TiO2 Nanocomposites with High Visible-Light

Luiz C. A. Oliveira , Eudes Lorençon , Adilson C. Silva , Lucas L. Nascimento .... Xiuqin Wu , Juan Zhao , Liping Wang , Mumei Han , Mengling Zha...
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Novel V2O5/BiVO4/TiO2 Nanocomposites with High Visible-LightInduced Photocatalytic Activity for the Degradation of Toluene Juanjuan Sun,† Xinyong Li,*,†,‡ Qidong Zhao,† Jun Ke,† and Dongke Zhang*,‡ †

State Key Laboratory of Fine Chemical, Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, Washington 6009, Australia ABSTRACT: In an effort to develop nanostructured photocatalysts to achieve high performance in heterogeneous photocatalysis, a novel composite V2O5/BiVO4/TiO2 photocatalyst was successfully synthesized by using a sequentially hydrothermal and adhering method. The structural and optical properties of the as-prepared samples were comparatively characterized. The formed ternary nanojunctions were composed of TiO2 nanobelts and V2O5/ BiVO4 nanorods which were self-assembled by smaller V2O5 and BiVO4 nanoparticles. Compared to pure TiO2 nanobelts and V2O5/BiVO4 nanorods, the V2O5/BiVO4/TiO2 composite exhibited higher photocatalytic activity in decomposition of gaseous toluene under visible light irradiation (λ > 400 nm). Electron spin resonance examination confirmed that the photoinduced active species (•OH and O2•−) were involved in the photocatalytic degradation of toluene. A detailed mechanism accounting for the enhanced photocatalytic activity of the V2O5/BiVO4/TiO2 nanocomposite was proposed in terms of the energy band structures of the components. The rationally designed ternary nanojunctions could effectively enhance the photocatalytic performance by increasing photoinduced charge carriers through the charges separation across their multiple interfaces.

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INTRODUCTION Semiconductor photocatalysis has emerged as one of the most promising ways for photoconversion-related technologies by utilizing the solar light in environment and energy fields. Particularly, photocatalytic oxidation has been applied extensively in the removal of organic pollutants in water and air as a cost-effective and environmentally friendly technology, which could completely decompose organic compounds into CO2 and H2O under mild conditions. Currently, general research interest in the related field has mainly focused on the preparation of photocatalysts with high activity, the exploration of photocatalytic reaction mechanism, and the design of practical reactors.1−3 The photocatalytic process is a consequence of the interaction between the semiconductor particles activated under light irradiation and the adsorbed substances. Electron−hole pairs are generated upon light absorption in these activated semiconductor particles, where they can then recombine or separate to participate in reductive and oxidative reactions, respectively.4 Therefore, the generation, efficient separation, and vectorial transfer of photoinduced charge carriers are essential prerequisite processes for photocatalytic reactions. Titania (TiO2) is a widely studied semiconductor photocatalyst because of its high chemical stability, low cost, and strong oxidizing ability, which has been involved a broad range of applications, such as water splitting, organic pollutants degradation, and photovoltaic devices.5,6 However, the wide © 2014 American Chemical Society

band gap of TiO2 (3.2 eV for anatase) causes its inactivity under visible light, which occupies the major part of solar light. Besides, the high recombination rate of the photoinduced charges hinders its practical applications. In order to overcome these limitations, many efforts have focused on the modification of the surface or bulk properties of TiO2 materials.7−9 A large number of TiO2-based composite materials have been designed and synthesized for obtaining improved photoconversion performance, by various strategies such as self-doping or doping with nonmetal atoms (C, N, and S),10−12 combination with noble metals particles (Ag, Pd, and Pt),13−15 or narrow band gap semiconductors (CdS, CdSe, WO3, and AgVO4)16−19 and dye sensitization.20,21 Among them, the formation of heterojunction structures between a narrow band gap semiconductor and TiO2 can efficiently extend the photosensitivity of TiO2 into the visible region. For such case, it is essential to rationally design multiple heterojunction structures with improved photogenerated electron−hole separation by energy band engineering. Vanadium oxide (V2O5, Eg = 2.3 eV) has drawn considerable interest recently because of its surface catalytic properties, optical properties,22,23 and especially its absorption capability of visible light. Recently, V2O5-based crystallites have been widely Received: February 6, 2014 Revised: March 27, 2014 Published: April 14, 2014 10113

