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Characterization of tin dioxide film for chemical vapors sensor
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Materials Science and Engineering C 28 (2008) 584 – 587 www.elsevier.com/locate/msec
Characterization of tin dioxide film for chemical vapors sensor I. Hafaiedh a,⁎, S. Helali a , K. Cherif a , A. Abdelghani a , G. Tournier b a
Unité de Recherche de Physique des Semi-conducteurs et Capteurs, IPEST, 2070 La Marsa, Tunisia b Ecole des Mines de Saint-Étienne, 158 cours Fauriel, 42023 Saint-Étienne, France Available online 18 October 2007
Abstract Recently, oxide semiconductor material used as transducer has been the central topic of many studies for gas sensor. In this paper we investigated the characteristic of a thick film of tin dioxide (SnO2) film for chemical vapor sensor. It has been prepared by screen-printing technology and deposited on alumina substrate provided with two gold electrodes. The morphology, the molecular composition and the electrical properties of this material have been characterized respectively by Atomic Force Spectroscopy (AFM), Fourier Transformed Infrared Spectroscopy (FTIR) and Impedance Spectroscopy (IS). The electrical properties showed a resistive behaviour of this material less than 300 °C which is the operating temperature of the sensor. The developed sensor can identify the nature of the detected gas, oxidizing or reducing. © 2007 Elsevier B.V. All rights reserved. Keywords: AFM; FTIR; Impedance Spectroscopy; Tin dioxide; Chemical vapors detection
1. Introduction
2. Experiments
Metal oxide semiconductor sensors based on electric conductivity measurement have been used extensively for gas detection. The sensing properties of the material are based on the adsorption of gas molecules on it surface which produce changes in their conductivity [1]. Recently, different materials such as WO3 and TiO2 have been used as electrochrome material for industrial applications. They offer many advantages over current technologies for detecting gases, such a low cost, long lifetime, high selectivity and sensitivity [2,3]. In this work, a thick film of tin dioxide (SnO2) prepared by serigraphy method has been charascterized by various techniques. Fourier Transformed Infrared Spectroscopy (FTIR) was used to identify the specific molecular components of the film. Atomic Force Microscopy (AFM) was used to investigate the surface morphology of the film. Finally impedance spectroscopy was used to study the electrical properties of the thick film under vapor.
2.1. Samples preparation
⁎ Corresponding author. Tel.: +216 71 74 18 36; fax: +216 7174 65 51. E-mail address: [email protected] (I. Hafaiedh). 0928-4931/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2007.10.064
Sintered sensing elements exhibit good stability and sensitivity but this kind of process is hardly transferable to a mass production. Thin film materials usually lead to weaker sensitive and stable devices. Hence, screen-printing technology constitutes a good compromise to obtain material with sensing properties similar to sintered ones, and to transfer the process towards industrial production. Samples have been prepared in the MICC Department of the SPIN Center of the Mining School of Saisnt-Etienne. In this study, a semi-automatic Aurel C890 machine was used, the procedure for preparing material (ink composition, annealing temperature) has been described elsewhere [4]. The ink is generally composed of the active material (SnO2) as well as of temporary and permanent binders in order to control the rheological properties for the deposition of the resulting material onto the substrate and its adhesion to it [5,6]. In this case, the SnO2 powder (Prolabo Company) is first mixed with a solvent and an organic vehicle without any mineral binder. An oxide film (4.4 ⁎ 2.2 mm2) with a thickness of 20 μm is then deposited by using a 180-mesh mask on an alpha-alumina substrate (38 ⁎ 5 ⁎ 0.4 mm3) provided with two gold electrodes
I. Hafaiedh et al. / Materials Science and Engineering C 28 (2008) 584–587
585
Fig. 1. Thick-film sensor.
