Formation mechanisms and aggregation behavior of borohydride reduced silver particles

In this work, we examine the formation mechanisms of nanoscale silver particles produced by the reduction of silver perchlorate with sodium borohydride. Evidence is presented that the reaction pathway does not follow classical nucleation and growth theory, but is dominated by colloidal interactions. Upon injection of silver into a sodium borohydride solution, a molecular species absorbing at 220 nm is produced in less than 1 s. We suggest that this species contains borohydride and small particles of reduced silver. The reaction mixture is initially dark as the result of the aggregation of the small silver particles into larger particles which have broad absorption spectra. During an "intermediate" stage, transmission electron microscopy and absorbance data show that even larger (similar to 6-10 nm) particles grow at the expense of the "monomeric" silver particles. Later in the reaction, electrochemical potential measurements show that the borohydride concentration suddenly decreases. Direct measurement of interparticle forces demonstrate that this change in the solution conditions drives the particle surface potential toward zero and results in increased adhesive forces. The resulting aggregation manifests itself in a darkening of the solution color. At low temperatures this corresponds to a 10 times increase in the particle size whereas, at high temperatures, the increase is minimal. This effect can be Linked to the number of monomeric silver particles remaining during the final transition.