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.