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Sensors and Actuators B 138 (2009) 368–373

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

humidity sensor based on KCl-doped SnO2 nanofibers

iaofeng Songa, Qi Qib, Tong Zhangb, Ce Wanga,∗

Alan G. Macdiarmid Institute, Jilin University, Changchun 130012, PR ChinaState Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China

r t i c l e i n f o

rticle history:eceived 5 December 2008eceived in revised form 12 February 2009

a b s t r a c t

A fast, simple and reliable humidity sensor based on ion-doped SnO2 nanofibers has been developed byscreen-printing on ceramic substrates with Ag–Pd interdigital electrodes in this research. The optimizedresults show that the impedance of the sensor decreases by more than five orders of magnitude with

ccepted 12 February 2009vailable online 24 February 2009

eywords:lectrospinningCl

increasing relative humidity (RH) from 11% to 95%, the response and recovery time of the sensor areabout 5 and 6 s, respectively. Compared to the previous electrical conductivity-based SnO2 humiditysensors, the proposed sensor exhibits higher humidity sensitivity, more rapid response and recovery,smaller hysteresis, and better linearity and stability.

© 2009 Elsevier B.V. All rights reserved.

nO2

umidity sensor

. Introduction

Water vapor present in the atmosphere is known as humidityhich is highly variable and changes according to seasons, land and

ea, etc. The controlling of humidity is important for human com-ort, storage of various goods, and industrial process control [1–4],o the design and fabrication of humidity sensors has become onef the most active research fields. Metal oxide semiconductors suchs SnO2, ZnO, WO3 or Fe2O3, usually representing a property thathe electrical conductivity varies with the composition of the gastmosphere surrounding them, are the popular and useful sensingaterials for gas and offer the potential for developing portable

nd inexpensive gas sensing device [5]. Among these metal oxideensors, most of the development work is centered on SnO2 mate-ial due to its lower working temperature and adaptability to senseifferent gases after dopant addition.

To improve the sensing characteristics, a general route is toake SnO2 material at the nanoscale, taking advantage of its

arge surface areas, and hence high surface activity, and gas sen-itivity [6,7]. Recently, one-dimensional (1D) SnO2 nanostructuresuch as nanobelts, nanowires, and nanofibers have attracted much

ttention because of their wide applications in photocatalysts, opto-lectronic devices, transducers, and sensors [8–10]. And many gasensors based on 1D SnO2 nanostructures have been successfullybtained. For example, Baratto’ research group has tested the gas

∗ Corresponding author at: Jilin University, Changchun 130012,R China. Tel.: +861 431 85168292.

E-mail address: cwang@jlu.edu.cn (C. Wang).

925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2009.02.027

sensing of SnO2 nanowires. The response of the sensor was highlyselective towards humidity and other polluting species, such as COand NH3 [11]. However, as a humidity sensor, their sensing char-acteristics (e.g., response, recovery, reproducibility, stability, andlinearity) still need to be improved.

In this work, we proposed a novel type of humidity sensorsbased on KCl-doped SnO2 nanofibers using electrospinning tech-nique [12]. Because alkali ions such as Li+ or K+ were often usedin fabricating humidity sensors [13,14], here KCl was selected fordopant to improve the vapor sensor characteristics. The preparedhumidity sensor shows excellent sensing characteristics, includingfast response time (e6 s) and recovery time (e7 s) for measuringrelative humidity (RH) in a wide range of 11–95% in air at room tem-perature (25 ◦C) with the impedance changing from 108 to 103 �.In addition, the sensor has desirable reproducibility, linearity, andstability.

2. Experimental

2.1. Chemicals and preparation of KCl-doped SnO2 nanofibers

All chemicals used in the experiment were of analytical gradeor better. The following was the abbreviation of the chemicalsand the corresponding manufacturers: Polyvinylpyrrolidone (PVP,Mn = 1,300,000, Aldrich), SnCl2·2H2O (Tianjin Fuchen Chemical

Regent Factory, China), KCl (Tianjin Huadong Chemical Regent Fac-tory, China), ethanol and N,N-dimethylformamide (DMF, TianjinTiantai Fine Chemicals Co., Ltd., China).

KCl-doped SnO2 nanofibers were synthesized by an electrospin-ning and calcination method described as the following: 0.4 g of

X. Song et al. / Sensors and Actuat

Fig. 1. Schematic diagram of experimental unit.

