2012), Mn 2 O 3 (Chandra et al. 2012), BiVO 4 (Yan et al. 2013), ZnWO 4 (Bai et al.
2012), Bi 2 WO 6 (Gao et al. 2011), Bi 2 MoO 6 (Wang et al. 2012b), CoFeO 4 (Fu et al.
2012), and BaCrO 4 (Gawande and Thakare 2013), have been used in combination
with graphene to serve as a photocatalyst. Theoretically, these semiconductor
photocatalysts are characterized by their electronic band structure, which composes
of an empty CB and filled VB (Xiang et al. 2012). The P25/graphene
nanocomposites, which was fabricated using the hydrothermal route exhibited a
higher capacity to degrade MB dye compared to pure P25 nanoparticles (Zhang et al.
2009). The graphene sheet enhances the ability to adsorb organic contaminants, as
well as improving the charge separation/migration rate, and visible light absorption
of P25/graphene nanocomposites (Zhang et al. 2009). Coupling a semiconductor
with graphene reduces its band gap through the hybridization of O np (n ¼ 2, 3, 4,
6, 7) and C 2p atomic orbitals to form a new VB (Li et al. 2013a). The amount of
graphene loading to the semiconductor should be carefully tuned to achieve efficient
photocatalyst–support interaction. Hence, the amount of graphene sheets plays a
significant role in the photocatalytic activity of the composites. Generally, increasing
the graphene sheet content in composites enhances the photocatalytic performance
but beyond the permissible limit reduces the photocatalytic performance (Yoo et al.
2011). The graphene/TiO 2 composite, which was prepared using the in situ method
showed 2.5 times improved photocatalytic degradation of MB dye with reference to
Degussa P25 TiO 2 (Wang et al. 2010). When the graphene content was increased to
20% in the ZnFe 2 O 4 /graphene composite, a higher degradation of MB dye was
observed compared to ZnFe 2 O 4 (Fu and Wang 2011b). Through a hydrothermal
reaction of CeO 2 /TiO 2 nanoparticles with graphene oxide in ethanol, a CeO 2 /TiO 2 /
graphene nanocomposite was prepared (Ghasemi et al. 2012). The performance of
CeO 2 /TiO 2 /graphene nanocomposites decreases with an increase in the graphene
sheet content. The CeO 2 /TiO 2 /graphene composite exhibited high photocatalytic
activity compared to the CeO 2 /TiO 2 /carbon composite owing to the unique electronic and structural properties of the graphene sheet. The influence of graphene
loading on the photocatalytic performance of TiO 2 /graphene composite showed that
a 0.05 threshold weight percent (wt%) of graphene exhibited the maximum activity
(Wang et al. 2012c). Using reduced graphene oxide and titanium isopropoxide as
Ti-precursors, a series of graphene/TiO 2 composites were fabricated by a sol–gel
route (Liu et al. 2013a). The as-fabricated composite showed improved degradation
of MB dye compared to pure TiO 2 with graphene/TiO 2 composite exhibiting a
higher activity than that of graphene/TiO 2 (P25). Through a facile sonochemical
approach, the Ag 2 Se/graphene/TiO 2 composite was fabricated (Meng et al. 2012).
The as-prepared composites demonstrated improved visible light absorption, high
adsorptivity of RhB dye, and excellent separation of charge carriers. The high
performance was ascribed to the synergetic effects of the absorption edge in the
visible light region and high charge carrier mobility of Ag 2 Se/graphene/TiO 2 composite. Khalid et al. (2012a) prepared a composite of Fe-doped TiO 2 with graphene
toward the photocatalytic degradation of MO dye. Due to the reduced charge carrier
recombination, the synergistic effect of enhanced adsorptivity of MO dye, and
improved visible light absorption, the as-synthesized showed tenfold photocatalytic
1 Nanotechnology for Water and Wastewater Treatment Using Graphene. . .
17
2012), Bi 2 WO 6 (Gao et al. 2011), Bi 2 MoO 6 (Wang et al. 2012b), CoFeO 4 (Fu et al.
2012), and BaCrO 4 (Gawande and Thakare 2013), have been used in combination
with graphene to serve as a photocatalyst. Theoretically, these semiconductor
photocatalysts are characterized by their electronic band structure, which composes
of an empty CB and filled VB (Xiang et al. 2012). The P25/graphene
nanocomposites, which was fabricated using the hydrothermal route exhibited a
higher capacity to degrade MB dye compared to pure P25 nanoparticles (Zhang et al.
2009). The graphene sheet enhances the ability to adsorb organic contaminants, as
well as improving the charge separation/migration rate, and visible light absorption
of P25/graphene nanocomposites (Zhang et al. 2009). Coupling a semiconductor
with graphene reduces its band gap through the hybridization of O np (n ¼ 2, 3, 4,
6, 7) and C 2p atomic orbitals to form a new VB (Li et al. 2013a). The amount of
graphene loading to the semiconductor should be carefully tuned to achieve efficient
photocatalyst–support interaction. Hence, the amount of graphene sheets plays a
significant role in the photocatalytic activity of the composites. Generally, increasing
the graphene sheet content in composites enhances the photocatalytic performance
but beyond the permissible limit reduces the photocatalytic performance (Yoo et al.
2011). The graphene/TiO 2 composite, which was prepared using the in situ method
showed 2.5 times improved photocatalytic degradation of MB dye with reference to
Degussa P25 TiO 2 (Wang et al. 2010). When the graphene content was increased to
20% in the ZnFe 2 O 4 /graphene composite, a higher degradation of MB dye was
observed compared to ZnFe 2 O 4 (Fu and Wang 2011b). Through a hydrothermal
reaction of CeO 2 /TiO 2 nanoparticles with graphene oxide in ethanol, a CeO 2 /TiO 2 /
graphene nanocomposite was prepared (Ghasemi et al. 2012). The performance of
CeO 2 /TiO 2 /graphene nanocomposites decreases with an increase in the graphene
sheet content. The CeO 2 /TiO 2 /graphene composite exhibited high photocatalytic
activity compared to the CeO 2 /TiO 2 /carbon composite owing to the unique electronic and structural properties of the graphene sheet. The influence of graphene
loading on the photocatalytic performance of TiO 2 /graphene composite showed that
a 0.05 threshold weight percent (wt%) of graphene exhibited the maximum activity
(Wang et al. 2012c). Using reduced graphene oxide and titanium isopropoxide as
Ti-precursors, a series of graphene/TiO 2 composites were fabricated by a sol–gel
route (Liu et al. 2013a). The as-fabricated composite showed improved degradation
of MB dye compared to pure TiO 2 with graphene/TiO 2 composite exhibiting a
higher activity than that of graphene/TiO 2 (P25). Through a facile sonochemical
approach, the Ag 2 Se/graphene/TiO 2 composite was fabricated (Meng et al. 2012).
The as-prepared composites demonstrated improved visible light absorption, high
adsorptivity of RhB dye, and excellent separation of charge carriers. The high
performance was ascribed to the synergetic effects of the absorption edge in the
visible light region and high charge carrier mobility of Ag 2 Se/graphene/TiO 2 composite. Khalid et al. (2012a) prepared a composite of Fe-doped TiO 2 with graphene
toward the photocatalytic degradation of MO dye. Due to the reduced charge carrier
recombination, the synergistic effect of enhanced adsorptivity of MO dye, and
improved visible light absorption, the as-synthesized showed tenfold photocatalytic
1 Nanotechnology for Water and Wastewater Treatment Using Graphene. . .
17