In2O3 hetero-nanofibers by a modified double jets electrospinning process

Sensors and Actuators B 166–167 (2012) 746–752

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Formaldehyde gas sensor based on SnO2 /In2 O3 hetero-nanofibers by a modified double jets electrospinning process Haiying Du a,b , Jing Wang a,∗ , Meiying Su a , Pengjun Yao a,c , Yangong Zheng a , Naisen Yu d a

School of Electronic Science and Technology, Dalian University of Technology, Dalian 116023, China Department of Electromechanical Engineering & Information, Dalian Nationalities University, Dalian 116600, China School of Educational Technology, Shenyang Normal University, Shenyang 110034, China d School of Science, Dalian Nationalities University, Dalian 116600, China b c

a r t i c l e

i n f o

Article history: Received 26 October 2011 Received in revised form 20 March 2012 Accepted 20 March 2012 Available online 28 March 2012 Keywords: Gas sensor Hetero-nanofibers Electrospinning Double jets

a b s t r a c t SnO2 /In2 O3 hetero-nanofibers composite was synthesized by using a modified electrospinning system with double jets of positive and negative polarity electric fields. The SnO2 /In2 O3 hetero-nanofibers with a netted structure composed of SnO2 and In2 O3 nanofibers were characterized by using X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM). Both SnO2 and In2 O3 nanofibers were hierarchical structures with many nanocrystallites. The SnO2 and In2 O3 showed very different nanocrystallites sizes in the SnO2 /In2 O3 hetero-nanofibers composite. A gas sensor was fabricated based on SnO2 /In2 O3 hetero-nanofibers composite. The operating temperature of the gas sensor was 375 ◦ C. The response value of the gas sensor based on SnO2 /In2 O3 hetero-nanofibers was higher than the ones of SnO2 nanofibers and In2 O3 nanofibers sensors, respectively, in formaldehyde concentration range of 0.5–50 ppm. Cross-responses of SnO2 /In2 O3 hetero-nanofibers sensor to formaldehyde, ethanol, ammonia, acetone, toluene and methanol were tested. The response value of the SnO2 /In2 O3 hetero-nanofibers sensor decreased when the relative humidity increased. The sensing mechanism of the SnO2 /In2 O3 hetero-nanofibers gas sensor was briefly analyzed. © 2012 Elsevier B.V. All rights reserved.

1. Introduction With the development of nanotechnology, the controllable synthesis of one-dimensional (1D) nanomaterials (including nanotubes, nanowires, nanofibers, and nanobelts) has attracted more attention due to the fact that the physical and chemical properties of nanomaterials by their dimensionality, size and specific surface [1,2]. Different nanostructures and morphology of one-dimensional nanomaterials have amazing characteristics, which has significant potential applications, such as optics [3], electronics [4], and gas sensors [5–7]. For example, semiconducting oxides nanobelts of tin oxide were obtained by vapor phase deposition as conductometric gas sensors [4]. The gas sensor based on Pd-doped ZnO nanofibers synthesized by electrospinning method exhibited a considerable sensitivity to CO gas in a concentration range of 1–20 ppm with a good selectivity [5]. The sensors based on electrospun Pd-doped TiO2 nanofibers showed promising gas sensing characteristics, such as low operation temperature (180 ◦ C) and sufficient gas response to NO2 [6]. Polyaniline nanotubes were synthesized by using Mn2 O3 nanofibers as oxidant template, and the gas sensor made of the

