Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes

Effects of Intercalation on ML-Ti3C2Tz MXene Properties and Friction Performance

Intercalation in two-dimensional (2D) materials can modify their physical, chemical, and electronic properties. This modification enables the tailoring of 2D material characteristics, enhancing their performance and expanding their applications in various fields. The friction performance of 2D materials such as MoS2 and graphite has a strong dependence on their interlayer spacing, and they exhibit an increase in d-spacing associated with a reduction in friction performance. The ability to control the interlayer spacing of Ti3C2Tz MXene has proven beneficial for energy storage applications such as batteries and supercapacitors, but no one has utilized this control of interlayer spacing for lubrication. In this study, we demonstrate that interlayer spacing of multilayer (ML) Ti3C2Tz MXene can be controlled through chemical intercalation and its direct effects on the electrical conductivity and friction performance. We observed a notable decrease in electrical conductivity in vacuum-filtered ML-Ti3C2Tz MXene films, which was attributed to an increased internal resistance resulting from the expansion of the interlayer gap. We also found a significant reduction in the coefficient of friction for ML-Ti3C2Tz MXene with an increased d-spacing. This reduction is attributed to a weakened attraction of individual ML-Ti3C2Tz MXene layers (intercalated). Under a tangential force, it becomes easier to slide within the larger interlayer gap with weakened van der Waals forces. This work provides insights into the tunability of MXene properties through interlayer spacing, offering potential applications requiring materials with specific electrical and friction characteristics.

Facilitating Ion Storage and Transport Pathways by In Situ Constructing 1D Carbon Nanotube Electric Bridges between 2D MXene Interlayers

Multiple van der Waals (vdW) gaps invoke abundant opportunities for contriving artificial architectures and tailoring desired properties via the intercalation route beyond the reach of conventional concepts. Intriguingly, the electrochemical intercalation strategy can precisely and reversibly tune the intercalation stage of charged functional species. This study presents a valid structural editing protocol facilitated by electrochemical intercalation to engineer MXene interlayers, ultimately incorporating in situ constructed carbon nanotube (CNT) electric bridges for enhanced ion storage and transport pathways. The method allows for the precise modulation of electrochemical forces to tailor materials for specific applications. Deep intercalation and in situ growth processes establish robust anchoring sites and connectivity hubs between MXenes and CNTs, ensuring structural homogeneity and stability in advanced electrode materials. The results demonstrate the effectiveness of electrochemistry-mediated interlayer nanoengineering in MXenes, offering a versatile approach to design vdW heterostructures with tailored functionalities for energy storage and conversion applications. This work highlights the potential of electrochemical modulation in advancing materials engineering strategies for next-generation energy storage technologies.

The Impact of Metal Ions on MXene Membranes: Critical Role of Titanium Vacancies

Two-dimensional transition metal carbides and nitrides (MXenes) and MXene-based membranes hold promise for applications including water purification and seawater desalination; however, their environmental behavior and fate in these matrices remain unknown. In this study, we systematically assessed the reaction efficiencies of Ti3C2Tx at varying important environmental conditions. Our experiments revealed that copper and iron ions accelerated the oxidation rate of Ti3C2Tx 55.4 and 33.4 times, respectively. TiO2 and amorphous carbon were identified as the primary solid products. Based on in situ water-phase atomic force microscopy, atomic high-angle annular dark-field scanning transmission electron microscopy, and theoretical results, we postulate that metal ions enhance Ti3C2Tx oxidation by spontaneously migrating and anchoring at Ti vacancies, which then become active sites for this reaction. This process increases the adsorption of H2O and oxygen, making the Ti vacancy-rich surface convex area the most vulnerable site to attack. The findings in this study provide useful information for a comprehensive understanding of the interaction between MXene structural defects and metal ions as well as for the design and modification of MXene membranes resistant to metal ion impact.

Molten Salt-Assisted Synthesis of Catalysts for Energy Conversion

A breakthrough in manufacturing procedures often enables people to obtain the desired functional materials. For the field of energy conversion, designing and constructing catalysts with high cost-effectiveness is urgently needed for commercial requirements. Herein, the molten salt-assisted synthesis (MSAS) strategy is emphasized, which combines the advantages of traditional solid and liquid phase synthesis of catalysts. It not only provides sufficient kinetic accessibility, but effectively controls the size, morphology, and crystal plane features of the product, thus possessing promising application prospects. Specifically, the selection and role of the molten salt system, as well as the mechanism of molten salt assistance are analyzed in depth. Then, the creation of the catalyst by the MSAS and the electrochemical energy conversion related application are introduced in detail. Finally, the key problems and countermeasures faced in breakthroughs are discussed and look forward to the future. Undoubtedly, this systematical review and insights here will promote the comprehensive understanding of the MSAS and further stimulate the generation of new and high efficiency catalysts.

