Preparation, characterization and photoluminescence of nanocrystalline calcium molybdate

Article abstract of DOI:10.1016/j.jallcom.2009.03.037

Nanocrystalline calcium molybdate was successfully synthesized from Ca(NO₃)₂ and Na₂MoO₄ in ethylene glycol using a microwave radiation method. Body-centered tetragonal structured calcium molybdate with narrow n

Full text of DOI:10.1016/j.jallcom.2009.03.037

Journal of Alloys and Compounds 481 (2009) 568–572
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Preparation, characterization and photoluminescence of nanocrystalline
calcium molybdate
Anukorn Phuruangrata,∗, Titipun Thongtemb,∗, Somchai Thongtema
a Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
b Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanocrystalline calcium molybdate was successfully synthesized from Ca(NO3)2 and Na2MoO4 in ethy-
lene glycol using a microwave radiation method. Body-centered tetragonal structured calcium molybdate
with narrow nanosized distribution was detected using XRD, SAED and TEM. A diffraction pattern was
also simulated and was found to be in accordance with those obtained from the experiment and JCPDS
standard. Raman and FTIR spectra show the Mo–O prominent stretching bands in the [MoO4]2− tetrahe-
drons at 879.59 and 743–895 cm−1, respectively. Photoluminescence emission of CaMoO4 was detected
at 477 nm, caused by the annihilation of a self-trapped excitons from the [MoO4]2− excited complex.
© 2009 Elsevier B.V. All rights reserved.
Received 1 January 2009
Received in revised form 4 March 2009
Accepted 5 March 2009
Available online 19 March 2009
Keywords:
Nanocrystalline calcium molybdate
Scheelite structure
Microwave radiation
1. Introduction
electric field intensity, leading to molecular vibration. The solu-
tion is able to attain high temperature at very rapid rate. Due to
Nanomaterials have very interesting optical and electrical prop-
erties which are different from their bulks and controlled by their
uniform shapes and narrow size distributions. Alkaline earth metal
molybdates are very interesting materials due to their structural
properties, and great potential and many promising applications.
They have attracted particular interest in a variety of applications,
such as laser host materials, luminescence materials, microwave
applications, microelectronics and catalysts [1–5]. There are a
variety of methods used to synthesize nanosized CaMoO4, such
as molten salt [4], a modified citrate complex method using
microwave irradiation [5] sonochemical method [6], and pulsed
laser ablation [7].
In recent years, microwave irradiation has been used for a
number of applications in preparing materials, including inorganic
complexes, oxides, sulfides and others. Microwave irradiation has
shown to be very rapid growth in its application to materials sci-
ence and engineering due to its unique reaction effect, such as
rapid volumetric heating and the consequent dramatic increase in
reaction rates [8,9]. A solvent with high dielectric constant is very
rapidly heated up to high temperature by microwave irradiation
at very short time. Ethylene glycol, which has a dielectric constant
of 40.3 at 298 K [10], is excellent in coupling with the oscillating
the vibration of a 2.45 GHz microwave radiation [11], nucleation
and growth proceeded in uniform liquid phase environment. The
microwave radiation has the advantage of short reaction time, sim-
ple and efficient over other conventional methods. The products
have small particle sizes, narrow particle size distributions and high
purity [9].
In 2005, Ryu et al. [5] reported the synthesis of nanosized
CaMoO4 powders at low temperatures by a modified citrate com-
plex method using microwave irradiation. The nanosized CaMoO4
powders were produced when the citrate complex precursors were
calcined at 500 ◦C and above for 3 h. For the present research, the
characterization and photoluminescence (PL) of nanocrystalline
CaMoO4 synthesized by a microwave irradiation method with no
calcination are reported.
2. Experimental procedure
To synthesize nanocrystalline CaMoO4, each 0.005 mol of Ca(NO3)2 and
Na2MoO4 was separately dissolved in 15.00 ml ethylene glycol. The two solutions
were mixed and stirred for 30 min. Then the mixture was transferred into a home-
made autoclave and heated at 50% of 600 W microwave for 20 min. For the 50%
of 600 W microwave-assisted synthesis, the power was on for 60 s and off for 60 s
interval every 120 s cycle−1. At the conclusion of the process, white precipitates were
produced, separated using a filtered paper, washed with distilled water and absolute
ethanol, and dried in air at 80 ◦C for 24 h. The product was then analyzed by X-ray
diffraction (XRD), Fourier transform inferred (FTIR) spectroscopy and Raman spec-
troscopy, transmission electron microscopy (TEM), selected area electron diffraction
(SAED) and a spectrophotometry.
∗
Corresponding authors. Tel.: +66 0 53 943341–45; fax: +66 0 53 892277.
E-mail addresses: phuruangrat@hotmail.com (A. Phuruangrat),
ttpthongtem@yahoo.com (T. Thongtem).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.03.037

