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ITO-film gas sensor for measuring photodecomposition of NO2 gas
Surface and Coatings Technology 142᎐144 Ž2001. 781᎐785
ITO-film gas sensor for measuring photodecomposition of NO 2 gas T. Sako a , A. Ohmi a , H. Yumoto a,U , K. Nishiyamab a
Department of Materials Science and Technology, Science Uni¨ ersity of Tokyo, Noda, Chiba 278-8510, Japan b Department of Mechanical Engineering, Science Uni¨ ersity of Tokyo, Noda, Chiba 278-8510, Japan
Abstract ITO-film NO 2 gas sensors Žthickness s 15 nm. were prepared by DC magnetron sputtering. The highest sensitivity was obtained at the sensor temperature of 523 K. When the NO 2 gas flow in a measurement tube was terminated, the as-prepared ITO-film sensor showed a gradual decrease of NO 2 concentration without NO 2 gas leak. We attribute this phenomenon to the desorption of decomposed NO 2 from the sensor surface. By annealing the sensor in NO 2 gas at 573 K, the sensor showed no decrease of the concentration and could be used for detecting the photodecomposition of NO 2 . XPS revealed that annealing in NO 2 gas for 0.5 h caused a CO-like bond to be formed on the sensor surface. This may become a stable adsorption site for NO 2 , thus inhibiting decomposition. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: ITO; Gas sensor; Photocatalysis; NO 2 ; DC magnetron sputtering
1. Introduction NO x gas causes environmental pollution and is reduced by photocatalysis. Outdoors, it is transformed from toxic gas to HNO3 when diluted by rain. The toxicity of the diluted HNO3 liquid is lower than NO x . In general, NO x gas is composed of NO and NO 2 , and the NO 2 is more toxic than NO. It is important to measure the reaction rate of NO 2 in order to study the photocatalytic efficiency. Gas chromatography is used for such concentration measurements, but the cost is high; a simple method is required for studying photocatalysis. If molecules such as NO 2 adsorb on the surface of n-type semiconductors, they will be acceptors and the surface states of the semiconductor may carry a negative charge, which is screened by positive charge Žspace
U
Corresponding author. Tel.: q81-471-24-1501; fax: q81-471-239362. E-mail address: [email protected] ŽH. Yumoto..
charge. inside the semiconductor. This screening length w1x is on the order of atomic distances in metals with free-electron concentrations of 10 22 rcm3 , but is on the order of 100 nm in semiconductors with concentrations of 10 17 rcm3. This space charge layer has a low density of free electrons. As this space charge layer decreases the conductive cross-sectional area of the semiconductor, the resistance of the semiconductor increases. This phenomenon is applied to gas sensors, a sensor in the form of a thin film or fine particles is found to show high sensitivity. In general, an indium tin oxide ŽITO. film is used for transparent electrodes. Since ITO is an n-type semiconductor, an ITO-film can also be used as a highly sensitive NO x gas sensor w2x. This sensor is convenient and the cost is low. We have studied ITO-film NO 2 gas sensors for the measurement of NO 2 gas concentration, particularly with respect to destruction of NO 2 gas by photocatalysis w3x. For the study of the sensing properties of a gas sensor, a NO 2 gas flow is needed, but the flow is stopped to measure the photocatalytic properties in most cases. The response curve of the
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 1 0 7 - 0
782
T. Sako et al. r Surface and Coatings Technology 142᎐144 (2001) 781᎐785
sensor is affected by the existence of the gas flow. The purpose of this work is to study the effect of the termination of NO x gas flow in detail, in order to apply the ITO-film gas sensor to the measurement of photocatalysis.
