First-principles study on point defects in hematite (α-Fe2O3)

Abstract

Hematite (α-Fe2O3) is receiving much attentions recently for the application to solar water splitting, because it is stable in water like other oxide materials and abundant on the earth crust. In addition, Fe2O3 has a low band gap (2 eV) which enables absorbtion of wide spectrum of sunlight. However, the first-principles study on this material is not sufficient yet. In particular, few studies have been done on the point defects although the material property critically depends on the presence of native defects. In this study, we carry out GGA+U calculations to investigate point defects in Fe2O3. In particular, we consider vacancies and interstitial defects of oxygen and iron, on which we determine relative stability by computing formation energies. The cell-size dependence of charged defects is carefully tested. The defect levels associated with each defect are examined and explained on the basis of bulk electronic structure, and we also consider electron polaron in defect-free structure. Subsequently, the transition levels of charged states are computed and we determine the defect densities and Fermi level positions by the charge neutrality condition. It is found that the intrinsic defects generally lead to n-type condition, which is mainly driven by Fe interstitial defects. This is consistent with the fact that Fe2O3 is used as photoanode in water splitting owing to its intrinsic n-type property. Interestingly, we find that the Fermi level in n-type condition is pinned to 0.55 eV below the conduction band minimum by the formation of electron polaron.