J.M. Vara

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.

The behaviour of electrodispersed platinum electrodes for the voltammetric electrooxidation of reduced carbon dioxide

The voltammetric electrooxidation of reduced CO2 adsorbate is studied on large active area (electrodispersed) platinum electrodes. This reaction is used as a test reaction in the absence of readsorption to establish a surface roughness/electrocatalytic activity relationship. In addition, the depolarizing effect of electrodispersed electrodes for the reaction is discussed in terms of the structural model recently derived for these electrodes from scanning tunneling microscopy imaging.

Kinetics and mechanism of “reduced CO2” electrooxidation on electrodispersed platinum in different acid solutions

Abstract The electrooxidation of “reduced CO2” electroadsorbates on electrodispersed platinum electrodes has been investigated in 0.05 M HClO4, 1 M HClO4, 0.5 M H2SO4 and 1 M H3PO4 at 25° C through voltammetry and potential step techniques. The overall reaction comprises three distinguishable processes, namely a first order triggering process, and two second order surface processes. The latter are influenced remarkably by the solution composition (anions). The second order reaction mechanism involves two distinguishable “ reduced CO2” electroadsorbates which react independently with the OH species formed from H2O electrooxidation on the bare platinum sites as the reaction proceeds. An interaction term has to be included in the rate equations to account for the experimental results. The mechanistic interpretation accounts for the values of the number of electrons per site ranging from 1 to 2.

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.

The influence of temperature on the kinetics and mechanism of reduced CO2 electrooxidation on electrodispersed Pt in 0.5 M H2SO4 and 1 M H3PO4

Abstract The kinetics of reduced CO2 electrooxidation on electrodispersed Pt have been investigated in 0.5 M H2SO4 and 1 M H3PO4 in the 2–80°C range by voltammetric and potentiostatic methods. Kinetic data can be interpreted through a complex reaction pathway involving two different reduced CO2 adsorbates. The reaction mechanism advanced earlier at 25°C is valid as a first approach for interpreting the reaction in the entire temperature range and in different electrolytes. The influence of temperature and potential sweep rate on the kinetic response of the system shows that an interconversion process involving the different CO2 adsorbates should be considered in the reaction mechanism. The interconversion process can explain the voltammetric peak multiplicity change with v and T. The apparent activation energy (80±15 kJ mol−1) is the same for the electrooxidation of both reduced CO2 adsorbates. This figure can be interpreted in terms of a modification of clathrate-type CO2 adsorbates into an intermediate which immediately decomposes electrochemically into H+, H2O and CO2. The influence of CO2 adsorbates on the H-atom electrosorption/electrodesorption reactions is also discussed.

Kinetics of CO2 electroadsorption on electrodispersed platinum electrodes in acid solutions

Abstract The kinetics of CO2 electroadsorption on electrodispersed Pt electrodes have been investigated in different acid electrolytes. The kinetic data obtained at different adsorption potentials, adsorption times and CO2 concentrations in solution have been interpreted in terms of two complex reaction mechanisms involving the formation of strongly and weakly bound reduced CO2 adsorbates through reactions involving s-H and w-H adatoms, respectively, and carbon dioxide. The reaction mechanism between H adatoms and CO2 implies the initial electroadsorption of H atoms, the transport of CO2 from the bulk of the solution to the reaction interface, and the formation of the reduced CO2 adsorbate. Under certain conditions, particularly when s-H adatoms are involved, the entire reaction approaches mass transport rate control. Conversely, the reaction between w-H adatoms and CO2 appears as a rather slow process.

Dynamics of roughness and surface reactions at solid electrodes

Abstract Surface disorder and random roughness of electrode surfaces are discussed in terms of Euclidean and fractal models. Roughness development at different metals results from two contributions operating in opposite directions, namely shadowing between growing clusters which leads to rough surfaces, and surface diffusion phenomena which tend to smooth surface irregularities. The second contribution depends on the specific mobility of the deposited metal atoms, the temperature, and the electrolyte composition. Monte Carlo simulations based upon nucleation and growth models show that the growth processes controlled by either surface reactions or mass transport of reacting species, including surface diffusion, produce compact deposits with a weak surface disorder. Conversely, the growth process under mass transport control involving a negligible surface diffusion generates open structures with a strong surface disorder. Rough metal deposits exhibiting a fractal behaviour are considered. The fractal dimension provides a quantitative description of the degree of disorder, the growth mechanism and the reactivity of those surfaces in relation to electrocatalysis.

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.