Plasma-Assisted Synthesis of Carbon Nanotubes

The application of plasma-enhanced chemical vapour deposition (PECVD) in the production and modification of carbon nanotubes (CNTs) will be reviewed. The challenges of PECVD methods to grow CNTs include low temperature synthesis, ion bombardment effects and directional growth of CNT within the plasma sheath. New strategies have been developed for low temperature synthesis of single-walled CNTs based the understanding of plasma chemistry and modelling. The modification of CNT surface properties and synthesis of CNT hybrid materials are possible with the utilization of plasma.     

Unzipping carbon nanotubes into nanoribbons upon oxidation: a first-principles study

First-principles calculations are performed to investigate the unzipping mechanism of carbon nanotubes (CNTs) into narrow graphene nanoribbons (GNRs) upon oxidation. By treating possible adsorptive structures, we found that, upon further oxidation, epoxy pairs tear the CNT up with an initial energy barrier of 0.59 eV (armchair) and 0.60 eV (zigzag), and the following steps of unzipping CNT become much easier because of the stress induced by the carbonyl pair. However, for zigzag CNTS, the unzipped edge structures of nanoribbons cannot be controlled, because of the similar stability of different oxidation process, which means zigzag CNTs should be avoided in producing high-quality GNRs.     

Stress simulation of carbon nanotubes in tension and compression

Based on a continuum shell model, a structural mechanics approach is presented to simulate stress-strain behavior of carbon nanotubes (CNTs). The nanoscale continuum theory is established to directly incorporate the Morse potential function into the constitutive model of CNTs. According to the present model, the mechanical properties of both zigzag and armchair tubes are investigated. The result shows that the atomic structures of CNTs have a significant influence on the stress-strain behavior. The armchair zigzag tube exhibits larger stress-strain response than the zigzag tube under tensile loading, but its relationship turns over between the tension and compression deformations. The theoretical approach supplies a set of very simple formulas and able be serve as a good approximation on the mechanical properties for CNTs.     

乙醇裂解制备碳纳米管及其生长机理研究

以乙醇作为碳源,Mo-Co/C和Mo-Fe/C作为催化剂,采用化学气相沉积法,高温裂解乙醇制备碳纳米管.利用SEM和TEM对碳纳米管形貌和结构进行表征.结果表明:乙醇在催化剂Mo-Co/C和Mo-Fe/C裂解产生的碳纳米管遵循顶部生长机理.并建立了乙醇制备碳纳米管的生长模型.     

Vinyl–carbon nanotubes for composite polymer materials

We report herein a simple method for attaching vinyl groups onto the sidewalls of carbon nanotubes (CNTs) and the application of vinyl–carbon nanotubes (CNT–C?C) in fabricating polymer composites. The synthesis of CNT–C?C was monitored with IR spectroscopy, Raman spectroscopy, and thermogravimetric analysis. The obtained CNT–C?C showed good compatibility with the in situ polymerization of poly(methyl methacrylate) (PMMA) and exhibited no tendency for phase separation in the final composite. A transmission electron microscopy study revealed a uniform coating on the CNT–C?C tubes, indicating good grafting efficiency of PMMA. The uniform dielectric PMMA coating was responsible for the lower electrical conductivity of the CNT–C?C/PMMA composites versus that of the CNTs without vinyl modification. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Carbon nanotube-based nanocomposites and their applications

The purpose of the current review article is to present a compherensive understanding regarding pros and cons of carbon nanotube–related nanocomposites and to find ways in order to improve the performance of nanocomposites with new designs. Nanomaterials including carbon nanotubes (CNTs) are employed in industrial applications such as supercapacitors, and biosensors, and etc. The present article has been prepared in three main categories. In the first part, carbon nanotube types have been presented, as single-walled carbon nanotubes, multi-walled carbon nanotubes, and also equivalent circuit models, which have been used to more clarify the experimental measurements of impedance. In the second part, nanocomposites with many carbon, inorganic and polymeric materials such as polymer/CNT, activated carbon/CNT, metal oxide/CNT, and carbon fiber/CNT have been investigated in more detail. In the third part, the focus in on the industrial applications of CNTs. including supercapacitors, biosensors, radar absorbing materials, solar cells, and corrosion protection studies. This review article explains the latest advances in carbon nanotubes and their applications in electrochemical, electrical and optical properties of nanocomposites.     

