Catalytic synthesis of bamboo-like multiwall BN nanotubes via SHS-annealing process

Article abstract of DOI:10.1016/j.jssc.2011.01.016

Bamboo-like multiwall boron nitride (BN) nanotubes were synthesized via annealing porous precursor prepared by self-propagation high temperature synthesis (SHS) method. The as-synthesized BN nanotubes were characterized by the field emission scanning elec

Full text of DOI:10.1016/j.jssc.2011.01.016

Journal of Solid State Chemistry 184 (2011) 633–636
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Catalytic synthesis of bamboo-like multiwall BN nanotubes
via SHS-annealing process
L.P. Zhang a, Y.L. Gu a,b,n, J.L. Wang a, G.W. Zhao a, Q.L. Qian a, J. Li a, X.Y. Pan a, Z.H. Zhang a,b
a School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China
b Nano and Ceramic Materials Research Center, Wuhan Institute of Technology, Wuhan 430073, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Bamboo-like multiwall boron nitride (BN) nanotubes were synthesized via annealing porous precursor
prepared by self-propagation high temperature synthesis (SHS) method. The as-synthesized BN
nanotubes were characterized by the field emission scanning electron microscopy (FE-SEM), transmis-
sion electron microscope (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), Raman and
Fourier transform infrared (FTIR) spectroscopy. These nanotubes have uniform diameters of about
Received 29 September 2010
Received in revised form
8 December 2010
Accepted 16 January 2011
Available online 25 January 2011
60 nm and an average length of about 10 mm. Four growth models, including tip, base, based tip and
Keywords:
base–tip growth models, are proposed based on the catalytic vapor–liquid–solid (VLS) growth
mechanism for explaining the formation of the as-synthesized bamboo-like BN nanotubes. Chemical
reactions and annealing mechanism are also discussed.
Boron nitride
Nanotubes
Synthesis
& 2011 Elsevier Inc. All rights reserved.
SHS
Bamboo-like
Growth mechanism
1. Introduction
that the bamboo-like BN nanotubes can be synthesized by heating
B/Fe2O3 [7] or barium metaborate [8] in flowing ammonia gas.
After the first successful synthesis of single-walled boron
nitride (BN) nanotubes in 1995 [1], different types of BN nano-
tubes have received increasing attention due to their remarkable
properties such as stable insulators, high thermal conductivity,
superb oxidation resistance, a band gap about 5.5 eV, distinguish-
able chemical stability and outstanding mechanical strength [2].
All of these unique properties give BN nanotubes great potential
applications in electronics, optoelectronics, energy storage and
nanoelectromechanical systems [3]. In addition, the BN nano-
tubes were also predicted to be applicable in gas adsorbents,
spintronic devices, UV lasers, high resistivity substrates, inter-
connects for nanoscale electronics, radiation stoppers and reinfor-
cing agents for functional airspace metals/ceramics [4]. Until now,
there have been many different synthetic techniques developed
to grow BN nanotubes, including arc-discharge, laser ablation,
ball-milling, plasma-jet, substitution reactions with carbon nano-
tubes as templates, and chemical vapor deposition (CVD) [5].
According to literature, the bamboo-like BN nanotubes have a
higher gravimetric hydrogen uptake capacity [6]. On the other
hand, they may exhibit more excellent elastic properties and
mechanical stiffness due to their unique wall-structures compared
to the other nano-structures of BN nanotubes. It has been reported
Bamboo-like BN nanotubes also can be fabricated by annealing
ball-milled graphite/BN powders [9] and B/B2O3/NH3 [10,11],
amorphous boron/Eu/NH3/N2 [12]. In addition, the bamboo-like
BN nanotubes were successfully prepared using CVD from B–N–O
precursor/NH3 [6], and Si/Al/Fe/B2H6/NH3/O2/Ar [3]. Although
these methods can synthesize bamboo-like BN nanotubes, the
yields and purity are disappointingly low.
In this communication, we report an effective CVD method
by annealing porous precursor to synthesize bulk bamboo-like
multiwall BN nanotubes. The porous precursor is prepared by
self-propagation high temperature synthesis (SHS) method. Based
on experimental results, four growth models of catalytic vapor–
liquid–solid (VLS) growth mechanism are proposed. Chemical
reactions and annealing mechanism are also discussed.
2. Experimental
The raw materials, 20.00 g CaB6 and 20.00 g Fe2O3 (both above
300 mesh, 99.5 wt%) were mixed adequately by a blender for
30 min. The mixture was put into a SHS-furnace and heated to
750 1C for 10 min in an argon atmosphere under normal pressure.
After combustion reaction, 38.47 g porous precursor was then
obtained. The porous precursor was placed in an alumina boat and
heated in the annealing furnace to 1150 1C at 6 1C/min keeping for
6 h in ammonia atmosphere at a flow rate of 0.8 L/min under
n Corresponding author. Fax: +86 27 8719 3199.
E-mail address: ncm@mail.wit.edu.cn (Y.L. Gu).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.01.016

