NSRRC Activity Report 2023

Physics and Materials Science 015 H igh-entropy alloy (HEA) nanocrystals made up of five or more elements have shown tremendous potential for applications in electrocatalysis, photocatalysis, and thermocatalysis. 1,2 Notably, the multi-principal elemental feature of HEAs contribute to a high mixing entropy and aids in the formation of a solid–solution phase instead of an intermetallic phase or a phase-separated mixture. Current synthetic methods usually involve a trial-and-error approach to optimize synthetic protocols, mainly because experimental parameters are built upon qualitative observations and empirical laws. In a pioneering study carried out by Tung-Han Yang (National Tsing Hua University) and his collaborators to develop a rational synthesis method, it was shown that the structure of HEA nanocrystals consisting of five platinum-group metals (PGMs; Pd, Pt, Rh, Ir, and Ru) with different spatial compositions can be reliably controlled through quantitative understanding of the metal precursors’ reduction kinetics and entropy of mixing in the nanocrystals. 3 Figure 1 schematically shows four methods (A–D) used by the authors to control reduction kinetics and entropy of mixing, enabling the preparation of solid–solution phase HEA nanocrystals with different spatial compositions and surface atomic structures. Figure 1(a) shows a control experiment, where a one-pot synthesis was used to demonstrate the resulting phase- separated PdPtRhIrRu nanocrystals formed through sequential reduction reactions of the different precursors. In contrast, a dropwise synthesis method ( Figs. 1(b) and 1(c) ) allowed the researchers to control the number of atoms generated from the different precursors, resulting in PdPtRhIrRu nanocrystals as a typical solid–solution alloy phase. Next, by reducing the time between drops to make the atom deposition rate ( R dep ) greater than the surface diffusion rate ( R diff ), dendritic solid–solution PdPtRhIrRu nanocrystals were obtained. Finally, by influencing the redox potentials of metal precursor ions at low temperatures, the difference between the time for Pd to reach the steady state compared to the other four Pt, Ir, Rh, and Ru precursors increases, and the authors were able to produce Pd@PdPtRhIrRu core-shell nanocrystals ( Fig. 1(d) ). In all four cases, the researchers carried out a very systematic and extensive set of experiments to synthesize the products and characterized them using X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle dark field scanning Synthesis of High-Entropy Alloy Nanocrystals: A Paradigm Shift The reduction kinetics and entropy of mixing experiments investigating the dropwise addition of a five-metal (quinary) precursor provide a novel and reliable method to design high-entropy alloy (HEA) nanocrystals. Fig. 1 : Schematic illustrating four methods (a–d) to manipulate the formation of solid–solution HEA nanocrystals with different spatial compositions and surface atomic structures. [Reproduced from Ref. 3]