Controllable‐Nitrogen Doped Carbon Layer Surrounding Carbon Nanotubes as Novel Carbon Support for Oxygen Reduction Reaction

Novel nitrogen‐doped carbon layer surrounding carbon nanotubes composite (NC‐CNT) (N/C ratio 3.3–14.3 wt.%) as catalyst support has been prepared using aniline as a dispersant to carbon nanotubes (CNTs) and as a source for both carbon and nitrogen coated on the surface of the CNTs, where the amount of doped nitrogen is controllable. The NC‐CNT so obtained were characterized with scanning electron microscopy (SEM), Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and nitrogen adsorption and desorption isotherms. A uniform dispersion of Pt nanoparticles (ca. 1.5–2.0 nm) was then anchored on the surface of NC‐CNT by using aromatic amine as a stabilizer. For these Pt/NC‐CNTs, cyclic voltammogram measurements show a high electrochemical activity surface area (up to 103.7 m² g^–1) compared to the commercial E‐TEK catalyst (55.3 m² g^–1). In single cell test, Pt/NC‐CNT catalyst has greatly enhanced catalytic activity toward the oxygen reduction reaction, resulting in an enhancement of ca. 37% in mass activity compared with that of E‐TEK.     

Carbon nanotubes based on anodic aluminum oxide nano-template

The effect of reaction gas and catalyst on the growth of carbon nanotubes (CNTs) in the anodic aluminum oxide (AAO) nano-template was investigated. A mechanism of CNT growth was proposed, which involves the competitive catalytic carbon deposition between on the Co catalyst particles electrodeposited at the bottom of the pores and on the AAO template itself. Presence of H₂ in the reacting gas mixture significantly affected the morphology and the wall structure of synthesized CNTs: CNTs of high crystallinity grew out of pores with H₂ while no CNTs overgrew in the absence of H₂. CNT synthesis by CO disproportionation showed a lower growth rate and a higher degree of ordering than those grown by C₂H₂ pyrolysis. The unified mechanism of CNT growth on AAO template is also proposed.     

Carbon nanotube induced polymer crystallization: The formation of nanohybrid shish-kebabs

Carbon nanotubes (CNTs) have attracted tremendous attention in recent years because of their superb optical, electronic and mechanical properties. In this article, we aim to discuss CNT-induced polymer crystallization with the focus on the newly discovered nanohybrid shish-kebab (NHSK) structure, wherein the CNT serves as the shish and polymer crystals are the kebabs. Polyethylene (PE) and Nylon 6,6 were successfully decorated on single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), and vapor grown carbon nanofibers (CNFs). The formation mechanism was attributed to “size-dependent soft epitaxy”. Polymer CNT nanocomposites (PCNs) containing PE, Nylon 6,6 were prepared using a solution blending technique. Both pristine CNTs and NHSKs were used as the precursors for the PCN preparation. The impact of CNTs on the polymer crystallization behavior will be discussed. Furthermore, four different polymers were decorated on CNTs using the physical vapor deposition method, forming a two-dimensional NHSK structure. These NHSKs represent a new type of nanoscale architecture. A variety of possible applications will be discussed.     

Determination of Geometrical Length of Airborne Carbon Nanotubes by Electron Microscopy,Model Calculation,and Filtration Method

The geometrical length of carbon nanotubes (CNTs) is of significant importance and this study aims at its fast determination. We evaluated a method to determine the geometrical length of CNTs by electrical mobility classification and developed a filtration based method. The measurement was carried out in air and the suitable generation method for airborne CNTs from liquid suspensions was investigated, followed by a proper control of the CNT dispersion degree by changing the CNT concentration in the liquid suspension. Airborne CNTs were generated by an atomizer, classified by a differential mobility analyzer and characterized by electron microscopy. The CNT geometrical lengths measured by electron microscopy served as the basis for comparison with other methods. Two theoretical models (Lall and Friedlander 2006; Li et al. 2012) for elongated particles were investigated and the comparison showed good agreement. This demonstrated the validity of the approach to determine the CNT length by the combination of mobility measurement and model calculation. The newly developed method, which employed a filtration model with uniform screens, showed comparable results with the CNT lengths as obtained from the electron microscopy.

Copyright 2013 American Association for Aerosol Research     

Ab initio density functional studies of the restructuring of graphene nanoribbons to form tailored single walled carbon nanotubes

A method based on density functional theory calculations is proposed for the preparation of chiral controlled single walled carbon nanotubes (SWCNTs) by tailoring the edges of bi-layered graphene nanoribbons (GNRs). We find that armchair edged bi-layered GNRs are highly stable and need to be compressed to overcome the energy barrier to form zigzag SWCNTs, while the zigzag edged bi-layered GNRs are intrinsically highly unstable and immediately form armchair SWCNTs. We have investigated the rehybridization of orbitals of carbon atoms in the process of nanotube formation. Nanotube formation is found to be assisted by the edge ripples along with the intrinsic edge reactivity of different types of bi-layered GNRs. Utilizing these results we show that it may be possible to produce high specificity chiral controlled SWCNTs and pattern them for nanoscale device applications.     

