Catalytic growth of nanotube and nanobamboo structures of boron nitride

Article abstract of DOI:10.1016/S0009-2614(01)00635-2

Nanotube and nanobamboo structures of boron nitride (BN) have been synthesized by heating a mixture of boron and iron oxide in flowing ammonia gas. The reaction temperature and the ratio of B/Fe₂O₃ are found to play an important role in determining the diameter and morphology of the product. The in situ generated nanoscale iron catalysts are responsible for the nanotube-like growth. The growth of BN nanotube and nanobamboo structure is ascribed to a catalytic growth within the framework of a vapor-liquid-solid catalytic growth mechanism.

Full text of DOI:10.1016/S0009-2614(01)00635-2

20 July 2001
Chemical Physics Letters 342 +2001) 492±496
www.elsevier.com/locate/cplett
Catalytic growth of nanotube and nanobamboo structures of
boron nitride
C.C. Tang *, M. Lamy de la Chapelle, P. Li, Y.M. Liu, H.Y. Dang, S.S. Fan
Department of Physics, Tsinghua University, Beijing 100084, People's Republic of China
Received 20 March 2001; in ®nal form 17 May 2001
Abstract
Nanotube and nanobamboo structures of boron nitride +BN) have been synthesized by heating a mixture of boron
and iron oxide in ¯owing ammonia gas. The reaction temperature and the ratio of B/Fe2O3 are found to play an
important role in determining the diameter and morphology of the product. The in situ generated nanoscale iron
catalysts are responsible for the nanotube-like growth. The growth of BN nanotube and nanobamboo structure is
ascribed to a catalytic growth within the framework of a vapor±liquid±solid catalytic growth mechanism. Ó 2001
Elsevier Science B.V. All rights reserved.
1. Introduction
ducting enough to support the arc. Chopra et al.
[4] synthesized BN nanotubes using a pressed BN
rod inserted into a hollow tungsten electrode. BN
powder packed in a tantalum tube was also used
by Terrones et al. [5] to produce BN nanotubes.
BN nanotubes prepared by the two methods
mentioned above were multi-layered, and we also
noted that the electrical properties of BN raw
materials were not changed. Recently, a macro-
scopically homogenous BN ingot containing co-
balt and nickel was synthesized by arc-melting
method and was directly used to produce double-
wall nanotubes [6]. In fact, single-walled BN na-
notubes can be synthesized by using conducting
HfB2 [7] or ZrB2 electrode [8]. Obviously the
constituent of the electrode helps to catalyze tu-
bular growth and determines the morphology of
BN nanotubes.
Boron nitride +BN) nanotubes are predicted [1]
to be semiconducting with a nearly invariant en-
ergy bands regardless of the diameter, chirality, or
the number of walls in the tube. This contrasts
markedly with the heterogeneity of electronic
properties of carbon nanotubes [2] and makes pure
BN nanotubes particularly useful for potential
device applications. Previous theoretical studies [3]
predicted that pure BN nanotube-like structures
should be energetically stable. Several experimen-
tal investigations have been made on the forma-
tion of BN nanotubes.
The arc-discharging method was ®rst employed
to synthesize BN nanotubes by using metal borides
or by packing BN into metals to generate BN
plasma, since BN and boron targets are not con-
The important e€ect on catalysts can also be
found in other techniques for synthesizing BN
nanotubes. Laser ablation has also turned out to
be an e€ective means for the synthesis of BN
* Corresponding author.
E-mail address: fss-dmp@mail.tsinghua.edu.cn +C.C. Tang).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S0 0 0 9 - 2 6 1 4 + 0 1 ) 0 0 6 3 5 - 2

