Formation and Atomic Structures of Boron Nitride Nanotubes with Cup

Materials Transactions, Vol. 48, No. 4 (2007) pp. 722 to 729 Special Issue on ACCMS Working Group Meeting on Clusters and Nanomaterials #2007 Society of Nano Science and Technology

Formation and Atomic Structures of Boron Nitride Nanotubes with Cup-Stacked and Fe Nanowire Encapsulated Structures Takeo Oku1; * , Naruhiro Koi1 , Ichihito Narita1 , Katsuaki Suganuma1 and Masahiko Nishijima2 1 2

Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki 567-0047, Japan Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Boron nitride (BN) nanotubes, nanohorns, nanocoils were synthesized by annealing Fe4 N and B powders at 1000 C in nitrogen gas atmosphere. Especially, Fe-filled BN nanotubes, bamboo-type and cup-stacked type BN nanotubes were produced. Formation mechanism and nanostructures were investigated by high-resolution electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, electron diffraction, energy dispersive X-ray spectroscopy and molecular mechanics calculations. [doi:10.2320/matertrans.48.722] (Received December 10, 2006; Accepted January 11, 2007; Published March 25, 2007) Keywords: nanotube, boron nitiride, high-resolution electron microscopy, Fe, nanowire

1.

Introduction

Carbon-based nanocage structures, such as fullerene clusters, nanotubes, nanohorns, nanocapsules and onions, have great potential for studying materials of low dimensions in an isolated environment.1–5) In addition to the carbon nanomaterials, boron nitride (BN) nanomaterials 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.3–6) Recently, several studies have been reported on BN nanomaterials such as BN nanotubes,6–9) BN nanocapsules,4,10) BN nanoparticles11,12) and BN nanohorns.13–15) Although BN nanohorns as a novel type of BN nanotubes were also reported by HREM image, there are still few reports on atomic structures and stability of BN nanotubes with a cup-stacked structure.9,13) Several studies have been reported on metal-filled BN nanomaterials. Nanowires constructed from magnetic materials, especially Fe, Co and some Fe-based alloys are of interest, because they are likely to be used in nanoelectronics devices, magnetic recording media and biological sensors. BN nanocables are of potential use for nanoscale electronic devices and nanostructured ceramic materials because of providing good stability at high temperatures with high electronic insulation in air. Therefore, metalfilled BN nanomaterials would have significant advantages for technological application. However, the oxidation- and corrosion resistances of surface are weak point of the metallic nanowires. Although it is reported that Fe-filled BN nanotube could be achieved,16) they still have some problems such as little production and low yield because it is difficult to exist in directly fabricating BN nanocable with metal cores to the poor wetting property of BN to metal. The purpose of the present work is twofold. The first is to synthesize BN nanotubes with cup-stacked and metal-filled structures. The second is to investigate the formation mechanism and atomic structures by high-resolution electron microscopy (HREM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), electron *Corresponding

author, E-mail: [email protected]

diffraction, energy dispersive X-ray spectroscopy (EDX) and molecular mechanics calculations. HREM is a powerful method for direct structure analysis on an atomic scale.17,18) It is also possible to use HAADF-STEM to detect single heavy atoms on alight support. Scattering is caused by the nucleus and follows roughly a Z2 dependence. Formation mechanism of Fe-filled BN nanotube and cup-stacked structures were proposed based on these results. The present study will give us a guideline for designing and synthesis of the BN nanotubes with various structures, which are expected as future nanoscale devices. 2.

Experimental Procedures

Fe4 N (99%, Kojundo Chemical Laboratory (KCL) Co. Ltd., Saitama, Japan) and boron (B) powders (99%, KCL) were used as raw materials. Their particle sizes were about 50 and 45 mm, respectively. After the Fe4 N and B (weight ratio WR = 1:1) were mixed by a triturator, the samples were set on an alumina boat and annealed in the furnace. The furnace was programmed to heat at 6 C/min from ambient to 1000 C and hold for 1–5 h and then cooled at 3 C/min to ambient temperature. Nitrogen pressure was 0.10 MPa, and its gas flow was 100 sccm. As-produced soot synthesized via the above method was centrifuged at 8000 rpm for 2 min, and supernatant liquid is removed.19) The remaining sediments were collected and observed. To observe the morphology of the samples before/after centrifugation, HREM and HAADF-STEM with a 300 kV electron microscope (JEM-3000F) were performed. Electron diffraction and EDX analysis which using the EDAX system were performed. For image processing of the observed HREM images, Digital Micrograph software (Gatan, Inc., California) was used. The digital images were masked and fast Fourier transformed. Basic structure models for BN nanotubes with a cupstacked structure were constructed by CS Chem3D (CambridgeSoft). For stability calculations, structural optimization of the BN nanotubes with a cup-stacked structure was performed by molecular mechanics calculations (MM2). To compare observed images with calculated ones, HREM

