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A Detailed Study on Carbon Nanotubes: Properties, Synthesis, and Characterization

Nishant Tripathi1, Prachi Sharma1,2, Vladimir Pavelyev1, Anastasiia Rymzhina1, and Prabhash Mishra1,3,4

1Nanoengineering Department, Samara National Research University, 34, Moskovskoye Shosse, Samara, 443086, Russia

2Department of Electrical, Electronics and Communication Engineering, GITAM School of Technology, Bengaluru Campus, GITAM (Deemed to be University) NH 207, Nagadenehalli, Doddaballapura, Karnataka 561203, India

3Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia, Jamia Nagar, New Delhi, 110025, India

4Center for Photonics & 2D Materials, Moscow Institute of Physics and Technology (MIPT), Dolgoprudny 141700, Russia

1.1 Introduction

The present chapter deals with the evolution of carbon allotropes, especially with carbon nanotubes (CNTs). CNTs are one of the most important nanomaterials of the carbon family. The invention of CNTs brought a real revolution in the field of nanoscience and nanotechnology. The astonishing properties of CNTs make them a suitable material for the development of applications in electronics, optoelectronics, medical, and in many other fields. Worldwide researchers are doing aggressive work on CNTs-based gas sensors, optical detectors, heat detectors, humidity sensors, transistors, nanoelectronics devices, and display applications [1, 2]. It has been observed that different types of applications based on CNTs require different types of CNTs. For example, sensors, especially for gas and optical applications, required horizontal aligned/network type of CNTs while field emission devices required vertically aligned CNTs. So, it has become very important to grow CNTs with selective structure and orientation [3]. Generally, CNTs can be grown by chemical vapor deposition (CVD), arc discharge, and laser ablation technique but among all the techniques CVD is the most preferable due to its capability to grow CNTs with selective properties. In general, the growth process of CNTs by using CVD system, a catalyst film deposited substrate is loaded into the chamber of CVD system, and then the chamber is heated around 800?°C in presence of a carrier gas; at 800?°C along with carrier gas, a hydrocarbon gas is supplied in CVD chamber for CNTs growth. CVD has lots of parameters to tune to decide CNTs' structural quality as well as morphology. The major parameters are the type of catalyst, catalyst deposition technique, catalyst engineering, growth temperature, growth duration as well as the type of carrier and hydrocarbon gas [3-5].

After synthesis of CNTs, its analysis became an important task. To analyze CNTs, researchers utilized various nanomaterial characterization tools such as scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM), Raman spectroscopy, and X-ray diffraction (XRD). SEM and AFM are useful for analyzing morphology of CNTs while TEM, Raman spectroscopy, and XRD are useful for the characterization of structural quality of CNTs.

All of the discussions mentioned motivated us to write a present book chapter. The chapter deals with evolution of Carbon from graphite, diamond, graphene to CNTs. It covers in detail various structures of CNTs, defects in CNTs, and their applications. It also covers the various synthesis and characterization techniques of CNTs. Finally, we made a broad discussion on the synthesis of selective CNTs.

1.2 Evolution of Carbon: Graphite to CNTs

The fundamental constituent or the building block of CNTs is Carbon. This group IV element of the periodic table is well known for its incredible ability to form crystalline solids and a variety of other compounds. It is placed in sixth position in the periodic table. Two out of its six electrons lie in 1s orbital, remaining electrons form sp3, sp2, or sp hybrid orbital. These four valence electrons constitute allotropes of Carbon such as CNT, diamond, graphite, graphene, and fullerenes [6]. It is an important fact that existing electronic bonds are very poor in the outer two orbitals as compared to the first orbital. Due to the weaker attraction of outer shell electrons than inner shell electrons, outer orbital electrons participate in electron hybridization. Small energy gradient between outer two orbitals (i.e. 2s and 2p) aids the overlapping of orbital wave functions favoring electron hybridization. There are three accessible mixing of atomic orbitals in carbon atoms making hybrid orbitals typically referred to as sp, sp2, and sp3 hybridization. In sp hybridization, one 2s and one 2p electrons participate in mixing (forming s-bonds) and leaving two 2p electrons free of mixing (p-bonds). sp2 hybridization involves the mixing of one's orbital electron and two 2p electrons. In sp3 hybridization, all outer shell electrons of carbon atoms take part in mixing. The orbitals are focused on corners of a tetrahedron restricted to carbon atoms. According to hybridization, a carbon atom makes bonds with minimum one to maximum of four partners to produce compounds. Structural quality of carbon-based compounds and allotropes also depend on type of hybridization. For spl hybridization, (l?+?1) s bonds take charge to form generally one-dimensional local structure.

