Hydrophilic and photocatalytic properties of the SiO2/TiO2 double layers

The hydrophilic and photocatalytic properties of the SiO2/TiO2 double layers composed of a 20-nm-thick porous SiO2 layer on the 200-nm-thick columnar anatase TiO2 layer were studied. The hydrophilicity of the double layers was strictly determined by the relative coverage of organic contaminants. The intrinsic hydrophilicity of 0° of SiO2 in terms of the water contact angle was restored by the photocatalytic decomposition of organic contaminants under the UV light irradiation.Electron spin resonance measurements revealed the generation of OH radicals under the UV light irradiation onto the SiO2/TiO2 double layers. Photoconductivity measurements showed that the current decay in O2 gas atmosphere was remarkably fast in comparison with that in H2O vapor. These observations support our view that the generation of OH radicals effective for decomposing organic contaminants on the surface begins with the reaction between O2 molecules and the photoexcited electrons. We propose together with other experimental facts herein that OH radicals would be generated via O2− and H2O2 in the double layer system.