This image combines a scanning electron microscopy (left) and a crystallographic fragment of the MXene structure, highlighting its ordered and finely tuned surface terminations. (Image Credit: B. Schröder/HZDR)
Researchers from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and other institutions have developed a new method to produce MXenes with exceptional purity and control. This gas-liquid-solid (GLS) process enables the synthesis of pure MXenes with halogen atoms evenly distributed across the surface, allowing for precise adjustment of the surface composition. Overall, the method significantly enhances the electrical conductivity, paving the way toward high-performance sensors, electronics, and energy technologies.
MXenes, identified in 2011, are an expanding class of inorganic 2D materials. These structures have layers---each one containing transition metals bonded with carbon or nitrogen. Meanwhile, the outer surfaces feature attached halogen atoms that influence the properties of the material. Usually, researchers rely on chemical etching to create MXenes, and these lead to a random distribution of surface terminations, like oxygen, fluorine, or chlorine.
The new approach uses MAX phases (solid starting materials), molten salts, and iodine vapor to create MXene sheets. Working together, the molten salts and iodine vapor control the binding of halogen atoms, like iodine, bromine, or chlorine, to the surface. With this method, MXenes have extremely uniform, well-ordered surface terminations and significantly reduced impurities.
The researchers used this method to synthesize MXenes from eight MAX phases (Ti3AlC2, Ti3AlCN, Ti2AlC, Ti2AlN, TiNbAlC, Nb2AlC, Nb4AlC3, and Mo2Ga2C), demonstrating its practicability. They also used density functional theory (DFT) calculations to examine how surface terminations affect MXenes’ stability and electrical properties.
Highlighting the significance of this technique, the team investigated Ti3C2 (titanium carbide MXene). Standard synthesis methods (chemical routes) produce Ti3C2 with a combination of chlorine and oxygen terminations that degrade the electrical properties. On the other hand, using the GLS technique to produce Ti3C2Cl2 contains only chlorine---arranged in an ordered structure with no impurities.
“The results were striking. The chlorine-terminated MXene variant showed a 160-fold increase in macroscopic conductivity and a 13-fold enhancement in terahertz conductivity compared with the same material made by traditional methods. In addition, a nearly fourfold increase in charge carrier mobility was observed, a key measure of how freely electrons move through a material,” Dr. Dongqi Li from TU Dresden said.
Performance improvements like these occur due to the more controlled surface chemistry. Uniformly arranging the chlorine atoms on the MXene surface allows the electrons to flow more freely due to fewer obstructions in their path. The team ran quantum transport simulations to verify that the smooth surfaces lowered electron trapping and scattering, which explains performance enhancements.
The team’s work demonstrates that controlling the surface halogen type affects the electromagnetic waves’ absorption characteristic of MXenes. Controlling the material like this enables it to be used in certain applications like electromagnetic shielding, radar-absorbing coatings, and next-gen wireless components. Chlorine-terminated MXenes demonstrate strong absorption in the 14-18 GHz frequency range. Bromine- and iodine-terminated MXenes have different frequency absorption ranges.
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