Two-dimensional (2D) MXenes are synthesized by a top-down etching of MAX phases, which could generates surface metal vacancies. However, the nature and impact of these vacancies remain unclear. We combine atomic force microscopy (AFM) nanoindentation, electrochemical studies, and density functional theory (DFT) to examine how titanium vacancies (VTi) influence the mechanical and electrochemical behavior of Ti3C2T x MXene. A moderate level of VTi increases the in-plane modulus of monolayer Ti3C2T x from 324 +/- 44 to 432 +/- 53 N m-1 and enhances the fracture force by similar to 60%. The calculated effective Young's modulus of 432 +/- 53 GPa is among the highest for 2D materials. Also, moderate VTi improves the electrochemical performance of MXenes. DFT indicates that partial occupation of VTi by H2O, H+, or Li+ redistributes charge and increases lattice stiffness, while coalescence impedes electron transport and suppresses capacitance at higher VTi. This study deepens the understanding of vacancies in MXenes and provides a route to tune their properties.

Incorporating MXenes as additives into polymeric ceramic precursors allows homogenous integration of these 2D carbides into ceramic matrices. In this study, Ti3C2Tx MXene flakes were functionalized using 3-(2-aminoethylamino)propyltrimethoxysilane (AEAPTMS) and stabilized in allyl-hydrido-polycarbosilane (SMP-10), a SiC polymeric precursor. Thermogravimetric analysis suggests the composites retain more residual mass ( 2.45–4.21

Over the past decade, 2D MXenes have garnered tremendous attention for membrane applications owing to their exceptional hydrophilicity, tunable surface chemistry, and antifouling properties. However, translating MXene-based membranes from laboratory demonstrations to practically relevant manufacturing remains challenging because scalable fabrication routes are still limited. Herein, we demonstrate a roll-to-roll nonsolvent-induced phase separation (R2R-NIPS) process for fabricating Ti3C2Tx/polysulfone (PSF) mixed-matrix ultrafiltration membranes in large sheet format (196 × 84 cm; geometric area ≈1.65 m2). Incorporation of 1 wt% MXene produced more open and interconnected membrane substructures, enhanced apparent wettability (contact angle reduced from 90° for PSF to 62°), a more negative surface charge, and subsurface flake embedding confirmed by characterization. These features yielded a water flux of 206 LMH (31% higher than pristine PSF) with 97.6% humic acid rejection (n = 3 batches, p < 0.01 for flux; p < 0.05 for rejection), while batch-to-batch variability remained <5%. Rather than claiming a measured thermodynamic phase diagram, we interpret the observed morphology as consistent with faster demixing during R2R-NIPS. This processing methodology-documenting practical parameter choices such as casting speed, wet thickness, and bath circulation-provides a feasible route toward larger-scale MXene membrane production for water purification.

Ordered double transition-metal (DTM) MXenes are a subfamily of two-dimensional (2D) carbides, nitrides, and carbonitrides, predicted to outperform single-metal MXenes in hydrogen evolution reaction (HER) catalysis due to the synergistic effect of two metals and their nonmetal (X) sublattice tailoring (X = C, N), resulting in tunable electronic structures. However, all synthesized DTM MXenes to date contain only carbon in the X sublattice. Here, we report the synthesis of a series of out-of-plane ordered DTM carbonitride MXenes (o-MXenes), Mo2Ti(CN)2Tx and Mo2Ti2(CN)3Tx, to systematically investigate the role of carbon to nitrogen ratio. To determine the optimal nitrogen content, we first evaluated the HER activity of the Mo2TiC2-yNyTx MXenes with density functional theory calculations and identified that 0.3 to 0.6 mol of nitrogen give enhanced performance compared to the carbide. We next synthesized and characterized 11 carbonitride MXenes with varying their C:N ratios and found nitrogen incorporation enhances HER activity compared to their carbide counterparts. Among them, Mo2TiC2-yNyTx MXene with 0.6 mol of nitrogen (y = 0.6) achieved the best performance, with an overpotential of ∼155 mV at 10 mA/cm2 under acidic conditions, compared with ∼236 mV for Mo2TiC2Tx. Our experimental and computational findings indicate that carbonitrides with ∼25-30 atom % nitrogen outperform all other o-MXene counterparts, with the improved performance arising from nitrogen-induced modulation of the electronic structure. This study identifies nonmetal sublattice control as a critical frontier in optimizing MXenes for sustainable energy applications.

