Chapter 1

Synthesis, Characterization and Functionalization of Carbon Nanotubes and Graphene: A Glimpse of Their Application

Mahe Talat and O.N. Srivastava*

Nanoscience and Nanotechnology Unit, Department of Physics, Banaras Hindu University, Varanasi, India

*Corresponding author: heponsphy@gmail.com

Abstract

Since the discovery of nanomaterials, carbon nanotubes structures have attracted great interest in most areas of science and engineering due to their unique physical and chemical properties and are supposed to be a key component of nanotechnology. The most recent addition to the family of carbon nanostructures is graphene. Graphene is a one-atom-thick material consisting of sp2-bonded carbon with a honeycomb structure. It resembles a large polyaromatic molecule of semi-infinite size. In the past five years, graphene-based nanomaterials have been the focus of not only material scientists but also engineers and medical scientists. The interesting and exciting properties of single-layer graphene sheets have excited the scientific community especially in the areas of materials, physics, chemistry and medical science. The state-of-the-art CNT production encompasses numerous methods and new routes are continuously being developed. The most common synthesis techniques are arc discharge, laser ablation, high pressure carbon monoxide (HiPCO) and chemical vapor deposition (CVD) with many variants. Most of these processes take place in vacuum or with process gases. By choosing appropriate experimental parameters, large quantities of nanotubes can be synthesized by these methods. It is possible to control some properties of the final product, such as type of CNTs synthesized (MWNTs vs. SWNTs), the quality of the nanotubes, the amount and type of impurities, and some structural CNT features. In this chapter we discuss some of the methods employed in our lab for the synthesis and characterization of the CNTs and graphene. For application in biomedical and targeted drug delivery, the major limitation of these nanomaterials is their poor solubility, agglomeration and processibility. Functionalization of CNTs and GS is, therefore, necessary to attach any desired compounds including drug and also to enhance the solubility and biocompatibility of these nanomaterials. Two types of functionalization methods, i.e., covalent and non-covalent methods are generally being adopted. We deliberate these two procedures of functionalization of CNTs and GS. The merits of these two modes of functionalization will also be discussed.

Keywords: Synthesis, characterization, application, CNT, graphene

1.1 Introduction

Carbon is the base element of all organic materials, one of the most abundant elements on earth and also the only element of the periodic table that occurs in allotropic forms from 0 dimensions to 3 dimensions due to its different hybridization capabilities. It has the unique ability to form allotropes, which can be described by valence bond hybridization, spn. The well-known forms are diamond, where n = 3, and graphite, having n = 2. It was shown that all other 1<n<3 are possible. The valence bond hybridization determines the physical and chemical properties of the allotropes of carbon. Fullerene (zero-dimensional), carbon nanotubes (one-dimensional) and graphene (single layer of graphite, two-dimensional) are all made of sp2-hybridized carbon atoms, whereas diamond (three-dimensional) is sp3 hybridized. The discovery of “fullerenes” added a new dimension to the knowledge of carbon science, which led to the Nobel Prize being awarded to R.F. Curl, H. Kroto and R.E. Smalley in 1996. Stimulated by fullerene discoveries, the Japanese scientist Ijima [1] discovered another new form of carbon-graphitic tubules and the CNT was born. He also suggested the possible structure of these tubes, which was later proven right. The subsequent discovery of “carbon nanotubes” (CNTs) added a new dimension to the science and technology of new carbons.

The most recent addition to the family of carbon nanostructures is graphene. Graphene is a one-atom-thick material consisting of sp2-bonded carbon with a honeycomb structure. It resembles a large polyaromatic molecule of semi-infinite size. In the past five years, graphene-based nanomaterials have been the focus of not only material scientists but also engineers and medical scientists. The interesting and exciting properties of single-layer graphene sheets, such as high mechanical strength, high elasticity and thermal conductivity, demonstration of the room-temperature quantum Hall effect, very high room temperature electron mobility, tunable optical properties, and a tunable band gap have excited the scientific community especially in the areas of materials, physics, chemistry and medical science. Following the series of Nobel Prizes awarded after the discovery of fullerenes, another nano star—graphene—received the 2010 Nobel Prize in Physics “for groundbreaking experiments regarding the two-dimensional material graphene” performed by Andre Geim and Konstantin Novoselov [2].

