- Email: [email protected]
Enhancement of hydrogen sensing response of ZnO nanowires for the decoration of WO3 nanoparticles
Accepted Manuscript Enhancement of hydrogen sensing response of ZnO nanowires for the decoration of WO3 nanoparticles Sunghoon Park PII: DOI: Reference:
S0167-577X(18)31517-9 https://doi.org/10.1016/j.matlet.2018.09.129 MLBLUE 24997
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
18 August 2018 18 September 2018 24 September 2018
Please cite this article as: S. Park, Enhancement of hydrogen sensing response of ZnO nanowires for the decoration of WO3 nanoparticles, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.09.129
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement of hydrogen sensing response of ZnO nanowires for the decoration of WO3 nanoparticles Sunghoon Park1 School of intelligent and mechatronic engineering, Sejong university, 209 Neungdong-ro, Gwangjin-gu, Seoul, South Korea WO3 nanoparticle-decorated ZnO nanowires are synthesized by the hydrothermal and vaporliquid solid (VLS) methods, respectively. WO3 nanoparticles are decorated on the surface of ZnO nanowires to fabricate the highly sensitive hydrogen gas sensor. This structured sensor presents a sensing response that is 4.3 times higher than its counterpart for 2000 ppm hydrogen gas. The variation in depletion layer width of the ZnO nanowires increases as the WO3 nanoparticles are decorated. This improves the sensing response of the WO3 nanoparticledecorated ZnO nanowire sensor compared with its counterpart. In this research, a synthetic method of WO3 nanoparticle-decorated ZnO nanowires is described, and its physical and chemical properties are analyzed. Moreover, the sensing responses are measured, and the sensing mechanism is discussed in this literature. Keywords: nanowires, nanoparticles, sensor, hydrogen, ZnO, WO3. Introduction Hydrogen gas is applied in various industrial fields, and the demand for it is currently increasing since being developed as a fuel for next-generation eco-friendly vehicles. However, since this gas has no color, smell, or taste, its detection is very important for its safe use [1]. Many sensors to detect this gas have been developed in previous decades [2-4], however, their sensitivities are not sufficient for the application of commercial hydrogen sensors.
1
Corresponding author. Tel.: +82 2 6935 2522. E-mail address: [email protected] (S. Park)
One-dimensional nanomaterials are applied in chemical gas sensors owing to their unique properties such as large surface area, high aspect ratio, and small diameters [5-7]. Among these nanomaterials, ZnO nanowire is well-known to be highly sensitive to hydrogen gas [8-10]. Moreover, many hydrogen sensors are developed for the conditions shown in Table 1. However, the sensitivity of the hydrogen sensor requires further improvement, and WO3 nanoparticles [16], which are also well-known highly sensitive hydrogen sensing materials, are decorated onto ZnO nanowires to improve the hydrogen-sensing properties of this sensor. In this literature, the method for the WO3 nanoparticle-decorated ZnO nanowire-based hydrogen gas sensor is described, and its sensing properties and mechanisms are also discussed. Experimental ZnO nanowires are synthesized by the VLS method. A 1-g mixture of ZnO and graphite (1:1) powder is put into an alumina crucible. A 1-nm Au-coated sapphire substrate is loaded onto a crucible, which is inserted in the center of the horizontal tube furnace. The furnace is sealed and a vacuum is developed to 1 mTorr. The chamber is heated to 800 °C, and 2 sccm O2 and 98 sccm N2 gas are supplied to the chamber over 1 h. Then, the gas supply is stopped and the chamber is cooled down to room temperature. A light gray velvet-like thin film can be observed when the sample is taken out. WO3 nanoparticles are synthesized using the hydrothermal method. 10 mM of ammonium tungstate hydrate and 10 mM nitric acid are mixed with the same ratio. A total of 50 ml of this solution is transferred to the hydrothermal kit and sealed tightly. This kit is inserted into a convection oven heated to 180 °C for 10 h. Then, the kit is cooled to room temperature, and some greenish blue colored precipitate can be found at the bottom of the solution. This solution is centrifuged to 3000 rpm over 5 min, and the precipitate is transferred to 100 ml of ethanol. This solution is ultrasonicated for 30 min, and 1 ml of solution is dropped onto a ZnO nanowire template. This template is spin coated at 5000 rpm for 100 s, and dried using a N2 gun.
