Chapter 2
Literature Survey
24
The application of semiconductor photocatalysis is an emerging area of science
and technology, which has grown significantly with industrial development for
effective removal of various pollutants in aqueous system as well as in the air. It
has received significant attention in the last couple of decades.1 Heterogeneous
photocatalysis is an advanced oxidation process which has been the subject of a
extensive area of studies related to air cleaning and water purification because of
following reasons;2
They are inexpensive.
They show low or no toxicity.
They are showing tunable properties that can be modified such as by size
reduction and their surface properties.
They show substantial stability and durability.
2.1. Zinc oxide (ZnO)
In addition to TiO2, there are many other binary metal oxides have been studied to
determine their photocatalytic oxidation properties. Among them ZnO has been
often considered as alternative to TiO2 because of its good optoelectronic, catalytic
and photochemical properties. The band gap energy of ZnO is 3.3 eV, it is lower
than most active TiO2 phase i.e. anatase. Upon the illumination of light, ZnO
generates holes that are strong enough to oxidized organic pollutants into less
harmful materials.3 So far, various ZnO nanostructures have been used to degrade
the harmful dyes into less harmful components by photocatalytic reaction under
UV and visible light illumination. Wang et al. successfully synthesized reduced
graphene coated ZnO composite via simple method and it was found that the
synthesized composite exhibited an improved RhB adsorption capacity and an
25
improved photocatalytic activity for degrading RhB in comparison to neat ZnO
NPs. The composite showed an excellent recycling performance for organic
pollutant removal up to 99% recovery over several cycles via simulated sunlight
irradiation.4 The photocatalytic activity of a material is directly influenced by its
crystal structure. Kislov et al. studied degradation of methyl orange over single
crystalline ZnO. The efficiency of dye degradation is strongly dependent upon the
orientation of different phases of ZnO photocatalyst.5 Interestingly, it has been
reported that ZnO absorb more fraction of solar spectrum than TiO2. Sakthivel et
al. studied the photocatalytic activity of commercial ZnO powder and compared
with that of Degussa P25 TiO2 over Acid Brown 14 as the model pollutant. These
catalysts were examined for surface area, particle size and crystallinity and studied
for the influence of the effect of initial dye concentration, amount of catalyst,
illumination time interval, pH of solution and adsorption of acid brown 14 on ZnO
and TiO2. The photodegradation rate was highest for ZnO which suggest that it
absorbs more fraction of solar spectrum in compared to TiO2.6 Behnajady et al. has
studied the photocatalytic reaction of C.I. Acid Yellow 23(AY23) mediated over
common semiconductor ZnO. The effects of process parameters such as, catalyst
loading, initial dye concentration, light intensity, and pH on the extent of
photodegradation have been investigated. The results show that the adsorption
constant (Kads) and rate constant (kL–H) in L–H model are dependent to the light
intensity, and increase with increasing the light intensity. With inserting the light
intensity parameter to L–H equation, this model can be used for predicting the
removal rate at different light intensities and initial concentrations of AY23.7 So, it
has been successfully reported in many papers, ZnO exhibited better photocatalytic
efficiency in compared to most widely studied photocatalyst TiO2.8
26
Mai et al. demonstrated facile degradation of Methyl green under visible light
illumination over ZnO. The effects of various factors viz. pH values, amount of
catalyst, initial dye concentration, and the presence of NaCl, Na2CO3, H2O2, and
Na2S2O8 on the degradation efficiency were studied. Thirty-two intermediates
were separated, identified, and characterized by high-performance liquid
chromatography photodiode arrayelectrospray ionization-mass spectrometry
(HPLC–ESI-DAD-MS) technology, giving insight into the pathways of the
degradation process.9 Usni H. reported the surfactant assisted chemical synthesis of
ZnO nanorods and used as-synthesized ZnO nanorods as photocatalyst for efficient
degradation of MB dye, also shown a ~90% degradation of MB in 7 hours.10 Pare
et al. have found ZnO experimentally to be a highly efficient photocatalyst for the
degradation of acridine orange dye. It observed that addition of an optimal amount
of hydrogen peroxide and potassium persulphate increase the degradation rate
while NaCl and Na2CO3 decrease degradation rate. The effect of addition of
cationic and anionic surfactants has also been investigated. Bubbling of nitrogen in
the reaction solution decreases the reaction rate.11
Dindar B. and lcli S. studied the photodegradations of phenol under concentrated
sunlight. ZnO exhibited most promising photoactivity and was as active as TiO2
under concentrated sunlight. The enhanced photocatalytic activity of ZnO under
these conditions may be related to absorption characteristics of ZnO in the 300-400
nm region. It was interesting to report that ZnO, exhibited better photocatalytic
activity in compared to either most commonly employed photocatalyst TiO2 or
Fe2O3.12 Pardeshi S. K. and Patil A. B. confirmed that phenol was degraded more
effectively under solar light in comparison to artificial visible light irradiation. It
was observed that photodegradation of phenol is favorable in weakly acidic or
neutral solutions. The demineralization of substrate was checked by Chemical
27
Oxygen Demand (COD) reduction method. It was good to observe that ZnO was
reused for five times as it underwent photocorrosion only to a negligible extent.
