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Micro gap effect on dilute H2S sensing properties of SnO2 thin film microsensors
Available online at www.sciencedirect.com
Sensors and Actuators B 130 (2008) 400–404
Micro gap effect on dilute H2S sensing properties of SnO2 thin film microsensors Jun Tamaki a,∗ , Yoshinori Nakataya a , Satoshi Konishi b a
b
Department of Applied Chemistry, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu-shi, Shiga 525-8577, Japan Department Micro System Technology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu-shi, Shiga 525-8577, Japan Available online 19 September 2007
Abstract The micro gap effect has been investigated on dilute H2 S sensing using SnO2 thin film microsensors equipped with Au or Pt micro gap electrode. The micro gap electrode was fabricated by means of MEMS techniques (photolithography and FIB) and the gap size was varied in the range of 0.1–1 m. For the SnO2 sensors equipped with Au micro gap electrode, the sensor response to dilute H2 S was slightly increased with decreasing gap size, suggesting the small micro gap effect. The sensor response was divided into that at oxide–electrode interface (Si ) and at oxide grain boundary (Sgb ). It was found that the small Sgb /Si ratio was responsible for the small micro gap effect and that the high sensor response at grain boundary was characteristic in SnO2 –H2 S system. On the other hand, the SnO2 sensors with Pt micro gap electrode, the clear and large micro gap effect was obtained on dilute H2 S sensing. In the Pt electrode sensor, the large Sgb /Si value induced the large micro gap effect. It was found that the Pt/SnO2 interface showed much higher sensor response than the Au/SnO2 interface. The electrode material strongly affected the sensing properties of semiconductor gas sensor with micro gap electrode. © 2007 Elsevier B.V. All rights reserved. Keywords: SnO2 thin film microsensor; Dilute H2 S detection; Micro gap effect; MEMS
1. Introduction Recently, we have found the micro gap effect in semiconductor gas sensors [1]. That is, the sensor response was increased with decreasing gap size of metallic electrode for resistance measurement of oxide sensing layer when the gap size was less than 1 m. This behavior has been observed in NO2 sensing using WO3 sensor [1,2] and Cl2 gas sensing using In2 O3 sensor [3,4] using the micro gap electrode made of Au. And this behavior resulted from the increasing contribution of oxide–electrode interface with decreasing gap size and the high sensor response at oxide–electrode interface. Moreover, the importance of electrode nano design was proposed for development of high sensitivity gas sensors. Since the previous results were the cases showing the response of resistance increase, the micro gap effect was extended to the case of resistance decrease in this study. We chose H2 S sensing using SnO2 thin film microsensors since high sensitivity H2 S sensors were needed for bad-smelling gas detection and SnO2
∗
Corresponding author. Tel.: +81 77 561 2862; fax: +81 77 561 2659. E-mail address: [email protected] (J. Tamaki).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.09.041
was excellent sensing material for H2 S sensing [5–8]. Further, the micro gap effect for Pt electrode was investigated in order to elucidate the nature of interface between SnO2 and metallic electrode. 2. Experimental The micro gap electrode was fabricated by means of MEMS techniques (photolithography and FIB). The Au or Pt line of 50 m in width was designed on SiO2 substrate by using photolithography. Prior to the lift-off process, the Au or Pt film was deposited on SiO2 substrate by means of evaporation or sputtering, respectively. Then, the Au or Pt line was cut by using FIB (focused ion beam) technique to be micro gap electrode with gap size of 0.1–1 m. The micro-drop of SnO2 sol solution (donated from Nissan Chemical Ind. Co. Ltd.) was dropped on the micro gap electrode by using micromanipulator, dried, and calcined at 600 ◦ C for 3 h in air to be oxide thin film microsensor. The detailed procedure of microsensor fabrication was described in our previous papers [1–4]. The resistance of oxide thin film was measured between electrodes with micro gap in air (Ra) as well as in H2 S-containing air (Rg) at 300 ◦ C. Ra and Rg mean the total sensor resistances
J. Tamaki et al. / Sensors and Actuators B 130 (2008) 400–404
401
Fig. 1. SEM images of SnO2 thin film microsensor equipped with 0.5 m micro gap electrode. Magnifications: (a) 10,000; (b) 400,000.
