Preparation of Ag-doped ZnO-SnO2 hollow nanofibers with an enhanced ethanol sensing performance by electrospinning

Preparation of Ag-doped ZnO-SnO2 hollow nanofibers with an enhanced ethanol sensing performance by electrospinning

Accepted Manuscript Preparation of Ag-doped ZnO-SnO2 hollow nanofibers with an enhanced ethanol sensing performance by electrospinning L. Ma, S.Y. Ma,...

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Accepted Manuscript Preparation of Ag-doped ZnO-SnO2 hollow nanofibers with an enhanced ethanol sensing performance by electrospinning L. Ma, S.Y. Ma, H. Kang, X.F. Shen, T.T. Wang, X.H. Jiang, Q. Chen PII: DOI: Reference:

S0167-577X(17)31183-7 http://dx.doi.org/10.1016/j.matlet.2017.08.004 MLBLUE 22981

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

22 June 2017 1 August 2017 1 August 2017

Please cite this article as: L. Ma, S.Y. Ma, H. Kang, X.F. Shen, T.T. Wang, X.H. Jiang, Q. Chen, Preparation of Ag-doped ZnO-SnO2 hollow nanofibers with an enhanced ethanol sensing performance by electrospinning, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.08.004

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Preparation of Ag-doped ZnO-SnO2 hollow nanofibers with an enhanced ethanol sensing performance by electrospinning L. Ma, S.Y. Ma, H. Kang, X.F. Shen, T.T. Wang, X.H. Jiang, Q. Chen Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China Abstract: The pure and Ag-doped ZnO-SnO2 hollow nanofibers had been synthesized via electrospinning method. The structure and morphology of the nanofibers were characterized by various techniques. The as-prepared products combined the two large advantages in morphology: surface roughness and porousness. Moreover, the sensor based on the Ag-doped ZnO-SnO2 hollow nanofibers exhibited excellent gas sensing performance at the low operating temperature of 200 °C and the fast response and recovery characteristics at a low concentration (1 ppm). It presented good selectivity and high response toward ethanol compared to pure nanofibers. The results demonstrated that Ag-doped ZnO-SnO2 nanofibers could be used as a kind of promising material for selective detection of low-concentration ethanol gas. Keywords: Semiconductor; Electrospinning; Nanofibers; Sensors; ZnO-SnO2 1. Introduction Hollow nanofibers as an interesting one-dimensional (1D) morphology have attracted increasing focus due to their unique properties and novel applications. As two important kinds of fundamental materials, SnO2 and ZnO have been widely studied due to their range of conductance variability and their response toward different gases [1]. Several approaches have been taken to improve their gas sensing performance, for example, doping with metal or rare earth element and synthesizing heterostructure nanomaterials. Previous studies reveal that ZnO-SnO2 heterostructure have received the most attention contributes to the sensing properties of the materials and much success on gas sensors [2-4]. In addition, it has been proven that nanoparticles of noble metals (Pd, Pt, Au, Ag) on the surface of metal oxides can act as sites for adsorbates, catalysts or promoters for surface reactions and as elements improving the thermal stability of 

Corresponding author: S.Y. Ma

Tel.: +86 18609479855 Fax: +86 9317971503

E-mail address: [email protected] (S.Y. Ma)

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the film nanostructure [5-7]. However, the effect of noble metals doping of ZnO-SnO2 on its gas-sensing has been only investigated in a limited number of reports. In this paper, on the basis of those good sensing properties of ZnO-SnO2 and the effects of noble metals Ag, we synthesized Ag-doped ZnO-SnO2 nanofibers and their gas sensing performances were investigated. According to the results, it was clearly seen that the Ag-doped ZnO-SnO2 hollow nanofibers exhibit a high response to low-concentration ethanol at 200°C, which was superior to the synthesized pure ZnO-SnO2 nanofibers.

