Activation of Oxygen on MgO: O_2~(·-) Radical Ion Formation on Thin, Metal-Supported MgO(OOl) Films

摘要

Thermodynamically stable, fully stoichiometric surfaces of simple oxides such as MgO(100), are generally believed to be inactive with respect to chemical reactions. Defects are thought to control the chemistry occurring on such surfaces. While this perception is certainly true for bulk oxides this may or may not hold if the oxide surface is in close proximity to a metal as it is found for example in supported ultrathin films. The idea that the supporting metal may influence the properties of adsorbates and thus their reactivity can be traced back to the model of Cabrera and Mott from the late 1940s explaining the growth of passivating (oxide) layers on Cu and ideas developed by Vol'kenshtein who pointed out a few decades ago that the chemical properties of the oxide might be altered significantly if used as a thin film of a few monolayer in thickness. This subject has recently found resurgence by theoretical calculations predicting altered properties for metal deposits as well as molecular adsorbates. In particular, it was predicted that metals with a rather high electron affinity, such as gold, will become negatively charged when adsorbed on thin oxide films with a rather low work function, such as MgO grown on Mo or Ag. By now there is experimental verification for this prediction.