The 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 

Let us consider the electrode kinetics associated with charge transfer from an n-type semiconductor particle to an electrode. As indicated by Albery et al. [164], the crucial difference between the electrochemistry of a colloidal particle and an ordinary electrochemically active solution phase species is the number of electrons transferred from the particle to the electrode may be large and will depend upon the potential of the electrode. Fig. 9.5 shows the model for an encounter of a particle with an electrode used by Albery and co-workers. kD is the mass-transfer coefficient for the transport of the particles to the electrode surface. In the simplest case, wherein it is assumed that the lifetime of the transferable electrons (majority carriers of thermal or photonic origin) is greater than the time taken by a particle to traverse the ORDE diffusion layer, this is given by 

Summing the transfer of electrons for each partition with these assumptions in mind, Albery et al. obtained for the total flux of electrons je  

To fully utilise the result expressed by equation (9.70), we must now consider the solution of the convective diffusion equation for a system of /i-type semiconductor particles as they diffuse towards an ORDE. Whilst 

The convective diffusion equation for a particle of concentration cm and which contains m electrons in the conduction band and surface states is given by  

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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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