Research Background

Two-dimensional nanosheets with attractive properties are the cornerstone of potential applications. Compared with bulk materials, two-dimensional nanomaterials are easy to assemble and are used for nanostructures with attractive electronic, chemical, physical and mechanical properties, high specific surface area and multi-purpose surface chemistry. So far, various two-dimensional materials such as graphene, hexagonal boron nitride (h-BN), graphite carbonitrides (g-C3N4), transition metal dihydroxy compounds (TMDs), black phosphorus (BP) and transition metals Oxides (TMOs) have attracted widespread attention and have proposed many strategies for developing them into macrostructures. For example, the development of macroscopic one-dimensional (1D) carbon-based fibers made from graphene oxide (GO) has made significant progress. Graphene fibers have received widespread attention because of their light weight, mechanical flexibility, bendability, stretchability, and versatility that can be woven into textiles for next-generation smart electronic devices. In particular, in order to realize the macro assembly of two-dimensional nanosheets into a fiber structure, the wet spinning process uses the phase transformation ability of a high-concentration colloidal dispersion (ie, liquid) to convert it into gel fiber components and solid fibers in a coagulation bath. It is a versatile way for continuous mass production of fibers for a long time. It is worth noting that understanding the molecular interactions between the flakes and systematically studying the parameters of the coagulation process are crucial for obtaining fibers from individual colloidal particles.

Ti3C2Tx-MXene is a two-dimensional material composed of transition metal nitrides and carbides (MXenes). Due to its excellent electrical conductivity, thermal conductivity, mechanical and chemical properties, and wide application prospects, it has been widely used as an emerging two-dimensional The material family explored. MXenes has the general structure of Mn+1XnTx, where M, X, and T represent transition metal, carbon/nitrogen, and surface terminal functional groups, such as O, F, and OH, respectively.

Article Introduction

Key points

Figure 1. Synthetic characterization diagram. The reconstructed as single MXene schematic MXene fiber points a:

This paper reports a direct and reliable synthetic route for the continuous control of wet spinning components to produce highly conductive, additive-free/binder-free, composite-free, and completely pure 1D MXene fibers (Figure 1) . The prepared MXene fiber has ultra-high electrical conductivity, high flexibility and excellent mechanical properties. The wet spinning strategy reported in this article provides a method for the continuous mass production of MXene fibers, which shows that MXene fibers are expected to be candidates for high-performance, flexible, portable, and wear-resistant electronic products. The use of scalable components to develop nanoscale features at the macro level represents the progress of these extraordinary two-dimensional materials in practical applications.

Figure 2. Synthesis and identification of Ti3C2Tx (MXene). a) MAX phase particles and b) MXene single-layer coating morphology on SiO2 wafer. C) AFM and d) C-AFM images of a single MXene in the same area. e) c) The height of the center lines #1 and #2 and the current line outline. Point two:

The largest phase (Ti3AlC2) powder with a graphite-like stacked layer structure was observed in the image obtained by scanning electron microscope (SEM) (Figure 2a and Supplementary Figure 1a). MXene (Ti3C2Tx) flakes are obtained by selectively etching Al from Ti3AlC2 powder with LiF and HCl.

The SEM image of the MXene single layer completely peeled off showed an average lateral dimension of 2.26 ± 0.95 μm (Figure 2b and supplementary Figures 1b, c). The height profile obtained by atomic force microscopy (AFM) mapping shows that the height of the MXene sheet is 1.35–1.81 nm, which corresponds to the single layer of MXene, which means that the flaking of the sheet is successful (Supplementary Figures 1d and e).

According to the AFM height profile, the folded MXene is a double layer with a height of 3.31–3.72 nm. Conductive atomic force microscopy (C-AFM) clearly shows that MXene has strong conductivity (Figure 2d, e).

Figure 3. Dispersibility and spinnability of MXene nanosheets in liquid crystal dispersions.

X-ray photoelectron spectroscopy (XPS) was used to further study the chemical function of the MXene peeling sheet. The deconvoluted C1s, O1s, and Ti2p XPS peaks indicate that there are inherent end groups on the MXene surface, such as C–Ti–Tx, C–Ti–(OH)x, and C–Ti–Ox, and most likely in MAX crystals Introduced during the aluminum etching process (Figure 3a, b, supplementary Figure 2a–C) . It is worth noting that these surface functionalities are very important for the formation of stable dispersions in aqueous media.

The negative surface charge value increases as the pH of MXene increases (Figure 3c), which is due to the ionization of the surface terminal groups, which indicates that there is a strong electrostatic repulsion between adjacent sheets.

The apparent dispersion of MXene was observed in different concentration ranges (Figure 3d). At different MXene concentrations, no sediment was formed at the bottom of the vial. At high concentration (25 mg mL-1), the MXene dispersion forms a viscous ink with a viscosity of 3.87 × 103 Pa s, and there is no aggregation and phase separation of solid particles and dispersion media (Figure 3e).

