The structural evolution of thin multi-walled carbon nanotubes during isothermal annealing

Systematic isothermal annealing has been carried out on catalytic chemical vapor deposition-derived highly disordered multi-walled carbon nanotubes in order to understand the structural evolution of carbon nanotubes kinetically. After specified isothermal time the crystallographic parameters strongly depend on annealing temperature, and can be divided into three stages. Rapid structural enhancement in the range from 1800 to 2200 °C originates from the abrupt evolution of the residual catalytic compounds and is seen as partially aligned small fringes along the tube length. The lack of further structural ordering at temperatures above 2400 °C and the low activation energy (ca. 90 kJ/mol) are thought to be the result of the limited space available for further structural development considering the nano-sized multi-walled carbon nanotubes.     

Surface and structural characterization of multi-walled carbon nanotubes following different oxidative treatments

Six commonly used wet chemical oxidants (HNO₃, KMnO₄, H₂SO₄/HNO₃, (NH₄)₂S₂O₈, H₂O₂, and O₃) were evaluated in terms of their effects on the surface chemistry and structure of MWCNTs using a combination of analytical techniques. X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDX) were used to characterize the extent of surface oxidation, while chemical derivatization techniques used in conjunction with XPS allowed the concentration of carboxyl, carbonyl, and hydroxyl groups at the surface to be quantified for each MWCNT sample. Our results indicate that the distribution of oxygen-containing functional groups was insensitive to the reaction conditions (e.g., w/w% of oxidant), but was sensitive to the identity of the oxidant. MWCNTs treated with (NH₄)₂S₂O₈, H₂O₂, and O₃ yielded higher concentrations of carbonyl and hydroxyl functional groups, while more aggressive oxidants (e.g., HNO₃, KMnO₄) formed higher fractional concentrations of carboxyl groups. IR spectroscopy was unable to identify oxygen-containing functional groups present on MWCNTs, while Raman spectra highlighted the frequently ambiguous nature of this technique for measuring CNT structural integrity. TEM was able to provide detailed structural information on oxidized MWCNT, including the extent of sidewall damage for different oxidative treatments.     

多壁碳纳米管的功能化及其应用研究

首先用混合酸纯化了原料碳纳米管,然后使用有机胺对碳纳米管进行了共价功能化。拉曼光谱和元素分析结果表明,烷基胺通过共价键接合于碳纳米管表面。从热重分析结果可知,碳纳米管表面附着有质量分数为6.9%～16.4%的有机物。透射电镜照片显示,在碳纳米管的外壳上覆盖有厚度为几纳米的无定形的材料,形成核—壳结构。力学性能结果表明,加入质量分数为1%的功能化纳米碳管可使环氧树脂的断裂韧性提高35%。     

Reduction of solubilized multi-walled carbon nanotubes

In this paper, the chemical reduction of solubilized multi-walled carbon nanotubes by LiAlH₄ was investigated. The amide groups on the nanotubes could be reduced to hydroxyl groups, which was confirmed by FTIR and XPS studies. The Raman spectroscopic investigation showed that the morphology of the nanotubes did not change after the reduction.     

Ni-deposited multi-walled carbon nanotubes by electrodeposition

Multi-walled carbon nanotubes (MWNTs), having a diameter around 100-200 nm, were used as host material for electrodeposition of Ni using a Ni plating bath containing homogeneously dispersed MWNTs. Ni-deposited or coated MWNTs with a skewered dumpling shape accumulated on the Cu cathode electrode. A model representing the growth process of these electrodeposits, which possess such a skewered dumpling shape, is presented. These electrodeposits were separated easily from the cathode electrode by ultrasonic irradiation in an acetone bath to yield a Ni-deposited or coated MWNTs powder. The amount of Ni deposited on the MWNTs can be controlled by selecting the appropriate electroplating conditions.     

