Light-Excited Superhydrophilicity of Amorphous TiO2 Thin Films

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Light-Excited Superhydrophilicity of Amorphous TiO2 Thin Films Deposited in an Aqueous Peroxotitanate Solution Yanfeng Gao, Yoshitake Masuda, and Kunihito Koumoto* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received August 5, 2003. In Final Form: January 14, 2004 We report on the photoinduced superhydrophilicity of the surface of amorphous TiO2. Amorphous TiO2 thin films were prepared on self-assembled monolayers by the peroxotitanate-complex deposition (PCD) and liquid-phase deposition (LPD) methods. The surface morphology and topography were characterized in detail. The contact angles were 34° and 66° for the as-deposited thin films through the PCD and LPD methods, respectively, which slowly increased to about 70° and 73° after being stored in air. After irradiation by UV light, the contact angle vanished and the surface exhibited superhydrophilicity. The superhydrophilicity and hydrophobicity could be switched by alternatively exposing the surface to UV light and drying in an atmosphere filled with organic gases. Although the oxidation of the contamination on the surface has effects on the increase in hydrophilicity, the X-ray photoelectron spectroscopy results suggested that the superhydrophilicity was also related to the transformation of the Ti-OH groups to groups that have dangling bonds. This paper indicates that an amorphous TiO2 thin film does not need to be heated to obtain superhydrophilicity; such a self-cleaning surface can be achieved at room temperature by our newly developed environmentally friendly method.

1. Introduction The understanding and fabrication of surfaces with superhydrophilicity or superhydrophobicity are of great interest for both fundamental research and practical applications.1-3 Both kinds of surfaces are approaches toward creating contamination-free or self-cleaning surfaces, which could be used as protective layers for roof or wall tiles, bathroom surfaces, or glass windows in highrise buildings. They can also be applied to miniaturized devices, where capillary forces significantly increase to unexpected levels. Whereas great efforts have been made to develop artificial self-cleaning surfaces, organisms have long employed such surfaces. The wings of a butterfly exhibit high hydrophobicity, which originates from their specific nanostructures.1k On these surfaces, water droplets * Author to whom correspondence should be addressed. E-mail: [email protected]. Fax: +81-52-789-3201. Tel: +8152-789-3327. (1) (a) Shibuchi, S.; Onda, T.; Satoh, N.; Tsuji, K. J. Phys. Chem. 1996, 100, 19512-19517. (b) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800-6806. (c) Wu, Y.; Sugimura, H.; Inoue, Y.; Takai, O. Chem. Vap. Deposition 2002, 8, 47-50. (d) Coupe, B.; Evangelista, M. E.; Yeung, R. M.; Chen, W. Langmuir 2001, 17, 19561960. (e) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 3213-3229. (f) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 70447047. (g) Aussillous, P.; Que´re´, D. Nature 2001, 411, 924. (h) Feng, L.; Li, S. H.; Li, H. J.; Zhai, J.; Song, Y. L.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2002, 41, 1221. (i) Wolansky, G.; Marmur, A. Langmuir 1998, 14, 5292. (j) Swain, P. S.; Lipowsky, R. Langmuir 1998, 14, 6772. (k) Gu, Z.-Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894-897. (2) Feng, L.; Song, Y.; Zhai, J.; Liu, B.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42, 800. (3) (a) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (b) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188. (c) Miyauchi, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Chem. Mater. 2000, 12, 3. (d) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 3023. (e) Stevens, N.; Priest, C. I.; Sedev, R.; Ralston, J. Langmuir 2003, 19, 3272.