C.C. Tang et al. / Chemical Physics Letters 342 +2001) 492±496
493
3. Results and discussion
nanotubes, and the morphology depends strongly
on the catalysts used [9±11]. BN nanotubes are
also produced by the annealing of ball-milled B or
BN powders in nitrogen, therein Fe and Fe aust-
enite phases were observed as possible catalysts
due to the contaminant from stainless-steel con-
tainer [12,13]. Moreover, BN nanotubes and B±N±
C nanotubes were also obtained by Rao et al.
using the pyrolysis of precursor molecules over Co
catalysts [14]. These experimental results suggest
that catalysts play an important role in synthesiz-
ing BN nanotubes.
Since these techniques mentioned above are not
ideally suited for the synthesis of BN nanotubes
for applications, greater attention needs to be paid
to devise new methods. In the present Letter, we
demonstrate a simple and e€ective method for
growing BN nanotubes, i.e., self-selected catalytic
growth. We believe that it is useful in under-
standing the role of catalyst in the growth of BN
nanotubes.
After reaction at temperature from 1200°C to
1500°C and with 15:1 weight ratio of B=Fe2O3, a
colorless wool-like product was found on the inner
walls of the tube and boat. The amount of the
product increased with the reaction temperature.
XRD results show that the wool-like product is a
mixture of hexagonal BN +h-BN) with two-layer
repeat units and rhombohedral BN +r-BN) with
three-layer repeat units. a-Fe di€raction peaks are
also observed in the XRD patterns. As the wool-
like product was collected far away from the raw
starting material, we believe that the formation of
the BN-phase must be due to a vapor-phase re-
action. In fact, when we excluded the existence of
oxygen by using iron instead of Fe2O3, the wool-
like product was not obtained. As boron has a low
vapor pressure, little boron vapor can be found
under these conditions. The vapor is likely to
contain boron oxide +B2O2 or B2O3) generated
from the reaction of boron with Fe2O3. Further-
more, TEM observations show that the wool-like
product is mainly composed of wool-like micro-
aggregates and growing in all directions like thorns
of a sea urchin. A typical HREM graph is shown
in Fig. 1a. Selected-area electron di€raction and
EELSshow that the BN aggregate is amorphous.
The amorphous-phase appears in incomplete hol-
low tube form. A little of Fe +about 5% weight
ratio) can also be detected in the aggregate from
EDSmeasurements. Unfortunately, actual mor-
phologies could not be observed since it is dicult
to disperse the aggregate in an ultrasonic bath.
It is easy to observe BN nanotubes inside the
wool-like product when the reaction temperature
is lower than 1350°C, as shown in Fig. 1b. The BN
nanotubes have diameters from 7 to 20 nm, and
usually have discontinuous and variable structure,
not typically straight and cylindrical. We did not
observe metal particles at the end of BN nanotu-
bes, which is a characteristic of vapor±liquid±solid
growth mechanism [15,16]. Previous reports indi-
cate that metastable disordered amorphous
BN-phase can be converted to tubular form at
high temperature [17]. We speculate that the
observed amorphous BN aggregate be responsible
for the growth of BN nanotubes.
2. Experimental
Fe2O3 and amorphous boron powder of a pu-
rity of 95% or better were mixed in an appropriate
ratio and homogenized in an agate pestle and
mortar. An alumina boat containing the powder
mixture was placed in the central hot zone inside
an alumina tube and heated in an argon gas ¯ow.
When the reaction temperature +from 1200°C to
1500°C in our experiments) was achieved, ¯owing
ammonia gas +50 standard cubic centimeters per
minute) was introduced and reacted for 1 h.
The products were identi®ed by means of X-ray
di€raction +XRD) analysis using CuKa radiation
at room temperature. Powder samples were dis-
persed in acetone using an ultrasonic bath, and a
drop of the suspension was deposited onto carbon
®lm coated grids for characterization by trans-
mission electron microscopy +TEM, Philips-
CM200), with micro-analysis via energy dispersive
spectrometry +EDS). High-resolution transmission
electron microscopy +HREM) study was carried
out in a JEM-2010 microscope operated under 200
kV, and with electron energy-loss spectrum system
+EELS).

494
C.C. Tang et al. / Chemical Physics Letters 342 +2001) 492±496
Fig. 2. The XRD patterns of the sponge-like layer covered the
starting reactants, the reaction was carried out at 1250°C and
with the B=Fe2O3 weight ratio of 15:1.
TEM observations show that in the gray sponge-
like layer, there is an abundant quantity of tubular
structures with diameters from 12 to 40 nm and
lengths up to several 10 lm. A typical TEM pic-
ture from the sample synthesized with the
B=Fe2O3 ˆ 15:1 and a reaction temperature of
1250°C is shown in Fig. 3. EELSspectrum shows
that the synthesized nanotubes contain only B and
N with a B:N ratio close to 1. Selected-area elec-
tron di€raction pattern of an individual tube is
also shown in Fig. 3 +inset). It is found that walls
are hexagonal BN+0 0 2) planes with a lattice
spacing of 0.341 nm. This distance of the BN
Fig. 1. +a) The HREM images of the wool-like product col-
lected on the inner walls of the tube and boat. +b) BN nanotubes
occurred in the wool-like product.
The yield of the BN nanotubes observed in the
wool-like product is still disappointingly low.
However, one-dimensional tubular BN with a
considerably high yield is obtained with an ap-
propriate B:Fe2O3 ratio +from about 5:1 to 15:1)
and a low reaction temperature +about 1250°C,
according to our experiments). In addition to the
wool-like product, a gray sponge-like layer cov-
ered the starting powder mixture. XRD patterns
+Fig. 2) show the layer to be a mixture of h-BN
and r-BN, with some a-Fe as impurity. No no-
ticeable peak of the starting Fe2O3 was observed
in the XRD patterns. This result strongly suggests
that reactions involving the reduction of Fe2O3
take place during this experiment. The formation
of BN probably occur from the following reac-
tions:
Fig. 3. TEM micrograph of the BN nanotubes occurred in the
sponge-like
6Bꢀs† ‡ 2Fe2O3ꢀs† ! 3B2O2ꢀg† ‡ 4Feꢀs; l†
B2O2ꢀg† ‡ 2NH3ꢀg†
ꢀ1†
ꢀ2†
sample
synthesized
with
the
starting
B:Fe2O3 ˆ 15:1 and the reaction temperature of 1250°C. The
electron di€raction from the BN nanotube area is shown in the
inset.
! 2BNꢀs† ‡ 2H2Oꢀg† ‡ H2ꢀg†

