hydrogenated boron nitride nanotubes (hBNNTs) exhibit excellent physisorption capability to store hydrogen molecules. They can absorb several times more hydrogen molecule than single-walled BNNTs and even more than double-walled BNNTs. The physisorption capacity of hBNNTs mainly depends on the number and the diameter of the tube walls and the size of the hydrogen molecules.

The synthesis of BNNTs can be carried out by high-temperature pressure (HTP) laser ablation, solid-state gasification and chemical vapor deposition (CVD). Depending on the type of boron precursor, catalyst, temperature and mode of heat, the size, length and structure of the BNNTs will vary.

It is also essential to understand the growth mechanism of BNNTs during the synthesis process in order to control the quality of the end products. Two growth modes coexist during the synthesis, a root growth mode and an open-end growth mode. The latter can be controlled by the height profile of the boron gas plume.

In the root growth mode, BNNTs are elongated by the continuous incorporation of BN radicals onto dangling bonds at the boron nanodroplets’ surface. Once the BN radicals reach supersaturation, heterogeneous nucleation of multilayer h-BN layers on the outer surface of the nitride nanodroplets occurs. This is followed by the physisorption of hydrogen molecules on the h-BN layers and the subsequent transformation into the NP 2 end structure in Fig. 3d.

As in CNTs, the atomic structure of BNNTs is hexagonal with sp2 hybridized boron and nitrogen atoms arranged in a hexagonal network. The electron density is primarily distributed on the nitrogen atoms due to the lower electronegativity of boron compared with carbon. This results in an insulating behavior with a wide band gap.