MXenes

In materials science, MXenes are a class of two-dimensional inorganic compounds along with MBorenes, that consist of atomically thin layers of transition metal carbides, nitrides, or carbonitrides. MXenes accept a variety of hydrophilic terminations. The first MXene was reported in 2011 at Drexel University's College of Engineering, and was named by combining the prefix "MAX" or "MX", with "ene" by analogy to graphene.

Structure

As-synthesized MXenes prepared via HF etching have an accordion-like morphology, which can be referred to as multi-layer MXene, or few-layer MXene for fewer than five layers. Because the surfaces of MXenes can be terminated by functional groups, the naming convention M_n+1X_nT_x can be used, where T is a functional group.

Mono transition

MXenes adopt three structures with one metal on the M site, as inherited from the parent MAX phases: M₂C, M₃C₂, and M₄C₃. They are produced by selectively etching out the A element from a MAX phase or other layered precursor, which has the general formula M_n+1AX_n, where M is an early transition metal, A is an element from group 13 or 14 of the periodic table, X is C and/or N, and n = 1–4. MAX phases have a layered hexagonal structure with P6₃/mmc symmetry, where M layers are nearly close packed and X atoms fill octahedral sites. Therefore, M_n+1X_n layers are interleaved with the A element, which is metallically bonded to the M element.

Double transition

Double transition metal MXenes can take two forms, ordered double transition metal MXenes or solid solution MXenes. Ordered double transition metal MXenes have the general formulas: M'₂M"C₂ or M'₂M"₂C₃ where M' and M" are transition metals. Double transition metal carbides that have been synthesized include Mo₂TiC₂, Mo₂Ti₂C₃, Cr₂TiC₂, and Mo₄VC₄. In some of these MXenes, the Mo or Cr atoms are on outer edges of the MXene and control electrochemical properties.
Solid-solution MXenes have the general formulas: C, C₂, C₃, or C₄, where the metals are randomly distributed throughout the structure in solid solutions leading to continuously tailorable properties.

Divacancy

By designing a parent 3D atomic laminate, ₂AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, 2D Mo_1.33C sheets with ordered metal divacancies may be possible.

Synthesis

MXenes are typically synthesized by a top-down selective etching process. This synthetic route is scalable, with no loss or change in properties with batch size. Producing a MXene by etching a MAX phase occurs mainly by using etching solutions that contain a fluoride ion, such as hydrofluoric acid, ammonium bifluoride, and a mixture of hydrochloric acid and lithium fluoride. For example, etching of Ti₃AlC₂ in aqueous HF at room temperature causes the A atoms to be selectively removed, and the surface of the carbide layers becomes terminated by O, OH, and/or F atoms. MXene can also be obtained in Lewis acid molten salts, such as ZnCl₂, and a Cl terminal can be realized. The Cl-terminated MXene is structurally stable up to 750 °C. A general Lewis acid molten salt approach can etch most of MAX phases members by some other melts.
The MXene Ti₄N₃ was the first nitride MXene reported, and is prepared by a different procedure than for carbide MXenes. To synthesize Ti₄N₃, the MAX phase Ti₄AlN₃ is mixed with a molten eutectic fluoride salt mixture of lithium fluoride, sodium fluoride, and potassium fluoride and treated at elevated temperatures. This procedure etches out Al, yielding multilayered Ti₄N₃, which can be delaminated into single and few layers by immersing the MXene in tetrabutylammonium hydroxide, followed by sonication.
MXenes can also be synthesized directly or via CVD processes. In 2024, single crystalline monolayer W5N6 was synthesized by CVD in wafer scale, which shows promise for electronic applications.
In a 2018 report, Peng et al. described a hydrothermal etching technique. In this etching method, the MAX phase is treated in the solution of acid and salt under high pressure and temperature conditions. The method is more effective in producing MXene dots and nano-sheets. Moreover, it is safer since HF fumes are not released during the etching process.

Types

Mono transition

2-1 MXenes: Ti₂C, V₂C, Nb₂C, Mo₂C
Mo₂N, Ti₂N, C, C, C, W_1.33C, Nb_1.33C, Mo_1.33C, Mo_1.33Y_0.67C
3-2 MXenes: Ti₃C₂, Ti₃CN, Zr₃C₂ and Hf₃C₂
4-3 MXenes: Ti₄N₃, Nb₄C₃, Ta₄C₃, V₄C₃, ₄C₃
5-4 MXenes: Mo₄VC₄

Double transition

2-1-2 MXenes: Mo₂TiC₂, Cr₂TiC₂, Mo₂ScC₂
2-2-3 MXenes: Mo₂Ti₂C₃

Covalent surface modification

2D transition-metal carbides surfaces can be chemically transformed with a variety of functional groups such as O, NH, S, Cl, Se, Br, and Te surface terminations as well as bare MXenes. The strategy involves installation and removal of the surface groups by performing substitution and elimination reactions in molten inorganic salts. Covalent bonding of organic molecules to MXene surfaces was demonstrated through reaction with aryl diazonium salts. Moreover, heating and re-termination experiments of Ti₃C₂T_x showed that H₂O, with a strong bonding to the Ti-Ti bridge-sites, can be considered as a termination species. An O and H₂O terminated Ti₃C₂T_x-surface restricts the CO₂ adsorption to the Ti on-top sites and may reduce the ability to store positive ions, such as Li⁺ and Na⁺. An O and H₂O terminated Ti₃C₂T_x-surface shows the capability to split water.

