Preparation and modification of well-aligned CNTs grown on AAO template

Anodic aluminum oxide (AAO) template was prepared by a two-step anodization method and cobalt particles were carefully deposited at the bottom of pores of the template. CVD method was used to grow carbon nanotubes (CNTs) on the template with this cobalt as catalyst and C₂H₂ as carbon source. Well-aligned carbon nanotubes were obtained perpendicular to the substrate. To modify the CNTs, ultrasonic treatment of the samples in alcoholic (or water) was applied and the results showed that the length of CNTs became equalized and their tips were opened without catalyst particles due to ultrasonically cutting off CNTs for the upper parts out of AAO pores. The mechanism is also discussed.     

Synthesis of carbon nanotubes by CVD: Effect of acetylene pressure on nanotubes characteristics

The effect of acetylene partial pressure on the structural and morphological properties of multi-walled carbon nanotubes (MWCNTs) synthesized by CVD on iron nanoparticles dispersed in a SiO₂ matrix as catalyst was investigated. The general growing conditions were: 110 cm³/min flow rate, 690 °C synthesis temperature, 180 Torr over pressure and two gas compositions: 2.5% and 10% C₂H₂/N₂. The catalyst and nanotubes were characterized by HR-TEM, SEM and DRX. TGA and DTA were also carried out to study degradation stages of synthesized CNTs. MWCNTs synthesized with low acetylene concentration are more regular and with a lower amount of amorphous carbon than those synthesized with a high concentration. During the synthesis of CNTs, amorphous carbon nanoparticles nucleate on the external wall of the nanotubes. At high acetylene concentration carbon nanoparticles grow, covering all CNTs’ surface, forming a compact coating. The combination of CNTs with this coating of amorphous carbon nanoparticles lead to a material with high decomposition temperature.     

Improved emission stability of HfC-coated carbon nanotubes field emitters

Plasma-enhanced chemical vapor deposition (PECVD) method was employed to synthesize the Fe-catalyzed carbon nanotubes (CNTs). Hf films were deposited onto the synthesized CNTs, followed by heat treatment at 1200 °C which could form HfC. Field emission properties indicate that the HfC-coated CNTs have good emission current density due to low work function of HfC and also keep stable emission characteristics under poor vacuum owing to the chemical inertness of HfC. Consequently, field emission characteristics of the CNTs can be improved by the HfC-coated surface treatment compared with the synthesized CNTs.     

Synthesis of nanocomposites based on nanotubes and silicates

In situ synthesis of nanocomposites based on carbon nanotubes and zeolite/montmorillonite was carried out in a hot filament CVD reactor where the precursors (methane and hydrogen) are activated by carbonized tungsten filaments heated up to 2200 °C. In nanocomposites formed both on zeolite and montmorillonite we observed cross-linking of the catalytic particles by nanotubes and creation of carbon nanotube bridges and three-dimensional networks. The length of nanotube bridges was in a range from several nm to nearly 10 μm. A high density of carbon nanotubes was observed in the whole volume of zeolite. The high catalytic efficiency of zeolite is most likely caused by its structure that allows anchoring of Fe³⁺ catalytic particles in the pores and prevents their migration from the sample. At the ends of the nanotubes grown on zeolite we observed particles of the catalyst. In montmorillonite, the particles catalyzing the growth of carbon nanotubes may be present not only on the external surface but also in the interlayer voids of the mineral. Its catalytic efficiency is enhanced as proved by the higher amount of CNTs and their bundles. In the course of CNTs synthesis probably also clumps of Fe³⁺ catalytic particles arise, which may be the reason for formation of bundles of nanotubes.     

