Langmuir, 13(4) [708ｰ713]（1997)
Langmuir, 13(4) [708ｰ713]（1997)
Electron Spin Resonance Study of Radicals
Produced by Photoirradiation on Quantized and Bulk ZnS Particles.
NAKAOKA,Y. and NOSAKA, Y.
(Abstract)
Quantized ZnS particles were prepared and isolated from aqueous solution with a capping agent, thioglycerol. The absorption peak of the particles located at 259−260 nm and the diameter was estimated to be 1.5−1.6 nm from X-ray diffractograms. Their excitation energy agrees well with that calculated with the finite depth potential well model. ESR spectra were measured for the quantized and bulk ZnS particles at 77K under photoirradiation. For the quantized ZnS, only surface hole radicals were observed, while several kinds of radicals consisting of trapped electron, inner and surface holes were observed for the bulk ZnS. A high activity of photocatalytic reactions for quantized semiconductor particles may arise from a large number of radicals photoproduced at the surface.
(Conclusion)
We observed the first step of ZnS photocatalysis and found that the valence band holes were trapped both at the surface and inside of the particles for the bulk materials. On the other hand, for quantized small particles, only surface radicals of holes were observed. The observation is consistent with that most Zn−S bonds of Q-ZnS locate at or near the surface. The holes trapped at the inner part of the particle have no ability to oxidize the surface molecules, and become probably inner trap sites to take a role of recombination center. Therefore, the high activity of Q-ZnS is responsible for the radicals formed at the surface, which participate in a particular photocatalytic reaction.
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