The role of isotropic and anisotropic Hubbard corrections for the magnetic ordering and absolute band alignment of hematite α-Fe2O3(0001) surfaces

Abstract

An isotropic (+U) and anisotropic [+(U−J)] corrected Density Functional Theory study for bulk hematite (α-Fe2O3) was carried out, and several competing terminations of its (0001) surface modeled via slabs of increasing thickness from twelve to thirty-six Fe-layers. In spite of small quantitative differences, the use of either U or (U-J) corrections showed not to qualitatively affect the results of the simulations both for bulk α-Fe2O3 and the lowest-energy α-Fe2O3(0001) surface studied, regardless of the thickness of the slab used. The energy favored antiferromagnetic ordering of bulk α-Fe2O3 was preserved in the relaxed slabs, with the largest surface-induced effects limited to the outermost three Fe-layers in the slabs. Mixed O- and Fe-terminations were found to be energetically favored and insulating. Conversely, fully O- or Fe-terminated surfaces were calculated to be energetically disfavored and metallic. Finally, the role of Fe- or O- termination for the semiconducting or metallic nature as well as absolute band alignment of α-Fe2O3(0001) surfaces was analyzed and discussed with respect to the challenges in enhancing the activity of α-Fe2O3 samples as photo-electrode for water splitting.