TiO2 Nanocrystalline

A nanocrystal heterojunction LaVO4/TiO2 visible light photocatalyst has been successfully prepared by a simple coupled method. The catalyst was charac...
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Environ. Sci. Technol. 2009, 43, 4164–4168

AnanocrystalheterojunctionLaVO4/TiO2 visiblelightphotocatalyst has been successfully prepared by a simple coupled method. The catalyst was characterized by powder X-ray diffraction, nitrogen adsorption-desorption, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectra, photoluminescence, and electrochemistry technology. The results showed that the prepared nanocomposite catalysts exhibited strong photocatalytic activity for decomposition of benzene under visible light irradiation with high photochemicalstability.Theenhancedphotocatalyticperformance of LaVO4/TiO2 may be attributed to not only the matched band potentials but also interconnected heterojunction of LaVO4 and TiO2 nanoparticles.

photocatalysts that can work effectively under visible light irradiation with photochemical stability is a very hot spot topic in photocatalysis research. The idea of the formation of a heterojunction structure between a narrow band gap semiconductor and TiO2 with matching band potentials provides an effective way to extend the photosensitivity of TiO2 into the visible region (9-11). Excited by visible light, the electrons or holes photogenerated from the narrow band gap semiconductor can be transferred to TiO2 and then initiate photocatalytic reaction. One common method for preparing these visible light photocatalysts was the simple physical mixing of them. Obviously, the efficiency of interparticle electron transfer is poor due to the momentary random collision of heterophase particles. Recently, semiconductor nanocrystal heterostructures consisting of two or more chemically distinct components have attracted increasing attention for their great potentialities to revolutionize nanomaterial research by providing a means to define multiple functionalities within a single nanostructure (12-14). It is also believed that it would lead to a more efficient interparticle electron transfer between two components because of the size effect of particles and the formed heterojunction of intimate contact heterophase nanoparticles (15, 16). LaVO4 has drawn considerable interest recently because of its surface catalytic properties, optical properties, and especially the absorption of visible light (17-19), but its utilization in the photocatalytic field has not been reported up to now. Herein we first report a heterojunction material consisting of LaVO4 and TiO2 nanoparticles prepared by a simple soft chemical method. The formed heterojunction LaVO4/TiO2 nanocomposite exhibited strong photocatalytic activity for decomposition of benzene with high photocatalytic stability in the process of gas-phase reaction under visible light irradiation. Additionally, its photocatalytic activity is also higher than TiO2 under UV light irradiation.

Introduction

Experimental Section

Benzene is a widespread volatile organic compound (VOC) found in polluted urban atmospheres. For its high toxicity, confirmed carcinogenicity, and environmental persistence, it is regarded as a priority hazardous substance for which efficient treatment technologies are needed (1). Semiconductor photocatalytic oxidation technology has been proven to be potentially advantageous for environmental pollutant remediation because it allows complete decomposition of VOC into CO2 and H2O at ambient conditions (2). Because of the nontoxicity, inexpensiveness, and chemical stability, TiO2 photocatalyst has been investigated extensively. Unfortunately, the wide band gap of TiO2 (3.2 eV for anatase) limits its possibility of employing visible light which occupies the major part of solar light (3). Meanwhile, the relatively high rate of electron-hole recombination often results in a low quantum yield and poor efficiency of photocatalytic reactions (4). There are many reports on metal and anion doped TiO2 catalysts, which show activity under visible light (5-7). However, they are impaired by increase in carrier-recombination facilities or thermal instability. Because of photocorrosion or rapid recombination of photogenerate electronhole pairs, traditional visible light photocatalysts with narrow band gaps are either unstable (CdS, CdSe, etc.) or have low activity (Fe2O3, WO3, etc.) (8). Therefore, the development of

