Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti3C2 MXene

Multifunctional Ti3C2Tx-alginate foams for energy harvesting and fire warning

Foams that combine seemingly opposite properties, such as high thermal insulation and electrical conductivity, are highly sought after for modern-day advanced applications. However, achieving a balance of these properties necessitates careful tuning of material compositions. Here, we prepared ice-templated Ti3C2Tx-alginate composite foams and investigated the role of Ti3C2Tx MXene in triboelectric energy production, thermal insulation, and flame retardancy. Our results show that adding 5 wt% Ti3C2Tx enhances the triboelectric output of 6 mm thick foams (380 V, 7.7 μA, 43 mW m-2) by 110%. Despite incorporating electrically conducting Ti3C2Tx, these macroporous composite foams have a thermal conductivity of only 62 mW m-1 K-1, while they also show flame-retardant properties, exhibiting self-extinguishing behavior. Finally, we demonstrate these composite foams for constructing smart fire alarm systems as they respond to small changes in electrical resistance induced by fire. Our findings prove that Ti3C2Tx is a versatile filler for biopolymer foams, introducing complementary functionalities that can be exploited in energy and safety applications.

Broadband Microwave Absorption of Nb2CTx Nanosheets by a One-Step Hydrothermal Method

As an emerging two-dimensional material, MXene holds great potential as a microwave-absorbing material due to its unique layered structure and flexibly tunable surface functional groups. Therefore, research on surface chemistry and interlayer engineering represents an effective strategy for optimizing the performance of MXene. In this work, we synthesized high-quality Nb2CTx nanosheets via a hydrothermal method. Detailed material characterization techniques have confirmed the synthesis of high-quality niobium-based nanosheets. The interlayer spacing, surface termination, and structural defects of the Nb2CTx nanosheets could be flexibly regulated by adjusting the duration of the hydrothermal etching process. This work reveals the etching time-dependent correlation of the electromagnetic parameters of Nb2CTx. It is noteworthy that the optimal impedance matching and microwave absorption performance were achieved after etching for 96 h, with a minimum reflection loss of -43.4 dB at an ultrathin thickness of 1.3 mm and an effective absorption bandwidth of 4.4 GHz at a thickness of 1.4 mm. This work presents a meaningful route to simultaneously adjust the nanoarchitecture and surface chemistry of MXene for advanced microwave absorption applications.

Material characterization methods for investigating charge storage processes in 2D and layered materials-based batteries and supercapacitors

Two-dimensional (2D) materials offer distinct advantages for electrochemical energy storage (EES) compared to bulk materials, including a high surface-to-volume ratio, tunable interlayer spacing, and excellent in-plane conductivity, making them highly attractive for applications in batteries and supercapacitors. Gaining a fundamental understanding of the energy storage processes in 2D material-based EES devices is essential for optimizing their chemical composition, surface chemistry, morphology, and interlayer structure to enhance ion transport, promote redox reactions, suppress side reactions, and ultimately improve overall performance. This review provides a comprehensive overview of the characterization techniques employed to probe charge storage mechanisms in 2D and thin-layered material-based EES systems, covering optical spectroscopy, imaging techniques, X-ray and neutron-based methods, mechanical probing, and nuclear magnetic resonance spectroscopy. We specifically highlight the application of these techniques in elucidating ion transport dynamics, tracking redox processes, identifying degradation pathways, and detecting interphase formation. Furthermore, we discuss the limitations, challenges, and potential pitfalls associated with each method, as well as future directions for advancing characterization techniques to better understand and optimize 2D material-based electrodes.

Characterization of MXene-Based Materials by X-Ray Computed Tomography

MXenes are a class of 2D materials that have gained significant attention for their potential applications in energy storage, electromagnetic interference shielding, biomedicine, and (opto)electronics. Despite their broad range of applications, a detailed understanding of the internal architecture of MXene-based materials remains limited due to the lack of effective 3D imaging techniques. This work demonstrates the application of X-ray micro-computed tomography (micro-CT) to investigate various MXene systems, including nanocomposites, coated textiles, and aerogels. Micro-CT enables high-resolution, 3D visualization of the internal microstructure, MXene distribution, infiltration patterns, and defect formations, which significantly influence the material's performance. Moreover, the typical technical challenges and limitations encountered during sample preparation, scanning, and post-processing of micro-CT data are discussed. The information obtained from optical and electron microscopy is also compared with micro-CT, highlighting the unique advantages of micro-CT in providing comprehensive 3D imaging and quantitative data. This study highlights micro-CT as a powerful and nondestructive imaging tool for characterizing MXene-based materials, providing insights into material optimization and guidelines for developing future advanced applications.

Lattice coherency engineering trigger rapid charge transport at the heterointerface of Te/In2O3@MXene photocatalysts for boosting photocatalytic hydrogen evolution

The establishment of heterojunctions has been demonstrated as an effective method to improve the efficiency of photocatalytic hydrogen production. Conventional heterojunctions usually have random orientation relationships, and heterointerfaces can hinder photogenerated carrier transport due to larger lattice mismatches, thus reducing the photoelectric conversion efficiency. In this study, a novel Te/In2O3@MXene lattice coherency heterojunction was prepared by leveraging the identical lattice spacing of In2O3 (222) and Te (021) crystal face. The lattice consistency facilitates enhanced photogenerated carrier transport rate between the heterostructure interface of In2O3 and Te. Furthermore, the incorporation of MXene, the electrons originating from Te 5p orbital achieve directional transfer in the heterojunction. This reduces the recombination of photogenerated electron - hole pairs and retains the photogenerated electrons with higher reducibility. The hydrogen production efficiency of Te/In2O3@MXene is 568.8 μmol/h g-1, which is 24 times higher than that of pristine In2O3, and it remains 90 % of its initial activity after six cycles. This study offers a novel approach to address the escalating carrier transfer resistance commonly observed in conventional heterojunctions.

Solid Intercalation and Exfoliation of Cl-Terminated Multilayered MXenes toward O-Functionalized Single Layers

Although ion intercalation is becoming a powerful strategy to produce expanded layered materials and atomic layers in aqueous or organic systems, it usually suffers from sluggish kinetics with a long intercalating time of several days. Here, we present a facile approach to produce O-functionalized single-layer MXenes by solid intercalation of Cl-terminated accordion-like MXenes in molten salts and subsequent exfoliation. The process involves the intercalation of metal cations (Li+, Na+ and K+) and anions (CO32-) in molten salts, resulting in substitution with -O surface groups and formation of gases (CO2). Such unique solid intercalation significantly expands multilayered MXenes in 30 min with an enhancement of interlayer spacing from 11.2 to 12.7 Å, facilitating their easy exfoliation to single layers. The resultant O-functionalized MXenes have a highly expanded structure and elevated permittivity, achieving a reflection loss value of -50.5 dB at a thickness of 1.35 mm for electromagnetic wave absorption.

Surface Tension-Driven Self-Planarization of MXene Liquid Crystalline Fiber for High-Performance Energy Storage

2D MXene-based liquid crystalline (LC) systems have emerged as promising precursors for constructing highly ordered functional materials, such as fibers, films, and aerogels via solution-based processing. In this study, we demonstrate surface tension-mediated self-planarization of MXene LC fibers by adjusting the solvent composition during wet-spinning, targeting improved electrochemical performance. Ethanol, a poor solvent for MXene, induced spontaneous parallel alignment of MXene platelets and facilitated densification into a ribbon-like geometry during coagulation. The resulting fibers featured a pore volume of 0.11 cm3 g-1 and an average pore diameter of 34 nm, enabling a volumetric capacitance of 1721.7 F cm-3 and an electrical conductivity of 9211.66 S cm-1. The mechanism underlying the self-planarization was investigated by using a range of solvents with varying physicochemical properties to identify key processing parameters. The MXene fibers were successfully implemented into LED-powered supercapacitor prototypes, demonstrating potential applicability for wearable energy applications.

