Synthesis of a Perpendicular TiO2 Nanosheet Film with the

A method was developed for preparing perpendicular TiO2 nanosheet films from titanate nanosheet films produced on a titanium metal sheet by hydrotherm...
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Langmuir 2007, 23, 7447-7450

7447

Synthesis of a Perpendicular TiO2 Nanosheet Film with the Superhydrophilic Property without UV Irradiation Eiji Hosono,† Hirofumi Matsuda,† Itaru Honma,† Masaki Ichihara,‡ and Haoshen Zhou*,† National Institute of AdVanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba 305-8568, Japan, and Material Design and Characterization Laboratory, Institute for Solid State Physics, UniVersity of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8581, Japan ReceiVed April 17, 2007. In Final Form: May 24, 2007 A method was developed for preparing perpendicular TiO2 nanosheet films from titanate nanosheet films produced on a titanium metal sheet by hydrothermal treatment with aqueous urea. The method is based on the consideration of some important points relating to the thermodynamics of nucleation and crystal growth. The resulting anatase TiO2 nanosheet films showed a specific superhydrophilicity without the need for any prior UV irradiation.

Nowadays, many researchers are attempting to improve of the properties of devices through the control of nanostructural properties. We have reported on the preparation by the selftemplating method through metal hydroxides of many kinds of metal oxide nanosheet films with perpendicular growth, such as ZnO,1,2 NiO,3 Co3O4,3 and LaOF.4 Elsewhere, many perpendicular nanosheet films, including Zn-Al hydrotalcite,5 copper compounds,6,7 cobalt compounds,7 manganese compounds,7 nickel compounds,8 and tin oxide,9 have been reported. Perpendicular nanosheet films have been prepared from elements covering a large area of the periodic table, from typical metals to transition metals and even rare earth metals. However, no perpendicular TiO2 nanosheet films have been reported, although TiO2 is one of the most important materials because of its widespread use in a wide variety of applications, including dye-sensitized solar cells,10 lithium ion batteries,11 photocatalysis,12 superhydrophobicity,13 superhydrophilicity,14-16 and proton conductors for fuel cells.17 In particular, the development of superhydrophilicity on TiO2 by UV irradiation14-16 is currently a topic of intensive research. The mechanism for the development of superhydrophilicity of TiO2 by UV irradiation is believed to involve the * Corresponding author. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ University of Tokyo. (1) Hosono, E.; Fujihara, S.; Kimura, T.; Imai, H. J. Colloid Interface Sci. 2004, 272, 391-398. (2) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. AdV. Mater. 2005, 17, 20912094. (3) Fujihara, S.; Hosono, E.; Kimura, T. J. Sol-Gel Sci. Technol. 2004, 31, 165-168. (4) Hosono, E.; Fujihara, S.; Kimura, T. Langmuir 2004, 20, 3769-3774. (5) Gao, Y. F.; Nagai, M.; Masuda, Y.; Sato, F.; Seo, W. S.; Koumoto, K. Langmuir 2006, 22, 3521-3527. (6) Wang, S.; Feng, L.; Jiang, L. AdV. Mater. 2006, 18, 767-770. (7) Schwenzer, B.; Roth, K. M.; Gomm, J. R.; Murr, M.; Morse, D. E. J. Mater. Chem. 2006, 16, 401-407. (8) Tan, Y.; Srinivasan, S.; Choi, K. S. J. Am. Chem. Soc. 2005, 127, 35963604. (9) Ohgi, H.; Maeda, T.; Hosono, E.; Fujihara, S.; Imai, H. Cryst. Growth Des. 2005, 5, 1079-1083. (10) Oregan, B.; Gratzel, M. Nature 1991, 353, 737-740. (11) Kavan, L.; Gratzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716-6723. (12) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (13) Feng, X.; Zhai, J.; Jiang. L. Angew. Chem., Int. Ed. 2005, 44, 5115-5118. (14) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431-432. (15) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. AdV. Mater. 1998, 10, 135-138. (16) Gao, Y. F.; Masuda, Y.; Koumoto, K. Langmuir 2004, 20, 3188-3194. (17) Yamada, M.; D. Li.; Honma, I.; Zhou, H. J. Am. Chem. Soc. 2005, 127, 13092-13093.