Preparation of Photocatalysts. All chemicals were analytical grade without further purification. The coupled bicomponent LaVO4/TiO2 nanocomposites were prepared using a simple sol-gel method. A mixture was formed by adding a small amount of LaVO4 nanocrystal powder to TiO2 sol and then gelatinized with heat treatment. In a typical synthesis procedure, 6.5 mmol of NaOH and 6.5 mmol of NH4VO3 were added to 10 mL of water to form NaVO3 aqueous solution. Then 13 mL of La(NO3)3 aqueous solution (0.5 mol L-1) was added dropwise under strong agitation. After the mixture was stirred for about 10 min, the obtained yellow suspension was poured into a 100 mL Teflon-lined autoclave, filled with H2O to about 80% of its capacity. The autoclave was maintained at 200 °C for 48 h and cooled to room temperature naturally. The products were filtered and then washed with distilled water and ethanol in sequence. Then the green products were dried at 100 °C to form pure monoclinic LaVO4 powder. A mixture of 1.3 mL of HNO3, 180 mL of H2O, and 15 mL of Ti(OC3H7)4 was peptized, dialyzed, and concentrated at room temperature to form a highly dispersed TiO2 colloidal solution. Finally, the LaVO4 powder was impregnated by selfmade TiO2 colloid (the initial ratio of LaVO4 to TiO2 was fixed at 1 wt %) with ultrasonic dispersed for 0.5 h and sustained stirring for 24 h, and then was heated in a microwave oven to remove solvents. The resultant xerogel was calcined at 500 °C for 6 h to obtain LaVO4/TiO2 nanocomposite catalysts. As a comparison, self-made pure TiO2 (T500 and T400), commercial TiO2 (P25, Degussa Co.), N-doped TiO2

Efficient Degradation of Benzene over LaVO4/TiO2 Nanocrystalline Heterojunction Photocatalyst under Visible Light Irradiation HANJIE HUANG, DANZHEN LI,* QIANG LIN, WENJUAN ZHANG, YU SHAO, YIBIN CHEN, MENG SUN, AND XIANZHI FU* Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou, 350002, P.R. China

Received February 7, 2009. Revised manuscript received April 8, 2009. Accepted April 13, 2009.

* Corresponding author phone/fax: (+86)591-83779256; e-mail: [email protected] (L. D.); [email protected] (F. X.). 4164

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10.1021/es900393h CCC: $40.75

 2009 American Chemical Society

Published on Web 05/01/2009

(TiO2-xNx), and a physical mixture of P25 and LaVO4 powders (LaVO4/TiO2(PM)) were also prepared. Self-made TiO2 colloid was treated with the same heating process as LaVO4/TiO2 to remove solvents and then was calcined at 500 and 400 °C for 6 h to prepare T500 and T400, respectively. TiO2-xNx was synthesized by a traditional method (6) of calcining P25 at 550 °C under dry NH3 flow for 3 h. The LaVO4/TiO2(PM) was prepared by mechanically mixing LaVO4 and P25 powders in the agate mortar without any further treatment. Characterization. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Cu KR radiation. Laser Raman spectra were recorded at room temperature in Renishaw inVia Raman systems and the laser line at 514 nm of argon ion laser was used as an excitation source. Transmission electron microscopy (TEM) images were collected by using a JEOL JEM 2010F microscope working at 200 kV. A Varian Cary 500 UV-vis spectrophotometer was used to record the UV-vis diffuse reflectance spectra of various samples. Nitrogen adsorption-desorption isothermals for the specific surface area and pore volume of the samples were collected at 77 K using Micromeritics ASAP2010 equipment. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scientific Inc.) at 3.0 × 10-10 mbar with monochromatic Al KR radiation (E ) 1486.2 eV). The photoluminescence (PL) spectra were surveyed by an Edinburgh FL/FS900 spectrophotometer. The generation of •OH radicals was investigated by the photoluminescence technique with terephthalic acid (TA-PL) (20, 21). The flatband potentials (Vfb) of TiO2 and LaVO4 were determined from Mott-Schottky plots by electrochemical method (22), which was carried out in conventional three-electrode cells using a PAR VMP3 Multi Potentiotat apparatus. (The experimental details of TA-PL and Vfb measurement by electrochemical method are present in the Supporting Information.) Photocatalytic Activity Measurement. The measurement of photocatalytic activity was carried out by the photocatalytic oxidation of benzene into carbon dioxide in the gas phase, which was in a 5 × 2 × 0.1 cm3 fixed-bed quartz reactor and operated in a single-pass mode. All catalysts were sieved to obtain particles 0.21-0.25 mm in size. A 500 W Xe-arc lamp equipped with an IR-cutoff filter (λ < 900 nm) and an UVcutoff filter (λ > 450 nm) was used as the visible light source, while that with a special cutoff filter (280 < λ < 400 nm) was used as the ultraviolet light source (shown in Figure S1). A bubbler that contained benzene was immersed in an ice-water bath and benzene (about 250 ppm) bubbled with oxygen from the bubbler was fed to 1.2 g of catalyst at a total flow rate of 20 cm3 min-1. The temperature of the reactions was controlled at 30 ( 1 °C by an air-cooling system. Analysis of the reactor effluent was conducted by a gas chromatograph (HP6890). The concentrations of benzene and carbon dioxide were determined by using the flame ionization detector (FID) and thermal conductivity detector (TCD), respectively. The adsorption/desorption equilibrium of benzene gas on the sample was obtained after 5 h in the dark before an activity measurement. Benzene was found to be thermally stable in the reactor without illumination.

