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SnO2 hybrid material prepared by hydrothermal route
Sensors and Actuators B 120 (2007) 568–572
Characterization and gas sensitivity study of polyaniline/SnO2 hybrid material prepared by hydrothermal route Lina Geng, Yingqiang Zhao, Xueliang Huang, Shurong Wang, Shoumin Zhang, Shihua Wu ∗ Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China Received 6 January 2006; received in revised form 10 March 2006; accepted 13 March 2006 Available online 5 May 2006
Abstract A polyaniline (PAni)/SnO2 hybrid material was prepared by a hydrothermal method and characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The XRD pattern suggested that PAni did not modify the crystal structure of SnO2 , but SnO2 affected the crystallization of PAni to some extent. The gas sensitivity of the PAni/SnO2 hybrid was also studied to ethanol and acetone at operation temperatures of 30, 60 and 90 ◦ C. It was found that the PAni/SnO2 hybrid material had gas sensitivity only when operated at 60 and 90 ◦ C, and it showed a linear relationship between the responses and the concentrations of ethanol and acetone at 90 ◦ C. The sensing mechanism was also discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Gas sensitivity; Polyaniline; SnO2 ; Hydrothermal route
1. Introduction At present, gas sensing materials can be classed mainly into two kinds: organic and inorganic materials. The requirements for useful gas sensing devices in a variety of applications are high sensitivity, fast response and recovery time, low cost and portability. Whilst the gas sensing devices based on inorganic materials are sensitive to a variety of organic species at low levels, the need for elevated operation temperatures increases power consumption, reduces sensor life and limits the portability. Doped or undoped SnO2 is most studied and used among the inorganic materials, but its operation temperature is about 250–450 ◦ C [1,2]. The gas sensing devices based on organic materials, such as polypyrrole, polyaniline and metalphthalocyanines, have gas sensitivity at room temperature, but their long response time [3,4] due to the orderly structure limits their usage. It has been reported that organic–inorganic hybrid materials can synergize or complement the properties of the pure organic or inorganic materials in electronics, optics, coating, catalysis and so on [5–8]. Recently the study of organic–inorganic
∗
Corresponding author. Tel.: +86 22 23505896; fax: +86 22 23502458. E-mail address: [email protected] (S. Wu).
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.03.009
hybrid materials for gas sensor application has proved that the hybridization improves the properties of pure organic and inorganic materials. Suri et al. [9] studied gas sensitivity of polypyrrole/iron oxide to CO2 , N2 and CH4 , and found that it had the highest sensitivity to CO2 gas. Nardis et al. [10] reported that cobalt porphyrin/tin dioxide had superior selectivity to methanol vapor and lower working temperature than pure SnO2 . Hosono et al. [11] and Matsubara et al. [12] synthesized PPy/MoO3 thin films and PPy/MoO3 pressed pellets and found that the PPy/MoO3 materials had better selectivity to polar volatile organic compounds (VOCs). In addition, Armes and Maeda [13] and Partch et al. [14] reported that this kind of hybrid materials possessed small grain size and high stability in air. Previously, our group reported the gas sensitivity of polypyrrole (PPy)/SnO2 and PPy/Fe2 O3 hybrid materials to toxic gases, such as NH3 , H2 S and NOx [15,16]. VOCs are also toxic; they can cause sick house syndrome, and are inflammable and explosive. In this paper, a PAni/SnO2 hybrid was prepared via a hydrothermal route in which PAni was polymerized in situ, and characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscope (SEM) and high-resolution electron microscope (HRTEM). The gas sensitivity study of the PAni/SnO2 material was carried out to ethanol and acetone at low operation temperatures. The sensing mechanism of the PAni/SnO2 hybrid material was also discussed.