This work envisages great potential towards sunlight mediated photocatalysis for
removal of phenol from waste water.13 Sobana N. and Swaminathan M. have
successfully reported that enhanced photocatalytic activity of ZnO by mixing it
with different proportion of activated carbon for the solar assisted photocatalytic
degradation of Direct Blue 53.The synergistic effect observed was to an extended
adsorption of DB53 on activated carbon followed by its transfer to ZnO where it
was photocatalytically degraded. The synergistic effect is responsible for the
enhanced photocatalytic activity of AC-ZnO in comparison to bare ZnO.14 Yu J.
and Yu X. discovered a facile route to synthesize hollow spheres of ZnO with
porous crystalline by hydrothermal treatment of glucose/ZnCl2 mixtures at 180 °C
for 24 h, and then calcined at different temperatures for 4 h. The photocatalytic
activity of the as-synthesized ZnO samples was evaluated by photocatalytic
decolorization of RhB. After many recycles, it was found that ZnO catalyst
exhibited efficient photodegradation of RhB and it does not lose it photocatalytic
activity which further confirms that as-synthesized ZnO hollow spheres show
stability and photocorrodibility.15
Zewei et al. present a facile method for fabrication of hollow ZnO spheres, in
which Zinc ion was first adsorbed on sulphonated polystyrene surface then reacted
with NaOH to form ZnO. During the formation of ZnO nanoshells, the temples
spheres were dissolved in the same medium to obtain ZnO hollow spheres directly.
Neither additional dissolution nor calcination process was needed in this method to
remove the templates, and the reaction conditions were very mild: neither high
temperature nor long time was needed. The as-synthesized ZnO hollow spheres
exhibited good photocatalytic activity.16
28
Comparelli et al. reported that the presence of passivating molecules i.e. different
organic molecules (surfactant) on the ZnO surface preserved the oxide from
photocorrosion and pH-dependent dissolution. The results demonstrate that
surfactant capped ZnO nanocrystals exhibit more facile photocatalytic degradation
than those of conventional ZnO-based because surface organic coating makes the
oxide resistant to photocorrosion and to pH changes which directly impact on the
photocatalytic activity.17 Zhang et al. reported that surface hybridization of ZnO
with graphite like carbon layers could significantly suppress the coalescence and
suppress growth of ZnO particles during high temperature treatment. The
Photocatalytic activity of ZnO was enhanced by hybridization with carbon layers
attributed to the improved adsorption ability and crystallinity.The as-prepared
samples exhibited high activity even after 720 h of photocatalysis reaction, while
pure ZnO almost deactivates its photoactivity in just 100 h due to serious
photocorrosion.18
It has been reported in many research papers that ZnO was quite active under
visible light illumination for the photodegradation of some organic compounds in
aqueous solution.19, 20 Etacheri et al. successfully synthesized Mg doped ZnO
nanoparticles through simple oxalate co-precipitation method. Textural properties
of as-synthesized Mg-doped ZnO studied at various calcined temperature were
superior in comparison to bare ZnO. In addition to this, Mg doped ZnO
nanoparticles exhibited a blue-shift in the near band edge photoluminescence (PL)
emission, decrease of PL intensities and superior sunlight-induced photocatalytic
decomposition of methylene blue in contrast to undoped ZnO.19 Lu et al.
demonstrated degradation of Basic Blue 11 under visible light illumination over
ZnO. They successfully studied the effect of various factors i.e. initial dye
concentration, catalyst dosage, and initial pH on the photocatalytic reaction.20
29
Becker et al. synthesized different crystallite and particle size of ZnO by
solvothermal process. It was found that particle size of ZnO nonmaterial could be
easily controlled by changing the nature of solvent. The photocatalytic activity of
ZnO nanoparticles was effectively demonstrated by measuring the discoloration of
RhB under visible light illumination.21 Zhang et al. prepared a hybrid material by
monomolecular-layer polyaniline dispersed on the surface of zinc oxide (ZnO).