A Review on Multifunctional Polymer-MXene Hybrid Materials for Electronic Applications

MXenes, a family of two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides, have emerged as a promising class of nanomaterials for interdisciplinary applications due to their unique physiochemical properties. The large surface area, excellent electrical conductivity, superior mechanical properties, and abundant possible functional groups make this layered nanomaterial an ideal candidate for multifunctional hybrid materials for electronic applications. This review highlights recent progress in MXene-based hybrid materials, focusing on their electrical, dielectric, and electromagnetic interference (EMI) shielding properties, with an emphasis on the development of multifunctionality required for advanced electronic devices. The review explores the multifunctional nature of MXene-based polymer nanocomposites and hybrid materials, covering the coexistence of a diverse range of properties, including sensory capabilities, electromagnetic interference shielding, energy storage, and the Joule heating phenomenon. Finally, the future outlook and key challenges are summarized, offering insights to guide future research aimed at improving the performance and functionality of MXene-polymer nanocomposites.

Ultrahigh Conductive MXene Films for Broadband Electromagnetic Interference Shielding

Broadband and ultrathin electromagnetic interference (EMI)-shielding materials are crucial for efficient high-frequency data transmission in emerging technologies. MXenes are renowned for their outstanding electrical conductivity and EMI-shielding capability. While substituting nitrogen (N) for carbon (C) atoms in the conventional MXene structure is theoretically expected to enhance these properties, synthesis challenges have hindered progress. Here, it is demonstrated that TixCyNx - y -1Tz MXene films with optimized N content achieve a record-high electrical conductivity of 35 000 S cm-1 and exceptional broadband EMI shielding across the X (8-12.4 GHz), Ka (26.5-40 GHz), and W (75-110 GHz) bands-outperforming all previously reported materials even at reduced thicknesses. By synthesizing a full series of high-stoichiometric TixAlCyNx - y -1 MAX phases without intermediate phases, the impact of N substitution on the physical and electrical properties of TixCyNx - y -1Tz MXene flakes is systematically explored, achieving complete composition tunability in both dispersion and film forms. These findings position TixCyNx - y -1Tz MXenes as promising candidates for applications spanning from conventional lower-frequency domains to next-generation sub-THz electronics.

Advances in MXene-based composites for next-generation flexible supercapacitors: From design and development to applications

Flexible supercapacitors (FSCs) are ubiquitously integrated into advancing miniaturized gadgets, wearables and portable electronic technologies. The allure of FSCs lies in their flexibility, compact size, and lightweight, which has compelled extensive investigation in the domain of FSCs. The performance of supercapacitor devices is largely determined by the choice of electrode material and the interaction at the electrode-electrolyte interface. In this context, MXene, an expeditiously expanding class of 2D materials, have garnered significant attention in the exciting field of flexible devices, owing to their high electrical conductivity, distinctive layered structure, substantial surface area, excellent hydrophilicity, and abundant surface terminal groups. These interesting attributes of MXene critically influence interfacial charge storage and transport mechanisms. This review strives to discuss the latest developments in MXene and MXene-based electrode materials for flexible supercapacitors. The review thoughtfully presents the aspects of flexibility, followed by discussions on device designing and fabrication. The role of substrate in fostering flexibility, requisite for solid-state electrolyte, and the influence of diverse device architecture on interfacial stability are closely scrutinized. The review incorporates a comprehensive discussion of the factors impacting the performance of MXene materials, with a particular focus on the features including composition, structure, electrode-electrolyte interaction, electrode morphology and device architecture. Besides, this review extensively investigates fabrication routes, electrochemical performance and mechanical resilience of MXene and MXene-based composites for FSCs. Armed with these insights, the review proposes a prospective roadmap delineating the challenges and opportunities in the advancement of MXene-based electrode materials for flexible supercapacitors.

Selectivity in Chemiresistive Gas Sensors: Strategies and Challenges

The demand for highly functional chemical gas sensors has surged due to the increasing awareness of human health to monitor metabolic disorders or noncommunicable diseases, safety measures against harmful greenhouse and/or explosive gases, and determination of food freshness. Over the years of dedicated research, several types of chemiresistive gas sensors have been realized with appreciable sensitivities toward various gases. However, critical issues such as poor selectivity and sluggish response/recovery speeds continue to impede their widespread commercialization. Specifically, the mechanisms behind the selective response of some chemiresistive materials toward specific gas analytes remain unclear. In this review, we discuss state-of-the-art strategies employed to attain gas-selective chemiresistive materials, with particular emphasis on materials design, surface modification or functionalization with catalysts, defect engineering, material structure control, and integration with physical/chemical gas filtration media. The nature of material surface-gas interactions and the supporting mechanisms are elucidated, opening opportunities for optimizing the materials design, fine-tuning the gas sensing performance, and guiding the selection of the most appropriate materials for the accurate detection of specific gases. This review concludes with recommendations for future research directions and potential opportunities for further selectivity improvements.

Comparative electromagnetic shielding performance of Ti3C2Tx-PVA composites in various structural forms: compact films, hydrogels, and aerogels

The structural design of light-weight MXene-polymer composites has attracted significant interest for enhancing both electromagnetic interference (EMI) shielding performance and mechanical strength, which are critical for practical applications. However, a systematic understanding of how various structural configurations of MXene composites affect EMI shielding is lacking. In this study, light-weight Ti3C2Tx-PVA composites were fabricated in three structural forms, hydrogel, aerogel, and compact film, while varying the Ti3C2Tx areal density (14 to 20 mg cm-2) to elucidate the role of structural design in X-band EMI shielding and mechanical properties. The EMI shielding performance depends on the structural configuration and areal density of the MXene in Ti3C2Tx-PVA composites. The shielding effectiveness increases with increasing Ti3C2Tx content in each configuration. At a fixed Ti3C2Tx areal density of 0.02 g cm-2, the Ti3C2Tx-PVA hydrogel demonstrated the highest shielding effectiveness (SE = 70 dB at 10 GHz), attributed to strong dipole polarization and efficient ionic conduction behavior, followed by the compact film (40 dB) and then the aerogel (21 dB). Notably, the aerogel achieved the highest absorption coefficient (A = 0.89) due to the improved impedance matching and pronounced internal reflections, whereas the hydrogel and compact film exhibited reflection-dominated shielding. Furthermore, the incorporation of PVA polymer molecules into Ti3C2Tx MXenes significantly enhanced their mechanical properties across all configurations: the hydrogel achieved high stretchability (636%), the aerogel displayed superior compressive strength (0.215 MPa), and the compact film reached a tensile strength of 56 MPa, each surpassing the performance of its pristine Ti3C2Tx MXene counterpart. Overall, tailoring the structural configuration into a hydrogel, aerogel, or compact film offers versatile routes for optimizing both EMI attenuation and mechanical performance of MXene-polymer composites.