Studies have shown that MXene tablets have lyotropic liquid crystal properties in the range of ~16 mg mL-1. As shown in Figure 3f, the MXene dispersion (25 mg mL−1) also exhibits birefringence between the two crossed polarizers, which indicates that the liquid crystal phase is formed due to the lack of local alignment of aggregation.

As is often observed in complex fluid systems containing rigid polymer chains, the viscosity of MXene increases with increasing concentration and decreases with increasing shear rate (Figure 3g). In addition, the shear stress of the MXene dispersion decreased significantly at the initial stage, and then gradually increased with the shear rate (Figure 3h), which indicates that the randomly oriented MXene sheets became aligned due to the shear-induced deformation. The reduction in shear stress is evident in the concentrated dispersion (above 15 mg mL−1).

Use the G‘/G‘‘ value of MXene to predict. Spinnability of MXene dispersion. Experimentally, at 5 mg mL-1, due to the weak gel strength, the MXene dispersion cannot form fibers, and the value of G′/G‘‘ is 13.33 (Supplementary Figure 4). When the G′/G‘‘ value of MXene is 6.64 at 12 mg mL−1, fibers are unstablely formed here, but the MXene dispersion exceeding 15 mg mL−1 (at 15 mg mL−1) was successfully G‘/G‘‘ value is 5.29) prepared as MXene fiber (Figure 3i).

Figure 4. Wet spinning of pure Ti3C2Tx MXene fibers. a Ti3C2Tx MXene dispersion is gelled by NH4+ ion. The sol-gel transition is determined by the vial inversion method. b Schematic diagram of the wet spinning process of Ti3C2Tx MXene fiber. c Wind the meter-length Ti3C2Tx MXene fiber on the spool. d Continuous wet spinning allows the production of Ti3C2Tx MXene fibers exceeding 1 μm (1.2 μm). SEM image of Ti3C2Tx MXene fiber: e overall morphology f cross-sectional view g side view. Ti3C2Tx MXene fiber is used as h wire and i headphone wire. Point four:

The colloidal stability of MXene tablets can be significantly affected by salt. The role of NH4 ions in the gelation of MXene dispersion has been confirmed by the vial inversion method (Figure 4a).

In fact, similar to the behavior of graphene and other 2D materials, the high exfoliation/delamination and gelation of MXene is critical for continuous fiber manufacturing. The prepared MXene liquid crystal dispersion was extruded into a coagulation solution with NH4 ions, and then washed by a reel in a water bath, thereby producing continuous fibers using a simple wet spinning method (Figure 4b and supplementary video). Extruded MXene does not form gel fibers without NH4 ions (Supplementary Figure 5).

Finally, the fiber was dried in air for 24 h to form a uniform, long and continuous axial MXene fiber. The 1 meter long MXene fiber mass produced by continuous spinning is wound on the bobbin (Figure 4c). The extruded 100% pure MXene fiber has a length of more than 1 m and is stable under continuous spinning (Figure 4d). The cross-section of MXene fibers shows a layered structure with highly dense nanosheets (Figure 4e-g). The rough morphology on the side of the fiber indicates drying and shrinkage (Figure 4g).

Highly conductive MXene fibers can be used in electrical applications, which successfully illuminate white diode (LED) lights (Figure 4h). In addition, MXene fiber replaces commercially available wires and is integrated into the earphone wires to transmit electrical signals (Figure 4i, supplementary audio).

Figure 5. Comparison of conductivity and Young‘s modulus. 

Figure 5 compares the electrical conductivity and Young‘s modulus of MXene fibers prepared in this experiment with MXene hybrid fibers and graphene fibers prepared in previous studies (Supplementary Figure 6 and Supplementary Table 2). It can be clearly seen from the Ashby diagram that our wet-spun pure MXene fiber is superior to other considered fibers in terms of electrical conductivity and Young‘s modulus. The conductivity of MXene fiber (7713 S cm-1) is almost 107 times and 27 times that of MXene/graphene hybrid fiber (72.3 and 290 S cm-1, respectively), which is MXen/PEDOT:PSS fiber (1490 S cm- 1) 5 times. In this study, the conductivity of MXene fiber is 12-220 times higher than that of graphene fiber. In addition, on the macro scale, the conductivity of pure MXene fiber is 3.2 times that of MXene film, indicating that MXene fiber has a good structure.

In summary, this paper effectively develops pure Ti3C2Tx MXene fiber without additives/binders or composite materials through a direct, continuous, large-scale wet spinning strategy. At a high concentration of 25 mg mL-1, the large Ti3C2Tx-MXene sheet has good dispersibility and shows the liquid crystal pattern and rheological characteristics of the lyotropic liquid crystal. The Ti3C2Tx-MXene fiber prepared by the wet spinning method has a very high conductivity of 7713 S cm-1, and successfully prepared a flexible, continuous, 1 meter long MXene fiber.

Article link:

https://www.nature.com/articles/s41467-020-16671-1

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