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KVMoO6-Clean-reddy-2015

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1
Jitta Raju Reddy
Sreenu Kurra
Naveen Kumar Veldurthi
Prathapuram Shrujana
Ravinder Guje
Muga Vithal
Department of Chemistry, Osmania
University, Hyderabad, India
Research Article
Preparation of Visible Light Active Ag- and
N-Doped KVMoO6: Photodegradation of
Methylene Blue
Brannerite-type layered vanadomolybdate, KVMoO6, was synthesized by sol–gel method
using ethylene glycol as gelating agent. Its silver- and nitrogen-doped analogues were
prepared by ion exchange and solid state methods, respectively. All compounds were
characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy
(XPS), energy dispersive spectroscopy (EDS), Fourier transform IR spectroscopy (FT-IR),
and UV–vis diffuse reflectance spectra. The bandgap energy of all compounds was
obtained from their diffuse reflectance spectra. Bandgap was reduced considerably
upon silver and nitrogen doping in KVMoO6. The photocatalytic activity of parent and
doped samples was studied by degradation of methylene blue. The nitrogen-doped
KVMoO6 shows higher photocatalytic activity against the degradation of methylene
blue. The stability and reusability of the nitrogen-doped KVMoO6 were investigated.
N-doped KVMoO6 is a potential catalytic material for cleaning wastewater.
Keywords: Bandgap energy; Dye degradation; Photocatalysis; Sol–gel synthesis
Received: February 28, 2014; revised: April 17, 2014; accepted: May 12, 2014
DOI: 10.1002/clen.201400164
1 Introduction
Recently, the demand for clean and green environment has
attracted extensive attention from academic and public sector
organizations due to rapid urbanization and industrial development which cause environmental pollution. Discharge of industrial
waste vitiates the natural aqueous environment in the vicinity of
industrial areas. Synthetic dyes are widely used in textile, cosmetic,
paper, leather, pharmaceutical, and food industries. Especially,
huge amounts of dye effluents from textile industries are discharged
into the aquatic habitats. Most of these dyes are dangerous and cause
severe environmental pollution by releasing toxic substances into
the aqueous phase and do not decompose quickly. Therefore,
investigations were directed toward the development of more ecofriendly materials for the remediation of harmful organic dye
pollutants [1, 2].
Semiconductor materials, such as binary and ternary oxides
belonging to pyrochlore, perovskite, spinel, delaffosite, and
brannerite families have been studied mostly because of their
thermal stability, wide ranging chemical, and physical
properties [3–5]. These semiconducting metal oxides are extensively
used in photo-induced processes. This is because, semiconductors
Correspondence: Prof. M. Vithal, Department of Chemistry, Osmania
University, Hyderabad 500 007, India
E-mail: [email protected]
Abbreviations: Ag-KVMoO6, Ag-doped KVMoO6; CA, citric acid; DRS,
diffuse reflectance spectroscopy; EDS, energy dispersive spectroscopy;
EG, ethylene glycol; FT-IR, Fourier transform IR spectroscopy; MB,
methylene blue; N-KVMoO6, N-doped KVMoO6; SEM, scanning electron
microscopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray
diffraction
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
can be excited by light having energy equal to or greater than its
bandgap and an electron are promoted to the conduction band,
leaving a hole in the valence band. This excited electron energy can
either be used directly to produce electricity as in photovoltaic solar
cells or drive a chemical reaction, as in photodegradation. In recent
years, the use of semiconductors for environmental purification has
become one of the most active research fields, because the toxic
organic pollutants can be completely mineralized to CO2 and H2O by
photocatalysis under mild conditions [6–9].
The photodegradation of pollutants over TiO2 is one of the most
promising heterogeneous photocatalytic applications [10, 11]. TiO2
has been intensively investigated as a photocatalyst, since the
reduction of CN in water reported by Frank and Bard (1977) which
is the first application of TiO2 in environmental purification [12].
However, with a relatively wide bandgap (3.2 eV), its practical
applications in the visible light region are limited [13]. Therefore, the
development of a photocatalyst with high activity under visible-light
irradiation is obligatory to meet the requirements of future
environmental and energy technologies driven by solar energy.
