THIOL SURFACE COMPLEXATION ON GROWING CDS CLUSTERS

The growth of small CdS colloidal particles has been initiated by pulse radiolytic release of sulfide from a thiol (3-mercapto-1,2-propanediol, RSH) in the presence of Cd2+ ions. The kinetics and stoichiometry of the ensuing reactions were followed by conductivity, absorption spectroscopy, and light-scattering techniques. The final CdS product has been identified by electron diffraction. The formation of Cd-thiolate complexes at the surface of the particles is indicated by conductivity and by energy dispersive analysis of X-ray (EDAX) results. The rate of formation of CdS clusters is strongly pH dependent due to the pH effect on the stability of Cd2+/HS– complexes. At low pHs (4.0–5.3) the growth mechanism is proposed to be primarily a cluster–molecule process. At this pH range Cd2+ ions at the CdS particle surface complex with thiolate ions stronger than in the bulk of the solution. The size control of the particles by thiols is proposed to result from a competition of thiolate ions with HS– ions for cadmium ions at the surface of the growing particles. At neutral and basic pHs growth is primarily a cluster–cluster aggregation process of CdS clusters within the cadmium–thiolate polynuclear complexes. The small particles seem to undergo weak clustering in solution, yet the spectroscopic properties of the samll particles is maintained. The utility of the thiol–controlled growth, both in size control and surface modification, is discussed. © 1990, American Chemical Society. All rights reserved.