of SnO2 thin film microsensor with micro gap electrode. The H2 S concentration was varied from 0.01 to 3 ppm. The sensor response (S) was defined as Rg/Ra for the easy calculation of sensor response at interface (Si ) and at grain boundary (Sgb ), which were mentioned after. 3. Results and discussion 3.1. Surface morphology of SnO2 thin film microsensors Fig. 1 shows the SEM images of SnO2 thin film deposited on micro gap electrode with 0.5 m gap. The SnO2 grains had spherical shape of 17 nm in diameter (Fig. 1(b)) and were uniformly deposited on whole of micro gap. The steps in SnO2 thin film depicted the edge of Au micro electrode. The 0.5 m gap was seen from the hollow of SnO2 thin film in Fig. 1(a). The crack was always observed in SnO2 thin film at the center of micro gap. 3.2. Sensing properties to dilute H2 S
Fig. 2. Response transient to 3 ppm H2 S of the SnO2 thin film microsensor with Au micro gap electrode of 0.1 m at 300 ◦ C.
Fig. 2 shows the response transient to 3 ppm H2 S of SnO2 thin film microsensor with 0.1 m gap electrode at 300 ◦ C. The sensor resistance was decreased upon exposure to 3 ppm H2 S, and the sensor response (Rg/Ra) was 1.4 × 10−3 . Since the SnO2 thin film microsensors showed the response of resistance decrease to H2 S, the sensor response (Rg/Ra) was less than unity and the larger deviation from unity (the smaller Rg/Ra value) means the higher response. If the sensor response is expressed as Ra/Rg, it is as high as 710. The 90% response time of output signal was as short as 1 min, and the 70% recovery time was 1 min. The calibration curve at 300 ◦ C was depicted in Fig. 3. The sensor response (Rg/Ra) was decreased with increasing H2 S concentration. And the linear relation between the sensor response and gas concentration was obtained in log–log plot in the concentration range of 0.01–3 ppm. 3.3. Micro gap effect on H2 S sensing The sensor responses (SR = g/Ra) of SnO2 thin film microsensors are shown in Fig. 4 as a function of gap size of Au
Fig. 3. Sensor response (Rg/Ra) of SnO2 thin film microsensor with Au micro gap electrode of 0.1 m at 300 ◦ C as a function of H2 S concentration.
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Fig. 4. Sensor responses to dilute H2 S of SnO2 thin film microsensors with Au micro gap electrode at 300 ◦ C as a function of gap size.
micro gap electrode. The measurements were carried out four times with different microsensors. The average was plotted and the error bar means the deviation. The cracks observed in SnO2 thin film may influence the deviation. As seen from Fig. 4, for all concentrations examined here, the sensor response (S = Rg/Ra) was slightly decreased when the gap size was decreased less than 0.5 m. The extent of decrease in sensor response, i.e., the micro gap effect, was decreased with decreasing H2 S concentration. To 0.1 ppm H2 S, the sensor response was almost constant irrespective of gap size. In order to explain the micro gap effect, the simple model where a few oxide grains are linearly packed between micro gap electrodes is considered as shown in Fig. 5. Between electrodes, there are two kinds of resistance which changes with gas exposure, i.e., the resistances at oxide–electrode interface (Ri ) and at grain boundary (Rgb ). Total sensor resistance (Rsensor ) is expressed by using Ri and Rgb , Rsensor = 2Ri + (N − 1)Rgb , where N is the number of oxide grains in the gap. From this equation, Rsensor in air is proportional to N. Indeed, Ra was plotted against N as shown in Fig. 6 and the linear relation was obtained between Ra and N. Thus, Rai and Ragb values were estimated from slope and intercept in Fig. 6, and the relation Rai = 5 Ragb was obtained. Rai and Ragb respond to dilute H2 S and the resistance changes at oxide–electrode interface and at grain boundary are defined as sensor response at oxide–electrode
Fig. 5. Model to explain the micro gap effect, where the oxide grains linearly packed between micro gap electrode.
