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) techni
que. Ag UPD was found to occur stepwise and similar in both systems. A
t relatively high underpotentials expanded and commensurate Ag adlayer
s 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 trans
ition. On Au(lll), the condensed Ag phase is preferentially created on
monatomic steps, whereas on Au(100) a simultaneous 2D nucleation of A
g on flat terraces is observed. Electrosorption valency measurements i
n the system Au(hkl)/Ag+, SO42- show gamma - z = 1, indicating that co
adsorption or competitive adsorption of anions can be excluded. Electr
ochemical and in situ STM results indicate similar charge-coverage beh
aviour in the system Au(hkl)/Ag+, ClO4-. Ag UPD on Au(hkl) occurs at p
ositive 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.