R. Córdova

Effect of anions on the nucleation and growth mechanism of polyaniline

Abstract In this work we show that the nucleation and growth mechanism (NGM) of polyaniline (PANI) films depends on the anions present in the electrolyte, the solubility of oligomers formed before the nucleation process and the stirring conditions. The NGM of PANI in an unstirred solution can be explained by assuming that this process is preceded by the formation of a high density oligomeric layer which is located near the electrode | solution interface. A thick oligomeric layer obtained with a high concentration of monomers or with anions of the type HSO4− or SO42−, which form more soluble species with the oligomers. Under these conditions, diffusion-controlled progressive nucleation with three-dimensional growth is found. However, when the monomer concentration is low or ClO4− anions are present, the thickness is decreased and the NGM is either instantaneous nucleation with three-dimensional growth or progressive nucleation with two-dimensional growth.

Nucleation and growth mechanisms of poly(thiophene) Part 1. Effect of electrolyte and monomer concentration in dichloromethane

Applying the potential step method, the effects of potential, monomer concentration, nature and concentration of electrolyte on the nucleation and growth processes of polythiophene on a Pt electrode in CH2Cl2 were studied. The current-time transients obtained were fitted using a mathematical equation that considers three contributions corresponding to 2D instantaneous and 3D instantaneous nucleation, both charge-transfer-controlled mechanisms, and 3D progressive nucleation with a diffusion-controlled mechanism. In any condition the principal contribution was always 3D instantaneous nucleation. However, the contribution of 2D instantaneous nucleation is important in the first stages of the nucleation process, indicating that the polymeric deposit initiates by the formation of a two-dimensional film. The contribution of 3D progressive nucleation appears at longer times and it is favoured with increasing monomer concentration. From the general nucleation and growth equation proposed, a relationship for the film thickness variation with time is obtained. Also, a general scheme of the nucleation and growth mechanism for poly(thiophene) is given.

Electrochemical and nanoelectrogravimetric studies of the nucleation and growth mechanisms of rhenium on polycrystalline gold electrode

The nucleation and growth mechanisms for rhenium on polycrystalline gold electrodes from an electrolytic bath containing 0.75 mM HReO4+0.1 M Na2SO4 (pH 2) have been studied. The potentiostatic step technique was simultaneously employed with measurements of mass changes in an electrochemical quartz crystal microbalance. Scanning electronic microscopy of rhenium electrodeposits were obtained. The mass–time transients were fitted with equations deduced from the current–time relationships of the conventional nucleation and growth models. The global equation that fitted the Δm/t transients indicated that the electrodeposition process of rhenium started with a two-dimensional progressive nucleation (PN2D), followed by another two contributions. The first of them corresponds to a progressive nucleation growing under diffusion control (PN3Ddif) and the second contribution, which is observed at longer times, corresponds to a progressive nucleation under charge transfer control (PN3DCT). From these three contributions, the PN2D corresponds to the charge of a monolayer and was attributed to 2D nuclei produced by the reduction of adsorbed perrhenate. The PN3Ddif was the most important contribution and represented 70–80% of the mass increase. The faradic efficiency for the electrodeposition process was in the range of 12–18% for the experimental conditions of this study.

Study of the electrochemical reduction of CO2 on electrodeposited rhenium electrodes in methanol media

Abstract The electrochemical reduction of carbon dioxide at a rhenium electrode electrodeposited onto a polycrystalline gold support was studied by means of polarization curves with simultaneous measurement of mass changes by an electrochemical quartz microbalance (ECQM). Potentiostatic electrolysis was done and the analyses of the reduction products were carried out by gas chromatography–mass spectrometry and Fourier transform infrared spectroscopy. The electrolyte was a methanol solution containing 0.1 M LiClO 4 +CO 2 , under atmospheric pressure. From the polarization curves it was observed that the CO 2 reduction presented a Tafel slope of −2 RT / F , indicating that the first electronation of the CO 2 molecule to form the radical anion (CO 2 − ), is the rate-controlling step. On the other hand, mass change measurements allow us to determine that CO 2 H and CO correspond to the intermediate species formed during CO 2 reduction. The product analysis indicated that the faradaic efficiency for this process depended strongly on the potential at which the electrolysis is carried out and on the hydrodynamic conditions of the solution. Under stirred conditions, the main products were CO (87%) and H 2 (13%) at −1.35 V. At this same potential and under quiescent conditions, the major products were CO (57%), CH 4 (10%) and H 2 (33%).

