In recent years, ternary layered carbide/nitride MAX phases and their derived two-dimensional nanolaminates MXenes have attracted extensive attention. The crystal structure of MAX phase is composed of M_n+1X_n unit interleaved with layers of A element. MAX phases combine good properties of metal and ceramic, which makes them promising candidates for high temperature structural materials, friction and wear devices, nuclear structural materials, etc. When etching the A-layer atoms of the MAX phase, the two-dimensional nanolaminates with the composition of M_n+1X_nT_x (T_x is surface termination), i.e. MXene, is obtained. MXenes have wide range of composition, and tunable physical and chemical properties, which endow them great potential in the applications of energy storage devices, electromagnetic shielding materials, and electronic devices, etc. In this paper, the research progress of MAX phase and MXene was introduced in terms of composition and structure, synthesis methods, and properties and application. Furthermore, the research prospects of this large family of materials were discussed.

Membrane-based gas separation is one of the critical technologies in filtration and separation industry, since it is more efficient, energy-saving and environmentally friendly compared with traditional separation technologies. Novel inorganic two-dimensional materials (2DMs) for gas separation are expected to achieve both high selectivity and high permeability, breaking through the trade-off between selectivity and permeability of commercial polymer membranes. This review begins with a brief explanation of gas separation mechanisms for membranes. Afterwards, special attention will be given to the recent advances in novel inorganic 2DMs including graphene and their derivatives, TMDs and MXene, about their design, fabrication and application in gas separation. The gas separation characteristics of different materials, their challenges and directions for future research are summarized. Moreover, the application of other novel inorganic 2DMs, such as LDH, h-BN and mica nanosheets in gas separation technology is also discussed. Finally, the perspectives and challenges for future research of novel inorganic 2DMs in gas separation field are outlined.

Two-dimensional materials have attracted broad interest because of their wide variety of properties. They can be used as photocatalysts and electrocatalysts due to their extremely high specific surface area, and have great potential application in the field of environment and renewable energy. This review focuses on the structure and properties of common two-dimensional materials such as 2D carbides and nitrides (MXenes), g-C₃N₄ and black phosphorus (BP). Furthermore, the latest research on the modification of two-dimensional materials in the area of photocatalysis and electrocatalysis are discussed and commented. Finally, research prospects for two-dimensional materials in the future are predicted.

With the development of wearable flexible electronic technology, the demand for flexible sensor with high sensitivity and wide sensing range is gradually increasing. The application of suitable conductive materials with high electrical conductivity and high flexibility as sensitive materials for sensors is the key to obtain high performance sensors. In recent years, MXene materials have become very promising sensitive materials due to their good conductivity, high flexibility, good hydrophilicity, and controllable synthesis. The types of MXene-based flexible force sensors, microstructure design of sensitive materials, sensing performance, and sensing mechanism analysis have been expound and summarized in this paper.

Electromagnetic interference (EMI) shielding films with excellent mechanical properties are highly promising for applications in flexible devices, automotive electronics and aerospace. Inspired by the excellent mechanical properties of nacre derived from its micro/nanoscale structure, high-performance MXene/Cellulose nanocrystals (CNC) composite films were prepared by simple solution blending and followed vacuum-assisted filtration process. The presence of CNC significantly improves the mechanical properties with tensile strength increasing from 18 MPa to 57 MPa and toughness improving from 70 kJ/m ³ to 313 kJ/m ³. Meanwhile, the composite film still exhibits high electrical conductivity (up to 10 ⁴ S/m) and excellent EMI shielding efficiency (over 40 dB) with a small thickness of 8 μm.

Ceramic matrix composites (CMCs) are promising candidates for application in aeroengine, aerospace aircraft thermal protection systems, nuclear power system, and other fields. At present, CMCs are developing from structural bearing materials to multi-functional composites. MAX phases are a group of layered ternary ceramics with excellent plastic deformation capacity, high electrical conductivity, good irradiation resistance and ablation resistance. Besides strengthening and toughening CMCs, the introducing MAX phases into CMCs can effectively improve the anti-irradiation, anti-ablation and electromagnetic interference shielding performance, meeting requirements of multi-functional CMCs. This paper reviewed the progress on MAX phases modified CMCs, design mechanism and application prospect.

Mo₂Ga₂C, a double-A-layer MAX, is reported to be films or powders. This paper researched the sintering properties of M₂Ga₂C powders to make dense bulk samples by vacuum hot pressing. It was found that 750 ℃ was a suitable sintering temperature, while higher temperature (850 ℃) resulted in decomposition of Mo₂Ga₂C yielding to main product of Mo₂C. During sintering process at 750 ℃, its grain size did not increase obviously with sintering time, meanwhile the size of pores decreased markedly and the relative density increased significantly with the increasing sintering time. Additionally, the hot-pressed samples had obvious texture. Due to layering, some grains changed their orientations during sintering, of which most of the (00l) planes in the hot-pressed samples preferred to be perpendicular to the direction of hot press. Almost fully densed Mo₂Ga₂C bulk (relative density: 98.8%) was obtained by hot pressing at 750 ℃ for 8 h. This advantage of the method suggested that it can serve as a promising preparation for Mo₂Ga₂C, a double-A-Layer MAX.

