Fe 2p photoemission line shape analysis for α-Fe₂O₃ (001) and (012) single crystal surfaces

We have investigated the Fe 2p core-level spectra of well-ordered α-Fe₂O₃ (001) and (012) single-crystal surfaces using X-ray photoemission spectroscopy (XPS), followed by an analysis based on the understandings obtained from our previous theoretical cluster-model calculations; a particular concern is for the energy splittings and XPS intensities of the various multiplets of the ionized configurations. We experimentally observed that, depending on crystallographic orientation and preparation conditions, the outermost Fe cations on the surface will have distinct and different oxygen coordinations; specifically Fe_6C³⁺ and Fe_5C³⁺. In particular, the differently coordinated Fe cations at the α-Fe₂O₃ surface have complex multiplet structures for the Fe 2p XPS which are examined in detail for the Fe 2p_3/2 spectral region. The Fe 2p photoemission region could be consistently assigned as arising from sets of multiplet split components which arise from the angular-momentum coupling between the open 2p and 3d shells, reflecting both the coordination environments and the nominal oxidation states. The line shapes given by the relative intensities and energies of the multiplets components are a direct consequence of the covalent mixing between the Fe 3d and O 2p shells. These findings suggest that precise analysis of the Fe 2p line shape can provide a means to determine the relative concentration of Fe cations on surfaces, offering an essential methodological tool for identifying the presence and concentration of the Fe cations on hematite-based materials, for instance, nanoparticles or films employed in heterogeneous catalysis, electrochemistry, or biochemistry-related applications.