Self assembled flower like CdS-ZnO nanocomposite and its photo catalytic activity

Article abstract of DOI:10.1016/j.jallcom.2013.08.184

Self assembled flower like CdS-ZnO nanocomposite has been synthesized by a facile chemical route and the prepared materials have been investigated as catalyst in the photodegradation of rhodamine B (RhB) in aqueous solution facilitated by the effective charge transfer mechanism in this coupled semiconductor system. CdS has been grown first and then petal-like ZnO has been produced to assemble into a flower like nanostructure. XRD, SEM, TEM, EDX have been employed to study the structural, morphological and compositional details of this unique nanostructure. It is observed that a strong quenching of band-edge emission of CdS following the growth of ZnO petal on it which is evidence for the occurrence of charge transfer between CdS and ZnO. The strong photocatalytic activity of this novel nanostructure on RhB has been investigated in details.

Full text of DOI:10.1016/j.jallcom.2013.08.184

Journal of Alloys and Compounds 583 (2014) 510–515
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Self assembled flower like CdS–ZnO nanocomposite and its photo
catalytic activity
⇑
T.K. Jana, A. Pal, K. Chatterjee
Department of Physics and Technophysics, Vidyasagar University, Midnapore 721102, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2013
Received in revised form 26 August 2013
Accepted 27 August 2013
Available online 6 September 2013
Self assembled flower like CdS–ZnO nanocomposite has been synthesized by a facile chemical route and
the prepared materials have been investigated as catalyst in the photodegradation of rhodamine B (RhB)
in aqueous solution facilitated by the effective charge transfer mechanism in this coupled semiconductor
system. CdS has been grown first and then petal-like ZnO has been produced to assemble into a flower
like nanostructure. XRD, SEM, TEM, EDX have been employed to study the structural, morphological
and compositional details of this unique nanostructure. It is observed that a strong quenching of band-
edge emission of CdS following the growth of ZnO petal on it which is evidence for the occurrence of
charge transfer between CdS and ZnO. The strong photocatalytic activity of this novel nanostructure
on RhB has been investigated in details.
Keywords:
CdS–ZnO nanocomposite
Optical behavior
Photocatalysis
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
Inorganic nanostructures are becoming ideal systems for
without having any harmful residues. It is found that to enhance
the efficiency, photogenerated charge carriers should be effectively
separated and that can be enhanced in the composite materials
according to their different band gap structures [18,19,24]. Charge
injection from a narrow or mid band gap semiconductor to a wide
band gap semiconductor can effectively enhance the charge sepa-
ration by decreasing the recombination possibilities [25]. Among
the different coupled semiconductor nanostructures, in the pursu-
ance of photocatalytic degradation, ZnO based nanocomposites
[18,19,26–28] gained extreme importance for its different advanta-
ges such as direct band gap [bulk : 3.37 eV], ease of crystallization,
anisotropic growth, nontoxicity, higher excitation binding energy
revealing novel phenomena at nanoscale leading towards wide
range of applications [1–7]. Depending upon the dimensionality
and the capability to tailor the morphology they are becoming
essential for smart and functional materials. Varity of geometrical
morphology [1,4,8–12] are being investigated for the different
nanostructures to explore novel properties out of those materials.
Specially the coupled semiconductor nanocomposite, having the
synergistic effects among the components, with different geometry
[
2,3,13–17] are attracting much attention due to the added advan-
2
À1 À1
tages such as improvement of charge separation, increase in the
life time of charge carrier and enhancement of the charge transfer
efficiency. The coupled semiconductors or the heterojunction
interface of semiconductors matching band potentials are particu-
larly attracting huge attention for their successful implementation
in photocatalytic degradation of organic pollutants [13,18–20].
