Hydrothermal synthesis of monodisperse WO 3 ·H 2 O square platelet particles, Yousei Mita b, a Graduate School of Science and Technology, Chiba University, Yayoicho 1-33, Inageku, Chiba
Trang 1Hydrothermal synthesis of monodisperse WO 3 ·H 2 O square platelet particles
, Yousei Mita b,
a Graduate School of Science and Technology, Chiba University, Yayoicho 1-33, Inageku, Chiba 263-8522, Japan
b Faculty of Engineering, Chiba University, Yayoicho 1-33, Inageku, Chiba 263-8522, Japan
Received 11 July 2006; accepted 22 July 2006 Available online 15 August 2006
Abstract
Monodisperse particles of tungsten(VI) oxide monohydrate were prepared in a hydrothermal system at 40 °C, where 20 ml of HCl solution (1.50 mol/l) was added to the same volume of Na2WO4aqueous solution (0.50 mol/l) with magnetic stirring, followed by standing in an air oven for 168 h The shape of the particles was square platelet and the mean size was 0.72μm with 10% of the coefficient of variation The XRD pattern was in good agreement with the standard JCPDS data for WO3·H2O The effects of counter cation and anion of the starting materials in addition to the preparation temperature and acid concentration were also examined
© 2006 Elsevier B.V All rights reserved
Keywords: Tungsten(VI) oxide monohydrate; Tungstic acid; Monodisperse particle; Hydrothermal synthesis
1 Introduction
Tungsten(VI) oxide and its hydrates, WO3·nH2O, are known
as electrochromic (EC) substances which alternate their color by
electrochemical redox reactions A deep blue color in the
reduced state is based on a mixed valence state of W(V) and W
(VI)[1] Thin films, prepared by vapor evaporation for instance,
are common in the WO3-based EC devices [2] However, if
particulate materials are applied, availabilities such as simple
production processes and hybrid compositions with functional
polymers[3,4]are expected In addition, printing processes are
applicable for patterning of the EC materials Monodisperse
particles are particularly expected to give sharp distributions of
properties such as response time, precise controls of properties,
regular arrangement of particles on electrode, etc
Hydrothermal synthesis from Na2WO4in a low pH region is
one of the typical ways to prepare tungsten oxide hydrates
Freedman[5]investigated influence of acid concentration and
temperature on preparation of WO3·H2O and WO3·2H2O from
Na2WO4solution Gerand et al.[6]prepared WO3·1 / 3H2O in
pure water at 120 °C by digesting washed precipitates of
WO3·nH2O gel or particles, preliminarily prepared from
Na2WO4in acidic conditions Instead of adding acid solutions,
an ion-exchange method was also applied to make Na2WO4 solution acidic to prepare the WO3·2H2O particles [7] How-ever, there seems to be few reports on narrowing the size distribution of these particles In the present study, the hydro-thermal condition was optimized to prepare monodisperse
WO3·H2O particles
2 Experimental
In the standard condition, 0.50 mol/l of Na2WO4solution was freshly prepared by dissolving 3.30 g of Na2WO4·2H2O in
20 ml of distilled water in a screw-capped bottle At 40 °C,
20 ml of HCl aqueous solution (1.50 mol/l) was added into the
Na2WO4solution by a transfer pipette under magnetic stirring, which was halted after 1 min from mixing After 168 h of storage in an air oven at 40 °C, the precipitates were centrifuged
at 2000 rpm for 15 min After removing the supernatant solution, they were re-dispersed in distilled water and centrifuged again Five sets of the centrifugation process were carried out to wash completely
The effects of the preparation condition were examined by varying either HCl concentration or temperature from the
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doi: 10.1016/j.matlet.2006.07.129
Trang 2standard condition Also, the influence of counter ions of the
starting materials was tested That is, Li2WO4and K2WO4were
used instead of Na2WO4 Alternatively, HCl was replaced by
HClO4, HNO3, or H2SO4 Because Li2WO4did not dissolve in
distilled water, it was used in a form of dispersion
Particle shape was observed by a scanning electron micro-scope (SEM, Hitachi S-2400) The mean size and coefficient of variation, COV, were obtained with a transmission electron microscope (TEM, JEOL JEM-1200EX) The particles were identified by X-ray diffractometry (XRD) using CuKα radiation with MacScience M18XHF-SRA The yield of the objective particles was estimated by a gravimetric analysis, in which precipitates fairly separated from soluble salts and gel-like residuals were heated to dehydrate at 750 °C for 30 min and then weighed as WO3 All chemicals were purchased from Wako Pure Chemicals except for Li2WO4which was purchased from Aldrich They were used as supplied
3 Results and discussion Particles prepared in the standard condition were in square platelet shape from SEM images, as shown inFig 1.Fig 1also indicates a histogram of their size distribution obtained from TEM observation The mean edge length was estimated as 0.72μm with 10% in COV from more than 200 particles The average thickness was about 0.2μm
Fig 1 SEM image of obtained particles in the standard condition and their size
distribution evaluated from TEM observations.
Fig 2 XRD patterns of (a) particles obtained in the standard condition and
(b) standard of WO ·H O on the basis of JCPDS data (JCPDS No 18-1418).
Fig 3 Time evolutions of (a) the mean size of particle and COV and corres-ponding TEM images at (b) 48 h, (c) 56 h, (d) 96 h, and (e) 168 h in the standard condition with the limited repetition of centrifugal washing.
