Pulse-Driven Semiconductor Gas Sensors Toward ppt Level Toluene

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Pulse-Driven Semiconductor Gas Sensors Toward ppt Level Toluene Detection Koichi Suematsu, Wataru Harano, Tokiharu Oyama, Yuka Shin, Ken Watanabe, and Kengo Shimanoe Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03076 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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Analytical Chemistry

Pulse-Driven Semiconductor Gas Sensors Toward ppt Level Toluene Detection

Koichi Suematsu †*, Wataru Harano ‡, Tokiharu Oyama ‡, Yuka Shin ‡, Ken Watanabe †, Kengo Shimanoe †

†

Department of Advanced Materials Science and Engineering, Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

‡

Department of Molecular and Material Science, Interdisciplinary Graduate School of Engineering Science, Kyushu University, Kasuga, Fukuoka, 816-8580, Japan

ACS Paragon Plus Environment

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Improvements in the responses of semiconductor gas sensors and reductions in their detection limits toward volatile organic compounds (VOCs) are required in order to facilitate the simple detection of diseases, such as cancer, through human-breath analysis. In this study, we introduce a heater-switching, pulse-driven, micro gas sensor composed of a microheater and a sensor electrode fabricated with Pd-SnO2-clustered nanoparticles as the sensing material. The sensor was repeatedly heated and allowed to cool by the application of voltage to the microheater; the VOC gases penetrate into the interior of the sensing layer during its unheated state. Consequently, the utility factor of the pulse-driven sensor was greater than that of a conventional, continuously heated sensor. As a result, the response of the sensor to toluene was enhanced; indeed, the sensor responded to toluene at levels of 1 ppb. In addition, according to the relationship between its response and concentration toluene, the pulse-driven sensor in this report can detect toluene at concentrations of 200 ppt and even lower. Therefore, the combination of a pulse-driven microheater and a suitable material designed to detect toluene resulted in improved sensor response, and facilitated ppt-level toluene detection. This sensor may play a key role in the development of medical diagnoses based on human breath.

Keyword: MEMS, pulse-driven, resistive-type gas sensor, SnO2, Pd loading, toluene, ppt level


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Resistive-type semiconductor gas sensors are widely used to detect flammable gases and to monitor air quality. In particular, SnO2-based gas sensors are attractive because of their high sensitivities to flammable gases (ppm level),1,2 and their potential to be both compact and low power consuming device.3-5 The analysis of human breath using semiconductor gas sensors, for the screening of cancers, has been anticipated in this decade.6-10 Itoh et al. reported that gaseous volatile organic compounds (VOCs), such as toluene, isoprene, and acetic acid, can be used as marker gases for lung-cancer screening.10 According to their report, the concentrations of these gases in human breath are at the ppb level, and that sub-ppb-level detection resolution is necessary for lung-cancer screening. However, to the best of our best knowledge, the detection limit for gaseous toluene using a semiconductor gas sensor is a few ppb.11,12 Hence, improving the gas-detection limit to sub-ppb levels represents a significant development for the screening of lung cancer based on human breath analysis without the need for an expensive and large analysis system and expert knowledge. SnO2-based semiconductor gas sensors detect flammable gases based on changes in electrical resistance that are caused by reactions between the chemisorbed oxygen on the SnO2 surface and the gas species. Enhancing catalytic activity toward gas oxidation on the particle surface and gas diffusion deep into the sensing layer through the introduction of large pores improve sensitivity to VOC gases.11,13-16 Generally, surface reactions between adsorbed oxygen and VOCs are activated by novel metals that are loaded on the particle surfaces.17,18 However, in contrast, activation of the gas-oxidation process hinders gas diffusion into the sensing layer because the flammable gases are oxidized on its surface.19,20 Consequently, there is a trade-off relationship


