22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
High-yield growth of small-diameter boron nitride nanotubes in radio-frequency
induction thermal plasma: role of hydrogen and quenching
K.S. Kim, C.T. Kingston, M. Plunkett and B. Simard
Security and Disruptive Technologies Portfolio, National Research Council Canada, 100 Sussex Drive, Ottawa, ON
K1A 0R6, Canada
Abstract: We report the high-yield production of small-diameter boron nitride nanotubes
(BNNTs) in an induction thermal plasma process. This new type of method, called a
hydrogen-assisted BNNT synthesis (HABS method) makes use of hydrogen, which
dramatically increases yield of small-diameter BNNT compared to the previous plasma
processes at atmospheric pressure. To understand the catalytic role of hydrogen in this
process, we investigate the hydrogen-mediated chemistry by examining reaction byproducts. The high selectivity of the process towards small-diameter BNNT (avg. ~5 nm)
is also discussed considering the effects of the rapid cooling of reaction stream.
Keywords: boron nitride nanotubes, small diameters, RF induction thermal plasma,
super-growth mechanism, rapid cooling
1. Introduction
Boron nitride nanotubes (BNNTs) are isoelectronic
analogues of carbon nanotubes (CNTs) and exhibit a
range of properties that are as compelling as those of
CNTs.1 Despite having been first synthesized in 1995,2
progress in science and technology of BNNTs is still
hampered by their very low production volume (~100
mg/h) and large diameters (> 50 nm). Historically, high
temperature plasmas (> 8,000 K) have demonstrated a
great potential in synthesizing high quality small-diameter
BNNTs at large scales from the direct decomposition of
boron (B)-containing feedstock;3,4 however, the growth of
BNNT in those processes turned out to be inefficient
resulting in low production rates or needs high pressure
up to 10 atm because the direct combination of B and N 2
into an hexagonal boron nitride (h-BN) phase (i.e., BNNT
precursors) is extremely slow owing to the strong triple
bond of N 2 .
Recently, we have addressed those obstacles by finding
molecular hydrogen (H 2 ) as an effective growth enhancer
of small diameter BNNT in a high temperature plasma
environment and demonstrated a yield rate approaching
30 g/h.5 In this study, we investigate the role of hydrogen
in this high-rate and high-yield growth of BNNT by
examining by-products extracted from as-produced
materials. The high selectivity of the process towards
small-diameter BNNT (avg. ~5 nm) is also discussed via
both material characterization and computational fluid
dynamics (CFD) simulations.
2. Methods
Raw BNNT materials were produced directly from pure
h-BN powder (99.5 %, avg. 70 nm, MK-hBN-N70, M K
Impex Corp.) by utilizing high-temperature induction
thermal plasma and hydrogen (21 vol. %).5 To understand
the reaction pathway in the presence of hydrogen, various
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by-products were extracted from as-produced materials by
using solvents such as methanol or tetrahydrofuran
(THF). Their compositions were analyzed by FT-IR
(Nicolet 6700 FT-IR). Numerical simulation was also
performed to obtain information of thermo-fluid fields
inside the reactor such as cooling rate. The detailed
method for the numerical simulation can be found
elsewhere.6
Fig. 1. Induction thermal plasma synthesis of BNNTs. a)
schematic of the process. b) and c) photos of as-produced
BNNT materials. d) SEM image. The scale bar is 2 μm. e)
TEM image of a five-walled BNNT.
3. Results and Discussion
Figures 1b and 1c show typical photos of the asproduced BNNT materials obtained from the HABS
process. Few-walled, high crystalline small-diameter
BNNT (~5 nm) were continuously produced from pure hBN powder in the presence of hydrogen (21 vol. %) at a
high rate of 20-30 g/h; however, in the absence of
hydrogen, the product contains large amounts of
1
amorphous boron powder released from the h-BN
feedstock with very few BNNT similar to the previous
studies.3 Most of B droplets solidified without reacting
with relatively inert N 2 ; although N radicals were also
generated from the vaporization of the feedstock mixture,
as temperature cools down, the majority of N atoms
recombined into N 2 at high temperatures above 7,000 K.
Therefore the main source of nitrogen during the BNNT
growth is inert N 2 and the BNNT growth is greatly
limited by the much too slow reaction between B and N 2 .
