Fluoride-Free Synthesis of Two-Dimensional Titanium Carbide (MXene) Using A Binary Aqueous System

Entropy Engineering of 2D Materials

Entropy, a measure of disorder or uncertainty in the thermodynamics system, has been widely used to confer desirable functions to alloys and ceramics. The incorporation of three or more principal elements into a single sublattice increases the entropy to medium and high levels, imparting these materials a mélange of advanced mechanical and catalytic properties. In particular, when scaling down the dimensionality of crystals from bulk to the 2D space, the interplay between entropy stabilization and quantum confinement offers enticing opportunities for exploring new fundamental science and applications, since the structural ordering, phase stability, and local electronic states of these distorted 2D materials get significantly reshaped. During the last few years, the large family of high-entropy 2D materials is rapidly expanding to host MXenes, hydrotalcites, chalcogenides, metal-organic frameworks (MOFs), and many other uncharted members. Here, the recent advances in this dynamic field are reviewed, elucidating the influence of entropy on the fundamental properties and underlying elementary mechanisms of 2D materials. In particular, their structure-property relationships resulting from theoretical predictions and experimental findings are discussed. Furthermore, an outlook on the key challenges and opportunities of such an emerging field of 2D materials is also provided.

MXenes for Wearable Physical Sensors toward Smart Healthcare

The gradual rise of personal healthcare awareness is accelerating the deployment of wearable sensors, whose ability of acquiring physiological vital signs depends on sensing materials. MXenes have distinct chemical and physical superiorities over other 2D nanomaterials for wearable sensors. This review presents a comprehensive summary of the latest advancements in MXenes-based materials for wearable physical sensors. It begins with an introduction to special structural features of MXenes for sensing performance, followed by an in-depth exploration of versatile functionalities. A detailed description of different sensing mechanisms is also included to illustrate the contribution of MXenes to the sensing performance and its improvement. In addition, the real-world applications of MXenes-based physical sensors for monitoring different physiological signs are included as well. The remaining challenges of MXenes-based materials for wearable physical sensors and their promising opportunities are finally narrated, in conjunction with a prospective for future development.

Evolution of Surface Chemistry in Two-Dimensional MXenes: From Mixed to Tunable Uniform Terminations

Surface chemistry of MXenes is of great interest as the terminations can define the intrinsic properties of this family of materials. The diverse and tunable terminations also distinguish MXenes from many other 2D materials. Conventional fluoride-containing reagents etching approaches resulted in MXenes with mixed fluoro-, oxo-, and hydroxyl surface groups. The relatively strong chemical bonding of MXenes' surface metal atoms with oxygen and fluorine makes post-synthetic covalent surface modifications of such MXenes unfavorable. In this minireview, we focus on the recent advances in MXenes with uniform surface terminations. Unconventional methods, including Lewis acidic molten salt etching (LAMS) and bottom-up direct synthesis, have been proven successful in producing halide-terminated MXenes. These synthetic strategies have opened new possibilities for MXenes because weaker surface chemical bonds in halide-terminated MXenes facilitate post-synthetic covalent surface modifications. Both computational and experimental results on surface termination-dependent properties are summarized and discussed. Finally, we offer our perspective on the opportunities and challenges in this exciting research field.

MXene Synthesis and Carbon Capture Applications: Mini-Review

MXenes, a class of two-dimensional transition metal carbides, nitrides, and carbonitrides, have garnered significant attention due to their remarkable potential for energy storage, electrocatalysis, and gas separation applications. The fabrication processes of MXene involve building up the MXene structure from constituent elements and the selective elimination of M-A bonds from the precursor MAX. However, considerable efforts are still required to design and develop efficient MXene-based technologies. This review article aims to briefly analyse the synthesis methods employed for MXene production, ranging from direct synthesis and conventional chemical wet etching approach to the more recent molten salt etching technique. The review highlights the advancements made in achieving precise control over the terminal groups, which is paramount for tailoring the properties of MXenes for specific applications. Furthermore, the potential of MXene-based materials for carbon capture applications, particularly in developing advanced adsorbents, is emphasized. The in-depth examination of MXene synthesis techniques and their implications for carbon capture applications provides a solid foundation for developing and optimizing these promising materials.

MXenes for Zinc-Based Electrochemical Energy Storage Devices

As an economical and safer alternative to lithium, zinc (Zn) is promising for realizing new high-performance electrochemical energy storage devices, such as Zn-ion batteries, Zn-ion hybrid capacitors, and Zn-air batteries. Well-designed electrodes are needed to enable efficient Zn electrochemistry for energy storage. Two-dimensional transition metal carbides and nitrides (MXenes) are emerging materials with unique electrical, mechanical, and electrochemical properties and versatile surface chemistry. They are potential material candidates for constructing high-performance electrodes of Zn-based energy storage devices. This review first briefly introduces the working mechanisms of the three Zn-based energy storage devices. Then, the recent progress on the synthesis, chemical functionalization, and structural design of MXene-based electrodes is summarized. Their performance in Zn-based devices is analyzed to establish relations between material properties, electrode structures, and device performance. Last, several research topics are proposed to be addressed for developing practical MXene-based electrodes for Zn-based energy storage devices to enable their commercialization and broad adoption in the near future.

Molten Salt Derived MXenes: Synthesis and Applications

About one decade after the first report on MXenes, these 2D early transition metal carbides or nitrides have become among the best-performing materials in electrode applications related to electrical energy storage devices and power-to-fuels conversion. MXenes are obtained by a top-down approach starting from the appropriate 3D MAX phase that undergoes etching of the A-site metal. Initial etching procedures are based on the use of concentrated HF or the in situ generation of this highly corrosive and poisonous reagent. Etching of the MAX phase is one of the major hurdles limiting the progress of the field. The present review summarizes an alternative, universal, and easily scalable etching procedure based on treating the MAX precursor with a Lewis acid molten salt. The review starts with presenting the current state of the art of the molten salt etching procedure to obtain or modify MXene, followed by a summary of the applications of these MXene samples. The aim of the review is to show the versatility and advantages of molten salt etching in terms of general applicability, control of the surface terminal groups, and uniform deposition of metal nanoparticles, among other features of the procedure.