disperse and easily remove dust particles. Structural observations and surface chemical analysis of the lotus leaf suggest that its self-cleaning process is dominated by the interdependence between the surface microstructure, reduced adhesion of dirty particles, and water repellency.4 Inspired by the lotus effect, researches have developed superhydrophobic surfaces by tuning surface forces through chemical modification or surface structure design.1,2 In comparison with superhydrophobic surfaces, the self-cleaning of which relies on the minuscule contact areas of the drops with the surface, superhydrophilic surfaces are easily wetted because of very small or vanished contact angles.5 Water drips off these surfaces, taking contaminants along with them. For most glass applications, not only wettability but also transparency or low scatter is essential. Recently, ultraviolet (UV) irradiation on titania has been found to induce a highly hydrophilic surface with a 0° contact angle for both water and oily liquid.3 The wettability is switchable between 0° (after UV irradiation) and 50-60° (after exposure to visible light or storage in the dark).6a Regardless of the high energy of the TiO2 surface, the clean TiO2 surface shows a water contact angle ranging from 10° to 72°.3e The superhydrophilicity of TiO2 has been reported to be ascribed to the chemisorption of the water molecules to the Ti3+ sites generated by photoreduction of surface Ti4+, which is closely related to the oxygen bridging sites of the crystal face.3 This hydrophilicity can be maintained by sunlight, so that contaminants are readily washed away by rainwater. The TiO2 thin film is transparent, making it possible to be used in glass or other applications requiring transparency, including self-cleaning and antifogging materials used in motor cars, buildings, and household glazing.6b (4) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (5) Blossey, R. Nat. Mater. 2003, 2, 301. (6) (a) Hattori, A.; Kawahara, T.; Uemoto, T.; Suzuki, F.; Tada, H.; Ito, S. J. Colloid Interface Sci. 2000, 232, 410. (b) O’Neill, S. A.; Parkin, I. P.; Clark, R. J. H.; Mills, A.; Elliott, N. J. Mater. Chem. 2003, 13, 56.

10.1021/la0303207 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/04/2004

Amorphous TiO2 Thin Films

TiO2 films are usually synthesized by vapor-phase deposition techniques7 such as chemical vapor deposition or atomic layer epitaxy, which require expensive highvacuum equipment and a great deal of energy. Novel direct low-temperature deposition processes for TiO2 thin films involve controlled hydrolysis of titanium species using an organic solvent8 or in an aqueous solution of TiCl4,9 TiF4,10 or (NH4)2TiF6.11 A complex peroxo precursor of titanium can also be employed for the deposition of TiO2 thin films by several methods, such as the electrochemical deposition12a and self-assembled monolayer technique.12b The peroxotitanium-type solution is usually prepared by adding droplets of pure TiCl4 to an ice-cooled aqueous solution containing H2O2 with or without excess acid.12 We invented a novel solution system and succeeded in direct deposition of a transparent, pure TiO2 thin film in an aqueous peroxotitanate solution, which was prepared by dissolving metatitanic acid (H2TiO3) in a mixture solvent of concentrated H2O2 and NH3‚H2O aqueous solutions.13,14 The dissolved peroxotitanate complexes could slowly form a solid by decomposition of the peroxo groups and/or release of the hydroxyl group by decreasing the pH of the solution. We call this process as peroxotitanate-complex deposition (PCD). The as-deposited thin film deposited by the PCD method was amorphous and crystallized into anatase TiO2 after heating at 300 °C for 1 h in air. For the as-deposited thin film, a chemical composition of TiO1.4(O2)0.5(OH)0.2‚1.34H2O was proposed based on X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, thermogravimetric analysis, and differential thermal analysis. The precipitate collected after deposition of the TiO2 thin film can be dissolved in the mixture of the H2O2 and NH3‚H2O aqueous solution and employed for TiO2 deposition again. The present method is simple, inexpensive, and environmentally friendly, showing the potential for industrial application. So far, almost all of the studies on the superhydrophilicity of TiO2 have examined a single crystal or polycrystal; there have been only a few studies on the superhydrophilicity of amorphous TiO2.15 The studies usually focused on wettability mechanism, almost ignoring the film preparation process for creating a practical, cheap selfcleaning surface. Amorphous, microporous titania modified with transition-metal salts has been found to generate photoelectrochemical and photochemical effects with visible light,16 and the effect of light irradiation on amorphous TiO2 is an interesting subject of study. Premodification of the substrate, typically Si with a native silica layer, by a hydrophilic self-assembled monolayer (SAM), was shown to improve the growth rate and (7) (a) Klaus, J. W.; Sneh, O.; Goerge, S. M. Science 1997, 278, 1934 and references therein. (b) Hitchman, M. L.; Alexandrov, S. E. Interface 2001, 10, 40. (8) Masuda, Y.; Jinbo, Y.; Yonezawa, T.; Koumoto, K. Chem. Mater. 2002, 14, 1236. (9) (a) Zheng, Y.; Shi, E.; Chen, Z.; Li, W.; Hu, X. J. Mater. Chem. 2001, 11, 1547. (b) Kim, K. J.; Benkstein, K. D.; Lagemaat, J. V. D.; Frank, A. J. Chem. Mater. 2002, 14, 1042. (10) Shimizu, K.; Imai, H.; Hirashima, H.; Tsukuma, K. Thin Solid Films 1999, 351, 220. (11) Deki, S.; Aoi, Y.; Hiroi, O.; Kajinami, A. Chem. Lett. 1996, 6, 433. (12) (a) Zhitomirsky, I.; Gal-Or, L.; Kohn, A.; Hennicke, H. W. J. Mater. Sci. 1995, 30, 5307. (b) Niesen, T. P.; Joachim, B.; Fritz, A. Chem. Mater. 2001, 13, 1552. (13) Gao, Y.-F.; Masuda, Y.; Peng, Z.-F.; Yonezawa, T.; Koumoto, K. J. Mater. Chem. 2003, 13, 608. (14) Gao, Y.-F.; Masuda, Y.; Koumoto, K. Chem. Mater., in press. (15) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F.; Diebold, U. J. Phys. Chem. B 1999, 103, 5328. (16) Kisch, H.; Zhang, L.; Lange, C.; Maier, W. F.; Antonius, C.; Meissner, D. Angew. Chem., Int. Ed. 1998, 37, 3034.

