Microwave preparation and remarkable ethanol sensing properties of ZnO particles with controlled morphologies in water-ethylene glycol binary solvent system

Accepted Manuscript Title: Microwave preparation and remarkable ethanol sensing properties of ZnO particles with controlled morphologies in water-ethylene glycol binary solvent system Authors: Tao Wang, Shusheng Xu, Nantao Hu, Jun Hu, Da Huang, Wenkai Jiang, Shuai Wang, Shimin Wu, Yafei Zhang, Zhi Yang PII: DOI: Reference:

S0925-4005(17)31506-X http://dx.doi.org/10.1016/j.snb.2017.08.099 SNB 22966

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

2-11-2016 3-8-2017 8-8-2017

Please cite this article as: Tao Wang, Shusheng Xu, Nantao Hu, Jun Hu, Da Huang, Wenkai Jiang, Shuai Wang, Shimin Wu, Yafei Zhang, Zhi Yang, Microwave preparation and remarkable ethanol sensing properties of ZnO particles with controlled morphologies in water-ethylene glycol binary solvent system, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.08.099 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.

Microwave preparation and remarkable ethanol sensing properties of ZnO particles with controlled morphologies in water-ethylene glycol binary solvent system Tao Wanga, Shusheng Xua, Nantao Hua, Jun Hua, Da Huanga, Wenkai Jianga, Shuai Wanga, Shimin Wub, Yafei Zhanga, Zhi Yanga,*

a

Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China b Key Laboratory of Urban Agriculture (South), Department of Food Science and Technology, School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China

*

Corresponding author. E-mail address: [email protected]

Highlights: 

ZnO particles with different morphologies were synthesized via a fast and simple microwave-assisted synthesis method.



The morphology of ZnO was controlled by the proportion of ethylene glycol in water-ethylene glycol binary solvent system.



The as prepared ZnO showed extraordinary ethanol sensing performance: low limit of detection (less than 1 ppm), very high response (about 250 at 300 °C), and excellent selectivity.

Abstract ZnO particles were synthesized via a fast (15 min) and simple (one step) microwave-assisted synthesis method in a water-ethylene glycol binary solvent system. By increasing the proportion of ethylene glycol, morphology of ZnO particles has an evolution from double-end clean-cut hexagonal prism to porous loose rough sphere. Furthermore, indirectly heated gas sensors were fabricated based on the materials to study their gas sensing properties to ethanol. The peanut-sharped ZnO showed the best ethanol sensing performance with low limit of detection (less than 1 ppm), very high response (about 250 at 300 °C), fast response and recovery time (10 and 17 s, respectively), excellent selectivity and good stability. Our work paves a simple, fast and environmentally friendly way to fabricate ethanol sensors with excellent sensing properties, which have potentials to be widely used in the future.

Keywords: Zinc oxide Microwave-assisted synthesis Binary solvent system Gas sensor Ethanol

1. Introduction As a typical n-type semiconductor, ZnO has always been keeping enduring charm in the field of electronics, photonics, acoustics and sensing for decades of years due to its unique characteristics like broad energy band (3.37 eV), high bond energy (60

meV) and noncentral symmetric wurtzite crystal structure [1-7]. Besides, compared with those new carbon materials like carbon nanotube and graphene [8-10], ZnO has its own irreplaceable advantages such as environmentally friendly, easily prepared and long-termly reliable, in the application of gas sensing to NO2 [11], CO [12], H2S [13], ethanol [14] and so on. In order to improve the sensing performance, dropping metal elements like Ag [15], Au [16], Cu [12], and Pd [17], is a common approach for reducing the surface barrier of ZnO and promoting the reaction of toxic gas. Control morphology of the material is another effective method to further increase the gas sensing properties. Various structures of ZnO, such as nanorods [18], nanowire [19], nanosheet [15] and nanoflower [20], have been synthesized and used for ethanol sensing applications. There are variety of methods to prepare ZnO, such as vapor deposition [21], hydrothermal synthesis [22], microwave-assisted synthesis [23], the sol-gel process [24], and mechanochemical process [25]. Due to the benefits of simple synthesis process, short time, and low cost, microwave-assisted synthesis has been widely applied in the preparation of ZnO particles. Herein, we have synthesized ZnO particles by a fast and simple microwave-assisted synthesis method in water-ethylene glycol binary solvent system， where the proportion of ethylene glycol was adjusted to obtain ZnO with different sizes and structures, leading to promotion of ethanol detection performance. To the best of our knowledge, there is no previous report on the microwave preparation and remarkable ethanol sensing properties of ZnO particles with controlled morphology in water-ethylene glycol binary solvent system.

