Metal dissolution and deposition

In the previous section we discussed outer sphere charge transfer. That process is characterized by the following features  

1) Both reactant and product are on the solution side of the interface 

2) The charge is carried by the electron, which is the only species crossing the interface 

3) There are no chemical bonds broken and no new bonds formed as a result of charge transfer. Indeed the reactant and the product are very similar, except for their charge 

Metal deposition is clearly not an outer-sphere charge-transfer process. The initial state is a solvated metal ion, while the final product is a neutral atom in the metal, for example  

---

The rotating hemispherical electrode (RHSE) was originally proposed by the author in 1971 as an analytical tool for studying high-rate corrosion and dissolution reactions [13]. Since then, much work has been published in the literature. The RHSE has a uniform primary current distribution, and its surface geometry is not easily deformed by metal deposition and dissolution reactions. These features have made the RHSE a complementary tool to the rotating disk electrode (RDE). 

The discussion of concentration polarization so far has centred on the depletion of electroactive material on the electrolyte side of the interface. If the metal deposition and dissolution processes involve metastable active surface atoms, then the rate of formation or disappearance of these may be the critical factor in the overall electrode kinetics. Equation (2.69) can be rewritten for crystallization overvoltage as... 

The direct transfer of metal ions to kink sites and/or to step edges, unambiguously found first in the deposition of silver, seems to be a general phenomenon in electrochemical metal deposition and dissolution. From a theoretical point of view, Gerischer [5.98] was the first to recognize the role of direct transfer in metal deposition processes. 

The nonperiodic structure of surface defects such as steps makes them very difficult, if not impossible, to investigate by commonly employed diffraction techniques, and real-space imaging becomes mandatory. In this respect, STM, with its capability of imaging electrode surfaces in situ with atomic resolution, provides a unique possibility of studying processes for which surface imperfections play a key role, such as metal deposition and dissolution [14-20], oxide formation [21-24], and corrosion [25-29]. The additional capability of STM to control material properties... 

It has been stated by several noted authors in electrochemistry that, in the case of metal deposition, charge is carried across the interface by ions rather than by electrons. " Unfortunately, the above authors did not implement the consequence of this difference in the analysis of the mechanism of metal deposition and dissolution. In one instance/ the author went as far as to state that... 

Metal deposition and dissolution (34) In the electrodeposition of solid metals such as silver and zinc, the cation is transported across the electrochemical interface to sites on the electrode surface (Figure 6-4). The positive charge of the cation is offset by electrons from the metal, and the adsorbed species becomes an adatom. These species have surface mobility and migrate along the electrode surface to an imperfection such as a step dislocation, where they enter into the crystal lattice. In the absence of sufficient step dislocations to accommodate the rate of deposition, the adatom surface concentration increases until two- or three-dimensional nucleation occurs. The rate of such nucleation and surface migration strongly influences the morphology of the electrocrystalhzation process. The reverse of this process is involved with electrodissolution of crystalline electro-deposits. 

The importance of measuring the imaginary component of the quartz crystal in order to study metal deposition and dissolution processes has also been noted by the authors of [26,88]. In particularly, in this way they [26] succeeded in separating contributions of mass loading and roughness to QCM response and to characterize the electrode roughness. 

Summary. Time-resolved, atomic-scale, in-situ STM studies of phase formation at metal electrode surfaces are described. Examples include structural phase transitions within the electrode surface layer and in anionic or metallic adsorbate layers as well as metal deposition and dissolution processes. 

Probe beam deflection can be observed during metal deposition and dissolution. The method is sensitive towards changes in the concentration gradient originating from dissolution or deposition of submonolayer amounts of material on the electrode surface [875]. A typical result of PBD measurements is displayed in Fig. 5.140. The negative PBD response observed during anodic stripping of the silver indicates concentration decay into the solution phase. The positive response... 

The electric double layer (edl) is fundamental for electrochemistry because the rate and mechanism of the various electrochemical reactions (hydrogen evolution, corrosion and corrosion inhibition by surfactants, metal deposition and dissolution, and so forth) depend on the... 

During the past few years, we have developed a method to combine our theory with DFT calculations [56, 57]. Basically, DFT provides several important parameters of our model, so that it has become possible to calculate free energy surfaces for the reactions and determine energies of activation. So far, our work has mainly been applied to electrocatalytic reactions, where we have focused on the effect of the electronic properties of the electrode on the reaction rate. However, the same procedure can be applied to metal deposition and dissolution, and we shall briefly return to this point at the end of this chapter. Here, we shall... 

Figure 1. Schematic representation of the metal deposition and dissolution reaction M, metal atom L, ligand S, solvent molecule z, charge (valency) of the metal in solution p, q, r, number of ligands per metal atom a, y, number of solvent molecules per metal atom.

This present part considers the mechanism of the metal deposition and dissolution processes as a whole. It analyzes both the thermodynamic and the kinetic consequences of the postulated mechanisms, as well as the repercussions of the mechanisms on the properties of the deposits obtained. The electrode is also viewed as an electrical system, and the effects of the electrode... 

Effect of Anions on the Kinetics of Metal Deposition and Dissolution... 

Anions can affect metal deposition and dissolution in several ways. 

All this is based on considerable fundamental knowledge of metal deposition and dissolution processes acquired during the boom of science of the last few decades[3]. Thus, the reasons for the formation of metal crystals of widely different appearance are well understood. They do not lie in the properties of the metals themselves (e.g, zinc and copper). 

E. Budevski, Metal deposition and dissolution. Part A Electrocrystal-... 

Bund A, Schwitzgebel G (2000) Investigations on metal depositions and dissolutions with an improved EQCMB based on quartz crystal impedance measurements. Electrochim Acta 45 3703—3710... 

IBM, written for the IBM/360 machine. CSMP was meant to simulate control processes. The authors used it to simulate CV of an adsorbed species. ESTYM PDE is a program based on orthogonal collocation (see Sect. 9.6) on moving finite elements [30]. ESTYM PDE was applied to simulate CVs complicated by homogeneous reactions [31-36], metal deposition and dissolution processes[37-39], hydrogen evolution and dissolution [40] and amalgam formation [41]. 

Electrode reactions can be divided into two major groups those in which only charge is transferred across the interface, and those in which both charge and mass are transferred. Outer-sphere charge transfer is a good example of the former, while metal deposition and dissolution is an example of the latter. 

---