Spectroscopy electrode/solution interface

In situ infrared spectroscopy allows one to obtain stracture-specific information at the electrode-solution interface. It is particularly useful in the study of electrocat-alytic reactions, molecular adsorption, and the adsorption of ions at metal surfaces. 

Interfacial water molecules play important roles in many physical, chemical and biological processes. A molecular-level understanding of the structural arrangement of water molecules at electrode/electrolyte solution interfaces is one of the most important issues in electrochemistry. The presence of oriented water molecules, induced by interactions between water dipoles and electrode and by the strong electric field within the double layer has been proposed [39-41]. It has also been proposed that water molecules are present at electrode surfaces in the form of clusters [42, 43]. Despite the numerous studies on the structure of water at metal electrode surfaces using various techniques such as surface enhanced Raman spectroscopy [44, 45], surface infrared spectroscopy [46, 47[, surface enhanced infrared spectroscopy [7, 8] and X-ray diffraction [48, 49[, the exact nature of the structure of water at an electrode/solution interface is still not fully understood. 

In recent years,3 4 however, there has been renewed interest in the study of the electrode/solution interface due in part to the development of new spectroscopic techniques such as surface-enhanced Raman spectroscopy,5-7 electrochemically modulated infrared reflectance spectroscopy and related techniques,8,9 second-harmonic generation,10-12 and others which give information about the identity and orientation of molecular species in the interfacial... 

As mentioned previously, this can be attributed in part to the lack of structure-sensitive techniques that can operate in the presence of a condensed phase. Ultrahigh-vacuum (UHV) surface spectroscopic techniques such as low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and others have been applied to the study of electrochemical interfaces, and a wealth of information has emerged from these ex situ studies on well-defined electrode surfaces.15"17 However, the fact that these techniques require the use of UHV precludes their use for in situ studies of the electrode/solution interface. In addition, transfer of the electrode from the electrolytic medium into UHV introduces the very serious question of whether the nature of the surface examined ex situ has the same structure as the surface in contact with the electrolyte and under potential control. Furthermore, any information on the solution side of the interface is, of necessity, lost. 

Materials that have a nonzero second-order susceptibility will produce light at twice the incident frequency. The magnitude of this effect is small, and has been a practical consideration only since the advent of lasers. If the symmetry of a crystal or other medium is such that it has a center of inversion, no SHG effect will be observed. However, surfaces by their very nature break this inversion symmetry. Hence, an SHG signal may arise at the electrode-solution interface even though both bulk phases may be considered centrosymmetric [66], The magnitude of the SHG signal is sensitive to surface conditions (e.g., electrode potential, ionic or molecular adsorption, etc.). Surface spectroscopy is also feasible since the SHG signal will be enhanced if either the incident frequency (to) or SHG (2co) corresponds to an electronic absorption of a surface species [66]. 

The increasing application of spectroscopic methods in electrochemistry has characterized the last decade and marked the beginning of new developments in electrochemical science [1]. Among these methods, in-situ infrared spectroscopy provides a very useful tool for characterizing the electrode-solution interface at a molecular level. First in-situ infrared (IR) electrochemical measurements were performed in 1966 [2] using the internal reflection form [3]. However, problems in obtaining very thin metal layers on the surface of the prisms used as IR windows, delayed the extensive application of in-situ IR spectroscopy until 1980, when the method was applied in the external reflection form [4]. The importance of this step does not need to be emphasized today. 

Another approach, internal reflection spectroscopy (IRS) or attenuated total reflectance (ATR) spectroscopy, takes advantage of the total reflection observed when a light beam is directed to the back of the OTE at an angle greater than the critical angle. Since the reflection takes place at the electrode-solution interface, the light beam is attenuated only by molecules close to the electrode surface. The penetration depth is about 1000 A in a typical experiment. Because of this very small effective cell length, the electrode is most frequently used in a multiple reflection mode [Fig. 44(b)]. 

Since the objective of the studies described herein is the characterisation of the solute species formed following redox reaction the very extensive research dealing with characterisation of the electrode/solute interface will not be discussed, excellent overviews of the experimental aspects of this subject are available. While this contribution focuses on applications involving IR, Raman spectroscopy has proved to be invaluable to many SEC studies where surface-enhanced Raman spectroscopy (SERS) and resonance Raman spectroscopy dominate. Reviews and recent studies attest to the value of these approaches. ... 

The combination of infrared (IR) spectroscopy and electrochemistry, IR spectro-electrochemistry, is a powerful tool for investigation of the electrode/solution interface. It is extremely useful for studies of the structure and bonding of species adsorbed on electrode surfaces. In situ IR-spectroelectrochemistry reveals important details of the potential-dependent surface chemistry of adsorbing species. 

With modern computerized frequency-analysis instrumentation and software, it is possible to acquire impedance data on cells and extract the values for all components of the circuit models of Figure 2.>7, This type of analysis, w hich is called electrochemical impedance spectroscopy, reveals the nature t>f the faradaic processes and often aids in the investigation of the mechanisms of electron-transfer reactions. In the section that follows, we explore the processes at the electrode-solution interface that give rise to the faradaic impedance. 

This chapter is devoted to describing the basic aspects of the measurement, instmmentation, measurement techniques, and practical applications of potential-modulated UV-visible spectroscopy as a representative spectroelectrochemi-cal tool to characterize thin organic films on electrode surfaces and to track the kinetics of the electrode surface processes. At the same time, miscellaneous features of the measurement, which may be important for those who intend to apply for the first time the potential-modulated UV-visible spectroscopic method in their experiments, will also be included. However, because of the Hmit to the chapter length as well as the existence of superior review articles on UV-visible reflectance spectroscopy at electrode/solution interfaces [2,6-9], detailed comprehensive description is minimized. With the intention of overviewing the UV-visible spectroscopic method for the benefit of experimental electrochemists, optical issues, especially optical reflection theory, are not detailed. 

The basic optical theory of reflection for a soHd surface covered with a thin film has already been well established. We do not intend to review it here in full detail. Instead, readers can refer to the textbook of optics [12] or review articles on UV-visible reflection spectroscopy at electrode surfaces [2, 6-8]. It must be noted that for some electrode/solution interfaces it is stiU an extremely difficult task to establish the modeling of the interface through the use of classical light reflection theory, as described in the later sections. 

Anions and neutral organic compounds are among other species whose adsorption on single-crystal electrodes has been probed with sensitivity by infrared spectroscopy. Because of their importance as common electrolytes, there has been long-standing interest in the behavior of simple oxoanions such as perchlorate, sulfates, and phosphates, at the electrode/solution interface (cf. Refs. 

In-situ Surface-enhanced Infrared Spectroscopy of the Electrode/Solution Interface... 

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