A Universal In-Situ Interfacial Growth Strategy for Various MXene-Based van der Waals Heterostructures with Uniform Heterointerfaces: The Efficient Conversion from 3D Composite to 2D Heterostructures

Two-dimensional (2D) van der Waals heterostructures endow individual 2D material with the novel functional structures, intriguing compositions, and fantastic interfaces, which efficiently provide a feasible route to overcome the intrinsic limitations of single 2D components and embrace the distinct features of different materials. However, the construction of 2D heterostructures with uniform heterointerfaces still poses significant challenges. Herein, a universal in-situ interfacial growth strategy is designed to controllably prepare a series of MXene-based tin selenides/sulfides with 2D van der Waals homogeneous heterostructures. Molten salt etching by-products that are usually recognized as undesirable impurities, are reasonably utilized by us to efficiently transform into different 2D nanostructures via in-situ interfacial growth. The obtained MXene-based 2D heterostructures present sandwiched structures and lamellar interlacing networks with uniform heterointerfaces, which demonstrate the efficient conversion from 3D composite to 2D heterostructures. Such 2D heterostructures significantly enhance charge transfer efficiency, chemical reversibility, and overall structural stability in the electrochemical process. Taking 2D-SnSe2/MXene anode as a representative, it delivers outstanding lithium storage performance with large reversible capacities and ultrahigh capacity retention of over 97% after numerous cycles at 0.2, 1.0, and 10.0 A g-1 current density, which suggests its tremendous application potential in lithium-ion batteries.

Large-scale conformal synthesis of one-dimensional MAX phases

MAX phases, a unique class of layered ternary compounds, along with their two-dimensional derivatives, MXenes, have drawn considerable attention in many fields. Notably, their one-dimensional (1D) counterpart exhibits more distinct properties and enhanced assemblability for broader applications. We propose a conformal synthetic route for 1D-MAX phases fabrication by integrating additional atoms into nanofibers template within a molten salt environment, enabling in-situ crystalline transformation. Several 1D-MAX phases are successfully synthesized on a large scale. Demonstrating its potential, a copper-based layer-by-layer composites containing 1% by volume of 1D-Ti2AlC reinforced phase achieves an impressive 98 IACS% conductivity and a friction coefficient of 0.08, while maintaining mechanical properties comparable to other Cu-MAX phase composites, making it suitable for advanced industrial areas. This strategy may promise opportunities for the fabrication of various 1D-MAX phases.

Coupling CoIr Nanoalloys with MXene by Lewis Acidic Molten Salt Etching for Wide-pH-Environment Hydrogen Evolution Reaction

Metal/MXene-based materials show broad prospects in energy conversation through the strong metal-support interaction (SMSI). However, the difficulty and harshness of synthesis heavily limit their further application. Herein, using Lewis acidic molten salt to etch MAX as a precursor of MXene, a more convenient and safer strategy is designed to in situ construct the MXene-supported CoIr nanoalloy (CoIr/MXene) catalyst through Ti─O─M bond. The special layered structure and oxygen-containing functional group of MXene regulate the SMSI upon CoIr nanoalloys. Moreover, the contact angle and in situ Raman test results exhibit good interface hydrophilicity of MXene, enhancing the water adsorption on interfaces, and accelerating the mass transfer process. As a result, CoIr/MXene shows high hydrogen evolution reaction (HER) performance, which only needs overpotentials of 34 and 50 mV to drive a current density of 10 mA cm-2 in alkaline and acidic media, respectively, with excellent stability. Especially, in alkaline media, CoIr/MXene possesses 6 times higher HER mass activity (4.297 A mgIr -1) than commercial Pt/C catalysts (0.686 A mgPt -1) at the potential of 50 mV, indicating larger active site density and intrinsic activity for CoIr/MXene. This work expands the application of the molten salt assist etching strategy and provides new insight for the development of metal/MXene-based catalysts.

Pivotal Role of Surface Terminations in MXene Thermodynamic Stability

MXenes, i.e., two-dimensional transition metal carbides and nitrides, have been reported as promising materials for various applications, including energy storage, biomedicine, and electronics. The family of MXenes has proliferated, and the chemical space of synthesized MXenes has expanded to 13 transition metals and a dozen elements in surface terminations. The diverse chemistry of MXenes enables systematical tuning of MXene properties to meet the needs of target applications. However, synthesizing new MXene compositions largely relies on a trial-and-error approach. To overcome it, computational predictions of MXene compositions that are thermodynamically stable are crucial to rationalize experimental efforts. Here, we report a comprehensive computational screening for thermodynamically stable MXenes across 29 transition metals and 11 surface terminations. Density functional theory calculations are employed to compute the energy above the convex energy hull as a descriptor of thermodynamic stability. The results are analyzed to explore factors crucial for determining the thermodynamic stability of MXenes, by which the chemistry of surface terminations is found to play a crucial role. The insights on the chemistry of 998 MXene compositions predicted to be (meta)stable are given to systematically guide further research on MXene synthesis and application.

Hydrogen Evolution Reaction Activity in Mo2TiC2Tx MXene Derived from Mo2TiAlC2 MAX Phase: Insights from Compositional Transformations

MAX phases represent a crucial building block for the synthesis of MXenes, which constitute an intriguing class of materials with significant application potential. This study investigates the catalytic properties of the Mo2TiAlC2 MAX phase and the corresponding Mo2TiC2T x MXene for the hydrogen evolution reaction (HER). Characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) revealed that despite the presence of secondary phases, the HER catalytic activity is primarily influenced by the MAX phase and its derived MXene. Interestingly, the catalytic activity of the MXene improves over time, attributed to the formation of MoO2 as identified by XPS. This work enhances the understanding of MXene-based materials for electrochemical applications, highlighting crucial structural and chemical transformations that optimize their performance in energy conversion technologies.

