synthesis of wo3tio2 nanocomposites via sol–gel method

Journal of Alloys and Compounds 398 (2005) 200–202
Synthesis of WO
3
/TiO
2
nanocomposites via sol–gel method
Huaming Yang
a,∗
, Rongrong Shi
a
, Ke Zhang
a
, Yuehua Hu
a
, Aidong Tang
b
, Xianwei Li
c
a
Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
b
Institute of Functional Materials, School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China
c
Institute of Resources and Environmental Engineering, Technology Centre, Baoshan Iron and Steel Co. Ltd., Shanghai 201900, China
Received 23 December 2004; received in revised form 31 January 2005; accepted 1 February 2005
Available online 4 March 2005
Abstract
Synthesis of WO
3
/TiO
2

nanocomposites by a sol–gel method was investigated using differential thermal analysis (DTA), X-ray diffraction
(XRD) and transmission electron microscopy (TEM) techniques. Thermal treatment of the precursor at 400
◦
C in air resulted in the formation
of WO
3
/TiO
2
nanocomposites with a particle size of about 60nm. The c-axis parameter of TiO
2
in the WO
3
/TiO
2
nanocomposites, lower than
that of pure TiO
2
, increased with increasing calcination time. Doping TiO
2
with WO
3
can lower its band gap and shift its optical response to
the visible region. This nanocomposite should be effective as a visible-light-driven photocatalyst.
© 2005 Elsevier B.V. All rights reserved.
Keywords: WO
3
/TiO
2
nanocomposites; Photocatalysis; Sol–gel method; Lattice parameters
1. Introduction

It is well known that nanosized TiO
2
powder is one of
the suitable semiconductors for photocatalyst and has been
widely applied in various photocatalytic fields, such as en-
vironmental purification, decomposition of organic contam-
inants and water photosplitting into H
2
and O
2
[1–5].How-
ever, its properties, not only the photo-efficiency or activity
but also the photoresponse, are not sufficient [6]. The vital
snag of TiO
2
semiconductor is that it only absorbs a small
portion of solar spectrum in the ultraviolet (UV) region (band
gap energy of pure TiO
2
is 3.2 eV). Hence, in order to absorb
maximum solar energy, it is necessary to shift the absorption
threshold towards the visible region. The high recombina-
tion ratio of photo-induced hole–electron pairs also reduces
its catalytic efficiency. Recently, various modifications have
been performed on nanosized TiO
2
to extend its optical ab-
sorption edge into the visible light region and to improve its
photocatalytic activity, including surface modification, metal
depositing, transition metal and transition metal oxide com-

plexes [7–13]. Coupling TiO
2
with WO
3
, which is a semi-
∗
Corresponding author. Tel.: +86 731 8830 549; fax: +86 731 8710 804.
E-mail address: (H. Yang).
conductor used as photocatalyst (E
g
= 2.8 eV), can achieve
an efficient charge separation. It is reported that WO
3
/TiO
2
nanocomposites have higher photocatalytic activity [14,15].
In this paper, synthesis of WO
3
/TiO
2
nanocomposites via the
sol–gel method was attempted.
2. Experimental details
The starting materials were AR-grade Ti(OBu
4
), am-
monium tungstate and anhydrous alcohol. Ten millilitres
Ti(OBu
4
) was dissolved in 10 ml anhydrous alcohol, and ul-

trasonicallydispersedtoformamixture.Five millilitres water
was slowly dripped into the mixture, which wasstirred for1 h
at room temperature. Then different additions of ammonium
tungstate solution were dripped into the mixture according to
the required amount of WO
3
in the WO
3
/TiO
2
nanocompos-
ites. The pH value of the solution was kept to be 10. The so-
lution was aged for 12h at ambient temperature, followed by
filtering, washing for several times with deionized water and
anhydrous alcohol, drying at 80
◦
C for 12h to produce a pre-
cursor. Subsequent calcination of the precursor at 400
◦
C for
different hours in air resulted in the formation of WO
3
/TiO
2
nanocomposites.
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.02.002
H. Yang et al. / Journal of Alloys and Compounds 398 (2005) 200–202 201
Differential thermal analysis (DTA) of the precursor was
carried out using an SDT2960 thermal analyzer at a heat-

ing rate of 10
◦
/min. The structure of the sample was ex-
amined using a D/max-␥A diffractometer (Cu K␣ radiation,
λ = 0.154056nm). The morphology of the nanocomposites
was observed using a JEM-200CX transmission electron mi-
croscope (TEM). The UV–vis absorption spectra of the sam-
ples were recorded on a Shimadzu UV-3101PC spectropho-
tometer.
3. Results and discussion
TheprecursorwassubjectedtoDTAanalysis.Thepurpose
was to determine thetemperatures of possible decomposition
and phase changes of the precursor during the thermal treat-
ment. Fig. 1shows the typicalDTAcurves forthe as-prepared
precursor. The exothermic peak round 280
◦
C is associated
with the decomposition of residual
OH groups and the con-
densationofnonbondedoxygen.Anexothermicpeakatabout
430
◦
Cwasclearlyobserved,whichpossiblycanbeattributed
to thecrystallization alterationof theanatase TiO
2
phase. But
there still exists a little difference in exothermic peak around
430
◦
C for the nanoparticle precursor with different amount

of WO
3
.
Fig. 2 shows the X-ray diffraction (XRD) patterns of
10 wt% and 20 wt% WO
3
/TiO
2
nanocomposites after cal-
cination at 400
◦
C for 12h. As can be seen, all the peaks can
be assigned to anatase TiO
2
and no crystalline WO
3
was de-
tected. Ma et al. [14] also reported the same result in their
Fig. 1. DTA curves of the precursor with different amount of WO
3
(a)
10 wt%, (b) 20wt% and (c) 40 wt%.
Fig. 2. XRD patterns of the WO
3
/TiO
2
nanocomposites with different
amounts of WO
3
(a) 10 wt% and (b) 20wt%, respectively.

