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MXenes have been considered as promising electrode materials for capacitive deionization (CDI). However, the availability of space for trapping and storing ions is usually inhibited in Mxene-based assemblies due to their special lamellar structure, which affects their desalination performance. Herein, a kind of N-doped Ti3C2Tx electrode with unique interconnected porous structure (N-3DM-S) are designed to improve the active site accessibility of the electrodes. The Ti3C2Tx shell layers with flaws are prepared on spherical sacrificial templates, which then lead to the formation of interconnected N-doped porous architecture upon the heat treatment under NH3 atmosphere. The interconnected porous feature improves the availability of electrode materials, while the doping of nitrogen further enhances the accessibility of electrode to electrolyte in addition to the chemical electrochemical properties. As a result, the as-obtained N-3DM-S samples with more valid surface active sites exhibit excellent CDI performance compared to the control samples with isolated pore structure and the ones without N-doping. Especially, the optimal CDI capacities of N-3DM-S electrodes can reach up to 53 ± 2 mg/g in NaCl aqueous of 5000 mg/L at 1.6V. Moreover, such N-3DM-S electrodes exhibit good regeneration stability as well, indicating their potential applications in the field of desalination.

Heavy metal contamination has caused serious impacts on the environments and risks towards human health, promoting intensive R&D efforts for removal of heavy metals from their primary sources (industrial and agricultural wastewaters). Among all developed techniques, the adsorption removing approach has attracted the most attention. This article reviews the recent developments of adsorbents for removing heavy metal ions from wastewaters in the past decade (2011-2020). It starts with design principles, followed by the evaluation of adsorbents and their performances for removing various heavy metals. Mainly developed adsorbents include carbon materials (activated carbon, nanotubes, and graphene), polymers, metal compounds (nanoparticles, MXenes, metal-organic frameworks, and magnetic materials), carbon nitrides, and zeolites. The challenges are also discussed as the perspectives.

Cost-effective electrocatalysts are imperative to the large-scale hydrogen evolution reaction. Herein, 1T/2H–MoSe2/Ti3C2 composites are integrated via a hydrothermal method by taking advantage of the tunable phases of MoSe2 and high conductivity of Ti3C2. Consequently, the 1T/2H–MoSe2/Ti3C2 composites exhibit an optimal overpotential of 150 mV at 10 mA cm−2, a low Tafel slope of 90 mV dec−1, and a steady catalytic activity for over 30 h in an alkaline environment. The improved HER performances can be ascribed to the introduction of the Ti3C2 support in a proper proportion, which not only makes the charge transfer process more convenient but also helps the dispersion of MoSe2. Furthermore, the formation of MoSe2/Ti3C2 heterogeneous interfaces and the synergistic effects stemming from the intimate interactions between the individual components ultimately boost the H2O dissociation in the Volmer step. This study indicates that Ti3C2 is an ideal conductive support for improving HER catalytic activity, and paves a new way for other transition metal selenides and MXenes to build highly efficient HER electrocatalysts.

The emergent of 2D MXenes composed of transition metal carbides, nitrides or carbonitrides has brought spurious scientific interests in the recent years. Its unique layered structure and highly configured surface functional groups in its outmost periphery benefit for the electrochemical supercapacitors, sensors, photo catalysts, electro catalysts, and photo(electro)catalysts. The interlayer spacing of MXenes can provide enough space for the intercalation agent to be embedded. The embedding of different intercalators can not only adjust the conductivity, flexibility and capacitance of MXenes, but also can apply the intercalation and deintercalation of intercalators between MXenes layers to ion batteries and kidney dialysis. Futhermore, the embedded intercalant will further increase the interlayer spacing of MXenes, which is beneficial to the formation and further modification of MXenes. In this review, we summarize several typical intercalation methods of MXenes and their relevant applications in the development of photo-, electro-, and photo(electro)catalytic hydrogen evolution reaction (HER) in recent years, which can provide significant guidance for expanding the interlayer spacing to get thin-layered MXenes, designing and synthesizing high-active MXenes catalysts towards highly efficient HER performance.

The regulation of surface O-termination of Ti3C2Tx MXenes is a fascinating problem in the development of anodes for sodium ion batteries. However, the preparation of bulk Ti3C2Tx anodes with O-rich termination (O-Ti3C2Tx) is still a great challenge, due to their poor antioxidant properties in air. Moreover, the electrochemical behaviors of bulk O-Ti3C2Tx are less clear. In this study, bulk O-Ti3C2Tx anodes were prepared by a simple “hot melting-decomposition” strategy. The melt organic matter during heat treatment isolates bulk Ti3C2Tx from air, resulting in bulk Ti3C2Tx remaining stable up to 300 °C in air. Furthermore, we revealed the Na+ storage mechanism of bulk O-Ti3C2Tx electrodes by studying the structural evolution of both O-Ti3C2Tx and bulk Ti3C2Tx electrodes during the charge and discharge process. Surprisingly, it was found that bulk O-Ti3C2Tx easily evolved into active Ti3C2Tx nanoparticles, while bulk Ti3C2Tx formed an amorphous matrix. The unique role of O-termination in stabilizing nanocrystals and the anti-aggregation effect of the as-formed Na16Ti10O28 nanocrystals produced at the edge of Ti3C2Tx nanoparticles resulted in enhanced sodium storage performance in bulk O-Ti3C2Tx electrodes. The bulk O-Ti3C2Tx electrode can still deliver a reversible capacity of 153 mA h g−1 after 2500 cycles even at a current density as high as 1 A g−1, showing superior sodium storage performance. The proposed method could be a general approach to improve the antioxidant properties of MXenes in air. Moreover, our work lays the foundation for O-termination regulation of MXenes and their electrochemical mechanism.

The development of emulsion templated functional materials has achieved great progress in the past decades in academic and industrial fields. Recently, new building blocks such as graphene, transition metal carbides and nitrides (MXenes) and nanocellulose have been used in the fabrication of emulsion templated functional materials (EFMs). Intrinsic characteristics of these nano building blocks along with the various material formats and structures endow the functional materials with excellent properties and diverse applications. Herein, the latest progresses in emulsion templating mechanisms, emerging nano building blocks, and material formats including monolith, film and microcapsule for EFMs are reviewed first. Typical applications of these functional materials are critically discussed in the fields of sensing, energy storage and conversion, fire resistance and thermal insulation, water purification, and microdroplet transportation. Current challenges, future prospects and research opportunities of EFMs are also discussed finally.

The exploration of MXenes, especially nitride MXenes, in the field of theranostic nanomedicine is still in its infancy. Here, towards synergistic chemo-photothermal oncotherapy, we demonstrate the first kind of 2D titanium nitride (Ti2N) MXene-based nanosystem (Ti2N@oSi) for dual-strategy synergistic oncotherapy. The unique structure of Ti2N nanosheets endows the drug carriers with an ultrahigh loading capacity of 796.3% and an excellent NIR photothermal conversion efficiency of 41.6% for chemo-photothermal therapy. After being coated with a biodegradable organosilica shell, the Ti2N@oSi nanocarriers show excellent characteristics of tumor targeting, pH/glutathione/photothermal-responsive drug release and dual-drug combination chemotherapy. Both in vitro and in vivo therapeutic evaluations demonstrate the pronounced tumor growth inhibition effect and superior biocompatibility of Ti2N@oSi nanocarriers. The excellent drug loading ability, photothermal conversion ability and surface modifiability of Ti2N open up new opportunities for tumor microenvironment-targeted synergistic oncotherapy. This work is supposed to broaden the application of MXenes in nanomedicine and, particularly, provide the first sight to the biomedical application of nitride MXenes.