Ultrafast Synthesis of MXenes in Minutes via Low-Temperature Molten Salt Etching

Developing green and efficient preparation strategies is a persistent pursuit in the field of 2D transition metal nitrides and/or carbides (MXenes). Traditional etching methods, such as HF-based or high-temperature Lewis-acid-molten-salt etching route, require harsher etching conditions and exhibit lower preparation efficiency with limited scalability, severely constraining their commercial production and practical application. Here, an ultrafast low-temperature molten salt (LTMS) etching method is presented for the large-scale synthesis of diverse MXenes within minutes by employing NH4HF2 as the etchant. The increased thermal motion and improved diffusion of molten NH4HF2 molecules significantly expedite the etching process of MAX phases, thus achieving the preparation of Ti3C2Tx MXene in just 5 minutes. The universality of the LTMS method renders it a valuable approach for the rapid synthesis of various MXenes, including V4C3Tx, Nb4C3Tx, Mo2TiC2Tx, and Mo2CTx. The LTMS method is easy to scale up and can yield more than 100 g Ti3C2Tx in a single reaction. The obtained LTMS-MXene exhibits excellent electrochemical performance for supercapacitors, evidently proving the effectiveness of the LTMS method. This work provides an ultrafast, universal, and scalable LTMS etching method for the large-scale commercial production of MXenes.

Physicochemical Modulations in MXenes for Carbon Dioxide Mitigation and Hydrogen Generation: Tandem Dialogue between Theoretical Anticipations and Experimental Evidences

The dawn of MXenes has fascinated researchers under their intriguing physicochemical attributes that govern their energy and environmental applications. Modifications in the physicochemical properties of MXenes pave the way for efficient energy-driven operations such as carbon capture and hydrogen generation. The physicochemical modulations such as interface engineering through van der Waals coupling with homo/hetero-junctions render the tunability of optoelectronic variables driving the photochemical and electrochemical processes. Herein, we have reviewed the recent achievements in physicochemical properties of MXenes by highlighting the role of intercalants/terminal groups, atomic defects, surface chemistry and few/mono-layer formation. Recent findings of MXenes-based materials are systematically surveyed in a tandem manner with the future outlook for constructing next-generation multi-functional catalytic systems. Theoretical modelling of MXenes surface engineering proffers the mechanistic comprehension of surface phenomena such as termination, interface formation, doping and functionalization, thereby enabling the researchers to exploit them for targeted applications. Therefore, theoretical anticipations and experimental evidences of electrochemical/photochemical carbon dioxide reduction and hydrogen evolution reactions are synergistically discussed.

XPS Binding Energy Shifts in 2D Ti3C2Tz MXene go largely Beyond Intuitive Explanations: Rationalization from DFT Simulations and Experiments

MXenes are prototypes of surface tunable 2D materials with vast potential for properties tuning. Accurately characterizing their surface functionalization and its role in electronic structure is crucial, X-ray photoelectron spectroscopy (XPS) being among the go-to methods to do so. Despite extensive use, XPS analysis remains however intricate. Focusing on the benchmark MXene Ti3C2Tz, Density Functional Theory (DFT) calculations of core-level binding energy shifts (BE.s.) are combined with experiments in order to provide a quantitative interpretation of XPS spectra. This approach demonstrates that BE.s. are driven by the complex interplay between chemical, structural, and subtle electronic structure effects preventing analysis from intuitive arguments or comparison with reference materials. In particular, it is shown that O terminations induce the largest BE.s. at Ti 2p levels despite lower electronegativity than F. Additionally, F 1s levels show weak sensitivity to the F local environment, explaining the single contribution in the spectrum, whereas O 1s states are significantly affected by the local surface chemistry. Finally, clear indicators of surface group vacancies are given at Ti 2p and O 1s levels. These results demonstrate the combination of calculations with experiments as a method of the highest value for MXenes XPS spectra analysis, providing guidelines for otherwise complex interpretations.

Recent developments of artificial intelligence in MXene-based devices: from synthesis to applications

Two-dimensional transition metal carbides, nitrides, or carbonitrides (MXenes) have garnered remarkable attention in various energy and environmental applications due to their high electrical conductivity, good thermal properties, large surface area, high mechanical strength, rapid charge transport mechanism, and tunable surface properties. Recently, artificial intelligence has been considered an emerging technology, and has been widely used in materials science, engineering, and biomedical applications due to its high efficiency and precision. In this review, we focus on the role of artificial intelligence-based technology in MXene-based devices and discuss the latest research directions of artificial intelligence in MXene-based devices, especially the use of artificial intelligence-based modeling tools for energy storage devices, sensors, and memristors. In addition, emphasis is given to recent progress made in synthesis methods for various MXenes and their advantages and disadvantages. Finally, the review ends with several recommendations and suggestions regarding the role of artificial intelligence in fabricating MXene-based devices. We anticipate that this review will provide guidelines on future research directions suitable for practical applications.

The synthesis of 3D organic-inorganic hybrid heterojunctions of g-C3N4 nanoneedles@Ti3C2 MXene with superior photocatalytic H2 evolution

3D organic-inorganic hybrid heterojunctions of 1D carbon nitride (g-C3N4) nanoneedles anchoring on 2D Ti3C2 MXene nanosheets have been constructed via a green molten salt treatment. The Ti3C2 MXene nanosheets and molten salt treatment guide the formation of ordered nanoneedle-like g-C3N4. Both the formed heterojunctions and the nanoneedle-like structures contribute charge separation and transfer for superior photocatalytic H2 evolution.

