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 functiona l cluster calculations. Both the neutral (Nb4+) and the singly ionized (Nb5 +) impurity charge states are taken into consideration. Nb is found to intr oduce a shallow donor level in bulk TiO2. According to X-ray absorption spe ctroscopy, the Nb charge is partially transferred to the nearest neighbors along the c axis. Close analogies are found between the electronic structur e modifications occurring in the bulk and on the surface. An impurity relat ed state is predicted to occur in the gap of Nb-doped TiO2(110) surfaces, i n contrast with photoemission experiments. Possible reasons for this discre pancy are discussed. Finally, structural changes undergone by the (110) sur face upon doping are found to be small (negligible for cationic sites where Ti atoms are not substituted), and insensitive to the impurity charge stat e.