A. Phuruangrat et al. / Journal of Alloys and Compounds 481 (2009) 568–572
569
Fig. 1. (a) XRD pattern of CaMoO4 synthesized using a microwave radiation and (b) the simulated diffraction pattern.
3. Results and discussion
Fig. 1a shows the XRD pattern of nanocrystalline CaMoO4, pre-
pared from Ca(NO3)2 and Na2MoO4 by a microwave irradiation
method. The pattern was compared to the JCPDS card no. 29-0351
[12] (a = b = 5.2260 Å and c = 11.4300 Å). All of the diffraction peaks
were specified that the product was tetragonal CaMoO4 structure
with no detection of any impurities, like MoO3, CaO, or others. Cal-
culated lattice parameters of the product are a = b = 5.2178 Å and
c = 11.4632 Å, which are very close to those of the corresponding
JCPDS card [12].
The crystallite size of nanocrystalline CaMoO4 was calculated
using the (1 1 2) peak of the XRD pattern and the Scherrer’s equa-
tion:
ꢀk
B =
,
L cos ꢁ
where ꢀ is the wavelength of Cu K␣ radiation, ꢁ the Bragg’s angle,
L the average crystallite size, k a constant (k = 0.89) and B is the full
width at half maximum (FWHM) of the (1 1 2) peak in radian [13],
and was found to be 15 nm.
Fig. 2. Unit cell of tetragonal CaMoO4.
To produce nanocrystalline CaMoO4, Ca(NO3)2 reacted with
Na2MoO4 in ethylene glycol under basic condition using
microwave radiation:
a
vibrations are classified into two types, the internal and external
modes. The first is caused by the oscillation inside the [MoO4]2−
molecular ionic groups with immobile mass center. The second is
the lattice phonon vibration, due to the motion of M2+ cations rela-
tive to the rigid molecular ionic units. In free space, [MoO4]2− ions
have Td-symmetry. By using factor group analysis, it follows that
3N = 15 degrees of freedom for [MoO4]2− molecular ionic groups are
composed of four internal modes denoted by v1(A1), v2(E), v3(F2)
Ca(NO3)2 + Na2MoO4 → CaMoO4 + 2NaNO3
Scheelite structured metal molybdates with respective ionic radii
of 1.12, 1.25, 1.42 and 1.29 Å for Ca, Sr, Ba and Pb [14] are body-
centered tetragonal symmetry C46h at room temperature, and have
two formula units per primitive cell [14–16]. Each Mo site is sur-
rounded by four equivalent O sites in tetrahedral symmetry and
each divalent metal shares corners with eight adjacent [MoO4]2−
tetrahedrons [1,15,16]. Calcium molybdate with I41/a space group
[12] was simulated using CaRIne version 3.1 program [17] and its
calculated lattice cell. The calcium molybdate unit cell (Fig. 2) shows
a complex structure composing of layers normal to the z-axis. Each
of them has a two-dimensional cesium chloride type arrangement
of Ca2+ divalent ion and [MoO4]2− tetrahedral configuration, which
are surrounded by eight ions of opposite sign [15,16,18]. The sim-
ulated XRD pattern using CaRIne version 3.1 program [17] and the
Cu K␣ line (1.5418 Å [9,11]) is shown in Fig. 1b. The 2ꢁ angles and
intensities of different peaks obtained from the simulation, exper-
iment, and JCPDS standard [12] is shown in Table 1. It is worth
noting that their values are in good accordance, although the (2 0 2)
low intensity peak of the JCPDS standard was not detected in the
experiment.
Table 1
The 2ꢁ diffraction angles (◦) and diffraction intensities of CaMoO4 obtained from the
JCPDS card no. 29-0351 [12], experiment and simulation.
Plane
JCPDS 29-0351
Experiment
Simulation
2ꢁ
Intensity
2ꢁ
Intensity
2ꢁ
Intensity
(1 0 1)
(1 1 2)
(0 0 4)
(2 0 0)
(2 0 2)
(2 1 1)
(1 1 4)
(2 1 3)
(2 0 4)
(2 2 0)
(1 1 6)
(2 1 5)
(3 1 2)
18.63
28.78
31.25
34.33
37.82
39.31
39.82
45.47
47.07
49.27
54.09
56.22
58.04
25.00
100.00
14.00
16.00
4.00
10.00
6.00
6.00
30.00
14.00
14.00
6.00
18.44
28.69
31.18
34.34
–
39.27
40.06
45.70
47.12
49.37
54.15
56.16
58.05
21.14
100.00
11.04
11.52
–
8.64
5.62
3.24
28.12
10.58
11.02
2.65
18.67
28.75
31.18
34.35
37.86
39.37
39.80
45.51
47.07
49.36
54.00
56.20
58.13
26.40
100.00
14.40
16.90
1.50
8.30
4.20
2.20
27.40
10.80
15.10
4.90
Metal molybdates have [MoO4]2− molecular ionic groups with
strong covalent Mo–O bonding, but M2+ cations such as Ca, Sr, Ba
and Pb are weakly couple with the ionic groups [14]. Their Raman
20.00
14.39
23.90

