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The unique water transport properties in nanospaces are essential for control of various chemical reactions, biochemical activities, and electrochemical systems. Fast water transport has been observed in one-dimensional nanospaces. However, water transport via zero-dimensional nanospaces has not yet been observed. Zero-dimensional nanospaces were obtained by extremely small and thin gate (zero-dimensional gate) insertion on graphene walls of single-walled carbon nanohorns. The water transport properties were examined by water vapor loading and release via the zero-dimensional gates, and molecular dynamics simulation. Although relatively large gates provided considerable adsorption hysteresis by long-term equilibrium, water vapor loading and release via the extremely small gates showed consecutive water loading and release. The molecular dynamics simulation showed consecutive water transport via the gates, probably because of lower energy barriers to water transport in the vicinity of the gates. The zero-d...

We explore the importance of the size and position of nanoholes on the electrochemical performance of single-walled carbon nanohorn (SWCNH)-based supercapacitors using an ionic liquid electrolyte. The oxidized sample at 673 K showed a low specific capacitance per unit of internal specific surface area (4.0 μF cm–2), as the nanoholes created on the tips of SWCNHs via a selective chemical attack are too small to introduce electrolyte ions. For a sample oxidized at 723 K, the enlarged diameter of the nanoholes on the tips allows electrolyte ions to penetrate into the internal spaces of the SWCNHs, thereby resulting in a 2-fold capacitance improvement (8.6 μF cm–2). However, the abrupt decrease in the capacitance of the oxidized SWCNHs at 823 K (3.8 μF cm–2) can be explained by the selective formation of nanoholes on the sidewalls of the SWCNHs, where the small interstitial pores restrict ion diffusion to deeply positioned nanoholes on the sidewalls of the SWCNHs. Our study clearly reveals that the size and p...

Single-walled carbon nanohorns containing Pd–Ni alloy nanoparticles (Pd–Ni/SWCNHs) were prepared by the gas-injected arc-in-water method, and high-pressure H2 absorption by these nanoparticles was investigated using a magnetic suspension balance at 30 °C and 1.2 MPa. The amount of H2 absorbed by them was larger than the predicted combined absorption contributed by Pd–Ni alloy and pure-carbon SWCNHs. This synergetic H2 absorption was induced by the combination of Pd–Ni alloy nanoparticles and SWCNHs and occurred because of a spillover effect. Remarkably, H2 absorption occurred by diffusing into as-grown Pd–Ni/SWCNHs. Opening the pores of Pd–Ni/SWCNHs by a mild oxidation treatment drastically improved the H2 absorption. Consequently, the diffusivity of H2 into Pd–Ni/SWCNHs was enhanced to the point where H2 absorption could reach saturation in an extremely short time.

We investigated the importance of the specific effective surface area through a detailed study on the relationship between electrical conductivity of single-walled carbon nanohorns (SWCNHs) and accessibility of the electrolyte ions in the SWCNH-based supercapacitor. After heat treatment of the SWCNHs, the ratio of sp2/sp3 carbons dramatically increased, suggesting an enhanced electrical conductivity of the SWCNHs. Even though the specific surface area (SSA) slightly decreased by 16% as a result of heat treatment, the specific capacitance per SSA of the SWCNH electrode remarkably increased from 22 to 47 μF cm–2. Such a result indicates an explicit increase in accessible effective surface area by electrolyte ions. Our result clearly showed that a higher degree of utilization for the interstitial pore of SWCNHs by solvated ions is a key factor in achieving a high volumetric capacitance of SWCNH-based supercapacitors.

High-density CH4 storage using adsorption techniques is an important issue in the use of CH4 as a clean energy source. The CH4 adsorption mechanism has to be understood to enable innovative improvements in CH4 adsorption storage. Here, we describe the adsorption mechanism, based on CH4 structure, and stabilities in the internal and external nanopores of single-walled carbon nanohorns, which have wide and narrow diameters, respectively. The adsorption of larger amounts of CH4 in the narrow nanopores at pressures lower than 3 MPa was the result of strong adsorption potential fields; in contrast, the wider nanopores achieve higher-density adsorption above 3 MPa, despite the relatively weak adsorption potential fields. In the wider nanopores, CH4 molecules were stabilized by trimer formation. Formation of CH4 clusters therefore compensates for the weak potential fields in the wider nanopores and enables high-density adsorption and adsorption of large amounts of CH4.

Finite carbon chains encapsulated into carbon nanotubes are investigated by using first-principles calculations based on spin density functional theory. Dimerization is observed in carbon chains formed by an even number of carbon atoms, but not in carbon chains containing an odd number of carbon atoms. All carbon chains inside the carbon nanotubes are spin-polarized due to spin-split energy levels around the Fermi level. By comparing the results of carbon chains inside the carbon nanotubes and isolated charged carbon chains, we found that the charge transfer from the carbon nanotubes to the carbon chains leads to the structure transformation and the spin-polarization of the carbon chains when encapsulated inside the carbon nanotubes, whereas the orbital hybridization removes the degeneracy of energy levels around the Fermi levels. Our results provide a deep understanding on the interaction between the carbon chains and the carbon nanotubes.

We investigated the inner structure of an aggregate made of single-wall carbon nanohorns (SWNHs). This structure has not been clarified because the thousands of SWNHs are densely packed and difficu...