- Email: [email protected]
hydrogen storage and magnetic properties of boron nitride nanotubes and nanocapsules
Diamond and Related Materials 12 (2003) 840–845
Synthesis, argonyhydrogen storage and magnetic properties of boron nitride nanotubes and nanocapsules Takeo Oku*, Masaki Kuno Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan
Abstract Boron nitride (BN) fullerene materials such as nanotubes, nanocapsules and nanocages were synthesized from LaB6, Co, Pd, Ti, Ni and Cu with boron powder by using an arc-melting method. For the BN nanocapsules with Co and CoOx nanoparticles, argon element was detected by energy dispersive X-ray spectroscopy, and the nanocapsules showed superparamagnetic property. Thermogravimetryydifferential thermogravimetric analysis of BN nanomaterials produced from LaB6 and Pdyboron powder showed possibility of hydrogen storage of ;3 wt.%. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Boron nitride; Nanotube; Hydrogen; Argon
1. Introduction Much work has been reported on carbon nanotubes w1x and nanocapsules w2x since the discovery of fullerenes w3x. 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 w4x. 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 w4x. Most of the studies on the BN fullerene materials were focused on the formation of BN nanotubes w4–10x. We have focused on the BN nanocapsules, nanotubes, nanocages, nanoparticles and clusters to discover new properties and structures by changing the encapsulated materials w4,8–12x, and on the possibility of gas storage in BN nanocapsules w4,13,14x. Since the BN nanocapsules consist of light elements, it is believed that they can store much gas per weight w4x. The present work has two aims. The first is to prepare BN nanotubes, nanocapsules and nanocages by arcmelting boron and metal powder compact in nitrogen and argon gas atmosphere. LaB6, Co, Pd, Ti, Ni and Cu were selected in order to take advantage of their catalytic *Corresponding author. Tel.: q81-6-6879-8521; fax: q81-6-68798522. E-mail address: [email protected] (T. Oku).
effect to produce the BN fullerene materials. La has shown excellent catalytic properties for producing a large number of single-walled carbon nanotubes and enlarging their diameter w15x. Co and Ni with ferromagnetism are expected as magnetic materials, and Pd is also expected to act as a hydrogen storage material. Secondly, we aim at investigating gas storage (Ar and H2) magnetic property of BN nanomaterials. Although gas storage of hydrogen and argon in carbon nanotubes has been reported w16,17x, carbon nanotubes are oxidized at 600 8C in air w18x. On the other hand, BN are stable up to 1000 8C in air w4x, 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) and electron dispersive X-ray spectroscopy (EDX) were carried out, which are very powerful methods for atomic structure analysis w19–21x and would be useful for life information science w22,23x. For some of BN nanomaterials, hydrogen gas storage and magnetization measurements were carried out using thermogravimetryy differential thermogravimetric analysis (TGyDTA) and a superconducting quantum interference device (SQUID) magnetometer. These studies provide a guideline for the formation and property of BN nanomaterials. 2. Experimental procedures Mixture powder compacts, as listed in Table 1, with the size of 3 mm in height and 30 mm in diameter were
0925-9635/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 3 2 6 - 6
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845 Table 1 Starting powder materials for BN fullerene materials produced in the present work Elements
Purity (%)
Particle size
Production company
B LaB6 Co Pd Ni Ti Cu
99 99 99 99.5 99 99.9 99.5
40 mm 1 mm 20 nm 0.1 mm 3 mm 325 mesh 0.05 mm
Niraco Co. Ltd Wako Co. Ltd ULVAC Co. Ltd Niraco Co. Ltd Koujyundokagaku Co. Ltd Niraco Co. Ltd Niraco Co. Ltd
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 2.0=10y3 Pa. After introducing a mixed gas of Ar 0.025 MPa and N2 0.025 MPa, arc-melting was applied to 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 (NEVAD03, Nissin Engineering Co. Ltd), and the whiteygray 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
841
Table 2 Atomic ratio and BN fullerene materials produced in the present work M:B
Nanocapsules
Nanotubes
Nanocages
La:Bs1:6 La:Bs1:7 La:Bs1:10 Co:Bs1:1 Pd:Bs1:4 Pd:Bs1:10 Ti:Bs1:4 Ti:Bs1:10 Cu:Bs1:4 Cu:Bs1:10 Ni:Bs1:10
Observed Rich Rich Rich Rich Rich Not observed Observed Observed Observed Rich
Observed Observed Not observed Not observed Not observed Not observed Rich Ti nanowires Rich Not observed Not observed Not observed
Not observed Not observed Observed Not observed Not observed Observed Not observed Observed Observed Not observed Not observed
microscope (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: vsd 2(syr)gy18h (hsvt; 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
Fig. 1. HREM image of BN nanotubes produced using powder with ratios of (a) La:Bs1:10; (b) La:Bs1:6 and (c) Ti:Bs1:10.
