liquid phase exfoliation, Li et al. (2011b) synthesized a monolayer graphene sheet
from wormlike graphite. The as-synthesized graphene was utilized to adsorb fluoride
from aqueous solution after 1-methyl-2-pyrrolidinone was removed at 200
C. The
epitaxial growth technique of graphene can be realized by heating a honeycomb
crystals of SiC at 2400 K (McAllister et al. 2007). For example, the vacuum
graphitization technique was used to fabricate epitaxial graphene via the thermal
decomposition of SiC at 2400 K (Berger et al. 2006). The mechanical exfoliation
method was the first method used for the synthesis of graphene (Novoselov et al.
2004). In this method, an extremely oriented pyrolytic graphite was entrenched in
photoresist materials, and the graphene layers were peeled off using adhesive tape.
Recently, the Hummers’ method/modified Hummers’ method is found as the most
highly and commonly used technique (Hummers and Offeman 1958). In this method
graphite is oxidized into graphene oxide using strong acids (e.g., KMnO 4 and
H 2 SO 4 ) to form a stable graphene oxide solution dispersed in water (Some et al.
2012), which are then reduced using hydrazine (Li et al. 2008). The CVD method
was used to fabricate graphene using a metal substrate under high temperature and
ultrahigh vacuum (Tung et al. 2009). In this process, graphene was deposited on the
metal substrate surface, such as Cu and Ni with a vapor-rich hydrocarbon heated at
~1073 K. Liquid-phase exfoliation of graphite oxide is among the most practical
methods for industrial fabrication of GR owing to its low-cost and scalability (Zhou
et al. 2014a). This process encompasses the sonication of graphite oxide/graphite
powders in aqueous solution.
1.6.4 Characterization of Graphene
The identification of graphene monolayers is a major issue faced during the characterization of graphene. Up to now, several techniques including the X-ray diffraction,
scanning electron microscopy, thermogravimetric analysis, Raman spectroscopy,
Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray
photoelectron spectroscopy, and atomic force microscopy have been used to characterize graphene. Since it is difficult for a single technique to offer all the essential
information, there is the need to combine two or more tools to precisely characterize
the intrinsic properties, texture, morphology, and crystal structure of graphene (Guo
and Dong 2011). Transmission electron microscopy (Hernandez et al. 2008) images
was used to measure the thickness and the number of graphene sheets. Raman
spectroscopy analysis is a nondestructive technique, which uses monochromatic
laser excitation to investigate the structure of graphene (Machado and Serp 2012).
There are three response peaks (D peak ¼ ~1350 cm
À1
, 2D peak ~2700 cm
À1
, and G
peak ¼ ~1580 cm
À1
) equal to different phonon/vibrational modes in graphene (Ferrari
et al. 2006). The D peak appears strongly in disorderly graphite, which indicates the
degree of graphene disorder. Based on the position, shape, and width of the 2D peak,
the quality and number of graphene layers can be investigated using Raman spectroscopy (Ferrari et al. 2006). The crystalline structure of graphene sheets is determined
12
F. Opoku et al.
from wormlike graphite. The as-synthesized graphene was utilized to adsorb fluoride
from aqueous solution after 1-methyl-2-pyrrolidinone was removed at 200
C. The
epitaxial growth technique of graphene can be realized by heating a honeycomb
crystals of SiC at 2400 K (McAllister et al. 2007). For example, the vacuum
graphitization technique was used to fabricate epitaxial graphene via the thermal
decomposition of SiC at 2400 K (Berger et al. 2006). The mechanical exfoliation
method was the first method used for the synthesis of graphene (Novoselov et al.
2004). In this method, an extremely oriented pyrolytic graphite was entrenched in
photoresist materials, and the graphene layers were peeled off using adhesive tape.
Recently, the Hummers’ method/modified Hummers’ method is found as the most
highly and commonly used technique (Hummers and Offeman 1958). In this method
graphite is oxidized into graphene oxide using strong acids (e.g., KMnO 4 and
H 2 SO 4 ) to form a stable graphene oxide solution dispersed in water (Some et al.
2012), which are then reduced using hydrazine (Li et al. 2008). The CVD method
was used to fabricate graphene using a metal substrate under high temperature and
ultrahigh vacuum (Tung et al. 2009). In this process, graphene was deposited on the
metal substrate surface, such as Cu and Ni with a vapor-rich hydrocarbon heated at
~1073 K. Liquid-phase exfoliation of graphite oxide is among the most practical
methods for industrial fabrication of GR owing to its low-cost and scalability (Zhou
et al. 2014a). This process encompasses the sonication of graphite oxide/graphite
powders in aqueous solution.
1.6.4 Characterization of Graphene
The identification of graphene monolayers is a major issue faced during the characterization of graphene. Up to now, several techniques including the X-ray diffraction,
scanning electron microscopy, thermogravimetric analysis, Raman spectroscopy,
Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray
photoelectron spectroscopy, and atomic force microscopy have been used to characterize graphene. Since it is difficult for a single technique to offer all the essential
information, there is the need to combine two or more tools to precisely characterize
the intrinsic properties, texture, morphology, and crystal structure of graphene (Guo
and Dong 2011). Transmission electron microscopy (Hernandez et al. 2008) images
was used to measure the thickness and the number of graphene sheets. Raman
spectroscopy analysis is a nondestructive technique, which uses monochromatic
laser excitation to investigate the structure of graphene (Machado and Serp 2012).
There are three response peaks (D peak ¼ ~1350 cm
À1
, 2D peak ~2700 cm
À1
, and G
peak ¼ ~1580 cm
À1
) equal to different phonon/vibrational modes in graphene (Ferrari
et al. 2006). The D peak appears strongly in disorderly graphite, which indicates the
degree of graphene disorder. Based on the position, shape, and width of the 2D peak,
the quality and number of graphene layers can be investigated using Raman spectroscopy (Ferrari et al. 2006). The crystalline structure of graphene sheets is determined
12
F. Opoku et al.