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used in photocatalytic degradation of organic pollutants.24−27 Wang et al.28 reported that 1D TiO2/V2O5 heterostructure shows much higher photocatalytic performance than pure TiO2 nanofibers, revealing that by coupling with a small band gap semiconductor material, the visible-light-driven catalytic activity of the branched heterostructures was greatly improved. Meanwhile, monoclinic BiVO4 (m-BiVO4) with narrow band gap (2.4 eV) as an attractive material has been reported to own excellent activity of photocatalytic O2 evolution and organic pollutants degradation under visible-light irradiation.29−33 Furthermore, a promising BiVO4-based composite of V2O5/ BiVO4 has demonstrated higher catalytic activity in comparison with bare BiVO4.34,35 However, the experimental realization and theoretically understanding of an effective ternary photocatalyst based on V2O5, BiVO4, and TiO2 remains a challenge. Herein, to utilize visible light more efficiently and promote the separation of charge carriers in photocatalytic reaction process, we have chosen orthorhombic V2O5 and m-BiVO4 as coupling semiconductors and built a novel ternary system of V2O5/BiVO4/TiO2 nanobelts. The V2O5/BiVO4 heterojunction was prepared first by a facile one-step hydrothermal method, and then the V2O5/BiVO4 nanorods were loaded onto the surface of TiO2 nanobelts. During the photocatalytic degradation of gaseous toluene under visible light irradiation, the V2O5/BiVO4/TiO2 nanocomposites exhibited much higher photocatalytic activity than TiO2 nanobelts and V2O5/BiVO4 nanorods. The photocatalytic mechanism of V2O5/BiVO4/ TiO2 under visible light was discussed in detail.

the mixture was sonicated for ca. 30 min. The dispersion was first stirred at room temperature for about 2 h and then stirred at 70 °C in a oil bath pot to evaporate the cyclohexane. The resulting light yellow powder was collected for further characterization. Photocatalyst Characterization. The morphology of the as-prepared samples was characterized using a scanning electron microscope (SEM) (JEOLJSM-6360LV microscope) with an accelerating voltage of 30.0 kV. Transmission electron microscopy (TEM) image was obtained with a JEM-2100F electron microscope (JEOL, Japan), using a 200 kV accelerating voltage. The crystalline phase of the samples was determined from a Rigaku D/Max 2550VB/PC X-ray diffractometer (XRD) with a Cu Kα radiation source (k = 0.154 056 nm), which operated at a voltage of 40 kV and a current of 30 mA. Xray photoelectron spectroscopy (XPS) data that determined the chemical composition of V2O5/BiVO4/TiO2 powder were recorded with a PerkinElmer PHI 5600 electron spectrometer. All of the binding energies were calibrated by using the contaminant carbon (C 1s) at 285.38 eV as a reference. The light absorption properties were measured using a UV−vis diffuse reflectance spectrophotometer (DRS) (JASCO, UV550) with a wavelength range of 200−800 nm. The photoluminescence (PL) characteristics were probed using a Hitachi F-4500 fluorescence spectrophotometer. The electron spin resonance (ESR) signals of radicals spin-trapped by spintrap reagent 5,5′-dimethyl-1-pirroline-N-oxide (DMPO) (purchased from Sigma Chemical Co.) were detected on a Bruker ECS106 X-band spectrometer. The irradiation source was a high-pressure xenon short arc lamp (a Phillips 500 W Xe lamp) with a UV-cutoff filter (λ > 400 nm), and the light intensity at the position of sample is 180 mW/cm2. Typical experimental conditions were as follows: center field = 3400 G, sweep width = 300 G, frequency = 9.865 GHz, temperature = 298 K, modulation amplitude = 1.00 G, and microwave power = 20 mW. To minimize measurement errors, the same quartz capillary tube was used throughout the ESR measurements. Photocatalytic Activity Evaluation. Photocatalytic activities of the catalysts were measured using the photocatalytic conversion of toluene under visible-light irradiation in a homebuilt quartz photoreaction cell about 120 mL, equipped with two KBr windows and a sample holder (diameter, 13 mm) for the catalyst wafer (0.03 g). The reaction atmosphere was air whose relative humidity was 45%, and the reaction temperature was about 35 °C. After the catalyst was placed in the sample holder, a small amount of toluene (4 μL) was injected into the reactor with a microsyringe. When the toluene reached adsorption equilibrium in the reactor for 1 h, the xenon lamp (XQ-500W) with a 400 nm UV-cutoff filter was turned on. The IR spectra were continuously collected on the VERTEX 70FTIR with a resolution of 1 cm−1 and 20 scans in the region of 4000−600 cm −1 during the course of reaction. The concentrations of toluene were analyzed by a gas chromatogram (Agilent 7890A) equipped with FID (HP-5 capillary column (30 m × 320 μm × 0.25 μm)) and TCD (Porapak Q).

