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Polypyrrole–ZnO hybrid sensor: Effect of camphor sulfonic acid doping on physical and gas sensing properties
Synthetic Metals 162 (2012) 1598–1603
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Polypyrrole–ZnO hybrid sensor: Effect of camphor sulfonic acid doping on physical and gas sensing properties M.A. Chougule a , Shashwati Sen b , V.B. Patil a,∗ a b
Materials Research Laboratory, School of Physical Sciences, Solapur University, Solapur 413255, MS, India Crystal Technology Section, Technical Physics Division, BARC, Mumbai, India
a r t i c l e
i n f o
Article history: Received 26 April 2012 Received in revised form 7 June 2012 Accepted 1 July 2012 Available online 19 August 2012 Keywords: CSA PPy–ZnO hybrid FESEM Selectivity Sensitivity NO2 sensor
a b s t r a c t The polypyrrole–zinc oxide (PPy–ZnO) hybrid sensor doped with different weight ratios of camphor sulfonic acid (CSA) were prepared by spin coating technique. These CSA doped PPy–ZnO hybrids were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Fourier transform infrared (FTIR) and UV–vis techniques which proved the formation of polypyrrole, PPy–ZnO and the interaction between polypyrrole–ZnO (PPy–ZnO) hybrid with CSA doping. The gas sensing properties of the PPy–ZnO hybrid films doped with CSA have been studied for oxidizing (NO2 ) as well as reducing (H2 S, NH3 , C2 H5 OH and CH3 OH) gases at room temperature. We demonstrate that CSA (30 wt.%) doped PPy–ZnO hybrid films are highly selective to NO2 along with high-sensitivity at low concentration (80% to 100 ppm), fast-response time (120 s) and better stability, which suggested that the CSA (30%) doped PPy–ZnO hybrid films are potential candidate for NO2 detection at room temperature. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx, NO and NO2 ) are typical air pollutants exhausting from car engines, boilers, or any other combustion processes. Nitrogen dioxide (NO2 ) is colorless, flammable, and dangerous, even at very low concentration. The interaction of NO2 and CO in sunlight tends to produce O3 , which is believed to be harmful to plants and the respiratory system of human beings and animals due to its strongly oxidizing behavior. Therefore, from the public health and environmental protection viewpoint, the sensitive detection of nitrogen dioxide is of great scientific importance. Therefore, it is needed to monitor NO2 gas at ppm levels at room temperature. In recent years, many efforts were reported on NO2 sensing using metal-oxides and complex metal-oxides; however, they operate at high temperatures (150–450 ◦ C) [1–4]. Conducting polymers, such as polypyrrole (PPy), polyaniline (Pani), polythiophene (PTh) and their derivatives, have been widely investigated as an active layer for the development of gas sensors operating at room temperature. The sensors made of conducting polymers have many improved characteristics, including high sensitivities and short response time at room temperature. Furthermore, conducting polymers have good mechanical properties, which allow a facile fabrication of sensors. As a result, a lot of
∗ Corresponding author. E-mail address: [email protected] (V.B. Patil). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.07.002
attentions were paid to the sensors fabricated from conducting polymers [5]. However, a major disadvantage of the conducting polymer based sensors is their lack in specificity i.e. they sense all the gases. This is because conducting polymers are doped/undoped by redox reactions; therefore, their doping level can be altered by transferring electrons from or to the gas. Electron-acceptor (oxidizing) gases, such as NO2 , Cl2 can remove electrons from the aromatic rings of conducting polymers. When this occurs at a ptype conducting polymer (e.g. PPy), the doping level as well as the electric conductance of the conducting polymer is enhanced. An opposite process will occur when detecting an electro-donating (reducing) gas e.g. H2 S, NH3 , CH4 and CO. Thus, a major challenge in conducting polymers based sensors is to enhance their specificity. Fortunately, the structural properties of conducting polymers can be modified conveniently by adding metals [6], metal-oxides [7,8] organic molecules [9] or carbon nanotubes [10], which in many cases improve the specificity. Shen and Wan studied the solubility in m-cresol, roomtemperature conductivity, morphology and thermal stability of PPy synthesized by in situ doping polymerization in the present of sulfonic acid [11]. It was noted that good solvating ability of sulfonic acid, such as DBSA and BNSA (5-butylnaphthalene), renders PPy soluble, while sulfonic acids only having large molecular size, such as CSA (camphor sulfonic acid) and MBSA (p-methylbenzenes sulfonic acid or p-toluene sulfonic acid), fail to make PPy soluble. The nature of sulfonic acid also has an influence on morphology of the resulting PPy. The images of PPy doped with CSA, DBSA and MBSA
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Fig. 1. Schematic view of room temperature gas sensor unit.
have typical granular morphology. But it has been found that doping PPy with mixed acid containing CSA and DBSA could get soluble conductive PPy with room temperature conductivity (2–18 S/cm) [12]. In this paper, we demonstrate that by modifying the PPy–ZnO hybrid films with CSA doping, one can make highly sensitive, stable and selective NO2 sensors operating at room temperature. We further demonstrate that CSA (30 wt.%) doped PPy–ZnO hybrid films show a linear response to NO2 in the concentration range 10–100 ppm, with a very high sensitivity of ∼80% and better stability (∼97.5%) at low concentration (100 ppm) operating at room temperature.
2. Experimental methods 2.1. Preparation of PPy–ZnO hybrid Polypyrrole (PPy) was synthesized by polymerization of pyrrole in the presence of ammonium persulphate as an oxidant by chemical oxidative polymerization method [13]. ZnO (NPs) was synthesized by sol–gel method using zinc acetate as a source material [14]. Such obtained PPy and ZnO (NPs) were mixed together in 50% weight ratio and grinded in agate mortar for 1 h to get PPy–ZnO (50%) nanohybrids [15]. In our previous report [15], it was found that the PPy nanohybrids filled with different loadings of ZnO nanoparticles, the particles are closely packed and no bare nanoparticles are observed even at the highest loading of 50 wt.%, which suggests the feasibility of this method to fabricate well dispersed nanoparticles with uniform coating layer.