Fig. 6. Relationship between Ra and N for SnO2 thin film microsensors with Au micro gap electrode of 0.1–1 m at 300 ◦ C.
interface (Si = Rgi /Rai ) and at grain boundary (Sgb = Rggb /Ragb ). The sensor response (S = Rg/Ra) means that of whole sensor system and is expressed as Eq. (1) by using Si , Sgb , and the relation Rai = 5 Ragb . S=
2Rgi + (N − 1)Rggb 10 Rg N −1 = = Si + Sgb Ra 2Rai + (N − 1)Ragb N +9 N +9
(1)
Si and Sgb can be calculated from experimental data set of S and N. The results are plotted in Fig. 7 as a function of H2 S concentration. Both Si and Sgb were linearly decreased with increasing H2 S concentration in log–log plot. Si was smaller than Sgb in all concentration range examined. The value of Sgb /Si ratio was 10 for 3 ppm H2 S and decreased to 3.4 for 0.1 ppm H2 S. These values were compared with that in previous results where the clear micro gap effect was obtained. Namely, Si /Sgb value was 32 in 0.5 ppm NO2 sensing using
Fig. 7. Calculated Si and Sgb values of SnO2 thin film microsensor with Au micro gap electrode as a function of H2 S concentration.
J. Tamaki et al. / Sensors and Actuators B 130 (2008) 400–404
Fig. 8. Sensor response to 3 ppm H2 S of the SnO2 thin film microsensors with Pt micro gap electrode of 0.1–1 m at 300 ◦ C as a function of gap size.
WO3 microsensor [1,2] and 42 in 1 ppm Cl2 sensing using In2 O3 microsensor [3,4] (note that in the cases of NO2 and Cl2 sensing, the ratio is reciprocal because of response of resistance increase). The Sgb /Si value was much smaller in SnO2 –H2 S system than in WO3 –NO2 and In2 O3 –Cl2 systems. It was found that the small Sgb /Si ratio was responsible for the weak micro gap effects in SnO2 –H2 S system. In this system, the small Sgb value (0.003 for 3 ppm H2 S) was obtained, inducing the small Sgb /Si ratio. Concerning the sensing mechanism, the resistance decrease results from the consumption of adsorbed oxygen on SnO2 grains in the oxidation of H2 S. It is considered that such an oxygen consumption induces the large amount of resistance change at grain boundary (Sgb ) as well as at oxide–electrode interface (Si ). 3.4. Effect of electrode material The electrode material was changed into Pt in order to investigate the interface properties between Pt and SnO2 . The micro gap electrodes made of Pt were fabricated with various gap sizes of 0.1–1 m, and the SnO2 thin film was deposited on the micro gap to be SnO2 thin film microsensors with Pt micro gap electrode. Fig. 8 shows the micro gap effect in the SnO2 microsensor with Pt electrode for the detection of 3 ppm H2 S at 300 ◦ C. The sensor response (Rg/Ra) was decreased with decreasing gap size and the clear and large micro gap effect was obtained for the
Table 1 Rai –Ragb relation, Si , Sgb , Sgb /Si values for SnO2 thin film microsensor
Rai –Ragb relation Si Sgb Sgb /Si
Pt electrode
Au electrode
Rai = 24 Ragb 0.000010 0.0080 770
Rai = 5 Ragb 0.00031 0.0031 10
403
SnO2 microsensors with Pt electrode. The same calculations as the case of SnO2 sensor with Au electrode were carried out, and the Rai –Ragb relation and the Si , Sgb parameters were calculated as listed in Table 1 for the SnO2 microsensor with Pt electrode. The results of the SnO2 microsensor with Au electrode were also listed for comparison. The solid line in Fig. 8 indicates the result of calculation due to Eq. (1) using Si , Sgb values in Table 1. The solid line fitted the experimental data very well, suggesting the successful calculation. As seen from Table 1, the large Sgb /Si value for Pt electrode induces the clear and large micro gap effect as discussed in Section 3.3. Comparing Si and Sgb values for both electrodes, Si value was much smaller for Pt electrode than Au electrode, indicating much higher sensor response at interface between Pt electrode and SnO2 grain to H2 S. The high activity in the H2 S oxidation of Pt would enhance the sensor response at interface. On the other hand, Sgb values were almost the same as each other, suggesting that the grain boundary conditions of SnO2 were the same between SnO2 sensor with Pt electrode and Au electrode. It was found that the electrode material influenced the interface properties between oxide grain and metallic electrode, and that the effect of electrode material strongly appeared in semiconductor gas sensor with micro gap electrode. 