Carbon Nanotubes

M. Endo, S. Iijima, M.S. Dresselhaus

• Publisher: Pergamon
• Year: 2013
• eBook ISBN: 9780080545530
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

Carbon nanotubes have been studied extensively in relation to fullerenes, and together with fullerenes have opened a new science and technology field on nano scale materials.A whole range of issues from the preparation, structure, properties and observation of quantum effects in carbon nanotubes in comparison with 0-D fullerenes are discussed.In addition, complementary reviews on carbon nanoparticles such as carbon nano-capsules, onion-like graphite particles and metal-coated fullerenes are covered.This book aims to cover recent research and development in this area, and so provide a convenient reference tool for all researchers in this field. It is also hoped that this book can serve to stimulate future work on carbon nanotubes.

PYROLYTIC CARBON NANOTUBES FROM VAPOR-GROWN CARBON FIBERS

ENDO MORINOBU¹, TAKEUCH KENJI¹, KIYOHARU KOBORI¹, TAKAHASHI A. KATSUSHI¹, W. KROTO HAROLD² and SARKAR², ¹Faculty of Engineering, Shinshu University, 500 Wakasato, Nagano 380, Japan; ²School of Chemistry and Molecular Sciences, University of Sussex, Brighton BNl 9QJ, U.K.

(Received 21 November 1994; accepted 10 February 1995)

Abstract

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.

Key Words

Carbon nanotubes

vapor-grown carbon fibers

high-resolution transmission electron microscope

graphite structure

nanotube growth mechanism

toroidal network

1. INTRODUCTION

Since Iijima’s original report[1], carbon nanotubes have been recognized as fascinating materials with nanometer dimensions promising exciting new areas of carbon chemistry and physics. From the viewpoint of fullerene science they also are interesting because they are forms of giant fullerenes[2]. The nanotubes prepared in a dc arc discharge using graphite electrodes at temperatures greater than 3000°C under helium were first reported by Iijima[1] and later by Ebbesen and Ajyayan[3]. Similar tubes, which we call pyrolytic carbon nanotubes (PCNTs), are produced by pyrolyzing hydrocarbons (e.g., benzene at ca. 1100°C)[4–9]. PCNTs can also be prepared using the same equipment as that used for the production of so called vapor-grown carbon fibers (VGCFs)[10]. The VGCFs are micron diameter fibers with circular cross-sections and central hollow cores with diameters ca. a few tens of nanometers. The graphitic networks are arranged in concentric cylinders. The intrinsic structures are rather like that of the annual growth of trees. The structure of VGCFs, especially those with hollow cores, are very similar to the structure of arc-formed carbon nanotubes (ACNTs). Both types of nanotubes, the ACNTs and the present PCNTs, appear to be essentially Russian Doll-like sets of elongated giant fullerenes[11,12]. Possible growth processes have been proposed involving both open-ended[13] and closed-cap[11,12] mechanisms for the primary tubules. Whether either of these mechanisms or some other occurs remains to be determined.

It is interesting to compare the formation process of fibrous forms of carbon with larger micron diameters and carbon nanotubes with nanometer diameters from the viewpoint of “one-dimensional” carbon structures as shown in Fig. 1. The first class consists of graphite whiskers and ACNTs produced by arc methods, whereas the second encompasses vapor-grown carbon fibers and PCNTs produced by pyrolytic processes. A third possible class would be polymer-based nanotubes and fibers such as PAN-based carbon fibers, which have yet to be formed with nanometer dimensions. In the present paper we compare and discuss the structures of PCNTs and VGCFs.

Fig. 1 Comparative preparation methods for micrometer size fibrous carbon and carbon nanotubes as one-dimensional forms of carbon.