Nanomaterials have emerged as an amazing class of materials that consists of a broad spectrum of examples with at least one dimension in the range of 1 to 100 nm. Exceptionally high surface areas can be achieved through the rational design of nanomaterials. Nanomaterials can be produced with outstanding magnetic, electrical, optical, mechanical, and catalytic properties that are substantially different from their bulk counterparts. The nanomaterial properties can be tuned as desired via precisely controlling the size, shape, synthesis conditions, and appropriate functionalization. This review discusses a brief history of nanomaterials and their use throughout history to trigger advances in nanotechnology development. In particular, we describe and define various terms relating to nanomaterials. Various nanomaterial synthesis methods, including top-down and bottom-up approaches, are discussed. The unique features of nanomaterials are highlighted throughout the review. This review describes advances in nanomaterials, specifically fullerenes, carbon nanotubes, graphene, carbon quantum dots, nanodiamonds, carbon nanohorns, nanoporous materials, core–shell nanoparticles, silicene, antimonene, MXenes, 2D MOF nanosheets, boron nitride nanosheets, layered double hydroxides, and metal-based nanomaterials. Finally, we conclude by discussing challenges and future perspectives relating to nanomaterials.

MXenes, since their discovery in 2011, have garnered significant research attention for a variety of applications due to their exciting physico-chemical properties. The presence of polar functional groups, hydrophilicity, tunable surface chemistry, 2D morphology with a large specific surface area, high negative zeta potential and environment-friendly characteristics are some of the important features of MXenes, which make them potential candidates for environmental remediation. This perspective encapsulates the recent progress in the use of MXenes for adsorptive remediation of water. In this work, an overview of the structure of MXenes along with their peculiar surface chemistry is given. Special attention is paid to their applications and the role of surface functional groups in removal of various contaminants including heavy metals, radionuclides, anions and dyes. Furthermore, future perspectives of using 2D MXenes for water-remediation are discussed in detail.

Developing ranking anode materials with sufficient electrical conductivity, ultrafast ion diffusion ability and considerable storage capacity is of great importance for rechargeable ion batteries but still challenging. Herein, using first-principles calculations, the potential of monolayer Nb2N as an anode material for alkali metal (e.g., Li, Na, K and Ca) ion batteries (LIBs, SIBs, PIBs and CIBs) has been explored. The calculated results indicate that the Nb2N monolayer is dynamically and thermally stable with excellent electronic conductivity. To be specific, the Li, Na, K and Ca atoms can be steadily adsorbed on the Nb2N monolayer with a low adsorption energy of −0.996, −1.263, −1.568, and −1.401 eV, respectively. Impressively, the calculated low diffusion barriers for Li, Na, K and Ca on the Nb2N monolayer are 0.047, 0.029, 0.015 and 0.051 eV, respectively, implying its high performance for the ultrafast charge and discharge processes. More importantly, the maximum storage capacities are 536 mA h g−1 for LIBs and 1072 mA h g−1 for CIBs, which are much larger than those of common anode materials. This work not only demonstrates that the Nb2N monolayer can be used as a promising anode material but also inspires the future rational design of other nitride MXenes in energy conversion and storage devices.

Due to the high conductivity and abundant active sites, the metallic 1T phase of a two-dimensional molybdenum sulfide monolayer (1T-MoS2) has witnessed a broad range of potential applications in catalysis, and spintronic and phase-switching devices, which, however, are greatly hampered by its poor stability. Thus, the development of particular strategies to realize the phase transition from the stable 2H phase to the metastable 1T phase for MoS2 nanosheets is highly desirable. Herein, by means of density functional theory (DFT) computations, we systematically explored the potential of the interfacial interaction of 2H- and 1T-MoS2 monolayers with a series of M2C MXenes (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) for achieving the 2H/1T phase transformation. Our results revealed that the 2H → 1T transition for MoS2 monolayers can occur thermodynamically by anchoring on Ti2C, Zr2C, or Hf2C substrates with the extremely strong metal–S interaction, which can be well rationalized by the analysis of the charge transfer, work function, and density of states. Specially, these obtained stable 1T-MoS2/M2C hybrid materials exhibit excellent metallic features, outstanding magnetism, and enhanced mechanical properties. Our findings provide a new avenue to tune the phase transformation for MoS2 monolayers by strong interfacial interactions, which helps to further widen the potential applications of MoS2 monolayers.

Two-dimensional MXenes have been extensively investigated in recent years and have resulted in the discovery of more than 30 compositions. The metallic conductivity and pseudocapacitive behavior of MXenes have demonstrated their exciting potential in electrochemical energy storage and conversion, electromagnetic shielding, wearable sensors, and personal thermotherapy. Driven by the surface termination functional groups, versatile chemistry, and outstanding solubility, MXenes have been integrated with various flexible and nanoporous substrates for wearable nanodevice applications. However, the available review mostly focuses on different substrates for multifunctional applications. No review intended for wearable nanodevices with a special focus on the breathability and comfort has been reported to date. In this review, we summarized the synthesis processes and their impact on MXene properties, and the structure–property relationship of MXenes. The current progress in MXene integration processes is comprehensively summarized and analyzed. We explored and evaluated the energy harvesting and storage, electromagnetic shielding, joule heating, as well as strain and pressure sensing applications. A brief overview of humidity and gas sensing of the MXene-based nanostructure is provided. Besides, the current challenges and perspectives in these burgeoning fields are highlighted for the development of wearable nanodevices.

Recently, transition metal borides (MBenes, analogous to MXenes) have attracted interest due to their potential applications in energy conversion and storage. In this work, we performed density functional theory calculations to systematically explore the exfoliation properties of 14 MAlB phases and their water splitting and N2 reduction reaction (NRR) performances. Results showed a linear relationship between the binding energy and exfoliation energy with the coefficient (R2) of 0.95, indicating that the lower the binding energy of element Al in MAlB (M2AlB2), the higher the exfoliation energy required to synthesize monolayer MB from MAlB (M2AlB2). NiB (B site) was predicted to possess the best electrocatalytic activity for water splitting, hydrogen evolution reaction (HER), and oxygen evolution reaction (OER) among the studied MBenes, and overpotentials on the NiB surface were calculated to be 0.08 V (for HER) and 0.37 V (for OER), respectively. The electronic properties and dynamic simulations indicated that NiB is the best candidate catalyst for water splitting. Conversely, the Fe site on FeB (FeB-Fe) was predicated to have the highest nitrogen reduction reaction (NRR) activity among the studied MBenes, with the overpotential ηNRR of 0.11 V. Furthermore, the B site of TaB (TaB-B) was identified as the best NRR catalyst against HER among the studied MBenes considering the HER side reaction.