4. Conclusions The micro gap effect is investigated on dilute H2 S sensing using SnO2 thin film microsensors equipped with micro gap electrode of 0.1–1 m. The micro gap effect is very weak for the SnO2 sensor with Au electrode, while it is clear and large for the sensor with Pt electrode. The ratio (Sgb /Si ) of sensor response at grain boundary between grains (Sgb ) to that at interface between oxide grain and metallic electrode (Si ) determines the extent of micro gap effect, i.e., the larger Sgb /Si ratio brings about the larger micro gap effect. For the H2 S sensing using the SnO2 microsensor with Au electrode, the small Sgb /Si value results from the small Sgb value, indicating that the response of resistance decrease due to consumption of adsorbed oxygen in H2 S oxidation induces the high sensor response at grain boundary. On the other hand, the SnO2 microsensor with Pt electrode shows the clear and large micro gap effect. This results from the large Sgb /Si value due to very small Si value, suggesting the high sensor response at interface between Pt and SnO2 probably because Pt has high activity in H2 S oxidation. Acknowledgment This work was partly supported by Grant-in-Aid for Scientific Research and The 21st Century COE Program “Micro- NanoScience Integrated System” from The Ministry of Education, Culture, Sport and Technology of Japan. References [1] J. Tamaki, A. Miyaji, J. Makinodan, S. Ogura, S. Konishi, Effect of microgap electrode on detection of dilute NO2 using WO3 thin film microsensors, Sens. Actuators B 108 (2005) 202–206.
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[2] J. Tamaki, Nano-design of oxide particles and electrode structure for high sensitivity NO2 sensor using WO3 thick film, MRS Proc. 828 (2005) 25–35. [3] J. Tamaki, J. Niimi, S. Ogura, S. Konishi, Effect of micro-gap electrode on sensing properties to dilute chlorine gas of indium oxide thin film microsensors, Sens. Actuators B 117 (2006) 353–358. [4] J. Tamaki, J. Niimi, S. Konishi, Effect of calcinations temperature on interface properties between In2 O3 and Au electrode in nanogap semiconductor gas sensors”, MRS Proc. 915 (2006) 197–202. [5] M. Ando, S. Suto, T. Suzuki, T. Tsuchida, C. Nakayama, N. Miura, N. Yamazoe, H2 S-sensitive thin film fabricated from hydrothermally synthesized SnO2 sol, J. Mater. Chem. 4 (1994) 631–633. [6] D.J. Yoo, J. Tamaki, S.J. Park, N. Miura, N. Yamazoe, Effects of thickness and calcination temperature on tin dioxide sol-derived thin-film sensor, J. Electrochem. Soc. 142 (1995) L105–L107. [7] D.J. Yoo, J. Tamaki, S.J. Park, N. Miura, N. Yamazoe, H2 S sensing characteristics of SnO2 thin film prepared from SnO2 sol by spin coating, J. Mater. Sci. Lett. 14 (1995) 1391–1393. [8] T. Mochida, K. Kikuchi, T. Kondo, H. Ueno, Y. Matsuura, Highly sensitive and selective H2 S gas sensor from r.f. sputtered SnO2 thin film, Sens. Actuators B 25 (1995) 433–437.
Biographies Jun Tamaki has been a professor at Ritsumeikan University since 2002. He received the B. Eng. Degree in chemical engineering in 1983 and the Dr. Eng. Degree in 1988 from Osaka University. His current research interests include semiconductor gas sensors, synthesis of oxide thin film by solution process, and heterogeneous catalysts. Yoshinori Nakataya received the B. Eng. Degree in applied chemistry in 2006 from Ritsumeikan University. He is student of Graduate School of Science and Engineering, Ritsumeikan University, and investigating on SnO2 thin film microsensors with micro gap electrode for detection of various reducing gases. Satoshi Konishi received the BS degree in 1991 in Electronics Engineering, the MS degree in 1993 and the PhD degree in 1996 in Electrical Engineering, from the University of Tokyo. He has been a professor at Ritsumeikan University since 2006. His study is devoted to microelectromechanical systems (MEMS), especially distributed MEMS. His current research focuses on microactuator for advanced positioner, acoustic MEMS, and biomedical MEMS.
