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CNT-based sensor array for selective and steady detection of SO2 and NO
Materials Research Bulletin 124 (2020) 110772
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CNT-based sensor array for selective and steady detection of SO2 and NO a,
b
Hui Song *, Quanfu Li , Yong Zhang a b c
T
c
School of Software Engineering, Qufu Normal University, Qufu, Shandong, 273165, China College of Electronic Engineering, Guangxi Normal University, Guilin, Guangxi, 541004, China School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sensor array CNT Non-self-sustaining discharge Mechanism Stability
The accurate detection of NO and SO2 emitted from fossil fuel power plants is critical for realizing the real-time control of clean combustion systems and environmental protection. An array of two CNT-based ionization sensors with different electrode separations is used to detect NO and SO2 in flue gas. The responses of each sensor show a monotone decreasing response, and are obviously separated but almost parallel. The decreasing response is attributed to strong consumption of N2(A3∑u+) and N2(a'1∑u+) in collision with SO2 or NO. And the separated and parallel responses in gas mixture indicates a good selectivity and the ability to simultaneously detect SO2 and NO concentrations with no other means. In addition, the array also have excellent long-term stability due to its non-self-sustaining discharge, which reducing the damage of CNTs caused by electrical breakdown. And the sensor has a fast response and recovery times of 8 s and 7 s, respectively.
1. Introduction Flue gas, emitted from fossil fuel power plants and industrial boilers contains NOX and SO2, are the main causes of photochemical smog, acid rain, and ozone layer depletion. At least half of the emissions come from fossil fuel power plants and industrial boilers [1–3]. NOX in flue gas are mainly NO, accounting for 90 %–95 %, and the remaining small amount is N2O and NO2 [4]. Thus, the detection and monitoring of SO2 and NO are important for realizing the real-time control of clean combustion systems and reducing their emissions. Carbon nanotubes (CNTs) have gained much attention in the field of gas sensing for their high surface area and hollow geometry [5–8]. In general, CNT-based gas sensors are classified into two categories: chemisorption and ionization according to theie working mechanisms. Due to the adsorption and desorption of gas molecules, the conductance or resistance of chemisorption gas sensors increases or decreases when exposed to gases [9–12]. The sensing properties of CNTs were improved by doping other elements or hybridization with other materials [13,14]. And the hollow or porous 3D carbon nanotube structure facilitates an enhanced gas diffusion leading to a higher gas response [15,16]. But the slow recovery at room temperature and the inability to detect gases with low adsorption energies are the main problems of this kind of sensor. In addition, how to differentiate between gases or mixed gases is also a challenge because the same changes in resistance could be caused by gases in different concentrations, or by a single pure gas [17,18].
⁎
Ionization gas sensor, working by fingerprint the ionization characteristics of distinct gases, is a closed chamber with which two electrodes are placed from each other at a certain distance. By applying high voltage, the electrons escaping from one electrode are accelerated by the electric field and obtained enough energy causing ionization by the collision between the electrons with the gas molecules. In 2003, Modi et al. first used CNTs as cathode to lower breakdown voltages by several-fold through the extremely high non-uniform electric fields generated near the sharp tips of CNTs [19]. The CNT-based sensors show good selectivity because of the unique breakdown voltage of a certain gas at constant temperature and pressure. And their response and recovery are also faster (shown in Table 1) than that of chemisorption sensors since they do not involve the adsorption and desorption of gases [19–23]. However CNTs are easily damaged since the high discharge current density under breakdown voltage generates excessive heat [20,21,24]. Because of the extremely non-uniform near the tips and high enhancement of the electric field, the CNT-based ionization sensor could be worked in a non-self-sustaining discharge, which reducing the damage of CNTs caused by electrical breakdown [20,21]. In this paper, we detect the concentration of SO2 and NO in mixed gases using an array of two ionization sensors. One 80 μm-electrodeseparation ionization sensor is used for SO2, detection and another with 100 μm-electrode-separation for NO. Each ionization sensor has a tripolar-electrode structure with a CNT cathode [Fig. 1(a)] and works in
Corresponding author. E-mail address: [email protected] (H. Song).
https://doi.org/10.1016/j.materresbull.2020.110772 Received 26 June 2019; Received in revised form 13 December 2019; Accepted 5 January 2020 Available online 09 January 2020 0025-5408/ © 2020 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 124 (2020) 110772
H. Song, et al.
Table 1 Performance comparison of the toxic gas sensors. Ref.