The synthesized samples are analyzed using scanning electron microscopy (SEM, Hitachi S4200) and X-ray diffraction (XRD, Philips X’pert MRD pro). The morphologies and crystal structures of the samples are analyzed using these pieces of apparatus. To measure the sensing properties of the samples, an interdigital electrode (IDE) patterned shadow mask is loaded onto samples, and a Ti/Au (10 nm/100 nm) electrode is sputtered. The shadow mask is removed after the sputtering process. The design gap of the electrodes is 0.2 mm. These samples are installed in the sensing chamber and connected to a sourcemeter. Hydrogen gas of 100, 200, 500, 1000, and 2000 ppm, respectively, is supplied to the samples to measure their response, and the samples are then recovered with synthetic air. To measure the response and recovery reaction, both hydrogen and air gas are supplied for a duration of 200 s, respectively. The flow rate of the supply gas is fixed at 200 sccm to measure the sensing properties accurately. The operation temperature is manipulated to 100, 150, 200, 250, and 300 °C to determine the optimum sensing condition. The responses of the sensors are defined as Ra/Rg, where Ra and Rg are the resistances when the sensors are exposed to air and hydrogen gas, respectively. Results and Discussion Figs. 1(a) and (b) are SEM images of bare ZnO nanowire (ZnO NW) and WO3 nanoparticledecorated ZnO nanowire (WO3-ZnO NW). In the figures, synthesized nanowires are presented at 30-80 nm in diameter and several tens of micrometers in length. The nanoparticles are decorated on the nanowires in Fig. 1(b), and the diameters of these nanoparticles are approximately 50 nm. Fig. 1(c) presents the XRD patterns of ZnO NW and WO3-ZnO NW. Both patterns include several peaks indicating that the synthesized nanowires are ZnO (JCPDS No. 89-1397). In contrast, the pattern of WO3-ZnO NW includes the additional three peaks, indicating that the nanoparticles are WO3 (89-4482). Fig. 2(a) presents the sensing responses of ZnO NW (black line) and WO3-ZnO NW (red line) based sensors as functions of operating temperature. Based on this figure, the optimum
operation temperature can be recognized as 200 °C in both sensors. Figs. 2(b)-(c) present the dynamic response curves of ZnO NW and WO3-ZnO NW, respectively, as a function of hydrogen concentration. The WO3-ZnO NW based sensor presents better responses at all hydrogen concentrations, and followed by Fig. 2(d), this sensor presents an enhanced response of 4.3 times when the sensors are exposed to 2000 ppm hydrogen gas compared with the ZnO NW based sensor. The resistance of nanowire-based hydrogen sensors shifts based on the following mechanisms [17]. When nanowires are exposed to air, O2 gas in the air is adsorbed to the nanowire surface. In the meantime, electrons in the nanowire are attracted to the adsorbed oxygen molecules and absorb to these molecules. As a result, the oxygen molecules change to oxygen ions. Since ZnO is an n-type semiconductor that uses electrons as electrical carriers, the conductive channel of ZnO reduces owing to these reactions. In contrast, if hydrogen gas is supplied to the sensor, it is adsorbed to the surface of the nanowires. Moreover, these adsorbed hydrogen molecules react with the oxygen ions adsorbed at the surface of the nanowires. H2O molecules are generated as a result of this reaction, and this gas is desorbed from the surface of the nanowires. In the meantime, the electrons absorbed to the oxygen ions revert to nanowires, and the concentration of the electrical carriers of nanowires increases again. As a result of these carriers, the resistance of the nanowires decreases again as presented in