- Email: [email protected]
Density-dependent of gas-sensing properties of Co3O4 nanowire arrays
Physica E 118 (2020) 113956
Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Density-dependent of gas-sensing properties of Co3O4 nanowire arrays Keng Xu a, **, Xing Yu a, Wei Zhao a, Wen Zeng b, c, * a
Jiangxi Key Laboratory of Nanomaterials and Sensors, Jiangxi Key Laboratory of Photoelectronics and Telecommunication, School of Physics, Communication and Electronics, Jiangxi Normal University, Nanchang 330022, Jiangxi, China b College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China c State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing, 400044, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Co3O4 Nanowire Array density Gas-sensing performance Hydrothermal method
Well-oriented Co3O4 nanowire arrays were synthesized in-situ on Al2O3 substrates via a simple hydrothermal method without seed layers. The phase structure and array morphology of Co3O4 nanowire arrays were inves tigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron mi croscopy (TEM). It was revealed that the array density of Co3O4 nanowire arrays could be controlled by the concentration of ammonium fluoride. The gas-sensing measurement revealed that the array structure on the surface of gas sensors exerted great impact on their performances. It was found the response value enhanced and then decayed with the increased array density of Co3O4 nanowires. We concluded that the Co3O4 nanowire arrays with moderate array density can possess highly exposed effective surface area to provide more pathways for gas diffusion than other samples. Our studies can provide a significant guidance on array design to fabricate high performance gas sensors. Moreover, the as-designed Co3O4 nanowire arrays have potential application in trimethylamine (TEA) detection.
1. Introduction Gas sensors based on metal oxide semiconductors offer great ad vantages with respect to others due to their low cost, simple fabrication, high stability, and good compatibility with portable instruments [1]. The gas-sensing mechanism of metal oxide semiconductors is achieved by the change in resistance caused by the surface interaction between the metal oxides and gas molecules [2]. In this case, the gas-sensing performances of metal oxide semiconductors are largely depended on their structures that can influence the surface interaction. With the development of nanotechnology, one-dimensional (1D) metal oxide nanomaterials such as nanorods, nanobelts, nanowires, and nanotubes, have received much attention due to their large surface-to-volume ratio and unique chemical properties [3]. A prerequisite for gas-sensing application is to integrate the 1D nanomaterials on the surface of Al2O3 substrates. Current developed strategies such as brush-coating and then sintering seem complicated and time-consuming [4]. Some inherent shortcomings such as particle agglomeration and loss of surface area due to the sintering in the fabrication process and sensor operation must be resolved [5]. As such, high-quality 1D nanostructure arrays are
much desirable [6]. The highly uniform morphology, stable orientated growing property, and large surface area as well as potential miniatur ization make the 1D nanoarray a promising structure for application in sensors. Nowadays, great progress has been achieved in the synthesis of 1D nanoarrays on the surface of Al2O3 substrates [4,7–9]. For example, uniform ZnO nanorod arrays were fabricated using a spontaneous-seeds-assisted growth method which exhibited enhanced gas-sensing properties [10]. However, up to now, it is still great chal lenge to design 1D array sensors with excellent gas-sensing perfor mances since the role of 1D nanoarray structures such as array density and array height on their gas-sensing performances was still lack of research. Co3O4 is an important metal oxide semiconductor with spinel structure, which was widely studied as a promising material in the area of lithium-ion batteries [11], supercapacitors [12], catalysis [13], etc. Among these numerous applications, Co3O4 based gas sensors have been paid much attention, which exhibited good gas-sensing characteristics [14]. For example, gas sensors based on Co3O4 rectangular porous rods were fabricated, which exhibited rapid response and recovery time, good selectivity and stability at a relatively low working temperature of
* Corresponding author. College of Materials Science and Engineering, Chongqing University, Chongqing, 400030, China. ** Corresponding author. E-mail addresses: [email protected] (K. Xu), [email protected] (W. Zeng). https://doi.org/10.1016/j.physe.2020.113956 Received 2 August 2019; Received in revised form 4 December 2019; Accepted 6 January 2020 Available online 8 January 2020 1386-9477/© 2020 Elsevier B.V. All rights reserved.
K. Xu et al.
Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113956
Fig. 1. Schematic diagram of the applied gas-sensing measurement system. The right is the structure of the sensor and the morphology of sensing film (Co(OH)2 arrays) on the surface of sensor.
200 � C due to the unique catalytic characteristics and porous structures of Co3O4 [15]. Though considerable efforts have been devoted, the gas-sensing behaviors of 1D Co3O4 nanoarrays were rarely studied. Only a few studies have reported the gas-sensing performances of 1D Co3O4 arrays [16–18]. What’s more, the effect of structure parameters of 1D Co3O4 nanoarrays on their gas-sensing performances was also lack of exploration. Herein, Co3O4 nanowire arrays were synthesized in-situ on Al2O3 substrates via a simple hydrothermal method without seed layers. The array density of Co3O4 nanowire arrays were controlled by the con centration of ammonium fluoride during the reaction. The gas-sensing performances of gas sensors based on different array density were measured. It was found the response value enhanced and then decayed with the increased array density of Co3O4 nanowires. This result was explained that the Co3O4 nanowire arrays with moderate array density can possess highly exposed effective surface area to provide more pathways for gas diffusion than other samples. 2. Experiment
Fig. 2. XRD patterns of Co3O4-1, Co3O4-2, Co3O4-3 and Co3O4-4.
