Microwave-assisted synthesis of barium molybdate by a citrate complex method and oriented aggregation

Article abstract of DOI:10.1016/j.materresbull.2005.04.032

BaMoO₄ powders, which have scheelite type structure, were successfully synthesized at low temperatures by a modified citrate complex method assisted by microwave irradiation. The citrate complex precursors were heat-treated at temperatures from 300 to 500 °C for 3 h. Crystallization of the BaMoO₄ powders were detected at 350 °C, and completed at a temperature of 400 °C. TEM image of the BaMoO₄ product obtained above 400 °C revealed spindle-rods-like or flake-like morphology. The anisotropic growth habit of BaMoO₄ leads to the oriented aggregation, which is attributed to the high chemical potentials of the intrinsic structure of BaMoO₄. The BaMoO₄ powder prepared at 500 °C showed the strongest photoluminescent intensity.

Full text of DOI:10.1016/j.materresbull.2005.04.032

Materials Research Bulletin 40 (2005) 1468–1476
www.elsevier.com/locate/matresbu
Microwave-assisted synthesis of barium molybdate by
a citrate complex method and oriented aggregation
a,
Jeong Ho Ryu , Jong-Won Yoon , Chang Sung Lim , Kwang Bo Shim
a
b
a
*
a
Department of Ceramic Engineering, CPRC, Hanyang University, 17 Haengdang-dong,
Seongdong-gu, Seoul 133-791, Korea
Department of Adv. Mat. Sci. & Eng., CPRC, Hanseo University, Seosan 356-706, Korea
b
Received 30 September 2004; received in revised form 17 March 2005; accepted 29 April 2005
Abstract
BaMoO powders, which have scheelite type structure, were successfully synthesized at low temperatures by a
4
modified citrate complex method assisted by microwave irradiation. The citrate complex precursors were heat-
treated at temperatures from 300 to 500 8C for 3 h. Crystallization of the BaMoO powders were detected at 350 8C,
4
and completed at a temperature of 400 8C. TEM image of the BaMoO product obtained above 400 8C revealed
4
spindle-rods-like or flake-like morphology. The anisotropic growth habit of BaMoO leads to the oriented
aggregation, which is attributed to the high chemical potentials of the intrinsic structure of BaMoO . The BaMoO
4
4
4
powder prepared at 500 8C showed the strongest photoluminescent intensity.
2005 Elsevier Ltd. All rights reserved.
#
Keywords: A. Ceramics; B. Chemical synthesis; C. Electron microscopy; D. Luminescence
1
. Introduction
BaMoO is important inorganic material of the metal molybdate families that have a high application
potential in various fields, such as in photoluminescence [1] and hosts for lanthanide activated lasers [2].
4
˚
Metal molybdates of relatively large bivalent cations (MMoO , ionic radius > 0.99 A, M = Ca, Ba, Pb,
4
Sr) exist in the so-called scheelite structure form (scheelite = CaWO ), where the molybdenum atom
4
*
Corresponding author. Tel.: +82 2 2220 0543; fax: +82 2 2299 2884.
E-mail address: jimihen@ihanyang.ac.kr (J.H. Ryu).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.04.032