One way of improving the efficiency of the catalyst is the
modification of its band structure by the partial substitution of
cation and/or anion [14, 15].
Brannerite, a naturally occurring mineral of composition (U,Ca)(Ti,
Fe)2O6 and often written as UTi2O6, exists in many uranium ore
bodies [16]. It crystallizes in monoclinic lattice with space group C2/m,
and both U and Ti occupy distorted octahedral coordination. The
structure is flexible for substitution at uranium site by Pb, Ca, Th, Y,
and Ce and titanium site by Si, Al, and Fe. This class of compounds
exhibit interesting physical properties, such as ferroelectricity and
fast ionic conductivity, which make some of them suitable choices for
applications in fuel cells and other similar devices [17]. The physical
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J. R. Reddy et al.
properties can be changed significantly by small variations in the
compositions leading to materials with the required set of characteristics for developing components of new devices. In this connection, the
layered type brannerites (containing vanadium, molybdenum, and
tungsten) have generated a new interest as electrode materials [18, 19].
Previous reports have shown that transition metal vanadates of
general composition AVMO6 (A ¼ Li, Na, K and M ¼ Mo, W) are used as
promising anode materials and catalysts due to their high-specific
capacity and layered crystal structure. One of the earliest works on
these materials, by Natividad et al., involved the study of partial
oxidation of benzyl alcohol into benzaldehyde under UV light [20]. The
research pertaining to the photocatalysis of these layered brannerites is
limited in spite of their flexible properties. Thus, the object of our
present work is to study the photocatalytic properties of brannerite
type oxides containing vanadium, molybdenum, and alkali metals.
These brannerite type oxides are flexible to undergo ion exchange with
guest ions and make them promising materials for photocatalytic
applications.
To date, a large number of scientific investigations about the
photocatalytic degradation of aqueous organic pollutants, such as
aliphatic alcohols, alkenes, phenols, dyes, and aromatic compounds
have been reported [21–24]. In the present investigation, we focused
our attention on the degradation of methylene blue (MB), which is
usually used as a probe contaminant to evaluate the activity of the
photocatalyst. It is widely used as a paper dye, microscopy stain,
chemical intermediate, medicinal agent, and cosmetic dye, which
may result in its release to the environment through various waste
streams. Toxic MB is chemically inert and stable in the environment,
and it is not easy to remove from wastewater because of the high
stability and solubility in water. It is harmful to the skin, eyes,
respiratory tract, and causes anemia. In the continuation of our
studies on photocatalytic degradation of organic pollutants,
attempts are made to vary the band structure of KVMoO6, a
brannerite-type layered oxide, by partial substitution of K and O
with Ag and N, respectively. Silver is an extremely attractive dopant
due to its remarkable catalytic activity. In the photocatalytic
process, Agþ acts as a sink for photo-induced charge carriers, could
promote interfacial charge transfer processes and inhibit the
recombination of photogenerated electron–hole pairs due to its
strong electron trap ability. In addition, nitrogen doping has been
proven to be very promising because the implantation of nitrogen
creates oxygen vacancies that are traps for excited electrons leading
to a reduction in recombination of photo-induced electron–hole
pairs. Therefore, silver and nitrogen were chosen as the dopants in
this study to enhance the photocatalytic activity of KVMoO6 [25, 26].
This paper deals with preparation, characterization and photocatalytic studies of Ag- and N-doped KVMoO6.
