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Effect of adding ZHS microcubes on ZnO nanorods for CO2 gas sensing applications
Solid State Electronics 164 (2020) 107711
Contents lists available at ScienceDirect
Solid State Electronics journal homepage: www.elsevier.com/locate/sse
Effect of adding ZHS microcubes on ZnO nanorods for CO2 gas sensing applications
T
⁎
Feng-Renn Juang , Bo-Yai Chen Department of Electrical Engineering, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon dioxide Gas sensor Heterojunctions Zinc hydroxystannate
Conventionally, the catalytic promotion of semiconductor metal oxide-based CO2 sensors response is simply based on chemical reaction. In most realistic applications, the sensors are operated in both sensing gas and light surrounding. Hence, we try to use a new type of catalyst, such as ZnSn(OH)6 (ZHS) has both chemical and photo catalytic functions to improve gas response more significantly. In this work, ZnO nanorods are deposited on ptype silicon substrate with and without ZHS microcubes covered on the top. The effects of adding the ZHS microcubes on CO2 sensing response have been studied in detail with experimental measurements. Experimental results show that the added ZHS microcubes promote CO2 response up to 350%, which is higher than the reported CO2 sensors with or without metal catalyst.
1. Introduction Carbon dioxide (CO2) gas sensors have been applied widely, such as early fire detection, medical diagnosis, air and food quality control, and industrial processes [1–4]. In these applications, the important concerns of sensors are high response and low cost. Thus the semiconductor-based sensing system is one of the most promising, compact, low-cost methods for CO2 detection. Among these systems, zinc oxide (ZnO) has become a favorable material for its wide band gap (~3.2–3.4 eV). It attracts more attention while it combines with nanorods structure for high surface/volume ratio [1]. On the other hand, some techniques such as addition of a catalyst and/or heterojunctions have proposed [5]. For example, addition of La in ZnO [6] and Ag in BaTiO3 [7] as catalysts show great improvements in response to CO2. The added catalyst can modify the electrical properties of base materials, while the heterojunctions enhance the depletion layer resistance modulation. However, the reported metal catalysts can only modify the base material chemically or physically [5]. Gas sensors are operated under both sensing gas and light condition in most practical cases. Hence we try to use a new type of catalyst, which has both chemical and optical catalytic functions to improve gas response more significantly. Recently, zinc hydroxystannate (ZnSn (OH)6, ZHS) has been widely used for gas sensing and CO2 dissociation activation applications [8–10]. The researches imply the material is suitable for this application.
⁎
This is a further research from our previously related work, which investigate ZHS microcubes on different ZnO nanorod structures [11]. In this work, we propose a band diagram model and have more detailed experiments to discuss the improvement of adding ZHS microcubes on ZnO nanorods. We have prepared ZnO nanorods with and without ZHS microcubes covered on p-type Si substrate by a low temperature hydrothermal synthesis. In order to study the effect of ZHS on CO2 sensing performances, the sensing ability of both structures are examined. Design concept of the devices is based on low leakage current of p-n junction. Therefore, we use p-Si substrates to fabricate the samples. The n-type materials (ZnO and ZHS) are synthesized on the substrates with nanorod or microcube structures. The effect of the heterojunction between ZnO nanorods and ZHS microcubes is discussed in this study. 2. Experimental details Hydrothermal method was used for synthesizing ZnO nanorods on a p-type silicon substrate [12,13]. Before starting the hydrothermal synthesis, a ZnO seed layer was dip-coated on the surface of the substrate. The mixture of solution for coating consisted of ethanol and 0.02 M Zn(CH3COO)2·2H2O and was heated up to 60 °C. For forming the ZnO film, a sintering process is followed by using a furnace under 400 °C for 30 min. After coating ZnO seed layer on the substrate, the sample was put into a beaker containing C6H12N4 and Zn(NO3)2·6H2O. The composition of these two chemicals had same concentration (0.07 M). The flower-like ZnO nanorods were grown on the ZnO seed
Corresponding author at: Department of Electrical Engineering, National Sun Yat-sen University, 70 Lienhai Rd., Kaohsiung 80424, Taiwan. E-mail address: [email protected] (F.-R. Juang).
https://doi.org/10.1016/j.sse.2019.107711 Received 8 May 2019; Received in revised form 24 October 2019; Accepted 18 November 2019 Available online 21 November 2019 0038-1101/ © 2019 Elsevier Ltd. All rights reserved.
