2D MXenes: Tunable Mechanical and Tribological Properties

Inverse Relationship between Thickness and Wear of Fluorinated Graphene: "Thinner Is Better"

Atomically thin two-dimensional (2D) materials are excellent candidates for utilization as a solid lubricant or additive at all length scales from macro-scale mechanical devices to micro/nano-electromechanical systems (MEMS/NEMS). In such applications, wear resistance of ultrathin 2D materials is critical for sustained lubrication performance. Here, we investigated the wear of fluorinated graphene (FG) nanosheets deposited on silicon surfaces using atomic force microscopy (AFM) and discovered that the wear resistance of FG improves as the FG thickness decreases from 4.2 to 0.8 nm (corresponding to seven layers to single layer) and the surface energy of the substrate underneath the FG nanosheets increases. On the basis of density function theory (DFT) calculations, the negative correlation of wear resistance to FG thickness and the positive correlation to substrate surface energy could be explained with the degree of interfacial charge transfer between FG and substrate which affects the strength of FG adhesion to the substrate.

The Emergence of 2D MXenes Based Zn-Ion Batteries: Recent Development and Prospects

Rechargeable zinc-ion batteries (ZIBs) with exceptional theoretical capacity have garnered significant interest in large-scale electrochemical energy storage devices due to their low cost, abundant material, inherent safety, high specific energy, and ecofriendly nature. Metal carbides/nitrides, known as MXenes, have emerged as a large family of 2D transition metal carbides or carbonitrides with excellent properties, e.g., high electrical conductivity, large surface functional groups (e.g., F, O, and OH), low energy barriers for the diffusion of electrolyte ions with wide interlayer spaces. After a decade of effort, significant development has been achieved in the synthesis, properties, and applications of MXenes. Thus, it has opened up various exciting opportunities to construct advanced MXene-based nanostructures for ZIBs with excellent specific energy and power. Herein, this review summarizes the advances across multiple synthesis routes, related properties, morphological and structural characteristics, and chemistries of MXenes for ZIBs. The recent development of MXene-based electrodes is introduced, and electrolytes for ZIBs are elucidated in detail. MXene-based rocking chair ZIBs, strategies to enhance the performance of MXene-based cathodes, suppress the dendrites in MXene-based anodes, and MXene-based flexible ZIBs are pointed out. A rational design and modification of the MXenes as well as the production of composites with metal oxides exhibits promise in solving issues and enhancing the electrochemical performance of ZIBs. Finally, the present challenges and future prospects for MXene-based ZIBs are discussed.

2D MBenes: A Novel Member in the Flatland

2D MBenes, early transition metal borides, are a very recent derivative of ternary or quaternary transition metal boride (MAB) phases and represent a new member in the flatland. Although holding great potential toward various applications, mainly theoretical knowledge about their potential properties is available. Theoretical calculations and preliminary experimental attempts demonstrate their rich chemistry, excellent reactivity, mechanical strength/stability, electrical conductivity, transition properties, and energy harvesting possibility. Compared to MXenes, MBenes' structure appears to be more complex due to multiple crystallographic arrangements, polymorphism, and structural transformations. This makes their synthesis and subsequent delamination into single flakes challenging. Overcoming this bottleneck will enable a rational control over MBenes' material-structure-property relationship. Innovations in MBenes' postprocessing approaches will allow for the design of new functional systems and devices with multipurpose functionalities thus opening a promising paradigm for the conscious design of high-performance 2D materials.

An Assessment of MXenes through Scanning Probe Microscopy

Recently, exploring the unique properties of 2D materials has constituted a new wave of research, which lead these materials to enormous applications ranging from optoelectronics to healthcare systems. Due to the profusion of surface terminated functionalities, MXenes have become an emerging class of 2D materials that can be easily integrated with other materials. The versatility of MXenes allows to tune their finest material properties for further device applications. This review initiates with the classification of preparation methods of MXenes, where the authors elaborate on the significance of top-down approaches including the exfoliation of solid layers. Next, the focus is diverted toward the materials analysis of MXenes including their terminations analysis as well as their intriguing electrical and mechanical behaviors through scanning probe microscopy. Finally, critical challenges and perspectives for MXenes analysis and applications are explored and discussed. Therefore, this comprehensive review can encourage researchers, and offer a precise direction to employ MXenes in various applications.

