Nonpolar Organic Dispersion of 2D Ti3C2Tx MXene Flakes via Simultaneous Interfacial Chemical Grafting and Phase Transfer Method

Advancements in Ti3C2Tx MXene Stability: Synergistic Antioxidant Strategies and Their Impact on Long-Lasting Flexible Sensors

Two-dimensional (2D) transition metal carbides (Ti3C2Tx MXene) have demonstrated substantial application potential across various fields, owing to their excellent metallic conductivity and solution processability. However, the rapid oxidation of Ti3C2Tx in aqueous environments, leading to a loss of stability within mere days, poses a significant obstacle for its practical applications. Herein, we introduce an antioxidant strategy that combines free radical scavenging with surface passivation, culminating in the design and synthesis of imidazolium-based ionic liquids (ILs) incorporating siloxane groups. By deploying a straightforward hydrolysis-addition reaction, we successfully fabricated IL-modified Ti3C2Tx materials (Ti3C2Tx-IL). The Ti3C2Tx -IL not only displayed exceptional conductivity exceeding 3.85 × 104 S/m and hydrophilic contact angles below 45° but also showcased its superior chemical stability and antioxidation mechanisms through various analyses, including visual color change experiments, spectroscopic and energy spectrum characterization, free radical scavenging tests, and density-functional-theory-based molecular simulations. Furthermore, when utilized as a conductive filler in the fabrication of a poly(vinyl alcohol)/nanocellulose fiber (PVA/CNF) composite hydrogel (PCMIL), the resultant sensors exhibited remarkable mechanical performance with up to 535% strain, 1.59 MPa strength, 4.35 MJ/m3 toughness, and a conductivity of 3.40 mS/cm, as well as a high sensitivity gauge factor of 3.3. Importantly, even after 45 days of storage, the PCMIL retained most of its functionalities, demonstrating superior performance in human-machine interaction applications compared to hydrogels made from unmodified Ti3C2Tx. This research establishes a robust antioxidant protection strategy for Ti3C2Tx, offering substantial technical reinforcement for its prospective applications in the realm of flexible electronics and sensing technologies.

Evolution of Surface Chemistry in Two-Dimensional MXenes: From Mixed to Tunable Uniform Terminations

Surface chemistry of MXenes is of great interest as the terminations can define the intrinsic properties of this family of materials. The diverse and tunable terminations also distinguish MXenes from many other 2D materials. Conventional fluoride-containing reagents etching approaches resulted in MXenes with mixed fluoro-, oxo-, and hydroxyl surface groups. The relatively strong chemical bonding of MXenes' surface metal atoms with oxygen and fluorine makes post-synthetic covalent surface modifications of such MXenes unfavorable. In this minireview, we focus on the recent advances in MXenes with uniform surface terminations. Unconventional methods, including Lewis acidic molten salt etching (LAMS) and bottom-up direct synthesis, have been proven successful in producing halide-terminated MXenes. These synthetic strategies have opened new possibilities for MXenes because weaker surface chemical bonds in halide-terminated MXenes facilitate post-synthetic covalent surface modifications. Both computational and experimental results on surface termination-dependent properties are summarized and discussed. Finally, we offer our perspective on the opportunities and challenges in this exciting research field.

Plasma-Activated Solutions Regulate Surface-Terminating Groups Enhancing Pseudocapacitive Ti3C2Tx Electrode Performance

2D transition metal carbides and nitrides (MXenes) are actively pursued as pseudocapacitive materials for supercapacitors owing to their advantages in electronic conductivity and surface reactivity. Increasing the fraction of ─O terminal groups in Ti3C2Tx is a promising approach to improve the pseudocapacitive charge storage in H2SO4 electrolytes, but it suffers from a lack of effective functionalization methods and stability of the groups in practical operation. Here a low-temperature and environment-friendly approach via the interaction of nonequilibrium plasmas with Ti3C2Tx dispersion is demonstrated to generate abundant and stable surface-terminating O groups. The impact of the discharge environment (Ar, O2, and H2) on the structural characteristics and electrochemical performance of Ti3C2Tx nanosheets is studied. The Ti3C2Tx modified in Ar and H2 maintains their original morphology but a significantly lower F content. Consequently, an extraordinarily high content (78.5%) of surface-terminating O groups is revealed by the high-resolution X-ray photoelectron spectroscopy spectra for the Ti3C2Tx samples modified in H2 plasma-treated solutions. Additionally, the Ti3C2Tx treated using H2 plasmas exhibits the best capacitive performance of 418.3 F g-1 at 2 mV s-1, which can maintain 95.88% capacity after 10 000 cycles. These results contribute to the development of advanced nanostructured pseudocapacitive electrode materials for renewable energy storage applications.

Recent Advancement in Emerging MXene-Based Photocatalytic Membrane for Revolutionizing Wastewater Treatment

MXene-based photocatalytic membranes provide significant benefits for wastewater treatment by effectively combining membrane separation and photocatalytic degradation processes. MXene represents a pioneering 2D photocatalyst with a variable elemental composition, substantial surface area, abundant surface terminations, and exceptional photoelectric performance, offering significant advantages in producing high-performance photocatalytic membranes. In this review, an in-depth overview of the latest scientific progress in MXene-based photocatalytic membranes is provided. Initially, a brief introduction to the structure and photocatalytic capabilities of MXene is provided, highlighting their pivotal role in promoting the photocatalytic process. Subsequently, in pursuit of the optimal MXene-based photocatalytic membrane, critical factors such as the morphology, hydrophilicity, and stability of MXenes are meticulously taken into account. Various preparation strategies for MXene-based photocatalytic membranes, including blending, vacuum filtration, and dip coating, are also discussed. Furthermore, the application and mechanism of MXene-based photocatalytic membranes in micropollutant removal, oil-water separation, and antibacterial are examined. Lastly, the challenges in the development and practical application of MXene-based photocatalytic membranes, as well as their future research direction are delineated.

Unveiling the power of MXenes: Solid lubrication perspectives and future directions

The interaction between two surfaces leads to the generation of friction and wear of material. Friction and wear are some of the major challenges that may readily be overcome by the third part of tribology called lubrication. Utilizing solid lubricants including polymers, carbon-based materials, soft metals, transition metal dichalcogenides, along with their potential benefits and drawbacks in dry environments can reduce friction. Recently, an emerging class of two-dimensional (2D) transition metal nitrides, carbides or carbonitrides commonly known as MXenes have emerged as an attractive alternative for solid lubrication because of their ability to establish wear-resistant tribo layers and well as low friction and shear strength. Furthermore, the inherent hydrophilic nature of these substances has led to limited dispersion stability and phase compatibility when combined with pure base oils. As a result, their potential use as solid lubricants and lubricant additives has been impeded. To address this issue and enhance the applicability of MXenes as solid lubricants, their surface modification can be an attractive tool. Therefore, this review provides a succinct summary of the current state-of-the-art in surface functionalization of MXenes, a subject that has not yet been thoroughly addressed. Further, the mechanical behavior of MXenes and composites has been discussed, followed by the potential of MXenes as a solid lubricant at micro- and macro-scale. Finally, the existing opportunities and challenges of the research area have been discussed with possible future research directions. We believe, this article will be a valuable resource for MXenes and opens the door to improve the chemical, physical and mechanical properties of MXenes in various applications, such as solid lubrication.

