Trans. Nonferrous Met. Soc. China 22(2012) 380-385

Low-temperature NO₂ sensors based on polythiophene/WO₃ organic-inorganic hybrids

GUO Xian-zhi, KANG Yan-fei, YANG Tai-li, WANG Shu-rong

TKL of Metal- and Molecule-based Material Chemistry, Key Laboratory of

Advanced Energy Materials Chemistry (MOE), Department of Chemistry, Nankai University, Tianjin 300071, China

Received 13 April 2011; accepted 23 June 2011

Abstract: Polythiophene (PTP) was prepared by a chemical oxidative polymerization and nanosized WO₃ was prepared by a colloidal chemical method. The organic–inorganic PTP/WO₃ hybrids with different mass fractions of PTP were obtained by a simple mechanically mixing the prepared PTP and WO₃. The as-prepared PTP/WO₃ hybrids have a higher thermal stability than the pure PTP. The gas sensing measurements demonstrate that the PTP/WO₃ hybrid sensors exhibit higher response for detecting NO₂ at low temperature than the pure PTP and WO₃ sensor. The sensing mechanism is suggested to be related to the existence of p–n heterojunctions in the PTP/WO₃ hybrids. The response of the PTP/WO₃ hybrids is markedly influenced by the PTP mass fraction. The 20% PTP/WO₃ hybrid shows high response and good selectivity to NO₂ at low temperature (<90 ^oC). Therefore, the PTP/WO₃ hybrids can be expected to be potentially used as gas sensor material for detecting NO₂ at low temperature.

Key words: NO₂ sensor; polythiophene/WO₃; low temperature

1 Introduction

As one of the potential conducting polymers, PTP and its derivatives have attracted considerable attention for their easy polymerization and good environmental and thermal stability [1, 2]. However, there are also some disadvantages such as low chemical stability and mechanical strength that are unfavorable for conducting polymer-related applications.

WO₃, an n-type semiconductor metal oxide, has received considerable attention for use as chemical sensor because of its unique sensing properties for a series of target gases, including H₂S [3, 4], NO₂ [5, 6], H₂ [7], O₃ [8], NH₃ [9] and volatile organic compounds (VOCs) [10]. NO₂ is toxic itself, badly harmful to human life and health, and, furthermore, is a main source of acid rain and photochemical smog [11]. WO₃ has been considered a promising sensing material of solid-state semiconductor gas sensors for NO₂ monitoring because of its excellent sensitivity [12]. However, similar to other semiconductor metal oxides, the high operating temperature and bad selectivity of WO₃ for detecting NO₂ restrict its actual application.

Inorganic–organic metal oxide/conducting polymer hybrid materials are currently of great interest for exploring enhanced sensor characteristics, due to their synergetic or complementary behaviors that are not available from their single counterparts [13]. Some effort has been paid to investigating this kind of hybrids for gas sensor applications. GUERNION et al [14] reported that the 3-alkylpolypyrrole-tin oxide composites exhibited much higher sensitivity and better selectivity to volatile organic compounds than the pure inorganic and organic materials. DESHPANDE et al [15] have synthesized the tin oxide-intercalated polyaniline nanocomposite, which had better sensitivity than the SnO₂ and polyaniline with respect to ammonia gas exposure. HOSONO et al [16] prepared intercalated polypyrrole/MoO₃ hybrids, which showed good sensing properties to VOCs.

In the present work, we prepared an organic- inorganic hybrid material containing PTP as the organic part and WO₃ nanocrystalline powders as the inorganic part by a simple mechanically mixing the prepared PTP and WO₃. The gas sensors based on the hybrids were fabricated and examined for gas sensing application for detecting NO₂. Obtained results showed that the hybrid materials exhibited high response and good selectivity for detecting NO₂ at low temperature.

