The synthesis and characterization of carbon nanotubes grown by chemical vapor deposition using a stainless steel catalyst

Multi wall carbon nanotubes (MWCNTs) were grown on a stainless steel (SS) sheet by chemical vapor deposition without the addition of external metal catalyst. We found that the key for highly efficient growth includes the nanoscale roughness of the SS surface, as shown by scanning tunneling microscopy, that acts as catalyst/template in the nanotube formation. Raman spectroscopy and electron microscopy were used to check the nature and quality of the synthesized nanotubes. We conclude that stainless steel favors a base-growth mechanism. Transmission electron energy loss spectroscopy performed on single metallic particles found inside the nanotubes clarified the atomic nature of the catalytic particles supplied by the steel. Only unoxidized iron was found and no traces of nickel and chromium were detected. In addition, the SS substrate has been used for a second growth process after carefully removing the synthesized CNTs, proving that a continuous production of CNTs from the same substrate is achievable.     

Effect of catalyst on carbon nanotubes synthesis on titanium diboride via chemical vapor deposition

Carbon nanotubes (CNTs) were in situ synthesized on titanium diboride (TiB₂) substrates using different catalysts (Fe, Co, and Ni) via ethylene chemical vapor deposition (CVD). The effects of various catalysts on the quality and quantity of grown CNTs were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman characterization techniques. Next, as-received powders were sintered by spark plasma sintering to obtain compact bulk composites. The effects of the metallic catalysts as sintering aids on the microstructural and mechanical properties of TiB₂-based composites were also studied. Results show that metal Ni not only plays a significant role in the catalyst to produce larger numbers of CNTs but also promotes the densification of TiB₂ ceramics in sintering and has been shown to provide good reinforcement. Maximum values of flexural strength (1093 ± 19 MPa) and toughness (13.9 ± 0.3 MPa ·m^1/2) were obtained using the Ni catalyst, which were significantly higher than the same values for Fe and Co catalysts.     

The chemical vapor deposition of carbon on carbon fibers

The relations between chemical vapor deposition (CVD) parameters and the resultant pyrolytic carbon microstructures have been examined for matrix deposition in fibrous carbon substrates. The parameters considered are temperature (1200–1450°C), pressure (20–630 Torr), C/H ratio ($\text{1}\text{4}\text{–}\text{1}\text{14}$), total flow rate (2–16) 1/min), and carbon felt density (0·12–0·23 g/cm³). Most of the data obtained are in agreement with a CVD model for carbon; where agreement is not obtained, it is surmised that the assumptions of the model may not be satisfied.     

Fabrication of Ni–B alloy coated vapor-grown carbon nanofibers by electroless deposition

Ni–B alloy coated vapor-grown carbon nanofibers (VGCNFs) were fabricated by electroless deposition and their microstructures were investigated. The effects of heat treatment on the coated VGCNFs were also studied. VGCNFs could be coated with a homogeneous Ni–B alloy film using a plating bath containing dimethylaminoborane (DMAB) as a reducing agent. The boron content of the Ni–B alloy film could be varied from 14 to 24 atom% B by varying the DMAB concentration of the plating bath. The VGCNFs were uniformly coated with a Ni–B alloy layer that was only several nanometers thick. The coating thickness on the VGCNFs could be controlled by varying the reaction time. The Ni–B alloy coatings formed in this study were semicrystalline or amorphous depending on the boron content of the alloy film. After heat treatment, the phase structure of the Ni–B alloy coatings changed to a stable crystalline structure consisting of a face-centered-cubic nickel phase and a Ni₃B phase. No cracks or exfoliation of the coatings were observed, even after heat treatment.     

CVD法制备碳纳米管的催化剂研究

CVD法制备单壁碳纳米管时有几个不可忽略的影响因素,其中催化剂的选取与制备极为重要,许多研究者采用不同的催化剂,获得了不同产量与质量的碳纳米管。本文主要从催化剂的选取和制备方法入手,综述了催化剂对碳纳米管制备的影响。     

Direct synthesis of fullerene-intercalated porous carbon nanofibers by chemical vapor deposition

Hybrid structures combining fullerenes and carbon nanotubes have exhibited exciting properties. However, the low efficiency and complex process of such assembly restrict their practical applications. We report a single-step procedure to synthesize the fullerene-intercalated (including endohedral metallofullerene (Y@Cn)) porous carbon nanofibers (pCNFs) by chemical vapor deposition (CVD) using a Fe/Y catalyst on a copper substrate. Fullerenes were simultaneously synthesized with the pCNF growth during the CVD process. Instead of attaching them on the surface of the CNFs, the fullerenes were inserted in the graphitic interlayer spacing, inducing micro- and mesopores in CNFs. The growth mechanism of the fullerene-intercalated pCNFs was discussed.     

