(337f) Small and Wide-Angle X-Ray Scattering for in Situ Monitoring of Particle Size Evolution During Platinum Nanoparticle Synthesis

The kinetics of metal synthesis at the onset of nanocrystal formation is of great interest due to reports of the unusual properties when matter is confined to the nanoscale. Particle structure in this critical size region impacts magnetics, electrocatalysis, optics, and heterogeneous synthesis.  Despite significant progress regarding the characterization of size and shape of Pt nanocrystals, data concerning nucleation and growth has proven elusive.  Current approaches relate equilibrated structures measured ex situ to initial synthesis parameters, missing the key nucleation and growth processes.  Here we use real-time, in-situ small-angle and wide-angle x-ray scattering (SAXS and WAXS) and a Pt synthesis scheme with slower particle growth relative to the use of strong reducing agents to observe the formation of Pt critical nuclei on a length scale approaching 0.6 nm and with a dynamic resolution of 20 seconds.  We correlate the autoreduction kinetics of precursor inorganic complexes with nanoparticle morphology, number density, polydispersity, and Pt volume fraction, thereby establishing the mechanism of particle size control at the onset of atomic coalescence. In addition, a hierarchical sub-structure is identified for the first time in the wide-angle scattering data related to the atomic structure of the growing Pt nanoparticles.