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Simulation of inter- and transgranular crack propagation in polycrystalline aggregates due to stress corrosion cracking
Accepted Manuscript Title: Hydrothermal synthesis of hierarchical flower-like ZnO nanostructure and its enhanced ethanol gas-sensing properties Authors: Ling Zhu, Yanqiong Li, Wen Zeng PII: DOI: Reference:
S0169-4332(17)32589-8 http://dx.doi.org/10.1016/j.apsusc.2017.08.229 APSUSC 37066
To appear in:
APSUSC
Received date: Revised date: Accepted date:
19-7-2017 13-8-2017 31-8-2017
Please cite this article as: Ling Zhu, Yanqiong Li, Wen Zeng, Hydrothermal synthesis of hierarchical flower-like ZnO nanostructure and its enhanced ethanol gas-sensing properties, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.08.229 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrothermal
synthesis
of
hierarchical
flower-like
ZnO
nanostructure and its enhanced ethanol gas-sensing properties
Ling Zhu1, Yanqiong Li2, Wen Zeng11*
1. College of Materials Science and Engineering, Chongqing University, Chongqing China 2. School of Electronic and Electrical Engineering, Chongqing University of Arts and Sciences, Chongqing China
1*Corresponding
author E-mail: [email protected] (W. Zeng)
Graphical abstract We report a hydrothermal synthesis of ZnO nanoparticles, nanoplates and nanoflowers, and investigate their gas-sensing behaviors. In addition, we also explore the effect of the concentration of surfactant CTAB on the ultimate morphology of the nanoflowers and propose the growth formation mechanism.
1
Highlights
ZnO nanoparticles, nanoplates and nanoflowers were successfully synthesized.
The nanoflowers architectures showed the best gas-sensing properties.
CTAB played a vital role in the ultimate morphology of the 3D hierarchical
architectures
Abstract ZnO nanoparticles, nanoplates and nanoflowers have been successfully synthesized via a facile hydrothermal route, and their microstructures and gas-sensing properties to ethanol were investigated. Among all the nanostructures, the nanoplates-assembled nanoflowers exhibited significantly higher gas-sensing performances than the others, which may ascribe to their hierarchical architectures with large specific area and abundant spaces for gas diffusion. Furthermore, we surprisingly found that the concentration of surfactant CTAB used had an essential effect on the ultimate morphology of the hierarchical nanoflowers. We hoped our findings could be in favor of further investigations on the fabrication of perfect hierarchical architectures.
Keywords: Hierarchical nanostructure; ZnO; Hydrothermal process; Gas sensing 1. Introduction Metal-oxide semiconductor nanomaterials has drawn extensive attention from researchers owing to their unique physicochemical properties and multifunctional applications [1-7]. Zinc oxide (ZnO), as an eminent n-type semiconductor with wide band gap of 3.37 eV and large exciton binding energy of 60 meV [8], which occupies an important position in various metal-oxide nanomaterials because of its preeminent electrical, optical and catalytic behaviors, has
2
been widely utilized in gas sensors [8], solar cells [3], photocatalyst [9, 10] and etc.. In recent years, countless endeavors have been poured on the development of gas sensors based on nanostructured ZnO to achieve the effective detection of hazardous gases. As is known to all, the sensing mechanism of ZnO is primarily controlled by its surface [11]. Therefore, tailoring the structure and morphology of ZnO becomes the key to optimizing the gas-sensing performances [12]. In particular, the elaborate design of three-dimension (3D) hierarchical nanostructures constructed from 1D and 2D building blocks can exactly achieve this, since such hierarchical architectures avoid the agglomerated configuration of low dimension nanostructures and are relevant for high specific area and fast gas diffusion, and thus become a hot topic at present [13-20]. Up to now, hydrothermal route has been verified to be one of the most extensively employed methods for fabricating the special 3D hierarchical ZnO nanostructures, which has an notable merits of simple, efficient, mild, cost-effective and high yields[21-24]. However, the specific morphology is strongly affected by a minor change in experimental parameters, such as zinc types [23], alkali types [25], solvent types [12], PH of the initial solution [26], reaction temperatures [24], reaction times [27], annealing temperatures [28] and so on. Consequently, developing a controllable method for preparation of unique 3D hierarchical architectures still remains a great challenge. In current work, we have successfully synthesized the ZnO nanostructures with different morphologies including nanoparticles, nanoplates and nanoflowers by a facile hydrothermal process. The gas sensing measurement revealed that the ethanol sensing properties of the nanoflowers was evidently enhanced as compared to the nanoparticles and nanoplates. Moreover, the concentration of surfactant CTAB was found to play a vital role in the morphological evolution
3
of the ZnO nanostructures. And the possible growth mechanism of the nanoflowers is proposed as well.
