Glass Nanoflake Array Films

It is interesting that the nanoflake products interlace to form a porous network with a large surface-to-volume ratio on the glass substrate. To the b...
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Langmuir 2005, 21, 3486-3492

Photocatalytic TiO2/Glass Nanoflake Array Films Wingkei Ho, Jimmy C. Yu,* and Jiaguo Yu Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China Received November 3, 2004. In Final Form: February 2, 2005 A new approach for the fabrication of oriented TiO2/glass nanoflake arrays has been developed. The ceramic nanoflake array was formed on a glass substrate via a simple, low temperature, and one-step hydrothermally induced phase separation approach without using any templates or additives. The factors affecting the formation of ceramic nanoflakes were examined by various characterization techniques. The results showed that the leaching of the soluble phase from the glass surface through hydrothermal processes resulted in oriented uniform ceramic nanoflake arrays. Electron microscope observations revealed that the nanoflakes formed a continuous porous three-dimensional-network array with a large surface-tovolume ratio. In addition, an anatase TiO2 film was successfully coated onto the nanoflake array by the sol-gel method. The TiO2/glass nanoflake array exhibited high activity for the photocatalytic degradation of acetone and for photoinduced hydrophilic conversion. Such enhancements were attributed to the beneficial effects of the new continuous porous three-dimensional-interconnected nanoflake network and its surface geometrical nanostructure. The present approach provides a convenient route to modify a photocatalytic coating with a porous nano-architectured substrate. This opens extensive new opportunities in the design of semiconductor/ceramic nanostructural array thin films with unusual properties for future optical and electronic applications.

Introduction Nanomaterials often exhibit unusual properties because of their unique structures and small dimensions. There has been considerable interest in the synthesis of nanoscale building blocks such as nanowires, nanorods, nanotubes, and nanobelts.1-4 However, control over orientation and lateral order of these nanomaterials is still a challenge. Thin films and coatings of oriented nanostructures are crucial and desirable for applications involving catalysis, filtration, sensing, photovoltaic cells, and high surface area electrodes. Recently, oriented nanostructure arrays have been prepared from a variety of materials including carbon, metal chalcogenides, oxides, and polymers.5-8 However, there have been very few reports on the direct growth of oriented ceramic nanostructure thin films such as oriented ceramic nano-architectural arrays.9 * Corresponding author. E-mail: [email protected]. Telephone: (852) 2609-6268. Fax: (852) 2603-5057. (1) (a) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (b) Iijima, S. Nature 1991, 354, 46. (c) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 123, 7961. (d) Han, W.; Fan, S.; Li, Q.; Hu, Y. Science 1997, 277, 1287. (e) Dai, H.; Wong, E. W.; Lu, Y. Z.; Fan, S.; Leiber, C. M. Nature 1995, 375, 769. (f) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444. (2) (a) Zhang, D.; Sun, L.; Yin, J.; Yan, C. Adv. Mater. 2003, 15, 1022. (b) Liu, B.; Zeng, H. J. Am. Chem. Soc. 2003, 125, 4430. (c) Guo, L.; Ji, Y. L.; Xu, H.; Simon, P.; Wu, Z. J. Am. Chem. Soc. 2002, 124, 14864. (d) Urban, J. J.; Yun, W. S.; Gu, Q.; Park, H. J. Am. Chem. Soc. 2002, 124, 1186. (e) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (3) (a) Krumeich, F.; Muhr, H. J.; Niederberger, M.; Bieri, F.; Schnyder, B.; Nesper, R. J. Am. Chem. Soc. 1999, 121, 8324. (b) Chen, Q.; Zhou, W.; Du, G.; Peng, L. Adv. Mater. 2002, 14, 1208. (c) Li, Y. D.; Li, X. L.; He, R. R.; Zhu, J.; Deng, Z. J. Am. Chem. Soc. 2002, 124, 1141. (d) Chen, J.; Li, S.; Tao, Z.; Shen, Y.; Cui, C. J. Am. Chem. Soc. 2003, 125, 5284. (4) (a) Yu, J.; Yu, J. C.; Ho, W.; Wu, L.; Wang, X. J. Am. Chem. Soc. 2004, 126, 3422. (b) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (c) Sun, X.; Chen, X.; Li, Y. Inorg. Chem. 2002, 41, 4996. (5) (a) Pan, Z.; Zhu, H.; Zhang, Z.; Im, H.; Dai, S.; Beach, D. B.; Lowndes, D. H. J. Phys. Chem. B 2003, 107, 1338. (b) Cao, A.; Ci, L.; Li, D.; Wei, B.; Xu, C.; Liang, J.; Wu, D. Chem. Phys. Lett. 2001, 335, 150. (c) Zhang, W. D.; Wen, Y.; Li, J.; Xu, G. Q.; Gan, L. M. Thin Solid Films 2002, 422, 120-125.