Therefore, the discovery and subsequent applications of these carbon nanomaterials have allowed the development of an entire branch of nanotechnology based on these versatile materials.

1.2 Synthesis and Characterization of Carbon Nanotubes

The first experimental evidence of carbon nanotubes (CNTs) came in 1991 [3] in the form of multi-wall nanotubes (MWNT), which motivated a sudden increase in nanotubes synthesis research. In 1993, the first experimental evidence of single-wall nanotubes (SWNT) was introduced [4]. Since then, the synthesis methods for CNTs have been developed tremendously. Production methods for carbon nanotubes (CNTs) can be broadly divided into two categories, chemical and physical, depending upon the process used to extract atomic carbon from the carbon-carrying precursor. Chemical methods rely upon the extraction of carbon solely through catalytic decomposition of precursors on the transition metal nanoparticles, whereas physical methods also use high energy sources, such as plasma or laser ablation to extract the atomic carbon. However, the most common synthesis techniques are arc discharge, laser ablation, high pressure carbon monoxide (HiPCO) and chemical vapor deposition (CVD) with many variants [5]. Most of these processes take place in vacuum or with process gases. By choosing appropriate experimental parameters, large quantities of nanotubes can be synthesized by these methods. It is possible to control some properties of the final product, such as type of CNTs synthesized (MWNTs vs SWNTs), the quality of the nanotubes, the amount and type of impurities, and some structural CNT features [6]. Other reported methods include plastic pyrolysis [7], diffusion flame synthesis and electrolysis using graphite electrodes immersed in molten ionic salts [8] and ball-milling of graphite [9].

In this chapter, we will discuss the method of synthesis of CNTs employed in our lab such as spray pyrolysis, arc discharge method and synthesis of CNTs by low pressure chemical vapor deposition (LPCVD) method, and catalytic decomposition of hydrocarbon gases, e.g., methane and ethylene, onto the Ferritin/Fe- SiO2- Si substrates.

Synthesis of CNTs was carried out using spray pyrolysis-assisted CVD method, where ferrocene (C10H10Fe) was used as a source of iron (Fe) which acts as a catalyst for the growth of CNTs. Castor oil was used as the carbon source; castor oil contains carbon, hydrogen and lower amount of oxygen. The spray pyrolysis setup consisted of a nozzle (inner diameter ~ 0.5 mm) attached to a ferrocene-castor oil supply used for releasing the solution into a quartz tube (700 mm long and inner diameter 25 mm), which was mounted inside a reaction furnace [10].

Spray pyrolysis of castor oil-ferrocene solution at ~ 850°C in Ar atmosphere leads to a uniform thick black deposition on the inner wall of the quartz tube at the reaction hot zone (~ 850°C). Figure 1.1(a) shows the SEM morphology of the as-grown CNTs. The length of CNTs was ~ 5–10 μm. Structural details of the as-grown CNTs sample were further investigated by TEM. Typical TEM image of the as-grown CNTs is shown in Figure 1.1(b). The TEM investigation of the as-grown CNTs confirms that the CNTs are multi-walled in nature. These nanotubes have varying diameters ranging from ~ 20–60 nm. In the spray pyrolysis reaction, the castor oil-ferrocene solution was atomized via spray nozzle and sprayed through carrier gas (Ar). The Fe particles (liberated by the decomposition of ferrocene) were deposited on the inner walls of the quartz tube. The carbon species released from decomposition of castor oil and also from ferrocene got adsorbed on the Fe particles and diffused rapidly along the axial direction leading to the formation of CNTs. A study was also done using castor oil-ferrocene with ammonia solution so as to develop CNTs containing nitrogen, i.e., C-N nanotubes. This was done keeping in view the fact that nitrogen-doped CNTs are considered as one of the important ingredients of CNT-based electronics. Figure 1.2(a) shows a typical SEM micrograph of as-grown C-N nanotubes, which reveals the wavy morphology of nanotubes. These wavy nanotubes are most likely due to pentagonal and heptagonal defects that are introduced in the hexagonal sheets. TEM images of the as-grown C-N nanotubes are shown in Figure 1.2(b). These CNTs have bamboo-shaped structures. The TEM image in Figure 1.2(b) shows that the nanotubes have a range of diameters varying from ~ 50–80 nm. It is suggested...