Reaction Routes at Small and Big Particles

Another interesting aspect of particle size effect is related to the density of photons absorbed by semiconductor particles, in comparison with the density of particles in a solution. Considering two solutions containing colloids of different sizes, in one case for instance 3-nm and in the other 4-(im particles, many more particles are present in the solution of 3-nm colloid than in that of 4-pm colloid, provided that the total concentration of the semiconductor material is identical in both solutions. The two solutions differ only insofar as the same material is distributed over a small density of large particles (4 pm) or over a high density of small particles (3 nm). As can be easily calculated, a time interval of 5.4 ms exists between the absorption events of two photons in one individual 3-nm particle for a photon flux of 4 x 10 cm s , assuming that all photons are absorbed in the colloidal solution [62]. In the case of the 4-pm particles, the time interval 

This problem has been analyzed by studying the oxidation of ethanol at ZnS [62]. This semiconductor was selected because the oxidation of alcohol to acetoaldehyde occurs entirely by hole transfer in two subsequent steps upon illumination. In the first step, a radical is formed by hole transfer at the surface of an individual particle after absorption of one photon (Eq. 9.26). Since it takes in the average several milliseconds before another hole is generated by photon absorption in the same individual 3-nm particle, the radical diffuses into the solution before another hole is created in the same particle. In the solution, the radicals formed at different particles can disproportionate (Fig. 9.25a) and subsequently dimerize. The whole sequence of possible reactions is given by 

The disproportionation leads to aldehyde and ethanol (Eq. 9.27a) whereas dimerization leads to butanediol (Eq. 9.27b and Fig. 9.25a). These products have indeed been found (concentration ratio of acetaldehyde to butanediol, 2.5) during illumination of a solution of 3-nm colloid [62]. Besides butanediol and acetaldehyde H2 was also detected, the latter being formed in the corresponding cathodic reaction. When the same experiment was performed with much larger particles (4 pm), no butanediol was found [62]. Since the time interval between the absorption events of two photons in one 4-pm particle is only 20 s, the radical can be oxidized to acetaldehyde by a further hole transfer at the same particle as illustrated in Fig. 9.25b [5]. Instead of Eq. (9.27) we have then 

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