Ag3VO4

Research Article pubs.acs.org/journal/ascecg

Synthesis and Characterization of Novel BiVO4/Ag3VO4 Heterojunction with Enhanced Visible-Light-Driven Photocatalytic Degradation of Dyes Ming Yan,† Yilin Wu,‡ Yan Yan,‡ Xu Yan,§ Fangfang Zhu,‡ Yinqun Hua,† and Weidong Shi*,‡ ACS Sustainable Chem. Eng. 2016.4:757-766. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/07/18. For personal use only.

†

School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China § School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China ‡

S Supporting Information *

ABSTRACT: Development of efficient photocatalysts for environmental remediation under visible light conditions has obtained much attention in recent years. In this study, the novel BiVO4/ Ag3VO4 heterojunction has been successfully fabricated via a hydrothermal process and a facile precipitation reaction. The organic dye Rhodamine B (RhB) was chosen to explore the photocatalytic performance, and it was found that the synthetic sample at 10:1 mol ratio of BiVO4:Ag3VO4 had the highest photocatalytic activity among all the photocatalysts. The RhB was completely degraded (95.9%) under visible light irradiation in 20 min, which was 10 times and 3.4 times higher than those of pristine BiVO4 and Ag3VO4, respectively. Furthermore, the A/10B sample also showed superior degradation activity on the other organic dyes such as methyl blue (MB), methyl red (MR), and methyl violet (MV). It is assumed that the enhanced photocatalytic property could be ascribed to the heterojunction, leading to an effective separation of the photogenerated charges carriers. The responsible photocatalytic mechanism is discussed based on the active species trapping experiments and ESR, and it was found that h+ and •OH are for the photocatalytic process. KEYWORDS: BiVO4/Ag3VO4, Heterojunction, Organic dyes, Visible light

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from water splitting20−23 and environmental remediation.24−28 Unfortunately, the photoinduced charge carriers of pure BiVO4 can easily recombine, leading to weak photocatalytic performance.29−31 It is worthwhile to note that an efficient visible-lightdriven photocatalyst with a narrow gap (2 eV), monoclinic scheelite silver vanadate (Ag3VO4), has received much attention because of its special band structures. Konta et al. have explored that Ag3VO4 and have found that it has wonderful photocatalytic performance for water splitting into H2 and O2.32 By comparison with a single-component semiconductor, heterojunction structures are beneficial to promote photocatalytic activity because of synergistic effects, such as improving light harvesting, increasing charge separation extent, and prolonging the lifetime of the charges carriers.33 In order to receive such ideal coupling effects, suitable band energy, requisite band gap energy, and close interfacial contact are essential elements for two constituent semiconductors.34,35 In recent years, many researchers have been interested in constructing a myriad of composites based on BiVO4, which displayed the superior photodecomposition of organic dyes compared to the individual material. For example, the BiOCl−

INTRODUCTION In recent decades, sewage release from industries generated a worrisome puzzle for environmental pollution. The discharge of wastewater often includes organic dyes, which are usually hazardous to human health.1,2 Hence, it is of great importance but still a challenge to eliminate organic dyes from aquatic environments. As is known, many technologies have been used to deal with this serious phenomenon, such as biodegradation, physical adsorption, and chemical oxidation methods; however, these technologies may be producing incomplete degradation, secondary pollution, and other dissatisfactory treatments.3,4 Very recently, one of the most promising technologies, photocatalytic oxidation, was abundantly applied in environment purification5−10 as well as to provide an efficient method to transform the dyes into harmless compounds.11−13 Until now, most reports about dye degradation were focused on conventional UV-driven titanium dioxide (TiO2). However, the wide band gap (3.2 eV for anatase) conspicuously confines its photocatalytic applications under visible light illumination.14,15 Therefore, it is indispensable to develop high efficiency visiblelight-driven photocatalysts.16−19 More recently, the ternary oxide semiconductor BiVO4 (monoclinic scheelite) has become a good candidate due to its narrow band gap (2.4 eV), high stability, nontoxicity, and sunlight utilization for applications both in oxygen evolution © 2015 American Chemical Society

Received: July 15, 2015 Revised: September 14, 2015 Published: December 14, 2015 757

