Understanding Hematite Doping with Group IV Elements:
A DFT+<i>U</i> Study
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Abstract
Si, Ge, or Sn doped hematite (α-Fe<sub>2</sub>O<sub>3</sub>) photoanodes show significantly enhanced efficiency
for photo-oxidization
of water. We employed DFT+<i>U</i> to study the doping of
α-Fe<sub>2</sub>O<sub>3</sub> 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 α-Fe<sub>2</sub>O<sub>3</sub> and provides guidelines for designing new efficient
photoanodes