Exfoliation of Quasi-Two-Dimensional Nanosheets of Metal Diborides

Metal diborides are a class of ceramic materials with crystal structures consisting of hexagonal sheets of boron atoms alternating with planes of metal atoms held together with mixed character ionic/covalent bonds. Many of the metal diborides are ultrahigh-temperature ceramics such as HfB2, TaB2, and ZrB2, which have melting points above 3000 °C, high mechanical hardness and strength at high temperatures, and high chemical resistance, while MgB2 is a superconductor with a transition temperature of 39 K. Here, we demonstrate that this diverse family of non-van der Waals (vdW) materials can be processed into stable dispersions of quasi-two-dimensional (2D) nanosheets using ultrasonication-assisted exfoliation. We generate quasi-2D nanosheets of the metal diborides AlB2, CrB2, HfB2, MgB2, NbB2, TaB2, TiB2, and ZrB2 and use electron and scanning probe microscopy techniques to characterize their structures, morphologies, and compositions. The exfoliated layers have a distribution of lateral dimensions from tens of nanometers up to several micrometers and a distribution of thicknesses from as low as 2-3 nm up to tens of nanometers, all while retaining their hexagonal atomic structure and chemical composition. We exploit the convenient solution-phase dispersions of exfoliated CrB2 nanosheets to incorporate them directly into polymer composites. In contrast to the hard and brittle bulk CrB2, we find that CrB2 nanocomposites remain very flexible and simultaneously provide increases in the elastic modulus and the ultimate tensile strength of the polymer. The successful liquid-phase production of quasi-2D metal diborides enables their processing using scalable low-temperature solution-phase methods, extending their use to previously unexplored applications, and reveals a new family of non-vdW materials that can be efficiently exfoliated into quasi-2D forms.