Production of mixed phase Ti³⁺-rich TiO₂ thin films by oxide defect engineered crystallization

Amorphous TiO₂ has insufficient chemical stability that can be enhanced with annealing induced crystallization. However, the crystalline structure is already predetermined by the defect 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. The films are thus stoichiometric containing a variable degree of interstitial O instead of lattice O. Defect-free TiO₂ crystallizes into microcrystalline anatase during vacuum annealing, whereas a moderate number density of defects causes crystallization into nanocrystalline rutile. An excessive number 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 number density of defects did not affect the crystallization temperature, which was 400 °C. All crystalline films, including the mixed amorphous/nanocrystalline rutile phase, were chemically stable in 1.0 M NaOH for 80 h. Unlike annealing treatments in oxidizing environments that are typically applied to improve stability, vacuum annealing improves the stability preserving also the Ti³⁺ gap states that are critical to the charge transfer in protective TiO₂-based photoelectrode coatings.