Ceramics are defined as inorganic, nonmetallic materials that are typically produced using clays and other minerals from the earth or chemically processed powders.10 These materials show attractive properties, including low density, high-temperature stability, and thermal shock resistance. They are, therefore, widely used as supports for substances possessing chemical or biological catalytic properties.11 Ceramics are often prepared with a surface layer that imparts a specific functionality or acts as a protective layer for the bulk materials.9a Certain treatment methods are available for modifying a ceramic surface to obtain specific structure and morphology. For example, a highly macroporous surface can be obtained by treating Pyrex glass under saturated-steam conditions at 470 °C.12 (6) (a) Zhang, L.; Yu, J. C.; Mo, M.; Wu, L.; Li, Q.; Kwong, K. W. J. Am. Chem. Soc. 2004, 126, 8116. (b) Berman, A.; Belman, N.; Golan, Y.; Ezersky, V.; Lifshitz, Y.; Golan, Y. Langmuir 2003, 19, 10962. (c) Huang, F.; Zhang, H. Z.; Banfield, J. F. J. Phys. Chem. B 2003, 107, 10470. (d) Gorer, S.; Ganske, J. A.; Hemminger, J. C.; Penner, R. M. J. Am. Chem. Soc. 1998, 120, 9584. (e) Meldrum, A.; Zuhr, R. A.; Sonder, E.; Budai, J. D.; White, C. W.; Boatner, L. A.; Ewing, R. C.; Henderson, D. O. Appl. Phys. Lett. 1999, 74, 697. (f) Pan, Z. Y.; Liu, X. J.; Zhang, S. Y.; Shen, G. J.; Zhang, L. G.; Lu, Z. H.; Liu, J. Z. J. Phys. Chem. B 1997, 101, 9703. (7) (a) Tian, Z. R.; Voigt, J. A.; Mckenzie, B.; Xu, H. J. Am. Chem. Soc. 2003, 125, 12384. (b) Li, Y.; Bando, Y.; Golberg. D. Adv. Mater. 2004, 16, 37. (c) Fan, R.; Wu, Y.; Li, D.; Yue, M.; Majumdar, A.; Yang, P. J. Am. Chem. Soc. 2003, 125, 5254. (d) Jiang, X.; Herricks, T.; Xia, Y. Nano Lett. 2002, 2, 1333. (e) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (f) Park, W. I.; Yi, G. C. Adv. Mater. 2004, 16, 87. (8) (a) Feng, L.; Song, Y.; Zhai, J.; Liu, B.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42, 800. (b) Liang, L.; Liu, Angew. Chem., Int. Ed. 2002, 41, 3665. (c) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (9) (a) Ishikawa, T.; Yamaoka, H.; Harada, Y.; Fujii, T.; Nagasawa, T. Nature 2002, 416, 64. (b) Chan, V. Z.; Hoffman, J.; Lee, V.; Latrou, H.; Avgeropoulos, A.; Hadjichristidis, N.; Miller, R. D.; Thomas, E. L. Science 1999, 286, 1716. (c) Li, D.; Wang, Y.; Xia, Y. Nano Lett. 2003, 8, 1167. (10) Rawson, P. Ceramics; Oxford University Press: London, New York, 1972. (11) (a) Choma, I.; Dawidowicz, A. L.; Lodkowski, R. J. Chromatogr. 1992, 600, 109. (b) Kennedy, J. F.; Cabral, J. M. S. In Solid Phase Biochemistry; Scouten, W. H., Eds.; Wiley: New York, 1983.

10.1021/la047308e CCC: $30.25 © 2005 American Chemical Society Published on Web 03/09/2005

