Morphology-controlled synthesis of BaWO4 crystals via biominmetic SLM system

Article abstract of DOI:10.1016/j.jallcom.2008.09.110

BaWO₄ crystals with various morphologies had been synthesized via a biominmetic system of supported liquid membrane in the presence of different additive reagents at room temperature. The X-ray diffraction patterns showed that the obtained BaWO

Full text of DOI:10.1016/j.jallcom.2008.09.110

Journal of Alloys and Compounds 476 (2009) 571–574
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Morphology-controlled synthesis of BaWO4 crystals via biominmetic
SLM system
Feng-Qiang Donga, Qing-Sheng Wua,∗, Ya-Ping Dingb
a Department of Chemistry, Tongji University, Shanghai 200092, PR China
b Department of Chemistry, Shanghai University, Shanghai 200444, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2008
Received in revised form 3 September 2008
Accepted 10 September 2008
Available online 18 November 2008
BaWO4 crystals with various morphologies had been synthesized via a biominmetic system of supported
liquid membrane in the presence of different additive reagents at room temperature. The X-ray diffrac-
tion patterns showed that the obtained BaWO4 crystals were tetragonal structure. Scanning electron
microscopy results of the samples revealed that additive reagents and reacting time could highly affect
the morphology and size of BaWO4 crystals. In fluorescence spectra, there was an excellent blue-shift of
luminescence. The growth processes of BaWO4 crystals with different morphologies were investigated
and a possible growth mechanism was discussed.
Keywords:
Chemical synthesis
Crystal growth
X-ray diffraction
Luminescence
© 2008 Published by Elsevier B.V.
1. Introduction
researches show that this system has a good command of crystals’
shape control [18,19].
Morphology-controlled synthesis has rapidly developed into a
promising field in material chemistry because of their potential
applications in the design of new materials and devices [1–7]. Bar-
ium tungstate, as a kind of important blue luminescence materials,
has been attracted increasing attentions. Recently, BaWO4 crys-
tals with different morphologies and structures, such as powders,
hollow/solid spheres, nano-rods/wires, films/nanofilms and other
nano/microcrystals, have been synthesized.
Herein BaWO4 crystals with different morphologies were syn-
thesized via the similar synthetic method at room temperature.
The effects of varieties of experimental conditions on the synthesis
were examined carefully, which shed new light on the formation
mechanism. This method will be used for preparing more materials
with different morphologies and structures.
2. Experimental procedure
At present, morphology-controlled synthesis methods are
mainly metathetic reactions [8], templates [9], Czochralski method
[10], microemulsions or reverse mice [11–14], microwave irradi-
ation [15] and electrochemical method [16], etc. But there is no
report about biomimetic method through supported liquid mem-
brane (SLM) system to synthesize this kind of materials.
The supported liquid membrane system is composed of mem-
brane solvent, mobile carrier and porous polymer membrane. It not
only plays a role in transporting reversing-concentration-grades
ions like “K–Na pump”, but also in assembling inorganic materials
like biomineralization process guided by collagen. The SLM system
was used to separate noble metal ions from waster water and enrich
samples. Recently, our research group introduced this system into
material synthesis and design for the first time [17]. The following
2.1. Synthesis and purification
All reagents were analytical grade and used without further purification. In a
typical procedure, 0.2 g o-phenanthroline (Phen), as mobile carrier, was added into
80 mL chloroform under vigorous stirring at 3000 rpm for 15 min to form an oil
phase. Then dried polymer membranes (a kind of micro-pore film made by esterified
fibrin and with the pore diameter about 200–250 nm) were immersed into the oil
phase for 48 h to form SLM. The SLM was treated with filter paper to eliminate
remnant oil on the surface. Then the SLM was solidified in a reactor to separate it into
two compartments, which were put in vertical way. In the vertical SLM system, 20 mL
of 0.1 M Na2WO4 solutions and BaCl2 solutions were put in upside and downside.
3 mL of 0.01 M edetic acid, ethylenediamine (ED) and citric acid was added into the
side of Na2WO4 as additive reagent, respectively. White precipitation generated in
the upper side was collected by centrifugation after 24 h. Then, it was washed by
distilled water, acetone and absolute ethanol in turn. Finally, it was dried under
vacuum at 60 ◦C for further characterization.
2.2. Characterizations
∗
Corresponding author. Tel.: +86 21 65981097.
E-mail address: qswu@tongji.edu.cn (Q.-S. Wu).
The crystal phase and purity of the product are characterized by X-ray pow-
der diffraction (XRD, Bruker AXS, D8 Focus, Germany) equipped with graphite
0925-8388/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2008.09.110

