Formation of colloidal silver nanoparticles: Capping action of citrate

Colloidal silver sols of long-time stability are formed in the gamma-irradi ation of 1.0 x 10(-4) M AgClO4 solutions, which also contain 0.3 M 2-propan ol, 2.5 x 10(-2) M N2O, and sodium citrate in various concentrations. The r eduction of Ag+ in these solutions is brought about by the I-hydroxyalkyl r adical generated in the radiolysis of 2-propanol; citrate does not act as a reductant but solely as a stabilizer of the colloidal particles formed. It s concentration is varied in the range from 5.0 x 10(-5) to 1.5 x 10(-3) M, and the size and size distribution of the silver particles are studied by electron microscopy. At low citrate concentration, partly agglomerated larg e particles are formed that have many imperfections. In an intermediate ran ge (a few 10(-4) M), well-separated particles with a rather narrow size dis tribution and little imperfections are formed, the size slightly decreasing with increasing citrate concentration. At high citrate concentrations, lar ge lumps of coalesced silver particles are present, due to destabilization by the high ionic strength of the solution. These findings are explained by two growth mechanisms: condensation of small silver clusters (type-I growt h), and reduction of Ag+ on silver particles via radical-to-particle electr on transfer (type-II growth). The particles formed in the intermediate rang e of citrate concentration were studied by high-resolution electron microsc opy and computer simulations. They constitute icosahedra and cuboctahedra.