COMBINED IN-SITU SPM AND EIS STUDIES OF PB UPD ON AG(111) AND AG(100)

Experimental results obtained by combined in situ scanning probe micro scopy (SPM) and electrochemical impedance spectroscopy (EIS) studies o f underpotential deposition (UPD) of Pb on Ag(lll) and Ag(100) are cor related and interpreted in terms of the kinetics of local electrode pr ocesses. Exchange current densities at different underpotentials are r elated to different low-dimensional (1D and 2D) Pb phases directly obs erved by in situ SPM. First order phase transitions between expanded ( gas-like) and condensed (liquid- or solid-like) 2D Pb phases as well a s surface alloy formation processes are taken into account for the int erpretation of impedance data, Relatively simple conditions are obtain ed on Ag(111) substrates where the formation of a condensed Pb monolay er starting exclusively at monatomic steps is observed by in situ SPM at low supersaturations with respect to the condensed 2D Pb phase. The analysis of SPM and EIS data indicates that the lateral growth of the condensed Pb monolayer preferentially occurs via a direct transfer of Pb2+ from the electrolyte to the step edges. (C) 1998 Published by El sevier Science Ltd. All rights reserved.