Preparation, characterization and gas sensitivity of polypyrrole/ g -Fe 2 O 3 hybrid materials § Lina Geng a, * , Shihua Wu b a Department of Chemistry, Hebei Normal University, Shijiazhuang 050016, People’s Republic of China b Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China 1. Introduction Organic–inorganic hybrid materials composed of oxides and conducting polymers that can synergize or complement the properties of pure organic and inorganic materials have been used in many applications, such as in electronics, optics, coating and catalysis [1–5]. Polymer/ferric oxide hybrids in particular have superior properties to those of pure polymer and ferric oxide, with both magnetic and polymer properties, and have demonstrated wide uses in medicine, biochemistry and industry [6,7]. Gas sensors have been developed to measure gas concentration, monitor emissions in combustion processes and provide feedback control [8]. The study of organic–inorganic hybrid materials for application in gas sensors is a current research hotspot, as these hybrids can compensate for the drawbacks of single inorganic sensors with high operating temperatures and low selectivity and of organic sensors with poor processability and long response– recovery time [9,10]. Itoh et al. [11] developed a (PNMA) x MoO 3 hybrid thin film and found in evaluating its VOC (volatile organic compound) – sensing properties that the selectivity of organic/ MoO 3 hybrids can be controlled by modifying the organic components. Nardis et al. [12] reported that cobalt porphyrin/ tin dioxide has superior selectivity to methanol vapor and lower working temperatures than pure SnO 2 . Meanwhile, Hosono and Matsubara [13,14] synthesized a PPy/MoO 3 thin film and PPy/ MoO 3 pressed pellet and found that PPy/MoO 3 materials have better selectivity compared with polar VOCs. Suri et al. [15] reported on a PPy/iron oxide material that is sensitivity to humidity and to N 2 , O 2 , CO 2 and CH 4 gases at different pressures. Our previous experiments confirmed that PPy/ZnO, PPy/WO 3 and PAni/SnO 2 hybrids are superior to single polymer and oxide sensing material in terms of selectivity and working temperature [16–18]. In this work, PPy/ g -Fe 2 O 3 hybrids were prepared by simultaneous gelation and polymerization processes and then characterized by FT-IR, XRD, TG–DTA and HRTEM. The gas sensitivities of PPy/ g -Fe 2 O 3 hybrids compared to pure PPy and g -Fe 2 O 3 under CO, H 2 , NH 3 , ethanol and acetone atmosphere at low operating temperatures (<100 8C) were evaluated. The sensing mechanism of polypyrrole/ g -Fe 2 O 3 is also discussed. 2. Experimental 2.1. Preparation and characterization of PPy/ g -Fe 2 O 3 Pyrrole monomers were distilled under reduced pressure, placed in a desiccator and stored at 4 8C until use. Methoxy ethanol was added to Fe(NO 3 ) 3 Á9H 2 O in a 100 ml round bottom flask Materials Research Bulletin 48 (2013) 4339–4343 A R T I C L E I N F O Article history: Received 7 April 2013 Received in revised form 1 July 2013 Accepted 7 July 2013 Available online 15 July 2013 Keywords: A. Composites B. Sol–gel chemistry C. Differential scanning calorimetry (DSC) C. Thermogravimetric analysis (TGA) A B S T R A C T Polypyrrole (PPy)/ g -Fe 2 O 3 hybrid materials were prepared by sol–gel polymerization in situ and characterized by Fourier transform infrared (FT-IR), X-ray powder diffraction (XRD), thermogravimetric and differential thermal analysis (TG–DTA) and high-resolution transmission electron microscope (HRTEM). The gas sensitivities in CO, H 2 , NH 3 , ethanol or acetone atmospheres were determined at 30 8C, 60 8C and 90 8C. FT-IR and XRD patterns suggest that ferric oxide in the hybrids was g -Fe 2 O 3 , with a diameter of approximately 5 nm. TG–DTA and HRTEM analyses showed that different reactant molar ratios of pyrrole monomer: Fe(NO 3 ) 3 Á9H 2 O resulted in different microstructures of g -Fe 2 O 3 and molecular