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Suspended CoPP-ZnO nanorods integrated with micro-heaters for highly sensitive VOC detection
Sensors and Actuators B 264 (2018) 249–254
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Suspended CoPP-ZnO nanorods integrated with micro-heaters for highly sensitive VOC detection Kyounghoon Lee a,1 , Dae-Hyun Baek a,1 , Jungwook Choi b , Jongbaeg Kim a,∗ a b
School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea School of Mechanical Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan, Gyeongbuk, 38541, Republic of Korea
a r t i c l e
i n f o
Article history: Received 1 October 2017 Received in revised form 20 January 2018 Accepted 23 February 2018 Keywords: VOC Micro-heaters ZnO nanorods Gas sensors Nanomaterials Porphyrins Functionalization
a b s t r a c t Suspended nanomaterials including nanowires and nanorods have attracted great interest as promising candidates for gas sensing materials because they can perform sensitive gas detection without being affected by the substrate. However, the suspended nanomaterial-based gas sensors reported so far have been developed in the absence of a heater, and given the fact that most sensors require high operating temperature, integration with the heater is essential for practical use of the sensor. This work demonstrates a suspended ZnO nanorod-based sensor integrated with a heater, based on a batch process. Depending on the degree of heating from the integrated heater, the suspended ZnO nanorods exhibited various sensitivities and the highest response was obtained when heated to 5.2 V. To further increase the response to volatile organic compounds (VOCs), the ZnO nanorods were functionalized with cobalt porphyrin and the functionalized ZnO nanorods exhibited response that was 2.6 times higher than that of the pristine one at 10 ppm toluene. The functionalized ZnO nanorods detected 2 ppb of toluene, which is lower than the concentration detectable by any of the previously reported chemoresistive VOC sensors. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Harmful substances emitted from artificial materials such as artificial adhesives and paints used in interior building materials reduce the quality of the indoor air and pose a risk to human health [1–3]. Volatile organic compounds (VOCs) are one of the most important substances known to degrade indoor air quality. One of the typical effects of VOCs is the sick building syndrome (SBS). SBS is a condition that results from exposure to harmful chemical toxins at home or in the workplace. For this reason, many countries have enacted legislation to limit the VOC content. Further, various sensors have been developed to mitigate the damage caused by VOCs and to ensure a comfortable indoor environment. Exposure to even low concentrations of VOCs, especially BTEX (benzene, toluene, xylene, and ethylbenzene), in these places is dangerous and carcinogenic [4–7]. Therefore, it is very important to detect a low concentration of VOCs, especially in the range of ppb to less than ppm. As a wide bandgap metal oxide semiconductor, Zinc oxide (ZnO) has attracted much attention as a chemical gas sensing mate-
∗ Corresponding author. E-mail address: [email protected] (J. Kim). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.snb.2018.02.161 0925-4005/© 2018 Elsevier B.V. All rights reserved.
rial with high performance, low cost and environmentally friendly attributes [8–11]. However, because of low exposure to reactive crystal planes, and easy recombination of electron-hole pairs, traditional ZnO-based sensing materials still have low response to low concentration VOCs, which limits their practical application. Continuous efforts have been made to overcome these shortcomings, such as structure-controlling to form porous nanostructures to increase exposed sites, and improving the sensing properties by increasing the mobility and separation efficiency of the electronhole pairs [12–17]. Nonetheless, even these improved ZnO-based sensors still have limits in sensing performance for low concentration VOC gases, such as undetectable response for sub-ppb level concentration or significant drift of sensing signals. Therefore, it is highly desirable to develop a highly sensitive gas sensor capable of detecting VOC gas at very low concentration, especially at the ppb level. Meanwhile, studies have been conducted to increase the response of the 1D nanomaterial-based sensors, and as a result, it has been found that more sensitive gas detection is possible when the sensing materials are present in a suspended form [18–24]. Suspended nanomaterials are less susceptible to noise or influence from the substrate than the nanomaterials attached to the substrate and can also escape from the boundary layer formed due to gas stagnation near the substrate, allowing more sensitive gas detection. Various suspended 1D nanomaterials such as functionalized tung-
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sten nanowires [18], carbon nanotubes [19], GaN nanowires [20], and Pd-carbon nanowires [21] have been used as gas sensing materials for sensitive gas detection. However, all the above-mentioned sensors were developed without heater integration, which is a large factor preventing practical use of the gas sensor, considering that most gas sensors require a high temperature at the time of sensing. We developed a suspended ZnO nanorod-based gas sensor for detecting low concentration of VOCs. Unlike conventional suspended 1D nanomaterial-based sensors, the suspended nanorods are integrated with a heater platform, which can be utilized in normal environment (not in high temperature furnace), because the operating temperature can be adjusted at low power without
any additional external heat source [19,20,23]. All fabrication processes, including the micro-electro-mechanical systems (MEMS) process for the heater-integrated sensor platform, and the growth of the suspended ZnO nanorods, consisted of batch-processes. To detect low concentration of VOCs by improving the response of the suspended ZnO nanorod-based gas sensor, porphyrin was used as a functional material. Porphyrins are known to provide more adsorption sites for VOCs and enable more sensitive detection of VOCs [25–28]. Among the various porphyrins, we chose cobalt porphyrin (CoPP), which is reported as the most efficient element for electrocatalytic reduction of oxygen ions used for VOC sensing [29].
Fig. 1. Schematic diagram showing fabrication process for the heater-integrated VOCs sensor.
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Fig. 2. SEM images of (a) the fabricated sensor and (b) ZnO nanorods grown between the two micro-bridge structures. (c) Optical image of the heater being driven.