2.3. Characterization and measurement methods The structural characterization of the CSA doped PPy–ZnO hybrid films were carried out using a Philips - PW 3710 (Almelo, ˚ Holland) X-ray diffractometer with CuK␣ radiation ( = 1.5406 A) in a 2 range from 10◦ to 80◦ at step width 0.02◦ and step time 1.25 s. The surface morphological study of the CSA doped PPy–ZnO hybrid films were carried out using field emission scanning electron microscopy (FESEM) (Model: MIRA3 TESCAN) with an acceleration voltage of 20 kV.The chemical structure of CSA doped PPy–ZnO hybrid films were carried out by Fourier transform infrared spectroscopy (FTIR) using Perkin-Elmer 100 spectrophotometer in reflectance mode. The thickness of the CSA (10–50%) doped PPy–ZnO hybrid films were measured using Ambios XP-1 surface profiler and it is in the range of 0.863–0.167 m. In the gas sensitivity measurements, reducing gases such as: C2 H5 OH, CH3 OH, NH3 , H2 S and oxidizing gases Cl2 , NO2 were used as detecting gases. The gas sensing behaviors of CSA doped PPy–ZnO hybrid were measured by custom fabricated gas sensing unit operating at room temperature. The sensing materials were deposited on glass substrate with silver electrodes, 1 mm wide and 1 mm apart from each other for the contacts. The schematic diagram of a typical gas sensor unit is shown in Fig. 1. The gas sensing measurement was in an airtight SS housing of 250 cc chamber and measured quantity of desired gas (from a standard canister of 1000 ppm concentration) was injected through syringe so as to yield desired gas concentration in the housing. The electrical response of the sensor was measured with a Keithley 6514 System Electrometer, which was controlled by a computer and to measure the resistance variation of the sensor films. The definition of gas response S (%) was the ratio of Ra − Rg /Ra × 100% where, Ra and Rg represented the resistance of the sensor in clear air and testing gas, respectively.
2.2. Preparation CSA doped PPy–ZnO hybrid films 3. Results and discussion The CSA doped PPy–ZnO hybrids were prepared by adding CSA with different weight ratios (10–50%) into the PPy–ZnO (50%) hybrid. These CSA doped PPy–ZnO hybrids were dissolved in mcresol and stirring it for 11 h at room temperature. Thin films of CSA doped nanohybrids were prepared on glass substrate by spin coating technique at 3000 rpm for 40 s and dried on hot plate at 100 ◦ C for 10 min.
3.1. Structural studies The X-ray diffraction patterns of pure PPy, PPy–ZnO (50%) and PPy–ZnO–CSA (30%) hybrids are displayed in Fig. 2. For pure PPy, the XRD pattern showed a broad, amorphous diffraction peak at approximately 2 = 20–30◦ in Fig. 2(a). In Fig. 2(b), new peaks
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Fig. 2. X-ray diffraction pattern of (a) pure PPy, (b) PPy–ZnO (50%), (c) PPy–ZnO–CSA (30%).
at 2 = 31.86◦ , 34.50◦ , 36.32◦ and 47.65◦ appeared, which corresponded well to (1 0 0), (0 0 2), (1 0 1) and (1 0 2) diffractions pattern of hexagonal wurtzite ZnO (JCPDS: 79.0208) [14]. Other peaks at 56.71◦ , 62.98◦ and 68.10◦ corresponded to the hexagonal wurtzite ZnO (1 1 0), (1 0 3) and (1 1 2) crystal planes, respectively. Compared with Fig. 2(b), new peaks at, 15.51◦ , and 17.59◦ in the crystal pattern of 30% CSA doped PPy–ZnO hybrid [PPy–ZnO–CSA(30%)] belong to CSA and more significant in the doping patterns appeared in Fig. 2(c). These results indicate the effect of camphor sulfonic acid doping on structural properties of PPy–ZnO hybrid. 3.2. Morphological studies Fig. 3 shows the field emission scanning electron micrographs (FESEM) of pure PPy, PPy–ZnO (50%) and PPy–ZnO–CSA (30%) hybrid films at 100,000× magnification, respectively. Fig. 3(a)
Fig. 4. FTIR spectra of (a) pure PPy, (b) PPy–ZnO (50%), (c) PPy–ZnO–CSA (30%).
shows the surface morphology of the PPy film has a uniform porous, large area granular morphology [13]. Fig. 3(b) shows FESEM of ZnO. Morphology of ZnO is aggregation of nanoparticles. The FESEM micrograph of hybrids of PPy–ZnO (50%) (Fig. 3(c)) clearly shows the uniform distribution of ZnO NPs into PPy matrix. Fig. 3(d) shows the surface morphology of the 30% CSA doped PPy–ZnO hybrids, which is uniform granular porous morphology attributed to the homogeneous dispersion of CSA in PPy–ZnO hybrids. It is also revealed that PPy–ZnO hybrids doped with 30% CSA shows homogeneous, large area, porous granular morphology suitable for gas sensing application. It promotes adsorption of gas molecules through the film surface, so excellent sensitivity can be expected [14,16].
Fig. 3. FESEM image of (a) pure PPy, (b) ZnO, (c) PPy–ZnO (50%), (d) PPy–ZnO–CSA (30%).
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Fig. 6. Photograph of experimental setup for gas sensor measurement. Fig. 5. UV–vis spectra of (a) pure PPy, (b) PPy–ZnO (50%), (c) PPy–ZnO–CSA (30%).
3.3. FTIR studies Fig. 4 shows Fourier transform infrared (FTIR) spectra of pure PPy, PPy–ZnO (50%) and CSA doped PPy–ZnO hybrid using KBr pellets were recorded in the range of 500–4000 cm−1 . For pure PPy, all the characteristics peaks are observed at 794 cm−1 ( C H wagging), 926 cm−1 (C C out of phase), 1048 cm−1 ( C H in plane vibration), 1292 cm−1 (C C bond), 1478 cm−1 (vibration of the pyrrole ring), 1558 cm−1 (C C bond), 1705 cm−1 (C N bond) and 3115 cm−1 (N H stretching vibrations), confirming the formation of polypyrrole [16,17]. In Fig. 3(b), FTIR spectra of PPy–ZnO (50%) hybrids show some shift toward lower wave number due to addition of ZnO contents. The most changes are shift of C H wagging, C C and C N bonds to lower values from 794–791 cm−1 , 1558–1555 cm−1 and 1705–1701 cm−1 , respectively. The peak shifts in PPy–ZnO hybrid which illustrated that PPy had incorporated with ZnO successfully and PPy gets highly reduced from oxidized form [15,18,19]. Fig. 3(c) shows FTIR spectra of 30% CSA doped PPy–ZnO hybrids, in which the shift of C H in plane vibration peak to high value from 1035 cm−1 in PPy–ZnO hybrid to 1046 cm−1 in CSA doped hybrids. The characteristics peak at 1701 cm−1 indicates the C N bond in the polymer chain. However the red shift in this band is observed from 1701 cm−1 to 1736.33 cm−1 . The blue shift of C H wagging is observed from 614.31 cm−1 to 604.89 cm−1 . This red shift or blue shift can be attributed to the interaction of CSA with PPy–ZnO hybrid. Note that the polymer is protonated in part by surface anions is demonstrated by the presence of the peak at 614.31 cm−1 , which is attributed to a stretching vibration in the surface anion [20]. The presence of CSA is confirmed by the bonds at 1046 cm−1 (SO3− ) [21–23].