Tuning Nanochannel Microenvironments of a Thermoresponsive MXene Membrane for Mixed Molecule Gradient Separation

Drawing inspiration from dynamic biological ion channels, researchers have developed various artificial membranes featuring responsive nanochannels. Typically, these membranes modify mass transport behaviors by manipulating the responsive layer on the inner surfaces of the intrinsic layer. In this study, we build two-dimensional lamellar membranes composed of titanium carbide MXene and poly(N-isopropylacrylamide), endowed with dual-level regulatable nanochannels, achieved through adjustments of nanochannel microenvironments. The size of these two-dimensional nanochannels can be altered by both the thermoresponsive polymer layer and the intrinsic MXene layer that could undergo spontaneous oxidation. The multilevel regulation strategy substantially enhances the molecular selectivity of MXene separation membranes, which is further applied for precise gradient separation toward multiple molecules. This advancement showcases the versatility and transformative capabilities of responsive nanochannel technology, setting the stage for innovative developments in diverse fields.

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

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

Facile synthesis of Eu3+-doped niobium carbide MXene quantum dots for parallel detection of hypoxanthine and fluoxetine via fluorescence quenching and enhancement mechanisms

A hydrothermal synthetic method is established to produce blue fluorescent Eu3+-doped niobium carbide MXene quantum dots (Eu3+-Nb2C MQDs). The synthesized Eu3+-Nb2C MQDs demonstrated a quantum yield of 20.61% and a maximum emission intensity at 405 nm. The as-prepared Eu3+-Nb2C MQDs acted as a sensor for the rapid and sensitive detection of hypoxanthine through fluorescence quenching, and of fluoxetine through fluorescence enhancement mechanisms. The emission peak of Eu3+-Nb2C MQDs at 405 nm exhibited a linear response for hypoxanthine and fluoxetine in the ranges of 0.5-25 µM and 0.125-2.5 µM, with detection limits of 15.0 and 3.7 nM, respectively. The newly developed probe was effectively used for the selective detection of hypoxanthine and fluoxetine in biofluids and pharmaceutical samples. Remarkably, the Eu3+-Nb2C MQDs exhibited minimal cytotoxicity towards A549 lung cancer cells and showed great potential as imaging agent for imaging of Saccharomyces cerevisiae cells.

Synergistic integration of MXene nanostructures into electrospun fibers for advanced biomedical engineering applications

MXene-based architectures have paved the way in various fields, particularly in healthcare area, owing to their remarkable physiochemical and electromagnetic characteristics. Moreover, the modification of MXene structures and their combination with polymeric networks have gained considerable prominence to further develop their features. The combination of electrospun fibers with MXenes would be promising in this regard since electrospinning is a well-established technique that is now being directed toward commercial biomedical applications. The introduction of MXenes into electrospun fibrous frameworks has highlighted outcomes in various biomedical applications, including cancer therapy, controlled drug delivery, antimicrobial targets, sensors, and tissue engineering. Correspondingly, this review describes the employed strategies for the preparation of electrospun configurations in tandem with MXene nanostructures with remarkable characteristics. Next, the advantages of MXene-decorated electrospun fibers for use in biomedical applications are comprehensively discussed. According to the investigations, rich surface functional groups, hydrophilicity, large surface area, photothermal features, and antimicrobial and antibacterial activities of MXenes could synergize the performance of electrospun layers to engineer versatile biomedical targets. Moreover, the future of this path is clarified to combat the challenges related to the electrospun fibers decorated with MXene nanosheets.

Rechargeable Seawater-Based Chloride-Ion Batteries Enabled by Covalent Surface Chemistry in MXenes

Rechargeable aqueous chloride-ion batteries (ACIBs) using Cl- ions as charge carriers represent a promising energy-storage technology, especially when natural seawater is introduced as the electrolyte, which can bring remarkable advantages in terms of cost-effectiveness, safety, and environmental sustainability. However, the implementation of this technology is hindered by the scarcity of electrodes capable of reversible chloride-anion storage. Here, we show that a Ti3C2Clx MXene with Cl surface terminations enables reversible Cl- ion storage in aqueous electrolytes. Further, we developed seawater-based ACIBs that show a high specific capacity and an exceptionally long lifespan (40000 cycles, more than 1 year) in natural seawater electrolyte. The pouch-type cells achieve a high energy density (50 Wh Lcell-1) and maintain stable performance across a broad temperature range (-20 to 50 °C). Our investigations reveal that the covalent interaction between Cl- ions and Cl-terminated MXene facilitates Cl- ion intercalation into the MXene interlayer, promoting rapid ion migration with a low energy barrier (0.10 eV). Moreover, this MXene variant also enables the reversible storage of Br- ions in an aqueous electrolyte with a long cycle life. This study may advance the design of anion storage electrodes and enable the development of sustainable aqueous batteries.

Heteroatom Immobilization Engineering toward High-Performance Metal Anodes

Heteroatom immobilization engineering (HAIE) is becoming a forefront approach in materials science and engineering, focusing on the precise control and manipulation of atomic-level interactions within heterogeneous systems. HAIE has emerged as an efficient strategy to fabricate single-atom sites for enhancing the performance of metal-based batteries. Despite the significant progress achieved through HAIE in metal anodes for metal-based batteries, several critical challenges such as metal dendrites, side reactions, and sluggish reaction kinetics are still present. In this review, we delve into the fundamental principles underlying heteroatom immobilization engineering in metal anodes, aiming to elucidate its role in enhancing the electrochemical performance in batteries. We systematically investigate how HAIE facilitates uniform nucleation of metal in anodes, how HAIE inhibits side reactions at the metal anode-electrolyte interface, and the role of HAIE in promoting the desolvation of metal ions and accelerating reaction kinetics within metal-based batteries. Finally, we discuss various strategies for implementing HAIE in electrode materials, such as high-temperature pyrolysis, vacancy reduction, and molten-salt etching and anchoring. These strategies include selecting appropriate heteroatoms, optimizing immobilization methods, and constructing material architectures. They can be utilized to further refine the performance to enhance the capabilities of HAIE and facilitate its widespread application in next-generation metal-based battery technologies.