634
L.P. Zhang et al. / Journal of Solid State Chemistry 184 (2011) 633–636
normal pressure. After the furnace cooled to room temperature, the
crude product was collected, and then stirred in 400 mL HCl solution
(15 wt%) at 80 1C for 24 h to remove byproducts and metal catalyst.
The precipitates were filtered, washed by distilled water and dried
in a vacuum oven at 80 1C for 12 h. Finally, 22.06 g gray product
powders were obtained with a yield of about 81.4 wt% with respect
to boron.
A corresponding HRTEM image (Fig. 1(b)) was recorded from a
part of the BN nanotube. It is demonstrated that the lattice fringes
are well-defined, suggesting that the nanotube wall has a high
degree of crystalline perfection. Fig. 1(c) shows the magnified
image of the BN nanotube (the frame in Fig. 1(c)). It clearly
reveals lattice fringes having an interlay distance of 0.34 nm,
which is close to the intershell spacings of the {002} lattice planes
of the hexagonal phase of BN.
The structures of the sample were investigated by X-ray diffrac-
tion analysis (XRD) using a Shimadzu XD-5A diffractometer with
˚
Fig. 2 shows the XRD pattern (a), Raman spectrum (b), wide-
scan FTIR spectrum (c) and the deconvulation FTIR spectrum (d) of
the as-synthesized BN nanotubes. The typical XRD pattern shown
in Fig. 2(a) reveals five diffraction peaks at d-spacings of 3.394,
CuKa radiation (wavelength l¼1.5406 A). The field emission scan-
ning electron microscopy (FE-SEM) analysis was employed to
examine sample morphology using a FEI Sirion 200 microscope.
For FE-SEM observation, the samples were prepared by directly
spreading the product powder on conductive tapes and spattering
Au or Pt on the surface. The morphology and structure of the sample
were further elucidated by transmission electron microscope (TEM)
and high-resolution TEM (HRTEM) in a JEOL JEM-2100 microscope.
For TEM and HRTEM observations, the samples were prepared by
ultrasonically dispersing the product in ethanol, and depositing
on copper grids coated with carbon film. Raman spectra were
recorded at room temperature on a Nicolet DXR Raman spectro-
meter using Nd:YAG laser at excitation of 532 nm. Fourier transform
infrared (FTIR) spectra were recorded on a Nicolet 6700 Fourier
transform infrared spectrometer in transmission mode using a
KBr wafer.
˚
2.162, 2.067, 1.641 and 1.251 A, that can be indexed as the (002),
(100), (101), (004) and (100) planes of hexagonal phase of BN. The
˚
lattice constants are a¼2.516 and c¼6.712 A, which is close to
˚
the value a¼2.503 and c¼6.661 A in PCPDF card no. 73-2095. The
fact that no other diffraction peaks are observed indicates that
the as-synthesized product has high purity of BN without impu-
rities such as CaB6, Fe2O3 and other crystalline phases. The XRD
analysis reveals that the as-grown product is pure hexagonal
structure BN.
Fig. 2(b) is the typical Raman spectrum of the as-synthesized
BN nanotubes, which shows a sharp peak at 1356 cmꢀ1, corre-
sponding to the hexagonal BN active mode, E2g, derived from the
in-plane atomic displacement of the B and N atoms toward each
other [6]. More information of the lattice vibrations can be gained
through FTIR characterization. A typical FTIR spectrum presented
in Fig. 2(c) shows strong vibrations at 793 and 1370 cmꢀ1
,
3. Results and discussion
indexed to the A2u (BN vibration out-of-plane polarization) and
E1u (BN vibration in-plane polarization) mode, respectively. The
broad absorption band near 3401 cmꢀ1 can be resulted from the
O–H bonds due to the absorbed water. Two peaks at about 1210
and 1530 cmꢀ1 emerge in the deconvulation spectrum (Fig. 2(d))
of the main peak around 1370 cmꢀ1. The former can be indexed
to vibrations like those of wurtzite BN possibly for structure
defects in the as-grown cylinder type of BN nanotubes. The later
is due to the stretching of the hexagonal BN network along the
tangential directions (T-mode), which is the characteristic absorp-
tion of well-crystallized BN nanotubes.
Fig. 1 displays the typical FE-SEM, TEM and HRTEM images of
the as-synthesized BN nanotubes. The typical FE-SEM image of
the as-synthesized sample in Fig. 1(a) shows the general mor-
phological characteristics. It presents a large quantity of one-
dimensional (1D) nano-structures with uniform diameters of
about 60 nm and an average length of about 10 mm. In addition,
a small amount of BN flakes can be observed. Nanotube content is
estimated as approximately 80 wt% according to the statistical
analyses by TEM and FE-SEM microscopy. To confirm the detailed
morphology and structure, a respective extensive TEM investiga-
tion was carried out. Fig. 1(d) reveals a representative TEM image
of an individual BN nanotube consisting of a row of periodic cup-
stack, bamboo-like structures. It also clearly displays that the tube
has an outer diameter of about 60 nm and an inner diameter of
20 nm.
In the growth process of the bamboo-like BN nanotubes, the
chemical reactions can be formulated as follows:
3CaB6+2Fe2O3¼B18Fe4Ca3O6
B18Fe4Ca3O6+18NH3¼18Bn+18Nn+Fe4Ca3O6+27H2
(1)
(2)
Fig. 1. FSEM (a), TEM (b) and HRTEM (c, d) images of the as-synthesized BN nanotubes.