DOI: 10.1021/acssuschemeng.5b00690 ACS Sustainable Chem. Eng. 2016, 4, 757−766

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. SEM images of BiVO4 (a), Ag3VO4 (b), and A/10B sample (c) and XRD patterns with different samples (d), TEM and HRTEM images (e), HAADF-STEM images of the A/10B sample (f), with maps of Bi-M (g), Ag-L (h), O-K(i), and V-K (j). dissolved in 30 mL of deionized water to make a hydrolyzed white floccule suspension. Then, 5 mmol NH4VO3 was dissolved in 30 mL of distilled water, which was heated at 75 °C with stirring for 15 min to produce a clear solution. The two solutions were mixed together to form the light yellow solution and stirred for 30 min. Then, a quantity of NaOH solution was added to adjust the PH value of 7. After stirring for another 30 min, the obtained precursor was transferred into a 100 mL Teflon-lined stainless autoclave at 140 °C for 20 h. Lastly, the obtained precipitates were collected by centrifugation, washed with deionized water and absolute ethanol several times, and dried at 60 °C for 12 h. Synthesis of BiVO4/Ag3VO4. The BiVO4/Ag3VO4 photocatalysts were obtained with using the process of deposition−precipitation. First, X (0.5, 1.5, 2.5, 5, and 10) mmol BiVO4 powder was dispersed in 40 mL of H2O via ultrasonication and vigorous stirring. Then, 1.5 mmol AgNO3 was added into the above solution with stirring for 0.5 h. After that, 40 mL of aqueous solution (containing 0.5 mmol Na3VO4) was slowly dropped onto the above solution, with stirring under darkness conditions for 7 h. Finally, the obtained samples were centrifuged and washed with deionized water and ethyl alcohol three times and dried at 60 °C for 12 h to obtain the BiVO4 /Ag3VO4 hybrid photocatalysts at different mole ratios (abbreviated as A/B, A/3B, A/ 5B, A/10B, A/20B). For comparison, bare Ag3VO4 was prepared by the same conditions but without BiVO4. Characterization. The D/MAX-2500 diffract meter (Rigaku, Japan) equipped with a nickel-filtered Cu Kα radiation source (λ = 1.54056 Å) was used to record X-ray diffraction (XRD) patterns. X-ray photoelectron spectroscopy (XPS) was performed on a PHI5000 Versa Probe electron spectrometer with the Al Kα source operated (ULVAC-PHI, Japan). Scanning electron microscopy (SEM) images was obtained using a Hitachi S-4800 field emission SEM (FESEM, Hitachi, Japan). Transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM), and scanning transmission electron microscope (STEM) mapping analyses were gathered on an F20 S-TWIN electron microscope (Tecnai G2, FEI Co.) at the accelerating voltage of 200 kV. The UV−vis diffuse reflectance spectra (DRS) was acquired by a spectrophotometer (Shimadzu UV2550), and BaSO4 was used as a reflectance standard.

BiVO4 heterojuction photocatalyst was reported and showed increased activity by improving the separation of the charge carrier.36 Wang and his co-workers have investigated the catalytic performance of the Ag3VO4/g-C3N4 heterostructure, finding that the coupled semiconductor exhibited higher photoactivity for triphenylmethane dye degradation than that of the single semiconductor under visible light illumination.37 From the above, it is likely that the enhanced photocatalytic activity of these hybrid materials is ascribed to the heterojunction structures, which improve the photoexcited electron−hole separation. As far as we know, BiVO4 hybridized with Ag3VO4 for photocatalytic activity has not been reported on previously. This strategy might induce a high-efficiency heterojunction to remove the organic pollutant. In this work, a novel BiVO4/Ag3VO4 heterojunction photocatalyst has been successfully prepared by developing a hydrothermal process and a precipitation method. The asprepared samples were explored by the degradation of Rhodamine B (RhB), which showed superior photocatalytic performance under visible light conditions (λ ≥ 420 nm). At the same time, the BiVO4/Ag3VO4 sample also can efficiently decompose other organic dyes such as methyl blue (MB), methyl red (MR), and methyl violet (MV). A tentative photocatalytic mechanism for the enhanced photocatalytic performance is also discussed in detail.

