Effective growth of boron nitride nanotubes

Article abstract of DOI:10.1016/S0009-2614(02)00325-1

An effective route of growing boron nitride nanotubes was developed. By reacting boron, silica and iron oxide catalysts supported on alumina, gas-phase reactants and liquid-phase catalyst could be generated in-situ. When ammonia was then introduced, boron

Full text of DOI:10.1016/S0009-2614(02)00325-1

2
2 April 2002
Chemical Physics Letters 356 (2002) 254–258
www.elsevier.com/locate/cplett
Effective growth of boron nitride nanotubes
a,
C.C. Tang , X.X. Ding , X.T. Huang , Z.W. Gan , S.R. Qi , W. Liu ,
a
a
a
a
a
*
b
S.S. Fan
a
Department of Physics, Nanotechnology Laboratory, Central China Normal University, Wuhan 430079, China
b
Department of Physics, Tsinghua University, Beijing 100080, China
Received 2 October 2001; in final form 22 February 2002
Abstract
An effective route of growing boron nitride nanotubes was developed. By reacting boron, silica and iron oxide
catalysts supported on alumina, gas-phase reactants and liquid-phase catalyst could be generated in-situ. When am-
monia was then introduced, boron nitride nanotubes could be formed by the reaction of boron oxide and ammonia
inside the liquid-phase catalysts. The synthesized boron nitride nanotubes are highly-crystallized and multi-walled
structures with 10–30 nm in diameter. The growth process was explained based on vapor–liquid–solid wire-like growth
mechanism. Ó 2002 Published by Elsevier Science B.V.
1
. Introduction
signed methods [3–9]. Unfortunately, these meth-
ods have not been effective for synthesizing bulk
BN nanotubes; for example, when considering the
insulating nature of boron nitride for arc-dis-
charge technique, low vapor pressure for laser
ablation and carbon contamination for template-
confined methods. Very recently, a hazardous
procedure of producing high-yield BN nanotubes
via chemical vapor deposition of the precursor
borazine on nickel boride catalysts has been re-
ported [10]. So far there is no reliable method to
produce bulk quantities of BN nanotubes and
therefore, the exploration on its synthesis is still a
challenging subject.
Original theoretical studies predict that pure
boron nitride (BN) nanotubes are energetically
stable in geometry and, in electronic properties,
are semiconductors with a uniform large bandgap
regardless of the radius and chirality, or the
number of walls of the tubes [1]. The characteristic
contrasts markedly with the heterogeneity of car-
bon nanotubes [2], for which electronic properties
range from metallic to semiconducting depending
on the diameter and chirality. BN nanotubes have
been fabricated by different techniques, such as
arc-discharge, laser ablation, carbon nanotubes
template-confinement and some specifically de-
Crystal growth of one-dimensional structure
requires a pathway to continuously provide gas- or
liquid-phase reactants in a specified site, in which
an anisotropic driving force exists to assure an
oriented growth [11,12]. When gas reactants and
*
Corresponding author.
E-mail address: cctang@phy.ccnu.edu.cn (C.C. Tang).
0
009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S0009-2614(02)00325-1

