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The intention of this monograph has been to assimilate key practical and theoretical aspects of those spectroelectrochemical techniques likely to become routine aids to electrochemical research and analysis. Many new methods for interphasial studies have been and are being developed. Accordingly, this book is restricted in scope primarily to in situ methods for studying metal! electrolyte or semiconductor! electrolyte systems; moreover, it is far from inclusive of the spectroelectrochemical techniques that have been devised. However, it is hoped that the practical descriptions provided are sufficiently explicit to encourage and enable the newcomer to establish the experimental facilities needed for a particular problem. The chapters in this text have been written by international authorities in their particular specialties. Each chapter is broadly organized to review the origins and historical background of the field, to provide sufficiently detailed theory for graduate student comprehension, to describe the practical design and experimental methodology, and to detail some representative application examples. Since publication of Volume 9 of the Advances in Electrochemistry and Electrochemical Engineering series (1973), a volume devoted specifically to spectroelectrochemistry, there has been unabated growth of these fields. A number of international symposia-such as those held at Snowmass, Colorado, in 1978, the proceedings of which were published by North-Holland (1980); at Logan, Utah in 1982, published by Elsevier (1983); or at the Fritz Haber Institute in 1986-have served as forums for the discussion of nontraditional methods to study interphases and as means for the dissemination of a diversity of specialist research papers.



1. Introduction

It has long been recognized by electrochemists that measurements of electrical currents, voltages, charges, or capacitances do not always provide unequivocal identification of electroactive molecules, i.e., although a diffusion current might be correlated to a particular species, with its peak or half-wave potentials for reduction or oxidation and a diffusion coefficient appropriate to the media, the molecular identity has to be inferred from the measured physical properties of standard systems. In more complex (multilayer) or natural (biochemical or environmental) systems, these properties may not always be resolvable. The ability, therefore, to utilize additional, perhaps more specific, physical characteristics of molecules to monitor electrode processes, in either dynamic or equilibrium conditions, would be immensely valuable. In the past several decades in particular, there have been considerable efforts expended to develop spectroelectrochemical techniques to aid electrochemical research. Molecular properties such as molar absorptivities, vibrational absorption frequencies, and electronic or magnetic resonance frequencies, in addition to the traditional electrical parameters, now are being used routinely to better our understanding of electron transfer reaction pathways and the fundamental molecular states at interfaces.
Robert J. Gale

2. X-Ray Techniques

The development of spectroelectrochemical techniques has been a major field of research for electrochemists since the late 1960s.(1) As can be seen from the other chapters in this book, there are now many such techniques being routinely applied in electrochemical laboratories around the world, while further developments are appearing all the time. One of the most significant successes of spectroelectrochemistry has been the elucidation of structure at the electrode/electrolyte solution interface. In particular, it is possible to investigate the electronic structure of the electrode surface, and of species near it, using UV-visible light as a probe, while vibrational spectroscopy can be used to study the molecular structure and local environment. Unfortunately, these optical techniques do not provide any direct information about either long-or short-range order parameters. Such information would clearly be very valuable both from a fundamental point of view as well as from a technological one. For example, it would be very interesting in metal deposition studies to be able to monitor the evolution of structure as nuclei are formed and as they subsequently grow into thick deposits. Similarly, the battery scientist would welcome the opportunity to follow structural changes occurring during the charge and discharge cycles of a battery system. While the electrochemist’s need for a technique, or techniques, capable of providing this type of information is readily apparent, it is not immediately clear what is the best approach to follow when attempting to develop one.
James Robinson

3. Photoemission Phenomena at Metallic and Semiconducting Electrodes

The original photoelectric effect was discovered by Hertz(1) in 1887 while he was investigating the properties of electromagnetic waves. It was observed that a spark would jump a gap between two electrodes more readily when the electrodes were illuminated with light, ultraviolet light having a greater effectiveness than light from the visible region of the spectrum. This simple observation, however, was to have profound impact on our conception of how radiation and matter interact.
Ricardo Borjas Severeyn, Robert J. Gale

