High-strength scalable MXene films through bridging-induced densification

Sustainable 3D printing by reversible salting-out effects with aqueous salt solutions

Achieving a simple yet sustainable printing technique with minimal instruments and energy remains challenging. Here, a facile and sustainable 3D printing technique is developed by utilizing a reversible salting-out effect. The salting-out effect induced by aqueous salt solutions lowers the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) solutions to below 10 °C. It enables the spontaneous and instant formation of physical crosslinks within PNIPAM chains at room temperature, thus allowing the PNIPAM solution to solidify upon contact with a salt solution. The PNIPAM solutions are extrudable through needles and can immediately solidify by salt ions, preserving printed structures, without rheological modifiers, chemical crosslinkers, and additional post-processing steps/equipment. The reversible physical crosslinking and de-crosslinking of the polymer through the salting-out effect demonstrate the recyclability of the polymeric ink. This printing approach extends to various PNIPAM-based composite solutions incorporating functional materials or other polymers, which offers great potential for developing water-soluble disposable electronic circuits, carriers for delivering small materials, and smart actuators.

Reversible Switching Between Microwave Absorption and EMI Shielding of VO2 Composite Foam

Developing lightweight composite with reversible switching between microwave (MW) absorption and electromagnetic interference (EMI) shielding is promising yet remains highly challenging due to the completely inconsistent attenuation mechanism for electromagnetic (EM) radiation. Here, a lightweight vanadium dioxide/expanded polymer microsphere composites foam (VO2/EPM) is designed and fabricated with porous structures and 3D VO2 interconnection, which possesses reversible switching function between MW absorption and EMI shielding under thermal stimulation. The VO2/EPM exhibits MW absorption with a broad effective absorption bandwidth of 3.25 GHz at room temperature (25 °C), while provides EMI shielding of 23.1 dB at moderately high temperature (100 °C). This reversible switching performance relies on the porous structure and tunability of electrical conductivity, complex permittivity, and impedance matching, which are substantially induced by the convertible crystal structure and electronic structure of VO2. Finite element simulation is employed to qualitatively investigate the change in interaction between EM waves and VO2/EPM before and after the phase transition. Moreover, the application of VO2/EPM is demonstrated with a reversible switching function in controlling wireless transmission on/off, showcasing its excellent cycling stability. This kind of smart material with a reversible switching function shows great potential in next-generation electronic devices.

Large-Scale, Mechanically Robust, Solvent-Resistant, and Antioxidant MXene-Based Composites for Reliable Long-Term Infrared Stealth

MXene-based thermal camouflage materials have gained increasing attention due to their low emissivity, however, the poor anti-oxidation restricts their potential applications under complex environments. Various modification methods and strategies, e.g., the addition of antioxidant molecules and fillers have been developed to overcome this, but the realization of long-term, reliable thermal camouflage using MXene network (coating) with excellent comprehensive performance remains a great challenge. Here, a MXene-based hybrid network comodified with hyaluronic acid (HA) and hyperbranched polysiloxane (HSi) molecules is designed and fabricated. Notably, the presence of appreciated HA molecules restricts the oxidation of MXene sheets without altering infrared stealth performance, superior to other water-soluble polymers; while the HSi molecules can act as efficient cross-linking agents to generate strong interactions between MXene sheets and HA molecules. The optimized MXene/HA/HSi composites exhibit excellent mechanical flexibility (folded into crane structure), good water/solvent resistance, and long-term stable thermal camouflage capability (with low infrared emissivity of ≈0.29). The long-term thermal camouflage reliability (≈8 months) under various outdoor weathers and the scalable coating capability of the MXene-coated textile enable them to disguise the IR signal of various targets in complex environments, indicating the great promise of achieved material for thermal camouflage, IR stealth, and counter surveillance.

Near-room-temperature water-mediated densification of bulk van der Waals materials from their nanosheets

The conventional fabrication of bulk van der Waals (vdW) materials requires a temperature above 1,000 °C to sinter from the corresponding particulates. Here we report the near-room-temperature densification (for example, ∼45 °C for 10 min) of two-dimensional nanosheets to form strong bulk materials with a porosity of <0.1%, which are mechanically stronger than the conventionally made ones. The mechanistic study shows that the water-mediated activation of van der Waals interactions accounts for the strong and dense bulk materials. Initially, water adsorbed on two-dimensional nanosheets lubricates and promotes alignment. The subsequent extrusion closes the gaps between the aligned nanosheets and densifies them into strong bulk materials. Water extrusion also generates stresses that increase with moulding temperature, and too high a temperature causes intersheet misalignment; therefore, a near-room-temperature moulding process is favoured. This technique provides an energy-efficient alternative to design a wide range of dense bulk van der Waals materials with tailored compositions and properties.

Few-layer V2C/MWCNT with high ionic accessibility for lithium-ion storage

A V2C MXene has a high theoretical capacity and low diffusion barrier, showing tremendous potential in lithium-ion batteries. However, most reports on V2C focus on a multilayered structure that is stacked, which diminishes the ionic accessibility and results in unsatisfactory cycling stability. Therefore, we synthesized a few-layer V2C (f-V2C) material and added multi-walled carbon nanotubes (MWCNTs). The formed f-V2C/MWCNT provides abundant pores, which enhance ionic accessibility, so that Li+ can easily enter the layer space. The introduction of MWCNTs can further separate the f-V2C, expand the specific surface area, reduce the charge transfer resistance, and heighten the structural stability. The experiments reveal that f-V2C/MWCNT has a high specific capacity of 531 mA h g-1 at 0.1 A g-1 after 100 cycles. Even at a high current density of 5.0 A g-1, the specific capacity can still reach 166 mA h g-1. Moreover, the f-V2C/MWCNT structure shows good cycling stability with a capacity retention rate of 95% after 1000 cycles at 5.0 A g-1. The above findings indicate that f-V2C/MWCNT has great application potential in the field of Li+ storage.

Wearable EMI Shielding Composite Films with Integrated Optimization of Electrical Safety, Biosafety and Thermal Safety

Biomaterial-based flexible electromagnetic interference (EMI) shielding composite films are desirable in many applications of wearable electronic devices. However, much research focuses on improving the EMI shielding performance of materials, while optimizing the comprehensive safety of wearable EMI shielding materials has been neglected. Herein, wearable cellulose nanofiber@boron nitride nanosheet/silver nanowire/bacterial cellulose (CNF@BNNS/AgNW/BC) EMI shielding composite films with sandwich structure are fabricated via a simple sequential vacuum filtration method. For the first time, the electrical safety, biosafety, and thermal safety of EMI shielding materials are optimized integratedly. Since both sides of the sandwich structure contain CNF and BC electrical insulation layers, the CNF@BNNS/AgNW/BC composite films exhibit excellent electrical safety. Furthermore, benefiting from the AgNW conductive networks in the middle layer, the CNF@BNNS/AgNW/BC exhibit excellent EMI shielding effectiveness of 49.95 dB and ultra-fast response Joule heating performance. More importantly, the antibacterial property of AgNW ensures the biosafety of the composite films. Meanwhile, the AgNW and the CNF@BNNS layers synergistically enhance the thermal conductivity of the CNF@BNNS/AgNW/BC composite film, reaching a high value of 8.85 W m‒1 K‒1, which significantly enhances its thermal safety when used in miniaturized electronic device. This work offers new ideas for fabricating biomaterial-based EMI shielding composite films with high comprehensive safety.

