Red Shift in Manganese- and Iron-Doped TiO2: A DFT+U Analysis

DFT (density functional theory) and DFT+U (DFT with Hubbard U correction for the on-site Coulomb repulsion) calculations have been carried out to study the effects of Mn or Fe doping to the energy band structures and optical properties of rutile TiO2. Both Fe and Mn doping reduces the overall energy band gap and introduces intermediate states/bands into the forbidden gap. It is observed that the Hubbard U correction to the 3d electrons of the host Ti atoms helps open up the energy gap of TiO2, On the other hand, the Hubbard U correction to the 3d electrons of the doping atom dictates the positions of intermediate states/bands with respect to the top of the valence band (VBM). The Hubbard correction to the 3d orbitals of a dopant, either Mn or Fe, shifts the up-spin states due to doping down to deeper levels, while it raises the down-spin states to higher energy levels. The doping induced up-spin states/bands are attributed to the hybridization between the dopant's 3d and the oxygen 2p orbitals, while the down-spin states are dictated mainly by the 3d orbitals of the dopants. Substitution of Ti sites with either Fe or Mn atoms causes significant red shift to the band gap and the optical absorption spectra, permitting the doped phase to absorb significantly longer wavelengths of the solar radiation. The doped materials are expected to be promising for photovoltaic applications.