Surface-diffusion mechanism for synthesis of substrate-free and catalyst-free boron nitride nanosheets

This study presents an innovative surface-diffusion mechanism for the growth of substrate-free and catalyst-free boron nitride nanosheets (BNNSs) by annealing an ammonium borate hydroxide hydrate precursor in a NH3 chemical vapor deposition system. At elevated temperatures, part of NH4B5O8 slowly decomposes and forms B2O3, and flowing NH3 gradually diffuses to the B2O3 surface to form vertically aligned BNNSs. The lateral dimension and crystallinity of the BNNSs increase, while their thickness decreases with the continuous surface-diffusion reaction. The residual NH4B5O8 and B2O3 absorbs moisture in the air to constitute NH4B5O8·4H2O and H3BO3 substrates. With increasing annealing temperature and soaking time (at 1400 °C for 7 h), all NH4B5O8 decomposes and the surface-diffusion reaction between B2O3 and NH3 completely occurs, forming substrate-free BNNSs with a lateral dimension of 1 μm and a thickness of 10 nm. This reliable approach for synthesizing BNNSs paves the way for future applications in advanced ceramic composites.