COVERAGE-DEPENDENT STRUCTURES OF SULFUR ON PT(111) STUDIED BY LOW-ENERGY-ELECTRON DIFFRACTION (LEED) AND SCANNING-TUNNELING-MICROSCOPY (STM)

The structures of sulfur at different coverages on the Pt(lll) surface have been examined using low-energy electron diffraction (LEED) and s canning tunneling microscopy (STM). The following ordered structures w ere identified as a function of coverage: (2 x 2) at 1/4 ML (monolayer ), (root 3 x root 3)R30 degrees at 1/3 ML and c(7 x root 3)rect. at 3/ 7 h-IL. Dynamical LEED calculations were used to determine the adsorpt ion geometry of the sulfur atom and examine the adsorbate-induced surf ace relaxations of the (2 x 2) and (root 3 x root 3)R30 degrees struct ures. The best agreement with calculated LEED I-V curves was obtained for sulfur adsorption in the fee hollow sites for both overlayers. For the (2 x 2) structure, the first- and the second-layer Pt atoms are r elaxed in the perpendicular and perhaps in the lateral directions. For the (root 3 x root 3)R30 degrees structure, perpendicular relaxations are not allowed by symmetry, while small lateral relaxations are poss ible. The S-Pt bond length is 2.24 Angstrom for the (2 x 2) and 2.25 A ngstrom for the (root 3 x root 3)R30 degrees structure. STM studies of the high-coverage c(7 x root 3)rect-3S structure indicate that there are three sulfur atoms per unit cell, with two sulfur atoms in fee hol low sites and one in an hcp hollow site. (C) 1997 Elsevier Science B.V .