Page 146 - Multifunctional Photocatalytic Materials for Energy

132 Multifunctional Photocatalytic Materials for Energy

Moreover, various technical complexities encountered in the direct depositions and
high costs are other key factors severely limiting commercial use of the CVD tech-
nique for photovoltaic applications.
Another method employed in the synthesis of graphene films is the epitaxial growth
of graphene on a hexagonal substrate such as silicon carbide (SiC), and this method has
been widely applied for the preparation of high-quality graphene [45]. This method
involves the thermal decomposition of SiC at extremely high temperatures (1273–
1773 K) under UHV atmosphere. This facilitates the separation of Si via sublimation,
which leads to the formation of carbon-rich surfaces that can be arranged themselves
in the form of a graphene layer by controlling the growth conditions [46]. This tech-
nique has been used to produce wafer-thin graphene layers that are widely utilized
in several microelectronic applications, including solar cells. Although high-quality,
superior-grade epitaxial graphene is obtained by this process, the transfer of graphene
from SiC to other substrates is difficult, and the cost of the process is too high, all of
which seriously restrict epitaxially grown graphene for solar cell applications [47].
CVD and epitaxial growth usually produce large-sized, defect-free graphene in
small quantities suitable for fundamental studies and electronic applications, and they
are more attractive than the mechanical cleavage method. However, these and other
methods previously mentioned are not suitable for the synthesis of graphene needed
for the preparation of graphene-based nanocomposites, which usually require large
amounts of graphene sheets, preferably with a modified surface structure. Although
mechanical exfoliation using the Scotch tape method is a laborious procedure and
rarely leads to good-quality individual graphene sheets, epitaxial growth requires
high-vacuum conditions and a specialized, expensive fabrication system to generate
films on small areas [48].
Therefore, for the manufacturing of graphene-based nanocomposites, which gen-
erally requires bulk quantities of homogeneously distributed graphene sheets, the top-
down approach (i.e., chemical and/or thermal reduction of graphite derivatives such as
graphite oxide (GO) and graphite fluoride) appears to be the most suitable and efficient
strategy. These techniques yield low-cost bulk amounts of graphene-like sheets, albeit
not defect-free, that are highly processable and that can be fabricated into a variety of
materials [49]. In this approach, graphite is chemically modified into its intermediary,
water-soluble GO using strong oxidants, such as H 2 SO 4 , HNO 3 , KMnO 4 , KClO 3 , and
ClO 2 , by applying different variations of the Brodie, Staudenmaier, and Hummers’
methods. Oxidation of graphite considerably increases the interlayer distance between
graphene sheets because of the accumulation of various oxygen-containing functional
groups. This ultimately facilitates the easy exfoliation of graphene oxide (GRO) sheets
required for the preparation of graphene via top-down approaches. Indeed, different
oxygen functionalities, such as hydroxyl, epoxide, carbonyl, and carboxyl groups, that
are present on the basal plane of GO promote various structural defects that lead GO
to deviate from the state of pristine graphene [49]. To restore the structure and prop-
erties of GO, the functional groups need to be removed from the surface and edges
of GRO sheets, which is typically achieved by different reduction methods, including
thermal, electrochemical, photochemical, chemical reduction, and various other green
reduction methods [50].