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Electrical conductivity response of polyaniline films to ethanol–water mixtures
Synthetic Metals 129 (2002) 303±308
Electrical conductivity response of polyaniline ®lms to ethanol±water mixtures L. Tarachiwina, P. Kiattibutra, L. Ruangchuaya, A. Sirivata,*, J. Schwankb a
The Petroleum and Petrochemical College, Chulalongkorn University, Phya Thai Road Soi Chulalongkorn, Bangkok 10330, Thailand b Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA Received 8 May 2001; received in revised form 23 January 2002; accepted 6 May 2002
Abstract Polyaniline emeraldine base was chemically synthesized and converted to polyaniline emeraldine salts through the protonation doping process using HCl and CH3COOH as the acid dopants. The doped polyanilines were characterized by ultraviolet±visible (UV±VIS) spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, TGA, GPC. The four-point probe technique was used to evaluate the effect of dopant type and doping molar ratio on the speci®c conductivity. Weak acid doping, by CH3COOH, produces ®lms with the speci®c conductivity which depends solely on the degree of protonation, or the number of charge carriers. On the other hand, stronger acid doping by HCl can induce crystalline domains, a greater electron mobility and hence a greater speci®c conductivity value. The speci®c conductivity of the HCl-doped and CH3COOH-doped polyaniline ®lms responds with positive increments upon exposed to water and ethanol. The interchain H-transfer is suggested to be a common mechanism which increases electron mobility upon exposure to water and ethanol, whereas additional protonation occurs only with the exposure to water. No evidence for ethanol molecules to interact chemically with the doped polyaniline ®lms was found. The ®lm electrical conductivity sensitivity is inversely proportional to ethanol concentration, with a higher sensitivity to concentration found in the ®lm doped with the acid with a lower pKa value. # 2002 Published by Elsevier Science B.V. Keywords: Polyaniline emeraldine salts; Protonic acid doping; Speci®c electrical conductivity; Exposures to water and ethanol
1. Introduction Polyaniline constitutes an important class of conductive polymers known and it can be synthesized either by chemical or electrochemical oxidative polymerizations [1,2]. In the emeraldine base form, or the half-oxidized form, the number of amine nitrogen atoms is equal to the number of imine nitrogen atoms; therefore, emeraldine base is also called poly(phenylene amine±imine) [1]. The structure of emeraldine base consists of two amine nitrogen atoms followed by two imine nitrogen atoms. The imine nitrogen indeed contributes a single p electron, with its lone pair lying within the molecular plane. Emeraldine base can be doped through a doping process called the protonation doping. Reaction of polyaniline emeraldine base in aqueous HCl solution results in a complete or partial protonation of the imine nitrogens to give a protonated emeraldine hydrochloride salt [3]. The protonated polyaniline is actually polysemiquinone radical cation [1,4]; one resonance form consists of two separated * Corresponding author. Tel.: 66-2-218-4131; fax: 66-2-611-7221. E-mail address: [email protected] (A. Sirivat).
0379-6779/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 1 1 2 - 1
polarons resulting from an ``internal'' redox reaction. The polarons are then separated into polaron lattice to minimize the eletrostatic repulsion between positive charges [1]. The overall structure is expected to have extensive spins and charge delocalization, resulting in a half-®lled polaron conduction band and an increase in the electrical conductivity by several orders of magnitude without any changes in the number of electrons on the polyaniline backbone [3,4]. In recent applications of polyaniline, in its conductive form of emeraldine salt, it has been used as a sensor material in solution sensor applications. Polyaniline was used to entrap enzyme glucose oxidase [5±11], used as a pH transducer [12], used as a chlorine biosensor [13], used as a deposited ferrocene-modi®ed polyaniline ®lm to detect hydrogen peroxide [14], used in fabricating a glucose amperometric sensor [15], used in fabricating a urea biosensor [16], and used as a sensor to detect aliphatic alcohol vapors [17,18]. Recently, interactions with water [19±21] and methanol [22] have been speci®cally investigated. In the present work, we are interested in utilizing conductive polymers, in particular doped polyaniline, in detecting ethanol concentration in ethanol±water mixtures.
