Highly Efficient Oxidation of Gaseous Benzene on Novel Ag3VO4

Jun 8, 2012 - Research Institute of Photocatalysis, National Research Center of ..... energy value of Ag in Ag3VO4/TiO2 nanocomposites was the same as...
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Highly Efficient Oxidation of Gaseous Benzene on Novel Ag3VO4/TiO2 Nanocomposite Photocatalysts under Visible and Simulated Solar Light Irradiation Jinxiu Wang, Hong Ruan, Wenjuan Li, Danzhen Li,* Yin Hu, Jing Chen, Yu Shao, and Yi Zheng Research Institute of Photocatalysis, National Research Center of Environmental Photocatalysis, Fujian Provincial Key Laboratory of Photocatalysis-State Key Laboratory Breeding Base, Fuzhou University, Fuzhou 350002, P. R. China S Supporting Information *

ABSTRACT: Novel Ag3VO4/TiO2 nanocomposites photocatalysts with high efficiency and broad spectrum response were first prepared by a facile and low-cost coupling method. The samples performed high photocatalytic activity and stability in decomposing continuous-flow gaseous benzene with high toxicity and stability under both visible and simulated solar light irradiation. When the mass fraction of Ag3VO4 in nanocomposites was 0.5%, the sample possessed the highest photocatalytic activity among those Ag3VO4/TiO2 nanocomposites in different proportions. The conversion and mineralization rate reached about 40 and 60% respectively, which was nearly two times higher than that of nitrogen-doped TiO2 (TiO2−xNx), when it was used to degrade continuous-flow gaseous benzene photocatalytically at an inlet concentration of 280 ppm and a gas hourly space velocity of 876 h−1 under visible light irradiation for 10 h. Moreover, under simulated solar light irradiation, the benzene could be nearly completely conversed and mineralized on the sample. The photoelectrochemical measurement confirmed that the interface charge separation efficiency was improved by coupling TiO2 with Ag3VO4, which contributed to the enhancement of photocatalytic activity. A variety of merits of Ag3VO4/TiO2 nanocomposites make it possess promising application value in industry.

1. INTRODUCTION In recent decades, the environment (air and water) we are living in has been seriously polluted by a variety of volatile organic compounds (VOCs). Among these, benzene is regarded as the most toxic one.1 Benzene, which does not easily photodecompose at relatively high concentrations due to its conjugated π-bond, is widely used not only as a solvent in industrial processes but also as an indoor air pollutant.2,3 Benzene has seriously threatened human health. Therefore, there is an urgent need to have an effective solution for the removal of benzene. Semiconductor photocatalytic oxidation technology has been proven to be potentially advantageous for environmental pollutant remediation because it allows complete decomposition of VOC into CO2 and H2O under ambient conditions.4 Titanium dioxide (TiO2) is the most extensively utilized semiconductor photocatalyst because of its appropriate band position, high chemical stability, low cost, and environmental friendliness. However, the wide band gap of TiO2 (3.2 eV for anatase) makes it inactive under visible light irradiation, which occupies the major part of solar light.5 Strategies for making TiO2 visible-light-active have been investigated extensively. By coupling TiO2 with a narrow band gap semionductor, its photoresponse is extended to the visible region, and charge carrier separation is achieved.6 TiO2 has been coupled with narrow band gap semiconductors like Bi2S3,7 CdS,7−9 CdSe,10 and V2O5,11 which are capable of absorbing visible light. The © 2012 American Chemical Society

results showed that the photocatalytic activity of coupled photocatalyst is higher than that of the single one. However, there are two prerequisites for the small band gap semiconductor: (i) the band gap value of the semiconductor should be near that for optimum utilization of solar radiant energy and (ii) its conduction band (CB) minimum should be higher than that of TiO2.12 Since Konta et al.13 reported that Ag3VO4 showed a photocatalytic activity for O2 evolution from an aqueous silver nitrate solution under visible light irradiation, silver vanadate (Ag3VO4) material has attracted much attention.14−17 Ag3VO4 also performed photocatalytic activity in photocatalytic degradation of gas benzene in visible light irradiation,2 yet the photocatalytic activity of pure Ag3VO4 is still low because of its difficult separation of electron−hole pairs. As one of techniques to improve the photocatalytic properties, coupled catalysts are used to accelerate the separation of electron−hole pairs. The p−n junction photocatalyst p-CaFe2O4/n-Ag3VO4,18 NiO/Ag3VO4,19 and La2O3/Ag3VO420 composite photocatalysts all performed superior photocatalytic activities to pure Ag3VO4 to some extent under visible light irradiation. However, these composites were utilized only to degrade dyes, and their Received: February 10, 2012 Revised: May 19, 2012 Published: June 8, 2012 13935

