Screen-Printed Tin-Doped Indium Oxide Films for Low
Temperature and Fast Response Methanol Gas Sensing
H. Mbarek, M. Saadoun, and B. Bessaïs∗
Nanomaterials and Energy Systems Laboratory, Research and Technology Centre of Energy,
Borj-Cedria Science and Technology Park, BP. 95, 2050 Hammam-Lif, Tunisia
(Received: 23 May 2007.Accepted: 17 August 2007)
In this work, an ITO based solid state gas sensor was developed using the screen printing method.
This technique consists of spreading out a viscous organometallic based paste (a dissolved
combination of metallic indium and tin) onto glass substrates. The screen printed paste, which
is~30μm thick, is submitted to a sintering procedure in an infrared furnace at temperatures ranging
between500°C and 600°C. The sintering step leads to the formation of transparent and conductive
ITO films, having a thickness in the range of 0.3–0.5μm. The screen printed ITO films were found to
be granular and porous. The screen printing method was found to be appropriate to form porous
films with rather high specific surface areas, features highly suitable to prepare gas sensing films.
The screen printed ITO films were found to be composed of nanocrystallites having small size
dimensions in the range of 120–150°C. Due to its porous structure and small grain size, screen
printed ITO films easily interact with oxygen and other gases. Depending on working temperature,
screen printed ITO films may be used for sensing both oxidizing and reducing gases. At rather low
working temperature (100°C–150°C), screen printed ITO films may be used for sensing methanol
(CH3OH), a highly toxic alcohol. We found that for various working temperatures, the ITO films
have almost the same response profile shapes versus methanol concentration. The highest ITO films
response (defined as the ratio of the resistance measured in ambient air on that measured in presence
of methanol) was pointed out at a working temperature around 130°C; at this temperature the
response reaches saturation at 100ppm of methanol gas. At a working temperature of 40°C, 10ppm
of methanol decreases the resistance of the ITO films by more than 3 times. At a working
temperature of 130°C and in presence of 100ppm of methanol, the resistance of the ITO films is 8
times smaller than that in ambient air.
Keywords: ITO, Methanol, Sensors, Screen Printing.
1. INTRODUCTION
Detection of hazardous gases and monitoring of combustion processes has encouraged the development of a
wide range of gas sensing devices. The number of volatile
organic solvents (VOCs) that may pose significant threats
to human health is very large.1The detection of such compounds is becoming an obligation in a wide range of
industry. Among VOCs, some alcohols are considered as
severe pollutants (i.e., methanol, butanol, acetone etc.).2 A
highly sensitive and selective alcohol sensors have
always been in great demand in biomedical, chemical,
drug and food industries and also for development of an
electronic nose.3In this framework, the development of
selective alcohol sensors having higher response that can

* Corresponding author; E-mail: brahim.bessais@crten.rnrt.tn
operate at low temperatures had a growing interest.4-5 In
order to facilitate toxic gaseous alcohol sensing, solidstate gas sensors must be used. Most of these sensors are
based on resistance variation as the semiconductor oxide
films are exposed to target gases. Methanol (CH3OH) is
known to be a very useful organic solvent with wide
spread applications in automotive fuel and several
manufactories. Most of the oxide semiconductor gas
sensors normally operate at elevated temperature. Moreover, reactive gas and vapors having low boiling point
require care to prevent the reaction sand fire in the
presence of the heater. The reduction of operating
temperature is one of the demanding factors of viable gas
sensor for industrial applications. Several works show
that ITO thin films prepared by different deposition
techniques6–8 have high response and good long-term
stability towards reducing gases, and then may be
designed as good gas sensors against methanol. The
present study is carried out to demonstrate that screen
printed ITO films may be used for sensing rather low
methanol concentration at low temperatures.
2. EXPERIMENTAL DETAILS
The ITO films were prepared by screen printing a viscous
organometallic paste (ESL # 3050) of a dissolved combination of metallic indium and tin onto glass substrates.
