- Email: [email protected]
Hydrothermal synthesis of nanoparticles-assembled NiO microspheres and their sensing properties
Journal Pre-proof Hydrothermal synthesis of nanoparticles-assembled NiO microspheres and their sensing properties
Shixiu Cao, Lingling Peng, Tao Han, Bitao Liu, Dachuan Zhu, Cong Zhao, Jing Xu, Yinyin Tang, Jinyu Wang, Shuangshuang He PII:
S1386-9477(19)30601-0
DOI:
https://doi.org/10.1016/j.physe.2019.113655
Reference:
PHYSE 113655
To appear in:
Physica E: Low-dimensional Systems and Nanostructures
Received Date:
19 April 2019
Accepted Date:
24 July 2019
Please cite this article as: Shixiu Cao, Lingling Peng, Tao Han, Bitao Liu, Dachuan Zhu, Cong Zhao, Jing Xu, Yinyin Tang, Jinyu Wang, Shuangshuang He, Hydrothermal synthesis of nanoparticles-assembled NiO microspheres and their sensing properties, Physica E: Lowdimensional Systems and Nanostructures (2019), https://doi.org/10.1016/j.physe.2019.113655
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.
Journal Pre-proof Graphical Abstract (for review) The NiO microspheres have been successfully synthesized via a facile hydrothermal method in the dissolvant of 1,2-Propanediol and H2O with a molar ratio of 1:1.. The NiO microspheres were self-assembled by uniform nanoparticles and exhibited the excellent gas-sensing performance. The possible growth mechanism of the NiO architectures nanostructures was discussed in detail.
Journal Pre-proof
Hydrothermal synthesis of nanoparticles-assembled NiO microspheres and their sensing properties Shixiu Cao1*, Lingling Peng1, Tao Han1, Bitao Liu1, Dachuan Zhu2*, Cong Zhao1, Jing Xu1, Yinyin Tang1, Jinyu Wang1, Shuangshuang He1 1, Research Institute for New Material Technology, Chongqing University of Arts and Science, Chongqing, 402160, China 2, College of Materials Science and Engineering, Sichuan University, Sichuan, 610000, China
Abstract: Three hierarchical NiO microspheres were successfully fabricated via a facile hydrothermal method in the dissolvant of 1,2-Propanediol and H2O with a molar ratio of 1:1. The NiO microspheres were self-assembled by uniform nanoparticles and exhibited excellent gas-sensing performance. A possible nucleation-growth mechanism was also proposed in detail. It is believed that this unique nanostructure has great potential to enhance the gas-sensing properties of nickel oxide and other semiconductor materials. Key words: NiO; microspheres; Crystal growth; Sensors; Hydrothermal synthesis 1.
Introduction
Nickel oxide (NiO) is a significant p-type semiconductor with wide band gap energy in the range of 3.6-4.0 eV, and has been extensively studied for its practical application in various areas, for instance, gas sensors, electrochromic, photocatalysts, electrocatalyst, energy devices
[1-8],
and so on. In these
applications, because of fast response, good stability, high sensitivity, and low cost, the gas sensors based on NiO nanomaterials caused the people’s deep concern, and was applied to various gases
Corresponding author at: Research institute for new material technology, Chongqing University of Arts and Science, Chongqing. E-mail address: [email protected] 1
Journal Pre-proof detection, such as ethanol (C2H5OH), acetone (C2H6CO), methanol (CH2O), ammonia (NH3), methanol (CH3OH), trimethylamine (N(CH3)3). The unique structures and morphologies play important roles in improving gas sensing performance, including changing the number of carriers, providing rich effective gas diffusion path as well as high surface area. Great efforts have been devoted to synthesizing NiO nanomaterials with various morphologies for gas sensitive materials, such as hollow spheres [1], nanoflowers [2], nanosheets [4], nanowires [5], octahedrons [8], and nanorods [9]. Recently, many studies have demonstrated that 3D nanostructures of the semiconductor metal oxides (such as SnO2, ZnO, Cr2O3 and CuO) showed outstanding sensing properties compared to nanorods, nanoblocks and nanowires [10-16]. It was owing to their high surface to volume ratio and broad internal contact area. The well-defined architectures provide effective gas diffusion channels along the surfaces and ensure the sufficient reactions. It is very significant to develop a facile, shape and size-controlled self-assembly route for the formation of 3D NiO architectures. Up to now, NiO nanomaterials have be fabricated by various methods such as hydrothermal method, solvothermal synthesis, thermal evaporation, electrospinning route, chemical vapor deposition [4-6, 9, 17]. In this work, we reported the uniform NiO microspheres self-assembled tightly by nanoparticles via a facile one-step hydrothermal method. The structures and morphologies of the as-prepared NiO microspheres were characterized. The gas-sensing performance was also investigated. Moreover, a possible nucleation-growth mechanism was also proposed. 2. Experimental Synthesis of samples: All the chemicals were of analytical purity and used directly without any further purification. Firstly, nickel nitrate (Ni(NO3)2 • 6H2O, 2mmol), urea(CO(NH2)2 (1mmol), and 1mmol citric acid, were dissolved in the 23ml mixed solution (1,2-Propanediol:H2O =1:1) under magnetic 2
