et al. 2019). The synthesized ternary nanocomposites (2 wt% TiO 2 /FeTiO 3 /WO 3 )
have verified highest photodegradation activity for reactive blue 160 with effective
reusability. The TiO 2 /FeTiO 3 /WO 3 (2 wt%) nanocomposites exhibited degradation
of reactive blue 160 in 120 min than bare photocatalyst. In WO 3 /FeTiO 3 /TiO 2
nanocomposites, FeTiO 3 accept electron and diminished electron–hole pair recombination and promoted degradation of dye through oxidation of holes. FeTiO 3
nanoparticles decorated on TiO 2 exhibit red shift toward visible light region owing
to small band gap of FeTiO 3 (2.0 eV). Moreover, the heterojunction of WO 3 , TiO 2,
and FeTiO 3 nanoparticles generates photocurrent via transporting e
À from TiO 2 to
FeTiO 3 when irradiated to solar light. Degradation results confirmed that WO 3 /
FeTiO 3 /TiO 2 ternary nanocomposites exhibited more photodegradation ability
than binary FeTiO 3 /TiO 2 and WO 3 /TiO 2 heterojunction at pH 3.
Recently, Hao et al. (2018) fabricated Z-scheme photocatalytic ternary system
comprised of WO 3 /Ag 3 PO 4 /Bi 2 WO 6 via hydrothermal method. The ternary composites exhibit superior photocatalytic property for rhodamine degradation in comparison with Bi 2 WO 6 and WO 3 /Ag 3 PO 4 . All three WO 3 , Ag 3 PO 4 , and Bi 2 WO 6 with
the band gap of 2.7, 2.32, and 2.75 eV, respectively, show absorption in visible
region. The efficient partition of charge carrier is due to double Z-scheme electron
transfer mechanism.
According to the conventional heterojunction, the electron migrates from conduction band of Bi 2 WO 6 , to Ag 3 PO 4 followed by WO 3 , whereas the holes transfer
from valence band of WO 3 , to Ag 3 PO 4 and finally toward Bi 2 WO 6 . All the electron
will assemble in the conduction band (0.74 eV) of WO 3 which could not reduce the
oxygen to superoxide radical (E O2/O
À ˙ ¼ 0.13 V vs. NHE), and holes assemble in
valence band (1.73 eV) of Bi 2 WO 6 which couldn’t oxidize hydroxyl ion to hydroxyl
radical (E OH-/OH˙ ¼ 1.99 V vs. NHE) (Hao et al. 2018). These results do not support
trapping experiment which showed that hydroxyl radical and superoxide radical
anion were the dominant active species accountable for the azo dye degradation. The
electron in the conduction band of WO 3 drifts toward valence band of Ag 3 PO 4
where, electron neutralizes the holes in the valence band of Ag 3 PO 4 ; thus all the
electrons assembled in the conduction band of Bi 2 WO 6 and holes in WO 3 valence
band. Now, these electron and holes easily reduce and oxidized the oxygen and
hydroxyl ion, respectively.
4.4 Coupled Semiconductor for Azo Dye Degradation
Vinodgopal and Kamat (1995) applied thin film of SnO 2 /TiO 2 -coupled semiconductor on electrode for azo dye degradation under ultraviolet light. Coupling two
semiconductors with different energy levels refines the charge separation accountable for higher efficiency. Bagheri and Mahjoub (2016) fabricated coupled
nanostructures of Fe 2 O 3 -Ga 2 O 3 via coprecipitation method. Among various metals,
oxide-based semiconductor Ga 2 O 3 is an eco-friendly material having very high band
gap of 4.8 eV. The coupling of these two semiconductors inhibits the rate of
4 Photocatalytic Degradation of Azo Dyes in Water
133
have verified highest photodegradation activity for reactive blue 160 with effective
reusability. The TiO 2 /FeTiO 3 /WO 3 (2 wt%) nanocomposites exhibited degradation
of reactive blue 160 in 120 min than bare photocatalyst. In WO 3 /FeTiO 3 /TiO 2
nanocomposites, FeTiO 3 accept electron and diminished electron–hole pair recombination and promoted degradation of dye through oxidation of holes. FeTiO 3
nanoparticles decorated on TiO 2 exhibit red shift toward visible light region owing
to small band gap of FeTiO 3 (2.0 eV). Moreover, the heterojunction of WO 3 , TiO 2,
and FeTiO 3 nanoparticles generates photocurrent via transporting e
À from TiO 2 to
FeTiO 3 when irradiated to solar light. Degradation results confirmed that WO 3 /
FeTiO 3 /TiO 2 ternary nanocomposites exhibited more photodegradation ability
than binary FeTiO 3 /TiO 2 and WO 3 /TiO 2 heterojunction at pH 3.
Recently, Hao et al. (2018) fabricated Z-scheme photocatalytic ternary system
comprised of WO 3 /Ag 3 PO 4 /Bi 2 WO 6 via hydrothermal method. The ternary composites exhibit superior photocatalytic property for rhodamine degradation in comparison with Bi 2 WO 6 and WO 3 /Ag 3 PO 4 . All three WO 3 , Ag 3 PO 4 , and Bi 2 WO 6 with
the band gap of 2.7, 2.32, and 2.75 eV, respectively, show absorption in visible
region. The efficient partition of charge carrier is due to double Z-scheme electron
transfer mechanism.
According to the conventional heterojunction, the electron migrates from conduction band of Bi 2 WO 6 , to Ag 3 PO 4 followed by WO 3 , whereas the holes transfer
from valence band of WO 3 , to Ag 3 PO 4 and finally toward Bi 2 WO 6 . All the electron
will assemble in the conduction band (0.74 eV) of WO 3 which could not reduce the
oxygen to superoxide radical (E O2/O
À ˙ ¼ 0.13 V vs. NHE), and holes assemble in
valence band (1.73 eV) of Bi 2 WO 6 which couldn’t oxidize hydroxyl ion to hydroxyl
radical (E OH-/OH˙ ¼ 1.99 V vs. NHE) (Hao et al. 2018). These results do not support
trapping experiment which showed that hydroxyl radical and superoxide radical
anion were the dominant active species accountable for the azo dye degradation. The
electron in the conduction band of WO 3 drifts toward valence band of Ag 3 PO 4
where, electron neutralizes the holes in the valence band of Ag 3 PO 4 ; thus all the
electrons assembled in the conduction band of Bi 2 WO 6 and holes in WO 3 valence
band. Now, these electron and holes easily reduce and oxidized the oxygen and
hydroxyl ion, respectively.
4.4 Coupled Semiconductor for Azo Dye Degradation
Vinodgopal and Kamat (1995) applied thin film of SnO 2 /TiO 2 -coupled semiconductor on electrode for azo dye degradation under ultraviolet light. Coupling two
semiconductors with different energy levels refines the charge separation accountable for higher efficiency. Bagheri and Mahjoub (2016) fabricated coupled
nanostructures of Fe 2 O 3 -Ga 2 O 3 via coprecipitation method. Among various metals,
oxide-based semiconductor Ga 2 O 3 is an eco-friendly material having very high band
gap of 4.8 eV. The coupling of these two semiconductors inhibits the rate of
4 Photocatalytic Degradation of Azo Dyes in Water
133