Sensors and Actuators B 142 (2009) 236–242
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Glucose-mediated hydrothermal synthesis and gas sensing characteristics of
WO
3
hollow microspheres
Choong-Yong Lee, Sun-Jung Kim, In-Sung Hwang, Jong-Heun Lee
∗
Department of Materials Science and Engineering, Korea University, Anam-Dong, Sungbuk-Gu, Seoul 136-713, Republic of Korea
article info
Article history:
Received 21 May 2009
Received in revised form 29 July 2009
Accepted 18 August 2009
Available online 21 August 2009
Keywords:
WO
3
NO
2
sensor
Hollow microspheres
Carbon template
abstract
Tungsten-coated carbon microspheres were prepared by one-pot hydrothermal reaction of an aqueous
solution containing glucose and sodium tungstate. The spheres were converted into WO
3
hollow micro-
spheres by the decomposition of their core carbon. The [glucose]/[sodium tungstate] ratio of the stock

effect [8]. Although one-step reaction is simple and convenient,
the precise control of wall thickness and/or hollow morphology
remains a challenging issue. By contrast, the thinness and porosity
of the wall and the size of the hollow spheres can be manipulated by
optimizing the use of well-defined templates. The chemical routes
include layer-by-layer assembly [9], heterocoagulation [10], and
controlled hydrolysis [11].
∗
Corresponding author. Tel.: +82 2 3290 3282; fax: +82 2 928 3584.
E-mail address: [email protected] (J H. Lee).
Sun and Li [12] suggested the synthetic route to prepare
monodisperse carbon microsphereswith hydrophilic surface by the
hydrothermal reaction of glucose and demonstrated the reactiv-
ity of carbon spheres through the uniform coating of Ag. Titirichi
[13] prepared various metal oxide (Fe
2
O
3
, NiO, Co
3
O
4
, MgO, CuO)
hollow spheres by one-pot hydrothermal reaction of a solution con-
taining metal ions and glucose and subsequent decomposition of
core carbon. In addition, the hollow structures of SnO
2
and ZnO
[14,15] have been prepared by similar chemical routes. However,
to the best of our knowledge, the preparation of WO

WO
4
·2H
2
O, 99% Kanto
Chemical Co., Ltd.) were used as carbon precursor and source
material, respectively. Table 1 shows the composition of the stock
solution for hydrothermal reaction. Glucose (3.963 g, 0.02 mol) was
0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2009.08.031
C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242 237
Fig. 1. (a) Electrode configuration, (b) sensor structure, and (c) schematic diagram of testing system.
dissolved in 20 ml of distilled water and a designated amount of
Na
2
WO
4
·2H
2
O was dissolved in 20 ml of distilled water. After mix-
ing the two solutions and subsequent mild stirring, the solutions
were transferred toa Teflon-lined, stainless steelautoclave (volume
100 cm
3
), sealed, and then heated at 200
◦
C for 24 h. After cooling,
the reaction products were washed with water and ethanol and
dried at 60
◦

ics Co., Ltd.).
2.3. Gas sensing characteristics
The WO
3
spherical powders after HT were prepared in a paste
form and applied to an alumina substrate having two Au electrodes
(Fig. 1(a) and (b), substrate: 1.5 mm × 1.5 mm, spacing between
two electrodes: 0.2 mm). The sensor element was heat treated at
400
◦
C for 30 min to decompose the organic component of the paste.
The films after HT were ∼15 ␮m thick. The sensor was installed
in a quartz tube and the furnace temperature was stabilized at a
constant sensing temperature (300
◦
C) (Fig. 1(c)). The gas concen-
tration was controlled by changing the mixing ratio of the parent
gases (5ppm NO
2
, 200ppm C
2
H
5
OH, 200ppm CH
3
COCH
3
, 100ppm
CO, 100 ppm C
3

