Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO
2
-sensing properties of tungsten oxide nanostructures,
Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.063
ARTICLE IN PRESS
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SNB-12446; No. of Pages 7
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Microstructure characterization and NO
2
-sensing properties of tungsten oxide
nanostructures
Yuxiang Qin
∗
, Ming Hu, Jie Zhang
School of Electronics and Information Engineering, Tianjin University, No 92, Weijjin Road, Nankai District, Tianjin 300072, PR China
article info
Article history:
Received 16 November 2009
Received in revised form 28 June 2010
Accepted 29 June 2010
Available online xxx
Keywords:
Tungsten oxide
Nanowires
Nanosheets
Solvothermal synthesis
Gas sensors

◦
C over NO
2
concentration ranging from 1 to 20 ppm. Both nanowires and nanosheets
exhibit reversible response to NO
2
gas at different concentrations. In comparison to WO
3
nanosheets,
W
18
O
49
nanowire bundles showed a much higher response value and faster response–recovery charac-
teristics to NO
2
gas, especially a much quicker response characteristic with response time of 19 s at the
concentration of 5 ppm.
© 2010 Published by Elsevier B.V.
1. Introduction
With theindustrialdevelopment, airpollutionis becomingmore
and moreserious. Especially, nitrogenoxide NO
x
(NO
2
or NO)which
results fromcombustionand automotiveemissions is amain source
of acid rain and photochemical smog [1]. So detection of toxic
NO
x

[10,11], ZnO [12]
and In
2
O
3
[13] with well-established nanostructure have exhib-
ited higher sensitivity and quicker response to detect gases at
low concentrations than the corresponding thin film materials
[14,15]. Likely, tungsten oxide in nanostructures like nanowires,
nanosheets and nanorods were investigated [16–18], and they
revealed good sensing properties while detecting toxic and haz-
ardous gases. For example, very good results for H
2
S gas sensor
based on tungsten oxide nanowires and nanosheets have been
reported. Chen co-workers [19] found that the single-crystalline
potassium-doped tungsten oxide nanosheets could exhibit high
sensitivity, fast response time and good stability to H
2
S, acetone
and Cl
2
. The investigation of Rao co-workers [20] indicated that
the WO
2.72
nanowires had much higher response value to H
2
S
than the nanoparticles or nanoplatelets of WO
3

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Furthermore, the nanowires exhibit a much quick response char-
acteristic with response time of 19 s to 5 ppm NO
2
.
2. Experimental
Nanowires and nanosheets of tungsten oxide were synthesized
by solvothermal method with tungsten hexachloride (WCl
6
)as
precursor and 1-propanol as solvent. First, a certain amount of
WCl
6
was dissolved in a little ethanol to form a solution in a
beaker. The ratio of WCl
6
mass to ethanol volume is 0.1 g/ml. Then,
1-propanol was added to the solution which was subsequently
transferred to and sealed in a 100 ml Teflon-lined stainless steel
autoclave. The volume of 1-propanol is80 mland the concentration
of WCl
6
in 1-propanol varied from 0.01 to 0.02M in our experi-
ments. The solvothermal reaction was conducted at 200
◦
C for 9 h
in an electric oven. After that, the autoclave was cooled naturally to

synthesized tungsten oxide nanostructureson the cleaned alumina
substrates which were attached with a pair of interdigitated Pt
electrodes with a thickness of 100 nm. Fig. 1(a and b) shows the
schematic diagrams of the interdigitated electrodes and the sensor
respectively. The electrodes were deposited using RF magnetron
sputtering method. The coating slurry was prepared by ultrasoni-
cally dispersing tungsten oxide powders in mixed organic solvents
of terpineol and ethanol with 2:1 volume ratio for 2 h. A physical
mask is used during spin coating to avoid the presence of slurry
at the end of the substrate. The coated sensing films were dried in
air for 30 min subsequently annealed at 300
◦
C for 1 h at ambient
atmosphere in order to burn out the organic solvent used in prepa-
ration of coating paste and to enhance the adherence of the sensing
film to the sensor substrates. Temperature was raised from ambi-
ent to 300
◦
C using a slow ramp of 2.5
◦
C/min in order to avoid the
occurrence of cracks in the films.
The NO
2
gas sensing measurements were carried out in a
home-built computer-controlled static gas sensing characteriza-
tion system consisting of a glass test chamber, a flat heating plate,
a professional digital multimeter and a data acquisition system.
The schematic diagram of the gas sensing test system is shown in
Fig. 1(c). The sensors were placed on the heating plate fixed in test