Results and Discussion Figure 1 showed the XRD results of the samples. The pattern of LaVO4 could be readily indexed to a pure monoclinic phase with lattice constants comparable with the values given in JCPDS (50-0367), and no traces of other phases are examined. It could be found that there were a main phase of anatase with a small amount of rutile and brookite of TiO2 present in the LaVO4/TiO2 sample. Moreover, the addition of LaVO4 strongly suppressed the phase transformation from anatase to rutile phase of TiO2 after the treatment at 500 °C, and the

FIGURE 1. X-ray diffraction patterns of T500, T400, LaVO4/TiO2, and LaVO4.

FIGURE 2. Nitrogen-sorption isotherms and the pore size distribution plots for LaVO4/TiO2. crystal phase of TiO2 in T400, which is treated at 400 °C, was very similar to that in LaVO4/TiO2, which is treated at 500 °C. However, because LaVO4 existed in a highly dispersed form in the TiO2 host or the amount of LaVO4 is so small (1 wt %) that it is beyond the detection of XRD, no distinct peaks corresponding to LaVO4 were observed in the LaVO4/TiO2 sample. In addition, when the amount of LaVO4 in TiO2 exceeded 2 wt %, the peaks corresponding to monoclinic LaVO4 appeared (shown in Figure S2). The Raman spectra also showed the existence of anatase and rutile phase of TiO2 in LaVO4/TiO2 sample, but no Raman shift peak of monoclinic LaVO4 was observed until the content of LaVO4 increased to 10 wt % (see Figure S3). Furthermore, the X-ray photoelectron spectroscopy (XPS) result also demonstrated the existence of Ti4+ [Ti(2p) peaks at 458.9 and 465.0 eV], La3+ [La(3d) peaks at 835.4 and 852.2 eV], and V5+ [V(2p3/2) peak at 517.0 eV] in LaVO4/TiO2 sample (see Figure S4). These results confirmed the existence of LaVO4 and TiO2 in LaVO4/TiO2 sample. The N2 sorption for the LaVO4/TiO2 sample was found to be of type IV isotherms, and the pore size distribution plot showed a narrow pore size distribution with average pore diameter of 8.6 nm (Figure 2). These results indicated the LaVO4/TiO2 sample was a porous solid. The BrunauerEmmett-Teller specific surface area of the LaVO4/TiO2 sample was found to be as much as ∼79.94 m2 g-1. A comparison of UV-vis diffuse reflection spectra (DRS) of T400, T500, P25, LaVO4/TiO2, and LaVO4 was shown in Figure 3. The results indicated that the absorption of visible light (λ > 400 nm) to LaVO4/TiO2 nanocomposite was clearly more than that of TiO2, which was due to the contribution of LaVO4. Moreover, the plot of transformed Kubelka-Munk function versus the energy of light afforded band gap energy (23) of 2.1 eV for monoclinic LaVO4 (shown in Figure S5). VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. DRS spectra of P25, T500, T400, LaVO4/TiO2, and LaVO4.

