Physical properties of CVD boron-doped multiwalled carbon nanotubes

The effects of boron doping and electron correlation on the transport properties of CVD boron-doped multiwalled carbon nanotubes are reported. The boron-doped multiwalled carbon nanotubes were characterized by TEM as well as Raman spectroscopy using different laser excitations (viz. 488, 514.5 and 647 nm). The intensity of the D-band laser excitation line increased after the boron incorporation into the carbon nanotubes. The G-band width increased on increasing the boron concentration, indicating the decrease of graphitization with increasing boron concentration. Electrical conductivity of the undoped and boron-doped carbon nanotubes reveal a 3-dimensional variable-range-hopping conductivity over a wide range of temperature, viz. from room temperature down to 2 K. The electrical conductivity is not found to be changed significantly by the present levels of B-doping. Electron Paramagnetic Resonance (EPR) results for the highest B-doped samples showed similarities with previously reported EPR literature measurements, but the low concentration sample gives a very broad ESR resonance line.     

Noncovalent functionalization of multi-walled carbon nanotubes with siloxane polyether copolymer

Siloxane polyether copolymer (PSPEO), a new amphiphilic macromolecule, which consists of hydrophobic –Si–O–Si– main chain and hydrophilic pendant parts of polyethylene oxide (PEO), has a comb-polymer molecular structure and exhibits excellent properties such as super-wettability and extremely low surface tension. Multi-walled carbon nanotubes (MWNTs) can be easily solubilized in water by noncovalent functionalization with PSPEO. TEM, HRTEM and Raman spectra were used to characterize the functionalized MWNTs. The results reveal that PSPEO is adsorbed on surface of MWNTs, creating a 5–10 nm adsorption layer to impede the aggregation of nanotubes. TEM measurement confirmed the single-tube nature of the MWNTs suspensions. The MWNTs/PSPEO suspensions were stable over 3 months.     

Fabrication of flat capped carbon nanotubes using an arc-discharge method assisted with a Sm-Co catalyst

In this study, carbon nanotubes (CNTs) were fabricated using an arc-discharge method assisted with samarium-cobalt (Sm-Co) chloride as a catalyst. The optimal fabrication condition was determined through a series of experiments on various ambient conditions. Observations were completed using scanning electron microscopy (SEM), Raman spectroscopy, and tunneling electron microscopy (TEM); the main products we observed are well-structured multi-walled carbon nanotubes. By identifying the radial breathing modes (RBMs) of the Raman spectra with a TEM micrograph, we also observed a small number of single-walled carbon nanotubes. With the assistance of the Sm-Co chloride catalyst, the RBMs of the Raman spectra were measured in the ambient pressure of 760 torr. The TEM observations revealed that our nanotubes have good graphitic structures and almost no bamboo defects, which agrees with their Raman measurements with a high I_G/I_D ratio (~88). A perfect graphitic flat cap was found to be attached at the end of the nanotube. Simulation shows that by incorporating 5 carbon pentagons, it is possible to construct a flat capped carbon nanotube. The results of our experiment offer a unique approach to growing high quality CNTs. Such a flat capped structure may useful for further advanced application in nano-electronics and nano-optics.     

Facile synthesis route of graphene-like structures from multiwall carbon nanotubes

Graphene-like nanostructures were synthesized from multiwall carbon nanotubes through chemical exfoliation route in mild conditions. For this purpose multiwall carbon nanotubes were synthesized by Chemical Vapor Deposition method using Al-Fe-Co catalyst and treated with KMnO₄. The obtained nanostructures were characterized by Raman spectroscopy, XRD, FTIR, EDX, SEM and TEM methods. FTIR results show that, treating the carbon nanotubes with KMnO₄ decorates their surface with oxygen containing functional groups. XRD and Raman spectroscopy results reveal that the outermost layers of the nanotubes were exfoliated during the treatment. The formation of graphene-like nanostructures was confirmed by SEM and TEM methods. The novelty of this work is the first time use of this type of mild and cheap condition for obtaining graphene-like nanostructures from MWCNTs without any other intermediate treatment.     

