Journal of Natural Gas Chemistry 20(2011)515–519

Gas sensing performance of polyaniline/ZnO organic-inorganichybrids for detecting VOCs at low temperature

Jing Huang1, Taili Yang2, Yanfei Kang2, Yao Wang2, Shurong Wang2∗1. Sinopec Research Institute of Petroleum Engineering, Beijing 100101, China;

2. Department of Chemistry, Nankai University, Tianjin 300071, China[Manuscript received April 1, 2011; revised April 11, 2011 ]

AbstractPolyaniline (PANI) was prepared by the chemical oxidative polymerization of aniline, and ZnO, with the mean particle size of 28 nm, wassynthesized by a non-aqueous solvent method. The organic-inorganic PANI/ZnO hybrids with different mass fractions of PANI were obtainedby mechanically mixing the prepared PANI and ZnO. The gas sensing properties of PANI/ZnO hybrids to different volatile organic compounds(VOCs) including methanol, ethanol and acetone were investigated at a low operating temperature of 90 ◦C. Compared with the pure PANI andZnO, the PANI/ZnO hybrids presented much higher response to VOCs. Meanwhile, the PANI/ZnO hybrid exhibited a good reversibility anda short response-recovery time, implying its potential application for gas sensors. The sensing mechanism was suggested to be related to theexistence of p-n heterojunctions in the PANI/ZnO hybrids.

Key wordsgas sensing performance; polyaniline/ZnO; volatile organic compounds; low temperature

1. Introduction

Owing to its unique optical and electrical properties, ZnO,as a functional n-type semiconductor, has been extensivelystudied in various applications, including piezoelectric, nano-generators, solar cells, nanolasers and gas sensors [1−6].However, similar to other inorganic semiconductors, such asSnO2 and Fe2O3, the significant disadvantage of ZnO is itshigh operating temperature (250−450 ◦C) [7,8], which in-creases power consumption and reduces sensors life.

Research on conducting polymers (mainly polypyrrole,polyaniline and polythiophene) has boomed wide applica-tions, such as electrical device and sensor material, dueto their facile preparation method, design flexibility, highand tunable conductivity, and environmental stability [9−14].These properties are unavailable from inorganic semiconduc-tors. However, conducting polymers also have their ownshortcomings such as low mechanical strength and low sen-sitivity for gas sensor application, which can restrict their po-tential applications in future.

It is well-known that the composition of sensor materialshas significant effects on their performances. Thus the forma-tion of an organic-inorganic hybrid may result in a synergiceffect and enhancing performance, which are helpful for im-proving sensor properties such as high response, low operat-ing temperature, and short response-recovery time. The studyof some organic-inorganic hybrids for gas sensor applicationhas proved that hybridization improves the properties of pureorganic and inorganic materials. For example, Guernion etal. [15] reported that the 3-alkylpolypyrrole/SnO2 hybrids ex-hibited high response to volatile organic compounds(VOCs).Hosono et al. [16] prepared intercalated polypyrrole/MoO3hybrids, which showed good sensing properties to VOCs.Dhawale et al. [17] found that the polyaniline/TiO2 sensorpresented the maximum response of 63% upon exposure to0.1 vol% liquefied petroleum gas (LPG) at room temperature.

In the present work, we have prepared a series of organic-inorganic PANI/ZnO hybrids. The gas sensors based on thehybrids were fabricated and examined for gas sensing appli-cation for detecting VOCs including methanol, ethanol andacetone at a low operation temperature.

∗ Corresponding author. Tel: +86-22-23505896; Fax: +86-22-23502458; E-mail: shrwang@nankai.edu.cnThis work was financially supported by the National Natural Science Foundation of China (No. 21171099) and Science and Technology Commission

Foundation of Tianjin ((Nos. 09JCYBJC03600 and 10JCYBJC03900).

Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10.1016/S1003-9953(10)60230-7

516 Jing Huang et al./ Journal of Natural Gas Chemistry Vol. 20 No. 5 2011

2. Experimental

2.1. Preparation of PANI

Polyaniline (PANI) was prepared by a chemical oxida-tive polymerization method. 0.4 mol/L aniline aqueous solu-tion, in which the concentration of H+ was adjusted to about1 mol/L by hydrochloric acid, was introduced in a flow of N2for 20 min to get rid of O2 in the solution. After that, ammo-nium peroxydisulfate (APS) was added with a APS : anilinemolar ratio of 1 : 1. The reaction was kept at N2 atmospherefor 4 h, and the obtained mixture was filtered. The precipi-tates were washed with hydrochloric acid, acetone and deion-ized water for several times, and then dried at 60 ◦C in vacuumfor 12 h.

