Effect of nitrogen in the reaction atmosphere on the microstructure of carbon nanofibers grown by thermal chemical vapor
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Carbon nanofibers (CNFs) with different microstructures were synthesized by thermal chemical vapor deposition using different growth temperatures and methane/nitrogen gas mixtures. High-resolution transmission electron microscopy images revealed that bamboolike structure could be formed both by increasing the growth temperature and by increasing the nitrogen content in the reaction atmosphere at a lower growth temperature. Elemental analysis results indicated that no significant change in the nitrogen concentration was found regardless of the increase of nitrogen flow in the feed gas. The formation of bamboolike structure of CNFs and the effect of nitrogen gas on the microstructure change of CNFs were discussed. Synthesis of carbon nanofibers (CNFs), one of carbon nanomaterials with a similar dimension as multiwalled carbon nanotubes (MWCNTs), has been studied extensively in recent years.1 Unlike the seamless tube structure of CNTs, CNFs were reported to have different arrangements of graphene layers.2–4 Three microstructures have been designated as platelet, herringbone, and tubular by Chambers et al.2 A new type of CNF with a cup-stacked-type microstructure was also reported by Endo et al.3 The various arrangements of graphene layers in CNFs give them many potential applications.5–7 Formation of bamboolike structure has been observed in CNFs synthesized using both thermal chemical vapor deposition (CVD)8–10 and plasma-enhanced chemical vapor deposition (PECVD)11–16 techniques. It is considered that the enhancement of bulk diffusion of carbon atoms in catalyst leads to the formation of bamboolike structure.8–10,12–14 As the bulk diffusion of carbon atoms were enhanced in catalyst, more precipitated graphene layers on the bottom of the catalyst leads to an accumulation of stress, also resulting in the formation of separation diaphragms, and ultimately the bamboo structure forms. It was pointed out that nitrogen plays an important role in the formation of bamboo structure synthesized by PECVD.12–16 Ma and Wang11 discussed the microstructure transition from hollow structure to bamboo structure by adding the excess nitrogen gas and found that the formation of bamboo structure would be triggered when inducing the nitrogen gas. Lin et al.12 showed that the presence of nitrogen can promote formation of the bamboolike CNTs and pointed out that the role of nitrogen is
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0350 J. Mater. Res., Vol. 24, No. 9, Sep 2009
http://journals.cambridge.org
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basically to prolong the passivation of the front catalyst surface to enhance carbon diffusion. Using optical emission spectroscopy, Wang et al.13 showed that the presence of nitrogen promotes the dissociation of CH4 and that with the participation of nitrogen, the CNTs presented a polymerized nanobell structure. Wang and Wang14 showed that the hollow and bamboo-structured CNTs were grown by plasma-enhanced hot filament CVD using reaction gases without and with a
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