2. Experiments 2.1 Synthesis of ZnO with different morphologies All the reagents were analytical-grade purity and used as-purchased without any further purification. ZnO nanostructures with different morphologies were prepared via a facile hydrothermal process. To synthesize the ZnO nanoflowers, 0.1 M Zn(NO3)2·6H2O and 0.1 M surfactant cetyltrim ethyl ammonium bromide (CTAB, (C16H33)N(CH3)3Br) were first dissolved into deionized water under intense magnetic stirring at room temperature for 3 h. Then, 0.6 M NaOH was slowly added into the obtained solution. After continuous stirring for further 0.5 h, the resultant solution was transferred into the Teflon-lined stainless steel autoclave, kept at 150oC for 16 h and subsequently cooled to room temperature naturally. The white precipitate was collected by centrifugation, repeatedly washed with deionized water and ethanol, and dried at 60oC for 24 h. Finally, the ZnO nanoflowers were obtained by calcinating the precipitate at 500oC for 2 h. The ZnO nanoplates and nanoparticles were prepared in the same way. The only differences were the absence of surfactant CTAB and the addition of NaOH (0.2 M), respectively.
2.2 Characterization The crystal phases of the as-prepared samples were characterized by X-ray diffraction (XRD) using a Rigaku D/Max-1200X diffractometer with the Cu Kα radiation (30 kV, 100 mA) in the 2θ range from 20o to 75o. The surface morphologies were recorded with a Nova 400 field emission scanning electron microscopy (FESEM).
2.3 Fabrication of gas sensor
4
The configuration of gas sensor is presented in Fig. 1a. To fabricate gas sensors, the as-synthesized ZnO powders were dispersed into deionized water to form a slurry which was then ultrasonicated for 0.5 h for homogenization. Subsequently, the slurry was coated onto an alumina tube on which a pair of Au electrodes and four Pt conducting wires had been previously equipped. The coated alumina tube was dried at 100oC for 2 h and annealed at 350oC for 1 h. Next, a Ni-Cr wire served as a heater was inserted into the alumina tube. Finally, further aging at 200oC for 240 h was needed to improve the stability and repeatability of gas sensor.
2.4 Gas sensing measurement The fabricated sensor was welded onto a pedestal and the gas-sensing properties were measured by a HW-30A gas sensitivity instruments (Hanwei Electronics Co., Ltd, Henan Province, China). In order to investigate the influence of gas concentration on the response, certain amount of target gas was measured by a needle and then evaporated into well-defined concentration of target gas in the reaction chamber. Fig. 1b shows the test electrical circuit. The operating temperature was adjusted by tuning the heating voltage (Vh). The sensor resistance (Rs) was estimated from the following formula: Rs=Rl(Vc-Vout)/Vout, where Vc and Vout referred to the circuit and output voltage, respectively. The gas response in this work was defined as S=Ra/Rg, in which Ra and Rg were the resistance in air and test gas, respectively. The response and recovery time were counted as the time taken by the sensors to reach 90% of its maximum and drop to 10% of its maximum, respectively.
3. Results and discussion 3.1 Structure and morphology To determine the phase of the as-prepared products, the representative XRD patterns of the
5
three samples with different morphologies are shown in Fig. 2. All of the diffraction peaks of the three samples are exactly matched with those of a standard ZnO with wurtzite hexagonal structure (JCPDS Card No. 36-1451). No other additional diffraction peaks from impurities are observed, implying the high purity of all the samples. The surface morphologies of the obtained products are investigated by SEM. Fig. 3a and b presents the SEM images of the ZnO nanoparticles, from which one can see that the nanoparticles are tiny and irregular, the average diameter of which is in the range of 40-80 nm. Unfortunately, there appears a serious particle aggregation originating from the Van der Waals force, which results in a sharp decrease in its specific area. Fig. 3c and d displays the SEM images of nanoplates, where one can observe that a great deal of nanoplates with a thickness of ~30 nm are randomly stacked together without any regularity, and no other morphology is found. The ZnO nanoflowers are illustrated in Fig. 3e and f. As one can see that many hierarchical flower-like nanostructures with a diameter of ~2 μm are uniformly distributed across the whole sample. Further magnification signifies that the individual nanoflower is comprised of large amount of nanoplates that have a thickness of ~30 nm (Fig. 3f). It seems that these nanoplates are assembled by intersecting and mixing with each other, forming the hierarchical flower-like structures. Different from the other two nanostructures, such one with vast intervals and pores is believed to be beneficial for the evident optimization in the gas-sensing performances.
