 4 is wasted and it is not consistent with the sustainable principles. In order to solve this problem, our group proposed a solar-to-H<sup>2</sup> energy conversion with SO<sup>2</sup> removal simultaneously via a solar water splitting process for the first time (Han et al., 2017). Since the oxidation of SO2<sup>−</sup> 3 (formed during desulfurization process) needs much lower activation energy and 2 electrons than that of water, acted as a sacrificing reagent during the process, thus it could significantly improve the evolution rate of H2. This method not only utilizes the waste energy during the desulfurization process, also realizes the energy production by using air pollutants, which could achieve the zero release of SO2.

Though we have reported a photoresponse semiconductor (BiVO4) for H<sup>2</sup> generation with SO<sup>2</sup> removal, but it is still a challenge to facilitate the performance due to the intrinsic low mobility of photogenerated charges of BiVO4. Lots of methods have been proposed to solve this problem, such as controlling the morphology (McDonald and Choi, 2012; Kim and Choi, 2014; Zhou et al., 2014), metal and nonmetal doping (Jo et al., 2012; Chen et al., 2015; Huang H. et al., 2015; Huang H. W. et al., 2015; Huang et al., 2017), forming heterojunctions (Hong et al., 2011; Luo et al., 2011; Seabold and Choi, 2012; Zhang et al., 2016). These modifications have improved the properties of BiVO<sup>4</sup> greatly by reducing the band-gap energy or improving the charge carrier transport. It has been reported that Mo6<sup>+</sup> ion substitute the V site in monoclinic sheelite BiVO<sup>4</sup> could improve the photoinduced carriers, which could facilitate the oxidation while in water splitting reaction system theoretically and experimentally (Luo et al., 2011, 2013; Parmar et al., 2012; Ding et al., 2014; Park et al., 2014; Seabold et al., 2014; Zhou et al., 2014; Jiang et al., 2015, 2017; Kuang et al., 2016; Nair et al., 2016; Pattengale and Huang, 2016). However, Mo-doped BiVO<sup>4</sup> has not been studied as photoanode for flue gas SO<sup>2</sup> removal. Here, we prepared Mo-doped BiVO<sup>4</sup> for enhancing H<sup>2</sup> generation with simultaneous SO<sup>2</sup> removal. Moreover, the importance of the amount of Mo-dopants on the performance of BiVO<sup>4</sup> with SO<sup>2</sup> removal was also studied. Besides we prepared a series of Mo-doped BiVO<sup>4</sup> films with different content. The structure characterizations of the obtained films are investigated by XRD, SEM, Raman, UV-vis. Furthermore, we studied the performance of H<sup>2</sup> generation and efficiency of SO<sup>2</sup> removal.

## EXPERIMENT

#### Synthesis of the Catalysts

In this work, the deionized water (DI water) was used throughout the whole experiment, and all the chemical reagents are analytical grade and used without any further purification.

F-doped SnO<sup>2</sup> coated glass (FTO) were purchased from China Southern Glass Co. Ltd, and the FTO glasses were sonicated by immersing in acetone, ethanol and DI water for removing the impurities on the surface of the glass. For comparison, the pristine BiVO<sup>4</sup> was also prepared. All of the electrodes were synthesized by drop-coating method. The precursor solution was dropped onto the conducting side of FTO, followed by annealing in air. The precursor solutions were synthesized by the following procedure (Zhang et al., 2014): diethylene-etriaminepentaacetic acid (DTPA) and ammonia in water (13.0 mol L−<sup>1</sup> ) were added into hot deionized water. After dissolution, the stoichiometric Bi(NO3)3•5H2O, V2O<sup>5</sup> powder and moderate ammonium molybdate tetrahydrate (H24Mo7N6O24·4H2O) were added into sequence as listed. The resulted mixture was stirred and heated for an hour to promote the dissolution and reaction (complexation of Bi3+, V5+, and Mo6<sup>+</sup> with DTPA) until the mixture turned into a transparent solution. Here, we prepared three different samples with the content of Mo ranging from 1, 3, to 5. The amounts of doped Mo were 1 atom%, 3 atom%, 5 atom%, and were denoted as BiVO4(Mo-1), BiVO4(Mo-3), BiVO4(Mo-5), respectively. Then, 40 µl prepared solutions were dropped onto the conducting side of FTO (1 × 2 cm) respectively. After dried at 60◦ C in oven, the films were annealed at 500◦ C for 3 h with a ramping rate of 2◦ C min−<sup>1</sup> in air. The above process was repeated by three times for the synthesis of the electrode.

