Underpotential deposition of Cd on Ag(111): an in situ STM study

摘要

The kinetics and mechanism of Cd underpotential deposition (UPD) and involved surface alloy formation processes the system Ag(1 1 1)/Cd2+,SO42- are studied by means of combined electrochemical measurements and in situ scanning tunneling microscopy (STM). The results show that the UPD process starts with a formation of an expanded (diluted) adlayer with a superlattice structure Ag(1 1 1)-(root3- x root19)R23.4degrees. In the underpotential range 50 mV < DeltaE < 80 mV this adlayer transforms to a condensed close packed Cd monolayer via a first order phase transition. At long polarization times the condensed monolayer undergoes structural changes involving place exchange processes between Cd atoms and surface Ag atoms. A formation of a second Cd monolayer and a significant Ag-Cd surface alloying take place at lower underpotentials (DeltaE < 50 mV). The kinetics of surface alloying are analyzed on the basis of a recently proposed diffusion model including a relatively fast initial formation of a very thin surface alloy film and a subsequent slow alloy growth controlled by solid state diffusion. The anodic dealloying results in an appearance of monatornically deep pits, which disappear quickly at relatively high underpotentials (DeltaE > 550 mV) indicating a high mobility of surface Ag atoms. (C) 2004 Elsevier B.V. All rights reserved.