2 Experimental

2.1 Preparation of PTP

PTP polymer was prepared via an in situ chemical oxidative polymerization. A specific amount of anhydrous FeCl₃ (A.R., Tianjin Guangfu Fine Chemical Research Institute) was added into 50 mL ethyl glycol methyl ether (A.R., Tianjin Guangfu Fine Chemical Research Institute) under vigorous stirring. Then a certain volume of thiophene (TP) monomers (A.R., Tianjin Guangfu Fine Chemical Research Institute), with a mole ratio of TP to FeCl₃ (1:3), was injected into the above stirred solution. After the reaction was carried out at room temperature for 3 h, the mixture was filtered. The obtained black precipitate was dipped in methanol for 24 h. The product was filtered and washed with methanol several times. The final product was dried at room temperature under vacuum for 24 h.

2.2 Preparation of WO₃

WO₃ was synthesized by a colloidal chemical method. Na₂WO₄·2H₂O (A.R., Tianjin Guangfu Fine Chemical Research Institute) was dissolved into a certain amount of deionized water. Then a certain concentration of aqueous solution of HCl was drop-wise added to the sodium tungstate solution under stirring at room temperature till no white precipitate was further formed. The pH of the solution was adjusted with an aqueous solution of HCl in the reaction process. Then the solution was aged for 24 h, after which 15 mL of 0.15 mol/L cetyltrimethyl ammoniumbromide (CTAB) (A.R., Tianjin Guangfu Fine Chemical Research Institute) solution was immediately added. The abundant white flocculent precipitate formed was treated by ultrasonication for 40 min. Then the precipitate was filtrated and centrifuged with deionized water to remove Cl^-, Br^- and any other possible remnants, dried at 80 ?C and calcined at 600 ?C for 2 h. Light yellow WO₃ powders were obtained.

2.3 Preparation of PTP/WO₃ hybrids

A series of PTP/WO₃ hybrids were prepared by mechanically mixing the above obtained PTP and WO₃ with the PTP mass fraction of 5%, 10%, 20% and 30%, respectively, denoted as PW-5, PW-10, PW-20 and PW-30.

2.4 Characterization

X-ray diffraction (XRD) analyses were performed on Rigaku D/max-2500 diffractometer operating at 40 kV and 100 mA, using Cu K_α radiation (scanning range 2θ: 10°-80°). Fourier transfer infrared (FTIR) spectra were recorded using Avatar380FT-IR spectrometer series in the wave number range of 4000-400 cm^-1. Thermal- gravimetry (TG) analyses were performed by ZRY-2P thermal analyzer at a linear heating rate of   10 °C/min. α-Al₂O₃ was used as reference.

2.5 Gas sensing properties test

The gas sensing measurements were performed on a commercial HW-30A system (a computer-controlled static gas sensing characterization system, Han Wei Electronics Co., Ltd., Henan Province, China). A proper account of sample powder was lightly grinded with several drops of terpineol in an agate mortar to form slurry. Then, the slurry was coated onto the outside surface of the alumina tube with 4 mm in length and    1 mm in diameter, as well as containing two Au electrodes and four Pt wires on both ends of the tube. A small Ni-Cr alloy filament was put through the tube to supply the operating temperatures by tuning the heating voltage. Finally, the Al₂O₃ tube was welded onto a pedestal with six probes. The sensor response to NO₂ gas is defined as the ratio of R_g/R_a, where R_g and R_a are the electrical resistances of the sensor in NO₂ gas and in air, respectively.

3 Results and discussion

Figure 1 shows XRD pattern of the prepared WO₃. All the diffraction peaks can be well indexed to orthorhombic WO₃ (JCPDS file no. 20—1324). No peaks for other impurities can be detected, indicating the formation of pure WO₃. The sharp peaks suggest that the crystal of WO₃ is perfect. The average crystallite size of WO₃ particles is about 29 nm, calculated by Scherrer’s equation.

Fig. 1 XRD pattern of prepared WO₃

FTIR spectrum of the prepared PTP is shown in  Fig. 2. Several weak peaks in the range of 2800-3100 cm^-1 and 1630 cm^-1 can be attributed to the C—H stretching vibrations and C=C characteristic peak, respectively. In the range of 600-1500 cm^-1, it is the fingerprint region of PTP. The peak at 784 cm^-1 is due to the C—H out-of-plane vibration of the 2, 5-substituted thiophene ring created by the polymerization of thiophene monomer. The peak at approximately 695 cm^-1 denotes the C—S stretching in the thiophene ring [17-20].

