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ZnO microflowers through a facile one-step hydrothermal approach
Materials Letters 256 (2019) 126649
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Synthesis of nanosheet-assembled porous NiO/ZnO microflowers through a facile one-step hydrothermal approach Limeng Qiu a, Wen Zeng a,⇑, Yanqiong Li b, Qu Zhou c,⇑ a
College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China School of Electronic and Electrical Engineering, Chongqing University of Arts and Sciences, Chongqing 400030, China c College of Engineering and Technology, Southwest University, Chongqing 400715, China b
a r t i c l e
i n f o
Article history: Received 30 June 2019 Received in revised form 23 August 2019 Accepted 7 September 2019 Available online 7 September 2019 Keywords: NiO/ZnO Porous microflower Electronic property Semiconductor Functional
a b s t r a c t The unique flower-like NiO/ZnO composite with porous nanosheets has been triumphantly manufactured via the one-step hydrothermal method initially. On the basis of morphology observations and ethanol gas measurements, it is considered that the construction of P-N junctions at the interface as well as the sheetlike porous microflowers architecture greatly contribute to the fantastic gas sensing properties of the prepared materials, which exhibit superb gas response value and outstanding repeatability. Furthermore, a possible formation mechanism is proposed, elucidating the process of the oriented-attachment, the selfassembly in addition to the pores-development. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction With the development of human civilization and the advancement of technology, the environmental problems generated by air pollution have turn into the primary difficulty faced by modern people [1–3]. In order to maintain health, gas detection utilized by the gas sensor is an effective approach to figure out the difficulty [4,5], which is capable to monitor combined with managing the air. After a long period of research, the gas sensor based on NiO/ ZnO P-N junction materials stands out among numerous gas sensor materials on account of excellent gas-sensing properties[6–8]. Meanwhile, several researchers have investigated about diverse structures of NiO/ZnO nanocomposites. For instance, a gas sensor constituted of NiO/ZnO heterojunction microflowers were triumphantly prepared [9], exhibiting high gas response towards formaldehyde. In addition, Zhu et al. [10] fabricated hierarchical hollow NiO/ZnO microspheres that displayed superior ethanol gas sensing properties compared with those of the ordinary NiO/ ZnO heterostructures. However, how to prepare NiO/ZnO nanocomposites via a simple but efficient way is a tough problem to be solved currently [11]. As far as we known, there has been rare report concentrated on one-step hydrothermal synthesis of NiO/ ZnO microflowers consisted of porous nanosheets. ⇑ Corresponding authors. E-mail addresses: [email protected] (W. Zeng), [email protected] (Q. Zhou). https://doi.org/10.1016/j.matlet.2019.126649 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
In this paper, a controlled flower-like NiO/ZnO composite with porous nanosheet was successfully manufactured via the onestep hydrothermal process. The consequences demonstrated the porous NiO/ZnO nanoflower possessed enhanced ethanol gas sensing performances with excellent gas response value and outstanding repeatability, which was related to the construction of P-N heterojunction as well as the distinct porous structure of NiO/ ZnO composite. Moreover, the possible mechanism of growth morphology has been explored. 2. Experiments Synthesis: The flower-like NiO/ZnO composite assembled from porous nanosheets was hydrothermally fabricated. Initially, 4 mmol Zn(NO3)26H2O and 4 mmol Ni(NO3)26H2O were mixed and stirred in 40 mL deionized water, so that the sources of zinc and nickel were completely dissolved. Then, 16 mmol NH4HCO3 was added into the prepared mixture [10]. After continuous stirring for 30 min, the mixed solution was transferred into a 50 mL volume of Teflon-lined stainless autoclave, which was heated at 120 °C for 3 h. Cooling naturally, the precipitate was obtained by centrifugation and washed for three times utilizing by deionized water and ethanol to remove excess salt and organics. Subsequently, the sample was dried in a vacuum oven at 60 °C. Finally, the product was attained via calcining at 400 °C for 2 h. Characterization: The compositions and phase analysis of the specimen were performed by an X-ray diffractometer (XRD, Rigaku
2
L. Qiu et al. / Materials Letters 256 (2019) 126649
D/Max-1200X) taking advantage of Cu Ka radiation. For the purpose of observing the micro-morphology, the product was characterized by a field emission scanning electron microscope (FESEM, Nava 400). Furthermore, field transmission electron microscopy (FETEM) images were investigated on Libra 200 using the oil-free vacuum system for more detailed structures. The fabricated sensor was installed on a CGS-1 intelligent gas sensitivity system. The definition of the gas response is Ra/Rg, where Ra and Rg represent the sensor resistances in air and target gas, respectively.
