Single Nanocrystals of Anatase-Type TiO2 Prepared from Layered

Oct 13, 2007 - The anatase nanocrystals formed by the in situ topotactic structural transformation reaction retain the sheetlike particle morphology o...
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Langmuir 2007, 23, 11782-11790

Single Nanocrystals of Anatase-Type TiO2 Prepared from Layered Titanate Nanosheets: Formation Mechanism and Characterization of Surface Properties Puhong Wen,†,‡ Hiroshi Itoh,† Weiping Tang,§ and Qi Feng*,† Department of AdVanced Materials Science, Faculty of Engineering, Kagawa UniVersity, 2217-20 Hayashi, Takamatsu 761-0396, Japan, Department of Chemistry and Chemical Engineering, Baoji UniVersity of Arts and Science, 44 Xibao Road, Baoji, Shanxi 721006, People’s Republic of China, and Research Institute for SolVothermal Technology, 2217-43 Hayashi, Takamatsu 761-0301, Japan ReceiVed June 4, 2007. In Final Form: August 4, 2007 Anatase-type TiO2 single nanocrystals with boatlike, comblike, sheetlike, leaflike, quadrate, rhombic, and wirelike particle morphologies were prepared by hydrothermal treatment of a layered titanate nanosheet colloidal solution. The formation reactions and surface properties of the TiO2 nanocrystals were investigated using XRD, TEM, TG-DTA analyses, and measurements of BET specific surface area, photocatalytic activity, and ruthenium dye (N719) adsorption. The crystal morphology can be controlled by reaction temperature, pH value of reaction solution, and exfoliating agent. The titanate nanosheets were transformed to the TiO2 nanocrystals by two types of reactions. One is an in situ topotactic structural transformation reaction, and the other is a dissolution-deposition reaction on the surface. The anatase nanocrystals formed by the in situ topotactic structural transformation reaction retain the sheetlike particle morphology of the precursor, and they preferentially expose the (010) plane of anatase structure. The crystal surface of anatase nanocrystals prepared in this study showed higher photocatalytic activity and higher ruthenium dye adsorption capacity than did the Ishihara ST-01 sample, a standard anatase nanocrystal sample. The results indicated the (010) plane of the anatase structure has high photocatalytic activity and high ruthenium dye adsorption ability.

Introduction Titanium dioxide is an important material that has wide applications as a white pigment in making paints and cosmetics, electroceramic materials, catalyst supports, and photocatalysts.1 Recently, much research attention is focused on the nanocrystals of titanium dioxide with high performance photocatalytic properties, which have potential applications in dye-sensitized solar cells,2 photodecomposition of organic species for environmental cleaning and photoantibacterial systems,3 photosplitting of water,4 photoself-cleaning,5 sensors,6 medical treatment for cancer tissues,7 etc. The photocatalytic performance of TiO2 can be optimized by microstructural and macrostructural control on the morphology of the material because of the intimately morphology-dependent characteristic of the photocatalytic properties.8 Nanostructured TiO2 materials can be prepared by dry and wet processes. Because a variety of reaction conditions, such as reactants, reaction medium, temperature, and pH of solution, * To whom correspondence should be addressed. E-mail: feng@ eng.kagawa-u.ac.jp. † Kagawa University. ‡ Baoji University of Arts and Science. § Research Institute for Solvothermal Technology. (1) (a) Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Phys. Chem. C 2007, 111, 5259-5275. (b) Wilson, G. J.; Will, G. D.; Frost, R. L.; Montgomery, S. A. J. Mater. Chem. 2002, 12, 1787-1791. (c) Wang, H.; Wang, T.; Xu, P. J. Mater. Sci.: Mater. Electron. 1998, 9, 327-330. (2) (a) Peter, L. M. J. Phys. Chem. C 2007, 111, 6601-6612. (b) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737-740. (c) Kambe, S.; Nakade, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2002, 106, 2967-2972. (3) Kominami, H.; Muratami, S.; Kato, J.; Kera, Y.; Ohtani, B. J. Phys. Chem. B 2002, 106, 10501-10507. (4) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (5) Wang, R.; Hashimoto, K.; Fujishima, A. Nature 1997, 388, 431-432. (6) Palomares, E.; Vilar, R.; Durrant, J. R. Chem. Commun. 2004, 4, 362-363. (7) Kubota, Y.; Shuin, T.; Kawasaki, C.; Hosaka, M.; Kitamura, H.; Cai, R.; Sakai, H.; Hashimoto, K.; Fujishima, A. Br. J. Cancer 1994, 70, 1107-1111. (8) Ruoff, R. S. Nature 1994, 372, 731-732.

can be chosen in the wet processes, the crystal size, crystal shape, and surface structure of the nanocrystals can be controlled more easily than that in the dry processes.9,10 Sol-gel and hydrothermal processes are the most popular methods in the synthesis of TiO2 nanocrystals, in which usually titanium salts are hydrolyzed in solution and then crystallized.11-14 In these processes, the crystal size can be controlled by crystal growth rate, while the crystal shape is dependent on the crystal growth direction that is not easy to control in normal cases. Although some studies on TiO2 nanocrystal shape control have been reported by adding crystal growth directing agents, such as surfactants and surface adsorption ions, into the reaction systems to control the crystal growth direction,15-17 it is not easy to achieve in normal cases. The photocatalytic activity of TiO2 nanocrystal is strongly dependent on the surface area, the exposing plane of crystal lattice, and the crystallinity.18 Up to now, most studies on the photocatalytic properties of TiO2 nanocrystals have been carried out on the effects of surface area and crystallinity, and the results have suggested that a larger surface area and higher crystallinity of TiO2 nanocrystals correspond to a higher photocatalytic (9) Chemseddine, A.; Jungblut, H.; Boulmaaz, S. J. Phys. Chem. 1996, 100, 12546-12551. (10) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. AdV. Mater. 1995, 8, 428-433. (11) Hosono, E.; Fujihara, S.; Kakiuchi, K.; Imai, H. J. Am. Chem. Soc. 2004, 126, 7790-7791. (12) Peng, T.; Zhao, D.; Dai, K.; Shi, W.; Hirao, K. J. Phys. Chem. B 2005, 109, 4947-4952. (13) Barringer, E. A.; Bowen, H. K. Langmuir 1985, 1, 414-420. (14) Park, S. I.; Jang, S. R.; Hong, J. S.; Vittal, R.; Kim, K. J. Chem. Mater. 2003, 15, 4633-4636. (15) Sreethawong, T.; Suzuki, Y.; Yoshikawa, S. Catal. Commun. 2005, 6, 119-124. (16) Li, X.; Zhang, Y.; Li, T.; Zhang, Z. Chem. Lett. 2006, 35, 506-507. (17) Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235-245. (18) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J.; Sakamoto, M.; Wang, F. J. Am. Chem. Soc. 2004, 126, 14943-14949.

