Pinhole-resistant nanocrystalline rutile TiO₂ photoelectrode coatings

Atomic layer deposited (ALD) TiO₂ thin films have a wide range of applications in photonics which are, however, limited by the chemical instability of the amorphous as-deposited TiO₂. Post-deposition annealing is required for improving the performance by inducing phase transitions and oxide defects. ALD precursor traces remaining in the TiO₂ film affect the thermally-induced processes but the understanding of the effect of growth temperature on precursor traces in the film as well as on the thermally-induced processes is weak. In this study 30 nm ALD TiO₂ was grown on Si wafer from tetrakis(dimethylamido)titanium and water at 100–200 °C. TiO₂ was subsequently annealed in vacuum at 200–500 °C. Increasing the growth temperature decreased the amount of N bearing precursor traces and thus makes the TiO₂ more easily reducible. The reduction takes place simultaneously with the crystallization and formation of O¹⁻ defects. Vacuum annealing of TiO₂ with less than 0.3 at% of N results in nanocrystalline rutile whereas samples with more N containing traces crystallized as microcrystalline anatase. Nanocrystalline rutile TiO₂ was chemically stable and resistant to the dissolution at the grain boundaries under alkaline conditions making it a suitable material for protective photoelectrode coatings used in artificial photosynthesis.