Enhanced Visible Light Photocatalytic Activity of V₂O₅ Cluster Modified N-Doped TiO₂ for Degradation of Toluene in Air

Abstract

V₂O₅ cluster-modified N-doped TiO₂ (N-TiO₂/V₂O₅) nanocomposites photocatalyst was prepared by a facile impregnation-calcination method. The effects of V₂O₅ cluster loading content on visible light photocatalytic activity of the as-prepared samples were investigated for degradation of toluene in air. The results showed that the visible light activity of N-doped TiO₂ was significantly enhanced by loading V₂O₅ clusters. The optimal V₂O₅ loading content was found to be 0.5 wt.%, reaching a removal ratio of 52.4% and a rate constant of 0.027 min⁻¹, far exceeding that of unmodified N-doped TiO₂. The enhanced activity is due to the deposition of V₂O₅ clusters on the surface of N-doped TiO₂. The conduction band (CB) potential of V₂O₅ (0.48 eV) is lower than the CB level of N-doped TiO₂ (−0.19 V), which favors the photogenerated electron transfer from CB of N-doped TiO₂ to V₂O₅ clusters. This function of V₂O₅ clusters helps promote the transfer and separation of photogenerated electrons and holes. The present work not only displays a feasible route for the utilization of low cost V₂O₅ clusters as a substitute for noble metals in enhancing the photocatalysis but also demonstrates a facile method for preparation of highly active composite photocatalyst for large-scale applications.

1. Introduction

Environmental pollution and energy crisis are the two major global challenges faced by human beings. Semiconductor photocatalysis is green technology that allows the use of sunlight for the destruction of pollutants and conversion of solar energy to hydrogen, thus providing an attractive route to potentially solve the both problems [1–3]. The widely used photocatalyst TiO₂ is, however, only UV active due to its relatively large band gap [4, 5]. As UV light accounts for a small fraction (4%) of the sun’s energy compared to visible light with a large fraction of 45%, the shift in the optical absorption of TiO₂ from the UV to the visible region is of significance for practical application of photocatalyst [6–8].

Visible light driven photocatalysis is one of the hottest topics worldwide [9–22]. Pioneered by Asahi et al. who reported that nitrogen doping can extend the optical absorption of TiO₂ into visible region and enhance the photocatalytic activity under visible light irradiation [9], many efforts have been devoted afterwards to developing nonmetal doped TiO₂ exhibiting enhanced visible light activities [10–14]. Among the various types of nonmetal doped TiO₂, N-doped TiO₂ is the most typical and has been intensively investigated for solar energy conversion and pollutants degradation [15–18]. However, nitrogen doping intrinsically favored the formation of defects that act as recombination center, which largely limited the photoactivity of N-doped TiO₂ under visible light illumination [19–22].

In order to improve the visible light photocatalytic performance of N-doped TiO₂, further modification has been employed, including codoping with nonmetals (C, S, F) [23–25], co-doping with metal ions (Fe, La, V) [26–28], noble metal deposition (Pt, Au) [29, 30] and metal oxide coupling (WO₃, PdO, ZrO₂) [31–33]. The modifications usually show promotive effects on visible light photocatalytic activity of N-doped TiO₂. The activity promotion is related to phase structure, optical absorption, charge transfer, and surface properties. Recently, nanoscale clusters have been utilized to modify different types of photocatalysts [34–42]. Highly enhanced photocatalytic activities over the investigated cluster/photocatalyst composites systems, including Ni(OH)₂/TiO₂, Cu(OH)₂/TiO₂, CuO/self-doped TiO₂, V₂O₅/C-doped TiO₂, V₂O₅/TiO₂, vanadium species/N-doped TiO₂, Fe₂O₃/WO₃, and CdS/graphene have been observed [34–42]. However, to our knowledge, there are few reports on the enhanced photocatalytic activity over V₂O₅ cluster-modified N-doped TiO₂ up to now.

