Michael E. Hyde

A review of the analysis of multiple nucleation with diffusion controlled growth

A review is given of the area of electrodeposition of materials via a mechanism of nucleation followed by diffusion controlled growth. A short historical background to the study of nucleation via potentiostatic current transient modelling is provided, followed by an outline of the major methods currently used, with some comments on their relative merits. An overview of the computer simulation of both nucleus distributions and diffusion to growing nuclei is given. Finally, methods, including optical microscopy and SPM, used for studying directly the development of surfaces on which nucleation is occurring are described. A table of some chemical systems to which the theoretical models have recently been applied is included.

How ultrasound influences the electrodeposition of metals

The mechanism of deposition of zinc, lead, cobalt and mercury on glassy carbon has been studied under the influence of power ultrasound (20 kHz), using cyclic voltammetry. Systems of relatively high metal concentration (1 M) and low concentration (10 mM) were compared, and the effects of the application of different ultrasonic intensities, from silent conditions to 238 W cm−2 investigated. Ultrasound was found to have a significant effect on the deposition part of the cycle in systems in which deposition is under diffusion control, while little, if any, effect was observed on systems in which deposition is under interfacial/charge transfer control. Ultrasound also affected the size of the stripping peak relative to the deposition peak, with the precise effect dependent on the deposition mechanism of the system. At high intensities, the ablative power of ultrasound was observed.

Electrochemical and AFM studies of the electrodeposition of cobalt on glassy carbon: an analysis of the effect of ultrasound

Abstract The mechanism of cobalt electrodeposition cobalt on glassy carbon from an aqueous sulphate solution has been studied using electrochemical techniques such as cyclic voltammetry, linear stripping voltammetry (LSV) and chronoamperometry, as well as contact mode atomic force microscopy (AFM) which has allowed the complementary determination of morphological detail of the resulting deposits. The influence of power ultrasound (20 kHz) was then studied. At suitably high overpotentials cobalt electrocrystallisation under silent conditions proceeds via a diffusion controlled mechanism and was analysed using the Scharifker and Hills model for potentiostatic growth: at low overpotentials (ca. −1.0 V vs. SCE) growth occurred under charge transfer control. Nucleation was found to be more ‘progressive’ in character at −1.0 V vs. SCE while at elevated overpotentials (more negative than −1.25 V) an ‘instantaneous’ nucleation behaviour was determined. Sonication favoured ‘instantaneous’ deposition and generally increased the rate of cobalt deposition, although ablation was dominant at small electrode—tip/horn separations. AFM images confirmed these findings, showing cobalt clusters of varying sizes at low overpotential and silent conditions, and more uniformly sized clusters at higher overpotentials, particularly when sonication was applied.

Nanoporous iron oxide membranes: layer-by-layer deposition and electrochemical characterisation of processes within nanopores

A versatile procedure for the formation of nanoporous metal oxide membranes is reported, based on a layer-by-layer deposition procedure (‘directed assembly’) of metal oxide nanoparticles with appropriate ‘linker’ molecules; here Fe2O3 particles and phytic acid. Two types of nanoporous Fe2O3 membranes have been prepared and characterised: (A) a nanofilm deposit composed of 4–5 nm diameter Fe2O3 particles linked by phytic acid and (B) a nanoporous film formed after calcination of the type A deposit at 500°C in air. The nanofilm deposits are characterised by microscopy (SEM and AFM) and by electrochemical methods. Mechanically stable and homogeneous nanofilm deposits with controlled thickness (ca. 3 nm per layer deposited) were obtained. Transport of small molecules or ions through the nanoporous structure and their electrochemical conversion are shown to be fast in the presence of a sufficiently high concentration of supporting electrolyte. During the electrochemical oxidation of ferrocyanide, Fe(CN)64−, the nanoporous structure of the type A deposit is shown to act as an ‘active’ membrane, which changes the electrode kinetics by ‘double-layer superposition’ effects. For the second type of nanofilm, type B, ferrocyanide is accumulated by adsorption within the porous structure.