J.H. Ryu et al. / Materials Research Bulletin 40 (2005) 1468–1476
1469
adopts tetrahedral coordination [3]. Scheelite-type molybdates of the same divalent metal are recipro-
cally soluble over the entire compositional range, which results in a rich family of solid-state solution
compounds.
BaMoO has usually been prepared by conventional methods consisting of repeated ball-milling and
4
solid-state reaction of BaCO and MoO powders [4]. However, BaMoO powders prepared by the solid-
3
3
4
state reaction are relatively large with inhomogeneous morphology and composition. Inhomogeneous
compounds of BaMoO might be easily formed because the MoO has a tendency to vaporize at high
temperatures [5].
4
3
These problems could be solved by applying advanced wet chemical methods. Polymerized complex
method as a modified Pechini method [6], where several metal ions in a solution could be first chelated to
form metal complexes and then polymerized to form a gel, seems to be most suitable among chemical
solution processes, because rigidly fixed cations are homogeneously dispersed in the polymer network
and have few chances to segregate even during pyrolysis. However, in spite of many advantages of the
polymeric complex method, the weakness of this method is the difficulty of the effective removal of the
large amount of organic substances [7]. Based on this consideration, the citrate complex method with
different solvents as chemical solution process was tried in this work for the synthesis of BaMoO4
powders. In this process, metal citrate complexes without a network structure are formed by using water
instead of ethylene glycol.
On the other hand, microwave irradiation as a heating source has been found and developed for a
number of applications in chemical and ceramic processing [8–10]. Compared with the usual methods,
microwave synthesis has the advantages of shortening reaction time, giving products with small particle
size, narrow particle size distribution and high purity. Jansen et al. [8] suggested that these advantages
could be attributed to fast homogeneous nucleation and easy dissolution of the gel.
In this work, we report the synthesis of BaMoO powders from citrate complex precursor using
4
microwave irradiation. The precursors and powders were evaluated through their crystallization process,
thermal decomposition and by identifying the particle morphology. Also, effects of crystallization
process and microstructure on the photoluminescent property were examined.
2
. Experimental
Barium carbonate (BaCO , Junsei Chemical Co. LTD., Japan) and ammonium molybdate
3
(
(NH ) Mo O Á4H O, Junsei Chemical Co. LTD., Japan) were used as the metallic cations. De-ionized
4
6
7
24
2
water(DW) and citric acid (HOC(CO H)(CH CO H) , CA, Yukiri Pure Chemical Co, LTD., Japan) were
2 2 2 2
used as the solvent and chealating agent for the process. Fig. 1 shows the schematic flow chart for the
synthesis of BaMoO powders by the citrate complex method using microwave irradiation. The citrate
4
solution was prepared by dissolving appropriate molar ratios of citric acid in de-ionized water (CA:DW
molar ratio = 1:4). After complete homogenization of the citrate solution, barium carbonate and
ammonium molybdate were dissolved in the molar ratio of total chelate metal cations (TO) and citric
acid (TO:CA molar ratio = 1:5). By keeping the solution at a temperature of 100 8C for 1 h under
constant stirring, the solution became viscous. A domestic microwave oven with 1200 W (Samsung
Electronic Corp., Korea 2.45 GHz) was conducted for the reactions in the solution. The solution was
placed in the microwave oven and the reaction was performed under ambient air for 30 min. The working
cycle of the microwave oven was set between 40 s on and 20 s off. The solution became more viscous

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J.H. Ryu et al. / Materials Research Bulletin 40 (2005) 1468–1476
Fig. 1. Flow chart for synthesis of BaMoO powders by the modified citrate complex method using microwave irradiation.
4
with time and changed its color to brown. No visible precipitation was observed during the heating
process. As this solution condensed, the brown product was converted into powders after grinding with a
teflon bar. Thermal analysis was performed on this powder, hereinafter referred to as the ‘precursor’.
Heat-treatment of the precursor was performed at various temperatures from 300 to 500 8C for 3 h.
The crystallization process of the polymeric precursor was evaluated by thermogravimetry-differential
thermal analysis (TG-DTA, SETRAM, France), using a sample weight of about 10 mg and a heating rate
of 5 8C/min. The existing phase in the particles after heat-treatment was identified by ordinary X-ray
diffraction (XRD, Cu Ka, 40 kV, 30 mA, Rigaku, Japan) with a scan rate of 3 8/min. The microstructure
and surface morphology of the powders were observed by transmission electron microscopy (TEM, JEM
2010, JEOL). The photoluminescent (PL) spectra were analyzed by a Japan Hitachi 850 Spectro-
photometer.
3
. Results and discussion
3
.1. General synthesis of BaMoO powders
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The crystallization process of the precursor was evaluated by XRD, TG-DTA and electronic
diffraction pattern (EDP). Fig. 2 shows the phase identification of the BaMoO powders heat-treated
4
for 3 h as a function of heating temperature in detail using XRD. In Fig. 2a, the powders of BaMoO at
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Fig. 2. XRD patterns of the BaMoO powders heat-treated at (a) 300; (b) 350; (c) 400; (d) 450 and (e) 500 8C for 3 h.
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3
be identified as BaMoO phases, and un-reacted or additional phases were not detected.
00 8C were amorphous without any crystallized phases. Above 350 8C in Fig. 2b–e, the powders could
4
Fig. 3 shows the TG-DTA curves for the BaMoO precursor. In Fig. 3, with the increase of
4
temperature, the weight loss occurs in the TG curve up to 470 8C. Thereafter, the weight remains
constant, indicating that the decomposition of all organic materials continued in the precursor, their
combustion and crystallization of BaMoO have been below 470 8C. No significant plateau, correspond-
4
ing to well-defined intermediate products, appeared in the heating process. The DTA curve in Fig. 3 could
be interpreted two physical meaning: (1) increase of DT curve from at 350 8C corresponds to initial
decomposition of the precursor and nucleation of the nanocrystlline BaMoO ; and (2) exothermic peak at
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4
brown and porous in structure, denoting an amorphous phase. It is attributed to contain a lot of carbons
45 8C corresponds to the crystallization of BaMoO . Below 350 8C, the resultant particles were dark
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4
Fig. 3. TG-DTA curves of the BaMoO precursor in flowing air.

