Wet process-based fabrication of WO 3 thin film for NO 2 detectionDepartment of Materials Science, Faculty of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan
Trang 1Wet process-based fabrication of WO 3 thin film for NO 2 detection
Department of Materials Science, Faculty of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan
Received 12 November 2003; received in revised form 6 February 2004; accepted 7 February 2004
Available online 8 April 2004
Abstract
It was tried to fabricate a WO3thin film device through a wet process starting from an aqueous sol of WO3·2H2O When mixed with polyethylene glycol (PEG, molecular weight 6000), the sol was made compatible to spin coating on an alumina substrate and the coating could be converted into a WO3thin film by calcination at 300◦C for 2 h Starting with a typical coating dispersion containing WO
3·2H2O
by 5 mass% on WO3basis and 2 mass% PEG, the WO3film obtained was 450 nm in mean thickness The film was a slim pack of square plates, each of which was a stack of thin lamellar crystals of WO3 The device was sensitive enough to detect 50 ppb NO2in air at 200
or 250◦C, although the response and recovery transients were rather sluggish Unexpectedly, the transients were found to be sharpened
drastically in humid atmosphere, while sensor response (sensitivity) to NO2was hardly degraded with humidity
© 2004 Elsevier B.V All rights reserved
Keywords: Tungsten oxide; Gas sensor; NO2 ; Thin film; Sol
1 Introduction
For semiconductor gas sensors, WO3is an important base
oxide which exhibits high sensitivity to non-hydrocarbon
gases, like NO2and NH3[1–3] So far various methods have
been adopted for preparing WO3 as a sensor material,
in-cluding pyrolysis of (NH4)10W12O41·5H2O[1–3],
sputter-ing or evaporation from a source of WO3[4–9], and sol–gel
processes starting from W-alkoxide[10] However, it is
dif-ficult to control the microstructure of WO3-based devices,
i.e the shape, size and stacking of WO3particles included,
by these methods Recently, we reported that an aqueous
colloidal dispersion (sol) of WO3·2H2O could be prepared
though wet processes starting from an ion-exchange
reac-tion of Na2WO4[11] The sol could be converted into a gel
by centrifuge, making it possible to fabricate a thick film by
a screen-printing method The resulting film of WO3 was
found to exhibit fairly excellent NO2 sensing properties
if the sol had been subjected to ultrasonic or centrifugal
treatments under proper conditions, which affected the
mor-phology of WO3 crystals[12,13] Apart from thick films,
it is also of interest to fabricate thin films of WO3from the
WO3·2H2O sol Unfortunately, this has been postponed
be-cause it was difficult to obtain a uniform thin film from the
sol by a spin-coating method However, it has been found
∗Corresponding author Tel.:+81-92-583-7537; fax: +81-92-583-7538.
E-mail address: yamazoe@mm.kyushu-u.ac.jp (N Yamazoe).
that, when mixed with an organic binder (polyethylene gly-col, PEG), the sol gives a thin film on an alumina substrate This paper aims at reporting morphology and NO2sensing properties of WO3thin films thus obtained
2 Experimental
A colloidal dispersion (sol) of WO3·2H2O was prepared
in the same way as reported elsewhere[11] An aqueous so-lution of Na2WO4(0.15 M) was let to pass through a column packed with protonated cation exchange resin (Diaion SK 1B) The effluent was kept standing for 3 days before it de-posited a gel The gel, collected by decantation, was washed with deionized water, and recollected by centrifuge and de-cantation The washed gel was dispersed in deionized water again to form a WO3·2H2O sol The sol contained colloidal particles of crystalline WO3·2H2O of about 30 nm in size The content of WO3·2H2O in the sol was set to be 5 mass%
on the WO3 basis unless otherwise noted PEG with aver-age molecular weight of 6000 was added to the above sol
by 2 mass% unless otherwise noted to obtain a spin-coating dispersion The dispersion was spin coated on an alumina substrate attached with a pair of interdigited gold electrodes (300m in separation between electrodes) under the rotation
speed of 1500 rpm The obtained thin film of WO3·2H2O was calcined at 300◦C for 2 h for conversion of WO
3·2H2O
to WO3as well as sintering Gas sensing experiments were carried out in a conventional flow apparatus equipped with a
0925-4005/$ – see front matter © 2004 Elsevier B.V All rights reserved.
