Materials Letters 63 (2009) 1975–1977

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Materials Letters

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Preparation, characterization, and gas-sensing properties of Pd-dopedIn2O3 nanofibers

Li Liu a,b,⁎, Tong Zhang c, Shouchun Li a,b, Lianyuan Wang a,b, Yunxia Tian 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 addresses: liul99@jlu.edu.cn, liuli_teacher@16

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.05.060

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 April 2009Accepted 24 May 2009Available online 31 May 2009

Keywords:Electrical propertiesCeramicsNanomaterialsSemiconductorsSensors

Pure and Pd-doped In2O3 nanofibers are synthesized via a simple electrospinning method and characterizedby scanning electron microscopy and X-ray diffraction. Comparing with pure In2O3 nanofibers, Pd-dopedIn2O3 nanofibers exhibit much higher sensitivity to ethanol at 200 °C. The sensor fabricated from Pd-dopedIn2O3 nanofibers can detect ethanol down to 1 ppm (the corresponding sensitivity is 4) with good selectivity,and the response and recovery times are 1 and 10 s, respectively. The sensing mechanism and the effect of Pddoping are discussed. The results indicate that the Pd-doped In2O3 nanofibers can be used to fabricate highperformance ethanol sensors.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The development of chemical sensors to flammable or toxic gaseshas been the focus of numerous research works [1]. Many kinds ofsensing materials including semiconductors (e.g., SnO2, TiO2, ZnO,In2O3, and WO3), polymers, and organic–inorganic hybrid compositeshave been produced to serve this kind of applications [2–5]. In2O3, asan n-type semiconductor, has proven to be a highly sensitive materialfor the detection of both reducing and oxidizing gases [6]. Extensivestudies have been put on improving the sensing performance of In2O3-based gas sensors. Doping metals or metal oxides on In2O3 has beenproved to be a simple and efficient route to enhance the sensing prop-erties [6]. Among the various additives, Pd is a widely used dopant forimproving the selectivity of gas sensors to ethanol, H2 and CO althoughthe sensitization mechanism is still controversial [7–9]. Moreover, Pddoping is also known to reduce the response time and decrease theworking temperature for the maximum sensitivity [7–9].

Recently, nanoscale science and technology are poised to take cen-ter stage with rapid development in the area of synthesis, character-ization, and novel application of the various forms of nanostructuresowing to their excellent optical, electrical, and chemical properties [6].In the sensor field, one-dimensional (1D) nanostructure sensingmate-rials have received considerable attention due to their high surface-to-volume ratio [6]. Although many papers on 1D nanostructure gassensors have been reported, the sensing properties based on doped 1DIn2O3 nanostructures have been rarely investigated.

3.com (L. Liu).

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In this letter, we report the preparation, characterization, and gas-sensing properties of Pd-doped In2O3 nanofibers. The sensor fabri-cated from Pd-doped In2O3 nanofibers exhibits excellent ethanolsensing properties at 200 °C. The sensing mechanism and the effect ofPd doping are discussed.

2. Experimental

Poly (vinyl pyrrolidone) (PVP, Mw=1,300,000) was purchasedfrom Aldrich. Indium nitrate (In(NO3)3·4.5H2O) and palladiumchloride (PdCl2) were obtained from Shanghai Chem-reagent Group(China). N, N-dimethylformamide (DMF) and ethanol (EtOH) (99.0%)were obtained from Tianjin Chemical Company (China). The abovechemical reagents used were of analytical grade and used withoutfurther purification.

In a typical procedure, 0.38 g In(NO3)3·4.5H2O and 0.0023 g PdCl2powder were added to 8.8 g mixed solvent containing DMF/EtOHwith the weight ratio of 1:1 and stirred for 2 h, and then 0.8 g PVPwas added to the above solution with stirring for 6 h. It was worthnoting that In(NO3)3·4.5H2O powder was not soluble in the PVP/EtOH solution. However, when a certain amount of DMF was added, adeep brown homogeneous solution could be prepared for continuouselectrospinning. The obtained solution was then loaded into a plasticsyringe in which the internal diameter of the pinhead is 0.8 mm. Thepinhead was connected to a high voltage supply that was capableof generating direct current (DC) voltages of up to 30 kV. In ourexperiment, a voltage of 15 kV was applied for electrospinning.An aluminum foil served as the counter electrode, and the distancebetween the capillary and the substrate electrode was 20 cm. The as-

Fig. 1. XRD patterns of pure and Pd-doped In2O3 nanofibers.

1976 L. Liu et al. / Materials Letters 63 (2009) 1975–1977

electrospun composite nanofibers were placed in a vacuum ovenfor 12 h at room temperature in order to remove the solvent residuals.In order to remove PVP completely, the composite nanofibers calcinedfor in air at 600 °C 4 h. Then, Pd-doped In2O3 ceramic nanofibers wereobtained. Pure In2O3 nanofibers were prepared via the same methodexcepting the addition of PdCl2.