The hybrid photocatalysts exhibited promising photocatalytic activity for the
degradation of the methylene blue (MB) because of high separation of
photogenerated electron and holes on the interface of hybrid ZnO. The
photocorrosion inhibition of ZnO could be attributed to the rapid transfer of
photogenerated holes by the polyaniline monolayer.22 El-Kemary et al.
Synthesized ZnO nanoparticles by simple heating of zinc acetate dihydrate and
triethylamine in ethanol at 50-60 °C for 60 min. The photocatalytic activity of ZnO
was executed for the degradation of ciprofloxacin under UV light irradiation. It
was good to note that the rate of discoloration was affected by change of pH,
degradation process was effective at pH 7 and 10, but it was rather slow at pH 4.23
Kansal et al. prepared ZnO nanoparticles using precipitation method by using zinc
acetate and triethylamine as template agent. Further the comparative evaluation of
the photocatalytic activity of the synthesized ZnO and commercial ZnO powder
was made. It was observed that the synthesized nanoparticles exhibited better
photocatalytic activity. Experiments were also performed to investigate the
reusability of the synthesized ZnO.24 Li B. and Wang Y. successfully synthesized
ZnO hierarchical microstructures with uniform flower-like morphology were
prepared on a large scale through a template- and surfactant-free low-temperature
(80 °C) aqueous solution route. The flower-like ZnO sample shows an enhanced
photocatalytic performance compared with the other nanostructure ZnO materials
30
like nanoparticles, nanosheets, and nanorods, which can be attributed to the special
structural feature with an open and porous nanostructured surface layer that
significantly facilitates photodegradation of RhB.25 Mohajerani et al. have
synthesized various shaped ZnO nanostructures like particle, rod, and flower by
using different schemes. He effectively demonstrated photocatalytic activity of as-
synthesized various shaped nanostructures for discoloration of CI acid red 27 under
direct sunlight irradiation. The photoactivity of the nanorods was slightly superior
to that of the nanoparticles while flower-like and microsphere 3D nanostructures
showed much lower photoactivity. 26
2.2. Strontium Titanate (SrTiO3)
Since, the discovery of water splitting on the surface of TiO2 semiconductor, the
photocatalytic properties of various metal oxide semiconductors have been
extensively studied. Among them, SrTiO3 is a typical ternary perovskite type oxide
with a wide band gap of ∼3.2eV that has been largely studied for the
photocatalytic water splitting because of its superior physical and chemical
properties, such as excellent thermal stability and photocorrosion resistance. The
perovskite characteristic of SrTiO3 enhances its physical properties like high
temperature chemical stability and photocorrodibility.
Hideki Kato and Kudo A. effectively synthesized codoped TiO2 and SrTiO3 with
antimony and chromium which successfully lower the band gap energy of these
materials by 2.2 and 2.4eV respectively. Cr and Sb codoped TiO2 exhibited
photocatalytic generation of Oxygen from an aqueous silver nitrate solution under
visible light irradiation, while SrTiO3 codoped with antimony and chromium
evolved H2 from an aqueous methanol solution. The activity of TiO2 photocatalyst
codoped with antimony and chromium was remarkably higher than that of TiO2
31
doped with only chromium. It was due to the charge balance by codoping of Sb5+
and Cr3+ ions, resulting in the suppression of formation of Cr6+ ions and oxygen
defects in the lattice which should work as effectively nonradiative recombination
centers between photogenerated electrons and holes.27 Konta et al. successfully
synthesized noble metal Mn, Ru, Rh, and Ir doped SrTiO3 with solid state reaction
method. Doped SrTiO3 possessed intense absorption bands in the visible light
region due to the excitation from the discontinuous levels formed by the dopants to
the conduction band of the SrTiO3 host. Mn and Ru doped SrTiO3 showed
photochemical evolution of O2 from aqueous silver nitrate solution, while Rh and
Ir doped SrTiO3 loaded with Pt cocatalysts produced H2 from an aqueous methanol
solution under visible light irradiation (λ > 440 nm). The Rh (1%)-doped SrTiO3
photocatalyst loaded with a Pt cocatalyst (0.1 wt %) gave 5.2% of the quantum
yield at 420 nm for the H2 evolution reaction.28 Wang et al. has synthesized La
and N codoped SrTiO3 by a mechano-chemical reaction method using SrTiO3,
urea and La2O3 as the raw materials. N-doped SrTiO3 could be prepared by
heating the mixture of SrTiO3 and La2O3 under flowing NH3 gas at 600 ºC. The
sample prepared with 0.2 mol% La2O3, 22 mol% urea and 77.8 mol% SrTiO3
