New materials drive future innovations and technologies
and 2D materials
Our materials of interest include a variety of solids ranging from carbides and nitrides to selected oxides and borides. Most of these ternary compounds crystallize in a layered structure which gives them interesting functionalities. Our main focus is the synthesis of new types of materials - e.g., in terms of chemical composition and morphology -, their structure and magnetic properties.
Some of the layered solids (MAX phases) can be chemically exfoliated into their 2D siblings (MXenes). Since MAX phases are the most common precursors for MXenes, new versions of MAX phases will open the path to new MXenes. They form as van der Waals-stacked multilayers and delaminated multi- and single-layer nanosheets stabilized by functional surface groups.
We take advantage of the breadth of synthesis techniques ranging from low-temperature to high-temperature as well as wet chemical to solid-state methods. We specialize in non-conventional techniques, such as microwave heating and spark plasma sintering. Both methods typically lead to high heating rates and high reaction temperatures due to the unique heating mechanisms (interaction between microwave radiation and the sample and/or susceptor and Joule heating of the graphite die and the sample).
To learn more about solid-state microwave heating and formation mechanisms, we developed a Raman spectrometer setup that allows for collecting in situ Raman spectroscopy data during heating (microwave and conventional).
We developed a sol gel-based synthesis for ternary carbides and particularly MAX phases, that allows us to access unique shapes and compounds with more "exotic" A-elements. For example, we prepared hollow amorphous carbon microspheres decorated with Cr2GaC particles, full and hollow Cr2GaC microspheres, thick films, and microwires. All products contain amorphous carbon that stems from the gel-building agent, e.g. citric acid.
An example for a MAX phase with a rare A-element, is the phosphorous-containing V2PC. In our lab, the solid-state reaction using elemental P failed, but the sol gel-based synthesis resulted in the desired product.
Beside extending this approach to futher MAX phases, e.g. Nb2PC, the reaction mechanism is also of interest.
Within the family of MAX phases (> 150 members), only a small fraction are nitrides and carbonitrides. They not only pose a challenge synthetically, but they also require detailed analysis to confirm their C/N content. We push towards the synthesis of new (carbo)nitrides, such as V2GaC1-xNx.
We are also interested in nitrides that crystallize in different structures, such as antiperovskites.
MAX phases are typically not (ferro)magnetically ordered because their M-element is an early-to-mid transition metal (Cr is the general cutoff element). We explore strategies, such as chemical composition/electronic structure engineering, to understand what drives magnetic ordering in these layered compounds, e.g. in the new solid solution (V/Cr)2GaC. In this example, the total spin susceptibility of Cr is responsible for the non-linear trend in susceptibility (with a maximum for x = 0.8). This gives us a strategy to induce ferromagnetic ordering by increasing the DOS further for this specific solid solution.
We also turn our attention to further antiperovskite compounds and layered borides (MAB phases) that are predicted to ferromagnetically order and possess magnetocaloric properties.
MXenes are a relatively young family of two-dimensional materials that are typically synthesized from their bulk siblings, MAX phases. The A-element in MAX phases, in most cases this is Al, can be chemically etched and removed from the layered structure. Commonly, this is done by treatment with aqueous hydrofluoric acid. New MAX phases can lead to new MXenes, as we have shown for V4AlC3 and the respective 2D V4C2Tx (Tx are the surface functional groups).
We can take advantage of the OH-surface groups and use them as anchors to attach small molecules and polymers. This opens the path to hybrid materials with, for example, switchable electronic properties.
In addition, we strive to synthesize magnetic MXenes.