Depletion Strategies for Crystallized Layers of Two-Dimensional Nanosheets to Enhance Lithium-Ion Conductivity in Polymer Nanocomposites

The assembly of long-range aligned structures of two-dimensional nanosheets (2DNSs) in polymer nanocomposites (PNCs) is in urgent need for the design of nanoelectronics and lightweight energy-storage materials of high conductivity for electricity or heat. These 2DNS are thin and exhibit thermal fluctuations, leading to an intricate interplay with polymers in which entropic effects can be exploited to facilitate a range of different assemblies. In molecular dynamics simulations of experimentally studied 2DNSs, we show that the layer-forming crystallization of 2DNSs is programmable by regulating the strengths and ranges of polymer-induced entropic depletion attractions between pairs of 2DNSs, as well as between single 2DNSs and a substrate surface, by exclusively tuning the temperature and size of the 2DNS. Enhancing the temperature supports the 2DNS-substrate depletion rather than crystallization of 2DNSs in the bulk, leading to crystallized layers of 2DNSs on the substrate surfaces. On the other hand, the interaction range of the 2DNS-2DNS depletion attraction extends further than the 2DNS-substrate attraction whenever the 2DNS size is well above the correlation length of the polymers, which results in a nonmonotonic dependence of the crystallization layer on the 2DNS size. It is demonstrated that the depletion-tuned crystallization layers of 2DNSs contribute to a conductive channel in which individual lithium ions (Li ions) migrate efficiently through the PNCs. This work provides statistical and dynamical insights into the balance between the 2DNS-2DNS and 2DNS-substrate depletion interactions in polymer-2DNS composites and highlights the possibilities to exploit depletion strategies in order to engineer crystallization processes of 2DNSs and thus to control electrical conductivity.

Sustainable liquid metal-induced conductive nacre

Nacre has inspired research to fabricate tough bulk composites for practical applications using inorganic nanomaterials as building blocks. However, with the considerable pressure to reduce global carbon emissions, preparing nacre-inspired composites remains a significant challenge using more economical and environmentally friendly building blocks. Here we demonstrate tough and conductive nacre by assembling aragonite platelets exfoliated from natural nacre, with liquid metal and sodium alginate used as the "mortar". The formation of GaOC coordination bonding between the gallium ions and sodium alginate molecules reduces the voids and improves compactness. The resultant conductive nacre exhibits much higher mechanical properties than natural nacre. It also shows excellent impact resistance attributed to the synergistic strengthening and toughening fracture mechanisms induced by liquid metal and sodium alginate. Furthermore, our conductive nacre exhibits exceptional self-monitoring sensitivity for maintaining structural integrity. The proposed strategy provides a novel avenue for turning natural nacre into a valuable green composite.

Reversible Constrained Dissociation and Reassembly of MXene Films

Enabling materials to undergo reversible dynamic transformations akin to the behaviors of living organisms represents a critical challenge in the field of material assembly. The pursuit of such capabilities using conventional materials has largely been met with limited success. Herein, the discovery of reversible constrained dissociation and reconfiguration in MXene films, offering an effective solution to overcome this obstacle is reported. Specifically, MXene films permit rapid intercalation of water molecules between their distinctive layers, resulting in a significant expansion and exhibiting confined dissociation within constrained spaces. Meanwhile, the process of capillary compression driven by water evaporation reinstates the dissociated MXene film to its original compact state. Further, the adhesive properties emerging from the confined disassociation of MXene films can spontaneously induce fusion between separate films. Utilizing this attribute, complex structures of MXene films can be effortlessly foamed and interlayer porosity precisely controlled, using only water as the inducer. Additionally, a parallel phenomenon has been identified in graphene oxide films. This work not only provides fresh insights into the microscopic mechanisms of 2D materials such as MXene but also paves a transformative path for their macroscopic assembly applications in the future.

γ-Ray Irradiation Significantly Enhances Capacitive Energy Storage Performance of Polymer Dielectric Films

Polymer dielectric capacitors are fundamental in advanced electronics and power grids but suffer from low energy density, hindering miniaturization of compact electrical systems. It is shown that high-energy and strong penetrating γ-irradiation significantly enhances capacitive energy storage performance of polymer dielectrics. γ-irradiated biaxially oriented polypropylene (BOPP) films exhibit an extraordinarily high energy density of 10.4 J cm-3 at 968 MV m-1 with an efficiency of 97.3%. In particular, an energy density of 4.06 J cm-3 with an ultrahigh efficiency of 98% is reliably maintained through 20 000 charge-discharge cycles under 600 MV m-1. At 125 °C, the γ-irradiated BOPP film still delivers a high discharged energy density of 5.88 J cm-3 with an efficiency of 90% at 770 MV m-1. Substantial improvements are also achieved for γ-irradiated cycloolefin copolymers at a high temperature of 150 °C, verifying the strategy generalizability. Experimental and theoretical analyses reveal that the excellent performance should be related to the γ-irradiation induced polar functional groups with high electron affinity in the molecular chain, which offer deep energy traps to impede charge transport. This work provides a simple and generally applicable strategy for developing high-performance polymer dielectrics.

Super-Stretchable and High-Energy Micro-Pseudocapacitors Based on MXene Embedded Ag Nanoparticles

The advancement of aqueous micro-supercapacitors offers an enticing prospect for a broad spectrum of applications, spanning from wearable electronics to micro-robotics and sensors. Unfortunately, conventional micro-supercapacitors are characterized by low capacity and slopy voltage profiles, limiting their energy density capabilities. To enhance the performance of these devices, the use of 2D MXene-based compounds has recently been proposed. Apart from their capacitive contributions, these structures can be loaded with redox-active nanowires which increase their energy density and stabilize their operation voltage. However, introducing rigid nanowires into MXene films typically leads to a significant decline in their mechanical properties, particularly in terms of flexibility. To overcome this issue, super stretchable micro-pseudocapacitor electrodes composed of MXene nanosheets and in situ reconstructed Ag nanoparticles (Ag-NP-MXene) are herein demonstrated, delivering high energy density, stable operation voltage of ≈1 V, and fast charging capabilities. Careful experimental analysis and theoretical simulations of the charging mechanism of the Ag-NP-MXene electrodes reveal a dual nature charge storage mechanism involving ad(de)sorption of ions and conversion reaction of Ag nanoparticles. The superior mechanical properties of synthesized films obtained through in situ construction of Ag-NP-MXene structure show an ultra stretchability, allowing the devices to provide stable voltage and energy output even at 100% elongation.

Multifunctional black phosphorus pressure sensors with bending angle monitoring and direction recognition characteristics

Flexible pressure sensors, an important class of intelligent sensing devices, are widely explored in body-motion and medical health monitoring, artificial intelligence and human-machine interaction. As a unique layered nanomaterial, black phosphorus (BP) has excellent electrical, mechanical, and flexible characteristics, which make it a promising candidate for fabricating high-performance pressure sensors. Herein, hierarchically structured BP-based pressure sensors were constructed. The sensors exhibit high sensitivity, stability and a wide sensing range and respond to various human motions including finger pressure, swallowing, and wrist bending. The sensors can identify different handwriting processes with featured signals. In particular, benefiting from the unique structure of loose-dense layers, the sensors show a distinctive response to bending angles and directions, revealing a characteristic of direction recognition. This feature facilitates the sensors to monitor human motions. The sensors have been successfully powered by a home-made Cu2ZnSn(S,Se)4 thin-film solar cell, which demonstrates the sustainability, flexibility and low power consumption of integrated devices. This work offers a strategy to construct hierarchically structured pressure/strain sensors with direction recognition and provides further insights into manufacturing portable sensing devices for realistic and innovative applications.