Solid State Electronics 164 (2020) 107711
F.-R. Juang and B.-Y. Chen
Fig. 1. Top view FESEM images of (a-b) ZnO nanorods at different magnifications; (c-e) ZHS microcubes/ZnO nanorods at different magnifications.
Fig. 1. (continued)
layer by immersing the sample into the solution under 90 °C for 5 h. In order to study the effect of adding ZnSn(OH)6 microcubes to the CO2 sensor, other group of samples were added some ZHS microcubes after ZnO nanorods synthesized on the substrate. ZHS microcubes were synthesized hydrothermally with 0.02 M SnCl4·5H2O and 0.2 M NaOH under 90 °C for 2 h. The microcubes spread on the nanorods with different sizes. Metal electrodes (Al) were evaporated respectively on the top of oxides (pure ZnO nanorods or ZHS/ZnO nanocomposites) and the bottom of the substrate to complete the sensing devices. Materials investigations and gas sensing properties of the devices were studied to confirm the improvement of adding ZHS microcubes. 3. Results and discussion 3.1. Material investigations To observe the morphology of ZnO nanorods, field emission scanning electron microscope (FESEM) was used from the top of the sample. The diameters of nanorods are all in nanoscale and the structure have a flower-like appearance as shown in Fig. 1(a) and (b). Moreover, the top view FESEM images of samples with nanocomposites are compared in Fig. 1(c)–(e) under different scales. As shown in the images, the ZHS
Fig. 1. (continued) 2
Solid State Electronics 164 (2020) 107711
F.-R. Juang and B.-Y. Chen
Temperature = 150°C
Current (A)
-4
10
ZnO
air 100 ppm CO2 500 ppm CO2 2500 ppm CO2
-5
10
-5
-4
-3
-2
-1
0
Bias voltage (V)
(a) Temperature = 150°C -4
10 Current (A)
Fig. 2. XRD pattern of ZHS microcubes on ZnO nanorods.
appears in a microcube structure with a larger volume than the nanorods. Beside, ZHS microcubes spread widely but not fully covered on ZnO nanorods. The X-ray diffraction (XRD) pattern shown in Fig. 2 gives a mixed phase of hexagonal ZnO crystals and cubic crystals of ZHS (JCPDS 201455) by using Cu-Kα radiation measurement. The ZHS microcubes grow preferentially along the (2 0 0) planes from the experimental result [14].
-5
10
ZHS/ZnO air 100 ppm CO2 500 ppm CO2 2500 ppm CO2
-6
10
-5
-4
-3
-2
-1
0
Bias voltage (V)
(b) Fig. 4. I-V characteristics of the (a) ZnO nanorods (b) ZHS-microcubes/ZnOnanorods devices with different CO2 concentrations under 150 °C.