Gas
TRes / TReca
Reusability
Condition
Material/Type
Ref. [1] Ref. [25] Ref. [26] Ref. [27] Ref. [28] Ref. [29] This paper
NOX、NH3 NO2、SO2 NH3、NO2 NH3、CO2 NO、NO2 NH3 NO、SO2
45 s/7 min —— 13.1 min/23 min —— 9 s/6 s gas diffusenessd 8 s/7 s
< 10 hour —— > 18 hour —— —— > 1 week > 1 month
350℃ Humidityc 150℃ RT RT RT RT
WO3/Chemisorption + PCAb CNTs/Chemisorption WS2/WO3/Chemisorption Y-doped ZnO/Ionization CNT cathode/Ionization CNT cathode/Ionization CNT cathode/Ionization
a
TRes and TRec are the response time and recovery time, respectively. PCA: Principal component analysis. c Here is humidity-assisted selective reactivity between NO2 and SO2 on CNTs. The resistance of SWCNT sensor increases at high humidity level (92 %) upon SO2 exposure and shows no obvious change at low humidity levels. While for NO2, the resistance always decreases independent of moisture levels. d The response/recovery time depends on the velocity of the gases diffuseness [29]. b
Fig. 1. (a) Schematic diagram of the gas-sensing test set-up; (b) SEM image of CVD CNT film; and (c) schematic of the three-electrode structure of the sensor.
non-self-sustaining discharges to expect a long life [30,31]. Here, the array of these two sensors with different electrode separations has been demonstrated the ability to directly detect SO2 and NO in a gas mixture at the same time. The gas-sensing mechanism is briefly discussed here. We also discuss the stability and transient response of the CNT-based ionization sensor.
maintaining the same gap distances between the extracting and collecting electrodes and between the cathode and extracting electrode. Thus the SO2 sensor with 80 μm electrode separations and the NO sensor with 100 μm electrode separations are fabricated.
2. Experimental
Stable voltages applied to the sensor array are supplied by power modules (NI PXI-4132). The measured currents are recorded by a precise digital multimeter (NI PXI-4071) controlled by computer via the MXI interface of the test system. As shown in Fig. 2, the sensor array was placed in a cylindrical test chamber with a 2.5-cm diameter and 5cm length. All the gases were well mixed in the mixture chamber, and then they flowed into the test chamber until atmospheric pressure was reached. The concentration of each component of the mixed gas could be controlled by three mass flow controllers (MFC Line Tech M3030 V with 1 % accuracy) with flow rates of 100 mL/min for NO, 50 mL/min for SO2, and 1000 mL/min for dry air. Before each measurement, the testing chamber was heated to 60 °C, a relative humidity of 50 % RH was maintained to simulate the condition of emissions from a power plant, and it was pumped to a vacuum of ∼5 kPa to minimize the impact of each concentration.
2.2. Gas-sensing test set-up
2.1. Sensor fabrication Three silicon slices, 450 μm × 8 mm × 27 mm, are processed through a mask, photolithography, and dry etching. A cathode with two cooling holes 4 mm in diameter was prepared, the same as the collecting electrode with an 8 mm × 6 mm rectangular area with a depth of 200 μm and an extracting electrode with one round hole with a 3 mm radius. Then Ti/Ni/Au (50 nm/125 nm/400 nm) was sputtered on the inner side of the cathode and the collecting electrode, and on both sides of the extracting electrode in turn. To enhance the bond strength of the substrate and the Ti/Ni/Au film, three electrodes were rapidly annealed at 450 °C for about 50 s. Finally, a vertically aligned CNT array grown by thermal chemical vapor deposition (TCVD) was transferred to the inner side of the cathode by wetting transfer [32]. Seen from an FEI Quanta 250 FEG transmission electron microscope image [Fig. 1(b)], the CNT film was homogeneous and dense, and the CNTs are ∼5–6 μm in length (marked in green) and ∼30 nm in diameter (insert of Fig. 1(b)). For their good electrical insulation properties and excellent resistance to high breakdown voltages, polyester films are used to separate the collecting electrode, extracting electrode, and cathode. Polyester films with a certain thickness separate the three electrodes,
3. Results and discussion Typical CNT-based ionization gas sensors identify the unknown gas with the unique breakdown voltage and determine the gas concentration by monitoring the self-sustaining discharge current [19]. And in this case, the high current density can damage CNT film in the case of electrical breakdown [33]. Because of the highly enhanced and non2
Materials Research Bulletin 124 (2020) 110772
H. Song, et al.
Fig. 2. Schematic diagram of mixed-gas distribution and gas-sensing experiments.
