Scheelite-type MWO4 (M = Ca, Sr, and Ba) nanophosphors: Facile synthesis, structural characterization, photoluminescence, and photocatalytic properties

Article abstract of DOI:10.1016/j.materresbull.2014.09.096

Scheelite-type MWO₄ (M = Ca, Sr, and Ba) nanophosphors were synthesized by the precipitation method. All compounds crystallized in the tetragonal structure with space group I41/a (No. 88). Scherrer's and TEM results revealed that the average crystallite size varies from 32 to 55 nm. FE-SEM illustrate the spherical (CaWO₄), bouquet (SrWO₄), and fish (BaWO₄) like morphologies. PL spectra indicate the broad emission peak maximum at 436 (CaWO₄), 440 (SrWO₄), and 433 nm (BaWO₄) under UV excitation. The calculated CIE color coordinates of MWO₄ nanophosphors are close to the commercial BAM and National Television System Committee blue phosphor. The photocatalytic activities of MWO₄ were investigated for the degradation of methylene blue dye under UV illumination. At pH 3, BaWO₄ nanocatalyst showed 100% dye degradation within 60 min. The photocatalytic activity was in the decreasing order of BaWO₄ > CaWO₄ > SrWO₄ under both neutral and acidic conditions.

Full text of DOI:10.1016/j.materresbull.2014.09.096

Accepted Manuscript
Title: Scheelite-type MWO4 (M = Ca, Sr and Ba)
nanophosphors: Facile synthesis, structural characterization,
photoluminescence and photocatalytic properties
Author: C. Shivakumara Rohit Saraf Sukanti Behera N.
Dhananjaya H. Nagabhushana
PII:
S0025-5408(14)00640-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2014.09.096
MRB 7779
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Revised date:
Accepted date:
15-5-2014
3-9-2014
26-9-2014
Please cite this article as: C.Shivakumara, Rohit Saraf, Sukanti Behera, N.Dhananjaya,
H.Nagabhushana, Scheelite-type MWO4 (M = Ca, Sr and Ba) nanophosphors: Facile
synthesis, structural characterization, photoluminescence and photocatalytic properties,
Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2014.09.096
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Scheelite-type MWO4 (M = Ca, Sr and Ba) nanophosphors: Facile synthesis,
structural characterization, photoluminescence and photocatalytic properties
C. Shivakumara a,*, Rohit Saraf b, Sukanti Behera a, N. Dhananjaya c, H. Nagabhushana d
a Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012,
India.
b Centre for Converging Technologies, University of Rajasthan, Jaipur 302 005, India.
c Department of Physics, B.M.S. Institute of Technology, Bangalore 560 064, India.
d C. N. R. Rao Center for Advanced Materials, Tumkur University, Tumkur 572 103, India.
_____________________________________________________________________________
* Corresponding Author
C. Shivakumara. E-mail: shiva@sscu.iisc.ernet.in
Telephone : +91 80 2293 2951; FAX : +91 80 2360 1310
Highlights
ꢀ Scheelite-type MWO4 nanphosphors were synthesized by wet chemical approach.
ꢀ CIE chromaticity diagram reveals that these nanophosphors emit intense blue
luminescence.

ꢀ Under UV illumination, we achieved 100% MB dye degradation over BaWO4
nanocatalysts within 60 min.
ꢀ MWO4 can be useful in removing industrial pollutants and fabricating blue component in
white LEDs.
Graphical abstract
Abstract
Scheelite-type MWO4 (M = Ca, Sr and Ba) nanophosphors were synthesized by the
precipitation method. All compounds crystallized in the tetragonal structure with space group
I41/a (No. 88). Scherrer’s and TEM results revealed that the average crystallite size varies from
32 to 55 nm. FE-SEM illustrate the spherical (CaWO4), bouquet (SrWO4) and fish (BaWO4) like
morphologies. PL spectra indicate the broad emission peak maximum at 436 (CaWO4), 440
(SrWO4) and 433 nm (BaWO4) under UV excitation. The calculated CIE color coordinates of
MWO4 nanophosphors are close to the commercial BAM and National Television System
Committee blue phosphor. The photocatalytic activities of MWO4 were investigated for the
degradation of methylene blue dye under UV illumination. At pH = 3, BaWO4 nanocatalyst
showed 100% dye degradation within 60 min. The photocatalytic activity was in the decreasing
order of BaWO4 > CaWO4 > SrWO4 under both neutral and acidic conditions.
Keywords: A. Oxides; B. Chemical synthesis; C. X-ray diffraction; D. Phosphors; D. Catalytic
properties.
1. Introduction

