Microstructure and spectroscopic properties of AMoO4 (A = Ca, Ba) particles synthesized via cyclic microwave-assisted metathetic route

Article abstract of DOI:10.14233/ajchem.2014.16519

AMoO4 (A=Ca, Ba) particles have been successfully synthesized via cyclic microwave-assisted metathetic route in ethylene glycol followed by further heat-treatment. The AMoO4 (A=Ca, Ba) particles were well crystallized after heat-treatment at 400-600 °C for 3 h. The microstructures exhibited fine morphologies with sizes of 0.5-1 and 1.5-2 μm for the CaMoO4 and BaMoO4 particles, respectively. The synthesized AMoO4 (A=Ca, Ba) particles were characterized by X-ray diffraction, scanning electron microscopy. Other spectroscopic properties were also examined using photoluminescence emission measurements and Raman spectroscopy.

Full text of DOI:10.14233/ajchem.2014.16519

Asian Journal of Chemistry; Vol. 26, No. 6 (2014), 1865-1868
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.16519
Microstructure and Spectroscopic Properties of AMoO4 (A = Ca, Ba)
Particles Synthesized via Cyclic Microwave-Assisted Metathetic Route
CHANG SUNG LIM
Department of Advanced Materials Science & Engineering, Hanseo University, Seosan 356-706, Republic of Korea
Corresponding author: Tel/Fax: +82 41 6601445; E-mail: cslim@hanseo.ac.kr
Received: 14 October 2013;
Accepted: 28 February 2014;
Published online: 10 March 2014;
AJC-14918
AMoO4 (A=Ca, Ba) particles have been successfully synthesized via cyclic microwave-assisted metathetic route in ethylene glycol
followed by further heat-treatment. The AMoO4 (A=Ca, Ba) particles were well crystallized after heat-treatment at 400-600 °C for 3 h.
The microstructures exhibited fine morphologies with sizes of 0.5-1 and 1.5-2 µm for the CaMoO4 and BaMoO4 particles, respectively.
The synthesizedAMoO4 (A=Ca, Ba) particles were characterized by X-ray diffraction, scanning electron microscopy. Other spectroscopic
properties were also examined using photoluminescence emission measurements and Raman spectroscopy.
Keywords: Microstructure, AMoO4 (A=Ca, Ba), Microwave-assisted metathetic route, Photoluminescence, Raman spectroscopy.
In this study, theAMoO4 (A=Ca, Ba) particles were synthe-
INTRODUCTION
sized by a microwave-assisted metathetic method in ethylene
Metal molybdates have attracted considerable attention for
glycol. The synthesized AMoO4 (A=Ca, Ba) particles were
potential applications in photoluminescence, hosts for lanthanide-
characterized by X-ray diffraction (XRD), scanning electron
activated lasers, photocatalysis and humidity sensors1-3. The
microscopy (SEM). Their spectroscopic properties were
physical, chemical and photochemical properties of metal
examined by photoluminescence (PL) emission and Raman
molybdates are dependent on the manufacturing method.
spectroscopy.
Several processes have been developed over the past decade
EXPERIMENTAL
to enhance the applications of metal molybdates, such as co-
precipitation4, the electrochemical technique5, aqueous minera-
lization process6, the hydrothermal method7,8, the microwave-
hydrothermal method9,10, the microwave-assisted polymerized
complex method11,12, pulsed laser ablation13 and the microwave
assisted synthesis in solution14. Among different methods,
solution-based chemical synthetic methods play the key role
in the design and production of fine oxide powders and are
successful in overcoming many limitations of traditional solid-
state, high-temperature methods.
CaCl2, BaCl2·2H2O and Na2MoO4·2H2O of analytic reagent
grade were used to prepare the AMoO4 (A=Ca, Ba) particles
by a cyclic microwave-assisted metathetic reaction in ethylene
glycol. For the metathetic reaction of CaMoO4, 0.01 mol each
of CaCl2 and Na2WO4·2H2O were dissolved in 30 mL ethylene
glycol, respectively. For the metathetic reaction of BaMoO4,
0.01 mol each of BaCl2·2H2O and Na2MoO4·2H2O were dissol-
ved in 30 mL of ethylene glycol, respectively.After the complete
homogeneous stirring of each solution, they were mixed together
and stirred until the solution became more viscous and trans-
parent. The solutions were mixed and adjusted to pH 9.5 using
NaOH. The aqueous solutions were then stirred at room tempe-
rature. In the sequence, the mixtures were transferred into a
Teflon vessel with a capacity of 120 mL. The Teflon vessel was
heated in a microwave oven operating at a frequency of 2.45
GHz and a maximum output power of 1250 W for 23 min.
The working cycle of the microwave-assisted metathetic reaction
was controlled precisely and consisted of 30 s on and 30 s off
for 10 min, followed by 30 s on and 60 s off for a further 20 min.
The resulting samples were treated with ultrasonic radiation
As compared to common methods, the microwave synthesis
technique provides such advantages as a short reaction time,
small particle size, narrow particle size distribution and it is
the high purity method suitable for the preparation of poly-
crystalline products. The microwave heating is delivered to
the surface of the material by radiant and/or convection heating,
which is transferred to the bulk of the material via conduction.
So, the microwave energy is delivered directly to the material
through the molecular interactions with electromagnetic field.
Heat can be generated through volumetric heating because
microwaves can penetrate the material and supply energy15-18
.