Fig. 3(a). In contrast, the variation in depletion layer width is enhanced as the WO3 nanoparticles are decorated on the ZnO nanowire surface. Since WO3 is an n-type semiconductor, the depletion layer expands as the WO3 nanoparticles are exposed to air; this is similar for the ZnO nanowire. Electrons in the ZnO nanowire transfer to the attached WO3 nanoparticles, and the depletion layer of the ZnO nanowire expands more compared to that of ZnO NW. As a result, the resistance of WO3-ZnO NW is higher than that of ZnO NW. In contrast, if WO3-ZnO NW is exposed to hydrogen gas, oxygen ions are eliminated in the same manner as with ZnO NW. Electrons absorbed to oxygen
ions revert to WO3 nanoparticles and ZnO nanowires, and electrons transferred to WO3 nanoparticles from the ZnO nanowire revert to the ZnO nanowire. As a result, the depletion layer width reduces as presented in Fig. 3(b), and resistance of WO3-ZnO NW decreases. As results of these reactions, the resistances of both sensors change. However, since the variation in depletion layer width of WO3-ZnO NW is larger than that of ZnO NW, the hydrogen sensing responses are better in the case of WO3-ZnO NW compared to ZnO NW. Conclusion Hydrogen gas is used in numerous industrial fields. However, since this gas can be a source of explosions, its detection is very important for its safe use. ZnO nanowires are well-known to be highly sensitive to hydrogen gas, and their sensing response can be improved by 4.3 times by decorating WO3 nanoparticles on their surfaces. These results show the origin of the variation in the depletion layer at the nanowire surface. The variation in width of the depletion layer of the ZnO nanowires increases for the decoration of WO3 nanoparticles on their surfaces, and therefore the hydrogen sensing response of WO3-ZnO NW is improved compared with ZnO NW. Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2017R1D1A1B03034567). References [1] N.D. Hoa, P.V. Tong, C.M. Hung, N.V. Duy, N.V. Hieu, Int. J. Hydrog. Energ. 43 (2018) 9446-9453. [2] K.-S. Choi, S.-P. Chang, Mater. Lett. 230 (2018) 48-52. [3] S. Park, Curr. Appl. Phys. 16 (2016) 2163-1269. [4] J.-S. Jang, S. Qiao, S.-J. Choi, G. Jha, A.F. Ogata, W.-T. Koo, D.-H. Kim, I.-D. Kim, M.
Penner, ACS Appl. Mater. Interfaces 9 (2017) 39464-39474. [5] S. Park, J. Alloys Compd. 696 (2017) 655-662. [6] K.K. Kim, D. Kim, S.-H. Kang, S. Park, Sens. Actuators B 248 (2017) 43-49. [7] K.S. Choi, S. Park, S.-P. Chang, Sens. Actuators B 238 (2017) 871-879. [8] M. Kumar, V.S. Bhati, S. Ranwa, J. Singh, M. Kumar, Sci. Rep. 7 (2017) 236. [9] Z.U. Abideen, H.W. Kim, S.S. Kim, Chem. Commun. 51 (2015) 15418-15421. [10] T.-R. Rashid, D.-T. Phan, G.-S. Chung, Sens. Actuators 193 (2014) 869-876. [11] X. San, G. Wang, B. Liang, Y. Song, S. Gao, J. Zhang, F. Meng, J. Alloys Compd. 622 (2015) 73-78. [12] M. Tonezzer, S. Iannotta, Talanta 122 (2014) 201-208. [13] D. Kathiravan, B.-R. Huang, A. Saravanan, ACS Appl. Mater. Interfaces 9 (2017) 1206412072. [14] M.T. Hosseinnejad, M. Ghoranneviss, M.R. Hantehzadeh, E. Darabi, J. Mater. Sci. 27 (2016) 11308-11318. [15] T.-R. Rashid, D.-T. Phan, G.-S. Chung, Sens. Actuators B 185 (2013) 777-784. [16] M. Horprathum, T. Srichaiyaperk, B. Samransuksamer, A. Wisitsoraat, P. Eiamchai, S. Limwichean, C. Chananonnawathorn, K. Aiempanakit, N. Nuntawong, V. Patthanasettakul, C. Oros, S. Porntheeraphat, P. Songsiriritthigul, H. Nakajima, A. Tuantranont, P. Chindaudom, ACS Appl. Mater. Interfaces 6 (2014) 22051-22060. [17] S. Park, S. Park, S. Lee, H.W. Kim, C. Lee, Sens. Actuators B 202 (2014) 840-845. Table 1. Sensing responses of ZnO nanostructure-based hydrogen gas sensors as reported in recent literatures.