X-ray diffraction (XRD) analysis was conducted on a ScintagXDS-2000 X-ray diffractometer with Cu Kα radiation (λ=1.5418 Å).Scanning electron microscopy (SEM) images were performed on aSHIMADZU SSX-550 (Japan) instrument.

The as-synthesized sample was mixed with deionized water(resistivity=18.0 MΩ cm−1) in a weight ratio of 100:25 to form apaste. The paste was coated on a ceramic tube on which a pair ofgold electrodes was previously printed, and then a Ni–Cr heating wirewas inserted 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 Lin volume) by a syringe through a rubber plug. After fully mixed withair (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 a mixture of target gas and air. The

Fig. 2. SEM images of pure (a) and

time taken by the sensor to achieve 90% of the total resistance changewas defined as the response time in the case of adsorption or therecovery time in the case of desorption.

3. Results and discussion

Fig. 1 shows the XRD patterns of pure and Pd-doped In2O3 nano-fibers. All of the diffraction peaks coincide with the correspondingpeaks of the cubic structure of In2O3 given in the standard data file(JCPD NO. 89-4595) [10]. No peaks corresponding to the dopant areobserved.

Fig. 2 (a) and (b) shows the SEM images of pure and Pd-dopedIn2O3 nanofibers, respectively. Both of the products are highly domi-nated by the nanofibers with lengths of several ten micrometers anddiameters ranging from 80 to 180 nm. The average diameter of thesetwo samples is about 120 nm. The results suggest that doping Pd inIn2O3 nanofibers does not change the fiber morphology evidently.

Fig. 3 (a) shows the sensitivities of pure and Pd-doped In2O3 nano-fibers to 1000 ppm ethanol at different operating temperatures. Thesensitivity of Pd-doped In2O3 nanofibers is found to increase withincreasing the operating temperature, which attains the maximum at200 °C, and then decreases with a further rise of the operating tem-perature. The same behaviors are observed in the case of pure In2O3

nanofibers. However the maximum sensitivity appears at 240 °C, andthe corresponding sensitivity is much lower than that of the Pd-dopedfibers. Accordingly, 200 °C is believed to be the optimum operatingtemperature for high sensitivity and is applied in all the investigationshereinafter. Fig. 3 (b) shows the dependence of the sensitivity on theethanol concentration of Pd-doped In2O3 nanofibers. The sensitivitylinearly increases with increasing the ethanol concentration below500 ppm (inset in Fig. 3 (b)). Above 500 ppm, the sensitivity slowlyincreases with increasing the ethanol concentration, which indicatesthat the sensor becomes more or less saturated. Finally the sensorreaches saturation at about 10000 ppm. Fig. 3 (c) shows the responseand recovery characteristics of Pd-doped In2O3 nanofibers. The sensi-tivities are 4, 18, 26, 90, and 154 to 1, 50, 100, 500, and 1000 ppmethanol, respectively. The response and recovery times are 1 and 10 s,respectively. The selectivity shown in Fig. 3 (d) indicates that the Pd-doped In2O3 nanofibers are less sensitive to NH3, H2, and H2S, andtotally insensitive to CO, C2H2, and CH4. Thus Pd-doped In2O3 nano-fibers exhibit prominently and highly selective, and can be put intovarious practical applications.

The sensing mechanism can be explained as follows [11]. WhenIn2O3 nanofibers are exposed to air, oxygen molecules adsorb on thesurface of the fibers and form chemisorbed oxygen species by captur-ing electrons from the conductance band. Thus In2O3 nanofibers will

Pd-doped (b) In2O3 nanofibers.

Fig. 3. (a) Dependence of sensitivity on operating temperature of pure and Pd-doped In2O3 nanofibers to 1000 ppm ethanol, (b) dependence of sensitivity on ethanol concentration,(c) response and recovery characteristics, and (d) selectivity of Pd-doped In2O3 nanofibers.

1977L. Liu et al. / Materials Letters 63 (2009) 1975–1977

show a high resistance state in air ambient. When the fibers areexposed to a reductive gas (such as ethanol) at moderate temperature,the gas may react with the surface oxygen species, which increasesthe electron concentration and eventually increases the conductivityof the In2O3 nanofibers. To explain the influence of dopants for sensingmaterials, two different mechanisms, that is, electronic and chemicalsensitization, have repeatedly been applied [12–13]. The sensing im-provement of Pd doping in our sensor may be explained by the elec-tronic sensitization. It is reported that Pd dopant in its oxidized state(PdO) acts as a strong acceptor for electrons of the host semiconduc-tor [14–15]. This induces an electron-depleted space-charge layer nearthe interface. By reacting with ethanol molecules, the PdO is reducedreleasing the electrons back to the semiconductor, and these pheno-mena will lead to a high sensitivity of the sensing materials.

4. Conclusions

In conclusion, Pd-doped In2O3 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 Pd-dopedIn2O3 nanofibers have excellent potential applications for fabricationhigh performance ethanol sensors.

Acknowledgement

This work was financially supported by the national innovationexperiment program for university students (No. 2009125).

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