Fig. 3. XRD patterns of 40 wt% WO
3
/TiO
2
nanocomposites after calcined
at 400
◦
C for different hours (a) 4h, (b) 8 h and (c) 12 h, respectively.
studies, they thought that amorphous tungsten oxide phase
covered the TiO
2
surface.
Fig. 3 shows the XRD patterns of 40wt% WO
3
/TiO
2
nanocomposites after calcination at 400
◦
C for (a) 4 h, (b) 8h
and (c) 12h, respectively. As shown in Fig. 3, the WO
3
/TiO
2
nanocomposites prepared by the sol–gel method was ob-
servedtohave theanatasestructureandtungstenoxide,show-
ing the presence of a sharp peak at 25.3
◦
of 2θ which is the
major peak for the anatase TiO
2

. Due to the increase in the
calcination time, the intensities of the peaks associated with
WO
3
increased to some extent. Fig. 4 shows a TEM micro-
graph of the 40 wt% WO
3
/TiO
2
nanocomposites after heat
treatmentat400
◦
Cfor12 h. The particle size of the nanocom-
posites observed in the TEM image was about 60nm indiam-
eter, while monodispersive particles with uniform size were
present.
The lattice parameters (a and c) of pure TiO
2
and
WO
3
/TiO
2
nanocomposites after calcination were calculated
using the formula:
1
d
2
=
h

2
+ k
2
a
2
+
l
2
c
2
(1)
where d is the interplane spacing, h, k and l are all Miller’s
indices. The values were listed in Table 1. The lattice param-
eters of pure anatase TiO
2
obtained by the sol–gel method
are a = b= 3.7523
˚
A, c = 10.0664
˚
A. It is clear that the lattice
parameters increase along the a- and b-axes while the c-axis
parameter decreases as tungsten oxide was doped. Longer
Fig. 4. TEM image of the 40 wt% WO
3
/TiO
2
nanocomposites.
202 H. Yang et al. / Journal of Alloys and Compounds 398 (2005) 200–202
Table 1

The lattice parameters of TiO
2
in WO
3
/TiO
2
nanocomposites
a
Samples Calcination time (h) a (=b)(
˚
A) c (
˚
A)
Pure TiO
2
12 3.7523 10.0664
10 wt% WO
3
/TiO
2
12 3.7961 9.3540
20 wt% WO
3
/TiO
2
12 3.7836 9.6761
40 wt% WO
3
/TiO
2

12 3.7903 9.5307
40 wt% WO
3
/TiO
2
8 3.7908 9.5172
40 wt% WO
3
/TiO
2
4 3.7914 9.5100
a
Calcination temperature was 400
◦
C.
Fig. 5. UV–vis absorption spectra of the sample.
calcination time resulted in an increase in c-axis parameter
of the 40 wt% WO
3
/TiO
2
nanocomposites. The decrease in
the lattice parameters of the WO
3
/TiO
2
nanocomposites in
comparison to pure TiO
2
may be attributed to the decrease

in the cation size in the octahedral site. The W
6+
ions have
a lower ionic radius (41 pm) than Ti
4+
(53 pm) in the octa-
hedral site of TiO
2
. This result also indicates that a doping
effect exists in the WO
3
/TiO
2
composite nanocrystallites.
Fig. 5 shows the absorption spectrum of the WO
3
/TiO
2
nanocomposites. The spectrum shows that the onset of ab-
sorption appears at about 475 nm. The onset of the optical
absorption of WO
3
/TiO
2
particles relative to the bulk anatase
TiO
2
(λ
E
= 387 nm) implies a red shift. The band gap energy

of the nanocomposites can be determined to be 2.67 eV from
the transformed Kubelka–Munk function, while the band gap
energy of pure TiO
2
is about 3.2eV, accordingly, this absorp-
tion feature suggests that the WO
3
/TiO
2
photocatalyst can
possibly be activated by the visible light, which can absorb
the maximum solar energy.
4. Conclusions
In summary, WO
3
/TiO
2
nanocomposites have been suc-
cessfully prepared by a sol–gel method. The intensities of the
XRD peaks associated with TiO
2
increased gradually with
increasing calcination time. Addition of WO
3
resulted in a
decrease in the c-axis parameter of the TiO
2
, which also in-
creasedwithincreasingcalcinationtime.Thisnanocomposite
is promising for high-performance visible-light-driven pho-

tocatalysts. A detailed study on the photocatalytic activity of
the nanocomposite is in progress.
Acknowledgement
This work was financially supported by the National
Natural Science Foundation of China (Nos. 50304014,
50474046).
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