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International Journal of Photoenergy
Volume 2012 (2012), Article ID 569716, 10 pages
http://dx.doi.org/10.1155/2012/569716
Research Article

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

College of Environmental and Biological Engineering, Key Laboratory of Catalysis Science and Technology of Chongqing Education Commission, Chongqing Technology and Business University, Chongqing 400067, China

Received 2 January 2012; Revised 19 February 2012; Accepted 20 February 2012

Academic Editor: Xuxu Wang

Copyright © 2012 Fan Dong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 [13]. 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 [68].

Visible light driven photocatalysis is one of the hottest topics worldwide [922]. 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 [1014]. 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 [1518]. 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 [1922].

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) [2325], co-doping with metal ions (Fe, La, V) [2628], noble metal deposition (Pt, Au) [29, 30] and metal oxide coupling (WO3, PdO, ZrO2) [3133]. 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 [3442]. 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 [3442]. 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/V2O5x, 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 ( 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 (/) 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 () of toluene was calculated as , where and 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 , where 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.

569716.fig.001
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 Eg, B1g, A1g, B2g, and Eg 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 B1g, two-phonon scattering, , and A1g 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.

fig2
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.

fig3
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 (Eb) for the N1s region of N-TiO2 and N-TiO2/V2O5-0.5% samples. For both samples, Eb of N1s at around 400 eV can be observed. This Eb 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.

fig4
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 Eb for Ti 2p3/2 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 Eb 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 Eb for V2p3/2 for both samples. The V 2p3/2 peak of V2O5 cluster samples can be fitted into two peaks, located at 517.3 and 516.2 eV. These two Eb 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 ( 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.

fig5
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 () 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 () and incident photon energy () can be written as for allowed transitions ( for indirect transition, direct transition), where is the absorption constants. Plot of and versus from the spectra data of N-TiO2 and V2O5 in Figure 5(a) presented in Figure 5(b) [44, 45]. The 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 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 [1518].

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 [1720]. 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)).

fig6
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 of 27.5% and 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 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 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].

fig7
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 () of a semiconductor at the point of zero charge (pHZPC) can be predicted by the equation [35, 50], where is the absolute electronegativity of the semiconductor (for V2O5, is 6.10 eV [51]; for P25 TiO2, is 5.81 eV [51]; is unknown for N-doped TiO2). EC is the energy of free electrons on the hydrogen scale (~4.5 eV). 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 of N-doped TiO2, as also shown in Table 1.

tab1
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 ( eV) is lower than the CB level of N-TiO2 ( 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 superoxide anion radicals. The process can be defined as V5+/V4+ redox cycle, as shown in the following equations (3)-(4):

569716.fig.008
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 radicals are known to be the most oxidizing species in photocatalysis reaction [13]. 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 [3436].

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, 5254]. 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).

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