Solid-phase synthesis of multi-walled carbon nanotubes from butadiynyl-ferrocene-containing compounds

Multi-walled carbon nanotubes (MWCNTs) have been synthesized from novel butadiynyl-ferrocene-containing compounds. The formation of the MWCNTs occurs in the solid phase at ambient pressure in a typical high-temperature furnace. Heat treatment of the various compounds to temperatures up to 1300 °C under atmospheric pressure resulted in the decomposition of the ferrocene units and the formation of Fe nanoparticles in the polymeric-to-carbon nanoparticle-to-carbon nanotube compositions. The Fe atoms, clusters, and/or nanoparticles are the key to the formation of the carbon nanotubes. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy studies show the presence of a large quantity of MWCNTs in the carbonaceous solid residue.     

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.     

Effect of multi-walled carbon nanotubes on methane hydrate formation

1 m³ of methane hydrate can be decomposed into a maximum of 216 m³ of methane gas under standard conditions. If these characteristics of hydrates are utilized in the opposite sense, natural gas can be fixed into water in the form of a hydrate solid. Therefore, the use of hydrates is considered to be a great way to transport and store natural gas in large quantities. However, when methane hydrate is formed artificially, the amount of gas that is consumed is relatively low, due to the slow reaction rate between water and methane gas. Therefore, for practical purposes in the application, the present investigation focuses on increasing the rate of formation of the hydrate and the amount of gas consumed by adding multi-walled carbon nanotubes (MWCNTs) to pure water. The results show that when 0.004 wt% of multi-walled carbon nanotubes was added to pure water, the amount of gas consumed was about 300% higher than that in pure water and the hydrate formation time decreased at a low subcooling temperature.     

Growth mechanism of vapor phase CVD-grown multi-walled carbon nanotubes

The formation mechanisms involved in the growth of carbon nanotubes (CNTs) by spray pyrolysis was studied. Both iron and nickel were used as catalysts for growth, and nanotubes were also produced using thermal chemical vapor deposition for comparison. Transmission electron microscopy was used to analyze the encapsulated metal catalyst particles found within the tubes, and the dimensions and location of these particles was recorded. CNTs grown by spray pyrolysis were found to have encapsulated particles in both the middle and end of tubes, with large length to diameter ratios. As a result of these observations, it is concluded that nanotubes grown using spray pyrolysis are formed via an open-ended, root growth mechanism. Additionally, the presence of multiple, high aspect ratio particles within single tubes is explained by an additional growth theory. During the continued growth of these CNTs, metal atoms or nanoscale metal catalyst particles deposit in the open ends of growing tubes, forming new particles and helping to prevent tube closure. CNTs grown with thermal CVD did not contain similar elongated particles or particles along the middle of the tubes, indicating that this new growth mechanism is only applicable in the case of tubes grown via spray pyrolysis or other vapor phase CVD growth methods.     

High-yield synthesis of multi-walled carbon nanotubes from graphite by molten salt electrolysis

This article presents a fundamentally redesigned molten salt electrolytic method of converting graphite directly into MWCNTs. By using optimized process parameters in terms of graphite microstructure, temperature and polarization potential, and by implementing a novel type of process control that changes the polarity of the graphite electrodes during electrolysis, it has become possible to produce gram quantities of a carbonaceous material that contains approximately 70% of MWCNTs in the form of extended agglomerates. The results indicate the true potential of this hitherto underrated method and suggest its suitability for MWCNT production at a larger scale.     

Polypropylene as a carbon source for the synthesis of multi-walled carbon nanotubes via catalytic combustion

Multi-walled carbon nanotubes (MWCNTs) were efficiently synthesized by catalytic combustion of polypropylene (PP) using nickel compounds (such as Ni₂O₃, NiO, Ni(OH)₂ and NiCO₃ · 2Ni(OH)₂) as catalysts in the presence of organic-modified montmorillonite (OMMT) at 630-830 °C. Morphologies of the sample undergoing different combustion times were observed to investigate actual process producing MWCNTs by this method. The obtained MWCNTs were characterized by X-ray diffraction (XRD), transmission electron microscope and Raman spectroscopy. The yield of MWCNTs was affected by the composition of PP mixtures with OMMT and nickel compounds and the combustion temperature. The proton acidic sites from the degraded OMMT layers due to the Hoffman reaction of the modifiers at high temperature played an important role in the catalytic degradation of PP to supply carbon sources that are easy to be catalyzed by nickel catalyst for the growth of MWCNTs. The XRD measurements demonstrated that the nickel compounds were in situ reduced into the Ni(0) state with the aid of hydrogen gas and/or hydrocarbons in the degradation products of PP, and the Ni(0) was really the active site for the growth of MWCNTs. The combination of nickel compounds with OMMT was a key factor to efficiently synthesize MWCNTs via catalytic combustion of PP.     

