- Email: [email protected]
recovery and selective gas detection at room temperature
Journal Pre-proofs Research paper Uniform hierarchical tetradecahedral SnO2/Zn2SnO4 composites for ultrafast response/recovery and selective gas detection at room temperature Zi-Yang Chen, Da-Peng Jiang, Shao-Hui Zhang, Chao Wang, Hui Huang, Long Zhang, Liu-Yi Ding, Lin-Jun Wang, Ge-Bo Pan PII: DOI: Reference:
S0009-2614(19)31048-6 https://doi.org/10.1016/j.cplett.2019.137067 CPLETT 137067
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
23 October 2019 20 December 2019 28 December 2019
Please cite this article as: Z-Y. Chen, D-P. Jiang, S-H. Zhang, C. Wang, H. Huang, L. Zhang, L-Y. Ding, L-J. Wang, G-B. Pan, Uniform hierarchical tetradecahedral SnO2/Zn2SnO4 composites for ultrafast response/recovery and selective gas detection at room temperature, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/ j.cplett.2019.137067
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 B.V.
Uniform hierarchical tetradecahedral SnO2/Zn2SnO4 composites
for
ultrafast
response/recovery
and
selective gas detection at room temperature Zi-Yang Chena,b, Da-Peng Jiangb,c, Shao-Hui Zhangb, Chao Wangb, Hui Huangb, Long Zhangb, Liu-Yi Dingb, Lin-Jun Wanga and Ge-Bo Pan*b a
School of Materials Science and Engineering, Shanghai University, No. 99 Shangda
Road, 200444 Shanghai, P. R. China b
Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences,
No. 398, Ruoshui Road, 215123 Suzhou, P. R. China c
Nano Science and Technology Institute, University of Science and Technology of
China, No. 166, Ren'ai Road, 215123 Suzhou, P. R. China
*Corresponding author at: Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 215123 Suzhou, P. R. China. Telephone: +86-0512-62872663. E-mail: [email protected] (G.-B. Pan)
Abstract In this work, hierarchical tetradecahedral SnO2/Zn2SnO4 composites were synthesized by a simple one-pot method without using surfactants and templates and have a uniform diameter of ~1 μm. Importantly, SnO2/Zn2SnO4 was used to manufacture gas sensor. After testing the sensor, it displays good selectivity to ethanol and excellent repeatability at room temperature (RT). It has ultra-fast response/recovery times with 9 s and 7 s at 200 ppm ethanol, respectively. Such enhanced gas-sensing performances are probably ascribed to the unique hierarchical tetradecahedral SnO2/Zn2SnO4 structure, the strong interfacial interaction and the existence of heterojunctions between Zn2SnO4 and SnO2. Keywords: Gas sensor; Zinc stannate; Tin oxide; Ethanol; Room temperature 1. Introduction Ethanol is a very important material in life and industry, and widely used in agriculture, industry, transportation and energy domains. Ethanol is volatile and flammable and may cause combustion or explosion when it leaks due to improper operation (the explosion limit of ethanol in air is 3.3% to 19.0%) [1]. In addition, ethanol vapor is also highly toxic, affecting the body's nervous system and blood system. The toxic reactions are caused through respiratory tract or skin, which can damage the respiratory mucosa and vision. Hence, it is very essential to detect the ethanol gas concentration in the environment. In the past few decades, metal oxide-based gas sensors have attracted a wide range of attention due to their fast response, low price, environmental friendliness and easy manufacture [2, 3]. Up to now, metal oxides as gas sensing materials have been developed and applied, such as WO3 [4, 5], ZnO [6], SnO2 [7], NiO [8], In2O3 [9] and so on. However, it still remains a scientific challenge to design and fabricate new sensing materials for increasing the selectivity and sensitivity, decreasing the detection limit [10, 11]. At present, in order to improve the gas sensing of materials, it is all effective to change the surface areas of materials, dope noble metals, or form heterostructures [12,13]. The former mainly increases the specific surface areas of materials through changing materials’ structures. The latter mainly increases the
sensitive properties of the materials by using the catalytic activity of the precious metal additive. Many researchers have composited two n/p-type metal oxide nanomaterials to form n-n or n-p heterostructures, which can greatly increase the gas sensing of sensitive components [14]. Some complex metal oxides have also raised a strong concern and been researched to change the surface areas of materials or form the heterostructures among the complex metal oxides in recent years, including ZnO/SnO2 [15], CuO/SnO2 [16], SnO2/CeO2 [17], ɑ-Fe2O3/NiO [18], Zn2SnO4/SnO2 [19], WO3/SnO2 [20] etc. The results show that gas sensing properties of these composite metal oxides are further improved in comparison with single metal oxides. Tin oxide (SnO2) is one of the most intensively investigated n-type semiconductor materials. Zinc stannate (Zn2SnO4) with an inverse spinel structure is an important ternary oxide, and the band gap is 3.6 eV [21]. The unique properties and huge potential applications have triggered extensive and profound researches in gas sensors [22, 23], photocatalysis [24] solar cells [25], and lithium-ion batteries [26, 27]. Reducing operating temperatures is critical to improve the safety of gas sensors, increase the life of portable or wearable devices, and build highly reliable wireless sensor networks [28]. Testing the sensors based on composite metal oxide materials at a certain test temperature (generally at high temperatures) has been carried out in recent years [29-31]. A conventional gas sensor generally uses a ceramic tube with an electrode and a heating wire coated by gas sensing materials and tests its properties after sintering. A problem occurs that the gas sensing layer is unevenly coated, which may affect the sensitivity of the sensors [32-34]. In this work, gas sensing material was transferred evenly to the interdigitated electrode by a screen-printing technique and tested the material’s gas sensing subsequently. Hierarchical tetradecahedral SnO2/Zn2SnO4 composites was successfully synthesized via a facile hydrothermal process. For the first time, we report a gas sensor based on hierarchical tetradecahedral SnO2/Zn2SnO4 material structures for detecting ethanol gas at RT. The structure and morphology of the products were characterized based on X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Furthermore, gas sensing properties of SnO2/Zn2SnO4 were investigated. 2. Experimental section
2.1 Materials Anhydrous alcohol (99.9%, Mw:46.07) reagent were purchased from Shanghai Titan Scientific Co., Ltd. NaOH (99.7%, Mw:40.00), Zn(NO3)2.6H2O (99.0%, Mw:297.49) and SnCl4.5H2O (99.0%, Mw:350.60) reagents all purchased from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). All chemicals are of analytical grade and were used as received without further purification. 2.2 Synthesis of hierarchical tetradecahedral SnO2/Zn2SnO4 composites The hierarchical tetradecahedral SnO2/Zn2SnO4 composites were synthesized by a facile hydrothermal process. In brief, 3.0 mmol Zn(NO3)2·6H2O and 3.0 mmol SnCl4·5H2O were dissolved in 33% ethanol solution to form solutions A and B, then mixed them with stirring all the time. 3.0 mmol NaOH solution was slowly added dropwise to the above solutions. After completing the above operation, it stired at 80 ℃ for 3 h. The solution was transferred into a 50 mL stainless steel teflon-lined autoclave, and then it was heated at 200 ◦C for 2 h. After this process, the reaction vessel was taken out and cooled to RT. The precipitates were collected by centrifugation and washed with absolute ethyl alcohol and deionized water several times. Finally, it was dried at 60 ℃ for 12 h. The synthesis process was displayed in Fig. 1.