L. Geng et al. / Sensors and Actuators B 120 (2007) 568–572
2. Experimental 2.1. Preparation and characterization of PAni/SnO2 Aniline (purchased, analytical purity) was distilled under reduced pressure before use. Ammonium peroxydisulfate (APS, (NH4 )2 S2 O8 ), HCl (38%) and SnO2 were all of analytical purity and used without further purification. Few drops of aniline were added to 50 mL water in which the concentration of H+ was adjusted to about 1 mol/L by HCl. The solution was ultrasonically treated for 30 min and then a specific mass of SnO2 (the mass ratio of aniline to SnO2 was 3:97) was added to it. The mixture was stirred and ultrasonically treated for 30 min each, then transferred to a Teflon autoclave, followed by the addition of APS at a molar ratio to aniline of 1:1. The autoclave was sealed and kept at 140 ◦ C for 4 h. When cooled to room temperature, the mixture was filtered. The precipitates were washed by deionized water, ethanol and acetone for several times, and then dried at 60 ◦ C in an oven. The crystal structure of the PAni/SnO2 hybrid material was examined by XRD (DMAX-2500 diffractometer with Cu K␣ radiation at 40 kV and 100 mA). The surface morphology was analyzed with the help of SEM (X-650, Japan, operated at 25 kV) and HRTEM (Philips T20ST, operated at 200 kV). FT-IR spectrum was recorded on an Avatar 380 FT-IR spectrophotometer using KBr pellets.
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by measuring the Vout . The response–recovery curves of the sensor, the curves of the measured voltage versus time, were recorded by a computer. The response (S) was defined as the ratio V(out)g /V(out)a , where V(out)a is the initial voltage of the sensor and V(out)g the voltage of the sensor when exposed to testing gases. The response or recovery time is the time for the voltage change to reach 90% of the total change from V(out)g to V(out)a or vice versa. 3. Results and discussions Fig. 2 shows the FT-IR spectrum of the obtained PAni/SnO2 hybrid material, in comparison with pure PAni, in the range of 400–4000 cm−1 . The pure PAni was prepared by chemical oxidation in an aqueous solution at room temperature. The acidity of the solvent was adjusted to about 1 mol/L by HCl. APS was used as an oxidant and its molar ratio to aniline was 1:1. The reaction equation was
2.2. Gas sensing characteristics measurement The gas sensitivity of PAni/SnO2 to ethanol and acetone was measured as described in ref. [17]. The sample was fabricated on an aluminum tube with Au electrodes and platinum wires. A Ni–Cr alloy wire through the tube was used as a heating filament. The volume of the testing chamber was 15 L, and a fan was used to make the test gas atmosphere homogeneous. The gas sensing test was carried out at a fixed humidity of 60% and the operation temperatures were 30, 60 and 90 ◦ C. Fig. 1 depicts the measuring electric circuit in this experiment. Vc is a circuit voltage, Vout a measured voltage, Vh a heating voltage and RL is the resistance of a loading resistor. The voltage across the sensor can be determined indirectly
Fig. 1. Electric circuit for gas sensing measurement.
Fig. 2. FT-IR spectra of: (1) pure PAni and (2) PAni/SnO2 hybrid material.
Fig. 3. XRD patterns of: (1) SnO2 , (2) PAni/SnO2 hybrid material and (3) PAni.
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Fig. 4. SEM image of PAni/SnO2 hybrid material.
. PAni exhibit characteristic bands at 1504, 1300, 1143 and 796 cm−1 [18,19], which are the C C stretching band in benzenoid rings, the C N stretching mode in Ar N, the vibration band of the dopant anion (HCl-PAni), and the C H bending vibration out of the plane of the para-disubstituted benzene rings, respectively. These characteristic peaks can also be seen in the PAni/SnO2 hybrid material, except that 796 cm−1 is overlapped by the stretching band of Sn O appeared at about 660 cm−1 . X-ray diffraction patterns of SnO2 (purchased, A.P., the particle size was about 15 nm), PAni and PAni/SnO2 hybrid material are shown in Fig. 3. It can be seen that the PAni/SnO2 hybrid material has the same profile as pure SnO2 , indicating that the crystal structure of SnO2 is not modified by PAni. Pure PAni has similarly strong peaks at 2θ, 20.44◦ (1 0 0 face) and 2θ, 25.10◦ (1 1 0 face) which were not observed in the XRD pattern of PAni/SnO2 , indicating that SnO2 nanoparticles hamper the crystallization of PAni. This is consistent with the reports of Xia and Wang [20] and He [21]. Fig. 4 shows the SEM image of the PAni/SnO2 hybrid material. It can be seen from Fig. 4 that the amorphous and spherical particles are uniformly dispersed, where the amorphous parti-
Fig. 5. HRTEM micrograph of PAni/SnO2 hybrid material.