L.P. Zhang et al. / Journal of Solid State Chemistry 184 (2011) 633–636
635
Fig. 2. XRD pattern (a), Raman spectrum (b), wide-scan FTIR spectrum (c) and the deconvulation FTIR spectrum (d) of the as-synthesized BN nanotubes.
x(Bn+Nn)+Fe4Ca3O6¼xBNNT+3FeO+3CaO+Fe
(3)
(4)
(18ꢀx)(Bn+Nn)¼(18ꢀx)BNFL
(x¼0–18, NT¼nanotubes, FL¼flakes BN)
As shown in Eq. (1), the porous precursor B18Fe4Ca3O6 is
prepared by SHS method from the raw material CaB6 and Fe2O3
powders. The SHS synthesis generally results in very high yields
above 95 wt%, so the chemical composition formula of the
precursor can be expressed approximately as B18Fe4Ca3O6. During
the annealing process, boron-catalyst-containing species in the
precursor reacts with NH3 and produced active boron (Bn) and
nitrogen (Nn) vapor (Eq. (2)). Liquid droplets of catalyst Fe or
Fe–Ca alloy support on CaO particles, expressed as Fe4Ca3O6 can
be simultaneously generated in-situ from metal borides and
oxides. According to the catalytic VLS growth mechanism [13],
BN nanotubes can be grown from the Bn and Nn vapor in the
presence of the catalyst (Eq. (3)). Small amount of BN flakes (as
seen in Fig. 1(a)), may also be produced through a vapor–solid
(VS) growth mechanism due to lacking of catalyst (Eq. (4)).
Therefore, the selectivity of BN nanotubes can be expressed as
x/18, where the value of x is estimated as approximately 14.5
according the selectivity of the as-synthesized BN nanotubes.
Fig. 3 is the schematic illustrations of the growth models of
the as-synthesized bamboo-like BN nanotubes. It includes four
growth models co-existing in the growth process, namely base,
tip, based tip and base–tip growth models shown in Fig. 3(a)–(d),
respectively.
According to the catalytic VLS growth mechanism [13], bam-
boo-like BN nanotubes can be grown depending on abruptly
changing of growth rate of individual BN nanotubes. Firstly,
reactive boron sources and catalyst particles are dispersed homo-
geneously in the porous precursors. Both supported and free
catalyst droplets can be formed in-situ during the annealing
process at 1150 1C (see Fig. 3(a1), (b1) and (c1)). Then NH3 is
decomposed to the active Nn and adsorbed on the surface of the
catalyst particle. As mentioned in Eq. (2), active boron (Bn) and
Fig. 3. Schematic illustrations of the as-synthesized BN nanotubes through
(a) base growth, (b) tip growth, (c) based tip growth, (d) base–tip growth and
corresponding features of growth rates. Black particles contain boron source and
catalyst. The arrows denote the movement direction of the tip catalyst droplets.

