Reaction of water with clean surfaces of MnO(100)

Water adsorption on the MnO(100) single crystal surface was quantitatively monitored using chemical shifts in the O 1s core level spectra. We find that the initial reactions at low water vapor pressures of 10(-8)-10(-4) Torr are limited to a small fraction of a monolayer and involve dissociation of water at or near defect centers resulting in surface hydroxyls. Even at these low pressures, there is an increase in water adsorption with increasing dosing time. Water uptake increases dramatically for vapor pressures between 10(-4) and 10(-2) Torr and then stabilizes above 10(-2) Torr. At this pressure, hydroxyl coverages slightly exceed a monolayer. Hydroxyl species formed on this surface are unstable and can be removed by annealing to 200 degrees C. These data are compared with those from similar studies of the interaction of water vapor with MgO(100) and the surfaces of iron oxides (alpha-Fe2O3 and Fe3O4), and some quantitative similarities and differences are discussed. In particular, the role of cooperative interactions in the condensed water overlayer in surface hydroxylation is emphasized.