3.2 Gas-sensing properties With an aim to figure out the influence of the ZnO with different morphologies on gas sensing, we systematically test the gas-sensing behaviors towards ethanol of fabricated gas sensors based on the as-prepared nanoparticles, nanoplates and nanoflowers. Fig. 4a shows the gas
6
response of the three sensors at various operating temperatures from 200oC to 450oC under the ethanol concentrition of 400 ppm, where one can observe an identical trend for the response change that increases firstly and then decreases with the temperature rising. The maximum gas responses for the nanoparticles, nanoplates and nanoflowers are 20.3, 23.3 and 30.4 at the optimum operating temperatures of 350oC, respectively. Apperantly, among all the three sensors, the one made of the nanoflowers exhibits the largest gas response towards ethanol at all the investigated temperatures. Next, the gas response of the sensors to ethnol ranging from 100 to 600 ppm at 350oC are measured, as presented in Fig. 4b. It can be seen that the gas responses of all the three sensors increase almost linearly with increasing the ethanol concentration in the measured range. Once again, the sensor made of the nanoflowers shows the higher gas response than the nanoparticles and nanoplates. Further, Fig. 4c depicts the response and recovery characteristics of the three sensors at 350oC towards 400 ppm ethanol, from which one can see that the voltages of the sensors goes up rapidly when the ethanol gas is in but returnes to its initial state when the ethanol gas is out. The visible difference of the three cases is that the voltage of the sensor made of the nanoflower is evidently higher than that of the other two sensors, indicating again the nanoflowers with enhanced gas response. In addtion, on account of the aforementioned defination for the response and recovery time, it is evaluated to be about 12 and 4 s for the nanoparticles, 12 and 5 s for the nanoplates, and 10 and 4 s for the nanoflowers.These further demonstrate that the nanoflowers is relatively more sensitive to ethanol than the nanoparticles and nanoplates. According to the results above, we can draw a conclusion that the ZnO nanoflowers assembled by nanoplates exhibits
7
superior ethanol gas-sensing behavious as compared to the nanoparticles and nanoplates, and thus are the potential materials for sensnig application.
3.3 Gas-sensing mechanism The gas-sensing mechanism of ZnO can be explained by a change in resistance arising from the adsorption and desorption of oxygen [27, 29-32]. As illustrated in Fig. 4d, when the sensor is deposited in air ambient, the oxygen molecules will be adsorbed on the surface of ZnO and then ionize into oxygen species in the form of O2-, O-, O2- by depriving electrons from the conduction band of ZnO, which is highly dependent on the operating temperatures [33, 34]. These transferred electrons induce a thick electron depletion layer, thereby leading to an increase in sensor resistance. However, once the ethanol gas is injected into the test chamber, ethanol molecules will react with the oxygen species on the surface, which releases the trapped electrons back to the conduction band of ZnO, resulting in a decrease in the width of electron depletion layer, finally causing the sensor resistance to be reduced. Generally, the specific area of ZnO determined by its particle size and surface morphology has a strong effect on its gas-sensing properties [35, 36]. The large specific area will contribute to excellent ethanol gas sensing due to more available reaction sites. In comparison, the ZnO nanoflowers possesses higher specific area than the nanoparticles and nanoplates, and hence exhibits the largest gas response towards ethanol. Moreover, such 3D hierarchical nanoflowers with vast intervals and pores avoids the serious aggregation of low dimension nanostructures, which can offer abundant spaces for mass gas diffusion, therefore showing a relatively fast reaction speed.
3.4 Growth mechanism of the nanoflowers
8
To gain more insight into the effect of the concentration of surfactant CTAB in the initial solution on the formation of such hierarchical ZnO nanoflowers, we present a range of SEM images of the final ZnO products prepared using various concentrations of CTAB (0.03 M; 0.05 M, 0.1 M; 0.3 M), as shown in Fig. 5. Seeing from the Fig. 5a and b, when introducing a little of CTAB (0.03 M), no flower-like structures appears, instead, a great deal of the nanoplates scattering randomly can be observed. Closer inspection reveals that the nanoplates have a tendency to assemble to the flower-like structures. Once more CTAB is further added (0.05 M), the nanoflowers has begun to take shape, just like the flower buds, although only a few nanoplates act as the flower petals (Fig. 5c and d). Continuous increasing in the concentration of CTAB (0.1 M), more nanoplates aggregate in a blooming flower-like fashion (Fig. 5e and f). However, the overdoes of CTAB (0.3 M) brings about the collapse of flower-like structures. The nanoplates are agglomerated in disorder (Fig. 5g and h). These has fully proved that the final morphology of the hierarchical flower-like structures can be governed by altering the concentration of CTAB, and an appropriate value is highly needed so as to obtain the perfect hierarchical ZnO nanoflowers. To understand the growth mechanism of the nanoflowers comprehensively, the possible chemical reactions under the hydrothermal conditions are presented as follows: Zn2+ + 2OH- → Zn(OH)2
(1)
Zn(OH)2 + 2OH- → Zn(OH)42-
(2)
CTAB → CTA+ +Br-
(3)
CTA+ + Zn(OH)42- → CTA+-Zn(OH)42-
(4)
CTA+-Zn(OH)42- → ZnO + CTA+ +H2O + 2OH-
(5)
On the basis of the experimental observation and analysis, we propose the plausible evolution
9
process of the ZnO nanoflowers, as shown in Fig. 6. The complex precursor Zn(OH)42- anions are formed from the Zn2+ of Zn(NO3)2·6H2O and OH- of NaOH under the hydrothermal. In the presence of cation surfactant CTAB, the positively charged CTA+ with a hydrophilic head and a hydrophobic tail clings to the negatively charged Zn(OH)42- nuclei to form the CTA+-Zn(OH)42ion-pairs due to electrostatic interaction [37]. Herein, CTAB serves as not only the ionic carrier contributing to fast chemical reaction, but also a templet on account of the formation of lamellar micelle. Capped by CTAB, the ZnO nanocrystals nucleate and then grow up. Afterwards, the nanoparticles undergo an oriented attachment, forming the nanoplates with the surface capping of CTAB micelle. These nanoplates finally self-assemble into the flower-like structures propelled by the descending of surface energy. In the case where the concentration of CTAB is low, it is most likely that the steric hindrance effect of CTAB as a stabilizer is not enough to against the Van der Waals forces, still remaining a degree of aggregation of the nanoplates. Further, the overdose of CTAB is also not favorable for ultimate formation of the hierarchical flower-like structures. The excess CTAB may be embedded between the nanoplates and hinders their free motion, that is, it impedes the progress for self-assembly of the nanoplates, which renders the collapse of the flower-like structures.