Fabrication Methods, Structural, Surface Morphology and Biomedical Applications of MXene: A Review

Recently, two-dimensional (2-D) layered materials have revealed outstanding properties and play a crucial role for numerous advanced applications. The emerging transition metal carbides and nitrides, known as MXene with empirical formula Mn+1XnTx, have generated widespread attention and demonstrated impressive potential in various fields. The fabrication of 2-D novel MXene and its composites and their characterizations are applicable to vast applications in different areas such as energy storage, gas sensors, catalysis, and biomedical applications. In this review, the main focus is on the various synthesis methods, their properties, and biomedical applications. This review provides detailed illustrations of MXenes for many biomedical applications, including bioimaging, drug delivery, therapies, biosensors, tissue engineering, and antibacterial reagents. The challenges and future prospects were highlighted in a comprehensive manner, and the existing problems and potential for MXene-based biomaterials were analyzed with the goal of accelerating their use in the biomedical field.

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.

Chemical Intercalation of Layered Materials: From Structure Tailoring to Applications

The intercalation of layered materials offers a flexible approach for tailoring their structures and generating unexpected properties. This review provides perspectives on the chemical intercalation of layered materials, including graphite/graphene, transition metal dichalcogenides, MXenes, and some particular materials. The characteristics of the different intercalation methods and their chemical mechanisms are discussed. The influence of intercalation on the structural changes of the host materials and the structural change how to affect the intrinsic properties of the intercalation compounds are discussed. Furthermore, a perspective on the applications of intercalation compounds in fields such as energy conversion and storage, catalysis, smart devices, biomedical applications, and environmental remediation is provided. Finally, brief insights into the challenges and future opportunities for the chemical intercalation of layered materials are provided.

Heterointerface engineering of MXene: Advanced applications in environmental remediation

Contemporary global industrialization, coupled with the relentless growth of the population, has led to a persistent escalation in the emission and accumulation of various toxic and harmful chemicals in the environment, severely disrupting the ecological balance. The development of efficient environmental cleanup materials is a crucial scientific and technological concern. Since the groundbreaking work on Ti3C2Tx in 2011, there has been a huge growing interest in MXene-based composites developed through heterointerface engineering due to its high surface area, hydrophilicity, eco-friendliness, biocompatibility, easy functionalization, excellent thermal/mechanical properties, metal conductivity and rich electronic density. In the area of environmental remediation, MXene-based composites obtained through heterointerface engineering strategies have the ability to effectively remove and systematically monitor contaminants in comparison to virgin MXene, thanks to the synergistic effects and complementary benefits. Heterointerface engineering strategy increases specific surface area, introduces catalytic sites, constructs heterojunctions/Schottky junctions, and facilitates carrier migration and electron-hole separation. These novel MXene-based composites represent significant advances in MXene research and deserve a comprehensive review. Although several excellent reviews and perspectives on the application of MXene-based composites in environmental remediation have been published, there is still a scarcity of comprehensive and systematic assessments on the reliable data and mechanisms of various MXene-based composite materials for pollutant removal and monitoring. In this focused review, the first part briefly introduces the common preparation strategies and characterization methods of single MXene and MXene-based composites, and the second part details the innovative application of MXene-based composites (involving the amalgamation of MXene with metal oxides, metal sulfide, g-C3N4, layered double hydroxides, metal-organic frameworks, single atom/quantum dots, polymers, etc.) in the field of environmental remediation, including carbon dioxide reduction, nitrogen monoxide and volatile organic compounds removal, antibiotic and heavy metal ions degradation, summarizing the relevant performance and mechanisms. Furthermore, the recent advancements in the utilization of MXene-based composites for the sensing of emerging environmental contaminants (antibiotic and antibiotic resistance genes) are summarized. Finally, an outline of the existing challenges and future prospects on this exciting field was narrated for plausible real-world use. This review will help to inspire the diverse design of MXene-based composites and to advance research related to their application in the environmental sector.

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 of MXene@MOF composites for catalytic water splitting and wastewater treatment approaches

MXenes are a group of 2D material which have been derived from the layered transition metal nitrides and carbides and have the characteristics like electrical conductivity, high surface area and variable surface chemical composition. Self-assembly of clusters/metal ions and organic linkers forms metal organic framework (MOF). Their advantages of ultrahigh porosity, highly exposed active sites and many pore architectures have garnered them a lot of attention. But poor conductivity and instability plague several conventional MOF. To address the issue, MOF can be linked with MXenes that have rich surface functional groups and excellent electrical conductivity. In this review, different etching methods for exfoliation of MXene along with the synthesis methods of MXene/MOF composites are reviewed, including hydrothermal method, solvothermal method, in-situ growth method, and self-assembly method. Moreover, application of these MXene/MOF composites for catalytic water splitting and wastewater treatment were also discussed in details. In addition to increasing a single MOF conductivity and stability, MXenes can add a variety of new features, such the template effect. Due to these benefits, MXene/MOF composites can be effectively used in several applications, including photocatalytic/electrocatalytic water splitting, adsorption and degradation of pollutants from wastewater. Finally, the authors explored the current challenges and the future opportunities to improve the efficiency of MXene/MOF composites.

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.

