Abstract
High-density phases of TiO2, such as rutile and high-pressure TiO2-II, have attracted interest as materials with high dielectric constant and refractive index values, while combinations of TiO2-II with anatase and rutile have been considered promising materials for catalytic applications. In this work, the atomic layer deposition of TiO2 on α-Al2O3 (0 0 0 1) (c-sapphire) was used to grow thin films containing different combinations of TiO2-II, anatase, and rutile, and to investigate the properties of the films. The results obtained demonstrate that in a temperature range of 300–400 °C, where transition from anatase to TiO2-II and rutile growth occurs in the films deposited on c-sapphire, the phase composition and other properties of a film depend significantly on the film thickness and ALD process time parameters. The changes in the phase composition, related to formation of the TiO2-II phase, caused an increase in the density and refractive index, minor narrowing of the optical bandgap, and an increase in the hardness of the films deposited on c-sapphire at TG ≥ 400 °C. These properties, together with high catalytic efficiency of mixed TiO2-II and anatase phases, as reported earlier, make the films promising for application in various functional coatings.
Highlights
In recent years, the number of attempts at applying TiO2 -II, a high-density metastable phase of titanium dioxide (TiO2 ), has markedly increased [1,2,3,4,5,6,7,8,9,10,11]
TiO2 -II has been obtained as an intermediate metastable phase during the anatase to rutile transformation, caused by ball milling [1,7,21,22], and in thin films deposited at process parameters corresponding to the transition from anatase to rutile growth [31,32,33]
We describe research aimed at revealing the effects of the atomic layer deposition (ALD) process parameters and film thicknesses on the growth and properties of TiO2 films deposited at temperatures corresponding to the transition from the anatase to TiO2 -II growth and, allowing the formation of different phase combinations in the films
Summary
The number of attempts at applying TiO2 -II, a high-density metastable phase of titanium dioxide (TiO2 ), has markedly increased [1,2,3,4,5,6,7,8,9,10,11] This phase had already been observed in high-pressure experiments in the 1960s [12,13,14], and a number of studies, focused on formation and structural studies of TiO2 -II [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34], have been published since that time. Further experimental studies of materials containing mixtures of TiO2 -II with rutile and/or anatase might be of even higher practical importance than those of phase-pure TiO2 -II
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