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between activation of the gas-oxidation process and improvements in gas diffusion; sensitivity toward VOC gases can be improved by judiciously considering these two factors. Recently, pulse-driven micro gas sensors that are compact and low power consuming have been reported.21,22 We previously introduced SnO2-based pulse-driven gas sensors that use microheaters and electrodes mounted by micro-electronic-mechanical-system (MEMS) techniques that repeatedly heat the microheater every second.23 In other words, this sensor is heated for one second and then allowed to cool for one second following heating, with these steps continuously repeated. In this driving mode, catalytic activation on the particle surfaces only proceeds during the heating-on phase, while flammable gases penetrate into the sensing layer during the heating-off phase. Schematic diagrams that depict this pulse-driven gas-diffusion behavior in the sensing layer using SnO2 nanoparticles (NPs) are displayed in Figure 1. In other words, this pulse-driven gas sensor operates by tucking the target gas into the sensing layer. In previous work Sasahara et al. reported that catalytic combustion sensor in pulse-heating mode toward VOC gases shows large response peak before reaching a steady state.24 Unfortunately, we did not focus on the phenomenon described above in our previous report.23 The pulse-driven gas sensor appears to combine the required two key factors, namely activation of the gas-oxidation process and gas penetration into the sensing layer, facilitating improvements in sensitivity toward flammable gases and decreasing the concentration detection limits. In this study, we enhanced the response of a sensor toward toluene through the use of a pulse-driven semiconductor gas sensor mounted with SnO2 NPs. In addition, we examined the


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possibility of decreasing the gas-detection limits of resistive-type semiconductor gas sensors using Pd-loaded SnO2-clustered nanoparticles (CNPs), which is the material that is most sensitive toward toluene gas.11 It is noteworthy that the pulse-driven Pd-SnO2-CNP gas sensor exhibited responses toward toluene at levels of a few ppb. Our results also reveal that the toluene detection limit of this gas sensor was at the few-hundred-ppt level. This indicates that the resistive-type semiconductor gas sensor has the potential to screen for lung cancer as a cost-effective and compact human-breath analyzer.

EXPERIMENTAL SECTION The details of the materials preparation can be found in the Supporting Information. Evaluation of gas sensing properties The MEMS-type microsensor device was provided by Figaro Engineering. A 1:1 (w/w) mixture of the powder and glycerin was prepared as the paste for the sensing layer, after which micro droplets of the paste were mounted onto the device with a capillary tube and a micro manipulation system (PatchMan NP2, Eppendorf), as shown in Figure S-1a. The sensor was dried at 180 °C for 12 h under vacuum to remove the glycerin. The microsensor was subsequently attached to the sensor measurement system as shown in Figure S-1b, and sintered at 450 °C for 12 h with a microheater under a flow of synthetic air. The sensor device obtained in this manner was examined by field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi). The gas-electrical properties of the sensor were examined using a conventional gas-flow


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apparatus, as shown in Figure S-1b. The temperature of the device was controlled by adjusting the voltage that was applied to the microheater using an analog output module (USB-9263, National Instruments). The relationship between the applied voltage and the heater temperature is displayed in Figure S-1c. The heater was driven in pulse-heating mode, in which it was repeatedly switched on and off, as shown in Figure S-1d. The electrical resistance was monitored every 0.05 second when the heater was on. The target gas was composed of 20 ppm toluene and 200 ppm hydrogen in synthetic air. Additionally, we tested the sensor response toward 1-8 ppb toluene in synthetic air, in which the base gas, 10 ppb toluene in nitrogen, was mixed with pure oxygen and nitrogen. The voltage across a standard resistor at an applied DC voltage of 4 V was measured to determine the electrical resistance of the sensor when pulse-driven at various applied heater voltages. The voltage across the standard resistor was measured, and the DC 4 V was applied using a multifunctional input/output device (USB-6289, National Instruments). The sensor response (S) is defined as the ratio of the electrical resistance in synthetic air (Ra) to that in the target gas (Rg); i.e. S = Ra/Rg; in addition, we also defined two different sensor responses at the beginnings and ends of heating pulses, as described in the Results and Discussion section.

RESULTS AND DISCUSSION The photographic and SEM images of the MEMS-type sensor device shown in Figure 2a clearly reveal that the device, which is constructed of a microheater and a sensor electrode, is square, with 100-µm side lengths. A small 40-µm-square sensor electrode is enclosed by a microheater.