10.1021/la701632t CCC: $37.00 © 2007 American Chemical Society Published on Web 10/13/2007

Single Nanocrystals of Anatase-Type TiO2

activity.19 It has been recognized that to understand the effects of crystal plane of the nanocrystals on the photocatalytic reaction and dye adsorption is very important from both the viewpoints of basic study and application. Only a few studies, however, have been focused on the effects of crystal plane, due to that the TiO2 nanocrystals that preferentially expose a specific crystal plane are very difficult to prepare. Computer simulation studies have suggested that the dye adsorption properties, which are a very significant point in fabricating dye-sensitized solar cells, would be strongly dependent on the plane of TiO2 crystal.20 Exfoliation technique for layered compounds is interesting and useful for the preparation of two-dimensional nanomaterials.21 Nanosheets of elementary host layer of layered compounds with uniform thickness can be obtained by using the exfoliation process. We think that the exfoliated nanosheets are not only useful twodimensional nanomaterials, but also promising precursors for the synthesis of new nanostructured materials, because the nanosheets can be transformed easily to other nanomaterials by slightly changing their structures and morphology. This technique would be of great use for size- and shape-controlled synthesis of nanocrystals. On the basis of this idea, we have prepared an MnO2/Ni(OH)2 sandwich layered nanostructure and manganese oxide nanofibers from exfoliated layered manganese oxide nanosheets.22,23 We have also prepared a thin film of layered titanate nanosheet by casting the titanate nanosheet solution on a substrate, and we transformed it into a rutile-type TiO2 thin film with a crystal-axis orientation to the (110) plane by heat treatment.24 The transformation reaction from the layered titanate nanosheet to the rutile is an in situ topotactic structural transformation reaction. These results imply that the TiO2 nanocrystals with a specific crystal plane can be prepared by using the titanate nanosheet precursor and the in situ topotactic structural transformation reaction. In the present Article, we describe a novel process for the synthesis of various TiO2 single nanocrystals with controllable morphology from layered titanate nanosheets, and nanostructural characterizations on the crystal surfaces, photocatalytic activities, and ruthenium dye adsorption properties of these TiO2 nanocrystals. The anatase-type TiO2 nanocrystals prepared by this process have specific morphology, and preferentially expose the (010) plane of anatase structure, which provide an opportunity for experimentally studying the photocatalytic and dye adsorption reactions on the definite crystal plane. Experimental Section Preparation of Layered Titanate Nanosheet Solutions. In the synthesis process, first a layered titanate of K0.8Ti1.73Li0.27O4 (KTLO) with lepidocrocite-like layered structure was prepared by a hydrothermal method. 5.1 g of KOH, 0.6 g of LiOH‚H2O, 6.9 g of TiO2 (anatase), and 25 mL of distilled water were sealed into a HastelloyC-lined vessel with internal volume of 45 mL, and then heated at 250 °C for 24 h under stirring conditions. After the hydrothermal treatment, the sample was washed with distilled water and dried at room temperature to obtain KTLO. KTLO (10 g) was treated with a 0.2 M HNO3 solution (1 L) for 1 day under stirring conditions to exchange K+ and Li+ in the layered structure with H+, and then the sample was washed with distilled water. After the acid treatment (19) Tsai, C. C.; Teng, H. Chem. Mater. 2006, 18, 367-373. (20) Nazeeruddin, Md. K.; Humphry-Baker, R.; Liska, P.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 8981-8987. (21) Omomo, Y.; Sasaki, T.; Wang, L. Z.; Watanabe, M. J. Am. Chem. Soc. 2003, 125, 3568-3575. (22) Xu, Y.; Feng, Q.; Kajiyoshi, K.; Yanagisawa, K.; Yang, X.; Makita, Y.; Kasaishi, S.; Ooi, K. Chem. Mater. 2002, 14, 3844-3851. (23) Tian, Z.; Feng, Q.; Sumida, N.; Makita, Y.; Ooi, K. Chem. Lett. 2004, 33, 952-953. (24) Feng, Q.; Kajiyashi, K.; Yanagisawa, K. Chem. Lett. 2003, 32, 48-49.