Infectious diseases spread rapidly among humans, and they are amid the major causes of human deaths globally. The world has seen several fatal infectious diseases in the past, for example, Spanish flu about a century ago, and now coronavirus disease-19 (COVID-19), the two major pandemics. While medicines and vaccines are the ultimate solutions to combat pandemics, material innovation and technological development are also crucial parts to fight against these highly infectious diseases. Materials and technological development can assist in developing high-performance and sensitive diagnostic devices, efficient antimicrobial components/personal protective equipment such as face masks and shields, aprons, sterilizers, touch-free components for sanitizations, and many other components for medical equipment. Due to the outstanding functional properties and feasibility to integrate with any systems, 2D materials are emerging solutions for biomedical applications. In particular, 2D materials beyond graphene such as MXenes, and composites of MXene–graphene show many promising properties such as high electrical conductivity, surface functionalization feasibility, excellent photocatalytic and photo-to-heat conversion properties, outstanding antimicrobial behavior, and many other tailorable properties. Here, we shed light on the properties of MXenes and MXene–graphene composites for healthcare applications and how they can contribute in the fight against the current and future pandemics.

For the first time, an “Evaporated-Nitrogen” Minimally Intensive Layer Delamination (EN-MILD) synthesis approach is reported to synthesize exceptionally high quality MXene sheets. In the EN-MILD method, the concentrations of acids and Li-ions are continuously increased during the etching process. By implementing the EN-MILD approach, the electrical conductivity increases up to 2.4 × 104 S cm−1, which is the highest reported value to date for Ti3C2Tx MXenes (a traditional MILD approach results in a conductivity of 5.8 × 103 S cm−1). This significant improvement in electrical conductivity arises from the high quality of the synthesized MXene sheets as well as a larger flake size. The EN-MILD synthesis approach also offers high yield of delaminated single MXene layers (up to ∼60% after the first round of washing/centrifugation) and high colloidal concentrations (up to 31 mg ml−1). The working electrode prepared from free-standing MXene paper shows an exceptional capacitance of ≈490 F g−1 at 1 A g−1 in a supercapacitor, which is among the highest values reported for MXene-based supercapacitor electrodes. The exceptional electrical conductivity, high yield of delaminated MXene single layers, and high colloidal concentration of the EN-MILD approach significantly expand the applications of MXenes.

A solvated network of two-dimensional materials in the form of hydrogel offers a unique platform for the full utilization of surface-dominated properties on a macroscopic scale. However, the development of such hydrogels with ceramic sheets of MXenes is highly challenging, and hence, they are often critically dried to aerogel form to make them self-standing. Herein, we report the preparation of a freestanding thin MXene hydrogel via spontaneous, quasi-ordered assembly over a metal plate and concurrent partial surface de-functionalization. We established that self-standing hydrogels can only be assembled from MXene solutions above a critical dispersion concentration (Cct) depending on the size of the sheets. On behalf of ordered porous structures with intrinsic hydrated ion-permeable channels, MXene hydrogels display very high surface capacitive contribution (>94%) in their pseudocapacitive energy storage performance. Consequently, the as-developed hydrogels display an excellent gravimetric capacitance of 391 F g−1 and rate performance, and continue to display an outstanding performance >337 F g−1 at a high enough mass loading up to ∼15 mg cm−2. As a versatility of hydrogel structures, an electrolyte-protected mechanical compression method was employed to obtain a compact yet ion channel–decorated electrode. As a result, compact MXene hydrogels achieved an excellent volumetric capacitance of 1120 F cm−3 and a rate capability of 73% at 1000 mV s−1.

The Zn metal anodes of aqueous zinc ion batteries (ZIBs) suffer from dendrites and severe side reactions, such as the hydrogen evolution reaction (HER) and passivation, which seriously restrict the shelf life of ZIBs. Herein, these issues were addressed with a solid polymer electrolyte (SPE) based on the poly(vinylidene fluoride-co-hexafluoropropylene) filled by the poly(methyl acrylate) grafted MXenes (denoted as PVHF/MXene-g-PMA) for the first time. Homogeneously dispersed MXenes were achieved benefiting from the intriguing interaction between the highly grafted PMA and PVHF matrix. The resulting SPE exhibits three orders of magnitude larger ionic conductivity than the PVHF matrix, reaching 2.69 × 10−4 S cm−1 at room temperature. Dendrite-free Zn plating/stripping with high reversibility was achieved (over 1000 h cycles at room temperature and 200 h at high temperature). Subsequently, the fabricated solid full cells with eliminated HER and suppressed anode dendrites exhibited excellent cycling performance of 10 000 cycles at 2C at room temperature and could work normally at temperatures ranging from −35 °C to 100 °C. Most importantly, over 90 days of shelf life was attained for the all-solid-state ZIBs after storage at low/high temperatures. Our work represents a substantial progress on the all-solid-state ZIBs with superior stability and reliability, reflected by the effectively suppressed dendrites and side reactions, excellent cycling performance and remarkable shelf life.

With the development of multifunction and miniaturization in modern electronics, polymeric films with strong mechanical performance and high thermal conductivity are urgently needed. Two-dimensional transition metal carbides and nitrides (MXenes) have attracted extensive attention due to their tunable surface chemistry, layered structure and charming properties. However, there are few studies on using MXenes as fillers to enhance polymer properties. In this paper, we fabricate a three-dimensional foam by the freeze-drying method to enhance the interfacial interaction between adjacent MXene sheets and polyimide (PI) macromolecules, and then a composite film with a dense and well-ordered layer-by-layer structure is produced by the hot-pressing process. Based on the secondary orientation strategy, the resultant MXene/PI film exhibits an enhanced thermal conductivity of 5.12 ± 0.37 W m−1 K−1 and tensile strength of 102 ± 3 MPa. Moreover, the composite film has good flexibility and flame retardancy owing to the synergistic effect of MXene sheets and PI chains. Hence, the MXene/PI composite film with the properties of flexibility, flame-retardancy, high mechanical strength and efficient heat transmission is expected to be used as the next thermal management material in a variety of applications.

The development of environmentally friendly, simple process and low cost synthesis methods for preparing graphene nanosheets (GNs) has attracted global interest. In this work, a simple and efficient method to synthesize GNs deriving from coal-tar pitch has been proposed from both experimental and theoretical point of views. The XRD, TEM and Raman results demonstrate that precursor Al4C3 could provide a growth environment for the final product of GNs. Innovatively, we have unraveled the microscopic origin for the decomposition of Al4C3 based on density functional theory calculations. It is highlighted that the surface energies and the analysis of elastic constants indicate the fact that the chemical etching process in Al4C3 can happen, which is similar to the exfoliation of well-known transition metal carbides MXenes. Furthermore, different bond breaking mechanisms have been found in Al4C3 at applied tensile and shear strains from the electron localization functions and stress–strain results. Our study not only offers an efficient method to synthesize GNs, but also to unravel the microscopic mechanism of fabrication by theoretical calculations.