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Top- and side-views of the microsensor on which the SnO2 NPs are mounted are shown in Figures 2b-d. The microheater and electrode are entirely covered by the SnO2 NPs, and the microsensor is dome-shaped to a height of about 40 µm. In addition, the sensing layer contains large pores that were probably produced by the evaporation of glycerine. The average pore radius, determined using a pore-size-distribution analyzer (BELSORP-mini II, Bel Japan), is about 5.3 nm,11 which is difficult to see in the SEM image. These pores are very important for the diffusion of large gas molecules, such as toluene.11,16 Figure 3a shows the transient response of the pulse-driven SnO2 microsensor to 20-ppm toluene when 1.04 V (~250 °C) was applied to the heater. During these measurements, the heater-on and -off phases were maintained for 5 and 10 s, respectively. The electrical resistance immediately decreased upon the introduction of toluene, and then increased again upon its removal from the atmosphere. The electrical-resistance-recovery behavior after exposure to toluene in the atmosphere was very slow. The transient curve resembles that of a conventional resistive-type sensor that is continuously heated by an electric furnace. The fundamental difference between the pulse-driven sensor and a continuous-heating sensor is the visual width of the transient electrical-resistance curve, especially in the toluene-included atmosphere. Expansions of the transient response curve in regions 1 and 2 are shown in Figure 3b. The electrical resistance in air remains constant during the heater-on phase, while it increases with time during the heater-on phase in the toluene-included atmosphere. It reveals that the electrical-resistance exhibits wide transient-response behavior in the toluene-included atmosphere as shown in Figure 3a. This electrical-resistance phenomenon is


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observed at other heater voltages, as shown in Figure S-2; the change in electrical resistance during the heater-on time period in the toluene-included atmosphere also increases with heater voltage (i.e., operating temperature). A strong rise in electrical resistance is especially observed during the initial 1 s at each temperature. According to a former report, this type of microheater completely reaches the desired temperature within 0.1 s.25 Hence, the observed increase in electrical resistance appears to be caused by the combustion of toluene deep within the sensing layer and the desorption of by-products such as H2O and CO2 from the sensing layer, as shown in the model in Figure 1b. Here, we determined two new sensor responses. The first sensor response (Si) is determined as the ratio of the electrical resistance in air (Ra) to the initial electrical resistance (Rg,i) determined 0.05 s following application of the heater voltage, as shown in Figure 3b (Si = Ra/Rg,i). The other sensor response (Se) is the ratio of the electrical resistance in air (Ra) to that (Rg.e) determined at the end of the heating phase, namely 5 s after the heater voltage was applied, as shown in Figure 3b (Se = Ra/Rg,e). Here, the latter sensor response (Se) is the same as that of a conventional sensor evaluated in continuous-heating mode, since equilibria exists between gas diffusion and surface reactions. The heater-voltage dependences of the responses of both sensors are shown in Figure 3c; Si is clearly larger than Se over the entire voltage range, and the differences between the two sensor responses increases with increasing heater voltage. Increasing the heater voltage reduces gas diffusion into the gas-sensing layer in the heater-on phase, which is why the difference between the sensor responses increases with heater voltage. Here the heater-voltage dependence of the responses to 200 ppm hydrogen are shown in Figure S-3. The differences between the two sensor responses toward


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hydrogen are smaller than that toward toluene over the entire voltage range, because hydrogen diffusion into the sensing layer is easier than the toluene diffusion.11,19 The Se values toward toluene and hydrogen are drastically decreased with increased heater voltage. Here, Se is the same as the conventional sensor response, and its behavior can be explained based on the surface reaction activity and gas diffusion. 26 If the operating temperature is higher than the appropriate temperature, gas diffusion into the sensing layer is impeded by the activation of the gas oxidizing reaction. Hence, the decrease in the Se value with increasing heater voltage is caused by the high temperature. On the other hand, the behavior of Si as a function of the heater voltage is different for the different gases; i.e., the value of Si toward toluene decreases gradually with increasing heater voltage but remains high, while the Si value toward hydrogen decreases drastically with increasing heater voltage, similarly to the Se value. This difference is probably caused by the reaction kinetics of toluene and hydrogen. This point is based on an understanding of the transient electrical-resistance curve, and unfortunately, has not yet been experimentally clarified. Hence, further investigation is necessary to clarify the phenomenon observed in the transient curve. Nevertheless, this difference in the behavior of Si may result in high selectivity toward VOC gases, which is important for resistive-type gas sensors. Figure 3d displays the effect of the heater-off time period on the sensor responses (Si and Se) at 1.04 V. The Se value is almost independent of the heater-off time period, while Si increases as this time period increases. A longer heater-off phase is associated with a longer heater-cooling period; hence, the longer the heater is off for, the cooler the material temperature becomes. These results are in good agreement with the model shown in Figure 1. We conclude that


