Hydrogen storage in boron nitride nanomaterials studied by TG/DTA and cluster calculation

Article abstract of DOI:10.1016/j.jpcs.2003.10.033

BN nanotubes, nanocages and nanocapsules were synthesized from LaB ₆ and Pd with boron powder by using an arc melting method. Thermogravimetry/differential thermogravimetric analysis of BN nanomaterials produced from LaB₆ and Pd/boron powder showed possibility of hydrogen storage of 1-3wt%. Conditions of H₂ gas storage in B ₃₆N₃₆ cluster, which was considered as a cap structure of B₉₉N₉₉ nanotube, were predicted by first principle single point energy calculations. H₂ molecules would be introduced from hexagonal rings of the cage structure. BN fullerene materials would store H ₂ molecule easier than carbon fullerene materials, and its stability for high temperature would be good.

Full text of DOI:10.1016/j.jpcs.2003.10.033

Journal of Physics and Chemistry of Solids 65 (2004) 549–552
www.elsevier.com/locate/jpcs
Hydrogen storage in boron nitride nanomaterials studied
by TG/DTA and cluster calculation
*
Takeo Oku , Masaki Kuno, Ichihito Narita
Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan
Abstract
BN nanotubes, nanocages and nanocapsules were synthesized from LaB
6
and Pd with boron powder by using an arc melting method.
Thermogravimetry/differential thermogravimetric analysis of BN nanomaterials produced from LaB6 and Pd/boron powder showed
possibility of hydrogen storage of 1–3 wt%. Conditions of H gas storage in B N cluster, which was considered as a cap structure of
2
36 36
99 2
B N99 nanotube, were predicted by first principle single point energy calculations. H molecules would be introduced from hexagonal rings
of the cage structure. BN fullerene materials would store H
temperature would be good.
2
molecule easier than carbon fullerene materials, and its stability for high
q 2003 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; C. Electron microscopy; D. Microstructure
1
. Introduction
advantage of their catalytic effect to produce the BN
nanomaterials. La has shown excellent catalytic properties
for producing a large number of single-walled carbon
nanotubes and enlarging their diameter [15], and Pd is also
expected to act as a hydrogen storage material. Although gas
storage of hydrogen and argon in carbon nanotubes has been
reported [16,17], carbon nanotubes are oxidized at 600 8C in
air [18]. On the other hand, BN are stable up to 1000 8C in
air [4], which indicates the excellent heat resistance
compared to the carbon materials for gas storage. To
understand the formation of BN nanostructures, high-
resolution electron microscopy (HREM) were carried
out, which are very powerful methods for atomic
structure analysis [19,20]. For the BN nanomaterials,
hydrogen gas storage measurements were carried out
using TG/DTA.
Many works have been reported on carbon nanotubes [1]
and nanocapsules [2] since the discovery of fullerenes [3].
Carbon nanostructures are intriguing for both scientific
research and future device applications, such as cluster
protection, nano-ball bearings, nano-opticalmagnetic
devices, catalysis, gas storage and biotechnology [4].
Recently, several studies on boron nitride (BN) fullerene
materials have been reported since they have excellent
properties such as heat resistance in air, insulation, wide
bandgap and structural stability [4,5]. Most of the studies on
the BN fullerene materials were focused on the formation of
BN nanotubes [4,6–10]. We have focused on the BN
nanocapsules, nanotubes, nanocages, nanoparticles and
clusters to discover new properties and structures by
changing the encapsulated materials [4,9–12], and on the
possibility of gas storage in BN nanocapsules [4,13,14].
Since the BN nanocapsules consist of light elements, it is
believed that they can store much gas per weight [4].
The purpose of the present work is to prepare BN
nanotubes, nanocapsules and nanocages, and to investigate
hydrogen gas storage by thermogravimetry/differential
thermogravimetric analysis (TG/DTA) and first principle
2. Experimental procedures
Mixture powder compacts made of boron particles (99%,
0 mm, Niraco Co. Ltd), LaB particles (99%, 1 mm, Wako
4
6
Co. Ltd) and Pd particles (99.5%, 0.1 mm, Niraco Co. Ltd),
with the size of 3 mm in height and 30 mm in diameter were
produced by pressing powder at 10 MPa. The atomic ratios
of metal (M) to boron (B) were in the range of 1:1–1:10.
The green compacts were set on a copper mold in an
electric-arc furnace, which was evacuated down to
calculation. LaB and Pd were selected in order to take
6
*
Corresponding author. Tel.: þ81-6-6879-8521; fax: þ81-6-6879-8522.
E-mail address: oku@sanken.osaka-u.ac.jp (T. Oku).
0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2003.10.033

