Covalent Surface Alteration of MXenes and Its Effect on Superconductivity

Surface functional groups such as F, O, and OH attached to MXenes have a profound impact on the electronic properties. Generally, MXenes are synthesized by the aqueous acidic etching of the MAX phase, and these functional groups are inherently incorporated during the synthesis. Recently, covalent surface modification of MXene has been demonstrated at low temperature using molten inorganic salts. Surface functional groups such as F, O, and OH attached to MXenes have a profound impact on the electronic properties. Generally, MXenes are synthesized by the aqueous acidic etching of the MAX phase, and these functional groups are inherently incorporated during the synthesis. Recently, covalent surface modification of MXene has been demonstrated at low temperature using molten inorganic salts. MXene is a promising two-dimensional (2D) inorganic compound consisting of hexagonal layered ternary transition metal carbides, carbonitrides, and nitrides. MXenes are synthesized by chemically etching the Mn+1AXn (MAX) phase in which M represents an early transition metal (e.g., Ti, V, Cr, Nb, etc.), A represents a group of elements (e.g., Al, Si, Sn, In, etc.), X represents carbon and/or nitrogen, and n = 1, 2, or 3 and the resulting MXenes are represented by Mn+1XnTx, where Tx is surface terminal groups.1Barsoum M.W. The MN+1AXN phases: A new class of solids.Progress in Solid State Chemistry. 2000; 28: 201-281Crossref Scopus (2831) Google Scholar,2Naguib M. Unocic R.R. Armstrong B.L. Nanda J. Large-scale delamination of multi-layers transition metal carbides and carbonitrides “MXenes”.Dalton Trans. 2015; 44: 9353-9358Crossref PubMed Google Scholar These materials were first discovered by Naguib, Barsoum, and coworkers in 2011.3Naguib M. Kurtoglu M. Presser V. Lu J. Niu J. Heon M. Hultman L. Gogotsi Y. Barsoum M.W. Two-dimensional nanocrystals produced by exfoliation of Ti3 AlC2.Adv. Mater. 2011; 23: 4248-4253Crossref PubMed Scopus (5573) Google Scholar There are several 2D materials like graphene4El-Kady M.F. Shao Y. Kaner R.B. Graphene for batteries, supercapacitors and beyond.Nat. Rev. Mater. 2016; 1: 16033Crossref Scopus (728) Google Scholar and perovskites5Leng K. Fu W. Liu Y. Chhowalla M. Loh K.P. From bulk to molecularly thin hybrid perovskites.Nat. Rev. Mater. 2020; 5: 482-500Crossref Scopus (107) Google Scholar,6Jana A. Ba Q. Kim K.S. Compositional and Dimensional Control of 2D and Quasi-2D Lead Halide Perovskites in Water.Adv. Funct. Mater. 2019; 29: 1900966Crossref Scopus (21) Google Scholar that have unique properties, but MXenes are the new members of the family with a wide range of chemistry. These materials have found profound applications in various fields such as energy storage applications, water purification, gas sensors, transparent conducting electrodes, batteries, supercapacitors, catalysts, electronics, optics, and many more.7Anasori B. Lukatskaya M.R. Gogotsi Y. 2D metal carbides and nitrides (MXenes) for energy storage.Nat. Rev. Mater. 2017; 2 (16098)Crossref Scopus (3807) Google Scholar,8Chaudhari N.K. Jin H. Kim B. San Baek D. Joo S.H. Lee K. MXene: An emerging two-dimensional material for future energy conversion and storage applications.J. Mater. Chem. A. 2017; 5: 24564-24579Crossref Google Scholar Generally, the MXenes are prepared by selective etching of MAX phases using aqueous HF solution. Recently, electrochemical9Pang S.-Y. Wong Y.-T. Yuan S. Liu Y. Tsang M.-K. Yang Z. Huang H. Wong W.-T. Hao J. Universal Strategy for HF-Free Facile and Rapid Synthesis of Two-dimensional MXenes as Multifunctional Energy Materials.J. Am. Chem. Soc. 2019; 141: 9610-9616Crossref PubMed Scopus (249) Google Scholar and hydrothermal methods10Li T. Yao L. Liu Q. Gu J. Luo R. Li J. Yan X. Wang W. Liu P. Chen B. et al.Fluorine-Free Synthesis of High-Purity Ti3 C2 Tx (T=OH, O) via Alkali Treatment.Angew. Chem. Int. Ed. Engl. 2018; 57: 6115-6119Crossref PubMed Scopus (472) Google Scholar are also used for etching MAX phases. However, a mixture of fluorine (F), oxygen (O), and hydroxide (OH) groups are inherently introduced at the surface of synthesized MXene. Using inorganic molten salts such as ZnCl2, a series of Zn-based MAX phases and Cl−-terminated MXenes were reported by Huang and co-workers.11Li M. Lu J. Luo K. Li Y. Chang K. Chen K. Zhou J. Rosen J. Hultman L. Eklund P. et al.Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes.J. Am. Chem. Soc. 2019; 141: 4730-4737Crossref PubMed Scopus (447) Google Scholar The MXene surface exchange reaction occurs at very high temperatures (300°C to 600°C), which is a cumbersome process to achieve through traditional solvents, so it remains a daunting task to systematically modify the surface of MXene. Recently, Talapin et al.12Kamysbayev V. Filatov A.S. Hu H. Rui X. Lagunas F. Wang D. Klie R.F. Talapin D.V. Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes.Science. 