In a typical procedure, the Co3O4 nanowire arrays were synthesized as follows: first, 0.291 g Co(NO3)2.6H2O as cobalt source, a certain amount of NH4F as substrate activation and 0.3 g of CO(NH2)2 as alkali sources were dissolved in 40 ml distilled water under stirring. To control the array density of Co3O4, the concentration of NH4F was set as 0, 0.074, 0.148 and 0.296 g, respectively. Then, the violet solution was transferred into a 50 mL Telfon-lined stainless-steel autoclave. A piece of alumina plate substrate which was placed with Pt interdigitated elec trodes and Pt heaters was immersed in the reaction solution. The auto clave was sealed and maintained in an electric oven at 110 � C for 5 h. After cooling down to room temperature, the alumina plate substrat was took out, rinsed several times, and dried at 80 � C under vacuum. After that, the as-prepared pink precursors were converted to Co3O4 via thermal decomposition at 400 � C in air for 2 h. The final sensors were named as Co3O4-1, Co3O4-2, Co3O4-3 and Co3O4-4, respectively. Gas-sensing performances were measured in a static testing instru ment which was purchased from Wuhan Hua Chuang Rui Ke Co. Ltd (as shown in Fig. 1). This equipment includes a test chamber (about 30 L in volume) and a personal computer. Phases of the as-prepared samples were measured by Philips X’pert X-ray diffractometer with Cu Kα1 ra diation. ZEISSEVO microscope and JEM-2100 microscope was
employed to examine the morphologies, which was operated at an ac celeration voltage of 15 kV. 3. Results and discussion 3.1. Structure and morphology X-ray powder diffraction (XRD) was employed to explore the crystal structure and composition of these samples. Fig. 2 showed the corre sponding typical XRD patterns. As can be seen, all the diffraction peaks was indexed to the cubic spinel Co3O4 (JCPDS file No. 43-1003). No signals from other secondary phases were observed in the patterns, revealing that Co3O4 with good crystallinity and purity were obtained under these conditions. Moreover, it also revealed that the phase and crystallinity of Co3O4 were not influenced by the concentration of NH4F. Fig. 3a presented a low-magnification FESEM image of Co3O4 on the alumina coplanar Al2O3 plate. As can be seen, the morphology of Co3O4 array film was strongly dependent on the concentration of NH4F. No visible deposit can be observed in the absence of NH4F as shown in Fig. 3a. It was revealed that only several sparse nanowires or nanosheets 2
K. Xu et al.
Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113956
Fig. 3. Typical SEM images of (a) Co3O4-1; (b) Co3O4-2; (c) Co3O4-3; (d) Co3O4-4.
covered on the plate and the Al2O3 substrate could be seen easily in Fig. 3a–. When the concentration of NH4F increased to 10 mmol, the Al2O3 plate was covered with black tint and the array density increased as shown in Fig. 3b. It can be observed that there were numerous Co3O4 nanowires stood on the surface of Al2O3 substrate and the nanowire density was higher than that of Co3O4-1. This trend was also observed when the concentration of NH4F increased to 20 and 40 mmol, which revealed the density of nanowire array further increased as shown in Fig. 3c and d, respectively. When the concentration of NH4F increased to 20 and 40 mmol, high array density was observed and the Co3O4 nanowires began crowed together to form nanoflowers. Namely, a high concentration of NH4F was favorable for the formation of dense nano wire arrays. To further explore the array structure of these samples, the crosssectional SEM images were showed in Fig. 4. It can be observed that all of the nanowires in these samples were homogeneously aligned on the ceramic substrates. As shown in Fig. 4a, the height of nanowires in Co3O4-1 was about 1 μm whose density was really low. With the increased NH4F concentration, the height of nanowires also increased. The nanowire height of Co3O4-2, Co3O4-3, and Co3O4-4 was about 2, 8, and 15 μm, respectively. Moreover, it also revealed that the array was getting more compact with the increased NH4F concentration. These results further indicated that the array density and array length increased from Co3O4-1 to Co3O4-4. TEM, SAED and HRTEM images were showed to investigate detailed structural information of Co3O4 nanowires in Fig. 5. Fig. 5a depicted a
low-magnification TEM image of Co3O4 nanowires. As can be seen, the Co3O4 nanowires exhibited 1D porous structure. The circles in the SAED patterns (Fig. 5b) were attributed to Co3O4 (111) and Co3O4 (311), respectively. Fig. 4c and d showed a high-resolution transmission elec tron micrograph (HRTEM) of a single Co3O4 nanowire and clear lattice fringes. The clear lattice stripes in the HRTEM images (Fig. 5d) indicated the good crystallinity of Co3O4 nanowires. As discussed above, the NH4F played a key role on the regulation of array density. During the reaction, cobalt nitrate provides Co2þ and urea provides OH and CO3 to form cobalt hydroxide carbonate pre 2)which cursors, while the NH4F can react with Co2þ to form CoF(x x 2þ acted as supersaturation controller to release the Co slowly to act with OH and CO3 or substrate activation to promote the formation of nuclei on alumina substrates [19]. Namely, at the initial stage of the hydrothermal reaction, the alumina substrates were activated by F and 2)formed CoF(x on the surface, which was favorable for the formation x of nuclei and ensured the good contact between arrays and substrates. Due to the assistance of F , the cobalt-hydroxide-carbonate began to heteronucleation on alumina substrates and assembly along the specific orientation preferentially to form 1D structure at the expense of 2)CoF(x . The number of active sites on the substrates increased with the x NH4F concentration. When the NH4F concentration increased to a certain value, the nucleus tended to aggregation to reduce the surface energy of system. After that, lots of growth units were preferentially supplied and led to the generation of flower-like cobalt-hydroxide-car bonate precursors. 3
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Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113956
Fig. 4. Typical cross-sectional SEM images of (a) Co3O4-1; (b) Co3O4-2; (c) Co3O4-3; (d) Co3O4-4.