1866 Lim
Asian J. Chem.
and washed many times with hot distilled water. The white
precipitates were collected and dried at 100 °C in a dry oven.
The final products were heat-treated at 400-600 °C for 3 h.
The existing phase in the particles after the metathetic
reactions and heat-treatment was identified by powder XRD
(CuKα, Rigaku D/MAX 2200, Japan). The microstructure and
surface morphology of theAMoO4 (A=Ca, Ba) particles were
observed by SEM (JSM-5600, JEOL, Japan). The photolumi-
nescence spectra were recorded using a spectrophotometer
(Perkin Elmer LS55, UK) at room temperature. Raman spectro-
scopy was performed using a LabRam HR (Jobin-Yvon, France).
The 514.5 nm line of anAr-ion laser was used as the excitation
source and the power applied to the samples was kept at 0.5
mW.
the BaMoO4 formed had Scheelite-type crystal structure with
lattice parameters of a = b = 5.573 Å and c = 12.786 Å18. The
metathetic reactions in ethylene glycol assisted by cyclic micro-
wave irradiation are suitable for the growth of the CaMoO4
and BaMoO4 crystallites responsible for the strongest intensity
peaks from the (112), (200) and (312) planes, which were the
major peaks of the CaMoO4 and BaMoO4.
Fig. 2 shows the SEM images of the synthesized (a)
CaMoO4 and (b) BaMoO4 particles after heat-treatment 600 °C
for 3 h. TheAMoO4 (A=Ca, Ba) particles were well crystallized
after heat-treatment at 600 °C for 3 h. The SEM image of the
CaMoO4 particles in Fig. 2a revealed fine morphology with a
size of 0.5-1 µm. The SEM image of the BaMoO4 particles in
Fig. 2b exhibited well crystallized morphologies with a size
of 1.5-2 µm. The microwave-assisted metathetic reaction
provides the exothermic energy required to synthesize the metal
molybdates. In addition, it enables the bulk of the material to
be heated uniformly, resulting in fine particles with a controlled
morphology and the product to be fabricated in a green manner
without the generation of solvent waste. Metathetic reactions,
such as CaCl2 + Na2MoO4 → CaMoO4 + 2NaCl and BaCl2 +
Na2MoO4 → BaMoO4 + 2NaCl, involve the exchange of atomic/
ionic species, in which the driving force is the exothermic
reaction accompanying the formation of NaCl20. The CaMoO4
and BaMoO4 particles were heated rapidly and uniformly by
RESULTS AND DISCUSSION
Fig. 1 shows the XRD patterns of the (a) CaMoO4 and (b)
BaMoO4 particles synthesized by the cyclic microwave-
assisted metathetic reaction in ethylene glycol. All the XRD
peaks were assigned to CaMoO4 and BaMoO4 with a Scheelite-
type structure, which are in good agreement with the crystallo-
graphic data of CaMoO4 (JCPDS: 85-0585) and BaMoO4
(JCPDS: 29-01930), respectively. The crystallized CaMoO4
and BaMoO4 particles were prepared using a metathetic reac-
tion in ethylene glycol assisted by cyclic microwave irradiation.
These XRD peaks were the same as those of the CaMoO4 and
BaMoO4 particles after heat-treatment at 400-600 °C for 3 h.
The CaMoO4 had a Scheelite-type crystal structure with lattice
parameters of a = b = 5.212 Å and c = 11.438 Å14, whereas
6000
(a)
5000
4000
3000
2000
1000
0
10
20
30
40
50
60
2
θ
(°)
5000
(b)
4500
4000
3500
3000
2500
2000
1500
1000
500
0
10
20
30
40
50
60
2θ (°)
Fig. 1. XRD patterns of the synthesized (a) CaMoO4 and (b) BaMoO4
particles
Fig. 2. SEM images of the synthesized (a) CaMoO4 and (b) BaMoO4
particles

