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Carbon nitride photocatalysts 105

with metallic and/or nonmetallic elements, hybridizing with carbonaceous materials
and other semiconductors, and sensitization with dyes. Then a review of modified
carbon nitride for Li-ion batteries and reduction of carbon dioxide to hydrocarbons
fuel is given. Last but not the least, we offer our perspectives on future research using
carbon- based photocatalyst for energy. This chapter aims at providing readers with
the instructive text and knowledge they need about the design and application of novel
carbon-based photocatalysts.

6.2 Graphitic carbon nitride for hydrogen evolution

The history of carbon nitride can be traced back to 1834, when it was initially syn-
thesized by Berzelius and then named melon by Liebig [17]. Afterward, theoretical
calculations predicted that carbon nitride existed in five allotropes of α-C 3 N 4 , β-C 3 N 4 ,
g-C 3 N 4 , cubic-C 3 N 4 , and pseudo cubic-C 3 N 4 , among which graphitic carbon nitride
(g-C 3 N 4 ) was found to be the most stable allotrope. The perfect structure of g-C 3 N 4
2
comprises only carbon and nitride elements, which are sp -hybridized to establish tra-
zine (C 3 N 3 ) or tri-s-triazine (C 6 N 7 ) rings. In comparison with trazine, the tri-s-triazine
system shows a lower energy, thus g-C 3 N 4 is considered as a π conjugated system
connecting a large number of tri-s-triazine rings in planar and stacking based on a Van
der Waals force interlayer (Fig. 6.2A and B) [12]. Whereas, because of undeveloped
experimental techniques, the unconfirmed molecular structure of C 3 N 4 was forgotten
for a long time. Carbon nitride was also first reported to be of no application because
of its insolubility and chemical inertness. In 2006 Goettmann et al. [19] for the first
time demonstrated that mesoporous carbon nitride can be employed for Friedel-Crafts
reactions. Three years later another milestone to applying carbon nitride in photocatal-
ysis water splitting was reported by Wang et al. [15] they synthesized graphitic carbon
nitride using the thermal polycondensation of a small organic molecular of cyanamide
and were first to test its performance in photocatalytic H 2 production. Both the CB and
VB matched the redox potentials of H 2 O for hydrogen and oxygen evolution, making
it an appealing candidate in the fields of energy storage and conversion. Thereafter,
an exponential increase in the number of carbon nitride-based photocatalysts in H 2
production and other energy applications were reported, which we review and discuss
in detail in the following section.

6.2.1 Tuning the reaction parameters and precursors
The photocatalytic performance of carbon nitride is dependent on its polymerization
degree varying from different precursors and calcination temperatures. Therefore
a surge of nitrogen-rich precursors, such as cyanamide [15], dicyanamide [20],
melamine [21], urea [22], and thiourea [18], have been employed for thermal conden-
sation at different temperatures to prepare carbon nitride photocatalysts.
Melamine is a common precursor for the synthesis of graphitic carbon nitride be-
cause of the relatively high polycondensation degree of the trazine structure, which
is prone to forming melem, the essential unit of carbon nitride. It is also regarded as