2024 MRS Communications Lecture

Yury Gogotsi, Drexel University

MXene in the Environment and for the Environment

Written by Molly McDonough

Two-dimensional (2D) materials have long been of interest to the scientific community due to their unique properties and their wide variety of applications in shrinking electronics and energy storage devices. One challenge of 2D materials is their environmental stability, since 2D materials are typically more reactive than their bulk counterparts and may adsorb molecules from the atmosphere like water or oxygen, which can negatively affect their properties. One family of 2D materials, called MXenes, have become of interest in recent years due to their unique properties such as high electrical conductivity. MXenes have the formula M_n+1X_nT_x, where M is a transition metal, X is carbon and/or nitrogen, and T represents the surface termination of the material. Yury Gogotsi seeks to study MXenes and examine their environmental stability to provide insight into functional applications of these 2D materials.

Gogtsi’s MRS Communications Lecture focused on the environmental stability of MXenes by intercalation of N-methylformamide (NMF). MXenes are 2D materials, typically about 1 nm thick, and have a transition metal/hydroxide like surface. By changing the surface termination of the MXene, properties like electrical conductivity can be tuned. In MXenes, the changes in surface termination do not eliminate the electrical conductivity, like in graphene. Additionally, MXenes can be comprised of up to 7 different transition metals to make high entropy MXenes. MXenes can be synthesized in a variety of ways, including in solution, by selective etching precursor materials, or in bulk by chemical vapor deposition (CVD).

One of the challenges with MXenes is that they are hydrophilic, which allows them to be easily processed in aqueous solutions, but also means they can degrade in the presence of water near defects like metal vacancies. The presence of water can also cause a reduction in the properties of the MXene, such as the electrical conductivity. Controlling the degradation of these materials is an important challenge to overcome to enable their applications.

Gogotsi’s group synthesized Ti₃C₂T_x by selective etching Al from Ti₃AlC₂ using a mixture of HCl and HF. Then, the MXene was delaminated into MXene nanosheets through Li⁺ intercalation. The MXene was then suspended in a water solution and an NMF solution. The MXene was far more stable in the NMF than in water, per UV-Vis absorption spectroscopy and x-ray diffraction measurements. The change in sheet resistance of MXenes in water and NMF was also tested by Gogotsi’s group. The research team demonstrated that in NMF, the loss in electrical conductivity over the span of 15 days is significantly lower than the loss observed in water. For long term storage, it is clear NMF is a better option than water.

Despite the challenges the hydrophilicity of MXenes causes, Gogotsi’s group and others have discovered ways to mitigate or reverse the effects of water on MXenes. One method that can be used is removing the water by heating up the MXenes, which has been shown to restore some of the electrical conductivity of these materials. Another option is to use an inert atmosphere, such as an argon-rich atmosphere, to reduce the effects of oxygen and water on the surface of these materials. Controlling the stability in these materials opens doors for applications of MXenes.

Overall, Gogotsi’s work demonstrates how the stability of MXenes can be optimized by changing the synthesis and storage methods. Additionally, this work demonstrates the possible applications, from energy storage to use as inks. The properties of MXenes and the simplicity of their synthesis indicate that MXenes could provide a solution to challenges in energy storage technology.

The MRS Communications Lecture recognizes excellence in the field of materials research through work published in MRS Communications during the previous year.