A. Hernández Creus

Electrochemical kinetics and growth modes of silver deposits on polyfaceted platinum spherical electrodes

Abstract The Ag electrodeposition on Pt spherical substrates was investigated over a wide range of experimental conditions to establish a correlation between the kinetics of the process and the different growth modes of the Ag overlayer. At E d ≊ E rev , the electrodeposition of the first Ag layer obeys a combined adsorption-desorption and nucleation and 2-D growth process under diffusion control. At E d E rev , the formation of the 2-D Ag layer proceeds through an adsorption mechanism, whereas the formation of the 3-D Ag layer fits a progressive nucleation and 3-D growth mechanism under diffusion control. Finally, when E d E φ , a potential threshold related to Ag dendritic growth, the kinetics of the reaction apparently obeys a progressive nucleation and 1-D growth. This process is triggered at edges and corners of large Ag crystals. Dendritic growth takes place outside the diffusion layer defined around the Pt substrate sphere. In this case migration plays a substantial role in the process.

Electrochemical and AFM characterization on gold and carbon electrodes of a high redox potential laccase from Fusarium proliferatum

The redox potential of the T1 copper site of laccase from Fusarium proliferatum was determined by titration to be about 510 mV vs. SCE (750 mV vs. NHE), which makes it a high redox potential enzyme. Anaerobic electron transfer reactions between laccase and carbon and gold electrodes were detected, both in solution and when the enzyme was adsorbed on these surfaces. In solution, a single high-potential signal (660 mV vs. SCE) was recorded at the carbon surfaces, attributable to the T1 copper site of the enzyme. However, a well-defined oxidative process at about 660 mV and an anodic wave at 350 mV vs. SCE were recorded at the gold electrode, respectively associated with the T1 and T2 copper sites. Laccase-modified carbon electrodes behaved analogously when the enzyme was in solution, unlike laccase adsorbed on gold, which showed only a low-potential signal. Laccase molecules were successfully imaged by AFM; obtaining a thick compact stable film on Au(111), and large aggregates forming a complex network of small branches leaving voids on the HOPG surface. Laccase-modified carbon electrodes retained significant enzymatic activity, efficiently oxidising violuric acid and reducing molecular oxygen. Explanations are proposed for how protein-film organisation affects the electrode function.

Mass-transport-induced kinetic transitions during the electrochemical formation of three-dimensional dendritic silver deposits under ohmic control

Abstract The influence of mass transport processes on the growth of three-dimensional dendritic Ag deposits on a Pt quasi-spherical substrate at a relatively large cathodic overpotential has been investigated. After a certain induction time a first stage of dendritic growth of Ag can be observed. The deposit formed at this stage exhibits a fractal surface and non-fractal mass—volume properties. For charge densities greater than a certain critical value, a second stage of growth can be distinguished. At this second stage the contribution of free convection changes the shape of the deposit. The fastest growth of the Ag electrodeposit is determined by the rate of migration of sulphate ions in the electric field. A model accounting for the parametric relationships derived from the experiments is presented.

The electrochemical faceting of copper in 85 % aqueous o-phosphoric acid by using a potential reversal technique

Abstract The electrochemical faceting of differently prepared polycrystalline Cu specimens in 85% H 3 PO 4 solution at 25°C was investigated using symmetric square-wave periodic-potential routines. The electrochemical characteristics of electrofaceted specimens were followed by Tl upd voltammetry and complemented by scanning electron microscopy observations. Depending on the experimental conditions the electrochemical faceting involves a preferred crystallographic orientation at the specimen surface and an increase in surface roughness. Electrofaceting at grain domains without a significant increase in surface roughness occurs preferentially at high frequencies, whereas Cu islands with a marked increase in surface roughness are produced at low frequencies. The interpretation of the overall effect is given in terms of the likely reaction mechanism already established for the Cu/Cu(II) and Cu/Cu(I) electrodes, including further kinetic aspects of the Cu(I) disproportionation reaction in the solution.

Changes in the growth mode of electrodeposited silver layers: Surface rearrangements induced by the presence of lead adatoms

Abstract The surface characteristics of Ag electrodeposits prepared on polyfaceted Pt(sc) electrodes have been followed through complete Pb upd/stripping voltammetry. The growth mode of Ag overlayers at constant temperature depends on both the cathodic electrodeposition overvoltage and the silver ion concentration in solution. Ag surface rearrangements can be induced by either holding the potential or Pb upd/stripping cycling in the potential range where the degree of surface coverage by Pb adatoms is between 0 and 1. The voltammograms of Ag overlayers at the monolayer level show considerable changes compared with those obtained for thicker Ag overlayers. The present results allow us to establish a correlation between the development of rough electrodeposits and the surface diffusion properties of metal atoms.

Metal patterned highly oriented pyrolytic graphite as a template for direct polymer molding

Fractal Pd islands have been used to demonstrate that a metal patterned highly oriented pyrolytic graphite (HOPG) is a suitable template for direct molding of cavities and channels on elastomeric polymeric materials. Due to the low reactivity of the surface the method requires neither substrate surface modification nor subsequent steps so that it is very simple and inexpensive. Features larger than 20 nm in size can be easily transferred from the template to the mold. Damage was observed neither in the patterned polymeric material nor in the HOPG template after release of the polymer film.