Titanium dioxide for solar-hydrogen III: Kinetic effects at elevated temperatures

The present work considers the equilibration kinetics for the O 2 / TiO 2 system and the related chemical diffusion coefficient. It is shown that the equilibration involves two kinetic regimes: Kinetic Regime I (rapid kinetics) and Kinetics Regime II (slow kinetics). The Kinetic Regime I is related to the diffusion of fast defects, including oxygen vacancies and Ti interstitials. The gas/solid equilibrium during routine testing of defect-related properties corresponds to the Kinetic Regime I. The Kinetic Regime II is related to gas/solid reactions that are rate-controlled by all kinds of point defects, including Ti vacancies. Therefore, the kinetic data within the Kinetic Regime II may be used for the imposition of a homogeneous distribution of all defects, while the Kinetic Regime I allows equilibration of only oxygen vacancies and titanium interstitials. The significance of the gas/solid kinetics, that is considered in terms of the chemical diffusion coefficient, in the processing of well-defined TiO 2 specimens with controlled nonstoichiometry and the related defect disorder, is outlined.