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Facile Synthesis of Single-Crystalline Ag2V4O11 Nanotube Material as a Novel Visible-Light-Sensitive Photocatalyst Haifeng Shi,*,†,‡,§ Zhaosheng Li,‡ Jiahui Kou,‡ Jinhua Ye,‡,§ and Zhigang Zou*,‡ School of Science, Jiangnan UniVersity, Wuxi, P. R. China, 214122, Eco-materials and Renewable Energy Research Center (ERERC), National Laboratory of Solid State Microstructures, Department of Physics, Nanjing UniVersity, Nanjing, P. R. China, 210093, and Photocatalytic Materials Center (PCMC), National Institute for Materials Science (NIMS) 1-2-1, Sengen, Tsukuba, Ibaraki, Japan, 305-0047 ReceiVed: March 24, 2010; ReVised Manuscript ReceiVed: NoVember 24, 2010
Ag2V4O11 nanotubes were synthesized by means of a facile surfactant-assisted hydrothermal process and developed as a novel visible-light-sensitive photocatalyst for 2-propanol (IPA) decomposition. The Ag2V4O11 nanotubes were single-crystalline and showed a uniform size with a diameter of about 10 nm in width and micrometer-sized length. The material can absorb a wide range of light irradiation up to 600 nm. The band gap of Ag2V4O11 was determined to be 2.0 eV according to the diffuse reflectance spectrum of the sample. The electronic structures and band edge positions of Ag2V4O11 were theoretically calculated on the basis of density functional theory and the constituent atom’s Mulliken electronegativity, respectively. It was revealed that the valence band consisted of Ag 4d and O 2p orbitals, while the conduction band was constructed by V 3d and Ag 5s5p orbitals. Compared with commercial N-TiO2, the Ag2V4O11 nanotube catalyst exhibited much higher photocatalytic activity for IPA degradation under visible-light irradiation. It was possibly ascribed to the narrow band gap, highly mobile charge carriers, and tubular structure. Introduction The increasing and urgent demand for environmental purification is becoming more and more important in view of humans’ continuable development. The photocatalysis technique using semiconductors and solar light has attracted tremendous attention as one of the potential and promising avenues to solve environmental issues facing mankind.1-9 As a green chemistry technology, photocatalysis can decompose organic pollutants completely into harmless chemicals such as H2O, CO2, and mineral acids (without the secondary waste products).10,11 Moreover, the photocatalytic reaction occurs at room temperature, which is a benign process that is undoubtedly superior to the traditional catalytic combustion treatment approach. Up to now, titanium dioxide (TiO2) has been studied the most extensively among a variety of photocatalysts due to its good activity and high stability. Unfortunately, the relative wide band gaps (Eg(anatase) ) 3.2 eV; Eg(rutile) ) 3.0 eV) limit their photocatalytic activities to only be active in the ultraviolet light region (4% of the total sunlight). Considering the efficient utilization of the visible light (accounting for 43% of the total sunlight) to generate electron-hole pairs for promoting photocatalytic redox reactions, therefore, it is highly desirable to develop a photocatalyst with efficient response under visiblelight irradiation. Since the early 1980s, many efforts have been expanded to develop photocatalysts operating effectively under visible-light irradiation. On one hand, the attempts are focused on the modification of TiO2, such as transition metal ions or aniondoped TiO2.4,12-16 These modified TiO2 photocatalysts are generally called the second generation of TiO2 photocatalyst.12 * Corresponding author. E-mail address:
[email protected] (HF Shi);
[email protected] (ZG Zou). † Jiangnan University. ‡ Nanjing University. § National Institute for Materials Science.