842
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845
Fig. 2. HREM images of BN nanocapsules produced using powder with ratios of (a); (b) La:Bs1:7; (c)–(e) Co:Bs1:10 and (f) Pd:Bs1:4. The corners of square BN cages are indicated by arrows.
the size and density of the produced BN nanocapsulesy 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 TGyDTA at temperatures in the range of 20–300 8C in H2 atmosphere. In addition, the magnetic properties of BN nanomaterials produced from CoyB were measured under zero-field cooling and field cooling (10 kG) by a SQUID magnetometer at temperatures in the range of 10–300 K. 3. Results and discussion BN fullerene materials produced in the present work are summarized as listed in Table 2. BN nanocapsules
were formed for most of the samples, and BN nanotubes were observed only in the samples with La and Ti. BN nanocages were observed in the samples with La, Ti and Cu. HREM images of BN nanotubes produced using LaB6 and Ti powder are shown in Fig. 1. In Fig. 1a, the diameter of the five-layered BN nanotube is changing from left to right, and amorphous patches are observed mostly at the right side. BN nanotubes produced from TiyB powder, as shown in Fig. 1c, have a bundled type structure, which would be due to the effect of Ti and are expected to act well as a gas storage material because of the space between the nanotubes. HREM images of BN nanocapsules produced from LaB6, Co and Pd powder are shown in Fig. 2. In Fig.
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845
843
Fig. 3. HREM images of BN naoncages produced using powder with ratios of (a) La:Bs1:10; (b) Ti:Bs1:10 and (c) Cu:Bs1:4. (d) Atomic structure model of B36N36 clusters. Four-membered rings of BN are indicated by star marks.
2a and b, the BN nanocapsules have square-like shape, with their corners indicated by arrows. A LaB6 nanoparticle was observed inside the BN layers in Fig. 2b. Co and CoOx nanoparticles were encapsulated in BN layers, as shown in Fig. 2c–e. BN nanocapsules with Pd nanoparticles were also produced, and Pd nanoparticles were covered by a few BN layers, as shown in Fig. 2f. HREM images of BN nanocages produced from LaB6, Ti and Cu are shown in Fig. 3. In Fig. 3a, 4membered rings of BN are indicated by arrows. In Fig. 3b and c, the BN nanocages have a network-like structure. Atomic structure model of B36N6 clusters w4x is shown in Fig. 3d, whose atomic arrangement is basically consistent with the structure of BN nanocapsules and nanocages in Fig. 2a, b and Fig. 3a. In order to confirm the formation of BN nanocapsules, EDX analysis was carried out. Fig. 4a is an EDX spectrum of BN nanocapsules with CoOx nanoparticles. In Fig. 4a, strong peaks of boron, nitrogen, oxygen, and cobalt are observed, which indicates the formation of BN nanocapsules with cobalt oxide nanoparticles. A weak peak of argon is also observed in Fig. 4a, as
indicated by an arrow. DTA and TG curve of BN nanomaterials produced from LaB6 powder is shown in Fig. 4b. At a temperature approximately 70 8C, an increase of sample weight of 0.3 mg is observed. For the samples of La:Bs1:6 and Pd:Bs1:4, weight increases of 3.2 and 1.6% were observed, respectively. Magnetization–temperature curves and magnetization–magnetic field curves of BN nanocapsules with Co and CoOx nanoparticles are shown in Fig. 5. A slight shoulder is observed approximately 60 K in Fig. 5a, and magnetization–magnetic field curves were measured at 25 and 100 K, and they showed similar behavior. A formation mechanism of BN nanotubes and nanocapsules synthesized in the present work is reported below. Metal and boron particles are melted by arcmelting, and during the solidification of the liquid into metal andyor boride nanoparticles, excess boron would react with nitrogen to form BN layers at the surface of the nanoparticles. Because of insulation, BN fullerene materials are usually fabricated by arc-discharge method with specific conducting electrodes such as HfB2 and ZrB2 w6x. The present arc-melting method from mixed powder has two advantages for BN nanomaterial pro-
844
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845
Fig. 4. (a) EDX spectrum of BN nanocapsules with Co and CoOx nanoparticles. (b) DTA and TG curve of BN nanocapsules and nanotubes produced using LaB6 powder.
