MAX/MAB phases are a series of non-van der Waals ternary layered ceramic materials with a hexagonal structure, rich in elemental composition and crystal structure, and embody physical properties of both ceramics and metals. They exhibit great potential for applications in extreme environments such as high temperature, strong corrosion, and irradiation. In recent years, two-dimensional (2D) materials derived from the MAX/MAB phase (MXene and MBene) have attracted enormous interest in the fields of materials physics and materials chemistry and become a new 2D van der Waals material after graphene and transition metal dichalcogenides. Therefore, structural modulation of MAX/MAB phase materials is essential for understanding the intrinsic properties of this broad class of layered ceramics and for investigating the functional properties of their derived structures. In this paper, we summarize new developments in MAX/MAB phases in recent years in terms of structural modulation, theoretical calculation, and fundamental application research and provide an outlook on the key challenges and prospects for the future development of these layered materials.

Excessive emission of greenhouse gases has serious adverse effects on global climate. How to reduce carbon emissions has become a global problem. Supercapacitors have advantages of long cycle life, high power density and relatively low carbon emissions. Developing supercapacitor energy storage is an effective measure to build the reliable future energy system. In recent years, MXene materials have achievedgreat popularity in the field of supercapacitor energy storage applications due to their excellent hydrophilicity, electrical conductivity, high electrochemical stability, and surface chemical tunability. However, the serious self-stacking problem of MXene limits its performance in energy storage. Developing advanced MXene materials is critical for next generation high-performance electrochemical energy storage devices. This paper reviews the research progress of MXene material in the field of supercapacitor energy storage. Firstly, the structure and energy storage properties of MXene are introduced, followed by analysis of the energy storage mechanism of MXene. Secondly, nanoengineering of structure design to improve the performance of MXene electrode is depicted. Thirdly, structure-performance relationship of MXene composite materials and its latest research progress in application of supercapacitor are summarized. Finally, research and development trends of MXene as an electrode for supercapacitor are broadly prospected.

Two-dimensional (2D) perovskite displays great potential in optoelectronic applications due to its inherent quantum well structure, large exciton binding energy and good stability. However, facile preparation of high-quality 2D perovskite films with low cost remains a huge challenge. In this work, high-quality two-dimensional perovskite (PEA)₂PbI₄ films were prepared by solution method at low annealing temperature(80 ℃) without other special treatments, and further applied in the field of photodetectors. The results show that this photodetector possessed a low dark current (10^-11 A), good responsiveness illuminated at a wavelength of 450 nm (107 mA·W^-1), high detection rate (2.05×10¹² Jones) and fast response time (250 μs/330 μs). After 1200 s continuous illumination, the device maintains 95% initial photocurrent. In addition, the photocurrent remains almost unchanged after storage for 30 d. This work provides promising strategy to develop stable and high-performance optoelectronic devices.

Lithium-sulfur batteries (LSBs) have attracted wide attention due to their high energy density, abundant raw material reserves and environmental friendliness. However, the shuttle effect of polysulfides, the large volume expansion during the reaction, and the poor electron conductivity of sulfur greatly limit their practical development. In this work, a ZIF-8 derived flower-like two-dimensional (2D) porous carbon nanosheet/sulfur composite (ZCN-SnS₂-S) combined with SnS₂ nanoparticles is designed as the cathode for LSBs. The unique 2D flower-shaped porous structure not only effectively alleviates the volume expansion during the reaction process, but also provides a fast channel for Li⁺ and electron transport. The presence of heteroatom N further promotes the adsorption of polysulfide. In particular, the polar SnS₂ enhances the chemical adsorption on polysulfides, resulting in excellent electrochemical performance. The ZCN-SnS₂-S electrode exhibits high reversible specific capacity of 948 mAh·g^-1 after 100 cycles at 0.2C (1C=1675 mA·g^-1), demonstrating the capacity retention rate of 83.7%. Even at a high current density of 2C for 300 cycles, it still has a reversible specific capacity of 546 mAh·g^-1.