#### Characterization of the Samples

The crystal structures of the as-prepared samples were determined using an X-Ray Diffraction (XRD) with Cu Kα radiation ranging from 10 to 60◦ . The crystallite sizes of the samples were calculated using the Scherrer formula:

$$D = \frac{K\lambda}{\beta \cos \theta} \tag{1}$$

Where D is the average crystallite size (nm), λ is the wavelength of the X-ray radiation (0.154 nm), K is the shape factor (0.9), β is the peak width at half-maximum height, corrected for instrumental broadening, and 2θ = 28.7◦ . The micromorphology and the microstructure of the samples were determined by using field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan). UV-vis transmission spectra of the as-prepared catalysts were measured using a UV-Vis spectrophotometer (SHIMADZU UV-2600) with an integrating sphere attachment. BaSO<sup>4</sup> used as a standard. Raman spectra were recorded with a Raman spectrometer (HORIBA, X-plo RA Plus), a green laser (532 nm) were used as excitation sources.

#### PEC Measurement

The apparatus for the PEC tests was a gastight photoreactor. The PEC performance were conducted with a typical threeelectrode configuration by using a potentiostat (CHI 660E, Shanghai Chenhua Co. Lid. China). The synthesized pure BiVO<sup>4</sup> or Mo-doped BiVO<sup>4</sup> electrode was used as working electrode, Pt wire was used as counter electrode and Hg/HgO electrode was used as reference electrode. The electrolyte solutions for the PEC tests were prepared by absorbing SO<sup>2</sup> gas with NaOH solutions of certain concentration (detailed in **Table 1**). We bubbled SO<sup>2</sup> of concentration is 1,000 ppm successively to NaOH aqueous with the flow rate of 200 ml min−<sup>1</sup> to form Na2SO<sup>3</sup> with a specific concentration. Eventually, the obtained electrolytes for PEC tests were 0.1 M NaOH−0.025 M Na2SO<sup>3</sup> [denoted as NaOH(aq)+SO2(g)-1], 0.1 M NaOH-0.05 M Na2SO<sup>3</sup> [denoted as NaOH(aq)+SO2(g)-2], 0.1 M NaOH-0.075 M Na2SO<sup>3</sup> [denoted as NaOH(aq)+SO2(g)- 3]. For comparison, the NaOH (aq, 0.1 M) solution was also prepared. The PEC tests were measured under illumination by using a 300 W Xe lamp solar simulator with AM 1.5 G filter (100 mW cm−<sup>2</sup> ) from the back side of the working electrode, as well as in dark conditions. Liner Sweep Voltammetry (LSV) was measured with the sweep rate of 10 mV s−<sup>1</sup> . The structures of prepared BiVO<sup>4</sup> and Mo-doped BiVO<sup>4</sup> bulk were optimized by using the CASTEP code (Ding et al., 2014). The primitive cell of pure monoclinic sheelite BiVO<sup>4</sup> was relaxed using 400 eV energy cut off for the plane-wave expansion. The structural model of Mo-doped monoclinic sheelite BiVO<sup>4</sup> was built by substituting one V atom in a relaxed (2 × 1 × 2) supercell of monoclinic sheelite BiVO<sup>4</sup> with one Mo atom.