Here, φ1 and φ2 refer to the concentration of one gas and the other in gas mixture, respectively. Thus, we could use an array of two these sensors with different electrode separations to simultaneously detect the concentration of two different gases in the mixture. That is, the collecting currents of these two sensors, are functions of their own electrode separations d1 and d2, and the concentration of each gas upon exposure to a two-gas mixture as shown in Eqs. (4) and (5). By measuring the collecting currents and solving these two Equations, we can get the concentration of two gases at the same time [37].
uniform electrical field near a CNT’s tips, the CNT-based ionization sensor could work in a non-self-sustaining discharge state when the applied voltage lower than the electrical breakdown voltage [20,34]. And the ionization sensor should be expected to prolong its lifetime by reducing CNT damage caused by electrical breakdown. Thus, we could steadily detect the concentration of gases with a non-self-sustaining ionization current. This had been reported previously using gold nanowires for the anode [20,21] and a CNT film cathode [22,29,35]. Here, when the applying an appropriate extracting voltage U1 (Fig.1(a)) lower than the breakdown voltage, an extremely non-uniform electrical field is generated in the vicinity of the cathode since that the tiny tips of the carbon nanotube arrays enhance the local field strength. Thus plenty of electrons emitted from the CNT film cathode and a nonself-sustaining discharge occurs near the cathode because more electrons and positive ions are produced by collisions between electrons with gas molecules. A great number of positive ions moved towards the collecting electrode by drift and diffusion to form ionization current. And the collecting current Ic, as part of ionization current, is greatly affected by the electrode separation d, and the first ionization coefficient α at a given gas temperature according to Townsend theory [36]:
Ic ∝ I0 e αd ,
(1)
(2)
And for a two-gas mixture, the collecting current could be expressed as follow.
Ic = f (d, φ1, φ2)
(4)
Ic2 = f (φ1, φ2 , d2)
(5)
In this work, an array of two CNT-based ionization sensors with d1 = 80 μm and d2 = 100 μm are used to simultaneously detect the concentrations of SO2 and NO in gas mixtures. As seen in Fig. 3(a), the collecting current of the CNT-based ionization sensor with 80 μm separations is 83.3 nA when there is no SO2 mixed with air, and decreases gradually to 53.2 nA with the SO2 concentration increasing to 740 ppm. For NO, the collecting current of the sensor with 100 μm electrode separations decreases gradually from 77.3 nA to 51 nA with the increase of NO concentration from 0 to 1120 ppm [Fig. 3(b)]. According to electron collision reactions R1 and R2 (Table S1), the two metastable states N2(a'1∑u+) and N2(A3∑u+) were plentiful between the extracting electrode and the cathode due to their dominance in N2 and N2-O2 discharges [38–41]. Associative ionizations, caused by the collisions between these two metastable states dominate the ionization channel through reactions R5 and R6 (Table S1), leading to the production of numerous positive N2+ ions due to their high rate constants [40,41]. Here, the rate constants were calculated from the Boltzmann equation for the electron energy distribution function [42–44]. However, the long-lived N2(a'1∑u+) and N2(A3∑u+) were greatly consumed in collision with NO (SO2) through R7 and R8 (R9 and R10), with higher rate constants [38–43], resulting in a dramatic decrease in positive N2+ ions. Accordingly, the collecting currents, mainly formed by N2+ diffusion and drift, reduced rapidly with the introduction of SO2 or NO. The ionization sensor exhibited a monotonic decreasing characteristic with increasing NO or SO2 concentration.
where, I0 is the initial current. Thus, the collecting current Ic is determined by the first ionization coefficient α at a constant separation d between the extracting electrode and CNT cathode. According to the definition, the first ionization coefficient α=Ape−BP/E is determined by gas constants A and B, pressure P and the applied electric field E. Here, A and B are constants related to gas species and temperature; while E is proportional to the electrode separation d at a constant extracting voltage U1, and P to gas concentration φ at a constant volume and temperature. Accordingly, the collecting current Ic can be expressed as a function of the electrode separation d, and the concentration φ for a given gas.