Sensor architecture
Operation temp. (oC)
H2 conc. (ppm)
Sensor resp.(Ra/Rg)
Ref.
Cat. free ZnO nanostructure
200
5000
8.5
[11]
2 dimensional ZnO
300
500
5.7
[12]
ZnO nanotube/graphene
Room temperature
100
1.28
[13]
Al doped ZnO thin film
300
1000
1.7
[14]
Pd nanoparticle decorated ZnO nanorod
Room temperature
1000
1.9
[15]
WO3-ZnO nanowire
200
2000
12.6
This work
Figure captions 1. SEM images of (a) bare ZnO nanowires, (b) WO3 nanoparticle-decorated ZnO nanowires, and (c) XRD patterns of bare (black line) and WO3 nanoparticle-decorated (red line) ZnO nanowires. 2. (a) Hydrogen sensing responses of bare (black line) and WO3 nanoparticle-decorated (red line) ZnO nanowires as a function of operating temperature; Dynamic response curves of (b) bare and (c) WO3 nanoparticle-decorated ZnO nanowires as a function of hydrogen concentrations; and (d) Summarized responses of corresponding sensors as a function of hydrogen concentration. 3. Schematic images of electrical structures of (a) bare and (b) WO3 nanoparticledecorated ZnO nanowire exposed to air and hydrogen gas, respectively. Figures Figure 1
S. Park Figure 2.
S. Park Figure 3.
S. Park ZnO NW and WO3 NP are synthesized VLS and hydrothermal method, respectively. WO3 nanoparticle are decorated on ZnO nanowire surface to fabricate high sensitive H2 sensor. H2 sensing responses of WO3-ZnO NW is improved compared with ZnO NW. Expansion of depletion layers of WO3-ZnO NW makes sensing responses improved.
S0167-577X(18)31517-9 https://doi.org/10.1016/j.matlet.2018.09.129 MLBLUE 24997
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
18 August 2018 18 September 2018 24 September 2018
Please cite this article as: S. Park, Enhancement of hydrogen sensing response of ZnO nanowires for the decoration of WO3 nanoparticles, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.09.129
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement of hydrogen sensing response of ZnO nanowires for the decoration of WO3 nanoparticles Sunghoon Park1 School of intelligent and mechatronic engineering, Sejong university, 209 Neungdong-ro, Gwangjin-gu, Seoul, South Korea WO3 nanoparticle-decorated ZnO nanowires are synthesized by the hydrothermal and vaporliquid solid (VLS) methods, respectively. WO3 nanoparticles are decorated on the surface of ZnO nanowires to fabricate the highly sensitive hydrogen gas sensor. This structured sensor presents a sensing response that is 4.3 times higher than its counterpart for 2000 ppm hydrogen gas. The variation in depletion layer width of the ZnO nanowires increases as the WO3 nanoparticles are decorated. This improves the sensing response of the WO3 nanoparticledecorated ZnO nanowire sensor compared with its counterpart. In this research, a synthetic method of WO3 nanoparticle-decorated ZnO nanowires is described, and its physical and chemical properties are analyzed. Moreover, the sensing responses are measured, and the sensing mechanism is discussed in this literature. Keywords: nanowires, nanoparticles, sensor, hydrogen, ZnO, WO3. Introduction Hydrogen gas is applied in various industrial fields, and the demand for it is currently increasing since being developed as a fuel for next-generation eco-friendly vehicles. However, since this gas has no color, smell, or taste, its detection is very important for its safe use [1]. Many sensors to detect this gas have been developed in previous decades [2-4], however, their sensitivities are not sufficient for the application of commercial hydrogen sensors.