Langmuir, Vol. 23, No. 23, 2007 11783 was done twice, an H+-form layered titanate H1.07Ti1.73O4‚nH2O (HTO) was obtained. Two kinds of the layered titanate nanosheet colloidal solutions were prepared by exfoliating HTO using exfoliating agents of tetrabutylammonium hydroxide (TBAOH) and n-propylamine (PA) solutions, respectively. In the preparation of TBA-HTO nanosheet colloidal solution, HTO (10 g) was treated in a 0.016 M TBAOH solution (2 L) under stirring conditions at room temperature for 7 days.21 A PA-HTO nanosheet colloidal solution was obtained by treating HTO (10 g) in a 0.1 M PA solution (1 L) under stirring conditions at room temperature for 24 h.24 Hydrothermal Treatment of HTO Nanosheet Solution. TiO2 nanocrystals were prepared by hydrothermal treatment of the HTO nanosheet colloidal solutions. The TBA-HTO nanosheet colloidal solution was adjusted to a desired pH value with a 3 M HCl solution and a 1 M TBAOH solution. The pH-adjusted TBA-HTO nanosheet solution (50 mL) was sealed into a Teflon-lined stainless steel vessel with an internal volume of 80 mL, and then was hydrothermally treated at a desired temperature for 24 h. After the hydrothermal treatment, the product was separated from the solution by centrifuge, then washed with distilled water, and finally dried using a freeze drier. The sample obtained is designated as TBA-X-Y, where TBA, X, and Y correspond to exfoliating agent used, pH value of nanosheet solution, and temperature of hydrothermal treatment, respectively. PA-X-Y sample was prepared by hydrothermal treatment of the PAHTO nanosheet colloidal solution in a manner similar to the case of the TBA-X-Y sample. HCl and PA solutions were used to adjust the pH value of the PA-HTO colloidal solution in a pH range below 13.5, and KOH solution was used in a pH range above 13.5. Physical Analysis. 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. Transmission electron microscope (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. Nitrogen gas adsorption was carried out on a QUANTACHROME AUTOSORB1-MP apparatus. The sample was degassed before the adsorption for 3.5 h at a temperature that is 5 °C lower than temperature of the hydrothermal treatment in its synthesis process. The specific surface area was calculated from the adsorption data using the BrunauerEmmett-Teller (BET) method. 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. Photocatalytic Characterization. The TiO2 nanocrystal sample (20 mg) was added in a 5 ppm methylene blue aqueous solution (MB, 100 mL), and the solution was irradiated with a 250 W black lamp located at 1 m from the MB solution. Degradation ratio of MB by the photocatalytic reaction was determined by measuring the concentration of MB before and after the black lamp irradiation using a SHIMADZU UV-2450 spectrophotometer. For the comparison, the Ishihara ST-01 sample (anatase, BET surface area 349 m2 g-1) was used as the standard sample for the photocatalytic activity. Adsorption of N719 Dye on TiO2 Nanocrystals. Adsorption experiment of dye was carried out by a batch method. A TiO2 nanocrystal sample (10 mg) was added into an ethanol solution (5 mL) of cis-di(thiocyanate)bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) bis-tetrabutylammonium (N719) dye in a concentration range of 1.0 × 10-3 to 1.0 × 10-4 M and was stirred at room temperature for 72 h. After the adsorption, the liquid phase was separated from the solid phase by centrifuge, and then the N719 dye concentration in the liquid phase was analyzed using a SHIMADZU UV-2450 spectrophotometer. The TiO2 nanocrystal sample was calcined at 450 °C for 30 min before the adsorption experiment.18 For the comparison, the Ishihara ST-01 sample was used also in the dye adsorption experiment.

Results and Discussion Preparation of HTO Nanosheet Solutions. The KTLO sample prepared by hydrothermal treatment of the mixture of TiO2, KOH, and LiOH‚H2O shows an XRD pattern of K0.8-

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Figure 1. XRD patterns of (a) KTLO, (b) HTO, (c) TBA-HTO, and (d) PA-HTO samples. Figure 3. XRD patterns of the samples obtained by hydrothermal treatment of TBA-HTO nanosheet colloidal solution (pH 11.5) at (a) 90, (b) 100, (c) 110, and (d) 120 °C, respectively.

Figure 2. Lepidocrocite-like layered structure of KTLO.

Ti1.73Li0.27O4‚nH2O (Figure 1a).25,26 This compound has a lepidocrocite-like layered structure with a basal spacing of 0.783 nm, as shown in Figure 2. The layered structure is composed of corrugated host layers of edge-shared TiO6 octahedra and interlayer exchangeable K+ ions compensating for the minus charge of the TiO6 octahedra layers. Li+ ions occupy Ti(IV) octahedral sites in the host layers. K+ and Li+ ions in the structure can be ion-exchanged with H+ ions by acid treatment. After the ion-exchange, the sample retained the lepidocrocite-like layered structure, the basal spacing changed to 0.922 nm, and H+-form titanate H1.07Ti1.73O4‚nH2O (HTO) with H3O+ ions and H2O in the interlayer space was obtained (Figure 1b).27 Because it has been reported that the H+-form layered titanate can be exfoliated in tetrabutylammonium hydroxide (TBAOH) and n-propylamine (PA) solutions, we used these two exfoliating agents to prepare HTO nanosheets.21,24 When HTO was treated in the TBAOH solution, TBA+ ions were intercalated into the interlayer by H+/ TBA+ ion-exchange reaction, resulting in exfoliation of the layered structure into HTO nanosheets and formation of TBAHTO nanosheet colloidal solution. PA-HTO nanosheet colloidal solution was obtained by using the PA solution as the exfoliating agent. If the nanosheet colloidal solutions were dried, the nanosheets restack to the layered structure again, and then TBA+ ions and PA molecules intercalated layered titanates were formed. The XRD patterns of these samples indicated that they have layered structures with basal spacings of 1.68 and 1.06 nm that correspond to the sizes of TBA+ and PA, respectively (Figure 1c and d). Formation of the titanate nanosheets after the exfoliation reactions was confirmed by a TEM study. The HTO sample has platelike particle morphology with a crystal size of about 2 µm in width before the exfoliation treatment (see Supporting Information), and the basal plane of the platelike particles is vertical to the [010] direction of the layered structure (see Figure 2). After the exfoliation reaction, nanosheet-like particles with particle dimensions of micrometer order in width and nanometer (25) Roth, R. S.; Parker, H. S.; Brower, W. S. Mater. Res. Bull. 1973, 8, 327-332. (26) Iida, M.; Sasaki, T.; Watanabe, M. Chem. Mater. 1998, 10, 3780-3782. (27) Feng, Q.; Hirasawa, M.; Yanagisawa, K. Chem. Mater. 2001, 13, 290296.