2. Experimental ITO-films for use as NO 2 gas sensors were prepared by DC magnetron sputtering. The base pressure of the chamber was 2.0= 10y3 to 3.0= 10y3 Pa. The Ar and O 2 gas pressures were 5.0 and 20.0 Pa, respectively. The sputtering power was 350 V and 0.5 A. The composition of the ITO target was 90 wt.% In 2 O 3 and 10 wt.% SnO 2 . White slide glass Žsoda glass. plates were used as substrates with a temperature of 573 K. The deposition time was 60 s and the film thickness was 15 nm. An ITO-film Žthickness of 15 nm. deposited on glass was cut to the size of 10 = 10 mm and a Au electrode was deposited on each end of the ITO surface by DC sputtering. The net surface area of the sensor was 1 = 10 mm. Pt wires connected the Au electrodes of the sensor and the power source. The sensor was put into a glass tube Ž 37 = 400 mm.. The current of 10 mA was applied and the voltage between the electrodes was measured, using a digital voltmeter. The voltage changed depending on the amount of NO 2 adsorption. NO 2 was diluted with dry air to 10᎐400 ppm and was introduced into the tube. The sensor was heated by a cartridge heater to 573 K. In general, the sensitivity S of a sensor is defined as
Fig. 1. Repeated measurements of ITO-film NO 2 gas sensors. The sensor temperatures were Ža. 473, Žb. 523 and Žc. 573 K.
tration of NO 2 gas was 200 ppm and the number of repetitions was five. In general, the resistance of the sensor was increased by mixing NO 2 gas in pure airflow, which then decreased to approximately to its original value by restoring the pure airflow. The sensitivity of the sensor at 523 K was the highest and the response time was the shortest. Fig. 2 shows the relationship between the NO 2 concentration and sensitivity. The temperature of the sen-
S s R grR a , where R g and R a are the resistances of the sensor in NO 2 gas and in air, respectively. In order to apply the sensor to the detection of photodecomposition of NO 2 gas, two valves at either side of the tube were closed to stop the gas flow. A photocatalytic ZnO film Ž20 = 20 mm. was put into the tube, and an ultraviolet lamp Ž0.6 W. was used to irradiate the film through the tube. The ZnO film was prepared by electroplating Ž45 mArcm2 . in 0.1 molrl ZnŽNO 3 . 2 solution ŽpH 5.0. at 323 K. 3. Results and discussion 3.1. Sensing properties of ITO-film NO2 gas sensor 3.1.1. Measurement during NO2 gas flow Fig. 1 shows measurement repetitions of ITO-film NO 2 gas sensors. The temperatures of the sensors were Ža. 473, Žb. 523 and Žc. 573 K, respectively. The concen-
Fig. 2. Relationship between NO 2 concentration and sensitivity of ITO-film NO 2 gas sensor.
T. Sako et al. r Surface and Coatings Technology 142᎐144 (2001) 781᎐785
Fig. 3. The effect of the termination of NO 2 gas flow on the response of ITO-film NO 2 gas sensor: annealed in Ža. air and Žb. 100 ppm NO 2 . The annealing temperature was 573 K.
sor was 523 K. The sensitivity increased with increasing NO 2 concentration. 3.1.2. Effect of termination of NO2 flow Fig. 3 shows the responses of gas sensors to terminate NO 2 flow. The temperature of the sensors was 573 K. The measurements were repeated after annealing the sensors at 573 K in Ža. air and Žb. 100 ppm NO 2 . The resistance of the sensor increased to a certain value in NO 2 flow. When the valves of the measurement glass tube were closed to stop NO 2 gas flow, the resistance of the sensor decreased in the case of
783
Ža., but there was no leakage in this system. This apparent leak phenomenon will be discussed below. Annealing for 3 h in air corresponded to three repetitions of measurement. In this case, the amount of apparent leakage was decreased. However, this sensor was not sufficiently stable to measure the photodecomposition of NO 2 . The sensor in case Žb. was annealed in NO 2 gas. In this case, the resistance increased and remained constant after the termination of NO 2 flow. The response of the sensor in NO 2 flow was excellent. Annealing in NO 2 enables this sensor to be used to measure the photodecomposition of NO 2 gas with the termination of the gas flow. Fig. 4 shows O1s and C1s spectra of ITO-film surfaces, which were measured by XPS. Ar etching of the sample was not carried out before the XPS measurements. The spectra were for samples Ža. as-prepared, and annealed Žb. in air and Žc. in 100 ppm NO 2 . The annealing temperature and time were 573 K and 0.5 h. The peaks at 529.9 and 285.0 eV corresponded to In 2 O 3 w4x and the C᎐C bond, respectively. The C᎐C bond corresponded to organic material w5x. We considered that this contamination by C was mainly caused by reversed flow of diffusion pump oil. The amount of C was decreased by annealing in air, but the bonding energies of the peaks were not changed. New peaks of O at 530.5 eV and C at 293.0 eV appeared after annealing in NO 2 gas. There were few data on XPS peaks of CO and CO 2 , but the new peak value of C was near the CO bonding energy Ž291.9 eV. w6x. The amount of O in sample Žc. was higher compared with samples Ža. and Žb.. We considered that the extra O originated from the decomposition of NO 2 during annealing in NO 2 , and formed new chemical bonds, such as the CO-like bond, on the sensor surface. There were no clear differences in N1s, in 3d 5r2 and Sn 3d 5r2 spectra
Fig. 4. XPS spectra of O1s and C1s of ITO-film NO 2 gas sensor surface: Ža. as prepared and annealed Žb. in air Ž573 K, 0.5 h. and Žc. in 100 ppm NO 2 Ž573 K, 0.5 h..
