V. Tsakova

Nucleation, growth and branching of polyaniline from microelectrode experiments

Abstract The kinetics of electrochemical formation of polyaniline were studied under potentiostatic conditions on gold and platinum microelectrodes. Two different stages of growth were observed, the first corresponding to the formation of a compact layer with average thickness of about 200 nm, the second corresponding to the growth of a branched structure. The initial parts of the current transients obtained at two different aniline concentrations were interpreted by means of the theory for progressive and instantaneous nucleation and growth. The study of the potential dependence at the very initial stage of film formation has shown that nucleation is not likely to be an electrochemical step in the overall polymerization process. A detailed investigation of the growth process alone yielded different apparent transfer coefficients corresponding to the two different stages of growth.

Electrochemical formation and stability of polyaniline films

Abstract A pulse potentiostatic method (PPM) is used for the electrochemical formation of polyaniline films. Under given parameters of the potentiostatic pulses an enhanced mode of growth of the polymer films is observed. Polyaniline films obtained by PPM and by the conventional potential-scan and potential-hold methods are compared with respect to their stability at two different electrochemical treatments.

Electrochemical incorporation of copper in polyaniline layers

The electrochemical deposition of copper is studied in polyaniline (PAN) layers with different redox states before the onset of metal deposition. Three characteristic oxidising peaks are found during dissolution of the copper deposit. Their charge contributions depend markedly on both thickness and the redox state of the polymer layers. The combined electrochemical, SEM and XPS studies allow to assign the different dissolution peaks to three types of copper deposits. The most negative peak is ascribed to macrodefects-induced deposition on polymer-free sites on the electrode surface. The middle peak should be connected to copper deposited in/on partially oxidised locations of the PAN coating. The most positive dissolution peak corresponds to copper incorporated in the polymer layer, prone to be dissolved only after driving the polymer layer in its oxidised state. Potentiostatic current transients corresponding to the third type of copper inclusions are studied as a function of deposition potential and PAN layer thickness. The current transients are interpreted by means of a model for instantaneous nucleation and three-dimensional (3D) growth under kinetic control. Data are obtained for the number and size of this type of copper inclusions.

Growth of polyaniline films under pulse potentiostatic conditions

Abstract Strong activation of the growth of polyaniline (PANI) films is found to occur when a pulse potential programme consisting of periodic cathodic and anodic rectangular pulses is applied. The potential interval for the cathodic pulses where growth activation is observed coincides with the potential interval where the polymerized material is almost fully reduced but is still conducting. It is found that the pulse programme does not influence the formation of the first nuclei but markedly affects the early stage of growth of the PANI films, i.e. the stage where different processes such as oligomerization, further nucleation and growth up to the formation of a full coverage layer take place on the electrode surface. The further growth of the compact PANI layer seems to be unaffected by the pulse procedure. It is shown that the activated polymer growth should not be related to the influence of the pulse procedure on the overoxidation (termination) reaction of the polymer chains. Different reasons for the activated PANI growth observed under pulse conditions, such as continuous redistribution of the redox centres in the PANI nuclei, periodic stretching and relaxing of the polymer chains etc., are discussed briefly.

Comparative studies of electrochemical phase formation by amperometric and microscopic methods: Part I. Nucleation kinetics in dilute solutions of mercury nitrate

Abstract The nucleation kinetics of mercury on platinum have been studied by measuring directly the “number of nuclei-time” relationship and by analysing potentiostatic current transients. The number density N0 of active sites on a platinum single-crystal substrate and the nucleation rate constant A were determined in the overpotential range 86–100 mV.

Influence of copper anion complexes on the incorporation of metal particles in polyaniline. Part I: Copper citrate complex

Copper electrodeposition in polyaniline-coated electrodes is studied using copper citrate complex anions as reducing species. Use of these complex anions allows shifting the potential window for metal deposition in the negative direction and resolving the polymer and metal reduction processes. As shown by galvanostatic experiments and SEM photographs, copper electrodeposition from citrate solution is highly inhibited and results in a small number (3.6 × 106 cm−2) of large hemispherical crystals located mainly on top of the polymer layer. Statistical analysis of distances between neighbouring crystals shows a random surface distribution of the copper hemispheres. Thus, the low number of crystals obtained cannot be related to the appearance and overlap of nucleation exclusion zones partly blocking the electrode surface. It is likely connected to the specific role of the metal anion complexes in the deposition process and more precisely to the inhibited diffusion of both the copper complex anions and the released (after reduction) citrate anions in and out of the polymer structure.

Electrochemical deposition of copper in polyaniline films: number density and spatial distribution of deposited metal clusters

The electrochemical deposition of copper in reduced polyaniline (PAN) films is studied at different polymer layer thicknesses. A saturation in the number density, n0, of active sites for metal deposition is found at 0.5 μm PAN thickness (n0=108 cm−2). The surface spatial distribution of the deposited copper crystals is analysed in two cases. It is found that nucleation exclusion zones play an appreciable role in the course of the deposition process in thin PAN films. In contrast, a random distribution and a larger number of deposited crystals are obtained in thicker PAN layers.

Role of polymer synthesis conditions for the copper electrodeposition in polyaniline

Abstract Polyaniline (PAN) layers are electrochemically synthesised by three different procedures – potentiostatic (PS), potentiodynamic (PD) and pulse potentiostatic (PP). Galvanostatic copper deposition and subsequent oxidation of copper species are studied on the three types of PAN layers. A marked difference in the deposition/dissolution behaviour is found depending on the type and redox state of PAN. Copper reduction is easily initiated at low overpotentials at oxidised PS and PP PAN specimens, whereas for oxidised PD layers copper deposition needs higher overpotentials. This difference is discussed in terms of various reduction behaviours of the three types of PAN. In general reduced PANs strongly inhibit copper nucleation and growth. Still a marked difference in the metal deposition on reduced PD and PP PANs on the one hand and reduced PS PAN on the other exists. This difference is related to the varying amount of defects in the three polymer structures, evidenced by SEM observation of Cu/PAN-coated electrodes.

Nucleation of silver on a polyaniline-coated platinum electrode

Abstract Polyaniline (PANI) films are electrodeposited on a platinum single crystal electrode by means of the sweep voltammetric techniqhe. The behaviour of the PANI films as subtrates for metal electrodeposition is investigated. The stationary nucleation rate of silver on PANI is studied as a function of the overpotential and the film thickness. Information is obtained on the number of pores in the PANI coating.

Spatial distribution of electrochemically deposited clusters : a simulation study

Abstract Monte Carlo simulations of electrochemical nucleation and growth have been carried out to demonstrate the specific features of spatial distributions of clusters obtained at different number densities of active sites. Different types of spatial distributions were obtained depending upon the factor dominating the nucleation kinetics: direct exhaustion of active sites or spreading of zones of reduced overpotential. Statistical criteria such as non-dimensional standard deviation ( σ j / r j ) and skewness (skew j ) were used to compare quantitatively the simulated distributions of the distances between nearest and far distant neighbours.