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Polyaniline–chitosan nanocomposite: High performance hydrogen sensor from new principle
Sensors and Actuators B 160 (2011) 1020–1025
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Polyaniline–chitosan nanocomposite: High performance hydrogen sensor from new principle Wei Li, Dong Mi Jang, Sea Yong An, Dojin Kim ∗ , Soon-Ku Hong, Hyojin Kim Department of Materials Science and Engineering, Chungnam National University, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 March 2011 Received in revised form 22 August 2011 Accepted 6 September 2011 Available online 12 September 2011 Keywords: Polyaniline Nanofiber Chitosan Sensor Hydrogen
a b s t r a c t A high selectivity and response towards hydrogen gas is realized with a composite of polyaniline nanofibers embedded in chitosan matrix. The enhanced response originated from the new sensing mechanism related with the composite nature of chitosan and polyaniline. Based on the high gas sensing performance through nanostructured resistive polyaniline nanofibers, the swelling of chitosan as the matrix distributing among the resistor network controls the hopping conduction between the nanofibers. This gate control of chitosan on the polyaniline conduction claims a new type of sensor mechanism resulting in a reverse polarity in conductivity change with respect to the conductivity change of polyaniline. The response-reversal can enhance the sensing response. Furthermore, the highly enhanced selectivity for hydrogen was also achieved through filtration action of chitosan for large size and polar molecules. We thus demonstrate a new material and structural design of solid state sensor for improvement in selectivity and response. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Based on semiconducting oxides, solid state gas sensors have been widely studied because of their small size, low cost, and compatibility with microelectronic processing, among others. The simplest and most important sensor type is based on the change of resistance due to the interaction of adsorbing gas molecules with defects in oxide material. The accompanying charge exchange between gas molecules and oxides causes a conductance change in the material, to be recorded as sensing signal. Reaction for sensing and transducing of signals in the sensor structure occurs in the same material, as schematically drawn in Fig. 1(a). The device usually requires high operation temperatures (above ∼200 ◦ C) for fast and reversible redox reactions on the material surfaces [1,2]. On the other hand, conducting polymers (CPs) such as polyaniline (PANI) have attracted considerable attention as a resistive type sensor, particularly due to the advantage of room temperature operation. Conductivity change in CPs can occur through various response mechanisms, such as changes in PANI conductivity by NH3 adsorption via deprotonation, chloroform adsorption via swelling, and methanol adsorption via uncoiling [3,4]. Response mechanisms originate from the polymer structure of long molecular chains with abundant functional groups and relatively spacious interchain and
intrachain gaps. Nevertheless, CP sensors can be categorized under Fig. 1(a). A new resistive type sensor structure of PANI nanofibers embedded in chitosan (CHIT) is proposed, as schematically drawn in Fig. 1(b), where the semiconducting PANI nanofibers network provides the default transducing path. However, current flow is controlled by a gating action of surrounding CHIT that interacts with analyte gas. Specifically, hydrogen molecules react with environmental oxygen to form water molecules, which in turn causes CHIT to swell, hence interrupting the conducting path between PANI nanofibers and raising resistance of the PANI nanofiber network, as schematically shown in Fig. 1(c). This type of indirect conduction control in sensors is preferred because of the resulting higher response signal and significantly improved gas selectivity. Poor gas selectivity or cross sensitivity of materials has long been the limitation of resistive type sensor materials [3–7]. A software dubbed “electronic nose” has been proposed to solve the poor selectivity issue of sensor materials in general [8,9]. Meanwhile, the concept of filter was proposed to improve device selectivity [1]. The CHIT layer in the composite structure is shown to reveal an excellent filtering action for gases other than hydrogen. 2. Experiment 2.1. Preparation of PANI nanofibers
∗ Corresponding author. Tel.: +82 42 821 6639, fax: +82 42 823 7648. E-mail address: [email protected] (D. Kim). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.09.020
Aniline and ammonium persulfate were used, as received from Sigma–Aldrich Co. PANI nanofibers were prepared using rapid
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Fig. 2. Schematic diagram of PANI/CHIT composite structure.
Fig. 1. (a) The conventional resistive type sensor like PANI which adsorb hydrogen to lower its resistance. (b) The newly proposed resistive type sensor made of series connection of conducting PANI and insulating CHIT. (c) Hydrogen can react with both PANI and CHIT, however in this case, the swelling of CHIT layer plays the major role to increase the resistance of the PANI conducting channel. The right-hand side of each panel shows the corresponding equivalent circuit. The standby resistance of PANI (RoPANI ) changes to RgPANI with gas adsorption. For the CHIT/PANI composite structure, the resistance changes from RoPANI + RoCHIT to RgPANI + RgCHIT with gas adsorption.
mixed reaction [10]. An ammonium persulfate acidic solution (50 ml, 1 M HCl) was quickly poured into aniline acidic solution (50 ml, 1 M HCl) at room temperature, followed immediately by magnetic stirring for 2 h. The solution was then left undisturbed for 12 h. Green precipitates were filtrated with filter paper and washed with a large amount of deionized water. According to previous studies [11], these PANI nanofibers are in highly doped emeraldine salt forms.
2.2. Preparation of PANI/CHIT composite film A solution of 0.07 ∼ 1.67 wt.% CHIT (75 ∼ 85% deacetylated, low molecular weight, produced by Aldrich, 30 ml) is prepared by dissolving 0.02–0.5 g CHIT in acetic acid solution (5%) with magnetic stirring. A PANI nanofiber (0.05 g) was added into this solution and dispersed by sonication, after which the green dispersion was spin coated (∼800 rpm) on the glass substrate using a pipette to apply a desired amount of solution onto the substrates. Film thickness was ∼5 m according to the cross section point of SEM view. The PANI and CHIT may form a strong hydrogen bonding through the abundant polar group (–OH, –NH2 , etc.) in PANI and CHIT (Fig. 2) [12].
Fig. 3. SEM images of PANI/CHIT composites made of different CHIT solution: (a) zero wt.%, (b) 0.07 wt.%, (c) 0.33 wt.%, and (d) 1.67 wt.%. Inset of (a) shows the high magnification of PANI nanofibers.
