Page 454 - Polymer-based Nanocomposites for Energy and Environmental Applications

Nanocomposite membrane for environmental remediation 411

against photocorrosion, low cost, and nontoxic in nature [35]. Mostly, titanium diox-
ide (TiO 2 ) is largely use for many applications, for instance, air and water purification,
because it acts as antifogging and self-cleaning surface agent, whereas for zinc oxide,
such applications have not been reported [36]. Wu et al. also has been conducted their
studied and found that TiO 2 has larger photocatalytic action than ZnO and SnO 2 [37].
To know the phase composition, size, and crystal structure, a lot of studies have been
carried out for investigating the photocatalytic activity of TiO 2 [38]. In the available
literature, three different types of TiO 2 are reported: [1] anatase, [2] rutile, and [3]
brookite [39]. Anatase and rutile phases of TiO 2 are the most studied types while still
very minimum literature is available on brookite [40,41]. Since the position of oxygen
ions on the exposed anatase TiO 2 particle surface exists at triangular arrangement, due
to which having very good absorption capacity of organic species [42], another pos-
itive reaction condition of anatase TiO 2 is the orientation of titanium ions with the
absorbed organic pollutants [43].

ZnO as a photocatalyst
Up to date, various types of semiconducting photocatalysis have been studied inclu-
ding TiO 2 , ZnO, ZrO 2 , CdS, and WO 3 [44-46]. Many of them have bandgap in the UV
region, that is, equal to or >3.36 eV (λ¼388 nm) [47]. Under the illumination of UV
radiation, these photocatalyses encourage the photocatalytic reactions [48]. In metal
oxide photocatalysts, the surface area, surface defects, and bandgap play a major
role in the photocatalysis [49]. It is reported that ZnO photocatalyst has better photo-
catalytic actions than normal photocatalysts [50].
During the last few decades, the photocatalytic degradation of different organic
pollutants exists in wastewaters that comes from different industrial and agricultural
waste and gains much more attention [51,52].
The photocatalytic reactions to absorb the organic pollutants are similar to hetero-
geneous catalysis that comprises the simultaneous adsorption of oxygen and organic
reactant species, followed by the oxidation on the ZnO photocatalyst surfaces [53,54].
Since zinc oxide provides more adsorption sites for the organic pollutants, ZnO is
considered a good photocatalyst, which means that an open porous structure with high
specific surface area is required [55].
It is confirmed that semiconducting photocatalyst arbitrated to photocatalytic
oxidative degradation of organic pollutants has good performance [56]. To show good
catalytic performance, in many potential applications, it is necessary that ZnO should
grip not only UV radiation but also visible light [57].
The use of nanocatalyst has many merits in one hand, while it may have also some
demerits, for instance, high cost of metal nanoparticles (Pt) and difficulties in recovery
for reuse. Thus, the problem is how to minimize the high cost and replenishment of
catalysts for continuing the treatment process. Zelmanov et al. carried out their study,
and they found that iron (III) oxide catalyst has strong effect of HCO 3 ,PO 4 /HPO 4 /
H 2 PO 4 , and H 2 O 2 on the phenol oxidation rate in wastewater. However, other ions
such as Cl, Na, SO 4 , Ca, and Mg did not display major effect on phenol oxidation
kinetics [58].