4. Conclusions In summary, we have used the facile hydrothermal method for the preparation of the ZnO nanoparticles, nanoplates and nanoflowers. The nanoflowers assembled by nanoplates exhibited the superior ethanol gas-sensing performances relative to the nanoparticles and nanoplates, displaying the highest gas response and fastest response and recovery speed. The enhanced properties may be attributed to the hierarchical flower-like architectures that possessed large
10
specific area and abundant spaces for gas diffusion. In addition, we also found that the concentration of CTAB obviously determined the final morphology of ZnO nanostructures, which was understood based upon the nucleation, growth and final self-assembly of ZnO nano-building blocks. Acknowledgment This work was supported by Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA0006) and Graduate Scientific Research and Innovation Foundation of Chongqing, China (Grant No. CYS16008).
11
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10847-10852. [19] S. Wang, J. Yang, H. Zhang, Y. Wang, X. Gao, L. Wang, Z. Zhu, One-pot synthesis of 3D hierarchical SnO2 nanostructures and their application for gas sensor, Sens. Actuators B: Chem. 207 (2015) 83-89. [20] W. Zeng, H. Zhang, Z. Wang, Effects of different petal thickness on gas sensing properties of flower-like WO3·H2O hierarchical architectures, Appl. Surf. Sci. 347 (2015) 73-78. [21] W. Guo, One-pot synthesis of urchin-like ZnO nanostructure and its enhanced acetone gas sensing properties, J. Mater. Sci.: Mater. Electron. 28 (2016) 963-972. [22] C. Peng, J. Guo, W. Yang, C. Shi, M. Liu, Y. Zheng, J. Xu, P. Chen, T. Huang, Y. Yang, Synthesis of three-dimensional flower-like hierarchical ZnO nanostructure and its enhanced acetone gas sensing properties, J. Alloy. Compd. 654 (2016) 371-378. [23] H. Zhang, R. Wu, Z. Chen, G. Liu, Z. Zhang, Z. Jiao, Self-assembly fabrication of 3D flower-like ZnO hierarchical nanostructures and their gas sensing properties, CrystEngComm 14 (2012) 1775. [24] X. Qu, M. Wang, W. Sun, R. Yang, Hierarchical flower-like ZnO microstructures: preparation, formation mechanism and application in gas sensor, J. Mater. Sci.: Mater. Electron. (2017). [25] X. Liu, J. Zhang, T. Yang, L. Wang, Y. Kang, S. Wang, S. Wu, Self-assembled hierarchical flowerlike ZnO architectures and their gas-sensing properties, Powder Technol. 217 (2012) 238-244. [26] K. Shingange, Z.P. Tshabalala, B.P. Dhonge, O.M. Ntwaeaborwa, D.E. Motaung, G.H. Mhlongo, 0D to 3D ZnO nanostructures and their luminescence, magnetic and sensing
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properties: Influence of pH and annealing, Mater. Res. Bull. 85 (2017) 52-63. [27] J. Luo, S.Y. Ma, A.M. Sun, L. Cheng, G.J. Yang, T. Wang, W.Q. Li, X.B. Li, Y.Z. Mao, D.J. Gz, Ethanol sensing enhancement by optimizing ZnO nanostructure: From 1D nanorods to 3D nanoflower, Mater. Lett. 137 (2014) 17-20. [28] F. Fan, Y. Feng, P. Tang, A. Chen, R. Luo, D. Li, Synthesis and Gas Sensing Performance of Dandelion-Like ZnO with Hierarchical Porous Structure, Ind. Eng. Chem. Res. 53 (2014) 12737-12743. [29] W. Guo, ZnO nanosheets assembled different hierarchical structures and their gas sensing properties, J. Mater. Sci.: Mater. Electron. 27 (2016) 7302-7310. [30] J. Xu, Z. Xue, N. Qin, Z. Cheng, Q. Xiang, The crystal facet-dependent gas sensing properties of ZnO nanosheets: Experimental and computational study, Sens. Actuators B: Chem. 242 (2017) 148-157. [31] J. Xu, J. Han, Y. Zhang, Y.a. Sun, B. Xie, Studies on alcohol sensing mechanism of ZnO based gas sensors, Sens. Actuators B: Chem. 132 (2008) 334-339. [32] W. Guo, T. Liu, H. Zhang, R. Sun, Y. Chen, W. Zeng, Z. Wang, Gas-sensing performance enhancement in ZnO nanostructures by hierarchical morphology, Sens. Actuators B: Chem. 166-167 (2012) 492-499. [33] N.G. Shimpi, S. Jain, N. Karmakar, A. Shah, D.C. Kothari, S. Mishra, Synthesis of ZnO nanopencils using wet chemical method and its investigation as LPG sensor, Appl. Surf. Sci. 390 (2016) 17-24. [34] Y. Zhang, T. Liu, S. Zhao, X. Kuang, S. Hussain, L. Lin, W. Zeng, X. Peng, Z. Wang, Hydrothermal synthesis of ZnO microcakes assembled by octahedrons and their gas-sensing
