Manipulating Local Ligand Environments for the Controlled Nucleation of Metal Nanoparticles and their Assembly into Nanodendrites

Abstract

One could argue that the synthesis of metal nanostructures with well-defined features remains an art, not a science, given our current understanding of how metal precursors are transformed into small metal clusters and eventually larger nanostructures. Here, we report a facile route to palladium nanodendrites and larger Pd bundles through the thermal decomposition of Pd precursors in only oleylamine. This simple, two-component system was designed to evaluate the effect of the local ligand environment on the nucleation of nanoparticles and their subsequent growth through either atomic addition or aggregation. It was found that both the nucleation and growth stages associated with the formation of the nanostructures, and thus the final morphologies, were controlled by the binding affinity of the ligands. Significantly, our analysis connects the formation of nanostructures directly to fundamental principles of coordination chemistry and provides universal guidelines that can be applied to the synthesis of other dendritic structures and high-quality nanomaterials in general. In particular, it was found that ligands with intermediate binding affinity towards Pd II ions regulate their gradual reduction and promote the aggregation-based assembly of the weakly passivated Pd nanoparticles into nanodendrites. In contrast, strongly coordinated ligands delay burst-like nucleation, and growth of quasispherical wellpassivated nanoparticles through atomic addition is favored. As metal nanostructures with branched morphologies often display distinct physicochemical properties, extracting the crucial parameters governing their formation will contribute to the future synthesis of structurally distinct metal dendrites. As we found, the principles that guided the ligand-controlled synthesis of Pd dendrites were general and also enabled the synthesis of Pt nanodendrites.Typical methods for producing nanoparticles composed of face-centered cubic (fcc) metals yield polyhedral shapes favored by thermodynamics; however, different growth mechanisms such as kinetically controlled overgrowth, aggregation-based growth, and template-mediated methods can be employed to synthesize branched metal nanoparticles in solution. [1] For example, Pt-on-Pd nanodendrites with superior catalytic activity for oxygen reduction were produced by using preformed Pd cuboctahedra as templates for the subsequent dendritic growth of Pt. [2] In this case, dendritic growth was attributed to a high rate of reduction for the Pt precursor facilitated by a seed-mediated autocatalytic process. Similar overgrowth processes have been reported in other syntheses of metal dendrites and are often coupled with the in situ generation of seeds. [3] Interestingly, Tilley and coworkers verified the importance of rapid growth kinetics in the formation of branched metal nanoparticles by monitoring the synthesis of Pd dendrites from PdCl 2 ·(CH 3 CN) 2 in the presence of toluene, oleylamine, oleic acid, and H 2 by an in situ synchrotron X-ray diffraction (XRD) technique. [4] The sign...

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