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MXenes are among the most promising families of 2D materials. These carbides and nitrides acquire surface terminations, which are a mix of O, OH, and F in the case of manufacturing from MAX phases using fluoride-containing solutions, or halogens, in the case of molten salt etching, gaseous halogen etching of MAX, or direct synthesis. These terminations make them thermodynamically stable or metastable.

Most MXenes (dozens of compositions) are produced and processed in aqueous solutions in open air, often at elevated temperatures, which by itself implies a certain level of environmental stability. However, like any non-oxide material, they can react with water and oxygen in the air. Hydrolysis in aqueous colloids accelerates by light, elevated temperature, and dissolved oxygen.  Oxidation or hydrolysis may also occur in wet air, leading to the formation of transition metal oxides and amorphous carbon, hydrocarbons in the case of hydrolysis, or carbon oxides in the case of oxidation.

Initial reports on the hydrolysis and oxidation of highly defective delaminated single-layer MXene flakes in dilute aqueous colloids created an impression of low environmental stability of MXenes. In fact, highly stoichiometric Ti3C2Tx (the most studied and frequently used MXene) or Nb4C3Tx can stay in deaerated aqueous colloids for over a year at room temperature, even when delaminated.  However, for prolonged storage, I recommend refrigerating a highly concentrated dispersion or a slurry in a sealed container filled with Ar or nitrogen.

Halogen-terminated MXenes have been less studied, but we know that Ti2CCl2 produced by direct gas phase synthesis didn’t change its composition after storage in a vial for a year with no special precautions.

To achieve high environmental stability, one should use stoichiometric MAX phase precursors, because oxygen in the carbon sublattice severely affects the environmental stability. Minimizing fluorine terminations that make Ti3C2Tx more prone to hydrolysis, increasing flake size, and decreasing the concentration of surface vacancies and pinholes caused by aggressive etching will ensure high environmental stability.

After processing into films/coatings and drying, Ti3C2Tx maintains its chemical composition, crystal structure, and high electrical conductivity for many years. 11-year-old films (the oldest we have in our lab) stored in the open air are still electrically conductive. However, hydrophilic surface of O/OH terminated MXenes makes them hygroscopic, and water can intercalate between the layers, especially at very high humidity and elevated temperature, decreasing the conductivity of the films due to swelling. The use of tertiary amines for delamination increases interlayer spacing in assembled films/coating. Unless large interlayer spacing is desired in applications, ion exchange and substitution with Li or Na ions is recommended. Removing intercalants and drying above 200°C can prevent water adsorption. Dry Ti3C2Tx MXene films are stable in air to ~400°C and in inert environment to ~800°C.

It is important to consider that thinner (M2X) structures are less stable compared to thicker ones (M3X2 or M4X3).  Metals that form soluble compounds in acidic and basic solutions, like V, make MXenes more susceptible to hydrolysis. Those are fundamental limitations that should be considered. Addition of antioxidants (e.g., ascorbates) or metal ions, such as Na⁺, that fill vacancies on the surface, covalent or non-covalent functionalization with polymers (e.g., polydopamine) can increase MXene shelf life, but one should consider the possible effect on material properties. Therefore, we prefer increasing the materials quality to using coatings or additives. Transferring MXene to a polar organic solvent (isopropanol or propylene carbonate) allows very long storage of samples with lower-than-average environmental stability.

The best way to deal with degradation-prone MXenes (e.g., Ti2CTx or over etched samples) is to process colloidal dispersions into final products (coatings, devices, etc.) right after synthesis. The produced MXene film surface can be further protected by polymer, graphene, or ALD oxide, if needed. This may work for some applications (e.g., EMI shielding).

Balancing stability vs. functionality is very important as additives, protective coatings, or surface modifications reduce conductivity or other properties, such as capacitance. We know that Ti3C2Tx can be cycled as a negative electrode in sulfuric acid electrolyte for 500,000 cycles at room temperature – longer than any other material, and can also operate at 70°C. Thus, the stability of Ti3C2Tx is quite sufficient for this application.

In summary, the stability of MXenes is not unlimited. Still, you deal with nanometer- or even sub nanometer-thin water-dispersible materials with a high density of states at the Fermi level that provide higher conductivity than reduced graphene oxide. Can any other 2D material accomplish this fit and offer a higher environmental stability?

Yury Gogotsi, Ph.D., D.Sc., Dr.h.c.

A.J. Drexel Nanomaterials Institute, Drexel University

3141 Chestnut St., Philadelphia, PA 19104, USA
https://nano.materials.drexel.edu

Yury Gogotsi | LinkedIn