The stringent safety protocols required for hydrofluoric acid (HF) based MAX phase etching and the reliance on material characterization tools such as XRD, SEM, EDS and XPS to study etching make the MXene research challenging. Here, we have employed 27Al NMR spectroscopy for the rapid detection of selective etching, directly from the etching supernatant, of soluble aluminum species generated during the MAX phase etching reaction. This technique was applied to the development of a new etching protocol for Ti3AlC2 MAX phase using the less hazardous hexafluorosilicic acid. The etching process was studied using a combination of 27Al and 19F NMR spectroscopies where it was demonstrated to be free of HF or free fluoride in quantities detectable by 19F NMR, and that the primary etching byproduct is H3AlF6. 19F NMR spectroscopy was additionally proven to be a viable technique to quantify the extent of etching using trifluoroacetic acid as an internal standard.

Point defects govern many important functional properties of two-dimensional (2D) materials. However, resolving the three-dimensional (3D) arrangement of these defects in multi-layer 2D materials remains a fundamental challenge, hindering rational defect engineering. Here, we overcome this limitation using an artificial intelligence-guided electron microscopy workflow to map the 3D topology and clustering of atomic vacancies in Ti3C2TX MXene. Our approach reconstructs the 3D coordinates of vacancies across hundreds of thousands of lattice sites, generating robust statistical insight into their distribution that can be correlated with specific synthesis pathways. This large-scale data enables us to classify a hierarchy of defect structures-from isolated vacancies to nanopores-revealing their preferred formation and interaction mechanisms, as corroborated by molecular dynamics simulations. This work provides a generalizable framework for understanding and ultimately controlling point defects across large volumes, paving the way for the rational design of defect-engineered functional 2D materials.

This study provides understanding of how mechanical degradation and residual solvents after solution processing collectively impact the electrical conductivity of MXene nanosheets.

MXenes are a rapidly expanding family of two-dimensional transition metal carbides and nitrides whose exceptional compositional, structural, and surface-chemical tunability has driven rapid growth across materials science, chemistry, physics, and engineering. This roadmap consolidates the current state of MXene research, spanning synthesis, processing, fundamental properties, computation, electrochemical energy storage, biomedical applications, electronics, optoelectronics, membranes, sensing, tribology, and extreme-environment technologies. By identifying key advances, persistent challenges, and emerging opportunities, the roadmap provides a forward-looking outlook for guiding MXenes from laboratory discovery toward transformative applications.

The widespread use of antibiotics has led to an increased number of antimicrobial-resistant (AMR) pathogens, highlighting the need for novel antibacterial nanomaterials with chemical and structural tunability. Here, we present the antibacterial properties/pathways of two molybdenum-based double transition metal (DTM) MXenes (Mo2TiC2Tx and Mo2Ti2C3Tx) and compare them with Ti3C2Tx MXene. We demonstrate that the antibacterial effectiveness of these MXenes is concentration- and time-dependent, with prolonged exposure time being more influential at lower concentration levels (<25 μg mL-1). Physical damage to E. coli cell walls by MXene nanoknives (sharp edges of MXene flakes), and disruption in metabolic functions through oxidative stress were key antibacterial pathways for Mo2TiC2Tx, Mo2Ti2C3Tx, and Ti3C2Tx MXenes. A 1 h sonication of MXene solutions reduced their flake sizes (average lateral size of 234 ± 163 nm) and led to substantial improvement of their antibacterial performance by bolstering the availability of nanoknives for physical damage to bacterial cells. However, prolonged sonication (2 h) resulted in reduced antibacterial effectiveness, potentially due to morphological defects of MXene flakes. We also studied the metal ion release and disc inhibition zone, which revealed no direct correlation between the MXenes' antibacterial properties and the leaching of ions or fragments. This study demonstrates the potential for improving the antibacterial effectiveness of molybdenum-containing DTM MXenes by controlling their chemical and structural characteristics.