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A. Phuruangrat et al. / Journal of Alloys and Compounds 481 (2009) 568–572
Fig. 3. (a) Raman and (b) FTIR spectra of CaMoO4 synthesized using a microwave radiation.
and v4(F2), one free rotation (f.r.) mode (F1) and one translation
mode (F2). In scheelite body-centered tetragonal structure, its sym-
metry is reduced to be S4. All degenerative modes are split, due
to the crystal field effect. The split is also caused by two formula
units per primitive cell of the scheelite structure, specified as the
effect of Davydov splitting. Group theory calculation shows that
there are 26 different modes, ꢂ = 3Ag + 5Au + 5Bg + 3Bu + 5Eg + 5Eu,
for zero wavevector of the scheelite primitive cell. The 3Ag, 5Bg and
5Eg modes are Raman active. Only four of the 5Au and 5Eu modes
are infrared (IR) active, and their remains are acoustic vibrations.
The 3Bu modes are silent [14]. CaMoO4 has [MoO4]2− tetrahedron
symmetry. Raman spectrum of CaMoO4 crystals was characterized
over 150–1000 cm−1 as shown in Fig. 3a. The vf.r.(Ag), v2(Ag), v4(Bg),
v3(Eg), v3(Bg) and v1(Ag) modes were detected at 203.55, 322.70,
392.38, 795.08, 847.50 and 879.59 cm−1, respectively. The present
results correlate well with those previously obtained [19]. Compar-
ing to Ar laser with 514.5 nm wavelength, a great deal of energy
was lost during the Raman analysis, due to the inelastic scattering
process. In addition, a transmittance mode of CaMoO4 was ana-
lyzed by FTIR as shown in Fig. 3b. For Td-symmetry, the scheelite
structure has four vibrations, specified as v1(A1), v2(E), v3(F2) and
v4(F2) [20]. In lattice space, its IR spectrum is in accordance with
the S4 site symmetry. The correlations of the Td ↔ S4 symmetries
are A1 ↔ A, E ↔ A + B and F2 ↔ B + E. There was no detection of the
v1 mode. Only the v2, v3 and v4 modes were detected [20]. For the
present analysis, a strong transmittance mode specified as Mo–O
anti-symmetric stretching vibration (v3) of [MoO4]2− tetrahedrons
[21] was detected at 743–895 cm−1. Additional weak peak of Mo–O
bending mode (v4) [21] was at 423 cm−1. The results are in accor-
dance with those previously reported [22].
TEM image, SAED pattern and particle size distribution of
CaMoO4 nanocrystalline are shown in Fig. 4. The product was
composed of a number of round nanosized particles. Its SAED
pattern shows a number of bright spots arranged in concentric
rings, with the calculated lattice planes obtained from the diam-
eters of the diffraction rings. For the present research, the product
is polycrystalline in nature. Among the diffraction planes, they
are (1 0 1), (1 1 2), (0 0 4), (2 0 0), (1 1 4), (2 0 4), (2 2 0), (1 1 6) and
Fig. 4. (a) TEM image, (b) SAED pattern and (c) particle size distribution of CaMoO4 synthesized using a microwave radiation.

