Ag3VO4 Nanoparticles Decorated Bi2O2CO3 Micro-Flowers: An Efficient Visible-Light-Driven Photocatalyst for the Removal of Toxic Contaminants

Semiconductor-based photocatalysis is of great potential for tackling the environmental pollution. Herein, a novel hierarchical heterostructure of Bi2O2CO3 micro-flowers in-situ decorated with Ag3VO4 nanoparticles was developed by a facile method. Various characterization techniques have been employed to study the physical and chemical property of the novel catalyst. The novel catalyst was utilized for the photocatalytic removal of industrial dyes (rhodamine B, methyl orange) and tetracycline antibiotic under visible-light irradiation. The results indicated that Ag3VO4/Bi2O2CO3 heterojunctions showed a remarkably enhanced activity, significantly higher than those of bare Ag3VO4, Bi2O2CO3, and the physical mixture of Ag3VO4 and Bi2O2CO3 samples. This could be ascribed to an enhanced visible-light harvesting capacity and effective separation of charge carriers by virtue of the construction of hierarchical Ag3VO4/Bi2O2CO3 heterojunction. Moreover, Ag3VO4/Bi2O2CO3 also possesses an excellent cycling stability. The outstanding performance of Ag3VO4/Bi2O2CO3 in removal of toxic pollutants indicates the potential of Ag3VO4/Bi2O2CO3 in real environmental remediation. Highlights Novel architectures of Ag3VO4 nanoparticles modified Bi2O2CO3 micro-flowers were constructed. Novel Ag3VO4/Bi2O2CO3 exhibited excellent photocatalytic activity and stability. Ag3VO4/Bi2O2CO3 heterojunctions significantly promote the charge separation.

The emergent Bi 2 O 2 CO 3 has attracted much interest for its good photocatalytic performance in the removal of toxic pollutants (Ni et al., 2016;Yu et al., 2018). However, the photocatalytic properties are still far from satisfactory owing to low solar utilization and fast recombination of electronhole pairs. To improve the visible-light photocatalytic activity of Bi 2 O 2 CO 3 , various strategies have been developed, such as design of microstructure (Zhao et al., 2011), deposition of metals (Yu et al., 2016), formation of heterojunction Huang et al., 2016;Feng et al., 2017;Hu et al., 2017), and doping with ions (Dong et al., 2014;Xiong et al., 2015). The rational construction of heterojunctions can effectively ameliorate the visible-light absorption ability and significantly suppress the electron-hole recombination (Han et al., 2017;Li et al., 2017b,c,e;Zhong et al., 2018). The further development of novel Bi 2 O 2 CO 3based catalysts is still required to offer more potential candidates for practical application and to figure out the reasons for the synergetic effect between the components. Ag 3 VO 4 , an active VLD photocatalyst, has drawn much attention in virtue of its unique band structures. Ag 3 VO 4 has been coupled with other semiconductors (e.g., Bi 2 WO 6 Li et al., 2017a;Zhang and Ma, 2017, BiOI Wang et al., 2015, BiOCl Wang et al., 2016, C 3 N 4 Wang et al., 2014, BiVO 4 Yan et al., 2016a, WO 3 Yan et al., 2016b to fabricate high-performance photocatalysts. To date, researches on Ag 3 VO 4 nanoparticles decorated Bi 2 O 2 CO 3 micro-flowers for the visible-light photodegradation of toxic contaminants have not been reported.
Herein, we report Ag 3 VO 4 nanoparticles evenly deposited on the surface of Bi 2 O 2 CO 3 micro-flowers by a simple precipitation method. Ag 3 VO 4 nanoparticles can optimize the visible-light response and facilitate the separation of charge carriers, endowing the novel heterojunction with excellent visible-light photocatalytic performance. The plausible visiblelight photocatalysis mechanism of Ag 3 VO 4 /Bi 2 O 2 CO 3 is also proposed.