Microwave-Absorbing Foams with Adjustable Absorption Frequency and Structural Coloration

Microwave-absorbing materials with regulatable absorption frequency and optical camouflage hold great significance in intelligent electronic devices and advanced stealth technology. Herein, we present an innovative microwave-absorbing foam that can dynamically tune microwave absorption frequencies via a simple mechanical compression while in parallel enabling optical camouflage over broad spectral ranges by adjusting the structural colors. The vivid colors spanning different color categories generated from thin-film interference can be precisely regulated by adjusting the thickness of the conformal TiO2 coatings on Ni/melamine foam. Enhanced interfacial and defect-induced polarizations resulting from the introduction of TiO2 coating synergistically contribute to the dielectric attenuation performance. Consequently, such a foam exhibits exceptional microwave absorption capabilities, and the absorption frequency can be dynamically tuned from the S band to the Ku band by manipulating its compression ratio. Additionally, simulation calculations validate the adjustable electromagnetic wave loss behavior, offering valuable insights for the development of next-generation intelligent electromagnetic devices across diverse fields.

Hydrogen Bond-Mediated Strong Plasticization for High-Performance Alginate Plastics

The increasingly severe plastic pollution has urged an inevitable trend to develop biodegradable plastic products that can take over synthetic plastics. As one of the most abundant natural polymers, polysaccharides are an ideal candidate to substitute synthetic plastics. The rigidity of polysaccharide chains principally allows for high strength and stiffness of their materials, however, challenges the facile orientation in material processing. Here, a general hydrogen bond-mediated plasticization strategy to regulate isotropic sodium alginate (SA) chains to a highly ordered state is developed, and alginate plastics with high performances are fabricated. It is revealed that hydroxyl groups in glycerol modulate the viscoelasticity of SA solids by forming strong hydrogen bonds with SA chains, achieving a large stretchability at a high solid content. Highly orientated alginate films exhibit a superior tensile strength of 575 MPa and toughness of 60.7 MJ m-3, outperforming most regenerated biomass films. The high solid content and large stretchability mediated by strong hydrogen bonding ensure plastic molding of solid-like SA with high fidelity. This hydrogen bond-mediated plasticity provides a facile but effective method to justify the high performances of polysaccharide-based plastics.

Universal Strategy for Metal-Organic Framework Growth: From Cascading-Functional Films to MOF-on-MOFs

Transformation of metal-organic framework (MOF) particles into thin films is urgently needed for the persistent development of well-applicable devices, and recently emerging functional-integrated hybrid frameworks. Although some flexible polymers and exclusive modification approaches have been proposed, the additive-free and widely applicable strategy has not been reported, hampering the deep investigation of the structure-performance relationship. A universal strategy for the in situ growth of large-area and continuous MOF films with controllable microstructures is introduced, through the modification of multi-scale and multi-structure substrates with poly(4-vinylpyridine) as the anchor to capture metal ions via Coulomb attraction. Based on the clarified structure-adsorption-separation mechanisms, the customized devices fabricated by in situ growth can achieve highly selective adsorption and excellently synergetic separation of various industrially relevant isomers. In addition, this strategy is also feasible for the construction of MOF-on-MOFs with varied lattice parameters. This strategy is easy to implement and will be widely applicable to the surface growth of diverse MOFs on desired substrates, and provides a new concept for developing hybrid MOFs integrating with customized functionalities.

Synergistic advancements in high-performance flexible capacitive pressure sensors: structural modifications, AI integration, and diverse applications

The development of flexible pressure sensors for monitoring human motion and physiological signals has attracted extensive scientific research. However, achieving low monitoring limits, a wide detection range, large bending stresses, and excellent mechanical stability simultaneously remains a serious challenge. With the aim of developing a high-performance capacitive pressure sensor (CPS), this paper introduces the successful preparation of a single-walled carbon nanotube (SWNT)/polydimethylsiloxane (S-PDMS) composite dielectric with a foam-like structure (high permittivity and low elasticity modulus) and MXene/SWNT (S-MXene) composite film electrodes with a micro-crumpled structure. The above structurally modified CPS (SMCPS) demonstrated an excellent response output during pressure loading, achieving a wide pressure detection range (up to 700 kPa), a low detection limit (16.55 Pa), fast response/recovery characteristics (48/60 ms), enhanced sensitivity across a wide pressure range, long-term stability under repeated heavy loading and unloading (40 kPa, >2000 cycles), and reliable performance under various temperature and humidity conditions. The SMCPS demonstrated a precise and stable capacitive response in monitoring subtle physiological signals and detecting motion, owing to its unique electrode structure. The flexible device was integrated with an Internet of Things module to create a smart glove system that enables real-time tracking of dynamic gestures. This system demonstrates exceptional performance in gesture recognition and prediction with artificial intelligence analysis, highlighting the potential of the SMCPS in human-machine interface applications.

Bottom-Up Film-to-Bulk Assembly Toward Bioinspired Bulk Structural Nanocomposites

Biological materials, although composed of meager minerals and biopolymers, often exhibit amazing mechanical properties far beyond their components due to hierarchically ordered structures. Understanding their structure-properties relationships and replicating them into artificial materials would boost the development of bulk structural nanocomposites. Layered microstructure widely exists in biological materials, serving as the fundamental structure in nanosheet-based nacres and nanofiber-based Bouligand tissues, and implying superior mechanical properties. High-efficient and scalable fabrication of bioinspired bulk structural nanocomposites with precise layered microstructure is therefore important yet remains difficult. Here, one straightforward bottom-up film-to-bulk assembly strategy is focused for fabricating bioinspired layered bulk structural nanocomposites. The bottom-up assembly strategy inherently offers a methodology for precise construction of bioinspired layered microstructure in bulk form, availability for fabrication of bioinspired bulk structural nanocomposites with large sizes and complex shapes, possibility for design of multiscale interfaces, feasibility for manipulation of diverse heterogeneities. Not limited to discussing what has been achieved by using the current bottom-up film-to-bulk assembly strategy, it is also envisioned how to promote such an assembly strategy to better benefit the development of bioinspired bulk structural nanocomposites. Compared to other assembly strategies, the highlighted strategy provides great opportunities for creating bioinspired bulk structural nanocomposites on demand.

Elastic properties and tensile strength of 2D Ti3C2Tx MXene monolayers

Two-dimensional (2D) transition metal nitrides and carbides (MXenes), represented by Ti3C2Tx, have broad applications in flexible electronics, electromechanical devices, and structural membranes due to their unique physical and chemical properties. Despite the Young's modulus of 2D Ti3C2Tx has been theoretically predicted to be 0.502 TPa, which has not been experimentally confirmed so far due to the measurement is extremely restricted. Here, by optimizing the sample preparation, cutting, and transfer protocols, we perform the direct in-situ tensile tests on monolayer Ti3C2Tx nanosheets using nanomechanical push-to-pull equipment under a scanning electron microscope. The effective Young's modulus is 0.484 ± 0.013 TPa, which is much closer to the theoretical value of 0.502 TPa than the previously reported 0.33 TPa by the disputed nanoindentation method, and the measured elastic stiffness is ~948 N/m. Moreover, during the process of tensile loading, the monolayer Ti3C2Tx shows an average elastic strain of ~3.2% and a tensile strength as large as ~15.4 GPa. This work corrects the previous reports by nanoindentation method and demonstrates that the Ti3C2Tx indeed keeps immense potential for broad range of applications.

