Real-time icosahedral to fcc structure transition during CoPt nanoparticle formation
Résumé
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 size, shape and correlation distance between nanoparticles, while GIXD allows the determination of the atomic structure. In this study we focused on the formation of ultrasmall CoPt nanoparticles, in the 1-4nm size range. A structural analysis method based on the Debye equation is presented coupled with cluster model calculations. These Monte Carlo simulations of relaxed CoPt cluster structures (disordered truncated octahedral, decahedral and icosahedral) were performed using a semi-empirical tight-binding potential to interpret diffraction spectra and structural transition. The experiment was performed at 500˚C in UHV conditions at the European Synchrotron Radiation Facility (ESRF) on the BM32 SUV beamline. Our results show that the evolution of the cluster structure during the growth is size-dependent and coalescence-dependent. In the early stage (N <300 atoms) of CoPt growth, coalescence events are the mainly responsible to the fcc nanoparticle formation, from icosahedral morphology.
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