Graphene and CNT Nanomaterials

In 2004, Novoselov and Geim fortunately isolated and analyzed a monolayer of graphene (MLG), which has led the scientists to elucidate the insights and the possibilities for altering the structure and properties of graphene and its other forms for desired applications. Graphene is a two-dimensional (2D) carbon form, that are constituted in a honeycomb lattice as shown in Fig. 2.1. The unit cell of MLG is made up of two carbon atoms, that are bonded together via sp² hybridization with 1.42 A of bond length and 2.46 A of lattice constant. In general, the graphene layers are arranged in a staging order of AB [Bernal) to form bilayer graphene (BLG), multilayer graphene (MULG), and graphite. In these graphene layers, carbon atoms exist in one layer will fit in the empty hexagon rings of the other layer in the bottom, where the inter-stacking distance between the layers will be 0.3354 A. Further, van der Waals attractions are exhibited in between the layers of BLG, MULG, and graphite, which keeps the layers together, due to я-cloud. Both MLG and BLG can be considered as zero-bandgap semiconductors

Figure 2.1 Structural representation of the 2D-graphene, ID-carbon nanotubes, and crystal lattice of graphene.

or semi-metals, which limits their practical applications only to electronics field [41, 42].

On the other hand, CNTs are one-dimensional (ID) tubular carbon forms with a length-to-diameter ratio of 1000:1 and are formed by rolling an MLG, BLG, MULG, and graphite. Hence, CNTs are structurally similar to graphene as illustrated in Fig. 2.1. Generally, CNTs are classified into two types based on the number of walls in CNTs they are single-wall CNTs (SWCNTs) and multiwall CNTs (MWCNTs). SWCNTs (diameter of 0.7-2 nm) are narrower than MWCNTs (diameter of 15 nm) and flexible to be curved. Additionally, CNTs are further classified into three types such as an armchair, zigzag, and chiral CNTs, based on the rolling of MLG or graphite during CNTs formation. Furthermore, the CNTs have the behavior of both metallic and semi-conductive properties with bandgap ranging from 1.8 eV [43, 44]. The structural arrangement of 2D-graphene and lD-CNTs are reported to be responsible for their exclusive properties to be beneficial in various applications [45].

Synthesis of Graphene

Since, the scientific breakthrough of graphene discovery, several physical and chemical approaches are reported for its synthesis to evaluate and demonstrate their industrial applications. In this section, the most common graphene synthesis approaches are

Figure 2.2 Common synthesis approaches to fabricate graphene structures.

discussed as illustrated in Fig. 2.2. Further, the advantages and the disadvantages of those synthesis approaches were mentioned in Table 2.1 [46-48].

Micromechanical exfoliation

Micromechanical exfoliation or scotch-tape approach was the first and an oldest physical approach for the synthesis of MLG on the surface of selected substrates. It is an eminent top-down approach, where the longitudinal or external force of ~300 nN/цт² was applied on the surface of layered materials to yield 2D-materials. This method generally yields high quality and few micrometers of graphene sheets [49].

Electrochemical exfoliation

Electrochemical exfoliation is another distinct method for the synthesis of graphene sheets from graphite rod in an electrochemical

Table 2.1 Advantages and disadvantages of the synthesis approaches used to fabricate graphene

Chemical synthesis

cell, where both anode and cathode were immersed in an electrolyte. In this method, 'graphite' rod will serve as an anode, 'Pt' wire will serve as a cathode, and the solutions of H₃P0₄ or H₂S0₄ will serve as an electrolyte. When applying electric voltage, graphene exfoliation can occur in an electrolyte and the accumulated graphene sheets can be washed and dried for further use [50].

2.2.1.3 Chemical synthesis

In the 20th century, synthetic organic scientists tried to synthesize polycyclic aromatic hydrocarbons (PAHs) via distinct chemical approaches under control reaction conditions, in which a definite length of graphene nanosheets can be prepared. In synthetic organic chemistry, PAHs can be recognized as 2D segments of graphene sheets with sp² hybridized carbon atoms [51].

Epitaxial growth

The word ‘epitaxy’ is obtained from the Greek language, where the meaning of 'epi' is'upon’ or ‘over’ and ‘taxis' is 'arrangement' or ‘order.’ This approach develops an epitaxial film of single-crystalline graphene on single-crystalline silicon carbide (SiC). It is considered as an extraordinary approach to synthesize graphene sheets on SiC substrate via higher temperatures of > 1100°C. The lateral size of graphene sheets varies with the ‘SiC’ thickness, as the silicon atoms are desorbed from the surface of the graphene sheet during the synthesis [52].

Chemical vapor deposition

Chemical vapor deposition (CVD) is one of the well-known methods to synthesize high quality solid-state materials such as MLG on various substances, including copper, nickel, iridium, ruthenium, rhodium, platinum, palladium and cobalt. In this approach, the volatile compounds will be decomposed or allowed to react together and can be deposited as a thin film of desired product on substrates under controlled pressure and temperature (750- 1200 C) conditions in a furnace. Generally, a mixture of hydrogen, methane, argon, and ethylene gas will be used for the synthesis of graphene in this approach [53].

Chemical reduction

The chemical reduction method is often employed to synthesize graphene sheets from MLG to MULG from graphene oxide (GO) via Hummer's method. Generally, chemical reduction of GO requires harmful, reducing agents such as sodium borohydride (NaBH₄), hydrazine, hydrazine hydrate, and dimethylhydrazine, that are highly toxic to humans and the environment [54].

Liquid-phase exfoliation

Recently, liquid-phase exfoliation is considered as one of the best affordable approaches to synthesize graphene sheets (both MLG and MULG) by exfoliating the bulk graphite in a liquid. It uses the sonication process, where mechanical shearing forces are applied to overcome van der Waals forces, that exist between the sheets of graphene in bulk graphite. However, it involves harmful chemical solvents and can lead to low yield and reaggregation of graphene sheets. Currently, researches are focused on developing green solvents and using surfactants such as Cetyl trimethyl ammonium bromide (СТАВ), sodium dodecyl sulfate (SDS), and tween-20 for an efficient exfoliation of bulk graphite in different solvents, to overcome the challenges of toxicity and low yield [55].

Biogenic reduction

Scientists are currently focusing on alternative methods such as reduction of GO by employing plant extracts, extracts from microbes, sugar molecules, and vitamin C, to overcome the disadvantages of using hazardous chemicals in chemical reduction and liquid-phase exfoliation [56].

Figure 2.3 Synthesis approaches for the fabrication of CNTs.