FIGURE 4. High-resolution TEM image of LaVO4/TiO2 nanocomposite. To obtain information about the structure of the samples, the LaVO4/TiO2 nanocomposite was characterized by highresolution transmission electron microscopy (HRTEM). As shown in Figure 4, many different lattice fringes can be found that allowed for identification of the crystallographic spacings of TiO2 and LaVO4. The fringes of d ) 0.352 nm matched the (101) crystallographic planes of anatase TiO2, while the fringes of d ) 0.296 nm and d ) 0.272 nm matched the (012) and (202j ) crystallographic planes of monoclinic LaVO4 nanoparticles, respectively. Furthermore, an interconnected fine nanoparticulate morphology was observed, indicating a LaVO4/TiO2 nanocrystal heterojunction was really formed in the composite catalyst. The photocatalytic activities of the LaVO4/TiO2 nanocomposite in decomposing benzene in the gas phase were

FIGURE 6. Photodegradation of benzene on LaVO4/TiO2 photocatalyst during repetition operation upon visible light irradiation. evaluated under visible light irradiation (450 < λ < 900 nm). As shown in Figure 5, the photocatalytic activities of T500, T400, P25, and LaVO4 were very low under visible light irradiation. By contrast, the LaVO4/TiO2 nanocomposite catalyst exhibited notably high visible-light photocatalytic activity. The conversion of benzene was about 57% and the produced CO2 was about 260 ppm after the reaction was steady. Such high visible photocatalytic activity can be maintained for 5 runs (50 h) as shown in Figure 6, which emphasized the photocatalytic stability of the LaVO4/TiO2 nanocomposite. To further understand the photocatalytic performance of the LaVO4/TiO2 nanocomposite catalyst, the nitrogen-doped TiO2 (TiO2-xNx) visible light photocatalyst and physical mixture of pure TiO2 (P25) and LaVO4 powders (LaVO4/TiO2(PM)) were chosen for comparison. As shown in Figure 7, the visible photocatalytic activity of LaVO4/TiO2 nanocomposite was remarkably higher than that of TiO2-xNx. LaVO4/TiO2(PM) also showed photocatalytic activity under visible light irradiation; however, the conversion of benzene and the produced CO2 on LaVO4/TiO2 nanocomposite were both about 4 times higher than these on LaVO4/TiO2(PM). Therefore, it is believed that the strong interactions between LaVO4 and TiO2 nanoparticles contributed to the efficient activity of LaVO4/TiO2 in the process of degradation of benzene. Moreover, under UV light (280 < λ < 400 nm) irradiation, the photocatalytic activity of LaVO4/TiO2 nanocomposite was also obviously higher than those of T400, T500, P25, TiO2-xNx, and LaVO4/TiO2(PM) (see Figure S6). Figure 8 showed the flat-band potentials (Vfb) of these semiconductors determined by an electrochemical method (10). It has been indicated from Mott-Schottky plots that the Vfb of T400, T500, and P25 were almost the same at -0.40

FIGURE 5. Conversion of C6H6 (a) and the amount of produced CO2 (b) on LaVO4/TiO2, T500, T400, P25, and LaVO4 under visible light irradiation and on LaVO4/TiO2 in the dark. 4166

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FIGURE 7. Photodegradation of benzene on TiO2-xNx (N-doped TiO2), LaVO4/TiO2(PM), and LaVO4/TiO2 nanocomposite under visible light irradiation.

FIGURE 8. Mott-Schottky plots of LaVO4, T400, T500, and P25 electrodes in 0.3 M LiClO4 at pH 3. V versus SCE at pH 3 (equivalent to -0.40 V vs NHE at pH 7), which was compatible with previous reports of TiO2 (24, 25). The Mott-Schottky plots also demonstrated that LaVO4 and TiO2 were both n-type semiconductors and the Vfb of LaVO4 was more negative than that of TiO2 by about 0.24 V at the same condition. It is generally known that the conduction band potentials (ECB) of n-type semiconductors is very close to (0.1-0.2 V more negative) the flat-band potentials (26), so it can be deduced that the conduction band (CB) position of LaVO4 was more negative than that of TiO2. Obviously, the difference of ECB between LaVO4 and TiO2 allowed the transfer of electron from the CB of LaVO4 to that of TiO2. To study the recombination of electron-hole pairs in photocatalysts, PL emission spectra of LaVO4 and LaVO4/ TiO2 were measured. It is known that LaVO4 absorbs visible light, but it is not active in visible light for photodegradation reactions. This is because of the rapid recombination of photogenerated electron-hole pairs as shown in Figure 9. The PL spectrum of LaVO4 shows a strong emission, which indicates that the electrons and holes recombine rapidly (27). By contrast, for LaVO4/TiO2, there was no PL peak observed, indicating that the electron-hole pairs recombination was very slow. From that, it was concluded that LaVO4 may function as a sensitizer to adsorb visible light and the heterojunction of LaVO4/TiO2 may act as an active center for hindering the rapid recombination of photoinduced electron-hole pairs generated by LaVO4. Hydroxyl radical (OH•) has been considered as a key species in the photodegradation of many hazardous chemical compounds for its high reaction ability to attack any organic molecule (2). As shown in Figure 10, the fluorescence intensity at 426 nm was linearly increasing along with the irradiation

