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.     

Catalytic CVD synthesis of double and triple-walled carbon nanotubes by the control of the catalyst preparation

We report the influence of catalyst preparation conditions for the synthesis of carbon nanotubes (CNTs) by catalytic chemical vapour deposition (CCVD). Catalysts were prepared by the combustion route using either urea or citric acid as the fuel. We found that the milder combustion conditions obtained in the case of citric acid can either limit the formation of carbon nanofibres (defined as carbon structures not composed of perfectly co-axial walls or only partially tubular) or increase the selectivity of the CCVD synthesis towards CNTs with fewer walls, depending on the catalyst composition. It is thus for example possible in the same CCVD conditions to prepare (with a catalyst of identical chemical composition) either a sample containing more than 90% double- and triple-walled CNTs, or a sample containing almost 80% double-walled CNTs.     

Synthesis of carbon nanotubes with and without catalyst particles

The initial development of carbon nanotube synthesis revolved heavily around the use of 3d valence transition metals such as Fe, Ni, and Co. More recently, noble metals (e.g. Au) and poor metals (e.g. In, Pb) have been shown to also yield carbon nanotubes. In addition, various ceramics and semiconductors can serve as catalytic particles suitable for tube formation and in some cases hybrid metal/metal oxide systems are possible. All-carbon systems for carbon nanotube growth without any catalytic particles have also been demonstrated. These different growth systems are briefly examined in this article and serve to highlight the breadth of avenues available for carbon nanotube synthesis.     

Co—Mo／MgO作催化剂CVD法制备碳纳米管

研究了催化剂的制备方法对合成碳纳米管的影响,分别采用溶胶凝胶法、浸渍法和燃烧法制备了Co-Mo／MgO催化剂,并以乙炔为碳源,Ar气为保护气,在750-950℃常压下生长碳纳米管。采用TEM对所得产物进行了表征,结果表明,对于Co-Mo／MgO体系,相对浸渍法和燃烧法,溶胶凝胶法是很好的选择,可以得到数量与质量均较好的碳纳米管,并讨论了溶胶凝胶法制备碳纳米管的过程中工艺参数的择优。     

Selective preparation of carbon nanoflakes,carbon nanospheres,and carbon nanotubes through carbonization of polymethacrylate by using different catalyst precursors

This work reports the selective preparation of different kinds of carbon nanomaterials through carbonization of polymethacrylate (PMA)/organophilic clay (Oclay) composite by just changing the catalyst precursors. The morphologies and structures of the carbon materials were characterized by Scanning and Transmission electron microscopy, X‐ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy. The result indicated that the catalyst precusors had a strong influence on the morphologies and yields of the obtained products. Carbon nanoflakes were produced when iron oxide was used as the catalyst precursor, cobalt oxide favored the formation of carbon nanospheres, while carbon nanotubes occurred over nickel oxide. The presence of Oclay plallets was determined to prevent the pyrolytic carbon species of PMA in the system from escaping, thus relatively more carbon nanomaterials were obtained. Based on the experimental observations, a possible mechanism was discussed for illustrating the growth process. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1029‐1037, 2013     

生长温度及催化剂结构对碳纳米管形貌的影响

采用热CVD制备了图形化碳纳米管阵列薄膜。着重研究了生长温度、催化剂结构对碳纳米管形貌的影响。结果表明,当温度<800℃时,生长的碳纳米管纯度较低;当温度为850℃时,生长为碳纤维;当生长温度为800℃时,生长纯度较好的碳纳米管,但碳纳米管垂直性较差。采用Al/Fe催化剂结构,生长出图形化的阵列化碳纳米管薄膜。     

生长温度及催化剂结构对碳纳米管形貌的影响

采用热CVD制备了图形化碳纳米管阵列薄膜。着重研究了生长温度、催化剂结构对碳纳米管形貌的影响。结果表明,当温度800℃时,生长的碳纳米管纯度较低;当温度为850℃时,生长为碳纤维;当生长温度为800℃时,生长纯度较好的碳纳米管,但碳纳米管垂直性较差。采用Al/Fe催化剂结构,生长出图形化的阵列化碳纳米管薄膜。     

