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.