Control of one-dimensional transport in multi-wall carbon nanotubes by wall destruction

We have investigated the damage in multi-wall carbon nanotubes (MWNTs) destroyed by electrical breakdown and focused ion beam bombardment (FIBB). The transport properties of a MWNT destroyed by electrical breakdown have been compared with those of a MWNT destroyed by FIBB. Also the Tomonaga–Luttinger transport (TLT) model has been applied to each type of destroyed MWNT. The MWNT destroyed by FIBB showed TLT behavior because of the weak destruction of the remaining walls. However, in the case of MWNTs destroyed by electrical breakdown, three-dimensional variable-range hopping (VRH) was observed in the low temperature transport. This suggests that the local damage has been caused by strong breakdown. There exists a clear difference between the effects of electrical breakdown and FIBB. Wall destruction by FIBB could be applied to control the one-dimensional transport of MWNTs.     

Fabrication of high-performance supercapacitors based on transversely oriented carbon nanotubes

High-performance supercapacitors with organic electrolyte 1 M TEABF₄ (tetraethyl ammonium tetrafluoroborate) in PC (propylene carbonate) were fabricated and tested, based on multiwall carbon nanotubes (MWNTs) deposited by electrophoresis on three types of alternative substrates: aluminium foil, ITO (indium tin oxide) coated PET (polyethylene terephthalate) film and PET film. In all cases, SEM (scanning electron microscopy) and STEM (scanning transmission electron microscopy) micrographs demonstrated that protruding, transversely oriented MWNT structures were formed, which should increase the transverse conductivity of these MWNT electrodes. The best supercapacitor cell of MWNT electrodes deposited on aluminium foil displayed good transverse orientation of the MWNT structures as well as an in-plane MWNT network at the feet of the protruding structures, which ensured good in-plane conductivity. Capacitor cells with MWNT electrodes deposited either on ITO-coated PET film or on PET film demonstrated lower but still very good performance due to the high density of transversely oriented MWNT structures (good transverse conductivity) but some in-plane inhomogeneities. Capacitor cells with drop-printed MWNTs on aluminium foil, without any transverse orientation, had 16–30 times lower specific capacitance and 5–40 times lower power density than the capacitor cells with the electrophoretically deposited MWNT electrodes.     

Supercritical fluid-assisted synthesis of a carbon nanotubes-grafted biocompatible polymer composite

A nanocomposite consisting of multi-wall nanotubes (MWNTs) grafted with a biocompatible polymer poly(2-hydroxyethyl methacrylate) was prepared by in situ polymerization in supercritical carbon dioxide. The surface of the MWNTs was first surface modified with hydroxyl groups in the solution of KMnO₄ and a phase-transfer catalyst. MWNT-OH was then functionalized with vinyl groups using a silane coupling agent, γ-methacryloxypropyltrimethoxysilane. The silane groups can improve the dipersion of MWNTs in supercritical carbon dioxide, while the terminal vinyl groups help fabricate polymer chains on the MWNT surface. The as-synthesized products were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermo-gravimetric analysis. The SEM and TEM images showed that the nanotubes were well coated with the polymer shell. The composite had higher thermal stability than the pure polymer and dispersed well in methanol. This biocompatible polymer composite was prepared using a green method and is expected to be useful as a biomaterial composite with potential applications in the biological field.     

浓硝酸处理前后多壁碳纳米管的荧光特性研究

对浓硝酸处理前后多壁碳纳米管的荧光特性进行了研究,结果发现多壁碳纳米管在浓硝酸处理前后都能产生荧光;与浓硝酸处理前相比,浓硝酸处理后的多壁碳纳米管的荧光有所增强,且荧光谱峰向短波长方向发生蓝移。碳纳米管产生的荧光与碳纳米管在其缺陷处捕获激发光的能量以及最低空轨道和最高占有轨道之间的能隙有关。多壁碳纳米管在浓硝酸处理前后都有缺陷,因而都能在其缺陷处捕获激发光能量而产生荧光;浓硝酸处理引起多壁碳纳米管的缺陷增多、捕获激发光能量增加因而荧光也随之增强。碳纳米管的最低空轨道和最高占有轨道之间的能隙随其长度减小而增大,浓硝酸处理引起多壁碳纳米管被削短导致其最低空轨道和最高占有轨道之间的能隙增大,因而引起多壁碳纳米管的荧光谱峰向短波方向蓝移。     

