Journal of Natural Gas Chemistry 20(2011)403–407

Preparation of polythiophene/WO3 organic-inorganic hybrids and theirgas sensing properties for NO2 detection at low temperature

Jing Huang1, Yanfei Kang2, Taili Yang2, 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 February 16, 2011; revised March 29, 2011 ]

AbstractPolythiophene/WO3 (PTP/WO3) organic-inorganic hybrids were synthesized by an in situ chemical oxidative polymerization method, and char-acterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermo-gravimetric analysis (TGA). The Polythiophene/WO3 hybrids have higher thermal stability than pure polythiophene, which is beneficial to potential application as chemical sensors. Gassensing measurements demonstrate that the gas sensor based on the Polythiophene/WO3 hybrids has high response and good selectivity for de-tecting NO2 of ppm level at low temperature. Both the operating temperature and PTP contents have an influence on the response of PTP/WO3hybrids to NO2. The 10 wt% PTP/WO3 hybrid showed the highest response at low operating temperature of 70 ◦C. It is expected that thePTP/WO3 hybrids can be potentially used as gas sensor material for detecting the low concentration of NO2 at low temperature.

Key wordspolythiophene/WO3 organic-inorganic hybrids; gas sensing properties; NO2 detection; low temperature

1. Introduction

NO2 itself is toxic, badly harmful to human life andhealth, and furthermore, is a main source of acid rain and pho-tochemical smog [1]. To protect the environment, the moni-toring and control of polluted gases have become increasinglyimportant. Thus, it is imperative to develop an applicable sen-sor for detecting NO2 gas. Many semiconductivemetal oxideslike SnO2, WO3, ZnO, TiO2, In2O3, γ-Fe2O3, etc. have beenwidely investigated as sensing materials for NO2 detection byeither thin or thick film fabrication technique [2−6].

Among various n-type semiconductor metal oxides, WO3has received considerable attention for use as chemical sensorbecause of its unique sensing properties for a series of targetgases, includingH2S [7,8], NO2 [9,10], H2 [11], O3 [12], NH3[13] and volatile organic compounds (VOC) [14]. WO3 hasbeen considered as a promising sensing material of solid-statesemiconductor gas sensors for NO2 monitoring because of itsexcellent sensitivity. However, similar to other semiconduc-tor metal oxides, the actual application of WO3 is restricteddue to its high operating temperature and bad selectivity fordetecting NO2.

Recently, conducting polymers such as polythiophene

(PTP), polypyrrole and polyaniline have received much atten-tion because of their potential applications in chemical andbiological sensors, electronic devices as well as efficient andlow cost solar cells, due to their remarkable mechanical andelectrical properties such as low operating temperature, lowcost, flexibility and easy processability and so on [15−18].As one kind of conducting polymers, polythiophene and itsderivatives have attracted considerable attention for their easypolymerization and good environmental and thermal stability[19,20]. However, there are also some disadvantages such aslow chemical stability and mechanical strength that are unfa-vorable for conducting polymer-related applications.

Although the development of inorganic and organic sens-ing materials has been considered alternative, it should be ob-served that their characteristics can be complementary and re-cently it has been considered the possibility to combine the ad-vantages of organic and inorganic counterparts in order to pre-pare hybrid materials having optimal properties. Inorganic-organic metal oxide/conducting polymer hybrid materials arecurrently of great interest in exploring enhanced sensor char-acteristics, due to their synergetic or complementary behav-iors that are not available from their single counterpart [21].Some efforts have been paid to investigate this kind of hybrids

∗ 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. 20871071) and the Science and Technology Commission

Foundation of Tianjin (No. 09JCYBJC03600 and 10JCYBJC03900).

Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10.1016/S1003-9953(10)60196-X

404 Jing Huang et al./ Journal of Natural Gas Chemistry Vol. 20 No. 4 2011

for gas sensor applications. Deshpande et al. [22] have syn-thesized the tin oxide-intercalated polyaniline nanocomposite,which had better sensitivity than individual SnO2 and polyani-line with respect to ammonia gas exposure. Guernion et al.[23] reported that the 3-alkylpolypyrrole-tin oxide compos-ites exhibited much higher sensitivity to volatile organic com-pounds than the pure inorganic and organic materials.

In the present work, we have prepared an organic-inorganic hybrid materials containing PTP as the organic partand WO3 nanocrystalline powders as the inorganic part byan in situ chemical oxidative polymerization of thiophenemonomers. The gas sensors based on the hybrids were fab-ricated and examined for gas sensing application in detectingNO2. Obtained results showed that the hybrid materials ex-hibited high response and good selectivity for detecting NO2of a ppm level at low temperature.

2. Experimental

All chemicals (analytical grade) used in this work werepurchased from Tianjin Guangfu Fine Chemical Research In-stitute. Thiophene monomers were distilled prior to use.Deionized water was used throughout the experiment.

