Photoinduced Hydrophilicity of Surfaces of Thin Films

The published communications devoted to studying the phenomenon of photoinduced hydrophilicity of the surfaces of thin metal oxide films have been reviewed. This phenomenon underlies the capability of photoactive materials for self-cleaning. The novelty of the work consists in the generalization and the critical analysis of the vast information obtained on the problem under consideration beginning from the discovery of the photoinduced superhydrophilicity of titanium dioxide in 1997 up to the present. The essence of the phenomenon and the mechanisms proposed for it have been described, and the examples of the influence of different factors on the kinetics and thermodynamics of this photoinduced process have been presented. A mechanism of the process has been suggested. It is based on the photoactivation and photodeactivation of surface active sites responsible for subsequent restructuring of adsorbed water molecules in a hydroxyl-hydrated multilayer on an oxide film surface, with the restructuring leading to a change in the surface free energy and, hence, the surface hydrophilicity. In conclusion, possible methods have been considered for controlling the wettability of the surfaces of photoactive materials with the use of light, with the method of creating heterostructured coatings being especially singled out among them.