order in thickness were obtained (see Supporting Information). The width is almost the same as that of the HTO platelike particle. The thickness of TBA-nanosheet is thinner than that of PAHTO nanosheet, in agreement with the exfoliating ability of TBAOH > PA. The nanosheets retain the lepidocrocite-like layered structure in the (010) plane direction. Hydrothermal Reaction in TBA-HTO Nanosheet System. TBA-HTO nanosheet colloidal solution was hydrothermally treated at 90, 100, 110, and 120 °C, respectively, to prepare TiO2 nanocrystals. Figure 3 shows the XRD patterns of the samples obtained by hydrothermal treatment of the nanosheet solution at pH 11.5 (without pH adjustment). Diffraction peaks of TBAHTO layered phase became broad, and diffraction peaks of anatase phase were observed after the reaction at 90 °C. After the reaction at 100 °C, only diffraction peaks of anatase phase were observed, indicating that TBA-HTO nanosheets were transformed to anatase phase completely at 100 °C. The intensity of the peaks increased with increasing reaction temperature, revealing an increase of crystallinity of anatase with increasing reaction temperature. A TG-DTA analysis on the hydrothermally treated TBA-HTO nanosheet samples indicated that the TBA+ content in the hydrothermally treated samples decreased with increasing reaction temperature, and without TBA+ was detected in the samples prepared above 110 °C (see Supporting Information). This fact reveals that the transformation from the layered phase to anatase phase accompanies loss of TBA+ from the titanate nanosheets. The hydrothermal treatment was also carried out at other pH conditions, and the temperature and pH dependences of the products are summarized in Figure 4. The dashed line delineates boundary between the layered phase and anatase phase, meaning that anatase phase is formed in the area above the dashed line. When the pH value is higher than 13, the TBA-HTO nanosheets are relatively stable. The TBA-HTO nanosheets show highest reactivity in the formation reaction of anatase under acidic conditions. The reactivity of the nanosheets under low basic, neutral, and acidic conditions is much higher than the HTO particles, which can be transformed to anatase phase also under hydrothermal conditions, but 200 °C is necessary.27,28 These facts indicate that the use of the nanosheet as precursor is an effective method for low-temperature synthesis of anatase. Hydrothermal Reaction in PA-HTO Nanosheet System. In the hydrothermal reaction system of PA-HTO nanosheet colloidal solution, except anatase phase, rutile phase can be obtained also. Figure 5 shows the XRD patterns of the samples (28) Feng, Q.; Hirasawa, M.; Kajiyoshi, K.; Yanagisawa, K. J. Am. Ceram. Soc. 2005, 88, 1415-1420.

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Figure 4. Phase diagram for the formation of anatase and rutile phases from HTO nanosheets under hydrothermal conditions: b, anatase phase; 9, layered phase in TBA-HTO nanosheet reaction system; O, anatase phase; ∆, rutile phase; 0, layered phase in PA-HTO nanosheet reaction system. The dashed line delineates boundary between the layered phase and anatase phase in TBA-HTO nanosheet reaction system.

Figure 5. XRD patterns of the samples obtained by hydrothermal treatment of PA-HTO nanosheet colloidal solution under different pH conditions: b, anatase phase; 2, rutile phase.

prepared by hydrothermal treatments at different pH conditions. PA-HTO nanosheet retained its layered structure at pH 14.1 and 135 °C and was transformed to single anatase phase at pH 11.3 (without pH adjustment) and 135 °C. A mixture of anatase and rutile phases was obtained at pH 1.2 and 130 °C, the content of rutile increased with decreasing pH value of the reaction solution, and finally single rutile phase was formed at pH 0.3. The fraction of rutile (FR) in the mixture can be estimated from XRD peak intensity data using formula 1.29

FR ) (1 + 0.8IA/IR)-1

(1)

where IA and IR represent the integrated intensities of anatase (101) and rutile (110) diffraction peaks, respectively. The FR value increases with decreasing pH value of the reaction solution (Table 1). The temperature and pH dependences of the products in this reaction system are summarized also in Figure 4. Similar to TBA-HTO nanosheet, PA-HTO nanosheet is stable under high basic conditions and transforms to anatase phase under low basic and neutral conditions. PA-HTO nanosheet can be transformed (29) Spurr, R. A.; Myers, H. Anal. Chem. 1957, 29, 760-762.

to rutile phase under acidic conditions (pH < 1.5), which is different from the TBA-HTO nanosheet where only anatase phase is formed. PA-HTO nanosheet shows also different reactivity from that of TBA-HTO nanosheet in the formation reaction of anatase phase. The reactivity of PA-HTO nanosheet is lower than that of TBA-HTO nanosheet under low basic and neutral conditions. For example, at pH 11.3, PA-HTO nanosheet starts the transformation reaction to anatase phase at 100 °C and completes the reaction at 135 °C, which is higher than the temperature (100 °C) for completing the transformation of TBAHTO nanosheet into anatase phase at same pH conditions. The reactivities are almost the same under acidic conditions, but the products are different. This may be due to different exfoliation abilities of TBA-OH and PA, causing formation of the nanosheets with different thickness (see Supporting Information). The thickness is thinner, and the reactivity is higher. The high solubility of titanium oxide in acidic solution would concern also the reactivity. Characterization of TiO2 Nanocrystals by TEM. A nanostructural study on the products of hydrothermal reaction was carried out by TEM observation. Particle morphology of the products changed dramatically with changing the reaction temperature and pH value of reaction solutions. In the TBAHTO nanosheet reaction system, at pH 11.5, nanocomb-like particles with a size of about 300 nm in width were obtained at 100 °C (Figure 6a). XRD analysis indicated that TBA-HTO nanosheets were transformed completely to anatase phase (Figure 3b). Many cracks were observed on the nanocomb-like anatase particles, suggesting the nanocomb-like anatase particles were formed by splitting the TBA-HTO nanosheets under the hydrothermal conditions. The nanocomb-like particles changed to nanoboat-like (inflexed nanoleaf-like) particles with a particle size of about 80 nm in width and 300 nm in length at 120 °C, and the SAED pattern indicated that the nanoboat-like particles were single crystal of anatase phase (Figure 6b). At pH 1.8, first the TBA-HTO nanosheets were split into nanobelt-like particles at 90 °C and then changed to small quadrate and rhombic particles with a particle size of about 20 nm in width at 120 °C (Figure 6c and d). The SAED result indicated that each particle is a