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property, J. Mater. Sci.: Mater. Electron. 26 (2015) 9529-9534. [35] J. Cui, L. Shi, T. Xie, D. Wang, Y. Lin, UV-light illumination room temperature HCHO gas-sensing mechanism of ZnO with different nanostructures, Sens. Actuators B: Chem. 227 (2016) 220-226. [36] Z. Jing, J. Zhan, Fabrication and gas-sensing properties of porous ZnO nanoplates, Adv. Mater. 20 (2008) 4547-4551. [37] H. Milani Moghaddam, H. Malkeshi, Self-assembly synthesis and ammonia gas-sensing properties of ZnO/Polythiophene nanofibers, J. Mater. Sci.: Mater. Electron. 27 (2016) 8807-8815.
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Figures
Fig. 1. (a) Schematic diagram of gas sensor. (b) Electric circuit of sensor device.
Fig. 2. XRD patterns of the obtained products with different morphologies: (a) nanoparticles, (b) nanoplates and (c) nanoflowers.
Fig. 3. SEM images of the ZnO: (a-b) nanoparticles, (c-d) nanoplates and (e-f) nanoflowers. 17
Fig. 4. (a) Response of the sensors made of the ZnO with different nanostructures exposed to 400 ppm ethanol at various temperatures. (b) Response of the three sensors under different ethanol concentration at 350oC. (c) Response-recovery curve of the three sensors at 350oC towards 400 ppm ethanol.
18
Fig. 5. SEM images of the ZnO samples synthesized with various concentrations of CTAB: (a-b) 0.03 M, (c-d) 0.05 M, (e-f) 0.1 M and (g-h) 0. 3 M.
Fig. 6. Schematic illustration for the possible evolution process of the hierarchical flower-like ZnO structures using different concentrations of CTAB. 19
S0169-4332(17)32589-8 http://dx.doi.org/10.1016/j.apsusc.2017.08.229 APSUSC 37066
To appear in:
APSUSC
Received date: Revised date: Accepted date:
19-7-2017 13-8-2017 31-8-2017
Please cite this article as: Ling Zhu, Yanqiong Li, Wen Zeng, Hydrothermal synthesis of hierarchical flower-like ZnO nanostructure and its enhanced ethanol gas-sensing properties, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.08.229 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrothermal
synthesis
of
hierarchical
flower-like
ZnO
nanostructure and its enhanced ethanol gas-sensing properties
Ling Zhu1, Yanqiong Li2, Wen Zeng11*
1. College of Materials Science and Engineering, Chongqing University, Chongqing China 2. School of Electronic and Electrical Engineering, Chongqing University of Arts and Sciences, Chongqing China
1*Corresponding
author E-mail: [email protected] (W. Zeng)
Graphical abstract We report a hydrothermal synthesis of ZnO nanoparticles, nanoplates and nanoflowers, and investigate their gas-sensing behaviors. In addition, we also explore the effect of the concentration of surfactant CTAB on the ultimate morphology of the nanoflowers and propose the growth formation mechanism.
1
Highlights
ZnO nanoparticles, nanoplates and nanoflowers were successfully synthesized.
The nanoflowers architectures showed the best gas-sensing properties.
CTAB played a vital role in the ultimate morphology of the 3D hierarchical
architectures
Abstract ZnO nanoparticles, nanoplates and nanoflowers have been successfully synthesized via a facile hydrothermal route, and their microstructures and gas-sensing properties to ethanol were investigated. Among all the nanostructures, the nanoplates-assembled nanoflowers exhibited significantly higher gas-sensing performances than the others, which may ascribe to their hierarchical architectures with large specific area and abundant spaces for gas diffusion. Furthermore, we surprisingly found that the concentration of surfactant CTAB used had an essential effect on the ultimate morphology of the hierarchical nanoflowers. We hoped our findings could be in favor of further investigations on the fabrication of perfect hierarchical architectures.
