Topotactic Transformation Reaction from Layered Titanate

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J. Phys. Chem. C 2009, 113, 20275–20280

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Topotactic Transformation Reaction from Layered Titanate Nanosheets into Anatase Nanocrystals Puhong Wen,†,‡ Yoshie Ishikawa,‡ Hiroshi Itoh,‡ and Qi Feng*,‡ Department of Chemistry and Chemical Engineering, Baoji UniVersity of Arts and Science, 1 Gaoxin Road, Baoji, Shaanxi 721013, People’s Republic of China, and Department of AdVanced Materials Science, Faculty of Engineering, Kagawa UniVersity, 2217-20 Hayashi, Takamatsu 761-0396, Japan ReceiVed: August 25, 2009; ReVised Manuscript ReceiVed: October 20, 2009

Anatase nanocrystals with a special crystal plane on the surface were prepared by hydrothermal treatment of layered titanate nanosheets. The formation reaction of the anatase phase from the layered titanate nanosheets was investigated using TEM, SAED, XRD, and TG-DTA. The transformation from the layered phase into the anatase phase is a topotactic reaction. In the topotactic reaction, the TiO6 octahedra in titanate nanosheets slightly and equably shift from the positions of the layered lattice to the positions of the anatase lattice. Two kinds of definite relationships between the crystal-axis orientations of the layered structure and anatase structure were found, meaning that there are two routes in the in situ topotactic structural transformation reaction. The anatase nanocrystals prepared by this method preferentially expose the (010) plane on their surfaces. 1. Introduction Recently, nanostructured titanium dioxide materials have received much attention due to their excellent photocatalytic activities that have potential applications to dye-sensitized solar cells,1 photoelectrochemical splitting of water for generation of hydrogen gas,2,3 and photocatalytic decomposition of hazardous organic species in environmental cleaning systems.4,5 In the photocatalytic reactions, the photocatalytic activity is dependent on the crystallinity, crystal size, and surface structure of TiO2 particles.6-11 Up to now, many studies have been carried out on the effects of the crystallinity, crystal size, and surface area on the photocatalytic activity.12 It is easy to understand that the surface structure will strongly affect the photocatalytic activity because the photocatalytic reaction occurs on the surface of TiO2 crystal. However, almost no study has been carried out on the effect of the crystal plane on the photocatalytic activity. Up to now, TiO2 nanocrystals have been prepared mainly by using sol-gel13,14 and hydrothermal methods.15 The crystallinity and crystal size can be controlled by these normal methods, but the crystal shape and crystal plane on the particle surface are difficult to control. The soft chemical process is a useful and effective method for fine synthesis of inorganic materials.16,17 In this process, usually a precursor with an open structure, such as layered structure and tunnel structure, is used, and the structure and chemical composition of the precursor can be modified by host-guest reaction, such as ion-exchange and intercalation reactions.18,19 By the soft chemical process, we have achieved the control of the tunnel structure and nanostructure in the synthesis of manganese oxides,20,21 the control of particle morphology in the synthesis of TiO2 and BaTiO3,22-24 and preparation of crystal-axis oriented TiO2 and BaTiO3 thin films on a polycrystal substrate.25 Very recently, we have developed a novel hydrothermal soft chemical process for the preparation of TiO2 nanocrystals.26,27 * To whom correspondence should be addressed. E-mail: feng@ eng.kagawa-u.ac.jp. † Baoji University of Arts and Science. ‡ Kagawa University.

