Ag UPD on Au(100) and Au(111)

The underpotential deposition (UPD) of Ag in the systems Au(hkl)/Ag+, ClO4- and Au(hkl)/ Ag+, SO42- with (hkl) = (100), (111) was studied by in situ STM and different electrochemical techniques including charge-coverage measurements with the twin-electrode thin-layer (TTL) technique. Ag UPD was found to occur stepwise and similar in both systems. At relatively high underpotentials expanded and commensurate Ag adlayers Au( 100)-c(root 2x5 root 2)R45 degrees Ag and Au(111)-(4 x 4)Ag are formed. At low underpotentials, condensed commensurate overlayers with Au(hkl)-(1 x 1)Ag structures are formed via a first order phase transition. On Au(lll), the condensed Ag phase is preferentially created on monatomic steps, whereas on Au(100) a simultaneous 2D nucleation of Ag on flat terraces is observed. Electrosorption valency measurements in the system Au(hkl)/Ag+, SO42- show gamma - z = 1, indicating that coadsorption or competitive adsorption of anions can be excluded. Electrochemical and in situ STM results indicate similar charge-coverage behaviour in the system Au(hkl)/Ag+, ClO4-. Ag UPD on Au(hkl) occurs at positive potentials with respect to the potentials of zero charge of Au(hkl) and Ag(hkl). Therefore, a nearly constant anion coverage in the entire Ag UPD range cannot be excluded. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.