Recent Advances in Non-Ti MXenes: Synthesis, Properties, and Novel Applications

One of the most fascinating 2D nanomaterials (NMs) ever found is various members of MXene family. Among them, the titanium-based MXenes, with more than 70% of publication-related investigations, are comparatively well studied, producing fundamental foundation for the 2D MXene family members with flexible properties, familiar with a variety of advanced novel technological applications. Nonetheless, there are still more candidates among transitional metals (TMs) that can function as MXene NMs in ways that go well beyond those that are now recognized. Systematized details of the preparations, characteristics, limitations, significant discoveries, and uses of the novel M-based MXenes (M-MXenes), where M stands for non-Ti TMs (M = Sc, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, and Lu), are given. The exceptional qualities of the 2D non-Ti MXene outperform standard Ti-MXene in several applications. There is many advancement in top-down as well as bottom-up production of MXenes family members, which allows for exact control of the M-characteristics MXene NMs to contain cutting-edge applications. This study offers a systematic evaluation of existing research, covering everything in producing complex M-MXenes from primary limitations to the characterization and selection of their applications in accordance with their novel features. The development of double metal combinations, extension of additional metal candidates beyond group-(III-VI)B family, and subsequent development of the 2D TM carbide/TMs nitride/TM carbonitrides to 2D metal boride family are also included in this overview. The possibilities and further recommendations for the way of non-Ti MXene NMs are in the synthesis of NMs will discuss in detail in this critical evaluation.

MXene/Carbon Nanocomposites for Water Treatment

One of the most critical problems faced by modern civilization is the depletion of freshwater resources due to their continuous consumption and contamination with different organic and inorganic pollutants. This paper considers the potential of already discovered MXenes in combination with carbon nanomaterials to address this problem. MXene appears to be a highly promising candidate for water purification due to its large surface area and electrochemical activity. However, the problems of swelling, stability, high cost, and scalability need to be overcome. The synthesis methods for MXene and its composites with graphene oxide, carbon nanotubes, carbon nanofibers, and cellulose nanofibers, along with their structure, properties, and mechanisms for removing various pollutants from water, are described. This review discusses the synthesis methods, properties, and mechanisms of water purification using MXene and its composites. It also explores the fundamental aspects of MXene/carbon nanocomposites in various forms, such as membranes, aerogels, and textiles. A comparative analysis of the latest research on this topic shows the progress in this field and the limitations for the practical application of MXene/carbon nanocomposites to solve the problem of drinking water scarcity. Consequently, this review demonstrates the relevance and promise of the material and underscores the importance of further research and development of MXene/carbon nanocomposites to provide effective water treatment solutions.

In-Situ Construction of Functional Multi-Dimensional MXene-based Composites Directly from MAX Phases through Gas-Solid Reactions

The weak bonding of A atoms with MX layers in MAX phases not only enables the selective etching of A layers for MXene preparation but brings about the chance to construct A derivatives/MXene composites via in situ conversion. Here, a facile and general gas-solid reaction systems are elegantly devised to construct multi-dimensional MXene based composites including AlF3 nanorods/MXene, AlF3 nanocrystals/MXene, amorphous AlF3/MXene, A filled carbon nanotubes/MXene, layered metal chalcogenides/MXene, MOF/MXene, and so on. The intrinsic effect mechanism of interlayer confinement towards crystal growth, catalytic behavior, van der Waals-heterostructure construction and coordination reaction are rationally put forward. The tight interface combination and synergistic effect from distinct components make them promising active materials for electrochemical applications. More particularly, the AlF3 nanorods/Nb2C MXene demonstrate bi-directional catalytic activity toward the conversion between Li2S and lithium polysulfides, which alleviates the shuttle effect in lithium-sulfur batteries.

MXenes with ordered triatomic-layer borate polyanion terminations

Surface terminations profoundly influence the intrinsic properties of MXenes, but existing terminations are limited to monoatomic layers or simple groups, showing disordered arrangements and inferior stability. Here we present the synthesis of MXenes with triatomic-layer borate polyanion terminations (OBO terminations) through a flux-assisted eutectic molten etching approach. During the synthesis, Lewis acidic salts act as the etching agent to obtain the MXene backbone, while borax generates BO2- species, which cap the MXene surface with an O-B-O configuration. In contrast to conventional chlorine/oxygen-terminated Nb2C with localized charge transport, OBO-terminated Nb2C features band transport described by the Drude model, exhibiting a 15-fold increase in electrical conductivity and a 10-fold improvement in charge mobility at the d.c. limit. This transition is attributed to surface ordering that effectively mitigates charge carrier backscattering and trapping. Additionally, OBO terminations provide Ti3C2 MXene with substantially enriched Li+-hosting sites and thereby a large charge-storage capacity of 420 mAh g-1. Our findings illustrate the potential of intricate termination configurations in MXenes and their applications for (opto)electronics and energy storage.

The Rise of Xene Hybrids

Xenes, mono-elemental atomic sheets, exhibit Dirac/Dirac-like quantum behavior. When interfaced with other 2D materials such as boron nitride, transition metal dichalcogenides, and metal carbides/nitrides/carbonitrides, it enables them with unique physicochemical properties, including structural stability, desirable bandgap, efficient charge carrier injection, flexibility/breaking stress, thermal conductivity, chemical reactivity, catalytic efficiency, molecular adsorption, and wettability. For example, BN acts as an anti-oxidative shield, MoS2 injects electrons upon laser excitation, and MXene provides mechanical flexibility. Beyond precise compositional modulations, stacking sequences, and inter-layer coupling controlled by parameters, achieving scalability and reproducibility in hybridization is crucial for implementing these quantum materials in consumer applications. However, realizing the full potential of these hybrid materials faces challenges such as air gaps, uneven interfaces, and the formation of defects and functional groups. Advanced synthesis techniques, a deep understanding of quantum behaviors, precise control over interfacial interactions, and awareness of cross-correlations among these factors are essential. Xene-based hybrids show immense promise for groundbreaking applications in quantum computing, flexible electronics, energy storage, and catalysis. In this timely perspective, recent discoveries of novel Xenes and their hybrids are highlighted, emphasizing correlations among synthetic parameters, structure, properties, and applications. It is anticipated that these insights will revolutionize diverse industries and technologies.

MXene: A Roadmap to Sustainable Energy Management, Synthesis Routes, Stabilization, and Economic Assessment

MXenes with their wide range of tunability and good surface chemistry provide unique and distinctive characteristics offering potential employment in various aspects of energy management applications. These high-performance materials have attracted considerable attention in recent decades due to their outstanding characteristics. In the literature, most of the work is related to specific methods for the preparation of MXenes. In this Review, we present a detailed discussion on the synthesis of MXenes through different etching routes involving acids, such as hydrochloric acid, hydrofluoric acid, and lithium fluoride, and non-acidic alkaline solution, electrochemical, and molten salt methods. Furthermore, a concise overview of the different structural, optical, electronic, and magnetic properties of MXenes is provided corresponding to their role in supporting high thermal, chemical, mechanical, environmental, and electrochemical stability. Additionally, the role of MXenes in maintaining the thermal management performance of photovoltaic thermal systems (PV/T), wearable light heaters, solar water desalination, batteries, and supercapacitors is also briefly discussed. A techno-economic and life cycle analysis of MXenes is provided to analyze their sustainability, scalability, and commercialization to facilitate a comprehensive array of energy management systems. Lastly, the technology readiness level of MXenes is defined, and future recommendations for MXenes are provided for their further utilization in niche applications. The present work strives to link the chemistry of MXenes to process economics for energy management applications.