In our previous study, N-doped TiO₂ (N-TiO₂) photocatalyst was prepared by partial oxidation of TiN in air [43]. In the present study, we report the facile preparation of V₂O₅ cluster-modified N-doped TiO₂ nanocomposite for the first time through an impregnation-calcination method and enhanced visible light photocatalytic activity for degradation of toluene in air. The microstructure, optical, and surface properties of the resulted nanocomposites photocatalysts were investigated systematically by XRD, Raman, TEM, XPS, UV-vis DRS, and PL. Based on the characterization results, a new mechanism on the promotive effects of V₂O₅ cluster modification on the charge transfer and visible light photocatalysis of N-doped TiO₂ was discussed and proposed.

2. Experimental Section

2.1. Preparation of Photocatalysts

N-doped TiO₂ (N-TiO₂) was prepared by partial oxidation of TiN. In a typical process, 3.0 g TiN powder was loaded in a ceramic crucible, and then placed in the middle of muffle furnace open to the atmosphere. The temperature was slowly ramped up to 450°C at a rate of 15°C/min and kept for 2 h to obtain N-doped TiO₂. V₂O₅ cluster modification was performed by incipient wetness impregnation of N-doped TiO₂ with aqueous solutions of NH₄VO₃ at room temperature, followed by stirring for 1 h, treating at 150°C for water evaporation and heating at 300°C for 1 h. The amount of loaded vanadium was controlled at 0, 0.01, 0.05, 0.2, 0.5, and 1.0 wt.%. The as-prepared samples were labeled as N-TiO₂/V₂O₅–x, where x represented the content of vanadium. For comparison, V₂O₅ (vanadium: 0.5 wt.%) was also loaded on SiO₂ instead of N-doped TiO₂ by the same process. The reference sample was labeled as SiO₂/V₂O₅–0.5%. Pure V₂O₅ was prepared accordingly in the absence of N-doped TiO₂ and SiO₂.

2.2. Characterization

The crystal phase of the samples was analyzed by X-ray diffraction with Cu Kα radiation (XRD: model D/max RA, Rigaku Co., Japan). Raman spectra were recorded at room temperature using a micro-Raman spectrometer (Raman: RAMANLOG 6, USA) with a 514.5 nm Ar⁺ laser as the excitation source in a backscattering geometry. The morphology and structure of the samples were examined by transmission electron microscopy (TEM: JEM-2010, Japan). X-ray photoelectron spectroscopy with Al Kα X-rays (ℎ𝜈=1486.6 eV) radiation (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties of the samples. The shift of the binding energy was corrected using the C1s level at 284.8 eV as an internal standard. The UV-vis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UV-vis spectrophotometer (UV-vis DRS: UV-2450, Japan) equipped with an integrating sphere assembly, using BaSO₄ as reflectance sample. The photoluminescence spectra were measured with a fluorospectrophotometer (PL: Fluorolog-3-Tau, France) using a Xe lamp as excitation source with optical filter. Nitrogen adsorption-desorption isotherms were obtained on a nitrogen adsorption apparatus (ASAP 2020, USA) with all samples degassed at 200°C prior to measurements. The BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (𝑃/𝑃₀) range of 0.05–0.3.

2.3. Visible Light Photocatalytic Activity

The visible light photocatalytic activity was evaluated by removal of toluene in air in a continuous flow reactor at ambient temperature. The volume of the rectangular reactor, made of stainless steel and covered with Saint-Glass, was 4.5 L (30 cm × 15 cm × 10 cm). A 300 W commercial Xe lamp was vertically placed outside the reactor. Four minifans were used to cool the lamp. For the visible light photocatalytic activity test, a UV cut-off filter (420 nm) was adopted to remove UV light in the light beam. Photocatalyst (0.20 g) was coated onto a dish with a diameter of 12.0 cm. The coated dish was then treated at 70°C to remove water in the suspension. The toluene gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of toluene. The initial concentration of toluene was diluted to 1.0 ppm at indoor level by the air stream supplied by compressed gas cylinder. The relative humidity (RH) level of the flow system was controlled at 50%. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 1.0 L/min by a mass flow controller. After the adsorption-desorption equilibrium was achieved, the lamp was turned on. The concentration of toluene was continuously measured by GC-FID (Shanghai, 7890II). The removal ratio (𝜂) of toluene was calculated as 𝜂(%)=(1−𝐶/𝐶₀)×100%, where 𝐶 and 𝐶₀ are concentrations of toluene in the outlet steam and the feeding stream, respectively. The kinetics of photocatalytic toluene removal reaction is a pseudo first-order reaction as ln(𝐶₀/𝐶)=𝑘𝑡, where 𝑘 is the initial apparent rate constant [44, 45].