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nitrogen atmosphere, and the heating rate was 5 °C/min. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific) at 3.0 × 10−10 mbar with monochromatic Al Kα radiation (E = 1486.2 eV). UV−vis diffuse reflectance spectra (DRS) of samples were collected by a Varian Cary 500 Scan UV−vis−NIR spectrometer with BaSO4 as the background and in the range 200−800 nm. Transmission electron microscopy (TEM) images were obtained by using a FEI Tecnai G2 F20 instrument operated at an accelerating voltage of 200 kV. The Brunauer−Emmett−Teller (BET) specific surface area and pore volume of the samples were measured on an Autosorb1C-TCD physical adsorption instrument (American Quantachrome) by N2 adsorption at 77 K. The photoelectrochemical experiment was conducted on CHI-660D electrochemical workstation (CH Instruments, USA). Photoelectrochemical measurements were carried out in a conventional threeelectrode cell with a quartz window. The sample was deposited on ITO glass to serve as the working electrode. The counter and reference electrodes were a platinum wire and an Ag/AgCl electrode, respectively. The electrolyte was 0.1 M Na2SO4 solution. The current polarity is cathodic in the system setup. The photoluminescence (PL) spectra were collected on an Edinburgh FL/FS900 spectrophotometer. The generation of hydroxyl radical was investigated by the PL technique with terephthalic acid (TA) as a probe molecule. The optimal concentration of TA solution was ∼5 × 10−4 M in a diluted NaOH aqueous solution (2 × 10−3 M).27−29 To detect the generation of activated species, spin-trapping electron spin resonance (ESR) measurement was conducted on a Bruker model A300 spectrometer. The settings were center field, 3512 G; microwave frequency, 9.86 GHz; microwave power, 20 mW. 2.4. Test of Photocatalytic Activity. The measurement of photocatalytic activity in oxidation of benzene on samples was carried out in 4.25 × 3.33 × 0.46 cm3 (internal capacity 1.37 cm3) fixed-bed quartz reactor operating in a continuous-flow single-pass mode and connected to an online gas chromatograph. The mass of photocatalysts loaded in the quartz reactor was 1.7 g, and all samples were passed through a stainless-steel sieve to obtain uniform size, which was 0.21 to 0.25 mm. The reactor was irradiated by a 500 W Xe-arc lamp. When the Xearc lamp was equipped with an IR-cutoff filter (λ < 900 nm) and a UV-cutoff filter (λ > 450 nm) at the same time, the visible light (450 nm < λ < 900 nm) was obtained. The simulated solar light was obtained by equipping Xe-arc lamp with an IR-cutoff filter (λ < 900 nm) only. A bubbler containing benzene was immersed in an ice−water bath. Benzene was bubbled with pure oxygen and then fed to catalyst at a total flow rate of 20 cm3 min−1 to maintain the gas hourly space velocity (GHSV) of 876 h−1. After the adsorption of benzene on photocatalysts in dark reached equilibrium, the photocatalysts were irradiated by light. The temperature of the reactions was controlled at 30 ± 1 °C by an air-cooling system. The inlet concentrations of benzene and carbon dioxide in the reaction stream were 280 and 0 ppm, respectively. The concentrations of benzene and CO2 from the outlet of the fixed-bed reactor was measured simultaneously by an online gas chromatograph (HP 6890) equipped with a flame ionization detector (FID), a thermal conductivity detector (TCD) and a Porapak R column. For comparison, the photocatalytic activities of self-made pure TiO2 (T500) and N-doped TiO2 (TiO2−xNx) were also measured under the same reaction conditions as those of Ag3VO4/TiO2 nanocomposites.

