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Enhancement of NH3 sensing performance in flower-like ZnO nanostructures and their growth mechanism
Accepted Manuscript Title: Enhancement of NH3 Sensing Performance in Flower-like ZnO Nanostructures and their Growth Mechanism Author: Yu Zhang Tianmo Liu Jinghua Hao Liyang Lin Wen Zeng Xianghe Peng Zhongchang Wang PII: DOI: Reference:
S0169-4332(15)01994-7 http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.170 APSUSC 31117
To appear in:
APSUSC
Received date: Revised date: Accepted date:
16-6-2015 8-8-2015 20-8-2015
Please cite this article as: Y. Zhang, T. Liu, J. Hao, L. Lin, W. Zeng, X. Peng, Z. Wang, Enhancement of NH3 Sensing Performance in Flower-like ZnO Nanostructures and their Growth Mechanism, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.170 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.
*Highlights (for review)
The sunflower-like ZnO nanostructures reveal superior NH3 sensing performance than ZnO nanoparticles.
The ZnO nanoflowers, looking like a sunflower with six petals and one pistil, consist of single
The sensitivity of ZnO nanoflowers exposure to NH3 is up to 49.5 under the concentration of 50
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ppm, which is better than that reported in the literatures.
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crystalline nanotriangles and nanosphere.
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Graphical Abstract (for review)
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*Manuscript
Enhancement of NH3 Sensing Performance in Flower-like ZnO Nanostructures and their Growth
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Mechanism
Yu Zhang1,2 , Tianmo Liu1,2*, Jinghua Hao1,2, Liyang Lin1,4, Wen Zeng1,2 , Xianghe Peng3,
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Zhongchang Wang4* 1
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College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China National Engineering Research Center for Mg Alloys, Chongqing University, Chongqing 400044,
Chongqing Key Laboratory of Heterogeneous Material Mechanics, Chongqing University, Chongqing,
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China
400044, China 4
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Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai
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980-8577, Japan
Abstract: ZnO nanostructures hold substantial promise for gas-sensing applications owning to their
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outstanding ethanol sensing performance, yet their sensing performance towards NH3 has rarely been reported. Here, we report on a successful preparation of sunflower-like ZnO nanostructures and ZnO nanoparticle via a facile hydrothermal method, and demonstrate that the ZnO nanoflowers have high gas-sensing performances towards NH3 under a low concentration of 10~50 ppm. Further structural characterization reveals that the sunflower-like nanostructure comprises six triangles-like and one sphere-like nanostructures, and the triangle-like nanostructure is single crystalline with {001} crystal face. As a consequence of their unique morphology, the nanoflowers show much improved NH3 sensing
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performances than the nanoparticles with a high sensitivity of 49.5. Keywords: ZnO, nanoflowers, single crystalline, gas sensor
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1. Introduction As a n-type semiconductor with a wide direct band gap of ~3.4 eV and a large exciton binding
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energy of ~60 meV at room temperature, ZnO has received a great deal of attentions for over decades.
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Cumulative attention is now being focused on the synthesis method of various ZnO nanostructures, including doping [1–3], composite [4–6] and template [7,8]. These methods are, however, very
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complicated and the preparation time is often rather tedious. Recently, an easy and convenient
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template-free hydrothermal approach has been utilized widely to synthesize ZnO [9,10]. In property, its gas-sensing performance and luminescence receive considerable interest. Moreover, it has been widely
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applied as supercapacitors [11,12], solar cells [13,14] and piezoelectric nanogenerators [15,16], and gas sensors [17,18]. Currently, much effort has been devoted to the study of gas-sensing performances of
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ZnO [3,19,20] due to the ever increased concerns over air quality and safety of human beings. Generally, gas-sensing performance of ZnO-based gas sensors is controlled by the morphology of
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ZnO nanostructures, which is tailored by both the area of exposed ZnO and the length of electron path. The fabrication of ZnO nanostructures with a high specific area and a short electron path is therefore of great importance. For example, Xie et al. reported the fabrication of different morphologies by a facile hydrothermal method [21] and compared the photocatalytic properties of nanospheres, nanoflowers, and nanosheets. Cheng et al. [22] prepared rugby-like, dart-like, and flower-like ZnO nanostructures in aqueous solution assisted by ultrasonication at low temperature of 80°C, and investigated luminescence properties. In addition, Xu et al. [23] prepared Comb-Like ZnO nanostructures via introducing Au
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catalysts, and Hussain et al. prepared ZnO nanocones and studied their gas-sensing properties toward formaldehyde [24]. Lian et al. fabricated ZnO micro-cups and micro-rings assembled by nanoparticles [25].