Synthesis of Catalysts
Bi 2 O 2 CO 3 was synthesized via a hydrothermal procedure. Briefly, 2 mmol of sodium carbonate (Na 2 CO 3 ) and 2 mmol of bismuth citrate (BiO 7 C 6 H 5 ) were sequentially dissolved in the solution containing 30 mL of deionized water and 5 mL of absolute ethanol with the assistance of ultra-sonication. The resulting solution was sealed in a 50 mL autoclave and heated at 160 • C for 25 h. After the reactor system was cooled down, the precipitants were washed thoroughly with de-ionized water and dried at 80 • C overnight.
Ag 3 VO 4 /Bi 2 O 2 CO 3 heterojunctions were constructed by a simple precipitation method. Briefly, an appropriate amount of Bi 2 O 2 CO 3 was ultrasonically suspended in 50 mL of H 2 O. Then, 3 mmol AgNO 3 was dissolved in the above solution under magnetical stirring. After that, Na 3 VO 4 (20 mL, 0.05 mol L −1 ) solution was slowly dropped into the mixture with vigorous stirring for 5 h. Lastly, the obtained solids were washed with deionized water four times and dried at 80 • C for 10 h to get the Ag 3 VO 4 /Bi 2 O 2 CO 3 heterojunctions. The heterojunctions with different Bi 2 O 2 CO 3 /Ag 3 VO 4 weight ratios of 0.05/1, 0.10/1, 0.30/1, and 0.50/1 are labeled as AVO/BOC-5, AVO/BOC-10, AVO/BOC-30, and AVO/BOC-50, respectively. Ag 3 VO 4 was prepared in the absence of Bi 2 O 2 CO 3 .

Characterization of Catalysts
The scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEM-2100 JEOL) were applied to characterize the morphology of the samples. Bruker Quantax 400 energy-dispersive X-ray spectroscopy (EDS) was used to identify the chemical composition. Powder X-ray diffractometer (XRD, MSAL XD2) was used to get the XRD patterns of the samples. UV-Vis diffuse reflectance spectra (DRS) were obtained on a spectrophotometer (Shimadzu UV−2600). Photoluminescence (PL) spectra of the samples were recorded on a Hitachi RF-6000 spectrophotometer.

Photocatalytic Tests
Pollutant [rhodamine B (RhB), methyl orange (MO), and tetracycline hydrochloride (TC)] removal performances were tested under visible-light irradiation, 300 W xenon lamp with filter (λ > 400 nm). The photocatalytic reaction was conducted   in a glass reactor containing 80 mL of RhB (5 mg L −1 ), MO (5 mg L −1 ), or TC (20 mg L −1 ) solution, and 40 mg of catalyst. The solution was first ultrasonically dispersed for 1 min and then magnetically stirred in the dark for 1 h. 1.5 mL of solution was taken at specified time, and centrifuged to remove the solids. The pollutant concentrations were determined using a Shimadzu UV-2600 spectrophotometer. Total organic carbon (TOC) value of the pollutant solutions during reaction was detected on a Shimadzu TOC analyzer.
When a small amount of Ag 3 VO 4 was introduced, no diffraction peaks of Ag 3 VO 4 can be observed in the XRD pattern of AVO/BOC-50. As the Ag 3 VO 4 content increases, AVO/BOC-30, AVO/BOC-10, and AVO/BOC-5 show the diffraction peaks of both Bi 2 O 2 CO 3 and Ag 3 VO 4 , indicating the successful fabrication of Ag 3 VO 4 /Bi 2 O 2 CO 3 heterojunctions.
The microstructures of Bi 2 O 2 CO 3 and Ag 3 VO 4 /Bi 2 O 2 CO 3 heterojunctions were investigated by using SEM. The SEM images in Figures 2A,B show that the obtained Bi 2 O 2 CO 3 exhibits flower-like microspheres constructed by countless nano-plates (Zhao et al., 2011). The SEM images in Figures 2C,D show the representative AVO/BOC-10 also possesses sphere-like morphology as that for pure Bi 2 O 2 CO 3 . Of note, numerous Ag 3 VO 4 nanoparticles were deposited on the surfaces of AVO/BOC-10.
The more detailed microstructures of AVO/BOC-10 were studied by TEM. As shown in Figures 2E,F, AVO/BOC-10 consists of Bi 2 O 2 CO 3 micro-flower (diameter: ∼1.1 µm) and Ag 3 VO 4 nanoparticles (size: ∼26 nm), and they were tightly combined with each other to generate closely hybrid hetero-structure, in favor of transfer and separation of charge carriers Zhang et al., 2016;Li et al., 2017a).
The corresponding EDS spectra of AVO/BOC-10 revealed that only signals for Ag, V, Bi, C, and O elements were detected, indicating the high purity of the sample (Figure 3). The above results verified that the facile precipitation method could successfully fabricate Ag 3 VO 4 /Bi 2 O 2 CO 3 heterojunctions with intimate contact between two constituents.
The sunlight absorption capability and band structures of a photocatalyst usually exert a significant effect on its photocatalytic performance. Thus, the UV-Vis DRS spectra of bare Bi 2 O 2 CO 3 , Ag 3 VO 4 , and Ag 3 VO 4 /Bi 2 O 2 CO 3 heterojunctions are measured and illustrated in Figure 4. Bi 2 O 2 CO 3 exhibited a strong absorption in the UV region with the absorption edge at 385 nm, in accordance with the previous reports (Zhao et al., 2011;Hu et al., 2017). Ag 3 VO 4 performed a 575 nm absorption edge in the VL region, consistent with the reported values (Wang et al., 2015;Yan et al., 2016b;Li et al., 2017a). Intriguingly, the combination of Ag 3 VO 4 and Bi 2 O 2 CO 3 substantially ameliorated the VL absorption properties of the heterojunctions, which is beneficial for the effective utilization of solar energy.
The bandgap width (Eg) was estimated according to the equation: Eg = 1240/λg (eV), and the Eg value of Ag 3 VO 4 and Bi 2 O 2 CO 3 are about 2.15 and 3.23 eV. The band positions (E VB and E CB ) of Ag 3 VO 4 and Bi 2 O 2 CO 3 can be calculated using the following formula: Frontiers in Chemistry | www.frontiersin.org Where X [ca. 6.36 eV for Ag 3 VO 4 (Li et al., 2017a), and ca. 6.54 eV for Bi 2 O 2 CO 3 Liang et al., 2014] is the electronegativity of the semiconductor. E e value equals to ∼4.5 eV. On the basis of above data, the E VB and E CB of Ag 3 VO 4 were determined as 0.01 and 2.14 eV, while those of Bi 2 O 2 CO 3 were 0.2 and 3.53 eV.