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EXPERIMENTAL SECTION Synthesis of TiO2 Nanobelts. All of the chemical reagents (Sinopharm Chemical Reagents Co., Ltd., China) were analytical grade and used as received without further purification. In a typical synthesis, 1.44 g of commercial TiO2 powders were dispersed in 96 mL of 10 M NaOH solution with constant stirring for 1 h, and then the mixed solution was put into a 120 mL Teflon-lined autoclave at 200 °C for 24 h. After hydrothermal processing, a white fluffy powder was collected and repeatedly washed with 0.1 M HCl solution and distilled water in sequence until the pH of the washing solution was less than 7 and then dried at 80 °C overnight. The samples were finally heated in a muffle furnace at 700 °C for 30 min at a ramp rate of 1 °C/min. As a result, the pristine anatase TiO2 nanobelts were obtained. Synthesis of V2O5/BiVO4 Self-Assembled Nanorods. Bi(NO3)3·5H2O, Na3VO4·12H2O, and sodium oleate were purchased from the Shanghai Reagent Company (China) and were used as purchased without further purification. In a typical synthesis, 2.88 mmol of sodium oleate was added to 48 mL of distilled water, and then (0.96 mmol) Bi(NO3)3·5H2O was put into the solution of sodium oleate with vigorously stirring for ca. 15 min. Then, a solution of Na3VO4·12H2O (0.96 mmol) in 48 mL of distilled water was then injected into the above solution. After vigorous stirring for 30 min, the mixture was transferred to a 120 mL Teflon-lined autoclave, sealed, and heated at 100 °C for 12 h. The system was then allowed to cool down to room temperature naturally; the supernatant was collected and extracted with acetone. The final product was washed with cyclohexane and absolute ethanol many times and then dried under vacuum overnight for further characterization. Synthesis of the V2O5/BiVO4/TiO2 Nanocomposites. 50 mg of V2O5/BiVO4 nanorods, 600 mg of TiO2 nanobelts, and 60 mL of cyclohexane were put into a 100 mL beaker, and then

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RESULTS AND DISCUSSION Characterizations of Photocatalysts. Figure 1 shows typical SEM, TEM, and HRTEM images of the TiO2 nanobelts, V2O5/BiVO4 self-assembled nanorods, and the V2O5/BiVO4/ TiO2 nanocomposites. In Figure 1a, it can be seen that the asprepared V2O5/BiVO4 appears as self-assembled nanorods with lengths in the range of 50−100 nm, consisting of V2O5 and 10114

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Figure 2. XRD patterns of (a) V2O5/BiVO4 and (b) TiO2 nanobelts and V2O5/BiVO4/TiO2 nanocomposites. Figure 1. Typical SEM and TEM images of the as-prepared samples. SEM (a) and HRTEM (b) images of the V2O5/BiVO4 self-assembled nanorods. (c) SEM image of the TiO2 nanobelts. Typical SEM (d), TEM, and HRTEM (e) images of the V2O5/BiVO4/TiO2 nanocomposites.