formation of either oxygen defects15,18 or dangling bonds.16 If a TiO2 surface could be constructed with a plane having a large number of oxygen defects or dangling bonds, then the resulting TiO2 should show superhydrophilicity without the need for any UV irradiation. The fabrication of perpendicular TiO2 nanosheet films is therefore of interest because it is considered that the large-area flat plane of the TiO2 nanosheet, which is vertical to the substrate, is thermodynamically stable and the edge plane, which has a thickness of several nanometers, is thermodynamically unstable. Therefore, a perpendicular TiO2 nanosheet would be expected to display superhydrophilicity without UV irradiation because the edge plane should contain a large number of defects or dangling bonds as a result of thermodynamic instability. Generally, the control of crystal growth is an important factor in relation to the fabrication of perpendicular nanosheet films. This control of crystal growth can be achieved by chemical bath deposition (CBD) through the heterogeneous nucleation of a metal oxide on a substrate as a result of a small degree of supersaturation of the solution19,20 because the occurrence of heterogeneous nucleation on a foreign surface is thermodynamically favored over homogeneous nucleation in a solution with a low degree of supersaturation. The solubility of solutes can change as a result of chemical reactions of starting metal salts in solution. When the solution reaches supersaturation, solid metal compounds are produced by nucleation and crystal growth. There are two potential routes for the synthesis of a perpendicular TiO2 nanosheet film. One is the direct synthesis of a perpendicular TiO2 nanosheet by the control of crystal growth. However, this is extremely difficult in the case of TiO2 with an anatase or rutile crystal structure. The second route is based on the observation that a SnO nanosheet with a layered structure can be converted into a SnO2 nanosheet by hydrothermal treatment or by heating.9 Similar treatment of a perpendicular titanate nanosheet with spontaneous 2D growth can result in a perpendicular TiO2 nanosheet. If a perpendicular titanate nanosheet film can be fabricated, then it should be readily converted into a perpendicular TiO2 nanosheet film. However, the fabrication of a perpendicular titanate nanosheet film is very difficult, although syntheses of titanate nanosheets and nanotube powders have been reported (18) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188-2194. (19) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, W.; McVay, G. L. Science 1994, 264, 48-55. (20) Gao, Y. F.; Koumoto, K. Cryst. Growth Des. 2005, 5, 1983-2017.

10.1021/la701117a CCC: $37.00 © 2007 American Chemical Society Published on Web 06/07/2007

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Figure 1. XPS spectrum in the N 1s region of the nanosheet films by hydrothermal synthesis of a titanium sheet in urea solution.

Figure 3. SEM images with 45° on the tilt of the ammonium titanate perpendicular nanosheet films on the titanium sheet by hydrothermal synthesis of a titanium sheet in (a) urea solution and (b) ammonium solution. Inset images are the view from the vertical direction. (c) TEM images of the ammonium titanate perpendicular nanosheet films by hydrothermal synthesis of a titanium sheet in urea solution. The sheet thickness is around 2.5 nm.

Figure 2. XRD patterns of the nanosheet films by hydrothermal synthesis of titanium sheet in urea solution (a, a′) and ammonium solution (b, b′). (|) Titanium, JCPDS 44-1294; (1) the unknown peak from the titanium substrate.

by several researchers.21-23 The difficulty in synthesizing perpendicular titanate nanosheets is the result of the low solubility of the titanium ion. In the concentration versus pH diagram, the zone of solubility of titanium ions is extremely narrow and is limited to conditions of high acidity.24 Hence, the titanate nanosheet, which is synthesized under basic condition, is fabricated as a powder by homogeneous nucleation because in the CBD method, which uses a solution of the metal ion as the precursor solution, it is not possible to control the supersaturation to the low degree necessary for heterogeneous nucleation. For heterogeneous nucleation of the titanate, the concentration of the titanium ion must be maintained at an extremely low level in the supersaturated solution. To solve this problem, we examined the use of a titanium metal sheet and an aqueous solution. When the titanium metal reacts with the solution, a very low concentration (21) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160-3163. (22) Rhee, C. H.; Bae, S. W.; Lee, J. S. Chem. Lett. 2005, 34, 660-661. (23) Takezawa, Y.; Imai, H. Small 2006, 2, 390-393. (24) Yamabi, S.; Imai, H. Chem. Mater. 2002, 14, 609-614.

