Photoinduced Superhydrophilicity of Titanium Dioxide: Effect of Heterovalent Doping with Metals

Abstract

The self-cleaning effect of titanium dioxide coatings is based on the photocatalytic oxidative ability and the phenomenon of photoinduced superhydrophilicity. Doping with metals is used to enhance the photocatalytic activity; however, its influence on the surface hydrophilicity remains to be studied. In this work, the effect of the heterovalent doping of titanium dioxide anatase on its hydrophilic properties has been investigated in detail. Thin xM–TiO₂ films, where the M symbol denotes Nb⁵⁺, Sc³⁺, and Al³⁺, with dopant concentrations of 0.0–1.0 at % have been obtained on glass substrates from solutions of corresponding sols by the deep coating method. The phase composition, surface dopant content, lattice microstress, surface acidity, and electron work function values have been determined and analyzed for three series of doped samples as functions of dopant concentrations. The surface hydrophilicity of xM–TiO₂ nanocoatings has been estimated by measuring water contact angle and surface free energy values. It has been shown that doping with niobium ions affects the wettability of titanium dioxide, while its hydrophilic state remains unchanged upon doping with scandium and aluminum ions. It has been found that the incorporation of niobium ions into anatase drastically increases the hydrophilicity of the surface with a simultaneous change in its acidity and work function. At the same time, as Nb content increases, the electronic factor prevails. The kinetic dependences obtained for the photoinduced water contact angles have shown an increase in the surface hydrophilicity of all investigated coatings irrespective of a dopant type within the studied dopant concentration range, thereby indicating their self-cleaning ability. At the same time, the final UV-induced hydrophilic state depends on a dopant type. The maximum surface hydrophilicity is achieved upon UV irradiation of TiO₂ doped with Nb regardless of its content. UV-irradiated Al-doped TiO₂ coatings exhibit small contact angles, while the photoinduced surface hydrophilicity of Sc-doped titanium dioxide films decreases with increasing scandium concentration.