The rapid growth of the 2D MXenes family is driven by the designer chemistry control of their composition and structures, including the transition metal and surface functional groups, non-metal X sublattice and atomic-layer configurations. This compositional diversity controls the chemical ordering, atomic-level defects and surface chemistry, ultimately shaping properties of the MXenes. In this Review, we discuss how variations in compositional diversity and atomic arrangement give rise to material properties that enable new applications and breakthroughs in technology. We review design strategies, including atomic vacancy control, intercalation engineering and surface functionalization, that fine-tune the composition–property relationships in MXenes. In addition, we present emerging areas of MXenes research, including advances in biomedicine, optoelectronics, environmental remediation and catalysis, communication and quantum technologies, space exploration and thermal management. The rapid growth of the 2D MXene family is driven by the designer chemistry control of their composition and structure. This Review discusses how compositional diversity, atomic arrangement, defects and surface chemistry govern properties, design strategies and emerging applications across technologies.

MXenes have shown great potential in electronic and optoelectronic applications. However, the optical properties of these highly conductive two-dimensional materials are not fully understood. The broad near-infrared (IR) optical extinction (similar to 1.5 eV) in Ti3C2Tx with mixed terminations (T-x: & boxH;O, -OH, -F, -Cl) has been widely attributed to a localized surface plasmon resonance (LSPR). However, previous simulations suggest this peak may be due to an interband transition (IBT). Here, we show that the real component of the dielectric constant of Ti3C2Tx at this peak is positive (epsilon(1) > 0), as measured by spectroscopic ellipsometry (SE), ruling out the possibility of LSPR. Moreover, this band nearly vanishes for experimentally synthesized chlorine-terminated Ti3C2Cl2. Density functional theory (DFT) calculations confirm an IBT origin for this band, specifically due to the oxygen terminations (Ti3C2O2). Metallic behavior is only experimentally observed below 1 eV (epsilon(1) < 0), and DFT calculations predict surface plasmon polaritons (SPPs) in the mid-IR (similar to 0.5 eV, outside the optical domain) and only for Ti3C2O2, but not for Ti3C2Cl2 or other terminations. Additionally, we demonstrate that making thicker Ti3C2Tx MXene films and/or removing the intercalated water can induce a blue shift in this IBT due to the influence of water in facilitating the out-of-plane conductivity. The IBT assignment is critical because its light-matter interaction is fundamentally different from that of an LSPR, providing a new foundation for designing innovative MXene-based optoelectronic materials, which can be tailored through their termination states, while an LSPR would be insensitive to such synthetic variations.

Mo-based DTM MXenes (Mo 2 TiC 2 T x and Mo 2 Ti 2 C 3 T x ) exhibit strong antibacterial performance through nanoknives and oxidative stress, offering a promising pathway to mitigate antibiotic-resistant Gram-negative bacteria.

MXenes, an emerging class of two-dimensional van der Waals materials, have become the focus of research, demonstrating exceptional potential in electrochemical, biochemical and electronic applications. This chapter provides a brief overview of MXenes, covering their fundamental characteristics, synthesis methods and current challenges, with particular attention on synthesis methods.

The stringent safety protocols required for hydrofluoric acid (HF) based MAX phase etching and the reliance on material characterization tools such as XRD, SEM, EDS and XPS to study etching make the MXene research challenging. Here, we have employed 27 Al NMR spectroscopy for the rapid detection of selective etching, directly from the etching supernatant, of soluble aluminum species generated during the MAX phase etching reaction. This technique was applied to the development of a new etching protocol for Ti 3 AlC 2 MAX phase using the less hazardous hexafluorosilicic acid. The etching process was studied using a combination of 27 Al and 19 F NMR spectroscopies where it was demonstrated to be free of HF or free fluoride in quantities detectable by 19 F NMR, and that the primary etching byproduct is H 3 AlF 6 . 19 F NMR spectroscopy was additionally proven to be a viable technique to quantify the extent of etching using trifluoroacetic acid as an internal standard.

