Prog. Mater. Sci. (IF:48.165) | 2D MXene and carbon

The importance of carbon, one of the most important elements of life, is self-evident. Since the Paleolithic era, humans have tried to use carbon to meet our most basic needs. Today, scientists have successfully developed full-dimensional carbon materials, from 0D carbon quantum dots, 1D carbon nanotubes or bands, 2D graphene and 3D carbon foam, and so on, have been widely used in many fields. Therefore, carbon materials also play an extremely important role in the world we live in, serving our lives. The unparalleled cost advantage makes it a pioneer and leader in many application fields, and it can even be integrated with other materials to have better performance in some specific areas.

Since the discovery of 2D graphene, many 2D materials have been extensively studied, such as Transition Metal oxides, transition metal chalcogenides, mainly including sulfides and selenides, and transition metal carbides/nitride, hexagonal boron nitride, layered LDH, 2D MOF, phosphene, germane, etc. Although these 2D materials have a common 2D structure as a whole, the specific physical and chemical properties vary depending on the chemical composition, which also gives the 2D material family a wide range of applications. Among them, the layered transition metal carbide is the hottest star in the past decade, the so-called MXene, due to its gold-like properties of electrical conductivity, surface rich functional groups, such as -F,-OH and =O, excellent hydrophilicity and surface electronegativity and other comprehensive excellent characteristics. Since it was first synthesized in 2011, it has become a hot research topic for researchers around the world, because we know that most 2D materials exhibit insulating, semiconductor or semi-metallic properties, so the discovery of MXene is the dawn of the 2D material system. Not only that, compared to other two-dimensional finite components, MXene has a very rich diversity based on different atomic layers, thanks to the in-plane or out-of-plane metal atomic ordering, from M2X, to M3X2, to M4X3 and more recently M5X4, which brings more than 100 possible components to the MXene material system. This is unmatched by any other material so far. More importantly, according to the different etching conditions and methods of the precursor MAX phase, a large number of functional groups on the surface of MXene can be regulated, so that it can be targeted according to the application scenario to design the most favorable surface structure. The van der Waals force between the MXene layers can be overcome by simple ultrasonic field assistance, coupled with the electrostatic repulsion generated by the electronegativity between the MXene layers, it is easy to obtain a highly dispersive colloidal solution, and a composite membrane material with ultra-flexibility and ultra-high conductivity can be obtained by simple extraction and filtration. Because of this excellent comprehensive nature, 2D MXene is a versatile building blocks material whose applications since its discovery range from initial energy storage to catalysis, sensing, solar cells and emerging electromagnetic shielding, water treatment and biomedicine.

There are two sides to everything, although MXene materials have many advantages like the above, but its disadvantages in specific application fields we must face. For energy storage applications, MXene's high electrical conductivity and chemical strength can store a large amount of charge through pseudocapacitance behavior. However, the actual electrochemical performance of MXene-based Electrode materials is strictly dependent on the surface properties controlled by the synthesis conditions, and often faces serious self-stacking phenomena during long-term charge and discharge cycles. It greatly hinders the diffusion behavior of ions. And its own higher molecular weight period usually has unsatisfactory theoretical capacity levels. MXene in its pure phase is also difficult to describe as having sufficient performance for catalytic applications. For other applications, a similar situation has been a major puzzle for researchers. Here, composite, is the most effective way, because it can combine the advantages of different components, the combination of the two has a potential "chemical reaction", may bring synergistic effects for MXene materials, take the essence, go the dross, and help the performance of MXene-based materials to a new level.

Here, we systematically summarize the structural integration of "1+1 > 2", that is, "xD carbon + 2D MXene = ∞". According to different dimensions of carbon matrix, from low dimension to high dimension, 0D quantum dots to 3D carbon skeleton, and the integration of 2D MXene composite structure, Firstly, according to different types of carbon matrix and corresponding different hybrid methods, the paper summarizes and compares. Secondly, the structure-activity relationships of xD/ 2D-carbon /MXene heterostructures were compared according to the synthesis-structure-performance logic. Based on the timeline of the multi-dimensional carbon matrix and MXene complex, from the initial 1D CNTs recombination, 2D graphene assembly, to the 3D-derived carbon loading, and more recently, the anchoring of 0D carbon quantum dots, the scientists can clearly understand the vein of exploration. From the number of published articles, it is not difficult to see that the number of research papers related to MXene materials is increasing year by year, among which, the proportion of research on MXene-based carbon composites is also getting higher and higher, and more importantly, it is more and more abundant in terms of types, which is gratifying for the research progress of MXene. In the end, the research progress of MXene-based carbon composites was summarized and the future research direction was prospected.