550
T. Oku et al. / Journal of Physics and Chemistry of Solids 65 (2004) 549–552
2
3
2
0
.0 £ 10 Pa. After introducing a mixed gas of Ar
.025 MPa and N 0.025 MPa, arc-melting was applied to
2
the samples at an accelerating voltage of 200 V and an arc
current of 125 A for 2 s. Arc-melting was performed with a
vacuum arc-melting furnace (NEV-AD03, Nisshin Engin-
eering Co. Ltd), and the white/gray BN nanomaterial
powders were collected from surface of the bulk.
Samples for HREM observation were prepared by
dispersing the materials on holy carbon grids. HREM
observation was performed with a 300 kV electron micro-
scope (JEM-3000F). To confirm the formation of BN
fullerene materials, EDX analysis was performed using the
EDAX system with a probe size of ,10 nm. In order to
measure hydrogen gas storage in BN nanomaterials, BN
nanomaterials were extracted from the obtained powder by a
supersonic dispersing method based on the Stokes equation
using ethanol. The Stokes equation is expressed as follows:
2
v ¼ d ðs 2 rÞg=18h (h ¼ vt; v; sedimentation rate, h;
viscosity of liquid, s; density of particles, r; density of
liquid, g; gravitational acceleration, d; diameter of particles,
t; subsidence time, h; height of liquid). Since there is a big
difference in the size and density of the produced BN
nanocapsules/nanotubes and other powders, it is believed
that this method would be effective for separation of BN
nanomaterials. After separation of BN nanomaterials,
hydrogen storage was measured by TG/DTA at tempera-
tures in the range of 20–300 8C in H atmosphere.
2
To search the optimized structure of B N and B N
36 36
9
9
99
with H2 molecules, semi-empirical molecular orbital
calculations (PM3) were performed. The energies of
B N with H molecules were calculated by first principle
3
6
36
2
single point energy calculation using Gaussian98. In the
calculation, 3-21G was used as ground function with
Hartree–Fock level.
3
. Results and discussion
A HREM image of a BN nanotube produced using LaB6
powder is shown in Fig. 1(a). In Fig. 1(a), the diameter of
the five-layered BN nanotube is changing from bottom to
top, and amorphous patches are observed mostly at the top.
Fig. 1. HREM images of BN nanotube and BN nanocapsules produced
using powder with ratios of (a) La:B ¼ 1:6, (b) La:B ¼ 1:7, and (c)
Pd:B ¼ 1:4.
A HREM image of BN nanocage produced from LaB /B
6
powder is shown in Fig. 1(b), which indicates square-like
shape, and four-membered rings of BN exist at the corner of
the cage. The BN nanocage has a network-like structure,
whose atomic arrangement is basically consistent with the
B N cluster structure [4,5]. BN nanocapsules with Pd
indicates the reversibility of hydrogen adsorption. It also
suggests that the hydrogen atoms would be physically
absorbed. For the samples of La:B ¼ 1:6 and Pd:B ¼ 1:4,
weight increases of 3.2 and 1.6% were observed, respect-
ively, as listed in Table 1.
3
6 36
nanoparticles were also produced as shown in Fig. 1(c), and
Pd nanoparticles were covered by a few BN layers. In order
to confirm the formation of BN nanocapsules, EDX analysis
was carried out, which showed the atomic ratio of B:N , 1.
DTA and TG curve of BN nanomaterials produced from
BN nanostructures would have energy barriers for H
molecules to pass through tetragonal and hexagonal rings.
Fig. 3 are structure models in which H molecule passes
from hexagonal rings of B99 36. Single point
2
2
N
99 and B36N
LaB powder is shown in Fig. 2. At a temperature around
6
energies were calculated with changing set point of H2
molecule from center of the cage at intervals of 0.1 nm. DE
is regarded as energy barrier that is given for H molecules
2
70 8C, an increase of sample weight of 0.3 mg is observed.
Weight change for this sample was almost reversible, which