C.C. Tang et al. / Chemical Physics Letters 356 (2002) 254–258
255
liquid impurity (catalyst) are used, the vapor–liq-
uid–solid (VLS) growth mechanism is responsible
for the synthesis of wire-like structure. The
mechanism was first proposed by Wagner and Ellis
in 1960s for synthesizing Si whisker [13]. Morales
and Lieber synthesized Si and Ge nanowires by
combining laser ablation and a vapor–liquid–solid
method [14]. This technique can produce nano-
scale liquid catalysts, which finally determine the
diameter of the final nanowires. Now it is com-
monly accepted that these factors, the formation
and activity of the ultra-fine liquid catalyst, the
absorption of the reacted gas, and the nucleation
and crystallization growth inside liquid catalyst,
play a crucial role in synthesizing nanowires.
ature in argon atmosphere, a gray product can be
obtained. The product was washed with diluted
hydrofluoric acid (1M) and then filtered for several
times to remove the residual silica. About 100 mg
of the product after purification could be obtained.
However, this purification procedure could not
fully remove the impurities remnant in the final
product. The remnant composition was identified
as the mixture of silica, alumina and BN by X-ray
diffraction analysis.
The product was checked by scanning electron
microscopy (SEM, JEM-6301F) and transmission
electron microscopy (TEM, JEOL-2010F) with
energy dispersive spectrometry system (EDS) and
electron energy loss spectrometer (EELS).
Recently, for growing nanowire-like structure,
we developed a simple and reliable technique
named as in-situ silica-assisted catalytic growth
method within the framework of VLS mechanism.
The method involved the in-situ formations of
reacting gases and catalysts being in liquid phase,
and turned out to be an effective procedure for
synthesizing some oxide nanowires and gallium
nitride nanowires [15,16]. We believe that it could
also be used to synthesize BN nanotubes by ra-
tionally choosing reaction routes. Indeed, here BN
nanotubes are successfully produced on a large
scale, by heating the mixture of boron, silica and
3. Results and discussion
SEM examinations indicate that a one-dimen-
sional structure has been synthesized, although
some silica, alumina and BN impurities with the
grain morphology coexist in the sample. However,
one-dimensional BN is dominant and its volume
percentage was estimated to be higher than 80%.
TEM image of the product shows that the one-
dimensional structure exhibits the straight nano-
tube-like morphology with 10 to 30 nm in diameter
and several micrometers in length (Fig. 1a). A
high-resolution TEM image from individual
nanotube further shows that the nanotubes are
highly crystallized in a multi-walled structure with
the layer spacing of 0.34 nm (Fig. 1a, inset). The
interlayer space is consistent with the interplanar
distance of 0.333 nm in bulk graphite-like BN.
EELS quantitative analysis allows us to determine
the chemical compositions of the synthesized na-
notubes and shows that they only contain B and N
with B:N ratio of 1:05 ꢀ 0:10, as shown in Fig. 1b.
EDS measurement for the part of BN nanotube
with pure tube-like structure indicates that no
contamination from silica, iron oxide and alumina
was observed (Fig. 1c).
iron oxide catalyst supported on Al
ammonia gas.
2 3
O under
2
. Experiments
The catalyst was prepared by impregnating,
then stirring and drying the appropriate amount
ferric nitrate and fumed alumina with methanol
solution. The size of Fe O catalysts are about 10
2 3
to 20 nm. Amorphous boron and silica powers
with molar ratio of 1:1 were thoroughly mixed and
2 3
homogenized with the pre-synthesized Fe O cat-
alyst. About 500 mg of the mixture was placed in a
quartz tube mounted in a tube furnace. An argon
flow passed through the quartz tube as the furnace
was heated to reach 1080 °C, and then was shut off
and pure ammonia gas was introduced at a flow
rate of 80 standard cubic centimeters per minute
and lasted for 1 h. After cooled to room temper-
Beside BN nanotubes with hollow structure,
some BN nano-bamboos could also be occasion-
ally observed from the TEM images. A typical
morphology of the bamboo structures was shown
in Fig. 2, which also exhibit a crystallized structure

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C.C. Tang et al. / Chemical Physics Letters 356 (2002) 254–258
with 40 to 60 nm in diameter. These bamboos
usually grow directly from the solid grains that are
comprised of boron, silicon and iron oxides, as
identified by EDS analysis. The morphology is
similar to the BN filaments synthesized by nitrid-
ing borides [17].
In addition, TEM images show that BN nano-
tubes have a closed circle tip at one end of the
nanotubes and terminate at another end in a
nanoparticle. Although having the nearly same
outer and inner diameters at most parts of an in-
dividual nanotube, the synthesized BN nanotubes
usually exhibit discontinuous or variable diameters
near their closed tips and the nanoparticles. A
gradually decreasing tendency of the tube diameter
near the closed tips could be observed, as shown in
Fig. 3. The existence of such unstable growth in-
dicates that the growth of BN nanotubes might go
through a non-equilibrium process, especially at
the beginning and ending of nanotubes growth.
Usually, two different morphologies of nano-
particles can be found from TEM images on the
tips of most nanotubes. One of the nanoparticle
exhibits the elongated fillings inside the nanotube
channels, as labeled in arrows in Fig. 1a, and the
circular particles encapsulated at the tip ends
shown in Fig. 4a. EDS analysis shows that they
contain only Fe and Si, and no Al content was
detected. Fig. 4b shows another nanoparticle with
irregular shape, which closely attaches on the
supported alumina and becomes a starting point of
Fig. 1. (a) TEM image of the BN nanotubes synthesized by in-
situ silica-assisted catalytic growth method, and a high-resolu-
tion TEM image from individual nanotube shown in the inset.
(b) Typical EELS determining the chemical compositions of the
synthesized nanotubes only containing B and N with B:N ratio
of 1.05. (c) EDS spectrum of BN nanotube with pure hollow-
like structure.
Fig. 3. TEM image showing the gradually decreasing diameter
of BN nanotubes near the closed tips (arrows).
Fig. 2. TEM image of the BN nano-bamboo.

