Two-dimensional MXenes are generally prepared by the etching of acid solutions. The as-synthesized MXenes are terminated by acid group anions (F^–, Cl^–, etc.), which affect the electrochemical performance of MXenes. Here, we report a novel method to prepare Mo₂C MXene from Mo₂Ga₂C by the hydrothermal etching of alkali solutions. Highly pure Mo₂C MXene was successfully synthesized by the etching of NaOH, while the etchings of LiOH and KOH were failed. The concentration of NaOH, temperature, and time strongly affect the purity of as-prepared MXene. Pure Mo₂C MXene could be synthesized by the etching of 20 M NaOH at 180 ℃ for 24 h. After intercalation by hexadecyl trimethyl ammonium bromide at 90 ℃ for 96 h, few-layer Mo₂C MXene was obtained. The Mo₂C MXene made by NaOH etching after intercalation exhibited excellent performance as anode of lithium-ion battery, compared with general Mo₂C MXene made by HF etching and the Mo₂C MXene reported in literature. The final discharge specific capacity was 266.73 mAh·g⁻¹ at 0.8 A·g⁻¹, which is 52% higher than that Mo₂C made by HF etching (175.77 mAh·g⁻¹). This is because Mo₂C MXene made by NaOH etching has lager specific surface area, lower resistance, and pure O/OH termination without acid anion termination. This is the first report to make Mo₂C MXene by alkali etching and the samples made by this method exhibited significantly better electrochemical performance than the samples made by general HF etching.

Nowadays, photocatalytic technologies are regarded as promising strategies to solve energy problems, and various photocatalysts have been synthesized and explored. In this paper, a novel CdS/MoO₂@Mo₂C-MXene photocatalyst for H₂ production was constructed by a two-step hydrothermal method, where MoO₂@Mo₂C-MXene acted as a binary co-catalyst. In the first hydrothermal step, MoO₂ crystals with an egged shape grew on the surface of two-dimensional (2D) Mo₂C MXene via an oxidation process in HCl aqueous solution. In the second hydrothermal step, CdS nanorods were uniformly assembled on the surface of MoO₂@Mo₂C-MXene in ethylenediamine with an inorganic cadmium source and organic sulfur source. The CdS/MoO₂@Mo₂C-MXene composite with MoO₂@Mo₂C-MXene of 5 wt% exhibits an ultrahigh visible-light photocatalytic H₂ production activity of 22,672 μmol/(g·h), which is ~21% higher than that of CdS/Mo₂C-MXene. In the CdS/MoO₂@Mo₂C-MXene composite, the MoO₂ with metallic nature separates CdS and Mo₂C MXene, which acts as an electron-transport bridge between CdS and Mo₂C MXene to accelerate the photoinduced electron transferring. Moreover, the energy band structure of CdS was changed by MoO₂@Mo₂C-MXene to suppress the recombination of photogenerated carriers. This novel compound delivers upgraded photocatalytic H₂ evolution performance and a new pathway of preparing the low-cost photocatalyst to solve energy problems in the future.

MAX phases (Ti₃SiC₂, Ti₃AlC₂, V₂AlC, Ti₄AlN₃, etc.) are layered ternary carbides/nitrides, which are generally processed and researched as structure ceramics. Selectively removing A layer from MAX phases, MXenes (Ti₃C₂, V₂C, Mo₂C, etc.) with two-dimensional (2D) structure can be prepared. The MXenes are electrically conductive and hydrophilic, which are promising as functional materials in many areas. This article reviews the milestones and the latest progress in the research of MAX phases and MXenes, from the perspective of ceramic science. Especially, this article focuses on the conversion from MAX phases to MXenes. First, we summarize the microstructure, preparation, properties, and applications of MAX phases. Among the various properties, the crack healing properties of MAX phase are highlighted. Thereafter, the critical issues on MXene research, including the preparation process, microstructure, MXene composites, and application of MXenes, are reviewed. Among the various applications, this review focuses on two selected applications: energy storage and electromagnetic interference shielding. Moreover, new research directions and future trends on MAX phases and MXenes are also discussed.

The effect of etching environment (opened or closed) on the synthesis and electrochemical properties of V₂C MXene was studied. V₂C MXene samples were synthesized by selectively etching of V₂AlC at 90 ℃ in two different environments: opened environment (OE) in oil bath pans under atmosphere pressure and closed environment (CE) in hydrothermal reaction kettles under higher pressures. In OE, only NaF (sodium fluoride) + HCl (hydrochloric acid) etching solution can be used to synthesize highly pure V₂C MXene. However, in CE, both LiF (lithium fluoride) + HCl and NaF+HCl etchant can be used to prepare V₂C MXene. Moreover, the V₂C MXene samples made in CE had higher purity and better-layered structure than those made in OE. Although the purity of V₂C obtained by LiF+HCl is lower than that of V₂C obtained using NaF+HCl, it shows better electrochemical performance as anodes of lithium-ion batteries (LIBs). Therefore, etching in CE is a better method for preparing highly pure V₂C MXene, which provides a reference for expanding the synthesis methods of V₂C with better electrochemical properties.

Two-dimensional carbide MXenes (Ti₃C₂T_x and V₂CT_x) were prepared by exfoliating MAX phases (Ti₃AlC₂ and V₂AlC) powders in the solution of sodium fluoride (NaF) and hydrochloric acid (HCl). The specific surface area (SSA) of as-prepared Ti₃C₂T_x was 21 m²/g, and that of V₂CT_x was 9 m²/g. After intercalation with dimethylsulfoxide, the SSA of Ti₃C₂T_x was increased to 66 m²/g; that of V₂CT_x was increased to 19 m²/g. Their adsorption properties on carbon dioxide (CO₂) were investigated under 0–4 MPa at room temperature (298 K). Intercalated Ti₃C₂T_x had the adsorption capacity of 5.79 mmol/g, which is close to the capacity of many common sorbents. The theoretical capacity of Ti₃C₂T_x with the SSA of 496 m²/g was up to 44.2 mmol/g. Additionally, due to high pack density, MXenes had very high volume-uptake capacity. The capacity of intercalated Ti₃C₂T_x measured in this paper was 502 V·v^–1. This value is already higher than volume capacity of most known sorbents. These results suggest that MXenes have some advantage features to be researched as novel CO₂ capture materials.

Two-dimensional (2D) carbide Ti₃C₂ was synthesized by exfoliating Ti₃AlC₂ in HF solution and used for supercapacitive performance investigation in 3 M KOH electrolyte. The specific surface area (SSA) of as-synthesized Ti₃C₂ was 22.35 m²/g. Ti₃C₂-based supercapacitor electrodes exhibited good energy storage ability and had a volumetric capacitance 119.8 F/cm³ at the current density of 2.5 A/g. Moreover, the addition of carbon black into Ti₃C₂ powders greatly improved the performance of Ti₃C₂-based capacitors because carbon black restrained the preferred orientation of 2D Ti₃C₂, providing fast ion transport channels, and in turn, decreasing electrical resistance from 16.7 Ω to 3.5 Ω.