Formation mechanism of monodisperse CdS particles from concentration solutions of Cd-EDTA complexes

The formation mechanism of monodisperse spherical CdS particles in a condensed homogeneous solution of Cd-EDTA complexes in the presence of thioacetamide (TAA), ammonium acetate, ammonia, and gelatin was studied. The monodisperse particles were formed through a consecutive process with an almost instantaneous nucleation and succeeding gradual growth without renucleation. The Cd-eDTA and TAA worked only as reservoirs of Cd²⁺ and S^2- 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 lowering the supersaturation below the critical level during their growth. The important role of CH₃COONH₄-NH₃ as a powerful pH buffer for maintaining 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 to 0.51 μm.