Ten Years of Progress in the Synthesis and Development of MXenes

Synergistic effect of Ti3C2Tx MXene/PAN nanofiber and LLZTO particles on high-performance PEO-based solid electrolyte for lithium metal battery

Solid polymer electrolytes (SPEs) have been considered the most promising separators for all-solid-state lithium metal batteries (ASSLMBs) due to their ease of processing and low cost. However, the practical applications of SPEs in ASSLMBs are limited by their low ionic conductivities and mechanical strength. Herein, we developed a three-dimensional (3D) interconnected MXene (Ti3C2Tx) network and Li6.4La3Zr1.4Ta0.6O12 (LLZTO) particles synergistically reinforced polyethylene oxide (PEO)-based SPE, where the association of Li+ with ether-oxygen in PEO could be significantly weakened through the Lewis acid-base interactions between the electron-absorbing group (Ti-F, -O-) of Ti3C2Tx and Li+. Besides, the TFSI- in lithium salts could be immobilized by hydrogen bonds from the Ti-OH of Ti3C2Tx. The 3D interconnected Ti3C2Tx network not only alleviated the agglomeration of inorganic fillers (LLZTO), but also improved the mechanical strength of composite solid electrolyte (CSE). Consequently, the assembled Li||CSE||Li symmetric battery showed excellent cycling stability at 35 ℃ (stable cycling over 3000 h at 0.1 mA cm-2, 0.1 mAh cm-2) and -2 ℃ (stable cycling over 2500 h at 0.05 mA cm-2, 0.05 mAh cm-2). Impressively, the LiFePO4||CSE||Li battery showed a high discharge capacity of 145.3 mAh/g at 0.3 C after 300 cycles at 35 ℃. This rational structural design provided a new strategy for the preparation of high-performance solid-state electrolytes for lithium metal batteries.

Layer-coupled corner states in two-dimensional topological multiferroics

The structural diversity and controllability in two-dimensional (2D) materials offers an intriguing platform for exploring a wide range of topological phenomena. The layer degree of freedom, as a novel technique for material manipulation, requires further investigation regarding its association with topological states. Here, using first-principles calculations and a tight-binding model, we propose a novel mechanism that couples the second-order topological corner states with the layer degree of freedom. By analyzing the edge states, topological indices, and spectra of nanoflakes, we identify ferromagnetic H'-Co2XF2 (X = C, N) as 2D second-order topological insulators with intrinsic ferroelectricity. Moreover, the topological corner states strongly couple with the layer degree of freedom, and, remarkably, ferroelectricity provides a nonvolatile handle to manipulate the layer-polarized corner states. These findings open an avenue for the manipulation of second-order topological states and establish a bridge between ferroelectricity and nontrivial topology.

Self-Thermal Management in Filtered Selenium-Terminated MXene Films for Flexible Safe Batteries

Li-ion batteries with superior interior thermal management are crucial to prevent thermal runaway and ensure safe, long-lasting operation at high temperatures or during rapid discharging and charging. Typically, such thermal management is achieved by focusing on the separator and electrolyte. Here, the study introduces a Se-terminated MXene free-standing electrode with exceptional electrical conductivity and low infrared emissivity, synergistically combining high-rate capacity with reduced heat radiation for safe, large, and fast Li+ storage. This is achieved through a one-step organic Lewis acid-assisted gas-phase reaction and vacuum filtration. The Se-terminated Nb2Se2C outperformed conventional disordered O/OH/F-terminated materials, enhancing Li+-storage capacity by ≈1.5 times in the fifth cycle (221 mAh·g-1 at 1 A·g-1) and improving mid-infrared adsorption with low thermal radiation. These benefits result from its superior electrical conductivity, excellent structural stability, and high permittivity in the infrared region. Calculations further reveal that increased permittivity and conductivity along the z-direction can reduce heat radiation from electrodes. This work highlights the potential of surface groups-terminated layered material-based free-standing flexible electrodes with self-thermal management ability for safe, fast energy storage.

Insights on updates in sodium alginate/MXenes composites as the designer matrix for various applications: A review

Advancements in two-dimensional materials, particularly MXenes, have spurred the development of innovative composites through their integration with natural polymers such as sodium alginate (SA). Mxenes exhibit a broad specific surface area, excellent electrical conductivity, and an abundance of surface terminations, which can be combined with SA to maximize the synergistic effect of the materials. This article provides a comprehensive review of state-of-the-art techniques in the fabrication of SA/MXene composites, analyzing the resulting structural and functional enhancements with a specific focus on advancing the design of these composites for practical applications. A detailed exploration of SA/MXene composites is provided, highlighting their utility in various sectors, such as wearable electronics, wastewater treatment, biomedical applications, and electromagnetic interference (EMI) shielding. The review identifies the unique advantages conferred by incorporating MXene in these composites, examines the current challenges, and proposes future research directions to understand and optimize these promising materials thoroughly. The remarkable properties of MXenes are emphasized as crucial for advancing the performance of SA-based composites, indicating significant potential for developing high-performance composite materials.

Monolayer Vacancy-Induced MXene Memory for Write-Verify-Free Programming

The fundamental logic states of 1 and 0 in Complementary Metal-Oxide-Semiconductor (CMOS) are essential for modern high-speed non-volatile solid-state memories. However, the accumulated storage signal in conventional physical components often leads to data distortion after multiple write operations. This necessitates a write-verify operation to ensure proper values within the 0/1 threshold ranges. In this work, a non-gradual switching memory with two distinct stable resistance levels is introduced, enabled by the asymmetric vertical structure of monolayer vacancy-induced oxidized Ti3C2Tx MXene for efficient carrier trapping and releasing. This non-cumulative resistance effect allows non-volatile memories to attain valid 0/1 logic levels through direct reprogramming, eliminating the need for a write-verify operation. The device exhibits superior performance characteristics, including short write/erase times (100 ns), a large switching ratio (≈3 × 104), long cyclic endurance (>104 cycles), extended retention (>4 × 106 s), and highly resistive stability (>104 continuous write operations). These findings present promising avenues for next-generation resistive memories, offering faster programming speed, exceptional write performance, and streamlined algorithms.

Creation of a boron carbide-based Ti3AlBC (312) MAX phase: a route to novel MXenes for energy storage

A novel boron carbide (B4C)-based Ti3AlBC (312) MAX phase was predicted for the first time via density functional theory (DFT). The stability of the MAX phase was confirmed by mechanical and thermal property analyses. The computational details revealed the attractive properties of Ti3AlBC, indicating its potential as an advanced material with improved characteristics. Its thermodynamic properties are reported as a function of temperature, indicating its potential for energy storage applications.