Functionalized 2D membranes for separations at the 1-nm scale

The ongoing evolution of two-dimensional (2D) material-based membranes has prompted the realization of mass separations at the 1-nm scale due to their well-defined selective nano- and subnanochannels. Strategic membrane functionalization is further found to be key to augmenting channel accuracy and efficiency in distinguishing ions, gases and molecules within this range and is thus trending as a research focus in energy-, resource-, environment- and pharmaceutical-related applications. In this review, we present the fundamentals underpinning functionalized 2D membranes in various separations, elucidating the critical "method-interaction-property" relationship. Starting with an introduction to various functionalization strategies, we focus our discussion on functionalization-induced channel-species interactions and reveal how they shape the transport- and operation-related features of the membrane in different scenarios. We also highlight the limitations and challenges of current functionalized 2D membranes and outline the necessary breakthroughs needed to apply them as reliable and high-performance separation units across industries in the future.

Dual Redox Reaction Sites for Pseudocapacitance Based on Ti and -P Functional Groups of Ti3C2PBrx MXene

MXenes have extensive applications due to their different properties determined by intrinsic structures and various functional groups. Exploring different functional groups of MXenes leads to improved performance or potential applications. In this work, we prepared new Ti3C2PBrx (x=0.4-0.6) MXene with phosphorus functional groups (-P) through a two-step gas-phase reaction. The acquisition of -P is achieved by replacing bromine functional groups (-Br) of Ti3C2Br2 in the phosphorus vapor. After -Br is replaced with -P, Ti3C2PBrx MXene shows an improved areal capacitance (360 mF cm-2) at 20 mV s-1 compared with Ti3C2Br2 MXene (102 mF cm-2). At a current density of 5 mA cm-2 after 10000 cycles, the capacitance retention of Ti3C2PBrx MXene has not decreased. The pseudocapacitive enhancement mechanism has been discovered based on the dual redox sites of the functional groups -P and Ti.

Advanced Insights on MXenes: Categories, Properties, Synthesis, and Applications in Alkali Metal Ion Batteries

The development and optimization of promising anode material for next-generation alkali metal ion batteries are significant for clean energy evolution. 2D MXenes have drawn extensive attention in electrochemical energy storage applications, due to their multiple advantages including excellent conductivity, robust mechanical properties, hydrophilicity of its functional terminations, and outstanding electrochemical storage capability. In this review, the categories, properties, and synthesis methods of MXenes are first outlined. Furthermore, the latest research and progress of MXenes and their composites in alkali metal ion storage are also summarized comprehensively. A special emphasis is placed on MXenes and their hybrids, ranging from material design and fabrication to fundamental understanding of the alkali ion storage mechanisms to battery performance optimization strategies. Lastly, the challenges and personal perspectives of the future research of MXenes and their composites for energy storage are presented.

One-step Synthesis of Organic Terminal 2D Ti3C2Tx MXene Nanosheets by Etching of Ti3AlC2 in an Organic Lewis Acid Solvent

The surface and interface chemistry are critical for controlling the properties of two-dimensional transition metal carbides and nitrides (MXenes). Numerous efforts have been devoted to the functionalization of MXenes with small inorganic ligands; however, few etching methods have been reported on the direct bonding of organic groups to MXene surfaces. In this work, we demonstrated an efficient and rapid strategy for the direct synthesis of 2D Ti3C2Tx MXene nanosheets with organic terminal groups in an organic Lewis acid (trifluoromethanesulfonic acid) solvent, without introducing additional intercalations. The dissolution of aluminum and the subsequent in situ introduction of trifluoromethanesulfonic acid resulted in the extraction of Ti3C2Tx MXene (T=CF3SO3 -) (denoted as CF3SO3H-Ti3C2Tx) flakes with sizes reaching 15 μm and high productivity (over 70 %) of monolayers or few layers. More importantly, the large CF3SO3H-Ti3C2Tx MXene nanosheets had high colloidal stability, making them promising as efficient electrocatalysts for the hydrogen evolution reaction.

Titanium Carbide (Ti3C2Tx) MXene for Sequestration of Aquatic Pollutants

The rapid expansion of industrialization has resulted in the release of multiple ecological contaminants in gaseous, liquid, and solid forms, which pose significant environmental risks to many different ecosystems. The efficient and cost-effective removal of these environmental pollutants has attracted global attention. This growing concern has prompted the synthesis and optimization of nanomaterials and their application as potential pollutant removal. In this context, MXene is considered an outstanding photocatalytic candidate due to its unique physicochemical and mechanical properties, which include high specific surface area, physiological compatibility, and robust electrodynamics. This review highlights recent advances in shaping titanium carbide (Ti3C2Tx) MXenes, emphasizing the importance of termination groups to boost photoactivity and product selectivity, with a primary focus on engineering aspects. First, a broad overview of Ti3C2Tx MXene is provided, delving into its catalytic properties and the formation of surface termination groups to establish a comprehensive understanding of its fundamental catalytic structure. Subsequently, the effects of engineering the morphology of Ti3C2Tx MXene into different structures, such as two-dimensional (2D) accordion-like forms, monolayers, hierarchies, quantum dots, and nanotubes. Finally, a concise overview of the removal of different environmental pollutants is presented, and the forthcoming challenges, along with their prospective outlooks, are delineated.

Fluorine-Free MXenes: Recent Advances, Synthesis Strategies, and Mechanisms

MXenes, an exceptional class of 2D materials, possess high conductivity, adaptable surface chemistry, mechanical strength, and tunable bandgaps, making them attractive for diverse applications. Unlocking the potential of MXenes requires precise control over synthesis methods and surface functionality. Conventionally, fluorine-based etchants are used in MXenes synthesis, posing both environmental concerns and alterations to surface properties, along with the introduction of certain defects. This prompts the exploration of innovative fluorine-free strategies for MXenes synthesis. This review focuses on environmentally friendly, fluorine-free techniques for MXene synthesis, emphasizing mechanisms and recent breakthroughs in alternative etching strategies. The comprehensive coverage includes electrochemical etching, Lewis acid-driven molten salt etching, alkaline/hydrothermal techniques, chemical vapor deposition (CVD), and recent innovative methods. Fluorine-free MXenes synthesis yields terminations such as ─O, ─OH, ─Cl, etc., influencing surface chemistry and improving their properties. The presence of ─OH groups in NaOH etched MXenes boosts their energy storage, while ─Cl functionality from Lewis acidic salts optimizes electrochemical performance. Fluorine-free methods mitigate adverse effects of ─F terminations on MXene conductivity, improving electronic properties and broadening their applications. In addition to traditional approaches, this review delves into novel fluorine-free methods for tailoring MXenes properties. It comprehensively addresses challenges, opportunities, and future perspectives in fluorine-free MXenes.