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Figure 7. TEM images of the samples obtained by hydrothermal treatment of PA-HTO nanosheet colloidal solution at (a) pH 11.3 and 135 °C, (b) and (c) pH 1.2 and 130 °C, and (d) pH 0.3 and 135 °C, respectively. The inset is the SAED pattern. Figure 6. TEM images of the samples obtained by hydrothermal treatment of TBA-HTO colloidal solution at (a) pH 11.5 and 100 °C, (b) pH 11.5 and 120 °C, (c) pH 1.8 and 90 °C, (d) pH 1.8 and 120 °C, (e) pH 13.0 and 90 °C, and (f) pH 13.0 and 120 °C, respectively. The insets are SAED patterns.

single crystal of anatase phase. At pH 13, sheetlike particles and some tubelike particles were observed at 90 °C (Figure 6e). XRD result indicated that this sample has layered structure (Figure 4). The nanotube-like particles were formed by curling the layered titanate nanosheet. Curled thin sheetlike anatase particles and some wirelike anatase particles were obtained at 120 °C (Figure 6f). In the PA-HTO nanosheet reaction system, at pH 11.3, the nanosheet-like particles of PA-HTO nanosheet were split into small nanocomb-like particles at 130 °C, and finally into nanoleaflike anatase particles with a particle size of about 30 nm in width and 200 nm in length at 135 °C (Figure 7a), similar to the case of TBA-HTO nanosheet. The nanoleaf-like particles have a flat surface and are thicker than the nanoboat-like particles prepared from the TBA-HTO nanosheets. At pH 1.2 and 130 °C, granular particles with a particle size of about 100 nm, quadrate particles with a particle size of about 20 nm in width, and some small particles were observed (Figure 7b). This sample is the mixture of anatase and rutile phases (Table 1). A high-resolution transmission electron microscopy (HRTEM) image indicates that the granular particles show lattice image with a lattice spacing of 0.32 that corresponds to the (110) lattice plane of rutile phase (Figure 7c). The SAED results reveal that the granular particles correspond to rutile phase, and the quadrate and small particles correspond to anatase phase. Almost only the granular particles were observed at pH 0.3 and 135 °C, and these particles showed SAED pattern of rutile phase (Figure 7d). A detail HRTEM study was carried out on the anatase nanocrystals. The lattice images of the nanoleaf-like, nanoboatlike, quadrate, and rhombic particles were clearly observed, indicating that these nanoparticles were the single nanocrystals with high crystallinity (Figure 8). From the distance between the adjacent lattice fringes, we can assign the lattice planes on the nanocrystals. The nanoleaf-like anatase nanocrystals show lattice

spacings of d101 ) 0.35 nm for the (101) plane, d100 ) 0.38 nm for the (100) plane, and d002 ) 0.48 nm for the (002) plane (Figure 8a). This result indicates that the basal plane of the nanoleaf-like anatase nanocrystals corresponds to the (010) plane that is vertical to the [010] direction. We observed that the basal planes of the nanoleaf-like, nanoboat-like, quadrate, and rhombic anatase crystals are always vertical to the [010] direction. This result suggested that these anatase nanocrystals have a large aspect ratio of width to thickness and attach on the microgrid surface by their basal planes. The above results reveal the anatase nanocrystals prepared in this study preferentially expose the (010) plane. Figure 8b shows the TEM image of nanocomb-like anatase particles. The lattice images of anatase can be observed, but are not as clear as the nanoboat-like crystals (Figure 8c), indicating lower crystallinity as compared to the nanoboat-like, which is in agreement with the XRD results of Figure 3. The cracks on the nanocomb-like particles direct always to the [001] direction of the anatase structure. Because the nanoleaf-like and the nanoboat-like anatase crystals are formed by splitting the nanocomb-like particles, their axis directions always correspond to the [001] direction. The axis direction of the nanobelt-like anatase particles agrees always also with the [001] direction (Figure 8d), revealing that the nanobelt-like particles are formed by splitting the sheetlike particles along the (100) plane. The sides of the quadrate anatase nanocrystals correspond to (100) and (001) planes, respectively (Figure 8e), indicating the quadrate anatase nanocrystals are formed by cutting the nanobelt-like particles along the (001) plane. Two types of rhombic anatase crystals were observed (Figure 8f and g). The interior angles of type I and II are in agreement with the angle of (101) and (100) planes, and angle of (101) and (001) planes, respectively, which can be calculated as 21.7° and 68.3° from the lattice constants of ananate (tetragonal S.G.: I41/amd, Z ) 4, a ) 0.37852 nm, c ) 0.95139 nm).30 (30) Howard, C. J.; Sabine, T. M.; Dickson, F. Acta Crystallogr., Sect. B 1991, 47, 462-468.

