The device using smaller WO3 lamellae prepared with a H2SO, solution pH —0.8 had the highest sensor response, exhibiting a lamellae resulted in a decrease in the electrical resistance o
Trang 1
A Eee
ELSEVIE
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Contents lists available at ScienceDirect
Highly sensitive NO sensors using lamellar-structured WO3
particles prepared by an acidification method
Tetsuya Kida?, Aya Nishiyama”, Masayoshi Yuasa?, Kengo Shimanoe?*, Noboru Yamazoe?
4 Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan
b Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan
Article history:
Received 13 May 2008
Received in revised form
24 September 2008
Accepted 28 September 2008
Available online 1 November 2008
Keywords:
NO;
WO;
Lamella
Acidification method
ABSTRACT
Tungsten trioxide (WO3) was prepared by acidification of NazW0O, with acid solutions such as H2SOz, HCl, and HNO3 (pH 0.5 to —0.8) and tested for its NO2z sensing properties Acidification with strong acid solutions (pH —0.5, —0.8) was found to produce lamellar-structured WO3 particles, which consisted of nano-sized crystalline plates that were 100-350 nm in lateral size and 20-50 nm in thickness, as observed by XRD and SEM analyses The sizes of the primary and secondary particles were decreased by decreasing the
PH of the acid solution used This was accompanied by an increase in the specific surface area The NO2z responses of the prepared WO3 lamellae were dependent on their morphology The device using smaller WO3 lamellae prepared with a H2SO, solution (pH —0.8) had the highest sensor response, exhibiting a
lamellae resulted in a decrease in the electrical resistance of the device, probably due to intimate contact between smaller lamellar particles, which allowed the detection of NO; in a rather wide concentration range In addition, the developed device showed high NO; selectivity without substantial interference from NO
1 Introduction
The continuous detection and monitoring of NOz in the atmo-
sphere have become highly important because of its toxic effects
on both animals and plants There has been a high demand for
compact, cheap, and preferably portable devices able to detect low
levels (ppb level) of NO2 in the atmosphere, since available analyti-
cal instruments based on Saltzman or chemiluminescence methods
are large and expensive Such a demand for high-performance NOz
sensors is rapidly growing for other applications It is envisaged that
in the next few years, an automatic damper (ventilation) system
will be introduced in cars This system needs a compact sensor that
can monitor NO> inside and outside in a rather wide concentration
range, from ppb to several ppm levels Thus far, several solid-
state NO» sensors, such as resistive [1-6], potentiometric [7-9],
amperometric [10-13], capacitive [14,15], optic [16,17], and surface
acoustic wave (SAW) types [18] have been developed In particu-
lar, resistive-type NO» sensors based on oxide-semiconductors are
well-suited for the above applications due to their superior prop-
erties and simple structure, and as such they have been intensively
studied for about 20 years [1-6] Through an extensive search for
* Corresponding author Tel.: +81 92 583 7876; fax: +81 92 583 7538
E-mail address: simanoe@mm.kyushu-u.ac.jp (K Shimanoe)
0925-4005/$ - see front matter © 2008 Elsevier B.V All rights reserved
doi:10.1016/j.snb.2008.09.056
NO>-sensitive materials, tungsten trioxide (WO3) has been found
to show very promising NO» sensing properties [4,5] Notably, WO3-based sensors can detect dilute NO in air without significant
interference from CO2, methane, CO, or Hz at low temperatures like
200 and 300°C [4] It is important to note that the sensor response
of WO3 depends significantly on the preparation method Many preparation routes for WO3 sensors have been reported, including sputtering [19], vacuum evaporation [20], pulsed-laser deposition [21], sol-gel [22-25], pyrolysis [4,5,26], photochemical [27], and ion-exchange methods [28-31 ]
We have previously reported in a series of papers that thick and thin film devices using lamellar-structured WO3 particles with nano-sized thickness, which were prepared by an ion-exchange method using a protonated cation-exchange resin and subsequent heat treatment, exhibited excellent NO2 sensing properties at 200-300 °C [28-31] It was found that the sensor response was sig- nificantly increased with a decrease in the thickness of the WO3 lamellae and was well-correlated with its thickness [30] Another important feature of the devices was the porous microstructure of the sensing layer packed with WO3 lamellae with a high anisotropic shape A sufficiently high sensor response was obtained, even to
10 ppb NO, in air, when WO3 lamellae that were ca 30 nm in thick- ness and 1 ym in lateral dimension were used for the sensing film [31] However, the electrical resistance of the sensing film was fairly high, exceeding 108 Q at 200°C in response to even low
Trang 2concen-trations of NOa, e.g., 500 ppb [30] Such a high electrical resistance