cles are PAni and the spherical particles are SnO2 and PAni/SnO2 hybrid. Excessively irregular PAni particles can be reduced by sufficient ultrasonic treatment and stirring [22]. In order to examine the structure of PAni/SnO2 hybrid material clearly, HRTEM was carried out (Fig. 5). From Fig. 5, we can see that some of the spherical particles with clear boundaries and crystal line are bared SnO2 , and other spherical particles with blurry boundaries are SnO2 enwrapped by PAni. In the gas sensing study, we found that the PAni/SnO2 hybrid material had no gas sensitivity to ethanol or acetone when operated at 30 ◦ C. But, when operated at 60 or 90 ◦ C, it was sensitive to low concentration of ethanol and acetone. Figs. 6 and 7 show the response–recovery curves of PAni/SnO2 hybrid material to ethanol and acetone of different concentrations. From Figs. 6 and 7, we can see that the PAni/SnO2 hybrid material has good reversibility when exposed to ethanol or acetone, and it has faster response time when operated at 90 ◦ C than at 60 ◦ C, and the measurement voltage levels off quickly after exposure to ethanol or acetone. The response time to ethanol and acetone was
Fig. 6. Response–recovery curves of PAni/SnO2 hybrid material to different concentrations of ethanol at: (a) 60 ◦ C and (b) 90 ◦ C.
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Fig. 7. Response–recovery curves of PAni/SnO2 hybrid material to different concentrations of acetone at: (a) 60 ◦ C and (b) 90 ◦ C.
within 23–43 and 16–20 s, respectively, at 90 ◦ C, and the recovery time was within 16–28 and 35–48 s, respectively. Fig. 8a and b show the relationship between the responses to ethanol and acetone of PAni/SnO2 hybrid material and the concentration of vapors, respectively. It can be seen that the responses to
ethanol and acetone of PAni/SnO2 increased linearly with the increasing concentrations. PAni is a p-type semiconductor, and SnO2 an n-type, so that there are two competitive mechanisms of electronic properties in the PAni/SnO2 hybrid material. When exposed to ethanol or acetone, PAni/SnO2 exhibits the properties of n-type semiconductors; that is, the resistance of n-type semiconductors decreases when exposed to reducing gases. It suggests that the sensing mechanism of PAni/SnO2 is governed by SnO2 . This maybe caused by the fact that SnO2 presents at a high level in PAni/SnO2 hybrid material. The PAni/SnO2 hybrid material is gas-sensitive at operating temperatures (60 and 90 ◦ C) much lower than those of SnO2 (about 250–450 ◦ C). This can be explained that the positively charged depletion layer on the surface of the SnO2 could be formed owing to inter-particle electron migration from SnO2 to PAni at the p–n heterojunctions, lowering the activation energy and enthalpy of physisorption for vapors with good electron-donating characteristics [23]. 4. Conclusions An organic–inorganic hybrid material PAni/SnO2 was prepared by a hydrothermal method and studied for gas sensing of ethanol and acetone. The PAni/SnO2 hybrid material was sensitive to ethanol and acetone when operated at 60 or 90 ◦ C and showed good reversibility. The sensing mechanism was suggested to be related to the existence of p–n heterojunctions in the PAni/SnO2 hybrid material. When operated at 90 ◦ C, the PAni/SnO2 hybrid material exhibited short response and recovery time (within 1 min). The material developed can overcome the shortcomings of long response time of PAni and the high operation temperature of SnO2 , thus presenting very important features for practical use. References
Fig. 8. Variations in response to: (a) ethanol and (b) acetone of PAn/iSnO2 hybrid material as a function of concentrations at 90 ◦ C.