2.1. Preparation of WO3 nanocrystalline powders

WO3 nanocrystalline powders were synthesized by a col-loidal chemical method. 3 mol/L HCl aqueous solution wasdropwise added into 0.5 mol/L Na2WO4 aqueous solution un-der stirring at room temperature till no white precipitate wasfurther formed. The pH value of the solution in the reactionprocess was kept at about 4−5. Then the solution was agedfor 24 h, after which 60 mL of 0.15 mol/L cetyltrimethylam-monium bromide (CTAB) solution was immediately added.Abundant white flocculent precipitate formed, which wastreated by ultrasonication for 40 min. Then the precipitate wasfiltrated and washed with deionized water to remove Cl−, Br−

and any other possible remnants, subsequently dried at 80 ◦Cand calcined at 600 ◦C for 2 h. Light yellow WO3 powderswere obtained.

2.2. Preparation of PTP/WO3 hybrids

PTP/WO3 hybrids were prepared via an in situ chemicaloxidative polymerization. 0.2 g WO3 was dispersed in 50 mLchloroform under ultrasonication for 1 h. Then a certain vol-ume (0.00188mL, 0.0188 mL, 0.0376 mL) of thiophene (TP)monomers was injected into the above solution and stirred for20 min. Subsequently, a requisite amount of anhydrous FeCl3with a molar ratio of TP/FeCl3 (1/3) was added into the solu-tion under vigorous stirring. The reaction was carried out atroom temperature for 3 h, with the color of the mixture chang-ing from gray to deep black. The product was filtered andwashed with methanol several times. The final product wasdried at 60 ◦C under vacuum for 24 h. A series of 1, 10 and

20 wt% PTP/WO3 hybrids were obtained. For comparison,pure PTP was also synthesized in the absence of WO3 by asimilar procedure.

2.3. Characterization of PTP/WO3 hybrids

The samples were characterized by X-ray diffractionanalysis (XRD, Rigaku D/max-2500, Cu Kα, λ = 1.5418 A),thermo-gravimetric analysis (TGA, ZRY-2P, 10 ◦C/min) andtransmission electron microscopy (TEM, Philips FEI Tecnai20ST, 200 kV).

2.4. Gas sensing property test

The gas sensing measurements were performed on a com-mercial HW-30A system (a computer-controlled static gassensing characterization system, Han Wei Electronics Co.,Ltd., Henan Province, China) at the working temperature of40, 70 and 90 ◦C with a relative humility (RH) of 25%–30%.A proper amount of sample powder was lightly ground withseveral drops of terpineol in an agate mortar to form slurry.Then, the slurry was coated onto the outside surface of thealumina tube with a length of 4 mm and a diameter of 1 mm,as well as containing two Au electrodes and four Pt wires onboth ends of the tube. A thin Ni-Cr alloy filament was putthrough the tube to supply the operating temperatures by tun-ing the heating voltage. Finally, the Al2O3 tube was weldedonto a pedestal with six probes. The sensors were aged at90 ◦C for several days in the gas chamber of HW-30A systembefore analysis. The sensor response to NO2 gas is defined asthe ratio of Ra/Rg, where Ra and Rg are the electrical resis-tance of the sensor in air and in NO2 gas, respectively.

3. Results and discussion

Figure 1 shows the XRD patterns of pure WO3,10 wt% PTP/WO3 hybrids and pure PTP. For pure WO3, allthe peaks are in accordance with those given by the JCPDSfile (No. 20-1324) for orthorhombic WO3. The sharp peaks

Figure 1. XRD patterns of WO3, 10 wt% PTP/WO3 and PTP

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suggested that the crystal of WO3 was perfect. The pure PTPpattern presented a broad, amorphous diffraction peak at ap-proximately 2θ = 20o–24o. The peak centered at around 22.8o

corresponded to the intermolecular π-π stacking emerges[24]. For 10 wt% PTP/WO3 hybrids, all the main peaks pre-sented in the pure WO3 were also observed, but the peaks areweaker than those of the pure WO3, which may result fromthe interaction between WO3 and PTP.

Thermal stability of the hybrids was examined by TGanalysis. Figure 2 shows the TG curves of pure PTP and10 wt% PTP/WO3 hybrids. In the case of pure PTP, the massdecrease started at approximately 290 ◦C, and almost decom-posed completely when the temperature was up to 650 ◦C.

Different from pure PTP, the 10 wt% PTP/WO3 hybridwas initially stable up to 320 ◦C, which was 30 ◦C higher thanthe pure PTP. The delay of decomposition process of the hy-brid indicates that the thermal stability of PTP/WO3 hybridis better than that of the pure PTP. As a result, these dataconfirm that the presence of WO3 in PTP/WO3 hybrid is re-sponsible for the higher thermal stability of the hybrid, whichis beneficial to the potential application of the hybrid as chem-ical sensors.

