12
2 Production of Advanced Materials in Molten Salts
acid [57]. Despite its popularity, however, this strategy of producing graphene suffers
from several basic limitations. It should be mentioned that in GO, the extended sp
2
conjugated networks of original graphene sheets are disturbed due to the presence
of covalent C–O bonds on the basal planes and edges. Consequently, the existence
of a high fraction of sp
3 hybridized carbon atoms (~0.6), makes GO basically an
insulator [58, 59].
Since conductivity is one of the most important properties of graphene for many
applications such as energy storage systems [60] and conductive polymers [61], the
maximum reduction of oxygen-containing groups from GO is a key step toward the
production of reduced GO (rGO) with appropriate properties for those applications
[62].
There are several methods reported for the reduction of GO, which are mainly
based on its chemical reduction using a reducing agent such as hydrazine, thermal
reduction or combination of both [63, 64]. Since the chemical reduction of GO is
usually time-consuming involving the use of highly toxic reducing agents that are
harmful to the environment [65, 66], the thermal reduction approach is considered
relatively simpler and more eco-friendly, during which the oxygen-containing functional groups attached on the carbon backbone is converted into gaseous species [67].
However, residual functional groups and defects dramatically alter the structure of the
carbon plane and also lead to the irreversible restacking or aggregation of graphene
sheets due to the π–π interaction. The carbon structure can also be damaged due to
the release of gaseous species [68].
Alkali and alkaline earth metals are strong reducing agents, but it is not practical to
use them for the reduction of GO due to their high reactivity with the environment.
Molten alkali and alkaline earth metal halides can provide an ideal medium for
the reduction process, thanks to their capability to dissolve the reactive metals and
oxygen. Abdelkader et al. [69] reported that GO (C/O = 2.2) can be reduced by
alkali and alkaline earth metals such as Li and Ca dissolved in the corresponding
molten salts, i.e., LiCl and CaCl 2 , to a C/O level of 10.4 and 14.5, after 2 and
4 h of the heat treatment, respectively. Furthermore, molten salts can prevent the
rGO from restacking (Fig. 2.6). Wang et al. [70] reported that the GO reduced at
600 °C for 2 h under N 2 could yield a rGO product with the surface area of only
90 m
2 g
−1 and an electrical conductivity of about 20 S m
−1 . However, when the
graphene oxide was heated under the same regime but in a mixture of LiCl–KCl,
the restacking phenomenon was greatly avoided, leading to a larger surface area of
392 m
2 g
−1 . A product with a much higher surface area of 750 m
2 g
−1 and enhanced
electrical conductivity of 500 S m
−1 was produced by adding KNO 3 to the molten
salt. The enhanced properties of the rGO material were attributed to the advantages
of the molten salt system in preventing graphene sheets to restacked, restoring the
conjugated networks and also providing a medium for KNO 3 activation and nitrogen
doping [70].
2 Production of Advanced Materials in Molten Salts
acid [57]. Despite its popularity, however, this strategy of producing graphene suffers
from several basic limitations. It should be mentioned that in GO, the extended sp
2
conjugated networks of original graphene sheets are disturbed due to the presence
of covalent C–O bonds on the basal planes and edges. Consequently, the existence
of a high fraction of sp
3 hybridized carbon atoms (~0.6), makes GO basically an
insulator [58, 59].
Since conductivity is one of the most important properties of graphene for many
applications such as energy storage systems [60] and conductive polymers [61], the
maximum reduction of oxygen-containing groups from GO is a key step toward the
production of reduced GO (rGO) with appropriate properties for those applications
[62].
There are several methods reported for the reduction of GO, which are mainly
based on its chemical reduction using a reducing agent such as hydrazine, thermal
reduction or combination of both [63, 64]. Since the chemical reduction of GO is
usually time-consuming involving the use of highly toxic reducing agents that are
harmful to the environment [65, 66], the thermal reduction approach is considered
relatively simpler and more eco-friendly, during which the oxygen-containing functional groups attached on the carbon backbone is converted into gaseous species [67].
However, residual functional groups and defects dramatically alter the structure of the
carbon plane and also lead to the irreversible restacking or aggregation of graphene
sheets due to the π–π interaction. The carbon structure can also be damaged due to
the release of gaseous species [68].
Alkali and alkaline earth metals are strong reducing agents, but it is not practical to
use them for the reduction of GO due to their high reactivity with the environment.
Molten alkali and alkaline earth metal halides can provide an ideal medium for
the reduction process, thanks to their capability to dissolve the reactive metals and
oxygen. Abdelkader et al. [69] reported that GO (C/O = 2.2) can be reduced by
alkali and alkaline earth metals such as Li and Ca dissolved in the corresponding
molten salts, i.e., LiCl and CaCl 2 , to a C/O level of 10.4 and 14.5, after 2 and
4 h of the heat treatment, respectively. Furthermore, molten salts can prevent the
rGO from restacking (Fig. 2.6). Wang et al. [70] reported that the GO reduced at
600 °C for 2 h under N 2 could yield a rGO product with the surface area of only
90 m
2 g
−1 and an electrical conductivity of about 20 S m
−1 . However, when the
graphene oxide was heated under the same regime but in a mixture of LiCl–KCl,
the restacking phenomenon was greatly avoided, leading to a larger surface area of
392 m
2 g
−1 . A product with a much higher surface area of 750 m
2 g
−1 and enhanced
electrical conductivity of 500 S m
−1 was produced by adding KNO 3 to the molten
salt. The enhanced properties of the rGO material were attributed to the advantages
of the molten salt system in preventing graphene sheets to restacked, restoring the
conjugated networks and also providing a medium for KNO 3 activation and nitrogen
doping [70].