Partially Dissociated Water Dimers at the Water-Hematite Interface

The oxygen evolution reaction (OER) is thought to occur via a four-step mechanism with *O, *OH, and *OOH as adsorbed intermediates. Linear scaling of the *OH and **OOH adsorption energies is proposed to limit the oxides' efficiency as OER catalysts, but the use of simple descriptors to screen candidate materials neglects potentially important water-water interactions. Here, we use a combination of temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), noncontact atomic force microscopy (ncAFM), and density functional theory (DFT)-based computations to show that highly stable HO-H2O dimer species form at the (1 (1) over bar 02) facet of hematite; a promising anode material for photoelectrochemical water splitting. The UHV-based results are complemented by measurements following exposure to liquid water and are consistent with prior X-ray scattering results. The presence of strongly bound water agglomerates is generally not taken into account in OER reaction schemes but may play a role in determining the required OER overpotential on metal oxides.