∗ Corresponding author. Tel.: +86 411 84708382; fax: +86 411 84706706. E-mail address: [email protected] (J. Wang). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.03.055

polyaniline nanotube could detect as low as 0.025 ppm NH3 at room temperature with good reversibility [7]. Electrospinning technique is a simple and easy way to fabricate nanofibers, and the nanofibers produced by this method have shown some excellent characteristics [8,9]. In the past years, electrospinning technique was mainly applied in a field of synthesis of pure organic polymer [10]. Recently, this technique has been extended to synthesis of inorganic materials, such as SnO2 [11], In2 O3 [12], TiO2 [13], WO3 [14], ZnO [15], Co3 O4 [16], BaTiO3 [17] and so on. Furthermore, electrospinning was proven to be a highly successful technique for controlling the synthesis of 1D morphology nanostructures including nanofibers [18,19], nanotubes [20], nanoribbons [21], and others novel structures [22]. Nanofibers mentioned above were synthesized based on single jet electrospinning technology. A special aligned multiple jets setup was designed to get high liquid throughput requirements and uniform fibers. The needles were disposed as a regular hexagon with an iron ring encircled. Different heights of the seven needles were used to receive uniform fibers [23]. The blend biodegradable nanofibrous nonwoven mats was fabricated via multi-jet electrospinning system [24]. A multi-jet with multi-syringe was connected a homopolar power, and one kind of electrospinning solution was injected into several syringes to increase both productivity and cover area [25]. A problem in above multi-jet electrospinning system is that the nanofibers

H. Du et al. / Sensors and Actuators B 166–167 (2012) 746–752

sprayed from different jets are separated by the repulsion of the jets with homopolar electric field [26]. In this report, a bipolar electrospinning setup of double jets with opposite electric field was designed, and SnO2 /In2 O3 heteronanofibers composite with a netted structure was prepared. A formaldehyde gas sensor based on SnO2 /In2 O3 hetero-nanofibers was fabricated, and the sensor showed favorable gas sensing properties. 2. Experimental 2.1. Preparation and characterization of SnO2 /In2 O3 hetero-nanofibers SnO2 /In2 O3 hetero-nanofibers were prepared by using a modified electrospinning system. Two syringes with jets containing different solutions were put on opposite electric field. Fig. 1 shows a sketch of the modified double jets electrospinning system. In this modified electrospinning system, two jets were connected to a positive polarity and a negative polarity, respectively, and collector is zero potential reference (Fig. 1). Under electric field force, Lorentz force and gravity of solutions, two kinds of nanofibers SnO2 and In2 O3 , respectively, were ejected from their nozzles and interlaced together at the same time. After a heat treatment, a netted structure SnO2 /In2 O3 hetero-nanofibers composite was obtained. Poly (vinyl pyrrolidone) (PVP, Mw = 1,300,000) was purchased from Aldrich, USA. Indium nitrate (In(NO3 )3 ·4.5H2 O), N,N-dimethylformamide (DMF) and ethanol (EtOH) (99.0%) were obtained from Sinopharm Chemical Reagent Co., Ltd., China. Stannous chloride (SnCl2 ·2H2 O) was obtained from Tianjin Kermel Chemical Company, China. The above chemical reagents were analytical grade and used without further purification. Two kinds of precursor solutions for synthesis of hetronanofibers were prepared. In2 O3 spinning solution was prepared as follows: 0.4 g In(NO3 )3 ·4.5H2 O were added into 4 ml ethanol under vigorous stirring for 30 min, and then 0.6 g PVP and 3 ml DMF were added into the as-prepared In(NO3 )3 solution in order to fully dissolve in the DMF/ethanol solution. The mixture of In2 O3 solution was stirred for 8 h at room temperature to attain sufficient viscosity required for electrospinning. While the spinning solution of SnO2 nanofibers was made by dissolving the mixture of 0.6 g SnCl2 and 4 ml ethanol in the mixture of 0.6 g PVP and 3 ml DMF under vigorous stirring for 8 h at room temperature. The two kinds of spinning solutions were loaded into the syringe A and syringe B (Fig. 1), respectively. Two D.C. high voltages applied