MXene chemistry, electrochemistry and energy storage applications

The diverse and tunable surface and bulk chemistry of MXenes affords valuable and distinctive properties, which can be useful across many components of energy storage devices. MXenes offer diverse functions in batteries and supercapacitors, including double-layer and redox-type ion storage, ion transfer regulation, steric hindrance, ion redistribution, electrocatalysts, electrodeposition substrates and so on. They have been utilized to enhance the stability and performance of electrodes, electrolytes and separators. In this Review, we present a discussion on the roles of MXene bulk and surface chemistries across various energy storage devices and clarify the correlations between their chemical properties and the required functions. We also provide guidelines for the utilization of MXene surface terminations to control the properties and improve the performance of batteries and supercapacitors. Finally, we conclude with a perspective on the challenges and opportunities of MXene-based energy storage components towards future practical applications.

A wood-mimetic porous MXene/gelatin hydrogel for electric field/sunlight bi-enhanced uranium adsorption

Although diverse uranium (U) adsorbents have been explored, it is still a great challenge for high-efficient uranium extraction form seawater. Herein a wood-mimetic oriented porous Ti3C2T x -MXene/gelatin hydrogel (MGH) has been explored through growing directional ice crystals cooled by liquid nitrogen and subsequently forming pores by freeze-dry (Ice-template) method, for ultrafast and high-efficient U-adsorption from seawater with great enhancement by both electric field and sunlight. Different from disperse Ti3C2T x -MXene powder, this MGH not only can be easily utilized but also can own ultrahigh specific surface area for high-efficient U-adsorption. The U-adsorbing capacity of this MGH (10 mg) can reach 4.17 mg·g−1 after only 1 week in 100 kg of seawater, which is outstanding in existing adsorbents. Furthermore, on the positive pole of 0.4 V direct current source or under 1-sun irradiation, the U-adsorbing capacity of the MGH can increase by 57.11% and 13.57%, respectively. Most importantly, the U-adsorption of this hydrogel can be greatly enhanced by simultaneously using the above two methods, which can increase the U-adsorbing capacity by 79.95% reaching 7.51 mg·g−1. This work provides a new biomimetic porous MXene-based hydrogel for electric field/sunlight bi-enhanced high-efficient U-extraction from seawater, which will inspire new strategy to design novel U-adsorbents and systems.

Friction of Ti3C2Tx MXenes

2D materials are well-known for their low-friction behavior by modifying the interfacial forces at atomic surfaces. Of the wide range of 2D materials, MXenes represent an emerging material class but their lubricating behavior has been scarcely investigated. Herein, the friction mechanisms of 2D Ti₃C₂T_x MXenes are demonstrated which are attributed to their surface terminations. We find that Ti₃C₂T_x MXenes do not exhibit the well-known frictional layer dependence of other 2D materials. Instead, the nanoscale lubricity of 2D MXenes is governed by the termination species resulting from synthesis. Annealing the MXenes demonstrate a 7% reduction in OH termination which translates to a 16-57% reduction of friction in agreement with DFT calculations. Finally, the stability of MXene flakes is demonstrated upon isolation from their aqueous environment. This work indicates that MXenes can provide sustainable lubricity at any thickness which makes them uniquely positioned among 2D material lubricants.

