Optical rotating disc electrode ORDE

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. 

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