Nanotubes and Nanowires

By C.N.R. Rao, A. Govindaraj

The Royal Society of Chemistry

Copyright © 2005 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-832-8

Contents

Abbreviations, xi,
Chapter 1 Carbon Nanotubes, 1,
Chapter 2 Inorganic Nanotubes,
Chapter 3 Inorganic Nanowires,
Subject Index, 265,

CHAPTER 1

Carbon Nanotubes

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 sp 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 twelve pentagonal rings and twenty hexagonal rings (Figure 1.1a). Fullerenes were discovered by Kroto et al. 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. 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. Iijima 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 ~3.4 Å. Unlike the 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 1.1(b). In this nanotube, graphite layers surround the central tubule. Figure 1.1 (c) 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.

Ever since the discovery of the carbon nanotubes, several ways of preparing them have been explored. Besides MWNTs, SWNTs have been prepared by various methods, including electrochemical synthesis and pyrolysis of precursor organic molecules. The structure of carbon nanotubes has been extensively investigated by high-resolution electron microscopy.” The nanotubes, prepared by arc vaporization of graphite, are closed at both ends, but can be opened by various oxidants. There has been considerable success in filling nanotubes with various materials. 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. One of the developments is the synthesis of aligned nanotube bundles for specific applications. Various properties and phenomena as well as several possible and likely applications of carbon nanotubes have been reported. Unsurprisingly, therefore, these nanomaterials have elicited great interest. Several review articles, special issues of journals and conference proceedings” have dealt with carbon nanotubes. Some of the reviews present possible technological applications, with focus on the electronic properties, the recent book of Reich et al. 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.

Since the discovery of the carbon nanotubes, there has been considerable work on inorganic layered materials such as MoS22, WS22 and BN to explore the formation of nanotubes of these materials. Indeed several have been synthesized and characterized. Inorganic nanotubes are discussed at length in Chapter 2. Here, we shall present several aspects of carbon nanotubes, such as their preparation, structure, mechanism of formation, chemical substitution, properties and applications. We briefly examine the three fundamental aspects of CNTs, namely, their electronic structure and related properties, their vibrational and thermal characteristics and their 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.

2 Synthesis

Multi-walled Nanotubes

Carbon nanotubes are readily prepared by striking an arc between graphite electrodes in ~0.7 atm (~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. Arc evaporation of graphite has been carried out in various kinds of ambient gases (He, Ar, and CH). 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. 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 the arc-discharge method in liquid nitrogen. 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 (8 mm diameter) and the cathode is a pure carbon rod (10 mm 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 1 mm, 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 gases and also lowers the temperature of the electrodes. Furthermore, CNTs do not stick to the chamber wall. The content of the MWNTs can be as high as 70% of the reaction product. Analysis with Auger-spectroscopy revealed that no nitrogen was incorporated in the MWNTs. Synthesis in a magnetic field gives defect-free and high purity (>95%) MWNTs, which can be used as nanosized electric wires for device fabrication. Here, the arc discharge is controlled by a magnetic field around the arc plasma, created by using extremely pure graphite (purity >99.999%) electrodes (Figure 1.3a and 1.3b). MWNTs can be mass produced economically by the plasma rotating arc discharge technique. The centrifugal force caused by the rotation generates turbulence and aaccelerate the carbon vapour perpendicular to the anode (Figure 1.3c). Rotation also distributes the micro discharges uniformly and generates a stable plasma. Consequently, it increases the plasma volume and raises the plasma temperature. At a rotations of 5000 rpm, a yield of 60% is obtained at 1025 °C (without the use of a catalyst). The yield increases up to 90% if the rotation speed is increased at 1150°C. The MWNTs obtained generally have an inner diameter of 1–3 nm and an outer diameter of ~10 nm.

Deposition of carbon vapour on cooled substrates of highly oriented pyrolytic graphite affords tube-like structures. Carbon nanotubes are also produced by electrolysis in molten halide salts with carbon electrodes under argon. MWNTs with well-ordered graphitic structures have also been obtained under hydrothermal conditions around 800 °C, under 60–100 MPa pressure, using a polyethylene-water mixture in the presence of a nickel catalyst. Besides the conventional arc-evaporation technique, carbon nanotubes are produced by chemical vapour deposition (CVD), by the decomposition of hydrocarbons such as C2H2 under inert conditions around 700 °C over Fe/graphite, Co/graphite or Fe/silica catalysts. Transition metal particles are essential for the formation of nanotubes by the CVD or pyrolysis process, and the diameter of the nanotube is generally determined by the size of the metal particles.