Next-Generation Intelligent MXene-Based Electrochemical Aptasensors for Point-of-Care Cancer Diagnostics

Delayed diagnosis of cancer using conventional diagnostic modalities needs to be addressed to reduce the mortality rate of cancer. Recently, 2D nanomaterial-enabled advanced biosensors have shown potential towards the early diagnosis of cancer. The high surface area, surface functional groups availability, and excellent electrical conductivity of MXene make it the 2D material of choice for the fabrication of advanced electrochemical biosensors for disease diagnostics. MXene-enabled electrochemical aptasensors have shown great promise for the detection of cancer biomarkers with a femtomolar limit of detection. Additionally, the stability, ease of synthesis, good reproducibility, and high specificity offered by MXene-enabled aptasensors hold promise to be the mainstream diagnostic approach. In this review, the design and fabrication of MXene-based electrochemical aptasensors for the detection of cancer biomarkers have been discussed. Besides, various synthetic processes and useful properties of MXenes which can be tuned and optimized easily and efficiently to fabricate sensitive biosensors have been elucidated. Further, futuristic sensing applications along with challenges will be deliberated herein.

Smart MXene Quantum Dot-Based Nanosystems for Biomedical Applications

MXene quantum dots (QDs), with their unique structural, optical, magnetic, and electronic characteristics, are promising contenders for various pharmaceutical and biomedical appliances including biological sensing/imaging, cancer diagnosis/therapy, regenerative medicine, tissue engineering, delivery of drugs/genes, and analytical biochemistry. Although functionalized MXene QDs have demonstrated high biocompatibility, superb optical properties, and stability, several challenging issues pertaining to their long-term toxicity, histopathology, biodistribution, biodegradability, and photoluminescence properties are still awaiting systematic study (especially the move towards the practical and clinical phases from the pre-clinical/lab-scale discoveries). The up-scalable and optimized synthesis methods need to be developed not only for the MXene QD-based nanosystems but also for other smart platforms and hybrid nanocomposites encompassing MXenes with vast clinical and biomedical potentials. Enhancing the functionalization strategies, improvement of synthesis methods, cytotoxicity/biosafety evaluations, enriching the biomedical applications, and exploring additional MXene QDs are crucial aspects for developing the smart MXene QD-based nanosystems with improved features. Herein, recent developments concerning the biomedical applications of MXene QDs are underscored with emphasis on current trends and future prospects.

2D MXene: A Potential Candidate for Photovoltaic Cells? A Critical Review

The 2D transition metal carbides/nitrides (2D MXenes) are a versatile class of 2D materials for photovoltaic (PV) systems. The numerous advantages of MXenes, including their excellent metallic conductivity, high optical transmittance, solution processability, tunable work-function, and hydrophilicity, make them suitable for deployment in PV technology. This comprehensive review focuses on the synthesis methodologies and properties of MXenes and MXene-based materials for PV systems. Titanium carbide MXene (Ti3 C2 Tx ), a well-known member of the MXene family, has been studied in many PV applications. Herein, the effectiveness of Ti3 C2 Tx as an additive in different types of PV cells, and the synergetic impact of Ti3 C2 Tx as an interfacial material on the photovoltaic performance of PV cells, are systematically examined. Subsequently, the utilization of Ti3 C2 Tx as a transparent conductive electrode, and its influence on the stability of the PV cells, are discussed. This review also considers problems that emerged from previous studies, and provides guidelines for the further exploration of Ti3 C2 Tx and other members of the 2D MXene family in PV technology. This timely study is expected to provide comprehensive understanding of the current status of MXenes, and to set the direction for the future development in 2D material design and processing for PVs.