Sensors and Actuators B 130 (2008) 400–404
Micro gap effect on dilute H2S sensing properties of SnO2 thin film microsensors Jun Tamaki a,∗ , Yoshinori Nakataya a , Satoshi Konishi b a
b
Department of Applied Chemistry, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu-shi, Shiga 525-8577, Japan Department Micro System Technology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu-shi, Shiga 525-8577, Japan Available online 19 September 2007
Abstract The micro gap effect has been investigated on dilute H2 S sensing using SnO2 thin film microsensors equipped with Au or Pt micro gap electrode. The micro gap electrode was fabricated by means of MEMS techniques (photolithography and FIB) and the gap size was varied in the range of 0.1–1 m. For the SnO2 sensors equipped with Au micro gap electrode, the sensor response to dilute H2 S was slightly increased with decreasing gap size, suggesting the small micro gap effect. The sensor response was divided into that at oxide–electrode interface (Si ) and at oxide grain boundary (Sgb ). It was found that the small Sgb /Si ratio was responsible for the small micro gap effect and that the high sensor response at grain boundary was characteristic in SnO2 –H2 S system. On the other hand, the SnO2 sensors with Pt micro gap electrode, the clear and large micro gap effect was obtained on dilute H2 S sensing. In the Pt electrode sensor, the large Sgb /Si value induced the large micro gap effect. It was found that the Pt/SnO2 interface showed much higher sensor response than the Au/SnO2 interface. The electrode material strongly affected the sensing properties of semiconductor gas sensor with micro gap electrode. © 2007 Elsevier B.V. All rights reserved. Keywords: SnO2 thin film microsensor; Dilute H2 S detection; Micro gap effect; MEMS
1. Introduction Recently, we have found the micro gap effect in semiconductor gas sensors [1]. That is, the sensor response was increased with decreasing gap size of metallic electrode for resistance measurement of oxide sensing layer when the gap size was less than 1 m. This behavior has been observed in NO2 sensing using WO3 sensor [1,2] and Cl2 gas sensing using In2 O3 sensor [3,4] using the micro gap electrode made of Au. And this behavior resulted from the increasing contribution of oxide–electrode interface with decreasing gap size and the high sensor response at oxide–electrode interface. Moreover, the importance of electrode nano design was proposed for development of high sensitivity gas sensors. Since the previous results were the cases showing the response of resistance increase, the micro gap effect was extended to the case of resistance decrease in this study. We chose H2 S sensing using SnO2 thin film microsensors since high sensitivity H2 S sensors were needed for bad-smelling gas detection and SnO2
∗
Corresponding author. Tel.: +81 77 561 2862; fax: +81 77 561 2659. E-mail address: [email protected] (J. Tamaki).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.09.041
was excellent sensing material for H2 S sensing [5–8]. Further, the micro gap effect for Pt electrode was investigated in order to elucidate the nature of interface between SnO2 and metallic electrode. 2. Experimental The micro gap electrode was fabricated by means of MEMS techniques (photolithography and FIB). The Au or Pt line of 50 m in width was designed on SiO2 substrate by using photolithography. Prior to the lift-off process, the Au or Pt film was deposited on SiO2 substrate by means of evaporation or sputtering, respectively. Then, the Au or Pt line was cut by using FIB (focused ion beam) technique to be micro gap electrode with gap size of 0.1–1 m. The micro-drop of SnO2 sol solution (donated from Nissan Chemical Ind. Co. Ltd.) was dropped on the micro gap electrode by using micromanipulator, dried, and calcined at 600 ◦ C for 3 h in air to be oxide thin film microsensor. The detailed procedure of microsensor fabrication was described in our previous papers [1–4]. The resistance of oxide thin film was measured between electrodes with micro gap in air (Ra) as well as in H2 S-containing air (Rg) at 300 ◦ C. Ra and Rg mean the total sensor resistances
J. Tamaki et al. / Sensors and Actuators B 130 (2008) 400–404
401
Fig. 1. SEM images of SnO2 thin film microsensor equipped with 0.5 m micro gap electrode. Magnifications: (a) 10,000; (b) 400,000.