Exploring Cr and molten salt interfacial interactions for molten salt applications

Molten salts play an important role in various energy-related applications such as high-temperature heat transfer fluids and reaction media. However, the extreme molten salt environment causes the degradation of materials, raising safety and sustainability challenges. A fundamental understanding of material-molten salt interfacial evolution is needed. This work studies the transformation of metallic Cr in molten 50/50 mol% KCl-MgCl2via multi-modal in situ synchrotron X-ray nano-tomography, diffraction and spectroscopy combined with density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Notably, in addition to the dissolution of Cr in the molten salt to form porous structures, a δ-A15 Cr phase was found to gradually form as a result of the metal-salt interaction. This phase change of Cr is associated with a change in the coordination environment of Cr at the interface. DFT and AIMD simulations provide a basis for understanding the enhanced stability of δ-A15 Cr vs. bcc Cr, by revealing their competitive phase thermodynamics at elevated temperatures and probing the interfacial behavior of the molten salt at relevant facets. This study provides critical insights into the morphological and chemical evolution of metal-molten salt interfaces. The combination of multimodal synchrotron analysis and atomic simulation also offers an opportunity to explore a broader range of systems critical to energy applications.

Universal Formation of Single Atoms from Molten Salt for Facilitating Selective CO2 Reduction

Clarifying the formation mechanism of single-atom sites guides the design of emerging single-atom catalysts (SACs) and facilitates the identification of the active sites at atomic scale. Herein, a molten-salt atomization strategy is developed for synthesizing zinc (Zn) SACs with temperature universality from 400 to 1000/1100 °C and an evolved coordination from Zn-N2Cl2 to Zn-N4. The electrochemical tests and in situ attenuated total reflectance-surface-enhanced infrared absorption spectroscopy confirm that the Zn-N4 atomic sites are active for electrochemical carbon dioxide (CO2) conversion to carbon monoxide (CO). In a strongly acidic medium (0.2 m K2SO4, pH = 1), the Zn SAC formed at 1000 °C (Zn1NC) containing Zn-N4 sites enables highly selective CO2 electroreduction to CO, with nearly 100% selectivity toward CO product in a wide current density range of 100-600 mA cm-2. During a 50 h continuous electrolysis at the industrial current density of 200 mA cm-2, Zn1NC achieves Faradaic efficiencies greater than 95% for CO product. The work presents a temperature-universal formation of single-atom sites, which provides a novel platform for unraveling the active sites in Zn SACs for CO2 electroreduction and extends the synthesis of SACs with controllable coordination sites.

Tuning the Surface Stability and Li/Na Storage of MXenes by Controlling the Surface Termination Coverage

2D transition metal carbides and/or nitrides, MXenes, are a class of widely studied materials with great potential for energy storage applications. The control of surface chemistry is an effective approach for preparing novel MXenes and modifying their electrochemical properties. However, an in-depth and systematic atomic-scale study of the effect of surface termination on MXene stability and electrochemical performance is scarce and thus is highly desired. Here, through high-throughput first-principles calculations, 28 stable chalcogen-functionalized M2CTz (M = V, Nb, and Ta, T = S, Se, and Te) under different chemical environments are identified. The reduction of termination coverage improves electrical conductivity but weakens in-plane stiffness. Intriguingly, based on charge transfer mechanism, the diffusion barrier of lithium/sodium atoms on the M2CTz exhibits a volcano-like relationship with termination coverage, and the ion diffusion channel formed in half termination coverage greatly accelerates lithium ion diffusion and returns to or exceeds sodium ion diffusion rate at full termination coverage. V2CSe2/Nb2CSz not only displays the large lithium/sodium capacity (592/409-466 mAhg-1) but also exhibits low barrier energy and open-circuit voltage, suggesting a promising candidate anode material for lithium/sodium-ion batteries. These findings provide insights into the design and fabrication of MXenes and tuning the electrochemical performance of MXenes by controlling termination coverage.

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.

Two-Dimensional MXene-Based Electrocatalysts: Challenges and Opportunities

MXene, regarded as cutting-edge two-dimensional (2D) materials, have been widely explored in various applications due to their remarkable flexibility, high specific surface area, good mechanical strength, and interesting electrical conductivity. Recently, 2D MXene has served as a ideal platform for the design and development of electrocatalysts with high activity, selectivity, and stability. This review article provides a detailed description of the structural engineering of MXene-based electrocatalysts and summarizes the uses of 2D MXene in hydrogen evolution reactions, nitrogen reduction reactions, oxygen evolution reactions, oxygen reduction reactions, and methanol/ethanol oxidation. Then, key issues and prospects for 2D MXene as a next-generation platform in fundamental research and real-world electrocatalysis applications are discussed. Emphasis will be given to material design and enhancement techniques. Finally, future research directions are suggested to improve the efficiency of MXene-based electrocatalysts.

Emerging 2D Nanomaterials-Integrated Hydrogels: Advancements in Designing Theragenerative Materials for Bone Regeneration and Disease Therapy

This review highlights recent advancements in the synthesis, processing, properties, and applications of 2D-material integrated hydrogels, with a focus on their performance in bone-related applications. Various synthesis methods and types of 2D nanomaterials, including graphene, graphene oxide, transition metal dichalcogenides, black phosphorus, and MXene are discussed, along with strategies for their incorporation into hydrogel matrices. These composite hydrogels exhibit tunable mechanical properties, high surface area, strong near-infrared (NIR) photon absorption and controlled release capabilities, making them suitable for a range of regeneration and therapeutic applications. In cancer therapy, 2D-material-based hydrogels show promise for photothermal and photodynamic therapies, and drug delivery (chemotherapy). The photothermal properties of these materials enable selective tumor ablation upon NIR irradiation, while their high drug-loading capacity facilitates targeted and controlled release of chemotherapeutic agents. Additionally, 2D-materials -infused hydrogels exhibit potent antibacterial activity, making them effective against multidrug-resistant infections and disruption of biofilm generated on implant surface. Moreover, their synergistic therapy approach combines multiple treatment modalities such as photothermal, chemo, and immunotherapy to enhance therapeutic outcomes. In bio-imaging, these materials serve as versatile contrast agents and imaging probes, enabling their real-time monitoring during tumor imaging. Furthermore, in bone regeneration, most 2D-materials incorporated hydrogels promote osteogenesis and tissue regeneration, offering potential solutions for bone defects repair. Overall, the integration of 2D materials into hydrogels presents a promising platform for developing multifunctional theragenerative biomaterials.