delocalized polaron free carrier tail absorption [24]. Such difference support the high conductivity of 30% CSA doped PPy–ZnO. 3.5. Gas sensitivity measurement Fig. 6 shows the experimental set up used for measurement of gas sensing properties of CSA doped PPy–ZnO hybrid films. 3.5.1. Gas sensitivity detection The conductivity of the polymer hybrids depends on the composition, conjugation length and chain length, level and type of doping, compactness of the sample, etc. [20]. For each of the conductivity values reported, the conductivity value of CSA doped PPy–ZnO hybrid was higher than that of pure PPy and PPy–ZnO hybrids [15]. In this experiment, CSA was used as surfactant and the polymer is protonated in part by surface anions in PPy–ZnO hybrids, so the CSA doped PPy–ZnO hybrids should have good conductivity and gas sensitivity. The sensitivity of PPy–ZnO–CSA (10%), PPy–ZnO–CSA (20%), PPy–ZnO–CSA (30%), PPy–ZnO–CSA (40%) and PPy–ZnO–CSA (50%) hybrids to 100 ppm NO2 concentration at room temperatures were 54%, 71%, 80%, 60% and 52%, respectively, which were much higher than that of pure PPy (14%) and PPy–ZnO (50%) hybrids (38%) as shown in Fig. 7 [25]. It was found that PPy–ZnO–CSA (30%) hybrid has better selectivity and maximum sensitivity (80%) than PPy, PPy–ZnO (50%) hybrid and other CSA doped PPy–ZnO hybrids to 100 ppm NO2 at room temperature. The 30% CSA doped PPy–ZnO hybrid provides uniform granular porous morphology and a high surface area to interact NO2 gas resulting in increased response.
3.4. UV–vis studies Fig. 5 shows UV–vis spectra of pure PPy, PPy–ZnO (50%) and 30% CSA doped PPy–ZnO hybrid films. The PPy film exhibit absorption peaks at 442 nm (2.81 eV). In case of PPy–ZnO (50%) hybrid film, peaks due to PPy shift to 429 nm (2.89 eV). It is observed that the band gap of PPy increased indicating the interaction between PPy and ZnO NPs and the peak shifts to higher energy that account to reduction of PPy after addition of ZnO NPs [12]. In case of 30% CSA doped PPy–ZnO hybrid, the shifting toward the higher wavelength side is observed that means peaks observed in PPy–ZnO hybrid at 429 nm is shifted to 381 nm. The shifting of the bands in the spectra of 30% CSA doped PPy–ZnO hybrid indicates that CSA interact strongly with PPy–ZnO hybrid and has the effect of doping on PPy–ZnO hybrids. However, UV spectra of 30% CSA doped PPy–ZnO hybrid exhibit the flat peak at 965 nm followed by free carrier tail indicates that the localized polaron band is converted to the
Fig. 7. The sensitivity variation of the sensors based on pure PPy, PPy–ZnO (50%) and CSA doped PPy–ZnO hybrids to 100 ppm NO2 at room temperature.
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Fig. 8. The sensitivity of PPy–ZnO (30%) sensor film to 100 ppm NO2 , NH3 , H2 S.
In order to test the cross sensitivity, the sensor based on CSA (30%) doped PPy–ZnO hybrid thin film was exposed to CH3 OH, C2 H5 OH, NH3 , H2 S and NO2 gases at room temperature. The film exhibits high sensitivity to NO2 , but no sensitivity to 100 ppm CH3 OH, C2 H5 OH and very less sensitivity to 100 ppm NH3 and H2 S as shown in Fig. 8. It was observed that the CSA (30%) doped PPy–ZnO hybrid film showed more selectivity and sensitivity for NO2 compared to H2 S and NH3 at room temperature. The higher sensitivity toward NO2 than NH3 , CH3 OH, C2 H5 OH, and H2 S, can be explained on the basis of different interactions between sensing film and adsorbed gas. CSA doped PPy–ZnO hybrid film is p-type material and when it interacts with NO2 (oxidizing gas) there is increase in charge carrier density. This results in increasing the conductivity of material and film resistance decreases. In case of reducing gases like NH3 , CH3 OH, C2 H5 OH and H2 S there is a decrease in charge carrier concentration and thereby decreasing conductivity. The interaction of H2 S, CH3 OH, C2 H5 OH and NH3 with CSA doped PPy–ZnO hybrid film is very less as compared to NO2 ; hence it shows very slow response and less sensitivity. The selectivity measured in terms of selectivity coefficient of a target gas to another gas is defined as K = SA /SB , where SA and SB are the responses of a sensor to a target gas A and an interference gas B, respectively. The CSA (30%) doped PPy–ZnO hybrid film showed more selectivity for NO2 over H2 S compared to NH3 (SNO2 /SNH3 = 6.66, SNO2 /SH2 S = 8.88) at room temperature. It revealed that NO2 is the more selective against H2 S and poor selective against NH3 . Therefore CSA (30%) doped PPy–ZnO hybrid film is utilized for further NO2 gas sensing study. Fig. 9 shows the dynamic variation of the response of the CSA doped PPy–ZnO hybrid sensor with time upon exposure to 10–100 ppm of NO2 at room temperature. From Fig. 9 it is clear that the initially the response of sensor film increases from 15 to 80% with increasing concentration of NO2 . At 100 ppm, the CSA doped PPy–ZnO sensor showed the maximum response of 80%. Such a higher value of response is believed to be due to the sensitivity of the hybrid sensors mainly determined by the interactions between the target gas and the surface of the sensor. So, it is obvious that for the materials of greater surface area, the interactions between the adsorbed gases and the sensor surface are significant [20,24]. The CSA (30%) doped PPy–ZnO hybrid porous interconnected granular morphology (Fig. 3) provided a high surface area to interact with NO2 molecules resulting in increased sensitivity. The gas sensitivity, S (%) of CSA (30%) doped PPy–ZnO hybrid at various concentrations of NO2 is shown in Fig. 10. The gas response of CSA (30%) doped PPy–ZnO hybrid was observed to increase continuously from 15 to 80% with increasing gas concentration in the
Fig. 9. The sensitivity variation of the sensor based on CSA (30%) doped PPy–ZnO hybrid to different conc. of NO2 gas at room temperature.
range 10–100 ppm NO2 and attain the maximum gas response. The higher gas response of CSA (30%) doped PPy–ZnO hybrid sensor is attributed due to the porous granular nature of polypyrrole grown on granular ZnO surface which offers more chemical reactions to occur at the interface and ultimately results into increased gas response [20,24]. It is also observed that the 30% CSA doped PPy–ZnO hybrid sensor has short response time (1–3 min), but full recovery is not achieved even after 24 h. This may be due to NO2 , a heavier gas than air which can block the interaction sites of CSA doped PPy–ZnO hybrid sensor [25]. 3.5.2. Stability and reproducibility of CSA doped PPy–ZnO hybrid sensor In order to check the stability and reproducibility of 30% CSA doped PPy–ZnO hybrid sensor, the change in resistance of the film as a function of time (response curve) was recorded at room temperature for fixed (100 ppm) concentration of NO2 gas for 45 days at an interval of 5 days is shown in Fig. 11. Initially CSA doped PPy–ZnO hybrid sensor showed relatively high response, it dropped from 80 to 75% and stable response obtained after 10 days with 97.5% stability. This is because in the initial stage CSA doped PPy–ZnO hybrid sensor may undergo interface modification during operation and then reaches to steady state indicating the stability of the CSA doped PPy–ZnO hybrid sensor operating at room temperature .
