Quantitative Measurement of Cooperative Binding in Partially Dissociated Water Dimers at the Hematite “R-Cut” Surface

Water-solid interfaces pervade the natural environment and modern technology. On some surfaces, water-water interactions induce the formation of partially dissociated interfacial layers; understanding why is important to model processes in catalysis or mineralogy. The complexity of the partially dissociated structures often makes it difficult to probe them quantitatively. Here, we utilize normal incidence X-ray standing waves (NIXSW) to study the structure of partially dissociated water dimers (H₂O-OH) at the α-Fe₂O₃(012) surface (also called the (11̅02) or "R-cut" surface): a system simple enough to be tractable yet complex enough to capture the essential physics. We find the H₂O and terminal OH groups to be the same height above the surface within experimental error (1.45 ± 0.04 and 1.47 ± 0.02 Å, respectively), in line with DFT-based calculations that predict comparable Fe-O bond lengths for both water and OH species. This result is understood in the context of cooperative binding, where the formation of the H-bond between adsorbed H₂O and OH induces the H₂O to bind more strongly and the OH to bind more weakly compared to when these species are isolated on the surface. The surface OH formed by the liberated proton is found to be in plane with a bulk truncated (012) surface (-0.01 ± 0.02 Å). DFT calculations based on various functionals correctly model the cooperative effect but overestimate the water-surface interaction.