Journal Pre-proof stirring at room temperature for 40 min. Then, the solution obtained was transferred into Teflon-lined stainless steel autoclave (80 ml) and sealed at 180 °C for 12 h. After cooling to room temperature, the product was collected by centrifugation and washed with distilled water and ethanol three times, respectively. Finally, the sample was dried at 400 °C for 4 h. Characterization of samples: The as-prepared samples was characterised by X-Ray Diffraction (XRD), a Rigaku D/Max-1200X diffractometry equipped with Cu Kα radiation. The morphologies and microstructures were observed by scanning electron microscopy (FE-SEM, Nova 400 Nano) and transmission electron microscope (TEM, ZEISS, LIBRA200). The gas sensing properties were measured with HW-30 A gas sensitivity instrument (Hanwei Electronics Co., Ltd.) Fabrication of the gas sensor: Firstly, the as-prepared NiO power was dispersed in distilled water to form paste. Next, the formed turbid liquid was then spin coated onto an alumina ceramic tube, positioned with a pair of Au electrodes and four Pt wires at each end point. In order to control the operating temperature, the Ni–Cr wire was inserted into the alumina tube as a heater. The schematic of the gas sensor is shown in Fig.1a. Finally, the gas sensor was dried at 350 °C for 3 h to evaporate the existed organic binder. Gas-sensing properties were measured using HW-30A gas sensitivity instrument (Hanwei Electronics Co., Ltd.). Gas concentration was controlled by injecting a given amount of target gas into the glass chamber. A load resistor (RL) was connected to the electric circuit of gas sensor (Fig. 1b). The operating temperature of gas sensor can be adjusted by varying the heating voltage (Vh). The resistance (RS) of gas sensor was estimated from RS = RL (Vc -Vout)/Vout, which was calculated automatically by the computer of the gas sensing test system, where the Vc and Vout were circuit and output voltage, respectively (Fig. 1b). The gas response (S) was defined as S = Rg/Ra, where Ra and Rg were 3
Journal Pre-proof resistances of the sensors in air and target gases, respectively. The response and recovery time was counted as the interval between when the response reached 90% of its maximum and when it dropped to 10% of its maximum [18].
Fig. 1. Schematic diagrams of the gas sensor (a) and measurement electric circuit (b).
3. Results and discussion XRD analysis: The XRD patterns of the product synthesized were presented in Fig.2.The five main diffraction peaks, namely (111), (200), (220), (311) and (222) of the XRD patterns, agreed well with the standard data file (JCPDS no. 04-0835). Moreover, no other impurity peaks were detected, indicating the good crystallinity of the obtained NiO product.
Intensity(a.u.)
(200)
(111) (220)
(311)
30
40
50
60
70
(222)
80
2-Theta(degree)
Fig. 2. XRD patterns of the as-prepared product. SEM observation: Figure 3 displayed SEM and TEM image of as-prepared samples. Fig. 3(a, b) showed NiO micropheres were disperse spherical shape with diameters of about 1μm~2μm. Fig.3c displayed the high-magnification SEM image of NiO micropheres. It clearly revealed that the NiO micropheres were assembled with nanoparticles and with relatively rough surface. Fig.3d showed the 4
Journal Pre-proof TEM morphology. And obviously the spheres were solid, and were in an average size of 1μm~2μm. From Fig.3e, the NiO microspheres were actually hierarchical structures made up of about 15-20nm nanoparticles. Fig.3f showed well-defined lattice fringes. And the interplanar distances of 0.24nm and 0.21 nm were corresponding to the (111) and (200) planes of the hexagonal phase of NiO, respectively. The inset of Fig. 3f showed the SAED pattern. The SAED pattern was consistent with strong ring patterns due to (111), (200), and (220) planes of hexagonal NiO, which was also consistent with XRD (Fig.2) analysis.
