Influence of SO42? anionic group substitution on the enhanced photoluminescence behaviour of red emitting CaMoO4:Eu3+ phosphor

Article abstract of DOI:10.1016/j.jallcom.2020.157022

CaMoO₄:xEu³⁺(x = 0.5, 1.0, 1.5, 2.0, and 2.5) powder phosphors incorporating SO₄^2? anions were synthesized at high temperature using the solid-state reaction technique. The structural, morphological and optical properties of these phosphors were analysed using X-ray diffraction (XRD), field emission scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and optical spectroscopy. The XRD results indicate that the incorporation of SO₄^2? anions and Eu³⁺ dopant ions did not affect the crystal structure of the CaMoO₄, but largely influenced the luminescence properties of the CaMoO₄–SO₄:Eu³⁺ phosphors.The optical properties of our materials were examined using the UV–vis absorption spectroscopy. The absorption edges of the phosphors with different concentrations of Eu³⁺ were less than the band gap energy of the CaMoO₄ and their values ranged from 3.30 to 4.75 eV. The intensity of the red photoluminescence (PL) from CaMoO₄:Eu³⁺ phosphors was enhanced considerably upon incorporation of SO₄^2? anions, suggesting that SO₄^2? acted to capture primary excitation energy and transfer it non-radiatively to Eu³⁺ ions. In addition, the incorporation of SO₄^2? ions also improved the fluorescence decay life-time values of the CaMoO₄:xEu³⁺ phosphors significantly. Tunable emission was observed when the Eu³⁺ concentration was varied. Our PL results indicated that the CaMoO₄–SO₄:Eu³⁺ phosphor exhibited the highest red emission intensity compared to CaMoO₄: Eu³⁺ phosphors, suggesting that CaMoO₄–SO₄:Eu³⁺ could be a promising red component material for potential application in white light-emitting diode devices.

Full text of DOI:10.1016/j.jallcom.2020.157022

Journal Pre-proof
2-
Influence of SO
anionic group substitution on the enhanced photoluminescence
3+
4
behaviour of red emitting CaMoO :Eu phosphor
4
M.L.A. Letswalo, L. Reddy, A. Balakrishna, H.C. Swart, O.M. Ntwaeaborwa
PII:
S0925-8388(20)33386-7
DOI:
https://doi.org/10.1016/j.jallcom.2020.157022
JALCOM 157022
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 21 June 2020
Revised Date: 31 August 2020
Accepted Date: 2 September 2020
Please cite this article as: M.L.A. Letswalo, L. Reddy, A. Balakrishna, H.C. Swart, O.M. Ntwaeaborwa,
2-
Influence of SO
anionic group substitution on the enhanced photoluminescence behaviour of red
3+
4
emitting CaMoO :Eu phosphor, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/
j.jallcom.2020.157022.
4
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Sample CRediT author statement
M.L.A Letswalo: Writing - Original Draft, Conceptualization, Methodology, Software
L. Reddy: Resource, Data curation, Funding acquisition.
A. Balakrishna: Software, Validation, Conceptualization, Writing - Review & Editing
H.C. Swart: Resource, Conceptualization, Writing - Review & Editing
O.M. Ntwaeaborwa: Resource, Validation, Conceptualization, Visualization Writing - Review &
Editing & Supervision

Graphical abstract:

2-
Influence of SO4 anionic group substitution on the enhanced
photoluminescence behaviour of red emitting CaMoO4:Eu3+ phosphor
M.L.A Letswalo1*, L. Reddy1, A. Balakrishna1*, H.C. Swart2 and O.M. Ntwaeaborwa1*
1Department of Physics, University of Johannesburg, Johannesburg, ZA 2006, South Africa
2Department of Physics, University of the Free State, Bloemfontein, ZA 9300, South Africa
Abstract
CaMoO4:xEu3+(x = 0.5, 1.0, 1.5, 2.0, and 2.5) powder phosphors incorporating SO42- anions
were synthesized at high temperature using the solid-state reaction technique. The
structural, morphological and optical properties of these phosphors were analyzed using X-
ray diffraction (XRD), field emission scanning electron microscopy, X-ray photoelectron
spectroscopy, Fourier transform infrared spectroscopy and optical spectroscopy. The XRD
results indicate that the incorporation of SO42- anions and Eu3+ dopant ions did not affect the
crystal structure of the CaMoO4, but largely influenced the luminescence properties of the
3+
The optical properties of our materials were examined using
CaMoO4-SO4:Eu phosphors.
the UV-vis absorption
spectroscopy. The absorption edges of the phosphors with different
3+
band gap energy of the CaMoO and their values
concentrations of Eu were less than the
4
ranged from 3.30 to 4.75 eV. The intensity of the red photoluminescence (PL) from
CaMoO4:Eu3+ phosphors was enhanced considerably upon incorporation of
2-
SO4 anions,
2-
suggesting that SO4 acted to capture primary excitation energy and transfer it non-
2-
radiatively to Eu3+ ions. In addition, the incorporation of SO4 ions also improved the
CaMoO :xEu3+ phosphors
. Tunable
significantly
fluorescence decay life-time values of the
4
emission was observed when the Eu3+ concentration was varied. Our PL results indicated
3+
that
the CaMoO4-SO4:Eu phosphor exhibited the highest red emission intensity compared
to CaMoO4: Eu3+ phosphors, suggesting that CaMoO4-SO4:Eu3+ could be a promising red
component material for potential application in white light-emitting diode devices.
2-
Keywords: solid-state reaction, CaMoO4phosphors, SO4 anionic group systems, Eu3+ ion,
photoluminescence and white light-emitting diode (W-LEDs)
*Corresponding Authors: E-mail
ntwaeab@gmail.com
,
leanyatsa.abram@gmail.com, bavula@uj.ac.za and
1

