It is one of the most significant trends in materials science: materials that consist of only a single layer of atoms, so-called "2D materials", often show completely different properties than thicker layers consisting of the same atoms. This field of research began with the Nobel Prize-winning material graphene. Now, research is being conducted into the material class of MXenes (pronounced Maxenes), which consist mainly of titanium and carbon, by TU Wien (Vienna) together with the companies CEST and AC2T.
These MXenes have properties that sound almost miraculous: they can be used for electromagnetic shielding, for energy storage or for novel sensors. At TU Wien, it was found that they are also amazingly suitable as solid lubricants, even under the harshest conditions, for example in space technology. The only problem so far has been that producing these MXenes was considered extremely dangerous and toxic. But now a new method has been developed: instead of a toxic acid, electricity is used. The new synthesis method has now been published in the renowned journal Small.
No more toxic hydrofluoric acid
"To produce MXenes, you first need so-called MAX phases. These are materials that can consist, for example, of layers of aluminium, titanium and carbon", explains Pierluigi Bilotto from the Research unit of Tribology at the Institute of Engineering Design and Product Development at TU Wien. "Until now, hydrofluoric acid was used to etch away the aluminium in the MAX, which then resulted in a system of atomically thin layers that can slide against each other with very little resistance. This makes these MXenes a great lubricant."
But handling hydrofluoric acid is no easy task. It is toxic and harmful to the environment, and there are strict regulations on how to handle this chemical. You need special, expensive laboratory equipment for it, and you get waste products that have to be disposed of in a costly way. "This is why MXenes have not yet made a major breakthrough in industry," says Pierluigi Bilotto. "It's hard to build up such a process on an industrial scale, and many companies understandably shy away from taking this step."
So Pierluigi Bilotto set out to find a better method – together with Prof. Carsten Gachot and Prof. Markus Valtiner from TU Wien, Dr. Markus Ostermann from CEST in Wiener Neustadt, Marko Pjlievic from AC2T and others.
Electrochemistry
"Electrochemistry offers an alternative route to break the aluminium bonds in the MAX phase, " says Pierluigi Bilotto. "When an electrical voltage is applied, the MAX phase experiences an electric current that initiates reactions at its interfaces. By precisely selecting the voltage, we are able to tune the reactions in a way that only Aluminium atoms are removed, leaving as product electrochemical MXenes (EC-MXenes).
The team found that a very specific electrochemical technique can be used to improve the electrochemical etching and EC-MXene's overall quality: well-dosed pulses of current. While the reactivity of the surface often drops quickly with other methods, short current pulses cause small hydrogen bubbles to form on the MAX phase materials, cleaning and reactivating the surface. This allows the electrochemical reaction to be sustained for longer periods of time and a large quantity of EC-MXenes to be produced.
The product obtained was then analysed with advanced techniques such as Atomic Force Microscopy, Scanning and Transmission Electron Microscopy, Raman and X-ray Photoelectron spectroscopy, and Low Energy Ion Scattering. – its properties are at least as good as those of MXenes previously produced using hydrofluoric acid. "My goal is to make the synthesis of MXene extremely simple. It should be possible in any kitchen", says Pierluigi Bilotto. "And we are very close to that."