Water-induced strong isotropic MXene-bridged graphene sheets for electrochemical energy storage

Graphene and two-dimensional transition metal carbides and/or nitrides (MXenes) are important materials for making flexible energy storage devices because of their electrical and mechanical properties. It remains a challenge to assemble nanoplatelets of these materials at room temperature into in-plane isotropic, free-standing sheets. Using nanoconfined water-induced basal-plane alignment and covalent and π-π interplatelet bridging, we fabricated Ti3C2Tx MXene-bridged graphene sheets at room temperature with isotropic in-plane tensile strength of 1.87 gigapascals and moduli of 98.7 gigapascals. The in-plane room temperature electrical conductivity reached 1423 siemens per centimeter, and volumetric specific capacity reached 828 coulombs per cubic centimeter. This nanoconfined water-induced alignment likely provides an important approach for making other aligned macroscopic assemblies of two-dimensional nanoplatelets.

Low-dimensional nanostructures for monolithic 3D-integrated flexible and stretchable electronics

Flexible/stretchable electronics, which are characterized by their ultrathin design, lightweight structure, and excellent mechanical robustness and conformability, have garnered significant attention due to their unprecedented potential in healthcare, advanced robotics, and human-machine interface technologies. An increasing number of low-dimensional nanostructures with exceptional mechanical, electronic, and/or optical properties are being developed for flexible/stretchable electronics to fulfill the functional and application requirements of information sensing, processing, and interactive loops. Compared to the traditional single-layer format, which has a restricted design space, a monolithic three-dimensional (M3D) integrated device architecture offers greater flexibility and stretchability for electronic devices, achieving a high-level of integration to accommodate the state-of-the-art design targets, such as skin-comfort, miniaturization, and multi-functionality. Low-dimensional nanostructures possess small size, unique characteristics, flexible/elastic adaptability, and effective vertical stacking capability, boosting the advancement of M3D-integrated flexible/stretchable systems. In this review, we provide a summary of the typical low-dimensional nanostructures found in semiconductor, interconnect, and substrate materials, and discuss the design rules of flexible/stretchable devices for intelligent sensing and data processing. Furthermore, artificial sensory systems in 3D integration have been reviewed, highlighting the advancements in flexible/stretchable electronics that are deployed with high-density, energy-efficiency, and multi-functionalities. Finally, we discuss the technical challenges and advanced methodologies involved in the design and optimization of low-dimensional nanostructures, to achieve monolithic 3D-integrated flexible/stretchable multi-sensory systems.

Robust and sensitive conductive nanocomposite hydrogel with bridge cross-linking-dominated hierarchical structural design

Conductive hydrogels have a remarkable potential for applications in soft electronics and robotics, owing to their noteworthy attributes, including electrical conductivity, stretchability, biocompatibility, etc. However, the limited strength and toughness of these hydrogels have traditionally impeded their practical implementation. Inspired by the hierarchical architecture of high-performance biological composites found in nature, we successfully fabricate a robust and sensitive conductive nanocomposite hydrogel through self-assembly-induced bridge cross-linking of MgB2 nanosheets and polyvinyl alcohol hydrogels. By combining the hierarchical lamellar microstructure with robust molecular B─O─C covalent bonds, the resulting conductive hydrogel exhibits an exceptional strength and toughness. Moreover, the hydrogel demonstrates exceptional sensitivity (response/relaxation time, 20 milliseconds; detection lower limit, ~1 Pascal) under external deformation. Such characteristics enable the conductive hydrogel to exhibit superior performance in soft sensing applications. This study introduces a high-performance conductive hydrogel and opens up exciting possibilities for the development of soft electronics.

Constructed MXene matrix composites as sensing material and applications thereof: A review

Most studies on MXene matrix composites for sensor development have primarily focused on synthesis and application. Nevertheless, there is currently a lack of research on how the introduction of different materials affects the sensing properties of these composites. The rapid development of MXene has raised intriguing questions about improving sensor performance by combining MXene with other materials such as polymers, metals and inorganic non-metals. This review will concentrate on the construction of MXene-based composites and explore ways to enhance their sensor applications. Specifically, this review describes why the introduction of materials to the system brings the advantage of low concentration and high sensitivity assays, as well as the MXene-based frameworks that have been recently investigated. Lastly, in order to capture the current trend of MXene-based composites in sensor applications and identify promising research directions, this review will critically evaluate the potential applications of newly developed MXene systems.

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.

Recent progress of MXene as an energy storage material

Thanks to its adjustable interlayer distance, large specific surface area, abundant active sites, and diverse surface functional groups, MXene has always been regarded as an excellent candidate for energy storage materials, including supercapacitors and ion batteries. Recent studies have also shown that MXene can serve as an efficient hydrogen storage catalyst. This review aims to summarize the latest research achievements in the field of MXene, especially its performance and application in energy storage. Different synthesis techniques have different effects on the energy storage performance of MXene. In this review, various common synthesis methods and the latest innovations in synthesis methods are discussed. MXene is prone to oxidation, and how to resist oxidation is also an important topic in MXene research. This article introduces the research results on improving the chemical stability of MXene through annealing. In addition, it aims to gain a deeper understanding of the future development and potential of MXene.

Quantitative analysis of printed nanostructured networks using high-resolution 3D FIB-SEM nanotomography

Networks of solution-processed nanomaterials are becoming increasingly important across applications in electronics, sensing and energy storage/generation. Although the physical properties of these devices are often completely dominated by network morphology, the network structure itself remains difficult to interrogate. Here, we utilise focused ion beam - scanning electron microscopy nanotomography (FIB-SEM-NT) to quantitatively characterise the morphology of printed nanostructured networks and their devices using nanometre-resolution 3D images. The influence of nanosheet/nanowire size on network structure in printed films of graphene, WS2 and silver nanosheets (AgNSs), as well as networks of silver nanowires (AgNWs), is investigated. We present a comprehensive toolkit to extract morphological characteristics including network porosity, tortuosity, specific surface area, pore dimensions and nanosheet orientation, which we link to network resistivity. By extending this technique to interrogate the structure and interfaces within printed vertical heterostacks, we demonstrate the potential of this technique for device characterisation and optimisation.

Functional MXenes: Progress and Perspectives on Synthetic Strategies and Structure-Property Interplay for Next-Generation Technologies

MXenes are a class of 2D materials that include layered transition metal carbides, nitrides, and carbonitrides. Since their inception in 2011, they have garnered significant attention due to their diverse compositions, unique structures, and extraordinary properties, such as high specific surface areas and excellent electrical conductivity. This versatility has opened up immense potential in various fields, catalyzing a surge in MXene research and leading to note worthy advancements. This review offers an in-depth overview of the evolution of MXenes over the past 5 years, with an emphasis on synthetic strategies, structure-property relationships, and technological prospects. A classification scheme for MXene structures based on entropy is presented and an updated summary of the elemental constituents of the MXene family is provided, as documented in recent literature. Delving into the microscopic structure and synthesis routes, the intricate structure-property relationships are explored at the nano/micro level that dictate the macroscopic applications of MXenes. Through an extensive review of the latest representative works, the utilization of MXenes in energy, environmental, electronic, and biomedical fields is showcased, offering a glimpse into the current technological bottlenecks, such asstability, scalability, and device integration. Moreover, potential pathways for advancing MXenes toward next-generation technologies are highlighted.

