ELECTRONIC NOSES APPLICATIONS AND TECHNOLOGIES
Nanostrutured thick film sensors for CO (g) based on Al doped SnO21Cabezas M.D.; 2Lamas D.G.; 3Baby R.E.; 4Cabanillas E.; 2Walsöe de Reca N.E.
1CITEFA,
2CITEFA-CONICET, 3CONICET-UTN (FRBA),4CONICET-CNEA
CINSO (Centro de INvestigaciones en SOlidos) CITEFA-CONICET,
Juan Bautista de La Salle 4397, Villa Martelli (B1603ALO), Buenos Aires, Argentina,
Fax: (54) (11)-4709-8241, E-Mail:walsoe@citefa.gov.ar
Received November 05, 2004. In final form May 30, 2005
Abstract
In this work, the aluminium doping of SnO2 lattice was studied,
synthesising a semiconductive nanocrystalline doped material by a gel-combustion
method with citric acid as fuel, to be used for thick film sensors to detect
CO (g). Aluminium concentration was varied from 1 to 5 at %, nanocrystalline
powders with crystallite mean size of 8-12 nm and low agglomeration were
obtained. The semiconductive material was characterised with X-rays diffraction,
energy dispersive electron microprobe and transmission electron microscopy.
Nanocrystalline powders were used to prepare conductive pastes with suitable
organic solvents employing our own formulations different from those of commercial
materials. Pastes were deposited as serigraphic thick films on electronic
purity alumina wafers provided with Pt electrodes. Sintering of thick sensing
films was performed in a range of medium temperatures to avoid important
crystallite growth since it is convenient to maintain the nanocrystalline
structure in order to increase sensor sensitivity. Sensors were characterised
in mixtures of [CO(g)+air] at temperatures from 300 to 400 °C. Results
indicated that aluminium was incorporated in the SnO2
lattice in low proportions leaving low concentrations of amorphous Al2O3
segregated. Sensors showed a high sensitivity to the target gas. In addition,
it was also found that the incorporation of aluminium to SnO2 lowers
the sensor operating temperature.
Resumen
En este trabajo de investigación, se estudió el dopado con
aluminio de SnO2, sintetizando el material semiconductor dopado
nanocristalino, mediante el método de gelificación-combustión empleando
ácido cítrico como combustible, con el objeto de usarlo en sensores
de película gruesa para detectar CO (g). La concentración de Al
se varió entre 1 y 5 at % y se obtuvieron polvos nanocristalinos, con
tamaño promedio de cristalitas entre 8 y 12 nm y bajo grado de aglomeración.
El material semiconductor fue caracterizado por difracción de rayos X,
microsonda electrónica dispersiva en energías y microscopía
electrónica de transmisión. Los polvos nanocristalinos fueron
usados para preparar pastas conductoras con solventes orgánicos convenientes,
empleando formulaciones propias diferentes de aquéllas de los materiales
comerciales. Las pastas se depositaron como películas gruesas por serigrafía
sobre sustratos de alúmina de pureza electrónica, provistos con
electrodos de Pt. El sinterizado de las películas gruesas sensoras se
realizó en un rango medio de temperaturas para evitar un crecimiento
importante de las cristalitas ya que es conveniente mantener la estructura
nanocristalina para aumentar la sensibilidad del sensor. Los sensores fueron
caracterizados en mezclas de [CO (g)+aire] en un rango de temperatura entre
300 y 400 oC.
Los resultados indicaron que el aluminio se había incorporado a la red
del SnO2 en bajas proporciones quedando, además, muy bajas
concentraciones de Al2O3 segregada amorfa. Los sensores
mostraron una alta sensibilidad al gas a detectar. Además, se encontró
que la incorporación del Al al SnO2 permitió reducir
la temperatura de operación del sensor.
Introduction
Tin dioxide (SnO2) is one of the most studied metal oxides because
of its interesting properties and applications. Perhaps the most important of
them are: coatings as transparent conductors and the fabrication of sensors
for reducing gases like CO, H2 or CH4. The sensors response
is based on measurements of surface electrical resistance, in comparison with
the resistance value in air, in the presence of the gas to be detected. Sensor
response is defined as the ratio S = Rair/Rair+gas. This
variation is due to the reducing gas reaction with the O- (oxygen
adsorbates on the SnO2 surface, formed in air at the typical working
temperatures, 300-500 ºC, which reduce the potential barrier produced by
the adsorbates limiting the electrons movement).
