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Is There Really a Photo-Induced Hydrophilic Effect on Titanium Dioxide

CAMD/CINF seminar by Donald A. Tryk, Kanagawa Academy of Science and Technology & Tokyo Metropolitan University, Japan.

Abstract
More than ten years ago, it was noticed that UV illumination of TiO2 made it hydrophilic, with the water contact angle approaching 0º. Initially, it was assumed that this was due to the well-known photocatalytic decomposition of adsorbed organic molecules. Then, various experiments were done that convinced people that there was an effect separate from photocatalysis. More recently, the research groups of Henderson and of Yates have re-examined this phenomenon and concluded that the original assumption is correct, i.e., there is just one effect-photocatalysis. The slow increase in the contact angle during storage in the dark, they propose, is due to the slow accumulation of an organic contaminant layer. Although this can certainly happen, this is not the end of the story. There is evidence that the contact angle can be cycled at will with alternating UV and visible illumination. Also, there are some very difficult-to-explain results, for example, the beneficial effect of tensile stress, the increased hardness of the TiO2 surface, and the increased lattice parameter. Recently, we have developed a new, modified model that could help to explain these results. This model takes into consideration the fact that both oxidation sites and reduction sites must exist side-by-side at the nanometer scale.