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

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

Recent Progress in Two-Dimensional Nanomaterials for Flame Retardance and Fire-Warning Applications

Graphene-like 2D nanomaterials, such as graphene, MXene, molybdenum disulfide, and boron nitride, present a promising avenue for eco-friendly flame retardants. Their inherent characteristics, including metal-like conductivity, high specific surface area, electron transport capacity, and solution processability, make them highly suitable for applications in both structural fire protection and fire alarm systems. This review offers an up-to-date exploration of advancements in flame retardant composites, utilizing pristine graphene-like nanosheets, versatile graphene-like nanosheets with multiple functions, and collaborative systems based on these nanomaterials. Moreover, graphene-like 2D nanomaterials exhibit considerable potential in the development of early fire alarm systems, enabling timely warnings. This review provides an overview of flame-retarding and fire-warning mechanisms, diverse multifunctional nanocomposites, and the evolving trends in the development of fire alarm systems anchored in graphene-like 2D nanomaterials and their derivatives. Ultimately, the existing challenges and prospective directions for the utilization of graphene-like 2D nanomaterials in flame retardant and fire-warning applications are put forward.

Fast access of the lattice thermal conductivity and phonon quasiparticle spectra of Mo2TiC2T2 (T = -O and -F) and Janus Mo2TiC2OF MXenes from machine learning potentials

The presence of strong anharmonic effects in surface functionalized MXenes greatly challenges the use of harmonic lattice dynamics calculations to predict their phonon spectra and lattice thermal conductivity at finite temperatures. Herein, we demonstrate the workflow for training and validating machine learning potentials in terms of moment tensor potential (MTP) for MXenes including Mo2TiC2, Mo2TiC2O2, Mo2TiC2F2 and Janus-Mo2TiC2OF monolayers. Then, the MTPs of MXenes are successfully combined with the harmonic lattice dynamics calculations to obtain the temperature renormalized phonon spectra, three-phonon scattering rates, phonon relaxation times and lattice thermal conductivity at finite temperatures. Furthermore, combining MTPs with classic molecular dynamics simulations at finite temperatures directly enables the calculation of phonon quasi-particle spectral energy density with a full inclusion of all anharmonic effects in MXenes. Our current results indicate that anharmonic effects are found to be relatively weak in Mo2TiC2 and Mo2TiC2O2 monolayers, whereas the phonon quasi-particle spectral energy densities largely resemble those of harmonic or renormalized lattice dynamics calculations. Significant broadening of spectral energy density at finite temperature is predicted for Mo2TiC2F2 and Janus-Mo2TiC2OF monolayers, implying strong anharmonic effects in those MXenes. Our work paves a new way for fast and reliable calculation of the phonon scattering process and lattice thermal conductivity of MXenes within MTPs trained from first-principles molecular dynamics simulations in the future.

Engineering Ti3C2-MXene Surface Composition for Excellent Li+ Storage Performance

Exploiting novel materials with high specific capacities is crucial for the progress of advanced energy storage devices. Intentionally constructing functional heterostructures based on a variety of two-dimensional (2D) substances proves to be an extremely efficient method for capitalizing on the shared benefits of these materials. By elaborately designing the structure, a greatly escalated steadiness can be achieved throughout electrochemical cycles, along with boosted electron transfer kinetics. In this study, chemical vapor deposition (CVD) was utilized to alter the surface composition of multilayer Ti3C2Tx MXene, contributing to contriving various layered heterostructure materials through a precise adjustment of the reaction temperature. The optimal composite materials at a reaction temperature of 500 °C (defined as MX500), incorporating MXene as the conductive substrate, exhibited outstanding stability and high coulombic efficiency during electrochemical cycling. Meanwhile, the reactive sites are increased by using TiS2 and TiO2 at the heterogeneous interfaces, which sustains a specific capacity of 449 mAh g-1 after 200 cycles at a current density of 0.1 A g-1 and further demonstrates their exceptional electrochemical characteristics. Additionally, the noted pseudocapacitive properties, like MXene materials, further highlight the diverse capabilities of intuitive material design. This study illuminates the complex details of surface modification in multilayer MXene and offers a crucial understanding of the strategic creation of heterostructures, significantly impacting sophisticated electrochemical applications.

Green Synthesis and Biosafety Assessment of MXene

The rise of MXene-based materials with fascinating physical and chemical properties has attracted wide attention in the field of biomedicine, because it can be exploited to regulate a variety of biological processes. The biomedical applications of MXene are still in its infancy, nevertheless, the comprehensive evaluation of MXene's biosafety is desperately needed. In this review, the composition and the synthetic methods of MXene materials are first introduced from the view of biosafety. The evaluation of the interaction between MXene and cells, as well as the safety of different forms of MXene applied in vivo are then discussed. This review provides a basic understanding of MXene biosafety and may bring new inspirations to the future applications of MXene-based materials in biomedicine.

MXene-supported MIL-88A(Fe) as persulfate activator for removal of tetracycline

The poor conductivity, poor stability, and agglomeration of iron-based metal organic framework MIL-88A(Fe) limit its application as persulfate (PS) activator in water purification. Herein, MXene-supported MIL-88A(Fe) composites (M88A/MX) were synthesized to enhance its adsorption and catalytic capability for tetracycline (TC) removal. Scanning electron microscope (SEM), X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FT-IR), and X-ray photoelectron spectroscopy (XPS) were used to characterize prepared materials, confirming the successful attachment of MIL-88A(Fe) to the surface of MXene. M88A/MX-0.2 composites, prepared with 0.2 g MXene addition, exhibit optimal degradation efficiency, reaching 98% under conditions of 0.2 g/L M88A/MX-0.2, 1.0 mM PS, 20 ppm TC, and pH 5. The degradation rate constants of M88A/MX-0.2 were 0.03217 min-1, which was much higher than that of MIL-88A(Fe) (0.00159 min-1) and MXene (0.00626 min-1). The removal effects of reaction parameters, such as dosage of M88A/MX-0.2 and PS; initial solution pH; and the presence of the common co-existing constituents (humic acid and the inorganic anions) were investigated in detail. Additionally, the reuse of M88A/MX-0.2 showed that the composites had good cycling stability by recurrent experiments. The results of electron paramagnetic resonance (EPR) and quenching experiments indicated that ·OH, ·SO4-, and ·O2- were involved in the M88A/MX-0.2/PS system where persulfate oxidation process was activated with prepared M88A/MX-0.2. In addition, the intermediates of photocatalytic degradation were determined by HPLC-MS, and the possible degradation pathways of the target molecules were inferred. This study offered a new avenue for sulfate-based degradation of Fe-based metal organic framework.

Conductive Peptide-Based MXene Hydrogel as a Piezoresistive Sensor

Wearable pressure sensors have become increasingly popular for personal healthcare and motion detection applications due to recent advances in materials science and functional nanomaterials. In this study, a novel composite hydrogel is presented as a sensitive piezoresistive sensor that can be utilized for various biomedical applications, such as wearable skin patches and integrated artificial skin that can measure pulse and blood pressure, as well as monitor sound as a self-powered microphone. The hydrogel is composed of self-assembled short peptides containing aromatic, positively- or negatively charged amino acids combined with 2D Ti3C2Tz MXene nanosheets. This material is low-cost, facile, reliable, and scalable for large areas while maintaining high sensitivity, a wide detection range, durability, oxidation stability, and biocompatibility. The bioinspired nanostructure, strong mechanical stability, and ease of functionalization make the assembled peptide-based composite MXene-hydrogel a promising and widely applicable material for use in bio-related wearable electronics.

