gas sensing properties of nanocrystalline tungsten oxide synthesized by acid

Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi-
tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid
precipitation method
T.D. Senguttuvan
a,∗
, Vibha Srivastava
a
, Jai S. Tawal
b
, Monika Mishra
a
, Shubhda Srivastava
a
, Kiran Jain
a
a
Electronic Materials Division, National Physical Laboratory, Dr. K.S. Kishnan Marg, New Delhi 110012, India
b
Materials Characterization Division, National Physical Laboratory, Dr. K.S. Kishnan Marg, New Delhi 110012, India
article info
Article history:
Received 9 December 2009
Received in revised form 23 June 2010

Accepted 26 June 2010
Available online xxx
Keywords:
WO
3
Metal oxide
Gas sensor
Ammonia sensor
Thick film
Chemical synthesis
abstract
WO
3
·2H
2
O samples were prepared by acidic precipitation of sodium tungstate solution. Nanocrystalline
WO
3
powders were obtained after 350 and 600
◦
C calcinations. XRD patterns of these samples showed a
diffraction profile similar to that of monoclinic WO
3
. Calcinations at 600
◦
C yield WO
3
powders with par-
ticle sizes ranging from 60 to170 nm whereas that for 350
◦

C calcinations are in the range of 30–150 nm.
The gas sensing properties of these powders in the form of thick film were investigated with and without
platinum doping. Gas response of thick films sintered at two different temperatures was measured at
four different operating temperatures. We confirm that the sensor elements made from 600
◦
C calcined
powders doped with platinum sintered at 800
◦
C are highly responsive and selective to ammonia vapour
at 350
◦
C operating temperature as compared to sensor elements made from commercial powders.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Gas sensors are used for many applications such as process
controls in chemical industries, detection of toxic environmental
pollutants, and for the prevention of hazardous gas leaks. Different
oxide semiconductors such as SnO
2
,WO
3
, ZnO, MoO
3
, TiO
2
,In
2
O
3
and mixed oxides have been studied and showed promising appli-

cations for detectingtarget gases suchas NO
x
,O
3
,NH
3
, CO, CO
2
,H
2
S
and So
x
[1–3] The working principle ofthese sensorsis based on the
detection of a change in resistance on exposure to a gas. Due to the
constraints of gas permeation only the surface layers are affected
by such reactions. Among various oxide sensors, WO
3
is responsive
to NO
x
,H
2
S, and NH
3
[4–6] In order to achieveimprovements in the
gas sensing properties such as enhancing their response, selectiv-
ity to a given target species and reduce the operating temperature,
small amounts of noble metals (Pt, Pd and Ag) are added to active
metal oxide layers [7,8]. The sensor characteristics strongly depend

on preparation techniques and the resulting material microstruc-
ture of active metal oxide layer [9,10]. Cantalini et al. reported
positive effects on the response cross-sensitivity by increases in
the annealing time for the WO
3
thin films [11,12]. Tamaki et al.
have shown that the response to NO
2
increases dramatically with
decreasing grain size of the tungsten oxide for sintered block type
∗
Corresponding author. Tel.: +91 11 45609461; fax: +91 11 25726938.
E-mail address: (T.D. Senguttuvan).
WO
3
-based gas sensors [13]. This generated a wide interest in
synthesizing nanosized powder and investigating gas sensor prop-
erties. Nanosized powders of WO
3
have been prepared by sol–gel,
spray-pyrolysis, radio frequency magnetron sputtering and aque-
ous chemicalroute etc [9–12,14]. Comparedwith other techniques,
aqueous chemical process is attractive because of its cost effective-
ness and the ease of material preparation. This technique was well
reported for NO
2
sensor fabrication [15]. However, it is not widely
explored for ammonia gas sensors preparation.
In the present work, we investigate ammonia gas sensing prop-
erties of nanocrystalline tungsten oxide with and withoutplatinum

doping under different processing conditions.
2. Experimental
Tungsten trioxide powder was prepared by acidic precipitation
route. 5 g Sodium Tungstate was dissolved in 200 ml of de-ionised
water to obtain atransparent solution. To this solution5% dilute HCl
is added drop-wise to obtain yellow tungstic acid precipitate. It was
then washed with water to remove sodium and chlorine ions till no
chlorine ion was detected on AgNO
3
test. At this stage the precipi-
tate was filtered and dried at 100
◦
C. These powders were calcined
at 350 and 600
◦
C to obtain powder A and powder B respectively.
For Pt doping, calcined powders A and B were mixed with 0.4 wt%
chloroplatinic acid solution in water and dried to obtain pow-
ders C and D. X-ray powder diffraction (XRD) analysis on powder
A and B were carried out using Bruker Analytical X-ray diffrac-
0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2010.06.053
Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi-
tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053
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tometer equippedwith graphite monochromatized CuK␣ radiation
( = 1.5418 Å). The nano-scale characterization was carried out by