However, the dopants not only induce the visible-light absorption but also act as the recombination centers, which unfortunately lead to the decreased photocatalytic activity. On the other hand, growing attention recently has been devoted to developing new multimetal oxide photocatalysts.17-23 These multimetal oxides are eminently attractive candidates as the photocatalytic materials, not only because of their higher chemical stability and easier preparation compared with nonoxide materials but also because they might possess fewer electron-hole pair recombination centers (defects) so that charge carrier separation and migration is more effective in contrast to the doped photocatalysts. Particularly, Ag-based oxides, with the unique hybridized valence bands (O 2p and Ag 4d orbitals), generally have a narrow band gap (e3 eV) and highly dispersed valence bands (VB), which result in good photoabsorption ability and high mobility of photoholes, respectively. Hence, the Ag-based materials have the potential and promising applications as visible-light-sensitive photocatalysts.24-27 However, the study on Ag-based photocatalysts is still limited, and herein, we envisioned developing some new Ag-based multimetal oxides as visible-light-sensitive photocatalyst candidates. Silver vanadate (Ag2V4O11) is commercially employed as a cathode material in Li/SVO battery for powering cardiac defibrillators due to its unique properties, such as high specific capacity and excellent rate capability.28 It is well-known that the surface morphology and microstructure could play a crucial role in determining material property. Therefore, much effort has been focused on fabricating various nanostructured Ag2V4O11 (such as nanobelts,29 nanowires,30 and nanorings31). Currently, one-dimensional nanoscaled tubular materials have attracted extraordinary attention for the promising application in photocatalysis32-34 because the nanotube photocatalysts generally provide larger specific surface areas available (so higher adsorption capacity and much more active sites) compared with nanoparticles and also possess efficient charge
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separation on their surface and hence promote the photocatalytic reactions.35 However, there have been no studies, to the best of our knowledge, about the synthesis of Ag2V4O11 nanotubes and the photocatalytic performance of Ag2V4O11 in the public literature. Herein, in this study, single-crystalline Ag2V4O11 nanotubes were prepared for the first time by means of a facile hydrothermal approach and developed as a novel visible-light-sensitive photocatalyst for 2-propanol (IPA) degradation. The physical characteristics of samples were examined by the techniques, such as XRD, BET measurement, TEM, XPS, and UV-vis diffuse reflectance spectroscopy. The possible growth mechanism is proposed. The material can absorb a wide range of light irradiation up to 600 nm, which almost covers the region from UV to all strong visible light in the sunlight. In contrast to the commercial N-TiO2 photocatalyst, the Ag2V4O11 nanotubes exhibited much higher photocatalytic activities for IPA degradation under visible-light irradiation. The electronic structures of Ag2V4O11 were investigated using plane-wave-based density functional theory calculations. Experimental Section Catalyst Synthesis. Single-crystalline silver vanadate (Ag2V4O11) nanotubes were prepared by a facial hydrothermal method. In a typical procedure, 0.3 g of ammonium metavanadate (NH4VO3, Aldrich) and 0.50 g of P123 were dissolved in 30 mL of distilled water containing 2 mL of 1 M HNO3. This mixture was stirred at room temperature for 7 h, followed by addition of 0.195 g of silver nitrate (AgNO3, Wako). After being stirred for 1 h, the mixture was transferred into a 50 mL Teflonlined autoclave and thermally treated at 150 °C for 24 h. The resulting products were recovered, washed with distilled water and acetone, and subsequently dried at 70 °C for 12 h. Sample Characterization. The crystal structure of Ag2V4O11 was determined by an X-ray diffractometer (Utilma III, Japan) using Cu KR radiation (λ ) 1.54056 Å) in the 2θ range of 20-80°. The UV-visible diffuse reflectance spectra of the sample were recorded at room temperature in the range of 250-700 nm using a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan) equipped with an integrating sphere attachment. The morphology of Ag2V4O11 powders was characterized by a transmission electron microscope (TEM, JEM-2010, operated at 200 kV, JEOL, Japan). The specific surface area was deduced according to the Brunauer-Emmett-Teller method using a nitrogen adsorption apparatus (TriStar-3000, Micrometrics, USA) at 77 K after a pretreatment at 473 K for 2 h. XPS analysis was taken on an X-ray photoelectron spectrometer (Thermo ESCALAB 250, power, 150 W; incident angle, 90°) using Al KR monochromatic X-ray radiation (1486.6 eV). The peak positions were calibrated against the referenced C 1s peak (285.0 eV) of contaminated carbon. Evaluation of Photocatalytic Activity. In this study, we applied the photocatalytic degradation of IPA to evaluate the property of Ag2V4O11 catalyst. Photocatalytic reaction was carried out in a cylindrical static Pyrex glass vessel. The catalyst powders (0.1 g) were evenly dispersed in a small glass dish (area ) 8.0 cm2) that was located in the center of the cylindrical vessel (0.5 L). After sealing the vessel, a certain amount of IPA was injected into the vessel using a syringe. Prior to light irradiation, the reactor was stored in dark conditions until an adsorption-desorption equilibrium was reached. Finally, the reactor was irradiated with the visible light, which was irradiated from a 300 W Xe lamp (ILC Technology, CERMAX LX-300) through a cutoff filter (providing the visible light with
Figure 1. (a) Ball-and-stick model and (b) the polyhedron model of Ag2V4O11.
