Chromium segregation in Cr-doped TiO2 (rutile): impact of oxygen activity

This work considers the effect of chromium surface segregation for polycrystalline Cr-doped TiO 2 on surface vs. bulk defect disorder. It is shown that annealing of Cr-doped TiO 2 (0.04 at% Cr) in the gas phase of variable oxygen activity at 1273 K results in a gradual transition in the valence of chromium at the surface from predominantly Cr 3+ species in reduced conditions, p(O 2 ) = 10 −12 Pa, to comparable concentrations of both Cr 3+ and Cr 6+ species in oxidising conditions, p(O 2 ) = 10 5 Pa. The reported data is considered in terms of defect equilibria leading to the formation of positively and negatively charged chromium in both the cation sub-lattice and interstitial sites. The derived theoretical models represent the effect of oxygen activity on the surface charge and the resulting electric field leading to migration mechanism of charged chromium species.