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2.3. Material characterization Surface morphology of the synthesized PANI nanofibers and PANI/CHIT composite was examined via scanning electron microscopy (SEM, JEOL-7000) at an accelerating voltage of 15 kV. 2.4. Sensor measurement Gas-sensing properties were measured in a stainless steel chamber equipped with a temperature-controlled chuck, as performed in previous experiments [13]. Pt electrodes were patterned on the glass substrate via sputter deposition of platinum through an interdigitated metal shadow mask. The comb-type electrodes consisted of four pairs of fingers. Each finger had a length of 4000 m and width of 200 m. The gap between each finger was 100 m. Dry air was used as both carrier and dilution gas. H2 , NH3 , and NO gases were diluted with dry air via a controlled flow through a mass flow controller. Because the volatile organic compounds (VOC) are in liquid form at room temperature, a bubbler evaporation system was used to deliver fixed concentration of volatile analytes into the detection chamber. All measurements were performed at room temperature. Current measurements on the sensor were obtained using a source meter (Keithley Model 2400) at a fixed voltage as a function of time. Resistance change is expressed by the following: Rg − Ro R (%) = 100 × Ro Ro
(1)
where Ro is the resistance in dry air, and Rg is the resistance upon exposure to gas molecules. 3. Results and discussion SEM image of a pure PANI nanofiber sensor is shown in Fig. 3(a), which reveals nanofibers with a diameter range of 40–70 nm and hundreds of nanometers in length, forming a network. PANI/CHIT composites, or PANI nanofibers coated by various thicknessess of CHIT layers, are shown in Fig. 3(b–d). Sensing property measurements were mainly conducted with the structure of Fig. 3(d) unless specified. The PANI is not exposed to out from CHIT matrix indicating that PANI is fully soaked in CHIT. Therefore, PANI fibers are distributed in the CHIT matrix as schematically depicted in Fig. 1(b). The other composite structures of Fig. 3(b) and (c) with thinner CHIT layers coating were tested only for comparison. Resistance of the PANI thin film [Fig. 3(a)] was approximately ∼103 . Film resistance was observed to increase in the composites due to intervention of insulating CHIT between the PANI nanofibers at their contacts. Resistance rise was drastic, up to ∼105 , with the highest content of CHIT [Fig. 3(d)]. Resistance of composite structure may be simplified to a series connection of a PANI and comp = RoPANI + RoCHIT , although the PANI network is CHIT resistor, Ro three-dimensional (Fig. 1). Resistance is directly proportional to CHIT concentration due to an increase in RoCHIT , as seen in a thick CHIT coating. Fig. 4 compares (a) the CHIT/PANI composite and (b) pure PANI structures in response to 4% H2 gas diluted in air at room temperature. The difference between the two are as follows: (i) resistance increased with the composite while it decreased with the PANI upon exposure; (ii) response with the composite film was higher (at ∼130%) than with the PANI at ∼28%; (iii) recovery did not completely occur with the composite; (iv) absolute resistance increased with repetition of exposure-and-recovery cycles; and (v) recovery level increased with repetition of the exposure-andrecovery cycles, and so on.
Fig. 4. The response of (a) PANI/CHIT(1.67 wt.%) composite and (b) pure PANI nanofiber to 4% H2 in dry air at room temperature.
A sign reversal of the response between composite and PANI structures can be expressed by the following: comp comp Rcomp = Rg − Ro > 0 while RPANI = RgPANI − RoPANI < 0. CHIT This indicates that R = RgCHIT − RoCHIT > −RPANI > 0 since comp PANI CHIT R = R + R . In short, upon exposure to H2 , resistance increase due to CHIT is far greater than resistance decrease in PANI. To determine how exposure of CHIT to H2 in the composite results in resistance increases, we need to reinvestigate the interaction between H2 and PANI, as well as between H2 and CHIT. Emeraldine salt, a form of polyaniline, is a p-type conductor that exhibits conductivity enhancement via interaction with hydrogen molecules. In addition, this is the origin of the negative RPANI . However, the explanation is not universally true; basically, two interpretations have been proposed in literature. MacDiarmid [14,15] proposed that hydrogen molecules adsorb on the polymer and dissociate into H atoms to bind on the backbone of the polymer chain and supply charge carriers. Conn et al. [16], for their part, proposed that hydrogen molecules convert to water molecules via catalytic oxidation, and their adsorption on the polymer increases its conductivity. Therefore, adsorbing species on PANI for conduction change is not definite between atomic hydrogen and water molecule, and it is important to figure out the species for the following discussion on the interaction between H2 gas and CHIT. To investigate the interaction of hydrogen with PANI, a structure [Fig. 3(a)] was mounted in a test chamber with ambient air until stabilization of the resistance is obtained. The examination focused on responses to H2 and water molecules. First, response to N2 , dry
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Fig. 5. (a) The response of PANI in various environments (a: N2 ; b: vacuum; c: N2 ; d: vacuum; e: dry air; f: vacuum; g: wet air; h: vacuum; i: N2 ; j: wet air; and k: dry air); (b) the intrinsic response of PANI to 4% H2 diluted in N2 and dry air; (c) the response of PANI/CHIT (1.67 wt%) composite to 4% H2 diluted in dry air and wet air; and (d) the response of PANI/CHIT composite of different CHIT contents to 4% H2 . All the experiments are carried out at room temperature.
air, vacuum, and wet air were examined, as shown in Fig. 5(a). Note that responses are all extremely slow processes. Exposure to water vapor exhibited enhanced conductivity in PANI (Regime g). Adsorbed water molecules were desorbed out of PANI by flushing gases such as nitrogen or air (Regime i and k), but not by evacuation (Regime h). Resistance increase with flowing of N2 or air (Regime a, c, and e) can then be understood as a similar process. The same goes with gases that flush preadsorbed moisture out of the PANI, if nitrogen does not interact with PANI [16,17]. Response in regime a revealed a long and slow upward drift as similarly reported by Conn et al. [16], who explained the behavior as a continuous desorption of moisture out of PANI. Therefore, recovery with pumping (Regime b, d, and f) occurred by re-adsorption of water molecules from the environment such as the chamber wall. Examinations that follow include the sensing of PANI to H2 in various recovery environments of vacuum, N2 , and air. Comparison of results between N2 and air environments is depicted in Fig. 5(b). While response was small in N2 (and vacuum), it was relatively large in ambient air. If hydrogen molecules directly adsorb and dissociate in PANI as per the MacDiarmid model, the response should not depend on ambient gas. This comparison suggests a role of oxygen in conductivity change of PANI along with H2 introduction, which supports Conn’s model. According to Barsukov et al. [18,19], oxygen molecules can chemisorb in PANI and participate in reduction reaction to form water molecules. Therefore, one conclusion is that reaction between H2 and O2 to form H2 O molecules may occur catalytically with PANI at room temperature. Behavior of H2 in the CHIT/PANI composite structures is then examined, where diffusion of hydrogen (and oxygen) molecules through CHIT is required to interact catalytically with PANI.