Yamazoe et al. [1] proved that the sensors response increases significantly
if the crystallite size (D) of the nanocrystalline tin dioxide is about twice
the adsorption depth, 2L (L is the depth of the space-charge layer), of oxygen
adsorbates. This fact means that the sensor performance improves not only when
D decreases but also if L increases, since a major proportion of material takes
place in the reaction of oxygen adsorbates with the target gas even if the grain
size is not excessively small.
Besides, Yamazoe et al. have found that doping with trivalent cations enables
to increase the response since the carriers concentration is reduced and consequently,
L increases [1]. As an example of this phenomenon, they showed Al2O3
doped SnO2 sensors which were built for H2 detection but
they did not report data on CO(g) sensing. It is important to point out that
these authors have prepared sensors material by coprecipitation, followed by
calcination at high temperature. They could not evaluate the Al solubility in
the SnO2 lattice, in spite of reporting that it had to be lower than
1% as determined by resistivity measurements.
The aim of this work was to study the possibility of increasing the Al solubility
in the SnO2 lattice because this fact could improve the gas sensor
sensitivity since carriers concentration could decrease even more. Nanocrystalline
SnO2 was synthesised by the nitrate-citrate gel-combustion method
[2, 3] since the employment of similar routes in other systems made possible
to increase the solubility limit of the dopant, in a metastable condition, while
retaining the homogeneity in composition of materials [4, 5]. This is due to
the fact that the powder is obtained by the rapid disintegration of the homogeneous
gel, so the system cannot evolve towards its equilibrium state.
Even though Al can be introduced in SnO2 lattice in low proportions,
results of the present work show that the solubility limit of Al in SnO2
can be extend by the use of the gel-combustion method in comparison with coprecipitation.
Crystallite sizes were considerably small, between 8 and 12 nm. Sensors showed
a high sensitivity, between 8 and 10 for 200 ppm of CO in air. It has also been
found that the incorporation of Al to SnO2 enables to decrease the
working temperature.
Experimental
Procedure
1.Powders and thick films preparation
SnO2 with 0, 2 and 5 at % Al powders have been obtained by the
nitrate-citrate gel-combustion method [2,3]. The employed raw materials were
pure metallic Sn and Al, dissolved in diluted HNO3. The use of chlorides
of both metals has been avoided because Cl- ions, if retained, can
damage the sensors performance. Citric acid was added in a proportion of six
acid moles per metal mole and neutrality was attained with NH4OH.
The solution was then thermally concentrated at a temperature of 80 ºC
on a heating plate until a dark gel was obtained. Finally, at temperature above
250 ºC, the dried gel burned due to a vigorous exothermic reaction between
nitric and citric acids. The ashes produced by combustion were calcined for
1 hour at 700 ºC.
Pastes were prepared with the synthesised powders and organic solvents to get
thick films for sensors, painted on electronic purity Al2O3
substrates, provided with Pt contacts, dried at 100 ºC with a final sintering
for an hour at 700 ºC.
2.
Powders and thick films characterisation
Powders and films were studied by X-Ray diffraction (XRD) with a Philips PW
3710 diffractometer, employing Cu-Ka radiation to
identify phases and to determine the crystallites size. For this last evaluation
the Scherrer equation: D = 0.9 l / b
cos q was used, where D is the crystallite size,
l is the wave length of the incident radiation (1.5418
Å for Cu-Ka radiation), b
is the half maximum peak width and q is the peak
position. The instrument width, necessary to correct b,
was evaluated employing a Al2O3 with a mean crystallite
size of 25 mm.
Powders morphology was studied by Transmission Electron Microscopy (TEM) with
a Philips CM200 electron microscope. Chemical composition and Al distribution
were evaluated by energy dispersive electron microprobe (EDS) using a EDAX CM20
microprobe.
3.
Sensors characterisation
To evaluate the sensors performance, the electrical resistance in synthetic
air and in a diluted solution of 200 ppm of CO in synthetic air was measured
with a 617 Keithley electrometer between 300 and 400 ºC. In both measurements
the gas flow was 200 ml/min.