Alkaline-earth metal tungstates represent an important class of inorganic materials [1, 2].
These metal tungstates have a scheelite-type tetragonal structure with space group I41/a (No. 88)
and point group C6 symmetry [3]. In the scheelite structure, alkaline earth metal ions
4h
coordinated with eight oxygen atoms and tungstate ions were connected with four oxygen atoms
in the tetrahedral symmetry. These class of oxides have a wide range of technological
applications as acousto-optic filters [4], solid state lasers [5, 6], light emitting diodes [7],
photocatalysts [8, 9], phosphors [10, 11, 12, 13] and cryogenic scintillation detectors [14]. The
scheelite-type tungstates exhibit self luminescence in almost entire visible spectrum which could
be decomposed in blue, green and red components. The emission in blue region was assigned to
the charge transfer transition within WO42- group [15]. Campos et al. [16, 17] showed that green
luminescence can be attributed to both oxygen complex vacancies and intrinsic slight distortion.
However, the position of emission peak was shown to be a function of the synthesis method (i.e.
morphology and particle size) and excitation wavelength. The excitation using shorter
wavelength (210–280 nm) gives blue emission, while longer wavelengths yield green and red
emission [18].
During the last three decades semiconductor metal oxides have been extensively used for
their photocatalytic activity, because of their advantages such as higher efficiency of generation,
mobility and separation of photoinduced electrons and holes, low cost and environmental
friendliness. As a well-known semiconductor, TiO2 has proven photocatalytic activity for
oxidative decomposition of organic compounds. Nevertheless, TiO2 was hampered by both
inherent slow reaction rates and poor solar efficiency in solar radiation. Several doped materials
have better photocatalytic performance, while the activity of TiO2 was rather limited [19].
Recently, some alternative oxide materials like tantalites, vanadates, molybdates, niobates, and

tungstates have been investigated for their photocatalytic activity towards water splitting and
organic pollutants degradation. In spite of some progress, achieving high photocatalytic activity
in the oxide semiconductors still remains a great challenge.
Several techniques have been employed to synthesize metal tungstates, such as the
conventional solid state reaction [20], solvothermal [21], hydrothermal [22], spray pyrolysis
[23], sol–gel [24], surfactant-assisted precipitation [25], galvanic [26, 27], microwave irradiation
[28], pulsed laser deposition [29] and vapor-deposited [30] method. All these methods require
high temperature, long processing time and sophisticated equipment with high maintenance
costs. They also lead to the formation of deleterious phases. However, room temperature
precipitation method was convenient, economical, energy saving, environmentally friendly and
less time consuming as compared to conventional preparative methods.
In the present work, scheelite-type metal tungstates nanophosphors were synthesized by
wet chemistry approach at room temperature. These nanophosphors were characterized using
powder X-ray diffraction (XRD), Field emission Scanning electron microscopy (FE-SEM),
Transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and UV–Visible
spectroscopy. Further, the photoluminescence and photocatalytic activity were carried out for
possible energy saving and environmental remedy applications.
2. Experimental
2.1. Materials
Ca(NO3)2.4H2O, Sr(NO3)2, Ba(NO3)2, Na2WO4.2H2O and Methylene blue dye were
analytical reagent grade and used without further purification.
2.2. Synthesis

MWO4 (M = Ca, Sr and Ba) compounds were prepared by the precipitation method at
room temperature without using any surfactant or stabilizing agent. In a typical synthesis of
CaWO4 compound, stoichiometric amount of Ca(NO3)2.4H2O (0.1 mol) and Na2WO4.2H2O (0.1
mol) were dissolved in 100 ml distilled water separately. The calcium nitrate solution was added
dropwise into the sodium tungstate solution with constant stirring for 1 h and the resulting white
suspension was aged for 2 h. The chemical reaction between calcium nitrate and sodium
tungstate in aqueous medium is as follows:
Ca(NO3)2.4H2O + Na2WO4.2H2O → CaWO4 + 2NaNO3 + 6H2O
(1)
Finally, the white precipitate was washed with distilled water for several times, filtered
and dried at 100 °C for 2 h. Similarly, SrWO4 and BaWO4 were synthesized by choosing
Sr(NO3)2 and Ba(NO3)2 as starting materials.
2.3. Characterization
The phase purity of MWO4 powder was examined by powder X-ray diffraction (XRD)
(PANalytical X’Pert Pro Powder diffractometer) using Cu Kα radiation (λ = 1.5418 Å, Ni filter).
For Rietveld refinement, data were collected at a scan rate of 2˚/ min (0.02˚ step size for 2θ from
10˚ to 80˚). Refinement was done using FullProf Suite-2000 version. Fourier transform infrared
(FTIR) spectra were recorded by Perkin Elmer Spectrometer, Frontier using KBr as a reference.
The surface morphology and crystallite size were examined using FE-SEM (FEI Sirion XL 30) at
accelerating voltage of 10 kV and TEM (JEOL 2100F microscope operated at 200 kV). UV–
Visible absorption spectra have been recorded for powders on Perkin Elmer Lambda 750
spectrophotometer. The photoluminescence studies have been carried out using Perkin Elmer
Luminescence Spectrometer (LS-55). The photocatalytic activity of methylene blue (MB)