Synthesis of carbon nanotubes on Ni-alloy and Si-substrates using counterflow methane–air diffusion flames

We have conducted an experimental study to investigate the synthesis of multi-walled carbon nanotubes (CNTs) in counterflow methane–air diffusion flames, with emphasis on effects of catalyst, temperature, and the air-side strain rate of the flow on CNTs growth. The counterflow flame was formed by fuel (CH₄ or CH₄ + N₂) and air streams impinging on each other. Two types of substrates were used to deposit CNTs. Ni-alloy (60% Ni + 26% Cr + 14% Fe) wire substrates synthesized curved and entangled CNTs, which have both straight and bamboo-like structures; Si-substrates with porous anodic aluminum oxide (AAO) nanotemplates synthesized well-aligned, self-assembled CNTs. These CNTs grown inside nanopores had a uniform geometry with controllable length and diameter. The axial temperature profiles of the flow were measured by a 125 μm diameter Pt/10% Rh–Pt thermocouple with a 0.3 mm bead junction. It was found that temperature could affect not only the success of CNTs synthesis, but also the morphology of synthesized CNTs. It was also found, against previous general belief, that there was a common temperature region (1023–1073 K) in chemical vapor deposition (CVD) and counterflow diffusion flames where CNTs could be produced. CNTs synthesized in counterflow flames were significantly affected by air-side strain rate not through the residence time, but through carbon sources available for CNTs growth. Off-symmetric counterflow flames could synthesize high-quality CNTs because with this configuration carbon sources at the fuel side could easily diffuse across the stagnation surface to support CNTs growth. These results show the feasibility of using counterflow flames to synthesize CNTs for particular applications such as fabricating nanoscale electronic devices.     

Aligned growth and alignment mechanism of carbon nanotubes by hot filament chemical vapor deposition

Carbon nanotubes (CNTs) growth on Inconel sheets was carried out using hot filament chemical vapor deposition (HFCVD) in a gas mixture of methane and hydrogen. Scanning electron microscopy, transmission electron microscopy and field electron emission (FEE) measurement were applied to study the structure and FEE properties of the deposited CNTs. The effect of bias voltage and substrate surface roughness on the growth of vertically aligned carbon nanotubes was investigated. Well-aligned CNTs were synthesized by bias enhanced HFCVD. The results show that a bias of −500 V generates the best alignment. It has been observed that at the early growth stage, aligned and non-aligned CNTs are growing simultaneously on the unscratched sheets, whereas only aligned CNTs are growing on the scratched sheets. The results indicate that tip growth is not necessary for the electric field to align the CNTs, and larger catalyst particles created by scratching before the heat treatment can induce alignment of CNTs at the early growth stage. In addition, tree-like CNTs bundles grown on the scratched substrates exhibit better FEE performances than dense carbon nanotube forest grown on the unscratched substrates due to the reduced screen effect.     

Synthesis of carbon nanotubes via Fe-catalyzed pyrolysis of phenolic resin

Carbon nanotubes (CNTs) with 40–100 nm in diameter and tens of micrometers in length were prepared via catalytic pyrolysis of phenol resin in Ar at 673–1273 K using ferric nitrate as a catalyst precursor. Structure and morphology of pyrolyzed resin were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Ferric nitrate was transformed to Fe₃O₄ at 673 K, and to metallic Fe and Fe_xC carbide at 873–1273 K. The optimal weight ratio of Fe catalyst to phenol resin for growing CNTs was 1.00 wt%, and the optimal temperature was 1073 K. In addition, use of a high pressure increased the yield of CNTs. Density functional theory (DFT) calculations suggest that Fe catalysts facilitate the CNTs growth by increasing the bond length and weakening the bond strength in C₂H₄ via donating electrons to the C atoms in it.     

Growth of carbon nanotubes on stainless steel substrates by DC-PECVD

We report on the fabrication of carbon nanotubes (CNTs) on Ni-coated stainless steel (SUS) substrates by using dc plasma enhanced chemical vapor deposition. The synthesized CNTs have the diameter of about 30 nm and the length of about 1.2 μm. To verify the effects of SUS substrates on the growth of CNTs, CNTs had also been grown on Ni-coated Si substrates. CNTs grown on the SUS substrates were more uniform compared with those grown on the Si substrates. Field emission properties of the CNT films were measured in the diode configuration, and the turn-on electric field of 3.87 V/μm and field enhancement factor β of about 1737 were obtained from the synthesized CNTs at the gap of 500 μm between the SUS substrate and the anode. These results have not only clarified the effects of the substrate on the growth of CNTs, but also shown the potential of CNTs in field emission applications, especially CNT-based cold-cathode X-ray tubes.     

