Received: 30 May 2017

| Accepted: 20 September 2017

DOI: 10.1111/jace.15250

RAPID COMMUNICATION

Facile synthesis of nanocrystalline hexagonal tungsten trioxidefrom metallic tungsten powder and hydrogen peroxide
Isao Tsuyumoto
Department of Applied Chemistry,
College of Bioscience and Chemistry,
Kanazawa Institute of Technology,
Nonoichi, Ishikawa, Japan
Correspondence
Isao Tsuyumoto, Department of Applied
Chemistry, College of Bioscience andChemistry, Kanazawa Institute ofTechnology, Ishikawa, Japan.
Email: tsuyu@neptune.kanazawa-it.ac.jp
Funding information
Grant-in-Aid for Scientific Research (C)from the Japan Society for the Promotionof Science (JSPS), Grant/Award Number:
No. 16K05946

Abstract
A hexagonal form of tungsten trioxide (h-WO3, particle size: 15.9-57.1 nm) was
found to be formed by a direct reaction between metallic tungsten powder(W, particle size: 0.45-0.59 lm) and 15%-30% hydrogen peroxide (H2O2) aq
solution. Oxide film on the powder surface having the similar crystal structure as
h-WO3 was essential for the formation, and the surface oxide film was formed byaging the powder in air at 45°C, a relative humidity of 100% (PH2O 96 hPa) for
3-28 days or in ambient atmosphere at room temperature for 12 years. The Rietveld analysis performed in the space group P63/mcm (Z = 6) indicated the crystal
structures were the same as those of the reported h-WO3 and that the crystallographic characteristic was as follows: a = 0.74219 nm, c = 0.77198 nm forh-WO3 from the 28-day aged powder, and a = 0.74538 nm, c = 0.77194 nm forh-WO3 from the 12-year aged powder.
KEYWORDS
nanoparticles, oxides, synthesis, tungsten/tungsten compounds, X-ray methods

1| INTRODUCTION

Tungsten trioxide, WO3, has received much attention for awide variety of applications such as photocatalyst in water
splitting,1-8 dye-sensitized solar cells,9 electrode materials
for lithium-ion batteries,10 photochromic films,11 and electrochromic displays.12,13 Many methods have been reported
for the preparation of WO3 nanoparticles as well as theirproperties and applications.14 Tungsten trioxide crystallizes
in several polymorphic forms, which can be categorized
into two general crystal structures. One is based on the
ReO3-type structure, i.e., WO6 octahedra link together by
corner sharing and form a three-dimensional framework. In
this case, distortions of the octahedra bring about lower
symmetries than ReO3, and these polymorphs have beenreported to be monoclinic phase (II) from 5 to 278 K, triclinic phase from 248 to 290-300 K, monoclinic phase (I)(c-phase) from 290-300 K to 600 K, orthorhombic phasefrom 600 to 1010 K, and tetragonal phase from 1010 K tothe melting point, 1746 K.15 The other general crystal
structure is a form with hexagonal symmetry first reported
J Am Ceram Soc. 2018;101:509–514.

by Gerand et al,16 i.e., WO6 octahedra share their corners
to form a framework with hexagonal channels along c-axis.
This hexagonal WO3 (h-WO3) is a metastable phase, which
has not been described in phase diagrams, and its syntheticroutes are limited to low-temperature processes because
high temperature causes transformation to the stable monoclinic phase (I).17 The h-WO3 was first prepared by dehydration of WO31/3H2O, and this hydrate was obtained byhydrothermal treatment of an aqueous suspension of either
tungstic acid gel or WO32H2O at 120°C.16 The h-WO3
has been also prepared by other methods such as hydrogen
peroxide oxidation of hexagonal ammonium tungsten
bronze,18 and thermal decomposition of ammonium peroxo-polytungstate.17 These preparation processes of h-WO3
require precise control of experimental conditions and are
not suitable for inexpensive large-scale production. As theh-WO3 as well as the monoclinic phase (I) has attracted a
lot of attention as an alternative material for photocatalyst,3a facile and inexpensive preparation method of h-WO3 ishighly desirable. Our research group has been investigatinga soft-chemical synthesis of metal oxides starting from awileyonlinelibrary.com/journal/jace© 2017 The American Ceramic Society