Photocatalytic processes in coupled semiconductors have started
to play an important role in environmental remediation as well
as in renewable energy generation [14,18–21]. While the organic
pollutants introduced into the water systems [22,23] are of great
concern for society, photocatalytic degradation can potentially be
used for water decontamination using visible or ultraviolet light
as the illuminating source. Therefore it is urgent to develop highly
efficient photocatalytic materials for pollutant degradation
of 60 meV, higher electron mobility [200 cm V
s ] and simplic-
ity in tailoring the morphology. However, owing to its large band
gap energy, its photoresponse is largely restricted to the ultraviolet
(UV) region which is only the 4% of the total solar spectrum. Cou-
pled semiconductors provide the opportunity to sensitize a wide
2
band gap semiconductor (ZnO, TiO ) by low/mid band gap semi-
conductors (CdS, In , Bi ) which are visible light active materi-
S
2 3
2 3
O
als. Particularly CdS (bulk band gap 2.42 eV), having high optical
absorption coefficient and similar lattice structure as ZnO, is one
of the most appropriate sensitizer for ZnO. Recently many groups
have attempted to address the efficient development of
heterojunctions for CdS–ZnO with different morphology
[19,21,29,16,30,31]. The study of CdS/ZnO system for photocata-
lytic effect not only varies in synthesis technique but also has wide
variety of efficiency according to the structure of the composite.
Different CdS–ZnO systems, e.g. urchin like CdS@ZnO [21], nano-
tubes arrays of CdS/ZnO [16], partially or fully covered CdS nano-
particles on ZnO nanorods [29], ZnO/CdS core shell nanorods
⇑
Corresponding author. Tel.: +91 9474816352.
E-mail addresses: kuntal2k@gmail.com,
K. Chatterjee).
kuntal@mail.vidyasagar.ac.in
(
0
925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.08.184

T.K. Jana et al. / Journal of Alloys and Compounds 583 (2014) 510–515
511
[
19], CdS nanoparticles/ZnO nanowires heterostructure [31] have
been reported addressing several relevant issues. But several key
factors like development of facile preparation method, generation
of proper lattice interface to facilitate the charge transfer, enhance-
ment of the total activation site i.e. surface area are still remain a
challenge for the scientific community.
Here we propose a novel method to tether CdS to ZnO in such a
fashion so that self assembled flower like unique nanostructures of
CdS–ZnO has been synthesized and the sample shows high photo-
catalytic activity employing effective charge transfer between two
coupled semiconductors. Simple chemical route is adopted for the
growth of these novel nanocomposite and structural, chemical and
morphological details of the specimen has been studied. A possible
growth mechanism has been demonstrated for the better under-
standing of the materials. Absorption and emission properties of
the nanocomposite have been investigated and finally the visible
light photocatalytic activity of flower like CdS–ZnO nanostructure
for degradation of RhB is studied in details. RhB is taken as model
organic dye as it is the important xanthene dye and dye pollutants
from the textile industry.
Fig. 1. X-ray diffraction patterns of as prepared CdS and CdS–ZnO nanocomposite.
peak intensity of pure CdS, the peak intensity of CdS–ZnO for CdS
phase is noticeably weak. The result is expected [32,33] because
the CdS is being shielded for the incoming X-ray by the ZnO in
the composite form. It is also very distinct from the peak intensity
that ZnO phase is much stronger than the CdS phase in the final
product. CdS phase (ICDD card No. 65-2887) has cubic zinc blend
structure with lattice constant a = 5.8320 Å where as ZnO phase
is matched with ICDD card No. 36-1451 having hexagonal wurtzite
crystal structure (a = b = 3.2495 Å, c = 5.2069 Å).
2
. Experimental section
All the chemicals are Merck made of analytical grade and used without further
purification.
2.1. Preparation of CdS
2 2
For preparation of CdS nanoseed, CdCl and Na S were used as the reactants
with molar ratio 1:1. These precursors were dissolved in distilled water under stir-
ring (20 min). Then Na S solution was added drop wise in CdCl solution keeping
2
2
The photo gallery of Fig. 2 shows the typical SEM and TEM
images of the synthesized CdS–ZnO nanocomposite structure.
Fig. 2(a)–(c) shows the SEM images at different magnifications
and Fig. 2(d)–(f) shows the TEM images at increasing magnifica-
tion. It is evident from Fig. 2(a) that the synthesized materials
has a unique flower like structure where the central part looks
much brighter and several petal like protrusion come out at differ-
ent directions from the center. Fig. 2(b) shows such few CdS–ZnO
nanoflowers where it seems the flower has grown partially. In
Fig. 2(c) one fully grown flower like structure has been shown.