In view of their unique planar structure and outstanding physical and chemical properties, two-dimensional (2D) materials have garnered the attention of interdisciplinary researchers in the domain of biomedical and clinical applications. MXenes are 2D transition metal carbides and nitrides with outstanding characteristics, comprising huge surface area, biocompatibility, low toxicity, significant electrical conductivity, antibacterial activity and hydrophilicity. Although numerous investigations have demonstrated the promising potential of MXenes for different biomedical applications that include biosensing, bioimaging, cancer therapy, tissue engineering, regenerative medicine, and drug delivery, there are still important challenging issues pertaining to their stability in physiological environments, sustained/controlled release of drugs, and biodegradability. The well-designed ultra-thin MXene nanosheets are deployable as promising biocompatible inorganic nanoplatforms for assorted biomedical applications via the clinical translation of nanomedicine; MXenes are good candidates for tissue engineering and regenerative medicine. Herein, recent progresses on 2D MXenes for state-of-the-art tissue engineering and regenerative medicine are discussed with emphasis on the significant challenges and future perspectives.

Due to the increasing market demand for graphene-based devices, van der Waals heterostructures based on 2D materials have increased rapidly worldwide during the last decade. Graphene-based applications are inadequate in some electronic devices such as field-effect transistors (FETs) and solar cells devices due to its intrinsic zero bandgap and complex mechanistic approach in related chemistry. Thus, it is highly desired to fill the gap and devote efforts to explore other 2D materials further, especially bi- and tri-elemental 2D materials, such as layered metal chalcogenides (LMDCs), metal oxides, and hexagonal boron nitride (hBN). During the past few years, a different research group contributed to improving the quality and performance of the mentioned 2D materials. Still, due to some deficiencies such as low conductivity and poor bandgap, there is a need to develop and investigate alternative materials. Recently, MXenes, which are compounds obtained due to the chemical delamination of quaternary or (ternary) layered nitrides or carbides, have been found as a great class of 2D elemental materials. The complex bonding in the MXene family (a mixture of metallic and covalent bonds) and other characteristics such as atomic stacking, electronic structures, surface terminal groups, and different synthetic routes are responsible for their unique properties such as hydrophilicity and metallicity, and they are predicted to have high elastic moduli in a single compound. On behalf of the rich chemistry and unique morphologies, MXene derivatives have delivered efficient sensors, energy storage materials, catalysts, and water purification materials. Although there is a great demand for MXene-based derivatives, there is further demand for their improvement such as the long-term stability, large scale production, as well as deposition and surface roughness, which create hurdles in their commercialization. Titanium-based MXene (Ti3C2Tx and Ti2CTx) heterostructures with other 2D nanomaterials are highly desired to solve the surface roughness problems and long-term stability as well as an alternative to overcome the deficiency of the scalable production of MXenes. The existence of different active functionalities (–O, –OH, –F, etc.) on the surface of Ti3C2Tx and Ti2CTx can be helpful for interacting with other 2D layered materials in the presence of solvents to fabricate heterostructures after using various deposition techniques such as spin coating, dip coating, and drop-casting for practical device fabrication processes. In the presence of weak interaction between the host (MXene derivatives) and guest (other nanostructures 2D materials), the intrinsic characteristics of both can be largely preserved for efficient energy storage applications. In this paper, we present a comprehensive literature library of the various strategies of 2D titanium-based MXenes such as Ti3C2Tx and Ti2CTx for the fabrication of their heterostructures with 2D materials using various deposition techniques and the application of the resultant structures in the fields of optoelectronics, catalysis, and energy storage devices.

First-principles calculations were performed to study the electrochemical performance of M2TiC2 (M = Cr or Mo) and M2TiC2Tx (T = O, F or OH) used as anode materials for sodium ion batteries (SIBs). The O functionalized MXenes (Cr2TiC2O2 and Mo2TiC2O2) are found to be more stable than F and OH terminated systems. The diffusion performance of sodium in MXene materials is mainly affected by the functional groups. The lowest diffusion barrier of functionalized MXenes is about one order larger in magnitude than that of bare MXenes. Although the introduction of O-groups hinders the diffusion of sodium, it can greatly improve the theoretical storage capacities. Meanwhile, the diffusion paths and diffusion energy barriers of Na are affected by Na concentration effects, while the interactions between terminations have little effect. Furthermore, multiple layers of sodium atoms are found to be adsorbed between the layers of M2TiC2O2, thus significantly increasing the theoretical capacities. The theoretical sodium storage capacities of M2TiC2O2 monolayers reach 515.70 mA h g−1 (M = Cr) and 362.46 mA h g−1 (M = Mo) and the OCVs can approach 0.034 V (M = Cr) and 0.042 V (M = Mo). Therefore, Cr2TiC2O2 and Mo2TiC2O2 are expected to be promising anode materials for SIBs due to their excellent properties, such as good electronic conductivity, low sodium diffusion barrier, and high theoretical sodium storage capacity.

With the wide applications of terahertz (THz) devices in future communication technology, THz protection materials are essential to overcome potential threats. Recently, THz metamaterials (MMs) based on two-dimensional (2D) materials (e.g., graphene, MXenes) have been extensively investigated due to their unique THz response properties. In this review, THz protection theories are briefly presented first, including reflection loss and shielding mechanisms. Then, the research progress of graphene and other 2D material-based THz MMs and intrinsic materials are reviewed. MMs absorbers in the forms of single layer, multiple layers, hybrid and tunable metasurfaces show excellent THz absorbing performance. These studies provide a sufficient theoretical and practical basis for THz protection, and superior properties promised the wide application prospects of 2D MMs. Three-dimensional intrinsic THz absorbing materials based on porous and ordered 2D materials also show exceptional THz protection performance and effectively integrate the advantages of intrinsic properties and the structural characteristics of 2D materials. These special structures can optimize the surface impedance matching and enable multiple THz scatterings and electric transmission loss, which can realize high-efficiency absorption loss and active controllable protection performance in ultra-wide THz wavebands. Finally, the advantages and existing problems of current THz protection materials are summarized, and their possible future development and applications are prospected.

The efficiency of green hydrogen production through photo- or electrochemical splitting of water depends strongly on the catalyst used. Several 2D materials have recently been proposed as effective catalysts or co-catalysts, and among them, oxygen-terminated MXenes deserve significant attention for both electro- and photo-induced water splitting. In this work, we described a hybrid photo-electrochemical approach based on the coupling of Ti3C2Tx MXene flakes on a plasmon-supported Au grating and light triggering of the electrocatalytic activity of the generated hybrid structure in a water splitting half reaction, namely, the hydrogen evolution reaction (HER). In particular, MXene flakes with both fluor and oxygen surface terminations were deposited on a periodic patterned gold surface that was capable of supporting surface plasmon-polariton (SPP) excitation under visible and near-infrared (NIR) light illumination. SPP excitation allows sub-diffraction focusing of light energy and effective enhancement of the electrocatalytic performance of Ti3C2Tx flakes. Under illumination, a significant enhancement of the HER kinetics was observed, as well as tuning of the HER rate-determining step from the Volmer step to the Heyrovsky step. In turn, we observed several-fold enhanced hydrogen evolution, which was attributed to plasmon-assisted hot charge carrier injection, with an additional contribution from the plasmon heating effect. The proposed Au grating/Ti3C2Tx hybrid structure allows utilization of the NIR part of the solar spectrum, which is commonly not used in water photolysis but achieves better efficiency in renewable energy-assisted green hydrogen production.

