Large-scale wet-spinning of highly electroconductive MXene fibers

Two-dimensional (2D) nanomaterials, such as graphene, have attracted significant attention due to their unique properties and potential applications in various fields. The development of macroscopic structures from these 2D materials is crucial for their practical utilization. Wet-spinning, a technique that transforms concentrated colloidal dispersions into fibers, has emerged as a promising method for continuous large-scale production of fibers from 2D materials. Ti3C2Tx MXene, a member of the 2D materials family, exhibits exceptional electroconductivity and electrochemical properties, making it a promising candidate for various applications. However, challenges remain in fabricating high-quality, pure MXene fibers using wet-spinning. Previous attempts have involved MXene composites with polymers or reduced graphene oxide (rGO), resulting in reduced electrical conductivity compared to pure MXene films. The main hurdle lies in the weak self-supporting organization of MXene sheets due to poor interlayer interactions and difficulties in achieving high concentrations of stable dispersions. This study aims to address these limitations by developing a novel wet-spinning method for the continuous fabrication of additive/binder-free pure MXene fibers with high electrical conductivity. The researchers hypothesize that by carefully controlling the concentration and the coagulation process, they can achieve stable, highly concentrated dispersions that can be wet-spun into high-performance fibers. The success of this approach is crucial for advancing the application of MXenes in next-generation flexible and wearable electronics.

The literature review focuses on existing methods for fabricating macroscopic structures from 2D materials, particularly graphene and MXene. Studies on graphene oxide (GO) fibers produced via wet-spinning are discussed, highlighting the importance of understanding molecular interactions and coagulation parameters for successful fiber formation. Several methods to prepare macroscopic 1D carbon-based fibers from graphene oxides (GO) are discussed. Graphene-related fibers have gained interest because of their versatile functionalities, such as lightweight, mechanical flexibility, bendability, stretchability, and the ability to be woven into textiles for the next generation of smart electronic gadgets. The authors also review prior work on MXene-based fibers, noting the use of wet-spinning and electrospinning techniques, often with polymer or rGO additives. However, these composite fibers typically exhibit lower electrical conductivity than pure MXene films, underscoring the need for a method to produce highly conductive pure MXene fibers. The key challenge identified in the literature is the weak self-supporting organization of MXene sheets due to poor interlayer interactions, hindering the formation of stable fibers from high-concentration dispersions.

The study begins with the synthesis of Ti3C2Tx MXene sheets from Ti3AlC2 MAX-phase powder through selective etching of the aluminum layer using LiF and HCl. The exfoliated MXene sheets are characterized using various techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM), conductive atomic force microscopy (C-AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These characterizations assess the size, thickness, morphology, crystallinity, and surface functionalities of the MXene sheets. The rheological properties of the MXene dispersion, crucial for wet-spinning, are examined using a rheometer at different concentrations. The shear viscosity, shear stress, and storage and loss moduli (G' and G'') are measured to determine the spinnability of the dispersion. The critical concentration for forming a lyotropic liquid-crystalline phase is determined experimentally and correlated to the Onsager model for predicting phase transitions in liquid crystals. The wet-spinning process itself involves extruding a highly concentrated MXene dispersion (25 mg/mL) through a nozzle into a coagulation bath containing ammonium ions (NH4Cl and NH4OH). The role of ammonium ions in facilitating the gelation and assembly of MXene sheets into fibers is investigated. The extruded fibers are washed and dried to obtain continuous, meter-long MXene fibers. The resulting MXene fibers are characterized in terms of their morphology (SEM), mechanical properties (tensile testing), and electrical conductivity (four-point probe method). The electrical conductivity is calculated using a formula considering the fiber diameter, resistance, and length. The performance of the fibers is demonstrated through practical applications, including their use as electrical wires to power an LED and as earphone wires for transmitting electrical signals.

The researchers successfully synthesized high-quality Ti3C2Tx MXene sheets with an average lateral size of ~2.26 μm. The XPS analysis confirmed the presence of surface termination groups (e.g., C–Ti–T, C–Ti–(OH), and C–Ti–Ox), crucial for stable dispersion in water. At a high concentration of 25 mg/mL, the MXene dispersion exhibited lyotropic liquid-crystalline behavior, as evidenced by birefringence under polarized light microscopy and rheological measurements. The viscosity of the dispersion increased with concentration and decreased with increasing shear rate, characteristic of liquid crystals. The spinnability of the dispersion was successfully predicted using the G'/G'' ratio from rheological measurements. The wet-spinning process, facilitated by the addition of ammonium ions in the coagulation bath, resulted in the continuous production of meter-long, flexible, pure MXene fibers. The fibers had a lamellar structure with highly compact nanosheets. Crucially, the fibers demonstrated an ultra-high electrical conductivity of 7713 S cm⁻¹, significantly exceeding the conductivity of previously reported MXene/graphene hybrid fibers and graphene fibers. The mechanical properties and electrical conductivity of the fibers were superior to those of other previously reported MXene and graphene fibers as shown in a comparison Ashby plot. The functionality of the MXene fibers was successfully demonstrated in practical applications such as switching on an LED light and transmitting audio signals through earphone wires.

The findings address the research question by successfully demonstrating a scalable and efficient method for producing high-performance pure MXene fibers. The high electrical conductivity of the fibers, significantly exceeding that of previously reported MXene-based composites, highlights the importance of avoiding additives and utilizing the intrinsic properties of pure MXene. The achievement of a stable, high-concentration MXene dispersion is key to the success of the wet-spinning process. The use of ammonium ions during coagulation plays a vital role in promoting the self-assembly of MXene sheets into a continuous fiber structure. The superior performance of the MXene fibers compared to previously reported graphene and MXene composite fibers suggests this approach may significantly advance the field of flexible and wearable electronics. The results support the hypothesis that carefully controlled wet-spinning can unlock the full potential of MXene for high-performance applications. The successful demonstration of the fibers in practical applications validates the potential of the approach for next-generation electronics.

This study presents a significant advance in the fabrication of MXene fibers. A scalable wet-spinning method yielded meter-long, flexible fibers with an exceptionally high electrical conductivity (7713 S cm⁻¹), surpassing previously reported results for MXene and graphene fibers. The successful integration of these fibers into functional devices such as LEDs and earphones demonstrates their potential for high-performance flexible and wearable electronics. Future research could explore different MXene compositions, further optimization of the wet-spinning parameters, and the integration of these fibers into more complex electronic systems for diverse applications.

The study focuses on a specific type of MXene (Ti3C2Tx). The generalizability of this wet-spinning method to other MXenes needs further investigation. The long-term stability of the fibers under various environmental conditions (e.g., humidity, temperature) is also an area that requires further study. A more detailed analysis of the fiber microstructure could provide deeper insights into the relationship between the fiber structure and its properties. Finally, scaling up the production process to an industrial level could face challenges that need further investigation.