weights of PPy. An increased amount of Fe(NO 3 ) 3 Á9H 2 O increased the degree of uniformity of the molecular weight of PPy and resulted in a change of g -Fe 2 O 3 microstructure from granular to stick particles. The results of gas sensitivities showed that the PPy/ g -Fe 2 O 3 hybrids exhibited high sensitivity to NH 3 at mild operating temperature (<100 8C). Furthermore, the sensing mechanism was also discussed. ß 2013 The Authors. Published by Elsevier Ltd. All rights reserved. § This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: +86 311 80787400; fax: +86 311 87881815. E-mail address: genglina0102@126.com (L. Geng). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.07.020 containing a magnetic stir bar, and then distilled pyrrole was added dropwise to the stirred solution for the molar ratio of pyrrole monomer: Fe(NO 3 ) 3 Á9H 2 O = 1:3 or 1:4 (the products are denoted as S1 and S2). The solution was continuously stirred and heated at a slow rate to evaporate the solvent, and a black powder precipitate was obtained. The chemical reaction equations were as follows: FeðNO 3 Þ 3 þ 3CH 3 OCH 2 CH 2 OH ¼ FeðOCH 2 CH 2 OCH 3 Þ 3 þ 3HNO 3 MðORÞ n þ H 2 O ¼ MðORÞ x ðOHÞ nÀx þ ðn À xÞHOR ½FeðOCH 2 CH 2 OCH 3 Þ 3 isabbreviatedasMðORÞ n  ÀÀMÀÀOH þ HOÀÀMÀÀ ¼ ÀÀMÀÀOÀÀMÀÀ þ H 2 O ÀÀMÀÀOR þ ROÀÀMÀÀ ¼ ÀÀMÀÀOÀÀMÀÀ þ ROH After washing with water followed by ethanol, the products were dried in an oven and then annealed at different temperatures of 100 8C, 130 8C, 150 8C or 180 8C. The properties of the PPy/ferric oxide hybrids were analyzed using several structural methods: FT-IR (Avatar 360 FT-IR spectrophotometer), XRD (DMAX-2500 diffractometer with Cu K a radiation at 40 kV and 100 mA), TG–DTA (ZRY-2P Simultaneous Thermal Analyzer) and HRTEM (Philips T20ST, operated at 200 kV). 2.2. Determination of gas sensing characteristics CO, H 2 , NH 3 , ethanol and acetone were selected for testing the gas sensitivity of the materials. The detection system and electric circuit have been described in our previously studies [15–17]. Briefly, the materials were fabricated on an aluminum tube with Au electrodes and platinum wires. A Ni–Cr alloy through the tube was used as a heating filament. The voltage of the sensor was measured indirectly by an external resistor in the testing circuit. Gas sensitivity is defined as S = V g /V a , where V a and V g are the voltages of the sensor in clear air and in the test gas, respectively [19,20]. All experiments were carried out at a fixed humidity of 60%. 3. Results and discussion The FT-IR spectra of S1 and S2 annealed at 150 8 C were compared with that of PPy in the range of 400–4000 cm À1 (Fig. 1). The characteristic bands of PPy were observed at 1560, 1398, 1298, 1211, 1047, 930 and 790 cm À1 , which were close to those reported in the literature [21]: stretching vibration (1560 cm À1 ) of the C55C bond, stretching vibration (1298 cm À1 ) of the C55C bond, stretching vibration (1211 cm À1 ) of the C–N bond, and the pyrrole ring bonds (1407, 1398, 1047, 930, 790 cm À1 ). In the S1 (150 8C) and S2 (150 8C) spectra, characteristic peaks of PPy were also found at 1407, 1398, 1047, 930 and 790 cm À1 , and the g -Fe 2 O 3 specific bands appeared in 681, 578 and 468 cm À1 . In the spectra, the C55O bond in pyrrolidone at about 1700 cm À1 due to the overoxidation of PPy was clearly seen. The obvious absorption peak at $1390 cm À1 corresponded to KBr. The XRD patterns revealed that the diffraction peaks of the S1 and S2 samples annealed at different temperatures appeared at the same crystal face (Fig. 2). These peaks were consistent with those from the Joint Committee on Powder Diffraction Standards (JCPDS) data file (25-1402) and, along with the FT-IR spectra analysis, indicated the iron oxide in S1 and S2 was g -Fe 2 O 3 . The diffraction 4000 3500 3000 2500 2000 1500 1000 500 0 10 20 30 40 50 60 70 S2 S1 PPy Transmittance (%) Waveleng th (nm -1 ) Fig. 1. FT-IR spectra of PPy, S1 (150 8C) and S2 (150 8C) . 