Identifying the Molecular Edge Termination of Exfoliated
Hexagonal Boron Nitride Nanosheets with Solid-State NMR Spectroscopy
and Plane-Wave DFT Calculations
posted on 2020-03-20, 18:34authored byRick W. Dorn, Matthew J. Ryan, Tae-Hoon Kim, Tian Wei Goh, Amrit Venkatesh, Patrick M. Heintz, Lin Zhou, Wenyu Huang, Aaron J. Rossini
Hexagonal
boron nitride nanosheets (h-BNNS), the
isoelectronic analog to graphene, have received interest over the
past decade due to their 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-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 molecular edge termination
in exfoliated h-BNNS. 1H → 11B cross-polarization MAS (CPMAS) SSNMR spectra of h-BNNS revealed multiple hydroxyl/oxygen coordinated 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 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 h-BNNS
molecular edge termination 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.