Synthesis and Raman characterization of boron-doped single-walled carbon nanotubes

A systematic study was carried out to dope single-walled carbon nanotube (SWNT) bundles with varying amounts of boron using the pulsed laser vaporization technique. Targets containing boron concentrations ranging from 0.5 to 10 at.% boron were prepared by mixing elemental boron with carbon paste and the Co/Ni catalysts. The laser-generated products that were obtained from these targets were characterized by high resolution transmission electron microscopy, electron energy loss spectroscopy (EELS), thermoelectric power (TEP) measurements, and Raman scattering experiments. Electron microscopy and Raman studies revealed that the presence of various levels of boron concentration in the target strongly affected the products that were prepared. SWNTs were found in the products prepared from targets containing up through 3 at.% boron, and high resolution EELS estimated that less than 0.05-0.1 at.% boron is present in the SWNT lattice. The absence of SWNT bundles in the products derived from targets containing more than 3 at.% boron implies that the presence of excess boron in the carbon plume severely inhibits the carbon nanotube growth. The overall effect of the boron incorporation primarily leads to: (i) a systematic increase in intensity of the disorder-induced band (D-band) upon boron doping, with increasing D-band intensity observed for higher doping levels, (ii) a systematic downshift in the G′-band frequency due the relatively weaker C-B bond, and (iii) a non-linear variation in the RBM and G′-band intensities which is attributed to shifts in resonance conditions in the doped tubes. Resonant Raman spectroscopy thus provides large changes in the intensity of prominent features even when the dopant concentration is below the detectable limit of EELS (0.05-0.1 at.%). Thermoelectric power data also provide complementary evidence for the presence of a small boron concentration in the SWNT lattice which transforms the SWNTs into a permanently p-type material.     

Translocation and fate of multi-walled carbon nanotubes in vivo

Carbon nanotube (CNT) mediated delivery system of drugs etc. has currently aroused a large interest. Because the delivery system will be ultimately introduced into the human body, the information about the in vivo biological behavior and consequences of CNTs becomes very important. Here, using [¹⁴C-taurine]-multi-walled CNTs (MWCNTs) as tracers, we show the biodistribution and translocation pathways of MWCNTs in mice by three different routes. After mice were exposed by intravenous injection, MWCNTs predominately accumulated in liver and retained for long time. Transmission electron micrographs clearly show the remarkable entrapment of MWCNTs in hepatic macrophages (Kupffer cells). The biological index examinations indicate low liver acute toxicity of MWCNTs. Some favorable aspects of MWCNTs being used as a drug nanovehicle are also discussed.     

Image analysis characterization of multi-walled carbon nanotubes

An original image analysis method is presented to characterize multi-walled carbon nanotubes from transmission electron microscopy images. The analysis is performed in three steps: (i) image preprocessing in order to isolate the nanotubes from the background, (ii) image segmentation, aiming at keeping only the measurable sections of nanotubes, and finally (iii) tube characteristics measurement. The measurement is based on a Lambert-like electron absorption law and is performed on the original gray level image itself. Two geometrical and one physical characteristics are determined for each tube, namely, its outer and inner radius and a linear electron absorption coefficient. The method is illustrated by comparing a pristine and an annealed carbon nanotube samples. The compaction of the tube walls during annealing is shown to result from a lowering of the external radius while the inner radius is left unchanged.     

Oxidative biodegradation of single- and multi-walled carbon nanotubes

In this study we compare the biodegradation of both single-walled (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) using two different oxidative conditions. In particular, we demonstrate that oxidized multi-walled carbon nanotubes are highly degraded, although not to completeness when treated with horseradish peroxidase (HRP) in the presence of hydrogen peroxide.     

Characterization and application of hydrophobin-dispersed multi-walled carbon nanotubes

Hydrophobins are amphiphilic proteins with high surface activity, which can readily adsorb on interfacial surfaces, especially on hydrophobic surfaces. Based on their properties, we used the class I hydrophobin isolated from Grifola frondosa (HGFI) to disperse multi-walled carbon nanotubes (MWCNTs) in water. MWCNTs could be effectively dispersed by 30-min sonication in a 0.1 mg/ml HGFI solution. Optical absorption and transmission electron microscopy provide evidence for individually stable dispersed MWCNTs. X-ray photoelectron, Fourier transform infrared, and Raman spectroscopies and thermogravimetric analysis suggest that HGFI can non-covalently bind to MWCNTs through hydrophobic interaction, rendering them hydrophilic. A quartz crystal microbalance and immunological sandwich assay were used to demonstrate that the HGFI-coated MWCNTs can be used to immobilize human immunoglobulin G in solution.     

