MXene, 作为新兴的二维纳米材料家族，具有优异的电化学、电子、光学和机械性能。基于as an emerging family of 2D nanomaterials, exhibits excellent electrochemical, electronic, optical, and mechanical properties. MXene 的异质结构已经在超级电容器、传感器、电池和光催化剂等应用中得到证明。如今，越来越多的研究关注基于-based heterostructures have already been demonstrated in applications such as supercapacitors, sensors, batteries, and photocatalysts. Nowadays, increasing research attention is attracted onto MXene 的异质结构，而对当前研究状况的总结却很少。-based heterostructures, while there is less effort spent to summarize the current research status.
To date, although MXene-based materials have been demonstrated to be widely used in different areas, there are still some challenges . For example, the application of MXenes in fabricating flexible energy storage devices is limited due to the difficulty in achieving a good balance between mechanical and electrochemical properties . The serious stacking phenomenon of MXenes impedes the diffusion of carriers in the vertical direction, lowering the specific capacity of MXenes under a high current density . The poor oxidation resistance of MXenes in the application of the water-based flexible battery seriously affects its conductivity and cycling stability . To overcome these shortcomings, different 2D nanomaterial structures are suggested to be spliced or stacked on top of each other, and as a result, many novel physical properties have been discovered.
In 2013, Geim and Grigoreva proposed for the first time a multilayer heterostructure, namely, van der Waals (vdW) heterostructure, which is formulated by using only the vertical vdW force between different layers to connect each 2D material and allow them to coexist in a stable way . The discovery of 2D materials has breathed new life into the construction of heterogeneous structures. Traditional heterostructures are constructed by either doping homogenous materials, such as PN junctions of semiconductor silicon, or by epitaxial growth on lattice-matched substrate materials . In this way, the material is severely limited, and serious dislocations and defects are easily formed at the interface, thus affecting the quality of the heterostructures. However, the 2D layered material has no dangling bonds on its surface, and different 2D atomic layers can be stacked together in selected order by means of weak van der Waals forces to form artificial heterostructures with atomically flat interfaces. Compared with traditional semiconductor heterostructures, 2D vdW heterostructures are not limited by lattice matching and material types, and can theoretically be stacked in any form (different types, angles, sequences, layers, etc.) like stacking wood [42、43、44]. The “arbitrary combination” of the van der Waals heterostructure allows these individual materials to be combined together while still maintaining the ultra-thin thickness . Therefore, the emergence of vdW heterostructures offers a new structural platform for exploring new electronic and optoelectronic devices.
The family of MXene materials has a great variety and excellent electrochemical, optical, and mechanical properties. However, the realization of the applications of MXene materials is often limited by some inherent drawbacks. To overcome these issues, many novel heterostructures have been constructed based on the special optical and electrical properties of an individual 2D crystal, generating synergetic photoelectric properties, and therefore, wide attention has been received from researchers on this topic [53、54、55、56]. Generally, 2D heterostructures can be divided into two types: vertical heterostructures and lateral heterostructures. Two kinds of MXene-based heterostructures are described as following.
Lateral MXene-based heterostructures are generally prepared by seamlessly integrating 2D materials into one plane through direct growth, which can accurately control the direction and quality of the interface inside the 2D lateral heterostructures . The 2D lateral heterostructure is connected by covalent bonds, which provide excellent intralaminar stability and improve the epitaxial quality.
Compared with vertical MXene-based heterostructures, the construction of 2D lateral heterostructures is more difficult in practice, and we cannot randomly choose the initial 2D materials to construct any heterostructure as we desire. Although the 2D lateral heterostructures are difficult to synthesize, the advantages of covalent bonding in the atomic plane and easy plane integration arouse people’s great interest. Zeng et al.  prepared 2D lateral WC-graphene (WC-G) heterostructures based on a versatile approach, which demonstrated excellent chemical stability and reactivity, as seen in Figure 3. Currently, there are limited studies on 2D lateral MXene heterostructures, but due to the special properties and significant application potential, more research efforts on this topic can be expected in the coming years.
