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

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 similar to 1.4 and similar to 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.