T. Oku et al. / Journal of Physics and Chemistry of Solids 65 (2004) 549–552
551
than that of B N hexagonal rings, and the H molecule
3
6
36
2
would pass from hexagonal rings of B N easier than from
6 36
3
hexagonal rings of C . It is known that H molecules are
6
0
2
adsorbed on walls of single-walled carbon nanotubes over
MPa as an experimental result [21]. As a result of
comparison of DE; H molecules enter into B N from
7
2
36 36
hexagonal rings easier than tetragonal rings B N and
6 36
3
hexagonal rings of C .
6
0
A formation mechanism of BN nanotubes and
nanocapsules synthesized in the present work is described
below. Metal and boron particles are melted by arc-
melting, and during the solidification of the liquid into
metal and/or boride nanoparticles, excess boron would
react with nitrogen to form BN layers at the surface of
the nanoparticles. Because of electrical insulation, BN
fullerene materials are usually fabricated by arc-discharge
method with specific conducting electrodes such as HfB2
and ZrB2 [7]. The present arc-melting method from
mixed powder has two advantages for BN nanomaterial
production. Since the powder becomes conducting by
pressing, special electrodes are not needed. In addition,
ordinary, commercial arc-melting furnaces can be used.
These advantages indicate a simpler fabrication method
compared to the ordinary arc-discharge methods [4,22].
Although gas storages of hydrogen and argon in carbon
nanotubes have been reported [16,17,23,24], there are few
reports for gas storage in BN fullerene materials [4,25] and
for calculations [13,14]. Weight increase of the sample in
TG measurements was observed as shown in Fig. 2. It might
be due to the hydrogen gas storage in the BN nanomaterials.
Fig. 2. DTA and TG curve of BN nanocapsules and nanotubes produced
6
using LaB powder.
Table 1
Atomic ratio of starting materials, produced structures and hydrogen
storage
M:B
Nanostructures
Weight change (wt%)
La:B ¼ 1:6
La:B ¼ 1:10
Pd:B ¼ 1:4
Nanotubes and nanocages
Nanotubes
þ3.2
þ0.58
þ1.6
Nanocapsules
to pass through hexagonal rings. The DE of B N
6 36
3
hexagonal rings showed the smallest value of 14 eV in the
present calculation, which is smaller than that of 27 eV at
tetragonal rings. The DE of C60 hexagonal rings was also
calculated to be 16 eV for comparison. This value is higher
2
Fig. 3. Structure models that H molecule passes through hexagonal rings of (a) (b) B99N99 nanotube and (c) B36N36 cluster.