photocatalytic activities of decomposing benzene were not measured. Our research team has been working on the study of semiconductor coupling and has successfully developed a series of highly efficient visible light photocatalysts with heterojuntion structure in the past few years.21−25 In this article, we first synthesized Ag3VO4/TiO2 nanocomposite photocatalyst by a convenient and mild approach. By coupling them, we try to improve the low solar energy utilization of TiO2 and difficult separation of electron−hole pairs of Ag3VO4 at the same time. Photodecomposition of benzene was selected as a model reaction to evaluate the photocatalytic performance of Ag3VO4/ TiO2 nanocomposite photocatalyst. The electrochemical analyses as an effective tool was used to study the interface charge separation efficiency. During the photocatalytic degradation of gaseous benzene, this catalyst performed much higher activity than TiO2−xNx under visible light irradiation, and the superiority became more apparent under simulated solar light irradiation. This simple and low-cost approach would be a highly efficient means for broadening its further industrial application.

2. EXPERIMENTAL SECTION 2.1. Preparation of Ag3VO4. All chemicals were analytical grade without further purification. The silver nitrate (AgNO3) and sodium metavanadate (NaVO3) were used as the silver and vanadium sources, respectively. 0.1 M AgNO3 (60 mL) solution was added dropwise to 0.1 M NaVO3 (20 mL) solution under constant stirring for ∼30 min. The pH value of the mixture was adjusted to 11 using 4 M NaOH solution. After it was stirred for 5 h with a constant rate at room temperature, the suspended solution was poured in a 100 mL Teflon-lined stainless autoclave and heated to 140 °C for 8 h. The products cooling to room temperature were washed with distilled water by centrifugation and dried at 60 °C. 2.2. Synthesis of Ag3VO4/TiO2 Nanocomposites. TiO2 sol was prepared by a typical method. Ti(OC3H7)4 (15 mL) was dripped slowly into 180 mL of 0.1 M HNO3 solution; then, the mixture was peptized, dialyzed, and concentrated at room temperature to form a highly dispersed TiO2 colloidal solution. Then, Ag3VO4 synthesized by the hydrothermal method was added to TiO2 sol at different mass ratios of Ag3VO4 to TiO2. After they were ultrasonic-dispersed for 0.5 h and stirred for 24 h, these mixed colloids were dried in a microwave oven to remove solvents. Finally, these precursors were calcined at 500 °C for 6 h to obtain Ag3VO4/TiO2 nanocomposite catalysts. X % AT denotes the as-prepared sample, and X% is the mass fraction of Ag3VO4. Self-made pure TiO2 (T500) and nitrogendoped TiO2 (TiO2−xNx) were used as references. TiO2 sol was dried in a microwave oven and calcined at 500 °C for 6 h to obtain T500. TiO2−xNx was synthesized by a traditional method.26 TiO2 xerogel, which was obtained by drying selfmade TiO2 sol in a microwave oven, was calcined at 550 °C under dry NH3 flow for 3 h to get TiO2−xNx. 2.3. Photocatalysts Characterization. The X-ray diffraction (XRD) patterns of the powders were measured by a Bruker D8 Advance X-ray diffractometer at 40 kV and 40 mA with Ni-filtered Cu Kα radiation. Laser Raman spectra were recorded at room temperature in Renishaw inVia Raman systems, and the laser line at 785 nm was used as an excitation source. Thermal stability was detected on a differential scanning calorimetry with thermogravimetry attachment (TG-DSC, STA 449C Jupiter, Netzsch, Germany) from 50 to 700 °C in a 13936

