Polyhedral Oligomeric Silsesquioxane (POSS)-Stabilized Pd Nanoparticles: Factors Governing Crystallite Morphology and Secondary Aggregate Structure

Polyhedral oligomeric silsesquioxane (POSS)-based amine ligands are used as capping agents in the reductive growth of Pd nanocrystallites, and the effects of the specific form of the ligand on the structure of both the secondary aggregate and the primary metal nanoparticle are explored. Secondary aggregates of the ligand capped Pd particles are seen to form a uniform structure only if (a) the POSS ligand is totally functionalized with amine groups and (b) the amine groups are in the hydrochloride form. By casting these results in the context of density functional theory calculations, we show that the morphology of Pd core-shell nanomaterials is determined by the ratio of self-interaction potentials of the ligands to their interaction with solvent. The structures of the primary metal crystallites are also shown to be sensitive to the amine ligand in the hydrochloride form. Our results further show that the POSS versions of a ligand will frequently form precipitating capped metal nanoparticle structures, where the discrete molecular forms produce no isolable metal nanoparticles at all on the time scale of the experiments. A study of the diffusion rates of the reactive species in the Pd/POSS-ligand system shows a correlation between the growth of observable metal crystallites and sufficiently slow ligand diffusion kinetics. Finally, we illustrate through a TEM time series that Pd nucleation seems to happen without a prior formation of observable self-assembled POSS ligand templates.