(112), (004), (200), (024), and (215) of monoclinic BiVO4 (JCPDS No. 75-2480), respectively. The peaks around 2θ of 17.65°, 26.37°, 27.88°, 32.33°, and 47.82° could be indexed to the characteristic peaks (002), (011), (111), (302), and (214) of orthorhombic V2O5 (JCPDS No. 85-2422), respectively. In Figure 2b, it can be seen that the prepared pure TiO2 powder is in good agreement with the anatase phase of TiO2 (JCPDS No. 71-1166). Moreover, the V2O5/BiVO4 modification on TiO2 nanobelts does not cause any change in the XRD peak positions compared with pure TiO2 nanobelts, which indicates that V2O5/BiVO4-modified TiO2 prepared by this adhering method retains the crystalline structure of neat TiO2 nanobelts. However, in the V2O5/BiVO4/TiO2 sample, the low amount (8 wt %) of V2O5/BiVO4 and relatively low diffraction intensity of BiVO4 resulted in no distinct peaks that could be observed corresponding to BiVO4. X-ray photoelectron spectroscopy (XPS) was employed to characterize the valence states and the surface chemical compositions. From Figure 3a, by comparing with pure TiO2 nanobelts, Bi and V elements are found in V2O5/BiVO4/TiO2 nanocomposites. In Figure 3b, two peaks for the Ti 2p at 464.8 and 459.2 eV are assigned to Ti 2p1/2 and Ti 2p3/2, respectively. These values agree well with XPS data in the literature and are assigned to Ti4+ in pure anatase titania.36 It seems that V2O5/ BiVO4 loading does not have significant effect on the position of Ti 2p peak. The XPS peak for C 1s at 285.38 eV is observed due to adventitious carbon from sample fabrication (Figure 3c). Two bands at 159.1 and 164.4 eV can be ascribed to the Bi 4f5/2 and Bi 4f7/2 binding energies, respectively, which confirms that the bismuth species are Bi3+ cations (Figure 3d).37 From Figure 3e, it is found that the binding energies of the vanadium peaks are centered at 517.2 and 524.5 eV, which is characteristic of

BiVO4 nanoparticles. Moreover, the HRTEM image in Figure 1b evidences the crystalline structure of the V2O5/BiVO4 nanorods. The lattice fringes of d = 0.337 and d = 0.259 nm match well with the crystallographic planes of orthorhombic V2O5 (011) and monoclinic BiVO4 (200), respectively. The SEM image of the pure TiO2 nanobelts is displayed in Figure 1c, with widths ranging from 50 to 200 nm and lengths from 500 nm to several micrometers. As shown in Figure 1d, the morphology of as-synthesized V2O5/BiVO4/TiO2 nanocomposites indicates that the V2O5/BiVO4 self-assembled nanorods has been deposited onto the TiO2 surfaces, which is further revealed by the low- and high-magnification TEM images (Figure 1e). In Figure 1e, the observed lattice fringes of 0.352, 0.259, and 0.200 nm correspond to the (101) plane of anatase TiO2, the (200) plane of monoclinic BiVO4, and the (214) plane of orthorhombic V2O5, respectively. Distinct borderlines between isolated units of V2O5, BiVO4, and TiO2 nanoparticles could be observed, which suggests that V2O5/BiVO4 nanorods have been successfully combined with the TiO2 nanobelts and the ternary heterojunction microstructure indeed formed from the three kinds of nanocrystallites. The XRD patterns of TiO2 nanobelts, V2O5/BiVO4 selfassembled nanorods, and V2O5/BiVO4/TiO2 nanocomposites are shown in Figure 2. The XRD pattern of V2O5/BiVO4 selfassembled nanorods displays diffraction peaks around 2θ of 18.98°, 28.80°, 30.53°, 34.50°, 47.27°, and 55.90° (Figure 2a), which could be indexed to the characteristic peaks (011), 10115

