High-entropy alloy (HEA) nanocrystals have attracted extensive attention in catalysis. However, there are no effective strategies for synthesizing them in a controllable and predictable manner. With quinary HEA nanocrystals made of platinum-group metals as an example, we demonstrate that their structures with spatial compositions can be predicted by quantitatively knowing the reduction kinetics of metal precursors and entropy of mixing in the nanocrystals under dropwise addition of the mixing five–metal precursor solution. The time to reach a steady state for each precursor plays a pivotal role in determining the structures of HEA nanocrystals with homogeneous alloy and core-shell features. Compared to the commercial platinum/carbon and phase-separated counterparts, the dendritic HEA nanocrystals with a defect-rich surface show substantial enhancement in catalytic activity and durability toward both hydrogen evolution and oxidation. This quantitative study will lead to a paradigm shift in the design of HEA nanocrystals, pushing away from the trial-and-error approach.

The crystal phase with a specific stacking sequence of atoms largely affects the catalytic performance of metal nanocrystals. Since the control of the phase at the same composition is extremely difficult, the phase-dependent performance of metal nanocrystals is studied rarely. Here, we show the synthesis of Ru nanocrystals with different percentages of face-centered cubic (FCC) and hexagonal close-packed (HCP) phases via kinetic control, further revealing a quantitative correlation between the phase percentage of Ru nanocrystals and the initial reduction rate of Ru(III) precursors. Specifically, we manipulate the single parameter-initial reduction rate by controlling the Ru(III) injection rate into the dropwise synthesis at a fixed reaction temperature and correlate the kinetic data with the Ru phase percentage analyzed by atomic-resolution electron microscopy and synchrotron X-ray scattering. Based on the quantitative analysis, the ranges of initial reduction rates of Ru precursors can be determined for synthesizing Ru nanocrystals with the percentages of unusual FCC phase from 9.0 to 55.1%. We demonstrate that a low initial reduction rate corresponds to the crystallization of the Ru HCP phase, while a high initial reduction rate favors the crystallization of the FCC lattice. Furthermore, we also systematically examine the catalytic performance of Ru nanocrystals with different phases.

With Pd as an example, a set of quantitative analyses is designed to shed light on the bromide‐mediated reduction kinetics and oxidative etching in determining the twin structure and facet of Pd nanocrystals. The success of this work relies on the kinetic measurements of Pd(II) precursor reduction and the close examinations of resultant Pd seeds and nanocrystals at different stages of synthesis. We observe there is a clear trend where low, moderate, and high initial Pd(II) reduction rates regulated by Br− ions correspond to the formation of Pd seeds with singly‐twinned, multiply‐twinned, and single‐crystal structures in the nucleation stage, respectively. Our quantitative analyses also suggest the oxidative etching induced by oxygen/Br− pair can selectively remove the multiply‐twinned Pd seeds from the products in the growth stage while leaving behind singly‐twinned or single‐crystal Pd seeds for the evolution into Pd nanocrystals with well‐defined facets in high purity. The mechanistic insights obtained in this work can be extended to the synthesis of Pd@Pd−Pt core−shell nanocubes with high‐index facets, which can be used as excellent electrocatalysts and photocatalysts for hydrogen generation.

High-entropy-alloy (HEA) nanocrystals consisting of a minimum of five elements have recently emerged as a versatile family of catalysts due to immense chemical space and tunability1-3. However, there are no effective strategies for synthesizing libraries of HEA nanocrystals with controlled surface atomic structures of exposed facets for boosting catalytic performance4-19. Due to the distinct nucleation and growth kinetics of constituent metals and their distinctive crystal structures, it is incredibly challenging to confine five or more different metal species situated on the nanocrystal surface with a specific arrangement but also in a high-entropy random mixing state. Here we present a straightforward strategy to craft a library of facet-controlled HEA nanocrystals with up to ten dissimilar metallic elements (Pt, Pd, Ir, Ru, Rh, Os, Au, Fe, Co, and Ni) by solution-phase layer-by-layer epitaxial growth, enabling the design of 638 distinct catalysts with 5 to 10 elements in equimolar ratios. The subnanometer-thick solid-solution HEA atomic layers can be deposited epitaxially on nanocrystal seeds with {100} and {111} facets, thus achieving HEA shells with square and hexagonal atomic arrangements, respectively. The hollow HEA nanocages with ultrathin walls along a specific direction can be further fabricated via post-synthetic chemical etching. Three facet-dependent catalytic actives of HEA nanocrystals are discovered in electrocatalysis and photocatalysis, and their catalytic facets with real active sites are also identified by in situ synchrotron X-ray absorption spectroscopy and density-functional theory calculations. Our work enables facet engineering in the multi-elemental space and unveils the critical needs for their future development toward catalysis.