solutions was analyzed by a Perkin Elmer UV – Visible spectrophotometer (Lambda 35) in the
range from 200 to 800 nm periodically.
2.4. Photocatalytic activity measurement
The photocatalytic performance of MWO4 nanocatalysts were evaluated by decomposing
the methylene blue under UV light irradiation at room temperature. In a typical dye degradation
experiment, about 50 mg of MWO4 nanocatalysts were dropped into the 50 ml of aqueous
solution of MB (1 x 10-5 M) to form suspension. Mercury - vapor lamp (125 W, Philips) was
used as the UV light source (λ = 365 nm). Prior to UV illumination, the suspension was
continuously stirred for about 30 minutes in the dark to develop adsorption – desorption
equilibrium between MB dye and the catalyst. Then, the stable aqueous suspension was exposed
to UV light from 0 to 60 minutes. During the photoreaction, about 3 ml of suspension was
collected at different time and centrifuged to remove the catalyst. The clear suspension was
analyzed using UV-Visible spectrophotometer. The percentage dye degradation rate was
calculated by the following equation [31]:
C0 − C
Photodegradation rate
(2)
=
× 100
C0
where C0 is the initial concentration at 0 min and C is the concentration at time t.
3. Results and discussion
3.1. Powder X-ray diffraction
Indexed powder X-ray diffraction patterns for MWO4 (M = Ca, Sr and Ba) compounds
were shown in Fig. S1. All the diffracted lines were agreed well with the JCPDS File Nos. 85-

0854 (CaWO4), 85-0587 (SrWO4) and 85-0588 (BaWO4). We did not observe any deleterious
diffracted lines; this confirms that all the compounds were single phase. The XRD peaks shifted
towards lower 2θ angles from CaWO4 to BaWO4 phosphors due to the increase in ionic radii of 8
2+
2+
2+
- coordinated alkaline earth metal ions, Ca (rCa = 1.12 Å), Sr (rSr = 1.26 Å) and Ba (rBa
=
1.42 Å). The structural parameters were obtained from the Rietveld refinement method using
powder XRD data. The patterns were typically refined for lattice parameters, scale factor,
backgrounds, pseudo-Voigt profile function (u, v and w), atomic coordinates and isothermal
temperature factors (Biso). The refinement result reveals that all the compounds crystallized in the
tetragonal scheelite-structure with space group I41/a (No. 88). The observed, calculated and the
difference XRD patterns for MWO4 compounds were shown in Fig. 1. There was a good
agreement between the observed and calculated patterns. In Table 1, we have summarized the
refined structural parameters for MWO4 compounds. The lattice parameters (a, c) and unit cell
volume increase with increasing ionic radii of alkaline earth ions (Fig. S2). The crystal structure
of MWO4 was modeled through VESTA program [33] using structural parameters obtained from
Rietveld method. In MWO4 structure (Fig. 2), the alkaline earth (M) atoms were coordinated to
eight oxygen (O) atoms having two types of M – O distances, which results in deltahedral [MO8]
clusters. The tungsten (W) atoms were coordinated to four oxygen atoms which form [WO4]
clusters. These [WO4] clusters were slightly distorted in the lattice due to difference in the O – W
– O bond angles. From Table 1, W – O bond lengths decrease with increasing ionic radii of M
ions. Whereas, M – O’ and M – O” increase with increasing ionic radii of M ions. On the other
hand, M – O – M, M – O’ – W bond angles decrease and M – O” – W increase with increasing
ionic radii of M ions.
The average crystallite size was estimated using Scherrer’s equation [34]:

k
λ
(3)
D =
β
cosθ
where λ is the wavelength (1.5418 Å) of X-rays, β is the full width at half maximum
(FWHM), θ is the diffraction angle, k is the shape factor (0.9) and D is the average crystallite
size. Based on this equation, the average crystallite sizes for CaWO4, SrWO4 and BaWO4
powders were found to be 32, 33 and 55 nm, respectively. The broadening of the XRD lines
decrease with increasing crystallite sizes. The effects of the lattice strain and crystallite size on
the FWHM were estimated by Williamson and Hall (W–H) plots using the relation [35]:
k
λ
(4)
β
cos
θ
=
+ 4ε sinθ
D
where λ is the wavelength of X-rays (1.5418 Å), β (FWHM in radians) is measured for
different XRD lines corresponding to different planes, θ is the diffraction angle, k is the shape
factor (0.9), D is the crystallite size and ε is the micro strain. For estimation of lattice strain,
βcosθ was plotted against 4sinθ (Fig. S3). The slope of line gives the strain (ε) and intercept
(kλ/D) on Y-axis gives crystallite size (D). The lattice strain increases with increasing
broadening of the X-ray diffraction line (Table 2). The crystallite size calculated from W–H plots
for MWO4 was in good agreement with those obtained from Scherrer’s method. This reveals that
the compounds were nanocrystalline in nature. The plot has a near zero slope for BaWO4
showing minimal or no contribution due to strain.
3.2. FE-SEM and TEM analysis
Fig. 3 shows FE-SEM images of MWO4 compounds obtained from precipitation method
at room temperature. The CaWO4 nanospheres processed in an aqueous medium exhibited an
average size distribution in the range from 25–50 nm (Fig. 3a). Fig. 3b illustrates the uniform

CaWO4 microspheres, with an average diameter of 5 µm, which comprise of agglomerated
nanoparticles. In Fig. 3d, the FESEM image revealed bouquet-like SrWO4 microcrystals with an
average size of about 3 µm, which were also composed of small spherical nanoparticles with the
size of 30–70 nm (Fig. 3c). Fig. 3e shows the morphology of aggregation of small nanocrystals
lead to the formation of fish-like microcrytals (Fig. 3f). These micrographs suggest that the
chemical reaction between alkaline earth metal and tungstate ions leads to fast nucleation,
growth and agglomeration of small particles. Fig. 4 shows the TEM images of MWO4
compounds. From Fig. 4, the nanoparticles were obtained in the range of 30–40 nm (CaWO4),
35–45 nm (SrWO4) and 50–60 nm (BaWO4), which was consistent with the calculated values of
the average crystallite size obtained from the Scherrer’s and W-H methods.
3.3. FTIR studies
For the alkaline earth tungstate MWO4 with scheelite structure, there are 26 different
vibration modes which are represented by the following equation:
Γ = 3Ag + 5Au + 5Bg + 3Bu + 5Eg + 5Eu
(5)
All even vibrations Ag, Bg and Eg are Raman-active modes, while the odd modes 4Au and 4Eu are
active only in infrared frequencies and remaining five vibrations [1Au and 1Eu are acoustic
modes and 3 Bu are forbidden] are IR inactive modes [36, 37].
The FTIR spectra (Fig. 5) of MWO4 nanophosphors measured in the wave number region
of 350–2000 cm-1. The characteristic strong and broad absorption bands have two vibration
modes around 800 cm-1 (CaWO4), 808 cm-1 (SrWO4), and 820 cm-1 (BaWO4) were related to
ν3[(1Eu) and (1Au)] internal modes which were assigned to O–W–O anti-symmetry stretching
vibrations in [WO4] tetrahedron. A sharp but less intense absorption peaks at 438 cm-1 (CaWO4),