Behavior of multi-walled carbon nanotubes on the porosity and microstructure of cement-based materials

The porosity and microstructure of a Portland cement-multi-walled carbon nanotube composite were investigated. Multi-walled carbon nanotubes (CNTs), up to 1 wt.% of cement, synthesized by infusion chemical vapor deposition, and Portland cement type I (PC) were used to produce pastes with a water to cement ratio of 0.5. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were used to characterize Portland cement-CNTs systems. MIP analysis of the results indicates that total porosity of the mixes with CNTs was found to decrease with increasing CNTs content. Moreover, an important effect of additional CNTs was a reduction in the number of mesopores, while SEM technique showed dispersion of CNTs between the hydration phases of Portland cement pastes.     

Synthesis of multi-wall carbon nanotubes by the pyrolysis of ethanol on Fe/MCM-41 mesoporous molecular sieves

Ordered hexagonal arrangement MCM-41 mesoporous molecular sieves were synthesized by the traditional hydrothermal method, and Fe-loaded MCM-41 mesoporous molecular sieves (Fe/MCM-41) were prepared by the wet impregnation method. Their mesoporous structures were testified by X-ray diffraction (XRD) and the N₂ physical adsorption technique. Carbon nanotubes (CNTs) were synthesized by the chemical vapor deposition (CVD) method via the pyrolysis of ethanol at atmospheric pressure using Fe/MCM-41 as a catalytic template. The effect of different reaction temperatures ranging from 600 to 800 ^°C on the formation of CNTs was investigated. The resulting carbon materials were characterized by various physicochemical techniques such as transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. The results show that multi-wall carbon nanotubes (MWCNTs) with an internal diameter of ca. 7.7 nm and an external diameter of ca. 16.9 nm were successfully obtained by the pyrolysis of ethanol at 800 ^°C utilizing Fe/MCM-41 as a catalytic template.     

CVD synthesis and radial deformations of large diameter single-walled CNTs

Single-walled carbon nanotubes (CNTs) were synthesized by a chemical vapor deposition (CVD) method on transmission electron microscopy (TEM) silica coated nickel grids using carbon monoxide as carbon source and iron nanoparticles as catalyst. The produced CNTs were as large as 11 nm in diameter. Investigations on the CNT deformations based on high-resolution TEM images showed that the deformation of CNTs due to their interaction with the substrate occurs at diameters larger than 2.7 nm. Small deformation of free standing tubes was found to occur at diameters above approximately 4.5 nm.     

A comparative study of field-emission from different one dimensional carbon nanostructures synthesized via thermal CVD system

Different one dimensional (1D) carbon nanostructures, such as carbon nanonoodles (CNNs), carbon nanospikes (CNSs) and carbon nanotubes (CNTs) have been synthesized via thermal chemical vapour deposition (TCVD) technique. The different 1D morphologies were synthesized by varying the substrate material and the deposition conditions. The as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). FESEM and TEM images showed that the diameters of the CNNs and CNTs were ∼40 nm while the diameters of the CNSs were around 100 nm. Field emission studies of the as-prepared samples showed that CNSs to be a better field emitter than CNNs, whereas CNTs are the best among the three producing large emission current. The variation of field emission properties with inter-electrode distance has been studied in detail. Also the time dependent field emission studies of all the nanostructures have been carried out.     

Formation and structure of boron nitride conical nanotubes

Nanotubes exhibiting a novel structure - boron nitride (BN) conical nanotubes whose walls consist of conical layers with their cone axis parallel to the tube axis, as opposed to ordinary nanotubes, composed of concentric cylindrical layers with their normal perpendicular to the tube axis - were synthesized simultaneously with BN nanotubes by using carbon nanotubes (CNTs) as templates. The diameters of the BN conical nanotubes are typically about 15 nm, which is similar to those of the starting CNTs. Apex angles and inner diameters of most BN conical nanotubes are about 40° and 1 nm, respectively. The lengths of the BN conical nanotubes range from 50 nm to up to several micrometers.     

