Effect of the Microstructure of Copper Films on the Damping of Oscillating Quartz Resonators*

An electrochemical procedure is described which allows the preparation of copper films of various crystallinity. Impedance spectra recorded for copper loaded quartz resonators were analysed in terms oft he lumped-element circuit of the Butterworth-Van Dyke type to obtain their electrical and mechanical properties. Plots of the damping resistance versus film thickness indicate that the film's dissipation factor is significantly smaller in the case of disordered films with a finer crystallinity (10—100nm) than in the case of more ordered structures having a grain size between 600—1500nm. This observations states, that the finely structured copper phase behaves more rigid than the coarse material. The suggested explanation relates this effect to energy losses which occur during oscillation at the phase boundary of the grains by wearless internal friction. No contributions to the damping from surface roughness were observed for films thicker 0.5pm. Thus, the damping of the quartz oscillator caused by different degrees of surface roughness of the generated copper films was of secondary importance, compared with the effect of the crystallinity

Hydrogen evolution reaction on electrodes coated with conducting-polymer films

The cathodic hydrogen evolution reaction has been studied on gold electrodes coated with poly(N-methylpyrrole)(PMPy), and with a polymer mixture of PMPy and poly(4-styrenesulfonate)(PMPy–PSS). In all cases, H+ ions were found to permeate the film and react at the metal-electrode surface. In particular, for low concentrations of HCl, the PMPy film hinders significantly the movement of the depolarizer to the electrode. On the other hand, in the case of PMPy–PSS the H+ transport across the film proceeds readily, because the H+ ions constitute counter-ions to the immobilized sulfonate groups, present at a concentration of ca. 1 mol dm–3. The effect of the partitioning equilibria and the Donnan potential on the observed potential dependence of the charge-transfer rate is discussed in detail. It is shown that the presence of the cation-exchanger coating (PMPy–PSS) enhances the rate of electrochemical H+ reduction (relative to the uncoated electrode) when the concentration of H+ in the electrolyte is small. The diffusion coefficient of H+ ions in the PMPy–PSS matrix was determined: D= 1 × 10–6 cm2 s–1

Kinetics and mechanism of charge-transfer reactions between conducting polymers and redox ions in electrolytes

Charge-transfer reactions of selected ionic redox couples were studied on rotating glassy carbon electrodes covered with conducting polymers of different membrane properties: poly-N-methylpyrrole as the anion exchanger or poly-N-methylpyrrole with immobilized poly(4-styrenesulfonate) ions as the cation exchanging matrix. The electrochemical and spectroscopic (EDAX) results obtained with the redox systems: Fe(CN)3-4-6, Ru(NH3)3+2+, Eu3+/Eu2+, Co(en)3+2+3 and Fe(C2O4)3-4-3 pointed to reactions proceeding at the polymer-solution interface. The data were analysed on the basis of a model in which the charge transfer was regarded as a redox reaction between polymeric sites in the film and the redox species in the solution. The rate of electron transfer was found to be: (i) proportional to the concentration of the oxidized or reduced polymeric sites; (ii) dependent to some extent on the Donnan potential prevailing at the interface; and (iii) correlated with the thermodynamic driving force of the reaction between the polymer and redox species

Application of the quartz microbalance in electrochemistry

An introduction is given into the basic physics of a microbalance, based on the piezoelectric properties of quartz crystals. The influence of a viscous medium in contact whith the oscillating quartz is discussed. A device is described which permits the application of such a balance in electrochemical research. A number of examples are given. They include: the current efficiency of galvanic metal deposition, the rate of uniform corrosion, the effect of corrosion inhibitors, the determination of alloy composition in thin layers, the mechanism of electroless metal deposition, the formation of self-assembled monolayers, and the elucidation of the mechanism of an autocatalyic reduction reaction. All examples are taken from the literature. The paper is aimed at graduate students, at organizers of electrochemical lab classes, and at all scientists from fundamental and applied research who want a simple, versatile tool for the study of mixed electrode reactions

EFFECT OF LOCALIZED ELECTRONIC STATES ON SIMPLE ELECTRON TRANSFER REACTIONS AT FILM-COVERED ELECTRODES

Nous avons discuté des différents mécanismes pour le transport électronique à travers des couches minces d'isolant et de semi-conducteur dans les systèmes métal-film-métal et métal-film-électrolyte. Pour les couches minces (≤ 50 Å) le mécanisme de transport est lié à l'effet tunnel élastique direct de résonance ou inélastique. Au contraire, le transfert électronique à travers des couches plus épaisses se fait par une bande de conduction ou par des sauts dus aux phonons à travers des états électroniques localisés. Nous avons aussi présenté des données expérimentales du transfert électronique à travers des couches minces de polymères d'acrylonitrile déposées sur des substrats d'acier inoxydable et de platine, soit dans des jonctions d'états-solides soit dans des systèmes électrochimiques impliquant des réactions simples d'oxydo-réduction. La comparaison avec les relations prévisibles entre densité de courant, surtension et les propriétés des films montre que le mécanisme de transfert d'électrons dans ces systèmes se comporterait comme un effet tunnel inélastique ou des sauts assistés par des phonons.Various mechanisms for electron transport across thin insulator or semiconductor films in metal-film-metal and metal-film-electrolyte systems are discussed. For thin layers (i. e. ≤ 50 Å) the transport mechanism may thus be direct and resonance elastic tunnelling, and inelastic tunnelling, whereas electron transfer across thicker layers proceeds by band conduction or phonon-assisted hopping via localized electronic states. Experimental data for electron transfer across thin polyacrylonitrile films on stainless steel or platinum substrates in both solid-state junctions and electrochemical systems involving simple redox reactions are presented. Comparison with predicted relationships between current density, overvoltage, and film properties shows that the electron transfer mechanism in these systems is likely to be inelastic tunnelling and/or phonon-assisted hopping