XPS analysis of an electrochemically modified electrode surface of natural enargite

A natural enargite (Cu 3 AsS 4 ) electrode has been studied by cyclic voltammetry (CV) and XPS. Cyclic voltammetry was carried out in a preparation chamber filled with Ar gas and coupled to the spectrometer in order to analyse by XPS, quasi-in situ, the electrochemically modified electrode surface. By CV, different potentials of oxidation and reduction were applied to the electrode in a disodium tetraborate decahydrate electrolyte solution of pH 9.2 at room temperature. Cycling in the anodic direction, XPS analysis has shown that at +500 mV the electrode surface is oxidized to CuO, CuSO 4 and As 2 O x (x = 3; 5) and the corresponding hydroxides and polysulphide are formed, whereas at lower oxidation potentials the Cu 2p and As 3d photoelectron peaks do not alter. However, with increasing oxidation potential up to +200 mV a new contribution in the S 2p signal and Cu loss indicate the formation of non-stoichiometric (metal-deficient) enargite and polysulphides in the electrode surface. Cycling then from +500 mV in the cathodic direction until -800 mV, we observe with increasing reduction potential a reverse process in the S 2p signal and Cu concentration. Furthermore, even at +200 mV the reduction from Cu(II) to Cu(I) occurs and no sulphate species are observed any more. These observations indicate that the copper concentration at the electrode surface is a sweep-direction-independent function of applied potential and that the oxide species formed at the electrode surface are only stable by application of high oxidation potentials. For comparison, the surfaces of the fractured mineral and the original polished electrode surface were also studied.

Electrochemical study concerning the deposition of copper on selenium covered gold electrodes

Abstract Voltammetry, chronoamperometry and anodic stripping voltammetry were used to study the processes involved in the electrodeposition of copper on polycrystalline gold electrodes previously covered with selenium layers. Through the analysis of the I–t transients it was possible to establish the potential edge where the initial stages of copper–selenium compounds starts. Taking into account the charges involved in the redissolution of deposits obtained by stepping the potential in a narrow interval, a general reaction scheme that considers the formation of CuSe and Cu2−xSe compounds is proposed.

Redox and solution chemistry of the SeSO32−–Zn–EDTA2− system and electrodeposition behavior of ZnSe from alkaline solutions

Abstract The specific redox and solution chemistry involved in the formulation of the plating solution for the electrodeposition of zinc selenide from the SeSO 3 2− /Zn 2+ /EDTA system was investigated. After a theoretical analysis which allowed the electrodeposition conditions to be fixed, based on potential–pH and solubility diagrams, an experimental study was conducted by varying the deposition parameters (composition, pH, potential). XRD, EDS, AFM and reflectance measurements were used to characterize the structure, morphology, composition, and optical properties of films deposited on conducting glasses. EDS analysis showed that the as grown films are almost stoichiometric in a wide potential interval and also that the film thicknesses are lower than the values estimated from the circulating charge. The possible reasons for the latter behavior are discussed.

ELECTRODEPOSITION AND CHARACTERIZATION OF ZnX (X=Se, Te) SEMICONDUCTOR THIN FILMS

El presente trabajo describe la sintesis y la caracterizacion de peliculas delgadas de ZnX (X = Se y Te) obtenidas por electrodeposicion a potencial constante en medio acido. Previamente, se realizo un estudio voltametrico y fotovoltametrico sobre diferentes substratos conductores los que permitieron determinar las mejores condiciones para la electro obtencion de estos compuestos. Las peliculas delgadas de ZnSe y de ZnTe fueron analizadas por diferentes tecnicas (SEM, EDS, XRD y medidas opticas). Las peliculas de ZnTe presentaron una composicion muy cercana a la estequiometrica, en tanto que las de ZnSe presentaron un exceso de Se el cual puede ser eliminado por un adecuado tratamiento termico. La caracterizacion optica de ambos semiconductores depositados sobre titanio arrojo valores de ancho de banda prohibida de transicion directa de 2,64 eV para ZnSe y 2,27 eV para ZnTe, muy cercanos a los aceptados en bibliografia.

Electrochemical behaviour of rhenium in aqueous solution

Abstract An open-circuit potential decay, quasi-stationary polarization curves and simple and complex potentiodynamic potential perturbation programmes were employed in order to analyse the electrochemical behaviour of rhenium in aqueous solution, within a wide pH range. The results show that the overall electro-oxidation of rhenium to perrhenate involves the participation of different intermediate oxides, which is inferred from the anodic and cathodic peak multiplicities observed through the potentiodynamic E - I displays. In order to propose an overall reaction scheme for the processes taking place at this interface, an attempt is made to correlate these peaks with different redox couples with the aid of the respective thermodynamic potential values.

Electrodeposition of Cu–Re alloy thin films

Abstract The electrodeposition of copper–rhenium alloys from Cu(II) and Re(IV) solutions was studied in an aqueous H 2 SO 4 matrix. Different substrates have been used (glassy carbon, titanium and gold), and the effect of them on the electrochemical behavior of the studied ions is shown. Cyclic voltammetry was used in conjunction with oscillating quartz crystal nanogravimetry to delineate features in the voltammograms accompanied by mass loss or gain at the electrode surface, from those attributable to the solution phase. The obtained Cu–Re films were subjected to scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD). The rhenium content increases as the potential was made more cathodic, reaching a maximum value of 10% atomic composition. The films were amorphous with a cauliflower structure.