Effects of the catalyst pretreatment on CO2 sensors made by carbon nanotubes

A novel CO₂ sensor was made by carbon nanotubes (CNTs). The CNTs were synthesized by catalystic thermal chemical vapour deposition at 700 °C. Prior to the synthesis, the Fe catalysts were pretreated by H₂ plasma for different times. Two terminal resistance of the as-grown CNTs mat was measured under different CO₂ concentrations. It was found that without the catalyst pretreatment, the sensitivity was about 4% when the CNTs mat was exposed to 800 mTorr CO₂ concentration. However, with various catalyst pretreatment times of 5, 10, 15 and 20 min, the sensitivity was 3.69%, 6.27%, 9.54%, and 12.1%, respectively. The Raman spectroscopy showed the I_D/I_G decreased from 0.668 to 0.539 as the catalyst pretreatment time increased. The XPS also showed the correlation of surface chemical components with the Raman spectroscopy. The Fe catalyst H₂ plasma pretreatment affected both the graphitization and surface binding sites of CNTs.     

Density control of carbon nanotubes and filaments films by wet etching of catalyst particles and effects on field emission properties

Carbon nanotubes and filaments (CNT&F) films with controlled density were grown by low pressure thermal chemical vapour deposition from acetylene on nickel nanoparticles. Density control was achieved by wet etching of the catalyst particles before carbon growth. Field emission measurements were carried out on several films with different CNT&F densities obtained with this method. Despite strong morphological changes, only slight differences in the field emission characteristics between the highest and the lowest density films were detected, suggesting that almost none of the CNT&F suppressed by the etching step took part to the field emission. However, a maximum field amplification factor was reached for the medium density film. Taking into account the field amplification factor distribution, a model is proposed to link the particles diameter distribution, the CNT&F film morphology and the field emission properties.     

The production of amorphous regions in carbon nanotubes by 140 keV He ion irradiation

The production of amorphous regions in carbon nanotubes irradiated with 140 keV He ions was studied using Raman spectroscopy and transmission electron microscopy (TEM). Intensity ratios of Raman D to G bands (I_D/I_G) initially increase and then decrease as a function of ion fluence at all investigated irradiation temperatures (room temperature, 200 and 400 °C). The critical ion fluences corresponding to the maximum in I_D/I_G ratios increase with increasing irradiation temperature because of the enhanced defect annealing. The displacement per atom (dpa) values, consistent with a maximum in I_D/I_G ratios, are determined to be 0.15 dpa at room temperature and 200 °C, and 0.3 dpa at 400 °C. TEM examination of all irradiated specimens supports Raman results indicating that the maximum in I_D/I_G correlates to the formation of amorphous regions. The study shows that after formation of amorphous regions at high fluences, I_D/I_G ratio can be no longer used to measure amorphous/graphitic content in CNTs.     

Synthesis of vertically-aligned carbon nanotubes without a catalyst by hydrogen arc discharge

Vertically-aligned carbon nanotubes (CNTs) were prepared by hydrogen arc discharge, using pure graphite powder as carbon source without catalysts added. Scanning and transmission electron microscopy characterizations show that the aligned CNTs have a bamboo-like structure, and their lengths and diameters are about 30 μm and 40–60 nm, respectively. No metallic impurities can be detected in the samples by careful X-ray photoelectron spectroscopy detection. The activation of hydrogen radicals, the heating effect of the arc, and the electric field surrounding the arc column area are considered to play important roles for the non-catalyst growth of the CNTs.     

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。     

Effects of catalyst pre-treatment on the growth of single-walled carbon nanotubes by microwave CVD

Layers of carbon nanotubes were deposited by microwave CVD on oxidized silicon substrates coated with Al-Fe-Mo catalyst films. To achieve a tube growth at about 973 K, the ion bombardment of the catalyst surface has to be avoided. The appropriate pre-treatment of the substrates is essential for the deposition of single-walled carbon nanotubes. Annealing in air is preferable to the frequently used reducing pre-treatment prior to the deposition as a higher area density of the tubes and a better reproducibility of deposition can be obtained. To figure out this finding, selected samples were investigated by analytical transmission electron microscopy and Raman spectroscopy. It is shown that the pre-treatment has a strong effect on the size and distribution of the catalyst particles.     