MXene-Based Pressure Sensor with a Self-Healing Property for Joule Heating and Friction Sliding

As a kind of flexible electronic device, flexible pressure sensor has attracted wide attention in medical monitoring and human-machine interaction. With the continuous deepening of research, high-sensitivity sensor is developing from single function to multi-function. However, Current multifunctional sensors lack the ability to integrate joule heating, detect sliding friction, and self-healing. Herein, a MXene/polyurethane (PU) flexible pressure sensor with a self-healing property for joule heating and friction sliding is fabricated. The MXene/PU sensitive layer with special spinosum structure is prepared by a simple spraying method. After face-to-face assembly of the sensitive layers, the MXene/PU flexible pressure sensor is obtained and showed excellent sensitivity (150.65 kPa-1), fast response/recovery speed (75.5/63.9 ms), and good stability (10 000 cycles). Based on the self-healing property of PU, the sensor also has the ability to heal after mechanical damage. In addition, the sensor realizes the joule heating function under low voltage, and has the real-time monitoring ability of sliding objects. Combined with low cost and simple manufacturing method, the multi-functional MXene/PU flexible sensor shows a wide range of application potential in human activity monitoring, thermal management, and slip recognition.

Facile Preparation of Photothermal Superhydrophobic Melamine Sponge Decorated with MXene and Lignin Particles for Efficient Oil/Water Separation, Fast Crude Oil Recovery, and Active Deicing

Frequent oil spills and the discharge of oily wastewaters have caused a serious threat to the environment, ecosystems, and human beings. Herein, a photothermal and superhydrophobic melamine sponge (MS) decorated with MXene and lignin particles has been prepared for the separation of oil/water mixtures, the recovery of crude oils, and active deicing. The obtained superhydrophobic melamine sponge shows a water contact angle (WCA) of 152.3° and an oil contact angle of ∼0° and possesses good chemical stability, thermal stability, and mechanical durability in terms of being immersed in various liquids (i.e., corrosive solutions, organic solvents, and boiling water) and being abrased by sandpapers. This superhydrophobic MS displays a high oil adsorption capacity of CCl4, up to 91.6 times its own weight and a high separation efficiency of 99.4%. Furthermore, the maximum surface temperature of the superhydrophobic MS reaches 57.5 °C under sunlight irradiation (1.0 kW/m2) due to the excellent photothermal heating conversion performance of MXene and lignin particles. When exposed to sunlight, the superhydrophobic MS can quickly absorb viscous crude oils up to 72 times its own weight. Also, the WCA of the superhydrophobic MS remains above 146° after 50 icing/deicing cycles, showing excellent photothermal anti-icing properties. Thus, this study presents an easy and low-cost method for designing photothermal superhydrophobic melamine sponges and opens a new avenue to the applications of efficient oil/water separation, fast crude oil recovery, and active deicing.

Comprehensive synthesis of Ti3C2Tx from MAX phase to MXene

MXenes are a large family of two-dimensional materials that have attracted attention across many fields due to their desirable optoelectronic, biological, mechanical and chemical properties. There currently exist many synthesis procedures that lead to differences in flake size, defects and surface chemistry, which in turn affect their properties. Herein, we describe the steps to synthesize Ti3C2Tx-the most important and widely used MXene, from a Ti3AlC2 MAX phase precursor. The procedure contains three main sections: synthesis of Ti3AlC2 MAX, wet chemical etching of the MAX in hydrofluoric acid/HCl solution to yield multilayer Ti3C2Tx and its delamination into single-layer flakes. Three delamination options are described; these use LiCl, tertiary amines (tetramethyl ammonium hydroxide/ tetrabutyl ammonium hydroxide) and dimethylsulfoxide respectively. These procedures can be adapted for the synthesis of MXenes beyond Ti3C2Tx. The MAX phase synthesis takes about 1 week, with the etching and delamination each requiring 2 d. This protocol requires users to have experience working with hydrofluoric acid, and it is recommended that users have experience with wet chemistry and centrifugation; characterization techniques such as X-ray diffraction and particle size analysis are also essential for the success of the protocol. While alternative synthesis methods, such as minimally intensive layer delamination, are desirable for certain MXenes (such as Ti2CTx) or specific applications, this protocol aims to standardize the more commonly used hydrofluoric acid/HCl etching method, which produces Ti3C2Tx with minimal concentration of defects and the highest conductivity and serves as a guideline for those working with MXenes for the first time.

Multiscale Structural Design of 2D Nanomaterials-based Flexible Electrodes for Wearable Energy Storage Applications

2D nanomaterials play a critical role in realizing high-performance flexible electrodes for wearable energy storge devices, owing to their merits of large surface area, high conductivity and high strength. The electrode is a complex system and the performance is determined by multiple and interrelated factors including the intrinsic properties of materials and the structures at different scales from macroscale to atomic scale. Multiscale design strategies have been developed to engineer the structures to exploit full potential and mitigate drawbacks of 2D materials. Analyzing the design strategies and understanding the working mechanisms are essential to facilitate the integration and harvest the synergistic effects. This review summarizes the multiscale design strategies from macroscale down to micro/nano-scale structures and atomic-scale structures for developing 2D nanomaterials-based flexible electrodes. It starts with brief introduction of 2D nanomaterials, followed by analysis of structural design strategies at different scales focusing on the elucidation of structure-property relationship, and ends with the presentation of challenges and future prospects. This review highlights the importance of integrating multiscale design strategies. Finding from this review may deepen the understanding of electrode performance and provide valuable guidelines for designing 2D nanomaterials-based flexible electrodes.

MXene-Based Elastomer Mimetic Stretchable Sensors: Design, Properties, and Applications

Flexible sensors based on MXene-polymer composites are highly prospective for next-generation wearable electronics used in human-machine interfaces. One of the motivating factors behind the progress of flexible sensors is the steady arrival of new conductive materials. MXenes, a new family of 2D nanomaterials, have been drawing attention since the last decade due to their high electronic conductivity, processability, mechanical robustness and chemical tunability. In this review, we encompass the fabrication of MXene-based polymeric nanocomposites, their structure-property relationship, and applications in the flexible sensor domain. Moreover, our discussion is not only limited to sensor design, their mechanism, and various modes of sensing platform, but also their future perspective and market throughout the world. With our article, we intend to fortify the bond between flexible matrices and MXenes thus promoting the swift advancement of flexible MXene-sensors for wearable technologies.

Interfacial Solar Evaporation: From Fundamental Research to Applications

In the last decade, interfacial solar steam generation (ISSG), powered by natural sunlight garnered significant attention due to its great potential for low-cost and environmentally friendly clean water production in alignment with the global decarbonization efforts. This review aims to share the knowledge and engage with a broader readership about the current progress of ISSG technology and the facing challenges to promote further advancements toward practical applications. The first part of this review assesses the current strategies for enhancing the energy efficiency of ISSG systems, including optimizing light absorption, reducing energy losses, harvesting additional energy, and lowering evaporation enthalpy. Subsequently, the current challenges faced by ISSG technologies, notably salt accumulation and bio-fouling issues in practical applications, are elucidated and contemporary methods are discussed to overcome these challenges. In the end, potential applications of ISSG, ranging from initial seawater desalination and industrial wastewater purification to power generation, sterilization, soil remediation, and innovative concept of solar sea farm, are introduced, highlighting the promising potential of ISSG technology in contributing to sustainable and environmentally conscious practices. Based on the review and in-depth understanding of these aspects, the future research focuses are proposed to address potential issues in both fundamental research and practical applications.