Fig. 5. (a) Low-resolution TEM image, (b) SAED pattern, (c and d) high-resolution TEM images of Co3O4 nanowires.
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Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113956
Fig. 6. (a) Response towards 100 ppm TEA at different temperatures, (b) dynamic response-recovery curve towards various TEA concentration, (c) the relationship between response and TEA concentration, (d) the cycling stability in one month.
3.2. Gas-sensing properties The response of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 towards 100 ppm triethylamine (TEA) as a function of operating temperature was carried out as shown in Fig. 6a. As can be seen, all samples exhibited the “increase-maximum-decrease” behavior, indicating the response values were largely controlled by the operating temperature. Such behavior was a normal phenomenon for semiconductor sensors, which was owing to the combined effect of surface chemical activation and dynamic adsorption-desorption balance. The optimal operating tem perature of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 was 200, 250, 250, and 250 � C, respectively. The increased optimal operating temperature with the array density and length was maybe due to the narrowed diffusion path in the Co3O4 arrays with high density. To diffuse into the inner arrays, gas molecules need higher activation energy to improve their weakened diffusion process due to the narrowed diffusion path. Therefore, the Co3O4 arrays with lowest array density (Co3O4-1) exhibited the lowest operating temperature. Moreover, it can be observed that Co3O4-2 showed the highest response than others at their optimal operating temperature. The highest response of Co3O4-2 was about 4 (Rg/Ra). Fig. 6b showed the representative dynamic response of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 towards with concentration ranging from 1 to 100 ppm TEA. Obviously, the response amplitude of these sensors gradationally increased with TEA concentration. During these circles, it was observed that the response and recovery performances were reproducible, indicating these sensors possessed a stable and repeatable behavior. Fig. 6c showed the correlation between response value and the TEA concentration. It was revealed that the response value of Co3O4-1 was higer than that of other samples at each concentration. The long-term stability of these samples was also evaluated by repeating the gas-sensing measurement every three days for one month. The
Fig. 7. Selectivity towards 100 ppm various gases of Co3O4-1, Co3O4-2, Co3O43, and Co3O4-4.
results in Fig. 6d indicated that all of these sensors exhibited good sta bility and reliability. Fig. 7 showed the response value of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 towards 100 ppm various gases, including ethanol, acetone, ethyl acetate, xylene, formaldehyde, TEA, ammonia, methanol, prop anol, benzene, and butanol, respectively. As can be seen, it was observed that Co3O4-2 exhibited an obvious high response towards TEA, revealing a good selectivity toward TEA. The varied response value towards different gases was possibly due to the different volatility and chemical 5
K. Xu et al.
Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113956
the high concentration and large electronegativity of oxygen (eqs. (1)– (3)) [24]. This electron-capture process leads to the formation of hole accumulation layer (HAL) [25,26]. For the p-type Co3O4, the as-formed HAL can decrease the resistance of Co3O4 [27]. When the Co3O4 is exposed to TEA, the TEA molecules can react with the adsorbed oxygen species and released the seized electrons back to the conduction band of Co3O4 (eq. (4)) [28]. The released electrons will recombine with the holes in Co3O4 and reduce the hole concentration in the HAL. This process eventually leads to an increased resistance for the p-type Co3O4. O2 ðgÞ ↔ O2 ðadsÞ
(1)
O2 ðadsÞ þ e ↔ O2 ðadsÞ
(2)
O2 ðadsÞ þ e ↔ 2O ðadsÞ
(3)
2NðC2 H5 Þ3 ðgasÞ þ 39O ðadsÞ→N2 þ 15H2 O þ 12CO2 þ 39e
(4)
According to the gas-sensing mechanism, the structures of Co3O4 nanowire arrays played a key role on its gas-sensing properties. When the array density is to low, the charge transfer is difficult between two adjacent nanowires. The resistance of Co3O4 nanowire arrays is much high, which is confirmed by Co3O4-1 whose resistance is as high as 30 MΩ as shown in Fig. 5b. The signal from gas is thus hard to be detected. In this case, the gas-sensing performances are improved with increased array density from Co3O4-1 to Co3O4-2. Moreover, the gas-sensing properties are also depended the effective surface area of Co3O4 nano wire arrays. A large effective surface area can provide more sites for gas adsorption and reaction [29–33]. As shown in Fig. 9, when the array density increased too high, the gas diffusion paths in the arrays become narrower and the gas molecular are hard to diffuse into the inner of Co3O4 nanowire arrays. The influence of gas adsorption and reaction thus become weak. Therefore, the gas-sensing performances of Co3O4-2 are superior to that of Co3O4-3 and Co3O4-4.