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adhesion of the thin film.14,17-23 In the present paper, we describe that a surface of high hydrophilicity can be realized by UV irradiation of an amorphous TiO2 film, which was fabricated on the SAMs directly in an aqueous solution. A comparative study suggests that the superhydrophilic TiO2 surface created by our environmentally friendly process is a promising candidate for self-cleaning layers. 2. Experimental Section Deposition of the TiO2 Thin Film by the PCD Method. SAMs of octadecyltrichlorosilane (Acros Organics, Fair Lawn, NJ) were prepared on the p-Si substrate (Shinetsu; resistivity ) 4-6 Ω cm) and UV-modified by the method described in other papers.14,17-23 The preparation of the samples has been described in our previous paper.13,14 Briefly, an appropriate amount of H2TiO3 (80%; Mitsuwa) was added to an ice-cooled solvent consisting of 25 cm3 of H2O2 (30% in H2O; Mitsubishi) and 5 cm3 of ammonia (25% in H2O; Kishida). The mixture was stirred for 90 min, a homogeneous pale yellow-green solution was obtained. This homogeneous solution was then diluted with deionized water (>18 MΩ cm) to 0.01 M Ti4+ at pH 2.0 (adjusted with the addition of an appropriate amount of HNO3). The cleaned substrate (ptype Si; Shinetsu) was then floated on the surface of the diluted solution for several hours to 120 h at room temperature (∼24 °C) to deposit a thin film. After being soaked, the substrate was carefully rinsed with distilled water before drying at 50 °C in a dark room under vacuum for 12 h. Deposition of the TiO2 Thin Film by the LPD Method. For comparison, TiO2 thin films were also prepared by the liquidphase deposition (LPD) method.24 The deposition was conducted in an aqueous solution consisting of 0.05 M (NH4)2TiF6 and 0.15 M H3BO3. The substrate was floated on the surface of the prepared solution with the SAM surface side facing down at room temperature for 48 h. The film was rinsed and dried in the same conditions as we used for creating the PCD-derived TiO2 thin film. Before characterization, the amorphous nature of the film was checked by XRD measurements. Characterization Techniques. The morphology of the film was measured with a scanning electron microscope (SEM; Hitachi). The chemical composition was characterized with an X-ray photoelectron spectroscope (ESCA-3200; Shimazu; 8 kV; 30 mA; pass energy ) 75 eV) using Mg KR as the x-ray source; all of the spectra were referenced to the C1s signal at 284.6 eV. A water contact angle (CA) was measured using CA-D (Kyowa Interface Science). For the formation of the superhydrophilic surfaces, the film was exposed to UV light for 1-5 min (wavelength centered at 184.9 and 253.7 nm, respectively; NLUV253, Nippon Laser & Electronics Lab; 1.8 mW cm2-) in atmospheric air at a relative humidity of 40% and room temperature. O2 decomposes after UV irradiation at 184.9 nm and partially forms O3. Most O3 is decomposed after UV irradiation at 253.7 eV. Friction-force microscopy (FFM) measurements were employed to distinguish the differences in the microstructure between hydrophilic and hydrophobic regions on the sample surface. When the cantilever scans a hydrophilic region, a water meniscus will form because of the capillary and superficial tension forces, which will generate an additional loading force and effectively increase the tip-sample friction force. However, a hydrophobic region usually demonstrates a (17) Gao, Y.-F.; Masuda, Y.; Koumoto, K. J. Korean Ceram. Soc. 2003, 40, 213-218. (18) Koumoto, K.; Seo, S.; Sugiyama, T.; Seo, W. S.; Dressick, W. J. Chem. Mater. 1999, 11, 2305. (19) Masuda, Y.; Sugiyama, T.; Koumoto, K. J. Mater. Chem. 2002, 12, 2643. (20) Gao, Y.-F.; Masuda, Y.; Yonezawa, T.; Koumoto, K. J. Ceram. Soc. Jpn. 2002, 110, 379. (21) Gao, Y.-F.; Masuda, Y.; Yonezawa, T.; Koumoto, K. Chem. Mater. 2002, 14, 5006. (22) Gao, Y.-F.; Masuda, Y.; Koumoto, K. Chem. Mater. 2003, 15, 2399. (23) Shirahata, N.; Masuda, Y.; Yonezawa, T.; Koumoto, K. Langmuir 2002, 18, 10379. (24) Pizem, H.; Sukenik, C. N.; Sampathkumaran, U.; McIlwain, A. K.; De Guire, M. R. Chem. Mater. 2002, 14, 2476.