Fluidized bed catalytic chemical vapor deposition synthesis of carbon nanotubes—A review

Carbon nanotubes (CNTs) are pure carbon in nanostructures with unique physico-chemical properties. They have brought significant breakthroughs in different fields such as materials, electronic devices, energy storage, separation, sensors, etc. If the CNTs are ever to fulfill their promise as an engineering material, commercial production will be required. Catalytic chemical vapor deposition (CCVD) technique coupled with a suitable reactor is considered as a scalable and relatively low-cost process enabling to produce high yield CNTs. Recent advances on CCVD of CNTs have shown that fluidized-bed reactors have a great potential for commercial production of this valuable material. However, the dominating process parameters which impact upon the CNT nucleation and growth need to be understood to control product morphology, optimize process productivity and scale up the process. This paper discusses a general overview of the key parameters in the CVD formation of CNT. The focus will be then shifted to the fluidized bed reactors as an alternative for commercial production of CNTs.     

A time‐dependent multiphysics,multiphase modeling framework for carbon nanotube synthesis using chemical vapor deposition

A time‐dependent multiphysics, multiphase model is proposed and fully developed here to describe carbon nanotubes (CNTs) fabrication using chemical vapor deposition (CVD). The fully integrated model accounts for chemical reaction as well as fluid, heat, and mass transport phenomena. The feed components for the CVD process are methane (CH₄), as the primary carbon source, and hydrogen (H₂). Numerous simulations are performed for a wide range of fabrication temperatures (973.15–1273.15 K) as well as different CH₄ (500–1000 sccm) and H₂ (250–750 sccm) flow rates. The effect of temperature, total flow rate, and feed mixture ratio on CNTs growth rate as well as the effect of amorphous carbon formation on the final product are calculated and compared with experimental results. The outcomes from this study provide a fundamental understanding and basis for the design of an efficient CNT fabrication process that is capable of producing a high yield of CNTs, with a minimum amount of amorphous carbon. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Low-temperature synthesis of graphene on Cu using plasma-assisted thermal chemical vapor deposition

Plasma-assisted thermal chemical vapor deposition (CVD) was carried out to synthesize high-quality graphene film at a low temperature of 600°C. Monolayer graphene films were thus synthesized on Cu foil using various ratios of hydrogen and methane in a gaseous mixture. The in situ plasma emission spectrum was measured to elucidate the mechanism of graphene growth in a plasma-assisted thermal CVD system. According to this process, a distance must be maintained between the plasma initial stage and the deposition stage to allow the plasma to diffuse to the substrate. Raman spectra revealed that a higher hydrogen concentration promoted the synthesis of a high-quality graphene film. The results demonstrate that plasma-assisted thermal CVD is a low-cost and effective way to synthesis high-quality graphene films at low temperature for graphene-based applications.     

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.     

Continuous synthesis of carbon spheres by a non-catalyst vertical chemical vapor deposition

High-purity carbon spheres were continuously produced by pyrolysis of acetylene at 1000 °C under a nitrogen atmosphere in a vertical chemical vapor deposition reactor. The produced carbon spheres had diameters in the range of 200–500 nm and were perfectly spherical in shape with rough surfaces. High resolution transmission electron microscopy analysis revealed that the carbon spheres were constructed of heavily distorted graphene layers. The results of the X-ray diffraction pattern and Raman spectra also confirmed that the presence of disordered graphene layers was due to a low graphitization degree. In addition, thermal stability and thermal oxidation of carbon spheres were studied. The results found that the surface of the carbon spheres could be modified and the amount of oxygen-containing functional groups increased after oxidation. In summary, the method provided a catalyst-free, substrate-free, hydrogen-free, and cost-effective synthesis for continuous production of carbon spheres.     

Laser assisted chemical vapor deposition synthesis of carbon nanotubes and their characterization

The deposition of carbon nanotubes using the laser assisted chemical vapor deposition process was studied to determine the effects of processing conditions on the quantity and quality of the tubes. A structured experimental design was utilized to test the effects of laser power, and concentration of the two precursors, acetylene and iron pentacarbonyl. Processing conditions were optimized with the assistance of heat and mass transport modeling. The synthesis of lines of carbon nanotubes as well as deposits formed under the influence of an electric field were also investigated.     

CVD法可控制备碳包覆金属纳米复合材料

以Fe质量分数分别为0.3%、1.6%、3.3%和5.2%的氯化钠担载铁为催化剂,化学气相沉积法催化裂解乙炔400℃下进行反应,系统探讨了碳包覆金属纳米颗粒的可控制备。通过扫描电子显微镜和高分辨透射电子显微镜对产物进行了表征,结果表明,w(Fe)=0.3%的催化剂制备的样品粒径在20～50 nm,平均直径约为30 nm;w(Fe)=1.6%的催化剂制备的样品粒径在35～60 nm,平均直径约为49 nm;而w(Fe)=3.3%和5.2%的催化剂制备的样品粒径差别不大,在40～100 nm,平均粒径约为65 nm。所制备的碳包覆金属纳米颗粒具有清晰的同心石墨壳层结构,并存在一定的结构缺陷。     

Kinetic study of carbon nanotubes synthesis by fluidized bed chemical vapor deposition

