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

Abstract

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 (TiO_2−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.