Reactive Additive-Induced Joining of Apolar Thiol-Coated Gold Nanoparticle Cores at Low Temperatures

Abstract

The agglomeration of apolar metallic nanoparticles (NPs) brings them within nanometer distances, enabling nonradiative coupling through electron conduction tunneling and plasmonic interactions. Even stronger coupling can be achieved with metallic connections between the particles. Thermally induced coarsening is often used to join particles in this way, but typically requires high temperatures. For example, the thermal coarsening of thiol-coated NPs in apolar dispersions requires heating above 120 °C to induce ligand desorption. This leads to side reactions, notably Ostwald Ripening and the formation of small particles. Here, we introduce a new method for controlled NP coarsening in apolar dispersions at near-ambient conditions. Small amounts of reactive chemical additives are added to the dispersion and thermally activated. Transmission electron microscopy and small-angle X-ray scattering reveal that adding the cyclic sulfide additive molecule tetrahydrothiophene (R5S) to a dispersion of hexadecanethiol (HDT)-coated gold nanoparticles (AuNPs) induces aggregation at temperatures as low as 40 to 60 °C. The cores form metallic joints, forming larger continuous gold bodies. An Arrhenius analysis of the temperature-dependent aggregation kinetics identifies the exchange of HDT to R5S on the gold surfaces as the rate-limiting step. Small-angle neutron scattering and thermogravimetry confirm the mechanism and indicate that ∼80% of HDT is replaced by R5S after 5 h at 50 °C. Progressive ligand exchange reduces the steric repulsion by the shells until aggregation sets in. A comparison with the larger cyclic sulfide thionane and the cyclic amine pyrrolidine, which do not induce coarsening, shows that the prerequisite for ligand desorption at moderate temperatures to enable AuNP coarsening is a small size and strong affinity of the additives to the gold surfaces.