Effect of the internal stress relaxation during the post-annealing on the photo-induced properties of TiO2 coatings reactively sputtered

Photocatalyst TiO₂ coatings have been reactively sputtered at high pressure on cold glass substrates pre-coated by a SiN_x sodium diffusion barrier. The as-deposited coatings were amorphous and the TiO₂/SiN_x/glass samples were subsequently heated at different temperatures under air. The TiO₂ films crystallise in the anatase structure above temperatures of 250 °C with a [001] preferential orientation. The structural analyses have demonstrated that the crystallites are elongated following the c axis direction, perpendicularly to the surface. No modifications of grain size and texture have been observed over the complete temperature range studied (250-550 °C). However, the lattice parameters evolution shows a decrease of the tensile stress with a rise in annealing temperature. The microstructure is then completely relaxed around 400 °C and finally compressive stress is observed at higher temperature. The study of the photo-induced (photocatalytic and hydrophilic) properties shows an activity maximum at 400 °C. These results suggest that the photo-induced properties would be favoured by a relaxed microstructural state of titanium dioxide.