C.C. Tang et al. / Chemical Physics Letters 356 (2002) 254–258
257
mediated to reach a specific range, in which a
liquid-state catalyst exists and catalytic activity
preserves. Under reducing atmosphere (boron and
ammonia), silica is reduced to silicon and then it
alloys with iron to form a low melting point FeSi
x
phase [18]. The observations of the nanoparticles
with variable ratio of iron and silicon indicate that
the composition of the liquid nanoparticles should
be FeSi
2
þ Fe or FeSi þ Si. Moreover, the melt-
2
ing point of nanoscale iron is lower than the cor-
responding bulk solid [19], therefore, we believe
that at our experimental temperature the catalyst
should be in liquid state.
Due to the existence of high Si content in raw
reactants, Si might occur in the final product as an
impurity. Indeed, except silica of incompletely re-
acted, we also observed another Si structure from
the TEM image and with EDS analysis. It is
amorphous Si nanowires with the variable diam-
eters and lengths up to several microns (Fig. 5).
Usually the Si nanowires have a slightly larger
diameter than BN nanotubes and shows a curved
morphology. We also noticed the recent reports on
the synthesis of the Si nanowires by oxide-assisted
method; therein Si nanowires can be obtained by
the direct sublimation of SiO gas at about 1200 °C
[20]. Therefore, we believe that SiO also appears
near the reaction area due to the reaction of silica
with boron or ammonia.
Fig. 4. (a) TEM image of the nanoparticles closely attached on
the supported alumina. (b) Another type of nanoparticles
wrapped into the BN nanotubes.
the BN nanotube growth. EDS analysis indicates
that the irregular pillar is the Al O used to sup-
2
3
port the catalyst, whereas the nanoparticle con-
tains Fe and Si. In addition, our EDS observations
for these nanoparticles show that and the molar
rates of Si and Fe are variable. These character-
istics of nanoparticles indicate that these BN
nanotubes were produced by a VLS growth
mechanism.
We will ascribe the in-situ silica-assisted cata-
lytic growth method to a VLS growth mechanism
to explain the large-scale growth of BN nanotubes.
First, our method takes into account of the dis-
tribution of the reacting vapors concentration.
Due to the involved redox reaction between boron
We performed further experiments in order to
confirm the above assumptions on the BN nano-
tubes growth. When ammonia is reacted with a
and oxides (SiO
boron oxide gas (B O or B O ) could be expected.
2 2 3
; Fe O ), the in-situ generation of
2
2
2
3
The method to produce the reactant gas guaran-
tees enough concentration of boron oxide atmo-
sphere in the reaction area. Therefore, when
ammonia was introduced, the continuous BN
crystal growth could be achieved. Second, we
2 3
consider the catalytic activities of the Fe O cata-
lyst. The high-temperature interaction of the
Fe O catalyst and its support Al O will reduce
2
3
2
3
iron catalytic effect and therefore, a low reaction
temperature is needed. However, VLS growth re-
quires a liquid-phase iron and consequently a high
reaction temperature. In our experiment, with the
Fig. 5. Amorphous Si nanowires existing in the BN nanotubes
(arrows).
2
addition of SiO , the temperature can effectively be

258
C.C. Tang et al. / Chemical Physics Letters 356 (2002) 254–258
mixture of boron and iron powder with the size
of about 1 lm at the temperatures from 1000 to
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