Fig. 2. SEM images of (a) as-electrospun fibers (the content of KCl = 15%), (b) the pure Sn(the contents of KCl are 5%, 10%, 15% and 20%, respectively). The inset in Fig. 2e is TEM im

ors B 138 (2009) 368–373 369

SnCl2 was mixed with a 1:1 weight ratio of DMF and ethanol ina glove-box under vigorous stirring for 10 min. Subsequently, thissolution was added to 0.8 g of PVP and a suitable amount of KClunder vigorous stirring for 30 min. Then, the mixture was loadedinto a glass syringe and connected to high-voltage power supply.10 kV was provided between the cathode (a flat foil) and anode(syringe) at a distance of 20 cm. The conversion of tin dichloride toSnO2 and the remove of PVP in the as-spun nanofibers were carriedout by calcination at 600 ◦C for 4 h in air.

The X-ray powder diffraction (XRD) data were collected on anX’Pert MPD Philips diffractometer (Cu K� X-radiation at 40 kV and50 mA). Scanning electron microscopy (SEM) images were recordedon a SHIMADZU SSX-550 instrument. Transmission electron micro-graphs (TEM) were performed on a Hitachi S-570 equipment.

2.2. Fabrication and measurement of humidity sensor

The KCl-doped SnO2 nanofibers were ground and mixed withdeionized water in a weight ratio of 100:35 to form a paste. Thepaste was screen-printed on a ceramic substrate (6 mm × 3 mm,0.5 mm in thick) with five pairs of Ag–Pd interdigital electrodes

O2 nanofibers, (c), (d), (e) and (f) different contents of KCl-doped SnO2 nanofibersage of 15% KCl-doped SnO2 nanofibers.

3 Actuators B 138 (2009) 368–373

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electrodes width and distance: 0.15 mm) to form a film with ahickness of about 10 �m, and then the film was dried in air at 60 ◦Cor 5 h.

The characteristic curves of humidity sensitivity were mea-ured on a ZL-5 model LCR analyzer (Shanghai, China). The voltagepplied in our studies was AC 1 V, and the frequency varied from0 Hz to 100 kHz. The RH range of 11–95% was obtained usingaturated salt solutions as the humidity generation sources. Theix different saturated salt solutions were LiCl, MgCl2, Mg(NO3)2,aCl, KCl, and KNO3, and their corresponding RH values were1%, 33%, 54%, 75%, 85% and 95% RH, respectively [15]. Thechematic diagram of the experimental unit is shown in Fig. 1.wo chambers were used in the measurements, and the sen-or was switched between them at a temperature of 25 ◦C. Theesponse–recovery process can be measured by switch betweenhambers. This method has been developed by Wang [16,17]. Thealibration procedure before measurements: we usually spend 10 ho ensure the air in chamber reaching equilibrate state duringhe investigations. There were also a standard humidity sensornd a thermometer in our system to monitor the RH in thehamber and the operating temperature of our sensor, respec-ively.

. Results and discussion

.1. Structure characteristics

The morphologies of the various fibers before and after cal-ination are shown in Fig. 2a–f. The as-electrospun fibers weremooth and even (Fig. 2a). The fibers obtained after the calcina-ion at 600 ◦C exhibited shrinkages and diameter was reduced dueo the decomposition of PVP and conversion of SnCl2 to SnO2 (Fig. 2and e). In contrast to the diameter of pure SnO2 nanofibers rang-ng from 80 to 150 nm (Fig. 2b), that of KCl-doped SnO2 nanofibersurn thicker (100–200 nm). The macroporous network resultingrom the intrinsic entanglement of nanofibers could be clearlybserved in Fig. 2b. Additionally, as shown in the inset of Fig. 2e,he surface of each ceramic nanofiber took on a rough porous struc-ure. These two characteristics should be particularly useful forpplications (such as catalysis, filtration, sensing, and solar cells)here surface area plays a key role in determining their perfor-ance.

Fig. 3 gives the XRD patterns of pure and 15% KCl-doped SnO2

anofibers. It can be seen that pure SnO2 nanofibers exhibited theetragonal rutile structure according to the database of JCPDS, whilehe addition of KCl did not change the structure of SnO2 nanofibersn calcination.

ig. 4. (a) Humidity sensitive properties of pure and different contents of KCl-doped SnOnd the frequency are 1 V and 100 Hz, respectively.

