Identifying the Molecular Edge Termination of Exfoliated Hexagonal Boron Nitride Nanosheets with Solid-State NMR Spectroscopy and Plane-Wave DFT Calculations

Hexagonal boron nitride nanosheets (h-BNNS), the isoelectric analog to graphene, have received much attention over the past decade due to their high thermal oxidative resistance, high bandgap, catalytic activity and low cost. The molecular functional groups that terminate boron and nitrogen zigzag and/or armchair edges directly affect their chemical, physical and electronic properties. However, an understanding of the exact molecular edge termination present in h-BNNS is lacking. Here, high-resolution magic-angle spinning (MAS) solid-state NMR (SSNMR) spectroscopy and plane-wave density-functional theory (DFT) calculations are used to determine the exact molecular edge termination in exfoliated h-BNNS. 1H→11B cross-polarization MAS (CPMAS) SSNMR spectra of h-BNNS revealed multiple hydroxyl/oxygen coordinate boron edge sites that were not detectable in direct excitation experiments. A dynamic nuclear polarization (DNP)-enhanced 1H→15N CPMAS spectrum of h-BNNS displayed four distinct 15N resonances while a 2D 1H{14N} dipolar-HMQC spectrum revealed three distinct 14N environments. Plane-wave DFT calculations were used to construct model edge structures and predict the corresponding 11B, 14N and 15N SSNMR spectra. Comparison of the experimental and predicted SSNMR spectra confirms that zigzag and armchair edges with both amine and boron hydroxide/oxide termination are present. The detailed characterization of h-BNNS molecular edge termination will provide usefulness for many material science applications and the techniques outlined here should be applicable to comprehensively understand the molecular edge terminations in other 2D materials.