Here the number of petals is higher than the number of petals in
the flower like structure shown in Fig. 2(b) and the figure repre-
sents the typical structure of a complete CdS–ZnO unit, evolves
in this synthesis mechanism. The TEM image shown in Fig. 2(d)
also supports the morphology of a flower like structure of the
grown composite materials. Here also the central part is much dar-
ker and it signifies that the central part is the densest region in
these three dimensionally grown structures. Fig. 2(e) shows the
magnified version of a petal in the flower structure where it is seen
that the petal is formed due to the congregation of several rod like
structures. In Fig. 2(f) high resolution TEM shows clearly the aggre-
gation of rods in a part of a petal. The atomic planes visible in the
rod shows the unidirectional growth of ZnO and it is evident that
the rods are actually made of ZnO to form the petal part of this un-
ique structure. The average total size of a typical CdS–ZnO flower is
around 400 nm and the average size of the petal is 100–150 nm.
But the rod inside the petal has an average size of 10 nm. The aver-
age size of the central core is 90 nm.
the solution under constant stirring. The whole system was maintained in ice-bath.
The precipitate was centrifuged and washed with distilled water for several times
until the pH becomes normal. The final water solution from where the precipitate
was taken was also tested with silver nitrate solution to discard the presence of
NaCl in the product. Finally, the obtained powder was dried at 100 °C for 2 h in a
vacuum furnace to get CdS nanoparticles which will be used as nanoseed for the
next synthesis part.
2.2. Preparation of CdS–ZnO nanocomposite
To produce CdS–ZnO nano composite, 0.03 gm CdS nanoparticles were well dis-
persed (sonicated for 30 min) in 25 ml distilled water (solution A). Separately zinc
nitrate solution (B) taking 1.48 gm Zn(NO in 15 ml DI water and NaOH solution
C) taking 0.4 gm NaOH in 10 ml DI water were also prepared.
Then solution B and solution C was added drop wise in solution A under stirring
3 2
)
(
condition for 30 min for the formation of ZnO crystal on the nucleating sites of CdS
nanoseeds. After the reaction is over the resultant precipitate was centrifuged and
washed with distilled water for several times until the pH becomes 7 and then dried
in a vacuum furnace at 100 °C for 2 h to produce CdS–ZnO nanocomposite.
Structural analysis of all the powdered samples were carried out by Rigaku
Mini–Flex X-ray diffractometer using Cu Ka radiation (k = 1.54178 Å) source. Mor-
phological analysis was done by both JEM 2100 Transmission Electron Microscope
at an accelerating voltage of 200 keV and FEI, Inspect F Scanning Electron Micros-
copy. EDX was carried out in S-4200, Hitachi. Optical absorption spectra of the pow-
dered samples were recorded in a UV–Vis 1700 Shimadzu Spectrophotometer and
to get these spectra the powdered samples were dispersed in ethyl alcohol and
mounted in the sample chamber while pure ethyl alcohol was taken in the refer-
ence beam position. In case of photocatalysis study the organic dye RhB was taken
in distilled water and the sample was dispersed in that solution. For the PL the sam-
ple was taken in powder form and the measurement was carried out in Perkin El-
mer LS55 fluorescence spectrophotometer.
3
. Result and discussion
As the lattice fringe pattern is not visible in the relatively thick
center part of the structure the SAED pattern was taken from that
region and the result, shown in Fig. 3(a), matches with the crystal
planes of CdS. So, it can be said that the central part of the structure
is formed from the seed materials i.e. CdS and the petal parts are
made of ZnO which was grown on the latter half of the synthesis
procedure. To corroborate the chemical analysis, EDX of the sample
was also carried out and the result shows the existence of Cd, S, Zn
and O elements. Here the strong Si line was originated from the Si
3.1. Structural, chemical and morphological study
Fig. 1 shows the XRD analyzed data for as prepared CdS and
CdS–ZnO samples both. It is evident from the peaks of the CdS–
ZnO sample that the product is well crystalline having no impurity
as there is no unmatched peak. All the peaks of the final product
are matched either with CdS or ZnO. Interestingly compare to the

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T.K. Jana et al. / Journal of Alloys and Compounds 583 (2014) 510–515
Fig. 2. Typical FESEM (a) – (c), TEM (d) and (e) and HRTEM (f) images of the as prepared CdS–ZnO nanocomposite sample taken at different magnification. The arrow in (e)
and (f) indicates the magnification region.
