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.

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.

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.

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.

Mn ²⁺ intercalation strategy to optimize the sodium storage performance of V₂C MXene was studied. The intercalated Mn ²⁺ not only enlarged the interlayer spacing of V₂C MXene but also formed a V-O-Mn covalent bond, which was beneficial to stabilize the structure of V₂C and inhibit the structural collapse caused by volume change during Na ⁺ decalation or intercalation. As a result, the intercalated V₂C MXene (V₂C@Mn) electrode showed a high specific capacity of 425 mAh·g ^-1 at the current density of 0.05 A·g ^-1, and 70% retention after 1200 cycles. This result clearly suggests that cations intercalated MXene has a great prospect in Na ⁺ storage.

Ge nanoparticles were synthesized uniformly on MXene sheets via a one-step chemical solution method. Morphology of Ge/MXene was characterized by SEM and TEM. Formation process and optimized synthesis condition was analyzed carefully. Ge/MXene was used as anode for lithium-ion batteries. Their electrochemical performances, including capacity, rate and cycling stability, were tested and evaluated. Ge/MXene exhibited a greatly improved capacity of 1200 mAh/g during the first hundred cycles at 0.2C with a loading of 1 mg/cm ². A capacity of 450 mAh/g at a higher loading of 2 mg/cm ² was obtained after 100 cycles. The excellence in electrochemistry is attributed to the high conductivity of MXene and its accommodable interlayer space.

Recently, a new type of 2D transition metal carbides or nitrides (MXene) has attracted wide attention due to its large specific surface area, good hydrophilicity, metallic conductivity and other physical and chemical properties. 2D Ti₃C₂T_x MXene was obtained by etching Al layer of Ti₃AlC₂ with LiF and HCl and then mechanically delaminated. And the monolayer Ti₃C₂T_x nanosheets with lateral dimension of 625 and 2562 nm can be prepared by changing the intensity and way of mechanically delamination, as well as the centrifugation rate and time. Then their morphology, structure, composition, and electrochemical performance of Ti₃C₂T_x were studied. The results showed that the specific capacitance of Ti₃C₂T_x with smaller lateral size (<1 μm) can reach 561.9 F/g, higher than that of reported graphene, carbon tube and MnO₂ in the repotted literatures. And the Ti₃C₂T_x electrode still remained 96% of the initial specific capacitance after 10 ⁴ testing cycles.

As a new class of two-dimensional transition metal carbon/nitride, MXenes have been proven to be a kind of pseudocapacitive supercapacitor electrode materials with excellent electrochemical property, and hold promise in practical use in the near future. In practical applications, it is required to make the electrode materials into planar porous electrodes for capacitor assembly. Herein, a simultaneous ammonization/carbonization method is proposed for the preparation of MXene planar porous electrode. Filter paper was used as a planar porous template, and MXene was coated on the fibers of the filter paper by means of dipping-drying, and then heat-treated in an ammonia atmosphere to obtain MXene/carbon planar porous composite electrodes. Analysis results show that the MXene nanosheets are uniformly coated on the carbonization-derived carbon fibers of the filter paper. When immersed 5 times, the areal capacitance reaches 403 mF/cm ² at a scan rate of 2 mV/s. After the composite electrode was tested for 2500 times in a galvanostatic charge-discharge cycle at a current density of 10 mA/cm ², the capacitance was almost the same as the initial capacitance, showing good rate performance and cycle stability. The MXene/carbon planar porous composite electrodes prepared by simultaneous ammonia/carbonization exhibit excellent electrochemical performance without using either polymer binder or metal current collector.

In order to rapidly remove Eu(III) from aqueous solution, an alkalized two-dimensional titanium carbide, Na-Ti₃C₂T_x, was successfully prepared by treating inorganic two-dimensional transition metal carbide (MXene) with NaOH. Adsorption behavior of Eu(III) on Na-Ti₃C₂T_x was systematically investigated by batch experiments. The results show that the adsorption process is greatly affected by pH and ionic strength of the solution, and reached equilibrium within 5 min. Based on Langmuir model fitting results, the maximum adsorption capacity of Eu(III) on Na-Ti₃C₂T_x was calculated to be 54.05 mg/g at pH 4.0 under 298 K. The thermodynamic results suggested that the adsorption process was a spontaneous and endothermic reaction. The adsorption mechanism was further analyzed by energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD) and extended X-ray absorption fine structure spectroscopy (EXAFS). These data revealed that Na ⁺ ions inside MXene galleries were exchanged by Eu ³⁺ ions and Eu(III) existed dominately in under outer-sphere surface complexation after adsorption under acidic pH conditions, but in inner-sphere surface complexation under near-neutral pH conditions. Due to its cost-effective prepatation and excellent sorption performance, Na-Ti₃C₂T_x may be a promising candidate for the efficient removal of trivalent minor actinides and lanthanides from radioactive wastewater.