2 Materials and methods
2.1 Synthesis of N- and Ag-doped KVMoO6
The preparation of potassium vanadomolybdate (KVMoO6) was carried
out by the modified sol–gel method which was earlier reported by
Mucha et al. [27]. NH4VO3 (S-D Fine), (NH4)6Mo7O24) 6H2O (Sigma–
Aldrich), KNO3 (Merck), citric acid (S-D Fine), and ethylene glycol are
used as the starting materials as received. In a typical synthetic process,
the stoichiometric amounts of NH4VO3 (1.24 g, 10.63 mmol),
(NH4)6Mo7O24 6H2O (1.87 g, 1.51 mmol), and KNO3 (1.07 g, 10.63 mmol)
were dissolved in water. Citric acid (CA) (9.56 g, 45.54 mmol) was added
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
to this solution such that the mole ratio of metal ions to CA is 1:2. The
mixture was stirred for several hours at room temperature followed by
slow evaporation till a viscous liquid was obtained. At this stage,
ethylene glycol (EG) (2.7 mL, 50.09 mmol) was added in the molar ratio
of metal ions to EG as 1:2.4. This mixture was heated on a hot plate at
160°C for 2–3 h. The temperature was increased to 200°C at the onset of
solidification to get dried powder. The resultant powder was heated in a
muffle furnace in air at 350°C/24 h, 400°C/24 h, and 450°C/24 h with
intermittent grindings. The N-doped KVMoO6 (N-KVMoO6) was
prepared using urea as reported earlier [28]. A thoroughly ground
mixture of KVMoO6 and urea (CO(NH2)2) in the weight ratio of 1:2 was
heated at 400°C for 2 h in a muffle furnace in air. The resultant material
was washed with distilled water to remove unreacted urea (or
byproducts) and dried in air. The silver doping of KVMoO6 (Ag-KVMoO6)
was performed by ion-exchange process. Firstly, 0.6 g (3.5 mmol) of
AgNO3 was dissolved in 50 mL of distilled water. To this solution 1 g
(3.5 mmol) of KVMoO6 was added under constant stirring. The color of
the powder changed slowly, and the stirring was continued for 12 h
under ambient condition. The solid was separated and washed with
distilled water several times and dried in air.
2.2 Characterization
The room temperature X-ray diffractograms of all samples were
recorded using Rigaku MiniFlex 600 X-ray diffractometer (Cu Ka,
l ¼ 1.5406 Å , 2u ¼ 10°–80°, step size (2u) ¼ 0.02°, and scan step
time ¼ 0.15 s) for phase confirmation. The chemical analysis of the
samples was carried out by X-ray photoelectron spectroscopy (XPS)
(model ESCALAB220I-XL) equipped with an Axis Ultra, Kratos
(1486.7 eV) monochromatic source for excitation. XPS spectra over
a binding energy range of 0–1200 eV were obtained using analyzer
pass energy of 117.4 eV. Scanning electron microscopy with energy
dispersive spectra (SEM-EDS) were recorded on the HITACHI SU-1500
variable pressure scanning electron microscope. Fourier transform
IR spectroscopy (FT-IR) was recorded using Shimadzu spectrometer
in the form of KBr pellets. JASCO V-650 UV–Vis spectrophotometer
was used for UV–Vis diffuse reflectance spectroscopy (DRS) in the
range of 200–800 nm. BaSO4 was used as the reflectance standard.
2.3 Photocatalytic experiments
The photocatalytic activity of all samples was evaluated by
photodegradation of MB using HEBER Visible Annular Type Photoreactor, model HVAR1234 (Haber Scientific, India), under visible
light irradiation using 500 W tungsten lamp as a light source. The
system was cooled by circulating water and maintained at room
temperature. In a typical process, 60 mL of aqueous MB solution
(10 mg L1) was stirred with 0.06 g of catalyst in a cylindrical-shaped
glass reactor at room temperature in air. The suspension was stirred
in the dark for 60 min to establish adsorption–desorption equilibrium. Then the solution was exposed to light with continuous
stirring. At regular 30 min intervals, about 3 mL of solution was
collected and centrifuged to remove the catalyst particles. The
change in the concentration of MB was obtained by recording the
absorbance at 664 nm using a UV–vis spectrophotometer.
3 Results and discussion
The structure and phase purity of all the prepared compositions
were analyzed by recording the powder X-ray diffraction (XRD).
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Figure 1. Powder X-ray diffraction patterns of KVMoO6, Ag-KVMoO6 and
N-KVMoO6.
