Effect of grain boundaries on semiconducting properties of TiO₂ at elevated temperatures

The present work represents the seminal study on the local properties of the grain boundaries of polycrystalline TiO₂ at elevated temperatures corresponding to the equilibrium with the gas phase of well-defined oxygen activity. These local properties have been determined from two sets of data obtained in two parallel projects on the determination of the defect-related electrical properties in identical conditions for both polycrystalline and single-crystal TiO₂. These properties, which have been determined in the gas/solid equilibrium, are independent of the applied experimental procedure and are determined only by the equilibrium conditions described by temperature and oxygen activity. Therefore, the reported grain boundary properties may be considered as material data. The data considered in this work include the electrical conductivity and thermoelectric power that were determined following the same experimental procedure. The present work also includes the equilibration kinetic data that are considered in terms of the chemical diffusion coefficient. Comparison between these two sets of data indicates that the local defect disorder and the related semiconducting properties of grain boundaries are different from those of the bulk phase. The analysis of the experimental data allows us to make the following conclusions: (1) Grain boundaries of TiO₂ act as donors of electrons. This effect seems to be related to the enrichment of the grain boundary layer in donor-type defects, such as oxygen vacancies. (2) The transport mechanisms for electrons and electron holes are the same for both SC-T_iO₂ and PC-T _iO₂. This indicates that the transport of electronic charge carriers is not affected by grain boundaries. (3) Grain boundaries exhibit weak links for ionic charge transport across polycrystalline TiO ₂. (4) The effective band gap for PC-TiO₂ is smaller than that for SC-TiO₂. This work shows that grain boundaries may be used to tailor semiconducting properties of polycrystalline TiO₂ in order to achieve the performance-related properties that are desired for specific applications.