2.2. Preparation of ZnO

ZnO was prepared by a non-aqueous solvent method.Zn(Ac)2·2H2O was dissolved into 100 mL diethylene glycol(DEG) to gain 0.02 mol/L solution. The system was kept at200 ◦C for 10 h, and then cooled to room temperature. Theobtained precipitate was centrifuged and washed with ethanolfor several times to remove DEG, dried at 80 ◦C and calcinedat 500 ◦C for 3 h.

2.3. Preparation of PANI/ZnO hybrids

A series of PANI/ZnO hybrids with the PANI mass per-cent of 1, 5, 10, 20, 30 and 40 wt% were prepared by a me-chanical mixing method. Appropriate amount of the aboveobtained PANI and ZnO powders were put in an agate mortoand then grinded adequately to gain a homogeneous mixture.The obtained mixtures were denoted as PZ-1, PZ-5, PZ-10,PZ-20, PZ-30 and PZ-40 with respect to the correspondingPANI mass percentages.

2.4. Characterization

X-ray diffraction (XRD) analyses were performed onRigaku D/max-2500 diffractometer operating at 40 kV and100 mA, using Cu Kα radiation (scanning range 2θ:10o−80o). Fourier transfer infrared (FT-IR) spectra wererecorded using Avatar380FT-IR spectrometer series in thewave number range of 4000−400 cm−1.

2.5. Gas sensing properties test

The gas sensing measurements were performed on a com-mercial HW-30A system (a computer-controlled static gassensing charaterization system, Han Wei Electronics Co.,Ltd., Henan Province, China). A proper amount of samplepowder was lightly grinded with several drops of terpineol toform a slurry in an agate mortar. Then, the slurry was coatedonto the outer surface of the alumina tube with 4 mm lengthand a diameter of 1 mm. The tube contains two Au electrodes

and four Pt wires on both ends of the tube. A small Ni-Cralloy filament was put through the tube to supply the operat-ing temperatures by tuning the heating voltage. Finally, theAl2O3 tube was welded onto a pedestal with six probes. Thesensor response is defined as Ra/Rg, where Ra is the sensorresistance in air and Rg is the sensor resistance in a target gas.

3. Results and discussion

Figure 1 shows the FT-IR spectrum of the prepared PANI.The characteristic absorption peaks of PANI appear at 1549,1503, 1300, 1143 and 796 cm−1, which correspond to theC = C stretching mode of the quinoid rings, C = C stretchingmode of benzenoid rings, the C–N stretching mode in Ar-NH-Ar, the vibration band of the dopant anion (HCl-PANI),and the C–H bending vibration out of the plane of the para-disubstituted benzene rings, respectively [18,19].

Figure 1. FT-IR spectrum of the prepared PANI

XRD pattern of the prepared ZnO is shown in Figure 2.All the diffraction peaks can be well indexed to the hexago-nal wurtzite ZnO (JCPDS card 36−1451). No peaks for otherimpurities can be detected, indicating the formation of pureZnO. The sharp peaks suggest that the crystal of ZnO is per-fect. The average crystallite size of ZnO particles is about28 nm, calculated by Scherrer’s equation. XRD pattern of theprepared PANI is also presented in Figure 2. It can be seenthat the PANI has two diffraction peaks at 2θ of 20.44o (100face) and 2θ of 25.10o (110 face), indicating the crystalliza-tion of PANI. This is consistent with the results of reportedreferences [20,21].

Figure 3 illustrates the response of the PANI/ZnO hy-brids with different PANI mass percentages (1, 5, 10, 20,30 and 40 wt%) to 0.1 vol% methanol, ethanol and ace-tone at the operating temperature of 90 ◦C. It can be seenthat the PANI/ZnO sensors show similar response patterns tomethanol, ethanol and acetone. The response of PANI/ZnOhybrids is markedly influenced by the PANI mass percent-age. The response of PANI/ZnO enhances with the increaseof the PANI mass percentage from 1 to 20 wt%, and the20 wt%PANI/ZnO exhibits the highest response. However,the further increase of the PANI mass percentage results in

Journal of Natural Gas Chemistry Vol. 20 No. 5 2011 517

the decrease of the response. It also can be noted that the20 wt%PANI/ZnO sensor shows higher response to ethanolthan to acetone and methanol. In the gas sensing study, itis found that neither the pure PANI nor ZnO show response

to methanol, ethanol and acetone at the low operating tem-perature of 90 ◦C. This illuminates that there is a synergisticeffect between PANI and ZnO, which strongly influences theresponse to VOCs at the low operating temperature.

