Structural, optical and photocatalytic properties of erbium (Er3+) and yttrium (Y3+) doped TiO2 thin films with remarkable self-cleaning super-hydrophilic properties

Abstract

The self-cleaning and super hydrophilic properties of pristine TiO₂ and of TiO₂ doped with Er³⁺ or Y³⁺ transparent thin films deposited onto glass substrates were investigated. The thin films prepared by multiple dipping and drying cycles of the glass substrate into the pristine TiO₂ sol and Er³⁺ or Y³⁺-doped TiO₂ sol were characterized by X-ray diffraction, UV-vis spectrophotometry, and atomic force microscopy (AFM). The self-cleaning photocatalytic activity of the thin films towards the removal of oleic acid deposited on the surface under UVA irradiation was evaluated. A remarkable enhancement was observed in the hydrophilic nature of the TiO₂ thin films under irradiation. The optical properties and wettability of TiO₂ were not affected by Er³⁺ or Y³⁺ doping. However, the photocatalytic degradation of oleic acid under UVA irradiation improved up to 1.83 or 1.95 fold as the Er³⁺ or Y³⁺ content increased, respectively, due to the enhanced separation of the photogenerated carriers and reduced crystallite size. AFM analysis showed that the surface roughness increased by increasing the Er³⁺ or Y³⁺ content due to the formation of large aggregates. This in turn contributes to the increase of the active surface area enhancing the photodegradation process. This study demonstrates that TiO₂ doped with low amounts of Er³⁺ or Y³⁺ down to 0.5 mol% can produce transparent, super-hydrophilic, thin film surfaces with remarkable self-cleaning properties.