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TSUYUMOTO

peroxo-metallic acid solution obtained by the direct reaction between metal powder and hydrogen peroxide.19-26
Using such new methods we have successfully prepared
KxWO3,27 BaTiO3,28 KNbO3,29 LiVO2,30 Ramsdellite
LixTiO2,31 etc. In this study, we found that a direct reaction
between tungsten (W) powder and 15%-30% hydrogen peroxide (H2O2) aq gave a precipitate of h-WO3 instead of aperoxo-polytungstic acid solution when metallic tungsten
powder with smaller particle size, 0.45-0.59 lm, was used.

2| EXPERIMENTAL PROCEDURE

2.1| Raw materials

Two types of metallic tungsten powders with different particle sizes were used: 0.45-0.59 lm (99.9% purity, W-H,
Japan New Metals Co., Ltd., Osaka, Japan), and 1.28 lm
on average (99.9% purity, No. 64401, Mitsuwa Chemicals
Co., Ltd., Osaka, Japan). As for the tungsten powder withparticle size of 0.45-0.59 lm, we also used an old sample
stored in a sample bottle in ambient atmosphere at room
temperature for 12 years along with the new samples.
Hydrogen peroxide solution (30.0 wt%, Kanto ChemicalCo., Inc., Tokyo, Japan) was used as purchased without
further purification.

2.2| Pretreatment

Tungsten powders were pretreated before the reaction toenhance the formation of surface oxide film having thesimilar crystal structure as h-WO3. We carried out either ofthe following pretreatments: heating in air at 45°C; water
vapor pressure (PH2O) 96 hPa (relative humidity, RH100%) for 1, 3, 7, 14, 21, or 28 day(s); heating in air at
275°C, PH2O 23 hPa for 12 hours; or keeping in water at
20°C for 7 days, 100°C for 12 hours, or 100°C at 1 MPafor 12 hours. The old tungsten powder stored for 12 years
was used without any pretreatment.

2.3| Synthesis

A vigorous reaction occurred by adding 11.0 g of tungsten
powder into 50 mL of 15% H2O2 aq, and a precipitate wasformed. After the vigorous reaction ceased, 20 mL of 30%
H2O2 aq was added more to complete the reaction. The
precipitate was filtrated using a membrane filter with poresize of 0.1 lm, rinsed with water, and dried at 45°C for
2 hours.

2.4| Characterization

The X-ray diffraction (XRD) patterns of the products were
measured with a diffractometer (XD-D1, Shimadzu, Kyoto,

Japan) using graphite-monochromatized CuKa radiation(k = 0.15406 nm). The X-ray profiles were collected
between 2° and 120° of 2h angles with a step interval of
0.02° and a count time of 4 s/step. The crystalline parameters were refined by the Rietveld method using the RIETAN-FP program,32,33 and the crystallite sizes were
estimated by the Scherrer equation. The morphology wasobserved by a field emission scanning electron microscope(FE-SEM, JSM-7100F, JEOL, Tokyo, Japan) with an accelerating voltage of 10 kV. The specific surface area ofh-WO3 was measured by the Brunauer-Emmett-Teller(BET) method using the adsorption of nitrogen gas at thetemperature of liquid nitrogen (Micrometrics Tristar 3020,
Shimadzu, Kyoto, Japan). Prior to the measurements, thesamples were degassed at 200°C for 2 hours under vacuum. The thermal behavior was investigated by thermogravimetry (TG) and differential thermal analysis (DTA;
DTG-50, Shimadzu, Kyoto, Japan) at a heating rate of
20°C/min in air.

