The present work considers the application of defect chemistry for engineering of semiconducting properties of metal oxides in general and TiO2 in particular. The performance-related functional properties of TiO2-based photoelectrode for hydrogen generation through water splitting using solar energy (solar-hydrogen) are considered in terms of (i) electronic structure, (ii) charge transport, (iii) near-surface charge distribution and the related electric fields, and (iv) defect disorder of the outermost surface layer. The present work considers the modification of these functional properties for TiO2 through the imposition of controlled defect disorder. The defect disorder is considered in terms of defect equilibria and the defect diagram describing the effect of oxygen activity on the concentration of both ionic and electronic defects.
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.
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