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nanocomposites, these precursors were calcined at 500 °C for 6 h to obtain Ag3VO4/TiO2 nanocomposite catalysts. During the process of heat treatment, Ag3VO4 melted and diffused in TiO2 matrix, so Ag3VO4 particles became smaller and highly dispersed in TiO2. When the amount of Ag3VO4 reached 30%, the characteristic diffraction peaks of Ag3VO4 which broadened because of the small size of particles, became apparent. The Raman shift peak of Ag3VO4 was not observed until the content of Ag3VO4 increased to 10 wt % (Figure S2 of the Supporting Information). 3.1.3. Form of Ag3VO4 Present in Ag3VO4/TiO2 Nanocomposites. The diffraction peaks of Ag3VO4 in Ag3VO4/TiO2 nanocomposites cannot be observed when the mass fraction of Ag3VO4 is less than 10% owing to detection limit of XRD analysis (shown in Figure 2). Therefore, 5% AT nanocomposites were selected to be determined by XPS due to its moderate proportion. In Figure 3a, two peaks were observed at 368 and 374 eV for the binding energy of Ag, and these data corresponded well to Ag 3d5/2 and Ag 3d3/2, respectively, which is the characteristic of Ag+ in Ag3VO4.20 The binding energy value of Ag in Ag3VO4/TiO2 nanocomposites was the same as that of pure Ag3VO4. In Figure 3b, only one peak at binding energy of 516.7 eV in V 2p3/2 was present in Ag3VO4/TiO2 nanocomposite owing to the small proportion of Ag3VO4, whereas two signals were observed at binding energies of 516.5 and 523.9 eV, corresponding to V 2p3/2 and V 2p5/2 in pure Ag3VO4. The binding energy of the V in Ag3VO4/TiO2 values shifted slightly to the positive by ∼0.2 eV in comparison with pure Ag3VO4. The shifting of the binding energy value to slightly higher value can be attributed to the smaller relaxation energy of the highly dispersed Ag3VO4 in the Ag3VO4/TiO2 nanocomposites or instrumental error.30 Therefore, it can be deduced by XPS analysis that the Ag and V coexisted as the form of pure Ag3VO4 in the Ag3VO4/TiO2 nanocomposites. 3.1.4. Morphology of Ag3VO4/TiO2 Nanocomposite. Further information on the sample was obtained by its highresolution TEM (HRTEM) image. In the 0.5% AT nanocomposite, the fringes of d = 0.35 nm and d = 0.23 nm observed in Figure 4a matched those of (101) and (112) crystallographic planes of anatase TiO2. The fringe of d = 0.32 nm corresponded to the (110) crystallographic plane of rutile TiO2. Because the interplanar spacing 0.289 nm corresponding to (−1, 2, 1) of Ag3VO4 located at 30.86° was near 0.29 nm matched with (1, 2, 1) of brookite TiO2 located at 30.81°, it was hard to determine to which crystallographic plane the fringe of d = 0.29 nm existing in the Figure 4a belonged. The small mass fraction makes the odds of the fringes of Ag3VO4 appearing in HRTEM image very low. Therefore, 1% AT nanocomposite was also detected by HRTEM. In addition to the distinct fringe of d = 0.35 nm mentioned above, the fringe with interplanar spacing 0.28 nm corresponding to (1, 2, 1) crystallographic plane of Ag3VO4 also was clearly observed in Figure 4b. The intimate contact took place between the coupling Ag3VO4 and TiO2. 3.2. Tests of Photocatalytic Activity. 3.2.1. Comparison among Photocatalytic Activities of Ag3VO4/TiO2 Nanocomposites. Photocatalytic activities of samples for decomposing benzene in a continuous flow gas-phase system were evaluated under visible light irradiation (450 < λ < 900 nm). The inlet concentration of benzene was 280 ppm at a GHSV of 876 h−1. The conversion of benzene and the production of CO2 were not observed when these samples were in the dark. Figure 5 showed the photocatalytic activities of Ag 3VO4/TiO2

3. RESULTS AND DISCUSSION 3.1. Composition and Structure of Ag 3VO4/TiO2 Nanocomposites. 3.1.1. Thermal Stability of Ag3VO4. As shown in Figure 1a, Ag3VO4 powder prepared by hydrothermal

Figure 1. Comparison of XRD patterns of hydrothermally synthesized Ag3VO4 (a) without heat treatment, (b) heat-treated at 400 °C for 6 h, and (c) heat-treated at 500 °C for 6 h.

method was completely in accord with the monoclinic αAg3VO4 (JCPDS 43-0542). After it was calcined at 400 or 500 °C for 6 h, Ag3VO4 powder did not decompose or transform into other crystalline phase. The sharp endothermic peak at ∼450 °C in TG-DSC curve of Ag3VO4 (Figure S1 of the Supporting Information) indicated that the solid Ag3VO4 melted because Ag3VO4 did not decompose and had little weight loss. Ag3VO4 is stable enough to endure the harsh heat treatment at 500 °C for 6 h, which is the last step of preparing the Ag3VO4/TiO2 nanocomposites. 3.1.2. XRD Analysis of Ag3VO4/TiO2 Nanocomposites. The XRD patterns of T500, Ag3VO4/TiO2 in different proportions, and pure Ag3VO4 are shown in Figure 2. The main phase of

Figure 2. XRD patterns of T500, Ag3VO4/TiO2 in different proportions, and pure Ag3VO4.