Fig. 1. Schematic diagram of the experimental process 2.3 Fabrication and measurement of gas sensor The gas sensor was fabricated as shown in Fig. S1. The sensor performance test uses
the WS-30A gas test system of Zhengzhou Yusheng Electronic Technology Co., Ltd. The fabricated sensor was welded onto a pedestal and the gas sensing properties. Our research team has done a lot of related tests and researches [35]. In order to investigate the relationship between gas concentrations and gas responses, certain amount of ethanol was measured by a pipette and then evaporated into well-defined concentration of target gas in the reaction chamber. The gas response in this work was defined as S=Ra/Rg, in which Ra and Rg are the resistances in air and test gas, respectively. The response time of the sensor is counted as the time required for the sensor to reach its corresponding maximum of 90%. In contrast, the recovery time is calculated by the time required for the sensor to reach its maximum value of 10%. 3. Results and Discussion 3.1 Characterization of hierarchical tetradecahedral SnO2/Zn2SnO4 composites The crystal structure and phase purity of the prepared SnO2/Zn2SnO4 composites were characterized by X-ray diffraction as shown in Fig. 2a. All characteristic peaks in the figure were consistent with standard SnO2 of tetragonal structure (JCPDS:41-1445) and standard Zn2SnO4 with spinel structure (JCPDS:24-1470). No other peaks belong to other impurities. In Fig. 2b, it clearly show the SEM morphology of the SnO2/Zn2SnO4 composites. The overall morphology of the samples were polyhedral shapes with uniform size of ~1 μm. Moreover, it can be seen that these polyhedrons were independent and uniformly dispersed in Fig. S7. As the high magnification SEM in the illustration of Fig. 2b presented, it shows the morphology of a single polyhedron. The polyhedron was confirmed to be a tetradecahedron. In addition, the tetradecahedral surfaces were evenly loaded with a kind of dense two-dimensional sheet structures. The EDS analysis confirmed the existence of Zn, Sn, and O elements from the heterojunction and these three elements were derived from two different substances, SnO2 and Zn2SnO4, respectively in Fig. S2. Furthermore, a single SnO2/Zn2SnO4 tetradecahedron was analyzed. Fig. S3a and b show SEM and TEM images of a single SnO2/Zn2SnO4 tetradecahedron, respectively.
Fig. 2. (a) XRD pattern of as-prepared SnO2/Zn2SnO4. (b) shows the SEM images of the SnO2/Zn2SnO4 The Zn2SnO4 was identified as a tetradecahedral spinel structure by XRD, SEM and TEM ( in Fig. S3). The tetradecahedron consists of six (100) faces and eight (111) faces [36]. In general, the primary driving force for single crystal growth is to reduce surface energy, thereby minimizing crystal surface energy. The crystal shape is determined by the relative order of surface energies [37]. For the tetradecahedral spinel structure, recent calculations show that the minimum surface energy is at the (100) plane. For (111) crystal faces, they are positively or negatively charged surfaces, which were composed of closed packed oxygen ions or tetra/hexa- coordinated metal- ions. This structure gives higher surface energy to the (111) crystal planes. For solutionbased reactions, a variety of experimental factors, including reactants, solvents, and surfactants, can alter the order of the (100) and (111) planes' surface energies. In this experiment, no surfactant was used, which resulted in both the (100) crystal planes and the (111) crystal planes to obtain a certain orientation growth, and the irregular Zn2SnO4 tetradecahedron structure was formed. The reaction produces a large amount of flaky SnO2, and the flaky SnO2 structure tends to adsorb to the surfaces of low surface energy, exhibiting a morphology as shown in Fig. S3a. In addition, the approximate generation process of SnO2/Zn2SnO4 is described in Fig. S4. 3.2 Gas-sensing properties of the as-prepared sensor To verify the performances of the SnO2/Zn2SnO4 composites on gas sensing, the gas sensing behaviors of the as-prepared sensor on ethanol have been studied. An important property of metal oxide semiconductor-based gas sensors is the operating temperature.
For demonstrating the performances of the sensor based on SnO2/Zn2SnO4 at lower temperatures, it was tested the responses with various temperatures at 100 ppm ethanol, as shown in Fig. 3a. As the test temperatures increases from 15 ℃ to 125 ℃, the responses first increase and then reach the maximum value, afterwards the responses decreased with further increasing temperature. The optimum operating temperature is 25 ℃ with response of 31.05 at 100 ppm ethanol. It is evident that at RT, the sensors still exhibit good response. Fig. 3(b-d) is the sensing behaviors of the sensors when orderly exposed to different concentrations of ethanol at RT. Fig. 3b is the linear curve of the sensors with the increasing of ethanol concentrations, obviously, the sensors showed nearly a linear increasing with ethanol concentrations from 10 to 100 ppm and 1 to 5 ppm. Fig. 3c and d is the corresponding dynamic response and recovery curves with ethanol concentrations increased from 1 to 5 ppm and 10 to 100 ppm. It can be seen that as ethanol concentrations increases, the responses of gas sensor based on SnO2/Zn2SnO4 are also gradually increased. The corresponding response values were 1.1, 1.80, 2.61, 3.62 for ethanol concentration from 1 to 5 ppm (Fig. 3d) and for concentration from 10 to 100 ppm, the response values were 5.35, 9.25, 17.64, 23.65, 31.05 (Fig. 3c), respectively. Besides, one can also be observed that the response amplitudes for the SnO2/Zn2SnO4 sensor to different ethanol concentrations are obviously higher than that of the pure SnO2 sensor, as shown in Fig. S5.
Fig. 3. (a) Responses of SnO2/Zn2SnO4 at different working temperature upon exposure to 100 ppm ethanol. (b) Responses of the sensors to ethanol with different
concentrations at RT. (c) and (d) Corresponding dynamic response curves of the sensor to different concentrations of ethanol at RT Repeatability is an important indicator for testing sensors. Fig. 4a shows five cycles of dynamic response curves of SnO2/Zn2SnO4 to 200 ppm ethanol at RT. The shapes of response and recovery curves is almost identical, which illustrate a good repeatability of the sensor. The response and recovery characteristics play crucial role in semiconductor metal oxide-based gas sensors.The response and recovery properties of the sensor to 200 ppm ethanol at RT was investigated as displayed in Fig. 4b. The sensor shows an excellent sensitivity with 9 s response time and 7 s recovery time at RT, which is faster than most of ethanol sensing materials in reports. Selectivity is another important property of gas sensor. Fig. 4c shows that the gas response value toward ethanol was about 30 times higher than other tested gases, which could indicate that the sensor shows a high response and excellent selectivity to ethanol. Otherwise, the sensor to several hydroxyl-containing VOC gases (methanol, isopropanol, n-butanol) have been tesed in Fig. S6. The results show that the responses of SnO2/Zn2SnO4 to methanol, isopropanol and n-butanol are very low, and the response value to ethanol is almost theirs 30 times, which could indicate that the sensor based on SnO2/Zn2SnO4 composites shows a high response and excellent selectivity to ethanol, but not sensitive to all hydroxyl-containing VOC gases. To further investigate the long-time reliability of SnO2/Zn2SnO4 sensor, the responses in air of the sensor to 100 ppm ethanol at RT during 45 days was measured. As depicted in Fig. 4d, there is no obvious fluctuation in the resistance and response values during the test days, illustrating good long-term reliability of the sensor.
Fig. 4. (a) Five cycles of dynamic response curves of SnO2/Zn2SnO4 to 200 ppm ethanol at RT. (b) Responses transient of SnO2/Zn2SnO4 to 200 ppm ethanol at RT. (c) Responses of the sensors to various test gases with a concentration of 200 ppm. (d) Resistances in air and responses to 100 ppm ethanol of SnO2/Zn2SnO4 as a function of the test days at RT
Anything else, there is a Table 1 summarized to prove the excel properties of the sensor based on hierarchical tetradecahedral SnO2/Zn2SnO4 compared with other ethanol sensing materials in reported literatures. By comparison, it can be seen that the SnO2/Zn2SnO4 sensor exhibits excellent performance for low operating temperature and ultra-sensitive characteristics.
