Time-Resolved, in Situ, Small- and Wide-Angle X-ray Scattering To Monitor Pt Nanoparticle Structure Evolution Stabilized by Adsorbed SnCl3- Ligands During Synthesis

Understanding and controlling nanoparticle formation mechanisms is important because of increasing reports of unusual material properties in this critical size region in fields ranging from magnetics, electrocatalysis, optics, and heterogeneous synthesis. Here we use real-time, in situ small-angle and wide-angle X-ray scattering to dynamically monitor the production of Pt critical nuclei from Sn-Pt complexes on a length scale approaching 0.6 nm. A time resolution of 20 s is achieved due to the slow reduction and growth processes of this unique system. The structure and evolution of inorganic Sn ligands on these Pt nanopartide cores is of particular interest because of the ability of Sn to mitigate poisoning of electrocatalyst surfaces by adsorbed CO intermediates (COads) on the anode of direct alcohol fuel cells. Coherent scattering from the stabilizing ligand shell is identified in the wide-angle scattering data and suggests segregation of Sn ligands at the surface of the nanoparticles synthesized using our approach. We correlate the autoreduction kinetics of precursor complexes with number density, polydispersity, and Pt volume fraction, thereby establishing the mechanism of particle size control at the onset of atomic coalescence. Particle growth is shown to terminate when a Sn shell of well-defined thickness (about 0.6 nm) is completed for a range of Pt core diameters in this critical size range.