Real-time icosahedral to fcc structure transition during CoPt nanoparticles formation

Nucleation and growth of supported CoPt nanoparticles were studied in situ and in real time by combined grazing incidence small-angle x-ray scattering (GISAXS) and grazing incidence x-ray diffraction (GIXD). GISAXS provides morphological features of nanoparticles as a function of size, shape and correlation distance between particles, while GIXD allows the determination of the atomic structure. We focus on the formation of ultrasmall CoPt nanoparticles, in the 1–4 nm size range at 500 ○ C. The structural analysis method based on the Debye equation is coupled with cluster model calculations performed by Monte Carlo simulations using a semi-empirical tight-binding potential to interpret diffraction spectra and structural transitions. Our results show that the cluster structure evolution during the growth is size-dependent and composition-dependent, yielding an icosahedral to fcc structure transition.