On the orbital anisotropy in hematite nanorod-based photoanodes

The orbital anisotropy of hematite (α-Fe₂O₃) nanorod arrays, an engineered structure commonly investigated for applications in solar water oxidation photoanodes, is probed using polarization-dependent soft X-ray absorption spectroscopy at the O K-edge and at the Fe L _2,3-edge. Thereby the unoccupied states of α-Fe ₂O₃ are examined. In the lowest energy region these are found to be strongly-hybridized Fe 3d (a_1g) orbitals and O ^2- ligand 2p orbitals, oriented along the c-axis. For [110]-oriented α-Fe₂O₃ nanocrystals the observed direction of strong hybridization is parallel to the substrate surface (perpendicular to the direction of electron conduction and light propagation in operating electrodes). The Fe L₃-edge line shape and aspects of polarization dependence can be reproduced by crystal field atomic multiplet calculations of 2p-to-3d transitions for Fe³⁺ in the D_3d point group symmetry of metal ions in the corundum structure. Both the O K-edge and Fe L ₃-edge spectra possess features that may be related to the high density of surface atoms in this nanoscale system. They are associated with partial coordination and therefore reduced symmetry compared to that for Fe ³⁺ in bulk crystals.