CVD synthesis of boron nitride nanotubes without metal catalysts

Article abstract of DOI:10.1016/S0009-2614(01)00194-4

An efficient CVD synthetic route for bulk quantities of boron nitride nanotubes (BN-NTs) was developed, where a B-N-O precursor generated from melamine diborate (C₃N₆H₆·2H₃BO₃) was employed as the precursor and no metal catalyst was used. The resultant tubes all show remarkable ordering of the concentric atomic layers and exhibit stoichiometric BN composition. It is commonly found that the nanotubes have bulbous tips showing B-N-O amorphous clusters encapsulated in BN cages. The amorphous clusters might play the catalytic role in the nanotube 'tip-growth' process as the metal catalysts do in the metal-catalyzed CVD method.

Full text of DOI:10.1016/S0009-2614(01)00194-4

3
0 March 2001
Chemical Physics Letters 337 +2001) 61±64
www.elsevier.nl/locate/cplett
CVD synthesis of boron nitride nanotubes without
metal catalysts
1
R. Ma , Y. Bando , T. Sato
*
National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
Received 12 December 2000; in ®nal form 6 February 2001
Abstract
An ecient CVD synthetic route for bulk quantities of boron nitride nanotubes +BN-NTs) was developed, where a
B±N±O precursor generated from melamine diborate ꢀC
3
N
6
H
6
Á 2H
3
BO
3
† was employed as the precursor and no metal
catalyst was used. The resultant tubes all show remarkable ordering of the concentric atomic layers and exhibit stoi-
chiometric BN composition. It is commonly found that the nanotubes have bulbous tips showing B±N±O amorphous
clusters encapsulated in BN cages. The amorphous clusters might play the catalytic role in the nanotube `tip-growth'
process as the metal catalysts do in the metal-catalyzed CVD method. Ó 2001 Elsevier Science B.V. All rights reserved.
1
. Introduction
CNTs can now be routinely prepared in bulk
amounts by the metal-catalyzed chemical vapor
deposition +CVD) method [10,11]. But the previ-
ous attempts to prepare BN-NTs by the same
pyrolysis process are disappointingly unsuccessful
[12,13]. Bourgeois et al. [14] previously reported
an iron-related growth of tubular ®bers by heat-
ing BN powder between 1750°C and 2100°C in a
Boron nitride nanotubes +BN-NTs), predicted
to have a uniform electronic bandgap independent
of the diameter and chirality, have been the subject
of intense recent investigations [1,2]. As BN-NTs
are analogous to carbon nanotubes +CNTs) [3],
many methods for BN-NTs synthesis were in-
spired from carbon, i.e. arc-discharge technique
using various electrodes [4±6] or laser ablation
method [7,8]. However, none of these methods can
make an equivalent of carbon production. A
process for substitution of CNTs into BN-NTs
with B O =N mixture has also been developed [9].
2
N atmosphere. The synthesized ®bers are ap-
peared in the bamboo-like rather than uniform
tube structures. Very recently, some success has
been achieved in growing BN-NTs on nickel bo-
ride catalysts from the precursor borazine
ꢀB N H † [15]. However, the yield is consistently
2
3
2
3
3
6
But, the usage of CNTs as the starting materials
hinders it from being a popular method.
low. In addition, the borazine generation is po-
tentially dangerous. In a word, BN-NTs are not
readily formed by the metal-catalyzed CVD pro-
cess. It is likely due to the poor wetting property
of BN with metal catalysts at relatively low
temperature. So the challenge still remains to
develop ecient CVD method for large synthesis
of BN-NTs.
*
Corresponding author. Fax: ꢀ81-298-51-6280.
E-mail addresses: rma@nirim.go.jp +R. Ma), bando@ni-
rim.go.jp +Y. Bando).
1
Fax: ꢀ81-298-51-6280.
0
009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 + 0 1 ) 0 0 1 9 4 - 4