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Ethanol molecule is relatively small compared to other aliphatic alcohols; it is used widely in foods and beverages and possibly as a substitute for regular gasoline. The essential requirements for an effective sensor material are: (a) sensitivity which includes the change in sensor speci®c conductivity for a given concentration, the lowest detectable concentration, and the highest saturated concentration; (b) selectivity for a particular chemical species; and (c) temporal response. We shall report our study on the effect of dopant type and doping molar ratio on the sensitivity and selectivity of the polyaniline electrical conductivity upon exposed to ethanol±water mixtures. The effect on temporal response shall be reported elsewhere. 2. Experimental 2.1. Materials Aniline monomer (Merck) was puri®ed prior to use. Ammonium peroxydisulphate ((NH4)2S2O8) (Merck) was used as an oxidant. HCl (BDH Laboratory) and CH3COOH (Lab-Scan) were used as received as acid dopants. Nmethyl-2-pyrrolidone (Lab-Scan) was used as the solvent in preparing polyaniline solutions for the ultraviolet±visible (UV±VIS) spectra and for ®lm casting. N-Methyl-2-pyrrolidone (Lab-Scan), HPLC grade, was used as the mobile phase in gel permeation chromatography. 2.2. Synthesis of polyaniline emeraldine base Polyaniline emeraldine base was synthesized according to the method described earlier [23]. Aniline monomer was puri®ed by distillation under reduced pressure at 50±60 8C. A 0.05 mol of puri®ed aniline monomers was dissolved in 300 ml of 1 M HCl aqueous solution and cooled at 0±5 8C in an ice bath. A 0.01 mol of (NH4)2S2O8 was dissolved in 100 ml of 1 M HCl aqueous solution, then it was slowly added into the previous aniline solution and the mixture was vigorously stirred for 2 h at 0±5 8C. The polyaniline hydrochloride, PAN/Cl , was ®ltrated and repeatedly washed with 750 ml of 1 M HCl aqueous solution, 750 ml of deionized water, and 750 ml of 0.1 M NH4OH until the ®ltrate was colorless. Polyaniline hydrochloride was treated in a 500 ml of 0.1 M NH4OH solution, by stirring vigorously for a period of 5 h and ®nally converted to emeraldine base. The product, the polyaniline emeraldine base, was ®ltrated and washed with 750 ml of 1 M NH4OH solution and deionized water until the ®ltrate was neutral. Polyaniline emeraldine base powder was dried in a vacuum oven at 40±50 8C for a period of 3 days and then stored in a dessicator at room temperature for later use. 2.3. Preparation of doped polyaniline emeraldine salt films Four grams of polyaniline emeraldine base was dissolved in 1000 ml of N-methyl-2-pyrrolidone and the solution was
stirred for 5 h at room temperature. Eight milliliters of the solution was then mixed with an acid dopant of a known concentration, and the mixture was stirred for 15±20 h at room temperature. Two acids were used as the dopants: HCl and CH3COOH with the corresponding pKa values of 6.10 and 4.76, respectively. A 0.5 ml of doped polyaniline solution was cast on a glass slide and the solvent was allowed to evaporate in a vacuum oven for a period between 96 and 100 h at 40±50 8C. The casting process was repeated several times and the typical ®lm thickness obtained was between 12 and 20 mm. 2.4. Characterizations of undoped and doped polyanilines The ultraviolet±visible (UV±VIS) spectra of polyaniline were obtained from a UV±VIS spectrometer (Perkin-Elmer, model Lambda 10) in the wavelength range of 300±900 nm. The light source was a deuterium lamp. N-methyl-2-pyrrolidone was used as the solvent to prepare the sample polyaniline solutions at the concentration of 0.1 g/l. The Fourier transform-infrared (FT-IR) spectra of polyaniline were obtained from a FT-IR spectrometer (Bruker, model FRA 106/S) in the absorbance mode with 20 scans at the resolution of 4 cm 1 covering the wavenumber between 400 and 4000 cm 1. A deuterated triglycine sulfate detector was used. Spectroscopy grade KBr (Carlo Erba) was used as the binding material. Polyaniline samples were mixed with dried KBr, ground and pelletized. A thermal gravimetric analyzer (Dupont, model TGA 2950) was used to study thermal stability and to determine the decomposition temperature of polyanilines synthesized at a heating rate of 20 8C/min. A gel permeation chromatography (Waters, 150C-Plus) was used to determine the molecular weight of polyaniline emeraldine base synthesized. We used an ultrastyragel column at 85 8C with NMP (Lab-Scan, HPLC grade) as the mobile phase, and known molecular weight polystyrenes were used as the standards for the calibration curve. Elution time was detected by a refractometer. Polyaniline emeraldine base powder was ®rst dissolved in NMP at the concentration of 0.6 wt.%, and the solution was ®ltered through a 0.2 mm PTFE ®lter. An X-ray diffractometer (Rigaku, model D/MAX-2000) was used to obtain the diffraction patterns and the degree of crystallinity. The measurements were carried out in the continuous mode with a scan speed of 58/min covering the angle 2y between 5 and 508. Cu Ka1 was used as the X-ray source. Custom made four-point probe and associated electrical units [24] were used to measure speci®c conductivity of polyaniline ®lms under atmospheric conditions or under the exposures to water, ethanol or their mixtures. The four-point probe built was of a rectangular array geometry. A current of 0.05 mA was applied between probes numbered 1 and 2 and the resultant voltage was measured across the probes numbered 3 and 4; the direction between the probes numbered 1