Mussel-Inspired Polynorepinephrine/MXene-Based Magnetic Nanohybrid for Electromagnetic Interference Shielding in X-Band and Strain-Sensing Performance

The current work delivers preparation of MXene-based magnetic nanohybrid coating for flexible electronic applications. Herein, we report carbon dot-triggered photopolymerized polynorepinepherene (PNE)-coated MXene and iron oxide hybrid deposited on the cellulose microporous membrane via a vacuum-assisted filtration strategy. The surface morphologies have been monitored by scanning electron microscopy analysis, and the coating thickness was evaluated by the gallium-ion-based focused ion beam method. Coated membranes have been tested against uniaxial tensile stretching and assessed by their fracture edges in order to assure flexibility and mechanical strength. Strain sensors and electromagnetic interference (EMI) shielding have both been tested on the material because of its electrical conductivity. The bending strain sensitivity has been stringent because of their fast 'rupture and reform' percolation network formation on the coated surface. Increased mechanical strength, solvent tolerance, cyclic deformation tolerance, and EMI shielding performance were achieved by decreasing interstitial membrane porosity. Considering a possible application, the membrane also has been tested against simulated static and dynamic water flow conditions that could infer its excellent robustness which also has been confirmed by elemental analysis via ICP-MS. Thus, as of nurturing the works of the literature, it could be believed that the developed material will be an ideal alternative of flexible lightweight cellulose for versatile electronic applications.

Metal-organic Framework ZIF-67 Functionalized MXene for Enhancing the Fire Safety of Thermoplastic Polyurethanes

In this work, a novel functionalization strategy for ZIF-67-modified layered MXene was proposed, aiming at improving the fire safety of thermoplastic polyurethanes (TPU). The ZIF-67@MXene was verified by microscopic morphology, elemental composition, functional group species and crystal structure, and then the successfully prepared ZIF-67@MXene was introduced into the TPU material. When ZIF-67@MXene content was only 0.5 wt%, the peak heat release rate, total heat release rate, peak smoke release rate, total smoke release rate, and CO yield of the TPU/ZIF-67@MXene composites were reduced by 26%, 9%, 50%, and 22%, respectively, compared with the pure TPU. The thermogravimetric tests showed that the residual char of TPU/ZIF-67@MXene composites was the most in all samples. In short, the high-quality carbon layer of TPU/ZIF-67@MXene composites acts as a physical barrier to the transfer of heat and toxic gases, greatly improving the flame retardant properties of the TPU polymer.

The 10th anniversary of MXenes: Challenges and prospects for their surface modification toward future biotechnological applications

A broad family of two-dimensional (2D) materials - carbides, nitrides, and carbonitrides of early transition metals, called MXenes, became a newcomer in the flatland at the turn of 2010 and 2011 (over ten years ago). Their unique physicochemical properties made them attractive for many applications, highly boosting the development of various fields, including biotechnological. However, MXenes' functional features that impact their bioactivity and toxicity are still not fully well understood. This study discusses the essentials for MXenes's surface modifications toward their application in modern biotechnology and nanomedicine. We survey modification strategies in context of cytotoxicity, biocompatibility, and most prospective applications ready to implement in medical practice. We put the discussion on the material-structure-chemistry-property relationship into perspective and concentrate on overarching challenges regarding incorporating MXenes into nanostructured organic/inorganic bioactive architectures. It is another emerging group of materials that are interesting from the biomedical point of view as well. Finally, we present an influential outlook on the growing demand for future research in this field.

Two-Dimensional Cobalt-Doped Ti3C2 MXene Nanozyme-Mediated Homogeneous Electrochemical Strategy for Pesticides Assay Based on In Situ Generation of Electroactive Substances

Common homogeneous electrochemical (HEC) sensors usually suffer from the drawbacks of high background signal, low signal-to-noise ratio, and even false positive results due to the preaddition of electroactive substances. Thus, it is necessary to develop novel HEC sensors based on in situ generation of electroactive substances to overcome these shortcomings, which, however, is underexplored. In this work, two-dimensional (2D) nanozymes, i.e., cobalt-doped 2D Ti₃C₂ MXene nanosheets (CMNSs), with excellent peroxidase-like properties were utilized to develop HEC sensors based on the in situ generation of electroactive substances for organophosphate pesticides (OPs) detection. The 2D CMNSs were synthesized via a template-directed wet chemical approach and displayed outstanding features of hydrophilia and water dispersibility, which could catalyze the oxidation of o-phenylenediamine (OPD) to generate significantly increased reduction current. Interestingly, the 2D CMNSs with peroxidase-like properties exhibited a unique response to thiol compounds and were thus employed as highly efficient catalysts to develop HEC sensors for OPs based on the hydrolysis of acetylthiocholine (ATCh) to form thiocholine catalyzed by acetylcholinesterase (AChE) and the inhibition of AChE activity by OPs. The recovery for OPs analysis of pakchoi extract solutions ranged from 97.4% to 103.3%. The as-proposed HEC sensor based on in situ generation of electroactive substances will provide a new way for the development of high-performance electrochemical sensors and demonstrate potential applicability for the determination of pesticide residues in real samples.