Vol. 26, No. 6 (2014)
Microstructure and Spectroscopic Properties of AMoO4 (A = Ca, Ba) Particles 1867
the cyclic microwave-assisted metathetic route. This classifies
the method among simple and cost-effective ones and, evidently,
the microwave-assisted metathetic technology is able to provide
high yields with an easy scale-up as a viable alternative for
the rapid synthesis of complex oxide composites19. The micro-
wave-assisted metathetic reaction and post heat-treatment are
interdependent and essential procedures to synthesize well
defined AMoO4 (A=Ca, Ba) particles. For the molybdate
materials to be used for practical applications, it is necessary
to control the size distribution and morphology of the particles.
Fig. 3 presents the photoluminescence emission spectra
of the CaMoO4 particles after heat-treatment at (a) 400 °C for
3 h, (b) 500 °C for 3 h and (c) 600 °C for 3 h excited at 250 nm
at room temperature. With excitation at 250 nm, the CaMoO4
particles exhibit a photoluminescence emission in the green
wavelength range of 480-500 nm. The intensity increases with
increasing heat-treatment temperature. The photoluminescence
intensity of phosphors strongly depends on the particle shape
and distribution. Generally, for similar morphological samples,
homogenized particles have better luminescent characteristics,
because of the lesser contamination or fewer dead layers on the
phosphor surface. The strong photoluminescence intensity was
attributed to their well-defined and homogeneous morphology
with sizes of 0.5-1 µm.
crystallinity plays an important role in improving the lumine-
scent efficiency. Therefore, the enhancement of the photolumi-
nescence intensity with increasing heat-treatment temperature
up to 600 °C is due to the increase in crystallinity. The photo-
luminescence intensity of energy-conversion materials depends
strongly on the particle shape and distribution. Generally, for
similar morphological samples, homogenized particles are
favorable to the luminescent characteristics, because of the
lesser contamination or fewer dead layers on the surface of the
energy-conversion materials.
(c)
(b)
(a)
200
300
400
500
600
700
800
900 1000
Wavelength (nm)
Fig. 4. Photoluminescence emission spectra of the BaMoO4 particles after
heat-treatment at (a) 400 °C for 3 h, (b) 500 °C for 3 h and (c) 600
°C for 3 h excited at 250 nm at room temperature
(c)
(a)
925
(b)
(a)
332
795
831
200
300
400
500
600
700
800
900 1000
344
Wavelength (nm)
352
148
189
Fig. 3. Photoluminescence emission spectra of the CaMoO4 particles after
heat-treatment at (a) 400 °C for 3 h, (b) 500 °C for 3 h and (c) 600
C for 3 h excited at 250 nm at room temperature
100
200 300
400 500 600
700 800 900 1000
Raman shift (cm–1)
Fig. 4 shows the photoluminescence emission spectra of
the BaMoO4 particles after heat-treatment at (a) 400 °C for 3 h,
(b) 500 °C for 3 h and (c) 600 °C for 3 h excited by 250 nm at
room temperature. The emission spectrum of metal molybdates
is due mainly to charge-transfer transitions within the [MoO4]2-
complex20,21. With excitation at 250 nm, the BaMoO4 particles
exhibit a photoluminescence emission in the blue wavelength
range of 370-420 nm. The photoluminescence spectra of the
CaMoO4 and BaMoO4 particles after heat-treatment at 400-
600 °C for 3 h shown in Figs. 4 and 5, respectively, show the
same peak positions. The photoluminescence intensities of the
CaMoO4 and BaMoO4 particles prepared at 600 °C are stronger
than those of the samples prepared at 400 and 500 °C. This
suggests that the photoluminescence intensity depends on the
crystallinity of the CaMoO4 and BaMoO4 particles. Their high
(b)
893
327
793
840
346
361
107
191
140
100
200
300
400
500
600
700
800
900 1000
Raman shift (cm–1)
Fig. 5. Raman spectra of the synthesized (a) CaMoO4 and (b) BaMoO4
particles

