Environmental Engineering Reference
In-Depth Information
aromatic rings on GO/RGO. The ionic interactions between MB and oxygen-containing
functional groups at the edges or on the surfaces of GO sheets are also thought to
increase the eficiency. It was also proposed in the study that GO is performing as the
adsorbent, electron acceptor, and photosensitizer to enhance the dye photodecomposi-
tion. Chen et al.
94
reported a strategy to prepare GO-TiO
2
composites from TiCl
3
and
GO as reactants. Visible light of wavelength >510 nm can excite the composite to induce
degradation of MO. A special variety of chemically bound TiO
2
(P25) was anchored on
the graphene surface by a hydrothermal process by Zhang et al.,
95
and the composite
was used for the photocatalytic degradation of dyes. Liang et al.
96
prepared GO-TiO
2
composites through the direct growth of TiO
2
NPs on GO sheets. A 3-fold increase in
photocatalytic degradation eficiency over conventional TiO
2
such as P25 was exhibited
by the composite for the degradation of 10 mg/L MO at pH 4.0 (Figure 34.2d and e).
Dong et al.
97
recently studied the shape dependence of photocatalytic activity of TiO
2
-
graphene composites where they anchored both spherical and rod-shaped TiO
2
NPs
on graphene and compared the photodegradation of MO. They found that graphene
composites having TiO
2
rods are more active than P25- and spherical NP-anchored
graphene composites. A highly photoactive graphene-wrapped amorphous TiO
2
was
prepared by Lee et al.
98
recently through hydrothermal process where GO reduction,
TiO
2
crystallization, and formation of GO-wrapped TiO
2
NPs occur in a single step. The
prepared hybrid exhibited excellent photocatalytic properties under visible light for the
degradation of MB, much higher than that of bare anatase TiO
2
NPs, graphene-TiO
2
NPs
(prepared by another two-step hydrothermal process), and P25 powder. Zhang et al.
99
recently suggested that the eficiency of graphene-TiO
2
composite for photocatalysis
can be improved by decreasing the defects on the graphene and increasing the interfa-
cial contact. Liu et al.
100
suggested that the use of TiO
2
NPs with speciic facet-exposed
structure will be beneicial to get better photocatalytic eficiency.
101
In their study, {001}
facet-exposed TiO
2
/graphene composite showed better photocatalyst properties than
P25 and other normal TiO
2
/graphene composites. This was explained to be due to the
formation of a Ti-O-C bond and the formation of nanoscale Schottky interfaces at the
contacts between TiO
2
and graphene. Also, it was found that positively charged dye
molecules are preferentially adsorbed onto the composites due to the photogenerated
charge gathered on graphene. A ternary composite containing graphene, TiO
2
, and
Fe
3
O
4
(GTF) is also reported to have enhanced photocatalytic ability.
102
The inclusion of
Fe
3
O
4
helped in the easy pot-treatment removal of the catalyst. Through an
in situ
depo-
sition strategy, TiO
2
NPs were deposited on GO by Jiang et al.
103
The composite showed
excellent utility in the photocatalytic removal of pollutants at optimum conditions of
solution pH, postcalcination, and at a deinite GO content. The maximum photooxida-
tive degradation rate of MO and the photoreductive conversion rate of Cr(VI) over the
composites were as high as 7.4× and 5.4× than that over P25. A sonochemically prepared
TiO
2
-graphene photocatalyst is reported for the photocatalytic degradation of MB.
104
It was found that the photocatalytic activity of composites with only 25 wt% TiO
2
is
better than that of commercial TiO
2
(100%). A TiO
2
-graphene composite (prepared by
a hydrothermal reaction between GO and TiO
2
in an ethanol-water mixture) for the
gas-phase photodegradation of volatile organic solvent was reported by Zhang et al.
105
The photocatalytic activity and stability of the composite for the gas-phase degrada-
tion of benzene was found to be higher compared with bare TiO
2
. Jiang and coworkers
reported the synthesis of graphene-TiO
2
composite by
in situ
growth of TiO
2
in the
interlayer of expanded graphite under solvothermal conditions.
106
Using phenol as the
model system, the study demonstrated the enhanced photocatalytic performance of
