Abstract
Electronic structures of rutile TiO2 doped with 4d transition metals were studied using the first principles method in the framework of the density functional theory. All 4d transition metal elements except for Zr were considered to substitute 1/16 of Ti lattice sites, for direct comparison of doping effect at the same level of impurity incorporation at the most stable sites, and the doping effect is expected to be positively correlated to the impurity concentration as is demonstrated through Cd-doped rutile. The results show that Y, Ag and Cd are p-type dopants, while Nb and Mo are n-type ones. These dopants also reduce the energy gap of TiO2, leading to some redshift in the optical absorption edge. Doping with Tc, Ru, Rh or Pd induces intermediate bands to divide the forbidden gap of the host phase into sub-gaps. These doping induced intermediate bands are of evident curvatures, and they are expected to act as stepping stones to relay valence electrons to the conduction band via multi-photon excitations, thus extending optical absorption of photons into the long wavelength regions of solar irradiance for enhanced photovoltaic and photo-catalytic functionalities. The effects of Hubbard U correction to the exchange-correlation functionals were also examined for some doped structures when the forbidden gap for the doped phase is from mid-gap states, while more extensive effort is expected to clarify the non-local effect associated with gap states. (C) 2012 Elsevier B. V. All rights reserved.