T500 was rutile phase, whereas that of Ag3VO4/TiO2 was anatase phase. As the amount of Ag3VO4 increased in Ag3VO4/ TiO2 nanocomposites, the intensities of rutile diffraction peaks gradually decreased, and that of anatase diffraction peaks increased. That is to say, Ag3VO4 can effectively inhibit the phase transformation from anatase to rutile phase of TiO2. Comparing 0.5% AT with T500, the effect was already very obviously observed. On the basis of the TG-DSC analysis of Ag3VO4, Ag3VO4 has melted when the temperature is higher than 450 °C. As mentioned in synthesis of Ag3VO4/TiO2 13937

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Figure 3. XPS spectra of (a) Ag 3d and (b) V 2p in Ag3VO4 and 5% AT.

Figure 4. High-resolution TEM images of (a) 0.5% and (b) 1% AT nanocomposite.

Figure 5. (a) Conversion of C6H6 and (b) the amount of produced CO2 on Ag3VO4/TiO2 nanocomposites, T500 and Ag3VO4, under visible light irradiation.

the reaction was steady. The 0.5% AT was the focus of study due to its optimal photocatalytic activity. 3.2.2. Comparison among Photocatalytic Activities of Different Photocatalysts including 0.5% AT, T500, and TiO2−xNx. 0.5% AT was chosen to be further investigated due to its outstanding activity among a variety of Ag3VO4/TiO2 nanocomposite photocatalysts. It is well known that TiO2−xNx is an efficient photocatalyst under visible light irradiation, so it was used as a reference.26 From Figure 6a, the visible photocatalytic activity of 0.5% AT toward degrading gas phase benzene was nearly two times higher than that of TiO2−xNx after 10 h of visible light irradiation (450 < λ < 900 nm). As shown in Figure 6b, 0.5% AT nanocomposite exhibited notably high photocatalytic activity that was three times higher than that of TiO2−xNx, when the system was illuminated with

nanocomposite with different concentrations of Ag3VO4 under visible light irradiation. A trace amount of nitrogen is probably doped into titanium oxide, which causes a little visible light photocatalytic activity of as-synthesized T500.31−33 Ag3VO4 had little visible photocatalytic activity. However, Ag3VO4/ TiO2 nanocomposites performed high visible photocatalytic activity. As the content of Ag3VO4 increased in Ag3VO4/TiO2 nanocomposites, their photocatalytic activities gradually decreased. The photocatalytic activity of 10% AT was even lower than that of T500. 0.5% AT sample performed optimum activity of decomposing gaseous benzene in visible light irradiation. For 0.5% AT nanocomposite, the conversion rate of benzene was 40%, and the produced CO2 was ∼380 ppm corresponding to the mineralization rate of benzene 60% after 13938

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Figure 6. Comparison of photocatalytic activities of decomposing benzene under (a) visible light and (b) simulated solar light irradiation on T500, TiO2−xNx, and 0.5% AT.

Figure 7. Cycling tests on the photocatalytic degradation of benzene on 0.5% AT nanocomposite under visible light irradiation: (a) conversion of C6H6; (b) the amount of produced CO2.

