We report the synthesis of high purity double-wall carbon nanotubes encapsulating GdCl₃ nanowires (GdCl₃@DWCNTs). The obtained GdCl₃@DWCNTs were characterized by transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis-NIR spectroscopy, and fluorescence spectroscopy. TEM measurements show that GdCl₃ impurities were absent on the exterior of the tubes and bundles. Furthermore, significant changes of the optical properties of the inner tubes were not triggered by being filled with GdCl₃ nanowires, which suggests that significant charge transfer does not occur between the encapsulated GdCl₃ and the inner tubes. Our results indicate that GdCl₃@DWCNTs could be useful for various applications using the expected magnetic properties of GdCl₃ and almost unchanged optical properties of DWCNTs.

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We have studied the supramolecular structures and photoelectronic properties of inclusion complexes of cyclic porphyrin dimers (M₂-CPD_Py, M=Ni, 2H) and fullerenes (C₆₀, C₇₀, PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) and Li⁺@C₆₀). The porphyrin dimers have self-assembling 4-pyridyl groups at the meso positions and include fullerenes in their cavities with high affinities (K_assoc≈10⁵ M^-1). In crystals, C₆₀⊂Ni₂-CPD_Py and PCBM⊂Ni₂-CPD_Py give self-assembled porphyrin nanotubes containing the linear arrays of fullerene molecules. These self-assemblies are induced mainly by the non-classical hydrogen bonds between pyridyl nitrogens and porphyin β C-H groups. On the other hand, in the crystals of C₆₀⊂H₄-CPD_Py and C₇₀⊂H₄-CPD_Py, the included fullerene molecules show zigzag chains through van der Waals contacts with each other. Due to these well-ordered arrays of C₆₀, both crystals of C₆₀⊂Ni₂-CPD_Py and C₆₀⊂H₄-CPD_Py have high charge mobilities (Σμ>0.1 cm² V^-1 s^-1). C₆₀⊂H₄-CPD_Py shows photoinduced electron transfer from the porphyrin moieties to C₆₀ and photovoltaic activity with IPCE_max=17% in photoelectrochemical cells. Moreover, the inclusion complexes of lithium-ion-encapsulated C₆₀(Li⁺@C₆₀) and M₂-CPD_Py exhibit photoinduced charge separation with very long lifetimes reaching submillisecond order.

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Elemental sulfur, one of the most important natural resources in the chemical industry, has been used mainly for the production of sulfuric acid, vulcanization of rubber, and fertilizers. Despite its importance, sulfur has been regarded as an abundant resource due to the fact that its use is limited to being an inexpensive reagent. Here we report the synthesis of one-dimensional crystals of sulfur using the quasi one-dimensional channels of carbon nanotubes. This one-dimensional phase is confirmed by high resolution transmission electron microscopy and X-ray diffraction, revealing monatomic zigzag and linear sulfur chains inside the constrained volume of the carbon nanotubes. In addition, X-ray diffraction exhibits asymmetric Bragg peaks, indicating the formation of one-dimensional sulfur “crystals”. Most importantly, the one-dimensional sulfur crystals exhibit a metallic character at ambient pressure, whereas bulk sulfur requires ultra-high pressures exceeding ∼90 GPa to become metallic. These results lead us to believe that elemental sulfur has great potential to be used as a metal wire in the nanoscale, thus paving a way for future applications.

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We have studied the structure around a metal ion such as zinc and copper restricted in the micropores of carbon materials with the X-ray absorption fine structure (XAFS) technique and adsorption isotherm analyses. We succeeded in revealing the dehydrated structure of zinc ions confined in the micropores of activated carbon fibers (ACFs) and carbon nanotubes (CNTs) whose pore widths are below 1 nm. XAFS spectra and adsorption isotherms of zinc ions at 303 K strongly indicate that the dehydrated structure can be stably formed even in a micropore whose pore width is less than the diameter of a spherically hydrated zinc ion, where the dehydrated ions can be stabilized by the strong potential well of the carbon micropores. Also, we studied the local structure around a copper ion of copper acetate restricted in the micropore of two kinds of ACFs by the XAFS technique. The results obtained indicate the elongation of the Cu-Cu distance of the dinuclear copper complex because of the distortion or the reduction in the number of bridging carboxylates inside the micropore. We could understand the specific formation of hydration or coordination structure formed in micropores by using d-block elements as probe metal ions.

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The single-wall carbon nanohorn (CNH) is a type of nanocarbon that is shown here to be potentially useful as a drug delivery system (DDS). Research results on the structure of CNHs, the incorporation of drugs in them, and their surface functionalization are introduced. The specific advantages of a CNH-DDS are shown using experimental results on animals. The low toxicity of CNHs is also briefly considered.

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Carbon nanofibers (CNFs) have unique structural characteristics, such as a nanometer diameter, a fibrous structure, and high crystallinity, and thus, are expected to be promising important materials for energy and environment applications. However, difficulties in handling and the high production costs of CNFs often hamper the industrialization of products from CNF alone. On the other hand, hybridization of CNF with other materials can improve their weaknesses, increase performance, and/or give them new abilities. In this paper, we introduce the effectiveness of CNFs as complementary functional materials especially for applications in the energy and environment fields, such as Li-ion secondary batteries, fuel cells, electric double-layer capacitors, and air purification.

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