TiO2(110) surface is known as a model of heterogeneous catalyst. For more realistic and practical applications, understanding of both local structures and properties induced by niobium dopants, calcium contaminants and oxygen defects is necessary. We investigated the effect of such impurities and defects on surface structure of Nb-doped TiO2(110) with different Nb concentrations (0.05 and 0.5 mass%) by scanning tunneling microscopy (STM) in ultra-high vacuum (UHV). A variety of Ca-induced structures were observed on both sample surfaces. After Ar sputtering, the 1×1 structure appeared on both samples. After annealing at 1273 K for half a day, the cross-linked 1×2 surface structure appeared on the 0.05 mass% sample, while no structural change was observed on the 0.5 mass% one. This result suggests that in the case of low Nb concentration, charge transfer by oxygen ions as well as electrons become considerable while major carriers are exclusively electrons in the sample with high Nb concentration.