of titanium ions diffuses into the solution, resulting in a concentration gradient. Supersaturation in the system is confined to an area near the substrate. This prevents homogeneous nucleation in solution and causes heterogeneous nucleation on the titanium metal sheet to give a titanate nanosheet film. We report here on the fabrication of perpendicular TiO2 nanosheet films by heat treatment of a titanate nanosheet film formed by hydrothermal treatment of a titanium metal sheet with aqueous urea. The resulting anatase TiO2 nanosheet films showed superhydrophilic properties without the need for any UV irradiation treatment. Experimental Section An aqueous solution of urea was prepared by dissolving 5 g of urea (Wako Chemical, 99.0%) in 40 mL of deionized water. This solution and the titanium sheet (Nilako, 99.5%) were placed in a Teflon-lined autoclave (model 4744, Parr Instrument Company). For the purpose of comparison, a 40 mL aqueous ammonia solution (Wako Chemical, 25-28%) was used instead of the aqueous urea solution. The autoclave was heated to 100 °C for 4 days. After the hydrothermal treatment, the sheet was washed with deionized water and dried at room temperature. The titanate coating on the sheet was then transformed to TiO2 by heating at 400 or 600 °C for 30 min in air. The crystal structure was examined by X-ray diffraction (XRD) analysis with a Bruker axs D8 Advance using Cu KR radiation. X-ray photoelectron spectroscopy (XPS) was carried out by using Al KR radiation with a Surface Science Instruments S-probe ESCA (model 2803). The morphology of the film was examined by field-

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Figure 4. (a, c) SEM images and (b, d) TEM images of the perpendicular TiO2 nanosheet films fabricated by heating the ammonium titanate films by hydrothermal synthesis in urea solution. (a, b) 400 °C for 30 min and (c, d) 600 °C for 30 min. The inset images in b and d are electron diffraction patterns. emission scanning electron microscopy (FESEM) and by highresolution transmission electron microscopy (HRTEM) using a Carl Zeiss Gemini Supra and a JEOL JEM-2010F, respectively. The measurement of the superhydrophilicity was conducted with a VCA Optima XE from AST Products Inc.

Results and Discussion Figure 1 shows the XPS spectrum in the N 1s region of the sheet after hydrothermal treatment of the titanium sheet in the aqueous urea solution. The spectrum can be separated into a large peak (402.2 eV) and a small peak (400.3 eV), which we assigned to N species in ammonium salts and to N-C, respectively. The sample therefore contains large numbers of ammonium groups. Figure 2(a and a′) shows XRD patterns of the sheet after hydrothermal treatment of the titanium sheet in the aqueous urea solution. The peak at around 10° corresponds to the layer structure of a typical titanate nanosheet.22,23 A calculation based on Scherrer’s equation indicates a crystallite size of the 11.5 nm, which corresponds in this case to the thickness of the nanosheet. Judging from the XPS and XRD results, we consider that the fabricated sample consists of ammonium titanate [(NH4)2TinO2n+1]. When the titanium sheet was subjected to hydrothermal treatment at 100 °C for 4 days in 25% aqueous ammonia, the XRD pattern of the sheet showed the same pattern of ammonium titanate as seen in Figure 2 (b and b′). In the case of the ammonium titanate sheet film produced by hydrothermal treatment with urea solution, the ammonium is a product of the decomposition of urea in the solution.2 Figure 3a,b shows SEM images of ammonium titanate films produced by hydrothermal treatment with aqueous urea and ammonium solutions, respectively. In both images, we can see

Figure 5. XRD patterns of the perpendicular TiO2 nanosheet films fabricated by heating the ammonium titanate films by hydrothermal synthesis in urea solution: (a) 400 °C for 30 min, (b) 600 °C for 30 min, (c) pattern of the titanium sheet (substrate), which is heated to 600 °C for 30 min.

the presence of perpendicular nanosheets on the titanium sheets. The sheet thickness from the urea solution is around 2.5 nm in Figure 3c. This value is different from that given by Scherrer’s equation (11.5 nm): this is a result of the XRD measurement method. XRD detects the flat crystalline plane along the substrate and does not detect the perpendicular crystalline plane on the substrate. The 11.5 nm given by Scherrer’s equation in Figure 1a is not the thickness of the perpendicular nanosheet but indicates the thickness of the titanate sheet covering the substrate. In terms of superhydrophilicity, the important part is the 2.5-nm-thick perpendicular nanosheet. In the case of the aqueous urea solution,

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Figure 6. Measurement of the contact angle of water on perpendicular TiO2 nanosheet films fabricated by heating to 600 °C through hydrothermal synthesis from urea solution.

