Electronic Structure of Excitons in Hematite Fe₂O₃

We present a computational study of the electronic structure of excitons in hematite Fe₂O₃ using density functional theory (DFT) and time-dependent DFT theory. Excitons resulting from solar light absorption are precursors to photogenerated charge carriers, which are the active species in solar-to-fuel redox conversions. Upon vertical excitation, the lowest energy exciton is found to have a multideterminantal wave function, describing an electron excited from O 2p states to Fe 3d states of the nearest basal plane, possibly supporting a recent XUV-derived exciton structure with a radius of a single Fe-O bond. Subsequently, the exciton self-traps into stable electron-hole pair structures whose wave functions are well-approximated by single excitations conveniently labeled “HOMO-LUMO”. Self-trapping yields electron-hole pair structures whereby the electron state (electron polaron-like) is separated from the hole state (hole polaron-like) by 3, 5, 7, 9, ... basal planes in structures referred to as Exc-3, Exc-5, Exc-7, Exc-9, ... The natural transition orbitals (electron-hole states) exhibit a strong “localized” character. The hole state is best described as a (FeO₆)⁺ octahedral moiety carrying ∼0.65 h⁺ charge, and overall ∼70% of the hole is assigned to O 2p atomic states, mostly on the four (4) equatorial O atoms of the moiety. The electron state is best described as a (FeFe)⁻ moiety carrying ∼0.80 e^- charge in Fe 3d atomic states. The lattice distortions around the hole site exhibit Fe-O bond shortening due to the removal of electron density from an orbital state with O-Fe-O antibonding character. The electron site shows Fe-Fe distance shortening due to excess electron density added to an orbital state with in-phase Fe-Fe interactions. These excitonic structures with increasing electron-hole separation can be viewed as the onset of exciton separation into isolated electrons and hole polarons.