Single-step synthesis of germanium nanowires encapsulated within multi-walled carbon nanotubes

We report the single-step synthesis of Ge nanowires encapsulated within multi-walled carbon nanotubes (MWCNTs) from a phenyltrimethylgermane (C₆H₅Ge(CH₃)₃) precursor, using a simple chemical vapor deposition (CVD) method. The MWCNT/germanium nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) measurements. TEM analysis reveals that the nanowires consist of well crystallized Ge cores which are completely encapsulated by the sheath-like MWCNTs, the latter corresponding to a layer thickness of 5-10 nm. SEM images, corresponding to various stages of nanowire growth, indicate that MWCNT growth occurs at Ge nanoparticles and that the growing MWCNTs carry Ge as nanowires away from the nanoparticles. By optimizing the CVD parameters, nanowires can be produced with uniform length and diameter in the range 6-10 μm and 200-300 nm, respectively.     

Dimensional control of multi-walled carbon nanotubes in floating-catalyst CVD synthesis

Dimensional control in CVD synthesis of MWNT’s is significant and critical to a number of different applications. This study examines the dimensional effect of a number of synthesis variables on the products of floating-catalyst CVD, including catalyst concentration in the feedstock, nanotube growth time, and deposition substrate selection. Extensive diameter surveys are performed by TEM and compared with results from thermo-gravimetric analyses to Raman spectroscopy, offering a novel dimensional analysis of nanotubes grown by FC-CVD methods. CNT diameters are inversely proportional to the catalyst concentration with weak correlation over the range examined and are directly proportional to growth time. Results are combined with prior art to develop a new theory regarding catalyst particle formation over a range of catalyst concentrations. Carbon deposition occurs in two stages, the first characterized by accelerating deposition and increases in CNT diameter and length, the second by etching of the array and carbon deposition at a constant rate. Deposition substrates interact directly with the catalysts to strongly influence the resulting nanotube diameters, based upon the mobility of the catalysts on the substrate surface.     

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.     

表面活性剂分散多壁碳纳米管机理及性能评价

十二烷基硫酸钠(SDS)、十六烷基三甲基溴化铵(CTAB)、壬基酚醚璜基琥珀酸单酯二钠盐(HT A-103)、辛基酚聚氧乙烯醚(OP-10)分别与质量分数为0.1％的多壁碳纳米管(MWCNTs)复配,制备出4种表面活性剂-MWCNTs分散体系.利用UV-Vis光谱、Zeta电位及SEM对其分散性进行了评价,分析了各类表...     

Catalytic metal-free formation of multi-walled carbon nanotubes in atmospheric arc discharge

Metallic-impurity-free, nano-sized, short multi-walled carbon nanotubes (MWCNTs) in the form of a tape have been synthesized using stabilized arc discharge under atmospheric conditions. The long, thin tape consisted of crystalline MWCNTs exhibiting indiscernibly blurred interior lattice images and a narrow, hollow core, as well as small and large nanoparticles. The disordered interior regions of the tubes were enlarged into hollow cores by thermal treatment at 2000 °C, suggesting that the elongated tubes crystallize via a super-cooling process. The proposed macroscopic model for the growth process of the tubes in the arc resembles the fiber formation of a recently reported electrospinning process; thermally activated carbon ion and vapor create viscous carbon clusters, and the built-up charge in the clusters leads to the elongation into tubules.     

Template synthesis of hollow carbon spheres anchored on carbon nanotubes for high rate performance supercapacitors

A carbon material consisting of hollow carbon spheres anchored on the surface of carbon nanotubes (CNT–HCS) has been synthesized by an easy chemical vapor deposition process using a CNT–MnO₂ hybrid as template. An electrode made of this material exhibits a maximum specific capacitance of 201.5 F g⁻¹ at 0.5 A g⁻¹ and excellent rate performance (69% retention ratio at 20 A g⁻¹). It has impressive cycling stability with 90% initial capacitance retained after 5000 cycles at 5 A g⁻¹ in 6 mol L⁻¹ KOH. Symmetric supercapacitors based on CNT–HCS achieve a maximum energy density of 11.3 W h kg⁻¹ and power density of 11.8 kW kg⁻¹ operated within a wide potential range of 0–1.6 V in 1.0 mol L⁻¹ Na₂SO₄ solution.     

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.