Cathodic deposition of metals

Often, we need only a qualitative estimate that is, we want to know whether the limiting current is raised or lowered by migration relative to the purely diffusion-limited current, or whether a, is larger or smaller than unity. It is evident that a, will be larger than unity when migration and diffusion are in the same direction. This is found in four cases for cations that are reactants in a cathodic reaction (as in the example above) or products in an anodic reaction, and for anions that are reactants in an anodic reaction or products in a cathodic reaction. In the other four cases (for cations that are reactants in an anodic or products in a cathodic reaction, and for anions that are reactants in a cathodic or products in an anodic reaction), we have a, < 1, a typical example being the cathodic deposition of metals from complex anions. 

Consider as an example the cathodic deposition of metal from a binary solution of the electrolyte MAof concentration q. The concentration changes from Cy (. to Cg, ... 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

Cathodic deposition of metal nanopaiticles on silicon typically proceeds via progressive formation of 3D nuclei, which gives opportunity to control electrodeposit properties at the nucleation stage. A complex AC and DC electrochemical characterization technique has been developed for fine tuning the nucleation stage in the formation of metal-silicon nanostructures. 

Fig. 4 Surface layers of an electrode during the cathodic deposition of metal.

The most generally useful method of metal-ion removal is cathodic deposition of metal which may be represented by ... 

Good cathodic deposition of metal powder or sponge is controlled by a number of factors which depend on the material itself and on the experimental conditions employed [14, 20, 21],... 

W. Traube and co-workers prepared cone. soln. of chromous chloride by the prolonged electrolytic reduction of aq. sola, of green chromic chloride, using lead plates as electrodes and employing a current desnity of 0-175 amp. per sq. dm. The electrolysis of chromous chloride soln., using an iron cathode, results in the formation of a cathodic deposit of metallic chromium mixed with chromium oxides. 

Typical applications comprise the cathodic deposition of metal and alloy layers for surface finishing applications. Many recipes can be found in Schlesinger and Paunovic (2010). 

During the treatment of dilute solutions, the cathodic deposition of metal is often under mass transport control, either initially or during longer batchprocessing times (section 2.5.2), In such cases, it was seen in Chapter 2 that the maximum duty of the reactor may be expressed in terms of the limiting current (which is proportional to the rate of metal deposition). From the definition of the mass transport coefficient ... 

This is a particularly useful approach in circumstances when these parameters are interrelated strongly, and perhaps time-dependent, e.g. if cathodic deposition of metal is carried out in the reactor near limiting-current conditions (Chapter 7), the rough deposit obtained may not only increase the electroactive area, but also provide effective turbulence promotion, so increasing ki. On the other hand, the use of a porous, insulating mesh next to an electrode may significantly improve via turbulence promotion, but it may obscure part of the electrode, i.e. A will be decreased. 

Schematic representation of steps in the cathodic deposition of metals. 

An electrolytic process for purifying cmde vanadium has been developed at the U.S. Bureau of Mines (16). It involves the cathodic deposition of vanadium from an electrolyte consisting of a solution of VCI2 in a fused KCl—LiCl eutectic. The vanadium content of the mixture is 2—5 wt % and the operating temperature of the cell is 650—675°C. Metal crystals or flakes of up to 99.995% purity have been obtained by this method. 

Vacuum Deposition-also vapor deposition or gas plating the deposition of metal coatings by means of precipitation (sometimes in vacuum) of metal vapor onto a treated surface. The vapor may be produced by thermal decomposition, cathode sputtering or evaporation of the molten metal in air or an inert gas. 

Oxygen overpotential is about 0.4-0.5 volt at a polished platinum anode in acid solution, and is of the order of 1 volt in alkaline solution with current densities of 0.02-0.03 A cm-2. As a rule the overpotential associated with the deposition of metals on the cathode is quite small (about 0.1-0.3 volt) because the depositions proceed nearly reversibly. 

Table 14.1 Deposition of metals at controlled potential of the mercury cathode...