3. Results and discussion Fig. 1a exhibits the XRD spectrum of the obtained sample. According to the standard cards, the diffraction peaks are identified as hexagonal wurtzite ZnO (JCPDS No. 76–0704) and face-centered cubic NiO (JCPDS No. 89–7130), indicating that the prepared product is a composite oxide of NiO/ZnO. Besides, no other characteristic peaks are found in the XRD pattern, which demonstrates that the highly pure NiO/ZnO composite is free of other impurities. As illustrated in Fig. 1b, the EDS spectrum only contains three elements of O, Zn and Ni (Cu is derived from the substrate), which further confirms the successful preparation of the NiO/ZnO composite. Fig. 2a reveals the microscopic morphology of the prepared NiO/ZnO composite by the observation of SEM. It is obvious to discover the product contains a number of uniformly distributed microflowers with the average diameter of about 4 lm. Remarkably, every single microflower cluster is composed of plenty of nanosheets that are stacked on each other. As displayed in Fig. 2b, it can be clearly seen that abundant pores are distributed densely on the whole petal from the magnified image of SEM. With the intention for further comprehensive characteristics, the nanosheet-assembled porous NiO/ZnO microflowers are confirmed
by the TEM image (Fig. 2c), which agrees well with the results of SEM. Fig. 3a presents the sensitivity of the gas sensor assembled by flower-like porous NiO/ZnO composites to 200 ppm ethanol gas at different working temperatures. A regularity ought to be noticed is that as the working temperature boosts, the gas response first rapidly increases and then sharply decreases, which exhibits a tendency of volcanic shape. The gas response at 300 °C reaches a maximum of 33. In addition, From the Fig. 3b, it can be discovered that the gas response rises linearly when the gas sensor is exposed to variable concentrations (50–400 ppm) of ethanol gas at 300 °C. Moreover, as illustrated in Fig. 3c, the sensor is tested for dynamic response towards 200 ppm of ethanol at 300 °C for 8 cycles, indicating that the gas sensor has excellent repeatability as well as recovery performances. Based on the above experimental results together with theoretical analysis, the reasons why the NiO/ZnO composite with the flaky porous microflowers has fantastic gas sensing performances to ethanol are as follows. On the one hand, the presence of the PN heterojunction which is formed at the interface plays a leading role in improving gas sensitivity. When the NiO/ZnO microflower consisted of porous nanosheets is placed in the air, due to the disparity of band gap combined with the Fermi level between NiO and ZnO [12], an additional depletion region is created [13], inducing a higher resistance of the gas sensor during the ionization of oxygen. However, when ethanol gas is introduced into the entire environment, the trapped electrons will release to the composite via chemical reactions, which results in a significant shrinkage of the P-N junction depletion region. Hence, the resistance of the gas sensor sharply drops, leading to the enhanced response to ethanol gas. On the other hand, the improvement of gas properties is ascribed to the unique porous microflower structure with a larger specific surface area, providing abundant surface active sites on the surface as well as more gas diffusion channels for the target gas.
Fig. 1. (a) XRD pattern of the NiO/ZnO composite. (b) EDS spectrum of the NiO/ZnO composite.
Fig. 2. (a, b) SEM images of the NiO/ZnO composite at different magnifications. (c) TEM image of the NiO/ZnO composite.
L. Qiu et al. / Materials Letters 256 (2019) 126649
3
Fig. 3. (a) The gas response of the sensor to 200 ppm ethanol gas at different working temperatures. (b) The gas response of the sensor versus variable concentrations of ethanol gas at 300 °C. (c) The dynamic response of the sensor to 200 ppm ethanol at 300 °C for 8 cycles.
Fig. 4. The alternation of the growth morphology.
Taking into characterization and analysis account, a probable formation mechanism is proposed as shown in Fig. 4.