Theoretical Study on the Electronic Structure and Magnetic Properties Regulation of Janus Structure of M’MCO2 2D MXenes

Motivated by the recent successful synthesis of Janus monolayer of transition metal (TM) dichalcogenides, MXenes with Janus structures are worthy of further study, concerning its electronic structure and magnetic properties. Here, we study the effect of different transition metal atoms on the structure stability and magnetic and electronic properties of M’MCO₂ (M’ and M = V, Cr and Mn). The result shows the output magnetic moment is contributed mainly by the d orbitals of the V, Cr, and Mn atoms. The total magnetic moments of ferromagnetic (FM) configuration and antiferromagnetic (AFM) configuration are affected by coupling types. FM has a large magnetic moment output, while the total magnetic moments of AFM2’s (intralayer AFM/interlayer FM) configuration and AFM3’s (interlayer AFM/intralayer AFM) configuration are close to 0. The band gap widths of VCrCO₂, VMnCO₂, CrMnCO₂, V₂CO₂, and Cr₂CO₂ are no more than 0.02 eV, showing metallic properties, while Mn₂CO₂ is a semiconductor with a 0.7071 eV band gap width. Janus MXenes can regulate the size of band gap, magnetic ground state, and output net magnetic moment. This work achieves the control of the magnetic properties of the available 2D materials, and provides theoretical guidance for the extensive design of novel Janus MXene materials.

A Semantic Annotation Pipeline towards the Generation of Knowledge Graphs in Tribology

Within the domain of tribology, enterprises and research institutions are constantly working on new concepts, materials, lubricants, or surface technologies for a wide range of applications. This is also reflected in the continuously growing number of publications, which in turn serve as guidance and benchmark for researchers and developers. Due to the lack of suited data and knowledge bases, knowledge acquisition and aggregation is still a manual process involving the time-consuming review of literature. Therefore, semantic annotation and natural language processing (NLP) techniques can decrease this manual effort by providing a semi-automatic support in knowledge acquisition. The generation of knowledge graphs as a structured information format from textual sources promises improved reuse and retrieval of information acquired from scientific literature. Motivated by this, the contribution introduces a novel semantic annotation pipeline for generating knowledge in the domain of tribology. The pipeline is built on Bidirectional Encoder Representations from Transformers (BERT)—a state-of-the-art language model—and involves classic NLP tasks like information extraction, named entity recognition and question answering. Within this contribution, the three modules of the pipeline for document extraction, annotation, and analysis are introduced. Based on a comparison with a manual annotation of publications on tribological model testing, satisfactory performance is verified.

Effects of polyacrylic acid molecular weights on V2C-MXene nanocoatings for obtaining ultralow friction and ultralow wear in an ambient working environment

Ultralow friction (μ ≈ 0.073 ± 0.024) is achieved for the LPAA@V2C vs. steel ball system through tribo-physicochemical interactions.

