Origin of the Enhanced Photocatalytic Activities of Semiconductors: A

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Origin of the Enhanced Photocatalytic Activities of Semiconductors: A Case Study of ZnO Doped with Mg2+ Xiaoqing Qiu,† Liping Li,† Jing Zheng,† Junjie Liu,† Xuefei Sun,†,‡ and Guangshe Li*,† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, and Graduate School of Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China and Research Institute of Photocatalysis, Fuzhou UniVersity, Fuzhou, 350002, People’s Republic of China ReceiVed: April 11, 2008; ReVised Manuscript ReceiVed: May 31, 2008

A series of Zn1-xMgxO samples with dopant content ranging from x ) 0 to 0.10 were prepared by a novel rheological phase reaction route. All Zn1-xMgxO samples were investigated by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, inductive coupled plasma optical emission spectroscopy, infrared and UV-vis absorption spectroscopy, and the Barrett-Emmett--Teller technique. The effects of Mg2+ doping in ZnO on the electronic structures and photogradation of methylene blue dye solution were investigated experimentally and theoretically. All Zn1-xMgxO samples exhibit high photoactivities comparable to Degussa P-25, which first increased with the Mg doing content up to x ) 0.05, and then slightly decreased with further doping of Mg to x ) 0.10. Density function theory calculations revealed that the substitutions of Mg for Zn ions in the wurtzite ZnO structure largely affected the conduction band, but left the valence band nearly unchanged. The bottom of the conduction band shifted toward higher energies and the contribution of Mg 3s orbitals to the conduction band became more pronounced with increasing Mg content, which explains the enhanced photocatalytic activities. The state density of interstitial Mg in ZnO showed a set of shallow acceptor levels above the valence band. These shallow levels could act as the trapping or recombination centers for photoinduced electrons and holes, accounting for the slightly decreased photodegradation efficiency with further increasing Mg content to x ) 0.10. The optimal doping content for photocatalytic performance was determined to be x ) 0.05, which is the consequence of the balance of two competing doping effects from lattice substitution and interstitial occupations on the electronic structures. 1. Introduction Semiconductor-based photocatalytic degradation of organic pollutants has attracted great interest since it provides a potential solution to many environmental pollution problems that mankind is facing nowadays.1,2 Principally, when a semiconductor absorbs a photon with energy greater than or equal to band gap energy, an electron would be promoted from the valence band to the conduction band, leaving a hole in the valence band. If this charge separation is valid, these electrons and holes can be used for efficient photocatalytic degradation of organic pollutants: holes can react with the surface-bound H2O or OH- to produce a powerful oxidant such as hydroxyl radicals, and therefore directly oxidize the organic pollutants. The conduction band electrons, on the other hand, may be picked up by the dissolved oxygen species to generate superoxide anion radicals, which are highly reactive for oxidizing organic compounds.3,4 To significantly improve the photocatalytic efficiency of oxide semiconductors (e.g., TiO2 and ZnO), many elaboration works have been done to tailor the bandgap energies by modifying the host structures with various transition metal dopants including V,5 Fe,6 Co,7 and Cu.8 For most of the semiconductors, doping strategies have shown both positive and negative impacts,2 because there exist very complicated factors like the electronic structure, dopant concentration, the energy level positions of the dopants within the band gap, d electronic * Corresponding author. Phone: +86-591-83792846. Fax: +86-59183714946. E-mail: [email protected]. † State Key Laboratory of Structural Chemistry. ‡ Research Institute of Photocatalysis.