Figure 8. Comparisons of (a) XRD and (b) Ag 3d XPS spectra between used sample and fresh sample.

simulated solar light (320 < λ < 900 nm). The conversion of benzene on 0.5% AT reached 95%, and more than 1420 ppm of CO2 was produced, corresponding to an high mineralization ratio of ∼84%. This meant that benzene was nearly completely photocatalytic degraded and mineralized into CO2 on 0.5% AT under simulated solar light irradiation and so 0.5% AT photocatalysts possessed a promising industrial application. All of these data indicated that the existence of Ag3VO4 in the

appropriate proportion is energetically favorable to enhance the photocatalytic activity of Ag3VO4/TiO2 nanocomposites. 3.2.3. Stability of Photocatalytic Performance of 0.5% AT Nanocomposite. As previously mentioned, 0.5% AT nanocomposite performed superior photocatalytic activity under both visible light and simulated solar light irradiation. Most importantly, excellent conversion and mineralization ratio of benzene on 0.5% AT could be maintained for >50 h under visible light irradiation, and there was no obvious catalyst 13939

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deactivation, as shown in Figure 7. On the basis of XRD and XPS analysis of used sample and fresh sample (Figure 8), the crystal structure and chemical state of 0.5% AT photocatalyst did not change after the photocatalytic reaction. Therefore, it is believed that 0.5% AT photocatalyst is stable. As we know, the activity stability is crucial to assess a photocatalyst and its application. Undoubtedly, the stability of 0.5% AT photocatalyst makes its extensive industrial application possible. 3.3. Role of Ag3VO4 in the Photocatalytic Activity of Ag3VO4/TiO2 Nanocomposites. 3.3.1. Components of Crystalline Phase. As mentioned in XRD analysis of Ag3VO4/TiO2 nanocomposites, Ag3VO4 can effectively inhibit the phase transformation from anatase to rutile phase of TiO2. The addition of Ag3VO4 made the main crystalline phase of TiO2 still anatase after it was heated to 500 °C. It is commonly believed that anatase is the active phase in photocatalytic reactions.34 An increasing number of studies have indicated that a bicrystalline framework of anatase and rutile shows much better photocatalytic activity than that of pure anatase TiO2.2 3.3.2. Optical Property. The UV−vis diffuse reflection spectra (DRS) of T500, Ag3VO4, and Ag3VO4/TiO2 nanocomposites are shown in Figure 9. The absorption edge of

Table 1. BET Specific Area of T500 and Ag3VO4/TiO2 Nanocomposites samples

T500

0.1% AT

0.5% AT

1% AT

2% AT

5% AT

SBET (m2·g−1)

28.24

35.93

48.98

54.56

58.34

38.33

decreased from 0.5% AT to 2% AT. N2-sorption isotherm of the 0.5% AT sample (Figure 10) belonged to type-IV isotherm. The pore size distribution was narrow, and the average pore diameter was 11 nm. The results indicated that 0.5% AT sample was a porous material.

Figure 10. Nitrogen adsorption−desorption isotherm and pore size distribution plot of the as-prepared 0.5% AT.

3.3.4. Enhancement of Interface Charge Separation Efficiency. A necessary step for semiconductor photocatalytic degradation is the generation and separation of electron hole pairs.35 The interface charge separation efficiency can be investigated by all kinds of photoelectrochemical tests, such as photocurrent spectra and electrochemical impedance spectroscopy (EIS).36 The photocurrent−time (I−t) profiles with zero bias electrode potential are shown in Figure 11. The photocurrent was rapidly generated at the beginning of illumination and soon reached a stable value on the interval time (20 s) of the pulsed visible light irradiation. The photocurrents of T500 and TiO2−xNx are too low to be observed and distinguished clearly in the Figure 11. Apparently,

Figure 9. UV−vis diffuse reflectance spectra of T500, Ag3VO4/TiO2 nanocomposites with different contents of Ag3VO4, and pure Ag3VO4.

Ag3VO4 was determined to be 560 nm, and the value of band gap estimated by the onset point of the absorption curve was 2.2 eV. The result was in agreement with that reported by Konta et al.13 As is known to all, the visible light absorption of materials is the precondition of some substance having visible photocatalytic activity. The visible light absorption of Ag3VO4/ TiO2 nanocomposites went up as the ratio of Ag3VO4 to TiO2 increased, but the increasing amplitude of visible light absorption gradually decreased. For 0.5% AT, its visible light absorption obviously enhanced compared with T500, which corresponded to its high visible photocatalytic activity. So the addition of Ag3VO4 contributed to enhancing visible light absorption of samples. 3.3.3. Analysis of Specific Surface Area. As we know, the specific area of material is closely related to the photocatalytic activity. High specific area is in favor of promoting photocatalytic activity. As displayed in Table1, the BET specific surface area of Ag3VO4/TiO2 increased as the content of Ag3VO4 increased, and then began to decrease when the content of Ag3VO4 exceeded the turning point 2%. The BET specific surface area of 0.5% AT was determined to be 48.98 m2·g−1, which was nearly twice as big as that of T500, but the amplitude of enhancement of visible light absorption gradually

