Hydrothermal synthesis of ZrW2O8 nanorods

Article abstract of DOI:10.1016/j.physb.2005.09.041

Negative thermal expansion ZrW₂O₈ nanorods were synthesized by a rapid low-temperature hydrothermal route, followed with a heat treatment at 500 °C for 6 h. Acidity of the HCl addition to the mixed solution in the hydrothermal condition strongly affects the formation of the precursor ZrW₂O₇(OH)₂(H₂O) ₂. High acidity of HCl in the range of 5-10 N facilitates the nucleation of ZrW₂O₇(OH)₂(H₂O) ₂, but constrains its growth. Low acidity of HCl addition (<5 N) only obtained the products ZrO₂ and WO₃ by firing the precursors. FE-SEM images revealed that the shape and size of ZrW ₂O₈ were modeled after its precursor ZrW₂O ₇(OH)₂(H₂O)₂.

Full text of DOI:10.1016/j.physb.2005.09.041

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Physica B 371 (2006) 81–84
www.elsevier.com/locate/physb
Hydrothermal synthesis of ZrW2O8 nanorods
Ã
Xianran Xinga, , Qifeng Xinga, Ranbo Yua, Jian Mengb, Jun Chena, Guirong Liua
aDepartment of Physical Chemistry, University of Science & Technology Beijing, Beijing 100083, China
bKey laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
Received 2 August 2005; received in revised form 30 September 2005; accepted 30 September 2005
Abstract
Negative thermal expansion ZrW2O8 nanorods were synthesized by a rapid low-temperature hydrothermal route, followed with a
heat treatment at 500 1C for 6 h. Acidity of the HCl addition to the mixed solution in the hydrothermal condition strongly affects
the formation of the precursor ZrW2O7(OH)2(H2O)2. High acidity of HCl in the range of 5–10 N facilitates the nucleation
of ZrW2O7(OH)2(H2O)2, but constrains its growth. Low acidity of HCl addition (o5 N) only obtained the products ZrO2 and WO3
by firing the precursors. FE-SEM images revealed that the shape and size of ZrW2O8 were modeled after its precursor
ZrW2O7(OH)2(H2O)2.
r 2005 Elsevier B.V. All rights reserved.
PACS: 81.05.–t; 81.10.–h; 81.07.–b
Keywords: ZrW2O8; Hydrothermal synthesis; Nanorod; Morphology
1. Introduction
firing at 1200 1C needs several days with intermediate
grindings. High-temperature sintering facilitates the for-
mation of volatile WO3 and results in non-stoichiometry
and impurity in the final product. The modified syntheses
with solution precursors also require a final heat treatment
at about 1200 1C. A sol–gel route was developed to
synthesize ZrW2O8 finally calcined at about 600 1C. The
solution usually was aged and gelatinized for several weeks
[6]. One rapid synthetic route [7], the hydrothermal
method, was approached, in which the mixed solutions
were contained in a Parr bomb for heating at 200 1C for
4 h, and the pure product was achieved by firing the
precursor at 600 1C for 10 h.
However, properties and applications of NTE materials
are highly correlated to their preparation processes, and
morphologies. To our knowledge, the effect of particle size
and shape of NTE compounds was studied less. In the
present study, the hydrothermal synthetic method was
employed to prepare nano-crystalline ZrW2O8. The pre-
cursors and the final products were investigated by thermo-
gravimetric-differential thermal analysis (TGA-DTA), and
their morphologies were studied by field emission scanning
electron microscopy (FE-SEM).
Due to potential applications of the strong isotropic
negative thermal expansion (NTE) of ZrW2O8, it attracts
extensive attention to its preparation, structure and
properties [1–4]. One obvious usage is to prepare compo-
sites with a desired thermal expansion coefficient (TEC)
to overcome the thermal shock. The TEC of ZrW2O8
(ꢀ8.7 ꢁ 10ꢀ6 Kꢀ1) is of the same order of magnitude as
that of the positive thermal expansion ceramic Al2O3
(9.0 ꢁ 10ꢀ6 Kꢀ1). This fact permits one to tailor the TEC of
the alumina matrix by doping with ZrW2O8, to obtain a
material that could be used as circuit boards. Needle-like or
rod dopants might enhance the composite of alumina
matrix with the similar shape particles oriented in the same
direction. The phase diagram of the ZrO2–WO3 system [5]
indicates that an intermediate compound ZrW2O8 is
thermodynamically stable in the temperature range of
1105–1267 1C. The conventional synthesis of ZrW2O8 by
Ã
Corresponding author.
E-mail address: xing@ustb.edu.cn (X. Xing).
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2005.09.041

