Nanotubes and Nanowires

C N Ram Rao, A Govindaraj

• Publisher: Royal Society of Chemistry
• Year: 2015
• ISBN:
  9781782626275
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Nanoscience

Research and literature on nanomaterials has exploded in volume in recent years. Nanotubes (both of carbon and inorganic materials) can be made in a variety of ways, and they demonstrate a wide range of interesting properties. Many of these properties, such as high mechanical strength and interesting electronic properties relate directly to potential applications. Nanowires have been made from a vast array of inorganic materials and provide great scope for further research into their properties and possible applications. This book provides a comprehensive and up-to-date survey of the research areas of carbon nanotubes, inorganic nanotubes and nanowires including: synthesis; characterisation; properties; applications Nanotubes and Nanowires includes an extensive list of references and is ideal both for graduates needing an introduction to the field of nanomaterials as well as for professionals and researchers in academia and industry.

CHAPTER 1

Carbon Nanotubes

1.1 Introduction

Diamond and graphite are the two well-known forms of crystalline carbon. Diamond has four-coordinate sp³ carbon atoms that form an extended three-dimensional network, whose motif is the chair conformation of cyclohexane. Graphite has three-coordinate sp² carbons that form planar sheets, whose motif is the flat six-membered benzene ring. The new carbon allotropes, the fullerenes, are closed-cage carbon molecules with three-coordinate carbon atoms tiling the spherical or nearly-spherical surfaces, the best known example being C₆₀, with a truncated icosahedral structure formed by 12 pentagonal rings and 20 hexagonal rings (Figure 1.1a). Fullerenes were discovered by Kroto et al.¹ in 1985 while investigating the nature of carbon present in interstellar space. The coordination at every carbon atom in fullerenes is not planar, but slightly pyramidalized, with some sp³ character present in the essentially sp² carbons. The key feature is the presence of five-membered rings, which provide the curvature necessary for forming a closed-cage structure. In 1990, Krätschmer et al.² found that the soot produced by arcing graphite electrodes contained C₆₀ and other fullerenes. The ability to generate fullerenes in gram quantities in the laboratory, using a relatively simple apparatus, gave rise to intense research activity on these molecules and caused a renaissance in the study of carbon. Iijima³ observed, in 1991, that nanotubules of graphite were deposited on the negative electrode during the direct current arcing of graphite for the preparation of fullerenes. These nanotubes are concentric graphitic cylinders closed at either end due to the presence of five-membered rings. Nanotubes can be multi-walled with a central tubule of nanometric diameter surrounded by graphitic layers separated by ∼3.4Å. Unlike in multi-walled nanotubes (MWNTs), in single-walled nanotubes (SWNTs), there is only the tubule and no graphitic layers. A transmission electron microscope (TEM) image of a MWNT is shown in Figure 1.1b. In this nanotube, graphite layers surround the central tubule. Figure 1.1c shows the structure of a nanotube formed by two concentric graphitic cylinders, obtained by force-field calculations. A single-walled nanotube can be visualized by cutting C₆₀ along the centre and spacing apart the hemispherical corannulene end-caps by a cylinder of graphite of the same diameter. Carbon nanotubes are the only form of carbon with extended bonding and yet with no dangling bonds. Since carbon nanotubes are derived from fullerenes, they are referred to as tubular fullerenes or bucky tubes.

Figure 1.1 (a) Schematic diagram of a C₆₀ molecule. (b) A TEM image of a multi-walled carbon nanotube. (c) Minimum energy structure of a double-walled carbon nanotube. (Reproduced from ref. 31a). (d) Electron diffraction pattern of a multi-walled carbon nanotube. (Reproduced from ref. 31c).