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To date, most of the studies are focused on ZnO-based gas sensors to ethanol. Nanorod, nanosheet and nanodiamond have been fabricated separately and the best sensitivity is reported to be ~10 at 100
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ppm concentration of ethanol [26]. In addition, nanoflowers assembled with porous nanosheets of ZnO
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have been synthesized and their gas sensitivity to ethanol is up to ~20 at 100 ppm [27]. However, the sensing performance of ZnO to NH3 is poor with a low sensitivity of less than 10 [28,29]. Although
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Suranan et al. [30] reported that the ZnO nanorod annealed in O2 at 650oC has the sensitivity of 22.6,
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its sensitivity value reduces to less than 5 once the nanorod is annealed at other atmospheres. To improve the gas sensitivity of ZnO to NH3 gas, here we fabricate two kinds of ZnO nanostructures via a
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hydrothermal approach and find that in comparison to nanoparticles, nanoflowers show much improved sensing performance to NH3. The nanoflowers are found to be assembled with six triangles-like
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nanostructures and sphere-like nanostructure in center, and the triangles-like nanostructures turn out to
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be single crystalline and grow along three symmetric parallel planes, namely, (001), (010) and (100).
2. Experimental
All chemicals were of analytic grade without further purification, including zinc nitrate
hexahydrate (Zn(NO3)2·6H2O), sodium citrate (C6H5Na3O7·2H2O), and sodium hydroxide (NaOH). To synthesize ZnO nanoflower, 1.19 g of Zn(NO3)2·6H2O and 0.14 g of sodium citrate were mixed and dissolved into 40 ml of distilled water under magnetic stirring for 30 min to form transparent solution. 0.08 g of NaOH were then added into the transparent solution, which was subsequently transferred into
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a Teflon-lined stainless steel autoclave (50 ml). The autoclave was sealed and heated at 180oC for 6 h, and then cooled to room temperature. The obtained white precipitates were harvested by centrifuging, washing with distilled water and ethanol several times, and drying at 60oC for 8 h. For comparison,
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ZnO nanoparticles were prepared as well with the similar process. The difference is that 0.16 g of NaOH was introduced into the transparent solution. Table 1 shows the preparation condition.
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Crystal phase of the as-synthesized products were investigated by X-ray diffraction (XRD, Rigaku
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D/max-1200X) with Cu Kα radiation (λ = 1.5406 Å) operated at 30 kV and 100 mA. Morphologies were characterized by field emission scanning electron microscopy (FE-SEM, Nova 400 Nano) at 10
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kV, transmission electron microscopy (TEM, ZEISS LIBRA200) at 200 kV, high-resolution TEM
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(HRTEM), and selected-area electron diffraction (SAED). Gas-sensing performances were investigated using a static flow system, CGS-8 intelligent test system (Beijing Elite Tech Co., Ltd.), which was
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consisted of a heater, a gas distributor, and a data acquisition system [31]. Gas-sensing performances were measured five times under each gas concentration of ethanol gas and NH 3. The gas concentration
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ranged from 10 ppm to 50 ppm and the sensor operation temperature was 250oC. The sensitivity was defined as Ra/Rg, where Ra and Rg were resistance of the sensors in air and target gas, respectively. All
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gas-sensing measurements were conducted at room temperature with humidity of 40%.
3. Results and discussion 3.1 Structure and Morphology Figure 1 shows XRD patterns of the as-synthesized samples. The peaks are sharp and strong, indicating a high degree of crystallization for the samples. All the diffraction peaks can be indexed as a pure hexagonal phase ZnO (JCPDS Card no. 36-1451 a = 3.25 Å, b = 3.25Å, c = 5.207 Å). No other
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diffraction peak is observed, which reveals that no intermediate products are produced during reaction process. Figure 2a shows the morphology of as-prepared ZnO. The oval particles are highly dispersed with an average diameter of 200-300 nm (Fig. 2b). From Fig. 2c, a large number of sunflower-like ZnO
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nanostructures are observed, and every nanoflower has six triangles-like nanostructures around one sphere-like nanostructure. The triangles-like and sphere-like nanostructures have an average size of 500
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and 800 nm, respectively (Fig. 2d).