3.2. Gas sensing analysis
device, there is another n-n heterojunction exists between ZHS and ZnO. Due to the depletion region and the barrier in the p-n heterojunction, Iair is small but some leakage current still can be produced as the bias increases. This phenomenon can be observed for both devices. But the n-n heterojunction in the ZHS covered sample forms an extra barrier to hinder the leakage. Hence, it leads Iair of the ZHS covered one lower than the pure ZnO nanorods device. During the measurement, CO2 gas flows in and contacts with the surface of the sensors. Gas molecules have some reactions with oxide materials. Adsorption of the molecules creates carbonate ions, which are negatively charged in metal oxides [15]. After the ions form on the surface, the electron concentration in the materials decreases and results in higher resistivity. As a result, the potential barrier between the p-n junction is also decreased and thus the sensors have larger current under exposure of CO2 gas. Same reactions are expected to occur in the p-n and the n-n heterojunctions for the ZHS microcubes covered sample. So the response current ICO2 of both devices are increased and over the value of Iair. It is the catalytic property and the extra n-n heterojunction make the deviation between ICO2 and Iair more obvious. To compare the ICO2 and Iair for both devices clearer, we plot their responses based on the definition of ((ICO2 − Iair)/Iair) × 100% in Fig. 6. The numerator of the fraction is the increment of current after CO2 flowed in and contacted the sensor. Sensor response will be higher if we lower the denominator (Iair) and increase the numerator (ICO2 − Iair) of the fraction. As mentioned above, Iair of ZHS covered device is smaller due to the extra n-n heterojunction. Besides, the increments of current in Fig. 4 under every CO2 concentration are larger in ZHS covered CO2 sensor. Because both ZnO and ZHS can react with CO2 and increase the number of electron in the materials. As we can see, under the optimal operation temperature (150 °C) and 2500 ppm CO2, the ZHS could promote the response from 80% to 350%. The improvement is very obvious and outstanding as we compare the response to other metal oxide based CO2 sensors [16–18]. Furthermore, the sensing transient of both devices to CO2
Both ZnO nanorods and ZnSn(OH)6-ZnO nanocomposites were developed on p-type Si substrates thus forming the devices with p-n junction. To realize the device structure more clearly, we illustrate the schematic diagram in Fig. 3. To test the response of the devices to CO2 gas, the current-voltage characteristics of pure ZnO nanorods and ZHS/ ZnO nanocomposites samples were measured. Both samples were biased negatively between top and bottom Al electrodes. The results of the I-V curves under reverse bias are shown in Fig. 4(a) and (b). All the measurements were tested at the temperature of 150 °C and changed the CO2 concentrations from 100 ppm to 2500 ppm. The currents of the devices in air before CO2 flowed in were also measured for comparison. All the measured currents including the response current under CO2 and air condition (ICO2 and Iair) increase with the operating voltage for both structures. As the gas concentration increases, ICO2 increases significantly, especially for the ZHS covered sample under high negative bias voltage. We can also find out that some of the measured currents of ZHS covered device are lower than the pure ZnO nanorods sample. ICO2 of ZHS covered device is smaller than Iair of pure ZnO nanorods even under 500 ppm CO2. We use the band diagrams for both devices under reverse bias as shown in Fig. 5 to illustrate these facts. For the pure ZnO nanorods device, only one heterojunction (p-n) exists between the p-type Si substrate and n-type ZnO nanorods. However, for the ZHS covered
Fig. 3. Schematic diagram of the ZHS-microcubes/ZnO-nanorods device structure. 3
Solid State Electronics 164 (2020) 107711
F.-R. Juang and B.-Y. Chen
Fig. 5. Gas sensing mechanism and band diagrams of the ZnO-nanorods devices with and without ZHS-microcubes under reverse bias.
400 ZnO ZHS/ZnO AZO [16] BaTiO3-CuO [17] CdO [18]
Response (%)
300 200 100 0 100
150
200
250
300
350
Temperature (°C) Fig. 6. Comparison of the responses of the devices under different operating temperatures.
(100–1000 ppm) at the optimal operating temperature of 150 °C are shown in Fig. 7(a) and (b). The definition of the response and recovery times are the time required for a change of the current to reach 90% of the equilibrium value [19]. Based on the results, the ZHS catalytic function makes the sensor have shorter response and recovery times (6 and 19 s). 4. Conclusions In this work, n-type ZnO nanorods are deposited on p-type silicon substrate with and without ZHS microcubes covered on the top to study the effect of ZHS on CO2 sensing response. Experimental results show that the addition of ZHS microcubes enhances its CO2 response up to 350%, which is higher than the reported metal oxide-based CO2 sensors with or without metal catalyst. ZHS has a good performance as a catalyst. Thus, we expect the enhancement of the sensor response will be more significant in real applications under CO2 gas surrounding.
