In 2011, chemists and engineers met the MXenes: a large family of two-dimensional nanomaterials whose members have already shown real talent for storing energy, purifying water and protecting against electromagnetic interference.
Knowing the family could find employment in those fields for years to come, researchers have since launched the equivalent of a background check into job performance, versatility, stability and quirks.
Scientists from the University of Nebraska-Lincoln and Drexel University recently published findings on an especially promising candidate that includes three titanium atoms and two carbon atoms. Their paper demonstrated that modifying the traditional method of synthesizing the MXene can substantially affect the structure and related properties of its individual, nanoscopic flakes.
The researchers then measured the electrical conductivity of their synthesized flakes, which substantially outperformed those previously reported. By doing so, they also established a threshold for conductivity that engineers could eventually target when incorporating the MXene in lithium-ion batteries, transistors, capacitors and other devices.
“MXenes are synthesized as thin sheets that are then processed for different applications,” said co-author Alexander Sinitskii, associate professor of chemistry at the University of Nebraska-Lincoln. “An important research direction in this field is to develop synthetic approaches to produce MXene sheets with high structural quality and electrical conductivity.”
MXenes begin their lives in the so-called MAX phase, whose name describes its signature components: the “M,” a transition metal such as titanium or chromium; an element such as aluminum from the “A” group of the periodic table; and the “X,” representing carbon or nitrogen atoms. To synthesize MXenes, chemists have used acidic solutions to etch away the “A” group while leaving the other layers intact – a relatively simple, high-yield technique.
But previous solutions have produced relatively small flakes peppered with nanoscopic pinholes that limit the movement of conductivity-driving electrons while offering plenty of opportunities for oxidation to degrade the material. By tweaking the ratio of solution to MAX phase, the Nebraska-Drexel team managed to synthesize defect-free flakes that were about 25 times larger, significantly more conductive and far less susceptible to degradation than those prepared via other approaches.
“We found that these slight variations in the chemical procedures result in pretty dramatic differences in the qualities of the products we obtain,” said Sinitskii, a member of the Nebraska Center for Materials and Nanoscience. “Our measurements revealed that electrical conductivity of MXene sheets is actually close to that of graphene, the two-dimensional material that holds the present record for conductivity. Information about the intrinsic properties of individual MXene sheets is important for optimizing the performance of energy-storage devices that consist of multiple sheets.”
The team’s paper appeared in the December issue of Advanced Energy Materials, which featured the study on its back cover. Sinitskii authored the paper with Alexey Lipatov, research assistant professor of chemistry; Alex Boson, graduate student in chemistry; along with Drexel University’s Yury Gogotsi, Mohamed Alhabeb and Maria Lukatskaya.
The team received support from the National Science Foundation, the NSF-funded Nebraska Materials Research Science and Engineering Center, and the U.S. Department of Energy’s Office of Science.