Thin-layer electrochemical metal electrodes

Figure 9.9 Assembly of sandwich-type optically transparent thin-layer electrochemical cell, a, Glass or quartz plates b, adhesive Teflon tape spacers c, minigrid working electrode d, metal thin-film working electrode, which may be used in place of (c) e, platinum wire auxiliary electrode f, silver-silver chloride reference electrode g, sample solution h, sample cup. [Adapted with permission from T.P. DeAngelis and W.R. Heineman, J. Chem. Educ. 53 594 (1976), Copyright 1976 American Chemical Society.]...

The electrode can be set directly over the reflection element. For the study of electrochemical reactions under current flow, this arrangement has the advantage that the electrode is in direct contact with the whole solution, i.e., no thin-layer configuration is used, thus avoiding its inherent problems. However, the preparation of thin layers of metals on the surface of reflection elements such as Ge or ZnSe may present some problems. The metal deposits must be thin enough (c. 10 nm) to allow the beam to reach the metal/solution interface. On the other hand, it is difficult to avoid island formation in such thin metal deposits and when this happens the film can have very poor conductivity. 

In situ FTIR " also had to overcome serious difficulties in its application to electrochemical problems. Unlike ellipsometry, where the wavelengths used are in a region of low solvent absorbance, IR is strongly absorbed by most familiar organic solvents and most particularly by water. This leads inevitably either to thin-layer cells or the development of internal reflection techniques. The former has the advantage of simplicity in interpreting spectral data, but it severely limits the type of electrochemistry that can be carried out. The latter requires not only a suitable high-refractive index substrate, such as Ge or Si, but also an adherent very thin layer of metal as the electrode. Technically this is difficult to fabricate so that the metal layer is continuous, and a substantial lateral resistance is inevitable. 

From the chemists point of view, PANI can be treated as a macromolecular polymer, lb elucidate its chemical structure, numerous spectroscopies have been employed. Vibrational spectroscopies have turned out to be most helpful. Infrared spectroscopy was initially employed ex situ (i.e., with dried samples outside the electrochemical cell) only later were adequate experimental designs developed for in situ studies with suitable thin layer electrochemical cells containing polymer-coated metal electrodes for external reflectance measurements. In many cases, IR spectroscopy is just used as a routine tool only those reports based on more intense applications are considered here [316-340]. A typical set of IR spectra of PANI exposed to an aqueous acidic electrolyte solution recorded in situ in the external reflection mode is shown in Figure 25. Interpretation of the spectra and assignment of the observed bands has been done by starting from two completely different points. In the classical approach, the polymer is treated as a collection of monomer units that show molecular connections. 

Before constructing an electrode for microwave electrochemical studies, the question of microwave penetration in relation to the geometry of the sample has to be evaluated carefully. Typically only moderately doped semiconductors can be well investigated by microwave electrochemical techniques. On the other hand, if the microwaves are interacting with thin layers of materials or liquids also highly doped or even metallic films can be used, provided an appropriate geometry is selected to allow interaction of the microwaves with a thin oxide-, Helmholtz-, or space-charge layer of the materials. 

While considering trends in further investigations, one has to pay special attention to the effect of electroreflection. So far, this effect has been used to obtain information on the structure of the near-the-surface region of a semiconductor, but the electroreflection method makes it possible, in principle, to study electrode reactions, adsorption, and the properties of thin surface layers. Let us note in this respect an important role of objects with semiconducting properties for electrochemistry and photoelectrochemistry as a whole. Here we mean oxide and other films, polylayers of adsorbed organic substances, and other materials on the surface of metallic electrodes. Anomalies in the electrochemical behavior of such systems are frequently explained by their semiconductor nature. Yet, there is a barrier between electrochemistry and photoelectrochemistry of crystalline semiconductors with electronic conductivity, on the one hand, and electrochemistry of oxide films, which usually are amorphous and have appreciable ionic conductivity, on the other hand. To overcome this barrier is the task of further investigations. 

In a typical spectroelectrochemical measurement, an optically transparent electrode (OTE) is used and the UV/vis absorption spectrum (or absorbance) of the substance participating in the reaction is measured. Various types of OTE exist, for example (i) a plate (glass, quartz or plastic) coated either with an optically transparent vapor-deposited metal (Pt or Au) film or with an optically transparent conductive tin oxide film (Fig. 5.26), and (ii) a fine micromesh (40-800 wires/cm) of electrically conductive material (Pt or Au). The electrochemical cell may be either a thin-layer cell with a solution-layer thickness of less than 0.2 mm (Fig. 9.2(a)) or a cell with a solution layer of conventional thickness ( 1 cm, Fig. 9.2(b)). The advantage of the thin-layer cell is that the electrolysis is complete within a short time ( 30 s). On the other hand, the cell with conventional solution thickness has the advantage that mass transport in the solution near the electrode surface can be treated mathematically by the theory of semi-infinite linear diffusion. 

The portable instrumentation and low power demands of stripping analysis satisfy many of the requirements for on-site and in situ measurements of trace metals. Stripping-based automated flow analyzers were developed for continuous on-line monitoring of trace metals since the mid-1970s [16,17]. These flow systems involve an electrochemical flow detector based on a wall-jet or thin-layer configuration along with a mercury-coated working electrode, and downstream reference and counter electrodes. 

Optically transparent electrode — (OTE), the electrode that is transparent to UV-visible light. Such an electrode is very useful to couple electrochemical and spectroscopic characterization of systems (- spectroelectro-chemistry). Usually the electrodes feature thin films of metals (Au, Pt) or semiconductors (In203, SnCb) deposited on transparent substrate (glass, quartz, plastic). Alternatively, they are in a form of fine wire mesh minigrids. OTE are usually used to obtain dependencies of spectra (or absorbance at given wavelengths) on applied potentials. When the -> diffusion layer is limited to a thin layer (i.e., by placing another, properly spaced, transparent substrate parallel to the OTE), bulk electrolysis can be completed in a few seconds and, for -> reversible or - quasireversible systems, equilibrium is reached for the whole solution with the electrode potential. Such OTEs are called optically transparent thin-layer electrodes or OTTLE s. 

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