Fig. 7. Transient response curves of (a) ZnO nanorods (b) ZHS-microcubes/ ZnO-nanorods sensors in 100–1000 ppm CO2 under 150 °C.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
Declaration of Competing Interest This work was supported by Ministry of Science and Technology, Taiwan, under contract MOST 104-2221-E-110-048. The authors would
The authors declare that they have no known competing financial 4
Solid State Electronics 164 (2020) 107711
F.-R. Juang and B.-Y. Chen
like to thank all the members of Thin Film Devices Laboratory (NSYSU).
Physica B 2008;403:3713–7. https://doi.org/10.1016/j.physb.2008.06.020. [14] Mahmood MA, Dutta J. Microwave assisted hydrothermal synthesis of zinc hydroxystannate films on glass substrates. J Sol-Gel Sci Technol 2012;62:495–504. https://doi.org/10.1007/s10971-012-2754-2. [15] Wang Y, Kovacik R, Meyer B, Kotsis K, Stodt D, Staemmler V, et al. CO2 activation by ZnO through the formation of an unusual tridentate surface carbonate. Angew Chem-Int Ed 2007;46:5624–7. https://doi.org/10.1002/anie.200700564. [16] Patil A, Dighavkar C, Borse R. Al doped ZnO thick films as CO2 gas sensors. J Optoelectron Adv Mater 2011;13:1331–7. [17] Herran J, Mandayo GG, Ayerdi I, Castano E. Influence of silver as an additive on BaTiO3-CuO thin film for CO2 monitoring. Sens Actuator B-Chem 2008;129:386–90. https://doi.org/10.1016/j.snb.2007.08.036. [18] Krishnakumar T, Jayaprakash R, Prakash T, Sathyaraj D, Donato N, Licoccia S, et al. CdO-based nanostructures as novel CO2 gas sensors. Nanotechnology 2011;22:325501https://doi.org/10.1088/0957-4484/22/32/325501. [19] Zouadi N, Messaci S, Sam S, Bradai D, Gabouze N. CO2 detection with CNx thin films deposited on porous silicon. Mater Sci Semicond Process 2015;29:367–71. https:// doi.org/10.1016/j.mssp.2014.07.023.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.sse.2019.107711. References [1] Jeong YJ, Balamurugan C, Lee DW. Enhanced CO2 gas-sensing performance of ZnO nanopowder by La loaded during simple hydrothermal method. Sens Actuator BChem 2016;229:288–96. https://doi.org/10.1016/j.snb.2015.11.093. [2] Liu Y, Lee D, Zhang X, Yoon J. Fluoride ion activated CO2 sensing using sol-gel system. Dyes Pigment 2017;139:658–63. https://doi.org/10.1016/j.dyepig.2016. 12.069. [3] Pérez de Vargas-Sansalvador IM, Erenas MM, Diamond D, Quilty B, Capitán-Vallvey LF. Water based-ionic liquid carbon dioxide sensor for applications in the food industry. Sens Actuator B-Chem 2017;253:302–9. https://doi.org/10.1016/j.snb. 2017.06.047. [4] Calvo LM, Domingo R. Influence of process operating parameters on CO2 emissions in continuous industrial plants. J Clean Prod 2015;96:253–62. https://doi.org/10. 1016/j.jclepro.2014.05.016. [5] Miller DR, Akbar SA, Morris PA. Nanoscale metal oxide-based heterojunctions for gas sensing: a review. Sens Actuator B-Chem 2014;204:250–72. https://doi.org/10. 