MXene as a frame for 2D water films shows new properties
Cooling down the MXene with enclosed water (left) leads to the formation of amorphous ice clusters (right), significantly increasing the distance between the MXene layers and the formerly metallic MXene becomes a semiconductor. When heated, the clusters dissolve, the distance between the layers decreases and the sample becomes metallic again. © HZB
An international team led by Dr. Tristan Petit and Prof. Yury Gogotsi has investigated MXene with confined water and ions at BESSY II. In the MXene samples, a transition between localised ice clusters to quasi-two-dimensional water films was identified by increasing temperature. The team also discovered that the intercalated water structure drives a reversible transition from metallic to semiconducting behaviour of the MXene film. This could enable the development of novel devices or sensors based on MXenes.
Water still has unknown sides. When water is forced into two dimensions by enclosing it in appropriate materials, new properties, phase transitions, and structures emerge. MXenes as a class of materials offer a unique platform for exploring these types of phenomena: MXenes consist of transition metal carbides and nitrides with a layered structure whose surfaces can help them absorb water easily. The water forms an extremely thin film between the individual layers.
A team led by Dr Tristan Petit, HZB, and Yury Gogotsi, Drexel University, USA, has investigated a series of MXene samples containing enclosed water and different ions at BESSY II using various analytical methods.
X-ray structural analysis revealed the formation of amorphous ice clusters in the enclosed water, which increases the distance between the MXene layers. The previously metallic MXene film then becomes a semiconductor. “When heated above 300 K, the clusters dissolve again, restoring the distance between the layers and their metallic behaviour,” says Petit. This metal-semiconductor transition is therefore reversible, unless the water layer is removed. Further X-ray investigations using different techniques revealed unique characteristics in the water hydrogen bond networks.
‘In the next step, we need computer-aided modelling to improve our understanding of the formation of amorphous ice and its impact on electronic transport,’ says Katherine Mazzio, who is co-first author of the study. She concludes that MXene is ideal for investigating the phase changes of confined water, providing new insights into how water behaves at the nanoscale. The use of MXene as a material for energy storage and catalysis is already being discussed, and the role of confined water on the unique MXene properties for these applications is currently being investigated.
- Peat as a sustainable precursor for fuel cell catalyst materialsIron-nitrogen-carbon catalysts have the potential to replace the more expensive platinum catalysts currently used in fuel cells. This is shown by a study conducted by researchers from the Helmholtz-Zentrum Berlin (HZB), Physikalisch-Technische Bundesanstalt (PTB) and universities in Tartu and Tallinn, Estonia. At BESSY II, the team observed the formation of complex microstructures within various samples. They then analysed which structural parameters were particularly important for fostering the preferred electrochemical reactions. The raw material for such catalysts is well decomposed peat.
- The future of corals – what X-rays can tell usThis summer, it was all over the media. Driven by the climate crisis, the oceans have now also passed a critical point, the absorption of CO2 is making the oceans increasingly acidic. The shells of certain sea snails are already showing the first signs of damage. But also the skeleton structures of coral reefs are deteriorating in more acidic conditions. This is especially concerning given that corals are already suffering from marine heatwaves and pollution, which are leading to bleaching and finally to the death of entire reefs worldwide. But how exactly does ocean acidification affect reef structures?
Prof. Dr. Tali Mass, a marine biologist from the University of Haifa, Israel, is an expert on stony corals. Together with Prof. Dr. Paul Zaslansky, X-ray imaging expert from Charité Berlin, she investigated at BESSY II the skeleton formation in baby corals, raised under different pH conditions. Antonia Rötger spoke online with the two experts about the results of their recent study and the future of coral reefs.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.