Pyrolytic carbon nanotubes from vapor-grown carbon fibers

• Published in: Carbon (New York) Vol. 33; no. 7; pp. 873 - 881
• Main Authors: Endo, Morinobu, Takeuchi, Kenji, Kobori, Kiyoharu, Takahashi, Katsushi, Kroto, Harold W., Sarkar, A.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 1995
• Subjects: Carbon nanotubes; graphite structure; high-resolution transmission electron microscope; nanotube growth mechanism; toroidal network; vapor-grown carbon fibers; Carbon nanotubes; nanotube growth mechanism; graphite structure; vapor-grown carbon fibers; high-resolution transmission electron microscope; toroidal network
• ISSN: 0008-6223; 1873-3891
• DOI: 10.1016/0008-6223(95)00016-7

The structure of as-grown and heat-treated pyrolytic carbon nanotubes (PCNTs) produced by hydrocarbon pyrolysis are discussed on the basis of a possible growth process. The structures are compared with those of nanotubes obtained by the arc method (ACNT; arc-formed carbon nanotubes). PCNTs, with and without secondary pyrolytic deposition (which results in diameter increase) are found to form during pyrolysis of benzene at temperatures ca. 1060 °C under hydrogen. PCNTs after heat treatment at above 2800 °C under argon exhibit have improved stability and can be studied by high-resolution transmission electron microscopy (HRTEM). The microstructures of PCNTs closely resemble those of vapor-grown carbon fibers (VGCFs). Some VGCFs that have micro-sized diameters appear to have nanotube inner cross-sections that have different mechanical properties from those of the outer pyrolytic sections. PCNTs initially appear to grow as ultra-thin graphene tubes with central hollow cores (diameter ca. 2 nm or more) and catalytic particles are not observed at the tip of these tubes. The secondary pyrolytic deposition, which results in characteristic thickening by addition of extra cylindrical carbon layers, appears to occur simultaneously with nanotube lengthening growth. After heat treatment, HRTEM studies indicate clearly that the hollow cores are closed at the ends of polygonized hemi-spherical carbon caps. The most commonly observed cone angle at the tip is generally ca. 20 °, which implies the presence of five pentagonal disclinations clustered near the tip of the hexagonal network. A structural model is proposed for PCNTs observed to have spindle-like shape and conical caps at both ends. Evidence is presented for the formation, during heat treatment, of hemi-toroidal rims linking adjacent concentric walls in PCNTs. A possible growth mechanism for PCNTs, in which the tip of the tube is the active reaction site, is proposed.