Photoinduced Hydrophilic Conversion of TiO₂/WO₃ Layered Thin Films

The photoinduced hydrophilicity of TiO 2 films with underlying WO 3 layers (TiO 2 /WO 3 layered thin film) was evaluated. Although the top surface was fully covered by TiO 2, the presence of the WO 3 layer enhanced the hydrophilic conversion rate under a 10-W cylindrical fluorescent light bulb. When the TiO 2 and WO 3 layers were separated by an insulating layer, the surface did not become highly hydrophilic under the same light bulb. On a TiO 2 /WO 3 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 WO 3 layer, however the reduction reaction was not. These results indicate that the photogenerated charge carriers can transfer between the TiO 2 and WO 3 layers, i.e., photogenerated holes accumulate in the TiO 2 layer, whereas electrons accumulate in the WO 3 layer. Because of the efficient charge separation, a TiO 2 /WO 3 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 TiO 2 layer could excite WO 3 . Thus, visible light can be used for the hydrophilic reaction in the present TiO 2 /WO 3 system.