1016/j.snb.2014.07.074. [6] Hieu NV, Khoang ND, Trung DD, Toan LD, Duy NV, Hoa ND. Comparative study on CO2 and CO sensing performance of LaOCl-coated ZnO nanowires. J Hazard Mater 2013;244:209–16. https://doi.org/10.1016/j.jhazmat.2012.11.023. [7] El-Sayed AM, Ismail FM, Yakout SM. Electrical conductivity and sensitive characteristics of Ag-added BaTiO3-CuO mixed oxide for CO2 gas sensing. J Mater Sci Technol 2011;27:35–40. https://doi.org/10.1016/S1005-0302(11)60022-4. [8] Wang H, Liu XX, Xie J, Duan M, Tang JL. Effect of humidity on the CO gas sensing of ZnSn(OH)6 film via quartz crystal microbalance technique. J Alloys Compd 2016;657:691–6. https://doi.org/10.1016/j.jallcom.2015.10.149. [9] Zhang HH, Song P, Han D, Yan HH, Yang ZX, Wang Q. Controllable synthesis of novel ZnSn(OH)6 hollow polyhedral structures with superior ethanol gas-sensing performance. Sens Actuator B-Chem 2015;209:384–90. https://doi.org/10.1016/j. snb.2014.11.140. [10] Tang LQ, Zhao ZY, Zhou Y, Lv BH, Li P, Ye JH, et al. Series of ZnSn(OH)6 polyhedra: enhanced CO2 dissociation activation and crystal facet-based homojunction boosting solar fuel synthesis. Inorg Chem 2017;56:5704–9. https://doi.org/10. 1021/acs.inorgchem.7b00219. [11] Juang FR, Chern WC, Chen BY. Carbon dioxide gas sensing properties of ZnSn (OH)6-ZnO nanocomposites with ZnO nanorod structures. Thin Solid Films 2018;660:771–6. https://doi.org/10.1016/j.tsf.2018.03.069. [12] Baruah S, Dutta J. Hydrothermal growth of ZnO nanostructures. Sci Technol Adv Mater 2009;10:013001https://doi.org/10.1088/1468-6996/10/1/013001. [13] Polsongkram D, Chamninok P, Pukird S, Chow L, Lupan O, Chai G, et al. Effect of synthesis conditions on the growth of ZnO nanorods via hydrothermal method.
Feng-Renn Juang received the B.S. and M.S. degrees in electrical engineering from National Cheng Kung University, Tainan, Taiwan, in 2006 and 2008, respectively. He also received the Ph.D. degree in microelectronics from National Cheng Kung University in 2011. He joined as a Faculty Member with National Sun Yat-sen University, Kaohsiung, Taiwan, in 2013, and is currently an Assistant Professor with the Department of Electrical Engineering. His current research interests include nanomaterials, gas sensors, and electrochemical sensors.
Bo-Yai Chen received the M.S. degrees in electrical engineering from National Sun Yat-sen University, Kaohsiung, Taiwan, in 2015. His current research interests include semiconductor materials and gas sensors.
5
Contents lists available at ScienceDirect
Solid State Electronics journal homepage: www.elsevier.com/locate/sse
Effect of adding ZHS microcubes on ZnO nanorods for CO2 gas sensing applications
T
⁎
Feng-Renn Juang , Bo-Yai Chen Department of Electrical Engineering, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon dioxide Gas sensor Heterojunctions Zinc hydroxystannate
Conventionally, the catalytic promotion of semiconductor metal oxide-based CO2 sensors response is simply based on chemical reaction. In most realistic applications, the sensors are operated in both sensing gas and light surrounding. Hence, we try to use a new type of catalyst, such as ZnSn(OH)6 (ZHS) has both chemical and photo catalytic functions to improve gas response more significantly. In this work, ZnO nanorods are deposited on ptype silicon substrate with and without ZHS microcubes covered on the top. The effects of adding the ZHS microcubes on CO2 sensing response have been studied in detail with experimental measurements. Experimental results show that the added ZHS microcubes promote CO2 response up to 350%, which is higher than the reported CO2 sensors with or without metal catalyst.
