PREPARATION OF METAL SULFIDE COMPOSITE ULTRAFINE PARTICLES IN REVERSE MICELLAR SYSTEMS AND THEIR PHOTOCATALYTIC PROPERTY

CdS, ZnS and their composite ultrafine particles were prepared using nanometer-sized waterpools in AOT/isooctane reverse micellar solutions as reaction media. The size of the ultrafine particles was controlled by changing the water content W-0 (= [H2O]/[AOT]) of the reverse micellar solution and this was almost independent of the feed concentrations of Cd2+, Zn2+ and S2-. Stable ultrafine particles, which do not cause excess aggregation and the band gap of which continues to be larger than that of the bulk semiconductor for a long time after formation, could be prepared at a value of W(0)less than 8. Mixing a micellar solution containing both Cd2+ and Zn2+ with a solution of S2- gave coprecipitated semiconductor ultrafine particles. Gradual precipitation of ZnS in a micellar solution which had contained CdS particles gave ZnS-coated CdS ultrafine particles. The use of an excess of S2- for precipitation was effective in making the composition of the resulting particles close to that of the initial solution, and also avoided the formation of mixed crystals. The composite ultrafine particles thus prepared could be directly applied to the photocatalytic reduction of water and were found to be improved in their photocatalytic activity compared to CdS particles. The photocatalytic properties of the coprecipitated particles varied according to the particle band gap.