configuration of transition ions, and the illumination light intensity that could contribute to the photocatalytic activities.9 Among all these factors, the electronic structure is well considered to play a crucial role in the photocatalytic properties.10 Apparently, clarifying the correlation between electronic structure and photoactivities is fundamentally important in this regard. It is well-known that the electronic structure of a given semiconductor is largely affected by doping effects and particle size. In the case of transition metal doping, the localized d-levels would be introduced in the band gap, which can decrease the photothreshold energy of the semiconductors, but also serve as the recombination centers for photoinduced charge carriers.11 Alternatively, alkaline earth metal ions have no localized d-levels, which can be taken as the candidate dopants for simplifying the correlation between structures and photocatalytic properties. Venkatachalam et al. found that doping of TiO2 nanoparticles with Mg2+ and Ba2+ produces higher photocatalytic activities than those of undoped TiO2 nanoparticles or commercial TiO2.12,13 Nevertheless, the entry of alkaline metal ions into the TiO2 lattice also results in the creation of significant lattice defects because of the charge compensation and the ionic radius mismatch between Mg2+ (or Ba2+) and Ti4+, which may put huge uncertainties to the origin of photoactivities.14 ZnO is a prototype semiconductor for achieving the correlation between electronic structure and photoactivities, since (1) it is well documented that ZnO is superior over TiO2 in producing hydrogen peroxide,15 which allows its uses in efficient photodegradation of organic acid16 and sterilization of bacteria

10.1021/jp803129e CCC: $40.75  2008 American Chemical Society Published on Web 07/19/2008

Origin of Photocatalytic Activities of Semiconductors and viruses,17,18 (2) Mg2+ has an ionic radius of 0.57 Å, which is very close to that of 0.60 Å for Zn2+ in tetrahedral coordination,19 as a result, the incorporation of Mg2+ in ZnO is not expected to induce significant change of lattice sizes20 dislike that in TiO2 doped with alkaline metal ions, and most importantly, (3) a continuous tailoring of electronic band gap is highly possible by varying the Mg2+ concentration in ZnO.21,22 Having these in mind, ZnO doped with Mg2+ becomes the target material to study in this work, which may give the picture about the origin of photocatalytic activity for a broad class of semiconductors. Herein, we first prepared Zn1-xMgxO nanoparticles by a novel rheological phase reaction precursor route.23,24 The doping effects were systematically studied with an attempt to investigate the possible reasons for the significantly improved photocatalytic activity toward degradation of methylene blue. Finally, we explored the correlation between the electronic structure and photocatalytic activity. 2. Experimental Section 2.1. Sample Preparation. Nanoparticles of Zn1-xMgxO (x ) 0, 0.025, 0.05, 0.075, 0.10, 0.15) were synthesized by a rheological phase reaction precursor method. All chemicals were analytical grade, used without further purification. Zinc acetate dihydrate (Zn(Ac)2 · 2H2O; 4.39 g, 0.02 mol) and given amounts of magnesium acetate (Mg(Ac)2 · 4H2O), and 3.78 g (0.03 mol) oxalic acid (H2C2O4 · 2H2O) were well mixed by grinding, in a mortar, and then a proper quantity of distilled water was added into the mixture to obtain the rheological bodies. This mixture was transferred to a 20-mL Teflon-lined stainless steel autoclave that was allowed to react at 120 °C for 6 h. A white precursor was obtained, which was then subjected to thermal treatment in air at 550 °C for 2 h in a muffle furnace with a step temperature controller. When the treatment was finished, the furnace was naturally cooled to room temperature, and the final sample was obtained. 2.2. Sample Characterization. Thermal behaviors of the precursors were examined with a Netzsch STA449C thermogravimetric analyzer (TGA) at a heating rate of 15 °C min-1 from 30 to 800 °C in a flow of air. The infrared optical properties were measured on a Perkin-Elmer IR spectrophotometer at a resolution of 4 cm-1, using a KBr pellet technique. The structural characteristics of the samples were measured by powder X-ray diffraction (XRD) at room temperature on a Rigaku D/MAX25000 diffractometer with a copper target. Ni powder served as an internal standard for peak-position determination. The morphologies of the samples were investigated by a field-emission scanning electron microscopy (SEM) using a JEOL JSM-6700 apparatus, and transition electron microscopy (TEM) on a JEOL JEM 2010 instrument under an acceleration voltage of 200 kV. The chemical compositions of the samples were quantitatively determined using inductive coupled plasma optical emission spectroscopy. The specific surface areas of the samples were determined from the nitrogen absorption data at liquid nitrogen temperature by using the BarrettEmmett-Teller (BET) technique on a Micromeritics ASAP 2000 Surface Area and Porosity Analyzer. The absorption spectra of the samples were recorded with a Perkin-Elmer UV WinLab Lambda 35 UV/vis spectrophotometer. The plane-wave based density functional theory (DFT) calculations of the Zn1-xMgxO samples were performed with the CASTEP program package25 with the core orbitals replaced by ultrasoft pseudopotentials, the generalized gradient approximation (GGA) with the PBE exchange correlation func-