Fig.3. SEM (a, b, c) and TEM (d, e, f) images of as-prepared samples.
Fig. 4. Schematic illustration the evolution processes of the as-prepared products. Growth mechanism: On the basis of the experimental observations and analysis, a possible growth 5
Journal Pre-proof mechanism for the morphologies evolution of the sample was proposed, as sketched in Fig. 4. It has been [11] reported that the hydrothermal reaction conditions such as temperature, pressure, and composition of the solution, play a crucial role in size, structure and growth units of metal oxide. From the above results, it is concluded that not only the shape and size of NiO nanostructures but also their preferred orientation can be controlled by citric acid in the initial solution. Generally speaking，a
crystal growth process consists of nucleation and growth. CO(NH2)2 + 3H2O → 2NH3•H2O + CO2
(1)
NH3•H2O → NH4+ + OHNi+ + OH-
Ni(OH)2
(2) NiO
(3)
According to Eqs. (1)-(3), the urea in aqueous solution hydrolyzed and formed NH3•H2O, and released the OH- ions. Meanwhile, Ni(NO3)2 • 6H2O reacted with OH- ions to form Ni(OH)2 nuclei.
Along with the rising temperature, tiny single crystals nucleated homogenously and grew gradually, which was ascribed to thermodynamic and dynamic effects. The citric acid had three carboxyl (–COOH) groups and one hydroxyl (–OH) group. Citric acid is known to form chelates with Ni species by –O–Ni–O– link in aqueous solution and therefore stabilized the colloidal Ni(OH)2 precursor particles so that these colloidal nuclei can grow during subsequent hydrothermal crystallization [20]. In the initial nucleation, it was suggested that the interaction between organic
acid and crystal surfaces may play a decisive role in controlling the resultant phase composition, crystal size and growth direction, and microstructure [19]. In the oriented growth period, the –COOH functional groups of citric acid may cap or selectively adsorb on the surfaces of Ni(OH)2 crystal nucleus in different directions. Citric acid could control the growth rate of various faces of Ni(OH)2 particles. As the reaction time increased, in order to reduce the surface energy, the nanoparticles 6
Journal Pre-proof gradually self-oriented and assembled to hierarchical Ni(OH)2 microspheres. Finally, the Ni(OH)2 microspheres were calcined and NiO microspheres were formed. Thus, the NiO microspheres assembled by nanoparticles were obtained. The shape of the nanocrystals can be made by using chemical reagents like organic inducer in the solution. (2018.12.15（ 4-1（ )
1.0
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Fig. 5. (a) Responses of NiO microspheres sensors to 60 ppm ethanol at different temperatures. (b) Responses of NiO microspheres sensors with different concentration at 260 °C. (c) Reproducibility characteristics at 260 °C under 100 ppm ethanol.
