Ti³⁺ Self-Doping-Mediated Optimization of TiO₂ Photocatalyst Coating Grown by Atomic Layer Deposition

Titanium dioxide (TiO2) thin films are being applied in various photonic applications where precise controlling of thin film morphology and crystal structure is required for optimum performance. Here, photocatalytic TiO2 thin films were fabricated by atomic layer deposition (ALD) using TDMAT and H2O precursors utilizing growth temperature (150–225 °C)-controlled self-doping with Ti3+. The performance was optimized in terms of post-deposition annealing (PDA) temperature (≤500 °C) and film thickness (20–50 nm) toward photoelectrochemical water oxidation in 1 M NaOH under 1 sun illumination. During the PDA, low ALD growth temperatures (150 and 175 °C) result in abrupt crystallization to anatase, whereas films grown at higher temperatures (200 and 225 °C) crystallize gradually to rutile. Unlike crystalline TiO2, as-deposited Ti3+ self-doped TiO2 films are amorphous and have low stability in 1 M NaOH. The best activity for water oxidation under 1 sun is obtained for the 30 nm post-annealed rutile TiO2 film with a maximum photocurrent of 0.3 mA/cm2. This benchmark performance can be attributed to the optimum TiO2 absorption with respect to carrier diffusion length and more efficient solar light absorption of rutile TiO2 compared to anatase TiO2. These results demonstrate a simple strategy to fabricate either anatase or rutile TiO2 thin films by controlling the ALD growth temperature.