NH4 HCO3 ! NH4 þ + HCO3 —
ð1Þ
3Zn2þ + 6HCO3 + 3H2 O ! Zn3 (CO3 )(OH)4 4H2 O + 5CO2
ð2Þ
3Ni2þ + 6HCO3 + 3H2 O ! Ni3 (CO3 )(OH)4 4H2 O + 5CO2
ð3Þ
Zn3 (CO3 )(OH)4 4H2 O ! 3ZnO + CO2 + 6H2 O
ð4Þ
Ni3 (CO3 )(OH)4 4H2 O ! 3NiO + CO2 + 6H2 O
ð5Þ
NH+4
HCO–3
At the beginning, and are released according to the Eq. (1). Next, Zn2+ and Ni2+ will further react with HCO–3 to form basic zinc carbonate and nickel carbonate as shown in Eqs. (2) and (3). With the temperature gradually increasing (Eqs. (4) and (5)), the tiny single crystals are uniformly nucleated along with progressively growing attributed to the influence of thermodynamics and kinetics [14]. With prolonged reaction time, the adjacent grains are repositioned into nanosheets in an oriented way. Owing to the minimization of the surface energy [15], the newly formed nanocrystals spontaneously fall on the pre-formed NiO/ZnO nanosheets to produce a new one, assembling into a sheet-like nanoflower NiO/ZnO architecture. Lastly, the poresdevelopment is ascribed to the elimination of the solute inorganic precursor material by calcination at elevated temperature, which generates the eventual morphology of nanoflowers assembled with porous nanosheets. 4. Conclusion In brief, the controlled nanosheet-assembled porous NiO/ZnO microflowers have been successfully manufactured through a facile one-step hydrothermal approach. Notably, the gas sensor comprised of the NiO/ZnO composites reveals enhanced gas sensing performances attached with superb gas response and terrific
repeatability to ethanol gas. What is more, the particular structure of the P-N junction with the sheet-like porous microflowers is of great importance to improve gas sensing properties. According to comprehensive analysis, a probable morphology growth mechanism is suggested as well. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJZD-K201800101). References [1] N. Kaur, D. Zappa, M. Ferroni, N. Poli, M. Campanini, R. Negrea, E. Comini, Sens. Actuators, B 262 (2018) 477–485. [2] Y. Xu, W. Zeng, Y. Li, Mater. Lett. 248 (2019) 86–88. [3] L. Sui, X. Zhang, X. Cheng, P. Wang, Y. Xu, S. Gao, H. Zhao, L. Huo, ACS Appl. Mater. Inter. 9 (2) (2017) 1661–1670. [4] S. Bai, H. Fu, X. Shu, R. Luo, A. Chen, D. Li, C. Liu, Mater. Lett. 210 (2018) 305– 308. [5] C. Liu, L.P. Zhao, B.Q. Wang, J. Colloid Interface Sci. 495 (2017) 207–215. [6] Y. Lu, Y. Ma, S. Ma, S. Yan, Ceram. Int. 43 (2017), 7508–7515. [7] Y. Zhang, W. Zeng, Y. Li, Appl. Surf. Sci. 495 (2019) 143619. [8] M. Patel, J. Kim, J. Alloy. Compd. 729 (2017) 796–801. [9] Y. Zhang, W. Zeng, Y.Q. Li, Ceram. Int. 45 (2019) 6043–6050. [10] Y.J. Zhang, W. Zeng, Y.Q. Li, Mater. Lett. 241 (2019) 223–226. [11] L. Zhu, W. Zeng, J. Yang, Y. Li, Ceram. Int. 44 (2018) 19825–19830. [12] J. Hu, J. Yang, W. Wang, Mater. Res. Bull. 102 (2018) 294–303. [13] T. Ming, W. Zeng, H. Long, Z. Wang, Sens. Actuators B: Chem. 231 (2016) 120– 128. [14] W. Dai, X. Pan, S. Chen, C. Chen, Z. Wen, H. Zhang, Z. Ye, J. Mater. Chem. C 2 (23) (2014) 4606. [15] Q. Zhou, W. Zeng, W. Chen, L. Xu, R. Kumar, A. Umar, Sens. Actuators, B 298 (2019) 126870.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Synthesis of nanosheet-assembled porous NiO/ZnO microflowers through a facile one-step hydrothermal approach Limeng Qiu a, Wen Zeng a,⇑, Yanqiong Li b, Qu Zhou c,⇑ a
College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China School of Electronic and Electrical Engineering, Chongqing University of Arts and Sciences, Chongqing 400030, China c College of Engineering and Technology, Southwest University, Chongqing 400715, China b
a r t i c l e
i n f o
Article history: Received 30 June 2019 Received in revised form 23 August 2019 Accepted 7 September 2019 Available online 7 September 2019 Keywords: NiO/ZnO Porous microflower Electronic property Semiconductor Functional
