Tomohiro Tojo

Enhanced electrical conductivities of N-doped carbon nanotubes by controlled heat treatment.

The thermal stability of nitrogen (N) functionalities on the sidewalls of N-doped multi-walled carbon nanotubes was investigated at temperatures ranging between 1000 °C and 2000 °C. The structural stability of the doped tubes was then correlated with the electrical conductivity both at the bulk and at the individual tube levels. When as-grown tubes were thermally treated at 1000 °C, we observed a very significant decrease in the electrical resistance of the individual nanotubes, from 54 kΩ to 0.5 kΩ, which is attributed to a low N doping level (e.g. 0.78 at% N). We noted that pyridine-type N was first decomposed whereas the substitutional N was stable up to 1500 °C. For nanotubes heat treated to 1800 °C and 2000 °C, the tubes exhibited an improved degree of crystallinity which was confirmed by both the low R value (I(D)/I(G)) in the Raman spectra and the presence of straight graphitic planes observed in TEM images. However, N atoms were not detected in these tubes and caused an increase in their electrical resistivity and resistance. These partially annealed doped tubes with enhanced electrical conductivities could be used in the fabrication of robust and electrically conducting composites, and these results could be extrapolated to N-doped graphene and other nanocarbons.

Chirality-dependent transport in double-walled carbon nanotube assemblies: the role of inner tubes.

A fundamental understanding of the electrical properties of carbon nanotubes is vital when fabricating high-performance polymeric composites as well as transparent conductive films. Herein, the chirality-dependent transport mechanisms in peapod- and chemical vapor deposition-grown double-walled carbon nanotubes (DWNTs) films are discussed by identifying the chiralities of the inner and the outer tubes using fast Fourier transform image processing, as well as optical studies (e.g., Raman, UV, and photoluminescence spectroscopies). The observed conduction mechanisms are strongly dependent on the total fraction of the metallic inner and outer tubes within the DWNT samples. Furthermore, the contribution of the inner tubes to the electronic transport properties of DWNT films is confirmed by photochemically deactivating the outer tubes in both types of DWNT samples.

