Structure of multiwalled carbon nanotubes grown by chemical vapor deposition

Carbon nanotubes have been grown by chemical vapor deposition at 650°C in an argon atmosphere using a butane-propane mixture and a nickel catalyst and have been characterized by scanning and transmission electron microscopy and Raman spectroscopy. The results indicate that the multiwalled nanotubes have an imperfect graphite-like structure with a conical supramolecular configuration. A phenomenological technique is proposed for statistical analysis of the state of carbon nanotubes in measurements of the intensity of the defect zone D in their Raman spectra.     

Defect structure of multiwalled carbon nanotubes studied by Raman spectroscopy

The defect structure of multiwalled carbon nanotubes has been studied by transmission electron microscopy and Raman spectroscopy, with particular attention to the shape and intensity of the defect band D and its overtone D*. Electron-microscopic results demonstrate that multiwalled nanotubes typically have multiple bends. The associated short- and long-range disorder influences the Raman spectrum of the nanotubes. The presence of several defect species, differing in scattering probability, results in a stochastic relationship between the intensities of the D*- and D-bands. This relationship is qualitatively interpreted in terms of general mechanisms of elastic/inelastic interactions of π-electrons with phonons and defects.     

Evaluating industrial grade functionalized multiwalled carbon nanotubes by X-ray photoelectron spectroscopy

Utilization of carbon nanotubes in various applications is a function of their dispersion in respective matrix which effectively depends on the functionalization employed. Functional group distribution on Carbon Nanotube surface is usually a complex mixture of groups depending on the oxidizing agent employed. In this regard, nine varieties of Industrial grade multiwalled carbon nanotubes belonging to 10–30, 20–40 and 50–80?nm outer diameter ranges but differing in functionality (carboxyl and hydroxyl) were analyzed. X-ray photoelectron spectroscopy was employed to quantify the different functionalities on pristine, hydroxyl and carboxyl functionalized multiwalled carbon nanotubes and high-resolution transmission electron microscopy was used to image the internal structure including the side wall damage in functionalized varieties. X-ray photoelectron spectroscopy measurements on nine batches have suggested the presence of carbonyl, carboxyl and hydroxyl functional groups on all multiwalled carbon nanotubes in different proportions. This implies that it is not possible to have exclusive hydroxyl or carboxyl functionalization on a CNT surface Additionally, comparison is drawn between already existing deconvolution procedures from literature.     

Preparation polystyrene/multiwalled carbon nanotubes nanocomposites by copolymerization of styrene and styryl-functionalized multiwalled carbon nanotubes

Styryl-functionalized multiwalled carbon nanotubes (p-MWNTs) were prepared by esterification based on the carboxylate salt of carbon nanotubes and p-chloromethylstyrene in toluene. Then in situ radical copolymerization of p-MWNTs and styrene initiated by 2,2′-azobis(isobutyronitrile) (AIBN) was applied to synthesize composites of styryl-functionalized multiwalled carbon nanotubes and polystyrene (PS) (p-MWNTs/PS). Characterizations carried out by FT-IR, ¹H NMR, UV–vis show that styryl group covalently bond to the surface of MWNTs. The results of UV showed that the solutions of p-MWNTs/PS in chloroform have the hyperchromic effect. Transmission electron microscopy (TEM) images of p-MWNTs/PS composites and scanning electron microscopy (SEM) images of fracture surface of p-MWNTs/PS composites showed the functionalized nanotubes had a better dispersion than that of the unfunctionalized MWNTs in the matrix. The results of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) suggested that the thermal stability of p-MWNTs/PS composites improved in the presence of MWNTs.     

Structural modification and enhanced electron emission from multiwalled carbon nanotubes grown on Ag/Fe catalysts coated Si-substrates