Fig. 8. The responses towards 100 ppm triethylamine under different rela tive humidities.
properties of gases that prompted different adsorption and catalytic performance [20,21]. The effect of relative humidity (RH) on triethyl amine sensing performance was also tested in different RH (relative humidity) conditions ranging from 20 to 80. The obtained relationship between RH and response value was depicted in Fig. 8. It can be observed that the response of all of these sensors were almost insensitive to humidity, which may be ascribed to its unique surficial property and high operating temperature. Similar phenomenon was also observed in other literature [22]. Gas-sensing mechanism of Co3O4 is based on the variation of con ductivity which is influenced by surface reaction with gas molecules during gas-sensing process [23]. When the Co3O4 is exposed to air, ox ygen molecules can adsorb on the Co3O4 surface and seize electrons from its conduction band to form chemisorbed oxygen species owing to
4. Conclusions In this study, Co3O4 nanowire arrays were synthesized in-situ on
Fig. 9. Gas-sensing mechanism of Co3O4 nanowire arrays with (a) low array density and (b) high array density. 6
K. Xu et al.
Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113956
Al2O3 substrates via a simple hydrothermal method without seed layers. The array density of Co3O4 nanowire arrays were controlled by the concentration of ammonium fluoride during the reaction. The gassensing performances of gas sensors based on different array density were measured. It was found the response value enhanced and then decayed with the increased array density of Co3O4 nanowires. This result was explained that the Co3O4 nanowire arrays with moderate array density can possess highly exposed effective surface area to pro vide more pathways for gas diffusion than other samples.
[7] D. Meng, D. Liu, G. Wang, Y. Shen, X. San, J. Si, F. Meng, Appl. Surf. Sci. 463 (2019) 348–356. [8] D. Barreca, D. Bekermann, E. Comini, A. Devi, R.A. Fischer, A. Gasparotto, C. Maccato, C. Sada, G. Sberveglieri, E. Tondello, CrystEngComm 12 (2010) 3419–3421. [9] S. Wang, P. Wang, C. Xiao, B. Yao, R. Zhao, M. Zhang, CrystEngComm 15 (2013) 9170–9175. [10] W. Kim, M. Choi, K. Yong, Sens. Actuators B Chem. 209 (2015) 989–996. [11] S. Chen, Y. Zhao, B. Sun, Z. Ao, X. Xie, Y. Wei, G. Wang, ACS Appl. Mater. Interfaces 7 (2015) 3306–3313. [12] J. Zhu, B. Huang, C. Zhao, H. Xu, S. Wang, Y. Chen, L. Xie, L. Chen, Electrochim. Acta 313 (2019) 194–204. [13] B. Zhu, G. Yang, D. Gao, Y. Wu, J. Zhao, Y. Fu, S. Ma, J. Alloy. Comp. 806 (2019) 163–169. [14] H. Jin, G. Sun, B. Zhang, N. Luo, Y. Li, L. Lin, H. Bala, J. Cao, Z. Zhang, Y. Wang, J. Alloy. Comp. 776 (2019) 782–790. [15] S. Wang, J. Cao, W. Cui, L. Fan, X. Li, D. Li, T. Zhang, Sens. Actuators B Chem. 297 (2019) 126746. [16] Z. Wen, L. Zhu, W. Mei, Y. Li, L. Hu, L. Sun, W. Wan, Z. Ye, J. Mater. Chem. A 1 (2013) 7511. [17] Z. Wen, L. Zhu, W. Mei, L. Hu, Y. Li, L. Sun, H. Cai, Z. Ye, Sens. Actuators B Chem. 186 (2013) 172–179. [18] L. Zhang, Z. Gao, C. Liu, Y. Zhang, Z. Tu, X. Yang, F. Yang, Z. Wen, L. Zhu, R. Liu, Y. Li, L. Cui, J. Mater. Chem. A 3 (2015) 2794–2801. [19] J. Jiang, J.P. Liu, X.T. Huang, Y.Y. Li, R.M. Ding, X.X. Ji, Y.Y. Hu, Q.B. Chi, Z. H. Zhu, Cryst. Growth Des. 10 (2010) 70–75. [20] S. Deng, X. Liu, N. Chen, D. Deng, X. Xiao, Y. Wang, Sens. Actuators B Chem. 233 (2016) 615–623. [21] H. Nguyen, S.A. El-Safty, J. Phys. Chem. C 115 (2011) 8466–8474. [22] J. Deng, R. Zhang, L. Wang, Z. Lou, T. Zhang, Sens. Actuators B Chem. 209 (2015) 449–455. [23] J.M. Xu, J.P. Cheng, J. Alloy. Comp. 686 (2016) 753–768. [24] K. Xu, J.P. Zou, S.Q. Tian, Y. Yang, F.Y. Zeng, T. Yu, Y.T. Zhang, X.M. Jie, C. L. Yuan, Sens. Actuators B Chem. 246 (2017) 68–77. [25] F. Qu, S. Zhang, B. Zhang, X. Zhou, S. Du, C.T. Lin, S. Ruan, M. Yang, Microchim. Acta 186 (2019) 222. [26] Y. Wang, F. Qu, J. Liu, Y. Wang, J. Zhou, S. Ruan, Sens. Actuators B Chem. 209 (2015) 515–523. [27] Q. Zhou, W. Zeng, Phys. E 95 (2018) 121–124. [28] X. Liu, K. Zhao, X. Sun, C. Zhang, X. Duan, P. Hou, G. Zhao, S. Zhang, H. Yang, R. Cao, X. Xu, Sens. Actuators B Chem. 285 (2019) 1–10. [29] J.H. Lee, Sens. Actuators B Chem. 140 (2009) 319–336. [30] L. Zhu, W. Zeng, Sens. Actuators A Phys. 267 (2017) 242–261. [31] W. Zeng, T. Liu, Physica B 405 (2010) 1345–1348. [32] L. Zhu, Y. Li, W. Zeng, Appl. Surf. Sci. 427 (2018) 281–287. [33] W. Zeng, T. Liu, D. Liu, E. Han, Sens. Actuators B Chem. 160 (2011) 455–462.
