Preparation andcharacterizationof SnO
2
and WO
x
±SnO
2
nanosized powders and thick ®lms for gas sensing
A. Chiorino
a
, G. Ghiotti
a,*
, F. Prinetto
a
, M.C. Carotta
b
,
C. Malagu
Á
b
, G. Martinelli
b
a
Dip. di Chimica, I.F.M. Universita
Á
di Torino, Via P. Giuria 7, 10125 Torino, Italy
b
INFM-Dip. di Fisica, Universita
Á
di Ferrara, Via Paradiso 12, 44100 Ferrara, Italy
Abstract
SnO
2
powders pure and added with W
6
at two different loadings (1 and 5 W mol%), were prepared via a sol±gel route. Thick ®lms
prepared from the powders were used as CO and NO
2
gas sensors. The morphology of the powders was analyzed by TEM, HRTEM and that
of ®lms by SEM. The goal of obtaining powders and ®lms made by nanosized particles, even after thermal treatments at 8508C was
attained. The effect of W on the response of powders and ®lms towards CO and NO
2
was studied by FT-IR and conductance measurements,
respectively. W markedly lowered the response of SnO
2
towards CO and markedly enhanced its ability to sense NO
2
. Surface species
formed by CO and NO
2
interaction were investigated. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Gas sensors; WO
x
±SnO
2
; CO; NO
2
1. Introduction
Research on gas sensors is aimed at obtaining new
materials to achieve highly sensitive and selective devices.
Grain size reduction is one of the main factors enhancing the
gas sensing properties of semiconducting oxides, while
addition of different catalysts has been demonstrated to
increase the sensitivity towards speci®c gases. Preparation
of oxidic powders via sol±gel methods has been proved to be
one of the best method to obtain nanosized materials. The
aim of this work is to obtain nanosized WO
x
±SnO
2
materials
and to test them as CO and NO
2
sensors. Indeed, WO
3
-doped
SnO
2
[1] or Sn
x
WO
3x
mixed oxides [2] have been proposed
as materials for gas sensors, on the basis of the well-known
outstanding sensitivity of WO
3
to NO
x
[3].
2. Experimental
2.1. Powders
Pure SnO
2
was prepared by a sol±gel process previously
described [4]. The resulting gel was dried overnight at 958C,
giving a powder subsequently calcined at 5508C, thereafter
named SN. The W
6
added materials were prepared by
impregnation of SN powder with given amounts of aqueous
solutions of ammonium tungstate (Merck) to obtain two
different W nominal loadings: 1 and 5 mol%. The impreg-
nated powders, dried 3 h at 1208C, are thereafter named W1
and W5. TEM and HRTEM analysis of SN, W1 and W5
powders was performed with a Jeol 2000 EX electron
microscope equipped with a top entry stage.
The powders were compressed in self-supporting pellets
of about 50 mg cm
À2
, and put in an IR cell, which allowed
thermal treatments in vacuo and in controlled atmospheres.
They were initially submitted to alternate outgassing-oxi-
dizing treatments in dry oxygen at 650 or 8508C, then cooled
in oxygen at the chosen temperature (room temperature
(RT), 150 or 3508C). FT-IR spectra were run before in
O
2
, and then in CO, NO
2
, CO/O
2
or NO
2
/O
2
mixtures on
a FT-IR spectrophotometer (Perkin-Elmer System 2000),
working in transmission/absorption mode, with resolution of
2cm
À1
. The ratio NO
2
/O
2
and CO/O
2
of the mixtures used
was 1/5. High purity gases (from Praxair) were used: O
2
and
CO without further puri®cation, while NO
2
was prepared in
laboratory, by contacting NO, freshly distilled, with O
2
during 4 weeks at RT. The outgassing-oxidizing pre-treat-
ment temperature is thereafter reported in the pellet label
(i.e. SN-650, W1-650, etc.).
BET surface areas of SN, SN-650, SN-850, W1-650 and
W5-650 pellets were determined by N
2
physisorption using
a Micromeritics ASAP (10
À4
Pa) apparatus.
Sensors and Actuators B 78 (2001) 89±97
*
Corresponding author. Tel.: 39-11-670-7539; fax: 39-11-670-7855.