Multiwalled carbon nanotubes and carbon nano-filaments were grown using Fe as the main catalyst and Ag as a co-catalyst by microwave plasma enhanced chemical vapour deposition. In this work we demonstrate the growth behaviour of carbon nanotubes (CNTs) grown on pure Fe-film and Ag–Fe films. We find that using Ag film beneath Fe film significantly abate the catalyst–substrate interactions by acting as a barrier layer as well as enhances the nucleation sites for the growth of CNTs due to the limited solubility with Fe and silicon. Scanning electron microscopy and transmission electron microscopy studies were carried out to image the microstructures of the samples. It was observed that the length of Fe catalyzed CNTs was ∼500 nm and Ag–Fe catalyzed CNTs varied from ∼600 nm to 1.7 μm. Micro Raman spectroscopy confirmed the improved crystalline nature of Ag–Fe CNTs. It was found that I_D/I_G ratio for Fe catalyzed CNTs was ∼1.08 and for Ag–Fe catalyzed CNTs was ∼0.7. The Ag–Fe catalyzed CNTs were found to be less defective as compared to Fe catalyzed CNTs. Field emission measurements using diode configuration, showed that electron emission from Ag–Fe catalyzed CNTs was much stronger as compared to Fe catalyzed CNTs. The threshold field for Ag–Fe catalyzed CNTs was (2.6 V μm⁻¹) smaller as compared to Fe catalyzed CNTs (3.8 V μm⁻¹) and thus shows better emission properties. This enhancement in electron emission mechanism as a result of introduction of Ag underlayer is attributed to the increased emitter sites and improved crystallinity.     

Synthesis and characterization of carbon nanotubes grown on montmorillonite clay catalysts

Multiwall carbon nanotubes have been grown on montmorillonite clay catalysts through anchoring on FeCo nanoparticles. The starting clay is a commercial sodium-rich montmorillonite in which the intercalated sodium ion was exchanged for cobalt(II) and iron(III) ions via mechanical agitation or sonication, both with and without subsequent centrifugation. The cobalt-iron intercalate clay was used as a catalyst for the synthesis of carbon nanotubes via decomposition of ethylene at 700 °C. The largest carbon deposit was obtained for catalysts prepared with 3 or 4 cation exchange equivalents. X-ray diffraction indicates both that the basal spacing of the clay increases from 12.43 Å to 16.4 Å upon intercalation of cobalt and iron. Atomic absorption analysis of the catalysts indicates that virtually all of the sodium ions originally present in the clay have been replaced by iron(III) and cobalt(II). Transmission electron micrographs show the presence of multiwall carbon nanotubes with inner and outer diameters of ca. 10 nm and 20 nm grown on metal particles present on the plates of catalysts. The iron-57 Mössbauer spectra indicate that the intercalated clay contains iron(III) in octahedral and tetrahedral sites and iron(II) in octahedral sites, the catalysts contain an extensive amount of small superparamagnetic particles of α-Fe₂O₃ and that the carbon-nanotube catalyst composites show the presence of iron(II) and iron(III) paramagnetic doublets, characteristic of a reduced montmorillonite, and of sextets that are characteristic of an FeCo alloy and of Fe₃C cementite. The Mössbauer spectra indicate that the carbon nanotubes grow on FeCo metallic nanoparticles and bond to these particles through the formation of cementite.     

Synthesis and photocatalysis study of multiwalled carbon nanotubes grown in a lead-based microspherical support

Plumbonacrite films were used as support material for growth of carbon nanotubes and the resulting nanocomposite was tested as photocatalysts for the degradation of methylene blue dye. The plumbonacrite films were deposited on silicon substrates by a simple alkaline chemical bath deposition process. After the CNT growth process, the resulting nanocomposite samples were analyzed by X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The high temperature during the CNT growth process decomposed the plumbonacrite material into lead microspheres, which were covered by the CNT constituting the nanocomposite films. Afterwards, the nanocomposite films were applied for the photodegradation of methylene blue molecules using natural sunlight. The nanocomposite films were reused for up to three cycles obtaining efficiencies in the degradation of the dye superior to 92% and a first order kinetics with rate constants of 8.9 × 10^?3 min^?1 in a maximum irradiation time of 240 minutes.     

Orientation relations between carbon nanotubes grown by chemical vapour deposition and residual iron-containing catalysts

Orientation relationships between the growth direction of carbon nanotubes and encapsulated residual iron-containing particles have been determined using transmission electron microscopy. The nanotubes that are prepared by Fe-catalysed chemical vapour deposition on sol–gel Fe(NO₃)₃-tetraethyl orthosilicate substrates are the helical multiwall type. Nanoscale particles of both the low-temperature α-Fe (ferrite) and high-temperature γ-Fe (austenite) were found in the cavity of the carbon nanotubes with , and parallel to the tube growth direction, respectively. Cementite Fe₃C, the most abundant Fe-containing phase in present samples was also found to be entrapped in nanotubes with or parallel to the tube axis. The metastable retention of γ-Fe particles at room temperature is ascribed to the strain energy induced at the particle-nanotube interface due to volume expansion upon the γ- → α-Fe phase transformation. The decomposition of initially high aspect-ratio, rod-shape particles into a string of ovulation, while encapsulated in carbon nanotubes is accounted for by the Rayleigh instability. Ovulation leading to reduced particle size has also contributed to increase the surface energy term that counterbalances the total free energy change of phase transformation from γ- to α-Fe and further aids to the metastable retention of γ-Fe.     

