“This process is like making a surgical cut of the MAX structure, peeling apart the layers and then reconstructing it with new and different metal layers. In addition to being able to produce new and unusual chemistries, which is interesting fundamentally, we can also make new and different MAX phases and use them to produce MXenes that are tailored to optimize various properties.”Yury Gogotsi, Drexel University
A new process that lets scientists chemically cut apart and stitch together nanoscopic layers of two-dimensional materials — like a tailor altering a suit — could be just the tool for designing the technology of a sustainable energy future. Researchers at Drexel University, together with colleagues from China and Sweden, have developed a method for structurally splitting, editing and reconstituting layered materials called MAX phases and MXenes. This method has the potential for producing new materials with very unusual compositions and exceptional properties.
A ‘chemical scissor’ is a chemical designed to break a chemical bond by reacting with a specific compound. 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 in Science, the international team has now reported a method to sharpen the scissors so they can cut through extremely strong and stable layered nanomaterials, in a way that breaks atomic bonds within a single atomic plane and then substitutes new elements. This offers a way to fundamentally alter the material’s composition in a single chemical ‘snip’.
“This research opens a new era of materials science, enabling atomistic engineering of two-dimensional and layered materials,” said Yury Gogotsi, a professor in Drexel’s College of Engineering and an author of the paper. “We are showing a way to assemble and disassemble these materials like LEGO blocks, which will lead to the development of exciting new materials that have not even been predicted to be able to exist until now.”
Gogotsi and his collaborators at Drexel have been studying the properties of a family of layered nanomaterials called MXenes that they discovered in 2011. MXenes begin as a precursor material called a MAX phase; ‘MAX’ is a chemical portmanteau signifying the three layers of the material – M, A and X. Applying a strong acid to the MAX phase chemically etches away the A layer, creating a more porously layered material – with an A-less moniker: MXene.
This discovery came on the heels of worldwide excitement about a two-dimensional nanomaterial called graphene, posited to be the strongest material in existence when the team of researchers who discovered it won the Nobel prize in 2010. Graphene’s discovery expanded the search for other atomically thin materials with extraordinary properties – like MXenes.
Drexel’s team has been assiduously exploring the properties of MXene materials, leading to discoveries about its exceptional electrical conductivity, durability, and ability to attract and filter chemical compounds, among others. But in some ways, the potential for MXenes has been capped from their inception by the way they’re 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 to etch it,” Gogotsi said. “While this allowed us to create dozens of MXenes, and predict that many dozen more could be created, the process did not allow for a great deal of control or precision.”
By contrast, the new process— ‘chemical scissor-mediated structural editing of layered transition metal carbides’ – is more like performing surgery, according to Gogotsi.
The first step involves using a Lewis acidic molten salt (LAMS) etching protocol to remove the A layer, as usual. But this protocol is also able to replace the A-layer with another element, such as chlorine. This is significant because it puts the material in a chemical state such that its layers can be sliced apart using a second set of chemical scissors, composed of a metal like zinc. These layers are the raw materials of MAX phases. This means that with the addition of a bit of chemical ‘mortar’ – a process called intercalation – the team can now build their own MAX phases, which can then be used to create new MXenes, tailored to enhance specific properties.
“This process is like making a surgical cut of the MAX structure, peeling apart the layers and then reconstructing it with new and different metal layers,” Gogotsi said. “In addition to being able to produce new and unusual chemistries, which is interesting fundamentally, we can also make new and different MAX phases and use them to produce MXenes that are tailored to optimize various properties.”
In addition to building new MAX phases, the team also used the method to create MXenes able to host new ‘guest atoms’ that it previously would not have been chemically able to accommodate – further expanding the family of MXene materials.
“We expect this work to lead to a major expansion of the already very large space of layered and two-dimensional materials,” Gogotsi said. “New MXenes that could not be produced from conventional MAX precursors are becoming 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 the delamination of two- and three-dimensional layered carbides, as well as metal-intercalated two-dimensional carbides, into single- and few-layer nanosheets. This will allow the researchers to characterize the fundamental properties of these new materials and then optimize them for use in energy storage, electronics and other applications.
This story is adapted from material from Drexel University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.