1
Corresponding author. Tel.: +82 2 6935 2522. E-mail address: [email protected] (S. Park)
One-dimensional nanomaterials are applied in chemical gas sensors owing to their unique properties such as large surface area, high aspect ratio, and small diameters [5-7]. Among these nanomaterials, ZnO nanowire is well-known to be highly sensitive to hydrogen gas [8-10]. Moreover, many hydrogen sensors are developed for the conditions shown in Table 1. However, the sensitivity of the hydrogen sensor requires further improvement, and WO3 nanoparticles [16], which are also well-known highly sensitive hydrogen sensing materials, are decorated onto ZnO nanowires to improve the hydrogen-sensing properties of this sensor. In this literature, the method for the WO3 nanoparticle-decorated ZnO nanowire-based hydrogen gas sensor is described, and its sensing properties and mechanisms are also discussed. Experimental ZnO nanowires are synthesized by the VLS method. A 1-g mixture of ZnO and graphite (1:1) powder is put into an alumina crucible. A 1-nm Au-coated sapphire substrate is loaded onto a crucible, which is inserted in the center of the horizontal tube furnace. The furnace is sealed and a vacuum is developed to 1 mTorr. The chamber is heated to 800 °C, and 2 sccm O2 and 98 sccm N2 gas are supplied to the chamber over 1 h. Then, the gas supply is stopped and the chamber is cooled down to room temperature. A light gray velvet-like thin film can be observed when the sample is taken out. WO3 nanoparticles are synthesized using the hydrothermal method. 10 mM of ammonium tungstate hydrate and 10 mM nitric acid are mixed with the same ratio. A total of 50 ml of this solution is transferred to the hydrothermal kit and sealed tightly. This kit is inserted into a convection oven heated to 180 °C for 10 h. Then, the kit is cooled to room temperature, and some greenish blue colored precipitate can be found at the bottom of the solution. This solution is centrifuged to 3000 rpm over 5 min, and the precipitate is transferred to 100 ml of ethanol. This solution is ultrasonicated for 30 min, and 1 ml of solution is dropped onto a ZnO nanowire template. This template is spin coated at 5000 rpm for 100 s, and dried using a N2 gun.
The synthesized samples are analyzed using scanning electron microscopy (SEM, Hitachi S4200) and X-ray diffraction (XRD, Philips X’pert MRD pro). The morphologies and crystal structures of the samples are analyzed using these pieces of apparatus. To measure the sensing properties of the samples, an interdigital electrode (IDE) patterned shadow mask is loaded onto samples, and a Ti/Au (10 nm/100 nm) electrode is sputtered. The shadow mask is removed after the sputtering process. The design gap of the electrodes is 0.2 mm. These samples are installed in the sensing chamber and connected to a sourcemeter. Hydrogen gas of 100, 200, 500, 1000, and 2000 ppm, respectively, is supplied to the samples to measure their response, and the samples are then recovered with synthetic air. To measure the response and recovery reaction, both hydrogen and air gas are supplied for a duration of 200 s, respectively. The flow rate of the supply gas is fixed at 200 sccm to measure the sensing properties accurately. The operation temperature is manipulated to 100, 150, 200, 250, and 300 °C to determine the optimum sensing condition. The responses of the sensors are defined as Ra/Rg, where Ra and Rg are the resistances when the sensors are exposed to air and hydrogen gas, respectively. Results and Discussion Figs. 1(a) and (b) are SEM images of bare ZnO nanowire (ZnO NW) and WO3 nanoparticledecorated ZnO nanowire (WO3-ZnO NW). In the figures, synthesized nanowires are presented at 30-80 nm in diameter and several tens of micrometers in length. The nanoparticles are decorated on the nanowires in Fig. 1(b), and the diameters of these nanoparticles are approximately 50 nm. Fig. 1(c) presents the XRD patterns of ZnO NW and WO3-ZnO NW. Both patterns include several peaks indicating that the synthesized nanowires are ZnO (JCPDS No. 89-1397). In contrast, the pattern of WO3-ZnO NW includes the additional three peaks, indicating that the nanoparticles are WO3 (89-4482). Fig. 2(a) presents the sensing responses of ZnO NW (black line) and WO3-ZnO NW (red line) based sensors as functions of operating temperature. Based on this figure, the optimum