TiO2/Glass Nanoflake Array Films

Here we report that an oriented ceramic nanoflake array on a soda-lime glass (SL-glass) substrate can be prepared by a one-step hydrothermally induced phase separation method. Conventionally, phase separation in ceramics is observed only at high temperatures and pressures. Phase separation takes place when a system reduces its free energy by separating into two or more phases.13 Our new approach makes it possible for phase separation to occur under a low temperature hydrothermal reaction condition. It is interesting that the nanoflake products interlace to form a porous network with a large surface-to-volume ratio on the glass substrate. To the best of our knowledge, the facial fabrication of ceramic nanostructural arrays directly on a ceramic surface by a low temperature phase separation approach has never been reported. To explore the potential of this nano-architectural ceramic substrate, we coated it with a TiO2 thin film by using the sol-gel method. Titanium dioxide based materials are believed to be the most promising photocatalysts because of their superior photoreactivity, nontoxicity, longterm stability, and low price.14 Many recent studies have shown that TiO2 films possess photocatalytically purifying,15 antibacterial,16 and self-cleaning17 properties. Commercial applications of TiO2 films on tiles and glass have already been reported.18 However, this is the first study of coating a TiO2 film on a highly porous glass nanoflake network. Our results show that both the photocatalytic activity and the photoinduced hydrophilicity of the TiO2 coating are enhanced. The increased surface roughness, high porosity, and large specific surface area are found to be beneficial for titanium dioxide photocatalysis. In addition, the formation mechanism of the ceramic nanoflake array film and its role in the surface reactions of the TiO2 coating are discussed. Experimental Section Preparation of Vertically Aligned Ceramic Nanoflake Array. SL-glass (72.5% SiO2, 13.7% Na2O, 9.80% CaO, 3.3% MgO, 0.4% Al2O3, 0.2% FeO/Fe2O3, and 0.1% K2O; Sail Brand, catalog no. 7101) was cleaned with 1.0 M HCl and absolute ethanol (12) Sigoli, F. A.; Feliciano, S.; Giotto, M. V.; Davolos, M. R.; Junior, M. J. J. Am. Ceram. Soc. 2003, 86, 1196. (13) Tomozawa, M.; McGahay, V.; Hyde, J. M. J. Non-Cryst. Solids 1990, 123, 197. (14) (a) Hoffmann, M. S.; Martin, T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69-96. (b) Fox, M. A.; Duby, M. T. Chem. Rev. 1993, 93, 341-357. (c) Kamat, P. V. Chem. Rev. 1993, 93, 267. (d) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735. (15) (a) Nam, H. J.; Amemiya, T.; Murabayashi, M.; Itoh, K. J. Phys. Chem. B 2004, 108, 8254. (b) Yu, J. C.; Ho, W.; Lin, J.; Yip, H.; Wong, P. K. Environ. Sci. Technol. 2003, 37, 2296. (c) Noguchu, T.; Fujishima, A.; Sawunyama, P.; Hashimoto, K. Environ. Sci. Technol. 1998, 32, 3831-3833. (d) Bount, M. C.; Kim, D. H.; Falconer, J. L. Environ. Sci. Technol. 2001, 35, 2988-2994. (e) Tada, H.; Tanaka, M. Langmuir 1997, 13, 360-364. (f) Blimes, S. A.; Mandelbaum, P.; Alvarez, F.; Victoria, N. M. J. Phys. Chem. B 2000, 104, 9851-9858. (16) (a) Sunada, K.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A. Environ. Sci. Technol. 1998, 32, 726-728. (b) Goswami, D. Y.; Trivedi, D. M.; Block, S. S. J. Sol. Energy Eng. 1997, 119 (1), 92-96. (c) Kikuchi, Y.; Sunada, K.; Iyoda, T.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 1997, 106, 51-56. (d) Hur, J. S.; Koh, Y. Biotechnol. Lett. 2002, 24, 23-25. (17) (a) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431-432. (b) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1992, 10, 135-138. (c) Sakai, N.; Wang, R.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Langmuir 1998, 14, 5918-5920. (d) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 3023-3026. (e) Watanabe, T.; Fukayama, S.; Miyauchi, M.; Fujishima, A.; Hashimoto, K. J. Sol.-Gel Sci. Technol. 2000, 19, 71-76. (f) Sun, R. D.; Nakajima, A.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 1984-1990. (18) (a) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1-26. (b) Ollis, D. F.; Al-Ekabi, H. Photocatalytic Purification and Treatment of Water and Air; Elsevier Sci. Pub.: New York, 1993.

Langmuir, Vol. 21, No. 8, 2005 3487 in an ultrasonic bath. The size of the SL-glass plate was 25.4 mm × 76.2 mm × 1.2 mm. After drying at 100 °C, the SL-glass was placed in a Teflon-lined stainless steel autoclave. Deionized water was added to the autoclave until 80% of its volume was filled. The autoclave was then heated to 180 °C, and the temperature was held for 3-12 h. After hydrothermal treatment, the SLglass was washed with ethanol and deionized water several times and then dried at room temperature. A quartz plate was also used instead of SL-glass as a reference. Preparation of TiO2 Coating on a Ceramic Nanoflake Array Support. The procedure for coating the TiO2 film on ceramic nanoflake arrays formed on SL-glass is similar to the one we previously used for the production of TiO2 thin films on glass.19 Titanium tetraisopropoxide (TTIP) was used as a titanium source. A total of 0.1 mol of TTIP was dissolved in 3.2 mol of absolute ethanol. After the alcohol solution was vigorously stirred for 2 h, 0.1 mol of acetylacetone and 0.1 mol of water were added. The resultant alkoxide solution was continuously stirred for 1 h at room temperature during hydrolysis and condensation of the titanium alkoxide. The chemical composition of the resulting TiO2 sol was TTIP/acetylacetone/EtOH/H2O ) 1:1:32:1. Ceramic nanoflake arrays formed on SL-glass were used as the substrate for dip coating. The TiO2 thin films deposited on the SL-glass were prepared from the above TiO2 sol solution by a dip-coating method in ambient atmosphere. The withdrawal speed was 4 mm/s. The thin films were dried at 100 °C for 60 min and then heat-treated in air at a heating rate of 3 °C/min to 500 °C. Characterization. XPS measurements were performed on a Phi Quantum 2000 system with a monochromatic Al KR source and a charge neutralizer. All the binding energies were referenced to a C(1s) peak at 284.8 eV of the surface adventitious carbon. The X-ray diffraction (XRD) patterns, obtained on a Philips MPD 18801 X-ray diffractometer using Cu KR radiation at a scan rate of 0.05° 2θ s-1, were used to identify the phase constitutions in the samples. The accelerating voltage and the applied current were 35 kV and 20 mA, respectively. High-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL 2010 transmission electron microscope operated with a 200-kV accelerating voltage. The sample was prepared on a carbon-coated copper grid by dispersing suspended nanoparticles onto the grid and evaporating the solvent. The elemental mapping was performed using a Gatan image filtering system. The general morphology and chemical composition of the thin films were examined by scanning electron microscope (SEM, LEO 1450VP) with an energy-dispersive X-ray spectrometer (Oxford Instrument) attached to the microscope. The samples were Au-coated before SEM imaging by a sputtering thin film coating system. The metal ions in the solution phase separated from the SLglass were analyzed by an inductively coupled plasma atomic emission spectrometer (ICP-AES; Thermal Jarrell Ash Atomscan 16 ICP spectrometer). Photocatalytic Activity Measurement. The photocatalytic activity experiments on TiO2 coatings for the oxidation of acetone in air were performed at ambient temperature using a 7000-mL reactor. Photocatalytic oxidation of acetone is based on the following reaction:20