Fig. 10. The sensitivity of CSA (30%) doped PPy–ZnO hybrids to different conc. NO2 gas at room temperature.
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materials offer excellent properties at room temperature, which suggest the potential application of the CSA doped PPy–ZnO hybrids in gas sensor field. Acknowledgments Authors (VBP) are grateful to DAE-BRNS, for financial support through the scheme no. 2010/37P/45/BRNS/1442. References
Fig. 11. Stability of CSA doped PPy–ZnO hybrid sensor.
3.5.3. Sensing mechanism of PPy–ZnO:CSA hybrids The CSA doped PPy–ZnO hybrid can be one of the most promising materials due to its high gas sensitivity at room temperature. Polypyrrole behaved as a p-type semiconductor, and ZnO was an ntype semiconductor. So the composites contained the properties of p–n junctions. The resistance of pure PPy, PPy–ZnO (50%) and 30% CSA doped PPy–ZnO hybrids at room temperature were 1.0531 M, 7.0596 M and 1.1784 M, respectively [12]. The results showed that the resistance of 30% CSA doped hybrid was smaller than that of PPy–ZnO (50%) hybrid, which suggested the formation of p–n junction. The n-type ZnO formed a hetero-p–n junction to polypyrrole with a depletion region [26]. When the CSA doped PPy–ZnO hybrids were exposed to NOx that acted as a dopant, the depletion region changed, and the resistance of conducting polymer decreased continuously. Therefore, the width of the depletion region decreased, and the conductivity of the polypyrrole channel increased [26]. The resistance of the CSA doped PPy–ZnO hybrids changed so dramatically that made it easily to detect small quantity of NOx with high sensitivity. 4. Conclusion We have fabricated CSA doped PPy–ZnO hybrid film prepared by spin coating technique showed high sensitivity, stability and better selectivity to NO2 gas at room temperature than PPy–ZnO(50%) hybrid at low concentration. Among all of the CSA doped PPy–ZnO hybrids, 30% CSA doped PPy–ZnO hybrid showed the highest sensitivity (80%) with stability (97.5%) at room temperature. Such
[1] T. Miyata, T. Hikosaka, T. Minami, Sensors and Actuators B 69 (2000) 16. [2] Y. Takao, K. Miyazaki, Y. Shimizu, M. Egashira, Journal of the Electrochemical Society 141 (1994) 1028. [3] A. Galdikas, Z. Martunas, A. Setkus, Sensors and Actuators B 7 (1992) 633. [4] T.V. Belysheva, L.P. Bogovtseva, Journal of Analytical Chemistry 61 (2006) 494. [5] H. Bai, G. Shi, Sensors 7 (3) (2007) 267–307. [6] A.A. Athawale, S.V. Bhagwat, P.P. Katre, Sensors and Actuators B 114 (2006) 263. [7] L. Geng, S.R. Wang, Y.Q. Zhao, P. Li, S.M. Zhang, W.P. Huang, S.H. Wu, Materials Chemistry and Physics 99 (2006) 15–19. [8] V. Saxena, D.K. Aswal, M. Kaur, S.P. Koiry, R.J. Kshirsagar, S.K. Deshpande, S.K. Gupta, J.V. Yakhmi, Applied Physics Letters 90 (2007) 043516. [9] C.N. Van, K. Potje-Kamloth, Thin Solid Films 392 (2001) 113–121. [10] K.H. An, S.Y. Jeong, H.R. Hwang, Y.H. Lee, Advanced Materials 16 (12) (2004) 1005–1009. [11] Y.Q. Shen, M.X. Wan, Synthetic Metals 96 (2) (1998) 127–132. [12] Y.Q. Shen, M.X. Wan, Journal of Applied Polymer Science 68 (8) (1998) 1277–1284. [13] M.A. Chougule, S.G. Pawar, S.L. Patil, B.T. Raut, P.R. Godse, Shashwati Sen, B.P. Vikas, IEEE Sensors Journal 11 (9) (2011) 2137–2141. [14] S.L. Patil, M.A. Chougule, S.G. Pawar, B.T. Raut, Shashwati Sen, V.B. Patil, Journal of Alloys and Compounds 509 (2011) 10055–10061. [15] M.A. Chougule, S. Sen, V.B. Patil, Journal of Applied Polymer Science (2012), http://dx.doi.org/10.1002/app.36475. [16] H. Shiigi, M. Kishimoto, H. Yakabe, B. Deore, T. Nagaoka, Analytical Sciences 18 (1) (2002) 41–44. [17] K. Cheah, M. Forsyth, V.T. Truong, Synthetic Metals 94 (2) (1998) 215–219. [18] Lin-Xia Wang, Xin-Gui Li, Yu-Liang Yang, Reactive & Functional Polymers 47 (2001) 125–139. [19] J.Y. Ouyang, Y.F. Li, Polymer 38 (1997) 3997–3999. [20] H. Tai, Y. Juang, G. Xie, J. Yu, X. Chen, Z. Ying, Sensors and Actuators B 129 (2008) 319–326. [21] C. He, Y. Tan, Y. Li, Journal of Applied Polymer Science 87 (9) (2003) 1537–1540. [22] J. Stejkal, I. Spurina, M. Trchova, J. Prokes, J. Krivka, E. Tobolkova, Macromolecules 31 (1998) 2218–2222. [23] E. Konyushenko, J. Stejkal, M. Trchova, J. Hradil, J. Kovarova, J. Prokes, M. Cieslar, J.Y. Hwang, K.H. Chen, I. Sapurina, Polymer 47 (2006) 5715–5723. [24] S.G. Pawar, S.L. Patil, M.A. Chougule, B.T. Raut, P.R. Godase, R.N. Mulik, V.B. Shashwati Sen, Patil, IEEE Sensor Journal 11 (11) (2011) 2980–2985. [25] M.A. Chougule, D.S. Dalvi, Sawanta Mali, P.S. Patil, A.V. Moholkar, G.L. Agawane, J.H. Kim, Shashwati Sen, V.B. Patil, Measurement, doi.org/10.1016/j.measurement.2012.04.023. [26] X. Ai, N. Anderson, J.C. Guo, J. Kowalik, L.M. Tolbert, T.Q. Lian, Journal of Applied Chemistry B 110 (2006) 25496–25503.
