Porous screen printed indium tin oxide (ITO) for NOx gas sensing

phys. stat. sol. (c) 4, No. 6, 1903–1907 (2007) / DOI 10.1002/pssc.200674315

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Porous screen printed indium tin oxide (ITO)

for NOx gas sensing

H. Mbarek, M. Saadoun, and B. Bessaïs*

Laboratoire de Photovoltaïque et des Semiconducteurs,

Centre de Recherches et des Technologies de l’Energie, BP 95, 2050 Hammam-Lif, Tunisia

Received 17 March 2006, revised 15 September 2006, accepted 15 November 2006

Published online 9 May 2007

PACS 07.07.df, 61.10.Nz, 61.43.Gt, 61.82.Rx, 68.37.Lp, 68.47.Fg

Tin-doped Indium Oxide (ITO) films were prepared by the screen printing method. Transparent and con-

ductive ITO thin films were obtained from an organometallic based paste fired in an Infrared furnace. The

Screen printed ITO films were found to be granular and porous. This specific morphology was found to

be suitable for sensing different gaseous species. This work investigates the possibility of using screen

printed (ITO) films as a specific material for efficient NOx gas sensing. It was found that screen printed

ITO is highly sensitive and stable towards NOx, especially for gas concentration higher than 50 ppm and

starting from a substrate working temperature of about 180 °C. The sensitivity of the ITO films increases

with increasing NOx concentration and temperature. The sensitivity and stability of the screen printed ITO

based sensors were studied within time. The ITO crystallite grain size dimension was found to be a key

parameter that influencesè the gas response characteristics. Maximum gas sensitivity and minimum res-

ponse time were observed for ITO films having lower crystallite size dimensions.

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

In recent years, there has been a growing interest in the reduction of air pollutant gases emitted during combustion processes in order to preserve the ecosystem. The increasing interest in pollution control, detection of hazardous gases and monitoring of combustion processes has encouraged the development of gas sensing devices. Solid state gas sensors based on metallic oxide thin films have become attractive owing to their simplicity and portability. A number of semiconductor oxides such as ZnO, SnO2, WO3

and ITO were used for different gas sensors [1–3]. Most of these sensors are based on resistance varia-tion as the semiconductor oxide films are exposed to target gases. Nitrogen oxide (NO, NO2, NOx) gases are known to cause dangerous environmental pollution, which can be reduced by photo-catalysis. Several works show that ITO thin films prepared by different deposition techniques [4–6] have high sensitivity and good long-term stability towards NO and NO2 gases. In this work we prepared ITO films by the simple and low cost screen printing method. We show that the morphology of the screen printed ITO films is highly suitable to develop stable and efficient NOx gas sensors.

2 Experimental

The ITO films were prepared by screen printing a viscous organometallic paste (ESL # 3050) of a dis-solved combination of metallic indium and tin onto glass substrates. To avoid the creation of flaking in the material, the samples are dried in air in an oven at a temperature of about 150 °C during 15 min. The

* Corresponding author: e-mail: brahim.bessais@inrst.rnrt.tn

1904 H. Mbarek et al.: Porous screen printed indium tin oxide for NOx gas sensing

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

ITO thin films are obtained after crystallization in an infrared furnace at a temperature ranging from 500 to 600 °C [7]. The firing time was varied from 15 to 70 min. In order to test the electrical behaviour of ITO films, the latter were introduced into a test chamber using a calibrated syringe having a minimum volume of 10 µl [8]. All measurements were performed in ambient atmosphere and at room temperature. In ambient atmosphere, the concentration of the test gas, in parts per million (ppm) can be estimated as:

6 6( ) 10 10g g

g

V VC ppm

V V V= ¥ @ ¥

+

, where g

V is the introduced gas volume and V the volume of the

test chamber. We varied the gas concentration from 50 ppm to 500 ppm. The sensitivity was defined as

air

gas

R

RS =

, wheregas

R and air

R are the electric resistances in presence of NOx and dry air, respectively.

