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Possible tactics to improve the growth of single-walled carbon nanotubes by chemical vapor deposition

Centre for Nanoscale Science and Technology (CNST), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
Carbon (Impact Factor: 6.16). 01/2002; 40(14):2693-2698. DOI: 10.1016/S0008-6223(02)00175-6

ABSTRACT The growth time, growth mode and the method of preparing the supported catalysts play an important role in the growth of single-walled nanotubes (SWNTs). Their effects on the chemical vapor deposition (CVD) growth of SWNTs with MgO-supported catalysts were investigated in this study. It is shown that the growth rate of SWNTs was large during the initial few minutes of growth, however the quality of the tubes was low owing to the formation of many defects. Long term growth may favor the formation of tubes with high quality and high yield, but the introduction of other forms of carbon (impurities) is also unavoidable. There was a balance between the increase in yield and quality and sacrifice of the purity during growth of SWNTs. MgO-supported catalysts prepared by the co-precipitation method were found to be more effective for the synthesis of SWNTs than those prepared by the widely used impregnation method. The size and dispersion state of the catalyst were found to be crucial in enhancing the growth of SWNTs. In addition, growth on the surface of SWNTs over nanosized catalyst films was shown to be more favorable for the synthesis of tube products with higher quality, yield and purity.

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    • "In particular the A 1g mode zone of CNT–Fe1%–Co1%-800-30, which is presented in the inset of Fig. 2b, exhibits two sharp peaks at 232 cm À1 and 273 cm À1 . The RBM of CNTs corresponds to the coherent vibration of the C atoms in the radial direction and it is characteristic of SWCNTs [8] [42] [43], DWCNTs [40] [44] [45] or a combination of those two [24] synthesized by CCVD and other methods. The RBM position is strongly depended on the CNT diameter and therefore, it can be used to estimate the value of the synthesized tube's diameter. "
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    ABSTRACT: Fe–Co bimetallic catalysts supported on MgO were studied for the catalytic chemical vapor deposition growth of carbon nanotubes (CNTs). Different wt.% metal loadings were investigated at various deposition temperatures and times. Characterization of the products involved thermal analysis (DTA–TGA), X-ray diffraction, spectroscopy (Raman, UPS, EELS and STS) and microscopy (SEM, TEM and STM) techniques. It was found that the metal content is critical, not only to the yield and the structural quality of the synthesized carbon nanotubes, but it can be also used to tune the desired type of synthesized nanotubes. Lower (2 wt.%) loadings of Fe–Co catalysts favor the formation of single- and/or double-wall CNTs for deposition time and temperature 30 min and 800 °C, respectively. Thermal analysis and Raman measurements showed that these thin CNTs were synthesized at high amounts (CNT-per-catalyst wt.% of more than 100%), exhibiting high graphitization degree with only traces of by-products (mainly amorphous carbon) among them. Microscopy results revealed the formation of CNTs bundles, consisting of individual nanotubes with less than 2 nm outer diameter, while additional energy loss measurements pointed out that the deposited CNTs are mainly single wall. Higher (10 wt.%) Fe–Co loadings resulted to the formation of multi-wall CNTs.
    Carbon 10/2010; DOI:10.1016/j.carbon.2010.05.039 · 6.16 Impact Factor
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    • "DWCNTs would find application in field emission devices [1] and super-tough fibers [2] [3] [4]. In the chemical vapor deposition (CVD) process to prepare DWCNTs, active components are usually deposited on porous supports, such as Al 2 O 3 , MgO, SiO 2 , etc. [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18], which is one of the most important processes due to easy mass production at low cost. Major focus has been put on keeping a high specific surface area (SSA, Brunauer–Emmett–Teller (BET)) and dispersing the metal nanoparticles (NPs) on the catalyst to control the nucleation [10] [13], so as to control the purity and yield of DWCNTs. "
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    ABSTRACT: Double-walled carbon nanotubes (DWCNTs) were prepared from methane using a Fe/MgO porous catalyst. A series of catalyst powders with different pore size distributions were obtained by compression at pressures of 0–233 MPa. These were used to decompose methane and synthesize DWCNTs which differed in activity, purity, yield and degree of perfection. Characterization by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, thermo-gravimetric analysis, N2 adsorption measurement (Brunauer–Emmett–Teller (BET)) and Hg penetration provided direct evidence that a compact catalyst structure is not good for the nucleation and growth of DWCNTs, e.g., a catalyst with a compact structure that did not have pores larger than 30–50 nm mostly produced multi-walled carbon nanotubes. The confined growth and buckling model of DWCNTs inside the porous catalysts are proposed to explain the growth behavior. These results suggest that a porous catalyst for DWCNT synthesis should have a large pore size distribution or loose stacked structure, which provides new guidelines for catalyst design.
    Carbon 11/2008; 46(14-46):1860-1868. DOI:10.1016/j.carbon.2008.07.040 · 6.16 Impact Factor
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    • "however, contradicts theoretical energy studies, which predict that smaller-diameter SWCNTs require higher formation energies [20]. The ratios of the G-to D-band intensity, which indicate the relative amount of amorphous carbon and sidewall and tube end defects compared to highly ordered graphitic carbon, have been used as a purity index to assess the purity of the SWCNTs synthesized [21] [22] [23]. A G-to D-band ratio much greater than unity indicates an abundance of highly ordered graphite and graphitic defects and a low amount of undesired carbon species. "
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    ABSTRACT: High-quality single-walled carbon nanotubes (SWCNTs) have been synthesized from H2–CH4 mixtures on a MgO-supported bimetallic Mo/Co catalyst using microwave plasma-enhanced chemical vapor deposition (PECVD). Reaction parameters including temperature, H2:CH4 ratio, plasma power, and synthesis time have been examined to assess their influence on SWCNT synthesis. Raman spectroscopy and high-resolution field emission scanning electron microscopy reveal that the quality, selectivity, density and predominant diameter of SWCNTs depend on the varied synthesis parameters. Results of this study can be used to optimize SWCNT synthesis conditions and products and to improve understanding of the growth of SWCNTs by PECVD.
    Carbon 01/2006; 44(1-44):10-18. DOI:10.1016/j.carbon.2005.07.027 · 6.16 Impact Factor
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