Surface Functionalization of Ti3C2Tx MXenes in Epoxy Nanocomposites: Enhancing Conductivity, EMI Shielding, Thermal Conductivity, and Mechanical Strength

MXenes have gained significant attention as multifunctional fillers in MXene-polymer nanocomposites. However, their inherently hydrophilic surfaces pose challenges in compatibility with hydrophobic polymers such as epoxy, potentially limiting composite performance. In this study, high-crystalline Ti3C2Tx MXenes were functionalized with alkylated 3,4-dihydroxy-l-phenylalanine ligands, transforming the hydrophilic MXene flakes into a more hydrophobic form, thus significantly enhancing compatibility with the epoxy matrix. This surface functionalization enabled uniform dispersion and supported the formation of a percolation network within the epoxy matrix at a low filler loading of just 0.12 vol %. Consequently, the functionalized MXene-epoxy nanocomposites exhibited remarkable performance, including an electrical conductivity of 8200 S m-1, outstanding electromagnetic interference (EMI) shielding effectiveness (SE) of 100 dB at 110 GHz (61 dB at 8.2 GHz), improved thermal conductivity of 1.37 W m-1 K-1, and a 300% increase in tensile toughness (271 KJ m-3). These properties substantially outperformed those of their nonfunctionalized counterparts and surpassed previously reported MXene-polymer nanocomposites. This study underscores the critical role of surface functionalization in unlocking the full potential of two-dimensional (2D) MXenes in polymer composites, providing a pathway to advanced multifunctional nanocomposite materials.

MXenes in healthcare: synthesis, fundamentals and applications

Since their discovery over a decade ago, MXenes have transformed the field of "materials for healthcare", stimulating growing interest in their healthcare-related applications. These developments have also driven significant advancements in MXenes' synthesis. This review systematically examines the synthesis of MXenes and their applications in sensing and biomedical fields, underscoring their pivotal role in addressing critical challenges in modern healthcare. We describe the experimental synthesis of MXenes by combining appropriate laboratory modules with the mechanistic principles underlying each synthesis step. In addition, we provide extensive details on the experimental parameters, critical considerations, and essential instructions for successful laboratory synthesis. Various healthcare applications including sensing, biomedical imaging, synergistic therapies, regenerative medicine, and wearable devices have been explored. We further highlight the emerging trends of MXenes, viz., their role as nanovehicles for drug delivery, vectors for gene therapy, and tools for immune profiling. By identifying the important parameters that define the utility of MXenes in biomedical applications, this review outlines strategies to regulate their biomedical profile, thereby serving as a valuable guide to design MXenes with application-specific properties. The final section integrates experimental research with theoretical studies to provide a comprehensive understanding of the field. It examines the role of emerging technologies, such as artificial intelligence (AI) and machine learning (ML), in accelerating material discovery, structure-property optimization, and automation. Complemented by detailed supplementary information on synthesis, stability, biocompatibility, environmental impact, and theoretical insights, this review offers a profound knowledge base for understanding this diverse family of 2D materials. Finally, we compared the potential of MXenes with that of other 2D materials to underscore the existing challenges and prioritize interdisciplinary collaboration. By synthesizing key studies from its discovery to current trends (especially from 2018 onward), this review provides a cohesive assessment of MXene synthesis with theoretical foundations and their prospects in the healthcare sector.

Ti3C2/CuWO4/Pt nanozyme: photothermal-enhanced chemodynamic antibacterial effects induced by NIR

With the growing issue of antibiotic resistance, it has become increasingly crucial to develop highly efficient antimicrobial materials. While the single-component nanozyme systems exhibited some catalytic activity, their efficiency remains suboptimal. This study presents a Ti3C2/CuWO4/Pt hybrid nanozyme composed of photothermal agents and nanozymes, which leverages the photothermal effect to enhance nanozyme activity and achieve efficient antimicrobial effects. The composite material exhibited peroxidase (POD)-like catalytic activity, effectively converting hydrogen peroxide (H2O2) into hydroxyl radicals (·OH). Meanwhile, the Ti3C2/CuWO4/Pt material demonstrated high photothermal conversion ability, which not only promoted the generation of ·OH under near-infrared (NIR) light irradiation, but also facilitated copper (Cu2+) ions release from the CuWO4 nanozyme, thereby further augmenting its catalytic activity. After 4 to 5 min of light irradiation, the Ti3C2/CuWO4/Pt nanozyme exhibited significant antimicrobial performance against both Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). In summary, this work presents a Ti3C2/CuWO4/Pt nanoplatform that utilizes the photothermal effect to enhance the chemodynamic antimicrobial activity, showcasing its potential applications in antibiotic-free antimicrobial fields.

Review of MXene synthesis and applications in electromagnetic shielding

With the ongoing advancements in wireless communication and electronic technology, the issue of electromagnetic radiation (EMR) pollution has become increasingly significant. Consequently, developing materials to mitigate EMR pollution is essential. The utility of 2D MXene (Ti3C2T x ) for electromagnetic interference (EMI) shielding was initially reported in 2016. Since then, MXenes have garnered substantial interest from the scientific community owing to their excellent metallic conductivity, low density, expansive specific surface area, and tunable interlayer spacing. In recent years, MXenes have demonstrated considerable promise in EMI shielding applications. This paper aims to examine the structural and chemical properties of MXenes, the methodologies for their synthesis, and summarize the advancements in MXene-based EMI shielding composites, highlighting their performance benefits. Additionally, this review will discuss the prospective developments in MXene-based materials for EMI shielding.

A high-sensitivity label-free electrochemical aptasensor for point-of-care measurements of low-density lipoprotein in plasma based on aptamer and MXene-CMCS-Hemin nanocomposites

Cardiovascular disease (CVD) remains a significant worldwide health challenge, with mortality rates rising rapidly. Low-density lipoprotein (LDL) is a crucial serum biomarker for the early diagnosis of CVD, which can significantly improve outcomes and reduce mortality. Herein, a label-free electrochemical aptasensor for rapid detection of LDL was developed based on the titanium carbide-carboxymethyl chitosan-hemin (MXene-CMCS-Hemin) nanocomposites as the electrochemical signal probe. Firstly, gold nanoparticles (Au NPs) were electrodeposited onto a screen-printed carbon electrode (SPCE) to form a conductive substrate. Subsequently, the MXene-CMCS-Hemin nanocomposites were anchored onto the Au NPs/SPCE surface. Then LDLApt was immobilized on the surface of MXene-CMCS-Hemin/Au NPs/SPCE to construct the electrochemical aptasensor. When LDL specifically bound with the LDLApt to form LDL-LDLApt complexes, hindering the electron transfer and reducing the Hemin oxidation current, LDL detection can be achieved via differential pulse voltammetry (DPV). Under optimal circumstances, the changes of Hemin's oxidation current showed a good linear response with LDL concentration in the range of 0.1-4.0 μmol/L with a detection limit of 0.095 μmol/L (S/N = 3). The aptasensor demonstrated good performance with the relative errors of 0.60 % to 6.58 % for the direct detection of LDL in human serum samples, which offers a novel tool for the clinical diagnosis of CVD.

Engineering the next generation of MXenes: challenges and strategies for scalable production and enhanced performance

Two-dimensional nanomaterials, such as MXenes, have garnered significant attention due to their excellent properties, including electrical conductivity, mechanical strength, and thermal stability. These properties make them promising candidates for energy storage and catalysis applications. However, several challenges impede their large-scale production and industrial application. Issues such as high production costs, safety concerns related to toxic etching agents, instability in oxidative environments, and the complex synthesis process must be addressed. In this review, we systematically analyze current methodologies for scaling up MXene production, focusing on the synthesis and etching of MAX phases, delamination strategies, and the production of MXene derivatives. We explore strategies for overcoming challenges like aggregation, oxidation, and cost, presenting optimization techniques for enhancing electrochemical performance and stability. The review also discusses the applications of MXenes in batteries and supercapacitors, emphasizing their potential for large-scale use. Finally, we provide an outlook on future research directions for MXene to develop safer and more cost-effective production methods to improve the performance of MXene in order to realize its commercial potential in energy technologies.

