A view from the inside: Complexity in the atomic scale ordering of supported metal nanoparticles

In this report, we describe the use of several analytical techniques, including X-ray absorption spectroscopy (XAS), electron microscopy, and electron diffraction, as tools for characterizing the structural dynamics of supported Pt nanoscale particles. We examined several carbon-supported samples. Electron microscopy shows that the particles in these samples (S1-S3) have average particle diameters of roughly 20, 40, and 60 A respectively, while electron microdiffraction data for these particles provided evidence of long-ranged ordering in the form of face centered cubic structures, This study highlights the use of advanced synchrotron X-ray absorption spectroscopies (XAS), in particular extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES), as powerful tools for studying the structural habits and dynamics of these prototypical nanoscale materials. Using state-of-the-art methods of measurement and computational modeling, we demonstrate that it is possible to develop a detailed understanding of the shape and morphology of the nanoscale clusters. We use these techniques to provide information about the nature of their surface texturing, establishing that they preferentially adopt oblate ("hemispherical") cuboctahedra cluster shapes truncated along the [I I I] basal plane. We further describe the use of temperature-dependent EXAFS measurements to investigate the nature of bond relaxation phenomenon occurring within the small metallic nanoparticles. To evaluate these complex structural behaviors, the disorder parameters are calculated from temperature-dependent EXAFS data and then subsequently compared to simple molecular graphics simulations of mechanisms involving either full cluster or surface relaxations. The average bond length and static disorder obtained by experiment appear to best fit a model involving dominant contributions made by surface atom bond relaxation.