duction. 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 w4x. Although gas storages of hydrogen and argon in carbon nanotubes have been reported w16,17,24,25x, there are few reports for gas storage in BN fullerene materials w4,26x and for calculation w13,14x. In Fig. 4a, the EDX spectrum taken from several BN nanocapsules shows an argon peak which indicates that argon atoms are encapsulated in BN nanocapsules. Since no change of BN lattice was observed, it is considered that Ar gas would be distributed not in the BN layers but around the interface between cobalt oxide and BN sheets during the BN sheet growth. Weight increase of the sample in TG measurements was observed as shown in Fig. 4b, which might be due to the hydrogen gas storage in the BN nanomaterials. 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. Size of the BN nanocapsules with CoOx nanoparticles and encapsulated nanocapsules are in the range of 10– 30 and 5–25 nm, respectively. The shape of the most BN nanocapsules is sphere, which would be due to the fact that BN layers grow around the solidified cobalt oxide and lattice defects are introduced. Co nanoparticles are easy to agglomerate, and they would show ferromagnetic properties. However, the magnetization–magnetic field curves at 25 and 100 K behave as superparamagnetics, which would be due to BN-layerseparation of Co and CoOx nanoparticles. This kind of BN nanocapsules is expected to be stable as magnetic refrigeration materials w4,27x. 4. Conclusions HREM observation and EDX analysis show the formation of BN fullerene materials such as nanotubes,
Fig. 5. (a) Magnetization–temperature curves and (b) magnetization–magnetic field curves of BN nanocapsules with Co and CoOx nanoparticles.
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845
nanocapsules and nanocages, which were synthesized from mixtures of LaB6, Co, Pd, Ti, Ni and Cu with boron powder by using an arc-melting method. Although samples produced with Co, Pd and Ni include only BN nanocapsule structures, samples produced with LaB6 present BN nanocapsule, nanotube and nanocage structures. BN nanotubes produced from TiyB powder have a bundled type structure. For the BN nanocapsules with CoOx nanoparticles, argon element was detected by EDX, and the nanocapsules showed superparamagnetic property. After separation of BN nanomaterials using ethanol, hydrogen storage was measured by TGyDTA at temperatures in the range of 20–300 8C, and the BN nanomaterials produced from LaB6 and Pdyboron powder showed possibility of hydrogen storage of ;3 wt.%. The present work indicates that BN fullerene materials could be one of the possible candidates as hydrogen and argon gas storage materials. Acknowledgments The authors would like to acknowledge Prof. K. Suganuma, Dr H. Tanaka and Dr M. Inoue for useful discussion and experimental help. This work is partly supported by Grant-in-Aid for Scientific Research, Ministry of Education, Science, Sports and Culture, Japan. References w1x S. Iijima, Nature 354 (1991) 56. w2x Y. Saito, T. Yoshikawa, M. Inagaki, M. Tomita, T. Hayashi, Chem. Phys. Lett. 204 (1993) 277. w3x H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature 318 (1985) 162.