Declaration of competing interest We confirm that our work have no known competing financial in terests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Keng Xu: Conceptualization, Methodology, Writing - original draft. Xing Yu: Data curation, Investigation. Wei Zhao: Writing - review & editing. Wen Zeng: Supervision. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51702140). References [1] W. Zeng, T. Liu, Z. Wang, S. Tsukimoto, M. Saito, Y. Ikuhara, Sensors 9 (2007) 9029–9038. [2] N.B.U. Weimar, J. Electroceram. 7 (2001) 143–167. [3] T.M. Li, W. Zeng, Z.C. Wang, Sens. Actuators B Chem. 221 (2015) 1570–1585. [4] Y. Zhu, Y. Wang, G. Duan, H. Zhang, Y. Li, G. Liu, L. Xu, W. Cai, Sens. Actuators B Chem. 221 (2015) 350–356. [5] C. Li, X. Qiao, J. Jian, F. Feng, H. Wang, L. Jia, Chem. Eng. J. 375 (2019) 121924. [6] D. Bao, P. Gao, L. Wang, Y. Wang, Y. Chen, G. Chen, G. Li, C. Chang, W. Qin, ChemPlusChem 78 (2013) 1266–1272.
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Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Density-dependent of gas-sensing properties of Co3O4 nanowire arrays Keng Xu a, **, Xing Yu a, Wei Zhao a, Wen Zeng b, c, * a
Jiangxi Key Laboratory of Nanomaterials and Sensors, Jiangxi Key Laboratory of Photoelectronics and Telecommunication, School of Physics, Communication and Electronics, Jiangxi Normal University, Nanchang 330022, Jiangxi, China b College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China c State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing, 400044, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Co3O4 Nanowire Array density Gas-sensing performance Hydrothermal method
Well-oriented Co3O4 nanowire arrays were synthesized in-situ on Al2O3 substrates via a simple hydrothermal method without seed layers. The phase structure and array morphology of Co3O4 nanowire arrays were inves tigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron mi croscopy (TEM). It was revealed that the array density of Co3O4 nanowire arrays could be controlled by the concentration of ammonium fluoride. The gas-sensing measurement revealed that the array structure on the surface of gas sensors exerted great impact on their performances. It was found the response value enhanced and then decayed with the increased array density of Co3O4 nanowires. We concluded that the Co3O4 nanowire arrays with moderate array density can possess highly exposed effective surface area to provide more pathways for gas diffusion than other samples. Our studies can provide a significant guidance on array design to fabricate high performance gas sensors. Moreover, the as-designed Co3O4 nanowire arrays have potential application in trimethylamine (TEA) detection.
1. Introduction Gas sensors based on metal oxide semiconductors offer great ad vantages with respect to others due to their low cost, simple fabrication, high stability, and good compatibility with portable instruments [1]. The gas-sensing mechanism of metal oxide semiconductors is achieved by the change in resistance caused by the surface interaction between the metal oxides and gas molecules [2]. In this case, the gas-sensing performances of metal oxide semiconductors are largely depended on their structures that can influence the surface interaction. With the development of nanotechnology, one-dimensional (1D) metal oxide nanomaterials such as nanorods, nanobelts, nanowires, and nanotubes, have received much attention due to their large surface-to-volume ratio and unique chemical properties [3]. A prerequisite for gas-sensing application is to integrate the 1D nanomaterials on the surface of Al2O3 substrates. Current developed strategies such as brush-coating and then sintering seem complicated and time-consuming [4]. Some inherent shortcomings such as particle agglomeration and loss of surface area due to the sintering in the fabrication process and sensor operation must be resolved [5]. As such, high-quality 1D nanostructure arrays are
much desirable [6]. The highly uniform morphology, stable orientated growing property, and large surface area as well as potential miniatur ization make the 1D nanoarray a promising structure for application in sensors. Nowadays, great progress has been achieved in the synthesis of 1D nanoarrays on the surface of Al2O3 substrates [4,7–9]. For example, uniform ZnO nanorod arrays were fabricated using a spontaneous-seeds-assisted growth method which exhibited enhanced gas-sensing properties [10]. However, up to now, it is still great chal lenge to design 1D array sensors with excellent gas-sensing perfor mances since the role of 1D nanoarray structures such as array density and array height on their gas-sensing performances was still lack of research. Co3O4 is an important metal oxide semiconductor with spinel structure, which was widely studied as a promising material in the area of lithium-ion batteries [11], supercapacitors [12], catalysis [13], etc. Among these numerous applications, Co3O4 based gas sensors have been paid much attention, which exhibited good gas-sensing characteristics [14]. For example, gas sensors based on Co3O4 rectangular porous rods were fabricated, which exhibited rapid response and recovery time, good selectivity and stability at a relatively low working temperature of
* Corresponding author. College of Materials Science and Engineering, Chongqing University, Chongqing, 400030, China. ** Corresponding author. E-mail addresses: [email protected] (K. Xu), [email protected] (W. Zeng). https://doi.org/10.1016/j.physe.2020.113956 Received 2 August 2019; Received in revised form 4 December 2019; Accepted 6 January 2020 Available online 8 January 2020 1386-9477/© 2020 Elsevier B.V. All rights reserved.