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Figure 8. HRTEM images of (a) leaflike, (b) comblike, (c) boatlike, (d) beltlike, (e) quadrate, and (f) and (g) rhombic anatase nanocrystals. Table 1. Crystal Shape, Size, and Surface Area for TiO2 Nanocrystals Prepared by Hydrothermal Reaction

sample TBA-1.8-120 TBA-11.5-120 TBA-13.0-120 TBA-14.1-120 PA-11.3-135 PA-1.2-130 PA-0.6-125 PA-0.3-135 ST-01 a

crystallite size/nma anatase phase [200] × [004] × [010]

SBET /m2 g-1

crystal phase

crystal shape

anatase anatase anatase layered anatase rutile/anatase (FR ) 0.6) rutile/anatase (FR ) 0.8) rutile anatase

quadrate and rhombic boatlike thin-sheet and wirelike platelike leaflike granular and quadrate

30 × 55 × 21 29 × 34 × 25

131 24.5 44.5 23.9 34.8 38.9

granular and quadrate

37 × 53 × 19

30.8

granular spherical

7.7b

24 × 21 × 11 43 × 61 × 16 40 × 71 × 16

25.5 349

Estimated from XRD data using the Sherrer formula (eq 2). b Estimated from the (101) peak.

Particle Size and Surface Area. Average crystallite sizes of anatase nanocrystals can be estimated from the line broadening of XRD diffraction peaks using the Sherrer formula (eq 2).31

L ) kλ/β cos θ

(2)

where L is the crystallite size, k is the Sherrer constant usually taken as 0.89, λ is the wavelength of X-ray radiation (0.15418 nm for Cu KR), and β is the full width at half-maximum (fwhm) of diffraction peak measured at 2θ. The average crystallite sizes of anatase calculated from the (200) and (004) diffraction data are given in Table 1. The average crystallite sizes calculated from the (200) and (004) diffraction data correspond to the width and length of the anatase nanocrystals, due to [100] and [001] directing to the width and length directions, respectively. Because all of the anatase nanocrystals prepared here have thin platelike morphology, they attach always on the microgrid surface by the basal planes, being vertical to the direction of (31) Cullity, B. D. Elements of X-ray Diffraction; Addison-Wesley Pub. Co.: Reading, MA, 1978.

TEM observation. The thickness of the crystals is difficult to observe directly by TEM analysis. We tried to estimate the thickness from the fwhm data of diffraction peak (211) using formula 3.

L(010) ) 0.364L(211)

(3)

where L(010) is crystallite size in the [010] direction, corresponding to the thickness of the platelike crystals; L(211) is the crystallite size calculated from the XRD diffraction peak of the (211) plane using the Sherrer formula (eq 2); the parameter 0.364 is obtained from the lattice parameters of anatase (tetragonal, a ) 0.37852 nm, c ) 0.95139 nm), where lattice vectors have a relationship of V[211] ) V[010] + V[200] + V[001]; and the length ratio (L(010)/ L(211)) of V[010] to V[211] can be calculated as 0.364. The calculated results of thickness L(010) for anatase single nanocrystals are shown in Table 1. The length (L(004)) of anatase nanocrystals prepared from TBAHTO nanosheet solution decreases with decreasing pH value of the reaction solution, agreeing with the TEM results. The length

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Figure 9. Scheme of the reaction mechanism for the formation of TiO2 nanocrystals from titanate nanosheets under hydrothermal reaction conditions.

and width of the nanoboat-like crystals prepared from TBAHTO nanosheets are larger than that of the nanoleaf-like crystals from PA-HTO nanosheets, agreeing also with the TEM results. The thickness of the nanocrystals decreases in an order of quadrate crystal (11 nm) < nanoboat-like crystal (16 nm) ) nanosheetlike crystal (16 nm) < leaf-like (21 nm), suggesting the thickness of the nanocrystals prepared from TBA-HTO nanosheets is thinner than that from PA-HTO nanosheets. The BET (Brunauer-Emmett-Teller) specific surface areas (SBET) of TiO2 nanoparticles are shown in Table 1. In the TBAHTO nanosheet reaction system at 120 °C, the anatase sample prepared in the acidic solution has the largest specific surface area, and that prepared in the neutral solution has the smallest specific surface area, which are almost in agreement with their crystal sizes. Although TBA-13.0-120 has almost the same crystal size as TBA-11.5-120, it has a larger specific surface area than that of TBA-11.5-120. This may be due to the porous structure formed by curled nanosheet-like particles in TBA-13.0-120 (Figure 6f). In the PA-HTO nanosheet reaction system, the specific surface area of TiO2 samples decreases with decreasing pH value of the reaction solution, corresponding to an increase of rutile phase content in the samples, in agreement with the particle size. Mechanism of Transformation Reaction from HTO Nanosheets to TiO2 Nanocrystals. On the basis of the above results, we proposed a reaction mechanism for the formation of TiO2 nanocrystals from HTO nanosheets in the novel hydrothermal process, as shown in Figure 9. In this process, first the layered structure of HTO is exfoliated into its elementary nanosheets by intercalating TBA+ ions or PA molecules into the interlayer space. Because TBA+ ions are intercalated into the interlayer space by H3O+/TBA+ ion-exchange reaction in the TBAOH solution, TBA+ and unreacted H3O+ ions locate on the surface of the TBA-HTO nanosheets. In the PA solution, PA molecules are intercalated by a protonation reaction of PA, and PAH+ ions are formed on the surface of the PA-HTO nanosheets. The transformation reaction from titanate nanosheet to TiO2 nanocrystal is a dehydration reaction that accompanies loss of

