Interface solution Spectroelectrochemical

The electrochemical behavior of a modified electrode ultimately depends on structural details at the molecular level. For example, the molecular-level interaction between the redox site in the film and the solvent from the contacting solution phase might play an important role in the electrochemical response. Molecular-level details are often difficult to infer from electrochemical methods alone, but do lend themselves to spectroscopic analyses. In recent years there has been an explosion of new spectroscopic techniques for characterizing modified electrodes and the electrode-solution interface in general [44,45]. In this section, we review some of these spectroelectrochemical methods. 

Laser Raman spectroscopy has played a major role in the study of electrochemical systems (see Section 3.4). The technique provides molecular-specific information on the structure of the solid-solution interfaces in situ and is particularly suited for spectroelectrochemical studies of corrosion and surface film formation. Metals such as Pb, Ag, Fe, Ni, Co, Cu, Cr, Ti, Au and Sn, stainless steel and other alloys in various solutions have been studied by the technique. 

Fig. 104. Equivalent circuits for the analysis of photocurrent-decay transients. The circuit elements are C, photocapacitor Rseries, total series resistance of the spectroelectrochemical cell Rl, load resistor Rin, internal leakage resistor R0, C , resistor and capacitor of counterelectrode solution interface Rd, resistance due to damaged surface layer.

F. In liquid ATR [27, 82-90], the IRE is a liquid poured into a specially configured cell (e.g., prism), with the sample surface at the base of the cell (Fig. 4.15). This technique can be used for studying both the solid-solution and solution-solution interfaces. Spectroelectrochemical experiments at the interface of two immiscible liquids can be conducted by utilizing the cell shown in Fig. 4.17, which was designed for the UV/Vis optical region [88]. The IRE liquid should be transparent in the spectral region of interest and its refractive index should be higher than that of the substrate or the second liquid. This is a substantial limitation of the liquid ATR technique. 

One possible point of confusion regarding the notion of "cluster surface capacitance is also worthy of comment here. For electrode surfaces, capacitance charging is usually denoted as a nonfaradaic process to distinguish it from faradaic events that involve necessarily electron transfer across the metal-solution interface. From the cluster solutes, however, all charge build-up must necessarily occur via electron transfer from a source present in, or in contact with, the same solution (such as the gold electrode utilized in the spectroelectrochemical measurements). Of course, charging of plane metal surfaces may also occur by this faradaic mechanism, but is more conveniently achieved by connection to an external electrical source, thereby acting as a polarizable electrode. 

In a typical UV-vis spectroelectrochemical experiment, a UV-vis light beam is directed through an electrode in an electrochemical cell, and the changes in absorbance, resulting from the species generated or consumed in the electrode process or being present at electrode-solution interface, are measured (Fig. 1). 

Figure 14.1 Common optical configurations for spectroelectrochemical cells showing path of incident light (thick line) and detected light (thin line) in (a) transmission mode normal to the electrode and (b) transmission mode parallel to the electrode, internal (c) and external (d) reflectance modes. The dashed line represents the electrode solution interface.

Surface-Enhance Raman Scattering (SERS) has been widely used to monitor electrode processes (1). The advantage of SERS over other spectroelectrochemical methods is that it makes possible the in situ vibrational characterization of the electrode/aqueous solution interface. Being so, SERS is used in the study of electrochemical solid-liquid interfaces to infer the orientation of adsorbed molecules and to characterize electrochemical products. In this work, the adsorption of pyrazine and some related species on silver electrodes was investigated. 

In situ spectroelectrochemical techniques may be regarded as a type of methodology in which spectroscopic information about the electrode, the electrode/electrolyte interface, and/or the electrolyte solution is sought under conditions in which the potential across the electrode/electrolyte interface is controlled during the data acquisition. There are some instances, however, in which because of intrinsic physical limitations, experiments cannot be conducted in a conventional in situ fashion. Two techniques that appear to fall in such a category, referred to hereafter as quasi in situ, will be presented in the following sections. 

---