3.2 Flower-like ZnO nanostructures
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Figure 3a shows SEM image of the nanoflowers, which comprises six petals and one sphere pistil. Figure 3b shows HRTEM image of a petal (from an area marked in red square in Fig. 3a), from which
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one can see that the petal is of single crystal. The spacing of lattice fringe is measured to be ~0.28 nm,
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which indicates that the petal grows along {001} crystal plane, consistent with selected-area electron diffraction (SAED) analysis showing that the petals of nanoflowers mainly grow along six symmetric
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parallel planes, ±(01-10), ±(10-10) and ±(1-100). This also agrees with the crystal structure and the stacking mode of ZnO in Figs. 3c and 3d. It is known that the (0001) surface of ZnO is terminated with Zn and the (000-1) surface is terminated with O. The former is catalytically active, while the latter is
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relatively inert [32].
Figure 4 sketches the growth process of ZnO nanoflower. Firstly, ZnO, OH- and C6H5O73- are
obtained by the reaction process. As aforementioned, the ZnO nuclei grow simultaneously along the {001}, resulting in the formation of the triangle-like ZnO nanostructures as petals, which indicates that the edges of ZnO nanostructures are positively charged. Secondly, the atoms in the angles of triangle-like nanostructures are arranged in a disordered way so that these nanostructures join together
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through their angles, which can be attributed to the high energy and large mobility of the disordered atoms. The C6H5O73-, which owns more negative charges than OH-, is preferentially attracted by triangle-like nanostructures with positive charges. Moreover, the empty region, which is surrounded by
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triangle-like nanostructures, is also filled with new ZnO nuclei under the driving of negative charges of C6H5O73-. As a bridge, C6H5O73- has a binding effect on triangle-like nanostructures and new nuclei.
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Finally, the ZnO new nuclei grow into the sphere-like nanostructures like the pistil of sunflower and the
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ZnO nanotriangles are transformed to petals of sunflower. Zn (NO3)2 ·6H2O ↔ Zn2+ + 2NO3- + 6H2O
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NaOH ↔Na+ + OH-
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C6H5Na3O7·2H2O↔ C6H5O73- + 3Na+ + 2H2O
3.3 Gas-sensing performance
(2) (3) (4)
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Zn2+ + 4OH- ↔ [Zn(OH)4]2- ↔ ZnO + H2O + 2OH-
(1)
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Figure 5a shows gas response of ZnO nanoflowers and nanoparticles as a function of heating current at 50 ppm. The ZnO nanoflowers and nanoparticles show a maximum gas response of 49.5 and
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33.2 at 160 mA, respectively. The stability of the two sensors is also investigated for 60 days, as shown in Fig. 5b, which reveals that the two sensors have a superior stability. Figure 6a shows NH3 response for the sensor made of ZnO nanoparticles under different concentrations. The sensitivity is increased with the rise of NH3 concentration, and the sensor is rather sensitive to the low NH3 concentration of 10 ppm. In Fig. 6b, the sensor made of ZnO nanoflower shows response to NH3 at 10 ppm, and its sensitivity is enhanced with the increase of the NH3 concentration. As shown in Fig. 6c, the sensitivity signal for the nanoflower stabilizes in 6 s when the sensor is exposed at NH3 atmosphere of 50 ppm and
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returns to its original state in 3 s once the sensor is exposed in air. The response and recovery time of the sensor made of the nanoparticle is 8 and 5 s. Moreover, it is found that the ZnO nanoflower has a higher gas-sensing performance than the ZnO nanoparticle under the same concentration of NH3 (Fig.
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6d), which can be ascribed to the morphologies. In fact, ZnO rarely suggests a perfect NH3 sensing performance. Few studies have reported that the
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sensitivity of ZnO to NH3 is more than 10. For example, Suranan et al. [30] reported that the ZnO
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nanorod has a sensitivity of 22.6. However, the nanorod must be annealed in O2 atmosphere at 650oC. When the nanorod is annealed in other atmosphere, its sensitivity value reduces to less than
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5. Table 2 summaries gas response of a variety of ZnO sensors to NH3 prepared with different methods
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[28-30]. The ZnO nanoflower in the present work shows the highest sensitivity to NH3, which can be attributed to its special morphology and single-crystalline structure.
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As a negative semiconductor, ZnO has more electrons on its surface. So, the oxygen in the air will adsorb on it and capture the electrons into negative oxygen ions (O (ads)-, O2(ads)– and O2(ads)2-).