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and ignitable organics. When the temperature further increases above 350 8C, crystal nuclei begin to
form, and consequently the primary crystallization process is completed accompanying the combustion
of the residual carbons and ignitable organics.
Fig. 4 shows electronic diffraction pattern (EDP) and morphology change of the BaMoO powders
4
prepared from 300 to 500 8C. The EDP of BaMoO powders heat-treated at 300 8C in Fig. 4a shows only
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diffuse hollow rings, corresponding to an amorphous phase. At 350 8C in Fig. 4b, dotted rings were
observed, signifying the nanocrystalline formation. In Fig. 4c, the products obtained at 400 8C reveal
spindle-rods-like aggregates, which are composed of tiny nanoparticles. In Fig. 4d, the BaMoO products
obtained at 500 8C clearly reveal flake-like aggregations with an average diameter of between 50 and
4
100 nm and lengths up to 500 nm. The selected area electron diffraction (SAED) pattern (Fig. 4e) taken
from a single flake-like granule reveals the oriented-crystalline nature of the sample with preferential
aggregating direction.
3
.2. Formation mechanism of oriented aggregation of BaMoO4
Fig. 5 shows that two-dimensional schematic diagram of possible arrangement process of BaMoO4
powders for the assembly morphology as a function of heating temperature. In general, when
synthesizing nanoparticles from precursor, nucleation is very fast, and subsequent growth occurs
by two primary mechanisms (Fig. 5): coarsening (also known as Ostwald ripening [11]) and
aggregation involving coarsening [12–16]. Coarsening processes contrive the growth of larger crystals
at the expense of smaller crystals. As the chemical potential of a particle increases with decreasing
particle size, the equilibrium concentration near a small particle becomes higher than near a large
particle, as described by the Gibbs–Thomson equation [17]. Crystal growth from aggregation can
occur, producing particle assemblies accumulated from randomly oriented to highly oriented nano-
particles [12–14]. Oriented aggregation [15–16] is an important mode for crystal growth. Primary
particles may aggregate in an oriented accumulation to produce a larger particle, or they may aggregate
randomly.
The present system is a typical oriented aggregation process involving coarsening process.
Amorphous fine particles in Fig. 4a acted as the precursor for the synthesis of crystallized BaMoO4.
Nanocrystalline nuclei were formed in the precursor following the coarsening at 350 8C in Fig. 4b. The
larger particles grew at the cost of the small ones, due to the difference of surface energy between the
large particles and the smaller particles, according to the well-known Gibbs–Thomson law [17]. At
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00 8C, in Fig. 4c the spindle-rods-like aggregates were coexisted with different lengths and irregular
morphology of nanoparticles. As the reaction proceeded at a higher temperature in Fig. 4d, the
irregular nanopaticles disappeared and flake-like granules were formed. It is considered that the flake-
like granules were formed by a horizontal assembly of the spindle-rods-like aggregates as shown in
Fig. 5.
The anisotropic aggregate habit of spindle-rods-like and flake-like granules can be considered under
the viewpoint of the intrinsic structure of the molybdates. Fig. 6 shows the BaMoO structure viewed
4
along the a-axis for the structural feature of the (1 0 0) faces. The alternative infinite chain structures can
be clearly seen from the molecule modeling results. In particular, the special feature of the (1 0 0) face
2
and all the negative [MoO4 ] chain lies on the (1 0 0) face. All the negative [MoO ] anions are
À
2À
4
2+
exposed, while all the positive Ba ions are shielded [3]. According to the Gibbs–Thomson theory, the
relative chemical potential of crystals is simply proportional to their surface-atom ratio, which is

