Kinetic Factors in the Synthesis of Silver Nanoparticles by Reduction of Ag+ with Hydrazine in Reverse Micelles of Triton N-42

Abstract

The growth kinetics of silver nanoparticles (AgNPs) during the reduction of AgNO3 by hydrazine in the droplets of dispersed aqueous phase encapsulated in the reverse micelles of oxyethylated surfactant Triton N-42 with decane as disperse medium was studied in situ by UV–vis spectroscopy. The mechanism of the process includes two steps that are slow, continuous nucleation and fast, autocatalytic surface growth. Both steps are under kinetic control and are limited by the rate of Ag+ reduction. The rate of nucleation is limited by reaction in the droplets of the aqueous phase forming the cores of reverse micelles, and the rate of the growth is limited by the reaction on the surface of AgNPs growing inside the micelles. The reduction of Ag+ is a second-order process with respect to N2H4. It includes the formation of the intermediate complex Ag(N2H4)+ and its reaction with another N2H4 molecule. The concentration effects of N2H4 (c′N2H4) and NH3 (c′NH3) as competing ligand, medium effects of ionic strength (I) and of the background salt in dispersed aqueous phase, the effect of solubilization capacity of the micellar solution (Vs/Vo), and the effect of temperature (T) on the observed rate constants for the steps were studied. An increase in c′N2H4, I, Vs/Vo and T can be used to accelerate the rates of both steps, whereas an increase in c′NH3 inhibits them. The background salts have a positive effect on the rate of nucleation, whereas their effect on the growth rate is small and has probably a negative trend. The size and composition of AgNPs were characterized by means of DLS, TEM, EDXA, XRD, UV–vis, and IR spectroscopy.