Two-dimensional metal carbides and nitrides, known as MXenes, are an emerging class of materials that are promising for a variety of applications. In this work, using time-dependent density functional theory calculations, we investigate the localized surface plasmon resonances and electric field confinement of pristine and surface-terminated [fluorinated (F) and/or oxidized (O)] mono-layered titanium carbide (Ti3C2) MXene nanoclusters. We found that the nanoclusters (Ti48C32, Ti48C32F32, and Ti48C32O32) exhibit broadband photoabsorption spectra and localized surface plasmon resonances even at low energy in the infrared region (a spectral range of interest for molecular sensing). In addition, the nanoclusters produce a sizable electric field confinement on the surface with a strength that varies with the F/O surface termination. Our findings provide significant theoretical insight into the optical and plasmonic properties of MXene nanoclusters.

The rapid development of human–machine interfaces and artificial intelligence is dependent on flexible and wearable soft devices such as sensors and energy storage systems. One of the key factors for these devices is the design of a flexible electrode with high sensitivity, fast response time, and a wide working range. Here, we report the fabrication of strain sensors and all-solid-state flexible supercapacitors using Co@N-CNT/MXenes as an electrode material. The manufactured sensor shows a high tensile range (strain up to 200%) and high stability. The resistance change caused by the fingers touching the sensor can be used to transmit the Morse code information. Flexible supercapacitors serving as power supply demonstrate excellent cycling stability (85 000 cycles) and coulombic efficiency (99.7%) for their high surface area and pseudocapacitance. A self-powered integrated system composed of the strain sensor and flexible supercapacitor is fabricated and operates stably in a wide strain sensing test range. Moreover, the flexible solar-charging self-powered integrated system could be attached to the human body for stable human motion detection. This study clearly shows that appropriate selection of a single functional material to enable it to be used in multi-functional sensors and supercapacitors can simplify the process and reduce the cost of manufacturing wearable devices.

Generating valley polarization for valleytronics applications requires breaking the inversion symmetry of two-dimensional (2D) hexagonal crystals. As such, 2D MXenes naturally lose their inversion symmetry upon surface functionalization, thereby opening up new design opportunities for controlling their physical and chemical properties. However, no spin-valley couplings have been proposed for magnetic MXenes thus far. Herein, we demonstrate that surface engineering not only breaks the inversion symmetry of 2D MXenes but also induces valley polarizations with diverse magnetic orders, including ferromagnetism, ferrimagnetism and antiferromagnetism. Using Cr2C-based MXenes as prototypes, our theoretical calculations showed that Janus MXenes Cr2COX (X = F, Cl and OH) are excellent candidates for ferrovalley materials, especially Cr2COF, which has a strong valley polarization of 334 meV and a high Curie temperature of 1146 K. Concurrently, Cr2C-based MXenes with mixed functionalizations (Cr2CO0.75F1.25 and Cr2CO1.25F0.75) displayed properties of ferrivalley and ferrovalley semiconductors, with 11 and 15 meV valley splitting, respectively. In other magnetic MXenes, surface engineering also induced valley properties, as shown by the new bipolar antiferrovalley identified in Cr2TiC2FCl MXene. Therefore, our study is the first proposal of an experimentally viable approach (i.e., surface engineering) for generating valley polarization in 2D MXenes by breaking their inversion symmetry while simultaneously providing a computational paradigm for probing other 2D nanomaterials with potential applications in valleytronics.

MXenes have been reported to have excellent electrical and thermal properties theoretically. However, the actual properties of the synthesized MXene are far less than expected. This discrepancy arises mostly from the structural defects formed during synthesis. This work demonstrates that the thermal, electrical, and mechanical properties of the Ti3C2 MXene can be readily improved via the vapor phase infiltration (VPI) phenomenon of platinum (Pt) which usually occurs in the conventional atomic layer deposition (ALD) process. The infiltrated Pt atoms are found to cure Ti-defects in the MXene and to build bridges that interconnect MXene nanoflakes by creating covalent Pt–C bonds, which act as highly efficient channels for electrical and thermal conduction. The Pt infiltrated MXene displays drastic enhancement in thermal, electrical, and mechanical properties, simultaneously. Notably, both in-plane electrical/thermal conductivity and cross-plane electrical/thermal conductivity are found to significantly increase (∼2.4/∼1.8 times and ∼6.6/∼5.0 times, respectively). Electrothermal heaters made by using the Pt-infiltrated MXene exhibit outstanding efficiency, high heating rate, and good stability.

Mo2C and Ti3C2 MXenes were investigated as earth-abundant electrocatalyts for the CO2 reduction reaction (CO2RR). Mo2C and Ti3C2 exhibited faradaic efficiencies of 90% (250 mV overpotential) and 65% (650 mV overpotential), respectively, for the reduction of CO2 to CO in acetonitrile using an ionic liquid electrolyte. The use of ionic liquid 1-ethyl-2-methylimidazolium tetrafluoroborate as an electrolyte in organic solvent suppressed the competing hydrogen evolution reaction. Density functional theory (DFT) calculations suggested that the catalytic active sites are oxygen vacancy sites on both MXene surfaces. Also, a spontaneous dissociation of adsorbed COOH species to a water molecule and adsorbed CO on Mo2C promote the CO2RR.

The practical applications of lithium–sulfur (Li–S) batteries are greatly hindered by the poor conductivity of sulfur, the shuttling of lithium polysulfides (LiPSs), and the sluggish kinetics in the charge–discharge process. In order to solve these problems, here we propose the surface-functionalized V2N MXenes as the host materials to improve the electrochemical performance of Li–S batteries. Based on the density functional theory (DFT) calculations, we found that both the bare and functionalized V2NT2 (T = O, F, OH, and S) exhibit metallicity, and three of them (V2NO2, V2NF2, and V2NS2) possess moderate LiPS adsorption strength, which thermodynamically benefits the suppression of the dissolution and shuttling of LiPSs. Besides, V2NS2 shows the lowest Gibbs free energy barrier for the sulfur reduction reaction (0.49 eV) during discharge, which kinetically suppresses the dissolution and shuttling of LiPSs by expediting the decomposition process from soluble LiPSs to insoluble ones. Moreover, surface functionalized V2NT2 also exhibits outstanding catalytic ability for Li2S decomposition during charge, which decreases the energy barrier from 3.64 eV (bare V2N) to 1.55 (V2NO2) and 1.19 eV (V2NS2), and increases the charging kinetics. Based on these results, V2NS2 monolayers are suggested as promising host materials for S cathodes due to the fast charge/discharge kinetics and effective suppression of LiPS shuttling. This theoretical study provides further insight into the application of nitride MXenes and other two-dimensional materials as conductive anchoring materials for Li–S batteries.

MXene/polymer nanocomposites simultaneously benefit from the attractive properties of MXenes and the flexibility and facile processability of polymers. These composites have shown superior properties such as high light-to-heat conversion, excellent electromagnetic interference shielding, and high charge storage, compared to other nanocomposites. They have applications in chemical, materials, electrical, environmental, mechanical, and biomedical engineering as well as medicine. This property-based review on MXene/polymer nanocomposites critically describes findings and achievements in these areas and puts future research directions into perspective. It surveys novel reported applications of MXene-based polymeric nanocomposites. It also covers surface modification approaches that expand the applications of MXenes in nanocomposites.

