Preparation and characterization of layered boron nitride nanosheets

Hexagonal boron nitride nanosheets (h-BNNSs) are two-dimensional nanomaterials whose structure, similar to graphene, is called white graphene. They have excellent physical and chemical properties. Few-layered and monolayer h-BNNSs have more extensive applications compared to block BNs due to their wider band gap and stronger insulation. However, the existing preparation methods have the disadvantages of uncontrollable product size, low yield, high cost, and pollution. Researchers are striving to develop an efficient and cheap method, and some have tried to prepared h-BNNSs through evaporation and recrystallization. However, it is difficult to control morphology in this method and prepared products are usually thick with uneven size distribution. Meanwhile, vacuum freeze-drying has been widely used in functional porous ceramics due its simple operation, controllability, and environmental friendliness. In this study, based on the two-step process of precursor synthesis and ammonia nitriding, few-layered h-BNNSs with large surface areas were successfully synthesized on a large scale by controlling key factors of precursor synthesis, such as ratio of atoms B to N in the raw material, dispersant, and drying method. The optimum preparation was using boric acid and urea with a molar ratio of 1∶30 as the source of B and N, and using methanol/deionized water as dispersant to obtain the precursors by vacuum freeze drying. Precursors were then transformed from ammonia vapor to nitride for 3 h at 900 ℃. The product was characterized by X-Ray diffraction, X-ray photoelectron spectroscopy, Raman spectra, thermogravimetric analysis, and differential thermal analysis for phase and molecular structure, and scanning electron microscope, atomic force microscope, transmission electron microscope, and N₂ adsorption-desorption isotherms for microstructure and specific surface area. Results indicate that the products are h-BNNSs with high purity, two to four atomic layers, 1 nm thickness, and with a high specific surface area of 871.8 m²·g⁻¹, similar to the microstructure of graphene. Single product mass averages 240 mg and average yield is steadily 96.7%. This method is economical, simple, and easy to operate. It could achieve macro-synthesis of few-layered h-BNNSs with a large area, conducive to research on boron nitride in various field applications, such as boron nitride/graphene composite materials, nano-electronic devices, pollutant adsorption, hydrogen storage, etc.