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at jet A and jet B were +9 kV and −11 kV, respectively. The distances between one jet and the collector, and between two jets were all 5 cm. The SnO2 and In2 O3 nanofibers were ejected from jets A and B simultaneously to form a wet netted polymer of SnO2 /In2 O3 hetero-nanofibers composite. The as-synthesized net of the heteronanofibers composite was heated at 600 ◦ C for 2 h in air. The PVP and water in the polymer composite volatilized during heating process, and finally, the netted SnO2 /In2 O3 hetero-nanofibers composite were obtained. For comparison, SnO2 and In2 O3 nanofibers were prepared by using single jet electrospinning system, respectively. The heating temperatures for both kinds of nanofibers were all 600 ◦ C. The structures of the nanofibers were characterized by an Xray diffraction instrument (XRD: D/Max 2400, Rigaku, Japan) in 2 region of 20–80◦ with Cu K␣1 radiation. The morphology images of nanofibers were obtained by using field emission scanning electron microscope (FE-SEM: Hitachi S-4800, Japan). 2.2. Sensors fabrication and measurements As-prepared SnO2 /In2 O3 hetero-nanofibers were mixed with deionized water to form a paste. The paste was coated onto a ceramic tube with a pair of gold electrodes to form a sensing film (250–300 ␮m in thickness) and dried at 100 ◦ C for 2 h, and subsequently annealed at 500 ◦ C for 2 h in air. Finally, a Ni–Cr heating wire was inserted into the ceramic tube to form an inside-heated gas sensor. The gas-sensing properties of the SnO2 /In2 O3 hetero-nanofibers sensor were measured using a static state gas-sensing test system. In the gas response measurement, a given amount of target gas was injected into a test chamber (50 L in volume) by a syringe through a rubber plug. For a required concentration, the volume of the injected gas (V) can be calculated as follows: V=

50 × C v%

(1)

where C is the concentration of the target gas (ppm), and v% is the volume fraction of bottled gas. Sensors were exposed to the atmospheric air by opening the chamber after the measurement. The export voltage on the sensor was measured by using a voltage division circuit on which the voltage was 10 V, and the sensor was connected in series with an external resistor (RL ). The gas response value (S) was defined as a ratio of the electrical resistance of the sensor in air (Ra ) to that in target gas (Rg ): S = Ra /Rg , and Ra = RL (10 − Vair )/Vair , Rg = RL (10 − Vgas )/Vgas , where Vair was the export voltage of RL in air, and Vgas was the voltage in target gas. 3. Results and discussions 3.1. Materials characterization

Fig. 1. Sketch of double jets electrospinning with opposite polarities.

The XRD patterns of the SnO2 nanofibers, In2 O3 nanofibers, and SnO2 /In2 O3 hetero-nanofibers are shown in Fig. 2 curves (a)–(c), respectively. We can see from curves (a) and (b) of Fig. 2 that the SnO2 nanofibers belong to tetragonal phase (square symbol) and the In2 O3 nanofibers belong to cubic phase (circle symbol). Fig. 2 curve (c) indicates that SnO2 (square symbol) and In2 O3 (circle symbol) exist simultaneously in the SnO2 /In2 O3 heteronanofibers composite. The average crystallite sizes are 22.6 nm for the SnO2 nanofibers (from curve (a)) and 37.3 nm for the In2 O3 nanofibers (from curve (b)), respectively, calculated by using Scherrer equation. Meanwhile, in the SnO2 /In2 O3 hetero-nanofibers composite, the average crystallite sizes are 21.2 nm and 40.2 nm for the SnO2 and the In2 O3 (from curve (c)), respectively. These results show that under the same synthesis conditions, the crystallite sizes in SnO2 nanofibers become smaller and the crystallite

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sizes in In2 O3 nanofibers become bigger in the SnO2 /In2 O3 heteronanofibers composite by double jets electrospinning process. These phenomena may be affected by two direction electric fields. During the double jets electrospinning process, the electric field force of SnO2 solution with positive voltage was relatively strong, causing the solution to overcome the surface tension and stretching into nanofibers with smaller size of the crystallites. While the electric field force of the negative voltage was relatively weak, and it might not sufficient enough to stretch the same size of the crystallites as in the positive side [27].

