TiO2 Hierarchical Heterostructure: Controllable Synthesis

Jul 28, 2009 - A three-dimensional (3D) multicomponent oxide, Bi2WO6/TiO2 as an example, hierarchical heterostructure was successfully synthesized as ...
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J. Phys. Chem. C 2009, 113, 14727–14731

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3D Bi2WO6/TiO2 Hierarchical Heterostructure: Controllable Synthesis and Enhanced Visible Photocatalytic Degradation Performances Meng Shang, Wenzhong Wang,* Ling Zhang, Songmei Sun, Lu Wang, and Lin Zhou State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, People’s Republic of China ReceiVed: May 16, 2009; ReVised Manuscript ReceiVed: June 30, 2009

A three-dimensional (3D) multicomponent oxide, Bi2WO6/TiO2 as an example, hierarchical heterostructure was successfully synthesized as a mat via a simple and practical electrospinning-assisted route. The asprepared hierarchical nanofibrous mat consisted of Bi2WO6 nanoplates growing aslant on the primary TiO2 nanofibers. Interestingly, the nanoplates were further composed of nanoparticles with a size of less than 20 nm. Bi2WO6 with different morphologies and microstructures could be obtained by adjusting the concentration of the precursor. From the mat (10 cm) to the nanoparticle (20 nm), this multicomponent oxide mat due to the self-supporting property could be layed or hung conveniently anywhere under solar irradiation and recycled easily. Due to the structure-property relationships, the 3D Bi2WO6/TiO2 hierarchical heterostructure exhibited enhanced visible photocatalytic activity over that of the bulk Bi2WO6/TiO2 powder (SSR), the Bi2WO6 nanoparticles (BWO), and the TiO2 sample in the decomposition of both acetaldehyde (CH3CHO) in air and rhodamine B (RhB) in water which are typical model pollutants. Close investigation revealed that the surface area, grain size, and hierarchical heterostructure of the as-prepared Bi2WO6/TiO2 mat could improve the photocatalytic activities. Introduction The applications on solar energy conversion and degradation of pollution by semiconductor photocatalysts have received extensive attention in modern society. Considering to utilize the solar energy more effectively, the development of efficient visible-light-active photocatalysts has attracted worldwide attentions.1 It has been proved that the photocatalytic activities were closely related to the structure of the photocatalysts.2 Especially, the hierarchical structure, a kind of nanostructure with specific morphology and high order, has been concerned due to its important role in the systematic study of structureproperty relationship and improved physical and chemical properties.3 In addition, the separation rate of photoinduced charge carriers in photocatalyst can be significantly increased by reducing its size to the nanoscale and thus the photocatalytic activity can be enhanced.4 Therefore, considerable effort has been taken to synthesize hierarchical photocatalysts with small size so as to achieve high activities.5 However, the recycling of the photocatalysts hindered their applications due to their small size. Though some researchers are working on other methods to make it practical, such efforts have not yet achieved satisfactory results.6 Therefore, it is still a big challenge to design efficient and practical visible-light-driven photocatalysts. Electrospinning, as an economical and simple method that is capable of fabricating nanofibers with high specific surface area and porous structure on a large scale, has been employed in many applications.7 One of the attractive features of this method is that the nanofibers can be readily prepared as mat with favorable recycling characteristics.8 More importantly, the technique could also be the new platform for fabricating complex nanostructures having controllable hierarchical features so as to design photocatalysts with high performance.9 Xia’s * To whom correspondence should be addressed. Phone: +86-21-52415295. Fax: +86-21-5241-3122. E-mail: [email protected].

