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 make it difficult to probe them in a quantitative manner. Here, we utilize normal incidence x-ray standing waves (NIXSW) to study the structure of partially dissociated water dimers (H2O-OH) at the Fe2O3(012) surface (also called (1-102) 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 Angstrom and 1.47 +/- 0.02 Angstrom, 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 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 Angstrom). DFT calculations based on various functionals correctly model the cooperative effect, but overestimate the water-surface interaction.