Structural superlubricity in 2D van der Waals heterojunctions

Structural superlubricity is a fundamentally important research topic in the area of tribology. Van der Waals heterojunctions of 2D materials are an ideal system for achieving structural superlubricity and possessing potentially a wide range of applications in the future due to their ultra-flat and incommensurate crystal interfaces. Here we briefly introduce the origin and mechanism of structural superlubricity and summarize the representative experimental results, in which the coefficient of friction has achieved the order of 10^-5. Furthermore, we analyze the factors affecting structural superlubricity of 2D materials, including dynamic reconstruction of interfaces, edge effects, interfacial adsorption, etc, and give a perspective on how to realize the macroscopic expansion and where it can be applied in practice.

Two-Dimensional Nanomaterials for the Removal of Pharmaceuticals from Wastewater: A Critical Review

The removal of pharmaceuticals from wastewater is critical due to their considerable risk on ecosystems and human health. Additionally, they are resistant to conventional chemical and biological remediation methods. Two-dimensional nanomaterials are a promising approach to face this challenge due to their combination of high surface areas, high electrical conductivities, and partially optical transparency. This review discusses the state-of-the-art concerning their use as adsorbents, oxidation catalysts or photocatalysts, and electrochemical catalysts for water treatment purposes. The bibliographic search bases upon academic databases including articles published until August 2021. Regarding adsorption, high removal capacities (>200 mg g−1) and short equilibrium times (<30 min) are reported for molybdenum disulfide, metal-organic frameworks, MXenes, and graphene oxide/magnetite nanocomposites, attributed to a strong adsorbate-adsorbent chemical interaction. Concerning photocatalysis, MXenes and carbon nitride heterostructures show enhanced charge carriers separation, favoring the generation of reactive oxygen species to degrade most pharmaceuticals. Peroxymonosulfate activation via pure or photo-assisted catalytic oxidation is promising to completely degrade many compounds in less than 30 min. Future work should be focused on the exploration of greener synthesis methods, regeneration, and recycling at the end-of-life of two-dimensional materials towards their successful large-scale production and application.

Graphene Confers Ultralow Friction on Nanogear Cogs

Friction-induced energy dissipation impedes the performance of nanomechanical devices. Nevertheless, the application of graphene is known to modulate frictional dissipation by inducing local strain. This work reports on the nanomechanics of graphene conformed on different textured silicon surfaces that mimic the cogs of a nanoscale gear. The variation in the pitch lengths regulates the strain induced in capped graphene revealed by scanning probe techniques, Raman spectroscopy, and molecular dynamics simulation. The atomistic visualization elucidates asymmetric straining of CC bonds over the corrugated architecture resulting in distinct friction dissipation with respect to the groove axis. Experimental results are reported for strain-dependent solid lubrication which can be regulated by the corrugation and leads to ultralow frictional forces. The results are applicable for graphene covered corrugated structures with movable components such as nanoelectromechanical systems, nanoscale gears, and robotics.

Recent Advances in Preparation and Testing Methods of Engine-Based Nanolubricants: A State-of-the-Art Review

Reducing power losses in engines is considered a key parameter of their efficiency improvement. Nanotechnology, as an interface technology, is considered one of the most promising strategies for this purpose. As a consumable liquid, researchers have studied nanolubricants through the last decade as potential engine oil. Nanolubricants were shown to cause a considerable reduction in the engine frictional and thermal losses, and fuel consumption as well. Despite that, numerous drawbacks regarding the quality of the processed nanolubricants were discerned. This includes the dispersion stability of these fluids and the lack of actual engine experiments. It has been shown that the selection criteria of nanoparticles to be used as lubricant additives for internal combustion engines is considered a complex process. Many factors have to be considered to investigate and follow up with their characteristics. The selection methodology includes tribological and rheological behaviours, thermal stability, dispersion stability, as well as engine performance. Through the last decade, studies on nanolubricants related to internal combustion engines focused only on one to three of these factors, with little concern towards the other factors that would have a considerable effect on their final behaviour. In this review study, recent works concerning nanolubricants are discussed and summarized. A complete image of the designing parameters for this approach is presented, to afford an effective product as engine lubricant.

