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Bartłomiej Streszewski, Wiktor Jaworski, Krzysztof Pacławski, Edit Csapó, Imre Dékány, Krzysztof Fitzner
Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 397, March 2012
Abstract
This work features the study of kinetics and mechanisms of redox reaction between [AuCl4]− and hydrazine sulfate, as well as the kinetics of gold nanoparticles (AuNPs) formation. UV–Vis spectrophotometry and dynamic light scattering (DLS) method were used to determine the influence of reductant concentration on the rate of Au(III) ions reduction and Au NPs formation. It was found that the reaction mechanism constitutes several steps. Homogeneous, bimolecular reduction of Au(III) complex ions comprise the first step. The second step consists of autocatalytic reduction of Au(I) ions to a metallic form and triggers the nucleation and autocatalytic growth of AuNPs. Using the modified Finke–Watzky model and the obtained kinetic data, the following values of the rate constants were determined: k1 = 1.188 (±0.019) M−1 s−1, k3 = 3.87 (±0.26) 10−2 M−1 s−1 and k4 = 7.53 (±0.21) 105 M−2 s−1. The study on DLS and transmission electron microscopy (TEM) indicates that the growth of AuNPs is a result of the autocatalytic redox reaction followed by the reaction-limited Ostwald ripening. The rate laws describing homogeneous reduction of gold(III) ions and the evolution of the hydrodynamic radius of AuNPs were also determined.
Additional Information
The redox reaction between gold(III) chloride complex ions and hydrazine sulfate leads to the gold nanoparticles (AuNPs) formation. From the kinetic studies it can be concluded that the mechanism of this proces is complex and consists of at least one intermediate step in which gold(I) complex ions appear in the system and each elementary step involves one electron transfer from reductant to precursor ions. To determine the rate equation of AuNPs formation, the Finke and Watzky model of reaction was modified by incorporating the additional step of reaction, i.e. gold(III) to gold(I) reduction, into the mechanism scheme.
Fig. 1: TEM micrographs of gold nanoparticles (AuNPs) taken at a different time of reaction advancement: 200s (a), 400 s (b) 600s (c).
Using UV–Vis spectrophotometry and DLS method, kinetics of gold(III) and gold(I) complex ions reduction as well as the kinetics of AuNPs nucleation and growth, have been determined. The values of corresponding rate constants indicate that the reduction of gold(I) complex ions to metallic gold is a determining step of AuNPs formation and appearing nanoparticles in the system are the sufficient catalyst during the reduction of gold(III) and gold(I) complex ions. Under these conditions, the stationary state in which the concentration of gold(I) complex ions remains constant can be reached.
From registered changes of the hydrodynamic radius value in time it is evidence that AuNPs are not stable. Their further growth, as a result of the autocatalytic redox reaction, is attributed to the reaction-limited Ostwald ripening process and can be suggested after microscopic studies. From the collected TEM images it is evident that within the first stage of AuNPs formation (200 s) particles reach 3 nm in diameter (Fig. 1a). However, the presence of autocatalytic reaction triggers the further growth of AuNPs and after 400 s the formation of relatively large aggregates with grainy structure and narrow size distribution is observed (Fig. 1b). At the same time, the Ostwald ripening-related phenomenon causes transformation of granular structure into polycrystals. With the agreement of Finke–Watzky–Finney mechanism, this structural change can be referred to the step of autocatalytic agglomeration associated with continuous redox reaction of gold ions on the surface of AuNPs.
Acknowledgments: This work was supported by the European Regional Development Fund under the Innovative Economy Operational Program (Grant No. 01.01.02-00-015/09-00).
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