group generated TiO2 nanofibers with controllable structures and V2O5 nanorods grown on the TiO2 nanofibers. Zhou’s group reported ZnO/TiO2 hierarchical structures. Unfortunately, most of them were based on the simple metal oxide and these photocatalysts could only be activated by UV light. Stimulated by the promising applications and the enhanced properties of novel structures, the synthesis of more efficient visible-lightdriven multicomponent oxide photocatalysts with hierarchical heterostructure is a subject with considerable research interest. Herein, for the first time we presented a simple and practical electrospinning-assisted approach to prepare a mat of multicomponent oxide Bi2WO6/TiO2 (BWO/TiO2) with hierarchical heterostructure. In the decomposition of both acetaldehyde (CH3CHO) in air and rhodamine B (RhB) in water which are typical model pollutants, the as-prepared mat exhibited a much higher photocatalytic activity than the powder prepared by traditional solid-state reaction (SSR) and the particles without hierarchical heterostructure. Experimental Section Material: Poly(vinyl pyrrolidone) (PVP) with a molecular weight (MW) of 1.3 × 106 was obtained from Aldrich, other reagents were of analytical purity and were used as received from Shanghai Chemical Company. Synthesis of TiO2 Nanofibrous Film (TiO2): Ti(OBu)4 (2.0 g) was added to a mixture of 7.5 g of absolute ethanol and 2.0 g of acetic acid in a capped bottle. Then 0.5 g of PVP was added to the solution. The mixture was stirred for 2 h. The feeding rate of the solution in the syringe was controlled as 2.5 mL h-1 by using a syringe pump. The voltage applied to the needle of the syringe was 15 kV and the distance between the tip of the needle and the patterned iron collector with square-shaped protrusions was 10 cm. The as-collected films were calcined at 500 °C for 2 h.

10.1021/jp9045808 CCC: $40.75  2009 American Chemical Society Published on Web 07/28/2009

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Synthesis of Hierarchical Bi2WO6/TiO2 Heterostructure (BWO/TiO2): Bi(NO3)3 · 5H2O and Na2WO4 · 2H2O, 2:1 in molar ratio, were mixed together in 40 mL of EG so as to form the solution. Then 1.2 mmol of TiO2 films and the desired amount of Bi2WO6 precursor were added into the Teflon-lined autoclave. Then the autoclave was sealed and heated to 160 °C for 15 h. The film was washed with deionized water and ethanol to remove any ionic residual then dried in oven at 80 °C for 4 h for further characterization. For comparison, the Bi2WO6 nanoparticles without TiO2 film (BWO) were also prepared via the solvothermal method. The bulk Bi2WO6/TiO2 powder (SSR) was prepared by traditional solid-state reaction. The bulk Bi2WO6 was first prepared according to the previous study,10 and then mixed with TiO2 sol to calcine at 500 °C for 2 h. Photocatalytic Experiments: Photocatalytic activities of the as-prepared samples were evaluated by the degradation of rhodamine B (RhB) and acetaldehyde (CH3CHO) under visiblelight irradiation of a 500 W Xe lamp with a 420 nm cutoff filter. For the degradation of RhB, 0.1 g of photocatalyst was added into 100 mL of RhB solution (1 × 10-5 to 1 × 10-4 M). Before illumination, the solution was slightly stirred for 60 min in the dark in order to reach adsorption-desorption equilibrium between the photocatalyst and RhB. At 10 min intervals, a 4 mL solution was sampled. Then the adsorption UV-visible spectrum of the centrifugated solution was recorded with use of a Hitachi U-3010 UV-visible spectrophotometer. Chemical oxygen demand (COD) was estimated before and after the treatment, using the K2Cr2O7 oxidation method. For the degradation of CH3CHO, 0.1 g of the as-prepared mat was layed at the bottom of a gas-closed reactor at room temperature (capacity 600 mL). This reactor is made of glass and has a quartz window. The reaction gas mixture (1 atm) consisted of 100 ppm of CH3CHO and N2 balance gas. Gaseous samples (1 mL) were periodically extracted and analyzed by a gas chromatograph (GC) equipped with a flame ionization detector (N2 carrier) and a catalytic conversion furnace. Characterization: The X-ray diffraction (XRD) patterns of the samples were measured on a D/MAX 2250 V diffractometer (Rigaku, Japan), using monochromatized Cu KR (λ ) 0.15418 nm) radiation under 40 kV and 100 mA and scanning over the range of 20° e 2θ e 70°. The morphologies and microstructures of as-prepared samples were analyzed by the Scanning Electron Microscope (SEM) (JEOL JSM-6700F) and Transmission Electron Microscopy (TEM) (JEOL JEM-2100F, accelerating voltage 200 kV). UV-vis diffuse reflectance spectra of the samples were obtained on an UV-vis spectrophotometer (Hitachi U-3010), using BaSO4 as the reference. Nitrogen adsorption-desorption measurements were conducted at 77.35 K on a Micromeritics Tristar 3000 analyzer after the samples were degassed at 200 °C for 6 h. The Brunauer-Emmett-Teller (BET) surface areas of the products were estimated by using the adsorption data. Results and Discussion The phase and composition of the BWO/TiO2 sample, as well as the TiO2 sample, were characterized by X-ray diffraction (XRD), as shown in Figure 1. The TiO2 sample used as the substrate was composed of anatase crystal phase combining with a small amount of rutile structure (Figure 1A). However, all of the diffraction peaks of the BWO/TiO2 sample can be indexed as well-crystallized orthorhombic Bi2WO6 (JCPDS 39-0256) and anatase TiO2 (Figure 1B). The broad diffraction peaks imply that the Bi2WO6/TiO2 heterostructure is constructed from nanoscale substructures.