a b s t r a c t The unique flower-like NiO/ZnO composite with porous nanosheets has been triumphantly manufactured via the one-step hydrothermal method initially. On the basis of morphology observations and ethanol gas measurements, it is considered that the construction of P-N junctions at the interface as well as the sheetlike porous microflowers architecture greatly contribute to the fantastic gas sensing properties of the prepared materials, which exhibit superb gas response value and outstanding repeatability. Furthermore, a possible formation mechanism is proposed, elucidating the process of the oriented-attachment, the selfassembly in addition to the pores-development. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction With the development of human civilization and the advancement of technology, the environmental problems generated by air pollution have turn into the primary difficulty faced by modern people [1–3]. In order to maintain health, gas detection utilized by the gas sensor is an effective approach to figure out the difficulty [4,5], which is capable to monitor combined with managing the air. After a long period of research, the gas sensor based on NiO/ ZnO P-N junction materials stands out among numerous gas sensor materials on account of excellent gas-sensing properties[6–8]. Meanwhile, several researchers have investigated about diverse structures of NiO/ZnO nanocomposites. For instance, a gas sensor constituted of NiO/ZnO heterojunction microflowers were triumphantly prepared [9], exhibiting high gas response towards formaldehyde. In addition, Zhu et al. [10] fabricated hierarchical hollow NiO/ZnO microspheres that displayed superior ethanol gas sensing properties compared with those of the ordinary NiO/ ZnO heterostructures. However, how to prepare NiO/ZnO nanocomposites via a simple but efficient way is a tough problem to be solved currently [11]. As far as we known, there has been rare report concentrated on one-step hydrothermal synthesis of NiO/ ZnO microflowers consisted of porous nanosheets. ⇑ Corresponding authors. E-mail addresses: [email protected] (W. Zeng), [email protected] (Q. Zhou). https://doi.org/10.1016/j.matlet.2019.126649 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
In this paper, a controlled flower-like NiO/ZnO composite with porous nanosheet was successfully manufactured via the onestep hydrothermal process. The consequences demonstrated the porous NiO/ZnO nanoflower possessed enhanced ethanol gas sensing performances with excellent gas response value and outstanding repeatability, which was related to the construction of P-N heterojunction as well as the distinct porous structure of NiO/ ZnO composite. Moreover, the possible mechanism of growth morphology has been explored. 2. Experiments Synthesis: The flower-like NiO/ZnO composite assembled from porous nanosheets was hydrothermally fabricated. Initially, 4 mmol Zn(NO3)26H2O and 4 mmol Ni(NO3)26H2O were mixed and stirred in 40 mL deionized water, so that the sources of zinc and nickel were completely dissolved. Then, 16 mmol NH4HCO3 was added into the prepared mixture [10]. After continuous stirring for 30 min, the mixed solution was transferred into a 50 mL volume of Teflon-lined stainless autoclave, which was heated at 120 °C for 3 h. Cooling naturally, the precipitate was obtained by centrifugation and washed for three times utilizing by deionized water and ethanol to remove excess salt and organics. Subsequently, the sample was dried in a vacuum oven at 60 °C. Finally, the product was attained via calcining at 400 °C for 2 h. Characterization: The compositions and phase analysis of the specimen were performed by an X-ray diffractometer (XRD, Rigaku
2
L. Qiu et al. / Materials Letters 256 (2019) 126649
D/Max-1200X) taking advantage of Cu Ka radiation. For the purpose of observing the micro-morphology, the product was characterized by a field emission scanning electron microscope (FESEM, Nava 400). Furthermore, field transmission electron microscopy (FETEM) images were investigated on Libra 200 using the oil-free vacuum system for more detailed structures. The fabricated sensor was installed on a CGS-1 intelligent gas sensitivity system. The definition of the gas response is Ra/Rg, where Ra and Rg represent the sensor resistances in air and target gas, respectively.