different cutoff wavelength) and a Pyrex filter with circulating water (removing the infrared ray light irradiation and preventing thermal catalytic effects). A gaseous sample (5 µL) was periodically extracted from the reaction vessel to analyze the concentrations of IPA ((CH3)2CHOH) and acetone (CH3COCH3), which were measured on a gas chromatograph (GC-14B, Shimadzu, Japan) equipped with a flame ionization detector (FID). Commercial nitrogen doped TiO2 (N-TiO2, TPS201; Sumitomo Corp., Japan) was used as a reference photocatalyst to compare the photocatalytic activity under the same experimental conditions. Calculation of the Electronic Structure. The ab initio calculations described here were performed with a CASTEP program package based on the density functional theory. A plane wave basis set was used to describe the electronic wave functions with a kinetic energy cutoff of 300 eV. The interactions between ionic cores and valence electrons were represented by ultrasoft pseudopotentials. Exchange-correlation potentials were described by generalized gradient approximations (GGARPBE). The Brillouin-zone (BZ) integrations of total energy were calculated using the special k point generated by the Monkhorst-Pack scheme with a 2 × 6 × 3 k point. Results and Discussion Figure 1 displays the ball-and-stick model and the polyhedron model of Ag2V4O11, respectively. It is found that Ag2V4O11 is a monoclinic crystal structure with the space group C2/m. The crystal structure of Ag2V4O11 (Figure 1b) consists of [V4O16]
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Figure 2. X-ray diffraction patterns of Ag2V4O11 before and after the photocatalytic gaseous IPA degradation.
units that are made of VO6 distorted octahedra sharing their apexes, which build infinite [V4O12] quadruple strings. These quadruple strings are further linked by the corner-shared oxygen atoms to provide the continuous [V4O11]n layers separated by AgO5 trigonal bipyramid layers.36 Figure 2 shows a comparison of the XRD patterns of Ag2V4O11 before and after the photocatalytic reaction. All diffraction peaks can be indexed to a monoclinic structured Ag2V4O11, in good agreement with those in the JCPDS Card (No. 49-0166). There is no obvious peak change in the XRD patterns of Ag2V4O11 before and after the photocatalytic gaseous IPA degradation reaction, indicating that the phase of the Ag2V4O11 catalyst is stable for the present photocatalytic reaction process. The BET surface area is measured to be 64 m2/g using the N2 adsorption-desorption measurement. The fine morphology details of the as-prepared sample were examined by means of a TEM technique. As shown in Figure 3a, the products are composed of a large quantity of nanotubes with a diameter of about 10 nm and micrometer-sized in length. Figure 3b is a representative HRTEM image of a single nanotube, which exhibits crystalline structure and clear lattice fringes. The interplanar spacing can be readily resolved to be 0.59 nm, corresponding to the (201j ) crystalline plane of the monoclinic Ag2V4O11. Combined with the SAED pattern (Figure 3c) recorded on an individual nanotube, it is revealed that the as-prepared Ag2V4O11 powders are single-crystalline nanotubes with a preferred orientation along the [302j] crystal plane. The chemical composition of Ag2V4O11 nanotubes is further analyzed using energy-dispersive spectroscopy (EDS, Figure 2d). The peaks of the elements Ag and V are detected in the EDS pattern, and the molar ratio is determined to be 1:2 (the signals of C and Cu elements come from the carbon-coated copper grid). The surface composition and state of the products were further investigated using X-ray photoelectron spectroscopy (XPS). As shown in the wide-scan XPS spectrum (Figure 4a), no obvious impurity peak can be detected. The two peaks (Figure 4b) located at 517.6 and 525.1 eV can be attributed to V 2p3/2 and V 2p1/2, respectively, while the other peak located at 530.6 eV corresponds to O 1s. The two strong peaks (Figure 4c) at the Ag region of 368.2 and 374.2 eV can be assigned to the binding energy of Ag 3d5/2 and Ag 3d3/2, respectively. The atomic ratio of the Ag:V:O in Ag2V4O11 calculated from the peak area is approximately 2:4:11.6, in fairly good agreement with the given formula for Ag2V4O11 with respect to the experimental errors. In this case, the rolling mechanism37-40 could be used to explain the formation of these nanotubes. The possible growth process proposed here mainly involved four steps in sequence:
Figure 3. (a) Low- and (b) high-magnification transmission electron microscopy (TEM) images of Ag2V4O11, (c) selective area electron diffraction (SAED) pattern, and (d) energy-dispersive X-ray analysis of the Ag2V4O11 sample.
