7.6 Molten Salt Graphitization of Amorphous Carbons
131
[69, 70], the possible dissolution of oxygen from the PET-derived amorphous carbon
into the molten NaCl may play an important role in promoting the graphitization.
The exfoliation of the graphitized carbon in contact with the molten salt [81] will
then form a nanostructured carbon containing exfoliated graphene nanosheets. The
graphitization process initiates preferentially at the edge sites of the PET-derived
amorphous carbon particles due to the higher surface energy and also a greater deal
of exposure to the molten salt in these areas.
Table 7.1 compares the properties of the nanostructured carbon material produced
by the reactive molten salt treatment of PET with those of carbon nanostructures
produced by a selected number of the state of art methods reported in the literature.
These methods can be divided into two groups, depending on whether graphite or nongraphitic carbon materials have been employed as the carbon source. Methods that
employ graphite as the carbon precursor include the chemical oxidation/exfoliation
of graphite followed by the reduction process [93–100], as well as the liquid phase
[101, 102] and molten salt exfoliation [83, 84] of graphite. Graphene-based materials
have been produced using natural and synthetic graphite as the carbon source by
chemical oxidation/exfoliation and the subsequent reduction of graphene oxide (GO).
This is the most used method in the literature for the preparation of atomically thin
sheets of carbon, in which graphite oxide is first produced by the chemical oxidation
of graphite using strong oxidizing agents. The graphite oxide formed can then be
exfoliated to produce GO. It should be noticed that GO is an insulating material,
since it is covalently decorated with oxygen-containing functional groups formed
due to the extensive use of strong oxidizing chemicals [103]. GO materials produced
by this approach can be activated [104] to provide a high specific surface area, which
can be as high as 3100 m
2 g
−1 . However, the electrical conductivity of GO is rather
poor, typically from values near zero to less than 250 S m
−1 [99, 104]. The reduction
of GO by strong reducing agents such as hydrazine can reduce the oxygen content
of the material, leading to the formation of reduced graphene oxide (rGO) with an
enhanced electrical conductivity [100]. However, the improvement of the electrical
conductivity of rGO brought about by the reduction process is limited, due to the
substantial structural damages to carbon layers caused by both the oxidation and
reduction processes. Therefore, the electrical conductivity obtained for rGO does
not exceed values such as about 34 [93], 700 [94] and 1000 [98] S m
−1 . By applying
more extensive treatments [96] or adding highly conductive materials, such as Ag
[98], a further improvement in the value of electrical conductivity, in the range of
3000–10,000 S m
−1 , can be achieved.
Graphite can also be exfoliated by non-oxidizing techniques, such as liquid phase
[101, 102] or molten salt electrochemical exfoliation [83, 84], and the resulting
graphene materials have higher values of electrical conductivity. Nevertheless, the
high conductivity of the graphene materials produced by the use of graphite precursors is not a big surprise, due to the high conductivity of the graphite raw material
itself. For example, the electrical conductivity (and the surface area) of natural and
synthetic graphite precursors were measured to be 2.3 × 10
4 S m
−1 (7.9 m
2 g
−1 ) and
1.2 × 10
4 S m
−1 (6 m
2 g
−1 ), respectively [94].
131
[69, 70], the possible dissolution of oxygen from the PET-derived amorphous carbon
into the molten NaCl may play an important role in promoting the graphitization.
The exfoliation of the graphitized carbon in contact with the molten salt [81] will
then form a nanostructured carbon containing exfoliated graphene nanosheets. The
graphitization process initiates preferentially at the edge sites of the PET-derived
amorphous carbon particles due to the higher surface energy and also a greater deal
of exposure to the molten salt in these areas.
Table 7.1 compares the properties of the nanostructured carbon material produced
by the reactive molten salt treatment of PET with those of carbon nanostructures
produced by a selected number of the state of art methods reported in the literature.
These methods can be divided into two groups, depending on whether graphite or nongraphitic carbon materials have been employed as the carbon source. Methods that
employ graphite as the carbon precursor include the chemical oxidation/exfoliation
of graphite followed by the reduction process [93–100], as well as the liquid phase
[101, 102] and molten salt exfoliation [83, 84] of graphite. Graphene-based materials
have been produced using natural and synthetic graphite as the carbon source by
chemical oxidation/exfoliation and the subsequent reduction of graphene oxide (GO).
This is the most used method in the literature for the preparation of atomically thin
sheets of carbon, in which graphite oxide is first produced by the chemical oxidation
of graphite using strong oxidizing agents. The graphite oxide formed can then be
exfoliated to produce GO. It should be noticed that GO is an insulating material,
since it is covalently decorated with oxygen-containing functional groups formed
due to the extensive use of strong oxidizing chemicals [103]. GO materials produced
by this approach can be activated [104] to provide a high specific surface area, which
can be as high as 3100 m
2 g
−1 . However, the electrical conductivity of GO is rather
poor, typically from values near zero to less than 250 S m
−1 [99, 104]. The reduction
of GO by strong reducing agents such as hydrazine can reduce the oxygen content
of the material, leading to the formation of reduced graphene oxide (rGO) with an
enhanced electrical conductivity [100]. However, the improvement of the electrical
conductivity of rGO brought about by the reduction process is limited, due to the
substantial structural damages to carbon layers caused by both the oxidation and
reduction processes. Therefore, the electrical conductivity obtained for rGO does
not exceed values such as about 34 [93], 700 [94] and 1000 [98] S m
−1 . By applying
more extensive treatments [96] or adding highly conductive materials, such as Ag
[98], a further improvement in the value of electrical conductivity, in the range of
3000–10,000 S m
−1 , can be achieved.
Graphite can also be exfoliated by non-oxidizing techniques, such as liquid phase
[101, 102] or molten salt electrochemical exfoliation [83, 84], and the resulting
graphene materials have higher values of electrical conductivity. Nevertheless, the
high conductivity of the graphene materials produced by the use of graphite precursors is not a big surprise, due to the high conductivity of the graphite raw material
itself. For example, the electrical conductivity (and the surface area) of natural and
synthetic graphite precursors were measured to be 2.3 × 10
4 S m
−1 (7.9 m
2 g
−1 ) and
1.2 × 10
4 S m
−1 (6 m
2 g
−1 ), respectively [94].