Ic = f (d, φgivengas )
Ic1 = f (φ1, φ2 , d1)
(3) 3
Materials Research Bulletin 124 (2020) 110772
H. Song, et al.
Fig. 3. (a) SO2 response of sensor with 80 μm separations; and (b) NO response of CNTs-based ionization sensor with 100 μm separations, at U1 = 150 V and U2 = 10 V.
[Fig. 4(b)]. This indicates that the array of these two sensors could simultaneously measure the concentrations of NO and SO2 in mixed gases without assistance. Moreover, the effect of NO on SO2 detection was obviously much stronger than that of SO2 on NO detection. It is considered that the rate constants of collision between N2(A3∑u+), N2(a'1∑u+), and NO (R7 and R8 in Table S1) were higher than those of N2(A3∑u+) and SO2 (R9 and R10 in Table S1), which should be responsible for the stronger effect of NO on SO2 than of SO2 on NO. The sensor arrays were used to repeat gas-sensing experiments both ten days and one month later to validate their long-term stability. The maximum collecting current variation was only 0.4 nA with the SO2 concentration in mixed gases increasing from 0 to 740 ppm [Fig. 5(a)]. For NO, the variation was within 0.6 nA [Fig. 5(b)]. The differences in the responses of the three tests could be neglected because the responses of the two sensors were at least 80 times the variation in the collecting current. The non-self-sustaining discharge of the ionization sensor reduces the CNT damage from the high current density passing in the case of electrical breakdown and increases the sensor life, which ensures the excellent long-term stability of the array.
The main problem is the potential interference from humidity when the sensor operates at less than 100 °C. The effect of relative humidity on ionization current has been studied with a NO sensor by increasing humidity from 30 % to 100 % at 25 ± 2 °C and 100 ± 5 kPa (Fig. S1). The collecting current increased with increasing relative humidity, while the change of the collecting current was almost negligible when the relative humidity did not exceed 70 %, as previously reported [45]. Considering the gas-detection condition of a power plant, all gas-sensing experiments were conducted at a temperature of 60 °C and relative humidity of 50 %RH. The dynamic response of the CNT-based ionization sensor is also presented with different concentrations of NO exposure (Supplemental Information, Fig. S2). We extracted the 90 % response and recovery times, as listed in Table S2. The average times were calculated at around 8 s and 7 s for response and recovery, respectively. Under exposure of NO and SO2 mixed with air, the collecting current of each sensor still decreased monotonically with the increase of SO2 or NO concentration [Fig. 4(a) and 4(b)]. The response curves of each sensor were almost parallel at different NO or SO2 concentrations. As shown in Fig. 4(a), when NO concentration in the mixed gases varied from 300 ppm to 820 ppm and from 820 ppm to 1120 ppm, the variations in collecting current were about 8 nA and 7 nA, respectively, with the increase of SO2 concentration from 0 to 740 ppm. For the NO sensor, the variation was about 1 nA, both when the SO2 concentration varied from 280 ppm to 420 ppm and from 420 ppm to 740 ppm
4. Conclusion Working in a non-self-sustaining discharge state, the CNT-based sensors have a long life for with reduced CNT damage caused by electrical breakdown, which ensures the excellent long-term stability of the
Fig. 4. Simultaneous measurement of SO2 and NO in the mixed gases using a sensor array at 150 V U1 and 10 V U2. (a) SO2 responses of the sensor with 80 μm separation; and (b) NO responses of the sensor with 100 μm separations, in NO, SO2, and dry air mixture. 4
Materials Research Bulletin 124 (2020) 110772
H. Song, et al.
Fig. 5. (a) Stability of SO2 sensor with 80 μm separation; and (b) NO sensor with 100 μm separation, in NO, SO2, and dry air mixture at U1 = 150 V and U2 = 10 V.
References
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Conflicts of interest The authors declare no conflicts of interest.
Author contribution statement Hui Song investigated the mechanism and prepared the manuscript. Quanfu Li grew the CNT film and performed the gas sensing experiments. Professor Yong Zhang conceived the idea.