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Figure 5. Photocatalytic acetone evolution concentrations over Ag2V4O11 and Com-N-TiO2 samples under visible-light irradiation. Reaction conditions: light source, a 300 W Xe lamp with Y44 filter and water filter; catalyst, 100 mg; the initial concentration of IPA is ∼2300 ppm.
Figure 4. XPS spectra of Ag2V4O11 nanotubes. (a) Survey spectrum, (b) high-resolution spectrum of O 1s and V 2p, and (c) high-resolution spectrum of Ag 3d.
(1) Ammonium metavanadate in aqueous solution formed NH4+ and VO3-, with increasing temperature. Some NH3 entered the gas phase, while metavanadate groups gradually agglutinate into [V4O12] quadruple strings. These quadruples are further linked through sharing the edges or the corners to form a V4O112- layer structure.41,42 (2) The P123 molecules condensed into aggregations with the V4O112- framework to form lamellar structures, and subsequently silver ions could assemble with these framework tunnels. (3) The condensation process continued and produced more lamellar assemblies. (4) These lamellar sheets became loose on the edges and then rolled up to form silver vanadium nanotubes. Gaseous IPA is commonly used as a model organic compound for evaluating the activity of a semiconductor photocatalyst. Because the direct photodegradation of IPA into CO2 is negligible, the evolved CO2 is usually from the mineralization of an intermediate (acetone). Thus, the photocatalytic activity is generally evaluated by the concentration of evolved acetone
at the initial stage.8,43,44 Figure 5 represents acetone evolution concentrations over Ag2V4O11 and Com-N-TiO2 samples under visible-light irradiation. One can observe that the acetone generates and increases with the prolongation of irradiation time over the Ag2V4O11 sample. As shown in Figure 5, the acetone generation rate of Ag2V4O11 is higher than that of commercial N-TiO2, indicating Ag2V4O11 is possibly a promising visiblelight-sensitive photocatalyst. In addition, there is hardly any acetone to be observed either under dark conditions or without any catalyst. This indicated that the reaction strongly depended on catalyst and light. Namely, the reaction was a photocatalytic oxidization process. It is worthwhile to note that the conversation rate of photooxidation of IPA into acetone reaches 43% after a 150 min photocatalytic degradation reaction, indicating that the Ag2V4O11 catalyst has fairly good activity under visible-light irradiation. The apparent quantum efficiency (QE) of IPA converting to acetone was calculated using the following equation: QE ) Nrp/Nip ) Nace/Nip,17 where Nrp is the number of photons involved in the reaction of acetone generation, Nace is the number of acetone molecules, and Nip is the number of incident photons. Using an interference filter (λ ) 419.1 nm, Tmax ) 40.4%), the apparent quantum efficiency (QE) of acetone evolution over Ag2V4O11 at λ ) 419.1 nm was measured to be ∼3.1%, indicating the Ag2V4O11 sample was active under visible-light irradiation. The photocatalytic mineralization of gaseous IPA was performed (can be seen in Supporting Information Figure S1). It was noted that the final concentration of CO2 (∼1120 ppm) over Ag2V4O11 samples after 30 h of reaction was nearly three times the initially injected IPA (∼378 ppm). Namely, almost all the injected IPA was mineralized so that a carbon balance was achieved eventually. The XRD (Figure 2), FE-SEM, and XPS techniques were applied for the stability test of the Ag2V4O11 sample (can be seen in the Supporting Information Figure S2 and Figure S3). The results show that the crystal structures, morphology, and surface chemical compositions of the as-prepared and postreaction samples are not obviously changed. Figure 6a shows the UV-vis diffuse reflectance spectrum of the sample. One can see that the absorption edge of Ag2V4O11 is up to about 600 nm, which almost covers the region from UV to all strong visible light in the sunlight. The optical band gap Eg of a semiconductor could be deduced according to the following equation
(Rhν)n ) A(hν - Eg)
(1)
where R is the absorption coefficient; hν is the incident photo energy; A is a proportionality constant related to the material;
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Figure 7. DFT calculations for Ag2V4O11. (a) Energy band dispersion and (b) projected density of states (DOS).