Chemically Tunable Ti3C2Tx MXene Surfaces

Modifying MXene surfaces is an effective strategy for controlling interactions with various chemical environments. Here, we demonstrate that treating Ti3C2Tx MXene surfaces with dilute acids and bases results in surfaces enriched with oxygen species capable of reversible proton exchange. These treatments produce surfaces with varying dispersibilities in different organic solvents that can be toggled between "on" and "off" states through successive treatments. Furthermore, we found that protonated MXenes exhibit slower ion kinetics due to surface passivation, whereas deprotonated MXenes show the opposite effect. This research opens avenues toward more versatile solution-based surface modification techniques that may rely less on specific MXene precursors and synthesis methods.

Au nanoparticle-engineered Ti3C2Tx MXenes as a high-performance SERS platform for detection of organic pollutants

The detection of organic pollutants at ultra-low concentrations is crucial for environmental monitoring, yet existing surface-enhanced Raman scattering (SERS) platforms often suffer from limited sensitivity, poor stability, and inconsistent signal reproducibility. To address these challenges, this study presents a high-performance SERS platform based on in situ gold (Au) nanoparticle-engineered Ti3C2Tx MXenes. This novel approach enhances signal amplification and ensures long-term stability for pollutant detection. The platform achieves an exceptional limit of detection (LOD) of 10-11 M with an enhancement factor of 1010 for Methylene Blue (MB), demonstrating its superior sensitivity. Additionally, signal repeatability has been validated by calculating the relative standard deviation (RSD), and the SERS substrate retains 83% of its signal intensity after 5 months of storage, confirming its durability. Furthermore, the platform effectively detects polybrominated diphenyl ether (BDE-47), a persistent organic pollutant, at concentrations below the regulatory threshold of 10-6 M. These results highlight the potential of the proposed SERS platform for reliable and long-term environmental monitoring of hazardous substances.

Record Efficiency of β-Phase PVDF-MXene Composites in Thin-Film Dielectric Capacitors

Polyvinylidene fluoride (PVDF) is a semicrystalline polymer used in thin-film dielectric capacitors because of its inherently high dielectric constant and low loss tangent. Its dielectric constant can be increased by the formation and alignment of its β-phase crystalline structure, which can be facilitated by 2D nanofillers. 2D carbides and nitrides, MXenes, are promising candidates due to their notable dielectric permittivity and ability to increase interfacial polarization. Still, their mixing is challenging due to weak interfacial interactions and poor dispersibility of MXenes in PVDF. This work explores a novel method for delaminating Ti3C2Tx MXene directly into organic solvents while maintaining flake size and quality, as well as the use of a non-solvent-induced phase separation method for producing both dense and porous PVDF-MXene composite films. A deeper understanding of dielectric behavior in these composites is reached by examining MXenes with both mixed and pure chlorine terminations in PVDF matrices. Thin-film capacitors fabricated from these composites display ultrahigh discharge energy density, exceeding 45 J cm-3 with 95% efficiency. The PVDF-MXene composites are also processed using a green and sustainable solvent, propylene carbonate.

2D MXenes: Synthesis, Properties, and Applications in Silicon-Based Optoelectronic Devices

MXenes, a rapidly emerging class of 2D transition metal carbides, nitrides, and carbonitrides, have attracted significant attention for their outstanding properties, including high electrical conductivity, tunable work function, and solution processability. These characteristics have made MXenes highly versatile and widely adopted in the next generation of optoelectronic devices, such as perovskite and organic solar cells. However, their integration into silicon-based optoelectronic devices remains relatively underexplored, despite silicon's dominance in the semiconductor industry. In this review, a timely summary of the recent progress in utilizing Ti-based MXenes, particularly Ti3C2Tx, in silicon-based optoelectronic devices is provided. The composition, synthesis methods, and key properties of MXenes that contribute to their potential for enhanced device performance are focused on. Furthermore, the latest advancements in MXene applications in silicon-based solar cells and photodetectors are discussed from fundamental and applied perspectives. Finally, the key challenges and future opportunities for the integration of MXenes in silicon-based optoelectronic devices are outlined.

Rational design of surface termination of Ti3C2T2 MXenes for lithium-ion battery anodes

Two-dimensional transition metal carbides, carbonitrides and nitrides (MXenes) have garnered increasing interest in the energy storage field due to their unique structural and electronic properties. However, the application performance is highly reliant on the surface termination, which is poorly understood from a chemical standpoint. In this work, the structural stability, chemical origin, electronic structure and lithium-ion (Li-ion) storage properties of 15 nonmetal terminated MXenes in the form of Ti3C2T2 (T = B, C, Si, N, P, As, O, S, Se, Te, F, Cl, Br, I and OH) were investigated using first-principles calculations. The results indicate that the partially occupied d-orbital and zero pseudogap lead to the high chemical activity of surface Ti, and that surface terminations can diminish its chemical activity. Furthermore, a large pseudogap of the d-orbital promotes the structural stability of Ti3C2T2. A useful descriptor, the antibonding state (Eσ*), was proposed to predict Li-ion adsorption energy. Combining the good electronic conductivity, high lithophilicity, low Li-ion diffusion barrier and high specific capacity, Ti3C2As2, Ti3C2S2 and Ti3C2Se2 are considered as promising anode candidates for Li-ion batteries. Additionally, S, Se and As doping can improve the Li-ion storage performance of oxygen terminated Ti3C2O2. This work offers insights into the chemical origin of the surface termination and paves the way for designing excellent Li-ion anode candidates based on MXenes.

Mechanical properties of two-dimensional material-based thin films: a comprehensive review

Two-dimensional (2D) materials are materials with a thickness of one or a few atoms with intriguing electrical, chemical, optical, electrochemical, and mechanical properties. Therefore, they are deemed candidates for ubiquitous engineering applications. Films and three-dimensional (3D) structures made from 2D materials introduce a distinct assembly structure that imparts the inherent properties of pristine 2D materials on a macroscopic scale. Acquiring the adequate strength and toughness of 2D material structures is of great interest due to their high demand for numerous industrial applications. This work presents a comprehensive review of the mechanical properties and deformation behavior of robust films composed of 2D materials that help them to attain other extraordinary properties. Moreover, the various key factors affecting the mechanical performance of such thin films, such as the lateral size of nanoflakes, fabrication technique of the film, thickness of the film, post-processing, and strain rate, are elucidated.

Investigations of the structural, electronic, and optical properties of Ti3XC2 (X = Ge, Pb, or Bi) by DFT

The global community currently faces significant challenges in meeting the rising demand for energy production. The development of clean energy technologies has gained substantial attention due to increasing energy shortages and worsening environmental degradation. Addressing these challenges requires the development of new materials. This study investigates the structural, electronic, and optical properties of Ti3XC2 (X = Ge, Pb, or Bi) MXenes, focusing on deintercalation and intercalation stages. The structural analysis demonstrates that the insertion and removal of Ge, Pb, and Bi within Ti3XC2 MXene significantly affect cell volume, with the second deintercalation stage exhibiting greater structural stability compared to the first. These MXenes exhibit metallic conductivity confirmed by density of states (DOS) calculations which reveal a zero band gap even with GGA corrections. The optical properties, including reflectivity, dielectric function and energy loss highlight distinct behavior among the intercalated structures, particularly for Ti3GeC2 which shows higher energy loss peaks in the 14-16 eV range. These findings provide valuable insights into the electronics and structural behavior of Ti3XC2 MXenes, making them promising candidates for advanced energy storage and electronic applications.