Keywords: Hierarchical nanostructure; ZnO; Hydrothermal process; Gas sensing 1. Introduction Metal-oxide semiconductor nanomaterials has drawn extensive attention from researchers owing to their unique physicochemical properties and multifunctional applications [1-7]. Zinc oxide (ZnO), as an eminent n-type semiconductor with wide band gap of 3.37 eV and large exciton binding energy of 60 meV [8], which occupies an important position in various metal-oxide nanomaterials because of its preeminent electrical, optical and catalytic behaviors, has
2
been widely utilized in gas sensors [8], solar cells [3], photocatalyst [9, 10] and etc.. In recent years, countless endeavors have been poured on the development of gas sensors based on nanostructured ZnO to achieve the effective detection of hazardous gases. As is known to all, the sensing mechanism of ZnO is primarily controlled by its surface [11]. Therefore, tailoring the structure and morphology of ZnO becomes the key to optimizing the gas-sensing performances [12]. In particular, the elaborate design of three-dimension (3D) hierarchical nanostructures constructed from 1D and 2D building blocks can exactly achieve this, since such hierarchical architectures avoid the agglomerated configuration of low dimension nanostructures and are relevant for high specific area and fast gas diffusion, and thus become a hot topic at present [13-20]. Up to now, hydrothermal route has been verified to be one of the most extensively employed methods for fabricating the special 3D hierarchical ZnO nanostructures, which has an notable merits of simple, efficient, mild, cost-effective and high yields[21-24]. However, the specific morphology is strongly affected by a minor change in experimental parameters, such as zinc types [23], alkali types [25], solvent types [12], PH of the initial solution [26], reaction temperatures [24], reaction times [27], annealing temperatures [28] and so on. Consequently, developing a controllable method for preparation of unique 3D hierarchical architectures still remains a great challenge. In current work, we have successfully synthesized the ZnO nanostructures with different morphologies including nanoparticles, nanoplates and nanoflowers by a facile hydrothermal process. The gas sensing measurement revealed that the ethanol sensing properties of the nanoflowers was evidently enhanced as compared to the nanoparticles and nanoplates. Moreover, the concentration of surfactant CTAB was found to play a vital role in the morphological evolution
3
of the ZnO nanostructures. And the possible growth mechanism of the nanoflowers is proposed as well.
2. Experiments 2.1 Synthesis of ZnO with different morphologies All the reagents were analytical-grade purity and used as-purchased without any further purification. ZnO nanostructures with different morphologies were prepared via a facile hydrothermal process. To synthesize the ZnO nanoflowers, 0.1 M Zn(NO3)2·6H2O and 0.1 M surfactant cetyltrim ethyl ammonium bromide (CTAB, (C16H33)N(CH3)3Br) were first dissolved into deionized water under intense magnetic stirring at room temperature for 3 h. Then, 0.6 M NaOH was slowly added into the obtained solution. After continuous stirring for further 0.5 h, the resultant solution was transferred into the Teflon-lined stainless steel autoclave, kept at 150oC for 16 h and subsequently cooled to room temperature naturally. The white precipitate was collected by centrifugation, repeatedly washed with deionized water and ethanol, and dried at 60oC for 24 h. Finally, the ZnO nanoflowers were obtained by calcinating the precipitate at 500oC for 2 h. The ZnO nanoplates and nanoparticles were prepared in the same way. The only differences were the absence of surfactant CTAB and the addition of NaOH (0.2 M), respectively.
2.2 Characterization The crystal phases of the as-prepared samples were characterized by X-ray diffraction (XRD) using a Rigaku D/Max-1200X diffractometer with the Cu Kα radiation (30 kV, 100 mA) in the 2θ range from 20o to 75o. The surface morphologies were recorded with a Nova 400 field emission scanning electron microscopy (FESEM).
2.3 Fabrication of gas sensor
4
The configuration of gas sensor is presented in Fig. 1a. To fabricate gas sensors, the as-synthesized ZnO powders were dispersed into deionized water to form a slurry which was then ultrasonicated for 0.5 h for homogenization. Subsequently, the slurry was coated onto an alumina tube on which a pair of Au electrodes and four Pt conducting wires had been previously equipped. The coated alumina tube was dried at 100oC for 2 h and annealed at 350oC for 1 h. Next, a Ni-Cr wire served as a heater was inserted into the alumina tube. Finally, further aging at 200oC for 240 h was needed to improve the stability and repeatability of gas sensor.
2.4 Gas sensing measurement The fabricated sensor was welded onto a pedestal and the gas-sensing properties were measured by a HW-30A gas sensitivity instruments (Hanwei Electronics Co., Ltd, Henan Province, China). In order to investigate the influence of gas concentration on the response, certain amount of target gas was measured by a needle and then evaporated into well-defined concentration of target gas in the reaction chamber. Fig. 1b shows the test electrical circuit. The operating temperature was adjusted by tuning the heating voltage (Vh). The sensor resistance (Rs) was estimated from the following formula: Rs=Rl(Vc-Vout)/Vout, where Vc and Vout referred to the circuit and output voltage, respectively. The gas response in this work was defined as S=Ra/Rg, in which Ra and Rg were the resistance in air and test gas, respectively. The response and recovery time were counted as the time taken by the sensors to reach 90% of its maximum and drop to 10% of its maximum, respectively.