CH3COCH3 + 4O2 f 3CO2 + 3H2O

(1)

A small amount of acetone was injected into the reactor. The reactor was connected to a dryer containing CaCl2 that was used for controlling the initial humidity in the reactor. The analysis of acetone, carbon dioxide, and water vapor concentration in the reactor was conducted online with a Photoacoustic IR Multigas Monitor (INNOVA Air Tech Instruments model 1312).21 The acetone vapor was allowed to reach adsorption equilibrium with the catalyst in the reactor prior to an experiment. The initial concentration of acetone after the adsorption equilibrium was about 400 ppm, which remained constant until a 15-W, 365-nm (19) (a) Yu, J. C.; Yu, J. G.; Zhao, J. C. Appl. Catal., B 2002, 36, 31-43. (b) Yu, J. C.; Ho, W.; Yu, J. G.; Hark, H. K.; Iu, K. S. Langmuir 2003, 19, 3889. (20) Zorn, M. E.; Tompkins, D. T.; Zeltner, W. A.; Anderson, M. A. Appl. Catal., B 1999, 23, 1-8. (21) Soria, J.; Conesa, J. C.; Augugliaro, V.; Palmisano, L.; Schiavello, M.; Sclafani, A. J. Phys. Chem. 1991, 95, 274-282.

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Figure 1. SEM images of large arrays of nanoflakes prepared by hydrothermal treatment at 180 °C for 6 h. (a) A face-on SEM image of nanoflake array films on the glass plate. (b) A SEM image with a 30° tilt toward the detector of the SEM. UV lamp (Cole-Parmer Instrument Co.) in the reactor was switched on. Integrated UV intensity in the range 310-400 nm striking the films measured with a UV radiometer (UVX, UVP, Inc., CA, U.S.A.) was 540 ( 10 µW/cm2, while the peak wavelength of UV light is 365 nm. The initial concentration of water vapor was 1.20 ( 0.02 vol %, and the initial temperature was 25 ( 1 °C. During the photocatalytic reaction, a near 3:1 ratio of carbon dioxide produced to acetone oxidized was observed,22 and the acetone concentration decreased steadily with increase in UV illumination time. Each reaction was followed for 60 min. The photocatalytic oxidation of acetone is a pseudo-first-order reaction, and its kinetics may be expressed as follows:14d,23 ln C/C0 ) -kt, where k is the apparent rate constant of pseudo-first order. C0 is the initial concentration (C0 ) 400 ppm ) 4.29 × 10-3 mol/L), and C is the reaction concentration of acetone. Measurements of Water Contact Angles. The water contact angles for freshly prepared TiO2 coatings on SL-glass were about 15-20°. However, when these samples were stored in the dark in air for 1 month, the water contact angles increased to 50-55°. To turn these slightly hydrophilic films into photoinduced hydrophilic ones, they were illuminated with a 15-W, 365-nm UV lamp (Cole-Parmer Instrument Co.) in an ambient environment. The intensity of UV light striking the films was about 540 ( 10 µW/cm2. The photoinduced hydrophilicity of films was evaluated by examining the change of the water contact angle. The sessile drop method was used for contact angle measurements with a contact angle meter (model CA-XP, Kyowa Interface Science Co., Ltd., Japan). The experimental error of the measurements was (1°. The droplet size used for the measurements was 5 µL. Water droplets were placed at five different positions for one sample, and the average value was adopted as the contact angle.

