For the reason that preliminary discovery of what has turn into a quickly rising household of two-dimensional layered supplies — known as MXenes — in 2011, Drexel College researchers have made regular progress in understanding the advanced chemical composition and construction, in addition to the bodily and electrochemical properties, of those exceptionally versatile supplies. Greater than a decade later, superior devices and a brand new method have allowed the staff to see inside the atomic layers to higher perceive the connection between the supplies’ kind and performance.
In a paper not too long ago revealed in Nature Nanotechnology, researchers from Drexel’s Faculty of Engineering and Poland’s Warsaw Institute of Know-how and Institute of Microelectronics and Photonics reported a brand new manner to take a look at the atoms that make up MXenes and their precursor supplies, MAX phases, utilizing a method known as secondary ion mass spectrometry. In doing so, the group found atoms in areas the place they weren’t anticipated and imperfections within the two-dimensional supplies that might clarify a few of their distinctive bodily properties. Additionally they demonstrated the existence of a wholly new subfamily of MXenes, known as oxycarbides, that are two-dimensional supplies the place as much as 30% of carbon atoms are changed by oxygen.
This discovery will allow researchers to construct new MXenes and different nanomaterials with tunable properties finest suited to particular functions from antennas for 5G and 6G wi-fi communication and shields for electromagnetic interference; to filters for hydrogen manufacturing, storage and separation; to wearable kidneys for dialysis sufferers.
“Higher understanding of the detailed construction and composition of two-dimensional supplies will permit us to unlock their full potential,” mentioned Yury Gogotsi, PhD, Distinguished College and Bach professor within the Faculty, who led the MXene characterization analysis. “We now have a clearer image of why MXenes behave the way in which they do and can be capable to tailor their construction and due to this fact behaviors for necessary new functions.”
Secondary-ion mass spectrometry (SIMS) is a generally used approach to review strong surfaces and skinny movies and the way their chemistry adjustments with depth. It really works by capturing a beam of charged particles at a pattern, which bombards the atoms on the floor of the fabric and ejects them — a course of known as sputtering. The ejected ions are detected, collected and recognized based mostly on their mass and function indicators of the composition of the fabric.
Whereas SIMS has been used to review multi-layered supplies over time, the depth decision has been restricted analyzing the floor of a fabric (a number of angstroms). A staff led by Pawel Michalowski, PhD, from Poland’s Institute of Microelectronics and Photonics, made quite a few enhancements to the approach, together with adjusting the angle and vitality of the beam, how the ejected ions are measured; and cleansing the floor of the samples, which allowed them to sputter samples layer by layer. This allowed the researchers to view the pattern with an atom-level decision that had not been beforehand attainable.
“The closest approach for evaluation of skinny layers and surfaces of MXenes is X-ray photoelectron spectroscopy, which now we have been utilizing at Drexel ranging from the invention of the primary MXene,” mentioned Mark Anayee, a doctoral candidate in Gogotsi’s group. “Whereas XPS solely gave us a have a look at the floor of the supplies, SIMS lets us analyze the layers beneath the floor. It permits us to ‘take away’ exactly one layer of atoms at a time with out disturbing those beneath it. This may give us a a lot clearer image that may not be attainable with every other laboratory approach.”
Because the staff peeled again the higher layer of atoms, like an archaeologist fastidiously unearthing a brand new discover, the researchers started to see the refined options of the chemical scaffolding inside the layers of supplies, revealing the sudden presence and positioning of atoms, and varied defects and imperfections.
“We demonstrated the formation of oxygen-containing MXenes, so-called oxycarbides. This represents a brand new subfamily of MXenes — which is an enormous discovery!” mentioned Gogotsi. “Our outcomes recommend that for each carbide MXene, there may be an oxycarbide MXene, the place oxygen replaces some carbon atoms within the lattice construction.”
Since MAX and MXenes symbolize a big household of supplies, the researchers additional explored extra advanced programs that embody a number of metallic components. They made a number of pathbreaking observations, together with the intermixing of atoms in chromium-titanium carbide MXene — which had been beforehand regarded as separated into distinct layers. They usually confirmed earlier findings, similar to the whole separation of molybdenum atoms to outer layers and titanium atoms to the internal layer in molybdenum-titanium carbide.
All of those findings are necessary for creating MXenes with a finely tuned construction and improved properties, in line with Gogotsi.
“We will now management not solely the overall elemental composition of MXenes, but additionally know through which atomic layers the precise components like carbon, oxygen, or metals are positioned,” mentioned Gogotsi. “We all know that eliminating oxygen helps to extend the environmental stability of titanium carbide MXene and improve its digital conductivity. Now that now we have a greater understanding of how a lot further oxygen is within the supplies, we will alter the recipe — so to talk — to provide MXenes that don’t have it, and because of this extra secure within the surroundings.”
The staff additionally plans to discover methods to separate layers of chromium and titanium, which can assist it develop MXenes with enticing magnetic properties. And now that the SIMS approach has confirmed to be efficient, Gogotsi plans to make use of it in future analysis, together with his current $3 million U.S. Division of Vitality-funded effort to discover MXenes for hydrogen storage — an necessary step towards the event of a brand new sustainable vitality supply.
“In some ways, finding out MXenes for the final decade has been mapping uncharted territory,” mentioned Gogotsi. “With this new method, now we have higher steerage on the place to search for new supplies and functions.”