Reactions in Suspensions and Colloidal Solutions

When electron-hole pairs are produced by light excitation in a small particle d d, electrons and holes can easily be transferred to an electron and a hole acceptor, respectively, provided that the energetic requirements are fulfilled. The quantum efficiency of the reaction depends on the transfer rate at the interface, on the recombination rate within the particle and on the transit time. The latter can be obtained by solving Fick s diffusion law. The average transit time within a particle of a radius R has then been obtained as [37] 

Taking typical values of D 0.1 cm and / = 10 nm, the average transit time is only about 1 ps. This time is much shorter than the recombination time so that electrons and holes can easily reach the surface. 

Charge transfer reactions have mainly been studied with regard to colloidal particles of Ti02, and to some extent with regard to metal sulfides such as CdS. Illumination of CdS colloids in aqueous solutions saturated with oxygen has led to a relatively fast anodic corrosion according to the reaction [38] 

These examples show clearly how reaction routes vary upon changes in the composition of the solution and that it is rather difficult to get information on the rate-limiting step. 

CdS particles or suspensions were the subject of many investigations for a period of time (1980-1985) because Gratzel etal. concluded from their experiments that photoelectrolysis of H2O occurred at CdS particles [42]. The relevant experiments were performed with particles loaded with two kinds of catalysts, namely 

As discussed in Chapter 7, fundamental aspects of charge transfer reactions at extended electrodes have mainly been studied by using simple one-step redox systems 

The role of O2 is here twofold first it acts as an electron acceptor, secondly it is involved in the anodic corrosion reaction leading to the formation of SO4. This is in agreement with the results obtained with CdS electrodes as discussed in Section 8.1.4. Interestingly, the corrosion rate was considerably reduced after treatment of the particles in a solution of Cd ions, which led to a blocking of S or HS radical sites at the surface, as already described in Section 9.1.1 [39]. Obviously, the formation of S radicals by holes is hindered. 

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