The Electrodeposition Process

Despite the vast quantity of data on the chemistry of electropolymerization, relatively little is known about the processes involved in the deposition of polymers on the electrode, i.e. the heterogeneous phase transition. Research — voltammetric 

A trace-crossing appears on the reverse sweep of the first cycle in all voltammograms, providing that the scan reversal lies close to the peak potential. 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

Galvanostatic, potentiostatic as well as potentiodynamic techniques can be used to electropolymerize suitable monomeric species and form the corresponding film on the electrode. Provided that the maximum formation potentials for all three techniques are the same, the resulting porperties of the films will be broadly similar. The potentiodynamic experiment in particular provides useful information on the growth rate of conducting polymers. The increase in current with each cycle of a multisweep CV is a direct measure of the increase in the surface of the redoxactive polymer and, hence, a suitable measure of relative growth rates (Fig. 5). 

The relative growth rate per cycle v is calculated from the peak current of the respective polymer oxidation using Eq. (3)  

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In the reductive regime, a strong, apparently irreversible, reduction peak is observed, located at -1510 mV vs. the quasi reference electrode used in this system. With in situ STM, a certain influence of the tip on the electrodeposition process was observed. The tip was therefore retracted, the electrode potential was set to -2000 mV, and after two hours the tip was reapproached. The surface topography that we obtained is presented in Figure 6.2-14. 

The metal to be plated is first cleaned carefully and then activated with a weak acid. Steel can be treated with 3-5% HCl, whilst a 10% fluoboric acid solution is suitable for copper alloys. It is then ready for the electrodeposition process. 

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. 

Compared with the CBD technique, electrodeposition requires some additional capital equipment (i.e., suitable power supplies and electrodes). Major advantages of the electrodeposition process include the insignificant amount of waste generation. The electrodeposition bath can be reused for an unlimited number of cycle times when salts are replenished in the bath. The major drawback for electrodeposition is that it requires conductive substrates, which limits the application of this process in several key technologies. 

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. 

Successful use of this cell for electrodeposition in the production of electrodeposits of desired properties depends on understanding each component specifically, components of the metal-solution interface. The metal-solution interface is the locus of the electrodeposition process and thus the most important component of an electrodeposition cell. 

In this chapter we discuss water and ionic solutions, in Chapter 3, structure of metals and metal surfaces and in Chapter 4, the formation and structure of the metal-solution interface. Discussion is limited to those topics that are directly relevant to the electrodeposition processes and the properties of electrodeposits. 

We show that the electric field in the metal-solution interphase is very high (e.g., 10 or lO V/cm). The importance of understanding the structure of the metal-solution interphase stems from the fact that the electrodepKJsition processes occur in this very thin region, where there is a very high electric field. Thus, the basic characteristics of the electrodeposition processes are that they proceed in a region of high electric field and that this field can be controlled by an external power source. In Chapter 6 we show how the rate of deposition varies with the potential and structure of the double layer. 

Atomic processes that constitute the electrodeposition process, Eq. (6.93), can be seen by presenting the structure of the initial, (solution), and the final state, (lattice). Since metal ions in the aqueous solution are hydrated, the structure of the initial state in Eq. (6.93) is represented by [M(H20)J". The structure of the final state is the M adion (adatom) at the kink site (Fig. 6.13), since it is generally assumed that atoms (ions) are attached to the crystal via a kink site (3). Thus, the final step of the overall reaction, Eq. (6.93), is the incorporation of the adion into the kink site. 

It is interesting to note that Brenner and Riddell (2-4) accidentally encountered electroless deposition of nickel and cobalt during electrodeposition of nickel-tungsten and cobalt-tungsten alloys (in the presence of sodium hypophosphite) on steel tubes in order to produce material with better hardness than that of steel. They found deposition efficiency higher than 100%, which was explained by an electroless deposition contribution to the electrodeposition process. 

In the discussion of atomistic aspects of electrodepKJsition of metals in Section 6.8 it was shown that in electrodeposition the transfer of a metal ion M"+ from the solution into the ionic metal lattice in the electrodeposition process may proceed via one of two mechanisms (1) a direct mechanism in which ion transfer takes place on a kink site of a step edge or on any site on the step edge (any growth site) or (2) the terrace-site ion mechanism. In the terrace-site transfer mechanism a metal ion is transferred from the solution (OHP) to the flat face of the terrace region. At this position the metal ion is in an adion state and is weakly bound to the crystal lattice. From this position it diffuses onto the surface, seeking a position with lower potential energy. The final position is a kink site. 

Electrodeposition of lead-tin alloy films is usually performed in the presence of peptone as an additive. Peptone is adsorbed on the metal surface during the electrodeposition process. The fractional surface coverage Q of the lead-tin electrode may be determined from the double-layer capacitance C measurements, and/or chronopotentiometric measurements. For a solution containing 9.0 g/L of tin and 13.0 g/L of lead, the following relationship between the concentration of peptone, the double-layer capacitance C, and the transition time At is observed (8). 

Electrodeposition presents, in principle, several advantages for the investigation and production of layered alloys. Among these are the tendency of electrodeposited materials to grow epitaxial and thus to form materials with a texture influenced by the substrate. Electrodeposition can be used in systems that do not lend themselves to vacuum deposition. The electrodeposition process is inexpensive and can be upscaled with relative ease for use on large parts further, it is a room-temperature technology. This last point may be important for systems in which undesirable interdiffusion between the adjacent layers may readily occur. 

Nanocrystalline materials have received extensive attention since they show unique mechanical, electronic and chemical properties. As the particle size approaches the nanoscale, the number of atoms in the grain boundaries increases, leading to dramatic effects on the physical properties and on the catalytic activity of the bulk material. Nowadays, there is a wide variety of methods for the preparation of nanocrystalline metals such as thermal spraying, sputter deposition, vapor deposition and electrodeposition. The electrodeposition process is commercially attractive since it can be performed at room temperature and the experimental set-up is less demanding. Furthermore, the particle size can be adjusted over a wide range by controlling the experimental parameters such as overvoltage, current density, composition, and temperature (see Chapter 8). 

As was shown in Chapter 4, elemental tantalum can be electrodeposited in the water- and air-stable ionic liquid [Pyi,4] TFSA at 200°C using TaFs as a source of tantalum [ 15,16]. The quality of the deposit was found to be improved upon addition of LiF to the deposition bath. At room temperature only ultrathin tantalum layers can be deposited as the element. The electrodeposition of tantalum was investigated by in situ STM to gain insight into the electrodeposition process. 

Since little is yet known about efficiency on the technical scale, future investigation should focus on (i) efficiency with respect to separation yield, energy demand and amount of mass separation agents required, (ii) long-term re-use options of auxiliary agents such as extractants or adsorbents and (iii) ease of scaling up. Furthermore, a crucial point for further development of regeneration will be to identify the pollutants that disturb the main process as well as their critical concentration levels in the electrodeposition process. 

A quartz round flask was used as an electrochemical cell with three electrodes. Al-wires (Alfa, 99.999%) were used as reference and counter electrodes. Mild steel sheets were employed as working electrodes. The working electrodes were mechanically polished with emery paper, cleaned with acetone in an ultrasonic bath, treated with dilute hydrochloric acid and rinsed with distilled water. Prior to the electrodeposition process the electrodes were anodically polarized in the employed ionic liquid to remove as far as possible the native oxide layer. Removal of the... 

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