Sulfur-Induced Reconstruction of Ag(111) Surfaces Studied by DFT

Density functional theory has been used to investigate the adsorption of sulfur atoms in the (root 7 x root 7)R19.1 degrees unit cell of Ag(111). For the coverages theta = 1/7 and 2/7 the S atoms adsorb at fcc and hcp hollow sites with negligible reconstruction of the surface. For theta = 3/7 a large surface reconstruction occurs. The three Ag atoms around the hcp site that hosts a S atom move vertically and in-plane away from the hollow center provoking two effects: (i) the S atom adsorbed at this site penetrates under the surface and (ii) a top site hosting the third S atom is transformed into a new 3-fold hollow site. A detailed analysis of the different contributions to the adsorption energy shows that the energy cost of deforming the lattice is paid off by the stronger and more favorable bonding of these two S atoms. This picture is confirmed by the new charge distribution after the reconstruction.