Multi‐walled carbon nanotubes (MWCNTs) have been produced with high selectivity by fluidized bed catalytic chemical vapor deposition from ethylene on Fe/Al₂O₃ catalysts. The influence of operating parameters such as deposition duration, temperature, ethylene and hydrogen partial pressures, and iron loading on MWCNT productivity, process selectivity, characteristics of final powders, and chemical composition of the outlet gases has been analyzed. Using gas phase chromatography, methane and ethane have been detected, whatever are the conditions used. Between 650 and 750°C, no catalyst deactivation occurs because nucleation remains active all along the synthesis, thanks to the explosion of the catalyst grains. Above 650°C, ethane itself produces MWCNTs, whereas methane does not react in the temperature range, 550–750°C. The formation of MWCNTs induces marked bed expansions and sharp decreases of grain density. Apparent kinetic laws have been deduced from the collected data. The apparent partial orders of reaction for ethylene, hydrogen, and iron were found to be 0.75, 0, and 0.28, respectively. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Synthesis of multi-walled carbon nanotubes by catalytic chemical vapor deposition using Cr2 − xFexO3 as catalyst

Chromium oxide and iron oxide solid solution was used as a catalyst for multi-walled carbon nanotubes synthesis by the catalytic chemical vapor deposition technique. The catalyst was prepared by the solution combustion synthesis method. Natural gas (NG) was employed as a carbon source for the carbon nanotube growth instead of methane, which is typically used. The carbon nanotube synthesis was carried out under H₂/NG and Ar/NG atmospheres at 950 °C. The Cr_2 − xFe_xO₃ catalyst was capable to produce carbon nanotubes only in H₂/NG atmospheres. Partial elimination of the catalyst after the synthesis was possible using a concentrated solution of HNO₃.     

The formation of stacked-cup carbon nanotubes using chemical vapor deposition from ethanol over silica

Processes involved in using SiO₂ particles as catalysts for stacked-cup carbon nanotube formation in a spray pyrolysis chemical deposition method from ethanol were investigated. In addition, the recyclability of the SiO₂ substrates is investigated. The SiO₂ particles are shown to reduce to SiC. Moreover, the addition of triethylborate to produce boron species, and extended reactions, through recycling, leads to higher yields by improving the availability of SiC species. The formation of the carbon nanostructures is best explained through a carbon dissolution mechanism.     

Kinetic modeling study of carbon nanotubes synthesis by fluidized bed chemical vapor deposition

The kinetic and physical laws developed in the first part of the study have been implemented in a modified version of the bubbling bed Kato and Wen model to represent multiwalled carbon nanotubes (MWCNTs) synthesis by catalytic chemical vapor deposition from ethylene as carbon source and using an Fe/Al₂O₃ catalyst. The absolute deviation for MWCNT productivity between experimental results of Part 1 and simulations is of 17.3% when only considering experiments for which the bed is mainly in bubbling regime. The influence of the main operating parameters on the evolutions with time of the species molar fractions, the weight of MWCNTs formed, and the bed characteristics has been numerically studied. Such capabilities can help designing new reactors. Finally, the model has been used for scale up purposes, by increasing the reactor diameter and catalyst weight. Simulations have shown that the process productivity could reach 74 tons/year of MWCNTs in a reactor 45 cm in diameter. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Synthesis of NiO-embedded carbon nanotubes using corona discharge enhanced chemical vapor deposition

Large-scale synthesis of NiO-embedded carbon nanotubes (CNTs) has been achieved using a nanoporous anodic aluminum (AAO) membrane as a template, with the aid of CH₄/H₂ corona discharge enhanced chemical vapor deposition (CVD). NiO nanoparticles are first introduced into the nanopores of the alumina template through wet impregnation method. The loading of NiO nanoparticles into the CNTs and the synthesis of the CNTs were simultaneously performed in the corona discharge reactor. Transmission electron microscopy characterization showed that the NiO nanoparticles are encapsulated into the walls of the CNTs, but not present on the outer surfaces.     

Selective area synthesis of aligned carbon nanofibers by laser-assisted catalytic chemical vapor deposition

Arrays of freestanding bamboo-type carbon nanofibers were grown on the surface of a porous alumina substrate by laser-assisted catalytic chemical vapor deposition. A continuous wave argon ion laser operated at a wavelength of 488 nm was used to thermally decompose pure ethylene over nickel catalysts. Two different catalyst preparation methods were used and are compared with respect to the synthesis of aligned nanofibers. First, a thin nickel film (50 nm) was evaporated on the substrate and was subsequently laser annealed into nanoparticles. This preparation produced non-aligned nanofiber films. Second, a 50 nm thick catalyst layer was electrochemically deposited within the pores of an alumina substrate. This preparation produced an array of vertically aligned nanofibers. A growth rate dependence on radial position within the irradiated area was observed. Average linear growth rates ranging from 554 nm/s to 25 μm/s are reported. The nanofibers were examined by scanning electron microscopy and Raman spectroscopy. Fiber texture and nanotexture were determined by lattice fringe analysis from high resolution transmission electron microscopy images. The alignment mechanism is also discussed.