2. Experimental 2.1. Chemicals and reagents Zinc acetate dihydrate (C4H6O4Zn2H2O), ethylene glycol (C2H6O2), and hexamethylene tetramine (HMTA) were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used without further purification. Deionized water with a resistivity of 18.2 MΩcm was used for all experiments.

2.2. Microwave preparation of ZnO particles The morphology controllable ZnO particles were prepared through a simple and fast microwave-assisted synthesis method in water-ethylene glycol binary solvent system. Firstly, a certain proportion of water and ethylene glycol were mixing sufficiently in a beaker. Then 0.15 g ZnAc2H2O and 0.075 g HMTA powders were dissolved in the binary solvent system under vigorous stirring for 15 min. Next, the as prepared solution was transferred into a single-mode microwave synthesis equipment (NOVA-2S, China), which can control the solution’s temperature and pressure in process of the microwave reaction. The heating scheme of NOVA-2S for the equipment was heating up to 180 °C in 5 min, holding the temperature for 10 min and cooling to room temperature spontaneously. After the microwave progress, the obtained materials were cleaned using deionized water and ethanol for 3 times respectively, followed by drying at 60 °C for 6 h. Finally, the as obtained powders were annealed in a muffle furnace at 400 °C for 2 h. The ZnO particles, which were

synthesized in different proportions of water and ethylene glycol binary solvent system (volume ratios were 6:0, 5:1, 4:2, 3:3, 2:4, 1:5 and 0:6), are numbered as ZnO-0, ZnO-1, ZnO-2, ZnO-3, ZnO-4, ZnO-5 and ZnO-6, respectively.

2.3. Characterization The microstructures and element components of the as-prepared ZnO materials were analyzed by a scanning electron microscope (SEM, Ultra Plus, Carl Zeiss, Germany). The crystal structure of the products was obtained using a D8 Advance X-ray diffractometer (XRD) (Bruker AXS Corporation, Germany) with Cu Kα source in a 2θ range of 10−90°.

2.4. Sensor fabrication and gas sensing performance test Alumina ceramic tubes, which are 1 mm in diameter and 4mm in length, with two inter digital Au electrodes were employed to construct gas sensors through a common approach [18,26-28]. The ZnO particles were transferred to the cleaned alumina ceramic tube through dipping and pulling the tube to the ZnO-dispersed ethanol suspension. After repeating this step several times, the surface of the alumina ceramic tube was covered with a dense and uniform film consisting of the sensitive material. Then, a Ni-Cr heating wire inserted into the tube was used to control the operating temperature of the gas sensor by adjusting the heating voltage (Uh). Fig. 1(a) illustrated structure of the gas sensor. The gas sensing elements were heated at 300 °C for 3 days in order to improve their stability and repeatability. ZnO-0, ZnO-2, ZnO-4

and ZnO-6 were chosen as sensitive materials to build gas sensors, which were named S1, S2, S3 and S4, respectively. Gas sensing tests were performed on a commercial WS-30A gas sensing measurement system (Weisheng Electronics Co., Ltd, Zhengzhou, China). All gas sensing tests throughout the work were performed under atmospheric pressure with the same relative humidity of (60 ± 1) %. In a typical gas sensing test progress [29], the specific gas with different concentrations was introduced into a glass chamber by a microsyringe. As demonstrated in Fig. 1(b), voltage of the test circuit is Uc, while the output voltage (Uo) is the terminal voltage of the load resistor (RL). The response (S) of the sensor is defined as the ratio of Ra and Rg, where Ra and Rg were the resistance of the sensor in air and in target gas, respectively. In this work, the voltage of the test circuit is 5V and the load resistor is 1MΩ. The relationship between the resistance of sensor (Rs) and Uo is illustrated in the equation below: 𝑅𝑠 = (

𝑈𝑐 − 1)𝑅𝐿 𝑈𝑜

Sometimes, different sensors may need different RL and Uc, in order to obtain the suitable measurement range. We can use the equation below to realize the convert of Uo under different test conditions. Only when Uo of different sensors are converted to the data under the same test conditions virtually, the comparison and evaluation of sensors’ performances can be carried out correctly. 𝑈𝑜2 =

