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 ZE, 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 ZE :-Z (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 thinlayer 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/T1 +, 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-steadystate 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)/T1 + can be explained by assuming the formation of superlattices on the electrode surface.Kinetic investigations in the systems Au/Ag +, Au/T1 +, 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.Crystallization phenomena play an important role in the case of cathodic metal deposition. They include different steps after the metal ion has passed through the electrochemical double layer and is still partially solvated. The final state can be described as the in-* Electrochemical Society Active Member. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 205.208.120.206 Downloaded on 2014-12-26 to IP ) unless CC L...

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

“…They also argued that the fact that measured charges in many UPD systems correspond just to a monolayer [42] is strong evidence for the lack of crystallisation process taking place, which would be more dependent on pretreatment of electrodes and purity of solutions. However, the surface coverage corresponding to a complete adsorbed layer are dependent on whether adsorbed atoms are restricted to interstitial sites between surface atoms in the electrode or whether a close-packed structure.…”

“…In addition the possibility of phase transformations occurring within the UPD monolayer during the underpotential deposition process was introduced and discussed. However, the work of some other authors [57][58][59][60][61][62][63] performed in the same systems, which soon followed, disputed these conclusions and showed that some of the peaked i-t transients could alternatively result from adsorption processes, although the experimental results were more or less confirmed. The main objection was that no really conclusive evidence for phase transformation through 2D nucleation and subsequent crystal growth.…”

“…Multipeak structure during linear sweep induced UPD on single crystals became a topic of investigation relatively recently [42,46,64,65] and immediately invited controversy. Some authors [64] claimed that no dependence of the deposition peak potential on substrate orientation was observed (indeed that only single deposition peak was increasing in the order Q (100) ≥ Q (110) ≥ Q (111) and the phenomenon was explained trough the superlattice model of adsorption.…”

“…Authors of the work done on UPD of lead and thallium on carefully prepared silver single crystals [47][48][49][50][51][57][58][59][60][61][62][63] although differing in explanation of the monolayer structure, do agree on the following: a) the L.S.V. peak potentials depend on crystal orientation of the substrate, b) the charge associated with the monolayer depends on the substrate, c) the multipeak structure arises from different states of the atoms, i.e.…”

The development of a micropatterned electrode for studies of zinc electrodeposition

“…These early results were perhaps limited in scope, but clearly showed that monolayer deposition at underpotentials results from a strong interaction between the monolayer and the substrate, which may be modelled empirically as an activity less than unity for the deposit, Lorenz used a twin electrode thin~layer (TTL) cell to study UPD [68], where the working and generator electrodes are 50 microns apart and have independent potential regulation. The reference and counter electrodes are held in a separate reservoir with supporting electrolyte.…”