Study on hydrophilic property of hydro-oxygenated amorphous TiOx:OH thin films

This paper is on hydrophilicity of amorphous TiOx thin films, which were prepared by plasma enhanced chemical groups. The as-deposited film showed a good hydrophilicity (water vapor deposition, including a large amount of OH contact angle below 5degrees) that deteriorated after long time storage in a dark ambient air (water contact angle of 59degrees). By UV light irradiation, the film surface recovered its initial hydrophilicity. Atomic force microscopic measurements of tip-sample adhesion force showed homogeneous distribution on the film surface before and after UV light irradiation. We suppose that the OH groups have a strong effect on the film hydrophilicity. Measurements of dark and photoexcited currents suggested that the deposited films have high photo-efficiency and very long lifetime of photo-excited carriers. Hence, we refer to them as hydro-oxygenated amorphous TiOx (a-TiOx:OH) films in analogy with a-Si:H. Furthermore, it was observed that the conductivity of a-TiOx:OH films was sensitive to the surrounding gases, especially to electron-withdrawing oxygen. In ambient air, it is deduced that the photo-excited electrons diffuse toward the film surface and react with oxygen to generate O-.(2)-. Considering the good hydrophilicity of the as-deposited a-TiOx:OH film and the strong oxidation power of O-.(2)-. it is concluded that the hydrophilicity of the a-TiOx:OH film was recovered by the photo-catalytic decomposition of hydrophobic adsorbates on the film surface. (C) 2002 Elsevier Science B.V. All rights reserved.