845
w4x T. Oku, M. Kuno, H. Kitahara, I. Narita, Int. J. Inorg. Mater. 3 (2001) 597. w5x N.G. Chopra, R.J. Luyken, K. Cherrey, et al., Science 269 (1995) 966. w6x A. Loiseau, F. Willaime, N. Demoncy, G. Hug, H. Pascard, Phys. Rev. Lett. 76 (1996) 4737. w7x D. Golberg, Y. Bando, K. Kurashima, T. Sasaki, Appl. Phys. Lett. 73 (1998) 2441. w8x T. Oku, T. Hirano, M. Kuno, T. Kusunose, K. Niihara, K. Suganuma, Mater. Sci. Eng., Part B 74 (2000) 206. w9x T. Oku, Physica B 323 (2002) 357. w10x I. Narita, T. Oku, Solid State Comm. 122 (2002) 465. w11x T. Oku, M. Kuno, I. Narita, Diamond Relat. Mater. 11 (2002) 940. w12x T. Oku, K. Hiraga, Diamond Relat. Mater. 10 (2001) 1398. w13x T. Oku, I. Narita, Physica B 323 (2002) 216. w14x I. Narita, T. Oku, Diamond Relat. Mater. 11 (2002) 945. w15x Y. Saito, K. Kawabata, M. Okuda, J. Phys. Chem. 99 (1995) 16076. w16x A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. w17x G.E. Gadd, M. Blackford, S. Moricca, et al., Science 277 (1997) 933. w18x T.W. Ebbesen, P.M. Ajayan, H. Hiura, K. Tanigaki, Nature 367 (1994) 519. w19x T. Oku, J. Ceram. Soc. Jpn. 109 (2001) S17. w20x T. Oku, Chem. Comm. (2002) 302. w21x T. Oku, R. Hatakeyama, T. Hirata, N. Sato, Physica B 323 (2002) 284. w22x T. Oku, H. Indo, J. Int. Soc. Life Inf. Sci. 20 (2002) 642. w23x T. Oku, E. Watanabe, S. Fukuda, T. Shirakawa, J. Int. Soc. Life Inf. Sci. 20 (2002) 616. w24x F. Lamari Darkrim, P. Malbrunot, G.P. Tartaglia, Int. J. Hydrogen Energy 27 (2002) 193. w25x M. Hirscher, M. Becher, M. Haluska, et al., J. Alloys Compounds 330–332 (2002) 654. w26x M. Kuno, T. Oku, K. Suganuma, Scripta Mater. 44 (2001) 1583. w27x H. Kitahara, T. Oku, T. Hirano, K. Suganuma, Diamond Relat. Mater. 10 (2001) 1210.
Synthesis, argonyhydrogen storage and magnetic properties of boron nitride nanotubes and nanocapsules Takeo Oku*, Masaki Kuno Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan
Abstract Boron nitride (BN) fullerene materials such as nanotubes, nanocapsules and nanocages were synthesized from LaB6, Co, Pd, Ti, Ni and Cu with boron powder by using an arc-melting method. For the BN nanocapsules with Co and CoOx nanoparticles, argon element was detected by energy dispersive X-ray spectroscopy, and the nanocapsules showed superparamagnetic property. Thermogravimetryydifferential thermogravimetric analysis of BN nanomaterials produced from LaB6 and Pdyboron powder showed possibility of hydrogen storage of ;3 wt.%. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Boron nitride; Nanotube; Hydrogen; Argon
1. Introduction Much work has been reported on carbon nanotubes w1x and nanocapsules w2x since the discovery of fullerenes w3x. 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 w4x. 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 w4x. Most of the studies on the BN fullerene materials were focused on the formation of BN nanotubes w4–10x. We have focused on the BN nanocapsules, nanotubes, nanocages, nanoparticles and clusters to discover new properties and structures by changing the encapsulated materials w4,8–12x, and on the possibility of gas storage in BN nanocapsules w4,13,14x. Since the BN nanocapsules consist of light elements, it is believed that they can store much gas per weight w4x. The present work has two aims. The first is to prepare BN nanotubes, nanocapsules and nanocages by arcmelting boron and metal powder compact in nitrogen and argon gas atmosphere. LaB6, Co, Pd, Ti, Ni and Cu were selected in order to take advantage of their catalytic *Corresponding author. Tel.: q81-6-6879-8521; fax: q81-6-68798522. E-mail address: [email protected] (T. Oku).
