Photoinduced hydrophilic conversion of TiO2/WO3 layered thin films

The photoinduced hydrophilicity of TiO2 films with underlying WO3 layers (TiO2/WO3 layered thin film) was evaluated. Although the top surface was fully covered by TiO2, the presence of the WO3 layer enhanced the hydrophilic conversion rate under a 10-W cylindrical fluorescent light bulb. When the TiO2 and WO3 layers were separated by an insulating layer, the surface did not become highly hydrophilic under the same light bulb. On a TiO2/WO3 layered film the photocatalytic oxidation of methylene blue adsorbed onto the surface and the photocatalytic reduction of silver nitrate in an aqueous solution were investigated under a 10-W cylindrical black light bulb. The oxidation reaction was enhanced by the presence of the WO3 layer, however the reduction reaction was not. These results indicate that the photogenerated charge carriers can transfer between the TiO2 and WO3 layers, i.e., photogenerated holes accumulate in the TiO2 layer, whereas electrons accumulate in the WO3 layer. Because of the efficient charge separation, a TiO2/WO3 layered film formed on a grounded electroconductive substrate displayed a higher hydrophilicizing rate than when coated onto an insulating substrate. It was also demonstrated that visible light passing through the TiO2 layer could excite WO3. Thus, visible light can be used for the hydrophilic reaction in the present TiO2/WO3 system.