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 vapor deposition, including a large amount of OH groups. The as-deposited film showed a good hydrophilicity (water contact angle below 5°) that deteriorated after long time storage in a dark ambient air (water contact angle of 59°). 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 photo-excited 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 O2−. Considering the good hydrophilicity of the as-deposited a-TiOx:OH film and the strong oxidation power of O2−, 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.