3. Results and Discussion

3.1. Phase Structure

The XRD patterns of as-prepared samples are shown in Figure 1. The phase structure of N-TiO₂ and V₂O₅ cluster-modified N-TiO₂ samples consists of mixed phases of anatase (JCPDS file No. 21–1272) and rutile (JCPDS, file no. 77–442). In the absence of substrate N-TiO₂, pure orthorhombic V₂O₅ phase (JSPD file no. 72–433) was produced. No characteristic diffraction peaks of V₂O₅ are observed in N-TiO₂/V₂O₅ composite samples because of its lower loading content, on the other hand, also indicating that V₂O₅ clusters were well dispersed on the N-TiO₂ surface [34, 35]. Figure 1 also shows that V₂O₅ cluster loading has almost no influence on the phase structure of N-TiO₂. By using the Debye-Scherrer equation, the crystallite sizes of anatase and rutile phase are calculated to be 18.3 and 22.8 nm, respectively. The BET surface areas of N-doped TiO₂ are measured to be 62.8 cm²/g, and V₂O₅ cluster loading is found to have little influence on the surface areas of N-doped TiO₂.

Figure 1 

XRD patterns of N-doped TiO2, V2O5 cluster modified N-TiO2 and V2O5 samples.

3.2. Raman Analysis

Figure 2(a) shows the Raman spectra of N-doped TiO₂, selected V₂O₅ cluster modified N-TiO₂, and V₂O₅ samples. The observed characteristic Raman bands at 144, 196, 395, 515, and 638 cm⁻¹ for samples containing N-TiO₂, assigned to the E_g, B_1g, A_1g, B_2g, and E_g vibrational modes of anatase phase TiO₂ [46]. The typical Raman bands of rutile phase TiO₂ appear at 143, 235, 447, and 612 cm⁻¹, which can be ascribed to the B_1g, two-phonon scattering, 𝐸_𝑔, and A_1g modes of rutile phase, respectively [47]. Raman bands of rutile phase at 143 cm⁻¹ are overlapped by 144 cm⁻¹ band of anatase phase. The inset in Figure 2(a) shows the enlarged Raman bands at 447 cm⁻¹ of rutile phase for samples containing N-TiO₂. As the content of rutile phase is low, other Raman bands of rutile phase cannot be observed directly in Figure 2(a). Raman mode of V₂O₅ is also shown in Figure 2(a) [48]. No Raman bands relevant to V₂O₅ are observed for the composite samples, which imply the absence of a separate crystalline V₂O₅ phase on the N-TiO₂/V₂O₅ samples, consistent with XRD results. The enlarged view of Raman bands in the range of 110–180 cm⁻¹ in Figure 2(b) shows that the bands at 144 cm⁻¹ shift to lower wave numbers as the loading content of V₂O₅ increases, suggesting the strong interaction between N-TiO₂ and V₂O₅ clusters.

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(b)

Figure 2 

Raman spectra of N-doped TiO2, selected V2O5 cluster modified N-TiO2 and V2O5 samples (A: anatase, R: rutile).