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EXPERIMENTAL SECTION

Chemicals. Silver nitrate (AgNO3), sodium orthovanadate (Na3VO4), bismuth nitrate (Bi(NO3)3·5H2O), sodium hydroxide (NaOH), ammonium metavanadate (NH4VO3), RhB, MB, MR, and MV were purchased from Sinopharm Chemical Reagent. All chemicals were of analytical reagent grade and used without further purification; deionized water was used in all experiments. Synthesis of BiVO4. The BiVO4 crystal was synthesized through the hydrothermal process. Typically, 5 mmol Bi(NO3)3·5H2O was 758


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Figure 2. Survey XPS spectra (a), Ag 3d XPS spectra (b), Bi 4f XPS spectra (c), and O 1S and V 2p XPS spectra of the A/10B sample (d). The electron spin resonance (ESR) signals were examined on the Bruker EPR A300-10/12 spectrometer. The photocurrent and electrochemical impedance spectroscopies (EIS) were performed by an electrochemical analyzer (CHI 660B Chenhua Instrument Company). Photocatalytic Activity. Photocatalytic performance of the BiVO4/Ag3VO4 sample was explored by the photodegradation of RhB, which was carried out at 308 K in a photochemical reactor under irradiation of visible light. In detail, 0.1 g of BiVO4/Ag3VO4 powder was mixed with 100 mL of organic dye solution (10 mg/L) in order to reach the absorption−desorption equilibrium between the photocatalyst and dye molecules; the suspensions were magnetically stirred in darkness for 0.5 h. The 250 W xenon lamp with a 420 nm cutoff filter provided visible light irradiation. At 5 min irradiation intervals, 6 mL of suspensions were collected and centrifuged to separate photocatalyst particles for further analysis. The photocatalytic degradation ratio (DR) was calculated via the following formula:

DR = (1 − A i /A 0) × 100%

The crystalline phases of different samples were detected by XRD analysis. Figure 1d exhibits the XRD patterns of BiVO4, BiVO4/Ag3VO4, and pure Ag3VO4 at different mole proportions. It could be found that the BiVO4 sample was well consistent with the structure of monoclinic scheelite (JCPDS No. 14-0688), and the Ag3VO4 diffraction peaks matched with its monoclinic phase (JCPDS No. 43-0542). While for the BiVO4/Ag3VO4 hybrid materials, the main diffraction XRD peaks are readily indexed on the basis of BiVO4 and Ag3VO4 catalysts. Results indicated coexistence of BiVO4 and Ag3VO4 catalysts in the hybrid materials. As an efficient method to prove the formation of heterostructure in the BiVO4/Ag3VO4 composite materials, TEM had also been employed. According to a low magnification TEM image of the BiVO4/Ag3VO4 sample in Figure 1e, it strongly revealed that irregular worm-like particles (BiVO4) and spherical nanoparticles (Ag3VO4) mixed with each other. As shown in the HRTEM images, the distinct lattice fringes with spacing of d = 0.162 nm and d = 0.191 nm are seen, which coincide with the (121) plane of BiVO4 and the (220) plane of Ag3VO4, respectively. The results clearly demonstrated that the composites between BiVO4 and Ag3VO4 have been formed. The structure and element distribution of the A/10B sample were also explored by STEM (Figure 1f−j). From the pictures with distinct color contrast, we conclude that a good spatial correspondence of Bi, Ag, V, and O elemental maps were formed with the intensities of their M lines, L lines, and K lines, demonstrating that Bi−Ag−V−O elements coexist in the A/ 10B composite, which is in agreement with the previous characterization. Furthermore, these pictures signify that the Ag3VO4 nanoparticles bind with the surface of BiVO4. XPS Analysis. The surface chemical composition of the A/ 10B sample was ultimately confirmed by XPS. According to the entire XPS spectrum observations in Figure 2a, the peaks of Bi, Ag, V, and O all existed in the A/10B sample. Figure 2b shows

(1)

where A0 is the foremost absorbency of organic dyes that arrived absorption−desorption equilibrium, and Ai is the absorbency after the sampling analysis. The concentration change of RhB was examined on a UV−vis absorption of the suspensions at its peak absorbency at 552 nm.