Photocatalytic Property
The VLD photocatalytic activity of Ag 3 VO 4 /Bi 2 O 2 CO 3 heterojunctions was studied through the degradation of RhB (Figure 5), MO ( Figure S1), and TC (Figure 6). Figure 5A displays the concentration change of RhB dye solution under visible light with the as-prepared catalysts. The blank test conducted without the presence of catalyst showed that RhB was not degraded after 60 min of irradiation. The photocatalytic activity of pristine Bi 2 O 2 CO 3 is much lower than other samples and the RhB degradation efficiency is 31.4%, mainly due to its large bandgap (Yu et al., 2016). Only 49.8% of RhB was removed by pure Ag 3 VO 4 due to the high recombination rate of charge carriers (Yan et al., 2016b). Inspiringly, when Bi 2 O 2 CO 3 was decorated with Ag 3 VO 4, the catalytic activity of these heterojunctions was substantially improved. After 60 min of irradiation, the RhB degradation efficiencies by using AVO/BOC-5, AVO/BOC-10, AVO/BOC-30 and AVO/BOC-50 were 85.8, 98.4, 76.9, and 71.1%, respectively, much higher than that by using the pristine Bi 2 O 2 CO 3 , Ag 3 VO 4 , or the mechanical mixture (91 wt% Ag 3 VO 4 + 9 wt% Bi 2 O 2 CO 3 ). The activity of Ag 3 VO 4 /Bi 2 O 2 CO 3 increases gradually and then declines regularly, while AVO/BOC-10 has the highest photocatalytic activity, indicating the vital role of Ag 3 VO 4 in enhancing the activity. The degradation rate constants (k) of RhB were presented in Figure 5B. The photocatalytic activity of AVO/BOC-10 achieved the maximum value of k = 0.0667 min −1 , it was about 8.9, 5.7, and 5.8-folds higher than pure Bi 2 O 2 CO 3 (0.0067 min −1 ), Ag 3 VO 4 (0.0100 min −1 ), and the mechanical mixture (0.0098 min −1 ). The degradation of antibiotic TC or industrial dye MO was also performed to further test the VL photocatalysis of AVO/BOC-10 (Figure 6 and Figure S1). Apparently, AVO/BOC-10 also showed high activity in the degradation of TC and MO ( Figure S1). The TC or MO degradation efficiency with AVO/BOC-10 as the catalyst was 83.7 or 94.2% after 180 min of reaction. These results demonstrate that AVO/BOC-10 exhibits extraordinary photocatalytic activity in the removal of toxic pollutants.
To assess the mineralization capability of Ag 3 VO 4 /Bi 2 O 2 CO 3 , the TOC data during RhB (50 mg L −1 ) degradation over AVO/BOC-10 (200 mg) was recorded and analyzed ( Figure 7A). It is found that the TOC removal efficiency of RhB with AVO/BOC-10 is 76.4% after 6 h of reaction, suggesting that AVO/BOC-10 possesses strong mineralization ability during the photocatalytic reaction.
For evaluating the stability of Ag 3 VO 4 /Bi 2 O 2 CO 3 , six successive cycles of RhB degradation with AVO/BOC-10 as a catalyst were carried out. Inspiringly, no apparent loss of activity of the catalyst in six times of reuse was observed, and the RhB removal efficiency retained 92.1% in the sixth run ( Figure 7B). In addition, the catalyst before and after six runs was characterized by XRD technique (Figure S2), and no obvious changes in the crystalline phases was detected, verifying the good stability of AVO/BOC-10. Moreover, the cycling degradation involving TC antibiotic further confirms the good stability of AVO/BOC-10 ( Figure S3). The photocatalytic tests demonstrate that AVO/BOC-10 endowed with high activity and stability is a kind of promising VLD photocatalysts, exhibiting great potential for wastewater treatment.