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Figure 1. The XRD patterns of the as-synthesized products: (A) TiO2 fibrous film and (B) Bi2WO6/TiO2 hierarchical heterostructure. 9 indicates the anatase phase of TiO2, 2 indicates the rutile phase of TiO2.

The hierarchical heterostructure of the BWO/TiO2 sample was shown in Figure 2. The macroscopic photograph (Figure 2A) reveals that the side length of the sample could reach ∼10 cm or more. By using the electrospinning method, mats of different sizes could be fabricated according to the practical requirement. This is one of the advantages of the electrospinning method. Moreover, considering the materials with ordered microstructures and patterns are due to specific physicochemical properties,11 a patterned collector with protrusions was thus employed to fabricate an electrospun mat with controllable architectures in our work. The scanning electron microscopy (SEM) image of the overall morphology is shown in Figure 2B. The ordered pattern with regular interspaces between nanofibers is demonstrated, which is also advantageous for photocatalysis because large surface area and more active sites to absorb or purify pollutant are supplied. Close observation on the nanofibers (Figure 2C) reveals that highly dense secondary Bi2WO6 nanoplates grew aslant on the primary TiO2 substrates and nanofibers with a diameter of about 200 nm were formed. The image of a single nanofiber (Figure 2D) further shows that the Bi2WO6 nanoplates possess an average side length of about 100 nm and a thickness of less than 20 nm. In addition, many pores with different diameter sizes, which may serve as transport paths for small molecules and utilize light sufficiently, are found among the nanoplates in this hierarchical structure. Interestingly, these two-dimensional layers are composed of substructure, in which densely packed nanoparticles orderly stacked or connected to each other. Further information about the Bi2WO6/TiO2 hierarchical heterostructure was obtained from transmission electron microscopy (TEM) images (Figure 2E). It is confirmed that nanofibers have diameters of about 200 nm, which agrees well with that revealed by the SEM images. Energy dispersive spectroscopy (EDS) microanalysis on selected areas (Figure 2F) indicates that the nanoplates are mainly made of Bi2WO6 and the parent nanofibers are still a mixture of TiO2 and Bi2WO6. Close inspection at the junction of nanoplates and nanofibers shows that the Bi2WO6 nanoplates have their roots inside the TiO2 nanofibers, suggesting that the Bi2WO6 nanoplates are not just loosely attached to the nanofibers surface. The highresolution transmission electron microscopy (HRTEM) investigation (Figure 2G) demonstrates the single-crystalline nature of the nanoparticles. The interlayer distances are consistent with the interplanar distance of the (131) plane of Bi2WO6 (0.314 nm) and the (101) plane of TiO2 (0.352 nm), respectively. These results also suggest that the prepared sample is considered to be a well-crystallized heterostructure with Bi2WO6 and TiO2 on nanoscale. Simultaneously, some fragments of the Bi2WO6 nanoplates further confirm the nanoplates consist of small nanoparticles with a size of less than 20 nm (Figure 2H).

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Figure 2. (A) The macroscopic photograph of Bi2WO6/TiO2 mat. (B) SEM image of the overall morphology of the mat. (C) The SEM image of a detailed view on Bi2WO6/TiO2 hierarchical nanofibers. (D) The SEM image of a single nanofiber. (E) The TEM image of Bi2WO6/TiO2 hierarchical nanofibers. (F) The EDS microanalysis on selected areas. (G) The HRTEM image of Bi2WO6/TiO2 nanofiber. (H) The TEM image of Bi2WO6 fragment after solvothermal treatment with TiO2 substrate.

Figure 3. The TEM image of Bi2WO6 nanoparticles after solvothermal treatment without TiO2 substrate: (A) lower magnification TEM image and (B) higher magnification TEM image.

