To this point, we have treated electrodeposition in a macroscopic way. However, deposit morphology and properties vary dramatically with the conditions under which the deposition takes place as illustrated in Figure 13.2, where current density is varied. In order to understand these different morphologies, we need to explore the fundamental processes that take place during deposition. The purpose of this section is to explore those fundamentals at an introductory level. Electrodeposition is a rich and interesting field about which much has been written, and we regret that we are limited by space to provide only a brief introduction. Please refer to the Further Reading section at the end of this chapter and to the literature for additional information.
Figure 13.2 Examples of morphologies of copper deposits grown at (a) 60, (b) 1200, (c) 2400, and (d) 5000 A·m−2. (From A. Ibañez and E. Fatás (2005) Surface & Coatings Technology, 191, 7–16. Reprinted with permission from Elsevier.)
The charge transfer that takes place during electrodeposition follows the same processes that we have discussed previously in other chapters. Consider a hydrated cation that approaches a surface. The cation must get sufficiently close to the surface to permit electron transfer from the surface in order to be reduced. However, electrodeposition offers an additional complexity that we have not yet considered—the formation of a new phase. After reduction, the metal atom must be incorporated into a metal lattice on the surface. In this section, we consider the physical processes by which this takes place and how these processes are influenced by the deposition conditions.
As we begin our discussion, imagine a perfect metal crystal, atomically flat with no defects. Since the lattice is complete, there is no room for an additional metal atom to be incorporated. Hence, a metal ion reduced on this surface would form a surface adion that is in a higher energy state than the metal atoms that are already incorporated into the lattice. We refer to the newly adsorbed species as an adion because evidence indicates that it retains a partial charge and some of the hydration water molecules rather than directly forming a charge-free surface atom. If there are other adions in close proximity and sufficient energy has been supplied, it is possible to form a stable nucleus from which deposition can continue. Nucleation is an important aspect of electrodeposition and is discussed below. However, before doing that, let’s consider the impact of having a real surface rather than the perfect, defect-free surface imagined above.
Real surfaces have a variety of defects, as illustrated schematically in Figure 13.3. These defects greatly reduce the energy required for incorporation of a new atom. Integration of a metal adion into the lattice occurs primarily at kink sites (see Figure 13.3) where half of metal atom is bound to the lattice. One of the important aspects of kink sites is that another kink site is produced as a result of lattice incorporation. Therefore, kink sites are not consumed until the edge of the surface is reached. In fact, for dislocation defects, such as the screw dislocation illustrated in Figure 13.4, deposition can continue indefinitely in a spiral form as shown in the figure. In contrast, vacancy sites are no longer available for adion incorporation once filled.
Figure 13.3 Schematic diagram of the structure of a crystal face with a simple cubic lattice. Source: Adapted from Budevski 1996.
Figure 13.4 Spiral growth around a screw dislocation during deposition of a metal.
Because of the availability of kink sites and other low-energy sites on real surfaces, deposition at low overpotentials takes place almost exclusively at those sites, and the nucleation of new sites does not play a role. Electron exchange takes place primarily on the flat portion of the surface, which represents the largest fraction of the total surface area and the location on the surface where less distortion of hydrated cation is required in order to get sufficiently close for electron transfer to occur. Adions then diffuse to step sites and eventually to kink sites where they are incorporated into the lattice as shown in Figure 13.3. The driving force for transport is the concentration gradient formed by the addition of adions away from the kink sites and consumption of the adions at the kink sites. Transfer of the adions to kink sites is the limiting resistance at low overpotentials where the concentration of adions on the surface is low. An increase in the overpotential leads to an increase in the adion concentration and a reduction in the transport resistance that must be overcome for incorporation into the lattice. At sufficiently high overpotentials, the process is no longer limited by adion diffusion.
What does this mean from a practical perspective? At low overpotentials growth occurs only at existing sites, and the result is a crystallographic deposit such as that shown in Figure 13.5. Growth can continue until all of the available sites on the surface are consumed. Any subsequent growth requires an overpotential that is large enough for nucleation to occur.
Figure 13.5 Crystallographic copper deposit showing macro-spiral growth. Source: Reproduced with permission of German Bunsen Society.
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