Functionalized MXene ink enables environmentally stable printed electronics

Establishing dependable, cost-effective electrical connections is vital for enhancing device performance and shrinking electronic circuits. MXenes, combining excellent electrical conductivity, high breakdown voltage, solution processability, and two-dimensional morphology, are promising candidates for contacts in microelectronics. However, their hydrophilic surfaces, which enable spontaneous environmental degradation and poor dispersion stability in organic solvents, have restricted certain electronic applications. Herein, electrohydrodynamic printing technique is used to fabricate fully solution-processed thin-film transistors with alkylated 3,4-dihydroxy-L-phenylalanine functionalized Ti3C2Tx (AD-MXene) as source, drain, and gate electrodes. The AD-MXene has excellent dispersion stability in ethanol, which is required for electrohydrodynamic printing, and maintains high electrical conductivity. It outperformed conventional vacuum-deposited Au and Al electrodes, providing thin-film transistors with good environmental stability due to its hydrophobicity. Further, thin-film transistors are integrated into logic gates and one-transistor-one-memory cells. This work, unveiling the ligand-functionalized MXenes' potential in printed electrical contacts, promotes environmentally robust MXene-based electronics (MXetronics).

High-permittivity Solvents Increase MXene Stability and Stacking Order Enabling Ultraefficient Terahertz Shielding

2D transition metal carbides and nitrides (MXenes) suggest an uncommonly broad combination of important functionalities amongst 2D materials. Nevertheless, MXene suffers from facile oxidation and colloidal instability upon conventional water-based processing, thus limiting applicability. By experiments and theory, It is suggested that for stability and dispersibility, it is critical to select uncommonly high permittivity solvents such as N-methylformamide (NMF) and formamide (FA) (εr  = 171, 109), unlike the classical solvents characterized by high dipole moment and polarity index. They also allow high MXene stacking order within thin films on carbon nanotube (CNT) substrates, showing very high Terahertz (THz) shielding effectiveness (SE) of 40-60 dB at 0.3-1.6 THz in spite of the film thinness < 2 µm. The stacking order and mesoscopic porosity turn relevant for THz-shielding as characterized by small-angle X-ray scattering (SAXS). The mechanistic understanding of stability and structural order allows guidance for generic MXene applications, in particular in telecommunication, and more generally processing of 2D materials.

Molecularly Confined Topochemical Transformation of MXene Enables Ultrathin Amorphous Metal-Oxide Nanosheets

Two-dimensional (2D) amorphous nanosheets with ultrathin thicknesses have properties that differ from their crystalline counterparts. However, conventional methods for growing 2D materials often produce either crystalline flakes or amorphous nanosheets with an uncontrollable thickness. Here, we report that ultrathin amorphous metal-oxide nanosheets featuring superior flatness can be realized through the molecularly confined topochemical transformation of MXene. Using MXene Ti2CTx as an example, we show that surface modification of Ti2CTx nanosheets with molecular ligands, such as oleylamine (OAm) and oleic acid (OA), not only imparts notable colloidal dispersity to Ti2CTx nanosheets in nonpolar organic solvents but also confines their subsequent oxidation to in-plane configurations. We demonstrate that unlike the drastic oxidation conventionally observed for pristine MXene, hydrophobizing MXene with OAm and OA ligands enables individual Ti2CTx nanosheets to undergo independent oxidation in a nondestructive manner, resulting in amorphous titanium oxide (am-TiO2) nanosheets that faithfully retain the dimension and flatness of pristine MXene. These am-TiO2 nanosheets exhibit exceptional activity as substrates for surface-enhanced Raman scattering. Importantly, this molecular confinement strategy can be extended to other MXene materials, providing a versatile approach for synthesizing ultrathin amorphous metal-oxide nanosheets with tailored compositions and functionalities.

Surface nitrided MXene sheets with outstanding electroconductivity and oxidation stability

Two-dimensional Ti3C2Tx MXenes are promising candidates for a wide range of film- or fiber-based devices owing to their solution processability, high electrical conductivity, and versatile surface chemistry. The surface terminal groups (Tx) of MXenes can be removed to increase their inherent electrical performance and ensure chemical stability. Therefore, understanding the chemical evolution during the removal of the terminal groups is crucial for guiding the production, processing, and application of MXenes. Herein, we investigate the effect of chemical modification on the electron-transfer behavior during the removal of the terminal groups by annealing Ti3C2Tx MXene single sheets under argon (Ar-MXene) and ammonia gas (NH3-MXene) conditions. Annealing in ammonia gas results in surface nitridation of MXenes and preserves the electron-abundant Ti3C2 structure, whereas annealing MXene single sheets in Ar gas results in the oxidation of the titanium layers. The surface-nitrided MXene film exhibits an electrical conductivity two times higher than that of the Ar-MXene film. The oxidation stability is quantified by calculating the oxidation rate constants for severe reactions with H2O2. The surface-nitrided MXene is 13 times more stable than Ar-MXene. The investigation of MXene single sheets provides fundamental insights that are valuable for designing electrically conductive and chemically stable MXenes.

2D Materials Beyond Post-AI Era: Smart Fibers, Soft Robotics, and Single Atom Catalysts

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

Effect of Sodium Dodecyl Sulfate on Stability of MXene Aqueous Dispersion

MXenes suffer from severe oxidation and progressive degradation in aqueous media due to its poor chemical stability. Herein, sodium dodecyl sulfate (SDS) is employed as an efficient protectant for long-term storage of Ti3 C2 Tx -MXene aqueous dispersion. Experimental data support SDS's capability to protect oxidation-prone sites on Ti3 C2 Tx nanosheets, providing extended colloidal stability of up to 213 days. Concentration-dependent anti-oxidation effect articulates that 1.5 mg mL-1 is deemed as an ideal SDS dose for Ti3 C2 Tx to achieve optimal oxidation-resistance in aqueous solution. Additionally, a chroma strategy is developed to instantly and precisely measure the oxidation degree of Ti3 C2 Tx . Adsorption-driven anti-oxidation efficacy of SDS is further confirmed by optimized conformations with interaction energies of SDS on termination-free and surface-defective Ti3 C2 Tx through multiscale simulations. This proposed route is a step forward in broadening the horizons of experimental and theoretical investigations of MXenes with promising implications for long-term storage and reliable applications.