Acknowledgments This study was financially supported by the National Natural Science Foundation of China (grant no. 61172040) and the National Natural Science Foundation of Shandong (ZR2016EEM39). Metal sputtering and CNT growth took place at the Institute of Vacuum Microelectronics & Microelectromechanical Systems, Xi’an Jiaotong University, Xi’an, China. The authors also thank Dr. Quanfu Li for his help in CNT growth and Senior Lab Manager, Prof. Kun Li, for his experimental guidance.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.materresbull.2020. 110772. 5
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CNT-based sensor array for selective and steady detection of SO2 and NO a,
b
Hui Song *, Quanfu Li , Yong Zhang a b c
T
c
School of Software Engineering, Qufu Normal University, Qufu, Shandong, 273165, China College of Electronic Engineering, Guangxi Normal University, Guilin, Guangxi, 541004, China School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sensor array CNT Non-self-sustaining discharge Mechanism Stability
The accurate detection of NO and SO2 emitted from fossil fuel power plants is critical for realizing the real-time control of clean combustion systems and environmental protection. An array of two CNT-based ionization sensors with different electrode separations is used to detect NO and SO2 in flue gas. The responses of each sensor show a monotone decreasing response, and are obviously separated but almost parallel. The decreasing response is attributed to strong consumption of N2(A3∑u+) and N2(a'1∑u+) in collision with SO2 or NO. And the separated and parallel responses in gas mixture indicates a good selectivity and the ability to simultaneously detect SO2 and NO concentrations with no other means. In addition, the array also have excellent long-term stability due to its non-self-sustaining discharge, which reducing the damage of CNTs caused by electrical breakdown. And the sensor has a fast response and recovery times of 8 s and 7 s, respectively.
1. Introduction Flue gas, emitted from fossil fuel power plants and industrial boilers contains NOX and SO2, are the main causes of photochemical smog, acid rain, and ozone layer depletion. At least half of the emissions come from fossil fuel power plants and industrial boilers [1–3]. NOX in flue gas are mainly NO, accounting for 90 %–95 %, and the remaining small amount is N2O and NO2 [4]. Thus, the detection and monitoring of SO2 and NO are important for realizing the real-time control of clean combustion systems and reducing their emissions. Carbon nanotubes (CNTs) have gained much attention in the field of gas sensing for their high surface area and hollow geometry [5–8]. In general, CNT-based gas sensors are classified into two categories: chemisorption and ionization according to theie working mechanisms. Due to the adsorption and desorption of gas molecules, the conductance or resistance of chemisorption gas sensors increases or decreases when exposed to gases [9–12]. The sensing properties of CNTs were improved by doping other elements or hybridization with other materials [13,14]. And the hollow or porous 3D carbon nanotube structure facilitates an enhanced gas diffusion leading to a higher gas response [15,16]. But the slow recovery at room temperature and the inability to detect gases with low adsorption energies are the main problems of this kind of sensor. In addition, how to differentiate between gases or mixed gases is also a challenge because the same changes in resistance could be caused by gases in different concentrations, or by a single pure gas [17,18].
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Ionization gas sensor, working by fingerprint the ionization characteristics of distinct gases, is a closed chamber with which two electrodes are placed from each other at a certain distance. By applying high voltage, the electrons escaping from one electrode are accelerated by the electric field and obtained enough energy causing ionization by the collision between the electrons with the gas molecules. In 2003, Modi et al. first used CNTs as cathode to lower breakdown voltages by several-fold through the extremely high non-uniform electric fields generated near the sharp tips of CNTs [19]. The CNT-based sensors show good selectivity because of the unique breakdown voltage of a certain gas at constant temperature and pressure. And their response and recovery are also faster (shown in Table 1) than that of chemisorption sensors since they do not involve the adsorption and desorption of gases [19–23]. However CNTs are easily damaged since the high discharge current density under breakdown voltage generates excessive heat [20,21,24]. Because of the extremely non-uniform near the tips and high enhancement of the electric field, the CNT-based ionization sensor could be worked in a non-self-sustaining discharge, which reducing the damage of CNTs caused by electrical breakdown [20,21]. In this paper, we detect the concentration of SO2 and NO in mixed gases using an array of two ionization sensors. One 80 μm-electrodeseparation ionization sensor is used for SO2, detection and another with 100 μm-electrode-separation for NO. Each ionization sensor has a tripolar-electrode structure with a CNT cathode [Fig. 1(a)] and works in
Corresponding author. E-mail address: [email protected] (H. Song).
https://doi.org/10.1016/j.materresbull.2020.110772 Received 26 June 2019; Received in revised form 13 December 2019; Accepted 5 January 2020 Available online 09 January 2020 0025-5408/ © 2020 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 124 (2020) 110772
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Table 1 Performance comparison of the toxic gas sensors. Ref.