This work establishes a comprehensive thermodynamic framework for vapor transport in hierarchical titanium carbide (Ti3C2T x ) MXene-poly-(vinylidene fluoride) (PVDF) coated polytetrafluoroethylene (PTFE) composite membranes through integrated physics-based analysis and experimental validation. The composite architecture leverages PTFE's exceptional hydrophobicity of 135° with PVDF's processability, creating triple interfaces governed by Gibbs excess surface thermodynamics. Controlled spinodal decomposition during nonsolvent-induced phase separation yielded β-phase-rich PVDF matrices with uniformly dispersed Ti3C2T x MXene nanosheets forming angstrom-precision transport channels. Vapor flux governed by Maxwell-Stefan multicomponent diffusion theory, coupled with molecular kinetic models for two-dimensional material interlayers, resulted in experimental vapor fluxes of 42 ± 3.1 kg·m-2·h-1 representing 80% enhancement over pristine PVDF membranes. The Ti3C2T x MXene-based nanochannels created thermodynamic selectivity barriers enabling >99.6% salt rejection over 36 h operation. The composite membranes exhibited substantially reduced NaCl crystallization/deposition compared to pristine PVDF, attributed to modified surface energetics and hierarchical pore architectures that disrupt salt nucleation. Thermal analysis revealed a compounding energy benefit: higher membrane porosity reduced the effective thermal conductivity and conductive heat loss while the enhanced vapor flux increased the evaporative heat flux, raising thermal efficiency from 49% for pristine PVDF to 68% for the optimized MXene-PVDF composite and demonstrating that flux enhancement and energy efficiency can be improved simultaneously rather than traded off against each other.

A wide range of strategies and additives have been examined to improve boron ignition and oxidation to realize complete utilization of boron's high volumetric and gravimetric energy content, but two-dimensional transition metal carbides, nitrides and carbonitrides (MXenes) remain relatively unexplored for this purpose. Here, we examine high-quality, single-to-few layer Ti3C2T x MXene nanosheets as additives in the preparation of Ti3C2T x MXene/boron clusters and report enhanced oxidation and ignition of these mixtures relative to neat boron. Boron with Ti3C2T x MXene additive content as low as 1 wt % show dramatic decreases in oxidation and ignition temperatures, with the greatest improvements observed at 20 wt % Ti3C2T x MXene loading. Differential scanning calorimetry and thermogravimetric analysis show a similar to 100 degrees C lower oxidation onset temperature, up to 25% additional mass gain, and similar to 50 degrees C decrease in the temperature required for boron oxide removal for the 20 wt % MXene composition relative to neat boron. Morphological and compositional characterization of the 20 wt % MXene mixture using scanning electron microscopy and energy-dispersive X-ray spectroscopy reveals 100-200 mu m clusters with nearly uniform distribution of the Ti3C2T x MXene and boron components. Determination of ignition temperature using heated wire ignition showed increased ignition reliability and decreased ignition temperatures for all compositions, with the 20 wt % MXene mixture showing ignition at as low as 333 degrees C.

