Understanding Hematite Doping with Group IV Elements: A DFT plus U Study

Si, Ge, or Sn doped hematite (alpha-Fe2O3) photoanodes show significantly enhanced efficiency for photo-oxidization of water. We employed DFT+U to study the doping of alpha-Fe2O3 with group IV elements, i.e., Si, Ge, and Sn. From the calculated formation energies and chemical potentials, three key points are concluded. (1) Low oxygen pressure is favored for doping both substitutional and interstitial dopants. (2) Substitutional doping of the Fe atom at the lattice site is more stable than interstitial doping in the octahedral vacancies. (3) Most interestingly, Ge doping is found to be easiest among the three dopants. This result contradicts intuition based on atomic size and indicates that Ge doping should be more efficient than Si and Sn doping in increasing the charge carrier concentration. Incorporation of the dopants at the Fe site generates an electron polaron and the dopant with the +4 valence state by spontaneous transfer of one electron from the dopant atom to a surrounding Fe atom, according to the analyses of charge transition energy levels and density of states. We identify the factors affecting the charge transfer process. The study elucidates the dopants role in increasing the electrical conductivity of alpha-Fe2O3 and provides guidelines for designing new efficient photoanodes.