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Figure 3. The whole XPS spectra (a) of pure TiO2 nanobelts and V2O5/BiVO4/TiO2 nanocomposites. (b) Ti 2p XPS spectrum, (c) C 1s XPS spectrum, (d) Bi 4f XPS spectrum, (e) V 2p XPS spectrum, and (f) O 1s XPS spectrum of V2O5/BiVO4/TiO2 nanocomposites.

the V5+ oxidation state.38 The asymmetric O 1s signal indicates the presence of different oxygen species on the surface of the sample (Figure 3f). After Gaussian−Lorentzian fitting of the peak shape of the O 1s signal, the bands at 529.7 and 532.1 eV could be attributed to the surface lattice oxygen and adsorbed oxygen species, respectively.39,40 The UV−vis diffuse reflection spectra of the prepared samples are displayed in Figure 4a, which indicate that the TiO2 nanobelts absorbed mainly UV light, whereas the absorption edge of the V2O5/BiVO4/TiO2 nanocomposites extend to visible wavelength of 520 nm. The optical band gap energy can be estimated from the Tauc’s plots by the equation αhν = A(hν − Eg)n/2, where α is the absorption coefficient near the absorption edge, h is the Planck constant, A is a constant, ν is light frequency, Eg is the absorption band gap energy, and n is 1 and 4 for a direct and indirect band gap semiconductor,

respectively.41,42 Plots of (αhν)2 versus hν of the samples are show in Figure 4b. The band gaps of TiO2 nanobelts and V2O5/BiVO4 nanorods are estimated to be about 3.16 and 2.12 eV, while the band gap of V2O5/BiVO4 loaded on TiO2 nanobelts to be about 2.34 eV. This comparison suggests that the loading of V2O5/BiVO4 significantly enhances the visible light absorption of TiO2. PL emission results from the recombination of excited charge carriers, and there is a strong correlation between PL emission intensity and photocatalytic performances.43,44 Semiconductors with lower PL intensities usually exhibit higher photocatalytic activity due to the lower charge recombination rate. Figure 5 presents the room temperature PL spectra of the as-prepared samples in the air with the excitation wavelength at 325 nm. By contrast, the V2O5/BiVO4/TiO2 nanocomposites exhibit much lower emission intensity than pure TiO2 nanobelts obviously. 10116