makes measuring the sensor signal with the simple electric circuits
that are typically used for commercial resistive sensors difficult,
which hinders the detection of NOz at higher concentrations (sub
ppm level) In addition, a high electrical resistance can sometimes
be a source of noise in the sensor signal Although the resistance
can be decreased by raising the operation temperature, this is coun-
terbalanced by a decrease in the sensor response In this study, in
order to decrease the sensor resistance and to extend the detectable
concentration range, we prepared smaller WO3 lamellae by the
acidification of WO,2~ in a strong acid solution We expected that
nano-sized lamellae would contact more closely to each other than
micro-sized lamellae, leading to a decrease in the sensor resistance
Acidification of W-polyanions is known to produce WO3:2H20 crys-
tals, a precursor of WO3 [32-35] The acidification method was
found to be able to tune the size of WO3 lamellae by changing the
pH of the acid solutions used, as described below
2 Experimental
WO3 particles were prepared by acidification of Na2WO, and
subsequent calcination of the resulting precipitates A NazWO4
solution was added drop by drop into an acidic solution under vig-
orous stirring The two solutions were mixed so as to set the molar
ratio of Na*/H* at 1/10 The pH of the acidic solution was controlled
with HạSOa, HNOa, and HCI to be between 0.5 and —0.8 The mix-
ing quickly produced a yellow gel (crystalline WO3-2H,2O), which
was aged for 1 day at 30°C The gel was washed thoroughly with
distilled water by centrifugation
The structure of the fabricated sensor device is shown in
Fig 1 The obtained precipitates were mixed with water to form
a paste The resulting paste was screen-printed on an alumina
substrate equipped with a pair of comb-type Au microelectrodes
(line width: 180,1m; distance between lines: 90,1m; sensing
layer area: 64mm7) The Au electrodes were also fabricated by a
screen-printing method using a commercial Au paste followed by
calcination at 850°C The paste deposited on the substrates was
calcined at 300°C for 2h in air to form a WO3 sensing layer via the
dehydration of the precursor, WO3-2H20
The surface morphology of the samples was analyzed witha field
emission scanning electron microscope (FE-SEM) The thickness
of the films was estimated to be 15-25 4m by FE-SEM observa-
tions The crystal structure and specific surface area of the samples
Al,O; substrate WO, layer
Au microelectrodes
gap size : 90 um
line width : 180 pum
Sensing layer area : 64 mm?
Fig 1 Schematic structure of a NOz sensor device, in which a WOs; thick film is
deposited on an alumina substrate equipped with a pair of comb-type microelec-
trodes,
were measured using an X-ray diffractometer (XRD) with copper
Ka radiation and a BET surface area analyzer, respectively The NO sensing properties of the devices were examined at an operating temperature of 200°C in a concentration range of 50-1000 ppb in air Measurements were performed using a conventional gas flow apparatus equipped with an electric furnace at a gas flow rate of
100 cm?/min The sensor response (S) was defined as the ratio of resistance in air containing NO2 (Rg) to that in dry air (Ra) (S=Rg/Ra)
3 Results and discussion 3.1 Crystal structure and microstructure Fig 2 shows the XRD patterns of WO3 particles prepared using the acidification method using H2SO, with different pH solutions (pH 0.5 to —0.8) and subsequent calcination at 300°C All XRD peaks were assigned to monoclinic WO3 (JCPDS 43-1035) for all samples Other peaks are ascribable to Si, which was mixed with samples as an internal standard for the XRD measurements It is noted that (001)-oriented WO3 particles were formed when a H2SO, solution with pH 0.5 was used On the other hand, such
a preferential orientation in the (001) plane became weak, i.e., the intensity of the (002) peak decreased with a decrease in the
pH of the solution while those of the (020) and (200) peaks increased In a previous report, we found that (00 1)-oriented WO3 particles were formed by dehydrating (01 0)-oriented WO3-2H2O crystallites, which were prepared via the acidification of NayWO, with a protonated cation-exchange resin and repeated washing- centrifugation treatments [28] Quite strikingly, the preferential orientation in layered WO3-2H20 was preserved even through the dehydration step leading to WO3 Thus, in the present case, the observed (00 1) orientation in WO3 at pH 0.5 also reflects the pref- erential (0 1 0) orientation in the precursor WO3-2H20 On the other hand, the observed loss in the crystal orientation at lower pH can be interpreted as follows: since WO3-2H20 is formed through the con- densation of W-polyanions by acidification, the condensation rate, which depends on the pH of the precursor solution, may affect the orientation of the WO3-2H20 crystallites It is thus suggested that with a significant decrease in pH of the solution from 0.5 to —0.8,
d
20 / degree
Fig 2 XRD patterns of WO3 particles prepared using H2SO, solutions with different
PH (a) 0.5, (b) 0, (c) —0.5, and (d) —0.8.