4
·2H
2
O
R30 30 0.5 M 0.0166 M
R5 5 0.5 M 0.1 M
R0.5 0.5 0.5 M 1 M
high-purity air (R
a
) with that in the target gases (R
g
). The electri-
cal resistance sensor was monitored using a Picoammeter/Voltage
Source (Keithley 6487) interfaced with a computer.
3. Results and discussion
3.1. X-ray diffraction analysis
Fig. 2 shows the XRD patterns of the R30, R5, and R0.5 powders
prepared by HT of precursors at 450
◦
C for 1 h. The R30 powders
Fig. 2. X-ray diffraction patterns of the powders prepared by heat treatment of (a)
R30, (b) R5, and (c) R0.5 precursors at 450
◦
C for 1 h.
238 C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242
showed a pure monoclinic phase of WO
3
, while the R5 and R0.5
powders were identified as Na
2

despite the removal of a large portion of Na by washing.
3.2. Particle morphology
The morphology of the precursors was closely dependent on
the R values. At R = 0.5 and 5, irregular precipitate morphologies
were prepared (Fig. 3(c) and (e)). In contrast, nearly mono-disperse,
spherical precursors with clean surface were prepared (Fig. 3(a)).
The morphology remained similar after HT at 450
◦
C for 1 h
(Fig. 3(b), (d) and (f)). Thus, the morphology of the powders could
be designed in the stage of hydrothermal reaction. The WO
3
micro-
spheres after HT (R30 powders) consisted of many small primary
particles (10–20 nm) (inset in Fig. 3(b)), whereas the average pri-
mary particle sizes of R5 and R0.5 powders were ∼400 nm and
∼1 ␮m (insets in Fig. 3(d) and (f)). These results indicated that the R
values determined the phase, morphology and the primary particle
sizes of the powders.
The scale bar of Fig. 3(b) is 10 times smaller than that of Fig. 3(a).
The sizes of more than 300 spheres were measured using the SEM
observation of well-dispersed precursors and powders. The aver-
age particle diameters of the R30 precursors and R30 powders were
1300 ± 249 nm and 226 ± 41 nm, respectively, indicating that the
spherical precursors shrank to 17% of their precursor’s diameter
during HT. The significant decrease of sphere sizes during HT in
the present study can be attributed to the decomposition of the
core carbon and subsequent sintering between primary particles.
To confirm this, the R30 precursors were heat treated at a more
rapid heating rate for a shorter duration at HTT (Fig. 4). The sphere

of the powders showed a solid and nano-porous morphology but
a few spheres showed a hollow morphology (arrow and inset in
Fig. 4(c)). In contrast, most of the spheres showed a hollow mor-
phology due to the rapid heating to HTT (R30-RH powders and
Fig. 4(b)), although solid and nano-porous spheres were also found.
The hollow shell was ∼30 nm thin (inset in Fig. 4(b)).
Sun and Li [12] suggested that monodisperse carbon micro-
spheres with hydrophilic surface can be prepared by the
hydrothermal reaction of glucose. The polymerization of glucose
into aromatic compounds and oligosaccharides and their subse-
quent carbonization into spheres were suggested as the formation
mechanism. The OH and CHO groups bonded to the surfaces of
the carbon spheres are advantageous for the reaction with metal
cations. Titirichi [13] prepared metal-ion-coated carbon spheres by
one-pot hydrothermalreaction of a solution containing glucose and
metal salts and transformed these spheres into metal oxide hollow
spheres by the decomposition of the core carbon.
In the present study, the hollow morphologies were found at
the R30-RH powders. This indicates that the carbon spheres with
hydrophilic surfaces are formed by the cross linking between glu-
cose in the beginning stage of reaction and W ions are coated on the
negatively charged surface of carbon spheres in the later stage of
reaction. Thus, the precursors in Fig. 4(a) can be regarded as carbon
spheres loosely coated with W-precursors. The precursor spheres
were transformed to hollow WO
3
spheres by the decomposition
of the core carbon (Fig. 4(b)) and then further transformed into
solid and nano-porous spheres by the sintering between the WO
3