1. Introduction:
Nowadays, host materials with good thermal and chemical stability [1–5], excellent
optical properties, and ease of incorporation of dopant ions are widely used to prepare
phosphors for lighting applications. In particular, phosphors that are used for solid state
lighting (SSL) applications in white light-emitting diodes (WLEDs). As a result, attention has
been focused on improving the efficiency and lifetime of these phosphors. Although W-
LEDs have been found to have high efficacy, relatively longer excited state life times, high
energy efficiency and high color stability; the quality of white light is low due to a lower
colour rendering index (CRI) and higher correlated colour temperature (CCT) due to
deficiency of the red light component in commercial tricolor LEDs[6-11]. This study was
therefore aimed at developing advanced red phosphor with high CRI and relatively low CCT
that could be used as a source of red light, which will in turn improve the quality of white
light output in tri-colour LEDs. Thus, this study was set out to prepare and evaluate
fundamental properties of red light emitting phosphors for such practical application.
Due to their excellent optical and electromagnetic properties, and chemical stability,
molybdate compounds are attractive host lattices for rare-earths (REs) dopant ions to prepare
chemically stable phosphors that could be used in applications such as display technology,
solid state lighting, Raman lasers, optical communication systems, and chemical
investigated in different fields of research
catalysis[13,14]. Molybdates have therefore been
including production of advanced light emitting materials. In particular, the sheelite calcium
molybdate is widely used as host for REs to prepare different types of phosphors. The
sheelite structure of CaMoO4 is composed of CaO8 polyhedral and MoO4 tetrahedron building
blocks. The central Ca atom or Mo atom are
properly aligned and are coordinated with eight
resulting in CaMoO4 exhibiting compositional stability [15]. Moreover,
or four O atoms,
CaMoO has
a broad higher absorption band range in the high energy region (i.e. ultraviolet
4
or UV region), and can preferably emit high intensity blue or green light under UV
RE3+ ions are incorporated in the CaMoO4 matrix and the material is
excitation. When
excited by high energy photons,
the captured primary excitation energy is transferred from
the MoO42- to the RE3+ ions, resulting in a dramatic increase in photoemission from the RE3+
ions [16]. In this study, we prepared red light emitting phosphor by doping CaMoO4 with
2-
Eu3+. In addition, we incorporated
anion group and we observed considerable
SO4
improvement in the PL intensity of red emission from Eu3+ ions due to non radiative energy
2-
and MoO42- to the Eu3+ ions. Phosphors prepared from Eu3+ complexes
SO4
transfer from
2

are used more often as sources of red light in a variety of displays and LEDs. These
phosphors exhibit higher luminescence efficiency compared to other red light emitting
example,
materials [17] and they are being investigated by many researchers worldwide. For
John Peteret.al.[18] investigated the PL properties of CaMoO4:Eu3+ (0.1 mol%) excited using
was due to the
near-UV radiation, and they observed a strong emission at 615 nm which
5
forced electric dipole transitions D →7F , which resulted in increased research interest
in
0
2
3+
CaMoO
order to fully understand the luminescence behavior of Eu in the
host. However,
4
the relatively low brightness and efficiency of CaMoO4:Eu3+ is not sufficient for application
in commercial W-LEDs. Therefore, it is necessary to conduct further research to find away to
3+
An investigation carried out by Wang et
enhance red emission from Eu in the CaMoO4 host.
al. [19, 20] in which Eu3+ was co-doped with Bi3+ and
3+
Sm showed an enhancement of the
PL intensity of the Eu3+ ion. Recently, several studies have been conducted in which anions
where used to substitute Mo6+ in the CaMoO4 lattice sites. For example, a study carried out
in the red PL emission intensity of
by Zhang et al. [21] showed an improvement
NaEu(MoO4)2 phosphor due to the incorporation of different anionic group systems.
Furthermore, an enhanced red emission was observed from NaLa(MoO4)2:Eu3+ due to the
2-
2-
3-
incorporation of SO4 and BO3 anionic groups[22]. In this study, SO4 anions, which
substituted MoO42- sites in CaMoO
CaMoO :Eu3+. In addition,
enhance the intensity of red emission from
4 were found to
3+
the effect of varying Eu concentration on the intensity of the red
4
PL emission was also investigated.
CaMoO4-SO4:xEu3+ (x = 0.5, 1, 1.5, 2, 2.5 mol. %) phosphor materials were
2-
SO42- was incorporated into the MoO4 sites
synthesized via the solid state reaction route.
Eu3+.We examined the
2-
resulting in enhanced red PL due to energy transfer from SO4 to
structure, morphological, and optical properties of the CaMoO4-SO4:xEu3+
. A
phosphors
mechanism
that explains the radiative transition is also discussed.
2. Experimental Procedure:
CaMoO4-SO4:xEu3+ (x = 0.5, 1.0, 1.5, 2.0, and 2.5) phosphors were prepared from
highly pure chemical compounds, namely CaCO3 (99.9% Sigma-Aldrich), MoO3 (99.9%
Sigma-Aldrich), (NH4)2SO4 (99.9% Sigma-Aldrich) and Eu2O3 (99.99% Sigma-Aldrich).
Stoichiometric amounts of the starting materials were mixed together in acetone using an
agate mortar and pestle. The mixture was calcined at 1100 ⁰C for 4 hours. The final mixture
3

was ground thoroughly using an agate mortar and pestle. The chemical reaction for the
synthesis is given by:
CaCO3 + MoO3+ (NH4)2HSO4 + Eu2O3 → CaMoO4SO4Eu2 + CO2 + H2O+ 7H2 + 2N2(1),
and the schematic representation of the synthesis procedure is given by:
Fig. 1: Schematic illustration of the sample preparation by the conventional solid-state
reaction route.
2.1
Characterization techniques:
Various techniques were used to characterize the samples. The crystal structure was analysed
using X-ray powder diffraction (XRD) using a Bruker AXS D8 X-ray diffractometer with
Cukα (1.5406Å). The data were recorded in 2θ range from 10⁰ to 80⁰. The particle
morphology and the chemical composition were examined using a field emission scanning
electron microscopy (FE-SEM) coupled with an Oxford Aztec 350 X-Max80 energy-
4