FIGURE 9. PL spectra of LaVO4 and LaVO4/TiO2.

FIGURE 10. (a) OH• trapping PL spectra of LaVO4/TiO2 on TA solution under visible irradiation. (b) Plots of the induced PL intensity (at 426 nm) vs irradiation time.

SCHEME 1. Proposed Mechanism for the Visible Light Photodegradation of Benzene on LaVO4/TiO2 Nanocomposite

time, which elucidated that OH• on LaVO4/TiO2 catalyst was really produced under visible light irradiation. Additionally, we could not detect the OH• in the blank test (no photocatalyst) under visible light irradiation or in the dark. On the basis of the above experimental results, a possible photocatalysis process (Scheme 1) for the degradation of benzene under visible light irradiation could conclude as follows: (1) Because of the high visible absorbance of LaVO4, when visible light was supplied to the LaVO4/TiO2 heterojunction, electrons and holes generated by LaVO4 were VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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separated. (2) Some electrons were injected into TiO2 nanoparticles quickly since the conduction band of LaVO4 was more negative than that of TiO2. Moreover, the formed nanostructure heterojunction on LaVO4/TiO2 composite also led to a more efficient interelectron transfer between the two components (16). (3) The photogenerated electrons were then captured by O2 to yield O2•- and H2O2, and then the OH• can be formed by reacting O2•- with H2O2 (28, 29). For its high reaction ability to attack any organic molecule, the generation of •OH was a key factor in the reaction of the photo-oxidation of benzene. (4) The photogenerated hole in LaVO4 also may activate some unsaturated organic pollutants (e.g., benzene), leading to subsequent decomposition (11). Furthermore, the large specific surface area of LaVO4/TiO2 nanocomposite was also favorable for photocatalytic reaction. To sum up, the improvement of charge separation, efficient interelectron transfer, the produced OH•, and large specific surface area were supposed to be responsible for the high efficient photocatalytic activity of the LaVO4/TiO2 nanocomposite. Further research of the explicit mechanism for the strong photocatalytic activity of the heterojunction LaVO4/TiO2 nanocomposite under visible light is now under investigation in our laboratory. In conclusion, a novel LaVO4/TiO2 nanocomposite material with interconnected nanocrystal heterojunction was prepared by a simple coupled method. This new photocatalyst exhibited strong photocatalytic activity for decomposition of benzene under visible light irradiation with high photocatalytic stability. Moreover, it also showed excellent photocatalytic performance under UV light irradiation. The enhanced photocatalytic performance of LaVO4/TiO2 may be attributed to not only the matched band potentials but also interconnected nanocrystal heterojunction of LaVO4 and TiO2. This work may present an important strategy to design and prepare visible-induced high-performance photocatalysts.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (20537010, 20677010, and 20873023), an “863” Project From the MOST of China (2006AA03Z340), National Basic Research Program of China (973 Program, 2007CB613306), and the Science Foundation of Fujian, China (2003F004, JA07001, and 0330-033070).

Supporting Information Available Experimental details of TA-PL and Vfb measurement, transmittance of the combined visible light filters and the ultraviolet light filter (Figure S1), XRD of LaVO4-TiO2 with different LaVO4 contents (Figure S2), laser Raman spectra of LaVO4 and LaVO4/TiO2 with different LaVO4 contents (Figure S3), XPS result of LaVO4/TiO2 (Figure S4), band gap of LaVO4 (Figure S5), and photodegradation of benzene on different catalysts under UV light irradiation (Figure S6). These materials are available free of charge via the Internet at http:// pubs.acs.org.

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