Hydrophilic property of SiO2/TiO2 double layer films

The hydrophilicity of the SiO2(top)/TiO2(under) double layer films prepared by vacuum evaporation was investigated. The as-deposited SiO2/TiO2 double layer films showed very good hydrophilicity (water contact angle: ∼0°). Their hydrophilicity, however, is deteriorated by the organic contamination adsorbed on the film but the hydrophilicity can be recovered by UV light irradiation. TOF-SIMS (time-of-flight secondary-ion-mass-spectrometry) measurements revealed that the amount of organic compounds adsorbed on the films decreased with the UV light irradiation. Secondarily, the amount of both Si–OH and Ti–OH groups changed little by the UV light irradiation. Thus, it can be concluded that the hydrophilicity of the SiO2/TiO2 double layer films is due to the stable Si–OH groups and the photo-catalytic TiO2 underlayer maintains the hydrophilicity of the double layer films by decomposing organic contaminants on the film surface. This SiO2/TiO2 double layer film has been in practical use for the automobiles' exterior rear view mirrors.