Microwave-assisted synthesis of BaMoO4 nanocrystallites by a citrate complex method and their anisotropic aggregation

Article abstract of DOI:10.1016/j.jallcom.2005.07.010

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 heat-treated at 400 °C revealed spindle-rods-like and flake-like morphology at 500 °C. The anisotropic aggregation habit of BaMoO₄ lead to two-dimensionally aggregated powder morphology, which was attributed to the high chemical potentials of the intrinsic structure of BaMoO₄.

Full text of DOI:10.1016/j.jallcom.2005.07.010

Journal of Alloys and Compounds 413 (2006) 144–149
Microwave-assisted synthesis of BaMoO4 nanocrystallites by a citrate
complex method and their anisotropic aggregation
Jeong Ho Ryu∗, Jong-Won Yoon, Kwang Bo Shim
Department of Ceramic Engineering, Ceramic Processing Research Center (CPRC), Hanyang University,
17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea
Received 13 November 2004; accepted 12 January 2005
Available online 22 August 2005
Abstract
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 heat-treated at 400 ◦C revealed spindle-rods-like and flake-like morphology at 500 ◦C. The anisotropic aggregation habit of BaMoO4
lead to two-dimensionally aggregated powder morphology, which was attributed to the high chemical potentials of the intrinsic structure of
BaMoO4.
© 2005 Elsevier B.V. All rights reserved.
Keywords: BaMoO4; Citrate complex method; Anisotropic aggregation; Assembling process
1. Introduction
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 polymerized complex
method, the weakness of this method is the difficulty of
the effective removal of the large amount of organic sub-
stances [6]. 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 nanopow-
ders. In this process, metal citrate complexes without a net-
work 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 appli-
cations in chemical and ceramic processing [7–9]. Compared
with the usual methods, microwave synthesis has the advan-
tages of shortening reaction time, giving products with small
particle size, narrow particle size distribution and high purity.
Jansen et al. [7] suggested that these advantages could be
attributed to fast homogeneous nucleation and easy dissolu-
tion of the gel.
BaMoO4 is an important inorganic material that has a high
application potential in various fields, such as in photolu-
minescence [1] and hosts for lanthanide-activated lasers [2].
BaMoO4 has usually been prepared by conventional methods
consisting of repeated ball-milling and solid-state reaction of
BaCO3 and MoO3 powders [3]. However, BaMoO4 pow-
ders prepared by the solid-state reaction are relatively large
with inhomogeneous morphology and composition. Inhomo-
geneous compounds of BaMoO4 might be easily formed
because MoO3 has a tendency to vaporize at high temper-
atures [4].
These problems could be solved via an advanced wet
chemical route. Polymerized complex method as a modified
Pechini method [5], 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
∗
Corresponding author. Tel.: +82 2 2220 0543; fax: +82 2 2299 2884.
E-mail address: jimihen@ihanyang.ac.kr (J.H. Ryu).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.07.010

J.H. Ryu et al. / Journal of Alloys and Compounds 413 (2006) 144–149
145
Inthiswork, wereportthesynthesisofBaMoO4 nanopow-
ders from citrate complex precursor assisted by microwave
irradiation and focus on the aggregation behavior of BaMoO4
nanopowders as a function of heating temperature. The pre-
cursors and powders were evaluated through their crystalliza-
tion process, thermal decomposition and by identifying the
particle morphology.
oven was set between 40 s on and 20 s off. The solution
became more viscous with time and its color changed to
brown. No visible precipitation was observed during the heat-
ing 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 ◦C
for 3 h.
The crystallization process of the polymeric precursor was
evaluated by thermogravimetry–differential thermal analy-
sis (TG–DTA, SETRAM, France), using a sample weight
of about 10 mg and a heating rate of 5 ◦C/min. The exist-
ing phase in the particles after heat-treatment was identified
by ordinary X-ray diffraction (XRD, Cu K␣, 40 kV, 30 mA,
Rigaku, Japan) with a scan rate of 3 ◦/min. The microstruc-
tureandsurfacemorphologyofthepowderswereobservedby
transmission electron microscopy (TEM, JEM 2010, JEOL).
2. Experimental
Barium carbonate (BaCO3, Junsei Chemical Co. Ltd.,
Japan) and ammonium molybdate tetrahydrate ((NH4)6Mo7
O24·4H2O, Fluka Chemical Co. Ltd., Japan) were used as
the metallic cations. De-ionized water (DW) and citric acid
(HOC(CO2H)(CH2CO2H)2, CA, Yukiri Pure Chemical Co.
Ltd., Japan) were used as the solvent and chelating agent for
the process. Fig. 1 shows the schematic flow chart for the
synthesis of BaMoO4 powders by the modified citrate com-
plex method assisted by microwave irradiation. The citrate
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, bar-
ium carbonate and ammonium molybdate tetrahydrate 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 ◦C for 1 h under con-
stant stirring, the solution became viscous. A microwave
irradiation system with 1200 W (2.45 GHz) was used 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
3. Results and discussion
3.1. Synthesis of BaMoO4 nanopowders and
crystallization process
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 BaMoO4 pow-
ders heat-treated for 3 h as a function of heating temperature
in detail using XRD. In Fig. 2(a), the powders of BaMoO4
at 300 ◦C were amorphous without any crystallized phases.
Above 350 ◦C in Fig. 2(b–e), the powders could be identified
as BaMoO4 phases, and unreacted or additional phases were
not detected.
Fig. 3 shows the TG–DTA curves for the BaMoO4 precur-
sor. In Fig. 3, with the increase of temperature, the weight loss
occurs in the TG curve up to 470 ◦C. Thereafter the weight
Fig. 1. Flow chart for synthesis of BaMoO4 powders by the modified citrate
complex method using microwave irradiation.
Fig. 2. XRD patterns of the BaMoO4 powders heat-treated at (a) 300, (b)
350, (c) 400, (d) 450 and (e) 500 ◦C for 3 h.