Formation and Atomic Structures of Boron Nitride Nanotubes with Cup-Stacked and Fe Nanowire Encapsulated Structures

a

723

b

200nm Fe

Fe

c

500nm

d

Fe

50nm

50nm

f

e

100nm

100nm

Fig. 1 (a) TEM image of the BN nanotubes and nanohorn. (b) BN nanotube with Fe nanoparticle. (c) Enlarged image of cap of (b). (d) BN nanocoil. (e) TEM and (b) HAADF images of bamboo-type nanotubes.

images were calculated by the multi-slice method using the MacTempas software (Total Resolution, CA, USA). The parameters used in the calculations are as follows: accelerating voltages = 300 kV, radius of the objective apertures = 5.9 nm1 , spherical aberration Cs = 0.6 mm, spread of focus

¼ 8 nm, semi-angle of divergence ¼ 0:55 mrad, under defocus values f ¼ 20 to 90 nm, unit cell (one cluster) = 5:0 5:0 7:0 nm3 , crystal thickness (unit-cell thickness, sixteen slices) t = 5.0 nm, space group P1 and assumed temperature factors of 0.02 nm2 .

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3.

T. Oku, N. Koi, I. Narita, K. Suganuma and M. Nishijima

Results and Discussion

Phases of the samples were determined by X-ray diffraction, which showed peaks of hexagonal BN and -Fe. Large amounts of BN nanotubes were produced, and Fig. 1(a) is a typical transmission electron microscope (TEM) image of the samples (WR = 9:1). BN nanohorn and nanotubes are observed, and lengths and widths of BN nanotubes were approximately 1–10 mm and 40–200 nm, respectively. A Fe nanoparticle is observed at the root area of a BN nanohorn. A nanotube shown by an arrow is a Fe-filled BN nanotube. Figure 1(b) is a TEM image of BN nanotube with a Fe nanoparticle, and the length is more than 2 mm. Figure 1(c) is a high magnification image of Fig. 1(b), and the BN nanotube has a bamboo-type structure, as indicated by an arrow. BN nanocoil was also produced, as shown Fig. 1(d), and a Fe nanoparticle is observed as indicated by an arrow. In the case of using magnetic materials as the catalysis metal for BN nanotubes, the magnetic nanoparticles move or rotate with the change of magnetic field, which arises from a coil heater, in the process of reaction. Therefore, it is considered that BN nanocoils were produced. High WR of Fe4 N would be suitable for synthesis of BN nanocoils because the frequency of moving is high with increasing of the amount of magnetic nanoparticles. Figures 1(e) and 1(f) are TEM and HAADF images of bamboo-type nanotubes, and bamboo structures are observed in both images. Aspect ratio of the bamboo structure is about 1, which is smaller than that in Fig. 1(c). This kind of structure would be useful for an isolationstructure of nanoparticles. Fe nanoparticles were often observed at the tip of nanotubes (WR = 1:1), as shown in Fig. 2(a). Enlarged images of a tip and an interface between the Fe nanoparticle and the nanotube are shown in Fig. 2(b) and 2(c), respectively. In Fig. 2(b), amorphous structures and lattice fringes of Fe2 B {2 0 0} are observed near the growth point of BN layers. The amorphous structure would be boron-rich phase formed from reaction with Fe4 N. At the interface between the Fe particle and BN nanotube in Fig. 2(c), lattice fringes of Fe {1 1 0} are observed, and the BN {0 0 2} layers are inclined from the nanotube axis indicated by z-axis. Figure 3(a) and 3(b) are TEM and HAADF-STEM images of Fe-filled BN nanotubes (WR = 9:1), which were remaining sediment after centrifugation. The contrast in the TEM image is weak and direct observation of Fe-filled BN nanotubes is difficult. The same area imaged by HAADFSTEM shows excellent contrast and the morphology of Fefilled BN nanotubes can be observed in detail. A great number of Fe-filled BN nanotubes were observed by HAADF-STEM. High WR of Fe4 N would be necessary for synthesis of Fe nanowires. TEM image of one of Fe-filled BN nanotubes is shown in Fig. 3(c). Figure 3(d) is an EDX spectrum of the Fe-filled BN nanotube. In Fig. 3(d), two peaks of boron, nitrogen are observed. This shows the atomic ratio of B:N = 46.5:53.5, which indicates formation of BN. A strong peak of Fe (0.70 keV) is also observed, while a Cu peak arises from the HREM grid. Figure 3(e) is an enlarged image of Fig. 3(c). Fig. 3(f) is an electron diffraction pattern of the Fe-filled BN nanotube. Strong peaks of BN nanotubes correspond to the planes of

a

Fe

30nm

b

amorphous

Fe2B{200}

BN {002}

Fe2B{200}

3nm

c

Fe{110}

BN{002}

z x 3nm Fig. 2 (a) Fe nanoparticle at a tip of nanotube. Enlarged images of (c) a tip and (d) an interface between the Fe nanoparticle and the nanotube.