Fig. 3(a)–(c) gives the SEM images of the as-synthesized SnO2 nanofibers, In2 O3 nanofibers and SnO2 /In2 O3 hetero-nanofibers, respectively. We can see from Fig. 3 that all nanofibers are composed of small nanocrystallites, indicating their hierarchical structures. Fig. 3(a) shows that the SnO2 nanofibers are relatively uniform with diameters of 200–250 nm, and the diameters of the nanocrystallites in this fiber are around 20 nm. Fig. 3(b) shows that the surfaces of the In2 O3 nanofibers are relatively irregular and slightly rough. The diameters of In2 O3 nanofibers are about 100–150 nm, which are composed of many nanocrystallites with 40–50 nm in diameters. We can obviously see from Fig. 3(c) that two kinds of nanofibers SnO2 and In2 O3 exist simultaneously in the SnO2 /In2 O3 composite, presenting a hetero-nanofibers composite system. Compare Fig. 3(a)–(c), we can see that the sizes of SnO2 nanocrystallites are smaller in the SnO2 /In2 O3 hetero-nanofibers than in the SnO2 nanofibers, while the sizes of In2 O3 nanocrystallites are bigger in SnO2 /In2 O3 hetero-nanofibers than in In2 O3 nanofibers. The average sizes of the nanocrystallites are similar to the ones calculated from XRD data. It is obviously that two kinds of different nanofibers are interlaced mutually to form a netted structure, and many porous and channels appear in this composite material. Fig. 3(d) shows a SEM image of the SnO2 /In2 O3 heteronanofibers in a high magnification for observing clearly. 3.2. Gas sensing properties The operating temperature strongly influences response property of a semiconductor gas sensor. Fig. 4 gives the responses of the gas sensors based on SnO2 nanofibers, In2 O3 nanofibers and SnO2 /In2 O3 hetero-nanofibers versus operating temperature, respectively, and the formaldehyde concentration is 10 ppm and

Fig. 3. SEM images of (a) SnO2 , (b) In2 O3 , (c) SnO2 /In2 O3 fibers, and (d) SnO2 /In2 O3 hetero-nanofibers in a high magnification.

H. Du et al. / Sensors and Actuators B 166–167 (2012) 746–752

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Fig. 4. Responses of SnO2 , In2 O3 and SnO2 /In2 O3 sensors to 10 ppm formaldehyde as a function of operating temperature.

Fig. 6. Response values of the gas sensors based on the SnO2 , In2 O3 and SnO2 /In2 O3 vs formaldehyde concentration.

relative humidity is 40%RH. We can see that the maximum response to formaldehyde is 7.5 at 375 ◦ C for the sensor of SnO2 /In2 O3 hetero-nanofibers. For the sensors fabricated with SnO2 and In2 O3 nanofibers, the maximum response values are 4.5 at 300 ◦ C and 4.4 at 375 ◦ C, respectively. Therefore, 375 ◦ C is chosen to be an operating temperature for further examine the properties of the SnO2 /In2 O3 hetero-nanofibers gas sensor. We can also see that the gas sensor fabricated using SnO2 /In2 O3 hetero-nanofibers represents higher response than the sensors based on SnO2 nanofibers and In2 O3 nanofibers, respectively. Fig. 5 shows the response and recover transient properties of the SnO2 nanofibers, In2 O3 nanofibers and SnO2 /In2 O3 hetero-nanofibers gas sensors, respectively, to formaldehyde in concentration range of 0.5–50 ppm at an operating temperature 375 ◦ C with relative humidity of 40% RH. We can see from the figure that the nine response cycles are successively recorded, and the gas sensors base on three kinds of nanofibers all show good sensitivities to formaldehyde. The sensor of SnO2 /In2 O3 hetero-nanofibers composite shows the highest response value (Ra /Rg ) to formaldehyde among these three kinds of sensors. The responses values of the sensors made of SnO2 nanofibers, In2 O3 nanofibers and SnO2 /In2 O3 hetero-nanofibers at operating

