Spectroelectrochemistry

Cells for Spectroelectrochemistry. Spectroscopic techniques have been used in conjunction with electrochemistry in a variety of ways, which can be grouped into three areas the direct optical study of the electrode interface, the 

There has been a great deal of theoretical and experimental work on photon-stimulated currents. Most of this centers around the questions of whether electrons can be photoejected from an electrode surface71 and whether photochemical excitation of the reactant or the electrode can reduce the barrier to electron transfer.71-73 In addition, there has been some study of reactive intermediates that are produced near an electrode by flash photolysis.74 

The third area of interest has been the observation by optical and ESR spectroscopy of intermediates that are produced electrochemically. Electron spin resonance is a useful technique for identifying species that have unpaired electrons, and reviews have documented the power of ESR for unraveling complicated reaction pathways.75-77 A number of cells have been described for use with this technique that fall into two categories—the flow cell in which the reactive intermediate is generated externally and flows into the cavity78 and the in situ generation system where electrodes are placed inside the resonant cavity of the spectrometer.79 

The principle of external generation also has been used for optical measurements of transient intermediates. Flow cells84,85 and cells that provide for the circulation of the solution past an electrode and into a spectrophotometric cell in a closed loop have been described.86,87 

Several reviews on the theory and applications of spectroelectrochemical methods have been published.88,89 

ESR spectroscopy directly proves the magnetic nature of charged conjugated molecules. Thus, combining in situ Vis-NIR and ESR spectroelectrochemistry gives a deep insight into the nature of the doped states. Indeed, this rather sophisticated technique was crucial for the assignment of the electronic absorption bands mentioned above [i25, 126]. 

At any given interface between two phases, the properties of both phases close to the interface, in particular those of the topmost 

1 Because of the extremely large number of original reports still appearing in growing numbers, this introduction is far from being a complete overview of the literature. Consequently, the quoted references do not represent an attempt to provide lists of the most recent or the most important publications they are, instead, selected with respect to the relevance for the topic of the current review or because of their value as introduction to a given method. 

Properties of interphases relevant for an understanding of the structures and dynamics present therein can be grouped into atomic (microscopic) and macroscopic. Classical electrochemical methods in most cases have provided only data pertaining to the latter. Nevertheless, the close relationships between both types of properties have allowed conclusions with respect to atomic models to be inferred from macroscopic information. Many spectroscopic methods applied to electrochemical problems in recent years have provided direct information on the atomic level. 

The application of spectroscopic methods to surface studies always involves a probe used to stimulate or perturb the interphase in a well defined way. This causes a signal to be emitted from the interphase. In many cases, the signal is simply the modulated or somehow modified probe. Special care has to be exercised to obtain information exclusively from parts of the interface as close as possible to the actual phase boundary. Many techniques are essentially surface sensitive. In some cases, methods or sample systems have to be modified to achieve this surface sensitivity. 

Obviously some probes and signals can be utilized only under ultra high vacuum conditions. Whereas spectroscopic studies of electro- 

In addition, the determination of metal-ligand bond distances in solution and their oxidation state dependence is critical to the application of electron transfer theories since such changes can contribute significantly to the energy of activation through the so-called inner-sphere reorganizational energy term. 

These authors have also developed a cell98 (Fig. 26) that employs reticulated vitreous carbon as a working electrode and they find that such a design allows for much faster electrolysis. Using such a cell, they have studied the [Ru(NH3)6]3+/2+ couple 

Most recently, they have developed a cell configuration for the study of modified electrodes that employs, as a working electrode, colloidal graphite deposited onto kapton tape (typically employed as a window material). Such an arrangement minimizes attenuation due to the electrolyte solution. 

Antonio et al100 have performed an in situ EXAFS spectro-electrochemical study of heteropolytungstate anions. 

