Control of hole opening in single-wall carbon nanotubes and single-wall carbon nanohorns using oxygen

Due to the simplicity of the process, holes in the graphene walls of single-wall carbon nanotubes (SWNTs) and single-wall carbon nanohorns (SWNHs) have often been opened using O2 gas at high temperatures, even though this contaminates the nanotubes with carbonaceous dust (C-dust). To open holes with less C-dust contamination, we found that a slow temperature increase of 1 degrees C/min or less, in air, was effective. We also found that SWNHs having little C-dust could store a large quantity of materials inside the tubes. We infer that the local temperature increase due to the exothermic reaction of combustion may have been suppressed in the slow combustion process, which was effective in reducing the C-dust.     

Micrometer-sized graphitic balls produced together with single-wall carbon nanohorns

We present in this report a new type of particles with micrometer-order sizes, which we called giant graphitic balls (GG balls). The GG balls are produced by CO2 laser ablation of graphite together with single-wall carbon nanohorns. They have graphitic structures whose layers tend to align parallel with the GG-ball surfaces, resulting in polygonal-like arrangements. Comparing the GG-ball structure with that of the previously reported polygonal graphite-particles, the growth mechanism of the GG ball is discussed briefly.     

Quantum effects on hydrogen isotope adsorption on single-wall carbon nanohorns

H(2) and D(2) adsorption on single-wall carbon nanohorns (SWNHs) have been measured at 77 K, and the experimental data were compared with grand canonical Monte Carlo simulations for adsorption of these hydrogen isotopes on a model SWNH. Quantum effects were included in the simulations through the Feynman-Hibbs effective potential. The simulation predictions show good agreement with the experimental results and suggest that the hydrogen isotope adsorption at 77 K can be successfully explained with the use of the effective potential. According to the simulations, the hydrogen isotopes are preferentially adsorbed in the cone part of the SWNH with a strong potential field, and quantum effects cause the density of adsorbed H(2) inside the SWNH to be 8-26% smaller than that of D(2). The difference between H(2) and D(2) adsorption increases as pressure decreases because the quantum spreading of H(2), which is wider than that of D(2), is fairly effective at the narrow conical part of the SWNH model. These facts indicate that quantum effects on hydrogen adsorption depend on pore structures and are very important even at 77 K.     

Isolating single-wall carbon nanohorns as small aggregates through a dispersion method

An approach to isolating small aggregates of single-wall carbon nanohorns (SWNHs) is presented. SWNHs are ultrasonically treated in an aqueous solution of surfactant, resulting in dispersion of SWNH aggregates. Subsequent centrifuging enables the separation of small aggregates from larger aggregates or agglomerations and removal of graphitic particles (GG balls), the main impurity. The SWNHs obtained in this way were purified and formed small aggregates, thus exhibiting characteristics superior to those of SWNHs before treatment. We believe that the ability to isolate small SWNH aggregates in an aqueous solution should contribute to their application in the fields of biological sensing and drug delivery systems.     

Synthesis of ultrafine Gd2O3 nanoparticles inside single-wall carbon nanohorns

The large diameter of single-wall carbon nanohorns (SWNHs) allows various molecules to be easily incorporated in hollow nanospaces. In this report, we prove that the nanospaces of SWNHs even work as the chemical reaction field at high temperature; that is, Gd-acetate clusters inside SWNHs were transformed into ultrafine Gd(2)O(3) nanoparticles with their particle size retained even after heat-treatment at 700 degrees C. This indicates that the confinement of the Gd-acetate clusters in a deep potential well of the SWNH nanospaces prevented a migration to form larger particles, giving rise to ultrafine Gd(2)O(3) nanoparticles of 2.3 nm in average diameter, which is much smaller than the case without SWNHs. The Gd(2)O(3) nanoparticles thus obtained were demonstrated to be actually useful to the magnetic resonance imaging. We believe that the presented effectiveness of the inner hollow spaces of SWNHs, therefore, also those of the carbon nanotubes, for high-temperature chemical reactions should be highlighted, and that the thus produced novel nanomaterials are promising to expand the fields of nanoscience.     