Fig. 2 FT-IR spectrum of prepared PPT

Thermal stability of the hybrid was examined by TG analysis. Figure 3 presents the TG curves of the prepared pure PTP and PW-20 hybrid. In the case of the pure PTP (Fig. 3(a)), the mass starts to decrease at approximately 170 °C, and almost decomposes completely when the temperature is up to 750 °C. Different from the pure PTP, the PW-20 hybrid is still stable at the temperature below 300 °C. The delay of decomposition process of the hybrid indicates that the thermal stability of the PTP/WO₃ hybrids is better than that of the pure PTP. As a result, these data confirm that the mass fraction of WO₃ in the PTP/WO₃ hybrids is responsible for the higher thermal stability of the hybrids, which is beneficial for the potential application of the hybrids as chemical sensors.

Fig. 3 TG curves of prepared pure PTP (a) and PW-20   hybrid (b)

To investigate the gas-sensing properties of the prepared PTP/WO₃ hybrids to NO₂ at low temperature, the gas-sensing test was carried at room temperature (RT), 60 and 90 °C, respectively. The gas sensing results are included in Figs. 4-8.

Fig. 4 Response of prepared PTP/WO₃ hybrids with different PTP mass fractions to NO₂ with different concentrations at room temperature

Fig. 5 Response of PW-20 hybrid to NO₂ with different concentrations at different operating temperatures

Fig. 6 Response of PTP/WO₃ hybrids with different PTP contents to 0.1% NO₂ (volume fraction) at different operating temperatures

Fig. 7 Response of PW-20 hybrid, pure WO₃ and pure PTP to 0.1% NO₂ (volume fraction) at different operating temperatures

Fig. 8 Response of PW-20 hybrid to various gases with same concentration of 0.1% at different operating temperatures

Figure 4 shows the response of the PTP/WO₃ hybrids with different PTP mass fractions (5%, 10%, 20% and 30%) to different concentrations (0.01%, 0.05%, 0.1% and 0.15%, volume fraction) of NO₂ at room temperature. All the four PTP/WO₃ hybrids show a similar response pattern to NO₂. From Fig. 4, it can be concluded that the gas response is a function of NO₂ concentration. The gas response increases generally to a certain extent with an increase in concentration of NO₂ from 0.01% to 0.15% (volume fraction). The maximum response can be observed when the NO₂ gas concentration is 0.15% (volume fraction). Meanwhile, the response of PTP/WO₃ hybrids enhances with the increase of PTP mass fraction from 5% to 20%, and the 20% PTP/WO₃ hybrid shows the highest response to NO₂. However, further increase of the PTP mass fraction results in the decrease of the response.

Figure 5 illustrates the response of the PW-20 hybrid to NO₂ with different concentrations at different operating temperatures. It can be observed that, similar to at room temperature, at the operating temperature of 60 and 90 °C the response increases with the increase of NO₂ concentration, and all the hybrids show the maximum response to 0.15% NO₂ (volume fraction). It is also found that the PTP/WO₃ hybrid exhibits much higher response to NO₂ at RT than at 60 or 90 °C. Therefore, it is suggested that this kind of hybrids can be potentially used for gas sensor material for detecting NO₂ at RT.

Figure 6 shows the response of 5%, 10%, 20% and 30% PTP/WO₃ hybrids to 0.1%NO₂ (volume fraction) at RT, 60 or 90 °C. It is found that the PW-20 hybrid has higher response to 0.1%NO₂ (volume fraction) at RT than at 60 °C or 90 °C, and at all the three operating temperatures, the PW-20 hybrid shows the highest response among the PTP/WO₃ hybrids.

