Number Density Of Active Sites Research Articles

Early stages of cobalt electrodeposition on FTO and n-type Si substrates in sulfate medium

The mechanism of cobalt electrocrystallization on fluorine-doped tin oxide (FTO)-coated conducting glass substrate has been studied and compared with results obtained on n-type Si electrodes. The initial stages of metal deposition from a solution of 0.25 M CoSO 4, 1 M Na 2SO 4 and 0.5 M H 3BO 3 (pH 3.5) were investigated using cyclic voltammetry, and chronoamperometry measurements. Important kinetics parameters were estimated from the analysis of current transients on the basis of the Scharifker–Hills model for electrochemical nucleation and diffusion-controlled growth. From the analysis of the experimental current transients, it was found that the process of cobalt deposition involves a three-dimensional (3D) electrochemical nucleation and diffusion-controlled growth. A strong dependence of the number density of active sites N 0 with applied potential was observed on both FTO and n-type Si electrodes.

Electrochemical nucleation and growth of copper deposition onto FTO and n-Si(1 0 0) electrodes

The initial stages of Cu electrodeposition onto fluor-tin-oxide (FTO) coated glass and n-Si(1 0 0) substrates from sulfate solutions containing respectively 5 × 10 −3 M CuSO 4, 1 M Na 2SO 4 and 0.5 M H 3BO 3 are studied using cyclic voltammetry and chronoamperometry and the Scharifker–Hills model is used to analyse the current transients. For electrodeposited Cu on both FTO and n-Si(1 0 0) electrodes, the nucleation is in a good agreement with the instantaneous nucleation and three-dimensional (3D) diffusion-limited growth. The values of kinetic parameters, number density of active sites N 0, and diffusion coefficient D for Cu 2+ ions are also calculated.

Electrodeposition Under Forced Convection Conditions

The copper electrodeposition process was studied under the influence of forced convection conditions, varying the electrode angular speed (ω) with the aid of a rotating disc electrode, using the potential step technique. From the analyses of the experimental current transients it was found that both: the nucleation rate constant (A) and the number density of active sites (N0) for ω > 200 rpm depending on the hydrodynamic conditions imposed. However, the current transient recorded in the 0 < ω < 200 rpm could not be analyzed with any of the theoretical formalisms proposed so far.

Inorganic Surface Nanostructuring by Atmospheric Pressure Plasma-Induced Graft Polymerization

Surface graft polymerization of 1-vinyl-2-pyrrolidone onto a silicon surface was accomplished by atmospheric pressure (AP) hydrogen plasma surface activation followed by graft polymerization in both N-methyl-2-pyrrolidone (NMP) and in an NMP/water solvent mixture. The formation of initiation sites was controlled by the plasma exposure period, radio frequency (rf) power, and adsorbed surface water. The surface number density of active sites was critically dependent on the presence of adsorbed surface water with a maximum observed at approximately a monolayer surface water coverage. The surface topology and morphology of the grafted polymer layer depended on the solvent mixture composition, initial monomer concentration, reaction temperature, and reaction time. Grafted polymer surfaces prepared in pure NMP resulted in a polymer feature spacing of as low as 5-10 nm (average feature diameter of about 17 nm), an rms surface roughness range of 0.18-0.72 nm, and a maximum grafted polymer layer thickness of 5.5 nm. Graft polymerization in an NMP/water solvent mixture, however, resulted in polymer feature sizes that increased up to a maximum average feature diameter of about 90 nm at [NMP] = 60% (v/v) with polymer feature spacing in the range of 10-50 nm. The surface topology of the polymer-modified silicon surfaces grafted in an NMP/water solvent mixture exhibited a bimodal feature height distribution. In constrast, graft polymerization in pure NMP resulted in a narrow feature height distribution of smaller-diameter surface features with smaller surface spacing. The results demonstrated that, with the present approach, the topology of the grafted polymer surface was tunable by adjusting the NMP/water ratio. The present surface graft polymerization method, which is carried out under AP conditions, is particularly advantageous for polymer surface structuring via radical polymerization and can, in principle, be scaled to large surfaces.

