Formation of loops on the surface of carbon nanofibers synthesized by plasma-enhanced chemical vapor deposition using an
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San-Der Chyou, Win-Tai Lee, and Chih-Shen Chen Taiwan Power Research Institute, Taiwan Power Company, Taipei 238, Taiwan, Republic of China (Received 23 March 2006; accepted 27 July 2006)
Carbon nanofibers (CNFs) were synthesized by both high-density plasma-enhanced chemical vapor deposition (CVD) and thermal CVD. The growth in the former was carried out in an inductively coupled plasma (ICP) reactor. The multilayer loop structure, which was reported to be found on both the inner and outer surfaces of cup-stacked-type CNFs grown using thermal CVD only after heat treatment above 1500 °C, was observed in the as-grown CNFs only on the outer surface using ICP-CVD. The dangling bonds caused by plasma etching and the bonding between edge carbon atoms aided by the high-density plasma are considered the main reasons of the formation of multilayer loops.
A variety of microstructures gives carbon nanofibers (CNFs) many potential applications. Three microstructures have been designated as platelet, herringbone, and tubular by Chambers et al.1 In a recent study, Endo et al. showed a new type of CNF with a cup-stacked-type microstructure.2 The structural feature of the cupstacked-type CNFs is the stacking morphology of truncated conical graphene layers, which exhibit a large portion of open edges both on the outer surface and in the inner core. These CNFs showed a different structure when heat treated at 1800–3000 °C.3 The most significant structural change after heat treatment is the formation of stable loops on both the inner and outer surfaces of CNFs between the adjacent open edges of graphene planes. Lower annealing temperature of the loop formation was also reported later by Endo et al.4 They suggested that a single-layer loop (SLL) forms at 900– 1200 °C between neighboring truncated conical graphene layers and that transition from SLL to multi-layer loop (MLL), which is an energetically stable structure, occurs above 1500 °C. Endo et al. take the release of hydrogen atoms, which are bonded on the edges of graphite layers during synthesis, as the reason for loop formation.4 When the hydrogen is released, the dangling bonds at the edges of graphite planes have to be stabilized through formation of the loop structure.4 Loop structure was also
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0330 2440
J. Mater. Res., Vol. 21, No. 10, Oct 2006
observed by Zheng et al.5 in CNFs with platelet and herringbone structures, heat treated at 2500 °C. In this paper, we report the observation of loop structure on the outer surface of the as-grown CNFs synthesized by plasma-enhanced chemical vapor deposition (CVD) using an inductively coupled plasma (ICP) reactor with low substrate temperature (800 °C). We synthesized CNFs using a Ni–Al2O3 composite catalyst6–8 by both ICP-CVD and thermal-CVD processes. In the ICP-CVD apparatus, the inductive coil was arranged cylindrically in the chamber. For the thermalCVD process, a three-zone tube furnace was used. The Ni–Al2O3 catalyst was derived
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