Scalable, High-Yield Monolayer MXene Preparation from Multilayer MXene for Many Applications

MXene (Ti3C2Tx) is renowned for its exceptional conductivity and hydrophilicity; however, the low yield of monolayers hinders its industrial scalability. Herein, we present a strategy to substantially enhance the monolayer yield by disrupting the hydrogen-bonding cage confinement of multilayer MXene using high-temperature ultrasound, challenging the conventional belief that monolayer MXene can only be prepared at lower temperatures. At approximately 70 °C, the weakened hydrogen bonding between the oxygen-containing terminal groups of multilayer MXene and surrounding water molecules weakens the hydrogen-bond cage confinement. This enables ultrasonic cavitation to generate more microbubbles that penetrate the interlayers of multilayer MXene, resulting in gentle and thorough delamination into larger monolayer nanosheets. Achieving up to a 95 % yield in just tens of minutes, these nanosheets exhibit properties comparable to those produced by traditional ice-bath methods. Furthermore, the high-concentration MXene ink produced on a large scale using this high-yield approach exhibits excellent printing and processing capabilities, and the corresponding products showcase superior infrared stealth and Joule heating characteristics. This work addresses a key technical bottleneck in MXene production, paving the way for its extensive technological and industrial applications.

The Future of MXenes: Exploring Oxidative Degradation Pathways and Coping with Surface/Edge Passivation Approach

The MXene, which is usually transition metal carbide, nitride, and carbonitride, is one of the emerging family of 2D materials, exhibiting considerable potential across various research areas. Despite theoretical versatility, practical application of MXene is prohibited due to its spontaneous oxidative degradation. This review meticulously discusses the factors influencing the oxidation of MXenes, considering both thermodynamic and kinetic point of view. The potential mechanisms of oxidation are systematically introduced, based on experimental and theoretical models. Typically, the surfaces and edges of MXenes are susceptible to oxidation, as the surface terminal groups are easily attacked by oxygen and water molecules, ultimately leading to structural deformation. To retard oxidative degradation, ligand mediated surface/edge passivation is suggested as a promising strategy. In this regard, detailed passivation strategies for MXenes are systematically explained based on the types of chemistry at the MXene-ligand interface-covalent bonding, electrostatic interactions, and hydrogen bonding-and the type of stabilizing moieties-organic, inorganic, biomolecules, and polymers. The retardation of oxidation is discussed in relation with the interaction type and passivating moiety. This review aims to catalyze future research to identify efficient and cost-effective ligands for the surface engineering of MXenes, enhancing their oxidation stability.

Transparent MXene Microelectrode Arrays for Multimodal Mapping of Neural Dynamics

Transparent microelectrode arrays have proven useful in neural sensing, offering a clear interface for monitoring brain activity without compromising high spatial and temporal resolution. The current landscape of transparent electrode technology faces challenges in developing durable, highly transparent electrodes while maintaining low interface impedance and prioritizing scalable processing and fabrication methods. To address these limitations, we introduce artifact-resistant transparent MXene microelectrode arrays optimized for high spatiotemporal resolution recording of neural activity. With 60% transmittance at 550 nm, these arrays enable simultaneous imaging and electrophysiology for multimodal neural mapping. Electrochemical characterization shows low impedance of 563 ± 99 kΩ at 1 kHz and a charge storage capacity of 58 mC cm⁻² without chemical doping. In vivo experiments in rodent models demonstrate the transparent arrays' functionality and performance. In a rodent model of chemically-induced epileptiform activity, we tracked ictal wavefronts via calcium imaging while simultaneously recording seizure onset. In the rat barrel cortex, we recorded multi-unit activity across cortical depths, showing the feasibility of recording high-frequency electrophysiological activity. The transparency and optical absorption properties of Ti₃C₂Tx MXene microelectrodes enable high-quality recordings and simultaneous light-based stimulation and imaging without contamination from light-induced artifacts.

Cr2TiC2Tx MXene as an adsorbent material in ultrasonic-assisted d-μ-solid phase extraction for trace detection of heavy metals

MXenes are a large family of two-dimensional transition metal carbides, nitrides, and carbonitrides. While MXenes have great potential for applications in analytical chemistry, most of the studies in this field are focused on Ti3C2Tx, the most popular MXene material. For example, several studies employed Ti3C2Tx as an adsorbent for the trace detection of toxic analytes, but there is limited knowledge on the utility of other MXene materials for this application. In this work, we investigated the potential of Cr2TiC2Tx, one of the least studied MXenes, for application as an adsorbent material in ultrasonic-assisted dispersive micro solid-phase extraction (d-μ-SPE) method for the detection of heavy metals at trace levels in food and soil samples. We synthesized large monolayer flakes of Cr2TiC2Tx and characterized it by a variety of microscopic and spectroscopic techniques. Cr2TiC2Tx MXene showed remarkable performance in the d-μ-SPE method with the detection limits of 0.09 and 1.9 ng mL-1, and dynamic ranges of 0.3-90 μg L-1 and 6-120 μg L-1 for cadmium (Cd2+) and lead (Pb2+) ions, respectively. The great performance of Cr2TiC2Tx MXene as an adsorbent for the trace detection of heavy metals highlights the importance of investigating other MXenes beyond Ti3C2Tx for analytical applications.

Enhancing Zn Deposition Reversibility on MXene Current Collectors by Forming ZnF2-Containing Solid-Electrolyte Interphase for Anode-Free Zinc Metal Batteries

Anode-free aqueous zinc metal batteries (AZMBs) offer significant potential for energy storage due to their low cost and environmental benefits. Ti3C2Tx MXene provides several advantages over traditional metallic current collectors like Cu and Ti, including better Zn plating affinity, lightweight, and flexibility. However, self-freestanding MXene current collectors in AZMBs remain underexplored, likely due to challenges with Zn deposition reversibility. This study investigates the combination of a Ti3C2Tx self-freestanding film with advanced electrolyte engineering, specifically examining the effects of Li-salt and propylene carbonate (PC) as additives on Zn plating reversibility. While using Li+ ions as an additive alone facilitates uniform Zn deposition on bulk metals through the electrostatic shielding effect, the addition of Li-salt negatively impacts Zn plating uniformity on Ti3C2Tx. Meanwhile, using PC additive alone forms an organic SEI layer on Ti3C2Tx and causes Zn agglomeration. The use of both additives together results in a ZnF2-containing hybrid SEI layer with improved interfacial kinetics, promoting more uniform Zn deposition. This approach achieves an average Coulombic efficiency (CE) of 96.8% over 150 cycles (a maximum CE of 97.8%). The study highlights the strategic difference in electrolyte design, emphasizing the need for tailored approaches to optimize Zn deposition on MXenes, contrasting with traditional metallic current collectors.

Two-Dimensional Nanostructured Ti3C2Tx MXene for Ceramic Materials: Preparation and Applications

Ti3C2Tx MXene, a novel two-dimensional transition metal carbide with nanoscale dimensions, has attracted significant attention due to its exceptional structural and performance characteristics. This review comprehensively examines various preparation methods for Ti3C2Tx MXene, including acid etching, acid-salt composite etching, alkali etching, and molten salt etching. It further discusses several strategies for interlayer exfoliation, highlighting the advantages and limitations of each method. The effects of these techniques on the nanostructure, surface functional groups, interlayer spacing, and overall performance of Ti3C2Tx MXene are evaluated. Additionally, this paper explores the diverse applications of Ti3C2Tx MXene in ceramic materials, particularly its role in enhancing mechanical properties, electrical and thermal conductivity, as well as oxidation and corrosion resistance. The primary objective of the review is to provide scientific insights and theoretical guidance for the preparation of Ti3C2Tx MXene and its further research and innovative applications in ceramic materials, advancing the development of high-performance, multifunctional ceramics.