Preparation and characterization of biocompatible magnetic carbon nanotubes

Magnetic carbon nanotubes consisting of multi-wall carbon nanotubes (MWNTs) core and Fe₃O₄ shell were successfully prepared by in situ thermal decomposition of Fe(acac)₃ or FeCl₃ or Fe(CO)₅ in 2-pyrrolidone containing acid treated MWNTs at 240 °C with the protection of nitrogen gas. The samples were characterized by TEM, XRD, SEAD, XPS and superconducting quantum interference device. Also, their biocompatibility was compared with naked carbon nanotubes. The results showed that after coated with Fe₃O₄ nanoparticles, the obtained magnetic carbon nanotubes show superparamagnetic characteristic at room temperature, and their blocking temperature is about 80 K. The magnetic properties of the nanotubes are relevant to the content of magnetic particles, increasing content of magnetic nanoparticles leads to higher blocking temperature and saturation magnetization. The results of antimicrobial activities to bacterial cells (Escherichia coli) showed that the MWNTs have antimicrobial activity, while the magnetic nanotubes are biocompatible even with a higher concentration than that of MWNTs.     

Multiwalled carbon nanotubes mass-produced by dc arc discharge in He–H2 gas mixture

Uniform cathode deposits (longer than 15 mm), containing multiwalled carbon nanotubes (MWNTs) inside, were produced by dc arc discharge evaporation with a computer-controlled feeder of a pure-carbon electrode without a metal catalyst in a He–H₂ gas mixture. The purification of MWNTs was carried out to remove amorphous carbon and carbon nanoparticles. High-resolution transmission electron microscopy observations and Raman scattering studies show that the MWNTs possess a high crystallinity and a mean outermost diameter of ∼ ∼10 nm. It has been confirmed that the current density in the electron field emission from a purified MWNT mat can reach 77.92 mA/cm², indicating that the purified MWNTs are a promising candidate electron source in a super high-luminance light-source tube or a miniature X-ray source.     

Preparation of Pt–CeO2/MWNT nano-composites by reverse micellar method for methanol oxidation

We report the preparation of Pt–CeO₂ nanoparticles on the multi-walled carbon nanotubes (MWNTs) by a reverse micellar method. Transmission electron microscopy (TEM) analysis indicated that well-dispersed small Pt–CeO₂ nanoparticles were formed on the MWCNTs. X-ray diffraction (XRD) analysis confirmed the formation of the Pt–CeO₂ nanoparticles on the MWNTs. Cyclic voltammetry (CV) results demonstrated that the Pt–CeO₂/MWNT exhibited a higher methanol oxidation than did the Pt/MWNT catalyst. The CO stripping test showed that CeO₂ can make CO stripped at a lower potential, which is helpful for CO and methanol electro-oxidation.     

磁场辅助热处理金属化碳纳米管场发射性能

利用化学镀方法对多壁碳纳米管(multi-walled carbon nanotubes,MWNTs)表面金属化镀镍(MWNTs/Ni),采用丝网印刷制备MWNTs/Ni场发射阴极,并在磁场辅助下热处理所得阴极,研究磁场辅助热处理对MWNTs/Ni阴极的场发射性能的影响,经300mT磁场辅助热处理的MWNTs/Ni的场发射阴极开启场强约为0.80V·μm~(-1),场增强因子β约为16068,对单根MWNTs/Ni在磁场中的受力情况进行建模分析,实验结果表明:磁场辅助热处理有助于提高MWNTs/Ni在阴极表面的直立分布,提高了MWNTs/Ni的场发射性能。     

Critical Exponents of Gelation and Conductivity in Polyacrylamide Gels Doped by Multiwalled Carbon Nanotubes

Polyacrylamide (PAM) doped by multiwalled carbon nanotube (MWNT) gels were prepared with different amounts of MWNTs varying in the range between 0.1 and 15 wt%. The PAM–MWNT composite gels were characterized by the steady state fluorescence technique (SSF). The alternative electrical conductivity (AC) of PAM–MWNT composite gels was measured by the dielectric spectroscopy technique. Observations around the gel point, t _gel for PAM–MWNTs composite gels showed that the gel fraction exponent β obeyed the percolation result. The critical exponent r of AC electrical conductivity for the composite PAM–MWNT gel was also measured and found to be about 2.0, which agrees with a random resistor network.     