In this process, exfoliated titanate nanosheets were used as the precursor, and TiO2 nanocrystals were obtained by hydrothermal treatment of the titanate nanosheet solution. We have found that shape-controlled nanocrystals of anatase-type TiO2 can be prepared by this method, and the anatase nanocrystals prepared by this method preferentially expose the (010) plane on the crystal surface; furthermore, these anatase nanocrystals show high photocatalytic activity and high ability to adsorb N719 dye used for the dye-sensitized solar cell. The present paper aims to study the reaction mechanism for the formation of TiO2 nanocrystals from the titanate nanosheet precursor under hydrothermal conditions. We investigate the structural transformation process from the titanate nanosheet structure to the anatase structure by nanostructural analysis in detail using TEM and SAED. From the analysis results, we found that the structural transformation reaction is a topotactic reaction, and there are definite relationships between the crystalaxis orientations of the titanate nanosheet precursor and the anatase nanocrystals. These results are useful to predict the crystal plane exposed on the nanocrystal surface and also to understand the topotactic structural transformation reactions in other soft chemical reaction systems. 2. Experimental Section Preparations of Titanate Nanosheet and PDDA-HTO Nanocomposite. A layered titanate nanosheet (PA-HTO) colloidal solution was prepared by exfoliating an H+-form layered titanate H1.07Ti1.73O4 · nH2O (HTO) using an exfoliating reagent of n-propylamine (PA) solution.24 The HTO (10 g) sample was treated in a 0.1 mol/L PA solution (1 L) under stirring conditions at room temperature for 24 h and then diluted 10 times with distilled water. The titanium content and pH value in the PAHTO nanosheet colloidal solution were 0.01 mol(Ti)/L and 11.3, respectively. A PDDA-HTO nanocomposite slurry solution (pH ) 9.2) was obtained by slowly dropping 500 mL of poly(diallyldimethylammonium chloride) (PDDA-Cl, Mw ) 2.0 × 105-3.5 × 105) solutions with monomer concentrations of 3.0 × 10-2 mol/L into 500 mL of the PA-HTO nanosheet solution under stirring condition at room temperature.

10.1021/jp908181e CCC: $40.75  2009 American Chemical Society Published on Web 11/03/2009

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Figure 1. Lepidocrocite-like layered structure of K0.8Ti1.73Li0.27O4.

Hydrothermal Treatments of PA-HTO Nanosheets and PDDA-HTO Nanocomposite. The PA-HTO nanosheet solution or the PDDA-HTO nanocomposite slurry solution (50 mL) was sealed in a Teflon-lined stainless steel vessel with an internal volume of 80 mL, which was then hydrothermally treated at the desired temperature for 24 h. After the hydrothermal treatment, the sample was separated from the solution by centrifugation, washed with distilled water, and finally dried using a freeze dryer. The samples obtained are designated as PA-X and PDDA-X, where PA and PDDA correspond to the samples prepared from the PA-HTO nanosheet solution and the PDDA-HTO nanocomposite slurry solution, respectively, and X corresponds to the temperature of hydrothermal treatment. Physical Analysis. Transmission electron microscopy (TEM) observation and selected area electron diffraction (SAED) were performed on a JEOL JEM-3010 at 300 kV, and the sample was supported on a microgrid. Simultaneous thermogravimetry and differential thermal analysis (TG-DTA) was carried out on a SHIMADZU DTG-60H at a heating rate of 10 °C · min-1. Powder X-ray diffraction (XRD) analysis of the samples was carried out on a SHIMADZU XRD-6100 X-ray diffractometer with Cu KR (λ ) 0.15418 nm) radiation. Atomic force microscopy (AFM) images were obtained using a JEOL-SPM4210 instrument. The titanate nanosheet sample for AFM observation was prepared by the method reported in ref 28. 3. Results and Discussion Structure of Layered Titanate H1.07Ti1.73O4 Nanosheet. An XRD study indicated that the layered titanate K0.8Ti1.73Li0.27O4 · nH2O (KTLO) used as the starting material has a lepidocrocitelike layered structure (orthorhombic; space group, Cmcm; a ) 0.376 nm, b ) 0.783 nm, c ) 0.297 nm), as shown in Figure 1.29 The layered structure is composed of corrugated host layers of edge-shared TiO6 octahedrons and interlayer ion-exchangeable K+ compensating for the minus charge of the TiO6 octahedral layers. Li+ occupies the Ti(IV) octahedral sites in the host layers. K+ and Li+ ions in the layered structure can be ion-exchanged with H+ ions by acid treatment. After the ionexchange, the sample retained the lepidocrocite-like layered structure, but the basal spacing changed from 0.783 to 0.922 nm.30 The H+-form titanate H1.07Ti1.73O4 · nH2O (HTO) with H3O+ ions and H2O in the interlayer space was obtained after the ion-exchange reaction. KTLO has a platelike crystal morphology with a size of about 0.2 µm in thickness and 3 µm in width. The basal plane of the platelike crystals corresponds to the (010) plane of the crystal structure (Figure 1). After the ion-exchange reaction, HTO keeps the platelike crystal morphology of KTLO. When HTO was treated with n-propylamine (PA) solution, PA molecules were intercalated into the interlayer space of the

