Satoshi Ohshima

Dispersion of metal nanoparticles for aligned carbon nanotube arrays

We report that Co metal nanoparticles (an average diameter of 4 nm) chemically synthesized by a reverse micelle method catalyzes the growth of multiwall carbon nanotubes (MWNTs) aligned perpendicular to a substrate. The surface of the nanoparticles is covered with surfactants so that the nanoparticles can be dispersed in organic solvent. The dispersion of the nanoparticles was cast directly onto a plane Si substrate for thermal pyrolysis of acetylene. We have found that the pretreatment of the metal nanoparticles with hydrogen sulfide before the pyrolysis straightens the MWNTs, suggesting sulfurization of the nanoparticle catalyst plays an important role in regular growth of the MWNTs. The dispersion of the nanoparticles offers a conventional and processible approach to synthesize large area aligned MWNT arrays.

Supercritical drying media modification for silica aerogel preparation

Abstract We modified a method of methanol supercritical drying by adding water, nitric acid and ammonia when preparing silica aerogels from tetramethoxysilane. We found that gel-component extraction during supercritical drying and the microstructure of prepared aerogels were affected by these additives. We compared the effects of the additives to the effects created during the normal supercritical drying process. Ethanol, 1-propanol, and carbon dioxide supercritical drying were the methods studied for comparison. We found that gel-component extraction decreased in the modified-method cases. We also found that adding water was the most effective modification to decrease gel-component extraction. The constituting particle and pore sizes were increased when water or base was added. The specific surface area was increased when acid was added. These results are discussed in this paper in terms of the sol–gel reaction during supercritical drying.

13C NMR study of 13C-enriched single-wall carbon nanotubes synthesized by catalytic decomposition of methane

Abstract 13 C NMR spectra and spin-lattice relaxation times were measured for single-wall carbon nanotubes with 99.9 and 50.0% 13 C enrichments and natural abundance (1.1% 13 C) prepared by catalytic decomposition of CH 4 . The 13 C isotropic shift is about 116 ppm from tetramethylsilane, being estimated from magic-angle-spinning (MAS) spectra. The value does not depend on the degree of the 13 C enrichment. The 13 C MAS NMR spectra show two additional small peaks at 171 and 152 ppm, which are ascribed to carbon species at defects or edges. The line widths of the main isotropic peak in MAS spectra are about 30 ppm, the origin of which is mostly chemical shift dispersion, reflecting a distribution of diameter and helicity. The line width in the 13 C static spectra originates from chemical shift dispersion, chemical shift anisotropy and dipole–dipole interactions between 13 C spins as well as between 13 C and 1 H spins at defects or edges. 1 H NMR spectra confirm the presence of H-containing species. The 13 C spin-lattice relaxation is dominated presumably by interaction with magnetic impurities.

Catalytic growth of carbon nanotubes and their patterning based on ink-jet and lithographic techniques

We report the catalytic synthesis of carbon nanotubes and two approaches to the catalyst patterning. Aligned multi-wall carbon nanotube (MWNT) arrays were synthesized in the presence of Co nanoparticles that were prepared by a reverse micelle method. By taking advantage of the fact that the Co nanoparticles are dispersible in organic solvent, an ink-jet technique has been applied to the Co nanoparticle dispersion for MWNT patterning. Single-wall carbon nanotubes (SWNTs) were synthesized by using Fe and Mo supported catalysts and the catalyst preparation process was combined with lithographic techniques, leading to formation of nanotubes bridging or crossing the catalyst patterns. These catalyst-patterning techniques, together with recent development of nanotube synthetic methods, may find applications in electronic and electrochemical applications, such as field-emission displays, nanodevices, and fuel cells.

Synthesis of crystalline boron nanowires by laser ablation

Crystalline boron nanowires with tetragonal structure have been synthesized based on laser ablation of a B/NiCo target; the nanowires are sometimes single crystals and have a droplet at one end of the nanowire; the droplet contains B, Ni and Co elements, which indicates that the vapor-liquid-solid (VLS) mechanism may play a key role in the growth of the boron nanowires.