Suffering from strong electrostatic interactions between divalent Zn²⁺ and host framework, molybdenum disulfide exhibits slow reaction kinetics as cathode for aqueous zinc-ion batteries. The narrow layer spacing of MoS₂ is difficulty in accommodating large size insertion of hydrated Zn²⁺, resulting in a lower discharge specific capacity. Here, NH₄⁺ expanded MoS₂-N was prepared by a simple ammonia-assisted hydrothermal. The result showed that the ammonia promoted hydrolysis of thioacetamide to provide reduced S^2- and generated a large amount of NH₄⁺ as intercalating particles. These particles expanded the layer spacing of pristine MoS₂ from 0.62 nm to 0.92 nm, greatly reducing the Zn²⁺ inserting energy barrier (with its charge transfer resistance of MoS₂-N only 35 Ω), and increased the discharge specific capacity to 149.9 mAh·g⁻¹ at the current density of 0.1 A·g^-1, 2 times that of MoS₂ electrode without NH₄⁺ expansion. Consequently, it exhibited a stable discharge capacity of about 110 mAh·g^-1 at the current density of 1.0 A·g^-1 with nearly 100% Coulombic efficiency after 200 cycles. The approach of ammonia-assisted layer expansion proposed in this study enriches the modification strategy to enhance the electrochemical performance of MoS₂ and provides a new idea for subsequent cathode development.

Ti₃C₂T_x MXene is a potential adsorbent of heavy metal ions due to its two-dimensional layered structure and abundant surface functional groups. However, it has disadvantages of limited layer spacing and poor stability in aqueous solution. Here, the modification strategy of Ti₃C₂T_x was explored to improve its chemical stability and ion adsorption capacity among which Fe₃O₄-Ti₃C₂T_x(FeMX) adsorbent with different doping amounts of Fe₃O₄ were prepared by one-step hydrothermal method. The results showed that the maximum theoretical Pb(II) adsorption capacity of FeMX adsorbent could reach 210.54 mg/g. Its adsorption mechanism was further revealed that Fe₃O₄ nanoparticles were evenly dispersed and intercalated between Ti₃C₂T_x nanosheets, which effectively increased specific surface area and layer spacing of Ti₃C₂T_x nanosheets, leading to improving Pb(II) removal ability. Therefore, this study provides a promising route for developing MXene matrix composites with excellent heavy metal ion adsorption properties.

Ag-based electrical contact plays a key role in low-voltage switches, which is intended to substitute the traditional and toxical “universal” contact of Ag/CdO. As a new kind of two-dimensional carbide material with good electrical conductivity and thermal conductivity, Ti₃C₂T_x, a representative of MXenes has showed exceptional potential in various fields, including being the reinforcement phase in electrical contact materials to substitute for the toxic CdO. In this work, we successfully prepared Ag/Ti₃C₂T_x composite by powder metallurgy. Phase and microstructure of the Ti₃C₂T_x and Ti₃AlC₂ were characterized, and their properties, such as electrical resistivity, microhardness, machinability, tensile strength, and anti-arc erosion performance were investigated and compared. The Ag/Ti₃C₂T_x has a resistivity of 30×10 ^-3 μΩ·m, 29% lower than that of Ag/Ti₃AlC₂ (42×10 ^-3 μΩ·m) and excellent machinability with intermediate microhardness (64 HV), showing broad application prospect as non-toxic electrical contact materials. Its improved conductivity is mainly attributed to the metallicity of Ti₃C₂T_x itself, the microstructural features, endowed by the deformability of Ti₃C₂T_x. However, the tensile strength (32.77 MPa) of Ag/Ti₃C₂T_x is inferior to that of Ag/Ti₃AlC₂ (145.52 MPa) due to lack of Al-Ag interdiffusion. The anti-arc erosion performance of Ag/Ti₃C₂T_x is also unmatchable with Ag/Ti₃AlC₂ due to absence of Al layer. Although the arc erosion resistance of Ag/Ti₃C₂T_x needs to be further improved uptill now, the significantly improved electrical conductivity makes it a potential substitute of current toxic Ag/CdO material. All results of this work provide an exploration direction for developing new environmentally friendly electrical contact material in the future.