2. Design and fabrication Fig. 1 shows the fabrication process of the VOC sensor integrated with the micro-heater. The sensors were batch-fabricated on a silicon-on-insulator (SOI) wafer with a 20 m-thick device layer, a 2 m-thick buried oxide layer, and a 450 m-thick handle layer. The device layer, which was heavily doped with Arsenide, had a resistivity value of 0.005 ·cm. Silicon micro-bridges were patterned on a SOI wafer using photolithography and deep reactiveion etching (DRIE) (Fig. 1a). Because conventional plate-like heaters are a structure that is difficult to integrate suspended nanowires, we fabricated micro-heaters with a belt shape, which allowed the nanowire to be easily suspended between the heaters. After patterning, the silicon surface was oxidized to passivate the Si heaters and their electrodes (Fig. 1b), and subsequently, the Au sensor electrodes were deposited (Fig. 1c) Metals, specifically, 10-nm thick Cr and 50-nm thick Au, were patterned on top of the SiO2 surface through the shadow mask to form the sensor electrodes. The patterned silicon was exposed through photolithography and reactive-ion etching (RIE) for use as heater electrodes (Fig. 1d). The size of the heater electrode is 700 by 700 m2 . A ZnO seed layer was deposited on the Au electrode surface through a shadow mask (Fig. 1e), followed by the growth of ZnO nanorods by hydrothermal synthesis (Fig. 1f). The hydrothermal synthesis was carried out by heating an aqueous solution of 25 mM zinc nitrate hexahydrate, 15 mM hexamethylenediamine, and 0.35 M ammonium hydroxide at 80 ◦ C for 7 h [30]. To improve the detection performance of toluene, the fabricated ZnO-nanorod-based sensor was functionalized with CoPP through the shadow mask. Thermal evaporation has been widely used as a method for the deposition of CoPP [31]. Here, the powder form of the product (Sigma Aldrich Co.) was purchased and deposited through thermal evaporation as well. Finally, 8 nm-thick CoPP was deposited on the ZnO nanorods by the thermal evaporation process through the shadow mask for the functionalization. (Fig. 1g). The grown ZnO nanorods on each side of the two separated sensor electrodes meet, electrically connecting them and forming two-terminal device for measuring the variation in signal when the VOC is adsorbed (Fig. 1h). The scanning electron microscope (SEM) images of the fabricated ZnO nanorod-based sensor integrated with micro-heaters are shown in Fig. 2(a) and (b). The bridge type micro-heater has 20 m in width and 900 m in length. ZnO nanorods were grown on the patterned ZnO seed layer through a shadow mask in the center of the fabricated device and two micro-bridge structures were connected by the grown ZnO nanorods, as shown in Fig. 2(b). The length of the nanorods ranged between 1 and 2 m, and the diameter was estimated to be approximately 100 nm.
3. Experimental details When voltage is applied to the silicon heater in the device layer, heat is transferred to the zinc oxide by conduction, and the heater
and the sensor are not electrically connected owing to the oxide insulating layer between the silicon and gold layers. Fig. 2(c) shows the optical image of the fabricated micro-heater driven by Joule heating. As shown in the figure, when the voltage is applied, heat is generated from the center of the heater where ZnO is present, and this high-temperature environment enables the ZnO nanorods to detect the VOCs with high response. To examine the gas-sensing performance of suspended ZnO nanorod-based sensors according to the voltage applied to the micro-heaters, gas sensing tests were performed by measuring the changes in electrical resistance of the sensors as the sensors were exposed to air-diluted toluene and dry air alternately at 1 atm pressure and room temperature. The concentration of toluene was adjusted by changing the mixing rate using a mass flow controller (MFC), while the total flow rate of toluene diluted in air was maintained as 500 sccm. Here, 10 ppm of air-diluted toluene gas in the gas bombe was purchased, and further diluted with air through a MFC. In particular, in the low-concentration gas detection experiment, a very precise MFC was used, and it was possible to flow a gas having a total flow rate of 500 sccm while containing 0.1–10 sccm of 10 ppm toluene gas. The exposure of the sensor to toluene gas led to a change in the sensor resistance, whereas, the exposure to air resulted in a resistance approximately equal to the initial value. The response is defined as Ra /Rg , where Ra and Rg are the resistances of the sensors before and after the exposure to toluene, respectively. For measuring Ra /Rg , a current flow through the ZnO nanorods was monitored by a sourcemeter (KEITHLEY 2400) under a fixed bias of 1 V applied. To evaluate repeatability, on/off cycles were performed with exposure to 10 ppm toluene. Prior to its exposure to toluene, only air flowed in the chamber. This was followed by exposure to toluene for 900 s. Subsequently, toluene was shut off and only air flowed into the test chamber to allow the sensor to recover.
4. Results and discussion Fig. 3 shows a graph of the response with respect to different heater voltages ranging from 0 to 6.9 V, in 10 ppm of toluene. As shown in the figure, adsorption of toluene molecules leads to the decrease in the resistance of ZnO nanorods. The response of the sensors increases at a higher temperature because the adsorption of gas molecules onto the surface of the nanorods becomes more active as the temperature increases. However, the increase in response saturates and subsequently decreases after a certain temperature. This is because the increased number of electrons that are thermally excited from the valence band to the conduction band abates the semiconducting properties of the materials. In this case, the CoPP-ZnO nanorod-based sensor did not react until the 3.5 V was applied to the heaters. The sensor showed the highest sensing performance for the heating voltage of 5.2 V, which was chosen as the heating voltage during the sensor operation. To determine the relationship between heating voltage and temperature, the resistance
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Fig. 3. Response with respect to different heater voltages from 0 to 6.9 V in 10 ppm toluene.