Gas sensing: as shown in fig.5, the gas response of the NiO microspheres fabricated sensors to ethanol was measured. The gas response was presented to 60 ppm ethanol at different temperatures (Fig. 5a). From Fig. 4a, the response reached the peak value at 260℃. Fig. 5b showed the relationship between response and ethanol concentrations (5~100ppm) for the sensor at 260℃. With increasing concentrations, the sensor response displayed a remarkable increase, indicating 7
Journal Pre-proof excellent selectivity to varied gas concentrations. Fig. 5c demonstrated the dynamic response and recovery characteristic towards 100 ppm ethanol at 260 °C. We saw that NiO microspheres displayed excellent stability and reproducibility. The excellent gas-sensing performances of NiO microspheres may be attributed to the well-defined architectures assembled by nanoparticles, which provide effective gas diffusion channels along the NiO surfaces and ensure the sufficient reactions [21]. 4. Conclusion In summary, we have successfully prepared novel NiO microspheres assembled by nanoparticles via the hydrothermal routes. The microspheres exhibited excellent gas-sensing performances. The results provided an innovative approach to effectively broaden the scope of ethanol gas sensors and exploit ideal candidates for next-generation gas-sensing devices. Acknowledgements The authors acknowledge the financial support to this work from the Basic and Frontier Research Program of Chongqing Municipality (cstc2016jcyjA0567, cstc2017jcyjBX0051), the Science and Technology Research Program of Chongqing Municipal Education Commission (KJ1711282, KJ1711283), the Natural Science Foundation of China (51702033). References [1] C. W. Kuang, W. Zeng, H. Ye, Y. Q. Li, Physica E. 97 (2018) 314-316. [2] Y. Zhang, W. Zeng, Mater. Lett. 195 (2017) 217-219. [3] Q. Wu, M. Wen, S. Chen, Q. Wu, J. Alloy. Compd. 646 (2015) 990-997. [4] Y. Lu, Y. H. M, S. Y. Ma, W. X. Jin, S. H. Yan, X. L. Xu, Q. Chen, Mater. Lett. 190 (2017) 252-255. [5] B. Liu, H. Q. Yang, H. Zhao, L. J. An, L.H. Zhang, R. Y. Shi, L. Wang, L. Bao, Y. Chen, Sensor. 8
Journal Pre-proof Actuat. B-Chem. 156 (2011) 251-262. [6] X. G. San, G. S. Wang, B. Liang, J. Ma, D. Meng, Y. B. Shen, J. Alloy. Compd. 636 (2015) 357-362. [7] L. Zhu, Y. Li, W. Zeng, Appl. Surf. Sci. 427 (2018) 281-287. [8] B. Liu, H. Yang, A. Wei, H. Zhao, L. Ning, C. Zhang, S. Liu, Appl. Catal. B172-173 (2015) 165-173. [9] T. Kavithaab and H. Yuvaraj, J. Mater. Chem. 21(2011) 15686-15691. [10] I. A. Abdel-Latif, M. M. Rahman, S. B. Khan, Results Phys. 8 (2018) 578-583. [11] S. B. Khan, K. S. Karimov, A. Din, K. Akhtar, Microchim. Acta 185 (2018) 2-10. [12] A. Din, K. S. Karimov, K. Akhtar,
M. I. Khan, M. T. S. Chani,
M. A. Khan, A. M. Asiri, S. B.
Khan, J. Mater. Sci-Mater. El. 28 (2017)4260-4266. [13] I. Ahmad, S. B. Khan, T. Kamal, A. M. Asiri, J. Mol. Liq. 229 (2017)429-435. [14] A. Din, S. B. Khan, M. I. Khan, S. A. B. Asif, M. A. Khan, S. Gul, K. Akhtar, A. M. Asiri, J. Mater. Sci-Mater. El. 1(2017) 1092-1100. [15] T. Kamal, Y. Anwar, S. B. Khan, M. T. S. Chani, A. M. Asiri, Carbohyd. Polym. 148 (2016) 153-160. [16] T. Kamal, S. B. Khan, A. M. Asiri, Environ. Pollut. 218(2016) 625-633. [17] J. Cao, H. M. Zhang, X. Q. Yan, Mater. Lett. 185 (2016) 40-42. [18] W. W. Guo, M. Fu, C. Z. Zhai, Z. C. Wang, Ceram. Int. 40(2014)2295-2298. [19] V. B. Patil, P. V. Adhyapak, S. S. SuryavanshiaI. S. Mull, J. Alloy. Compd.590 (2014) 283-288 [20] X. G. Wang, H. L. Zhang, L. L. Liu, W. J. Li, P. Cao, Mater. Lett. 130 (2014) 248-251 [21] Y. Yu, Y. Xia, W. Zeng, R. Liu, Mater. Lett. 206 (2017) 80-83. 9
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Conflict of interest
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Journal Pre-proof Figure Captions Fig. 1. Schematic diagrams of the gas sensor (a) and measurement electric circuit (b). Fig. 2. XRD patterns of the as-prepared product. Fig. 3. SEM (a, b, c) and TEM (d, e, f) images of as-prepared samples. Fig. 4. Schematic illustration the evolution processes of the as-prepared products. Fig. 5. (a) Responses of NiO microspheres sensors to 60 ppm ethanol at different temperatures. (b) Responses of NiO microspheres sensors with different concentration at 260°C. (c) Reproducibility characteristics at 260 °C under 100 ppm ethanol.
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Highlights
The NiO microspheres were synthesized by a hydrothermal route. The NiO microspheres show excellent gas sensing performances to ethanol. It is important for preparations of other metallic oxide as gas sensitive material.
Shixiu Cao, Lingling Peng, Tao Han, Bitao Liu, Dachuan Zhu, Cong Zhao, Jing Xu, Yinyin Tang, Jinyu Wang, Shuangshuang He PII:
S1386-9477(19)30601-0
DOI:
https://doi.org/10.1016/j.physe.2019.113655
Reference:
PHYSE 113655
To appear in:
Physica E: Low-dimensional Systems and Nanostructures
Received Date:
19 April 2019
Accepted Date:
24 July 2019
Please cite this article as: Shixiu Cao, Lingling Peng, Tao Han, Bitao Liu, Dachuan Zhu, Cong Zhao, Jing Xu, Yinyin Tang, Jinyu Wang, Shuangshuang He, Hydrothermal synthesis of nanoparticles-assembled NiO microspheres and their sensing properties, Physica E: Lowdimensional Systems and Nanostructures (2019), https://doi.org/10.1016/j.physe.2019.113655
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.
Journal Pre-proof Graphical Abstract (for review) The NiO microspheres have been successfully synthesized via a facile hydrothermal method in the dissolvant of 1,2-Propanediol and H2O with a molar ratio of 1:1.. The NiO microspheres were self-assembled by uniform nanoparticles and exhibited the excellent gas-sensing performance. The possible growth mechanism of the NiO architectures nanostructures was discussed in detail.
Journal Pre-proof
Hydrothermal synthesis of nanoparticles-assembled NiO microspheres and their sensing properties Shixiu Cao1*, Lingling Peng1, Tao Han1, Bitao Liu1, Dachuan Zhu2*, Cong Zhao1, Jing Xu1, Yinyin Tang1, Jinyu Wang1, Shuangshuang He1 1, Research Institute for New Material Technology, Chongqing University of Arts and Science, Chongqing, 402160, China 2, College of Materials Science and Engineering, Sichuan University, Sichuan, 610000, China
Abstract: Three hierarchical NiO microspheres were successfully fabricated via a facile hydrothermal method in the dissolvant of 1,2-Propanediol and H2O with a molar ratio of 1:1. The NiO microspheres were self-assembled by uniform nanoparticles and exhibited excellent gas-sensing performance. A possible nucleation-growth mechanism was also proposed in detail. It is believed that this unique nanostructure has great potential to enhance the gas-sensing properties of nickel oxide and other semiconductor materials. Key words: NiO; microspheres; Crystal growth; Sensors; Hydrothermal synthesis 1.
Introduction
Nickel oxide (NiO) is a significant p-type semiconductor with wide band gap energy in the range of 3.6-4.0 eV, and has been extensively studied for its practical application in various areas, for instance, gas sensors, electrochromic, photocatalysts, electrocatalyst, energy devices
[1-8],
and so on. In these
applications, because of fast response, good stability, high sensitivity, and low cost, the gas sensors based on NiO nanomaterials caused the people’s deep concern, and was applied to various gases
Corresponding author at: Research institute for new material technology, Chongqing University of Arts and Science, Chongqing. E-mail address: [email protected] 1
Journal Pre-proof detection, such as ethanol (C2H5OH), acetone (C2H6CO), methanol (CH2O), ammonia (NH3), methanol (CH3OH), trimethylamine (N(CH3)3). The unique structures and morphologies play important roles in improving gas sensing performance, including changing the number of carriers, providing rich effective gas diffusion path as well as high surface area. Great efforts have been devoted to synthesizing NiO nanomaterials with various morphologies for gas sensitive materials, such as hollow spheres [1], nanoflowers [2], nanosheets [4], nanowires [5], octahedrons [8], and nanorods [9]. Recently, many studies have demonstrated that 3D nanostructures of the semiconductor metal oxides (such as SnO2, ZnO, Cr2O3 and CuO) showed outstanding sensing properties compared to nanorods, nanoblocks and nanowires [10-16]. It was owing to their high surface to volume ratio and broad internal contact area. The well-defined architectures provide effective gas diffusion channels along the surfaces and ensure the sufficient reactions. It is very significant to develop a facile, shape and size-controlled self-assembly route for the formation of 3D NiO architectures. Up to now, NiO nanomaterials have be fabricated by various methods such as hydrothermal method, solvothermal synthesis, thermal evaporation, electrospinning route, chemical vapor deposition [4-6, 9, 17]. In this work, we reported the uniform NiO microspheres self-assembled tightly by nanoparticles via a facile one-step hydrothermal method. The structures and morphologies of the as-prepared NiO microspheres were characterized. The gas-sensing performance was also investigated. Moreover, a possible nucleation-growth mechanism was also proposed. 2. Experimental Synthesis of samples: All the chemicals were of analytical purity and used directly without any further purification. Firstly, nickel nitrate (Ni(NO3)2 • 6H2O, 2mmol), urea(CO(NH2)2 (1mmol), and 1mmol citric acid, were dissolved in the 23ml mixed solution (1,2-Propanediol:H2O =1:1) under magnetic 2