Chemical Vapour Deposition (CVD)

The chemical vapour deposition (CVD) method uses a carbon source in the gas phase and a plasma or a resistively heated coil, to transfer the energy to the gaseous carbon molecule. Commonly used carbon sources are methane, carbon monoxide and acetylene. The energy source cracks the molecule into atomic carbon. The carbon then diffuses towards the substrate, which is heated and coated with a catalyst (usually a first row transition metal such as Ni, Fe or Co) and binds to it. Carbon nanotubes are formed in this procedure if the proper parameters are maintained. Good alignment as well as positional control on a nanometric scaleare achieved by using CVD. Control over the diameter, as well as the growth rate of the nanotubes is also achieved. Use of an appropriate metal catalyst permits preferential growth of single-walled rather than multi-walled nanotubes.

CVD synthesis of nanotubes is essentially a two-step process, consisting of a catalyst preparation step followed by synthesis of the nanotube. The catalyst is generally prepared by sputtering a transition metal onto a substrate, followed by etching by chemicals such as ammonia, or thermal annealing, to induce the nucleation of catalyst particles. Thermal annealing results in metal cluster formation on the substrate, from which the nanotubes grow. The temperature for the synthesis of nanotubes by CVD is generally in the 650–900 °C range. Typical nanotube yields from CVD are around 30%. Various CVD processes have been used for carbon nanotubes synthesis, including plasma-enhanced CVD, thermal chemical CVD, alcohol catalytic CVD, aerogel-supported CVD and laser-assisted CVD.

Plasma-enhanced Chemical Vapour Deposition

The plasma-enhanced CVD method involves a glow discharge in a chamber or a reaction furnace through a high-frequency voltage applied to both the electrodes. Figure 1.4 shows a schematic diagram of a typical plasma CVD apparatus with a parallel plate electrode structure. A substrate is placed on the grounded electrode. To form a uniform film, the reaction gas is supplied from the opposite plate. Catalytic metals such as Fe, Ni and Co are deposited on a Si, SiO2, or glass substrate using thermal CVD or sputtering. After the nanoscopic fine metal particles are formed, the carbon nanotubes grow on the metal particles on the substrate by the glow discharge generated from a high frequency power source. A carbon-containing gas, such as C2H2, CH4, C2H4, C2H6 or CO is supplied to the chamber during discharge. The catalyst has a strong effect on the nanotube diameter, growth rate, wall thickness, morphology and microstructure. Nickel seems to be the most suitable catalyst for the growth of aligned MWNTs by this technique. The diameter of the MWNTs is around 15 nm. The highest yield of carbon nanotubes achieved by Chen et al. was about 50%, at a relatively low temperature (<330 °C)

Thermal Chemical Vapour Deposition

In this method, Fe, Ni, Co or an alloy of these metals is initially deposited on a substrate. After the substrate is etched by a dilute HF solution, it is placed in a quartz boat, positioned in a CVD reaction furnace. Nanometre-sized catalytic metal particles are formed after an additional etching of the catalytic metal film using NH3 gas at 750–1050 °C. Nanotubes grow on the fine catalytic metal particles by the CVD process. When growing MWNTs on a Fe catalytic film by thermal CVD, the diameter range of the carbon nanotubes depends on the thickness of the catalytic film. When the film thickness was 13 nm, the diameter was between 30 and 40 nm. When a thickness of 27 nm was used, the diameter range was between 100–200 nm.

Vapour Phase Growth

In the vapour phase growth, pyrolysis or the floating catalyst method the carbon vapour and the catalytic metal particles are both deposited in the reaction chamber without a substrate. The diameter of the carbon nanotubes by vapour phase growth is in the range 2–4 nm for SWNTs and 70 and 100 nm for MWNTs. Using this technique, Sen et al. prepared MWNTs as well as metal-filled onion-like and nanotube structures by the pyrolysis of metallocenes such as ferrocene, cobaltocene, and nickelocene under reducing conditions, wherein the precursor acts as the source of the metal as well as carbon. The pyrolysis set-up consists of stainless steel gas flow lines and a two-stage furnace system fitted with a quartz tube (Figure 1.5), the flow rate of the gases being controlled by the use of mass flow controllers. In a typical preparation, a known quantity (100 mg) of the metallocene (presublimed 99.99% purity) is taken in a quartz boat and placed at the centre of the first furnace, and a mixture of Ar and H2 of the desired composition is passed through the quartz tube. The metallocene is sublimed by heating the first furnace to 200 °C at a controlled heating rate (20 °C/min-1). The metallocene vapour generated is carried by the Ar-H2 gas stream into the second furnace, maintained at 900 °C, where pyrolysis occurs. The main variables in the experiment are the heating rate of ferrocene, the flow rate of Ar, and the pyrolysis temperature. Figure 1.6(a) shows a TEM image of MWNTs obtained by the pyrolysis of mixture of C2H2 and ferrocene. Ferrocene vapour carried by a 75% Ar + 25% H2 mixture at 900 sccm (sccm = standard cubic centimetre per minute) into the furnace yields large quantities of carbon deposits, mainly containing carbon nanotubes. Under similar conditions, nickelocene with benzene gave MWNTs (Figure 1.6b). This procedure has been employed for large-scale production of carbon nanotubes.

