9.3 The Mechanistic Aspects of Visible/Sunlight
Photoactivity
As aforementioned notes, a semiconductor photocatalyst absorbs the energetic
photons that lead to the generation of electron–hole pairs with electron photoexcitation through heavy valence band to empty conduction band. As we know, very
quick recombination of the generated electron–hole give rise to energy destruction
and diminishing of quantum efficiency. Accordingly, novel-improving mechanisms
for spare recombination are constantly pursued. The key issue for spare recombination is to stretch the photo-absorption region along with separation performance of
electron–hole pairs. Producing of heterojunction kind of crystalline semiconductors
is proposed as an operational solution. The effectiveness of a semiconductor within
photocatalytic behavior crucially belongs to the energy alignment of the band gap.
Interfaces of semiconductor heterojunction can be categorized into three types:
straddling gap, type I; staggered gap, type II; and broken gap, type III (Fig. 9.6a).
The great improvement can be achieved by conversion of traditional type II into
direct Z- and S-schemes. S-scheme is built up as a combination of two n-type
semiconductor photocatalysts. (Di et al. 2017; Low et al. 2017; Zhu et al. 2017;
Fu et al. 2018, 2019; Tan et al. 2018; Li et al. 2019e) (Fig. 9.6b).
The effective developed photocatalysts can be categorized in four main classes:
metal oxides (Zhu et al. 2017; Tan et al. 2018; Li et al. 2019e), sulfides (Tada et al.
2011; Zhang et al. 2012, 2016; Bai et al. 2013; Wei et al. 2018b), valuable metal
semiconductors (Miao et al. 2013; Cai et al. 2017; Li et al. 2018a; Zhang et al.
2018b), and non-metallic semiconductors (Feng et al. 2018; Wu et al. 2018; Zheng
et al. 2018; Qi et al. 2019; Reddy et al. 2019; Wang et al. 2019b). However, each
photocatalyst has some disutility such as heavy metal or harmful leaching pollution,
expensive, high thermal treatment, and low stability within catalytic reactions. Wang
et al. (2008) reported that an applicable synthesized organic conjugated
photocatalyst, named graphite carbon nitride (g-C 3 N 4 ), has the capability of visible
light absorption with band gap ¼ 2.7 eV and λ > 420 nm for water splitting. g-C 3 N 4
has various advantages such as easy preparation route, high stability, low cost, and
visible frequencies sensitivity (Nayak et al. 2015; Jiang et al. 2018a; Li et al. 2019d;
Xu et al. 2019b; Zhu et al. 2019). Therefore, recently huge attentions have been
grown for preparation of pure g-C 3 N 4 (Ma et al. 2018; Wang et al. 2018; Zhao et al.
2018), elemental loading modification (Wang et al. 2017; Bellardita et al. 2018; Da
Silva et al. 2018; Deng et al. 2018; Shanker et al. 2018), heterogeneous composites
(Tian et al. 2013; Zhou et al. 2014; Ran et al. 2018a), and diverse morphology
preparation (Yang et al. 2015; Yu et al. 2016; Shakeel et al. 2019). The notable issue
has many defects which exist with pure bulk g-C 3 N 4 including small specific surface
area (Sun and Liang 2017; Jiang et al. 2018b), low performance in solar irradiation
ranges due to low absorption of wavelengths longer than 460 nm (Ye et al. 2015;
Naseri et al. 2017; Shen et al. 2018; Zhang et al. 2018a), difficult film forming, and
rapid electron–hole recombination (Hao et al. 2018; Jin et al. 2018; Shi et al. 2018)
(see Fig. 9.7). Therefore, new composite compounds with specific morphology can
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M. Chahkandi and M. Zargazi
Photoactivity
As aforementioned notes, a semiconductor photocatalyst absorbs the energetic
photons that lead to the generation of electron–hole pairs with electron photoexcitation through heavy valence band to empty conduction band. As we know, very
quick recombination of the generated electron–hole give rise to energy destruction
and diminishing of quantum efficiency. Accordingly, novel-improving mechanisms
for spare recombination are constantly pursued. The key issue for spare recombination is to stretch the photo-absorption region along with separation performance of
electron–hole pairs. Producing of heterojunction kind of crystalline semiconductors
is proposed as an operational solution. The effectiveness of a semiconductor within
photocatalytic behavior crucially belongs to the energy alignment of the band gap.
Interfaces of semiconductor heterojunction can be categorized into three types:
straddling gap, type I; staggered gap, type II; and broken gap, type III (Fig. 9.6a).
The great improvement can be achieved by conversion of traditional type II into
direct Z- and S-schemes. S-scheme is built up as a combination of two n-type
semiconductor photocatalysts. (Di et al. 2017; Low et al. 2017; Zhu et al. 2017;
Fu et al. 2018, 2019; Tan et al. 2018; Li et al. 2019e) (Fig. 9.6b).
The effective developed photocatalysts can be categorized in four main classes:
metal oxides (Zhu et al. 2017; Tan et al. 2018; Li et al. 2019e), sulfides (Tada et al.
2011; Zhang et al. 2012, 2016; Bai et al. 2013; Wei et al. 2018b), valuable metal
semiconductors (Miao et al. 2013; Cai et al. 2017; Li et al. 2018a; Zhang et al.
2018b), and non-metallic semiconductors (Feng et al. 2018; Wu et al. 2018; Zheng
et al. 2018; Qi et al. 2019; Reddy et al. 2019; Wang et al. 2019b). However, each
photocatalyst has some disutility such as heavy metal or harmful leaching pollution,
expensive, high thermal treatment, and low stability within catalytic reactions. Wang
et al. (2008) reported that an applicable synthesized organic conjugated
photocatalyst, named graphite carbon nitride (g-C 3 N 4 ), has the capability of visible
light absorption with band gap ¼ 2.7 eV and λ > 420 nm for water splitting. g-C 3 N 4
has various advantages such as easy preparation route, high stability, low cost, and
visible frequencies sensitivity (Nayak et al. 2015; Jiang et al. 2018a; Li et al. 2019d;
Xu et al. 2019b; Zhu et al. 2019). Therefore, recently huge attentions have been
grown for preparation of pure g-C 3 N 4 (Ma et al. 2018; Wang et al. 2018; Zhao et al.
2018), elemental loading modification (Wang et al. 2017; Bellardita et al. 2018; Da
Silva et al. 2018; Deng et al. 2018; Shanker et al. 2018), heterogeneous composites
(Tian et al. 2013; Zhou et al. 2014; Ran et al. 2018a), and diverse morphology
preparation (Yang et al. 2015; Yu et al. 2016; Shakeel et al. 2019). The notable issue
has many defects which exist with pure bulk g-C 3 N 4 including small specific surface
area (Sun and Liang 2017; Jiang et al. 2018b), low performance in solar irradiation
ranges due to low absorption of wavelengths longer than 460 nm (Ye et al. 2015;
Naseri et al. 2017; Shen et al. 2018; Zhang et al. 2018a), difficult film forming, and
rapid electron–hole recombination (Hao et al. 2018; Jin et al. 2018; Shi et al. 2018)
(see Fig. 9.7). Therefore, new composite compounds with specific morphology can
292
M. Chahkandi and M. Zargazi