Results and Discussion The morphology of the ceramic film was examined under a SEM. When viewed from an angle perpendicular to the surface (Figure 1a), the film appears to contain uniform white stripes all across the surface. It is revealed that the white stripes are actually the sharp edges of uniform nanoflakes, mostly oriented perpendicular to the substrate. In contrast, no nano-architecture can be detected in the original SL-glass (not shown here). The oriented texture of the film can be better observed from the SEM image of the nanoflake array films on the glass plate with a 30° tilt toward the detector of the SEM (Figure 1b). Figure 2a clearly shows the morphology of the nanoflake array on the hydrothermally treated SL-glass before (the upper left corner) and after (the rest of the image) peeling off the top layer by using a cutter. The three-dimensional interconnected nanoflake network can be better observed from the exposed region. A higher magnification SEM (22) Yu, J. C.; Lin, J.; Lo, D.; Lam, S. K. Langmuir 2000, 16 (18), 7304-7308. (23) Fernandez, A.; Lassaletta, G.; Jimenez, V. M.; Justo, A.; Gonzalez-Elipe, A. R.; Herrmann, J.-M.; Tahiri, H.; Ait-Ichou, Y. Appl. Catal., B 1995, 7, 49-63.

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Figure 2. (a) SEM image of the nanoflake array on a hydrothermally treated SL-glass before (the upper left corner) and after (the rest of the image) peeling off the top layer by using a cutter. (b) A higher magnification SEM image of the exposed hydrothermally treated SL-glass surface.

image of the peeled surface (Figure 2b) shows that the film consists of a large amount of small clusters with the shape of leaves or petals. It should be emphasized that the nanoflakes interlace perpendicularly to form a threedimensional interconnected nanoflake network with a large surface-to-volume ratio. These very sharp and thinedged nanoflakes possess a porous structure, which is potentially a good adsorption site. To investigate how the oriented nanoflake array film was formed, we examined a sample prepared with a shorter hydrothermal reaction time (3 h). At this stage, only part of the glass surface contains the nanoflake structure. When the reaction time was increased to 12 h, the growth of nanoflakes on the entire surface formed a thick film, which could be easily removed from the substrate as a free-standing film. In contrast, the nanoflake array film formed in a shorter reaction time (6 h) adheres to the substrate strongly. Obviously, this result suggests that a hydrothermal process plays a key role in the nanoflake array formation. TEM allows us to gain an insight into the size, structure, and morphology of ceramic nanoflakes. TEM images with different magnifications (Figure 3a-b) indicate the filmlike morphology and uniformity of these high-quality nanoflake arrays. The nanoflakes show uniform size (200 nm in diameter) over the entire array and form a nestlike morphology. Figure 3c shows a HRTEM image looking down onto the sharp edge of a single nanoflake. The thickness of the nanoflake is about 4-5 nm at the center. The selected area electron diffraction pattern (Figure 3d) reveals the amorphous structure of the nanoflakes. Moreover, in the absence of Bragg peaks in the XRD (not shown here), the amorphous nature of the material before and after treatment was confirmed. The spatial distribution of different compositional elements in the nanoflake array was clarified by elemental mappings, which was performed using the O K edge (532 eV) and Si K edge (1839 eV), respectively. Figure 4 depicts the bright-field images together with the Si and O mappings of the same region from the nanoflake arrays. It shows that the nanoflake array structure is mainly composed of Si and O, which are homogeneously distributed over the nanoflakes. Noticeably, the hydrothermal process induces the formation of ceramic nanoflake arrays only by means of its influence on the surface structure and morphology of the SL-glass but not on the phase constitution and main composition. Chemical composition analysis using energy-dispersive X-ray (EDX) spectroscopy (Figure 5) illustrates that the original SL-glass is mainly composed of Si and O, with trace amounts of other metals. After hydrothermal treat-

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Figure 3. (a, b) TEM images of the ceramic nanoflake array prepared by hydrothermal treatment at 180 °C for 6 h. (c) HRTEM image of an individual nanoflake (3-4 nm in thickness). (d) Electron diffraction pattern.

ment, the EDX spectrum shows a slight increase in the O, Ca, Al, and Mg contents. It is important to note that no sodium was detected in the newly formed nanoflake arrays. These results are consistent with those of highresolution X-ray photoelectron spectroscopy (Figure 6), in which sodium is absent in the nanoflake array on the glass surface after the hydrothermal process. This suggests that the formation of the nanoflake array involves the change of metal content on the glass surface during the hydrothermal process. Conventionally, hydrothermal reaction is often used for the synthesis of nano-architectural materials with different compositions, structures, and morphologies at relatively lower reaction temperatures (generally below 250 °C). Because the properties of water, such as density, viscosity, and dielectric constant, are directly influenced by the high pressure and temperature conditions of the hydrothermal process, the process also allows the hydration of vitreous structure by absorption or chemical reactions between silica and water.24 Under hydrothermal conditions, water becomes supercritical.25 Therefore, water molecules are extremely reactive and can diffuse into the vitreous network, leading to the glass hydration. Increasing the amount of water in the glass matrix markedly decreases both glass-transition temperature and viscosity. Hence, ion exchange and phase separation of glass can occur at a relatively low temperature. When the reaction temperature is kept higher than the glass-transition temperature in a hydrothermal process, ion exchange takes place.25,26

