Bromide‐Mediated Reduction Kinetics and Oxidative Etching for Manipulating the Twin Structure and Facet of Pd Nanocrystals for Catalysis

Abstract

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.

Toward a Quantitative Understanding of Crystal-Phase Engineering of Ru Nanocrystals

“…We injected the Ru­(III) precursor into the reaction in one shot at 180 °C and then measured their concentrations with time through inductively coupled plasma optical emission spectrometry (ICP–OES). We further determined the rate constant k by the fitting using the pseudo-first-order kinetics since the concentration of reducing agent AA was in significant excess compared to the concentration of the Ru­(III) precursor. ,− The rate constant of Ru­(III) reduction was around 3.35 × 10 –3 min –1 . Under the values of n 0 = 1.045 × 10 –4 mmol and k = 3.35 × 10 –3 min –1 , we simulated the reaction kinetics with the different injection rates for the estimation of the instantaneous number of Ru­(III) precursors by eq .…”

“…In contrast, the reduction of the Ru­(III) precursor underwent slower reduction kinetics while decreasing the injection rates. In general, kinetics or thermodynamics in the early nucleation stage could determine the formation of nanocrystal seeds with a certain phase. ,− In the later growth stage, the tiny seeds could govern the growth of nanocrystals with the same phase as the seeds. Therefore, to realize the reduction kinetics of the Ru­(III) precursor on the phase formation, we plot the proportions of HCP/FCC phases as a function of the initial reduction rates in the nucleation stage, as shown in Figure D.…”

Anisotropic Growth of Silver Nanocubes: The Role of Bromide Adsorption and Hydrophilic Polymers

“…Shape-controlled synthesis of metal nanocrystals offers a pathway to regulate their optical, catalytic, and biological properties. − This approach typically involves the use of capping agents, which selectively bind to specific facets to influence the atomic deposition rate or surface energy, thereby yielding nanocrystals with different aspect ratios or bounded by distinct facets. − The unique ability of capping agents to drive anisotropic growth makes a deep understanding of their roles essential for synthesizing metal nanomaterials with the desired shapes and properties. Often, these roles are inferred from the shape of the product synthesized.…”

Quantitative Study on the Influence of Bromide Ions toward the Reduction Kinetics for Size-Tunable Palladium Nanocubes

“…Bromide ions have been reported as a capping agent toward the {100} facet of Pd nanocrystals, , and the size of Pd NCs can be tuned by changing the dosage of bromide ions . It is generally accepted that the concentration of bromide ions in the reaction system can impact the reduction kinetics of the Pd precursor, further controlling the outcome of a synthesis. , However, nearly 2 decades since the first time this perspective was proposed, there is still a lack of quantitative understanding or description of their kinetic characteristics under different reaction conditions. Earlier studies favor qualitative observations and often use qualitative terms, such as “slow” and “fast”, to describe the reduction process .…”