#### Hydrogen Evolution

The reactor for hydrogen evolution experiments is identical with the apparatus used for PEC tests, which use a two-electrode configuration. The electrolyte was NaOH solution bubbled with SO<sup>2</sup> gas. The experiments were conducted under illumination


using a 300 W Xe lamp with AM 1.5 G filter (100 mW cm−<sup>2</sup> ) from the back side of the photoanodes in a 150 ml reactor with 100 ml electrolytes filled in and the external bias was 1.6 V. The amount of H<sup>2</sup> was analyzed by gas chromatography using a thermal conduction detector (TCD) once an hour. The pH values of the solution during the PEC tests were detected by Ohaus (STARTER 2100). The theoretical evolution rate of H<sup>2</sup> is calculated according to the following equation:

$$\nu = \frac{I \times t}{Z \times F \times A \times 3600} \tag{2}$$

Where, v indicates the evolution rate of H<sup>2</sup> (mol cm−<sup>2</sup> h −1 ); I indicates the average current (A); t indicates the time (s); Z indicates the transferred electron number (1); F is the Faraday's constant (96,500 C mol−<sup>1</sup> ); A is the area of the film (cm<sup>2</sup> ).

The equation for the calculation of Faradaic efficiency is:

$$\text{Faradaic efficiency } (\%) = \frac{m \times n \times F}{I \times t} \times 100\% \tag{3}$$

Where, m is the experimental value of H<sup>2</sup> (mol); n is the reacted electron number (1); F is the Faraday's constant (96,500 C mol−<sup>1</sup> ); I is the average current (A); t is the time (s).

# RESULTS AND DISCUSSION

# Structure and Physical Properties of the Photoelectrode

The SEM (Scanning Electron Microscopy) images of pristine BiVO<sup>4</sup> and Mo-doped BiVO<sup>4</sup> films are shown in **Figure 1**. All of the films present nanoparticle structure while a rougher, more disordered structure can be observed in the BiVO<sup>4</sup> films. Besides, the incorporation of Mo could decrease the average size of BiVO<sup>4</sup> with decreasing aggregation of the particles. The average diameter of pure BiVO<sup>4</sup> is about 280 nm (**Figure 1a**), and the average diameter of Mo-doped BiVO<sup>4</sup> are ranging from 200 to 250 nm, which are smaller than that of pure BiVO<sup>4</sup> (**Figures 1b–d**). Besides, the smaller particle size of the asprepared photocatalyst could provide more available active sites, which could be in favor of the PEC performance.

**Figure 2A** compares the XRD (X-Ray Diffraction) patterns of pure and Mo-doped BiVO<sup>4</sup> films with different concentrations of Mo dopant. All the diffraction peaks are assigned to phase-pure monoclinic sheelite BiVO<sup>4</sup> structure (JCPDS No. 14-0688). Since the films are very thin, the diffraction peaks of FTO substrate are also observed in XRD. No noticeable peaks of MoO<sup>3</sup> is detected in the Mo-doped BiVO4, the results are consistent with previous study (Berglund et al., 2012; Luo et al., 2013; Chen et al., 2015; Nair et al., 2016; Thalluri et al., 2016). Additionally, the main peaks of the monoclinic BiVO<sup>4</sup> structure shift to lower intensity and higher scattering angles in the Mo-doped BiVO<sup>4</sup> films since the radius of Mo is higher than that of V. This represents a shrinkage or an enlargement of the d spacing of corresponding crystal planes due to incorporation of dopant cations into V sites of BiVO<sup>4</sup> (Parmar et al., 2012). As the pure phase and modification in d spacing, it can be calculated that the Mo have been effectively incorporated into the crystal lattice of BiVO<sup>4</sup> with the monoclinic phase unchanged (Parmar et al., 2012). Furthermore, the crystallite sizes calculated by using the Scherrer formula are 110, 117, 83, 85 nm for the BiVO4, BiVO4(Mo-1), BiVO4(Mo-3), BiVO4(Mo-5), respectively.