3

| RESULTS AND DISCUSSION

A pale blue precipitate was found to be formed after a vigorous reaction between the old tungsten powder (particle
size: 0.45-0.59 lm) stored for 12 years and hydrogen peroxide, while the reaction between the tungsten powder(particle size: 1.28 lm) and hydrogen peroxide yielded yellow transparent peroxo-polytungstic acid solution asreported in literature.21,22 As shown in Figure 1, the XRD
pattern of the precipitate was in good agreement with thatof h-WO3 (PDF-000331387), and interestingly the oldtungsten powder for the raw material showed weak broad
peaks at the same angles with h-WO3 as well as strong
sharp peaks of metallic tungsten. Oxide film having thesimilar crystal structure as h-WO3 was formed on the powder surface during the storage for 12 years, and this suggests that the oxide film grown on tungsten surface inambient atmosphere at room temperature is the phase ofh-WO3. On the other hand, the pristine tungsten powder(particle size: 0.45-0.59 lm) showed no broad peaks at theangles of h-WO3 in the XRD pattern, and the precipitate of
the same reaction became the mixture of tetragonal WO2.90(PDF-000181417) and other oxides. These results suggestthat the oxide film on the powder surface is an essential
seed crystal for the formation of h-WO3.
In order to prepare the surface oxide film artificially ina short time and to reproduce the formation of h-WO3, weaged the pristine tungsten powders (particle size:
0.45-0.59 lm) in various conditions such as steaming andboiling at ordinary and high pressures. As shown in
Table 1 and Figure 2, we successfully obtained h-WO3 asa single-phase precipitate in the reaction with hydrogen| 511TSUYUMOTOF I G U R E 1 XRD patterns of (A) h-WO3 obtained as a precipitate by the reaction between tungsten powder (particle size: 0.45-0.59 lm)
stored in ambient atmosphere at room temperature for 12 years and hydrogen peroxide, (B) tungsten powder stored in ambient atmosphere atroom temperature for 12 years, and (C) magnified view (9100) of (B) [Color figure can be viewed at wileyonlinelibrary.com]

peroxide by using the tungsten powder after aging at 45°C,
RH 100% (PH2O 9.58 kPa) for 3 days or more. Small
amount of h-WO3 (PDF-000331387) was observed alongwith WO3H2O (PDF-000430679) and metallic tungsten(PDF-000040806) in the XRD patterns of the samples after
aging at 45°C, RH 100% for 3 days or more, while thesample after aging in the same condition for 1 day showedno peaks other than those of metallic tungsten. This indicates that surface oxide film consisting of WO3H2O andh-WO3 was formed on the powder surface at 45°C in
humid air and that this acted as a seed crystal for the formation of h-WO3. After the reaction with hydrogen peroxide, WO3H2O and metallic tungsten completelytransformed into h-WO3, and they were not observed in the
XRD patterns of the product. When tungsten powder wasaged in water at 100°C, small amount of highly crystalline
WO3H2O was observed together with small amount of
h-WO3 in the XRD pattern, and the highly crystalline

WO3H2O on the powder surface presumably caused the
predominant formation of WO3H2O compared withh-WO3. On the other hand, when the tungsten powder wasaged at 275°C in humidified air for 12 hours, small amountof triclinic WO3 (PDF-000321395) was observed on the
powder surface and the triclinic WO3 was formed afterreaction with hydrogen peroxide. Furthermore, when weused tungsten powder with larger particle size, 1.28 lm,
only small amount of highly crystalline WO3H2O wasobserved together with those of W after aging at 45°C, RH
100% for 3 or 7 days, and WO3H2O was obtained alone
as a precipitate by the reaction with hydrogen peroxide.
The precipitation of h-WO3 occurred only when ultrafine
tungsten powder with particle size of 0.45-0.59 lm wasused. The surface oxide film consisting of h-WO3 wasformed only on the surface of ultrafine tungsten powder,and the film was an essential seed crystal for the formationof h-WO3. It can be considered that the crystal growth ofT A B L E 1 Aging conditions of tungsten powders (particle size: 0.45-0.59 lm) and the products after aging and reaction with hydrogen
peroxidet (°C)

Atmosphere/solvent

PH2O (hPa)