Gas
TRes / TReca
Reusability
Condition
Material/Type
Ref. [1] Ref. [25] Ref. [26] Ref. [27] Ref. [28] Ref. [29] This paper
NOX、NH3 NO2、SO2 NH3、NO2 NH3、CO2 NO、NO2 NH3 NO、SO2
45 s/7 min —— 13.1 min/23 min —— 9 s/6 s gas diffusenessd 8 s/7 s
< 10 hour —— > 18 hour —— —— > 1 week > 1 month
350℃ Humidityc 150℃ RT RT RT RT
WO3/Chemisorption + PCAb CNTs/Chemisorption WS2/WO3/Chemisorption Y-doped ZnO/Ionization CNT cathode/Ionization CNT cathode/Ionization CNT cathode/Ionization
a
TRes and TRec are the response time and recovery time, respectively. PCA: Principal component analysis. c Here is humidity-assisted selective reactivity between NO2 and SO2 on CNTs. The resistance of SWCNT sensor increases at high humidity level (92 %) upon SO2 exposure and shows no obvious change at low humidity levels. While for NO2, the resistance always decreases independent of moisture levels. d The response/recovery time depends on the velocity of the gases diffuseness [29]. b
Fig. 1. (a) Schematic diagram of the gas-sensing test set-up; (b) SEM image of CVD CNT film; and (c) schematic of the three-electrode structure of the sensor.
non-self-sustaining discharges to expect a long life [30,31]. Here, the array of these two sensors with different electrode separations has been demonstrated the ability to directly detect SO2 and NO in a gas mixture at the same time. The gas-sensing mechanism is briefly discussed here. We also discuss the stability and transient response of the CNT-based ionization sensor.
maintaining the same gap distances between the extracting and collecting electrodes and between the cathode and extracting electrode. Thus the SO2 sensor with 80 μm electrode separations and the NO sensor with 100 μm electrode separations are fabricated.
2. Experimental
Stable voltages applied to the sensor array are supplied by power modules (NI PXI-4132). The measured currents are recorded by a precise digital multimeter (NI PXI-4071) controlled by computer via the MXI interface of the test system. As shown in Fig. 2, the sensor array was placed in a cylindrical test chamber with a 2.5-cm diameter and 5cm length. All the gases were well mixed in the mixture chamber, and then they flowed into the test chamber until atmospheric pressure was reached. The concentration of each component of the mixed gas could be controlled by three mass flow controllers (MFC Line Tech M3030 V with 1 % accuracy) with flow rates of 100 mL/min for NO, 50 mL/min for SO2, and 1000 mL/min for dry air. Before each measurement, the testing chamber was heated to 60 °C, a relative humidity of 50 % RH was maintained to simulate the condition of emissions from a power plant, and it was pumped to a vacuum of ∼5 kPa to minimize the impact of each concentration.
2.2. Gas-sensing test set-up
2.1. Sensor fabrication Three silicon slices, 450 μm × 8 mm × 27 mm, are processed through a mask, photolithography, and dry etching. A cathode with two cooling holes 4 mm in diameter was prepared, the same as the collecting electrode with an 8 mm × 6 mm rectangular area with a depth of 200 μm and an extracting electrode with one round hole with a 3 mm radius. Then Ti/Ni/Au (50 nm/125 nm/400 nm) was sputtered on the inner side of the cathode and the collecting electrode, and on both sides of the extracting electrode in turn. To enhance the bond strength of the substrate and the Ti/Ni/Au film, three electrodes were rapidly annealed at 450 °C for about 50 s. Finally, a vertically aligned CNT array grown by thermal chemical vapor deposition (TCVD) was transferred to the inner side of the cathode by wetting transfer [32]. Seen from an FEI Quanta 250 FEG transmission electron microscope image [Fig. 1(b)], the CNT film was homogeneous and dense, and the CNTs are ∼5–6 μm in length (marked in green) and ∼30 nm in diameter (insert of Fig. 1(b)). For their good electrical insulation properties and excellent resistance to high breakdown voltages, polyester films are used to separate the collecting electrode, extracting electrode, and cathode. Polyester films with a certain thickness separate the three electrodes,
3. Results and discussion Typical CNT-based ionization gas sensors identify the unknown gas with the unique breakdown voltage and determine the gas concentration by monitoring the self-sustaining discharge current [19]. And in this case, the high current density can damage CNT film in the case of electrical breakdown [33]. Because of the highly enhanced and non2
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Fig. 2. Schematic diagram of mixed-gas distribution and gas-sensing experiments.