the resistance of the sensor in the measuring gas and that in clean
air, respectively. The response time is defined as the time required
for the resistance rising to 90% of the equilibrium value since the
test gas is injected. Conversely, therecovery time is the time for the
resistance in equilibrium to go down to 10% of the original value in
air since the test gas is released.
3. Results and discussion
3.1. Structural characterization
The morphologies of tungsten oxide nanostructures synthe-
sized at different WCl
6
concentrations after annealing at 300
◦
C
for 1 h were shown in Fig. 2(a–d). It can be seen from Fig. 2(a),
Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO
2
-sensing properties of tungsten oxide nanostructures,
Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.063
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Fig. 2. (a, c, d) SEM images of tungsten oxide nanostructures synthesized at WCl
6
concentration of 0.01M, 0.015M and 0.02 M, respectively, after annealing at 300
◦
C for 1h.
(b) TEM image of the annealed tungsten oxide synthesized at 0.01M. The insets in (a) and (d) are the SEM images of the corresponding product before annealing.
the product synthesized at the WCl

At higher concentration, the highly saturated WCl
6
could prohibit
the growth of tungsten oxide nanowires along the main growth
direction.
XRD analysis was carried out to identify the crystalline struc-
ture of the tungsten oxide before and after annealing at 300
◦
C for
1h. Fig. 3(a, c, e) and (b, d, f) respectively shows the XRD pat-
terns of the as-synthesized and annealed samples. As shown in
Fig. 3(a), the main diffraction peaks of the bundled nanowires syn-
thesized at WCl
6
concentration of 0.01 M can be well indexed as
the monoclinic cell of W
18
O
49
with cell parameters of a =18.32 Å,
b =3.79 Å, c = 14.04 Å and ˇ = 115.04
◦
(JCPDS No. 65-1291). The
strongest peak intensity of (0 10) plane indicates that the crys-
Fig. 3. XRD patterns of tungsten oxide nanostructures: (a, c, e) nanowires, mixture
and nanosheets synthesized at WCl
6
concentrations of 0.01 M, 0.015 M and 0.02 M,
respectively, before annealing, (b, d, f) nanowires, mixture and nanosheets after
annealing.

(M)
Equilibrium
resistance
(M)
Sensor
response
100 43.9 ± 0.8 631.7 ± 9.7 13.4 ± 0.5 44.2 ± 0.7 535.2 ± 31.0 11.1 ± 0.9
125 9.6 ± 0.4 1020.7 ± 39.1 105.1 ± 0.5 10.1 ± 0.7 665.6 ± 40.6 65.2 ± 0.5
150 3.1 ± 0.1 468.8 ± 12.7 151.2 ± 0.5 3.3 ± 0.4 354.1 ± 43.1 107.3 ± 0.3
175 2.1 ± 0.1 307.1 ± 9.8 144.6 ± 0.4 1.3 ± 0.1 121.3 ± 15.5 95.2 ± 0.4
200 1.8 ± 0.04 226.7 ± 4.6 123.6 ± 0.3 0.6 ± 0.1 42.5 ± 9.2 73.5 ± 0.4
225 1.4 ± 0.1 68.6 ± 2.9 48.0 ± 0.2 0.6 ± 0.1 13.7 ± 1.7 21.1 ± 0.2
250 1.1 ± 0.1 2.4 ± 0.1 1.2 ± 0.1 0.7 ± 0.1 8.0 ± 0.7 10.2 ± 0.1
Fig. 4. BET plots of N
2
adsorption isothermsfor tungstenoxide nanostructures after
annealing at 300
◦
C for 1 h.
tal grows preferentially along the b-axis, i.e. the [0 10] direction.
The XRD pattern of the nanosheets obtained at WCl
6
concentration
of 0.02 M corresponds to the monoclinic structure of WO
3
with
lattice of a= 7.297Å, b = 7.539 Å, c =7.688 Å and ˇ =90.91
◦
(JCPDS
No. 43-1035), seen in Fig. 3(e). From this XRD pattern, the two