[1] R. Ionescu, A. Vancu, F. Buta, Diode-like SnO2 gas detection devices, Sens. Actuators B 43 (1997) 126–131. [2] C. Bittencourt, E. Llobet, P. Ivanox, X. Correrg, X. Vilanova, J. Brezmes, J. Hubalek, K. Malysz, J.J. Pireaux, J. Calderer, Influence of the doping method on the sensitivity of Pt-doped SnO2 , Sens. Actuators B 97 (2004) 67–73.
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[19] A.P. Monkman, P. Adams, Optical and electronic properties of stretchoriented solution-cast polyaniline films, Synth. Met. 40 (1991) 87– 96. [20] H.S. Xia, Q. Wang, Ultrasonic irradiation: a novel approach to prepare conductive polyaniline/nanocrystalline titanium oxide composites, Chem. Mater. 14 (2002) 2158–2165. [21] Y.J. He, Synthesis of polyaniline/nano-CeO2 composite microspheres via a solid-stabilized emulsion route, Mater. Chem. Phys. 92 (2005) 134– 137. [22] J. Xu, X.L. Li, J.F. Liu, X. Wang, Q. Peng, Y.D. Li, Solusion route to inorganic nanobelt-conducting organic polymer core-shell nanocomposites, J. Polym. Sci. A: Polym. Chem. 43 (2005) 2892–2900. [23] P.J. Benjamin, E. Phillip, J.E. Richard, L.H. Colin, M.R. Norman, Novel composite organic–inorganic semiconductor sensors for the quantitative detection of target organic vapours, J. Mater. Chem. 6 (1996) 289–294.
Biographies Lina Geng is currently a PhD candidate at Nankai University. She received her BS degree majoring in chemistry education in 1999, MS degree majoring in bioinorganic chemistry in 2003 from Hebei Normal University, Shijiazhuang, China, respectively. Her interests cover material chemistry and chemical sensors. Yingqiang Zhao received BS degree in chemistry from Tianjin Normal University in 2004. He is currently a postgraduate in the Department of Chemistry in Nankai University. His research is focused on the development and application of gas-sensitive materials. Xueliang Huang is a postgraduate in Nankai University. He received his BS degree majoring in chemistry from Xiangtan Normal University in 2003. His interests cover nanomaterials and catalysis. Shurong Wang is working at Nankai University. She received her MS degree in chemistry from Nankai University in 2004. Her research covers nanomaterials, catalysis and gas sensors. Shoumin Zhang is an associate professor of chemistry in Nankai University. He received his PhD from Nankai University in 1999. His current research fields are inorganic chemistry and materials. Shihua Wu received his BS degree in inorganic chemistry from Nankai University, Tianjin, China. He is a professor of inorganic chemistry since 1998 and head of department of chemistry, where he directs a multidisciplinary research group working on nanomaterials, catalysis, gas sensors and other new functional materials.
Characterization and gas sensitivity study of polyaniline/SnO2 hybrid material prepared by hydrothermal route Lina Geng, Yingqiang Zhao, Xueliang Huang, Shurong Wang, Shoumin Zhang, Shihua Wu ∗ Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China Received 6 January 2006; received in revised form 10 March 2006; accepted 13 March 2006 Available online 5 May 2006
Abstract A polyaniline (PAni)/SnO2 hybrid material was prepared by a hydrothermal method and characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The XRD pattern suggested that PAni did not modify the crystal structure of SnO2 , but SnO2 affected the crystallization of PAni to some extent. The gas sensitivity of the PAni/SnO2 hybrid was also studied to ethanol and acetone at operation temperatures of 30, 60 and 90 ◦ C. It was found that the PAni/SnO2 hybrid material had gas sensitivity only when operated at 60 and 90 ◦ C, and it showed a linear relationship between the responses and the concentrations of ethanol and acetone at 90 ◦ C. The sensing mechanism was also discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Gas sensitivity; Polyaniline; SnO2 ; Hydrothermal route
1. Introduction At present, gas sensing materials can be classed mainly into two kinds: organic and inorganic materials. The requirements for useful gas sensing devices in a variety of applications are high sensitivity, fast response and recovery time, low cost and portability. Whilst the gas sensing devices based on inorganic materials are sensitive to a variety of organic species at low levels, the need for elevated operation temperatures increases power consumption, reduces sensor life and limits the portability. Doped or undoped SnO2 is most studied and used among the inorganic materials, but its operation temperature is about 250–450 ◦ C [1,2]. The gas sensing devices based on organic materials, such as polypyrrole, polyaniline and metalphthalocyanines, have gas sensitivity at room temperature, but their long response time [3,4] due to the orderly structure limits their usage. It has been reported that organic–inorganic hybrid materials can synergize or complement the properties of the pure organic or inorganic materials in electronics, optics, coating, catalysis and so on [5–8]. Recently the study of organic–inorganic
∗
Corresponding author. Tel.: +86 22 23505896; fax: +86 22 23502458. E-mail address: [email protected] (S. Wu).
