Mixed phase Ti3+-rich TiO2 thin films by oxide defect engineered crystallization

Abstract Amorphous TiO2 thin films have an insufficient chemical stability for photocatalytic applications. Annealing treatments induce crystallization improving the chemical stability. However, the crystalline structure is already predetermined by the composition of the amorphous phase. In this paper, we demonstrate that the oxide defects, i.e., oxygen vacancies and Ti3+ states, can be created by O2 deficiency during ion-beam sputter deposition without affecting the O/Ti ratio of TiO2 that thus contains a variable degree of under- and over-coordinated Ti atoms. TiO2 with only a few defects crystallizes into microcrystalline anatase during vacuum annealing whereas a moderate density of defects causes crystallization into nanocrystalline rutile. An excessive density of defects results in a mixed amorphous/nanocrystalline rutile phase that was analyzed by near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The density of defects did not affect the crystallization temperature that was 400 °C. Unlike the as deposited amorphous TiO2 thin films, all crystalline films including the mixed phase one were chemically stable in 1.0 M NaOH. The vacuum annealing preserves the Ti3+ gap states that are critical to the charge transfer in protective TiO2-based photoelectrode coatings.