Si-O-Na+ + H2O(g) T Si-OH + Na+ + OH-

(2)

Hydroxide ions are released into solutions as a result of alkali ion exchange with protons from supercritical water (reaction 2). This phenomenon was confirmed by monitoring the exchanged ions during the hydrothermal process by an ICP-AES. A significant increase of Na compared to (24) Doremus, R. H. Glass Science, 2nd ed.; Wiley: New York, 1994. (25) (a) Charles, R. J. J. Appl. Phys. 1958, 29, 1549. (b) Douglas, R. W.; El-Shamy, T. M. M. J. Am. Ceram. Soc. 1967, 50, 1.

other metal ions (e.g., Ca, Mg, and Al) was observed in the solution after the hydrothermal process. The results suggest the replacement of Na ions by protons because the proton can form a stronger bond with oxygen. This explains why no Na could be detected in the nanoflake array on the SL-glass surface. In addition, when the temperature is higher than the glass-transition temperature, the glass constituents are sufficiently mobile for the glass phase separation to take place. It has been shown that the water content of the gel formed by the glass-water reaction usually increases with the alkali content of the glass.27 Therefore, the gel layer formed on SL-glass satisfies a necessary condition of water content for phase separation during hydration. Once phase separation occurs, the soluble phase formed during the hydrothermal process can be leached by supercritical water that has a very high fluidity.28 There is evidence that the viscosity of water in the supercritical state can be 10-20 times less than that of water under normal temperature and pressure conditions.28 Such an increase in mobility causes water to become an etchant under supercritical conditions. After the leaching of soluble phases from glass, the remaining insoluble phases assemble into a threedimensional interconnected nanoflake network on the glass surface. It is important to note that the nanoflake array is not deposited on the substrate but is formed by the leaching of the soluble phase from glass during the hydrothermally induced phase separation process. Thus, most of the nanoflakes formed are perpendicular to the glass substrate. To clarify whether the change in surface content of glass by the hydrothermal process is a crucial step in the formation of the nanostructure on the SLglass surface, a control experiment was carried out using a pure silica substrate (quartz) instead of SL-glass as the starting material. It was found that no nanoflake array could be formed on the quartz surface after the hydrothermal process. This implies that the metal ions in the glass substrate are the dominant factors influencing the process. Although the formation mechanism for the nanoflakes is not yet fully understood at this stage, hydrothermally induced ion exchange between the glass and water as well as phase separation from glass by supercritical water are believed to play an important role in the growth of the ceramic nanoflake array. Because the present method allows the formation of an oriented porous nanostructural ceramic thin film on a glass substrate, the resulting materials may have potential applications as catalyst supports, vessels for drug delivery, and components in optoelectronic nanodevices. To test their applicability as a catalyst support, we coated a TiO2 thin film on a porous nanoflake array using the sol-gel method and studied its photocatalytic properties. The peaks in the XRD patterns (Figure 7a) correspond to the anatase TiO2 with different crystal planes (JCPDS 211272). This shows that an anatase TiO2 film was successfully coated on the nanoflake array support. The average crystallite size is 17.2 nm for the TiO2/ceramic nanoflake array, as calculated by applying the Scherrer equation to the full width at half-maximum of the (101) crystal plane of anatase TiO2. Obviously, the morphology of the nanoflake array was preserved (Figure 7b) even after calcination at 500 °C for the crystallization of TiO2. Furthermore, examination of the TiO2 coated ceramic nanoflake array using TEM indicates that the TiO2 (26) Tomozawa, M. J. Non-Cryst. Solids 1985, 73, 197. (27) Tomozawa, M.; Capella, S. J. Am. Ceram. Soc. 1983, C24. (28) (a) Franck, E. U. Pure Appl. Chem. 1970, 24, 13. (b) Franck, E. U. Pure Appl. Chem. 1987, 59, 25.

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Figure 4. (a, d) TEM image of ceramic nanoflake array. (b, e) Elemental O mapping of the images shown in parts a and d, respectively. (c, f) Elemental Si mapping of the images shown in parts a and d, respectively.

Figure 5. EDX spectra take of the silica matrix nanoflakes array (a) before and (b) after hydrothermal treatment at 180 °C for 6 h.

Figure 6. High-resolution XPS spectra of the (a) Al(2p), (b) Mg(2s), (c) Na(1s), (d) O(1s), (e) Si(2p), and (f) Ca(2p) regions for the ceramic nanoflake array after hydrothermally induced phase separation.

particles are coated on the surface of the ceramic nanoflake array (Figure 7b,d). In Figure 7c, a boundary was clearly observed between the regions with and without TiO2 coating. A HRTEM image of an individual TiO2-coated ceramic nanoflake is depicted in Figure 7e. Fringes corresponding to the anatase TiO2 are observed under higher magnification (Figure 7f). The fringe spacing of

Figure 7. (a) XRD patterns recorded from the TiO2/SL-glass and TiO2/ceramic nanoflake array/SL-glass. (b) SEM images of TiO2/ceramic nanoflake array after calcination at 500 °C. (c, d) Low-magnification TEM image of the TiO2/ceramic nanoflake array. (e) HRTEM image of an individual TiO2 coated ceramic nanoflake. (f) Higher-magnification TEM image of part e.

