Electronic structure of Nb impurities in and on TiO2

The electronic properties of Nb substitutional impurities in the bulk phase and at the (110) surface of TiO2 are studied by means of density functional cluster calculations. Both the neutral (Nb4+) and the singly ionized (Nb5+) impurity charge states are taken into consideration. Nb is found to introduce a shallow donor level in bulk TiO2. According to X-ray absorption spectroscopy, the Nb charge is partially transferred to the nearest neighbors along the c axis. Close analogies are found between the electronic structure modifications occurring in the bulk and on the surface. An impurity related state is predicted to occur in the gap of Nb-doped TiO2(110) surfaces, in contrast with photoemission experiments. Possible reasons for this discrepancy are discussed. Finally, structural changes undergone by the (110) surface upon doping are found to be small (negligible for cationic sites where Ti atoms are not substituted), and insensitive to the impurity charge state.