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Figure 2. TEM and HRTEM images of a titanate nanosheet sample. The inset shows the SAED pattern. aL and cL correspond to the a and c axes of the layered structure.

layered structure by a protonation reaction of PA with H3O+ in the interlayer space. This intercalation caused exfoliation of HTO, and then a PA-HTO nanosheet colloidal solution was obtained.24 Figure 2 shows the TEM images and selected area electron diffraction (SAED) pattern of the exfoliated PA-HTO nanosheet sample. The PA-HTO nanosheet sample has a nanosheet-like particle morphology with dimensions of micrometer order in width and nanometer order in thickness. Diffraction spots with d values of 0.376 and 0.149 nm were observed in the SAED pattern. These diffraction spots correspond to the (100) and (002) planes of the lepidocrocite-like layered structure, respectively (Figure 1). In the HRTEM image (Figure 2b), lattice spacings of d ) 0.38 and 0.30 nm can be assigned to the (100) and (001) planes of the layered structure, respectively, by measuring the distance between the adjacent lattice fringes. The formation of the single-layer nanosheets with a thickness of about 0.7 nm was confirmed by AFM study in the exfoliation reaction (see Figure S1, Supporting Information). The above results indicate that the exfoliated nanosheets retained the lepidocrocite-like structure in two-dimension of the (010) plane, meaning the nanosheet (two-dimensional crystal) has an ordering atom arrangement in its basal plane along the direction that is perpendicular to the b axis of the lepidocrocitelike layered structure (Figure 1). Formation of Anatase in PA-HTO Nanosheet System. If the PA-HTO nanosheet colloidal solution was hydrothermally treated, the layered structure can be transformed into the anatase structure (tetragonal; space group, I41/amd; Z ) 4, a ) 0.37852 nm, c ) 0.95139 nm). The layered phase of the PA-HTO nanosheet was partially transformed to the anatase phase at 130 °C and then completed the transformation reaction above 135 °C (see Figure S2, Supporting Information). The transformation reaction from the layered structure into the anatase structure was investigated by TEM. Figure 3 shows the TEM images of PA-130 and PA-135 samples. Two kinds of typical particle morphologies were observed in the PA130 sample that was a mixed phase of the titanate and anatase. One is a nanocomb-like particle morphology, and the other is a nanosheet-like particle morphology (Figure 3a,c). The HRTEM image of the nanocomb-like particles revealed that it was anatase phase with d(101) ) 0.35 nm (Figure 3b). The nanosheet-like particles have a distorted layered structure with d(001) ) 0.30 nm and d(100) ) 0.36 nm, as shown in Figure 3d. Many cracks were observed on the nanocomb-like particles, suggesting that the nanocomb-like particles were formed by splitting the nanosheet-like particles in the structural transformation reaction from the layered structure to the anatase structure. It is interesting that the cracks on the nanocomb-like particles direct always to the [001] direction of the anatase structure and their basal plane is vertical always to the [010] direction of anatase

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Figure 4. DTA (top) and TG (bottom) curves of the (a) PDDA-HTO nanocomposite sample and products obtained by hydrothermal treatment of the nanocomposite at (b) 100, (c) 130, (d) 140, and (e) 150 °C.

