Molecular modeling of water adsorption on hematite

Abstract

This paper describes the results of modeling the surface hydration configurations 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 values. The planes chosen were the {111}, {011} and {210} planes expressed in rhombohedral 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 stabilised by reaction with water. The Fe-terminated {111} plane was found to be unstable in the presence of excess water (67% coverage). The surface iron atoms relax away from the simulation cell to leave the O-terminated hydrated layer behind. Chemisorption may be energetically feasible at low surface coverages (<67% coverage). The {011} plane of hematite showed a preference for 100% water coverage (full coordination of the surface iron atoms). The surface energy of adsorbing water on this plane was lower than for the {210} plane particularly at high water coverages. The {210} plane was not stabilised by reaction with water at any coverage. The surfaces underwent relaxations depending on the water coverage. Large relaxations were observed at lower coverages for the {011} plane while the largest relaxations were observed at higher coverages on the {210} plane.