The formation of sodium citrate stabilized spherical silver nanoparticles synthesized by homogeneous nucleation in aqua solutions was monitored by the method of direct potentiometry. It was observed that the kinetic curve of Ag+ ions reduction can be described by the Finke–Watzky (FW) autocatalytic two–step mechanism in the case of a large excess of hydrazine and NaOH. To expand the FW minimalistic model, the impact of the starting concentrations of the reagents on the kinetics of Ag nanoparticles synthesis was studied. It was determined that the nucleation stage is limited by the homogeneous process of ions Ag+ reduction, and the reaction orders for the all reagents are calculated. A surface–dependent model of Ag nanoparticle growth is proposed, which takes into account the change in the hydrazine and alkali concentrations; the rate constants of the pseudoelementary reactions are also calculated.

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The kinetics of formation of silver nanoparticles consisting of nearly 300 metal atoms is investigated, which were prepared by reduction of silver nitrate with hydrazine in ethylene glycol at 25 • C without any stabilizer other than the glycol solvent. The resulting sigmoidal kinetic curves are analyzed by using the 1997 Finke-Watzky two-step mechanism of slow continuous nucleation with subsequent fast autocatalytic surface growth. The kinetics of homogeneous nucleation of metal nanoparticles was analyzed using the assumption about the stepwise adjunction of precursor and the quasi steadystate approximation. The equations were proposed to calculate the concentration of the formed metal nanoparticles and their mean size from the experimentally determined values of the Finke-Watzky rate constants. It is shown that a stepwise nucleation process can be described in the terms of the catalytically effective nucleus concept and that the number of atoms in the catalytically effective nucleus can be estimated. K E Y W O R D Scatalytically effective nucleus concept, homogeneous nucleation, kinetics, silver nanoparticles 266

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