Nebulized spray pyrolysis has been successfully employed for the synthesis of MWNTs. Figure 1.7 shows the schematic diagram of the experimental set-up. Silicon substrates were placed in the regions I–IV of the reactor to collect the product. In a typical synthesis, 2 g of metallocene was dissolved in 100 mL of a hydrocarbon and the solution nebulized using a 1.54 MHz ultrasonic beam carried into a 25 mm quartz tube, in a SiC furnace maintained at the required temperature (800–1000 °C). Pure argon was used as the carrier gas and the gas flow rate was controlled using UNIT mass flow controllers. In a typical procedure, the carrier gas flow rate was kept at 1000 sccm and pyrolysis was carried for 30 min. After the reaction, the furnace was allowed to cool. Products were collected after the tube cooled to room temperature. The average droplet size for the various solvents for 1.54 MHz frequency is ~2.2 µm. Ferrocene, cobaltocene, nickelocene and iron pentacarbonyl were used as both catalysts and carbon sources. Acetylene, benzene, toluene, xylene, mesitylene and n-hexane, used as solvents for the catalysts, act as additional carbon sources. The TEM images in Figure 1.8 show the MWNTs obtained by this method. Random or aligned bundles of MWNTs with fairly uniform diameters are obtained, depending on the flow rate and hydrocarbon solvent used for dissolving the metallocene. Well-graphitized MWNTs were obtained with a solution of ferrocene in xylene (inset in Figure 1.8d). The product quality depends on the pyrolysis temperature, carrier gas flow rate, additional carbon sources used and the catalyst precursor concentration. This procedure can be scaled up for continuous production of MWNTs.

Aligned Nanotube Bundles and Micropatterning

Some applications of carbon nanotubes, e.g. as electron emitters, require that they are aligned or micropatterned. Aligned nanotube bundles have been obtained by CVD over transition metal catalysts embedded in the pores of mesoporous silica or the channels of alumina membranes. Terrones et al. prepared aligned nanotubes over silica substrates, laser-patterned with cobalt. Ren et al. employed plasma-enhanced CVD on nickel-coated glass, using acetylene and ammonia mixtures, for this purpose. The mechanism of growth of nanotubes by this method and the exact role of the metal particles are not clear, although a nucleation process involving the metal particles is considered important. Fan et al. obtained aligned nanotubes by employing CVD on porous silicon and plain silicon substrates patterned with Fe films. The role of the transition metal particles assumes significance since aligned nanotubes are obtained by the pyrolysis of acetylene over iron/silica catalyst surfaces. Knowing that carbon nanotubes can be prepared by the pyrolysis of mixtures of organometallic precursors and hydrocarbons, one would expect that transition metal nanoparticles produced in situ in the pyrolysis may not only nucleate the formation of carbon nanotubes but also align them. This has been examined by pyrolysing metallocenes along with additional hydrocarbon sources, in a suitably designed apparatus (Figure 1.5). Figure 1.9 (a–c) shows scanning electron microscope (SEM) images of aligned nanotubes obtained by the pyrolysis of ferrocene. The image in Figure 1.9(a) shows large bundles of aligned nanotubes. Figure 1.9(b) shows the side-view whereas Figure 1.9(c) shows the top-view of the aligned nanotubes, wherein the nanotube tips are seen. A TEM image of a part of an aligned nanotube bundle obtained from the pyrolysis of an acetylene–ferrocene mixture is shown in Figure 1.9(d). The average length of the nanotubes is generally around 60 mm with methane and acetylene. Andrews et al. have carried out the pyrolysis of ferrocene–xylene mixtures to obtain aligned carbon nanotubes. Pyrolysis of Fe(II)phthalocyanine also yields aligned nanotubes. Hexagonally ordered arrays of nanotubes are produced by using alumina templates with ordered pores. Quasi-aligned carbon nanotubes are obtained by using metal impregnated zeolite templates. This method offers a control of the diameter of nanotubes through the use of zeolite templates of known diameter. The advantage of the precursor method is that aligned nanotube bundles are produced in one step, at a relatively low cost, without prior preparation of substrates. Rao and Govindaraj have reviewed the precursor route to carbon nanotubes.

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