微量水对碳纳米管形貌的影响及其机理研究

利用介质阻挡放电等离子体化学气相沉积技术,在蒸镀有25nm Ni催化剂层的Si基片上,以CH₄和H₂作为反应气体,在973K下制备了碳纳米管,并研究了微量水的引入对碳纳米管形貌的影响.场发射扫描电子显微镜结果表明,不加水时,制备出的碳纳米管直径不均匀,分布在40—90nm之间,呈链节状的结构;加入少量水时,制备出的碳纳米管直径比较均匀,集中在70nm左右,表面为瘤状结构;当水的流量进一步增加时,得到的碳纳米管表面光滑,出现了枝状结构.原位测量了加水前后等离子体区的发射光谱,分析了微量水的引入对于碳纳米管形貌变化的影响机理. 关键词： 碳纳米管 介质阻挡放电 水 发射光谱     

Preparation and luminescence study of Eu(III) titanate nanotubes and nanowires using carbon nanotubes as removable templates

Eu(III) titanate nanotubes and nanowires have been successfully synthesized by solvothermal method using carbon nanotubes (CNTs) as removable templates. The products were characterized by X-ray diffraction spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectrometry, thermogravimetric and differential thermal analysis. It is demonstrated that CNTs are fully coated with an amorphous Eu₂(TiO₃)₃ layer, which is about 10 nm thick and almost continuous and uniform. After the Eu₂(TiO₃)₃/CNTs composites have been calcined at various temperatures, Eu₂(TiO₃)₃ nanotubes and nanowires are obtained by removing the CNTs templates. The diameter of the Eu₂(TiO₃)₃ nanotubes is 40–60 nm, which is consistent with that of CNTs. Both nanotubes and nanowires have a narrow distribution of diameters. The fluorescence properties of the Eu₂(TiO₃)₃ nanotubes and nanowires calcined at various temperatures have been investigated. The results indicate that when the Eu₂(TiO₃)₃/CNTs composites were calcined at 700 °C for 5 h, the Eu₂(TiO₃)₃ nanotubes obtained can be effectively excited by 395 nm light, and exhibit strong red emission around 616 nm. It is very interesting to discover that a few residual carbons doped in Eu₂(TiO₃)₃ nanotubes and many oxygen vacancies could promote the intensity of red emission peak of Eu³⁺ ions. In addition, Eu₂(TiO₃)₃ nanowires calcined at 900 °C for 5 h also have a strong red emission peak due to many oxygen vacancies and defects formed on the surface of the nanowires and inside them.     

Electrical,structural and thermal studies of carbon nanotubes from natural legume seeds: kala chana

ABSTRACT

Carbon nanotubes (CNTs) are the carbon materials measured at nanoscale level and they are defined in two types according to the number of concentric layers, i.e. single-layer tube is single-walled nanotubes, while multi-layer tube structure is called multi-walled nanotubes. The green method synthesis for the preparation of CNTs begins with the smashing of legume seeds kala chana, and then they form complex with cobalt salt. Desiccation of the complex compound forms cobalt salt and seed protein. The complex is then decomposed at 625 ºC in muffle furnace for 20 min. Purification of the decomposed sample is done through acid wash treatment and dried in vacuum oven. The confirmations of CNTs are done by nuclear magnetic resonance and Fourier transform infrared, which analyzes the denatured protein, reacted to the metal salt. X-Ray diffraction determines the MWNTs with transmission electron microscope (TEM) reports the network structure of CNTs. thermal gravimetric analysis (TGA)–differential thermal analysis (DTA)–thermogravimetric analysis (DTG) tests the amount of sample under thermal treatment. Vibrating sample magnetometer determines the paramagnetic nature of CNTs. CNTs thus prepared can be used in mechanical fields, in solar cells, in electronics fields, etc. because of their multidisciplinary properties. The synthesized CNTs are eco-friendly in nature, prepared by the legume seed natural precursor.     

