Evaluating the Exfoliation Efficiency of Quasi-2D Metal Diboride Nanosheets Using Hansen Solubility Parameters

Non-van der Waals (non-vdW) solids are emerging sources of two-dimensional (2D) nanosheets that can be produced via liquid-phase exfoliation (LPE), and are beginning to expand our understanding of 2D and quasi-2D materials. Recently, nanosheets formed by LPE processing of bulk metal diborides, a diverse family of layered non-vdW ceramic materials, have been reported. However, detailed knowledge of the exfoliation efficiency of these nanomaterials is lacking, and is important for their effective solution-phase processing and for understanding their fundamental surface chemistry, since they have significant differences from more conventional nanosheets produced from layered vdW compounds. Here in this paper we use Hansen solubility theory to investigate nanosheets of the metal borides CrB₂ and MgB₂ derived from LPE. By preparing dispersions in 33 different solvents, we determine Hansen solubility parameters (δ_D, δ_P, δ_H) for both these metal diborides. We find that they exhibit notably higher δ_P and δ_H values compared to conventional vdW materials such as graphene and MoS₂, likely as a result of the types of bonds broken in such materials from exfoliation which allows for more favorable interactions with more polar and hydrogen-bonding solvents. We apply the solubility parameters to identify cosolvent blends suitable for CrB₂ and MgB₂ that produce dispersions with concentrations that match or exceed those of the top-performing individual solvents for each material and that have markedly higher stability compared to the constituent solvents of the blends alone. This work provides insight into the exfoliation effectiveness of different solvents for preparation of nanosheets from metal diborides and non-vdW materials in general. Such knowledge will be crucial for developing liquid-phase exfoliation strategies for incorporating these materials in applications such as nanocomposites, inks, and coatings.