Effective Surface Modification of 2D MXene toward Thermal Energy Conversion and Management

Thermal energy management is a crucial aspect of many research developments, such as hybrid and soft electronics, aerospace, and electric vehicles. The selection of materials is of critical importance in these applications to manage thermal energy effectively. From this perspective, MXene, a new type of 2D material, has attracted considerable attention in thermal energy management, including thermal conduction and conversion, owing to its unique electrical and thermal properties. However, tailored surface modification of 2D MXenes is required to meet the application requirements or overcome specific limitations. Herein, a comprehensive review of surface modification of 2D MXenes for thermal energy management is discussed. First, this work discusses the current progress in the surface modification of 2D MXenes, including termination with functional groups, small-molecule organic compound functionalization, and polymer modification and composites. Subsequently, an in situ analysis of surface-modified 2D MXenes is presented. This is followed by an overview of the recent progress in the thermal energy management of 2D MXenes and their composites, such as Joule heating, heat dissipation, thermoelectric energy conversion, and photothermal conversion. Finally, some challenges facing the application of 2D MXenes are discussed, and an outlook on surface-modified 2D MXenes is provided.

Surface and Interface Regulation of MXenes: Methods and Properties

Since the discovery of Ti3 C2 Tx in 2011, 2D transition metal carbides, nitrides, and carbonitrides, known as MXenes, have been attracting great attention as the emerging member of 2D materials. The surface terminations, intercalants, and the interfaces between MXenes and other substances are of importance for tuning the properties of MXenes. For instance, surface termination of MXenes can change the density of states at the Fermi levels to make MXenes electronically tunable. Different terminations can lead to band opening and changes in behavior from metallic to semiconducting, as well as dramatic changes in the work function of MXenes. On the other hand, electron transfer occurring at the interface between MXenes and other substances due to the physical interaction/chemical bonding, changes the electron configuration of MXenes and realizes the functionalization. In this review, the most up-to-date progress of the surface and interface regulation of MXenes is comprehensively summarized, introducing the effect of various synthesis methods on the surface and interface chemistry, the routes on tuning the surface and interface chemistry, and the related potential applications. Finally, the perspective of the future research directions and challenges on surface and interface regulation is outlined.

Biocompatible and Oxidation-Resistant Ti3 C2 Tx MXene with Halogen-Free Surface Terminations

Surface chemistry influences not only physicochemical properties but also safety and applications of MXene nanomaterials. Fluorinated Ti3 C2 Tx MXene, synthesized using conventional HF-based etchants, raises concerns regarding harmful effects on electronics and toxicity to living organisms. In this study, well-delaminated halogen-free Ti3 C2 Tx flakes are synthesized using NaOH-based etching solution. The transversal surface plasmon mode of halogen-free Ti3 C2 Tx MXene (833 nm) confirmed red-shift compared to conventional Ti3 C2 Tx (752 nm), and the halogen-free Ti3 C2 Tx MXene has a different density of state by the high proportion of -O and -OH terminations. The synthesized halogen-free Ti3 C2 Tx exhibits a lower water contact angle (34.5°) and work function (3.6 eV) than those of fluorinated Ti3 C2 Tx (49.8° and 4.14 eV, respectively). The synthesized halogen-free Ti3 C2 Tx exhibits high biocompatibility with the living cells, as evidenced by no noticeable cytotoxicity, even at very high concentrations (2000 µg mL⁻1 ), at which fluorinated Ti3 C2 Tx caused ≈50% reduction in cell viability upon its oxidation. Additionally, the oxidation stability of halogen-free Ti3 C2 Tx is enhanced unexpectedly, which cumulatively provides a good rationale for pursuing the halogen-free routes for synthesizing MXene materials for their uses in biomedical and therapeutic applications.

MXene Contact Engineering for Printed Electronics

MXenes emerging as an amazing class of 2D layered materials, have drawn great attention in the past decade. Recent progress suggest that MXene-based materials have been widely explored as conductive electrodes for printed electronics, including electronic and optoelectronic devices, sensors, and energy storage systems. Here, the critical factors impacting device performance are comprehensively interpreted from the viewpoint of contact engineering, thereby giving a deep understanding of surface microstructures, contact defects, and energy level matching as well as their interaction principles. This review also summarizes the existing challenges of MXene inks and the related printing techniques, aiming at inspiring researchers to develop novel large-area and high-resolution printing integration methods. Moreover, to effectually tune the states of contact interface and meet the urgent demands of printed electronics, the significance of MXene contact engineering in reducing defects, matching energy levels, and regulating performance is highlighted. Finally, the printed electronics constructed by the collaborative combination of the printing process and contact engineering are discussed.

Grafting Behavior of Amine Ligands for Surface Modification of MXene

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.

Two-Dimensional Janus MXene Inks for Versatile Functional Coatings on Arbitrary Substrates

Solution processing of two-dimensional nanomaterial inks guarantees efficient, straightforward fabrication of functional films, coatings, flexible devices, etc. Despite the excellent solution processibility and viscoelasticity of MXene aqueous inks, formulation of nonaqueous MXene inks with great affinity to both hydrophilic and hydrophobic substrates has proven quite challenging, limiting the practical applications of MXenes in printing/coatings on various substrates. Here, MXene surface chemistry is manipulated by asymmetrically grafting polystyrene and further concentrating the flakes into additive-free Janus MXene organic inks. The modified MXene nanosheets exhibit hydrophilicity on one side and hydrophobicity on the other. As a result, Janus MXene nanosheets ensure broad dispersibility in polar and nonpolar solvents, which in turn greatly extends the ink shelf life by slowing down the oxidation kinetics. Janus MXene sheets dispersed in toluene at room temperature remain at 90% of the initial solids after 1 month of storage. Janus surface engineering on MXene flakes guarantees the straightforward formation of uniform yet firm, large-area coatings on hydrophilic or hydrophobic substrates. These coatings demonstrate improved photothermal properties and chemical stability as well as good electromagnetic interference shielding performance. This strategy provides a simple and cost-effective way to promote the performance of MXene electronics in a variety of applications.

2D MXene-Based Biosensing: A Review

MXene emerged as decent 2D material and has been exploited for numerous applications in the last decade. The remunerations of the ideal metallic conductivity, optical absorbance, mechanical stability, higher heterogeneous electron transfer rate, and good redox capability have made MXene a potential candidate for biosensing applications. The hydrophilic nature, biocompatibility, antifouling, and anti-toxicity properties have opened avenues for MXene to perform in vitro and in vivo analysis. In this review, the concept, operating principle, detailed mechanism, and characteristic properties are comprehensively assessed and compiled along with breakthroughs in MXene fabrication and conjugation strategies for the development of unique electrochemical and optical biosensors. Further, the current challenges are summarized and suggested future aspects. This review article is believed to shed some light on the development of MXene for biosensing and will open new opportunities for the future advanced translational application of MXene bioassays.