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Polypyrrole–ZnO hybrid sensor: Effect of camphor sulfonic acid doping on physical and gas sensing properties M.A. Chougule a , Shashwati Sen b , V.B. Patil a,∗ a b
Materials Research Laboratory, School of Physical Sciences, Solapur University, Solapur 413255, MS, India Crystal Technology Section, Technical Physics Division, BARC, Mumbai, India
a r t i c l e
i n f o
Article history: Received 26 April 2012 Received in revised form 7 June 2012 Accepted 1 July 2012 Available online 19 August 2012 Keywords: CSA PPy–ZnO hybrid FESEM Selectivity Sensitivity NO2 sensor
a b s t r a c t The polypyrrole–zinc oxide (PPy–ZnO) hybrid sensor doped with different weight ratios of camphor sulfonic acid (CSA) were prepared by spin coating technique. These CSA doped PPy–ZnO hybrids were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Fourier transform infrared (FTIR) and UV–vis techniques which proved the formation of polypyrrole, PPy–ZnO and the interaction between polypyrrole–ZnO (PPy–ZnO) hybrid with CSA doping. The gas sensing properties of the PPy–ZnO hybrid films doped with CSA have been studied for oxidizing (NO2 ) as well as reducing (H2 S, NH3 , C2 H5 OH and CH3 OH) gases at room temperature. We demonstrate that CSA (30 wt.%) doped PPy–ZnO hybrid films are highly selective to NO2 along with high-sensitivity at low concentration (80% to 100 ppm), fast-response time (120 s) and better stability, which suggested that the CSA (30%) doped PPy–ZnO hybrid films are potential candidate for NO2 detection at room temperature. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx, NO and NO2 ) are typical air pollutants exhausting from car engines, boilers, or any other combustion processes. Nitrogen dioxide (NO2 ) is colorless, flammable, and dangerous, even at very low concentration. The interaction of NO2 and CO in sunlight tends to produce O3 , which is believed to be harmful to plants and the respiratory system of human beings and animals due to its strongly oxidizing behavior. Therefore, from the public health and environmental protection viewpoint, the sensitive detection of nitrogen dioxide is of great scientific importance. Therefore, it is needed to monitor NO2 gas at ppm levels at room temperature. In recent years, many efforts were reported on NO2 sensing using metal-oxides and complex metal-oxides; however, they operate at high temperatures (150–450 ◦ C) [1–4]. Conducting polymers, such as polypyrrole (PPy), polyaniline (Pani), polythiophene (PTh) and their derivatives, have been widely investigated as an active layer for the development of gas sensors operating at room temperature. The sensors made of conducting polymers have many improved characteristics, including high sensitivities and short response time at room temperature. Furthermore, conducting polymers have good mechanical properties, which allow a facile fabrication of sensors. As a result, a lot of
∗ Corresponding author. E-mail address: [email protected] (V.B. Patil). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.07.002
attentions were paid to the sensors fabricated from conducting polymers [5]. However, a major disadvantage of the conducting polymer based sensors is their lack in specificity i.e. they sense all the gases. This is because conducting polymers are doped/undoped by redox reactions; therefore, their doping level can be altered by transferring electrons from or to the gas. Electron-acceptor (oxidizing) gases, such as NO2 , Cl2 can remove electrons from the aromatic rings of conducting polymers. When this occurs at a ptype conducting polymer (e.g. PPy), the doping level as well as the electric conductance of the conducting polymer is enhanced. An opposite process will occur when detecting an electro-donating (reducing) gas e.g. H2 S, NH3 , CH4 and CO. Thus, a major challenge in conducting polymers based sensors is to enhance their specificity. Fortunately, the structural properties of conducting polymers can be modified conveniently by adding metals [6], metal-oxides [7,8] organic molecules [9] or carbon nanotubes [10], which in many cases improve the specificity. Shen and Wan studied the solubility in m-cresol, roomtemperature conductivity, morphology and thermal stability of PPy synthesized by in situ doping polymerization in the present of sulfonic acid [11]. It was noted that good solvating ability of sulfonic acid, such as DBSA and BNSA (5-butylnaphthalene), renders PPy soluble, while sulfonic acids only having large molecular size, such as CSA (camphor sulfonic acid) and MBSA (p-methylbenzenes sulfonic acid or p-toluene sulfonic acid), fail to make PPy soluble. The nature of sulfonic acid also has an influence on morphology of the resulting PPy. The images of PPy doped with CSA, DBSA and MBSA
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Fig. 1. Schematic view of room temperature gas sensor unit.
have typical granular morphology. But it has been found that doping PPy with mixed acid containing CSA and DBSA could get soluble conductive PPy with room temperature conductivity (2–18 S/cm) [12]. In this paper, we demonstrate that by modifying the PPy–ZnO hybrid films with CSA doping, one can make highly sensitive, stable and selective NO2 sensors operating at room temperature. We further demonstrate that CSA (30 wt.%) doped PPy–ZnO hybrid films show a linear response to NO2 in the concentration range 10–100 ppm, with a very high sensitivity of ∼80% and better stability (∼97.5%) at low concentration (100 ppm) operating at room temperature.
2. Experimental methods 2.1. Preparation of PPy–ZnO hybrid Polypyrrole (PPy) was synthesized by polymerization of pyrrole in the presence of ammonium persulphate as an oxidant by chemical oxidative polymerization method [13]. ZnO (NPs) was synthesized by sol–gel method using zinc acetate as a source material [14]. Such obtained PPy and ZnO (NPs) were mixed together in 50% weight ratio and grinded in agate mortar for 1 h to get PPy–ZnO (50%) nanohybrids [15]. In our previous report [15], it was found that the PPy nanohybrids filled with different loadings of ZnO nanoparticles, the particles are closely packed and no bare nanoparticles are observed even at the highest loading of 50 wt.%, which suggests the feasibility of this method to fabricate well dispersed nanoparticles with uniform coating layer.