of the sensor electrode just above the heater bridge was calibrated at various known temperatures. Then, the resistance was measured when driving the heaters and converted to temperature. As a result, we found that the temperature was 17, 80, 158, 258, 323 and 350◦ C when the heating voltage was 0, 2.5, 3.5, 4.5, 5 and 5.2 V, respectively. The detailed correlation between the voltage and the temperature is shown in Fig. S1. The toluene-sensing mechanism of ZnO functionalized with CoPP involves chemisorbed oxygen at the oxide surface; interaction with gaseous analytes can release electrons in conduction bands, thereby modifying the electrical conductivity of the oxide. As is widely known, in the case of bare ZnO, O2− is formed on the surface of ZnO under air, thereby forming a depletion layer along the surface of ZnO. When toluene is to be adsorbed onto the bare ZnO, it bonds with the O2− formed on the surface of ZnO, and the resistance changes as the thickness of the ZnO depletion layer changes. On the other hand, it is well known that the natural functions played by CoPP include binding and activation of molecular oxygen, and synthetic CoPPs have been exploited as electrocatalysts for the reduction of O2 [32]. The working temperature necessary for ZnO sensors may affect or deteriorate the organic compounds coated on ZnO for functionalization; however, CoPPs are stable compounds and can resist the influence of the working temperature without decomposing [33]. CoPPs also improve the response to VOCs by providing a variety of interacting adsorption sites. Hydrogen bonding, polarization and polar interactions are expected to occur between VOC and CoPP [25]. The effect of
porphyrin functionalization is shown in Fig. 4 by comparing the response at exposure to different toluene concentrations from 2 to 10 ppm. As shown in Fig. 4(b), the effect of functionalization was further improved when the sensor was exposed to a higher concentration of toluene. In the case of 10 ppm, the response of the functionalized ZnO nanorods was 2.6 times larger than that of pristine one at 10 ppm of toluene. The results indicate that the various sites of adsorption of CoPPs are effective in detecting VOCs, and can help detect even lower concentrations of VOCs by further maximizing the effects of the existing suspended structure. In addition, the heterojunction would be formed between ZnO and CoPP. Once the heterojunction formed, the electrons in conduction band of the ZnO nanorods move into the CoPPs as the ZnO has a higher Fermi level than CoPP. Then, the depletion layer will become larger inside the ZnO, and this increased initial resistance will have positive effect on the toluene sensing performance [34]. We have also fabricated a sensor using a gas-sensing material consisting of only CoPP without ZnO. In this case, the electrical conductivity of CoPP was too low to be used as a sensing material. Fig. 5(a) shows the response of the suspended CoPP-ZnO nanorod-based sensor for 17 different cycles. The gas response was 2.78 in each cycle. The response time, defined as the rise time to reach 90% of the maximum resistance change, was approximately 16 s and the sensor recovered to its original value of resistance within 5 min without any significant drift, after the toluene was turned off and the chamber was purged with air. The CoPP-ZnO nanorod-based sensor had good repeatability and reversibility demonstrating relatively fast and stable on/off switching in each cycle. Sensing experiments upon exposure to various gases were performed to determine the selectivity of the sensor. In addition to toluene, the sensor was exposed to 10 ppm of H2 , NO2 , and CO2 , and each gas is a gas that can be exposed in real life together with toluene. As shown in Fig. S2, there was little reaction to the other two gases except H2 , for which ZnO is known to respond very sensitively [35,36]. According to the previous reports, it has been found that porphyrins have different sensitivities to various gases depending on the metal in the center [27,37]. Therefore, selective gas detection will be possible in future works by arranging sensor arrays that are functionalized with other porphyrins. The influence of humidity on many gas sensors is known to be a disadvantage. To investigate the effect of the fabricated sensors on humidity, gas sensing experiments were performed under various humidity conditions. The humidity level was controlled by introducing water vapor using a bubbler and measured by a humidity detector(KIMO HQ210). Fig. S3 shows the gas responses upon exposure to 1 ppm toluene under various relative humidities from 0 to 85 RH%. As shown in the fig-
Fig. 4. Toluene sensing properties of ZnO nanorod-based sensors before and after functionalization with CoPP. The effect of CoPP-functionalization is compared by the response at the exposure to different toluene concentration, from 2 to 10 ppm.
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Fig. 5. (a) Repeatability test of the MPP-ZnO nanorod-based VOCs sensor with exposure to 10 ppm toluene. (b) Response change upon exposure to different concentration of toluene ranging from 2 to 200 ppb.