4. UV-Visible Reflectance Spectroscopy

Within the last two decades, interfacial electrochemistry has become very much an integral part of modern surface science.(1–6) In the search for a microscopic description of the electrochemical double layer, which obviously is necessary for a detailed understanding of electrode processes, electrochemists have increasingly recognized the potential power of surface science concepts and techniques for obtaining information which would complement the thermodynamic description derived from classical electrochemical methods. As a consequence, (1) new, nonelectrochemical techniques have been introduced and combined with the traditional electrochemical methods, (2) atomically well-defined single-crystal surfaces have been increasingly used as electrodes rather than polycrystalline material, and (3) structure-sensitive methods such as low-energy electron diffraction (LEED), developed for surface characterization under ultrahigh-vacuum conditions, have been employed to study electrode surfaces before and after electrochemical experiments.(4,5) In addition, atomistic concepts for interactions at surfaces have been introduced from surface science into electrochemistry to modify or replace the macroscopic and highly phenomenological models of the electrochemical double layer, which were developed from the analysis of thermodynamic data.
Dieter M. Kolb

5. Infrared Reflectance Spectroscopy

The structure of the electrode/electrolyte interface plays an important, growing role in electrochemistry and electrocatalysis. On one hand, the distribution of charged particles and dipolar molecules in the double layer (i.e., the transition region between the surface of the metal electrode and the bulk of the electrolyte solution) under the combined influence of diffusion and potential gradients determines the potential barrier, which strongly influences the rate of electrochemical reactions, i.e., reactions involving charge transfer through the interface.(1) On the other hand, many electrode reactions proceed through adsorbed intermediates, which are produced during chemisorption of the reacting molecules (the so-called electroactive species) on the electrode surface. The mechanisms of these electrocatalytic reactions will therefore greatly depend on the nature of the electrode material, which determines the structure of the adsorbed intermediates(2).
Bernard Beden, Claude Lamy

6. Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has been observed at solid/solution, solid/gas, solid/vacuum, and solid/solid interfaces, and it is possibly the most sensitive surface high-resolution vibrational spectrosopic technique available as an analytical probe. The great interest in SERS is evidenced by the virtual explosion in the number of publications dealing with this technique since 1977 from the physics, chemistry, and materials science research communities. One reason for this interest in SERS is the generality of the technique with regard to the nature of the material phase in contact with the solid surface. Another reason is that there are several mechanisms that contribute to SERS, and a considerable scientific challenge exists in determining the extent of the contributions of various mechanisms to the overall enhancement.
Ronald L. Birke, John R. Lombardi

7. ESR Spectroscopy of Electrode Processes

Electron spin resonance (ESR) is an attractive technique for the identification and study of species containing an odd number of electrons (radicals, radical cations and anions, and certain transition metal species). The experiment is based on the fact that whereas in the absence of a magnetic field, the two possible spin states, \( + \frac{1} {2},\, - \frac{1} {2} \), of an unpaired electron have identical energies, this degeneracy is lost when a field is applied. ESR spectroscopy involves the flipping of the spin between the two now different energy levels, an act which is brought about by the absorption of microwave radiation. As will be shown below, the presence of magnetic nuclei in the molecule is revealed as hyperfine structure in the ESR transition, which thus may be used to provide a positive identification of the odd-electron species. Since each magnetic nucleus contributes (at least in principle) to the hyperfine structure, a rather more intimate insight into molecular identity emerges than from, for example, UV-visible spectroscopy. It is this high information content of ESR spectroscopy, together with the sensitivity of the technique (radical concentrations on the order of 10−8 M may be observed with standard equipment(1)), that has made ESR the method of choice for investigating complex electrode reactions which proceed via radical intermediates. Other advantages are that the specificity towards paramagnetic species is advantageous in some cases, that the technique is nonperturbing (there is no activation or destruction of the sample), and that absolute concentrations may be measured (albeit with limited accuracy).
Richard G. Compton, Andrew M. Waller

8. Mössbauer Spectroscopy

Mössbauer spectroscopy is a technique that relies on the phenomenon of recoilless emission and absorption of γ-rays for the investigation of nuclear quantum states. The energy associated with such quantum states is modified by the interactions of the nucleus with the surrounding electric and magnetic fields. Hence, the analysis of information derived from such measurements may be expected to afford considerable insight into the structural, electronic, and magnetic properties of a variety of condensed-phase materials. Although restricted to only a few elements, this methodology has found wide application in a number of research areas, including chemistry, biology, and metallurgy, and has recently emerged as a powerful tool in the investigation of systems of interest to physical electrochemistry.
Daniel A. Scherson


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