Defect disorder and semiconducting propertes of titanium dioxide single crystal

The semiconducting properties of TiO₂ single crystal and their changes during oxidation and reduction at elevated temperatures (1073 - 1323 K) under controlled oxygen activity (10^-9 - 10⁵ Pa) were monitored using measurements of electrical conductivity and thermoelectric power. The experimental data obtained in equilibrium led to a TiO₂ defect disorder model. According to this model, oxygen vacancies are the predominant defect species in TiO₂ across a wide range of oxygen activities. This work has discovered the diffusion of Ti vacancies, which are formed during prolonged oxidation at elevated temperatures and in a gas phase of high oxygen activity. Observations indicate that appreciable concentrations of Ti vacancies are formed on the TiO₂ surface and then are very slowly incorporated into the bulk. The obtained diffusion data has shown that in the commonly studied temperature range (1000-1400 K) the Ti vacancy concentration is quenched and can be considered as constant. Prolonged oxidation involves two kinetic regimes that are related to the transport of defects of different mobilities. The defect disorder model derived in this work may be beneficial for engineering TiO₂ for enhanced water splitting through the selection of optimal processing conditions, including temperature and oxygen activity.