Advanced perspectives on MXene composite nanomaterials: Types synthetic methods, thermal energy utilization and 3D-printed techniques

MXene, 2D material, can be synthesized as single flake with 1 nm thickness by using phase change material, polymer and graphene oxide. Meanwhile, the MXene and its composite derivative materials have been applied widely in electro-to-thermal conversion, photo-to-thermal conversion, thermal energy storage, and 3D printing ink aspects. Furthermore, the forward-looking utilization of the MXene nanomaterials in hydrogen energy storage, radio frequency field application, CO₂ capture and remediation of environmental pollution, is explored. This article reveals that the efficiencies of the photo-to-thermal and electro-to-thermal energy conversions with the MXene nanomaterials could reach about 80-90%. In parallel, it is demonstrated that the MXene printed ink has the excellent rheological property and high viscosity and stability of liquid, which contribute to arranging the multi-dimensional architectures with functional materials and controlling the flow rate of the MXene ink in the range of 0.03-0.15 mL/min for speedily printing and various printing structures.

Surface-Functionalized Ti3C2Tx MXene as a Kind of Efficient Lubricating Additive for Supramolecular Gel

Low interlaminar shear stress, high mechanical strength, and tunable structure make Ti₃C₂T_x MXene a burgeoning star as solid lubricants and lubricant additives. Although surface modification strategy can improve its compatibility with base oils, it will eventually settle due to gravity. Additionally, base oils are prone to leakage, creep, and volatilization, which limit their application. To address these issues, supramolecular gels with surface-modified Ti₃C₂T_x were conceived. Apart from the lubrication effect, the high thermal conductivity of Ti₃C₂T_x MXene accelerated the phase transition rate of supramolecular gels. The thermal-reversible and creep-resistant properties distinguish them from other conventional lubricants. The tribological tests showed that the 500 solvent neutral (SN) supramolecular gel with 0.10 wt % Ti₃C₂T_x-octadecylphosphonic acid (Ti₃C₂T_x-ODPA) reduced the coefficient of friction (COF) by 46.32% and wear volume by 81.18% compared with pure 500SN oil. Moreover, they also performed well in load-carrying capacity, temperature tolerance, and speed adaptability. This work puts forward a new approach to prepare MXene-based lubricants tailored for some severe lubrication conditions. These exceptional features enable their application in rolling bearings, some gears, and other low maintenance mechanisms.

Regenerating MXene by a Facile Chemical Treatment Method

A popular substance in the MXene family, titanium carbide (Ti₃C₂T_x), has received substantial attention mainly due to its high metallic conductivity, easy solution processability, and environment friendliness. However, the poor oxygen resistance nature of MXene has prevented its practical applications from being realized. Despite significant attempts to improve the oxidative stability of MXenes, a comprehensive understanding of the oxidation mechanism is still elusive, thus leaving an optimal strategy for recycling oxidized MXene in question. Here, by developing a facile hydrofluoric acid (HF) post-treatment, we have unraveled the regeneration kinetics of the oxidized Ti₃C₂T_x. A systematic and extensive investigation using a combination of Raman spectroscopy, scanning electron microscopy, X-ray diffractometer, and X-ray photoelectron spectroscopy revealed that HF post-treatment is critical for restoring the structure/morphology and surface composition of MXene nanosheets. These are ascribed to the oxidizing agent removal kinetics, while the generation of amorphous carbon and Ti(III) in fluorinated derivatives provides efficient electrical conductivity. Our findings suggested that HF post-treatment is sufficient to evade and reduce the degradation process while maintaining the conductivity for a longer time, which will not only be economically advantageous but also a step forward for the rational design of Ti₃C₂T_x-based devices and functional coatings.

Universal Ligands for Dispersion of Two-Dimensional MXene in Organic Solvents

Ligands can control the surface chemistry, physicochemical properties, processing, and applications of nanomaterials. MXenes are the fastest growing family of two-dimensional (2D) nanomaterials, showing promise for energy, electronic, and environmental applications. However, complex oxidation states, surface terminal groups, and interaction with the environment have hindered the development of organic ligands suitable for MXenes. Here, we demonstrate a simple, fast, scalable, and universally applicable ligand chemistry for MXenes using alkylated 3,4-dihydroxy-l-phenylalanine (ADOPA). Due to the strong hydrogen-bonding and π-electron interactions between the catechol head and surface terminal groups of MXenes and the presence of a hydrophobic fluorinated alkyl tail compatible with organic solvents, the ADOPA ligands functionalize MXene surfaces under mild reaction conditions without sacrificing their properties. Stable colloidal solutions and highly concentrated liquid crystals of various MXenes, including Ti2CTx, Nb2CTx, V2CTx, Mo2CTx, Ti3C2Tx, Ti3CNTx, Mo2TiC2Tx, Mo2Ti2C3Tx, and Ti4N3Tx, have been produced in various organic solvents. Such products offer excellent electrical conductivity, improved oxidation stability, and excellent processability, enabling applications in flexible electrodes and electromagnetic interference shielding.

Tuning thermal and electrical properties of MXenes via dehydration

Recently, MXenes (a class of two-dimensional transition metal carbides) have attracted great attention in various applications such as humidity sensors, owing to their unique electrical and thermal properties. However, previous studies of MXenes mostly focus on their humidity-sensing characteristics such as the mechanical response, and only few reports on their electrical and thermal response are available. Herein, we present novel transient electrothermal experiments to demonstrate that a transition from a negative to a positive resistance-temperature relationship can take place when the MXene sample becomes fully dehydrated. This surprising and unusual phenomenon was elucidated through non-equilibrium molecular dynamics simulations and attributed to water absorption/desorption onto the chemically active MXene surface. A linear relationship was also found between electrical/thermal properties and environmental humidity, which could be related to water adsorption on the surface of the MXene sensor. We further decomposed the total measured thermal conductivity and found that phonons were the dominant thermal carriers in the MXene sample. The main breakthrough of this work is the discovery of the unusual resistance-temperature relationship, which should be applicable to the design of MXene-based sensors for various applications.