Figure 6. (a) Wavelength dependence of acetone evolution with different cutoff filters after light irradiation for 60 min over Ag2V4O11 catalyst. Reaction conditions: light source, a 300 W Xe lamp; with the different wavelength cutoff filter and circulating water filter; catalyst, 100 mg; the initial concentration of IPA, about 2300 ppm. (b) A plot of (Rhν)1/2 versus energy (eV) for Ag2V4O11.
and Eg is the band gap energy of the semiconductor. The value of the index n depends on the electronic transition of the semiconductor (ndirect ) 2; nindirect ) 1/2), respectively. In this case, the calculated energy band structure results (Figure 7a) reveal that Ag2V4O11 is an indirect band gap semiconductor. Thus, the band gap energy is obtained from the intercept of the tangent line in the plot of (Rhν)1/2 versus energy (Figure 6b), and the value is determined to be 2.0 eV for Ag2V4O11. The wavelength dependence of the catalytic reaction is generally applied to distinguish if the reaction is really driven by light.45-47 Figure 6a shows the wavelength dependence of the evolved acetone concentration over Ag2V4O11 catalysts, whose data points are collected from 60 min of light irradiation (with a circulating water filter and different optical cutoff filters). One can observe that the evolved acetone concentration decreases with increasing wavelength of the irradiation light, and the acetone is hardly evolved when a cutoff filter of 640 nm (HOYA, R64, Japan) and a circulating water filter are used, which is in good agreement with the optical absorption property of Ag2V4O11 (Figure 6a). This indicates that the performance of the present catalytic reaction strongly depends on the light absorption ability. That is, the evolved acetone rate is governed by the optical absorption property of Ag2V4O11. It is reliable to conclude that the photodecomposition of IPA over the Ag2V4O11 catalyst is a photocatalytic reaction. The optical properties and the photocatalytic performances of semiconductor photocatalysts are closely related to their electronic structures. For an efficient visible-light-sensitive
photocatalyst, there are three key factors to be satisfied:18 (1) narrow band gap, (2) high mobility and long lifetime of photogenerated charge carriers, and (3) suitable potential positions of the valence band (VB) and conduction band (CB). The UV-visible absorption spectrum has revealed the good visible-light absorption and narrow band gap. We next applied plane-wave-based density functional theory calculations (CASTEP program package) to calculate the band structures of Ag2V4O11 to study the characteristics of the photogenerated charge carriers. As shown in Figure 7a, the valence band maximum (VBM) is located near the middle point on the GY line in the first Brillouin zone, while the conduction band minimum (CBM) is located at the Z point. This implies that Ag2V4O11 is an indirect band gap semiconductor. Generally, as for a direct semiconductor, the photoinduced electron-hole pairs could separate more easily in contrast to that in the indirect cases; however, the rate of the photoinduced carriers recombination would definitely increase simultaneously. In other words, the lifetime of photogenerated charge carriers in the indirect band gap semiconductor may be longer than that in the direct case. Actually, many efficient indirect semiconductor photocatalysts have been reported, such as AgGaO2,23 Ag2ZnGeO4,26 AgInO2,27 NaBi(WO4),48 and so on. The DOS (Figure 7b) displays that the conduction bands of Ag2V4O11 are composed of the V 3d and Ag 5s5p orbitals, while the valence bands are constructed by the hybridized O 2p and Ag 4d orbitals. Actually, in several previously reported Agcontaining photocatalysts, the hybridization between O 2p and Ag 4d orbitals has also been observed. The results indicate that the position of the hybridized orbitals (Ag 4d + O 2p) is located at a more positive energy level relative to that of O 2p orbitals, generally leading to the materials having a narrower band gap and hence displaying visible-light absorption. The correlation of E and V is known as V ) ∆kE(k)/p, where E is the energy of electron and V is the velocity of electron. If ∆kE(k) is small, namely, the energy band is relatively weakly
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Figure 8. Schematic illustration for the calculated conduction and valence band potentials of Ag2V4O11 with respect to the vacuum level with an error of 0.2 eV.
