Colloidally prepared core@shell nanoparticles (NPs) were
converted
to monodisperse high entropy alloy (HEA) NPs by annealing, including
quinary, senary, and septenary phases comprised of PdCuPtNi with Co,
Ir, Rh, Fe, and/or Ru. Intraparticle heterogeneity, i.e., subdomains
within individual NPs with different metal distributions, was observed
for NPs containing Ir and Ru, with the phase stabilities of the HEAs
studied by atomistic simulations. The quinary HEA NPs were found to
be durable catalysts for the oxygen reduction reaction, with all but
the PdCuPtNiIr NPs presenting better activities than commercial Pt.
Density functional theory (DFT) calculations for PdCuPtNiCo and PdCuPtNiIr
surfaces (the two extremes in performance) found agreement with experiment
by weighting the adsorption energy contributions by the probabilities
of each active site based on their DFT energies. This finding highlights
how intraparticle heterogeneity, which we show is likely overlooked
in many systems due to analytical limitations, can be leveraged toward
efficient catalysis.
Chemical design of multicomponent nanocrystals requires atomic-level understanding of reaction kinetics. Here, we apply single-particle imaging coupled with atomistic simulation to study reaction pathways and rates of Pd@Au and Cu@Au core-shell nanocubes undergoing oxidative dissolution. Quantitative analysis of etching kinetics using in situ transmission electron microscopy (TEM) imaging reveals that the dissolution mechanism changes from predominantly edge-selective to layer-by-layer removal of Au atoms as the reaction progresses. Dissolution of the Au shell slows down when both metals are exposed, which we attribute to galvanic corrosion protection. Morphological transformations are determined by intrinsic anisotropy due to coordination-number-dependent atom removal rates and extrinsic anisotropy induced by the graphene window. Our work demonstrates that bimetallic core-shell nanocrystals are excellent probes for the local physicochemical conditions inside TEM liquid cells. Furthermore, single-particle TEM imaging and atomistic simulation of reaction trajectories can inform future design strategies for compositionally and architecturally sophisticated nanocrystals.
Pt catalysts are widely studied for the oxygen reduction reaction, but their cost and susceptibility to poisoning limit their use. A strategy to address both problems is to incorporate a second transition metal to form a bimetallic alloy; however, the durability of such catalysts can be hampered by leaching of non-noble metal components. Here, we show that random alloyed surfaces can be stabilized to achieve high durability by depositing the alloyed phase on top of intermetallic seeds using a model system with PdCu cores and PtCu shells. Specifically, random alloyed PtCu shells were deposited on PdCu seeds that were either the atomically random face-centered cubic phase (FCC A1, Fm3m) or the atomically ordered CsCl-like phase (B2, Pm3m). Precise control over crystallite size, particle shape, and composition allowed for comparison of these two core@shell PdCu@PtCu catalysts and the effects of the core phase on electrocatalytic durability. Indeed, the nanocatalyst with the intermetallic core saw only an 18% decrease in activity after stability testing (and minimal Cu leaching), whereas the nanocatalyst with the random alloy core saw a 58% decrease (and greater Cu leaching). The origin of this enhanced durability was probed by classical molecular dynamics simulations of model catalysts, with good agreement between model and experiment. Although many random alloy and intermetallic nanocatalysts have been evaluated, this study directly compares random alloy and intermetallic cores for electrocatalysis with the enhanced durability achieved with the intermetallic cores likely general to other core@shell nanocatalysts.
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