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the sensor response to toluene can be systematically improved through control of the heater-driving mode. To further improve Si, we fabricated the most suitable material for toluene detection, namely Pd-SnO2 CNPs, which have large pores (pore radius ~10 nm) and high catalytic activities.11 The larger pores improve gas diffusion deep into the sensing layer during the heater-off phase, despite the high catalytic activity, as depicted schematically in Figure S-4. Figures 4a and b display transient response curves toward 1–8 ppb toluene when heated at 1.04 V. Here, the heater-on and -off timespans are 5 and 100 s, respectively. The 100-s heater-off phase was applied to confirm the capacity of the resistive-type gas sensor, as the sensor completely cools to room temperature over this heater-off time period. The operating curve is visually very wide and is similar to that depicted in Figure 3a, especially in the toluene-containing atmosphere; wide electrical resistance is also observed in air atmosphere. According to the expansion shown in Figure 4b, electrical resistance increases with time not only in the toluene-included atmosphere, but also in air. Unfortunately, we cannot explain this phenomenon at present. Nevertheless, the pulse-driven Pd-SnO2-CNP sensor exhibits a change in electrical resistance even at an extremely low toluene concentration, such as 1 ppb. Figure 4c shows the toluene concentration dependence of Si and Se. Here, the Se value at 1 ppb is almost unity, which corresponds to our previously determined toluene-detection limit.11 On the other hand, the Si toward 1 and 8 ppb toluene are 3 and 4.5, respectively, which are over the detection limit of a conventional resistive-type sensor. The resistance in air during the heater-on time period changed by a factor of about 2, which is the ratio of the initial resistance of the


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heater-on phase (Ra,i) to that at end of this phase (Ra), as shown in Figure 4c. According to these results, the Si detection limit is about 200 ppt; such a low toluene-sensor detection limit has not been previously reported. However, as discussed above, this sensor exhibits changes in electrical resistance during the heater-on phase even in air. The electrical resistance in air is required to be constant in order to detect toluene at levels less than 100 ppt. Despite this, combining the optimized sensor material with pulse-driven heating facilitates the detection of sub-ppb levels of toluene. Finally, to confirm the effect of the heater-off time period, the toluene concentration in the sensing layer was simulated based on the gas-diffusion and surface-reaction rates from the Knudsen diffusion model. The gas-diffusion depth in the sensing layer can be modeled using the following equations:19 = =

!"

, =

(1), (2),

where C is the concentration of toluene gas, CS is the concentration of the toluene gas at the surface of the sensing layer, x is the depth from the surface of the sensing layer, Dk is the diffusion coefficient, k is the rate constant for the surface reaction, L is the thickness of the sensing layer (40 µm), r is the pore radius (about 10 nm)23, R is the molar gas constant, T is the temperature (heater temperature, shown in Figure S-1c), and M is the molecular mass of toluene (0.092 kg∙mol-1). Heater-off and -on refers to the unheated and heated phases, and the temperature influences to Dk and k. Here, Dk and k increase with increasing temperature because Dk is determined by equation (2), while k depends on the catalytic activity. Here, the Dk values at 25 and 250 °C are 1.7 × 10-6 and 2.3


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× 10-6 m2∙s-1, respectively; therefore Dk is not strongly influenced by temperature. In contrast, the value of k should be significantly different between 25 and 250 °C because toluene combustion on Pd-SnO2 occurs at temperatures over 200 °C;27 consequently k toward toluene at 25 °C should be extremely low, while it should be larger by several orders of magnitude at 250 °C. Figures S-5a and b display simulated normalized toluene-concentration profiles in the sensing layer as functions of the distance from the surface of the layer at various k values, at 25 and 250 °C, respectively. Here, k was set to be in the 1–100000 s-1 range. Clearly, the normalized toluene concentration at the bottom of the sensing layer (40 µm) is significantly enhanced at lower k values, i.e., lower catalytic activities. Hence, lowering the catalytic activity results in improved toluene diffusion deep into the sensing layer, which is facilitated by the cooling of the material during the heater-off phase.

