Covalent functionalization of boron nitride nanosheets via reductive activation

Hexagonal boron nitride is well known for its unique structure and excellent physical properties, particularly in hexagonal boron nitride nanosheets (BNNSs) with high potential in multiple technological applications. However, its severe layer-by-layer aggregation and incompatibility with processing liquids or condensed phase materials pose a great challenge. Covalent functionalization of BNNSs has been a common approach to address these critical issues, yet it is extremely difficult to carry out due to the chemical inertness of BNNSs. In this study, we report a novel and general route to covalently functionalize BNNSs via a simple reduction reaction. This involves initial negative charging through effective reductive activation which enables subsequent reactions with various organic alkyl halides. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) results confirm that linear alkyl chains with varying lengths are successfully grafted onto BNNSs, which leads to matched compatibility with organic media and the exfoliation level of few-layer thickness. The increase of the alkyl chain length considerably promotes their solubility in organic solvents with iodoalkanes as the most efficient grafting agents. Incorporation of alkylated BNNSs into a polymer matrix at low filler loadings leads to significant enhancements in mechanical properties over neat polymers, suggesting their exceptional reinforcement for polymer nanocomposites. This facile and scalable reductive chemistry route is applicable to versatile chemical modifications of BNNSs with diverse functional groups and grafting agents by reactions with suitable electrophiles.