3.3. TEM-EDS Analysis

TEM observation (Figures 3(a) and 3(b)) reveals that the N-TiO₂ and N-TiO₂/V₂O₅-0.5% sample consists of agglomerates of primary particles of 20–30 nm in diameter. For V₂O₅ modified sample, some V₂O₅ clusters with size of ca. 1–3 nm are observed and dispersed on the surface of N-TiO₂ (Figure 3(b) and see Figure S1 in supplementary material available on line at doi: 10.1155/2012/569716). The direct contact between V₂O₅ cluster and N-TiO₂ favors the formation of heterojunction between the two components. Figure 3(c) illustrates the energy-dispersive X-ray spectroscopy (EDS) spectra of N-TiO₂/V₂O₅-0.5% sample. The amount of vanadium content obtained from EDS (Table inset in Figure 3(c)) is in agreement with the modification content. To further reveal the dispersity of V₂O₅ clusters, elemental mapping images of O, Ti, and V are demonstrated in Figures 3(d)–3(f). It can be seen that V₂O₅ clusters are uniformly deposited on the surface of N-TiO₂ nanoparticles, which is also favorable for the interaction between V₂O₅ clusters and N-TiO₂.

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(b)

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(f)

Figure 3 

TEM image of N-doped TiO2 sample (a), TEM image (b), EDS (c) and elemental mapping image (d, e, f) of of N-TiO2/V2O5-0.5% sample.

3.4. XPS Analysis

XPS is used to determine the chemical composition and surface properties of catalysts. Figure 4(a) shows the binding energy (E_b) for the N1s region of N-TiO₂ and N-TiO₂/V₂O₅-0.5% samples. For both samples, E_b of N1s at around 400 eV can be observed. This E_b is a typical feature of substitutional lattice nitrogen (N³⁻) for oxygen, forming N-Ti-O structure [43]. The formation of N-Ti-O bond is the natural result of partial oxidation of TiN by oxygen. The content of doped nitrogen is determined to be 0.87 at % and have little change after modification with V₂O₅ clusters according to the XPS result.

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(b)

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(d)

Figure 4 

XPS spectra for N-doped TiO2 and N-TiO2/V2O5-0.5% samples of N1s (a), O1s (b), Ti2p, (c) and V2p (d).

Figure 4(b) shows the XPS spectra for O1s region. It can be seen that the O1s region can be fitted into three peaks for both samples, which can be attributed to Ti-O (529.9 eV), -OH hydroxyl groups (531.3 eV), and chemisorbed H₂O (532.7 eV), respectively [44]. Further observation in Figure 4(b) indicates that the molar ratio of oxygen in hydroxyl groups to all kinds of oxygen contributions increases after V₂O₅ cluster modification.

Figure 4(c) shows the E_b for Ti 2p_3/2 for both samples. It can be seen that Ti2p peak at 458.85 eV of N-TiO₂/V₂O₅-0.5% sample shifted positively by 0.25 eV in comparison with that of the Ti2p peak in N-TiO₂ sample (458.60 eV). The shifting of E_b can be ascribed to the interaction between host N-TiO₂ and guest V₂O₅ clusters, as also confirmed by Raman spectra [20].

Figure 4(d) shows the E_b for V2p_3/2 for both samples. The V 2p_3/2 peak of V₂O₅ cluster samples can be fitted into two peaks, located at 517.3 and 516.2 eV. These two E_b can be assigned to V⁵⁺ and V⁴⁺, respectively [49]. The variation of vanadium oxidation state is frequently observed in catalysis. In XPS measurement, N-TiO₂ can be exited by the high energy of X-rays (ℎ𝜈=1486.6 eV) to produced electrons in conduction band (CB). The V⁵⁺ then accepts the CB electrons to produce V⁴⁺. These results also confirm that the electrons transfer from the CB of N-TiO₂ to V₂O₅ clusters on the surface and the partial reduction of V⁵⁺ to V⁴⁺ [37].