temperature 375 ◦ C vs formaldehyde concentration in humidity of 40% RH, respectively, are illustrated in Fig. 6. The response curves of the sensors are basically linearity when the formaldehyde concentration is in a range of 0.5–50 ppm. We can see from the figure that the response of the SnO2 /In2 O3 hetero-nanofibers sensor is much higher than the ones of the SnO2 and In2 O3 sensors alone. The response of the SnO2 /In2 O3 hetero-nanofibers sensor to 50 ppm formaldehyde is 18.9. The lowest concentration of formaldehyde detected by the sensor of SnO2 /In2 O3 hetero-nanofibers is 0.5 ppm with a response of 2.2. The cross-responses of SnO2 /In2 O3 sensor to the concentration range of 0.5–50 ppm formaldehyde, ethanol, ammonia, acetone, toluene and methanol at an operating temperature 375 ◦ C are demonstrated in Fig. 7, respectively. The selectivity property of the sensor indicates that SnO2 /In2 O3 hetero-nanofibers got higher response to formaldehyde than to ethanol and acetone vapors, and hardly sensitive to toluene, ammonia and methanol in a concentration range of 0.5–50 ppm. The sensing properties of the hetero-nanofibers sensor were influenced by relative humidity (RH). Fig. 8 gives the transient

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Fig. 8. Transient properties of SnO2 /In2 O3 hetero-nanofibers gas sensor to 10 ppm formaldehyde in different relative humidities at an operating temperature 375 ◦ C.

properties of the gas sensor based on SnO2 /In2 O3 hetero-nanofibers to 10 ppm formaldehyde in humidities of 40% RH, 50% RH and 60% RH at an operating temperature 375 ◦ C, respectively. Table 1 gives the resistances and the responses of the SnO2 /In2 O3 heteronanofibers sensor at different relative humidities with an operating temperature 375 ◦ C. We can see from the Table 1 that both the resistances Ra and Rg decrease with the rise of relative humidity, and the response values of the gas sensor decreases from 5.1 to 4.3 and 3.7 with the rise of RH from 40% to 50% and 60%, respectively. We try to explain Fig. 8 and Table 1 as follows: when the sensing materials are in air, the O2 could chemisorb on the surface of the materials and reaction with electrons in n-type materials (SnO2 and In2 O3 ): 1 O2 + e → (O− 2 )ads 2

(2)

−  (O− 2 )ads + e → 2(O )ads

(3)

When n-type material meets reducing formaldehyde gas (CHOH), there might happen the following reaction [28]: → CHOOH(g) + e CHOH(g) + O− ads

(4)

The reactions produce electrons, and the resistance of the sensors in formaldehyde vapor decreases. As relative humidity rises, more water molecular on the surface of the sensing material decomposes to H3 O+ and H+ ions and the resistance of the material in air (Ra in Table 1) decrease. When the target gas appears in testing chamber, the adsorbed oxygen atoms/ions enhance the water chemisorption on the surfaces of sensing material [29]. The water molecular decomposes and then more ions appear, which make the resistance of the sensor in target gas decrease (Rg in Table 1). On the other hand, more and more water vapor adsorbs on the surface of the composite nanofibers as relative humidity Table 1 Resistance and response of the SnO2 /In2 O3 sensor at different relative humidities with an operating temperature 375 ◦ C. RH (%) 40 50 60

Resistance in air (Ra ) 165 k 132.3 k 116.5 k

Resistance in formaldehyde (Rg )

Response (Ra /Rg )

32.4 k 31.1 k 30.8 k

5.1 4.3 3.7

increasing, the potential barrier height at surface of sensing material may increase by water vapor adsorption when reducing vapor exists around sensing material, resulting a decrease of water adsorption capacity of sensing material [30,31]. In the SnO2 /In2 O3 hetero-nanofibers system, increasing humidity restricts the adsorption of water molecular on the surface of the nanofibers when reducing vapor formaldehyde exists. As the analysis above, with the relative humidity rising, both resistances of Ra and Rg decrease. But the extent of decrease of the Ra is larger than the Rg . As a result, the response values of the gas sensor (Ra /Rg ) decrease as rise of relative humidity.

