Self-assembly of two-dimensional MXene sheets is used in various fields to create multiscale structures due to their electrical, mechanical, and chemical properties. In principle, MXene nanosheets are assembled by molecular interactions, including hydrogen bonds, electrostatic interactions, and van der Waals forces. This study describes how MXene colloid nanosheets can form self-supporting MXene hydrogels. Three-dimensional network structures of MXene gels are strengthened by reinforced electrostatic interactions between nanosheets. Stable gel networks are beneficial for fabricating highly aligned fibers because MXene gel can endure structural deformation. During wet spinning of highly concentrated MXene colloids in a coagulation bath, MXene sheets can be transformed into perfectly aligned fibers under a mechanical drawing force. Oriented MXene fibers exhibit a 1.5-fold increase in electrical conductivity (12 504 S cm–1) and Young’s modulus (122 GPa) compared with other fibers. The oriented MXene fibers are expected to have widespread applications, including electrical wiring and signal transmission.

Surface modification to improve the oxidation stability and dispersibility of MXene in diverse organic media is a facile strategy for broadening its application. Among the various ligands that can be grafted on the MXene surface, oleylamine (OAm), with amine functionalities, is an advantageous candidate owing to its strong interactions and commercial viability. OAms are grafted onto MXene through covalent bonds induced by nucleophilic reactions and H bonds in liquid interface reactions at room temperature. In addition, this grafting behavior of the ligand was characterized by a reduction in the slope with an increase in the ligand concentration (C l), confirming that the OAms were grafted via Langmuir-like behavior, and the monolayer of OAms was developed via two distinct steps (I: lying-down phase; II: ordered monolayer). MXene nanosheets modified by OAm (OAm-MX) are highly dispersible in a wide range of organic solvents owing to the alkyl chain of the OAms, which induces hydrophobic properties on the surface of MXene. The OAm-MX dispersion exhibits outstanding oxidation and dispersion stability and remarkable coating performance on a wide range of substrates owing to their excellent solution processability. Therefore, this study provides fundamental insights into the adsorption behavior and interaction between amine ligands and MXene nanosheets for the surface chemistry of MXene.

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Controlling the microstructures in fibers, such as crystalline structures and microvoids, is a crucial challenge for the development of mechanically strong graphene fibers (GFs). To date, although GFs graphitized at high temperatures have exhibited high tensile strength, GFs still have limited the ultimate mechanical strength owing to the presence due to the structural defects, including the imperfect alignment of graphitic crystallites and the presence of microsized voids. In this study, we significantly enhanced the mechanical strength of GF by controlling microstructures of fibers. GF was hybridized by incorporating polyacrylonitrile (PAN) in the graphene oxide (GO) dope solution. In addition, we controlled the orientation of the inner structure by applying a tensile force at 800 °C. The results suggest that PAN can act as a binder for graphene sheets and can facilitate the rearrangement of the fiber’s microstructure. PAN was directionally carbonized between graphene sheets due to the catalytic effect of graphene. The resulting hybrid GFs successfully displayed a high strength of 1.10 GPa without undergoing graphitization at extremely high temperatures. We believe that controlling the alignment of nanoassembled structure is an efficient strategy for achieving the inherent performance characteristics of graphene at the level of multidimensional structures including films and fibers.

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