多壁碳纳米管的有机修饰与表征

为了增加多壁碳纳米管(MWCNTs)表面活性,通过浓H2SO4和浓HNO3处理过的MWCNTs与SOCl2回流进而与合成的N-乙基-3,6-二氨基咔唑反应,得到了有机修饰的MWCNTs.用傅立叶变换红外(FTIR)光谱对有机修饰的MWCNTs结构进行研究.研究结果结构表明:有机修饰的MWCNTs红外光谱在1617和16...     

Low-temperature catalytic oxidation of multi-walled carbon nanotubes

When in a pure form, carbon nanotubes are known to be stable in air up to ∼800 K making them attractive for a large variety of applications. In this work, we report a significant decrease of ignition temperature (in some cases occurring at ∼500 K) and a reduction in the apparent activation energy for oxidation in air as a result of impregnation with nanoparticles (<2 nm) of metal (Pt, Pd, Ni and Co) acetylacetonates or by decoration with corresponding oxides. Surprisingly, defects introduced by partial oxidation of the carbon nanotubes do not in practice have any influence on the enhancement of further oxidation. Reduction temperatures of metal oxides with H₂ were close to those of other carbon supported catalyst materials. However, the carbon nanotubes showed a tendency for low temperature gasification in the presence of hydrogenation catalyst metals (Pt, Pd).     

Thermal oxidative cutting of multi-walled carbon nanotubes

Commercially available, multi-walled carbon nanotubes grown by CVD are usually inherently entangled, but can be separated by cutting. However, most cutting methods both cause damage to the nanotubes and involve a lengthy work-up procedure. The use of abrupt, repeated exposure to oxidising conditions in air proved to be an efficient (68% yield) means of producing material with open ends, moderate functionalisation, and enhanced solvent dispersibility; the average lengths were reduced from over 5 μm to approximately 650 nm. Additionally, the character of the surface oxides can be tuned to have either an acidic or basic character by using a simple thermal treatment. These approaches could be deliberately integrated into conventional CVD processes, but also have implications for the products of standard nanotube syntheses. Raman spectroscopy and electron microscopy were used to study the impact of cutting on the intrinsic graphitic structure and the length distribution. X-ray photoelectron spectroscopy was used to determine the extent of functionalisation. The cut carbon nanotubes were dispersed in dimethylformamide (DMF), a Lewis basic solvent, and chloroform, a Lewis acidic solvent, using mild sonication. Through the use of an experimentally determined extinction coefficient (ε = 35.10 ml mg⁻¹ cm⁻¹), the relative dispersibility of the cut and functionalised carbon nanotubes in DMF and chloroform was determined.     

Coalescence of single-walled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments

Electric arc-discharge single-wall carbon nanotubes are annealed between 1600 and 2800 °C under argon flow. Their stability and evolution are studied by coupling TEM, X-ray diffraction and Raman spectroscopy. The first modifications appear at 1800 °C with a significant decrease of the crystalline order. It is due to SWNTs coalescence leading to smaller bundles but with an increase of the tube diameters from 2 to 4 nm. From 2200 °C, SWNTs progressively disappear to the benefit of MWNTs having at first two to three carbon layers then reaching 7 nm external diameter. The possible mechanisms responsible for the SWNTs coalescence and instability and their transformation in MWNTs are discussed.     

Synthesis and characterization of double-walled carbon nanotubes from multi-walled carbon nanotubes by hydrogen-arc discharge

Double-walled carbon nanotubes (DWNTs) were synthesized in a large scale by a hydrogen arc discharge method using graphite powders or multi-walled carbon nanotubes/carbon nanofibers (MWNTs/CNFs) as carbon feedstock. The yield of DWNTs reached about 4 g/h. We found that the DWNT product synthesized from MWNTs/CNFs has higher purity than that from graphite powders. The results from high-resolution transmission electron microscopy observations revealed that more than 80% of the carbon nanotubes were DWNTs and the rest were single-walled carbon nanotubes (SWNTs), and their outer and inner diameters ranged from 1.75 to 4.87 nm and 1.06 to 3.93 nm, respectively. It was observed that the ends of the isolated DWNTs were uncapped and it was also found that cobalt as the dominant composition of the catalyst played a vital role in the growth of DWNTs by this method. In addition, the pore structures of the DWNTs obtained were investigated by cryogenic nitrogen adsorption measurements.