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Figure 1. AFM images of the amorphous TiO2 thin films deposited by the LPD method (a) and the PCD method (b). The measured areas were 10 × 10 µm2 for the LPD-derived film and 500 × 500 nm2 for the PCD-derived TiO2 thin film. The images clearly show the difference in topography; the PCD-derived film demonstrated nanostructure characteristics and had a flat surface. However, the LPD-derived film composed of micrometer particles gave a much larger roughness. lower friction force. Atomic force microscopy (AFM)/FFM measurements (Seiko Instruments Inc., SPI 3800N) were performed with an Si3N4 cantilever (NP-S, Nanoprobes Digital Instruments Inc.; torsion spring constant k ) 0.03 N/m) and a sharpened tip (nominal curvature radius ) 5-40 nm) at a relative humidity of about 40% in ambient air. The typical normal load applied was 0.1 nN, which was much lower than the additional loading force generated by the capillary meniscus (about 6 nN). Hence, the total loading force depended on the capillary-meniscusinduced one, which would be further reflected on the difference in the friction force.

3. Results and Discussion Surface Topography and Morphology. Figure 1 shows the difference in the surface topography of the LPDand PCD-derived TiO2 thin films. The particle size for the LPD-derived thin film ranged from 0.5 to 1 µm in diameter, which was much larger than 10-30 nm for the PCDderived TiO2 thin film. The difference in the particle size reflected various deposition behaviors under the present conditions; the nuclei preferentially grew by consumption of ions or molecules generated in the solution, whereas a large quantity of the nuclei formed in the PCD solution and filled in the voids rather than coalescing to the alreadyformed particles to make them grow. The roughness, rootmean-square (rms), of the LPD-derived thin film was larger than that of the PCD-derived one. For the same measured area of 10 × 10 µm2, rms were 30-50 nm and 10-15 nm, respectively. For the various measured areas of 500 × 500 nm2, the rms for the PCD-derived thin film was as low as 4-8 nm. The different surface characteristics should affect the hydrophilicity. The film morphology was further characterized by SEM. Small cracks could be observed on the surface of the PCDderived TiO2 thin film (Figure 2a). However, no cracks were observed at low magnifications (see inset images). Conversely, large cracks appeared in the LPD-derived thin film, which separated the film into several unattached areas (Figure 2b). The film thickness directly obtained by the SEM observation (Figure 2c,d) indicated that the PCDderived TiO2 thin film was about 550 nm thick (Figure 2c), which was slightly thicker than 480 nm for the LPDderived one (Figure 2d). Superhydrophilicity. The contact angles for the asobtained thin film were about 66° (LPD, result not shown) and 34° (PCD, Figure 3a), respectively. It increased to 70-75° after being stored in air for 2-3 days. The release

