Metal minigrid

Another approach is to employ a metal minigrid or a thin metal or semiconductor film for the working electrode (Chap. 11). These electrodes permit transmittance of the optical beam through the electrode and are readily adapted for use in thin-layer cells. Fast heterogeneous or homogeneous electron transfer reactions, or homogeneous chemical reactions initiated by the electrode... 

WE is a metallic minigrid, RE is a Ag/AgCl electrode, and CE, separated by the solution by means of a porous flit, is made of a Pt spiral wire or grid these requirements are necessary to increase the CE surface and, thereby, to increase the current flowing between WE and CE and to decrease the electrolysis time. 

The cell is constituted by two metallic holders both of them have a eentral hole through which the light beam passes, and two other small holes that are used to fill the cell with the solution. These two holders sandwieh two quartz windows and then they are screwed together. By means of suitable spacers it is possible to leave a thin cavity between the quartz windows that is filled with the solution. In this thin cavity a polyethylene spacer (ca. 150 pm thick) with melt-sealed eleetrodes is also contained. WE and CE are metallic minigrids and a Ag wire plays the role of quasi reference electrode (QRE), the potential of whieh ean be considered constant for a short time interval. 

The best choice of metal for making a minigrid is gold, because it is so malleable. 

The development of electrodes that exhibit optical transparency has enabled spectral observations to be made directly through the electrode simultaneously with electrochemical perturbations [19-21]. These electrodes typically consist of a very thin film of conductive material such as Pt, Au, carbon, or a semiconductor such as doped tin oxide that is deposited on a glass or quartz substrate. Miniature metal screens, minigrid electrodes in which the presence of very small holes (6-40 fim) lends transparency, have also been used. Optically transparent electrodes (OTE) and the cells that incorporate them are discussed in Chapters 9 and 11. 

Figure 9.7 Three configurations for sandwich-type thin-layer cells. (A) Minigrid suspended between two spacers. (B) Twin-electrode cell using metal films on glass. (C) Single-electrode cell, barrier plate and electrode plate, s, Sample solution.

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.]...

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. 

In addition to the modified electrodes described in the previous sections, which usually involve a conductive substrate and a single film of modifying material, more complicated structures have been described. Typical examples (Figure 14.2.4) include multiple films of different polymers (e.g., bilayer structures), metal films formed on the polymer layer (sandwich structures), multiple conductive substrates under the polymer film (electrode arrays), intermixed films of ionic and electronic conductor (biconductive layers), and polymer layers with porous metal or minigrid supports (solid polymer electrolyte or ion-gate structures) (6,7). These often show different electrochemical properties than the simpler modified electrodes and may be useful in applications such as switches, amplifiers, and sensors. 

A gold or platinum wire mesh or minigrid acting as the working electrode and located in the beam path, an additional metal grid serving as the counter electrode... 

Figure 1. Gold microelectrode array (a) Construction of gold microelectrode array. M, metal wire C, silver paint connection G, gold minigrid S, microscope slides E, epoxy resin.

---