What’s Graphene?

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Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer carbon foils in 1962. The carbon-carbon bond length in graphene is about 0.142 nanometers. Graphene sheets stack to form graphite with an interplanar spacing of 0.335 nm, which means that a stack of 3 million sheets would be only one millimeter thick. Graphene is the basic structural element of some carbon allotropes including graphite, charcoal, carbon nanotubes, and fullerenes. It can also be considered as an indefinitely large aromatic molecule, the limiting case of the family of flat polycyclic aromatic hydrocarbons. The Nobel Prize in Physics for 2010 was awarded to Andre Geim and Konstantin Novoselov "for groundbreaking experiments regarding the two-dimensional material graphene". Graphene is a new carbon material, distinctly different from 1-D carbon nanotube (CNTs), 0-D fullerences and 3-D bulk graphite [1]. It is a kind of ideal two-dimensional atomic crystal, which was successfully prepared and identified in recent years. Graphene has attracted more and more attention from scientists in diverse areas [2-5], because it has exhibited potential applications in microelectric devices, sensors, biomedicines and mechanic resonators.

The Functionalization of Graphene

Graphene can now be prepared in many methods, such as intercalation [6], sonication in various solvents [7], solvothermal synthesis [8], and chemical vapour deposition (CVD) [9]. Among these methods, chemical methods for the production of graphenes are both versatitle and scalable [10]. The chemical methods will afford the possibility of high-volume produc?tion, and versatile in terms of being well-suited to chemical func?tionalization. Due to the rich hydrophilic oxygencontaining groups such as carboxyl, hydroxyl, and epoxide, the graphene oxide readily suspends in water and polar organic solvents, such as ethylene glycol, DMF, NMP and THF at about 0.5mg ml?1 [8]. In order to enhance the solubility of graphene oxide nanosheets in water, the graphene oxide nanosheets were functionalized with allylamine [9]. The maximum solubility for graphene oxide–allylamine powders in water has been determined to be 1.55 mg ml-1, which is more than two times that of bare graphene oxide nanosheets. Si et al. introduce a small number of p-phenyl-SO3H groups into the graphene oxide before it was fully reduced and the resulting graphene remained soluble in water and did not aggregate [10]. Chen et al prepared stable graphene colloid using phenylene diamine as the reducing agent and stabilizer, and the as-made graphene could be dispersed well in ethanol, glycol, N-methyl-2-pyrrolidone (NMP), but not in N,N-dimethylformamide (DMF) [11] (figure 1).

Figure 1.       Photos of G dispersed in different solvents: (a) ethanol, (b) glycol, (c) NMP and (d) DMF.

Zhu et al employed triblock copolymers (PEO-b-PPO-b-PEO) as the solubilizing agent for chemically exfoliated graphite oxide, and graphene formed through in situ reduction by hydrazine [12]. The formation of the stable aqueous copolymer-coated graphene solution is due to the noncovalent interaction between the hydrophobic PPO segments of the triblock copolymer and the hydrophobic graphene surface, whereas the hydrophilic PEO chains extend into water (figure 2).

Figure 2.       Proposed structure of the copolymer coated graphene (a)  and supramolecular well-dispersed graphene sheet containing hybrid hydrogel (b).

Figure 3.       Schematic diagram of graphene-PLL synthesis and assembly process of graphene-PLL and HRP at a gold electrode

Shan et al prepared water-soluble graphene sheets functionalized by biocompatible poly-L-lysine as a linker through a covalent amide group [13]. PLL-functionalized graphene is water-soluble and biocompatible, which makes it a novel material promising for biological applications. Park et al [14] have used KOH to produce an aqueous homogeneous suspension containing conducting chemically modified graphene sheets from a precursor dispersion of graphene oxide in water. They suggested that KOH, a strong base, can confer a large negative charge through reactions with reactive hydroxyl, epoxy, and carboxylic acid groups on the graphene oxide sheets, resulting in reduced graphene oxide sheets that remain dispersed in water for at least 4 months (figure 4).

Figure 4.        (a) Aqueous colloidal suspension from left: graphene oxide, K-modified graphene oxide, hKMG. (b) AFM image of hKMG sheets on a mica substrate. (c) BF TEM image of hKMG sheets; inset, selected area diffraction pattern of what were found to be two overlapping hKMG sheets.

Graphene suspension could be prepared by simply heating an exfoliated graphite oxide suspension under strongly alkaline conditions at moderate temperature, and they found exfoliated graphite oxide would undergo quickly deoxygenation in strong alkali solutions (figure 5) [15].

Figure 5.       a) Illustration for the deoxygenation of exfoliated GO under alkaline conditions and b) images of the exfoliated-GO suspension (0.5mg mL-1) before and after reaction. The control experiment in b) was carried out by heating the pristine exfoliated-GO suspension without NaOH and KOH at 90 8C for 5 h with the aid of sonication.

Xu et al synthesized the amphiphilic graphite oxide, graphite oxide was modified by an excess amount of toluene-2, 4-diisocynate [16]. Stankovich et al prepared a number of functionalized graphite oxides by treatment of graphite oxide (GO) with organic isocyanates. These isocyanate-treated GOs (iGOs) can then be exfoliated into functionalized graphene oxide nanoplatelets that can form a stable dispersion in polar aprotic solvents [17] (figure 6).