Journal Pre-proof stirring at room temperature for 40 min. Then, the solution obtained was transferred into Teflon-lined stainless steel autoclave (80 ml) and sealed at 180 °C for 12 h. After cooling to room temperature, the product was collected by centrifugation and washed with distilled water and ethanol three times, respectively. Finally, the sample was dried at 400 °C for 4 h. Characterization of samples: The as-prepared samples was characterised by X-Ray Diffraction (XRD), a Rigaku D/Max-1200X diffractometry equipped with Cu Kα radiation. The morphologies and microstructures were observed by scanning electron microscopy (FE-SEM, Nova 400 Nano) and transmission electron microscope (TEM, ZEISS, LIBRA200). The gas sensing properties were measured with HW-30 A gas sensitivity instrument (Hanwei Electronics Co., Ltd.) Fabrication of the gas sensor: Firstly, the as-prepared NiO power was dispersed in distilled water to form paste. Next, the formed turbid liquid was then spin coated onto an alumina ceramic tube, positioned with a pair of Au electrodes and four Pt wires at each end point. In order to control the operating temperature, the Ni–Cr wire was inserted into the alumina tube as a heater. The schematic of the gas sensor is shown in Fig.1a. Finally, the gas sensor was dried at 350 °C for 3 h to evaporate the existed organic binder. Gas-sensing properties were measured using HW-30A gas sensitivity instrument (Hanwei Electronics Co., Ltd.). Gas concentration was controlled by injecting a given amount of target gas into the glass chamber. A load resistor (RL) was connected to the electric circuit of gas sensor (Fig. 1b). The operating temperature of gas sensor can be adjusted by varying the heating voltage (Vh). The resistance (RS) of gas sensor was estimated from RS = RL (Vc -Vout)/Vout, which was calculated automatically by the computer of the gas sensing test system, where the Vc and Vout were circuit and output voltage, respectively (Fig. 1b). The gas response (S) was defined as S = Rg/Ra, where Ra and Rg were 3
Journal Pre-proof resistances of the sensors in air and target gases, respectively. The response and recovery time was counted as the interval between when the response reached 90% of its maximum and when it dropped to 10% of its maximum [18].
Fig. 1. Schematic diagrams of the gas sensor (a) and measurement electric circuit (b).
3. Results and discussion XRD analysis: The XRD patterns of the product synthesized were presented in Fig.2.The five main diffraction peaks, namely (111), (200), (220), (311) and (222) of the XRD patterns, agreed well with the standard data file (JCPDS no. 04-0835). Moreover, no other impurity peaks were detected, indicating the good crystallinity of the obtained NiO product.
Intensity(a.u.)
(200)
(111) (220)
(311)
30
40
50
60
70
(222)
80
2-Theta(degree)
Fig. 2. XRD patterns of the as-prepared product. SEM observation: Figure 3 displayed SEM and TEM image of as-prepared samples. Fig. 3(a, b) showed NiO micropheres were disperse spherical shape with diameters of about 1μm~2μm. Fig.3c displayed the high-magnification SEM image of NiO micropheres. It clearly revealed that the NiO micropheres were assembled with nanoparticles and with relatively rough surface. Fig.3d showed the 4
Journal Pre-proof TEM morphology. And obviously the spheres were solid, and were in an average size of 1μm~2μm. From Fig.3e, the NiO microspheres were actually hierarchical structures made up of about 15-20nm nanoparticles. Fig.3f showed well-defined lattice fringes. And the interplanar distances of 0.24nm and 0.21 nm were corresponding to the (111) and (200) planes of the hexagonal phase of NiO, respectively. The inset of Fig. 3f showed the SAED pattern. The SAED pattern was consistent with strong ring patterns due to (111), (200), and (220) planes of hexagonal NiO, which was also consistent with XRD (Fig.2) analysis.