dispersive (flat), V will be low. That is, the more flat the energy is, the more localized the charge carriers are. As for the present Ag2V4O11, both calculated VB and CB are abrupt; namely, V of the holes and the electron is high. Hence, the mobilities of both charge carriers are high in Ag2V4O11, leading to promotion of the photocatalytic activity. On the basis of DFT results, one can see that Ag2V4O11 has a narrow band gap and highly mobile charge carriers, which is helpful to the visible-light absorption and carrier transposition in the photocatalysts. In addition to the electronic structure characteristics, nanotube structure is considered to be helpful for promoting the photocatalytic reactions since nanotube photocatalysts generally provide larger available surface areas (so higher adsorption capacity and much more active sites) and possess efficient charge separation on their surface.35 All these suggest some useful information to design an efficient photocatalytic material. The potential positions of the valence band (VB) and conduction band (CB) of a photocatalyst are also strongly related to the process of photocatalytic oxidation of organic compounds. Herein, we speculated theoretically the band edge positions of Ag2V4O11 using the equation related to Mulliken electronegativity and the band gap of a semiconductor, which is described by49,50
ECB ) X - 0.5Eg
(2)
where ECB and Eg are the bottom position of CB relative to the vacuum level and the band gap energy of the semiconductor, respectively, and X is the geometric mean of the Mulliken electronegativity of the constituent atoms in the semiconductor. The Mulliken electronegativity of an atom is the arithmetic mean of the atomic electron affinity and the first ionization energy. Generally, the atomic electron affinity is one order of magnitude smaller than the first ionization energy. Thus, Mulliken electronegativity is approximately equal to half of the first ionization energy. Subsequently, ECB is determined to be 4.4 eV relative to the vacuum level, which is just around the H2 evolution potential (4.5 eV) or O2/O2•- (4.454 eV) in the view of the method inherent error (0.2 eV),50 and the VB edge of the semiconductor is obtained to be 6.4 eV from the band gap. It is well-known that the active species H2O2 can oxidize many organic compounds due to its strong oxidative potential (6.27 eV). Considering that Ag2V4O11 has the suitable VB potential, it possibly generates such active species (H2O2) to oxidize the organic compounds. On the basis of the present cognition, the band potential of Ag2V4O11 is illustrated in Figure 8. On the basis of knowledge about band potential, the fundamental
In this study, single-crystalline Ag2V4O11 nanotubes were successfully prepared for the first time by means of a facile surfactant-assisted hydrothermal process and developed as a new visible-light-sensitive photocatalyst for IPA photodegradation. The Ag2V4O11 nanotubes were single-crystalline conformed by HRTEM and SAED and displayed a uniform size with a diameter of about 10 nm in width and several micrometers in length. The material can absorb a wide range of light up to 600 nm, which almost includes the region from UV to all strong visible light in the sunlight. Ag2V4O11 was an indirect band gap semiconductor with the band gap of 2.0 eV. The activity of the Ag2V4O11 photocatalyst showed obvious wavelength dependence, in good agreement with the light absorption property of Ag2V4O11. The DFT calculations revealed that the top of the VB consisted of the hybridized Ag 4d and O 2p orbitals, while the bottom of the CB was constructed by V 3d and Ag 5s5p orbitals, respectively. A band structure calculation showed that the charge carriers in the CB and VB were highly mobile. Compared with commercial N-TiO2, Ag2V4O11 nanotubes displayed much higher photocatalytic activity for IPA degradation under visible-light irradiation. It is revealed that the high photocatalytic activity was possibly ascribed to the narrow band gap, highly mobile charge carriers, and nanotube structure. In summary, the present research is positive and is expected to provide some useful information for preparing the nanotube materials at lower temperature and developing a new series of photocatalysts for photodecomposing organic compounds. Acknowledgment. The authors would like to acknowledge financial support from National Natural Science Foundation of China (No 11047118), the Fundamental Research Funds for the Central Universities JUSRP11010, as well as the National Basic Research Program of China 973 Program (No 2007CB613305). One of the authors (Dr. Shi) would like to thank China Postdoctoral Science Foundation (No 20100481103). Supporting Information Available: Figures S1-S3. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69–96. (2) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735–758. (3) Fujishima, A T. N.; Rao, D. A.; Tryk, J. Photochem. Photobiol. C 2000, 1, 1–21. (4) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (5) Yang, H C.; Sun, S.; Qiao, J.; Zou, G.; Liu, S.; Smith, H.; Cheng, G.; Lu, Nature 2008, 453, 638–641. (6) Zhang, M.; Chen, C.; Ma, W.; Zhao, J. C. Angew. Chem., Int. Ed. 2008, 47, 4516–4520. (7) Lucky, R. A.; Charpentier, P. A. AdV. Mater. 2008, 20, 1755–1759. (8) Wang, D.; Kako, T.; Ye, J. J. Am. Chem. Soc. 2008, 130, 2724– 2725. (9) Shi, H. F.; Li, Z. S.; Ye, J. H.; Zou, Z. G. J. Phys. D: Appl. Phys. 2010, 43 (1-7), 085402. (10) Bansal, A.; Madhavi, S.; Tan, T.; Lim, T. M. Catal. Today 2008, 131, 250–254.
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