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

Abstract Amorphous columnar TiO 2 films were synthesised by reactive sputtering on cold soda–lime glass substrates (TiO 2 /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 TiO 2 films deposited in same conditions, but on glass pre-coated with a SiN x diffusion barrier (TiO 2 /SiN x /glass films). Once crystallised, both series of TiO 2 films exhibit [001] preferential orientation corresponding to the columnar growth. Grain coalescence associated to a modification of the grain shape is only observed in TiO 2 /glass films for annealing temperatures higher than 450 °C, whereas neither microstructural nor structural change is observed in TiO 2 /SiN x /glass films. The Na-contaminated TiO 2 films exhibit different photocatalytic behaviour with the annealing temperature compared to the Na-free TiO 2 films. A discussion is finally based on these differences.