Table1. Comparison of previously reported materials-based ethanol sensing Sensor type
Operating
Concentration
Response/recovery
Response
temperature
(ppm)
time (S)
(Ra/Rg)
Ref
(℃） Zn2SnO4 Porous
280
200
11/20
82.2
[21]
180
50
18/45
23.4
[38]
200
100
76/139
14
[39]
500
20
120/200
4.9
[40]
Compact Zn2SnO4
380
50
13/15
12.8
[33]
SnO2/Zn2SnO4
RT
200
9/7
134.7
This
hollow cubes Zn2SnO4 Nanosphere Zn2SnO4/SnO2 Octahedral-like Zn2SnO4 Nanowires
Tetradecahedron
work
3.3 Gas sensing mechanism Analysis of the above experimental phenomena and results, we propose that the gassensing mechanism of the ethanol sensing materials SnO2/Zn2SnO4 composites. One is based on a change in the electrical resistance after a reaction between the ionosorbed oxygen species and ethanol molecules [41]. According to the ionosorption model, when the sensor is exposed in air, oxygen molecules will adsorb on the surface of SnO2/Zn2SnO4 to ionize into the Ox− (O2−, O−, O2−) by depriving electrons from the conduction band of SnO2/Zn2SnO4 composites (reaction (1)), As shown in Fig. 5. O2(Gas) → O2(Ads) + e- → O2−(Ads) + e- → O− (Ads) + e- → O2−(Ads)
(1)
It forms a thick electron depletion layer on the surface of SnO2/Zn2SnO4 and a high barrier between adjacent crystal grains, resulting in an increase in the resistance of the sensor. Ethanol gas is a reducing gas, and when the sensor is exposed to a reducing gas, the ethanol molecules reacts with the surface-adsorbed oxygen species, thereby releasing the trapped electrons back into the conduction band of the SnO2/Zn2SnO4,
which results in the conductivity increasing and the sensor’s resistance reducing (reaction (2)). The reaction process between the surface adsorbed oxygen species and ethanol is described as the following equation. C2H5OH + Ox− → 2CO2 + 3H2O + xe-
(2)
The gas sensors based on SnO2/Zn2SnO4 have ultra-sensitive and low operating temperature properties mainly due to hierarchical structure and heterojunctions formed between SnO2 and Zn2SnO4. First of all, hierarchical tetradecahedron structure provides higher specific surface area causing faster diffusion and adsorption of the gas. It also provides more surface active sites for the reaction of adsorbed oxygen and ethanol gases, which can lead to ultra-sensitive [42]. On the other hand, another consideration is the formation of a heterojunction at the interface between Zn2SnO4 and SnO2 due to their different work functions [19, 21]. Heterojunctions formed between different semiconductor oxides have a significant effect on gas sensing performance. Since the conduction band edge of Zn2SnO4 locates at higher potential than SnO2, electrons in the conduction band of Zn2SnO4 will migrate to the conduction band of SnO2 until their Fermi energy level is up to equilibrium when SnO2 is doped with Zn2SnO4. The process will deplete the extra electrons of SnO2/Zn2SnO4 composite interface, which will enhance the gas sensing properties.
Fig. 5. Schematic illustration of ethanol gas sensing mechanism of hierarchical
tetradecahedron SnO2/Zn2SnO4
4. Conclusion In summary, hierarchical tetradecahedral SnO2/Zn2SnO4 composites was successfully synthesized by a facile one-pot method. The structure and morphology of the SnO2/Zn2SnO4 composites are characterized by SEM, TEM, XRD and EDS. The results show that the SnO2 nanoplates are uniformly dispersed on the surfaces of tetradecahedral Zn2SnO4. The gas sensing performances of the as-fabricated gas sensor were systematically investigated by using SnO2/Zn2SnO4 composite as gas sensing material. It exhibits a significant enhancement in ethanol gas-sensing performances with a much higher response, good selectivity and good repeatability. It is worth mentioning that the manufactured sensors display praiseworthy low operating temperature (RT) and ultra-sensitive with 9 s response time and 7 s recovery time to 200 ppm ethanol. Other than that, reasonable gas sensing mechanisms are discussed. The good sensing performances can be mainly attributed to the SnO2/Zn2SnO4 composites’ unique hierarchical tetradecahedral loaded with sheet-like structure and heterojunctions formed between SnO2 and Zn2SnO4. This work indicates that the SnO2/Zn2SnO4 composites are very promising sensing material for the application of ethanol gas sensor.
Conflicts of interest There are no competitive economic benefits to declare. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 61773372), the Jiangsu Province Outstanding Youth Foundation (No. BK20160058), the Jiangsu Basic Research Program (Natural Science Foundation, No. BK20170427), Chinese Academy of Sciences Technology Service Network Program (STS, KFJ-STS-QYZX-081) and the Chinese Academy of Sciences.
References [1] C. Hagleitner, A. Hierlemann, D. Lauge, A. Kimmer, N. Kerness, O. Brand, H.Baltes, Smart single-chip gas sensor microsystem, Nature 414(2001) 293-296. [2] F. Yang, Z. Guo, Comparison of the enhanced gas sensing properties of tin dioxide samples doped with different catalytic transition elements, J. Colloid Interf. Sci., 448(2015) 265-274. [3] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators, B, 140(2009) 319-336. [4] J. Ma, Y. Ren, X. Zhou, L. Liu, Y. Zhu, X. Cheng, P. Xu, X. Li, Y. Deng, D. Zhao, Pt nanoparticles sensitized ordered mesoporous WO3 semiconductor: gas sensing performance and mechanism study, Adv. Funct. Mater., 28(2018) 1705268. [5] Y. Shen, H. Bi, T. Li, X. Zhong, X. Chen, A. Fan, D. Wei, Low-temperature and highly enhanced NO2 sensing performance of Au-functionalized WO3 microspheres with a hierarchical nanostructure, Appl. Surf. Sci., 434(2018) 922-931. [6] V.K. Guptaa, R. Sadeghic, F. Karimid, A novel electrochemical sensor based on ZnO nanoparticle and ionic liquid binder for square wave voltammetric determination of droxidopa in pharmaceutical and urine samples, Sens. Actuators, B, 186(2013) 603609. [7] A. Gurlo, Nanosensors: towards morphological control of gas sensing activity. SnO2, In2O3, ZnO and WO3 case studies, Nanoscale, 3(2011) 154-0. [8] C. Wang, X.B. Cui, J.Y. Liu, X. Zhou, X.Y. Cheng, P. Sun, X.L. Hu, X.W. Li, J. Zheng, G.Y. Lu, Design of superior ethanol gas sensor based on Al-doped NiO nanorod-flowers, ACS Sens., 1(2016) 131-136. [9] S. Park, G.J. Sun, H. Kheel, W.I. Lee, S. Lee, S.B. Choi, C. Lee, Synergistic effects of codecoration of oxide nanoparticles on the gas sensing performance of In2O3 nanorods, Sens. Actuators, B, 227(2016) 591-599. [10] J.W. Deng, J.M. Ma, L. Mei, Y.J. Tang, Y.J. Chen, T. Lv, Z. Xu, T.H. Wang, Porous ɑ-Fe2O3 nanosphere-based H2S sensor with fast response, high selectivity and enhanced sensitivity, J. Mater. Chem. A, 1(2013) 12400-12403.