Fig. 3. SAED pattern of the core region (indicated by arrow) of CdS–ZnO nanocomposite with matched crystal planes (a); EDX taken from the as prepared nanocompsite
samples deposited on Si wafer (b); and the schematic representation of the growth mechanism of flowerlike CdS–ZnO sample (c).
wafer which was taken as the substrate for the powdered sample
to carry out EDX measurement.
assembled in such a way that few petal like protrusions form
around the central part. This is how the CdS–ZnO nanocomposite
starts to take a flower like structure. As the time passes in the reac-
tion several new ZnO nanorods may start to nucleate from the
same CdS seed and they eventually grow and assemble just to
add new petals in the structure. In the model it is schematically
shown that how ZnO nanorods are growing around the seed and
how they got assembled to form petals. Eventually the number
of such petal increases as the reaction goes on to develop a fully
grown flower like CdS–ZnO structure. The last part of the sche-
matic diagram shows SEM image of such typical fully grown
CdS–ZnO structure to resemble the model figure. It is to be noticed
that Fig. 2(b) and (d) shows some of the typical structure of par-
tially grown nanocomposite. In this case somehow the growth
mechanism has been stopped after the formation of first few ZnO
3.2. Growth mechanism
Now it is important to understand the evolution of growth pro-
cess of such unique structure. On the perspective of the results ob-
tained so far, we have proposed a model of growth mechanism
which is shown in Fig. 3(c). The model starts with the CdS nanopar-
ticle, grown in the first phase of the synthesis part and such CdS
nanoparticles are the nucleating points for the next phase of syn-
thesis. In the next part of synthesis several ZnO nanorods will start
to grow initiating from the CdS seeds. The growth of these ZnO
nanorods is in the outward directions from the central nucleating
site of CdS. As the reaction proceeds the nearby nanorods are self

T.K. Jana et al. / Journal of Alloys and Compounds 583 (2014) 510–515
513
petals. It is found that both fully and partially grown flower like
formations are available in the sample morphology. Further inves-
tigation is required to reveal the effect and control of different
reaction parameters on the evolution of this nanocomposite struc-
ture. Though charge transfer characteristic of CdS quantum dot
sensitized ZnO flower-like nanostructure have been reported ear-
lier [34] in the category of these coupled semiconductors system
but the growth mechanism was not addressed properly. In the cat-
egory of single semiconductor system, direct observation of growth
of MgO nanoflowers by chemical vapor deposition method was
also addressed earlier [5]. Here, in our case, the system and struc-
ture are different and the development of the structure is given to
have the in-depth understanding towards engineering the mor-
phology of such materials.
dispersed in DI water taken in UV light (254 nm). The photograph
for the CdS–ZnO sample taken in same UV source has been shown
near to the CdS–ZnO spectrum in Fig. 4(b). Here also it is clear that
the luminescence of CdS has been quenched after the formation of
CdS–ZnO nanocomposite. These available separated electrons and
holes now can effectively be guided towards the activation of other
chemical elements and this observation pave the foundation of the
possibility that the materials can be utilized in photocatalytic
applications.