1. Introduction Carbon dioxide (CO2) gas sensors have been applied widely, such as early fire detection, medical diagnosis, air and food quality control, and industrial processes [1–4]. In these applications, the important concerns of sensors are high response and low cost. Thus the semiconductor-based sensing system is one of the most promising, compact, low-cost methods for CO2 detection. Among these systems, zinc oxide (ZnO) has become a favorable material for its wide band gap (~3.2–3.4 eV). It attracts more attention while it combines with nanorods structure for high surface/volume ratio [1]. On the other hand, some techniques such as addition of a catalyst and/or heterojunctions have proposed [5]. For example, addition of La in ZnO [6] and Ag in BaTiO3 [7] as catalysts show great improvements in response to CO2. The added catalyst can modify the electrical properties of base materials, while the heterojunctions enhance the depletion layer resistance modulation. However, the reported metal catalysts can only modify the base material chemically or physically [5]. Gas sensors are operated under both sensing gas and light condition in most practical cases. Hence we try to use a new type of catalyst, which has both chemical and optical catalytic functions to improve gas response more significantly. Recently, zinc hydroxystannate (ZnSn (OH)6, ZHS) has been widely used for gas sensing and CO2 dissociation activation applications [8–10]. The researches imply the material is suitable for this application.
⁎
This is a further research from our previously related work, which investigate ZHS microcubes on different ZnO nanorod structures [11]. In this work, we propose a band diagram model and have more detailed experiments to discuss the improvement of adding ZHS microcubes on ZnO nanorods. We have prepared ZnO nanorods with and without ZHS microcubes covered on p-type Si substrate by a low temperature hydrothermal synthesis. In order to study the effect of ZHS on CO2 sensing performances, the sensing ability of both structures are examined. Design concept of the devices is based on low leakage current of p-n junction. Therefore, we use p-Si substrates to fabricate the samples. The n-type materials (ZnO and ZHS) are synthesized on the substrates with nanorod or microcube structures. The effect of the heterojunction between ZnO nanorods and ZHS microcubes is discussed in this study. 2. Experimental details Hydrothermal method was used for synthesizing ZnO nanorods on a p-type silicon substrate [12,13]. Before starting the hydrothermal synthesis, a ZnO seed layer was dip-coated on the surface of the substrate. The mixture of solution for coating consisted of ethanol and 0.02 M Zn(CH3COO)2·2H2O and was heated up to 60 °C. For forming the ZnO film, a sintering process is followed by using a furnace under 400 °C for 30 min. After coating ZnO seed layer on the substrate, the sample was put into a beaker containing C6H12N4 and Zn(NO3)2·6H2O. The composition of these two chemicals had same concentration (0.07 M). The flower-like ZnO nanorods were grown on the ZnO seed
Corresponding author at: Department of Electrical Engineering, National Sun Yat-sen University, 70 Lienhai Rd., Kaohsiung 80424, Taiwan. E-mail address: [email protected] (F.-R. Juang).
https://doi.org/10.1016/j.sse.2019.107711 Received 8 May 2019; Received in revised form 24 October 2019; Accepted 18 November 2019 Available online 21 November 2019 0038-1101/ © 2019 Elsevier Ltd. All rights reserved.
Solid State Electronics 164 (2020) 107711
F.-R. Juang and B.-Y. Chen
Fig. 1. Top view FESEM images of (a-b) ZnO nanorods at different magnifications; (c-e) ZHS microcubes/ZnO nanorods at different magnifications.