Figure 3. TEM and HRTEM images of the samples obtained by hydrothermal treatment of the PA-HTO nanosheet solution at (a-d) 130 and (e, f) 135 °C. aL, cL, aA, and cA correspond to the a and c axes of the layered structure and anatase structure, respectively.

(Figure 3b). These results reveal that the transformation from the layered structure to the anatase structure under hydrothermal conditions is a topotactic reaction, where there is definite relationship between the crystal-axis orientations of the layered structure and anatase structure. In the PA-135 sample, uniform nanoleaf-like particles were observed (Figure 3e). The HRTEM image indicated that nanoleaf-like particles were single crystal of anatase and their axis directions always correspond to the [001] direction of the anatase structure (Figure 3f), revealing that the nanoleaf-like particles were formed by splitting the nanocomb-like particles under the hydrothermal conditions. Formation of Anatase in PDDA-HTO Nanocomposite System. The PDDA-HTO nanocomposite can be transformed also into the anatase phase by the hydrothermal treatment similar to the case of PA-HTO nanosheets. When poly(diallyldimethylammonium chloride) (PDDA-Cl) solution was added into the PA-HTO nanosheet solution, the positively charged PDDA polycations attracted the negatively charged HTO nanosheets together, and then the precipitate of PDDA-HTO nanocomposite was formed. An XRD study (see Figure S3, Supporting Information) indicated that the PDDA-HTO nanocomposite sample has a layered structure with a basal spacing of 1.60 nm, suggesting that the PDDA polycations were intercalated into the interlayer, which caused an increase of basal spacing from 0.922 to 1.60 nm. Under the hydrothermal conditions, the layered phase starts the transformation reaction to anatase phase at 130 °C and completes the transformation reaction above 150 °C. This temperature is obviously higher than that of 135 °C to complete the transformation of the PA-HTO nanosheets into the anatase phase. This result reveals that the PDDA-HTO nanocomposite sample is more stable than the PA-HTO nanosheet sample under hydrothermal conditions.

Figure 4 shows the TG-DTA curves of the PDDA-HTO sample and products obtained by hydrothermal treatment of the PDDA-HTO nanocomposite. For the PDDA-HTO sample, the weight loss up to 200 °C is due to dissipation of interlayer water, and the exothermic peaks around 330 and 500 °C with a large weight loss can be attributed to the decomposition of PDDA polycations in the interlayer to CO2 gas. After the hydrothermal treatment at 100 and 130 °C, the TG curves were almost the same as that of the PDDA-HTO sample, but the exothermic peak around 330 °C became sharper and more intense. This result agrees with the result of XRD, where the diffraction peaks of the layered phase became sharper and more intense with increasing hydrothermal temperature up to 130 °C (see Figure S3, Supporting Information). These results suggest that the HTO nanosheets are stacked in a low ordering arrangement in the PDDA-HTO sample. After the hydrothermal treatment up to 130 °C, it changes to an ordered stacking structure. When the hydrothermal temperature was higher than 130 °C, the weight loss and the intensity of exothermic peaks around 330 and 500 °C decreased with increasing the reaction temperature. This result reveals that the transformation reaction of the layered phase into the anatase phase accompanies release of PDDA polycations from the interlayer into the solution phase. Figure 5 shows typical TEM images of the PDDA-HTO nanocomposite sample and products obtained by hydrothermal treatment of the nanocomposite. The PDDA-HTO nanocomposite had a nanosheet-like particle morphology (Figure 5a), and the thickness of the nanosheet-like particles was thicker than that of PA-HTO nanosheet sample (Figure 2a), indicating that the restacking reaction of the HTO nanosheets occurred when PDDA was added into the PA-HTO nanosheet solution. Some cracks on the nanosheet-like particles and some nanostriplike particles were observed after the hydrothermal treatment at 130 °C (Figure 5b). The XRD result (see Figure S3, pattern c, Supporting Information) suggests that the crack formation corresponds to the beginning of the transformation reaction from the layered structure into the anatase structure. The nanosheetlike particles were split into nanostrip-like particles at 140 °C,

