Researchers at the U.S. Department of Energy have observed 2D materials with a unique property that could help improve the design of fast-charging electronic and energy-storage devices. The team, which is from the DoE’s Oak Ridge National Laboratory (ORNL), discovered 2D materials that actually cannibalize themselves to create atomic “building blocks” for forming stable structures, explained Xiahan Sang, a post-doc researcher at ORNL who worked on the project, in an ORNL news release.
Sang and Raymond Unocic, another ORNL researcher, led a team that used state-of-the-art scanning transmission electron microscopy (STEM), along with theory-based simulations, to discover new atomic properties of the starting material—a 2D ceramic called a MXene, pronounced “max-een.”
|After a monolayer MXene is heated, functional groups are removed from both surfaces. Titanium and carbon atoms migrate from one area to both surfaces, creating a pore and forming new structures. (Image source: Xiahan Sang and Andy Sproles, Oak Ridge National Laboratory, U.S. Department of Energy)|
MXenes are unique among ceramics in that they are good electrical conductors, according to researchers. This is because they are made from alternating atomic layers of carbon or nitrogen sandwiched between transition metals like titanium. “Under our experimental conditions, titanium and carbon atoms can spontaneously form an atomically thin layer of 2D transition-metal carbide, which was never observed before,” Sang said in the news release.
The work—which bodes well for the future design of electronic devices—required a number of collaborators from various institutions to achieve its result, according to the ORNL team. Drexel University students synthesized the high-quality material required for the experiments using five-ply, single-crystal monolayer flakes of MXene. They took the flakes from a parent crystal called MAX, which comprises the following: a transition metal denoted by “M;" an element such as aluminum or silicon, denoted by “A;" and either a carbon or nitrogen atom, denoted by “X.”
To create the material, the team used an acidic solution to etch out the monoatomic aluminum layers, exfoliate the material, and delaminate it into individual monolayers of a titanium carbide MXene as the base material for the experiments, researchers said. Once the material was established, the ORNL scientists suspended a large MXene flake on a heating chip with holes drilled in it. This prevented any support materials or substrates from interfering with the MXene. Then, under vacuum, the suspended flake was exposed to heat and irradiated with an electron beam to clean the MXene surface and fully expose the layer of titanium atoms, researchers said.
Typically inert materials, MXenes can be activated after protective groups of elements—such as oxygen, hydrogen, and fluorine atoms that remain after acid exfoliation—are removed. “Once those functional groups are gone, now you’re left with a bare titanium layer (and underneath, alternating carbon, titanium, carbon, titanium) that’s free to reconstruct and form new structures on top of existing structures,” Sang explained.
By conducting high-resolution STEM imaging, researchers proved that titanium and carbon atoms moved from one part of the material to another, creating a pore and forming new structures in a cannibalistic way because the material feeds on itself, researchers said. The team published a paper on its work in the journal Nature Communications.
Researchers said the material they discovered can pave the way for new types of designs for fast-charging electronic applications because of the way ions move within it, Unocic said. “These materials are efficient at ionic transport, which lends itself well to battery and supercapacitor applications,” he noted. Unocic added that the team also aims to explore how ionic transport changes when researchers add more layers to nanometer-thin MXene sheets.
The team hopes that other scientists will build on this work to develop advanced materials and generate useful nanoscale structures.
Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.
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