Materials Transactions, Vol. 49, No. 11 (2008) pp. 2461 to 2464
Special Issue on Advances in Computational Materials Science and Engineering V
#2008 The Japan Institute of Metals
Growth of Boron Nitride Nanohorn Structures
Takeo Oku1; * , Kenji Hiraga2 and Toshitsugu Matsuda3
1
Department of Materials Science, The University of Shiga Prefecture, Hikone 522-8533, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3
JMC New Materials, Inc., Tokyo 103-0013, Japan
2
Boron nitride (BN) nanohorns were fabricated, and their structures were investigated by transmission electron microscopy and molecular
mechanics calculation. The multi-walled BN nanohorns would be stabilized by stacking of nanohorn structures. Growth of the BN nanohorn was
observed at elevated temperatures, and the activation energy for the nanohorn growth was estimated to be 2.3 eV.
[doi:10.2320/matertrans.MB200806]
(Received July 23, 2008; Accepted September 3, 2008; Published October 16, 2008)
Keywords: nanostructures, chemical vapor deposition, transmission electron microscopy
1.
Introduction
Various kinds of carbon-based nanocage structures, such
as fullerene clusters, nanotubes, nanopolyhedra, cones, cubes
and onions, have great potential for studying materials of low
dimensions in an isolated environment.1) Especially, carbon
nanohorns as a novel type of carbon nanotubes are expected
as catalyst electrode materials for next-generation fuel cells,
which separate hydrogen and electrons from methanol.2,3)
Boron nitride (BN) nanostructured materials with a bandgap
energy of 6 eV and non-magnetism are also expected to
show various electronic, optical and magnetic properties such
as Coulomb blockade, photoluminescence, and supermagnetism.4,5) Recently, several studies have been reported on
BN nanomaterials such as BN nanotubes,6–8) BN clusters,9,10)
BN nanocapsules4,5,11) and BN nanoparticles,12,13) which are
expected to be useful as electronic devices, high heatresistance semiconductors and insulator lubricants. In addition, some BN nanohorns as a novel type of BN nanotubes
were also reported by HREM images.14,15)
The purpose of the present work is to synthesize stable
BN nanohorns by using a chemical vapor deposition method.
The second purpose is to investigate the atomic structures
and stability of BN nanohorns by transmission electron
microscopy (TEM) and molecular mechanics calculations.
The present study will give us a guideline for designing and
synthesis of the BN nanohorns, which are expected as future
nanoscale devices.
2.
Experimental Procedures
Chemically vapor deposited boron nitride (CVD-BN)
plates were synthesized from BCl3 -NH3 -H2 gas systems at
temperatures of 1400–2000 C and total gas pressures of
5 Torr on graphite substrates.16,17) These BN plates were
thinned to 100 mm in thickness with emery papers, and then
punched into discs of 2.3 mm diameter with a supersonic
wave cutter. The discs were polished with a dimple grinder to
less than 50 mm in thickness, and thinned by argon ionmilling at an accelerating voltage of 3–5 kV. TEM observa*Corresponding
author, E-mail: [email protected]
tions were performed with a 200 kV electron microscope
(JEM-200CX) equipped with a top entry goniometer having
a point-to-point resolution of 0.23 nm. Structural optimization of the BN nanohorns was performed by semiempirical molecular mechanics calculations (CS Chem3D,
CambridgeSoft).
3.
Results and Discussion
Figure 1(a) shows a TEM image of BN nanohorns grown
from the BCl3 -NH3 -H2 at a deposition temperature of
1800 C, and a number of particles (nanohorns) are visible
with the structures of stars. It should be noted that only those
nanohorns which satisfy certain diffraction conditions are
visible in the images. A considerable number of particles are
contained in the samples, as confirmed by tilting. Enlarged
images of BN nanohorns synthesized at 1800 and 2000 C are
shown in Fig. 1(b) and (c), respectively. Five boundaries are
observed in the particles in Fig. 1(c) and 1(d). The strain
contrast results from some defects in the particles. Growth
sizes of nanohorns at elevated temperatures are summarized
as Table 1.
Figure 2(a) is a TEM image of a BN nanohorn tilted 37
away from the star axis (fivefold axis). The fivefold axis is
indicated by the interface. A high-resolution image of a part
of Fig. 2(a) is shown in Fig. 2(b). {002} planes of h-BN are
observed, and the angle between them is 140 .
From the results of TEM observation, a three-dimensional
structure model is proposed, as shown in Fig. 3. Figure 3(a)
and 3(b) are schematic illustrations of BN {002}-layer
stacking viewed along the fivefold axis [201] and [110]
direction, respectively. The angle between the two neighboring {002} of h-BN is 136 , which almost agrees with the
experimental data of Fig. 2(b). The present model agrees
well with a surface structure of pyramidal pentagonal facets
observed by scanning electron micrographs in the previous
work.16) Atomic structure models of the BN nanohorn with
stacking structures are shown in Fig. 3(c) and 3(d). The
nanohorn angle is 112 , which agrees with the angle of
136 that is 37 -inclined from the fivefold axis.