3.4. Photocatalytic experiments
The photo catalytic activity of CdS–ZnO system was measured
by the degradation of RhB in aqueous solution under halogen lamp
radiation. In a typical process 10 lM solution of RhB in DI water
3.3. Optical characteristics
was taken and the CdS–ZnO sample was added to it to maintain
a catalyst concentration of 1 gm L . To achieve equilibrium
À1
Fig. 4(a) shows the UV–Vis absorption characteristics of the
adsorption the organic dye with catalyst solution was stirred in
dark for 1 h and after 1 h of stirring no significant change was ob-
served. The solution was then exposed to 1000 W halogen lamp
radiation. The beaker was kept in cold water bath to maintain
the reaction temperature at 290 K. Actually the radiation from hal-
ogen lamp in the NIR region was balanced by keeping the solution
always in the room temperature and ensuring that the degradation
was only the result of photocatalysis without having any thermal
effect. The zero time reading was obtained from the sample solu-
tion taken just before turning on the light [37]. Henceforth the
sample was taken from the mother solution under the light radia-
tion in regular interval to measure the optical absorbance. Fig. 5(a)
shows different absorption spectrum of the sample solution taken
at different times. At starting time (t = 0) the spectrum presents the
normal intense absorption characteristics of RhB at 554 nm. As
time passes the photocatalysis reaction goes on and the gradual
degradation of the organic dye leads to the subsequent reduction
of the absorption peak intensity. The inset in this figure shows
the photograph of the dye with catalyst at starting time and after
190 min of light exposure. It is seen that the color of the dye is
completely faded away confirming the degradation of the dye mol-
ecules. The percentage of degradation measured at different time
was calculated by normalizing the peak intensity in term of the
starting (t = 0) peak intensity of RhB with catalyst. Experiments
without radiation of light in presence of catalyst and with radiation
of light without the presence of catalyst have also been done. All
these data in terms of degradation percentage is presented in
pure CdS and CdS–ZnO nanocomposite. In the lower panel of
Fig. 4(a) the absorption peak for prepared CdS sample is found to
be near 470 nm. After formation of the nanocomposite the distinct
signature of CdS has been lost and there is no such peak at 470 nm
observed in the absorption spectrum shown in the upper panel of
Fig. 4(a). The sharp peak in the UV region at around 353 nm is orig-
inated from the band edge transition of ZnO. Photoluminescence
spectra of CdS and CdS–ZnO nanocomposite samples are shown
in Fig. 4(b). It is interesting to note that CdS has strong emission
peak near 561 nm along with two other peaks at 530 nm and
5
44 nm. It is established that surface defects as well as shallow
and deep trap states play important role in the luminescence prop-
erty of CdS. Here the strong emission peak near 561 (2.19 eV) is ex-
pected to be linked to the trapped electron/holes at surface defects
[
35] and peak originated at 530 nm (2.32 eV) corresponds to the
defects caused by the interstitial sulfur [36]. But in case of CdS–
ZnO nanocomposite the spectrum shows almost quenching of
emission peak intensity at 561 nm while the other two peaks in
position and intensity are almost remaining same as the pure
CdS sample. It signifies that the transfer of electrons from CdS con-
duction band to ZnO conduction band in the present CdS–ZnO sys-
tem may inhibit the radiative relaxation of the electrons in CdS
energy levels and cause the effective quenching of emission band
at 2.19 eV. To make the matter visibly clear the UV-photography
of both samples are presented here. Inset near to CdS spectrum
in Fig. 4(b) shows yellow luminescence of the CdS sample
Fig. 4. Optical absorbance spectrum for CdS sample [(a) lower panel] and for CdS–ZnO nanocomposite sample [(a) upper panel]; emission spectra for CdS and CdS–ZnO
samples (b). The excitation wavelength is 310 nm for both the case. Inset of (b) panel shows the photograph of respective sample at UV light (254 nm).

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T.K. Jana et al. / Journal of Alloys and Compounds 583 (2014) 510–515
Fig. 5. Absorption spectra of rhodamine B aqueous solution in presence of CdS–ZnO during illumination (a); Decrease in normalized absorption intensity of rhodamine B with
respect to time for different cases. Inset in (a) panel shows the photograph of RhB in presence of CdS–ZnO at starting and ending time of illumination.
