Effect of the TiO ₂ –ZnO Heterostructure on the Photoinduced Hydrophilic Conversion of TiO ₂ and ZnO Surfaces

The present study demonstrates the effect of the composition and structure of TiO 2 –ZnO heterostructured coatings on the photoinduced hydrophilic conversion of TiO 2 and ZnO surfaces. It has been observed that the kinetic parameters of the photoinduced hydrophilic conversion can be significantly changed. Based on the analysis of the experimental data and the proposed mechanism, it is concluded that the ratio between the surface concentrations of electrons and holes participating in the photoinduced generation and deactivation of hydrophilic surface sites plays the major role in this alteration of the surface hydrophilicity of such heterostructured coatings. The ratio between the surface concentrations of electrons and holes can be changed considerably by the formation of the so-called type II heterojunction between TiO 2 and ZnO, yielding an effective charge separation. The proposed approach based on the creation of these type II heterostructures seems to be promising and productive both for fundamental studies of the photoinduced surface superhydrophilicity and for the application of self-cleaning coatings with controlled wettability.