Ti4O7/g-C3N4 for Visible Light Photocatalytic Oxidation of Hypophosphite: Effect of Mass Ratio of Ti4O7/g-C3N4

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 (·${\text{O}}_2^{-}$) generated during the photocatalytic process (Hao et al., 2017). The most commonly used TiO₂ 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-C₃N₄ (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 (Ti_nO_2n−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 Ti_nO_2n−1, Ti₄O₇ has the properties of best conductivity (1,500 S cm⁻¹) and the robust resistance to aggressive chemical conditions (Ganiyu et al., 2016). Construction of g-C₃N₄/Ti₄O₇ 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 Ti₄O₇ (m): g-C₃N₄ (m) of the g-C₃N₄/Ti₄O₇ heterojunction on photocatalytic performance is still unknown. In this study, the effect of the mass ratio of Ti₄O₇/g-C₃N₄ (Ti₄O₇ (m): g-C₃N₄ (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 ·${\text{O}}_2^{-}$ radicals generated during the photocatalytic process. Our results indicated that Ti₄O₇/g-C₃N₄ 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-C₃N₄ and Ti₄O₇, respectively).

Materials and Methods

Chemicals

The reagents used for the preparation and performance characterization of g-C₃N₄/Ti₄O₇ 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 MΩ cm).

Preparation of Ti₄O₇/g-C₃N₄ Photocatalysts

Graphite-like carbon nitride was prepared first using a liquid-based growth method (Sun et al., 2018). Then, g-C₃N₄ (2 g) and Ti₄O₇ (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 as-prepared g-C₃N₄/Ti₄O₇ with various mass ratios (Ti₄O₇ (m): g-C₃N₄ (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-C₃N₄ and 1:20-Ti₄O₇/g-C₃N₄, respectively.

Analysis and Test Methods

The as-prepared g-C₃N₄/Ti₄O₇ 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/Ti₄O₇ were calculated by Brunauer-Emmett-Teller (BET) equation and Barrett-Joyner-Halenda (BJH) method according to the N₂ adsorption/desorption isotherms. The optical and electrochemical properties of the as-prepared g-C₃N₄/Ti₄O₇ 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 ·${\text{O}}_2^{-}$ 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 N₂ purging.

Analysis of Photocatalytic Performance

The photocatalytic performance of g-C₃N₄/Ti₄O₇ 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⁻² 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/Ti₄O₇ with various mass ratios (10 mg) dispersed in 100 mg L⁻¹ 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 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):

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

where C₀ and C_t represent the concentrations of hypophosphite at initial and given time, respectively.

Results and Discussion

Materials Characterization

The SEM images of pure g-C₃N₄ and Ti₄O₇/g-C₃N₄ photocatalysts were shown in Figure 1 that g-C₃N₄ had a sheet-like structure (Figure 1A) and spheroidal Ti₄O₇ crystals were deposited on the surface of C₃N₄ (Figure 1B)_. The corresponding XPS high resolution spectra of Ti₄O₇/g-C₃N₄ 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 sp² bonded C in N=C(–N)₂ (288.3 eV) (Jo and Natarajan, 2015). With regard to the N 1s spectra, there were three peaks (Figure 1D), the main peak at 398.8 eV assigned to sp² 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)₃ (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-C₃N₄ reported previously. Meanwhile, Ti₄O₇ is a mixed-valence compound with two evenly occupied Ti⁴⁺ (3d⁰) and Ti³⁺ (3d¹) 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 2p_1/2 and Ti 2p_3/2 peaks of Ti⁴⁺, and another two peaks at 457.9 eV and 463.8 eV were assigned to Ti³⁺ (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 Ti₄O₇ on the surface of g-C₃N₄ with covalent bonds.

FIGURE 1

Figure 1. The SEM images of (A) g-C₃N₄, (B) Ti₄O₇/g-C₃N₄ photocatalysts; and XPS spectrum of (C) C 1s, (D) N 1s, (E) Ti 2p, (F) O 1s of Ti₄O₇/g-C₃N₄ photocatalyst.

The XRD phase structures of Ti₄O₇/g-C₃N₄ 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 Ti₄O₇ 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 Ti₄O₇ (m): g-C₃N₄ (m), the intensity of the Ti₄O₇ peaks became stronger while that of the C₃N₄ peaks became weaker. Furthermore, Ti₄O₇/g-C₃N₄ especially 1:2-Ti₄O₇/g-C₃N₄ (mass ratio of Ti₄O₇/g-C₃N₄ of 0.5) matched well with the reference of pure Ti₄O₇ and g-C₃N₄, indicating that the main structures of Ti₄O₇ and g-C₃N₄ were not destroyed during the synthesis process of Ti₄O₇/g-C₃N₄.