ure, the response of the sensor decreased gradually with increasing humidity, and showed a similar response at humidities above 65 RH%. The existing ZnO-based sensors also have such a tendency [38,39], and the fabricated sensor is considered to have some influence on humidity, like other ZnO-based sensors. Fig. 5(b) shows the response variation when the sensor was exposed to different toluene concentrations ranging from 2 to 200 ppb. As the concentration of toluene increased, the response also increased, and the sensor exhibited an observable resistance change with a low noise level at concentrations as low as 2 ppb. In this experiment, 2 ppb is the lowest concentration of toluene that can be diluted using the laboratory setup. In many conventional gas sensor studies, the signal-to-noise ratio is used to derive the detection limit, in which up to three signal-to-noise ratios are considered actual sensing signals [40,41]. For this sensor, we also calculated the noise to determine whether 2 ppb could be considered an actual detectable signal, and the possibility of sensing toluene at concentrations below 2 ppb. To calculate the noise, the standard deviation of 100 points on the baseline before the sensor was exposed to toluene was calculated, and the calculated signal-to-noise ratio when exposed to 2 ppb of toluene was 79.83. The results indicate that our sensor can sensitively detect 2 ppb of toluene and that if a setup that can be exposed to lower concentrations of toluene is used, lower toluene concentrations could be detected. Based on these results, we concluded that the minimum detectable concentration of the suspended CoPP-ZnO nanorod sensor is the lowest compared to the previously reported similar toluene sensors based on ZnO, while having a relatively long recovery time [16,42–45]. 5. Conclusion In summary, we demonstrated the suspended CoPP-ZnO nanorod-based VOC sensor integrated with micro-heaters for the first time. The micro-bridges were fabricated by MEMS process, and the ZnO nanorods were synthesized between the bridges, in the suspended form. Unlike previous researches on suspended 1D nanomaterial-based sensors, which were fabricated in a heater-free form and could not be used in normal environment (not in high temperature furnace), the developed sensors were fabricated in a form integrated with micro-heaters. The synthesized ZnO nanorods were functionalized with CoPP and confirmed the improvement of toluene-sensing performance owing to the provision of various adsorption sites for VOCs by CoPP. Our emphasis is mostly placed on improved response of ZnO-based nanostructures for detecting low concentrations of VOC at the ppb–ppm range. The developed VOC sensors achieved the minimum detection concentration of toluene as 2 ppb, which is the lowest concentration of all chemoresistive VOC sensors ever developed. The good performance of the sen-
sor is likely due to the suspended structure of the nanorods along with the functionalization effect of CoPPs. The fabricated sensor can be applied to indoor environmental monitoring systems to detect very low concentrations of VOCs at an early stage of outgasing. In addition, our approach can also be used to develop suspended nanomaterial-based sensors made of various materials other than ZnO, which can be used in the development of highly sensitive gas sensors for various gases. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (NRF2015R1A2A1A01005496). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.snb.2018.02.161. References [1] C. Warneke, J.A. De Gouw, W.C. Kuster, P.D. Goldan, R. Fall, Validation of atmospheric VOC measurements by proton-transfer-reaction mass spectrometry using a gas-chromatographic preseparation method, Environ. Sci. Technol. 37 (11) (2003) 2494–2501. [2] A. Colomb, N. Yassaa, J. Williams, I. Peeken, K. Lochte, Screening volatile organic compounds (VOCs) emissions from five marine phytoplankton species by head space gas chromatography/mass spectrometry (HS-GC/MS), J. Environ. Monitor 10 (3) (2008) 325–330. [3] R.M. Cavalcante, M.V.F. de Andrade, R.V. Marins, L.D.M. Oliveira, Development of a headspace-gas chromatography (HS-GC-PID-FID) method for the determination of VOCs in environmental aqueous matrices: optimization, verification and elimination of matrix effect and VOC distribution on the Fortaleza Coast, Brazil, Microchem. J. 96 (2) (2010) 337–343. [4] M.T. Ke, M.T. Lee, C.Y. Lee, L.M. Fu, A MEMS-based benzene gas sensor with a self-heating WO3 sensing layer, Sens.-Basel 9 (4) (2009) 2895–2906. [5] B. Ghaddab, F. Berger, J.B. Sanchez, P. Menini, C. Mavon, P. Yoboue, V. Potin, Benzene monitoring by micro-machined sensors with SnO2 layer obtained by using micro-droplet deposition technique, Sens. Actuators B-Chem. 152 (1) (2011) 68–72. [6] V.S. Vaishnav, S.G. Patel, J.N. Panchal, Development of ITO thin film sensor for detection of benzene, Sens. Actuators B-Chem. 206 (2015) 381–388. [7] N.H. Kim, S.J. Choi, D.J. Yang, J. Bae, J. Park, I.D. Kim, Highly sensitive and selective hydrogen sulfide and toluene sensors using Pd functionalized WO3 nanofibers for potential diagnosis of halitosis and lung cancer, Sens. Actuators B-Chem. 193 (2014) 574–581. [8] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett. 84 (18) (2004) 3654–3656. [9] Z.H. Jing, J.H. Zhan, Fabrication and gas-sensing properties of porous ZnO nanoplates, Adv. Mater. 20 (23) (2008) 4547–4551. [10] H.Y. Huang, P.C. Xu, D. Zheng, C.Z. Chen, X.X. Li, Sulfuration-desulfuration reaction sensing effect of intrinsic ZnO nanowires for high-performance H2 S detection, J. Mater. Chem. A 3 (12) (2015) 6330–6339.
