CARBON NANOTUBES

CNTs are nano-architectured allotropes of carbon which are few nanometers in diameter and form one-dimensional structures because the length is orders of magnitude larger than the diameter. Based on the way in which carbon nanotubes are rolled, they are divided into SWCNTs and MWCNTs (Fig. 1.4). SWNTs are formed when single graphene sheets are rolled in the form of a tube whereas MCWNTs are the result of multiple rolled single graphene sheets rolled up to form a tube, while the MWCNTs are the result of concentric tubes of gr aphene. Due to some unusual properties like high degree of stiffness, a large length-to-diameter ratio and exceptional resilience, CNTs are used for a variety of applications.⁸⁹ The report of Sumio Iijima in 1991 introduced CNTs into the scientific community. It was followed by individual publications on the methods to obtain SWNTs by Iijima and Donald Bethune in 1993. All fields of sciences welcomed CNTs with both hands and later it was the era of CNTs.

Several fossil fuel related carbon sources like acetylene, benzene, xylene, toluene, etc. have been used to synthesize CNTs.^90-92 These precursors are not renewable and may vanish with time in the near future. Not only the crisis for fossil fuel-associated carbon precursors but also the destruction to nature highlight the need for natural precursors. Lot of naturally occurring sources are emerging as a new hope with an immense potential of efficiency to synthesize CNTs. Environmentally benign and those synthesis methods of CNTs following principles of green chemistry is of enormous importance. In this section, general methods of synthesis of CNTs use of natural precursors, natural catalysts employed, and the properties are discussed in detail.

FIGURE 1.4 Representation of (a) single-walled carbon nanotube (SWCNT) and (b) multi-walled carbon nanotube (MWCNT).

SYNTHESIS OF CARBON NANOTUBES

Various ways can be used for the fabrication of carbon nanotubes with required properties for a desired application. The main synthesis methods which produce CNTs with minimum chemical and structural defects are described below.

ARC DISCHARGE METHOD

This was the foremost and familiar method for the synthesis of CNTs. In this method, an electric arc discharge is produced between a cathode and graphite anode in a steel chamber with an inert gas. The chamber containing vaporized carbon molecules and catalysts is heated to 4000 К under applied pressure. During this process, a part of the vaporized carbon solidifies as hard cylindrical deposit on the tip of cathode and the remaining part of carbon condenses forming cathode soot and chamber soot. CNTs are formed by cathode soot and anode soot and the selection of inert gas and metallic catalyst added decides whether the CNTs formed are SWCNTs or MWCNTs.⁹³ Even though this method facilitates the high yield of CNTs, slight control over the orientation of formed CNTs is a major drawback, which eventually affect their activity.

LASER ABLATION METHOD

The only difference between laser ablation technique and arc discharge method is in the input energy sources. In this method, a laser acts as an input energy source. A quartz tube containing graphite block is initially heated in a furnace at high temperature using a laser source in the presence of metal particles as catalyst. Argon, which is maintained during the course of reaction carries away the vaporized carbon that finally condenses on the cooler walls of quartz. Studies shows that laser pulse power could influence the diameter and yield of CNTs.⁹⁴-⁹⁵ High yield and relatively low metallic impurity are the advantages of this method. A major limitation of this method is that the synthesized nanotubes may not be straight on a regular basis and have branching to some extent.

CHEMICAL VAPOR DEPOSITION (CVD] METHOD

By using CVD approach CNTs can be grown on a variety of materials. This is a widely used method for the synthesis of CNTs by using natural precursors as carbon source. The characteristic growth mechanism of nanotubes in the process of CVD involves the catalyzed dissociation of hydrocarbon molecules by the transition metal and saturation of carbon atoms in the metal nanoparticle. This method can be classified into different types, including catalytic chemical vapor deposition (CCVD)⁹⁶ plasma-enhanced chemical vapor deposition (PECVD)⁹⁷ microwave plasma-enhanced chemical vapor deposition (MPECVD)⁹⁸ and oxygen-assisted CVD. In this technique, catalyst is implanted in a ceramic boat that is planted into a quartz tube. The reaction mixture containing a source of hydrocarbon is passed through a catalyst bed at high temperature and then cooled to room temperature. This technique allows CNTs to grow in an orderly manner with nanostructures designed specifically for some applications. The generation of tubular carbon solids with sp^: structure occurs when the metal particle precipitates out carbon. Working conditions such as pressure, temperature, concentration, nature of support, and time of reaction have great impact on the characteristics of CNTs.⁹⁹¹⁰⁰

SPRAY PYROLYSIS METHOD

This is an advanced form of chemical vapor deposition method for the production of CNTs on different surfaces. In this method, the system is sealed completely to prevent leaking and consists of thennolyne single zone split tube furnace with a quartz tube. Samples are placed about nine inches from the center of tube furnace inside quartz tube. Pure argon is passed through the reactor and the furnace is heated. At a particular temperature and a specific flow rate of argon, the reaction mixture was injected rapidly. CNTs are then obtained in the form of powder after successive chemical treatment. This technique is superior to CVD method since the reaction mixture can be continuously introduced with adequate control into the zone of reaction thereby making the synthesis of MWCNTs continuous and cheaper.¹⁰¹

SYNTHESIS OF CNTS USING NATURAL PRECURSORS

Use of natural precursors for the synthesis of CNTs is of immense importance hi the current scenario. Green alternatives to fossil fuel-related precursors are safe, environment friendly, renewable, and inexpensive. Several green precursors are reported for the successive production of CNTs.

FIGURE 1.5 Type of natural precursors used for the synthesis of CNTs.