of SnO2 thin film microsensor with micro gap electrode. The H2 S concentration was varied from 0.01 to 3 ppm. The sensor response (S) was defined as Rg/Ra for the easy calculation of sensor response at interface (Si ) and at grain boundary (Sgb ), which were mentioned after. 3. Results and discussion 3.1. Surface morphology of SnO2 thin film microsensors Fig. 1 shows the SEM images of SnO2 thin film deposited on micro gap electrode with 0.5 m gap. The SnO2 grains had spherical shape of 17 nm in diameter (Fig. 1(b)) and were uniformly deposited on whole of micro gap. The steps in SnO2 thin film depicted the edge of Au micro electrode. The 0.5 m gap was seen from the hollow of SnO2 thin film in Fig. 1(a). The crack was always observed in SnO2 thin film at the center of micro gap. 3.2. Sensing properties to dilute H2 S
Fig. 2. Response transient to 3 ppm H2 S of the SnO2 thin film microsensor with Au micro gap electrode of 0.1 m at 300 ◦ C.
Fig. 2 shows the response transient to 3 ppm H2 S of SnO2 thin film microsensor with 0.1 m gap electrode at 300 ◦ C. The sensor resistance was decreased upon exposure to 3 ppm H2 S, and the sensor response (Rg/Ra) was 1.4 × 10−3 . Since the SnO2 thin film microsensors showed the response of resistance decrease to H2 S, the sensor response (Rg/Ra) was less than unity and the larger deviation from unity (the smaller Rg/Ra value) means the higher response. If the sensor response is expressed as Ra/Rg, it is as high as 710. The 90% response time of output signal was as short as 1 min, and the 70% recovery time was 1 min. The calibration curve at 300 ◦ C was depicted in Fig. 3. The sensor response (Rg/Ra) was decreased with increasing H2 S concentration. And the linear relation between the sensor response and gas concentration was obtained in log–log plot in the concentration range of 0.01–3 ppm. 3.3. Micro gap effect on H2 S sensing The sensor responses (SR = g/Ra) of SnO2 thin film microsensors are shown in Fig. 4 as a function of gap size of Au
Fig. 3. Sensor response (Rg/Ra) of SnO2 thin film microsensor with Au micro gap electrode of 0.1 m at 300 ◦ C as a function of H2 S concentration.
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Fig. 4. Sensor responses to dilute H2 S of SnO2 thin film microsensors with Au micro gap electrode at 300 ◦ C as a function of gap size.
micro gap electrode. The measurements were carried out four times with different microsensors. The average was plotted and the error bar means the deviation. The cracks observed in SnO2 thin film may influence the deviation. As seen from Fig. 4, for all concentrations examined here, the sensor response (S = Rg/Ra) was slightly decreased when the gap size was decreased less than 0.5 m. The extent of decrease in sensor response, i.e., the micro gap effect, was decreased with decreasing H2 S concentration. To 0.1 ppm H2 S, the sensor response was almost constant irrespective of gap size. In order to explain the micro gap effect, the simple model where a few oxide grains are linearly packed between micro gap electrodes is considered as shown in Fig. 5. Between electrodes, there are two kinds of resistance which changes with gas exposure, i.e., the resistances at oxide–electrode interface (Ri ) and at grain boundary (Rgb ). Total sensor resistance (Rsensor ) is expressed by using Ri and Rgb , Rsensor = 2Ri + (N − 1)Rgb , where N is the number of oxide grains in the gap. From this equation, Rsensor in air is proportional to N. Indeed, Ra was plotted against N as shown in Fig. 6 and the linear relation was obtained between Ra and N. Thus, Rai and Ragb values were estimated from slope and intercept in Fig. 6, and the relation Rai = 5 Ragb was obtained. Rai and Ragb respond to dilute H2 S and the resistance changes at oxide–electrode interface and at grain boundary are defined as sensor response at oxide–electrode
Fig. 5. Model to explain the micro gap effect, where the oxide grains linearly packed between micro gap electrode.
