Page 93 - Multifunctional Photocatalytic Materials for Energy

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electron mobility. Note that the presence of foreign atoms (i.e., O, N, B, P, and S) and
defect sites on the lattice of graphene will influence its electrical properties [26].
In fact, the presence of heteroatoms in graphene changes its properties, such as elec-
tric conductivity, thermal stability, and chemical reactivity, improving the efficiency
and selectivity of the photocatalytic process and avoiding the need for noble metals
in the photocatalytic process [37]. For example, the band structure of graphene can
be tailored by chemical doping with electron-withdrawing oxygen functionalities or
electron-donating nitrogen functionalities, which usually makes graphene a p-type or
an n-type semiconductor, respectively (Fig. 5.2B) [38,39]. A tunable band gap, from
insulating to conducting, can be achieved by controlling the reduction degree of rGO,
as the band gap energy is strongly correlated with the number of oxidized sites, and
the oxidization degree of rGO [40]. Understanding the consequences of doping on
graphene's electrical performance is thus key to discovering its possible applications
in photocatalysis.
Generally, the advantages of graphene and its derivative-based photocatalysts can
be categorized as (i) ideal electron sink and/or electron transport bridge to suppress
photogenerated carrier recombination; (ii) band gap tuning acting as a photosensi-
tizer to extend absorption of light; (iii) remarkable specific surface areas that can sig-
nificantly increase the specific area of graphene-based photocatalysts; and (iv) good
stability for long-term photocatalytic application. Given their remarkable properties,
graphene and its derivatives provide a wide range of opportunities to prepare diverse
forms of composite materials with extraordinary properties for the photocatalytic
splitting of water to H 2 and photocatalytic reduction of CO 2 to hydrocarbon fuels.
Particularly, the preparation of fine-tuned and robust graphene-based photocatalytic
materials is necessary to meet the practical requirements for solar fuels generation.

5.3 Graphene-based semiconductor photocatalysts

Graphene and its derivatives have been incorporated into many different semiconduc-
tors to fabricate graphene-based composites for various photocatalytic applications.
These photocatalysts include inorganic and organic semiconductors, among others.
Many preparation protocols have been carried out to prepare graphene-based compos-
ite photocatalysts, such as mixing and/or sonication, sol-gel, liquid phase deposition,
UV-assisted photoreduction, self-assembling, and hydrothermal and solvothermal meth-
ods [20]. In this section, we discuss the synthesis of different graphene-based compos-
ites and consider different types of semiconductor photocatalysts and synthesis methods.

5.3.1 Synthesis of graphene-based titanium dioxide
photocatalysts
Since the pioneering work of Fujishima and Honda in 1972 [8], titanium dioxide (TiO 2 )
has been the most widely studied material for synthesizing composites for photocat-
alytic applications because of its superior photocatalytic properties, easy availability,
long-term stability, and low toxicity. Thus composites of graphene derivatives and