552
T. Oku et al. / Journal of Physics and Chemistry of Solids 65 (2004) 549–552
Since there would be metal and boron nanoparticles in the
separated BN nanomaterials even after the separation,
further qualification and evaluation of the samples are
needed for hydrogen storage.
[7] A. Loiseau, F. Willaime, N. Demoncy, G. Hug, H. Pascard, Boron
nitride nanotubes with reduced numbers of layers synthesized by arc
discharge, Phys. Rev. Lett. 76 (1996) 4737–4740.
[
8] D. Golberg, F.-F. Xu, Y. Bando, Filling boron nitride nanotubes with
metals, Appl. Phys. A 76 (2003) 479–485.
Carbon fullerenes and boron nitride fullerenes are
sublimed at 600 and 1000 8C, respectively. Boron nitiride
fulerenes would storage H molecules with smaller energy
[
9] T. Oku, T. Hirano, M. Kuno, T. Kusunose, K. Niihara, K. Suganuma,
Synthesis, atomic structures and properties of carbon and boron
nitride fullerene materials, Mater, Sci. Engng B74 (2000) 206–217.
2
[
[
10] T. Oku, Synthesis and atomic structures of boron nitride nanotubes,
Physica, B 323 (2002) 357–359.
than carbon fullerenes [13,14], and would give good
stability at high temperature. Boron nitride fullerene
materials would be better candidate for H2 storage
materials.
11] I. Narita, T. Oku, Effects of catalytic metals for synthesis of BN
fullerene nanomaterials, Diamond Related Mater. 12 (2003)
1
146–1150.
12] T. Oku, K. Suganuma, High-resolution electron microscopy and
structural optimization of C36, B36 36 and Fe@B36 36 clusters,
Diamond Related Mater. 10 (2001) 1205–1209.
13] T. Oku, I. Narita, Calculation of H gas storage for boron nitride and
carbon nanotubes studied from the cluster calculation, Physica, B 323
2002) 216–218.
14] I. Narita, T. Oku, Molecular dynamics calculation of H
[
[
[
[
[
N
N
4
. Conclusions
2
HREM observation showed the formation of BN
nanotubes, nanocapsules and nanocages, which were
(
2
gas storage in
synthesized from mixtures of LaB , Pd, and boron powder
6
C60 and B36N36 clusters, Diamond Related Mater. 11 (2002)
945–948.
by using an arc melting method. Although samples
produced with Pd include only BN nanocapsule structures,
samples produced with LaB6 present BN nanocapsule,
nanotube and nanocage structures. After separation of BN
nanomaterials using ethanol, hydrogen storage was
measured by TG/DTA, and the BN nanomaterials produced
15] Y. Saito, K. Kawabata, M. Okuda, Single-layered carbon nanotubes
synthesized by catalytic assistance of rare-earths in a carbon arc,
J. Phys. Chem. 99 (1995) 16076–16079.
16] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune,
M.J. Heben, Storage of hydrogen in single-walled carbon nanotubes,
Nature 386 (1997) 377–379.
from LaB and Pd/boron powder showed possibility of
6
[17] G.E. Gadd, M. Blackford, S. Moricca, N. Webb, P.J. Evans, A.M.
Smith, G. Jacobesen, S. Leung, A. Day, Q. Hua, The world’s smallest
gas cylinders?, Science 277 (1997) 933–936.
hydrogen storage of ,3 wt%. Theoretical calculation also
showed H gas storage in B N cluster. The present work
indicates that BN fullerene materials could be a one of the
possible candidates as hydrogen gas storage materials.
2
36 36
[
[
[
18] T.W. Ebbesen, P.M. Ajayan, H. Hiura, K. Tanigaki, Purification of
nanotubes, Nature 367 (1994) 519.
19] T. Oku, Three-dimensional imaging of YB56 by high-resolution
electron microscopy, Chem. Commun. (2002) 302–303.
20] T. Oku, S. Nakajima, Atomic structures of surface and interface in
(
Hg, Tl, Pb)-based superconducting oxides studied by high-resolution
References
electron microscopy, Solid Stat. Commun. 124 (2002) 305–309.
21] Y. Ye, C.C. Ahn, C. Witham, B. Fultz, J. Liu, A.G. Rinzler, D.
Colbert, K.A. Smith, R.E. Smalley, Hydrogen adsorption and
cohesive energy of single-walled carbon nanotubes, Appl. Phys.
Lett. 74 (1999) 2307–2309.
[
[
[
1] S. Iijima, Helical microtubles of graphitic carbon, Nature 354 (1991)
6–58.
5
2] Y. Saito, T. Yoshikawa, M. Inagaki, M. Tomita, T. Hayashi, Growth
and structure of graphitic tubules and polyhedral particles in arc-
discharge, Chem. Phys. Lett. 204 (1993) 277–282.
[
22] I. Narita, T. Oku, Synthesis of boron nitride nanotubes by using YB
powder, Solid State Commun. 122 (2002) 465–468.
6
[
[
3] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, C60:
Backminsterfullerene, Nature 318 (1985) 162–163.
[
23] F. Lamari Darkrim, P. Malbrunot, G.P. Tartaglia, Review of hydrogen
storage by adsorption in carbon nanotubes, Int. J. Hydrogen Energy 27
4] T. Oku, M. Kuno, H. Kitahara, I. Narita, Formation, atomic structures
and properties of boron nitride and carbon nanocage fullerene
materials, Int. J. Inorg. Mater. 3 (2001) 597–612.
(
2002) 193–202.
[
24] M. Hirscher, M. Becher, M. Haluska, A. Quintel, V. Skakalova, Y.-M.
Choi, U. Dettlaff-Weglikowska, S. Roth, I. Stepanek, P. Bernier, A.
Leonhardt, J. Fink, Hydrogen storage in carbon nanostructures,
J. Alloys Compd 330-332 (2002) 654–658.
[5] T. Oku, M. Kuno, I. Narita, High-resolution electron microscopy and
electronic structures of endohedral La@B36N36 clusters, Diamond
Related Mater. 11 (2002) 940–944.
[
6] N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G.
Louie, A. Zettl, Boron nitride nanotubes, Science 269 (1995)
[25] T. Oku, M. Kuno, Synthesis, argon/hydrogen storage and magnetic
properties of boron nitride nanotubes and nanocapsules, Diamond
Related Mater. 12 (2003) 840–845.
966–967.