初始温度及碳源对碳纳米管气相爆轰法合成的影响

改变初始温度以及分别使用甲烷和乙炔气体作碳源时气相爆轰合成碳纳米管,研究了初始温度与不同碳源对碳纳米管的影响。利用X射线衍射(XRD)、透射电镜(TEM)、拉曼(Raman)光谱等对碳纳米管进行表征。结果表明,随着初始温度的升高,所合成的碳纳米管的产量减少且石墨化程度降低,但管壁会变得光滑且管径有所增加。当使用乙炔时,所合成的产物中没有碳纳米管,而是合成了石墨化程度较高的无定形碳,随着催化剂量的增加,产物中碳包覆颗粒增多且包覆层清晰可见,但存在结构缺陷。当初始温度在110~130 ℃时,使用甲烷气体运用气相爆轰的手段是合成碳纳米管的较佳方案。     

Synthesis and crystallinity of carbon nanotubes produced by a vapor-phase growth method

We have studied the effect of temperature on the growth and crystallinity of carbon nanotubes (CNTs), synthesized by a vapor-phase growth method using a catalytic reaction of iron pentacarbonyl (Fe(CO)₅) and acetylene (C₂H₂) gas. By increasing the growth temperature from 750 °C to 950 °C, both the growth rate and the diameter of the CNTs increase. Moreover, the crystallinity of the graphite sheets improves progressively with increasing growth temperature. Adjustment of the growth temperature gives potential for controlled growth of CNTs in a large-scale synthesis of CNTs. PACS 61.46.+w; 68.37.-d; 81.07.De     

Detailed profiling of CNTs arrays along the growth window in a floating catalyst reactor

We report a detailed longitudinal and depth profiles of multi-wall carbon nanotubes (CNTs) arrays synthesized using xylene and ferrocene in a floating catalyst reactor. Point to point analyses of the CNTs grown in a “growth window” with CNTs arrays longer than 0.5 mm were performed using optical microscopy, Raman spectroscopy, FESEM, high-resolution TGA/DTA, and TEM techniques. The heights of the CNTs arrays show a maximum at a mid point of the growth window, while a reverse trend of minimum is observed for iron-to-CNTs atomic ratios. The ratio of amorphous carbon to CNTs sharply increases along the growth window and from the bottom to top of CNTs arrays. The CNTs diameter also increases along the growth window, due to deposition of the amorphous carbon, which can be almost removed by temperature programmed oxidation up to around 500 °C. A base growth mechanism, the variations of catalyst content, residence time and temperature profile along the growth window, the adsorption and decomposition of polycyclic aromatic hydrocarbons to amorphous carbon, and a limited diffusion of hydrocarbon species through the arrays covered by excessive amorphous carbon may explain the results.     

Fabrication of completely filled carbon nanotubes with copper nanowires in a confined space

Carbon nanotubes (CNTs) filled completely with polycrystalline Cu nanowires were synthesized by laser vaporization of Cu and graphite under high-pressure Ar gas atmosphere. Depending on the Ar gas pressure (0.1–0.9 MPa) and the Cu content (1–40 at.%) in graphite targets for laser vaporization, various products with different morphologies were observed by scanning and transmission electron microscopy. The ratios of the Cu-filled CNTs and carbon nanocapsules particularly increased as Ar gas pressure was increased. The maximum ∼60% fraction of Cu-filled CNTs with outer diameter of 10–50 nm and length of 0.3–3 μm was achieved at 0.9 MPa from graphite containing 20 at.% Cu. Most of the encapsulated Cu-nanowires were surrounded by single, double, or triple graphitic layers. Although the yield of the Cu-filled CNTs was also dependent on the Cu content in the graphite targets, no unfilled CNTs were produced even for low Cu content. The growth of Cu-filled CNTs is explained by the formation of molten Cu–C composite particles with an unusually C-rich composition in a space confined by high-pressure Ar gas, followed by precipitating Cu and C from the particles and subjecting them to phase separation.