terns of the annealed tungsten oxides indicated an increase degree
of crystallinity.
3.2. Physical adsorption–desorption measurements
To examine the porous structure of the W
18
O
49
nanowire bun-
dles and WO
3
nanosheets, the specific surface area and pore size
distribution of the samples annealed at 300
◦
C for 1 h are deter-
mined by the physical adsorption–desorption measurements in N
2
gas. BET plots of N
2
gas adsorption isotherms for tungsten oxide
nanostructures are shown in Fig. 4. Here, W is the weight of N
2
gas adsorbed at a relative pressure P/P
0
. P/P
0
is the pressure of N
2
gas divided by its saturation vapor pressure. It can be seen that
the data points are on a straight line for every sample, suggest-
Fig. 5. Pore size distributions per unit mass of tungsten oxide nanowires and

nanowires, the pore size
distribution has a peak at about 2.1 nm, while the pore size distri-
bution peak shifts to a larger size of 2.3 nm for WO
3
nanosheets.
The measured specific surface area and pore volume of
the annealed bundled W
18
O
49
nanowires are 72.03 m
2
/g and
0.13 cc/g, whilst for the WO
3
nanosheets they are 41.85 m
2
/g
and 0.12 cc/g, respectively. These values are slight lower than
those obtained from the samples before annealing. For the as-
synthesized W
18
O
49
nanowires and WO
3
nanosheets, the specific
surface area and pore volume are respectively 89.89 m
2
/g/0.14 cc/g

2
-sensing properties
The gas sensing properties of the sensors based on tungsten
oxide nanostructures towards 5 ppm NO
2
were tested at operating
temperatures ranging from 100 to 250
◦
C. Table 1 shows the mea-
sured baseline resistances, equilibrium resistances and calculated
sensor responses for the W
18
O
49
nanowires and WO
3
nanosheets,
respectively, as a function of operating temperature. It should be
noted that three continual tests were preformed to the same sen-
sor sample at every operating temperature in order to ensure the
reliability of the testing data. The baseline resistances and the equi-
librium resistances in three tests were found to be similar, and
every data shown in Table 1 is the average value of three data
obtained from three tests respectively. The standard deviations for
each valuewere also shownin the table. From Table1, it canbe seen
that the baseline resistances of tungsten oxide decrease along with
increasing temperature, which is consistent with typical semicon-
ductor materials. It is well known that the response of the sensor
is much dependent on the operating temperatures. Such relation
is illustrated in Table 1. The response tests’ results show maxi-

and detected NO
2
gas. Thus, the response of the tungsten oxide
film is low. Conversely, some of the adsorbed oxygen species may
be desorbed from the film surface at high temperature, which also
leads to low response value. As a result, there should be an optimal
operating temperature to balance the above two effects in order
to achieve the maximum gas response. It is clear from Table 1 that
the measurement carried out at temperatures ranging from 150
to 200
◦
C can obtain relatively high NO
2
response, and the highest
response is achieved at 150
◦
C. From Table 1,W
18
O
49
nanowires
exhibited much higher NO
2
response than WO
3
nanosheets at dif-
ferent operating temperature ranging from 100 to 200
◦
C. While it
is noteworthy that, at 250

18
O
49
nanowires
show fasterresponse–recoverythan theWO
3
nanosheets atvarious
operating temperatures.
Fig. 7 shows the dynamic responses of tungsten oxide nanos-
tructures to NO
2
gas in varying concentration. The operating
temperature is 200
◦
C. Fig. 7(a and b) shows the results of the
W
18
O
49
nanowires and WO
3
nanosheets synthesized at WCl
6
con-
centrations of 0.01 M and 0.02 M, respectively. As shown in this
figure, the measured resistances increased upon exposure to NO
2
gas. This result is expected because the oxidizing analyte NO
2
withdraws electrons from the n-type tungsten oxide surface and

