Low-temperature synthesis of metal tungstates nanocrystallites in ethylene glycol

Article abstract of DOI:10.1016/j.materresbull.2003.09.004

In this paper, we report the low-temperature synthesis of metal tungstate, MWO₄ (M=Ca, Sr, Ba, Cd, Zn, Pb) nanocrystallites. By reaction between metal chloride and sodium tungstate in ethylene glycol at 180°C for 10h, well-crystallized tungstate particles were successfully obtained. Characterization by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) shows that the product powders consist of nanosize particles. Photoluminescence measurement reveals that the as-obtained CaWO₄, CdWO₄, and PbWO₄ show excitonic peaks at about 430, 500 and 500nm, respectively. The solvent and reaction conditions are important in the formation of the products.

Full text of DOI:10.1016/j.materresbull.2003.09.004

Materials Research Bulletin 38 (2003) 1783–1789
Low-temperature synthesis of metal tungstates
nanocrystallites in ethylene glycol
*
Di Chen, Guozhen Shen, Kaibin Tang , Huagui Zheng, Yitai Qian
Structure Research Laboratory, Department of Chemistry, University of Science and
Technology of China, Hefei 230026, PR China
Received 17 May 2003; received in revised form 25 August 2003; accepted 10 September 2003
Abstract
In this paper, we report the low-temperature synthesis of metal tungstate, MWO (M ¼ Ca, Sr, Ba, Cd, Zn, Pb)
4
nanocrystallites. By reaction between metal chloride and sodium tungstate in ethylene glycol at 180 8C for 10 h,
well-crystallized tungstate particles were successfully obtained. Characterization by X-ray powder diffraction
(
particles. Photoluminescence measurement reveals that the as-obtained CaWO , CdWO , and PbWO show
XRD) and transmission electron microscopy (TEM) shows that the product powders consist of nanosize
4
4
4
excitonic peaks at about 430, 500 and 500 nm, respectively. The solvent and reaction conditions are important in
the formation of the products.
#
2003 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; B. Chemical synthesis; C. X-ray diffraction
1
. Introduction
Powders with particles of uniform shape and narrow size distribution lying in the nanometer range
have been shown to possess interesting properties. Nanoscale oxide particles or mixed oxides
particles are gaining increasing technical importance for classic areas of application such as
catalysts, passive electronic components, or ceramic materials [1,2]. Metal oxide nanoparticles are
also widely used in industrial applications as catalysts, ceramic, pigments and so on. With regard to
all of the applications, numerous approaches have been explored for the preparation of spherical
nanosized particles ranging from lithographic technologies to chemical methods [3–5]. Compared
with the conventional solid-state reaction methods, solution-based synthesis results in higher levels
*
Corresponding author. Tel.: þ86-551-360-1600; fax: þ86-551-360-1600.
E-mail address: gzsh@mail.ustc.edu.cn (K. Tang).
0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2003.09.004