3.5. UV-Vis DRS

Figure 5(a) shows UV-vis DRS of N-doped TiO₂, N-TiO₂/V₂O₅, and V₂O₅ samples and P25. Compared with undoped TiO₂ P25, optical absorption of N-doped TiO₂ shifts into visible light region (400–550 nm) as the localized N doping level was formed in the band gap. When the amount of vanadium loaded was less than 0.20 wt.%, there was no obvious change in visible light absorption compared with N-TiO₂. The optical absorption in visible region was significantly increased when the content of vanadium loaded was higher than 0.50 wt.%. Pure V₂O₅ exhibits broad visible light absorption. The enhanced absorption of V₂O₅ cluster-modified N-TiO₂ nanocomposite can be ascribed to d-d transition of vanadium species.

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(b)

Figure 5 

UV-vis DRS of N-doped TiO2, N-TiO2/V2O5, V2O5 samples and P25 (a). Plot of (αhν)2 versus photon energy of V2O5 (direct semiconductor) (b), inset in (b) shows the plot of (αhν)1/2 versus photon energy of N-TiO2 (indirect semiconductor).

Bandgap (𝐸_𝑔) energies can be estimated from UV-vis DRS spectra. Semiconductors are classified to be indirect or direct according to the lowest allowed electronic transition. The relation between absorption coefficient (𝛼) and incident photon energy (ℎ𝜈) can be written as 𝛼ℎ𝜈=𝐵_𝑑(ℎ𝜈−𝐸_𝑔)^𝑛 for allowed transitions (𝑛=2 for indirect transition, 𝑛=1/2 direct transition), where 𝐵_𝑑 is the absorption constants. Plot of (𝛼ℎ𝜈)^1/2 and (𝛼ℎ𝜈)² versus ℎ𝜈 from the spectra data of N-TiO₂ and V₂O₅ in Figure 5(a) presented in Figure 5(b) [44, 45]. The 𝐸_𝑔 estimated from the intercept of the tangents to the plots is 2.75 and 2.24 eV for N-TiO₂ and V₂O₅, respectively. The estimated 𝐸_𝑔 for V₂O₅ is consistent with previous report [50]. It is generally recognized that nitrogen doping could reduce the bandgap of TiO₂ by uplifting the position of valence band (VB) while keeping the position of CB unchanged [15–18].

3.6. PL Spectra

Photoluminescence (PL) emission spectra have been widely used to investigate the efficiency of charge carrier trapping, migration, and transfer in order to understand the fate of electron/hole pairs in semiconductors since PL emission results from the recombination of photogenerated charge carriers. Figure 6 shows the room-temperature PL spectra of selected V₂O₅ cluster-modified N-TiO₂ samples under the excitation of 300 nm light and 425 nm light. As the PL emission reflects the recombination rate of excited electrons and holes, a lower PL intensity indicates decreased charge recombination rate and enhanced charge separation rate [17–20]. It can be seen from Figure 6 that the PL intensity of V₂O₅ cluster modified N-TiO₂ is lower than that of N-TiO₂. As the loading content of V₂O₅ increases, the PL intensity decreases correspondingly. This result indicates that V₂O₅ loaded on N-TiO₂ surface (Inset in Figure 2(a)) can effectively enhance the separation of electron/hole pairs. This also implies that N-TiO₂/V₂O₅ has a lower recombination rate of electron/hole pairs under both UV and visible light irradiation. This is ascribed to the fact that the electrons are excited from the VB to the CB of N-TiO₂ and then migrate to V₂O₅ clusters, which prevents the direct recombination of electrons and holes, as also confirmed by XPS result on V2p (Figure 4(d)).

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Figure 6 

PL spectra of N-TiO2 and selected V2O5 cluster modified N-TiO2 samples under the excitation of 300 nm light (a) and 425 nm light (b).