𝑈𝑐2 𝑈 𝑅 (𝑈𝑐1 − 1) × 𝑅𝐿1 + 1 𝑜1

𝐿2

Where Uo1 is the output voltage of the sensor, when the voltage of the test circuit is chosen as Uc1 and the load resistor is chosen as RL1. Similarly, Uo2 is the output

voltage of the sensor under a different test condition, when the voltage of the test circuit is chosen as Uc2 and the load resistor is chosen as RL2. However, due to the preparation process is unstable, initial resistances of these sensors, which are tested at the same time, are sometimes very different. Comparison of 𝑅𝑠 or Uo is valueless, while it is generally necessary to study changes in response values of different sensors. The following equation illustrates the relationship between S and Uo: S=

(𝑈𝑐 − 𝑈𝑜𝑎 ) × 𝑈𝑜𝑔 (𝑈𝑐 − 𝑈𝑜𝑔 ) × 𝑈𝑜𝑎

Uoa is the output voltage of WS-30A, when the sensor is in air. Uog is the output voltage of WS-30A, when the sensor is in target gas. In this paper, original data of the gas sensing experiments is converted into sensor response data for plotting and analysis. In single response and recovery test, the response progress and recovery progress were provided with enough time to ensure the resistance of sensor reach to stable state. The response time (Tres) is defined as period of time from gas sensor contact with gas to be detected to variation of resistance reach to 90 % of |𝑅𝑎 − 𝑅𝑔 |. Similarly, the recovery time (Trec) is defined as period of time from gas sensor away from gas to be detected to variation of resistance reach to 90 % of |𝑅𝑎 − 𝑅𝑔 |. We use consistent time interval to determine the process of response and recovery in gradient concentration response test and cyclic response test. The target gas with certain concentration was injected into the test chamber. After two minutes, the chamber was opened and sensors was exposed to clean air. One minute later, the chamber was closed and the

target gas was injected into the test chamber. Follow this process repeatedly.

3. Results and discussion 3.1. Structure and morphology of the ZnO particles The morphologies and microstructures of the as-prepared ZnO materials were analyzed by SEM. Fig. 2 is SEM images with different magnification of the ZnO-0, ZnO-1, ZnO-2, ZnO-3, ZnO-4, ZnO-5 and ZnO-6, from which an evident evolution of morphology of ZnO particles is observed. With the increasing of the proportion of ethylene glycol, morphology of ZnO particles transits from double-end clean-cut hexagonal prism structure to porous loose rough sphere gradually. When the reaction solvent is pure water, ZnO presents double-end clean-cut hexagonal prism form. When the content of ethylene glycol is increasing gradually, the double-end hexagonal prisms become rounder and rounder, and their surface are coarser and coarser. As shown in Fig. 2(e), when volume ratio of water and ethylene glycol reaches to 2:4, the ZnO particles exhibit peanut shape, which is thin in middle and thick in both ends. As shown in Fig. 2(f), when the volume ratio comes to 1:5, the material is seemed like two sphere stick together, whose surface is very coarse and both ends are full of small particles. At last, when the reaction solvent is pure ethylene glycol, ZnO becomes single porous loose rough sphere completely, which is illustrated in Fig. 2(g). In conclusion, in the condition of microwave irradiation, water may let ZnO tend to form hexagonal prism, while ethylene glycol may prompt ZnO to form sphere, in the binary

solvent system.

The element components of as prepared material have been confirmed by EDS analyses. As shown in Fig. 3(a), there are only zinc and oxygen in ZnO-0, ZnO-2, ZnO-4 and ZnO-6. The crystal phases of these four materials are characterized by XRD. The peaks match well with Bragg reflections of the standard hexagonal wurtzite ZnO structure (JCPDS card no. 36-1451) [30], and no characteristic peaks are observed for any other impurities such as Zn or Zn(OH)2 in the XRD patterns, which are demonstrated in Fig. 3(b). From the above, pure ZnO with different kinds of morphologies have been prepared through a fast and simple microwave-assisted approach. As illustrated in Fig. 3(b) and (c), it is noteworthy that XRD pattern of ZnO-6 is obviously different with patterns of ZnO-0, ZnO-2 and ZnO-4, whose half peak width is larger and peak intensity is weaker. According to Scherrer's equation, the grain size of ZnO-6 may decrease intensely compared with ZnO-0, ZnO-2 and ZnO-4, which is proved by high-magnification SEM image of ZnO-6 in Fig. 3(d) [31,32].