E-mail address: ghiotti@ch.unito.it (G. Ghiotti).
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0925-4005(01)00795-X
2.2. Films
The sensors were obtained starting from miniaturized
laser pre-cut 96% alumina substrates (2:5mm 2:5mm
Â0:25 mm for each device) provided with a heater element
on the backside, a Pt-100 resistor for the control of the sensor
operating temperature and a gold front interdigitated con-
tacts. The thick ®lms were prepared starting from pastes
obtained by adding to the above powders an organic vehicle
together with a small percentage of glass fritt for improving
the adhesion of the layers to the substrates. The ®lms were
then ®red for 1 h at 650 or 8508C in air. The thickness of the
deposited layers was in the 20±30 mm. The conductance
measurements were performed in a closed test chamber at a
¯ow rate of 0.5 l min
À1
, at temperature up to 4008C in wet
(40% RH) air and in 100 ppm of CO or in 10 ppm of NO
2
in
wet air. The sensors were biased at 5 V constant voltage and
the conductance was obtained measuring the voltage drop on
calibrated resistors. Six sample series have been tested,
obtained from the SN, W1 and W5 powders ®red at 650
and 8508C. For sake of brevity, the labels used for powder
pellets were also used for the ®red thick ®lms (SN-650, W1-
650, etc.).
The morphology of all ®lms ®red at 8508C was analyzed
by a 360-Cambridge scanning electron microscope.
3. Results and discussion
3.1. Morphology and texture
TEM images of SN powder showed aggregates of parti-
cles, rounded shaped and homogeneous in size (diameter
10±20 nm). W1 and W5 samples showed more densely
packed aggregates of particles with very indented borders
and homogeneous in size (20 nm). HRTEM images of the
three powders showed particles with fringe distances of the
cassiterite (1 1 0) and (1 0 1) planes. However, small spots
of 0.5±1 nm in size could be seen on some particles of W5
sample. They could be due to the presence of small aggre-
gates of tungsten-oxide units, even if too small and/or too
poorly crystalline to give speci®c re¯exes in the electron
diffraction pattern. In Fig. 1a and b HRTEM images of SN
and W5 powders, respectively, are shown.
On powders treated at 650 and 8508C no HRTEM images
were done. Only scanning electron microscopies of SN-850,
W1-850 and W5-850 ®lms were performed. SN-850 ®lms
showed densely packed aggregates of rounded particles
homogeneous in size (diameter 40±60 nm) (see Fig. 2a).
At variance, W1-850 and W5-850 ®lms showed aggregates of
particles with irregular shape, less homogeneous in size, their
diameter ranging between 40 and 90 nm (see Fig. 2b and c).
SN, SN-650 and SN-850 powders showed speci®c surface
areas of 31, 31 and 17 m
2
g
À1
, respectively. W1-650 and
W5-650 pellets showed speci®c surface area of 25 and
27 m
2
g
À1
, respectively.
3.2. FT-IR characterization
The FT-IR spectra of both W1-650/850 and W5-650/850
in dry O
2
differed from those of SN-650/850 by two main
features: (i) the overall IR transmittance was higher after W
addition, as shown in Fig. 3 for SN-650 and W1-650 samples
and (ii) new peaks, of medium and low intensity in the low
frequency side of the spectrum (1150±780 cm
À1
region)
appeared. These peaks are well evident in the inset of
Fig. 3, where the spectra in absorbance of SN-650, W1-
650 and W5-650 samples are compared (curves 1±3, respec-
tively) in the more restricted 1150±780 cm
À1
region. The
increase in transmittance can be easily interpreted as due to a
decreased concentration of free electrons andof electrons
trapped in oxygen vacancies, the W
6
ions acting evidently
as deeper acceptor levels. The new peaks, whose intensity
increased with W loading, fall in a region where the vibra-
tion modes of W=O, (W±O±W)
n
and (Sn±O±W)
n
are
expected to fall [5±7].