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2. Experimental procedures
precursor ZrW2O7(OH)2(H2O)2, an intermediate of the
final product ZrW2O8. The crystallinity of the precursor
lightly increases with decreasing acidity (see Fig. 1).
However, further reduction of the HCl acidity (o5 N)
leads to the formation of amorphous precursors, and the
consequent heat treatment of 500 1C produces mixed
powders of ZrO2 and WO3.
The thermal conversion behavior of ZrW2O7(OH)2
(H2O)2 was followed by thermal analysis, which was
similar to that of the single crystal preparation of ZrW2O8
[8]. The first endothermic peak at about 270 1C revealed
that there occurred a transformation reaction from
ZrW2O7(OH)2(H2O)2 to ZrW2O8 with about 7.7% weight
loss. It is approximately in accordance with the theoretic
calculation of 8.4% weight loss.
The starting materials were zirconium oxychloride
ammonium
ZrOCl2 ꢂ 8H2O
and
metatungstate
N5H37W6O24 ꢂ H2O (Beijing Chemical reagent Co.). Solu-
tion A with 0.25 M Zr and solution B with 0.25 M W were
prepared, respectively. Fifty millilitre solution A and
100 ml solution B were added to 25 ml deioned water
simultaneously by dropwise addition with continuous
stirring and heating at 60 1C for 2 h. And then 50 ml of
different aqueous HCl solutions (1, 3, 5, 7 and 10 N) was
added to the mixed solution, continuously stirred for
another 3 h. After that the mixture was transferred to a
Teflon-lined Parr bomb and heated at 180 1C for 6 h.
Finally, the product was filtered, washed with deioned
water, and dried at 60 1C as a precursor for various
measurements.
ZrW2O7ðOHÞ2ðH2OÞ2 ! ZrW2O8 þ 3H2O " :
Phase identification and structure characterization were
carried out using a 21 kW extra power X-ray diffractometer
(Model M21XVHF22, MAC science Co., Ltd.) with Cu
Ka radiation, curved crystal graphite monochromators
and scintillation detectors. The scanning speed of 2-y angle
is 41/min. The particle size and shape were observed by a
field emission scanning electron microscopy (FE-SEM,
Hitachi, S-4200). TGA-DTA measurements were per-
formed using a thermal analysis system (Model STA 409,
Netzsch Geraetebau GmbH), at a heating rate of 10 1C/
min, in an alumina crucible, with a-Al2O3 powder as a
reference.
This assumption was confirmed by the XRD patterns of
the pure phase ZrW2O8, which was derived from the
precursor ZrW2O7(OH)2(H2O)2 sintered at 300, 400 or
500 1C for 6 h, respectively [8]. From 270 1C up to 700 1C,
no any transparent thermal effect was detected, implying
the existence of ZrW2O8 without any new phase formation.
Above 700 1C, the slope of the DTA curve changes sharply
with absorption of heats, related to the decomposition of
ZrW2O8 into ZrO2 and WO3 due to its thermodynamic
metastability.
FE-SEM images of precursors, prepared by the hydro-
thermal treatment (180 1C, 6 h) with the mixture of the
different aqueous HCl additions, were shown in Fig. 2. All
precursors crystallized in nanorods with the exception of
Fig. 2c, and the magnitudes vary from about
40 nm ꢁ 200 nm for the addition of 10 N HCl solution(Fig.
2a), to 50 nm ꢁ 600 nm for the addition of 7 N HCl solution
(Fig. 2b), finally up to 500 nm ꢁ 10 mm for the addition of
5 N HCl solution (Fig. 2c). The tendency of crystallinity of
the precursor with the feed acidity could be observed,
slightly increasing with the additions of aqueous HCl from
10 to 7 N (see Figs. 2a and b), and well improved in the 5 N
HCl addition (see Fig. 2c), which was also confirmed by
XRD measurements (see Fig. 1). The crystal growth of the
precursors from nanometer to micron level reveals that
maintaining some acidity of HCl (^5 N), as discussed
previously, is necessary to synthesize pure ZrW2O7(OH)2
(H2O)2, whereas, the high acidity solution restrains the
growth of the ZrW2O7(OH)2(H2O)2 crystal.
3. Results and discussion
Fig. 1 shows XRD patterns of the precursors prepared in
the additions of the different aqueous HCl solutions (1, 3,
5, 7 and 10 N). In the present case, the HCl was added to
the mixed solution with continuous stirring for 3 h, and
then the mixture was transferred to a Teflon-lined Parr
bomb and heated at 180 1C for 6 h, yielding the crystallized
The shapes of the ZrW2O8 nanoparticles, achieved by
heating the precursor derived in the addition of 10 N HCl
solution, were correlated to the heating temperature (see
Fig. 3). Sintering temperature increment from 500 to
550 1C enhances the crystal growth of ZrW2O8 from
30 nm ꢁ 200 nm to about 40 nm ꢁ 400 nm. However, an-
other 501 increment leads to a rapid growth of crystalline
ZrW2O8 (in the dimension of 500 nm ꢁ 10 mm). However,
much difference was found in the case of 5 N HCl addition
(see Fig. 4). The shapes and magnitudes of the crystalline
ZrW2O8 particles (about 500 nm ꢁ 10 mm) sintered at
Fig. 1. XRD patterns of precursors prepared in different HCl additions
(CHCl ¼ 1,3,5,7,10 N).