Two-dimensional (2D) materials have brought about significant technological advancements in the field of biomaterials. Transition metal carbides and/or nitrides (MXenes) have a planar structure educed from their corresponding parent MAX phase by selective etching of ‘A’ and further delamination. Since the first MXene was reported in 2011, MXenes now comprise a rapidly growing family of 2D materials, having attracted extensive attention from researchers. Owing to their excellent electronic properties, outstanding photothermal conversion performance, high specific surface area, good biocompatibility, and low toxicity, MXenes have shown a good application prospect in tumor theranostics. This paper reviews substantive findings of the original researches focused on the preparation, property and application in tumor theranotics, including recent advances, challenges and future perspectives of MXenes. Firstly, we briefly summarize the preparation methods and property of MXenes, including HF acid method, fluoride salt method, molten salt method, alkali-assisted hydrothermal method, and chemical vapor deposition method, as well as stability, mechanical, optical, and electrical properties. Secondly, we focus on the application of MXenes in photothermal therapy and combined therapy. The usual method is to combine photothermal therapy, photodynamic therapy and chemotherapy to carry out multi-modal combined treatment of tumors. The combined therapy can also be improved by constructing surface nanopores of MXenes and loading chemotherapy drugs in them. Furthermore, enhanced MXenes synergistic therapeutic effect on tumor and reduced toxic side effects on normal tissue can be endued by active targeting technology. In addition, the preparation of multifunctional MXenes composite nanomaterials to obtain radiation treatment and imaging capabilities such as computed tomography scans and magnetic resonance imaging, can establish an integrated platform for MXenes theranostics. Finally, we briefly introduced other applications of MXenes in biomedicine which are beneficial to tumor theranostics, and elaborate the current challenges and future development prospects of MXenes in cancer theranostics.

MXenes have been widely studied for their excellent specific surface area, high conductivity and composition tunability, which have been used as a highly efficient electrode material for lithium-ion batteries (LIBs). However, limited storage capacity and severe lattice expansion caused by Li-ions diffusion restrict the application of MXenes as electrode materials. Here, Ti₃C₂ MXenes with surface halogenation (fluorination, chlorination and bromination) as representative MXene materials were designed. Effects of surface functionalization on the atomic structures, electronic properties, mechanical properties, and electrochemical performance of Ti₃C₂T₂ (T = F, Cl and Br) anode in LIBs were investigated using first-principles calculations based on density functional theory with van der Waals correction. The results reveal that Ti₃C₂T₂ MXenes exhibit metallic conductivity with improved structural stability and mechanical strength. Compared with Ti₃C₂F₂ and Ti₃C₂Br₂, Ti₃C₂Cl₂ exhibits the large elastic modulus (321.70 and 329.43 N/m along x and y directions, respectively), low diffusion barrier (0.275 eV), high open circuit voltage (0.54 eV), and storage capacity (674.21 mA·h/g) with stoichiometric ratio of Ti₃C₂Cl₂Li₆, which renders the enhanced rate performance and endures the repeated lattice expansion and contraction during the charge/ discharge process. Moreover, surface chlorination yields expanded interlayer spacing, which can improve Li-ion accessibility and fast charge-discharge rate in Ti₃C₂Cl₂. The research demonstrates that Cl^- terminated Ti₃C₂ is a promising anode material, and provides effective and reversible routes to engineering other MXenes as anode materials for LIBs.

Defects at the surface and grain boundary of the three-dimensional (3D) organic-inorganic metal halide perovskite film incline to cause non-radiative recombination of charge carriers and accelerate decomposition of 3D perovskite, in turn deteriorating the power conversion efficiency (PCE) and stability of the perovskite solar cells (PSCs). In this study, the organic 4-chlorobenzylamine cation was applied to react with 3D perovskite and the residual PbI₂ to in-situ form a two-dimensional (2D) perovskite top layer, which can passivate the surface and grain boundary defects of the 3D perovskite film, and improve the surface hydrophobicity. Based on this strategy, 2D/3D-PSCs with higher PCE and better stability were successfully obtained. Their structure, morphology photoelectric propery and stability of PSCs were systematically studied. All results show that 2D/3D-PSCs achieve PCEs up to 20.88%, much higher than that of 18.70% for the 3D-PSCs. In addition, 2D/3D-PSCs can maintain 82% of the initial PCE after 200 h continuous operation under 1-sun illumination in N₂ atmosphere, exhibiting excellent stability.

