High-temperature chemical vapor deposition (CVD) is a prominent method for synthesizing boron nitride nanotubes (BNNTs), offering control over structural parameters such as diameter, wall number, and crystallinity. The process relies on carefully selected precursors and catalysts to achieve sp2-bonded hexagonal boron nitride (h-BN) structures, which differ significantly from carbon nanotube (CNT) growth mechanisms despite superficial similarities in tubular morphology.

The most common precursor system for BNNT synthesis involves boron oxide (B2O3) and ammonia (NH3). Boron oxide serves as the boron source, while ammonia provides nitrogen and acts as a reducing agent. At temperatures exceeding 1200°C, B2O3 vaporizes and reacts with NH3 to form intermediate species such as boron suboxide (BOx) and aminoborane (H2N-BH2), which subsequently nucleate into BNNTs. The reaction pathway can be summarized as follows:
B2O3 (g) + 2NH3 (g) → 2BN (s) + 3H2O (g)

The presence of metal oxide catalysts, typically magnesium oxide (MgO) or iron oxide (Fe2O3), is critical for lowering the activation energy of BNNT formation. These catalysts facilitate the decomposition of precursors and promote the assembly of sp2-bonded BN layers. MgO, for instance, forms a molten borate phase at high temperatures, which dissolves boron and nitrogen species, enabling their reorganization into hexagonal BN networks. The catalyst particles often remain at the tips of growing BNNTs, suggesting a vapor-liquid-solid (VLS) growth mechanism analogous to CNT synthesis. However, unlike CNTs, where metallic catalysts like iron or cobalt are used, BNNT growth often employs oxides due to the strong affinity between boron and oxygen.

Achieving high-purity sp2-bonded BNNTs presents several challenges. First, the high thermodynamic stability of B-N bonds compared to C-C bonds necessitates extreme synthesis conditions, typically above 1400°C, to ensure proper crystallinity. Lower temperatures often result in amorphous boron nitride or poorly ordered structures. Second, the competition between BNNT growth and the formation of other boron nitride phases, such as cubic BN (c-BN) or turbostratic BN, must be carefully managed through precise control of temperature gradients and gas flow rates. Turbostratic BN, characterized by misaligned h-BN layers, is a common byproduct when precursor concentrations or reaction kinetics are suboptimal.

A key distinction from CNT growth lies in the role of hydrogen. In CNT CVD, hydrogen is often used to etch amorphous carbon and promote graphitization. In BNNT synthesis, however, excessive hydrogen can lead to the formation of borazine (B3N3H6) or other hydrogenated intermediates, disrupting the sp2 network. Conversely, insufficient hydrogen may result in oxygen incorporation into the BN lattice, creating defects. Balancing the NH3 flow rate is therefore crucial, as it serves as both a nitrogen source and a hydrogen supplier.

The diameter and wall number of BNNTs are influenced by catalyst particle size and precursor supersaturation. Smaller catalyst particles (<50 nm) tend to produce single- or few-walled BNNTs, while larger particles yield multi-walled structures. Unlike CNTs, where chirality dictates electronic properties, BNNTs are uniformly wide-bandgap insulators regardless of chirality, simplifying the focus on structural perfection rather than electronic tailoring.

Comparative analysis with CNT growth reveals fundamental differences in nucleation and growth dynamics. CNTs often grow via a base-growth or tip-growth mechanism depending on catalyst-substrate interactions, while BNNTs predominantly exhibit tip-growth due to the stronger adhesion of BN to metal oxide catalysts. Additionally, CNT synthesis benefits from a wider range of carbon precursors (e.g., methane, ethylene), whereas BNNT synthesis is limited to a narrower selection of boron and nitrogen sources due to thermodynamic constraints.

Contamination is another critical issue. Oxygen impurities from incomplete B2O3 reduction can incorporate into the BN lattice, disrupting the sp2 hybridization. Post-synthesis treatments, such as high-temperature annealing in nitrogen or ammonia, are often required to remove residual oxygen and improve crystallinity. In contrast, CNT synthesis faces fewer challenges with oxygen contamination, as carbon precursors are typically oxygen-free.

In summary, high-temperature CVD of BNNTs demands precise control over precursors, catalysts, and reaction conditions to achieve sp2-bonded structures. The interplay between boron oxide, ammonia, and metal oxide catalysts governs nucleation and growth, while challenges such as phase competition and impurity incorporation necessitate rigorous optimization. Comparisons with CNT growth highlight the unique complexities of BNNT synthesis, particularly the need for higher temperatures and the sensitivity to oxygen and hydrogen balance. Advances in understanding these process-structure relationships are essential for scaling up BNNT production with consistent quality.