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After losing the electrons, the conductive band of ZnO will become narrower and its resistance will enhance. Once the NH3 gas comes up, it will react with negative oxygen ions and release the
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captured electrons back to the conductive band of ZnO. Then, the ZnO will have wider conductive band and its resistance will recover. The principle of the NH3 is shown as in Figure 7. When ZnO is in the air, the oxygen captured on its surface will react successively into negative oxygen ions (O (ads) ,
O2(ads)– and O2(ads)2-). While the NH3 gas is introduced, it will react with these negative
oxygen ions into nitrogen and release the electrons captured by oxygen. The reaction equation is appended below.
O2 + 2e-↔ 2O (ads)-
(1)
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(2)
O2 + 2e-↔ O2 (ads)2-
(3)
2NH3 + 3O (ads)- →N2 + 3H2O + 3e-
(4)
4NH3 + 3O2 (ads)- → 2N2 + 6H2O + 3e-
(5)
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O2 + e-↔ O2 (ads)-
4NH3 + 3O2 (ads)2- →2N2 + 6H2O + 6e-
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(6)
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4. Conclusions
We have successfully synthesized ZnO nanoparticles and nanoflowers via a simple hydrothermal
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strategy. The nanoparticles have an oval-like shape with an average diameter of 200-300 nm, and the
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nanoflowers show a sunflower-like shape with petals and pistils. As a consequence of these special morphologies, the nanoflowers exhibit a much improved gas-sensing performance to NH3 than the
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nanoparticles. In particular, the gas sensor made of the nanoflower shows a high sensitivity of 49.5 and a short response and recovery time of 6 s and 3 s, respectively, under NH3 of 50 ppm.
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Acknowledgments
The authors gratefully acknowledge the financial support to this work from the National Natural Science Foundation of China under Grant No. 11332013 and Fundamental Research Funds for Central
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Universities under Grant NO. 106112015CDJXY130013. Z.C.W. thanks supports from the Scientific Research (B) (grant no. 15H04114), Challenging Exploratory Research (grant no. 15K14117), JSPS and CAS under Japan-China Scientific Cooperation Program, and Shorai Foundation for Science and Technology.
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Corresponding authors TEL: +86 65102465; fax: +86 65102465 E-mail addresses: [email protected] (T.L.); [email protected] (Z.W.)
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Figure
Fig. 1 XRD patterns of the as-synthesized nanoparticle and nanoflower.
Fig. 2 SEM characterization of the samples. (a,b) SEM images of the ZnO nanoparticles with different magnification. (c,d) SEM images of ZnO nanoflowers with different magnification.
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Fig. 3 (a) SEM image of the ZnO nanoflowers. (b) HRTEM image of the ZnO nanoflower. Insets
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show the TEM image of a single ZnO nanoflower and corresponding SAED pattern.
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Fig. 4 Schematic illustration of the fabrication process of a single ZnO nanoflower.
Fig. 5 (a) Sensitivity of the ZnO nanoflower and nanoparticle under the NH3 concentration of 50
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ppm. The heating current ranges from 120 mA to 200 mA. (b) Long-term stability of the sensors made of the ZnO nanflower and nanoparticle under the NH3 concentration of 50 ppm. The heating
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current is 160 mA.
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Fig. 6 Gas response of the sensors fabricated with (a) ZnO particle and (b) ZnO nanoflower under different concentration of NH3. The optimum heating current of 160 mA is used. (c) Gas response
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of the ZnO nanoflower and nanoparticle to NH3 at 50 ppm. (d) Sensitivity of the two sensors to NH3 at 50 ppm.
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Fig. 7 Sketch showing the working principle of the ZnO-based gas sensor.
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Table
Table 1 Experimental parameters to prepare as-synthesized nanoparticle and nanoflower. Zn2+ (mM)
Samples
OH- (mM)
C6H5Na3O7·2H2O (g)
T (℃)
t (h)
4
2
0.14
180
6
Nanoflower
4
1
0.14
180
6
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Nanoparticle
comparison.