Fig. 1. (continued)
layer by immersing the sample into the solution under 90 °C for 5 h. In order to study the effect of adding ZnSn(OH)6 microcubes to the CO2 sensor, other group of samples were added some ZHS microcubes after ZnO nanorods synthesized on the substrate. ZHS microcubes were synthesized hydrothermally with 0.02 M SnCl4·5H2O and 0.2 M NaOH under 90 °C for 2 h. The microcubes spread on the nanorods with different sizes. Metal electrodes (Al) were evaporated respectively on the top of oxides (pure ZnO nanorods or ZHS/ZnO nanocomposites) and the bottom of the substrate to complete the sensing devices. Materials investigations and gas sensing properties of the devices were studied to confirm the improvement of adding ZHS microcubes. 3. Results and discussion 3.1. Material investigations To observe the morphology of ZnO nanorods, field emission scanning electron microscope (FESEM) was used from the top of the sample. The diameters of nanorods are all in nanoscale and the structure have a flower-like appearance as shown in Fig. 1(a) and (b). Moreover, the top view FESEM images of samples with nanocomposites are compared in Fig. 1(c)–(e) under different scales. As shown in the images, the ZHS
Fig. 1. (continued) 2
Solid State Electronics 164 (2020) 107711
F.-R. Juang and B.-Y. Chen
Temperature = 150°C
Current (A)
-4
10
ZnO
air 100 ppm CO2 500 ppm CO2 2500 ppm CO2
-5
10
-5
-4
-3
-2
-1
0
Bias voltage (V)
(a) Temperature = 150°C -4
10 Current (A)
Fig. 2. XRD pattern of ZHS microcubes on ZnO nanorods.
appears in a microcube structure with a larger volume than the nanorods. Beside, ZHS microcubes spread widely but not fully covered on ZnO nanorods. The X-ray diffraction (XRD) pattern shown in Fig. 2 gives a mixed phase of hexagonal ZnO crystals and cubic crystals of ZHS (JCPDS 201455) by using Cu-Kα radiation measurement. The ZHS microcubes grow preferentially along the (2 0 0) planes from the experimental result [14].
-5
10
ZHS/ZnO air 100 ppm CO2 500 ppm CO2 2500 ppm CO2
-6
10
-5
-4
-3
-2
-1
0
Bias voltage (V)
(b) Fig. 4. I-V characteristics of the (a) ZnO nanorods (b) ZHS-microcubes/ZnOnanorods devices with different CO2 concentrations under 150 °C.
3.2. Gas sensing analysis
device, there is another n-n heterojunction exists between ZHS and ZnO. Due to the depletion region and the barrier in the p-n heterojunction, Iair is small but some leakage current still can be produced as the bias increases. This phenomenon can be observed for both devices. But the n-n heterojunction in the ZHS covered sample forms an extra barrier to hinder the leakage. Hence, it leads Iair of the ZHS covered one lower than the pure ZnO nanorods device. During the measurement, CO2 gas flows in and contacts with the surface of the sensors. Gas molecules have some reactions with oxide materials. Adsorption of the molecules creates carbonate ions, which are negatively charged in metal oxides [15]. After the ions form on the surface, the electron concentration in the materials decreases and results in higher resistivity. As a result, the potential barrier between the p-n junction is also decreased and thus the sensors have larger current under exposure of CO2 gas. Same reactions are expected to occur in the p-n and the n-n heterojunctions for the ZHS microcubes covered sample. So the response current ICO2 of both devices are increased and over the value of Iair. It is the catalytic property and the extra n-n heterojunction make the deviation between ICO2 and Iair more obvious. To compare the ICO2 and Iair for both devices clearer, we plot their responses based on the definition of ((ICO2 − Iair)/Iair) × 100% in Fig. 6. The numerator of the fraction is the increment of current after CO2 flowed in and contacted the sensor. Sensor response will be higher if we lower the denominator (Iair) and increase the numerator (ICO2 − Iair) of the fraction. As mentioned above, Iair of ZHS covered device is smaller due to the extra n-n heterojunction. Besides, the increments of current in Fig. 4 under every CO2 concentration are larger in ZHS covered CO2 sensor. Because both ZnO and ZHS can react with CO2 and increase the number of electron in the materials. As we can see, under the optimal operation temperature (150 °C) and 2500 ppm CO2, the ZHS could promote the response from 80% to 350%. The improvement is very obvious and outstanding as we compare the response to other metal oxide based CO2 sensors [16–18]. Furthermore, the sensing transient of both devices to CO2