Fig. 3. XRD of pure and 15% KCl-doped SnO2 nanofibers.

3.2. Humidity sensing properties

Fig. 4a shows measured impedance responses of the pure andKCl-doped SnO2 nanofibers containing the different contents ofKCl in the range of RH of 11–95% at 25 ◦C. Under an electricalfield at 100 Hz, the pure SnO2 nanofibers did not showed a visi-ble change of impedance until humidity reached as high as 54% RH,but the KCl-doped SnO2 nanofibers presented obvious decrease ofimpedance in the range of 11–95% RH, and the optimal ratio of KCl-to-SnO2 was found to be 15%. For the case of 15% KCl-doped SnO2nanofibers, the impedance variations of the sensor showed nearlyfive-order change during the RH variation of 11–95%, indicating thehigh humidity sensitivity.

When the sensing material is measured at increasing anddecreasing RH at the same temperature, a hysteresis effect can beobserved. The humidity hysteresis characteristic of humidity sensorbased on 15% KCl-doped SnO2 nanofibers at 25 ◦C is given in Fig. 4b,where the solid triangle line stands for the course from low to highRH (corresponding to the adsorption process), while the hollow tri-angle line designs for the opposite direction (corresponding to thedesorption process). The lines for adsorption and desorption are

close to the maximum hysteresis being less than 3% under 80% RH.

To find out the optimization relationship of sample betweenimpedance and RH, we measured the impedance at different fre-quencies. The RH dependencies of the impedance under various

2 nanofibers. (b) Hysteresis of the 15% KCl-doped SnO2 nanofibers. The AC voltage

X. Song et al. / Sensors and Actuat

Fq

fdoibqaod

Ff

ig. 5. Resistance vs. 15% KCl-doped SnO2 nanofibers measured at different fre-uency.

requencies are given in Fig. 5. It can be seen that impedanceecreased with increasing the frequency, and the best linearityf the impedance versus RH curve appeared at 100 Hz, while thempedance was almost flat above 100 kHz. Thus the impedance

ecame independent of the humidity with increasing the fre-uency. This is because the electrical field direction changes slowlyt low frequency and there appears the space-charge polarizationf adsorbed water. When the frequency is high, the electrical fieldirection changes fast, the polarization of the water cannot catch up

ig. 6. Response and recovery curve of the 15% KCl-doped SnO2 nanofibers measured at 1or 30 days.

ors B 138 (2009) 368–373 371

with it, and hence the dielectric constant is small and independentof RH [18,19].

The response and recovery behavior is one of the most importantcharacteristics for evaluating the performance of humidity sensors.Fig. 6 gives the response–recovery properties and stability of thesensor based on 15% KCl-doped SnO2 nanofibers. The response andrecovery time were about 5 and 6 s respectively when RH rangesfrom 11% to 95% RH (Fig. 6a). The results indicate our product takingon rapid response and recovery characteristics, which are fast thanmany humidity sensors reported before [20]. It can be seen fromthe curves for 10 cycles (Fig. 6b), that the peak values almost neverchange, implying the good reproducibility of the sensor. In addition,as shown in Fig. 6c, there are few changes in impedances, con-firming the good stability of our sensors further. Moreover, sensorsynthesis process and performance show desirable reproducibilityand stability during this study. Certainly, to broaden application,the influence of the other factors including high temperatureon the sensor performance will be investigated farther in nextwork.

3.3. Discussion on sensing mechanism

In order to discovering the sensing mechanism, the compleximpedance has been measured and shown in Fig. 7. In our work,the frequency varies from 20 to 100 Hz and the RH ranges from11% to 95%. We measured the complex impedance and complex

angle at different frequencies and different RH, and then calculatedthe real part and the imaginary part of complex impedance. Thefigures show that when RH is low (11%, 33% and 54% RH), a halfsemicircle is observed and becomes a semicircle with increasingof RH. When RH increases unceasingly (75%, 85% and 95% RH), a

00 Hz for (a) 1 cycle and (b) 10 cycles. (c) Stability of the sensor after exposing in air

372 X. Song et al. / Sensors and Actuators B 138 (2009) 368–373

ts of th

lhibcn[Fntfinlfitfifiahiis

Fig. 7. The complex impedance plo

ine appears after semicircle at the low frequency range, and theigher the RH is, the longer the line is. The semicircle plot of the

mpedance represents a kind of sensing mechanism, which cane modeled by an equivalent parallel circuit of a resistor and aapacitor [21–23], as shown in Fig. 8a. The curve of semicircle con-ected with a line represents another kind of sensing mechanism24–26], of which the equivalent circuit has been also shown inig. 8b. Here Rf represents the resistance of the KCl-doped SnO2anofiber film, which decreases as RH increases; Cf the capaci-ance of the film and Zi the impedance at the electrode/sensinglm interface. Thus there is different humidity sensing mecha-ism at low and high RH for the KCl-doped SnO2 nanofibers. At

ow humidity, only a few water molecules are adsorbed onto theber film. Since the coverage of water on the surface is not con-inuous, the ion transport is hard to happen. As the pores on thebers present a high local charge density and a strong electrostaticeld, which promotes chemical molecules adsorption and dissoci-tion. The dissociation provides protons as charge carriers of the

opping transport [27]. At high humidity, based on the model of

on transport mechanism reported by Casalbore-Miceli et al. [28],ons could dissolve in the adsorbed water, and the dielectric con-tant, which is a function of the adsorbed water, makes them free

Fig. 8. An equivalent circuit of such complex impedance plots.

e 15% KCl-doped SnO2 nanofibers.

from the interaction of the opposite charges. Thus more ions couldtransfer freely. The quick transfer of ions on the water layers, whichare formed among SnO2 nanofibers, results in a sharp decrease ofthe impedance.

Certainly, the excellent humidity sensing characteristics of thesensor described here can be attributed to its structures, suchas macropores in the fiber network, the rough porous structureand large surface area of KCl-doped SnO2 nanofibers, which eas-ily enable water molecules to adsorb on the surfaces of SnO2nanofibers. Simultaneously, 1D structure can facilitate fast masstransfer of the water molecules to or from the interaction region aswell as improve charge carriers to traverse the barriers introducedby molecular recognition along the 1D nanostructure. Moreover,the addition of alkali ions can provide more active sites for watermolecules resulting in the improvement of the sensitivity asreported before [29]. So the impedance continuously decreasesby more than five orders of magnitude compared to the initialimpedance.

4. Conclusions

A humidity sensor has been developed based on KCl-doped SnO2electrospun nanofibers. Under the optimal conditions, it showsthe good sensing characteristics such as super sensitivity, rapidresponse and recovery behavior, wide range of response, good lin-earity and stability. Moreover, the simple, effective, and low-costroute to synthesis process shows a good application prospect forhumidity determination in daily life, industry and other fields.

Acknowledgements

The authors would like to thank the editor and the anony-mous reviewers for their valuable comments on this manuscript.

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his work has been supported by the National 973 project (no.007CD936203), National 863 project (no. 2007AA03z324) andNSF of China (No. 10672139).

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Biographies

Xiaofeng Song is an associate professor of Department of Chemistry in ChangchunUniversity of Technology. He received his BS degree in Department of Chemistry,Changchun University of Technology in 1995. He received his PhD degree in Depart-ment of Chemistry, Jilin University, Changchun, China in 2008. His main researchfocuses on inorganic and organic materials.

Qi Qi received his BS degree from the College of Electronics Science and Engineer-ing, Jilin University, China in 2004. He entered the PhD course in 2004, majored inmicroelectronics and solid state electronics.

Tong Zhang received her MS degree in major of semiconductor materials in 1992,and PhD degree in the field of microelectronics and solid state electronics in 2001from Jilin University. She was appointed a full professor in College of ElectronicsScience and Engineering, Jilin University in 2001. Now, she is interested in the fieldof sensing functional materials and gas sensors and humidity sensors.

Ce Wang is a professor Changchun University of Technology in Jilin University, China.She received her BS degree in Department of Chemistry, Jilin University in 1982. Shereceived her PhD degree in Technische Universitaet Wien, Austria in 1995. And sheheld her postdoctoral position in Drexel University, from 1995 to 1997. Her currentresearch interests are electrospinning and polymer chemistry.