Exfoliation of layered MgB2 by integrating solid state and solvent based strategies

Abstract

The main objective of this research is to find a new technique to exfoliate layered metal borides to yield boron rich nanomaterials. The science of 2D nanomaterials has expanded significantly beyond graphene over the past decade. The high aspect ratio and quantum confinement impart this family of nanomaterials properties unprecedented in bulk materials. This has led researchers to investigate the delamination of inorganic layered compounds and realize newer analogous of graphene. Transition metal dichalcogenides (TMDs), transition metal carbides, metal halides, metal oxides, layered double hydroxides, CaSi2, and CaGe2 are few ofthe examples which have been successfully exfoliated [1] . Here we present the exfoliation of magnesium diboride (MgB2), an inorganic layered compound comprising boron honeycomb planes alternated by Mg atoms, by integrating solid state reaction with solvent dispersion. It has been recently demonstrated by our research group that MgB2 crystals can be exfoliated by ultrasonication in the presence of water to yield boron-rich nanosheets [2]. Although not all pristine MgB2 flakes undergo exfoliation; some flakes undergo degradation to form insoluble products due to a chemical reaction with water. In order to minimize the degradation of nanosheets formed in the aqueous medium, we investigated a new strategy that utilizes a solid state reaction. We show that reacting MgB2 powder with NiCl2 at high temperatures followed by ultrasonication in solvent results in a dispersion of low-dimensional boron rich nanostructures.These nanostructures are accompanied by the presence of Ni, NiO, MgO, and magnesium or nickel boride compounds as evidenced by XRD spectra, FESEM, and TEM. We show that the mechanism underlying exfoliation of the layered MgB2 is based on the selective extraction of Mg from MgB2 promoted by the formation of MgCl2 that loosens the boron honeycomb planes. Ultrasonication of the Mg deficient layered structure in acetonitrile results in delamination to yield low dimensional nanostructures with lateral sizes in the range of 200 nm–3 µm. This first report of exfoliation of MgB2 using solid state reaction presents a facile way to obtain dispersions of boron rich low dimensional nanostructures presenting novel nanointerfaces.