[11] Y. Cui, Q.Q. Wie, H.K, Park, C.M. Lieber, Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species, Science, 293(2001) 1289-1292. [12] R. Malik, V.K. Tomer , L. Kienle , V. Chaudhary, S. Nehra, S. Duhan, Ordered Mesoporous Ag-ZnO@g-CN Nanohybrid as Highly Efficient Bifunctional Sensing Material, Adv. Mater. Interfaces, 5(2018) 1701357. [13] D. Xue, P. Wang, Z. Zhang, Y. Wang, Enhanced methane sensing property of flower-like SnO2 doped by Pt nanoparticles: A combined experimental and firstprinciple study, Sens. Actuators, B, 2019: 126710. [14] T.M.N. Quan, V.D. Nguyen, T.P. Nguyen ， N.T. Nguyen, M.H. Chu, D.H. Nguyen,V.H. Nguyen, Superior enhancement of NO2 gas response using n-p-n transition of carbon nanotubes/SnO2 nanowires heterojunctions, Sens. Actuators, B, 238(2017) 1120-1127. [15] X. Yang, S. Zhang, Q. Yu, L. Zhao, P. Sun, T. Wang, One step synthesis of branched SnO2/ZnO heterostructures and their enhanced gas-sensing properties, Sens. Actuators, B, 281(2019) 415-423. [16] Y.G. Zheng, J. Wang, P.J. Yao, Formaldehyde sensing properties of electrospun NiO-doped SnO2 nanofibers, Sens. Actuators, B, 156(2011) 723-730. [17] J.Y. Liu, M.J. Dai, T.S. Wang, P. Sun, X.S. Liang, G.Y. Lu, K. Shimanoe, N. Yamazoe, Enhanced gas sensing properties of SnO2 hollow spheres decorated with CeO2 nanoparticles heterostructure composite materials, ACS Appl. Mater. Interfaces, 8(2016) 6669-6677. [18] M. Haq, Z. Wen, Z. Zhang, S. Khan, Z. Lou, Z. Ye, L. Zhu, A two-step synthesis of nanosheet-coveredfibers based on alpha-Fe2O3/NiO composites towards enhanced acetone sensing, Sci. Rep. 8(2018) 1705. [19] S.S. Zhang, G. Sun, Y.W. Li, B. Zhang, Continuously improved gas-sensing performance of SnO2/Zn2SnO4 porous cubes by structure evolution and further NiO decoration, Sens. Actuators, B, 255(2018) 2936-2943. [20] D. Xue, Y. Wang, J. Cao, G. Sun, Z. Zhang, Improving methane gas sensing performance of flower-like SnO2 decorated by WO3 nanoplates, Talanta 199(2019) 603-611. [21] Z. Chen, M. Cao, C. Hu, Novel Zn2SnO4 hierarchical nanostructures and their gas sensing properties toward ethanol, J. Phys. Chem. C, 115(2011) 5522-5529. [22] H.K. Wang, A.L. Rogach, Hierarchical SnO2 nanostructures: recent advances in
design synthesis, and applications, Chem. Mater., 26(2014) 123-133. [23] X. Chu, R. Hu, J. Wang, Y. Dong, W. Zhang, L. Bai, W. Sun, Preparation and gas sensing properties of graphene-Zn2SnO4 composite materials, Sens. Actuators, B, 251(2017) 120-126. [24] D.P. Pratim, A. Roy, S. Das, P.S. Devi, Enhanced stability of Zn2SnO4 with N719 N3 and eosin Y dye molecules for DSSC application, Phys. Chem. Chem. Phys.,18(2015) 1429-1438. [25] J. Yang, S. Wang, L. Zhang, R. Dong, Z. Zhu, X. Gao, Zn2SnO4−doped SnO2 hollow spheres for phenylamine gas sensor application, Sens. Actuators, B, 239(2017) 857-864. [26] Z.X. Chen, M. Zhou, Y.L. Cao, X.P. Ai, H.X. Yang, J. Liu, In situ generation of few-layer graphene coatings on SnO2−SiC core-shell nanoparticles for highperformance lithium-ion storage, Adv. Energy Mater., 2(2012) 95-102. [27] L. Qin, S. Liang, X. Tan, A. Pan, Zn2SnO4/graphene composites as anode materials for high performance lithium-ion batteries, J. Alloy Compd., 692(2017) 124-130. [28] X.F. Chu, R.X. Hu, J.L. Wang, Y.P. Dong, W.B. Zhang, L.S. Bai, W.Q. Sun, Preparation and gas sensing properties of graphene-Zn2SnO4 composite materials, Sens. Actuators, B, 251(2017) 120-126. [29] T.V. Dang, N.D. Hoa, N.V. Duy, N.V. Hieu, Chlorine gas sensing performance of on-chip grown ZnO, WO3, and SnO2 nanowire sensors, ACS Appl. Mater. Interfaces, 8(2016) 4828-4837. [30] H.N. Hai, T.D. Duc, H.N. Thanh, P.N. Huy, T.P. Duy, H.H. Si, Fast response of carbon monoxide gas sensors using a highly porous network of ZnO nanoparticles decorated on 3D reduced graphene oxide, Appl. Surf. Sci., 434(2018) 1048-1054. [31] K. Tian, X.X. Wang, Z.Y. Yu, H.Y. Li, X. Guo, Hierarchical and hollow Fe2O3 nanoboxes derived from metal-organic frameworks with excellent sensitivity to H2S, ACS Appl. Mater. Interfaces, 9(2017) 29669-29676. [32] L. Zhu, Y. Li, W. Zeng, Hydrothermal synthesis of hierarchical flower-like ZnO nanostructure and its enhanced ethanol gas-sensing properties, Appl. Surf. Sci., 427(2018) 281-287 [33] Y.F. Bing, Y. Zeng, C. Liu, L. Qiao, Y.M. Sui, B. Zou, W.T. Zheng, G.T. Zou, Assembly of hierarchical ZnSnO3 hollow microspheres from ultra-thin nanorods and the enhanced ethanol-sensing performances, Sens. Actuators, B, 190(2014) 370-377. [34] T. Zhai, H.G. Xu, W.R. Li, H.Q. Yu, Z.R. Chen, J.Q. Wang, B.Q. Cao, Low-
temperature in-situ growth of SnO2 nanosheets and its high triethylamine sensing response by constructing Au-loaded ZnO/SnO2 heterostructure, J. Alloy. Compd., 737(2018) 603-612. [35] C. Wang, L.J.Wang, L. Zhang, R. Xi, H. Huang, S.H. Zhang, G.B. Pan, Electrodeposition of ZnO nanorods onto GaN towards enhanced H2S sensing, J. Alloy. Compd., 790(2019) 363-369. [36] Y.Q. Jiang, X.X. Chen, R. Sun, Z. Xiong, L.S. Zhang, Hydrothermal syntheses and gas sensing properties of cubic and quasi-cubic Zn2SnO4, Mater. Chem. Phys., 129(2011) 53-61. [37] C.M. Fang, S.C. Parker, G.D. With, Atomistic Simulation of the Surface Energy of Spinel MgAl2O4, J. Am. Ceram. Soc., 83(2000) 2082-2084. [38] D.M. An, N. Mao, G.Z. Deng, Y.L. Zou, Y. Li, T. Wei, X.X. Lian, Ethanol gassensing characteristic of the Zn2SnO4 nanospheres, Ceram. Int., 42(2016) 3535-3541. [39] S. Shu, M. Wang, Y. Wei, S. Liu, Synthesis of surface layered hierarchical octahedral-like structured Zn2SnO4/SnO2 with excellent sensing properties toward HCHO, Sens. Actuators, B, 243(2017) 1171-1180. [40] T. Tharsika, A.S.M.A. Haseeb, S.A. Akbar, M.F.M. Sabri, Y.H. Wong, Gas sensing properties of zinc stannate (Zn2SnO4) nanowires prepared by carbon assisted thermal evaporation process, J. Alloy. Compd., 618(2015) 455-462. [41] C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal oxide gas sensors: sensitivity and influencing factors, Sensors, 10(2010) 2088-2106. [42] K.D. Nguyen, T.D. Do, D.V. Nguyen, H.D. Nguyen, H.V. Nguyen, Design of SnO2/ZnO hierarchical nanostructures for enhanced ethanol gas-sensing performance, Sens. Actuators, B, 174(2012) 594-601.
Highlights
The hierarchical tetradecahedral SnO2/Zn2SnO4 composites were synthesized by a simple one-pot method.
The hierarchical tetradecahedral SnO2/Zn2SnO4 composites exhibited excellent ethanol gas sensing properties.
The hierarchical tetradecahedral SnO2/Zn2SnO4 composites can work at room temperature.