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.

Advances in MXene-based luminescence sensing strategies

MXenes have attracted the attention of many researchers as one of the latest two-dimensional (2D) materials over the last decade. Their great potential for biosensing has also been fully exploited after the discovery of their unique properties such as superior optical properties, excellent hydrophilicity, good thermal stability, excellent mechanical property, high electrical conductivity, biocompatibility, large surface area, and ease of surface functionalization. In the MXene-based luminescence sensing strategy, MXenes typically appear in the form of nanosheets, quantum dots and modified MXene nanocomposites, and they are utilized as different sensing platforms or as a luminescence source. In this review, we focused on the MXene-based luminescence sensing strategies, including fluorescence, electrochemiluminescence and chemiluminescence sensors and the comparison of their performance. Finally, the perspectives of the MXene-based luminescence sensors are discussed.

Electrochemical exfoliation of 2D materials beyond graphene

After the discovery of graphene in 2004, the field of atomically thin crystals has exploded with the discovery of thousands of 2-dimensional materials (2DMs) with unique electronic and optical properties, by making them very attractive for a broad range of applications, from electronics to energy storage and harvesting, and from sensing to biomedical applications. In order to integrate 2DMs into practical applications, it is crucial to develop mass scalable techniques providing crystals of high quality and in large yield. Electrochemical exfoliation is one of the most promising methods for producing 2DMs, as it enables quick and large-scale production of solution processable nanosheets with a thickness well below 10 layers and lateral size above 1 μm. Originally, this technique was developed for the production of graphene; however, in the last few years, this approach has been successfully extended to other 2DMs, such as transition metal dichalcogenides, black phosphorous, hexagonal boron nitride, MXenes and many other emerging 2D materials. This review first provides an introduction to the fundamentals of electrochemical exfoliation and then it discusses the production of each class of 2DMs, by introducing their properties and giving examples of applications. Finally, a summary and perspective are given to address some of the challenges in this research area.

MXene-Based Chemo-Sensors and Other Sensing Devices

MXenes have received worldwide attention across various scientific and technological fields since the first report of the synthesis of Ti3C2 nanostructures in 2011. The unique characteristics of MXenes, such as superior mechanical strength and flexibility, liquid-phase processability, tunable surface functionality, high electrical conductivity, and the ability to customize their properties, have led to the widespread development and exploration of their applications in energy storage, electronics, biomedicine, catalysis, and environmental technologies. The significant growth in publications related to MXenes over the past decade highlights the extensive research interest in this material. One area that has a great potential for improvement through the integration of MXenes is sensor design. Strain sensors, temperature sensors, pressure sensors, biosensors (both optical and electrochemical), gas sensors, and environmental pollution sensors targeted at volatile organic compounds (VOCs) could all gain numerous improvements from the inclusion of MXenes. This report delves into the current research landscape, exploring the advancements in MXene-based chemo-sensor technologies and examining potential future applications across diverse sensor types.

Top-down design of high-performance V-based MBene anode for Li/Na-ion batteries

Two-dimensional (2D) MBenes have enormous potential in energy applications. Vanadium metal, with its versatile and tunable electronic states, can further enhance the electrochemical performance of MBenes. However, most MBenes are composed of a few atomic layers as the metal boron (MB) block, e.g., M2B2, which might lead to instability and poor mechanical response. Herein, we designed and predicted 2D V4B6 associated with different terminations (T = Cl, O, S) using a top-down method and global search for parental V4AB6. Among the A element candidates, the P-glued MAB phase exhibited high stability and easy synthesizability. Moreover, 2D V4B6 was feasibly formed and easily exfoliated owing to its weak V-P bonding. Most of the surface functionalization could improve both the mechanical and electrochemical properties of the V4B6 monolayer. In particular, 2D V4B6S2 exhibited a high potential as an anode material for lithium-ion batteries (LIBs) with high theoretical capacity (297 mA h g-1), low diffusion barrier (0.166 eV), and low open circuit voltage (0.136 V), outperforming a majority of MXenes and transition metal sulfide layers. This work offers a new strategy for designing desirable 2D layers from parental materials, and tuning their properties via composition and surface functionalization, which could shed light on the development of other 2D metal-ion anodes.

Recent Advances in Photothermal Therapy at Near-Infrared-II Based on 2D MXenes

The use of photothermal therapy (PTT) with the near-infrared II region (NIR-II: 1000-1700 nm) is expected to be a powerful cancer treatment strategy. It retains the noninvasive nature and excellent temporal and spatial controllability of the traditional PTT, and offers significant advantages in terms of tissue penetration depth, background noise, and the maximum permissible exposure standards for skin. MXenes, transition-metal carbides, nitrides, and carbonitrides are emerging inorganic nanomaterials with natural biocompatibility, wide spectral absorption, and a high photothermal conversion efficiency. The PTT of MXenes in the NIR-II region not only provides a valuable reference for exploring photothermal agents that respond to NIR-II in 2D inorganic nanomaterials, but also be considered as a promising biomedical therapy. First, the synthesis methods of 2D MXenes are briefly summarized, and the laser light source, mechanism of photothermal conversion, and evaluation criteria of photothermal performance are introduced. Second, the latest progress of PTT based on 2D MXenes in NIR-II are reviewed, including titanium carbide (Ti3 C2 ), niobium carbide (Nb2 C), and molybdenum carbide (Mo2 C). Finally, the main problems in the PTT application of 2D MXenes to NIR-II and future research directions are discussed.

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.

Fluoride-Free Synthesis of 2D Titanium Carbide (MXenes) Assisted by scCO2 -Based Ternary Solution

2D MXene-Ti3 C2 Tx holds great promise in various electronic applications, especially for electromagnetic interference (EMI) shielding devices and supercapacitors. Ti3 C2 Tx synthesis typically involves the use of hazardous fluorine-containing chemicals that can result in the formation of inert fluoride functional groups on the surface of Ti3 C2 Tx , severely degrading its properties and posing a threat to the performance of electron transfer among electrical devices. Herein, a supercritical carbon dioxide-based ternary solution (scCO2 /DMSO/HCl) to produce fluoride-free Ti3 C2 Tx in mild conditions (via 0.5 m HCl, 20 MPa, 32 °C) is reported. The fluorine-free Ti3 C2 Tx films electrode presents an excellent gravimetric capacitance of 320 F g-1 at 2 mV s-1 in 1 m H2 SO4 . Besides, it is demonstrated that fluorine-free Ti3 C2 Tx films exhibit outstanding EMI shielding efficiency of 53.12 dB at 2.5 µm thickness. The findings offer a mild and practical approach to producing fluoride-free Ti3 C2 Tx and open opportunities for exploring MXenes' potential applications in various fields.