High-aligned carbon nanotube film with netlike bulges

High-aligned carbon nanotubes film with netlike bulges made of catalyst particles has been synthesized on a silica wafer by pyrolyzing ferrocene/melamine mixtures. The structure and composition of carbon nanotubes are investigated by scanning electron microscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electron energy-loss spectroscopy (EELS). It is found that these nanotubes have uniform outer diameters of about 25 nm and lengths of about 40 μm. High-resolution TEM images show that each carbon nanotube is composed of graphite-like layers arranged in a stacked-cup-like structure. XPS spectrum shows that the crust covering the tops of the aligned carbon nanotube film consists of carbon, iron and ferric oxide. The EELS spectrum shows that these nanotubes are pure-carbon tubes. The formation mechanism of the netlike bulges has been provided.     

Synthesis of carbon nanotubes via toluene-thermal reduction process at moderate temperature

Carbon nanotubes were synthesized with toluene as carbon source and solvent at 350^∘C under stiring situation. In this process, La/Ni were used as catalyst, which were reduced via LaCl₃ and NiCl₂ with sodium as reductant. The products were characterized with X-ray powder diffractometer (XRD), transmission electronic microscopy (TEM) and Raman spectroscopy. The carbon nanotubes had diameters ranging from 100–200 nm and lengths ranging from hundreds of nanometers to several micrometers. The yield of carbon nanotubes, in accordance with TEM observations was estimated to be about 70%.     

Electrodeposited carbon nanotube thin films

A successful attempt to grow carbon nanotubes (CNTs) by electrodeposition technique for the first time is reported here. Carbon nanotubes were grown on Si (001) substrate using acetonitrile (1% v/v) and water as electrolyte at an applied d.c. potential ∼20 V. The films were characterized by X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), Raman, optical absorbance, Fourier Transform Infra Red spectroscopy (FTIR) and Electron Spin Resonance (ESR) measurements. The effect of magnetic field on the growth of nanotubes was studied critically. It was found that the presence of magnetic field during electro-deposition played a crucial role on the growth of carbon nanotubes and hence the electronic properties. Photoluminescence (PL) studies indicated band edge luminescence ∼0.72-0.83 eV. Field emission studies indicated lower turn-on voltage and higher current density for films deposited with magnetic field.     

Calorimetric study of carbon nanotubes and aluminum

Differential scanning calorimeter (DSC) was used to investigate apparent activation energy and reaction order of the reaction of carbon nanotubes and aluminum by Kissinger equation and Crane equation under non-isothermal condition. The reaction product was examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results show that carbon nanotubes react with aluminum and form Al₄C₃ phases with needle shape. The peak temperature of the reaction of carbon nanotubes and aluminum is found to depend on the heating rate during the continuous heating. Apparent activation energy and reaction order of the reaction of carbon nanotubes and aluminum are 194.01 and 0.92 kJ/mol, respectively.     

Preparation temperature effect on the synthesis of various carbon nanostructures

Various carbon nanostructures (CNs) have been prepared by a simple deposition technique based on the pyrolysis of a new carbon source material tetrahydrofuran (THF) mixed with ferrocene using quartz tube reactor in the temperature range 700–1100 °C. A detailed study of how the synthesis parameter such as growth temperature affects the morphology of the carbon nanostructures is presented. The obtained CNs are investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), electron dispersive scattering (EDS), thermogravimetry analysis (TGA), Raman and transmission electron microscope (TEM). It is observed that at 700 °C, normal CNTs are formed. Iron filled multi-walled carbon nanotubes (MWCNTs) and carbon nanoribbons (CNRs) are formed at 950 °C. Magnetic characterization of iron filled MWCNTs and CNRs studied at 300 K by superconducting quantum interference device (SQUID) reveals that these nanostructures have an enhanced coercivity (Hc = 1049 Oe) higher than that of bulk Fe. The large shape anisotropy of MWCNTs, which act on the encapsulated material (Fe), is attributed for the contribution of the higher coercivity. Coiled carbon nanotubes (CCNTs) were obtained as main products in large quantities at temperature 1100 °C.     