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Fig. 2. FE-SEM images of precursors ZrW2O7(OH)2(H2O)2 prepared in
the hydrothermal conditions with different HCl additions: (a) 10 N HCl;
(b) 7 N HCl; (c) 5 N HCl.
Fig. 3. FE-SEM images of ZrW2O8 by heating precursor ZrW2O7(OH)2
(H2O)2 prepared in the addition of 10 N HCl solution: (a) 500 1C; (b)
550 1C; (c) 600 1C.
500 1C for 6 h are almost the same as those of the particles
sintered at 550 1C (see Figs. 4a and b). Further heat
treatment at 700 1C for another 6 h destructed the ZrW2O8
nanorods into mixed powders (see Fig. 4c). XRD
measurements demonstrated the decomposition of ZrW2O8
into ZrO2 and WO3, which agrees well with the results
of TGA–DTA measurements due to its thermodynamic
metastability.

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morphologies of ZrW2O8 are very similar to those of the
precursors ZrW2O7(OH)2(H2O)2. The frame of structure of
ZrW2O7(OH)2(H2O)2 perhaps plays a basic role to
construct the frame of ZrW2O8 at 500 1C, even though
they are different structures. Thus, the original shape of the
precursor is maintained in the final product ZrW2O8.
4. Conclusions
ZrW2O8 nanorods, varying from 40 to 500 nm in
diameter, were prepared by heating the precursor
ZrW2O7(OH)2(H2O)2 at 500 1C for 6 h. The magnitude of
well crystallized ZrW2O8 particles was in the dimension of
500 nm ꢁ 10 mm. The synthesis of the crystalline precursor
was highly correlated to the feed acidity of HCl addition to
the mixed solution in present hydrothermal conditions.
Some acidity must be maintained and low acidity
(CHClo5 N) will give rise to the products of ZrO2 and
WO3 when the precursors are fired. The shapes and
magnitudes of the crystalline ZrW2O8 particles were
modeled after their precursors. Raising the sintering
temperature increases the crystal growth of ZrW2O8
derived from precursors in the additions of 10 N HCl and
7 N HCl, and almost does not affect the well-crystallized
ZrW2O8. Further heat treatment at 700 1C destructed the
crystalline ZrW2O8 particles and resulted in decomposition
of ZrW2O8 into ZrO2 and WO3, due to its thermodynamic
metastability. ZrW2O8 nanorods with the cubic phase are
promising materials for possible application as enhanced
composites for overcoming the thermal shock.
Acknowledgments
This work was financially supported by National
Natural Science Foundation of China (No. 20331030,
50374009), and Funds of Ministry of Education of China
for TRAPOYT program.
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