dispersive X-ray spectroscopy (EDS). The ultraviolet visible (UV-Vis) reflectance spectra
were measured using a Perkin Elmer Lambda 950 spectrophotometer. Fourier transform
infrared spectra (FTIR) were recorded using a Nicolet 6700 FTIR spectrometer in the spectral
range of 4000– 400 cm−1. The PL and the lifetime measurements were recorded in the
fluorescence mode using the Cary eclipse spectrofluorometer using
a
150W
monochromatized xenon lamp as the excitation source. All characterizations were measured
at room temperature.
3. Results and Discussion
3.1 Powder X-Ray diffraction analysis:
Fig 2 (a) shows the XRD profiles of CaMoO4 and CaMoO4-SO4:xEu3+ (x = 0.5, 1.0,
1.5, 2.0 and 2.5 mol. %) measured in the range of 10-80o.
All the diffraction peaks resemble
CaMoO The peaks were indexed
those of the standard scheelite tetragonal structure of
.
4
according to a standard XRD spectrum referenced in JCPDS file no. 29-351[23] of a pure
tetragonal phase of CaMoO4. The crystal structure as determined using
software confirmed that our materials resemble the scheelite structure of
the diamond
CaMoO4
with
space group I41/a (no.88) as shown in Fig. 2(b). The crystal structure showsCa2+ and
MoO42- ionic sites, which can be occupied by Eu3+ and SO42- ions, respectively, as reported
elsewhere [24]. The enlarged version of the (112) reflection for different concentrations of
Eu3+, shown in Fig. 2(c), illustrates the peak shift for varying concentration of Eu3+. At 1.5
and 2.0 mol.% of Eu3+, the peaks became narrow, with increased intensity and reduced
widths. The narrowing of the diffraction peaks and increased intensity indicate an
improvement in crystallinity. In addition, anion groups have also been reported to
contribute positively tocrystallization of CaMoO4[25]. The shift in the peak positions was
2-
most likely due to micro strains resulting from the incorporation of Eu3+ and SO4 . The
average crystallite size (D) of the CaMoO4 and CaMoO4-SO4:xEu3+ phosphors were
calculated for the prominent (112) XRD peak using Scherrer's formula [26]. The average
crystallite size of CaMoO4 was ~ 46 nm, the sizes of Eu3+ doped CaMoO4-SO4 phosphors
increased from ~50 nm to ~70 nm, and the FWHM of the diffraction peak decreased with
increasing Eu3+concentration as shown in Fig. 2(c).
5

Fig.2: (a) X-ray diffraction patterns of CaMoO4 and CaMoO4-SO4:xEu3+ phosphors and (b) crystal
structure of CaMoO4-SO4:Eu3+ and (c) magnified view of the (112) diffraction peak.
3.2
Particle Morphology and Chemical composition analyses:
Fig 3(a) shows the EDS spectrum of CaMoO4-SO4:Eu3+ confirming the presence of all the
elements including Mo, O, Ca, S and Eu and the inset, Fig 3(b), shows the weight
6

percentages of these elements. The weight percentage of Mo is greater than that of O, Ca, Eu
and S. Fig. 3(c) shows the SEM image of the CaMoO4-SO4:(1 mol%) Eu3+ powder phosphor
The powder was made of irregularly shaped particles agglomerated together, with different
sizes and well-defined grain boundaries. The need for prolonged heat treatment in the solid-
state route leads to aggressive grain growth and agglomeration, thus forming large
micrometric particles [27].
Fig. 3: (a) EDS spectra of CaMoO4-SO4:Eu3+ phosphor b) Map sum spectra and (c) SEM
image showing the morphology.
3.3
Fourier-transformation infrared analysis:
The FT-IR spectra of the CaMoO4, CaMoO4:Eu3+ and CaMoO4-SO4:Eu3+ phosphors
are displayed in Fig. 4 (a). All the spectra have a broad band around 1131 cm-1 which is
attributed to O-H-O absorption. Weak absorption peaks at 1798 and 2409 cm-1 are ascribed to
the vibrational bending of O-H bonds, resulting from atmospheric moisture absorbed on the
surface. The bands at 911 and 811cm-1are attributed to stretching vibration of O-Mo-O in
2-
2-
MoO4 [28]. Weak broad vibration peaks around 1128, 1041 and 1005 cm-1 are due to SO4
pointing to successful incorporation of SO4 in the host lattice [29]. The absorption bands
around 424 and 475 cm-1 are associated with the bending vibration of Mo-O groups. Fig. 4(b)
showed the schematic diagram of the bending and stretching vibrations of MoO4 and SO4.
7

Fig. 4 (a): FTIR spectrum of CaMoO4:Eu3+ 1.0 mol %, CaMoO4-SO4:Eu3+1.0 mol% and (b)
Schematic diagram of MoO4 and SO4 structural groups.
3.4
X-ray Photoelectron Spectroscopy: Surface Chemical Composition analysis
The XPS survey spectrum shown in Fig. 5 displays the electronic and chemical
composition of the 1.0 mol% Eu3+ doped CaMoO4-SO4 phosphor. All observed peaks in the
spectrum confirmed the presence of Ca, Mo, O, Eu, S and C in the CaMoO4-SO4:Eu3+
phosphor. C was from adsorbed atmospheric hydrocarbons and/or from the carbon tape [30,
31] on which the samples were mounted. Fig. 6 (a) and (b) present the high resolution XPS
spectrum of Ca before and after sputtering respectively. Before sputtering the XPS peak for
the Ca (2p) has a core binding energies (BE) of 350.8 eV and 347.2 eV for the 2p1/2 and 2p3/2,
respectively. These peaks have the FWHM of 3.0 and 2.5 eV, respectively, for the 2p1/2 and
2p3/2 doublets. These results show that the Ca oxidation state was stabilized into the +2 state.
The XPS spectrum in Fig. 7 (a) shows the high resolution peaks of the Mo 3d with the
8