784
T. Sako et al. r Surface and Coatings Technology 142᎐144 (2001) 781᎐785
among samples Ža. ᎐ Žc.. Since a stable response of the sensor was obtained only with sample Žc., it is considered that the new CO-like bonding sites become stable adsorption sites for NO 2 , which prevents the desorption of decomposed NO 2 gas. We discuss the difference in the response curves of the sensors. NO 2 molecules are very active and may decompose into other materials on adsorption sites w7x. If there are no stable adsorption sites for NO 2 on the as-prepared ITO-film sensor, some NO 2 molecules are decomposed and desorb from the sensor. This desorption effect inhibits the net adsorption of NO 2 . Therefore, the resistance increases very slowly in NO 2 flow. When the valves of the tube are closed to stop the NO 2 flow, the NO 2 concentration in the tube decreases due to the decomposition of NO 2 , and the resistance of the sensor decreases without gas leakage from the tube. Annealing in NO 2 gas results in stable adsorption sites on the sensor, consequently the decomposition of NO 2 molecules is reduced and can be neglected. The resistance increases rapidly when NO 2 mixing gas begins to flow and reaches a saturated value. The NO 2 concentration was not changed by terminating the NO 2 flow. When NO 2 gas flows into the tube, shear stress occurs within the gas due to viscosity flow, which enhances the desorption of NO 2 gas. Therefore, stopping the flow increases the number of NO 2 molecules adsorbed on the sensor in the tube, and the resistance of the sensor increases further and eventually saturates. This explains the response curve of the sample annealed in NO 2 gas. The resistance curves were improved by repeating the measurement after annealing in air, as shown in Fig. 4a. During one measurement, the sensor was held at 573 K in NO 2 for 300 s, which is equivalent to
annealing in NO 2 gas. The repetition of measurement cause, the stable adsorption sites for NO 2 to gradually form so that the resistance curve becomes stable. 3.2. Application to photocatalysis Fig. 5a shows the typical response of an annealed sensor to a change in the concentration of NO 2 with the termination of NO 2 flow. A photocatalytic ZnO as well as an ITO gas sensor was placed in the glass tube. After NO 2 gas flow becomes steady at 50 ppm, the valves at either side of the tube were closed. Then, the resistance increased and saturated. The same experiment was done with 75 and 100 ppm NO 2 . The calibration curve between the resistance and the concentration was obtained using the three saturated points. The starting point was the time at which ultraviolet illumination for photodecomposition of NO 2 commenced. Fig. 5b shows the photodecomposition curve of NO 2 , which was obtained from the curve of Fig. 5a. If the calibration curve is always obtained before the measurement of photocatalysis, the ITO-film NO 2 gas sensor can be used for measuring the photo-decomposition of NO 2 .
4. Conclusion
1. An ITO-film NO 2 gas sensor Žthickness of 15 nm. was prepared by DC magnetron sputtering. The highest sensitivity was obtained at the sensor temperature of 523 K. Reproducibility of the response of the sensor was improved by the repeating the measurement.
