Synthesis, Characterization and Optical Properties of BaMoO4 Synthesized by Microwave Induced Plasma Method

Article abstract of DOI:10.1134/S0036023618060141

BaMoO₄ crystals were synthesized by a 900 W microwave induced plasma process (MIP) for 40, 60, 120 and 140 min. Phase, morphology, vibrational mode and energy gap were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and UV-visible spectroscopy. In this research, the phase and morphology of product were influenced by microwave heating time. The sample processed for 140 min shows spherical particles tetragonal BaMoO₄ phase with size of 200–700 nm in diameter. BaMoO₄ with band gap of 3.35 eV shows a blue emission wavelength at 440 nm.

Full text of DOI:10.1134/S0036023618060141

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2018, Vol. 63, No. 6, pp. 725–731. © Pleiades Publishing, Ltd., 2018.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Synthesis, Characterization and Optical Properties of BaMoO
4
1
Synthesized by Microwave Induced Plasma Method
a,
b
c
d, e
Arrak Klinbumrung *, Anukorn Phuruangrat , Titipun Thongtem , and Somchai Thongtem
a
Division of Materials Science, Faculty of Science, University of Phayao, Phayao, 56000 Thailand
b
Department of Materials Science and Technology, Faculty of Science,
Prince of Songkla University, Hat Yai, Songkhla, 90112 Thailand
c
Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai, 50200 Thailand
d
Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai, 50200 Thailand
e
Center of Excellence in Materials Science and Technology, Chiang Mai University, Chiang Mai, 50200 Thailand
*
e-mail: arrak.kl@up.ac.th
Received November 17, 2017
Abstract⎯ BaMoO crystals were synthesized by a 900 W microwave induced plasma process (MIP) for 40,
4
6
0, 120 and 140 min. Phase, morphology, vibrational mode and energy gap were characterized by X-ray dif-
fraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photolu-
minescence (PL) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and UV-visible spectros-
copy. In this research, the phase and morphology of product were influenced by microwave heating time. The
sample processed for 140 min shows spherical particles tetragonal BaMoO phase with size of 200–700 nm
4
in diameter. BaMoO with band gap of 3.35 eV shows a blue emission wavelength at 440 nm.
4
DOI: 10.1134/S0036023618060141
INTRODUCTION
procedure, short reaction time and environmental
friendliness. This method can be applied for large
scale production.
Synthesis of inorganic materials with controlled
shapes and sizes has been a challenging topic in mate-
rials science research [1–4] because the physical and
In this work, BaMoO crystalline structures were
4
chemical properties are influenced by their different synthesized by microwave plasma method. Phase,
morphologies. Thus, scientists and researchers pay morphology and optical properties of the products
much attention to develop novel synthesis methods to were characterized by different techniques.
obtain micro/nano-structure materials with differ-
ence morphologies.
EXPERIMENTAL PROCEDURE
Barium molybdate (BaMoO ) has two crystalline
4
structures, tetragonal scheelite (β) and cubic (α), con-
(NH ) Mo O ⋅ 4H O and Ba(CH COO) were
4 6 7 24 2 3 2
trolled by processing temperature to form crystallite used as precursor reactants without further treatment.
[
5]. BaMoO with scheelite tetragonal structure of To produce BaMoO crystals, 1 : 1 molar ratio of the
4
4
general formula AXO (A = Ca, Sr, Ba and X = Mo) starting material with a total mass of 0.5 g was thor-
4
[
6, 7] is very attractive for a number of technological oughly mixed under dry condition. The as-synthesized
applications. Electro-optical devices have been solid mixture was loaded into a 14 mm I.D. × 100 mm
showed high potential due to their strong lumines- long silica tube which was placed in a horizontal
cence [8]. In addition, this material was frequently quartz tube. The horizontal quartz tube was tightly
applied as photocatalyst [9] and solid state laser [10]. closed and evacuated until the system reached 3.7 ±
A variety of processing techniques has been used to 0.1 kPa absolute pressure. Subsequently, the system
synthesize BaMoO powder, including solid-state containing solid mixture was heated by 900 W micro-
4
reaction [11], electrochemical method [12], Czochral- wave plasma for 5 min and let the system cool down to
ski method [13], calcination [14] and microwave radi- ambient temperature. The powders were thoroughly
ation [15]. Among them, microwave induced plasma mixed in air and repeatedly heated at the same condi-
(
MIP) is a promising method used to grow both nano- tion until completion a total synthesizing time of 40,
and micro-scale products due to simple experimental 60, 120 and 140 min. During processing, the system
was continuously evacuated to eliminate the evolved
1
The article is published in the original.
gases.
725

