Towards sustainable energy : photocatalysis of Cr-doped TiO2. 5. : effect of segregation on surface versus bulk composition

The present chain of five papers considers the concept of solar-to-chemical energy conversion using TiO 2-based semiconductors. The series reports the effect of chromium on the key performance-related properties of polycrystalline TiO 2 (rutile), including electronic structure, photocatalytic activity, intrinsic defect disorder, electrochemical coupling and surface versus bulk properties. This work considers the effect of oxygen activity on segregation-induced surface versus bulk composition for both polycrystalline and single-crystal specimens of Cr-doped TiO 2. It has been documented that annealing of Cr-doped TiO 2 at 1273 K in oxidising conditions results in an enrichment and depletion of the surface layer with chromium. It is shown that the segregation-induced enrichment factor for single crystal is substantially larger than that for polycrystalline specimen. The effect is considered in terms of a theoretical model showing that surface segregation of solute in polycrystalline specimen is encumbered by its segregation to grain boundaries. It is also shown that the segregation-induced enrichment is profoundly influenced by oxygen activity. The new insight of this work involves (i) the determination of well-defined chromium segregation in Cr-doped TiO 2, including single-crystal and polycrystalline specimens, after annealing in the gas phase of controlled oxygen activity, and (ii) identification of the predominant driving force of segregation of chromium in Cr-doped TiO 2 that is based on electrostatic interactions between the low-dimensional surface structure (LDSS) and electrically charge segregating species. [Figure not available: see fulltext.].