0 102030405060708090 0 500 1000 1500 2000 2500 3000 4 3 2 1 440 513 426 400 220 313 Intensity (a.u.) 2 The ta (deg .) 1.10 0 o C 2.13 0 o C 3.15 0 o C 4.18 0 o C 0 10 20 30 40 50 60 70 80 90 0 500 1000 1500 2000 2500 3000 4 3 2 1 440 513 426 400 220 313 Intensity (a.u.) 2 The ta (deg .) 1.10 0 o C 2.13 0 o C 3.15 0 o C 4.18 0 o C (a) (b) Fig. 2. XRD patterns of sample S1 (a) and S2 (b) annealed at different temperatures. L. Geng, S. Wu / Materials Research Bulletin 48 (2013) 4339–4343 4340 intensity increased with increasing annealing temperature, and the particle sizes of S1 and S2 samples annealed at 150 8C were 5.3 nm and 4.7 nm, respectively, according to the Scherrer formula. The S1 and S2 samples annealed at 100 8C were heated at the rate of 10 8C/min. The TG–DTA curve for S1 showed three exothermic and two weight loss processes in the range of 20– 550 8C (Fig. 3a). The two points of weight losses in the TG curve corresponded to the first and second exothermic processes in the DTA curve, while the third exothermic process had no quality change. These two exothermic peaks coupled with weight losses near 219 8C and 308 8C were caused by the degradation of PPy, as the molecular weight of PPy in S1 was not uniform, the small forms degraded first, while the larger ones degraded later. The total weight loss percentage of S1 (100 8C) was 31.9%. The third exother- mic peak near 441 8C was the crystal phase transition of g -Fe 2 O 3 to a -Fe 2 O 3 and therefore caused no weight loss. The two exothermic peaks and one point of weight loss in the TG–DTA curve of S2 (100 8C) occurred in the range of 20–600 8C (Fig. 3b). The base of the first exothermic peak near 261 8C was wide, which spanned the temperature range of the two exothermic processes near 219 8C and 308 8C of S1 (100 8C) . This result could be explained by the molecular weights of the PPy species in S2 (100 8C) being close, and therefore degradation of PPy appeared continuous during heating. The total weight loss percentage of S2 (100 8C) was 29%, similar to that of S1 (100 8C) . As with S2 (100 8C) , the second exothermic peak of S2 (100 8C) near 458 8C was the crystal phase transition of g -Fe 2 O 3 to a -Fe 2 O 3 , which did not result in weight loss. Although the percentages of weight loss of S1 (100 8C) and S2 (100 8C) were similar, the number of exothermic peaks were different due to differences in molecular weights of PPy in the two samples (Fig. 3a and b). In addition, the phase-transition temperature of S2 (100 8C) was 17 8C higher than that of S1 (100 8C) . These results could be explained further from the TEM and HRTEM micrographs. Fig. 4a shows the polymer characteristics (i.e., amorphous particles and blurry boundaries) of S1 (150 8C) , even though it was the hybrid of PPy and g -Fe 2 O 3 . The crystal lines of g - Fe 2 O 3 were not obvious even with HRTEM (Fig. 4b), which was due to the g -Fe 2 O 3 being enwrapped by PPy (Fig. 5). From the TEM and HRTEM micrographs of S2 (150 8C) , the amorphous polymer, granular and stick g -Fe 2 O 3 particles could be seen, and the length and width of the stick form was about 200 nm and 15 nm, respectively. Only g -Fe 2 O 3 diffraction peaks appeared in the XRD patterns of S2, indicating that the granular and stick particles were all g -type ferric oxide. This result indicates that the different molar ratios of py to Fe(NO 3 ) 3 Á9H 2 O can affect the morphology of iron oxide. Brezoi and Ion [22] had reported that the amount of py could influence the crystal phase of iron oxide in PPy/ iron oxide hybrids. Xia and Wang [23] and He [24] all reported that polymer conformation does influence the crystal shape of inorganic oxide. However, these authors did not discuss the specific effect of proportion of reactants on the morphology of inorganic oxide in the crystal phase. Fig. 3. TG–DTA curves of sample S1 (100 8C) (a) and S2 (100 