photocatalytic ability may be improved by some suitable metal cation coupling, such
as La and Ga (Qin et al. 2007). Also, perovskite crystallite structure of CaTiO 3 with
band gap ¼ 3.5 doped with Zr shows higher photocatalytic performance under UV
light (Sun et al. 2007). K 4 Nb 6 O 17 with layered structure show excellent
photosplitting of water in an aqueous methanolic solution. The structure modified
with cocatalysts of NiO, Au, Pt, and Cs perform increased photocatalytic activity for
H 2 production (Sayama et al. 1998). Tantalate metal oxides like LiTaO 3 with band
gap ¼ 4.7 eV, KTaO 3 with band gap ¼ 3.6 eV, and perovskite NaTaO 3 with band
gap ¼ 4.0 eV have high water splitting yields that mainly depend on band angles of
Ta–O–Ta. Opening the angles near to 180
caused more easily transportation of
electron–hole pairs and much reduction of the band gap. Some of W- and Mo-based
heterogeneous materials show photoactive performance of water splitting just under
UV light such as PbWO 4 with band gap ¼ 3.9 eV and PbMoO 4 with band
gap ¼ 3.31 eV (Akihiko et al. 1990).
Visible Light-Activated Catalysts
The pure metal oxide usually bears some disadvantages of great resistivity and fast
recombination pace of photo-produced charges. For example, WO 3 , Bi 2 WO 6 ,
Bi 2 MoO 6 , and α-Fe 2 O 3 have band gaps 2.8, 2.8, 2.7, and 2.2 eV, respectively,
because positions of low conduction band do not have photoactivity about H 2
evolution (Aroutiounian et al. 2002; Ingler et al. 2004; Satsangi et al. 2008).
Therefore, recent investigations try to improve the photoconductivity and low
recombination rate of charges. One route is metal or non-metal doping to engineer
the band gap energy. The electron donor species with higher levels of band gap than
valence band of original photocatalyst, or electron acceptor ones with lower levels of
band gap than original conduction band, provide wide ranges of band gap of metal
oxides with visible light photoactivity. Coupling of TiO 2 with Pt
4+ and Ag
+
increases the photocatalytic performance underneath both visible and UV irradiations (Kim et al. 2005b; Rengaraj and Li 2006). Pt
4+ and Ag
+ metal ions participating in visible light absorption resulted in reducing of recombination rate. Using dye
for sensitizing of metal oxides caused in reduction of wide band gap is another
approaching method within improving the visible light sensitivity of water splitting.
The process progresses with shift of excited electron of HOMO to LUMO of dye
molecule and next transferring to conduction band of original photocatalysts. TiO 2
loaded with dye and K 4 Nb 6 O 17 show enhanced capability of H 2 evolution. Moreover, numerous coordination compounds Co(II), Zn(II), Pt(II), and Cr(II) with
polypyridine, phthalocyanine, alizarine, and metalloporphyrins perform
photocatalytic efficiency within H 2 generation (Shimidzu et al. 1985). A new
heptazine-based porous organic polymer named POP–HE show intense visible
light catalytic activity of oxidative conversion of benzyl alcohol to benzaldehyde.
The researchers claimed that POP–HE compound has higher photocatalytic efficiency than graphite carbon nitride (Xu et al. 2019a). Another new research shows
that anchoring of Pd nanoparticles to TiO 2 can permanently improve the
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M. Chahkandi and M. Zargazi
as La and Ga (Qin et al. 2007). Also, perovskite crystallite structure of CaTiO 3 with
band gap ¼ 3.5 doped with Zr shows higher photocatalytic performance under UV
light (Sun et al. 2007). K 4 Nb 6 O 17 with layered structure show excellent
photosplitting of water in an aqueous methanolic solution. The structure modified
with cocatalysts of NiO, Au, Pt, and Cs perform increased photocatalytic activity for
H 2 production (Sayama et al. 1998). Tantalate metal oxides like LiTaO 3 with band
gap ¼ 4.7 eV, KTaO 3 with band gap ¼ 3.6 eV, and perovskite NaTaO 3 with band
gap ¼ 4.0 eV have high water splitting yields that mainly depend on band angles of
Ta–O–Ta. Opening the angles near to 180
caused more easily transportation of
electron–hole pairs and much reduction of the band gap. Some of W- and Mo-based
heterogeneous materials show photoactive performance of water splitting just under
UV light such as PbWO 4 with band gap ¼ 3.9 eV and PbMoO 4 with band
gap ¼ 3.31 eV (Akihiko et al. 1990).
Visible Light-Activated Catalysts
The pure metal oxide usually bears some disadvantages of great resistivity and fast
recombination pace of photo-produced charges. For example, WO 3 , Bi 2 WO 6 ,
Bi 2 MoO 6 , and α-Fe 2 O 3 have band gaps 2.8, 2.8, 2.7, and 2.2 eV, respectively,
because positions of low conduction band do not have photoactivity about H 2
evolution (Aroutiounian et al. 2002; Ingler et al. 2004; Satsangi et al. 2008).
Therefore, recent investigations try to improve the photoconductivity and low
recombination rate of charges. One route is metal or non-metal doping to engineer
the band gap energy. The electron donor species with higher levels of band gap than
valence band of original photocatalyst, or electron acceptor ones with lower levels of
band gap than original conduction band, provide wide ranges of band gap of metal
oxides with visible light photoactivity. Coupling of TiO 2 with Pt
4+ and Ag
+
increases the photocatalytic performance underneath both visible and UV irradiations (Kim et al. 2005b; Rengaraj and Li 2006). Pt
4+ and Ag
+ metal ions participating in visible light absorption resulted in reducing of recombination rate. Using dye
for sensitizing of metal oxides caused in reduction of wide band gap is another
approaching method within improving the visible light sensitivity of water splitting.
The process progresses with shift of excited electron of HOMO to LUMO of dye
molecule and next transferring to conduction band of original photocatalysts. TiO 2
loaded with dye and K 4 Nb 6 O 17 show enhanced capability of H 2 evolution. Moreover, numerous coordination compounds Co(II), Zn(II), Pt(II), and Cr(II) with
polypyridine, phthalocyanine, alizarine, and metalloporphyrins perform
photocatalytic efficiency within H 2 generation (Shimidzu et al. 1985). A new
heptazine-based porous organic polymer named POP–HE show intense visible
light catalytic activity of oxidative conversion of benzyl alcohol to benzaldehyde.
The researchers claimed that POP–HE compound has higher photocatalytic efficiency than graphite carbon nitride (Xu et al. 2019a). Another new research shows
that anchoring of Pd nanoparticles to TiO 2 can permanently improve the
296
M. Chahkandi and M. Zargazi