Furthermore, one can monitor changes in oxidation state by the shift in the edge position. For example, Fig. 28 shows that upon 

The electrochemical cell would need quartz windows. A typical three-electrode system, with WE, CE, and reference electrode, is used. Eor IR work, the source and spectrometer would have to be suitable and the windows of the cell would have to be IR transparent, for example, CaFj. Suitable materials have been discussed for both techniques in Chapters 4 and 5. Commercial systems are available from BioLogic Science Instruments, Claix, Erance (www.bio-Iogic.info), and from ZAHNER-Elektrik GmbH Co., KG, Kronach, Germany (www.zahner.de). Their websites contain pictures and detailed instrument descriptions as well as a number of application notes and technical notes. 

Optically transparent electrodes (OTEs) can be used to construct optically transparent cells for use in a conventional UV/VIS or IR spectrometer (Plieth et al. 1998). OTEs are of various types, depending on the application, and can include the following  

The optically transparent electrode (OTE) is used as a single WE or a stack of WEs and is combined with a reference electrode and a CE (also called an anxUiary electrode) in a spectrochemical cell. Variations of the OTE inclnde thin-layer cells and long optical path length thin-layer cells. Perforated and reflective electrodes are also used as WEs. 

Plots derived from SEC experiments can include, amraig others, absorbance as a function of potential at a constant wavelength, absorbance as a function of the wavelength at a constant potential, or absorbance as a function of time after a potential step or scan, which gives a derivative of the signal with respect to time in the shape of a cyclic voltammogram. In general, the tpproach taken is to set a potential, allow the system under study to equilibrate, collect a spectrum, and then repeat at different potentials. 

Actuator for automatic change between reference and measurement cells 

National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland 

Electrochemistry can provide both thermodynamic and kinetic information on a range of chemical processes driven by electron transfer. However, electrochemistry can rarely unequivocally identify electroactive species the molecular identity of a new electrogenerated material is typically inferred from the measured physical properties of a known standard system. In addition, electrochemistry provides only limited and indirect information on structural changes accompanying redox events. 

Conventional spectroelectrochemistry involves bulk electrolysis of an analyte in a low volume cell combined with simultaneous or subsequent in situ spectroscopic investigation. The key point is that investigations are in situ with spectroscopic studies undertaken within an electrochemical cell that is under potential control. Spectroelectrochemical experiments are frequently qualitative and are used to structurally characterize an intermediate redox state. Quantitative measuronents can be experimentally challenging because they require rigorous geometric arrangement of the cell to avoid problems such as iR drop and low current densities as a consequence of the relative size of the working electrode and its orientation with respect to the other electrodes. 

Pharr and Griffiths [20] studied the electrolytic oxidation of ferrocyanide ion on a platinum electrode by modulating the potential of the electrode around the standard 

In summary, we have described in this chapter how the dynamics of three of the types of reversible chemical and physical changes can be investigated using a step-scan interferometer by modulating some property of the sample. It can be expected that several more reports of analogous processes will be reported in the future. 

SAMPLE MODULATION SPECTROMETRY WITH A STEP-SCAN INTERFEROMETER 

Dowrey, and C. Marcott, Characterization of polymers using polarization-modualtion infrared techniques dynamic infrared linear dichroism (DIRLD) spectroscopy, in Fourier-Tran orm Infrared Characterization of Polymers, H. Ishida, Ed., Plenum Press, New York, 1987, p. 33. 

Manning, G. L. Pariente, B. D. Lemer, J. H. Perkins, R. S. Jackson, and P. R. Griffiths, Multiple modulation step-scanning Fourier transform infrared spectroscopy, in Computer Assisted Analytical Spectroscopy, S. D. Brown, Ed., Wiley, New York, 19%, Chap. 1. 