(thickness 1 μm) deposited by reactive sputtering. The SnO2 material is finally annealed for 12 h at 700 °C in air. An example of this type of sensor is shown in Fig. 1. 2.2. Atomic Force Microscopy (AFM) The surface morphology of the tin dioxide film has been investigated by Atomic Force Microscopy (AFM) technique with a Nanoscope IV (Bourj Cedria, Tunisia) equipped with a contact mode and using silicon nitride tips. 2.3. Fourier Transformed Infrared Spectroscopy (FTIR) The specific molecular components and structures of this sample have been identified by Fourier Transform Infrared Spectroscopy (FTIR). The spectrometer used is a Brucker IFS66 V/S spectrometer equipped with MIR (Middle infrared) source, DTGS detector and KBr separating mirror. The infra-red
Fig. 3. FTIR spectrum of SnO2 thick film.
spectroscopy is obtained in reflexion mode with a resolution of 0.1 cm− 1 and is collected with 32 scans for the reference and the sample. 2.4. Impedance spectroscopy Impedance spectroscopy (EIS) has emerged as a useful analytical tool for the development of sensor devices in a wide variety of configurations [7–11]. In this study we used the impedance spectroscopy as a tool to characterize the electrical properties of the thick film of SnO2 and to study the mechanism of gas oxide-semiconductor interaction. The HP 4192A impedance Analyzer was used to obtain impedance spectra. It provided the applied sinewave voltage signals. A perturbation amplitude of ± 50 mV (peak to peak) was chosen and the frequency range was varied in the range of
Fig. 2. AFM images (2 × 2 μm2) showing the surface morphology of SnO2 thick film before annealing.
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I. Hafaiedh et al. / Materials Science and Engineering C 28 (2008) 584–587
Fig. 4. The impedance spectrum taken between 5 Hz and 1 × 107 Hz, at 300 °C.
5 Hz to 1 × 107 Hz. The impedance of SnO2 film was derived from the measurements of Bode's diagrams.
Fig. 6. Resistance variation of the tin dioxide film versus temperature under air, oxygen and ethanol.
associated to the SnO family which shows the quality and the purity of the film.
3. Results and discussions 3.3. Electrical properties 3.1. Morphological characterization
3.2. Molecular characterization
3.3.1. Electric model Fig. 4 shows the impedance spectrum (bode diagram) of tin dioxide film under air in the frequency range between 5 Hz and 1 × 107 Hz. We observe a resistive behaviour of the material at low and intermediate frequency. A capacitive behaviour can be observed beyond 1 MHz. This capacity can be neglected (0.06 nF) which can be caused by the heating. The tin dioxide films can be represented by a simple resistance.
Fig. 3 presents the FTIR spectrum of SnO2 thick film between 400 and 4000 cm − 1 . The spectrum shows an absorption peak around 600–615 cm − 1 , 1100 cm − 1 , 800 cm− 1 and 450 cm− 1 that is assigned to the fundamental vibration of Sn–O (Sn–O–Sn). The absorption band situated towards 550 cm− 1 is likely associated to Sn–OH vibration. The molecular vibrations of the chemical bonds detected are
3.3.2. Temperature effect Fig. 5 shows the resistance variation of the thick tin dioxide film versus temperature (from 150 to 400 °C). We observe an increase in the resistance up 350 °C, then a decrease. This particular behaviour is associated to a transformation phase of the SnO2 thick film that can be associated to the structural modification of the material. A similar phenomenon has been
Fig. 5. Resistance variation of tin dioxide film under air versus temperature.
Fig 7. The sensitivity of the sensor as function of ethanol concentration at different operating temperature.
Fig. 2 present the AFM photos (2 × 2 μm2) of the SnO2 film after annealing. The film appears with crystalline grains shape relatively dense and homogeneous on a micrometer scale. The size of these grains showed by AFM photo argues the thickness of the material layer.
I. Hafaiedh et al. / Materials Science and Engineering C 28 (2008) 584–587
587
3.3.4. Calibration curve The sensitivity of tin dioxide to chemical vapor is generally defined as (Ra − Rg) / Ra, where Ra and Rg are the resistance of the film under air and vapor, respectively. Fig. 7 presents the sensitivity of the sensor versus various concentrations of ethanol, at various temperatures ranging from 300 to 400 °C. We notice that the material is more sensitive for high ethanol concentration. The increase of ethanol concentrations causes an increase in sensor sensitivity. The higher sensitivity of the sensor was obtained at 300 °C. Fig. 8 shows the resistance of the tin dioxide layer under different ethanol concentrations. This curve shows that the conductivity of the material is more interesting at 300 °C which is due to a change in the transformation phase. Fig. 8. The resistance variation of the tin dioxide film versus ethanol concentration at different temperature.
