Formation of gold nanoparticles during the reduction of HAuBr 4 in reverse micelles of oxyethylated surfactant: Influenc
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The growth kinetics of gold nanoparticles (NPs) during the reduction of HAuBr4 by hydrazine in the reverse micelles of oxyethylated surfactant Tergitol NP 4 was studied in situ by UV–vis spectroscopy. Kinetic mechanism includes the steps of slow, continuous nucleation and fast, autocatalytic surface growth. Both steps are under kinetic control of the precursor reduction. The rate of nucleation is limited by reaction in the droplets of the aqueous phase forming the cores of reverse micelles, and growth rate is limited by the reaction on the surface of gold NPs growing inside the micelles. The chemical mechanism of reduction of halogenated forms of gold AuX4– by hydrazine is the same in the case of X 5 Cl, Br and includes the equilibria of formation and redox decomposition of the intermediate complexes AuIII(N2H4)X3 and AuI(N2H4)X. The initial form of AuX4– (X 5 Cl, Br) does not affect the size of the final NPs synthesized in micellar solution of oxyethylated surfactant.
I. INTRODUCTION
After the dramatic advances in the nanoscience over the past 15 years, particularly in synthetic methods of nanoparticles (NPs), a transition from studying NPs as model objects to the development of advanced materials based on NPs was made.1 However, for the evolution of nanomaterials into viable commercial products there required further development of cost-effective reproducible synthetic procedures, providing narrow NP size distributions and control over mean size and the chemical composition of NPs.1 In spite of the great number of known synthetic procedures, the majority of them are just empirically chosen conditions for the synthesis of different kinds of NPs.2 So far, it is impossible to predict the result of the synthesis of NPs exclusively on the basis of experimental conditions, in contrast to modern organic synthesis. It is clear that only the understanding of growth mechanisms of NPs allows one to develop the effective synthetic strategies to generate large quantities of NPs that are monodisperse, pure, and well-characterized. Unfortunately, only few studies were focused on the kinetics and regularities of the growth of NPs in comparison with diverse empirical synthetic procedures,2 however, substantial advance has been achieved in this direction during the recent years, as evidenced by reviews.2,3 Contributing Editor: Sanjay Mathur a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.121 J. Mater. Res., Vol. 30, No. 12, Jun 28, 2015
Quantitative studies of the growth kinetics of NPs of precious metals (M 5 Au, Ag, Ir, Pt, Pd, Ru, and Rh)2–14 showed that the formation of primary metal particles is limited by the rate of reduction of the metal precursor and proceeds according to the mechanism (1–2), known as the two step Watzky–Finke kinetics model.6 The mechanism includes the steps of nucleation (1) and autocatalytic growth (2). nMZþ ! M0 n ; k1 ; ð1Þ 0 M n þ MZþ ! M0 nþ1 ; k2
:
ð2Þ
The first step includes slow homogeneous reduction of Mz1 species to M0 atoms fol
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