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Figure 5. TEM images of the (a) PDDA-HTO nanocomposite sample and products obtained by hydrothermal treatment of the nanocomposite at (b) 130, (c) 140, and (d) 150 °C. The inset shows the SAED pattern.

accompanying formation of the anatase phase (Figure 5c). Subsequently the nanostrip-like particles were split further into nanorod-like particles at 150 °C (Figure 5d). Nanostructural Study on Structural Transformation Reaction. To investigate the mechanism for the transformation reaction from the layered structure of the HTO nanosheets into the anatase structure, a nanostructural analysis was carried on the intermediate products in the transformation process in detail. We tried to find the intermediate product in the PA-HTO nanosheet reaction system. However, it is very difficult. In this reaction system, the intermediate product is unstable and transforms into the anatase phase immediately as it is formed. The mixture of nanocomb-like anatase particles and nanosheetlike HTO particles was observed in the PA-130 sample (Figure 3). On the other hand, the intermediate product can be observed in the PDDA-HTO reaction system due to its relatively high stability. Figure 6 shows the HRTEM images of PDDA-HTO and products obtained after the hydrothermal reaction. The layered lattice structure with d(001) ) 0.30 nm and d(100) ) 0.38 nm was observed in the PDDA-HTO sample, but it is not very clear (Figure 6a) due to its low crystallinity. The lattice of the layered structure can be observed more clearly in the PDDA-130 sample (Figure 6b). This result agrees with the XRD result that the layered phase has higher crystallinity in the PDDA-130 sample than that in the PDDA-HTO sample (see Figure S3, Supporting Information). It is very interesting that two types of crystal lattices were observed simultaneously in one nanosheet-like particle in the PDDA-130 sample, as shown in Figure 6c,d. One corresponds to the layered structure and other to a distorted anatase structure, meaning the intermediate product. In the crystal lattice of the distorted anatase structure in Figure 6c, the angle between the (101) and (101j) planes (45°) was larger than that (43.4°) of a normal anatase lattice, and the lattice spacings of (101), (100), and (002) were 0.37, 0.39, and 0.49 nm, which were slightly larger than those (0.35, 0.38, and 0.48 nm) of the normal anatase, respectively. In the crystal lattice of the distorted anatase in Figure 6d, the angle between the (101) and (101j) planes was 46°, and the lattice spacings of (101) and (100) were 0.36 and 0.40 nm, respectively, which were larger

Figure 6. HRTEM images of the (a) PDDA-HTO nanocomposite and samples obtained by hydrothermal treatment of the nanocomposite at (b-e) 130 and (f) 150 °C: (a, b) layered phase, (c, d) mixed phase of the layered structure and anatase structure, and (e, f) anatase phase. aL, cL, aA, and cA correspond to the a and c axes of the layered structure and anatase structure, respectively.

than those of a normal anatase lattice, whereas the lattice spacing of (002) (0.41 nm) was smaller than that (0.48 nm) of normal anatase. The distorted anatase structure is observed usually in the PDDA-130 sample, and different degrees of distorted lattices can be observed at different areas in one nanosheet-like particle, as shown in Figure 6e, where distorting degree increases in an order of D area < C area < B area < A area, indicating that the transformation from the layered structure to the anatase structure is uncompleted in this particle. The anatase lattice with high crystallinity was observed in the PDDA-150 sample, as shown in Figure 6f, revealing that the transformation from the layered structure to the anatase structure was completed under the hightemperature conditions. The above results reveal that, first, the layered structure is transformed to the distorted anatase phase and then to the normal anatase phase in the formation process of the anatase phase. Topotactic Structural Transformation Reaction. Because the transformation reaction from the layered structure to the anatase structure is a topotactic reaction, there will be a definite relationship between the crystal-axis orientations of the layered structure and the anatase structure. This relationship can be confirmed from the selected area electron diffraction (SAED) patterns of the PDDA-130 sample, as shown in Figure 7. The PDDA-HTO shows an SAED pattern of a rectangular lattice with a ) 0.38 nm and c ) 0.30 nm, which corresponds to the two-dimensional atomic arrangement of a lepidocrocite-like