Multi-Directional Growth of Aligned Carbon Nanotubes Over Catalyst Film Prepared by Atomic Layer Deposition

The structure of vertically aligned carbon nanotubes (CNTs) severely depends on the properties of pre-prepared catalyst films. Aiming for the preparation of precisely controlled catalyst film, atomic layer deposition (ALD) was employed to deposit uniform Fe₂O₃ film for the growth of CNT arrays on planar substrate surfaces as well as the curved ones. Iron acetylacetonate and ozone were introduced into the reactor alternately as precursors to realize the formation of catalyst films. By varying the deposition cycles, uniform and smooth Fe₂O₃ catalyst films with different thicknesses were obtained on Si/SiO₂ substrate, which supported the growth of highly oriented few-walled CNT arrays. Utilizing the advantage of ALD process in coating non-planar surfaces, uniform catalyst films can also be successfully deposited onto quartz fibers. Aligned few-walled CNTs can be grafted on the quartz fibers, and they self-organized into a leaf-shaped structure due to the curved surface morphology. The growth of aligned CNTs on non-planar surfaces holds promise in constructing hierarchical CNT architectures in future.     

Synthesis and Enhanced Field-Emission of Thin-Walled,Open-Ended,and Well-Aligned N-Doped Carbon Nanotubes

Thin-walled, open-ended, and well-aligned N-doped carbon nanotubes (CNTs) on the quartz slides were synthesized by using acetonitrile as carbon sources. As-obtained products possess large thin-walled index (TWI, defined as the ratio of inner diameter and wall thickness of a CNT). The effect of temperature on the growth of CNTs using acetonitrile as the carbon source was also investigated. It is found that the diameter, the TWI of CNTs increase and the Fe encapsulation in CNTs decreases as the growth temperature rises in the range of 780–860°C. When the growth temperature is kept at 860°C, CNTs with TWI = 6.2 can be obtained. It was found that the filed-emission properties became better as CNT growth temperatures increased from 780 to 860°C. The lowest turn-on and threshold field was 0.27 and 0.49 V/μm, respectively. And the best field-enhancement factors reached 1.09 × 10⁵, which is significantly improved about an order of magnitude compared with previous reports. In this study, about 30 × 50 mm² free-standing film of thin-walled open-ended well-aligned N-doped carbon nanotubes was also prepared. The free-standing film can be transferred easily to other substrates, which would promote their applications in different fields.     

Synthesis of carbon nanotubes on graphene quantum dot surface by catalyst free chemical vapor deposition

The growth of carbon nanotubes (CNTs) on graphene quantum dot surface has been explored using acetylene as the carbon source in a catalyst free chemical vapor deposition process. Dynamic studies were conducted to observe the CNT growth. The obtained nanotubes have a diameter distribution of 10–30 nm and show medium graphitic quality. Transmission electron microscopy observations and dynamic studies indicate that the formation of CNTs follows a different mechanism from traditional growth models, in which a wire-to-tube process and self-assembling of CNTs are involved. On the basis of these observations, a tentative continuous growth model is proposed for the CNT growth.     

Flame nanotube synthesis in moderate electric fields: From alignment and growth rate effects to structural variations and branching phenomena

The flame synthesis of carbon nanotubes (CNTs) coupled with application of moderate electric fields is studied experimentally as a means to control CNTs growth rates and morphology. The nanotubes are grown on a conductive metal-based catalytic probe positioned at the fuel side of the opposed flow oxy-flame. The probe was connected to an external voltage source to generate radial electric fields on its surface. At low applied voltages (from 0.3 to 2 V), the effect of the electric field on alignment and growth rate enhancement revealed the generation of vertically aligned carbon nanotube (VACNT) arrays with uniform distribution of CNT diameters. Further increases of the applied voltage resulted in structural modifications of the generated nanotubes. In particular, helically coiled CNTs were observed at applied voltages of 3 V. At higher voltages the arrays contained multi-walled CNTs with fascinating modified morphologies such as Y, T, and multi-junction patterns. Analysis of the samples generated at applied voltage of 5 V showed the presence of particle sprouting and early CNT junctions in the form of small bumps extruding from the outer surface of the CNTs. Analysis of material samples synthesized at 12 and 25 V showed the presence of fully branched CNT structures.     

