Competitive reactivities determine the size and composition of multimetallic nanocrystals

 

Multimetallic nanocrystals (NCs) have attracted considerable attention due to their physical, chemical, and catalytic properties, which often surpass those of their monometallic counterparts. The distinctive properties of NCs are determined by the synergistic interactions among their constituent metals.

Synthesizing these materials with precise control over size and composition remains a major challenge because of the differences in the reactivities of the metal precursors. Owing to these differences, one would expect that increasing the number of metal precursors would enhance the formation of heterogeneous products (mixtures of particles with different sizes and compositions).

However, a multinational team of researchers demonstrated a counterintuitive effect in the synthesis of multimetallic nanocrystals: differences in the reactivities of metal precursors can actually promote the formation of highly uniform multimetallic nanocrystals.

Ru nanoparticles (≈ 4.5 nm) and precursor solutions of Fe, Co, Ni, and Cu were used as seeds. Upon introducing five metals (RuFeCoNiCu), a uniform product was obtained: pentametallic nanocrystals of ≈ 14.1 ± 1.4 nm with a narrow size distribution. This effect persisted even when the seed size, precursor ratios, and additional metals (Cr, In) were varied.

The mechanism underlying this remarkable process was elucidated through time-lapse analysis of intermediate products and tomography. As shown in the Figure, the formation of pentametallic nanocrystals proceeded through three distinct stages: (i) predominant reduction of Cu on previously formed Ru seeds, (ii) onset of Co, Ni, and Fe reduction accompanied by partial surface-layer formation, and (iii) complete reduction and integration of all constituent metals into fully formed RuFeCoNiCu nanocrystals.

When the pentametallic nanocrystals supported on Al2O3 were used as catalysts, they exhibited a reaction rate more than four times higher than that of monometallic Ru in ammonia decomposition (NH3 → N2 + 3H2), while maintaining comparable activation energy and thermal stability.

This work proposes a new principle for the design of complex multimetallic nanocrystals: rather than suppressing the competition among metal precursor reactivities, it can be harnessed. This finding leads the way toward libraries of nanomaterials with unique synergistic properties for applications such as catalysis and sustainable energy technologies.

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