7
26
ARRAK KLINBUMRUNG et al.
The products were characterized by an X-ray dif- known melting point sealed in silica capillaries were
fractometer (XRD, Siemens D500) operating at 20 kV immersed in the generated plasmas. Melting point of
1
5 mA and using CuK line in combination with the each salt was estimated to be or exceed the melting
α
database of the Joint Committee on Powder Diffrac- point of the salt. The obtained temperature was
tion Standards (JCPDS) [16]. The morphologies of between 770°C for KCl and 801°C for NaCl for all gas-
product were analyzed by a scanning electron micro- eous conditions of dry air containing 78084 mol. %
scope (SEM, JEOL JSM-6335F) operating at 15 kV
N , 20.947 mol % O , 0.934 mol. % Ar and other gases
2 2
and a transmission electron microscope (TEM, JEOL [18]. The air plasma generating in vacuum by a 900 W
JEM-2010). The FTIR spectra were operated in Fou- MIP gave the temperature range of 770–801°C. Thus
rier transform infrared spectrometer (FTIR, Bruker the temperature of the present experiment was
Tensor 27) with KBr as a diluting agent and operated between 770–800°C depending on total mass of the
−1
in the range of 400–4000 cm . The optical properties processed system.
were investigated by UV-VIS spectrophotometer
Previously, the synthesis of BaMoO by other
4
(Lambda 25 Perkin Elmer) and photoluminescence
(PL) spectrometer (LS 50B Perkin Elmer) using a
3
methods was reported. The spontaneous crystalline
structure was prepared by Czochralski heating at
50 nm excitation wavelength at room temperature.
1
.100–1.200°C for 30–40 h [13]. The calcination at
9
00°C for 24 h was used to produce a binary oxide
14]. Microwave irradiation without plasma heat-
treated at 600°C for 3 h was reported to produce
[
RESULTS AND DISCUSSION
XRD patterns of the products synthesized by a 900 W BaMoO particles [15]. Comparing with the present
4
microwave plasma for different reaction time are
shown in Fig. 1. For the XRD peaks of 40 and 60 min
processing time, they revealed the presence of solid
work, MIP was shown as a low cost synthesis of
BaMoO due to the short reaction time and low pro-
4
cessing temperature. The generated radiation of the
plasma system gains higher energy to raise a tempera-
ture than that of the chemical process [19] and the
mixture phases of orthorhombic BaCO (JCPDS
3
no. 05-0378 [16]), monoclinic MoO (JCPDS no. 76-
2
1
807 [16]) and tetragonal BaMoO (JCPDS no. 29- well-defined crystallization products obtained from
4
0
193 [16]). The reduction of XRD intensity of BaCO
MIP method were no longer required post heat-treat-
3
at 23.9° of the 40 and 60 min synthesis was due to the ment.
prolonged heating time for synthesizing of BaMoO .
The crystallographic constants of tetragonal
4
At 120 min, the XRD analysis revealed that the prod- BaMoO pure phase were calculated by a least square
4
uct still contained some impurities of both BaCO and refinement method through the UNITCELL-97 pro-
3
MoO . Until at 140 min heating time, pure crystalline gram [20]. The calculated lattice parameters of a = b =
2
BaMoO was successfully synthesized without impu- 5.58286 ± 0.00029 ꢀ and c = 12.82319 ± 0.00105 ꢀ are
4
in accordance with the report of Cavalcante et al. (a =
rity detection. The (112) XRD peak of BaMoO shows
4
5
.5696 ꢀ and c = 12.7865 ꢀ) [21] and the JCPDS
the highest intensity comparing with other peaks of
the same synthesizing condition. Upon increasing the
prolong of processing time from 40 min to 140 min,
their XRD patterns became dramatically sharper and
stronger, including crystalline degree of the product
was much improved. In this research, the prolonged
no. 029-0193 (a = 5.5802 ꢀ and c = 12.8210 ꢀ) [16].
The constants show small difference, implying that
synthesizing process with rapid temperature increase
can distort the crystalline structure.
The crystallite size (G ) of the as-synthesized
hkl
operating time is the main influence for crystal BaMoO was estimated by the classical Scherrer for-
4
arrangement. According to the XRD results, solid
mixture of BaCO and MoO existed in the products.
mula as express below:
3
2
0
.9λ
Subsequently, these constituents completely disap-
peared in the final stage. Thus the presence of the end
Ghkl
=
,
(2)
D cos θ
product was the results of the reaction between BaCO
where λ is the wavelength of the incident X-ray (CuK
3
α
and MoO solid phases. A possible chemical reaction radiation with 0.15418 nm), D is the full width at half
2
maximum (FWHM) and θ is the Bragg’s angle. The
112) strongest peak of 140 min processing time was
selected for calculation. Its estimated crystallite size is
04.1 nm.
SEM images of BaMoO are showed in Fig. 2. The
for the formation of BaMoO can be described as fol-
lows
4
(
BaCO (s) + MoO (s) → BaMoO (s) + CO(g). (1)
3
2
4
1
Temperature of the products in during microwave-
4
induced plasma (MIP) was observed by Brooks and as-synthesized particles show different shapes and
Douthwaite [17]. A 900 W MIP experiment in differ- sizes for different heating times. The agglomeration
ent gases of Ar, N , 5% H /N , NH and O as sur- of polygonal shape particles with average diameter of
rounding gases was reported. Before microwave 182 nm was obtained when the processing time was
induction, the closed chamber was evacuated until the 40 min. The particle shape slightly changes when the
inside pressure reach the 1–50 mbar range. Salts of operating time was prolonged to 60 min. It can be
2
2
2
3
2
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SYNTHESIS, CHARACTERIZATION AND OPTICAL PROPERTIES
727
BaCO3
MoO2
BaMoO4
140 min
120 min
60 min
40 min
20
30
40
50
60
2θ, deg
Fig. 1. XRD patterns of BaMoO processed for 40, 60, 120, and 140 min.
4
explained that the particles attempt to reduce their the [–110] direction. A high resolution TEM
surface energy by increasing surface curvature. Within (HRTEM) image was used to determine a lattice
6
0 min heating time, the agglomerated particles were fringe of 3.45 ꢀ which can be specified as the preferred
enlarged to about 255 nm in diameter and more open. (112) crystallographic growth. Crystalline structure of
At higher temperature, smaller grains merge with 140 min processing time was simulated by VESTA ver-
larger neighboring grains by diffusing process. The sion 3.0 program. The atomic coordinates from the
increase of processing time can lead to grain size report of Takai et al. (Table 1) [22] and lattice param-
increase. During the thermal synthesizing process, eters (a = b = 5.58286 ± 0.00029 ꢀ and c = 12.82319 ±
gases emerged from the system. Some diffused out as 0.00105 ꢀ) obtained from the above XRD analysis
gas phases and pores generated. For pure BaMoO
were used for model development. The lattice param-
4
phase synthesized for 140 min, the particles appeared eters obtained from this experiment should be used for
as sphere with 200–700 nm in diameter.
unit cell simulation because the simulated cell was
used to explain the pure BaMoO product. The gener-
4
TEM was used to characterize the pure BaMoO
4
ated unit cell of BaMoO is shown in Fig. 3d. The
4
sample processed for 140 min as shown in Fig. 3. The
as-synthesized particles are spherical shape with 200–
scheelite is composed of MoO tetrahedrons sur-
4
rounding Ba atoms with eight-fold coordination. Each
Mo ion is inside tetrahedrons composing of O ions
and Ba ions are surrounded with eight O ions.
3
00 nm in diameter. The synthesizing method is diffi-
cult to control the product with narrow-size distribu-
tion due to heterogeneous crystals growth rate. The
SAED pattern of a single crystal processed for 140 min
The photoluminescence (PL) spectra of the as-
was indexed to the (004), (116) and (112) crystallo- synthesized BaMoO using excited wavelength of 350 nm
4
graphic planes with an electron beam projected along at room temperature shows in Fig. 4. The emission
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 63 No. 6 2018