The photocatalysts deactivation is one of the major issues, which lowers the usefulness of photocatalytic oxidation technology for the removal of low content volatile organic compounds (VOCs). Here, we carried out a series of experiments to demonstrate that the photocatalysts stability could be significantly improved via coupling the oxide base semiconductors, i.e., TiO₂ with 2D materials such as graphitic carbon nitride (g-C₃N₄). Initially, when Ag modified TiO₂ (AT) was used for the gaseous acetaldehyde degradation, a robust deactivation was observed within 60 min. The AT catalyst completely lost its activity when the reaction time was extended to 400 min. On the contrary, the g-C₃N₄ modified AT (CAT) showed superior photocatalytic performance and improved stability (600 min). The in-situ FT-IR, PL, and photocurrent studies suggested that the accumulation of reaction intermediates in the case of AT fundamentally caused the deactivation. However, the g-C₃N₄ provided excessive adsorption sites for the reaction by-products which improved the stability. Additionally, the PL and ESR studies suggested that the existence of g-C₃N₄ improved the charge separation and production of reactive oxygen species, which facilitated the photodegradation of acetaldehyde and ultimate reaction products. This study realizes the usefulness of 2D materials for developing stable and visible light active photocatalysts for applications in sustainable VOC abatement technology.

Graphitic carbon nitride (g-C₃N₄) is widely used in the field of photocatalysis due to its unique two-dimensional planar structure and suitable energy band structure. However, it has some disadvantages such as fast recombination of the electron-hole, low visible-light utilization efficiency and poor dispersion in water, which hinder its application. In this study, the hydrogel prepared by sodium alginate was used as matrix to improve the dispersion of Ag@C₃N₄ composite in water, and at the same time enhanced the separation efficiency of photoelectron-holes pairs, thus improving its photocatalytic performance. Firstly, g-C₃N₄ was synthesized by thermal polymerization and then exfoliated into nanosheets by ultrasound. Then, Ag nanoparticles were deposited in situ on the surface of g-C₃N₄ by solution method to prepare Ag@C₃N₄. Finally, hydrogel loaded with Ag@C₃N₄ (SA/Ag@C₃N₄) was obtained by using calcium ion as crosslinker and sodium alginate (SA) as precursor. The morphology, microstructure and phase composition of the as-prepared photocatalyst were characterized. The as-prepared SA/Ag@C₃N₄ exhibited a 1.5 times higher photocatalytic degradation rate of methyl orange than that of Ag@C₃N₄. The catalytic mechanism was investigated by photoluminescence spectrum, time resolved photoluminescence spectrum and electron paramagnetic resonance spectrum. The results showed that the surface plasmon resonance effect of silver nanoparticles together with the porous structure and mass transfer channel of sodium alginate hydrogel plays a synergistic role in the enhancement of photocatalytic performance.

Oxygen reduction reaction (ORR) is the key reaction in cathode for fuel cells. Because of the sluggish kinetics, platinum (Pt) is widely used as the electrocatalysts for ORR. However, the high cost of Pt and poor stability of carbon black support under high voltage limit the commercialization and durability of fuel cells. Two-dimensional transition metal dichalcogenides (2D TMDs) possess large specific area, tunable electronic structure, and high chemical stability, making them a good candidate for ORR catalysts with high activity and durability. This paper reviews the recent progress of 2D TMDs-based ORR electrocatalysts. First, crystal structure, electronic properties, and ORR reaction mechanism are briefly introduced. Then some strategies for adjusting ORR performance of 2D TMDs are summarized, including heteroatom doping, phase conversion, defect engineering, and strain engineering. Meanwhile, the ORR activity enhancement arising from 2D TMDs-based heterostructures is also analyzed. Finally, perspectives are given for current issues and their possible solutions.