K. Xu et al.
Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113956
Fig. 1. Schematic diagram of the applied gas-sensing measurement system. The right is the structure of the sensor and the morphology of sensing film (Co(OH)2 arrays) on the surface of sensor.
200 � C due to the unique catalytic characteristics and porous structures of Co3O4 [15]. Though considerable efforts have been devoted, the gas-sensing behaviors of 1D Co3O4 nanoarrays were rarely studied. Only a few studies have reported the gas-sensing performances of 1D Co3O4 arrays [16–18]. What’s more, the effect of structure parameters of 1D Co3O4 nanoarrays on their gas-sensing performances was also lack of exploration. Herein, Co3O4 nanowire arrays were synthesized in-situ on Al2O3 substrates via a simple hydrothermal method without seed layers. The array density of Co3O4 nanowire arrays were controlled by the con centration of ammonium fluoride during the reaction. The gas-sensing performances of gas sensors based on different array density were measured. It was found the response value enhanced and then decayed with the increased array density of Co3O4 nanowires. This result was explained that the Co3O4 nanowire arrays with moderate array density can possess highly exposed effective surface area to provide more pathways for gas diffusion than other samples. 2. Experiment
Fig. 2. XRD patterns of Co3O4-1, Co3O4-2, Co3O4-3 and Co3O4-4.
In a typical procedure, the Co3O4 nanowire arrays were synthesized as follows: first, 0.291 g Co(NO3)2.6H2O as cobalt source, a certain amount of NH4F as substrate activation and 0.3 g of CO(NH2)2 as alkali sources were dissolved in 40 ml distilled water under stirring. To control the array density of Co3O4, the concentration of NH4F was set as 0, 0.074, 0.148 and 0.296 g, respectively. Then, the violet solution was transferred into a 50 mL Telfon-lined stainless-steel autoclave. A piece of alumina plate substrate which was placed with Pt interdigitated elec trodes and Pt heaters was immersed in the reaction solution. The auto clave was sealed and maintained in an electric oven at 110 � C for 5 h. After cooling down to room temperature, the alumina plate substrat was took out, rinsed several times, and dried at 80 � C under vacuum. After that, the as-prepared pink precursors were converted to Co3O4 via thermal decomposition at 400 � C in air for 2 h. The final sensors were named as Co3O4-1, Co3O4-2, Co3O4-3 and Co3O4-4, respectively. Gas-sensing performances were measured in a static testing instru ment which was purchased from Wuhan Hua Chuang Rui Ke Co. Ltd (as shown in Fig. 1). This equipment includes a test chamber (about 30 L in volume) and a personal computer. Phases of the as-prepared samples were measured by Philips X’pert X-ray diffractometer with Cu Kα1 ra diation. ZEISSEVO microscope and JEM-2100 microscope was
employed to examine the morphologies, which was operated at an ac celeration voltage of 15 kV. 3. Results and discussion 3.1. Structure and morphology X-ray powder diffraction (XRD) was employed to explore the crystal structure and composition of these samples. Fig. 2 showed the corre sponding typical XRD patterns. As can be seen, all the diffraction peaks was indexed to the cubic spinel Co3O4 (JCPDS file No. 43-1003). No signals from other secondary phases were observed in the patterns, revealing that Co3O4 with good crystallinity and purity were obtained under these conditions. Moreover, it also revealed that the phase and crystallinity of Co3O4 were not influenced by the concentration of NH4F. Fig. 3a presented a low-magnification FESEM image of Co3O4 on the alumina coplanar Al2O3 plate. As can be seen, the morphology of Co3O4 array film was strongly dependent on the concentration of NH4F. No visible deposit can be observed in the absence of NH4F as shown in Fig. 3a. It was revealed that only several sparse nanowires or nanosheets 2
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Fig. 3. Typical SEM images of (a) Co3O4-1; (b) Co3O4-2; (c) Co3O4-3; (d) Co3O4-4.