Direct methanol fuel cells have good application prospects due to their advantages of convenient operation, high conversion efficiency, low operating temperature, low pollution, and easy storage and easy transportation of liquid fuel. However, existing anode catalysts have shortcomings such as low catalytic activity and poor resistance to CO toxicity which restrict its commercial application. In this study, a series of PtRu/(Ti₃C₂T_x)_0.5-(MWCNTs)_0.5 anode catalyst materials with different Pt and Ru ratios were prepared by three-step method. Ti₃C₂T_x was obtained by HF corrosion of Ti₃AlC₂, and acidified multi-walled carbon nanotubes (MWCNTs). After the compounding, Pt and Ru particles are supported by a solvothermal method. The synergistic relationship of Ru and Pt atoms was analyzed by XRD, SEM, EDS, TEM, and XPS. The results show that the Ru atoms are mixed with the Pt atoms to form PtRu bimetallic alloy with a particle size of about 3.6 nm. The electrochemical results show that the Pt₁Ru_0.5/(Ti₃C₂T_x)_0.5-(MWCNTs)_0.5 catalyst has the best electrochemical performance. Its electrochemical active area (ECSA) is 139.5 m ²/g, and positive peak current density is 36.4 mA/cm ².

As a new two-dimensional transition metal carbides, MXene has various potential applications, such as energy storage, catalyst, composite material, and luminescent materials for their excellent physical and chemical properties. The element doping, geometrical defect, surficial functionalization, external electric field, and external strain can be used as effective methods for modulation of their properties. Ti₂CO₂, the thinnest Ti-based MXene, exhibits semiconducting character. The effects of electric field on the band structure of perfect primitive Ti₂CO₂ were explored in this work. The results revealed that the band gap of perfect primitive Ti₂CO₂ decreased with the increasing electric field. Carbon (C) vacancy in Ti₂CO₂ MXene was easily produced during the preparation process. Further investigation showed that the tensile strain could be used to regulate the conductivity of this system as the bands around the Fermi energy become smoother with increasing tensile strain. The investigation of charged C vacancy doped 2×2×1 Ti₂CO₂ indicated that its Fermi energy decreased with the increase of charge state. When it was +2 charged, the C vacancy doped 2×2×1 Ti₂CO₂ exhibited semiconducting character and owned a direct band gap of 0.489 eV.

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.

The three-dimensional layered compound MAX phase has excellent mechanical property of both metals and ceramics, which is generally considered as a kind of high safety structural materials. In recent reports, V₂(Sn, A)C (A = Fe, Co, Ni and Mn) materials showed that antiferromagnetic property can be obtained by inserting the subgroup elements into A layer of MAX phase by molten salt method. However, how to further regulate magnetic properties of MAX phase through design of its crystal structure has attracted the attention of scholars in the field of spintronics and other fields. In this work, four new MAX phases of (V, Nb)₂(Sn, A)C (A = Fe, Co, Ni and Mn) were synthesized based on M/A double solid solution by molten salt method, and proved to be synthesized successfully. Magnetic property of the MAX phases was checked by SQUID (superconducting quantum interference device magnetometer). It is found that the change of Curie temperature is correlated with tetragonal ratio (c/a) and elemental composition. Changes of lattice parameters, tetragonal rate and magnetic results before and after introducing Nb element into M site were further compared. Besides, the H_c and M_r of (V, Nb)₂(Sn, Fe)C, (V, Nb)₂(Sn, Ni)C, and (V, Nb)₂(Sn, Mn)C decreased and the M_r increased compared with V₂(Sn, A)C (A = Fe, Ni, Mn) before introducing Nb element into M site. All these results were opposite after introducing Nb element into M site of V₂(Sn, Co)C which reveals the influence of M/A-site doble solid solution to the magnetic property of MAX phase, and provides a new way for tailoring magnetic property of MAX phase.