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Figure 7. SAED patterns of the (a) PDDA-HTO sample and (b-d) PDDA-130 sample: (a, b) layered phase of titanate and (c, d) mixed phase of the titanate and anatase. aL, cL, aA, and cA correspond to the a and c axes of the layered structure and anatase structure, respectively.

layered structure (Figure 7a). The PDDA-130 sample showed mainly the pattern of the two-dimensional layered structure similar to that of PDDA-HTO, whereas diffraction spots were more clear than those of the PDDA-HTO sample (Figure 7b), indicating that the crystallinity of the layered structure increased after the hydrothermal treatment. Except for the SAED pattern of the layered structure, the SAED pattern of the anatase phase was observed also in the PDDA-130 sample due to that the layered phase was partially transformed to the anatase phase. The SAED patterns of the layered structure and anatase structure were observed simultaneously, meaning two sets of SAED spots for the layered phase and anatase phase appeared in one SAED picture, as shown in Figure 7c,d, where the spots marked with a circle correspond to the diffractions of the anatase phase and other to the layered phase. The SAED analysis indicates that there are two kinds of orientation relationships between the crystal axes of the layered structure and anatase structure, suggesting that there are two routes in the structural transformation reaction from the layered structure to the anatase structure. In both the two routes, the b-axis direction of the layered structure (orthorhombic system), that is, observing direction, corresponds to the b-axis direction of the anatase structure (tetragonal system). In route I, the c-axis direction of the layered structure corresponds to the [101] direction of the anatase structure, as shown in Figure 7c. This reveals that the a- and c-axis orientations of the layered structure are transformed to the a- and c-axis orientations of the anatase structure by rotating an angle of 66° on the (010) plane in route I, which can be confirmed also directly from the HRTEM image of Figure 6c. In route II, the crystal-axis orientations of the layered structure are transformed to the crystal-axis orientations of the anatase structure by rotating an angle of 35° on the (010) plane, as shown in Figure 7d, which corresponds the HRTEM image of Figure 6d.

Figure 8. Scheme of the structural transformation reaction from the titanate nanosheet structure into the anatase structure in the hydrothermal process.

On the basis of the above results, we proposed a reaction model for the topotactic structural transformation from the layered structure to the anatase structure, as shown in Figure 8. Under the hydrothermal conditions, first, the distorted layered structure is formed and then transformed to the distorted anatase structure. In this process, the TiO6 octahedra in the titanate nanosheet slightly and equably shift from the positions of the layered lattice to the positions of the anatase lattice. The (010) plane (the basal plane of titanate nanosheets) of the lepidocroc-

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ite-like layered structure is transformed to the (010) plane (the basal plane of sheet-like anatase nanocrystals) of anatase in the topotactic reaction. There are two routes for the position shifts of TiO6 octahedra. One route is route I, where a- and c-axis orientations of the layered structure transform to a- and c-axis orientations of the anatase structure by tilting 66° on the (010) plane. The other route is route II, where a- and c-axis orientations of the layered structure transform to a- and c-axis orientations of the anatase structure by tilting 36° on the (010) plane. Finally, the distorted anatase structure is transformed to the normal anatase structure. The anatase nanocrystals formed by this mechanism expose the (010) plane on their basal plane. Therefore, anatase nanocrystals preferentially expose the (010) plane on their surfaces, even when they are split into small particles. 4. Conclusion We have studied the formation reaction of anatase from the layered titanate nanosheets. The layered titanate nanosheets can be transformed into the anatase phase under hydrothermal conditions. The structural transformation from the layered structure to the anatase structure is a topotactic reaction. There are two kinds of definite relationships between the crystal-axis orientations of the layered structure and anatase structure, meaning that there are two routes in the structural transformation reaction. In these two routes, a- and c-axis orientations of the layered structure transform to a- and c-axis orientations of the anatase structure by tilting 66° and 36°, respectively, on the (010) plane. The anatase nanocrystals formed by this mechanism preferentially expose the (010) plane on their surfaces. Acknowledgment. This work was supported by the Grantsin-Aid for Scientific Research (B) (No. 20350096) from the Japan Society for the Promotion of Science, Scientific Research Project (No. 09JK334) from Shaanxi Province Office of Education, and Key Research Project (No. ZK0843) from Baoji University of Arts and Sciences. Supporting Information Available: The characterization results of the layered titanate nanosheet sample by AFM image and XRD patterns of the PA-HTO nanosheet sample, PDDA-