Supercapacitor carbon electrodes with high capacitance

Electrochemical behavior of electrodes on the basis of CH900-20 activated carbon (AC) cloth has been studied in concentrated sulfuric acid solution. Cyclic voltammetric curves have been studied in the reversibility range (from 0.1 to 0.9 V RHE) and in the deep cathodic charging potential range (from –0.8 to 1 V RHE). It has been shown that electric double layer (EDL) charging occurs in the reversibility range, while faradaic processes of hydrogen intercalation into AC carbon take place in the range of negative potentials (←0.1 V). The intercalation process is governed by slow solid-phase hydrogen diffusion. The specific charge value grows at an increase in concentrated sulfuric acid solution. The mechanism of double intercalation of sulfuric acid and hydrogen into the AC material is suggested. On the basis of the reached specific discharge capacitance of 1,560 C/g (or 1,110 F/g) and Faraday's law, it has been concluded that the compound of C₆H is formed in the limiting case of deepest cathodic charging. The obtained data have been used in a mathematical charge–discharge model for an AC electrode taking into account the EDL charging and the hydrogen intercalation. The galvanostatic recharge curves have been calculated in the diapason of currents by the developed model.     

Optimum hole-opening condition for Cisplatin incorporation in single-wall carbon nanohorns and its release

We enclosed cisplatin (CDDP), an anticancer drug, inside single-wall carbon nanohorns (SWNH) with holes opened by being heated from room temperature to a target temperature (475-580 degrees C) in flowing dry air, with an increase rate of 1 degrees C/min. The optimum target temperature was found to be 500 degrees C, in terms of the least amount of CDDP deposited outside the SWNH, when the quantity of CDDP encapsulated inside the SWNH was 12 wt %. The incorporated CDDP was slowly released from the SWNH in phosphate buffer saline, and the released quantity was 80%, which was greatly improved from the previous value of 15%. This indicated that a CDDP-containing SWNH could become more potentially useful for biological applications.     

Effect of functional groups at hole edges on cisplatin release from inside single-wall carbon nanohorns

We incorporated cisplatin inside single-wall carbon nanohorns (NHs) and revealed that 70% of the cisplatin was released from NHs having holes with hydrogen-terminated edges when they were immersed in phosphate-buffered saline (PBS). However, only 15% was released from NHs having holes with oxygen-containing functional groups at the hole edges (NHox). Elemental analysis indicated that -COOH and -OH groups at the hole edges of NHox changed mainly to -COONa and -ONa groups by immersion in PBS. These groups decreased the practical hole diameters, which resulted in hindering the cisplatin release from NHox. This means that the release of the material from inside NHox would be controlled by chemically modifying the functional groups attached to the hole edges of NHox; thus the potential applicability of NHox to a material carrier would be enhanced.     

Electrochemical and conductivity measurements of single-wall carbon nanotube network electrodes

The electrochemical response of two-dimensional networks of pristine single-wall carbon nanotubes (SWNTs) has been investigated. SWNTs were grown by catalyzed chemical vapor deposition on an insulating SiO2 substrate, and then electrically contacted by lithographically defined Au electrodes. Subsequent insulation of the contact electrodes enabled the electrochemical properties of the SWNT network to be isolated and directly studied for the first time. The electrochemical activity of the SWNT network was found to be strongly dependent on the applied potential. For the same SWNT electrode, the limiting current for the oxidation of 5 mM Fe(phen)32+ was found to be much greater than expected based on the signal for the reduction of 5 mM Ru(NH3)63+. Simultaneous conductance and electrochemical measurements demonstrated decreasing conductance as the potential was scanned negative (versus Ag/AgCl) with the minimum conductance at around the reduction potential for Ru(NH3)63+. These results are consistent with the presence of both metallic and semiconducting SWNTs in the SWNT network electrode. Moreover, these results show that through appropriate choice of mediator and electrode potential, metallic SWNTs can be electrochemically addressed independently of semiconducting SWNTs.     

Controlling the incorporation and release of C60 in nanometer-scale hollow spaces inside single-wall carbon nanohorns

We succeeded in large-scale preparation of single-wall carbon nanohorns (SWNH) encapsulating C60 molecules in a liquid phase at room temperature using a "nano-precipitation" method, that is, complete evaporation of the toluene from a C60-SWNH-toluene mixture. The C60 molecules were found to occupy 6-36% of the hollow space inside the SWNH, depending on the initial quantity of C60. We showed that the C60 in C60@SWNHox was quickly released in toluene, and the release rate decreased by adding ethanol to toluene. Numerical analysis of the release profiles indicated that there were fast and slow release processes. We consider that the incorporation quantity and the release rate of C60 were controllable in/from SWNHs because SWNHs have large diameters, 2-5 nm.     

Electrodeposition of polyaniline in long TiO2 nanotube arrays for high-areal capacitance supercapacitor electrodes

Journal of Solid State Electrochemistry - Anodic TiO2 nanotube arrays (TNTAs) were found to be a suitable scaffold for the loading of other active materials for supercapacitors. The prepared...     