3.5 Å measured well matches the distance between the (101) crystal planes of TiO2 confirming the existence of TiO2.14

TiO2/Glass Nanoflake Array Films

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Table 1. Photocatalytic Activities of Samples samples

rate constant,a s-1 (10-5)

ceramic nanoflake array/SL-glass TiO2/SL-glass TiO2/ceramic nanoflake array/SL-glass

0 0.7 1.7

a Calculated from the average degradation rate (∆C/C ) of acetone 0 after 1 h of photocatalytic reaction; surface area of the samples ) 160 cm2.

To explore the effects of the morphology of the nanoflake array on the functionality of a TiO2 coating, the photocatalytic and hydrophilic properties of the samples were studied. It can be seen from Table 1 that the photocatalytic activity of the TiO2/glass nanoflake array is significantly higher than that without the ceramic nanoflake interlayer. The difference is approximately 2.4 times. This is probably due to the modification of the surface morphology of TiO2 from the ceramic nanoflake array interlayer. It is wellknown that excitation of TiO2 with photon energy higher than its band gap gives rise to excited-state electrons and holes at the conduction band and valence band, respectively. This can initiate various redox reactions producing strong oxidizing species such as hydroxyl radicals to decompose organic compounds. A porous structure is favorable in heterogeneous photocatalysis because the enlarged surface area facilitates the surface adsorption of organic species while multiple scattering of the porous network enhances light harvesting.29 The TiO2 coated nanoflake array has continuous pore channels, which are located throughout the film. This porous TiO2 framework allows fast diffusion of various gaseous reactants and products during photocatalytic reactions. Chemical reactions are most effective when the transport paths through which molecules move into or out of the nanostructured materials are included as an integral part of the architectural design.30 The three-dimensional interconnected nanoflake array channels in the porous TiO2 films serve as efficient transport paths for gaseous reactants and products in photocatalytic reactions, thus improving the photocatalytic activity. In addition, it should be mentioned that diffused sodium was found in the TiO2 films on both untreated and hydrothermally treated SL-glasses with concentrations of 3.9 atom % and 4.2 atom %, respectively. This indicated that both samples suffered from the same deteriorating effect of the diffused sodium.31 Thus, the diffused sodium was not the main reason for the lower activity of the untreated samples. Furthermore, utilizing a porous silica matrix as a support for TiO2 allows more effective adsorption of intermediates formed during the reaction. A coupled system of TiO2/SiO2 is, therefore, considered to be a more efficient photocatalyst for photocatalytic decomposition than TiO2 alone.32 The increase in photocatalytic efficiency is attributed to the presence of an adsorbent (SiO2). The adsorbent phase can trap the partially degraded products near the TiO2 sites. This can effectively prevent the discharge of harmful substances out of the system and allows more time for the photocatalytic reaction to proceed. Apart from the photocatalytic degradation of organic pollutants, wettability is also a very important property (29) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269. (30) Rolison, D. R. Science 2003, 14, 1698. (31) (a) Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A. J. Mater. Res. 1995, 10, 2842. (b) Paz, Y.; Heller, A. J. Mater. Res. 1997, 12, 2759. (32) (a) Chuan, X. Y.; Hirano, M.; Inagaki, M. Appl. Catal. B: Environ. 2004, 51, 255. (b) Anderson, C.; Bard, A. J. J. Phys. Chem. B 1997, 101, 2611. (c) Ding, Z.; Hu, X. J.; Lu, G. Q.; Yue, P. L.; Greenfield, P. F. Langmuir 2000, 16, 6216. (d) Miller, L. W.; Tejedor-Tejedor, M. I.; Anderson, M. A. Environ. Sci. Technol. 1999, 33, 2070.

Figure 8. Plots of contact angles of TiO2 films coated on SLglass and ceramic nanoflake array/SL-glass versus UV irradiation time: illumination intensity, 540 ( 10 µW/cm2 under ambient conditions (23 °C, relative humidity 75%, in air). The contact angle was measured at five different positions for each sample, and the average value was recorded.

