Figure 3.2 Synthesis approaches for graphene.
The synthetic routes of graphene preparation can be categorized into top-down and bottom-up methodologies as presented in Figure 3.2.
Top-down Approaches:
To start with, mechanical exfoliation, a peeling method where a commercially available graphite sheet is dry-etched in the presence of oxygen plasma, then by the use of photoresist, the layers are peeled off by means of scotch tape. In the meanwhile, it is observed that the appearance of thin flakes, is found on the photoresist and is generally washed off in acetone and can be then be relocated to a Si wafer. Then after characterization, it is often investigated in the literature that these thin flakes are governed of few layers of graphene. Despite the fact, Geim and coworkers also employed the use of mechanical exfoliation, which led to many stimulating investigations on graphene and its electronic and mechanical properties [5]. Recently, a facile and green synthetic strategy for the large scale production of graphene by means of ball-milling of graphite flakes with carbohydrates namely sucrose, as an assisting ball milling reagent was investigated in detail by Balasubramanyan S. and coworkers, that credibly aided in the synthesis of graphene by the active reaction of the sucrose with the graphite by means of CH/π or OH/π molecular interactions. Followed by the calcination of this reaction mixture, i.e. graphene–sucrose, produced few-layered graphene nanosheets. These formed graphene sheets resulted in the stable suspensions made in numerous organic solvents, aiding its additional processing and fabrication methodologies [6]. On the contrary, the chemical oxidation of graphitic layers, followed by their successive exfoliation results in the huge quantity of synthesized graphite oxide monolayers. This approach impedes the invasive chemical reaction which engenders the structural defects in graphene sheets that disrupt its electronic properties and transforms it to semi-conductive nature. As a consequence, the large-scale synthesis of graphene sheets with optimized size is technologically advanced and commercial with the potential use of chemical exfoliation approach. The development of chemical-oxidation approach for synthesizing low-defect graphene sheets, is lately, investigated by Badri M. and coworkers by introducing a facile synthetic method vital for vastly conductive graphene thin film formation. This work described the synthesis of graphene thin films by means of a cost-effective route of chemical exfoliation of graphene dispersion and alkaline lignin (AL), used as a surfactant, to harvest an outcome yield of 0.72 ± 0.05 mg/mL without any structural defects. These as-synthesized graphene thin films revealed an electrical conductivity of 615.8 S/m achieved at room temperature, and amazingly, an improvement in electrical conductivity as high as 5376.3 S/m was also detailed in their results after post-synthesis annealing processes [7].
Bottom-Up Approaches:
Producing graphite through epitaxial growth under the effect of ultra-high vacuum (UHV) annealing conditions of SiC surface has attracted many researchers and technologists for the semiconductor industry [8]. The practice of epitaxial growth of graphene layers on silicon carbide (SiC) substrates gives an impression of being highly capable approach for the fabrication of electronic devices. Likewise, Berger and De Heer in the early days of graphene research provoked the usage of epitaxial graphene on SiC substrates [9]. The epitaxial growth procedure of graphene embraces firstly, annealing of SiC substrate under UHV conditions where silicon atoms channel from the substrate, followed by the exclusion of Si atoms from the substrate surface and result in carbon atoms which rearrange into graphene layers. The thickness of epitaxially grown graphene layers relies on the time and temperature of annealing. Largely, from the literature, the annealing of SiC substrates at 1200°C leads to the formation of ‘‘few-layer graphene’’ (FLG) in the span of 1-2 minutes [10]. Lately, the practice known vapour phase annealing has been largely investigated to grow few layers of graphene on SiC. At the outlay of employing high annealing temperatures classically above 400°C central to the foundation of a few layers of graphene with improved thickness and homogeneity on SiC [11]. While, chemical vapour deposition (CVD) growth, known to be the utmost promising approach of today for extensive production of graphene films. Even though the foundation of ‘‘monolayer graphite’’ was mentioned in early CVD studies on single metal crystals the first successful synthesis of few-layer graphene films using CVD was reported in 2006 by Somani and coworkers using camphor as the precursor on Ni foils [12]. This investigation fetched an innovative synthetic route for graphene growth for overpowering the inexplicable matter, i.e. control over number of graphene layers. Thereafter, much advancements have been known to acquire one-few layers of graphene onto numerous substrates like metallic with precise thickness. Followed by the growth of graphene layers, a process of chemical etching is favored for detaching out these layers from the substrate and transferring to another, deprived of any complex procedures. Lately, a continuous-phase, porous graphene was synthesized by CVD method was investigated by N Yazici M. and coworkers and evaluated for the appropriateness of catalyst and its support for polymer electrolyte membrane (PEM) fuel cells [13]. The CVD chamber used ultralow Pt sputter which was deposited onto porous, continuous phase N-doped graphene. Similarly, plasma-enhanced chemical vapour deposition (PECVD) or arc-discharge growth assists a new synthetic route for graphene growth at a temperature much lower to CVD. According to the literature, the first available report focusses on the growth of mono- and few-layers of graphene by PECVD, which employed the use of the radio-frequency system to grow graphene layers on various substrates [14]. Later, much research efforts have been dedicated to recognizing the synthetic mechanistic of graphene growth and detailed experiential studies to optimize the thickness of graphene layers. The merits of the PECVD embraces the short deposition time around <5 min and a temperature of 650°C for the growth of graphene layers (contrary to 1000°C for CVD) [15]. In recent times, an in-situ encapsulation strategy devised by Wang X. and coworkers, reported PECVD for the growth of ultrathin graphene shells over T-Nb2O5 nanowires (Gr-Nb2O5 composites), targeting a highly conductive anode material for sodium-ion hybrid supercapacitors [16]. This synthetic growth involves the steadiness perceived between the few-layer of graphene and chemical etching instigated by atomic hydrogen. The vertical alignment of the obtained graphene layers formed by this method is triggered by the effect of the plasma electric field direction [17].
3.3 Reduced Graphene Oxide
The structure of graphene oxide (GO) constitutes to hydroxyl and epoxy (1,2-ether) functional moieties present at its basal plane and ketone-rich carbonyl groups at the edges. These functionalities offer reactive sites for the effectual potential of its surface-modification [18]. GO is electrically insulating, and constitutes of irreversible defects due to disruption in the conjugated electronic structure by these functional groups. For this reason, GO is chemically reduced to form reduced GO (rGO) by means of various reducing agents, namely hydrazine, sodium borohydride, etc. which reinstate its conductivity [19]. With the chemical reduction process of transforming GO to rGO, the visual colour changes are observed in which the brown coloured water-dispersed GO turned to the formation of black colour aggregates forming rGO. The reason for the aggregation of rGO attributes to the removal of oxygen atoms; thus, attaining hydrophobic nature [20]. Inline to this, Li and coworkers investigated the synthesis of reduced graphene oxide by employing hydrazine as a reducing agent and regulating the pH of the solution by means of ammonia solution. The aim of regulating pH with ammonia solution resulted in deprotonation of the carboxylic acid functionalities