Fig. 6. Relationship between Ra and N for SnO2 thin film microsensors with Au micro gap electrode of 0.1–1 m at 300 ◦ C.
interface (Si = Rgi /Rai ) and at grain boundary (Sgb = Rggb /Ragb ). The sensor response (S = Rg/Ra) means that of whole sensor system and is expressed as Eq. (1) by using Si , Sgb , and the relation Rai = 5 Ragb . S=
2Rgi + (N − 1)Rggb 10 Rg N −1 = = Si + Sgb Ra 2Rai + (N − 1)Ragb N +9 N +9
(1)
Si and Sgb can be calculated from experimental data set of S and N. The results are plotted in Fig. 7 as a function of H2 S concentration. Both Si and Sgb were linearly decreased with increasing H2 S concentration in log–log plot. Si was smaller than Sgb in all concentration range examined. The value of Sgb /Si ratio was 10 for 3 ppm H2 S and decreased to 3.4 for 0.1 ppm H2 S. These values were compared with that in previous results where the clear micro gap effect was obtained. Namely, Si /Sgb value was 32 in 0.5 ppm NO2 sensing using
Fig. 7. Calculated Si and Sgb values of SnO2 thin film microsensor with Au micro gap electrode as a function of H2 S concentration.
J. Tamaki et al. / Sensors and Actuators B 130 (2008) 400–404
Fig. 8. Sensor response to 3 ppm H2 S of the SnO2 thin film microsensors with Pt micro gap electrode of 0.1–1 m at 300 ◦ C as a function of gap size.
WO3 microsensor [1,2] and 42 in 1 ppm Cl2 sensing using In2 O3 microsensor [3,4] (note that in the cases of NO2 and Cl2 sensing, the ratio is reciprocal because of response of resistance increase). The Sgb /Si value was much smaller in SnO2 –H2 S system than in WO3 –NO2 and In2 O3 –Cl2 systems. It was found that the small Sgb /Si ratio was responsible for the weak micro gap effects in SnO2 –H2 S system. In this system, the small Sgb value (0.003 for 3 ppm H2 S) was obtained, inducing the small Sgb /Si ratio. Concerning the sensing mechanism, the resistance decrease results from the consumption of adsorbed oxygen on SnO2 grains in the oxidation of H2 S. It is considered that such an oxygen consumption induces the large amount of resistance change at grain boundary (Sgb ) as well as at oxide–electrode interface (Si ). 3.4. Effect of electrode material The electrode material was changed into Pt in order to investigate the interface properties between Pt and SnO2 . The micro gap electrodes made of Pt were fabricated with various gap sizes of 0.1–1 m, and the SnO2 thin film was deposited on the micro gap to be SnO2 thin film microsensors with Pt micro gap electrode. Fig. 8 shows the micro gap effect in the SnO2 microsensor with Pt electrode for the detection of 3 ppm H2 S at 300 ◦ C. The sensor response (Rg/Ra) was decreased with decreasing gap size and the clear and large micro gap effect was obtained for the
Table 1 Rai –Ragb relation, Si , Sgb , Sgb /Si values for SnO2 thin film microsensor
Rai –Ragb relation Si Sgb Sgb /Si
Pt electrode
Au electrode
Rai = 24 Ragb 0.000010 0.0080 770
Rai = 5 Ragb 0.00031 0.0031 10
403
SnO2 microsensors with Pt electrode. The same calculations as the case of SnO2 sensor with Au electrode were carried out, and the Rai –Ragb relation and the Si , Sgb parameters were calculated as listed in Table 1 for the SnO2 microsensor with Pt electrode. The results of the SnO2 microsensor with Au electrode were also listed for comparison. The solid line in Fig. 8 indicates the result of calculation due to Eq. (1) using Si , Sgb values in Table 1. The solid line fitted the experimental data very well, suggesting the successful calculation. As seen from Table 1, the large Sgb /Si value for Pt electrode induces the clear and large micro gap effect as discussed in Section 3.3. Comparing Si and Sgb values for both electrodes, Si value was much smaller for Pt electrode than Au electrode, indicating much higher sensor response at interface between Pt electrode and SnO2 grain to H2 S. The high activity in the H2 S oxidation of Pt would enhance the sensor response at interface. On the other hand, Sgb values were almost the same as each other, suggesting that the grain boundary conditions of SnO2 were the same between SnO2 sensor with Pt electrode and Au electrode. It was found that the electrode material influenced the interface properties between oxide grain and metallic electrode, and that the effect of electrode material strongly appeared in semiconductor gas sensor with micro gap electrode. 