covered on the plate and the Al2O3 substrate could be seen easily in Fig. 3a–. When the concentration of NH4F increased to 10 mmol, the Al2O3 plate was covered with black tint and the array density increased as shown in Fig. 3b. It can be observed that there were numerous Co3O4 nanowires stood on the surface of Al2O3 substrate and the nanowire density was higher than that of Co3O4-1. This trend was also observed when the concentration of NH4F increased to 20 and 40 mmol, which revealed the density of nanowire array further increased as shown in Fig. 3c and d, respectively. When the concentration of NH4F increased to 20 and 40 mmol, high array density was observed and the Co3O4 nanowires began crowed together to form nanoflowers. Namely, a high concentration of NH4F was favorable for the formation of dense nano wire arrays. To further explore the array structure of these samples, the crosssectional SEM images were showed in Fig. 4. It can be observed that all of the nanowires in these samples were homogeneously aligned on the ceramic substrates. As shown in Fig. 4a, the height of nanowires in Co3O4-1 was about 1 μm whose density was really low. With the increased NH4F concentration, the height of nanowires also increased. The nanowire height of Co3O4-2, Co3O4-3, and Co3O4-4 was about 2, 8, and 15 μm, respectively. Moreover, it also revealed that the array was getting more compact with the increased NH4F concentration. These results further indicated that the array density and array length increased from Co3O4-1 to Co3O4-4. TEM, SAED and HRTEM images were showed to investigate detailed structural information of Co3O4 nanowires in Fig. 5. Fig. 5a depicted a
low-magnification TEM image of Co3O4 nanowires. As can be seen, the Co3O4 nanowires exhibited 1D porous structure. The circles in the SAED patterns (Fig. 5b) were attributed to Co3O4 (111) and Co3O4 (311), respectively. Fig. 4c and d showed a high-resolution transmission elec tron micrograph (HRTEM) of a single Co3O4 nanowire and clear lattice fringes. The clear lattice stripes in the HRTEM images (Fig. 5d) indicated the good crystallinity of Co3O4 nanowires. As discussed above, the NH4F played a key role on the regulation of array density. During the reaction, cobalt nitrate provides Co2þ and urea provides OH and CO3 to form cobalt hydroxide carbonate pre 2)which cursors, while the NH4F can react with Co2þ to form CoF(x x 2þ acted as supersaturation controller to release the Co slowly to act with OH and CO3 or substrate activation to promote the formation of nuclei on alumina substrates [19]. Namely, at the initial stage of the hydrothermal reaction, the alumina substrates were activated by F and 2)formed CoF(x on the surface, which was favorable for the formation x of nuclei and ensured the good contact between arrays and substrates. Due to the assistance of F , the cobalt-hydroxide-carbonate began to heteronucleation on alumina substrates and assembly along the specific orientation preferentially to form 1D structure at the expense of 2)CoF(x . The number of active sites on the substrates increased with the x NH4F concentration. When the NH4F concentration increased to a certain value, the nucleus tended to aggregation to reduce the surface energy of system. After that, lots of growth units were preferentially supplied and led to the generation of flower-like cobalt-hydroxide-car bonate precursors. 3
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Fig. 4. Typical cross-sectional SEM images of (a) Co3O4-1; (b) Co3O4-2; (c) Co3O4-3; (d) Co3O4-4.
Fig. 5. (a) Low-resolution TEM image, (b) SAED pattern, (c and d) high-resolution TEM images of Co3O4 nanowires.
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Fig. 6. (a) Response towards 100 ppm TEA at different temperatures, (b) dynamic response-recovery curve towards various TEA concentration, (c) the relationship between response and TEA concentration, (d) the cycling stability in one month.
3.2. Gas-sensing properties The response of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 towards 100 ppm triethylamine (TEA) as a function of operating temperature was carried out as shown in Fig. 6a. As can be seen, all samples exhibited the “increase-maximum-decrease” behavior, indicating the response values were largely controlled by the operating temperature. Such behavior was a normal phenomenon for semiconductor sensors, which was owing to the combined effect of surface chemical activation and dynamic adsorption-desorption balance. The optimal operating tem perature of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 was 200, 250, 250, and 250 � C, respectively. The increased optimal operating temperature with the array density and length was maybe due to the narrowed diffusion path in the Co3O4 arrays with high density. To diffuse into the inner arrays, gas molecules need higher activation energy to improve their weakened diffusion process due to the narrowed diffusion path. Therefore, the Co3O4 arrays with lowest array density (Co3O4-1) exhibited the lowest operating temperature. Moreover, it can be observed that Co3O4-2 showed the highest response than others at their optimal operating temperature. The highest response of Co3O4-2 was about 4 (Rg/Ra). Fig. 6b showed the representative dynamic response of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 towards with concentration ranging from 1 to 100 ppm TEA. Obviously, the response amplitude of these sensors gradationally increased with TEA concentration. During these circles, it was observed that the response and recovery performances were reproducible, indicating these sensors possessed a stable and repeatable behavior. Fig. 6c showed the correlation between response value and the TEA concentration. It was revealed that the response value of Co3O4-1 was higer than that of other samples at each concentration. The long-term stability of these samples was also evaluated by repeating the gas-sensing measurement every three days for one month. The
Fig. 7. Selectivity towards 100 ppm various gases of Co3O4-1, Co3O4-2, Co3O43, and Co3O4-4.
