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A notable advancement in materials science is the exploration of two-dimensional (2D) materials, which exhibit unique properties distinct from their bulk counterparts. This fascinating area began with graphene, a material celebrated for its remarkable characteristics. Currently, research is expanding to a category known as MXenes (pronounced Maxenes), primarily composed of titanium and carbon, with significant contributions from TU Wien (Vienna) in collaboration with the companies CEST and AC2T.
MXenes are garnering attention for their extraordinary capabilities, making them suitable for applications such as electromagnetic shielding, energy storage, and innovative sensor technologies. Notably, TU Wien’s research has revealed that MXenes also excel as solid lubricants, even in challenging environments like those found in space technology. A major hurdle, however, has been the production method that previously relied on highly toxic acids. Fortunately, a safer alternative has emerged through a novel approach that utilizes electricity for synthesis. This breakthrough has been detailed in a recent publication in the journal Small.
Eliminating Toxic Hydrofluoric Acid
“The synthesis of MXenes begins with materials known as MAX phases, which consist of layers such as aluminum, titanium, and carbon,” states Pierluigi Bilotto from the Research unit of Tribology at TU Wien’s Institute of Engineering Design and Product Development. “Traditionally, hydrofluoric acid was employed to etch away the aluminum from the MAX phases, resulting in atomically thin layers capable of sliding against one another with minimal friction, making these MXenes excellent lubricants.”
However, the handling of hydrofluoric acid poses significant challenges. It is not only toxic but also environmentally hazardous, necessitating stringent regulations and specialized, costly laboratory equipment. The waste generated also demands careful disposal, complicating the process further. “These issues have hindered the widespread adoption of MXenes in industrial applications,” Bilotto notes. “Setting up such a process on a commercial scale has proven difficult, leading many companies to hesitate in investing.”
In response, Bilotto, along with Prof. Carsten Gachot, Prof. Markus Valtiner, Dr. Markus Ostermann from CEST, and Marko Pjlievic from AC2T, sought to devise a more efficient method.
Harnessing Electrochemistry
“Electrochemistry presents an innovative pathway to disrupt the aluminum bonds within the MAX phases,” Bilotto explains. “By applying an electric voltage, the MAX phase generates current at its interfaces, which can trigger selective reactions. By carefully adjusting the voltage, we can optimally remove aluminum atoms, resulting in the creation of electrochemical MXenes (EC-MXenes).
The research team identified that utilizing a specific electrochemical technique can enhance the etching process and improve the quality of EC-MXenes: well-timed current pulses. Unlike traditional methods where surface reactivity diminishes quickly, short bursts of electricity lead to the formation of small hydrogen bubbles on the MAX phase materials. This catalytic effect helps cleanse and reactivate the surface, ensuring that the electrochemical reaction can continue over extended periods, thus facilitating the production of a significant quantity of EC-MXenes.
The resulting product underwent comprehensive analysis using advanced techniques, including Atomic Force Microscopy, Scanning and Transmission Electron Microscopy, as well as Raman and X-ray Photoelectron spectroscopy. The characteristics of these new MXenes were found to be at least comparable to those synthesized using hydrofluoric acid. “My objective is to simplify the MXene synthesis process so much that it could be performed in a standard kitchen,” Bilotto states. “And we are making significant progress towards that goal.”
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