In Situ Surface Exafs Study of an Underpotentially Deposited Silver Monolayer on Gold (111)

Fluorescence detected surface extended x-ray absorption fine structure (EXAFS) has been used to probe the structure of underpotentially deposited Ag on a Au(111) surface from an aqueous solution using sodium perchlorate as the electrolyte. Silver is shown to be fully reduced on the surface with observed bond distances of 2.88 + 0.025 A and 2.91 + 0.025 A for silver-silver and silver-gold near neighbors, respectively. Data is obtained with the polarization of the incident x-ray beam both parallel and perpendicular to the electrode surface. From this, we show that the silver monolayer is (1 x 1) commensurate with the substrate and that the Ag ad-atoms occupy three-fold hollow sites. In addition to backscattering from gold and silver near neighbors, backscattering from oxygen, presumably adsorbed water is observed. We have studied this at different electrode potentials (+0.7, +0.5, -0.1 V us. Ag/AgCl). At all three potentials the measured silver-oxygen distance is the same: 2.21 ± 0.025 A. This is contrasted to another system where this previously has been studied, Pb on Ag(111) where the metal-oxygen distance varies with applied potential. © 1993, The Electrochemical Society, Inc. All rights reserved.