Optically transparent electrode types

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. 

We have studied the photoelectrochemical behavior of Chi a and Chi b on an n-type Sn02 (60) optically transparent electrode (OTE thickness of the Sn02 layer on the glass substrate, ca. 2000 A donor density, 1020-21cm-3). Chi monolayer assemblies, deposited by means of the Langmuir-Blodgett technique (20,21), were employed. The use of such monolayer assemblies as interfacial dye layers... 

AG93) AQ80, AR36, see also Optically transparent electrodes p-type (p-type SnOTE), AR36... 

Optically transparent electrodes (OTE) have been used for many kinds of spectro-electrochemical investigations several aspects of this type of research have been covered in reviews [87]. 

The obvious prerequisite is an optically transparent electrode (OTE). Several types of OTEs have been reported (6-13). They may be thin films of a semiconductor (e.g., Sn02 or... 

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. 

There are two types of OTEs [384], A metal microgrid with small (10-30-[xm) holes, which allows >50% of the radiation to be transmitted, or an interdigitized array metal electrode [385] may be used for identification of products or intermediates in redox systems [386, 387], The other type is comprised of a metal film deposited on a transparent support. The thickness of this film is a compromise between its electrical conductivity and optical transparency. This type of OTE can be used for surface analysis, particularly when the metal of the working electrode can provide the surface enhancement (Section 3.9.4). 

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 ... 

Fujihira et al. fabricated the accep-tor/sensitizer/donor (A/S/D) type LB multilayer system as a molecular photodiode based on Kuhn s idea of the light-driven electron pump [32] on a gold optically transparent electrode (AuOTE) [33]. The structure and function of the molecular photodiode of A/S/D are shown in Fig. 18(a), where hydrophilic parts and hydrophobic units are indicated by circles and squares, respectively. Three functional compounds tend to orient regularly in the heterogeneous LB films due to their am-phiphihc properties. 

In many spectroelectrochemical studies, optically transparent electrodes, which are transparent to radiation in a particular spectral region, have been widely used. One type of transparent electrode consists of a very thin film of conductive material such as platinum, gold, tin oxide, indium oxide, or carbon, which is deposited on a transparent substrate such as glass (visible), quartz (UV-visible), or germanium (IR). A second type of transparent electrode is the minigrid electrode. 

An optically transparent thin-layer electrode (OTTLE) study18 revealed that the visible spectra of the reduced forms of [Ru(bipy)3]2+ derivatives can be separated into two classes. Type A complexes, such as [Ru(bipy)3]2+, [Ru(L7)3]2+, and [Ru(L )3]2+ show spectra on reduction which contain low-intensity (e< 2,500 dm3 mol-1 cm-1) bands these spectra are similar to those of the reduced free ligand and are clearly associated with ligand radical anions. In contrast, type B complexes such as [Ru(L8)3]2+ and [Ru(L9)3]2+ on reduction exhibit spectra containing broad bands of greater intensity (1,000 [Pg.584]

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.]...

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.]...

Photoexcitation of n-type semiconductors renders the surface highly activated toward electron transfer reactions. Capture of the photogenerated oxidizing equivalent (hole) by an adsorbed oxidizable organic molecule initiates a redox sequence which ultimately produces unique oxidation products. Furthermore, specific one electron routes can be observed on such irradiated surfaces. The irradiated semiconductor employed as a single crystalline electrode, as an amorphous powder, or as an optically transparent colloid, thus acts as both a reaction template and as a directed electron acceptor. Recent examples from our laboratory will be presented to illustrate the control of oxidative cleavage reactions which can be achieved with these heterogeneous photocatalysts. 

Metal and semiconductor materials (borides, carbides, nitrides, and silicides). Tin oxide-coated glass has been used as an electrode material in electrochemical spectroscopy. By doping of the tin oxide with antimony, an n-type semiconductor is formed. The surface is chemically inert and is transparent in the visible region of the spectrum. However, it is more useful for its optical transparency than as an electrode material. 

In the case of pure electrical measurements for substrates and contact materials Au, Ag, or Pt are preferred due to the p-type behaviour of as-prepared oligothiophenes. If simple band models are assumed for otnT and the contacts, materials like the noble metals with a workfunction of 5.3 eV (Au) or 5.6 eV (Pt) should lead to ohmic contacts whereas materials with low workfunction such as Al (4.28 eV) or Mg (3.66 eV) should form Schottky barriers. (For n-type behaviour, i.e. after n-doping or annealing in air, compare Section 4.2.2, the opposite is true.) Both types of contacts are necessary for electro-optical measurements. Here also one electrode has to be optically transparent. The most common material for the latter purpose is indium-oxide doped tin-oxide (ITO). This material is highly transparent and highly conductive but has the problem that the substrate always exhibits several spikes standing out of the surface. The other type of semi-transparent electrodes are ultra-thin metal films evaporated onto the organic film. 

So far, five types of mercury electrode have been used in specular reflection measurements (i) hanging mercury drop electrode (HMDE) [38, 39], (ii) mercury film deposited on a platinum or gold substrate [40-42], (iii) mercury pool electrode with or without an amalgamated platinum ring guide [43, 44], (iv) mercury drop bottom electrode placed on the optical window [45—47], and (v) mercury drop bottom electrode placed on an underlying ion-conductive optically transparent polymer film [48]. To avoid the difficulty due to mechanical vibration and shape change, the mercury drop bottom electrode would be useful. 

More recent results on CNTs were much more promising [227,228]. Two types of CNT thin films were tested functionalized P3-single walled nanotubes (SWNTs) and raw (non-functionalized) SWNTs. Both films provided excellent actuation characteristics (on par with carbon grease), had a negligible influence on mechanical properties of the film, and remained stable over longer periods of time. In addition, the carbon nanotube films could be made thin enough to remain optically transparent for use as transparent compliant electrodes [229]. 

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