(002) of BN. Strong peaks are also indexed as metallic Fe with a bcc structure, and the incident beam is parallel to the [111] zone axis of -Fe. Figure 4(a) is an enlarged HREM image of Fig. 3(e),


a

725

b

300nm

300nm

c Intensity (Arb. unit)

B N

Cu

0

50nm

Fe

d

0.5

1

1.5

2

2.5

Energy (keV)

e

f

Fe 211

BN Fe 110

Fe 101

BN 002 Fe 011 Fe nanowire 000

BN

10nm Fig. 3 (a) TEM and (b) HAADF images of Fe-filled BN nanotubes. (c) TEM image of Fe-filled BN nanotube. (d) EDX spectrum of Fefilled BN nanotube. (e) Enlarged image of (c). (f) Electron-diffraction pattern obtained from (e).

and Fig. 4(b) is filtered Fourier transform of Fig. 4(a). Figure 4(c) is inverse Fourier transform of 4(b), and Fig. 4(d) is an enlarged image of 4(c). Figure 4(d) shows a lattice image of the bcc Fe-filled BN nanotube. The nanotube axis is parallel to the [110] direction of Fe, which indicates the bcc

Fe is epitaxially grown to the [110] zone axis. The tubular layers around the nanowire have an average interlayer spacing of 0.34 nm, which corresponds to the (002) spacing of BN. Figure 4(e) is inverse Fourier transform of Fig. 4(b) using

726


a

b Fe 101

Fe 110

BN {002}

BN 002 Fe 011 000

Fe {112} Fe {110}

1nm

c

d BN {002}

BN {002} 0.34nm

Fe {112} Fe {110}

Fe

1nm

f

e

Fe {110} Fe {110}

1nm

1nm

Fig. 4 (a) HREM image of Fe-filled BN nanotube. (b) Filtered Fourier transform of (a). (c) Inverse Fourier transform of (b). (d) Enlarge image of square in (c). (e) Inverse Fourier transform of (b) using 000, Fe 011 and Fe 01 1 reflections. (f) Enlarged image of square in (e).

000, Fe (011 and 01 1) reflections, and Fig. 4(f) is an enlarged image of 4(e). Several edge-on dislocations are observed as indicated by arrows, which would be due to lattice distortion produced during Fe-filled nanotube growth. This lattice distortion is also observed as expansion in the electron

diffraction pattern of Fig. 3(f), as indicated by arrows. As shown in Fig. 2(c), BN layers are often inclined compared to nanotube axis, which are called cup-stacked nanotubes. A HREM image and Fourier filtered image of nanotube wall of bamboo-type BN nanotube with cup-stacked


a

727

b

BN{002}

z

z x

x

2nm

c

2nm

d

z

z x

e

x

1nm

1nm

f

BN{002}

-40nm

-50nm

Fig. 5 (a) HREM image of nanotube wall of bamboo-type BN nanotube with cup-stacked structures. (b) Processed image after Fourier filtering of (a). (c) HREM image of nanotube center. (d) Processed image after Fourier filtering of (c). (e) Processed image after Fourier filtering of (c). (e) Structure model of four-fold walled B1976 N1976 nanotube with a cup-stacked structure. (f) Calculated HREM images as a function of defocus values.

structures (WR = 1:1) is shown in Fig. 5(a) and 5(b), respectively. The nanotube axis is indicated by z-axis. BN {0 0 2} layers are inclined compared to the nanotube axis, and the

cone angle between the BN layers at both nanotube walls is 36 . An enlarged image of nanotube center is shown in Fig. 5(c), and a HREM image with clear contrast was proc-

