Effect of Na diffusion from glass substrate on the microstructural and photocatalytic properties of post-annealed TiO2 films synthesised by reactive sputtering

Amorphous columnar TiO2 films were synthesised by reactive sputtering on cold soda–lime glass substrates (TiO2/glass films). The films were subsequently heated in order to crystallise the photoactive anatase phase. The surface chemical composition assessment demonstrates the occurrence of metallic Na, the amount of which increases with the annealing temperature. The evolution of the structural, microstructural and photocatalytic properties of the films with the annealing temperature was investigated and compared to that of TiO2 films deposited in same conditions, but on glass pre-coated with a SiNx diffusion barrier (TiO2/SiNx/glass films). Once crystallised, both series of TiO2 films exhibit [001] preferential orientation corresponding to the columnar growth. Grain coalescence associated to a modification of the grain shape is only observed in TiO2/glass films for annealing temperatures higher than 450 °C, whereas neither microstructural nor structural change is observed in TiO2/SiNx/glass films. The Na-contaminated TiO2 films exhibit different photocatalytic behaviour with the annealing temperature compared to the Na-free TiO2 films. A discussion is finally based on these differences.