Janusz Nowotny

This work, which overviews defect chemistry of TiO₂ (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 TiO₂-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 in situ surface characterization in operational conditions, expansion of bulk defect chemistry to surface defect chemistry requires a defect-related surface-sensitive tool for in situ 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 in situ surface monitoring of TiO₂ during gas/solid equilibration, along with in situ 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 TiO₂, in operational conditions.

Rational design of surface properties of oxide semiconductors for energy conversion requires in situ surface characterization.

Surface properties of crystalline solids and the related defect disorder of the surface layer have a critical effect on the reactivity and performance of materials, including energy materials. It is shown here that a high-temperature electron probe enables unequivocal surface characterization of energy oxide materials in a gas/solid equilibrium, including the affinity-related charge transfer and segregation-affected defect disorder. As an example, this work considers in situ surface monitoring of barium titanate at elevated temperatures during oxidation to determine a quantity (described as work function) that is reflective of the chemical potential of electrons during gas/solid reactions. The probe enables insight into local surface structures and opens up new fields of surface defect chemistry and surface defect engineering of materials for clean energy conversion.