Microwave Enhancement of Autocatalytic Growth of Nanometals

Abstract

The desire for designing efficient synthetic methods that lead to industrially important nanomaterials has led a desire to more fully understand the mechanism of growth and how modern synthetic techniques can be employed. Microwave (MW) synthesis is one such technique that has attracted attention as a green, sustainable method. The reports of enhancement of formation rates and improved quality for MW driven reactions are intriguing, but the lack of understanding of the reaction mechanism and how coupling to the MW field leads to these observations is concerning. In this manuscript, the growth of a metal nanoparticles (NPs) in a microwave cavity is spectroscopically analyzed and compared with the classical autocatalytic method of NP growth to elucidate the underpinnings for the observed enhanced growth behavior for metal NPs prepared in a MW field. The study illustrates that microwave synthesis of nickel and gold NPs below saturation conditions follows the Finke-Watzky mechanism of nucleation and growth. The enhancement of the reaction arises from the size-dependent increase in MW absorption cross section for the metal NPs. For Ni, the presence of oxides is considered via theoretical computations and compared to dielectric measurements of isolated nickel NPs. The study definitively shows that MW growth can be modeled by an autocatalytic mechanism that directly leads to the observed enhanced rates and improved quality widely reported in the nanomaterial community when MW irradiation is employed.

Dielectric Properties for Nanocomposites Comparing Commercial and Synthetic Ni- and Fe3O4-Loaded Polystyrene

“…The larger value for the Ni samples is consistent with the metallic character of Ni and consistent with our early analysis of Ni permittivity. 78…”

Synthesis of Highly Uniform Nickel Multipods with Tunable Aspect Ratio by Microwave Power Control

“…MW heating can give rise to selective heating due to the fact that MW absorption by a nanoparticle is size-and shape-dependent. 2,24,46 The interaction of the MW field will be largest at sharp edges, reflecting the properties of the dielectric absorption process. 28,31,33−35 In systems exhibiting overgrowth, this may lead to selective heating of the tips and potentially increased reduction rates at the overgrowth sites owing to the lightning rod effect, 25−32 leading to enhanced anisotropic growth at the superheated tips as hypothesized in Figure 1.…”

“…Enhancing tip growth requires control of surface energy, which can be achieved by ligand binding or potentially selective heating. MW heating can give rise to selective heating due to the fact that MW absorption by a nanoparticle is size- and shape-dependent. ,, The interaction of the MW field will be largest at sharp edges, reflecting the properties of the dielectric absorption process. ,,− In systems exhibiting overgrowth, this may lead to selective heating of the tips and potentially increased reduction rates at the overgrowth sites owing to the lightning rod effect, − leading to enhanced anisotropic growth at the superheated tips as hypothesized in Figure . Of course, under continuous MW absorption, as the nanoparticle reaches thermal equilibrium, this effect will be diminished, leading to the reported spherical shapes.…”

“…The growth of metal nanoparticles is known to follow an autocatalytic two-step Finke–Watzky mechanism, where the first step involves a slow reduction step ( k 1 ) of the cationic precursor in solution by a weak reducing agent, such as oleylamine, followed by a second fast reduction step ( k 2 ) of the cation precursor at the surface of the growing nanoparticle. − In the autocatalytic mechanism, k 2 ≫ k 1 , leading to growth being dominated by the nanoparticle surface. The nuclei morphology, the facet stability for a given metal, the reaction conditions, and face-selective binding of ligands can lead to hyper-branching.…”

Drastic Microwave Heating of Percolated Pt Metal Nanoparticles Supported on Al2O3 Substrate

“…The suitable conductivity of the Au NPs layer provided the resonant interaction between the electrons and MWs, leading to the best heating property. For the isolated NPs system, there are only a few reports referring to the insight of the MW heating [30][31][32]. The supported NPs are often highly dispersed on the catalyst support that is also the isolated NPs system.…”