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RESULTS AND DISCUSSION Morphology and Structure. The morphological evolution of various samples was examined by SEM (Figure 1a−c). From Figure 1a, large numbers of the irregular worm-like particles with the size about 300−700 nm are clearly seen in the pure BiVO4 samples. As shown in Figure 1b, the Ag3VO4 samples are mainly composed of irregular spherical nanoparticles with diameters of 30−100 nm. Both two types of materials are found in the hybrid samples in Figure 1c, which correspond to Ag3VO4 and BiVO4, respectively. Furthermore, we find that the Ag3VO4 nanoparticles bind with the surface of BiVO4 and were well dispersed. 759


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Figure 3. UV−vis adsorption spectra (a) and calculated band gap of BiVO4, Ag3VO4, and BiVO4/Ag3VO4 composites at different mole proportions (b).

Figure 4. Photocatalytic degradation of RhB with different samples (a, b), time-dependent UV−vis absorption spectra of the RhB solution in the presence of A/10B sample (C), kinetics curves of the photocatalytic degradation ratio of RhB with different samples (d), cycle experiments (e), and photocatalytic degradation of MV, MR, and MB with A/10B sample (f).

V5+ in the A/10B sample. The results of XPS images further confirm the coexistence of BiVO4 and Ag3VO4 in the hybrid materials, which is consistent with the XRD analysis. The binding energies (BE), full width at half-maximum (fwhm)b and relative intensity from A/10B after deconvolution are shown in Table S1. UV−Vis Absorption Spectra. The optical properties for the different samples were tested by UV−vis diffuse reflectance spectroscopy and are shown in Figure 3a. According to the spectra, all the samples expressed absorbance in the visible

that the two strong peaks at 367.9 and 374.1 eV are assigned to Ag 3d5/2 and Ag 3d3/2 respectively, which verified that the silver species are Ag+ cations. The Bi 4f peaks located at 164.4 and 159.1 eV should originate from BiVO4 (Figure 2c), which correspond to Bi 4f5/2 and Bi 4f7/2, respectively. The bands are attributed to Bi3+. In Figure 2d, the O 1s is fitted to two symmetrical peaks at 530.0 and 532.6 eV, which may be assigned to the lattice oxygen in the A/10B sample and the adsorbed H2O, respectively.38,39The XPS signals of V 2p3/2 was observed at the binding energy of 517.2 eV, corresponding to 760


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Table 1. Apparent Rate Constant Value for Photodegradation of RhB over BiVO4, Ag3VO4, and BiVO4/Ag3VO4 Composites at Different Mole Proportions in 20 min under Visible Light Irradiation samples

BiVO4

Ag3VO4

A/B

A/3B

A/5B

A/10B

A/20B

Kapp (min−1)

0.00497

0.0168

0.03645

0.05211

0.06994

0.15967

0.03658

Figure 5. Typical LC−MS chromatogram at the irradiation for 20 min.

performed a much higher degradation capability than those of the single BiVO4 and Ag3VO4. Sample A/10B exhibited the highest activity with the RhB degradation ratio (DR) of 95.9% in 20 min. In contrast, only 9.4% and 28.4% of RhB dye molecules could be decomposed for the BiVO4 and Ag3VO4, respectively. Therefore, we envision that the enhanced photocatalytic activity for composites can be obtained because of the existence of heterojunction structures between the two parts (BiVO4 and Ag3VO4) of the coupled semiconductors. Figure 4c shows the temporal evolution of the spectral variations during the degradation of RhB over the A/10B sample with visible-light condition. From Figure 4c, we conclude that as the degradation time increased the absorption peaks at 552 nm of the RhB gradually declined, accompanyed by a slight concomitant blue shift. After irradiation for 20 min, the photodegradation rate of the RhB solution reached about 95.9%, indicating that RhB was hardly completely degraded. The inset shows the relevant color variations of the RhB solution. This blue shift of the absorption peaks at 552 nm can be assigned to generate a new derivative in the process of RhB reduction. The kinetic behavior was carried out to further illustrate the photocatalytic performance. As shown in Figure

regions, and the absorption edges of BiVO4/Ag3VO4 with different mole ratios somehow changed in an orderly pattern. Energy band gaps (Eg) of the different samples were also calculated according to the reported equation:38 α = A(hv − E)g n/2 /hv