High-strength nacre-like composite films based on pre-polymerised polydopamine and polyethyleneimine cross-linked MXene layers via multi-bonding interactions

Demands for high-strength flexible electrodes have significantly increased across various fields, especially in wearable electronics. Inspired by the strong integrated layered structure of the natural nacre via multi-bonding interactions, we report the fabrication of the strong integrated nacre-like composite films based on pre-polymerised polydopamine and polyethyleneimine cross-linked MXene layers (p-DEM), achieving the synergic effect of hydrogen bonding, covalent bonding and electrostatic interactions. As a result, a high-level tensile strength of ∼302 MPa, 10.8 times higher than that of the plain MXene film, is obtained for the prepared p-DE0.5M composite film. Meanwhile, the composite film also delivers superior energy storage (∼1218F cm-3 at 5 mV s-1) and rate performances (capacitance retention of 81.3% at 1000 mV s-1). To demonstrate the practical application of the composite films, a symmetrical supercapacitor based on p-DE0.5M electrodes is assembled, which shows stable energy storage performances under different deformation states such as bending angles at 0, 60, 90 and 180°, or withstand repeated bending times (1000 cycles). This type of multi-bonding interactions induced strong integrated MXene assembly may promote the wide applications of MXene-based films in flexible electronics, artificial intelligence, and tissue engineering, to name a few.

Robust and Multifunctional Ti3 C2 Tx /Modified Sawdust Composite Paper for Electromagnetic Interference Shielding and Wearable Thermal Management

Robust, ultrathin, and environmental-friendliness papers that synergize high-efficiency electromagnetic interference (EMI) shielding, personal thermal management, and wearable heaters are essential for next-generation smart wearable devices. Herein, MXene nanocomposite paper with a nacre-like structure for EMI shielding and electrothermal/photothermal conversion is fabricated by vacuum filtration of Ti3 C2 Tx MXene and modified sawdust. The hydrogen bonding and highly oriented structure enhance the mechanical properties of the modified sawdust/MXene composite paper (SM paper). The SM paper with 50 wt% MXene content shows a strength of 23 MPa and a toughness of 13 MJ·M-3 . The conductivity of the SM paper is 10 195 S·m-1 , resulting in an EMI shielding effectiveness (SE) of 67.9 dB and a specific SE value (SSE/t) of 8486 dB·cm2 ·g-1 . In addition, the SM paper exhibits excellent thermal management performance including high light/electro-to-thermal conversion, rapid Joule heating and photothermal response, and sufficient heating stability. Notably, the SM paper exhibits low infrared emissivity and distinguished infrared stealth performance, camouflaging a high-temperature heater surface of 147-81 °C. The SM-based e-skin achieves visualization of Joule heating and realizes human motions monitoring. This work presents a new strategy for designing MXene-based wearable devices with great EMI shielding, artificial intelligence, and thermal management applications.

Core-Sheath Heterogeneous Interlocked Conductive Fiber Enables Smart Textile for Personalized Healthcare and Thermal Management

Whereas thermal comfort and healthcare management during long-term wear are essentially required for wearable system, simultaneously achieving them remains challenge. Herein, a highly comfortable and breathable smart textile for personal healthcare and thermal management is developed, via assembling stimuli-responsive core-sheath dual network that silver nanowires(AgNWs) core interlocked graphene sheath induced by MXene. Small MXene nanosheets with abundant groups is proposed as a novel "dispersant" to graphene according to "like dissolves like" theory, while simultaneously acting as "cross-linker" between AgNWs and graphene networks by filling the voids between them. The core-sheath heterogeneous interlocked conductive fiber induced by MXene "cross-linking" exhibits a reliable response to various mechanical/electrical/light stimuli, even under large mechanical deformations(100%). The core-sheath conductive fiber-enabled smart textile can adapt to movements of human body seamlessly, and convert these mechanical deformations into character signals for accurate healthcare monitoring with rapid response(440 ms). Moreover, smart textile with excellent Joule heating and photothermal effect exhibits instant thermal energy harvesting/storage during the stimuli-response process, which can be developed as self-powered thermal management and dynamic camouflage when integrated with phase change and thermochromic layer. The smart fibers/textiles with core-sheath heterogeneous interlocked structures hold great promise in personalized healthcare and thermal management.

Printed High-Adhesion Flexible Electrodes Based on an Interlocking Structure for Self-Powered Intelligent Movement Monitoring

Two-dimensional transition metal carbide nitrides (MXenes) have been extensively explored in diverse areas, such as electrochemical energy storage and flexible electronics. Although the solution-processed MXene-based device has made significant achievements, it is still a challenge to develop large-scale and high-resolution printing methods for flexible printed electronics. In this work, we reported a novel strategy of a porous interlocking structure to obtain flexible MXene/laser-induced graphene (LMX) composite electrodes with enhanced adhesion and high printing resolution. In comparison to traditional printed MXene electrodes, the LMX electrode with an interlocking interface possesses enhanced mechanical properties (adhesive strength of 2.17 MPa) and comparable electrical properties (0.68 S/mm). Furthermore, owing to the outstanding stability and flexibility, the LMX-based triboelectric nanogenerator (TENG) can be used as a self-powered sensor to monitor finger-bending movement. A support vector machine (SVM)-assisted self-powered motion sensor can distinguish the bending angle with high recognition accuracy and can effectively identify different angles. The successful experience of directly printing flexible electrodes with excellent mechanical and electrical properties can be promoted to other solution-processed two-dimensional materials. Our strategy opens up a promising perspective to develop flexible and printed electronics.

Highly Thermally Conductive and Structurally Ultra-Stable Graphitic Films with Seamless Heterointerfaces for Extreme Thermal Management

Highly thermally conductive graphitic film (GF) materials have become a competitive solution for the thermal management of high-power electronic devices. However, their catastrophic structural failure under extreme alternating thermal/cold shock poses a significant challenge to reliability and safety. Here, we present the first investigation into the structural failure mechanism of GF during cyclic liquid nitrogen shocks (LNS), which reveals a bubbling process characterized by "permeation-diffusion-deformation" phenomenon. To overcome this long-standing structural weakness, a novel metal-nanoarmor strategy is proposed to construct a Cu-modified graphitic film (GF@Cu) with seamless heterointerface. This well-designed interface ensures superior structural stability for GF@Cu after hundreds of LNS cycles from 77 to 300 K. Moreover, GF@Cu maintains high thermal conductivity up to 1088 W m-1 K-1 with degradation of less than 5% even after 150 LNS cycles, superior to that of pure GF (50% degradation). Our work not only offers an opportunity to improve the robustness of graphitic films by the rational structural design but also facilitates the applications of thermally conductive carbon-based materials for future extreme thermal management in complex aerospace electronics.

