Thermodynamics and kinetics of underpotential deposition of metal monolayers on polycrystalline substrates

The equilibrium and dynamic properties of underpotential deposits are discussed, as are some problems of methodology in estimating these properties. In particular, there is a special emphasis on interpreting the ring-disc electrode response during the underpotential deposition, UPD, of silver on polycrystalline gold. The underpotential-coverage relationship as a function of solution composition provides an insight into particle-substrate, particle-particle and particle-electric field interactions. Some of the features that emerge from the analysis of equilibrium UPD behaviour are: (a) the predominant influence of the particle-substrate interaction through work function changes, (b) the absence of discreteness of charge effects due to any partially charged UPD species over wide coverage ranges, and (c) the importance of particle-particle interactions in causing multiple energy states. The dynamic charge flux during the UPD process is analysed in terms of dynamic changes in double layer capacitance and the potential of zero charge, and other parallel processes through the formalism of a dynamic electrosorption valency, which includes both charge separation and charge-transfer components. The dynamic material flux of the UPD species is found to be under mixed control by solution mass transport, surface adsorption and charge-transfer processes. The phenomenological rate parameters and double layer capacitance changes suggest the formation of ordered structures at higher UPD coverages and random aggregates at lower coverages.