(300b) Ligands Dominate Agglomerate Morphologies of Unpolar Nanoparticles

Agglomeration is a ubiquitous feature of nanoparticles. Surprisingly little is known on the factors that dominate the morphology of agglomerates formed by technologically relevant, unpolar nanoparticles. In this contribution, we discuss experiments that suggest a dominating effect on agglomerate morphologies. Temperature-dependant agglomeration studies on alkylthiol-stabilized gold nanoparticles show that ligand solubility dominates the kinetics of agglomeration and the structure at the micrometer scale. Precipitation induced by adding poor solvents at different temperatures indicate that the molecular structure of the ligand shell affects kinetics and structure at the nanometer scale.

We used dynamic light scattering and small-angle x-ray scattering (SAXS) to follow the agglomeration of alkylthiol-coated gold nanoparticles with 6 nm core diameter in situ and analyzed the data using modified Smoluchowski models. Systematic variations of the ligand chain lengths between 12 to 18 carbon atoms indicated a temperature-dependant solvation of the ligands. We identified critical temperatures at which the agglomeration changed from reaction- to diffusion limited mechanisms [1]. The resulting agglomerates had markedly different geometries as visible in SAXS and transmission electron microscopy (TEM).

The same particles were precipitated by the addition of a more polar solvent, 1-propanol, that is less solvating towards the alkyl chains of the ligands. We found a sharp transition in the nanoscale geometry of the formed agglomerates: at temperatures below the melting point of the dry ligand monolayer, randomly dense packed agglomerates formed. At temperatures above the melting point, the agglomerates were dense and crystalline, with sharp facets of the superlattice [2]. Brownian dynamics simulations were performed to interpret this behavior. We believe that the short-range interactions between the particles are strongly affected by the molecular structure of the ligand layer. Molten ligands facilitate the relative motion or particles inside agglomerates.

In summary, this contribution presents experimental results indicating a crucial role of the ligands in the structure formation of unpolar nanoparticle agglomerates. The core material has less influence on the structure of the superstructures. We therefore believe that rational design of agglomerate morphologies for the use in hybrid materials is possible.

[1] P. Born and T. Kraus: Ligand-dominated temperature dependence of agglomeration kinetics and morphology in alkyl thiol-coated gold nanoparticles. Physical Review E, under review (2013)

[2] T. Geyer, P. Born and T. Kraus: Switching between crystallization and amorphous agglomeration of alkyl thiol-coated gold nanoparticles. Physical Review Letters 109 (2012) 128302