Response of the CHIT/PANI composite to H2 (as well as to H2 O for comparison purposes) is shown in Fig. 5(c). The following are observed: (i) response polarity reversal with composite was observed not only for H2 , but for H2 O molecules as well, and (ii) response-and-recovery kinetics was considerably slower with H2 O than with H2 . While response polarity reversal for both H2 and H2 O in the composite again strongly supports that the origin of H2 sensing is same as that of H2 O sensing, polarity changes definitely attribute to CHIT, which is known to swell by absorbing water molecules [12,20]. Spin-coating of a mixture of PANI nanofibers in viscous CHIT solution produces a structure [Fig. 1(b)], and stacking of CHIT-covered PANI nanofibers results in a thin CHIT layer jammed at PANI fiber-to-fiber contacts. When the jammed CHIT swells and separates fiber-to-fiber contacts [Fig. 1(c)], transport of carriers from fiber-to-fiber is interrupted to yield a positive RCHIT . Although adsorption of H2 O into PANI reduces PANI resistance (or negative RPANI ), the considerably larger positive RCHIT increases overall resistance of the composite, Rcomp . The thicker jammed CHIT layer will produce greater swelling, and in turn, larger separation of fiber-to-fiber gap to produce larger RCHIT . Comparison between composites with different thicknessess of jammed CHIT layers [Fig. 3(b)–(d)] is depicted in Fig. 5(d). As expected, relative contribution of RCHIT over RPANI is larger with a thicker CHIT sample. The second observation that regards the difference in the response-and-recovery kinetics between H2 and H2 O [Fig. 5(c)] may be discussed as follows. The relatively much slower response for H2 O may be explained by the polar nature of water molecules, whose diffusion can be immensely obstructed by strong hydrogen bonding with significant amounts of amine and hydroxyl groups
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Fig. 6. The concentration dependence of response of the PANI/CHIT (1.67 wt.%) composite sensor.
of CHIT. On the contrary, the considerably smaller kinetic diameter of H2 and O2 molecules in comparison with the inter- and intrachain distance of CHIT [21,22], along with their nonpolar nature, may allow faster diffusion in CHIT at the response stage. However, if the real species operating at the PANI–CHIT interface is H2 O for both H2 and H2 O analytes, different recovery behaviors between H2 and H2 O in Fig. 5(c) cannot be distinguished. One may expect similarly slow recoveries in the hydrophilic CHIT for both cases because recovery kinetics is related with diffusion out of the same water molecules. In the recovery cycle for H2 , one observes two clearly distinguished kinetic regions: initial fast recovery (several minutes), followed by an extremely slow recovery (more than hundreds of minutes). The latter process was manifested by an offset in the recovery resistance level, which accumulated in the following repeated cycles [Fig. 4(a)]; this suggested that either two different molecular species or two different binding energies of a species are involved in the recovery stage. The first case can be possible if, in addition to binding with the catalytically produced H2 O, H2 directly interacts as well with amino/hydroxyl functional groups at the interface. For the latter case, one may imagine the different binding energy of H2 O at the PANI–CHIT interface from that in the CHIT bulk since PANI–CHIT interface offers more variable functional groups for the binding of H2 O compared to pure CHIT. In any case, the Raman and FTIR analyses did not reveal any remarkable indication that H2 or H2 O molecules are bound in the composite. Thus, further detailed microscopic examination is necessary to figure out the sensing mechanism. The sensor response to the H2 gas concentration ranging from 0.3 to 4% was quite linear, as shown in Fig. 6. Another remarkable feature of the composite structure exists in its high selectivity towards hydrogen. Sensing properties of the composite to other oxidizing, reducing, and organic vapor gases are summarized in Fig. 7. NO gas is known to oxidize the emeraldine salt PANI to increase its resistance [7], and the PANI nanofiber sensor showed approximately 25% response to 40 ppm NO while the composite sensor showed practically no response. Therefore, null response with the composite indicates that NO gas could neither reach the PANI nanofibers nor contribute to volume change of CHIT. Evidently, NO gas could not penetrate the CHIT layer, possibly due to its capture by the functional groups of CHIT through H-abstraction reaction [23,24]. The composite did not show any response to a reducing gas, or 60 ppm NH3 , again indicating total blockage of NH3 by CHIT. High polarity of molecules can induce strong dipole–dipole interactions with the functional groups. The filtering action of CHIT was observed as well with certain important volatile organic compounds of chloroform (CHCl3 ),
Fig. 7. The response of PANI and PANI/CHIT (1.67 wt.%) composite to NH3 , NO, CCl3 , CH3 OH, C6 H6 , CH3 COCH3 , CH3 COOH, C6 H14 , and H2 at room temperature. All the gases and vapors are diluted in dry air.
methanol (CH3 OH), benzene (C6 H6 ), acetone (CH3 COCH3 ), acetic acid (CH3 COOH), and hexane (C6 H14 ). A number of neutral volatile organic compounds are known to change conductivity of PANI via the swelling effect [3,25], but CHIT almost blocked the six common organic vapors used in laboratory or industry. High selectivity of the composite structure relies on CHIT, which filters out large and polar molecules while admitting small and nonpolar ones [20]. Upon filtration, hydroxyl and amino functional groups of CHIT catch polar gas molecules through strong dipole–dipole interaction and/or hydrogen bonding. 4. Conclusions In conclusion, nanocomposite structures of the PANI nanofibers network, coated with controlled thicknesses of CHIT, are fabricated. The composite structure sensor showed higher response towards hydrogen gas than the pure PANI nanofiber sensor, but with reversal in response polarity. The result was explained by modeling the composite structure as a network of conducting PANI nanofibers, which are coated with insulating CHIT film. When exposed to H2 diluted in air, H2 catalytically react with O2 to form H2 O on PANI, whose adsorption enhances PANI conductivity. However, H2 O likewise causes swelling of CHIT at contact and physically separates the distance between conducting PANI nanofibers, hence interrupting current flow between PANIs. The latter effect overrides the former so that the overall conductivity in the composite decreases. In other words, CHIT exhibits a gating action on current flow from PANI to PANI, similar to the depletion-mode junction field-effect transistor; this unique conduction control mechanism in the PANI–CHIT composite induced the apparent conductivity-type reversal. Another observed important function of CHIT is the physicochemical filter: finite inter- and intra-chain space and numerous functional groups of CHIT can block large size and/or polar molecules. The most dramatic effect is observed with selective sensing of the smallest and nonpolar molecule of H2 . Therefore, a new operating principle is discovered for hydrogen sensors with high sensitivity and high selectivity, given that they operate at room temperature. Acknowledgements This study was supported by the National Research Laboratory and R32-20026 programs of the Ministry of Education, Science, and
