Photoinduced Superhydrophilicity of Titanium Dioxide: Effect of Heterovalent Metal Doping

The self-cleaning effect of titanium dioxide coatings is based on the photocatalytic oxidizing ability and the effect of photoinduced superhydrophilicity. Metal doping is used to enhance photocatalytic activity, while its effect on surface hydrophilicity is practically not studied. In this work, the influence of heterovalent doping of anatase titanium dioxide on its hydrophilic properties was investigated in detail. Thin films x-M-TiO2, where M – Nb5+, Sc3+, Al3+, with dopant concentration in the range of 0.0–1.0 at. %, were obtained on glass substrates from solutions of the corresponding sols by dip-coating method. The phase composition, surface dopant content, lattice microstress, surface acidity and electron work function values were determined and analyzed for three series of doped samples as a function of dopant concentration. The surface hydrophilicity of x-M-TiO2 nanocoatings was evaluated using the water contact angle and surface free energy values. It was shown that doping with niobium ions changes the wettability of titanium dioxide, while its hydrophilic state does not change when doped with scandium and aluminum ions. It was found that the appearance of niobium ions in anatase leads to a sharp increase in the hydrophilicity of the surface with a simultaneous change in the acidity and work function, but with increasing Nb content the electronic factor becomes dominant. The obtained kinetic dependences of the photoinduced water contact angle showed an increase in the surface hydrophilicity of all investigated coatings irrespective of the dopant type within the given concentrations, which demonstrates their self-cleaning ability. At the same time, the final UV-induced hydrophilic state depends on the dopant type. Maximum surface hydrophilicity is achieved with UV irradiation of Nb-doped TiO2 regardless of its content, the Al-doped series of coatings exhibit small contact angles, and the photoinduced surface hydrophilicity of Sc-doped titanium dioxide films decreases with increasing scandium content. Maximum surface hydrophilicity was achieved with UV irradiation of Nb-doped TiO2 regardless of its content, the Al-doped series of coatings exhibit small water contact angle values, and the photoinduced surface hydrophilicity of Sc-doped titanium dioxide films decreases with increasing scandium content.