As a member of the MBene family, 2D MoB MBene has attracted much attention due to its fascinating properties, and is expected to be widely used in electrochemistry, energy storage, and catalysis fields. So far, work on the synthesis of 2D MoB nanosheets by acid etching or dealloying strategy of MoAlB precursor has not been very successful. Acid etching leads to the simultaneous dissolution of Mo and Al ions in MoAlB in acid solution, making it impossible to synthesize 2D MoB. Dealloying strategy at high temperatures transforms MoAlB into MoB particles rather than 2D MoB nanosheets. A study reported that two MoB nanosheets were partially delaminated from the etched Al region of Mo2AlB2 after etching a MoAlB single crystal in NaOH solution, and suggested that Mo2AlB2 is an ideal precursor for MoB MBene compared to MoAlB. However, Mo2AlB2 is a metastable compound that is difficult to synthesize at high temperatures with conventional solid-state synthesis methods. Therefore, it is imperative to explore a fast and efficient route for the synthesis of Mo2AlB2 and MoB MBene.
Recently, a research team led by Shibo Li from Beijing Jiaotong University, China, first reported the topochemical synthesis of Mo2AlB2 and 2D MoB nanosheets by deintercalating Al from MoAlB through a ZnCl2 molten salt etching method. The influence of etching temperature, etching time, and starting mixtures on the formation of the final products have been investigated. This work demonstrates that molten salt etching is an effective and safe method for preparing Mo2AlB2 and 2D MoB MBene.
The team published their work in Journal of Advanced Ceramics on April 11, 2023.
"In this report, we synthesized phase-pure Mo2AlB2 powder by the deintercalation of Al from MoAlB through a ZnCl2 molten salt etching approach below 600 °C. The experimental result was in good agreement with Mo2AlB2 theoretical prediction. SEM images showed that Mo2AlB2 has a typical layered structure with a few etching cavities, and TEM identified the good crystallinity of Mo2AlB2 with orthorhombic lattice," said Shibo Li, professor at the School of Mechanical, Electronic and Control Engineering at Beijing Jiaotong University (China), a senior expert whose research interests focus on the field of MAX and MAB phases and corresponding 2D MXene and MBene nanomaterials.
"When increase to 700 °C, MoB MBene was obtained. The formed MoB has a sheet structure with a preferred (112) plane, which was further identified by SEM. However, XPS analysis shows that no -Cl surface group terminates on the Mo-B layer." said Shibo Li.
The LiF-HCl acid solution was also chosen to etch the Mo2AlB2 precursor, and MoB formed gradually with increasing temperature and time. However, there is still residual Mo2AlB2 phase even after etching at 40 °C for 42 hours, indicating an inefficient process for MoB MBene synthesis. "The molten salt method is preferred to synthesize 2D MoB MBene because it is a safe and more efficient approach when compared to acid etching." Shibo Li said.
Upon salt etching of MoAlB at temperatures below 600 °C, the removal of half of Al atoms in MoAlB with Al bilayer resulted in the formation of Mo2AlB2 with Al monolayer. As the temperature was increased to 600 ℃, the energy is enough to trigger the deintercalation of Al from Mo2AlB2 by ZnCl2 etching. The structure of Mo-B slabs was rearranged to form 2D MoB MBene when all Al atoms were removed. However, these nanosheets are more likely to agglomerate at high temperatures due to the lack of -Cl terminations, forming a stacked structure. "2D MBene was first obtained by molten salt etching, which promotes the fundamental research of MBene." said Shibo Li.
As the novel nanomaterials, more research is still needed to explore the properties of Mo2AlB2 and MoB MBene. Li suggests that energy storage, microwave absorption, and catalysis properties are the main future research directions.
Other contributors include Junji Mou, Weiwei Zhang, Weimin Xu from the School of Mechanical, Electronic and Control Engineering at Beijing Jiaotong University in Beijing, China; Shukai Fan, Guoping Bei from the Porcelain Fuchi High Tech Nano Materials Co., Ltd in Suzhou, China.
This work was supported by the Fundamental Research Funds for the Central Universities (Grant No. 2022YJS088) and the National Natural Science Foundation of China (Grant No. 52275171).
About Author
Shibo Li, PhD, Professor, is now working at Beijing Jiaotong University. His research interests focus on the synthesis and characterization of ternary layered MAX and MAB phases, 2D MXenes and MBenes, crack healing of advanced ceramics, radar/infrared integrated stealth materials, and applications of advanced materials in high seed railway and high temperature coatings.
Li has published over 170 academic papers in journals such as Carbon Energy, ACS Appl Material Inter, J Adv Ceramic, J Euro Ceramic Soc, and J Am Ceramic Soc, and obtained more than 20 patents.
He serves as vice editor-in-chief of the International Journal of Applied Ceramic Technology, and the editorial board member of Coating.
About Journal of Advanced Ceramics
Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen. JAC's 2023 IF is 18.6, ranking in Top 1 (1/31, Q1) among all journals in "Materials Science, Ceramics" category, and its 2023 CiteScore is 21.0 (top 5%) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508
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