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Fig 3 FE-SEM images of WO; particles prepared using H2SO, solutions with different pHs (a) 0.5, (b) 0, (c) —0.5, and (d) —0.8
the crystal growth rate in other directions such as (200) and (020)
became dominant, as observed in the XRD patterns However, it
remains uncertain how such a difference in the direction of the
crystal growth occurred at different pHs Further studies are thus
necessary to draw conclusions
Fig 3 shows FE-SEM images of WO3 particles prepared using
H2SO, solutions with different pHs (pH 0.5 to —0.8) and subsequent
calcination at 300°C The morphology of the particles differed con-
siderably depending on the pH of the precursor solution Particles
prepared at pH 0.5 were leaf shaped and were ca 2.5 jm in lat-
eral size and 0.2-0.5 Lm in thickness, as estimated from the image
(Fig 3(a)) The size of the particles is in almost the same range
as those prepared by the ion-exchange method [28] Balazsi and
Pfeifer also reported a similar morphology of WO3-2H20 crystals
prepared by an acidification method [33] Conversely, particles pre-
pared at lower pHs were of a rectangular platelet shape (lamellar)
and the size of the particles decreased with a decrease in the pH
of the precursor solution In accordance with the decrease in the
lateral size, the thickness also decreased, as observed in the images
(Fig 3(b-d)) The estimated lateral sizes and thicknesses of lamellar
Table 1
particles are summarized in Table 1, together with the specific sur- face areas and the crystalline sizes obtained from XRD peaks using Scherrer’s equation:
0.9A
where D represents mean crystalline size, B stands for full width at half maximum of the peak, A is the wavelength of the X-ray, and @
is the center angle of the peak In the estimation of crystalline size, XRD peaks around 22-—26° are decomposed into three components resulting from the (00 2), (020), and (200) planes Apparently, the surface area of lamellar particles was increased with a decrease in the lateral size and thickness The crystal size was decreased with decreasing precursor solution pH It appears that acidification of WO,” in a strong acid solution yields nano-sized lamellar WO3 particles The obtained results suggest that, in a highly acidic solu- tion, the rate of crystal nucleation may be faster than that of crystal growth, resulting in a decrease in the crystalline size
WO3 particles were also prepared using different acid solutions including HNO3 and HCl Figs 4 and 5 show the XRD patterns
Particle size, crystalline size, specific surface area, and resistance in air of WO3 particles prepared by the acidification method
Acid solution Particle size (nm) Crystallite size (nm) Specific surface area (m?/g) Resistance in air (&2)
Trang 4
5
CS
=
œ
=
2
=
(b)
(c)
20 / degree
Fig 4 XRD patterns of WO3 particles prepared using different acid solutions (pH
—0.5) (a) HaSƠa, (b) HNOs, and (c) HCI
and FE-SEM images, respectively, of WO3 particles prepared by the
acidification method using HCl and HNO3 solutions (pH —0.5) and
subsequent calcination at 300°C Their specific surface area and
estimated crystal size are also summarized in Table 1 No significant
changes in the physical properties such as morphology, crystalline
size, and surface area were observed when WO3 was prepared using different acid solutions with the same pH It can be concluded that
the presence of anions such as SO4~, Cl~, and NO37 has no drastic
influence on the physical properties of the WO3 particles
3.2 Electrical properties The value of electrical resistance is one of the important param- eters for resistive-type semiconductor gas sensors If the electrical resistance is too high, reliable gas detection becomes difficult due
to the generation of noise in sensor devices based on bridge cir- cuits Such a problem becomes more serious for the detection of oxidizing gases, such as NO and O3, using n-type semiconductors,
in which target gases are detected by a sharp increase in resistance This is in contrast to the detection of reducing gases such as H2 and CO, in which target gases are detected by a sharp decrease in
resistance Thus, it is difficult to evaluate higher NO» concentra-
tions with a conventional WO3-based sensor with a high electrical resistance
Fig 6 shows the dependence of the electrical resistance on NOz concentration at 200 °C for various devices using WO3 particles pre- pared with different acid solutions (pH —0.5), together with that for the device using WO3 particles prepared by the ion-exchange method The electrical resistances in air of the WO3 particles used are listed in Table 1 Obviously, the resistances were successfully decreased by almost one order of magnitude when smaller WO3 lamellae, prepared by the acidification method, were used The decreases in the resistance were observed for all devices using WO3 lamellae in the same size range The observed decrease in the resistance is suggested to originate from the formation of intimate contacts among lamellae by decreasing their lateral size
Fig 5 FE-SEM images of WO3 particles prepared using different acid solutions (pH —0.5) (a) H2SOx4, (b) HNOs, and (c) HCl.
