Transformations during Sintering of Small (D_core < 2 nm) Ligand-Stabilized Gold Nanoparticles: Influence of Ligand Functionality and Core Size

Abstract

Ligand-stabilized gold nanoparticles have been investigated as both discrete entities with size-dependent properties and as precursor inks for low-temperature deposition of thin films and patterns. In the first instance it is important to preserve the nanoparticle core, whereas for thin film applications it is desirable for the nanoparticles to sinter at relatively low temperatures. In each case, a detailed understanding of the factors that govern nanoparticle sintering will lead to improved nanomaterial design. An investigation of the sintering behavior of ∼1.4 and ∼0.9 nm gold nanoparticle cores, each passivated with two different ligands, by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) illustrates a clear size and ligand dependency on the sintering process. TGA reveals that free ligand volatilizes at lower temperatures than when bound to the nanoparticle core. Ligands of the same chain length with different terminal functionality show distinctly different volatilities and rates of ligand loss, revealing that volatility is derived from composition rather than merely ligand chain length. Conducting TGA and DSC measurements on nanoparticles of the same ligand passivation but of different core size shows that larger nanoparticles lose ligands and sinter more readily than smaller nanoparticles, suggesting a greater stability of the ligand shell on smaller nanoparticles. TGA, DSC, and X-ray photoelectron spectroscopy (XPS) analyses show that sintering is triggered by a very small amount of ligand loss. Once initiated, the sintering process rapidly excludes ligand from the gold surface, forming a porous film, as shown by scanning electron microscopy (SEM). These studies suggest that both the nanoparticle core size and ligand identity need to be considered together when selecting nanoparticles to either prevent or promote nanoparticle sintering.

Fine structure of gold nanoparticles stabilized by buthyldithiol: Species identified by Mössbauer spectroscopy

“…4A) Taking into account the shape of the curve as well as the size of the synthesized nanoparticles the registered spectrum is in good agreement with the experimental results published previously [25,26]. The characteristic plasmonic band appears at around 500 nm, indicating a decrease in particle size [39].…”

“…On thiol protected small gold particles (1.5-5 nm range) the weight loss may amount to 15-35 wt% [19,25] depending on the molecular weight of the thiol molecules. The TG curve obtained on our sample is presented in Fig.…”

“…On well-developed crystal faces of larger particles thiol molecules may form self-assembled monolayers [24]. In the transient size range (1-3 nm) the encapsulated particles may easily aggregate and sinter, by preserving simultaneously their protecting layers in part [25]. Dithiols exhibit a particular feature, namely their terminal thiol groups on the same molecule may be linked to different Au NPs stabilizing thereby large clusters of them.…”

Green nanomaterials for psoriatic lesions

“…80.76 %. TGA and DSC analysis of nanomaterials based on metallic nanoparticles are very commonly studied with respect to sintering nanoparticle inks for printed electronics [96][97][98][99][100]. Yu et al [99] presented TGA results and DSC profiles of AgNPs-organic molecules (in their case, the silver nanoparticles were obtained using sodium polyacrylate and ascorbic acid) with sizes of 20-30, 30-40 and 100-120 nm.…”

“…The research of Smith and Hutchinson [100] indicated the dependency of the sintering process on the size of the nanoparticle and ligand shell. TGA revealed that the free ligand decomposed at lower temperatures in comparison with the ligand bound to the nanoparticle.…”