Materials Letters 63 (2009) 2041–2043

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Materials Letters

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Improved ethanol sensing properties of Cu-doped SnO2 nanofibers

Li Liu a,b,⁎, Tong Zhang c, Lianyuan Wang a,b, Shouchun Li a,b

a College of Physics, Jilin University, Changchun 130012, PR Chinab National Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR Chinac State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China

⁎ Corresponding author. Tel.: +86 431 8502260.E-mail address: liul99@jlu.edu.cn (L. Liu).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.06.048

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 April 2009Accepted 18 June 2009Available online 25 June 2009

Keywords:Electrical propertiesCeramicsNanomaterialsSemiconductorsSensors

Pure and Cu-doped SnO2 nanofibers are synthesized via a simple electrospinning method, and characterizedby transmission electron microscopy and X-ray diffraction. The sensor fabricated from Cu-doped SnO2

nanofibers exhibits improved sensing properties to ethanol at 300 °C. The sensitivity is up to 3 when thissensor is exposed to 5 ppm ethanol. The response and recovery times are about 1 and 10 s, respectively. Thelinear dependence of the sensitivity on the ethanol concentration is observed in the range of 5–500 ppm.Good selectivity is also observed in our studies. The results make Cu-doped SnO2 nanofibers good candidatesfor fabricating high performance ethanol sensors.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Gas sensors based on metal-oxide semiconductors (MOS) find awide variety of applications in different branches of industry, ecology,medicine, etc [1–4]. Recently, interest in this field has shifted to one-dimensional (1D) MOS nanostructures which are more attractivewhen compared with powder samples due to their high surface-to-volume ratio (a higher surface area provides more sites for analytemolecules adsorption) [5]. Hitherto, promising sensing results ofthese nanostructures have been reported [5]. However, developingnovel 1D nanostructures with high sensitivity and rapid response isstill in great demand.

SnO2 is recognized as one of the key functional MOS owing to itsexcellent chemical and electrical properties. In the sensor field, SnO2 iswidely investigated for solid-state gas sensors of resistor type [6–8].However, wide application of SnO2-based gas sensors is limited by lowsensitivity, slow response, lack of selectivity, and effect of aging.Extensive studies have been put on improving the sensing perfor-mance of SnO2-based gas sensors [6–8]. Doping Cu has been proved tobe an effective route for the sensing improvement of SnO2 to ethanol,H2S, and CO2 [9]. However, the sensing properties of 1D Cu-dopedSnO2 nanostructures have been rarely reported.

In this letter, we report the synthesis and gas sensing properties ofpure and Cu-doped SnO2 nanofibers. The sensor fabricated from Cu-doped SnO2 nanofibers exhibits improved ethanol sensing propertiesat 300 °C. The results demonstrate that Cu-doped SnO2 nanofibers are

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very promising materials for fabricating high performance ethanolsensors.

2. Experimental

All chemicals (analytical grade reagents) were purchased fromTianjin Chemicals Co. Ltd. and used as received without furtherpurification. Cu-doped SnO2 nanofibers were synthesized via a simpleelectrospinning method. Typically, 0.36 g of SnCl2·2H2O was dissolvein a 1:1 weight ratio of N, N-dimethylformamide/ethanol undervigorous stirring for 30 min. Subsequently, this solution was addedto 0.8 g of poly(vinyl pyrrolidone) (PVP) and different weights ofCuCl2·2H2O (0.015, 0.018, and 0.021 g) under vigorous stirring for 1 h.Then, the mixture was loaded into a glass syringe and connected tohigh-voltage power supply. 10 kV was applied between the cathode (aflat aluminum foil) and anode (syringe) at a distance of 20 cm. Theconversion of SnCl2·2H2O to SnO2 and the complete removal of PVP inthe as-spun fibers were achieved by calcining at 500 °C for 4 h in air.All the measurements were carried out on the calcined fibers. Theproducts were designated as Cu-doped SnO2 nanofibers (X), where Xwas theweight of CuCl2·2H2O in the precursor. In our experiment, thevalue of X was 0.015, 0.018, and 0.021 for three Cu-doped SnO2

nanofibers, respectively. Pure SnO2 nanofibers were prepared via thesame method excepting the addition of CuCl2·2H2O.

The component of the sample was determined by an energydispersion X-ray spectroscopy (EDX) equipped in scanning electronmicroscopy system (HITACHI S-4700). Transmission electron micro-scope (TEM) images and selected area electron diffraction (SAED)patterns were obtained on a HITACHI S-570 microscope with anaccelerating voltage of 200 kV. X-ray diffraction (XRD) analysis was

2042 L. Liu et al. / Materials Letters 63 (2009) 2041–2043

conducted on a Scintag XDS-2000 X-ray diffractometer with Cu Kαradiation (λ=1.5418 Å).

The as-synthesized sample was mixed with deionized water(resistivity=18.0MΩ cm−1) in a weight ratio of 100:25 to form apaste. The paste was coated on a ceramic tube on which a pair of goldelectrodes was previously printed, and then a Ni–Cr heating wire wasinserted in the tube to form a side-heated gas sensor.

Gas sensing properties were measured using a static test system.Saturated target vapor was injected into a test chamber (about 1 L involume) by a syringe through a rubber plug. After fully mixed with air(relative humidity was about 25%), the sensor was put into the testchamber. When the sensitivity reached a constant value, the sensorwas taken out to recover in air. The electrical properties of the sensorwere measured by a RQ2 intelligent test meter (Qingdao, China). Thesensitivity value (S) was defined as S=Ra/Rg, where Ra was thesensor resistance in air and Rg was that in a mixture of target gas andair. The time taken by the sensor to achieve 90% of the total resistancechange was defined as the response time in the case of adsorption orthe recovery time in the case of desorption.