the negative charge of the nanosheet and the cations of TBA+ or PAH+ and H3O+ on the surface. In the structure transformation process, two types of reactions will occur simultaneously.27 One is an in situ topotactic transformation reaction, in which the structure of titanate nanosheets is transformed to TiO2 structure by an in situ topotactic dehydration reaction, where the particle morphology of nanosheet precursor is retained after the topotactic reaction. Another is a dissolution-deposition reaction on the surface of titanate nanosheets, similar to the normal hydrothermal reaction, resulting in a morphology change of the precursor. In the pH range of 2-12, the titanate nanosheets can be transformed into anatase phase mainly by the in situ topotactic structural transformation reaction, forming sheetlike anatase particles (reaction 1 in Figure 9). In this reaction, the TiO6 octahedra in titanate nanosheets slightly and equably shift from the positions of lepidocrocite-like layered structure to the positions of anatase structure. The (010) plane (the basal plane of titanate nanosheet) of lepidocrocite-like layered structure is transformed to the (010) plane (the basal plane of sheet-like anatase nanocrystals) of anatase structure in the topotactic reaction. This is the reason why the basal planes of sheet-like anatase nanocrystals are always vertical to the [010] direction of anatase structure. The formation of cracks along the (100) plane of anatase structure is due to the dissolution reaction (reaction 2 in Figure 9), which splits finally the sheetlike particles into the nanoboatlike and nanoleaf-like anatase nanocrystals. Under acidic conditions (pH < 2), sheetlike anatase particles are formed first by the in situ topotactic structural transformation reaction (reaction 1 in Figure 9), and then split into the nanobeltlike particles by the dissolution reaction along the (100) plane (reaction 3 in Figure 9), similar to the case in the pH range of 2-12. The dissolution reaction along the (001) plane results in the cutting of the nanobelt-like particles into the quadrate anatase nanocrystals. Furthermore, the dissolution reactions along the (001) and (101) planes cut the nanobelt-like particles into the rhombic anatase nanocrystals. The dissolution reactions along the (001) and (101) planes occur mainly in the acidic solution,

Single Nanocrystals of Anatase-Type TiO2

suggesting the solubilities along the (001) and (101) planes are lower than that along the (100) plane that can occur even in the pH range of 2-12. We think that the formation of rutile phase in the acidic solution is due to the dissolution-deposition reaction, similar to the normal hydrothermal reaction. It has been reported that rutile phase is formed easily in the acidic solution by the normal hydrothermal reaction.32-34 The rutile crystals prepared here show the granular particle morphology similar to the normal dissolution-deposition reaction, without relativity to the morphology of nanosheet precursor. The formation of small particles of anatase phase in the acid solution (Figure 7b) is due to the dissolution reaction of anatase phase. Therefore, we conclude that rutile phase is formed by the dissolution-deposition reaction. The formation of rutile phase in PA solution, but not in TBA solution, may be due to higher solubility of titanium oxide in PA solution than in TBA solution, because PA could coordinate to Ti(IV) with its lone pair electrons. TBA-HTO nanosheet shows the higher reactivity in the formation reaction of anatase phase than PA-HTO nanosheet because of the higher exfoliation ability of TBA+ ion than that of PA molecule. We think that a restacking reaction of a small number of titanate nanosheets together occurs in the transformation reaction, accompanying loss of TBA+ or PAH+ ions on the surface of the nanosheets. The restacking reaction occurs more easily in the PA-HTO nanosheet reaction system than that in the TBA-HTO nanosheet reaction system, because of the lower exfoliation ability of PA molecule, resulting in thicker sheetlike particles. This idea is supported by the fact of the nanoboat-like anatase crystals prepared from TBA-HTO nanosheets are thinner than the nanoleaf-like particles prepared from PA-HTO nanosheets (Table 1). The thicker sheetlike particle shows lower reactivity in the formation process of TiO2. The high reactivity of titanate nanosheets in the acidic solution may be due to the high solubility of the titanium oxides, which breaks the nanosheets into small flinders that show high reactivity in the formation of TiO2. Because the dissolution reaction of the nanosheet is inhibited in the alkaline solution (at the high pH conditions), the sheetlike anatase particles are formed in TBA-HTO nanosheet solution at pH 13 (reaction 4 of Figure 9). The nanowire-like anatase particles are formed at pH 13 by the topotactic transformation of the nanotube-like particles with layered structure to anatase phase. Photocatalytic Activity of TiO2 Nanocrystals. Because anatase nanocrystals have potential application as the photocatalyst, we investigated the photocatalytic activity of anatase nanocrystals with typical crystal morphology. Figure 10 shows the results of degradation rate of methylene blue by the photocatalytic reaction under black light lamp irradiating conditions. Ishihara ST-01, a typical commercial anatase nanocrystal sample, was used as the standard sample for the comparison. A TEM study indicated that ST-01 sample has spherical crystal morphology with a crystal size of about 5 nm, and the spherical particles expose various crystal planes (see Supporting Information), which are similar to the normal anatase nanocrystals with spherical particle morphology.35 It was found that TBA-13.0120 with the nanosheet-like and nanowire-like crystal morphology (32) Li, G.; Gray, K. A. Chem. Mater. 2007, 19, 1143-1146. (33) Feng, X.; Zhai, J.; Jiang, L. Angew. Chem., Int. Ed. 2005, 44, 5115-5118. (34) Zheng, Y.; Shi, E.; Chen, Z.; Li, W.; Hu, X. J. Mater. Chem. 2001, 11, 1547-1551. (35) Shklover, V.; Nazeeruddin, M.-K.; Zakeeruddin, S. M.; Barbe, C.; Kay, A.; Haibach, T.; Steurer, W.; Hermann, R.; Nissen, H.-U.; Gro¨tzel, M. Chem. Mater. 1997, 9, 430-439.