reported for WO3 thin film [12]. Furthermore, the decreasing of the resistance beyond 350 °C is associated to a semiconductorlike behaviour, according to the following Eq. (1): R ¼ R0 exp
EG kT
ð1Þ
where R0 represent the resistance of the material at room temperature, K is the Boltzmann constant; T is the temperature and EG represent the activation energy. 3.3.3. Detection The resistance variation of the tin oxide layer under air, oxygen and ethanol (C2H5OH) versus temperature is reported in Fig. 6. Firstly, we observed an increase in the resistance under oxygen compared to air. This phenomenon is explained by the oxidizing character of oxygen. The oxygen molecules adsorbed on the semiconductor surface, remove electrons from the conduction band which include an increase in the resistance of SnO2 [13] (see Eq. (2)): 1=2 O2 þ e− þ s→O− −s
ð2Þ
where s is an adsorption site on the SnO2 surface and erepresents an electron from the conduction band. This result enables us to notice that temperature increases when oxygen ions adsorbed easily on SnO2 surface. This induce gradually an increase in the resistance. Beyond 300 °C, oxygen starts to be desorbed, which induces a decrease in the resistance. The introduction of ethanol modifies mainly the resistance of tin oxide for higher temperature. The presence of a reducing gas (ethanol) decreases the resistance of the material. This phenomenon is explained by the reducing character of ethanol reacting with oxygen, release electrons which became free to conduct according to Eq. (3): 2C2 H5 OH þ O−2 ðChemÞ→2CH3 CHO þ H2 O þ e
ð3Þ
According to the nature of gas, oxidizing or reducing, conductivity of the material can be changed. It increases when the gas is oxidizing, and decreases when is reducing.
4. Conclusion The characterization of tin dioxide thick film, prepared by serigraphy method, was studied with various techniques, such as AFM, FTIR and impedance spectroscopy. The results shows that the surface of the tin dioxide appears in the shape of the crystalline grains relatively dense and homogeneous on a micrometer scale. The obtained FTIR spectra show that the material does not present any impurities. Impedance spectroscopy is a powerful technique to study the chemical vapors tin dioxide interaction. The sensor can be used with high reproductibility to identify the nature of the detected gas as an oxidizing or reducing. For future work, sol-gel membrane can be deposited on the tin dioxide film to perform the selectivity. References [1] E. Cominia, M. Ferronib, V. Guidib, G. Fagliaa, G. Martinellib, G. Sberveglieria, Sensors and Actuators B 84 (2002) 26. [2] H. Meixner, U. Lampe, Sensors and Actuators B 33 (1996) 198. [3] L. Gajdosık, Sensors and Actuators B 106 (2005) 691. [4] B. Rivière, J.P. Viricelle, C. Pijolat, Sensors and Actuators B 93 (2003) 531. [5] L. Montanaro, A. Tersalvi, Journal of Electroceramics 5 (3) (2000) 253. [6] N. Guillet, R. Lalauze, J.P. Viricelle, C. Pijolat, L. Montanaro, Materials Sciences and Engineering C 21 (2002) 97. [7] N. Bukuna, A. Vinokurov, M. Vinokurova, L. Derlyukova, Yu. Dobrovolsky, A. Levchenko, Sensors and Actuators B 106 (2005) 153. [8] T.G. Maffeïes, G.T. Owen, M.W. Penny, T.K.H. Starke, S.A. Clark, H. Ferkel, S.P. Wilks, Surface Science 520 (2002) 29. [9] J.R. MacDonald, Impedance Spectroscopy, Wiley, New York, 1987. [10] D. Macdonald, Electrochemical Acta 51 (2006) 1376. [11] C.F. Sanchez, C.J. McNeil, K. Rawson, Trends in Analytical Chemistry 24 (1) (2005). [12] A. Laabidi, C. Jacolin, M. Bendahan, A. Abdelghani, J. Guerin, K. Aguira, M. Maaref, Sensors and Actuators B 106 (2005) 713. [13] P. Montmeat, C. Pijolat, B. Riviere, G. Tournier, J.P. Viricelle, Materials Science and Engineering C 21 (1-2) (2002) 113.
