Keiji Kurashima

Synthesis of boron nitride nanotubes from carbon nanotubes by a substitution reaction

A method involving carbon nanotubes substituted reaction was developed for the synthesis of mass quantities of boron nitride nanotubes. Boron oxide vapor was reacted with nitrogen gas in the presence of carbon nanotubes to form boron nitride nanotubes, whose diameters and lengths are similar to those of the starting carbon nanotubes. It is proposed that carbon atoms of carbon nanotubes can be fully substituted by boron and nitrogen atoms through a general chemical reaction. The results suggest that the synthesis methodology developed here may also be extended to form nanotubes from other novel materials.

Nanotubes in boron nitride laser heated at high pressure

Here we report on the finding of pure boron nitride (BN) nanotubes that do not contain any additional inclusions and on a new method for their growth: laser heating of boron nitrides at high nitrogen pressure (5–15 GPa). The multiwalled nanotubes were observed using high resolution electron microscopy and were chemically characterized by electron energy loss spectroscopy. The circular or polygonal cross‐sectional nanotubes, which have 3–8 shells and a characteristic outer dimension cross section of 3–15 nm, were found to have grown either in melted cubic BN or in hexagonal+amorphous BN that had recrystallized on the specimen’s surface from the fluid phase.

Octahedral boron nitride fullerenes formed by electron beam irradiation

Here we report on the formation of fullerenes with a reduced number of layers (typically ⩽3) in boron nitride (BN) which was subjected to in situ electron irradiation at 20 and 490 °C in a high resolution 300 kV transmission electron microscope (HRTEM). The BN fullerenes exhibited B/N stoichiometry of ∼1 as confirmed by electron energy loss spectroscopy using a 1 nm electron probe. The fullerene HRTEM images revealed rectangle-like shapes when viewed in specific projections, unlike the quasispherical carbon fullerene morphology. The octahedral BN fullerene model [O. Stephan, Y. Bando, A. Loiseau, F. Willaime, N. Shramchenko, T. Tamiya, and T. Sato, Appl. Phys. A 67, 107 (1998)] is verified by the BN fullerene observations at different viewing angles.

ZNO NANOBELTS GROWN ON SI SUBSTRATE

Using infrared irradiation to heat an industrial brass (Cu–Zn alloy) disk in moderate vacuum, ZnO nanobelts were directly prepared on a Si substrate. The nanobelts had a single-crystal hexagonal structure and grew along the [0001] direction. The nanobelts had two distinct widths along their entire length. Photoluminescence measurement showed that the nanobelts had an intensive near-band ultraviolet emission at 379 nm. Large-area growth and high quality indicate that the prepared ZnO nanobelts have potential application in optoelectronic devices.

Ultrafast Viscous Permeation of Organic Solvents Through Diamond-Like Carbon Nanosheets

Porous Membranes Thin semi-permeable membranes are commonly used as chemical barriers or for filtration purposes. While the size of the pores will influence which molecules are able to pass, other factors—including the surface chemistry of the pore walls, electrostatic interactions, and differences in solubility—can also affect the diffusion rates. There is also a trade-off between the thickness of the membrane regarding strength and permeation rates (see the Perspective by Paul). Karan et al. (p. 444) fabricated membranes from amorphous carbon, which showed excellent strength and could be used for filtrations involving organic solvents. Nair et al. (p. 442) observed unusual behavior in graphene-based membranes which were able to prevent the diffusion of many small-molecule gases, including helium, but showed almost barrier-free movement of water. Membranes made from diamond-like carbon are used to rapidly separate organic compounds. Chemical, petrochemical, energy, and environment-related industries strongly require high-performance nanofiltration membranes applicable to organic solvents. To achieve high solvent permeability, filtration membranes must be as thin as possible, while retaining mechanical strength and solvent resistance. Here, we report on the preparation of ultrathin free-standing amorphous carbon membranes with Young’s moduli of 90 to 170 gigapascals. The membranes can separate organic dyes at a rate three orders of magnitude greater than that of commercially available membranes. Permeation experiments revealed that the hard carbon layer has hydrophobic pores of ~1 nanometer, which allow the ultrafast viscous permeation of organic solvents through the membrane.