1
784
D. Chen et al. / Materials Research Bulletin 38 (2003) 1783–1789
of chemical homogeneity. Also, mixing of the starting materials at the molecular level is achieved in
solution-based systems, this is especially important when multi-component oxides are being
prepared.
In this paper, we report the preparation of several nanosized metal tungstate powders [such as:
CaWO , SrWO , BaWO , CdWO , ZnWO and PbWO ] from the reaction between metal chloride
4
4
4
4
4
4
and sodium tungstate in the ethylene glycol system at low temperature. Previous investigation had
shown that ethylene glycol can efficiently complex and stabilize the surface of the nanoparticles,
accordingly, the particles exist almost non-agglomerated in polyol media [1]. By using ethylene
glycol as the solvent, a lot of inorganic materials, such as chalcogenides, metal oxides, have
been successfully prepared. But the report on the synthesis of metal tungstate has seldom been
found.
Calcium tungstates (CaWO ) scheelite structural family, for example are of commercial interest for
4
applications in lasers, and fluorescent lamps because of their attractive photoluminescence properties
[6–8]. Cadmium tungstate (CdWO ) crystals with a monoclinic wolframite structure [9] considered to
4
be highly functional materials because of their high average refractive index [10], low radiation
damage, low aftergrow to luminescence and high X-ray absorption coefficient [11]. And some of the
other transition metal tungstates (TMT) are of especial importance because of their electrical
conductivity [12,13] and magnetic properties [14]. These materials also find application as catalysts
[15] and humidity sensors [16].
Although many methods have been reported on the preparation and characterization of metal
tungstate [17–21], our present route is relatively simple, efficient and can be applied to synthesis of a
host of metal tungstates nanocrystalline at low temperature.
2
. Experimental
Metal tungstates were synthesized in ethylene glycol (EG, bp ¼ 198 8C) with the reaction of
MCl ꢀnH O (M ¼ Ca, Sr, Ba, Cd, Zn, Pb) and Na WO powders in an autoclave at 180 8C under self-
2
2
2
4
generated pressure.
In a typical procedure, 0.001 mol MCl ꢀnH O and slightly excessive sodium tungstate were put
2
2
into a Teflon-lined stainless steel autoclave of 50 ml capacity, then the autoclave was filled with
ethylene glycol to 80% of the total volume. After being sealed, the autoclave was maintained at
1
80 8C for 10 h without shaking or stirring during the heating period and then allowed to cool to
room temperature. The precipitates were collected then washed with absolute alcohol and distilled
water to remove the residue of ethylene glycol. The final product was dried in a vacuum at 50 8C for
about 4 h.
The obtained samples were characterized by the X-ray powder diffraction (XRD) method. The XRD
was carried out with Japan Rigaku D/-max-gA diffractometer equipped with a graphite mono-
˚
chromatized Cu Ka radiation (l ¼ 1:5418A). A scan rate of 0.058/s was applied to record the patterns
in the 2y range 10–708. The morphology and particle sizes of as-obtained samples were determined by
transmission electron microscopy (TEM). The images were taken with Hitachi H-800 transmission
electron microscope, using an accelerating voltage of 200 kV. The luminescence and excitation spectra
of the samples were determined by a Hitachi 850 fluorescence spectrometer with a Xe lamp at room
temperature.

D. Chen et al. / Materials Research Bulletin 38 (2003) 1783–1789
1785
3
. Results and discussion
CaWO , PbWO , etc. with the formula MWO are characterized by the tetragonal space group
4
4
4
6
4 h
I4 /a or C (Fig. 1). In this structure, the primitive unit cell has been described to have two MWO4
1
units [22], each of which has an inversion center. The M and W sites have S point symmetry. There
4
have three crystal parameters {x, y, z} to describe the location of the O sites, which have only a little
symmetry, and are arranged in nearly tetrahedral coordination about each W site. Some literature
þa
ꢁa
[
23] has described the crystal structure as highly ionic with M cations and tetrahedral WO4
anions, where a ꢂ 2. The W and O with formal changes of ꢁb and ꢁg, respectively, where
ꢁ
a
4
bonding.
g ꢁ bBa, consist of highly ionic WO anions. b and g would be 6 or 2 if there were no covalent
4
XRD patterns of the as-obtained nanocrystalline tungstates are shown in Fig. 2. The product phases,
which were obtained from the diffraction peaks in the XRD patterns, and the cell constants derived
from the XRD patterns are given in Table 1. The cell constants are consistent with the reported values.
The broadening of the peaks indicates small grain sizes, according to the Scherrer equation. By means
of XRD, no peaks of impurities could be detected.
Fig. 3 shows typical TEM microphotographs of the as-obtained nanocrystalline samples. Nearly all
the products obtained were spherical particles. The grain sizes were directly observed and it is clearly
seen that almost all the samples are non-aggregated and the sizes of these samples are in the nanoscale
range. And the average sizes from the TEM observation are also listed in Table 1. The results agree well
with those obtained from the XRD patterns.
The PL spectra of the as-obtained tungstates are shown in Fig. 4. Fig. 4a is the PL spectrum of as-
obtained CaWO , which were measured using a 283.4 nm excitation line. It exhibited only green peak
4
at 430 nm, which was similar to the result reported in literature [24]. The PL spectra of as-obtained
CdWO (Fig. 4b) and PbWO (Fig. 4c) also exhibited a peak at 500 and 500 nm, respectively. The
corresponding excitation line is 290 nm for CdWO and 300 nm for PbWO , respectively. The results
4
4
4
4
are also in agreement with the literatures [21,25].
Fig. 1. The structure of MWO4.