7
28
ARRAK KLINBUMRUNG et al.
(a)
(b)
(c)
(d)
Fig. 2. SEM images of the BaMoO products processed for (a–d) 40, 60, 120 and 140 min, respectively.
4
peaks revealed a broad PL spectrum over the range of higher intermediary energy stage. The electron from
00–550 nm. They show a strong blue emission peak shallow oxygen 2p state (O-2pσ) and deep oxygen 2p
at around 440 nm with according to Wu et al. report state (O-2pπ) near valence band absorbs the photon
4
[
23]. The shoulder peaks at around 520 nm wavelength energy and transport to higher energy level of molyb-
were caused by defects containing in the BaMoO
sample [24].
4
denum 4d state (Mo-4d) near conduction band. When
the electron falls back to original lower energy state,
The energy state of BaMoO composed of oxygen photon energy was released. In case of (Mo-4d) →
4
(
O-2pσ) electronic transition, released photon energy
2
p state located at the upper of valence band (VB) and
molybdenum 4d state located at the lower of the con- corresponds with the blue emission. Green emission
duction band (CB) was studied by Sczancoski et al. relates with the electron falling from (Mo-4d) to (O-2pπ)
[
25]. The photon energy of 350 nm excitation wave- [26]. The emission band position is a little difference
length (3.54 eV) is high enough to activate electron to from one experiment to other experiment. The differ-
ence is not only due to the synthesizing process but
due to the size and morphology of the as-synthesized
Table 1. Atomic coordinates used to simulate the unit cell
of BaMoO processed for 140 min
product as well. The photoluminescence intensity
4
increases with the increase of the processing time. The
Atom
Site
x
y
z
photoluminescence intensity of the sample prepared
for 140 min is the strongest, in accordance with the
above XRD analysis. The analysis indicates that the
prolonged heating time improves degree of crystallin-
Barium
Molybdenum
Oxygen
4b
4a
16f
0
0
0
0
0.5
0
0.76731 0.14013 0.08188
ity of the BaMoO particles. They can be concluded
4
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SYNTHESIS, CHARACTERIZATION AND OPTICAL PROPERTIES
729
200 nm
(a)
(b)
Ba
Mo
Ba
Mo
Ba
Mo
Mo
Ba
c
Ba
Mo
d(112)
Mo
Ba
a
b
10 nm
(c)
(d)
Fig. 3. TEM image, SAED pattern and HRTEM image of BaMoO processed for 140 min, respectively (a–c); simulated crystal
4
structure of BaMoO synthesized for 140 min (d).
4
that the high crystallinity plays an important role in absorption band at 3435 cm− is assigned to O–H
1
promoting the PL efficiency.
stretching vibration of residual water on the surface of
BaMoO [30].
4
Figure 5 shows the FTIR spectrum of BaMoO
4
−1
processed for 140 min over the 400–4000 cm wave-
Theory of interband absorption is a simple method
length range. BaMoO structure with tetrahedral sym- to explain the relationship between band structure and
4
energy band gap of the material [31]. The optical
absorption in UV-vis region was used to determine an
optical energy band gap which is related to the absorp-
tion coefficient (α) and the photonic energy (hν) as
follows
metry can be described the vibration modes as Γ =
A (ν ) + E(ν ) + F (ν ) + F (ν ) [27]. The strongest
1
1
2
2
3
2
4
−1
absorption band at 828 cm is identified to the F (ν )
internal mode, associated with antisymmetric stretch-
2
3
2
4
−
ing of Mo–O stretching in MoO tetrahedrons [28].
−1
n
The band at 1428 cm is attributed to the deformation
of C–H bond [29], implying that hydrocarbon still
remains as a small fraction in the final product. The
(αhν) = B(hν − E ) ,
(3)
0
where B is a constant related to the material and the
hydrocarbon is not investigated by XRD study because matrix element of the transition, E is the optical
0
its peaks were covered by XRD background. The broad energy gap, and n is an integer which corresponds with
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 63 No. 6 2018