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Fig.1. (a) XRD patterns; (b) EDX pattern of S3 and (e, f) Nitrogen adsorption–desorption isotherm and pore size distribution plot (inset) of S1 and S3. 2. Experimental All the chemicals were analytical purity and used without further purification. Specimens in this experiment were synthesized as follows, 0.43 g Zn(CH3COO)2·2H2O and 0.37 g SnCl2·2H2O (molar ratio of 1: 1) were dissolved in the 3.2 ml DMF and stirred adequately for 4 h at 25°C. At the same time, 0.36 g poly vinyl pyrrolidine (PVP, Mw = 13,000,000) and a moderate amount of ethanol were mixed together followed by stirring for 3 h at 25°C. Finally, two solutions and certain amount of AgNO3 were mixed and stirred, then loaded into a glass syringe for electrospinning. A voltage of 15 kV was applied. Pure (S1) and

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different amount of Ag-doped ZnO-SnO2 nanofibers S2 (1 wt.%), S3 (3 wt.%) and S4 (5 wt.%) were obtained by calcining the electrospun samples at 600°C for 2 h.

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Fig. 2. The SEM images of S1 (a) and S3 (b); The TEM images (c, d) and HRTEM images (e, f) of S3 nanofibers. The morphology, structure and gas-sensing properties of the two samples were characterized via scanning electron microscopy (SEM, S-4800), X-ray diffraction (XRD, D/Max-2400), transmission

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electron microscopy (TEM, USA FEI Tecnai G2 TF20) and WS-60A gas-sensing measurement device (Weisheng Electronics Science and Technology Co. Ltd. China). The response (R) was defined as Ra/Rg, where Ra and Rg were the resistance of the sensor in air and in test gas, respectively [8]. 3. Results and discussion Fig. 1(a) shows the XRD patterns of S1and S3. All diffraction peaks can be perfectly indexed as the tetragonal rutile structure for SnO2 (JCPDS 41-1445) and the hexagonal wurtzite structure (JCPDS 36-1451) for ZnO. The XRD pattern of S3 composites exhibits three additional peaks at 38.2º, 44.1º and 64.9º, which are readily assigned to the (111), (200) and (220) planes of face center cubic (fcc) structure of silver (JCPDS File No.04-783), respectively. No other peaks are observed, which confirms samples with relatively high crystal purity. In addition, Fig. 1(b) shows the corresponding EDX of S3. It can be observed that the as-synthesized product is consisted of Ag, Zn, Sn and O elements. As can be viewed, Ag ions have been successfully doped into samples, and the peaks of C and Cu are caused by the detection instrument and environment. Furthermore, as shown in Fig. 1(c) and Fig. 1(d), the BJH Adsorption average pore width of S1 and S3 are 18.7 and 25.6 nm. Their BET surface areas are calculated to be 33.58 and 38.12 m2/g, respectively. Fig. 2(a) and (b) show SEM images of S1 and S3. It can be seen that the surface of samples is rough and obvious porous structure. It should be noted that compared with two samples, the porous of S3 is more than S1. The increase in pore structure may be caused by the dopants of materials that can induce some strain and defects in the lattice, further impedes the grain growth (The calculation confirms that the reduction of the grain size with doping) and the surface energy of the sample decrease [9, 10], which makes the reunite phenomena of the particle decreased and formation of pore structure. Fig. 2(c) and Fig. 2(d) show the hollow feature of S3 at low magnification and high magnification, respectively, which exhibits the hollow nanofibers with uniform diameter and thin layer, the diameter is 247.6 nm and the thickness of the tube wall is 24.3 nm. In addition, it can also be observed clearly that the sample is porous structure by TEM which were consistent with the SEM characterization. This rough and porous structure indicates a large surface active region which holds advantages for gas-sensing properties [12]. Meanwhile, high-resolution images (HRTEM) are shown in Fig. 2(e) and (f) with detailed lattice structure. The interplanar distances are 2.38Å, 2.82 and 2.46 Å, 3.43 and 2.64 Å, corresponding to the (111) planes of silver, the (100) and (101)

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planes of hexagonal wurtzite ZnO, the (110) and (101) planes of rutile SnO2, respectively. The result is well matched with the XRD analysis.