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the surface of the V2O5/BiVO4/TiO2 wafer, the reaction cell was irradiated with visible light. The IR characteristic peaks of various species yielded by toluene degradation are shown in Figure 6. As can be seen in Figure 6a, before visible-light irradiation (t = 0), the absorption bands are strong at 3075, 3038, 2937, and 2884 cm−1, which are assigned to the C−H stretching (3075 and 3038 cm−1) mode and methyl stretching vibration (2937 and 2884 cm−1) of toluene, respectively. The bands of 1611 and 1500 cm−1 are associated with vibrations of an aromatic ring.48,49 As the irradiation time extended, the intensity of bands at 3075, 3038, 2937, and 2884 cm−1 began to decrease significantly. After 6 h, most of toluene was decomposed. Meanwhile, the bands at 2360 and 2340 cm−1 (Figure 6c) corresponding to CO2 increased as the reaction proceeded, and some new surface species were formed, as evidenced by the FTIR spectra in Figure 6b. New bands appeared at 1455, 1473, 1508, 1520, 1540, 1635, and 1650 cm−1. Among them, bands at 1473 and 1455 cm−1 are assigned to the benzyl alcohol. The bands located at 1540 and 1508 cm−1 account for the formation of CO bonding, implying the attack on the carbon structures by the activated oxygen species or hydroxyl radical and the formation of benzaldehyde species during the reaction process. In addition, the bands at 1520 and 1560 cm−1 are assigned to the asymmetric stretching vibration of the carboxylate group COO−, while the bands at 1635 and 1650 cm−1 are associated with CC of benzoic acid.49,50 The hydroxyl groups on the photocatalyst play different roles in the photo-oxidation process: the surface hydroxyls with bands at 3688, 3672, and 3648 cm−1 act as adsorption sites, while the surface hydroxyls with bands at 3734, 3723, and 3715 cm−1 act as the sources of • OH radicals, which are formed by photoholes reacting with hydroxyl groups adsorbed on the surface of the photocatalyst. The bands at 3630 and 3615 cm−1 are assigned to water species adsorbed on the surface.51 According to the above results and previous report,52 toluene is initially partially photooxidized to benzyl alcohol by the •OH radical. Benzyl alcohol is then further oxidized into benzaldehyde and then benzoic acid, which is finally oxidized into carbon dioxide and water. Photocatalytic Activity Test. The photocatalytic conversion of toluene over different samples under visible light irradiation is shown in Figure 7a. The initial concentration of 120 ppm for toluene was used for reaction. Without the presence of a catalyst, the rate of toluene degradation could be neglected under visible light irradiation. The conversion of toluene was about 12%, 39%, and 70% over P25, pure TiO2 nanobelts, and V2O5/BiVO4 nanorods after 6 h reaction, respectively. After the introduction of V2O5/BiVO4, the photocatalytic activity improved tremendously compared with original TiO2 nanobelts. The conversion of toluene over the ternary nanocomposite was as high as 91% under the same conditions. A pseudo-first-order kinetic model was employed to fit the degradation data by using the linear transformation: ln(C0/Ct) = F(t) = kt (k is the kinetic constant), as shown in Figure 7b. The kinetic constants of toluene degradation over the V2O5/BiVO4/TiO2 nanocomposites, V2O5/BiVO4 nanorods, pure TiO2 nanobelts, P25, and without photocatalyst are 0.332, 0.182, 0.081, 0.023, and 0.004 min−1, respectively. Mechanism of Enhanced Photocatalytic Activity. The electron spin resonance (ESR) technique was used to explore the reactive species evolved during the photocatalytic reaction process, such as hydroxyl radical and superoxide radical species. As shown in Figure 8, four characteristic peaks of DMPO−•OH

Figure 4. Optical absorption properties of the as-prepared samples: (a) UV−vis spectra and (b) energy gap calculated by plotting (αhν)2 versus hν.

Figure 5. PL spectra of as-prepared TiO2 nanobelts, V2O5/BiVO4 nanorods, and the V2O5/BiVO4/TiO2 composite with the excitation wavelength of 325 nm.

The lowering of emission intensity is an indicative of a decrease in irradiative recombination of the charges for the V2O5/ BiVO4-modified TiO2 nanobelts. The typical emission peaks appear at 400 and 468 nm. Herein, the PL signal at 400 nm can be attributed to band edge emission of self-trapped excitons localized on TiO2 nanobelts. A strong 468 nm luminescence peak might be caused by the surface states related recombination of the electron−hole pairs in TiO2 nanobelts and V2O5/BiVO4 nanorods.45−47 FTIR Analysis. In-situ infrared spectra could provide a realtime monitoring of transient events occurring on the catalyst during the foreign species adsorption and their reactions on the surface, which is very useful in clarifying catalytic mechanisms. In this work, when toluene reached adsorption equilibrium on 10117

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Figure 6. (a−d) FTIR spectra recorded as a function of irradiation time following the photo-oxidation of toluene over the V2O5/BiVO4/TiO2 composite under the visible-light irradiation.

Figure 7. (a) Comparison of photocatalytic performance of the samples by testing the degradation of toluene under visible-light irradiation. (b) Kinetics of toluene decomposition over the different samples.

Figure 8. ESR spectra of radical adducts trapped by DMPO in TiO2, V2O5/BiVO4 and V2O5/BiVO4/TiO2 dispersions after 60 s visible-light irradiation (λ > 400 nm): (a) DMPO−•OH formed in irradiated aqueous dispersions; (b) DMPO−O2•− formed in irradiated methanol dispersions.

