Formation mechanism of uniform CdS particles from condensed Cd(OH)(2) suspension

The formation mechanism of uniform cadmium sulfide particles obtained in large quantities from condensed Cd(OH)(2) suspension in the presence of thioacetamide (TAA), gelatin and NH3-NH4+ buffer system was investigated. Basically, the reaction proceeded through rapid dissolution of Cd(OH)(2) particles induced by the reaction of Cd2+ ions with S2- ions liberated from TAA in the solution phase. It was finished in ca. 1 min under the standard conditions, 0.5 mol dm(-3) in Cd(OH)(2), 0.55 mol dm(-3) in TAA, 1 wt% in deionized gelatin, and 1.0 mol dm(-3) NH4NO3 at 20 degrees C and at pH 8.50 adjusted with NH3. The resulting uniform and nearly spherical particles of CdS prepared under the standard conditions were 40 nm in mean diameter and were polycrystalline particles consisting of randomly oriented much smaller subcrystals (8.60 nm). Besides using the NH3-NH4+ system in generating uniform CdS particles, employing borax/NaOH buffer system in the formation of spherical CdS particles was also possible. It was also found that S2- ions were furnished through a reaction, CH3CSNH2 --> CH3CN + 2H(+) + S2-. This reaction was excessively accelerated by the rapid consumption of S2- ions so that it is thought to be a reversible reaction. The reaction was virtually stoichiometric and the contribution of hydrolysis of TAA was not observed. The uniform growth of the CdS particles was achieved by the controlled dissolution of the Cd(OH)(2) without aggregation of the growing CdS particles after an instantaneous nucleation. (C) 1995 Academic Press, Inc.