Controllable Synthesis and VLS Growth Mechanism of Boron Nitride Nanotubes Catalyzed by Lithium Carbonate

Controllable synthesis of boron nitride nanotubes (BNNTs) remains challenging due to the difficulty of stabilizing reactive boron species and regulating their conversion pathways during high-temperature reactions. In this study, lithium carbonate (Li₂CO₃) is employed as a controlled Li-O releasing precursor, which decomposes in situ to promote the formation of lithium borate-rich Li-B-O liquid phases. These lithium borates act as the true reactive species that activate boron and facilitate vapor-liquid-solid (VLS) growth of BNNTs. The effects of reaction temperature and Li₂CO₃/B molar ratio on the morphology, structure, and purity of BNNTs were systematically investigated. An optimal Li₂CO₃/B ratio of 0.06 produced uniform, high-purity BNNTs, whereas insufficient or excessive Li₂CO₃ led to incomplete nitridation or the formation of Li-B-O byproducts. Structural analysesincluding FT-IR, XRD, Raman spectroscopy, and XPSreveal that the optimized conditions suppress defect formation and enhance B-N coordination, yielding BNNTs with well-defined tubular architecture. A refined VLS growth mechanism is proposed, in which the size, shape, and spatial distribution of Li-containing liquid droplets govern BNNT diameter, crystallinity, and nucleation density. This work establishes a mechanistic framework for Li₂CO₃-assisted BNNT synthesis, clarifying the precursor role of Li₂CO₃ in generating active lithium borate species, and provides a controllable strategy for achieving BNNTs with uniform morphology and well-defined crystalline structures through liquid-phase-mediated growth.