146
J.H. Ryu et al. / Journal of Alloys and Compounds 413 (2006) 144–149
curve in Fig. 3 could be interpreted by two physical mean-
ings: (1) increase of DT curve from 350 ◦C corresponds to
initial decomposition of the precursor and nucleation of the
nanocrystalline BaMoO4 and (2) exothermic peak at 445 ◦C
corresponds to the crystallization of BaMoO4. Below 350 ◦C,
the resultant particles were dark brown and porous in struc-
ture, denoting an amorphous phase. It is attributed to contain
a lot of carbons and ignitable organics. When the temperature
further increases above 350 ◦C, crystal nuclei begin to form
and consequently the primary crystallization process is com-
pleted accompanying the combustion of the residual carbons
and ignitable organics.
Fig. 4 shows electronic diffraction pattern and morphol-
ogy change of the BaMoO4 powders prepared from 300 to
500 ◦C. The EDP of BaMoO4 powders heat-treated at 300 ◦C
in Fig. 4(a) shows only diffuse hollow rings, corresponding
to an amorphous phase. At 350 ◦C in Fig. 4(b), dotted rings
were observed, signifying the nanocrystalline formation. In
Fig. 4(c), the products obtained at 400 ◦C reveal spindle-rods-
like aggregated morphology, which are composed of tiny
nanoparticles. In Fig. 4(d), the BaMoO4 products obtained
at 500 ◦C clearly reveal flake-like morphology with an aver-
Fig. 3. TG–DTA curves of the BaMoO4 precursor in flowing air.
remains constant, indicating that the decomposition of all
organic materials continued in the precursor, their combus-
tion and crystallization of BaMoO4 have been below 470 ◦C.
No significant plateau, corresponding to well-defined inter-
mediate products, appeared in the heating process. The DTA
Fig. 4. TEM and EDP of BaMoO4 powders heat-treated at (a) 300, (b) 350, (c) 400 and (d) 500 ◦C for 3 h.

J.H. Ryu et al. / Journal of Alloys and Compounds 413 (2006) 144–149
147
age diameter of between 50 and 100 nm and lengths up to
500 nm.
At 400 ◦C, in Fig. 4(c) the spindle-rods-like aggregates
coexisted with different lengths and irregular morphology
of nanoparticles. This anisotropic aggregation habit can be
considered under the viewpoint of the intrinsic structure of
the BaMoO4. Fig. 6 shows the BaMoO4 structure viewed
along the a-axis for the structural feature of the (1 0 0) faces.
In particular, the special feature of the (1 0 0) face and all the
negative [MoO42−] chain lies on the (1 0 0) face. All the nega-
tive [MoO42−] anions are exposed, while all the positive Ba2+
ions are shielded [17]. The (1 0 0) faces of BaMoO4 contain
units fully composed of highly distorted tetrahedral-Mo 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) [18].
This structural feature makes the (1 0 0) faces very active to
the surrounding growth conditions. It is this structural fea-
ture that plays a key role in the highly intrinsic preferential
growth along the axis of the BaMoO4 aggregates.
As the reaction proceeded at a higher temperature in
Fig. 4(d), the irregular nanoparticles disappeared and flake-
like aggregates were formed. It is considered that the flake-
like aggregates were formed by a horizontally assembling
process of the spindle-rods-like aggregates as shown in
Fig. 5(d) and (e). Fig. 7(a) shows a typical TEM image of
aggregated BaMoO4 particles prepared at 400 ◦C. The upper
image shows a spindle-rods-like crystal constructed from at
least several primary BaMoO4 nanocrystallites. Schematic
white-dotted outlines in below image show one possible
attachment scheme that could give rise to the observed one-
dimensionally anisotropic aggregation morphology. Fig. 7(b)
showsanelongatedandassembledcrystalpreparedat500 ◦C.
It is constructed of three primary aggregated building blocks.
The white-dotted lines in the bottom image highlight the four
regions of the assembled flake-like aggregate.
3.2. Formation mechanism of anisotropic aggregation in
BaMoO4 system
In general, when synthesizing nanoparticles from precur-
sor, nucleation is very fast and subsequent growth occurs by
twoprimarymechanisms:coarsening(alsoknownasOstwald
ripening [10]) and aggregation process [11–15]. Coarsening
processes contrive the growth of larger crystals at the expense
of smaller crystals as described by the Gibbs–Thomson equa-
tion [16]. Crystal growth from aggregation can occur, produc-
ing particle assemblies accumulated from randomly oriented
to highly oriented nanoparticles [11–13]. Anisotropic aggre-
gation is an important mode for crystal growth. Primary par-
ticles may aggregate in an oriented accumulation to produce
larger particles, or they may aggregate randomly [14,15].
The present system is a typical oriented aggregation
process involving coarsening process. Fig. 5 shows that
schematic diagram of a possible arranging process of
BaMoO4 nanopowders for the two-dimensionally assembled
morphology as a function of heating temperature. Amor-
phous nanoparticles in Fig. 4(a) acted as the precursor for the
synthesis of BaMoO4 nanopowders. Nanocrystalline nuclei
were formed in the precursor following the coarsening at
350 ◦C in Fig. 4(b). And 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, accord-
ing to the well-known Gibbs–Thomson law [16].
Fig. 5. Schematic diagram oftwo-dimensional representationofaggregation
growth mechanism.
Fig. 6. BaMoO4 structure viewed along the a-axis for the structural feature
of the (1 0 0) faces.