effect to produce the BN fullerene materials. La has shown excellent catalytic properties for producing a large number of single-walled carbon nanotubes and enlarging their diameter w15x. Co and Ni with ferromagnetism are expected as magnetic materials, and Pd is also expected to act as a hydrogen storage material. Secondly, we aim at investigating gas storage (Ar and H2) magnetic property of BN nanomaterials. Although gas storage of hydrogen and argon in carbon nanotubes has been reported w16,17x, carbon nanotubes are oxidized at 600 8C in air w18x. On the other hand, BN are stable up to 1000 8C in air w4x, 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) and electron dispersive X-ray spectroscopy (EDX) were carried out, which are very powerful methods for atomic structure analysis w19–21x and would be useful for life information science w22,23x. For some of BN nanomaterials, hydrogen gas storage and magnetization measurements were carried out using thermogravimetryy differential thermogravimetric analysis (TGyDTA) and a superconducting quantum interference device (SQUID) magnetometer. These studies provide a guideline for the formation and property of BN nanomaterials. 2. Experimental procedures Mixture powder compacts, as listed in Table 1, with the size of 3 mm in height and 30 mm in diameter were
0925-9635/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 3 2 6 - 6
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845 Table 1 Starting powder materials for BN fullerene materials produced in the present work Elements
Purity (%)
Particle size
Production company
B LaB6 Co Pd Ni Ti Cu
99 99 99 99.5 99 99.9 99.5
40 mm 1 mm 20 nm 0.1 mm 3 mm 325 mesh 0.05 mm
Niraco Co. Ltd Wako Co. Ltd ULVAC Co. Ltd Niraco Co. Ltd Koujyundokagaku Co. Ltd Niraco Co. Ltd Niraco Co. Ltd
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 2.0=10y3 Pa. After introducing a mixed gas of Ar 0.025 MPa and N2 0.025 MPa, arc-melting was applied to 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 (NEVAD03, Nissin Engineering Co. Ltd), and the whiteygray 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
841
Table 2 Atomic ratio and BN fullerene materials produced in the present work M:B
Nanocapsules
Nanotubes
Nanocages
La:Bs1:6 La:Bs1:7 La:Bs1:10 Co:Bs1:1 Pd:Bs1:4 Pd:Bs1:10 Ti:Bs1:4 Ti:Bs1:10 Cu:Bs1:4 Cu:Bs1:10 Ni:Bs1:10
Observed Rich Rich Rich Rich Rich Not observed Observed Observed Observed Rich
Observed Observed Not observed Not observed Not observed Not observed Rich Ti nanowires Rich Not observed Not observed Not observed
Not observed Not observed Observed Not observed Not observed Observed Not observed Observed Observed Not observed Not observed
microscope (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: vsd 2(syr)gy18h (hsvt; 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
Fig. 1. HREM image of BN nanotubes produced using powder with ratios of (a) La:Bs1:10; (b) La:Bs1:6 and (c) Ti:Bs1:10.
842
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845
Fig. 2. HREM images of BN nanocapsules produced using powder with ratios of (a); (b) La:Bs1:7; (c)–(e) Co:Bs1:10 and (f) Pd:Bs1:4. The corners of square BN cages are indicated by arrows.
the size and density of the produced BN nanocapsulesy 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 TGyDTA at temperatures in the range of 20–300 8C in H2 atmosphere. In addition, the magnetic properties of BN nanomaterials produced from CoyB were measured under zero-field cooling and field cooling (10 kG) by a SQUID magnetometer at temperatures in the range of 10–300 K. 3. Results and discussion BN fullerene materials produced in the present work are summarized as listed in Table 2. BN nanocapsules
were formed for most of the samples, and BN nanotubes were observed only in the samples with La and Ti. BN nanocages were observed in the samples with La, Ti and Cu. HREM images of BN nanotubes produced using LaB6 and Ti powder are shown in Fig. 1. In Fig. 1a, the diameter of the five-layered BN nanotube is changing from left to right, and amorphous patches are observed mostly at the right side. BN nanotubes produced from TiyB powder, as shown in Fig. 1c, have a bundled type structure, which would be due to the effect of Ti and are expected to act well as a gas storage material because of the space between the nanotubes. HREM images of BN nanocapsules produced from LaB6, Co and Pd powder are shown in Fig. 2. In Fig.
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845
843
Fig. 3. HREM images of BN naoncages produced using powder with ratios of (a) La:Bs1:10; (b) Ti:Bs1:10 and (c) Cu:Bs1:4. (d) Atomic structure model of B36N36 clusters. Four-membered rings of BN are indicated by star marks.