MXene-Bimetallic Hybrids via Mixed Molten Salts Etching for Kinetics-Enhanced and Dendrite-Free Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries with high theoretical energy density (2600 Wh kg-1) are considered to be one of the most promising secondary batteries. However, the practical application of Li-S batteries is limited by the polysulfides shuttling and unstable lithium metal anodes. Herein, an asymmetric separator (CACNM@PP), composed of Co-Ni/MXene (CNM) on the cathode and Cu-Ag/MXene (CAM) on the anode for high-performance Li-S batteries is reported. For the cathode, CNM provides a synergistic effect by integrating Co, Ni, and MXene, resulting in strong chemical interactions and fast conversion kinetics for polysulfides. For the anode, CAM with abundant lithiophilicity active sites can lower the nucleation barrier of Li. Moreover, LiCl/LiF layers are generated in situ as an ion conductor layer during charging and discharging, inducing a uniform deposition of Li. Therefore, the assembled cells with the CACNM@PP separators harvest excellent electrochemical performance. This work provides novel insights into the development of commercially available high-energy density Li-S batteries with asymmetric separators.

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.

First-principles study of electronic, mechanical, and optical properties of M3GaB2 (M = Ti, Hf) MAX phases

Integrating ceramic and metallic properties in MAX phases makes them highly desirable for diverse technological applications. In this study, through first-principles density functional theory (DFT), we investigated the physical properties of two new 312 MAX compounds, M3GaB2 (M = Ti, Hf). Chemical stability is confirmed via formation energy assessment, while mechanical stability is established by determining elastic stiffness constants. A thorough analysis of mechanical behaviors includes bulk modulus, shear modulus, Young's modulus, and hardness parameters. M3GaB2 demonstrates elastic constants and moduli closely aligned with other 312 carbides. Understanding the electronic band structure and density of states (DOS) sheds light on metallic properties, with anisotropy in electrical conductivity clarified through energy dispersion analysis. Investigation of photon interaction with titled compounds, including dielectric constants (real and imaginary parts), refractive index, absorption coefficient, photoconductivity, reflectivity, and energy loss function, has been carried out. The potential of M3GaB2 borides as a coating to reduce solar is evaluated based on the reflectivity spectra. These findings deepen our understanding of material properties and suggest diverse applications for M3GaB2 in various technological domains.

A review of etching methods and applications of two-dimensional MXenes

MXenes have been attracting much attention since their introduction due to their amazing properties such as unique structure, good hydrophilicity, metal-grade electrical conductivity, rich surface chemistry, low ionic diffusion resistance, and excellent mechanical strength. It is noteworthy that different synthesis methods have a great influence on the structure and properties of MXenes. In recent years, some modification strategies of MXenes with unique insights have been developed with the increasing research. In summary, this paper reviews and summarizes the recent research progress of MXenes from the perspective of preparation processes (including hydrofluoric acid direct etching, fluoride/concentrated acid hybrid etching, fluoride melt etching, electrochemical etching, alkali-assisted etching and Lewis acid etching strategies), which can provide valuable guidance for the preparation and application of high-performance MXenes-based materials.

Phase transition and electrochemical properties of S-functionalized MXene anodes for Li-ion batteries: a first-principles investigation

The advancement of anode materials for achieving high energy storage is a crucial topic for high-performance Li-ion batteries (LIBs). Here, first-principles calculations were used to conduct a thorough and systematic investigation into lithium storage properties of MXenes with new S functional groups as LIB anode materials. Density of states, diffusion energy barriers, open circuit voltages and storage capacities were calculated to comprehensively evaluate the lithium storage properties of S-functionalized MXenes. Based on the computational results, Ti2CS2 and V2CS2 were selected as excellent candidates from ten M2CS2 MXenes. The diffusion energy barriers of M2CS2 within the range of 0.26-0.32 eV are lower than those of M2CO2 and M2CF2, indicating that M2CS2 anodes exhibit faster charge/discharge rates. By examining the stable crystal structures and comparing atomic positions before and after Li adsorptions, structural phase transitions during Li-ion adsorptions could happen for nearly all M2CS2 MXenes. The phase transitions predicted were directly observed using ab initio molecular dynamic simulations. The cycle stability, storage capacity and other lithium storage properties were enhanced by the reversible structural phase transition.

