DYNAMICS OF SURFACE ALLOYING PROMOTED BY CADMIUM UNDERPOTENTIAL DEPOSITION ANODIC-STRIPPING CYCLES ON IRREGULAR SILVER ELECTRODEPOSITS

The underpotential deposition of Cd from dilute aqueous 5 x 10(-3) M CdSO4 + 0.5 M Na2SO4 + 0.01 M H2SO4 on Ag electrode-posits grown on polyfaceted Pt single-crystal microspheres was studied using potentiodynamic and potentiostatic techniques, transmission electron diffraction, scanning tunneling microscopy and X-ray photoelectron spectroscopy. Ag electrodeposits were grown in 5 x 10(-3) M Ag2SO4 + 0.5 M Na2SO4 + 0.01 M H2SO4 at cathodic overpotentials eta(c) of -0.06 V and -0.58 V to form a columnar and dendritic Ag surface respectively. Surface rearrangement processes involving the formation of Cd + Ag alloys were studied by stripping voltammetry after the potential was held for a certain time at different preset values covering the potential range in which the degree of surface coverage by Cd atoms was less than or close to unity (theta(c) less-than-or-equal-to 1) on both Ag surfaces. Surface diffusion and penetration of Cd atoms into bulk Ag for theta(c) less-than-or-equal-to 1 are discussed. A place-exchange mechanism is advanced to account for the kinetics of Cd + Ag alloy formation.