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Technology, Korea. We also acknowledge the financial support from the Grant No. 2010-0004327 and 205-2005-1-D00078. References [1] S. Ahlers, G. Muller, Th. Doll, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 3, American Scientific Publishing, Stevenson Ranch, CA, USA, 2006, p. 413. [2] S. Basu, S. Roy, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 6, American Scientific Publishing, Stevenson Ranch, CA, USA, 2006, p. 15. [3] S. Virji, J. Huang, R.B. Kaner, B.H. Weiller, Polyaniline nanofiber gas sensors: examination of response mechanisms, Nano Lett. 4 (2004) 491–496. [4] C.K. Tan, D.J. Blackwood, Interactions between polyaniline and methanol vapour, Sens. Actuators B 71 (2000) 184–191. [5] D.S. Sutar, N. Padma, D.K. Aswal, S.K. Deshpande, S.K. Gupta, J.V. Yakhmi, Preparation of nanofibrous polyaniline films and their application as ammonia gas sensor, Sens. Actuators B 128 (2007) 286–292. [6] X.B. Yan, Z.J. Han, Y. Yang, B.K. Tay, NO2 gas sensing with polyaniline nanofibers synthesized by a facile aqueous/organic interfacial polymerization, Sens. Actuators B 123 (2007) 107–113. [7] U. Lange, N.V. Roznyatovskaya, V.M. Mirsky, Conducting polymers in chemical sensors and arrays, Anal. Chim. Acta 614 (2008) 1–26. [8] T. Eklöv, I. Lundström, Distributed sensor system for quantification of individual components in a multiple gas mixture, Anal. Chem. 71 (1999) 3544–3550. [9] C.D. Natale, R. Paolesse, A. Macagnano, A. Mantini, C. Goletti, A. D’Amico, Characterization and design of porphyrins-based broad selectivity chemical sensors for electronic nose applications, Sens. Actuators B 52 (1998) 162–168. [10] J. Huang, R.B. Kaner, Nanofiber formation in the chemical polymerization of aniline: a mechanistic study, Angew. Chem. 43 (2004) 5817–5821. [11] W. Li, D.H. Nguyen, Y. Cho, D. Kim, J. Kim, Nanofibers of conducting polyaniline for aromatic organic compound sensor, Sens. Actuators B 143 (2009) 132–138. [12] Y. He, B. Zhu, Y. Inoue, Hydrogen bonds in polymer blends, Prog. Polym. Sci. 29 (2004) 1021–1051. [13] D.H. Nguyen, V.Q. Nguyen, D. Kim, Nanowire structured SnOx –SWNT composites: high performance sensor for NOx detection, Sens. Actuators B 142 (2009) 253–259. [14] A.G. MacDiarmid, Presentation at Department of Energy, USA, May 24, 2005. http://www.hydrogen.energy.gov/pdfs/review05/stp 42 macdiarmid.pdf. [15] A.Z. Sadek, W. Wlodarski, K. Kalantar-Zadeh, C. Baker, R.B. Kaner, Doped and dedoped polyaniline nanofiber based conductometric hydrogen gas sensors, Sens. Actuators A 139 (2007) 53–57. [16] C. Conn, S. Sestak, A.T. Baker, J. Unsworth, A polyaniline based selective hydrogen sensor, Electroanalysis 10 (1998) 1137–1141. [17] S. Virji, R.B. Kaner, B.H. Weiller, Hydrogen sensors based on conductivity changes in polyaniline nanofibers, J. Phys. Chem. B 110 (2006) 22266–22270. [18] V.Z. Barsukov, V.G. Khomenko, S.V. Chivikov, I.V. Barsukov, T.I. Motronyuk, On the faradaic and non-faradaic mechanisms of electrochemical processes in conducting polymers and some other reversible systems with solid-phase reagents, Electrochim. Acta 46 (2001) 4083–4094. [19] V.Z. Barsukov, V.G. Khomenko, A.S. Katashinskii, T.I. Motronyuk, Catalytic activity of polyaniline in the molecular oxygen reduction: its nature and mechanism, Russ. J. Electrochem. 40 (2004) 1170–1173. [20] T. Wang, S. Gunasekaran, State of water in chitosan–PVA hydrogel, J. Appl. Polym. Sci. 101 (2006) 3227–3232.
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[21] J.A. Seo, J.H. Koh, D.K. Roh, J.H. Kim, Preparation and characterization of crosslinked proton conducting membranes based on chitosan and PSSA-MA copolymer, Solid State Ionics 180 (2009) 998–1002. [22] R.L. David, Handbook of Chemistry and Physics, 85th ed., Boca Raton, CRC Press, 1995. [23] T. Feng, J. Gong, Y. Du, Z. Huang, Free radical scavenging activity of cellulosetreated chitosan, J. Appl. Polym. Sci. 111 (2009) 545–550. [24] L.J. Ignarro, Nitric Oxide: Biology and Pathobiology, Academic Press, San Diego, CA, 2000. [25] J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, Nanostructured polyaniline sensors, Chem. Eur. J. 10 (2004) 1314–1319.
Biographies Wei Li received the degree of Master of Science in Materials Science at Shenyang University of Chemical Technology, China in 2005. He is now in Ph.D. course in Materials Science and Engineering, Chungnam Nat. Univ., Korea. His current research interests are fabrication of nanostructure conducting polymers and their applications to electronic devices, gas sensors, etc. Dong Mi Jang received her BS in Materials Science and Engineering at Chungnam National University, Korea, in 2009. She is now in Master course in Materials Science and Engineering, Chungnam National University, Korea. Her current interest is synthesis of metal oxide nanostructures. Sea Yong An received his BS in Materials Science and Engineering at Chungnam National University, Korea, in 2009. He is now in Master course in Materials Science and Engineering, Chungnam National University, Korea. His current interest is synthesis of metal oxide nanostructures. Dojin Kim received his Ph.D. in Materials Science and Engineering from the University of Southern California, USA, in 1989. He is a professor at the School of Nanotechnology and the Department of Materials Science and Engineering in Chungnam National University, Korea. His current research interests are synthesis of carbon nanotube and metal oxides and applications to electronic devices of field emitters, solar cells, gas sensors, and so on. He is also interested in semiconductor nanostructures growth and applications. Soon-Ku Hong received his Ph.D. in Applied Physics from Tohoku University, Japan, in 2001. He is an associate professor at the School of Nanotechnology and the Department of Materials Science and Engineering in Chungnam National University, Korea. His current research interests are growth and applications of oxide and nitride semiconductors for light emitting diodes, sensors, and solar cells. Major fabrication techniques for his researches are molecular-beam epitaxy, chemical vapor deposition, hydrothermal growth, and sol–gel. He is also interested in defect and interface engineering in hetero-epitaxial films and characterization of structural defects by transmission electron microscopy. Hyojin Kim received his Ph.D. in Materials Science and Engineering from Korea Advanced Institute of Science and Technology, Korea, in 1993. He is a professor at the Department of Materials Science and Engineering in Chungnam National University, Korea. He is currently interested in synthesis and applications of ZnO-based oxide materials, including ZnO-based gas sensors and diluted magnetic semiconductors.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Polyaniline–chitosan nanocomposite: High performance hydrogen sensor from new principle Wei Li, Dong Mi Jang, Sea Yong An, Dojin Kim ∗ , Soon-Ku Hong, Hyojin Kim Department of Materials Science and Engineering, Chungnam National University, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 March 2011 Received in revised form 22 August 2011 Accepted 6 September 2011 Available online 12 September 2011 Keywords: Polyaniline Nanofiber Chitosan Sensor Hydrogen