Thermodynamic properties of multiwalled carbon nanotubes

The experimental study of the heat capacity of multiwalled carbon nanotubes has been conducted at a constant pressure and a temperature in the range from 60 to 300 K. The derived temperature dependence of the heat capacity has been shown to differ from that of graphite. The explanation of the fact has been given in terms of the special features of phonon spectra of the above materials. Based on the experimental results and reliable literature data standard values of the basic thermodynamic functions of multiwalled carbon nanotubes (enthalpy, entropy, and Gibbs reduced energy) have been calculated.     

Physical properties of multiwalled carbon nanotubes

Multiwalled carbon nanotubes (MWNTs), which were prepared by hydrogen arc discharge, were purified by using an infrared radiation heating system. The morphology, structure, vibrational modes and crystalline perfection of purified MWNTs were investigated by using scanning electron microscopy, high-resolution transmission electron microscopy, an X-ray diffractometer and a Raman spectrometer. Moreover, the electrical conductivity of individual purified MWNTs was measured using a two-probe method using a micro manipulator system. It turned out that the MWNTs had a high degree of graphitization, an electrical conductivity of about 1.85×10³ S cm⁻¹ along the long axis, and an enormous current density of more than 10⁷ A cm⁻².     

电场诱导多壁碳纳米管有序排列对多壁碳纳米管/环氧树脂复合材料性能的影响

采用交流（AC）电场诱导法制备了多壁碳纳米管（MWCNTs）均匀分散且定向有序排列的MWCNTs/环氧树脂复合材料。采用SEM、偏振拉曼光谱等研究了电场强度、MWCNTs含量、加电时间及温度（黏度）等因素对MWCNTs定向排列的影响,讨论了MWCNTs有序排列对MWCNTs/环氧树脂复合材料电学和力学性能的影响。结果表明：MWCNTs沿电场方向有序排列;MWCNTs/环氧树脂复合材料施加AC电场后的拉曼强度明显高于未施加电场的情况;当MWCNTs含量从0wt%增加到0.025wt%时,MWCNTs/环氧树脂复合材料导电率从2.3×10^-12 S/cm增加到1.3×10^-8 S/cm,增加了约4个数量级;MWCNTs含量为2.5wt%时,MWCNTs/环氧树脂复合材料拉伸强度提高了26.3%。     

Preparation and structure of multiwalled carbon nanotubes

The structure of oxidized multiwalled carbon nanotubes has been studied by transmission electron microscopy and X-ray diffraction. The results demonstrate that oxidation disrupts nanotubes. Subsequent heat treatment at 300°C also changes the structure of the nanotubes, increasing their inner diameter and reducing their outer diameter.     

1.6-己二胺化学修饰多壁碳纳米管

为获得两端具有活性端胺基的碳纳米管,通过化学方法对多壁碳纳米管(MWCNTs)接枝改性,利用氯化亚砜将MWCNTs酸处理产生的羧基转化为酰氯,进一步与1.6-己二胺发生亲核取代反应,将1.6-己二胺接枝到碳纳米管端部.采用X射线光电子能谱(XPS)、透射电镜(TEM)及扫描电镜(SEM)对化学修饰过程中各反应产物进行表征.结果表明:酸的切割作用,使碳纳米管从端帽下及管壁缺陷处打开,碳管长度由十几微米变为500nm左右;1.6-己二胺成功地接枝到碳纳米管端部,能谱分析(EDS)结果表明N元素接枝率达3.29%,接枝后的碳纳米管可以均匀分散在丙酮溶液中,并含有活性端胺基.     

Dispersion of multiwalled carbon nanotubes in water by lignin

Multiwalled carbon nanotubes (MWCNT) can be stably dispersed in water with small amount of lignin. One-step dispersion in the 20.0 g/L concentration range is achieved at room temperature with excellent electrical properties of MWCNT. Lignin is depicted to act as an anti-static agent.     