Binding energies of BN nanohorns with various nanohorn
angles were calculated by molecular mechanics calculation,
2462
T. Oku, K. Hiraga and T. Matsuda
a
150nm
b
c
50nm
40nm
TEM images of BN nanohorns grown at deposition temperatures of (a) 1800 C, (b) 1800 C and (c) 2000 C.
Fig. 1
a
b
30nm
{002}
1nm
(a) TEM image of BN nanohorn tilted 37 away from the nanohorn axis. (b) HREM image of a part of (a).
Fig. 2
Table 1 Growth sizes of nanohorns at elevated temperatures.
Deposition temperature [ C]
1600
1700
1800
2000
Nanohorn size [nm]
25
50
100
300
and the calculated values for these stacking BN nanohorns
are summarized in Table 2. A BN nanohorn with a nanohorn
angle of 112 has the lowest binding energy as listed in
Table 2, which agrees with a large growth size of the present
results. Distance between BN layers of nanohorn in HREM
images were measured to be 0:35 nm, and the basic
structure models were constructed based on them. After
molecular mechanics calculation, the layer distances were
optimized as 0:37 nm. Since the hexagonal BN structure
has atomic stacking of B-N-B-N along c-axis, stacking
structures of two and four layers were also calculated. Total
energies per atommol of BN clusters were reduced by
Growth of Boron Nitride Nanohorn Structures
<002>
(a)
2463
(c)
{002}
(b)
(d)
{002}
{002}
Fig. 3 (a) Schematic illustration of BN {002}-layer stacking observed along (a) the nanohorn axis and (b) the h110i directions of
hexagonal BN. Atomic arrangement of BN nanohorns (c) along and (d) perpendicular to the nanohorn axis.
Table 2 Binding energies of BN nanohorns with various horn angles. Numbers of the nanohorn layers are three.
B1641 N1560
B1635 N1635
B1671 N1623
B1605 N1605
B1608 N1575
Nanohorn angle
[degrees]
20
40
60
84
112
Binding energy
[kcal/molatom]
0.642
0.470
0.513
0.384
0:442
Table 3
Binding energies of BN nanohorns with various layer structures.
B164 N156
B328 N312
B656 N624
B731 N713
B1462 N1426
Number of
stacking layers
1
2
4
1
2
Binding energy
[kcal/molatom]
0.0506
0:268
0:450
0.0294
0:317
stacking of BN layers (Table 3), and it is believed that the
electrons below and above hexagonal BN networks would
have a role of van der Waals bonding between stacking
layers, and the structure of BN nanohorns with fivefold
symmetry would be stabilized by multiplying hexagonal BN
ring networks.
From the results of Table 1, growth sizes were plotted as
a function of deposition temperatures in the range of 1600
to 2000 C, as shown in Fig. 4. Activation energy of the
nanohorn growth was calculated to be 2.3 eV, which might
be due to the lattice diffusion of boron and nitrogen atoms
in BN. BN nanohorns with various tip angles have been
reported, and the present pentagonal structure with a tip angle
of 112 showed fairly low energy as listed in Table 2.
It is believed that this low energy would be due to its
small curvature and multiply twinned h-BN structure.
Binding energy decreases by stacking BN nanohorn layers
as listed in Table 3, which results in stabilization and growth
of the multiply-twinned h-BN structure.
Taking account of the lattice parameters of h-BN, the
misfit of the fivefold h-BN structure is 1.6 , which is much
smaller than the 7.3 of ordinary multiply-twinned fcc
2464
T. Oku, K. Hiraga and T. Matsuda
Growth size (nm)
proposed, and the activation energy for BN nanohorn growth
was estimated to be 2.3 eV.
Acknowledgements
The authors would like to thank N. Koi and A. Nishiwaki
for calculation help.
REFERENCES
1000/T (1/K)
Fig. 4 Growth size of BN nanohorns at temperatures ranging from 1600 to
2000 C.
nanoparticles. This small amount of misfit allows the growth
of relatively large sized particles with little inner distortion.
The present BN nanohorns have B-B bonding between the
h-BN rings, which results in a distorted pentagonal shape as
observed in the experimental data of Fig. 1.
4.
Conclusions
BN nanohorns with a fivefold symmetry were synthesized
by a chemical vapor deposition method. Growth and
structures of the BN nanohorns were investigated by TEM
and molecular mechanics calculation. Sizes of the BN
nanohorns were strongly dependent on the deposition temperatures. A structure model for the fivefold BN nanohorns is
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