Role of Hydrogen in High-Yield Growth of Boron Nitride Nanotubes at Atmospheric Pressure by Induction Thermal Plasma

Abstract

We recently demonstrated scalable manufacturing of boron nitride nanotubes (BNNTs) directly from hexagonal BN (hBN) powder by using induction thermal plasma, with a high-yield rate approaching 20 g/h. The main finding was that the presence of hydrogen is crucial for the high-yield growth of BNNTs. Here we investigate the detailed role of hydrogen by numerical modeling and in situ optical emission spectroscopy (OES) and reveal that both the thermofluidic fields and chemical pathways are significantly altered by hydrogen in favor of rapid growth of BNNTs. The numerical simulation indicated improved particle heating and quenching rates (∼10 K/s) due to the high thermal conductivity of hydrogen over the temperature range of 3500-4000 K. These are crucial for the complete vaporization of the hBN feedstock and rapid formation of nanosized B droplets for the subsequent BNNT growth. Hydrogen is also found to extend the active BNNT growth zone toward the reactor downstream, maintaining the gas temperature above the B solidification limit (∼2300 K) by releasing the recombination heat of H atoms, which starts at 3800 K. The OES study revealed that H radicals also stabilize B or N radicals from dissociation of the feedstock as BH and NH radicals while suppressing the formation of N or N species. Our density functional theory calculations showed that such radicals can provide faster chemical pathways for the formation of BN compared with relatively inert N.

Single- and double-walled boron nitride nanotubes: Controlled synthesis and application for water purification

“…Figure S3 shows FE-SEM images of the BNNTs synthesized at various pressures, which indicated that a large amount of BNNTs was synthesized at each pressure condition. As shown in Figure S3c, amorphous boron and a minimal amount of h-BN flakes were observed as impurities in our grown BNNTs through FE-SEM, which is similar to previous reports 34,37,45 . Figure S4 shows the HRTEM images of BNNTs synthesized under different pressure conditions, which corresponded to the change in BNNT color shown in Figure 2d.…”

“…Thus, researchers in the BNNT field are interested in the selective synthesis of SWBNNTs and DWBNNTs because they have superior properties to those of MWBNNTs due to structural perfection and quantum confinement effects. However, studies on controlling the number of BNNT layers and reducing various impurities such as amorphous boron and h-BN remain in the early stages 20,34,37,40,45 .…”

Dual growth mode of boron nitride nanotubes in high temperature pressure laser ablation

“…at NRC produced high-quality BNNTs at the rate of 20 g/h, without using a metal catalyst 35 . They also suggested a growth mechanism of BNNTs in this method involving the catalytic activity of hydrogen 37 . An extended-pressure inductively coupled plasma system that offers excellent flexibility in the use of various boron precursors, developed by Zettle’s group from UC Berkeley, can yield BNNTs at an even higher rate (35 g/h); however, the purity of the final products is slightly inferior, because a minority of other boron nitride byproducts (nanoribbon, nanococoon) are also attained 36 .…”

Boron nitride nanotubes: synthesis and applications

“…However, in doing so, it requires a profound understanding of the growth mechanism of BNNT in every single method. Recently, computer simulation and in situ diagnostic tools to evaluate every critical factor from several research groups have initially provided some insight into growing process of BNNT [ 51 , 53 , 58 ]. On the other hand, the widespread availability of BNNT has triggered great interest in the development of BNNT applications.…”

“…With the addition of hydrogen, the B–N–H was formed and promoted the formation of BNNT by efficiently reacting with the boron nano-droplet rather than nitrogen molecules. Hydrogen effect was investigated through additional simulations and in situ optical emission spectroscopy (OES) study [ 58 ]. Zettle et al obtained double-walled BNNTs at very high production rate (35 g/h) while developing the extended pressure inductively-coupled plasma system (EPIC) method in high-pressure conditions based on induction plasma method (Fig.…”