Photoemission and STM study of the electronic structure of Nb-doped ${\mathrm{TiO}}_2$

High-resolution core- and valence-level photoemission spectra of Nb-doped ${\mathrm{TiO}}_2$ ceramics $({\mathrm{Ti}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_2$ with $0.01<x<\mathrm{}0.8)$ 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 ${\mathrm{Ti}}_{0.9}{\mathrm{Nb}}_{0.1}{\mathrm{O}}_2$ 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 ${\mathrm{TiO}}_{2-x}$ but is shifted from the resonance profile for the Nb $4d$ states of ${\mathrm{NbO}}_2.$ STM images on Nb-doped ${\mathrm{TiO}}_2\mathrm{}\left(110\right)\mathrm{}$ 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 ${\mathrm{TiO}}_2$ clusters. These calculations show that Nb doping of ${\mathrm{TiO}}_2$ introduces new states of mixed $\mathrm{Nb}4d–\mathrm{Ti}3d$ 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 $\mathrm{Nb}4d/\mathrm{O}2p$ 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.