2. VAPOR-GROWN CARBON FIBERS AND PYROLYTIC CARBON NANOTUBES

Vapor-grown carbon fibers have been prepared by catalyzed carbonization of aromatic carbon species using ultra-fine metal particles, such as iron. The particles, with diameters less than 10 nm may be dispersed on a substrate (substrate method), or allowed to float in the reaction chamber (fluidized method). Both methods give similar structures, in which ultra-fine catalytic particles are encapsulated in the tubule tips (Fig. 2). Continued pyrolytic deposition occurs on the initially formed thin carbon fibers causing thickening (ca. 10 μ m diameter, Fig. 3a). Substrate catalyzed fibers tend to be thicker and the floating technique produces thinner fibers (ca. 1 μ m diameter). This is due to the shorter reaction time that occurs in the fluidized method (Fig. 3b). Later floating catalytic methods are useful for large-scale fiber production and, thus, VGCFs should offer a most cost-effective means of producing discontinuous carbon fibers. These VGCFs offer great promise as valuable functional carbon filler materials and should also be useful in carbon fiber-reinforced plastic (CFRP) production. As seen in Fig. 3b even in the “as-grown” state, carbon particles are eliminated by controlling the reaction conditions. This promises the possibility of producing pure ACNTs without the need for separating spheroidal carbon particles. Hitherto, large amounts of carbon particles have always been a byproduct of nanotube production and, so far, they have only been eliminated by selective oxidation [14]. This has led to the loss of significant amounts of nanotubes – ca. 99%.

Fig. 2 Vapour-grown carbon fiber showing relatively early stage of growth; at the tip the seeded Fe catalytic particle is encapsulated.

Fig. 3 Vapor-grown carbon fibers obtained by substrate method with diameter ca. 10 μ m (a) and those by floating catalyst method (b) (inserted, low magnification).

3. PREPARATION OF VGCFs AND PCNTs

The PCNTs in this study were prepared using the same apparatus[9] as that employed to produce VGCFs by the substrate method[10,15]. Benzene vapor was introduced, together with hydrogen, into a ceramic reaction tube in which the substrate consisted of a centrally placed artificial graphite rod. The temperature of the furnace was maintained in the 1000°C range. The partial pressure of benzene was adjusted to be much lower than that generally used for the preparation of VGCFs[10,15] and, after one hour decomposition, the furnace was allowed to attain room temperature and the hydrogen was replaced by argon. After taking out the substrate, its surface wa...

Table of contents

• Cover image
• Title page
• Table of Contents
• Elsevier Journals of Related Interest
• Copyright
• EDITORIAL
• PREFACE
• Chapter 1: PYROLYTIC CARBON NANOTUBES FROM VAPOR-GROWN CARBON FIBERS
• Chapter 2: ELECTRIC EFFECTS IN NANOTUBE GROWTH
• Chapter 3: CATALYTIC PRODUCTION AND PURIFICATION OF NANOTUBULES HAVING FULLERENE-SCALE DIAMETERS
• Chapter 4: PHYSICS OF CARBON NANOTUBES
• Chapter 5: ELECTRONIC AND STRUCTURAL PROPERTIES OF CARBON NANOTUBES
• Chapter 6: CARBON NANOTUBES WITH SINGLE-LAYER WALLS
• Chapter 7: CARBON NANOTUBES: I. GEOMETRICAL CONSIDERATIONS
• Chapter 8: SCANNING TUNNELING MICROSCOPY OF CARBON NANOTUBES AND NANOCONES
• Chapter 9: TOPOLOGICAL AND SP3 DEFECT STRUCTURES IN NANOTUBES
• Chapter 10: HELICALLY COILED AND TOROIDAL CAGE FORMS OF GRAPHITIC CARBON
• Chapter 11: MODEL STRUCTURE OF PERFECTLY GRAPHITIZABLE COILED CARBON NANOTUBES
• Chapter 12: HEMI-TOROIDAL NETWORKS IN PYROLYTIC CARBON NANOTUBES
• Chapter 13: PROPERTIES OF BUCKYTUBES AND DERIVATIVES
• Chapter 14: ELECTRONIC PROPERTIES OF CARBON NANOTUBES: EXPERIMENTAL RESULTS
• Chapter 15: VIBRATIONAL MODES OF CARBON NANOTUBES; SPECTROSCOPY AND THEORY
• Chapter 16: MECHANICAL AND THERMAL PROPERTIES OF CARBON NANOTUBES
• Chapter 17: Flexibility of graphene layers in carbon nanotubes
• Chapter 18: NANOPARTICLES AND FILLED NANOCAPSULES
• Chapter 19: ONION-LIKE GRAPHITIC PARTICLES
• Chapter 20: METAL-COATED FULLERENES
• SUBJECT INDEX
• AUTHOR INDEX
• CARBON NANOTUBES