Wettability of photoresponsive titanium dioxide surfaces

According to K. Hashimoto and co-workers, the wettability of TiO2 surfaces can be altered by irradiation. Surfaces become hydrophilic (water contact angle, similar to0degrees) after UV irradiation and gradually revert to a more hydrophobic state (contact angles, 50-70degrees) when left in the dark or exposed to visible light. Such changes have been observed for both anatase and rutile surfaces (single crystals and polyerystals) 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 (similar to50degrees) 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 mum in diameter) on a hydrophilic TiO2 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.