2020; 369: 979-983Crossref PubMed Scopus (438) Google Scholar pioneered a general synthetic methodology to install and remove surface functional groups using molten inorganic salts to synthesize O, imidogen (NH), sulfur (S), chlorine (Cl), selenium (Se), bromine (Br), and tellurium (Te) surface-terminated MXenes as well as a bare one (Figure 1).12Kamysbayev V. Filatov A.S. Hu H. Rui X. Lagunas F. Wang D. Klie R.F. Talapin D.V. Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes.Science. 2020; 369: 979-983Crossref PubMed Scopus (438) Google Scholar Their synthetic method is highly promising as different types of MXenes can be synthesized without the use of aqueous solution. The team has also explored that surface groups play a pivotal role in controlling the interatomic distances in the MXene lattice and that the superconductivity is largely governed by the surface groups. The authors synthesize dihalide-terminated Ti3C2X2, Ti2CX2 and Nb2CX2 MXenes in Lewis acidic CdX2 molten salt (X = Cl/Br) without any undesired oxidation and hydrolysis. This method is highly efficient for tuning the surface chemistry, structure, as well as properties of these materials. The halide-terminated surface of these materials is labile because of the weak M-Br and M-Cl bonds making them facile for exchange reactions at relatively low temperatures. The surface exchange reactions of MXenes were performed in molten alkali metal halide salts, e.g., LiCl/KCl and CsBr/KBr/LiBr, which were used as solvents. The exchange reactions of Ti3C2Cl2 and Ti3C2Br2 with Li2Se, Li2O, and NaNH2 yielded Ti3C2Se, Ti3C2O, and Ti3C2(NH) MXenes, respectively, (Figure 1B) and its further reaction with n-BuLi gave intercalated sheets with negative surface charge. Interestingly, MXene 2D sheets remained intact during all stages of the transformation. The reactions of Ti3C2Br2 and Ti2CBr2 with LiH at 300°C produced bare Ti3C2◌2 where ◌ stands for the vacancy site. It undergoes reductive elimination of hydride followed by exchange reaction. The surface reactions of MXenes are expected to be difficult due to the larger size of entering ions than the leaving ones. Surprisingly, comparable exchange reaction rates were observed during the exchange reactions of Ti3C2Cl2 and Ti3C2Br2 with O2−, S2−, Se2−, and Te2− at the same temperature. In molten states, MXene sheets are unstacked, and this process greatly accelerated the diffusion of ions allowing the MXene surfaces to be sterically accessible. MXene structure is strongly affected by the nature of the surface groups. For Ti2CBr2, “a” lattice constant was equal to 3.32 Å, whereas it changed to 3.01 Å upon exchanging Br− with O2− and the reaction with Te2− produced MXene with “a” = 3.62 Å. This is attributed to the van der Waals radii and packing density of surface atoms. Cl- and Br-terminated MXenes showed larger “a” than the S- and Se-terminated MXenes, respectively. This is due to the fact that mononionic halides were replaced by dichalcogenide ions for balancing the charges at surface of MXenes. Bare (◌) and NH-terminated MXenes were nearly strain-free, whereas Cl-, S-, Se-, and Br-terminated MXenes all had strain. The team also extended the synthetic procedure to prepare the Nb2CCl2 MXene from the Nb2AlC MAX phase and then performed the surface exchange reaction to synthesize Nb2CTn (T = ◌, Cl, O, S, Se) (Figure 1C). The surface groups, which affects the biaxial lattice strain, phonon frequencies, and the strength of the electron-phonon coupling, play a pivotal role in tuning the electronic properties of these MXenes. Both the Nb2AlC and Nb2CCl2 displayed comparable resistivity above 30 K, which decreased upon cooling the sample (Figure 1Di). This is attributed to the metallic conductivity of MXenes associated with non-zero electronic states at the Fermi energy level. However, when the Nb2CCl2 MXene was cooled below 30 K, the resistivity began to increase due to the onset of localization. At a critical temperature (Tc) ∼6.0 K, a rapid decrease in resistivity of many orders of magnitude was observed, indicating the superconductivity of the Nb2CCl2 MXene. Compared to the Nb2CCl2 MXene, the chalcogenide ions (O, S, Se)-terminated MXenes showed a gradual increase in resistivity upon cooling the sample. In the low-temperature region, superconducting transitions were observed for Nb2CS2 (Tc ∼6.4 K), Nb2CSe (Tc ∼4.5 K), and Nb2C(NH) (Tc ∼7.1 K), whereas Nb2COx did not exhibit the superconducting state (Figure 1Dii). The current synthetic method developed by Talapin et al.12Kamysbayev V. Filatov A.S. Hu H. Rui X. Lagunas F. Wang D. Klie R.F. Talapin D.V. Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes.Science. 2020; 369: 979-983Crossref PubMed Scopus (438) Google Scholar offers an exciting opportunity to modify the surface of MXenes in solid states. Like molecular compounds, chemical bonds could be rationally designed within an expanded MXene stack. The combination of etching and substitution reactions using Lewis acidic and basic molten salts would allow us to synthesize desired MXene structures with better electronic properties.