NH3 (ppm)
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Morphology and synthesis method
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Table 2 NH3 response of the ZnO nanoflower. Previously reported data are also given for
Ra/Rg
T/C
Reference
50
49.5
160 (mA)
This work
ZnO nanorod (thermal evaporation)
100
22.6
300 (℃)
[30]
ZnO nanospheres (hydrothermal)
50
9
300 (℃)
[28]
Nanowire-like network ZnO (SPMIC)
100
2.8
300 (℃)
[29]
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Sunflower-like ZnO (hydrothermal)
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S0169-4332(15)01994-7 http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.170 APSUSC 31117
To appear in:
APSUSC
Received date: Revised date: Accepted date:
16-6-2015 8-8-2015 20-8-2015
Please cite this article as: Y. Zhang, T. Liu, J. Hao, L. Lin, W. Zeng, X. Peng, Z. Wang, Enhancement of NH3 Sensing Performance in Flower-like ZnO Nanostructures and their Growth Mechanism, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.170 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.
*Highlights (for review)
The sunflower-like ZnO nanostructures reveal superior NH3 sensing performance than ZnO nanoparticles.
The ZnO nanoflowers, looking like a sunflower with six petals and one pistil, consist of single
The sensitivity of ZnO nanoflowers exposure to NH3 is up to 49.5 under the concentration of 50
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ppm, which is better than that reported in the literatures.
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crystalline nanotriangles and nanosphere.
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Graphical Abstract (for review)
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*Manuscript
Enhancement of NH3 Sensing Performance in Flower-like ZnO Nanostructures and their Growth
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Mechanism
Yu Zhang1,2 , Tianmo Liu1,2*, Jinghua Hao1,2, Liyang Lin1,4, Wen Zeng1,2 , Xianghe Peng3,
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Zhongchang Wang4* 1
2
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College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China National Engineering Research Center for Mg Alloys, Chongqing University, Chongqing 400044,
Chongqing Key Laboratory of Heterogeneous Material Mechanics, Chongqing University, Chongqing,
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China
400044, China 4
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Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai
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980-8577, Japan
Abstract: ZnO nanostructures hold substantial promise for gas-sensing applications owning to their
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outstanding ethanol sensing performance, yet their sensing performance towards NH3 has rarely been reported. Here, we report on a successful preparation of sunflower-like ZnO nanostructures and ZnO nanoparticle via a facile hydrothermal method, and demonstrate that the ZnO nanoflowers have high gas-sensing performances towards NH3 under a low concentration of 10~50 ppm. Further structural characterization reveals that the sunflower-like nanostructure comprises six triangles-like and one sphere-like nanostructures, and the triangle-like nanostructure is single crystalline with {001} crystal face. As a consequence of their unique morphology, the nanoflowers show much improved NH3 sensing
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performances than the nanoparticles with a high sensitivity of 49.5. Keywords: ZnO, nanoflowers, single crystalline, gas sensor
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1. Introduction As a n-type semiconductor with a wide direct band gap of ~3.4 eV and a large exciton binding
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energy of ~60 meV at room temperature, ZnO has received a great deal of attentions for over decades.
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Cumulative attention is now being focused on the synthesis method of various ZnO nanostructures, including doping [1–3], composite [4–6] and template [7,8]. These methods are, however, very
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complicated and the preparation time is often rather tedious. Recently, an easy and convenient
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template-free hydrothermal approach has been utilized widely to synthesize ZnO [9,10]. In property, its gas-sensing performance and luminescence receive considerable interest. Moreover, it has been widely
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applied as supercapacitors [11,12], solar cells [13,14] and piezoelectric nanogenerators [15,16], and gas sensors [17,18]. Currently, much effort has been devoted to the study of gas-sensing performances of
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ZnO [3,19,20] due to the ever increased concerns over air quality and safety of human beings. Generally, gas-sensing performance of ZnO-based gas sensors is controlled by the morphology of
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ZnO nanostructures, which is tailored by both the area of exposed ZnO and the length of electron path. The fabrication of ZnO nanostructures with a high specific area and a short electron path is therefore of great importance. For example, Xie et al. reported the fabrication of different morphologies by a facile hydrothermal method [21] and compared the photocatalytic properties of nanospheres, nanoflowers, and nanosheets. Cheng et al. [22] prepared rugby-like, dart-like, and flower-like ZnO nanostructures in aqueous solution assisted by ultrasonication at low temperature of 80°C, and investigated luminescence properties. In addition, Xu et al. [23] prepared Comb-Like ZnO nanostructures via introducing Au
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catalysts, and Hussain et al. prepared ZnO nanocones and studied their gas-sensing properties toward formaldehyde [24]. Lian et al. fabricated ZnO micro-cups and micro-rings assembled by nanoparticles [25].