The hierarchical tetradecahedral SnO2/Zn2SnO4 composites exhibited a rapid response time of 9 s and recovery time of 7 s.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
CRediT author statement Zi-Yang Chen: literature search, figures, study design, data collection, data analysis, data interpretation Da-Peng Jiang: literature search, data collection Shao-Hui Zhang: literature search, software, data collection Chao Wang: literature search, software Hui Huang: literature search, Long Zhang: literature search Liu-Yi Ding: literature search Lin-Jun Wang: literature search Ge-Bo Pan: literature search, study design
S0009-2614(19)31048-6 https://doi.org/10.1016/j.cplett.2019.137067 CPLETT 137067
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
23 October 2019 20 December 2019 28 December 2019
Please cite this article as: Z-Y. Chen, D-P. Jiang, S-H. Zhang, C. Wang, H. Huang, L. Zhang, L-Y. Ding, L-J. Wang, G-B. Pan, Uniform hierarchical tetradecahedral SnO2/Zn2SnO4 composites for ultrafast response/recovery and selective gas detection at room temperature, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/ j.cplett.2019.137067
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 B.V.
Uniform hierarchical tetradecahedral SnO2/Zn2SnO4 composites
for
ultrafast
response/recovery
and
selective gas detection at room temperature Zi-Yang Chena,b, Da-Peng Jiangb,c, Shao-Hui Zhangb, Chao Wangb, Hui Huangb, Long Zhangb, Liu-Yi Dingb, Lin-Jun Wanga and Ge-Bo Pan*b a
School of Materials Science and Engineering, Shanghai University, No. 99 Shangda
Road, 200444 Shanghai, P. R. China b
Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences,
No. 398, Ruoshui Road, 215123 Suzhou, P. R. China c
Nano Science and Technology Institute, University of Science and Technology of
China, No. 166, Ren'ai Road, 215123 Suzhou, P. R. China
*Corresponding author at: Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 215123 Suzhou, P. R. China. Telephone: +86-0512-62872663. E-mail: [email protected] (G.-B. Pan)
Abstract In this work, hierarchical tetradecahedral SnO2/Zn2SnO4 composites were synthesized by a simple one-pot method without using surfactants and templates and have a uniform diameter of ~1 μm. Importantly, SnO2/Zn2SnO4 was used to manufacture gas sensor. After testing the sensor, it displays good selectivity to ethanol and excellent repeatability at room temperature (RT). It has ultra-fast response/recovery times with 9 s and 7 s at 200 ppm ethanol, respectively. Such enhanced gas-sensing performances are probably ascribed to the unique hierarchical tetradecahedral SnO2/Zn2SnO4 structure, the strong interfacial interaction and the existence of heterojunctions between Zn2SnO4 and SnO2. Keywords: Gas sensor; Zinc stannate; Tin oxide; Ethanol; Room temperature 1. Introduction Ethanol is a very important material in life and industry, and widely used in agriculture, industry, transportation and energy domains. Ethanol is volatile and flammable and may cause combustion or explosion when it leaks due to improper operation (the explosion limit of ethanol in air is 3.3% to 19.0%) [1]. In addition, ethanol vapor is also highly toxic, affecting the body's nervous system and blood system. The toxic reactions are caused through respiratory tract or skin, which can damage the respiratory mucosa and vision. Hence, it is very essential to detect the ethanol gas concentration in the environment. In the past few decades, metal oxide-based gas sensors have attracted a wide range of attention due to their fast response, low price, environmental friendliness and easy manufacture [2, 3]. Up to now, metal oxides as gas sensing materials have been developed and applied, such as WO3 [4, 5], ZnO [6], SnO2 [7], NiO [8], In2O3 [9] and so on. However, it still remains a scientific challenge to design and fabricate new sensing materials for increasing the selectivity and sensitivity, decreasing the detection limit [10, 11]. At present, in order to improve the gas sensing of materials, it is all effective to change the surface areas of materials, dope noble metals, or form heterostructures [12,13]. The former mainly increases the specific surface areas of materials through changing materials’ structures. The latter mainly increases the
sensitive properties of the materials by using the catalytic activity of the precious metal additive. Many researchers have composited two n/p-type metal oxide nanomaterials to form n-n or n-p heterostructures, which can greatly increase the gas sensing of sensitive components [14]. Some complex metal oxides have also raised a strong concern and been researched to change the surface areas of materials or form the heterostructures among the complex metal oxides in recent years, including ZnO/SnO2 [15], CuO/SnO2 [16], SnO2/CeO2 [17], ɑ-Fe2O3/NiO [18], Zn2SnO4/SnO2 [19], WO3/SnO2 [20] etc. The results show that gas sensing properties of these composite metal oxides are further improved in comparison with single metal oxides. Tin oxide (SnO2) is one of the most intensively investigated n-type semiconductor materials. Zinc stannate (Zn2SnO4) with an inverse spinel structure is an important ternary oxide, and the band gap is 3.6 eV [21]. The unique properties and huge potential applications have triggered extensive and profound researches in gas sensors [22, 23], photocatalysis [24] solar cells [25], and lithium-ion batteries [26, 27]. Reducing operating temperatures is critical to improve the safety of gas sensors, increase the life of portable or wearable devices, and build highly reliable wireless sensor networks [28]. Testing the sensors based on composite metal oxide materials at a certain test temperature (generally at high temperatures) has been carried out in recent years [29-31]. A conventional gas sensor generally uses a ceramic tube with an electrode and a heating wire coated by gas sensing materials and tests its properties after sintering. A problem occurs that the gas sensing layer is unevenly coated, which may affect the sensitivity of the sensors [32-34]. In this work, gas sensing material was transferred evenly to the interdigitated electrode by a screen-printing technique and tested the material’s gas sensing subsequently. Hierarchical tetradecahedral SnO2/Zn2SnO4 composites was successfully synthesized via a facile hydrothermal process. For the first time, we report a gas sensor based on hierarchical tetradecahedral SnO2/Zn2SnO4 material structures for detecting ethanol gas at RT. The structure and morphology of the products were characterized based on X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Furthermore, gas sensing properties of SnO2/Zn2SnO4 were investigated. 2. Experimental section
2.1 Materials Anhydrous alcohol (99.9%, Mw:46.07) reagent were purchased from Shanghai Titan Scientific Co., Ltd. NaOH (99.7%, Mw:40.00), Zn(NO3)2.6H2O (99.0%, Mw:297.49) and SnCl4.5H2O (99.0%, Mw:350.60) reagents all purchased from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). All chemicals are of analytical grade and were used as received without further purification. 2.2 Synthesis of hierarchical tetradecahedral SnO2/Zn2SnO4 composites The hierarchical tetradecahedral SnO2/Zn2SnO4 composites were synthesized by a facile hydrothermal process. In brief, 3.0 mmol Zn(NO3)2·6H2O and 3.0 mmol SnCl4·5H2O were dissolved in 33% ethanol solution to form solutions A and B, then mixed them with stirring all the time. 3.0 mmol NaOH solution was slowly added dropwise to the above solutions. After completing the above operation, it stired at 80 ℃ for 3 h. The solution was transferred into a 50 mL stainless steel teflon-lined autoclave, and then it was heated at 200 ◦C for 2 h. After this process, the reaction vessel was taken out and cooled to RT. The precipitates were collected by centrifugation and washed with absolute ethyl alcohol and deionized water several times. Finally, it was dried at 60 ℃ for 12 h. The synthesis process was displayed in Fig. 1.
Fig. 1. Schematic diagram of the experimental process 2.3 Fabrication and measurement of gas sensor The gas sensor was fabricated as shown in Fig. S1. The sensor performance test uses
the WS-30A gas test system of Zhengzhou Yusheng Electronic Technology Co., Ltd. The fabricated sensor was welded onto a pedestal and the gas sensing properties. Our research team has done a lot of related tests and researches [35]. In order to investigate the relationship between gas concentrations and gas responses, certain amount of ethanol was measured by a pipette and then evaporated into well-defined concentration of target gas in the reaction chamber. The gas response in this work was defined as S=Ra/Rg, in which Ra and Rg are the resistances in air and test gas, respectively. The response time of the sensor is counted as the time required for the sensor to reach its corresponding maximum of 90%. In contrast, the recovery time is calculated by the time required for the sensor to reach its maximum value of 10%. 3. Results and Discussion 3.1 Characterization of hierarchical tetradecahedral SnO2/Zn2SnO4 composites The crystal structure and phase purity of the prepared SnO2/Zn2SnO4 composites were characterized by X-ray diffraction as shown in Fig. 2a. All characteristic peaks in the figure were consistent with standard SnO2 of tetragonal structure (JCPDS:41-1445) and standard Zn2SnO4 with spinel structure (JCPDS:24-1470). No other peaks belong to other impurities. In Fig. 2b, it clearly show the SEM morphology of the SnO2/Zn2SnO4 composites. The overall morphology of the samples were polyhedral shapes with uniform size of ~1 μm. Moreover, it can be seen that these polyhedrons were independent and uniformly dispersed in Fig. S7. As the high magnification SEM in the illustration of Fig. 2b presented, it shows the morphology of a single polyhedron. The polyhedron was confirmed to be a tetradecahedron. In addition, the tetradecahedral surfaces were evenly loaded with a kind of dense two-dimensional sheet structures. The EDS analysis confirmed the existence of Zn, Sn, and O elements from the heterojunction and these three elements were derived from two different substances, SnO2 and Zn2SnO4, respectively in Fig. S2. Furthermore, a single SnO2/Zn2SnO4 tetradecahedron was analyzed. Fig. S3a and b show SEM and TEM images of a single SnO2/Zn2SnO4 tetradecahedron, respectively.
Fig. 2. (a) XRD pattern of as-prepared SnO2/Zn2SnO4. (b) shows the SEM images of the SnO2/Zn2SnO4 The Zn2SnO4 was identified as a tetradecahedral spinel structure by XRD, SEM and TEM ( in Fig. S3). The tetradecahedron consists of six (100) faces and eight (111) faces [36]. In general, the primary driving force for single crystal growth is to reduce surface energy, thereby minimizing crystal surface energy. The crystal shape is determined by the relative order of surface energies [37]. For the tetradecahedral spinel structure, recent calculations show that the minimum surface energy is at the (100) plane. For (111) crystal faces, they are positively or negatively charged surfaces, which were composed of closed packed oxygen ions or tetra/hexa- coordinated metal- ions. This structure gives higher surface energy to the (111) crystal planes. For solutionbased reactions, a variety of experimental factors, including reactants, solvents, and surfactants, can alter the order of the (100) and (111) planes' surface energies. In this experiment, no surfactant was used, which resulted in both the (100) crystal planes and the (111) crystal planes to obtain a certain orientation growth, and the irregular Zn2SnO4 tetradecahedron structure was formed. The reaction produces a large amount of flaky SnO2, and the flaky SnO2 structure tends to adsorb to the surfaces of low surface energy, exhibiting a morphology as shown in Fig. S3a. In addition, the approximate generation process of SnO2/Zn2SnO4 is described in Fig. S4. 3.2 Gas-sensing properties of the as-prepared sensor To verify the performances of the SnO2/Zn2SnO4 composites on gas sensing, the gas sensing behaviors of the as-prepared sensor on ethanol have been studied. An important property of metal oxide semiconductor-based gas sensors is the operating temperature.
For demonstrating the performances of the sensor based on SnO2/Zn2SnO4 at lower temperatures, it was tested the responses with various temperatures at 100 ppm ethanol, as shown in Fig. 3a. As the test temperatures increases from 15 ℃ to 125 ℃, the responses first increase and then reach the maximum value, afterwards the responses decreased with further increasing temperature. The optimum operating temperature is 25 ℃ with response of 31.05 at 100 ppm ethanol. It is evident that at RT, the sensors still exhibit good response. Fig. 3(b-d) is the sensing behaviors of the sensors when orderly exposed to different concentrations of ethanol at RT. Fig. 3b is the linear curve of the sensors with the increasing of ethanol concentrations, obviously, the sensors showed nearly a linear increasing with ethanol concentrations from 10 to 100 ppm and 1 to 5 ppm. Fig. 3c and d is the corresponding dynamic response and recovery curves with ethanol concentrations increased from 1 to 5 ppm and 10 to 100 ppm. It can be seen that as ethanol concentrations increases, the responses of gas sensor based on SnO2/Zn2SnO4 are also gradually increased. The corresponding response values were 1.1, 1.80, 2.61, 3.62 for ethanol concentration from 1 to 5 ppm (Fig. 3d) and for concentration from 10 to 100 ppm, the response values were 5.35, 9.25, 17.64, 23.65, 31.05 (Fig. 3c), respectively. Besides, one can also be observed that the response amplitudes for the SnO2/Zn2SnO4 sensor to different ethanol concentrations are obviously higher than that of the pure SnO2 sensor, as shown in Fig. S5.
Fig. 3. (a) Responses of SnO2/Zn2SnO4 at different working temperature upon exposure to 100 ppm ethanol. (b) Responses of the sensors to ethanol with different
concentrations at RT. (c) and (d) Corresponding dynamic response curves of the sensor to different concentrations of ethanol at RT Repeatability is an important indicator for testing sensors. Fig. 4a shows five cycles of dynamic response curves of SnO2/Zn2SnO4 to 200 ppm ethanol at RT. The shapes of response and recovery curves is almost identical, which illustrate a good repeatability of the sensor. The response and recovery characteristics play crucial role in semiconductor metal oxide-based gas sensors.The response and recovery properties of the sensor to 200 ppm ethanol at RT was investigated as displayed in Fig. 4b. The sensor shows an excellent sensitivity with 9 s response time and 7 s recovery time at RT, which is faster than most of ethanol sensing materials in reports. Selectivity is another important property of gas sensor. Fig. 4c shows that the gas response value toward ethanol was about 30 times higher than other tested gases, which could indicate that the sensor shows a high response and excellent selectivity to ethanol. Otherwise, the sensor to several hydroxyl-containing VOC gases (methanol, isopropanol, n-butanol) have been tesed in Fig. S6. The results show that the responses of SnO2/Zn2SnO4 to methanol, isopropanol and n-butanol are very low, and the response value to ethanol is almost theirs 30 times, which could indicate that the sensor based on SnO2/Zn2SnO4 composites shows a high response and excellent selectivity to ethanol, but not sensitive to all hydroxyl-containing VOC gases. To further investigate the long-time reliability of SnO2/Zn2SnO4 sensor, the responses in air of the sensor to 100 ppm ethanol at RT during 45 days was measured. As depicted in Fig. 4d, there is no obvious fluctuation in the resistance and response values during the test days, illustrating good long-term reliability of the sensor.
Fig. 4. (a) Five cycles of dynamic response curves of SnO2/Zn2SnO4 to 200 ppm ethanol at RT. (b) Responses transient of SnO2/Zn2SnO4 to 200 ppm ethanol at RT. (c) Responses of the sensors to various test gases with a concentration of 200 ppm. (d) Resistances in air and responses to 100 ppm ethanol of SnO2/Zn2SnO4 as a function of the test days at RT
Anything else, there is a Table 1 summarized to prove the excel properties of the sensor based on hierarchical tetradecahedral SnO2/Zn2SnO4 compared with other ethanol sensing materials in reported literatures. By comparison, it can be seen that the SnO2/Zn2SnO4 sensor exhibits excellent performance for low operating temperature and ultra-sensitive characteristics.
Table1. Comparison of previously reported materials-based ethanol sensing Sensor type
Operating
Concentration
Response/recovery
Response
temperature
(ppm)
time (S)
(Ra/Rg)
Ref
(℃） Zn2SnO4 Porous
280
200
11/20
82.2
[21]
180
50
18/45
23.4
[38]
200
100
76/139
14
[39]
500
20
120/200
4.9
[40]
Compact Zn2SnO4
380
50
13/15
12.8
[33]
SnO2/Zn2SnO4
RT
200
9/7
134.7
This
hollow cubes Zn2SnO4 Nanosphere Zn2SnO4/SnO2 Octahedral-like Zn2SnO4 Nanowires
Tetradecahedron
work
3.3 Gas sensing mechanism Analysis of the above experimental phenomena and results, we propose that the gassensing mechanism of the ethanol sensing materials SnO2/Zn2SnO4 composites. One is based on a change in the electrical resistance after a reaction between the ionosorbed oxygen species and ethanol molecules [41]. According to the ionosorption model, when the sensor is exposed in air, oxygen molecules will adsorb on the surface of SnO2/Zn2SnO4 to ionize into the Ox− (O2−, O−, O2−) by depriving electrons from the conduction band of SnO2/Zn2SnO4 composites (reaction (1)), As shown in Fig. 5. O2(Gas) → O2(Ads) + e- → O2−(Ads) + e- → O− (Ads) + e- → O2−(Ads)
(1)
It forms a thick electron depletion layer on the surface of SnO2/Zn2SnO4 and a high barrier between adjacent crystal grains, resulting in an increase in the resistance of the sensor. Ethanol gas is a reducing gas, and when the sensor is exposed to a reducing gas, the ethanol molecules reacts with the surface-adsorbed oxygen species, thereby releasing the trapped electrons back into the conduction band of the SnO2/Zn2SnO4,
which results in the conductivity increasing and the sensor’s resistance reducing (reaction (2)). The reaction process between the surface adsorbed oxygen species and ethanol is described as the following equation. C2H5OH + Ox− → 2CO2 + 3H2O + xe-
(2)
The gas sensors based on SnO2/Zn2SnO4 have ultra-sensitive and low operating temperature properties mainly due to hierarchical structure and heterojunctions formed between SnO2 and Zn2SnO4. First of all, hierarchical tetradecahedron structure provides higher specific surface area causing faster diffusion and adsorption of the gas. It also provides more surface active sites for the reaction of adsorbed oxygen and ethanol gases, which can lead to ultra-sensitive [42]. On the other hand, another consideration is the formation of a heterojunction at the interface between Zn2SnO4 and SnO2 due to their different work functions [19, 21]. Heterojunctions formed between different semiconductor oxides have a significant effect on gas sensing performance. Since the conduction band edge of Zn2SnO4 locates at higher potential than SnO2, electrons in the conduction band of Zn2SnO4 will migrate to the conduction band of SnO2 until their Fermi energy level is up to equilibrium when SnO2 is doped with Zn2SnO4. The process will deplete the extra electrons of SnO2/Zn2SnO4 composite interface, which will enhance the gas sensing properties.
Fig. 5. Schematic illustration of ethanol gas sensing mechanism of hierarchical
tetradecahedron SnO2/Zn2SnO4
4. Conclusion In summary, hierarchical tetradecahedral SnO2/Zn2SnO4 composites was successfully synthesized by a facile one-pot method. The structure and morphology of the SnO2/Zn2SnO4 composites are characterized by SEM, TEM, XRD and EDS. The results show that the SnO2 nanoplates are uniformly dispersed on the surfaces of tetradecahedral Zn2SnO4. The gas sensing performances of the as-fabricated gas sensor were systematically investigated by using SnO2/Zn2SnO4 composite as gas sensing material. It exhibits a significant enhancement in ethanol gas-sensing performances with a much higher response, good selectivity and good repeatability. It is worth mentioning that the manufactured sensors display praiseworthy low operating temperature (RT) and ultra-sensitive with 9 s response time and 7 s recovery time to 200 ppm ethanol. Other than that, reasonable gas sensing mechanisms are discussed. The good sensing performances can be mainly attributed to the SnO2/Zn2SnO4 composites’ unique hierarchical tetradecahedral loaded with sheet-like structure and heterojunctions formed between SnO2 and Zn2SnO4. This work indicates that the SnO2/Zn2SnO4 composites are very promising sensing material for the application of ethanol gas sensor.
Conflicts of interest There are no competitive economic benefits to declare. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 61773372), the Jiangsu Province Outstanding Youth Foundation (No. BK20160058), the Jiangsu Basic Research Program (Natural Science Foundation, No. BK20170427), Chinese Academy of Sciences Technology Service Network Program (STS, KFJ-STS-QYZX-081) and the Chinese Academy of Sciences.
References [1] C. Hagleitner, A. Hierlemann, D. Lauge, A. Kimmer, N. Kerness, O. Brand, H.Baltes, Smart single-chip gas sensor microsystem, Nature 414(2001) 293-296. [2] F. Yang, Z. Guo, Comparison of the enhanced gas sensing properties of tin dioxide samples doped with different catalytic transition elements, J. Colloid Interf. Sci., 448(2015) 265-274. [3] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators, B, 140(2009) 319-336. [4] J. Ma, Y. Ren, X. Zhou, L. Liu, Y. Zhu, X. Cheng, P. Xu, X. Li, Y. Deng, D. Zhao, Pt nanoparticles sensitized ordered mesoporous WO3 semiconductor: gas sensing performance and mechanism study, Adv. Funct. Mater., 28(2018) 1705268. [5] Y. Shen, H. Bi, T. Li, X. Zhong, X. Chen, A. Fan, D. Wei, Low-temperature and highly enhanced NO2 sensing performance of Au-functionalized WO3 microspheres with a hierarchical nanostructure, Appl. Surf. Sci., 434(2018) 922-931. [6] V.K. Guptaa, R. Sadeghic, F. Karimid, A novel electrochemical sensor based on ZnO nanoparticle and ionic liquid binder for square wave voltammetric determination of droxidopa in pharmaceutical and urine samples, Sens. Actuators, B, 186(2013) 603609. [7] A. Gurlo, Nanosensors: towards morphological control of gas sensing activity. SnO2, In2O3, ZnO and WO3 case studies, Nanoscale, 3(2011) 154-0. [8] C. Wang, X.B. Cui, J.Y. Liu, X. Zhou, X.Y. Cheng, P. Sun, X.L. Hu, X.W. Li, J. Zheng, G.Y. Lu, Design of superior ethanol gas sensor based on Al-doped NiO nanorod-flowers, ACS Sens., 1(2016) 131-136. [9] S. Park, G.J. Sun, H. Kheel, W.I. Lee, S. Lee, S.B. Choi, C. Lee, Synergistic effects of codecoration of oxide nanoparticles on the gas sensing performance of In2O3 nanorods, Sens. Actuators, B, 227(2016) 591-599. [10] J.W. Deng, J.M. Ma, L. Mei, Y.J. Tang, Y.J. Chen, T. Lv, Z. Xu, T.H. Wang, Porous ɑ-Fe2O3 nanosphere-based H2S sensor with fast response, high selectivity and enhanced sensitivity, J. Mater. Chem. A, 1(2013) 12400-12403.