The gas sensing device consists typically of a screen printed ITO film on a glass substrate, on which two Ag metallic electrodes were deposited by thermal evaporation.

3 Results and discussion

3.1 Microstructure of the screen printed ITO thin films

Figure 1 shows a surface TEM micrograph of a screen-printed ITO film, which was found to be granular and porous. In a previous work, ITO films fired at 600 °C for 40 min have a mean grain size dimension of about 120 Å and a porosity of about 35% [7].

Fig. 1 Surface TEM micrograph of a screen-printed ITO film. The firing temperature and time are respectively

600 °C and 40 min.

Figure 2 shows XRD patterns of ITO films for three different firing temperatures. One may notice (Fig. 2) that as the firing temperature increases, the XRD lines become progressively more intense and sharp.

30 40 50 60

0

50

100

150

200

250

300

350

(622)(440)

(400)

(222)

(b)

(c)

(a)

CPS

2θ (degrees) Fig. 2 X-ray diffraction patterns of screen-printed ITO films prepared at different firing temperatures; (a) 500 °C, (b)

550 °C and (c) 600 °C for 60 min.

50 nm

phys. stat. sol. (c) 4, No. 6 (2007) 1905

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In order to obtain good crystallinity, the firing temperature should be higher than 450 °C. XRD patterns confirm that an optimum crystallinity may be obtained starting from a firing time of 60 min. Figure 3 shows the variation of the sheet resistance (Rs) versus working temperature in ambient atmos-phere. One can notice the rather sheet resistance of the screen printed ITO films as regard to those pre-pared by conventional techniques. The small grain size of the screen- printed ITO associated with its granular and porous nature (Fig. 1) may be responsible for this rather high sheet resistance. The variation of the sheet resistance versus annealing (working) temperature let us point out three zones (Fig. 3). In the first zone (25 °C–100 °C), the sheet resistance decreases slowly. In the second zone (100 °C–200 °C), a fast decrease of the sheet resistance was observed.

50 100 150 200 250 300 350

0

5

10

15

20

25

Shee

t R

esis

tance

(KΩ

)

Temperature (°C)

Fig. 3 Sheet resistance of the screen printed ITO films measured at various temperatures in ambient air.

In the third zone (200 °C–330 °C), a stabilization of the sheet resistance was obtained. When the tem-perature increases from the ambient to about 100 °C, we observe a slight decrease of the sheet resistance, which could be due to oxygen and water vapour desorption from grain surface (physisorbed species) and pores (quasi-free species). At this stage a non equilibrium between subsurfacic and bulk oxygen occurs leading to a segregation of bulk oxygen to the surface, thus producing further oxygen vacancies [9]. This may explain the fast variation of the resistivity in the 100 °C–200 °C temperature range. Above this tem-perature range, a quasi stabilisation of the resistivity was observed. This behaviour could be due to the low segregation of the deep unstable bulk oxygen and then to rather long delay in oxygen desorption [9].

3.2 Screen printed ITO based gas sensors

The mean grain size dimension is an important factor as regard to the sensitivity of the designed gas sensor. Thus it would be essential to estimate the mean grain size as a function of firing temperature. In Table 1, we report the variation of the mean grain size as a function of firing temperature. Table 1 Dependence on firing temperature of the crystallite grain size dimensions of the screen-printed ITO films.

The grain size dimensions were calculated from for the major (222) XRD (Fig. 2)

line of the ITO films using the Scherrer’s formulae.

600 580 550 500 Temperature (°C)

15.76 14.80 14.04 12.15 Grain size (nm)

1906 H. Mbarek et al.: Porous screen printed indium tin oxide for NOx gas sensing

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

It is well known that the decrease of the crystallite sizes increases the specific surface area of the porous films, then the solid-gas interactions giving rise to significant sensitivities. Figure 4 shows the variation of the sensitivity for various firing temperatures. From Fig. 4, it is clear that higher sensitivity is obtained for 500 °C, which is the lower firing temperature leading to crystallised ITO films (Fig. 2).