Endogenous Nb2CTx/Nb2O5 Schottky heterostructures for superior lithium-ion storage

Schottky heterostructures have significant advantages for exciting charge transfer kinetics at material interfaces. In this work, endogenous Nb2CTx/Nb2O5 Schottky heterostructures with a large active surface area were constructed using an in-situ architectural strategy. The semiconductor Nb2O5 has a low work function, and during the construction of Nb2CTx/Nb2O5 Schottky heterostructures, there was an interfacial electron transfer, which resulted in a built-in electric field. The electrochemical reaction kinetics of Nb2CTx/Nb2O5 Schottky heterostructures were enhanced due to the rapid transfer of charge driven by the electric field. The Nb2CTx/Nb2O5 Schottky heterostructures have a large active surface area, which contributes to excellent electrolyte diffusion kinetics. Therefore, Nb2CTx/Nb2O5 Schottky heterostructures have excellent lithium-ion storage capacity with 575 mAh/g after 200 cycles at 0.10 A/g, and 290 mAh/g after 1000 cycles at 2.00 A/g, without capacity fading. Furthermore, in-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy analyses reveal the mechanisms for structure evolution and lithium-ion storage optimization of Nb2CTx/Nb2O5 Schottky heterostructures during the electrochemical reaction. The construction of Schottky heterostructures with excited charge transport kinetics provides a novel idea for optimizing the lithium-ion storage activity of MXenes materials.

High-Performance Graphene Biocomposite Enabled by Fe3+ Coordination for Thermal Management

Emerging biocomposites with excellent heat dissipation capabilities and inherent sustainability are urgently needed to address the cooling issues of modern electronics and growing environmental concerns. However, the moisture stability, mechanical performance, thermal conductivity, and even flame retardancy of biomass-based materials are generally insufficient for practical thermal management applications. Herein, we present a high-performance graphene biocomposite consisting of carboxylated cellulose nanofibers and graphene nanosheets through an evaporation-induced self-assembly and subsequent Fe3+ cross-linking strategy. The Fe3+ coordination plays a critical role in stabilizing the material structure, thereby improving the mechanical strength and water stability of the biocomposite films, and its effect is revealed by density functional theory calculations. The hierarchical structure of the biocomposite films also leads to a high in-plane thermal conductivity of 42.5 W m-1 K-1, enabling a superior heat transfer performance. Furthermore, the resultant biocomposite films exhibit outstanding Joule heating performance with a fast thermal response and long-term stability, improved thermal stability, and flame retardancy. Therefore, such a general strategy and the desired overall properties of the biocomposite films offer wide application prospects for functional and safe thermal management.

An Ultra-Thin MXene Film for Multimodal Sensing of Neuroelectrical Signals with Artifacts Removal

Neuroelectrical signals transmitted onto the skin tend to decay to an extremely weak level, making them highly susceptible to interference from the environment and body movement. Meanwhile, for comprehensively understanding cognitive nerve conduction, multimodal sensing of neural signals, such as magnetic resonance imaging (MRI) and functional near-infrared spectroscopy (fNIRS), is highly required. Previous metal or polymer conductors cannot either provide a seamless on-skin feature for accurate sensing of neuroelectrical signals or be compatible with multimodal imaging techniques without opto- and magnet- artifacts. Herein, a ≈20 nm thick MXene film that is able to simultaneously detect electrophysiological signals and perform imaging by MRI and fNIRS with high fidelity is reported. The ultrathin film is made of crosslinked Ti3 C2 Tx film via poly (3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS), showing a record high electroconductivity and transparency combination (11 000 S cm-1 @89%). Among them, PEDOT: PSS not only plays a cross-linking role to stabilize MXene film but also shortens the interlayer distance for effective charge transfer and high transparency. Thus, it can achieve a low interfacial impedance with skin or neural surfaces for accurate recording of electrophysiological signals with low motion artifacts. Besides, the high transparency originating from the ultrathin feature leads to good compatibility with fNIRS and MRI without optical and magnetic artifacts, enabling multimodal cognitive neural monitoring during prolonged use.

Ultrastrong Ionotronic Films Showing Electrochemical Osmotic Actuation

A multifunctional soft material with high ionic and electrical conductivity, combined with high mechanical properties and the ability to change shape can enable bioinspired responsive devices and systems. The incorporation of all these characteristics in a single material is very challenging, as the improvement of one property tends to reduce other properties. Here, a nanocomposite film based on charged, high-aspect-ratio 1D flexible nanocellulose fibrils, and 2D Ti3 C2 Tx MXene is presented. The self-assembly process results in a stratified structure with the nanoparticles aligned in-plane, providing high ionotronic conductivity and mechanical strength, as well as large water uptake. In hydrogel form with 20 wt% liquid, the electrical conductivity is over 200 S cm-1 and the in-plane tensile strength is close to 100 MPa. This multifunctional performance results from the uniquely layered composite structure at nano- and mesoscales. A new type of electrical soft actuator is assembled where voltage as low as ±1 V resulted in osmotic effects and giant reversible out-of-plane swelling, reaching 85% strain.

Scalable Production of Catecholamine-Densified MXene Coatings for Electromagnetic Shielding and Infrared Stealth

Processing transition metal carbides/nitrides (MXenes) inks into large-area functional coatings expects promising potential for electromagnetic interference (EMI) shielding and infrared stealth. However, the coating performances, especially for scalable fabrication techniques, are greatly constrained by the flake size and stacking manner of MXene. Herein, the large-area production of highly densified and oriented MXene coatings is demonstrated by engineering interfacial interactions of small MXene flakes with catecholamine molecules. The catecholamine molecules can micro-crosslink MXene nanosheets, significantly improving the ink's rheological properties. It favors the shear-induced sheet arrangement and inhibition of structural defects in the blade coating process, making it possible to achieve high orientation and densification of MXene assembly by either large-area coating or patterned printing. Interestingly, the MXene/catecholamine coating exhibits high conductivity of up to 12 247 S cm-1 and ultrahigh specific EMI shielding effectiveness of 2.0 ×10 5  dB cm2 g-1 , obviously superior to most of the reported MXene materials. Furthermore, the regularly assembled structure also endows the MXene coatings with low infrared emissivities for infrared stealth applications. Therefore, MXene/catecholamine coatings with ultraefficient EMI shielding and low infrared emissivity prove the feasibility of applications in aerospace, military, and wearable devices.

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.

Metal ion assistant transformation strategy to synthesize catechol-based metal-organic frameworks from Ti3C2Tx precursors

Chemical transformation strategy is capable of fabricating nanomaterials with well-defined structures and fascinating performance via controllable crystallization kinetics in the phase transformation. V2CTx MXene has been used as precursors to fabricate vanadium porphyrin metal-organic frameworks (V-PMOFs) via the coordination of deprotonated carboxylic acid ligands. However, the rational and in-depth exploration of synthesis mechanism with the aim of enriching the variety of MXene (i.e., Ti3C2Tx) and organic ligands (i.e., catechol-based) to design new MOFs is rarely reported. Herein, we have first developed a metal ion assistant transformation strategy to synthesize three-dimensional catechol-based TiCu-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) MOFs with a non-interpenetrating SrSi2 (srs) framework using two-dimensional Ti3C2Tx as precursors. The unique synergetic transformation mechanism involves the electron transfer from Ti3C2Tx to electrostatically adsorbed Cu2+ ion for redox reaction, the subsequent Ti-C bond rupture for Ti4+ ion release, and the continuous chelation coordination between Ti4+/Cu2+ and HHTP. Ti3C2Tx precursors and auxiliary metal ion could be rationally substituted by V2CTx and Mn+ (e.g., Ni2+, Co2+, Mn2+, and Zn2+), respectively. This strategy lays the foundation for the design and synthesis of innovative and multifarious MOFs derived from MXene or other unconventional metal precursors.