Though XRD shows some trace that Mo have been doped in the crystal of pure BiVO4. However, it is too rough to identify the doping sites in the crystal lattice because of the low doping concentration. To probe the doping sites and local distortions of Mo-doped BiVO4, the Raman spectra were measured and the results are shown in **Figure 2B**. The Raman mode located at 829 cm−<sup>1</sup> is assigned to the symmetric stretching mode of VO3<sup>−</sup> 4 units. It is clear that the symmetric stretching mode in Mo-doped BiVO<sup>4</sup> shifts to a lower wave number, which suggests Mo6<sup>+</sup> substitutes V5<sup>+</sup> in the VO3<sup>−</sup> 4 tetrahedron (Luo et al., 2013; Zhang et al., 2014).

Optical properties of the films are very important for the PEC performance. **Figure 3A** displays the UV-Vis DRS absorption

spectra of the four films. All of the films present a strong absorption in the UV-Vis range, and the incorporation of Mo can hardly affect the absorption edges of BiVO4. Besides, there isn't any peak shift in the UV-Vis spectra of Mo-doped films, which indicates there is not existence of the phase transfer from monoclinic to tetragonal structures with doping of Mo (**Figure 3A**; Pattengale and Huang, 2016). As the pure BiVO<sup>4</sup> shows a larger optical absorbance at wavelengths (>325 nm), thus the pure BiVO<sup>4</sup> are less porous than the Mo-doped BiVO<sup>4</sup> (Nair et al., 2016).

The optical band gap energy can be calculated by the following equation:

$$
\alpha h\nu = A(h\nu - E\_{\text{g}})^{n/2} \tag{4}
$$

where, α, hν, A, and E<sup>g</sup> are the absorption coefficient, photo energy, constant and band gap energy, respectively. The value of n depends on whether the transition is direct (n = 1) or indirect (n = 4). The bandgap of all the films are about 2.5 eV (**Figure 3B**), which is consistent with the reported band gap of BiVO<sup>4</sup> (Zhang et al., 2014). This result indicates that the incorporation of low amount of Mo could hardly influence the bandgap of BiVO4, and it is considered to be the characteristic band gap of monoclinic phase of BiVO4.

In the experiments, the SO<sup>2</sup> gas was absorbed by NaOH solutions and the concentration of SO2<sup>−</sup> <sup>3</sup> was detected. Through analyzing, the removal efficiencies of SO<sup>2</sup> gas were about 98%, it indicated that the absorption method could removal SO<sup>2</sup> completely. In order to determine the PEC properties of the photoelectrodes, the Linear Sweep Voltammograms (LSV) were measured both in dark and under AM 1.5 G illumination (100 mW cm−<sup>2</sup> ; **Figure 4**). After analyzing the photocurrent densities of the photoanodes in different electrolyte systems after absorbing SO<sup>2</sup> (**Figure 4A**), it is concluded: (1) the photocurrent densities are negligible under dark conditions in different electrolyte systems; (2) the photocurrent densities could be significantly enhanced after inletting SO<sup>2</sup> into electrolyte. Since the introduction of SO<sup>2</sup> into electrolyte, the concentration of SO2<sup>−</sup> 3 could be increased, and the oxidation reaction of SO2<sup>−</sup> 3 needs lower activation energy and kinetically faster than that of water, thus the formed SO2<sup>−</sup> 3 consumes the photogenerated holes instantaneously and generates a higher photocurrent density than that of water. Though the doping of Mo could decrease the light-absorbing slightly (**Figure 3A**), but it could significantly enhance the bulk charge carrier transportation of BiVO4. Therefore, the PEC performance of Mo-doping BiVO<sup>4</sup> is higher than that of BiVO4. The photocurrent density is greatly dependent on the amount of SO<sup>2</sup> absorbed in the electrolyte. In NaOH(aq)+SO2(g)-3, the photocurrent density is improved by 1.2 times, 1.6 times, and 5 times than that of NaOH(aq)+SO2(g)-2, NaOH(aq)+SO2(g)-1, NaOH(aq) at 0.5 V vs. Hg/HgO, respectively. The photocurrent density is significantly improved by 5 times as compared with our previous research on porous BiVO<sup>4</sup> (Han et al., 2017).

