Influence of Metal–Ligand Coordination on the Elemental Growth and Alloying Composition of Pt–Ni Octahedral Nanoparticles for Oxygen Reduction Electrocatalysis

Abstract

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.

Visualizing Ligand-Mediated Bimetallic Nanocrystal Formation Pathways with in Situ Liquid-Phase Transmission Electron Microscopy Synthesis

“…23 Selective binding of metal ions to ligands can also be used to rationally manipulate and harmonize the reduction kinetics of metal precursors. 25 Use of complex ions as singlesource precursors points to a facile approach to nanoparticle synthesis, where capping ligands actively control nanocrystal properties and final nanoparticle characteristics (e.g., composition and alloy structure) are directly encoded into the molecular scale precursor chemistry via rationally designed coordinate bonds, reduction potentials, and metal−metal bonds.…”

Surface Modification for Promoting Durable, Efficient, and Selective Electrocatalysts

“…In some reports, alloys are protected by diverse molecules, or polymers, but without studying their effects on the electrocatalytic performance, even if these stabilizing ligands may have an impact as described in this review. While one work describes a negative effect of ligand coordination on Pt−Ni nanoparticles shaping that leads to lower ORR activity, some examples show the synergy between alloying and metal‐ligand interfacial bonding effect for increasing the catalysts’ performance.…”

Capping agent‐free synthesis of surface engineered Pt nanocube for direct ammonia fuel cell

“…This again points to the critical role of acetylacetone in the formation of the (100) facet (Figure S5). Surface faceting using acetylacetone has previously been used in the synthesis of octahedral PtNi nanoparticles (ie, (111) PtNi) 45,46 . However, to the best of our knowledge, this is the first report on the synthesis of faceted Pt nanoparticles by hot‐separation.…”