Wettability of Photoresponsive Titanium Dioxide Surfaces N. Stevens, C. I. Priest, R. Sedev, and J. Ralston* Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia Received July 22, 2002. In Final Form: November 18, 2002 According to K. Hashimoto and co-workers, the wettability of TiO2 surfaces can be altered by irradiation. Surfaces become hydrophilic (water contact angle, ∼0°) after UV irradiation and gradually revert to a more hydrophobic state (contact angles, 50-70°) when left in the dark or exposed to visible light. Such changes have been observed for both anatase and rutile surfaces (single crystals and polycrystals) and presumably are not directly related to the photocatalytic activity of TiO2. We report here similar changes of the contact angle of water on flat titania wafers and on densely packed layers of micron-sized titania particles. Wettability changes can be reversibly cycled, and the effect is rather robust. The hydrophobic-to-hydrophilic conversion is faster than the inverse one (at comparable UV and visible light intensities). The contact angle change observed on wafers (∼50°) is twice as large as that on the particle layer, but this is mainly related to the effect of roughness. Using a photomask, we have patterned successfully regular arrays of hydrophobic circles (10 μm in diameter) on a hydrophilic TiO 2 matrix and vice versa. These findings are of significant interest for the design of intelligent surfaces even though the detailed mechanism of the transition is yet to be elucidated. Introduction The wettability of materials plays an important role in nature and technology. There is a growing necessity to understand and possibly control the contact angle between a liquid and a solid surface. Since devices are progressively miniaturized, capillary forces and hence contact angle are becoming more and more important. Surfactants are widely used to modify interfacial properties, but they often adsorb on different interfaces, not just the target one, could modify the bulk solution properties, and may interact with other components thus creating side effects. This explains the strong interest in solid surfaces whose wettability can be controlled reversibly by an external stimulus (e.g., radiation, electric field, etc.). Recently Hashimoto and co-workers have published extensively on the hydrophilic-hydrophobic conversion of flat titanium dioxide layers induced by irradiating the dry sample with light of a specific wavelength. 1-8 The contact angle of water on a clean TiO 2 surface can be repeatedly cycled between practically zero (after UV irradiation 1-8 ) and 50-60° (after irradiation with visible light or storage in the dark 4,6,8 ). The wettability changes occur for both anatase and rutile surfaces (single crystal 4 or polycrystalline 5 ) and appear to be independent of their photocatalytic activity. 5 These findings are rather unexpected. The TiO 2 surface as a metal oxide surface is a high-energy one and therefore should be fully wetted by most liquids. 9,10 However, reports about the contact angle of water on a clean (freshly prepared) titania surface are contradictory: 72°, 1 60°, 11 52°, 12 50°, 2 15°, 3,13 less than 10°, 14 and 0°. 15 A useful measure of the wettability of high-energy surfaces is the heat of wetting (defined as the difference between the total surface energy of the wetted and dry solid 16 ). The heat of wetting of fully hydroxylated amorphous silica in water is 160 mJ/m 2 , 17 and the surface of clean silica is undoubtedly hydrophilic. The heat of wetting of anatase in water is 520 mJ/m 2 , 18 thus confirming the above expectation. A very simple, if somewhat crude, explanation can be given to the influence of light irradiation as follows. Organic contaminants are always accumulating at high- energy surfaces, 19 and TiO 2 is no exception to this rule. 15 Under UV irradiation of the (photocatalytic) TiO 2 surface, these are decomposed. The surface becomes more wettable simply because it is cleaner. During storage in a laboratory atmosphere, contaminants are gradually adsorbing at the surface, thus making it more hydrophobic (titania surfaces are easily contaminated within few days; 15 as a matter of fact, they remain hydrophilic only a few minutes after cleaning 14 ). Hashimoto and co-workers argued that this is not the case since (i) the contact angle before and after cleaning * Corresponding author. Tel: +61 8 8302 3066. Fax: +61 8 8302 3683. E-mail: john.ralston@unisa.edu.au. (1) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. 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