High-Entropy 2D Carbide MXenes: TiVNbMoC3 and TiVCrMoC3

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are a fast-growing family of 2D materials. MXenes 2D flakes have n + 1 (n = 1-4) atomic layers of transition metals interleaved by carbon/nitrogen layers, but to-date remain limited in composition to one or two transition metals. In this study, by implementing four transition metals, we report the synthesis of multi-principal-element high-entropy M₄C₃T_x MXenes. Specifically, we introduce two high-entropy MXenes, TiVNbMoC₃T_x and TiVCrMoC₃T_x, as well as their precursor TiVNbMoAlC₃ and TiVCrMoAlC₃ high-entropy MAX phases. We used a combination of real and reciprocal space characterization (X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, and scanning transmission electron microscopy) to establish the structure, phase purity, and equimolar distribution of the four transition metals in high-entropy MAX and MXene phases. We use first-principles calculations to compute the formation energies and explore synthesizability of these high-entropy MAX phases. We also show that when three transition metals are used instead of four, under similar synthesis conditions to those of the four-transition-metal MAX phase, two different MAX phases can be formed (i.e., no pure single-phase forms). This finding indicates the importance of configurational entropy in stabilizing the desired single-phase high-entropy MAX over multiphases of MAX, which is essential for the synthesis of phase-pure high-entropy MXenes. The synthesis of high-entropy MXenes significantly expands the compositional variety of the MXene family to further tune their properties, including electronic, magnetic, electrochemical, catalytic, high temperature stability, and mechanical behavior.

Nacre-Mimetic, Mechanically Flexible, and Electrically Conductive Silk Fibroin-MXene Composite Foams as Piezoresistive Pressure Sensors

The hierarchical nacre-like three-dimensional (3D) assembly of porous and lightweight materials is in high demand for applications such as sensors, flexible energy storage and harvesting devices, electromagnetic interference shielding, and biomedical applications. However, designing such a biomimetic hierarchical architecture is highly challenging due to the lack of experimental approaches to achieve the necessary control over the materials' microstructure on the multilength scale. Aerogels and foam-based materials have recently been developed as attractive candidates for pressure-sensing applications. However, despite recent progress, the bottleneck for these materials to achieve electrical conductivity combined with high mechanical flexibility and fast strain recovery remains. In this study, for the first time, inspired by the multiscale architecture of nacre, we fabricated a series of ultra-lightweight, flexible, electrically conductive, and relatively high-strength composite foams through hybridizing the cross-linked silk fibroin (SF) biopolymer, extracted from Bombyx mori silkworm cocoon, reinforced with two-dimensional graphene oxide (GO) and Ti₃C₂ MXene nanosheets. Nacre is a naturally porous material with a lightweight, mechanically robust network structure, thanks to its 3D interconnected lamella-bridge micromorphology. Inspired by this material, we assemble a cross-linked SF fibrous solution with MXene and GO nanosheets into nacre-like architecture using a bidirectional freeze-casting technique. Subsequent freeze-drying and gas-phase hydrophobization resulted in composite foams with 3D hierarchical porous architectures with a unique combination of mechanical resilience, electrical conductance, and ultra-lightness. The developed composite presented excellent performances as piezoresistive pressure-sensing devices and sorbents for oil/water separation, which indicated great potential in mechanically switchable electronics.

Enhanced terahertz shielding by adding rare Ag nanoparticles to Ti3C2TxMXene fiber membranes

Polyacrylonitrile/Ti₃C₂T_xMXene/silver nanoparticles fiber membranes with different silver nanoparticles contents and thickness of porous structure have been successfully prepared by electrospinning. Through the measurement of terahertz time domain spectrum, the shielding effect of the fiber membrane with 1% silver nanoparticles content can reach up to 12 dB. Moreover, the thickness of the spinning fiber membranes is controlled by adjusting the spinning time, so as to better analyze the influence of the thickness of the shielding performance in terahertz band. We attribute this excellent phenomenon to porous structure of the spun fiber membrane and combination of Ti₃C₂T_xMXene with few-layers and silver nanoparticles to increase the absorption and conductivity of the fiber membrane, thereby enhancing the shielding effect in terahertz range. Meanwhile, the prepared polyacrylonitrile/Ti₃C₂T_xMXene/silver nanoparticles fiber membranes show good stability and little change in terahertz shielding effect after high temperature annealing. This may provide potential ideas about the development of high-performance terahertz shielding materials, which are of great significance of terahertz electromagnetic shielding.