a transmission electron microscope (TEM, model JEOL JFM 200x).
The morphologies of thick films were observed by using scanning
electron microscope (LEO 440 SEM).
Pure powders A and B, platinium doped powders C and D, micron
sized commercial powder (Sigma–Aldrich) and platinum doped
commercial powders were used tomake thickfilm paste. Thick film
paste was prepared in butyl carbitol medium containing a small
amount of ethyl cellulose and terpineol oil. These pastes were then
screen printed on alumina substrates that had gold finger contacts.
Sensor films were coated overthe gold finger contacts, and sintered
at 600
◦
C and 800
◦
C for 10 min. Thus we have obtained eleven dif-
ferent sensor elements. They are designated as AP
600 (for sensor
prepared from pure powder A sintered at 600
◦
C), BP 600 (for sensor
prepared from pure powder B sintered at 600
◦
C), AP 800 (for sensor
prepared from pure powder A sintered at 800
◦
C), BP 800 (for sensor
prepared from pure powder B sintered at 800
◦
C), CPt 600 (for sen-
sor prepared from Pt doped powder C sintered at 600

◦
C), DPt 600
(for sensor prepared from Pt doped powder D sintered at 600
◦
C),
CPt
800 (for sensor prepared from Pt doped powder C sintered at
800
◦
C), DPt
800 (for sensorprepared fromPt doped powderD csin-
tered at 800
◦
C), Comm Powder (sensor prepared from commercial
powder Sintered at 800
◦
C, Comm-600 (sensor prepared from Pt
doped commercial C sintered at 600
◦
C) and Comm-800 (sensor
prepared from Pt doped commercial C sintered at 800
◦
C). Resistiv-
ity and gas response for CNG, NO
2
,NH
3
, CO, LPG and Ethanol gas
was measured using Keithley 2000 multimeter in a static system.
The sensor element was placed onto an externally heated sample

holder and the working temperature of the thick films was deter-
mined with a thermocouple attached near the sensor element. The
gas or vapour of required amount was injected into the chamber
using a 1 ml syringe. The sample’s resistance was measured in air
and after exposure to targeted gas or vapour. After completing the
measurement, the gas was leaked out. To get uniform temperature
distribution throughout the sensor elements, they were heated to
targeted temperature in dry air for 1 h before measurements were
carried out. Gas sensing properties of the thick films were carried
out at operating temperatures ranging from 250 to 450
◦
C. The gas
response (S) was definedas the ratioR
a
/R
g
or R
g
/R
a
, for reducing and
oxidizing gases, respectively; where R
a
is the resistance in pure air
and R
g
is the sensor resistance in the presence of a species diluted
in air
3. Results and discussion
Fig. 1 shows the XRD pattern of powders A and B. The charac-

teristic peaks observed at 2Â values 23.2, 28.88 and 34.17 confirms
that these powders are monoclinic WO
3
[JCPDS 43-1035]. The sharp
diffraction peaks imply good crystallinityof WO
3
powders. Absence
of characteristicpeaks corresponding to other impurities suchas W
or W(OH)
6
indicates the phase purity of WO
3
. The tungsten triox-
ide can exists in several polymorphic forms such as monoclinic,
hexagonal and pyrochlore around room temperature. Choi et al.
have shown in the case of WO
3
derived from sol prepared by ion
exchange method both hexagonal and pyrochlore crystals switch
completely to monoclinic phase if it is heated to more than 500
◦
C
and cooled downto room temperature[16,17]. However inour case
both the powders A and B are calcined at 350 and 600
◦
C show only
monoclinic WO
3
Fig. 2 shows the TEM microstructure of the powders A and
B respectively. It is evident from the micrographs that the pow-

der A has particles in the size range of 30–150 nm and powder
B has particles in the range of 60–170 nm. This increase in parti-
cle size is understandable since higher calcination temperatures
Fig. 1. XRD pattern of powders calcined at 350 and 600
◦
C.
lead to increased diffusion rate that in turn coalesce the adjacent
grains.
3.1. Gas Response—pure WO
3
Response to ammonia gas was measured for AP 600, BP 600,
AP
800 and BP 800 sensor elements. The sensor response was eval-
uated asa function of operating temperature. Wecould not observe
Fig. 2. TEM micrograph of powders calcined at 350 and 600
◦
C.
Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi-
tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053
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Fig. 3. Response of AP 800 (sensor prepared from pure powder A), BP
800 (sensor
prepared from pure powder B) and Comm Powder (sensor prepared from commer-
cial powder) all sintered at 800–4000 ppm NH3 gas.
noticeable sensor response for AP 600 andBP 600 sensor elements.
Fig. 3 shows the sensor response for AP
800 and BP