However, the presence of H 2 or H atoms can
significantly change the reaction pathway by promoting
the creation of B-N-H intermediates (e.g., ammonia
borane or borazine). Such compounds are known as
effective precursors for BNNT growth and provide faster
and more efficient chemical pathways to the in-flight
nitridation of B particles than the direct B-N 2 reaction due
to the relatively weak bonds of B-H or N-H. The FT-IR
spectrum of the reaction by-product extracted is shown in
Fig.2. Absorption peaks corresponding to N-H and B-H
bonds are clearly seen near 3,200 cm-1 and 2,260 cm-1,
respectively, along with the in-plane B-N stretching mode
at 1,390 cm-1. In addition, the out-of-plane B-N-B
bending mode at 780 cm-1 is observed implying the
presence of B-N-H compound in the as-produced
materials. The observation of these B-N-H compounds as
by-products is a strong implication that such compounds
were produced and stabilized by hydrogen in the process.
In comparison to nitridation from N 2 , faster formation of
an h-BN phase is expected from these B-N-H species
which in turn accelerates the growth BNNTs.
Fig. 2. FT-IR spectra of as-produced BNNT material and
by-product extracted.
For the effective nucleation of small-diameter BNNTs,
formation of nano-sized B droplets (i.e., seed particles for
BNNTs, < 10 nm) is important. The numerical simulation
and TEM results suggest that exceptionally rapid cooling
of the reaction stream (~105 K/s) in both axial and radial
directions facilitates the condensation of B into small
sized droplets as shown in Fig. 3. The B vapors generated
2
by the hot plasma undergo rapid cooling from 8,000 K to
4,000 K within less than 1 ms due to the plasma jet
expansion at the entrance of the reactor and the
entrainment of cold molecular gases of H 2 and N 2
injected from the sheath gas channel. This rapid cooling
provides a strong driving force for abundant nucleation of
nano-sized B droplets from supersaturation of B vapor
and also limits their subsequent growth. Figure 3e shows
a small-diameter BNNT grown from nano-sized B
nanoparticles (~10 nm) suggesting that rapid cooling of
the reaction stream is responsible for the nucleation of
small diameter BNNT exclusively.
Fig. 3. a) calculated temperature distribution inside the
reactor. b) temperature profile along the axis. c)
temperature profiles in radial direction. d) TEM image of
as-produce BNNT material showing B nanoparticles. The
scale bar is 1μm. e) TEM image showing BNNT grown
from B nanoparticles. The scale bar is 10 nm.
4. Conclusion
We found that the presence of hydrogen in induction
thermal plasma dramatically increases yield of smalldiameter BNNTs compared to the previous plasma
processes. Through the studies of by-products, it is
suggests that molecular hydrogen promotes the formation
of B-N-H intermediate species during the process, which
opens new and competitive chemical pathways to the
reformation of an h-BN-like phase in B droplets in
comparison to nitridation from N 2 . In addition, numerical
simulation and TEM results show that exceptionally rapid
cooling of the reaction stream (~105 K/s) favors the
condensation of B into small sized droplets is responsible
for the nucleation of small diameter BNNT exclusively.
5. References
[1] R. Arenal, X. Blase and A. Loiseau, Adv. Phys., 2010,
59, 101-179.
[2] N. G. Chopra, R. J. Luyken, K. Cherrey, V. H.
Crespi1, M. L. Cohen, S. G. Louie and A. Zettl, Science,
1995, 269, 966-967.
[3] C. M. Lee, S. I. Choi, S. S. Choi and S. H. Hong,
Appl. Phys., 2006, 6, 166-170.
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[4] A. Fathalizadeh, T. Pham, W. Mickelson, A. Zettl,
Nano Lett., 2014, 14, 4881-4886.
[5] K. S. Kim, C. T. Kingston, A. Hrdina, M. B.
Jakubinek, J. Guan, M. Plunkett and B. Simard, ACS
Nano, 2014, 8, 6211-6220.
[6] K. S. Kim, A. Moradian, J. Mostaghimi, Y. Alinejad,
A. Shahverdi, B. Simard and G. Soucy, Nano Res., 2009,
2, 800-817.
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