3.3. The gas sensing mechanism of the SnO2 /In2 O3 hetero-nanofibers The experiments show that the gas sensors made of SnO2 nanofibers, In2 O3 nanofibers and SnO2 /In2 O3 hetero-nanofibers are all sensitive to formaldehyde. And the response value of SnO2 /In2 O3 hetero-nanofibers sensor is higher than those of both SnO2 and In2 O3 nanofibers sensors. That means the adsorption capability of SnO2 /In2 O3 hetero-nanofibers to formaldehyde was greatly enhanced. The reason maybe as follows: gas sensing properties could be improved by adjusting nanocrystallite size and the porosity of sensing materials [32]. There are more channels among nanofibers in SnO2 /In2 O3 hetero-nanofibers system because two kinds of fibers with different nanocrystallite sizes exist in one material. Which provide more opportunity for inner sensing material to contact with formaldehyde, and then, the response value of SnO2 /In2 O3 hetero-nanofibers gas sensor increases. Secondly, two kinds of boundaries exist in SnO2 /In2 O3 hetero-nanofibers composite: the boundaries between homogeneous nanocrystallites of SnO2 and/or In2 O3 , respectively, and the boundary between two kinds of nanofibers. The boundary between SnO2 and In2 O3 nanofibers is an n–n homotype hetero-junction in SnO2 /In2 O3 composite material. The nanocrystallites boundary barrier of a hetero-junction may decreases when the composite materials meet formaldehyde, and more electrons transfer from the gas to the sensing material, and so the response of the sensor increases [33–35]. The sensing mechanism of hetero-nanofibers sensing material needs further investigations.