3. Results and discussion Fig. 1a exhibits the XRD spectrum of the obtained sample. According to the standard cards, the diffraction peaks are identified as hexagonal wurtzite ZnO (JCPDS No. 76–0704) and face-centered cubic NiO (JCPDS No. 89–7130), indicating that the prepared product is a composite oxide of NiO/ZnO. Besides, no other characteristic peaks are found in the XRD pattern, which demonstrates that the highly pure NiO/ZnO composite is free of other impurities. As illustrated in Fig. 1b, the EDS spectrum only contains three elements of O, Zn and Ni (Cu is derived from the substrate), which further confirms the successful preparation of the NiO/ZnO composite. Fig. 2a reveals the microscopic morphology of the prepared NiO/ZnO composite by the observation of SEM. It is obvious to discover the product contains a number of uniformly distributed microflowers with the average diameter of about 4 lm. Remarkably, every single microflower cluster is composed of plenty of nanosheets that are stacked on each other. As displayed in Fig. 2b, it can be clearly seen that abundant pores are distributed densely on the whole petal from the magnified image of SEM. With the intention for further comprehensive characteristics, the nanosheet-assembled porous NiO/ZnO microflowers are confirmed
by the TEM image (Fig. 2c), which agrees well with the results of SEM. Fig. 3a presents the sensitivity of the gas sensor assembled by flower-like porous NiO/ZnO composites to 200 ppm ethanol gas at different working temperatures. A regularity ought to be noticed is that as the working temperature boosts, the gas response first rapidly increases and then sharply decreases, which exhibits a tendency of volcanic shape. The gas response at 300 °C reaches a maximum of 33. In addition, From the Fig. 3b, it can be discovered that the gas response rises linearly when the gas sensor is exposed to variable concentrations (50–400 ppm) of ethanol gas at 300 °C. Moreover, as illustrated in Fig. 3c, the sensor is tested for dynamic response towards 200 ppm of ethanol at 300 °C for 8 cycles, indicating that the gas sensor has excellent repeatability as well as recovery performances. Based on the above experimental results together with theoretical analysis, the reasons why the NiO/ZnO composite with the flaky porous microflowers has fantastic gas sensing performances to ethanol are as follows. On the one hand, the presence of the PN heterojunction which is formed at the interface plays a leading role in improving gas sensitivity. When the NiO/ZnO microflower consisted of porous nanosheets is placed in the air, due to the disparity of band gap combined with the Fermi level between NiO and ZnO [12], an additional depletion region is created [13], inducing a higher resistance of the gas sensor during the ionization of oxygen. However, when ethanol gas is introduced into the entire environment, the trapped electrons will release to the composite via chemical reactions, which results in a significant shrinkage of the P-N junction depletion region. Hence, the resistance of the gas sensor sharply drops, leading to the enhanced response to ethanol gas. On the other hand, the improvement of gas properties is ascribed to the unique porous microflower structure with a larger specific surface area, providing abundant surface active sites on the surface as well as more gas diffusion channels for the target gas.
Fig. 1. (a) XRD pattern of the NiO/ZnO composite. (b) EDS spectrum of the NiO/ZnO composite.
Fig. 2. (a, b) SEM images of the NiO/ZnO composite at different magnifications. (c) TEM image of the NiO/ZnO composite.
L. Qiu et al. / Materials Letters 256 (2019) 126649
3
Fig. 3. (a) The gas response of the sensor to 200 ppm ethanol gas at different working temperatures. (b) The gas response of the sensor versus variable concentrations of ethanol gas at 300 °C. (c) The dynamic response of the sensor to 200 ppm ethanol at 300 °C for 8 cycles.
Fig. 4. The alternation of the growth morphology.
Taking into characterization and analysis account, a probable formation mechanism is proposed as shown in Fig. 4.