Subtle 2D/2D MXene-Based Heterostructures for High-Performance Electrocatalytic Water Splitting

Developing efficient electrocatalysts is significant for the commercial application of electrocatalytic water splitting. 2D materials have presented great prospects in electrocatalysis for their high surface-to-volume ratio and tunable electronic properties. Particularly, MXene emerges as one of the most promising candidates for electrocatalysts, exhibiting unique advantages of hydrophilicity, outstanding conductivity, and exceptional stability. However, it suffers from lacking catalytic active sites, poor oxidation resistance, and easy stacking, leading to a significant suppression of the catalytic performance. Combining MXene with other 2D materials is an effective way to tackle the aforementioned drawbacks. In this review, the focus is on the accurate synthesis of 2D/2D MXene-based catalysts toward electrocatalytic water splitting. First, the mechanisms of electrocatalytic water splitting and the relative properties and preparation methods of MXenes are introduced to offer the basis for accurate synthesis of 2D/2D MXene-based catalysts. Then, the accurate synthesis methods for various categories of 2D/2D MXene-based catalysts, such as wet-chemical, phase-transformation, electrodeposition, etc., are systematically elaborated. Furthermore, in-depth investigations are conducted into the internal interactions and structure-performance relationship of 2D/2D MXene-based catalysts. Finally, the current challenges and future opportunities are proposed for the development of 2D/2D MXene-based catalysts, aiming to enlighten these promising nanomaterials for electrocatalytic water splitting.

Recent advances using MXenes in biomedical applications

An MXene is a novel two-dimensional transition metal carbide or nitride, with a typical formula of Mn+1XnTx (M = transition metals, X = carbon or nitrogen, and T = functional groups). MXenes have found wide application in biomedicine and biosensing, owing to their high biocompatibility, abundant reactive surface groups, good conductivity, and photothermal properties. Applications include photo- and electrochemical sensors, energy storage, and electronics. This review will highlight recent applications of MXene and MXene-derived materials in drug delivery, tissue engineering, antimicrobial activity, and biosensors (optical and electrochemical). We further elaborate on recent developments in utilizing MXenes for photothermal cancer therapy, and we explore multimodal treatments, including the integration of chemotherapeutic agents or magnetic nanoparticles for enhanced therapeutic efficacy. The high surface area and reactivity of MXenes provide an interface to respond to the changes in the environment, allowing MXene-based drug carriers to respond to changes in pH, reactive oxygen species (ROS), and electrical signals for controlled release applications. Furthermore, the conductivity of MXene enables it to provide electrical stimulation for cultured cells and endows it with photocatalytic capabilities that can be used in antibiotic applications. Wearable and in situ sensors incorporating MXenes are also included. Major challenges and future development directions of MXenes in biomedical applications are also discussed. The remarkable properties of MXenes will undoubtedly lead to their increasing use in the applications discussed here, as well as many others.

MXene-Mediated Catalytic Redox Reactions for Biomedical Applications

Reactive oxygen species (ROS) play a crucial role in orchestrating a myriad of physiological processes within living systems. With the advent of materdicine, an array of nanomaterials has been intricately engineered to influence the redox equilibrium in biological milieus, thereby pioneering a distinctive therapeutic paradigm predicated on ROS-centric biochemistry. Among these, two-dimensional carbides, nitrides, and carbonitrides, collectively known as MXenes, stand out due to their multi-valent and multi-elemental compositions, large surface area, high conductivity, and pronounced local surface plasmon resonance effects, positioning them as prominent contributors in ROS modulation. This review aims to provide an overview of the advancements in harnessing MXenes for catalytic redox reactions in various biological applications, including tumor, anti-infective, and anti-inflammatory therapies. The emphasis lies on elucidating the therapeutic mechanism of MXenes, involving both pro-oxidation and anti-oxidation processes, underscoring the redox-related therapeutic applications facilitated by self-catalysis, photo-excitation, and sono-excitation properties of MXenes. Furthermore, this review highlights the existing challenges and outlines future development trends in leveraging MXenes for ROS-involving disease treatments, marking a significant step towards the integration of these nanomaterials into clinical practice.

Stacking-induced phonon transport engineering of siligene

Tunable phonon transport properties of two-dimensional materials are desirable for effective heat management in various application scenarios. Here, we demonstrate by first-principles calculations and Boltzmann transport theory that the lattice thermal conductivity of siligene could be efficiently engineered by forming various stacking configurations. Unlike few-layer graphene, the stacked siligenes are found to be covalently bonded along the out-of-plane direction, which leads to unique dependence of the thermal conductivity on both the stacking order and layer number. Due to the restricted flexural phonon scattering induced by the horizontal reflection symmetry, the AA stacking configuration of bilayer siligene exhibits obviously higher thermal conductivity compared with the AB stacking. In addition, we observe increasing thermal conductivity with the layer number, as evidenced by the reduced phonon scattering phase space and Grüneisen parameter. Interestingly, the Fuchs-Sondheimer model works well for the thickness-dependent thermal conductivity of stacked siligenes.

Sustainable Dual-Ion Batteries beyond Li

The limitations of resources used in current Li-ion batteries may hinder their widespread use in grid-scale energy storage systems, prompting the search for low-cost and resource-abundant alternatives. "Beyond-Li cation" batteries have emerged as promising contenders; however, they confront noteworthy challenges due to the scarcity of suitable host materials for these cations. In contrast, anions, the other crucial component in electrolytes, demonstrate reversible intercalation capacity in specific materials like graphite. The convergence of anion and cation storage has given rise to a new battery technology known as dual-ion batteries (DIBs). This comprehensive review presents the current status, advancements, and future prospects of sustainable DIBs beyond Li. Notably, most DIBs exhibit similar cathode reaction mechanisms involving anion intercalation, while the distinguishing factor lies in the cation types functioning at the anode. Accordingly, the review is organized into sections by various cation types, including Na-, K-, Mg-, Zn-, Ca-, Al-, NH4 + -, and proton-based DIBs. Moreover, a perspective on these novel DIBs is presented, along with proposed protocols for investigating DIBs and promising future research directions. It is envisioned that this review will inspire fresh concepts, ideas, and research directions, while raising important questions to further tailor and understand sustainable DIBs, ultimately facilitating their practical realization.

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

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

Mo-based MXenes: Synthesis, properties, and applications

Ti-MXene allows a range of possibilities to tune their compositional stoichiometry due to their electronic and electrochemical properties. Other than conventionally explored Ti-MXene, there have been ample opportunities for the non-Ti-based MXenes, especially the emerging Mo-based MXenes. Mo-MXenes are established to be remarkable with optoelectronic and electrochemical properties, tuned energy, catalysis, and sensing applications. In this timely review, we systematically discuss the various organized synthesis procedures, associated experimental tunning parameters, physiochemical properties, structural evaluation, stability challenges, key findings, and a wide range of applications of emerging Mo-MXene over Ti-MXenes. We also critically examined the precise control of Mo-MXenes to cater to advanced applications by comprehensively evaluating the summary of recent studies using artificial intelligence and machine learning tools. The critical future perspectives, significant challenges, and possible outlooks for successfully developing and using Mo-MXenes for various practical applications are highlighted.