1868 Lim
Asian J. Chem.
The four narrow shoulders in the emission spectra at
346 and 327 cm-1 in addition to the free rotation mode at 191
and 140 cm-1 and the external modes at 107 cm-1, indicating
highly crystalline particles.
approximately 480, 500, 520 and 600 nm are believed to be
due to a defect structure22. Such peaks, namely the "spread-
eagle" shape of the blue emission, can be explained by the
influence of the Jahn-Teller effect23,24 on the degenerated excited
state of the [MoO4]2- tetrahedron. Generally, the presence of
Gaussian components indicates that the electronic levels
corresponding to the relaxed excited state of an emission centre
belong to a degenerate excited state influenced by some pertu-
rbation, e.g. a local low symmetry crystal field22. The Jahn-
Teller splitting effect essentially determines the emission shape
of the AMoO4 (A= Ca, Ba) particles. The additional emission
bands can be explained by the existence of a Frenkel defect
structure (oxygen ion shifted to the inter-position with the
simultaneous creation of vacancies) in the surface layers of
the BaMoO4 particles25.
ACKNOWLEDGEMENTS
This study was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and Tech-
nology (2013-054508).
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Fig. 5 shows the Raman spectra of the (a) CaMoO4 and
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Conclusion
AMoO4 (A=Ca, Ba) particles were successfully synthesized
via microwave-assisted metathetic route in ethylene glycol
followed by further heat treatment.After heat-treatment at 600 °C
for 3 h, the CaMoO4 particles exhibited fine morphologies
with sizes of 0.5-1 µm, whereas the BaMoO4 particles exhibited
well crystallized morphologies with sizes of 1.5-2 µm. With
excitation at 250 nm, the CaMoO4 particles exhibited strong
photoluminescence emissions in the green wavelength range
of 480-500 nm, whereas the BaMoO4 particles exhibited photo-
luminescence emissions in the blue wavelength range of 370-
420 nm. The photoluminescence intensities of the CaMoO4
and BaMoO4 particles prepared at 600 °C are stronger than
those of the samples prepared at 400 and 500 °C. The well-
resolved internal mode peaks in the Raman spectra for the
CaMoO4 particles at 925, 831, 795, 352, 344 and 332 cm-1
were observed in addition to the free rotation mode at 189 cm-1
and the external mode at 148 cm-1, whereas those in the spectra
of the BaMoO4 particles were observed at 893, 840, 793, 361,
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