4. Conclusions The micro gap effect is investigated on dilute H2 S sensing using SnO2 thin film microsensors equipped with micro gap electrode of 0.1–1 m. The micro gap effect is very weak for the SnO2 sensor with Au electrode, while it is clear and large for the sensor with Pt electrode. The ratio (Sgb /Si ) of sensor response at grain boundary between grains (Sgb ) to that at interface between oxide grain and metallic electrode (Si ) determines the extent of micro gap effect, i.e., the larger Sgb /Si ratio brings about the larger micro gap effect. For the H2 S sensing using the SnO2 microsensor with Au electrode, the small Sgb /Si value results from the small Sgb value, indicating that the response of resistance decrease due to consumption of adsorbed oxygen in H2 S oxidation induces the high sensor response at grain boundary. On the other hand, the SnO2 microsensor with Pt electrode shows the clear and large micro gap effect. This results from the large Sgb /Si value due to very small Si value, suggesting the high sensor response at interface between Pt and SnO2 probably because Pt has high activity in H2 S oxidation. Acknowledgment This work was partly supported by Grant-in-Aid for Scientific Research and The 21st Century COE Program “Micro- NanoScience Integrated System” from The Ministry of Education, Culture, Sport and Technology of Japan. References [1] J. Tamaki, A. Miyaji, J. Makinodan, S. Ogura, S. Konishi, Effect of microgap electrode on detection of dilute NO2 using WO3 thin film microsensors, Sens. Actuators B 108 (2005) 202–206.
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[2] J. Tamaki, Nano-design of oxide particles and electrode structure for high sensitivity NO2 sensor using WO3 thick film, MRS Proc. 828 (2005) 25–35. [3] J. Tamaki, J. Niimi, S. Ogura, S. Konishi, Effect of micro-gap electrode on sensing properties to dilute chlorine gas of indium oxide thin film microsensors, Sens. Actuators B 117 (2006) 353–358. [4] J. Tamaki, J. Niimi, S. Konishi, Effect of calcinations temperature on interface properties between In2 O3 and Au electrode in nanogap semiconductor gas sensors”, MRS Proc. 915 (2006) 197–202. [5] M. Ando, S. Suto, T. Suzuki, T. Tsuchida, C. Nakayama, N. Miura, N. Yamazoe, H2 S-sensitive thin film fabricated from hydrothermally synthesized SnO2 sol, J. Mater. Chem. 4 (1994) 631–633. [6] D.J. Yoo, J. Tamaki, S.J. Park, N. Miura, N. Yamazoe, Effects of thickness and calcination temperature on tin dioxide sol-derived thin-film sensor, J. Electrochem. Soc. 142 (1995) L105–L107. [7] D.J. Yoo, J. Tamaki, S.J. Park, N. Miura, N. Yamazoe, H2 S sensing characteristics of SnO2 thin film prepared from SnO2 sol by spin coating, J. Mater. Sci. Lett. 14 (1995) 1391–1393. [8] T. Mochida, K. Kikuchi, T. Kondo, H. Ueno, Y. Matsuura, Highly sensitive and selective H2 S gas sensor from r.f. sputtered SnO2 thin film, Sens. Actuators B 25 (1995) 433–437.
Biographies Jun Tamaki has been a professor at Ritsumeikan University since 2002. He received the B. Eng. Degree in chemical engineering in 1983 and the Dr. Eng. Degree in 1988 from Osaka University. His current research interests include semiconductor gas sensors, synthesis of oxide thin film by solution process, and heterogeneous catalysts. Yoshinori Nakataya received the B. Eng. Degree in applied chemistry in 2006 from Ritsumeikan University. He is student of Graduate School of Science and Engineering, Ritsumeikan University, and investigating on SnO2 thin film microsensors with micro gap electrode for detection of various reducing gases. Satoshi Konishi received the BS degree in 1991 in Electronics Engineering, the MS degree in 1993 and the PhD degree in 1996 in Electrical Engineering, from the University of Tokyo. He has been a professor at Ritsumeikan University since 2006. His study is devoted to microelectromechanical systems (MEMS), especially distributed MEMS. His current research focuses on microactuator for advanced positioner, acoustic MEMS, and biomedical MEMS.