FWHM of 1.8 for the 3d5/2 and 1.8 eV for the 3d3/2 with the core BE of 233.3 eV and 235.5
eV respectively, consistent with similar peaks reported previously[30]. There is a significant
change in the FWHM, peak positions, and intensity of the Mo 3d spectrum after sputtering as
shown in Fig.7(b). Mo (3d) XPS peaks illustrates the BE shift of 3d3/2 for 2.1 eV and 3d5/2 for
1.4eV with the corresponding FWHM of 4.8 eV and 1.0 eV. A minor peak was observed at
229.2 eV, which is associated with the lower oxidation state or the Mo metallic peak for the
Mo 3d [31]. Fig 8 shows the high resolution XPS spectra of O1s (a) before and (b) after
sputerring. The spectra were fitted with three- peaks using a multipack software. The peaks
are labelled O1 (532.1eV) O2 (530.4 eV) and O3 (529.8eV) and they are respectively assigned
to chemisorbed oxygen, oxygen ion vacancies in the lattice and adsorbed water molecules on
the surface of the CaMoO4-SO4:Eu3+ phosphor [32-34]. After sputtering, the O2peak became
less intense and shifted to the higher binding energy side. The high resolution XPS spectrum
for S 2p core-level shown in Fig. 9(a) consists of two peaks located at 162.1 and 166.5eV and
they are attributed to the S 2p3/2 and S 2p1/2, respectively[35, 36]. The S 2p peak is acribedto
S in the SO4 ions as reported previously [37]. The high resolution XPS spectrum of the
Eu3dis shown in Fig.9 (b). The high-signal-to-noise-ratio is due to relatively lower
concentration levels of Eu3+[38]. The peak around 1135 eV corresponds to Eu3+3d5/2 while
the peak at 1165 eV corresponds to Eu3+ 3d3/2 and these peaks are consistent with the reported
values for the +3 oxidation states of the Eu ions [39]
9

Fig. 5: XPS spectra of the CaMoO4-SO4:Eu3+ phosphor before and after sputtering.
Fig. 6: (a) XPS Spectra of Ca 2p before and (b) after sputtering.
10

Fig. 7: (a) XPS spectra of Mo 3d before and (b) after sputtering.
Fig. 8: The high resolution XPS scan spectrum of O 1s of CaMoO4-SO4:Eu3+(1.0 mol % of
Eu3+) before and after sputtering.
11

Fig. 9 The high resolution XPS scan spectra of (a)S 2p and (b)Eu3d for of CaMoO4-
SO4:Eu3+(1.0 mol %) phosphors.
3.4
Ultraviolet-visible absorption spectroscopy:
Fig.10 shows the optical diffuse reflectance spectra (DRS) of the CaMoO4host and
CaMoO4-SO4:xEu3+ (x = 0.5, 1.0, 1.5, 2.0 and 2.5 mol. %). All the DRS spectra of CaMoO4
host and CaMoO4-SO4:xEu3+ showed similar pattern except additional absorption bands from
CaMoO4-SO4:xEu3+ which are attributed to the f-f absorptions of Eu3+ dopants. The
absorption of the CaMoO4 host is located at ~450 nm, which has been associated with the O2–
→Mo6+charge transfer absorption band [40]. This indicates that our phosphors were capable
of absorbing in the near ultraviolet region efficiently. This broad absorption peak
wavelengths shifted towards the shorter wavelengths with increasing Eu3+ concentration.
Other than the MoO66– group absorption band, additional absorption bands attributed to the
typical f-f transitions of the Eu3+ ion were detected at 457, 535, 1385, 1962, 2061 and 2200
nm. The optical band gap of the CaMoO4 and CaMoO4-SO4:xEu3+(x = 0.5, 1.0, 1.5, 2.0 and
2.5 mol.%) phosphor materials were calculated by using the Kubelka–Munk function (R) as
shown in Fig. 11[41]. The optical band gap for these CaMoO4, and CaMoO4-SO4 doped with
0.5, 1.0, 1.5, 2.0 and 2.5 mol% of Eu3+ were found to be 3.72, 4.66, 4.60, 4.34, 4.30 and 4.57
eV respectively. The increased bandgap were found in Eu3+ doped CaMoO4-SO4 phosphors
2-
and this attributed to incorporation of Eu3+ and SO4 ions. As can be seen in Fig. 8, the
energy band gap values were progressively suppressed with increasing Eu3+concentration.
The variation in optical band gap energy values are associated with the changes in the local
atomic structure or lattice. These band gap values were greater than those reported previously
[42, 43].
12

Fig.10: Diffuse reflection spectra for CaMoO4-SO4:xEu3+(x = 0.5, 1.0, 1.5, 2.0 and 2.5 mol.
%) phosphor materials.
Fig.11: Optical band gap determination of the CaMoO4-SO4:xEu3+(x = 0.5, 1.0, 1.5, 2.0 and 2.5
mol. %) phosphor materials.
3.5
Photoluminescence properties of CaMoO4-SO4:Eu3+
Fig.12 shows the PL excitation spectra of the CaMoO4 host and the CaMoO4-SO4:xEu3+ (x =
0.5, 1.0, 1.5, 2.0 and 2.5 mol. %) phosphors recorded when monitoring the emission peak of
Eu3+ (5D0→7F2) at the wavelength of 615 nm. The excitation peaks located at 361, 382, 395
7
7
and 416 nm correspond to the 7F0→5G4, 7F0→5G2, F0→5L6 and F0→5D3 transitions of Eu3+
ions, respectively. Under excitation at 395 nm, the emission spectra of CaMoO4-SO4:xEu3+ (x =
0.5, 1.0, 1.5, 2.0 and 2.5 mol. %) phosphors were recorded and are shown in Fig. 13. The
13

