Large-scale synthesis of highly aligned nitrogen doped carbon nanotubes by injection chemical vapor deposition methods
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In this study, we compare the effects of pyridine (C5H5N) and pyrimidine (C4H4N2) precursors, using ferrocene as a metal source, in the production of nitrogen containing multiwalled carbon nanotubes. Using standard chemical vapor deposition techniques, highly aligned mats of carbon-nitrogen carbon nanotube were synthesized. The maximum nitrogen concentration in these materials is between 1% and 2% when pyridine is used as the precursor and can be increased to 3.2% when pyrimidine is used as the precursor. However, the electronic structure of both materials, as determined using scanning tunneling spectroscopy, suggests that the nitrogen is incorporated into the nanotube lattice in the same way for both precursors.
I. INTRODUCTION 1
Since the first observation by Ijima, there has been much interest in preparing carbon nanotubes and related materials. Modification of nanotube properties is of significant interest to the materials science community. For example, C3N4 and CN nanotubes have been theoretically predicted to be superhard and metallic respectively.2–4 Efficient field emission has recently been shown using nitrogen doped multi-walled nanotubes.5 Thus, control over the electronic and structural properties of nanotubes, through substitutional doping schemes, provides an interesting route to broadening the potential applications of these materials. So far, many methods have been exploited to synthesize CN nanotubes, such as magnetron sputtering,6 chemical deposition,7–9 and pyrolysis.10–17 In spite of these efforts, control over N content and large-scale production viability is still beyond reach. This comes from a fundamental lack of understanding of growth mechanisms for doped materials. For instance, it is not clear how the bonding configuration of the substitutional dopant might change with the growth temperature and, through this, how the different precursors might affect the nitrogen concentration of carbon nanotubes. In this study, we present a comparison of the large scale synthesis of CNx nanotubes using injection-based chemical vapor deposition (CVD) of ferrocene in pyridine at different temperatures13,18 with the use of
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0069 538
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 2, Feb 2005 Downloaded: 10 Jan 2016
ferrocene in pyrimidine as a precursor to understand how the doping level of carbon nanotubes might be better controlled. Ferrocene is used as the catalyst throughout the study.15,16,19,20 X-ray photoelectron spectroscopy (XPS) was used to check the overall nitrogen concentration of carbon nanotubes and how the different doping changes with the growth temperature. The structure of the CNx nanotubes is characterized by transmission electron microcopy (TEM) and the electronic structure by scanning tunneling spectroscopy (STS). II. EXPERIMENTAL PROCEDURE
The experimental set up used to synthesize CNx nanotubes consists of a two-stage tubular quartz furnace (diameter ∼45mm, work length ∼4
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