Defect‐Enhanced Dispersion of Carbon Nanotubes in DNA Solutions

Since DNA, a well-known biopolymer, has proven to be effective for dispersing and sorting carbon nanotubes, intensive studies have been carried out in order to obtain both theoretical and experimental understanding of the DNA–nanotube interaction. The ultrasonication process has been applied to dispersing a strongly bundled aggregate of carbon nanotubes in a nanotube suspension containing DNA in order to overcome the attractive van der Waals forces between adjacent tubes. The strong hydrodynamic shear forces generated by the ultrasonication process create a space within the bundled nanotubes, which allows the DNA molecules to impinge and become adsorbed on the outer surface of the nanotubes through a hydrophobic interaction, thereby leading to a stable nanotube emulsion through the formation of DNA– nanotube hybrid structures. Until now, single-stranded DNA has proven to be more effective for dispersing nanotubes than double-stranded DNA, and the combination of the short complementary oligonucleotides d(GT)3 and d(AC)3 exhibit an even superior dispersing ability. Moreover, theoretical studies have assumed that the shape of carbon nanotubes constitutes a perfect cylinder. However, the catalytic growth of carbon nanotubes in the reaction chamber inevitably involves the formation of defects (e.g. vacancies, nonhexagonal rings, and foreign atoms), which modify the electronic structure and surface properties of the tubes 14] and are expected to ultimately alter the interaction between nanotubes and DNA. In order to be able to reproducibly assemble nanotube–DNA hybrid structures as bio-medical devices, it is essential to consider the effects of defects within carbon nanotubes. Herein, the role of defects generated on the sidewall of carbon nanotubes is studied by directly comparing the dispersibility of defective and crystalline thin (small outer diameter) multi-walled carbon nanotubes (MWNTs) in aqueous DNA solutions. The dispersibility of nanotubes depend strongly on the density of the defects on the sidewall. More specifically, crystalline tubes thermally treated at 2300 8C exhibit a dispersibility twice as low as that of the as-grown tubes in aqueous DNA solutions. Theoretical calculations shown herein support the conclusion that defect sites exhibit high reactivity toward the adsorption of DNA. There are three reasons for selecting thin MWNTs rather than single-walled carbon nanotubes (SWNTs) for this study: 1) Thin MWNTs exhibit a strongly bundled structure like SWNTs because of their small diameter below 10 nm, 2) they are structurally stable up to 2800 8C, while SWNTs and DWNTs are easily transformed into graphitic material at these high temperatures, 17] and 3) their diameter is too large (above 2 nm) for the manifestation of detailed quantum effects associated with 1D dispersion relations. As a general metric for comparing the structural integrity of SWNTs or DWNTs, the intensity of the Dband, which is explained by double-resonance theory, 19] is suitable for evaluating the quality of SWNTs containing only a small number of defects. However, due to the breakdown of the van Hove singularities, this approach is not applicable to SWNTs or MWNTs containing many defects. As a tool for controlling the defect density in as-grown defective thin MWNTs, we choose high-temperature thermal treatment 21] instead of the commonly used chemical treatment. Oxidative chemical treatment can introduce both chemical moieties and defects on the sidewall of the tubes, which might affect the tube–tube interactions as well as their dispersibility. In other words, it is difficult to separate the role of defects within tubes from the effects of chemical treatment in evaluating the dispersibility of tubes in DNA solutions. The effects of high-temperature annealing on the defect density of carbon nanotubes are clearly observed as consecutive changes in the Raman spectra (Figure 1 a). Although there are no distinct changes in the G-band located at 1582 cm 1 (E2g2 graphite mode), the intensities of the D-band (a defect-induced mode) at 1350 cm 1 decrease with increasing thermal treatment temperatures, and saturate for samples prepared at 2300 8C in argon. This temperature range is consistent with the region in conventional carbon materials where the mobility of carbon atoms increases abruptly. Therefore, a drastic decrease in the R value (the intensity of the D-band divided by the intensity of the G-band) from 0.42 to 0.1 (see inset in Figure 1 a) indicates the effective removal (or annealing) of defects within carbon nanotubes by the high-temperature thermal treatment. In order to visualize the improvement of the structural integrity, we carried out detailed SEM and TEM studies on pristine tubes (Figures 1 b, c) and tubes thermally treated at 2300 8C (FigACHTUNGTRENNUNGures 1 d, e). Pristine tubes consisting of undulated fringes and amorphous carbon layers (Figure 1 c) were transformed into crystalline tubes (Figure 1 e) featuring straight linear fringes lengthwise along the tube. Fortunately, there were no appa[a] J. H. Kim, Prof. M. Kataoka, Dr. D. Shimamoto, Prof. H. Muramatsu, Dr. Y. C. Jung, T. Tojo, Prof. T. Hayashi, Prof. Y. A. Kim, Prof. M. Endo Faculty of Engineering Shinshu University, 4-17-1 Wakasato, Nagano-shi 380-8553 (Japan) Fax: (+ 81) 26-269-5208 E-mail : yak@endomoribu.shinshu-u.ac.jp [b] Prof. M. Terrones Advanced Materials Department IPICYT, San Luis Potosi 78210 (M xico) [c] Prof. M. S. Dresselhaus Massachusetts Institute of Technology Cambridge, Massachusetts 02139-4307 (USA)

Development of Lithium-Stuffed Garnet-Type Oxide Solid Electrolytes with High Ionic Conductivity for Application to All-Solid-State Batteries

All-solid-state lithium-ion battery (LiB) is expected as one of the next generation energy storage devices because of their high energy density, high safety and excellent cycle stability. Although oxide-based solid electrolyte materials have rather lower conductivity and poor deformability than sulfide-based one, they have other advantages such as their chemical stability and easiness for handling. Among the various oxide-based SEs, lithium stuffed garnet-type oxide with the formula of Li7La3Zr2O12 (LLZ) have been widely studied because of their high conductivity above 10-4 Scm-1 at room temperature, excellent thermal performance and stability against Li metal anode.Here, we present our recent progress for the development of garnet-type solid electrolytes with high conductivity by simultaneous substitution of Ta5+ into Zr4+ site and Ba2+ into La3+ site in LLZ. Li+ concentration was fixed to 6.5 per chemical formulae, so that the formulae of our Li garnet-type oxide is expressed as Li6.5La3-xBaxZr1.5-xTa0.5+xO12 (LLBZT) and Ba contents x are changed from 0 to 0.3. As results, all LLBZT samples have cubic garnet structure without containing any secondary phases. The lattice parameters of LLBZT decrease with increasing Ba2+ contents x < 0.10 while increase with x from 0.10 to 0.30, possibly due to the simultaneous change of Ba2+ and Ta5+ substitution levels. Relative densities of LLBZT are in the range between 89% and 93% and not influenced so much by the compositions. From AC impedance spectroscopy measurements, the total (bulk + grain) conductivity at 27oC of LLBZT shows its maximum value of 8.34 x 10-4 S cm-1 at x = 0.10, which is slightly higher than the conductivity (= 7.94 x 10-4 S cm-1) of LLZT without substituting Ba (x = 0). Activation energy of the conductivity tends to become lower by Ba substation, while excess Ba substitution degrades the conductivity in LLBZT. LLBZT has wide electrochemical potential window of 0-6 V vs. Li+/Li and reversible Li+ insertion and extraction reactions of TiNb2O7 film electrode formed on LLBZT by aerosol deposition are successfully demonstrated at 60oC. The results indicate that LLBZT can potentially be used as a solid electrolyte in all-solid-state batteries.

