Electrodeposition Kinetics

The thermodynamic equilibrium potential of an electrode surface in contact with its ions in the solution is defined by the well-known Nernst equation. 

represents the equilibrium potential of the electrode surface at standard conditions P° = 101 kPa, T° = 298 K), R is the universal gas constant, F is Faraday s constant, and T is the absolute temperature. The term represents the activity of the metal ions in solution, with n being their oxidation state. In dilute solutions, the ion activity is approximated by their concentration, i.e. Q/ + 

The above expression suggests that equilibrium potential of metal electrode shifts for 0.059 V/n in negative direction if the metal ion concentration in the solution is decreased ten times. This means that the actual value of the equilibrium potential can be effectively changed by adjusting the concentration of the corresponding metal ions in the solution. If the applied potential E) to the metal electrode is more negative than Ej jnr/ f, ( = E — Ej + 0) the electrode is at overpotential conditions, and X] is called overpotential. Under this condition, metal electrodeposition occurs, (A/ + + ne M). 

During metal electtodeposition, for a given value of overpotential, the corresponding cathodic current density [A m ] is observed which can be correlated to the deposition flux [mol m s ] using the expression, flux = j/nF. The relation between the deposition current density j) and electrode surface overpotential JjsfU] is described by Butler V /rwer equation defined below  

However, the transport limitations for the ions from the bulk solution, going through the diffusion layer, towards the electrode surface also play an important role in overally vs n dependence. The maximum deposition current density (flux) that can be reached through the diffusion layer,yx [A-m ], is the function of the diffusion layer thickness [m, ion diffusivity Z)Af +[m -s ], and concentration of depositing ions in the bulk solution [mol  

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Dhamo N (1994) An electrochemical hydrocyclone cell for the treatment of dilute solutions approximate plug-flow model for electrodeposition kinetics. J Appl Electrochem 24 745-750... 

We have studied a range of different metals and metal oxides with widely varying degrees of electrochemical reversibility and electrodeposition kinetics, hi contrast to silver, many important noble metals and metal oxides electrodeposit irreversibly, hi Figure 16.1.6a, for example, irreversible cyclic voltammograms acquired in plating solutions for MoOj (a metallic oxide of molybdenum) and platinum are compared with the reversible CVs seen for silver. 

A1 is more noble than Ti, and so at room temperature only codeposits and alloys can be obtained. Furthermore, kinetic factors also play a role in the electrodeposition of the element. 

Tellurium and cadmium Electrodeposition of Te has been reported [33] in basic chloroaluminates the element is formed from the [TeCl ] complex in one four-electron reduction step, furthermore, metallic Te can be reduced to Te species. Electrodeposition of the element on glassy carbon involves three-dimensional nucleation. A systematic study of the electrodeposition in different ionic liquids would be of interest because - as with InSb - a defined codeposition with cadmium could produce the direct semiconductor CdTe. Although this semiconductor can be deposited from aqueous solutions in a layer-by-layer process [34], variation of the temperature over a wide range would be interesting since the grain sizes and the kinetics of the reaction would be influenced. 

As noted earlier, the kinetics of electrochemical processes are inflnenced by the microstractnre of the electrolyte in the electrode boundary layer. This zone is populated by a large number of species, including the solvent, reactants, intermediates, ions, inhibitors, promoters, and imparities. The way in which these species interact with each other is poorly understood. Major improvements in the performance of batteries, electrodeposition systems, and electroorganic synthesis cells, as well as other electrochemical processes, conld be achieved through a detailed understanding of boundaiy layer stracture. 

Given the thermodynamic properties of a system, judicious variation of the different plating parameters to assist in manufacturing the desired electrodeposit should be based on an accurate kinetic model. Engelken and Van Doren [6, 7] proposed... 

In searching to formulate a mechanism of CuInSc2 phase formation by one-step electrodeposition from acid (pH 1-3) aqueous solutions containing millimolar concentrations of selenous acid and indium and copper sulfates, Kois et al. [178] considered a number of consecutive reactions involving the formation of Se, CuSe, and Cu2Se phases as a pre-requisite for the formation of CIS (Table 3.2). Thermodynamic and kinetic analyses on this basis were used to calculate a potential-pH diagram (Fig. 3.10) for the aqueous Cu+In-i-Se system and construct a distribution diagram of the final products in terms of deposition potential and composition ratio of Se(lV)/Cu(ll) in solution. 

I. Development of a simple, Butler-Volmer equation-based kinetic model for MiXi (CdTe) electrodeposition. J Electrochem Soc 132 2904-2909... 

The composition of the electrolyte is quite important in controlling the electrolytic deposition of the pertinent metal, the chemical interaction of the deposit with the electrolyte, and the electrical conductivity of the electrolyte. In the case of molten salts, the solvent cations and the solvent anions influence the electrodeposition process through the formation of complexes. The stability of these complexes determines the extent of the reversibility of the overall electroreduction process and, hence, the type of the deposit formed. By selecting a suitable mixture of solvent cations to produce a chemically stable solution with strong solute cation-anion interactions, it is possible to optimize the stability of the complexes so as to obtain the best deposition kinetics. In the case of refractory and reactive metals, the presence of a reasonably stable complex is necessary in order to yield a coherent deposition rather than a dendritic type of deposition. 

Kim and Jorne [37] have used a rotating zinc hemisphere to study the kinetics of zinc dissolution and deposition reactions in concentrated zinc chloride solutions. The electrodeposition reaction of cadmium on mercury was used by Mortko and Cover [43] in their investigation of a rotating dropping mercury electrode their data behaved according to Eqs. (74)-(76). 

Electroless deposition as we know it today has had many applications, e.g., in corrosion prevention [5-8], and electronics [9]. Although it yields a limited number of metals and alloys compared to electrodeposition, materials with unique properties, such as Ni-P (corrosion resistance) and Co-P (magnetic properties), are readily obtained by electroless deposition. It is in principle easier to obtain coatings of uniform thickness and composition using the electroless process, since one does not have the current density uniformity problem of electrodeposition. However, as we shall see, the practitioner of electroless deposition needs to be aware of the actions of solution additives and dissolved O2 gas on deposition kinetics, which affect deposit thickness and composition uniformity. Nevertheless, electroless deposition is experiencing increased interest in microelectronics, in part due to the need to replace expensive vacuum metallization methods with less expensive and selective deposition methods. The need to find creative deposition methods in the emerging field of nanofabrication is generating much interest in electroless deposition, at the present time more so as a useful process however, than as a subject of serious research. 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

Modeling the Mn-Al system is particularly difficult because the kinetics of the Mn and A1 deposition reactions can not be measured directly. Although it is possible to estimate the current-potential relationships for both Mn and A1 from electrodeposit composition, no examination along these lines appears in the literature. A close ex-... 

Radiometric methods are unique for their ability to provide directly the surface concentration of the adsorbate. A method for in situ study of electrochemical reactions on solid electrodes was invented by Joliot. ° He used a thin gold foil as an electrode which at the same time served as the window of the radiation counter. Johot determined the kinetics and the effect of tartaric acid on polonium electrodeposition on gold. The method was later further developed and improved (e.g.. Refs. 102,103). 

Since the kinetics of the doping processes is expected to depend upon the nature of the counterion, particularly its size (which may influence the mobility throughout the polymer host), it is possible to control the diffusion kinetics by selecting the nature of the supporting electrolyte employed in the electrodeposition process. 

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