Tantalum Segregation in Ta-Doped TiO₂ and the Related Impact on Charge Separation during Illumination

This investigation was aimed at identifying the influence of applied processing conditions, temperature and oxygen activity, on the segregation of Ta in Ta-doped TiO₂. 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 (<i>p</i>(O₂) = 101 kPa) than that under reducing conditions (<i>p</i>(O₂) = ∼10^–10 Pa). This respectively manifests as an accumulation and depletion of Ta at the surface of Ta–TiO₂ 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 TiO₂ despite maintaining the bulk dopant loading. In so doing, a TiO₂-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 TiO₂-based materials for improved solar harvesting