How are MXene Nanomaterials Synthesized?

By Osama AliJan 25 2022

The world has been revolutionized by using different kinds of materials to its advantage; two-dimensional (2D) materials have been on the horizon of material science due to their aspect ratio and few atomic layers thickness.

Image Credit: Konstantin Kolosov/Shutterstock.com

These 2-D materials can be optimized in their chemical, electronic and other properties, enhancing their importance in many well-known fields.

2-D Materials such as - MXenes

There have been multiple new and noteworthy 2D materials discovered after the 2004 discovery of graphene. This includes hexagonal boron nitrides, transition metal dichalcogenides (TMDs), and various single-element 2D compounds like silicene and borophene.

 Graphene and TMDs appear to be the two most common two-dimensional materials that feature in research at this time.

By etching an elemental layer of 'A' from the MAX phases, Naguib et al. in (2011) examined the exfoliation of 2D transition metal carbides. The suffix '-ene' indicates that these 2D-layered MXenes are akin to graphene.

MXenes are two-dimensional (2D) materials with different characteristics than their three-dimensional (3D) parent predecessors, created by selectively etching A layers from MAX phases. To represent MXenes, the formula M_n+1X_n T_x is now being used, where T denotes the functional groups (-O, -F, or -OH) that form during the etching process as a result of the compound's interaction with acids.

Three potential lattice structures of MXene are available because of the n values ranging from 1 to 3 in the phases of MAX. The first MXene was determined to be Ti₃C₂T_x,.

Structure of MXenes

MXenes synthesis involves acid etching of its surface, which results in the addition of fluorine functional groups, oxygen and hydroxyl. Density functional theory (DFT) simulations led to the first proposed multilayer (ML)-MXene (Ti₃C₂T_x) structure.

MXenes, possess closely packed hexagonal structures known as HCP. In M₂X, the HCP sequence of ABABAB is exhibited by M atoms. On the other hand, the other variations, M3X2 and M4X3, tend towards that FCC sequence, which is ABCABC.

To synthesize MXenes with HCP ordering in the bulk state, such as Mo and Cr carbides, this atomic arrangement is critical.

From the DFT calculations, it has been shown that hexagonal molybdenum carbides have greater stability than their FCC counterparts.

MXenes such as Mo₃C₂T_x and Mo₄C₃T_x, which have an arrangement of the M atoms similar to that of rock salt, are unstable through the formation energies.

Synthesis of MXenes

Synthesis of MXenes is achieved by removal of Layer 'A' from the MAX parents through selective etching. These acids, which contain aqueous fluoride, have been extensively utilized in the past as etchants for this purpose.

Powders are mixed with an acidic solution at room temperature and then allowed to cool down for a predetermined period of time. Thus, the MAX phase's A layers are selectively etched, and weak surface termination bonds such as oxygen, fluoride or hydroxyl are used to replace the metallic connections present through the MX layers on the surface ML-MXene.

A high-temperature etching process has also yielded MXenes from the MAX phase. To make the first nitride-based MXene in 2016, researchers used a fluoride salt molten solution (59 percent KF, 29 percent LiF and 12 percent NaF), heated to 550 degrees Celsius to remove the Al layer from Ti₄AlN₃ powder. Tetrabutylammonium hydroxide (TBAOH) was then delaminated to obtain a monolayer of MXene (Ti₄N₃T_x).

Properties

MXenes have exceptional metallic conductivity, hydrophilicity, and mechanical qualities that are not found in any other compound. It has already been demonstrated that the electron mobilities of various MXenes can exceed graphene (2.5x 10⁵cm² V^-1 s^-1) with values as high as 10⁶ cm² V^-1 s^-1.

The metallic conductance of Ti₃C₂T_x sheets has been measured at roughly 6500 S cm^-1, which is higher than that of other 2D materials, such as CNT and graphene.

Excellent bending stiffness goes hand in hand with wear resistance, high strength and hardness; M₂X MXenes have greater stiffness and strength qualities than M₃X₂ and M₄X₃ structures, which are both based on M₃X₂.

Further Developments and Future of MXenes

Around 70 MAX phases have been discovered, including solid solutions and ordered-M layers. Of those 70 phases, 20 are those that have been etched into a 2D MXene material.

Al had been successfully etched from Ti₃SiC₂ to generate Ti₃C₂ MXene, but the other 11 A elements in MAX phases had not been etched until 2018.

Since novel MAX phases and related two-dimensional MXenes are now a major research focus, MXenes are still in their infancy when it comes to a wide range of applications, such as hydrogen storage and supercapacitors. Its commercial and scientific applications require a lot of research and are an open field for further investigations.

Continue reading: Advancements in Synthesis Techniques of Mxene Nanomaterials.

References and Further Reading

Michael Naguib, et al (2011). Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. ADVANCED MATERIALS, 4248-4253. Available at: https://doi.org/10.1002/adma.201102306

Mochalin, S. H. (2019). Hydrolysis of 2D Transition-Metal Carbides (MXenes) in Colloidal Solutions. Inorganic Chemistry, 1721-2230. Available at: https://doi.org/10.1021/acs.inorgchem.8b02890

O. Salim, et al (2019). Introduction to MXenes: synthesis and characteristics. Materials Today Chemistry. Available at: https://doi.org/10.1016/j.mtchem.2019.08.010

Patrick Urbankowski, et al (2016). Synthesis of two-dimensional titanium nitride Ti4N3 (MXene). Nanoscale. Available at: https://doi.org/10.1039/C6NR02253G

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