MXene-Based Current Collectors for Advanced Rechargeable Batteries

As an indispensable component of rechargeable batteries, the current collector plays a crucial role in supporting the electrode materials and collecting the accumulated electrical energy. However, some key issues, like uneven resources, high weight percentage, electrolytic corrosion, and high-voltage instability, cannot meet the growing need for rechargeable batteries. In recent years, MXene-based current collectors have achieved considerable achievements due to its unique structure, large surface area, and high conductivity. The related research has increased significantly. Nonetheless, a comprehensive review of this area is seldom. Herein the applications and progress of MXene in current collector are systematically summarized and discussed. Meanwhile, some challenges and future directions are presented.

Unleashing the Potential of MXene-Based Flexible Materials for High-Performance Energy Storage Devices

Since the initial discovery of Ti3 C2 a decade ago, there has been a significant surge of interest in 2D MXenes and MXene-based composites. This can be attributed to the remarkable intrinsic properties exhibited by MXenes, including metallic conductivity, abundant functional groups, unique layered microstructure, and the ability to control interlayer spacing. These properties contribute to the exceptional electrical and mechanical performance of MXenes, rendering them highly suitable for implementation as candidate materials in flexible and wearable energy storage devices. Recently, a substantial number of novel research has been dedicated to exploring MXene-based flexible materials with diverse functionalities and specifically designed structures, aiming to enhance the efficiency of energy storage systems. In this review, a comprehensive overview of the synthesis and fabrication strategies employed in the development of these diverse MXene-based materials is provided. Furthermore, an in-depth analysis of the energy storage applications exhibited by these innovative flexible materials, encompassing supercapacitors, Li-ion batteries, Li-S batteries, and other potential avenues, is conducted. In addition to presenting the current state of the field, the challenges encountered in the implementation of MXene-based flexible materials are also highlighted and insights are provided into future research directions and prospects.

Molecularly Imprinted Polymer-Based Sensors Integrated with Transition Metal Dichalcogenides (TMDs) and MXenes: A Review

Molecularly imprinted polymer (MIP)-based electrochemical sensors have been extensively researched due to their higher sensitivity, quick response, and operational ease. To develop more advanced sensing devices with enhanced properties, MIPs have been integrated with two-dimensional (2D) layered materials such as transition metal dichalcogenides (TMDs) and MXenes. These 2D materials have unique electronic properties and an extended surface area, making them promising sensing materials that can improve the performance of MIPs. In this review article, we describe the methods used for the synthesis of TMDs and MXenes integrated MIP-based electrochemical sensors. Furthermore, we have provided a critical review of a wide range of analytes determined through the application of these electrochemical sensors. We also go over the influence of TMDs and MXenes on the binding kinetics and adsorption capacity which has enhanced binding recognition and sensing abilities. The combination of TMDs and MXenes with MIPs shows promising synergy in the development of highly efficient recognition materials. In the future, these sensors could be explored for a wider range of applications in environmental remediation, drug delivery, energy storage, and more. Finally, we address the challenges and future perspectives of using TMDs and MXenes integrated MIPs. We conclude with a focus on future development and the scope of integrating these materials in sensing technology.

Synthesis of environmentally friendly neonicotinoid insecticide with proper functional properties by theoretical methodologies

Modern insecticide substitutes using acetylcholine receptors (nAChR) as biochemical targets, such as neonicotinoid insecticides (NNIs), have been extensively researched. Only 12 compounds have been experimentally realized since the initial discovery of imidacloprid. Increasingly, the bottleneck in this field is to rapidly determine the synthesizability of NNI substitutes. Here, we designed a coupled evaluation system for synthesis prediction and validation, including the synthesis probability, reaction path difficulty, and electron transfer characteristics of NNIs and their substitutes. Firstly, a total of 1475 eigenvalues were generated and 52 critical eigenvalues were screened out through the Pearson's correlation coefficient. The positive and unlabeled (PU) machine learning was constructed using the critical eigenvalues NNIs, including 12 experimentally synthesized NNIs (positive samples) and 73 unsynthesized NNI substitutes (unlabeled samples). Results identified 3 NNI substitutes that were highly promising candidates for synthesis (synthesis probability > 0.5). The results of density functional theory demonstrated the ranking of their reaction ease was UN-1 (31.4 kcal/mol) > UN-2 (81.6 kcal/mol) > UN-3 (3.35 ×103 kcal/mol). Time-dependent density functional theory revealed that changes in the electron distribution and electron excitation type were critical factors affecting their synthesizability, and the local excitation type was more favorable for the synthesizability of NNI substituents. The findings provide significant guidance for NNIs synthesis, reducing the possible space of unlabeled samples to 95.89% of their original size, while also minimizing the cost of research on subsequent NNI substitutes.

In situ synthesis of g-C3N4/Ti3C2Tx nano-heterostructures for enhanced photocatalytic H2 generation via water splitting

Herein, we demonstrated the in situ synthesis of g-C3N4/Ti3C2Tx nano-heterostructures for hydrogen generation under UV visible light irradiation. The formation of the g-C3N4/Ti3C2Tx nano-heterostructures was confirmed via powder X-ray diffraction and supported by XPS. The FE-SEM images indicated the formation of layered structures of MXene and g-C3N4. HR-TEM images and SAED patterns confirmed the presence of g-C3N4 together with Ti3C2Tx nanosheets, i.e., the formation of nano-heterostructures of g-C3N4/Ti3C2Tx. The absorption spectra clearly showed the distinct band gaps of g-C3N4 and Ti3C2Tx in the nano-heterostructure. The increase in PL intensity and broadening of the peak with an increase in g-C3N4 indicated the suppression of electron-hole recombination. Furthermore, the nano-heterostructure was used as a photocatalyst for H2 generation from water and methylene blue dye degradation. The highest H2 evolution (1912.25 μmol/0.1 g) with good apparent quantum yield (3.1%) and an efficient degradation of MB were obtained for gCT-0.75, which was much higher compared to that of the pristine materials. The gCT-0.75 nano-heterostructure possessed a high surface area and abundant vacancy defects, facilitating the separation of charge carriers, which was ultimately responsible for this high photocatalytic activity. Additionally, TRPL clearly showed a higher decay time, which supports the enhancement in the photocatalytic activity of the gCT-0.75 nano-heterostructure. The nano-heterostructure with the optimum concentration of g-C3N4 formed a hetero-junction with the linked catalytic system, which facilitated efficient charge carrier separation also responsible for the enhanced photocatalytic activity.