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Figure 1. XRD patterns of Zn1-xMgxO with given dopant concentration, x. Vertical bars below the patterns represent the standard diffraction data from JCPDS file for bulk ZnO (No. 36-1451) and MgO (No. 7-239). The asterisk (/) represents the internal standard nickel.

Figure 2. Lattice parameters (a, c, V )
3ja2c/2) as a function of Mg2+ dopant content, x.

tional, and a kinetic energy cutoff of 300 eV. Thirty-two atom supercells were constructed in order to explore the doping effects by 2 × 2 × 1 expansion of the unit cell of wurtzite ZnO. The Broyden, Fletcher, Goldfarb, and Shannon minimizer is used to perform the structural optimization and the convergence. All band structures and the density of states were calculated on the corresponding optimized crystal geometries. 2.3. Photocatalytic Activity Test. A heteropolyaromatic dye, Methylene blue dye (MB), was used as a probe molecule to evaluate the photocatalytic reactivity of the samples in response to ultraviolet light at room temperature. The experiments were carried out as follows: 50 mg of the samples was dispersed in 80 mL of 2 × 10-5 M MB aqueous solution in a 100-mL beaker. Prior to illumination, the suspensions were magnetically stirred in the dark for 12 h to ensure the establishment of absorption/ desorption equilibrium of MB on the sample surfaces. Subsequently, the suspension was irradiated under a 20-W lowpressure Hg lamp with a peak wavelength of 254 nm positioned about 15 cm away from the breaker. At given intervals, 3 mL of the suspensions was extracted and subsequently centrifuged at a rate of 9000 rpm for 15 min. UV-vis absorption spectra of the supernatant were then measured with a Perkin-Elmer UV WinLab Lambda 35 spectrophotometer. 3. Results and Discussion Figure 1 shows XRD patterns of the Zn1-xMgxO samples that were obtained after high-temperature calcinations of the precursor. The precursor is an oxalate as confirmed by IR and TGA

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Figure 3. (a) TEM image of Zn1-xMgxO at x ) 0, (b) high-resolution TEM image of an individual particle for x ) 0, (c) large scale photograph for x ) 0 by SEM, and (d) SEM photograph for x ) 0.05.

Figure 4. (a) UV-vis reflectance spectra and (b) relationships between (Rhν)2 and photon energy of Zn1-xMgxO samples.

(Figure S1, Supporting Information). The standard data for bulk ZnO (JCPDS Card No. 36-1451) and MgO (No. 7-239) are also shown in Figure 1 for comparison. All samples are highly crystallized as indicated by the sharp XRD peaks. With varying the dopant content x e 0.10, XRD data matched well the standard data for the ZnO wurtzite structure, while when further increasing the Mg content up to x ) 0.15, several additional peaks appeared, which are assigned to the secondary phase of MgO. Therefore, the solubility limit of Mg2+ in ZnO was determined to be lower than x ) 0.15 in this work.

This solubility limit is apparently higher than that of about 4% reported in the literature,26 but lower than those of 33% with pulsed laser deposition,20 16.5% by metalorganic chemical vapor deposition,27 46% or 49% reported by Minemoto et al.28 or Park et al.,29 while it is close to that of 10% when using a solution chemistry with subsequent treatment procedures.30 The substitution of Mg2+ in ZnO was confirmed by lattice parameters as a function of the dopant content of Mg. As indicated in Figure 2, with increasing the Mg2+ concentration, the lattice parameter, a, decreased monotonously, while c increased slightly, in

Origin of Photocatalytic Activities of Semiconductors

Figure 5. Infrared spectra of Zn1-xMgxO samples at given Mg concentration, x.