FIGURE 2

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

The as-prepared Ti₄O₇/g-C₃N₄ photocatalysts with various mass ratios were further characterized by FTIR as shown in Figure 2B. Typical absorption peaks of 1,230–1,630 cm⁻¹ were attributed to tri-s-triazine ring moieties of g-C₃N₄. For example, the absorption peaks of 1,638 and 1,570 cm⁻¹ were related to C = N stretching, and the peaks of 1,474, 1,410, 1,322, 1,241 cm⁻¹ were attributed to C–N stretching. Meanwhile, the sharp peak of 810 cm⁻¹ was due to the bending vibration of heptazine rings, indicating that the heptazine units might exist in C₃N₄ (Hatamie et al., 2018). With increase of the mass ratio of Ti₄O₇/g-C₃N₄, the absorption intensity related to the vibrational bands of g-C₃N₄ became weaker. The presence of Ti₄O₇ in Ti₄O₇/g-C₃N₄ restricted the evolution trend of g-C₃N₄ and thus alleviated the severe stacking of aromatic units of g-C₃N₄. Additionally, the delocalized π-π conjugated electronic system of g-C₃N₄ 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-C₃N₄ and 1:2-Ti₄O₇/g-C₃N₄ photocatalysts were shown in Figure 3. The specific surface area calculated according to Figure 3A was 116.0, 56.5, and 174.0 m²·g⁻¹ for Ti₄O₇, g-C₃N₄ 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 Ti₄O₇ and g-C₃N₄, 1:2-Ti₄O₇/g-C₃N₄ with larger specific surface area would facilitate photocatalytic oxidation of hypophosphite. Moreover, the pore size distribution of 1:2-Ti₄O₇/g-C₃N₄ was between 5 and 15 nm and that of pure Ti₄O₇ was mainly in the range of 10–25 nm, while no obvious mesopores were observed for g-C₃N₄ (Figure 3B).

FIGURE 3

Figure 3. (A) N₂ adsorption-desorption isotherms and (B) pore size distribution of g-C₃N₄, pure Ti₄O₇ and 1:2-Ti₄O₇/g-C₃N₄ photocatalysts.

Photocatalytic Performance Analysis

The photocatalytic performance of different photocatalysts was analyzed and compared as shown in Figure 4A. Ti₄O₇/g-C₃N₄ exhibited the significantly improved efficiency in hypophosphite oxidation under visible light irradiation (λ > 420 nm) in comparison to the pure g-C₃N₄ and Ti₄O₇. Additionally, the oxidation efficiency of hypophosphite was increased with increase of the mass ratio of Ti₄O₇/g-C₃N₄ and 1:2-Ti₄O₇/g-C₃N₄ 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 (R² > 0.95) as shown in Figure 4B and the rate constant of 1:2-Ti₄O₇/g-C₃N₄ was 17.7-fold and 91.0-fold higher than that of pure g-C₃N₄ and Ti₄O₇, respectively. The low efficiency of photocatalytic oxidation of hypophosphite for pure g-C₃N₄ was possibly because of the rapid combination of electron-hole pairs (Yan et al., 2018). Similarly, the photocatalytic performance of pure Ti₄O₇ was extremely limited herein, which was mainly due to the wide band gap of 2.9 eV of pure Ti₄O₇ (Maragatha et al., 2017). Nevertheless, the efficiency of photocatalytic oxidation of hypophosphite was remarkably improved especially for 1:2-Ti₄O₇/g-C₃N₄ possibly as a result of the Ti₄O₇/g-C₃N₄ 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.

FIGURE 4

Figure 4. (A) The oxidation efficiency of hypophosphite for different photocatalysts; (B) Plot of ln(C₀/C_t) against reaction time for the catalytic oxidation of hypophosphite using different photocatalysts.

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-C₃N₄ showed a distinct red-shift in comparison to pure g-C₃N₄ 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 Ti₄O₇ (m): g-C₃N₄ (m), indicating that the Ti₄O₇/g-C₃N₄ heterojunction with good interaction between Ti₄O₇ and g-C₃N₄ facilitated the enhancement of visible-light harvesting (Chang et al., 2018). The band gap of Ti₄O₇/g-C₃N₄ was determined such that (Ai et al., 2018):

${(\alpha \text{h}\nu )}^2=A(\text{h}\nu -{\text{E}}_{\text{g}})(2)$

where α is the optical absorption coefficient; h is the Plank's constant; ν is the photonic frequency; A is the proportionality constant; E_g is the band gap.

FIGURE 5

Figure 5. UV–vis DRS spectra of different photocatalysts.