5
emission peaks at 575 nm and 750 nm are due to the D0→7FJ (J = 1, 2, 3, 4) transitions of
5
Eu3+ ions, respectively. The strongest emission peak at 615 nm is due to the D0→7F2
transitions of Eu3+.The weak emissions located at 592, 655 and 702 nm, are associated,
respectively, with the 5D0→7F1, 5D0→7F3 and 5D0→7F4, transitions of Eu3+[44]. The 5D0→7F2
transition is associated with the electric dipole, that is insensitive to the lattice site symmetry
5
5
of the Eu3+ ions. The D0→7F2 transition is much stronger than the D0→7F1 transition,
indicating that the Eu3+ ion is placed in a distorted local environment [45]. Partially
substituting Ti with SiO2 reduced the density of defects and improve the crystal surroundings
of Pr3+ which resulted in the increase of the rate of energy transfer from the CaTiO2 to Pr3+,
and a subsequent increase in emission intensity from Pr3+ [46]. In this study, the same
2-
behavior was observed where the substitution of SO4 anions and the defect densities were
reduced as non-radiative centres and improved the local distortion of the crystal field
surrounding the Eu3+in CaMoO4 during calcination. This phenomenon has tendency to
2-
increase the rate of energy transfer from the CaMoO4hostto Eu3+ ions. In addition, the SO4
ion act as charge compensator or sensitizer [47]. Fig.14 (a) shows the PL maximum
intensities of the CaMoO4-SO4:xEu3+ phosphor material as a function of different Eu3+ ion
5
concentrations for the major emission peak associated with the D0→7F2 transition. The PL
intensity increased with increasing Eu3+ ion concentration from 0 mol%, maximized at 1.0
mol% and then decreased when the concentration was increased to 2.5 mol.%. The decrease
of the PL intensity at Eu3+ concentrations above1.0 mol% is ascribed to concentration
quenching effect [48]. By using the Van Uitert equation (5), the interaction type responsible
for the energy transfer was determined [49].
ꢀ
ꢂ
=
,
)
(5)
⁄
ꢆ ꢇ
ꢁ
(ꢃꢄꢅ(ꢁ)
where I/x is the emission intensity per ion concentration (x), β and k are the constants for a
given host lattice, and θ is a function of multipole-multipole interaction. When the values of θ
are 6, 8, and 10, it shows respectively the d–d (dipole-dipole), d–q (dipole-quadrupole), and
q–q (quadrupole-quadrupole) interactions. Fig. 14(b) shows a graph of the I/x versus x (where
I is the emission intensity and x is the doping concentration) on the logarithmic scale. The
graph is of a linear type with a slope of -2.024, which is equal to –θ/3. Therefore, the value of
θ is ~6, suggesting that the d–d is the type of interation responsible for the quenching effect
of the PL emission of Eu3+ ions in the CaMoO4-SO4 phosphors.
14

Fig. 12: PL excitation spectra of CaMoO4-SO4:xEu3+(x = 0.5, 1.0, 1.5, 2.0 and 2.5mol. %)
monitored under 615 nm emission wavelength.
Fig. 13: PL emission spectra of CaMoO4-SO4:Eu3+ x mol%. (x = 0.5, 1.0, 1.5 2.0 and 2.5)
monitored under 395 nm excitation wavelength.
15

Fig. 14: (a) Photoluminescence emission intensity as a function of Eu3+ ion concentration
and (b) Log (I/x) versus log(x) plot of CaMoO4-SO4:Eu3+phosphors
2-
3.6. Energy transfer mechanism between MoO4 to Eu3+ ion
Fig. 15 shows the schematic diagram explaining the mechanism of energy transfer
from MoO42- to the 4f ground level of Eu3+. A similar mechanism was proposed by Lee et
1
al.[50]. Under UV excitation, the host electrons make a transition from the A1 ground state
to the 1T1 excited state. When the Eu3+ ions are incorporated in the CaMoO4-SO4, the energy
is transferred from MoO42- to the unexcited 4f levels of the Eu3+ ions. Therefore, an electron
may relax non-radiactively as it makes a transition from the higher excited energy states to
the lower energy states and multiple photon emission occur during this transition. This
2-
absorbed primary energy is transferred from the MoO4 to the Eu3+ ion, resulting in the
7
5
electronic transition from the F0 ground state to the L6 excited states of the Eu3+ ion. The
5
5
5L6, and Dj (j= 1, 2, 3) levels decays non-radioactively to the D0 level followed by the
5
7
radiative emission of visible photons during a transition from the D0 to FJ (J=1, 2, 3, 4)
levels of Eu3+ ions.
16

Fig. 15: The energy level diagram and the energy transfer process in CaMoO4-SO4: Eu3+
phosphor.
3.7.
Phosphorescence decay time curves:
5
Fig. 16 depicts the decay behaviour of the D0→7F2 transition emission line for Eu3+
in the CaMoO4:Eu3+ and the CaMoO4-SO4:Eu3+ samples. The experimental curves were fitted
using a double-exponential function:
ꢍ
ꢍ
( )
ꢈ ꢉ = ꢊꢃ exp ꢋꢌ ꢎ ꢐ ꢑ ꢊꢒ exp ꢋꢌ ꢎ ꢐ,
(6)
ꢏ
ꢓ
where I(t) is the phosphorescence intensity, A1 and A2 are the constants, and ꢔ1 and ꢔ2 are the
lifetimes or decay constants. The lifetime of CaMoO4:Eu3+ is 430 ms, while that CaMoO4-
SO4:Eu3+ is 442 ms (i.e. 12 ms longer than the former). These results provides direct evidence
2-
that SO4 does have a positive effect on the PL intensity and lifetime of CaMoO4:Eu3+
2-
material. It is therefore reasonable to conclude that SO4 ions acted toharvest and transfer
17

primary excitation energy to Eu3+ resulting in improved red photoluminescence of Eu3+ at the
wavelength of 625 nm.
Fig. 16: The fluorescence decay curves of CaMoO4:Eu3+(1mol%) and CaMoO4-SO4:Eu3+(1mol%)
phosphors.
3.8. CIE-color-coordinates and CCT of CaMoO4-SO4:Eu3+ phosphors:
The CIE chromaticity coordinates diagram of CaMoO4-SO4:xEu3+ is shown in Fig. 17.
The colour was tuned from orange-yellow to red, as we vary the molar concentration of Eu3+.
The CCT was determined using McCamy empirical formula: [51]
CCT= − 437n3 + 3601n2 + 6861n + 5514.1
(7)
where n = (x-xe)/(y-ye) and xe = 0.332 and ye = 0.1858
The CIE coordinates and the CCT values are presented in Table 1. The CIE coordinates show
that our phosphors changed colour from orange-red to red colour with incorporation of the
2-
SO4 anionic groups and different concentrations of Eu3+. This confirmed that the emission
colours were tunable. The CCT values of our phosphors were found in the range of 3879 K-
7391 K for different Eu3+ ion concentrations. For the purpose of commercial lighting, the
CCT value lower than 5000 K is certainly the cold red light. The obtained CCT values of
CaMoO4-SO4:xEu3+ (x= 0.5, 1.0 and 1.5) are in close agreement with the CCT value of the at
18