Fig. 5. Ža. Typical response of annealed ITO gas sensor. When the valves at either side of the photocatalytic tube were closed at NO 2 concentration of 50, 75 and 100 ppm, the resistance saturated. The calibration curve was obtained using these three points. The starting time was the time at which ultraviolet ŽUV. illumination commenced. Žb. Typical photodecomposition curve of NO 2 . This was obtained from the curve of Ža.. ZnO film was used as a photocatalyst.
T. Sako et al. r Surface and Coatings Technology 142᎐144 (2001) 781᎐785
2. When the NO 2 gas flow was terminated, the asprepared ITO-film sensor showed a gradual decrease of the NO 2 concentration without NO 2 gas leakage. This apparent gas leakage did not occur after annealing the sensor in 100 ppm NO 2 gas at 573 K for 0.5 h, and the sensor could be used for detecting photo-decomposition of NO 2 . XPS revealed that the CO-like bond sites formed by annealing in NO 2 gas were stable adsorption sites for NO 2 , which prevented the desorption of NO 2 gas from the sensor.
785
References w1x H. Luth, Surfaces and Interfaces of Solid Materials, Springer, Berlin, 1995, p. 316. w2x G. Sberveglieri, P. Benussi, G. Coccoli, S. Gropplli, P. Nelli, Thin Solid Films 186 Ž1990. 349. w3x H. Yumoto, T. Inoue, S.J. Li, T. Sako, K. Nishiyama, Thin Solid Films 345 Ž1999. 38. w4x J.C.C. Fan, J.B. Goodenough, J. Appl. Phys. 3527 Ž1977. 48. w5x N. Ikeo, Y. Iijima, N. Niimura et al., Handbook of X-Ray Photoelectron Spectroscopy, JEOL, Tokyo, 1991. w6x U. Gelius, P.F. Heden, J. Hedman et al., Phys. Scr. 2 Ž1970. 70. w7x C.R. Brundle, J. Vac. Sci. Technol. 13 Ž1976. 301.
ITO-film gas sensor for measuring photodecomposition of NO 2 gas T. Sako a , A. Ohmi a , H. Yumoto a,U , K. Nishiyamab a
Department of Materials Science and Technology, Science Uni¨ ersity of Tokyo, Noda, Chiba 278-8510, Japan b Department of Mechanical Engineering, Science Uni¨ ersity of Tokyo, Noda, Chiba 278-8510, Japan
Abstract ITO-film NO 2 gas sensors Žthickness s 15 nm. were prepared by DC magnetron sputtering. The highest sensitivity was obtained at the sensor temperature of 523 K. When the NO 2 gas flow in a measurement tube was terminated, the as-prepared ITO-film sensor showed a gradual decrease of NO 2 concentration without NO 2 gas leak. We attribute this phenomenon to the desorption of decomposed NO 2 from the sensor surface. By annealing the sensor in NO 2 gas at 573 K, the sensor showed no decrease of the concentration and could be used for detecting the photodecomposition of NO 2 . XPS revealed that annealing in NO 2 gas for 0.5 h caused a CO-like bond to be formed on the sensor surface. This may become a stable adsorption site for NO 2 , thus inhibiting decomposition. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: ITO; Gas sensor; Photocatalysis; NO 2 ; DC magnetron sputtering
1. Introduction NO x gas causes environmental pollution and is reduced by photocatalysis. Outdoors, it is transformed from toxic gas to HNO3 when diluted by rain. The toxicity of the diluted HNO3 liquid is lower than NO x . In general, NO x gas is composed of NO and NO 2 , and the NO 2 is more toxic than NO. It is important to measure the reaction rate of NO 2 in order to study the photocatalytic efficiency. Gas chromatography is used for such concentration measurements, but the cost is high; a simple method is required for studying photocatalysis. If molecules such as NO 2 adsorb on the surface of n-type semiconductors, they will be acceptors and the surface states of the semiconductor may carry a negative charge, which is screened by positive charge Žspace
U
Corresponding author. Tel.: q81-471-24-1501; fax: q81-471-239362. E-mail address: [email protected] ŽH. Yumoto..
