Page 145 - Multifunctional Photocatalytic Materials for Energy

Graphene-based nanomaterials for solar cells 131

their overlapping p z orbitals inhibit complete elimination of bulk graphite into indi-
vidual graphene layers. Often the attempts to mechanically exfoliate graphite result in
stacks of sheets or in only a few isolated sheets, which is this method’s major draw-
back. Thus this method can be used only for fundamental analysis of pure graphene’s
characteristics. On the other hand, chemical exfoliation methods often result in a class
of graphene-like materials best described as highly reduced graphene oxide (HRG),
with graphene domains, defects, and residual oxygen-containing groups on the surface
of the sheets. Notably, most of the currently available methods are not capable of pro-
ducing defect-free, single-layer graphene in bulk quantities. Moreover, controlling the
size, shape, edge, and number of layers of graphene by random exfoliation, growth,
or assembly process is also very challenging [33]. Despite these drawbacks, serious
efforts are ongoing, and considerable success has been achieved in obtaining bulk
amounts of controlled and defect-free graphene [34].
So far a number of methods are reported for the preparation of graphene, and they
are broadly classified according to two approaches: the bottom-up and the top-down
approaches [35]. Bottom-up growth of graphene sheets is an alternative to the mechan-
ical exfoliation of bulk graphite. In bottom-up approaches, the production of graphene
is performed by using alternative carbon-containing sources as precursors, whereas the
top-down methods involve sequential oxidation and reduction of graphite. The most
popular methods under the bottom-up approaches include chemical vapor deposition
(CVD), chemical conversion, arc discharge, unzipping of CNTs, and so on [36]. The
bottom-up methods are usually simple and are efficient in producing graphene with a
defined number of layers; however, these methods usually require high temperatures.
Also, with these methods, large-area graphene films can be produced and can also be
grown on alternative substrates. In addition, the bottom-up approaches can be used to
fabricate graphene with atomic-level precision using synthetic chemical techniques
that have been developed over decades [37,38]. Although bottom-up strategies can
yield high-quality, defect-free graphene, the graphene obtained from such methods
usually has several defects [39]. Furthermore, these methods are not suitable for the
production of bulk quantities of graphene, which is commonly required for various
applications, including the production of graphene-based nanocomposites [15,40].
Among various bottom-up approaches, chemical vapor deposition (CVD) has
emerged as an important technique for the preparation and production of graphene
since it was first reported few years ago [41]. The CVD approach to producing
graphene relies on dissolving carbon into metal surfaces, such as Ni and Cu that act
as catalysts, and then forcing it to precipitate out by cooling the metal [42]. Although
graphene films possess poor transparency, usually graphene films obtained from CVD
methods exhibit superb conductivities. Indeed, CVD is the primary technique used to
obtain large-area graphene sheets, which are usually in high demand for various solar
cell applications [43]. Several studies have been reported on using the CVD method
for the deposition of single-layer graphene films on different substrates (e.g., copper
and nickel foils) that are later transferred onto an inverted photovoltaic device with
glass/ITO as cathode and graphene as anode [44]. However, testing the applicabil-
ity of a large graphene film as an electrode in solar cells is still in its infancy; thus
CVD-grown graphene electrodes are not yet at the point of being used commercially.