[11] Y. Cui, Q.Q. Wie, H.K, Park, C.M. Lieber, Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species, Science, 293(2001) 1289-1292. [12] R. Malik, V.K. Tomer , L. Kienle , V. Chaudhary, S. Nehra, S. Duhan, Ordered Mesoporous Ag-ZnO@g-CN Nanohybrid as Highly Efficient Bifunctional Sensing Material, Adv. Mater. Interfaces, 5(2018) 1701357. [13] D. Xue, P. Wang, Z. Zhang, Y. Wang, Enhanced methane sensing property of flower-like SnO2 doped by Pt nanoparticles: A combined experimental and firstprinciple study, Sens. Actuators, B, 2019: 126710. [14] T.M.N. Quan, V.D. Nguyen, T.P. Nguyen ， N.T. Nguyen, M.H. Chu, D.H. Nguyen,V.H. Nguyen, Superior enhancement of NO2 gas response using n-p-n transition of carbon nanotubes/SnO2 nanowires heterojunctions, Sens. Actuators, B, 238(2017) 1120-1127. [15] X. Yang, S. Zhang, Q. Yu, L. Zhao, P. Sun, T. Wang, One step synthesis of branched SnO2/ZnO heterostructures and their enhanced gas-sensing properties, Sens. Actuators, B, 281(2019) 415-423. [16] Y.G. Zheng, J. Wang, P.J. Yao, Formaldehyde sensing properties of electrospun NiO-doped SnO2 nanofibers, Sens. Actuators, B, 156(2011) 723-730. [17] J.Y. Liu, M.J. Dai, T.S. Wang, P. Sun, X.S. Liang, G.Y. Lu, K. Shimanoe, N. Yamazoe, Enhanced gas sensing properties of SnO2 hollow spheres decorated with CeO2 nanoparticles heterostructure composite materials, ACS Appl. Mater. Interfaces, 8(2016) 6669-6677. [18] M. Haq, Z. Wen, Z. Zhang, S. Khan, Z. Lou, Z. Ye, L. Zhu, A two-step synthesis of nanosheet-coveredfibers based on alpha-Fe2O3/NiO composites towards enhanced acetone sensing, Sci. Rep. 8(2018) 1705. [19] S.S. Zhang, G. Sun, Y.W. Li, B. Zhang, Continuously improved gas-sensing performance of SnO2/Zn2SnO4 porous cubes by structure evolution and further NiO decoration, Sens. Actuators, B, 255(2018) 2936-2943. [20] D. Xue, Y. Wang, J. Cao, G. Sun, Z. Zhang, Improving methane gas sensing performance of flower-like SnO2 decorated by WO3 nanoplates, Talanta 199(2019) 603-611. [21] Z. Chen, M. Cao, C. Hu, Novel Zn2SnO4 hierarchical nanostructures and their gas sensing properties toward ethanol, J. Phys. Chem. C, 115(2011) 5522-5529. [22] H.K. Wang, A.L. Rogach, Hierarchical SnO2 nanostructures: recent advances in
design synthesis, and applications, Chem. Mater., 26(2014) 123-133. [23] X. Chu, R. Hu, J. Wang, Y. Dong, W. Zhang, L. Bai, W. Sun, Preparation and gas sensing properties of graphene-Zn2SnO4 composite materials, Sens. Actuators, B, 251(2017) 120-126. [24] D.P. Pratim, A. Roy, S. Das, P.S. Devi, Enhanced stability of Zn2SnO4 with N719 N3 and eosin Y dye molecules for DSSC application, Phys. Chem. Chem. Phys.,18(2015) 1429-1438. [25] J. Yang, S. Wang, L. Zhang, R. Dong, Z. Zhu, X. Gao, Zn2SnO4−doped SnO2 hollow spheres for phenylamine gas sensor application, Sens. Actuators, B, 239(2017) 857-864. [26] Z.X. Chen, M. Zhou, Y.L. Cao, X.P. Ai, H.X. Yang, J. Liu, In situ generation of few-layer graphene coatings on SnO2−SiC core-shell nanoparticles for highperformance lithium-ion storage, Adv. Energy Mater., 2(2012) 95-102. [27] L. Qin, S. Liang, X. Tan, A. Pan, Zn2SnO4/graphene composites as anode materials for high performance lithium-ion batteries, J. Alloy Compd., 692(2017) 124-130. [28] X.F. Chu, R.X. Hu, J.L. Wang, Y.P. Dong, W.B. Zhang, L.S. Bai, W.Q. Sun, Preparation and gas sensing properties of graphene-Zn2SnO4 composite materials, Sens. Actuators, B, 251(2017) 120-126. [29] T.V. Dang, N.D. Hoa, N.V. Duy, N.V. Hieu, Chlorine gas sensing performance of on-chip grown ZnO, WO3, and SnO2 nanowire sensors, ACS Appl. Mater. Interfaces, 8(2016) 4828-4837. [30] H.N. Hai, T.D. Duc, H.N. Thanh, P.N. Huy, T.P. Duy, H.H. Si, Fast response of carbon monoxide gas sensors using a highly porous network of ZnO nanoparticles decorated on 3D reduced graphene oxide, Appl. Surf. Sci., 434(2018) 1048-1054. [31] K. Tian, X.X. Wang, Z.Y. Yu, H.Y. Li, X. Guo, Hierarchical and hollow Fe2O3 nanoboxes derived from metal-organic frameworks with excellent sensitivity to H2S, ACS Appl. Mater. Interfaces, 9(2017) 29669-29676. [32] L. Zhu, Y. Li, W. Zeng, Hydrothermal synthesis of hierarchical flower-like ZnO nanostructure and its enhanced ethanol gas-sensing properties, Appl. Surf. Sci., 427(2018) 281-287 [33] Y.F. Bing, Y. Zeng, C. Liu, L. Qiao, Y.M. Sui, B. Zou, W.T. Zheng, G.T. Zou, Assembly of hierarchical ZnSnO3 hollow microspheres from ultra-thin nanorods and the enhanced ethanol-sensing performances, Sens. Actuators, B, 190(2014) 370-377. [34] T. Zhai, H.G. Xu, W.R. Li, H.Q. Yu, Z.R. Chen, J.Q. Wang, B.Q. Cao, Low-
temperature in-situ growth of SnO2 nanosheets and its high triethylamine sensing response by constructing Au-loaded ZnO/SnO2 heterostructure, J. Alloy. Compd., 737(2018) 603-612. [35] C. Wang, L.J.Wang, L. Zhang, R. Xi, H. Huang, S.H. Zhang, G.B. Pan, Electrodeposition of ZnO nanorods onto GaN towards enhanced H2S sensing, J. Alloy. Compd., 790(2019) 363-369. [36] Y.Q. Jiang, X.X. Chen, R. Sun, Z. Xiong, L.S. Zhang, Hydrothermal syntheses and gas sensing properties of cubic and quasi-cubic Zn2SnO4, Mater. Chem. Phys., 129(2011) 53-61. [37] C.M. Fang, S.C. Parker, G.D. With, Atomistic Simulation of the Surface Energy of Spinel MgAl2O4, J. Am. Ceram. Soc., 83(2000) 2082-2084. [38] D.M. An, N. Mao, G.Z. Deng, Y.L. Zou, Y. Li, T. Wei, X.X. Lian, Ethanol gassensing characteristic of the Zn2SnO4 nanospheres, Ceram. Int., 42(2016) 3535-3541. [39] S. Shu, M. Wang, Y. Wei, S. Liu, Synthesis of surface layered hierarchical octahedral-like structured Zn2SnO4/SnO2 with excellent sensing properties toward HCHO, Sens. Actuators, B, 243(2017) 1171-1180. [40] T. Tharsika, A.S.M.A. Haseeb, S.A. Akbar, M.F.M. Sabri, Y.H. Wong, Gas sensing properties of zinc stannate (Zn2SnO4) nanowires prepared by carbon assisted thermal evaporation process, J. Alloy. Compd., 618(2015) 455-462. [41] C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal oxide gas sensors: sensitivity and influencing factors, Sensors, 10(2010) 2088-2106. [42] K.D. Nguyen, T.D. Do, D.V. Nguyen, H.D. Nguyen, H.V. Nguyen, Design of SnO2/ZnO hierarchical nanostructures for enhanced ethanol gas-sensing performance, Sens. Actuators, B, 174(2012) 594-601.
Highlights
The hierarchical tetradecahedral SnO2/Zn2SnO4 composites were synthesized by a simple one-pot method.
The hierarchical tetradecahedral SnO2/Zn2SnO4 composites exhibited excellent ethanol gas sensing properties.
The hierarchical tetradecahedral SnO2/Zn2SnO4 composites can work at room temperature.
The hierarchical tetradecahedral SnO2/Zn2SnO4 composites exhibited a rapid response time of 9 s and recovery time of 7 s.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
CRediT author statement Zi-Yang Chen: literature search, figures, study design, data collection, data analysis, data interpretation Da-Peng Jiang: literature search, data collection Shao-Hui Zhang: literature search, software, data collection Chao Wang: literature search, software Hui Huang: literature search, Long Zhang: literature search Liu-Yi Ding: literature search Lin-Jun Wang: literature search Ge-Bo Pan: literature search, study design