(Nanowork News) In the decade since their discovery at Drexel University, a family of two-dimensional materials called MXenes have shown great promise in applications ranging from desalination and energy storage to electromagnetic shielding and communications. While researchers have long speculated about the origins of MXenes’ versatility, a recent study led by Drexel and the University of California, Los Angeles, provides the first clear glimpse into the surface chemistry underlying their capabilities.
The team, which also included researchers from the University of California, Northridge, and Lawrence Berkeley National Laboratory, used advanced imaging techniques called scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to map the electrochemical surface topography of titanium carbide MXene, one of the most studied and widely used materials in this family.
Their research results were published in the 5th anniversary issue of the journal Cell Press. problem (“Atomic-scale investigation of TithreeSeed2teaX “MXene Surfaces” will help explain the diverse properties exhibited by MXene-based materials and will aid researchers in developing new materials for specific applications.
![Injection tunneling microscope mapping MXene surfaces](https://www.nanowerk.com/nanotechnology-news3/id65470_1.jpg)
“Many of the potential outcomes of MXene arise from its rich surface chemistry,” says lead author Dr. Yury Gogotsi, Distinguished University and Bach Professor in Drexel’s College of Engineering, whose research group helped discover the material in 2011. “This first atomic-scale view of a surface using scanning tunneling microscopy is an exciting development that opens up new possibilities for controlling material surfaces and for applying MXene in advanced technologies.”
Although MXene is a two-dimensional material, the interactions that underlie its chemical, electrochemical, and catalytic properties—including ultrafast electrical energy storage, water splitting to produce hydrogen, and separation of urea from blood—are initiated by the atoms that form its surface layer.
Previous studies have investigated the chemical structure of MXene surfaces at low resolution using techniques such as scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), and tip-enhanced Raman spectroscopy (TERS). These tools provide an indirect readout of the material’s composition, but provide little information about the complexity of the surface organization.
In contrast, scanning tunneling microscopy and scanning tunneling spectroscopy provide more direct information about the shape and composition of material surface structures, as well as surface chemistry and properties.
These instruments are sensitive enough to distinguish one atom from another when scanning a flat surface using a very sharp probe. The tip of the probe is charged so that it can interact with each atom as it passes by. This interaction, called quantum tunneling, provides information about the atoms on the surface of the material. The spectroscopic scan provides information about the surface composition at the atomic and molecular level. The scan is converted into an image, forming a topographical map of the material surface.
“STM/STS allows us to see the atomic arrangement on the MXene surface and study its conductivity at atomic resolution,” Gogotsi said. “This is key to understanding why MXenes have such extreme properties and outperform other materials in many applications. It will also help us explore the quantum properties of MXenes and uncover new opportunities for this rapidly expanding class of materials.”
According to the researchers, being able to identify and pinpoint groups of atoms (called functional groups) and measure their properties on a surface based on their specific locations and attachments is a key advance in understanding how MXene interacts with other chemicals and materials.
“MXene surfaces are chemically heterogeneous, which is what makes them so interesting and also what makes them so difficult to study,” said Dr. Paul Weiss, a distinguished professor at UCLA and UC Chancellor’s Chair in Chemistry who led the research with Gogotsi. “We believe that’s the key to their amazing properties, but we don’t yet know which chemical functions are important for which applications.”
The researchers’ STM/STS imaging revealed 10-nanometer-sized features on the MXene surface, likely titanium dioxide clusters, with tiny protrusions arranged in a distorted hexagonal symmetry. The researchers thought these were functional groups, and confirmed them chemically.
The results of this study are consistent with previous theory, low-resolution microscopy, and spectral data on the surfaces of titanium carbide MXenes, including predictions that the surfaces are metallic. However, a closer look at the nature of the surface defects and heterogeneities is a key step toward understanding how these defects affect the material's behavior, the team said.
“In this work, we’ve started to pull the strings. We’ve been able to image and begin to assign some of the chemical functions,” Weiss said. “One of the most intriguing unknown aspects of MXene is how defects and heterogeneities play a role in its function and environmental stability. We now have a platform to explore those roles.”
Drawing on the collective expertise of materials scientists at Drexel University, STM groups at UCLA and Lawrence Berkeley National Laboratory, and theoretical scientists at the University of California, Northridge, the group will continue rigorous analysis of the material while establishing processes to tune its chemical composition to tailor its functionality for a variety of applications.