A study of the initial stages of the electrochemical deposition of metals on foreign substrates: Lead and thallium on copper and silver surfaces: General discussion

Abstract

The basic applicability of the UPD theory of Gerischer et al. has been confirmed in the case of lead and thallium deposition on copper and silver single crystals, but it was shown also that this is only one of the important factors. It was established that the effect of substrate structure on the mechanism of the UPD monolayer formation plays an important role in the addition to the effect of electronegativity difference between the substrate and depositing atoms. An effect of changing the concentration of depositing metal and specifically adsorbing anion were found and attributed to the changes in the electronegativity of the substrate and underpotential layer as a result of changing the electrode potential. It was conclusively proven by the analysis of the peaked current-time transients obtained by potential step measurements and the very sharp voltammetry peaks, that first order 2D phase transformations are possible in UPD systems. It was also found that the L.S.V. peaks reflecting such processes must not be expected to be free from the influence of slow kinetics. It was established that the 2D crystalline metal-like phases with the closest-packed epitaxial structure exist as a stable, final product in UPD. Moreover, the possibility of higher order 2D transformations in UPD has been given strong support by the evidence of gradual monolayer density change with increasing driving force. Lead UPD on vitreous carbon was found to be the result of substrate reconstruction induced by the repeated deposition and dissolution leading to surface condition changes favoring deposition of the first layer. UPD monolayers on copper and silver surfaces always preceded OPD and had a profound effect on its nucleation overpotential (making it very small indeed) or even changing its character from 3D into 2D (in the case of vitreous carbon substrate). It was also shown that lead and thallium OPD on copper and silver single crystals starts off by charge transfer controlled instantaneous 3D nucleation and subsequent growth of 3D centers. Finally, the results obtained in the experiments performed have shown that electrochemical techniques can prove very suitable in the study of 2D phase transformation kinetics and that their further employment in this field should prove particularly rewarding.