a b s t r a c t A high selectivity and response towards hydrogen gas is realized with a composite of polyaniline nanofibers embedded in chitosan matrix. The enhanced response originated from the new sensing mechanism related with the composite nature of chitosan and polyaniline. Based on the high gas sensing performance through nanostructured resistive polyaniline nanofibers, the swelling of chitosan as the matrix distributing among the resistor network controls the hopping conduction between the nanofibers. This gate control of chitosan on the polyaniline conduction claims a new type of sensor mechanism resulting in a reverse polarity in conductivity change with respect to the conductivity change of polyaniline. The response-reversal can enhance the sensing response. Furthermore, the highly enhanced selectivity for hydrogen was also achieved through filtration action of chitosan for large size and polar molecules. We thus demonstrate a new material and structural design of solid state sensor for improvement in selectivity and response. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Based on semiconducting oxides, solid state gas sensors have been widely studied because of their small size, low cost, and compatibility with microelectronic processing, among others. The simplest and most important sensor type is based on the change of resistance due to the interaction of adsorbing gas molecules with defects in oxide material. The accompanying charge exchange between gas molecules and oxides causes a conductance change in the material, to be recorded as sensing signal. Reaction for sensing and transducing of signals in the sensor structure occurs in the same material, as schematically drawn in Fig. 1(a). The device usually requires high operation temperatures (above ∼200 ◦ C) for fast and reversible redox reactions on the material surfaces [1,2]. On the other hand, conducting polymers (CPs) such as polyaniline (PANI) have attracted considerable attention as a resistive type sensor, particularly due to the advantage of room temperature operation. Conductivity change in CPs can occur through various response mechanisms, such as changes in PANI conductivity by NH3 adsorption via deprotonation, chloroform adsorption via swelling, and methanol adsorption via uncoiling [3,4]. Response mechanisms originate from the polymer structure of long molecular chains with abundant functional groups and relatively spacious interchain and
intrachain gaps. Nevertheless, CP sensors can be categorized under Fig. 1(a). A new resistive type sensor structure of PANI nanofibers embedded in chitosan (CHIT) is proposed, as schematically drawn in Fig. 1(b), where the semiconducting PANI nanofibers network provides the default transducing path. However, current flow is controlled by a gating action of surrounding CHIT that interacts with analyte gas. Specifically, hydrogen molecules react with environmental oxygen to form water molecules, which in turn causes CHIT to swell, hence interrupting the conducting path between PANI nanofibers and raising resistance of the PANI nanofiber network, as schematically shown in Fig. 1(c). This type of indirect conduction control in sensors is preferred because of the resulting higher response signal and significantly improved gas selectivity. Poor gas selectivity or cross sensitivity of materials has long been the limitation of resistive type sensor materials [3–7]. A software dubbed “electronic nose” has been proposed to solve the poor selectivity issue of sensor materials in general [8,9]. Meanwhile, the concept of filter was proposed to improve device selectivity [1]. The CHIT layer in the composite structure is shown to reveal an excellent filtering action for gases other than hydrogen. 2. Experiment 2.1. Preparation of PANI nanofibers
∗ Corresponding author. Tel.: +82 42 821 6639, fax: +82 42 823 7648. E-mail address: [email protected] (D. Kim). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.09.020
Aniline and ammonium persulfate were used, as received from Sigma–Aldrich Co. PANI nanofibers were prepared using rapid
W. Li et al. / Sensors and Actuators B 160 (2011) 1020–1025
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Fig. 2. Schematic diagram of PANI/CHIT composite structure.
Fig. 1. (a) The conventional resistive type sensor like PANI which adsorb hydrogen to lower its resistance. (b) The newly proposed resistive type sensor made of series connection of conducting PANI and insulating CHIT. (c) Hydrogen can react with both PANI and CHIT, however in this case, the swelling of CHIT layer plays the major role to increase the resistance of the PANI conducting channel. The right-hand side of each panel shows the corresponding equivalent circuit. The standby resistance of PANI (RoPANI ) changes to RgPANI with gas adsorption. For the CHIT/PANI composite structure, the resistance changes from RoPANI + RoCHIT to RgPANI + RgCHIT with gas adsorption.
mixed reaction [10]. An ammonium persulfate acidic solution (50 ml, 1 M HCl) was quickly poured into aniline acidic solution (50 ml, 1 M HCl) at room temperature, followed immediately by magnetic stirring for 2 h. The solution was then left undisturbed for 12 h. Green precipitates were filtrated with filter paper and washed with a large amount of deionized water. According to previous studies [11], these PANI nanofibers are in highly doped emeraldine salt forms.
2.2. Preparation of PANI/CHIT composite film A solution of 0.07 ∼ 1.67 wt.% CHIT (75 ∼ 85% deacetylated, low molecular weight, produced by Aldrich, 30 ml) is prepared by dissolving 0.02–0.5 g CHIT in acetic acid solution (5%) with magnetic stirring. A PANI nanofiber (0.05 g) was added into this solution and dispersed by sonication, after which the green dispersion was spin coated (∼800 rpm) on the glass substrate using a pipette to apply a desired amount of solution onto the substrates. Film thickness was ∼5 m according to the cross section point of SEM view. The PANI and CHIT may form a strong hydrogen bonding through the abundant polar group (–OH, –NH2 , etc.) in PANI and CHIT (Fig. 2) [12].
Fig. 3. SEM images of PANI/CHIT composites made of different CHIT solution: (a) zero wt.%, (b) 0.07 wt.%, (c) 0.33 wt.%, and (d) 1.67 wt.%. Inset of (a) shows the high magnification of PANI nanofibers.