Materials Science and Engineering C 28 (2008) 584 – 587 www.elsevier.com/locate/msec
Characterization of tin dioxide film for chemical vapors sensor I. Hafaiedh a,⁎, S. Helali a , K. Cherif a , A. Abdelghani a , G. Tournier b a
Unité de Recherche de Physique des Semi-conducteurs et Capteurs, IPEST, 2070 La Marsa, Tunisia b Ecole des Mines de Saint-Étienne, 158 cours Fauriel, 42023 Saint-Étienne, France Available online 18 October 2007
Abstract Recently, oxide semiconductor material used as transducer has been the central topic of many studies for gas sensor. In this paper we investigated the characteristic of a thick film of tin dioxide (SnO2) film for chemical vapor sensor. It has been prepared by screen-printing technology and deposited on alumina substrate provided with two gold electrodes. The morphology, the molecular composition and the electrical properties of this material have been characterized respectively by Atomic Force Spectroscopy (AFM), Fourier Transformed Infrared Spectroscopy (FTIR) and Impedance Spectroscopy (IS). The electrical properties showed a resistive behaviour of this material less than 300 °C which is the operating temperature of the sensor. The developed sensor can identify the nature of the detected gas, oxidizing or reducing. © 2007 Elsevier B.V. All rights reserved. Keywords: AFM; FTIR; Impedance Spectroscopy; Tin dioxide; Chemical vapors detection
1. Introduction
2. Experiments
Metal oxide semiconductor sensors based on electric conductivity measurement have been used extensively for gas detection. The sensing properties of the material are based on the adsorption of gas molecules on it surface which produce changes in their conductivity [1]. Recently, different materials such as WO3 and TiO2 have been used as electrochrome material for industrial applications. They offer many advantages over current technologies for detecting gases, such a low cost, long lifetime, high selectivity and sensitivity [2,3]. In this work, a thick film of tin dioxide (SnO2) prepared by serigraphy method has been charascterized by various techniques. Fourier Transformed Infrared Spectroscopy (FTIR) was used to identify the specific molecular components of the film. Atomic Force Microscopy (AFM) was used to investigate the surface morphology of the film. Finally impedance spectroscopy was used to study the electrical properties of the thick film under vapor.
2.1. Samples preparation
⁎ Corresponding author. Tel.: +216 71 74 18 36; fax: +216 7174 65 51. E-mail address: [email protected] (I. Hafaiedh). 0928-4931/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2007.10.064
Sintered sensing elements exhibit good stability and sensitivity but this kind of process is hardly transferable to a mass production. Thin film materials usually lead to weaker sensitive and stable devices. Hence, screen-printing technology constitutes a good compromise to obtain material with sensing properties similar to sintered ones, and to transfer the process towards industrial production. Samples have been prepared in the MICC Department of the SPIN Center of the Mining School of Saisnt-Etienne. In this study, a semi-automatic Aurel C890 machine was used, the procedure for preparing material (ink composition, annealing temperature) has been described elsewhere [4]. The ink is generally composed of the active material (SnO2) as well as of temporary and permanent binders in order to control the rheological properties for the deposition of the resulting material onto the substrate and its adhesion to it [5,6]. In this case, the SnO2 powder (Prolabo Company) is first mixed with a solvent and an organic vehicle without any mineral binder. An oxide film (4.4 ⁎ 2.2 mm2) with a thickness of 20 μm is then deposited by using a 180-mesh mask on an alpha-alumina substrate (38 ⁎ 5 ⁎ 0.4 mm3) provided with two gold electrodes
I. Hafaiedh et al. / Materials Science and Engineering C 28 (2008) 584–587
585
Fig. 1. Thick-film sensor.