Figure 7 presents the response of the PW-20 hybrid, pure WO₃ and pure PTP to 0.1% NO₂ (volume fraction) at different operating temperatures. Obviously, compared with the pure WO₃ and PTP, the sensor response has been significantly improved by constructing hybrid materials. Therefore, the PTP/WO₃ hybrid can be one of the most promising materials due to its high response at low temperature. It is well known that PTP behaves as a p-type semiconductor and WO₃ behaves an n-type semiconductor. It is expected that the p–n heterojunctions are formed in the PTP/WO₃ hybrids [21], which could generate a unique electron donor–acceptor system, increasing the depletion barrier height and thus improving the response of the sensor [22]. The test gas can adjust the conductivity of the junction by changing the depletion region. When the NO₂ gas is introduced, the width of the depletion region decreases and the conductivity of the PTP channel increases [23-25]. Therefore, the PTP/WO₃ hybrid shows much higher response in comparison to pure PTP and WO₃ at low operating temperature. However, it can also be observed from Figs. 4-6 that the concentration of the PTP is as high as 30% which leads to the decrease in the response of the PTP/WO₃ to NO₂, which should be related to the fact that the p-n heterojunctions are covered with the excessive PTP. As a result, the effective p-n heterojunctions are reduced and the response decreases.

For practical use, the selectivity of the chemical sensor is also an important consideration. Figure 8 shows the response of the PW-20 hybrid to various gases including NO₂, H₂S, CO, H₂, NH₃, ethanol, methanol and acetone with the same concentration of 0.1% NO₂ (volume fraction) at RT, 60 and 90 °C, respectively. It is found in Fig. 8 that the PW-20 hybrid exhibits high response to NO₂, but less to H₂S, and no gas response to CO, H₂, NH₃, ethanol, methanol and acetone, illuminating that the sensor based on the as-obtained PTP/WO₃ hybrid has good selectivity to NO₂.

4 Conclusions

1) The as-prepared PTP/WO₃ hybrids have a higher thermal stability than the pure PTP, confirming that the presence of WO₃ in the PTP/WO₃ hybrid is responsible for the higher thermal stability of the hybrid, due to a certain synergetic interaction between the inorganic WO₃ and organic PTP components, which is beneficial for the potential application of the hybrid as chemical sensors.

2) The gas sensing measurements demonstrate that the sensors based on the PTP/WO₃ hybrids exhibit higher gas response for detecting NO₂ at low temperature than the sensor based on pure PTP or WO₃. The sensing mechanism is suggested to be related to the existence of p–n heterojunctions in the PTP/WO₃ hybrids.

3) The response of the PTP/WO₃ hybrids is markedly influenced by the PTP mass fraction. Among the hybrids, the 20% PTP/WO₃ hybrid shows the highest response to NO₂. Moreover, the 20% PTP/WO₃ hybrid shows higher response at room temperature than at 60 or 90^oC. Meanwhile, the PTP/WO₃ hybrid exhibits much higher sensor response to NO₂ compared with other gases including H₂S, CO, H₂, NH₃, ethanol, methanol and acetone, illuminating that the sensors based on the as-obtained PTP/WO₃ hybrids have good selectivity to NO₂.

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基于聚噻吩/WO₃的有机-无机复合材料低温NO₂传感器

郭先芝，康艳飞，杨太利，王淑荣

南开大学 化学系，先进能源材料化学教育部重点实验室，金属与分子基材料化学天津市重点实验室，天津 300071

摘  要：通过化学氧化聚合法和胶溶法分别制备了聚噻吩(PTP)和纳米WO₃，通过简单机械共混和PTP和WO₃制备不同PTP质量分数的有机-无机PTP/WO₃ 复合物。所制备的PTP/WO₃复合物比纯的PTP具有更高的热稳定性。气敏测试结果表明，在低温检测NO₂时，PTP/WO₃复合物具有比纯的PTP和WO₃ 更高的响应，这可能与PTP/WO₃复合物中p–n异质节的存在有关系。PTP/WO₃复合物的响应受PTP质量分数的影响显著，20% PTP/WO₃复合物在低温(<90 °C)时对NO₂ 显示出高的响应和好的选择性。因此，PTP/WO₃传感器有望被用于低温检测NO₂

关键词：NO₂传感器；聚噻吩/WO₃；低温

(Edited by LI Xiang-qun)

Foundation item: Project (21171099) supported by the National Natural Science Foundation of China; Projects (09JCYBJC03600, 10JCYBJC03900) supported by Technology Commission Foundation of Tianjin, China

Corresponding author: WANG Shu-rong; Tel: +86-22-23505896; Fax: +86-22-23502458; E-mail: shrwang@nankai.edu.cn

DOI: 10.1016/S1003-6326(11)61187-4