which has nearly the same nitrogen and lanthanum doping atomic fractions could
be obtained by a mechano-chemical method and exhibited high photocatalytic
activities. A new absorption edge formed in the visible reason. La and N codoped
SrTiO3 exhibited enhanced photocatalytic activity for NO destruction as compared
to bare SrTiO3 under visible light.29 S, C cation-codoped strontium titanium
dioxide (SrTiO3) was synthesized by Ohno et al. by mixing of thiourea and SrTiO3
powders in appropriate amount then calcined at appropriate temperatures (400,
500, or 600 oC). After successful calcinations C and S ions were doped into
SrTiO3, which are responsible for shift in absorption edge of SrTiO3 powder. The
S and C codoped SrTiO3 showed better photocatalytic activity for oxidation of 2-
32
propanol in comparison to bare SrTiO3 under a wide range of light irradiation at
wavelengths longer than 350 nm. The formation of this new absorption edge might
be the reason for the high level of visible-light photocatalytic activity of this
substance.30
SrTiO3 is usually synthesized by solid state reaction method at high temperature
but the main drawback of this method is possibility of segregation and does not
show reproducibility, which diminishes the photocatalytic activity of synthesized
photocatalyst. Puangpetch et al. synthesized mesoporous SrTiO3 nanocrystal via
simple sol-gel method by using strontium nitrate Sr(NO3)2 and tetraisopropyl
orthotitanate (TIPT) as precursors and CTAB as structure-directing agent in
anhydrous ethanol, ethylene glycol (EtOH/EG) was selected as a solvent. The
mesoporous SrTiO3 shows excellent crystallinity, specific surface area, and pore
size of material which directly show impact on the photocatalytic activity of
SrTiO3. The photocatalytic activity for the degradation of Methyl Orange exhibited
by the sample obtained at a calcination temperature of 700 ºC was much higher
than that of a nonmesoporous commercial SrTiO3.31 Wang et al. have successfully
synthesized SrTiO3 nanocrystallines by a solvothermal method using H2TiO3 as
starting material and found more active for the photodegradation of MB than
commercial SrTiO3 nanoparticles.32 Chen et al. synthesized SrTiO3 via a simple
sol-gel process and successfully demonstrated their photocatalytic activities for NO
degradation under either UV light and sunshine.33
Zheng et al. synthesized SrTiO3 hollow microspheres built by regular nanocubes
by a general and facile hydrothermal method. The synthesized hollow
SrTiO3 microspheres exhibit excellent photocatalytic activity in photoreduction of
Cr(VI). The SrTiO3 possess higher reducing ability in compare to TiO2 because
the conduction band edge position of SrTiO3 (−1.4 V vs. SCE) is more negative
33
than that of anatase TiO2 (−1.2 V vs. SCE). So, the photocatalytic activity of
SrTiO3 is higher than TiO2.34 Jia et al. synthesized a series of highly reactive Ni
and La codoped SrTiO3 photocatalysts via simple sol-gel method by mixing
strontium chloride hexahydrate, nickel chloride and lanthanum chloride in proper
stoichiometry. Successful incorporation of Ni and La into the SrTiO3, were
supported by the presence of extended absorption shifted from 380 nm to 700 nm.
Under a 100W incandescent lamp irradiating for 14 h, a 100% of MB was
degraded, which is much higher than those of either bare SrTiO3 or commercial
Degussa P25. The optimal range of Ni and La dopants is 0.1 - 1.0 mol%. The
formation of a new absorption edge and the large surface area may be the main
reasons for the high activity.35
Jia et al. synthesized Ni and La codoped SrTiO3 via simple sol-gel method and it
was found that the calcination temperature strongly affects not only the structural
properties and the visible light photocatalytic activities but also the stability of the
synthesized photocatalysts. It was executed that rate of discoloration of malachite
green over synthesized photocatalyst decreases with increasing calcination
temperature because high temperature causes loss in specific area, pore volume
and initial adsorption rate as well as the large lattice distortion. While, the catalysts
calcined at lower temperatures were more stable and exhibited extended broad
absorption tail in visible region after co-doping of Ni and La. Although bare
SrTiO3 shows a high activity due to dye-sensitization, codoping of Ni and La did
enhance the visible light activity, especially for the catalysts calcined at high
temperature. Moreover, the optimal doping amount of Ni and La is 1.0%. The
photocatalytic activities of all synthesized photocatalysts are found to be higher
than that of the commercial P25 under visible light irradiation.36 Sulaeman et al.
