XPS and STM Study of Nb-Doped TiO2(110)-(1 x 1) Surfaces

The Nb-doped rutile titanium dioxide TiO2(110)-(1 x 1) surface, which was obtained after cleaning by cycles of Ar+ sputtering and annealing in ultrahigh vacuum, was examined by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscope (STM) techniques. The angle-resolved XPS measurement revealed that Nb in a pentavalent state was concentrated in the near-surface region. Preferential removal of 0 by the Ar+ sputtering induced enrichment of Nb and Ti on the surface, and the Nb cations with a low diffusivity in bulk TiO2 remained in the near-surface region in the subsequent annealing. Atom-sized spots assignable to either Nb atoms incorporated into the rutile lattice or Nb adatoms were observed in the STM images. The density of the spots was almost one-third of that of the surface Nb atoms estimated from the XPS results, which indicated that a large part of the near-surface Nb atoms was in the interstitial sites and was invisible in the STM images. The Nb interstitials were segregated to the surface to form oxide particles when the surface was annealed in O-2. The height of the Nb adatom in STM images was reversibly changed, dependent on the adatom-adatom distances. The change in the image height of the adatoms was attributed to the change in the oxidation state.