Microscopic mechanisms for the catalyst assisted growth of single-wall carbon nanotubes

Whatever the synthesis technique used, the growth of ropes of single-wall carbon nanotubes requires the assistance of a metallic catalyst. In this paper, the role played by the catalyst is studied both experimentally and theoretically. Experimentally, the similarities between the samples synthesized from different techniques suggest a common growth mechanism proceeding via the precipitation of excess carbon on metallic nanoparticles. In this paper, the correlation between ropes and catalytic particles is investigated in detail in the case of the Ni-Y catalyst used in the arc discharge technique by combining high resolution transmission electron microscopy, X-ray and electron energy loss spectroscopy. It is shown that the ropes are always found attached to metallic particles about ten times larger than the tube diameter. A further remarkable proof of this relationship is provided by the chemical analyses of the metallic particles. These are found to be free of carbon and to always display the same Ni:Y composition range, whatever the initial Ni:Y composition of the catalyst mixture used in the synthesis, whereas the composition of other particles is highly dispersed. These experimental results support a mechanism of formation based on a vapor-liquid-solid model, in which the tubes of a given bundle nucleate in a cooperative manner and grow at the surface of a same metallic particle. This phenomenological scheme is supported by quantum molecular dynamics simulations which show that carbon atoms are incorporated at the root of a growing tube by a diffusion-segregation process occurring at the surface of the catalytic particle.     

Effect of the catalyst structure on the formation of carbon nanotubes over Ni/MgO catalyst

Ni/MgO solid solution catalyst was prepared by decomposition of nickel and magnesium nitrate using dielectric barrier discharge (DBD) plasma operated at atmospheric pressure and less than 175 °C. Well-defined lattice fringes of the Ni (111) plane are clearly observed in the plasma prepared Ni/MgO catalyst. The plasma prepared catalyst possesses fewer defects, compared to the catalyst prepared by thermal decomposition at elevated temperature. It results in a better balance between the carbon formation and the carbon nanotube (CNT) growth. The crystallinity of the Ni particle from thermal decomposition is more complex. It is difficult to distinguish the Ni planes with the thermal decomposed catalyst. CNTs from CO decomposition over the plasma made catalyst show a narrow diameter distribution with a high aspect ratio. The DBD plasma decomposition is a facile, simple and effective way for the preparation of Ni catalysts to fabricate high quality CNTs.     

Temperature and catalyst effects on the production of amorphous carbon nanotubes by a modified arc discharge

Large amounts of amorphous carbon nanotubes (ACNTs) were prepared with Co-Ni alloy powders as catalyst in hydrogen gas atmosphere by a modified arc discharging furnace which can control temperature during the electric arcing process. The experimental results indicate that the cooperative function of temperature and catalyst plays an important role in the soot production rate and the relative ACNT purity. When temperature increases from 25 °C to 700 °C, the soot production rate increases from around 1 g/h to 8 g/h, the best relative ACNT purity at 600 °C can reach up to 99% compared to the room temperature sample. Without catalyst, only plate graphite is formed at 25 °C and very few carbon nanotubes are found when temperature increases to 600 °C. TEM, SEM, HRTEM and XRD analysis showed that the as-prepared carbon nanotubes are almost amorphous. The soot production rate is 8 g/h and diameter range of amorphous carbon nanotubes is about 7-20 nm, respectively.     

Influence of catalytic particle size on the performance of fluidized-bed chemical vapor deposition synthesis of carbon nanotubes

In this paper, the impacts of catalytic particle size on the overall reactor performance for carbon nanotubes (CNTs) production using a fluidized-bed chemical vapor deposition (FBCVD) process have been studied. Six different particle size fractions (10-20 μm, 20-53 μm, 53-75 μm, 75-100 μm, 100-200 μm, and 200-300 μm) were selected. It was observed that the smaller the catalytic particle diameter, the greater the carbon deposition efficiency and the greater CNT synthesis selectivity. The 10-20 μm catalytic particles exhibited 30% higher carbon deposition efficiency than the 200-300 μm catalytic particles. The selectivity toward CNTs formation was also approximately 100%. These observations could be explained by the fact that when the diameter of the catalytic particle gets smaller, the breakthrough capacities during frontal diffusion will be bigger due to a shorter diffusion path length within the particle. Moreover, the fine particles ensured high interstitial velocity which subsequently enhances the heat and mass transfer, and consequently improves the CVD reaction.