Molecular modeling of water adsorption on hematite

This paper describes the results of modeling the surface hydration configur ations formed when different planes of the hematite crystal were exposed to water using empirically derived potentials able to replicate the hematite, goethite and lepidocrocite structures to within 2% of their measured value s. The planes chosen were the {111}, {011} and {210} planes expressed in rh ombohedral coordinates. It was found that of all the surfaces studied there was a preference for hydration on the O-terminated basal {111} plane. This plane had the lowest hydrated surface energy and it was also the most stab ilised by reaction with water. The Fe-terminated {111} plane was found to b e unstable in the presence of excess water (greater than or equal to 67% co verage). The surface iron atoms relax away from the simulation cell to leav e the O-terminated hydrated layer behind. Chemisorption may be energeticall y feasible at low surface coverages (< 67% coverage). The {011} plane of he matite showed a preference for 100% water coverage (full coordination of th e surface iron atoms). The surface energy of adsorbing water on this plane was lower than for the {210} plane particularly at high water coverages. Th e {210} plane was not stabilised by reaction with water at any coverage. Th e surfaces underwent relaxations depending on the water coverage. Large rel axations were observed at lower coverages for the {011} plane while the lar gest relaxations were observed at higher coverages on the {210} plane.