High Impact Resistance of 2D MXene with Multiple Fracture Modes

Two-dimensional (2D) transition metal carbides/nitrides (MXenes) are promising nanomaterials due to their remarkable mechanical and electrical properties. However, the out-of-plane mechanical properties of MXene under impact loading remain unclear. Here, particular impact-resistant fracture behaviors and energy dissipation mechanisms of MXene were systemically investigated via molecular dynamics (MD) simulation. Specifically, it was found that the specific penetration energy of MXene exceeds most conventional impact-resistant materials, such as aluminum and polycarbonate. Two kinds of novel energy dissipation mechanisms, including radial fracture and crushed fracture under different impact velocities, are revealed. In addition, the sandwiched atomic-layer structure of MXene can deflect cracks and restrain their propagation to some extent, enabling the cracked MXene to retain remarkable resistance. This work provides in-depth insights into the impact-resistance of MXene, laying a foundation for its future applications.

Upscaled Synthesis Protocol for Phase-Pure, Colloidally Stable MXenes with Long Shelf Lives

MXenes are electrically conductive 2D transition metal carbides/nitrides obtained by the etching of nanolaminated MAX phase compounds, followed by exfoliation to single- or few-layered nanosheets. The mainstream chemical etching processes have evolved from pure hydrofluoric acid (HF) etching into the innovative "minimally intensive layer delamination" (MILD) route. Despite their current popularity and remarkable application potential, the scalability of MILD-produced MXenes remains unproven, excluding MXenes from industrial applications. This work proposes a "next-generation MILD" (NGMILD) synthesis protocol for phase-pure, colloidally stable MXenes that withstand long periods of dry storage. NGMILD incorporates the synergistic effects of a secondary salt, a richer lithium (Li) environment, and iterative alcohol-based washing to achieve high-purity MXenes, while improving etching efficiency, intercalation, and shelf life. Moreover, NGMILD comprises a sulfuric acid (H2 SO4 ) post-treatment for the selective removal of the Li3 AlF6 impurity that commonly persists in MILD-produced MXenes. This work demonstrates the upscaled NGMILD synthesis of (50 g) phase-pure Ti3 C2 Tz MXene clays with high extraction yields (>22%) of supernatant dispersions. Finally, NGMILD-produced MXene clays dry-stored for six months under ambient conditions experience minimal degradation, while retaining excellent redispersibility. Overall, the NGMILD protocol is a leap forward toward the industrial production of MXenes and their subsequent market deployment.

Thin lamellar films with enhanced mechanical properties for durable radiative cooling

Passive daytime radiative cooling is a promising path to tackle energy, environment and security issues originated from global warming. However, the contradiction between desired high solar reflectivity and necessary applicable performance is a major limitation at this stage. Herein, we demonstrate a "Solvent exchange-Reprotonation" processing strategy to fabricate a lamellar structure integrating aramid nanofibers with core-shell TiO2-coated Mica microplatelets for enhanced strength and durability without compromising optical performance. Such approach enables a slow but complete two-step protonation transition and the formation of three-dimensional dendritic networks with strong fibrillar joints, where overloaded scatterers are stably grasped and anchored in alignment, thereby resulting in a high strength of ~112 MPa as well as excellent environmental durability including ultraviolet aging, high temperature, scratches, etc. Notably, the strong backward scattering excited by multiple core-shell and shell-air interfaces guarantees a balanced reflectivity (~92%) and thickness (~25 μm), which is further revealed by outdoor tests where attainable subambient temperature drops are ~3.35 °C for daytime and ~6.11 °C for nighttime. Consequently, both the cooling capacity and comprehensive outdoor-services performance, greatly push radiative cooling towards real-world applications.

A synergistic interfacial and topological strategy for reinforcing aramid nanofiber films

While nanomaterials possess impressive mechanical properties at the microscale level, their macroscopic assemblies usually exhibit inferior properties due to ineffective stress transfer among individual nanomaterials. This issue is addressed in this work by achieving strong interfacial interactions between aramid nanofibers and graphene oxide nanosheets through a neutralization reaction in a dipolar solvent and regulating the topological properties using polymer micelles to form a compact structure, leading to the formation of a super-strong and super-tough nanofiber film. The film was prepared through a sol-gel-film transition process and possesses a nacre-like microstructure that deflects microcracks and prevents them from propagating straight through the film. Remarkably, it demonstrates a tensile strength of 599.0 MPa and a toughness of 37.7 MJ m-3, which are 491.0% and 1094.5% that of a pristine aramid nanofiber film, respectively. In addition, it exhibits excellent tolerance to extreme temperatures (-196 to 300 °C) and fatigue resistance to folding 10 000 times. Overall, this study presents a synergistic interfacial and topological enhancement strategy for constructing nanomaterial-based composites with inherited properties from the nanoscale building blocks to the macroscale structural material.

Strong Ti3 C2 Tx MXene-Based Composite Films Fabricated through Bioinspired Bridging for Flexible Energy Storage Devices

Flexible energy storage device is one of the most critical components as power source for wearable electronics. The emergence of MXenes, a growing family of 2D nanomaterials, has demonstrated a brand-new possibility for flexible energy storage. However, the fabrication of MXene films with satisfactory mechanical, electrical, and electrochemical reliabilities remains challenging due to the weak interlayer interactions and self-restacking of MXene sheets. Sequential bridging of polydopamine/polyethyleneimine-functionalized (PDA/PEI)-coated MXene sheets to induce synergistically covalent and hydrogen binding connections of MXene-based films is demonstrated here. By interrupting self-hydrogen bonding and π-π stacking interactions, the introduction of long-chain PEI can not only inhibit the massive aggregation of PDA, but also improve the continuity of the interconnection network of PDA/PEI between MXene layers. Hence, the as-prepared MXene/PDA/PEI composite film displays high mechanical strength (≈366 MPa) which achieves 12-fold improvement compared with pure MXene film, as well as superior energy storage capability (≈454 F g-1  at 5 mV s-1 ) and rate performance of ≈48% at 10 000 mV s-1 . This modulation of inserted polymer between MXene layers can provide an avenue for assembling high performance MXene films, and can even be extended to the fabrication of other 2D platelets for varied applications.

Effect of the deposition process on the stability of Ti3C2Tx MXene films for bioelectronics

Ti₃C₂T_x MXene is emerging as the enabling material in a broad range of wearable and implantable medical technologies, thanks to its outstanding electrical, electrochemical, and optoelectronic properties, and its compatibility with high-throughput solution-based processing. While the prevalence of Ti₃C₂T_x MXene in biomedical research, and in particular bioelectronics, has steadily increased, the long-term stability and degradation of Ti₃C₂T_x MXene films have not yet been thoroughly investigated, limiting its use for chronic applications. Here, we investigate the stability of Ti₃C₂T_x films and electrodes under environmental conditions that are relevant to medical and bioelectronic technologies: storage in ambient atmosphere (shelf-life), submersion in saline (akin to the in vivo environment), and storage in a desiccator (low-humidity). Furthermore, to evaluate the effect of the MXene deposition method and thickness on the film stability in the different conditions, we compare thin (25 nm), and thick (1.0 μm) films and electrodes fabricated via spray-coating and blade-coating. Our findings indicate that film processing method and thickness play a significant role in determining the long-term performance of Ti₃C₂T_x films and electrodes, with highly aligned, thick films from blade coating remarkably retaining their conductivity, electrochemical impedance, and morphological integrity even after 30 days in saline. Our extensive spectroscopic analysis reveals that the degradation of Ti₃C₂T_x films in high-humidity environments is primarily driven by moisture intercalation, ingress, and film delamination, with evidence of only minimal to moderate oxidation.

