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Abstract
The underpotential deposition of Cd from dilute aqueous 5 × 10 −3 M CdSO 4 + 0.5 M Na 2SO 4 + 0.01 M H 2SO 4 on Ag electrodeposits 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 × 10 −3 M Ag 2SO 4 + 0.5 M Na 2SO 4 + 0.01 M H 2SO 4 at cathodic overpotentials η 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 (θ c⩽ 1) on both Ag surfaces. Surface diffusion and penetration of Cd atoms into bulk Ag for θ c ⩽ 1 are discussed. A place-exchange mechanism is advanced to account for the kinetics of Cd + Ag alloy formation.
Highlights
The early stages of metal phase growth of an epitaxial film imply the formation of an interfacial region which can be described by different models involving a discontinuous monolayer, a transition monolayer or a diffusive interface [1,2]
Pb underpotential deposition Columnar and dendritic Ag surfaces were characterized by Pb UPD-anodic stripping cyclic voltammetry in 0.1 M NaClO, + 5 >G10h3 M HCIO, + 10m2 M Pb(ClO,), at 0.01 V s-l in the 0 V to -0.43 V range
Pb underpotential deposition The Pb UPD-anodic stripping voltammograms recorded on columnar Ag electrodes (Fig. l(a)) show a pair of small broad peaks covering the -0.24 V to - 0.32 V range [17]
Summary
The early stages of metal phase growth of an epitaxial film imply the formation of an interfacial region which can be described by different models involving a discontinuous monolayer, a transition monolayer or a diffusive interface [1,2]. Alloy formation is possible even in the UPD potential range in those systems forming stable intermetallic phases or exhibiting sufficient miscibility between substrate and adsorbate metals. This behavior has been observed in a number of polycrystalline [6-81 and single-crystal [9-161 substrates. Martins et al / Surface alloying promoted by Cd underpotential deposition-stripping on Ag spectively [17,18] Such electrodeposits have recently been considered as reference systems for describing irregular surface growth patterns predicted by theoretical models [19]. Dermalloy kinetics appear to be determined by the mobility of Ag surface atoms into a mobile Cd surface layer
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