MXene-Reinforced Composite Cryogel Scaffold for Neural Tissue Repair

The development of effective materials for neural tissue repair remains a major challenge in regenerative medicine. In this study, we present a novel MXene-reinforced composite cryogel scaffold designed for neural tissue regeneration. MXenes, a class of two-dimensional materials with high conductivity and biocompatibility, were integrated into a polyvinyl alcohol (PVA) matrix via cryopolymerization to form a macroporous, mechanically stable scaffold. The morphology, mechanical properties, and swelling behavior of the cryogel with different MXene contents have been assessed. The effects of MXene on the viability/proliferation and differentiation of neural cells (PC-12) cultured in the composite cryogel were elucidated. The MXene/PVA cryogel demonstrated excellent cell-supporting potential, with MXene not only showing no toxicity but also promoting the proliferation of cultured PC-12. Additionally, MXene induced a neuritogenesis-like process in the cells as evidenced by morphological changes and the enhanced expression of the neural marker β-III-tubulin. The neuroprotective properties of the MXene component were revealed by the alleviation of oxidative stress and reduction of intracellular ROS levels. These findings highlight the potential of MXene-embedded PVA cryogel as a promising material that can be further used in conjunction with electrostimulation therapy for advancing strategies in neural tissue engineering.

MXene-based multilayered and ultrawideband absorber for solar cell and photovoltaic applications

We proposed the ultrawideband solar absorber using the multisized metal resonator oriented on the top of the multilayered Metal-SiO₂-MXene-MgF₂-Tungsten structure. We have carried out a numerical investigation of this structure for the 100-2500 THz frequency, which covers the infrared, visible, and UV spectra. The proposed solar absorber is numerically investigated for the different physical parameters, such as the height of the layers, unit cell size, and resonator orientation, to identify optimized results for the high absorption capacity. The structure presented in the study shows promise, with an average absorption of 80% over the large frequency spectrum of 100-2500 THz. This structure was also investigated for the variation in oblique incident angle, which showcases the absorption stability up to 60⁰ of the incident angle. We have also reported the comparative analysis for this proposed absorber structure with other designs, demonstrating the absorption efficiency over infrared, visible, and UV spectra. The proposed structure and discrete resonator length can offer a better solution for trapping the different frequency ranges, resulting in high absorption over a wideband frequency. This study can be applied to designing highly efficient parasitic solar absorber structures, which are essential to highly efficient photovoltaic and solar cell design.

Enhanced Acoustoelectric Energy Harvesting with Ti3C2Tx MXene in an All-Fiber Nanogenerator

Materials and devices that harvest acoustic energy can enable autonomous powering of microdevices and wireless sensors. However, traditional acoustic energy harvesters rely on brittle piezoceramics, which have restricted their use in wearable electronic devices. To address these limitations, this study involves the fabrication of acoustic harvesters using electrospinning of the piezoelectric polymer PVDF-TrFE onto fabric-based electrodes. Two-dimensional (2D) Ti3C2Tx MXene flakes were used to induce polarization locking of the electrospun PVDF-TrFE for optimal electromechanical performance of PVDF-TrFE. The mechanically robust, lightweight, and flexible device was demonstrated to detect and harvest energy in the sound frequency range of 50 to 1000 Hz at sound levels between 60 and 95 dB, while exhibiting a high sensitivity of 37 VPa-1, which is higher than previously reported values for PVDF-based sound harvesters. The maximum output power can reach 19 mW/cm3 under 200 Hz and 95 dB. The development of this material opens a future pathway for powering small electronic devices, such as implantable biomedical devices, smart wearable technology, and remote Internet-of-Things devices.

Ti3C2T x MXene Thin Films and Intercalated Species Characterized by IR-to-UV Broadband Ellipsometry

MXenes are two-dimensional (2D) materials with versatile applications in optoelectronics, batteries, and catalysis. To unlock their full potential, it is crucial to characterize MXene interfaces and intercalated species in more detail than is currently possible with conventional optical spectroscopies. Here, we combine ultra-broadband ellipsometry and transmission spectroscopy from the mid-infrared (IR) to the deep-ultraviolet (UV) to probe quantitatively the composition, structure, transport, and optical properties of spray-coated Ti3C2T x MXene thin films with varying material properties. We find film thickness heterogeneity and surface roughness in the low-nanometer range as well as depth-dependent conductivity properties, which we quantify with a graded Drude model. The optically determined sheet resistance is confirmed by four-point probe measurements. Furthermore, we employ density-functional-theory calculations to assign the observed absorption bands in the MXene dielectric function to various interband transitions from mixed MXene surface terminations. The prominent 1.48 eV (833 nm) spectral feature is found to be related to oxygen termination. Additional plasmonic effects are also suggested. Finally, we leverage the chemical sensitivity of state-of-the-art IR ellipsometry to separate the fingerprints of intercalated species within the MXene from the dominant Drude contributions, presenting for the first time a set of infrared optical constants of intercalated water. This work lays the foundation for optical metrology for interface engineering of MXene and other 2D materials.

Carbon Thin-Film Electrodes as High-Performing Substrates for Correlative Single Entity Electrochemistry

Correlative methods to characterize single entities by electrochemistry and microscopy/spectroscopy are increasingly needed to elucidate structure-function relationships of nanomaterials. However, the technical constraints often differ depending on the characterization techniques to be applied in combination. One of the cornerstones of correlative single-entity electrochemistry (SEE) is the substrate, which needs to achieve a high conductivity, low roughness, and electrochemical inertness. This work shows that graphitized sputtered carbon thin films constitute excellent electrodes for SEE while enabling characterization with scanning probe, optical, electron, and X-ray microscopies. Three different correlative SEE experiments using nanoparticles, nanocubes, and 2D Ti3C2Tx MXene materials are reported to illustrate the potential of using carbon thin film substrates for SEE characterization. The advantages and unique capabilities of SEE correlative strategies are further demonstrated by showing that electrochemically oxidized Ti3C2Tx MXene display changes in chemical bonding and electrolyte ion distribution.

Thermoelectric Modulation of Neat Ti3C2Tx MXenes by Finely Regulating the Stacking of Nanosheets

Emerging two-dimensional MXenes have been extensively studied in a wide range of fields thanks to their superior electrical and hydrophilic attributes as well as excellent chemical stability and mechanical flexibility. Among them, the ultrahigh electrical conductivity (σ) and tunable band structures of benchmark Ti3C2Tx MXene demonstrate its good potential as thermoelectric (TE) materials. However, both the large variation of σ reported in the literature and the intrinsically low Seebeck coefficient (S) hinder the practical applications. Herein, this study has for the first time systematically investigated the TE properties of neat Ti3C2Tx films, which are finely modulated by exploiting different dispersing solvents, controlling nanosheet sizes and constructing composites. First, deionized water is found to be superior for obtaining closely packed MXene sheets relative to other polar solvents. Second, a simultaneous increase in both S and σ is realized via elevating centrifugal speed on MXene aqueous suspensions to obtain small-sized nanosheets, thus yielding an ultrahigh power factor up to ~ 156 μW m-1 K-2. Third, S is significantly enhanced yet accompanied by a reduction in σ when constructing MXene-based nanocomposites, the latter of which is originated from the damage to the intimate stackings of MXene nanosheets. Together, a correlation between the TE properties of neat Ti3C2Tx films and the stacking of nanosheets is elucidated, which would stimulate further exploration of MXene TEs.