Duration

Product after aging

Main product after reaction

45

Air

96

1d

WO3H2OWO3H2O2H2O

3d

WO3H2O + h-WO3

h-WO3

7d

WO3H2O + h-WO3

h-WO3

14 d

WO3H2O + h-WO3

h-WO3

21 d

WO3H2O + h-WO3

h-WO3

28 d

WO3H2O + h-WO3

h-WO3

275

Air

23

12 h

Triclinic WO3Triclinic WO3

20

Water

—

7d

WO3H2OWO3H2O

100

Water

—

12 h

WO3H2O + h-WO3

WO3H2O + h-WO3

100

Water, 1 MPa

—

12 h

WO3H2OWO3H2O|512

TSUYUMOTOF I G U R E 2 XRD patterns of (A) hWO3 obtained as a precipitate by thereaction between tungsten powder (particle
size: 0.45-0.59 lm) aged in air at 45°C anda relative humidity of 100% (PH2O
9.58 kPa) for 28 days and hydrogen
peroxide, and (B) tungsten powder (particle
size: 0.45-0.59 lm) aged in air at 45°C anda relative humidity of 100% (PH2O
9.58 kPa) for 28 days. ▼: W, ▽:WO3H2O, ↓: h-WO3 [Color figure can beviewed at wileyonlinelibrary.com]

the metastable phase, h-WO3, is easy to occur on the low
crystalline metal surface of ultrafine powder rather than thehigh crystalline surface of the larger particles. The Rietveld
refined structural parameters for h-WO3 from two types oftungsten powders after storage for 12 years and after aging
for 28 days at 45°C, RH 100% are summarized in Table 2.
All the data were well fitted with the space group P63/mcm(Z = 6), and the crystal structures proved to be similar to
those in the earlier reports.11,12 Gerand et al described thestructure using the space group P6/mmm (Z = 3), and theO(1) atoms between the W layers were fixed at 3 g sites(1/2, 0, and 1/2) with high symmetries. We located the O(1) atoms at 6g sites (x, 0, 1/4) with lower symmetries inthe space group P63/mcm (Z = 6) and obtained the structural parameter, x: 0.500, as fitting results for the both samples. The O(1) atoms positions converged to the sameatomic positions as the 3 g sites (1/2, 0, and 1/2) in the(A)

T A B L E 2 Refined crystallographic data of h-WO3 obtained fromtungsten powder aged for 12 years (A) and aged at 45°C, RH 100%for 28 days (B)(a) Hexagonal; space group; P63/mcm (193)
a = 0.74538(16) nm, c = 0.77194(20) nm Z = 6
Atom

Site

W

6f (1/2, 0, 0)

Atomic position

OccupancyO(1)6g (x, 0, 1/4)

x: 0.500(7)

1.0

O(2)

12i (x, 2x, 0)

x: 0.1982(16)

1.0

1.0

(B)

Interatomic distances/nm
W-O(1)

2 9 0.1930

O(1)-O(2)8 9 0.2765

W-O(2)

4 9 0.1980

O(2)-O(2)2 9 0.2559
2 9 0.3021(b) Hexagonal; space group; P63/mcm (193)
a = 0.74219(13) nm, c = 0.77198(13) nm Z = 6
Atom

Site

W

6f (1/2, 0, 0)

Atomic position

OccupancyO(1)6g (x, 0, 1/4)

x: 0.500(3)

1.0

O(2)

12i (x, 2x, 0)

x: 0.1913(15)

1.0

1.0

Interatomic distances/nm
W-O(1)

2 9 0.1930

O(1)-O(2)8 9 0.2783

W-O(2)

