Substrate-termination and H2O-coverage dependent dissociation of H2O on Fe3O4(1Â 1Â 1)

The reaction of Fe3O4(1 1 1) with water vapour has been studied with scanning tunnelling microscopy (STM) and with X-ray and UV-photoemission as a function of water partial pressure and temperature. The photoemission results point to dissociation to form surface hydroxyls at a partial pressure of 10−6 mbar H2O and a substrate temperature of about 200 K. At 298 K it is known that dissociation occurs at around 10−3 mbar [Kendelewicz et al., Surf. Sci. 453 (2000) 32]. This difference suggests that an intermolecular mechanism of dissociation is involved. It also suggests that the pressure dependence arises from a coverage term rather than differences in the Gibbs Free Energies of the oxide and hydroxide, as previously proposed. The STM results indicate that dissociation takes place on a termination of Fe3O4(1 1 1) thought to contain a 1/4 monolayer (ML) of Fe3+ ions on top of a close-packed oxygen monolayer.