2.3. Characterization and measurement methods The structural characterization of the CSA doped PPy–ZnO hybrid films were carried out using a Philips - PW 3710 (Almelo, ˚ Holland) X-ray diffractometer with CuK␣ radiation ( = 1.5406 A) in a 2 range from 10◦ to 80◦ at step width 0.02◦ and step time 1.25 s. The surface morphological study of the CSA doped PPy–ZnO hybrid films were carried out using field emission scanning electron microscopy (FESEM) (Model: MIRA3 TESCAN) with an acceleration voltage of 20 kV.The chemical structure of CSA doped PPy–ZnO hybrid films were carried out by Fourier transform infrared spectroscopy (FTIR) using Perkin-Elmer 100 spectrophotometer in reflectance mode. The thickness of the CSA (10–50%) doped PPy–ZnO hybrid films were measured using Ambios XP-1 surface profiler and it is in the range of 0.863–0.167 m. In the gas sensitivity measurements, reducing gases such as: C2 H5 OH, CH3 OH, NH3 , H2 S and oxidizing gases Cl2 , NO2 were used as detecting gases. The gas sensing behaviors of CSA doped PPy–ZnO hybrid were measured by custom fabricated gas sensing unit operating at room temperature. The sensing materials were deposited on glass substrate with silver electrodes, 1 mm wide and 1 mm apart from each other for the contacts. The schematic diagram of a typical gas sensor unit is shown in Fig. 1. The gas sensing measurement was in an airtight SS housing of 250 cc chamber and measured quantity of desired gas (from a standard canister of 1000 ppm concentration) was injected through syringe so as to yield desired gas concentration in the housing. The electrical response of the sensor was measured with a Keithley 6514 System Electrometer, which was controlled by a computer and to measure the resistance variation of the sensor films. The definition of gas response S (%) was the ratio of Ra − Rg /Ra × 100% where, Ra and Rg represented the resistance of the sensor in clear air and testing gas, respectively.
2.2. Preparation CSA doped PPy–ZnO hybrid films 3. Results and discussion The CSA doped PPy–ZnO hybrids were prepared by adding CSA with different weight ratios (10–50%) into the PPy–ZnO (50%) hybrid. These CSA doped PPy–ZnO hybrids were dissolved in mcresol and stirring it for 11 h at room temperature. Thin films of CSA doped nanohybrids were prepared on glass substrate by spin coating technique at 3000 rpm for 40 s and dried on hot plate at 100 ◦ C for 10 min.
3.1. Structural studies The X-ray diffraction patterns of pure PPy, PPy–ZnO (50%) and PPy–ZnO–CSA (30%) hybrids are displayed in Fig. 2. For pure PPy, the XRD pattern showed a broad, amorphous diffraction peak at approximately 2 = 20–30◦ in Fig. 2(a). In Fig. 2(b), new peaks
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Fig. 2. X-ray diffraction pattern of (a) pure PPy, (b) PPy–ZnO (50%), (c) PPy–ZnO–CSA (30%).
at 2 = 31.86◦ , 34.50◦ , 36.32◦ and 47.65◦ appeared, which corresponded well to (1 0 0), (0 0 2), (1 0 1) and (1 0 2) diffractions pattern of hexagonal wurtzite ZnO (JCPDS: 79.0208) [14]. Other peaks at 56.71◦ , 62.98◦ and 68.10◦ corresponded to the hexagonal wurtzite ZnO (1 1 0), (1 0 3) and (1 1 2) crystal planes, respectively. Compared with Fig. 2(b), new peaks at, 15.51◦ , and 17.59◦ in the crystal pattern of 30% CSA doped PPy–ZnO hybrid [PPy–ZnO–CSA(30%)] belong to CSA and more significant in the doping patterns appeared in Fig. 2(c). These results indicate the effect of camphor sulfonic acid doping on structural properties of PPy–ZnO hybrid. 3.2. Morphological studies Fig. 3 shows the field emission scanning electron micrographs (FESEM) of pure PPy, PPy–ZnO (50%) and PPy–ZnO–CSA (30%) hybrid films at 100,000× magnification, respectively. Fig. 3(a)
Fig. 4. FTIR spectra of (a) pure PPy, (b) PPy–ZnO (50%), (c) PPy–ZnO–CSA (30%).
shows the surface morphology of the PPy film has a uniform porous, large area granular morphology [13]. Fig. 3(b) shows FESEM of ZnO. Morphology of ZnO is aggregation of nanoparticles. The FESEM micrograph of hybrids of PPy–ZnO (50%) (Fig. 3(c)) clearly shows the uniform distribution of ZnO NPs into PPy matrix. Fig. 3(d) shows the surface morphology of the 30% CSA doped PPy–ZnO hybrids, which is uniform granular porous morphology attributed to the homogeneous dispersion of CSA in PPy–ZnO hybrids. It is also revealed that PPy–ZnO hybrids doped with 30% CSA shows homogeneous, large area, porous granular morphology suitable for gas sensing application. It promotes adsorption of gas molecules through the film surface, so excellent sensitivity can be expected [14,16].
Fig. 3. FESEM image of (a) pure PPy, (b) ZnO, (c) PPy–ZnO (50%), (d) PPy–ZnO–CSA (30%).
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Fig. 6. Photograph of experimental setup for gas sensor measurement. Fig. 5. UV–vis spectra of (a) pure PPy, (b) PPy–ZnO (50%), (c) PPy–ZnO–CSA (30%).
3.3. FTIR studies Fig. 4 shows Fourier transform infrared (FTIR) spectra of pure PPy, PPy–ZnO (50%) and CSA doped PPy–ZnO hybrid using KBr pellets were recorded in the range of 500–4000 cm−1 . For pure PPy, all the characteristics peaks are observed at 794 cm−1 ( C H wagging), 926 cm−1 (C C out of phase), 1048 cm−1 ( C H in plane vibration), 1292 cm−1 (C C bond), 1478 cm−1 (vibration of the pyrrole ring), 1558 cm−1 (C C bond), 1705 cm−1 (C N bond) and 3115 cm−1 (N H stretching vibrations), confirming the formation of polypyrrole [16,17]. In Fig. 3(b), FTIR spectra of PPy–ZnO (50%) hybrids show some shift toward lower wave number due to addition of ZnO contents. The most changes are shift of C H wagging, C C and C N bonds to lower values from 794–791 cm−1 , 1558–1555 cm−1 and 1705–1701 cm−1 , respectively. The peak shifts in PPy–ZnO hybrid which illustrated that PPy had incorporated with ZnO successfully and PPy gets highly reduced from oxidized form [15,18,19]. Fig. 3(c) shows FTIR spectra of 30% CSA doped PPy–ZnO hybrids, in which the shift of C H in plane vibration peak to high value from 1035 cm−1 in PPy–ZnO hybrid to 1046 cm−1 in CSA doped hybrids. The characteristics peak at 1701 cm−1 indicates the C N bond in the polymer chain. However the red shift in this band is observed from 1701 cm−1 to 1736.33 cm−1 . The blue shift of C H wagging is observed from 614.31 cm−1 to 604.89 cm−1 . This red shift or blue shift can be attributed to the interaction of CSA with PPy–ZnO hybrid. Note that the polymer is protonated in part by surface anions is demonstrated by the presence of the peak at 614.31 cm−1 , which is attributed to a stretching vibration in the surface anion [20]. The presence of CSA is confirmed by the bonds at 1046 cm−1 (SO3− ) [21–23].