800 sensor
elements to 4000 ppm NH
3
gas. For both the sensors elements
maximum response was observed at 400
◦
C operating tempera-
ture, where as for the sensor elements prepared from micron sized
commercial highest response was observed at 400
◦
C operating
temperature. WO
3
powders calcined at 600
◦
C better response as
compared to powders calcined at 350
◦
C. The TEM results have con-
firmed lower particle size for powder A than for powder B. As per
linear extrapolation one would expect lower grain size for AP
800
sensor element and hence a higher response which is contrary to
our results. The reason for this discrepancy can be understood by
seeing the SEM micrographs of AP
800 sensor element (Fig. 4) and
BP
800, sensor element (Fig. 5). AP 800 sensor element showed
polycrystalline grain morphology consisting of spherical grains of
grain size 100nm together with plate like grains of grain size in the

range 300–500 nm. The average grain size of this sensor element
was calculated by linear intercept method (EN 623-3) andit is found
Fig. 4. SEM micrograph of AP 800 (sensor prepared from pure powder A sintered
at 800
◦
C).
Fig. 5. SEM micrograph of BP
800 (sensor prepared from pure powder B sintered at
800
◦
C).
to be 680 nm. BP
800, sensor element showed polycrystalline grain
morphology consisting of mostly of spherical grains with grainsizes
ranging from 130 to 600 nm. The average grain size of this sen-
sor element is 300 nm. We observe better a response in smaller
grain size sensor element. These results are similar to increase in
response of tungsten oxide sensor with decrease in grain size as
reported by Tamaki et al. [13]. The sensing properties of WO
3
film
are also controlled by the surface defects [18]. The higher response
to NH
3
for BP 800 sensor element can also be because of its large
irregular voids. Further,more oxygen vacanciesare expected onthe
surface of tungsten oxide since it is calcined at higher temperature
in accordance with the results obtained by Liu et al. [19].
3.2. Gas response—Pt doped WO
3

The highest magnitude of response to 4000 ppm NH
3
observed
for BP
800 sensor element was 9. We have tried to improve the
response by doping with platinum as explained earlier. Response to
ammonia gas was measured for sensor elements CPt
600, DPt 600,
Fig. 6. CPt 600 (sensor prepared from Pt doped powder C sintered at 600
◦
C),
CPt
800 (sensor prepared from Pt doped powder C sintered at 800
◦
C) sensor,
Comm-600 (sensor prepared from Pt doped commercial C sintered at 600
◦
C)
and Comm-800 (sensor prepared from Pt doped commercial C sintered at 800
◦
C)
response for to 4000 ppm NH
3
gas.
Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi-
tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053
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Fig. 7. DPt
600 (sensor prepared from Pt doped powder D sintered at 600
◦
C)
DPt
800(sensor prepared from Pt doped powder D sintered at 800
◦
C) Comm-600
(sensor prepared from Pt doped commercial C sintered at 600
◦
C) and Comm-800
(sensor prepared from Pt doped commercial C sintered at 800
◦
C) response for to
4000 ppm NH
3
gas.
Fig. 8. The response of DPt
800(sensor prepared from Pt doped powder D sintered
at 800
◦
C) to NH
3
gas operating at 350
◦
C.
CPt 800 and DPt 800. Fig. 6 shows the sensor response for CPt 600,
CPt
800, Comm-600 and Comm-800 sensor element to 4000 ppm
NH

3
gas. The maximum response was observed at 350
◦
C for all
sensor elements except for Comm-600. However, the response
magnitude was better for CPt
800 sensor element and it was found
to be 82 as compared to all other elements. Fig. 7 shows the
sensor response for DPt
600 sensor element and DPt 800 sensor
element along with Comm-600 and Comm-800 sensor elements to
4000 ppm NH
3
gas. The maximum response wasobserved at 350
◦
C,
and was highest for sensor annealed at 800
◦
C. Above 400
◦
C oper-
ating temperature both sensor elements (DPt
600 and DPt 800)
show same response to 4000 ppm NH
3
gas. The response magni-
tude of 125 was observed for DPt
800 sensor element as compared
to a magnitude of 9 for BP
800 sensor element. Fig. 8 shows the