(thickness 1 μm) deposited by reactive sputtering. The SnO2 material is finally annealed for 12 h at 700 °C in air. An example of this type of sensor is shown in Fig. 1. 2.2. Atomic Force Microscopy (AFM) The surface morphology of the tin dioxide film has been investigated by Atomic Force Microscopy (AFM) technique with a Nanoscope IV (Bourj Cedria, Tunisia) equipped with a contact mode and using silicon nitride tips. 2.3. Fourier Transformed Infrared Spectroscopy (FTIR) The specific molecular components and structures of this sample have been identified by Fourier Transform Infrared Spectroscopy (FTIR). The spectrometer used is a Brucker IFS66 V/S spectrometer equipped with MIR (Middle infrared) source, DTGS detector and KBr separating mirror. The infra-red
Fig. 3. FTIR spectrum of SnO2 thick film.
spectroscopy is obtained in reflexion mode with a resolution of 0.1 cm− 1 and is collected with 32 scans for the reference and the sample. 2.4. Impedance spectroscopy Impedance spectroscopy (EIS) has emerged as a useful analytical tool for the development of sensor devices in a wide variety of configurations [7–11]. In this study we used the impedance spectroscopy as a tool to characterize the electrical properties of the thick film of SnO2 and to study the mechanism of gas oxide-semiconductor interaction. The HP 4192A impedance Analyzer was used to obtain impedance spectra. It provided the applied sinewave voltage signals. A perturbation amplitude of ± 50 mV (peak to peak) was chosen and the frequency range was varied in the range of
Fig. 2. AFM images (2 × 2 μm2) showing the surface morphology of SnO2 thick film before annealing.
586
I. Hafaiedh et al. / Materials Science and Engineering C 28 (2008) 584–587
Fig. 4. The impedance spectrum taken between 5 Hz and 1 × 107 Hz, at 300 °C.
5 Hz to 1 × 107 Hz. The impedance of SnO2 film was derived from the measurements of Bode's diagrams.
Fig. 6. Resistance variation of the tin dioxide film versus temperature under air, oxygen and ethanol.
associated to the SnO family which shows the quality and the purity of the film.
3. Results and discussions 3.3. Electrical properties 3.1. Morphological characterization
3.2. Molecular characterization
3.3.1. Electric model Fig. 4 shows the impedance spectrum (bode diagram) of tin dioxide film under air in the frequency range between 5 Hz and 1 × 107 Hz. We observe a resistive behaviour of the material at low and intermediate frequency. A capacitive behaviour can be observed beyond 1 MHz. This capacity can be neglected (0.06 nF) which can be caused by the heating. The tin dioxide films can be represented by a simple resistance.
Fig. 3 presents the FTIR spectrum of SnO2 thick film between 400 and 4000 cm − 1 . The spectrum shows an absorption peak around 600–615 cm − 1 , 1100 cm − 1 , 800 cm− 1 and 450 cm− 1 that is assigned to the fundamental vibration of Sn–O (Sn–O–Sn). The absorption band situated towards 550 cm− 1 is likely associated to Sn–OH vibration. The molecular vibrations of the chemical bonds detected are
3.3.2. Temperature effect Fig. 5 shows the resistance variation of the thick tin dioxide film versus temperature (from 150 to 400 °C). We observe an increase in the resistance up 350 °C, then a decrease. This particular behaviour is associated to a transformation phase of the SnO2 thick film that can be associated to the structural modification of the material. A similar phenomenon has been
Fig. 5. Resistance variation of tin dioxide film under air versus temperature.
Fig 7. The sensitivity of the sensor as function of ethanol concentration at different operating temperature.
Fig. 2 present the AFM photos (2 × 2 μm2) of the SnO2 film after annealing. The film appears with crystalline grains shape relatively dense and homogeneous on a micrometer scale. The size of these grains showed by AFM photo argues the thickness of the material layer.
I. Hafaiedh et al. / Materials Science and Engineering C 28 (2008) 584–587
587
3.3.4. Calibration curve The sensitivity of tin dioxide to chemical vapor is generally defined as (Ra − Rg) / Ra, where Ra and Rg are the resistance of the film under air and vapor, respectively. Fig. 7 presents the sensitivity of the sensor versus various concentrations of ethanol, at various temperatures ranging from 300 to 400 °C. We notice that the material is more sensitive for high ethanol concentration. The increase of ethanol concentrations causes an increase in sensor sensitivity. The higher sensitivity of the sensor was obtained at 300 °C. Fig. 8 shows the resistance of the tin dioxide layer under different ethanol concentrations. This curve shows that the conductivity of the material is more interesting at 300 °C which is due to a change in the transformation phase. Fig. 8. The resistance variation of the tin dioxide film versus ethanol concentration at different temperature.
reported for WO3 thin film [12]. Furthermore, the decreasing of the resistance beyond 350 °C is associated to a semiconductorlike behaviour, according to the following Eq. (1): R ¼ R0 exp
EG kT
ð1Þ
where R0 represent the resistance of the material at room temperature, K is the Boltzmann constant; T is the temperature and EG represent the activation energy. 3.3.3. Detection The resistance variation of the tin oxide layer under air, oxygen and ethanol (C2H5OH) versus temperature is reported in Fig. 6. Firstly, we observed an increase in the resistance under oxygen compared to air. This phenomenon is explained by the oxidizing character of oxygen. The oxygen molecules adsorbed on the semiconductor surface, remove electrons from the conduction band which include an increase in the resistance of SnO2 [13] (see Eq. (2)): 1=2 O2 þ e− þ s→O− −s
ð2Þ
where s is an adsorption site on the SnO2 surface and erepresents an electron from the conduction band. This result enables us to notice that temperature increases when oxygen ions adsorbed easily on SnO2 surface. This induce gradually an increase in the resistance. Beyond 300 °C, oxygen starts to be desorbed, which induces a decrease in the resistance. The introduction of ethanol modifies mainly the resistance of tin oxide for higher temperature. The presence of a reducing gas (ethanol) decreases the resistance of the material. This phenomenon is explained by the reducing character of ethanol reacting with oxygen, release electrons which became free to conduct according to Eq. (3): 2C2 H5 OH þ O−2 ðChemÞ→2CH3 CHO þ H2 O þ e
ð3Þ
According to the nature of gas, oxidizing or reducing, conductivity of the material can be changed. It increases when the gas is oxidizing, and decreases when is reducing.
4. Conclusion The characterization of tin dioxide thick film, prepared by serigraphy method, was studied with various techniques, such as AFM, FTIR and impedance spectroscopy. The results shows that the surface of the tin dioxide appears in the shape of the crystalline grains relatively dense and homogeneous on a micrometer scale. The obtained FTIR spectra show that the material does not present any impurities. Impedance spectroscopy is a powerful technique to study the chemical vapors tin dioxide interaction. The sensor can be used with high reproductibility to identify the nature of the detected gas as an oxidizing or reducing. For future work, sol-gel membrane can be deposited on the tin dioxide film to perform the selectivity. References [1] E. Cominia, M. Ferronib, V. Guidib, G. Fagliaa, G. Martinellib, G. Sberveglieria, Sensors and Actuators B 84 (2002) 26. [2] H. Meixner, U. Lampe, Sensors and Actuators B 33 (1996) 198. [3] L. Gajdosık, Sensors and Actuators B 106 (2005) 691. [4] B. Rivière, J.P. Viricelle, C. Pijolat, Sensors and Actuators B 93 (2003) 531. [5] L. Montanaro, A. Tersalvi, Journal of Electroceramics 5 (3) (2000) 253. [6] N. Guillet, R. Lalauze, J.P. Viricelle, C. Pijolat, L. Montanaro, Materials Sciences and Engineering C 21 (2002) 97. [7] N. Bukuna, A. Vinokurov, M. Vinokurova, L. Derlyukova, Yu. Dobrovolsky, A. Levchenko, Sensors and Actuators B 106 (2005) 153. [8] T.G. Maffeïes, G.T. Owen, M.W. Penny, T.K.H. Starke, S.A. Clark, H. Ferkel, S.P. Wilks, Surface Science 520 (2002) 29. [9] J.R. MacDonald, Impedance Spectroscopy, Wiley, New York, 1987. [10] D. Macdonald, Electrochemical Acta 51 (2006) 1376. [11] C.F. Sanchez, C.J. McNeil, K. Rawson, Trends in Analytical Chemistry 24 (1) (2005). [12] A. Laabidi, C. Jacolin, M. Bendahan, A. Abdelghani, J. Guerin, K. Aguira, M. Maaref, Sensors and Actuators B 106 (2005) 713. [13] P. Montmeat, C. Pijolat, B. Riviere, G. Tournier, J.P. Viricelle, Materials Science and Engineering C 21 (1-2) (2002) 113.