Gold Nanoparticle Internal Structure and Symmetry Probed by Unified Small-Angle X-ray Scattering and X-ray Diffraction Coupled with Molecular Dynamics Analysis.

Shape and size are known to determine a nanoparticle's properties. Hardly ever studied in synthesis, the internal crystal structure (i.e., particle defects, crystallinity, and symmetry) is just as critical as shape and size since it directly impacts catalytic efficiency, plasmon resonance, and orients anisotropic growth of metallic nanoparticles. Hence, its control cannot be ignored any longer in today's research and applications in nanotechnology. This study implemented an unprecedented reliable measurement combining these three structural aspects. The unified small-angle X-ray scattering and diffraction measurement (SAXS/XRD) was coupled with molecular dynamics to allow simultaneous determination of nanoparticles' shape, size, and crystallinity at the atomic scale. Symmetry distribution (icosahedra-Ih, decahedra-Dh, and truncated octahedra-TOh) of 2-6 nm colloidal gold nanoparticles synthesized in organic solvents was quantified. Nanoparticle number density showed the predominance of Ih, followed by Dh, and little, if any, TOh. This result contradicts some theoretical predictions and highlights the strong effect of the synthesis environment on structure stability. We foresee that this unified SAXS/XRD analysis, yielding both statistical and quantitative counts of nanoparticles' symmetry distribution, will provide new insights into nanoparticle formation, growth, and assembly.