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130 Multifunctional Photocatalytic Materials for Energy

7.2 Properties of graphene

Pristine graphene is generally viewed as a key building block of various graphitic car-
bon allotropes, including 0D buckminsterfullerene, 1D CNTs, and graphite. Although
graphite is a naturally occurring material and has been in use for centuries, the first
reported method for producing graphene can be traced only as far back as 1970, and
free-standing, single-layer graphene was first obtained in 2004 by isolating graphene
from graphite via micromechanical cleavage [24–26]. The delay in the discovery of
free-standing graphene sheets is partially attributed to their one-atom-thick nature,
which was initially believed to be thermodynamically unstable. However, graphene
is not only stable but also has excellent electronic and mechanical properties, such
2
−1 −1
as a charge-carrier mobility of 250,000 cm V s at room temperature, a thermal
−1
−1
−1
conductivity of 5000 Wm K , an electrical conductivity of up to 6000 S cm , and a
2 −1
large theoretical specific surface area of 2630 m g . In addition, graphene is highly
transparent, with absorption of <2.3% toward visible light, which is due mainly to
the presence of a single layer of atoms. The optical absorption of graphene is found
to increase linearly with an increase in the number of atomic layers A = 1−T = πα.
In addition, with a Young's modulus of 1 TPa and an ultimate strength of 130 GPa,
single-layer graphene is the strongest material ever measured [27]. Among its vari-
ous properties, the most remarkable one is its excellent charge carrier mobility that
causes exceptional phenomena in graphene. The charge carriers in graphene behave
as massless relativistic particles or Dirac fermions that can move with little scattering
[28,29]. For instance, graphene demonstrates a superb zero-band gap, 2D semicon-
ducting properties, and an unusual half-integer quantum Hall effect that produces a
strong ambipolar electric field effect at room temperature and enables tuning of charge
carriers between electrons and holes by applying required voltages [26]. The high
electronic conductivity in graphene is mainly attributed to the small number of defects
in its crystal structure, which significantly increases the electron transport properties
2
of graphene without scattering; and also sp hybridization provides free electrons in its
third dimension. These exceptional electrical, optical, and mechanical properties have
led to graphene and other graphene-based materials being incorporated into various
aspects of photovoltaic cells.

7.3 Synthesis of graphene-based materials

Fabrication, growth, and synthesis of a graphene layer can be carried out by various
physical and chemical approaches and have been intensively investigated in the litera-
ture. [30–32]. Mechanical exfoliation of graphene from graphite crystals was the orig-
inal method for synthesizing graphene sheets. In this method, pristine graphene layers
are separated from graphite crystals by mechanical exfoliation using Scotch tape. The
graphene planes are bound by various Van der Waal forces in the graphite lattice [26].
Using this method, high-quality graphene layers can be produced with the lowest
number of defects and high electronic mobility. However, under typical mechanical
actions, in many cases the interactions between adjacent 2D layers of graphene and