Two-dimensional heterostructures can be prepared by deterministic transfer methods, CVD epitaxial growth methods, and self-assembly . The various synthesis approaches of 2D heterostructures directly affect their physical and chemical properties, thus affecting their application fields . Generally, the deterministic transfer method and CVD epitaxial growth method are most often used to construct 2D heterostructures . PDMS, PPC, and PMMA are commonly used in deterministic transfer methods. As for the CVD epitaxial growth method, it is suitable for both vertical heterostructures and lateral heterostructures . By adjusting the temperature, composition, velocity, and direction of the flow, different types of heterostructures can be prepared. Currently, three major preparing methods have been proposed for constructing MXene-based heterostructures, namely, the hydrothermal method , electrostatic self-assembly method , and chemical vapor deposition .
The hydrothermal method  refers to the method of preparing materials by dissolving and recrystallizing powders with water as the solvent in a sealed pressure vessel. The hydrothermal method has the advantages of relatively mild operating conditions, high crystallinity of products, environmental friendliness, and good dispersity. In addition, the cost of hydrothermal synthesis is lower in terms of instrumentation, energy, and material precursors compared to gas and solid-phase methods. MXene is dispersed in the liquid phase with another material to obtain a heterostructure under hydrothermal conditions . Under the conditions of high temperature and high pressure, this method is able to improve the activity and manipulate the functional groups at the surface of MXene.
In practical applications, MXene-based heterostructures with rich functions are usually required. A hydrothermal environment can control the functional groups on the surface of MXene-based heterostructures, so as to improve their activity. Qiao et al.  designed and fabricated Ti3C2/CdS heterostructures for use as highly efficient co-catalysts by a hydrothermal strategy. The characterization results showed that the Ti3C2/CdS heterostructure was spontaneously decorated with a large number of hydrophilic functional groups (-OH and -O). In addition, the CdS/Ti3C2 heterostructure with a cauliflower structure showed ultra-high visible light photocatalytic activity and had great application potential in the field of photocatalysis. Wang et al.  constructed a 1T-MoS2/Ti3C2 MXene heterostructure for a supercapacitor via the hydrothermal method and studied the electrochemical storage mechanism of the heterostructure. The experimental results showed that the supercapacitor based on 1T-MoS2/Ti3C2 MXene heterostructure had a high capacitance ratio and excellent rate performance, and maintained an excellent cycling stability after tens of thousands of cycles because of the synergistic effect between MoS2 and MXene.在实际应用中，通常需要具有丰富功能的基于Under the hydrothermal MXene 的异质结构。水热环境可以控制基于 MXene 的异质结构表面的官能团，从而提高其活性。乔等人。[environment, the functional groups on the surface of MXenes are improved. Due 73to ]the 设计并制造了 Tielectrostatic interaction 3and other effects, the second phase dispersed Cin 2the /CliquidS 异质结构，通过水热策略用作高效助催化剂。表征结果表明，Ti phase can grow in situ 3on the surface Cof 2MXene, /CandS异质结构自发地被大量亲水性官能团（-OH和-O）修饰。此外，CdS/Ti the two kinds of materials 3are in Cclose 2具有花椰菜结构的异质结构显示出超高的可见光光催化活性，在光催化领域具有巨大的应用潜力。王等人。[contact 74to ]通过水热法构建了用于超级电容器的1T-MfoSrm a 2heterostructure, /Twhich 3has strong Cinterface 2interaction, MXexcellene异质结构，并研究了异质结构的电化学存储机理。实验结果表明，基于1T-MoSt electron 2transfer /Tability, 3and can Cprovide 2 MXene异质结构的超级电容器具有较高的电容比和优异的倍率性能，由于MoS 2之间的协同作用，在数万次循环后仍保持优异的循环稳定性。和a MXlargene。 在水热环境下，MX interface cones表面的官能团得到改善。由于静电相互作用等作用，分散在液相中的第二相可以在MXene表面原位生长，两种材料紧密接触形成异质结构，具有强界面相互作用，优异的电子传递能力，并能在界面处提供较大的界面接触面积。曹等人。tact area at the interface. Cao et al.  75successfully prepared ]a 通过水热策略成功制备了一种新型的novel 2D/2D Ti 3 C 2 /Bi 2 WO6 heterostructure 6异质结构。合成的through a hydrothermal strategy. The synthesized Ti3 C 2 /Bi 2 WO6 6异质结构表现出优异的光催化还原heterostructure showed an excellent ability forphotocatalytic reduction of CO2, 2的能力，这主要是由于合成的异质结构的比表面积和孔结构的改善，以及电荷转移距离短和界面接触面积大。which was mainly due to the improvement of the specific surface area and pore structure of the synthesized heterostructure, as well as the short charge-transfer distance and large interface contact area.
