The impact of niobium surface segregation on charge separation in niobium-doped titanium dioxide

The present investigation has studied the impact of Nb surface segregation on charge carrier separation in polycrystalline Nb-doped (0.65 at % Nb) TiO2. By use of secondary ion mass spectrometry, the effect of applied processing conditions (temperature and oxygen activity) on the segregation of Nb has been obtained and correlated against surface photovoltage data obtained using surface photovoltage spectroscopy (SPS). It has been observed that within the range of the applied processing conditions, the segregation of Nb toward or away from the surface can be achieved. The corresponding SPS results demonstrate that, when Nb is removed from the surface and near-surface regions of Nb-TiO2, charge separation increases, as reflected by an increase in the surface photovoltage. After consideration of the impact of oxygen activity on the defect disorder of Nb-TiO2, it is concluded that the application of very low oxygen activities (p(O2) in the vicinity of 10-15 Pa) at temperatures of 1473 K and above leads to the removal of Nb from the surface and near-surface region. This provides additional upward band bending and is believed to be responsible for the improvement in charge separation.