NANOTUBES *
CHAPTER 7 CHAPTER 7
7.1 INTRODUCTION
Carbon nanotubes (CNTs) are one of the most important materials under investigation for nano
technology. Their tensile strength is one hundred times that of steel, their thermal conductivity is found to be better than all but the purest diamond, and their electrical conductivity is similar to copper but with the ability to carry much higher currents (Holister, Harper, & Vas, 2003). NTs come in different shapes:
long, short, single walled, multiwalled, open, closed, with different types of spiral structure, etc. Each type has its own production costs and applications. Some have been produced in large quantities while others are only now being produced commercially with average purity and in quantities greater than a few grams. NTs have been constructed with length to diameter ratio of up to 132,000,000:1, which is much larger than any other material. They are considered as members of the fullerene structural family, which also includes the spherical buckyballs. Their name is derived from their size, since the diameter of an NT is on the order of only a few nanometers which is approximately 1/50,000th of the width of a human hair, while their length can be up to 18 cm. NTs are categorized as singlewalled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs) (Holister et al., 2003). Fig. 7.1 shows the two types of NTs.
The term “nanotube” is usually used to refer to the CNT, which has received high attention from researchers over the last few years and promises a host of interesting applications. There are many other types of NTs, from various inorganic kinds, such as those made from boron nitride, to organic ones, such as those made from selfassembling cyclic peptides or from naturally occurring heat shock proteins that are extracted from bacteria that thrive in extreme environments. However, CNTs excite the most interest, promise the greatest variety of applications, and currently appear to have by far the highest industrial potential (Holister et al., 2003).
7.1.1 HISTORY
In 1952, Radushkevich and Lukyanovich published clear images of 50 nm diameter tubes made of carbon in the Soviet Journal of Physical Chemistry. Later, a paper by Oberlin, Endo, and Koyama published in 1976 showed hollow carbon tubes with nanometersized diameters using a vapor growth technique. In 1979, John Abrahamson presented evidence of CNTs at the 14th Biennial Conference of Carbon at Penn State University. The paper described how CNTs were produced on carbon anodes
* By Yaser Dahman, Ahmad Bayan, Bohdan Volynets, and Navid Ghaffari.
146 CHAPTER 7 NANOTUBES
using the arc discharge apparatus. Later on, in 1981, a group of Soviet scientists published papers about carbon nanoparticles produced using the thermocatalytical disproportionation of carbon monox
ide method. The authors mentioned in their paper that the described carbon multilayer tubular crystals were created by rolling graphene layers into cylinders. Most of the academic literature credits the discovery of hollow, nanometersized tubes composed of graphitic carbon to Sumio Iijima of Nippon Electric Company in 1991 (Hirlekar et al., 2009).
7.1.2 CLASSIFICATION OF CNTs
CNTs are classified into the following two types: SWNT and MWNT.
Comparison between SWNT and MWNT is presented in Table 7.1.
7.1.3 MOLECULAR STRUCTURE
A CNT can be described as a cylindrical molecule composed of carbon atoms. A typical singlewalled carbon nanotube (SWCNT) structure is shown in Fig. 7.2. A major feature of the structure is the hexa
gon pattern that repeats itself periodically in space. As a result of the recurrence, each atom is bonded FIGURE 7.1
Single-walled and multi-walled nanotubes.
From Holister, P., Harper, T. E., & Vas, C. R. (2003). Nanotubes white paper. CMP Cientifica, 5–7.
147 7.1 INTrOdUcTION
to three neighboring atoms. The resulting structure is mainly due to the sp2 hybridization bonding during which one sorbital and two porbitals combine to form three hybrid sp2 orbitals at an angle of 120° to each other within the same plane. This covalent bond which is referred to as the σ bond and is shown in Fig. 7.3 plays an important role in the remarkable mechanical properties of CNTs as a result of this strong chemical bond. Also, the πbond is out of the plane and is relatively weak which contrib
utes to the interaction between the layers in multiwalled carbon nanotubes (MWCNTs) and between SWCNTs in SWCNT bundles (Ruoff et al., 2003).
7.1.4 STRUCTURES OF SWCNTs
The bonding in the CNTs is similar to the graphene sheet. A widely used approach to identify the types of SWCNT is by reference to rolling up the graphene sheet. The key geometric parameter associated
Table 7.1 Comparison Between SWNT and MWNT
SWNT MWNT
Single layer of graphene Multiple layer of graphene
Catalyst is required for synthesis Can be produced without catalyst Bulk synthesis is difficult as it requires proper control
over growth and atmospheric condition
Bulk synthesis is easy
Purity is poor Purity is high
A chance of defect is more during functionalization A chance of defect is less but once occurred it is difficult to improve
Less accumulation in body More accumulation in body
Characterization and evaluation is easy It has very complex structure It can be easily twisted and is more pliable It cannot be easily twisted
From Hirlekar, R., et al. (2009). Carbon nanotubes and its applications: a review. Asian Journal of Pharmaceutical and Clinical Research, 1–11.
FIGURE 7.2
Scheme of sp2 hybridization in graphene; σ bonds, π bonds, and their energies with respect to Fermi level.
From Ruoff, R., et al. (2003). Mechanical properties of carbon nanotubes: theoretical predictions and experimental measurements.
C. R. Physique, 4, 993–1008.
148 CHAPTER 7 NANOTUBES
with this process is the rollup vector r as shown in Fig. 7.4 which is made to be equal to na + mb, and therefore can be expressed as the linear combination of the lattice basis a and b. It is then possible to associate a particular integer pair (n,m) with each SWCNT. The values of n and m define three catego
ries of CNTs, if: m=0, “Zigzag”; n = m, “Armchair”; other, “Chiral” (Ruoff et al., 2003). Fig. 7.5 shows the different types of NTs.
FIGURE 7.3
Structure of a section of (10, 10) cNT. Each node shown is a carbon atom and lines represent the chemical bonds.
From Ruoff, R., et al. (2003). Mechanical properties of carbon nanotubes: theoretical predictions and experimental measurements.
C. R. Physique, 4, 993–1008.
FIGURE 7.4
definition of roll-up vector as linear combinations of base vectors a and b.
From Ruoff, R., et al. (2003). Mechanical properties of carbon nanotubes: theoretical predictions and experimental measurements.
C. R. Physique, 4, 993–1008.
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