An AFM Study of the Correlation of Lead Dioxide Electrocatalytic Activity with Observed Morphology.

PbO2 is widely employed as an electrocatalyst for anodic oxidation processes including the generation of oxygen and the degradation of various organic species in aqueous solution. However, despite extensive investigation, the precise mechanism of action remains obscure. In this paper we establish a previously unrecognized strong correlation between the morphology of the PbO2 deposits and their electrocatalytic activity. Cyclic voltammetric results are described for the electrodeposition of PbO2 on boron-doped diamond (BDD) electrodes from 2.5 mM solutions of lead(II) nitrate in nitric acid at pH values between approximately 0 and 7. A likely change in mechanism is observed around pH 4, consistent with the Pourbaix diagram of lead. The morphology of the PbO2 films is observed as a function of time and potential, using in situ AFM in each of the lead solutions. Information on the growth rates of the films is extracted, and the limitations of using AFM in such an application are discussed. It is shown that the deposit morphology depends strongly on the specific conditions used. The oxidation of a 100 mM glucose solution on electrodes modified by PbO2 deposition at a range of potentials and pH values is used as an indicator of the catalytic activity of the corresponding films, leading to the observation of the correlation between deposit morphology and catalytic activity mentioned above.

Boron doped diamond electrode modified with iridium oxide for amperometic detection of ultra trace amounts of arsenic(III)

Boron doped diamond (BDD) electrodes modified by electrodeposition from hydrous iridium oxide (IrOx) have been developed for the detection of arsenic(III). Potential cycling is used to deposit films of hydrous iridium oxide onto boron doped diamond electrode from a saturated solution of alkaline iridium(III) solution. A stable reversible redox couple was observed at the surface of modified electrode in both acidic and basic solutions. The properties of iridium oxide films, stability and its electrochemical properties were investigated by atomic force microscopy (AFM) and cyclic voltammetry. The modified electrodes showed excellent electrocatalytic activity toward oxidation arsenic(III) over a wide pH range (2–8); also they showed an excellent analytical performance for the amperometric detection of arsenic(III). The detection limit, sensitivity, response time and linearity are 2 nM, 4.2 nA nM−1, 60 ms and 20 nM–50 µM. The precision for 10 replicate determinations of 40 µM arsenic was 0.80% (RSD). These analytical parameters compare favourably with those obtained with modern analytical techniques such as inductively coupled plasma mass spectrometry and hydride generation atomic fluorescence spectrometry. The advantageous properties of this modified electrode for arsenic determination are its inherent stability, excellent catalytic activity over a wide pH range, high sensitivity and simplicity.

The oxidation of trivalent chromium at polycrystalline gold electrodes

The electrochemical oxidation of Cr(III) to Cr(VI) species was examined in aqueous solution. The responses of boron doped diamond, glassy carbon and gold electrodes were probed towards the oxidation of trivalent chromium over a wide pH range (1.0–13.0). High quality voltammetric profiles were found to appear only at a gold electrode and in solutions of pH greater than 12. It was found that the oxidation reaction proceeds via a multi-step mechanism, where the first electron transfer is electrochemically irreversible and rate-determining, followed by two fast electron transfers. DIGISIM was successfully utilized to model the experimentally obtained data. The oxidation was additionally found to involve OH− ions, at potentials where these are adsorbed at the gold electrode surface. AFM measurements were carried out to complement these findings.

The theory of electrodeposition in the presence of forced convection:: Transport controlled nucleation of hemispheres

Abstract A new equation to describe the diffusion controlled nucleation of hemispherical centres under conditions of forced convection is derived, in a manner similar to that of the Scharifker and Mostany equation (J. Electroanal. Chem. 177 (1984) 13). It has been shown that the transients measured under these conditions should show a monotonically rising transient, followed by convergence to a limiting current. The accuracy and validity of the model is tested by comparison with experimental data obtained using a 5 mM Co/Co 2+ system, sonicated at 66 W cm −2 . Excellent fits are obtained, and the parameters derived are in agreement with those derived from independent experiments.