Dynamic column buckling of multi-walled carbon nanotubes under axial impact load

This paper studies the dynamic column buckling of multi-walled carbon nanotubes (MWNTs) under axial impact load. The analysis is based on the continuum mechanics model and a simplified model for the van der Waals forces between adjacent layers. By introducing initial imperfections for MWNTs and applying the method of preferred mode, a buckling condition is derived for the buckling load and associated buckling mode. In particular, explicit expressions are obtained for double-walled carbon nanotubes (DWNTs). Finally, numerical calculations are worked out for a DWNT and a five-layer MWNT with different length-to-radius ratios.     

Raman spectroscopy and field-emission properties of CVD-grown carbon-nanotube films

Carbon-nanotube films are very efficient cathodes for field-emission devices. This study presents a comprehensive comparison between structural, spectroscopic and field-emission properties of films of aligned and non-aligned multi-wall nanotubes (MWNTs) which are grown by thermal chemical vapour deposition. Three types of films are investigated: vertically aligned MWNTs with clean and coated nanotube side walls as well as non-aligned MWNT films. Raman spectra taken on the aligned MWNT films consist of many lines of first-, second- and third-order signals. Several lines are reported here for the first time for MWNTs. The presence of the surface coating leads to a decrease and broadening of the higher-order signals as well as an increase in the disorder-induced contributions in the first-order regime. The aligned MWNT films have excellent field-emission properties with very high emission current densities and low turn-on and threshold fields. The presence of a surface coating has no impact on the efficiency of the field-emission process. Films of non-aligned MWNTs show considerably reduced electron-emission current densities and larger critical fields. Received: 25 April 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Transparent carbon nanotube-based driving electrodes for liquid crystal dispersion display devices

We report the fabrication of transparent rigid or flexible liquid crystal dispersion displays driven by electrodes prepared from coating inert substrates with stable aqueous suspensions of single-walled carbon nanotubes (SWNTs) or multiple-walled carbon nanotubes (MWNTs). The SWNT-coated electrodes exhibit a dense nanotube network with a sheet resistivity ( 25 k) about one decade lower than that of their MWNT counterparts. The electro-optical performance of the SWNT-based devices is at least equivalent to that of an indium tin oxide-based display, whereas those driven by MWNT electrodes operate at slightly higher threshold and saturation voltages. The novel, flexible display devices can be repeatedly bent without any noticeable loss of physical characteristics. PACS 42.79.Kr; 61.46.+w; 81.07.De     

Covalent attachment of polystyrene on multi-walled carbon nanotubes via nitroxide mediated polymerization

A new method to attach polymers on carbon nanotubes has been studied. We used nitroxide mediated polymerization (NMP) to graft polystyrene from multi-walled carbon nanotubes (MWNTs). Carboxyl groups on MWNTs were activated with thionyl chloride (SOCl₂) to give acyl chloride derivative (MWNT-COCl). NMP initiator was introduced to MWNT by esterification of 1-hydroxy-2-phenyl-2-(2′,2′, 6′, 6′-tetramethyl-1′-piperidinyloxy)ethane (HO-PE-TEMPO) with acyl chloride groups of MWNTs. The obtained MWNT-PE-TEMPO was used for initiation of bulk polymerization of styrene, yielding polystyrene-grafted MWNTs (MWNT-g-pSt). The resulting composites of MWNT-g-pSt were analyzed by TEM, SEM, FT-IR and TGA.     

Simulation and fabrication of carbon nanotubes field emission pressure sensors

A novel field emission pressure sensor has been achieved utilizing carbon nanotubes (CNTs) as the electron source. The sensor consists of the anode sensing film fabricated by wet etching process and multi-wall carbon nanotubes (MWNTs) cathode in the micro-vacuum chamber. MWNTs on the silicon substrate were grown by thermal CVD. The prototype pressure sensor has a measured sensitivity of about 0.17-0.77 nA/Pa (101-550 KPa). The work shows the potential use of CNTs-based field-emitter in microsensors, such as accelerometers and tactile sensors.     