A Comprehensive Review of Electromagnetic Interference Shielding Composite Materials

The interaction between matter and microwaves assumes critical significance due to the ubiquity of wireless communication technology. The selective shielding of microwaves represents the only way to achieve the control on crucial technological sectors. The implementation of microwave shielding ensures the proper functioning of electronic devices. By preventing electromagnetic pollution, shielding safeguards the integrity and optimal performances of devices, contributing to the reliability and efficiency of technological systems in various sectors and allowing the further step forwards in a safe and secure society. Nevertheless, the microwave shielding research is vast and can be quite hard to approach due to the large number and variety of studies regarding both theory and experiments. In this review, we focused our attention on the comprehensive discussion of the current state of the art of materials used for the production of electromagnetic interference shielding composites, with the aim of providing a solid reference point to explore this research field.

High yield and wide lateral size growth of α-Mo2C: exploring the boundaries of CVD growth of bare MXene analogues

Synthesis of Mo2C bare MXenes, without surface terminations groups, via chemical vapor deposition (CVD) on metal foils is scientifically a very intriguing crystal growth process, and there are still challenges and limited fundamental understanding to overcome to obtain high yield and wide crystal size lateral growth. Achieving large area coverage via direct growth is scientifically vital to utilize the full potential of their unique properties in different applications. In this study, we sought to expand the boundaries of the current CVD growth approach for Mo2C MXenes and gain insights into the possibilities and limitations of large area growth, with a particular focus on controlling Mo concentration. We report a facile modification of their typical CVD growth protocol and show its influence on the Mo2C synthesis, with growth times spanning up to 3 h. Specifically, prior to initiating the CVD growth process, we introduced a holding step in temperature at 1095 °C. This proved to be beneficial in increasing the Mo concentration on the liquid Cu growth surface. We achieved an average Mo2C crystals coverage of approximately 50% of the growth substrate area, increased tendency of coalescence and merging of individual flakes, and lateral flake sizes up to 170μm wide. To gain deeper understanding into their CVD growth behavior, we conducted a systematic investigation of the effect of several factors, including (i) a holding step time on Mo diffusion rate through molten Cu, (ii) the Cu foil thickness over the Mo foil, and (iii) the CVD growth time. Phase, chemical and microstructural characterization by x-ray diffraction, x-ray photon spectroscopy, SEM and scanning/transmission electron microscopy revealed that the grown crystals are single phaseα-Mo2C. Furthermore, insights gained from this study sheds light on crucial factors and inherent limitations that are essential to consider and may help guide future research progress in CVD growth of bare MXenes.

Current advances in biomaterials for inner ear cell regeneration

Inner ear cell regeneration from stem/progenitor cells provides potential therapeutic strategies for the restoration of sensorineural hearing loss (SNHL), however, the efficiency of regeneration is low and the functions of differentiated cells are not yet mature. Biomaterials have been used in inner ear cell regeneration to construct a more physiologically relevant 3D culture system which mimics the stem cell microenvironment and facilitates cellular interactions. Currently, these biomaterials include hydrogel, conductive materials, magneto-responsive materials, photo-responsive materials, etc. We analyzed the characteristics and described the advantages and limitations of these materials. Furthermore, we reviewed the mechanisms by which biomaterials with different physicochemical properties act on the inner ear cell regeneration and depicted the current status of the material selection based on their characteristics to achieve the reconstruction of the auditory circuits. The application of biomaterials in inner ear cell regeneration offers promising opportunities for the reconstruction of the auditory circuits and the restoration of hearing, yet biomaterials should be strategically explored and combined according to the obstacles to be solved in the inner ear cell regeneration research.

MXene-based electrochemical devices applied for healthcare applications

The initial part of the review provides an extensive overview about MXenes as novel and exciting 2D nanomaterials describing their basic physico-chemical features, methods of their synthesis, and possible interfacial modifications and techniques, which could be applied to the characterization of MXenes. Unique physico-chemical parameters of MXenes make them attractive for many practical applications, which are shortly discussed. Use of MXenes for healthcare applications is a hot scientific discipline which is discussed in detail. The article focuses on determination of low molecular weight analytes (metabolites), high molecular weight analytes (DNA/RNA and proteins), or even cells, exosomes, and viruses detected using electrochemical sensors and biosensors. Separate chapters are provided to show the potential of MXene-based devices for determination of cancer biomarkers and as wearable sensors and biosensors for monitoring of a wide range of human activities.

Effective boosting of halogenated α, β-unsaturated C4-dicarbonyl electrocatalytic hydrodehalogenation by 1 T'-MoS2/Ti3C2T2 (T = O, OH, F) heterojunctions: A theoretical study

Halogenated α, β-unsaturated C4-dicarbonyl (X-BDA), a novel family of high-toxicity ring cleavage products, is produced during the disinfection of phenolic compounds. The technique of electrocatalytic hydrodehalogenation (ECH) is efficient in rupturing carbon-halogen bonds and generating useful chemicals. This study used first principles to examine the ECH reaction mechanism of X-BDA and the subsequent hydrogenation reaction of the toxic derivative BDA over the 1 T'-MoS2/Ti3C2T2 (T = O, OH, F) catalysts. The catalytic activity of Ti3C2T2 (T = O, OH, F) catalysts decreases gradually with -OH, -F, -O functional group. The loading of 1 T'-MoS2 onto the Ti3C2T2 surface improves the stability and selectivity of Ti3C2T2. In particular, 1 T'-MoS2/Ti3C2(OH)2 is most conducive to the ECH reaction of X-BDA via a direct-indirect continuous reduction process. It exhibits excellent removal capability towards Cl-BDA, with decreasing reactivity in the order of the Cl-, Br-, and I-BDA. The material offers a solution to the challenging dechlorination issue. The dehalogenated product BDA can be hydrogenated to produce 1,4-butanedial, 1,4-butanediol, and 1,4-butenediol. Three valuable chemicals can be obtained by exerting an applied potential of - 0.65 V. This work suggests that the formation of heterojunction catalyst may lead to new strategies to improve ECH for environmental remediation applications.

Acid-assisted ultrasonic preparation of nitrogen-doped MXene quantum dots for the efficient fluorescence "off-on-off" detection of Zn(II) in water and oxalic acid in vegetables

A novel fluorescence "off-on-off" probe was presented to detect Zn(II) and oxalic acid (OA) based on nitrogen-doped MXene quantum dots (N-MQDs), which were synthesized by an ultrasound approach at room temperature with nitric acid and ethylenediamine. These N-MQDs displayed small size (<10 nm), water dispersibility, and good photoluminescence. Furthermore, the N-MQDs showed an selective response towards Zn(II) through fluorescence enhancement, with a limit of detection (LOD) calculated as 0.127 μM in the linear range of 0-20 μM. Then, the fluorescence of N-MQDs/Zn(II) system could be selectively quenched after adding OA, with an effective response in the range from 0 to 20 μM (LOD: 0.883 μM). The fluorescence "turn-on" and "turn-off" properties of N-MQDs were resulted from the intramolecular charge transfer (ICT) of Zn(II) and the coordination between OA and Zn(II), respectively. This sensing platform was successfully applied for Zn(II) and OA detection in actual environmental and vegetable samples.