NH4 HCO3 ! NH4 þ + HCO3 —
ð1Þ
3Zn2þ + 6HCO3 + 3H2 O ! Zn3 (CO3 )(OH)4 4H2 O + 5CO2
ð2Þ
3Ni2þ + 6HCO3 + 3H2 O ! Ni3 (CO3 )(OH)4 4H2 O + 5CO2
ð3Þ
Zn3 (CO3 )(OH)4 4H2 O ! 3ZnO + CO2 + 6H2 O
ð4Þ
Ni3 (CO3 )(OH)4 4H2 O ! 3NiO + CO2 + 6H2 O
ð5Þ
NH+4
HCO–3
At the beginning, and are released according to the Eq. (1). Next, Zn2+ and Ni2+ will further react with HCO–3 to form basic zinc carbonate and nickel carbonate as shown in Eqs. (2) and (3). With the temperature gradually increasing (Eqs. (4) and (5)), the tiny single crystals are uniformly nucleated along with progressively growing attributed to the influence of thermodynamics and kinetics [14]. With prolonged reaction time, the adjacent grains are repositioned into nanosheets in an oriented way. Owing to the minimization of the surface energy [15], the newly formed nanocrystals spontaneously fall on the pre-formed NiO/ZnO nanosheets to produce a new one, assembling into a sheet-like nanoflower NiO/ZnO architecture. Lastly, the poresdevelopment is ascribed to the elimination of the solute inorganic precursor material by calcination at elevated temperature, which generates the eventual morphology of nanoflowers assembled with porous nanosheets. 4. Conclusion In brief, the controlled nanosheet-assembled porous NiO/ZnO microflowers have been successfully manufactured through a facile one-step hydrothermal approach. Notably, the gas sensor comprised of the NiO/ZnO composites reveals enhanced gas sensing performances attached with superb gas response and terrific
repeatability to ethanol gas. What is more, the particular structure of the P-N junction with the sheet-like porous microflowers is of great importance to improve gas sensing properties. According to comprehensive analysis, a probable morphology growth mechanism is suggested as well. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJZD-K201800101). References [1] N. Kaur, D. Zappa, M. Ferroni, N. Poli, M. Campanini, R. Negrea, E. Comini, Sens. Actuators, B 262 (2018) 477–485. [2] Y. Xu, W. Zeng, Y. Li, Mater. Lett. 248 (2019) 86–88. [3] L. Sui, X. Zhang, X. Cheng, P. Wang, Y. Xu, S. Gao, H. Zhao, L. Huo, ACS Appl. Mater. Inter. 9 (2) (2017) 1661–1670. [4] S. Bai, H. Fu, X. Shu, R. Luo, A. Chen, D. Li, C. Liu, Mater. Lett. 210 (2018) 305– 308. [5] C. Liu, L.P. Zhao, B.Q. Wang, J. Colloid Interface Sci. 495 (2017) 207–215. [6] Y. Lu, Y. Ma, S. Ma, S. Yan, Ceram. Int. 43 (2017), 7508–7515. [7] Y. Zhang, W. Zeng, Y. Li, Appl. Surf. Sci. 495 (2019) 143619. [8] M. Patel, J. Kim, J. Alloy. Compd. 729 (2017) 796–801. [9] Y. Zhang, W. Zeng, Y.Q. Li, Ceram. Int. 45 (2019) 6043–6050. [10] Y.J. Zhang, W. Zeng, Y.Q. Li, Mater. Lett. 241 (2019) 223–226. [11] L. Zhu, W. Zeng, J. Yang, Y. Li, Ceram. Int. 44 (2018) 19825–19830. [12] J. Hu, J. Yang, W. Wang, Mater. Res. Bull. 102 (2018) 294–303. [13] T. Ming, W. Zeng, H. Long, Z. Wang, Sens. Actuators B: Chem. 231 (2016) 120– 128. [14] W. Dai, X. Pan, S. Chen, C. Chen, Z. Wen, H. Zhang, Z. Ye, J. Mater. Chem. C 2 (23) (2014) 4606. [15] Q. Zhou, W. Zeng, W. Chen, L. Xu, R. Kumar, A. Umar, Sens. Actuators, B 298 (2019) 126870.