A Ti3C2Tx@PANI core-shell heterostructure assembled into a 3D porous hydrogel as a free-standing electrode for high-energy supercapacitors

Although Ti3C2Tx MXenes have attracted attention in electrochemical energy storage devices due to their excellent electronic conductivity, controllable layer structure, and huge redox active surface area, the application of Ti3C2Tx as supercapacitor (SC) electrode materials is severely limited by the ineffective chemical ion transport kinetics caused by self-restacking. In order to increase the interlayer spacing of Ti3C2Tx, the intercalation method is hailed as an effective process. Herein, polyaniline (PANI) nanorods as intercalators were synthesized by the polymerization of an aniline (ANI) monomer chemisorbed onto Ti3C2Tx wrinkled nanosheets, and the formation of a Ti3C2Tx@PANI heterostructure is conducive to the large interlayer voids. Then, the heterostructure was integrated into a three-dimensional (3D) porous cross-linked framework via a simple graphene oxide (GO)-assisted self-convergence hydrothermal strategy with low temperatures. Due to the synergistic effect among each component and 3D porous interconnected structure, the hierarchical Ti3C2Tx@PANI-reduced graphene oxide (RGO) heterostructure hydrogel possesses the advantages of excellent electrical conductivity, high specific capacitance, repressive aggregation, and large electrochemical active area. Heterostructure hydrogel electrodes (without binders) display excellent electrochemical performance with a specific capacitance as high as 301.0 F g-1 at 1 A g-1, 90.74% capacitance retention over 10 000 cycles, and a maximum energy density of 44.6 W h kg-1 at a power density of 504.7 W kg-1. Our study provides a fresh strategy for constructing a 3D Ti3C2Tx-based framework applicable to other MXenes in the design of hybrid structures for maximizing their potential applications in energy storage.

Graphene-like emerging 2D materials: recent progress, challenges and future outlook

Owing to the unique physical and chemical properties of 2D materials and the great success of graphene in various applications, the scientific community has been influenced to explore a new class of graphene-like 2D materials for next-generation technological applications. Consequently, many alternative layered and non-layered 2D materials, including h-BN, TMDs, and MXenes, have been synthesized recently for applications related to the 4th industrial revolution. In this review, recent progress in state-of-the-art research on 2D materials, including their synthesis routes, characterization and application-oriented properties, has been highlighted. The evolving applications of 2D materials in the areas of electronics, optoelectronics, spintronic devices, sensors, high-performance and transparent electrodes, energy conversion and storage, electromagnetic interference shielding, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nanocomposites are discussed. In particular, the state-of-the-art applications, challenges, and outlook of every class of 2D material are also presented as concluding remarks to guide this fast-progressing class of 2D materials beyond graphene for scientific research into next-generation materials.

Progress in the Synthesis Process and Electrocatalytic Application of MXene Materials

With their rich surface chemistry, high electrical conductivity, variable bandgap, and thermal stability, 2D materials have been developed for effective electrochemical energy conversion systems over the past decade. Due to the diversity brought about by the use of transition metals and C/N pairings, the 2D material MXene has found excellent applications in many fields. Among the various applications, many breakthroughs have been made in electrocatalytic applications. Nevertheless, related studies on topics such as the factors affecting the material properties and safer and greener preparation methods have not been reported in detail. Therefore, in this paper, we review the relevant preparation methods of MXene and the safer, more environmentally friendly preparation techniques in detail, and summarize the progress of research on MXene-based materials as highly efficient electrocatalysts in the electrocatalytic field of hydrogen precipitation reaction, nitrogen reduction reaction, oxygen precipitation reaction, oxygen reduction reaction, and carbon dioxide reduction reaction. We also discuss the technology related to MXene materials for hydrogen storage. The main challenges and opportunities for MXene-based materials, which constitute a platform for next-generation electrocatalysis in basic research and practical applications, are highlighted. This paper aims to promote the further development of MXenes and related materials for electrocatalytic applications.

Transparent Electronics for Wearable Electronics Application

Recent advancements in wearable electronics offer seamless integration with the human body for extracting various biophysical and biochemical information for real-time health monitoring, clinical diagnostics, and augmented reality. Enormous efforts have been dedicated to imparting stretchability/flexibility and softness to electronic devices through materials science and structural modifications that enable stable and comfortable integration of these devices with the curvilinear and soft human body. However, the optical properties of these devices are still in the early stages of consideration. By incorporating transparency, visual information from interfacing biological systems can be preserved and utilized for comprehensive clinical diagnosis with image analysis techniques. Additionally, transparency provides optical imperceptibility, alleviating reluctance to wear the device on exposed skin. This review discusses the recent advancement of transparent wearable electronics in a comprehensive way that includes materials, processing, devices, and applications. Materials for transparent wearable electronics are discussed regarding their characteristics, synthesis, and engineering strategies for property enhancements. We also examine bridging techniques for stable integration with the soft human body. Building blocks for wearable electronic systems, including sensors, energy devices, actuators, and displays, are discussed with their mechanisms and performances. Lastly, we summarize the potential applications and conclude with the remaining challenges and prospects.

Extending the Chemistry of Layered Solids and Nanosheets: Chemistry and Structure of MAX Phases, MAB Phases and MXenes

MAX phases are layered solids with unique properties combining characteristics of ceramics and metals. MXenes are their two-dimensional siblings that can be synthesized as van der Waals-stacked and multi-/single-layer nanosheets, which possess chemical and physical properties that make them interesting for a plethora of applications. Both families of materials are highly versatile in terms of their chemical composition and theoretical studies suggest that many more members are stable and can be synthesized. This is very intriguing because new combinations of elements, and potentially new structures, can lead to further (tunable) properties. In this review, we focus on the synthesis science (including non-conventional approaches) and structure of members less investigated, namely compounds with more exotic M-, A-, and X-elements, for example nitrides and (carbo)nitrides, and the related family of MAB phases.

MXene Derivatives for Energy Storage and Conversions

Associated with the rapid development of 2D transition metal carbides, nitrides, and carbonitrides (MXenes), MXene derivatives have been recently exploited and exhibited unique physical/chemical properties, holding promising applications in the areas of energy storage and conversions. This review provides a comprehensive summarization of the latest research and progress on MXene derivatives, including termination-tailored MXenes, single-atom implanted MXenes, intercalated MXenes, van der Waals atomic layers, and non-van der Waals heterostructures. The intrinsic relationship between structure, properties, and corresponding applications for MXene derivatives are then emphasized. Finally, the essential challenges are addressed and perspectives for the MXene derivatives are also discussed.