Metal hydroxides and oxides have emerged as fascinating materials and key structures for electrocatalysis however, they have rarely been investigated for the HER. Herein, we introduce unique transition-metal hydroxides@MXene (TMHs@MXene) hybrids, including Co(OH)2@MXene, Ni(OH)2@MXene, and FeOOH@MXene, with well-defined components and hierarchical sheet-like architectures for the alkaline HER. By virtue of their novel structure and the strong interfacial interactions between transition-metal hydroxides (TMHs) and MXene nanosheets, the obtained nanohybrids not only provide sufficient active sites and a robust structure, but also ensure favourable electrochemical kinetics and superior catalytic activity. Significantly, both theoretical calculations and the electrochemical test prove that the interfacial electronic coupling between the two different components could optimize the adsorption energy of water and hydrogen, thereby resulting in Pt-like catalytic activity including a low Tafel slope (31.7 mV dec−1), a small overpotential (21.0 mV@10 mA cm−2), and excellent stability for Co(OH)2@MXene. As expected, an alkaline water electrolyzer was built using Co(OH)2@MXene as the cathode for overall water splitting, which achieves a current density of 10 mA cm−2 at 1.46 V with outstanding stability over 100 h. Our discovery highlights the great potential of interfacial electronic coupling to optimize advanced electrocatalysts for application in energy-related fields.

MXenes are two-dimensional nanomaterials, which can be constructed from different elements. The rich interlayer groups, surface groups, and the flexible layer spacing of MXenes make them ideal catalysts. Among these, Ti3C2Tx has gained particular attention as a photocatalyst for photocatalytic CO2 reduction reactions (CO2RR), hydrogen evolution reactions (HER), and photocatalytic degradation reactions. The structure of Ti3C2Tx, hydrophilic surface functional groups, and the Gibbs free energy for hydrogen adsorption lead to the excellent photocatalytic HER performance of this material. Numerous surface defects on Ti3C2Tx also provide plentiful CO2 adsorption sites for CO2RR. It is the structure of two-dimensional nanomaterials and their high-speed electron transport channels that enable their excellent catalytic oxidation activity. However, at present, there are still challenges that limit their further application, the most significant of which is the material stability. In order to overcome this, the synthetic routes to prepare these photocatalysts need to be adapted.

Constructing heterostructure solar cells using donor and acceptor semiconductors has attracted global interest owing to their high power conversion efficiency. In this paper, based on density functional theory calculations, we comprehensively evaluated 64 two-dimensional transition metal carbides (MXenes) to explore them as appropriate semiconductors for solar cells by material screening. The results highlight that Zr2CO2 (Hf2CO2) and Ti2CO2 are promising donor and acceptor materials, respectively. Interestingly, the heterostructures have a moderate band gap of 1.22 eV and exhibit a noticeable absorbance coefficient of 105 cm−1 in the visible light region. Moreover, the type-II nature of the heterostructures could induce effective electron–hole separation. Furthermore, the photocurrents of Ti2CO2/Zr2CO2 and Ti2CO2/Hf2CO2 heterostructure solar cell devices are competitive with those of silicon devices. In particular, Ti2CO2/Zr2CO2 and Ti2CO2/Hf2CO2 heterostructure solar cells deliver a very high power conversion efficiency of 22.74% and 19.56%, respectively. Our present study paves the way for facilitating the potential application of MXenes as photovoltaic materials.

To promote the development of supercapacitors and their applications in modern electronics, it is crucial to explore novel supercapacitor electrode materials. As a representative member of the rising 2D MXenes, Ti3C2Tx MXene has shown tremendous potential for supercapacitor electrodes owing to its unique physicochemical properties. Here, the most recent advances in Ti3C2Tx-based supercapacitor electrodes are comprehensively reviewed, with an emphasis on the vital role that Ti3C2Tx MXene plays in the remarkable electrochemical performance and related mechanisms. The fabrication methods, electrode structures, working mechanisms, electrochemical performance and related influencing factors, mechanical properties and applications, as well as the associated advantages/disadvantages of Ti3C2Tx-based supercapacitor electrodes are thoroughly and exhaustively summarized and discussed. Based on the recent progress, the existing challenges along with the corresponding possible solutions, and the future prospects of Ti3C2Tx-based materials for supercapacitors are also outlined and discussed.

Graphene and related materials have been widely studied due to their superior properties in a wide range of applications. However, large-scale production remains a critical challenge to enable commercial acceptance. Here, we present a facile, scalable, one-step electrochemical method for producing hybrid transition metal oxide (V, Fe, Ti, or Mn)/graphene materials (TMO-EGs) as active materials for supercapacitors. Therein, we have designed and developed a continuous flow reactor with a high production rate (>4 g h−1) of TMO-EGs, where the TMO accounts for 36 weight%. TMO-EG flakes demonstrate a moderate lateral size of up to 5 μm and a specific surface area of 64 m2 g−1. Notably, TMO-EGs present a capacitance of up to 188 F g−1 as single electrodes in 4 M LiCl. The most promising material, MnOx-EG, has been used for the large-scale production of thin-film supercapacitor devices (40 × 40 × 0.25 mm) in a commercial pilot line. Using 1 M Na2SO4 as the electrolyte, the as-fabricated devices deliver a capacitance of 52 mF cm−2, with 83% capacitance retention after 6000 charge–discharge cycles, comparable to recent reports of similar devices. The simplicity, scalability, and versatility of our method are highly promising to promote the commercial applications of graphene-based materials and can be further developed for the upscalable production of other 2D materials, such as transition metal dichalcogenides and MXenes.

Due to the low cost, high element abundance and intrinsic safety, potassium-ion batteries (KIBs) have attracted a surge of interest in recent years. Currently, the key challenge and obstacle to the development of KIBs is to find suitable anode materials with large capacity, high rate capability and small lattice changes during the charge/discharge process. MXenes with excellent energy storage properties are promising anode materials for KIBs and their energy performance largely depends on the surface termination. Here, two-dimensional O- and S-terminated V2C MXene anode materials are designed to model high performance potassium-ion batteries. Using first-principles calculations, the structural properties and potential battery performance in KIBs of V2CO2 and V2CS2 are systematically investigated. The inherent metallic nature, a small diffusion barrier, a low average open circuit voltage, and a high theoretical specific capacity (489.93 mA h g-1 of V2CO2 and 200.24 mA h g-1 of V2CS2) demonstrate that both of them are ideal anode materials for KIBs. Meanwhile, we also investigated the mechanism of the difference in energy performance between V2CO2 and V2CS2 at atomic and electronic levels, in other words, the energy performance difference introduced by surface O- and S-terminations.

The exciting combination of high electrical conductivity, high specific capacitance and colloidal stability of two-dimensional Ti3C2Tx MXene (referred to as MXene) has shown great potential in a wide range of applications including wearable electronics, energy storage, sensors, and electromagnetic interference shielding. To realize its full potential, recent literature has reported a variety of solution-based processing methodologies to develop MXenes into multifunctional architectures, such as fibres, films and aerogels. In response to these recent critical advances, this review provides a comprehensive analysis of the diverse solution-based processing methodologies currently being used for MXene-architecture fabrication. A critical evaluation of the processing challenges directly affecting macroscale material properties and ultimately, the performance of the resulting prototype devices is also provided. Opportunities arising from the observed and foreseen challenges regarding their use are discussed to provide avenues for new designs and realise practical use in high performance applications.