of TiO2. The photoinduced hydrophilicity of TiO2 is a process distinct from the conventional photocatalytic oxidation reactions.15b,18a,33 Although a TiO2 surface is originally less hydrophilic, it becomes highly hydrophilic by its band gap excitation via UV light irradiation and afterward gradually reverts to the original state in the dark. Figure 8 shows the time dependence of water contact angles of TiO2 films coated on SL-glass and ceramic nanoflake array/SL-glass under UV irradiation of 540 µW/ cm2. The water contact angle of the TiO2/glass nanoflake array sample drops sharply from 51.4 to 16.8° within 20 min and then gradually decreases to a critical contact angle (the lowest value of the contact angle under UV irradiation) of 2-3° after 140 min. However, the contact angle of a reference TiO2 sample (without the ceramic nanoflake array) drops from 50.6 to only 32.5° and levels off at 6-7° after 140 min. To characterize the conversion quantitatively, the changes in contact angles during the first 20 min under UV illumination are compared. As shown in Figure 9, the hydrophilicizing rate (∆θ) of TiO2 films with a ceramic nanoflake array interlayer is about 1.5-2°/min, while the sample without an interlayer has a lower hydrophilicizing rate of 0.8-1.0°/min. Shown in Figure 10 are the reconversion processes, from a hydrophilic state to a hydrophobic sate, with dark storage time. During the first few days, the water contact angle of the TiO2/glass nanoflake array increases more slowly than that of the reference sample. Further increase is not observed from both samples after the water contact angles reach values close to those of their original states. Based on the above results, we conclude that the TiO2 coated ceramic nanoflake array exhibits a better photoinduced hydrophilicity, i.e., a lower critical contact angle, higher hydrophilicizing rate and lower reconversion rate to the hydrophobic state. UV irradiation generates electron-hole pairs in the TiO2 surface, and some of the holes can react with lattice oxygen to form surface oxygen vacancies. Meanwhile, water and oxygen may compete for surface adsorption. The defect sites are kinetically more favorable for water adsorption than oxygen adsorption.15b,17f At a result, the surface hydrophilicity is improved, leading to the water contact angle of TiO2 surfaces in all samples changing from about 50 to below 7°. It is important to note that the wettability of solid surfaces with water is governed not only by the (33) Miyauchi, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Chem. Mater. 2000, 12, 3-5.

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as the surface of the TiO2 coated on a glass nanoflake array is much rougher than that coated on bare SL-glass. The availability of surface hydroxyl groups as well as the rough surface nanostructures (or porous structure) favor the adsorption of water molecules and also reduce the rate of conversion from the hydrophilic to the hydrophobic state.38 Conclusion

Figure 9. Hydrophilicizing rate versus UV irradiation time interval of TiO2 films coated on the SL-glass substrate with and without the ceramic nanoflake array. The rates were estimated from the changes in the water contact angle in the first 20 min upon a 365-nm irradiation of 540 ( 10 µW/cm2 under ambient conditions (23 °C, relative humidity 75%, in air).

Figure 10. Changes in water contact angles of the photoinduced hydrophilic TiO2 films coated on the SL-glass substrate with and without a ceramic nanoflake array versus storage time in the dark under ambient conditions (23 °C, relative humidity 75%, in air). Water droplets were placed at five different positions for one sample, and the average value was adopted as the contact angle.

chemical properties of a surface but also by its geometry. According to Wenzel, as far as the geometry of a surface is concerned, the hydrophilic properties are enhanced by roughness.34-37 This explains why the TiO2/glass nanoflake array exhibits an enhanced photoinduced hydrophilicity

We have demonstrated the fabrication of well-aligned silica matrix nanoflake arrays supported on a glass substrate via a slow temperature hydrothermally induced phase separation approach. The leaching of the soluble phase from glass through a hydrothermal process results in the formation of uniform ceramic nanoflakes, which form a continuous porous three-dimensional network array with a large surface-to-volume ratio. More significantly, we believe that by means of this facile hydrothermal route, it is possible to prepare various silica matrix nanoarchitectural films on a solid substrate from ceramic glasses with different constituents. Furthermore, these nanostructural films can be used directly as a support for semiconductor coatings. Adapting this strategy, we successfully coated anatase TiO2 on the nanoflake array using the sol-gel method. Modification of the surface morphology of a photocatalyst has proven to be beneficial. The enhancements in photocatalytic activity and photoinduced hydrophilic conversion are attributed to the continuous porous threedimensional interconnected nanoflake network and its surface geometrical nanostructure. The former enhances light harvesting, facilitates the surface adsorption of organic species, and serves as an efficient transport path for photocatalytic reactions. The latter increases the surface roughness and enhances the photoinduced hydrophilicity. The novelty of the present approach is that it provides a simple route for the modification of semiconductor coatings with an oriented porous nano-architecture. This can be extended to prepare other oriented semiconductor/ ceramic nanostructural arrays and opens up opportunities for the design of novel optical/electronic materials. Acknowledgment. This work was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CUHK402904). LA047308E (34) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990. (35) (a) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466. (b) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988-994. (36) Miwa, M.; Nakaima, A.; Fujishima, A. Watanabe, T.; Hashimoto, K. Langmuir 2000, 16, 5754. (37) Nakajima, A.; Koizumi, S.; Watanabe, T.; Hashimoto, K. Langmuir 2000, 16, 7048. (38) Yu, J. C.; Yu, J. G.; Tang, H. Y.; Zhang, L. Z. J. Mater. Chem. 2002, 12, 81.