Low-temperature synthesis of cation-ordered bulk Zn3WN4 semiconductor via heterovalent solid-state metathesis

Metathesis reactions are widely used in synthetic chemistry. While state-of-the-art organic metathesis involves highly controlled processes where specific bonds are broken and formed, inorganic metathesis reactions are often extremely exothermic and, consequently, poorly controlled. Ternary nitrides offer a technologically relevant platform for expanding synthetic control of inorganic metathesis reactions. Here, we show that energy-controlled metathesis reactions involving a heterovalent exchange are possible in inorganic nitrides. We synthesized Zn3WN4 by swapping Zn2+ and Li+ between Li6WN4 and ZnX2 (X = Br, Cl, F) precursors. The in situ synchrotron powder X-ray diffraction and differential scanning calorimetry show that the reaction onset is correlated with the ZnX2 melting point and that product purity is inversely correlated with the reaction's exothermicity. Therefore, careful choice of the halide counterion (i.e., ZnBr2) allows the synthesis to proceed in a swift but controlled manner at a surprisingly low temperature for an inorganic nitride (300 °C). High resolution synchrotron powder X-ray diffraction and diffuse reflectance spectroscopy confirm the synthesis of a cation-ordered Zn3WN4 semiconducting material. We hypothesize that this synthesis strategy is generalizable because many Li-M-N phases are known (where M is a metal) and could therefore serve as precursors for metathesis reactions targeting new ternary nitrides. This work expands the synthetic control of inorganic metathesis reactions in a way that will accelerate the discovery of novel functional ternary nitrides and other currently inaccessible materials.

MXene Key Composites: A New Arena for Gas Sensors

With the development of science and technology, the scale of industrial production continues to grow, and the types and quantities of gas raw materials used in industrial production and produced during the production process are also constantly increasing. These gases include flammable and explosive gases, and even contain toxic gases. Therefore, it is very important and necessary for gas sensors to detect and monitor these gases quickly and accurately. In recent years, a new two-dimensional material called MXene has attracted widespread attention in various applications. Their abundant surface functional groups and sites, excellent current conductivity, tunable surface chemistry, and outstanding stability make them promising for gas sensor applications. Since the birth of MXene materials, researchers have utilized the efficient and convenient solution etching preparation, high flexibility, and easily functionalize MXene with other materials to prepare composites for gas sensing. This has opened a new chapter in high-performance gas sensing materials and provided a new approach for advanced sensor research. However, previous reviews on MXene-based composite materials in gas sensing only focused on the performance of gas sensing, without systematically explaining the gas sensing mechanisms generated by different gases, as well as summarizing and predicting the advantages and disadvantages of MXene-based composite materials. This article reviews the latest progress in the application of MXene-based composite materials in gas sensing. Firstly, a brief summary was given of the commonly used methods for preparing gas sensing device structures, followed by an introduction to the key attributes of MXene related to gas sensing performance. This article focuses on the performance of MXene-based composite materials used for gas sensing, such as MXene/graphene, MXene/Metal oxide, MXene/Transition metal sulfides (TMDs), MXene/Metal-organic framework (MOF), MXene/Polymer. It summarizes the advantages and disadvantages of MXene composite materials with different composites and discusses the possible gas sensing mechanisms of MXene-based composite materials for different gases. Finally, future directions and inroads of MXenes-based composites in gas sensing are presented and discussed.

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.

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.

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.

Halogen-powered static conversion chemistry

Halogen-powered static conversion batteries (HSCBs) thrive in energy storage applications. They fall into the category of secondary non-flow batteries and operate by reversibly changing the chemical valence of halogens in the electrodes or/and electrolytes to transfer electrons, distinguishing them from the classic rocking-chair batteries. The active halide chemicals developed for these purposes include organic halides, halide salts, halogenated inorganics, organic-inorganic halides and the most widely studied elemental halogens. Aside from this, various redox mechanisms have been discovered based on multi-electron transfer and effective reaction pathways, contributing to improved electrochemical performances and stabilities of HSCBs. In this Review, we discuss the status of HSCBs and their electrochemical mechanism-performance correlations. We first provide a detailed exposition of the fundamental redox mechanisms, thermodynamics, conversion and catalysis chemistry, and mass or electron transfer modes involved in HSCBs. We conclude with a perspective on the challenges faced by the community and opportunities towards practical applications of high-energy halogen cathodes in energy-storage devices.

Chemical Conversions within the Mo-Ga-C System: Layered Solids with Variable Ga Content

Layered carbides are fascinating compounds due to their enormous structural and chemical diversity, as well as their potential to possess useful and tunable functional properties. Their preparation, however, is challenging and forces synthesis scientists to develop creative and innovative strategies to access high-quality materials. One unique compound among carbides is Mo2Ga2C. Its structure is related to the large and steadily growing family of 211 MAX phases that crystallize in a hexagonal structure (space group P63/mmc) with alternating layers of edge-sharing M6X octahedra and layers of the A-element. Mo2Ga2C also crystallizes in the same space group, with the difference that the A-element layer is occupied by two A-elements, here Ga, that sit right on top of each other (hence named "221" compound). Here, we propose that the Ga content in this compound is variable between 2:2, 2:1, and 2: ≤1 (and 2:0) Mo/Ga ratios. We demonstrate that one Ga layer can be selectively removed from Mo2Ga2C without jeopardizing the hexagonal P63/mmc structure. This is realized by chemical treatment of the 221 phase Mo2Ga2C with a Lewis acid, leading to the "conventional" 211 MAX phase Mo2GaC. Upon further reaction with CuCl2, more Ga is removed and replaced with Cu (instead of fully exfoliating into the Ga-free Mo2CTx MXene), leading to Mo2Ga1-xCuxC still crystallizing with space group P63/mmc, however, with a significantly larger c-lattice parameter. Furthermore, 211 Mo2GaC can be reacted with Ga to recover the initial 221 Mo2Ga2C. All three reaction pathways have not been reported previously and are supported by powder X-ray diffraction (PXRD), electron microscopy, X-ray spectroscopy, and density functional theory (DFT) calculations.