Near-Atomic-Scale Perspective on the Oxidation of Ti3 C2 Tx MXenes: Insights from Atom Probe Tomography

MXenes are a family of 2D transition metal carbides and nitrides with remarkable properties, bearing great potential for energy storage and catalysis applications. However, their oxidation behavior is not yet fully understood, and there are still open questions regarding the spatial distribution and precise quantification of surface terminations, intercalated ions, and possible uncontrolled impurities incorporated during synthesis and processing. Here, atom probe tomography (APT) analysis of as-synthesized Ti3 C2 Tx MXenes reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as unetched Al. Following oxidation of the colloidal solution of MXenes, it is observed that the alkalis are enriched in TiO2 nanowires. Although these elements are tolerated through the incorporation by wet chemical synthesis, they are often overlooked when the activity of these materials is considered, particularly during catalytic testing. This work demonstrates how the capability of APT to image these elements in 3D at the near-atomic scale can help to better understand the activity and degradation of MXenes, in order to guide their synthesis for superior functional properties.

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

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

Multifunctional Flexible MXene/AgNW Composite Thin Film with Ultrahigh Conductivity Enabled by a Sandwich-Structured Assembly Strategy

Flexible composite films have attracted considerable attention due to great potential for healthcare, telecommunication, and aerospace. However, it is still challenging to achieve high conductivity and multifunctional integration, mainly due to poorly designed composite structures of these films. Herein, a novel sandwich-structured assembly strategy is proposed to fabricate flexible composite thin films made of Ag nanowire (AgNW) core and MXene layers by combination of spray coating and vacuum filtration process. In this case, ultrathin MXene layers play crucial roles in constructing compact composite structures strongly anchored to substrate with extensive hydrogen-bonding interactions. The resultant sandwich-structured MXene/AgNW composite thin films (SMAFs) exhibit ultrahigh electrical conductivity (up to 27193 S cm-1 ), resulting in exceptional electromagnetic interference shielding effectiveness of 16 223.3 dB cm2 g-1 and impressive Joule heating performance with rapid heating rate of 10.4 °C s-1 . Moreover, the uniform SMAFs can also be facilely cut into kirigami-patterned interconnects, which indicate superior strain-insensitive conductance even after long-term exposure to extreme temperatures. The demonstrated strategy offers a significant paradigm to construct multifunctional composite thin films for next-generation integrated flexible electronics with practical applications.

Application Scopes of Miniaturized MXene-Functionalized Electrospun Nanofibers-Based Electrochemical Energy Devices

Exploring combinatorial materials, as well as rational device configuration design, are assumed to be the key strategies for deploying versatile electrochemical devices. MXene sheets have revealed a high hydrophilic surface with proper mechanical and electrical characteristics, rendering them supreme additive candidates to integrate in electrospun electrochemical power tools. The synergetic effects of MXene 2D layers with the nanofibrous networks can boost actuator responsive ability, battery capacity retention, fuel cell stability, sensor sensitivity, and supercapacitor areal capacitance. Their superior mechanical features can be endowed to the electrospun layers through the embedding of the MXene additive. In this review, the preparation and inherent features of the MXene configurations are briefly evaluated. The fabrication and overall performance of the MXene-loaded nanofibers applicable in electrochemical actuators, batteries, fuel cells, sensors, and supercapacitors are comprehensively figured out. Eventually, an outlook on the future development of MXene-based electrospun composites is presented. A substantial focus has been devoted to date to engineering conjugated MXene and electrospun fibrous frames. The potential performance of the MXene-decorated nanofibers presents a bright future of nanoengineering toward technological growth. Meanwhile, a balance between the pros and cons of the synthesized MXene composite layers is worthwhile to consider in the future.

Surface-Enhanced Raman Spectroscopy for Boosting Electrochemical CO2 Reduction on Amorphous-Surfaced Tin Oxide Supported by MXene

Amorphous materials disrupt the intrinsic linear scalar dependence seen in their crystalline counterparts, typically exhibiting enhanced catalytic characteristics. Nevertheless, substantial obstacles remain in terms of boosting their stability, enhancing their conductivity, and elucidating distinct catalytic mechanisms. Herein, a core-shell catalyst, comprising a crystalline SnO2 core and an amorphous SnOx shell supported on MXene (denoted as SnO2@SnOx/MXene), was prepared utilizing hydrothermal and solution reduction methods. The SnO2@SnOx/MXene catalyst excels in the electrocatalytic conversion of CO2 to formate, yielding a Faradaic efficiency (FE) as high as 93% for formate production at -1.17 V vs RHE and demonstrating exceptional durability. Both density functional theory (DFT) calculations and experimental results indicate that the SnOx shell bolsters formate formation by fine-tuning the adsorption energy of the *OCHO intermediate. In SnO2@SnOx/MXene, MXene plays a vital role in enhancing the conductivity and stability of the amorphous shell and especially amplifying Raman signals of catalyst components. The ex/in situ surface-enhanced Raman scattering (SERS) application further confirms the formation of amorphous SnOx and further enables the direct detection of the formation of the intermediate species. This work provides the basis for the application of amorphous materials in practical electrocatalytic reduction of CO2 reduction.

Recent Advances and Strategies in MXene-Based Electrodes for Supercapacitors: Applications, Challenges and Future Prospects

With the growing demand for technologies to sustain high energy consumption, supercapacitors are gaining prominence as efficient energy storage solutions beyond conventional batteries. MXene-based electrodes have gained recognition as a promising material for supercapacitor applications because of their superior electrical conductivity, extensive surface area, and chemical stability. This review provides a comprehensive analysis of the recent progress and strategies in the development of MXene-based electrodes for supercapacitors. It covers various synthesis methods, characterization techniques, and performance parameters of these electrodes. The review also highlights the current challenges and limitations, including scalability and stability issues, and suggests potential solutions. The future outlooks and directions for further research in this field are also discussed, including the creation of new synthesis methods and the exploration of novel applications. The aim of the review is to offer a current and up-to-date understanding of the state-of-the-art in MXene-based electrodes for supercapacitors and to stimulate further research in the field.

An Overview of Electrochemical Sensors Based on Transition Metal Carbides and Oxides: Synthesis and Applications

Sensors play vital roles in industry and healthcare due to the significance of controlling the presence of different substances in industrial processes, human organs, and the environment. Electrochemical sensors have gained more attention recently than conventional sensors, including optical fibers, chromatography devices, and chemiresistors, due to their better versatility, higher sensitivity and selectivity, and lower complexity. Herein, we review transition metal carbides (TMCs) and transition metal oxides (TMOs) as outstanding materials for electrochemical sensors. We navigate through the fabrication processes of TMCs and TMOs and reveal the relationships among their synthesis processes, morphological structures, and sensing performance. The state-of-the-art biological, gas, and hydrogen peroxide electrochemical sensors based on TMCs and TMOs are reviewed, and potential challenges in the field are suggested. This review can help others to understand recent advancements in electrochemical sensors based on transition metal oxides and carbides.