The samples are dried in air in an oven at a temperature
of about 150°C during 15min, in order to remove a bit of
solvent and to prepare the samples to the sintering
process, where cracking may occur. The ITO thin films
were crystallized in an infrared furnace at a temperature
ranging from 500 to 600°C.9 The firing time was varied
from 15 to 70min. In order to test the electrical behavior
of ITO films, the latter were introduced into a test
chamber using a calibrated syringe having a minimum
volume of 10μl.10All measurements were performed in
ambient atmosphere and at room temperature. We varied
the methanol gas concentration from 10ppm to 100ppm.
The response was defined as R0/Rg, where R0 is the
resistance in ambient dried air and Rg the resistance in
presence of methanol gas. The gas sensing device is very
simple; it consists of a screen printed ITO film deposited
on a Pyrex glass substrate; two Ag metallic contacts were
thermally evaporated onto the films to ensure electric
resistance measurements.
Figure 1 shows the variation of the resistance of the
ITO film versus working temperature in ambient atomsphere. The observed behavior (Fig.1) was explained
elsewhere.10 It is worth noting two working temperature
zones; at low temperature (<130°C) the resistance is
rather high (>10kΩ), while for higher temperatures
Fig.1. Resistance of the screen printed ITO films measured
at various temperatures in ambient air.
(>200°C), he resistance decreases and stabilizes around
2kΩ. It is well known that when reducing gases are
adsorbed on the surface of n-type semiconductors (such
as ITO, SnO2, ZnO etc.) they react with previously
chemisorbed oxygen, inducing electron transfer to the
conduction band of the sensing material leading to a
decrease of the resistivity (i.e., resistance) of the base
material. Now, knowing that methanol (CH3OH) is a
reducing gas, it would be preferable to operate in a
temperature range where the resistance is high (i.e.,
<130°C), in order to get larger resistance variation sand
then higher responses. In the contrary, operating at
oxidizing ambient leads to a resistance increase; in that
case, and following Figure 1, higher responses are
obtained at high operating temperatures (>200°C).
As the response is measured in terms of resistance
variation, it is clear that for screen printed ITO, the low
working temperature zone is suitable for sensing reducing
gas (such as methanol), while higher temperature zone is
appropriate for sensing oxidizing gases (such as NO).All
the features make screen printing a simple and low cost
technology to prepare ITO films for gas sensing (both
reducing and oxidizing gases).
3. RESULTS AND DISCUSSION
Figure 2 shows a cross-section TEM view of a screen
printed ITO film grown on a silicon substrate; dark zones
represent ITO crystallites, while clear zones correspond
to pores. The growth of the SiO2 layer at the interface
ITO/Silicon has been discussed elsewhere.9The screen
printed ITO film was found to be granular and porous.
Ina previous work, ITO films fired at 600°C for 30min
Fig.2. TEM cross-section view showing screen printed ITO
film on silicon. The sintering temperature and time are 600 °C
and 30 min, respectively.
have a mean grain size dimension of about 120Å and a
porosity of about 35%.9 In a previous work,11 we showed
that screen printed ITO films having good crystallinity
should be sintered at temperatures higher than 450°C for
60min. In porous materials based sensors, it is well
established that the decrease of the crystallite size
dimensions increases the specific surface area, which is
an important factor as regard to the solid–gas interactions
giving rise to significant sensitivities.
The mean crystallite size dimension varies from 120 Å
to more than 150Å, for sintering temperatures ranging
from 500 to 600°C, respectively.10 In order to get higher
sensitivities against methanol gas, we chosen preparing
crystalline ITO films having the lowest grain size
dimension (i.e., 120 Å, the sintering temperature is
500°C).Therefore, in the following, all investigations will
be made on ITO films sintered at 500°C.