CONCLUSIONS We introduced pulse-driven resistive-type semiconductor gas sensors for improving the toluene-concentration detection limit. The sensing layer was fabricated by injection onto a microsensor device composed of a microheater and a sensor electrode by the MEMS technique. The sensor was heated in pulse-mode, and the electrical resistance during the initial heater-on phase was significantly larger than that at the end of the heater-on phase in a toluene-included atmosphere. Using differences in electrical resistance, we determined two types of sensor response; one is the sensor response based on the initial electrical resistance (Si), and the other is that using the final electrical resistance (Se) during the heater-on timespan; Si was consistently higher than Se. The


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pulse-driven Pd-SnO2-CNPs responded even at toluene levels of a few ppb. Moreover, they facilitated the detection of toluene gas at levels of 200 ppt; such a low detection limit toward toluene has not been previously reported. The toluene-diffusion simulation revealed that toluene gas diffuses deep into the sensing layer during the heater-off phase when the catalytic activity is low. Hence, in order to enhance the sensor response and decrease the detection limit toward toluene using pulse-driven gas sensors, a large difference in the catalytic activities between the heater-on and -off phases is important. Therefore, we propose that pulse-driven resistive-type semiconductor gas sensors have the abilities to detect sub-ppb levels of VOC gases, and have advantages such as compactness and low power consumption.

ACKNOWLEDGMENTS

We thank Figaro Engineering Inc. for providing the microsensor device. This work was partially supported by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI), grant Number JP16H04219 and JP17K17941.

ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS publications web site. Materials preparation; Schematic image of the measurement system and operation profiles;


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transient response curves, response to hydrogen, schematic images of gas diffusion, and simulation of the toluene concentration profile of sensing layer. (PDF)

AUTHOR INFORMATION Corresponding author *Koichi Suematsu E-mail: [email protected] Fax: +81-92-583-7538

Notes The authors declare no competing financial interest.


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(17) Takeguchi, T.; Takeoh, O.; Aoyama, S.; Ueda, J.; Kikuchi, R.; Eguchi, K. Appl. Catal. A 2003, 252, 205-214. (18) Kamiuchi, N.; Mitsui, T.; Yamaguchi, N.; Muroyama, H.; Matsui, T.; Kikuchi, R.; Eguchi, K. Catal. Today 2010, 157, 415-419. (19) Sakai, G.; Matsunaga, N.; Shimanoe, K.; Yamazoe, N. Sens. Actuator B 2001, 80, 125-131. (20) Suematsu, K.; Watanabe, K.; Tou, A.; Sun, Y.; Shimanoe, K. Anal. Chem. 2018, 90, 1959-1966. (21) Ruiz, A.M.; Illa, X.; Diaz, R.; Romano-Rodriguez, A.; Morante, J.R. Sens. Actuator B 2006, 118, 318-322. (22) Triantafyllopoulou, R.; Tsamis, C. phys. stat. sol. a 2008, 205, 2643-2646. (23) Suematsu, K.; Shin, Y.; Ma, Nan.; Oyama, T.; Sasaki, M.; Yuasa, M.; Kida, T.; Shimanoe, K. Anal. Chem. 2015, 87, 8407-8415. (24) Sasahara, T.; Nishimura, M.; Ishihara, H.; Toyoda, K.; Sunayama, T.; Uematsu, S.; Ozawa, T.; Ogino, K.; Egashira, M. Electrochem. 2003, 71, 457-462. (25) Yoshioka, K.; Tanihira, T.; Shinnishi, K.; Kaneyasu, K. Chem. Sens. 2007, 23 (suppl. B), 19-21 [in Japanese]. (26) Yamazoe, N.; Sakai, G.; Shimanoe, K. Catal. Surv. Asia 2003, 7, 63-75. (27) Takeguchi, T.; Aoyama, S.; Ueda, J.; Kikuchi, R.; Eguchi, K. Top. Catal. 2003, 23, 159-162.


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Figure 1. Schematic diagrams of the gas-diffusion behavior in: (a) the heating-off and (b) heating-on phases.

Figure 2. (a) Photographic and SEM images of a blank MEMS sensor device. (b–d) top and side views of the SnO2 microsensor device.


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Figure 3. (a) Transient response curve of the SnO2 microsensor toward 20 ppm toluene at an applied voltage of 1.04 V (~250 °C) and (b) an expanded region of the transient response curve. (c) Responses of the sensor as functions of applied heater voltage. (d) Dependence of the heater-off timespan on sensor response to 20 ppm toluene at 1.04 V.


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Figure 4. (a) Transient response curve of the Pd-SnO2 CNP microsensor to various toluene concentrations at 1.04 V (~250 °C) and (b) an expanded region of the transient response curve. (c) Relationship between Pd-SnO2-CNP-sensor response and toluene concentration.


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ToC only


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Pulse-Driven Semiconductor Gas Sensors Toward ppt Level Toluene

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