the film thickness was around 500 nm, which means that the perpendicular growth length of the ammonium titanate film from the aqueous urea solution is longer than that from the ammonium solution (around 250 nm). The probable reaction mechanism is as follows:

nTi + 2NH4OH + (2n - 1)H2O f (NH4)2TinO2n+1 + 2nH2 (1) There is a small degree of supersaturation based on the slow reaction in eq 1, which is important for the heterogeneous nucleation and crystal growth. The weak basicity of NH4OH, compared with that of NaOH or KOH is suitable for slow reaction. The small difference in the reaction rates causes differences in the rate of the reaction and crystal growth. Two research groups have reported on the fabrication of titanate nanowires from titanium.25,26 Equation 1 is based on a report25 on the fabrication of K2Ti8O17 by hydrothermal treatment of titanium with KOH. Another paper26 reports the fabrication of TiO2 by sequential heating and proton exchange of Na2TiO13‚ xH2O, synthesized by hydrothermal treatment of titanium with H2O2 and NaOH. Indeed, the fabrication of TiO2 by the heating of proton-exchanged titanate is a well-known synthetic method. However, if the alkali metal in the titanate film on titanium is exchanged for protons, then the strength of adhesion of the film decreases, and the film is unsuitable for a variety of applications. However, we used ammonium ions for the heterogeneous nucleation, resulting in gradual crystal growth through a slow reaction at the lower temperature of 100 °C to give an ammonium titanate film that could be converted to TiO2 solely by heating, without a proton-exchange step. Parts a and b of Figure 4 show SEM micrographs of TiO2 films obtained from ammonium titanate film prepared from the urea solution by heating for 30 min to 400 and 600 °C, respectively. Parts c and d of Figure 4 show the corresponding TEM micrographs. The film morphology and nanosheet thickness of ammonium titanate are maintained. From the XRD patterns, both the film heated to 400 °C (Figure 5a) and the film heated to 600 °C showed anatase peaks; the film heated to 600 °C also showed a rutile peak (Figure 5b). The rutile peak was derived from a rutile phase formed by heating the titanium substrate because the XRD pattern of a bare titanium sheet heated to 600 °C also showed a rutile peak (Figure 5c). Therefore, TiO2 nanosheets heated to 400 and 600 °C both show anatase phases. (25) Wang, B. L.; Chen, Q.; Hu, J.; Li, H.; Hu, Y. F.; Peng, L. M. Chem. Phys. Lett. 2005, 406, 95-100. (26) Zhao, Y.; Lee, U. H.; Suh, M.; Kwon, Y. U. Bull. Korean Chem. Soc. 2004, 25, 1341-1345.

Figure S1(a,b) shows high-resolution TEM images of the TiO2 film obtained by heating the ammonium titanate film from the urea solution to 400 and 600 °C for 30 min, respectively (Supporting Information). In Figure S1a, we can see the lattice fringe of anatase (101). This is a similar result to the electron diffraction pattern, as shown in the insets of Figures 4b and S1b, whereas the inset images of Figure 4d show the lattice image of anatase (101) and the electron diffraction pattern. Judging from the XRD and TEM results, the synthesized TiO2 nanosheet film is an anatase phase. We checked the water contact angle of the perpendicular anatase nanosheet film (Figure 6). Generally, non-UV-irradiated TiO2 films with a large convexo-concave structure show superhydrophobic or hydrophobic properties.13,27 After UV irradiation, the films develop superhydrophilicity.13,15,16,27 However, our perpendicular anatase nanosheet film showed superhydrophilicity without UV irradiation. Within 0.078 s of the dropping of the water, the contact angle was around 10°. After that time, the contact angle could not be measured because it was extremely low, indicating the existence of superhydrophilicity, which is a specific property of the film. It has been reported that the superhydrophilicity of UV-irradiated TiO2 is caused by the presence of hydroxyl groups produced by the formation of oxygen defects.15,18 Elsewhere, on the basis of an XPS study, it has been proposed that the formation of dangling bonds is responsible for the superhydrophilicity of UV-irradiated TiO2.16 In this work, the top side of the TiO2 nanosheet film is formed by the edges of perpendicular nanosheets that are several nanometers thick. We therefore considered that the densities of defects or dangling bonds at the edge surface are larger than those on a flat plane of the nanosheet. The high concentration of defects or dangling bonds is a result of the specific morphology, which is that of a perpendicular nanosheet anatase film, and is responsible for the superhydrophilicity without prior UV irradiation. In summary, we report the fabrication of perpendicular TiO2 nanosheet films from titanate nanosheet films prepared on a titanium metal sheet by hydrothermal treatment in aqueous urea. Moreover, the anatase TiO2 nanosheet films showed superhydrophilicity without prior UV irradiation. Supporting Information Available: High-resolution TEM images of perpendicular TiO2 nanosheet films fabricated by heating ammonium titanane films by hydrothermal synthesis in urea solution. This material is available free of charge via the Internet at http://pubs.acs.org. LA701117A (27) Irie, H.; Ping, T. S.; Shibata, T.; Hashimoto, K. Electrochem. Solid-State Lett. 2005, 8, D23-D25.