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Biographies Kyounghoon Lee received the BS degree in Mechanical Engineering from Yonsei University, Seoul, Korea, in 2009. He is currently working toward the PhD degree in Mechanical Engineering at the Yonsei University as a researcher of Nano Transducers Laboratory. His research interests are fabrication of metal oxide-based VOCs sensors. Dae-Hyun Baek received the BS and PhD degrees in Mechanical Engineering from Yonsei University, Seoul, Korea, in 2008 and 2018, respectively. He is currently working as a postdoctoral research associate at the Yonsei University. His research interests are fabrication of nanomaterials and NEMS gas sensors. Jungwook Choi received the BS and PhD degrees in Mechanical Engineering from Yonsei University, Korea in 2006 and 2013, respectively. He worked as a postdoctoral research associate at Purdue University from 2014 to 2016. He then joined the School of Mechanical Engineering at Yeungnam University as an assistant professor. His current research focuses on modeling, design and fabrication of nanomaterialintegrated micro/nanosystems. Jongbaeg Kim received the BS degree in Mechanical Engineering from Yonsei University, Seoul, Korea, in 1997, the MS degree in Mechanical Engineering from the University of Texas, Austin, TX, in 1999, and the PhD degree in Mechanical Engineering from the University of California, Berkeley, in 2004, He was with Dicon Fiberoptics Inc., Richmond, CA, from 2004 to 2005, where he designed and developed high-performance optical MEMS components for telecommunication applications. He then joined the Yonsei University, where he is currently a professor with the School of Mechanical Engineering. His research interests are modeling, design and fabrication of microsystems, and integrated nanostructures on MEMS.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Suspended CoPP-ZnO nanorods integrated with micro-heaters for highly sensitive VOC detection Kyounghoon Lee a,1 , Dae-Hyun Baek a,1 , Jungwook Choi b , Jongbaeg Kim a,∗ a b
School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea School of Mechanical Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan, Gyeongbuk, 38541, Republic of Korea
a r t i c l e
i n f o
Article history: Received 1 October 2017 Received in revised form 20 January 2018 Accepted 23 February 2018 Keywords: VOC Micro-heaters ZnO nanorods Gas sensors Nanomaterials Porphyrins Functionalization
a b s t r a c t Suspended nanomaterials including nanowires and nanorods have attracted great interest as promising candidates for gas sensing materials because they can perform sensitive gas detection without being affected by the substrate. However, the suspended nanomaterial-based gas sensors reported so far have been developed in the absence of a heater, and given the fact that most sensors require high operating temperature, integration with the heater is essential for practical use of the sensor. This work demonstrates a suspended ZnO nanorod-based sensor integrated with a heater, based on a batch process. Depending on the degree of heating from the integrated heater, the suspended ZnO nanorods exhibited various sensitivities and the highest response was obtained when heated to 5.2 V. To further increase the response to volatile organic compounds (VOCs), the ZnO nanorods were functionalized with cobalt porphyrin and the functionalized ZnO nanorods exhibited response that was 2.6 times higher than that of the pristine one at 10 ppm toluene. The functionalized ZnO nanorods detected 2 ppb of toluene, which is lower than the concentration detectable by any of the previously reported chemoresistive VOC sensors. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Harmful substances emitted from artificial materials such as artificial adhesives and paints used in interior building materials reduce the quality of the indoor air and pose a risk to human health [1–3]. Volatile organic compounds (VOCs) are one of the most important substances known to degrade indoor air quality. One of the typical effects of VOCs is the sick building syndrome (SBS). SBS is a condition that results from exposure to harmful chemical toxins at home or in the workplace. For this reason, many countries have enacted legislation to limit the VOC content. Further, various sensors have been developed to mitigate the damage caused by VOCs and to ensure a comfortable indoor environment. Exposure to even low concentrations of VOCs, especially BTEX (benzene, toluene, xylene, and ethylbenzene), in these places is dangerous and carcinogenic [4–7]. Therefore, it is very important to detect a low concentration of VOCs, especially in the range of ppb to less than ppm. As a wide bandgap metal oxide semiconductor, Zinc oxide (ZnO) has attracted much attention as a chemical gas sensing mate-
∗ Corresponding author. E-mail address: [email protected] (J. Kim). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.snb.2018.02.161 0925-4005/© 2018 Elsevier B.V. All rights reserved.
rial with high performance, low cost and environmentally friendly attributes [8–11]. However, because of low exposure to reactive crystal planes, and easy recombination of electron-hole pairs, traditional ZnO-based sensing materials still have low response to low concentration VOCs, which limits their practical application. Continuous efforts have been made to overcome these shortcomings, such as structure-controlling to form porous nanostructures to increase exposed sites, and improving the sensing properties by increasing the mobility and separation efficiency of the electronhole pairs [12–17]. Nonetheless, even these improved ZnO-based sensors still have limits in sensing performance for low concentration VOC gases, such as undetectable response for sub-ppb level concentration or significant drift of sensing signals. Therefore, it is highly desirable to develop a highly sensitive gas sensor capable of detecting VOC gas at very low concentration, especially at the ppb level. Meanwhile, studies have been conducted to increase the response of the 1D nanomaterial-based sensors, and as a result, it has been found that more sensitive gas detection is possible when the sensing materials are present in a suspended form [18–24]. Suspended nanomaterials are less susceptible to noise or influence from the substrate than the nanomaterials attached to the substrate and can also escape from the boundary layer formed due to gas stagnation near the substrate, allowing more sensitive gas detection. Various suspended 1D nanomaterials such as functionalized tung-
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sten nanowires [18], carbon nanotubes [19], GaN nanowires [20], and Pd-carbon nanowires [21] have been used as gas sensing materials for sensitive gas detection. However, all the above-mentioned sensors were developed without heater integration, which is a large factor preventing practical use of the gas sensor, considering that most gas sensors require a high temperature at the time of sensing. We developed a suspended ZnO nanorod-based gas sensor for detecting low concentration of VOCs. Unlike conventional suspended 1D nanomaterial-based sensors, the suspended nanorods are integrated with a heater platform, which can be utilized in normal environment (not in high temperature furnace), because the operating temperature can be adjusted at low power without
any additional external heat source [19,20,23]. All fabrication processes, including the micro-electro-mechanical systems (MEMS) process for the heater-integrated sensor platform, and the growth of the suspended ZnO nanorods, consisted of batch-processes. To detect low concentration of VOCs by improving the response of the suspended ZnO nanorod-based gas sensor, porphyrin was used as a functional material. Porphyrins are known to provide more adsorption sites for VOCs and enable more sensitive detection of VOCs [25–28]. Among the various porphyrins, we chose cobalt porphyrin (CoPP), which is reported as the most efficient element for electrocatalytic reduction of oxygen ions used for VOC sensing [29].
Fig. 1. Schematic diagram showing fabrication process for the heater-integrated VOCs sensor.
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Fig. 2. SEM images of (a) the fabricated sensor and (b) ZnO nanorods grown between the two micro-bridge structures. (c) Optical image of the heater being driven.