C.C. Tang et al. / Chemical Physics Letters 342 +2001) 492±496
495
layers indicates that the BN nanotube has a similar
structure to the products synthesized by arc-dis-
charge and laser ablation techniques. TEM images
also show that some BN nanotubes terminate at a
nanoparticle. EDSanalysis reveals that the nano-
particles are dominantly Fe. The characteristic
ending nanoparticles indicate that vapor±liquid±
solid growth is the most likely mechanism for the
BN nanotubes formation.
Both the B=Fe2O3 ratio and the reaction tem-
perature have noticeable e€ect on the nanotubes
formed. When a temperature higher than 1300°C
was used, ¯ake-like Fe grains and wool-like BN
aggregates were obtained. Few of the nanotubes
have their ends buried in BN aggregates and others
are closed and do not contain metal particles. These
BN nanotubes should have a similar origin to those
found in the wool-like product away from raw re-
actant. We also ®nd another tubular BN with a di-
ameter ranging from 60 to 110 nm and a length of
several micrometers +Fig. 4a,b). Most of the tubes
are divided into several segments by the connection
of inner tube walls and show bamboo-like shape.
The kink crystallizes in cylindrical structure +Fig.
4a) or exhibits irregular grain structure +Fig. 4b).
Such bamboo-like structure has been observed
earlier [18]. Usually metal particles adhere tothe end
of such ®laments. BN bamboo structures were ob-
served more frequently when a lower B=Fe2O3 ratio
was used.
We speculate that the state of iron at the
reaction temperature play a crucial role in the
formation of BN nanotubes or bamboo. The
formation of nanoscale Fe in liquid-phase is an
important factor for the one-dimensional growth.
Considering that the melting point of nanoscale
Fe is lower than the corresponding bulk Fe
[19,20], we believe that the catalytic activity of
Fe is related to the reaction temperature and the
size of Fe grains. Nanoscale Fe grains are
formed by reaction +1) or from the collision and
condensation of Fe vapor at high temperatures.
We notice that only nanoscale Fe can be an
e€ective catalyst for synthesizing BN nanoscale
one-dimensional structure. The grain sizes de-
pend on the reaction temperature and the con-
centration of Fe vapor. The high reaction
temperature and Fe content in reactants conduce
Fig. 4. BN bamboo-like structure occurred in the sample syn-
thesized at 1350°C. The kink exhibits: +a) a regular cylindrical
structure or +b) irregular grains.
to forming large Fe grain, and consequently
lower the catalytic activity or lose it fully. This is
also the reason that the BN nanotubes within
the framework of vapor±liquid±solid mechanism
can only be synthesized in a narrow range of
about 1250°C.
In conclusion, BN nanotubes can be synthe-
sized from the reaction of ammonia gas with a
mixture of boron and iron oxide. The synthesis
depends on the used B=Fe2O3 weight ratio and
reaction temperature. High temperature and high
Fe2O3 concentrations decrease the formation of
BN nanotubes and give rise to the occurrence of
BN bamboo. The growth of BN nanotubes and
bamboo can be attributed to a vapor±liquid±solid
growth mechanism. In addition, some amorphous
BN aggregates are observed away from the raw
reactant. Few BN nanotubes can also be found in
the aggregates, and their growth is related to the
existence of the amorphous BN phase.

496
C.C. Tang et al. / Chemical Physics Letters 342 +2001) 492±496
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