TiO2-impregnated SiO2 aerogels by alcohol supercritical drying with zeolite

Abstract Preparation of monolithic TiO2-impregnated SiO2 aerogels (SiO2–TiO2 aerogels) with controlled microstructure was investigated. Tetraisopropylorthotitanate–acetylacetone mixture was impregnated to microstructure-designed silica alcogels and was decomposed during alcohol supercritical drying (SCD). The microstructure of these SiO2–TiO2 aerogels reflected that of the corresponding SiO2 aerogels. Anatase was detected by X-ray diffraction. Some of titanium elements were included in silica matrix, and the others existed as particles with 100–150 nm on the surface of the matrix. Titanium content was 25–34 at.% in SCD without zeolite, whereas SCD with zeolite increased it to over 40 at.% and decreased the amount of the particles on the surface. The SCD with zeolite was considered to prevent titanium components from being removed from the matrix with silica alcogel components.

Geometrical structure and electronic properties of atomically resolved multiwall carbon nanotubes

Multiwall carbon nanotubes (MWNTs) are predicated to exhibit various electronic properties depending on their diameters and chiralities. The existence of multishells offers the possibility of switching between high and low conducting states within the same nanotube. So far, it has been a big challenge to probe the structure and the electronic properties of different MWNT shells. Here we report the nanostructure measurements of MWNT together with their electronic properties as revealed by a scanning tunneling microscope. The images show tubes with different chiralities (0°–30°) and diameters (4–9 nm). Upon thermal oxidation in air, we have observed diameter-dependent tube opening. MWNTs with large diameters (7–9 nm) show open ends while smaller diameters have closed ends. The structure of open end shows unsaturated bonds, which offers rich chemistry to develop molecular technology based on carbon nanotubes. Finally, we present the structure of the inner shells of MWNT.

Angular-resolved EELS of a carbon nanotube

Abstract Electron energy loss spectroscopy (EELS) and electron spectroscopic imaging techniques are applied to a carbon nanotube (NT) to reveal the fine electronic state in the cylindrical structure of graphite. Although NTs with large diameters show similar EELS patterns at the carbon K-edge (285 eV) to those of graphite, a fine molecular straw with a diameter less than 5 nm exhibits different features. Especially it was found that the angular-resolved EELS formed by electrons scattered normal to the longitudinal axis of the NT indicates a strong contribution of π∗ excitation compared with σ∗ excitation. It confirms that the anisotropy of the NT exists in the structure and electronic structure.

Carbon nanotube synthesis using colloidal solution of metal nanoparticles

Nanoparticles of transition metal are important in the formation process of carbon nanotubes. We have prepared the nanoparticles containing Co and Mo by a reverse micelle method and applied them to the catalytic growth of nanotubes. The gas-phase catalytic reaction of the colloidal solution of the nanoparticles produced single-wall carbon nanotubes (SWNTs) and multi-wall carbon nanotubes (MWNTs).

Supercritical drying with zeolite for the preparation of silica aerogels

Adding zeolite during supercritical drying (SCD) process (methanol) when preparing silica aerogel was effective to make an aerogel with less shrinkage and without cracking. Silica aerogels prepared by adding zeolite had higher densities, despite less shrinkage, than those prepared without zeolite. This difference was mainly due to the promotion of the hydrolysis of unreacted silica alcogel alcoxyl groups with water produced when zeolite catalyzed methanol dehydration during SCD. The hydrolysis was most likely to make the silica alcogel component stable enough to resist extraction. Shrinkage differences that were dependent on the amount of the base catalyst were eliminated by zeolite use. Silica aerogels having about the same density and different microstructures were thus obtained from the same starting solution by changing the amount of the base catalyst. Effect of zeolite was considered from the viewpoint of the composition of supercritical solvent.