Figure 1 shows the XRD pattern of KVMoO6 along with the silver- and
nitrogen-doped KVMoO6. The XRD patterns of all the samples are
consistent with the published Powder Diffraction data of KVMoO6
which crystallizes in orthorhombic lattice (JCPDF: 89-4563). All
powder patterns are free from impurities and confirm the phase
formation. The powder patterns of Ag- and N-doped KVMoO6 are
similar with parent KVMoO6 suggesting that the doping did not alter
the original crystal structure. However, a careful observation of the
diffraction pattern of Ag- and N-doped KVMoO6 shows a shift in the
position of d-lines with respect to the parent KVMoO6 (Fig. 1, inset).
The systematic peak shifting indicates that silver and nitrogen are
successfully doped into the KVMoO6 lattice.
The substitution of silver and nitrogen into KVMoO6 is supported
by XPS and SEM-EDS measurements. The X-ray photoelectron
spectrum of KVMoO6 is shown in Fig. 2. The survey spectrum of
KVMoO6 reveals the presence of peaks belonging to K2p
(293.0 0.5 eV), V2p (517 0.5 eV), Mo3d (233 0.5 eV), and O1s
(530 0.5 eV). The observed peak positions of constituent elements
Figure 2. XPS survey spectrum of KVMoO6.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3
of KVMoO6 are in agreement with previous reports [20, 29]. The X-ray
photoelectron survey spectrum of silver- and nitrogen-doped
KVMoO6 also shows these peaks along with additional peaks of
silver and nitrogen, respectively. The presence of silver/nitrogen in
doped samples is examined by analyzing their high resolution X-ray
photoelectron spectra. Figure 3a and b displays the Ag3d and N1s
spectra of Ag-KVMoO6 and N-KVMoO6, respectively. Generally, the
existence of Agþ ion can be identified by the observation of Ag3d
peak in the binding energy 367–372 eV range [20]. The spectrum
(Fig. 3a) shows two peaks centered at 367.95 and 373.85 eV
corresponding to 3d5/2 and 3d3/2 states, respectively [30]. The spin–
orbit splitting is 5.90 eV which is close to the reported value. The Xray photoelectron spectrum of N-KVMoO6 for the N1s peak exhibits a
symmetrical peak centered at 396.95 eV (Fig. 3b), which is attributed
to the substitutional nitrogen in place of oxygen in the lattice [31].
Thus, the XPS results authenticate the presence of silver and
nitrogen in Ag-KVMoO6 and N-KVMoO6, respectively.
The presence of doped elements in the KVMoO6 lattice was further
substantiated by their SEM-EDS profiles. The SEM figures of all the
samples are shown in Fig. 4a, and revealed that the morphology of
the crystallites were not altered even after (Ag and N) doping.
Figure 4b shows the energy dispersive spectra (EDS) of parent and
doped samples. The appearance of Ag and N peaks in Ag-KVMoO6 and
N-KVMoO6, respectively, clearly indicates their substitution in the
lattice. From these EDS profiles, the content (weight ratio, wt%) of
the silver and nitrogen in Ag-KVMoO6 and N-KVMoO6 was found to be
5.93 and 2.93 wt%, respectively. Therefore, these results further
support the presence of silver and nitrogen in the KVMoO6 lattice.
Figure 5 shows the FT-IR Spectra of KVMoO6, Ag-KVMoO6, and
N-KVMoO6. The vibrational spectra of all investigated vanadates are
similar to each other and comparable with reported systems [29, 32].
The structure of KVMoO6 is characterized by distorted VO6 and MoO6
octahedra sharing edges. Alkali metal ions are located in interlayers
and form M0 O6 (M0 ¼ Li, Na, and K) coordination polyhedra [33]. The
bands seen in the high frequency region (800–1000 cm1) were
assigned to (V/Mo)O6 stretching vibrations. The strong bands
observed around 960 cm1 were attributed to vibrations of the
terminal (V/Mo)¼O groups and very weak bands found near
939 cm1 were assigned to some overtone or combination bands.
The strong and weak bands found in the regions 800–930 cm1 were
recognized as asymmetric stretching vibrations of (V/Mo)–O–(V/Mo)
bridges. Similarly, strong and very broad bands seen in the regions
660–730 and 450–650 cm1, respectively, are described as symmetric
stretching of (V/Mo)¼O. The bands identified below 450 cm1 with
medium and weak intensity were represented to both the bending
modes of (V/Mo)¼O and translational modes of M0 O6 polyhedra.