doi:10.1016/j.snb.2004.02.031
Trang 2heating facility at a gas flow rate of 100 cm3/min The
con-centration of NO2was varied between 10 and 1000 ppb by
diluting a parent NO2 gas (1 ppm in dry air) with dry air
When necessary, part of the air was humidified by babbling
through water phase
3 Results and discussion
3.1 Morphology of WO3thin films
Thin films of WO3could be derived from the WO3·2H2O
sol added with PEG (2 wt.%) by one time spin coating and
calcination at 300◦C for 2 h.Fig 1shows SEM images of
the thin films obtained The films were an irregular packing
of square plates of WO3, 0.5–1m in width and 0.2–0.5 m
in thickness Each square plate was a stack of thin plates
(lamellae) of less than 100 nm in thickness This morphology
was unchanged regardless of the WO3·2H2O contents in the
spin-coating dispersion, i.e 1 mass%Fig 1aand 5 mass%
Fig 1b Actually the outlook of WO3crystals, featured by
lamellar structure, was essentially the same as what we
ob-served for the WO3thick films prepared by a screen-printing
method[12,13] The lamellar structure has its origin in the
crystal habit of WO3·2H2O, which tends to grow into thin
Fig 1 SEM images (top and cross-sectional views) of WO 3 thin films derived from WO 3 ·2H2 O sols (PEG 2 mass%) WO 3·2H2 O content: (a) 1 mass%, (b) and (c) 5 mass%.
plates in the sol During the drying process, the thin plates are stacked together to form square plates having lamel-lar structure The lamellamel-lar structure is preserved even after
WO3·2H2O is dehydrated finally into WO3 by calcination (dehydration temperature about 200◦C) We have shown that topotaxy holds well between the crystals of WO3·2H2O and that of WO3: The basal plane (0 1 0) of WO3·2H2O is converted into the basal plane (0 0 2) of WO3[12]
As shown by the cross-sectional view Fig 1c, the thin films were a slim layer of discrete square plates of WO3 ly-ing on the substrate In the direction normal to the substrate,
a few plates were stacked in some spots, while only a single plate existed in other spots With such a gross population, the films were hardly uniform in thickness Nevertheless, the thickness of the layer was measured visually at arbitrarily selected five spots to obtain their mean value for each film
Fig 2 shows the mean thickness thus obtained as a func-tion of WO3·2H2O content (1–5 mass%) of the spin-coating dispersion, together with the maximum and minimum in thickness measured Although the thickness fluctuated sig-nificantly depending on spots within a single film, the mean thickness tended to increase monotonically with increasing
WO3 content The film derived from 5 mass% WO3·2H2O dispersion, 450 nm in the mean thickness, was supplied to the subsequent gas sensing experiments
Trang 3Fig 2 Mean thickness of WO 3 thin films derived from WO 3·2H2 O sols
as a function of WO 3 ·2H2 O content (mass% on the WO 3 basis) Vertical
bars show maxima and minima in thickness.
Remarks are given here to the role and fate of PEG added
to the spin-coating dispersion As stated earlier, it was unable
to deposit a layer of WO3·2H2O crystals on the substrate
from a neat WO3·2H2O dispersion by spin coating
Obvi-ously, the deposition of WO3·2H2O crystals in the present
case was assisted greatly by PEG It is known that PEG
in-creases the viscosity of the dispersion In fact, the viscosity
of the spin-coating dispersions containing 0 and 2 mass%
PEG was 1.0 and 1.4 cP, respectively An increase in
viscos-ity would make the dispersion more adhesive to the substrate,
giving the WO3·2H2O crystals more chances to deposit in
the spin-coating process It is thus considered at present that
the primary role of PEG is to increase the viscosity of the
dis-persion Part of PEG would be left behind in the spin-coated
film As revealed by thermogravimetric analysis, however,
PEG molecules began to decompose at about 200◦C, and
no residues remained after calcination at 300◦C for 2 h.