Multiscale numerical simulation of in-plane mechanical properties of two-dimensional monolayers

Many applications of two dimensional (2D) materials are often achieved through strain engineering, which is directly dependent on their in-plane mechanical characteristics. Therefore, understanding the in-plane mechanical characteristics of the 2D monolayers becomes imperative. Nevertheless, direct experimental measurements of in-plane mechanical properties of 2D monolayers face great difficulties due to the issues related to the availability of high-quality 2D materials and sophisticated facilities. As an alternative, numerical simulation has the potential to theoretically predict such properties. This review presents some recent progress in numerically exploring the in-plane mechanical properties of 2D materials, including first-principles density functional theory, force-field based classical molecular dynamics, and the finite-element method. The relevant case studies are provided to describe the applications of these methods along with their pros and cons. We hope that the multiscale simulation methods discussed in this review will inspire new ideas and boost further advances of the computational study on the in-plane mechanical properties of 2D materials.

The recent progress of multiscale numeric methods for investigating in-plane mechanical properties of 2D monolayers is reviewed.

2D transition metal carbides (MXenes) in metal and ceramic matrix composites

Two-dimensional transition metal carbides, nitrides, and carbonitrides (known as MXenes) have evolved as competitive materials and fillers for developing composites and hybrids for applications ranging from catalysis, energy storage, selective ion filtration, electromagnetic wave attenuation, and electronic/piezoelectric behavior. MXenes' incorporation into metal matrix and ceramic matrix composites is a growing field with significant potential due to their impressive mechanical, electrical, and chemical behavior. With about 50 synthesized MXene compositions, the degree of control over their composition and structure paired with their high-temperature stability is unique in the field of 2D materials. As a result, MXenes offer a new avenue for application driven design of functional and structural composites with tailorable mechanical, electrical, and thermochemical properties. In this article, we review recent developments for use of MXenes in metal and ceramic composites and provide an outlook for future research in this field.

Superior Wear-Resistance of Ti3C2Tx Multilayer Coatings

Owing to MXenes' tunable mechanical properties induced by their structural and chemical diversity, MXenes are believed to compete with state-of-the-art 2D nanomaterials such as graphene regarding their tribological performance. Their nanolaminate structure offers weak interlayer interactions and an easy-to-shear ability to render them excellent candidates for solid lubrication. However, the acting friction and wear mechanisms are yet to be explored. To elucidate these mechanisms, 100-nm-thick homogeneous multilayer Ti₃C₂T_x coatings are deposited on technologically relevant stainless steel by electrospraying. Using ball-on-disk tribometry (Si₃N₄ counterbody) with acting contact pressures of about 300 MPa, their long-term friction and wear performance under dry conditions are studied. MXene-coated specimens demonstrate a 6-fold friction reduction and an ultralow wear rate (4 × 10^-9 mm³ N^-1 m^-1) over 100 000 sliding cycles, outperforming state-of-the-art 2D nanomaterials by at least 200% regarding their wear life. High-resolution characterization verified the formation of a beneficial tribolayer consisting of thermally/mechanically degraded MXenes and amorphous/nanocrystalline iron oxides. The transfer of this tribolayer to the counterbody transforms the initial steel/Si₃N₄ contact to tribolayer/tribolayer contact with low shear resistance. MXene pileups at the wear track's reversal points continuously supply the tribological contact with fresh, lubricious nanosheets, thus enabling an ultra-wear-resistant and low-friction performance.