Spectroelectrochemistry electrodes

Because silver, gold and copper electrodes are easily activated for SERS by roughening by use of reduction-oxidation cycles, SERS has been widely applied in electrochemistry to monitor the adsorption, orientation, and reactions of molecules at those electrodes in-situ. Special cells for SERS spectroelectrochemistry have been manufactured from chemically resistant materials and with a working electrode accessible to the laser radiation. The versatility of such a cell has been demonstrated in electrochemical reactions of corrosive, moisture-sensitive materials such as oxyhalide electrolytes [4.299]. 

Infrared spectroelectrochemical methods, particularly those based on Fourier transform infrared (FTIR) spectroscopy can provide structural information that UV-visible absorbance techniques do not. FTIR spectroelectrochemistry has thus been fruitful in the characterization of reactions occurring on electrode surfaces. The technique requires very thin cells to overcome solvent absorption problems. 

W Heineman, F. Hawkridge and H. Blount, Spectroelectrochemistry at Optically Transparent Electrodes in A.J. Bard, Ed., Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1986. 

Jeanmaire, D. L. and Van Duyne, R. P. (1977) Surface Raman spectroelectrochemistry Part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode. J. Electroanal. Chem., 84, 1—20. 

Figure 12.1 Schematic of the spectroelectrochemistry apparatus at the University of Dlinois. The thin-layer spectroelectrochemical cell (TLE cell) has a 25 p.m thick spacer between the electrode and window to control the electrolyte layer thickness and allow for reproducible refilbng of the gap. The broadband infrared (BBIR) and narrowband visible (NBVIS) pulses used for BB-SFG spectroscopy are generated by a femtosecond laser (see Fig. 12.3). Voltammetric and spectrometric data are acquired simultaneously.

Dederichs E, Eriedrich KA, Daum W. 2000. Sum-frequency vibrational spectroscopy of CO adsorption on Pt(lll) and Pt(llO) electrode surfaces in perchloric acid solution Effects of thin-layer electrol3des in spectroelectrochemistry. J Phys Chem B 104 6626-6632. 

Semiconductors. In Sections 2.4.1, 4.5 and 5.10.4 basic physical and electrochemical properties of semiconductors are discussed so that the present paragraph only deals with practically important electrode materials. The most common semiconductors are Si, Ge, CdS, and GaAs. They can be doped to p- or n-state, and used as electrodes for various electrochemical and photoelectrochemical studies. Germanium has also found application as an infrared transparent electrode for the in situ infrared spectroelectrochemistry, where it is used either pure or coated with thin transparent films of Au or C (Section 5.5.6). The common disadvantage of Ge and other semiconductors mentioned is their relatively high chemical reactivity, which causes the practical electrodes to be almost always covered with an oxide (hydrated oxide) film. 

The other popular approach to in situ spectroelectrochemistry is based on the use of an OTE electrode in a thin-layer, optically transparent thin layer electrode (OTTLE), cell. A schematic representation of one design of OTTLE cell is shown in Figure 2.105. 

We can deduce the meaning of the word spectroelectrochemistry by dissecting it piece by piece. Spectroelectrochemistry follows an electrochemical process by the use of electromagnetic radiation (hence spectra- ). In principle, any form of spectroscopy can be used to follow the progress of an electrode reaction, but in practice we tend to concentrate on two, namely UV—visible ( UV—vis ) spectroscopy and a form of microwave spectroscopy known as electron paramagnetic resonance (EPR), as described below. 

Experimentally, ex situ spectroelectrochemistry is very easy to carry out. We generate a chromophore at an electrode, and then transfer it to a standard UV-vis spectroscopy cell and measure the absorbance. (We may need a modified spectroscopy cell if the product is air-sensitive or otherwise too reactive.)... 

Figure 8.2 Schematic representation of a cell used for in situ spectroelectrochemistry. Notice how the counter electrode (CE) has two prongs, one either side of the optically transparent working electrode (WE). Neither the reference electrode (RE), nor the CE can be allowed to interrupt the path of the light beam (as indicated by the circle on the front view).

Figure 8.3 Illustration of in situ spectroelectrochemistry, showing a set of UV-vis ( electronic ) spectra of solid-state Prussian Blue (iron(ii,iii) hexacyanoferrate(ii)) adhered to an ITO-coated optically transparent electrode. The spectra are shown as a function of applied potential (i) —0.2 (ii) -1-0.5 (iii) -1-0.8 (iv) -1-0.85 (v) -1-0.9 (vi) +1.2 V (all vs. SCE). From Mortimer, R. J. and Rosseinsky, D. R., J. Chem. Soc., Dalton Trans., 2059-2061 (1984). Reproduced by permission of The Royal Society of Chemistry.

Figure 8.5 Schematic representation of a typical EPR cell - in this case, the Compton-Waller flow cell - used for in situ spectroelectrochemistry. The working electrode is placed outside of the cavity of the EPR spectrometer, with the counter elecUode being normally an SCE or AgCl,Ag . The working electrode is a flat polished plate of platinum, positioned parallel to the direction of the electric field. Reproduced from Compton, R. G. and Waller, A. M., Comprehensive Chemical Kinetics, Vol. 29, p. 173, Copyright (1989), with permission from Elsevier Science.

Having defined in situ and ex situ methodology, we have seen that in situ spectroelectrochemistry (simultaneous electrochemistry and spectroscopy) is a powerful technique for studying electrode processes. 

Optically transparent electrodes. In situ spectroelectrochemistry was discussed in the previous chapter. The most common materials for constructing optically transparent electrodes for use in such analyses are thin films of semiconducting oxide deposited on to glass. Such materials are readily available commercially. 

The electrochemical approach discussed here relies on a number of special properties of indium tin-oxide (ITO) electrodes, which had been used in particular for spectroelectrochemistry since ITO is optically transparent and can be fabricated on glass [28, 29]. The first important attribute of ITO is the ability to access potentials up to about 1.4 V (all potentials versus SSCE) in neutral solution [29]. Second, ITO electrodes do not adsorb DNA appreciably [30], which could be anticipated from the ability of metal oxides to adsorb cationic proteins [31] polyanionic nucleic acids were therefore not expected to adsorb. This property makes ITO quite different from carbon, which allows access to relatively high potentials but strongly adsorbs DNA [32]. Third, the direct oxidation of guanine at ITO is extremely slow, even... 

Spectroelectrochemistry [3] is the field in which electrochemistry is combined with spectroscopy. Spectroelectrochemical techniques are useful in studying the electrochemical phenomena that occur both in solutions and at electrode surfaces. Here, only the phenomena in solutions are considered. 

Figure 9.10 Assembly of sandwich-type optically transparent electrochemical cell for extended x-ray absorbance fine structure (EXAFS) spectroelectrochemistry. Cell body is of MACOR working electrode is reticulated vitreous carbon (RVC). [From Ref. 64, with permission.]...

The special properties of OTEs that permit the use of transmission spectro-electrochemical techniques are often at cross purposes with the acquisition of reliable electrochemical data. The desire to have the superior electrical properties of bulk conducting materials, and thereby reliable electrochemical data, together with the power of a coupled optical probe led groups to develop various diffraction and reflection approaches to spectroelectrochemistry. Light diffracted by a laser beam passing parallel to a planar bulk electrode can be used to significantly increase the effective path length and sensitivity in spectroelectrochemistry [66]. 

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