4 9 0.2003

O(2)-O(2)2 9 0.2459
2 9 0.3163

F I G U R E 3 FE-SEM images of h-WO3 powders prepared fromtungsten powders stored for 12 years (A) and aged at 45°C, and RH
100% for 28 days (B)| 513TSUYUMOTO

space group P6/mmm, and consequently we obtained thesame fitting results for the two space groups P63/mcm andP6/mmm. The result for the space group P63/mcm is presented here because this is more supportive of the atomic
position of O(1). The interatomic distances between W andO were calculated at 0.1980-0.2003 nm along the ab plane
and 0.1930 nm along the c-axis, and these are in good
agreement with the reported W-O single bonding distances.12,22 The O-O distances between neighboring oxygens vary from 0.2459 nm to 0.3163 nm, and this brings
about the distortion of the WO6 octahedra, which arestacked parallelly to the c-axis. Additionally, Oi et al used
more general sites of the same space group P63/mcm andreported remarkably distorted WO6 octahedra. We also
considered such atomic arrangement, but the fitting results
were either almost the same as the above results or divergent. Thus, we adopted the present atomic arrangement.
The crystallite sizes of h-WO3 estimated from the peak
profile parameters using the Scherrer equation were
14.2 nm and 12.6 nm, respectively, for the samples from
12-year-old tungsten powder and from 28-day aged tungsten powder. The BET specific surface areas of h-WO3
were 16.9 m2/g and 60.3 m2/g, respectively, for the samples from 12-year-old tungsten powder and from 28-day
aged tungsten powder. If we assume spherical particles, theaverage particle sizes are calculated at 57.1 nm and15.9 nm, respectively. Figure 3 shows the FE-SEM imagesof h-WO3 powders prepared from 12-year-old tungsten
powder and from 28-day aged tungsten powder. The aggregates of nanocrystalline particles are observed for both of
the samples. The crystallite size of h-WO3 from 28-day
aged tungsten powder was smaller than that from 12-yearold tungsten powder, and this is in good agreement withthe results obtained from the BET analysis. The crystallite
sizes of both of h-WO3 were much smaller than the particle
size of the raw material tungsten powder (0.45-0.59 lm)
and varied depending on the preparation method. This suggests that the formation process of h-WO3 is a topochemical oxidation through powder surface involving disruption
to nanocrystalline particles. This room-temperature process
allowed the formation of nanocrystalline and metastablephase by suppressing further crystal growth and transformation into more stable phase. This method is advantageous in preparation of nanocrystalline and metastable
metal oxides because a high-temperature crystal growth
process is not involved in the reaction unlike the solid-state
reactions and the hydrothermal methods.

4| CONCLUSIONS

Nanocrystalline h-WO3 with crystallite size of 12.6-14.2 nm
was facilely obtained as a precipitate in the direct reaction

between tungsten powder and hydrogen peroxide. This new
synthetic route necessitates surface oxide film having thesimilar crystal structure as h-WO3, which can be obtained byaging the tungsten powder under conditions such as in air at
45°C, RH 100% for 3-28 days. The surface oxide film acts
as a seed crystal for the crystal growth and leads to the formation of the metastable phase. The crystal structure of thepresent h-WO3 was well fitted with the space groups P63/
mcm and P6/mmm in the Rietveld refinement and confirmed
to be the same as that first reported by Gerand et al. Our
method is advantageous over the conventional ones in that it
is a facile room-temperature process and allows the formation of metastable and nanocrystalline phases. We consider
the method will be applied to the preparation of the othermetal oxides by using direct reactions of metallic powders
such as titanium, molybdenum, niobium, and vanadium withhydrogen peroxide.
ACKNOWLEDGMENTS
The author is sincerely grateful to Mr. A. Edano and Mr.
T. Kousaka for experimental assistance and to Prof. M.
Sakamoto for the FE-SEM measurement. This work was
supported by a Grant-in-Aid for Scientific Research (C),No. 16K05946, from the Japan Society for the Promotionof Science (JSPS).
ORCID
Isao Tsuyumoto

http://orcid.org/0000-0003-1644-6077

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How to cite this article: Tsuyumoto I. Facile
synthesis of nanocrystalline hexagonal tungsten
trioxide from metallic tungsten powder and hydrogen
peroxide. J Am Ceram Soc. 2018;101:509–514.
https://doi.org/10.1111/jace.15250