碳源对碳纳米管形态的影响

以苯和甲苯为碳源,二茂铁为催化剂,含硫化合物为助催化剂,采用浮游催化裂解法制备了碳纳米管,并采用TEM对不同条件下所得碳纳米管进行了形态分析。结果发现,碳源中苯和甲苯的配比对碳纳米管的形态有着重要的影响。以纯苯为碳源时,产物主要为直线型碳纳米管,并存在极少量短的弯曲型碳纳米管。随着碳源中甲苯比例的增加,产物中折线型碳纳米管增加。以纯甲苯为碳源,产物中仍有少量直线型碳纳米管,而不完全是折线型碳纳米管;此外,产物中还发现了极少量分支型碳纳米管。根据所得结果讨论分析了甲苯的加入对碳纳米管形态的影响以及各种碳纳米管的形成机理,认为可能是由于甲苯在催化热解过程中产生的碳种不同于苯催化热解所产生的碳种,造成碳在催化剂颗粒各处浓度不同,从而在碳纳米管的不同部位引入五元环和七元环而形成各种形态的碳纳米管。     

Synthesis and characterization of large area well-aligned carbon nanotubes by ECR-CVD without substrate bias

Large area, well-aligned carbon nanotubes (CNTs) were synthesized on porous silicon by electron cyclotron resonance chemical vapor deposition (ECR-CVD). No bias was applied on the substrate in this experiment. CH₄ and H₂ were used as source gases and Fe₃O₄ nanoparticles as the catalyst. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), and Raman spectrum were used to evaluate the structure and composition. The results show that these CNTs have varying outer diameters from 10 to 90 nm and uniform length over 10 μm. They display hollow tubular and chain structures. The possible formation mechanism of aligned CNTs is discussed.     

Nanofibres of CA/PAN with high amount of carbon nanotubes by core–shell electrospinning

We prepared polyacrylonitrile (PAN) and cellulose acetate (CA) based nanofibres with high amount of carbon nanotubes (CNTs) by core–shell electrospinning. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to evaluate the morphology and structure of the electrospun nanofibres. Raman spectroscopy (Raman) and TEM indicate alignment of CNTs in the polymer fibres. Core–shell electrospinning improved the distribution and uniformity of the fibres. The loading of carbon nanotubes showed better thermal stability.     

Influence of catalyst particles on multi-walled carbon nanotubes morphology and structure

Multi-walled carbon nanotubes (MWCNTs) have been successfully grown by Chemical Vapor Deposition (CVD) method. Elucidating the key characteristics of catalyst sources that affect carbon nanotubes growth is of great importance for improving and control MWCNTs morphology and structure. In this work we present a systematically study of CVD parameters, such as catalyst source, substrate morphology and temperature and how it affects carbon nanotubes synthesis. The novelty of this work lies on the catalyst composition. Two specific catalyst sources were analyzed: (i) Fe₂Co and (ii) Fe₂Co with ferrocene. Cyclic Voltammetry results confirmed the presence of Fe²⁺ in the Fe₂Co with ferrocene solution. X-Ray Diffraction analysis confirmed the presence of iron particles on the substrate surface after its submission to growth conditions. Raman results suggested an improvement in carbon nanotubes crystalline quality catalyzed by Fe₂Co with ferrocene. For tridimensional substrates such as fibers, the Fe₂Co with ferrocene provided aligned CNTs with lower defects density noticed in Raman spectra and SEM micrographs. Finally, we corroborated the Fe²⁺ encapsulation relation with the growth mechanism and MWCNTs formation.     

An attempt to prepare carbon nanotubes by carbonizing polyphosphazene nanotubes with high carbon content

Polyphosphazene nanotubes with about 20 nm in inner diameter and 100-200 nm in outer diameter were fabricated easily and then carbonized at 800 °C in a nitrogen atmosphere. Scanning electron microscopy and transmission electron microscope results showed that the bulk morphology of polyphosphazene nanotubes was retained after carbonization. The carbon content of the carbonized samples reached 93.28%. X-ray diffraction and Raman spectrum showed that the carbonized samples had low graphitization state. The present method can be used for a mass production of carbon nanotubes.     

Structural characterization of a mechanically milled carbon nanotube/aluminum mixture

The structural evolution of carbon nanotubes (CNTs) during mechanical milling was investigated using SEM, TEM, XRD, XPS and Raman spectroscopy. The study showed that milling of the CNTs alone introduces defects but preserves the tubular structure. When milling the CNTs with aluminum (Al) powder in order to produce a composite, Raman spectroscopy has shown that most of the nanotubes are destroyed. During sintering of the CNT/Al milled mixture, the carbon atoms available from the destruction of the nanotubes react with the Al to form aluminum carbide (Al₄C₃). The effect of milling on the Al matrix was also studied.     