delocalized polaron free carrier tail absorption [24]. Such difference support the high conductivity of 30% CSA doped PPy–ZnO. 3.5. Gas sensitivity measurement Fig. 6 shows the experimental set up used for measurement of gas sensing properties of CSA doped PPy–ZnO hybrid films. 3.5.1. Gas sensitivity detection The conductivity of the polymer hybrids depends on the composition, conjugation length and chain length, level and type of doping, compactness of the sample, etc. [20]. For each of the conductivity values reported, the conductivity value of CSA doped PPy–ZnO hybrid was higher than that of pure PPy and PPy–ZnO hybrids [15]. In this experiment, CSA was used as surfactant and the polymer is protonated in part by surface anions in PPy–ZnO hybrids, so the CSA doped PPy–ZnO hybrids should have good conductivity and gas sensitivity. The sensitivity of PPy–ZnO–CSA (10%), PPy–ZnO–CSA (20%), PPy–ZnO–CSA (30%), PPy–ZnO–CSA (40%) and PPy–ZnO–CSA (50%) hybrids to 100 ppm NO2 concentration at room temperatures were 54%, 71%, 80%, 60% and 52%, respectively, which were much higher than that of pure PPy (14%) and PPy–ZnO (50%) hybrids (38%) as shown in Fig. 7 [25]. It was found that PPy–ZnO–CSA (30%) hybrid has better selectivity and maximum sensitivity (80%) than PPy, PPy–ZnO (50%) hybrid and other CSA doped PPy–ZnO hybrids to 100 ppm NO2 at room temperature. The 30% CSA doped PPy–ZnO hybrid provides uniform granular porous morphology and a high surface area to interact NO2 gas resulting in increased response.
3.4. UV–vis studies Fig. 5 shows UV–vis spectra of pure PPy, PPy–ZnO (50%) and 30% CSA doped PPy–ZnO hybrid films. The PPy film exhibit absorption peaks at 442 nm (2.81 eV). In case of PPy–ZnO (50%) hybrid film, peaks due to PPy shift to 429 nm (2.89 eV). It is observed that the band gap of PPy increased indicating the interaction between PPy and ZnO NPs and the peak shifts to higher energy that account to reduction of PPy after addition of ZnO NPs [12]. In case of 30% CSA doped PPy–ZnO hybrid, the shifting toward the higher wavelength side is observed that means peaks observed in PPy–ZnO hybrid at 429 nm is shifted to 381 nm. The shifting of the bands in the spectra of 30% CSA doped PPy–ZnO hybrid indicates that CSA interact strongly with PPy–ZnO hybrid and has the effect of doping on PPy–ZnO hybrids. However, UV spectra of 30% CSA doped PPy–ZnO hybrid exhibit the flat peak at 965 nm followed by free carrier tail indicates that the localized polaron band is converted to the
Fig. 7. The sensitivity variation of the sensors based on pure PPy, PPy–ZnO (50%) and CSA doped PPy–ZnO hybrids to 100 ppm NO2 at room temperature.
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Fig. 8. The sensitivity of PPy–ZnO (30%) sensor film to 100 ppm NO2 , NH3 , H2 S.
In order to test the cross sensitivity, the sensor based on CSA (30%) doped PPy–ZnO hybrid thin film was exposed to CH3 OH, C2 H5 OH, NH3 , H2 S and NO2 gases at room temperature. The film exhibits high sensitivity to NO2 , but no sensitivity to 100 ppm CH3 OH, C2 H5 OH and very less sensitivity to 100 ppm NH3 and H2 S as shown in Fig. 8. It was observed that the CSA (30%) doped PPy–ZnO hybrid film showed more selectivity and sensitivity for NO2 compared to H2 S and NH3 at room temperature. The higher sensitivity toward NO2 than NH3 , CH3 OH, C2 H5 OH, and H2 S, can be explained on the basis of different interactions between sensing film and adsorbed gas. CSA doped PPy–ZnO hybrid film is p-type material and when it interacts with NO2 (oxidizing gas) there is increase in charge carrier density. This results in increasing the conductivity of material and film resistance decreases. In case of reducing gases like NH3 , CH3 OH, C2 H5 OH and H2 S there is a decrease in charge carrier concentration and thereby decreasing conductivity. The interaction of H2 S, CH3 OH, C2 H5 OH and NH3 with CSA doped PPy–ZnO hybrid film is very less as compared to NO2 ; hence it shows very slow response and less sensitivity. The selectivity measured in terms of selectivity coefficient of a target gas to another gas is defined as K = SA /SB , where SA and SB are the responses of a sensor to a target gas A and an interference gas B, respectively. The CSA (30%) doped PPy–ZnO hybrid film showed more selectivity for NO2 over H2 S compared to NH3 (SNO2 /SNH3 = 6.66, SNO2 /SH2 S = 8.88) at room temperature. It revealed that NO2 is the more selective against H2 S and poor selective against NH3 . Therefore CSA (30%) doped PPy–ZnO hybrid film is utilized for further NO2 gas sensing study. Fig. 9 shows the dynamic variation of the response of the CSA doped PPy–ZnO hybrid sensor with time upon exposure to 10–100 ppm of NO2 at room temperature. From Fig. 9 it is clear that the initially the response of sensor film increases from 15 to 80% with increasing concentration of NO2 . At 100 ppm, the CSA doped PPy–ZnO sensor showed the maximum response of 80%. Such a higher value of response is believed to be due to the sensitivity of the hybrid sensors mainly determined by the interactions between the target gas and the surface of the sensor. So, it is obvious that for the materials of greater surface area, the interactions between the adsorbed gases and the sensor surface are significant [20,24]. The CSA (30%) doped PPy–ZnO hybrid porous interconnected granular morphology (Fig. 3) provided a high surface area to interact with NO2 molecules resulting in increased sensitivity. The gas sensitivity, S (%) of CSA (30%) doped PPy–ZnO hybrid at various concentrations of NO2 is shown in Fig. 10. The gas response of CSA (30%) doped PPy–ZnO hybrid was observed to increase continuously from 15 to 80% with increasing gas concentration in the
Fig. 9. The sensitivity variation of the sensor based on CSA (30%) doped PPy–ZnO hybrid to different conc. of NO2 gas at room temperature.
range 10–100 ppm NO2 and attain the maximum gas response. The higher gas response of CSA (30%) doped PPy–ZnO hybrid sensor is attributed due to the porous granular nature of polypyrrole grown on granular ZnO surface which offers more chemical reactions to occur at the interface and ultimately results into increased gas response [20,24]. It is also observed that the 30% CSA doped PPy–ZnO hybrid sensor has short response time (1–3 min), but full recovery is not achieved even after 24 h. This may be due to NO2 , a heavier gas than air which can block the interaction sites of CSA doped PPy–ZnO hybrid sensor [25]. 3.5.2. Stability and reproducibility of CSA doped PPy–ZnO hybrid sensor In order to check the stability and reproducibility of 30% CSA doped PPy–ZnO hybrid sensor, the change in resistance of the film as a function of time (response curve) was recorded at room temperature for fixed (100 ppm) concentration of NO2 gas for 45 days at an interval of 5 days is shown in Fig. 11. Initially CSA doped PPy–ZnO hybrid sensor showed relatively high response, it dropped from 80 to 75% and stable response obtained after 10 days with 97.5% stability. This is because in the initial stage CSA doped PPy–ZnO hybrid sensor may undergo interface modification during operation and then reaches to steady state indicating the stability of the CSA doped PPy–ZnO hybrid sensor operating at room temperature .
