Hematite surfaces: Band bending and local electronic states

Abstract

Hematite is a promising material for photoelectrochemical water splitting applications and understanding the complete mechanism of water oxidation reaction close to its surfaces is of great importance. Herein, we present a theoretical investigation of the electronic states of nonstoichiometric hematite(0001) films in the presence of absorbed H on the surfaces, as well as Ti doping. The calculations, performed at the $\text{DFT}+U$ level, indicate that iron can appear in different oxidation states, from ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{4+}$ and ${\mathrm{Fe}}^{5+}$ in the vicinity of impurities or surfaces. The electron and hole distributions in hematite slabs could control the bending and local state of valence and conduction bands. The projected density of states in different atomic positions shows an upward band bending close to the surfaces, which varies with the number of absorbed H or with Ti doping. In the dark, the width of the depletion layer is computed as less than 12 \AA{} for neutral slabs, while adding excess electrons expands it to involve more atomic layers, depending on the number of absorbed H on the surface. In the neutral slabs in the presence of H, we obtain an upward band bending of 0.18, 0.42, and 0.24 eV by increasing the number of H from one to three on each side, respectively. The evolution of bands in neutral slabs, with/without Ti doping, occurs in a few layers in nearby the surface and also Fe with an oxidation state rather than ${\mathrm{Fe}}^{3+}$. According to our results, doping Ti creates a polaron and locally affects the band structure. In the charged surface with excess electrons, a metallic behavior is observed on the surface and a large band bending in the middle layers. This work shows that a substantial upward bending of the bands in hematite can exist even at neutral surfaces in contact with a vacuum.