Using small and wide angle X-ray scattering, scanning transmission electron microscopy coupled with X-ray energy dispersive spectroscopy and electrochemical methods, we unveil the processes at stake during the formation of porous hollow PtNi/C nanoparticles. We show that Ni-rich nanoparticles covered by a NiXBYOZ shell are first formed before alloying with Pt atoms (t = 3 min). Due to galvanic displacement, a thick Pt-rich shell then develops (t = 20 min). The resulting gradient in chemical potential becomes a strong driving force for the outwards diffusion of Ni atoms, yielding ultimately to porous hollow PtNi/C nanoparticles. These results show that hollow PtNi/C nanoparticles are the ultimate stage of a dissolution process involving galvanic replacement and the nanoscale Kirkendall effect, similarly to what was observed during the ageing of bi-metallic catalysts in real proton exchange membrane fuel cells 7. Beyond its interest for hollow NPs, this study also provides a general combination of techniques capable to unveil the mechanism of formation and growth of preferentially-shaped metal NPs.
Figure 1: Schematic representation and X-ray energy dispersive spectroscopy elemental maps of the different nanostructures sequentially formed during the synthesis of hollow PtNi/C NPs. The images are recorded for t = 1, 2, 3, 4, 10, 20, 40 and 60 min after the addition of the first drop of the NaBH4 solution. The non-charged plain circles are representative of NiXBYOZ(grey), Ni (green) and Pt-rich (gold) crystallites.
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