Niobium Surface Segregation in Polycrystalline Niobium-Doped Titanium Dioxide

In the current investigation, Nb surface segregation has been studied in polycrystalline Nb-doped TiO2 (0.65 at %) at two extremes of oxygen activity (p(O-2) = 101 kPa and p(O-2) = 10(-15) Pa) over the temperature range of 1173 to 1673 K. The aim has been to establish the effect of changes in ambient oxygen activity and temperature on surface Nb enrichment. Using X-ray photoelectron spectroscopy and secondary ion mass spectrometry, the concentration of Nb at the surface has been determined along with Nb depth profiles. It has been found that the ambient oxygen activity during processing at elevated temperatures has a strong influence on the activity of Nb at the surface and the extent of Nb segregation. Specifically, it was found that the application of low oxygen activity during processing (p(O-2) = 10(-15) Pa) substantially increases the surface activity of Nb, resulting in the removal of Nb from the surface and near-surface region. In contrast, processing under conditions of high oxygen activity (p(O-2) = 101 k Pa) was found to promote the enrichment of Nb in the surface due to an apparent drop in the surface activity of Nb. Temperature was observed to have a weaker influence on Nb segregation than oxygen activity, but increased temperature was observed to clearly decrease the surface concentration of Nb. The obtained results indicate that the driving force for Nb segregation can be "tuned" through manipulation of p(O-2) and temperature and protnise to deliver a mechanism for controlling both the composition of the surface and the near-surface region.