On the growth mechanism of single-walled carbon nanotubes by catalytic carbon vapor deposition on supported metal catalysts

A comprehensive kinetic study was performed to throw light on the formation mechanism of single walled carbon nanotubes (SWNTs) in chemical vapor deposition processes. SWNTs were synthesized by catalytic decomposition of methane or ethylene on supported transition metal catalysts. Kinetic curves (the amount of SWNT as a function of time) were obtained as a function of the nature and the preparation of the supported catalysts, temperature, the fluxes of the gases (the reagent hydrocarbon and the carrying gas), and the partial pressure of the hydrocarbon. The final products were characterized by transmission electron microscopy, Raman spectroscopy, chemical analysis, and thermogravimetric measurements. The fundamental factors determining the SWNT formation are discussed in detail, taking into consideration several observations from the literature as well.     

Synthesis of multiwalled carbon nanotubes by high-temperature vacuum annealing of amorphous carbon

Vacuum annealing of a mixture of amorphous carbon and cobalt nanoparticles supported on microporous zeolite at high-temperature results in the formation of multiwalled carbon nanotubes which are essentially filled with metal nanoparticles or nanowires as observed by transmission electron microscopy. The electronic properties of nanotubes by variable temperature ESR techniques shows that g values show little change with temperature from 77 to 327 K but the line width (ΔH_pp) of the ESR signal for nanotubes synthesized from amorphous carbon increases from 7.9 G at 77 K to 9.5 G at 327 K.     

Synthesis and characterization of phospholipid-modified multiwalled carbon nanotubes

A simple three-step strategy to functionalize multiwalled carbon nanotubes using 1,2-distearoyl-sn-glycero-3-phosphoethanolamine phospholipids has been described. The resulting phospholipid-modified multiwalled carbon nanotubes were analyzed by TEM, AFM, NMR, IR, UV-vis and TGA techniques. The experimental results show that the use of amine-terminated phospholipids not only improves the dispersity of multiwalled carbon nanotubes in both aqueous and organic solvents greatly, but also results in the significant enhancement of biocompatibility. These findings will serve as a future biological platform for new devices ranging from biosensors to nano-detectors.     

Light-scattering and dispersion behavior of multiwalled carbon nanotubes

Elliptically polarized light-scattering measurements were performed to investigate the dispersion behavior of multiwalled carbon nanotubes (MWNT). Xylene- and pyridine-derived MWNT powders were dispersed in water and ethanol in separate optic cells and allowed to sit undisturbed over a two-week time period after probe sonication. Continuous light-scattering measurements taken between scattering angles of 10-170 deg and repeated over several days showed that the nanotubes formed fractal-like networks. The pyridine-derived MWNTs showed greater dispersion variation over time, tending to aggregate and clump much faster than the xylene-derived tubes. The water suspensions appeared much more stable than the ethanol suspensions, which transformed into nonfractal morphology after a few hours. We relate the dispersion stability to size and fringe patterns on the outer surface of the nanotubes. Measured values of fractal dimension were distinctly lower than those in previous studies of single-walled carbon nanotubes. Profiles of both diagonal and off-diagonal scattering matrix elements are presented.     

Functionalization and solubilization of thin multiwalled carbon nanotubes

The reaction of thin multiwalled carbon nanotubes with a mixture of concentrated HNO₃ and H₂SO₄ has been studied by IR absorption and x-ray photoelectron spectroscopies. The results indicate the attachment of-C(O)OH groups to nanotubes and subsequent conversion of these groups to-C(O)Cl groups via reaction with SOCl₂ and then to-C(O)NR₂ via reaction with didodecylamine. The yield of the carboxylated nanotubes is 53%, and that of the amidated nanotubes is 28%. The O:C atomic ratio in the carboxylated tubes is 1.0:9.0. The solubility of the carboxylated tubes in water is 3.13 g/l, and that of the amidated tubes in chloroform is 1.30 g/l.     

聚乙烯/马来酸酐接枝聚合物修饰多壁碳纳米管

多壁碳纳米管(MWCNTs)与混酸(浓硫酸∶浓硝酸体积比为3∶1)和无水乙二胺进行酸化、胺化反应使MWCNTs表面产生羧基和氨基基团,进而与聚乙烯/马来酸酐接枝聚合物(PE-g-MA)发生开环反应制备PE-g-MWCNTs,以提高MWCNTs在聚乙烯基体中的分散性。采用红外光谱、X-射线光电子能谱(XPS)和拉曼光谱对MWCNTs的化学修饰进行定性表征。结果表明:当MWCNTs的体积分数为0.67%时,MWCNT/PE复合材料的体积电阻率发生渗流突变。MWCNTs的体积分数在0.1%～1.2%时,MWCNT/PE复合材料体积电阻率由1016Ω.m下降至105Ω.m。