Synthesis Landscapes for Ammonia Borane Chemical Vapor Deposition of -BN and BNNT: Unraveling Reactions and Intermediates from First-Principles.

Planar hexagonal boron nitride (-BN) and tubular BN nanotube (BNNT), known for their superior mechanical and thermal properties, as well as wide electronic band gap, hold great potential for nanoelectronic and optoelectronic devices. Chemical vapor deposition has demonstrated the best way to scalable synthesis of high-quality BN nanomaterials. Yet, the atomistic understanding of reactions from precursors to product-material remains elusive, posing challenges for experimental design. Here, performing first-principles calculations and molecular simulations, we explore pyrolytic decomposition pathways of the most used precursor ammonia borane (HBNH, AB) to BN, in gas-phase and on Ni(111) or amorphous boron (for BNNT growth) surfaces, for comparison. It reveals that in the gas phase, a pair of AB molecules cooperate to form intermediate NH and ammonia diborane, which further dissociates into HBNH, accompanied by critical BH and NH ions. These ions act as H scavengers facilitating HBNH dehydrogenation into HBNH. The consequent HBNH directly feeds BN flake growth by reacting with the crystal edge, while the addition of HBNH to the edge is prohibited at 1500 K. In contrast, on Ni and boron surfaces, AB monomer dehydrogenates stepwise, deeper, yielding BNH and BN dimer as the primary building unit. Our study maps out three typical experimental conditions regarding the dissociation of AB-precursor, providing insights into the underlying reaction mechanisms of gas-phase precursors, to help as guidelines for the experimental growth of BN nanomaterials.