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essed after Fourier noise filtering as shown in Fig. 5(d), which shows hexagonal arrangements of white dots. A structure model for B494 N494 cup-layer was proposed, which consists only of hexagonal BN rings. A structure model and calculated HREM images of four-fold walled B1976 N1976 nanotube with a cup-stacked structure are shown in Fig. 5(e) and 5(f), respectively. The calculated images (Fig. 5(f)) at defocus values of 40 and 50 nm have similar contrast of the HREM images in Fig. 5(b) and 5(d). Total energies of B494 N494 (1 layer), B988 N988 (2 layers) and B1976 N1976 (4 layers) cup-stacked BN layers were calculated as 0.906, 0.642 and 0.522 kcal/molatom, respectively, which indicates that the structure of BN multi-layered nanotubes with a cup-stacked structure would be stabilized by stacking hexagonal BN networks. Distance between BN layers of nanotubes with a cup-stacked structure in a HREM image was measured to be 0.35 nm, and the basic structure model was constructed based on it. After molecular mechanics calculations, the layer distances were optimized as 0.38 nm. Cup-stacked carbon nanotubes with Pt nanoparticles in inner surface of the hollow core had been reported.20) The BN nanotubes with cup-stacked structures in the present work would also be one of the candidates for atomic and gas storage, as well as carbon nanotubes. Cone angles of BN cupstacks were measured to be 36 , which agreed well with that of the model in Fig. 5(e) (38 ). Cone angles of carbon nanotubes with a cup-stacked structure were reported to be in the range of 45–80 .20) The cause of the different cone angles of the present cup-stacked BN nanotubes would be due to the different stacking of BN layers along c-axis (B-N-B-N. . .) from carbon layers. The cone angles might also depend on the shape of catalysis particles, as shown in Fig. 2(a). A small amount of nanocrystalline Fe2 B compounds were observed at the tip of the BN nanotube (Fig. 6). Chemical formulas that Fe4 N reacts with B, and generates Fe and BN in our experiments can be proposed as follows: Fe4 N þ 3B ! BN þ 2Fe2 B

N2

N2

N2

N2 Fe2B

N2

N2

B

N2

N2 amorphous FeB N2

N2 B N2

B

B Fe2B

N2

B

Fe

Fe Fe

BN

BN

Fe [110] BN

[WR = 9:1 (Fe4N:B)]

[WR = 1:1 (Fe4N:B)]

Fig. 6 Schematic illustration of the formation mechanism of Fe-filled BN nanotube and bamboo-type structure.

ð1Þ

Fe2 B and dissolution of boron were obtained, and BN was produced in the reaction expressed as eq. (1) because Fe2 B is thermodynamically more stable than Fe4 N. Although the Fe2 B is stable to 1389 C, the Gibbs-Thompson effect shown that the melting occurs at a significantly lower temperature compared to values in the standard phase diagram. Therefore, fluid-like Fe2 B can be attained more easily. In the next process, the reaction expressed as eq. (2) would take place. 2Fe2 B þ N2 (g) ! 2BN þ 4Fe

Fe4N

ð2Þ

Boron in liquid-like Fe2 B started to segregate on the surface of the particle. The boron would react with N2 gas, and BN was produced. -Fe in liquid-like Fe2 B is epitaxially grown to the [110] direction, and Fe nanowires were produced in the reaction of eq. (2). In adeition, high WR would be mandatory for the formation of Fe-filled BN nanotubes. As the results of these reactions, the [110] of Fe is parallel to the BN nanotube axis. Gibb’s energy on each formula is calculated as 89:4 and 23:2 kcal for the formulas (1) and (2) at 1000 C, respectively. These negative values would stand for correctness of

the proposed formulas. It is considered that a formation of FeB compounds might plays an important role for growth of the BN nanotubes, and that amorphous boron might change to BN and Fe2 B on the surface of the Fe4 N nanoparticles. When magnetic materials are used as catalysis metals for BN nanotube formation, the magnetic nanoparticles would move around by magnetic field of a coil heater during the reaction process. Then, segments of BN {002} layers were produced in the tubes, which results in formation of bamboo structures (Fig. 6). The interval of the BN layer segments might be related to the amount of iron nanoparticles, and further studies are expected on the control of the bamboo structure. 4.

Conclusion

The present work provides the process for cup-stacked BN nanotubes, nanohorns, nanocoils and Fe-filled BN nanotubes. These BN nanomaterials were synthesized by annealing Fe4 N and B powders at 1000 C for 1–5 h in nitrogen gas atmosphere. Atomic structure models and formation mech-


anism of the BN nanotubes were proposed from HREM, image simulations and molecular mechanics calculation, which showed that the present multilayered structures with hexagonal BN networks would be stabilized by stacking of BN cup-layers. Fe-filled BN nanotubes were also observed, and the [110] direction of the Fe nanowires was parallel to the BN nanotube axis. These unique structures would be suitable materials for nanoelectronics devices, magnetic recording media and biological sensors with excellent protection against oxidation and wear. Acknowledgements The authors would like to thank A. Nishiwaki for calculation help. This work was partly supported by Hosokawa Powder Technology Foundation, and Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science and Technology, Japan. REFERENCES 1) H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl and R. E. Smalley: Nature 318 (1985) 162–163. 2) S. Iijima: Nature 354 (1991) 56–58. 3) T. Oku, T. Hirano, M. Kuno, T. Kusunose, K. Niihara and K. Suganuma: Mater. Sci. Eng. B74 (2000) 206–217. 4) T. Oku, M. Kuno, H. Kitahara and I. Narita: Int. J. Inorg. Mater. 3

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