7
30
ARRAK KLINBUMRUNG et al.
140 min
120 min
1428
3
435
60 min
40 min
828
400
450
500
Wavelength, nm
550
600
4
000 3500 3000 2500 2000 1500 1000 500
–1
Wavenumber, cm
Fig. 4. PL spectra of BaMoO4 processed by a 900 W
microwave induced plasma for different lengths of heating
time.
Fig. 5. FTIR spectrum of BaMoO processed for 140 min.
4
the characteristic of the transition process: n = 1/2 for
direct allowed, n = 2 for indirect allowed, n = 3/2 for
direct forbidden, and n = 3 for indirect forbidden tran-
sitions.
2
The plot of (αhν) versus (hν) of BaMoO synthe-
4
sized for 140 min shows in Fig. 6. The energy gap can
be determined by extrapolating the linear portion of
the plot to zero absorbance. The estimate E value of
0
BaMoO sample synthesized for 140 min heating time
4
is 3.35 eV which is closely to Thomas et al. report
(
3.20 eV) [32]. There are several factors that can play
the role in the energy gap, such as crystalline degree,
structure, impurities, crystallite size, product mor-
phologies and others.
2
3
4
5
Photon energy, eV
2
Fig. 6. The (αhν) versus photon energy plot of BaMoO
4
CONCLUSIONS
processed for 140 min.
BaMoO crystals were successfully synthesized by a
4
9
00 W microwave induced plasma method for 140 min
financial support under the administration of Materi-
als Science Research Center, Faculty of Science, Chi-
ang Mai University, and the Faculty of Science, Uni-
versity of Phayao through a general support.
heating time. The XRD pattern revealed that the as-
synthesized powder presents a tetragonal structure
without impurity detection. SEM image of 140 min
heating time shows that the particles were 200–700 nm
in diameter. HRTEM investigation exhibited the pre-
ferred (112) crystallographic growth. PL emission
peak of the pure product is in the blue region at around
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