Electrochemistry of Titanium Carbide MXenes in Supercapacitor

Novel electrode materials are always explored to achieve better performance of supercapacitors. Titanium carbide MXenes, Ti3 C2 Tx , are one of the very promising candidates for electrode materials in supercapacitors due to their unique structural and ion storage properties as 2D materials. Their large specific surface area, adjustable functionalized surface terminals, high electrical conductivities, hydrophilicity, and high Faradaic capacitance, also known widely but confusingly as pseudocapacitance, are highly desirable for making high-performance electrodes with increased dis-/charging rates and capacities. Herein, some selective electrochemical considerations of Ti3 C2 Tx MXenes for uses in supercapacitors are critically reviewed and assessed, aiming at a better fundamental understanding of the electrochemical basics and processes in Ti3 C2 Tx MXene-based electrode materials for supercapacitor applications.

Effective Surface Modification of 2D MXene toward Thermal Energy Conversion and Management

Thermal energy management is a crucial aspect of many research developments, such as hybrid and soft electronics, aerospace, and electric vehicles. The selection of materials is of critical importance in these applications to manage thermal energy effectively. From this perspective, MXene, a new type of 2D material, has attracted considerable attention in thermal energy management, including thermal conduction and conversion, owing to its unique electrical and thermal properties. However, tailored surface modification of 2D MXenes is required to meet the application requirements or overcome specific limitations. Herein, a comprehensive review of surface modification of 2D MXenes for thermal energy management is discussed. First, this work discusses the current progress in the surface modification of 2D MXenes, including termination with functional groups, small-molecule organic compound functionalization, and polymer modification and composites. Subsequently, an in situ analysis of surface-modified 2D MXenes is presented. This is followed by an overview of the recent progress in the thermal energy management of 2D MXenes and their composites, such as Joule heating, heat dissipation, thermoelectric energy conversion, and photothermal conversion. Finally, some challenges facing the application of 2D MXenes are discussed, and an outlook on surface-modified 2D MXenes is provided.

Surface and Interface Regulation of MXenes: Methods and Properties

Since the discovery of Ti3 C2 Tx in 2011, 2D transition metal carbides, nitrides, and carbonitrides, known as MXenes, have been attracting great attention as the emerging member of 2D materials. The surface terminations, intercalants, and the interfaces between MXenes and other substances are of importance for tuning the properties of MXenes. For instance, surface termination of MXenes can change the density of states at the Fermi levels to make MXenes electronically tunable. Different terminations can lead to band opening and changes in behavior from metallic to semiconducting, as well as dramatic changes in the work function of MXenes. On the other hand, electron transfer occurring at the interface between MXenes and other substances due to the physical interaction/chemical bonding, changes the electron configuration of MXenes and realizes the functionalization. In this review, the most up-to-date progress of the surface and interface regulation of MXenes is comprehensively summarized, introducing the effect of various synthesis methods on the surface and interface chemistry, the routes on tuning the surface and interface chemistry, and the related potential applications. Finally, the perspective of the future research directions and challenges on surface and interface regulation is outlined.

In Situ Formed Ti/Nb Nanocatalysts within a Bimetal 3D MXene Nanostructure Realizing Long Cyclic Lifetime and Faster Kinetic Rates of MgH2

Magnesium hydride (MGH) is a high-capacity and low-cost hydrogen storage material; however, slow kinetic rates, high dehydrogenation temperature, and short cycle life hindered its large-scale applications. We proposed a strategy of designing novel delaminated 3D bimetal MXene (d-TiNbCT_x) nanostructure to solve these problems. The on-set dehydrogenation temperature of MGH@d-TiNbCT_x composition was reduced to 150 °C, achieving 7.2 wt % of hydrogen releasing capacity within the range of 150-250 °C. This composition absorbed 7.2 wt % hydrogen within 5 min at 200 °C and 5.5 wt % at 30 °C within 2 h, while the desorption capacity (6.0 wt %) was measured at 275 °C within 7 min. After 150 cycles at 250 °C, the 6.5 wt % capacity was retained with negligible loss of hydrogen content. These results were attributed to the catalytic effect of in situ-formed TiH₂/NbH₂ nanocatalysts, which lead to dissociate the Mg-H bonds and promote of kinetic rates. This unique structure paves great opportunities for designing of highly efficient MGHs/MXene nanocomposites to improve the hydrogen storage performance of MGHs.

Microenvironment Restruction of Emerging 2D Materials and their Roles in Therapeutic and Diagnostic Nano-Bio-Platforms

Engineering advanced therapeutic and diagnostic nano-bio-platforms (NBPFs) have emerged as rapidly-developed pathways against a wide range of challenges in antitumor, antipathogen, tissue regeneration, bioimaging, and biosensing applications. Emerged 2D materials have attracted extensive scientific interest as fundamental building blocks or nanostructures among material scientists, chemists, biologists, and doctors due to their advantageous physicochemical and biological properties. This timely review provides a comprehensive summary of creating advanced NBPFs via emerging 2D materials (2D-NBPFs) with unique insights into the corresponding molecularly restructured microenvironments and biofunctionalities. First, it is focused on an up-to-date overview of the synthetic strategies for designing 2D-NBPFs with a cross-comparison of their advantages and disadvantages. After that, the recent key achievements are summarized in tuning the biofunctionalities of 2D-NBPFs via molecularly programmed microenvironments, including physiological stability, biocompatibility, bio-adhesiveness, specific binding to pathogens, broad-spectrum pathogen inhibitors, stimuli-responsive systems, and enzyme-mimetics. Moreover, the representative therapeutic and diagnostic applications of 2D-NBPFs are also discussed with detailed disclosure of their critical design principles and parameters. Finally, current challenges and future research directions are also discussed. Overall, this review will provide cutting-edge and multidisciplinary guidance for accelerating future developments and therapeutic/diagnostic applications of 2D-NBPFs.