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Fig. 1. SEM images of BaWO4 crystals prepared with different additive reagents: (a) no additive applied; (b) edetic acid applied; (c) ED applied; (d) citric acid applied.
monochromatized Cu K␣ radiation (ꢀ = 1.54056 Å), employing a scanning rate of
0.02◦ s−1. The operation voltage and current were maintained at 40 kV and 40 mA,
respectively. The morphologies and nanostructure of the as-synthesized products
are recorded by scanning electron microscopy (SEM, Philip XL30, Holand) at an accel-
erating voltage of 20 kV. Room temperature fluorescence spectra of the samples were
obtained by a Perkin-Elmer luminescence spectrophotometer (LS-55, USA) with a
Xe lamp as the excitation source.
were introduced, a lot of noises appear showed that crystal phase
became weaken and that the crystallinity was not very well. The
relative intensities of crystal plane changed greatly. The changing
relative intensity of corresponding diffraction crystal planes indi-
cated that the growth tendencies of crystal planes were changed.
Increases in relative intensity meant the growth of crystal plane
was promoted. Otherwise, the growth of crystal plane was con-
fined. On the other hand, the aggregation of crystal unit cells was
also affected by the space structure of modifiers. Consequently, the
crystal morphologies became different.
3. Results and discussion
Fig. 1 shows the effects of additive reagents on the morphologies
and sizes of BaWO4 crystallites. In the control experiments with-
out any crystal modifiers, the BaWO4 crystal was backbone-like
with regular folds on the surface and with the length of ca. 100 ␮m
(Fig. 1(a)). The morphologies and sizes of as-obtained BaWO4 crys-
tals changed remarkably when different additive reagents were
introduced (shown in Fig. 1(b–d)). Bamboo shoot-like BaWO4
crystal with length of ca. 20 ␮m was obtained when EDTA was
introduced as the modifier. Crystal surfaces were smoother and the
size became shorter and wider than those obtained without any
modifier. When ethylenediamine was used as the modifier, rod-
like crystals were typically synthesized. These crystals have length
of ca. 20–30 ␮m and the morphology became slim and sharp. The
crystal was flower cluster-like and with the length of ca. 1 ␮m when
citric acid was introduced.
In order to learn more about the crystal growth process, the
influences of additive reagents on crystal planes’ growth and its
formation mechanism, the effects of a variety of experimental
conditions on the synthesis were examined carefully. Here reac-
tion time and additive reagent (EDTA)’s dosage were varied in the
range of 12–60 h and 2–6 mL of 0.01 M, respectively. As shown
in Fig. 3(a–d), the products’ morphologies changed from shuttle-
like to ear-like with the increase of EDTA’s dosage and reaction
time. Meanwhile, the structures became more complex. At first,
From the above SEM images, the morphologies were changed
when different reagents were introduced. It was believed that the
changes of morphologies were related to additive reagents’ func-
tional groups and space structures. Therefore, morphology and size
could be tuned to meet the need of device design by introduc-
ing additive reagents with different functional groups and space
structures.
Fig. 2 presents the X-ray diffraction patterns of the samples.
According to the Powder Diffraction Standards (JCPDS) card No:
85–0588, all of them belonged to tetragonal structure. From SEM
images, it could be seen that when modifiers like EDTA, ethylene-
diamine and citric acid were introduced, some crystal planes
became ambiguous, which was consistent with what the XRD
curves showed. Especially when ethylenediamine and citric acid
Fig. 2. XRD patterns of BaWO4 crystals prepared with different additive reagents:
(a) no additive applied; (b) edetic acid applied; (c) ethylenediamine applied; (d)
citric acid applied.