MXenes have emerged as promising solid lubricants due to their layered structure, tunable chemistry, and ability to form mechanically robust, wear-resistant tribo-films. However, most studies have focused on single-metal MXenes such as Ti3C2Tx, Ti3CNTx, or V2CTx, leaving multimetal MXenes largely unexplored. Here, we present a comprehensive tribological and mechanochemical evaluation of ordered double-transition metal Mo2TiC2Tx and Mo2Ti2C3Tx coatings under dry sliding in ambient conditions. Using nanoindentation mapping, X-ray photoelectron spectroscopy, Raman spectroscopy, and electron microscopy, we demonstrate that Mo2Ti2C3Tx tends to form dense, chemically stabilized, and mechanically robust tribo-layers thus maintaining a low and stable coefficient of friction (∼0.1) and wear rate (∼0.1 × 10-3 mm3/N·m) under a contact pressure of 0.55 GPa. These tribolayers exhibit improved mechanical properties (hardness ∼ 4.2 GPa; Young's modulus ∼ 103 GPa), along with increased carbide retention and reduced surface oxidation. In contrast, Mo2TiC2Tx coatings display a less favorable behavior, resulting in a higher COF (∼0.5), greater wear rate (∼1.3 × 10-3 mm3/N·m), and the formation of thinner, chemically degraded tribo-layers under comparable conditions. Mo2Ti2C3Tx exhibited the best tribological and mechanical performance under comparable conditions, clearly outperforming Ti3C2Tx, Ti3CNTx, and Mo2TiC2Tx. Our study introduces Mo-based MXenes as an emerging frontier in solid lubrication and the importance of MXene structure and composition in their tribo-layer evolution and stress accommodation mechanisms.

Carbon dioxide (CO2) capture under elevated pressure conditions is of particular relevance for pre-combustion capture and syngas purification processes. Here, we report CO2 adsorption in a nickel-intercalated titanium carbide Ni-Ti3C2Tx MXene-fluorohectorite clay heterostructure, designed to modify the high-pressure adsorption behavior characteristic of pristine MXenes. The heterostructure exhibits a CO2 adsorption capacity of 1.909 mmol g-1 at 50 bar and retains measurable uptake upon pressure release, with 0.602 mmol g-1 remaining at 1 bar after desorption. These results indicate that MXene-clay heterostructures are promising candidates for high-pressure CO2 separation, while also providing a platform for future exploration of CO2 conversion strategies beyond the scope of the present study.

Two-dimensional MXenes are promising solid lubricants, but the roles of compositional complexity and surface chemistry in governing interfacial friction remain unclear. Here, we systematically investigate the adhesion and friction behaviors of medium-entropy (ME) MXenes, TiVNbMoC3 and TiVCrMoC3, and compare them with conventional titanium carbide MXenes, Ti2C and Ti3C2, using a SiO2 colloidal atomic force microscopy probe. Thermal annealing at 200 C converts OH surface terminations to O terminations, leading to pronounced reductions in adhesion energy and friction force across all MXenes studied. ME MXenes exhibit larger adhesion reductions because of their higher initial OH contents and more extensive OH-to-O conversion. In addition, their intrinsically higher out-of-plane bending stiffness suppresses energy dissipation during sliding, enabling ultralow friction. Notably, superlubricity is achieved in ME MXenes, with annealed TiVCrMoC3 exhibiting a coefficient of friction as low as 0.0022, outperforming graphene, MoSe2, and other MXenes evaluated using the same experimental approach. These findings identify compositional complexity as a powerful strategy for engineering MXenes with exceptional tribological performance and establish ME MXenes as a new class of solid lubricants.

Ti3C2Tx MXene nanosheets (hereafter "MXene") demonstrate great potential for applications in wearable electronics, sensors, energy storage, biomedical devices, and electromagnetic interference shielding. Their richly functionalized surfaces enable redispersion from water into polar organic solvents, offering flexibility for processing using non-aqueous inks. However, this solvent exchange process, typically achieved through vigorous vortex mixing, reduces the electrical conductivity of the resulting material. While this decrease has been attributed to increased interlayer spacing due to solvent intercalation, we show that mechanical agitation during solvent exchange directly causes sheet damage. Our results indicate that different redispersion conditions produce MXene sheets with varying degrees of damage and oxidation, which in turn lead to decreased conductivity. This drop persists even with volatile solvents and after annealing up to 200 degrees C. Furthermore, reversing the process by exchanging the organic solvent back to water, thereby restoring the interlayer spacing, still yields lower conductivity than pristine MXene. Collectively, these results indicate that mechanical sheet degradation is irreversible. We anticipate that recognizing the critical role of processing-induced damage will guide researchers in developing optimized MXene processing conditions to achieve desired material properties for specific applications.