noon (i.e. standard daylight at noon) (5000 - 5400 K) that is suitable for cold light emission
[52]. The present results suggest that our materials are promising candidates for producing
cool red light illumination. The colour purity is one of the important features for evaluating
performance of phosphors, and it can be determined using equation (8):
ꢓ
ꢓ
ꢕ(ꢖꢗꢖ ) ꢄ(ꢙꢗꢙ
)
ꢘꢘ
ꢘꢘ
Color purity =
ꢓ ꢛ 100%,
(8)
ꢓ
ꢕ(ꢖ ꢗꢖ ) ꢄ(ꢙ ꢗꢙ
)
ꢘꢘ
ꢘꢘ
ꢚ
ꢚ
where (x, y), (xee, yee), and (xd, yd) are the CIE color chromaticity coordinates of the sample
point, the standard equal-energy point (0.3333, 0.3333), and the dominant-wavelength point
of the sample, respectively. The color purity values of the CaMoO4-SO4:Eu3+phosphorsare
listed in table 1, and were found to be in the range of 89–97%, which is greater than the
standard red emitting phosphor (CaMoO4:Eu3+). The values shown in table 1 indicate that the
phosphors having a composition of the CaMoO4-SO4:xEu3+ (0.5 and 1 mol. %) showed the
maximum red color emission and an exceptional colour chromaticity coordinates with an
improved colour purity.
Fig. 17: CIE chromaticity diagram of the CaMoO4-SO4: xEu3+ phosphors.
19

Samples
CIE 1931 Coordinates
CCT
Colour
Colour
x
y
values (K) purity (%)
CaMoO4:1.0Eu3+
CaMoO4-SO4:0.5Eu3+
CaMoO4-SO4:1.0Eu3+
CaMoO4-SO4:1.5Eu3+
CaMoO4-SO4:2.0Eu3+
0.57
0.69
0.69
0.67
0.66
0.42
0.31
0.31
0.30
0.32
1805
5194
5194
7391
3879
68
97
97
92
89
Orange-red
Red
Red
Red
Red
Table 1: CIE1931 chromaticity coordinate, CCT and colour purity values of CaMoO4-
+(x = 0.5, 1.0, 1.5, 2.0, and 2.5)
SO4:xEu3
and the corresponding colors.
Conclusion:
CaMoO4-SO4:xEu3+ (x = 0.5, 1.0, 1.5, 2.0 and 2.5 mol. %) phosphor powders were
successfully synthesized by the solid-state reaction method. The structural, morphological,
and optical properties were investigated using different techniques. The XRD diffraction
patterns were consistent with those reported in the standard JCPDS file no. 29-351 of the pure
tetragonal phase of CaMoO4. The DRS absorption spectra showed that the optical absorption
2-
band gap of our phosphors shifted toward the lower energies due to incorporation of the SO4
. The PL properties showed the tunable emission from orange-red to red emission colour that
2-
was dependent on the Eu3+ concentration. Incorporation of the SO4 into the host lattice
improved the emission intensity of CaMoO4:Eu3+, which confirms that SO4 ions are
2-
excellent sensitizer of the red emission. The purity of the red emission was analysed and the
CIE coordinates, CCT and color purity analysis results suggest that our materials are suitable
for use as sources of red light in displays and light emitting devices of different types.
.
Acknowledgments
The South African National Nanoscience Postgraduate Teaching and Training Platform
(NNPTTP) of the Department of Science and Technology support this work financially.OMN
would like to acknowledge financial support from the Competitive Programme for Rated
Researchers (CPRR) (Grant no. CPR20110724000021870) of the National Research
Foundation (NRF)-South Africa. This research was also financially supported by PDRFs,
Global Excellence and Stature Funding (GES) programme, University of Johannesburg and
Faculty of Science, University of Johannesburg, South Africa. HCS acknowledge the Sarchi
chair initiative of the Department of Science and Technology (84415) for equipment and
financial support.
20

References
[1] Y. Liu, G. Liu, J. Wang, X. Dong, W. Yu, Single-component and warm-white-emitting
phosphor NaGd(WO4)2:Tm3+, Dy3+, Eu3+: synthesis, luminescence, energy transfer, and
tunablecolor, Inorg. Chem. 53 (2014) 11457–11466.
[2] M.E. Foley, R.W. Meulenberg, J.R. Mcbride, G.F. Strouse, Eu3+-doped ZnB2O4(B
=Al3+, Ga3+) nanospinels: an efficient red phosphor, Chem. Mater.27 (2015) 8362–
8374.
[3] X. Huang, B. Li, H. Guo, D. Chen, Molybdenum-doping-induced photoluminescence
enhancement in Eu3+-activated CaWO4 red-emitting phosphors for white light-emitting
diodes, Dyes Pigm.143 (2017) 86–94.
[4] S.K. Gupta, P.S. Ghosh, A.K. Yadav, N. Pathak, A. Arya, S.N. Jha, D. Bhattacharyya,
R.M. Kadam, Luminescence properties of SrZrO3/Tb3+ perovskite: host-dopant energy-
transfer dynamics and local structure of Tb3+, Inorg. Chem. 55 (2016) 1728–1740.
[5] Y. Zhai, X. Zhao, C. Liu, P. Song, X. Jing, Y. Han, J. Wang,
CaMoO4:Dy3+,Eu3+ phosphors: microwave synthesis, characterization, tunable
luminescence properties and energy transfer mechanism, Optik, 164 (2018) 433-442
[6] N. Narendran, Y. Gu, J. Freyssinier, H. Yu and L. Deng, Solid-state lighting: failure
analysis of white LEDs, J. Cryst. Growth, 268 (2004) 449-456.
[7] A. Parchur, A. Prasad, A. Ansari, S. Rai and R. Ningthoujam, Luminescence properties
of Tb3+-doped CaMoO4 nanoparticles: annealing effect, polar medium dispersible,
polymer film and core-shell formation, Dalton Trans.41 (2012)11032.
[8] E. Schubert, Solid-State Light Sources Getting Smart, Science, 308 (2005)1274-1278.
[9] J. Park, H. Jung, G. Seeta Rama Raju, B. Moon, J. Jeong and J. Kim, Tunable
luminescence and energy transfer process between Tb3+ and Eu3+ in GYAG:Bi3+, Tb3+,
Eu3+ phosphors, Solid State Sci., 12 (2010) 719-724.
[10] H. Jang, Y. Won and D. Jeon, Improvement of the electroluminescent property of blue
LED coated with highly luminescent yellow-emitting phosphors, Applied Phys. B, 95
(2009) 715-720.
[11] S. Wang, Q. Sun. B. Devakumar, J. Liang, L. Sun,X. Huang, Novel highly efficient and
thermally stable Ca2GdTaO6:Eu3+ red-emitting phosphors with high color purity for
UV/blue-excited WLEDs, J.AlloysCompd.804 (2019) 93-99.
[12] Y. He, M. Zhao, Y. Song, G. Zhao and X. Ai, Effect of Bi3+ on fluorescence properties
of YPO4:Dy3+ phosphors synthesized by a modified chemical co-precipitation method,
J. Lumin., 131 (2011) 1144-1148.
21