Origin of the Improved Performance
The photocatalytic activity depends strongly on the separation efficiency of photo-induced charge carriers Li et al., 2017a), thus, photoluminescence (PL) spectrum of the samples were acquired to illustrate the electron-hole separation (Figure 8).
Apparently, the PL intensity of AVO/BOC-10 is much weaker than that of pristine Bi 2 O 2 CO 3 . Since a weaker PL intensity signifies higher separation rate of charge carriers, AVO/BOC-10 should possess a higher separation efficiency compared with Bi 2 O 2 CO 3 . That is to say, the photo-induced electron-hole pairs were efficiently separated in AVO/BOC-10 system due to FIGURE 10 | Photocatalytic degradation mechanism of toxic pollutant over Ag 3 VO 4 /Bi 2 O 2 CO 3 .
Frontiers in Chemistry | www.frontiersin.org the interfacial charge transfer, resulting in the elevated photocatalytic activity.
For mechanistic study, various additives were employed to research the main active species in the photocatalytic reaction process (Figure 9) (Zhang and Ma, 2017;Li et al., 2018). When 1 mmol of BQ (benzoquinone, O •− 2 scavenger) or AO (ammonium oxalate, h + scavenger) was introduced, the activity of AVO/BOC-10 was substantially quenched, and the RhB degradation efficiency declined from 98.4 to 29.7% or 20.2%, revealing that O •− 2 and h + should play vital roles in the photocatalysis. On the contrary, no obvious decrease in the activity was observed with adding 1 mmol of IPA (isopropyl alcohol, •OH scavenger), signifying that •OH plays a minor role.
Based on this systematic investigation, a possible visiblelight photocatalytic mechanism for pollutant degradation over Ag 3 VO 4 /Bi 2 O 2 CO 3 is proposed (Figure 10). Apparently, the position of CB and VB between Ag 3 VO 4 and Bi 2 O 2 CO 3 are beneficial to achieve effective separation of photo-generated charge carriers (Wang F. F. et al., 2017;Ye et al., 2018). Photogenerated electrons and holes are produced on Ag 3 VO 4 under visible-light illumination. Since the CB of Bi 2 O 2 CO 3 is lower than that of Ag 3 VO 4 , the electrons can be injected readily from Ag 3 VO 4 to Bi 2 O 2 CO 3 . The electrons on the CB of Bi 2 O 2 CO 3 can react with O 2 to form active O •− 2 , degrading toxic pollutants such as RhB/MO/TC. Simultaneously, the holes with strong oxide capability in the VB of Ag 3 VO 4 are available to take part in the decomposition of pollutants. In such a way, the electrons and holes can be effectively utilized, as evidenced by the result of PL spectra of Bi 2 O 2 CO 3 and Ag 3 VO 4 /Bi 2 O 2 CO 3 (Figure 8). In summary, the combination of Ag 3 VO 4 and Bi 2 O 2 CO 3 enhances the charge separation, leading to the high activity.

CONCLUSIONS
In this study, innovative Ag 3 VO 4 /Bi 2 O 2 CO 3 heterojunction photocatalysts were synthesized via a simple procedure. The Ag 3 VO 4 /Bi 2 O 2 CO 3 heterojunction (AVO/BOC-10) displayed the optimal photocatalytic properties toward the degradation of toxic pollutants (RhB dye, MO dye, and TC antibiotic), much higher than pristine Bi 2 O 2 CO 3 and Ag 3 VO 4 . The close contact and the match of bandgap structure between both constituents boost the separation of electron-hole pairs, mainly accounting for the activity enhancement. The holes and O •− 2 were determined as the primary active species responsible for the efficient removal and mineralization of the toxic pollutants. Therefore, Ag 3 VO 4 /Bi 2 O 2 CO 3 holds huge potential for real wastewater treatment.

AUTHOR CONTRIBUTIONS
SL designed and performed the experiments, and data analysis. SH, WJ, YuL, YaL, YZ, JL, and LM assisted with some of the tests. SL wrote the main content of the paper. All authors have read and approved the paper to be submitted.