L. Vázquez

STM-SEM combination study on the electrochemical growth mechanism and structure of gold overlayers: A quantitative approach to electrochemical metal surface roughening

Abstract The growth mode and structure of gold overlayers resulting from the electroreduction of thick oxide films are studied using potentiodynamic and potentiostatic techniques combined with ex-situ STM and SEM. Electroreduction of the thick gold oxide at low overpotentials, that is slowly grown gold overlayers, results in a close-packed array of grains exhibiting a low roughness factor. Grains in the order of 100 nm of radii are formed by aggregation of small monomers. At higher overpotentials, fast grown gold overlayers, we suggest that the monomer growth results in a columnar structure terminated on rounded domes with radii ranging between 10 and 20 nm. This overlayer exhibits a roughness factor which increases according to electrodeposit height. The columnar structure is unstable decreasing its surface free energy by coalescence of small columns to form large units leading to a drastic decrease in the surface area with ageing time. A mechanism for the growth mode is proposed where the crystallite size depends on the diffusion length of the electrodepositing particles which is controlled by the applied overpotential.

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.

Fractal characterisation of electrodispersed gold electrodes

Abstract The fractal dimension of the surface, D s , of thin columnar gold electrodeposits (surface roughness factor 50–100) grown on gold wire cathodes by electroreducing hydrous gold oxide layers has been determined by measuring the diffusion controlled current of the Fe(CN) 4− 6 /Fe(CN) 3− 6 reaction. The diffusion current ( I ) vs. time ( t ) relationships obeyed a I α t −α dependence with D s = 2α + 1. The initial D s , value is 2.5 ± 0.1, and decreases to 2.3 ± 0.1 by keeping the deposit in contact with the electrolyte solution at 298 K due to the smoothing of the rough metal surface by surface diffusion.

Scanning tunneling microscopy of platinum electrode surfaces with different preferred crystallographic orientations

Abstract We have used scanning tunneling microscopy to study the surface microtopography of platinum electrodes preferentially oriented by a procedure denoted as electrochemical faceting. The treated specimens show clear reoriented patches on the surface in the form of steps with well-defined orientations separated by terraces of different sizes. The microscopic data obtained by STM correspond well with voltammetric analysis.

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-SEM and impedance characterization of columnar structured gold electrodes

Abstract The impedance spectra of thin gold electrodeposits with 10−6−10−5 cm average columnar radius and 4×10−6−3×10−4 cm average height have been measured in acid solutions. Frequency dispersion has been observed for film thicknesses greater than 3 × 10−5 cm and electrolyte resistivities larger than 300 Ω cm−1. STM-SEM observations reveal a self-affine surface structure with a size distribution of pores ranging from 10−6 to 10−4 cm between the columnar elements. In diluted solutions the contribution of the smallest pores to the sampled electrode area decreases as ω increases explaining the observed frequency dispersion. This result confirms that self-affine surfaces exhibit a fractional-power-law impedance. However, no simple correlation was found between the α value and the D value of the self-affine surface measured by an independent method. The present results also show that the electrolyte resistivity plays a certain role in determining the α value.

Fuzzy algorithm control effectiveness on drum boiler simulated dynamics

The optimal control of parameters in a system assumes an important role in industrial processes. Models based on boiler-turbine plant are proposed in various applications. The target of this paper is to apply intelligent techniques on a boiler simulator to improve the speed and precision of control. In order to release such task, genetic-fuzzy controllers able of tuning the time duration of specific boiler drum signals, are designed. The results show good precision and relevant speed of control system improving the control performances of classical control structures.