Concerning the CO interaction at RT with the materials,
the results can be resumed as follows:
1. The response to pure CO of the SN-650/850 pellets was
that already reported in previous papers for SnO
2
prepared in different way [4,8]: CO reacted with oxygen
surface ions (O
2
À
,O
À
,O
2À
) giving carbonate-like
species, which partially decomposed to CO
2
releasing
electrons to the tin oxide; the electrons released
repopulated the conduction band (CB) and the oxygen
vacancies (V
O
2
). The electronic repopulation of the
V
O
2
could be revealed by the intensity increase of a
very broad absorption extending all over near and
medium IR regions, previously assigned to the photo-
ionization of the monoionized oxygen vacancies,
V
O
hn 3 V
O
2
e
À
(CB) [7±9]. The surface re-
duction increased increasing the contact times and/or the
equilibrium pressure. Taking the integrated intensity of
the broad electronic absorption as a measure of the
material response to CO, the SN-650 pellet appeared
more sensitive than the SN-850 one.
2. As for the W-added materials, W1-650/850 and W5-
650/850 showed a behavior similar to that of SN-650/
850 ones. However, within the same pre-treatment
temperature, SN appeared the most sensitive material,
the sensitivity being progressively lower increasing the
W loading. Furthermore, the CO interaction caused the
intensity decrease of the peaks assigned to tungstenyls
(W=O) groups, revealing they were at the surface.
The behavior above described is illustrated in Fig. 4a±c
for SN-650, W1-650 and W5-650 materials (taken as an
example): the reported curves are the differences between
absorption spectra after and before the interaction with CO
at different contact times and/or equilibrium pressures and
they clearly show the increase of the very broad absorption
previously discussed. The species formed at the surface were
90 A. Chiorino et al. / Sensors and Actuators B 78 (2001) 89±97
also detected, the sharp peaks superimposed to the broad
electronic absorption are, indeed, the vibration modes of the
surface species formed by CO interaction. On all materials a
weak band at 2203 cm
À1
is present, assigned to CO linearly
bound to coordinatively unsaturated surface Sn
4
ions. The
various peaks observed in the 1800±1000 cm
À1
spectral
region could be assigned to different surface carbonate-like
species formed by the reductive adsorption of CO on
different sites. The main species formed, in absolute and
relative amounts depending on the materials and on the
Fig. 1. HRTEM images of (a) SN and (b) W5 powders.
A. Chiorino et al. / Sensors and Actuators B 78 (2001) 89±97 91
pre-treatment temperatures, were as follows: (i) chelating
and bridging bidentates carbonates (n(C=O) at 1650±
1550 cm
À1
, n
asym
(OCO) and n
sym
(OCO) at 1270 and
1020 cm
À1
, respectively), (ii) ``strongly perturbed CO
2
''
or ``carboxylate species'' (n
asym
(OCO) and n
sym
(OCO) at
1750±1730 and 1270 cm
À1
, respectively) and (iii) CO
2
linearly coordinated (n
asym
(OCO) at 2330 cm
À1
). On W1
and W5 the negative peaks at 1040±1030 cm
À1
account for
the erosion of the (W=O) surface groups.
At 150 or 3508C in CO/O
2
mixture all the samples
responded to CO. However, while it was possible to compare
the spectroscopic response at 1508C of the different materi-
als, the SN-650/850 being again the most active samples,
this was not possible at 3508C because they completely
loose their transparency to the radiation. We could only
conclude that at 3508C all materials better respond to CO
than at 1508C. No surface species could be detected at
1508C, and obviously, at 3508C.
As for NO
2
or NO
2
/O
2
mixture, we only examined the
interaction with SN-650, W1-650 and W5-650 pellets.
Concerning interaction at RT we examined the response to
5 mbar of NO
2
, following the spectra evolution with the
contact time. The results can be resumed as follows:
1. By interaction with SN-650 pellet, a quick reduction
process at the surface immediately occurred with
electron release to the CB and the subsequent V
O
2
repopulation to V
O
, revealed by the quick increase of
the very broad absorption already described extending
all over the near and medium IR regions (see Fig. 5a),
which reached its maximum intensity in 2 s. Sharp
vibrational bands appeared, but of very weak intensity,
the most intense being a peak at 1178 cm
À1
, assignable
to surface n
asym
(ONO) mode of chelating nitrites. At
higher contact time, the electron population of the V
O
Fig. 2. SEM images of (a) SN-850, (b) W1-850 and (c) W5-850 films.
