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.

Introduction

Semiconductor photocatalysis has been regarded as one of the most promising nanotechnologies for the treatment of environmental pollution (Bora and Mewada, 2017; Cates, 2017; Wang W. et al., 2017; Zhang and Ma, 2017; Zhu and Wang, 2017). A significant research topic of photocatalysis is the exploration of highly active photocatalysts (Li et al., 2014, 2017d, 2018b; Adhikari et al., 2015, 2016, 2017; Martin et al., 2015; Zhang et al., 2016, 2017; Mousavi et al., 2018; Yu et al., 2018).

The emergent Bi₂O₂CO₃ 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 electron-hole pairs. To improve the visible-light photocatalytic activity of Bi₂O₂CO₃, various strategies have been developed, such as design of microstructure (Zhao et al., 2011), deposition of metals (Yu et al., 2016), formation of heterojunction (Chen et al., 2016, 2017; 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 Bi₂O₂CO₃-based 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.

Ag₃VO₄, an active VLD photocatalyst, has drawn much attention in virtue of its unique band structures. Ag₃VO₄ has been coupled with other semiconductors (e.g., Bi₂WO₆ Li et al., 2017a; Zhang and Ma, 2017, BiOI Wang et al., 2015, BiOCl Wang et al., 2016, C₃N₄ Wang et al., 2014, BiVO₄ Yan et al., 2016a, WO₃ Yan et al., 2016b) to fabricate high-performance photocatalysts. To date, researches on Ag₃VO₄ nanoparticles decorated Bi₂O₂CO₃ micro-flowers for the visible-light photo-degradation of toxic contaminants have not been reported.

Herein, we report Ag₃VO₄ nanoparticles evenly deposited on the surface of Bi₂O₂CO₃ micro-flowers by a simple precipitation method. Ag₃VO₄ 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 visible-light photocatalysis mechanism of Ag₃VO₄/Bi₂O₂CO₃ is also proposed.

Experiment

Chemicals

Bismuth citrate (BiO₇C₆H₅), sodium carbonate (Na₂CO₃), ethanol (CH₃CH₂OH), silver nitrate (AgNO₃), sodium vanadate (Na₃VO₄), rhodamine B (RhB), ammonium oxalate (AO), AgNO₃, tetracycline hydrochloride (TC), p-benzoquinone (BQ), methyl orange (MO), and iso-propanol (IPA) were bought from Shanghai Chemical Reagent factory (China). All the reagents were analytic grade and used without further treatment.

Synthesis of Catalysts

Bi₂O₂CO₃ was synthesized via a hydrothermal procedure. Briefly, 2 mmol of sodium carbonate (Na₂CO₃) and 2 mmol of bismuth citrate (BiO₇C₆H₅) 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.

Ag₃VO₄/Bi₂O₂CO₃ heterojunctions were constructed by a simple precipitation method. Briefly, an appropriate amount of Bi₂O₂CO₃ was ultrasonically suspended in 50 mL of H₂O. Then, 3 mmol AgNO₃ was dissolved in the above solution under magnetical stirring. After that, Na₃VO₄ (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 Ag₃VO₄/Bi₂O₂CO₃ heterojunctions. The heterojunctions with different Bi₂O₂CO₃/Ag₃VO₄ 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. Ag₃VO₄ was prepared in the absence of Bi₂O₂CO₃.

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.

Results and Discussion

Characterization

Figure 1 displays the XRD patterns of the as-prepared pure Ag₃VO₄, Bi₂O₂CO₃, and their heterojunctions (AVO/BOC-5, AVO/BOC-10, AVO/BOC-30, and AVO/BOC-50). The diffraction peaks of Ag₃VO₄ and Bi₂O₂CO₃ prepared match well with monoclinic phase of Ag₃VO₄ (JCPDS 43-0542) and tetragonal phase of Bi₂O₂CO₃ (JCPDS 41-1488), respectively.

