synthesis and characterization of wo3 nanostructures prepared by an aged-hydrothermal method

Synthesis and characterization of WO
3
nanostructures prepared
by an aged-hydrothermal method
R. Huirache-Acuña
a,b,
⁎
, F. Paraguay-Delgado
c,1
,M.A.Albiter
d
,J.Lara-Romero
d
,
R. Martínez-Sánchez
c
a
CFATA-UNAM, Boulevard Juriquilla 3001, Juriquilla Querétaro, 76230, Mexico
b
Universidad La Salle Morelia, Av. Universidad 500, Mpio. Tarímbaro Mich., 58880, Mexico
c
Centro de Investigación en Materiales Avanzados, S.C. CIMAV, Laboratorio Nacional de Nanotecnología-Chihuahua, Miguel de Cervantes
120, Complejo Industrial Chihuahua, Chih., 31109, Mexico
d
Facultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, Morelia Mich., 58000, Mexico
ARTICLE DATA ABSTRACT
Article history:
Received 7 November 2008
Receivedin revised form 5 March 2009
Accepted 9 March 2009
Nanostructures of tungsten trioxide (WO

3
) have been successfully synthesized by using an
aged route at low temperature (60 °C) followed by a hydrothermal method at 200 °C for 48 h
under well controlled conditions. The material was studied by X-ray diffraction (XRD),
scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS),
transmission electron microscopy (TEM) and high-resolution transmission electron
microscopy (HRTEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS).
Specific Surface Area (S
BET
) were measured by using the BET method. The lengths of the WO
3
nanostructures obtained are between 30 and 200 nm and their diameters are from 20 to
70 nm. The growth direction of the tungste n oxide nanostructures was determined along
[010] axis with an inter-planar distance of 0.38 nm.
© 2009 Elsevier Inc. All rights reserved.
Keywords:
Nanostructures
Tungsten trioxide (WO
3
)
Hydrothermal method
1. Introduction
In recent years, studies of transition metal oxides nanos-
tructures have become impor tant due to the different
potential applications, such as nanoelectronics [1], gas sensors
[2,3], optical devices [4], electrochromic windows [5,6], and
catalysts [7 , 8] . Nanostructures of tungsten oxide having
nanometer scale had been formed by using different condi-
tions and preparation methods: electrospinning [9], oxidation
of a substrate under appropriate conditions and the deposition

of tungsten oxide from a tungsten foil heated in the presence
of oxygen [10], by heating a tungsten filament in a partial
oxygen atmosphere [11], by reacting WO(OMe)
4
under auto-
genic pressure at elevated temperature followed by annealing
[12], by hot filament chemical vapor deposition [13], physical
vapor deposition process [14] and by ultrasonic spray and laser
pyrolysis techniques [15–17]. Recently, Therese et al. [18] and
Ha et al. [19] reported the synthesis of WO
3
nanostructures by
following a hydrothermal route. They used as raw materials
ammonium salts and some additives to control the formation
of nanomaterials. In comparison with the methods mentioned
before, the hydrothermal route is an economical preparation
method of nanostructures since it does not require an
expensive experimental setup. In this work, we report the
synthesis and characterization of WO
3
nanostructures pre-
pared by following an easy two step aged-hydrothermal
MATERIALS CHARACTERIZATION 60 (2009) 932– 937
⁎ Corresponding author. CFATA-UNAM, Boulevard Juriquilla 3001, Juriquilla Querétaro, 76230, Mexico. Tel.: +52 442 238 11 43;
fax: +52 442 238 1 1 65.
E-mail address: (R. Huirache-Acuña).
1
Present address: National Institute for Nanotechnology, 11421 Saskatchewan Drive Edmonton (AB) Canada T6G 2M9.
1044-5803/$ – see front matter © 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.matchar.2009.03.006

method at low temperature by using ammonium metatung-
state as tungsten source and without the presence of
additives.
2. Experimental
2.1. Synthesis of WO
3
Nanostructures
WO
3
nanostructures were synthesized by using an aged-
hydrothermal route. A saturated aqueous solution of ammo-
nium metatungstate [(NH
4
)
10
W
12
O
41
xH
2
O] (0.15 mol of W) was
prepared and acidified with HNO
3
2.2 N (Normal) to produce a
pH around 5 and then kept in a flask hermetically sealed with
stirring by one week at 60 °C. Then, 5 ml of the aged solution
was deposited into a Teflon-lined stainless steel autoclave and
heated at 200 °C for 48 h. The material obtained was filtered
and washed with deionized water and dried in the presence of