agreement with that previously reported in the literature.31 As a consequence, the lattice volume, V )
3ja2c/2, increased slightly (Figure 2). Considering the similar ionic radius of Zn2+ and Mg2+,19 such variations in lattice parameters should stem from the dipole-dipole interactions on the surfaces of nanoparticles and the resulting negative pressures as reported in our recent work on ZnO nanostructures doped with smaller Co2+.32 Morphologies and microstructures of the Zn1-xMgxO samples were investigated by TEM and SEM. As demonstrated in Figures 3a, sample Zn1-xMgxO at x ) 0 consisted of tiny single crystals with grain sizes 60 to 100 nm. The corresponding highresolution TEM image is illustrated in Figure 3b. Lattice fringes revealed the single crystalline nature of these nanoparticles, demonstrating a high sample uniformity of the as-obtained ZnO nanoparticles. SEM photographs in Figure 3c,d give a direct view of the morphologies of Zn1-xMgxO samples. All Zn1-xMgxO samples are an assembly of ellipsoidal nanoparticles. Introduction of Mg2+ into the ZnO matrix did not change the particle shape and particle size. These observations are of great significance, since the impacts of morphology and size variations could be first excluded as the causes for the photocatalytic activity,33,34 which will be described latter. The chemical compositions of the samples Zn1-xMgxO at x ) 0.05 was identified by EDS, which indicates the presence of Zn and Mg (Figure S2, Supporting Information). The exact Mg content was precisely determined by inductive coupled plasma (Figure S3, Supporting Information), which demonstrates that the atomic ratio of the samples was very close to the initial one. Optical absorption properties of Zn1-xMgxO nanoparticles were measured by diffuse reflectance spectroscopy at room temperature. As demonstrated in Figure 4a, Zn1-xMgxO samples are transparent in the visible region, which, however, shows an intense band-to-band absorption in the UV region. This absorption edge shifted toward higher energies as the Mg content increased. The absorbance coefficient (R) was calculated from the raw absorbance data to obtain the optical band gap, Eg. The Eg values were thus determined by extrapolation of the linear portion of the (Rhν)2 curve versus the photon energy hν to (Rhν)2 ) 0. As illustrated in Figure 4b, with increasing the Mg2+ content from x ) 0 to 0.10, the band gap energies increased continuously from 3.27 to 3.44 eV. The observation of a systematic blue shift in band gap implies that Mg2+ was successfully incorporated into the wurtzite ZnO lattices.22 There are two primary causes that may contribute to the variations in band gap energies, namely, quantum size effect and electronic structure modifications. With regards to the quantum size effects, as stated above, all Zn1-xMgxO nanoparticles have compatible particle sizes in the range of 60 to 100 nm, which are far beyond

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12245 the region (i.e., 0, which can be ascribed to the bond Mg-O-Zn. Therefore, the evolution of the diagnostic bands of ZnO in our work is apparently associated with the substitution of Mg2+ for Zn2+. In Figure 5, the broad absorption bands centered at 3455.9 and 1635.6 cm-1 are assigned to the hydroxyl groups of the chemisorbed and/or physisorbed water molecules43 on sample surfaces. Because the photochemical reactions mainly take place on the sample surfaces, the absorbed surface species would play an important role in the photocatalytic properties.44 From the infrared spectra in Figure 5, all Zn1-xMgxO samples were indicated to contain plenty of surface water molecules at the absence of the contaminated species. Therefore, the effect of surface species45 except for absorbed water could be ignored from the causes of the photocatalytic activity of Zn1-xMgxO. Methylene blue (MB) degradation was used to evaluate the photocatalytic activity of the Zn1-xMgxO samples. The timedependent absorption spectra of MB aqueous solution during the UV light irradiation in the presence of the samples Zn1-xMgxO at x ) 0 and 0.05 are displayed in Figure 6a,b. It is seen that the characteristic absorption peaks of MB at approximately 665 nm became weaker as the irradiation time increases. No new absorption peaks appeared, and no obvious hypsochromic shift was observed, which indicate that the degradation of MB occurred under UV exposure. MB solution over Zn1-xMgxO at x ) 0 was completely degraded after irradiation for 7.5 h (Figure 6a), whereas the sample at x ) 0.05 showed an excellent activity with a short degradation time of 3.5 h. Further comparative studies of the concentration changes of MB with irradiation time for all Zn1-xMgxO samples are shown in Figure 6c. It is clear that all Mg-doped ZnO samples (0.10 g x > 0) possessed higher photocatalytic activities