FIGURE 1

Figure 1. XRD patterns of Ag₃VO₄/Bi₂O₂CO₃ heterojunctions (AVO/BOC-5, AVO/BOC-10, AVO/BOC-30, and AVO/BOC-50), pure Bi₂O₂CO₃, and Ag₃VO₄.

When a small amount of Ag₃VO₄ was introduced, no diffraction peaks of Ag₃VO₄ can be observed in the XRD pattern of AVO/BOC-50. As the Ag₃VO₄ content increases, AVO/BOC-30, AVO/BOC-10, and AVO/BOC-5 show the diffraction peaks of both Bi₂O₂CO₃ and Ag₃VO₄, indicating the successful fabrication of Ag₃VO₄/Bi₂O₂CO₃ heterojunctions.

The microstructures of Bi₂O₂CO₃ and Ag₃VO₄/Bi₂O₂CO₃ heterojunctions were investigated by using SEM. The SEM images in Figures 2A,B show that the obtained Bi₂O₂CO₃ 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 Bi₂O₂CO₃. Of note, numerous Ag₃VO₄ nanoparticles were deposited on the surfaces of AVO/BOC-10.

FIGURE 2

Figure 2. SEM images of pristine Bi₂O₂CO₃ (A,B) and AVO/BOC-30 (C,D); TEM images of AVO/BOC-10 (E,F).

The more detailed microstructures of AVO/BOC-10 were studied by TEM. As shown in Figures 2E,F, AVO/BOC-10 consists of Bi₂O₂CO₃ micro-flower (diameter: ~1.1 μm) and Ag₃VO₄ 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 (Huang et al., 2015; 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 Ag₃VO₄/Bi₂O₂CO₃ heterojunctions with intimate contact between two constituents.

FIGURE 3

Figure 3. EDS spectra of AVO/BOC-10.

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 Bi₂O₂CO₃, Ag₃VO₄, and Ag₃VO₄/Bi₂O₂CO₃ heterojunctions are measured and illustrated in Figure 4. Bi₂O₂CO₃ 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). Ag₃VO₄ 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 Ag₃VO₄ and Bi₂O₂CO₃ substantially ameliorated the VL absorption properties of the heterojunctions, which is beneficial for the effective utilization of solar energy.

FIGURE 4

Figure 4. The UV–Vis DRS of bare Bi₂O₂CO₃, Ag₃VO₄ and Ag₃VO₄/Bi₂O₂CO₃ heterojunctions.

The bandgap width (Eg) was estimated according to the equation: Eg = 1240/λg (eV), and the Eg value of Ag₃VO₄ and Bi₂O₂CO₃ are about 2.15 and 3.23 eV. The band positions (E_VB and E_CB) of Ag₃VO₄ and Bi₂O₂CO₃ can be calculated using the following formula:

$\begin{array}{rc}{\mathit{\text{E}}}_{\text{VB}}=\mathit{\text{X}}-{\mathit{\text{E}}}^{\text{e}}+0.5{\mathit{\text{E}}}_{\text{g}}& (1)\end{array}$

$\begin{array}{rc}{\mathit{\text{E}}}_{\text{CB}}={\mathit{\text{E}}}_{\text{VB}}-{\mathit{\text{E}}}_{\text{g}}& (2)\end{array}$

Where X [ca. 6.36 eV for Ag₃VO₄ (Li et al., 2017a), and ca. 6.54 eV for Bi₂O₂CO₃ 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 Ag₃VO₄ were determined as 0.01 and 2.14 eV, while those of Bi₂O₂CO₃ were 0.2 and 3.53 eV.

Photocatalytic Property

The VLD photocatalytic activity of Ag₃VO₄/Bi₂O₂CO₃ 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 Bi₂O₂CO₃ 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 Ag₃VO₄ due to the high recombination rate of charge carriers (Yan et al., 2016b). Inspiringly, when Bi₂O₂CO₃ was decorated with Ag₃VO₄, 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 Bi₂O₂CO₃, Ag₃VO₄, or the mechanical mixture (91 wt% Ag₃VO₄ + 9 wt% Bi₂O₂CO₃). The activity of Ag₃VO₄/Bi₂O₂CO₃ increases gradually and then declines regularly, while AVO/BOC-10 has the highest photocatalytic activity, indicating the vital role of Ag₃VO₄ in enhancing the activity.