3. Results and discussion 3.1 Structure and morphology To determine the phase of the as-prepared products, the representative XRD patterns of the
5
three samples with different morphologies are shown in Fig. 2. All of the diffraction peaks of the three samples are exactly matched with those of a standard ZnO with wurtzite hexagonal structure (JCPDS Card No. 36-1451). No other additional diffraction peaks from impurities are observed, implying the high purity of all the samples. The surface morphologies of the obtained products are investigated by SEM. Fig. 3a and b presents the SEM images of the ZnO nanoparticles, from which one can see that the nanoparticles are tiny and irregular, the average diameter of which is in the range of 40-80 nm. Unfortunately, there appears a serious particle aggregation originating from the Van der Waals force, which results in a sharp decrease in its specific area. Fig. 3c and d displays the SEM images of nanoplates, where one can observe that a great deal of nanoplates with a thickness of ~30 nm are randomly stacked together without any regularity, and no other morphology is found. The ZnO nanoflowers are illustrated in Fig. 3e and f. As one can see that many hierarchical flower-like nanostructures with a diameter of ~2 μm are uniformly distributed across the whole sample. Further magnification signifies that the individual nanoflower is comprised of large amount of nanoplates that have a thickness of ~30 nm (Fig. 3f). It seems that these nanoplates are assembled by intersecting and mixing with each other, forming the hierarchical flower-like structures. Different from the other two nanostructures, such one with vast intervals and pores is believed to be beneficial for the evident optimization in the gas-sensing performances.
3.2 Gas-sensing properties With an aim to figure out the influence of the ZnO with different morphologies on gas sensing, we systematically test the gas-sensing behaviors towards ethanol of fabricated gas sensors based on the as-prepared nanoparticles, nanoplates and nanoflowers. Fig. 4a shows the gas
6
response of the three sensors at various operating temperatures from 200oC to 450oC under the ethanol concentrition of 400 ppm, where one can observe an identical trend for the response change that increases firstly and then decreases with the temperature rising. The maximum gas responses for the nanoparticles, nanoplates and nanoflowers are 20.3, 23.3 and 30.4 at the optimum operating temperatures of 350oC, respectively. Apperantly, among all the three sensors, the one made of the nanoflowers exhibits the largest gas response towards ethanol at all the investigated temperatures. Next, the gas response of the sensors to ethnol ranging from 100 to 600 ppm at 350oC are measured, as presented in Fig. 4b. It can be seen that the gas responses of all the three sensors increase almost linearly with increasing the ethanol concentration in the measured range. Once again, the sensor made of the nanoflowers shows the higher gas response than the nanoparticles and nanoplates. Further, Fig. 4c depicts the response and recovery characteristics of the three sensors at 350oC towards 400 ppm ethanol, from which one can see that the voltages of the sensors goes up rapidly when the ethanol gas is in but returnes to its initial state when the ethanol gas is out. The visible difference of the three cases is that the voltage of the sensor made of the nanoflower is evidently higher than that of the other two sensors, indicating again the nanoflowers with enhanced gas response. In addtion, on account of the aforementioned defination for the response and recovery time, it is evaluated to be about 12 and 4 s for the nanoparticles, 12 and 5 s for the nanoplates, and 10 and 4 s for the nanoflowers.These further demonstrate that the nanoflowers is relatively more sensitive to ethanol than the nanoparticles and nanoplates. According to the results above, we can draw a conclusion that the ZnO nanoflowers assembled by nanoplates exhibits
7
superior ethanol gas-sensing behavious as compared to the nanoparticles and nanoplates, and thus are the potential materials for sensnig application.
3.3 Gas-sensing mechanism The gas-sensing mechanism of ZnO can be explained by a change in resistance arising from the adsorption and desorption of oxygen [27, 29-32]. As illustrated in Fig. 4d, when the sensor is deposited in air ambient, the oxygen molecules will be adsorbed on the surface of ZnO and then ionize into oxygen species in the form of O2-, O-, O2- by depriving electrons from the conduction band of ZnO, which is highly dependent on the operating temperatures [33, 34]. These transferred electrons induce a thick electron depletion layer, thereby leading to an increase in sensor resistance. However, once the ethanol gas is injected into the test chamber, ethanol molecules will react with the oxygen species on the surface, which releases the trapped electrons back to the conduction band of ZnO, resulting in a decrease in the width of electron depletion layer, finally causing the sensor resistance to be reduced. Generally, the specific area of ZnO determined by its particle size and surface morphology has a strong effect on its gas-sensing properties [35, 36]. The large specific area will contribute to excellent ethanol gas sensing due to more available reaction sites. In comparison, the ZnO nanoflowers possesses higher specific area than the nanoparticles and nanoplates, and hence exhibits the largest gas response towards ethanol. Moreover, such 3D hierarchical nanoflowers with vast intervals and pores avoids the serious aggregation of low dimension nanostructures, which can offer abundant spaces for mass gas diffusion, therefore showing a relatively fast reaction speed.