3.7. Photocatalytic Activity and Proposed Mechanism

To elucidate the effects of V₂O₅ cluster modification on the photoactivity of N-doped TiO₂, visible light photocatalytic degradation of gaseous toluene at indoor level was performed. Figure 7 shows the visible light photocatalytic degradation curves (Figure 7(a)) and apparent reaction rate constant k (Figure 7(b)) of SiO₂/V₂O₅, N-TiO₂, and N-TiO₂/V₂O₅ samples with different loading content of V₂O₅. SiO₂/V₂O₅ exhibits negligible activity, which implies that V₂O₅ alone is not active under visible light probably due to the rapid recombination between CB electrons and VB holes. N-doped TiO₂ shows decent visible light activity toward toluene degradation with removal ratio 𝜂 of 27.5% and 𝑘 of 0.009 min⁻¹. With the loading of V₂O₅ clusters in rang of 0.01~1.0 wt.%, N-TiO₂/V₂O₅ samples exhibit significantly enhanced visible light photocatalytic activity than that of unmodified N-TiO₂. After loading only 0.01 wt.% of V₂O₅ on N-TiO₂, the visible light activity of N-TiO₂/V₂O₅-0.01% is markedly enhanced with a η 39.7% and 𝑘 of 0.0165 min⁻¹. With further increasing V₂O₅ loading from 0.01 wt.% to 0.5 wt.%, the visible light activity on N-TiO₂/V₂O₅ is further increased and achieves a maximum η of 52.4% and 𝑘 of 0.027 min⁻¹. When the V₂O₅ loading content is higher than 0.5 wt.%, a further increase in V₂O₅ loading leads to a obvious reduction of the photocatalytic activity. This is probably due to the following reasons: (i) deposition of excessive V₂O₅ clusters resulted in the decrease (or shield) of the N-TiO₂ surface active sites; (ii) disappearance of surface effect due to the increase of particle size [34, 35].

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Figure 7 

Visible light photocatalytic degradation curves (a) and comparison of the apparent rate constant (k) of as-prepared samples (b), (1) SiO2/V2O5 sample, (2) N-TiO2, (3)–(7) V2O5 cluster modified N-TiO2 samples.

From what has been observed and discussed above, several conclusions can be drawn: (1) V₂O₅ is inactive for photocatalytic degradation of toluene under visible light irradiation although the band gap of V₂O₅ is suitable for visible light excitation. (2) After V₂O₅ cluster modification, the visible light activities of N-TiO₂/V₂O₅ samples are highly enhanced. (3) The content of V₂O₅ cluster significantly influences visible light activity of N-TiO₂. Obviously, the enhanced separation of electrons and holes pairs on N-TiO₂/V₂O₅ nanocomposite due to the CB electron migration from N-TiO₂ to V₂O₅ cluster is directly responsible for the highly enhanced visible light photocatalytic activity.

Here comes a fundamental issue. What is the driven force for CB electron of N-TiO₂ to migrate to V₂O₅ clusters? The band edge positions of CB and VB of semiconductor can be determined with the following approach. The CB edge (𝐸⁰_𝐶𝐵) of a semiconductor at the point of zero charge (pH_ZPC) can be predicted by the equation 𝐸⁰_𝐶𝐵=𝑋−𝐸^𝐶−1/2𝐸_𝑔 [35, 50], where 𝑋 is the absolute electronegativity of the semiconductor (for V₂O₅, 𝑋 is 6.10 eV [51]; for P25 TiO₂, 𝑋 is 5.81 eV [51]; 𝑋 is unknown for N-doped TiO₂). E_C is the energy of free electrons on the hydrogen scale (~4.5 eV). 𝐸_𝑔 is the band gap energy of the semiconductor. The calculated positions of CB and VB of V₂O₅ and P25 are listed in Table 1. It is well known that nonmetal doping does not change the CB position of TiO₂. The VB position of N-doped TiO₂ can be determined based on the calculated CB position of undoped TiO₂ P25 and 𝐸_𝑔 of N-doped TiO₂, as also shown in Table 1.

Table 1 

Absolute electronegativity, calculated conduction band (CB) edge, calculated valence band (VB) position, and bandgap energy for P25, V₂O₅, and N-doped TiO₂ at the point of zero charge.

