FORMATION MECHANISM OF MONODISPERSE CDS PARTICLES FROM CONCENTRATED-SOLUTIONS OF CD-EDTA COMPLEXES

The formation mechanism of monodisperse spherical CdS particles in a c ondensed homogeneous solution of Cd-EDTA complexes in the presence of thioacetamide (TAA), ammonium acetate, ammonia, and gelatin was studie d. The monodisperse particles were formed through a consecutive proces s with an almost instantaneous nucleation and succeeding gradual growt h without renucleation, The Cd-EDTA and TAA worked only as reservoirs of Cd2+ and S2- ions, but the rate-determining step of particle growth was found to be the dissociation process of the Cd-EDTA complexes and not of the TAA, Ammonia played a decisive role as a complexing agent at high concentrations of Cd(NH3)(n)(2+), which could promptly release free Cd2+ ions, in promoting nucleation as well as growth even under a low supersaturation, and thus achieved excellent monodispersity by l owering the supersaturation below the critical level during their grow th. The important role of CH3COONH4-NH3 as a powerful pH buffer for ma intaining the activity of both NH3 and TAA was also proved. Moreover, size control of the final product was achieved in a range from 0.038 t o 0.51 mu m. (C) 1996 Academic Press, Inc.