Determining the Edge Termination of h-BN Nanosheets

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Illustration of nanosheets
(Top) Illustration of the sonication assisted exfoliation of h-BN to h-BN nanosheets. Zigzag and armchair edges are shown. (Right, Bottom) Edge-selective 11B and 14N solid-state NMR spectra of h-BN nanosheets. (Left, Bottom) DFT optimized structural edge models.

Hexagonal boron nitride nanosheets, the isoelectronic analog to graphene, have received interest over the past decade due to their 2D structure, high thermal oxidative resistance, high bandgap, catalytic activity and low cost. The 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 molecular edge termination present in h-BN nanosheets 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 molecular edge termination in exfoliated h-BN. 1H→11B cross-polarization MAS (CPMAS) SSNMR spectra 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 the h-BN nanosheets displayed four distinct 15N resonances while a 2D 1H{14N} dipolar-HMQC spectrum acquired with fast MAS 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 molecular edge termination of h-BN will prove useful for many material science applications. The techniques outlined here should also be applicable to understand the molecular edge terminations in other 2D materials.

R.W. Dorn, M.J. Ryan, T.H. Kim, T.W. Goh, A. Venkatesh, P.M. Heintz, L. Zhou, W. Huang,  A.J. Rossini.* Identifying the Molecular Edge Termination of Exfoliated Hexagonal Boron Nitride Nanosheets with Solid-State NMR Spectroscopy and Plane-wave DFT Calculations, Chem. Mater. 2020, In Press, DOI: 10.1021/acs.chemmater.0c00104.