The schematic enhanced charge transfer and separation in the V₂O₅ cluster modified N-doped TiO₂ system are illustrated in Figure 8. Under visible light irradiation, electrons can be excited to the CB of N-TiO₂, leaving holes in the VB (1). These holes will react with OH⁻ on the catalyst surface to form •OH radicals (2). Because the potential of V₂O₅ (𝐸⁰_CB=0.48 eV) is lower than the CB level of N-TiO₂ (𝐸⁰_CB=−0.19 V), the photogenerated CB electrons in N-TiO₂ can transfer rapidly to V₂O₅ clusters and then effectively reduce partial V⁵⁺ to V⁴⁺ (see Figure 4(d)), thus promoting the separation of and transfer of photogenerated electrons and holes pairs. The PL experiments further confirm the transfer of photogenerated electrons from N-TiO₂ to V₂O₅ clusters (Figure 6). The electrons accepted by V⁵⁺ can then be transferred quickly to oxygen molecules under the aerated condition to regenerate V⁴⁺ to produce O₂^•− superoxide anion radicals. The process can be defined as V⁵⁺/V⁴⁺ redox cycle, as shown in the following equations (3)-(4): N-TiO₂+ℎ𝜈(visible)⟶h⁺+e⁻OH⁻+h⁺⟶•VOH₅₊V₂O₅+e⁻⟶V⁴⁺V⁴⁺+O₂⟶V⁵⁺V₂O₅+O₂^•−. (1)(2)(3)(4)

Figure 8 

Schematic illustration for the enhanced charge transfer and separation in the V2O5 cluster modified N-doped TiO2 system under visible light irradiation.

The •OH and O₂^•− radicals are known to be the most oxidizing species in photocatalysis reaction [1–3]. These results suggest that the separation of electrons and holes can be effectively enhanced by V₂O₅ clusters, which will greatly enhance the visible light photocatalytic activity of N-doped TiO₂. Similar phenomena have also been observed on CuO, Cu(OH)₂, and Ni(OH)₂ cluster modified TiO₂ with highly enhanced photocatalytic activity [34–36].

The effect of V₂O₅ cluster modification on the charge transfer and separation of other semiconductor is, to some extent, similar to that of the role of noble metals in photocatalysis [45, 52–54]. The present work not only displays a feasible route for the utilization of low cost V₂O₅ clusters as a substitute for noble metals in enhancing the photocatalysis but also demonstrates a facile method for preparation of highly active composite photocatalysts for environmental application.

4. Conclusion

In order to enhance the visible light photocatalytic activity of N-doped TiO₂ for practical environmental application, a facile impregnation-calcination method is developed for the preparation of highly active V₂O₅ cluster modified N-doped TiO₂ nanocomposites photocatalyst for degradation of toluene in air. The V₂O₅ clusters can act as electron mediator to effectively inhibit the recombination of photogenerated electron/hole pairs. The optimal V₂O₅ loading content is determined to be 0.5 wt.%, and the corresponding toluene removal ratio is 52.4%, which largely exceeds that of unmodified N-doped TiO₂. The CB potential of V₂O₅ (0.48 eV) is lower than the CB level of N-doped TiO₂ (−0.19 V), which is the driven force for the electron transfer from CB of N-doped TiO₂ to V₂O₅ clusters, thus greatly promoting the visible light activity of N-doped TiO₂. This work not only provides a feasible route for utilizing low-cost V₂O₅ clusters as a substitute for noble metals in enhancing the visible light photocatalysis but also demonstrates a facile method for preparation of highly active composites photocatalyst for large scale application.

Acknowledgment

This research is financially supported by the National Natural Science Foundation of China (51108487), the Program for Young Talented Teachers in Universities (Chongqing, 2011), the National High Technology Research and Development Program (863 Program) of China (2010AA064905), The key discipline development project of CTBU (1252001), Projects from Chongqing Education Commission (KJTD201020, KJZH11214, KJ090727), and Natural Science Foundation of Chongqing (CSTC, 2010BB0260).

Supplementary Materials

Supplementary Figure S1: shows enlarged view of V₂O₅ cluster modified N-doped TiO₂.

1. Supplementary Material