The optical properties of KVMoO6, Ag-KVMoO6, and N-KVMoO6 are
examined from their UV-Vis diffuse reflectance spectroscopy (DRS)
(Fig. 6). All the DRS profiles have absorption edges in the visible
region, which is in agreement with their colors (Fig. 6, inset). The
absorption edges of KVMoO6, Ag-KVMoO6, and N-KVMoO6 are located
at 454, 480, and 516 nm, respectively. The significant feature of
doped samples is the red shift of the absorption edge into mid visible
region. Bandgap energy of all the compounds is calculated from the
Kubelka Munk (KM) plots, [F(Ra) hn]1/2 versus hn (eV) where F(Ra) is
absorption coefficient and hn is a photon energy (Fig. 6, inset). From
these plots, the bandgap energy of KVMoO6, Ag-KVMoO6, and NKVMoO6 are found to be 2.73, 2.58, and 2.40 eV, respectively. The
lowering of the bandgap energy of silver- and nitrogen-doped
photocatalysts compared to parent KVMoO6 is due to the
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Figure 3. XPS spectrum of (a) core level Ag3d of Ag-KVMoO6 and (b) core level N1s of N-KVMoO6.
modification of the valence band and/or conduction band upon
doping [34]. These results clearly show that all the materials have
suitable bandgap energy for photocatalytic applications in the
visible range. Additionally, the nitrogen-doped KVMoO6 possessed
lower bandgap energy compared to the parent- and silver-doped
KVMoO6. Therefore, N-KVMoO6 is expected to show more photoactivity compared to the other two compounds.
3.1 Photocatalytic measurements
The photoactivity of a material depends on several factors, such as
surface area, extent of crystallinity, bandgap of catalyst, nature of
pollutant, the number of electron–hole pairs generated, the
recombination of electron and hole pair, etc. Among these,
the number of electron–hole pair generation and recombination
rate plays important roles in the degradation of the pollutant.
The initiating step in the photocatalysis is the excitation of the
semiconductor by the radiation of sufficient energy to produce
electron–hole pairs. The generation of these electron–hole pairs is
proportional to the amount of light absorbed by the photocatalyst.
For better absorption of the light in the visible region, the catalyst
should have lower bandgap energy. Accordingly, in the present
investigation all the photocatalysts have suitable bandgap energy
for absorption of visible light.
Figure 4. (A) SEM images of (a) KVMoO6, (b) Ag-KVMoO6, and (c) N-KVMoO6, (B) EDS profiles of (a) KVMoO6, (b) Ag-KVMoO6, and (c) N-KVMoO6.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
Figure 7. Photocatalytic degradation of MB using KVMoO6, Ag-KVMoO6,
N-KVMoO6, and Degussa P25 (reaction conditions: catalyst load:
60 mg/60 mL and initial concentration of dye: 10 mg/L).
Figure 5. FT-IR spectra of KVMoO6, Ag-KVMoO6, and N-KVMoO6.
The photocatalytic activity of parent and doped KVMoO6 was
investigated by visible light driven photodegradation of MB aqueous
solution. Experiments were carried out in presence and absence of
light and also with and without the catalyst. The dark experiments
were carried out to establish an adsorption–desorption equilibrium
before start of visible light irradiation. After 1 h of dark experiment,
the reaction mixture was exposed to visible light. The extent of
degradation was monitored by measuring the change in the optical
absorption of MB dye. The variation in the concentrations of MB
with irradiation time in the presence of KVMoO6, Ag-KVMoO6, and
N-KVMoO6 is shown in Fig. 7. Degussa P25 was used as a reference.
The results indicate that all the three samples show photoactivity in
the degradation of MB. It is noticed that N-KVMoO6 effectively
degrades the MB dye compared to parent KVMoO6, silver-doped
KVMoO6, and Degussa P25. The higher activity of N-KVMoO6 may
Figure 8. The absorption spectra of aqueous MB solution in the presence
of N-KVMoO6. The inset shows the color of MB with irradiation time.