The effects of confinement inside carbon nanotubes on catalysis

The unique tubular morphology of carbon nanotubes (CNTs) has triggered wide research interest. These structures can be used as nanoreactors and to create novel composites through the encapsulation of guest materials in their well-defined channels. The rigid nanotubes restrict the size of the encapsulated materials down to the nanometer and even the sub-nanometer scale. In addition, interactions may develop between the encapsulated molecules and nanomaterials and the CNT surfaces. The curvature of CNT walls causes the π electron density of the graphene layers to shift from the concave inner to the convex outer surface, which results in an electric potential difference. As a result, the molecules and nanomaterials on the exterior walls of CNTs likely display different properties and chemical reactivities from those confined within CNTs. Catalysis that utilizes the interior surface of CNTs was only explored recently. An increasing number of studies have demonstrated that confining metal or metal oxide nanoparticles inside CNTs often leads to a different catalytic activity with respect to the same metals deposited on the CNT exterior surface. Furthermore, this inside and outside activity difference varies based on the metals used and the reactions catalyzed. In this Account, we describe the efforts toward understanding the fundamental effects of confining metal nanoparticles inside the CNT channels. This research may provide a novel approach to modulate their catalytic performance and promote rational design of catalysts. To achieve this, we have developed strategies for homogeneous dispersion of nanoparticles inside nanotubes. Because researchers have previously demonstrated the insertion of nanoparticles within larger nanotubes, we focused specifically on multiwalled carbon nanotubes (MWCNTs) with an inner diameter (i.d.) smaller than 10 nm and double-walled carbon nanotubes (DWCNTs) with 1.0-1.5 nm i.d. The results show that CNTs with well-defined morphology and unique electronic structure of CNTs provide an intriguing confinement environment for catalysis.     

锌/碳纳米管复合电沉积薄膜性能研究

通过复合电沉积技术制备了纳米叠层锌/碳纳米管和光亮锌/碳纳米管2种复合薄膜,薄膜的拉曼光谱验证了锌与碳纳米管的共沉积。薄膜表面的场发射扫描电子显微镜观测显示碳纳米管表面的金属包覆层连续且均匀,预示着良好的界面结合。在2种薄膜的断口和裂纹处分别发现了被拔出基体和桥联的碳纳米管,证实了碳管对基体具有有效的增强作用。     

Carbon nanotubes grown in situ on graphene nanosheets as superior anodes for Li-ion batteries

Graphene and carbon nanotubes are intriguing alternative anode materials for lithium ion batteries. The prevention of graphene restacking and facilitation of lithium diffusion into CNTs with large aspect ratio are highly desirable for the performance enhancements including capacity, cycliability and rate capability. In this work, we demonstrated that a multilayered graphene-CNT hybrid nanostructure was able to hold such merits. GNS were separated and stabilized by CNTs grown in situ on GNS surface. The length of CNTs was found to be a key factor to the electrochemical performances. The GNS-CNT composite with the shortest CNT decoration displayed highly reversible capacities of 573 mAh g(-1) at a small current of 0.2C and 520 mAh g(-1) at a large current of 2C. The growth and lithium storage mechanism for graphene-CNT composite was also proposed.     

Nano carbon hybrids from the simultaneous synthesis of CNT/NCD by MPCVD

A carbon hybrid composed by nanocrystalline diamond (NCD) and carbon nanotubes (CNT) was successfully synthesized by microwave plasma chemical vapor deposition (MPCVD), assisted by a continuous delivery of a Fe catalyst. This way, both carbon forms were grown simultaneously, originating a composite structure mainly formed by nanodiamond conglomerates well interconnected by multiwall carbon nanotubes. NCD clusters often develop at the crossing points of different CNTs. A good NCD/CNT mechanical link is predicted based on the observation of carbon nanotubes penetrating and/or involving isolated nanocrystalline diamond clusters. The overall appearance of the NCD/CNT material resembles that of a 2D neuronal network. The nanocrystalline diamond and carbon nanotube phases were structurally confirmed by μ-Raman spectroscopy, the morphological features were characterized by SEM and EDS was used to appraise the catalyst surface distribution.     

Synthesis of carbon nanotubes by sequential pyrolysis and combustion of polyethylene

Carbon nanotubes (CNTs) were synthesized from polyethylene using a pyrolysis-combustion technique. Pre- or post-consumer polyethylene (in strips, pellets or ground particles) was pyrolyzed to gaseous products, which were then combined with an oxygen-containing gas and auto-ignited to create a fuel-rich flame. The combustion effluent was used to synthesize multi-wall CNTs using stainless-steel wire mesh as both catalyst and substrate. An overall CNT yield in the order of 10%, by mass relative to the initial carbon in the fuel, was typically achieved before purification. The viability of partial conversion of a readily-available waste stream to the value-added product of CNTs was demonstrated, combining the economy and process safety aspects of flame synthesis with the control of synthesis temperature that chemical vapor deposition methods typically afford.