Formation of Colloidal Silver Nanoparticles: Capping Action of Citrate

Colloidal silver sols of long-time stability are formed in the γ-irradiation of 1.0 × 10-4 M AgClO4 solutions, which also contain 0.3 M 2-propanol, 2.5 × 10-2 M N2O, and sodium citrate in various concentrations. The reduction of Ag+ in these solutions is brought about by the 1-hydroxyalkyl radical generated in the radiolysis of 2-propanol; citrate does not act as a reductant but solely as a stabilizer of the colloidal particles formed. Its concentration is varied in the range from 5.0 × 10-5 to 1.5 × 10-3 M, and the size and size distribution of the silver particles are studied by electron microscopy. At low citrate concentration, partly agglomerated large particles are formed that have many imperfections. In an intermediate range (a few 10-4 M), well-separated particles with a rather narrow size distribution and little imperfections are formed, the size slightly decreasing with increasing citrate concentration. At high citrate concentrations, large 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 growth), and reduction of Ag+ on silver particles via radical-to-particle electron transfer (type-II growth). The particles formed in the intermediate range of citrate concentration were studied by high-resolution electron microscopy and computer simulations. They constitute icosahedra and cuboctahedra.