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.03.009
hybrid materials for gas sensor application has proved that the hybridization improves the properties of pure organic and inorganic materials. Suri et al. [9] studied gas sensitivity of polypyrrole/iron oxide to CO2 , N2 and CH4 , and found that it had the highest sensitivity to CO2 gas. Nardis et al. [10] reported that cobalt porphyrin/tin dioxide had superior selectivity to methanol vapor and lower working temperature than pure SnO2 . Hosono et al. [11] and Matsubara et al. [12] synthesized PPy/MoO3 thin films and PPy/MoO3 pressed pellets and found that the PPy/MoO3 materials had better selectivity to polar volatile organic compounds (VOCs). In addition, Armes and Maeda [13] and Partch et al. [14] reported that this kind of hybrid materials possessed small grain size and high stability in air. Previously, our group reported the gas sensitivity of polypyrrole (PPy)/SnO2 and PPy/Fe2 O3 hybrid materials to toxic gases, such as NH3 , H2 S and NOx [15,16]. VOCs are also toxic; they can cause sick house syndrome, and are inflammable and explosive. In this paper, a PAni/SnO2 hybrid was prepared via a hydrothermal route in which PAni was polymerized in situ, and characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscope (SEM) and high-resolution electron microscope (HRTEM). The gas sensitivity study of the PAni/SnO2 material was carried out to ethanol and acetone at low operation temperatures. The sensing mechanism of the PAni/SnO2 hybrid material was also discussed.
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2. Experimental 2.1. Preparation and characterization of PAni/SnO2 Aniline (purchased, analytical purity) was distilled under reduced pressure before use. Ammonium peroxydisulfate (APS, (NH4 )2 S2 O8 ), HCl (38%) and SnO2 were all of analytical purity and used without further purification. Few drops of aniline were added to 50 mL water in which the concentration of H+ was adjusted to about 1 mol/L by HCl. The solution was ultrasonically treated for 30 min and then a specific mass of SnO2 (the mass ratio of aniline to SnO2 was 3:97) was added to it. The mixture was stirred and ultrasonically treated for 30 min each, then transferred to a Teflon autoclave, followed by the addition of APS at a molar ratio to aniline of 1:1. The autoclave was sealed and kept at 140 ◦ C for 4 h. When cooled to room temperature, the mixture was filtered. The precipitates were washed by deionized water, ethanol and acetone for several times, and then dried at 60 ◦ C in an oven. The crystal structure of the PAni/SnO2 hybrid material was examined by XRD (DMAX-2500 diffractometer with Cu K␣ radiation at 40 kV and 100 mA). The surface morphology was analyzed with the help of SEM (X-650, Japan, operated at 25 kV) and HRTEM (Philips T20ST, operated at 200 kV). FT-IR spectrum was recorded on an Avatar 380 FT-IR spectrophotometer using KBr pellets.
