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 nanocomposite materials 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 nanosheet (MXenes) is a new type of two-dimensional material discovered in recent years. A large number of pioneering researches have been conducted by Michel Barsoum of the University of Drexel in the United States. The laboratory has been successively acquired. Structures such as Ti3C2Tz, Ti2CTz, Ta4C3Tz, TiNbCTz, (V0.5, Cr0.5)3C2Tz, Ti3CNTz, Nb2CTz, V2CTz, Nb4C3Tz, Mo2TiC2Tz, Mo2Ti2C3Tz, Cr2TiC2Tz, Mo2CTz, Ti4N3Tz, etc. 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, Nb2CTz, etc. 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 as 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, the preparation of MXenes is mainly carried out in HF acid, NH4HF2 solution, LiF and HCl mixed solution and eutectic mixed salt medium for Al material in the MAX phase A material (a material system for more than 70 members) Al Atoms are selectively etched. Since the transition metals Zr and Hf hardly form a MAX phase with an A-site Al, the MXenes materials of the Zr-based and Hf-based systems have not been reported so far. The special fiber and nuclear energy materials engineering laboratory of Ningbo Institute of Materials, Chinese Academy of Sciences adopts high-purity new type Zr3Al3C5 layered carbide obtained by in-situ reaction discharge plasma sintering (SPS) as a precursor, and uses HF acid as an etchant to selectively peel bonds. Weak, easily hydrolyzed Al-C structural units, for the first time obtained Zr-based two-dimensional MXenes materials. This work has been published in the international journal Angewandte Chemie-International Edition (128, 5092-5097, 2016).
Compared to Zr-based materials, Hf-based layered carbides are more difficult to obtain a single phase, and the mixed phases of Hf3Al3C5, Hf3Al4C6 and Hf2Al4C5 ternary compounds are generally obtained, and due to the strong intersublayer interface bonding, the study found that Directly using ternary Hf-Al-C composites as precursors, it is difficult to obtain Hf-based two-dimensional materials by selective etching, and the resulting exfoliation products are mainly cubic phase HfC. Existing studies have shown that single-phase solid solutions based on these ternary phases are relatively more readily available and contribute to improved phase purity. In addition, taking into account the strong interaction between Hf-C and Al-C laminae, in order to further achieve effective stripping, the interface between Hf-C and Al-C sublayers within the unit cells is regulated to weaken Hf- The interface between C and Al-C laminae is very important. The researchers used a solid solution method to tune the intracellular sublayers. A small amount of Si was introduced at the Al site. The new Hf2[Al(Si)]4C5 and Hf3[Al(Si)]4C6 solid solution materials were synthesized by the SPS method. The solid solution is a precursor, and the selective stripping of Al(Si)-C structural units was achieved using HF acid as an etchant. For the first time, a two-dimensional Hf series MXenes material was obtained. With the aid of binding energy and atomic charge calculations, the microscopic mechanism of the Si-doping promoting hydrofluoric acid stripping process was elucidated. Since Si has one more valence electron than Al, doping Al atoms can effectively weaken the Hf atomic layer and the stripped sheet. The interfacial bonding between the layers of Al(Si)4C4 reduces the corresponding binding energy directly from 8.60 eV to 4.05 eV, so that the introduction of Si achieves the effective tuning of the intra-monomer HfC and Al(Si)-C plate interfaces. , significantly weakened the interface binding, and thus achieved the peeling. Hf series novel two-dimensional carbide materials have potential applications in energy storage, wave absorption, and optoelectronic devices. The laboratory found that it has excellent electrochemical energy storage characteristics, and the volumetric specific capacity was 1567mAhcm-3and 504mAhcm-3 after 200 cycles of 200mAg-1 in the lithium battery and sodium battery tests, respectively. The high volumetric material is expected to be applied to miniaturized energy supply systems that can be applied to space vehicles and mobile equipment. The work on the new Hf-based MXene two-dimensional material has recently been received and published by the international journal ACS Nano (DOI: 10.1021/acsnano.7b00030).
In addition, the laboratory collaborated with Chunyi Chung, a professor at City University of Hong Kong, to obtain Ti3C2 type MXene materials with quantum dots using conventional hydrothermal methods. The quantum dot material has good fluorescence characteristics and biocompatibility, and is expected to be widely used in non-rare earth light-emitting display materials, biomarkers, and photothermal therapy and the like. This work will also be published in the 2017 issue of Advanced Materials DOI: 10.1002/adma.201604847.
At present, research on MXene materials in the international arena is in the ascendant and is gradually becoming a new research hotspot following two-dimensional materials such as graphene, molybdenum disulfide, and black scales. Chinese scientists' breakthroughs in the synthesis of MXene materials corresponding to Zr and Hf systems will effectively expand people's horizons for understanding two-dimensional materials and provide new materials for the research of nano-energy devices and optoelectronic devices.
The above work was supported by the National Natural Science Foundation of China (21671195, 11604346, 51502210, 21577144, and 91426304) and the Nuclear Energy Materials Innovation Team of the Chinese Academy of Sciences.
Hf is a schematic diagram of the synthesis of MXene materials and an atomic force microscope topography. At present, the transition zone metal region of the periodic table has been synthesized corresponding MXene materials, of which Zr and Hf are synthesized by the Chinese Academy of Sciences.
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