2a and b, the BN nanocapsules have square-like shape, with their corners indicated by arrows. A LaB6 nanoparticle was observed inside the BN layers in Fig. 2b. Co and CoOx nanoparticles were encapsulated in BN layers, as shown in Fig. 2c–e. BN nanocapsules with Pd nanoparticles were also produced, and Pd nanoparticles were covered by a few BN layers, as shown in Fig. 2f. HREM images of BN nanocages produced from LaB6, Ti and Cu are shown in Fig. 3. In Fig. 3a, 4membered rings of BN are indicated by arrows. In Fig. 3b and c, the BN nanocages have a network-like structure. Atomic structure model of B36N6 clusters w4x is shown in Fig. 3d, whose atomic arrangement is basically consistent with the structure of BN nanocapsules and nanocages in Fig. 2a, b and Fig. 3a. In order to confirm the formation of BN nanocapsules, EDX analysis was carried out. Fig. 4a is an EDX spectrum of BN nanocapsules with CoOx nanoparticles. In Fig. 4a, strong peaks of boron, nitrogen, oxygen, and cobalt are observed, which indicates the formation of BN nanocapsules with cobalt oxide nanoparticles. A weak peak of argon is also observed in Fig. 4a, as
indicated by an arrow. DTA and TG curve of BN nanomaterials produced from LaB6 powder is shown in Fig. 4b. At a temperature approximately 70 8C, an increase of sample weight of 0.3 mg is observed. For the samples of La:Bs1:6 and Pd:Bs1:4, weight increases of 3.2 and 1.6% were observed, respectively. Magnetization–temperature curves and magnetization–magnetic field curves of BN nanocapsules with Co and CoOx nanoparticles are shown in Fig. 5. A slight shoulder is observed approximately 60 K in Fig. 5a, and magnetization–magnetic field curves were measured at 25 and 100 K, and they showed similar behavior. A formation mechanism of BN nanotubes and nanocapsules synthesized in the present work is reported below. Metal and boron particles are melted by arcmelting, and during the solidification of the liquid into metal andyor boride nanoparticles, excess boron would react with nitrogen to form BN layers at the surface of the nanoparticles. Because of insulation, BN fullerene materials are usually fabricated by arc-discharge method with specific conducting electrodes such as HfB2 and ZrB2 w6x. The present arc-melting method from mixed powder has two advantages for BN nanomaterial pro-
844
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845
Fig. 4. (a) EDX spectrum of BN nanocapsules with Co and CoOx nanoparticles. (b) DTA and TG curve of BN nanocapsules and nanotubes produced using LaB6 powder.
duction. 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 w4x. Although gas storages of hydrogen and argon in carbon nanotubes have been reported w16,17,24,25x, there are few reports for gas storage in BN fullerene materials w4,26x and for calculation w13,14x. In Fig. 4a, the EDX spectrum taken from several BN nanocapsules shows an argon peak which indicates that argon atoms are encapsulated in BN nanocapsules. Since no change of BN lattice was observed, it is considered that Ar gas would be distributed not in the BN layers but around the interface between cobalt oxide and BN sheets during the BN sheet growth. Weight increase of the sample in TG measurements was observed as shown in Fig. 4b, which might be due to the hydrogen gas storage in the BN nanomaterials. 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. Size of the BN nanocapsules with CoOx nanoparticles and encapsulated nanocapsules are in the range of 10– 30 and 5–25 nm, respectively. The shape of the most BN nanocapsules is sphere, which would be due to the fact that BN layers grow around the solidified cobalt oxide and lattice defects are introduced. Co nanoparticles are easy to agglomerate, and they would show ferromagnetic properties. However, the magnetization–magnetic field curves at 25 and 100 K behave as superparamagnetics, which would be due to BN-layerseparation of Co and CoOx nanoparticles. This kind of BN nanocapsules is expected to be stable as magnetic refrigeration materials w4,27x. 4. Conclusions HREM observation and EDX analysis show the formation of BN fullerene materials such as nanotubes,
Fig. 5. (a) Magnetization–temperature curves and (b) magnetization–magnetic field curves of BN nanocapsules with Co and CoOx nanoparticles.