Controlled interlayer spacing of scrolled reduced graphene nanotubes by thermal annealing

We have demonstrated the ability to control the interlayer spacing of scrolled reduced graphene nanotubes through a high-temperature thermal treatment. The thermal annealing-induced variation of the interlayer spacing from 0.385 to 0.339 nm allowed us to study the change in the electronic and transport properties of the scrolled tubes.

Unusually High Dispersion of Nitrogen-Doped Carbon Nanotubes in DNA Solution

The dispersibility in a DNA solution of bundled multiwalled carbon nanotubes (MWCNTs), having different chemical functional groups on the CNT sidewall, was investigated by optical spectroscopy. We observed that the dispersibility of nitrogen (N)-doped MWCNTs was significantly higher than that of pure MWCNTs and MWCNTs synthesized in the presence of ethanol. This result is supported by the larger amount of adsorbed DNA on N-doped MWCNTs, as well as by the higher binding energy established between nucleobases and the N-doped CNTs. Pure MWCNTs are dispersed in DNA solution via van der Waals and hydrophobic interactions; in contrast, the nitrogenated sites within N-doped MWCNTs provided additional sites for interactions that are important to disperse nanotubes in DNA solutions.

Electrochemical role of oxygen containing functional groups on activated carbon electrode

We have experimentally and theoretically clarified the effect of oxygen functional groups on the capacitive performance of a photochemically treated activated carbon electrode. A high density of CO groups at the mouth of the micropores, where the chemically active edge sites are predominantly available, increases the energy barrier for ions to enter the pores, thereby resulting in a large decrease in the specific capacitance.

Boron-assisted coalescence of parallel multi-walled carbon nanotubes

Coalescing carbon nanotubes is a major challenge for designing structures with novel physical and chemical properties and for creating three-dimensional carbon networks with improved mechanical and transport properties. We have coalesced adjacent triple walled carbon nanotubes (TWNTs) covalently, using catalytic boron atoms at high temperatures. The two outermost and then the two inner nanotubes of adjacent TWNTs merged in order to create an enlarged flattened double-walled carbon nanotube which encapsulated the two innermost single-walled carbon nanotubes.

Properties of Lithium Trivanadate Film Electrodes Formed on Garnet-Type Oxide Solid Electrolyte by Aerosol Deposition

We fabricated lithium trivanadate LiV3O8 (LVO) film electrodes for the first time on a garnet-type Ta-doped Li7La3Zr2O12 (LLZT) solid electrolyte using the aerosol deposition (AD) method. Ball-milled LVO powder with sizes in the range of 0.5–2 µm was used as a raw material for LVO film fabrication via impact consolidation at room temperature. LVO film (thickness = 5 µm) formed by AD has a dense structure composed of deformed and fractured LVO particles and pores were not observed at the LVO/LLZT interface. For electrochemical characterization of LVO film electrodes, lithium (Li) metal foil was attached on the other end face of a LLZT pellet to comprise a LVO/LLZT/Li all-solid-state cell. From impedance measurements, the charge transfer resistance at the LVO/LLZT interface is estimated to be around 103 Ω cm2 at room temperature, which is much higher than at the Li/LLZT interface. Reversible charge and discharge reactions in the LVO/LLZT/Li cell were demonstrated and the specific capacities were 100 and 290 mAh g−1 at 50 and 100 °C. Good cycling stability of electrode reaction indicates strong adhesion between the LVO film electrode formed via impact consolidation and LLZT.

A fundamental study on carbon composites of FeF3·0.33H2O as open-framework cathode materials for calcium-ion batteries

Carbon composites of open-framework iron fluoride (FeF3·0.33H2O/C) was investigated as a new cathode material for calcium ion batteries for the first time. FeF3·0.33H2O/C delivers a relatively large capacity of ca. 110mAhg-1. Its reversible capacity was greatly improved over non-composite FeF3·0.33H2O. During the first discharge and discharge-charge, insertion/extraction of Ca2+ into/from FeF3·0.33H2O/C were confirmed by an ex-situ X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) analysis. From the ex-situ analysis results, it was confirmed that Ca2+ was inserted and extracted with redox of Fe.