Materials Letters 63 (2009) 917–919

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Materials Letters

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Synthesis and ethanol sensing properties of Fe-doped SnO2 nanofibers

Zhixue Wang, Lei Liu ⁎College of Computer Science and Technology, Jilin University, Changchun 130012, PR China

⁎ Corresponding author. Tel./fax: +86 431 85159373.E-mail address: liuleijlu2008@yahoo.com.cn (L. Liu).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.matlet.2009.01.051

a b s t r a c t

a r t i c l e i n f o

Article history:

Fe-doped SnO2 nanofibers Received 28 September 2008Accepted 7 January 2009Available online 24 January 2009

Keywords:SnO2

EthanolSemiconductorsNanomaterialsSensors

are synthesized through an electrospinning method and characterized byscanning electron microscopy and transmission electron microscopy. The sensor fabricated from thesenanofibers exhibits high sensitivity and rapid response/recovery to ethanol at 300 °C. The sensitivity is up to15.3 when the sensor is exposed to 100 ppm ethanol, and the response and recovery time is about 1 and 3 s,respectively. The linear dependence of the sensitivity on the ethanol concentration is observed in the range of10–300 ppm. These results demonstrate that Fe-doped SnO2 nanofibers can be used as the sensing materialfor fabricating high performance ethanol sensors.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Design and fabrication of chemical sensors has become one of themost active research fields due to their wide applications in manyfields, such as industrial production, process control, environmentalmonitoring, healthcare, defense and security [1–4]. Since the phe-nomenon that the charge-carrier concentration on the surface of asemiconductor metal oxide changes in accordance with the composi-tion of the surrounding atmosphere is discovered nearly half a centuryago [5]. Considerable research has been carried out on the develop-ment of chemical sensors based on semiconductor metal oxides suchas SnO2, ZnO, and TiO2 [6]. Recently, inspired by the advantages ofsmall size, high density of surface sites and increased surface-to-volume ratios, synthesis of these semiconductor metal oxides withone-dimensional (1D) nanostructures and exploration of their pro-perties are of current interest [7]. Up to now, many 1D nanostructuresincluding nanorods, nanotubes, nanofibers, nanowires, and nanobeltshave been successfully developed and applied in chemical sensors [8].Their excellent sensing performances are based on the nanometerdiameter that may great influence on their chemical and physicalcharacteristics [9].

Herein, we present a simple and effective route for the synthesis ofFe-doped SnO2 nanofibers with excellent ethanol sensing properties.The sensor fabricated from these nanofibers exhibits high sensitivityto ethanol, and the response and recovery time is about 1 and 3 s,respectively. These high sensing performances are based on the 1Dstructure of the nanofibers combining with the improvement broughtby Fe doping.

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2. Experimental

2.1. Preparation of materials

In a typical procedure, 0.4 g of SnCl2·2H2Owasmixedwith 4.42 g N,N-dimethylformamide (DMF) and 4.42 g of ethanol under vigorousstirring for 30 min. Subsequently, this solution was added to 0.8 g ofpoly (vinyl pyrrolidone) (PVP) and 0.035 g of FeCl3·9H2O undervigorous stirring for 1 h. Then, the mixture was loaded into a glasssyringe and connected to high-voltage power supply. 10 kV wasapplied between the cathode (a flat aluminum foil) and anode(syringe) at a distance of 20 cm. The complete removal of PVP in theas-spun nanofibers was achieved by calcining at 500 °C for 4 h in air.

2.2. Measurement

The above calcined sample was mixed with deionized water in aweight ratio of 100:30 to form a paste. The paste was coated on aceramic tube onwhich a pair of gold electrodeswas previously printed,and then a Ni–Cr heating wire was inserted in the tube to form a side-heated gas sensor.

Gas sensing properties of the as-fabricated sensor were measuredusing a static test system. Special application software was developedto collect the data and govern the whole system by computers auto-matically [10]. Saturated target vapor was injected into a test chamber(about 1 L in volume) by a syringe through a rubber plug. The sensorsensitivity was measured by comparing the resistance of the sensor inair (Ra) with that in target gas (Rg) by using a SQC-100 test system(Ningbo, China) at 300 °C. The response timewas specified as the timeto rise to 90% of the equilibrium value of sensor resistance after targetvaporwas injected. The recovery timewas defined as the time to fall to10% of the final resistance value after the removal of target vapor. The

Fig. 1. (a) SEM and (b) TEM images of the Fe-doped SnO2 nanofibers.

918 Z. Wang, L. Liu / Materials Letters 63 (2009) 917–919

laboratory atmosphere was maintained at 25% relative humidity by anautomatic drier.

Scanning electron microscopy (SEM) images were recorded on aSHIMADZU SSX-550 (Japan) instrument. Transmission electronmicroscopy (TEM) images were obtained on a HITACHI S-570microscope with an accelerating voltage of 200 kV.