Ever since the discovery of carbon nanotubes,³ several ways of preparing them have been explored.^4,5 Multi-walled (MWNTs), double-walled (DWNTs) and single-walled (SWNTs) carbon nanotubes have been synthesised using methods which include arc evaporation of graphite, laser ablation, chemical vapour deposition (CVD) and vapour phase decomposition or disproportionation of carbon-containing molecules.^6,7 Kumar and Ando⁸ have reviewed the synthesis, growth mechanism and mass production of carbon nanotubes by CVD. MWNTs and SWNTs have also been prepared by techniques such as electrochemical synthesis⁹ and pyrolysis of precursor organic molecules.¹⁰ Among the various types of carbon nanotubes, SWNTs are of special interest because of their unique properties and potential applications. Specific methods have been found to obtain long SWNTs¹¹ as well as diameter-control of the nanotubes.¹² SWNTs of various forms such as rings,¹³ brushes^14a and films^14b have been prepared. Vertically aligned¹⁵ as well as horizontally aligned¹⁶ carbon nanotubes have been prepared on different substrates by several workers. Various methods as well as new strategies employed for the enrichment or selective generation of SWNTs with metallic, semiconducting and other unique electronic properties has been reported.¹⁷ Diameter selective dispersions of SWNTs are reported.¹⁸

The structure of carbon nanotubes has been extensively investigated by high-resolution electron microscopy.^19–21 Nanotubes, prepared by arc vaporization of graphite, are closed at both ends, but can be opened by various oxidants.^22,23 There has been considerable success in filling nanotubes with various materials.²⁴ Apart from opening and filling, carbon nanotubes have been doped with boron and nitrogen, giving rise to p-type and n-type materials respectively. By employing carbon nanotubes as removable templates, oxidic, carbidic and other nanostructures have been prepared. Aligned nanotube bundles have been grown for specific applications. Properties and phenomena as well as several possible and likely applications of carbon nanotubes have been reported extensively. Unsurprisingly, therefore, these nanomaterials have elicited great interest. Several review articles, special issues of journals and conference proceedings^25–31 have dealt with carbon nanotubes. Some of the reviews present possible technological applications, with focus on the electronic properties,^30,31 the book of Reich et al.³² being devoted to a detailed presentation of the basic physics of carbon nanotubes. There are several other reviews and books as well, some of which are cited as references.^33–38

Since the discovery of the carbon nanotubes, there has been considerable work on inorganic layered materials such as MoS₂, WS₂ and BN to explore the formation of nanotubes of these materials. Indeed several have been synthesized and characterized.^39–42 Inorganic nanotubes are discussed at length in Chapter 2. Here, we shall present various aspects of carbon nanotubes, which include their preparation, structure, mechanism of formation, chemical modification, functionalization, properties and applications. Specifically, we discuss their electronic structure and related properties, vibrational, thermal characteristics and mechanical properties. These aspects are interrelated, since both thermal and mechanical properties reflect the chemical bonding in the carbon network, which controls their electronic structure as well.

1.2 Synthesis

1.2.1 Multi-walled Nanotubes

Carbon nanotubes are readily prepared by striking an arc between graphite electrodes in ∼0.7atm (∼500 Torr) of helium, which is considerably larger than the helium pressure used to produce fullerene soot. The schematic diagram of the apparatus is shown in Figure 1.2. A current of 60–100 A across a potential drop of about 25 V gives high yields of carbon nanotubes. The arcing process can be optimized such that the major portion of the carbon anode gets deposited on the cathode as carbon nanotubes and graphitic nanoparticles.^43a Arc evaporation of graphite has been carried out in various kinds of ambient gases (He, Ar and CH₄).^43b

Figure 1.2 Schematic diagram of an arc discharge apparatus. (Reproduced from ref. 43b).

Hydrogen appears to be effective in producing MWNTs of high crystallinity. Arc-produced MWNTs in hydrogen also contain very few carbon nanoparticles. Carbon nanotubes have been produced in large quantities by using plasma arc-jets by optimizing the quenching process in an arc between a graphite anode and a cooled copper electrode.^44,45 If both the electrodes are of graphite, MWNTs are the main products, along with side products such as fullerenes, amorphous carbon, and graphite sheets.

A route to highly crystalline MWNTs is by arc discharge in liquid nitrogen.⁴⁶ In this method, vacuum is replaced with liquid nitrogen in the arc discharge chamber. Typically, direct current was supplied to the apparatus using a power supply. The anode is a pure carbon rod (8mm diameter) and the cathode is a pure carbon rod (10mm diameter). The Dewar flask is filled with liquid nitrogen and the electrode assembly immersed in nitrogen. Arc discharge occurs as the distance between the electrodes became less than 1mm, and a current of ∼80 A flows between them. When the arc discharge is over, carbon deposits near the cathode are recovered for analysis. Liquid nitrogen prevents the electrodes from contamination with unwanted gase...