8C) (b). Fig. 4. TEM (a) and HRTEM (b) micrographs of sample S1 (150 8C) . L. Geng, S. Wu / Materials Research Bulletin 48 (2013) 4339–4343 4341 In the gas sensing study, S1 and S2 were made for thick film sensors, and their sensitivities for CO, H 2 , NH 3 , ethanol and acetone gases were tested at 30 8C, 60 8C and 90 8C. PPy/ g -Fe 2 O 3 hybrids prepared with reactants at two differ ent ratios and four annealing temperatures all showed no gas sensitivity to 3000 ppm CO, H 2 , ethanol and acetone at 30 8C, 60 8C or 90 8C, but showed good response to 2000 ppm NH 3 under the three operating temperatures. In addition, S1 and S2 showed similar sensitivity characteristics. The response–recovery curves of S1 (150 8C) (Fig. 6a) and S2 (150 8C) (Fig. 6b) showed that they had good reversible and quick response– recovery times to 2000 ppm NH 3 at the operating temperatures of 30 8C, 60 8C and 90 8C (Fig. 6). The response and recovery times of S1 (150 8C) were 12–36 s and 20–22 s, respectively, at different working temperatures, and those of S2 (150 8C) were 17–40 s and 20–23 s. The results shown in Fig. 6 also suggest that the testing voltage increased when the NH 3 gas was inputted, which was due to the increase in resistances and decreased conductivities of 0 50 10 015 020 025 030 0 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 gas ou t gas in 3 2 1 Voltage (V) Time (s) 1.30 o C 2.60 o C 3.90 o C 0 50 10 015 020 025 030 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 gas ou t gas in 3 2 1 Voltage (V) Time (s) 1.30 o C 2.60 o C 3.90 o C (b) (a) Fig. 6. Response–recovery curves of S1 (150 8C) (a) and S2 (150 8C) (b) to 2000 ppm NH 3 at different working temperatures. Fig. 7. Varied sensitivities of S1 (a) and S2 (b) (annealed at different temperatures) with different concentrations of NH 3 at 90 8C. Fig. 5. TEM (a) and HRTEM (b) micrographs of sample S2 (100 8C) . L. Geng, S. Wu / Materials Research Bulletin 48 (2013) 4339–4343 4342 S1 (150 8C) and S2 (150 8C) in the NH 3 atmosphere. Thus, the PPy/ g - Fe 2 O 3 hybrids showed characteristics of an n-type semiconductor, although they contained both p- and n-type semiconductors. This finding may be attributed to the relatively high content of g -Fe 2 O 3 in the hybrids. However, the pure g -Fe 2 O 3 prepared as described in reference [25] showed no gas sensitivity at the operating temperatures of 30 8C, 60 8C or 90 8C, which was due to g -Fe 2 O 3 being an insulator at normal temperatures. The sensitivities of S1 and S2 annealed at 100 8C, 130 8C, 150 8C and 180 8C under different concentrations of NH 3 gases at 90 8C increased linearly with increasing concentrations of NH 3 (Fig. 7). The sensitivity curves of S1 and S2 tested at 30 8C and 60 8C were similar with that at 90 8C (data not shown). These results suggest that S1 and S2 based sensors can be used in low operating temperatures (<100 8C) to detect a wide testing range NH 3 gas concentrations. Our previous studies reported that PPy/ZnO and PPy/WO 3 had good selectivity to NO x and H 2 S respectively, but had no sensitivity to NH 3 at the high concentration of 2000 ppm [16,26]. Therefore, PPy/ g -Fe 2 O 3 hybrids can be developed in further applications as NH 3 selectivity sensors. 4. Conclusions The reactant ratio of pyrrole monomer: Fe(NO 3 ) 3 Á9H 2 O and annealing temperature of PPy/ g -Fe 2 O 3 hybrids prepared by sol–gel polymerization in situ were shown here to influence their micro- structure and gas sensitivity. Increasing amounts of Fe(NO 3 ) 3 Á9H 2 O increased the degree of PPy uniformity and resulted in the microstructure change of g -Fe 2 O 3 from granular to stick particle form. Furthermore, the PPy/ g -Fe 2 O 3 hybrids were all selectively sensitive to NH 3 gas at low temperatures (<100 8C) and could overcome the shortcomings of the long response time of PPy and high operating temperature of g -Fe 2 O 3 . 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