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by measuring the Vout . The response–recovery curves of the sensor, the curves of the measured voltage versus time, were recorded by a computer. The response (S) was defined as the ratio V(out)g /V(out)a , where V(out)a is the initial voltage of the sensor and V(out)g the voltage of the sensor when exposed to testing gases. The response or recovery time is the time for the voltage change to reach 90% of the total change from V(out)g to V(out)a or vice versa. 3. Results and discussions Fig. 2 shows the FT-IR spectrum of the obtained PAni/SnO2 hybrid material, in comparison with pure PAni, in the range of 400–4000 cm−1 . The pure PAni was prepared by chemical oxidation in an aqueous solution at room temperature. The acidity of the solvent was adjusted to about 1 mol/L by HCl. APS was used as an oxidant and its molar ratio to aniline was 1:1. The reaction equation was
2.2. Gas sensing characteristics measurement The gas sensitivity of PAni/SnO2 to ethanol and acetone was measured as described in ref. [17]. The sample was fabricated on an aluminum tube with Au electrodes and platinum wires. A Ni–Cr alloy wire through the tube was used as a heating filament. The volume of the testing chamber was 15 L, and a fan was used to make the test gas atmosphere homogeneous. The gas sensing test was carried out at a fixed humidity of 60% and the operation temperatures were 30, 60 and 90 ◦ C. Fig. 1 depicts the measuring electric circuit in this experiment. Vc is a circuit voltage, Vout a measured voltage, Vh a heating voltage and RL is the resistance of a loading resistor. The voltage across the sensor can be determined indirectly
Fig. 1. Electric circuit for gas sensing measurement.
Fig. 2. FT-IR spectra of: (1) pure PAni and (2) PAni/SnO2 hybrid material.
Fig. 3. XRD patterns of: (1) SnO2 , (2) PAni/SnO2 hybrid material and (3) PAni.
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Fig. 4. SEM image of PAni/SnO2 hybrid material.
. PAni exhibit characteristic bands at 1504, 1300, 1143 and 796 cm−1 [18,19], which are the C C stretching band in benzenoid rings, the C N stretching mode in Ar N, the vibration band of the dopant anion (HCl-PAni), and the C H bending vibration out of the plane of the para-disubstituted benzene rings, respectively. These characteristic peaks can also be seen in the PAni/SnO2 hybrid material, except that 796 cm−1 is overlapped by the stretching band of Sn O appeared at about 660 cm−1 . X-ray diffraction patterns of SnO2 (purchased, A.P., the particle size was about 15 nm), PAni and PAni/SnO2 hybrid material are shown in Fig. 3. It can be seen that the PAni/SnO2 hybrid material has the same profile as pure SnO2 , indicating that the crystal structure of SnO2 is not modified by PAni. Pure PAni has similarly strong peaks at 2θ, 20.44◦ (1 0 0 face) and 2θ, 25.10◦ (1 1 0 face) which were not observed in the XRD pattern of PAni/SnO2 , indicating that SnO2 nanoparticles hamper the crystallization of PAni. This is consistent with the reports of Xia and Wang [20] and He [21]. Fig. 4 shows the SEM image of the PAni/SnO2 hybrid material. It can be seen from Fig. 4 that the amorphous and spherical particles are uniformly dispersed, where the amorphous parti-
Fig. 5. HRTEM micrograph of PAni/SnO2 hybrid material.
cles are PAni and the spherical particles are SnO2 and PAni/SnO2 hybrid. Excessively irregular PAni particles can be reduced by sufficient ultrasonic treatment and stirring [22]. In order to examine the structure of PAni/SnO2 hybrid material clearly, HRTEM was carried out (Fig. 5). From Fig. 5, we can see that some of the spherical particles with clear boundaries and crystal line are bared SnO2 , and other spherical particles with blurry boundaries are SnO2 enwrapped by PAni. In the gas sensing study, we found that the PAni/SnO2 hybrid material had no gas sensitivity to ethanol or acetone when operated at 30 ◦ C. But, when operated at 60 or 90 ◦ C, it was sensitive to low concentration of ethanol and acetone. Figs. 6 and 7 show the response–recovery curves of PAni/SnO2 hybrid material to ethanol and acetone of different concentrations. From Figs. 6 and 7, we can see that the PAni/SnO2 hybrid material has good reversibility when exposed to ethanol or acetone, and it has faster response time when operated at 90 ◦ C than at 60 ◦ C, and the measurement voltage levels off quickly after exposure to ethanol or acetone. The response time to ethanol and acetone was
Fig. 6. Response–recovery curves of PAni/SnO2 hybrid material to different concentrations of ethanol at: (a) 60 ◦ C and (b) 90 ◦ C.
