Water ordering and surface relaxations at the hematite (1 1 0)–water interface

Structural characterization of iron oxide–water interfaces provides insight into the mechanisms through which these minerals control contaminant fate and element cycling in soil, sedimentary, and groundwater systems. Ordering of interfacial water and structural relaxations at the hematite (1 1 0) surface have been investigated in situ using high-resolution specular X-ray reflectivity. These measurements demonstrate that relaxations are constrained to primarily the top ∼5 Å of the surface. Near-surface iron atoms do not relax substantially, although the uppermost layer displays an increased distribution width, while the undercoordinated oxygens on the surface uniformly relaxed outward. Two sites of adsorbed water and additional layering of water farther from the surface were observed. Water fully covers the (1 1 0) surface and appears to form a continuous network extending into bulk solution, with positional order decreasing to that of a disordered bulk fluid within 1 nm. The arrangement of water is similar to that on the hematite (0 1 2) surface, which has a similar surface topography, although these surfaces display different vibrational amplitudes or positional disorder of adsorbed water molecules and average spacings of near-surface layered water. Comparison between these surfaces suggests that interfacial water ordering on hematite is controlled primarily by surface structure and steric constraints and that highly ordered water is likely common to most hematite–water interfaces.