Jorge Mostany

Three-dimensional nucleation with diffusion controlled growth: Part I. Number density of active sites and nucleation rates per site

Abstract The current transient for three-dimensional nucleation on a finite number of active sites, followed by diffusion controlled growth, has been analysed. Both the number density of active sites and the true nucleation rate per site can be obtained from the current maximum of single-step potentiostatic experiments. Instantaneous and progressive nucleation are shown to be special cases of the more general situation described.

Three-dimensional nucleation with diffusion controlled growth: Part II. The nucleation of lead on vitreous carbon

Abstract The nucleation of lead onto vitreous carbon electrodes has been investigated. The number density of active sites for nucleation was found to vary with the overpotential but not with the concentration of lead ions in solution. The overpotential and concentration dependence of the nucleation rate were also studied, and the results interpreted according to both atomistic and classical theories. Due to the small size obtained for the critical nuclei, the latter was refined to incorporate the effect of the line tension along the contact line between the three phases involved. However, the atomistic theory seems to afford a better representation of this electrochemical nucleation process.

IN SITU FTIR STUDY OF REDOX AND OVEROXIDATION PROCESSES IN POLYPYRROLE FILMS

In situ infrared reflection spectra of thin polypyrrole films electrodeposited on platinum were obtained in aqueous KNO3, KCl and KF solutions, within a wide potential interval ranging from the reduced, insulating state, to the oxidative degradation of the material. These spectra provide information on the molecular changes resulting from the generation of charge carriers (radical cations and dications) in this conjugated heterocyclic system, as well as the irreversible chemical changes accompanying its electrochemical oxidation at very positive potentials. The spectral region between 1000 and 1700 cm−1 contains information on the relative populations of charge defects as a function of potential, and the results confirm that electronic charge transport at moderate potentials is due to formation of radical cations, which oxidize further or recombine to form dications as the potential increases. At more positive potentials the polymer degrades with formation of carbonyl and hydroxyl groups as well as CO2. The overoxidation process occurs at less positive potential in F− solution due to nucleophilic attack of OH− ions originating from dissociation of the aqueous solvent, as shown by the infrared spectra and previous calorimetric evidence. In the absence of irreversible chemical changes due to overoxidation, the infrared spectra obtained in the different solutions were similar, indicating weak interactions between ions and charge defects in the backbone.

On the Theory of the Potentiostatic Current Transient for Diffusion‐Controlled Three‐Dimensional Electrocrystallization Processes

Several available theoretical descriptions of the potentiostatic current transient that allow the determination of the rates of the nucleation process, A, and the number densities of sites for nucleation, N0, with three-dimensional, diffusion-controlled growth of clusters are critically examined and compared with experimental and computer-simulated data. All these obtain the overall current f rom the material flux to free, noninteracting growing centers, considering the overlap of circular diffusion zones expanding around them. It is shown that even though when compared to experimental data the diverse theories appear almost indistinguishable, esse ntial differences among them arise from their distinct calculation of the growth rate of diffusion zones, yielding significantly different values for A and N0. Computer simulations of the process for which the values of A and N0 are accurately known allowed meaningful comparisons between theories and an assessment of the relative accuracy with which the different available formulations yield estimates of the kinetic parameters.

Thermodynamic studies of anion adsorption at the Pt(111) electrode surface in sulfuric acid solutions

Abstract The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a Pt(111) electrode in solutions of H 2 SO 4 with an excess of an inert electrolyte (0.1 M HClO 4 ). A high quality Pt(111) single crystal electrode with a low defect density was employed in this study. The total charge densities were calculated by integration of cyclic voltammetry curves. A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The best results were obtained when the charge was the independent variable. These results show that the maximum Gibbs excess of (bi)sulfate attains a value of ∼3×10 14 ions cm −2 , which corresponds to the packing density of ∼0.2 monolayer. This number agrees well with the results of a radioactive labeling method (Kolics and Wieckowski, J. Phys. Chem. Sect. B 105 (2001) 2588) and with recent STM studies (Itaya, Prog. Surf. Sci. 58 (1998) 121; Funtikov, Linke, Stimming and Vogel, Surf. Sci. 324 (1995) L343). The calculated Esin–Markov coefficient and electrosorption valencies exhibit an important dependence on the electrode potential (or charge).