Layered MXene Films via Self-Assembly

MXene has attracted significant attention as a 2D material family due to its metallic conductivity and abundant surface functional groups and has been extensively studied and applied as bulk materials and microscale thin films. MXene possesses ionizable surfaces and edges, as well as high surface area. Its customizable dispersibility demonstrates unique advantages in self-assembly solution processing. Recent studies have demonstrated the application value of layered MXene films at the nanoscale thickness and the reliance of processing on self-assembly techniques. However, this field currently lacks sufficient attention. Here, the regulatory mechanisms are summarized for the preparation of layered MXene films through self-assembly techniques, as well as introduce their applications. Moreover, the future challenges of large-scale applications of MXene self-assembly techniques are proposed. It is believed that this review would provide a dynamic and promising path for the development of layered MXene self-assembly techniques.

All-In-One Epoxy/MXene Nanocomposites with Bead-Type Polymeric Imidazole Latent Curing Agent for Enhancing Storage Stability and Flame Retardancy

Developing a single-component epoxy system is challenging but crucial for advanced thermoset applications. Unfortunately, conventional latent curing agents using chemical or physical passivation do not provide satisfactory storage stability and the necessary property requirements. Here, it is demonstrated that all-in-one epoxy/MXene nanocomposite system, comprising epoxy resin, polymeric imidazole latent curing agent beads (PILCAB), and Ti3C2Tx MXene, exhibits excellent storage stability, improved flame retardancy, and enhanced mechanical strength. PILCABs, prepared through a Diels-Alder (DA) crosslinking reaction between furan groups of poly(imidazolyl methacrylate)-random poly (furfuryl methacrylate) (PIm-r-PFu) copolymer and bismaleimide (BMI), exhibit excellent storage stability, as stable as under 60 °C storage due to the imidazole reactivity being suppressed synergistically by both physical and chemical passivation mechanisms. Ti3C2Tx MXene flakes, surface-functionalized with alkylated 3,4-dihydroxyl-L-phenylalanine, exhibit excellent compatibility with the epoxy matrix. Consequently, the enhanced storage stability, flame retardancy, and mechanical strength of the all-in-one epoxy/MXene nanocomposite are attributed to the strong DA bond formation in latent curing agent, efficient charring capability of MXene and BMI, and the catalytic effect of the MXene. This study opens new avenues for designing and developing single-component epoxy systems that satisfy demanding requirements, including storage stability, mechanical strength, and flame retardancy, which are essential for practical applications.

Direct Fabrication of Atomically Defined Pores in MXenes Using Feedback-Driven STEM

Controlled fabrication of nanopores in 2D materials offer the means to create robust membranes needed for ion transport and nanofiltration. Techniques for creating nanopores have relied upon either plasma etching or direct irradiation; however, aberration-corrected scanning transmission electron microscopy (STEM) offers the advantage of combining a sub-Å sized electron beam for atomic manipulation along with atomic resolution imaging. Here, a method for automated nanopore fabrication is utilized with real-time atomic visualization to enhance the mechanistic understanding of beam-induced transformations. Additionally, an electron beam simulation technique, Electron-Beam Simulator (E-BeamSim) is developed to observe the atomic movements and interactions resulting from electron beam irradiation. Using the MXene Ti3C2Tx, the influence of temperature on nanopore fabrication is explored by tracking atomic transformations and find that at room temperature the electron beam irradiation induces random displacement and results in titanium pileups at the nanopore edge, which is confirmed by E-BeamSim. At elevated temperatures, after removal of the surface functional groups and with the increased mobility of atoms results in atomic transformations that lead to the selective removal of atoms layer by layer. This work can lead to the development of defect engineering techniques within functionalized MXene layers and other 2D materials.

Enhanced Oxidation-Resistant and Conductivity in MXene Films with Seamless Heterostructure

MXene-based films have garnered significant attention for their remarkable electrical and mechanical properties. Nevertheless, the practical application of MXene is impeded by its intrinsic instability caused by spontaneous oxidation. The traditional anti-oxidative strategies frequently lead to a compromise in stability, electrical conductivity, and mechanical properties. In this study, a novel approach is proposed involving metal nano-armoring, wherein a copper layer with nano thickness is deposited onto MXene film surfaces to establish a uniform and seamless heterogeneous interface (MXene@Cu). The precise tunability and uniformity of this heterostructure are consistently demonstrated through both theoretical calculations and experimental results. The MXene@Cu films exhibit exceptional electrical conductivity of 1.17 × 106 S m-1, electromagnetic interference shielding effectiveness of 77.1 dB, and tensile strength of 43.4 MPa. More importantly, this heterostructure significantly improves MXene's stability against oxidation. After exposure to air for 30 days, the resultant MXene@Cu films exhibit a remarkable conductivity retention of 72.0%, significantly exceeding that of pristine MXene films (44.3%). This scalable synthesis approach holds significant promise for electronic device applications, particularly in electromagnetic shielding and thermal management.

Nanoscale Surface and Bulk Electronic Properties of Ti3C2Tx MXene Unraveled by Multimodal X-Ray Spectromicroscopy

2D layered materials, such as transition metal carbides or nitrides, known as MXenes, offer an ideal platform to investigate charge transfer processes in confined environment, relevant for energy conversion and storage applications. Their rich surface chemistry plays an essential role in the pseudocapacitive behavior of MXenes. However, the local distribution of surface functional groups over single flakes and within few- or multilayered flakes remains unclear. In this work, scanning X-ray microscopy (SXM) is introduced with simultaneous transmission and electron yield detection, enabling multimodal nanoscale chemical imaging with bulk and surface sensitivity, respectively, of individual MXene flakes. The Ti chemical bonding environment is found to significantly vary between few-layered hydrofluoric acid-etched Ti3C2Tx MXenes and multilayered molten salt (MS)-etched Ti3C2Tx MXenes. Postmortem analysis of MS-etched Ti3C2Tx electrodes cycled in a Li-ion battery further illustrates that simultaneous bulk and surface chemical imaging using SXM offers a method well adapted to the characterization of the electrode-electrolyte interactions at the nanoscale.

Violation of the Wiedemann-Franz Law and Ultralow Thermal Conductivity of Ti3C2Tx MXene

The high electrical conductivity and good chemical stability of MXenes offer hopes for their use in many applications, such as wearable electronics, energy storage, and electromagnetic interference shielding. While their optical, electronic, and electrochemical properties have been widely studied, information on the thermal properties of MXenes is scarce. In this study, we investigate the heat transport properties of Ti3C2Tx MXene single flakes using scanning thermal microscopy and find exceptionally low anisotropic thermal conductivities within the Ti3C2Tx flakes, leading to an effective thermal conductivity of 0.78 ± 0.21 W m-1 K-1. This observation is in stark contrast to the predictions of the Wiedemann-Franz law, as the estimated Lorenz number is only 0.25 of the classical value. Due to the combination of low thermal conductivity and low emissivity of Ti3C2Tx, the heat loss from it is 2 orders of magnitude smaller than that from common metals. Our study explores the heat transport mechanisms of MXenes and highlights a promising approach for developing thermal insulation, two-dimensional thermoelectric, or infrared stealth materials.