[13] M. Piatkowska, E. Tomaszewicz Synthesis, structure, and thermal stability of new
scheelite-type Pb(1-3x)hxPr2x(MoO4)1-3x(WO4)3x ceramic materials, J Therm. Anal
Calorim 126 (2016) 111–119.
[14] Y. Liu, H. Li, S. Tang, Q. Zhou, K. Wang, H. Tang, Z. Wang, A red-emitting phosphor
K2[MoO2F4] H2O:Mn4+ for warm white light-emitting diodes with a high color
rendering index, Mater. Res. Bull., 122(2020) 110675.
[15] F.A. Rabuffetti, S.P. Culver, L. Suescun, R.L. Brutchey, Structural disorder in
AMoO4(A=Ca, Sr, Ba) scheelite nanocrystals, Inorg. Chem. 53 (2014) 1056–1061.
[16] S.K. Gupta, M. Sahu, P.S. Ghosh, D. Tyagi, M.K. Saxena, R.M. Kadam, Energy
transfer dynamics and luminescence properties of Eu3+ in CaMoO4 and SrMoO4,
Dalton Trans. 44 (2015) 18957–18969.
[17] Y. Hatefi, N. Shahtahmasebi, A. Moghimi,andE.Attaran, Frequency-conversion
properties of Eu3+ doped chlorophosphateglass ceramics containing CaCl2nanocrystals,
J. Lumin.131 (2011) 114-118.
[18] A. Peter, Photoluminescence Studies of CaMoO4: Eu3+ Phosphors, Asian J. Mater.
Chem.1 (2016) 84-86..
[19] Z. Wang, H. Liang, M. Gong and Q. Su, Novel red phosphor of Bi3+, Sm3+ co-activated
NaEu(MoO4)2, Opt. Mater. 29 (2007) 896-900.
[20] P. Jaffe, Eu2+ Luminescence in the Ternary EuO-Al2O3-SiO2ꢀSystem, J.Electrochem.
Soc. 116(1969) 629-633.
4−
3−
[21] Y. Zhang, S. Shi, J. Gao and J. Zhou, Effects of SO2 or SiO2 Doping on the
Photoluminescence of NaEu(MoO4)2 Nanophosphors for Light-Emitting Diodes, J
Nanosci.Nanotechno.10 (2010) 2156-2160.
[22] K. Ariga and M. Aono, Supra-materials nanoarchitectonics. Oxford: William Andrew,
2017.
[23] E. Gürmen, E. Daniels and J. King, Crystal Structure Refinement of SrMoO4, SrWO4,
CaMoO4, and BaWO4 by Neutron Diffraction, J. Chem. Phys. 55 (1971) 1093-1097.
[24] L.G. Van Uitert, Characterization of energy transfer interactions between rare earth
ions, J. Electrochem. Soc. 114 (1967) 1048–1053.
[25] W. Zhang, J. Li, Y. Wang, J. Long, and K. Qiu, Synthesis and luminescence properties
2-
3-
(MoO4)2-xAGx:Eu3+(AG = SO4 , BO3 ) red phosphors for white light emitting diodes,
J. AlloysCompd.635 (2015) 16–20.
22

[26] O. Ntwaeaborwa, S. Mofokeng, V. Kumar and R. Kroon, Structural, optical and
photoluminescence properties of Eu3+ doped ZnO nanoparticles, Spectrochim. Acta A
182 (2017) 42-49.
[27] A. Varma, A.S. Mukasyan, A.S. Rogachev, K.V. Manukyan, Solution combustion
synthesis of nanoscale materials, Chem. Rev. 116 (2016,) 14493−14586.
[28] Y. Huang, L. Zhou, L. Yang and Z. Tang, Self-assembled 3D flower-like
NaY(MoO4)2:Eu3+
microarchitectures:
Hydrothermal
synthesis,
formation
mechanismand luminescence properties, Opt. Mater.33 (2011) 777-782.
2−
[29] Y. Wang, W. Zhang, J. Li, and J. Long, Effect of MoO4 partial substitution on the
optical enhancement of LaNa(MoO4) 2: Dy3+ phosphors for white light-emitting diodes,
Mater. Sci. Semicond. Process.41 (2016) 277-281.
[30] B. Singh, A. Parchur, R. Ningthoujam, A. Ansari, P. Singh and S. Rai, Influence of
Gd3+ co-doping on the structural property of CaMoO4Eu nanoparticles, Dalton Trans.,
43 (2014) 4770-4778.
[31] S. Gupta, M. Sahu, P. Ghosh, D. Tyagi, M. Saxenaand R. Kadam, Energy transfer
dynamics and luminescence properties of Eu3+ in CaMoO4 and SrMoO4, Dalton
Trans.,44 (2015) 18957-18969.
[32] A. Parchur, A. Prasad, S. Rai, R. Tewari, R. Sahu, G. Okram, R. Singh and R.
Ningthoujam, Observation of intermediate bands in Eu3+ doped YPO4 host: Li+ ion
effect and blue to pink light emitter, AIP Advances, 2 (2012) 032119.
[33] B. Singh, M. Maheshwary, P. Ramakrishna, S. Singh, V. Sonu, S. Singh, P. Singh, A.
Bahadur, R. Singh and S. Rai, Improved photoluminescencebehavior of Eu3+ activated
CaMoO4 nanoparticles via Zn2+ incorporation, RSC Advances5 (2015) 55977-55985.
[34] R. Karthik, J. Kumar, S. Chen, T. Kokulnathan, H. Yang and V. Muthuraj, Design of
novel ytterbium molybdate nanoflakes anchored carbon nanofibers: challenging
sustainable catalyst for the detection and degradation of assassination weapon
(Paraoxon-Ethyl), ACS Sustain. Chem. Eng. 6 (2018) 8615-8630.
[35] X. Wang, Q. Xiang, B. Liu, L. Wang, T. Luo, D. Chen and G. Shen, TiO2 modified FeS
nanostructures with enhanced electrochemical performance for lithium-ion batteries,
Sci. Rep. 3 (2013).
[36] G. Tai, T. Zeng, J. Yu, J. Zhou, Y. You are, X. Wang, H. Wu, X. Sun, T. Hu and W.
Guo, Fast and large-area growth of uniform MoS2 monolayers on molybdenum foils,
Nanoscale, 8 (2016) 2234-2241.
23