Equal channel angular pressing (ECAP) followed by heat treatment was carried out to prepare Ag/Ti₃AlC₂ composites. Effects of heat treatment on the electrical resistivities and mechanical properties of the Ag/Ti₃AlC₂ composites were investigated. Results show that ECAP effectively densifies the Ag/Ti₃AlC₂ compacts, and layered Ti₃AlC₂ particles are delaminated and aligned due to shearing effect during ECAP. Alignment of Ti₃AlC₂ particles resulted in anisotropy of electrical and mechanical properties of the composites. Perpendicular to the alignment of Ti₃AlC₂ particles displayed high resistivity and compressive strength. Moreover, resistivity and compressive strength increased with following heat treatment, yielding the maximum at 800 ℃. These increments are attributed to the enhanced interfacial reactions between Ag and Ti₃AlC₂ at high temperatures. Findings in this study indicate that densification and microstructural control of Ag/MAX composites can be achieved simultaneously by ECAP, while the following heat treatment can tailor their properties.

The MAX phases are a family of ternary layered material with both metal and ceramic properties, and it is also precursor materials for synthesis of two-dimensional MXenes. The theory predicts that there are more than 600 kinds of stable ternary layered MAX phase materials. Now, more than 80 kinds of ternary layered MAX phases that the M-site elements are mainly from early transition metal have been experimental synthesized, but few researches are reported on MAX phases where M is a rare earth element. In this study, Sc, Sn and C powders were used as raw materials to synthesize a novel ternary Sc₂SnC MAX phase via molten salt method. Phase composition and microstructure of Sc₂SnC were confirmed by X-ray diffraction, scanning electron microscope and X-ray energy spectrum analysis. And, structural stability, lattice parameters, mechanical and electronic properties of Sc₂SnC were investigated via density functional theory. The theoretical results show that Sc₂SnC is thermodynamically stable, and the Sc₂SnC is metallic in nature where the contribution from Sc-3d states dominates the electronic conductivity at the Fermi level. This study provides a route to explore more unknown ternary layered rare earth compounds Re_n+1SnC_n (Re=Sc, Y, La-Nd, n=1) and corresponding rare earth MXenes.

Phase diagrams are used as an indicator to estimate the thermodynamic stabilities of the novel MAX phases (Ti₃AuC₂, Ti₃IrC₂, Ti₃ZnC₂, Ti₂ZnC). The phase diagrams of the Ti-Au-C, Ti-Ir-C, and Ti-Zn-C systems were obtained using the CALPHAD (Calculation of Phase Diagrams) approach coupled with ab initio calculations. The calculated results confirmed thermodynamic stabilities of the synthesized Ti₃AuC₂, Ti₃IrC₂, Ti₃ZnC₂, and Ti₂ZnC MAX phases, which is in great agreement with the experiment information. The present work shows a systematic method to calculate the thermodynamic stability of the novel MAX phases, which can be used as guidance to synthesize more undiscovered MAX phases.

Two-dimensional MXene nanosheets with vertical junction structure was employed for easy immobilization of horse radish peroxidase enzymes to fabricate the electrochemical hydrogen peroxide (H₂O₂) biosensor. The synthesized MXene nanosheets exhibited large specific area, excellent electronic conductivity and good dispersion in aqueous phase. Horse Radish Peroxidase (HRP) enzymes molecules immobilized on MXene/chitosan/GCE electrode demonstrated good electrocatalytic activity toward reduction of H₂O₂. The fabricated HRP@MXene/chitosan/GCE biosensor exhibited a wide linear range from 5 to 1650 μmol?L ^-1, a limit of detection of 0.74 μmol?L ^-1 and good operation stability. The fabricated biosensor was successfully employed for detection of trace level of H₂O₂ in both solid and liquid food.

To explore the interface engineering on the carrier recombination in two-dimensional (2D) van der Waals (vdW) heterostructures, we developed a theoretical model to address the size-dependent interlayer and Auger recombination rates in MoS₂/WSe₂ in terms of interface bond relaxation method and Fermi's golden rule. It is found that the Auger recombination lifetime in MoS₂/WSe₂ increases with increasing thickness due to the weakening of Coulomb interaction between holes and electrons, as well as the Auger recombination rate is much smaller than that of MoS₂ and WSe₂ units. However, when the thin h-BN layer is introduced into the MoS₂/WSe₂, the interlayer and Auger recombination rates show opposite trends as the h-BN thickness increases. When the thickness of h-BN reaches 9.1 nm under the condition of 1L MoS₂/h-BN/1L WSe₂, the Auger recombination rate approaches 5.3 ns ^-1. These results indicate that the relevant recombination processes can be tuned by interface and dimension. Therefore, our results provide a useful guidance for the optimal design of 2D transition metal dichalcogenides-based optoelectronic nanodevices.

The shortage of fresh water resources resultes from the rapid growth of population and industrial development. Desalination of seawater and brackish water is an effective way to alleviate the freshwater crisis. In this work, the TiO₂/Ti₃C₂T_x composites were prepared by directly calcinating Ti₃C₂T_x for hybrid capacitive deionization (HCDI). The results show that the calcination temperature has a significant impact on morphology, structure, electrochemical and desalination behavior of the TiO₂/Ti₃C₂T_x composites. To constitute the full HCDI device, the optimized TiO₂/Ti₃C₂T_x and acid treated activated carbon (AC) was employed as cathode and anode, respectively. In the constant voltage mode, the salt removal capacity of TiO₂/Ti₃C₂T_x‖AC reached 23.8 mg·g ^-1 under the cell voltage of 1.2 V in NaCl solution with an initial conductivity of 3000 μS·cm ^-1. After 20 cycles, the capacity retention rate remains at 78%. Besides, through exploring the morphology and crystal texture evolution of TiO₂/Ti₃C₂T_x electrodes before and after desalination, it is found that the desalination of TiO₂/Ti₃C₂T_x electrodes may be achieved due to the intercalation of sodium ions into the Ti₃C₂T_x interlayer.