(2)

where α, h, v, A, Eg are the absorption coefficient, Planck’s constant, incident light frequency, constant, and band gap energy, respectively. However, n depends on the characteristics of the optical transition of the semiconductors, n = 1 for direct and 4 for indirect transition semiconductors. BiVO4 and Ag3VO4 are typical kinds of direct transition semiconductors. The energy band gaps of BiVO4, Ag3VO4, A/B, A/3B, A/5B, A/ 10B, and A/20B from the plots of (αhν)2 versus hv can be estimated and were 2.4, 2, 2.09, 2.13 2.2 2.25, and 2.35, respectively (Figure 3b). However, as the content of BiVO4 increased, the energy band gaps of BiVO4/Ag3VO4 composites gradually increased. Photocatalytic Behaviors. The photocatalytic performances of the BiVO4/Ag3VO4 hybrid materials were evaluated via the RhB degradation with visible-light irradiation. In order to make a comparison, the single BiVO4 and Ag3VO4 were also offered. As shown in Figure 4a and b, all the hybrid samples 761


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ACS Sustainable Chemistry & Engineering 4d, the plots of RhB decomposition matched the pseudo-firstorder kinetic correlation: ln(C0/C) = kappt

(3)

where C and C0 represent the RhB concentrations at different irradiation times t and the initial concentration at t = 0, respectively. kapp expresses the constant of degradation rate, and the values are shown in Table 1. Obviously, the value for A/10B (0.15967) was the highest among all the samples, which is consistent with the conclusion of photocatalytic degradation. The repeating experiments were applied to prove the stability of the A/10B sample. After each run, the photocatalysts were washed by centrifuge followed with ultrasonic cleaning. In Figure 4e, it is clearly observed that the A/10B sample exhibited a high stability, resulting in a high decomposition ratio even after five cycles. Meanwhile, the BET values of the A/10B sample were 13.42 and 13.40 m2 g−1 before and after five cycles of experiments, respectively. The data have no obvious changes. In order to further certify the stability of the A/10B sample, the XRD pattern and SEM image after five cycles are also provided in Figure S2. From Figure S2a, it is clearly observed that the SEM image of the A/10B was barely changed after five cycles. Moreover, the phase and structure of the A/10B remained unchanged (Figure S2b), which suggested that the sample was stable after five cycles of photocatalytic degradation processes. In order to acquire better insight about the catalytic activities of the A/10B sample, we selected MB, MR, and MV as target pollutants for photodegradation (Figure 4f). Under visible light irradiation, 93.8% MV could be discolored within 40 min, while 71.1% MR and 64.3% MB were degraded within the same time, suggesting that the BiVO4/Ag3VO4 composites have highly efficient photocatalytic activity for the organic dyes degradation. As the colorless organic pollutant, TC was further studied under visible light for eliminating the indirect dye photosensitization in the photocatalytic system. As shown in Figure S3, compared to pure Ag3VO4 (29.2%) and BiVO4 (20.7%), the Ag3VO4/BiVO4 composites exhibited the higher photocatalytic performance. Among all the prepared samples, A/10B showed the highest photocatalytic performance; 72.9% of colorless TC molecules could be decomposed within 40 min. The results were similar to the conclusion of degradation of RhB, excluding the possible sensitized effect of the dyes on the photocatalytic performance of the catalysts. Photodegradation of RhB and Identification of the Intermediates. The intermediates of RhB were investigated via LC-MS, and the results are exhibited in Figure 5. It is clearly observed that there is an intense prominent ion with m/z = 443 (Figure 5a), which can be attributed to RhB. From the analysis of MS and the previous studies,40−42 under visible light illumination, the dye is N-de-ethylated in a stepwise manner accompanying a color change of the dispersion from initial red to colorless, and it is degraded by a series of successive deethylation reaction from N,N,N′,N′-tetraethylated rhodamine to rhodamine. In Figure 5b, the major peak was located at m/z = 415, owing to the loss of the ethyl group on the RhB dye structures. By the further removal of the residual ethyl groups on the RhB structures, the m/z values of 387, 359, and 331 were gradually detected in Figure 5(c−f). On the basis of all the above experimental results and the previous studies,43−45 the possible pathways of degradation of RhB under visible light illumination are depicted in Figure 6.