[37] A. Balakrishna, H. Swart, R. Ramaraghavulu, A. Bedyal, R. Kroon and O.
Ntwaeaborwa, Structural evolution induced by substitution of designated molybdate
−2
−3
−3
−2
sites (MoO4 )with different anionic groups (BO3 , PO4 and SO4 ) in CaMoO4
:Sm3+ phosphors-A study on colortunable luminescent properties, J. AlloysCompd. 727
(2017) 224-237.
[38] J. F. Moulder, W. F. Stickle, P. E. Sobel and K. D. Bomben, Handbook of X-ray photo
electron spectroscopy, PerkinElmer Corporation, Eden Prairie, MN, USA, 1992.
[39] S. Gupta, M. Sahu, P. Ghosh, D. Tyagi, M. Saxena, and R. Kadam, Energy transfer
dynamics and luminescence properties of Eu3+ in CaMoO4 and SrMoO4, Dalton Trans.
44 (2015) 18957-18969.
[40] Y. Zhai, X. Zhao, C. Liu, P. Song, X. Jing, Y. Han and J. Wang, CaMoO4 Dy3+, Eu3+
phosphors: microwave synthesis, characterization, tunable luminescence propertiesand
energy transfer mechanism, Optik164 (2018) 433-442.
[41] H. Philipp, Optical properties of non-crystalline Si, SiO, SiOxand SiO2, J. Phys. Chem.
Solids32 (1971) 1935-1945.
[42] S. Dutta, S. Som, A. Kunti, S. Sharma, V. Kumar, H. Swart and H. Visser, Ag7+ ion
induced modification of morphology, optical and luminescence behavior of charge
compensated CaMoO4nanophosphor, Nucl. Instrum. Meth. B 384 (2016) 76-85.
[43] C. Bouzidi, K. Horchani-Naifer, Z. Khadraoui, H. Elhouichetand M. Ferid, Synthesis,
characterization and DFT calculations of electronic and optical properties of
CaMoO4,Physica B497 (2016) 34-38.
[44] A. K. Parchura and R. S. Ningthoujam, Behaviour of electric and magnetic dipole
5
transitions of Eu3+, D0→7F0 and Eu–O charge transfer band in Li+ co-doped
YPO4:Eu3+, RSC Advances 2 (2012) 10859–10868.
[45] C. Chiu, C. Liu, S. Huang and T. Chen, White-Light-Emitting Diodes Using Red-
Emitting LiEu(WO4)2−x(MoO4)x Phosphors,J. Electrochem. Soc. 154 (2007) J181-J184.
[46] X. Zhang, C. Cao, C. Zhang, L. Chen, J. Zhang, X.J. Wang, Improved
photoluminescence and afterglow in CaTiO3:Pr3+ with addition of nanosized SiO2,
Physica B406 (2011) 3891–3895.
[47] X. Zhang et al., Photoluminescence and energy storage traps in CaTiO3:Pr3+, Mater.
Res. Bull. 45 (2010) 1832-1836.
[48] G. Mhlongo, M. Dhlamini, H. Swart, O. Ntwaeaborwa and K. Hillie, Dependence of
photoluminescence (PL) emission intensity on Eu3+ and ZnO concentrations in
Y2O3Eu3+andZnO·Y2O3Eu3+nanophosphors, Opt. Mater. 33 (2011) 1495-1499.
24

[49] L.G. Van Uitert, Characterization of energy transfer interactions between rare earth
ions, J. Electrochem. Soc. 114 (1967) 1048–1053.
[50] G. Lee and S. Kang, Solid-solution red phosphors for white LEDs, J. Lumin. 131
(2011) 2582-2588.
[51] C.S. McCamy, Correlated color temperature as an explicit function of chromaticity
coordinates, Color Res. Appl. 17 (2) (1992) 142–144.
[52] J. Han, W. Im, G. Lee and D. Jeon, Near UV-pumped yellow-emitting Eu2+-doped
Na3K(Si1−xAlx)8O16±δ phosphor for white-emitting LEDs, J. Mater. Chem. 22 (2012)
8793.
25

‹ CaMoO4:Eu3+ phosphors substituted with SO4 groups were prepared via solid state reaction
2-
method.
2-
‹
Photoluminescence (PL)properties of CaMoO4:Eu3+ were improved by SO4 ions
‹ The energy transfer from MoO42- to Eu3+ was showed in CaMoO4 host.
‹ CaMoO4-SO4:Eu3+ (1 mol %) phosphor reported the strongest PL emission.
‹ Orange-red to red color tuneable emission has been achieved by the SO42- substitution.
‹ CaMoO4-SO4:Eu3+ are potential candidates for UV excited red emission LED applications.

Declaration of interests
(X)The authors declare that they have no known competing financialinterestsor personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: