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Water–oxygen interplay on tin dioxide surface: Implication on gas sensing
Chemical Physics Letters 410 (2005) 321–323 www.elsevier.com/locate/cplett
Water–oxygen interplay on tin dioxide surface: Implication on gas sensing Dorota Koziej a
a,b,*
, Nicolae Baˆrsan a, Udo Weimar a, Jacek Szuber b, Kengo Shimanoe c, Noboru Yamazoe c
Department of Chemistry and Pharmacy, University of Tu¨bingen, Institute of Physical and Theoretical Chemistry, Auf der Morgenstelle 15, 72076 Tu¨bingen, Germany b Silesian University of Technology, Department of Microelectronics, ul.Krzywoustego 2, 44100 Gliwice, Poland c Faculty of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan Received 22 March 2005; in final form 28 April 2005 Available online 20 June 2005
Abstract Resistance measurements and direct spectroscopic investigations were used to monitor the surface reaction path between oxygen and water at the surface of SnO2. The experiments were carried out at high sensor operation temperature (330 and 400 °C) and at a constant background of water vapour. We found that there is a significant interaction between adsorbed oxygen ions and water vapour, which results in formation of terminal hydroxyl groups on tin dioxide surface. This observation is an evidence of water– oxygen interaction and so brings a new insight to the mechanistic modelling of the sensing with tin dioxide based sensors. Ó 2005 Elsevier B.V. All rights reserved.
The basic understanding of gas sensing with metal oxides based gas sensors is still far from being achieved even for SnO2, which is the most widely studied material [1,2]. The main reason is that spectroscopic investigations are mostly performed in conditions far away from the ones normally encountered in real world applications, namely in UHV on single crystals or on thin layers [3] and at low temperatures. Real sensors are consisting of polycrystalline materials and are operated between 200 and 400 °C in humid air. Recently we proposed an investigation approach based on a combination of phenomenological and spectroscopic techniques all applied in realistic test conditions [4]. Using it, we explained the influence of water vapour in the CO detection with Pd-doped gas sensors; the main input allowing selecting among possible explanations was the spectroscopic one – in that case provided by Diffuse Reflectance Infrared *
Corresponding author. Fax: +49 7071 295960. E-mail address: [email protected] (D. Koziej).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.107
Fourier Transformed spectroscopy (DRIFT). We concentrated on that specific problem because, in general, the influence of water vapour in the gas sensing with metal oxides based gas sensors is of paramount importance in most applications [5,6]. It was demonstrated that the effects depend on target gas, base material, fabrication technology, doping, etc. [7–9] and that they range from enhancement down to inhibition of sensing. Alone for SnO2, there are a few proposed mechanisms for surface reactions such as: (a) homolytic dissociation of water and reaction with lattice oxygen resulting in the formation of two types of hydroxyl groups (terminal and rooted) [10]; (b) homolytic dissociation of water and the formation of terminal hydroxyl groups and oxygen vacancies [11]; (c) competitive reaction of water with pre-adsorbed oxygen ions [11]; (d) dissociative adsorption of water and reaction with oxygen ad atom (at low temperature) or with oxygen ions (high temperature) [12,13]. Empirically, it was found that the conductance is increased in the presence of water vapour and
322
D. Koziej et al. / Chemical Physics Letters 410 (2005) 321–323
that the induced conductivity changes were always reversible. The goal of our research was the investigation of the interaction of water vapour with the surface of undoped SnO2 at different oxygen concentrations. We recognised that there is a significant interaction between adsorbed oxygen ions and water vapour, which results in formation of terminal hydroxyl groups on tin dioxide surface. This observation is an evidence of water–oxygen interaction and so brings a new insight to the mechanistic modelling of the sensing with tin dioxide based sensors. All studies were performed as close as possible to the operation conditions in real sensing application, namely on real sensors, at normal operation temperature and at normal ambient pressure; the exception was the humidity. In order to be able to observe the surface water related species and for understanding the initial steps of the interaction with water vapour, the humidity level was kept extremely low (ppm). The samples examined in this work are in the form thick films printed onto alumina substrates provided with electrodes and heaters [14]. The base material is a SnO2 powder – mean grain size 7 nm – obtained in a hydrothermal process from aqueous solutions of tin chloride and ammonium hydrogen carbonate [15]. The measurements were performed at sensor operation temperatures of 330 and 400 °C in a flow of nitrogen (200 ml/min). The concentration of residual gases in nitrogen was monitored and kept constant during measurements (O2 – 70 ppm, humidity 3 ± 1 ppm). The changes of the surface species concentrations were directly monitored by means of Diffuse Reflectance Fourier Transformed Spectroscopy. The spectra were recorded by a Bruker 66v spectrometer with a resolution of 2 cm1. Simultaneously, conductance measurements were performed. Fig. 1 presents the SnO2 sensor surface band bending changes induced by the increase of oxygen concentration
Fig. 1. Changes of the band bending of tin dioxide surface under exposure to oxygen (2000–50 000 ppm) at 400 °C sensor operation temperature and at constant humidity level (3 ppm).
in the ambient atmosphere. The changes of the barrier height were estimated on the basis of a simplified Schottky model by using the measured sensor resistance values [16]. It is widely accepted that the consequence of oxygen ionosorption on a n-type semiconductor material is an increase of the negative charge at the surface determining a band bending and consequently the formation of a depletion layer: O2 þ 2e $ 2O ad
ð1Þ
We observed, as expected, that with increasing oxygen concentrations the surface barrier height is increasing without reaching saturation even at high oxygen concentrations (50 000 ppm); the tendency is, however, detectable in the major slope decrease over 1% of oxygen. Fig. 2 shows the changes of the surface species coverage upon increasing of oxygen concentration from 70 up to 50 000 ppm; the results were obtained at 400 °C sensor operation temperature. One observes that the oxygen presence in the ambient atmosphere increases the concentration of hydroxyl groups (peaks at 3640 cm1). The effect is the most spectacular for the lowest oxygen concentration (2000 ppm) and evolves towards saturation. The single channel units were chosen in order to demonstrate that there are other OH bands on the SnO2 surface, not sensitive to the changes of oxygen concentration. The same qualitative behaviour was observed at 330 °C. Furthermore, the reversibility of the reaction between water and oxygen ions was investigated. In (Fig. 3) it can be observed that the hydroxyl groups built upon oxygen exposure (line 1), are decreasing after removing the oxygen from the ambient atmosphere (line 2); this fact indicates the reversibility of the process. The spectrum at 70 ppm O2 was used as a reference spectrum in the apparent absorbance units.
Fig. 2. Single channel DRIFT spectra of the tin dioxide sensor operated at 400 °C exposed to oxygen: (0) 70 ppm; (1) 2000 ppm; (2) 5000 ppm; (3) 10 000 ppm; (4) 50 000 ppm at 400 °C and at constant humidity level (3 ppm).
D. Koziej et al. / Chemical Physics Letters 410 (2005) 321–323
323
ation all contributions and their balance that will determine the recorded signals. Also, as a consequence of the decrease of adsorbed oxygen ions determined by the reaction with reducing gases, the surface concentration of hydroxyl groups will decrease. This might explain the already reported experimental results indicating such findings [19] without the need of a direct CO–hydroxyl group interaction.
Acknowledgement One of the authors (D.K.) gratefully acknowledged the scientific discussion with Prof. V. Hoffmann. Fig. 3. DRIFT spectra of the tin dioxide sensor operated at 400 °C: line 1 – spectrum recorded during exposure of 50 000 ppm of oxygen for 3 h, line 2 – spectrum recorded 2 h after exposure to 50 000 ppm of oxygen in the flow of 70 ppm of oxygen in nitrogen in apparent absorbance units (log I/I0). As a reference (I0) a spectrum recorded before exposure to oxygen was used (Fig. 2, line 0).
The above presented experimental findings strongly suggest that an interaction between oxygen and water takes place on the surface of tin dioxide. Taking into account that: the ionosorbed oxygen atoms are the dominant form of adsorbed oxygen species on SnO2; no formation of other OH groups besides of the terminal one is observed; one can conclude that during water adsorption onto the surface of tin oxide the interaction between O species and water related species – path (d) [12,13] – plays an important role. One of the most possible scenarios (already suggested on the basis of conductivity measurements but never proved until now [12,13]) is the reaction of pre-adsorbed oxygen, namely O, with water that results in the formation of terminal hydroxyl groups and the release of an electron to the conduction band: H2 O þ O ad þ 2Sn $ 2ðSn–OHÞ þ e
ð2Þ
Such an interaction mechanism indicates that water, under conditions of dynamic adsorption–desorption equilibrium, compete with reducing gases for the preadsorbed oxygen species. Consequently, the higher sensor signal towards, e.g., CO in the absence of water generally reported for undoped SnO2 based sensors [4,16,17] and observed also for the actual sensors [18] can be better understood and explained; nevertheless, for the full understanding one has to take into consider-
References [1] D. Kohl, J. Phys. D: Appl. Phys. 34 (2001) R125. [2] V. Lantto, T.R. Rantala, T.S. Rantala, J. Eur. Ceram. Soc. 21 (10–11) (2001) 1961. [3] T.G.G. Maffeı¨s, G.T. Owen, M.W. Penny, T.K.H. Starke, A. Clark, H. Ferkel, S.P. Wilks, Surf. Sci. 520 (2002) 29. [4] N. Barsan, U. Weimar, J. Phys-Condens. Mat. 15 (20) (2003) 813. [5] G. Neri, A. Bonavita, G. Rizzo, S. Galvagno, N. Donato, L.S. Caput, Sensor. Actuator. B 101 (2004) 90. [6] J. Giber, I.V. Perczel, J. Gerblinger, U. Lampe, M. Fleischer, Sensor. Actuator. B 18–19 (1994) 113. [7] P.A. Thiel, T.E. Madey, Surf. Sci. Rep. 7 (1987) 211. [8] M.A. Henderson, Surf. Sci. Rep. 46 (2002) 1. [9] D.E. Williams, K.F.E. Pratt, J. Chem. Soc., Faraday Trans. 94 (1998) 3493. [10] G. Heiland, D. Kohl, in: T. Seiyama (Ed.), Chemical Sensor Technology, vol. 1, Kodansha, Tokyo, 1988. [11] V.A. Henrich, P.A. Cox, The Surface Science of Metal Oxides, University Press, Cambridge, 1994, p. 312. [12] K.D. Schierbaum, U. Weimar, W. Go¨pel, R. Kowalkowski, Sensor. Actuator. B 3 (1991) 205. [13] Y. Matsuura, K. Takahata, K. Ihokura, Sensor. Actuator. 14 (1988) 223. [14] J. Kappler, N. Barsan, U. Weimar, A. Dieguez, J.L. Alay, A. Romano-Rodrigez, J.R. Morante, Fresen. J. Anal. Chem. 361 (1998) 110. [15] N.S. Baik, G. Sakai, N. Miura, N. Yamazoe, Sensor. Actuator. B 63 (2000) 74. [16] W. Schmid, N. Baˆrsan, U. Weimar, Sensor. Actuator. B 103 (1–2) (2004) 362. [17] M. Schweizer, J.G. Zheng, U. Weimar, W. Go¨pel, N. Barsan, E. Pentia, A. Tomescu, Sensor. Actuator. B 31 (1996) 71. [18] T. Sahm, A. Gurlo, N. Baˆrsan, A. Oprea, U. Weimar, N. Yamazoe, G. Sakai, K. Shimanoe, Chemical Sensors, 20 B, 160. [19] S. Hahn, N. Baˆrsan, U. Weimar, S.G. Ejakov, J.H. Visser, R.E. Soltis, Thin Solid Films 436 (2003) 17.
Water–oxygen interplay on tin dioxide surface: Implication on gas sensing Dorota Koziej a
a,b,*
, Nicolae Baˆrsan a, Udo Weimar a, Jacek Szuber b, Kengo Shimanoe c, Noboru Yamazoe c
Department of Chemistry and Pharmacy, University of Tu¨bingen, Institute of Physical and Theoretical Chemistry, Auf der Morgenstelle 15, 72076 Tu¨bingen, Germany b Silesian University of Technology, Department of Microelectronics, ul.Krzywoustego 2, 44100 Gliwice, Poland c Faculty of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan Received 22 March 2005; in final form 28 April 2005 Available online 20 June 2005
Abstract Resistance measurements and direct spectroscopic investigations were used to monitor the surface reaction path between oxygen and water at the surface of SnO2. The experiments were carried out at high sensor operation temperature (330 and 400 °C) and at a constant background of water vapour. We found that there is a significant interaction between adsorbed oxygen ions and water vapour, which results in formation of terminal hydroxyl groups on tin dioxide surface. This observation is an evidence of water– oxygen interaction and so brings a new insight to the mechanistic modelling of the sensing with tin dioxide based sensors. Ó 2005 Elsevier B.V. All rights reserved.
The basic understanding of gas sensing with metal oxides based gas sensors is still far from being achieved even for SnO2, which is the most widely studied material [1,2]. The main reason is that spectroscopic investigations are mostly performed in conditions far away from the ones normally encountered in real world applications, namely in UHV on single crystals or on thin layers [3] and at low temperatures. Real sensors are consisting of polycrystalline materials and are operated between 200 and 400 °C in humid air. Recently we proposed an investigation approach based on a combination of phenomenological and spectroscopic techniques all applied in realistic test conditions [4]. Using it, we explained the influence of water vapour in the CO detection with Pd-doped gas sensors; the main input allowing selecting among possible explanations was the spectroscopic one – in that case provided by Diffuse Reflectance Infrared *
Corresponding author. Fax: +49 7071 295960. E-mail address: [email protected] (D. Koziej).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.107
Fourier Transformed spectroscopy (DRIFT). We concentrated on that specific problem because, in general, the influence of water vapour in the gas sensing with metal oxides based gas sensors is of paramount importance in most applications [5,6]. It was demonstrated that the effects depend on target gas, base material, fabrication technology, doping, etc. [7–9] and that they range from enhancement down to inhibition of sensing. Alone for SnO2, there are a few proposed mechanisms for surface reactions such as: (a) homolytic dissociation of water and reaction with lattice oxygen resulting in the formation of two types of hydroxyl groups (terminal and rooted) [10]; (b) homolytic dissociation of water and the formation of terminal hydroxyl groups and oxygen vacancies [11]; (c) competitive reaction of water with pre-adsorbed oxygen ions [11]; (d) dissociative adsorption of water and reaction with oxygen ad atom (at low temperature) or with oxygen ions (high temperature) [12,13]. Empirically, it was found that the conductance is increased in the presence of water vapour and
322
D. Koziej et al. / Chemical Physics Letters 410 (2005) 321–323
that the induced conductivity changes were always reversible. The goal of our research was the investigation of the interaction of water vapour with the surface of undoped SnO2 at different oxygen concentrations. We recognised that there is a significant interaction between adsorbed oxygen ions and water vapour, which results in formation of terminal hydroxyl groups on tin dioxide surface. This observation is an evidence of water–oxygen interaction and so brings a new insight to the mechanistic modelling of the sensing with tin dioxide based sensors. All studies were performed as close as possible to the operation conditions in real sensing application, namely on real sensors, at normal operation temperature and at normal ambient pressure; the exception was the humidity. In order to be able to observe the surface water related species and for understanding the initial steps of the interaction with water vapour, the humidity level was kept extremely low (ppm). The samples examined in this work are in the form thick films printed onto alumina substrates provided with electrodes and heaters [14]. The base material is a SnO2 powder – mean grain size 7 nm – obtained in a hydrothermal process from aqueous solutions of tin chloride and ammonium hydrogen carbonate [15]. The measurements were performed at sensor operation temperatures of 330 and 400 °C in a flow of nitrogen (200 ml/min). The concentration of residual gases in nitrogen was monitored and kept constant during measurements (O2 – 70 ppm, humidity 3 ± 1 ppm). The changes of the surface species concentrations were directly monitored by means of Diffuse Reflectance Fourier Transformed Spectroscopy. The spectra were recorded by a Bruker 66v spectrometer with a resolution of 2 cm1. Simultaneously, conductance measurements were performed. Fig. 1 presents the SnO2 sensor surface band bending changes induced by the increase of oxygen concentration
Fig. 1. Changes of the band bending of tin dioxide surface under exposure to oxygen (2000–50 000 ppm) at 400 °C sensor operation temperature and at constant humidity level (3 ppm).
in the ambient atmosphere. The changes of the barrier height were estimated on the basis of a simplified Schottky model by using the measured sensor resistance values [16]. It is widely accepted that the consequence of oxygen ionosorption on a n-type semiconductor material is an increase of the negative charge at the surface determining a band bending and consequently the formation of a depletion layer: O2 þ 2e $ 2O ad
ð1Þ
We observed, as expected, that with increasing oxygen concentrations the surface barrier height is increasing without reaching saturation even at high oxygen concentrations (50 000 ppm); the tendency is, however, detectable in the major slope decrease over 1% of oxygen. Fig. 2 shows the changes of the surface species coverage upon increasing of oxygen concentration from 70 up to 50 000 ppm; the results were obtained at 400 °C sensor operation temperature. One observes that the oxygen presence in the ambient atmosphere increases the concentration of hydroxyl groups (peaks at 3640 cm1). The effect is the most spectacular for the lowest oxygen concentration (2000 ppm) and evolves towards saturation. The single channel units were chosen in order to demonstrate that there are other OH bands on the SnO2 surface, not sensitive to the changes of oxygen concentration. The same qualitative behaviour was observed at 330 °C. Furthermore, the reversibility of the reaction between water and oxygen ions was investigated. In (Fig. 3) it can be observed that the hydroxyl groups built upon oxygen exposure (line 1), are decreasing after removing the oxygen from the ambient atmosphere (line 2); this fact indicates the reversibility of the process. The spectrum at 70 ppm O2 was used as a reference spectrum in the apparent absorbance units.
Fig. 2. Single channel DRIFT spectra of the tin dioxide sensor operated at 400 °C exposed to oxygen: (0) 70 ppm; (1) 2000 ppm; (2) 5000 ppm; (3) 10 000 ppm; (4) 50 000 ppm at 400 °C and at constant humidity level (3 ppm).
D. Koziej et al. / Chemical Physics Letters 410 (2005) 321–323
323
ation all contributions and their balance that will determine the recorded signals. Also, as a consequence of the decrease of adsorbed oxygen ions determined by the reaction with reducing gases, the surface concentration of hydroxyl groups will decrease. This might explain the already reported experimental results indicating such findings [19] without the need of a direct CO–hydroxyl group interaction.
Acknowledgement One of the authors (D.K.) gratefully acknowledged the scientific discussion with Prof. V. Hoffmann. Fig. 3. DRIFT spectra of the tin dioxide sensor operated at 400 °C: line 1 – spectrum recorded during exposure of 50 000 ppm of oxygen for 3 h, line 2 – spectrum recorded 2 h after exposure to 50 000 ppm of oxygen in the flow of 70 ppm of oxygen in nitrogen in apparent absorbance units (log I/I0). As a reference (I0) a spectrum recorded before exposure to oxygen was used (Fig. 2, line 0).
The above presented experimental findings strongly suggest that an interaction between oxygen and water takes place on the surface of tin dioxide. Taking into account that: the ionosorbed oxygen atoms are the dominant form of adsorbed oxygen species on SnO2; no formation of other OH groups besides of the terminal one is observed; one can conclude that during water adsorption onto the surface of tin oxide the interaction between O species and water related species – path (d) [12,13] – plays an important role. One of the most possible scenarios (already suggested on the basis of conductivity measurements but never proved until now [12,13]) is the reaction of pre-adsorbed oxygen, namely O, with water that results in the formation of terminal hydroxyl groups and the release of an electron to the conduction band: H2 O þ O ad þ 2Sn $ 2ðSn–OHÞ þ e
ð2Þ
Such an interaction mechanism indicates that water, under conditions of dynamic adsorption–desorption equilibrium, compete with reducing gases for the preadsorbed oxygen species. Consequently, the higher sensor signal towards, e.g., CO in the absence of water generally reported for undoped SnO2 based sensors [4,16,17] and observed also for the actual sensors [18] can be better understood and explained; nevertheless, for the full understanding one has to take into consider-
References [1] D. Kohl, J. Phys. D: Appl. Phys. 34 (2001) R125. [2] V. Lantto, T.R. Rantala, T.S. Rantala, J. Eur. Ceram. Soc. 21 (10–11) (2001) 1961. [3] T.G.G. Maffeı¨s, G.T. Owen, M.W. Penny, T.K.H. Starke, A. Clark, H. Ferkel, S.P. Wilks, Surf. Sci. 520 (2002) 29. [4] N. Barsan, U. Weimar, J. Phys-Condens. Mat. 15 (20) (2003) 813. [5] G. Neri, A. Bonavita, G. Rizzo, S. Galvagno, N. Donato, L.S. Caput, Sensor. Actuator. B 101 (2004) 90. [6] J. Giber, I.V. Perczel, J. Gerblinger, U. Lampe, M. Fleischer, Sensor. Actuator. B 18–19 (1994) 113. [7] P.A. Thiel, T.E. Madey, Surf. Sci. Rep. 7 (1987) 211. [8] M.A. Henderson, Surf. Sci. Rep. 46 (2002) 1. [9] D.E. Williams, K.F.E. Pratt, J. Chem. Soc., Faraday Trans. 94 (1998) 3493. [10] G. Heiland, D. Kohl, in: T. Seiyama (Ed.), Chemical Sensor Technology, vol. 1, Kodansha, Tokyo, 1988. [11] V.A. Henrich, P.A. Cox, The Surface Science of Metal Oxides, University Press, Cambridge, 1994, p. 312. [12] K.D. Schierbaum, U. Weimar, W. Go¨pel, R. Kowalkowski, Sensor. Actuator. B 3 (1991) 205. [13] Y. Matsuura, K. Takahata, K. Ihokura, Sensor. Actuator. 14 (1988) 223. [14] J. Kappler, N. Barsan, U. Weimar, A. Dieguez, J.L. Alay, A. Romano-Rodrigez, J.R. Morante, Fresen. J. Anal. Chem. 361 (1998) 110. [15] N.S. Baik, G. Sakai, N. Miura, N. Yamazoe, Sensor. Actuator. B 63 (2000) 74. [16] W. Schmid, N. Baˆrsan, U. Weimar, Sensor. Actuator. B 103 (1–2) (2004) 362. [17] M. Schweizer, J.G. Zheng, U. Weimar, W. Go¨pel, N. Barsan, E. Pentia, A. Tomescu, Sensor. Actuator. B 31 (1996) 71. [18] T. Sahm, A. Gurlo, N. Baˆrsan, A. Oprea, U. Weimar, N. Yamazoe, G. Sakai, K. Shimanoe, Chemical Sensors, 20 B, 160. [19] S. Hahn, N. Baˆrsan, U. Weimar, S.G. Ejakov, J.H. Visser, R.E. Soltis, Thin Solid Films 436 (2003) 17.