Sekundær ionmassespektrometri afslører atomer, der udgør MXener og deres forløbermaterialer

Siden den første opdagelse af, hvad der er blevet en hurtigt voksende familie af todimensionelle lagdelte materialer - kaldet MXenes - i 2011, har Drexel University-forskere gjort støt fremskridt med at forstå den komplekse kemiske sammensætning og struktur, såvel som de fysiske og elektrokemiske egenskaber, af disse usædvanligt alsidige materialer. Mere end et årti senere har avancerede instrumenter og en ny tilgang gjort det muligt for teamet at peere inden for atomlagene for bedre at forstå sammenhængen mellem materialernes form og funktion.

I et papir for nylig offentliggjort i Nature Nanotechnology , rapporterede forskere fra Drexel's College of Engineering og Polens Warszawa Institute of Technology og Institute of Microelectronics and Photonics en ny måde at se på de atomer, der udgør MXener og deres forløbermaterialer, MAX faser, ved hjælp af en teknik kaldet sekundær ion massespektrometri. Derved opdagede gruppen atomer på steder, hvor de ikke var forventet, og ufuldkommenheder i de todimensionelle materialer, der kunne forklare nogle af deres unikke fysiske egenskaber. De demonstrerede også eksistensen af ​​en helt ny underfamilie af MXener, kaldet oxycarbider, som er todimensionelle materialer, hvor op til 30 % af kulstofatomerne er erstattet af oxygen.

Denne opdagelse vil gøre det muligt for forskere at bygge nye MXener og andre nanomaterialer med justerbare egenskaber, der er bedst egnede til specifikke applikationer fra antenner til 5G og 6G trådløs kommunikation og skjolde til elektromagnetisk interferens; til filtre til brintproduktion, lagring og separation; til bærbare nyrer til dialysepatienter.

"Bedre forståelse af den detaljerede struktur og sammensætning af todimensionelle materialer vil give os mulighed for at låse op for deres fulde potentiale," sagde Yury Gogotsi, Ph.D., Distinguished University og Bach professor i College, som ledede MXene karakteriseringsforskningen. "Vi har nu et klarere billede af, hvorfor MXenes opfører sig, som de gør, og vil være i stand til at skræddersy deres struktur og derfor adfærd til vigtige nye applikationer."

Sekundær-ion massespektrometri (SIMS) er en almindeligt anvendt teknik til at studere faste overflader og tynde film, og hvordan deres kemi ændres med dybden. Det virker ved at skyde en stråle af ladede partikler mod en prøve, som bombarderer atomerne på overfladen af ​​materialet og udstøder dem - en proces kaldet sputtering. De udstødte ioner detekteres, opsamles og identificeres baseret på deres masse og tjener som indikatorer for materialets sammensætning.

Mens SIMS er blevet brugt til at studere flerlagsmaterialer gennem årene, har dybdeopløsningen været begrænset ved at undersøge overfladen af ​​et materiale (flere ångstrøm). A team led by Pawel Michalowski, Ph.D., from Poland's Institute of Microelectronics and Photonics, made a number of improvements to the technique, including adjusting the angle and energy of the beam, how the ejected ions are measured; and cleaning the surface of the samples, which allowed them to sputter samples layer by layer. This allowed the researchers to view the sample with an atom-level resolution that had not been previously possible.

"The closest technique for analysis of thin layers and surfaces of MXenes is X-ray photoelectron spectroscopy, which we have been using at Drexel starting from the discovery of the first MXene," said Mark Anayee, a doctoral candidate in Gogotsi's group. "While XPS only gave us a look at the surface of the materials, SIMS lets us analyze the layers beneath the surface. It allows us to 'remove' precisely one layer of atoms at a time without disturbing the ones beneath it. This can give us a much clearer picture that would not be possible with any other laboratory technique."

As the team peeled back the upper layer of atoms, like an archaeologist carefully unearthing a new find, the researchers began to see the subtle features of the chemical scaffolding within the layers of materials, revealing the unexpected presence and positioning of atoms, and various defects and imperfections.

"We demonstrated the formation of oxygen-containing MXenes, so-called oxycarbides. This represents a new subfamily of MXenes—which is a big discovery." said Gogotsi. "Our results suggest that for every carbide MXene, there is an oxycarbide MXene, where oxygen replaces some carbon atoms in the lattice structure."

Since MAX and MXenes represent a large family of materials, the researchers further explored more complex systems that include multiple metal elements. They made several pathbreaking observations, including the intermixing of atoms in chromium-titanium carbide MXene—which were previously thought to be separated into distinct layers. And they confirmed previous findings, such as the complete separation of molybdenum atoms to outer layers and titanium atoms to the inner layer in molybdenum-titanium carbide.

All of these findings are important for developing MXenes with a finely tuned structure and improved properties, according to Gogotsi.

"We can now control not only the total elemental composition of MXenes, but also know in which atomic layers the specific elements like carbon, oxygen, or metals are located," said Gogotsi. "We know that eliminating oxygen helps to increase the environmental stability of titanium carbide MXene and increase its electronic conductivity. Now that we have a better understanding of how much additional oxygen is in the materials, we can adjust the recipe—so to speak—to produce MXenes that do not have it, and as a result more stable in the environment."

The team also plans to explore ways to separate layers of chromium and titanium, which will help it develop MXenes with attractive magnetic properties. And now that the SIMS technique has proven to be effective, Gogotsi plans to use it in future research, including his recent $3 million U.S. Department of Energy-funded effort to explore MXenes for hydrogen storage—an important step toward the development of a new sustainable energy source.

"In many ways, studying MXenes for the last decade has been mapping uncharted territory," said Gogotsi. "With this new approach, we have better guidance on where to look for new materials and applications." + Udforsk yderligere

Titanium carbide flakes obtained by selective etching of titanium silicon carbide