Langmuir, Vol. 23, No. 23, 2007 11789

Figure 10. Changes of degradation amount of methylene blue with irradiation time for anatase nanocrystal samples under black light irradiating conditions.

showed the highest photocatalytic activity that is about 3 times higher as compared to that of ST-01. TBA-1.8-120 with the quadrate morphology and PA-11.3-135 with the nanoleaf-like morphology showed slightly higher and lower photocatalytic activity than that of ST-01, respectively. To characterize the photocatalytic activity of crystal surface, we estimated the degradation rate of methylene blue per BET surface area. The degradation ratios at 180 min were 0.74, 0.23, 0.069, and 0.029 mg(MB)/m2 for TBA-13-120, PA-11.3-135, TBA-1.8-120, and ST-01, respectively. This result reveals that the surface photocatalytic activity increases in an order of TBA-13-120 > PA11.3-135 > TBA-1.8-120 > ST-01, and the anatase nanocrystals prepared in this study have very high photocatalytic active surface. Becuase the anatase nanocrystals prepared from the titanate nanosheets preferentially expose the (010) plane, we think that the (010) plane is the one of the highest photocatalytic active surface. The increasing order of TBA-13-120 > PA-11.3-135 > TBA-1.8-120 for the surface activity also suggests that, except the exposing plane, the crystallinity and the particle morphology of the nanocrystals also affect the photocatalytic activity. The lower surface activity for TBA-1.8-120 than that for PA-11.3135 may be due to its lower crystallinity. The higher surface activity for TBA-13-120 than that for PA-11.3-135 may be due to its curled nanosheet-like particle morphology (Figure 6f). The curled nanosheet-like particles will result in the formation of porous structure that will help the adsorption of methylene blue molecules on the surface of the photocatalyst. Adsorptive Property of N719 Dye. The characterization of the dye adsorptive behavior on the anatase nanocrystal surface is very important for the application of the nanocrystals in dyesensitized solar cells. Figure 11 shows the adsorption isotherms of N719, a typical ruthenium dye used in dye-sensitized solar cells, on PA-11.3-135 and ST-01 samples at room temperature; to understand the surface adsorption properties, we show the dye uptake amounts on per BET surface area of anatase nanocrystals. We chose the PA-11.3-135 sample in the dye adsorptive study, because this sample has uniform particle morphology with a flat surface. The adsorptive results from such sample will reflect a clear adsorptive property of the exposing lattice plane. The experimental data fit Langmuir isotherm; the N719 adsorptions on PA-11.3-135 and ST-01 can be explained by the Langmuir monolayer adsorption model.36 Langmuir equation can be represented in the linear form (eq 4).

Cs/Qs ) 1/(Qmb) + Cs/Qm

(4)

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1.0 and 0.24 molecule/nm2, respectively. The result reveals that the surface of the nanowire-like crystals of ref 18 has higher adsorption capacity than that of ST-01, but lower than that of PA-11.3-135 exposing mainly the (010) plane. It is known that the (101) plane of anatase phase is the one being mainly exposed for the spherical particle prepared by the normal methods.35,37 It is revealed that the (010) plane has higher ability for the adsorption of N719 dye than the (101) plane, maybe the best surface for the adsorption of the ruthenium dyes.

Conclusions Figure 11. Isotherms for N719 dye adsorption on (a) PA-11.3-135 and (b) ST-01 samples.

where Qs is the N719 uptake (mol m-2), Cs is the equilibrium N719 concentration in the ethanol solution (mol dm-3), Qm is the saturation (maximum) adsorption capacity (mol m-2), and b is the adsorption constant (dm3 mol-1). From the fitting of experimental data by plotting Cs/Qs against Cs, the saturation adsorption capacity (Qm) and the adsorption constant (b) for the N719 absorptions are evaluated as 5.0 × 10-6 mol m-2 and 1.3 × 103 dm3 mol-1 with a correlation coefficient of 0.980 for PA-11.3-135, and as 5.0 × 10-7 mol m-2 and 6.4 × 103 dm3 mol-1 with a correlation coefficient of 0.997 for ST-01, respectively. The adsorption constant for ST-01 is slightly larger than that for PA-11.3-135, suggesting that the adsorption of N719 on ST-01 surface is slightly stronger than that on PA-11.3-135 surface. The saturation adsorption capacity for PA-11.3-135 is 10 times larger than that for ST-01, revealing PA-11.3-135 has more adsorption sites on its surface than that on ST-01 surface. It has been reported that nanowire-like anatase crystals exposing mainly the (101) plane show a high adsorption capacity of 0.63 molecule/nm2 for N719 dye in a 3 × 10-4 mol dm-3 N719 dye solution.18 For the comparison, we calculated also the adsorption capacities of PA-11.3-135 and ST-01 at the same concentration of N719 dye from the data of Figure 11, and the capacities were (36) Xue, M.; Chitrakar, R.; Sakane, K.; Hirotsu, T.; Ooi, K.; Yoshimara, Y.; Feng, Q.; Sumida, N. J. Colloid Interface Sci. 2005, 285, 487-492.

The single anatase nanocrystals with boatlike, comblike, sheetlike, leaflike, quadrate, rhombic, and wirelike particle morphologies can be prepared by hydrothermal treatment of titanate nanosheets. The nanocrystal morphology is dependent on the reaction temperature, pH value of the reacting solution, and exfoliating agent. The titanate nanosheets are transformed to the TiO2 nanocrystals by two types of reactions. One is the in situ topotactic structural transformation reaction, and another is the dissolution-deposition reaction. The anatase nanocrystals formed by the in situ topotactic structural transformation reaction retain the sheetlike particle morphology of the precursor and preferentially expose the (010) plane of anatase structure. The rutile nanocrystals are formed by the dissolution-deposition reaction under low pH conditions. The (010) plane of anatase structure shows high photocatalytic activity and high N719 dye adsorptive capacity. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research (B) (No. 17310074) from the Japan Society for the Promotion of Science. Supporting Information Available: Experimental results of DTA, TEM, and SEM on the characterization of titanate nanosheet, anatase nanocrystal, and ST-01 samples. This material is available free of charge via the Internet at http://pubs.acs.org. LA701632T (37) Shklover, M.; Ovchinnikov, Y. E.; Braginsky, L. S.; Zakeeruddin, S. M.; Gro¨tzel, M. Chem. Mater. 1998, 10, 2533-2541.