Intercalation and delamination

Since MXenes are layered solids and interlayer bonding is weak, intercalation of guest molecules is possible. Guest molecules include dimethyl sulfoxide, hydrazine, and urea. For example, N₂H₄ can be intercalated into Ti₃C₂₂ with the molecules parallel to the MXene basal planes to form a monolayer. Intercalaction increases the MXene c lattice parameter, which weakens the bonding between MX layers. Ions, including Li⁺, Pb²⁺, and Al³⁺, can also be intercalated into MXenes, either spontaneously or when a negative potential is applied to a MXene electrode.

Delamination

Ti₃C₂ MXene produced by HF etching has accordion-like morphology with residual forces that keep MXene layers together, preventing separation into individual layers. Despite the weakness of these forces, ultrasound treatment produces only low yields of single-layer flakes. For large scale delamination, DMSO is intercalated into ML-MXene powders under constant stirring to further weaken the interlayer bonding and then delaminated with ultrasound treatment. This results in large scale layer separation and formation of the colloidal solutions of the FL-MXene. These solutions can later be filtered to prepare MXene "paper".

MXene clay

For the case of Ti₃C₂T_x and Ti₂CT_x, etching with concentrated hydrofluoric acid leads to an open, accordion-like morphology with a short distance between layers. To be dispersed in suspension, the material must be pre-intercalated with something like DMSO. However, when etching is conducted with hydrochloric acid and LiF as a fluoride source, morphology is more compact with a larger inter-layer spacing, presumably due to intercalated water. The material has been found to be 'clay-like': as seen in clay materials, Ti₃C₂T_x demonstrates the ability to expand its interlayer distance hydration and can reversibly exchange charge-balancing Group I and Group II cations. Further, when hydrated, the MXene clay becomes pliable and can be molded into desired shapes, becoming a hard solid upon drying. Unlike most clays, however, MXene clay shows high electrical conductivity upon drying and is hydrophilic. It disperses into single layer two-dimensional sheets in water without surfactants. Further, due to these properties, it can be rolled into free-standing, additive-free electrodes for energy storage applications.

Material processing

MXenes can be solution-processed in aqueous or polar organic solvents, such as water, ethanol, dimethyl formamide, propylene carbonate, etc., enabling various types of deposition via vacuum filtration, spin coating, spray coating, dip coating, and roll casting. Studies considered ink-jet printing of additive free Ti₃C₂T_x inks and inks composed of Ti₃C₂T_x and proteins.
Lateral flake size plays a role in the observed properties. Several synthetic routes produce varying flake sizes. For example, when HF is used as an etchant, the intercalation and delamination step require sonication to exfoliate material into single flakes. The resulting flakes are several hundreds of nanometers in lateral size. This is beneficial for applications such as catalysis and specific biomedical and electrochemical applications. However, if larger flakes are warranted, especially for electronic or optical applications, defect-free, large area flakes are necessary. This can be achieved by Minimally Intensive Layer Delamination, where the quantity of LiF to MAX phase is scaled up resulting in flakes that can be delminated in situ when washing to neutral pH.
Post-synthesis processing techniques to tailor the flake size include sonication, differential centrifugation, and density gradient centrifugation procedures. Post processing methods rely heavily on the as-produced flake size.
Sonication can decrease flake size from 4.4 μm, to an average of 1.0 μm after 15 minutes of bath sonication, down to 350 nm after 3 hours of bath sonication. By utilizing probe sonication, flakes can be reduced to an average of 130 nm in lateral size. Differential centrifugation, also known as cascading centrifugation, can be used to select flakes based on lateral size by increasing the centrifuge speed sequentially from low speeds to high speeds and collecting the sediment. Large, "medium" and "small" flakes can be obtained. Density gradient centrifugation applies a density gradient in the centrifuge tube. Flakes move through the centrifuge tube at different rates based on the relative flake density. To sort MXenes, a sucrose and water density gradient can be used from 10 to 66 w/v %. Using density gradients allows for more mono-disperse distributions in flake sizes and studies show the flake distribution can be varied from 100 to 10 μm without employing sonication.