62
R. Ma et al. / Chemical Physics Letters 337 *2001) 61±64
Here we describe an ecient CVD synthetic
route to bulk production of BN-NTs, where a
ternary B±N±O compound generated from mela-
3. Results and discussion
Fig. 1 shows the SEM examination result of the
raw white powders taken from walls. The image
reveals abundant entangled ®brous structures. The
straight ®brous structures are nanometer dimen-
sion in diameter and several micrometers in length.
The most interesting feature of these ®bers is that
they commonly exhibit bulbous tips. TEM analy-
sis demonstrates that the synthesized ®bers are
multi-walled nanotubes. Fig. 2 shows a low mag-
ni®cation TEM image of the nanotubes. The na-
notubes may be partially ®lled with amorphous
materials. It also indicates that the tubes are likely
to grow out of the bulbous tips.
mine diborate ꢀC
3
N
6
H
6
Á 2H
3
BO
3
† [16] was chosen
as the precursor and no metal catalyst was used.
An alternative growth mechanism will also be
discussed based on the morphological analysis of
these nanotubes.
2
. Experimental
Melamine diborate was prepared as white
powder by cooling a hot aqueous solution of
melamine and boric acid in a molar ratio of 1:2. It
was then calcined at 500°C in air for 3 h and an-
nealed at 800°C in N
chemical reaction takes place [16]: 2ꢀC
Á2H BO †ˆB H‡4H NCN‡2CO ‡NH
O. The obtained powder precursor is a B±N±O
intermediate product, B N O H as empirical for-
2
for 1 h. The following
3
6 6
N H
3
3
4
N
3
O
2
2
2
3
‡
6
H
2
4
3
2
mula, containing BN skeleton and about 27 wt%
of oxygen.
The powder precursor was charged into a
graphite crucible, itself placed into a graphite
susceptor. Then the system was heated rapidly by
induction heating and held at 1700°C for 2 h in a
2 2
N gas stream. The N gas stream, ®rst bubbling
from distilled water ꢀH O† at 25°C and then
2
Fig. 1. SEM image of the collected white products. The straight
®bers are nanometer dimension in diameter and several mi-
crometers in length.
passing through carbon black at 1500°C, was in-
troduced into the susceptor at the ¯owing rate of
1
l/min.
After the reaction, white products were ob-
served on the outwall of the crucible and inner
wall of the susceptor, where the temperature is in
the order of 1200°C. Samples of raw materials
removed from the walls were observed by scan-
ning electron microscopy +SEM). Some powders
were ultrasonically dispersed in CCl and dripped
4
onto a holey carbon-coated-copper grid for
characterization with a JEM-3000F +JEOL) high
resolution transmission ®eld emission electron
microscopy +HRTEM) operated at 300 kV at-
tached with parallel detection Gatan-666 electron
energy loss spectrometer +EELS). EELS experi-
ments were performed by using a stationary
focused 1 nm electron probe with a Gatan
spectrometer.
Fig. 2. Low magnifaction TEM image of the synthesized na-
notubes, showing the tubes may be growing out of the bulbous
tips.

R. Ma et al. / Chemical Physics Letters 337 *2001) 61±64
63
EELS spectra showed that the nanotubes in the
product were all BN-NTs. Under HRTEM ex-
amination, a typical well-grown BN nanotube is
shown in Fig. 3a. The dark bands in the ®gure are
considered as the BN layers. The perfectly straight
fringes exhibit that the tubes are well ordered. The
particular tube has 12 concentric layers, with inner
and outer diameters of 5.2 and 13.1 nm, respec-
tively, indicating an inter-layer spacing of about
table absence of carbon K-edge at 284 eV. The B/N
ratios calculated from our spectra obtained from
many tubes range from 0.8 to 1.15. Taking into
consideration a typical error of ꢁ20% due to the
background subtraction, the synthesized nanotu-
bes exhibit a stoichiometric BN composition.
Carefully examining the bulbous tips of the
tubes, we found that the tips are BN cages with
amorphous B±N±O clusters encapsulated inside. A
HRTEM image of the tip morphology is shown in
Fig. 4a. The peculiar tip morphology, which to our
knowledge had not been observed in prior studies,
may be related to the growth process of the BN-
NTs. In terms of the growth mechanism of BN-
NTs in the present CVD method without adding
metal catalysts, the following process is proposed.
At the experimental temperatures +1700°C), some
0
.33 nm. The data are in good agreement with the
inplanar distance of 0.333 nm in bulk h-BN
17,18]. A representative EELS spectrum taken
[
from the tubes is shown in Fig. 3b. It demonstrates
distinct absorption peaks of B and N characteristic
K-edge at 188 and 401 eV, respectively, with no-
2 3
species such as B O and some amorphous B±N±O
Fig. 3. HRTEM image of a multi-walled nanotube and the
typical EELS spectrum. +a) A well-grown nanotube with 12
layers on either side of a hollow and uniform layer separation
Fig. 4. The peculiar tip morphology and proposed growth
mechanism: +a) HRTEM image of a bulbous tip, showing BN
cage ®lled with amorphous materials; +b) a postulated model for
the nanotube growth under the present condition, similar to the
nanotube `tip-growth' in the metal-catalyzed CVD method.
+
D ˆ 13:1 nm, d ˆ 5:2 nm). +b) A typical EELS spectrum taken
from the nanotubes. The distinct K-edge peaks at 188 and 401
eV identi®es that the composition of the tube is boron and ni-
trogen.