Two-dimensional (2D) monolayer MoSi₂N₄ has attracted wide attention due to its excellent carrier transport capacity and chemical stability. However, the relationship between its photoelectric properties and applied plane strain has not been thoroughly explored. The effect of plane strain on band structures and photoelectric properties of 2D monolayer MoSi₂N₄ is revealed by the plane-wave ultrasoft pseudopotentials. The results show that the monolayer MoSi₂N₄ is an indirect band gap semiconductor. Its top of valance band is dominated by Mo4d orbitals and partly contributed by N2p orbitals, while its bottom of conduction band is mainly contributed by Mo4d orbitals. Under tensile strain, band gap of monolayer MoSi₂N₄ narrows gradually and effective mass of photogenerated carriers decreases continuously. Under compressive strain, the band gap widens gradually and the effective mass increases slowly. It is worth noting that a compressive strain (ε=-2.8%) results in transition form indirect to direct band gap. Optical absorption of monolayer MoSi₂N₄ exhibits obvious anisotropy, which edge shifts in different degree under the plane strain, effectively expanding the spectral response range of the system and beneficial to the photoelectric properties. These results provide a theoretical guidance for further research on the application of 2D monolayer MoSi₂N₄ in the field of new tunable nano optoelectronic devices.

Black phosphorus (BP) with excellent and unique physical and chemical properties has emerged as the most promising semiconductor for energy storage and conversion, micro-nano devices, photo- and electro-catalysis, biomedicine, and so on. It is crucial to synthesize high-quality precursors of orthorhombic BP for realizing the applications of two-dimensional BP and zero-dimensional BP quantum dots. Herein, the effects of mineralizer components and ratios on BP growth were studied by the chemical vapor transport (CVT) method without temperature gradient. The results indicate that orthorhombic BP can be synthesized under some experimental combinations that can be considered viable only when tin (or lead) and iodine coexist together with the appropriate ratio. And the mass ratio ranges of tin and iodine w(Sn/I₂) for BP preparation is wide, and the size of BP crystal obtained at w(Sn/I₂)=0.47 is up to 1.2 cm, of which yield and crystal quality are superior. Combined with the nucleation and growth mechanism of BP, tin and iodine are severely significant for the nucleation and growth of BP, which has been widely accepted. Mineralization effect of iodine is more obvious than that of tin, and sufficient tin contributes to the synthesis of large-size bulk BP crystals without temperature gradient. As a result, w(Sn/I₂)=0.47 is the optimal minera lizer ratia for fabricating orthorhombic BP in this work.

Two-dimensional transition metal dichalcogenides are appealing materials for the preparation of nanoelectronic devices, and the development of memristors for information storage and neuromorphic computing using such materials is of particular interest. However, memristor arrays based on two-dimensional transition metal dichalcogenides are rarely reported due to low yield and high device-to-device variability. Herein, the 2D MoTe₂ film was prepared by the chemical vapor deposition method. Then the memristive devices based on 2D MoTe₂ film were fabricated through the polymethyl methacrylate transfer method and the lift-off process. The as-prepared MoTe₂ devices perform stable bipolar resistive switching, including superior retention characteristics (>500 s), fast switching (~60 ns for SET and ~280 ns for RESET), and excellent endurance (>2000 cycles). More importantly, the MoTe₂ devices exhibit high yield (96%), low cycle-to-cycle variability (6.6% for SET and 5.2% for RESET), and low device-to-device variability (19.9% for SET and 15.6% for RESET). In addition, a 3×3 memristor array with 1R scheme is successfully demonstrated based on 2D MoTe₂ film. And, high recognition accuracy (91.3%) is realized by simulation in the artificial neural network with the MoTe₂ devices working as synapses. It is found that the formation/rupture of metallic filaments is the dominating switching mechanism based on the investigations of the electron transport characteristics of high and low resistance states in the present MoTe₂ devices. This work demonstrates that large-scale two-dimensional transition metal dichalcogenides film is of great potential for future applications in neuromorphic computing.