2.Super-hydrophilic phenomenon with the photocatalyst In the usual environment, a surface of material repels the water to a certain extent. Fig. 1 shows the shape of the waterdrops on the surfaces of glass, resin and hydrophobic resin. Hydrophilicity of a material can be represented with a contact angle of water with the material. The contact angles of water with inorganic materials, such as glass are 20-30 degrees. Also, the contact angles of water with usual resins and hydrophobic resins, such as silicone resin or fluorocarbon polymer, are 70-90 degrees and more than 90 degrees, respectively. Until now, there are few materials that have the contact angle of water less than 10 degrees, except water absorptional materials or activated surface using interface activator. However, these materials have little durability and the low contact angle values do not last for a long time. Our modified TiO2 photocatalyst thin film has interesting property of super-hydrophilicity. At first, the contact angle of water with the TiO2 thin film is several tens degrees. Then, by the irradiation of UV light, the contact angle decreases gradually, and at last, it reaches 0 degree. After that, the contact angle remains only several degrees for scores of hours without the irradiation of UV light. Besides, even if the contact angle increases, it easily decreases again only by the irradiation of UV light. Therefore, the modified TiO2 thin film is the first truly practical material that realizes the super-hydrophilicity. Fig. 2 shows the change of the contact angle of water with a TiO2-silicone thin film by UV irradiation. Before UV irradiation, the contact angle with the film is more than 90 degrees, then it gradually decreases to 0 degree with UV irradiation and the film never repels water. Now, we are theoretically studying the rationale of these phenomena. The super-hydrophilicity should be caused by the function of photo-catalyst. As TiO2 photocatalyst decomposes hydrophobic molecules those originally existing on the surface of material, very thin film of physisorbed water forms on the surface. We presume that the water thin film is the origin of the super-hydrophilicity. Fig. 3 shows the schematic diagram of presumed super-hydrophilicizing mechanism. If the conventional photocatalyst technology is used with the organic matrix, the conventional photocatalyst decomposes not only dirty marks but also the organic matrix.However, our novel super-catalystic photocatalyst exhibits the super-hydrophilicity with little photocatalystic decomposability. Therefore, the super-hydrophilic photocatalyst has much more application than the conventional photocatalyst.
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