Fast and High-Yield Anhydrous Synthesis of Ti3 C2 Tx MXene with High Electrical Conductivity and Exceptional Mechanical Strength

2D transition metal carbides or nitrides (MXenes) have attracted considerable attention from materials scientists and engineers owing to their physicochemical properties. Currently, MXenes are synthesized from MAX-phase precursors using aqueous HF. Here, in order to enhance the production of MXenes, an anhydrous etching solution is proposed, consisting of dimethylsulfoxide as solvent with its high boiling point, NH₄ HF₂ as an etchant, CH₃ SO₃ H as an acid, and NH₄ PF₆ as an intercalant. The reaction temperature can be increased up to 100 °C to accelerate the etching and delamination of Ti₃ AlC₂ MAX crystals; in addition, the destructive side reaction of the produced Ti₃ C₂ T_x MXene is suppressed in the etchant. Consequently, the etching reaction is completed in 4 h at 100 °C and produces high-quality monolayer Ti₃ C₂ T_x with an electrical conductivity of 8200 S cm^-1 and yield of over 70%. The Ti₃ C₂ T_x MXene fabricated via this modified synthesis exhibits different surface structures and properties arising from more F-terminations than those of Ti₃ C₂ T_x synthesized in aqueous HF₂ T. The atypical surface structure of Ti₃ C₂ T_x MXene results in an exceptionally high ultimate tensile strength (167 ± 8 MPa), which is five times larger than those of Ti₃ C₂ T_x MXenes synthesized in aqueous HF solution (31.7 ± 7.8 MPa).

A Review on MXene Synthesis, Stability, and Photocatalytic Applications

Photocatalytic water splitting, CO₂ reduction, and pollutant degradation have emerged as promising strategies to remedy the existing environmental and energy crises. However, grafting of expensive and less abundant noble-metal cocatalysts on photocatalyst materials is a mandatory practice to achieve enhanced photocatalytic performance owing to the ability of the cocatalysts to extract electrons efficiently from the photocatalyst and enable rapid/enhanced catalytic reaction. Hence, developing highly efficient, inexpensive, and noble-metal-free cocatalysts composed of earth-abundant elements is considered as a noteworthy step toward considering photocatalysis as a more economical strategy. Recently, MXenes (two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides) have shown huge potential as alternatives for noble-metal cocatalysts. MXenes have several excellent properties, including atomically thin 2D morphology, metallic electrical conductivity, hydrophilic surface, and high specific surface area. In addition, they exhibit Gibbs free energy of intermediate H atom adsorption as close to zero and less than that of a commercial Pt-based cocatalyst, a Fermi level position above the H₂ generation potential, and an excellent ability to capture and activate CO₂ molecules. Therefore, there is a growing interest in MXene-based photocatalyst materials for various photocatalytic events. In this review, we focus on the recent advances in the synthesis of MXenes with 2D and 0D morphologies, the stability of MXenes, and MXene-based photocatalysts for H₂ evolution, CO₂ reduction, and pollutant degradation. The existing challenges and the possible future directions to enhance the photocatalytic performance of MXene-based photocatalysts are also discussed.

Ionic liquid decorated MXene/Poly (N-isopropylacrylamide) composite hydrogel with high strength, chemical stability and strong adsorption

Organic phenolic pollutants in industrial wastewater cause severe environmental pollution and physiological damage. Poly (N-isopropylacrylamide) (PNIPAM) hydrogels generally have poor mechanical strength and are also intrinsically frangible, limiting their widespread applications in wastewater treatment. Combining them with 2-dimensional materials can also only improve the mechanical properties of hydrogels. Here, we report a high-strength, chemical stability and strong adsorption MXene/poly (N-isopropylacrylamide) (PNIPAM) thermosensitive composite hydrogel for efficient removal of phenolic pollutants from industrial wastewater. Ionic liquids (ILs) were grafted onto the surface of MXenes and introduced into NIPAM monomer solution to obtain composite hydrogels by in-situ polymerization for improved mechanical strength and adsorption capacity of the composite hydrogel. Compared with the MXene/PNIPAM composite hydrogel, the introduction of ILs simultaneously improves the mechanical and adsorption properties of the composite hydrogel. The ILs bind to the surface of MXene flakes through electrostatic interactions, which improved the thermal stability and oxidation resistance of MXenes while maintaining its good dispersion. Using 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) modified MXene (MXene-EMIMBF4) did not change significantly were observed after aging for 45 days. As-prepared composite hydrogels demonstrated excellent mechanical properties, reusability, and high adsorption capacity for p-Nitrophenol (4-NP). The MXene-EMIMBF4/PNIPAM hydrogel could recover after ten 95% strain compression cycles under the synergistic effect of chemical bonding and electrostatic attraction. Its maximum adsorption capacity for 4-NP was 200.29 mg g^-1 at room temperature, and the adsorption capacity maintained at ∼90% of its initial value after five adsorption cycles, which was related to the introduction of EMIMBF4 to form a denser network structure. The adsorption data followed the pseudo-second-order kinetics and Freundlich models.

Influencing Factors on Synthesis and Properties of MXene: A Review

MXene has a unique two-dimensional layered structure, a large specific surface area, and good electrical conductivity, stability, magnetic properties, and mechanical properties. Attributed to its structure and properties, MXene has entered various application fields, such as tumors, antibiotics, batteries, hydrogen storage, sensors, electromagnetic shielding, etc. Studies on MXene are implemented in various fields. The preparation methods, surface functional groups, and the preparation technology of composite materials directly affect the performance of MXene and determine its application fields. As significantly important issues, many influencing factors exist in the preparation process of MXene and the process of composite material design, which directly impact the properties and applications of MXene. This paper investigates and analyzes the factors influencing the preparation, properties, and application of MXene, explores the research progress of MXene, and provides critical insights for synthesizing and modifying effective MXene adsorbents. Moreover, this review discusses the influencing factors on applications, i.e., CO2 reduction, energy storage, heavy metal removal, etc., and compares key factors, including surface modification, pH, reaction time, and adsorption capacity, etc. Furthermore, this review provides an overview on current status and offers recommendations.

Development of Highly Sensitive and Humidity Independent Room Temeprature NO2 Gas Sensor Using Two Dimensional Ti3C2Tx Nanosheets and One Dimensional WO3 Nanorods Nanocomposite

Room temperature gas sensors have been widely explored in gas sensor technology for real-time applications. However, humidity has found to affect the room temperature sensing and the sensor life, necessitating the development of novel sensing materials with high sensitivity and stability under humid conditions at room temperature. In this work, the room temperature sensing performance of a Ti₃C₂T_x decorated, WO₃ nanorods based nanocomposite has been investigated. The hydrothermally synthesized WO₃/Ti₃C₂T_x nanocomposite has been investigated for structural, morphological, and electrical studies using X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and Brunanuer-Emmett-Teller techniques. The WO₃/Ti₃C₂T_x sensors have been found to be highly selective to NO₂ at room temperature and exhibit much higher sensitivity in comparison to pristine WO₃ nanorods. Furthermore, sodium l-ascorbate treated Ti₃C₂T_x sheets in WO₃/Ti₃C₂T_x enhanced the stability and reversibility of the sensor toward NO₂ even under variable humidity conditions (0-99% relative humidity). This study shows the potential room temperature sensing application of a WO₃/Ti₃C₂T_x nanocomposite-based sensor for detecting NO₂ at sub-ppb level. Further, a plausible sensing mechanism based on WO₃/Ti₃C₂T_x nanocomposite has been proposed to explain the improved sensing characteristics.