Field emission from MoO3 nanobelts

Single-crystalline MoO3 nanobelts having an orthorhombic structure were prepared on a Si wafer via heating a Mo foil in air. The nanobelts were 50–300 nm wide and tens of nanometers thick. The nanobelt lengths lie in the [001] direction. Field-emission measurements showed that the threshold field decreased with the anode–sample separation increasing. Typically, a threshold field of 12.9 V/μm was determined at a spacing of 80 μm. The nanobelts exhibited a sharp increase in emission current density near the threshold field and, thus, reached a high current density at a relatively low field. Emission from both sharp corners and edges of the nanobelts is assumed to contribute to the high emission current. The high-current emission paired with high stability indicates that the prepared MoO3 nanobelt films are excellent field emitters.

Large-scale synthesis and HRTEM analysis of single-walled B- and N-doped carbon nanotube bundles

Bundles of B- and N-doped single-walled carbon nanotubes (SWNTs) containing up to ∼10 at% B and up to ∼2 at% N were synthesized at high yields under thermo–chemical treatment of pure C SWNT bundles and B2O3 in a flowing nitrogen atmosphere. The bundles were characterized by means of high-resolution transmission electron microscopy and electron energy loss spectroscopy. The effects of synthesis temperature (1503–1773 K) and time (30–240 min) on the B and N contents and yield of the SWNT bundles were determined. The maximum yield of the B- and N-doped SWNT bundles was obtained under synthesis at 1553 K over 30 min. Atomic structure and morphology of individual SWNTs in the bundles, in particular, packing of doped SWNTs, helicity distribution, encapsulation of fullerene-like clusters, diameter and shell number variations were studied. The synthesized SWNTs in the bundles were stacked in a honeycomb array with the uniform inter-tube spacing of ∼0.3 nm. No preferable orientation for the graphene-like tubular shells was found, i.e. both zigzag and armchair edges were observed with approximately equal proportions. Frequently, diameter increase took place for the outer tubes in a bundle and for isolated SWNTs. C-based or BN-based fullerene-like encapsulates were observed in individual SWNTs. Carbon oxidation by the B2O3 vapor and B and N substitution for C is thought to underlie the doping of C SWNTs. The substitution reaction temperature–time limits with respect to the morphological stability of B- and N-doped SWNT bundles are finally elucidated.

Boron-doped carbon nanotubes prepared through a substitution reaction

Abstract Boron-doped carbon nanotubes (CNTs) have been prepared through a partial substitution reaction, where some carbon atoms of CNTs are substituted by boron atoms. Boron oxide vapor reacts with CNTs to form B x C ( x ≤0.10) nanotubes at 1373 K in 4 h under an argon atmosphere. The B x C nanotubes have diameters and lengths similar to those of the starting CNTs. Boron is seen to enhance the graphitization of CNTs. B 4 C and B 13 C 2 crystalline nanorods are also formed, with typical diameters between 6 and 30 nm. It is suggested the synthetic method described here might be used to produce a large class of new doping CNTs.

Insights into the structure of BN nanotubes

The following features of multiwalled BN nanotubes were discovered using a field emission high-resolution analytical transmission electron microscope: (i) coexistence of hexagonal and rhombohedral stacking in nanotube shell assembly; (ii) flattening of nanotube cross section, which makes possible clear atomic resolution of the core structure in a three-shelled nanotube; and (iii) change in chirality of tubular layers from armchair to zigzag arrangement in a 30° double-walled nanotube kink, as revealed by atomically resolved images of tube wall segments.

MoO3-promoted synthesis of multi-walled BN nanotubes from C nanotube templates

Abstract Large-scale synthesis of BN multi-walled nanotube (MWNT) ropes from C MWNT templates was carried out. A mixture of B 2 O 3 and MoO 3 was used as an oxidizing agent during a chemical reaction involving C MWNT oxidation in a flowing nitrogen atmosphere at 1773 K. Encapsulation of Mo-based clusters into the BN MWNTs was simultaneously achieved. The morphology, atomic structure and chemical composition of MWNTs synthesized at 1773 K with and without MoO 3 promoter were compared by means of high-resolution transmission electron microscopy and electron energy loss spectroscopy. A striking increase in BN MWNT yield and self-assemblage of tubes into ropes were discovered under MoO 3 -promoted synthesis.