Fig.3. SEM (a, b, c) and TEM (d, e, f) images of as-prepared samples.
Fig. 4. Schematic illustration the evolution processes of the as-prepared products. Growth mechanism: On the basis of the experimental observations and analysis, a possible growth 5
Journal Pre-proof mechanism for the morphologies evolution of the sample was proposed, as sketched in Fig. 4. It has been [11] reported that the hydrothermal reaction conditions such as temperature, pressure, and composition of the solution, play a crucial role in size, structure and growth units of metal oxide. From the above results, it is concluded that not only the shape and size of NiO nanostructures but also their preferred orientation can be controlled by citric acid in the initial solution. Generally speaking，a
crystal growth process consists of nucleation and growth. CO(NH2)2 + 3H2O → 2NH3•H2O + CO2
(1)
NH3•H2O → NH4+ + OHNi+ + OH-
Ni(OH)2
(2) NiO
(3)
According to Eqs. (1)-(3), the urea in aqueous solution hydrolyzed and formed NH3•H2O, and released the OH- ions. Meanwhile, Ni(NO3)2 • 6H2O reacted with OH- ions to form Ni(OH)2 nuclei.
Along with the rising temperature, tiny single crystals nucleated homogenously and grew gradually, which was ascribed to thermodynamic and dynamic effects. The citric acid had three carboxyl (–COOH) groups and one hydroxyl (–OH) group. Citric acid is known to form chelates with Ni species by –O–Ni–O– link in aqueous solution and therefore stabilized the colloidal Ni(OH)2 precursor particles so that these colloidal nuclei can grow during subsequent hydrothermal crystallization [20]. In the initial nucleation, it was suggested that the interaction between organic
acid and crystal surfaces may play a decisive role in controlling the resultant phase composition, crystal size and growth direction, and microstructure [19]. In the oriented growth period, the –COOH functional groups of citric acid may cap or selectively adsorb on the surfaces of Ni(OH)2 crystal nucleus in different directions. Citric acid could control the growth rate of various faces of Ni(OH)2 particles. As the reaction time increased, in order to reduce the surface energy, the nanoparticles 6
Journal Pre-proof gradually self-oriented and assembled to hierarchical Ni(OH)2 microspheres. Finally, the Ni(OH)2 microspheres were calcined and NiO microspheres were formed. Thus, the NiO microspheres assembled by nanoparticles were obtained. The shape of the nanocrystals can be made by using chemical reagents like organic inducer in the solution. (2018.12.15（ 4-1（ )
1.0
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0.8
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Fig. 5. (a) Responses of NiO microspheres sensors to 60 ppm ethanol at different temperatures. (b) Responses of NiO microspheres sensors with different concentration at 260 °C. (c) Reproducibility characteristics at 260 °C under 100 ppm ethanol.
Gas sensing: as shown in fig.5, the gas response of the NiO microspheres fabricated sensors to ethanol was measured. The gas response was presented to 60 ppm ethanol at different temperatures (Fig. 5a). From Fig. 4a, the response reached the peak value at 260℃. Fig. 5b showed the relationship between response and ethanol concentrations (5~100ppm) for the sensor at 260℃. With increasing concentrations, the sensor response displayed a remarkable increase, indicating 7