410 cm-1 (SrWO4), and 380 cm-1 (BaWO4) have appeared, which arises due to symmetric
bending vibrational mode ν4[1(Au + Eu)] within the [WO4] tetrahedron. Adsorbed water
molecules on the surface of the sample at 1631 cm-1, 1631 cm-1 and 1617 cm-1 were also detected
for CaWO4, SrWO4 and BaWO4 respectively. The photocatalytic activity was closely related to
the number of –OH groups present on the surface of catalyst because the photogenerated holes
(h+) react with water and generate •OH radicals, which can oxidize the organic pollutants.
Therefore, an increase in the number of surface –OH groups could improve the photocatalytic
activity.
3.4. UV-Visible absorption spectroscopy
The UV-Vis absorption spectra of MWO4 nanophosphors (Fig. 6a) indicate a blue-shift of
the UV−Visible absorption edges with increasing ionic radii of M ions. CaWO4, SrWO4 and
BaWO4 nanophosphors have intense absorption edges at 290, 266 and 259 nm which lie in the
UV region. The optical band gap energy (Eg) was estimated by the method proposed by Wood
and Tauc. The optical band gap was calculated with absorbance and photon energy by the
following equation [38]:
n
(6)
α
h
ν
= A
(
h
ν
− Eg
)
where α = Absorption coefficient, hν = Photon energy, Eg = Band gap energy, A is a
constant that is different for different types of transitions, indicated by different values of n (n =
1/2, 2, 3/2, or 3 for allowed direct, allowed indirect, forbidden direct, and forbidden indirect
electronic transitions, respectively). Fig. 6b depicts the plots of (αhν)2 versus the photon energy
(hν) for MWO4 nanophosphors. The intercept of the tangent to the x-axis gives good
approximation of the band gap energies of CaWO4, SrWO4 and BaWO4 nanophosphors as 4.27,
4.66, and 4.79 eV, respectively. These band gap energy values nearly matched with the literature

[39, 40] (Table 2). It was clearly seen that the band gap energy increases with increasing ionic
radii of M ions.
3.5. Photoluminescence properties
Photoluminescence spectra were often used to examine the efficiency of charge carrier
trapping, immigration and transfer, as well as to understand the fate of photogenerated e-/h+ pairs
in semiconductor particles. Fig. 7 exhibits the excitation spectra of CaWO4, SrWO4 and BaWO4
nanophosphors, which shows the broad band with a maximum at 350, 355 and 350 nm, when
monitored at 436, 440 and 433 nm emission wavelengths, respectively. For CaWO4, an intrinsic
broad emission peak was observed centered at 436 nm upon excitation at 350 nm (Fig. 7a). In
case of SrWO4 (λex = 375 nm), the emission peak was highest at 440 nm (Fig. 7b). The emission
spectrum of BaWO4 nanophosphor shown in Fig. 7c revealed the peak at 433 nm under
excitation of 350 nm. It was generally accepted that the excitation and emission of MWO4
nanophosphors mainly arise from the charge-transfer transitions from the oxygen ligands to the
central tungsten atom within the WO42- complex between the last fully occupied t1 orbital and the
first empty 2e orbital [26, 41].
The broad band suggests that the emission process was a typical multi phonon or multi
level process, i.e., a solid system in which the relaxation occurs by several paths, involving the
participation of numerous energy states within the band gap. It can be observed that MWO4
emission curves exhibit a broad luminescence in the blue wavelength range. The excitation from
1
3
3
the ground state (1A1) of WO42- gives rise to four excited states, i.e., 1T2, T1, T2, and T1, from
1
1
which only the transition between A1 and T2 is electric dipole allowed [42]. However, the
2-
WO4 complexes in scheelite tungstates are slightly distorted from Td to D2d symmetry by the
influence of the Jahn-Teller effect and electron-phonon interactions in general [43, 44]. Hence,

the orbitally degenerate T state undergoes distortion and symmetry lowering to a D2d symmetry
1
1
1
state, which strongly enhances the number of transitions observed. Both A1 → T2, T1 and
1A1 → 3T2, 3T1 transitions become partially allowed, thereby rendering radiative transitions from
1
low-lying triplet excited states to the A1 ground state as clearly detectable emission peaks (i.e.,
at 437 nm) in the blue luminescent spectra.
The color clarity of any luminescent material was expressed in terms of chromaticity
coordinates, called Commission International De I’Eclairage (CIE). The CIE chromaticity
coordinates (x, y) were calculated from the PL spectra. Fig. 8 shows the CIE chromaticity
diagram of MWO4 nanophosphors. The CIE coordinates of CaWO4, SrWO4 and BaWO4 were
found to be (0.150, 0.056), (0.149, 0.076) and (0.151, 0.059), respectively, which fall into the
blue region of the CIE diagram. The evaluated color coordinates are close to the commercial
BAM (0.144, 0.072) and National Television System Committee (NTSC) blue phosphor (0.140,
0.080) [45]. Therefore, the present nanophosphors can be useful for the production of blue
component in white LEDs.
3.6. Photocatalytic activity
Methylene Blue was a cationic dye and releases MB+ ions on dissolution in water,
absorbing the light in the 500–700 nm range. The MB dye degradation was demonstrated at
neutral (pH = 7) and acidic (pH = 3) conditions over MWO4 nanocatalysts under UV
illumination. UV–Vis spectra of decomposed MB dye with the variation of time from 0 to 60
min exhibits the maximum absorption wavelength at 664 nm (Fig. S4). Under both neutral and
acidic conditions, the absorbance intensities of MB were gradually decreased in the presence of
MWO4 nanocatalysts with time.