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To date, most of the studies are focused on ZnO-based gas sensors to ethanol. Nanorod, nanosheet and nanodiamond have been fabricated separately and the best sensitivity is reported to be ~10 at 100
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ppm concentration of ethanol [26]. In addition, nanoflowers assembled with porous nanosheets of ZnO
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have been synthesized and their gas sensitivity to ethanol is up to ~20 at 100 ppm [27]. However, the sensing performance of ZnO to NH3 is poor with a low sensitivity of less than 10 [28,29]. Although
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Suranan et al. [30] reported that the ZnO nanorod annealed in O2 at 650oC has the sensitivity of 22.6,
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its sensitivity value reduces to less than 5 once the nanorod is annealed at other atmospheres. To improve the gas sensitivity of ZnO to NH3 gas, here we fabricate two kinds of ZnO nanostructures via a
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hydrothermal approach and find that in comparison to nanoparticles, nanoflowers show much improved sensing performance to NH3. The nanoflowers are found to be assembled with six triangles-like
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nanostructures and sphere-like nanostructure in center, and the triangles-like nanostructures turn out to
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be single crystalline and grow along three symmetric parallel planes, namely, (001), (010) and (100).
2. Experimental
All chemicals were of analytic grade without further purification, including zinc nitrate
hexahydrate (Zn(NO3)2·6H2O), sodium citrate (C6H5Na3O7·2H2O), and sodium hydroxide (NaOH). To synthesize ZnO nanoflower, 1.19 g of Zn(NO3)2·6H2O and 0.14 g of sodium citrate were mixed and dissolved into 40 ml of distilled water under magnetic stirring for 30 min to form transparent solution. 0.08 g of NaOH were then added into the transparent solution, which was subsequently transferred into
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a Teflon-lined stainless steel autoclave (50 ml). The autoclave was sealed and heated at 180oC for 6 h, and then cooled to room temperature. The obtained white precipitates were harvested by centrifuging, washing with distilled water and ethanol several times, and drying at 60oC for 8 h. For comparison,
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ZnO nanoparticles were prepared as well with the similar process. The difference is that 0.16 g of NaOH was introduced into the transparent solution. Table 1 shows the preparation condition.
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Crystal phase of the as-synthesized products were investigated by X-ray diffraction (XRD, Rigaku
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D/max-1200X) with Cu Kα radiation (λ = 1.5406 Å) operated at 30 kV and 100 mA. Morphologies were characterized by field emission scanning electron microscopy (FE-SEM, Nova 400 Nano) at 10
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kV, transmission electron microscopy (TEM, ZEISS LIBRA200) at 200 kV, high-resolution TEM
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(HRTEM), and selected-area electron diffraction (SAED). Gas-sensing performances were investigated using a static flow system, CGS-8 intelligent test system (Beijing Elite Tech Co., Ltd.), which was
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consisted of a heater, a gas distributor, and a data acquisition system [31]. Gas-sensing performances were measured five times under each gas concentration of ethanol gas and NH 3. The gas concentration
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ranged from 10 ppm to 50 ppm and the sensor operation temperature was 250oC. The sensitivity was defined as Ra/Rg, where Ra and Rg were resistance of the sensors in air and target gas, respectively. All
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gas-sensing measurements were conducted at room temperature with humidity of 40%.
3. Results and discussion 3.1 Structure and Morphology Figure 1 shows XRD patterns of the as-synthesized samples. The peaks are sharp and strong, indicating a high degree of crystallization for the samples. All the diffraction peaks can be indexed as a pure hexagonal phase ZnO (JCPDS Card no. 36-1451 a = 3.25 Å, b = 3.25Å, c = 5.207 Å). No other
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diffraction peak is observed, which reveals that no intermediate products are produced during reaction process. Figure 2a shows the morphology of as-prepared ZnO. The oval particles are highly dispersed with an average diameter of 200-300 nm (Fig. 2b). From Fig. 2c, a large number of sunflower-like ZnO
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nanostructures are observed, and every nanoflower has six triangles-like nanostructures around one sphere-like nanostructure. The triangles-like and sphere-like nanostructures have an average size of 500
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and 800 nm, respectively (Fig. 2d).
