Recently, the National Institute of Materials Technology and Engineering, Chinese Academy of Sciences, special fiber and nuclear energy materials engineering laboratory has synthesized a new two-dimensional nanocrystalline single crystal material of pre-transition metal carbide. This work was published online by the international journal Angewandte Chemie-International Edition as a VIP (very important paper, top 5%) article (DOI: 10.1002/anie.201510432).
Due to its high specific surface area, unique electronic structure and physical and chemical properties, two-dimensional materials have attracted widespread attention. As the most widely studied two-dimensional material, graphene exhibits a wide range of applications in electrochemical energy storage, transparent electrode materials, and nanocomposites due to its high mechanical strength, excellent electrical conductivity, and thermal conductivity. However, the intrinsic zero band gap and single chemical composition limit its application in fields such as field effect transistors. The study of binary and ternary two-dimensional materials such as metal oxides, layered metal chalcogenides, hexagonal boron nitride, and layered hydroxides has attracted increasing attention.
The two-dimensional layered transition metal carbide nanosheets (MXenes) are a new type of two-dimensional material discovered in recent years. Michel Barsoum, a professor at the University of Drexel in the United States, has done a lot of pioneering research in this field, and the laboratory has been successively acquired. Ti3C2Tz, Ti2CTz, Ta4C3Tz, TiNbCTz, (V0.5, Cr0.5)3C2Tz, Ti3CNTz, Nb2CTz, V2CTz, Nb4C3Tz, Mo2TiC2Tz, Mo2Ti2C3Tz, Mo2C and other MXene structures. MXenes have high specific surface area, good conductivity and hydrophilicity. Theoretical predictions of such materials have high elastic modulus and high carrier mobility, and have good application prospects in conductive materials and functionally reinforced composites.
Previous studies have found that a variety of cations can be spontaneously inserted into the MXenes material layer, and therefore have a good application prospect in the field of energy storage. As reported in previous studies, Ti3C2Tz, Ti2CTz, V2CTz, and Nb2CTz can be used as electrode materials for lithium-ion batteries and supercapacitors. They have high specific capacitance (up to 410 mAh/g @ 1 C) and volumetric capacitance ( Up to 900F/cm3) and good charge and discharge cycle stability (Science, 2013, 341, 1502-1505; Nature 2014, 516, 78-81). Therefore, MXenes is considered to be a new generation of two-dimensional nano-functional materials with great development potential.
Because of this, how to preemptively synthesize transition metal carbide materials with rich d-electronic structure has become the focus of worldwide attention. At present, MXenes are mainly prepared by using HF acid, NH4HF2 solution, LiF and HCl mixed solution at room temperature or slightly higher than room temperature, and for the MAX phase material with Al at A position (a material system of more than 70 members). Al atoms are selectively etched and obtained. Since the transition metals Zr and Hf hardly form the MAX phase with the A-site Al, the MXenes of Zr-based and Hf-based materials have not been reported so far.
The special fiber and nuclear energy materials engineering laboratory of Ningbo Institute of Materials Co., Ltd. uses high-purity new-type Zr3Al3C5 layered carbide obtained by SPS (sintered-in-place plasma sintering) as a precursor, and HF acid as an etchant. The selective peeling bond is weaker. The easily hydrolyzed Al-C structural unit, for the first time, obtained a Zr-based two-dimensional MXenes material (as shown in the figure). Since Zr3Al3C5 is a member of a new type of ternary MnAl3C2 or quaternary Mn[Al(Si)]4C3 carbides (where M = Zr or Hf, n = 1,2,3), nearly 20 species have been found in experimental studies. Ternary or quaternary carbides, so this study provides new ideas for the synthesis of new two-dimensional carbide materials.
The researchers of the engineering laboratory used the experimental research and theoretical calculation (DFT) method to finally determine that the chemical composition of the Zr-based two-dimensional material is Zr3C2Tx, with a large interlayer spacing (1.63 nm), which can be obtained after ultrasonic treatment. The material exhibits graphene-like curls and wrinkles. In argon or vacuum environments, compared with Ti3C2Tz, Zr3C2Tx materials exhibit excellent structural stability, and can maintain their two-dimensional properties at 1200°C. However, the two-dimensional structure of Ti3C2Tz is destroyed and the phase transformation of TiC is established. The direct parameter binding energy, which reflects the structural stability, was calculated to reveal the source of the structural stability differences between the Zr3C2Tz and Ti3C2Tz materials.
In addition, researchers at the Ningbo Institute of Materials further predicted the structural, elastic, and electrical properties of the synthesized Zr3C2Tz material. Zr3C2T2 (T=O, F, OH) exhibited metallic properties with good conductivity; the C11 value of Zr3C2O2 was as high as 392.9 GPa. This study provides a new perspective for expanding the chemistry, controllable synthesis and application of MXenes materials. The new type of Zr3C2Tz MXenes is expected to be used in high-temperature radiation-damaged nuclear energy structural materials, energy storage (lithium batteries, lithium-sulfur batteries, super capacitors, etc.) and intelligent polymers. Materials, gas sensors and catalysis are gaining application.
This work was funded by the National Natural Science Foundation of China (91226202 and 91426304) and the Cross Innovation Team Project of the Chinese Academy of Sciences.
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