operation temperature can be recognized as 200 °C in both sensors. Figs. 2(b)-(c) present the dynamic response curves of ZnO NW and WO3-ZnO NW, respectively, as a function of hydrogen concentration. The WO3-ZnO NW based sensor presents better responses at all hydrogen concentrations, and followed by Fig. 2(d), this sensor presents an enhanced response of 4.3 times when the sensors are exposed to 2000 ppm hydrogen gas compared with the ZnO NW based sensor. The resistance of nanowire-based hydrogen sensors shifts based on the following mechanisms [17]. When nanowires are exposed to air, O2 gas in the air is adsorbed to the nanowire surface. In the meantime, electrons in the nanowire are attracted to the adsorbed oxygen molecules and absorb to these molecules. As a result, the oxygen molecules change to oxygen ions. Since ZnO is an n-type semiconductor that uses electrons as electrical carriers, the conductive channel of ZnO reduces owing to these reactions. In contrast, if hydrogen gas is supplied to the sensor, it is adsorbed to the surface of the nanowires. Moreover, these adsorbed hydrogen molecules react with the oxygen ions adsorbed at the surface of the nanowires. H2O molecules are generated as a result of this reaction, and this gas is desorbed from the surface of the nanowires. In the meantime, the electrons absorbed to the oxygen ions revert to nanowires, and the concentration of the electrical carriers of nanowires increases again. As a result of these carriers, the resistance of the nanowires decreases again as presented in Fig. 3(a). In contrast, the variation in depletion layer width is enhanced as the WO3 nanoparticles are decorated on the ZnO nanowire surface. Since WO3 is an n-type semiconductor, the depletion layer expands as the WO3 nanoparticles are exposed to air; this is similar for the ZnO nanowire. Electrons in the ZnO nanowire transfer to the attached WO3 nanoparticles, and the depletion layer of the ZnO nanowire expands more compared to that of ZnO NW. As a result, the resistance of WO3-ZnO NW is higher than that of ZnO NW. In contrast, if WO3-ZnO NW is exposed to hydrogen gas, oxygen ions are eliminated in the same manner as with ZnO NW. Electrons absorbed to oxygen
ions revert to WO3 nanoparticles and ZnO nanowires, and electrons transferred to WO3 nanoparticles from the ZnO nanowire revert to the ZnO nanowire. As a result, the depletion layer width reduces as presented in Fig. 3(b), and resistance of WO3-ZnO NW decreases. As results of these reactions, the resistances of both sensors change. However, since the variation in depletion layer width of WO3-ZnO NW is larger than that of ZnO NW, the hydrogen sensing responses are better in the case of WO3-ZnO NW compared to ZnO NW. Conclusion Hydrogen gas is used in numerous industrial fields. However, since this gas can be a source of explosions, its detection is very important for its safe use. ZnO nanowires are well-known to be highly sensitive to hydrogen gas, and their sensing response can be improved by 4.3 times by decorating WO3 nanoparticles on their surfaces. These results show the origin of the variation in the depletion layer at the nanowire surface. The variation in width of the depletion layer of the ZnO nanowires increases for the decoration of WO3 nanoparticles on their surfaces, and therefore the hydrogen sensing response of WO3-ZnO NW is improved compared with ZnO NW. Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2017R1D1A1B03034567). References [1] N.D. Hoa, P.V. Tong, C.M. Hung, N.V. Duy, N.V. Hieu, Int. J. Hydrog. Energ. 43 (2018) 9446-9453. [2] K.-S. Choi, S.-P. Chang, Mater. Lett. 230 (2018) 48-52. [3] S. Park, Curr. Appl. Phys. 16 (2016) 2163-1269. [4] J.-S. Jang, S. Qiao, S.-J. Choi, G. Jha, A.F. Ogata, W.-T. Koo, D.-H. Kim, I.-D. Kim, M.
