P. Ocón

Polypyrrole layers for steel protection

Abstract This report describes the behaviour of polypyrrole and polypyrrole/carboximethylcellulose (Ppy/CMC) prepared by different methods in order to prevent the corrosion of steel. Potentiodynamic polarization curves and open circuit potential were used to evaluate the capacity of these materials to protect the surface. The results obtained allowed to assume that Ppy/CMC is better material to prevent corrosion than polypyrrole (Ppy) and potentiostatic method better than galvanostatic to obtain the deposit.

Self-affine fractal vapour-deposited gold surfaces characterization by scanning tunnelling microscopy

The morphological evolution of the surfaces of gold deposits grown from the vapour on smooth glass under nonequilibrium conditions and incident angle near substrate normal is studied at the nanometer level by scanning tunnelling microscopy. For an average film thickness equal to or greater than 500 nm, the interface thickness (ξ) reaches a steady state. Under these conditions, ξ depends on the scan length (L) as ξ ∝ Lα with α = 0.35 ± 0.05 for L > ds, where ds is the columnar size, and α = 0.89 ± 0.05 for L ds agrees with the prediction of ballistic deposition models without restructuring, whereas that for L < ds exceeds the prediction of ballistic models including restructuring.

XPS and ARXPS study of silver underpotential deposition on platinum in acid solution

Abstract The underpotential deposition (upd) of silver on polycrystalline platinum electrodes has been studied using electrochemical techniques and X-ray photoelectron spectroscopy (XPS). Under the electrochemical experimental conditions used for deposition, the cathodic ( q c ) voltammetric charge density is around 200 μC m −2 which is equivalent to ∼1.2 ML of deposited silver. XPS and angle-resolved X-ray photoelectron spectroscopy (ARXPS) were used to determine the chemical composition of the deposited films. Two different approaches have been used to extract the concentration depth profiles from ARXPS measurements: (a) a simple parametric model with some hypotheses about the form of the depth profile and (b) a general algorithm that uses regularization and singular value decomposition (SVD) techniques. The structure and composition of the films is rather complex. The upd deposition of silver yields films with concentration gradients that can be modeled by a multilayer structure involving silver islands. Starting from the outer surface, the predominant species are C, O+S+Ag, and the platinum substrate.

Electrogeneration of polypyrrole-carboxymethylcellulose composites: electrochemical, microgravimetric and morphological studies

Abstract The electrochemical behaviour of platinum electrodes in aqueous solutions containing carboxymethylcellulose, CMC and pyrrole was followed by voltammetry and chronopotentiometry. The electrogeneration of a composite polypyrrole-CMC at potentials above 500xa0mV vs SCE is influenced by the relative concentrations of pyrrole and polyelectrolyte. Electrolyte adsorption prevails on the electrode at concentrations of pyrrole below 0.1xa0M. The monomer oxidation is diffusion controlled. The polymerization process was followed by “ ex situ ” gravimetric determinations at different temperatures ranging between 5 and 50°C. Productivity of the consumed charges and charge storage efficiencies in the electrogenerated films were obtained. Selected electrodeposition variables are typically: [Pyrrole], [CMC], Txa0 °C, E vs SCE.

Scale-dependent roughening kinetics in vapor deposited gold

Abstract The roughening kinetics of gold deposits grown from vapor was studied by scanning tunneling microscopy. The dynamic scaling yielded the following growth exponents α(I)=0.90±0.06 and β(I)=0.25±0.06 for Ls ds, where Ls is the scan length and ds is the average diameter of columns. The scaling properties of the domain-dependent-surface roughness exponents allowed us to give the rationale for experimental data on the fractal behavior of thin metal films.

A comparative study of electrodeposited and vapour deposited gold films: Fractal surface characterization through scanning tunnelling microscopy

Abstract The surfaces of Au deposits grown under non-equilibrium conditions from either the electroreduction of Au oxide or from the vapour have been analysed as fractals by measuring the perimeter (P) and the area (A) of intergranular voids. The values of P and A were determined from scanning tunnelling microscopy (STM) topographic imaging of the deposit surfaces. A fractal behaviour P ∝ A D 2 was found with D = 1.5 ± 0.1 and D = 1.7 ± 0.1 for the electrodeposited and vapour deposited Au films, respectively. These figures remain constant for film thicknesses between 100 and 1000 nm. The value of Ds, the fractal dimension of the surfaces, is 2.5 ± 0.1 for the Au electrodeposits, and 2.7 ± 0.1 for the Au vapour deposited films. The former value is consistent with either a diffusion or an electric field controlled growth model, whereas the latter is in agreement with a ballistic growth model.

Structural configurations of thin dried polyaniline films on gold(111) from scanning tunneling microscopy

Abstract The early stages of growth of polyaniline films electrodeposited on Au were studied by conventional electrochemical techniques, X-ray diffraction and ex situ scanning tunneling microscopy (STM). From high resolution STM imaging it appears that the first step of polyaniline formation involves the adsorption of aniline monomers on Au(111) terraces leading to domains with hexagonal arrays involving a phenyl-phenyl distance in the range 0.70 nm ⩽ d ⩽ 1.0 nm. The second step corresponds to the initiation of the polymer growth yielding typically 10 × 10 nm 2 crystalline domains consisting of either 0.45 × 0.6 nm 2 rectangular arrays or 0.75 × 0.45 nm 2 arrays forming 120 ° angles. The crystalline domains coexist with highly disordered polymer domains. At advanced stages of growth, the formation of polymer fibers 2 nm in average width takes place leading to a full substrate coverage by a polymer deposit with an irregular surface.

The role of slow surface-atom reordering processes in the underpotential deposition of metals: Silver underpotential deposition on platinum in acid solutions

Abstract The role played by slow surface-atom reordering processes in the underpotential deposition of Ag on polycrystalline Pt and polyfacetted Pt single crystal substrates from 10 −3 M Ag salt in aqueous acid solutions has been studied by using potentiostatic and potentiodynamic techniques complemented with Auger spectroscopy. The potential cycling between E r , the reversible potential of the Ag/Ag + electrode reaction, and 1.10 V (SHE) for 24 h resulted in the formation of a Ag + Pt surface alloy and the incorporation of Ag into the second layer of Pt atoms. The Ag electrodeposition/ Ag anodic stripping on the Ag + Pt alloy gives rise to two new pairs of voltammetric peaks in the range 0.93-0.75 V, whereas the complete stripping of Ag occurs at 1.15 V. On the basis of surface-atom reordering processes which initiate through a place exchange mechanism involving Ag and Pt surface atoms followed by a slow Ag atom diffusion into bulk Pt, the entire cyclic voltammetric features and Auger data can be accounted for.

STM study of fractal scaling in evaporated gold films

Fractal characterization of vacuum-evaporated gold films on glass substrates with thicknesses (h) comprised between 30 and 850 nm is made from STM data by using the dynamic scaling approach for surface growth. The rms roughness (ξ) and the scan length (L) obey a ξ versus L α relationship with a depending on h. For h≥500 nm and L>d s , the average column diameter, α≃1/3 in agreement with the predictions of ballistic deposition models without restructuring. For L<D s , α≃0.9 approaching a euclidean value. The STM method is verified through its application to several computer-generated surfaces, leading to a good agreement with the theoretical values

Methods of fractal analysis applied to STM imaging

Abstract Vapor deposited gold films have been characterized by applying methods of fractal analysis to scanning tunneling microscopy (STM) images. Results from the use of five different methods are in agreement within the limitation of each method. Those methods are suitable to characterize rough surfaces at the nanometer level provided that a large number of images is considered.