Photoemission and STM Study of the Electronic Structure of Nb-Doped TiO2

High-resolution core- and valence-level photoemission spectra of Nb-doped TiO2 ceramics (Ti1-xNbxO2 with 0.01<x<0.8) have been measured using monochromatic x-ray excitation. Nb doping produces a well-defined photoemission peak in the bulk band gap of rutile, whose intensity increases with increasing doping level. Core-level spectroscopy shows that the Nb is incorporated within the rutile lattice at low doping levels mainly as Nb(V) and that the gap state is associated with Ti(III) ions. This conclusion is reinforced by variable energy photoemission measurements on Ti0.9Nb0.1O2 in the vicinity of the Ti 3p and Nb 4p core thresholds. The photoemission resonance profile for the gap states reaches half maximum intensity at the same energy as found for oxygen-deficient TiO2-x but is shifted from the resonance profile for the Nb 4d states of NbO2. STM images on Nb-doped TiO2(110) are considered in relation to the spectroscopic measurements. Nb dopant atoms are imaged as “bright spot” clusters, implying delocalization of charge from Nb onto neighboring Ti ions. The experimental x-ray photoelectron spectroscopy data are compared with density-of-states profiles derived from local-density approximation calculations on pure and Nb-doped TiO2 clusters. These calculations show that Nb doping of TiO2 introduces new states of mixed Nb4d–Ti3d character above the O 2p valence band of the host material. In addition, there is increased x-ray photoemission intensity across the O 2p valence band owing to strong Nb4d/O2p hybridization and a cross section for ionization of Nb 4d states that is an order of magnitude larger than that for O 2p or Ti 3d states.