Figure 6.       Proposed reactions during the isocyanate treatment of GO where organic isocyanates react with the hydroxyl (left oval) and carboxyl groups

Niyogi et al gained graphene oxides mod?ified by long alkyl chains (octadecylamine) [18]. Worsley et al produced alkyl-chain-modified graphene sheets that could be dispersed in organic solvents after sonication [19]. Wang et al reported the synthesis of hydrophobic graphene oxide nanosheets by a solvothermal method [9], and then they prepared organophilic graphene nanosheets by reacting with octadecylamine. Lomeda et al prepared surfactant-wrapped chemically converted graphene sheets through the reduction of graphene oxide with hydrazine were functionalized by treatment with aryl diazonium salts (figure 7).

Figure 7.       Starting with SDBS-wrapped GO, reduction, and functionalization of intermediate SDBS-wrapped CCG with diazonium salts

Soluble graphene layers in THF can be generated by the covalent attachment of alkyl chains to graphene layers by the reduction of graphite fluoride with alkyl lithium reagents [20]. Such covalent functionalization enables solubilization in organic solvents, such as CCl4, CH2Cl2, and THF.

Figure 8.      Photographs of a) dispersions of the amide-functionalized EG in THF, CCl4, and dichloromethane, b) water soluble EG, c) dispersion of HDTMS-treated EG in CCl4, d) dispersion of DBDT-treated EG in CCl4, e) dispersion of PYBS-treated EG in DMF and f) water dispersions of EG treated with CTAB, SDS, and IGP.

Qian et al [21] reported a solvothermal-assisted exfoliation process to produce monolayer and bilayer graphene sheets in a highly polar organic solvent, using expanded graphite (EG) as the starting material. It is proposed that the dipole-induced dipole interactions between graphene and acetonitrile facilitate the exfoliation and dispersion of graphene (figure 9).

Figure 9.       Schematic illustration of solvothermal-assisted exfoliation and dispersion of graphene sheets in ACN: (a) pristine expandable graphite; (b) EG; (c) insertion of CAN molecules into the interlayers of EG; (d) exfoliated graphene sheets dispersed in ACN; (e) optical images of four samples obtained under the different conditions

The Potential Applications of Graphene

Dai et al [22] found that chemically derived and noncovalently functionalized graphene sheets could self-assemble onto patterned gold structures via electrostatic interactions between the functional groups and the gold surfaces (figure 10). The self-assembled graphene sheets may be used as the molecular sensors for highly sensitive gas detection.

Figure 10.      Self-assembly of graphene sheets (GS) on gold: (a) an AFM image of as-made GS; (b) a schematic drawing of noncovalently functionalized GS; (c) a schematic drawing of selective adsorption of GS on a gold pattern on silicon dioxide, mediated by electrostatic interactions between positively charged groups on GS and negative ions adsorbed on Au

Highly conducting graphene sheets produced by the exfoliation–reintercalation–expansion of graphite are readily suspended in organic solvents [23]. The sheets in organic solvents can be made into large, transparent, conducting films by Langmuir–Blodgett assembly in a layer-by-layer Manner (figure 11). Sun et al [24] found that the intrinsic photoluminescence of graphene oxide was used for live cell imaging in the near-infrared with little background. Owing to its small size, intrinsic optical properties, large specific surface area, low cost, and useful non-covalent interactions with aromatic drug molecules, graphene oxide is a promising new material for biological and medical applications (figure 12).

Figure 11.      a) Schematic representation of the exfoliated graphite reintercalated with sulphuric acid molecules (spheres) between the layers. b) Schematic of tetrabutyl ammoniumhydroxide (TBA; dark blue spheres) in the intercalated graphite. c) Schematic of single-layer graphene coated with DSPE–mPEG molecules also shown is a photograph of the solution of single-layer graphene.

Figure 12.      A schematic illustration of doxorubicin (DOX) loading onto NGO PEG Rituxan via π-stacking

Figure 13.      Structure of 6THIOP-NH-SPFGraphene.

Ultraviolet–visible absorption and fluorescence emission data of functionalized graphene hybrid material with oligothiophene show that the attachment of the lectron-acceptor group (graphene oxide sheet) onto the oligothiophene molecules results in an improved absorption than its parent compound in the whole spectral region and an efficient quenching of photoluminescence[25] (figure 13).

Figure 14.      Charge and discharge curves of graphene nanosheets as anode in lithium-ion cells. The inset is the cyclic voltammograms of graphene nanosheet electrode.

Wang et al [26] found that the nanosheets exhibited an enhanced lithium storage capacity as anodes in lithium-ion cells and good cyclic performance (figure 14). Cao et al. [27] prepared a graphene-CdS nanocomposite material with good structural and optoelectronic properties by a facile one-step reaction. Graphene oxide has been simultaneously reduced to graphene during the deposition of CdS. This simple approach takes advantage of the stable single-layer property of graphene oxide to guarantee the final graphene-CdS product in a single-layer form (figure 15).

Figure 15.      a) Scheme of the one-step synthesis of G-CdS. The CdS QDs are not shown at their actual size. b) Scheme of the solvothermal reduction of GO to graphene in DMSO.

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