charge. inside the semiconductor. This screening length w1x is on the order of atomic distances in metals with free-electron concentrations of 10 22 rcm3 , but is on the order of 100 nm in semiconductors with concentrations of 10 17 rcm3. This space charge layer has a low density of free electrons. As this space charge layer decreases the conductive cross-sectional area of the semiconductor, the resistance of the semiconductor increases. This phenomenon is applied to gas sensors, a sensor in the form of a thin film or fine particles is found to show high sensitivity. In general, an indium tin oxide ŽITO. film is used for transparent electrodes. Since ITO is an n-type semiconductor, an ITO-film can also be used as a highly sensitive NO x gas sensor w2x. This sensor is convenient and the cost is low. We have studied ITO-film NO 2 gas sensors for the measurement of NO 2 gas concentration, particularly with respect to destruction of NO 2 gas by photocatalysis w3x. For the study of the sensing properties of a gas sensor, a NO 2 gas flow is needed, but the flow is stopped to measure the photocatalytic properties in most cases. The response curve of the
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 1 0 7 - 0
782
T. Sako et al. r Surface and Coatings Technology 142᎐144 (2001) 781᎐785
sensor is affected by the existence of the gas flow. The purpose of this work is to study the effect of the termination of NO x gas flow in detail, in order to apply the ITO-film gas sensor to the measurement of photocatalysis.
2. Experimental ITO-films for use as NO 2 gas sensors were prepared by DC magnetron sputtering. The base pressure of the chamber was 2.0= 10y3 to 3.0= 10y3 Pa. The Ar and O 2 gas pressures were 5.0 and 20.0 Pa, respectively. The sputtering power was 350 V and 0.5 A. The composition of the ITO target was 90 wt.% In 2 O 3 and 10 wt.% SnO 2 . White slide glass Žsoda glass. plates were used as substrates with a temperature of 573 K. The deposition time was 60 s and the film thickness was 15 nm. An ITO-film Žthickness of 15 nm. deposited on glass was cut to the size of 10 = 10 mm and a Au electrode was deposited on each end of the ITO surface by DC sputtering. The net surface area of the sensor was 1 = 10 mm. Pt wires connected the Au electrodes of the sensor and the power source. The sensor was put into a glass tube Ž 37 = 400 mm.. The current of 10 mA was applied and the voltage between the electrodes was measured, using a digital voltmeter. The voltage changed depending on the amount of NO 2 adsorption. NO 2 was diluted with dry air to 10᎐400 ppm and was introduced into the tube. The sensor was heated by a cartridge heater to 573 K. In general, the sensitivity S of a sensor is defined as
Fig. 1. Repeated measurements of ITO-film NO 2 gas sensors. The sensor temperatures were Ža. 473, Žb. 523 and Žc. 573 K.
tration of NO 2 gas was 200 ppm and the number of repetitions was five. In general, the resistance of the sensor was increased by mixing NO 2 gas in pure airflow, which then decreased to approximately to its original value by restoring the pure airflow. The sensitivity of the sensor at 523 K was the highest and the response time was the shortest. Fig. 2 shows the relationship between the NO 2 concentration and sensitivity. The temperature of the sen-
S s R grR a , where R g and R a are the resistances of the sensor in NO 2 gas and in air, respectively. In order to apply the sensor to the detection of photodecomposition of NO 2 gas, two valves at either side of the tube were closed to stop the gas flow. A photocatalytic ZnO film Ž20 = 20 mm. was put into the tube, and an ultraviolet lamp Ž0.6 W. was used to irradiate the film through the tube. The ZnO film was prepared by electroplating Ž45 mArcm2 . in 0.1 molrl ZnŽNO 3 . 2 solution ŽpH 5.0. at 323 K. 3. Results and discussion 3.1. Sensing properties of ITO-film NO2 gas sensor 3.1.1. Measurement during NO2 gas flow Fig. 1 shows measurement repetitions of ITO-film NO 2 gas sensors. The temperatures of the sensors were Ža. 473, Žb. 523 and Žc. 573 K, respectively. The concen-
Fig. 2. Relationship between NO 2 concentration and sensitivity of ITO-film NO 2 gas sensor.