2. Design and fabrication Fig. 1 shows the fabrication process of the VOC sensor integrated with the micro-heater. The sensors were batch-fabricated on a silicon-on-insulator (SOI) wafer with a 20 m-thick device layer, a 2 m-thick buried oxide layer, and a 450 m-thick handle layer. The device layer, which was heavily doped with Arsenide, had a resistivity value of 0.005 ·cm. Silicon micro-bridges were patterned on a SOI wafer using photolithography and deep reactiveion etching (DRIE) (Fig. 1a). Because conventional plate-like heaters are a structure that is difficult to integrate suspended nanowires, we fabricated micro-heaters with a belt shape, which allowed the nanowire to be easily suspended between the heaters. After patterning, the silicon surface was oxidized to passivate the Si heaters and their electrodes (Fig. 1b), and subsequently, the Au sensor electrodes were deposited (Fig. 1c) Metals, specifically, 10-nm thick Cr and 50-nm thick Au, were patterned on top of the SiO2 surface through the shadow mask to form the sensor electrodes. The patterned silicon was exposed through photolithography and reactive-ion etching (RIE) for use as heater electrodes (Fig. 1d). The size of the heater electrode is 700 by 700 m2 . A ZnO seed layer was deposited on the Au electrode surface through a shadow mask (Fig. 1e), followed by the growth of ZnO nanorods by hydrothermal synthesis (Fig. 1f). The hydrothermal synthesis was carried out by heating an aqueous solution of 25 mM zinc nitrate hexahydrate, 15 mM hexamethylenediamine, and 0.35 M ammonium hydroxide at 80 ◦ C for 7 h [30]. To improve the detection performance of toluene, the fabricated ZnO-nanorod-based sensor was functionalized with CoPP through the shadow mask. Thermal evaporation has been widely used as a method for the deposition of CoPP [31]. Here, the powder form of the product (Sigma Aldrich Co.) was purchased and deposited through thermal evaporation as well. Finally, 8 nm-thick CoPP was deposited on the ZnO nanorods by the thermal evaporation process through the shadow mask for the functionalization. (Fig. 1g). The grown ZnO nanorods on each side of the two separated sensor electrodes meet, electrically connecting them and forming two-terminal device for measuring the variation in signal when the VOC is adsorbed (Fig. 1h). The scanning electron microscope (SEM) images of the fabricated ZnO nanorod-based sensor integrated with micro-heaters are shown in Fig. 2(a) and (b). The bridge type micro-heater has 20 m in width and 900 m in length. ZnO nanorods were grown on the patterned ZnO seed layer through a shadow mask in the center of the fabricated device and two micro-bridge structures were connected by the grown ZnO nanorods, as shown in Fig. 2(b). The length of the nanorods ranged between 1 and 2 m, and the diameter was estimated to be approximately 100 nm.
3. Experimental details When voltage is applied to the silicon heater in the device layer, heat is transferred to the zinc oxide by conduction, and the heater
and the sensor are not electrically connected owing to the oxide insulating layer between the silicon and gold layers. Fig. 2(c) shows the optical image of the fabricated micro-heater driven by Joule heating. As shown in the figure, when the voltage is applied, heat is generated from the center of the heater where ZnO is present, and this high-temperature environment enables the ZnO nanorods to detect the VOCs with high response. To examine the gas-sensing performance of suspended ZnO nanorod-based sensors according to the voltage applied to the micro-heaters, gas sensing tests were performed by measuring the changes in electrical resistance of the sensors as the sensors were exposed to air-diluted toluene and dry air alternately at 1 atm pressure and room temperature. The concentration of toluene was adjusted by changing the mixing rate using a mass flow controller (MFC), while the total flow rate of toluene diluted in air was maintained as 500 sccm. Here, 10 ppm of air-diluted toluene gas in the gas bombe was purchased, and further diluted with air through a MFC. In particular, in the low-concentration gas detection experiment, a very precise MFC was used, and it was possible to flow a gas having a total flow rate of 500 sccm while containing 0.1–10 sccm of 10 ppm toluene gas. The exposure of the sensor to toluene gas led to a change in the sensor resistance, whereas, the exposure to air resulted in a resistance approximately equal to the initial value. The response is defined as Ra /Rg , where Ra and Rg are the resistances of the sensors before and after the exposure to toluene, respectively. For measuring Ra /Rg , a current flow through the ZnO nanorods was monitored by a sourcemeter (KEITHLEY 2400) under a fixed bias of 1 V applied. To evaluate repeatability, on/off cycles were performed with exposure to 10 ppm toluene. Prior to its exposure to toluene, only air flowed in the chamber. This was followed by exposure to toluene for 900 s. Subsequently, toluene was shut off and only air flowed into the test chamber to allow the sensor to recover.
4. Results and discussion Fig. 3 shows a graph of the response with respect to different heater voltages ranging from 0 to 6.9 V, in 10 ppm of toluene. As shown in the figure, adsorption of toluene molecules leads to the decrease in the resistance of ZnO nanorods. The response of the sensors increases at a higher temperature because the adsorption of gas molecules onto the surface of the nanorods becomes more active as the temperature increases. However, the increase in response saturates and subsequently decreases after a certain temperature. This is because the increased number of electrons that are thermally excited from the valence band to the conduction band abates the semiconducting properties of the materials. In this case, the CoPP-ZnO nanorod-based sensor did not react until the 3.5 V was applied to the heaters. The sensor showed the highest sensing performance for the heating voltage of 5.2 V, which was chosen as the heating voltage during the sensor operation. To determine the relationship between heating voltage and temperature, the resistance
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Fig. 3. Response with respect to different heater voltages from 0 to 6.9 V in 10 ppm toluene.