3. Results and discussion

Fig.1(a) shows the SEM image of the as-synthesized Fe-doped SnO2 nanofibers. Theproduct is highly dominated by the nanofibers with lengths of several ten micrometersand diameters ranging from 60 to 150 nm. The average diameter of the nanofibers isabout 90 nm. Feature of the nanofibers was also examined by TEM, the result in Fig.1(b)shows a typical characteristic of the nanofibers, which agrees with the SEM results.

Here,we focus our investigations on the ethanol sensing properties of the as-synthesizedFe-doped SnO2 nanofibers. The insert of Fig. 2 shows a schematic image of the sensorstructures. The Fe-doped SnO2 nanofibers exhibit high sensitivity and rapid response/recoverycharacteristics to ethanol at 300 °C, as shown in Fig. 2. The sensitivity is about 2.8, 7.7,15.3, and 28.7 to 10, 50, 100, and 200 ppm ethanol, respectively. These results reveal that thesensor can detect ethanol of concentrations down to 10 ppm. Furthermore, it can also be seenthat the electrical signal from the sensor becomes stable within 1 s after it is exposed toethanol, and returns to the original valueswithin 3 s after the tested vapor is replacedwith air.The rapid response and recovery of the sensor is based on the story that 1D structures of ourproducts can facilitate fast mass transfer of the analytemolecules to and from the interactionregion and also require charge carriers to traverse the barriers introduced by molecularrecognition along the nanofibers [7]. Simultaneously, comparingwith 2Dnanoscalefilms, theinterfacial areas between the active sensing region of the nanofibers and the underlying

Fig. 2. Response and recovery characteristics of the Fe-doped SnO2 nanofibers toethanol at 300 °C. The left insert shows a schematic image of sensor structure, in whicha Ni–Cr heater inside a small ceramic tube is used to control the operating temperature.

substrate is greatly reduced. Those advantages lead to significant gain in the sensingperformances of our nanofibers [7].

The sensitivity of the Fe-doped SnO2 nanofibers versus ethanol concentration isshown in Fig. 3. The sensitivity rapidly increases with increasing ethanol concentrationbelow 300 ppm. Above 300 ppm, the sensitivity slowly increases with the ethanolconcentration, indicating the sensor becomes more or less saturated. Finally thesensitivity reaches saturation at about 20000 ppm. Moreover, the insert in Fig. 3 showsthe linear calibration curve in the range of 10–300 ppm, which further confirms that theFe-doped SnO2 nanofibers can be used as a promising material for ethanol sensors. Infact, the sensitivity of a semiconductor metal oxide is usually depicted as S=A[C]N+B,where A and B are constants and [C] is the concentration of the target gas or vapor. Nusually has a value between 0.5 and 1.0, depending on the change of the surface speciesand the stoichiometry of the elementary reactions on the surface [11]. As show in theinsert of Fig. 3, a linear relationship between sensitivity and the ethanol concentrationcan be observed, indicating that N=1.0 for the Fe-doped SnO2 nanofibers.

Most of the semiconductormetal oxide sensingmaterials operate on the basis of themodification of the electrical properties of an active element, which is brought about bythe adsorption of an analyte on the surface of the sensor [5]. Normally, the O2molecules,which are chemisorbed and dissociated on the surface of semiconductor metal oxides,can generate oxygen species. These oxygen species lead to a decrease in the conductanceof the sensing layer, resulting in a high resistance of the sensor. When the sensor isexposed to a reducing gas such as ethanol, the reducing gasmay reactwith the adsorbedoxygen molecule and increase the conductance of the sensing layer, thereby the sensorresponse can easily be found by comparing the resistance of the sensing layer in air andthe target gas. The effect of Fe doping on the gas sensing performances of SnO2 can beexplained by the defect chemistry model of acceptor-doped SnO2 suggested by Fukuiand Nakane [12]. Fe3+ ions act as an acceptor to SnO2, leading to an increase of filmresistivity, which eventually improve the sensing properties of our product. Addition-ally, taking the large surface to volume ratio, effective electron transport, and greatly

Fig. 3. Sensitivity of the Fe-doped SnO2 nanofibers versus ethanol concentration, the rightinsert is the calibration curve in the range of 10–300 ppm.

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reduced interfacial areas between the active sensing regions of the nanofibers, excellentethanol sensing properties can be also found.

4. Conclusions

In summary, Fe-doped SnO2 nanofibers with an average diameter of90 nm are synthesized by an electrospinning method, and their ethanolsensing properties are also investigated by exposing the correspondingsensor todifferent concentrationsof ethanol at300 °C.Highsensitivityandrapid response/recovery are observed in our investigations, suggestingthat Fe-doped SnO2 nanofibers are good candidates for fabricating highperformance gas sensors.

Acknowledgement

The authors acknowledge Dr Dianwei Qian (Chinese Academy ofSciences) for generous help on sensor measurement. This work was

financially supported by the National Natural Science Foundationof China (Grant No. 60873044/F0202) and Research Fund for theDoctoral Program of Higher Education (Grant No. 20060183044).

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