New insights on the Cd UPD on Au(111)

Abstract

The Cd underpotential deposition (UPD) process on Au(111) was analyzed by means of combined electrochemical measurements and in situ scanning tunneling microscopy (STM). In the underpotential range 300 ≤ ΔE (mV) ≤400, 2D Cd islands are formed on the fcc regions of the Au(111)-(√3 × 22) reconstructed surface without lifting the reconstruction. At lower underpotentials, the 2D Cd islands grow and, simultaneously, new 2D islands nucleate and coalesce with the previous ones forming a complete condensed Cd monolayer (ML). STM images and long time polarization experiments performed at ΔE = 70 mV demonstrate the formation of an Au-Cd surface alloy. At ΔE = 10 mV, the formation of the complete Cd ML is accompanied by a significant Au-Cd surface alloying and the kinetic results reveal two different solid-state diffusion processes. The first one, with a diffusion coefficient D1 = 4 × 10-17 cm2 s -1, could be ascribed to the mutual diffusion of Au and Cd atoms through a highly distorted (vacancy-rich) Au-Cd alloy layer. The second and faster diffusion process (D2 = 7 × 10-16 cm2 s -1) is associated with the appearance of an additional peak in the anodic stripping curves and could be attributed to the formation of another CdzAux alloy phase.