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ion-exchanged method
10 8 l l I ] | l | l | l l 1 l
NO, concentration / ppb
Fig 6 Dependence of the electrical resistance on NO concentration at 200°C for
the devices using WO3 lamellae prepared by the acidification method using different
acid solutions (pH —0.5) (a: solid triangle) H2SO,, (b: solid circle) HNO3, and (c:
mark) HCl For comparison, the electrical resistance of the device using WO3 lamellae
prepared by the ion-exchange method is also shown
3.3 NO» sensing properties
The NO, sensing properties of the sensor devices were exam-
ined at 200°C Fig 7 shows the sensor response as a function of
NO» concentration for the devices fabricated with a H2SQO, solu-
tion at different pH values For comparison, the sensor response of
the device fabricated by the ion-exchange method is also plotted
It was found that a large NOz response was obtained using nano-
sized WO3 lamellae, as compared to the case using micro-sized
lamellae prepared via the ion-exchange method The device using
the smallest WO3 lamellae prepared with a H2SO, solution at pH
—0.8 showed the highest sensor response (Fig 7(d)) It has been
1 | 1 | 1 | 1 | I
(d) O
&
on
“fi
i ra ⁄ °
NQ, concentration / ppb
Fig 7 Sensor response (S=R,/Ra) as a function of NO2 concentration at 200°C for
the devices using WO3 lamellae prepared using H2SO, solutions with different pH
values (a: mark) pH 0.5, (b: open triangle) 0, (c: solid circle) —0.5, and (d: open
circle) —0.8 For comparison, the sensor response of the device using WO3 lamellae
prepared by the ion-exchange method is also shown
proposed that the adsorption of NO» on the n-type semiconductor WO3 induces electron-depleted space-charge layers inside the WO3 surfaces [36] For nano-sized thin WO3 lamellae, a whole region of the lamellae can be occupied by the space-charge layer upon NO> adsorption (full depletion) This significantly increases the dou- ble Schottky barrier heights at the boundaries between lamellae, resulting in a large increase in the electrical resistance Such an effect can explain why thinner nano-sized lamellae exhibit a greater response, ie., a large resistance change upon NO, adsorption
It is noteworthy that the developed devices with lower electri- cal resistances can detect NO» in a wide concentration range of 50-1000 ppb This feature is very important when the sensor is applied to environmental monitoring or air-quality control How- ever, the sensor response was gradually saturated at higher NO» concentrations This may be explained in terms of the full depletion
of small WO3 lamellar crystals induced by formation of space- charge layers in the whole region of the crystals upon higher NOa adsorption It is thought that the ratio of the number of fully depleted crystals to that of non-fully depleted crystals increases rapidly with decreasing crystalline size Consequently, the proba- bility of finding pairs of neighboring non-fully depleted crystals, and of connecting a conductive path to the next, would decrease sharply with decreasing crystalline size In this case, although some small WO3 lamellae have the capability of NO» adsorption, most conductive paths are interrupted by fully depleted crystals This situation may bring about the observed saturation of the sensor response (resistance change) at higher NOz concentrations How- ever, the sensor response of the device (c) showed such saturation
at higher NO»z concentration than 500 ppb The possible explana- tion seems that the NO» adsorption sites on WO3 lamellae were decreased by the agglomeration of WO3 lamellae As can be seen
in Table 1, the surface area of the device (c) was about halves of the device (d), although the crystallite size of the device (c) was the almost same as that of the device (d) In short, it is expected that the surface of agglomerated lamellae would not supply more sites for NO» adsorption, meaning that further extension of space charge layer was difficult
Fig 8 shows the sensor response of devices fabricated with three different acid solutions at pH —0.5 as a function of NO» concen- tration These devices also responded to dilute NO» and showed a
250 1 | I | i 1 | I
(b) 200K
ap
I
so pu ~ 4
NO, concentration / ppb
Fig 8 Sensor response (S=R,/Ra) as a function of NOz concentration at 200°C for the devices using WO3 lamellae prepared with different acid solutions (pH —0.5) (a: solid circle) Ha SÓa, (b: solid triangle) HNO3, and (c: mark) HCl.
