Spectroelectrochemistry optically transparent electrodes

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. 

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.

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. 

Figure 17.10 Gas-tight transmission cell for IR spectroelectrochemistry in moderate-melting salts (A) optically transparent electrode (OTE) port, (B) reference electrode and auxiliary electrode ports, (C) Si windows, (D) vacuum valve, (E) light path. [From P. A. Flowers and G. Mamantov, J. Electrochem. Soc. 136 2944 (1989), with permission.]...

Figure 6.22 Cell system for spectroelectrochemistry by use of optically transparent electrodes (OTEs).

Spectroelectrochemistry has become a valued technique coupling spectroscopy and electrochemistry. Spectroelectrochemistry is a bulk electrochemical technique and as such many of the cell requirements discussed above that pertain to BE apply for spectroelectrochemistry. Often concentrations for spectroelectrochemistry are much lower than most electrochemical techniques due to the spectroscopic absorbance requirements. The bulk solution must still be oxi-dized/reduced in spectroelectrochemistry. Large surface area working and auxiliary electrodes are employed as in the bulk methods described above. Cells designed with optically transparent electrodes like thin films of Sn02 or In203 or optically transparent mesh electrodes are employed, otherwise the electrode must be manually removed to record spectra. Optically transparent electrodes can be constructed such that the solution volume to electrode surface area ratio is very small making the BE occm rapidly. 

Figure 28. Configurations for spectroelectrochemistry. A) optically transparent electrode B) optically transparent thin-layer electrode (OTTLE) C) Internal reflection spectroscopy, and D) specular reflectance spectroscopy.

Figure 44. Electrode systems for spectroelectrochemistry (a) optically transparent electrode, (b) electrode for internal reflection spectroscopy, and (c) electrode for specular reflectance spectroscopy.

Spectroelectrochemistry [99] Is a hybrid technique resulting from the association of electrochemistry with spectroscopy via the use of cells with optically transparent electrodes [100-103]. The potential of this technique lies in the possibility of Identifying both the type and the amount of the species generated In an electrochemical step. The Intrinsic characteristics of spectroelectrochemistry require the use of fast measuring systems —spectroscopic image detectors in most cases [104-107]— and the consequent acquisition of the large number of data provided by the detection system In a short time by means of an oscilloscope or, even better, of a computer also allowing the subsequent exhaustive treatment of the raw data. 

These coated glasses can be used as working electrodes [optically transparent electrodes (OTE)] in standard three-electrode arrangements provided that both glass and coating are chemically and electrochemically stable and inert in the used electrolyte solution and the applied range of electrode potentials. The use of a modified infrared spectroscopy transmission cell equipped with quartz windows for UV-Vis spectroelectrochemistry has been described [18]. Platinum layers deposited onto the quartz served as an optically transparent working electrode and an additional platinum layer served as a pseudo-reference electrode. A counter electrode outside the thin layer zone (in one of the tubes used for solution supply) served as a counter... 

Fig. 5.4. Cell arrangement for in situ UV-Vis spectroelectrochemistry with optically transparent electrodes...

A combination of transmission and external reflectance spectroscopy resulting in a cell for bidimensional UV-Vis spectroelectrochemistry has been described [61]. With an optically transparent electrode (OTL), the schematic setup shown in Fig. 5.8 illustrates the different pathways of the light. One beam passes through the electrode and the electrolyte solution in front of it and the second beam passes only through the solution in front of the electrode close to it, guided strictly in parallel to the surface. Thus the former beam carries information pertaining to both the solution and the electrochemical interface (e.g. polymer films or other modifications on the electrode surface), whereas the latter beam carries only information about the solution phase. Proper data treatment enables separation of both parts. Identification of... 

