Synthesis of uniform particles of CdS, ZnS, Pbs and CuS from concentrated solutions of the metal chelates

A series of uniform metal sulfide particles including CdS, ZnS, PbS and CuS have been synthesized by aging relatively concentrated solutions of Cd²⁺, Zn²⁺ Pb²⁺ or Cu²⁺ ions in combination with a wide variety of chelating agents, thioacetamide (CH₃CSNH₂), ammonia and gelatin at 25°C. With Cd²⁺, Zn²⁺ and Pb²⁺, the yields of the metal sulfides were lowered with increasing stability constant of the metal chelates. When the stability constant in logK₁ or logK₁K₂ was lower than 10, the reaction was too fast to produce completely uniform particles in the present system. On the other hand, when the stability constant exceeded ca. 18, it was almost impossible to produce the metal sulfide with a sufficiently high yield. Hence, appropriate chelates were chosen from those with stability constants between 10 and 18. A new theory of the growth kinetics was formulated and used for the analysis of the experimental results. It was found that the first-order kinetics of particle growth was the best in achievement of monodispersity. For this purpose, ammonia was useful as an accelerator of the growth of the metal sulfides. The formation of CuS particles was often found to be complete within 2 min at 25°C irrespective of the stability constants of Cu²⁺ chelates, owing to the exceptionally high dissociation rate constants of these chelates. In many systems, CuS was found to be dissolved after the complete precipitation due to the direct oxidization of S^2- ions of the CuS particles by dissolved oxygen, yielding uniform hollow particles of CuS.