reported a facile method to synthesize Strontium titanate nanoparticles by a
34
microwave-assisted solvothermal reaction of SrCl2·6H2O, Ti(OC3H7)4 in KOH
methanol–oleic acid solution. The particle size of synthesized SrTiO3 nanoparticles
were about 15-18 nm. The photocatalytic activity of SrTiO3 under visible light
irradiation could be generated by modification of the surface with the carboxyl
group (–COO-) from oleic acid which enabled the absorption of visible light.37
2.3. Copper oxide (CuO)
CuO is an important p-type transition-metal-oxide semiconductor, with a narrow
band gap (eg =1.2 eV) and exhibiting a versatile range of applications such as
fabrication of solar cell38, optical and photovoltaic devices39, gas sensing40, and
many others. These splendid properties make CuO a useful material.41, 42 Xu et al.
synthesized octahedral Cu2O crystals via effective and facile method by reducing
copper hydroxide with hydrazine without using any surfactant. The length of
octahedral crystal can be adjusted by change of concentration of OH- and Cu2+ ions
in solution. It was found that the as synthesized octahedral Cu2O crystals shows
better photodiscoloration for methyl orange as compared to cubic Cu2O particles.43
Later Zhang et al. further support Xu’s work by successfully preparing Cu2O
microcrystals by a hydrothermal process with use of stearic acid as a structure-
directing agent. They compared the photodiscoloration for methyl orange, the
photocatalytic properties of Cu2O are strongly dependent on the shape of the
crystals i.e. number of atoms located at the edges, corners or surfaces. 44
Ma et al. Synthesized flower like Cu2O architecture by simple polyol process in
the presence of acetamide. The size of petal of flower is about 5-6 nm. The novel
architecture shows a blue shift of absorption edge compared to Cu2O nanocubes
and good photocatalytic activity for the degradation of dye brilliant red X-3B
under simulated solar light.45 Vaseem et al. successfully synthesized flower-shaped
35
CuO nanostructures consisting of triangular-shaped leaves, having sharpened tips
with the wider bases, grown by simple aqueous solution process and demonstrated
its photocatalytic activity over methylene blue.46 Liu et al. reported facile synthesis
of various morphologies of CuO nanostructures by hydrothermal process, PEG200
as a structure directing reagent to tailor the morphology of CuO nanostructures.
The photocatalytic activity has been correlated with the different nanostructures of
CuO. The 1D CuO nanoribbons exhibit the best performance on the RhB
photodecomposition because of the exposed high surface energy crystal plane.47
Wang et al. successfully synthesized hollow CuO microspheres through a simple
hydrothermal method in the presence of cetyltrimethylammonium bromide
(CTAB). The effects of reaction temperature, surfactant, and the molar ratio of
Urea/Cu2+ on the morphologies of the resulting products were investigated. The
CuO hollow microspheres show good photocatalytic activity for decolorization of
RhB under UV-light illumination.48
Mukherjee et al. synthesized CuO nanowhiskers like structure via electrochemical
route by using metallic copper as precursor. The Structural characterization
showed the formation of cubic phase for both the Cu and CuO films, whereas, the
grains were found to change their shapes from cubic to nano-whiskers as an effect
of annealing (in air at 600 °C for 30 min). The photocatalytic activity of the as-
synthesized nanomaterial CuO films was determined by measuring the degradation
of Rose Bengal (RB) dye, it was found the synthesized CuO showed good
photocatalytic activity and it can be used for its potential application in waste water
treatment.49 Liu et al. prepared CuO nanowires using Cu foil as substrate via a
solution route i.e. simple, low cost, and can be completed in the absence of any
surfactant. The morphological investigation of synthesized CuO products wire-
shaped and are grown in large quantity. The as-grown CuO exhibited excellent
36
photocatalytic activity, it was successfully measured that 90% of the methyl orange
was degraded after 180 min under nature light.50
37
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