Fig. 5(b). It is seen that as expected there is no change in degrada-
tion of RhB when it is exposed to light without catalyst. In case of
RhB with CdS–ZnO catalyst without light radiation there is little
change and the dye degraded up to 24% within the measurement
duration of 190 min. It seems the catalytic effect of the prepared
compound semiconductor is responsible for the slight degradation
of this organic dye. But the noteworthy thing is that when the dye
with sample is exposed in light the degradation rate is extremely
fast and dye is almost completely (90%) decomposed within
1
90 min. Here it is clear that CdS–ZnO works as strong photo-cat-
alytic agent decolorizing RhB taken as model organic pollutant. The
degradation rate constant was calculated from the slope obtained
by linear regression from a plot of the natural logarithm of the nor-
malized absorbance as a function of irradiation time [37]. The rate
constant is found to be 0.0133/min which is better [38] or compa-
2
rable [39] to the reported TiO based highly photocatalytic materi-
als. In case of ZnO/CdS system, Ravishankar and his team have
shown ZnO/CdS heterostructures with engineered interfaces for
high photocatalytic activity and our degradation rate constant is
in close match to that of the best sample in the reported series
[
29]. ZnO/CdS core shell nanorods have also been attempted with
variable shell thicknesses to tune the photocatalytic efficiency
and it was found that the photodegradation of RhB is better for
the higher thickness of CdS shell around ZnO nanorods [19]. Nozaki
et al. have demonstrated photocatalytic degradation of 3-4-dihy-
droxy benzoic acid by CdS–ZnO nanorods, chemically grown on in-
dium tin oxide, but their also it takes comparatively long time
(
ꢀ20 h) to reach the sensible degradation of the dye [40]. ITO/
CdS/ZnO composite films were employed for the degradation of
methyl orange and the result exhibited that the films prepared un-
der specific conditions have shown higher photocatalytic activity
following pseudo first order kinetics of degradation [41]. In our
system the structure is unique and the photodegradation capacity
is also impressive.
Fig. 6. The schematic photodegradation process in the CdS–ZnO/RhB aqueous
solution (a); N-deethylation of rhodamine B to rhodamine (b) [33].
To address the photocatalytic effect in details we propose a pos-
sible charge transfer mechanism in our synthesized system and it
is shown in Fig. 6(a) as a modified energy band diagram, presented
on the canvas of the synthesized materials. The band position of
CdS–ZnO system [19,21,34] clearly elucidate the feasibility of
charge transfer from CdS to ZnO due to more negative potential
of CB and VB edges of CdS than those of ZnO. Furthermore for en-
hanced photoresponse a faster rate of charge transport is needed
and it is facilitated by the intimate and effective contact of the hy-
brid materials. Here the flower like structure of CdS–ZnO provides
the opportunity for better intersystem electron transfer. According
to Anderson’s model type II heterojunction is formed between CdS
and ZnO. Visible light from halogen lamp generates electron–hole
pair in CdS and the photogenerated electrons in CdS core can easily
move to ZnO petals by the process of ballistic diffusion [40]. Once
the electrons diffuse into the conduction band of ZnO the recombi-
nation probability becomes small as there can be no free hole in
ZnO under visible excitation. On the other hand remaining part
of the photogenerated excitons i.e. holes are accumulated in the
VB of CdS as they cannot move towards more positive VB of ZnO.
Now the holes in VB of CdS can react with water adhering to the
ꢁ
surface to convert the OH into highly reactive OH and electrons
in CB of ZnO can potentially convert the dissolved oxygen into
ÅÀ
Å
highly oxidative radicals O which in turn produces OH [39,42].
2
ÅÀ
+
Å
This O can react with a H of solvent water to form OOH and this
2
Å
Å
Å
OOH can eventually generate H O and finally OH . The strong OH
2
2
radicals decompose the organic dye. It is also reported [43] that

T.K. Jana et al. / Journal of Alloys and Compounds 583 (2014) 510–515
515
Å
Å
OOH and OH are necessary to N-deethylation of RhB which is the
basic requirement for the complete degradation of the dye.
Fig. 6(b) shows the N-deethylation of RhB to Rh [44]. In this way
photogenerated electrons and holes in the synthesized CdS–ZnO
nanoflower system effectively degrade the organic dye and present
the system as a potential material for future study.
[16] X. Qi, G. She, Y. Liu, L. Mu, W. Shi, Electrochemical synthesis of CdS/ZnO
nanotube arrays with excellent photoelectrochemical properties, Chem.
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In conclusion we have demonstrated simple route to prepare
unique flower like CdS–ZnO nanostructure. The evolution of com-
plete self assembled growth mechanism has been proposed on
the basis of the results obtained from different chemical and struc-
tural investigations. The excellent coupling of the two semiconduc-
tors enhanced the charge separation capability and was supported
by the photoluminescence study. The novel structure manifests
strong photocatalytic activity and is explained on the basis of effec-
tive charge separation and charge transfer mechanism in the pre-
pared system. The synthesis method is general and applicable to
a wide range of coupled system for photocatalytic and for other
possible photo conversion applications.
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This work was financially supported by the DST-FASTTRACK, In-
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