Trang 3The particles were identified as tungsten(VI) oxide monohydrate by a
good agreement in the XRD pattern with a JCPDS data for WO3·H2O
(JCPDS No.18-1418) as shown in Fig 2 The crystal structure of
WO3·H2O is anisotropic, where layers consisting of W and O atoms are
stacked via hydrogen bonds[8] As explained by Livage and Guzman
[9], the W atom in the precursor has four coordinated OH groups in an
equatorial xy plane with a W_O bond and water molecule in the
vertical z-axis The platelet shape of particles suggests a large
dif-ference in the growth rates of the crystal orientations depending on the
precursor structure
The change of appearance during the formation process was as
follows At first, a yellow transparent solution was formed by mixing of
the Na2WO4and HCl solutions due to formation of soluble complexes
The mixed solution soon became turbid by the generation of gel-like
precipitates and lost fluidity within a few minutes Reflecting the
decrease of complexes, the yellowish color was once reduced until 48 h
but gradually increased again by growth of WO3·H2O particles, which
are also yellow The apparent volume of precipitate started to decrease
at∼72 h and reached to ca 1/3 of the whole volume at 168 h
Fig 3shows time evolutions of the mean edge length of objective
particles and COV with some corresponding TEM images at 48, 56, 96,
and 168 h, where the results were obtained from precipitates discretely
prepared for each time Besides, the repetition of centrifugal washing
was limited to two times to retain the gel-like precipitates Only gel was
observed before 48 h but objected particles were formed in the
gel-network at least at 56 h Fig 3(a) suggests ongoing particle growth
even at 168 h In addition, the gel-like precipitates seemed amorphous
in the earlier stage of the generation process and somewhat crystalline
at 168 h from dark-field observations with TEM The standard
deviation of particle size was almost constant (ca 0.07μm) during the
growth period Thus COV tended to decrease from 22% at 56 h to 10%
at 168 h
Due to the gel-like residuals existing at the end of precipitation, as
shown inFig 3(e), the yield of the objective particles was unfortunately
low as 25% It was raised to 75% by using Li2WO4instead of Na2WO4,
although particles were polydisperse ones with about 0.4μm and 25%
in mean size and COV, respectively K2WO4generated only gel-like
precipitates without any objective particles Moreover, when the
concentration of Na2WO4solution was reduced to 0.1 mol/l, coupled
with 0.85 mol/l of HCl, the yield was raised to 80% with a declined
monodispersity (12% in COV with 0.46μm in the mean size) Univalent
acids, HClO4and HNO3, gave almost the same results as that of HCl
On the contrary, H2SO4formed only gel-like precipitates Balázsi and
Pfeifer [10] observed structural and morphological changes of
WO3·2H2O particles by a decrease of Na+ion content through a
re-peated washing treatment Therefore, counter ions of tungstates and
acids anyway play important roles on the formation process in this
system, although the mechanism has not been clear yet
The effect of the concentration of HCl solution was tested in the
range of 1.40–1.80 mol/l at 40 °C with Na2WO4 When increasing the
HCl concentration, the mean size was reduced from 0.95 to 0.62μm
and COV tended to decline from 10% to 19% When temperature was
varied in 35–55 °C with 1.50 mol/l of HCl solution, the mean size was
decreased from 0.80 to 0.59 μm by the increasing temperature A
minimum COV was shown at 40 °C (20, 10, 13, 12, and 15% in COV at
35, 40, 45, 50, and 55 °C, respectively) Except for 35 °C, the apparent
volumes of yellow precipitates at 168 h were almost the same as that of
the standard condition (40 °C), suggesting similar yields in these
conditions since the apparent volume reflects a degree of
transforma-tion to the WO3·H2O particles from bulky gels At 35 °C, the apparent
volume of the precipitate was almost the same as that of the whole volume The larger mean size at 35 °C, even the small conversion to
WO3·H2O, is due to an inhibited nucleation Hence this temperature is likely a critical one to form WO3·H2O in the system This observation may be supported by the results of Freedman[5], who has reported that only WO3·2H2O was obtained at 25 °C while mixtures of WO3·H2O and WO3·2H2O were obtained at 50 °C
Generally in monodisperse particle preparations, monodispersity indicated by COV is narrowed by the progress of growth with at least a constant absolute size distribution[11] Thus longer growth periods relative to nucleation ones are preferred in addition to a clear separation
of both steps In agreement with the mechanisms, COV in the standard condition was decreased to reach the monodisperse particles with the almost constant standard deviation and increasing particle size, as mentioned above Hence size distribution at the end of the nucleation period has an importance in the final monodispersity In other words, higher HCl concentration and higher temperature than the standard condition kept a high supersaturation ratio to advance the nucleation period, giving smaller size and declined monodispersity in the present system
4 Conclusions Monodisperse WO3·H2O particles in a square platelet shape were prepared in an optimized hydrothermal system The narrow size distribution, 10% in COV with 0.72μm in mean size, was achieved in the standard condition
Acknowledgement
We would like to thank Dr T Kojima (Department of Applied Chemistry and Biotechnology, Chiba University) for the XRD measurements and Mr Y Funabashi (Department of Information and Image Sciences, Chiba University) for the SEM observa-tions The research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Young Scientists (B) and by the Hosokawa Powder Technology Foundation
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