Trang 6
3
5
Time / min Fig 9 Response transients to NO» at 200°C of the device using WO3 lamellae pre-
pared with a H2SO, solution (pH —0.8)
sufficient ability to detect ppb level NOz in the atmosphere How-
ever, the sensor response of the devices differed depending on the
acid solutions used for preparing the WO3 particles The sensor fab-
ricated with HNO3 showed the best NO» response, but the device
made using HCl showed a lower response Since the physical prop-
erties of the sensing layers, such as particle size, crystalline size,
and surface area, are not very different among the three devices,
it is difficult to account for the observed difference on the basis of
these properties As possible reasons, differences in surface proper-
ties such as composition, porosity, and remaining impurities such
as Cl~ can be considered However, further experimental evidence
is required to elucidate the reasons for this
As noted above, the preparation using a strong acid solution
(H2SO4, pH —0.8) produced the most NOz-sensitive WO3 lamellae,
which were 30-50 nm in thickness To explore the possibilities of
using this material, other NOz sensing properties, such as response
speed and selectivity, were examined Response transients of the
device using WO3 lamellae prepared with a H2SQO, solution at pH
—0.8 are shown in Fig 9 The device responded reversibly to NO2
with changes in the resistance at 200°C However, the response
was rather sluggish, suggesting slow adsorption and desorption
rates of NO» Times for 90% response and recovery were 4 and
250
200
150
100
NOx concentration / ppm
Fig 10 Sensor response (S=R,/Ra) as a function of NO, ((a) NO2 and (b) NO) concen-
tration at 200°C for the device using WO3 lamellae prepared with a H2SOz, solution
11 min, respectively, for 1000 ppb NO»z at 200°C Such a sluggish response seems to be due to close packing of the particles As can
be seen in Fig 3(d), there are few meso- and macro-pores use- ful for gas diffusion in the film for WO3 lamellae prepared at pH
—0.8 To improve the response and recovery response behaviors, the sizes and distribution of the pores and length of gas diffusion path (thickness or radius of secondary particles of grains) need to
be optimized Fig 10 shows the dependence of the sensor response
on NO, (NO and NO>) concentration at 200°C for the device using WO3 lamellae prepared with a H2SO, solution (pH —0.8) The device showed a much lower sensor response to NO (S<2.3) than NO> in the 200-4000 ppb concentration range As highlighted in the liter- ature [5], NO is detected through the conversion of NO to NO» and subsequent NO» adsorption onto WO3 Thus, the obtained results suggest that WO3 lamellae prepared by the acidification method have little catalytic activity for converting NO to NO, at 200°C
It is also possible that the improved response to NO>z successfully diminished the interference from NO
4 Conclusion
The acidification of NaWO, with a strong acid solution (pH —0.5
or —0.8) produced lamellar-structured WO3 particles that were 100-350 nm in lateral size and 20-50 nm in thickness The electri- cal resistance of the device was successfully reduced by one order
of magnitude by using the nano-sized WO3 lamellae, probably due
to improved contact among lamellae The device using the nano- sized WO3 lamellae showed excellent NO sensing properties at the low temperature of 200 °C The sensor response exceeded S= 150 to dilute NO, (500 ppb in air) The device also exhibited a high NO» selectivity with little NO response (S=2.3-4 ppm NO at 200°C) Acknowledgment
This work was financially supported in part by a Grant-in-Aid for Scientific Research (B) (no 18350075) from the Ministry of Educa- tion, Science, Sports and Culture of Japan
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Biographies
Tetsuya Kida has been an Associate Professor at Kyushu University since 2006 He received his M Eng Degree in materials science in 1996 and his Dr Eng Degree
in 2001 from Kyushu University His current research interests include the devel- opment of chemical sensors, nano-particle synthesis methods, and self-assembled inorganic-organic hybrid materials
Aya Nishiyama received her B Eng Degree in materials science in 2007 from Kyushu University She is currently a Masters course student at the Department of Molecular and Material Sciences in Kyushu University
Masayoshi Yuasa has been an Assistant Professor at Kyushu University since 2005
He received his M Eng Degree in materials science in 2003 His current research interests include the development of chemical sensors and active electrocatalysts for oxygen reduction
Kengo Shimanoe has been a Professor at Kyushu University since 2005 He received
a BE degree in Applied Chemistry in 1983 and a M Eng Degree in 1985 from Kagoshima University and Kyushu University, respectively He joined Nippon Steel Corp in 1985, and received a Dr Eng Degree in 1993 from Kyushu University His cur- rent research interests include the development of gas sensors and other functional
devices
Noboru Yamazoe had been a professor at Kyushu University since 1981 until he retired in 2004 He received his M Eng Degree in Applied Chemistry in 1963 and Dr Eng Degree in 1969 from Kyushu University His research interests were directed mostly to the development and application of functional inorganic materials.