H. Gómez

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.

Silver nanowire arrays electrochemically grown into nanoporous anodic alumina templates

Silver nanowire arrays with high aspect ratios have been prepared using potentiostatic electrodeposition within the confined nanochannels of a commercial porous anodic aluminium oxide template. The nucleation and growth processes are intensively studied by current versus time transients. Scanning electron microscopy results show that the nanowires have a highly anisotropic structure with diameters and lengths of 170?nm and 58??m, respectively, which coincide with the dimensions of the template used. Structural characterization using x-ray diffraction shows that the Ag nanowires are highly crystalline, and those obtained at higher overpotentials present a very strong [220] preferred crystallographic orientation. The optical properties of the silver nanowires embedded in the alumina template show a clear edge close to 320?nm, that is an expected value for a silver?alumina composite material.

Angular dependence of magnetic properties in Ni nanowire arrays

The angular dependence of the remanence and coercivity of Ni nanowire arrays produced inside the pores of anodic alumina membranes has been studied. By comparing our analytical calculations with our measurements, we conclude that the magnetization reversal in this array is driven by means of the nucleation and propagation of a transverse wall. A simple model based on an adapted Stoner–Wohlfarth model is used to explain the angular dependence of the coercivity.

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.

Geometry dependence of coercivity in Ni nanowire arrays.

Magnetic properties of arrays of nanowires produced inside the pores of anodic alumina membranes have been studied by means of vibrating sample magnetometer techniques. In these systems the length of the wires strongly influences the coercivity of the array. A simple model for the coercivity as a function of the geometry is presented which exhibits good agreement with experimental results. Magnetostatic interactions between the wires are responsible for a decrease of the coercive field.

XPS, SEM, EDX and EIS study of an electrochemically modified electrode surface of natural chalcocite (Cu2S)

Abstract An electrode surface of a natural chalcocite mineral obtained from the ‘Chuquicamata’ mine in Chile has been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy including microanalysis (SEM/EDX). Different potentials were applied via CV to the electrode surface in a specially designed preparation chamber of inert (Ar gas) atmosphere. This chamber was coupled, in the case of XPS analysis, to the X-ray photoelectron spectrometer and, in the case of SEM/EDX analysis, to the electron microscope in order to study quasi-in-situ changes in the chemical composition and morphology at the electrode surface induced by the electrochemical treatment and to relate them to the EIS results obtained for the same applied potential. CV was performed in an aqueous 0.05 M borax electrolyte solution of pH 9.2 at 300 K (cycles: 0→−850 mV (cathodic sense: reduction)→+200 mV (anodic sense: oxidation)→−400 mV, all vs. SCE). Along the cyclic voltammogram, which shows two anodic peaks, ten points of applied potential (for 1 or 400 s) have been studied by XPS. Partial oxidation from Cu(I) to Cu(II) is observed at +100 mV versus SCE applied for 400 s which is assigned to the formation of CuO, Cu(OH) 2 and possibly Cu 3 (SO 4 )(OH) 4 . For comparison, the fractured mineral surface was also studied by XPS. The measurements of SEM and EDX did not show any relevant alterations, except for the potential of +100 mV versus SCE in the positive-going potential, in which the formation of protrusions with a high concentration of oxygen (44%) was detected. The modifications detected by XPS, SEM and EDX reflect large changes in the electrochemical parameters obtained using EIS and are characteristic of a partially covered electrode; the charge-transfer resistance across the irregularly formed layer on the surface increases for the point +100 mV versus SCE in the positive-going potential and its capacitance is reduced.

Characterization of Natural Enargite/Aqueous Solution Systems by Electrochemical Techniques

The electrochemical behavior of natural enargite electrodes in stirred and quiescent aqueous solutions covering a wide pH range was investigated using ac and dc electrochemical techniques. It is proposed that in the first stage of enargite oxidation the release of copper ions takes place and a copper-depleted passivating film is formed on the electrode surface. At more positive potential the film becomes oxidized but depending on solution pH, a secondary passivation can be attained due to the precipitation of cupric arsenate. The electroreduction of enargite or a possible oxidized film formed during the preliminary treatment of the mineral surface initially releases S(II-) species and generates a surface film containing As 2 S 2 and Cu 2 S. At more negative potentials the electroreduction of As 2 S 2 to metallic As predominates. A dynamic system analysis carried out at pH 9.2 using electrochemical impedance spectroscopy gave additional information about the processes occurring on the enargite surface under anodic polarization.

SEM, EDX and EIS study of an electrochemically modified electrode surface of natural enargite (Cu3AsS4)

Abstract Enargite electrodes, made from mineral originating in the ‘Salvador’ mine (Chanaral, Chile), have been studied using SEM, EDX, CV and EIS in an alkaline solution for a range of different potentials in the positive sweep direction (PSD) and negative sweep direction (NSD). The results of SEM and EDX have shown that the principal changes in morphology and chemical composition take place on the surface of the electrode for oxidation potentials close to the value that corresponds to the most intense peak in the PSD, where there is clearly formation of a layer of oxidised material with areas of morphology which are irregular and high in oxygen content. These results are in accordance with the findings from XPS studies published previously. The charge transfer resistance and the resistance of the anodic layer of the electrode were determined by EIS measurements; the values depend heavily on the applied potential in the PSD or NSD. These changes are related to the changes on the surface of the electrode studied by SEM and EDX

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%).