Water on FeO(111) and Fe3O4(111):: Adsorption behavior on different surface terminations

The adsorption of water on ordered epitaxial FeO(111) and Fe3O4(111) films was investigated by thermal desorption spectroscopy (TDS) and photoelectron spectroscopy (UPS, XPS) under adsorption-desorption equilibrium conditions. On the purely oxygen-terminated FeO(111) surface, water monomers get physisorbed first, followed by the formation of a hydrogen-bonded bilayer with an ice-like structure and condensation of ice multilayers as the coverage is increased. On the Fe3O4(111) surface exposing both iron and oxygen atoms, water dissociates resulting in adsorbed hydroxyl groups, followed by coadsorption of water monomers and condensation of ice multilayers. A quantitative comparison between the hydroxyl saturation coverage and the defect concentrations deduced from LEED and STM measurements rules out a purely defect related dissociation of water. It is proposed that OH- groups are bound to iron cations and the H+ species to oxygen anions exposed in the topmost layer of the regular Fe3O4(111) surface. The comparison between the FeO(111) and Fe3O4(111) surface chemistry demonstrates that the chemical reactivity of metal oxides is related to surface metal sites. The saturation coverages, isosteric heats of adsorption, preexponential frequency factors, and initial dipole moments of the different species were determined quantitatively. On the basis of these data, structural models for the adsorbed phases on the iron oxide surfaces are proposed.