At pH = 7, when the reaction was performed under dark for 30 min, we observed 57%,
30%, and 27% MB dye degradation for BaWO4, CaWO4, and SrWO4 nanocatalysts respectively
(Figs. S5a, 9a). However, there was a negligible change in MB dye concentration in acidic
condition (Figs. S5b, 9b). Under neutral condition, MB dye was significantly degraded by 88%,
85%, and 54% within 60 minutes for BaWO4, CaWO4, and SrWO4 nanocatalysts respectively. In
case of acidic condition, 100% dye degradation was observed over BaWO4 within 60 min which
was higher than 94% for CaWO4 and 74% for SrWO4. This implies that acidic conditions were
favorable towards the formation of the reactive intermediates that is hydroxyl radicals was
significantly enhanced, which further help in enhancing the reaction rate. On the other hand in
neutral medium conditions, the formation of reactive intermediates was relatively less favourable
and hence less spontaneous.
The pH-dependent photodegradation can be mainly attributed to the variations of surface
charge properties of the photocatalyst. Consequently, this changes the absorption behavior of a
dye on the catalyst surface. The differences in the photocatalytic activity in synthesized
tungstates can be ascribed to different factors, such as the amount of adsorbed dyes, the position
of the valence and conductive bands and the rate of the electron–hole recombination. The good
results of photocatalytic activity were obtained in the decreasing order of BaWO4 > CaWO4 >
SrWO4 under both neutral and acidic conditions.
The kinetics of MB degradation reaction was presented in Fig. 10, which follows
apparent first order kinetics in agreement with a general Langmuir–Hinshelwood mechanism
[46]:
− dC
kKC
(7)
R =
=
dt
1 + KC

where R is the degradation rate of reactant (mg/l min), C is the concentration of reactant
(mg/l), t the illumination time, K is the adsorption coefficient of reactant (l/mg) and k is the
reaction rate constant (mg/l min). If C is very small then the above equation could be simplified
to
C




0
(8)
ln
= kKt = kapp t
C


where C0 is the initial concentration (0 min) of the MB aqueous solution and C is the
concentration of the MB aqueous solution for different times of UV illumination.
From the plot of ln(C0/C) vs. the irradiation time (t) (Fig. 10a), straight lines can be fitted
in the first part of the plots, allowing to estimate apparent first order kinetic constants of 0.029
min-1 (BaWO4), 0.026 min-1 (CaWO4) and 0.07 min-1 (SrWO4) under neutral conditions. We
have noted that this rate constant values are smaller than the values kBaWO4 = 0.086 min-1, kCaWO4
= 0.044 min-1, and kSrWO4 = 0.022 min-1 for the degradation of MB dye under acidic condition
(Fig. 10b). As the decolorization reactions proceed, the reaction rates significantly deviate from
the straight line, suggesting that the presence of adsorbed intermediates and products of the
complete mineralization adsorbed on the photocatalysts surface affects the process. For instance,
formation of carbonates from the interaction with the CO2 produced from dyes degradation was
previously indicated as major drawback of active Bi2O3 photocatalyst [47]. Further, we have also
compared our photocatalytic results with the reported catalysts for the degradation of MB dye
and were shown in Table 3. The table reveals that BaWO4 nanocatalyst shows 100% MB dye
degradation in shorter duration (60 min.) and its rate constant under acidic condition is higher as
compared to TiO2 [48], 5% Ag-ZnO [49], SnO2: Zn2+ [50] and ZnWO4 [51] catalysts.

