Revealing High-Temperature Reduction Dynamics of High-Entropy Alloy Nanoparticles via In Situ Transmission Electron Microscopy

Abstract

Understanding the behavior of high-entropy alloy (HEA) materials under hydrogen (H 2 ) environment is of utmost importance for their promising applications in structural materials, catalysis, and energy-related reactions. Herein, the reduction behavior of oxidized FeCoNiCuPt HEA nanoparticles (NPs) in atmospheric pressure H 2 environment was investigated by in situ gas-cell transmission electron microscopy (TEM). The reduction reaction front was maintained at the external surface of the oxide. During reduction, the oxide layer expanded and transformed into porous structures where oxidized Cu was fully reduced to Cu NPs while Fe, Co, and Ni remained in the oxidized form. In situ chemical analysis showed that the expansion of the oxide layer resulted from the outward diffusion flux of all transition metals (Fe, Co, Ni, Cu). Revealing the H 2 reduction behavior of HEA NPs facilitates the development of advanced multicomponent alloys for applications targeting H 2 formation and storage, catalytic hydrogenation, and corrosion removal.

High‐entropy alloy catalysts: From bulk to nano toward highly efficient carbon and nitrogen catalysis

“…So far, only a few studies are reporting the in situ study of HEA nanoparticles' behaviors under either their synthesis conditions or working environment. [136][137][138][139] In situ microscopic technique, like transmission electron microscopy (TEM), shows the ability to observe the process of catalyst formation at unprecedentedly high spatial resolutions. Using a Jouleheating method enabled by a TEM biasing sample holder, the formation process of Pt nanoparticles on a carbon nanofiber (CNF) substrate was observed by Huang et al 140 Before the heating procedure, H 2 PtCl 6 salt precursor was loaded to CNF and then transferred into the TEM chamber.…”

“…In addition to the oxidation, the same research group also studied the reduction behavior of oxidized HEA nanoparticles in an H 2 environment using a similar in situ gas TEM method at 400°C. 137 When H 2 penetrates the oxide layer at a constant temperature of 400°C, a porous structure is observed; the reaction front, however, remains mainly on the outer surface of the oxide (Figure 7D). Only copper oxide was seen to be fully reduced and further separated into copper nanoparticles, while Fe, Co, and Ni remain in their oxide phases.…”

Colloidal synthesis of nanoparticles: from bimetallic to high entropy alloys

“…Typical examples include quinary PtPdRhRuCe NPs as ammonia oxidation catalysts 9 or oxidized FeCoNiCuPt HEA for applications targeting H 2 formation and storage. 10 As a result, this spectrum of impressive properties makes this new broad class of structural materials attractive. [11][12][13][14][15][16][17][18] To take full advantage of their potential as structural and functional materials for future nanotechnologies, it is crucial to thoroughly understand the structure-property relationship in HEA-NPs.…”

Grand challenges in nuclear materials new insights into nuclear materials using ion irradiation, CryoEM, and in-situ liquid cell microscopy

“…The initial lamellar microstructure consisting of rod-like α-U precipitates embedded in a hexagonal δ-U-Zr matrix underwent spinodal decomposition at 600 °C to a body centered cubic form. High temperature gas phase TEM holders are being used to study the reaction of high-entropy alloys with hydrogen (Song et al, 2021). High-entropy alloys are being considered in advanced reactor designs owing to their potential radiation resistance.…”

“…High-entropy alloys are being considered in advanced reactor designs owing to their potential radiation resistance. Song et al (2021) used an in-situ gas-flow TEM holder to view the formation of a porous structure in a high-entropy alloy as hydrogen penetrated the material. Conroy et al (2017) used LC TEM to study the t dissolution of boehmite in water, as influenced by the electron beam, a phase found in alkaline nuclear waste sludge from the high temperature reaction of Al in solution.…”