Mechanistic Prediction of Nucleate Boiling Heat Transfer–Achievable or a Hopeless Task?

Over the last half of the twentieth century, a number of purely empirical and mechanism-based correlations have been developed for pool nucleate boiling. Empirical correlations differ from each other substantially with respect to the functional dependence of heat flux on fluid and surface properties, including gravity. The mechanism-based correlations require knowledge of the number density of active sites, bubble diameter at departure, and bubble-release frequency. However, because of the complex nature of the subprocesses involved, it has not been possible to develop comprehensive models or correlations for these parameters. This, in turn, has led to the pessimistic view that mechanistic prediction of nucleate boiling is a hopeless task. However, there is an alternative to the past approaches—complete numerical simulation of the boiling process. Value of this approach for bubble dynamics and associated heat transfer is shown through excellent agreement of predictions with data obtained on microfabricated surfaces on which active nucleation sites can be controlled.

The effect of temperature on the kinetics and mechanism of silver electrodeposition

The influence of temperature on the kinetics of the electrochemical phase formation of silver onto glassy carbon was studied by means of potentiodynamic and potentiostatic techniques. It was found that as the temperature increased, a smaller overpotential was required to initiate the deposition process of silver. The experimental current density transients were adequately described by a theoretical model involving the simultaneous presence of two processes, namely adsorption of silver ions and multiple 3D nucleation and growth limited by diffusion. It was also shown that for all the temperatures considered, there exists an overpotential (−0.25 V), at which the maximum values for the nucleation rate and the number density of active sites were obtained.

Gold Electrocrystallization on Carbon and Highly Oriented Pyrolytic Graphite from Concentrated Solutions of LiCl

The early stages of the electrolytic deposition of gold on carbon electrodes from concentrated LiCl electrolytes have been investigated by voltammetry, chronoamperometry, and microscopy. The analysis of current‐time transients according to existing theories indicates that this process occurs by multiple three‐dimensional nucleation, followed by diffusion‐controlled growth of nuclei. Diffusion coefficients calculated on the basis of nucleation theory were found to be higher than those from rotating disk experiments (RDE), suggesting that gold species are adsorbed on the electrode surface. Gold nucleation from 6 M LiCl cannot be classified as either instantaneous or progressive nucleation. The kinetic parameters for nucleation, aNo (nucleation rate) and No (number density of active sites on the substrate surface), were estimated by using a general Poisson nucleation law. Both quantities were found to vary with potential and with concentration of gold. The potential dependence of the nucleation rate, aNo, was interpreted according to classical and atomistic theory. For low gold concentrations, where adsorption of AuCl or might occur prior to nucleation, the number of atoms in the critical nucleus (Nc) was less than unity over the entire potential range analyzed. For high gold concentrations the number of atoms forming the critical nucleus depends on overpotential. In all solutions studied nucleation takes place on a limited number of active sites.

Silver Electrocrystallization on Vitreous Carbon from Ammonium Hydroxide Solutions

The early stages of the electrolytic deposition of silver onto vitreous carbon electrodes from ammonium hydroxide solutions have been investigated by the potential step technique. The analysis of the experimental current transients according to existing theories indicates that this process occurs by multiple three‐dimensional nucleation, followed by diffusion controlled growth of nuclei. It is shown that treatments that involve classifying the process as either instantaneous or progressive nucleation are not always adequate for the quantitative analysis of electrochemical nucleation phenomena. The nucleation kinetics parameters A (nucleation rate constant per site) and No (number density of active sites on the substrate surface) were estimated separately from the current transient maxima by two different approaches. Both quantities were found to vary with the potential and with the concentration of silver ions, except at very high overpotentials for silver deposition. The potential dependence of the nucleation rate A was interpreted according to the atomistic theory and in all cases it was found that the number of atoms in the critical nucleus (nk) was one over the entire potential range analyzed.

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

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

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.