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L.P. Zhang et al. / Journal of Solid State Chemistry 184 (2011) 633–636
Fig. 4. Sketch of bulk synthesis of BN nanotubes by annealing porous precursor: (a) original state of porous precursors prepared by SHS, (b) catalytic sites and the initial
growth stage of BN nanotubes, and (c) the final stage of fully annealed precursor. Black particles contain boron source and catalyst, while the gray denote catalyst support.
The wool-like structures represent BN nanotubes.
nitrogen (Nn) vapor form BN. The newly formed BN species
precipitate layer by layer to form BN shells, as described
in Figs. 3(a2), (b2) and (c2). The BN layers grow from the interface
between the catalyst and BN shells (see Fig. 3(a3), (b3) and (c3)).
With the gradual growth of BN nanotube, negative pressure of
inner void is formed and liquid catalyst droplets are reshaped.
While the pulling force generated by the negative pressure is too
big to withstand, an instant disjoint occurs at the interface. At
that moment, outside vapor containing Bn and Nn fly inside to
balance the pressure, leaving an achieved bamboo knot structure
(see Figs. 3(a4), (b4) and (c4)). And finally, the bamboo-like tube
can grow up through repeating the process. Similar to the other
three growth models described above, the bamboo-like BN
nanotubes can also grow with base–tip growth mechanism
(Fig. 3(d)). In fact, it has two growth points combining the base
and tip growth mechanism. The characteristic growth rates for
the bamboo-like BN nanotubes can be expressed as a periodic
changing of growth rate, as shown in Fig. 3(d).
1150 1C in a flowing ammonia atmosphere using porous precur-
sor prepared by SHS method. The as-synthesized bamboo-like BN
nanotubes have uniform diameters of about 60 nm and a length
of about 10 mm in average. Four growth models based on the
catalytic VLS growth mechanism are proposed, so-called base, tip,
based tip and base–tip growth models. Chemical reactions and
annealing mechanism demonstrate that the porous precursor
plays a crucial role in bulk synthesis of the as-synthesized
bamboo-like BN nanotubes.
Acknowledgments
The authors acknowledge the financial support from the
Government of Hubei Province of the People’s Republic of China
for this research work. The authors acknowledge Prof. Xu Liqiang
(School of Chemistry and Chemical Engineering, Shandong Uni-
versity) for their help in assisting microscopy analyses.
Fig. 4 is a sketch of bulk synthesis of BN nanotubes by
annealing porous precursor. The structure of the porous precursor
consists of boron source and catalyst sources (black particles) and
inert support particles (gray, Fig. 4(a)). In the initial annealing
stage, the BN nanotubes begin to grow mainly by base, based tip
and base–tip growth models from boron source and catalyst
particles (Fig. 4(b)). Importantly, the porous structure is kept
well-structured during the quick and random growth of BN
nanotubes. Bulk synthesis of BN nanotubes can be easily reached
via unique SHS-annealing method accordingly. When the pre-
cursor is completely reacted, BN nanotubes push away and
separate from the support particles, resulting in local agglomer-
ates of wool-like BN nanotubes (Fig. 4(c)). Therefore, the porous
structure of the solid precursor plays a key role in bulk synthesis
of the as-grown BN nanotubes process.
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In conclusion, bamboo-like multiwall BN nanotubes were
successfully synthesized by a CVD method via annealing at