Contemporary innovations in two-dimensional transition metal dichalcogenide-based P-N junctions for optoelectronics

Two-dimensional transition metal dichalcogenides (2D-TMDCs) with various physical characteristics have attracted significant interest from the scientific and industrial worlds in the years following Moore's law. The p-n junction is one of the earliest electrical components to be utilized in electronics and optoelectronics, and modern research on 2D materials has renewed interest in it. In this regard, device preparation and application have evolved substantially in this decade. 2D TMDCs provide unprecedented flexibility in the construction of innovative p-n junction device designs, which is not achievable with traditional bulk semiconductors. It has been investigated using 2D TMDCs for various junctions, including homojunctions, heterojunctions, P-I-N junctions, and broken gap junctions. To achieve high-performance p-n junctions, several issues still need to be resolved, such as developing 2D TMDCs of superior quality, raising the rectification ratio and quantum efficiency, and successfully separating the photogenerated electron-hole pairs, among other things. This review comprehensively details the various 2D-based p-n junction geometries investigated with an emphasis on 2D junctions. We investigated the 2D p-n junctions utilized in current rectifiers and photodetectors. To make a comparison of various devices easier, important optoelectronic and electronic features are presented. We thoroughly assessed the review's prospects and challenges for this emerging field of study. This study will serve as a roadmap for more real-world photodetection technology applications.

Liquid metal assisted fabrication of MXene-based films: Toward superior electromagnetic interference shielding and thermal management

The development of thin and flexible films that possess both electromagnetic interference (EMI) shielding and thermal management capabilities has always been an intriguing pursuit, but itisnevertheless a crucialproblemtoaddress. Inspired by the deformability of liquid metal (LM) and film forming capacity of MXene, here we present a series of ternary compositing films prepared via cellulose nanofiber (CNF) assisted vacuum filtration technology. Originating from the highly conductive LM/MXene network, the MLMC film presents a maximum EMI shielding effectiness (EMI SE) of 78 dB at a tiny thickness of 45 μm, together with a high specific EMI SE of 3046 dB mm-1. Meanwhile, these compositing films also deliver excellent flexibility and mechanical reliability, showing no obvious decline in EMI shielding performance even after 1000 bending and 500 folding cycles, respectively. Moreover, notable anisotropic thermal conductive property was successfully achieved, allowing for a highly desirable in-plane thermal conductivity of 7.8 W m-1 K-1. This accomplishment also yielded an exceptional electro-thermal conversion capacity, enabling efficient low-voltage (3 V) heating capabilities. These captivating features are expected to greatly drive the widespread adoption of LM-based films in future flexible electronic and wearable technologies.

Tunable Electronic Properties, Carrier Mobility, and Contact Characteristics in Type-II BSe/Sc2CF2 Heterostructures toward Next-Generation Optoelectronic Devices

Conducting heterostructures have emerged as a promising strategy to enhance physical properties and unlock the potential application of such materials. Herein, we conduct and investigate the electronic and transport properties of the BSe/Sc2CF2 heterostructure using first-principles calculations. The BSe/Sc2CF2 heterostructure is structurally and thermodynamically stable, indicating that it can be feasible for further experiments. The BSe/Sc2CF2 heterostructure exhibits a semiconducting behavior with an indirect band gap and possesses type-II band alignment. This unique alignment promotes efficient charge separation, making it highly promising for device applications, including solar cells and photodetectors. Furthermore, type-II band alignment in the BSe/Sc2CF2 heterostructure leads to a reduced band gap compared to the individual BSe and Sc2CF2 monolayers, leading to enhanced charge carrier mobility and light absorption. Additionally, the generation of the BSe/Sc2CF2 heterostructure enhances the transport properties of the BSe and Sc2CF2 monolayers. The electric fields and strains can modify the electronic properties, thus expanding the potential application possibilities. Both the electric fields and strains can tune the band gap and lead to the type-II to type-I conversion in the BSe/Sc2CF2 heterostructure. These findings shed light on the versatile nature of the BSe/Sc2CF2 heterostructure and its potential for advanced nanoelectronic and optoelectronic devices.

Application of MXene-Based Materials for Cathode in Lithium-Sulfur Batteries

The lithium-sulfur (Li-S) batteries have a high theoretical specific capacity of 1675 mAh ⋅ g-1 and have become the most promising high-energy storage system for the next generation batteries technology. However, their applications are hindered by insulated feature and volume expansion of sulfur, as well as the "shuttle effect" of polysulfides. MXenes own metallic conductivity and strong ability of polysulfides adsorption. Besides, their unique two-dimensional (2D) structure, large specific surface area, abundant functional groups, and adjustability are beneficial to overcome the drawbacks of the sulfur cathode. In this review, different mainstream preparation methods and excellent properties of MXenes are summarized. Significant achievements and recent progress of MXene-based cathodes and interlayers applied to Li-S cathodes are concluded later. Finally, the challenges, possible solutions and potential applications of MXenes for Li-S batteries are also presented.

Elucidation of the synergistic effects of 3d metal (M = Cu, Co, and Ni) dopants and terminations (T = -O- and -OH) of Ti3C2Tx MXenes for urea adsorption ability via DFT calculations and experiments

Dialysis is an artificial process to remove excess urea toxins from the body through adsorption or conversion. Urea adsorption by emergent 2D materials such as MXenes is one probable approach. Based on density functional theory (DFT) studies, the surface of Ti3C2Tx (T = -O- and -OH) MXenes is not optimum for urea adsorption. Therefore, functionalization with 3d metal dopants (Cu, Co, and Ni) is proposed to improve their urea adsorption ability. DFT calculations indicate that oxygen-terminated Ti3C2O2 has a much better urea adsorption ability when doped with Cu, Co, and Ni, with adsorption energy (Eads) values of -2.11 eV, -1.90 eV and -1.72 eV, respectively. These adsorption energies are much more favourable than that of the undoped one (Eads = -0.52 eV). To verify the calculation results, MILD Ti3C2Tx, or MXenes synthesized via the safer and easier minimally intensive layer delamination (MILD) method, were utilized to simulate Ti3C2O2 since they have -O- termination as the dominant species. Experimentally, the adsorption studies found that low concentration of Cu, Co, and Ni on MILD Ti3C2Tx showed a urea removal efficiency of 21.9%, 6.0% and 0.2%, respectively, much better than 0% removal efficiency of unfunctionalized Ti3C2Tx. Therefore, both DFT calculations and experiments showed that various metal functionalized MXenes have a similar trend for urea adsorption, highlighting the feasibility of using the computational approach to predict urea adsorption and further opening a new promising direction for the urea adsorption. Finally, this study is also the first to examine synergistic effects of metal dopants and surface terminations on MXenes for urea adsorption.