Wen et al. HTO nanocomposite sample, and products obtained by hydrothermal treatment of these samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (2) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (3) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (4) Minabe, T.; Tryk, D. A.; Sawunyama, P.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 2000, 137, 53. (5) Fu, G.; Vary, P. S.; Lin, C.-T. J. Phys.Chem. B 2005, 109, 8889. (6) Tsai, C. C.; Teng, H. Chem. Mater. 2006, 18, 367. (7) Sakata, T.; Kawai, T.; Hashimoto, K. Chem. Phys. Lett. 1982, 88, 50. (8) Ohtani, B.; Hanada, J. I.; Nishimoto, S. I.; Kagiya, T. Chem. Phys. Lett. 1985, 120, 292. (9) Sclafani, A.; Palmisano, L.; Schiavello, M. J. Phys. Chem. 1990, 94, 829. (10) Ohtani, B.; Okugawa, Y.; Nishimoto, S. I.; Kagiya, T. J. Phys. Chem. 1987, 91, 3350. (11) Oosawa, Y.; Gra¨tzel, M. J. Chem. Soc., Faraday Trans. 1 1988, 84, 197. (12) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (13) Sreethawong, T.; Suzuki, Y.; Yoshikawa, S. Int. J. Hydrogen Energy 2005, 30, 1053. (14) Li, G.; Gray, K. A. Chem. Mater. 2007, 19, 1143. (15) Yue, Y.; Gao, Z. Chem. Commun. 2000, 1755. (16) Stein, A.; Keller, S. W.; Mallouk, T. E. Science 1993, 259, 1559. (17) Kopnov, F.; Tenne, R. Digest J. Nanomaterials and Biostructures 2008, 3, 123. (18) Rouxel, J.; Tournoux, M.; Brec, R. Soft Chemical Routes to New Materials; Trans Tech Publications: Aedermansdorf, Switzerland, 1994. (19) Omomo, Y.; Sasaki, T.; Watanabe, M. Solid State Ionics 2002, 151, 243. (20) Feng, Q.; Kanoh, H.; Ooi, K. J. Mater. Chem. 1999, 9, 319. (21) Tian, Z.; Feng, Q.; Sumida, N.; Makita, Y.; Ooi, K. Chem. Lett. 2004, 33, 952. (22) Feng, Q.; Hirasawa, M.; Yanagisawa, K. Chem. Mater. 2001, 13, 290. (23) Feng, Q.; Hirasawa, M.; Kajiyoshi, K.; Yanagisawa, K. J. Am. Ceram. Soc. 2005, 88, 1415. (24) Wen, P.; Itoh, H.; Feng, Q. Chem. Lett. 2006, 35, 1226. (25) Feng, Q.; Kajiyoshi, K.; Yanagisawa, K. Chem. Lett. 2003, 32, 48. (26) Wen, P.; Itoh, H.; Tang, W.; Feng, Q. Langmuir 2007, 23, 11782. (27) Wen, P.; Tang, W.; Itoh, H.; Feng, Q. Microporous Mesoporous Mater. 2008, 116, 147. (28) Tanaka, T.; Fukuda, K.; Ebina, Y.; Takada, K.; Sasaki, T. AdV. Mater. 2004, 16, 872. (29) Roth, R. S.; Parker, H. S.; Brower, W. S. Mater. Res. Bull. 1973, 8, 327. (30) Iida, M.; Sasaki, T.; Watanabe, M. Chem. Mater. 1998, 10, 3780.

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