A. Phuruangrat et al. / Journal of Alloys and Compounds 481 (2009) 568–572
571
[23–25]. The emission is caused by the recombination of elec-
trons in the 1T2 excited state and holes in the 1A1 ground state.
Fig. 6 shows photoluminescence of CaMoO4 nanocrystalline using
a 212 nm excitation wavelength. It shows the maximum emis-
sion peak at 477 nm, specified as the intrinsic emission, and was
caused by the annihilation of a self-trapped excitons from the
[MoO4]2− excited complex. It can be excited either in the excitonic
absorption band or in the recombination process of the scheel-
ite structured CaMoO4 [26]. At room temperature, the 3T1 and
3T2 excited states are also involved in the intrinsic emission by
a spin–forbidden transition to the 1A1 ground state in optically
detected electron paramagnetic resonance of CaMoO4. The tran-
sition from the triplets at the excited states to the ground state was
clearly detected, and was the cause to broaden the PL spectrum
[23].
Fig. 5. Schematic diagram of the crystal-field splitting and hybridization of the
molecular orbitals of [MoO4]2− tetrahedrons.
4. Conclusions
CaMoO4 nanocrystallines were successfully produced from
Ca(NO3)2 and Na2MoO4 in ethylene glycol by a one step microwave
radiation method with no further calcination. The body-centered
tetragonal CaMoO4 structure with a narrow particle size distri-
bution of 16–44 nm was detected using XRD, SAED and TEM.
The vf.r.(Ag), v2(Ag), v4(Bg), v3(Eg), v3(Bg) and v1(Ag) modes were
detected using Raman spectroscopy, and very strong Mo–O anti-
symmetric stretching mode (v3) in [MoO4]2− molecular ionic
groups using FTIR. The PL intrinsic emission of CaMoO4 nanocrys-
talline shows the narrow central peak of the [MoO4]2− excited
complex at 477 nm.
(2 1 5), which are in accordance with those of the JCPDS standard
for CaMoO4 [12]. The size distribution was determined from 150
particles of the TEM image. The CaMoO4 nanoparticles have nar-
row size distribution, and are in favor their luminescent properties.
The particle sizes are in the range of 16–44 nm with the average of
28 nm.
To produce CaMoO4 round nanoparticles, Ca(NO3)2 reacted with
Na2MoO4 in ethylene glycol under a microwave radiation, and
CaMoO4 molecules began to exist. Simultaneously, several of the
molecules nucleated to form nuclei. As the time passed, the nuclei
grew by the diffusion of nearby molecules to form nanoparticles.
Their sizes were limited by the diffusion rate and concentration of
the nearby molecules, length of time, microwave power and others,
but their round shape was controlled by their isotropic growth in
all directions. Some nanoparticles clustered together in groups as
well. This is the cause of the difference in the particle sizes obtained
by the calculation using Scherrer’s equation and the measurement
from TEM image.
Acknowledgements
We are extremely grateful to the Thailand Research Fund (TRF),
and NANOTEC, a member of NSTDA, Ministry of Science and Tech-
nology, Thailand for financial support, and the Graduate School of
Chiang Mai University for general funding.
The emission shape of CaMoO4 nanoparticles may be explained
by considering Jahn–Teller splitting effect on the excited states
of tetrahedral [MoO4]2− anions. The crystal-field splitting and
hybridization of the molecular orbitals of [MoO4]2− tetrahedrons
are shown in Fig. 5. For the ground state, all one-electron states
below the energy band gap (Eg) are filled, resulting in the 1A1
many-electron ground states. At the lowest excited states, there
are one-hole in the t1 states of the valence band and one-
electron in the e states of the conduction band, corresponding
to the 1T1, 1T2, 3T1 and 3T2 many-electron excited states. Only
the electronic transition between 1T2 and 1A1 states is allowed
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