Impedance spectroscopy of undoped, doped and overoxidized polypyrrole films

Abstract The frequency response of polypyrrole thin films was studied in the presence of NO 3 − , F − , Cl − and Br − in the potential region of interest for the study of the insulator—conductor transition. The impedance spectra have been interpreted using the Randles equivalent circuit modified with a constant phase element (CPE), using nonlinear least-squares fitting of the parameters of the circuit to experimental data. The results show that the distribution of relaxation times widens with oxidation of the polymer, indicating the presence of domains with different electrical properties, consistent with hopping of charge carriers in the oxidized state. It is also shown that in F − solution and at moderate potentials, overoxidation leads to irreversible degradation of the electroactive properties, affecting the electronic conductivity and diffusion coefficients of ionic species within the film.

The kinetics of mercury nucleation from Hg22+ and Hg2+ solutions on vitreous carbon electrodes

The kinetics of the electrochemical nucleation of Hg onto vitreous carbon were studied from analysis of potentiostatic current transients obtained at different overpotentials in Hg22+ and Hg2+ solutions. Similar sizes of critical nuclei and number densities of sites on the surface were found for nucleation of Hg from both solutions, indicating, in agreement with nucleation theory, that the rates of nucleation are controlled by surface energies and deposit–substrate interactions. Saturation number densities of nuclei were deduced from the kinetic analysis, and their relationship with those obtained from direct observation of the electrode surface is discussed. It was found that at low overpotentials, the nuclei deposited from both solutions were uniformly distributed on the electrode surface, whereas their distribution was affected by inhibition of the nucleation rates close to already established nuclei at high overpotentials. This inhibition was enhanced during deposition from Hg2+ solution.

The nucleation of lead from halide-containing solutions

The density of active sites and the nucleation rates per site for the nucleation of lead onto vitreous carbon from lead ion solutions in the presence of halides (Cl−, Br−, I−) have been investigated. Mass transport to growing nuclei was not found to be affected by the formation of soluble complexes of lead with the halides. No systematic variation of the true nucleation rates per site was observed upon addition of halides to the electrolyte. The density of active sites for nucleation, on the other hand, increased in the presence of halides. It is also shown that, in the absence of halide ions, the density of active sites for the nucleation of both lead and silver are equal to and independent of the concentration of metal ions in solution.

Spatial distributions and saturation number densities of lead nuclei deposited on vitreous carbon electrodes

The nucleation of lead onto vitreous carbon electrodes has been investigated. Diffusion-controlled growth of nuclei inhibits nucleation, giving rise to saturation with nuclei and introducing correlations in their location on the electrode surface. Similar nuclear number densities were obtained from direct, microscopic observation of the surface and from analysis of potentiostatic current transients considering the development of nucleation exclusion zones around already established nuclei. At low overpotentials nuclei were found to be uniformly distributed on the surface, whereas at high overpotentials the results were found to be consistent with simulations of the process that allowed for the existence of exclusion zones around nuclei, with radii proportional to the square root of their age.

On the spatial distribution of nuclei on electrode surfaces

Abstract Interactions among growing nuclei diminish the nucleation rate, giving rise to saturation nuclear number densities and introducting correlations in their location on the electrode surface. These have been investigated considering the existence of nucleation exclusion zones around each nucleus, with radii r e growing with the age u of nuclei according to laws of the type r e  ku x , with 0 ⩽ x ⩽ ∞. From digital simulations of the process, the nearest-neighbour distributions corresponding to the cases x =0, 1/2, 1 or ∞ were obtained and compared with expressions available for x = ∞ and x = 0. The results show that the propagation rates of the exclusion zones can be obtained from microscopic studies of the electrode surfaces in single-step potentiostatic experiments. It is also shown that the analysis of spatial distributions of nuclei on the surface also provides additional criteria for the identification of nucleation rate laws.