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Real time UV-visible absorption measurements of stopped flows of AgNO 3 and (NH 4 ) 2 S are used to study the nucleation and growth of silver sulfide nanoparticles (Ag 2 S NP). Ag 2 S NP in the size range of 2 to 10 nm are formed a few seconds after the flow containing the reactants stops. The absorbance near the band edge of the semiconductor nanoparticle is used to monitor the number of particles formed with time and study the nucleation process. Transmission electron microscopy measurements are used to correlate particle size and the indirect band gap energy, determined from the onset of light absorption. A linear relation is established between Ag 2 S NP particle size and indirect band gap energy. The nucleation and growth process are not well separated in time. The initial nucleation and growth rates are found to increase with initial [AgNO 3 ] o /[(NH 4 ) 2 S] o ratios larger than 1. Silver-rich sulfides are proposed to be involved in the nucleation stage and growth process of Ag 2 S NP. Density functional calculations are consistent with that interpretation: Ag 3 S + is found to have a lower energy than the Ag 2 S and AgSH molecules or the AgSand Ag 2 SH + ions. The results are discussed in terms of classic nucleation theory and the possible growth mechanisms are discussed.

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A model for metal spherical particle formation is proposed, guided by optical kinetic data of monodisperse metal colloid synthesis. The dynamic optical response of these systems is characterized by broad bands, making their analysis difficult to interpret in terms of particles sizes. To overcome this problem, the data are analyzed in terms of a new strategy called simulated dynamic optical response. Kinetic data were generated using a silver-ethylenediamine complex reduced with isoascorbic acid as a model system. Using this strategy, it was found that autocatalytic formation of primary particles followed by a zone of very fast aggregation mechanism can describe the dominant dynamics during early stages. In later stages, the dominant mechanism switches to slower aggregation modulated by a stability factor. The presented model and identification strategy may be applied to other reaction precipitation systems to produce metal particles.