Factors Affecting UV-Induced Superhydrophilic Conversion of a TiO2 Surface

The present study explored the effects of several factors (wetting, light intensity, spectral variation of the actinic light, heating, and surface acidity) on the hydrophilic conversion of the surface of TiO2 nanocoatings. The experimental dependencies of the efficiencies of photoinduced hydrophilic surface conversion on the intensity and wavelength of the actinic light clearly indicate the role of electronic photoexcitation in hydrophilic surface transformation. Particularly, the maximum extrema in spectral dependence of the efficiency of photoinduced hydrophilic conversion correspond to the energies of the first indirect and first direct electronic band-to-band transitions in TiO2. At the same time, temperature dependence and the effect of the surface acidity on the hydrophilic behavior of the TiO2 surface demonstrate the importance of the multilayer hydrate structure in both the original hydrophilicity of the surface and the direction of the photoinduced hydrophilic conversion. Estimation of the surface energy alteration under photoexcitation suggests that only specific surface sites (10(-3)-10(-4) monolayer) are responsible for the effect of photoinduced superhydrophilicity of a TiO2 surface.