Figure 11. Photocurrent of (a) 0.5% AT and (b) T500 or TiO2−xNx under visible light irradiation. 13940

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the photocurrent intensity of 0.5% AT was dramatically stronger than them. The photocurrent intensity mainly depended on electron generation capacity and electron transfer effectiveness between Ag3VO4/TiO2 nanoparticles and working electrode. The amount of excited electrons and the efficiency of interfacial electron transfer for 0.5% AT both were enhanced resulting from the existence of Ag3VO4. EIS was also used to investigate the separation process of photogenerated charges on T500, TiO2−xNx, and 0.5% AT under visible light irradiation (Figure S3 of the Supporting Information). The radius of the arc on the EIS Nynquist plot reflects the reaction rate occurring at the surface of the electrode.35 The arc radius on the EIS Nynquist plot of 0.5% AT electrode was smaller than that of the T500 electrode, which meant a more effective separation of photogenerated electron−hole pairs and faster interfacial charge transfer. 3.4. Discussion of Possible Photocatalytic Mechanism. Hydroxyl radical (OH•) has been considered to be a key species in the photocatalytic degradation of many hazardous chemical compounds for its high reaction ability to attack any organic molecule.4 The formation of OH• on the surface of 0.5% AT was detected by a PL technique with TA as a probe molecule. TA reacts with OH• readily to produce a highly fluorescent product, 2-hydroxyterephthalic acid, whose PL peak intensity is in proportion to the amount of OH radicals produced in water.29 As shown in Figure 12, the fluorescence

Figure 13. DMPO spin-trapping ESR spectra on 0.5% AT in aqueous dispersion for DMPO−OH•.

photoelectron spectroscopy (VB-XPS). As shown in Figure 14, for the Ag3VO4, its EF was ca. 1.09 eV above the VB top. The

Figure 14. VB-XPS spectra of the (a) Ag3VO4 and (b) TiO2.

band gap of Ag3VO4 was evaluated as 2.2 eV from the UV−vis DRS, which fairly accorded with experimental results reported in the literature.13 So, the EF in Ag3VO4 lied in the middle of CB and VB. Ag3VO4 was identified as intrinsic semiconductor. For the TiO2, its EF was located at 2.25 eV above the VB top. When two types of semiconductor materials are closely joined together, the heterojunction structure forms. Their respective Fermi levels shift with each other due to the effect of build-in internal electric field between them until the two semiconductors have the uniform Fermi level. On the basis of the above measurements and analysis, the relative positions of energy bands of Ag3VO4 and TiO2 were known and are shown in Scheme 1. As reported by Rehman et al., the efficient electron and hole transfer between the sensitizer and TiO2 depends on the difference between the respective CB and VB potentials of the two semiconductors.6 Obviously, the difference between energy bands of Ag3VO4 and TiO2 (Scheme 1) allows the efficient transfer of electron and hole between them. On the basis of the above experimental results, a possible photocatalysis mechanism (Scheme 1) for the degradation of benzene on Ag3VO4/TiO2 nanocomposite photocatalysts under visible light and simulated solar light irradiation was

Figure 12. OH• trapping PL spectra of 0.5% AT in TA solution under visible light irradiation.

intensity at 426 nm gradually increased with the irradiation time, which elucidated that OH• on 0.5% AT was really produced under visible light irradiation. Additionally, the OH• was not detected in the blank test (no photocatalyst) under visible light irradiation. Meanwhile, the existence of the activated species in the Ag3VO4/TiO2 system was detected by spin-trapping ESR technique. The results are shown in Figure 13. Four characteristic peaks with intensity 1:2:2:1 for DMPO−OH• were observed on 0.5% AT under visible light irradiation, indicating that the OH• radical formed. There was no signal in it when the Ag3VO4/TiO2 suspension was in the dark. Fermi level (EF), the chemical potential of electron in a semiconductor, is usually applied to index the direction of electron transfer between two semiconductors with intimate contact. To determine the position of EF with respect to valence band (VB) top of semiconductor, the total densities of VB states of the samples were measured by valence band X-ray 13941

