The present work considers the application of defect chemistry for engineering of semiconducting properties of metal oxides in general and TiO<sub>2</sub> in particular. The performance-related functional properties of TiO<sub>2</sub>-based photoelectrode for hydrogen generation through water splitting using solar energy (solar-hydrogen) are considered in terms of (i) electronic structure, (ii) charge transport, (iii) near-surface charge distribution and the related electric fields, and (iv) defect disorder of the outermost surface layer. The present work considers the modification of these functional properties for TiO<sub>2</sub> through the imposition of controlled defect disorder. The defect disorder is considered in terms of defect equilibria and the defect diagram describing the effect of oxygen activity on the concentration of both ionic and electronic defects.
The present paper considers the effect of segregation on the performance of photo-electrode materials for
photo-electrochemical water splitting. This phenomenon, which alters the surface composition of a material
during processing at elevated temperatures, has the capacity to dominate interfacial charge transfer between
the photo-electrode and the electrolyte. As the present understanding of segregation in metal oxides is
limited, this paper aims at addressing the need to collect empirical data which can be used for the
development of novel materials.
In the present investigation, Nb surface segregation was investigated at 1273 K under high and low oxygen
activity using secondary ion mass spectrometry (SIMS). A calibration procedure was used to enable
quantifiable data and Nb was observed to segregate strongly, especially at high oxygen activity. While this
was attributed to the defect disorder, it remained unclear whether gas/solid equilibrium was achieved, and
consequently whether the observed behaviour represents equilibrium segregation. Irrespectively, the
observed behaviour clearly illustrates how the surface composition of a metal oxide can be altered through
the control of segregation. This must be considered in the pursuit of high performance photo-electrode
materials for water splitting under sunlight.