1
786
D. Chen et al. / Materials Research Bulletin 38 (2003) 1783–1789
PbWO4
ZnWO4
CdWO4
BaWO4
SrWO4
CaWO4
10
20
30
40
50
60
70
2θ/degree
Fig. 2. XRD patterns of as-obtained nanocrystalline tungstates: CaWO
4
, SrWO
4
, BaWO
4 4 4 4
, CdWO , ZnWO , and PbWO .
In the present experiments, ethylene glycol was selected as the solvent because it can effectively
complex and stabilize the surface of the nanoparticles. Accordingly, the nanoparticles exist almost non-
agglomerated in ethylene glycol media [1], which were benefit for the preparation of stable and uniform
tungstates. To make a comparison, the synthesis of nanocrystalline metal tungstates was also carried out
Table 1
Characterization of as-obtained products
Products
CaWO4
Phase
Cell constants
Observed
TEM observed grain size (nm)
JCPDS Card
Reported
Tetragonal
Tetragonal
Tetragonal
Monoclinic
a ¼ 5.30
a ¼ 5.174
c ¼ 11.19
40
30
77-2236
85-0587
85-0588
14-0676
c ¼ 11.213
SrWO
4
a ¼ 5.434
a ¼ 5.416
c ¼ 11.970
c ¼ 11.95
BaWO4
a ¼ 5.627
a ¼ 5.613
c ¼ 12.72
a ¼ 5.029
b ¼ 5.859
c ¼ 5.074
140
10
c ¼ 12.731
CdWO
4
a ¼ 5.031
b ¼ 5.871
c ¼ 5.079
ZnWO
4
Monoclinic
Tetragonal
a ¼ 4.729
b ¼ 5.714
c ¼ 4.965
a ¼ 4.720
b ¼ 5.700
c ¼ 4.950
70
60
73-0554
19-0708
PbWO4
a ¼ 5.452
a ¼ 5.461
c ¼ 12.04
c ¼ 12.04

D. Chen et al. / Materials Research Bulletin 38 (2003) 1783–1789
1787
Fig. 3. TEM images of typical samples of nanocrystalline tungstates: (a) CaWO , (b) SrWO , (c) BaWO , (d) CdWO , (e)
4
4
4
4
4 4
ZnWO , (f) PbWO .
in ethanol, distilled water or their mixtures with different ratio at 180 8C for 10 h, respectively. The
XRD patterns show that there were many characteristic peaks of other impurity phases besides the
samples of tungstates.
The influences of reaction temperature and reaction time were also studied. In our experiments, we
changed the synthetic temperature in the ethylene glycol system from 10 to 200 8C keeping the other
reaction condition constant. The XRD results show that no tungstates were formed below 100 8C. And
temperature higher than 200 8C or time longer than 10 h, the grain sizes will grow bigger. The optimum
conditions for synthesizing pure phase of metal tungstates were at 180 8C for 10 h.

1
788
D. Chen et al. / Materials Research Bulletin 38 (2003) 1783–1789
3
00 350 400 450 500 550 600
350 400 450 500 550 600 650 700
(a)
Wavelength (nm)
(b)
Wavelength (nm)
4
00
500
600
700
800
(c)
Wavelength (nm)
Fig. 4. Photoluminescence (PL) spectrum of as-prepared nanocrystalline tungstates: (a) CaWO
4 4 4
, (b) CdWO , (c) PbWO .