Compared with the four different electrode films in NaOH(aq)+SO2(g)-3 electrolyte, it is clear that BiVO4(Mo-3) displayed an outstanding PEC performance. The photocurrent density of BiVO4(Mo-3) is 1.7 times than that of BiVO4(Mo-1), 2 times than that of BiVO4(Mo-5), and 5 times than that of BiVO<sup>4</sup> at 0.5 V vs. Hg/HgO (**Figure 4B**). The current densities of BiVO4(Mo-3) in NaOH(aq) with and without SO2(g)-3 at 0.6 V vs. Hg/HgO are ca. 0.6 and 0.1 mA cm−<sup>2</sup> , respectively; the current density in NaOH(aq)+SO2(g)-3 is 6 times higher than that in NaOH(aq). On the other hand, the evolution rate of H<sup>2</sup> in NaOH(aq)+SO2(g)-3 (39.4 µmol h−<sup>1</sup> cm−<sup>2</sup> ) is more than 40 times higher than that in NaOH (0.92 µmol h−<sup>1</sup> cm−<sup>2</sup> ) in the **Table 2**. The above results indicate that the amount of Mo at 3 atom% could be an optimal choice.

The experiments for H<sup>2</sup> evolution in different systems were measured with a two-electrode configuration at a bias of 1.6 V under AM 1.5G irradiation. **Figure 5A** shows the amount of H<sup>2</sup> evolved in different systems and **Table 2** summarized the data of current densities, theoretical/ experimental evolution rates of H<sup>2</sup> and the Faradaic efficiencies in the different systems. Comparing each photoanodes in different solution systems, it is clear that the SO<sup>2</sup> removal could significantly facilitate the current density and evolution rate of H2, which is in good agreement with our previous research (Han et al., 2017). Besides, the current density and evolution rates for BiVO4(Mo-1, 3, 5) under light irradiation are higher than that under the dark condition. Furthermore, all of the Mo doped BiVO<sup>4</sup> photoanodes show more attractive performance than that of pure BiVO4, and the BiVO4(Mo-3) performed the best H<sup>2</sup> generation activities. As calculated, a highest H<sup>2</sup> evolution rate of 39.4 µmol h−<sup>1</sup> cm−<sup>2</sup> is realized in NaOH(aq)+SO2(g)-3 with BiVO4(Mo-3) as photoanode, and the H<sup>2</sup> evolution rate is only 0.19 µmol h−<sup>1</sup> cm−<sup>2</sup> in NaOH(aq) with the BiVO<sup>4</sup> as photoanode, suggesting the H<sup>2</sup> production can be enhanced about 200 times with the removal of SO<sup>2</sup> simultaneously with 3 atom% Mo-doped BiVO4. Furthermore, the theoretical evolution rates of H<sup>2</sup> are close to theoretical rates in each system, indicating a high Faradaic efficiency of H<sup>2</sup> production (higher than 95%). Besides, the catalysts could not dissolve although the solution is a strong basic aqueous solution.

FIGURE 5 | (A) Hydrogen generation at an applied potential of 1.6 V in two-electrode configuration for 3 h; (B) The variation of pH values of electrolytes during the process.

TABLE 2 | The current density, theoretical and experimental evolution rate of H2, and the Faradaic efficiency in different electrolytes [A−0.1 M NaOH; B—NaOH(aq)+SO2(g)-3].


In order to better understand the structure reconstruction with/without Mo doping, the geometric structures of pristine and the Mo-doped BiVO<sup>4</sup> were compared in **Figure 6**. It is clear that the doping of Mo could increase the length of Bi-O bonds. This phenomenon indicates that the original coordinated O atoms are "shifted" toward the doped Mo sites, and introduce "oxygen vacancies." As well-known, the oxygen vacancies could play an important role in improving the photocatalytic performance. Besides, the Mo6<sup>+</sup> ion substitute the V site in monoclinic sheelite BiVO<sup>4</sup> could improve the transportation of photoinduced carriers, which could facilitate the charge carrier separation and leads to suppressed bulk recombination (Ding et al., 2014). Therefore, the photocatalytic activity is enhanced significantly after Mo-doping, as shown in **Figure 4**.