Cathodic deposition of lead sulfide from acidic aqueous solutions of Pb(II) ions (nitrate salts mainly) and Na2S203 on various metallic substrates at room temperature has been reported. Stoichiometric PbS films composed of small crystallites (estimated XRD diameter 13 nm) of RS structure were obtained at constant potential on Ti [204]. Also, single-phase, polycrystalline thin films of RS PbS were electrode-posited potentiostatically on Ti, Al, and stainless steel (SS) [205]. It was found that the Al and Ti substrates promoted growth of PbS with prominent (200) and (111)... 

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

Underpotential Deposition of Metal Atoms Because of the energy of interaction between a foreign substrate and the adsorbed metal atoms formed by discharge, cathodic discharge of a limited amount of metal ions producing adatoms is possible at potentials more positive than the equilibrium potential of the particular system, and also more positive than the potential of steady metal deposition. 

Electrogravimetry is one of the oldest electroanalytical methods and generally consists in the selective cathodic deposition of the analyte metal on an electrode (usually platinum), followed by weighing. Although preferably high, the current efficiency does not need to be 100%, provided that the electrodeposition is complete, i.e., exhaustive electrolysis of the metal of interest this contrasts with coulometry, which in addition to exhaustive electrolysis requires 100% current efficiency. 

The significance of the supporting electrolyte cation depends crucially on whether a divided or an undivided cell is used. In a divided cell, the choice of cation is of minor importance but in an undivided cell the cathode process should not lead to formation of base and thereby to buffering of the solution. Metal cations such as Li+, Na+ or Mg + are often the choice since in aprotic solvents the metal cation may be the most easily reduced component. This has been observed as deposits of metal on the surface of the cathode arising from... 

Synthetic lipids and peptides have been found to self-assemble into tubules [51,52]. Several groups have used these tubules as templates [17,51,53-56]. Much of this work has been the electroless deposition of metals [51,54]. Electrolessly plated Ni tubules were found to be effective field emission cathode sources [55]. Other materials templated in or on self-assembled lipid tubules include conducting polymer [56] and inorganic oxides [53]. Nanotubules from cellular cytoskeletons have also been used for electroless deposition of metals [57]. 

Voltz and Holt determined the optimum conditions necessary for the electrodeposition of macroamounts of technetium from an aqueous bath of NH TcO. The results reveal that technetium can be deposited in macro-amounts as a bright metal from aqueous solutions. A bath containing 1 M (NH )2S04 and 0.006-0.024 M NH TcO with HjSO added to give a pH of about 1.0 can be electrolyzed at 1-2 A/dm. Thus, a metallic cathodic deposit of technetium with a current efficiency range of 18-30% is obtained. 

The conclusion of Huang et al. was supported by Lin et al., who used a three-electrode cell design to monitor the voltage profile of both the anode and cathode in a full lithium ion cell during cycling at low temperatures and found that these cyclings resulted in the deposition of metallic lithium on the... 

Based on Faraday s law (equation 8.160), the of the reaction is in both cases negative reactions then proceed spontaneously toward the right, with the dissolution of Fe and Zn electrodes, respectively, and deposition of metallic Cu at the cathode. 

Let us now consider a galvanic cell with the redox couples of equation 8.164. This cell may be composed of a Cu electrode immersed in a one-molal solution of CUSO4 and a Zn electrode immersed in a one-molal solution of ZnS04 ( Dan-iell cell or Daniell element ). Equation 8.170 shows that the galvanic potential is positive the AG of the reaction is negative and the reaction proceeds toward the right. If we short-circuit the cell to annul the potential, we observe dissolution of the Zn electrode and deposition of metallic Cu at the opposite electrode. The flow of electrons is from left to right thus, the Zn electrode is the anode (metallic Zn is oxidized to Zn cf eq. 8.167), and the Cu electrode is the cathode (Cu ions are reduced to metallic Cu eq. 8.168) ... 

In consideration of the M /M electrode. Ox would be replaced by and Figure 6.10 would represent variation of concentration of the ions with distance from the cathode in case of the non-steady-state deposition of metal M. The variation of the concentration of the reactant Ox at the electrode (x = 0), Cq (x = 0,0, is given by the equation... 

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