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Figure 6. Time-dependent absorption spectra of RhB solution during UV light irradiation in the presence of the samples at (a) x ) 0 and (b) x ) 0.05, (c) relative photocatalytic activity of the samples as well as Degussa P-25 and pure anatase TiO2 under UV light irradiation, and (d) representative nitrogen desorption/adsorption isotherms for samples at x ) 0.05. Inset is the relationship between the specific area and Mg content of the samples.

than that at x ) 0. The photocatalytic activities of Zn1-xMgxO samples first increased with the Mg doping content up to x ) 0.05, and then slightly decreased with further doping Mg to x ) 0.10. The order of photocatalytic activities was Zn0.95Mg0.05O > Zn0.925Mg0.075O > Zn0.9Mg0.1O > Zn0.975Mg0.025O > ZnO. Among all samples, Zn0.95Mg0.05O was found to be the most active, which shows a photocatalytic activity about 86% higher than that of x ) 0. Experiments were also performed for photodegradation of Rhodamine B dye solution under identical conditions for other dopant contents. A similar trend of photocatalytic activity was observed (not shown). We also compared the photocatalytic properties of our Zn1-xMgxO samples with those of Degussa P-25 and pure anatase TiO2.46 Interestingly, as shown in Figure 6c, all Zn1-xMgxO samples exhibited photocatalytic activity much higher than those of pure anatase TiO2, but comparable to that of Degussa P-25. Here we address the photocatalytic acitivity as a function of the dopant concentration. In general, the photocatalytic activity of an oxide semiconductor is closely related to the particle sizes,30 morphologies,34,47 and surface properties.44,45 For our Zn1-xMgxO samples, the dependence of activity on Mg content could not be explained by these three factors because of the similar particle sizes, morphologies, and surface properties as discussed above. The specific surface area is another factor that has to be considered.48 Figure 6d shows the representative nitrogen desorption/adsorption isotherms of Zn1-xMgxO at x ) 0.05, wherein the BET surface areas for all samples as a function of Mg content in Zn1-xMgxO are also given. The N2 isotherm is a type III isotherm with a large type H3 hysteresis hoop.49 As shown in Figure 6d, the specific areas of the samples are comparable with each other. Therefore, the

enhanced photocatalytic activity of ZnO by doping Mg should be ascribed to some other factors rather than surface area. To specify the role of electronic structure on the photocatalytic activities of Zn1-xMgxO, the partial density of states (DOS) were calculated on the basis of plane-wave density function theory program package CASTEP.25 Figure 7 displays the variation of DOS with the energy. The zero energy corresponds to the Fermi-level (EF). For wurtzite ZnO, the top of valence band maximum (VBM) below EF mainly consists of O 2p like states, while the bottom of the conduction band minimum (CBM) above EF is predominantly composed of Zn 4s like states (Figure 7a). To get an insight into the qualitative trend in DOS of Zn1-xMgxO as Mg content increases, the unit cells of Zn1-xMgxO were first constructed by substituting Mg at the Zn lattice site in wurtzite ZnO structures. The DOS of Zn0.75Mg0.25O and Zn0.5Mg0.5O were illustrated in Figure 7b,c. It is seen that Mg substitution at the Zn site in ZnO structures results in an apparent variation of the conduction band, but the valence band remains almost the same as that of ZnO. As Mg content increases, the contribution of Mg 3s like states becomes dominant at the bottom of the conduction band. Since Mg 3s like states are higher than the Zn 4s level,50 the band gap of Zn1-xMgxO is broaden with increasing Mg concentration by heightening the bottom of the conduction band, in good agreement with our UV-vis spectra analysis (Figure 4b). A similar trend has also been reported recently by Maeda et al.51 and Huang et al.52 This theoretical analysis is very valuable to explore the correlation between the electronic structure and photocatalytic activity. The Mg 3s orbitals occurring in the conduction band may provide a favorable channel for the photoinduced charge carrier transport.53 The band gap of

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Figure 7. Local density of states for (a) pure ZnO, (b) Zn0.75Mg0.25O, and (c) Zn0.5Mg0.5O, and (d) ZnO with 1/8 Mg interstitial doping every ZnO unit cell. The vertical dash line represents the Fermi level.

