MXene Research Team Paves the Way for 2D Materials

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An international research team has developed a "gas-liquid-solid" process that can be used to produce MXenes—a family of two-dimensional materials—with unprecedented purity and controllability.

Research Team Paves the Way for 2D Materials

MXenes were discovered in 2011 and are a rapidly growing class of inorganic two-dimensional materials. Each structural unit consists of layers of transition metals combined with carbon or nitrogen and is terminated by atoms bonded to the outermost surfaces. These surface terminations play a crucial role in determining the material properties. "They strongly influence how electrons move through the material, how stable it is and how it interacts with light, heat and chemical environments," explains Dr. Mahdi Ghorbani-Asl from the Institute of Ion Beam Physics and Materials Research at the HZDR.

Until now, most MXenes have been produced using chemical etching processes that lead to mixed and randomly distributed surface terminations with elements such as oxygen, fluorine or chlorine. "This atomic disorder limits performance because it traps and scatters electrons, much like potholes slow down traffic on a highway," describes Dr. Dongqi Li from TU Dresden, Germany. In the new gas-liquid-solid (GLS) process, solid starting materials, known as MAX phases, are used together with molten salts and iodine vapor to produce MXene films. Crucially, the molten salts and iodine interact to control which halogen atoms such as chlorine, bromine or iodine attach to the surface. The result is MXenes with highly uniform and well-ordered surface terminations and a greatly reduced level of impurities.

By combining theory with our experimental ability to precisely control surface terminations, we open a new pathway to MXenes with improved stability and tailored functional properties.

Dr. Mahdi Ghorbani-Asl

Using this approach, the international research team from TU Dresden, the Max Planck Institute of Microstructure Physics Halle, the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and other partner institutions in Europe succeeded in synthesizing MXenes from eight different MAX phases. In addition, the researchers used density functional theory (DFT) calculations to gain deeper insights into the influence of surface end groups on the stability and electronic properties of MXenes. "By combining theory and our experimental ability to precisely control surface terminations, we open a new route to MXenes with improved stability and tailored functional properties," summarizes Ghorbani-Asl.

Conductivity Due to Perfectly Organized Surfaces

To illustrate the potential of the new method, the team focused on one of the most frequently studied representatives of this class of compounds: the titanium carbide MXene Ti₃C₂. When produced using conventional chemical processes, Ti₃C₂ usually contains a mixture of chlorine and oxygen end groups, which impair its electrical properties. In contrast, Ti₃C₂Cl₂ produced using the GLS method contains only chlorine, which is incorporated into a highly ordered structure with no detectable impurities. "The results were impressive. The MXene variant, with only chlorine atoms covering the surface, showed a 160-fold increase in macroscopic conductivity and a 13-fold improvement in terahertz conductivity compared to the same material prepared using conventional methods. In addition, an almost four-fold increase in charge carrier mobility was observed, an important measure of how freely electrons can move through a material," summarizes Li. These performance improvements are directly attributable to the cleaner surface chemistry. As all chlorine atoms are arranged in an orderly fashion on the MXene surface, electrons encounter fewer obstacles and can flow more smoothly.

Customized 2D Materials for the Technologies of Tomorrow

The study shows that by adjusting the nature of the surface halogen, not only the electrical transport but also the absorption of electromagnetic waves by MXenes changes. This means that the materials can be developed for specific applications such as radar-absorbing coatings, electromagnetic shielding and next-generation wireless components. For example, MXenes with chlorine end groups show strong absorption in the frequency range of 14 to 18 GHz, while MXenes with bromine and iodine end groups absorb in other frequency ranges.

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