(1:2:2:1 quartet pattern) can be observed with visible-lightirradiated aqueous dispersions of V2O5/BiVO4/TiO2 nano10118

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which have strong oxidizing power, are more favorable to react with adsorbed H2O to produce reactive •OH radicals; the photogenerated electrons are good reductants, which could be captured by the adsorbed O2 molecules on the surface of the catalyst and reduce them to O2•− radicals.56 These radicals are main active species to attack the toluene molecules adsorbed on the surface of nanocomposites during the degradation process. Therefore, the charges transfer between different components should be very beneficial for promoting the photocatalytic reactivity.

composites, while weak peaks are detected for V2O5/BiVO4 dispersion and no such signals for TiO2 nanobelts dispersion under identical conditions. Similarly, the stronger six characteristic peaks of the DMPO−O2•− adducts are also observed with visible-light-irradiated methanol dispersions of V2O5/BiVO4/ TiO2 nanocomposites. ESR results indicate that certain visible light irradiation is crucial to the generation of •OH radical and O2•− radical species, and it is confirmed that both •OH and O2•− are produced on the surface of V2O5/BiVO4/TiO2 nanocomposites and •OH radicals with strong oxidation capability act as the predominant species. To clarify the photocatalytic mechanism based on the electronic structures, an illustration of interparticle electron transfer behavior is proposed in Figure 9. The conduction band,

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CONCLUSIONS In summary, novel V2O5/BiVO4/TiO2 ternary nanocomposites have been successfully synthesized. The introduction of V2O5/ BiVO4 has been found to extend the spectral response of TiO2 from the UV to visible region. The V2O5/BiVO4/TiO2 nanocomposites exhibit a higher photocatalytic activity for the decomposition of gaseous toluene compared to pure TiO2 and V2O5/BiVO4 under visible light irradiation. Such V2O5/ BiVO4/TiO2 nanocomposite fabricated may be utilized as a new visible-light-driven photocatalyst for removing volatile organic pollutants in the ambient environment. We demonstrate that the rationally designed ternary nanojunctions could effectively enhance photocatalytic performance by increasing photoinduced charge carriers through the charge separation across their multiple interfaces.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (X.L.). *E-mail [email protected] (D.Z.). Notes

The authors declare no competing financial interest.

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Figure 9. Schematic of energy bands matching and electron−hole pairs separation of the V2O5/BiVO4/TiO2 nanocomposites under visible-light irradiation.

ACKNOWLEDGMENTS This work was supported financially by the National Nature Science Foundation of China (21377015, 21207015, N_HKUST646/10), the Major State Basic Research Development Program of China (973 Program) (No. 2011CB936002), and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.

valence band, and Fermi level of BiVO4, V2O5, and TiO2 are shown in Figure 9a. When the three semiconductor materials (BiVO4, V2O5, and TiO2) are joined together, Fermi levels of three components have the trend to reach a balanced value due to the system thermal equilibrium.28,33,53−55 As a result, the conduction bands and valence bands of V2O5 and BiVO4 shift upward, lying above the conduction band bottom of TiO2 (Figure 9b). When the ternary system is irradiated under visible light (>400 nm), electrons in the valence band (VB) of BiVO4 and V2O5 are excited to the conduction band (CB), creating holes in their VB. Because the CB edge potential of BiVO4 is more negative than that of V2O5 and TiO2, the photogenerated electrons of BiVO4 tend to migrate toward the CB of V2O5 and TiO2. Simultaneously, at the interface between V2O5 and BiVO4, the photogenerated holes in the VB of V2O5 could flow into the VB of BiVO4 due to the more negative VB edge of BiVO4 than that of V2O5. Meanwhile, as the CB edge potential of V2O5 is more negative than that of TiO2, the photogenerated electrons from V2O5 can also be injected into the conduction band of the TiO2 across their interface. Thus, the V2O5/BiVO4/ TiO2 nanocomposite has a much higher performance of photogenerated carriers separation compared with the V2O5/ BiVO4 nanorods, which has also been revealed by the results of PL and ESR. Thermodynamically, the photogenerated holes,

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