Page 134 - Polymer-based Nanocomposites for Energy and Environmental Applications

Energy and environmental applications of graphene and its derivatives 109

2 1
Brunauer-Emmett-Teller surface area of 2630 m g while experimental value
2 1
of 640 m g [15,16]. Graphene owing to its unique structure enjoys unusual phys-
icochemical properties, such as promising elasticity, high thermal conductivity, high
carrier mobility, and great stiffness [17].
Andre Geim and Konstantin Novoselov were awarded the Nobel Prize in Physics in
2010 for “groundbreaking experiments” regarding the 2-D material graphene in 2004
[18,19]. After that, graphene and derivative become an emerging field, and several
synthesis methods have been explored, including physical, mechanical, and chemical
synthesis [20]. They include intercalation and exfoliation of graphite, arc discharge of
graphite in the presence of helium and hydrogen gases, chemical vapor deposition
(CVDs), GO reduction, and epitaxial graphene growth via sublimation of silicon from
silicon carbide. Researchers however classified the synthesis approaches in two major
ways, the bottom-up approach and top-down approach, as illustrated in Fig. 4.6 [21].
Bottom-up approach involves CVDs, but it produces small amount of graphene film
and is quite time-consuming. Mechanical exfoliation, a top-down approach, also
presents difficulty in manufacturing large amount of graphene film for practical appli-
cations. Although chemical exfoliation of graphite flakes is the usual method
(Hummers’ method) with relatively low cost and high yield, in this graphite, flakes
are heavily oxygenated into GO nanosheets [22], by using sonication in both the
dissolution and intercalation stages [10]. Currently, graphene massive production
obtained from CVDs [10] and graphite precursors through oxidation-exfoliation-
reduction method because of its diversity for functionalization [23], by following
direct route from graphite to graphene, a top-down approach, is displayed in
Fig. 4.7. The oxidative treatment of graphite increases the interlayer distance between
graphene sheets in graphite for an easy exfoliation [24]. The oxygen functionalities of
graphite oxide moieties, such as carbonyl, epoxide, hydroxyl and carboxyl groups,
account for the structural defects that lead graphite oxide to deviate from the state
of pristine graphene. GO is then reduced to graphene by electrochemical thermal
annealing, chemical reduction, and other unconventional methods such as microwave

Fig. 4.6 Top-down and bottom-up methodologies for graphene synthesis [21].