of water from the as-obtained thin film after drying is probably attributable to the slight increase in the contact angle, whereas the adsorption of contaminants during storage resulted in a further increase in the contact angle. Note that the contact angle for the as-obtained film could still be measured, suggesting that the surface was not completely wetted. UV irradiation for 5 min induced a superhydrophilic surface with the vanished contact angle (Figure 3b). After storage in a drybox filled with organic gases such as tolune and silanes, the contact angle increased to 90° (Figure 3c). However, the contact angle of zero was still obtained after UV irradiation for 1 min (Figure 3d). Hence, a superhydrophilic surface was created just by UV irradiation, and the superhydrophilicity could be controlled reversibly by alternate UV irradiation and contaminant adsorption. The solid surface with switchable wettability enables us to selectively modify the target surfaces, while leaving other surfaces or components unchanged. This is more beneficial than the liquid surfactants, which are widely used for surface/interfacial modification. The surfactant can adsorb on different interfaces and targets, making selective modification difficult. The present results suggest that crystalline TiO2 is not always necessary for a self-cleaning or antifogging surface; that is, the crystallization process can be eliminated to achieve superhydrophilicity by heating the amorphous film to high temperatures. Wenzel25 was the first to conclude that the contact angle of a liquid varied on surfaces with different roughness, which suggested that a rough surface made of a material with a contact angle of less than 90° was more hydrophilic than a flat one. Regardless of their different surface morphology, the LPD-derived TiO2 thin films also showed UV-induced superhydrophilicity, revealing that wettability can be generated for the different amorphous TiO2 thin films with various chemical compositions and microstructures. However, one should still note that the surface morphology might affect the flow of water off of the surface. When a similar volume of deionized water was dripped onto the surfaces of LPD-derived TiO2 thin film, the area of water spread out was smaller than that of the PCD-derived TiO2 thin film. This finding suggests that a rough surface of TiO2 might show a low cleaning ability and be unable to let water flow off of it. (25) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.

Amorphous TiO2 Thin Films

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Figure 2. SEM photographs of the PCD- (a, c) and LPD-derived (b, d) TiO2 thin films deposited at room temperature for 120 h (PCD) and 48 h (LPD). For cross-sectional observation, the specimens were tilted at about 75°. The film thickness was directly measured in the cross-sectional photographs to be about 550 and 480 nm for PCD- and LPD-derived thin films, respectively. Both of the two films were observed with cracks in the surface, while the cracks in the PCD-derived thin films were superficial and small. Conversely, the cracks in the LPD-derived TiO2 thin film were large and grew across the film (b, d).

Figure 3. Water contact angles (WCAs) for the PCD-derived TiO2 thin film (a) and after UV irradiation for 5 min in air (b). The as-obtained TiO2 thin film possessed a WCA of about 34°. However, after UV irradiation, the WCA vanished, and the water drop spread out on the surface of the film. After the film was stored in a drybox filled with organic gases at 150 °C for 5 min, the WCA increased to about 93°, suggesting that the surface was hydrophobic (c). However, a highly superhydrophilic surface was still achieved after UV irradiation, even for as short as 1 min, implying that the hydrophilicity of the surface is switchable (d).

Changes of Surface Morphology and FrictionForce Distribution after UV Irradiation. FFM was further employed to clarify the UV-induced microstructure differences for the PCD-derived TiO2 thin film (Figure 4b,d). For comparison, the AFM images (Figure 4a,c) for the corresponding areas are also shown. In the measured area, the film was composed of nanosized particles of 1015 nm in diameter and exhibited a dense surface (Figure

4a). After UV irradiation, the mean particle size seems to increase slightly. However, because of the effect of the static charge distribution on the surface and/or shape and size of the cantilever, the AFM image does not always reveal the actual size and shape of the particles. It is difficult to directly observe the surface change by SEM in such a small dimension (particle size ) 10-15 nm in diameter). Therefore, we cannot clarify whether the change in size can be attributed to the effect of the friction force (Figure 4c). The rms roughness was 3.0-4.1 nm for the measurement area of 150 × 150 nm2. The distribution of the friction force is shown in the corresponding FFM images (Figure 4b,d). The FFM images reveal that the friction force seems homogeneous for the measured areas regardless of UV irradiation. Although the distribution of the friction force was heterogeneous for a particle, it appears to be a little small along the boundaries between the particles. However, the presence of discrete rectangular hydrophilic (30-80 nm) domains along a specific direction that have been observed for a rutile TiO2(110) single crystal were not observed in our amorphous TiO2 thin film.3a Both contact-angle measurements and FFM results suggested that the superhydrophilicity effects were not specific to crystalline TiO2 but were perhaps more general. Effect of UV Irradiation on the Chemical Changes of the Surface. XPS measurements were conducted to analyze the changes of the chemical groups before and after UV irradiation (Figure 5). According to the former studies,13,14 the dissolution of H2TiO3 was a result of the formation of peroxotitanate ions represented by [TiO(O2)(OH) 2]2- at high pH (>10). The existence of hydroxyl and peroxo groups in the chemical composition of the peroxotitanate complex suggests that low temperatures and high pH values are favorable for the stability of the

