A new process that allows scientists to snap together nanolayers of two-dimensional materials and stitch them together—like a tailor changing a suit—may just be a tool for designing the technology of a sustainable energy future. Researchers from Drexel University, China and Sweden, have developed a method for structural partitioning, editing and reshaping of layered materials, called MAX and MXenes phases, with the potential to yield new materials with highly unusual compositions and exceptional properties.
“Chemical scissors” are chemical substances designed to interact with a specific compound to break a chemical bond. The original set of chemical shears, designed to break carbon and hydrogen bonds in organic molecules, was reported more than a decade ago. In research recently published in Sciencesthe international team reported a method for sharpening scissors so that they could cut layered, extremely strong and stable nanomaterials in such a way that they break atomic bonds within a single atomic level, then substitute new elements – fundamentally changing the composition of the material in a single chemical “snip”.
“This research opens a new era of materials science, enabling atomic engineering of two-dimensional, layered materials,” said Yuri Gogotsi, Ph.D., Bach Distinguished Professor and Chair in the Drexel School of Engineering, who was the author of the paper. “We are demonstrating a way to assemble and disassemble these materials like LEGO blocks, which will lead to the development of exciting new materials that were not even expected to exist until now.”
Gogotsi and his collaborators at Drexel have been studying the properties of a family of layered nanomaterials called MXenes, which they discovered in 2011. MXenes start out as starting materials called the MAX phase; “MAX” is a chemical etchant referring to the three layers of the material: M, A, and X. Applying a strong acid to the MAX phase chemically etches the A layer, creating a more porous layered material—with an A-less moniker: MXene.
The discovery followed worldwide excitement over a two-dimensional nanomaterial called graphene, purported to be the strongest material in existence when the team of researchers who discovered it won a Nobel Prize in 2010. The discovery of graphene broadened the search for another atomically thin material with unusual properties – Like MXenes.
Drexel’s team has been diligently exploring the properties of MXene materials, which has led to discoveries about exceptional electrical conductivity, durability, and the ability to attract and filter chemical compounds, among other things. But in some ways, the potential of MXenes has been limited since their inception by the way they are produced and the limited range of MAX stages and skins that can be used to create them.
“Previously we could only produce new MXenes by modifying the chemistry of the MAX phase or the acid used to etch them,” Gogotsi said. “While this allowed us to create dozens of MXenes, and predict that dozens more could be created, the process did not allow for a great deal of control or precision.”
By contrast, the process that the team–led by Gugutsi and Cheng Huang, Ph.D., professor at the Chinese Academy of Sciences–reported in Sciences The paper explains, “Scissor-mediated chemical structural editing of layered transition metal carbidesAnd“It’s like having surgery,” according to Gogotsi.
The first step is to use a molten Lewis acid salt (LAMS) etching protocol that removes the A layer, as usual, but is also able to replace it with another element, such as chlorine. This is important because it puts the material in a chemical state so that its layers can be cut with a second set of chemical shears, made of metal, such as zinc. These layers are the raw materials for the MAX stages, which means adding a little chemical “slurry” — a process called intercalation — allows the team to create their own MAX stages, which can then be used to create new MXenes, specifically designed to enhance certain properties.
“This process is like making a surgical cut to Max’s skeletal structure, peeling the layers back and then rebuilding them with new, different layers of metal,” Gogozzi said. “In addition to being able to produce new and unusual chemistries, which is fundamentally interesting, we can also create new and different MAX phases and use them to produce MXenes that are designed to improve various properties.”
In addition to building new MAX phases, the team also reported using a method to create MXenes that can host new “guest atoms” that were not previously able to chemically assimilate – further expanding the MXene family of materials.
“We expect this work to lead to a significant expansion of the already large space of layered and two-dimensional materials,” Gogozzi said. “New MXenes that cannot be produced from conventional MAX precursors are made possible. Of course, new materials with unusual structure and properties are expected to enable new technologies.”
The next step for this research, according to Gogotsi, is to separate the two-dimensional and three-dimensional carbides, as well as the metal-segmented two-dimensional carbides, into single-layer and few-layer nanosheets. This will allow researchers to characterize their essential properties to improve new materials for use in energy storage, electronics, and other applications.
