Novel Triatomic-Layer Borate Polyanion Terminations for MXenes

Two-dimensional (2D) transition metal carbides/nitrides, known as MXenes, have captured intensive attention owing to their promising applications in the areas of energy storage, (opto)electronics, environmental and future quantum technologies. Unlike other 2D materials, MXenes typically possess a layer of terminal groups capping the exposed surface metal atoms, which substantially influence the properties of MXenes. However, the terminations for hitherto developed MXenes are limited to monolayers or simple groups, showing disordered arrangements and inferior stability. The full potential of termination engineering in MXenes still calls for further investigation and exploration. Researchers from the group of Prof. Xinliang Feng and Dr. Minghao Yu, together with collaborators, show the synthesis of MXenes (Nb₂C and Ti₃C₂) with ordered triatomic-layer borate polyanion terminations through a flux-assisted eutectic molten etching approach. Lewis acidic salts in the eutectic molten state act as the etching agent to obtain the MXene backbone. Meanwhile, borax, as the flux, undergoes thermal decomposition to generate anionic BO₂⁻ species, which cap the MXene surface with an O-B-O configuration (denoted OBO-terminations). Contrasting with conventional Cl/O-terminated Nb₂C with strongly localized charge transport, OBO-terminated Nb₂C features band transport properties described by the Drude model. This transition in the conduction mechanism is attributed to the surface structure ordering enabled by OBO-terminations, which effectively mitigates charge carrier backscattering and trapping. Consequently, it results in a notable 15-fold enhancement in electrical conductivity and a tenfold improvement in charge mobility at the dc limit. Furthermore, triatomic-layer OBO-terminations provide substantially enriched Li⁺-hosting sites, enabling a high charge storage capacity for Ti₃C₂ MXenes (420 mAh g⁻¹), nearly double that of the Cl/O-terminated Ti₃C₂ (212 mAh g⁻¹). This work not only illustrate the potential for intricate termination configurations in MXenes, but also provide inspiration for further advancements in their potential applications like (opto)electronics and energy storage.

Reference: Dongqi Li, Wenhao Zheng, Sai Manoj Gali, Kamil Sobczak, Michal Horák, Josef Polčák, Nikolaj Lopatik, Zichao Li, Jiaxu Zhang, Davood Sabaghi, Shengqiang Zhou, Paweł P. Michałowski, Ehrenfried Zschech, Eike Brunner, Mikołaj Donten, Tomáš Šikola, Mischa Bonn, Hai I. Wang^*, David Beljonne^*, Minghao Yu^*. Xinliang Feng^*, MXenes with ordered triatomic-layer borate polyanion terminations, Nat. Mater. (2024). DOI: 10.1038/s41563-024-01911-2.

Acknowledgements: This work was financially supported by European Union’s Horizon 2020 research and innovation programme (GrapheneCore3 881603, LIGHT-CAP 101017821, GREENCAP 101091572), M-ERA.NET and Sächsisches Staatsministerium für Wissenschaft und Kunst (HYSUCAP 100478697), and German Research Foundation (DFG) within the Cluster of Excellence, CRC 1415 (Grant No. 417590517). TAČR EPSILON project (Nr. TH71020004), GAČR project (Nr. 23-07617S) and CzechNanoLab project (LM2023051) funded by MEYS CR are gratefully acknowledged for the financial support of the measurements in CEITEC Nano Research Infrastructure. The computational resources in Mons were supported by the FNRS “Consortium des Equipements de Calcul Intensif−CECI” program (Grant No. 2.5020.11) and by the Walloon Region (ZENOBE Tier-1 supercomputer, 1117545). D.L. was funded by China Scholarships Council (CSC). P.P.M. was supported by the National Science Centre (Project No. 2018/31/D/ST5/00399) and National Centre for Research and Development (Project No. LIDER/8/0055/L-12/20/NCBR/2021). The authors acknowledge the use of the facilities in the Dresden Center for Nanoanalysis (DCN) at Technische Universität Dresden, the GWK support for providing computing time through the Center for Information Services and High-Performance Computing (ZIH) at TU Dresden, and beam time allocation at beamline P65 at the PETRA III synchrotron (DESY, Hamburg, Germany) and beamline BL04 at ALBA synchrotron (Barcelona, Spain).