1. Overview
1.1. INTRODUCTION
Electrochemical deposition of metals and alloys involves the reduction of metal ions from aqueous, organic, and fused-salt electrolytes. In this book we treat deposition from aqueous solutions only. The reduction of metal ions M z+ in aqueous solution is represented by
$$M_{solution}^{z+}+ze\to M_{lattice}$$(1.1)
This can be accomplished by means of two different processes: (1) an electrodeposition process in which z electrons (e) are provided by an external power supply, and (2) an electroless (autocatalytic) deposition process in which a reducing agent in the solution is the electron source (no external power supply is involved). These two processes, electrodeposition and electroless deposition, constitute the electrochemical deposition. In this book we treat both of these processes. In either case our interest is in a metal electrode in contact with an aqueous ionic solution. Depositionreaction presented by Eq. (1.1) is a reaction of charged particles at the interface between a solid metal electrode and a liquid solution. The two types of charged particles, a metalion and an electron, can cross the interface.
Four types of fundamental subjects are involved in the process represented by Eq. (1.1): (1) metal–solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice (M lattice ), and (4) structure and properties of the deposits. The material in this book is arranged according to these four fundamental issues. We start by considering in the first three chapters the basic components of an electrochemical cell for deposition. Chapter 2 treats water and ionic solutions; Chapter 3, metal and metal surfaces; and Chapter 4, the metal–solution interface. In Chapter 5 we discuss the potential difference across an interface, and in Chapter 6, the kinetics and mechanisms of electrodeposition. Nucleation and growth of thin films and formation of the bulk phase are treated in Chapter 7. Electroless deposition and deposition by displacement are the subject of Chapters 8 and 9, respectively. In Chapter 10 we discuss the effects of additives in the deposition and nucleation and growth processes. Simultaneous deposition of two or more metals, alloy deposition, is discussed in Chapter 11. The manner in which current and metal are distributed on the substrate is the subject of Chapter 12. Characterization of metal surfaces before and/or after deposition and during the deposition process is treated in Chapters 13 and 14. Chapter 15 treats modeling of the deposition process. Structure and properties of deposits are treated in Chapters 16 to 18.
It is seen from the above that the present book contains a number of different types of material, and it is likely that on first reading, some readers, will want to use some chapters, whereas others may want to use different ones. For this reason the chapters and their various sections have been made independent of each other as far as pos- sible. Certain chapters can be omitted without causing difficulties in reading suc- ceeding chapters. For example, Chapters 3 (on metals and metal surfaces), 7 (on nucleation and growth models), 14 (on in situ characterization of deposition processes), and 15 (mathematical modeling in electrochemistry) can be omitted on first reading. Thus, the book can be used in a variety of ways to serve the needs of different readers.
1.2. RELATION OF ELECTROCHEMICAL DEPOSITION TO OTHER SCIENCES
The relation of electrochemical deposition to other sciences may be appreciated by considering the above-mentioned four types of fundamental problems associated with Eq. (1.1)
1. The metal–solution interface as the locus of the deposition processes. This interface has two components: a metal and an aqueous ionic solution. To understand this interface, it is necessary to have a basic knowledge of the structure and electronic properties of metals, the molecular structure of water, and the structure and proper- ties of ionic solutions. The structure and electronic properties of metals are the subject matter of solid-state physics. The structure and properties of water and ionic solutions are (mainly) subjects related to chemical physics (and physical chemistry). Thus, to study and understand the structure of the metal–solution interface, it is nec- essary to have some knowledge of solid-state physics as well as of chemical physics. Relevant presentations of these subjects are given in Chapters 2 and 3.
2. Kinetics and mechanism of the deposition process. The rate of the deposition reactionn [Eq. (1.1)] is defined as the number of moles of M z? depositing per second and per unit area of the electrode surface:
$$v = k[M^{z+}]$$(1.2)
where $k$ is the rate constant of the reduction reaction and [$M^{z+}$] represents the activity of $M^{z+}$. The rate constant $k$ of electrochemical processes is interpreted on the basis of the statistical mechanics and is given by the expression
$$k=\frac{k_{B}T}{h}\left ( -\frac{\Delta G_{e}^{\ddagger}}{RT} \right )$$(1.3)
where $k_{B}$ is the Boltzmann constant, $T$ is the absolute temperature, $h$ is the Planck constant, $\Delta G_{e}^{\ddagger}$ is the electrochemical activation energy, and $R$ is the gas constant. The electrochemical activation energy is a function of the electrode potential E:
$$\Delta G_{e}^{\ddagger}=f(E)$$(1.4)
This account of electrochemical kinetics shows why understanding and development of electrochemical deposition depends on statistical mechanics, which itself was developed by both physicists and chemists. The interpretation of $\Delta G_{e}^{\ddagger}$ is connected also to quantum mechanics.
3. Nucleation and growth processes of the metal lattice. Understanding of the nucleation and growth of surface nuclei, formation of monolayers and multilayers, and growth of coherent bulk deposit is based on knowledge of condensed-matter physics and physical chemistry of surfaces.
4. Structure and properties of deposits. These can be understood and interpreted on the basis of a variety of surface and bulk analytic techniques and methods that reveal electrical, magnetic, and physical properties of metals and alloys.
The authors of this book believe that this review of the relationship between the subject of electrochemical deposition and other sciences justifies one important general conclusion: that electrochemical deposition is a fascinating field. We hope that the readers will agree with this and work diligently on understanding, development, and/or applications of the fundamentals of electrochemical deposition.