However, the Bi2WO6 nanoparticles (BWO) prepared via a similar solvothermal process except that TiO2 substrate was not introduced present a larger size (Figure 3). The particles with a size of about 30 nm are not assembled to the regular nanoplates, indicating that the shape and size of the secondary structure were affected by the presence of TiO2.9b Because the Bi2WO6 nanoplates rooted inside the TiO2 nanofibers and in combination with the HRTEM image, the interfacial energy of Bi2WO6 formation should decrease in the presence of heterostructure. Therefore, the size of the Bi2WO6 nanoparticles in BWO/TiO2 is smaller than that in the BWO sample. In other words, the size should increase if the heterostructure was not formed. To prove this conclusion, the concentration of the Bi2WO6 precursor which could also affect the morphologies of the hierarchical Bi2WO6/TiO2 nanofibers was increased (Figure 4). It was found that no heterostructure was formed, the TiO2 nanofibers were just coated by the as-grown nanoparticles with a larger size of about 100 nm, and the density of the nanoparticles also dramatically increased. To illuminate this desirable structure clearly, the sketch map is given in Scheme 1. From the mat (10 cm) to the nanoparticle (20 nm), this multicomponent oxide hierarchical heterostructure due to the self-supporting property could be layed or hung conveniently anywhere under solar irradiation and recycled

Figure 4. The SEM image of solvothermally treated TiO2 nanofibers with the Bi2WO6 precursor of 0.4 mmol: (A) lower magnification SEM image and (B) higher magnification SEM image.

SCHEME 1: The Sketch Map of the Bi2WO6/TiO2 Hierarchical Heterostructure

easily. More importantly, due to the structure-property relationship, this mat may possess enhanced activities for a broad variety of applications in photocatalysis, catalysis, solar-cell, separation, and purification. To prove the enhanced photocatalytic activity of the threedimensional hierarchical heterostructure, the decomposition of a widely used dye, RhB, in water and CH3CHO in air under visible-light (λ > 420 nm) irradiation was investigated. Figure 5A shows the photocatalytic degradation rate of different photocatalysts under visible-light irradiation, where C was the absorption of RhB at the wavelength of 552 nm and C0 was the absorption after the adsorption equilibrium on the samples before irradiation. A blank test (RhB without any catalyst) under visible light exhibited little photolysis. The decrease of RhB with the samples in the dark for 30 min was similar to that of the blank test. With the hierarchical Bi2WO6/TiO2 heterostruc-

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Figure 7. Variations of chemical oxygen demand (COD) and transmittance of RhB (10-4 M) aqueous solutions with irradiation time (catalyst, 0.1 g).

Figure 5. (A) Photocatalytic degradation of RhB in the presence of Bi2WO6/TiO2 hierarchical heterostructure under visible-light (λ > 420 nm) irradiation. (B) The comparison of rate constant k. Figure 8. Photocatalytic oxidative decomposition of CH3CHO (100 ppm) over 0.1 g of catalyst under visible-light (λ > 420 nm) irradiation.

Figure 6. The comparisons of photocatalytic activities among the samples with different concentrations of Bi2WO6 precursor under visible-light irradiation. (0.1 mmol of Bi2WO6 precursor: 0.1BWO/ TiO2; 0.2 mmol of Bi2WO6 precursor: 0.2BWO/TiO2; 0.4 mmol of Bi2WO6 precursor: 0.4BWO/TiO2).

ture, the photodegradation efficiency of RhB reaches nearly 100% after 30 min of visible-light irradiation, which was much higher than that of the SSR, BWO, and TiO2 samples. Via the first-order linear fit, the determined reaction rate constant, k, for the hierarchical Bi2WO6/TiO2 heterostructure (0.0927) was more than 12-fold as fast as that of the SSR sample (0.0077) (Figure 5B). Especially, the BWO/TiO2 sample exhibited much higher photodegradation efficiency than that of the other samples with different concentrations of precursor due to the specific structure (Figure 6). As the reduction of chemical oxygen demand (COD) reflects the extent of degradation or mineralization of an organic species along with the color removal, the change of COD values in the photodegradation of RhB with the sample of BWO/TiO2 was studied as a function of irradiation time under visible light, as shown in Figure 7. The initial COD concentration of the RhB solution is 247.6 mg/L, and the T % (measured at 500 nm) is 0.3%. After visible-light irradiation for 4 h, the COD concentration decreased to 68.76 mg/L, and the T % at 500 nm reached 98%. The significant decrease in the COD values and the increase of the transmittance further confirm that RhB was truly photodegraded by the Bi2WO6/TiO2 hierarchical heterostructure. In addition to the decoloration of dye, acetaldehyde (CH3CHO), a typical contamination that has no photolysis and no light absorption in the photodegradation process, was selected

