Ultrasmall high‐entropy alloy nanoparticles with 1 nm size by continuous‐flow reactor
Li Li, Zhicheng Zhang
Abstract
High-entropy alloys (HEAs) have been widely applied in the field of catalysis due to their unique physicochemical properties. Nevertheless, it still remains a big challenge to prepare HEA nanoparticles (NPs) with ultrasmall particle size and uniform size distribution. Recently, Kohei Kusada and Hiroshi Kitagawa's team reported a continuous-flow reactor-based liquid-phase reduction method for the preparation of ultrafine homogeneous HEA NPs, providing an effective strategy for the rational construction of high-efficiency HEA catalysts. HEAs have a wide range of application prospects in the field of catalysis due to their unique characteristics, including a rich composition of elements (typically composed of at least five elements) and adjustable element ratios (each element content ranging from 5 to 35 at%).1, 2 During the alloying reaction, large differences in the chemical and physical properties of the mixed elements lead to immiscibility (phase separation or element separation). It is extremely necessary to develop a synthetic strategy with high tolerance to synthetic conditions and a wide selection of elements. Currently, the preparation methods of HEAs mainly include the carbon thermal shock method, rapid bed pyrolysis method, solvothermal method, laser scanning ablation, and other methods.3-7 However, these methods are difficult to synthesize small-sized HEA NPs, thereby greatly limiting their widespread application in the field of catalysis.8, 9 As is well known, small-sized HEA catalysts have a larger specific surface area, which can provide more active sites for catalytic reactions and thus show more excellent performance.10-13 Therefore, it is crucial to develop new synthetic methods that can precisely control the size of HEA NPs. Recently, the team of Kohei Kusada and Hiroshi Kitagawa from Kyoto University successfully synthesized ultrasmall IrPdPtRhRu HEA NPs with an average size of 1.32 ± 0.41 nm using a self-developed continuous flow reactor under the action of a strong reducing agent, and verified that the five elements were evenly distributed in the ultrafine NPs with face-centered cubic (fcc) structure by scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDX), and powder X-ray diffraction (PXRD).14 Excitingly, the IrPdPtRhRu HEA NPs exhibited excellent electrocatalytic activity toward hydrogen evolution reaction (HER) under acidic conditions. Significantly, the overpotential at a current density of 10 mA/cm2 over the IrPdPtRhRu HEA NPs is only 6 mV, which is 1/3 that of commercial Pt/C. The developed continuous-flow reactor, as shown in Figure 1A, enables uniform mixing of metal precursors (such as metal acetylacetonate) with strong reducing agents (lithium naphthalenide) via diaphragm pumps and a stainless-steel tube. It is worth noting that metal lithium and naphthalene were mixed in deoxytetrahydrofuran in advance and stirred in a glove box for 1 day to prepare a strong reducing agent of lithium naphthalate.15 Specifically, accompanied by the presence of deoxidized tetrahydrofuran, the metal precursors (Solution 1) and reducing agent (Solution 2) flowed at 21.0 mL/min for approximately 15 s to converge at the collection point. Finally, under the protection of argon, the black solution obtained from the outlet of the reactor was collected in a flask containing a carbon support, and HEA NPs were collected by centrifugation and drying at room temperature, and stored in hydrogen. It is worth mentioning that the continuous-flow reactor-assisted synthesis in this work can effectively improve productivity and reproducibility, which is more suitable for the preparation of HEA NPs in industrialized large-scale production. Currently, the verification of HEAs typically involves assessing the uniformity of element distribution using STEM and confirming the phase structure through XRD. In this work, the high-angle annular dark-field STEM (Figure 1B) indicates that the particle size of HEA NPs is ultrasmall, and each element of Ir, Pd, Pt, Rh, and Ru is evenly distributed from the corresponding EDX maps, indicating the successful preparation of HEA NPs. Furthermore, the EDX result demonstrates that the proportion of each element was about 20%, meaning the nearly equal to the atomic ratio distribution. As shown in Figure 1C, PXRD was carried out to determine the structure of the HEA NPs. The XRD pattern was analyzed by the Rietveld refinement method, which showed that the 1.32 nm HEA NPs of IrPdPtRhRu exhibited a fcc structure with a lattice constant of 3.8843(8) Å. It is worth emphasizing that the achievement of ultrasmall size and uniform structure in HEA NPs can be attributed to the utilization of a continuous flow reactor and a potent reducing agent, such as lithium naphthalenide, which could accelerate the simultaneous reduction and nucleation of all precursors at room temperature, enabling the production of ultrasmall NPs with uniform size. The ultrasmall-sized HEA NPs could facilitate maximum exposure of active sites, while their multicomponent nature optimizes the synergy between these sites. This leads to effective optimization of the electronic structure of alloy materials, ultimately reducing barriers in electrocatalytic processes, and thereby improving catalytic performance. In conclusion, this study introduces a novel liquid-phase reduction method using a continuous-flow reactor and a potent reducing agent, lithium naphthalenide, to successfully prepare ultrasmall HEA NPs (IrPdPtRhRu) with an average size of 1.32 ± 0.41 nm, which exhibited superior performance in the HER compared to commercial Pt/C with a remarkably small overpotential of only 6 mV at 10 mA/cm2 under acidic conditions. It is believed that the developed continuous-flow reactor would offer a promising platform for the efficient synthesis of ultrasmall HEA NPs with practical application value. It is noteworthy that most HEA catalysts are particles at present, so it is necessary to develop efficient methods to prepare HEA materials with different morphologies and phase structures. In summary, HEA nanomaterials hold great promise as new catalytic materials, warranting further exploration and investigation by researchers. This work was supported by the National Natural Science Foundation of China (22071172). The authors declare no conflicts of interest.