T. Oku, M. Kuno / Diamond and Related Materials 12 (2003) 840–845
nanocapsules and nanocages, which were synthesized from mixtures of LaB6, Co, Pd, Ti, Ni and Cu with boron powder by using an arc-melting method. Although samples produced with Co, Pd and Ni include only BN nanocapsule structures, samples produced with LaB6 present BN nanocapsule, nanotube and nanocage structures. BN nanotubes produced from TiyB powder have a bundled type structure. For the BN nanocapsules with CoOx nanoparticles, argon element was detected by EDX, and the nanocapsules showed superparamagnetic property. After separation of BN nanomaterials using ethanol, hydrogen storage was measured by TGyDTA at temperatures in the range of 20–300 8C, and the BN nanomaterials produced from LaB6 and Pdyboron powder showed possibility of hydrogen storage of ;3 wt.%. The present work indicates that BN fullerene materials could be one of the possible candidates as hydrogen and argon gas storage materials. Acknowledgments The authors would like to acknowledge Prof. K. Suganuma, Dr H. Tanaka and Dr M. Inoue for useful discussion and experimental help. This work is partly supported by Grant-in-Aid for Scientific Research, Ministry of Education, Science, Sports and Culture, Japan. References w1x S. Iijima, Nature 354 (1991) 56. w2x Y. Saito, T. Yoshikawa, M. Inagaki, M. Tomita, T. Hayashi, Chem. Phys. Lett. 204 (1993) 277. w3x H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature 318 (1985) 162.
845
w4x T. Oku, M. Kuno, H. Kitahara, I. Narita, Int. J. Inorg. Mater. 3 (2001) 597. w5x N.G. Chopra, R.J. Luyken, K. Cherrey, et al., Science 269 (1995) 966. w6x A. Loiseau, F. Willaime, N. Demoncy, G. Hug, H. Pascard, Phys. Rev. Lett. 76 (1996) 4737. w7x D. Golberg, Y. Bando, K. Kurashima, T. Sasaki, Appl. Phys. Lett. 73 (1998) 2441. w8x T. Oku, T. Hirano, M. Kuno, T. Kusunose, K. Niihara, K. Suganuma, Mater. Sci. Eng., Part B 74 (2000) 206. w9x T. Oku, Physica B 323 (2002) 357. w10x I. Narita, T. Oku, Solid State Comm. 122 (2002) 465. w11x T. Oku, M. Kuno, I. Narita, Diamond Relat. Mater. 11 (2002) 940. w12x T. Oku, K. Hiraga, Diamond Relat. Mater. 10 (2001) 1398. w13x T. Oku, I. Narita, Physica B 323 (2002) 216. w14x I. Narita, T. Oku, Diamond Relat. Mater. 11 (2002) 945. w15x Y. Saito, K. Kawabata, M. Okuda, J. Phys. Chem. 99 (1995) 16076. w16x A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. w17x G.E. Gadd, M. Blackford, S. Moricca, et al., Science 277 (1997) 933. w18x T.W. Ebbesen, P.M. Ajayan, H. Hiura, K. Tanigaki, Nature 367 (1994) 519. w19x T. Oku, J. Ceram. Soc. Jpn. 109 (2001) S17. w20x T. Oku, Chem. Comm. (2002) 302. w21x T. Oku, R. Hatakeyama, T. Hirata, N. Sato, Physica B 323 (2002) 284. w22x T. Oku, H. Indo, J. Int. Soc. Life Inf. Sci. 20 (2002) 642. w23x T. Oku, E. Watanabe, S. Fukuda, T. Shirakawa, J. Int. Soc. Life Inf. Sci. 20 (2002) 616. w24x F. Lamari Darkrim, P. Malbrunot, G.P. Tartaglia, Int. J. Hydrogen Energy 27 (2002) 193. w25x M. Hirscher, M. Becher, M. Haluska, et al., J. Alloys Compounds 330–332 (2002) 654. w26x M. Kuno, T. Oku, K. Suganuma, Scripta Mater. 44 (2001) 1583. w27x H. Kitahara, T. Oku, T. Hirano, K. Suganuma, Diamond Relat. Mater. 10 (2001) 1210.