D. Chen et al. / Materials Research Bulletin 38 (2003) 1783–1789
1789
4
. Conclusion
Nanocrystalline tungstates (CaWO , SrWO , BaWO , CdWO , ZnWO , and PbWO ) were
4
4
4
4
4
4
successfully synthesized in ethylene glycol at 180 8C. The solvent ethylene glycol played a key role
and the reaction temperature also affected the crystallization and purity of the final products. With
optimum reaction conditions selected, these nanocrystalline tungstates with high qualities were
successfully prepared. The simplicity of the process and high yield make it possible for industrial
application.
Acknowledgements
This work is supported by the National Natural Science Foundation of China and the 973 Projects of
China.
References
[
[
1] C. Feldmann, H.-O. Jungk, Angew. Chem. Int. Ed. 40 (2001) 359.
2] C.A. Leech, L.E. Champbell, Spinel solid solution catalysts for automotive applications, in: R.F. Gould (Ed.), Catalysts
for Control of Automotive Pollutants, American Ceramic Society, Washington, DC, 1985, p. 161.
3] A. Hagfeldt, M. Gr a¨ tzel, Chem. Rev. 95 (1995) 49.
[
[
[
[
[
[
[
4] Engineering a small world: from atomic manipulation to microfabrication, Spec. Sec. Sci. 254 (1991) 3355.
5] M.S. Whittingham, Curr. Opin. Solid State Mater. Sci. 1 (1996) 227.
6] L.F. Johnson, G.D. Boyd, K. Nassau, R.R. Soden, Phys. Rev. 126 (1962) 1406.
7] R. Grasser, E. Pitt, A. Scharmann, G. Zimmerer, Phys. Status Solidi B 69 (1975) 359.
8] W.S. Cho, M. Yashima, M. Kakihana, A. Kudo, T. Sakata, M. Yoshimura, Appl. Phys. Lett. 66 (1995) 1027.
9] A.P. Chicagov, V.V. Ilyukhin, N. Belov, Sov. Phys. Dokl. 11 (1966) 11.
[
[
[
[
[
[
10] H. Lotem, Z. Burshtein, Opt. Lett. 12 (1987) 561.
11] V.A. Pustovarov, A.L. Krymov, B. Shulgin, Rev. Sci. Instrum. 63 (1992) 3521.
12] T. Mathew, N.M. Batra, S.K. Arora, J. Mater. Sci. 27 (1992) 4003.
13] K.C. Li, C.Y. Wang, Tungsten, Chapman and Hall, London, 1956.
14] B. Lake, R.A. Cowley, D.A. Tennant, J. Phys. Condens. Mater. 49 (1997) 1051.
15] M. Grayon, D. Eckroth, E. Graber, A. Klingberg, P.M. Siegel, Kirk-Concise Encyclopaedia of Chemical Technology, 2nd
ed., Wiley-Interscience, New York, 1985.
[
[
[
[
[
[
[
[
[
[
16] A.K. Bhattacharya, R.G. Biswas, A. hartridge, J. Mater. Sci. 32 (1997) 353.
17] L.P. Dorfman, D.L. Houck, M.J. Scheithauer, J. Mater. Res. 16 (2001) 1096.
18] A. Sen, P. Pramanik, J. Eur. Ceram. Soc. 21 (2001) 745.
19] Z.D. Lou, M. Cocivera, Mater. Res. Bull. 37 (2002) 1573.
20] V. Marchev, G. Gospodinov, D. Georgiev, Zh. Obshch. Khim. 66 (1996) 1062.
21] H.W. Liao, Y.F. Wang, X.M. Liu, Y.D. Li, Y.T. Qian, Chem. Mater. 12 (2000) 2819.
22] Y. Zhang, N.A.W. Holzwarth, R.T. Williams, Phys. Rev. B 57 (1998) 12738.
23] L.G. Sillen, A.L. Nylander, Ark. Kemi. Miner. Geol. 17A (1943) 1.
24] A. Shmilevich, D. Weiss, R. Chen, N. Kristianpoller, Radiat. Prot. Dosim. 84 (1999) 131.
25] C.S. Shi, Y.G. Wei, X.Y. Yang, D.F. Zhou, C.X. Guo, J.Y. Liao, H.G. Tang, Chem. Phys. Lett. 328 (2000) 1.