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Fig. 3. (a) Four sensors response to 50 ppm ethanol at different testing temperatures (100-300°C), (b) sensors responses to different concentrations of ethanol at 200°C (the inset shows the calibration curve in the range of 1-100 ppm), (c) the response/recovery curves of S1 and S3 to ethanol at 200°C (the inset shows the dynamic sensing transient to 1 ppm), (d) the response of S1 and S3 to 50 ppm different of gases at 200°C. The responses of sensors based on pure and Ag-doped nanofibers versus operating temperatures are tested, and it found that the optimum detecting gas is ethanol. In order to determine the optimum operating temperature, the responses of sensors based on all samples to 50 ppm ethanol at different operating temperatures all samples are tested as shown in Fig. 3(a). It is clearly observed that the responses of four sensors increasing as the temperature increases firstly, and reach to their maximum value at 200°C, then

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decrease gradually with a further increasing of temperature, that is to say the optimal operating temperature is 200°C. The responses of four sensors for different concentrations of ethanol (1–6000 ppm) at 200°C are shown by Fig. 3(b), the response increases rapidly with the increase of ethanol concentration. Above 2000 ppm, the response increases slowly with the concentration further increasing, due to the sensor being saturated. Obviously, the gas response reaches almost linearly when the gas concentration ranges from 1 to 200 ppm (inset of Fig. 3(b)), which reveals the sensors are well suited for low concentration detecting. Fig. 3 (c) depicts the response of the two samples to ethanol at 200°C. The responses of S1 and S3 sensors are 1.5 and 7.6, 7.4 and 20.9, 14.6 and 33.2, 39.2 and 72.3, 81.4 and 128.6 for 1, 10, 20, 50 and 100 ppm, respectively. It is noteworthy that when the concentration is as low as 1 ppm (inset of Fig. 3(c)), the sensor made of this material can still deliver a high response to be 7.6 for ethanol and the response and recovery time are about 5 s and 5 s, respectively, which can meet the practical application. Moreover, selectivity is an important parameter of sensors. Fig. 3(d) shows the response of sensors to 50 ppm various gases at 200°C: the response value to ethanol is about 2.5 times higher than acetone (about 28.8), and 33 times higher than benzene (about 2.2), which indicates that S3 exhibits good selectivity for ethanol. All these results further prove that S3 is more appropriate for using as ethanol sensing materials than S1. Moreover, a comparison of gas sensing properties between our work and previous literatures [2-4, 11] is summarized in Table 1.

Table 1. Comparison of gas-sensing performance of various ZnO and SnO2 materials based gas sensors towards ethanol.

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4. Conclusions In conclusion, pure and 3 wt.% Ag-doped ZnO-SnO2 hollow nanofibers have been successfully synthesized by the electrospinning. XRD patterns and HRTEM images indicate that Ag have been successfully doped into ZnO-SnO2. When used to detect ethanol, the sensor based on Ag-doped ZnO-SnO2 exhibited good gas sensing performance at 200°C to 1 ppm ethanol, and the response and recovery time are about 5 and 5 s, respectively. It is believed that the sensor based on Ag-doped ZnO-SnO2 hollow nanofibers is a promising candidate for the efficient detection to ethanol in the environment. Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant Nos. 10874140, 51562035 and 51462031), and the Foundations of Northwest Normal University (Grant Nos. NWNU-LKQN-12-11 and 13-18).

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Highlights: 

Pure and Ag doped ZnO/SnO2 hollow nanofibers are synthesized by electrospinning method.



Ag-doping ZnO/SnO2 nanofibers improved the gas sensing properties.



Ag-doped ZnO/SnO2 sensor exhibits high response and good selectivity to low-concentration ethanol (1 ppm) at 200 °C.

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