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Figure 4. AFM (a, c) and FFM (b, d) images of the as-deposited PCD-derived TiO2 thin film before (a, b) and after (c, d) UV irradiation for 5 min. All of the scanning areas were 150 × 150 nm2.

peroxotitanate complex. In fact, this species can be stable for several months at low temperatures, usually 10. However, the slow decomposition to the solid phase still occurred even under these conditions. When the temperature or pH is increased or lowered, respectively, the OH and peroxo groups will be released. As a result, the corresponding peroxotitanate complex decomposes and forms a solid phase. The Ti2p spectra consisted of two peaks of Ti2p3/2 and Ti2p1/2 located at 458.7-458.9 eV and 464.6-464.9 eV, respectively, for both LPD- and PCD-derived TiO2 thin films (Figure 5). One should note that residual fluorine was also detected for the LPD-derived TiO2 thin film even after UV irradiation. Conversely, no toxicant was employed during film deposition, and no residue was observed in the film using the PCD method. Wang et al. measured a single crystal by XPS and proposed that the existence of Ti3+ after UV irradiation is important for the formation of superhydrophilicity.3b In fact, the photoinduced chargetransfer process usually resulted in the reduction of Ti4+ to Ti3+.26 Our as-deposited thin films are amorphous; therefore, there should be a larger concentration of broken (26) Anpo, M.; Che, M.; Fubini, B.; Garrone, E.; Giamello, E.; Paganini, M. C. Top. Catal. 1999, 8, 189-198.

bonds and other defects related to Ti3+ rather than those found in the annealed crystalline films, although no peak fitting was conducted for our as-deposited film because the equipment broke down. Detailed analysis was conducted to clarify the UV-induced changes of the oxygen content. For the as-deposited TiO2 thin film by the PCD method, three states of oxygen bound to titanium were detected. Peak separation of the O1s spectrum clearly showed three kinds of oxygen with binding energies of 530.4, 532.3, and 533.6 eV, with respective contributions of 50.3%, 39.6%, and 10.1% (Figure 6a). The peak attributed to the Si substrate was not observed, suggesting that no O1s peak could be ascribed to the native silica layer of the substrate. The peak located at about 530.4 eV, which decreased slightly to 530.2 eV after UV irradiation, could be assigned to the TiIV-O bonds (529.7530.4 eV for TiO2).17,27,28 The peaks located at about 532 eV (532.3 and 531.6 eV for the surfaces before and after UV irradiation, respectively) could be assigned to the hydroxyl groups.13,14,29 The highest binding energies, 533.6 and 532.9 eV, for the surfaces before and after UV (27) Huang, D.; Xiao, Z. D.; Gu, J.-H.; Huang, N.-P.; Yuan, C. W. Thin Solid Films 1997, 305, 110. (28) Zhang, F.; Jin, S.; Mao, Y.; Zheng, Z.; Chen, Y.; Liu, X. Thin Solid Films 1997, 310, 29.

Amorphous TiO2 Thin Films

Figure 5. XPS spectra of the Ti2p range for the PCD- (a, b) and LPD-derived (c, d) TiO2 thin films before (a, c) and after (b, d) UV irradiation.

