Photo-driven oxidation of water on alpha-Fe2O3 surfaces: An ab initio study

Adopting the theoretical scheme developed by the Norskov group [see, for example, Norskov et al., J. Phys. Chem. B 108, 17886 (2004)], we conducted a density functional theory study of photo-driven oxidation processes of water on various terminations of the clean hematite (alpha-Fe2O3) (0001) surface, explicitly taking into account the strong correlation among the 3d states of iron through the Hubbard U parameter. Six best-known terminations, namely, Fe - Fe - O-3 - (we call S-1), O - Fe - Fe(S-2), O-2 - Fe - Fe-(S-3), O-3 - Fe - Fe-(S-4), Fe - O-3 -Fe-(S-5), and O - Fe - O-3-(S-6), are first exposed to water, the stability of resulting surfaces is investigated under photoelectrochemical conditions by considering different chemical reactions (and their reaction free energies) that lead to surfaces covered by O atoms or/and OH groups. Assuming that the water splitting reaction is driven by the redox potential for photogenerated holes with respect to the normal hydrogen electrode, U-VB, at voltage larger than U-VB, most 3-oxygen terminated substrates are stable. These results thus suggest that the surface, hydroxylated in the dark, should release protons under illumination. Considering the surface free energy of all the possible terminations shows that O-3-S-5 and O-3-S-1 are the most thermodynamically stable. While water oxidation process on the former requires an overpotential of 1.22 V, only 0.84 V is needed on the latter. (C) 2014 AIP Publishing LLC.