Computational Studies of Experimentally Observed Structures of Sulfur on Metal Surfaces

First-principles electronic structure calculations were carried out to examine the experimentally observed structures of sulfur on close-packed surfaces of a number of important metals -Ag(111), Cu(111), Ni(111), Pt(111), Rh(111), Re(0001), and Ru(0001). At low coverages (<= 1/3 ML), the prediction is consistent with the typical pattern of preferred sulfur occupancy of 3-fold hollow sites, notably the fcc site on the (111) surfaces and the hcp site on the (0001) surfaces. Theoretical confirmation for the existence of pure sulfur overlayer phases on Pt(111), Rh(111), Re(0001), and Ru(0001) at higher coverages (>1/3 ML) was provided. For the (root 7 x root 7) phase seen on Ag(111), the most preferred structure identified for the adsorbed S trimer consists of a S atom on the top site bonded to two S atoms situated on the nearest neighbor off-bridge site positions. Among the different densely packed mixed sulfur-metal overlayer models suggested for the (root 7 x root 7) phase on Cu(111), the structure which consists of metal and S atoms in a hexagonal-like arrangement on the top substrate was found to be the most energetically favorable. For the (5 root 3 x 2) phase on Ni(111), the calculations confirm the existence of clock-reconstructed top layer metal atoms onto which sulfur atoms are adsorbed.