Electrostatic self-assembly  uses the electrostatic interaction of two kinds of nanomaterials with opposite charges in an aqueous solution for self-assembly, so as to form nanoscale ultra-thin polymer materials. Among many self-assembly methods, electrostatic self-assembly has a wide range of applications, owing to its simplicity and controllable thickness . As a common method for constructing two-dimensional heterostructures, a variety of MXene-based heterostructures have been constructed via electrostatic self-assembly and have been applied in many fields . However, electrostatic self-assembly is less stable due to the electrostatic interaction and hydrogen bonding.
In recent years, due to its simplicity, electrostatic self-assembly has also been adopted to synthesize photocatalysts with high photocatalytic activity. Hu et al.  synthesized 2D/2D Ti3C2/porous g-C3N4 (TC/PCN) photocatalysts through a facile electrostatic self-assembly method by integrating the merits of g-C3N4 and Ti3C2. The synthesized heterostructures exhibited exceptional performance compared with pure PCN and the observed activity had no significant decrease after four cyclic experiments. In another experiment, boron-doped graphite carbonitride (g-C3N4) and few-layer Ti3C2 MXene were combined to construct heterostructures by electrostatic self-assembly for enhanced photocatalytic reduction of CO2 . The optimized composite structure had excellent photocatalytic activity and stability. The yields of CO and CH4 were 3.2 times and 8.9 times higher than that of a bare g-C3N4, respectively. Zhuang et al.  successfully prepared TiO2/Ti3C2 heterostructures by the electrostatic self-assembly technique. The maximum hydrogen production rate was 2.8 times larger than that of pure TiO2 nanofibers, and the nanocomposite maintained a good hydrogen production cycle capacity, owing to the heterogeneous interface between TiO2 and Ti3C2 nanosheets.
At present, chemical vapor deposition (CVD) has been widely used to prepare vertical and lateral heterostructures. Compared with the stacking method, the MXene-based heterostructures prepared by CVD can obtain a very clean interface. In addition, high-quality MXene-based heterostructures can be synthesized by carefully controlling the preparation parameters. What’s more, the synthesized heterostructures have a strong interface interaction.目前，化学气相沉积（CVD）已广泛用于制备垂直和横向异质结构。与堆叠方法相比，CVD制备的Among the three common synthesis approaches of MXene基异质结构可以获得非常干净的界面。此外，通过仔细控制制备参数，可以合成高质量的基于-based heterostructures, the hydrothermal method has the advantages of relatively mild operating conditions, environmental friendliness, good dispersion, and low cost. At the same time, the activity of heterostructures can be improved and the functional groups on the surface of MXene 的异质结构。更重要的是，合成后的异质结构具有很强的界面相互作用。 在MXs can be manipulated in the hydrothermal environment. Electrostatic self-assembly is widely used because of its simple preparing procedures and controllable thickness. However, the surface of the cone基异质结构的三种常见合成方法中，水热法具有操作条件相对温和、环境友好、分散性好、成本低等优点。同时，可以提高异质结构的活性，并且可以在水热环境中操纵 MXenes 表面的官能团。静电自组装因其制备程序简单、厚度可控而得到广泛应用。但是，需要对构成材料的表面进行预处理。在CVD工艺中，可以调节压力、温度、气体流速、催化剂类型等参数，实现对MXene基异质结构尺寸、层数、形貌和质量的精细控制，CVD适用于合成垂直和横向异质结构。通过上述三种方法合成的高质量异质结构能够提供较大的界面接触面积和较短的界面电荷转移距离，并防止stituent materials needs to be pretreated. In the CVD process, the parameters such as pressure, temperature, gas flow rate, and catalyst type can be adjusted to realize the fine control of the size, layer number, morphology, and quality of MXene-based heterostructures, and CVD is applicable to synthesize both vertical and lateral heterostructures. The high-quality heterostructures synthesized by the aforementioned three methods are able to provide a large interface contact area and a short charge-transfer distance at the interface, as well as prevent the stacking of MXene 层的堆叠，从而产生改进的界面载流子传输。layers, generating an improved interfacial carrier transport.