Confined Growth of Boron Nitride Nanotubes Enabled by Varying Mg-to-Al Ratios in a MgAl₂O₄ Catalyst

Currently, the synthesis of boron nitride nanotubes (BNNTs) based on gaseous reactions results in the dispersion of the products throughout the reaction chamber, posing challenges for collection and follow-up applications. For this reason, this work selects MgAl2O4 as the catalyst and achieves the confined growth of BNNTs. Subsequent replacement of the catalyst with Mg(NO3)2 and Al(NO3)3 further revealed that the key lies in the Mg-to-Al ratio. The confined growth was recreated when it matched the stoichiometry of MgAl2O4; otherwise, it resulted in floating growth; i.e., a large number of nanotubes were collected by the substrate above the precursor. This means that controlled switching between confined growth and floating growth of BNNTs was achieved by manipulating the Mg-to-Al ratio. This not only verifies the role of MgAl2O4 in confined growth but also provides a solid foundation for possible fixed-point growth and opens up new possibilities for low-cost and large-scale production of BNNTs.