The linear chain compound of carbon i.e. "Carbyne" is an example of sp hybridization. sp2 hybridization leads to the formation of two-dimensional structures such as graphene. 3D-structures such as diamond are formed by sp3 hybridization. It is also noticed that in sp and sp2 mixing one or two p orbital does not involve hybridization, instead showing their presence in the form p-bonds. Depending on orbital mixing carbon has many allotropes and among these amorphous carbon, diamond, graphite, graphene, and CNTs have received a great amount of attention [7]. These allotropes of carbon exhibit a part or completely different properties in nature. For their commendable and extraordinary multidimensional properties be it CNTs, graphene or diamond, and others, allotropes of carbon have become a hot cake for research investigations.

1.2.1 Graphite

The word graphite is taken from a Greek word "graphein" i.e. "write." Pencil, which is an attractive and widely used tool for writing and drawing, is made from it. It was invented by Debye et al. in 1917 [8]. Graphite is a combined structure of layers. Each layer is called Graphene [9]. In graphite structure, carbon atoms are situated in hexagonal fashion on the XY plane [10]. The distance between carbon atoms in a single layer of graphite is 0.142?nm. The separation distance between layers of graphite is approximately 0.335?nm [11].

The graphene layers are held together by weak van der Waals forces to form solid structure graphite. Each carbon atom is situated on edge of hexagon having three s-bond in sp2 hybridization form in which three valence electrons participate in hybridization and one valence electron exists in pz orbital creating two-dimensional electron gas in the form of p-bond or cloud. It is spread all over on individual XY planes of graphite. Due to the mobility of said electron gas, graphite shows electrical conductivity. It is more reactive than diamond. The separation distance between the nearest two carbon atoms on an individual layer of graphite is around 0.142?nm that is approximately equal to band order of 1.5 and two times larger than the aromatic carbon atom's covalent radius. The separation gap among the nearest two layers of graphite is approximately two times of van der Waals radius. The weak van der Waals interaction within layers of graphite causes the layers of graphite to easily move along the XY plane. Generally, two types of graphite are found in nature abundantly: a stable form that is hexagonal or a-graphite and rhombohedral or ß-graphite. ß-graphite is more abundant in nature than a-graphite. By heat treatment, ß-graphite can be changed into a-graphite. Other than these two, a few more types of graphite are found in nature. Sometimes, due to some disorder in stacking, no more attraction exists between layers and hence individual layers of graphene or graphite randomly turn around the z-axis and move in the XY plane resulting in a turbostratic structure.

1.2.2 Diamond

Diamond is an allotrope of carbon with sp3 hybridization. Every carbon atom in diamond is bonded to four nearest carbon atoms. Each diamond cell has a tetrahedron structure with a bond length of 154.45?pm. Mainly, two types of structures of diamond are found: cubic and hexagonal. Cubic structure is more available than hexagonal [12, 13]. Hexagonal-shaped diamonds are very precious and rare to find. It was first found in 1967 in a meteorite. Still, there is a possible way to synthesize artificially hexagonal type diamond from graphite by heat treatment of graphite under high pressure with ambient temperature along the vertical axis. Some other structures are also found in which nitrogen molecules are mixed with carbon atoms. Some noticed nitrogen-contained diamonds are Ia, Ib, IIa, and IIb. If...