Ten Years of Progress in the Synthesis and Development of MXenes

Since their discovery in 2011, the number of 2D transition metal carbides and nitrides (MXenes) has steadily increased. Currently more than 40 MXene compositions exist. The ultimate number is far greater and in time they may develop into the largest family of 2D materials known. MXenes’ unique properties, such as their metal-like electrical conductivity reaching ≈20 000 S cm−1, render them quite useful in a large number of applications, including energy storage, optoelectronic, biomedical, communications, and environmental. The number of MXene papers and patents published has been growing quickly. The first MXene generation is synthesized using selective etching of metal layers from the MAX phases, layered transition metal carbides and carbonitrides using hydrofluoric acid. Since then, multiple synthesis approaches have been developed, including selective etching in a mixture of fluoride salts and various acids, non-aqueous etchants, halogens, and molten salts, allowing for the synthesis of new MXenes with better control over their surface chemistries. Herein, a brief historical overview of the first 10 years of MXene research and a perspective on their synthesis and future development are provided. The fact that their production is readily scalable in aqueous environments, with high yields bodes well for their commercialization. 

Properties Of MAX Phase

MAX phase exhibits outstanding performance of metals and ceramics, possessing the following properties:

• Higher thermal conductivity

• Thermal shock resistance and good damage tolerance

• Strength of corrosion resistance

• High elastic modulus

• High –temperature oxidation resistance similar to ceramics

• Some MAX phases show exceedingly good oxidation and oxidation induced self-healing ability resistance and can be used as high-temperature structure material.

Electrical and electronic properties

 The electrical and electronic properties of various types of MXene vary primarily with surface terminal functionalities and stoichiometry. Further, Schematic illustration of synthesis of MXene via selective etching of “A” element from MAX phase using various etching agents, followed by intercalation and delamination.2D metal carbides and their hybrid nanostructure: fundamental, synthesis, and applications 241 the electrical conductivity of the pressed (disc-shaped) MXene is similar to that of layered graphene and more than the carbon nanotubes (CNTs) and reduced graphene oxide. Moreover, it is observed that the number of layers and functional groups on the MXene increases resistivity value . The measured electrical conductivity of Ti3C2Tx varies from 850 to 9880 S cm21 , which is mainly attributed to (i) the surface terminal functional group (type and extent); (ii) the interlayer distance between the flakes; and (iii) the lateral size of the MXene sheets by different etching process. Less etching time with low concentration of HF creates fewer defect sites and more lateral size furnishes better electrical conductivity . Furthermore, humidity in the environment might affect their electrical conductivity which could be useful in developing humidity sensors. Further, to improve the electrical and electronic properties of MXene sheets, various treatments, including thermal and chemical, on the surface can be carried out using different methods.

Chemical properties

A number of functional moieties, such as 2 F, 5 O, and 2 OH, are present on the surface of MXene after selective etching of the “A” layer from the MAX phase. MXene with surface terminal groups was considered as stable, due to the possibility of replacement of the terminal -F upon washing with water or water storage [36]. Further, Naguib et al. reported that Ti3C2Tx oxidizes into CO2 under pressurized water and air. On the other hand, the anatase form of TiO2 was found after oxidation of the MXene (Ti3C2Tx) and this was studied by Ghassemi et al. through TEM analysis. It was anticipated that there is formation of anatase and rutile forms of the TiO2 during flash and slow oxidation. In addition, another report suggests the formation of anatase or rutile phases of the TiO2, when MXene reacts with air (O2) and the subsequent uniform segregation of TiO2 on the surface and between the layers of the MXene, resulting in increased d-spacing.

 Electrochemical properties 

MXenes are considered to be one of the most promising candidates for energy storage and conversion processes, including batteries and supercapacitors, due to their unique properties. The MXenes and their related composites exhibit high surface area, large specific capacities, high energy Fundamentals and Supercapacitor Applications of 2D Materials and power densities, and hence these properties make them capable materials for electrochemical energy storage applications. 

• Application

MXenes are a new family of two-dimensional (2-D) materials that show promising properties for sensors. MXenes have been modified with the enzyme AChE and chitosan to create a biosensor for detecting organophosphate pesticides (OPs) in water. MXenes-Ti3C2 is an excellent immobilization matrix with biocompatibility for proteins. O-terminated MXene nanosheet can be used as ammonia (NH3) sensors. MXenes can also be used to detect common volatile organic molecules such as acetone, ethanol, methanol, and ammonia.

In other words, MXenes are demonstrated to be a promising novel 2-D nanomaterial with significant potential applications in sensing due to the high surface area to volume ratio and particular surface structure with practicable surface functionalization, Electrical storage devices According to several researches conducted today, the identical MAX phases can create various MXene Nano sheets with varied material characteristics. This is a result of using several paths that have an impact on the final material, such as exfoliation chemicals and post-treatments. The conductivity of the bulk material may alter as a result. It is also shown that different exfoliates may be used to alter the terminal groups of the nanosheets throughout the production operations. The characteristics of MXenes can be improved by taking into account all these variables for uses in electrical storage or chemical sensing.

https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201702196

https://www.jmst.org/article/2020/1005-0302/1005-0302-37-0-77.shtml