The separation of different conducting multi-walled carbon nanotubes by AC dielectrophoresis

A microfluidic device was fabricated to separate different conducting parts of multi-walled carbon nanotubes (MWCNTs). The device consists of a curing PDMS fluidic-flow chamber covered on electrode coated glass. The electrode was designed to generate non-uniform electric field by patterning via lithography. A range of frequencies with low applied voltage was utilized to induce different sign of dielectrophoresis force. Two different separation schemes based on positive and negative dielectrophoresis were employed to separate different conducting parts in unsorted MWCNTs. From Raman spectroscopy, conducting MWCNTs collected by positive dielectrophoresis showed little variation in the intensities of D-band and G-band ratio while the less conducting MWCNTs collected by negative dielectrophoresis showed decreasing intensities in these positions. The ID/IG ratio in the samples collected by both separation schemes is decreasing compared to the unsorted samples. The electric properties of the samples were characterized by a dielectrophoresis frequency spectra method. The conductance in positive dielectrophoresis collected sample is the greatest while the conductance in negative dielectrophoresis collected sample is the smallest. The trend in the conductance in unsorted and sorted samples is confirmed by current-voltage measurements.     

Magnetic separation of Fe catalyst from single-walled carbon nanotubes in an aqueous surfactant solution

We report an efficient technique to separate ferromagnetic catalyst particles from an aqueous surfactant solution of single-walled carbon nanotubes (SWNTs) by the use of a 1.3 T permanent magnet. High resolution transmission electron microscopy (HRTEM) demonstrates that SWNTs are coated with a surfactant layer that stabilises the aqueous dispersions of SWNTs. The residual quantities of Fe catalyst (∼3%) can be effectively removed from a colloid solution of SWNTs in a magnetic field while absorbance spectra of the initial and purified solutions show that the nanotube diameter distribution remains unchanged.     

The characteristics of multi-walled carbon nanotubes by a two-step separation scheme via dielectrophoresis

The unique properties of carbon nanotubes (CNTs) have led to demonstration of many applications. For good performance, the uniformity in the CNT property is very important. Among many purification schemes, dielectrophoresis (DEP) separation is non-destructive and often employed to separate CNTs. In this study, a two-step DEP separation scheme was developed to purify multi-walled carbon nanotubes (MWCNTs). This process is performed by either positive or negative DEP force followed successively by another positive or negative DEP force. The purification degree is characterized by the I_D/I_G ratio. The results show that the affinity in the first step primarily determines the overall separation degree. This indicates the operation frequency (DEP affinity) rather than the number of steps is the critical factor in DEP separation.     

Single-chirality separation of ultra-thin semiconducting arc discharge single-walled carbon nanotubes

Ultra-thin single-walled carbon nanotubes (SWCNTs) with a narrow diameter distribution are prepared with a Fe catalyst under an optimized H₂/He atmosphere by arc discharge first time. These SWCNTs were enriched with a pH–temperature controlled gel chromatography single-chirality separation process. The separation process uses the electrostatic interaction between the gel and the functional groups on the sidewalls of nanotubes, which is defined by the curvature of the C–C bond and the chiral vector (n, m). Tuning the interaction through varying the pH and temperature enables the separation of five species from the sodium dodecyl sulfate (SDS) wrapped SWCNTs in aqueous solution, namely (7, 5), (9, 4), (7, 6), (8, 6), and (8, 7), as evidenced by several optical spectroscopy techniques.     

Production of carbon nanotubes from coal

We have demonstrated that the production of carbon nanotubes in large quantities is possible with inexpensive coal as the starting carbon source by the arc discharge technique. It has been found that a large amount of carbon nanotubes of good quality can be obtained in the cathode deposits in which carbon nanotubes are present in nest-like bundles. For the growth of carbon nanotubes, the buffer gas pressure in the reactor is one of the crucial factors. The mineral matter in raw coals may also play an important part in the formation process of carbon nanotubes.     

煤基碳纳米管的制备

碳纳米管是一类新型纳米炭，具有很多潜在的应用价值，用煤为碳源制备碳纳米管可以降低其成本，本文介绍了电弧放电法和等离子体法制备煤基碳纳米管研究情况，以及相应的煤基碳纳米管生长机制。     

Synthesis of carbon nanotubes from acetone

The process of synthesizing carbon nanotubes (CNTs) using the method of catalytic gas-phase pyrolysis has been studied using acetone as a source of carbon. CNTs with outer diameters of 8–10 nm were prepared. The highest yield of the CNTs with the best quality is achieved when (Co, Mo)/MgO-Al₂O₃ catalyst is used. When (Fe, Co, Mo)/Al₂O₃ is used, the yield and quality of CNTs are lower. For comparison, CNTs obtained on the same catalysts but with propylene as the source of carbon have been investigated. It has been shown that, in this case, the best yield is achieved if (Fe, Co, Mo)/Al₂O₃ catalyst is used. According to the thermogravimetric data, CNT prepared at optimal conditions from acetone have fewer structural defects than those prepared from polypropylene. The optimal temperature and concentration conditions of the CNT synthesis from acetone have been determined. Based on the kinetic data, it has been assumed that the growth of CNTs takes place due to the ketene formed under the thermal decomposition of acetone. The ecological aspects of the CNT preparation from hydrocarbons and acetone are considered.     