Fig 3 Response and recovery transients of electrical resistance to switching-on and -off 100 and 500 ppb NO 2 in dry air at three selected temperatures (Film derived from a dispersion of 5 mass% WO 3·2H2 O and 2 mass% PEG.)
3.2 NO2sensing properties in dry atmosphere
The WO3 thin film derived from the dispersion of
5 mass% WO3·2H2O was examined for NO2sensing prop-erties at 200, 250 and 300◦C Response and recovery transients in electrical resistance to switching-on and -off
100 and 500 ppb NO2 are shown in Fig 3 Response and recovery were both rather sluggish, taking about 10 min
or more for 90% of full response or of full recovery at all temperatures This was in contrast to the case of thick films (about 6m in thickness) reported previously[12,13], where the 90% response or recovery times were less than
2 min for the films calcined at 300◦C The electrical
re-sistance of the thin film device in air (Ra) was fairly high, exceeding 107 at 200◦C Accordingly, the resistance under exposure to NO2 in air (Rg) easily went beyond the limit of reliable measurement range (∼109) even
for small concentrations (e.g 500 ppb) of NO2 The upper limiting concentration of NO2 practically acceptable was rather low, for example, being smaller than a few hundred ppb at 200◦C Within this limitation, the present film was quite sensitive to NO2 As seen fromFig 4, where sensor
response as expressed by normalized resistance (Rg/Ra) is shown as a function of NO2concentration at three different temperatures At 200◦C, sensor response was as large as 7–50 ppb NO2 With a rise in temperature, sensor response reduced rather sharply, showing 50 and 10–500 ppb NO2at
250 and 300◦C, respectively Nevertheless, sensor response
to 50 ppb NO2at 250◦C was still 3, assuring that the device was sensitive enough even at this temperature to meet the detection of environmental NO2 (environmental standard: 40–60 ppb in Japan) For comparison, sensor response of the screen-printed thick film device was also shown in the same figure It is seen that the present device is more sensitive
to NO2than the thick film device at 200◦C, although such
Trang 4Fig 4 Normalized resistance (Rg/Ra ) as a function of NO 2 concentration
for a thin film device (full lines and filled marks) and a thick film
device (broken lines and open marks) at three selected temperatures (dry
atmosphere).
superiority is reduced and almost lost at 250 and 300◦C,
respectively
3.3 Preparation under different conditions
The thin film device prepared above was found to have a
problem that the response and recovery transients were too
sluggish Thus, thin film devices were prepared under
dif-ferent conditions First, the content of PEG in the dispersion
was increased from 2 mass% to 7, 15 or 20 mass%, while
keeping the WO3·2H2O content the same (5 mass%) The
viscosity of the spin-coating dispersion was increased from
1.4 to 3.3, 8.0 or 11.3 cP, correspondingly, so that the film
thickness would be expected to increase in this order.Fig 5a
shows the response and recovery transients to 50 ppb NO2
for the resulting thin film devices, calcined at 300◦C The
rate of response tended to be quicker with 15 and 20 mass%
PEG (transient (3) and (4)) than with 2 and 7 mass% PEG
((1) and (2)), whereas the rate of recovery was almost
un-changed regardless of the PEG content As also seen from
the figure, the magnitudes of sensor response were about
the same, being large enough to safely detect 50 ppb NO2
for all the devices These results indicate the necessity of
investigating the influences of high PEG content on the
re-sulting thin films in more detail in the future Second, the
calcination temperature was raised from 300 to 400◦C
Re-sulting transients are shown in Fig 5b The rates of both
response and recovery were made more sluggish as
com-pared with the case ofFig 5a The deterioration of response
and recovery rates with increasing calcination temperature
has been observed for the thick film devices, and the
phe-nomenon has been interpreted as reflecting an increase of
micro-pores in the WO3lamellae included[13] In the case
of the thick films, however, fairly quick transients, taking less
than 2 min for 90% response or recovery, have been obtained
even after calcination at 400◦C The sluggish transients of
the present thin films, shown inFig 5a and b, suggest that
the micropore structure may differ significantly from that
Fig 5 Response and recovery transients to 50 ppb NO 2 in dry air for
WO 3 thin film devices, derived from WO 3 ·2H2 O sols different in PEG content and calcined at 300 ◦C (a) and 400◦C (b) PEG content in mass%:
(1) 2, (2) 7, (3) 15 and (4) 20.
