Adion surface diffusion

Fig. 7.132. Following surface diffusion to a step site, the adion diffuses along the step to a kink site and to final lattice incorporation.

The discovery of the heterogeneity of surfaces, and in particular of dislocations (see Section 7.12.12), was made in the 1930s (Taylor, 1936), but there had been theoretical work on metal deposition at an earlier time. The model of the surface employed by these earlier workers (Kossel, 1927 Stranski, 1928 Erdey-Gruz, and Volmer, 193 l)was a flat plane without steps and edges to which the adions produced by ion transfer from the double layer could surface diffuse. The only way a metal could grow on a perfect planar surface without growth sites was by nucleation of the deposited atoms, rather than diffusion to the growth sites shown in Fig. 7.134. 

The treatment of this surface diffusion is of interest, among other reasons, because it allows a calculation of the profile of the surface-diffusing adions—where their concentration is greatest and how its rate depends on the dislocation density and other elements of the heterogeneous metal structure. 

The kinetics by which UPD layers form are qualitatively the processes already discussed. There are the electron transfer kinetics from the metal substrate to the depositing ion and the surface diffusion of the adions formed to edge sites on terraces. Complications occur, however, for there is the adsorption of ions to take care of and that brings up questions of which isotherm to use (Section 6.8). Three kinds of UPD formations are shown in Fig. 7.146. Thus Fig. 7.146 (c) shows ID phase formation along a monatomic step in the terraces on the single ciystal Fig. 7.146 (b) shows 2D nucleation at a step, and Fig. 7.146 (a) shows 2D nucleation on an atomically flat plane. 

This problem will be side-stepped for a moment, and a simpler one tackled. Consider two steps that are parallel to each other, the type of steps considered in the analysis of the constant-current transient. As ions transfer across the electrified interface and the adions thus formed surface diffuse and become incorporated in the steps, there is an advance of the steps toward each other (Fig. 7.156). Eventually the two steps approach each other, some closely, so that all one is left with is a one-atom-wide and one-atom-deep chasm. The moment this is filled in, the two steps disappear. The collision of the two steps moving toward each other has resulted in their mutual annihilation. 

The relevance of crystal faces to the subject of electrociystalhzation comes up as follows Each of the crystal faces just described contains all the microfeatures that have been described in previous sections, steps, kinks, etc. Further, the same phenomena of deposition—the ions crossing the electrified interface to form adions, the surface diffusion, lattice incorporation of adions, screw dislocation, growth spirals, etc.—occur on all the facets. 

Terrace Ion-Transfer Mechanism, In the terrace siteion-transfer mechanism a metal ion is transferred from the solution (OHP) to the flat face of the terrace region (Fig. 6.15). At this position the metal ion is in the adion (adsorbed-like) state, having most of its water of hydration. It is weakly bound to the crystal lattice. From this position it diffuses on the surface, seeking a position of lower energy. The final position is a kink site. 

Fig. 7.131. Since the surface adion is unaffected by the electric field normal to the electrode surface, in order to reach the step site, the adion diffuses in a random-walk manner.

All solid surfaces exhibit structural features that can have significant effects on the kinetics of charge transfer reactions and on the stability of the interfacial region. In the case of metals, the most significant structural features for "smooth" surfaces are emergent dislocations, kink sites, steps, and ledges. It has long been known, for example, that the kinetics of some electrodissolution and electrodeposition reactions depend on the density of such sites at the surface, but the exact mechanisms by which the effects occur have not been established. The role of "adion" in these processes is also unclear, as is the sequence of the dehydration-electronation-adsorption-diffusion-incorporation processes, even for the simplest of metals. 

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