The biocompatibility of carbon nanotubes

Carbon nanotubes (CNT) are well-ordered, high aspect ratio allotropes of carbon. The two main variants, single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) both possess a high tensile strength, are ultra-light weight, and have excellent chemical and thermal stability. They also possess semi- and metallic-conductive properties. This startling array of features has led to many proposed applications in the biomedical field, including biosensors, drug and vaccine delivery and the preparation of unique biomaterials such as reinforced and/or conductive polymer nanocomposites. Despite an explosion of research into potential devices and applications, it is only recently that information on toxicity and biocompatibility has become available. This review presents a summary of the performance of existing carbon biomaterials and gives an outline of the emerging field of nanotoxicology, before reviewing the available and often conflicting investigations into the cytotoxicity and biocompatibility of CNT. Finally, future areas of investigation and possible solutions to current problems are proposed.     

Vacuum annealing of carbon nanotubes produced from amorphous carbon

In the vacuum annealing of carbon nanotubes formed by the mechanical activation of amorphous carbon (from sphagnum moss), the yield of purified nanotubes is 19–75 wt %. The potential of materials derived from sphagnum moss as enteric sorbents is considered.     

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.     

Asymmetric oxygen-functionalized carbon nanotubes dispersed in polysulfone for CO2 separation

Carbon dioxide separation from flue gases is an important challenge to be faced. Membrane processes are a promising alternative to increase technical and economical constraints once the development of materials with superior characteristics are attained. Integrally asymmetric mixed matrix membranes (MMMs) were prepared by dry/wet phase inversion process of polysulfone (PSF) containing oxygen-functionalized multiwalled carbon nanotubes (MWNT-O). Fourier transform infrared (FTIR) spectroscopy confirmed the presence of MWNT-O in MMMs. Thermal gravimetric analysis (TGA) showed that MMMs are stable up to 150°C. Photomicrographs from scanning electron microscopy (SEM) revealed that MMMs consist of an asymmetric structure with a skin layer supported on a sponge-like substructure. The pore size of the support of MMMs increased with MWNT-O content from 0.4 to 0.8 wt.% and the thickness of the dense layer decreased. However, when the content of MWNT-O increased to 1 wt.%, the pore size decreased, and the dense layer increased. Therefore, MMMs changed CO₂ separation performance. For 1 wt.% MWNT-O loading compared to the neat polymer, CO₂ permeance and CO₂/N₂ selectivity was increased from 1.5 to 2.7 GPU, and from 9.5 to 14.3, respectively.     

Bulk enrichment and separation of multi-walled carbon nanotubes by density gradient centrifugation

A convenient and efficient method was developed to sort water soluble multi-walled carbon nanotubes (MWCNTs) by length using sucrose gradient centrifugation. MWCNTs with narrow length distribution were obtained.     

The role of sodium dodecyl sulfate concentration in the separation of carbon nanotubes using gel chromatography

Gel chromatography has been demonstrated as an effective method for generating separated fractions of metallic and semiconducting carbon nanotubes when starting with a heterogeneous dispersion in sodium dodecyl sulfate (SDS). The influence of the surfactant concentration in this process has been examined here for chromatographic separation using a dextran-based gel as the stationary phase. Decreasing the concentration of SDS from 4 to 0.5 wt.% caused a gradual increase in the adsorption of semiconducting nanotubes to the gel in a species-selective manner, with low concentrations of SDS (around 0.5%) found to provide the best semiconductor–metal separation. Elution using a stepwise concentration gradient was able to produce fractions of reduced diameter population from the polydisperse HiPCO starting material, where a good correlation between the concentration of elution and local bond curvature for each nanotube species was observed. Since bleaching of optical absorbance through protonation in the presence of dissolved oxygen was found to mask the presence of nanotubes with large diameters, it was deemed necessary to reverse the protonation effect through hydroxide addition in order to detect these species in optical measurements of nanotube dispersions.     

Ejection of DNA molecules from carbon nanotubes

Ejection of DNA molecule from carbon nanotubes subjected to torsion is investigated with molecular dynamics simulations. The wall of a carbon nanotube undergoes a collapse and a corresponding propagation of the collapse when a torsion loading applied to the tube is beyond a critical value. The van der Waals force between the encapsulated DNA molecule and the collapsed wall of the nanotube on one hand is a driving force to push the molecule out of the tube, while may also prevent the ejection when the onset of the collapsed wall is in front of the molecule. To ensure a complete DNA ejection, a design of a stopper, a small fixed portion on the carbon nanotube to prevent the collapsed wall from propagating in front of the DNA molecule, is developed. The effects of the environmental temperature, the torsion loading, and the size of the nanotube on the delivering process of the DNA molecule are also examined. Simulation results demonstrate the feasibility of ejection of DNA molecules from carbon nanotubes subjected to torsion and provide guidance on designs of the DNA delivery in biology and medical applications.     

洋葱状碳纳米颗粒转化为碳纳米管的研究

以热固性酚醛树脂为碳源,以硝酸铁为催化剂前驱体制备洋葱状碳纳米颗粒。采用酸化方法使洋葱状纳米颗粒变成不稳定的球冠状态,在高温下使球冠进行重新组装,转化为形状不规则的碳纳米管,实现了材料转化。并详细分析了洋葱状碳纳米颗粒形成与转化为碳纳米管的机理。