Exploring highly active centers for NN triple-bond activation and the suppression of the competing H2 evolution reaction (HER) are key considerations for photocatalytic N2 fixation. As a novel 2D transition-metal carbide, Ti3C2 nanosheets (MXenes) have been recently considered as a promising N2 reduction reaction (NRR) active center. However, little of the expected progress in the use of Ti3C2 for photocatalytic N2 fixation has been achieved. Herein, we report a novel 2D/2D Ti3C2/N-defect g-C3N4 heterostructure photocatalyst exhibiting highly enhanced photocatalytic nitrogen fixation activity, with an NH3 yield of 5.792 mg h−1 g−1. This study demonstrates that the heterostructure was constructed by filling the oxygen-terminals of Ti3C2 in the N-defects of g-C3N4 to form C–O–Ti interactions. Together, the construction of the hetero-interface and the introduction of N-defects contribute to rapid interfacial charge transfer to the active sites. Importantly, the exposed edge Ti of Ti3C2 was confirmed to be the active site for N2 adsorption and activation, and these active Ti sites exhibit desirable NRR selectivity via suppressing the competing HER. Finally, a mechanism for photocatalytic N2 fixation was proposed to reveal the evolution of the redox circle that originated from the multi-valence Ti species during the N2 adsorption, activation, and dissociation process.

Electrochemical CO2/CO conversion to valuable chemical products is an attractive strategy for storage of clean energy and control of greenhouse gas emission. Currently, CO2 reduction to CO is relatively mature, whereas the deep reduction and further conversion of CO into multi-carbon products, such as ethylene (C2H4) and ethanol (C2H5OH), are highly challenging. Based on the density functional theory (DFT) calculations, we explored the possibility of CO reduction reaction (CORR), to obtain C2 products, with defective MXenes in which the defect is created by removing two neighboring oxygen atoms on the surface. Our results revealed that the dual-oxygen vacancy in defective Mo2TiC2O2 (labeled as Mo2TiC2O2-2OV) can offer a unique environment that confines and enriches the active *COH species, significantly promoting the reduction process as well as C–C bond coupling. The thermodynamic barrier of the potential-determining step (PDS) for Mo2TiC2O2-2OV is 0.32 eV with promising selectivity of C2 products over the competing hydrogen evolution reaction (HER). This work provides a feasible strategy for designing MXene-based electrocatalysts for highly efficient CO2/CO reduction to C2 products.

MXenes have received much attention as promising candidates for noble metal-free hydrogen evolution reaction (HER) electrocatalysts due to their high electrical conductivity, surface hydrophilicity, abundant surface functional groups, and great potential for rational hybridization with other materials. Herein, a novel porous monolayered-Ti3C2Tx@NiCoP (P-Ti3C2Tx@NiCoP) nanostructure was synthesized with uniform distribution of bimetallic compounds for improved charge transfer capability and electrocatalytic activity. In experiments, H2O2-utilized oxidation formed a highly mesoporous structure with a maximized surface area of monolayered MXenes as the support. A subsequent solvothermal process followed by phosphidation enabled successful anchoring of highly HER-active NiCoP nanoclusters onto abundantly exposed terminal edges of the P-Ti3C2Tx support. The structural porosity of the P-Ti3C2Tx nanoflakes played an important role in creating additional room for embedding catalytically active species while stably imparting high electrical conductivity to accelerate charge transfer to NiCoP nanoclusters. With structural modification and effective hybridization, P-Ti3C2Tx@NiCoP showed highly enhanced HER activity with significantly lower overpotentials of 115 and 101 mV at a current density of -10 mA cm-2 in 0.5 M H2SO4 and 1.0 M KOH, respectively, along with showing long-term stability over 60 h. As such, our approach of designing structurally modified-Ti3C2Tx and hybridizing with other electrocatalytically active species would function as a solid platform for implementing Ti3C2Tx-based hetero-nanostructures to achieve state-of-the-art performance in next-generation energy conversion applications.

Herein a rapid and sensitive fluorometric bioanalysis platform for mercury(ii) (Hg2+) detection was innovatively developed using ultrathin two-dimensional MXenes (Ti3C2) as fluorescence quencher and Hg2+-induced exonuclease III (Exo III)-assisted target recycling strategy for efficient signal amplification. Initially, fluorophore-labeled single-stranded DNA (FAM-labeled probe) can be easily adsorbed onto the surface of ultrathin Ti3C2 nanosheets by hydrogen bonding and metal chelating interaction, and the fluorescence signal emitted by the FAM-labeled probe is quenched strongly owing to the fluorescence resonance energy transfer between the FAM and ultrathin Ti3C2 nanosheets. Upon sensing the target Hg2+, the protruding DNA fragment at the 3' end of hairpin will hybridize with primer (hairpin-Hg2+-primer), and then further digested by Exo III to produce a probe (nicker). The released target Hg2+ and primer continue to participate in the next recycling, resulting in more hairpin probes becoming nickers. The combination of a large number of nickers and FAM-probe resulted in a significant increase in the fluorescence signal of the system, which was attributed to the fact that the double helix DNA was more rigid and separated from the surface of the ultrathin Ti3C2 nanosheets. The obvious fluorescence signal change of the Ti3C2-based Exo III-assisted target recycling can be accurately monitored by fluorescence spectrometry, which is also proportional to the concentration of Hg2+. Under optimum operating conditions, the peak intensity (520 nm wavelength) of fluorescence increased with increasing Hg2+ within a wide dynamic working range from 0.05 nM to 50 nM (R2 = 0.9913) with a limit of detection down to 42.5 pM. The proposed strategy uses ultrathin MXenes as a platform for binding nucleic acids, which contributes to its potential in nucleic acid hybridization-based biosensing and/or nucleic acid signal amplification bio-applications.

In recent years, a new family of two-dimensional (2D) transition metal carbides and/or carbonitrides labeled MXenes have greatly attracted worldwide attention. Due to their unique structure and excellent properties, such as unusual hydrophilicity, electrical conductivity, flexibility, and pseudo-capacitance, MXenes have great potential applications in energy storage, water desalination, catalysis, electromagnetic interference shielding, transparent conductive films and so on. However, MXenes exhibit poor stability especially under oxygen-containing conditions because of the structural defects, active transition metals and termination groups, which restrain their application fields. In this review, we focus on the current research on the stability of MXenes including oxidation and thermal stability under various conditions. The mechanism is also discussed. Based on this, the methods to improve the stability of MXenes are proposed, aiming to expand the application scope. Firstly, the structure and synthesis methods of MXenes are briefly introduced. Secondly, the oxidation process and mechanism of MXenes in different environments are introduced, including gaseous, liquid, solid, UV irradiation, hydrothermal, anodic oxidation and so on. Thirdly, the thermal stability of MXenes under an oxygen-containing atmosphere, neutral atmosphere or reducing atmosphere at different temperatures is also introduced. Finally, the methods of improving the stability of MXenes including controlling the storage environment, improving the synthesis method, regulating functional terminations, chemical edge capping methods and physical packaging methods are further discussed.