results in Fig. 6d indicated that all of these sensors exhibited good sta bility and reliability. Fig. 7 showed the response value of Co3O4-1, Co3O4-2, Co3O4-3, and Co3O4-4 towards 100 ppm various gases, including ethanol, acetone, ethyl acetate, xylene, formaldehyde, TEA, ammonia, methanol, prop anol, benzene, and butanol, respectively. As can be seen, it was observed that Co3O4-2 exhibited an obvious high response towards TEA, revealing a good selectivity toward TEA. The varied response value towards different gases was possibly due to the different volatility and chemical 5
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the high concentration and large electronegativity of oxygen (eqs. (1)– (3)) [24]. This electron-capture process leads to the formation of hole accumulation layer (HAL) [25,26]. For the p-type Co3O4, the as-formed HAL can decrease the resistance of Co3O4 [27]. When the Co3O4 is exposed to TEA, the TEA molecules can react with the adsorbed oxygen species and released the seized electrons back to the conduction band of Co3O4 (eq. (4)) [28]. The released electrons will recombine with the holes in Co3O4 and reduce the hole concentration in the HAL. This process eventually leads to an increased resistance for the p-type Co3O4. O2 ðgÞ ↔ O2 ðadsÞ
(1)
O2 ðadsÞ þ e ↔ O2 ðadsÞ
(2)
O2 ðadsÞ þ e ↔ 2O ðadsÞ
(3)
2NðC2 H5 Þ3 ðgasÞ þ 39O ðadsÞ→N2 þ 15H2 O þ 12CO2 þ 39e
(4)
According to the gas-sensing mechanism, the structures of Co3O4 nanowire arrays played a key role on its gas-sensing properties. When the array density is to low, the charge transfer is difficult between two adjacent nanowires. The resistance of Co3O4 nanowire arrays is much high, which is confirmed by Co3O4-1 whose resistance is as high as 30 MΩ as shown in Fig. 5b. The signal from gas is thus hard to be detected. In this case, the gas-sensing performances are improved with increased array density from Co3O4-1 to Co3O4-2. Moreover, the gas-sensing properties are also depended the effective surface area of Co3O4 nano wire arrays. A large effective surface area can provide more sites for gas adsorption and reaction [29–33]. As shown in Fig. 9, when the array density increased too high, the gas diffusion paths in the arrays become narrower and the gas molecular are hard to diffuse into the inner of Co3O4 nanowire arrays. The influence of gas adsorption and reaction thus become weak. Therefore, the gas-sensing performances of Co3O4-2 are superior to that of Co3O4-3 and Co3O4-4.
Fig. 8. The responses towards 100 ppm triethylamine under different rela tive humidities.
properties of gases that prompted different adsorption and catalytic performance [20,21]. The effect of relative humidity (RH) on triethyl amine sensing performance was also tested in different RH (relative humidity) conditions ranging from 20 to 80. The obtained relationship between RH and response value was depicted in Fig. 8. It can be observed that the response of all of these sensors were almost insensitive to humidity, which may be ascribed to its unique surficial property and high operating temperature. Similar phenomenon was also observed in other literature [22]. Gas-sensing mechanism of Co3O4 is based on the variation of con ductivity which is influenced by surface reaction with gas molecules during gas-sensing process [23]. When the Co3O4 is exposed to air, ox ygen molecules can adsorb on the Co3O4 surface and seize electrons from its conduction band to form chemisorbed oxygen species owing to
4. Conclusions In this study, Co3O4 nanowire arrays were synthesized in-situ on
Fig. 9. Gas-sensing mechanism of Co3O4 nanowire arrays with (a) low array density and (b) high array density. 6
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Al2O3 substrates via a simple hydrothermal method without seed layers. The array density of Co3O4 nanowire arrays were controlled by the concentration of ammonium fluoride during the reaction. The gassensing performances of gas sensors based on different array density were measured. It was found the response value enhanced and then decayed with the increased array density of Co3O4 nanowires. This result was explained that the Co3O4 nanowire arrays with moderate array density can possess highly exposed effective surface area to pro vide more pathways for gas diffusion than other samples.
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Declaration of competing interest We confirm that our work have no known competing financial in terests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Keng Xu: Conceptualization, Methodology, Writing - original draft. Xing Yu: Data curation, Investigation. Wei Zhao: Writing - review & editing. Wen Zeng: Supervision. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51702140). References [1] W. Zeng, T. Liu, Z. Wang, S. Tsukimoto, M. Saito, Y. Ikuhara, Sensors 9 (2007) 9029–9038. [2] N.B.U. Weimar, J. Electroceram. 7 (2001) 143–167. [3] T.M. Li, W. Zeng, Z.C. Wang, Sens. Actuators B Chem. 221 (2015) 1570–1585. [4] Y. Zhu, Y. Wang, G. Duan, H. Zhang, Y. Li, G. Liu, L. Xu, W. Cai, Sens. Actuators B Chem. 221 (2015) 350–356. [5] C. Li, X. Qiao, J. Jian, F. Feng, H. Wang, L. Jia, Chem. Eng. J. 375 (2019) 121924. [6] D. Bao, P. Gao, L. Wang, Y. Wang, Y. Chen, G. Chen, G. Li, C. Chang, W. Qin, ChemPlusChem 78 (2013) 1266–1272.
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