A seminal study was made of 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 were 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 were determined in the gas/solid equilibrium, were independent of the applied experimental procedure and were 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 were 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 were different from those of the bulk phase. Analysis of the experimental data led to the conclusions that: (1) Grain boundaries of TiO2 acted 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 were the same for both SC-TiO2 and PC-T iO2. This indicates that the transport of electronic charge carriers was not affected by grain boundaries. (3) Grain boundaries exhibit weak links for ionic charge transport across polycrystalline TiO2. (4) The effective band gap for PC-TiO2 was smaller than that for SC-TiO2. This work showed that grain boundaries might be used to tailor semiconducting properties of polycrystalline TiO2 in order to achieve the performance-related properties that were desired for specific applications.

Effect of Grain Boundaries on Semiconducting Properties of TiO2 at Elevated Temperatures. Nowotny, J., Bak, T., Burg, T., Nowotny, M.K., Sheppard, L.R.: Journal of Physical Chemistry C, 2007, 111[27], 9769-78