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Carbon nitride photocatalysts 6

Jinqiang Zhang, Hongqi Sun
Edith Cowan University, Joondalup, WA, Australia

6.1 Introduction

The fast industrial pace, which currently relies heavily on fossil fuels, continues to
increase the energy crisis and to cause environmental deterioration, making these two
issues major concerns in countries all over the world. As a result, the successful ex-
ploitation of alternative energy resources plays a major role in enhancing the future of
human beings. Hydrogen, the most widely distributed element on Earth, has become
one of the most discussed energy resources. Hydrogen energy has the potential to yield
greater economic benefits than conventional fossil fuels and, because of its intrinsic
nature, to reduce environmental pollution and the greenhouse effect [1]. However,
the current production of hydrogen still relies heavily on traditional gas reforming
technology, which requires highly critical conditions such as high temperature and
pressure. Therefore the increasing energy demands together with the requirement for
sustainable energy sources are driving research activities toward producing more hy-
drogen via cost-effective and environmentally friendly technologies [2].
In 1972 Fujishima and Honda published a pioneering report on using TiO 2 as an
electrode in photoelectrochemical hydrogen evolution. Since then hydrogen produc-
tion via photocatalysis has become a hot topic as an innovative way to convert solar
+
energy to clean chemical energy [3]. In the photocatalytic process, H in water is
−
reduced to H 2 , whereas OH is oxidized into O 2 over the surface of a semiconductor
with an appropriate band gap energy [4]. As a result, seeking stable and efficient pho-
tocatalysts has been at the frontier in the solar energy storage and conversion fields.
Semiconductor materials considered for hydrogen evolution are placed in three cat-
egories: metal oxides and sulfides (TiO 2 [5], ZnO [6], CdS [7], MoS 2 [8]), complex
metal semiconductors (Bi 2 MoO 6 [9], TaON [10], Ag 3 PO 4 [11]), and nonmetallic semi-
conductors (graphitic carbon nitride [12] and black phosphorus [13]). TiO 2 (3.2 eV)
and ZnO (3.3 eV), which have been widely used in hydrogen evolution as typical pho-
tocatalysts, however, they respond only to ultraviolet light, which accounts for less
than 5% of solar spectrum energy. CdS is considered to be a fascinating candidate be-
cause of its moderate band gap (2.4 eV) and relatively high photocatalytic efficiency.
2−
Nevertheless, its poor stability, which is due to easy self-oxidation of S by photo-
generated holes, is still a big issue [14]. Moreover, metal-based photocatalysts cannot
meet the requirement of sustainability because of their high cost, which are due to
their scarcity and to contamination caused by incorrect disposal. Additionally, visible
light, with about 42% of solar energy, has been insufficiently utilized to date because
of the lack of suitable photocatalyst materials. Therefore it can be concluded that pho-
tocatalytic hydrogen evolution has been restricted mainly because an eco-friendly

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