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2.3. Material characterization Surface morphology of the synthesized PANI nanofibers and PANI/CHIT composite was examined via scanning electron microscopy (SEM, JEOL-7000) at an accelerating voltage of 15 kV. 2.4. Sensor measurement Gas-sensing properties were measured in a stainless steel chamber equipped with a temperature-controlled chuck, as performed in previous experiments [13]. Pt electrodes were patterned on the glass substrate via sputter deposition of platinum through an interdigitated metal shadow mask. The comb-type electrodes consisted of four pairs of fingers. Each finger had a length of 4000 m and width of 200 m. The gap between each finger was 100 m. Dry air was used as both carrier and dilution gas. H2 , NH3 , and NO gases were diluted with dry air via a controlled flow through a mass flow controller. Because the volatile organic compounds (VOC) are in liquid form at room temperature, a bubbler evaporation system was used to deliver fixed concentration of volatile analytes into the detection chamber. All measurements were performed at room temperature. Current measurements on the sensor were obtained using a source meter (Keithley Model 2400) at a fixed voltage as a function of time. Resistance change is expressed by the following: Rg − Ro R (%) = 100 × Ro Ro
(1)
where Ro is the resistance in dry air, and Rg is the resistance upon exposure to gas molecules. 3. Results and discussion SEM image of a pure PANI nanofiber sensor is shown in Fig. 3(a), which reveals nanofibers with a diameter range of 40–70 nm and hundreds of nanometers in length, forming a network. PANI/CHIT composites, or PANI nanofibers coated by various thicknessess of CHIT layers, are shown in Fig. 3(b–d). Sensing property measurements were mainly conducted with the structure of Fig. 3(d) unless specified. The PANI is not exposed to out from CHIT matrix indicating that PANI is fully soaked in CHIT. Therefore, PANI fibers are distributed in the CHIT matrix as schematically depicted in Fig. 1(b). The other composite structures of Fig. 3(b) and (c) with thinner CHIT layers coating were tested only for comparison. Resistance of the PANI thin film [Fig. 3(a)] was approximately ∼103 . Film resistance was observed to increase in the composites due to intervention of insulating CHIT between the PANI nanofibers at their contacts. Resistance rise was drastic, up to ∼105 , with the highest content of CHIT [Fig. 3(d)]. Resistance of composite structure may be simplified to a series connection of a PANI and comp = RoPANI + RoCHIT , although the PANI network is CHIT resistor, Ro three-dimensional (Fig. 1). Resistance is directly proportional to CHIT concentration due to an increase in RoCHIT , as seen in a thick CHIT coating. Fig. 4 compares (a) the CHIT/PANI composite and (b) pure PANI structures in response to 4% H2 gas diluted in air at room temperature. The difference between the two are as follows: (i) resistance increased with the composite while it decreased with the PANI upon exposure; (ii) response with the composite film was higher (at ∼130%) than with the PANI at ∼28%; (iii) recovery did not completely occur with the composite; (iv) absolute resistance increased with repetition of exposure-and-recovery cycles; and (v) recovery level increased with repetition of the exposure-andrecovery cycles, and so on.
Fig. 4. The response of (a) PANI/CHIT(1.67 wt.%) composite and (b) pure PANI nanofiber to 4% H2 in dry air at room temperature.
A sign reversal of the response between composite and PANI structures can be expressed by the following: comp comp Rcomp = Rg − Ro > 0 while RPANI = RgPANI − RoPANI < 0. CHIT This indicates that R = RgCHIT − RoCHIT > −RPANI > 0 since comp PANI CHIT R = R + R . In short, upon exposure to H2 , resistance increase due to CHIT is far greater than resistance decrease in PANI. To determine how exposure of CHIT to H2 in the composite results in resistance increases, we need to reinvestigate the interaction between H2 and PANI, as well as between H2 and CHIT. Emeraldine salt, a form of polyaniline, is a p-type conductor that exhibits conductivity enhancement via interaction with hydrogen molecules. In addition, this is the origin of the negative RPANI . However, the explanation is not universally true; basically, two interpretations have been proposed in literature. MacDiarmid [14,15] proposed that hydrogen molecules adsorb on the polymer and dissociate into H atoms to bind on the backbone of the polymer chain and supply charge carriers. Conn et al. [16], for their part, proposed that hydrogen molecules convert to water molecules via catalytic oxidation, and their adsorption on the polymer increases its conductivity. Therefore, adsorbing species on PANI for conduction change is not definite between atomic hydrogen and water molecule, and it is important to figure out the species for the following discussion on the interaction between H2 gas and CHIT. To investigate the interaction of hydrogen with PANI, a structure [Fig. 3(a)] was mounted in a test chamber with ambient air until stabilization of the resistance is obtained. The examination focused on responses to H2 and water molecules. First, response to N2 , dry
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Fig. 5. (a) The response of PANI in various environments (a: N2 ; b: vacuum; c: N2 ; d: vacuum; e: dry air; f: vacuum; g: wet air; h: vacuum; i: N2 ; j: wet air; and k: dry air); (b) the intrinsic response of PANI to 4% H2 diluted in N2 and dry air; (c) the response of PANI/CHIT (1.67 wt%) composite to 4% H2 diluted in dry air and wet air; and (d) the response of PANI/CHIT composite of different CHIT contents to 4% H2 . All the experiments are carried out at room temperature.
air, vacuum, and wet air were examined, as shown in Fig. 5(a). Note that responses are all extremely slow processes. Exposure to water vapor exhibited enhanced conductivity in PANI (Regime g). Adsorbed water molecules were desorbed out of PANI by flushing gases such as nitrogen or air (Regime i and k), but not by evacuation (Regime h). Resistance increase with flowing of N2 or air (Regime a, c, and e) can then be understood as a similar process. The same goes with gases that flush preadsorbed moisture out of the PANI, if nitrogen does not interact with PANI [16,17]. Response in regime a revealed a long and slow upward drift as similarly reported by Conn et al. [16], who explained the behavior as a continuous desorption of moisture out of PANI. Therefore, recovery with pumping (Regime b, d, and f) occurred by re-adsorption of water molecules from the environment such as the chamber wall. Examinations that follow include the sensing of PANI to H2 in various recovery environments of vacuum, N2 , and air. Comparison of results between N2 and air environments is depicted in Fig. 5(b). While response was small in N2 (and vacuum), it was relatively large in ambient air. If hydrogen molecules directly adsorb and dissociate in PANI as per the MacDiarmid model, the response should not depend on ambient gas. This comparison suggests a role of oxygen in conductivity change of PANI along with H2 introduction, which supports Conn’s model. According to Barsukov et al. [18,19], oxygen molecules can chemisorb in PANI and participate in reduction reaction to form water molecules. Therefore, one conclusion is that reaction between H2 and O2 to form H2 O molecules may occur catalytically with PANI at room temperature. Behavior of H2 in the CHIT/PANI composite structures is then examined, where diffusion of hydrogen (and oxygen) molecules through CHIT is required to interact catalytically with PANI.