Results and Discussion
XRD patterns of the studied materials did not present any shift lattice parameters
with increasing doping concentration, suggesting that aluminium solubility in
SnO2 was low. In spite of that, Al2O3 peaks
were not observed.This fact was assigned to the low doping concentration but,
experiments with high Al concentrations suggested that it was more probable
that this phase had remained amorphous. The powders crystallite size resulted
between 8 and 12 nm, considerably smaller than that obtained for coprecipitated
material, Yamazoe et al. [1]. As an example, Figure 1 shows
the diffractogram corresponding to undoped SnO2 powder exhibiting
a crystallite mean size of 12 nm. It was also found that the crystallite size
was slightly reduced by an increasing of aluminium content. The thick films
exhibited similar crystallite size to that of powders because the calcination
temperature was relatively low.
Figure
1: Diffractogram corresponding to non-doped SnO2 powder.
Figure
2 : TEM micrograph of nanocrystalline 5 at% Al doped SnO2
powders (bright field).
TEM studies enabled to confirm the crystallite size as determined by XRD. For example, the Figure 2 shows a TEM bright field image of SnO2 – 5 at% Al powder with a crystallite size of 10 nm. Besides, it is important to point out that powders are formed by spherical particles, characteristic of powders obtained by gel-combustion method with a very low state of agglomeration. The EDS study of powder demonstrated that a part of the Al is not incorporated into the SnO2 lattice since small segregated Al-riched regions were observed as well.
Figure
3. Response (for 200ppm CO + air) of thick film pure SnO2
and of
doped - 2 and 5 at% Al doped SnO2 sensors.
To evaluate the Al solubility in SnO2 lattice, the
variation of the electrical resistance with the Al concentration was measured.
The resistance of undoped SnO2 films was about 107 W.
The resistance of 0.2 at% Al doped SnO2 films resulted within the
same magnitude order but ~3 times more resistive while the 5 at% Al doped SnO2
films exhibited a difference of more than one order of magnitude higher. This
continuos increase of resistance is consistent with the decrease of the carriers
concentration and with the increase of L (which is possible if Al is incorporated
into the SnO2 lattice). Yamazoe et al. [1] reported a higher resistance
variation for coprecipitated powders. These authors found a resistance change
three orders of magnitude higher for 1 at% Al doped SnO2 but they
did not observe an important resistance increase with higher doping concentration.
This fact was associated to Al2O3 segregation even for
very low Al concentrations, concluding that the Al solubility limit in SnO2
was lower than 1at%. The considerable difference in the behaviour of materials
according to their synthesis (coprecipitation or gel-combustion) suggests that
the gel-combustion method enables to increase significantly the Al solubility
in SnO2.
In Figure 3, the response of pure, 2 at% and 5 at% Al doped
SnO2 is plotted versus temperature (°C) for 200 ppm CO in air.
If pure SnO2 sensors were compared with 2 at% Al doped SnO2
sensors, it was observed that the latter exhibited a higher sensitivity for
the whole considered temperature range. The maximal sensitivity resulted: ~8
for non-doped SnO2 and 10 for 2 at% Al doped SnO2. The
optimal working temperature (Tw) resulted ~375 °C for both types
of sensors. However, 5 at% Al doped SnO2 sensors exhibited a lower
optimal working temperature: 325 ºC and their sensitivity resulted of
8. This decrease of Tw may be assigned to a shift of the formation temperature
characteristic of adsorbates O- by the incorporation of Al in the
SnO2 lattice. This phenomenon has been reported in the technical
literature for doping with trivalent elements like Bi [6].
Conclusions
Results indicate that the gel-combustion method enables to increase the Al solubility
in SnO2 in comparison with the coprecipitation method and, at the
same time, it reduces the crystallite size. Both facts improve the sensitivity
of the sensors built with the gel-combustion synthesised materials as it was
proved with the obtained thick films sensors. The response of the sensors reported
in this work was considerably high, between 8 and 10 for 200 ppm CO in synthetic
air. Furthermore, it is possible to reduce the temperature of operation of sensors
by increasing the Al concentration.
Acknowledgements
Authors are indebted to the Agencia Nacional de Promoción Científica
y Tecnológica by Grant PICT No. 8688 which enabled to carry out this
research.
References
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