3.2. Gas sensing properties The result of gas sensing tests showed a significant difference towards gas sensing characteristics of S1, S2, S3 and S4, which were based on different morphologies of ZnO particles (ZnO-0, ZnO-2, ZnO-4 and ZnO-6). Heating temperature usually brings enormous influence to gas sensing process [33,34], as a

result the gas sensing performances of S1, S2, S3 and S4 at different working temperatures were studied. Fig. 4(a) illustrates the response of S1, S2, S3 and S4 to 100 ppm ethanol at different temperatures. The response of S1, S2, S3 and S4 generally exhibits a tendency to increase first and then decrease at the same operating temperature. When working temperature is 300 °C, the trend is particularly evident: SS1SS4. For the same sensor, as the operating temperature gradually increases, the response increases first and then decreases. S3 has a distinguish ethanol sensing performance at relative low working temperature (response to 100 ppm ethanol is 55 at 200 °C), while S1, S2 and S4 have smaller response. Response of S3 to ethanol increases obviously with heating temperature increasing until the highest response (about 250) is achieved at 300 °C. As a result, 300 °C is chosen as working temperature of S3. Single response-recovery curves of S1, S2, S3 and S4 to 100 ppm ethanol at 300 °C is presented in Fig. 4(b), which evidently shows excellent sensing properties of S3 with very high response and fast recovery, compared with S1, S2 and S4. Response-recovery curves of S1, S2, S3 and S4 to 20, 40, 60, 80, 100 and 120 ppm ethanol at 300 °C are shown in Fig. 4(c). With concentration of ethanol increasing, response of S3 is rising regularly, while other sensors show inconspicuously change of response. Meanwhile, as demonstrated in Fig. 4(d), the response time and recovery time of S3 to 100 ppm ethanol are very short (10 and 17 s, respectively) at working temperature, which means that the gas sensor can stay the best condition with the heating temperature of 300 °C.

S3 not only has distinguished sensing properties to ethanol at low working

temperature, but also shows excellent response to low-concentration ethanol, whose limit of detection (LOD) is far less than 1 ppm. Response-recovery curves of S3 to 1, 3, 5, 7, 9 and 11 ppm ethanol at 300 °C are shown in Fig. 5(a), from which we can see S3 exhibits excellent low-concentration ethanol-detection ability. Fig. 5(b) illustrates that response rises gradually with concentration of ethanol increasing and no signs of saturation are observed, which means that the sensor can detect a very wide range of concentration of ethanol. Selectivity is an important parameter for gas sensor. The responses of S3 to 100 ppm ethanol and some toxic gases (formaldehyde, acetone, ammonia and methanol) at 300 °C are shown in Fig. 5(c). Obviously, response of the sensor to ethanol is much higher than those to other gases, which means the ZnO sensor exhibits good selectivity to ethanol. Repeatability is one of the reasons that hinder gas sensors’ practical applications. Fig. 5(d) demonstrates five times of response and recovery process of S3 to 100 ppm ethanol at 300 °C. The response-recovery curves are almost same which reflect that S3 has a superior repeatability.

Stability is an important parameter of gas sensors. Fig. 6 is the single response-recovery curve of S2 to 100 ppm ethanol at 300 °C, after being put in the air for six months. Although the response process is not smooth, the response value can still up to 180. It is worth noting that the sensor is exposed to the air without any protection for six months. The attenuation of the response and the shake of the response curve are mainly due to the adsorption of a lot of gas molecules in ZnO

surface. At the same time, the microstructure of ZnO has been damaged slightly in the presence of water vapor and CO2 [35,36]. Degradation of contacts and heaters, cracks in ZnO layer and fluctuations of temperature in the surrounding atmosphere all contribute to a decline in the performance of the sensor [37]. In order to avoid the attenuation of the sensors’ performances in practical applications, these ZnO based sensors should be well encapsulated and avoid contacting with moist air for long-term storage [38,39].

A comparison of the performances of different kinds of sensitive materials based ethanol sensors is made in Table 1. From the table, we can see clearly that our ZnO based ethanol sensor exhibits superior properties with high response and short response time and recovery time. Meanwhile, the sensor shows excellent selectivity to ethanol and good stability. Our work paves a very fast and simple path to fabricate ethanol sensors with excellent sensing properties, which have potentials to be widely used in the future.

