Monitoring Ligand-Mediated Growth and Aggregation of Metal Nanoparticles and Nanodendrites by In Situ Synchrotron Scattering Techniques

In situ synchrotron wide-angle X-ray scattering (WAXS) and ultra-small angle X-ray scattering (USAXS) were used to study the growth of Pd nanodendrites and quasi-spherical Pd nanoparticles. The metal-ligand environments of both the precursors and growing nanoparticles contribute to nanostructure formation. Specifically, the use of Pd(hfac)(2) (hfac=hexafluoroacetylacetonate) as a precursor facilitated nanostructure formation more facilely compared to Pd(acac)(2) (acac=acetylacetonate), as quantified by in situ WAXS. This enhanced rate is attributed to the better leaving group properties of the hfac(-) ligands, and this result demonstrates that fundamental principles from coordination chemistry can guide the selection of precursors in nanomaterial syntheses. This in situ approach also revealed concomitant nucleation, coalescence, and growth processes during the synthesis of monodisperse Pd nanoparticles. Significantly, this finding identifies self-limited growth mediated by the local ligand environment of the particles as the central means of size distribution control and contradicts the typically cited mechanism (LaMer-like growth) for such chemical systems. These observations highlight the importance of applying in situ approaches to study nanostructure formation as intertwined processes can be revealed.