On-surface and subsurface adsorption of oxygen on stepped Ag(2 1 0) and Ag(4 1 0) surfaces

The adsorption of atomic oxygen and its inclusion into subsurface sites on Ag(2 1 0) and Ag(4 1 0) surfaces have been investigated using density functional theory. We find that––in the absence of adatoms on the first metal layer––subsurface adsorption results in strong lattice distortion which makes it energetically unfavoured. However subsurface sites are significantly stabilised when a sufficient amount of O adatoms is present on the surface. At high enough O coverage on the Ag(2 1 0) surface the mixed on-surface + subsurface O adsorption is energetically favoured with respect to the on-surface only adsorption. Instead, on the Ag(4 1 0) surface, at the coverage we have considered (3/8 ML), the existence of stable terrace sites makes the subsurface O incorporation less favourable. These findings are compatible with the results of recent HREEL experiments which have actually motivated this work.