Penner, ACS Appl. Mater. Interfaces 9 (2017) 39464-39474. [5] S. Park, J. Alloys Compd. 696 (2017) 655-662. [6] K.K. Kim, D. Kim, S.-H. Kang, S. Park, Sens. Actuators B 248 (2017) 43-49. [7] K.S. Choi, S. Park, S.-P. Chang, Sens. Actuators B 238 (2017) 871-879. [8] M. Kumar, V.S. Bhati, S. Ranwa, J. Singh, M. Kumar, Sci. Rep. 7 (2017) 236. [9] Z.U. Abideen, H.W. Kim, S.S. Kim, Chem. Commun. 51 (2015) 15418-15421. [10] T.-R. Rashid, D.-T. Phan, G.-S. Chung, Sens. Actuators 193 (2014) 869-876. [11] X. San, G. Wang, B. Liang, Y. Song, S. Gao, J. Zhang, F. Meng, J. Alloys Compd. 622 (2015) 73-78. [12] M. Tonezzer, S. Iannotta, Talanta 122 (2014) 201-208. [13] D. Kathiravan, B.-R. Huang, A. Saravanan, ACS Appl. Mater. Interfaces 9 (2017) 1206412072. [14] M.T. Hosseinnejad, M. Ghoranneviss, M.R. Hantehzadeh, E. Darabi, J. Mater. Sci. 27 (2016) 11308-11318. [15] T.-R. Rashid, D.-T. Phan, G.-S. Chung, Sens. Actuators B 185 (2013) 777-784. [16] M. Horprathum, T. Srichaiyaperk, B. Samransuksamer, A. Wisitsoraat, P. Eiamchai, S. Limwichean, C. Chananonnawathorn, K. Aiempanakit, N. Nuntawong, V. Patthanasettakul, C. Oros, S. Porntheeraphat, P. Songsiriritthigul, H. Nakajima, A. Tuantranont, P. Chindaudom, ACS Appl. Mater. Interfaces 6 (2014) 22051-22060. [17] S. Park, S. Park, S. Lee, H.W. Kim, C. Lee, Sens. Actuators B 202 (2014) 840-845. Table 1. Sensing responses of ZnO nanostructure-based hydrogen gas sensors as reported in recent literatures.
Sensor architecture
Operation temp. (oC)
H2 conc. (ppm)
Sensor resp.(Ra/Rg)
Ref.
Cat. free ZnO nanostructure
200
5000
8.5
[11]
2 dimensional ZnO
300
500
5.7
[12]
ZnO nanotube/graphene
Room temperature
100
1.28
[13]
Al doped ZnO thin film
300
1000
1.7
[14]
Pd nanoparticle decorated ZnO nanorod
Room temperature
1000
1.9
[15]
WO3-ZnO nanowire
200
2000
12.6
This work
Figure captions 1. SEM images of (a) bare ZnO nanowires, (b) WO3 nanoparticle-decorated ZnO nanowires, and (c) XRD patterns of bare (black line) and WO3 nanoparticle-decorated (red line) ZnO nanowires. 2. (a) Hydrogen sensing responses of bare (black line) and WO3 nanoparticle-decorated (red line) ZnO nanowires as a function of operating temperature; Dynamic response curves of (b) bare and (c) WO3 nanoparticle-decorated ZnO nanowires as a function of hydrogen concentrations; and (d) Summarized responses of corresponding sensors as a function of hydrogen concentration. 3. Schematic images of electrical structures of (a) bare and (b) WO3 nanoparticledecorated ZnO nanowire exposed to air and hydrogen gas, respectively. Figures Figure 1
S. Park Figure 2.
S. Park Figure 3.
S. Park ZnO NW and WO3 NP are synthesized VLS and hydrothermal method, respectively. WO3 nanoparticle are decorated on ZnO nanowire surface to fabricate high sensitive H2 sensor. H2 sensing responses of WO3-ZnO NW is improved compared with ZnO NW. Expansion of depletion layers of WO3-ZnO NW makes sensing responses improved.