T. Sako et al. r Surface and Coatings Technology 142᎐144 (2001) 781᎐785
Fig. 3. The effect of the termination of NO 2 gas flow on the response of ITO-film NO 2 gas sensor: annealed in Ža. air and Žb. 100 ppm NO 2 . The annealing temperature was 573 K.
sor was 523 K. The sensitivity increased with increasing NO 2 concentration. 3.1.2. Effect of termination of NO2 flow Fig. 3 shows the responses of gas sensors to terminate NO 2 flow. The temperature of the sensors was 573 K. The measurements were repeated after annealing the sensors at 573 K in Ža. air and Žb. 100 ppm NO 2 . The resistance of the sensor increased to a certain value in NO 2 flow. When the valves of the measurement glass tube were closed to stop NO 2 gas flow, the resistance of the sensor decreased in the case of
783
Ža., but there was no leakage in this system. This apparent leak phenomenon will be discussed below. Annealing for 3 h in air corresponded to three repetitions of measurement. In this case, the amount of apparent leakage was decreased. However, this sensor was not sufficiently stable to measure the photodecomposition of NO 2 . The sensor in case Žb. was annealed in NO 2 gas. In this case, the resistance increased and remained constant after the termination of NO 2 flow. The response of the sensor in NO 2 flow was excellent. Annealing in NO 2 enables this sensor to be used to measure the photodecomposition of NO 2 gas with the termination of the gas flow. Fig. 4 shows O1s and C1s spectra of ITO-film surfaces, which were measured by XPS. Ar etching of the sample was not carried out before the XPS measurements. The spectra were for samples Ža. as-prepared, and annealed Žb. in air and Žc. in 100 ppm NO 2 . The annealing temperature and time were 573 K and 0.5 h. The peaks at 529.9 and 285.0 eV corresponded to In 2 O 3 w4x and the C᎐C bond, respectively. The C᎐C bond corresponded to organic material w5x. We considered that this contamination by C was mainly caused by reversed flow of diffusion pump oil. The amount of C was decreased by annealing in air, but the bonding energies of the peaks were not changed. New peaks of O at 530.5 eV and C at 293.0 eV appeared after annealing in NO 2 gas. There were few data on XPS peaks of CO and CO 2 , but the new peak value of C was near the CO bonding energy Ž291.9 eV. w6x. The amount of O in sample Žc. was higher compared with samples Ža. and Žb.. We considered that the extra O originated from the decomposition of NO 2 during annealing in NO 2 , and formed new chemical bonds, such as the CO-like bond, on the sensor surface. There were no clear differences in N1s, in 3d 5r2 and Sn 3d 5r2 spectra
Fig. 4. XPS spectra of O1s and C1s of ITO-film NO 2 gas sensor surface: Ža. as prepared and annealed Žb. in air Ž573 K, 0.5 h. and Žc. in 100 ppm NO 2 Ž573 K, 0.5 h..
784
T. Sako et al. r Surface and Coatings Technology 142᎐144 (2001) 781᎐785
among samples Ža. ᎐ Žc.. Since a stable response of the sensor was obtained only with sample Žc., it is considered that the new CO-like bonding sites become stable adsorption sites for NO 2 , which prevents the desorption of decomposed NO 2 gas. We discuss the difference in the response curves of the sensors. NO 2 molecules are very active and may decompose into other materials on adsorption sites w7x. If there are no stable adsorption sites for NO 2 on the as-prepared ITO-film sensor, some NO 2 molecules are decomposed and desorb from the sensor. This desorption effect inhibits the net adsorption of NO 2 . Therefore, the resistance increases very slowly in NO 2 flow. When the valves of the tube are closed to stop the NO 2 flow, the NO 2 concentration in the tube decreases due to the decomposition of NO 2 , and the resistance of the sensor decreases without gas leakage from the tube. Annealing in NO 2 gas results in stable adsorption sites on the sensor, consequently the decomposition of NO 2 molecules is reduced and can be neglected. The resistance increases rapidly when NO 2 mixing gas begins to flow and reaches a saturated value. The NO 2 concentration was not changed by terminating the NO 2 flow. When NO 2 gas flows into the tube, shear stress occurs within the gas due to viscosity flow, which enhances the desorption of NO 2 gas. Therefore, stopping the flow increases the number of NO 2 molecules adsorbed on the sensor in the tube, and the resistance of the sensor increases further and eventually saturates. This explains the response curve of the sample annealed in NO 2 gas. The resistance curves were improved by repeating the measurement after annealing in air, as shown in Fig. 4a. During one measurement, the sensor was held at 573 K in NO 2 for 300 s, which is equivalent to
annealing in NO 2 gas. The repetition of measurement cause, the stable adsorption sites for NO 2 to gradually form so that the resistance curve becomes stable. 3.2. Application to photocatalysis Fig. 5a shows the typical response of an annealed sensor to a change in the concentration of NO 2 with the termination of NO 2 flow. A photocatalytic ZnO as well as an ITO gas sensor was placed in the glass tube. After NO 2 gas flow becomes steady at 50 ppm, the valves at either side of the tube were closed. Then, the resistance increased and saturated. The same experiment was done with 75 and 100 ppm NO 2 . The calibration curve between the resistance and the concentration was obtained using the three saturated points. The starting point was the time at which ultraviolet illumination for photodecomposition of NO 2 commenced. Fig. 5b shows the photodecomposition curve of NO 2 , which was obtained from the curve of Fig. 5a. If the calibration curve is always obtained before the measurement of photocatalysis, the ITO-film NO 2 gas sensor can be used for measuring the photo-decomposition of NO 2 .
4. Conclusion
1. An ITO-film NO 2 gas sensor Žthickness of 15 nm. was prepared by DC magnetron sputtering. The highest sensitivity was obtained at the sensor temperature of 523 K. Reproducibility of the response of the sensor was improved by the repeating the measurement.
Fig. 5. Ža. Typical response of annealed ITO gas sensor. When the valves at either side of the photocatalytic tube were closed at NO 2 concentration of 50, 75 and 100 ppm, the resistance saturated. The calibration curve was obtained using these three points. The starting time was the time at which ultraviolet ŽUV. illumination commenced. Žb. Typical photodecomposition curve of NO 2 . This was obtained from the curve of Ža.. ZnO film was used as a photocatalyst.
T. Sako et al. r Surface and Coatings Technology 142᎐144 (2001) 781᎐785
2. When the NO 2 gas flow was terminated, the asprepared ITO-film sensor showed a gradual decrease of the NO 2 concentration without NO 2 gas leakage. This apparent gas leakage did not occur after annealing the sensor in 100 ppm NO 2 gas at 573 K for 0.5 h, and the sensor could be used for detecting photo-decomposition of NO 2 . XPS revealed that the CO-like bond sites formed by annealing in NO 2 gas were stable adsorption sites for NO 2 , which prevented the desorption of NO 2 gas from the sensor.
785
References w1x H. Luth, Surfaces and Interfaces of Solid Materials, Springer, Berlin, 1995, p. 316. w2x G. Sberveglieri, P. Benussi, G. Coccoli, S. Gropplli, P. Nelli, Thin Solid Films 186 Ž1990. 349. w3x H. Yumoto, T. Inoue, S.J. Li, T. Sako, K. Nishiyama, Thin Solid Films 345 Ž1999. 38. w4x J.C.C. Fan, J.B. Goodenough, J. Appl. Phys. 3527 Ž1977. 48. w5x N. Ikeo, Y. Iijima, N. Niimura et al., Handbook of X-Ray Photoelectron Spectroscopy, JEOL, Tokyo, 1991. w6x U. Gelius, P.F. Heden, J. Hedman et al., Phys. Scr. 2 Ž1970. 70. w7x C.R. Brundle, J. Vac. Sci. Technol. 13 Ž1976. 301.