of the sensor electrode just above the heater bridge was calibrated at various known temperatures. Then, the resistance was measured when driving the heaters and converted to temperature. As a result, we found that the temperature was 17, 80, 158, 258, 323 and 350◦ C when the heating voltage was 0, 2.5, 3.5, 4.5, 5 and 5.2 V, respectively. The detailed correlation between the voltage and the temperature is shown in Fig. S1. The toluene-sensing mechanism of ZnO functionalized with CoPP involves chemisorbed oxygen at the oxide surface; interaction with gaseous analytes can release electrons in conduction bands, thereby modifying the electrical conductivity of the oxide. As is widely known, in the case of bare ZnO, O2− is formed on the surface of ZnO under air, thereby forming a depletion layer along the surface of ZnO. When toluene is to be adsorbed onto the bare ZnO, it bonds with the O2− formed on the surface of ZnO, and the resistance changes as the thickness of the ZnO depletion layer changes. On the other hand, it is well known that the natural functions played by CoPP include binding and activation of molecular oxygen, and synthetic CoPPs have been exploited as electrocatalysts for the reduction of O2 [32]. The working temperature necessary for ZnO sensors may affect or deteriorate the organic compounds coated on ZnO for functionalization; however, CoPPs are stable compounds and can resist the influence of the working temperature without decomposing [33]. CoPPs also improve the response to VOCs by providing a variety of interacting adsorption sites. Hydrogen bonding, polarization and polar interactions are expected to occur between VOC and CoPP [25]. The effect of
porphyrin functionalization is shown in Fig. 4 by comparing the response at exposure to different toluene concentrations from 2 to 10 ppm. As shown in Fig. 4(b), the effect of functionalization was further improved when the sensor was exposed to a higher concentration of toluene. In the case of 10 ppm, the response of the functionalized ZnO nanorods was 2.6 times larger than that of pristine one at 10 ppm of toluene. The results indicate that the various sites of adsorption of CoPPs are effective in detecting VOCs, and can help detect even lower concentrations of VOCs by further maximizing the effects of the existing suspended structure. In addition, the heterojunction would be formed between ZnO and CoPP. Once the heterojunction formed, the electrons in conduction band of the ZnO nanorods move into the CoPPs as the ZnO has a higher Fermi level than CoPP. Then, the depletion layer will become larger inside the ZnO, and this increased initial resistance will have positive effect on the toluene sensing performance [34]. We have also fabricated a sensor using a gas-sensing material consisting of only CoPP without ZnO. In this case, the electrical conductivity of CoPP was too low to be used as a sensing material. Fig. 5(a) shows the response of the suspended CoPP-ZnO nanorod-based sensor for 17 different cycles. The gas response was 2.78 in each cycle. The response time, defined as the rise time to reach 90% of the maximum resistance change, was approximately 16 s and the sensor recovered to its original value of resistance within 5 min without any significant drift, after the toluene was turned off and the chamber was purged with air. The CoPP-ZnO nanorod-based sensor had good repeatability and reversibility demonstrating relatively fast and stable on/off switching in each cycle. Sensing experiments upon exposure to various gases were performed to determine the selectivity of the sensor. In addition to toluene, the sensor was exposed to 10 ppm of H2 , NO2 , and CO2 , and each gas is a gas that can be exposed in real life together with toluene. As shown in Fig. S2, there was little reaction to the other two gases except H2 , for which ZnO is known to respond very sensitively [35,36]. According to the previous reports, it has been found that porphyrins have different sensitivities to various gases depending on the metal in the center [27,37]. Therefore, selective gas detection will be possible in future works by arranging sensor arrays that are functionalized with other porphyrins. The influence of humidity on many gas sensors is known to be a disadvantage. To investigate the effect of the fabricated sensors on humidity, gas sensing experiments were performed under various humidity conditions. The humidity level was controlled by introducing water vapor using a bubbler and measured by a humidity detector(KIMO HQ210). Fig. S3 shows the gas responses upon exposure to 1 ppm toluene under various relative humidities from 0 to 85 RH%. As shown in the fig-
Fig. 4. Toluene sensing properties of ZnO nanorod-based sensors before and after functionalization with CoPP. The effect of CoPP-functionalization is compared by the response at the exposure to different toluene concentration, from 2 to 10 ppm.
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Fig. 5. (a) Repeatability test of the MPP-ZnO nanorod-based VOCs sensor with exposure to 10 ppm toluene. (b) Response change upon exposure to different concentration of toluene ranging from 2 to 200 ppb.