Recent Advances in Two-Dimensional MXene-Based Electrochemical Biosensors for Sweat Analysis

Sweat, a biofluid secreted naturally from the eccrine glands of the human body, is rich in several electrolytes, metabolites, biomolecules, and even xenobiotics that enter the body through other means. Recent studies indicate a high correlation between the analytes' concentrations in the sweat and the blood, opening up sweat as a medium for disease diagnosis and other general health monitoring applications. However, low concentration of analytes in sweat is a significant limitation, requiring high-performing sensors for this application. Electrochemical sensors, due to their high sensitivity, low cost, and miniaturization, play a crucial role in realizing the potential of sweat as a key sensing medium. MXenes, recently developed anisotropic two-dimensional atomic-layered nanomaterials composed of early transition metal carbides or nitrides, are currently being explored as a material of choice for electrochemical sensors. Their large surface area, tunable electrical properties, excellent mechanical strength, good dispersibility, and biocompatibility make them attractive for bio-electrochemical sensing platforms. This review presents the recent progress made in MXene-based bio-electrochemical sensors such as wearable, implantable, and microfluidic sensors and their applications in disease diagnosis and developing point-of-care sensing platforms. Finally, the paper discusses the challenges and limitations of MXenes as a material of choice in bio-electrochemical sensors and future perspectives on this exciting material for sweat-sensing applications.

Progress and challenges of emerging MXene based materials for thermoelectric applications

To realize sustainable development, more and more countries forwarded carbon neutrality goal. Accordingly, improving the utilization efficiency of traditional fossil fuel is an effective strategy for this great goal. Keeping this in mind, developing thermoelectric devices to recover waste heat energy resulted in the consumption process of fuel is demonstrated to be promising. High performance thermoelectric devices require advanced materials. MXenes are a kind of 2D materials with a layered structure, which demonstrate excellent thermoelectric performance owing to their unique physical, mechanical, and chemical properties. Also, substantial achievement has been gained during the past few years in synthesizing MXene based materials for thermoelectric devices. In this review, the mainstream synthetic routes of MXene from etching MAX were summarized. Significantly, the current state and challenges of research on improving the performance of MXene based thermoelectrics are explored, including pristine MXene and MXene based composites.

Recent Progress in MXene Hydrogel for Wearable Electronics

Recently, hydrogels have attracted great attention because of their unique properties, including stretchability, self-adhesion, transparency, and biocompatibility. They can transmit electrical signals for potential applications in flexible electronics, human-machine interfaces, sensors, actuators, et al. MXene, a newly emerged two-dimensional (2D) nanomaterial, is an ideal candidate for wearable sensors, benefitting from its surface's negatively charged hydrophilic nature, biocompatibility, high specific surface area, facile functionalization, and high metallic conductivity. However, stability has been a limiting factor for MXene-based applications, and fabricating MXene into hydrogels has been proven to significantly improve their stability. The unique and complex gel structure and gelation mechanism of MXene hydrogels require intensive research and engineering at nanoscale. Although the application of MXene-based composites in sensors has been widely studied, the preparation methods and applications of MXene-based hydrogels in wearable electronics is relatively rare. Thus, in order to facilitate the effective evolution of MXene hydrogel sensors, the design strategies, preparation methods, and applications of MXene hydrogels for flexible and wearable electronics are comprehensively discussed and summarized in this work.

Retrospective insights into recent MXene-based catalysts for CO2 electro/photoreduction: how far have we gone?

The electro/photocatalytic CO₂ reduction reaction (CO₂RR) is a long-term avenue toward synthesizing renewable fuels and value-added chemicals, as well as addressing the global energy crisis and environmental challenges. As a result, current research studies have focused on investigating new materials and implementing numerous fabrication approaches to increase the catalytic performances of electro/photocatalysts toward the CO₂RR. MXenes, also known as 2D transition metal carbides, nitrides, and carbonitrides, are intriguing materials with outstanding traits. Since their discovery in 2011, there has been a flurry of interest in MXenes in electrocatalysis and photocatalysis, owing to their several benefits, including high mechanical strength, tunable structure, surface functionality, high specific surface area, and remarkable electrical conductivity. Herein, this review serves as a milestone for the most recent development of MXene-based catalysts for the electrocatalytic and photocatalytic CO₂RR. The overall structure of MXenes is described, followed by a summary of several synthesis pathways classified as top-down and bottom-up approaches, including HF-etching, in situ HF-formation, electrochemical etching, and halogen etching. Additionally, the state-of-the-art development in the field of both the electrocatalytic and photocatalytic CO₂RR is systematically reviewed. Surface termination modulation and heterostructure engineering of MXene-based electro/photocatalysts, and insights into the reaction mechanism for the comprehension of the structure-performance relationship from the CO₂RR via density functional theory (DFT) have been underlined toward activity enhancement. Finally, imperative issues together with future perspectives associated with MXene-based electro/photocatalysts are proposed.

Recent Advances and Perspectives of Lewis Acidic Etching Route: An Emerging Preparation Strategy for MXenes

Since the discovery in 2011, MXenes have become the rising star in the field of two-dimensional materials. Benefiting from the metallic-level conductivity, large and adjustable gallery spacing, low ion diffusion barrier, rich surface chemistry, superior mechanical strength, MXenes exhibit great application prospects in energy storage and conversion, sensors, optoelectronics, electromagnetic interference shielding and biomedicine. Nevertheless, two issues seriously deteriorate the further development of MXenes. One is the high experimental risk of common preparation methods such as HF etching, and the other is the difficulty in obtaining MXenes with controllable surface groups. Recently, Lewis acidic etching, as a brand-new preparation strategy for MXenes, has attracted intensive attention due to its high safety and the ability to endow MXenes with uniform terminations. However, a comprehensive review of Lewis acidic etching method has not been reported yet. Herein, we first introduce the Lewis acidic etching from the following four aspects: etching mechanism, terminations regulation, in-situ formed metals and delamination of multi-layered MXenes. Further, the applications of MXenes and MXene-based hybrids obtained by Lewis acidic etching route in energy storage and conversion, sensors and microwave absorption are carefully summarized. Finally, some challenges and opportunities of Lewis acidic etching strategy are also presented.

The Rise of MXene: A Wonder 2D Material, from Its Synthesis and Properties to Its Versatile Applications-A Comprehensive Review

MXene, a new member of 2D material, unites the eminence of hydrophilicity, large surface groups, superb flexibility and excellent conductivity. Because of its prodigious characteristics, MXene has gained much approbation among researchers worldwide. MXene's noteworthy features, such as its electrical conductivity, structural property, magnetic behaviour, etc., manifest a broad spectrum of applications, including environment, catalytic, wireless communications, electromagnetic interference (EMI) shielding, drug delivery, wound dressing, bio-imaging, antimicrobial and biosensor. In this review article, an overview of the latest advancements in the applications of MXene has been reported. First, various synthesis strategies of MXene will be summarized, followed by the different structural, physical and chemical properties. The current advances in versatile applications have been discussed. The article aims to incorporate all the possible applications of MXene, making it a versatile material that juxtaposes it with other 2D materials. A separate section is dedicated to the bottlenecks for future developments and recommendations.