Nucleation control in size and dispersity of metallic nanoparticles: The prominent role of particle aggregation

The aim of this project is to investigate the effect of aggregative nucleation and growth on the final size and dispersity in metallic nanoparticle systems. Aggregative nucleation functions are determined for the first time for three nanocrystal systems, namely gold (Au), silver (Ag) and bismuth (Bi). These nucleation functions give critical information that correlates closely with the size and dispersity of the nanocrystals synthesized.;The aggregative nucleation functions and growth kinetics of pre-synthesized Au nanoparticles as a function of tetra-n-octylammonium bromide (n-octyl4NBr) is investigated. For each kinetic trial, the time dependence of the aggregative nucleation rate is extracted from the early-time nanocrystal size distributions (CSDs), and fitted by a Gaussian profile. The height of the profile is the maximum nucleation rate, Gamma max, and the 2sigma width is the time window for nucleation, Delta tn. These nucleation parameters control the final mean size and size distribution of the coarsened nanocrystals. The coarsening kinetics are influenced by tetraoctylammonium bromide concentration because the nanocrystals are partially electrostatically stabilized.;A mechanistic study of Ag-nanoparticle growth by reaction of [(PPh 3)2Ag(O2CC13H27)] and AIBN is reported. The half-life for precursor disappearance at 130.0 +/- 0.1 ºC under the reaction conditions is determined to be 3.65 +/- 0.42 min, which defines the time scale for classical (LaMer) nucleation and growth to be within the first 15 min (4 half-lives). The nanoparticle-growth kinetics are separately determined by TEM monitoring and UV-visible spectroscopy. Fits to the kinetic data establish that the active-growth regime extends to 58 min, and that Ostwald ripening ensues shortly thereafter. Evidence for an aggregative nucleation and growth process is obtained. The quantitative data indicate that classical nucleation and growth, aggregative nucleation and growth, and Ostwald ripening occur in consecutive time regimes with little overlap, and that nanoparticle growth is dominated by the aggregative regime.;The kinetics and mechanism of Bi-nanocrystal growth from the precursor Bi[N(SiMe3)2]3 are determined at various Na[N(SiMe3)2] additive concentrations. The results establish that aggregative nucleation and growth processes dominate Bi-nanocrystal formation. The time dependence of the aggregative nucleation rate -- the nucleation function -- is determined over the range of Na[N(SiMe3) 2] concentrations studied. The time width of aggregative nucleation (Deltatn) is shown to remain reasonably narrow, and to correlate with the final Bi-nanocrystal size distribution. The maximum aggregative nucleation rate (Gammamax) is shown to vary systematically with Na[N(SiMe3)2] concentration, producing a systematic variation in the final nanocrystal mean size. The Na[N(SiMe3) 2] additive functions as both a nucleation-control agent and an Ostwald-ripening agent.