air at room temperature.
2.2. Characterization
A scanning electron microscope (JEOL JSM 5800 LV) was used
to perform morphological analysis. Several fields were analyzed
at different magnifications in order to get information of the
prevalent features. The elemental composition was determined
using energy dispersive spectroscopy (EDS) (Oxford Inca X-
Sight). Specific surface area (S
BET
) determination was made with
a Quantachrome AUTOSORB-1 model by nitrogen adsorption at
− 196 °C using the BET isotherm. Samples were degassed under
flowing argon at 200 °C for 2 h before nitrogen adsorption. TEM
and HRTEM micrographs were obtained in a Philips TECNAI F20
FEG transmission electron microscope operated at 200 kV. X-ray
analysis was made with a Philips X Pert MPD diffractometer,
equipped with a graphite monochromator, copper Kα radiation
with wavelength λ=1.54056 Å, operated at 43 kV and 30 mA.
Raman spectroscopy was performed using a Labram system
model Dilor micro-Raman equipped with a 20 mW He–Ne laser
emitting at 632.8 nm and a holographic notch filter made by
Kaiser Optical Systems, Inc. (model supertNotch-Plus) with a
256×1024-pixel char ge-coupled device (CCD) used as the
detector; and a computer-controlled XY stage with a spatial
resolution of 0.1 µm with two interchangeable gratings (600 and
1800 g mm
− 1
) and a confocal microscope with 10, 50, and 100×
objectives. All measurements were collected at room tempera-
ture with no special sample preparation. Oxidation state and

surface composition were analyzed by an X-ray photoelectron
spectroscope, Energy Spectrometer EA 11 MCD, using Mg
monoenergetic soft X-ray (Kα =1253.6 eV).
3. Results and Discussion
The synthesis method presented in this work for procuring
WO
3
nanostructures is a modification of the method reported
by Albiter et al. for MoO
3
nanorods [20] and Therese et al. for
WO
3
nanostructures [18]. The main difference between
Therese et al. method and ours is that we did not use additives
to form the nanostructures. On the other hand, nitric acid
(HNO
3
) was not used before the hydrothermal treatment as
reported by Albiter et al.
To convert W
12
O
41
10−
anions to neutral W
12
O
36
, excess

divalent oxygen anions must be removed. Stoichiometrically,
five divalent oxygen anions per W
12
O
41
10−
must be combined
with protons from the acidic medium:
W
12
O
10−
41
þ 10H
þ
→12WO
3
þ 5H
2
O ð1Þ
According to reaction (1), high concentrations of both
W
12
O
41
10−
and H
+
would shift the reaction to the right ensuring
the formation of WO

3
, although many intermediate steps and
Fig. 1 – XRD pattern for WO
3
nanostructures. Where all
reflections are indexed based on a hexagonal WO
3
cell.
Table 1 – Crystallite size (ϕ) determi ned by Scherer
equation (ϕ=Kλ/βCosθ), where K=0.9, λ=1.54 Å, β is
FWHM in radians and θ is the glancing angle.
(hkl) 100 001 200
t (Å) 209.221 443.202 210.485
Fig. 2 – N
2
adsorption–desorption isotherm at − 196 °C of
tungsten oxide nanostructures.
933MATERIALS CHARACTERIZATION 60 (2009) 932– 937
thus compounds and phases may exist. It is thus anticipated
that the formation of WO
3
should have a strong dependence
on the acid medium. Furthermore, the aging time in solution
and the hydrothermal treatment time inside the autoclave
have a strong influence in the formation of nanostructures.
Paraguay-Delgado et al. [21], concluded that two conditions
are important for procuring nanostructures, the first being
that for an extended-time-aged solution a short hydrothermal
treatment is required (about 24 h) and the second being that
for a short-time-aged solution (1 week) at least 36 h of

hydrothermal treatment is required.
The XRD pattern from WO
3
nanostructures is reported in
Fig. 1 where a well crystallized phase was observed. The half-
widths reflection indicates the presence of nanoscale tung-
sten oxide which was corroborated measuring the crystallite
size (ϕ) for more intense and representative reflections (100),
(001) and (200) (Table 1) by using the Scherer equation:
/ =
Kk
bCosh
ð2Þ
The observed peaks could be indexed based in a hex-
agonal cell with inter-planar spacings for tungsten t rioxide
(ICSD 32,001, J CPDS 33-1387; a =7.298 Å, c=3.899 Å, space
group P6/mm) [18,19]. It was also observed that there are not
other impurity phase peaks.
As observed, Fig. 2 shows the N
2
adsorption–desorption
curve corresponding to a type IV isotherm (IUPAC Classifi-
cation) with desorption step characteristic of mesoporous
materials above t he re lative p ressure (P/Po) of 0.4 a nd
specific surface area (S
BET
) values between 34 and 35 m
2
(Table 2). The formation of a mesoporous material is due to
the water vapor pressure inside the autoclave at 200 °C, at