Theoretical insights on potential-dependent oxidation behaviors and antioxidant strategies of MXenes

Significant efforts have been devoted to investigating the oxidation of MXenes in various environments. However, the underlying mechanism of MXene oxidation and its dependence on the electrode potential remain poorly understood. Here we show the oxidation behavior of MXenes under the working conditions of electrochemical processes in terms of kinetics and thermodynamics by using constant-potential ab initio simulations. The theoretical results indicate that the potential effects can be attributed to the nucleophilic attack of water molecules on metal atoms, similar to that taking place in the Oxygen Evolution Reaction. Building upon these findings, we deduced the oxidation potential of the common MXenes, and proposed antioxidant strategies for MXene. Finally, we demonstrated that MBenes, the boron analogs of MXenes, may undergo a similar nucleophilic attack in water and inferred that molecule-induced Walden inversion is widely present in material reconstructions. This work contributes to a fundamental understanding MXene stability at the atomic level, and promotes the transition in materials discovery from trial-and-error synthesis to rational design.

Lignin as a bioderived modular surfactant and intercalant for Ti3C2Tx MXene stabilization and tunable functions

Controlled tailoring of atomically thin MXene interlayer spacings by surfactant/intercalants (e.g., polymers, ligands, small molecules) is important to maximize their potential for application. However, challenges persist in achieving precise spacing tunability in a well-defined stacking, combining long-term stability and dispersibility in various solvents. Here, we discovered that lignin can be used as surfactants/intercalants of Ti3C2Tx MXenes. The resulting MXene@lignin complexes exhibit superior colloidal stability and oxidation resistance in both water and different organic solvents. More important, we reveal a dynamic interaction between MXene and lignin that enables a wide-range fine interlayer distance tuning at a sub-nanometer scale. Such dynamic interaction is sparse in the reported organic surfactants/intercalants containing single types of functional groups. We also demonstrate the tunability of electrical conductivity, infrared emissivity, and electromagnetic interference shielding effectiveness. Our approach offers a starting point to explore the potential of MXene-biomacromolecule composites for electronics and photonics applications.

Recent progress in Si/Ti3C2Tx MXene anode materials for lithium-ion batteries

Cardiovascular diseases (CVDs) are a major global health issue, causing significant morbidity and mortality worldwide. Early diagnosis and continuous monitoring of physiological signals are crucial for managing cardiovascular diseases, necessitating the development of lightweight and cost-effective wearable devices. These devices should incorporate portable energy storage systems, such as lithium-ion batteries (LIBs). To enhance the durability and consistency of the monitoring systems, there is a need to develop LIBs with high energy density. Silicon-based materials hold great promise for future LIBs anodes due to their high theoretical capacity and cost-efficiency. Despite their potential, silicon-based materials encounter challenges like substantial volume fluctuations and sluggish kinetics. Transition metal carbide, MXene, features a two-dimensional structure, offering advantages in silicon-based anode materials. This review initially presents the potential of silicon-based anodes and then addresses their challenges. Subsequently, the advantages of MXene are systematically reviewed, including unique structure, abundant surface functional groups, excellent electrical conductivity, and excellent ion transport performance. Next, the detailed discussion covers recent advancements in Si/Ti3C2Tx MXene anode materials for LIBs, with a focus on their synthesis methods. Finally, the challenges and future perspectives of synthesizing Si/Ti3C2Tx nanocomposites are examined, aiming to provide a foundational resource for designing advanced materials for high-energy LIBs.

Long-Term Storage of Ti3C2Tx Aqueous Dispersion with Stable Electrochemical Properties

MXenes possess high metallic conductivity and excellent dispersion quality and pseudocapcitance. Their good hydrophilicity makes them particularly suitable as eco-friendly inks for printing applications. However, MXenes are prone to oxidization in aqueous dispersions, and it is very important to improve their stability. Here, the long-term storage of MXene aqueous dispersions was realized by the introduction of sodium L-ascorbate (NaAsc) as the antioxidant. The preserved MXenes exhibited very stable electrochemical properties. Even after 60-day storage, the supercapacitor with preserved MXenes as the electrode still demonstrated an excellent specific capacitance of 381.1 F/g at a scan rate of 5 mV/s and a good retention rate of 92.6% after 10,000 consecutive cyclic voltammetry measurements, which was nearly the same as that of fresh MXenes. The results indicate a facile and efficient method to realize the long-term storage of MXene aqueous dispersions for mass use in future energy storage.

Impact of Asymmetric Microstructure on Ion Transport in Ti3C2Tx Membranes

Consolidation or densification of low-dimensional MXene materials into membranes can result in the formation of asymmetric membrane structures. Nanostructural (short-range) and microstructural (long-range) heterogeneity can influence mass transport and separation mechanisms. Short-range structural dynamics include the presence of water confined between the 2D layers, while long-range structural properties include the formation of defects, micropores, and mesopores. Herein, it is demonstrated that structural heterogeneity in Ti3C2Tx membranes fabricated via vacuum-assisted filtration significantly affects ion transport. Higher ion permeabilities are achieved when the dense "bottom" side of the membrane, rather than the porous "top" side, faces the feed solution. Characterization of the membrane reveals distinct differences in flake alignment, surface roughness, and porosity across the membrane. The directional dependence on permeability suggests that one region of the membrane experiences stronger internal concentration polarization, potentially suppressing permeability through the porous side of the membrane.

Improvement of Electrical and Thermal Properties of Carbon Nanotube Sheets by Adding Silver Nanowire and Mxene for an Electromagnetic-Interference-Shielding Property Study

Hybrid carbon nanotube (CNT) sheets were fabricated by mixing CNTs with silver nanowires (AgNWs) and MXene to study their electromagnetic-interference (EMI)-shielding properties. CNT/AgNW and CNT/MXene hybrid sheets were produced by ultrasonic homogenization and vacuum filtration, resulting in free-standing CNT sheets. Three different weight ratios of AgNW and MXene were added to the CNT dispersions to produce hybrid CNT sheets. Microstructure characterization was performed using scanning electron microscopy, and the Wiedemann-Franz law was used to characterize transport properties. The resulting hybrid sheets exhibited improved electrical conductivity, thermal conductivity, and EMI-shielding effectiveness compared to pristine CNT sheets. X-band EMI-shielding effectiveness improved by over 200%, while electrical conductivity improved by more than 1500% in the hybrid sheets due to a higher charge-carrier density and synergistic effects between nanomaterials. The addition of AgNW to CNT sheets resulted in a large improvement in electrical conductivity and EMI shielding; however, this may also result in increased weight and sample thickness. Similarly, the addition of MXene to CNT sheets may result in an increase in weight due to the presence of the denser MXene flakes.

Multi-Interface Engineering of MXenes for Self-Powered Wearable Devices

Self-powered wearable devices with integrated energy supply module and sensitive sensors have significantly blossomed for continuous monitoring of human activity and the surrounding environment in healthcare sectors. The emerging of MXene-based materials has brought research upsurge in the fields of energy and electronics, owing to their excellent electrochemical performance, large surface area, superior mechanical performance, and tunable interfacial properties, where their performance can be further boosted via multi-interface engineering. Herein, a comprehensive review of recent progress in MXenes for self-powered wearable devices is discussed from the aspects of multi-interface engineering. The fundamental properties of MXenes including electronic, mechanical, optical, and thermal characteristics are discussed in detail. Different from previous review works on MXenes, multi-interface engineering of MXenes from termination regulation to surface modification and their impact on the performance of materials and energy storage/conversion devices are summarized. Based on the interfacial manipulation strategies, potential applications of MXene-based self-powered wearable devices are outlined. Finally, proposals and perspectives are provided on the current challenges and future directions in MXene-based self-powered wearable devices.