Figure 6. Proposed degradation pathways for photocatalytic degradation of RhB with Ag3VO4/BiVO4 photocatalysts.

N2 Adsorption−desorption Isotherms. The nitrogen adsorption−desorption isotherm and pore size distribution plot of Ag3VO4, BiVO4, and the A/10B samples are shown in Figure S4. It is clearly observed that the isotherms belonged to type-IV isotherms, indicating that the as-prepared samples were mesoporous materials. From Table S5, we see that the BET surface areas (SBET) of the Ag3VO4/BiVO4 composites (from A/B to A/20B, the SBET were found to be 9.45, 10.24, 12.04, 13.42, and 9.87 m2g−1) were higher than those of pure Ag3VO4 (4.86 m2g−1) and BiVO4 (6.91 m2g−1), respectively. Moreover, the SBET of the Ag3VO4/BiVO4 composites increased as the content of BiVO4 increased and then began to decrease when the content of BiVO4 exceeded the mole ratio of 1/10. The results are consistent with the conclusion of the constant of degradation rate (k). In addition, the SBET and average pore diameter of all the samples are listed in Table S5. Electrochemistry Analysis. Electrochemical impedance spectroscopy (EIS) was carried out to explore the process of their charge transfer resistance. As shown in Figure 7a, the radius of the arc on EIS of the A/10B sample was smaller than those of pure BiVO4 and Ag3VO4, indicating that the A/10B sample has the lowest resistance, which means a faster chargetransfer process. Therefore, the coupling of BiVO4 with Ag3VO4 could improve the separation of the photogenerated charge carrier, leading to highly efficient photocatalytic performance. For better understanding about the process of electron transfer, the photocurrent was then measured (Figure 7b). The photocurrent intensity of the A/10B sample was higher than those of pure Ag3VO4 and BiVO4. The conclusion was consistent with the EIS analysis, which clearly suggested that the heterojunction structure between Ag3VO4 and BiVO4 762


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reduction potential of −0.33 eV/NHE (O2 /•O 2−) by electrons.46 However, because the reduction potential of O2/ H2O2 was 0.695 eV,39 the electrons can react to generate •OH via the O2 and H+ by the equation: O2 + 2e− + 2H+ = H2O2, H2O2 + e− = •OH + OH−. In comparison to the potential of •OH/OH− (2.38 eV/NHE) and •OH/H2O (2.72 eV/ NHE),37 Ag3VO4 (EVB = 2.14 eV) is more negative, so the •OH could not produce with the reaction of h+, OH−, and H2O on the BiVO4/Ag3VO4 surface. It can just react with organic dyes immediately. The electron spin resonance (ESR) technique was carried out to reveal the reactive species evolved in the process of photocatalytic reaction with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in water. From Figure 8a, it is shown that no ESR signals generated under the dark conditions. On the contrary, when the light is turned on, the four characteristic peaks of DMPO−•OH with intensity 1:2:2:1 were observed on BiVO4/ Ag3VO4, indicating the •OH radical produced in the reaction system.47 A suite of active species trapping experiments were used to probe the photocatalytic oxidation mechanism (Figure 8b,c). As 1 mM triethanolamine (TEA)48−51 was added to trap the holes (h+), the photodegradation ratio significantly declined compared to the reaction without TEA, revealing that the photogenerated holes (h+) play a major role in the selective oxidation of the pollutant. When 1 mM iso-propanol (IPA)51 was added to trap •OH, the photodegradation ratio decreased 14.5%; therefore, it is concluded that •OH took part in the oxidation reaction. On the contrary, the degradation ratio increased a little with the addition of AgNO3 (1 mM) for e−,48−51 which demonstrated that the scavenger of e− had less opportunity to join recombination of electron−hole pairs and produced more holes to take part in reaction process. According to the obtained active species trapping experiments and ESR, we envision that h+ and •OH were the major active species for organic dyes degradation. The holes on VB of Ag3VO4 can easily activate organic pollutants, leading to subsequent decomposition. The •OH on the CB is obtained

Figure 7. EIS (a) and transient photocurrent response (b) for the pure BiVO4, Ag3VO4, and A/10B samples.

