Environmental Engineering Reference
In-Depth Information
the composite and the enhanced activity was explained based on the increased charge
separation, improved light absorbance and light absorption width, and high adsorptiv-
ity for pollutants in the composite.
Other semiconductor composites of graphene have also been found to be highly appli-
cable for the photocatalytic decontamination of polluted water. Li and Cao
107
recently
reported a ZnO-graphene composite and their eficient photocatalyst activity for the deg-
radation and iltered removal of RhB. A composite containing ZnO, graphene, and CNT is
also reported.
108
This study suggested that for this composite, the enhanced photocatalytic
activity strongly depends on the presence of CNT, owing to the increased light absorp-
tion and the reduced charge recombination due to CNTs. A hybrid structure comprising
ZnO@ZnS hollow dumbbells and graphene was fabricated through a polymer-assisted
hydrothermal reaction and sulfurization treatment by Yu et al.
109
Owing to the hybrid
structure and the presence of a new transfer pathway of electrons from ZnS to graphene, the
composites exhibited superior photocatalytic activities to ZnO dumbbells and ZnO@ZnS
hollow dumbbells. A facile in situ reduction strategy of GO and ZnWO
4
in water to form
an eficient photocatalytic composite ZnWO
4
-graphene hybrid (GAW-X) was reported by
Bai et al.
110
An enhanced eficiency (~7.1× and 2.3× eficiency compared with pure ZnWO
4
in the presence of visible and UV light irradiation) was demonstrated by the GAW-X for
the photodegradation of MO. However, visible and UV light-initiated photocatalytic pro-
cesses followed different mechanisms. Visible light-induced photocatalytic activity, due to
the formation of
OH andO
2
−
via photosensitization of graphene in ZnWO
4
-graphene. UV
light induced enhanced photocatalytic activity of the composite, due to the high separation
eficiency of photoinduced electron-hole pairs resulting from the promotion of an HOMO
orbit of graphene in the composite.
110
Liu et al.
111
fabricated a graphene-CdS composite via a
solvothermal method having signiicantly enhanced photocatalytic degradation eficiency
toward RhB compared with bare graphene and CdS nanorods. Ye et al.
112
compared the
eficiency of graphene-CdS and CNT-CdS composites for the degradation of organic dyes
and found that graphene-based composites are better candidates for the application. Khan
et al.
113
reported a ternary composite containing graphene, CdS, and ZnO/Al
2
O
3
with supe-
rior photocatalytic activity. This study also attributed the enhancement to the enhanced
surface area and effective separation of photo-induced charge carriers by the presence of
GO in the composite. Different graphene-based composites containing WO
3
nanorods,
114
BiVO
4
,
115
BiOBr,
116,117
Bi
2
O
2
CO
3
,
118
BiFeO
3
,
119
Fe(III),
120
NiFe
2
O
4
,
121
Ag
3
PO
4
,
122,123
and Ag/AgX
(X = Br, Cl),
124
InNbO
4
,
125
have been reported to show good photocatalytic degradation
capacity against various pollutants. A graphene-enwrapped plasmonic Ag/AgX (X = Br,
Cl) nanocomposite photocatalyst at the water-oil interface was reported by Zhu et al.
126
The photodegredation capacity of the catalyst was tested against MO under visible light
irradiation. The formation of the composite led to an increase in adsorptive capacity due
to the presence of GO in the composite. The smaller size of Ag/AgX NPs facilitated charge
transfer, and also suppressed recombination of electron-hole pairs in Ag/AgX/GO, lead-
ing to the enhanced photocatalytic activity of the composite. Xiong et al.
127
reported the vis-
ible light-induced photocatalytic degradation of dyes over a graphene-gold NP (RGO-Au)
hybrid. Spontaneous reduction of HAuCl
4
by RGO resulted in the anchoring of Au NPs on
the graphene substrate. RhB was used as the model system to evaluate the photoactivity
of the RGO-Au composite. A mechanism was proposed for the degradation as well. First,
the dye gets excited to dye*, followed by an electron transfer from dye* to graphene. Later,
the electron is moved to a Au NP where it gets trapped by O
2
to produce various reactive
oxygen species (ROS). This ROS degrades the dye. Various graphene-based semiconductor
nanocomposites have been reported to have high photocatalytic activity.