F.-Q. Dong et al. / Journal of Alloys and Compounds 476 (2009) 571–574
573
Fig. 3. SEM images of BaWO4 crystals prepared by different reaction times and dosage of EDTA: (a) 24 h, 2 mL of 0.01 M; (b) 48 h, 2 mL of 0.01 M; (c) 48 h, 4 mL of 0.01 M; (d)
60 h, 6 mL of 0.01 M.
the BaWO4 crystal was shuttle-shape with smooth surface and with
the length of ca. 40 ␮m and the width 20 ␮m. When the reaction
time increased to 48 h, the product’s morphology became irregular
and the length–width ratio increases. When the additive reagent’s
dosage increased to 4 mL of 0.01 M for 48 h, the product’s mor-
phology became bamboo shoot-like. With the further increase of
reaction time and additive reagent’s dosage, the product’s mor-
phology became complex ear-like and size increases obviously to
80 ␮m. Therefore, Fig. 3(b and c) could be taken as the intermediate
products of Fig. 3(d). The ear-shaped BaWO4 macro-crystals form
from shuttle-shaped particles through an oriented particle aggre-
gation process. Therefore, we proposed the structures of BaWO4
crystals were self-assembled by lots of BaWO4 nanoparticles and
the BaWO4 crystals aggregated bigger with the increase of reac-
tion time [20], which resulted in the weakening control of additive
reagent EDTA [21].
plexes were transported to the other side of SLM system, and
2−
then captured by WO4
ions. This would make the solution
supersaturated easily, which in turn affected BaWO4’s nucle-
ation procedure. The formation process of BaWO4 crystals was
a typical “Oswald ripening process” [22]. The formation of tiny
crystalline nuclei in a supersaturated medium occurs at first
and then was followed by crystal growth, in which the larger
particles would grow at the cost of the small ones due to the
energy difference between large particles and small particles of
a higher solubility based on the Gibbs–Thompson law.
(2) The cooperation template effect of SLM and crystal modi-
fiers also had some effects on the BaWO4 nucleation process.
When the additive reagents were introduced into the SLM sys-
tem, chelation would prevent them from growing and “Oswald
ripening process”. The formation process of BaWO4 crystals
turned to aggregation and “oriented attachment”. Under the
presence of additive reagents, the functional groups of additive
reagent affected the crystal cell’s growth tendency which could
be seen in XRD data; the space structure of additive reagent
affect the unit cells aggregation resulted in crystal morpholo-
gies change which could be seen in SEM images in Figs. 2 and 3.
Fig. 4 presents the space structure of additive reagents.
Based on the above-obtained SEM and XRD results, it’s clear that
the morphologies and sizes of BaWO4 crystals could be controlled
by adding different reagents into the SLM system and a possible
mechanism was shown as follows:
(1) The SLM system itself had some effect on the BaWO4 nucleation
process, which might be due to the transportation and selec-
tive combination of biomembrane. At first, Ba2+ ions combine
with the Phen carriers to form Ba2+–Phen complex. The com-
EDTA and citric acid belonged to carboxylic compounds while
EDTA and ethylenediamine contain N atoms, and their space con-
Fig. 4. The ball and stick model of the crystal modifiers: (a) EDTA; (b) ethylenediamine; (c) citric acid.

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4. Conclusion
In summary, BaWO4 crystals with tetragonal structure were syn-
thesized by SLM system at room temperature. The morphologies
and sizes of BaWO4 crystals could be controlled through adding
different reagents into SLM system, which could be further mod-
ified by adjusting additive reagents’ concentration and reaction
time. The as-synthesized BaWO4 crystals had some potential appli-
cations because of their excellent blue-shift luminescence. This
method was important to widen biomineralization field, and also
provided a new route for materials’ control synthesis.
Acknowledgements
We acknowledge the financial support of the National Natu-
ral Science Foundation (No. 50772074) of China, the State Major
Research Plan (973) of China (No. 2006CB932302), and the Nano-
Foundation of Shanghai in China (No. 0852nm01200).
Fig. 5. PL spectra of as-prepared BaWO4 crystals (ex = 300 nm).
figuration was different. EDTA was like two pincers of a crab. Citric
acid was just like a triangle taper whose bottom was an isosce-
les triangle [23]. Ethylenediamine was like a linear-shape. When
carboxylic group or amido group was chelated with Ba2+ under
coordination or electrostatic effect, the single bonding in the chain
nearby would rotate to get the lowest energy, which resulted in
different inhibition of space matching. In other words, the above-
mentioned differences gave rise to the diversity on the degree
and position of crystal plane development. Their difference led to
the morphology change certainly; the effect was due to molecular
recognition, space structure complement, or static effect. There-
fore, the product’s morphology was rod-shape under the present
of ethylenediamine, which was also reported in the literature [24].
On the other hand, the product’s morphology was complex under
the present of EDTA and citric acid.
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crystals with the additive reagent of 3 mL of 0.01 M EDTA for 48 h
reaction time. As shown in Fig. 5, it could be seen that the blue-
shifted emitting peaks were at 328 nm and 396 nm when excited
at 300 nm, whereas those of BaWO4 nanospheres with diameter of
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to the quantum size effect or crystalline imperfection and disorder
[23]. From the XRD data in Fig. 1(b), the average crystallite size cal-
culated from Scherrer’s formula was 46 nm. We supposed that the
as-prepared BaWO4 crystals were self-assembled by nanoparticles,
which was in good agreement with the proposed growth mecha-
nism and the calculated results of XRD data. The as-synthesized
BaWO4 crystals with excellent blue-shift of luminescence might be
have some potential applications in photoelectric filed and device
design.
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