Potential drop metal deposit

Anodic stripping voltammetry consists of two steps (Figure 11.37). The first is a controlled potential electrolysis in which the working electrode, usually a hanging mercury drop or mercury film, is held at a cathodic potential sufficient to deposit the metal ion on the electrode. For example, with Cu + the deposition reaction is... 

Figure 5.13 (a) Potential drop across a SAM and the adjacent double layer (DL) in a region free of defects shown in top part. Solid and dashed lines are two examples of different positions of the Fermi level Ep. Dash-dot line indicates the course of an equipotential line representing the potential at the outer surface of the SAM including defects, (b) Illustration of processes involved in electrochemical metal deposition. For details, see text. 

Fig. 5.1 Schematic energy band bending for (A) large particle, (B) small particle, and (C) metal-deposited particle. R, radius of the particle Lsc, space charge layer E(red/ox), redox level in solution E , Fermi level in semiconductor

The potential that is fixed is that of the metallic particles making up the bed. However, there is an IR drop through the solution which increases with the length ofthe bed, so that the potential available for deposition becomes significantly less as the length of the bed increases. This limits the effective value of L in Eq. 

A continuous metal deposit layer may behave as an ohmic contact or a Schottky barrier. For a relatively thick metal film the silicon can still behave like a semiconductor before the onset of current. For example, for n-Si deposited electrochemically with 150nm An, the electrode behavior is similar to that of bare silicon electrode At positive potentials the anodic current is small whereas at cathodic potentials current from hydrogen evolution increases with increasing polarization. " In the potential region before the onset potential for the cathodic current a linear Mott-Schottky plot is obtained giving a flatband potential similar to that of bare silicon sample. In the potential region where hydrogen evolution occurs, it behaves like a metal with potential drops mostly in the Helmholtz layer. 

E25.27(b) No reduction of cations to metal will occur until the cathode potential is dropped below the zero-current potential for the reduction of Ni2+ (-0,23 Vat unit activity), Deposition of Ni will occur at an appreciable rate after the potential drops significantly below this value however, the deposition of Fe will begin (albeit slowly) after the potential is brought below —0.44 V. If the goal is to deposit pure Ni, then the Ni will be deposited rather slowly at just above —0.44 V then the Fe can be deposited rapidly by dropping the potential well below —0.44 V. 

When there is a net current through the electrolytic cell, the rates of deposition and dissolution are not equal. As a result, the potential drop at the electrode surface is different from the equilibrium potential. The difference is called the overpotential. If the magnitude of the external current density is small compared to the exchange current density, the departure from equilibrium is also small. In this case, the electrode potential is close to the equilibrium value given by the Nernst equation and for practical purposes, the overpotential can be neglected. It should be noted that two or more electrode processes that have different equilibrium potentials may occur independently of each other at the same metal surface (Newman 1991). 

These interfering effects can be minimised or avoided by reducing the deposition time and the total amount of metal deposited. This would of course mean a loss of sensitivity, but use of differential pulse stripping voltammetry can offset this. The use of a hanging mercury drop electrode with its larger mercury volume offers less intermetallic interference than does the mercury film electrode. Careful choice of a deposition potential can also sometimes prevent the codeposition of metals forming intermetallic compounds. 

Ohmic inhibitors may increase the resistance by forming a nonconducting thick film on the metal surface or by potential drop in the solution. They increase the overvoltage of the anodic or cathodic reactions depending on where the film is deposited (Fig. 14.9). 

In the second phase of pit growth corrosion products of dissolved metal ions and chloride anions are deposited in the hole. This film further stabilizes the potential drop and provides the conditions for an electrochemical polishing of the surface in the hole. This finally creates the larger semi-spherical forms of the pits. 

The practical situation is more complex than the above suggests, e.g. gas evolution at one of the electrodes may cause an appreciable ohmic potential drop, due to the hold-up of (poorly conducting) gas bubbles in such cases, problems may be minimized by using open, porous electrodes (which allow gas to escape from their back surface) or by suitable electrolyte-dectrode movement, e.g. an increased electrolyte flow. Alternatively, electrodes may produce poorly conductive (e.g. polymeric or oxide) films, or separators may be fouled (by, for example, deposition of organic films or metals hydroxides). 

In the electrolytic cell, the cupric ions and sulfate ions both contribute to the conduction mechanisms. But only cupric ions enter into the electrode reaction and pass through the electrode-solution interface. The electrode therefore acts like a semipermeable membrane which is permeable to the Cu ions but impermeable to the 80 ions. Anions accumulate near the anode and become depleted near the cathode, resulting in concentration gradients in the solution near the electrodes of both ions. This is termed as concentration polarization. Let us determine the current-voltage characteristic of the cell, that is, the concentration polarization. To do this, we must calculate the flux of metal ions (cations) arriving at the cathode and depositing on it. We assume that the overall rate of the electrode reaction is determined by this flux. Once the cation distribution is known, the potential drop can be calculated. Note that anions are effectively motionless and do not produce a current. Let us assume that electrodes of the electrolytic cell are infinite planes at the anode (y = 0) and cathode (y = h) (Figure 6.3). The electrolyte velocity is zero. The definition of the current densities is... 

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