3.3. Formation mechanism of the ZnO particles The formation mechanism of the ZnO particles under microwave irradiation in water-ethylene glycol binary solvent system may be explained as follows [46-50]: firstly, OH− ions formed as a result of the reaction of H2O with NH3 which was produced by decomposition of HMTA. Secondly, nucleation of ZnO proceeds rapidly. When the solvent is pure water, the in situ generated ZnO nuclei grew preferentially

along the [0001] direction, therefore single crystalline ZnO rods is generated. Subsequently, polar face of the ZnO rods has a tendency to link together, result in the formation of double-end clean-cut hexagonal prism. When the proportion of ethylene glycol in water-ethylene glycol binary solvent system is increasing, growing along the [0001] direction is suppressed due to the surface energy of crystal growth is changed by the binary solvent system. Hence, longitudinal growth is inhibited and radial growth was promoted. The ZnO is becoming fatter, rounder, and coarser. When the solvent is full of ethylene glycol, the growth of ZnO nuclei is almost stopped for the reason of low surface energy and lack of OH− ions. Tiny ZnO nanoparticles flock together to form porous loose rough sphere. Major chemical reactions for the formation of ZnO in the binary solvent system can be summarized as follows: (𝐶𝐻2 )6 𝑁4 + 6𝐻2 𝑂 → 4𝑁𝐻3 + 6𝐻𝐶𝐻𝑂 NH3 + 𝐻2 𝑂 → 𝑁𝐻4+ + 𝑂𝐻 − 𝑍𝑛2+ + 2𝑂𝐻 − → 𝑍𝑛𝑂 + 𝐻2 𝑂

(1) (2) (3)

3.4. Gas Sensing mechanism of the ZnO particles ZnO is an n-type semiconductor, whose conductivity is decided by the concentration of electrons. The fundamental gas sensing mechanism of ZnO is that target gas reacts with oxygen on ZnO surface, which changes the concentration of electrons by the trapping and detrapping process [51-55]. As illustrated in Fig. 7, the progress of ethanol sensing of ZnO can be demonstrated as follows: firstly, reactive oxygen species such as 𝑂2− , 𝑂2− , 𝑂− are adsorbed on ZnO surface, which are strongly depend on temperature. 𝑂2− is usually chemisorbed at low temperature, while 𝑂2− , 𝑂− are chemisorbed at high temperature. The reaction process can be

described as follows: 𝑂2 (𝑔𝑎𝑠) ↔ 𝑂2 (𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑) 𝑂2 (𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑) + 𝑒 − ↔ 𝑂2− 𝑂2− + 𝑒 − ↔ 2𝑂−

(4) (5) (6)

As a result, a thin oxygen anion layer is formed on the surface of ZnO, which decreases concentration of electrons inside ZnO and rises surface potential barrier. Secondly, when ethanol is added into the atmosphere, ethanol molecules are absorbed in ZnO surfaced, followed by a series of reactions with oxygen anion: 𝐶2 𝐻5 𝑂𝐻 (𝑔𝑎𝑠) ↔ 𝐶2 𝐻5 𝑂𝐻 (𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑) 𝐶2 𝐻5 𝑂𝐻 (𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑) + 6𝑂− → 2𝐶𝑂2 + 3𝐻2 𝑂 + 6𝑒 −

(7) (8)

The decomposition of the ethanol consumes oxygen anions and release electrons, led to decreasing of oxygen anion layer thickness and increasing of carrier concentration. Finally, surface potential barrier of ZnO is down and conductivity is up, which means a response of ZnO to ethanol has been occurred.

Microstructure, grain size, and crystal defect have huge influences on ZnO surface states [36,56-58]. ZnO thin film in the sensor is connected by necks and the interconnected grains and the larger aggregates are connected by grain boundaries. The relationship between the grain size (D) and the width of the depletion layer (L) produced around the surface of the crystallites can be used to distinguish three situations [57-61]: (1) When D >> 2L, the sensitivity is controlled by grain boundary barrier for inter-crystallite charge transport. (2) When D > 2L, the effect of cross section area of the depletion region surrounding each neck tops up the grain boundary barrier one. (3) When D < 2L, the sensitivity is essentially controlled by the intra-crystallite conductivity (grain control). The donor defects (zinc interstitial and

