*Storage of Natural Gas by CNTs DOI: http://dx.doi.org/10.5772/intechopen.103814*

irradiation, and solar approaches. Laser abrasion and arc discharge are common methods for producing CNTs from carbon vapor. Carbon nanotubes made by these two methods can maintain good quality with less structural defects due to the presence of impurities in their constituents. In both methods, the growth process is performed at high temperatures, and re-firing properly ensures that the defects in the shape of the tubular graphene are reduced. In these techniques, it is difficult to control growth on patterned substrates at a reasonable rate. It should be noted that these production methods are expensive due to the need for high temperatures. To solve such problems, CVD production methods have been considered and used to allow the growth of various CNT structures. CVD seems to be a practical process due to its low required temperature range (500–1200 °C). In addition, because CVD provides better control over the diameter, length, and number of CNT walls, their application can be extended to nanoelectronics, field diffusion, and more. However, researchers continue to emphasize the development of more efficient, cost-effective, and environmentally friendly alternatives to large-scale CNTs. Recycling waste or disposable materials into higher-value products (such as ceramics and steel) has encouraged researchers to synthesize CNTs from waste sources for a variety of applications. The use of waste as a source for the synthesis of carbon nanotubes can simultaneously reduce solid waste and construction costs [6]. But the most popular and most widely used method of synthesis is the chemical vapor deposition method [2]. Today, many advances have been made to obtain carbon materials with very fine porous pores that have very high adsorption properties for most gases. Pores at the molecular scale can absorb large amounts of gas, the adsorption potential of porous walls increases the density of the adsorbed material inside the pores. The major advantage of CNTs is related to the fact that the carbon structure is practically known. This aspect has permitted the correlation of experimental data with theoretical predictions [5]. Since the discovery of CNTs to date, scientists have made great efforts to design, synthesize, and characterize CNT layouts, including single-walled, double-walled, and multiwalled CNTs (SWCNTs, DWCNTs, and MWCNTs). Due to the strong van der Waals (VDW) forces between the carbon atoms of adjacent pipes, CNTs tend to form in stable molds on their own. This geometric shape creates different adsorption sites that differ in the amount of energy required to absorb the gas. Most scientists agree that the so-called groove area (g) between the two CNTs is the best absorption region (see **Figure 3**). The best CNT geometry for maximum adsorption depends strongly on the applied pressure. Thus it can happen that the adsorption strength does not change monotonically as a function of the nanotube diameter D and the inter-tube distanced [7].

Undoubtedly, yeast fermentation plays a critical role in the formation of CNTs. Therefore, an in-depth understanding of the CNT formation mechanism is required [8]. Molecular surface engineering is based on the construction of CNTs through the chemical route. This is related to the adjustable structure of CNTs, for example, crystallinity, number of walls, cavities, and length with the help of variables such as proper growth control, use of suitable catalysts and carbon sources. The cost of manufacturing CNTs can be easily calculated with the help of the selected chemical route and process control [6]. There are two types of carbon nanotube based on the number of layers or walls: the single-wall carbon nanotubes (SWCNTs) and the multi-wall carbon nanotubes (MWCNTs). The SWCNT can be described as a graphene sheet rolled into single cylindrical shape so that the structure is onedimensional with axial symmetry. Most SWCNTs typically have diameters in the range of 1–1.3 nm and a few micrometers long. SWNT can be formed in three different designs: armchair, chiral, and Zig-Zag. The design of nanotubes depends on the complexity of the graphene in a tube, which can be represented by an index pair (n, m). The integers n and m represent the number of unit vectors in two

#### **Figure 3.**

*Schematic arrangement of a parallel aligned three dimensionally SWCNT array in a simulation box (i.e. the black framework) of volume Lx* � *Ly* � *Lz nm<sup>3</sup> . D is the nanotube diameter. The parameter d is the surface to surface intertube distance beyond VDW diameter σC-C = 0.34 nm of carbon atoms. It is defined as d = dCNT σC-C with dCNT denoting the shortest separation between carbon atoms of adjacent tubes. Interstitial and groove regions are represented by i and g, respectively. Note that there is some ambiguity on how to discriminate between i and g [7].*

directions in the graphene crystal lattice. If m = 0, this type of nanotube is called zigzag; if *n* = m, those nanotubes produced are called chair nanotubes, and in the third case, if m 6¼ n, it is called chiral nanotube. The values of the integers n and m greatly affect the property of SWNT. The MWCNT is a multilayer of graphene sheets rolled and superimposed on each other. The outer diameters are typically in the range of 2–100 nm while the inner diameters are in the range of 1–3 nm, and the length is one to several micrometers. SWCNTs are more flexible than MWCNTs. They can be twisted, flattened, and bent into small circles or around a sharp bend without breaking, thereby increasing its applicability. SWCNTs have the unique electronic and mechanical properties, which can be used in applications such as field emission displays, nanocomposite material, nanosensors, and logical elements. MWCNTs exhibit some advantages over SWCNTs such as higher surface-to-volume ratio, they are easier to produce in high volume quantities, the product cost per unit is low, and its thermal stability and chemical stability are enhanced. However, MWCNTs have regions of structural imperfection, which may reduce its desirability for application. CNTs are light in weight and have the strongest tensile strength as compared with any synthetic fiber. This strength results from the covalent Sp2 bonds formed between the individual carbon atoms. A standard SWCNT can withstand a pressure of 25 GPa without deformation. In terms of thermal conductivity, nanotubes are good conductors along the tube axis but good insulators lateral to the tube axis. Measurements carried out show that SWCNTs have better thermal conductivity compared with copper under the same conditions. MWCNTs exhibit a striking telescoping property whereby an inner nanotube core can slide almost without friction within its outer nanotube shell, thus creating an atomically perfect linear or rotational bearing. The symmetry and unique electronic structure of

*Storage of Natural Gas by CNTs DOI: http://dx.doi.org/10.5772/intechopen.103814*

graphene strongly affect its electrical properties. All CNTs have a large surface area and a high level of adsorption [2]. The type of adsorption in CNTs can be easily divided into three modes: (1) internal adsorption in which only the CNT interior space can be used, (2) external adsorption where adsorption can only be done in the space between CNTs (e.g., areas intermediate and groove; see **Figure 1** and (3) unlimited absorption in which both sides of the CNT, i.e., inside and outside, can be used. Among them, unlimited recruitment is the only thing that matters in the industry [7].
