The role of isotropic and anisotropic Hubbard corrections for the magnetic ordering and absolute band alignment of hematite α-Fe_2O_3(0001)surfaces

An isotropic (+U) and anisotropic [+(U-J)] corrected Density Functional Theory study for bulk hematite (α-Fe_2O_3) 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 bulkα-Fe_2O_3 and the lowestenergyα-Fe_2O_3(0001) surface studied,regardless of the thickness of the slab used.The energy favored antiferromagnetic ordering of bulkα-Fe_2O_3 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 ofα-Fe_2O_3 (0001) surfaces was analyzed and discussed with respect to the challenges in enhancing the activity ofα-Fe_2O_3 samples as photo-electrode for water splitting.