Response of the CHIT/PANI composite to H2 (as well as to H2 O for comparison purposes) is shown in Fig. 5(c). The following are observed: (i) response polarity reversal with composite was observed not only for H2 , but for H2 O molecules as well, and (ii) response-and-recovery kinetics was considerably slower with H2 O than with H2 . While response polarity reversal for both H2 and H2 O in the composite again strongly supports that the origin of H2 sensing is same as that of H2 O sensing, polarity changes definitely attribute to CHIT, which is known to swell by absorbing water molecules [12,20]. Spin-coating of a mixture of PANI nanofibers in viscous CHIT solution produces a structure [Fig. 1(b)], and stacking of CHIT-covered PANI nanofibers results in a thin CHIT layer jammed at PANI fiber-to-fiber contacts. When the jammed CHIT swells and separates fiber-to-fiber contacts [Fig. 1(c)], transport of carriers from fiber-to-fiber is interrupted to yield a positive RCHIT . Although adsorption of H2 O into PANI reduces PANI resistance (or negative RPANI ), the considerably larger positive RCHIT increases overall resistance of the composite, Rcomp . The thicker jammed CHIT layer will produce greater swelling, and in turn, larger separation of fiber-to-fiber gap to produce larger RCHIT . Comparison between composites with different thicknessess of jammed CHIT layers [Fig. 3(b)–(d)] is depicted in Fig. 5(d). As expected, relative contribution of RCHIT over RPANI is larger with a thicker CHIT sample. The second observation that regards the difference in the response-and-recovery kinetics between H2 and H2 O [Fig. 5(c)] may be discussed as follows. The relatively much slower response for H2 O may be explained by the polar nature of water molecules, whose diffusion can be immensely obstructed by strong hydrogen bonding with significant amounts of amine and hydroxyl groups
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Fig. 6. The concentration dependence of response of the PANI/CHIT (1.67 wt.%) composite sensor.
of CHIT. On the contrary, the considerably smaller kinetic diameter of H2 and O2 molecules in comparison with the inter- and intrachain distance of CHIT [21,22], along with their nonpolar nature, may allow faster diffusion in CHIT at the response stage. However, if the real species operating at the PANI–CHIT interface is H2 O for both H2 and H2 O analytes, different recovery behaviors between H2 and H2 O in Fig. 5(c) cannot be distinguished. One may expect similarly slow recoveries in the hydrophilic CHIT for both cases because recovery kinetics is related with diffusion out of the same water molecules. In the recovery cycle for H2 , one observes two clearly distinguished kinetic regions: initial fast recovery (several minutes), followed by an extremely slow recovery (more than hundreds of minutes). The latter process was manifested by an offset in the recovery resistance level, which accumulated in the following repeated cycles [Fig. 4(a)]; this suggested that either two different molecular species or two different binding energies of a species are involved in the recovery stage. The first case can be possible if, in addition to binding with the catalytically produced H2 O, H2 directly interacts as well with amino/hydroxyl functional groups at the interface. For the latter case, one may imagine the different binding energy of H2 O at the PANI–CHIT interface from that in the CHIT bulk since PANI–CHIT interface offers more variable functional groups for the binding of H2 O compared to pure CHIT. In any case, the Raman and FTIR analyses did not reveal any remarkable indication that H2 or H2 O molecules are bound in the composite. Thus, further detailed microscopic examination is necessary to figure out the sensing mechanism. The sensor response to the H2 gas concentration ranging from 0.3 to 4% was quite linear, as shown in Fig. 6. Another remarkable feature of the composite structure exists in its high selectivity towards hydrogen. Sensing properties of the composite to other oxidizing, reducing, and organic vapor gases are summarized in Fig. 7. NO gas is known to oxidize the emeraldine salt PANI to increase its resistance [7], and the PANI nanofiber sensor showed approximately 25% response to 40 ppm NO while the composite sensor showed practically no response. Therefore, null response with the composite indicates that NO gas could neither reach the PANI nanofibers nor contribute to volume change of CHIT. Evidently, NO gas could not penetrate the CHIT layer, possibly due to its capture by the functional groups of CHIT through H-abstraction reaction [23,24]. The composite did not show any response to a reducing gas, or 60 ppm NH3 , again indicating total blockage of NH3 by CHIT. High polarity of molecules can induce strong dipole–dipole interactions with the functional groups. The filtering action of CHIT was observed as well with certain important volatile organic compounds of chloroform (CHCl3 ),
Fig. 7. The response of PANI and PANI/CHIT (1.67 wt.%) composite to NH3 , NO, CCl3 , CH3 OH, C6 H6 , CH3 COCH3 , CH3 COOH, C6 H14 , and H2 at room temperature. All the gases and vapors are diluted in dry air.
methanol (CH3 OH), benzene (C6 H6 ), acetone (CH3 COCH3 ), acetic acid (CH3 COOH), and hexane (C6 H14 ). A number of neutral volatile organic compounds are known to change conductivity of PANI via the swelling effect [3,25], but CHIT almost blocked the six common organic vapors used in laboratory or industry. High selectivity of the composite structure relies on CHIT, which filters out large and polar molecules while admitting small and nonpolar ones [20]. Upon filtration, hydroxyl and amino functional groups of CHIT catch polar gas molecules through strong dipole–dipole interaction and/or hydrogen bonding. 4. Conclusions In conclusion, nanocomposite structures of the PANI nanofibers network, coated with controlled thicknesses of CHIT, are fabricated. The composite structure sensor showed higher response towards hydrogen gas than the pure PANI nanofiber sensor, but with reversal in response polarity. The result was explained by modeling the composite structure as a network of conducting PANI nanofibers, which are coated with insulating CHIT film. When exposed to H2 diluted in air, H2 catalytically react with O2 to form H2 O on PANI, whose adsorption enhances PANI conductivity. However, H2 O likewise causes swelling of CHIT at contact and physically separates the distance between conducting PANI nanofibers, hence interrupting current flow between PANIs. The latter effect overrides the former so that the overall conductivity in the composite decreases. In other words, CHIT exhibits a gating action on current flow from PANI to PANI, similar to the depletion-mode junction field-effect transistor; this unique conduction control mechanism in the PANI–CHIT composite induced the apparent conductivity-type reversal. Another observed important function of CHIT is the physicochemical filter: finite inter- and intra-chain space and numerous functional groups of CHIT can block large size and/or polar molecules. The most dramatic effect is observed with selective sensing of the smallest and nonpolar molecule of H2 . Therefore, a new operating principle is discovered for hydrogen sensors with high sensitivity and high selectivity, given that they operate at room temperature. Acknowledgements This study was supported by the National Research Laboratory and R32-20026 programs of the Ministry of Education, Science, and