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OH (531.9 and 531.7 eV before and after UV irradiation, respectively) and Ti-O (530.0 and 530.2 eV before and after UV irradiation, respectively). Possible Formation Mechanism of Superhydrophilicity. The oxidation of surface contaminants has effects on the decrease in the water contact angle. However, it is difficult to induce superhydrophilicity, that is, a zero contact angle. Two obvious changes occurred after UV irradiation for both LPD- and PCD-derived TiO2 thin films based on XPS results. The peaks for Ti-OH shifted to the lower-binding-energy side, and their intensity weakened after UV irradiation. Quantitative analysis (by area) showed that the atomic ratio of Ti-OH decreased typically from 39.6% to 28.4% (PCD). Meanwhile, the ratio for Ti-O increased from 50.3% to 61.8% (Figure 6a,b). A similar trend was also observed for the LPD-derived thin film (see the corresponding values in Figure 6c,d). This result does not comply with the conventional understanding about the origin of hydrophilicity. Generally, hydrophilicity was considered to be generated through the formation of hydrophilic groups on the surface of the oxide, such as Si-OH and Ti-OH for SiO2 or TiO2, respectively. The typically hydrophobic surface of poly(ethylene terephthalate) (PET) can therefore be modified to be hydrophilic by hydrolysis, reduction, and aminolysis.31 These treatments involve the formation of the hydrophilic groups on the PET surface. The changes of the relative amount of the Ti-OH and Ti-O bonds show that the photoirradiation resulted in condensation of the hydroxyl groups, implying that the irradiation-induced wettability conversion is related to the transformation of Ti-OH to the Ti-O-Ti groups or the formation of the dangling bond. Annealing the as-deposited thin film results in the cut of the Ti-OH bond and generation of the Ti-O-Ti bond, while the superhydrophilicity cannot be induced for this surface in the absence of UV irradiation. Furthermore, the Ti-OH group will regenerate after wetting a UV-irradiated TiO2 surface, but the superhydrophilicity can still be preserved. These results suggest that the formation of the dangling bond rather than that of Ti-O-Ti may be a more possible reason for the generation of photoinduced superhydrophilicity. A water molecule easily adsorbs to the dangling bond,32 and the surface exhibits superhydrophilicity. However, such a dangling bond is unstable, and the superhydrophilicity will become invalid, implying that repeat irradiation is needed to transform the surface to superhydrophilic again. 4. Conclusions Amorphous TiO2 was found to exhibit UV-light-induced superhydrophilicity. TiO2 thin films were prepared on selfassembled monolayers by the LPD and our newly developed PCD methods at room temperature. The PCD method employed a peroxotitanate solution, which was prepared by dissolving H2TiO3 in a solvent mixture of ammonia and peroxohydrogen aqueous solutions. The microstructure, topography, and thickness were characterized for the as-deposited thin films and those after UV irradiation in detail. The contact angle was 34° and 66° for the asdeposited films prepared by the PCD and LPD methods, respectively. After a period of time in air, the corresponding

Figure 6. XPS spectra of the O1s range for the PCD- (a, b) and LPD-derived (c, d) TiO2 thin films before (a, c) and after (b, d) UV irradiation.

irradiation, respectively, could be assigned to the O22groups (532-533 eV).30 Compared to the PCD-derived TiO2 thin film, O1s of the LPD-derived TiO2 thin film can be separated into two peaks (Figure 6c,d), attributed to Ti-

(29) Yu, J. C.; Zhang, L.-Z.; Zheng, Z.; Zhao, J.-C. Chem. Mater. 2003, 15, 2280. (30) Rao, C. N. R.; Ganguly, P.; Hegde, M. S.; Sarma, D. D. J. Am. Chem. Soc. 1987, 109, 6893. (31) Fadeev, A. F.; McCarthy T. J. Langmuir 1998, 14, 5586 and references therein. (32) Jolivet, J.-P.; Henry, M.; Livage, J. Metal Oxide Chemistry and Synthesis; John Wiley & Sons: Chichester, U.K., 2000; p 211.

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contact angle increased to 70° (PCD) and 73° (LPD). However, superhydrophilic surfaces with vanished contact angles were achieved after UV irradiation (wavelength ) 253 nm) for as short as 1 min, suggesting that the superhydrophilicity was not specific to crystalline TiO2. The superhydrophilicity could be controlled reversibly. Direct observation of the microstructure changes using FFM failed to confirm the presence of the hydrophilic domains, which were reported for single-crystal TiO2, implying that the proposed mechanism is not universally applicable but is more complex than previously thought. X-ray photoelectron spectra detected a decrease in the amount of the Ti-OH bonds and an increase in the Ti-O

Gao et al.

bonds (dangling bond rather than that of the Ti-O-Ti group), suggesting that the superhydrophilicity may be closely related to this transformation. Note Added after ASAP Posting. This article was released ASAP with errors in text citations to Figure 2, in the second paragraph of Results and Discussion, and Figure 6, in the last paragraph of the subheading Effect of UV Irradiation on the Chemical Changes of the Surface in Results and Discussion. The correct version was posted March 18, 2004. LA0303207