R.G. Barradas

Investigations of copper in aqueous alkaline solutions by cyclic voltammetry

Summary Cyclic voltammetric investigations of copper electrodes in aqueous KOH were carried out systematically. Nine peaks on the voltammograms were analysed as stated in the Conclusions section. It should be stressed that some of the peaks can be assigned with a good degree of certainty by comparison with previously published works, but a few of them are somewhat speculative and may require further work to establish the identities of the species proposed.

Rotating copper disk electrode studies of the mechanism of the dissolution-passivation step on copper in alkaline solutions

Summary From rotating copper disk electrochemistry and analysis of potentiostatic transients it was demonstrated that copper dissolves reversibly as Cu(OH) 4 −2 . It was also shown that passivation of the electrode surface occurred by a nucleation and growth mechanism similar in principle to that proposed by Armstrong et al. . The passivating film formed under potentiostatic conditions on the electrode appears to be Cu(OH) 2 .

A kinetic model of multilayer adsorption in electrochemical phase formation

Abstract A kinetic model of multilayer adsorption in electrochemical phase formation has been developed along the same lines as the multilayer model of nucleation and growth in electrocrystallisation. A novel expression was obtained for the current-time transient due to a potential step. Unlike the nucleation and growth version, the kinetic adsorption model predicts, in general, a zero steady state current and passivation. However, if irreversible adsorption is assumed, one obtains the results of a non-steady state current and the system never passivates.

A computer simulation of the voltammogram describing the active-passive transition of the Pb(Hg)/PbCl2 interface

Abstract A computer simulation of the voltammogram corresponding to a three-dimensional nucleation and growth mechanism is described. Theory is compared to experiment for the Pb(Hg)/PbCl 2 interface. Good agreement is found.

The anodic behaviour of lead amalgam electrodes in HCl solution

Abstract The anodic behaviour of the lead amalgam electrode has been investigated in aqueous hydrochloric acid solutions. Both voltammetric and potential pulse results are described. The mechanism of passivation is shown to be the nucleation and growth of three-dimensional nuclei of PbCl2 which progressively block the electrode. The nuclei are considered to be right circular cones, distributed at random on the electrode surface. As in the case of solid lead electrodes, a simultaneous dissolution of PbCln2−n complexes is observed, which diffuse away from the electrode under mass transport control. Evidence is also presented that the first stage in the growth of the anodic film is the two-dimensional nucleation and growth of a monolayer of PhCl2. Unfortunately, this process is partially obscured by the dissolution reaction. A reaction scheme is proposed.

Studies in electrocrystallisation: Part V. Monte Carlo simulations of the 2-dimensional instantaneous nucleation and growth model

Abstract A new computer simulation model whereby the nuclei and the electrode surface are represented by random points is described and applied to the case of 2-D instantaneous nucleation and growth under potentiostatic conditions. Six different electrode geometries are investigated using this simulation method. It is found that for small numbers of nuclei, interactions of the growing film with the electrode edges can cause serious deviations from conventional theory. Fluctuations in the spatial distribution of the nuclei can cause the apparent nuclear density to vary considerably.

Studies in electrocrystallization: Part III. The effect of ohmic and finite risetime distortions on data acquisition in 2D nucleation and growth

Abstract The effect of uncompensated iR-drop on two-dimensional nucleation and growth is described. Both instantaneous and progressive nucleation laws are considered. Also the effect of the finite risetime of potential square-waves on the experimental data acquisition is presented. Data are examined in terms of current-time transients and also in terms of the transient maximum parameters im and tm. It is demonstrated that neglect of ohmic and finite risetime factors causes complex distortions of rate data, particularly at short times. Prospects of minimising these distortions are briefly considered.

A comparative potentiostatic study of the electrocrystallization of PbO2 on platinum and glassy carbon electrodes

Abstract The deposition of PbO 2 on platinum and glassy carbon electrodes was investigated experimentally. The comparison of the different behaviour on the two types of electrodes is presented. The effects of variation of temperature, potential, nucleation order and induction time-lags on the electrocrystallisation process are discussed. The present investigations have confirmed that electrocrystallisation of PbO 2 occurs through 3-D nucleation and growth on glassy carbon and most probably also on platinum. The data on platinum contains considerable noise from simultaneous chemical processes and is not amenable for analysis in a simple way. The steady state behaviour is unusual because it has generally been assumed that charge transfer is rate determining under most conditions. However, as shown here, if lattice incorporation is slow even at the steady state, it provides a method for separating the activation energies associated with the nucleation and growth rate constants. Finally, the dissolution of PbO 2 into the platinum substrate and its implication on transient rates merits more attention. In this context the Pt-PbO 2 system is probably not a good choice because of the large contribution of substrate oxidation to the measured transient rates.

Double layer studies of aqueous sodium citrate solution at the mercury electrode

Summary The homogeneous and heterogeneous electrochemical behaviour of aqueous sodium citrate has been examined. Bulk solution properties such as viscosity and cation activity can give useful information towards the interpretation of double layer results from electrocapillary and capacitance studies as a function of concentration and pH. Our experimental findings on the nature and behaviour of adsorbed species in the interfacial region could be accounted for by consideration of field assisted dissociation followed by water structure enforced ion-pairing effects according to the theoretical treatment of Jenard and Hurwitz.

Temporal statistics of single nucleus electrocrystallisation on microelectrodes

Abstract The stochastic temporal component of the nucleation of single two-dimensional centres is considered. The model is steady-state, layer-by-layer metal deposition under conditions where nucleation events are well separated in time, on average. The difference between the mean frequency of appearance of nuclei and the nucleation frequency is clearly delineated. Results of the present study are expressed in terms of the statistics of the induction times between successive nucleation events. and a method of determining those times from experimental data is derived. The role of the spatial component of nucleation as a stochastic “lowest common denominator” is discussed in terms of determining the statistical properties of the observed time lag between nucleation events.