With the existence of flue gas SO<sup>2</sup> removal, SO2<sup>−</sup> <sup>3</sup> was formed in electrolyte solution. Then the photo-generated holes could be more quickly consumed by the formed SO2<sup>−</sup> 3 than that without SO2<sup>−</sup> 3 (pure NaOH solution), because oxidation of SO2<sup>−</sup> 3 has a much lower activation energy and kinetically much faster than water oxidation or OH<sup>−</sup> oxidation (McDonald and Choi, 2012; Seabold and Choi, 2012). The oxidation of SO<sup>2</sup> is thus proposed to replace the oxygen evolution reaction in water splitting, which can improve the water splitting efficiency and reduce the cost of H<sup>2</sup> production. The whole process for SO<sup>2</sup> removal with simultaneous production of H<sup>2</sup> is thus summarized and illustrated in **Figure 7**. Mo-doped BiVO<sup>4</sup> is first excited to generate e <sup>−</sup> in conduction band (CB) and create hole (h <sup>+</sup>) in the valence band (VB) at the same time. With the assistance of space charge layer and extra bias, the e <sup>−</sup> transfer to the cathode and participate in cathode reaction for H<sup>2</sup> production. While the formed of SO2<sup>−</sup> 3 after flue gas SO<sup>2</sup> absorption is oxidized by the holes on the photoanode. As the Mo incorporated in BiVO<sup>4</sup> could improve the electronic conductivity of pure BiVO<sup>4</sup> (Luo et al., 2011; Pilli et al., 2011), therefore, the e <sup>−</sup> produced in Mo doped

BiVO<sup>4</sup> moves faster than that produced in BiVO4, and shows a significantly increased photocurrent density (**Figure 4**) and rate of H<sup>2</sup> production (**Figure 5**). The reactions are summarized as following:

$$\text{SO}\_2\text{(g)} + \text{OH}^- \rightarrow \text{HSO}\_3^- \tag{5}$$

$$\mathrm{HSO\_3^-} + \mathrm{OH^-} \rightarrow \mathrm{SO\_3^{2-}} + \mathrm{H\_2O} \tag{6}$$

Reaction on photoanode (Mo-doped BiVO<sup>4</sup> film):

$$\rm SO\_3^{2-} + 2OH^- \rightarrow SO\_4^{2-} + H\_2O + 2e^- \tag{7}$$

Reaction on cathode (Pt wire):

$$2\text{H}\_2\text{O} + 2e^- \rightarrow \text{H}\_2\text{(g)} + 2\text{OH}^- \tag{8}$$

The total reaction:

$$\text{SO}\_2\text{(g)} + 2\text{OH}^- \overset{\text{hy}}{\rightarrow} \text{H}\_2\text{(g)} + \text{SO}\_4^{2-} \tag{9}$$

#### CONCLUSIONS

In summary, the effect of Mo doping on BiVO<sup>4</sup> used as photoanode for H<sup>2</sup> production with simultaneously flue gas

#### REFERENCES


SO<sup>2</sup> removal is investigated. The 3 atom% Mo-doped BiVO<sup>4</sup> (BiVO4(Mo-3)) possessed the best PEC activity due to its better charge carrier transportation. With the help of Mo, the H<sup>2</sup> evolution rate and SO<sup>2</sup> removal rate of Mo-doped BiVO<sup>4</sup> almost 3 times higher than pristine BiVO4. Through this process, the SO<sup>2</sup> in flue gas is removed and collected to produce H2, which could greatly reduce the cost of desulfurization process, and even make it profitable.

#### AUTHOR CONTRIBUTIONS

JH: assisted in design of the experiments and wrote the manuscript; KL: performed the experiments and wrote the manuscript; HC: assisted in the analysis and interpretation of the data; LZ: planned the project, designed the experiments, and also wrote the manuscript.

#### ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. 21507011 and No. 21677037), and Ministry of Science and Technology of the People's Republic of China (2016YFE0112200 and 2016YFC0203700).

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**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer, GJ, and handling Editor declared their shared affiliation.

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