3.4 Growth mechanism of the nanoflowers
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To gain more insight into the effect of the concentration of surfactant CTAB in the initial solution on the formation of such hierarchical ZnO nanoflowers, we present a range of SEM images of the final ZnO products prepared using various concentrations of CTAB (0.03 M; 0.05 M, 0.1 M; 0.3 M), as shown in Fig. 5. Seeing from the Fig. 5a and b, when introducing a little of CTAB (0.03 M), no flower-like structures appears, instead, a great deal of the nanoplates scattering randomly can be observed. Closer inspection reveals that the nanoplates have a tendency to assemble to the flower-like structures. Once more CTAB is further added (0.05 M), the nanoflowers has begun to take shape, just like the flower buds, although only a few nanoplates act as the flower petals (Fig. 5c and d). Continuous increasing in the concentration of CTAB (0.1 M), more nanoplates aggregate in a blooming flower-like fashion (Fig. 5e and f). However, the overdoes of CTAB (0.3 M) brings about the collapse of flower-like structures. The nanoplates are agglomerated in disorder (Fig. 5g and h). These has fully proved that the final morphology of the hierarchical flower-like structures can be governed by altering the concentration of CTAB, and an appropriate value is highly needed so as to obtain the perfect hierarchical ZnO nanoflowers. To understand the growth mechanism of the nanoflowers comprehensively, the possible chemical reactions under the hydrothermal conditions are presented as follows: Zn2+ + 2OH- → Zn(OH)2
(1)
Zn(OH)2 + 2OH- → Zn(OH)42-
(2)
CTAB → CTA+ +Br-
(3)
CTA+ + Zn(OH)42- → CTA+-Zn(OH)42-
(4)
CTA+-Zn(OH)42- → ZnO + CTA+ +H2O + 2OH-
(5)
On the basis of the experimental observation and analysis, we propose the plausible evolution
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process of the ZnO nanoflowers, as shown in Fig. 6. The complex precursor Zn(OH)42- anions are formed from the Zn2+ of Zn(NO3)2·6H2O and OH- of NaOH under the hydrothermal. In the presence of cation surfactant CTAB, the positively charged CTA+ with a hydrophilic head and a hydrophobic tail clings to the negatively charged Zn(OH)42- nuclei to form the CTA+-Zn(OH)42ion-pairs due to electrostatic interaction [37]. Herein, CTAB serves as not only the ionic carrier contributing to fast chemical reaction, but also a templet on account of the formation of lamellar micelle. Capped by CTAB, the ZnO nanocrystals nucleate and then grow up. Afterwards, the nanoparticles undergo an oriented attachment, forming the nanoplates with the surface capping of CTAB micelle. These nanoplates finally self-assemble into the flower-like structures propelled by the descending of surface energy. In the case where the concentration of CTAB is low, it is most likely that the steric hindrance effect of CTAB as a stabilizer is not enough to against the Van der Waals forces, still remaining a degree of aggregation of the nanoplates. Further, the overdose of CTAB is also not favorable for ultimate formation of the hierarchical flower-like structures. The excess CTAB may be embedded between the nanoplates and hinders their free motion, that is, it impedes the progress for self-assembly of the nanoplates, which renders the collapse of the flower-like structures.
4. Conclusions In summary, we have used the facile hydrothermal method for the preparation of the ZnO nanoparticles, nanoplates and nanoflowers. The nanoflowers assembled by nanoplates exhibited the superior ethanol gas-sensing performances relative to the nanoparticles and nanoplates, displaying the highest gas response and fastest response and recovery speed. The enhanced properties may be attributed to the hierarchical flower-like architectures that possessed large
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specific area and abundant spaces for gas diffusion. In addition, we also found that the concentration of CTAB obviously determined the final morphology of ZnO nanostructures, which was understood based upon the nucleation, growth and final self-assembly of ZnO nano-building blocks. Acknowledgment This work was supported by Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA0006) and Graduate Scientific Research and Innovation Foundation of Chongqing, China (Grant No. CYS16008).
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Figures
Fig. 1. (a) Schematic diagram of gas sensor. (b) Electric circuit of sensor device.
Fig. 2. XRD patterns of the obtained products with different morphologies: (a) nanoparticles, (b) nanoplates and (c) nanoflowers.
Fig. 3. SEM images of the ZnO: (a-b) nanoparticles, (c-d) nanoplates and (e-f) nanoflowers. 17
Fig. 4. (a) Response of the sensors made of the ZnO with different nanostructures exposed to 400 ppm ethanol at various temperatures. (b) Response of the three sensors under different ethanol concentration at 350oC. (c) Response-recovery curve of the three sensors at 350oC towards 400 ppm ethanol.
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Fig. 5. SEM images of the ZnO samples synthesized with various concentrations of CTAB: (a-b) 0.03 M, (c-d) 0.05 M, (e-f) 0.1 M and (g-h) 0. 3 M.
Fig. 6. Schematic illustration for the possible evolution process of the hierarchical flower-like ZnO structures using different concentrations of CTAB. 19