Compositional segregations in shaped alloy nanoparticles can significantly affect their catalytic activity and are largely dependent on their elemental anisotropic growth and diffusion during nanoparticle synthesis. An efficient approach to control the surface segregations while keeping the nanoparticle shape are highly desired for fine-tuning their catalytic properties. Using octahedral Pt−Ni nanoparticles as a typical example, we report a new strategy to modulate the surface composition of shaped bimetallic nanoparticles by microwave-enhanced surface diffusion during solvothermal synthesis. Compared to traditional solvothermal synthesis, the application of microwave significantly promotes atomic diffusion, particularly surface diffusion, within the Pt−Ni octahedrons, leading to Pt segregation on the {111} facets while largely keeping the octahedral shape. The obtained segregated Pt−Ni octahedral nanoparticles performed excellent activity toward oxygen reduction reaction. The revealed microwave-enhanced surface diffusion in a liquid phase provides a new way to modulate surface compositions of bimetallic alloy nanoparticles at relatively lower temperatures compared to the widely adopted high-temperature gas-phase thermal annealing.
Understanding
the role of surfactants or ligands on the growth
mechanism of metal/alloy nanoparticles (NPs) is important for controlled
synthesis of functional metallic NPs with tailored structures and
properties. There have been a number of works showing the significant
impact of surfactants/ligands on the shape-controlled synthesis of
nanocrystals with well-defined surfaces. Beyond the morphological
shape control, impact of the surfactants/ligands on the alloying structure
of bimetallic nanocrystals, however, still remains largely unaddressed.
We reveal here a significant effect of benzoic acid ligand on the
elemental growth and alloying phase structure of octahedral Pt–Ni
NPs, a class of highly active electrocatalyst for oxygen reduction
reaction in fuel cells. Contrary to previous reports showing the critical
role of benzoic acid in directing the growth of octahedral Pt–Ni
NPs, we found that benzoic acid played a minor role in forming the
octahedral shape; instead, it can strongly coordinate with Ni cation
and significantly slows down its reduction rate, leading to a phase
separation in the Pt–Ni NP products (a mixture of Pt-rich octahedral
NPs and nearly pure Ni NPs). Such phase separation further resulted
in a lower catalytic activity and stability. These results help us
comprehensively understand the effect of metal–ligand coordination
chemistry on the elemental growth mechanism and alloying phase structure
of bimetallic NPs, complementing previous emphasis on the role of
surfactants in purely morphological shape control.
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