by A new process that allows scientists to chemically cut and sew nanoscopic layers of two-dimensional materials — like a tailor altering a suit — could be just the tool to design the technology of a sustainable energy future. Researchers from Drexel University, China and Sweden have developed a method to structurally separate, process and reconstitute multilayer materials, called MAX phases and MXenes, with the potential to produce new materials with highly unusual compositions and extraordinary properties.
“Chemical scissor” is a chemical substance designed to react with a specific compound to break a chemical bond. The original set of chemical scissors, designed to break carbon-hydrogen bonds in organic molecules, was reported more than a decade ago. In a paper recently published in Scienceinternational team reported a method for sharpening the scissors so that they can cut extremely strong and stable layered nanomaterials in a way that breaks atomic bonds at a single atomic level and then replaces new elements — fundamentally changing the composition of the material in single chemical “piece”.
“This research opens a new era in materials science, enabling the atomic engineering of two-dimensional and multilayered materials,” said Yury Gogotsi, PhD, University Distinguished Professor and Bach Chair in Drexel’s College of Engineering, who was an author of the research. “We’re showing a way to assemble and disassemble these materials like LEGO blocks, which will lead to the development of exciting new materials that weren’t even predicted to exist until now.”
Gogotsi and his colleagues at Drexel have been studying the properties of a family of nanomaterials called MXenes, which they discovered in 2011. MXenes start out as a precursor material called the MAX phase. “MAX” is a chemical portmanteau for the material’s three layers: M, A, and X. Applying a strong acid to the MAX phase chemically removes the A layer, creating a material with a more porous layer — with a less A name : MXene.
The discovery came after global excitement over a two-dimensional nanomaterial called graphene, which was supposed to be the strongest material in existence, when the team of researchers who discovered it won the Nobel Prize in 2010. The discovery of graphene extended the search for other thin atoms materials with exceptional properties — such as MXenes.
The Drexel team diligently investigates the properties of MXene materials, leading to discoveries about its excellent electrical conductivity, durability, and ability to attract and filter chemical compounds, among other things. But somehow, the possibilities for MXenes have been limited since their inception by the way they are produced and the limited set of MAX phases and etchants that can be used to create them.
“Previously we could only produce new MXenes by adjusting the chemistry of the MAX phase or the acid used for etching,” said Gogotsi. “While this allowed us to create dozens of MXen, and predict that many dozens more could be created, the process did not allow for much control or precision.”
Instead, the process reported by the team — led by Gogotsi and Qing Huang, PhD, a professor at the Chinese Academy of Sciences — Science The publication explains that, “chemical scissor-mediated structuring of layered transition metal carbides,“It’s more like surgery, according to Gogotsi.
The first step is to use a Lewis acid molten salt (LAMS) etching protocol that removes the A layer, as usual, but can also replace it with another element, such as chlorine. This is important because it puts the material into a chemical state so that its layers can be sliced using a second set of chemical scissors, composed of a metal such as zinc. These layers are the raw materials of MAX phases, meaning that adding a little chemical “mortar” — a process called intercalation — allows the team to create their own MAX phases, which can then be used to creating new MXenes, tailored to enhance specific properties.
“This process is like making a surgical cut of the MAX structure, peeling back the layers, and then rebuilding it with new and different metal layers,” Gogotsi said. “In addition to being able to make new and unusual chemicals, which is fundamentally interesting, we can also make new and different MAX phases and use them to make MXenes that are tailored to optimize various properties.”
In addition to making new MAX phases, the team also reported using the method to create MXenes that can host new “guest atoms” that previously could not host chemically—further expanding the MXene family of materials.
“We expect that this work will lead to a significant expansion of the already very large space of multilayer and two-dimensional materials,” Gogotsi said. “New MXenes that could not be produced from conventional MAX precursors are becoming possible. Naturally, new materials with unusual structure and properties are expected to enable new technologies.”
The next step for this research, according to Gogotsi, is to exfoliate the 2D and 3D layered carbides, as well as metal intercalated 2D carbides, into single- and few-layer nanosheets. This will allow researchers to characterize their fundamental properties to optimize the new materials for use in energy storage, electronics and other applications.