Past efforts allow us to formulate three objectives for the present work. First we would like a technique that is roughly 2 to 4 (or more) orders of magnitude more sensitive than existing spectro-electrochemical methods. If this were achieved, the techniques could be applied to high-sensitivity analysis where one has a complex mixture and one makes use of the selectivity of spectroelectrochemis-try. Second, it would be valuable to lower the usable time scale of spectroelectrochemistry down into the microsecond region for a variety of chemical systems. With an optically transparent electrode and virtually all spectroelectrochemical methods, the response is limited by an effective path length which decreases with the time scale. Therefore, it is very difficult to monitor species on a microsecond time scale simply due to the low sensitivity of the techniques. The third objective is spatial.resolution of the diffusion layer. It would be very informative from both fundamental and practical standpoints to be able to accurately observe concentration vs. distance profiles. 

Optically transparent electrodes are used in spectroelectrochemistry They can be made by evaporating 10-100 nm thick layers of platinum, gold, tin dioxide, silver, copper, mercury and carbon onto glass, or quartz substrates [42] (for more details see Chap. II.7). 

Since the comprehensive reviews published some time ago in Electroanalytical chemistry [5, 6], and a concise version of these in 1976 [7], there have been many recent reviews covering spectroelectrochemistry, using radiation other than in the visible region [8-11]. However, there are far fewer reviews exclusively covering UV-visible spectroelectrochemistry. The most recent and complete review (with 390 references) was published in 1996 [12]. Other surveys include those by Pragst [13], McCreery and coworkers [14] and Plieth and coworkers [15]. There are also the well-known biennial reviews in Analytical Chemistry within the Dynamic Electrochemistry sections [16, 17]. Various book chapters and monographs provide excellent summaries on the techniques/theory and the applications of optically transparent electrodes [18-21]. Although we shall... 

Barbante GJ, Hogan CF, Hughes AB (2009) Solid state spectroelectrochemistry of microparticles of ruthenium diimine complexes immobilized on optically transparent electrodes. J Solid State Electrochem 13 599-608... 

X. OPTICALLY TRANSPARENT ELECTRODES FOR SPECTROELECTROCHEMISTRY 239 XL ADVANCED ELECTROCATALYST SUPPORT MATERIALS 251... 

Spectroelectrochemistry at Optically Transparent Electrodes I. Electrodes Under Semi-infinite Diffusion Conditions, Theodore Kuwana and Nicholas Winograd... 

FIGURE A4 Electrochemical cell used for spectroelectrochemistry. An optically transparent electrode such as ITO is shown inside the cuvette. 

According to their size, in particular to their typical dimensions, electrodes are called macroelectrodes with typical dimension (e.g., diameter of a disc-shaped electrode, length of the edge of a sheet electrode) in the range of mm or cm, microelectrodes (with pm), and nanoelectrodes (with nm). Electrodes for particular methods are called rotating disc electrodes (see entry Controlled How Methods for Electrochemical Measurements ), optically transparent electrodes (OTL, see entry UV-Vis Spectroelectrochemistry ), and thin-layer electrodes (TLE for electrolysis with a limited solution volume present as a thin layer of liquid). 

UV-Vis Spectroelectrochemistry, Fig. 4 Diagram of the ATR spectroelectrochemical cell demonstrating the placement of the optically transparent electrode and the coupling prisms thereon... 

Richardstm JN, Aguilar Z, Kaval N, Andria SE, Shtoyko T, Seliskar CJ, Heineman WR (2003) Optical and electrochemical evaluation of colloidal An nanoparticle-rrO hybrid optically transparent electrodes and their application to attenuated total reflectance spectroelectrochemistry. Electrochim Acta 48 4291-4299... 

Spectroelectrochemistry at Optically Transparent Electrodes I. Electrodes under Semi-Infinite Diffusion Conditions, Theodore Kuwana and Nicholas Winograd Organometallic Electrochemistry, Michael D. Morris Faradaic Rectification Method and Its Applications in the Study of Electrode Processes, H.P. Agarwal... 

Spectroelectrochemistry at Optically Transparent Electrodes, II. Electrodes under Thin-Layer and Semi-Infinite Diffusion Conditions and Indirect Coulometric Iterations, William H. Heineman, Fred M. Hawkridge, and Henry N. Blount Polynomial Approximation Techniques for Differential Equations in Electrochemical Problems, Stanley Pons Chemically Modified Electrodes, Royce W. Murray... 

---