Fig. 3. FT-IR transmittance spectra of SN-650 (solid line) and W1-650 (dotted line) samples in dry oxygen. Inset: FT-IR absorbance spectra of SN-650, W1-
650 and W5-650 samples (curves 1±3, respectively) in the 1150±750 cm
À1
region; the spectra have been translated along the absorbance axis to allow a better
comparison.
92 A. Chiorino et al. / Sensors and Actuators B 78 (2001) 89±97
started to decrease revealing an oxidation process with a
kinetic slower than the reductive one. The electron
population decrease was paralleled by the growth of
sharp peaks related to the formation a variety of NO
x
surface species: chelating nitrites (n
asym
(ONO) mode at
1178 cm
À1
), several types of chelating and bridging
nitrates (n(N=O) modes in the range 1700±1500 cm
À1
,
n
asym
(ONO) modes in the range 1250±1180 cm
À1
and
n
sym
(ONO) modes in the range 1000±900 cm
À1
, respec-
tively), NO
dÀ
and NO
2
d
weakly adsorbed species
(bands in the range 2050±1800 cm
À1
). When the
reactions stopped after 1 min, the sample was slightly
oxidized, that is the electron concentration in the V
O
was lower than before the NO
2
admission: in fact, taking
as reference the dotted line representing the line of
absorbance zero in Fig. 5a, a broad, weak, and negative
absorption is evident, showing the same shape of the
electronic absorption assigned to V
O
hn 3 V
O
2
e
À
(CB).
2. At variance, W1-650 pellet was immediately oxidized
by NO
2
(see Fig. 5b), with decrease of the electron
population of the V
O
: a broad, weak, and negative
absorption is already evident after a contact time of 2 s,
at the same time weak vibrational peaks related to the
formation of a variety of NO
x
surface species appeared.
At higher contact times the electronic population
remained unchanged, while the vibrational peaks related
to the NO
x
surface species continued to increase during
1 min. The surface species formed were similar to those
present at the surface of the SN-650 material but in
different amounts. In particular chelating nitrites, NO
dÀ
and NO
2
d
weakly adsorbed species were markedly
Fig. 4. Effects of CO at RT on FT-IR spectra of (a) SN-650, (b) W1-650
and (c) W5-650 samples. (a) Equilibrium pressures: <10
À2
mbar,
1 Â 10
À2
mbar and 2 or 7 mbar (curves 1±3, respectively). (b) and (c)
Equilibrium pressures: 0.1, 2 and 7 mbar (curves 1±3, respectively). The
dotted line represents the zero of absorbance.
Fig. 5. Time resolved effects of 5 mbar of NO
2
at RT on FT-IR spectra of
(a) SN-650, (b) W1-650 and (c) W5-650 pellets. Spectra after 2 s, 10 s and
1 min (curves 1±3, respectively). The dotted line represents the zero of
absorbance.
A. Chiorino et al. / Sensors and Actuators B 78 (2001) 89±97 93
decreased. Chelating and bridging nitrates were the
species predominant at the surface, their vibration
frequencies being slightly different from those present
on SN-650, indicating that tungsten ions were involved
in the coordination of these species. Concerning W5-
650 sample (Fig. 5c), again the electron population of
the V
O
started to decrease immediately after NO
2
interaction, revealing oxidation processes, that stopped
after 10 s. Also in this case the electron concentration
decrease was paralleled by the growth of sharp peaks
related to the formation of NO
x
surface species. However,
chelating nitrites and NO
dÀ
or NO
2
d
weakly adsorbed
species were completely absent, only one type of bridging
nitrate was still present (n(N=O) mode at 1700 cm
À1
,
n
asym
(ONO) modes at 1300 cm
À1
and n
sym
(ONO) at
1000 cm
À1
, respectively), while the main surface species
were nitrate of ionic types not detected on the other two
materials (n
asym
(ONO) mode at 1450 cm
À1
and n
sym
(O-
NO) mode at 1370 cm
À1
). This indicates that the surface
chemistry was markedly changed.