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degrading flowing benzene steam under visible and simulated solar light irradiation. Its photocatalytic activity outdistanced that of TiO2−xNx; moreover, it was almost completely able to degrade and mineralize benzene under simulated solar light irradiation. In our Ag3VO4/TiO2 system, the advantages to increasing the photocatalytic activity of the degradation of benzene can be explained in the four facets: First, Ag3VO4 can effectively inhibit the phase transformation from anatase to rutile phase of TiO2 and so the appropriate framework of them contributed to the high photocatalytic efficiency of Ag3VO4/ TiO2 nanocomposite. Second, the large specific surface area of Ag3VO4/TiO2 nanocomposite was also favorable for photocatalytic activity. Third, the composite increased the visible light absorption and made the absorption edge shift to long wavelength region. Therefore, the essential role of the Ag3VO4/TiO2 semiconductors must be the transfer of electrons, injected by the excited Ag3VO4, to a suitable oxidizing agent (O2) at a separate position on the surface. Therefore, the recombination of the excited electrons and holes was effectively inhibited. Finally, the difference between energy bands of Ag3VO4 and TiO2 allowed the efficient electron and hole transfer between them, which meant a more effective separation of photogenerated electron−hole pairs.

Scheme 1. Proposed Mechanism for the Photocatalytic Degradation of Benzene on Ag3VO4/TiO2 Nanocomposite Photocatalyst under Visible Light Irradiation (Solid Line) and Simulated Solar Light Irradiation (Solid Line and Dashed Line)

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ASSOCIATED CONTENT

S Supporting Information *

proposed. Under visible light irradiation, Ag3VO4 was excited, and subsequently electrons were transferred from Ag3VO4 to CB of TiO2. A more effective separation of photogenerated electron−hole pairs and faster interfacial charge transfer in the Ag3VO4/TiO2 nanocomposite were verified by the photoelectrochemical measurement. The recombination of the excited electrons and holes was effectively inhibited. The excited electrons and transferred electrons can be trapped by surface oxygen to form O2•− and H2O2;37 then, the OH• can be formed by reacting O2•−or electrons with H2O2.28 Because the reaction ability of OH• is high enough to attack any organic molecules, it has been assigned as a key species in the mineralization mechanism of the photooxidation of benzene. Although, it is impossible to generate OH• radical through the direct oxidation of oxide hydroxyl, the photogenerated holes on Ag3VO4 can easily activate benzene, leading to a subsequent decomposition.23 When the sample was irradiated by simulated solar light, Ag3VO4 and TiO2 were both excited; then, a large number of electrons and holes were produced in the system. Electrons transferred from Ag3VO4 to CB of TiO2 and holes shifted from TiO2 to VB of Ag3VO4 at the same time. The recombination of the excited electrons and holes was more effectively inhibited than that under visible light irradiation. Moreover, the photogenerated holes on TiO2 can directly oxidate water to generate OH• radical. Compared with that under visible light irradiation, there were more pathways to produce OH• radical in the photocatalytic system under simulated solar light. No wonder that 0.5% AT performed high photocatalytic activity under simulated solar light irradiation. Further research of the explicit mechanism for the high photocatalytic activity of the Ag3VO4/TiO2 nanocomposite under visible light irradiation is now going on in our laboratory.

TG-DSC curves of hydrothermal prepared Ag3VO4; Raman spectra of (a) Ag3VO4, (b) T500, (c) 0.5% AT, and (d) 10% AT; Nyquist plots for T500, TiO2−xNx, and 0.5% AT nanocomposite under visible light irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NNSF of China (21173047, 21073036, and 21033003), National Basic Research Program of China (973 Program, 2007CB613306).

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REFERENCES

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4. CONCLUSIONS A series of Ag3VO4/TiO2 nanocomposite photocatalysts were synthesized by improved sol−gel coupling method. 0.5% AT nanocomposite performed optimal photocatalytic activity of 13942

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