- Email: [email protected]
Sprayed SnO2 thin films for NO2 sensors
Sensors and Actuators B 58 Ž1999. 370–374 www.elsevier.nlrlocatersensorb
Sprayed SnO 2 thin films for NO 2 sensors G. Leo a , R. Rella a
a,)
, P. Siciliano a , S. Capone b, J.C. Alonso c , V. Pankov c , A. Ortiz
c
Istituto per lo studio di nuoÕi Materiali per l’Elettronica (IME-CNR), Via Arnesano, 73100 Lecce, Italy b INFM Unita’ di Lecce, Õia per Arnesano, 73 100 Lecce, Italy c Instituto de InÕestigaciones en Materiales, UNAM. A.P. 70-360, Coyoacan 04510, D.F., Mexico Accepted 11 January 1999
Abstract SnO 2 thin films as NO 2 sensor in low concentration range have been obtained by chemical spray deposition technique. The AFM analysis showed that the as-deposited films are constituted of islands of rectangular shape and elongated in the same direction. The electrical characterisation results and their variations in the presence of the NO 2 gas mixed in low concentration with dry air are reported. The electrical response to NO 2 gas was studied in order to find the optimum detection temperature. The results have shown a resistance change of about 6000% at a working temperature of 3508C in the presence of 500 ppm of NO 2 toxic gas and a sensitivity threshold of about 5 ppm at the same working temperature. q 1999 Elsevier Science S.A. All rights reserved. Keywords: SnO 2 thin films; NO 2 sensors; Chemical spray deposition
1. Introduction Toxic gas detection management by sensitive and reversible sensors, has became a field of vital importance in environment-conscious modern life. Control and detection of toxic gases such as carbon monoxide ŽCO., methane ŽCH 4 ., nitrogen oxide ŽNO x . are very important because of the destructive effects these gases have on the respiratory system of human beings. This is the reason for more and more increasing requirements to monitor the gas pollution in urban agglomerates or in the work ambient atmosphere are being made. To this purpose there is an increasing need for low-cost, selective, sensitive and reliable gas sensors. Actually, environmental monitoring is carried out using typical analytical techniques which use very expansive equipments. The use of devices based on solid-state gas sensors should consent to realise an environmental monitoring network of low cost. Naturally, it is necessary to obtain high sensitive, stable and reliable gas-sensors. A simple method to detect gases is measuring the electrical conductivity change induced on a suitable active layer by the absorption of gas molecules on its surface w1x.
)
Corresponding author. Tel.: q39-832-320-244; Fax: q39-832-325299; E-mail: [email protected]
Traditional gas-sensing materials are inorganic oxide semiconductors, such as SnO 2 , TiO 2 , WO 3 etc. w2–4x. Among the n-type semiconducting oxides, SnO 2 is currently chosen because of its good chemical stability and its high sensitivity at low operating temperature Žaround 4008C. w5x. SnO 2 films can be obtained by various technique but thin film technology seems to be the most suitable for preparing gas sensors. Different physical and chemical deposition method are actually used, like reactive sputtering w6x, electron beam deposition, thermal evaporation w7x, sol–gel and spray pyrolysis w8,9x. In this work we report a study of the physical properties and the preliminary NO 2 sensing characteristics of SnO 2 thin films prepared by the spray pyrolysis method. This technique offers several advantages to control the morphology of the sample. The high porosity and large surface area of the films obtained by using this deposition technique may enhance sensitivity mechanism.
2. Experimental details The spray pyrolysis technique consists of spraying a solution containing a soluble salt of the cation of interest onto a heated substrate. In the present work the spray pyrolysis apparatus was used to produce the films similar to those previously reported w10x. The SnCl 4 q 5H 2 O Žfrom
0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 0 9 8 - 2
G. Leo et al.r Sensors and Actuators B 58 (1999) 370–374
Aldrich 24,467-8. was used as tin source material. The starting solution was 0.1 M SnCl 4 in deionized water. The values of the deposition parameters such as substrate temperature Ts s 5108C, air flow rate Fg s 9 lrmin and solution flow rate Fs s 6 mlrmin were kept constant. The deposition time was varied in the range from 4 to 10 min. The films were deposited onto different substrates, such as: Pyrex glass, Corning 7059 glass and fused quartz. The film morphology was investigated in air by atomic force microscopy ŽAFM. operating in contact mode with a commercial Si 2 N3 tip of radius 20–60 nm. We analysed data from three different scan size: 2 mm = 2 mm; 5 mm = 5 mm and 10 mm = 10 mm each with 256 = 256 pointsrlines. AFM images were taken from different region of the sample in order to check the lateral uniformity of the film morphology. The standard deviation of height values, RMS Žroot mean square. roughness, calculated by the AFM software for the individual scans were then averaged. Pixel counting is performed with NIH-IMAGE software. The microstructure of the films was analysed by means of X-ray diffraction measurements. The diffractograms were obtained with a Siemens D500 diffractometer, using the CuK a1 radiation with a wavelength of 1.5406 A. The gas effect on electrical conductivity was analysed by performing electrical measurements in a controlled atmosphere. Gas sensing test of the tin dioxide thin films were carried out in a dynamic flow system implemented in our laboratory. The gas to be tested and dry air, coming from certified bottles, were injected in a stainless steel cell; the desired composition of the mixture and the flow rate can be varied by a MKS mass flow control system Žmod. 647B.. The samples were placed onto a heated stage in the chamber and exposed to different gas concentration. A Eurotherm Termoregolator controlled the temperature of the stage, while a Pt thermometer, near the sensor, measured the working temperature of the sensor. This was connected, by means of ohmic gold electrodes, to two gold points and the d.c. resistance was continuously monitored by an Keithley electrometer Žmod. 617.. The whole experimental apparatus was interfaced to a PC equipped with a Pentium 133 microprocessor and a IEEE 488 board for acquiring and plotting the electrical response of each sensor in real time.
3. Results and discussion 3.1. X-ray results X-ray diffraction measurements performed onto our samples indicate that the as-deposited films are polycrystalline and the crystalline structure corresponds to the cassiterite tetragonal phase of the SnO 2 . A typical X-ray diffraction pattern of the films obtained on the various substrates is given in Fig. 1. The diffraction peaks and
371
Fig. 1. X-ray diffraction pattern for as deposited SnO 2 thin film.
their positions are associated with the reflections reported in the 41-1445 ASTM file. These results show that the crystalline structure of the deposited material is independent of the used substrate material. 3.2. AFM results The topographic plan views of the as-deposited and after gas expositions SnO 2 thin film are shown in Fig. 2Ža. and Žb.. The RMS roughness is Ž16.4 " 0.4. nm for the as-grown sample and it does not change after gas expositions and thermal treatment being Ž16.7 " 0.6. nm. On the contrary some difference can be revealed in the morphology of the two samples. The morphology of the as-deposited film consists of islands of rectangular shape Žlamellae. all elongated in the same direction ŽFig. 2a.. Hence, the sample surface seems to be composed by several layers of superposed lamellae each height about 10 nm. The mean dimensions of these lamellae are Ž0.20 " 0.01.mm for the major side and Ž0.10 " 0.01.mm for the minor side. The ratio of white pixel to black pixel, that is the ratio between film and voids in the sample, is 50%. This value gives a measure of the film density. After exposing the sample surface to various NO 2 gas concentration up to 5008C the surface morphology of the SnO 2 thin film appears to change. In fact, as it can be observed in the AFM image ŽFig. 2b. the lateral dimension of the lamellae are slightly decreased and the shape appear to be more rounded. Moreover the ratio filmrvoids increases from 50% up to 60% indicating an apparent increase in the film density. This is probably due to change in the morphology of the film when the sample works at temperature close to the deposition temperature. Further transmission electron microscopy measurements have
372
G. Leo et al.r Sensors and Actuators B 58 (1999) 370–374
Fig. 3. Relative resistance variation D R r R 0 as a function of the operating temperature for a typical sprayed SnO 2 thin films.
Fig. 2. AFM images of as-deposited SnO 2 thin films Ža. and after expositions to NO 2 gas at 5008C working temperature Žb..
absorption process is sufficiently fast and the relative resistance variation, defined as D RrR 0 s Ž R y R 0 .rR 0 , is most favourable. Here we define R and R 0 as the resistance of the sample in the presence of NO 2 gas and in dry-air respectively. Fig. 3 shows the response D RrR 0 to 500 ppm NO 2 of a sprayed SnO 2 thin film 230 nm thick as a function of the operating temperature. As one can see, the relative resistance variation increases with the temperature and reaches a maximum value in correspondence of T s 3508C. If the temperature increases again, the sensitivity decreases. This behaviour is currently explained taking into account that the oxide surface is populated by a variety of physisorbed and chemisorbed molecules. In y y particular, H 2 O, OHy, O 2 , Oy and Oy 2 , O , where O 2
shown that these lamellae are constitute of grains having dimension of about 10 nm. Systematic observations about the average dimension and structure of these grains by using selected area diffraction measurements will be object of the next paper. 3.3. NO2 sensing characteristics All the samples were submitted to the same experimental procedure. They were exposed to a dry air flow until their resistance had not reach an equilibrium value. Then the gas to be detected, diluted in dry air at a fixed concentration, was put into the cell and the samples left in the gas–air mixture until their resistance didn’t vary further. The active gas was then switched off and the sample left to recover in air. During the recovering time the resistance returns to its initial value proving the reversibility of the process. It is well known that for each coupling of material and gas, there is an operating temperature range in which the
Fig. 4. Electrical response of sprayed SnO 2 thin films vs. NO 2 concentration at 3508C working temperature.
G. Leo et al.r Sensors and Actuators B 58 (1999) 370–374
373
sensitivity temperature. As one can see, the response of the analysed films in a log–log plot is linear in the investigated NO 2 concentration range. Consequently, the resistance R gas is related to the gas concentration or, equivalently, to the partial pressure by means of a power law R gasrR air s k Ž PNO 2 .
n
Where P NO 2 is the partial pressure of the oxidising gas and n is a positive exponent which does not depend on P NO 2 but it is probably linked with the stoichiometry of the previous proposed reaction, whereas k is a constant. The exponent n, calculated from the slope of the straight line of Fig. 5, was found to be Ž0.34 " 0.01.. In reality, we have observed that the active layer undergoes a diminution in the response after repeated expositions to NO 2 gas and measurements cycles, then it became stable according to the calibration curve reported in Fig. 5. Fig. 5. Resistance change vs. NO 2 concentration for a typical sprayed SnO 2 film working at the maximum sensitivity temperature.
are likely to predominate. The presence of these species modulates the electrical behaviour of the active layer in the presence of the oxidising or reducing gases. At the temperature of the active layer of about 300–4008C the species y are dominant in the sensing mechanism. At Oy 2 and O these temperatures NO 2 can be absorbed or to interact with the oxygen adsorbed onto the sensing layer according to the reactions: NO 2 gas q ey™ NOy 2 y y y NO 2 gas q Oy 2 q 2e ™ NO 2 q 2O
These reactions reduce the electron concentration near the surface and consequently the resistance of the layer increases w11,12x. Moreover, NO 2 ions are desorbed as NO 2 gas flow is stopped and consequently a recovery of the initial condition takes place. Fig. 4 reports the dynamic change in the electrical resistance of a typical SnO 2 sprayed film at a working temperature of 3508C resulting from different NO 2 concentration in the mixture with dry-air. It is evident that the resistance of the film reaches its initial value when the sensor is exposed to pure dry-air. It is interesting to note that the response time and recovery time Ždefined as the times required to reach 90% of the steady-state values of R gas and R air , respectively. to 50 ppm NO 2 gas present in the mixture with dry air at 3508C working temperature, results to be 6 and 5 min respectively. Moreover we have observed that the response and recovering process are dependent on working temperature of the sensing layer. Fig. 5 shows a typical calibration curve, namely the ratio R gasrR air as a function of the gas concentration for a typical sprayed SnO 2 thin film working at a maximum
4. Conclusions In this work a preliminary characterisation shows that it is possible to obtain SnO 2 thin films with a high specific area by using the spray pyrolysis deposition technique. X-ray diffraction measurements have shown that the films are polycrystalline with a cassiterite tetragonal phase. The average dimension of the constituent grains having a rectangular shape was of about 0.20 mm for the major side and 0.10 mm for the minor side as evidenced by AFM observations. The films have shown encouraging NO 2 sensing characteristics. However, further analysis will be necessary in order to increase the performance of the active layer as a function of the deposition parameters.
Acknowledgements The authors wish to thank Mr. F. Casino for technical assistance during the measurements. This work was partially supported by CEE-FESR subprogram II, project ‘Materiali, Processi e Dispositivi per sensoristica, optoelettronica ed elettronica di potenza’.
References w1x S.R. Morrison, M.J. Madon, Chemical Sensing with Solid State Devices Academic Press, London, 1989. w2x T. Oyabu, Sensing characteristics of SnO 2 thin film gas sensors, J. Appl. Phys. 53 Ž1982. 2785. w3x H.M. Lin, C.M. Hsu, H.Y. Yang, P.Y. Lee, C.C. Yang, Nanocrystalline WO 3 -based H 2 S sensors, Sensors and Actuators B 22 Ž1994. 63–68. w4x F. Boccuzzi, E. Guglielmetti, A. Chiorino, IR study of gas sensor materials: NO interaction on ZnO and TiO 2 pure or modified by metals, Sensors and Actuators B 7 Ž1992. 645. w5x K.D. Schierbaum, U. Weimar, W. Gopel, Comparison of ceramics,
374
G. Leo et al.r Sensors and Actuators B 58 (1999) 370–374
thick and thin film chemical sensors based upon SnO 2 , Sensors and Actuators B 7 Ž1992. 709–716. w6x M.C. Horrillo, P. Serrini, J. Santos, L. Manes, Influence of the deposition conditions of SnO 2 thin films by reactive sputtering on the sensitivity to urban pollutants, Sensors and Actuators B 45 Ž1997. 193. w7x M. Di Giulio, G. Micocci, A. Serra, A. Tepore, R. Rella, P. Siciliano, SnO 2 thin films for gas sensors prepared by r.f. reactive sputtering, Sensors and Actuators B 24–25 Ž1995. 465–468. w8x A. Wilson, J. Wright, J. Murphy, Ma.M. Stroud, S.C. Thorpe, Sol–gel materials for gas sensing applications, Sensors and Actuators B 18–19 Ž1994. 506.
w9x M. de la L. Olvera, R. Asamoza, SnO 2 and SnO 2 : Pt thin films used as gas sensors, Sensors and Actuators B 45 Ž1997. 49–53. w10x J. Aranovich, A. Ortiz, R.H. Bube, Optical and electrical properties of ZnO films prepared by spray pyrolysis for solar cell applications, J. Vac. Sci. Technol. 16 Ž1979. 994–1003. w11x I. Sayago, J. Gutierrez, L. Ares, J.I. Robla, M.C. Horrillo, J. Getino, J.A. Agapito, The interaction of different oxidising agents on doped tin oxide, Sensors and Actuators B 24–25 Ž1995. 512–515. w12x B. Ruhland, Th. Becker, G. Muller, Gas-kinetic interactions of nitrous oxides with SnO 2 surfaces, Sensors and Actuators B 50 Ž1998. 85.
Sprayed SnO 2 thin films for NO 2 sensors G. Leo a , R. Rella a
a,)
, P. Siciliano a , S. Capone b, J.C. Alonso c , V. Pankov c , A. Ortiz
c
Istituto per lo studio di nuoÕi Materiali per l’Elettronica (IME-CNR), Via Arnesano, 73100 Lecce, Italy b INFM Unita’ di Lecce, Õia per Arnesano, 73 100 Lecce, Italy c Instituto de InÕestigaciones en Materiales, UNAM. A.P. 70-360, Coyoacan 04510, D.F., Mexico Accepted 11 January 1999
Abstract SnO 2 thin films as NO 2 sensor in low concentration range have been obtained by chemical spray deposition technique. The AFM analysis showed that the as-deposited films are constituted of islands of rectangular shape and elongated in the same direction. The electrical characterisation results and their variations in the presence of the NO 2 gas mixed in low concentration with dry air are reported. The electrical response to NO 2 gas was studied in order to find the optimum detection temperature. The results have shown a resistance change of about 6000% at a working temperature of 3508C in the presence of 500 ppm of NO 2 toxic gas and a sensitivity threshold of about 5 ppm at the same working temperature. q 1999 Elsevier Science S.A. All rights reserved. Keywords: SnO 2 thin films; NO 2 sensors; Chemical spray deposition
1. Introduction Toxic gas detection management by sensitive and reversible sensors, has became a field of vital importance in environment-conscious modern life. Control and detection of toxic gases such as carbon monoxide ŽCO., methane ŽCH 4 ., nitrogen oxide ŽNO x . are very important because of the destructive effects these gases have on the respiratory system of human beings. This is the reason for more and more increasing requirements to monitor the gas pollution in urban agglomerates or in the work ambient atmosphere are being made. To this purpose there is an increasing need for low-cost, selective, sensitive and reliable gas sensors. Actually, environmental monitoring is carried out using typical analytical techniques which use very expansive equipments. The use of devices based on solid-state gas sensors should consent to realise an environmental monitoring network of low cost. Naturally, it is necessary to obtain high sensitive, stable and reliable gas-sensors. A simple method to detect gases is measuring the electrical conductivity change induced on a suitable active layer by the absorption of gas molecules on its surface w1x.
)
Corresponding author. Tel.: q39-832-320-244; Fax: q39-832-325299; E-mail: [email protected]
Traditional gas-sensing materials are inorganic oxide semiconductors, such as SnO 2 , TiO 2 , WO 3 etc. w2–4x. Among the n-type semiconducting oxides, SnO 2 is currently chosen because of its good chemical stability and its high sensitivity at low operating temperature Žaround 4008C. w5x. SnO 2 films can be obtained by various technique but thin film technology seems to be the most suitable for preparing gas sensors. Different physical and chemical deposition method are actually used, like reactive sputtering w6x, electron beam deposition, thermal evaporation w7x, sol–gel and spray pyrolysis w8,9x. In this work we report a study of the physical properties and the preliminary NO 2 sensing characteristics of SnO 2 thin films prepared by the spray pyrolysis method. This technique offers several advantages to control the morphology of the sample. The high porosity and large surface area of the films obtained by using this deposition technique may enhance sensitivity mechanism.
2. Experimental details The spray pyrolysis technique consists of spraying a solution containing a soluble salt of the cation of interest onto a heated substrate. In the present work the spray pyrolysis apparatus was used to produce the films similar to those previously reported w10x. The SnCl 4 q 5H 2 O Žfrom
0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 0 9 8 - 2
G. Leo et al.r Sensors and Actuators B 58 (1999) 370–374
Aldrich 24,467-8. was used as tin source material. The starting solution was 0.1 M SnCl 4 in deionized water. The values of the deposition parameters such as substrate temperature Ts s 5108C, air flow rate Fg s 9 lrmin and solution flow rate Fs s 6 mlrmin were kept constant. The deposition time was varied in the range from 4 to 10 min. The films were deposited onto different substrates, such as: Pyrex glass, Corning 7059 glass and fused quartz. The film morphology was investigated in air by atomic force microscopy ŽAFM. operating in contact mode with a commercial Si 2 N3 tip of radius 20–60 nm. We analysed data from three different scan size: 2 mm = 2 mm; 5 mm = 5 mm and 10 mm = 10 mm each with 256 = 256 pointsrlines. AFM images were taken from different region of the sample in order to check the lateral uniformity of the film morphology. The standard deviation of height values, RMS Žroot mean square. roughness, calculated by the AFM software for the individual scans were then averaged. Pixel counting is performed with NIH-IMAGE software. The microstructure of the films was analysed by means of X-ray diffraction measurements. The diffractograms were obtained with a Siemens D500 diffractometer, using the CuK a1 radiation with a wavelength of 1.5406 A. The gas effect on electrical conductivity was analysed by performing electrical measurements in a controlled atmosphere. Gas sensing test of the tin dioxide thin films were carried out in a dynamic flow system implemented in our laboratory. The gas to be tested and dry air, coming from certified bottles, were injected in a stainless steel cell; the desired composition of the mixture and the flow rate can be varied by a MKS mass flow control system Žmod. 647B.. The samples were placed onto a heated stage in the chamber and exposed to different gas concentration. A Eurotherm Termoregolator controlled the temperature of the stage, while a Pt thermometer, near the sensor, measured the working temperature of the sensor. This was connected, by means of ohmic gold electrodes, to two gold points and the d.c. resistance was continuously monitored by an Keithley electrometer Žmod. 617.. The whole experimental apparatus was interfaced to a PC equipped with a Pentium 133 microprocessor and a IEEE 488 board for acquiring and plotting the electrical response of each sensor in real time.
3. Results and discussion 3.1. X-ray results X-ray diffraction measurements performed onto our samples indicate that the as-deposited films are polycrystalline and the crystalline structure corresponds to the cassiterite tetragonal phase of the SnO 2 . A typical X-ray diffraction pattern of the films obtained on the various substrates is given in Fig. 1. The diffraction peaks and
371
Fig. 1. X-ray diffraction pattern for as deposited SnO 2 thin film.
their positions are associated with the reflections reported in the 41-1445 ASTM file. These results show that the crystalline structure of the deposited material is independent of the used substrate material. 3.2. AFM results The topographic plan views of the as-deposited and after gas expositions SnO 2 thin film are shown in Fig. 2Ža. and Žb.. The RMS roughness is Ž16.4 " 0.4. nm for the as-grown sample and it does not change after gas expositions and thermal treatment being Ž16.7 " 0.6. nm. On the contrary some difference can be revealed in the morphology of the two samples. The morphology of the as-deposited film consists of islands of rectangular shape Žlamellae. all elongated in the same direction ŽFig. 2a.. Hence, the sample surface seems to be composed by several layers of superposed lamellae each height about 10 nm. The mean dimensions of these lamellae are Ž0.20 " 0.01.mm for the major side and Ž0.10 " 0.01.mm for the minor side. The ratio of white pixel to black pixel, that is the ratio between film and voids in the sample, is 50%. This value gives a measure of the film density. After exposing the sample surface to various NO 2 gas concentration up to 5008C the surface morphology of the SnO 2 thin film appears to change. In fact, as it can be observed in the AFM image ŽFig. 2b. the lateral dimension of the lamellae are slightly decreased and the shape appear to be more rounded. Moreover the ratio filmrvoids increases from 50% up to 60% indicating an apparent increase in the film density. This is probably due to change in the morphology of the film when the sample works at temperature close to the deposition temperature. Further transmission electron microscopy measurements have
372
G. Leo et al.r Sensors and Actuators B 58 (1999) 370–374
Fig. 3. Relative resistance variation D R r R 0 as a function of the operating temperature for a typical sprayed SnO 2 thin films.
Fig. 2. AFM images of as-deposited SnO 2 thin films Ža. and after expositions to NO 2 gas at 5008C working temperature Žb..
absorption process is sufficiently fast and the relative resistance variation, defined as D RrR 0 s Ž R y R 0 .rR 0 , is most favourable. Here we define R and R 0 as the resistance of the sample in the presence of NO 2 gas and in dry-air respectively. Fig. 3 shows the response D RrR 0 to 500 ppm NO 2 of a sprayed SnO 2 thin film 230 nm thick as a function of the operating temperature. As one can see, the relative resistance variation increases with the temperature and reaches a maximum value in correspondence of T s 3508C. If the temperature increases again, the sensitivity decreases. This behaviour is currently explained taking into account that the oxide surface is populated by a variety of physisorbed and chemisorbed molecules. In y y particular, H 2 O, OHy, O 2 , Oy and Oy 2 , O , where O 2
shown that these lamellae are constitute of grains having dimension of about 10 nm. Systematic observations about the average dimension and structure of these grains by using selected area diffraction measurements will be object of the next paper. 3.3. NO2 sensing characteristics All the samples were submitted to the same experimental procedure. They were exposed to a dry air flow until their resistance had not reach an equilibrium value. Then the gas to be detected, diluted in dry air at a fixed concentration, was put into the cell and the samples left in the gas–air mixture until their resistance didn’t vary further. The active gas was then switched off and the sample left to recover in air. During the recovering time the resistance returns to its initial value proving the reversibility of the process. It is well known that for each coupling of material and gas, there is an operating temperature range in which the
Fig. 4. Electrical response of sprayed SnO 2 thin films vs. NO 2 concentration at 3508C working temperature.
G. Leo et al.r Sensors and Actuators B 58 (1999) 370–374
373
sensitivity temperature. As one can see, the response of the analysed films in a log–log plot is linear in the investigated NO 2 concentration range. Consequently, the resistance R gas is related to the gas concentration or, equivalently, to the partial pressure by means of a power law R gasrR air s k Ž PNO 2 .
n
Where P NO 2 is the partial pressure of the oxidising gas and n is a positive exponent which does not depend on P NO 2 but it is probably linked with the stoichiometry of the previous proposed reaction, whereas k is a constant. The exponent n, calculated from the slope of the straight line of Fig. 5, was found to be Ž0.34 " 0.01.. In reality, we have observed that the active layer undergoes a diminution in the response after repeated expositions to NO 2 gas and measurements cycles, then it became stable according to the calibration curve reported in Fig. 5. Fig. 5. Resistance change vs. NO 2 concentration for a typical sprayed SnO 2 film working at the maximum sensitivity temperature.
are likely to predominate. The presence of these species modulates the electrical behaviour of the active layer in the presence of the oxidising or reducing gases. At the temperature of the active layer of about 300–4008C the species y are dominant in the sensing mechanism. At Oy 2 and O these temperatures NO 2 can be absorbed or to interact with the oxygen adsorbed onto the sensing layer according to the reactions: NO 2 gas q ey™ NOy 2 y y y NO 2 gas q Oy 2 q 2e ™ NO 2 q 2O
These reactions reduce the electron concentration near the surface and consequently the resistance of the layer increases w11,12x. Moreover, NO 2 ions are desorbed as NO 2 gas flow is stopped and consequently a recovery of the initial condition takes place. Fig. 4 reports the dynamic change in the electrical resistance of a typical SnO 2 sprayed film at a working temperature of 3508C resulting from different NO 2 concentration in the mixture with dry-air. It is evident that the resistance of the film reaches its initial value when the sensor is exposed to pure dry-air. It is interesting to note that the response time and recovery time Ždefined as the times required to reach 90% of the steady-state values of R gas and R air , respectively. to 50 ppm NO 2 gas present in the mixture with dry air at 3508C working temperature, results to be 6 and 5 min respectively. Moreover we have observed that the response and recovering process are dependent on working temperature of the sensing layer. Fig. 5 shows a typical calibration curve, namely the ratio R gasrR air as a function of the gas concentration for a typical sprayed SnO 2 thin film working at a maximum
4. Conclusions In this work a preliminary characterisation shows that it is possible to obtain SnO 2 thin films with a high specific area by using the spray pyrolysis deposition technique. X-ray diffraction measurements have shown that the films are polycrystalline with a cassiterite tetragonal phase. The average dimension of the constituent grains having a rectangular shape was of about 0.20 mm for the major side and 0.10 mm for the minor side as evidenced by AFM observations. The films have shown encouraging NO 2 sensing characteristics. However, further analysis will be necessary in order to increase the performance of the active layer as a function of the deposition parameters.
Acknowledgements The authors wish to thank Mr. F. Casino for technical assistance during the measurements. This work was partially supported by CEE-FESR subprogram II, project ‘Materiali, Processi e Dispositivi per sensoristica, optoelettronica ed elettronica di potenza’.
References w1x S.R. Morrison, M.J. Madon, Chemical Sensing with Solid State Devices Academic Press, London, 1989. w2x T. Oyabu, Sensing characteristics of SnO 2 thin film gas sensors, J. Appl. Phys. 53 Ž1982. 2785. w3x H.M. Lin, C.M. Hsu, H.Y. Yang, P.Y. Lee, C.C. Yang, Nanocrystalline WO 3 -based H 2 S sensors, Sensors and Actuators B 22 Ž1994. 63–68. w4x F. Boccuzzi, E. Guglielmetti, A. Chiorino, IR study of gas sensor materials: NO interaction on ZnO and TiO 2 pure or modified by metals, Sensors and Actuators B 7 Ž1992. 645. w5x K.D. Schierbaum, U. Weimar, W. Gopel, Comparison of ceramics,
374
G. Leo et al.r Sensors and Actuators B 58 (1999) 370–374
thick and thin film chemical sensors based upon SnO 2 , Sensors and Actuators B 7 Ž1992. 709–716. w6x M.C. Horrillo, P. Serrini, J. Santos, L. Manes, Influence of the deposition conditions of SnO 2 thin films by reactive sputtering on the sensitivity to urban pollutants, Sensors and Actuators B 45 Ž1997. 193. w7x M. Di Giulio, G. Micocci, A. Serra, A. Tepore, R. Rella, P. Siciliano, SnO 2 thin films for gas sensors prepared by r.f. reactive sputtering, Sensors and Actuators B 24–25 Ž1995. 465–468. w8x A. Wilson, J. Wright, J. Murphy, Ma.M. Stroud, S.C. Thorpe, Sol–gel materials for gas sensing applications, Sensors and Actuators B 18–19 Ž1994. 506.
w9x M. de la L. Olvera, R. Asamoza, SnO 2 and SnO 2 : Pt thin films used as gas sensors, Sensors and Actuators B 45 Ž1997. 49–53. w10x J. Aranovich, A. Ortiz, R.H. Bube, Optical and electrical properties of ZnO films prepared by spray pyrolysis for solar cell applications, J. Vac. Sci. Technol. 16 Ž1979. 994–1003. w11x I. Sayago, J. Gutierrez, L. Ares, J.I. Robla, M.C. Horrillo, J. Getino, J.A. Agapito, The interaction of different oxidising agents on doped tin oxide, Sensors and Actuators B 24–25 Ž1995. 512–515. w12x B. Ruhland, Th. Becker, G. Muller, Gas-kinetic interactions of nitrous oxides with SnO 2 surfaces, Sensors and Actuators B 50 Ž1998. 85.