Atomic-Level Structure of Mesoporous Hexagonal Boron Nitride Determined by High-Resolution Solid-State Multinuclear Magnetic Resonance Spectroscopy and Density Functional Theory Calculations

Mesoporous hexagonal boron nitride (p-BN) has received significant attention over the last decade as a promising candidate for water cleaning/pollutant removal and hydrogen storage applications. Here, high-resolution solid-state NMR spectroscopy and plane-wave density-functional theory (DFT) calculations are used to obtain an atomic-level description of p-BN. H-1-N-15 or H-1-N-14 heteronuclear (HETCOR) correlation experiments recorded with either conventional NMR at room temperature or dynamic nuclear polarization surface-enhanced spectroscopy (DNP-SENS) at ca. 100 K reveal NB2H, NBH2, NBH3+ species residing on the edges of BN sheets. Ultra-high field 35.2 T B-11 NMR spectroscopy was used to resolve B-11 NMR signals from BN3, BN2Ox(OH)(1-x) (x = 0-1), BNOx(OH)(2-x) (x = 0-2), BOx(OH)(3-x) (x = 0-3), and BOx(OH)(4-x)(-) (x = 0-4). Importantly, 2D B-11 dipolar double-quantum-single-quantum homonuclear correlation spectra reveal that many pore/defect sites are composed of boron oxide/hydroxide clusters connected to the BN framework through BN2O units. 1D and 2D B-11{N-15} HETCOR NMR experiments, in addition to plane-wave DFT calculations of nine different structural models, further confirm the assignment of all NMR signals. The detailed structure determination of the pore and edge/defect sites within p-BN should further enable the rational design and development of next-generation p-BN-based materials. In addition, the techniques outlined here should be applicable to determine structure within other porous and/or boron-based materials.

Automated summary

In this study, high-resolution solid-state NMR spectroscopy and plane-wave density-functional theory (DFT) calculations were used to describe the atomic-level structure of mesoporous hexagonal boron nitride (p-BN). The results revealed the presence of various B and N signals in p-BN and provided a detailed characterization of the pore and edge/defect sites within the material. This is of great importance for the design and development of next-generation p-BN-based materials.