Quantitative Measurement\nof Cooperative Binding in\nPartially Dissociated Water Dimers at the Hematite “R-Cut”\nSurface

Water–solid\ninterfaces pervade the natural environment\nand\nmodern technology. On some surfaces, water–water interactions\ninduce the formation of partially dissociated interfacial layers;\nunderstanding why is important to model processes in catalysis or\nmineralogy. The complexity of the partially dissociated structures\noften makes it difficult to probe them quantitatively. Here, we utilize\nnormal incidence X-ray standing waves (NIXSW) to study the structure\nof partially dissociated water dimers (H<sub>2</sub>O–OH) at\nthe α-Fe<sub>2</sub>O<sub>3</sub>(012) surface (also called\nthe (11̅02) or “R-cut” surface): a system simple\nenough to be tractable yet complex enough to capture the essential\nphysics. We find the H<sub>2</sub>O and terminal OH groups to be the\nsame height above the surface within experimental error (1.45 ±\n0.04 and 1.47 ± 0.02 Å, respectively), in line with DFT-based\ncalculations that predict comparable Fe–O bond lengths for\nboth water and OH species. This result is understood in the context\nof cooperative binding, where the formation of the H-bond between\nadsorbed H<sub>2</sub>O and OH induces the H<sub>2</sub>O to bind\nmore strongly and the OH to bind more weakly compared to when these\nspecies are isolated on the surface. The surface OH formed by the\nliberated proton is found to be in plane with a bulk truncated (012)\nsurface (−0.01 ± 0.02 Å). DFT calculations based\non various functionals correctly model the cooperative effect but\noverestimate the water–surface interaction.