Boron Nitride Nanotube Nucleation during Ni-Catalyzed Boron Oxide Chemical Vapor Deposition

摘要

Boron oxide chemical vapor deposition (BOCVD) has proved to be a valuable synthetic technique for the catalytic synthesis of boron nitride nanotubes (BNNTs) for almost two decades. However, the nucleation mechanism of BNNTs during BOCVD remains largely unknown. Here, we report a mechanism to explain BNNT nucleation on Ni catalyst nanoparticles during BOCVD with an ammonia precursor using nonequilibrium molecular dynamics simulations. The presence of oxygen is a significant impediment to the formation of BN hexagonal ring networks, due to the B-O bond strength and the rapid adsorption kinetics of BO monomers to the Ni catalyst. Despite H2O production being assumed to accompany the formation of BN during BOCVD, we do not observe Ni-catalyzed evolution of H2O, although significant amounts of H-2 is evident. At low oxygen chemical potentials, defect-free BN ring networks are produced following the oligomerization of BN chain structures and the Ni-catalyzed cleavage of homoelemental B-B and N-N bonds. The BNNT tip structures align perpendicular to the surface via the direct fusion of adjacent BN ring networks via a mechanism that is a stark departure from that observed for carbon nanotube nucleation.