The use of electrochemical techniques coupled with spectrophotometric measurements in the ultraviolet (UV), visible (Vis), and infrared (NIR) region, where the electronic transitions occur, enables to monitor the absorption (see Chap. 3) and/or emission (see Chap. 5) spectmm of oxidized or reduced species [1, 2]. To reach this goal it is necessary to electrolyze all the electroactive species contained 

Dipartimento di Chimica G. Ciamician , Alma Mater Studiorum, Universiti di Bologna, Via Selmi n. 2, 40126 Bologna, Italy e-mail margherita.venturi unibo.it 

Ceroni (ed.). The Exploration of Supratnolecular Systems and Nanostructures by Photochemical Techniques, Lecture Notes in Chemistry 78, 

Such a technique is very useful, for example to study the spectroscopic (absorption and emission) properties of metal complexes in unusual oxidation states, non reachable by means of the traditional synthetic routes [3]. In this regard Fig. 9.1 shows the absorption spectra of the [Ru(bpy)3] complex and of its [Ru(bpy)3], [Ru(bpy)3], and [Ru(bpy)3] reduced species obtained in dimethylformamide at 232 K by electrolysis at suitable potential values [4]. 

In the case of supramolecular species [5, 6], spectroelectrochemical measurements are even more important. They, indeed, enable to establish the more stable isomer among the possible ones, and to evidence the occurrence of electro-chemically induced molecular movements or conformational rearrangements. In this regard, three studies performed on catenanes (supramolecular systems minimally formed by two interlocked rings) are illustrated in the following. 

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In this chapter, transient teclmiques, steady-state teclmiques, electrochemical impedance, photoelectrochemistry and spectroelectrochemistry are discussed. 

Plieth W, Wilson G S and de la Fe C 1998 Spectroelectrochemistry a survey of in situ spectroscopic techniques Pure Appi. Chem. 70 1395... 

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. 

Besides its widespread use for investigating the mechanism of redox processes, spectroelectrochemistry can be usefiil for analytical purposes. In particular, the simultaneous profiling of optical and electrochemical properties can enhance the overall selectivity of different sensing (30) and detection (31) applications. Such coupling of two modes of selectivity is facilitated by the judicious choice of the operating potential and wavelength. 

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. 

Example 2-4 A potential-step spectroelectrochemistry experiment using a reactant concentration of 2 mM generated a product with an absorbance (sampled after 25 s) of 0.8. Calculate the reactant concentration that yielded an absorbance of 0.4 upon sampling at 16 s. 

Exp lam and demonstrate clearly how spectroelectrochemistry can provide useful information about a reaction mechanism involving a redox process followed by a chemical reaction (EC mechanism), involving decomposition of the reaction product. Draw an absorbance-time plot for different rate constants of the decomposition reaction. 

Describe clearly how thin-layer spectroelectrochemistry is used for measuring the values of E° and n. 

Radical ions, 33, 44 Raman spectroelectrochemistry, 45 Randles-Sevcik equation, 31 Rate constant, 12, 18 Rate determining step, 4, 14 Reaction mechanism, 33, 36, 113 Reaction pathway, 4, 33 Reaction rate, 12 Receptor-based sensors, 186 Redox recycling, 135... 

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. 

Radish KM, Shao J, Ou Z, Zhan R, Burdet E, Barhe J-M, Gros CP, Guilard R. 2005. Electrochemistry and spectroelectrochemistry of heterobimetalUc porphyrin-corrole dyads. Influence of the spacer, metal ion, and oxidation state on the pyridine binding ability. Inorg Chem 44, 9023-9038. 

Kobayashi N, Lam H, Nevin WA, Janda P, Leznoff CC, Lever ABP. 1990. Electrochemistry and spectroelectrochemistry of 1,8-naphthalene- and 1,8-anthracene-hnked cofacial binuclear metallophthalocyanines. New mixed-valence metaUophthalocyanines. Inorg Chem 29 3415. 

Z. Ding. Spectroelectrochemistry and Photoelectrochemistry of Charge Transfer at Liquid/ Liquid Interfaces. PhD Thesis, Ecole Polytechnique Federale de Lausanne, Lausanne, 1999. V. J. Cunnane, D. J. Schiffrin, C. Beltran, G. Geblewicz, and T. Solomon. J. Electroanal. Chem. 247 203 (1988). 

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. 

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