Zn1-xMgxO shifts toward higher energies as Mg content increases. It is well-known that the reduction ability of the photoinduced electron is determined by the position of the conduction bands relative to the Fermi level.48 Therefore, photocatalytic activity (or reduction ability) of photoinduced electrons should increase with the bandgap energies. Namely, when more Zn2+ is substituted by Mg2+, the band gap will increase, seemingly to give the most active photocatalyst for Zn0.9Mg0.1O. Nevertheless, Figure 6c clearly indicates that the sample Zn0.95Mg0.05O shows a superior activity. What is the reason for this usual observation? Here, we addressed the microstructural factors and the resulting electronic structures. Generally, there are two primary forms of Mg occupations that may contribute the positions of the conduction bands, i.e., lattice substitution and interstitial occupations. From the phase diagram of ZnO-MgO binary systems, the thermodynamic solubility limit of Mg ions in the ZnO lattice was about 4%,26 as is confirmed by our recent preparation using a thermodynamic equilibrium system.21 For the present samples, when the Mg content is less than x ) 0.04, the majority of Mg ions are likely substituted in the Zn sites of ZnO, while when more Mg ions were dissolved in the ZnO lattice, an excess of Mg ions might enter into the interstitial sites. Therefore, the interstitial doping model along with the substitution model of the lattice site54 should be taken into account for the photocatalytic activities. Having these considerations, the density of states (DOS) for Mg in the interstitial site of ZnO was calculated. As shown in Figure 7d, the valence band is almost the same as the situation of Mg in the lattice site of ZnO. However, unlike the substitution of the lattice site, a set of shallow acceptor levels was observed above the EF when Mg2+ was incorporated in the interstitial sites (inset of Figure 7d). These shallow levels resulting from Mg interstitial doping would act as the trapping or recombination centers for photoinduced electrons and holes, which accounts for the slightly decreased photodegradation efficiency as x > 0.05 (Figure 6c). Consequently, the optimal doping content for photocatalytic performance is the consequence of the balance

of two competing doping effects from lattice substitution and interstitial occupations on the electronic structures. 4. Conclusions This work describes the preparation and photocatalytic performance of highly crystalline Zn1-xMgxO samples. All Zn1-xMgxO samples exhibited similar particle sizes, morphologies, and surface area. The effects of Mg2+ doping on photocatalytic degradation of methylene blue were explored experimentally. The order of photocatalytic activities was clearly demonstrated: Zn0.95Mg0.05O > Zn0.925Mg0.075O > Zn0.9Mg0.1O > Zn0.975Mg0.025O > ZnO. Density function theory calculations showed that the photocatalytic activities of Zn1-xMgxO were highly relevant to the Mg doping. The substitution of Mg ions at Zn sites shifted the conduction band toward higher energies and enhanced the photocatalytic activities, while the incorporation of Mg ions at interstitial sites produced the impurity levels between conduction band and EF, which acted as the trapping or recombination centers for the photoinduced electrons and holes, and consequently reduced the photocatalytic activities. The balance of the competing doping effects from lattice substitution and interstitial occupations on the electronic structures explains the optimized photocatalytic activity for Zn1-xMgxO. Such a finding is fundamentally important, which may help to provide hints for developing and designing new photocatalytic semiconductors. Acknowledgment. This work was financially supported by NSFC (under the contract Nos. 20671092, 20773132, and 20771101), the National Basic Research Program of China (No. 2007CB613301), the Directional program (KJCXZ-YW-MO5), the Knowledge Innovation Program of the Chinese Academy of Sciences, and FJIRSM key program (No. SZD-07004-3). Supporting Information Available: Figures showing the IR spectrum of the oxalate precursor of Zn0.95Mg0.05O, the EDX

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