J.H. Ryu et al. / Materials Research Bulletin 40 (2005) 1468–1476
1473
Fig. 4. TEM and EDP of BaMoO
electron diffraction pattern of the flake-like granule.
4
powders heat-treated at (a) 300; (b) 350; (c) 400; and (d) 500 8C for 3 h. (e) The selected area

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J.H. Ryu et al. / Materials Research Bulletin 40 (2005) 1468–1476
Fig. 5. Schematic diagram of two-dimensional representation of aggregation growth mechanism.
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Fig. 6. BaMoO structure viewed along the a-axis for the structural feature of the (1 0 0) faces.

J.H. Ryu et al. / Materials Research Bulletin 40 (2005) 1468–1476
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determined by the average dangling bonds per atom over the entire crystal. It is considered that one-
dimensional growth and aggregate occur only if the chemical potential of the monomers in the
precursor is much higher than the highest chemical potential of the atoms on the surface of the
nanocrystals, which are the two unique facets perpendicular to the c-axis in the case of CdSe
nanocrystals [18]. The (1 0 0) faces of BaMoO contain units fully composed of highly distorted
4
tetrahedral molybdenum with dangling bonds, whereas the deformed octahedral Ba-chain was
shielded by the chains of tetrahedral-Mo. These chains exist along the a-axis alternatively, which
makes the (1 0 0) face perpendicular to the a-axis with the highest chemical potential among other
facets, such as (0 0 1) and (0 1 0) [19]. This structural feature makes the (1 0 0) faces very active to the
surrounding growth conditions.
3
.3. Photoluminescent properties of BaMoO powders
4
Room-temperature photoluminescent properties of the prepared BaMoO were studied in Fig. 7. The
4
photoluminescence of BaMoO powders has been discussed in the frame of molecular orbital models of
4
2À
the MoO4 radicals [20]. It is generally assumed that the measured emission spectrum of BaMoO is
4
2À
mainly attributed to the charge-transfer transitions within the MoO4 complex. With the excited
wavelength at 240 nm, the powders exhibited blue emission peaks of 385 nm in Fig. 7. The intensity of
PL emission in BaMoO powders prepared at 500 8C was stronger than those of other temperatures,
4
indicating the great dependence of the PL properties on the morphology and crystallinity of the BaMoO4
powders. The PL intensity of phosphors depends strongly on the particle shape and crystallinity.
Generally, for the similar morphological samples, the homogenized particle must be favorable to
luminescent characteristics because of less contamination or fewer dead layers on the phosphor surface
[21]. The enhancement of PL intensity at 500 8C may be because the sample treated at 500 8C has a
higher crystallinity than those of the other temperatures. It may also be because the sample at 500 8C has
higher uniformity and homogeneously oriented aggregation morphology.
Fig. 7. Emission spectra of the BaMoO heat-treated at (a) 350; (b) 400; and (c) 500 8C for 3 h.
4

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. Summary
BaMoO powders were successfully synthesized at low temperature by the modified citrate complex
4
method using microwave irradiation. Crystallization of BaMoO powders were detected at 350 8C. Most
4
of the BaMoO powders heat-treated between 400 and 500 8C showed primarily aggregated morphology.
4
The oriented aggregation was attributed to the extremely high anisotropic chemical potentials of the
intrinsic structure of the barium molybdate. The powders exhibited blue PL emission peaks of 385 nm
and the sample prepared at 500 8C showed the strongest intensity, which was attributed to the higher
crystallinity and homogeneously oriented aggregation morphology.
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