Environmental implications of metal-organic frameworks and MXenes in biomedical applications: a perspective

Metal-organic frameworks (MOFs) and MXenes have demonstrated immense potential for biomedical applications, offering a plethora of advantages. MXenes, in particular, exhibit robust mechanical strength, hydrophilicity, large surface areas, significant light absorption potential, and tunable surface terminations, among other remarkable characteristics. Meanwhile, MOFs possess high porosity and large surface area, making them ideal for protecting active biomolecules and serving as carriers for drug delivery, hence their extensive study in the field of biomedicine. However, akin to other (nano)materials, concerns regarding their environmental implications persist. The number of studies investigating the toxicity and biocompatibility of MXenes and MOFs is growing, albeit further systematic research is needed to thoroughly understand their biosafety issues and biological effects prior to clinical trials. The synthesis of MXenes often involves the use of strong acids and high temperatures, which, if not properly managed, can have adverse effects on the environment. Efforts should be made to minimize the release of harmful byproducts and ensure proper waste management during the production process. In addition, it is crucial to assess the potential release of MXenes into the environment during their use in biomedical applications. For the biomedical applications of MOFs, several challenges exist. These include high fabrication costs, poor selectivity, low capacity, the quest for stable and water-resistant MOFs, as well as difficulties in recycling/regeneration and maintaining chemical/thermal/mechanical stability. Thus, careful consideration of the biosafety issues associated with their fabrication and utilization is vital. In addition to the synthesis and manufacturing processes, the ultimate utilization and fate of MOFs and MXenes in biomedical applications must be taken into account. While numerous reviews have been published regarding the biomedical applications of MOFs and MXenes, this perspective aims to shed light on the key environmental implications and biosafety issues, urging researchers to conduct further research in this field. Thus, the crucial aspects of the environmental implications and biosafety of MOFs and MXenes in biomedicine are thoroughly discussed, focusing on the main challenges and outlining future directions.

MXene/AgNW composite material for selective and efficient removal of radioactive cesium and iodine from water

Toxic fission products, such as cesium (137Cs) and iodine (129I) are of great concern because of their long half-lives and high solubility in water. The simultaneous removal of Cs and I using a single adsorbent is an area of increasing interest. In this study, MXene/silver nanowire (AgNW) composite was synthesized through physical mixing and employed for simultaneous removal of iodide (I-) and cesium (Cs+) ions from contaminated water. The MXene/AgNW composite demonstrated excellent adsorption capacities of 84.70 and 26.22 mg/g for I- and Cs+, respectively. The experimental data supported the hypothesis of multilayer adsorption of Cs+ owing to the inter-lamellar structures and the presence of heterogeneous adsorption sites in MXene. The interaction between I- and the AgNW involved chemisorption followed by monolayer adsorption. MXene/AgNW composite material exhibited promising results in the presence of competitive ions under extreme pH conditions. Thus, synthesized composite materials holds promising potential as an adsorbent for the remediation of radioactive liquid waste.

Strain and electric field induced electronic property modifications in two-dimensional Janus SZrAZ2 (A = Si, Ge; Z = P, As) monolayers

Recently, significant attention has been directed towards two-dimensional Janus materials owing to their unique structure and novel properties. In this work, we have introduced novel two-dimensional Janus monolayers, SZrAZ2 (A = Si, Ge; Z = P, As), through first principles. Our primary focus was the investigation of the controllable electronic properties exhibited by the Janus SZrAZ2 structures under the influence of strain and an external electric field. Our research findings indicate the dynamic and thermodynamic stability of Janus SZrAZ2 (A = Si, Ge; Z = P, As) monolayers. In the equilibrium state, these monolayers exhibit properties of an indirect band gap semiconductor. When subjected to biaxial strain and an external electric field, we observed that the dependency of SZrSiAs2 and SZrGeAs2 monolayers on an external electric field is very weak. Their electronic properties can only be modulated by applying biaxial strain. For SZrSiP2 and SZrGeP2 monolayers, their electronic properties can be modulated under biaxial strain and an external electric field, resulting in a transition from semiconducting to metallic behavior. Finally, we calculated the carrier mobility of these four structures and observed that the SZrGeAs2 monolayer exhibits a hole mobility of up to 597.52 cm2 s-1 V-1 in the x-direction, whereas the SZrSiP2 monolayer demonstrates an electron mobility of up to 479.30 cm2 s-1 V-1 in the y-direction. In the x-direction, the electron mobility of SZrSiAs2 and SZrGeP2 monolayers was measured to be 189.88 and 528.44 cm2 s-1 V-1, respectively. These values are greater than or equivalent to that of experimentally synthesized MoS2 (∼200 cm2 s-1 V-1). Our research lays the foundation for utilizing two-dimensional Janus materials in electronic devices.

Preparation and properties of novel binary and ternary highly amorphous poly(vinyl alcohol)-based composites with hybrid nanofillers

Smart protective coatings and devices are currently of great interest. In particular, they can absorb or reflect harmful waves of electromagnetic interference (EMI). In this work, novel binary and ternary composites with highly amorphous poly(vinyl alcohol) (HAVOH) as a matrix and single-walled carbon nanotubes (SWCNTs) and MXenes as nanofillers were prepared. HAVOH is a recently patented kind of poly(vinyl alcohol) (PVOH) that was modified with diol monomers. MXenes are a new type of inorganic two-dimensional (2D) nanoparticle consisting of carbides, nitrides and carbonitrides. Three series of composites, HAVOH/SWCNTs, HAVOH/MXenes and HAVOH/SWCNTs/MXenes, were prepared using the solvent casting method. Samples were tested with various methods to study their structure, electrical properties, thermal behavior and EMI-shielding properties. HAVOH/3.0 wt.% SWCNTs/3.0 wt.% MXene specimens revealed a shielding effectiveness of 55 dB, which is 122 times better than that of the neat matrix. These results are promising for the fabrication of films with protective effects against EMI.

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

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

Computationally Guided Synthesis of MXenes by Dry Selective Extraction

MXenes are a rapidly growing family of 2D transition metal carbides and nitrides that are promising for various applications, including energy storage and conversion, electronics, and healthcare. Hydrofluoric-acid-based etchants are typically used for large-scale and high-throughput synthesis of MXenes, which also leads to a mixture of surface terminations that impede MXene properties. Herein, a computational thermodynamic model with experimental validation is presented to explore the feasibility of fluorine-free synthesis of MXenes with uniform surface terminations by dry selective extraction (DSE) from precursor MAX phases using iodine vapors. A range of MXenes and respective precursor compositions are systematically screened using first-principles calculations to find candidates with high phase stability and low etching energy. A thermodynamic model based on the "CALculation of PHAse Diagrams" (CALPHAD) approach is further demonstrated, using Ti3 C2 I2 as an example, to assess the Gibbs free energy of the DSE reaction and the state of the byproducts as a function of temperature and pressure. Based on the assessment, the optimal synthesis temperature and vapor pressure are predicted and further verified by experiments. This work opens an avenue for scalable, fluorine-free dry synthesis of MXenes with compositions and surface chemistries that are not accessible using wet chemical etching.