A systematic density functional study of ordered sulfur overlayers on Cu(1 1 1) and Ag(1 1 1): Influence of the adsorbate coverage

The interaction of atomic S with the Cu(1 1 1) and Ag(1 1 1) surfaces has been studied using density functional calculations and periodic slab models. Relevant properties were obtained as a function of the S coverage reaching values as low as θ = 1/28 ML. Some properties such as perpendicular distance from the S atom to the surface, the adsorbate net charge or the vibrational frequency for the normal model corresponding to the perpendicular distance to the surface can be considered as converged a relatively high coverage (θ = 1/4 ML). However, the adsorption energy and properties related to the electronic structure of the metal substrate exhibit small but noticeable variations depending on the coverage. A particularly interesting finding is that presence of a small amount of S on the metal surface provokes significant distortions of the topmost atomic layer and induces a significant variation of the surface work function. The implications of these findings for the reactivity of the S covered Cu(1 1 1) and Ag(1 1 1) metal surfaces are discussed.