Figure 2. TG curves of pure PTP and 10 wt% PTP/WO3 hybrid

TEMmicrographs of pureWO3 and PTP/WO3 hybrid areshown in Figure 3. From these figures, it can be seen thatthe WO3 has an average particle size of about 20−30 nm. Itis also observed from Figure 3(b)–(c) that WO3 particles arewell encapsulated by PTP with a thickness of about severalnanometers.

Figure 3. TEM micrographs of pure WO3 (a) and PTP/WO3 hybrid (b and c)

We have systematically studied the gas sensing propertiesof the PTP/WO3 hybrids with different PTP contents (1, 10and 20 wt%) to detect NO2 gas with different concentrations(10, 20, 50 and 100 ppm) at different operating temperatures(40, 70 and 90 ◦C). The gas sensing results are included inFigure 4. At all the three different operating temperatures,it can be observed that, with the increase of the NO2 con-centration, the sensor response was improved generally to acertain extent. When the gas concentration was 100 ppm, thePTP/WO3 hybrids showed the maximum response. Amongthe hybrids, 10 wt% PTP/WO3 hybrid showed much higherresponse to NO2 than 1 and 20 wt% PTP/WO3 hybrids. FromFigure 4, one can easily see that 10 wt% PTP/WO3 hybridshowed higher response at 70 ◦C than that at 40 and 90 ◦C.

For comparison, Figure 5 presents the response of10 wt% PTP/WO3 hybrid, pure WO3 and pure PTP to NO2with different concentrations at 70 ◦C. Obviously, compared

with pure WO3 and PTP, the response of PTP/WO3 sensorhas been significantly improved by constructing hybrid ma-terials. PTP/WO3 hybrids can be one of the most promisingmaterials due to their high gas sensitivity at low temperature.Polythiophene behaves as a p-type semiconductor, and WO3is a n-type semiconductor. So the PTP/WO3 hybrids containthe properties of p-n junctions [25]. The electronic proper-ties of PTP/WO3 materials are mainly governed by WO3 dueto the low content of PTP in PTP/WO3 materials. A posi-tively charged depletion layer is formed on the WO3 surfaceowing to the inter-particle electron migration from WO3 toPTP at p-n heterojunctions. This will cause the decrease ofthe activation energy and enthalpy of physisorption for gaseswith good electron-donating characteristics [26]. When thePTP/WO3 hybrids are exposed to NO2 that acted as an elec-tron dopant, the depletion region changed, and the resistanceof the conducting polymer decreases continuously. Therefore,

406 Jing Huang et al./ Journal of Natural Gas Chemistry Vol. 20 No. 4 2011

Figure 4. Response of (1, 10 and 20 wt%) PTP/WO3 hybrids to different concentrations of NO2 at different operating temperatures

the width of the depletion region decreases, and the conduc-tivity of the polythiophene channel increases [27−29]. Theresistance of PTP/WO3 hybrids changes so dramatically thatit is easy to detect small quantity of NO2 with high sensitivity.

For practical use, the selectivity of chemical sensor is alsoan important factor for consideration. It is found in Figure 6that the 10 wt% PTP/WO3 hybrid exhibits high sensor re-sponse to NO2, but less or no gas response to H2S, ethanol,methanol and acetone with the same concentration of

Figure 5. Response of 10 wt% PTP/WO3 hybrid, pure WO3 and pure PTPto different concentrations of NO2 at 70 ◦C

100 ppm, indicating that the sensor based on the as-obtainedPTP/WO3 hybrid have good selectivity to NO2.

Figure 6. Response of 10 wt% PTP/WO3 hybrid to various gases with thesame concentration of 100 ppm at 90 ◦C

4. Conclusions

The as-prepared PTP/WO3 hybrids have a higher thermalstability than the pure PTP, confirming that the presence ofWO3 in PTP/WO3 hybrid is responsible for the higher thermalstability of the hybrid, due to a certain synergetic interaction

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between the inorganic WO3 and organic PTP components,which is beneficial to the potential application of the hybridas chemical sensors. The gas sensing measurements demon-strate that the sensors based on the PTP/WO3 hybrids exhibithigher gas response for detecting NO2 of ppm level at lowtemperature than those based on pure PTP and WO3. Amongthe hybrids, the 10 wt% PTP/WO3 hybrid shows much higherresponse to NO2 than 1 and 20 wt% PTP/WO3 hybrids. More-over, the 10 wt% PTP/WO3 hybrid shows higher response at70 ◦C than that at 40 and 90 ◦C. The PTP/WO3 hybrid ex-hibits much higher sensor response to NO2 compared withother gases including H2S, ethanol, methanol and acetone, in-dicating that the sensors based on the as-obtained PTP/WO3hybrids have good selectivity to NO2. Therefore, it is expectedthat the PTP/WO3 hybrids can be potentially used as gas sen-sor materials for detecting low concentration of NO2 at lowtemperature.

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