of the thick films, although this is also to be confirmed experimentally
3.4 Response to NO2in humid atmosphere
Finally, NO2 sensing properties in humid atmosphere were tested briefly by using a fresh thin film device, which was derived from the dispersion added with 2 mass% PEG and calcined at 300◦C.Fig 6shows response and recovery transients to 1000 and 500 ppb NO2in air of 10% relative hu-midity (RH) at 250 and 300◦C Unexpectedly, the response and recovery transients were found to be improved drasti-cally under the humid atmosphere The 90% response (re-covery) times were about 1 min (about 0.5 min) at both tem-peratures It is remarkable that the recovery is quicker than the response, because the reverse is the case usually Almost the same conclusion was drawn when relative humidity was increased to 40 and 70% (data not shown here) The mech-anism by which the humidity improves the transients is not known well at present It may be possible that the adsorption
of NO2in the micro-pores of WO3lamellae, which is consid-ered to be responsible for the sluggish transients, is prevented
Trang 5Fig 6 Response and recovery transients to 1000 and 500 ppb NO 2 in
humid air (10% relative humidity) at 250 and 300 ◦C (Device derived
from a dispersion of 5 mass% WO 3·2H2 O and 2 mass% PEG and calcined
at 300 ◦C.)
from occurring due to preferential condensation of water
va-por Any way, it turned out that the problem of the present
de-vice, i.e being sluggish in response and recovery, could thus
be eliminated in humid atmospheres In addition, although
the resistance of the device in air (Ra) was lowered by some
extent in the humid conditions, sensor response (Rg/Ra)
hardly degraded with humidity, as seen from the comparison
betweenFigs 3 and 6 This suggests that the NO2sensing
may be carried out more favorably in humid atmospheres
than in dry atmosphere provided that the humidity level is
known separately More detailed investigations are desired
for the NO2sensing properties in humid atmospheres
4 Conclusions
An aqueous sol of WO3·2H2O, added with PEG, could
be applied successfully for fabricating a thin film of WO3
on an alumina substrate by spin coating and calcination
The thin film device obtained was sensitive enough to detect
dilute NO2in environments, though the response and
recov-ery transients were rather sluggish Remarkably, the
tran-sients were made very sharp in humid atmospheres, while
sensor response (sensitivity) to NO2 hardly degraded with
humidity
Acknowledgements
This study was partially supported by a grant-in-aid for
Scientific Research from The Ministry of Education,
Cul-ture, Sport, Science and Technology of Japan
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Biographies
Yong-Gyu Choi received his BE degree in Materials Science and
Engi-neering in 1996 and ME degree in 1998 from Kyungsung University in Korea He received PhD in Engineering in 2003 from Kyushu Univer-sity His current research interest is development of a NOx sensor by spin-coating method with WO 3 sol provided by ion exchange method.
Go Sakai has been a research associate at Kyushu University since 1996.
He received his BE degree in Applied Chemistry in 1991, ME degree
in 1993 and PhD in Engineering in 1996 from Kyushu University His current research work is focused on development of chemical sensors as well as functional inorganic materials.
Kengo Shimanoe has been an Associate Professor at Kyushu University
since 1999 He received his BE degree in Applied Chemistry in 1983 and
ME degree in 1985 from Kagoshima University and Kyushu University, respectively He joined the advanced materials and technology labora-tory in Nippon Steel Corp and studied the electronic characterization on semiconductor surface and the electrochemical reaction on materials He received PhD in Engineering in 1993 from Kyushu University His cur-rent research interests include the development of chemical sensors and the analysis of solid surface.
Noboru Yamazoe has been a Professor at Kyushu University since 1981.
He received his BE degree in Applied Chemistry in 1963 and PhD in Engineering in 1969 from Kyushu University His current research inter-ests include the development and application of the functional inorganic materials.