ACS Nano: Ultra-high conductivity, ultra-high strength MXene fiber!

Two-dimensional (2D) nanomaterials have become the subject of intensive research in recent years due to their outstanding physical, chemical and electronic properties. The inherent anisotropy of two-dimensional nanomaterials makes it easy to assemble in a preferential direction and becomes a promising base material for building multilayer macrostructures such as films and fibers. However, structural defects (including microscopic voids, nanoscale boundaries, and local disordered arrangement of building units) can reduce the performance of components. Achieving a perfectly aligned, highly oriented, and defect-free assembly structure is essential for achieving ideal electrical conductivity and mechanical properties. Ti3C2Tx MXenes are composed of transition metal nitrides and carbides. Due to their electrical and thermal conductivity, mechanical and chemical properties, they have a wide range of potential applications (such as supercapacitors, sensors and transparent electrodes). The defect-free MXene structure can increase the conductivity, which can be achieved by controlling the fluid gradient by inducing shear (ie stretching) on the gel phase. However, the strength of the Ti3C2Tx MXenes gel network is not strong enough to withstand shear deformation, and due to the lack of strong interaction (such as chemical bonding) and the short length of its components, it is easily broken due to mechanical disturbance.
Based on this, Tae Hee Han of Hanyang University in South Korea developed a strategy to strengthen the MXene gel network to make it stable enough to withstand mechanical interference. Controlling pH can enhance the electrostatic interaction between MXene plates. MXene gel deforms stably in the direction of external shear stress, and the gel has sufficient mechanical strength to withstand the deformation. This enhanced gel network is very beneficial for making perfectly aligned fibers. During MXene solution wet spinning, the MXene sheet is assembled into gel fibers, which spontaneously transform into highly aligned fibers under the action of mechanical stretching force. Oriented MXene fibers have high electrical conductivity (12504 S cm –1) and strong Youngs modulus (122 GPa). This structural design method provides a general strategy for preparing high-performance fibers and controlling the molecular interaction between colloidal particles to induce a strong and deformable gel network. The result "Highly Electroconductive and Mechanically Strong Ti3C2Tx MXene Fibers Using a Deformable MXene Gel" was published in the journal ACS Nano. Synthetic MXene tablets and gelation of MXene
The author synthesized MXene flakes from MAX phase powder by selectively etching the Al layer, with an average lateral size of 3.99±2.62μm and a thickness of approximately 2.14 nm. There are C–Ti–Fx, C–Ti-(OH)x and C–Ti–Ox terminal groups on the surface of MXene, so Ti3C2Tx is negatively charged and exhibits hydrophilic properties. Its Zeta potential is related to pH, and there is a strong electrostatic repulsion between adjacent sheets. Controlling the electrostatic interaction can induce the bonding of MXene sheets. NH4+ ions interact with the MXene sheet, and the MXene dispersion forms a hydrogel through electrostatic interaction (Figure 1d). MXene gel exhibits a stable self-supporting structure (Figure 1E).

Figure 1. Synthetic MXene sheet and gelation of MXene. (A) SEM and (b) AFM images of MXene monoliths coated on SiO2 wafers using Langmuir-Blodgett technology. (C) TEM image and EDX element mapping of MXene single layer. (D) Gelation of MXene. (E) MXene gel image. The mechanical properties of MXene and its gel can enhance the MXene gel network by enhancing electrostatic interaction. The author synthesized MXene gel under different pH conditions (5 and 9), placed a 10 g weight on the gel, MG 5 collapsed, and MG 9 had no structural fracture, indicating the strong attraction between MXene tablets at high pH Enhance the structural stability of the gel (Figure a). MG9 is not only stable in structure, but also capable of deforming under external force. The internal structure can be well aligned through the shear stress generated by the extrusion of the plastic nozzle (Figure 2 b and c).

Figure 2. (a) MXene dispersion and images of MG 5 and MG 9. Each MXene gel coagulated in a pH 5 or 9 solution. (B) Image and SEM image of MG 9. (C) The image of the extruded MG 9 and the SEM image of the internal structure. MXene dispersions, MG 5 and MG 9 exhibited typical shear thinning behavior related to the orientation of the nanosheets under shear stress (Figure 3a). The viscosity of MXene gel gradually increases with the increase of the pH of the coagulation solution, which means that the network of MXene gel increases with the increase of pH. The G value of MG 5 is 2 orders of magnitude higher than that of MXene, and the G value of MG 9 is 2.5 times that of MG 5. The yield stresses of MXene dispersion, MG 5 and MG 9 were 1.6, 149.0, and 370.4 Pa, respectively (Figure 3C). It shows that the coagulation solution with high pH value will strengthen the structural network of MXene gel, which will not deform and break under high shear stress.



Figure 3. Rheological properties of MXene dispersion and gel. The (a) viscosity, (b) storage, loss modulus, and (h) frequency dependence of the Casson diagram of the MXene dispersion and each MG. MXene fiber preparation and research The author then puts the MXene dispersion into a syringe and extrudes it into a coagulation bath containing NH4Cl solution with pH values of 5 and 9 through a nozzle to prepare gel fibers by wet spinning. Small angle X-ray scattering (SAXS) shows that MXene fibers spun at a high draw ratio exhibit excellent orientation and a highly stacked layered structure. The axial performance of MXene fiber depends on the compactness and arrangement of MXene nanosheets in the fiber. The mechanical stretching process through external flow is an effective strategy for manufacturing fibers with excellent mechanical properties. The tightness and arrangement of MXene sheets in the fiber can affect their mechanical properties and electrical conductivity. Fibers with high draw ratios have higher Youngs modulus, tensile strength and conductivity (Figure 5a). Among them, the Youngs modulus of MF-3 with a stretch ratio of 3 is 122 GPa, the tensile strength is 344 MPa, and the electrical conductivity is as high as 12504 S cm-1 (Figure 5b). Compared with graphene and its composite materials, MXene fiber has better performance.

Figure 4. Morphological and structural parameters of MXene fibers. (A) SEM image and SAXS mode of MF-1, (b) MF-2 and (c) MF-3. (D) The change in the orientation of the micropores in the arc of the MXene fiber, (e) the diameter, (f) the density, and the porosity of the MXene fiber for each draw ratio.





Figure 5. (a) Tensile strength-strain curve and (b) conductivity of MF-1, MF-2 and MF-3. (C) The ultimate strength of MF and previously reported MXene and composite fibers. (D) The relationship between the Youngs modulus of MF and MXene, graphene and its composite materials on the conductivity. Conclusion The author synthesized a strong and deformable MXene gel by studying the molecular interactions in MXene nanomaterials and the assembly of MXene fibers with high electrical conductivity and mechanical strength. Due to the strong network of MXene gel, the MXene gel fibers can be mechanically stretched under the application of shear force during the wet spinning process, resulting in a highly aligned and dense multi-scale structure. MXene fiber is expected to be used to control white LED wires and keyboard signal transmission.