Single-walled carbon nanotubes synthesized by the pyrolysis of pyridine over catalysts
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J. Cech and S. Roth Max-Planck-Institute for Solid State Research, 70569 Stuttgart, Germany (Received 17 April 2006; accepted 1 June 2006)
Single-walled carbon nanotubes (SWNTs) were successfully synthesized by pyrolysis of pyridine over MgO supported Fe–Mo or Co–Mo catalysts in the presence of pure H2 or a mixture of H2 and NH3 atmospheres. The average diameters of SWNTs are ∼1.5 and ∼3 nm for pure H2 and the H2 and HN3 mixture, respectively. Scanning tunneling spectroscopy (STS) studies show that the SWNTs grown in both atmospheres are doped with nitrogen substituted into the lattice in a pyridine-type structure. This results in a donor feature in the local density of states with an energy that depends on the nitrogen doping concentration.
I. INTRODUCTION
Single-walled carbon nanotubes (SWNTs) have been an area of intense research since their discovery in 19931,2 because of their exceptional mechanical and unique electronic properties.3,4 Based on these attributes, many potential applications, such as field emission devices,5 field-effect transistors,6 sensors,7 and supercapacitors8 have been proposed, and many research groups have worked on the production of SWNTs over the past decade. Arc discharge,9 laser vaporization,10 and catalytic chemical vapor deposition (CVD)11 have been studied to synthesize SWNTs. To date, the CVD method appears to be a promising technique because it has a potential for a large-scale synthesis of high-quality SWNTs at relatively low cost. In CVD methods, the SWNT growth is affected by catalyst (composition, type of support material and the morphology of the metal), carbon source, and growth temperature. So far, there have been many reports of the synthesis of SWNTs using various carbon-containing molecules in the CVD method.11–29 In these studies, CH4 and C2H2 have been used as the main carbon source for the synthesis of SWNTs. Nevertheless, to our knowledge, the use of pyridine has not been studied for SWNT synthesis, although it can be used to synthesize nitrogen-doped multi-walled carbon nanotubes (MWNTs).30–33 However, as is well known, as-prepared SWNTs are a mixture of metallic
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0346 J. Mater. Res., Vol. 21, No. 11, Nov 2006
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and semi-conductive tubes, and tailoring/controlling their electronic properties remains a major challenge. One method to tailor and modify the electronic properties of SWNTs may be to dope boron or nitrogen atoms into the carbon lattice. This is analogous to the boron and nitrogen doping of MWNTs, which can lead to p-type and n-type doping, respectively.34 Very recently, even SWNTs doped with both boron and nitrogen have been fulfilled via a bias-assisted hot filament (HF)CVD process.35 In this work, we used pyridine as a new carbon source and Fe–Mo catalysts prepared on MgO support by the combustion synthesis process. In general, materials such as silica (SiO2), alumina (Al2O3), zeolite, and magnesium oxi
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