Predictive Modelling of the Growth of Nanoscale Silver Particles: Verification of an Aggregative Growth Mechanism

Abstract

The formation mechanisms of nanoscale particles are poorly understood. Classical nucleation and growth models fail to describe many systems because they neglect important particle-particle interactions. In this study, we examine the formation of nanoscale silver particles from the reduction of silver salt using sodium borohydride. Within seconds of mixing silver salt solution with borohydride, >95% of the ionic silver is consumed, leaving essentially no silver for classical growth by molecular addition. These particles are stabilized by the charge associated with adsorbed borohydride anions, but continue to grow by aggregation and coalescence. The surface charge is extremely sensitive to solution conditions, and was measured by two independent techniques which showed the same qualitative trends. The surface charge decreases with solution temperature and increases with borohydride concentration. The ability of typical colloidal interaction potentials to describe this proposed aggregative growth process was tested through the development of a predictive growth model. In a series of test experiments, it is clearly shown that the aggregative growth model using colloidal interaction potentials accurately describes the growth process using values of the surface potential similar to those measured. This study clearly demonstrates the importance of particle-particle interactions in nanoscale crystallization processes and shows the ability of colloidal interaction models to describe the silver-borohydride system.