Optical rotating disc electrode

Unfortunately, the to-electrode precipitation required for conventional (photo)electrochemical measurements on colloidal semiconductors necessarily perturbs the (assumed) spherical diffusion fields and surface adsorption equilibria that obtain at particles in the free solution state, phenomena which are instrumental in determining the dynamic and static charge transfer characteristics of the semiconductor. Consequently, there is a requirement for photoelectrochemical techniques capable of in situ, non-per-turbative investigations of the mechanistic details and catalytic properties of colloidal semiconductors in solution conditions typical of their intended ultimate application. Two such techniques are photoelectrophoresis and the Optical Rotating Disc Electrode (ORDE, developed by Albery et al.). As mentioned above, the former technique has already been reviewed by this author elsewhere [47]. Thus, the remainder of this review will concentrate on measurements that can be made with the latter... 

Whilst this may initially appear to be in opposition to the results of the optical rotating disc electrode study on colloidal CdS (Fig. 9.9), this may be readily explained by consideration of the relatively low illumination intensities used in the ORDE experiments, and the high surface state concentrations typical of the samples employed therein. The former precludes the generation of a Burstein shift while the latter, with a quantum yield of 0.77 for (S )surf generation from S2 ions at the CdS particle surface [115, 116], provides a highly efficient mechanism for positive charge accumulation at the particle surface. 

Transient photoelectrochemical behaviour of colloidal CdS The experiments described in this section are performed by recording light-on transient photocurrents from aqueous dispersions of 2-12 nm radii CdS particles (prepared as above) at a stationary optical rotating disc electrode. However, to be able to interpret the results from these experiments, it was first necessary to model the time-dependent behaviour of the mass transport limited photocurrent at the ORDE. 

In this appendix we review the general theoretical aspects of the semitransparent Optical Rotating Disc Electrode. The electrode is assumed to be uniformly illuminated by parallel light passing through the electrode and into the solution where it generates, from species A, a photoproduct... 

A small, horizontally mounted electrolysis cell of volume 8 cm was fitted with a rotating disc electrode (RDE) or stirrer and an optical-fiber probe inserted to monitor the electrolysis. The first-order decay of electrogenerated [Fe(bpy)3] + was monitored at open circuit [43]. 

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). 

For the investigator who wants to study electrode processes at depth, a number of more physically oriented methods are available, such as double layer capacitance measurements19 rotating disc and ring disc techniques 25 and radio-. active tracer methods 40a Spectroscopical methods in conjunction with optically transparent electrodes can be used for the study of intermediates 40b), as can also total reflectance spectroscopy 40c). 

An armoury of powerful electrochemical methods is available. Potential step techniques such as differential pulse DP or square-wave SW voltammetry offer advantages in sensitivity and resolution. Hydrodynamic techniques involving use of rotating disc or rotating ring-disc electrodes allow the chemical steps of the electrode process to be separated from mass transport. Electrochemical transformations may be monitored optically with spectroelectrochemical methods. Even the electrode interface itself is amenable to study by in situ spectroscopic techniques. Detailed descriptions of these methods are to be found in appropriate texts [1-4]. 

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