Photoemission and STM study of the electronic structure of Nb-doped TiO2

High-resolution core- and valence-level photoemission spectra of Nb-doped T iO2 ceramics (Ti1-xNbxO2 with 0,01 < x < 0.8) have been measured using mono chromatic x-ray excitation. Nb doping produces a well-defined photoemission peak in the bulk band gap of rutile, whose intensity increases with increa sing doping level. Core-level spectroscopy shows that the Nb is incorporate d within the rutile lattice at low doping levels mainly as Nb(V) and that t he gap state is associated with TL(III) ions. This conclusion is reinforced by variable energy photoemission measurements on Ti0.9Nb0.1O2 in the vicin ity of the Ti 3p and Nb 4p core thresholds. The photoemission resonance pro file for the gap states reaches half maximum intensity at the same energy a s found for oxygen-deficient TiO2-x, but is shifted from the resonance prof ile for the Nb 4d states of NbO2. STM images on Nb-doped TiO2(110) are cons idered in relation to the spectroscopic measurements. Nh dopant atoms are i maged as "bright spot" clusters, implying delocalization of charge from No onto neighboring Ti ions. The experimental x-ray photoelectron spectroscopy data are compared with density-of-states profiles derived from local-densi ty approximation calculations on pure and Nb-doped TiO2 clusters. These cal culations show that Nh doping of TiO2 introduces new states of mixed Nb 4d- Ti 3d character above the O 2p valence band of the host material. In additi on, there is increased x-ray photoemission intensity across the O 2p valenc e band owing to strong Nb 4d/O 2p hybridization and a cross section for ion ization of Nb 4d states that is an order of magnitude larger than that for O 2p or Ti 3d states.