3.2 Flower-like ZnO nanostructures
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Figure 3a shows SEM image of the nanoflowers, which comprises six petals and one sphere pistil. Figure 3b shows HRTEM image of a petal (from an area marked in red square in Fig. 3a), from which
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one can see that the petal is of single crystal. The spacing of lattice fringe is measured to be ~0.28 nm,
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which indicates that the petal grows along {001} crystal plane, consistent with selected-area electron diffraction (SAED) analysis showing that the petals of nanoflowers mainly grow along six symmetric
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parallel planes, ±(01-10), ±(10-10) and ±(1-100). This also agrees with the crystal structure and the stacking mode of ZnO in Figs. 3c and 3d. It is known that the (0001) surface of ZnO is terminated with Zn and the (000-1) surface is terminated with O. The former is catalytically active, while the latter is
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relatively inert [32].
Figure 4 sketches the growth process of ZnO nanoflower. Firstly, ZnO, OH- and C6H5O73- are
obtained by the reaction process. As aforementioned, the ZnO nuclei grow simultaneously along the {001}, resulting in the formation of the triangle-like ZnO nanostructures as petals, which indicates that the edges of ZnO nanostructures are positively charged. Secondly, the atoms in the angles of triangle-like nanostructures are arranged in a disordered way so that these nanostructures join together
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through their angles, which can be attributed to the high energy and large mobility of the disordered atoms. The C6H5O73-, which owns more negative charges than OH-, is preferentially attracted by triangle-like nanostructures with positive charges. Moreover, the empty region, which is surrounded by
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triangle-like nanostructures, is also filled with new ZnO nuclei under the driving of negative charges of C6H5O73-. As a bridge, C6H5O73- has a binding effect on triangle-like nanostructures and new nuclei.
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Finally, the ZnO new nuclei grow into the sphere-like nanostructures like the pistil of sunflower and the
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ZnO nanotriangles are transformed to petals of sunflower. Zn (NO3)2 ·6H2O ↔ Zn2+ + 2NO3- + 6H2O
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NaOH ↔Na+ + OH-
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C6H5Na3O7·2H2O↔ C6H5O73- + 3Na+ + 2H2O
3.3 Gas-sensing performance
(2) (3) (4)
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Zn2+ + 4OH- ↔ [Zn(OH)4]2- ↔ ZnO + H2O + 2OH-
(1)
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Figure 5a shows gas response of ZnO nanoflowers and nanoparticles as a function of heating current at 50 ppm. The ZnO nanoflowers and nanoparticles show a maximum gas response of 49.5 and
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33.2 at 160 mA, respectively. The stability of the two sensors is also investigated for 60 days, as shown in Fig. 5b, which reveals that the two sensors have a superior stability. Figure 6a shows NH3 response for the sensor made of ZnO nanoparticles under different concentrations. The sensitivity is increased with the rise of NH3 concentration, and the sensor is rather sensitive to the low NH3 concentration of 10 ppm. In Fig. 6b, the sensor made of ZnO nanoflower shows response to NH3 at 10 ppm, and its sensitivity is enhanced with the increase of the NH3 concentration. As shown in Fig. 6c, the sensitivity signal for the nanoflower stabilizes in 6 s when the sensor is exposed at NH3 atmosphere of 50 ppm and
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returns to its original state in 3 s once the sensor is exposed in air. The response and recovery time of the sensor made of the nanoparticle is 8 and 5 s. Moreover, it is found that the ZnO nanoflower has a higher gas-sensing performance than the ZnO nanoparticle under the same concentration of NH3 (Fig.
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6d), which can be ascribed to the morphologies. In fact, ZnO rarely suggests a perfect NH3 sensing performance. Few studies have reported that the
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sensitivity of ZnO to NH3 is more than 10. For example, Suranan et al. [30] reported that the ZnO
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nanorod has a sensitivity of 22.6. However, the nanorod must be annealed in O2 atmosphere at 650oC. When the nanorod is annealed in other atmosphere, its sensitivity value reduces to less than
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5. Table 2 summaries gas response of a variety of ZnO sensors to NH3 prepared with different methods
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[28-30]. The ZnO nanoflower in the present work shows the highest sensitivity to NH3, which can be attributed to its special morphology and single-crystalline structure.
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As a negative semiconductor, ZnO has more electrons on its surface. So, the oxygen in the air will adsorb on it and capture the electrons into negative oxygen ions (O (ads)-, O2(ads)– and O2(ads)2-).