Here, φ1 and φ2 refer to the concentration of one gas and the other in gas mixture, respectively. Thus, we could use an array of two these sensors with different electrode separations to simultaneously detect the concentration of two different gases in the mixture. That is, the collecting currents of these two sensors, are functions of their own electrode separations d1 and d2, and the concentration of each gas upon exposure to a two-gas mixture as shown in Eqs. (4) and (5). By measuring the collecting currents and solving these two Equations, we can get the concentration of two gases at the same time [37].
uniform electrical field near a CNT’s tips, the CNT-based ionization sensor could work in a non-self-sustaining discharge state when the applied voltage lower than the electrical breakdown voltage [20,34]. And the ionization sensor should be expected to prolong its lifetime by reducing CNT damage caused by electrical breakdown. Thus, we could steadily detect the concentration of gases with a non-self-sustaining ionization current. This had been reported previously using gold nanowires for the anode [20,21] and a CNT film cathode [22,29,35]. Here, when the applying an appropriate extracting voltage U1 (Fig.1(a)) lower than the breakdown voltage, an extremely non-uniform electrical field is generated in the vicinity of the cathode since that the tiny tips of the carbon nanotube arrays enhance the local field strength. Thus plenty of electrons emitted from the CNT film cathode and a nonself-sustaining discharge occurs near the cathode because more electrons and positive ions are produced by collisions between electrons with gas molecules. A great number of positive ions moved towards the collecting electrode by drift and diffusion to form ionization current. And the collecting current Ic, as part of ionization current, is greatly affected by the electrode separation d, and the first ionization coefficient α at a given gas temperature according to Townsend theory [36]:
Ic ∝ I0 e αd ,
(1)
(2)
And for a two-gas mixture, the collecting current could be expressed as follow.
Ic = f (d, φ1, φ2)
(4)
Ic2 = f (φ1, φ2 , d2)
(5)
In this work, an array of two CNT-based ionization sensors with d1 = 80 μm and d2 = 100 μm are used to simultaneously detect the concentrations of SO2 and NO in gas mixtures. As seen in Fig. 3(a), the collecting current of the CNT-based ionization sensor with 80 μm separations is 83.3 nA when there is no SO2 mixed with air, and decreases gradually to 53.2 nA with the SO2 concentration increasing to 740 ppm. For NO, the collecting current of the sensor with 100 μm electrode separations decreases gradually from 77.3 nA to 51 nA with the increase of NO concentration from 0 to 1120 ppm [Fig. 3(b)]. According to electron collision reactions R1 and R2 (Table S1), the two metastable states N2(a'1∑u+) and N2(A3∑u+) were plentiful between the extracting electrode and the cathode due to their dominance in N2 and N2-O2 discharges [38–41]. Associative ionizations, caused by the collisions between these two metastable states dominate the ionization channel through reactions R5 and R6 (Table S1), leading to the production of numerous positive N2+ ions due to their high rate constants [40,41]. Here, the rate constants were calculated from the Boltzmann equation for the electron energy distribution function [42–44]. However, the long-lived N2(a'1∑u+) and N2(A3∑u+) were greatly consumed in collision with NO (SO2) through R7 and R8 (R9 and R10), with higher rate constants [38–43], resulting in a dramatic decrease in positive N2+ ions. Accordingly, the collecting currents, mainly formed by N2+ diffusion and drift, reduced rapidly with the introduction of SO2 or NO. The ionization sensor exhibited a monotonic decreasing characteristic with increasing NO or SO2 concentration.
where, I0 is the initial current. Thus, the collecting current Ic is determined by the first ionization coefficient α at a constant separation d between the extracting electrode and CNT cathode. According to the definition, the first ionization coefficient α=Ape−BP/E is determined by gas constants A and B, pressure P and the applied electric field E. Here, A and B are constants related to gas species and temperature; while E is proportional to the electrode separation d at a constant extracting voltage U1, and P to gas concentration φ at a constant volume and temperature. Accordingly, the collecting current Ic can be expressed as a function of the electrode separation d, and the concentration φ for a given gas.
