This investigation was aimed at identifying the influence of applied processing conditions, temperature and oxygen activity, on the segregation of Ta in Tadoped TiO 2 . Over the temperature range of 1173−1523 K, it has been observed that the tendency for Ta to segregate is greater under the application of oxidizing conditions (p(O 2 ) = 101 kPa) than that under reducing conditions (p(O 2 ) = ∼10 −10 Pa). This respectively manifests as an accumulation and depletion of Ta at the surface of Ta−TiO 2 under the respective applied oxygen activities. This behavior has been interpreted in terms of the relative activities of Ta at the surface and bulk during processing. The results indicate that it is possible to substantially alter the concentration of a donor dopant at the surface of TiO 2 despite maintaining the bulk dopant loading. In so doing, a TiO 2 -based homojunction is formed that can be engineered to favor charge separation, as demonstrated by preliminary surface photovoltage measurements. The optimization of this homojunction provides a promising approach for developing a novel photoelectrode material for solar-driven water splitting. This investigation has provided a fundamental basis for further investigations of compositionally graded TiO 2 -based materials for improved solar harvesting.

This investigation has assessed the behavior of Ta enrichment in Ta-doped TiO 2 under various conditions of controlled oxygen activity and temperature. The aim has been to establish the relationships between specific processing conditions and the resulting compositional variation within the surface and nearsurface region. Under the application of oxidizing conditions [p(O 2 ) = 101 kPa], it has been observed that Ta will strongly enrich the surface of Ta-doped TiO 2 irrespective of the annealing temperature (over the range of 1173-1523 K). However, under reducing conditions [p(O 2 ) in the vicinity of 10 -10 Pa], Ta enrichment is observed at 1173 K, but Ta depletion from the surface and near-surface is observed at 1348 and 1523 K. This is attributed to an apparent lack of stability of the surface phase, which could possibly be TiTa 2 O 7 . The results for the investigation contribute to the engineering of TiO 2 -based photoelectrode materials that possess improved charge separation properties. J ournalwas to observe the behavior of tantalum surface enrichment in Ta-doped TiO 2 in response to the applied conditions of processing. These results will then be built upon to study the influence of Ta concentration gradients on charge separation in Ta-doped TiO 2 . Comparisons will be made to the similarly prepared Nb-doped TiO 2 to identify the differences resulting from the selection of donor dopant.

195Pt solute diffusion coefficients in undoped single‐crystal rutile TiO2 have been determined parallel to the c‐axis over the temperature range of 1073–1523 K and oxygen partial pressure of 101 kPa. This has yielded the following temperature dependence: D195Pt=1.3±3.2×10−13m2s−1exp(−122.0±13.6italickJmol−1normalRT) On the basis of this result, it has been concluded that 195Pt diffuses via an interstitial mechanism involving rapid transport parallel to the c‐axis through open interstitial diffusion channels. While other divalent cations have also displayed diffusion via the same mechanism, and report diffusion rates that greatly exceed that for cation self‐diffusion, the overall rate of 195Pt diffusion is slow. This has been attributed to the size of Pt2+ (0.80 Å) which is comparative to the dimension of interstitial channels (0.77 Å) through which transport is believed to be taking place.