Functional multi-walled carbon nanotube/polyaniline composite films as supports of platinum for formic acid electrooxidation

Embedding of carbon nanotubes in conducting polymeric matrices for various nanocomposites material is now a popular area. In this article, a concise chemical method has been described for the preparation of homogeneous nanocomposite of multi-walled carbon nanotube (MWNT)/polyaniline (PANI) by electrochemical codeposition. For this we functionalized the MWNTs via the diazotization reaction. This helped to disperse the nanotubes in aniline. The composite films were dispersed Pt by electrodeposition technique. The presence of MWNTs and platinum in the composite films was confirmed by XRD analysis and transmission electron microscopy (TEM). Four-point probe investigations revealed that the MWNT/PANI composite films exhibited a good conductivity. Cyclic voltammograms (CV) showed that Pt-modified MWNT/PANI composite films perform higher electrocatalytic activity and better long-term stability than Pt-modified pure PANI film toward formic acid oxidation. The results imply that the MWNT/PANI composite films as a promising support material improves the electrocatalytic activity for formic acid oxidation greatly.     

Electron field emission from magnetic nanomaterial encapsulated multi-walled carbon nanotubes

The present work describes the field emission characteristics of nanoscale magnetic nanomaterial encapsulated multi-walled carbon nanotubes (MWNTs) fabricated over flexible graphitized carbon cloth. Ni/MWNTs, NiFe/MWNTs and NiFeCo/MWNTs have been synthesized by catalytic chemical vapor decomposition of methane over Mischmetal (Mm)-based AB₃ (MmNi₃, MmFe_1.5Ni_1.5 and MmFeCoNi) alloy hydride catalysts. Metal-encapsulated MWNTs exhibited superior field emission performance than pure MWNT-based field emitters over the same substrate. The results indicate that a Ni-filled MWNT field emitter is a promising material for practical field emission application with a lowest turn-on field of 0.6 V/μm and a high emission current density of 0.3 mA/cm² at 0.9 V/μm.     

The synthesis of multi-walled carbon nanotubes (MWNTs) by catalytic pyrolysis of the phenol-formaldehyde resins

A series of carbon nanomaterials, particularly multi-walled carbon nanotubes (MWNT), are obtained as products from catalytic pyrolysis of the cross-linked phenol-formaldehyde resins with different ferrocene under inert atmosphere. The morphology and structure of the samples were evaluated by TEM and XRD techniques. CNTs morphology is dependent on the iron nanoparticles and their forms (Fe, Fe₃C) resulted from ferrocene decomposition. The amount of nanotubes increases with iron content released from ferrocene catalyst during the pyrolysis process. Fe₃C nanoparticles drive the nucleation and the growth of carbon nanotubes during the pyrolysis process. Long (up to microns) well-defined MWNTs with small defects, ropes and disordered carbon are representatives in the pyrolyzed resins composition.     

Hydrogen storage in multi-wall carbon nanotubes using samples up to 85 g

Hydrogen storage in carbon nanotubes (CNTs) is investigated at ambient temperature and pressures of 0–12 MPa, using 35–85 g multi-wall carbon nanotube (MWNT) samples that were synthesized in a nano-agglomerate fluidized bed reactor. The volume of hydrogen gas released by the CNTs was measured by a volumetric flow meter. The capability of H₂ storage in the CNT samples of mass of up to 85 g can be obtained with a precision of 0.01 wt.%. MWNTs with average diameters ranging from 10–30 nm and were pretreated using nitric acid or a sodium hydroxide solution wash and a high temperature treatment. The influence of the hydrogen pressure, hydrogen storage time and treatment method were studied. All data show that the amount of hydrogen released by the MWNTs at room temperature is no more than 0.30 wt.%, while hydrogen released by MWNT at 77 K can reach 2.27 wt.%. PACS 61.46.+w; 68.43.-h; 84.60.Ve     

Electronic properties and field emission of carbon nanotube films treated by hydrogen plasma

Large-scale and very even multi-wall carbon nanotube (MWNT) films have been obtained at room temperature by an electrophoresis deposition technique. The characterization, by means of scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and micro-Raman spectroscopy, shows that the MWNTs with hydrogen-plasma (HP) treatment are covered by onion-like nanolumps, and three-dimensional multiple-way-connected nanotube webs are formed. The electronic property of the treated MWNT film is converted from semiconducting to metallic. The field-emission test indicates that the HP-treated MWNT film has a low threshold of 1.1 V/m (at 0.1 A/cm²), a high emission light spot density of about 10⁵ cm^-2, and a stable and suitable emission current. The conversion mechanism of the treated carbon nanotube structure and the reason for the change of the electronic and field-emission characteristics of the MWNT films are discussed. PACS 81.07.De; 82.33.Xj; 85.45.Db