The Formation of Monolayer Metal Films on Electrodes

Abstract

The two‐ and three‐dimensional nucleation and the metal ion adsorption process are discussed as initial steps for electrocrystallization on the same or on a foreign substrate. The metal ion adsorption takes place in the underpotential range, which is more positive than the equilibrium potential of the nucleated metal/metal ion electrode. Thermodynamic considerations show that an overlapping of disturbing processes in the underpotential range can be characterized by a factor , which is called charge‐coverage coefficient or electrosorption valency. It is a function of different independent variables. The metal monolayer model is satisfied only in the simplest case for (valency of metal ions in solution).The different experimental methods for determining the thermodynamics, kinetics, and structure of metal ion adsorbates are critically discussed. The most reliable thermodynamic results are obtained by the twin‐electrode thin‐layer technique. Kinetic data can be obtained by pulse measurements under potentiostatic‐galvanostatic conditions. The structure of metal films can be determined by combining different methods including optical investigations.Results are given for different systems. In the systems Au/Ag+ , Au/Tl+, and Au/Pb++ the metal monolayer model is practically valid. In the case of Cu++ ion adsorption on Au the redox reaction forming Cu+ ions acts as a disturbing process. A cosorption of anions takes place in the system Ag/Pb++. The formation of alloys in the underpotential range was observed in the systems Pt/Pb++ and Bi/Pb++.The structure of metal ion adsorbates must be in correlation with the nucleation phenomena at more negative potentials. In many systems no three‐dimensional nucleation overvoltage is observed by using a quasi‐steady‐state experimental technique. The density of the crystal imperfections and the crystallographic orientation of the surface planes influence the metal ion adsorption process and the structure of films. The results obtained in the systems Ag (poly‐ and monocrystalline)/Tl+ can be explained by assuming the formation of superlattices on the electrode surface.Kinetic investigations in the systems Au/Ag+, Au/Tl+, and Au/Pb++ show relatively high exchange current densities for the metal ion adsorption process. In these cases the experimental results can be explained by assuming surface diffusion of adions as the rate determining step in the adsorption process. In the system Au/Cu++, a charge transfer step is rate determining, which probably forms precedingly Cu+ ions.