Stretchable and Self-Adhesive Humidity-Sensing Patch for Multiplexed Non-Contact Sensing

The slippage of moisture-sensitive materials from substrates during bending or stretching is a common issue that causes baseline drift and even failure of the flexible humidity sensors, which are essential components of wearable electronic devices. In this study, we report a stretchable, self-adhesive, and transparent humidity-sensing electronic patch comprising liquid metal particle electrodes with a stretchable serpentine structure and a humidity-sensing layer made of Ti3C2Tx MXene/carboxymethyl cellulose. This patch is constructed on a soft-hard integrated heterostructure substrate and demonstrates stable humidity-sensitive response performance at 100% tensile strain, along with autonomous adhesion to human skin. Additionally, it exhibits maximum response (1145.4%) at 90% relative humidity (RH), fast response and recovery time (1.4/5.9 s), elevated sensitivity (64.63%/% RH), and preserved humidity sensing under deformation, as well as easy scalability for multiplexed detection. We further illustrate the patch's potential applications in healthcare and environmental monitoring through a non-contact security door control system and wind monitor system. Our proposed strain-isolation strategy can be extended to other rigid conductive materials and stretchable substrates, providing a feasible mechanism for producing stretchable electronic skin patches.

Two-dimensional MXene membranes with biomimetic sub-nanochannels for enhanced cation sieving

Membranes with high ion permeability and selectivity are of considerable interest for sustainable water treatment, resource extraction and energy storage. Herein, inspired by K+ channel of streptomyces A (KcsA K+), we have constructed cation sieving membranes using MXene nanosheets and Ethylenediaminetetraacetic acid (EDTA) molecules as building blocks. Numerous negatively charged oxygen atoms of EDTA molecules and 6.0 Å two-dimensional (2D) sub-nanochannel of MXene nanosheets enable biomimetic channel size, chemical groups and tunable charge density for the resulting membranes. The membranes show the capability to recognize monovalent/divalent cations, achieving excellent K+/Mg2+ selectivity of 121.2 using mixed salt solution as the feed, which outperforms other reported membranes under similar testing conditions and transcends the current upper limit. Characterization and simulations indicate that the cation recognition effect of EDTA and partial dehydration effects play critical roles in cations selective sieving and increasing the local charge density within the sub-nanochannel significantly improves cation selectivity. Our findings provide a theoretical basis for ions transport in sub-nanochannels and an alternative strategy for design ions separation membranes.

Multifunctional MXene/C Aerogels for Enhanced Microwave Absorption and Thermal Insulation

Two-dimensional transition metal carbides and nitrides (MXene) have emerged as promising candidates for microwave absorption (MA) materials. However, they also have some drawbacks, such as poor impedance matching, high self-stacking tendency, and high density. To tackle these challenges, MXene nanosheets were incorporated into polyacrylonitrile (PAN) nanofibers and subsequently assembled into a three-dimensional (3D) network structure through PAN carbonization, yielding MXene/C aerogels. The 3D network effectively extends the path of microcurrent transmission, leading to enhanced conductive loss of electromagnetic (EM) waves. Moreover, the aerogel's rich pore structure significantly improves the impedance matching while effectively reducing the density of the MXene-based absorbers. EM parameter analysis shows that the MXene/C aerogels exhibit a minimum reflection loss (RLmin) value of - 53.02 dB (f = 4.44 GHz, t = 3.8 mm), and an effective absorption bandwidth (EAB) of 5.3 GHz (t = 2.4 mm, 7.44-12.72 GHz). Radar cross-sectional (RCS) simulations were employed to assess the radar stealth effect of the aerogels, revealing that the maximum RCS reduction value of the perfect electric conductor covered by the MXene/C aerogel reaches 12.02 dB m2. In addition to the MA performance, the MXene/C aerogel also demonstrates good thermal insulation performance, and a 5-mm-thick aerogel can generate a temperature gradient of over 30 °C at 82 °C. This study provides a feasible design approach for creating lightweight, efficient, and multifunctional MXene-based MA materials.

Applications of X-Ray-Based Characterization in MXene Research

X-rays are a penetrating form of high-energy electromagnetic radiation with wavelengths ranging from 10 pm to 10 nm. Similar to visible light, X-rays provide a powerful tool to study the atoms and elemental information of objects. Different characterization methods based on X-rays are established, such as X-ray diffraction, small- and wide-angle X-ray scattering, and X-ray-based spectroscopies, to explore the structural and elemental information of varied materials including low-dimensional nanomaterials. This review summarizes the recent progress of using X-ray related characterization methods in MXenes, a new family of 2D nanomaterials. These methods provide key information on the nanomaterials, covering synthesis, elemental composition, and the assembly of MXene sheets and their composites. Additionally, new characterization methods are proposed as future research directions in the outlook section to enhance understanding of MXene surface and chemical properties. This review is expected to provide a guideline for characterization method selection and aid in precise interpretation of the experimental data in MXene research.

Nanoconfinement enabled non-covalently decorated MXene membranes for ion-sieving

Covalent modification is commonly used to tune the channel size and functionality of 2D membranes. However, common synthesis strategies used to produce such modifications are known to disrupt the structure of the membranes. Herein, we report less intrusive yet equally effective non-covalent modifications on Ti₃C₂T_x MXene membranes by a solvent treatment, where the channels are robustly decorated by protic solvents via hydrogen bond network. The densely functionalized (-O, -F, -OH) Ti₃C₂T_x channel allows multiple hydrogen bond establishment and its sub-1-nm size induces a nanoconfinement effect to greatly strengthen these interactions by maintaining solvent-MXene distance and solvent orientation. In sub-1-nm ion sieving and separation, as-decorated membranes exhibit stable ion rejection, and proton-cation (H⁺/Mⁿ⁺) selectivity that is up to 50 times and 30 times, respectively, higher than that of pristine membranes. It demonstrates the feasibility of non-covalent methods as a broad modification alternative for nanochannels integrated in energy-, resource- and environment-related applications.

Ultrastrong yet Ductile 2D Titanium Nanomaterial for On-Skin Conformal Triboelectric Sensing

Conventional titanium (e.g., bulk or thin films) is well-known for its relatively high mechanical strength, excellent corrosion resistance, and superior biocompatibility, which are suitable for biomedical engineering and wearable devices. However, the strength of conventional titanium often trades off its ductility, and their use in wearable devices has not been explored yet. In this work, we fabricated a series of large-sized 2D titanium nanomaterials with the method of polymer surface buckling enabled exfoliation (PSBEE), which possess a unique heterogeneous nanostructure containing nanosized titanium, titanium oxide, and MXene-like phases. As a result, these 2D titaniums exhibit both superb mechanical strength (6-13 GPa) and remarkable ductility (25-35%) at room temperature, outperforming all other titanium-based materials reported so far. More interestingly, we demonstrate that the 2D titanium nanomaterials also showed good performance in triboelectric sensing and can be used to fabricate self-powered, on-skin conformal triboelectric sensors with good mechanical reliability.