Impact of variations in ALD procedure on nanomorphology, protecting properties and chemical stability of thin TiO2 Films

Thin TiO2 films were deposited by atomic layer deposition (ALD) at 150 and 250 °C on FTO and Si/SiO2 substrates to examine the effect of deposition conditions on morphology, structure, chemical stability, and photoelectrochemical performance. Films grown at 150 °C were amorphous and crystallised into anatase after annealing at 500 °C, accompanied by nanoscale morphological rearrangement. In contrast, films deposited at 250 °C were amorphous and non-stoichiometric (TiO2−x) with Ti3+ self-doping; annealing reduced the doping level without inducing crystallisation. The films degraded in 0.1 M HClO4 within 72 h but remained stable in alkaline media (pH 8). Electrochemical studies using the [Fe(CN)6]3−/4− redox couple showed that low-temperature ALD TiO2 layers (8–50 nm) effectively blocked charge transfer, whereas this approach was unsuitable for high-temperature ALD films due to self-doping. The as-deposited high-temperature ALD TiO2/FTO exhibits higher photoelectrochemical (PEC) efficiency than low-temperature films due to Ti3+ self-doping. The as-deposited low-temperature ALD TiO2/FTO shows negligible PEC efficiency, which increases significantly after annealing owing to the formation of the anatase phase.