Recent Advances in Oxidation Stable Chemistry of 2D MXenes

As an emerging star of 2D nanomaterials, 2D transition metal carbides and nitrides, named MXenes, present a large potential in various research areas owing to their intrinsic multilayer structure and intriguing physico-chemical properties. However, the fabrication and application of functional MXene-based devices still remain challenging as they are prone to oxidative degradation under ambient environment. Within this review, the preparation methods of MXenes focusing on the recent investigations on their thermal structure-stability relationships in inert, oxidizing, and aqueous environments are systematically introduced. Moreover, the key factors that affect the oxidation of MXenes, such as, atmosphere, temperature, composition, microstructure, and aqueous environment, are reviewed. Based on different scenarios, strategies for avoiding or delaying the oxidation of MXenes are proposed to encourage the utilization of MXenes in complicated environments, especially at high temperature. Furthermore, the chemistry of MXene-derived oxides is analyzed, which can offer perspectives on the further design and fabrication of novel 2D composites with the unique structures of MXenes being preserved.

Comb-type polymer-hybridized MXene nanosheets dispersible in arbitrary polar, nonpolar, and ionic solvents

Solution-based processing of two-dimensional (2D) nanomaterials is highly desirable, especially for the low-temperature large-area fabrication of flexible multifunctional devices. MXenes, an emerging family of 2D materials composed of transition metal carbides, carbonitrides, or nitrides, provide excellent electrical and electrochemical properties through aqueous processing. Here, we further expand the horizon of MXene processing by introducing a polymeric superdispersant for MXene nanosheets. Segmented anchor-spacer structures of a comb-type polymer, polycarboxylate ether (PCE), provide polymer grafting-like steric spacings over the van der Waals range of MXene surfaces, thereby reducing the colloidal interactions by the order of 10³, regardless of solvent. An unprecedented broad dispersibility window for Ti₃C₂T_x MXene, covering polar, nonpolar, and even ionic solvents, was achieved. Furthermore, close PCE entanglements in MXene@PCE composite films resulted in highly robust properties upon prolonged mechanical and humidity stresses.

Covalently grafting first-generation PAMAM dendrimers onto MXenes with self-adsorbed AuNPs for use as a functional nanoplatform for highly sensitive electrochemical biosensing of cTnT

2D MXene-Ti₃C₂T_χ has demonstrated promising application prospects in various fields; however, it fails to function properly in biosensor setups due to restacking and anodic oxidation problems. To expand beyond these existing limitations, an effective strategy to for modifying the MXene by covalently grafting first-generation poly(amidoamine) dendrimers onto an MXene in situ (MXene@PAMAM) was reported herein. When used as a conjugated template, the MXene not only preserved the high conductivity but also conferred a specific 2D architecture and large specific surface areas for anchoring PAMAM. The PAMAM, an efficient spacer and stabilizer, simultaneously suppressed the substantial restacking and oxidation of the MXene, which endowed this hybrid with improved electrochemical performance compared to that of the bare MXene in terms of favorable conductivity and stability under anodic potential. Moreover, the massive amino terminals of PAMAM offer abundant active sites for adsorbing Au nanoparticles (AuNPs). The resulting 3D hierarchical nanoarchitecture, AuNPs/MXene@PAMAM, had advanced structural merits that led to its superior electrochemical performance in biosensing. As a proof of concept, this MXene@PAMAM-based nanobiosensing platform was applied to develop an immunosensor for detecting human cardiac troponin T (cTnT). A fast, sensitive, and highly selective response toward the target in the presence of a [Fe(CN)₆]^3-/4- redox marker was realized, ensuring a wide detection of 0.1-1000 ng/mL with an LOD of 0.069 ng/mL. The sensor's signal only decreased by 4.38% after 3 weeks, demonstrating that it exhibited satisfactory stability and better results than previously reported MXene-based biosensors. This work has potential applicability in the bioanalysis of cTnT and other biomarkers and paves a new path for fabricating high-performance MXenes for biomedical applications and electrochemical engineering.

Covalent Surface Modification of Ti3C2Tx MXene with Chemically Active Polymeric Ligands Producing Highly Conductive and Ordered Microstructure Films

As interest continues to grow in Ti₃C₂T_x and other related MXenes, advancement in methods of manipulation of their surface functional groups beyond synthesis-based surface terminations (T_x: -F, -OH, and ═O) can provide mechanisms to enhance solution processability as well as produce improved solid-state device architectures and coatings. Here, we report a chemically important surface modification approach in which "solvent-like" polymers, polyethylene glycol carboxylic acid (PEG6-COOH), are covalently attached onto MXenes via esterification chemistry. Surface modification of Ti₃C₂T_x with PEG6-COOH with large ligand loading (up to 14% by mass) greatly enhances dispersibility in a wide range of nonpolar organic solvents (e.g., 2.88 mg/mL in chloroform) without oxidation of Ti₃C₂T_x two-dimensional flakes or changes in the structure ordering. Furthermore, cooperative interactions between polymer chains improve the nanoscale assembly of uniform microstructures of stacked MXene-PEG6 flakes into ordered thin films with excellent electrical conductivity (∼16,200 S·cm^-1). Most importantly, our covalent surface modification approach with ω-functionalized PEG6 ligands (ω-PEG6-COOH, where ω: -NH₂, -N₃, -CH═CH₂) allows for control over the degree of functionalization (incorporation of valency) of MXene. We believe that installing valency onto MXenes through short, ion conducting PEG ligands without compromising MXenes' features such as solution processability, structural stability, and electrical conductivity further enhance MXenes surface chemistry tunability and performance and widens their applications.

Bifunctional Smart Hydrogel Dressing with Strain Sensitivity and NIR-Responsive Performance

Smart response hydrogel has a broad application prospect in human health real-time monitoring due to its responses to a variety of stimuli. In this study, we developed a novel smart hydrogel dressing based on conductive MXene nanosheets and a temperature-sensitive PNIPAm polymer. γ-Methacryloxypropyltrimethoxysilane (KH570) was selected to functionalize the surface of MXene further to improve the interface compatibility between MXene and PNIPAm. Our prepared K-M/PNIPAm hydrogel was found to have a strain-sensitive property, as well as a respond to NIR phase change and volume change. When applied as a strain flexible sensor, this K-M/PNIPAm hydrogel exhibited a high strain sensitivity with a gauge factor (GF) of 4.491, a broad working strain range of ≈250%, a fast response of ∼160 ms, and good cycle stability (i.e., 3000 s at 20% strain). Besides, this K-M/PNIPAm hydrogel can be used as an efficient NIR light-controlled drug release carrier to achieve on-demand drug release. This work paved the way for the application of smart response hydrogel in human health real-time monitoring and NIR-controlled drug release functions.