oxygen vacancy) in ZnO are beneficial to the formation of active oxygen and ethoxy. The active oxygen would further interact with the ethoxy, and released electrons which resulted in the decrease of the resistance of sensor [36]. More donor defects mean more chemisorbed oxygen and ethoxy, which lead to higher sensing response. Increasing surface area may provide more sites for chemisorption and reactions, therefore gas detection property would be promoted too. It is well known that the sensing mechanism of ZnO belongs to the surface-controlled type. Its sensitivity is relative to grain size, surface state, oxygen adsorption quantity, active energy of oxygen adsorption and lattice defects. As mentioned above, on the one hand, when the reaction solvent is pure water, ZnO with morphology of double-end clean-cut hexagonal prism is formed. This is a typical structure of wurtzite, which has stable crystal structure and high degree of crystallization. On the other hand, porous loose rough sphere will be synthesized in pure ethylene glycol, which is made up of tiny particles with high specific surface area. Meanwhile, ZnO with such structure may have more defects. Hence, the gas sensor’s final performance is the result of many synergistic or antagonistic effects, which are discussed above. In the progress of evolution of morphology of ZnO particles from double-end clean-cut hexagonal prism to porous loose rough sphere, the performance of sensors show the law of ascending and descending. An extremum point is existed, where the sensor can achieve its greatest response and the ZnO with morphology of peanut shows best ethanol sensing performance.

4. Conclusions The morphology controllable ZnO particles were prepared through a simple and fast microwave-assisted synthesis method in water-ethylene glycol binary solvent system. By changing the proportion of ethylene glycol, morphology of ZnO particles has an evolution from double-end clean-cut hexagonal prism to porous loose rough sphere. The ZnO with morphology of peanut (volume ratios of water and ethylene were 2:4) shows best ethanol sensing performance with low limit of detection (less than 1 ppm), very high response (about 250 at 300 °C), fast response and recovery time (10 and 17 s, respectively), excellent selectivity and good stability. This paper paves a new path to fabricate ethanol sensors with excellent sensing properties, which have potentials to be widely used in the future.

Acknowledgments The authors gratefully acknowledge financial supports by the National Key Research and Development Program of China (2016YFC0102700), the National Natural Science Foundation of China (61671299), Shanghai Science and Technology Grant (16JC1402000), the Program of Shanghai Academic/Technology Research Leader (15XD1525200), Shanghai Jiao Tong University Agri-X Funding (Agri-X2015007), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (GZ2016005). We also

acknowledge analysis support from the Instrumental Analysis Center of Shanghai Jiao Tong University and the Center for Advanced Electronic Materials and Devices of Shanghai Jiao Tong University.

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Biographies Tao Wang is a PhD candidate in Shanghai Jiao Tong University, China. His research focus now includes nanomaterials and their application for gas sensors. Shusheng Xu is a PhD candidate in Shanghai Jiao Tong University, China. His research focus now includes nanomaterials and their application for gas sensors. Nantao Hu is currently an associate professor in Shanghai Jiao Tong University, China. His research interests include synthesis of nanomaterials and their applications in nanodevices. Jun Hu is a MS candidate in Shanghai Jiao Tong University, China. His research focus now includes nanomaterials and their application for gas sensors. Da Huang is a PhD candidate in Shanghai Jiao Tong University, China. His research focus now includes nanomaterials and their application for gas sensors. Wenkai Jiang is a PhD candidate in Shanghai Jiao Tong University, China. His research focus now includes nanomaterials and their application for gas sensors. Shuai Wang is a MS candidate in Shanghai Jiao Tong University, China. His research focus now includes nanomaterials and their application for gas sensors. Shimin Wu is currently a professor in Shanghai Jiao Tong University, China. His research interests include synthesis of nanomaterials and their applications in nanodevices. Yafei Zhang is currently a professor in Shanghai Jiao Tong University, China. His research interests include synthesis of nanomaterials and their applications in nanodevices. Zhi Yang is currently a professor in Shanghai Jiao Tong University, China. His research interests include synthesis of nanomaterials and their applications in nanodevices.

