MXenes are versatile nanomaterials that were initially discovered about 10 years ago. Researchers have been evaluating them in a wide range of applications including energy storage, functional textiles and telecommunications.

Michel Barsoum, distinguished professor of materials science and engineering at Drexel University in Philadelphia, Pa., says, “MXenes are two-dimensional substances that have the general formula M_n+1X_nT_z where M is an early transition metal, X stands for carbon and/or nitrogen and T represents the various termination groups, which typically are oxygen, hydroxyl and/or fluorine.”

Most MXenes are prepared by etching a single layer of ‘A’ atoms from layered ternary carbide materials known as MAX phases. Barsoum says, “MAX phases are early transition metal carbides and nitrides where M is an early transition metal, A is a group 13 or 14 element (typically aluminum), and X represents carbon, nitrogen and/or boron.”

In a previous TLT article,¹ work by Barsoum and his colleagues was discussed that applies the concept of the MAX strategy to produce an oxidation-resistant molybdenum boride. The result of this work was the synthesis of a layered boride based on molybdenum, boron and aluminum known as MoAlB. This material exhibited excellent oxidation resistance under extremely high temperature conditions.

The traditional process for producing MXenes involved etching the MAX phases using an aqueous approach initially with hydrofluoric acid. Barsoum says, “The etching process removes aluminum, but the hazardous nature of hydrofluoric acid makes this problematic. An improved process was then developed where lithium fluoride and hydrochloric acid were used instead, which proved to be just as effective. This process proved to be very facile because lithium ions ended up between MXene layers that, in turn, were easily delaminated into single sheets when the pH of the suspension approached neutral.”

The aqueous process works well, and water is a fantastic solvent to use, according to Barsoum. But residual water present in MXenes can be a hindrance in applications, which will not tolerate an aqueous medium. Barsoum says, “Two applications that come to mind are batteries and polymer MXene composites. In the latter case, addition of a small percentage of a MXene into a polymer to make polymer composites results in a dramatic improvement in the material’s stiffness and tensile strengths. An improvement in physical properties is very helpful in applications such as automobile bumpers.”

A new approach has now been developed for producing MXenes in the absence of water.

Ammonium bifluoride
Barsoum and his colleagues have now developed a procedure for synthesizing MXenes by a non-aqueous method that uses ammonium bifluoride and a polar organic solvent. He says, “We found that mixing a MAX phase with ammonium bifluoride in a polar organic solvent, followed by heating at 35 C for a week, led to the formation of MXene in quantitative yield. The aluminum etched from the MAX phase reacted to form the byproducts, aluminum trifluoride and ammonium hexafluoroaluminate. Both were removed during an ensuing washing step that utilized acidic propanol.”

A number of solvents, including propylene carbonate, acetonitrile and N, N-dimethylformamide were used. Barsoum says, “We focused on using propylene carbonate because this solvent is used as an electrolyte in lithium-ion batteries.” 

The researchers prepared the titanium carbide MXene by this procedure. X-ray diffraction was used to characterize the MXene. This analytical technique showed that ammonium cation complexes associated with organic solvent molecules were present in the interlayer space between the MXene.

Delamination of the MXenes into single to few layers was achieved through sonification in the presence of propylene carbonate, followed by centrifugation that separated out the byproducts as sediment. Figure 3 shows a transmission electron micrograph of a delaminated MXene flake produced by the non-aqueous procedure.


Figure 3. A transmission electron micrograph shows a delaminated MXene flake produced by a non-aqueous procedure. Figure courtesy of Drexel University.

To demonstrate the potential for the MXene produced without water, the researchers incorporated them as anodes into sodium-ion batteries. A battery capacity that stabilized around 160 milliampere hours per gram was obtained, which was nearly double what was found when the same MXene was prepared through the original aqueous procedure.

X-ray photoelectron analysis was conducted on sodium metal foils exposed to MXenes generated by both the aqueous and non-aqueous pathways. No deterioration of the sodium was detected in the MXene prepared using propylene carbonate as the solvent. In contrast, when the sodium was exposed to the MXene prepared using the aqueous approach, sodium oxide and sodium fluoride were prepared indicating that deterioration occurred.

Barsoum says, “In this application, it is well established that the presence of water messes things up.”

Future work will focus on what Barsoum considers the “Holy Grail” for MXenes, which is to develop a process that does not require fluorine etching. He says, “Our objective is to figure out how to sidestep the use of fluorine, which can be hazardous.” A second objective is to use MXenes in polymer composites that can be utilized in additive manufacturing.

Additional information on this work can be found in a recent article² or by contacting Barsoum at barsoumw@drexel.edu.

REFERENCES
1. Canter, N. (2016), “Oxidation-resistant molybdenum boride,” TLT, 72 (11), pp. 12-13.
2. Natu, V., Pai, R., Sokol, M., Carey, M., Kalra, V. and Barsoum, M. (2020), “2D Ti₃C₂T_z MXene Synthesized by Water-free Etching of Ti₃AlC₂ in Polar Organic Solvents,” Chem, 6 (3), pp. 616-630.