When the energy of UV light in terms of photon is equal to or greater than the band gap
of MWO4, the electrons receive energy and transfer of electrons takes place from the valence
band (VB) to the conduction band (CB) which result in the formation of a hole (h+) in the VB
and an electron (e−) in the CB. The holes react with water and generate •OH radicals, which can
oxidize the organic pollutants. The conduction band electron reacts with oxygen in the reduction
process and produces •OH radicals. These radicals reduce the organic pollutants. These oxidation
and reduction processes were capable of degrading the organic pollutants under UV light
irradiation. According to this, the relevant reactions at the semiconductor surface causing the
degradation of MB dye can be expressed as follows:
+
MWO4 + hν (UV) → MWO4 (eCB− + hVB
)
(9)
MWO4 (hVB+) + H2O → MWO4 + H+ + •OH
MWO4 (hVB+) + OH−→ MWO4 + •OH
(10)
(11)
(12)
(13)
(14)
(15)
(16)
–
MWO4 (eCB−) + O2 → MWO4 + •O2 (or •O–)
–
•O2 (or •O–) + H+ → •O2H (or •OH)
C16H18N3SCl + •OH → CO2, NH4 , NO3 , SO42−, Cl− and H2O
C16H18N3SCl + hVB+→ oxidation products
+
−
C16H18N3SCl + eCB−→ reduction products
where hv is photon energy required to excite the semiconductor electron from the valence
band (VB) region to conduction band (CB) region.
4. Conclusions
MWO4 (M = Ca, Sr and Ba) nanophosphors were synthesized by the room temperature
precipitation method. Powder XRD and Rietveld refinement data indicate that these

nanophosphors have a monophasic scheelite-type tetragonal structure. FE-SEM images of
CaWO4, SrWO4 and BaWO4 nanophosphors suggest the spherical, bouquet and fish-like
morphology, respectively. PL spectra exhibit the broad intense blue emission peak. It was
observed that the measured CIE coordinates of MWO4 nanophosphors are close to the
commercial BAM (0.144, 0.072) and NTSC blue phosphor (0.140, 0.080). At pH = 3, 100% MB
dye degradation was observed within 60 minutes over BaWO4 nanocatalysts under UV
illumination.The photocatalytic activity was in the decreasing order of BaWO4 > CaWO4 >
SrWO4 under both neutral and acidic conditions. All these results indicate that MWO4 is a
promising candidate for the development of blue phosphor in white LEDs as well as for
removing industrial pollutants.
Acknowledgements
The authors like to thank Mr. Surender Kumar, IPC for providing solid UV-Visible data
and acknowledge for chemical division TEM facility, IISc. One of the authors (R.S.) greatly
acknowledges Professor Ashok K. Nagawat and Dr. K.V.R. Rao, Centre for Converging
Technologies, University of Rajasthan for encouragement.
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Figure Captions
Fig. 1. Observed, calculated and the difference XRD patterns for (a) CaWO4, (b) SrWO4 and (c)
BaWO4 compounds.
Fig. 2. Crystal structures of MWO4 (M = Ca, Sr and Ba) nanophosphors.
Fig. 3. FE-SEM images of (a, b) CaWO4, (c, d) SrWO4 and (e, f) BaWO4 nanophosphors.
Fig. 4. TEM images of (a) CaWO4, (b) SrWO4 and (c) BaWO4 nanophosphors.
Fig. 5. FTIR spectra of (a) CaWO4, (b) SrWO4 and (c) BaWO4 nanophosphors.
Fig. 6. (a) UV−Vis absorption spectra and (b) plots of (αhν)2 versus the photon energy (hν) of
CaWO4, SrWO4 and BaWO4 nanophosphors.
Fig. 7. Excitation and emission spectra of (a) CaWO4, (b) SrWO4 and (c) BaWO4
nanophosphors.

Fig. 8. CIE chromaticity diagram of CaWO4, SrWO4 and BaWO4 nanophosphors.
Fig. 9. Relative concentration variation of MB over BaWO4, SrWO4 and CaWO4 nanocatalysts
under (a) pH = 7 and (b) pH=3 with irradiation time.
Fig. 10. First order kinetics of MB dye degradation as a function of time over BaWO4, SrWO4
and CaWO4 nanocatalysts (a) pH = 7 and (b) pH=3.
Supplementary Figure Captions
Fig. S1. XRD patterns of (a) CaWO4, (b) SrWO4 and (c) BaWO4 nanophosphors.
Fig. S2. Plots of (a) lattice parameters and (b) cell volumes as a function of alkaline earth ions.
Fig. S3. W–H plots of (a) CaWO4, (b) SrWO4 and (c) BaWO4 nanophosphors.
Fig. S4. UV–Vis absorbance spectra of MB dye as a function of time and pH over (a, b) BaWO4,
(c, d) SrWO4 and (e, f) CaWO4 nanocatalysts under UV illumination.
Fig. S5. Percentage of MB dye degradation over CaWO4, SrWO4, and BaWO4 nanocatalysts
under (a) pH = 7, and (b) pH=3 as a function of irradiation time.
Table 1. Rietveld refined structural parameters, selected bond lengths and bond angles for
CaWO4, SrWO4 and BaWO4 nanophosphors.