Fig. 10. The sensitivity of CSA (30%) doped PPy–ZnO hybrids to different conc. NO2 gas at room temperature.
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materials offer excellent properties at room temperature, which suggest the potential application of the CSA doped PPy–ZnO hybrids in gas sensor field. Acknowledgments Authors (VBP) are grateful to DAE-BRNS, for financial support through the scheme no. 2010/37P/45/BRNS/1442. References
Fig. 11. Stability of CSA doped PPy–ZnO hybrid sensor.
3.5.3. Sensing mechanism of PPy–ZnO:CSA hybrids The CSA doped PPy–ZnO hybrid can be one of the most promising materials due to its high gas sensitivity at room temperature. Polypyrrole behaved as a p-type semiconductor, and ZnO was an ntype semiconductor. So the composites contained the properties of p–n junctions. The resistance of pure PPy, PPy–ZnO (50%) and 30% CSA doped PPy–ZnO hybrids at room temperature were 1.0531 M, 7.0596 M and 1.1784 M, respectively [12]. The results showed that the resistance of 30% CSA doped hybrid was smaller than that of PPy–ZnO (50%) hybrid, which suggested the formation of p–n junction. The n-type ZnO formed a hetero-p–n junction to polypyrrole with a depletion region [26]. When the CSA doped PPy–ZnO hybrids were exposed to NOx that acted as a dopant, the depletion region changed, and the resistance of conducting polymer decreased continuously. Therefore, the width of the depletion region decreased, and the conductivity of the polypyrrole channel increased [26]. The resistance of the CSA doped PPy–ZnO hybrids changed so dramatically that made it easily to detect small quantity of NOx with high sensitivity. 4. Conclusion We have fabricated CSA doped PPy–ZnO hybrid film prepared by spin coating technique showed high sensitivity, stability and better selectivity to NO2 gas at room temperature than PPy–ZnO(50%) hybrid at low concentration. Among all of the CSA doped PPy–ZnO hybrids, 30% CSA doped PPy–ZnO hybrid showed the highest sensitivity (80%) with stability (97.5%) at room temperature. Such
[1] T. Miyata, T. Hikosaka, T. Minami, Sensors and Actuators B 69 (2000) 16. [2] Y. Takao, K. Miyazaki, Y. Shimizu, M. Egashira, Journal of the Electrochemical Society 141 (1994) 1028. [3] A. Galdikas, Z. Martunas, A. Setkus, Sensors and Actuators B 7 (1992) 633. [4] T.V. Belysheva, L.P. Bogovtseva, Journal of Analytical Chemistry 61 (2006) 494. [5] H. Bai, G. Shi, Sensors 7 (3) (2007) 267–307. [6] A.A. Athawale, S.V. Bhagwat, P.P. Katre, Sensors and Actuators B 114 (2006) 263. [7] L. Geng, S.R. Wang, Y.Q. Zhao, P. Li, S.M. Zhang, W.P. Huang, S.H. Wu, Materials Chemistry and Physics 99 (2006) 15–19. [8] V. Saxena, D.K. Aswal, M. Kaur, S.P. Koiry, R.J. Kshirsagar, S.K. Deshpande, S.K. Gupta, J.V. Yakhmi, Applied Physics Letters 90 (2007) 043516. [9] C.N. Van, K. Potje-Kamloth, Thin Solid Films 392 (2001) 113–121. [10] K.H. An, S.Y. Jeong, H.R. Hwang, Y.H. Lee, Advanced Materials 16 (12) (2004) 1005–1009. [11] Y.Q. Shen, M.X. Wan, Synthetic Metals 96 (2) (1998) 127–132. [12] Y.Q. Shen, M.X. Wan, Journal of Applied Polymer Science 68 (8) (1998) 1277–1284. [13] M.A. Chougule, S.G. Pawar, S.L. Patil, B.T. Raut, P.R. Godse, Shashwati Sen, B.P. Vikas, IEEE Sensors Journal 11 (9) (2011) 2137–2141. [14] S.L. Patil, M.A. Chougule, S.G. Pawar, B.T. Raut, Shashwati Sen, V.B. Patil, Journal of Alloys and Compounds 509 (2011) 10055–10061. [15] M.A. Chougule, S. Sen, V.B. Patil, Journal of Applied Polymer Science (2012), http://dx.doi.org/10.1002/app.36475. [16] H. Shiigi, M. Kishimoto, H. Yakabe, B. Deore, T. Nagaoka, Analytical Sciences 18 (1) (2002) 41–44. [17] K. Cheah, M. Forsyth, V.T. Truong, Synthetic Metals 94 (2) (1998) 215–219. [18] Lin-Xia Wang, Xin-Gui Li, Yu-Liang Yang, Reactive & Functional Polymers 47 (2001) 125–139. [19] J.Y. Ouyang, Y.F. Li, Polymer 38 (1997) 3997–3999. [20] H. Tai, Y. Juang, G. Xie, J. Yu, X. Chen, Z. Ying, Sensors and Actuators B 129 (2008) 319–326. [21] C. He, Y. Tan, Y. Li, Journal of Applied Polymer Science 87 (9) (2003) 1537–1540. [22] J. Stejkal, I. Spurina, M. Trchova, J. Prokes, J. Krivka, E. Tobolkova, Macromolecules 31 (1998) 2218–2222. [23] E. Konyushenko, J. Stejkal, M. Trchova, J. Hradil, J. Kovarova, J. Prokes, M. Cieslar, J.Y. Hwang, K.H. Chen, I. Sapurina, Polymer 47 (2006) 5715–5723. [24] S.G. Pawar, S.L. Patil, M.A. Chougule, B.T. Raut, P.R. Godase, R.N. Mulik, V.B. Shashwati Sen, Patil, IEEE Sensor Journal 11 (11) (2011) 2980–2985. [25] M.A. Chougule, D.S. Dalvi, Sawanta Mali, P.S. Patil, A.V. Moholkar, G.L. Agawane, J.H. Kim, Shashwati Sen, V.B. Patil, Measurement, doi.org/10.1016/j.measurement.2012.04.023. [26] X. Ai, N. Anderson, J.C. Guo, J. Kowalik, L.M. Tolbert, T.Q. Lian, Journal of Applied Chemistry B 110 (2006) 25496–25503.