148
J.H. Ryu et al. / Journal of Alloys and Compounds 413 (2006) 144–149
Fig. 7. TEM image of aggregated BaMoO4 particles heat-treated at (a) 400 and (b) 500 ◦C for 3 h.
4. Summary
[2] A.M. Lejus, R. Collongues, in: E. Kaldis (Ed.), Current Topics in
Materials Science, vol. 4, North-Holland Publishing Co., Amsterdam,
1980, p. 481.
BaMoO4 powders were successfully synthesized at low
temperature by the modified citrate complex method using
microwave irradiation. Crystallization of BaMoO4 powders
were detected at 350 ◦C and completed at 400 ◦C. The prod-
ucts obtained at 400 ◦C revealed spindle-rods-like aggre-
gation. The anisotropic aggregation was attributed to the
extremely high anisotropic chemical potentials of the intrin-
sic scheelite structure. As the reaction proceeded at a higher
temperature, 500 ◦C, flake-like aggregates were observed,
which were formed by a horizontal assembly of the spindle-
rods-like aggregates.
[3] C. Pupp, R. Yamdagni, R.F. Porter, J. Inorg. Nucl. Chem. 31 (1969)
2021–2029.
[4] W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M.
Yoshimura, J. Am. Ceram. Soc. 80 (3) (1997) 765–769.
[5] M.P. Pechini, U.S. Patent No. 3330697, July 11, 1967.
[6] S. Yamamoto, M. Kakihana, S. Kato, J. Alloys Compd. 297 (2000)
81–86.
[7] C. Jansen, A. Arafat, A.K. Barakat, H. Van Bekkum, in: M.L.
Occelli, H. Robson (Eds.), Synthesis of Microporous Materi-
als, vol. 1, Van Nostrand Reinhold, New York, 1992, pp. 507–
508.
[8] T. Ohgushi, K. Ishimaru, J. Am. Ceram. Soc. 84 (2) (2001) 321–
327.
[9] R. Roy, S. Komarneni, J.L. Yang, J. Am. Ceram. Soc. 68 (1985)
392–395.
References
[10] E.M. Wong, J.E. Bonevich, P.C. Searson, J. Phys. Chem. B 102
(1998) 7770–7775.
[11] V. Privman, D.V. Goia, J. Park, E. Matijevic, J. Colloid Interface
Sci. 213 (1999) 36–45.
[1] F.A. Kroger, Some Aspect of the Luminescence of Solids, Elsevier,
Amsterdam, 1948, p. 107.

J.H. Ryu et al. / Journal of Alloys and Compounds 413 (2006) 144–149
149
[12] M. Ocana, M.P. Morales, C.J. Serna, J. Colloid Interface Sci. 171
(1995) 85–91.
[16] J.W. Mullin, Crystallization, third ed., Butterworth–Heinemann,
Oxford, 1997.
[13] R.L. Penn, J.F. Banfield, Science 281 (1998) 969–971.
[14] J.F. Banfield, S.A. Welch, H.Z. Zhang, T.T. Ebert, R.L. Penn, Science
289 (2000) 751–754.
[17] S.H. Yu, B. Liu, M.S. Mo, J.H. Huang, X.M. Liu, Y.T. Qian, Adv.
Funct. Mater. 13 (8) (2003) 639–647.
[18] T.G. Cooper, N.H. de Leeuw, Surf. Sci. 531 (2003) 159–
176.
[15] A.P. Alivisatos, Science 289 (2000) 736–737.