ure, the response of the sensor decreased gradually with increasing humidity, and showed a similar response at humidities above 65 RH%. The existing ZnO-based sensors also have such a tendency [38,39], and the fabricated sensor is considered to have some influence on humidity, like other ZnO-based sensors. Fig. 5(b) shows the response variation when the sensor was exposed to different toluene concentrations ranging from 2 to 200 ppb. As the concentration of toluene increased, the response also increased, and the sensor exhibited an observable resistance change with a low noise level at concentrations as low as 2 ppb. In this experiment, 2 ppb is the lowest concentration of toluene that can be diluted using the laboratory setup. In many conventional gas sensor studies, the signal-to-noise ratio is used to derive the detection limit, in which up to three signal-to-noise ratios are considered actual sensing signals [40,41]. For this sensor, we also calculated the noise to determine whether 2 ppb could be considered an actual detectable signal, and the possibility of sensing toluene at concentrations below 2 ppb. To calculate the noise, the standard deviation of 100 points on the baseline before the sensor was exposed to toluene was calculated, and the calculated signal-to-noise ratio when exposed to 2 ppb of toluene was 79.83. The results indicate that our sensor can sensitively detect 2 ppb of toluene and that if a setup that can be exposed to lower concentrations of toluene is used, lower toluene concentrations could be detected. Based on these results, we concluded that the minimum detectable concentration of the suspended CoPP-ZnO nanorod sensor is the lowest compared to the previously reported similar toluene sensors based on ZnO, while having a relatively long recovery time [16,42–45]. 5. Conclusion In summary, we demonstrated the suspended CoPP-ZnO nanorod-based VOC sensor integrated with micro-heaters for the first time. The micro-bridges were fabricated by MEMS process, and the ZnO nanorods were synthesized between the bridges, in the suspended form. Unlike previous researches on suspended 1D nanomaterial-based sensors, which were fabricated in a heater-free form and could not be used in normal environment (not in high temperature furnace), the developed sensors were fabricated in a form integrated with micro-heaters. The synthesized ZnO nanorods were functionalized with CoPP and confirmed the improvement of toluene-sensing performance owing to the provision of various adsorption sites for VOCs by CoPP. Our emphasis is mostly placed on improved response of ZnO-based nanostructures for detecting low concentrations of VOC at the ppb–ppm range. The developed VOC sensors achieved the minimum detection concentration of toluene as 2 ppb, which is the lowest concentration of all chemoresistive VOC sensors ever developed. The good performance of the sen-
sor is likely due to the suspended structure of the nanorods along with the functionalization effect of CoPPs. The fabricated sensor can be applied to indoor environmental monitoring systems to detect very low concentrations of VOCs at an early stage of outgasing. In addition, our approach can also be used to develop suspended nanomaterial-based sensors made of various materials other than ZnO, which can be used in the development of highly sensitive gas sensors for various gases. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (NRF2015R1A2A1A01005496). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.snb.2018.02.161. References [1] C. Warneke, J.A. De Gouw, W.C. Kuster, P.D. Goldan, R. Fall, Validation of atmospheric VOC measurements by proton-transfer-reaction mass spectrometry using a gas-chromatographic preseparation method, Environ. Sci. Technol. 37 (11) (2003) 2494–2501. [2] A. Colomb, N. Yassaa, J. Williams, I. Peeken, K. Lochte, Screening volatile organic compounds (VOCs) emissions from five marine phytoplankton species by head space gas chromatography/mass spectrometry (HS-GC/MS), J. Environ. Monitor 10 (3) (2008) 325–330. [3] R.M. Cavalcante, M.V.F. de Andrade, R.V. Marins, L.D.M. Oliveira, Development of a headspace-gas chromatography (HS-GC-PID-FID) method for the determination of VOCs in environmental aqueous matrices: optimization, verification and elimination of matrix effect and VOC distribution on the Fortaleza Coast, Brazil, Microchem. J. 96 (2) (2010) 337–343. [4] M.T. Ke, M.T. Lee, C.Y. Lee, L.M. Fu, A MEMS-based benzene gas sensor with a self-heating WO3 sensing layer, Sens.-Basel 9 (4) (2009) 2895–2906. [5] B. Ghaddab, F. Berger, J.B. Sanchez, P. Menini, C. Mavon, P. Yoboue, V. Potin, Benzene monitoring by micro-machined sensors with SnO2 layer obtained by using micro-droplet deposition technique, Sens. Actuators B-Chem. 152 (1) (2011) 68–72. [6] V.S. Vaishnav, S.G. Patel, J.N. Panchal, Development of ITO thin film sensor for detection of benzene, Sens. Actuators B-Chem. 206 (2015) 381–388. [7] N.H. Kim, S.J. Choi, D.J. Yang, J. Bae, J. Park, I.D. Kim, Highly sensitive and selective hydrogen sulfide and toluene sensors using Pd functionalized WO3 nanofibers for potential diagnosis of halitosis and lung cancer, Sens. Actuators B-Chem. 193 (2014) 574–581. [8] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett. 84 (18) (2004) 3654–3656. [9] Z.H. Jing, J.H. Zhan, Fabrication and gas-sensing properties of porous ZnO nanoplates, Adv. Mater. 20 (23) (2008) 4547–4551. [10] H.Y. Huang, P.C. Xu, D. Zheng, C.Z. Chen, X.X. Li, Sulfuration-desulfuration reaction sensing effect of intrinsic ZnO nanowires for high-performance H2 S detection, J. Mater. Chem. A 3 (12) (2015) 6330–6339.
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Biographies Kyounghoon Lee received the BS degree in Mechanical Engineering from Yonsei University, Seoul, Korea, in 2009. He is currently working toward the PhD degree in Mechanical Engineering at the Yonsei University as a researcher of Nano Transducers Laboratory. His research interests are fabrication of metal oxide-based VOCs sensors. Dae-Hyun Baek received the BS and PhD degrees in Mechanical Engineering from Yonsei University, Seoul, Korea, in 2008 and 2018, respectively. He is currently working as a postdoctoral research associate at the Yonsei University. His research interests are fabrication of nanomaterials and NEMS gas sensors. Jungwook Choi received the BS and PhD degrees in Mechanical Engineering from Yonsei University, Korea in 2006 and 2013, respectively. He worked as a postdoctoral research associate at Purdue University from 2014 to 2016. He then joined the School of Mechanical Engineering at Yeungnam University as an assistant professor. His current research focuses on modeling, design and fabrication of nanomaterialintegrated micro/nanosystems. Jongbaeg Kim received the BS degree in Mechanical Engineering from Yonsei University, Seoul, Korea, in 1997, the MS degree in Mechanical Engineering from the University of Texas, Austin, TX, in 1999, and the PhD degree in Mechanical Engineering from the University of California, Berkeley, in 2004, He was with Dicon Fiberoptics Inc., Richmond, CA, from 2004 to 2005, where he designed and developed high-performance optical MEMS components for telecommunication applications. He then joined the Yonsei University, where he is currently a professor with the School of Mechanical Engineering. His research interests are modeling, design and fabrication of microsystems, and integrated nanostructures on MEMS.