At the equilibrium (after 1 min), the values of the inte-
grated negative intensity of the broad electronic absorption
were in the following order: W5-650 < W1-650 < SN-650
sample (compare the intensities of the broad negative
absorption of the curve 3 in Fig. 5). Taking this integrated
negative intensity as a measure of the electronic response to
NO
2
, W5 appeared the most sensitive one, the sensitivity
decreasing with the W loading.
Concerning the NO
2
/O
2
interaction at 150 or 3508C, we
only studied SN-650 and W5-650 pellets. By interaction
with the gas mixture both the materials were quickly oxi-
dized, the oxidation degree being higher for the W5-650
than for SN-650. The highest oxidation degree was obtained
at 1508C. At this temperature, taking the integrated negative
intensity of the broad electronic absorption as a measure of
the material response, W5 appeared more sensitive than SN
material (see Fig. 6). At variance, no marked difference
could be detected between the two samples at 3508 C.
3.3. Electrical characterization
Carbon monoxide and nitrogen dioxide are two of the
most dangerous gases polluting the atmosphere in the urban
areas. For obtaining actual responses to CO and NO
2
,we
tested the sensors in conditions as much as possible close to
the operating ones. With this aim in view, the in¯uence of
tungsten addition onto the electrical properties of tin oxide
as host material was investigated performing conductance
measurements as a function of temperature in wet air at 40%
RH. Fig. 7 shows the Arrhenius plots of pure and tungsten-
added SnO
2
thick ®lms ®red at 8508C (SN-850, W1-850 and
W5-850) obtained changing the temperature from 100 up to
5008C at the heating rate of 3 K min
À1
. The samples exhib-
ited three regions of conductivity; this behavior is quite
usual in thick ®lm sensors of SnO
2
differently synthesized as
reported in previous works [10,11]. Moreover, an addition of
5% of tungsten caused a signi®cant decrease in conductance,
till about one order of magnitude at the temperature of
3008C; besides the W1-850 samples showed a behavior in
between the SN-850 and W5-850 ones. All the electrical
characteristics investigated showed the same trend being in
very good agreement with FT-IR analyses.
At the same time the W loading increased the intergrain
energy barriers as a function of temperature, as it is shown in
Fig. 8 (the experimental method to determine them is
described in [12]). Since the difference between the mini-
mum and the maximum values of the energy barrier (DE)is
lower in the case of W-added materials, the above result
cannot be associated with a greater amount of oxygen ions
Fig. 6. Effects of 5 mbar of the NO
2
/O
2
mixture at 1508C on FT-IR spectra
of SN-650 and W5-650 pellets (curves 1 and 2, respectively). The dotted
line represents the zero of absorbance.
Fig. 7. Temperature dependence of the conductance in wet (40% RH) air
of SN-850, W1-850 and W5-850 thick films heated to 8508C.
94 A. Chiorino et al. / Sensors and Actuators B 78 (2001) 89±97
adsorbed which is at contrary expected to increase DE [13].
In fact from the Fig. 8 it is possible to evaluate DE as 0.15 eV
in the case of SN-850, 0.13 eV for W1-850 and ®nally
0.11 eV for W5-850. Therefore, the decreasing conductance
has to be attributed to the increased number of holes in the
valence band injected by the acceptor-like W ions.
As suggested by the behavior of the energy barrier, the
temperature interval in which the oxygen adsorption may
occur is ranging from 150 up to 3608C, as well as the
gas±surface reactions [12]. Moreover, in n-type semicon-
ductor oxides as SnO
2
, at low temperature, when the energy
barrier is low, we expect the oxidizing gases more reactive;
an opposite behavior occurs in the case of reducing gases.
According to the above observation, the SnO
2
and W-
added SnO
2
-based ®lms were tested as gas sensor versus
carbon monoxide and nitrogen dioxide in the 150±3508C
range of temperature. In agreement with previous works of
the same authors, the maximum response to NO
2
has been
found at 1508C, whilst the CO presented its maximum
response at about 3008C (see for instance [4]).