to further confirm and reveal the structure-property relationships. A 0.1 g sample of the as-prepared mat was layed at the bottom of a gas-closed reactor. The reaction gas mixture (1 atm) consisted of 100 ppm CH3CHO and N2 balance gas. It was found that CH3CHO was degraded by the samples with an obvious production of CO2 under visible-light irradiation (Figure 8). Similarly, due to the structure, the rate of CO2 evolution over BWO/TiO2 sample was much higher (8 times) than that of SSR sample and BWO sample after irradiating for 120 min, and the rate over TiO2 could be neglected under visible light. The hierarchical Bi2WO6/TiO2 photocatalyst could be easily recycled without deactivation, indicating that the mat is fairly stable. Furthermore, the mineralization of dye and the degradation of CH3CHO to CO2 strongly prove that the visible-light-driven reaction is assuredly “photocatalytic”.12 The photocatalysis in both liquid and gas proved that the three-dimensional hierarchical Bi2WO6/TiO2 heterostructure prepared via the electrospinning-assisted method presents the enhanced visible-light-driven photocatalytic activity, which may be due to the following reasons. First, the Brunauer-EmmettTeller (BET) surface area of the hierarchical heterostructure was estimated to be about 29.6 m2 g-1, which was much higher than that of the SSR (1.7 m2 g-1). The high surface area and plenty of pores in this hierarchical structure bring not only more surface reached by the visible light but also more active catalytic sites, which results in good photocatalytic performance.5a Second, decreased grain radius could reduce the recombination opportunities of the photogenerated electron-hole pairs.1a The hierarchical Bi2WO6/TiO2 are built of small nanoparticles, which is beneficial for promoting the photocatalytic efficiency. Last but not least, the hierarchical heterostructure prepared via the electrospinning-assisted method provides a heterogeneous structure. After the calculation, the band gap (Eg) of Bi2WO6 is estimated to be about 2.68 eV from the onset of the absorption edge (Figure 9), which indicates that the mat with the yellow color has the ability to absorb visible light for photocatalytic decomposition of organic contaminants. According to the

3D Bi2WO6/TiO2 Hierarchical Heterostructure

J. Phys. Chem. C, Vol. 113, No. 33, 2009 14731 photocatalytic activity was ascribed to the grain size, the surface area, and the hierarchical heterostructure. We believe that this approach provides a new way to prepare various 3D multicomponent oxide hierarchical heterostructure on a large scale, which possesses a wide range of applied potential. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 50672117 and 50732004) and the Nanotechnology Programs of Science and Technology Commission of Shanghai (0852nm00500).

Figure 9. The band gap (Eg) of the BWO sample is estimated to be about 2.8 eV from the absorption edge.

Figure 10. Schematic diagram for energy band matching and electron-hole separations.

estimated Eg values of the BWO sample and TiO2, the calculated conduction band (CB) and valence band (VB) edge potentials of Bi2WO6 and TiO2 are shown in Figure 10.13 Under visiblelight irradiation, the electrons in the VB of Bi2WO6 are excited to its CB. Thereby the VB of Bi2WO6 is rendered partially holes. Due to the VB level Bi2WO6 is lower by 0.353 V than that of TiO2, thus holes in the VB of Bi2WO6 can be transferred to that of TiO2. Simultaneously, anatase TiO2 with a loosely packed structure is used mainly as a hole-accepting semiconductor. As a result, the semiconductors with matching band potentials are tightly bonded to construct the efficient heterostructure. The migration of photogenerated carriers can be promoted by the internal field, so less of a barrier exists. Therefore, the probability of electron-hole recombination can be reduced, a larger number of electrons on the Bi2WO6 surface and holes on the TiO2 surface, respectively, can participate in photocatalytic reactions to directly or indirectly mineralize organic pollution, and thus the photocatalytic reaction can be enhanced greatly. Conclusions We have presented for the first time an economic and practical method of preparing 3D Bi2WO6/TiO2 hierarchical heterostructure. From the mat in the macrostructure to the nanoparticles in nanoscale, this Bi2WO6/TiO2 photocatalyst possessed favorable recycling characteristics, and exhibited enhanced photocatalytic activity in the decomposition of CH3CHO and RhB under visible-light (λ > 420 nm) irradiation. The higher

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