3. Results and discussion

The Cu content in the Cu-doped SnO2 nanofibers (0.018) is about3.8 wt.%, as determined by EDX. Fig. 1(a) and (b) show the TEMimages of pure and Cu-doped SnO2 nanofibers (0.018), respectively.The products are highly dominated by the nanofibers with an averagediameter of about 50 nm. No obvious difference can be found in theTEM images, indicating doping Cu does not change the morphology ofSnO2 nanofibers evidently. The TEM images of other samples are alsosimilar with these two fibers. The SAED patterns (inset in Fig. 1(a)and (b)) show that both the pure and Cu-doped SnO2 nanofibers(0.018) are polycrystalline in structure. Fig. 1(c) shows the XRD pat-tern of the pure and Cu-doped SnO2 nanofibers (0.018). All the dif-

Fig. 1. (a) TEM image and SAED pattern (insert) of SnO2 nanofibers, (b) TEM image andSAED pattern (insert) of Cu-doped SnO2 nanofibers(0.018) and (c) XRD patterns of pureand Cu-doped SnO2 nanofibers(0.018).

Fig. 2. (a) Sensitivities of pure and Cu-doped SnO2 nanofibers to 100 ppm ethanol atdifferent operating temperatures and (b) response and recovery characteristics of Cu-doped SnO2 nanofibers (0.018) to different concentrations of ethanol at 300 °C.

fraction peaks can be indexed to the tetragonal rutile structure ofSnO2, which agree well with the reported values from JCPDS card (41-1445) [6].

The sensitivities of pure and Cu-doped SnO2 nanofibers to 100 ppmethanol at different operating temperatures are shown in Fig. 2 (a).The sensitivities of all samples are found to increase with increasingthe operating temperature, which attain the maximum at 300 °C,and then decrease with a further rise of the operating temperature.Among these four fibers, Cu-doped SnO2 nanofibers (0.018) exhibitthe highest sensitivity at 300 °C, thus all the discussions below arefocused on this sample. The response and recovery characteristics ofCu-doped SnO2 nanofibers (0.018) to ethanol at 300 °C are shown inFig. 2 (b). When exposed to 5 ppm ethanol, the sensitivity is about 3.With increasing ethanol concentration, the sensitivity increasessignificantly. For ethanol at level of 50, 100, 200, and 500 ppm, thesensitivities are about 8, 13, 20, and 56, respectively. The response andrecovery times are about 1 and 10 s, respectively.

Fig. 3. (a) Sensitivities of Cu-doped SnO2 nanofibers (0.018) to different concentrationsof ethanol at 300 °C (the insert shows the calibration curve in the range of 5–500 ppm)and (b) selectivity of Cu-doped SnO2 nanofibers (0.018) at 300 °C (the gas concen-tration is 100 ppm).

2043L. Liu et al. / Materials Letters 63 (2009) 2041–2043

Fig. 3(a) shows the sensitivity of Cu-doped SnO2 nanofibers(0.018)versus ethanol concentration at 300 °C. The sensitivity rapidly in-creases with increasing the ethanol concentration up to 500 ppm.Above 500 ppm, the sensitivity slowly increases with increasing theethanol concentration, which indicates that the sensor becomesmore or less saturated. Finally the sensor reaches saturation at about20000 ppm. Moreover, the insert in Fig. 3(a) shows the linear cali-bration curve in the range of 5–500 ppm. Such a linear dependencefurther shows that the Cu-doped SnO2 nanofibers (0.018) can be usedas promising materials for ethanol sensors. The selectivity shown inFig. 3 (b) indicates that the Cu-doped SnO2 nanofibers (0.018) are less

sensitive to CH3COCH3 and NH3, and totally insensitive to H2S, CO,C2H2, and CH4 at 300 °C. Thus Cu-doped SnO2 nanofibers(0.018)exhibit prominently and highly selective, and can be put into variouspractical applications.

The sensing mechanism can be explained as follows [10]. SnO2 is anonstoichiometric oxide having oxygen vacancies and electron donorstates. When the SnO2 nanofibers are surrounded by air, oxygenmolecules will adsorb on the fiber surface to generate chemisorbedoxygen species (O− is believed to be dominant), and resulting in adecrease of carrier concentration [11]. Consequently, SnO2 will show ahigh resistance. When ethanol is introduced at a moderate tempera-ture, these nanofibers are exposed to the traces of reductive gas. Byreacting with the oxygen species on the SnO2 surface, the reductiveethanol will reduce the concentration of oxygen species on the SnO2

surface and thus increase the electron concentration, which even-tually increases the conductivity of our sensor. To explain the in-fluence of CuO doping, many different mechanisms have been appliedin previous papers [12]. In our case, CuO may act as a strong acceptorfor electrons of SnO2, which induces an electron-depleted space-charge layer near the inter face. By reacting with ethanol, CuO isreduced releasing the electrons back to the semiconductor, whicheventually improve the sensing performance of SnO2 nanofibers.

4. Conclusions

In conclusion, Cu-doped SnO2 nanofibers are synthesized throughan electrospinning method and investigated as the ethanol sensingmaterials. High sensitivity, rapid response, and good selectivity areobserved in our investigations. The results demonstrate that Cu-dopedSnO2 nanofibers have excellent potential applications for fabricationhigh performance ethanol sensors.

Acknowledgement

This work was financially supported by NNSF of China (No.10672139).

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