Fig. 1. (a) Structure of the gas sensor, and (b) measuring electric circuit of gas sensing measurement system. Fig. 2. SEM images of ZnO particles, which were synthesized in binary solvent system, where the proportions of water and ethylene glycol were (a) 6:0, (b) 5:1, (c) 4:2, (d) 3:3, (e) 2:4, (f) 1:5 and (g) 0:6, respectively. Magnification SEM images of ZnO particles, which were synthesized in binary solvent system, where the proportions of water and ethylene glycol were (h) 6:0, (i) 4:2, (j) 2:4 and (k) 0:6, respectively. Fig. 3. (a) EDS spectrums and (b) XRD patterns of ZnO-0, ZnO-2, ZnO-4 and ZnO-6. (c) Magnification of partial XRD patterns of ZnO-0, ZnO-2, ZnO-4 and ZnO-6. (d) High-magnification SEM image of ZnO-6. Fig. 4. (a) Response of S1, S2, S3 and S4 to 100 ppm ethanol at different temperatures. (b) Single response-recovery curves of S1, S2, S3 and S4 to 100 ppm ethanol at 300 °C. (c) Response-recovery curves of S1, S2, S3 and S4 to 20, 40, 60, 80, 100 and 120 ppm ethanol at 300 °C. (d) Response and recovery time of S1, S2, S3 and S4 to 100 ppm ethanol at 300 °C Fig. 5. (a) Response-recovery curves of S3 to 1, 3, 5, 7, 9 and 11 ppm ethanol at 300 °C. (b) Response of S3 to different concentrations of ethanol at 300 °C. (c) Selectivity of S3 to different kinds of gases. (d) Repeatability of S3 to 100 ppm ethanol at 300 °C. Fig. 6. Single response-recovery curve of S2 to 100 ppm ethanol at 300 °C, after being put in the air for six months. Fig. 7. The progress of ethanol sensing of ZnO: (a) ZnO in the air, (b) oxygen species are adsorbed on ZnO surface, (c) ethanol molecules react with oxygen anion, and (d) ZnO returns to the initial state.

Fig. 1. (a) Structure of the gas sensor, and (b) measuring electric circuit of gas sensing measurement system.

Fig. 2. SEM images of ZnO particles, which were synthesized in binary solvent system, where the proportions of water and ethylene glycol were (a) 6:0, (b) 5:1, (c) 4:2, (d) 3:3, (e) 2:4, (f) 1:5 and (g) 0:6, respectively. Magnification SEM images of ZnO particles, which were synthesized in binary solvent system, where the proportions of water and ethylene glycol were (h) 6:0, (i) 4:2, (j) 2:4 and (k) 0:6, respectively.

Fig. 3. (a) EDS spectrums and (b) XRD patterns of ZnO-0, ZnO-2, ZnO-4 and ZnO-6. (c) Magnification of partial XRD patterns of ZnO-0, ZnO-2, ZnO-4 and ZnO-6. (d) High-magnification SEM image of ZnO-6.

Fig. 4. (a) Response of S1, S2, S3 and S4 to 100 ppm ethanol at different temperatures. (b) Single response-recovery curves of S1, S2, S3 and S4 to 100 ppm ethanol at 300 °C. (c) Response-recovery curves of S1, S2, S3 and S4 to 20, 40, 60, 80, 100 and 120 ppm ethanol at 300 °C. (d) Response and recovery time of S1, S2, S3 and S4 to 100 ppm ethanol at 300 °C.

Fig. 5. (a) Response-recovery curves of S3 to 1, 3, 5, 7, 9 and 11 ppm ethanol at 300 °C. (b) Response of S3 to different concentrations of ethanol at 300 °C. (c) Selectivity of S3 to different kinds of gases. (d) Repeatability of S3 to 100 ppm ethanol at 300 °C.

Fig. 6. Single response-recovery curve of S2 to 100 ppm ethanol at 300 °C, after being put in the air for six months.

Fig. 7. The progress of ethanol sensing of ZnO: (a) ZnO in the air, (b) oxygen species are adsorbed on ZnO surface, (c) ethanol molecules react with oxygen anion, and (d) ZnO returns to the initial state.

Table 1. A comparison of the performances of different kinds of sensitive materials based ethanol sensors Material

Concentration (ppm)

Response

Response time (s)

Recovery time (s)

Ref.

ZnO

50

9.7

35

37

[40]

ZnO

300

275

36

35

[41]

SnO2

50

30

13

16

[42]

SnO2

100

48

8

-

[43]

SnO2/ZnO

300

23

150

1100

[44]

SnO2/ZnO

100

14.7

10

23

[45] This

ZnO

100

250

10

17 work