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Technology, Korea. We also acknowledge the financial support from the Grant No. 2010-0004327 and 205-2005-1-D00078. References [1] S. Ahlers, G. Muller, Th. Doll, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 3, American Scientific Publishing, Stevenson Ranch, CA, USA, 2006, p. 413. [2] S. Basu, S. Roy, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 6, American Scientific Publishing, Stevenson Ranch, CA, USA, 2006, p. 15. [3] S. Virji, J. Huang, R.B. Kaner, B.H. Weiller, Polyaniline nanofiber gas sensors: examination of response mechanisms, Nano Lett. 4 (2004) 491–496. [4] C.K. Tan, D.J. Blackwood, Interactions between polyaniline and methanol vapour, Sens. Actuators B 71 (2000) 184–191. [5] D.S. Sutar, N. Padma, D.K. Aswal, S.K. Deshpande, S.K. Gupta, J.V. Yakhmi, Preparation of nanofibrous polyaniline films and their application as ammonia gas sensor, Sens. Actuators B 128 (2007) 286–292. [6] X.B. Yan, Z.J. Han, Y. Yang, B.K. Tay, NO2 gas sensing with polyaniline nanofibers synthesized by a facile aqueous/organic interfacial polymerization, Sens. Actuators B 123 (2007) 107–113. [7] U. Lange, N.V. Roznyatovskaya, V.M. Mirsky, Conducting polymers in chemical sensors and arrays, Anal. Chim. Acta 614 (2008) 1–26. [8] T. Eklöv, I. Lundström, Distributed sensor system for quantification of individual components in a multiple gas mixture, Anal. Chem. 71 (1999) 3544–3550. [9] C.D. Natale, R. Paolesse, A. Macagnano, A. Mantini, C. Goletti, A. D’Amico, Characterization and design of porphyrins-based broad selectivity chemical sensors for electronic nose applications, Sens. Actuators B 52 (1998) 162–168. [10] J. Huang, R.B. Kaner, Nanofiber formation in the chemical polymerization of aniline: a mechanistic study, Angew. Chem. 43 (2004) 5817–5821. [11] W. Li, D.H. Nguyen, Y. Cho, D. Kim, J. Kim, Nanofibers of conducting polyaniline for aromatic organic compound sensor, Sens. Actuators B 143 (2009) 132–138. [12] Y. He, B. Zhu, Y. Inoue, Hydrogen bonds in polymer blends, Prog. Polym. Sci. 29 (2004) 1021–1051. [13] D.H. Nguyen, V.Q. Nguyen, D. Kim, Nanowire structured SnOx –SWNT composites: high performance sensor for NOx detection, Sens. Actuators B 142 (2009) 253–259. [14] A.G. MacDiarmid, Presentation at Department of Energy, USA, May 24, 2005. http://www.hydrogen.energy.gov/pdfs/review05/stp 42 macdiarmid.pdf. [15] A.Z. Sadek, W. Wlodarski, K. Kalantar-Zadeh, C. Baker, R.B. Kaner, Doped and dedoped polyaniline nanofiber based conductometric hydrogen gas sensors, Sens. Actuators A 139 (2007) 53–57. [16] C. Conn, S. Sestak, A.T. Baker, J. Unsworth, A polyaniline based selective hydrogen sensor, Electroanalysis 10 (1998) 1137–1141. [17] S. Virji, R.B. Kaner, B.H. Weiller, Hydrogen sensors based on conductivity changes in polyaniline nanofibers, J. Phys. Chem. B 110 (2006) 22266–22270. [18] V.Z. Barsukov, V.G. Khomenko, S.V. Chivikov, I.V. Barsukov, T.I. Motronyuk, On the faradaic and non-faradaic mechanisms of electrochemical processes in conducting polymers and some other reversible systems with solid-phase reagents, Electrochim. Acta 46 (2001) 4083–4094. [19] V.Z. Barsukov, V.G. Khomenko, A.S. Katashinskii, T.I. Motronyuk, Catalytic activity of polyaniline in the molecular oxygen reduction: its nature and mechanism, Russ. J. Electrochem. 40 (2004) 1170–1173. [20] T. Wang, S. Gunasekaran, State of water in chitosan–PVA hydrogel, J. Appl. Polym. Sci. 101 (2006) 3227–3232.
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[21] J.A. Seo, J.H. Koh, D.K. Roh, J.H. Kim, Preparation and characterization of crosslinked proton conducting membranes based on chitosan and PSSA-MA copolymer, Solid State Ionics 180 (2009) 998–1002. [22] R.L. David, Handbook of Chemistry and Physics, 85th ed., Boca Raton, CRC Press, 1995. [23] T. Feng, J. Gong, Y. Du, Z. Huang, Free radical scavenging activity of cellulosetreated chitosan, J. Appl. Polym. Sci. 111 (2009) 545–550. [24] L.J. Ignarro, Nitric Oxide: Biology and Pathobiology, Academic Press, San Diego, CA, 2000. [25] J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, Nanostructured polyaniline sensors, Chem. Eur. J. 10 (2004) 1314–1319.
Biographies Wei Li received the degree of Master of Science in Materials Science at Shenyang University of Chemical Technology, China in 2005. He is now in Ph.D. course in Materials Science and Engineering, Chungnam Nat. Univ., Korea. His current research interests are fabrication of nanostructure conducting polymers and their applications to electronic devices, gas sensors, etc. Dong Mi Jang received her BS in Materials Science and Engineering at Chungnam National University, Korea, in 2009. She is now in Master course in Materials Science and Engineering, Chungnam National University, Korea. Her current interest is synthesis of metal oxide nanostructures. Sea Yong An received his BS in Materials Science and Engineering at Chungnam National University, Korea, in 2009. He is now in Master course in Materials Science and Engineering, Chungnam National University, Korea. His current interest is synthesis of metal oxide nanostructures. Dojin Kim received his Ph.D. in Materials Science and Engineering from the University of Southern California, USA, in 1989. He is a professor at the School of Nanotechnology and the Department of Materials Science and Engineering in Chungnam National University, Korea. His current research interests are synthesis of carbon nanotube and metal oxides and applications to electronic devices of field emitters, solar cells, gas sensors, and so on. He is also interested in semiconductor nanostructures growth and applications. Soon-Ku Hong received his Ph.D. in Applied Physics from Tohoku University, Japan, in 2001. He is an associate professor at the School of Nanotechnology and the Department of Materials Science and Engineering in Chungnam National University, Korea. His current research interests are growth and applications of oxide and nitride semiconductors for light emitting diodes, sensors, and solar cells. Major fabrication techniques for his researches are molecular-beam epitaxy, chemical vapor deposition, hydrothermal growth, and sol–gel. He is also interested in defect and interface engineering in hetero-epitaxial films and characterization of structural defects by transmission electron microscopy. Hyojin Kim received his Ph.D. in Materials Science and Engineering from Korea Advanced Institute of Science and Technology, Korea, in 1993. He is a professor at the Department of Materials Science and Engineering in Chungnam National University, Korea. He is currently interested in synthesis and applications of ZnO-based oxide materials, including ZnO-based gas sensors and diluted magnetic semiconductors.