this time the exactly mechanism of formation is not right
known [22].
Fig. 3a–c shows SEM micrographs at different magnifica-
tions from separable WO
3
nanostructures with different size
protruding out. The oxide nanostructures had smooth sur-
faces and a not well-defined rectangular cross section. These
particles were about 0.1 to 3 µm long, and 50–200 nm wide as
determined from SEM images. As illustrated in Fig. 3a–c this
method led to the formation of nanostructures in a wide range
of thicknesses, most of which had shown an irregular shape.
The oxygen (O) and tungsten (W) atomic contents were
determined by Energy Dispersive Spectroscopy (EDS) analysis
(1% error) and the results are reported in Table 2. The EDS
spectrum presented in Fig. 3d reveals a 3:1 atomic ratio for
oxygen and tungsten elements, which solely constitute the
composition of WO
3
.
Transmission electron microscopy (TEM) micrographs of
WO
3
nanostructures are reported (Fig. 4a–b). A representative
TEM image of the tungsten oxide nanostructures is given in
Table 2 – Elemental analysis determined by EDS (% atomic)
and specific surface area (S
BET
).
Sample % at. O % at. W S

BET
(m
2
/g)
WO
3
(I) 75 25 35
WO
3
(II) 75 25 34.7
Fig. 3 – Scanning electron microscopy images of WO
3
nanostructures at different magnifications: (a) 5000×, b) 14,000× and
c) 15,000× (d) EDS spectrum.
934 MATERIALS CHARACTERIZATION 60 (2009) 932– 937
Fig. 4a. This sample consists of very well separated particles
with irregular shape and lengths between 30 and 200 nm and
wide from 20 to 70 nm, aggregated together due to the high
surface energy owing to their nanosize. Fig. 4b shows a TEM
micrograph at a higher magnification. By using HRTEM we
observed that the growth direction of the tungsten oxide
nanostructures is along [010] axis with an inter-planar
distance of 0.38 nm (Fig. 4b). It seems that the WO
3
growth
proceeds layer by layer increasing the thickness and the width
of the nanostructures.
Raman spectroscopy was used to characterize this
material since this technique is suitable to obtain details
of the WO

3
chemical structure (Fig. 5). Three broad bands
were clearly detected: high in the 900–1000 cm
− 1
region,
medium in the 600–800 cm
− 1
region and low in the 200–
400 cm
− 1
region. The most intense peak is centered at
780 cm
− 1
with a shoulder at 730 cm
− 1
and they are attributed
to the symmetric and asymmetric vibrations of W
6+
–O
bonds (O–W–O st re tch ing mod es ). T wo pea ks cen te red at
320 and 270 c m
− 1
can be found in the 200–400 range and
correspond to W–O–W bending modes of the bridging
oxygen [23–25]. A peak at 910 cm
− 1
with a s houlder
positioned at 960 cm
− 1
in the 900–1 000 cm

− 1
can be
observed. These peaks correspond to the WfOstretching
mode of terminal oxyg en atoms th at are present on th e
surface of the cluster (dangling bonds) or at the boundaries
of nanometre grains [15,26].Thesmallfeatureobservedat
435 cm
− 1
is attributed to the characteristic band of crystal-
line WO
3
[27]. Note that these results confirm the formation
of hexagonal WO
3
since the main features corresponding to
monoclinic WO
3
typically reported at 807 and 715 cm
− 1
are
absent in the R aman spectrum [28–30].
The XPS collected spectra of the material for the peaks O
1 s
and W
4f
are shown in Fig. 6a and b respectively. Peaks position
for O
1 s
is 530.3 eV and binding energy peak located at 35.4 and
40.6 eV is attributed to W

4 f
. According to the literature [31] it is
WO
3
, this means that the surface of the material contains W
6+
and not other oxidation state for this metal was detected. This
was also verified by calculating the O/W ratio using their
relative peak areas (I) and atomic sensitivity factors (S), which
is shown below (Eq. (3)):
Atomic Ratio
O
W
=
I
0
S
0
I
W
S
W
=
47483:97
0:66
63692:91
2:75
=3:1: ð3Þ
4. Conclusions
WO

3
nanostructures with hexagonal phase and mesopor-
osity were obtained by using a two step aged-hydrothermal
method. By using HRTEM we observed that the growth
direction of the tungsten oxide nanostructures is along [010]
Fig. 4 – Transmission electron microscopy (TEM) micrographs
of WO
3
nanostructures.
Fig. 5 – Typical Raman spectrum of hexagonal WO
3
nanostructures obtained by the aged-hydrothermal method.
935MATERIALS CHARACTERIZATION 60 (2009) 932– 937
axis with an inter-planar distance of 0.38 nm. Raman, EDS and
XPS analysis confirmed that the chemical structure and
oxidation states belong to tungsten oxide (WO
3
).
Acknowledgements
The authors appreciate the valuable technical assistance
of M.C. W. Antúnez, M. I.Q. Alicia d el Real, M.C. E. Torres, Ing.
C. Ornelas, Dr. A. Medina, Dr . Ismeli Alfonso and M.C. F.
Rodríguez Melgarejo. This work was fin anciall y supported by
CIMAV, S.C., Universidad La Salle Morelia and Postdoctoral
fellowship-UNAM.
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