Figure 6. UV–vis diffuse reflectance spectra of KVMoO6, Ag-KVMoO6,
and N-KVMoO6. The inset shows the corresponding KM function versus
energy and color of the respective samples.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 9. Cyclic runs in the Photocatalytic degradation of the MB
solution in the presence of N-KVMoO6 (reaction conditions: catalyst load:
60 mg/60 mL and initial concentration of dye: 10 mg/L).
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Figure 10. Powder X-ray diffraction patterns and FT-IR spectra of N-KVMoO6 (a) before and (b) after MB degradation.
be due to its lower bandgap energy and inhibition of photoinduced electron–hole recombination. The photocatalyst with lower
bandgap energy can absorb more number of photons of light in the
visible region and facilitates the generation of a large number of
electron–hole pairs in the photocatalytic reaction [26]. Figure 8 shows
the typical UV–Vis spectra of MB aqueous solution in the presence of
N-KVMoO6 under visible light irradiation. The spectrum of MB is
characterized by two bands (245 and 292 nm) in the UV and two bands
(614 and 664 nm) in the visible regions corresponding to the
characteristic aromatic rings of MB and conjugated p-system,
respectively. It is observed that with increase in the irradiation time,
the intensity of these peaks decreased indicating the destruction of
MB structure (Fig. 8). The change in the color of MB with irradiation
time is shown in the inset of Fig. 8. It is obvious that its color
almost disappears in about 120 min of visible light irradiation. Thus,
N-KVMoO6 can be used to clean the contaminated water under visible
and/or sunlight.
Generally, in photocatalysis, photons of incident light are
absorbed by the semiconductor. If the energy of the incident light
is equivalent and/or greater than the bandgap energy of the
semiconductor, electrons are promoted from the valence band to
the conduction band, thereby creating electron–hole pair. These
electron–hole pairs react with the surface adsorbed species to
generate radicals that are responsible for the degradation of dye
molecules to yield byproducts [15]. At the end of the reaction, the
catalyst can be regenerated and reused for further reaction. Thus,
the stability and reusability of catalysts are very important issues for
practical applications. The stability and reusability of N-KVMoO6
were tested for MB photodegradation for four times. As shown in
Fig. 9, the catalyst N-KVMoO6 exhibits same activity for all the
cycles of photo-degradation of MB. The chemical stability of
the photocatalyst (N-KVMoO6) was verified by recording its powder
XRD pattern and FT-IR spectrum after MB decomposition experiment. The powder XRD pattern and FT-IR spectrum are found to be
similar before and after the photocatalytic experiment (Fig. 10).
Hence, N-KVMoO6 is stable and can be repeatedly used for MB
decomposition.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
4 Concluding remarks
Phase pure layered brannerite, KVMoO6, has been successfully
prepared by the sol–gel method. Silver-doped KVMoO6 was obtained
from parent KVMoO6 by ion exchange method. The nitrogen-doped
KVMoO6 was prepared by heating the mixture of KVMoO6 and urea at
400°C/2 h. Phase formation of all the samples was confirmed by
powder XRD patterns. The appearance of Ag3d and N1s peaks in (i)
XPS and (ii) EDS confirms the incorporation of Ag and N into KVMoO6
lattice. The bandgap energy is reduced considerably in the Ag- and
N-doped KVMoO6 compared to its parent oxide. All compounds show
photocatalytic activity against the degradation of MB. Compared to
Degussa P25 and other photocatalysts (parent- and silver-doped
KVMoO6), nitrogen-doped KVMoO6 exhibits higher photocatalytic
activity may be due to its lower bandgap energy and the reduction in
the photo-induced electron–hole recombination. Even after the
fourth cycle of photodegradation of MB, N-KVMoO6 has been found
to be stable and its photocatalytic activity is unchanged.
Acknowledgements
The authors would like to thank Department of Science and
Technology (DST), New Delhi, under PURSE and FIST schemes, and
UGC, New Delhi, under UPE programme. JRR thanks the Council of
Scientific and Industrial Research (CSIR), New Delhi, for the award of
Senior Research Fellowship.
The authors have declared no conflicts of interest.
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