generated rapid separation of the photogenerated charge carrier as well as accelerated the process of interfacial charges transfer. Therefore, the A/10B composite exhibited enhanced photocatalytic activity. Mechanism of BiVO4/Ag3VO4 Heterojunction Structure. As is well known, many reactive species such as h+, •O2−, and •OH play major roles on decomposing dyes.37,45 In our work, •O2− could not be produced because of a more positive

Figure 8. DMPO spin-trapping ESR spectra (a), photocatalytic degradation ratios of RhB using different radical scavengers over A/10B sample (b, c), and photocatalytic mechanistic (d). 763


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ACS Sustainable Chemistry & Engineering from the reduction of e− and then photodegrade the organic dyes. To illustrate the photocatalytic process for decomposing organic dyes by the BiVO4/Ag3VO4 composite, we carried out band position calculations by a plain method according to the previous paper.37 We can evaluate the conduction band of semiconductor by the formula: ECB = X − E e − 0.5Eg

(4)

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where ECB is the CB edge potential; X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms (X value for BiVO4 is 6.03552 and for Ag3VO4 is 5.64537); Ee is the energy of free electrons on the hydrogen scale (4.5 eV); and Eg is the band gap energy of the semiconductor. The EVB value is obtained by EVB = ECB + Eg. As the band gaps of BiVO4 and Ag3VO4 were 2 and 2.4 eV by the UV−vis diffuse reflectance absorption spectra, the conduction band (CB) of BiVO4 and Ag3VO4 were calculated to be 0.335 and 0.145 eV, and the valence bands (VB) of BiVO4 and Ag3VO4 were calculated to be 2.735 and 2.145 eV, respectively. On the basis of ESR, active species trapping experiments, and calculated energy bands, a possible photocatalytic mechanism of BiVO4/Ag3VO4 heterojunction structure is drawn as shown in Figure 8d. When the BiVO4/ Ag3VO4 sample was vsubjected to visible-light-irradiation of visible light, electrons are excited form the VB to the CB with simultaneous generation of holes in the VB. Due to the difference of band gaps, electrons on Ag3VO4 CB can easily migrate to the CB of BiVO4; on the other hand, holes on BiVO4 VB can also be injected into Ag3VO4 VB, which effectively inhibited recombination and prolonged the lifetime of photogenerated electrons and holes, accelerating the photocatalysis process.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +86-51188791800. Fax: +86 0511-88791800. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21276116, 21477050, 21301076, and 21303074), Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011), Chinese-German Cooperation Research Project (GZ1091), Program for High-Level Innovative and Entrepreneurial Talents in Jiangsu Province, Program for New Century Excellent Talents in University (NCET −13-0835), Henry Fok Education Foundation (141068), and Six Talents Peak Project in Jiangsu Province (XCL-025).

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REFERENCES

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CONCLUSIONS In summary, we have proposed a novel visible-light-driven BiVO4/Ag3VO4 heterojunction photocatalyst for the first time by developing several straightforward synthesis techniques (hydrothermal process and precipitation reaction), which were innovatively used as a highly efficient system for photocatalytic degradation of dyes. The mole ratio at 10:1 of the BiVO4:Ag3VO4 sample performed optimal photocatalytic properties for degradation of RhB (95.9%), MB, MR, and MV under irradiation of visible light (λ ≥ 420 nm). On the basis of the active species trapping experiments, ESR, and calculated energy band positions, the mechanism of enhanced photocatalytic activity for the BiVO4/Ag3VO4 composite was discussed. Enhancement of decomposition of dye is attributed to the heterojunction structure and widely improved the separation of the photogenerated electrons and holes. In general, the BiVO4 hybridized with Ag3VO4 could be used for solving the issue of low photocatalytic performance. Such a composite photocatalyst may provide a promising way for environmental remediation.

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Table S1: Binding energies (BE), full width at halfmaximum (fwhm), and relative intensity from A/10B after deconvolution. Figure S2: SEM (a) and XRD pattern (b) of the A/10B sample after five cycles of photocatalytic experiments. Figure S3: Photocatalytic degradation of TC with as-prepared samples. Figure S4: Nitrogen adsorption−desorption isotherm and pore size distribution plot of the BiVO4 (a,b), Ag3VO4 (c,d), and A/10B (e,f) samples. Table S5: Physical properties of asprepared samples. (DOC)

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