The band gap followed the order: 1:2-Ti₄O₇/g-C₃N₄ (2.07 eV) < 1:5-Ti₄O₇/g-C₃N₄ (2.25 eV) < 1:10-Ti₄O₇/g-C₃N₄ (2.33 eV) < 1:20-Ti₄O₇/g-C₃N₄ (2.43 eV) < pure g-C₃N₄ (2.70 eV). As such, the narrowed band gap of Ti₄O₇/g-C₃N₄ 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-C₃N₄ increased with the increase of the mass ratio of Ti₄O₇/g-C₃N₄. 1:2-Ti₄O₇/g-C₃N₄ had the highest photocurrent density of 0.30 μA cm⁻², while the photocurrent density for pure g-C₃N₄ was only 0.10 μA cm⁻². The enlarged photocurrent density of Ti₄O₇/g-C₃N₄ suggested the more efficient charge carrier (e⁻-h⁺) generation on the Ti₄O₇/g-C₃N₄ surface (Liu et al., 2017). Herein, the electrons in the valence band of g-C₃N₄ were excited upon visible light irradiation and then migrated to the conduction band of Ti₄O₇ with effectively enhanced efficiency of the charge carrier (e⁻-h⁺) 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-C₃N₄ switched reversibly and was unchanged after repetitive ON/OFF illumination cycles, indicating the good photoelectrochemical stability of Ti₄O₇/g-C₃N₄.

FIGURE 6

Figure 6. Electrochemical properties analysis: (A) photocurrent density and (B) EIS Nyquist plots of different photocatalysts.

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-C₃N₄ 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-C₃N₄ indicated that 1:2-Ti₄O₇/g-C₃N₄ in comparison to pure g-C₃N₄ would also facilitate the photocatalytic oxidation of hypophosphite as evident in Figure 4.

Proposed Mechanism

Reactive oxygen species (ROS) including ·${\text{O}}_2^{-}$ and ·OH would be generated during the photocatalytic system (Huang et al., 2017a,b). Herein, the contributions of ·OH and ·${\text{O}}_2^{-}$ 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 N₂ purging applied to reduce the superoxide ·${\text{O}}_2^{-}$ 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 ·${\text{O}}_2^{-}$ 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).

FIGURE 7

Figure 7. Effect of radical scavengers on the photocatalytic oxidation of hypophosphite by the 1:2-Ti₄O₇/g-C₃N₄ photocatalyst.

The photocatalytic mechanism of Ti₄O₇/g-C₃N₄ was proposed as shown in Figure 8. Under visible light irradiation, electrons in the valence band of g-C₃N₄ were excited and then the excited electrons migrated to the conduction band of Ti₄O₇ through the heterojunction surface of Ti₄O₇/g-C₃N₄ with the generation of holes in the valence band of g-C₃N₄ (Zhu et al., 2018). The electrons on the surface of Ti₄O₇ could easily react with O₂ adsorbed on the Ti₄O₇/g-C₃N₄ surface to generate ·${\text{O}}_2^{-}$ radicals for hypophosphite oxidation. Meanwhile, the active ·OH radicals with high redox potential were generated through ·${\text{O}}_2^{-}$ 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 ·${\text{O}}_2^{-}$ radicals generated during the photocatalytic process as shown in Figure 8.

FIGURE 8

Figure 8. Schematic of the mechanism of photocatalytic oxidation of hypophosphite by Ti₄O₇/g-C₃N₄ photocatalysts.

Analysis of Stability

The stability of photocatalyst is another vital consideration to evaluate the photocatalytic performance. As shown in Figure 9, relatively robust reusability was exhibited by 1:2-Ti₄O₇/g-C₃N₄ 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.

FIGURE 9

Figure 9. The stability analysis of the 1:2-Ti₄O₇/g-C₃N₄ photocatalyst.

Conclusion

In this work, Ti₄O₇/g-C₃N₄ with various mass ratios (Ti₄O₇ (m): g-C₃N₄ (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-C₃N₄ on Ti₄O₇/g-C₃N₄ visible light photocatalytic oxidation of hypophosphite was evaluated. Ti₄O₇/g-C₃N₄ exhibited remarkably improved photocatalytic performance on hypophosphite oxidation in comparison to pure g-C₃N₄ and 1:2-Ti₄O₇/g-C₃N₄ 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-C₃N₄ and Ti₄O₇, respectively). The heterojunction structure of Ti₄O₇/g-C₃N₄ with broader light absorption significantly enhanced the efficiency of the charge carrier (e⁻-h⁺) generation and separation. ·OH and ·${\text{O}}_2^{-}$ 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).

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.

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