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4. Conclusion SnO2 /In2 O3 hetero-nanofibers were synthesized by using a modified electrospinning system with double jets of positive and negative polarity electric fields. The nanofibers SnO2 and nanofibers In2 O3 existed simultaneously in SnO2 /In2 O3 hetero-nanofibers composite. Every nanofiber was composed of small nanocrystallites. Gas sensors were fabricated based on SnO2 nanofibers, In2 O3 nanofibers, and SnO2 /In2 O3 hetero-nanofibers, respectively. The gas sensing properties of the sensors were tested with a formaldehyde concentration range of 0.5–50 ppm at operating temperature 375 ◦ C. The gas sensor based on SnO2 /In2 O3 heteronanofibers exhibited high response value to formaldehyde. The response values were 18.9 and 2.2 for formaldehyde concentrations of 50 ppm and 0.5 ppm, respectively. The response value of SnO2 /In2 O3 hetero-nanofibers sensor was higher than the ones of SnO2 nanofibers and In2 O3 nanofibers sensors, respectively. The SnO2 /In2 O3 hetero-nanofibers sensor show good selectivity to formaldehyde in the interfere gases of ethanol, ammonia, acetone, toluene and methanol. With the rise of the relative humidity, the response of the gas sensor decreased. The gas sensing mechanism of the SnO2 /In2 O3 hetero-nanofibers sensor is analyzed. More channels among nanofibers in SnO2 /In2 O3 system due to two kinds of fibers with different nanocrystallite sizes exist in one material system. The response value of the gas sensor increases by more opportunity for inner sensing materials to contact with formaldehyde. An n–n homotype hetero-junction of SnO2 /In2 O3 may exist in the composite material, and the nanocrystallite boundary barrier of the hetero-junction may decrease when the composite materials meet formaldehyde. More electrons transfer from the gas to the sensing material, and so the response of the sensor increases. Acknowledgments The authors thank The National Natural Science Foundation of China (61176068, 61131004) and 973 Projects (2011CB302105) for financial support. References [1] W.E. Teo, S. Ramakrishna, Electrospun nanofibers as a platform for multifunctional, hierarchically organized nanocomposite, Composites Science and Technology 69 (2009) 1804–1817. [2] X.J. Wu, F. Zhu, C. Mu, Y.Q. Liang, L.F. Xu, Q.W. Chen, R.Z. Chen, D.S. Xu, Electrochemical synthesis and applications of oriented and hierarchically quasi-1D semiconducting nanostructures, Coordination Chemistry Reviews 254 (2010) 1135–1150. [3] G. Nixon Samuel Vijayakumar, S. Devashankar, M. Rathnakumari, P. Sureshkumar, Synthesis of electrospun ZnO/CuO nanocomposite fibers and their dielectric and non-linear optic studies, Journal of Alloys and Compounds 507 (2010) 225–229. [4] E. Comini, G. Faglia, G. Sberveglieri, D. Calestani, L. Zanotti, M. Zha, Tin oxide nanobelts electrical and sensing properties, Sensors and Actuators B 111 (112) (2005) 2–6. [5] S.H. Wei, Y. Yu, M.H. Zhou, CO gas sensing of Pd-doped ZnO nanofibers synthesized by electrospinning method, Materials Letters 64 (2010) 2284–2286. [6] J. Moona, J.A. Park, S.J. Lee, T. Zyung, I.D. Kim, Pd-doped TiO2 nanofiber networks for gas sensor applications, Sensors and Actuators B 149 (2010) 301–305. [7] Y.H. Li, J. Gong, G.H. He, Y.L. Deng, Synthesis of polyaniline nanotubes using Mn2 O3 nanofibers as oxidant and their ammonia sensing properties, Synthetic Metals 161 (2011) 56–61. [8] N. Bhardwaj, S. Kundu, Electrospinning: a fascinating fiber fabrication technique, Biotechnology Advances 28 (2010) 325–347. [9] Z.M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Composites Science and Technology 63 (2003) 2223–2253. [10] Y. Zhang, J.P. Li, G.M. An, X.L. He, Highly porous SnO2 fibers by electrospinning and oxygen plasma etching and its ethanol-sensing properties, Sensors and Actuators B 144 (2010) 43–48. [11] Y. Zheng, J. Wang, P. Yao, Formaldehyde sensing properties of electrospun NiOdoped SnO2 nanofibers, Sensors and Actuators B 156 (2011) 723–730.

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Biographies Haiying Du is a doctoral student in the School of Electronic Science and Technology, Dalian University of Technology, China. She works in the Department of Electromechanical Engineering & Information, Dalian Nationalities University, China. Her current scientific interest is chemical sensors and detection technique. Jing Wang is a professor in the School of Electronic Science and Technology, Dalian University of Technology, China. She received her master degree from the Department of Electronic Engineering, Jilin University, China in 1981. Her current scientific interests are chemical sensors and sensing materials.

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Meiying Su is a doctoral student in the School of Electronic Science and Technology, Dalian University of Technology, China. Her current scientific interest is humidity sensors.

Yangong Zheng is a doctoral student in the School of Electronic Science and Technology, Dalian University of Technology, China. His current scientific interest is chemical sensors.

Pengjun Yao is a doctoral student in the School of Electronic Science and Technology, Dalian University of Technology, China. He works in the School of Educational Technology, Shenyang Normal University, China. His current scientific interest is chemical sensors.

Naisen Yu works in the Department of Electromechanical Engineering & Information, Dalian Nationalities University, China. His current scientific interest is preparation and characterization of the semiconductor light-emitting materials.