Compounds
CaWO4
SrWO4
BaWO4
Crystal System
Space group
Tetragonal
I 4 1 / a (No. 88)
Tetragonal
I 4 1 / a (No. 88)
Tetragonal
I 4 1 / a (No. 88)
Lattice Parameters (Å)
a
c
5.250(1)
11.393(3)
314.03(11)
5.421(5)
11.952(9)
351.27(6)
5.615(2)
12.722(4)
401.04(3)
Cell volume (Å3)
Atomic positions*
M (4b)
x
y
z
0.0000
0.2500
0.6250
0.0000
0.2500
0.6250
0.0000
0.2500
0.6250
W (4a)
x
y
z
0.0000
0.2500
0.1250
0.0000
0.2500
0.1250
0.0000
0.2500
0.1250
O (16f)
x
y
z
0.2752(3)
0.0964(5)
0.0440(6)
0.2683(2)
0.1093(2)
0.0496(7)
0.2422(2)
0.1403(3)
0.0369(9)
R Factors
Rp
2.73
3.38
5.39
Rwp
3.57
4.16
7.28
Rexp
6.51
0.30
6.58
0.40
7.39
0.97
χ2
RBragg
2.23
3.76
7.56
RF
3.24
5.22
9.46
Bond lengths (Å)*
W – O x 4
M – O’ x 4
M – O” x 4
Bond angles (°)*
M – O – M
M – O’ – W
M – O” – W
O’ – W – O’
O” – W – O”
1.894 (6)
2.356 (3)
2.398 (1)
109.12 (5)
121.31 (4)
128.55 (7)
103.73 (1)
121.70 (2)
1.873 (3)
2.487 (8)
2.552 (3)
106.37 (3)
122.46 (3)
130.82 (4)
103.38 (7)
122.49 (6)
1.846 (4)
2.619 (2)
2.842 (3)
101.86 (2)
114.04 (2)
143.04 (1)
107.91 (1)
110.26 (6)

*Table 2. Crystallite size, lattice strain and energy band gap of CaWO4, SrWO4 and BaWO4
nanophosphors.
Band gap (Eg)
Crystallite size (nm)
Lattice
(eV)
Literatures
Compounds
strain (10-4)
Present
work
Scherrer’s
W – H
[39, 40]
CaWO4
SrWO4
BaWO4
32
33
55
34
32
57
7.5
4.2
1.8
4.27
4.66
4.79
4.2 – 6.8
4.5 – 4.9
4.7 – 5.2

Table 3. Comparison of MWO4 (M = Ca, Sr and Ba) with the reported catalysts for MB dye
degradation.
Catalyst
Crystallite Initial MB
Catalyst
conc.
(mg/L)
Reaction
condition
MB degradation
Rate
Reference
Size (nm)
conc.
constan
Percentage
Time
t (min-1)
taken
(min.)
300
60
60
TiO2
ZnO
1% Ag-ZnO
3% Ag-ZnO
5% Ag-ZnO
SnO2
12
-
10 mg/L
10 ppm
125
300
300
300
300
1000
125 W, UV
8 W, UV
8 W, UV
8 W, UV
8 W, UV
20 W, UV
45
51
57
0.0019
0.0122
0.0143
0.0130
0.0122
0.008 ±
0.001
[48]
[49]
[49]
[49]
[49]
[50]
-
10 ppm
-
10 ppm
54
60
-
12
10 ppm
2 x 10-5 M
52
60
100
600
SnO2: Zn2+
2.4
2 x 10-5 M
1000
20 W, UV
100
80
0.040 ±
0.001
[50]
CoWO4
CuWO4
ZnWO4
CaWO4
30
32
44
32
5 x 10-5 M
5 x 10-5 M
5 x 10-5 M
1 x 10-5 M
2857
2857
2857
1000
Visible
Visible
Visible
40
70
95
94
120
120
120
60
0.0033
0.0104
0.0272
0.0440
[51]
[51]
[51]
Present
work
Present
work
125 W, UV
SrWO4
BaWO4
33
55
1 x 10-5 M
1 x 10-5 M
1000
1000
125 W, UV
125 W, UV
74
60
60
0.0220
0.0860
100
Present
work

M = Ca, Sr and Ba

(a)
(b)
(c)
Fig. 1

Fig. 2
(a)
(b)

(d)
(c)
(e)
(f)
Fig. 3
(b)
(a)