Defect Chemistry of Titanium Dioxide (Rutile). Progress Toward Sustainable Energy.

This work, which overviews defect chemistry of TiO (rutile), is focused on atomic-size structural defects that are thermodynamically reversible. Here it is shown that thermodynamics can be used in defect engineering of TiO-based energy materials, such as photoelectrodes and photocatalysts. We show that surface segregation of defects leads to the building-up of new surface structures that are responsible for reactivity. Since rational design of surface properties requires surface characterization in operational conditions, expansion of bulk defect chemistry to surface defect chemistry requires a defect-related surface-sensitive tool for monitoring of defect-related properties at elevated temperatures corresponding to defect equilibria and in a controlled gas-phase environment. Here we show that the high-temperature electron probe is a defect-related surface-sensitive tool that is uniquely positioned to aid surface defect engineering and determine unequivocal surface properties. The related applied aspects are considered for photoelectrochemical water splitting and the performance of solid oxide fuel cells. Here we report that trail-blazing studies on surface monitoring of TiO during gas/solid equilibration, along with characterization of surface semiconducting properties, leads to the discovery of a segregation-induced low-dimensional surface structure that is responsible for stable performance of oxide semiconductors, such as TiO, in operational conditions.