Ic = f (d, φgivengas )
Ic1 = f (φ1, φ2 , d1)
(3) 3
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Fig. 3. (a) SO2 response of sensor with 80 μm separations; and (b) NO response of CNTs-based ionization sensor with 100 μm separations, at U1 = 150 V and U2 = 10 V.
[Fig. 4(b)]. This indicates that the array of these two sensors could simultaneously measure the concentrations of NO and SO2 in mixed gases without assistance. Moreover, the effect of NO on SO2 detection was obviously much stronger than that of SO2 on NO detection. It is considered that the rate constants of collision between N2(A3∑u+), N2(a'1∑u+), and NO (R7 and R8 in Table S1) were higher than those of N2(A3∑u+) and SO2 (R9 and R10 in Table S1), which should be responsible for the stronger effect of NO on SO2 than of SO2 on NO. The sensor arrays were used to repeat gas-sensing experiments both ten days and one month later to validate their long-term stability. The maximum collecting current variation was only 0.4 nA with the SO2 concentration in mixed gases increasing from 0 to 740 ppm [Fig. 5(a)]. For NO, the variation was within 0.6 nA [Fig. 5(b)]. The differences in the responses of the three tests could be neglected because the responses of the two sensors were at least 80 times the variation in the collecting current. The non-self-sustaining discharge of the ionization sensor reduces the CNT damage from the high current density passing in the case of electrical breakdown and increases the sensor life, which ensures the excellent long-term stability of the array.
The main problem is the potential interference from humidity when the sensor operates at less than 100 °C. The effect of relative humidity on ionization current has been studied with a NO sensor by increasing humidity from 30 % to 100 % at 25 ± 2 °C and 100 ± 5 kPa (Fig. S1). The collecting current increased with increasing relative humidity, while the change of the collecting current was almost negligible when the relative humidity did not exceed 70 %, as previously reported [45]. Considering the gas-detection condition of a power plant, all gas-sensing experiments were conducted at a temperature of 60 °C and relative humidity of 50 %RH. The dynamic response of the CNT-based ionization sensor is also presented with different concentrations of NO exposure (Supplemental Information, Fig. S2). We extracted the 90 % response and recovery times, as listed in Table S2. The average times were calculated at around 8 s and 7 s for response and recovery, respectively. Under exposure of NO and SO2 mixed with air, the collecting current of each sensor still decreased monotonically with the increase of SO2 or NO concentration [Fig. 4(a) and 4(b)]. The response curves of each sensor were almost parallel at different NO or SO2 concentrations. As shown in Fig. 4(a), when NO concentration in the mixed gases varied from 300 ppm to 820 ppm and from 820 ppm to 1120 ppm, the variations in collecting current were about 8 nA and 7 nA, respectively, with the increase of SO2 concentration from 0 to 740 ppm. For the NO sensor, the variation was about 1 nA, both when the SO2 concentration varied from 280 ppm to 420 ppm and from 420 ppm to 740 ppm
4. Conclusion Working in a non-self-sustaining discharge state, the CNT-based sensors have a long life for with reduced CNT damage caused by electrical breakdown, which ensures the excellent long-term stability of the
Fig. 4. Simultaneous measurement of SO2 and NO in the mixed gases using a sensor array at 150 V U1 and 10 V U2. (a) SO2 responses of the sensor with 80 μm separation; and (b) NO responses of the sensor with 100 μm separations, in NO, SO2, and dry air mixture. 4
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Fig. 5. (a) Stability of SO2 sensor with 80 μm separation; and (b) NO sensor with 100 μm separation, in NO, SO2, and dry air mixture at U1 = 150 V and U2 = 10 V.
References
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Conflicts of interest The authors declare no conflicts of interest.
Author contribution statement Hui Song investigated the mechanism and prepared the manuscript. Quanfu Li grew the CNT film and performed the gas sensing experiments. Professor Yong Zhang conceived the idea.
Acknowledgments This study was financially supported by the National Natural Science Foundation of China (grant no. 61172040) and the National Natural Science Foundation of Shandong (ZR2016EEM39). Metal sputtering and CNT growth took place at the Institute of Vacuum Microelectronics & Microelectromechanical Systems, Xi’an Jiaotong University, Xi’an, China. The authors also thank Dr. Quanfu Li for his help in CNT growth and Senior Lab Manager, Prof. Kun Li, for his experimental guidance.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.materresbull.2020. 110772. 5
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