Figure 3 depicts the variation of the response of the
ITO based sensor versus working temperature for two
different methanol gas concentrations (10ppm and
50ppm). It is interesting to note that the shape of the
response variation versus temperature is independent of
gas concentration; a maximum response was obtained at a
working temperature of about 130°C, independent of
methanol gas concentration. In a previous work10,
focusing on sensing NH3 (another reducing gas), screen
printed ITO presents a maximum response at a working
temperature of about 120°C, also independent of gas
concentration. This seems to indicate that there could be
similar reaction pathways while sensing methanol and
ammonia. However, it is more realistic to say that the best
operating temperature is the result of the combination of
different factors, which in the case of methanol and
ammonia accidentally may lead to such similarities. At
this point we can say that screen printed ITO has a
maximum response for reducing gases at a working
temperature around 120–130°C.
Fig.3. Variation of the response of screen printed ITO films
versus working temperature for two methanol gas
concentration (a) 10ppm, (b) 50ppm.
Fig.4. Evolution of the response of screen printed ITO based
sensor with methanol concentration.
Figure 4 shows the variation of the response versus
methanol concentration at a working temperature of
130°C (which corresponds to the maximum response at
any reducing gas concentration).
First, it is interesting to note that screen printed ITO is
sensitive at rather low methanol concentration (<20ppm).
The response increases (i.e., Rg decreases) linearly versus
methanol gas concentration and reaches a step at 100ppm
(Fig.4). Therefore the sensor saturates at rather low
methanol gas concentration. At the same working
temperature (130°C), screen printed ITO is less sensitive
toNH3 (Ref. [10]) than to methanol; in the case of NH3
gas, the response reaches saturation at rather high gas
concentration (~1000ppm).10 This disparity may be due to
the difference in the oxidation process of the adsorbed
chemical species, leading to injection of electrons into the
conduction band of the material. The reason for a
decrease in the resistance of ITO may be due to oxidation
of the methanol vapor upon coming into contact with the
ITO surface, which liberates free electrons and H2O.The
atmospheric oxygen chemisorbs on the surface of the
oxide semiconductor as O2− or O−, removing an electron
from the conduction band of the n-type semiconductor,
developing a depletion region on the surface. Methanol
vapors react with the chemisorbed oxygen and re-inject
the carrier, thereby reducing the resistance of the material.
The oxidation of methanol takes place via two routes, one
the dehydrogenate onto formaldehyde as shown in Eq.(1)
and the other the formation of formic acid as shown in
Eq.(2).12
Methanol vapor reacts with chemisorbed oxygen,
re-injects the carriers and increases the conductance of
oxide. Two possibilities of reactions are:
CH3OH  O  (Ads.)  HCOH  H 2 O  e 
(1)
or

CH3OH  O2 (Ads.)  HCOOH  H2O  e
(2)
methanol concentration may be. Good response, stability
and reproducibility were observed at 130°C for methanol
concentration of about 50ppm. The intrinsic properties of
screen-printed ITO films certainly play an important role
regarding the best conditions of methanol sensing. It is
obvious that further investigations are needed to
understand the various kinetics that govern adsorption
and desorption phenomena of methanol on the porous
surface of screen printed ITO.
References and Notes
1.
2.
Fig.5. Repetitive response measurements of screen-printed ITO
for methanol vapors sensing. Measurements were done at
130°C, for a methanol concentration of 50ppm.
Hence, the product of this reaction may be
formaldehyde and H2O in one case and formic acid and
H2O in the other case.
Figure 5 shows time evolution of the response of
screen printed ITO based sensor under various methanol
gas excitations at a working temperature of 130°C. It is
worth noting the fast response of the sensor and its good
reproducibility towards methanol vapors. No memory
effects were observed at 130°C.
4. CONCLUSION
Screen-printed ITO thin films were prepared and tested
for methanol vapor sensing. It was found that screen
printed ITO is very sensitive to methanol vapors. A sensor
working temperature of 130°C was pointed out as the
temperature giving the highest response, whatever
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