linear response of DPt
800 sensor element to NH
3
gas within the
concentration range 100–4000 ppm operating at 350
◦
C.
The response time represents the time required by the response
factor to undergo 90% variation with respect to its equilibrium value
following a step increase in the test gas concentration. The recovery
time represents the time required by the sensitivity factor to return
to 10% below its equilibrium value in air following the zeroing of
Fig. 9. Resistance vs time graph for DPt
800 (sensor prepared from Pt doped powder
D sintered at 800
◦
C) at different operating temperatures.
Fig. 10. The response of DPt 800 (sensor prepared from Pt doped powder D sintered
at 800
◦
C) to 800 ppm of different gases (NH
3
, LPG, CNG, CO ethanol and NO
2
.)
the test gas Fig. 9 shows response time graph for DPt 800 sensor
element at different operating temperatures. It took less than 20 s
at an operating temperature of 400
◦
C and 60s for 200

◦
C operating
temperature. It should also be noted that tungsten oxide nanopar-
ticles prepared by gas deposition yield films that can be used even
at room temperature with similar and greater sensitivities for H
2
S
gas [20]. Our results indicate the limitations posed by fluid-based
chemistry that was used to fabricate the nanoparticle films.
The Pt–WO
3
(DPt 800 sensor element) sensor was highly selec-
tive towards ammonia at an operating temperature of 350
◦
C.
Fig. 10 shows the response to NH
3
, LPG, CNG, CO ethanol and NO
2
were recorded.
4. Conclusions
Nanocrystalline tungsten oxide powder was produced by an
acid precipitation of sodium tungstate suitable for ammonia gas
sensors. These powders were monoclinic WO
3
. The particles with
size distribution of 30–150 and 60–170 nm resulted from 350 and
600
◦
C calcinations respectively. Sensor element obtained from

350
◦
C calcined powders showed a polycrystalline grain morphol-
ogy consisting ofspherical grains ofsize 100nm together withplate
like grains of size 300–500 nm. Whereas 600
◦
C calcined powder
resulted in a sensor elements with polycrystalline grain morphol-
ogy consisting only of spherical grains of size ranging from 130 to
600 nm. Sensor elements made with pure WO
3
powders calcined
at 600
◦
C are better than those calcined at 350
◦
C. Platinum doping
resulted in better response irrespective of calcination temperature.
The maximum response to NH
3
was achieved at an operating tem-
perature of 350
◦
C for all the platinum dopedsensor elements and at
400
◦
C for undoped sensors. The best response for NH
3
is observed
Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi-

tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053
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in the sensor element annealed at 800 after it was green formed
from Pt doped powder calcined at 600
◦
C. The response magni-
tude of 125 observed for the best Pt doped WO
3
sensor element
was significantly than a magnitude 9 for the best pure WO
3
sensor
element.
Acknowledgements
We are thankful to Mr Jain, CEERI Pilani for providing the sensor
substrates. We are also thankful to Dr. S.K. Halder for measurement
of XRD. One of the authors (VS) thanks Council of scientific and
industrial research for the award of Research Associateship.
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Biographies
T.D. Sengutavan obtained his M.E. from REC Trichy in the field of Materials Science
with specialization in Ceramics and Ph.D. in Sol–gel processing from IIT Delhi. He is
working as scientist in National Physical Laboratory, New Delhi, for the past 13 years.
His research interest includes ceramic structures, powder processing, microwave
sintering and metal oxide gas sensors.
Vibha Srivastava obtained her Ph.D. in 2004 from Gorakhpur University in materials

science. Presently she is working as research associate at National Physical Labo-
ratory, New Delhi. Her current research interests are nanoscience, nanostructure
mesoporous materials and gas sensors.
Jai S.Tawala obtained his M.Sc. in 2006 from NagpurUniversity in Physics. Presently
he is working as technical assistant at National Physical Laboratory, New Delhi. His
current research interests are nanostructure materials and characterization.
Monika Mishra obtained her M.Sc. in 2006 from Bundelkhand University in Physics.
Presently she is working as research intern at National Physical Laboratory, New
Delhi. Her current research interests are nanostructure materials and gas sensors.
Shubhda Srivastava obtained her M.Sc. in 2005 from Avadh University in Electron-
ics. Presently she is doing her M.Tech. Project at National Physical Laboratory, New
delhi. Her current research interests are nanostructure material and gas sensors.
Kiran Jain did her Ph.D. in the field of high Tc superconductivity from Delhi Uni-
versity. She is working as scientist in National Physical Laboratory, New Delhi, for
the past 25 years on the diverse area of research material science such as high Tc
superconductors, ceramics, nanocrystalline semiconductors, thin film photovoltaics
and metal oxide gas sensors etc.