Preparation and characterization of titanate nanotubes/carbon composites

Titanate nanotubes/carbon composites(TNT/CCs) were synthesized by allowing carbon-coated TiO₂ (CCT) powder to react with a dense aqueous solution of NaOH at 120 °C for a proper period of time. As-prepared CCT and TNT/CCs were characterized by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectrometry. The processes for formation of titanate nanotubes/carbon composites were discussed. It was found that the TiO₂ particles in TiO₂-carbon composite were enwrapped by a fine layer of carbon with a thickness of about 4 nm. This carbon layer functioned to inhibit the transformation from anatase TiO₂ to orthorhombic titanate. As a result, the anatase TiO₂ in CCT was incompletely transformed into orthorhombic titanate nanotubes upon 24 h of reaction in the dense and hot NaOH solution. When the carbon layers were gradually peeled off along with the formation of more orthorhombic titanate nanotubes at extended reaction durations (e.g., 72 h), anatase TiO₂ particles in CCT were completely transformed into orthorhombic titanate nanotubes, yielding TNT/CCs whose morphology was highly dependent on the reaction time and temperature.     

Dissolution, characterization and photofunctionalization of carbon nanotubes

A simple method to disperse carbon nanotubes (CNTs) has been achieved, which gives two photofunctionalized CNTs, hydrazine nanotubes (h-CNTs) and 1,3,4-oxadiazole nanotubes (o-CNTs). Results from FTIR, ¹H NMR spectroscopy and TEM observations showed that the functionalization was successful. The modified nanotubes can dissolve in most of the nonpolar organic solvents and no precipitate was observed in the solution of the nanotubes even after 2 months. The functionalized nanotubes showed photo-electronic properties, which is due to the attachment of the function groups to them as proved by steady-state fluorescence spectroscopy. Both h-CNTs and o-CNTs showed good thermal stability below 300 °C and might be used as functional materials.     

Effective chemical oxidation on the structure of multiwalled carbon nanotubes

A two-step chemical oxidation of multiwalled carbon nanotubes (MWCNTs) with different oxidation reagents was particularly studied. The reagents used in first step were the acidic mixture of sulfuric acid and nitric acid, and the reagents used in the second step were a mixture of sulfuric acid and hydrogen peroxide for different time. After each treatment, the functionalization yield of the oxygen-containing groups such as carboxylic group, hydroxyl functional groups and other functional groups on the surface of the carbon nanotubes (CNTs), was quantified by the analysis of XPS measurements. Two-step for a short period of time treated MWCNTs exhibited a larger fraction of carboxyl (COOH) groups compared to one step and two step for longer-time oxidative treatments, and the defect formation on the CNTs was verified mild by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy and transmission electron microscopy (TEM) graphs, which is necessary for derived reaction and better manipulation of the material and use in the application of the carbon nanotubes.     

Moderate temperature synthesis of single-walled carbon nanotubes on alumina supported nickel oxide catalyst

A simple nickel oxide catalyst has been developed in synthesizing single-walled carbon nanotubes (SWNTs) at moderate temperature. The catalyst used in the experiment was without a preceding reduction in hydrogen flow. The synthesis of SWNTs was performed at a temperature of 700 °C, which represents a moderate reaction temperature. The presence of SWNTs on the catalyst was confirmed by transmission electron microscope (TEM) and Raman spectroscope. The Raman spectrum shows a strong intensity at the radial breathing mode, indicating that the occurrence of SWNTs was dominant. Raman data further reveals that the synthesized SWNTs had the diameters in the range from 0.58 to 2.02 nm.     

Growth of carbon nanotubes on carbon fibre substrates to produce hybrid/phenolic composites with improved mechanical properties

Carbon nanotubes were grown by chemical vapor deposition (CVD) on different carbon fibre substrates namely, unidirectional (UD) carbon fibre tows, bi-directional (2D) carbon fibre cloth and three dimensional (3D) carbon fibre felt. These substrates were used as the reinforcement in phenolic resin matrix to develop hybrid CF–CNT composites. The growth morphology and other characteristics of the as grown tubes were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal gravimetry (TGA) which confirmed a copious growth of multiwalled carbon nanotubes (MWNTs) on these substrates. The mechanical properties of the hybrid composites was found to increase with the increasing amount of deposited carbon nanotubes. The flexural strength (FS) improved by 20% for UD, 75% for 2D and 66% for 3D hybrid composites as compared to that prepared by neat reinforcements (without CNT growth) under identical conditions. Flexural modulus (FM) of these composites also improved by 28%, 54% and 46%, respectively.