In recent years, the utilization of gas sensors has increased tremendously in daily life and industry. Importantly, appropriate material selection should be made for gas sensors in order to achieve outstanding gas sensing performance, such as high sensitivity, good selectivity, a fast response/recovery time, and long-term stability. Numerous studies have shown that neither pure metal oxide semiconductor nor individual 2D nanomaterial (graphene, transition metal dichalcogenides, metal–organic frameworks, metal oxide nanosheets, MXenes, and phosphorene) based gas sensors are capable of showing excellent gas-sensing performance towards gas molecules. However, synergistic combinations of metal oxides and 2D nanomaterials have demonstrated enhanced gas-sensing performance in many studies. This review aims at providing a comprehensive summary of the current advancements in 2D/metal-oxide based heterostructures as gas sensors. Additionally, the underlying sensing mechanisms of various kinds of gas sensors are systematically described, and the device architectures and their corresponding sensing performances are summarized. Finally, the challenges and future prospects of 2D/metal-oxide nanocomposite-based gas sensors for sensing applications have been outlined.

Synthesis of D-xylonic acid, a high value chemical feedstock, from biomass chemicals through a photocatalytic process is of great significance in respect of economical and sustainable concerns, and therefore has aroused considerable attention. Herein, a TiO2/Ti3C2(TT) photocatalyst was designed for the oxidation of xylose to D-xylonic acid under ambient conditions. The photocatalyst is fabricated by in situ growth of TiO2 nano-particles on Ti3C2 nanosheets and exhibits 64.2% of D-xylonic acid yield with excellent stability. The mechanistic study indicates that the electron transfer from TiO2 to Ti3C2 significantly increases the life time of light-induced charge carriers, and hence improved photocatalytic performance.

Recently, a new class of 2D materials called MXenes have attracted massive attention in a variety of applications. The abundant active sites, metallic conductivity, tunable surface chemistry and outstanding stability of MXenes make them desirable for gas sensing applications. In this context, an impactful amount of research has been performed on MXene based sensors and they can be considered one of the potential future materials for gas sensing. In a focused way, properties like high flexibility, convenient solution processability and easy functionalization of MXenes open the door to make their composites with other nanomaterials and provide a new avenue for advanced sensor research. Previously reported reviews on MXenes and their nanocomposites were not fully focused on the gas sensing application and covered a wide area of applications in energy storage, biomedicine, and photocatalysis. In this review, we focused on the latest research and advancement of pristine MXenes and their nanocomposites for the sensing of gases, volatile organic compounds (VOCs) and humidity. In this review, we first investigated the synthesis procedure of pure MXenes from the MAX phase by selective etching with the help of suitable etchants and delaminating agents. Subsequently, the synthesis procedure of MXene nanocomposites with other nanomaterials like metal oxides, polymers, 2D nanomaterials and other sensing materials has been reported. Later on, we briefly discuss the properties of MXenes involved in sensing performance and the modulation of properties after functionalization due to the synergistic effect of both materials. Finally, the detailed sensing performance and related sensing mechanism have been discussed for MXenes and their nanocomposite-based sensors for a variety of gases, VOCs and humidity.

Flexible supercapacitors (FSCs) are promising energy storage devices in wearable electronic systems. They have attracted tremendous attention owing to their unique properties of excellent flexibility, fast charging and discharging capabilities, and durable service life. Herein, the recent developments of electrode materials and device structures of FSCs are introduced in detail. Based on the energy storage mechanisms, the characteristics and research progress of electrode materials, such as carbon materials, transition metal oxides/hydroxides, conductive polymers, and MXenes, are classified and analyzed. The improvement strategy of these materials is emphasized. In recent years, the numerous novel architectures of FSCs have been established for better wearable applications. Thus, this review summarizes the latest progress in the planar, fibrous, and stereo architectures of FSCs in wearable electronics. The influence of the substrate, electrolyte, and structure design on the performance of FSCs is discussed. Finally, the challenges and prospects of FSCs in wearable electronic products and the future development direction are prospected.

As an important family of 2D materials, MXenes show great potential in various applications due to their specific physical and chemical properties, such as high conductivity and surface activity. Here, we investigate 54 phosphorus-functionalized MXenes (P-MXenes) and screen out 10 stable structures by analyzing their dynamical, thermal, and mechanical stabilities based on first-principles calculations. We find that these stable P-MXenes remain metallic and show weak magnetism. We show that they are effective as electrocatalysts for the hydrogen evolution reaction (HER) with near-zero Gibbs free energies of hydrogen adsorption, which are comparable to that of Pt. To understand the microkinetic process in the HER, an explicit water model is used to examine the Volmer–Heyrovsky (V–H) and Volmer–Tafel (V–T) pathways. We find that P-MXenes show a high HER performance along the V–H process due to the low activation barrier. Our systematic study confirms the possibility for the phosphorus-functionalization of MXenes, guides the design of novel electrocatalysts for the HER via surface-modification, and provides deep understanding on the HER mechanism.

A new class of two-dimensional (2D) materials known as MXene became one of the most promising candidates under investigation after the discovery of the first member of this group, Ti3C2Tx, in 2011. These layered transition metal nitrides, carbonitrides, and carbides have offered an attractive new area in inorganic 2D materials with their fascinating structural and chemical characteristics due to the versatility in elemental compositions and surface terminations. Significantly, the surface hydrophilicity of MXenes facilitates the easy composite formation with carbonaceous materials, polymers, and metal oxides that further opens up an efficient pathway for tuning the characteristics of MXenes for different applications. Despite exponentially increasing interest, MXenes lack proper systematic information. This article is focused on reviewing the most applicable aspects of MXenes including structural features with their relevant characteristics and applicability by describing both experimental and theoretical perspectives. A critical survey on current trends, status, and possibilities of MXenes and MXene-based materials have been represented for energy-related applications as well as electromagnetic interference (EMI) shielding, blends and composites, sensors, organic light-emitting diodes (OLED), and actuators and robotics associated with the limitations and future potentialities.

MXenes, an emerging class of two-dimensional (2D) materials, have driven a tremendous amount of research due to their fascinating properties including excellent metallic conductivities, favorable mechanical stability and easily tailorable surface chemistry. However, the daunting drawbacks of MXene flake restacking and oxidation instability hinder their further applications. To this end, we proposed a universal approach to transform Ti3C2Tx MXenes into nanoscale ionic materials (NIMs) using an electronic interaction strategy, where the Ti3C2Tx MXenes were firstly modified using an organosilane (OS) with acidic moieties, before electrostatic grafting of an alkaline oligomer to form the resultant stable electron-balanced MXene NIMs. The as-prepared Ti3C2Tx MXene NIMs presented rather stable antioxidant ability even after standing for 570 days. Moreover, MXene NIMs exhibit macroscopic flow behaviors at room temperature, which greatly improve processability and show extensive potential in addressing the daunting restacking issue when blending Ti3C2Tx MXene flakes with other polymer matrixes. Furthermore, the generality of this strategy was demonstrated by using various classes of alkaline oligomer species. As a proof of concept, Ti3C2Tx MXene NIMs (e.g., MX-I-M2070) were incorporated into the Pebax-1657 matrix to prepare mixed matrix membranes (MMMs), where the CO2 selective separation was investigated using experimental and molecular simulation (MS) methods. More notably, the rich library of alkaline oligomer species provides more convenience for further structural optimization, thus opening up plentiful opportunities for advancing the development of MXene NIMs towards other task-specific applications, such as catalysis and photothermal conversion.