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
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