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Fig. 7. Response–recovery curves of PAni/SnO2 hybrid material to different concentrations of acetone at: (a) 60 ◦ C and (b) 90 ◦ C.
within 23–43 and 16–20 s, respectively, at 90 ◦ C, and the recovery time was within 16–28 and 35–48 s, respectively. Fig. 8a and b show the relationship between the responses to ethanol and acetone of PAni/SnO2 hybrid material and the concentration of vapors, respectively. It can be seen that the responses to
ethanol and acetone of PAni/SnO2 increased linearly with the increasing concentrations. PAni is a p-type semiconductor, and SnO2 an n-type, so that there are two competitive mechanisms of electronic properties in the PAni/SnO2 hybrid material. When exposed to ethanol or acetone, PAni/SnO2 exhibits the properties of n-type semiconductors; that is, the resistance of n-type semiconductors decreases when exposed to reducing gases. It suggests that the sensing mechanism of PAni/SnO2 is governed by SnO2 . This maybe caused by the fact that SnO2 presents at a high level in PAni/SnO2 hybrid material. The PAni/SnO2 hybrid material is gas-sensitive at operating temperatures (60 and 90 ◦ C) much lower than those of SnO2 (about 250–450 ◦ C). This can be explained that the positively charged depletion layer on the surface of the SnO2 could be formed owing to inter-particle electron migration from SnO2 to PAni at the p–n heterojunctions, lowering the activation energy and enthalpy of physisorption for vapors with good electron-donating characteristics [23]. 4. Conclusions An organic–inorganic hybrid material PAni/SnO2 was prepared by a hydrothermal method and studied for gas sensing of ethanol and acetone. The PAni/SnO2 hybrid material was sensitive to ethanol and acetone when operated at 60 or 90 ◦ C and showed good reversibility. The sensing mechanism was suggested to be related to the existence of p–n heterojunctions in the PAni/SnO2 hybrid material. When operated at 90 ◦ C, the PAni/SnO2 hybrid material exhibited short response and recovery time (within 1 min). The material developed can overcome the shortcomings of long response time of PAni and the high operation temperature of SnO2 , thus presenting very important features for practical use. References
Fig. 8. Variations in response to: (a) ethanol and (b) acetone of PAn/iSnO2 hybrid material as a function of concentrations at 90 ◦ C.
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Biographies Lina Geng is currently a PhD candidate at Nankai University. She received her BS degree majoring in chemistry education in 1999, MS degree majoring in bioinorganic chemistry in 2003 from Hebei Normal University, Shijiazhuang, China, respectively. Her interests cover material chemistry and chemical sensors. Yingqiang Zhao received BS degree in chemistry from Tianjin Normal University in 2004. He is currently a postgraduate in the Department of Chemistry in Nankai University. His research is focused on the development and application of gas-sensitive materials. Xueliang Huang is a postgraduate in Nankai University. He received his BS degree majoring in chemistry from Xiangtan Normal University in 2003. His interests cover nanomaterials and catalysis. Shurong Wang is working at Nankai University. She received her MS degree in chemistry from Nankai University in 2004. Her research covers nanomaterials, catalysis and gas sensors. Shoumin Zhang is an associate professor of chemistry in Nankai University. He received his PhD from Nankai University in 1999. His current research fields are inorganic chemistry and materials. Shihua Wu received his BS degree in inorganic chemistry from Nankai University, Tianjin, China. He is a professor of inorganic chemistry since 1998 and head of department of chemistry, where he directs a multidisciplinary research group working on nanomaterials, catalysis, gas sensors and other new functional materials.