Figure 2. XRD patterns of the prepared ZnO and PANI

Figure 3. Response of PANI/ZnO hybrids with different PANI mass percent-ages to 0.1 vol% methanol, ethanol and acetone at the operating temperatureof 90 ◦C

Figure 4 presents the response of PANI/ZnO hybrids withthe different PANI contents (1, 5, 10, 20, 30 and 40 wt%)to different concentrations of methanol (0.05, 0.1, 0.15, 0.2and 0.25 vol%) at the operating temperature of 90 ◦C. Allthe six PANI/ZnO hybrids show a similar response pattern tomethanol. From the figure, it can be concluded that the gasresponse is a function of methanol concentration. The gas re-sponse increases generally to a certain extent with an increasein the concentration of methanol from 0.05 to 0.25 vol%. Themaximal response can be observed when the gas concentra-tion is 0.25 vol%.

Figures 5 shows the response-recovery curves of thePZ-20 hybrid to different concentrations of methanol, ethanoland acetone at the operating temperature of 90 ◦C. From thesecurves, it can be seen that the PZ-20 hybrid has good re-versibility when exposed to methanol, ethanol or acetone, andfast response-recovery characteristic. The response/recovery

Figure 4. Response of PANI/ZnO hybrids with different PANI mass percent-ages to different concentrations of methanol at the operating temperature of90 ◦C

time is an important parameter used for characterizing a sen-sor. It is defined as the time for 90% of the final change involtage, when the gas is in and out. In this study, the responsetime to methanol, ethanol and acetone are within 148, 32 and49 s, respectively, and the recovery time are within 118, 109and 160 s, respectively. They are short enough to meet thepractical requirement.

It can also be observed from Figure 5 that the voltage in-creases upon exposure to the three reducing VOCs with theelectron-donating characteristic, indicating the resistance de-creases. The decrease of the sensor resistance can be hypoth-esized that the composite sensing layer behaves as an n-typesemiconductor. It is well-known that PANI is a p-type semi-conductor, and ZnO is an n-type semiconductor, so there aretwo competitive mechanisms of electronic properties in thePANI/ZnO hybrid. Meanwhile, the PANI/ZnO hybrid con-tains the properties of p-n heterojunction [22,23].

518 Jing Huang et al./ Journal of Natural Gas Chemistry Vol. 20 No. 5 2011

Figure 5. Response-recovery curves of PZ-20 hybrid to different concentra-tions of (a) methanol, (b) ethanol and (c) acetone at the operating temperatureof 90 ◦C

Compared with the pure PANI and ZnO, the as-synthesized PANI/ZnO hybrids showed a much higher re-sponse to VOCs at the low operating temperature of 90 ◦C.It can be proposed that the enhancement of the response ofthe hybrid sensors may be related to the p-n heterojunctionformed by PANI and ZnO nanoparticles. In the gas sensingmeasurement, when exposed to methanol, ethanol or acetone,the PANI/ZnO hybrids exhibit the properties of n-type semi-conductors, that is, the resistance of n-type semiconductorsdecreases when exposed to reducing gases. It suggests thatthe sensing mechanism of the PANI/ZnO hybids is governed

by ZnO. This may be caused by the fact that ZnO was presentat a high level in the PANI/ZnO hybrids. A positively chargeddepletion layer is formed on the ZnO surface owing to theinter-particle electron migration from ZnO to PANI at the p-n heterojunctions. This will decrease the activation energyand enthalpy of physisorption for gases with good electron-donating characteristics [24].

4. Conclusions

In summary, polyaniline (PANI) was prepared by thechemical oxidative polymerization of aniline. ZnO, with themean particle size of 28 nm, was prepared by a non-aqueoussolvent method using Zn(Ac)2·2H2O as the precursor. A se-ries of PANI/ZnO hybrids, with different PANI mass percent-age, were prepared by mechanically mixing the above ob-tained PANI and ZnO. Compared with the pure PANI andZnO, the PANI/ZnO hybrids showed markedly enhanced re-sponse at the low operating temperature of 90 ◦C. Meanwhile,the response of PANI/ZnO hybrids was markedly influencedby the PANI mass percentage. The 20 wt%PANI/ZnO hybridexhibited the highest response to VOCs, along with a goodreversibility and a short response-recovery time. The sensingmechanism was suggested to be related to the existence of p-nheterojunctions in the PANI/ZnO hybrids.

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