Electrodes external reflectance cells

Figure 2.40 Schematic representation of the external reflectance cell design commonly employed in in situ IR experiments, if the working electrode is a semiconductor, then the semiconductor/ electrolyte interface can be studied under illumination with, for example, UV light by directing the beam perpendicular to the IR beam, as shown.

Relative to the transmission cells described in Section 1.3, external reflectance cells are less simple to construct and require additional optics for incorporation into the optical path of the spectrometer. The advantages associated with the approach relate to the well-defined nature of the working electrode, the wider range of materials suitable for this purpose, and the greater control over the thickness of the thin layer of solution. These factors contribute to a much faster (>10x) rate of electrosynthesis for external reflectance compared to transmission cells. 

External reflectance. The most commonly applied in situ IR techniques involve the external reflectance approach. These methods seek to minimise the strong solvent absorption by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The result is a thin layer of electrolyte trapped between electrode and window usually 1 to 50 pm. A typical thin layer cell is shown in Figure 2.40. 

The first in situ Infrared Reflectance Spectroscopy rmder electrochemical control of the working electrode in a three-electrode cell was realized by Beden et al. using the so-called Electrochemical-ly Modulated Infrared Reflectance Spectroscopy (EMIRS). Experimental details of this external reflection technique are fully described in text books. °... 

Figure 1.3 Working electrode designs for external reflectance SEC cells, (a) Single element, (b) temperature controlled, " and (c) multielectrode assembly. ...

With temperature control and anaerobic considerations in mind, we designed a variable-temperature thin-layer specular (external) reflectance spectroelectro-chemical cell (Figure 5.1). The cell design is such that the incident light from the spectrometer reflects off the working electrode, ensuring that the species detected... 

Besides cell designs that employ metals or other electrode materials in disc shape for external reflection spectroscopy, Robinson and McCreery have successfully employed cylindrical carbon fibers of 12 pm diameter [58, 59]. The carbon fiber was illuminated with a tunable dye laser. Scattered light was collected with fiber optics and guided to a photomultiplier detector. Because no thin layer arrangement and consequently poor electrochemical cell response were involved, fast experiments on a microsecond time scale were possible. Studies of polyaniline films deposited on platinum discs have been described [60]. 

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... 

Fig. 5.47. Optical path of IR light in an external reflection attachment equipped with an electrochemical cell a polarizer b, e,f plane mirrors c concave mirror d cell with working electrode...

The above geometry (termed external reflection) is the only one usable in practice for bulk metal electrodes (see Fig. 6.2 a) [7j. It has also been used for semiconducting electrodes [19]. Because of the non-negUgible absorbance of electrolytes in the IR, it requires the use of thin-layer cells (1-10 pm), which may be impractical in the presence of Faradaic processes (high series resistance of the electrolyte, non-uniform accessibility of the electrode) or if dynamic information is of interest (large response time of the cell). 

FIGURE 7.2. Experimental arrangement of an external reflectance electrochemical cell. PTFE=polytetrafluoroethene SCE=saturated calomel electrode IR=infrared PVDF=poly-vinylidenedifluoride. 

Reflection techniques have been reviewed [23, 157]. Many of these cells are based on the established FTIR reflection cell design using a cylindrical cell and fiber-optic or external reflection accessory to deliver the light to the electrode surface. For example, the Salbeck cell has proved quite popular, as it allows operation under both thin-layer (15 pm, 1 s electrolysis time) and semi-infinite... 

FTIR gives important molecular information on the species formed on the electrode surface [186,233,271,274,288,290,305,328,332,334,349-361], IR radiation is strongly absorbed by most organic solvents and particularly by water, which distinguishes it from the UVATS radiation used in these spectroscopies and also in ellipsometry. This leads inevitably to the use of a thin-layer cell in transmittance mode however, the severe attenuation of the IR beam still remains a serious problem. Therefore, in the majority of cases internal or external reflection techniques have been applied. 

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