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After losing the electrons, the conductive band of ZnO will become narrower and its resistance will enhance. Once the NH3 gas comes up, it will react with negative oxygen ions and release the
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captured electrons back to the conductive band of ZnO. Then, the ZnO will have wider conductive band and its resistance will recover. The principle of the NH3 is shown as in Figure 7. When ZnO is in the air, the oxygen captured on its surface will react successively into negative oxygen ions (O (ads) ,
O2(ads)– and O2(ads)2-). While the NH3 gas is introduced, it will react with these negative
oxygen ions into nitrogen and release the electrons captured by oxygen. The reaction equation is appended below.
O2 + 2e-↔ 2O (ads)-
(1)
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(2)
O2 + 2e-↔ O2 (ads)2-
(3)
2NH3 + 3O (ads)- →N2 + 3H2O + 3e-
(4)
4NH3 + 3O2 (ads)- → 2N2 + 6H2O + 3e-
(5)
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O2 + e-↔ O2 (ads)-
4NH3 + 3O2 (ads)2- →2N2 + 6H2O + 6e-
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(6)
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4. Conclusions
We have successfully synthesized ZnO nanoparticles and nanoflowers via a simple hydrothermal
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strategy. The nanoparticles have an oval-like shape with an average diameter of 200-300 nm, and the
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nanoflowers show a sunflower-like shape with petals and pistils. As a consequence of these special morphologies, the nanoflowers exhibit a much improved gas-sensing performance to NH3 than the
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nanoparticles. In particular, the gas sensor made of the nanoflower shows a high sensitivity of 49.5 and a short response and recovery time of 6 s and 3 s, respectively, under NH3 of 50 ppm.
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Acknowledgments
The authors gratefully acknowledge the financial support to this work from the National Natural Science Foundation of China under Grant No. 11332013 and Fundamental Research Funds for Central
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Universities under Grant NO. 106112015CDJXY130013. Z.C.W. thanks supports from the Scientific Research (B) (grant no. 15H04114), Challenging Exploratory Research (grant no. 15K14117), JSPS and CAS under Japan-China Scientific Cooperation Program, and Shorai Foundation for Science and Technology.
*
Corresponding authors TEL: +86 65102465; fax: +86 65102465 E-mail addresses: [email protected] (T.L.); [email protected] (Z.W.)
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Figure
Fig. 1 XRD patterns of the as-synthesized nanoparticle and nanoflower.
Fig. 2 SEM characterization of the samples. (a,b) SEM images of the ZnO nanoparticles with different magnification. (c,d) SEM images of ZnO nanoflowers with different magnification.
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Fig. 3 (a) SEM image of the ZnO nanoflowers. (b) HRTEM image of the ZnO nanoflower. Insets
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show the TEM image of a single ZnO nanoflower and corresponding SAED pattern.
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Fig. 4 Schematic illustration of the fabrication process of a single ZnO nanoflower.
Fig. 5 (a) Sensitivity of the ZnO nanoflower and nanoparticle under the NH3 concentration of 50
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ppm. The heating current ranges from 120 mA to 200 mA. (b) Long-term stability of the sensors made of the ZnO nanflower and nanoparticle under the NH3 concentration of 50 ppm. The heating
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current is 160 mA.
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Fig. 6 Gas response of the sensors fabricated with (a) ZnO particle and (b) ZnO nanoflower under different concentration of NH3. The optimum heating current of 160 mA is used. (c) Gas response
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of the ZnO nanoflower and nanoparticle to NH3 at 50 ppm. (d) Sensitivity of the two sensors to NH3 at 50 ppm.
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Fig. 7 Sketch showing the working principle of the ZnO-based gas sensor.
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Table
Table 1 Experimental parameters to prepare as-synthesized nanoparticle and nanoflower. Zn2+ (mM)
Samples
OH- (mM)
C6H5Na3O7·2H2O (g)
T (℃)
t (h)
4
2
0.14
180
6
Nanoflower
4
1
0.14
180
6
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Nanoparticle
comparison.
NH3 (ppm)
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Morphology and synthesis method
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Table 2 NH3 response of the ZnO nanoflower. Previously reported data are also given for
Ra/Rg
T/C
Reference
50
49.5
160 (mA)
This work
ZnO nanorod (thermal evaporation)
100
22.6
300 (℃)
[30]
ZnO nanospheres (hydrothermal)
50
9
300 (℃)
[28]
Nanowire-like network ZnO (SPMIC)
100
2.8
300 (℃)
[29]
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Sunflower-like ZnO (hydrothermal)
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