Some of the main natural precursors used for the synthesis of CNTs are solid natural hydrocarbon precursors (camphor), liquid natural hydrocarbon precursors (turpentine oil, eucalyptus, palm oil, neem oil, sunflower oil, jatropha-derived bio-diesel, castor oil, sesame oil, camphor oil, tea tree extract) (Fig. 1.5). In addition to these, several plant parts and plant products were also used for the synthesis of carbon nanotubes. The type of precursor, catalyst, and reaction temperature are found to be the most important parameters determining the quantity and quality of nanotube formed. High temperature prefers the formation of SWNTs than MWNTs. Also as the temperature increases the yield of product increases due to the complete cracking of carbon containing compounds. The synthesis method adopted for most of these are either CVD or spray pyrolysis method. A table summarizing the source, type of CNT formed, catalyst used and reaction conditions are listed below (Table 1.2).

TABLE 1.2 Some of the Natural Precursors used for CNT Synthesis. Type of CNTs Formed, Condition, and Catalyst for CNT formation.

NATURAL RESOURCES AS CATALYST

The metal catalysts used in the synthesis of CNTs cause serious hazards to living organisms,¹²⁰ since it remains as such in the grown CNTs. The catalyst particle can enter into the human body during medical applications. Also in agricultural fields, there are chances for this catalyst to reach plant body resulting in cell division inhibition.¹²¹ The production of CNTs using metal catalyst requires expensive instrumentation and high temperature. All these circumstances points out the need for the development of cheap and nonmetallic catalyst for the safe application in different fields. As a result of intense research in this area, scientists have developed some green approaches for the same. Catalyst-free carbon nanotube synthesis, use of green plant extracts, and naturally available resources as catalysts are some of the best initiative steps taken for the sustainable development.

GREEN PLANT EXTRACT AS CATALYST

Plant extracts of neem (Azadirachta indica), walnut (Juglans regia), garden grass (Cynodon dactyl on), and rose (Rosa) were used as catalyst for the CNT synthesis. Comparatively high yield than that of other reported metal catalyst, requirement of inexpensive systems for synthesis, and low growth temperature were the mam benefits obtained using nontoxic plant extracts.¹²²

NATURAL MATERIALS AS CATALYST

All natural materials including minerals which contain metals or metal oxides in trace amounts are ideal for the use as catalysts in the synthesis of CNTs. The minerals cannot be classified as a renewable type but they are abundant, inexpensive and are apt to be used as natural catalyst for the preparation of CNTs. Volcanic lava rock (trachybasalt and alkali basalt),¹²³ garnet (beach sand),¹²⁴ bentonite,¹²⁵ and red soil (terra rosa)¹²⁶ are some of the minerals used for the CNT synthesis. All these examples employed either acetylene or natural gas as carbon precursors. Compared to toxic metal catalyst-mediated CNT synthesis, this approach is environment friendly as it enables the production of CNTs in large scale at low cost. A well-crystallized structure and easy separation are the further plus points of adopting this method.

CATALYST-FREE SYNTHESIS OF CNTS

In some cases, the precursor used for the synthesis itself contains certain minerals which can act as catalyst. These minerals are capable of adjusting the size of pores, modifying pyrolytic, and process thereby helping in the gr owth of CNTs. CNTs were successfully synthesized from bamboo charcoal,¹¹⁶ sunflower seed hulls and sago,¹²⁷ black Jews ear fungus, and black sesame seeds¹²⁸ without any external catalyst. The minerals inherently present in these precursors promotes the growth of CNTs in an aligned maimer. The use of catalyst-free synthetic precursors does not make any harm on living organisms and also promotes harmless.

PROPERTIES OF CNTS

CNTs are beehive-shaped tubes comprising only of carbon and having a thickness of approximately 1/50,000* of human hair. The ratio between length and diameter of graphene sheets, which are rolled into tube is in such a way that they have unique one-dimensional structures. The exceptional properties such as high conductivity, mechanical strength, elasticity, chemical, thermal, and structural stability make CNTs an excellent candidate for a wide range of applications. The properties of CNTs are dependent on the diameter, length, and morphology of tube.^129-131 SWCNTs and MWCNTs differ from each other in various aspects. There are three different forms of SWCNTs such as armchair, chiral, and zigzag, depending on the way in which it is wrapped to form a cylinder (Table 1.3).

TABLE 1.3 Comparison Between SWCNT and MWCNT.

The high electrical conductivity of CNTs is due to high electron transfer rate over its sidewall. The curvature of sidewall and structural defects account for the chemical reactivity of CNTs. Increasing the curvature of the sidewall increases the exohedral chemical reactivity at the convex surface of CNTs. This is mainly attributed to the distortions of planar C-C bonds of sp² hybridization and misalignment of p orbitals.¹³² The available surface area and porous nature of nanotube determines the adsorption properties of CNTs. The higher condensation pressures and minimum heat of adsorption than graphite make CNTs as a better adsorbent.¹³³ The tube ends and sidewall of CNTs contribute to their EC behavior. Another important property of CNT is their elasticity. When exposed to compressive forces, it can bend and twist without damaging the nanotube and will return to its original state after removing the force. But defects in structure of nanotube can weaken its strength. The modulus of elasticity determines the elasticity in both SWCNTs and MWCNTs.

Carbon nanotubes are an inevitable part of technology with a huge potential of future applications. Green approach is the only answer to solve the dilemma of fossil fuel related source and the pollution it cause to nature. The use of natural resource as precursor and catalyst will not only minimize the cost and utilization of restricted fossil fuels but will also help us to take care of our environment in a more benign way. In this effort, the various synthesis methods employed for the preparation of CNTs involved the common natural precursors used, different green approaches adopted for replacing toxic metal catalysts, and the general properties of CNTs are discussed. The study of properties and applications of CNTs from natural sources is still in progress. Therefore it can be concluded that the CNTs synthesized from green precursors have a very high potential for future applications.