64
R. Ma et al. / Chemical Physics Letters 337 *2001) 61±64
clusters may originate from the decomposition of
the precursor. Then the cloud of B vapor and
amorphous B±N±O clusters is carried away by the
owing N stream to the lower temperature region,
Golberg, K. Kurashima, Y. Gao, and F. Xu for
great help in transmission electron microscope
characterization and valuable suggestions.
2
O
3
¯
2
here is in the order of 1200°C, where amorphous
B±N±O clusters condense onto the walls of the
graphite crucible and susceptor. Almost simulta-
References
neously, the vapor phase reaction between B
and other species in the atmosphere may take
place, expressed as: B O ‡ N
‡ 4C ‡ H
BN ‡ 4CO ‡ H [19±21]. The formed BN possi-
2
O
3
[1] A. Rubio, J. Corkill, M.L. Cohen, Phys. Rev. B 49 +1994)
5
081.
[2] X. Blase, A. Rubio, S.G. Louie, M.L. Cohen, Europhys.
Lett. 28 +1994) 335.
2
O
3
2
2
!
2
2
[3] S. Iijima, Nature +London) 354 +1991) 56.
bly precipitates onto or di€uses through the
amorphous clusters, forming nanotubes [22]. The
nanotube growth continues with the amorphous
cluster detach and moves at the head of the
growing tubes. The process stops when the amor-
phous cluster becomes inactive +for example the
temperature decreases or itself is fully covered by
BN layers as in Fig. 4a). Such a postulated
mechanism, somewhat similar to the nanotube
[4] N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L.
Cohen, S.G. Louie, A. Zettle, Science 269 +1995) 966.
[5] A. Loiseau, F. Willaime, N. Demoncy, G. Hug, H.
Pascard, Phys. Rev. Lett. 76 +1996) 4737.
[
6] M. Terrones, W.K. Hsu, H. Terrones, J.P. Zhang, S.
Ramos, J.P. Hare, R. Castillo, K. Prassides, A.K. Chee-
tham, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 259
+1996) 568.
[
[
7] D.P. Yu, X.S. Sun, C.S. Lee, I. Bello, S.T. Lee, H.D. Gu,
K.M. Leung, G.W. Zhou, Z.F. Dong, Z. Zhang, Appl.
Phys. Lett. 72 +1998) 1966.
`
[
tip-growth' in the metal-catalyzed CVD method
10,11,23,24], is illustrated graphically in Fig. 4b.
8] D. Golberg, Y. Bando, M. Eremets, K. Takemura, K.
Kurashima, H. Yusa, Appl. Phys. Lett. 69 +1996) 2045.
In this model, the amorphous clusters play an ac-
tive role as the metal catalysts do.
[9] W. Han, Y. Bando, K. Kurashima, T. Sato, Appl. Phys.
Lett. 73 +1998) 3085.
[
10] V. Ivanov, J.B. Nagy, Ph. Lambin, A. Lucas, X.B. Zhang,
X.F. Zhang, D. Bernaerts, G. Van Tendeloo, S. Ame-
linckx, J. Van Landuyt, Chem. Phys. Lett. 223 +1994) 329.
11] H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert,
R.E. Smalley, Chem. Phys. Lett. 260 +1996) 471.
12] P. Gleize, M.C. Schouler, P. Gadelle, M. Caillet, J. Mater.
Sci. 29 +1994) 1575.
4
. Conclusions
[
[
[
To summarize, BN-NTs were successfully pre-
pared in good yield with melamine diborate as the
starting material. The bulbous tips of these tubes,
BN cages ®lled with amorphous clusters, appear to
be related to the growth process. The CVD
method present here may provide a solution to
bulk production of pure BN-NTs. Furthermore, as
the nanotubes are deposited in the temperature
region of 1200°C and no metal catalyst is needed,
it may also be employed to grow BN-NTs ®lms on
some substrates +e.g., Si substrate) for potential
device application.
13] R. Sen, B.C. Satishkumar, A. Govindaraj, K.R. Hariku-
mar, G. Raina, J.P. Zhang, A.K. Cheetham, C.N.R. Rao,
Chem. Phys. Lett. 287 +1998) 671.
[14] L. Bourgeois, Y. Bando, T. Sato, J. Phys. D 33 +2000) 1902.
[
[
[
15] O.R. Lourie, C.R. Jones, B.M. Bartlett, P.C. Gibbons,
R.S. Ruo€, W.E. Buhro, Chem. Mater. 12 +2000) 1808.
16] T. Hagio, K. Kobayashi, T. Sato, J. Ceram. Soc. Jpn. 102
+
1994) 1051.
17] A. H ꢀe rold, B. Marzluf, P. P ꢀe rio, Combt. Rend. 246 +1958)
1886.
[
[
18] R.T. Paine, C.K. Narula, Chem. Rev. 90 +1990) 73.
19] T. Ishii, T. Sato, Research Report of National Institute for
Research in Inorg. Mater. +Jpn.) 27 +1981) 16.
20] T. Sato, Proc. Jpn. Acad. Ser. B 61 +1985) 459.
21] T. Ishii, T. Sato, Y. Sekikaw, M. Iwata, J. Cryst. Growth
52 +1981) 285.
[
[
Acknowledgements
[22] G.W. Zhou, Z. Zhang, Z.G. Bai, D.P. Yu, Solid State
Commun. 109 +1999) 555.
This work was supported by the Science and
Technology Agency +STA) Fellowship at the Na-
tional Institute for Research in Inorganic Materi-
als +NIRIM), Tsukuba, Japan. R.M. thanks D.
[
[
23] R.T.K. Baker, Carbon 27 +1989) 315.
24] S.B. Sinnott, R. Andrews, D. Qian, A.M. Rao, Z. Mao,
E.C. Dickey, F. Derbyshire, Chem. Phys. Lett. 315 +1999)
25.