An Organic Solvent-Assisted Intercalation and Collection (OAIC) for Ti3C2Tx MXene with Controllable Sizes and Improved Yield

A good method of synthesizing Ti₃C₂T_x (MXene) is critical for ensuring its success in practical applications, e.g., electromagnetic interference shielding, electrochemical energy storage, catalysis, sensors, and biomedicine. The main concerns focus on the moderation of the approach, yield, and product quality. Herein, a modified approach, organic solvent-assisted intercalation and collection, was developed to prepare Ti₃C₂T_x flakes. The new approach simultaneously solves all the concerns, featuring a low requirement for facility (centrifugation speed < 4000 rpm in whole process), gram-level preparation with remarkable yield (46.3%), a good electrical conductivity (8672 S cm^-1), an outstanding capacitive performance (352 F g^-1), and easy control over the dimension of Ti₃C₂T_x flakes (0.47-4.60 μm²). This approach not only gives a superb example for the synthesis of other MXene materials in laboratory, but sheds new light for the future mass production of Ti₃C₂T_x MXene.

Preparation and Application of 2D MXene-Based Gas Sensors: A Review

Since MXene (a two-dimensional material) was discovered in 2011, it has been favored in all aspects due to its rich surface functional groups, large specific surface area, high conductivity, large porosity, rich organic bonds, and high hydrophilicity. In this paper, the preparation of MXene is introduced first. HF etching was the first etching method for MXene; however, HF is corrosive, resulting in the development of the in situ HF method (fluoride + HCl). Due to the harmful effects of fluorine terminal on the performance of MXene, a fluorine-free preparation method was developed. The increase in interlayer spacing brought about by adding an intercalator can affect MXene’s performance. The usual preparation methods render MXene inevitably agglomerate and the resulting yields are insufficient. Many new preparation methods were researched in order to solve the problems of agglomeration and yield. Secondly, the application of MXene-based materials in gas sensors was discussed. MXene is often regarded as a flexible gas sensor, and the detection of ppb-level acetone at room temperature was observed for the first time. After the formation of composite materials, the increasing interlayer spacing and the specific surface area increased the number of active sites of gas adsorption and the gas sensitivity performance improved. Moreover, this paper discusses the gas-sensing mechanism of MXene. The gas-sensing mechanism of metallic MXene is affected by the expansion of the lamellae and will be doped with H2O and oxygen during the etching process in order to become a p-type semiconductor. A p-n heterojunction and a Schottky barrier forms due to combinations with other semiconductors; thus, the gas sensitivities of composite materials are regulated and controlled by them. Although there are only several reports on the application of MXene materials to gas sensors, MXene and its composite materials are expected to become materials that can effectively detect gases at room temperature, especially for the detection of NH3 and VOC gas. Finally, the challenges and opportunities of MXene as a gas sensor are discussed.

Preparation Strategies and Applications of MXene-Polymer Composites: A Review

As a new member of the 2D material family, MXene integrates high metallic conductivity and hydrophilic property simultaneously. It shows tremendous potential in fields of energy storage, sensing, electromagnetic shielding, and so forth. Due to the abundant surface functional groups, the physical and chemical properties of MXene can be tuned by the formation of MXene-polymer composites. The introduction of polymers can expand the interlayer spacing, reduce the distance of ion/electron transport, improve the surface hydrophilicity, and thus guide the assembly of MXene-polymer structures. Herein, the preparation strategies of MXene-polymer composites including physical mixing, surface modification, such as anchoring through TiN and Ti-O-C bonds, bonding through esterification, grafting functional groups through TiOSi/TiOP bonds, photograft reaction, as well as in situ polymerization are highlighted. In addition, the possible mechanisms for each strategy are explained. Furthermore, the applications of MXene-polymer composites obtained by different preparation strategies are summarized. Finally, perspectives and challenges are presented for the designs of MXene-polymer composites.

A transport channel-regulated MXene membrane via organic phosphonic acids for efficient water permeation

A series of organic phosphonic acids (OPAs) were applied as multifunctional spacers to enlarge the inner space of carbide MXene (Ti3C2Tx) laminates. A synergistic improvement in permeance, rejection and stability is achieved via introducing OPA to create pillared laminates. This strategy provides a universal way to regulate transport channels of MXene-based membranes.

Polymer-Laminated Ti3C2TX MXene Electrodes for Transparent and Flexible Field-Driven Electronics

MXenes (Ti₃C₂T_X) are two-dimensional transition-metal carbides and carbonitrides with high conductivity and optical transparency. However, transparent MXene electrodes with high environmental stability suitable for various flexible organic electronic devices have rarely been demonstrated. By laminating a thin polymer film onto a solution-processed MXene layer to protect the MXene film from harsh environmental conditions, we present transparent and flexible MXene electronic devices. A thin polymer layer spin-coated onto a transparent MXene electrode provides environmental stability even under air exposure longer than 7 d at high temperatures (up to 70 °C) and humidity levels (up to 50%) without degrading the transparency of the electrode. The resulting polymer-laminated (PL) MXene electrode facilitates the development of a variety of field-driven photoelectronic devices by exploiting the electric field exerted between the MXene layer and the counter electrode through the insulating polymer. Field-induced electroluminescent displays, based on both organic and inorganic phosphors, with PL-MXene electrodes are demonstrated with high transparency and mechanical flexibility. Furthermore, our PL-MXene electrode exhibits high versatility through successful implementation in capacitive-type pressure sensors and triboelectric nanogenerators, resulting in field-driven sensing and energy harvesting electronic devices with excellent operation reliability.