Functionalisation of carbon nanohorns

The functionalisation of single wall carbon nanohorns via 1,3-dipolar cycloaddition has been achieved, and the products have been characterised by spectroscopy, microscopy and thermogravimetry.     

Nanosized Fe3O4-modified activated carbon for supercapacitor electrodes

Nanosized Fe₃O₄-modified activated carbon composites for supercapacitor electrodes have been investigated. Structural and morphological characterizations of activated materials are carried out using X-ray diffraction and scanning electron microscopy, respectively. The electrochemical performances of the composite electrodes are evaluated by cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy. The experimental results show that the specific capacitances of the 10 wt % Fe₃O₄-modified activated carbon composite electrode (154.3 F g^?1) is highly improved compared with that of Fe₃O₄ (78.5 F g^?1) and AC (79.2 F g^?1) at the current density of 5 mA cm^?2, respectively. The charge/discharge tests show that it could retain 79.6% of its initial capacitance over 1000 cycles, suggesting its potential application for the fabrication of high-quality supercapacitors.     

MnO2/polyaniline hybrid nanostructures on carbon cloth for supercapacitor electrodes

Journal of Solid State Electrochemistry - A facile two-step strategy is developed for synthesis of MnO2/polyaniline (PANI) hybrid nanostructures on carbon cloth (CC). Vertically aligned PANI...     

Peroxidase activity of enzymes bound to the ends of single-wall carbon nanotube forest electrodes

This communication reports the first example, to our knowledge, of enzymes covalently attached onto the ends of vertically oriented single-wall carbon nanotube (SWNT) forest arrays used as electrodes. Quasi-reversible Fe^III/Fe^II voltammetry was observed for the iron heme enzymes myoglobin and horseradish peroxidase coupled to carboxylated ends of the nanotube forests by amide linkages. Results suggest that the “trees” in the nanotube forest behaved electrically similar to a metal, conducting electrons from the external circuit to the redox sites of the enzymes. Electrochemically manifested peroxidase activity of myoglobin and horseradish peroxidase attached to the SWNT forests was demonstrated, with detection limits for hydrogen peroxide in buffer solutions of ∼100 nM. These prototype SWNT-forest biosensors are easy to prepare, and enzyme layers were stable for weeks.     

Synthesis of NiCo2O4/mesoporous carbon composites for supercapacitor electrodes

Journal of Solid State Electrochemistry - In this study, nickel cobaltite/mesoporous carbon composites are synthesized by reacting CoCl2, Ni (NO3)2 and nitric acid-treated biogas slurry mesoporous...     

Preparation of activated carbon from polyaniline by zinc chloride activation as supercapacitor electrodes

Activated carbon for supercapacitor electrode was prepared from polyaniline using chemical activation with ZnCl₂. The morphology, surface chemical composition, and surface area of the as-prepared carbon materials were investigated by scanning electron microscope, atomic force microscopy, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller measurement, respectively. Electrochemical characteristics were evaluated by cyclic voltammograms, galvanostatic charge/discharge, and electrochemical impedance spectroscopy tests in 6.0 mol L⁻¹ KOH aqueous solution. The electrochemical measurements showed that ZnCl₂ activation led to better capacitive performances. The activated carbon presented a high-specific gravimetric capacitance of 174 F g⁻¹, with rectangular cyclic voltammetry curves at a scan rate of 2 mV s⁻¹, and it remained 93% even at a high scan rate of 50 mV s⁻¹. These demonstrated that activated carbon would be a promising electrode material for supercapacitors.

     

Preparation of activated carbon from polyaniline by zinc chloride activation as supercapacitor electrodes

Activated carbon for supercapacitor electrode was prepared from polyaniline using chemical activation with ZnCl₂. The morphology, surface chemical composition, and surface area of the as-prepared carbon materials were investigated by scanning electron microscope, atomic force microscopy, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller measurement, respectively. Electrochemical characteristics were evaluated by cyclic voltammograms, galvanostatic charge/discharge, and electrochemical impedance spectroscopy tests in 6.0?mol?L^?? KOH aqueous solution. The electrochemical measurements showed that ZnCl₂ activation led to better capacitive performances. The activated carbon presented a high-specific gravimetric capacitance of 174?F?g^??, with rectangular cyclic voltammetry curves at a scan rate of 2?mV?s^??, and it remained 93% even at a high scan rate of 50?mV?s^??. These demonstrated that activated carbon would be a promising electrode material for supercapacitors.