Both ZnO nanorods and ZnSn(OH)6-ZnO nanocomposites were developed on p-type Si substrates thus forming the devices with p-n junction. To realize the device structure more clearly, we illustrate the schematic diagram in Fig. 3. To test the response of the devices to CO2 gas, the current-voltage characteristics of pure ZnO nanorods and ZHS/ ZnO nanocomposites samples were measured. Both samples were biased negatively between top and bottom Al electrodes. The results of the I-V curves under reverse bias are shown in Fig. 4(a) and (b). All the measurements were tested at the temperature of 150 °C and changed the CO2 concentrations from 100 ppm to 2500 ppm. The currents of the devices in air before CO2 flowed in were also measured for comparison. All the measured currents including the response current under CO2 and air condition (ICO2 and Iair) increase with the operating voltage for both structures. As the gas concentration increases, ICO2 increases significantly, especially for the ZHS covered sample under high negative bias voltage. We can also find out that some of the measured currents of ZHS covered device are lower than the pure ZnO nanorods sample. ICO2 of ZHS covered device is smaller than Iair of pure ZnO nanorods even under 500 ppm CO2. We use the band diagrams for both devices under reverse bias as shown in Fig. 5 to illustrate these facts. For the pure ZnO nanorods device, only one heterojunction (p-n) exists between the p-type Si substrate and n-type ZnO nanorods. However, for the ZHS covered
Fig. 3. Schematic diagram of the ZHS-microcubes/ZnO-nanorods device structure. 3
Solid State Electronics 164 (2020) 107711
F.-R. Juang and B.-Y. Chen
Fig. 5. Gas sensing mechanism and band diagrams of the ZnO-nanorods devices with and without ZHS-microcubes under reverse bias.
400 ZnO ZHS/ZnO AZO [16] BaTiO3-CuO [17] CdO [18]
Response (%)
300 200 100 0 100
150
200
250
300
350
Temperature (°C) Fig. 6. Comparison of the responses of the devices under different operating temperatures.
(100–1000 ppm) at the optimal operating temperature of 150 °C are shown in Fig. 7(a) and (b). The definition of the response and recovery times are the time required for a change of the current to reach 90% of the equilibrium value [19]. Based on the results, the ZHS catalytic function makes the sensor have shorter response and recovery times (6 and 19 s). 4. Conclusions In this work, n-type ZnO nanorods are deposited on p-type silicon substrate with and without ZHS microcubes covered on the top to study the effect of ZHS on CO2 sensing response. Experimental results show that the addition of ZHS microcubes enhances its CO2 response up to 350%, which is higher than the reported metal oxide-based CO2 sensors with or without metal catalyst. ZHS has a good performance as a catalyst. Thus, we expect the enhancement of the sensor response will be more significant in real applications under CO2 gas surrounding.
Fig. 7. Transient response curves of (a) ZnO nanorods (b) ZHS-microcubes/ ZnO-nanorods sensors in 100–1000 ppm CO2 under 150 °C.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
Declaration of Competing Interest This work was supported by Ministry of Science and Technology, Taiwan, under contract MOST 104-2221-E-110-048. The authors would
The authors declare that they have no known competing financial 4
Solid State Electronics 164 (2020) 107711
F.-R. Juang and B.-Y. Chen
like to thank all the members of Thin Film Devices Laboratory (NSYSU).
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Feng-Renn Juang received the B.S. and M.S. degrees in electrical engineering from National Cheng Kung University, Tainan, Taiwan, in 2006 and 2008, respectively. He also received the Ph.D. degree in microelectronics from National Cheng Kung University in 2011. He joined as a Faculty Member with National Sun Yat-sen University, Kaohsiung, Taiwan, in 2013, and is currently an Assistant Professor with the Department of Electrical Engineering. His current research interests include nanomaterials, gas sensors, and electrochemical sensors.
Bo-Yai Chen received the M.S. degrees in electrical engineering from National Sun Yat-sen University, Kaohsiung, Taiwan, in 2015. His current research interests include semiconductor materials and gas sensors.
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