Influence of the Deposition Parameters on the Properties of TiO₂ Thin Films on Spherical Substrates

Wastewater treatment targeting reuse may limit water scarcity. Photocatalysis is an advanced oxidation process that may be employed in the removal of traces of organic pollutants, where the material choice is important. Titanium dioxide (TiO₂) is a highly efficient photocatalyst with good aqueous stability. TiO₂ powder has a high surface area, thus allowing good pollutant adsorption, but it is difficult to filter for reuse. Thin films have a significantly lower surface area but are easier to regenerate and reuse. In this paper, we report on obtaining sol-gel TiO₂ thin films on spherical beads (2 mm diameter) with high surface area and easy recovery from wastewater. The complex influence of the substrate morphology (etched up to 48 h in concentrated H₂SO₄), of the sol dilution with ethanol (1:0 or 1:1), and the number of layers (1 or 2) on the structure, morphology, chemical composition, and photocatalytic performance of the TiO₂ thin films is investigated. Etching the substrate for 2 h in H₂SO₄ leads to uniform, smooth surfaces on which crystalline, homogeneous TiO₂ thin films are grown. Films deposited using an undiluted sol are stable in water, with some surface reorganization of the TiO₂ aggregates occurring, while the films obtained using diluted sol are partially washed out. By increasing the film thickness through the deposition of a second layer, the roughness increases (from ~50 nm to ~100 nm), but this increase is not high enough to promote higher adsorption or overall photocatalytic efficiency in methylene blue photodegradation (both about 40% after 8 h of UV-Vis irradiation at 55 W/m²). The most promising thin film, deposited on spherical bead substrates (etched for 2 h in H₂SO₄) using the undiluted sol, with one layer, is highly crystalline, uniform, water-stable, and proves to have good photocatalytic activity.