49
nanowires upon exposure to 1,
5, 10 and 20 ppm NO
2
are 13.4, 123.6, 203.4 and 332.3, while those
of the WO
3
nanosheets are 13.3,73.5, 144.2 and279.8, respectively.
As has been reported, the gas sensing mechanism of tung-
sten oxide belongs to the surface-controlled type in which the
surface states and oxygen adsorption play an important role
[26,28]. Atmospheric oxygen adsorbs electrons from the conduc-
tion band of the sensing metal oxide and occurs on the surface
Fig. 7. Dynamic response of (a) bundled W
18
O
49
nanowires, (b) WO
3
nanosheets to
varying NO
2
concentration at an operating temperature of 200
◦
C.
Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO
2
-sensing properties of tungsten oxide nanostructures,
Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.063
ARTICLE IN PRESS

2−
, etc.) induces
the formation of electron-depleted space-charge layers inside the
tungsten oxide surfaces and thus the increase in the resistance
[29]. According to the BET measurements, the W
18
O
49
nanowires
showed much larger specific surface area (72.03 m
2
/g) than the
WO
3
nanosheets (41.85 m
2
/g). The larger surface area can provide
more adsorption–desorption sites and a larger amount of surface
adsorbed oxygen species interacting with detected gas molecules.
Thus, W
18
O
49
nanowires with higher specific surface area show
much larger change in resistance upon exposure to NO
2
than the
WO
3
nanosheets with lower specific surface area. Besides, non-

O
49
nanowires for almost 10 times at
the operating temperature of 250
◦
C. This result implies that some
other factor dominates the gassensing performance ofthe one- and
two-dimension tungstenoxidenanostructure. Itis foundfrom Fig.2
that thermal treatment resulted in a more evident change in the
microstructure of W
18
O
49
nanowires than that of WO
3
nanosheets,
indicating that the microstructure of W
18
O
49
nanowires is much
more sensitive to temperature than WO
3
nanosheets. Therefore, it
is possible that the microstructure change of W
18
O
49
nanowires
(e.g. further agglomeration) at high operating temperature affects

O
49
nanowires compared to that of the WO
3
nanosheets when both
expose to the same NO
2
concentration. Especially, the W
18
O
49
nanowires exhibit very fast responsecharacteristic to NO
2
gas with
the response times of42, 19, 16 and 14 s to 1, 5, 10 and 20ppm NO
2
,
respectively.
4. Conclusions
One- and two-dimensional tungsten oxidenanostructures were
synthesized at 200
◦
C by solvothermal method with tungsten hex-
achloride (WCl
6
) as precursor and 1-propanol as solvent. The
synthesis processes were preformed at different WCl
6
concen-
trations (0.01, 0.015 and 0.02 M, respectively). One-dimensional

O
49
nanowires and
WO
3
nanosheets exhibit reversible response to different concen-
trations of NO
2
. Compared to WO
3
nanosheets, W
18
O
49
nanowires
showed quicker response–recovery and higher response value to
different concentration of NO
2
gas due to the high specific sur-
face area and the non-stoichiometric crystal structure, and their
response time, recovery time andresponse value are 19 s, 112 s and
123.6 to 5 ppm NO
2
at 200
◦
C, respectively. These results indicate
the one-dimensional W
18
O
49

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Ming Hu received a M.S. in microelectronics and solid-stateelectronics from Tianjin
University in 1991. She is now a professor in Department of Electronics Science and
Technology in Tianjin University. Her research interests include MEMS, gas sensor,
functional film devices.
Jie Zhang received her Bachelor degree in microelectronics and solid-state elec-
tronics from Tianjin University in 2008. She is now a graduate student at Tianjin
University. Her current research is focused on the tungsten oxide based gas sensor
and material adsorption properties simulation.