FIGURE 5

Figure 5. (A) Photocatalytic degradation efficiency of RhB over different catalysts under visible light. (B) Rate constants of RhB degradation during reaction over different catalysts.

FIGURE 6

Figure 6. Photocatalytic degradation efficiency of TC over AVO/BOC-10.

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 Bi₂O₂CO₃ (0.0067 min⁻¹), Ag₃VO₄ (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 Ag₃VO₄/Bi₂O₂CO₃, 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.

FIGURE 7

Figure 7. (A) TOC removal rate during RhB degradation with AVO/BOC-10; (B) The cycling performance of AVO/BOC-10.

For evaluating the stability of Ag₃VO₄/Bi₂O₂CO₃, 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 (Hu et al., 2017; 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 Bi₂O₂CO₃. Since a weaker PL intensity signifies higher separation rate of charge carriers, AVO/BOC-10 should possess a higher separation efficiency compared with Bi₂O₂CO₃. That is to say, the photo-induced electron-hole pairs were efficiently separated in AVO/BOC-10 system due to the interfacial charge transfer, resulting in the elevated photocatalytic activity.

FIGURE 8

Figure 8. PL spectra of pristine Bi₂O₂CO₃ and AVO/BOC-10.

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${}_2^{\bullet -}$ 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${}_2^{\bullet -}$ 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.

FIGURE 9

Figure 9. Radical scavenge tests with AVO/BOC-10 as the catalyst.

Based on this systematic investigation, a possible visible-light photocatalytic mechanism for pollutant degradation over Ag₃VO₄/Bi₂O₂CO₃ is proposed (Figure 10). Apparently, the position of CB and VB between Ag₃VO₄ and Bi₂O₂CO₃ are beneficial to achieve effective separation of photo-generated charge carriers (Wang F. F. et al., 2017; Ye et al., 2018). Photo-generated electrons and holes are produced on Ag₃VO₄ under visible-light illumination. Since the CB of Bi₂O₂CO₃ is lower than that of Ag₃VO₄, the electrons can be injected readily from Ag₃VO₄ to Bi₂O₂CO₃. The electrons on the CB of Bi₂O₂CO₃ can react with O₂ to form active O${}_2^{\bullet -}$, degrading toxic pollutants such as RhB/MO/TC. Simultaneously, the holes with strong oxide capability in the VB of Ag₃VO₄ 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 Bi₂O₂CO₃ and Ag₃VO₄/Bi₂O₂CO₃ (Figure 8). In summary, the combination of Ag₃VO₄ and Bi₂O₂CO₃ enhances the charge separation, leading to the high activity.

FIGURE 10

Figure 10. Photocatalytic degradation mechanism of toxic pollutant over Ag₃VO₄/Bi₂O₂CO₃.

Conclusions

In this study, innovative Ag₃VO₄/Bi₂O₂CO₃ heterojunction photocatalysts were synthesized via a simple procedure. The Ag₃VO₄/Bi₂O₂CO₃ 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 Bi₂O₂CO₃ and Ag₃VO₄. 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${}_2^{\bullet -}$ were determined as the primary active species responsible for the efficient removal and mineralization of the toxic pollutants. Therefore, Ag₃VO₄/Bi₂O₂CO₃ 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.

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.

Acknowledgments

This work has been financially supported by the National Natural Science Foundation of China (51708504 and 31501573), the Public Projects of Zhejiang Province (2017C32079 and LGN18E080003), the Science and Technology Project of Zhoushan (2017C41006, 2016C41012, 2015C21014, and 2015C21013), the National Key Research Development Program of China (2016YFC0400501), and the Research Startup Foundation of Zhejiang Ocean University (12215090117).

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2018.00255/full#supplementary-material

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