Masafumi Ata

Work function of carbon nanotubes

Abstract The work functions of multi- and single-walled carbon nanotubes are found to be 4.95 and 5.05 eV, respectively. The measurements were carried out using the photoelectron emission (PEE) method, which allows easy and precise measurements to be made in air. We have found that the work function of the nanotubes is 0.1–0.2 eV larger than that of highly oriented pyrolytic graphite (HOPG) of which the valence state is σ–π orthogonal. This result is ascribable to reflection of the σ–π mixed valence state in the case of carbon nanotubes. The experimental data were well reproduced in ab-initio calculations on planar and cylindrical conjugated states.

Hydrogen storage capacity of commercially available carbon materials at room temperature

The hydrogen storage capacity of five types of commercially available carbon materials with different nanostructures was measured at up to 8 MPa at room temperature using an apparatus based on a volumetric method with an error of less than 0.04 wt %/gr. The highest storage capacity of 0.43 wt % was obtained for purified HiPco™ single-walled carbon nanotubes (SWNTs). In the SWNTs, the hydrogen density in pores with a diameter of less than 1 nm was estimated to be a 0.022 g/ml, which corresponds to 31% of the density of liquid hydrogen. Issues in the development of carbon-based hydrogen storage media are discussed.

Fullerene proton conductors

Abstract AC impedance of electronically insulative polyhydroxyfullerene C60(OH)12 showed conductivity of 7×10−6 S/cm at 295 K, which is attributable to proton conductivity. Activation energy of proton conduction in this material was 0.33 eV and it may correspond to the proton-hopping energy. Drift of the water molecules through the bulk was negligible because proton conductivity did not change for over 10 h, while a direct current of protons was flowing in dry hydrogen gas.

Gas–solid interactions in the hydrogen/single-walled carbon nanotube system

Abstract Gas–solid interactions between hydrogen and single-walled carbon nanotubes (SWNTs) were investigated using highly purified SWNTs. The activation energy of hydrogen desorption, measured to be 0.21 eV, indicates that hydrogen is physisorbed in the pores and that inter-tube pores have an adsorption potential of about −0.21 eV, which induces hydrogen physisorption at ambient temperature. The total amount of adsorbed hydrogen, about 0.3 wt% at 9 MPa, shows that 38% of the inter-tube sites are occupied. These findings are interpreted in terms of the chemical potential of hydrogen and the adsorption potential of the inter-tube pores.

Plasma‐polymerized C60/C70 mixture films: Electric conductivity and structure

Polymerization of a mixture of C60/C70 in rf plasma is reported. The electric dark‐current conductivity of the plasma‐polymerized mixture of C60/C70, which is approximately 10−7 S/cm in the atmosphere, does not depend on the applied voltage at least in the range of −25 to 25 V. A semiconductor‐type temperature dependence of the conductivity in the higher‐temperature domain was observed, and the band‐gap energy was estimated to be 2.1 eV. The conductivity increased with increasing temperature from 25 to 230 °C in vacuum, whereas in the atmosphere the conductivity increased upon decreasing the temperature below 80 °C. It is supposed that in this temperature domain the electric conductivity is enhanced by the existence of water molecules on the film, the surface of which is characterized by a high hydrophilicity. The surface morphology of the polymerized film was characterized by the presence of aggregates with diameters of about 300 A and the surface was highly hydrophilic, polar, and heterogeneous. The sur...

Studies of amorphous carbon using X-ray photoelectron spectroscopy, near-edge X-ray-absorption fine structure and Raman spectroscopy

Abstract We report core level and valence band X-ray photoelectron spectroscopy (XPS), carbon K and oxygen K near-edge X-ray-absorption fine structure spectroscopy (NEXAFS), and Raman spectroscopy results of plasma-deposited amorphous carbon generated from fullerene C 60 . In comparison with evaporated C 60 , the C 1 s peak is broader and asymmetric for the amorphous carbon film and its shake-up satellites disappear. The valence band shows three fairly broad peaks. Only one prominent π * resonance occurs in the NEXAFS spectrum. Recognizable structures appear in the σ * region indicating the formation of new bonds. In the Raman spectrum the typical D and G bands were observed. The amorphization of C 60 and post-plasma functionalization of the surface after exposure to atmosphere cause changes in the electronic structure.

Large-scale rooted growth of aligned super bundles of single-walled carbon nanotubes using a directed arc plasma method

We have modified the conventional arc-discharge method for the production of single-walled carbon nanotubes (SWNTs) by using a bowl-shaped cathode that can direct the arc plasma in a specified direction and using a combination of cobalt, nickel, iron and cerium as an efficient catalyst, with sulfur as a promoter and tin as a nanowelding element. Under optimized conditions, SWNT super bundles can be efficiently produced with carbon-to-tube yields of over 70%.

Purification and alignment of arc-synthesis single-walled carbon nanotube bundles

We report here a scalable method for purification and alignment of single-walled carbon nanotubes (SWNT) in an aqueous solution. Arc-synthesis soot containing SWNTs is first treated with a concentrated nitric acid. After removal of most of the impurities and water, macroscopic and well-aligned SWNT bundles up to several centimeters long are formed in a rotary evaporator. Alignment of the SWNT bundles is ascribed to the liquid flow induced by rotary evaporation and van der Waals interactions among the bundles. The aligned SWNT bundles are further purified by ultrasonic Soxhlet extraction and annealing.

Proton Conduction in Polyhydroxy Hydrogensulfated Fullerenes

Proton conductivity of polyhydroxy hydrogensulfated fullerene C 60 (OSO 3 H) x (OH) y (x ∼ 6, y ∼ 6), and partially hydrolyzed polycyclosulfated fullerene, C 60 (OSO 2 O) 6-z (OSO 3 H) z (OH) z (z < 1), was investigated. The ac impedance measurements revealed that these materials showed proton conductivities of 10 and 0.1 mS/cm, respectively, at room temperature. Those conductivity values are higher than that of a fullerenol-type proton conductor. This suggests that introduction of highly acidic groups, especially OSO 3 H, surrounding the fullerene cage leads to higher conductivity. The activation energy of the conduction in C 60 (OSO 3 H) x (OH) y was estimated to be 0.35 eV from a variable-temperature conductivity measurement. The conductivity is constant for IS h in a dry-gas condition, indicating that no water molecule drifts together with a proton in this material. It is thus suggested that protons in a polyhydroxy hydrogensulfated fullerene are transported by a hopping mechanism as is the case for fullerenol C 60 (OH) 12 .

Hydrogen storage in C70 encapsulated single-walled carbon nanotube

Molecular hydrogen desorption from C 70 encapsulated single-walled carbon nanotube (C 70 -peapod) has been investigated by Temperature-Programmed-Desorption (TPD) method. The single-walled carbon nanotube (SWNT) is in the spotlight recently due to reports of high-density hydrogen storage capability at moderate temperature. Here, we find that C 70 -peapod loose hydrogen molecule at about 365 K and the NaOH treatment is crucial for the withdrawal of hydrogen at moderate temperature. This result fairly agrees with the previous our report in SWNT-bundles and C 60 -peapod.