4
were very
low (not shown), while the R30 and R30-RH powders showed very
high gas responses (as presented below). The large pore volume
of the R30 powders (Fig. 5), despite their apparently solid interior
structure (Fig. 4(c)), was attributed to their evolution from hollow
spheres. The further increase of pore volumes in the size range of
5–20 nm by the employment of rapid heating indicates that the
hollow morphology of R30-RH powders is advantageous to achieve
both of high surface area and large pore volume.
3.4. Gas sensing characteristics
As stated above, the gas responses of the R5 and R0.5 powders
were negligible. Thus, the gas responses of the R30-RH and R30
240 C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242
Fig. 5. Nitrogen adsorption–desorptionisotherm plots and corresponding pore-size
distribution plots of R30-RH, R30, R5, and R0.5 powders.
powders at 300
◦
C were measured and the results are shown in
Fig. 6. Note that the gas response to NO
2
is the R
g
/R
a
value and
those to other gases are the R
a
/R
g

3
H
8
,CH
4
, and C
2
H
5
OH were very low
(1.1–3.3) (Fig. 6(b)). The concentration of NO
2
was 1/100 of those of
the other gases. These results demonstrated the high gas response
and selective detection of WO
3
nano-porous microspheres to NO
2
.
The selective detection and gas responses were enhanced further
by the use of hollow R30-RH powders. The R
g
/R
a
value to 1 ppm NO
2
was increased to 53.9 whereas those to the other gases decreased
to 1.1–1.9 (Fig. 6(a)).
Fig. 6. Gas responses to 1 ppm NO
2

g
/R
a
value and those to other gases are
the R
a
/R
g
values.
The sensing transients to 0.5–2.5 ppm NO
2
of the R30-RH and
R30 powders at 300
◦
C are shown in Fig. 7(a) and (b), respectively.
The sensor resistance in air (R
a
) was rather high (40–50 M). When
exposed to NO
2
, the sensor resistance greatly increased up to a few
G level. The gas responses of the R30-RH sensor to 0.5–2.5 ppm
NO
2
were 38.6–81.5, which were 2.2–2.9 times higher than those
of the R30 sensor (13.4–36.5) (Fig. 7(c)).
Table 2 summarizesthe NO
2
responses of theundoped WO
3

C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242 241
Table 2
Gas responses to NO
2
in the present study and those reported in the literature [16–27].
WO
3
sensing materials (preparation) [NO
2
] R
g
/R
a
Sensing temperature (
◦
C) Reference
Hollow microspheres 1 ppm 53.9 300 Present study
Thin film (thermal evaporation) 0.5 ppm ∼22 100 [16]
Thin film (rf sputtering) 1 ppm ∼5 370 [17]
Thin film (vacuum thermal deposition) 1 ppm ∼4 200 [18]
Thin film (Aerosol-assisted CVD) 0.2 ppm ∼80 150 [19]
Thin film (reactive magnetron sputtering) 1 ppm ∼450 200 [20]
Nanocrystalline powders 5 ppm ∼75 150 [21]
Lamellar structures (acidification) 1 ppm ∼280 200 [22]
Nanostructures (thermal evaporation) 5 ppm ∼3 250 [23]
WO
3
nanopetals (dealloying of W–Al alloy and thermal oxidation) 5 ppm ∼4 250 [24]
NWs network (thermal evaporation) 1 ppm ∼150 300 [25]
NWs array (thermal evaporation) 1 ppm ∼10 180 [26]

hollow microspheres were prepared by the glucose-
mediated, hydrothermal synthesis of W-coated carbon spheres and
their calcination at 450
◦
C. With the input of increasing thermal
energy during calcination, the hollow WO
3
microspheres were
gradually transformed into solid microspheres. Both hollow and
solid WO
3
microspheres showed high response and selective detec-
tion to 0.5–2.5 ppm NO
2
. In particular, the responses to NO
2
were
increased 2.2–2.9 times by using hollow morphology, which was
attributed to the high surface area for gas sensing and the effec-
tive diffusion of NO
2
toward all the primary particles through the
nano-porous and thin shell layers.
Acknowledgements
This work was supported by KOSEF NRL program grant funded
by the Korean Government (MEST) (No. R0A-2008-000-20032-0)
and a grant from the Fundamental R&D program for Core Tech-
nology of Materials (M2008010013) funded by the Ministry of
Knowledge Economy, Republic of Korea.
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Biographies
Choong-Yong Lee studied materials science and engineering and received his BS
degree from Korea University in 2007. He is currently a master course student at