Concerning the SN-850, W1-850, W5-850 samples, the
tests showed that the ability of the thick ®lms to detect CO
was lowered increasing the W loading, in agreement with the
barrier energy behavior. In fact it has been demonstrated that
DE is strongly related to the sensitivity to reducing gases,
like carbon monoxide [13]. The W content also affected the
temperature of the maximum response. In particular W
addition stabilized the response at the different working
temperatures: W5 thick ®lms showed a weak response
almost unaltered in all the examined range (200±3508C).
Concerning NO
2
, conductance measurements showed that
the ability of the material to detect the gas was enhanced
increasing the W loading. The enhancement was particularly
marked at 2508C (three times more passing from SN-850 to
W5-850). It is worthy to note that at this working tempera-
ture the SN-850 samples showed the maximum response to
CO, being more than three times higher than that of the W5
®lms (see Fig. 9). Furthermore, all three materials showed
the maximum response to NO
2
at 1508C, temperature at
which the response of the three ®lms to CO was very low.
Fig. 8. Temperature dependence of the barrier energy in wet (40% RH) air
of SN-850, W1-850 and W5-850 thick films heated to 8508C.
Fig. 9. Electrical response to (a) 100 ppm of CO (G
gas
/G
air
) and (b) 10 ppm of NO
2
(G
air
/G
gas
) in wet air (40% RH) of SN-850, W1-850 and W5-850 thick
films, measured at 2508C.
A. Chiorino et al. / Sensors and Actuators B 78 (2001) 89±97 95
The SN-650, W1-650, W5-650 samples behaved in the
same way as the samples ®red at 8508C; the only difference
was the temperature of the maximum response to CO (300
instead of 2508C). More generally the samples ®red at 6508C
showed a higher response to the tested gases probably
because of a greater speci®c surface area. As an example,
the tests at the working temperatures of 3008C for 100 ppm
of CO (Fig. 10) and at 2008C for 10 ppm of NO
2
(Fig. 11) in
wet air (40% RH) are reported. We have shown these tests
(300 instead of 2508C for CO and 200 instead of 1508C for
NO
2
) to highlight the differences between the behavior
of various samples taking also in to account that higher
temperatures are always to prefer both to improve the
response time and to avoid poisoning phenomena.
4. Conclusions
SEM, TEM and HRTEM microscopies show that the goal
of obtaining powders and ®lms made by nanosized particles
even after thermal treatments at 8508C, is attained.
FT-IR spectroscopic and electrical measurements have
been employed on powders and ®lms, respectively, for
obtaining information on the electronic effect due to the
tungsten addition. FT-IR results show that W addition
increases the sample transmittance and this phenomenon
has been attributed to a decreased concentration of free
electrons andof electrons trapped in oxygen vacancies, the
W
6
ions acting as deeper acceptor levels. According to this,
the electrical data show that W markedly lowers the con-
ductance of the ®lms in air.
Electrical measurements show that W lowers the response
of tin oxide to CO and also affects the temperature of the
maximum response. At variance the W addition enhances its
ability to sense NO
2
, leaving unaltered the temperature of
the maximum response: 1508C.
FT-IR transmission changes of the three materials, pre-
treated either at 650 or 8508C, contacted with pure CO at RT
or with CO/O
2
mixture at 150 and 3508C, are qualitatively in
good agreement with the electrical measurements. The
agreement between FT-IR and conductance measurements
has been also con®rmed for NO
2
and NO
2
/O
2
mixtures, at
least for the samples studied with both the techniques. FT-IR
spectroscopy was also employed to obtain information on
the nature of the surface species formed by interaction with
the two examined gases.
Acknowledgements
Financial support was provided by the Italian CNR
(Progetti Finalizzati MADESS II).
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. used as CO and NO
2
gas sensors. The morphology of the powders was analyzed by TEM, HRTEM and that
of ®lms by SEM. The goal of obtaining powders and ®lms. area of 25 and
27 m
2
g
À1
, respectively.
3.2. FT-IR characterization
The FT-IR spectra of both W1-650/850 and W5-650/850
in dry O
2
differed from those of