Mechanism and Kinetics of Oxidation Reaction of Aqueous Ti3C2Tx Suspensions at Different pHs and Temperatures

Understanding the oxidation reaction of aqueous Ti₃C₂T_x MXene suspensions is very important for fostering fundamental academic studies as well as widespread industrial applications. Herein, we investigated the mechanism and kinetics of the oxidation reaction of aqueous Ti₃C₂T_x suspensions at various pH and temperature conditions. Through comprehensive analysis, the mechanism of the chemical oxidative degradation of aqueous Ti₃C₂T_x colloids was established. Chemical oxidation produces solid products such as TiO₂ and amorphous carbon as well as various gaseous species including CH₄, CO, CO₂, and HF. Additionally, our comprehensive kinetic study proposes that aqueous Ti₃C₂T_x dispersions are degraded via an acid-catalyzed oxidation reaction, where, under acidic conditions, the protonation of the hydroxyl terminal groups on the Ti₃C₂T_x flakes induces electron localization on titanium atoms and accelerates their oxidation reaction. In contrast, under basic conditions, the electrostatically alkali-metalized hydroxyl intermediates forming a bulky solvent cage results in less electron localization on titanium atoms, and thus retards their oxidative degradation.

Ambient-Stable Two-Dimensional Titanium Carbide (MXene) Enabled by Iodine Etching

MXene (e.g., Ti3 C2 ) represents an important class of two-dimensional (2D) materials owing to its unique metallic conductivity and tunable surface chemistry. However, the mainstream synthetic methods rely on the chemical etching of MAX powders (e.g., Ti3 AlC2 ) using hazardous HF or alike, leading to MXene sheets with fluorine termination and poor ambient stability in colloidal dispersions. Here, we demonstrate a fluoride-free, iodine (I2 ) assisted etching route for preparing 2D MXene (Ti3 C2 Tx , T=O, OH) with oxygen-rich terminal groups and intact lattice structure. More than 71 % of sheets are thinner than 5 nm with an average size of 1.8 μm. They present excellent thin-film conductivity of 1250 S cm-1 and great ambient stability in water for at least 2 weeks. 2D MXene sheets with abundant oxygen surface groups are excellent electrode materials for supercapacitors, delivering a high gravimetric capacitance of 293 F g-1 at a scan rate of 1 mV s-1 , superior to those made from fluoride-based etchants (<290 F g-1 at 1 mV s-1 ). Our strategy provides a promising pathway for the facile and sustainable production of highly stable MXene materials.

High Concentration of Ti3C2Tx MXene in Organic Solvent

MXenes are currently one of the most widely studied two-dimensional materials due to their properties. However, obtaining highly dispersed MXene materials in organic solvent remains a significant challenge for current research. Here, we have developed a method called the tuned microenvironment method (TMM) to prepare a highly concentrated Ti₃C₂T_x organic solvent dispersion by tuning the microenvironment of Ti₃C₂T_x. The as-proposed TMM is a simple and efficient approach, as Ti₃C₂T_x can be dispersed in N,N-dimethylformamide and other solvents by stirring and shaking for a short time, without the need for a sonication step. The delaminated single-layer MXene yield can reach 90% or greater, and a large-scale synthesis has also been demonstrated with TMM by delaminating 30 g of multilayer Ti₃C₂T_x raw powder in a one-pot synthesis. The synthesized Ti₃C₂T_x nanosheets dispersed in an organic solvent possess a clean surface, uniform thickness, and large size. The Ti₃C₂T_x dispersed in an organic solvent exhibits excellent oxidation resistance even under aerobic conditions at room temperature. Through the experimental investigation, the successful preparation of a highly concentrated Ti₃C₂T_x organic solvent dispersion via TMM can be attributed to the following factors: (1) the intercalation of the cation can lead to the change in the hydrophobicity and surface functionalization of the material; (2) proper solvent properties are required in order to disperse MXene nanosheets well. To demonstrate the applicability of the highly concentrated Ti₃C₂T_x organic solvent dispersion, a composite fiber with excellent electrical conductivity is prepared via the wet-spinning of a Ti₃C₂T_x (dispersed in DMF) and polyacrylonitrile mixture. Finally, various types of MXenes, such as Nb₂CT_x, Nb₄C₃T_x, and Mo₂Ti₂C₃T_x, can also be prepared as highly concentrated MXene organic solvent dispersions via TMM, which proves the universality of this method. Thus, it is expected that this work demonstrates promising potential in the research of the MXene material family.

Recent Advanced on the MXene-Organic Hybrids: Design, Synthesis, and Their Applications

With increasing research interest in the field of flexible electronics and wearable devices, intensive efforts have been paid to the development of novel inorganic-organic hybrid materials. As a newly developed two-dimensional (2D) material family, MXenes present many advantages compared with other 2D analogs, especially the variable surface terminal groups, thus the infinite possibility for the regulation of surface physicochemical properties. However, there is still less attention paid to the interfacial compatibility of the MXene-organic hybrids. To this end, this review will briefly summarize the recent progress on MXene-organic hybrids, offers a deeper understanding of the interaction and collaborative mechanism between the MXenes and organic component. After the discussion of the structure and surface characters of MXenes, strategies towards MXene-organic hybrids are introduced based on the interfacial interactions. Based on different application scenarios, the advantages of MXene-organic hybrids in constructing flexible devices are then discussed. The challenges and outlook on MXene-organic hybrids are also presented.

Natural bamboo leaves as dielectric layers for flexible capacitive pressure sensors with adjustable sensitivity and a broad detection range

Developing a high-performance capacitive sensor for diverse application scenarios has posed requirements for the sensor to have high sensitivity, broad detection range, and cost-effectiveness. In this experiment, a flexible pressure sensor with a high sensitivity of 2.08 kPa⁻¹ at pressure lower than 1 kPa, as well as a wide working range of 0–600 kPa and remarkable stability (for at least 4000 cycles), was designed. In the device structure, silver nanowires (Ag NWs)/MXene-composite-coated polydimethylsiloxane (PDMS) and natural bamboo leaves at different growth stages were used as the electrode and the micro-structured dielectric layers, respectively. The rough surface of the composite conductive materials and the hierarchical microstructure of the bamboo leaves ensured a high sensitivity and broad pressure range of the sandwich-structured sensor, and the different sizes of the microstructures yielded adjustable sensitivity of the sensor. Furthermore, the outstanding performance of the proposed device made it possible to detect the actual object load, human physical stimuli, and proximity distance, demonstrating applications of flexible and wearable devices in various fields, such as weight/force tapping, breath/wrist pulse/speech, joint bending, and approach distance.

Capacitive pressure sensors based on bamboo leaves endow adjustable sensitivity, wide working range and remarkable stability, indicating promising applications in diverse application scenarios.

2D Transition Metal Carbides (MXenes): Applications as an Electrically Conducting Material

Since their discovery in 2011, 2D transition metal carbides, nitrides, and carbonitrides, known as MXenes, have attracted considerable global research interest owing to their outstanding electrical conductivity coupled with light weight, flexibility, transparency, surface chemistry tunability, and easy solution processability. Here, the promising abilities of 2D MXenes, from both experimental and theoretical perspectives, for designing conductive materials for a range of applications, including electromagnetic interference shielding, flexible optoelectronics, sensors, and thermal heaters, are presented.