Journal Pre-proof excellent selectivity to varied gas concentrations. Fig. 5c demonstrated the dynamic response and recovery characteristic towards 100 ppm ethanol at 260 °C. We saw that NiO microspheres displayed excellent stability and reproducibility. The excellent gas-sensing performances of NiO microspheres may be attributed to the well-defined architectures assembled by nanoparticles, which provide effective gas diffusion channels along the NiO surfaces and ensure the sufficient reactions [21]. 4. Conclusion In summary, we have successfully prepared novel NiO microspheres assembled by nanoparticles via the hydrothermal routes. The microspheres exhibited excellent gas-sensing performances. The results provided an innovative approach to effectively broaden the scope of ethanol gas sensors and exploit ideal candidates for next-generation gas-sensing devices. Acknowledgements The authors acknowledge the financial support to this work from the Basic and Frontier Research Program of Chongqing Municipality (cstc2016jcyjA0567, cstc2017jcyjBX0051), the Science and Technology Research Program of Chongqing Municipal Education Commission (KJ1711282, KJ1711283), the Natural Science Foundation of China (51702033). References [1] C. W. Kuang, W. Zeng, H. Ye, Y. Q. Li, Physica E. 97 (2018) 314-316. [2] Y. Zhang, W. Zeng, Mater. Lett. 195 (2017) 217-219. [3] Q. Wu, M. Wen, S. Chen, Q. Wu, J. Alloy. Compd. 646 (2015) 990-997. [4] Y. Lu, Y. H. M, S. Y. Ma, W. X. Jin, S. H. Yan, X. L. Xu, Q. Chen, Mater. Lett. 190 (2017) 252-255. [5] B. Liu, H. Q. Yang, H. Zhao, L. J. An, L.H. Zhang, R. Y. Shi, L. Wang, L. Bao, Y. Chen, Sensor. 8
Journal Pre-proof Actuat. B-Chem. 156 (2011) 251-262. [6] X. G. San, G. S. Wang, B. Liang, J. Ma, D. Meng, Y. B. Shen, J. Alloy. Compd. 636 (2015) 357-362. [7] L. Zhu, Y. Li, W. Zeng, Appl. Surf. Sci. 427 (2018) 281-287. [8] B. Liu, H. Yang, A. Wei, H. Zhao, L. Ning, C. Zhang, S. Liu, Appl. Catal. B172-173 (2015) 165-173. [9] T. Kavithaab and H. Yuvaraj, J. Mater. Chem. 21(2011) 15686-15691. [10] I. A. Abdel-Latif, M. M. Rahman, S. B. Khan, Results Phys. 8 (2018) 578-583. [11] S. B. Khan, K. S. Karimov, A. Din, K. Akhtar, Microchim. Acta 185 (2018) 2-10. [12] A. Din, K. S. Karimov, K. Akhtar,
M. I. Khan, M. T. S. Chani,
M. A. Khan, A. M. Asiri, S. B.
Khan, J. Mater. Sci-Mater. El. 28 (2017)4260-4266. [13] I. Ahmad, S. B. Khan, T. Kamal, A. M. Asiri, J. Mol. Liq. 229 (2017)429-435. [14] A. Din, S. B. Khan, M. I. Khan, S. A. B. Asif, M. A. Khan, S. Gul, K. Akhtar, A. M. Asiri, J. Mater. Sci-Mater. El. 1(2017) 1092-1100. [15] T. Kamal, Y. Anwar, S. B. Khan, M. T. S. Chani, A. M. Asiri, Carbohyd. Polym. 148 (2016) 153-160. [16] T. Kamal, S. B. Khan, A. M. Asiri, Environ. Pollut. 218(2016) 625-633. [17] J. Cao, H. M. Zhang, X. Q. Yan, Mater. Lett. 185 (2016) 40-42. [18] W. W. Guo, M. Fu, C. Z. Zhai, Z. C. Wang, Ceram. Int. 40(2014)2295-2298. [19] V. B. Patil, P. V. Adhyapak, S. S. SuryavanshiaI. S. Mull, J. Alloy. Compd.590 (2014) 283-288 [20] X. G. Wang, H. L. Zhang, L. L. Liu, W. J. Li, P. Cao, Mater. Lett. 130 (2014) 248-251 [21] Y. Yu, Y. Xia, W. Zeng, R. Liu, Mater. Lett. 206 (2017) 80-83. 9
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Conflict of interest
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Journal Pre-proof Figure Captions Fig. 1. Schematic diagrams of the gas sensor (a) and measurement electric circuit (b). Fig. 2. XRD patterns of the as-prepared product. Fig. 3. SEM (a, b, c) and TEM (d, e, f) images of as-prepared samples. Fig. 4. Schematic illustration the evolution processes of the as-prepared products. Fig. 5. (a) Responses of NiO microspheres sensors to 60 ppm ethanol at different temperatures. (b) Responses of NiO microspheres sensors with different concentration at 260°C. (c) Reproducibility characteristics at 260 °C under 100 ppm ethanol.
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Highlights
The NiO microspheres were synthesized by a hydrothermal route. The NiO microspheres show excellent gas sensing performances to ethanol. It is important for preparations of other metallic oxide as gas sensitive material.