500 520 540 560 580 600

6

8

10

12

14

16

Sen

siti

vit

y (

Rg/

Rair)

Firing temperature (°C)

Fig. 4 Variation of the sensitivity of the ITO films versus firing temperature.

In the following all screen printed ITO sensing films were fired at 500 °C. The electrical conductivity of the ITO films varies when a change in the ambient occurs. We found that this property is based on gas detection. Figure 5 shows transient responses of the ITO film-based sensor while varying NOx concentra-tions at different temperatures. The sensitivity of the ITO film increases as the operating temperature rises. However, for all prospected temperatures the sensitivity almost saturates at near 1000 ppm. The NOx is known to be an oxidising gas, as a result the resistance of the ITO film-based sensor increases when NOx vapours enter into contact with the ITO porous surface. The mechanisms normally accepted for semiconductor-based sensors assume that adsorbed oxygen on the oxide surface removes some of the gap electronic density and thus decreases the materials conductivity.

0 200 400 600 800 1000 1200

0

2

4

6

8

10

12

14

16

T=30 °C

T=130 °C

T=210 °C

T=60 °C

T=180 °C

Sen

siti

vit

y (

Rg/

R0)

Concentration of NO (ppm)

Fig. 5 Dependence of ITO film sensitivity on NO

x concentration for various working temperatures.

In fact, adsorption of NOx on this type of materials is rather complex and may lead to various surface species including nitrites/nitrates and possibly NO-species. If there are no stable adsorption sites for NO in the screen-printed ITO thin film, some NO related molecules may decompose and desorbs from the sensor, which in turn inhibits NOx adsorption. Therefore, higher working temperature can accelerate the decomposition and desorption of nitrites or related compounds. By adjusting the working temperature,

phys. stat. sol. (c) 4, No. 6 (2007) 1907

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the ITO gas sensor sensitivity may be reproducible at different NO concentrations. Figure 6 shows re-petitive sensitivity measurement of the screen-printed ITO film.

0 10 20 30 40

1

2

3

4

5

6

7

After 10 excitations

T = 130°C

Sen

siti

vit

y (

Rg/

R0)

Time (min)

(a)

0 5 10 15 20 25

1

2

3

4

5

6

7T=180°C

After 10 excitations

Sen

siti

vit

y (

Rg/

R0)

Time (min)

(b)

0 5 10 15 20 25 30

0

2

4

6

8

10

12T = 210 °C

After 10 excitations

Sen

siti

vit

y (

Rg/

R0)

Time (min)

(c)

Fig. 6 Repetitive sensitivity measurements of a screen-printed ITO based NOx gas sensor at various temperatures: a)

130 °C, (b) 180 °C, (c) 210 °C. The NO concentration is 500 ppm.

The NO

x concentration was fixed at 500 ppm and 10 repetitive measurements were performed at 130 °C,

180 °C and 210 °C. In general, the sensitivity of the sensor increases by mixing NOx gas with pure air,

which almost instantaneously decreases to its initial value (in pure air). At 130 °C, a memory effect was

detected. However, at 180 °C and 210 °C, a reproducible sensitivity was pointed out, even after 10 exci-

tations; no instability nor memory effects were observed. Thus, 180°C is an appropriate low working

temperature to sense NOx by porous screen-printed ITO thin films. These results seems to be efficient as

compared to those obtained for ITO films prepared by sol-gel, where higher working temperatures and

long desorbing times were needed [10, 11].

4 Conclusions

Screen-printed ITO thin films were prepared and tested for NOx gas sensing. Highest sensitivity, stability

and reproducibility were observed at approximately 180 °C for NOx concentration of about 500 ppm.

The intrinsic porosity of the screen-printed ITO films certainly plays an important role in their gas sens-

ing properties. It is obvious that further investigations are needed to understand the various kinetics that

govern the adsorption and desorption phenomena of NOx on the porous surface of the ITO films. Lower

NOx concentration will be tested in the future.

Acknowledgements This work was supported by the Ministry of Scientific Research, Technology and Competency

Development.

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