Page 572 - Polymer-based Nanocomposites for Energy and Environmental Applications

Hybrid materials based on polymer nanocomposites for environmental applications 525

this material reside in high-photocatalytic properties and a larger UV absorption as
compared with TiO 2 . Moreover, defects can be easily introduced in ZnO, acting as
carrier traps that can improve the photocatalytic performance of composites [86].

Other hybrid conjugated polymer-based nanocomposites
Besides metal oxides, many other materials have been used with conjugated poly-
mers to produce high-photocatalytic performance. These materials have porous
nanostructures and assure a large contact surface with the polymer matrix. The
photocatalytic performance of the composites is expected to be high because the
charge-transfer rate is enhanced. Graphene is a particularly interesting material that
has been recently investigated for photocatalysis. It has high electric and thermal
conductivity and a large specific area (which is defined as the surface per gram
2
of material). Theoretically, it reaches 2600 g/cm for graphene. Such properties
would highly favor the charge carrier transport and transfer in composites. Hybrid
PANI/graphene composites have been prepared by polymerization of aniline in
the presence of GO [87]. Their high-photocatalytic efficiency is explained by the
large contact area between graphene sheet and PANI that increases the charge sep-
aration under light illumination. The photocatalytic activities of conjugated polymers
are however limited because of the high-recombination rate of charge carriers, which
reduce the charge separation. Graphene can also be used then in conjugated polymer/
metal oxide composites for facilitating the transport of charge carriers after their sep-
aration at the polymer/metal oxide interface. High performance of hybrid P3HT/
TiO 2 /graphene composites [88] in degrading rhodamine B solution has been
observed. Due to the lower energy level Ec at the bottom of the CB ( 4.42 eV),
graphene can collect electrons from the oxide ( 4.20 eV) and transport them to
the catalyst surface for degrading the pollutant.
Finally, it is possible to modify the structure of conjugated polymers for obtaining a
porous morphology, which will increase the contact area with the catalyst in order to
improve catalytic reactions and hence the efficiency. For instance, composites made
of polyvinyl alcohol and TiO 2 have been thermally treated giving a porous structure,
in which the transformed conjugated polymer is coated on the oxide nanoparticles.
High efficiency of such composites in degrading phenol solutions under visible light
has been obtained [89].

19.3.1.2 Composites used in heavy metal removal

Heavy metals as pollutants in water are ions such as copper (Cu), Ag, chromium (Cr),
3
Cd, Pb, and arsenic (As). They are elements having high-atomic density (>5 g/cm )
[90] and are present in wastewater, produced by industrial, agricultural, and domestic
consumption and discharged into the environment. These pollutants when incorpora-
ted into drinking water are toxic for human and should be removed to prevent deceases
by intoxication. One of the most known effects of heavy metals on human health is the
damage on nervous system caused by high-Pb concentration that contaminates drink-
ing water. Treatments of wastewater can be realized by several techniques [91],