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and 2 is parallel to that of the probe numbered 3 and 4. The speci®c conductivity was calculated from s 1=KVt, where K is the geometric correction factor or the ratio of the probe width (distance between probes numbered 1 and 3) divided by the probe length (distance between probes numbered 1 and 2), V the measured voltage drop over the probe length, and t the polyaniline ®lm thickness. In preliminary tests, the current used was varied in order to verify that the voltage drop was in the Ohmic regime. The geometric correction factor K was calibrated and obtained from SiO2 sheets of known resistance values. In the study of the effect of exposures to water, ethanol and their mixtures, 2.0 ml of prepared solution was pipetted and dropped onto a polyaniline ®lm without allowing any contact with the four probes. The ®lm and solution temperatures were maintained at 25 8C before measurements were taken. 3. Results and discussion 3.1. Characterizations of polyaniline emeraldine base and salts We veri®ed that the undoped and doped polymers synthesized were polyaniline emearaldine base and salts by investigating their UV±VIS spectra. At a low doping molar ratio (Na/NEB) for HCl-doped polyaniline, two absorption bands were observed: the bands at 325 and 625 nm. These two absorption peaks corresponded to the p±p transition of the benzenoid ring and the exciton absorption of the quinoid ring, respectively [25]. These two absorption peaks were also found in an undoped polyaniline emeraldine base synthesized. At a higher doping molar ratio, two new absorption peaks were identi®ed. The peak at 865 nm represented the polaron species, replacing the absorbance peak of 625 nm which disappeared completely. The shoulder peak at 420 nm represented the bipolaron species which were also present, but in a smaller proportion relative to the polaron species. For the other protonic acid used, CH3COOH, the absorption bands in the UV±VIS spectra at 420 and 865 nm appeared in all of the doped polyanilines, provided doping molar ratio was suf®ciently large. Our UV±VIS spectra results are consistent with those reported previously [4]. A FT-IR spectrum of the polyaniline emeraldine base was measured. There were ®ve important absorption bands present. The band at 1590 cm 1 represented the CN stretching of the quinoid ring. The band at 1495 cm 1 indicated the stretching of the benzenoid ring. The C±N stretching of the benzenoid ring appeared as the 1299 cm 1 band. The band at 1163 cm 1 re¯ected the in-plane C±H bending motion of the quinoid ring and the band at 827 cm 1 was identi®ed with the out-of-plane bending of C±H bond in the aromatic ring. The peaks identi®ed are consistent with previously published data [26,27]. FT-IR spectra of the PANI/HCl ®lms of various molar ratios were measured. There were ®ve new bands present:
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1735, 1154, 1054, 1014, and 854 cm 1. The band at 1735 cm 1 represented the CO stretching of NMP molecules in the polyaniline ®lms [23]. The band at 1154 cm 1, appearing at high values of the molar ratio, indicated the broken symmetry mode of the quinoid ring [28]. The bands of 1054 and 1014 cm 1, not present in the FT-IR spectra at the low value of Na/NEB but visible at high value of Na/NEB, represented the substitutions of excess chloride ions onto the meta-position and ortho-position, respectively [29]. The band at 854 cm 1, representing the out of plane bending of the aromatic ring [27], resulted from the shift of the band at 827 cm 1 of the emeraldine base due to the change from the quinoid structure to the benzenoid structure upon doping. FT-IR spectra of the PANI/CH3COOH ®lms were also measured and the peaks were identical to those of the undoped PANI ®lms because CH3COOH is a weak acid and has less ef®ciency to protonate H on the polymer chain even at relatively high doping molar ratios used. Thermal gravimetric analysis thermogram of polyaniline emeraldine base was recorded in N2 atmosphere showing a two-step weight loss behavior. The ®rst step, with 1±2 wt.% loss occurring at temperature around 70 8C, can be attributed to the loss of water molecules [30]. The second step occurred at 510 8C where the skeletal of emeraldine base decomposed [31]. For both PANI/HCl and PANI/CH3COOH polyaniline ®lms, a three-step weight loss behavior was observed. The ®rst step, occurring between 50 and 100 8C, was due to loss of water. The second step, at approximately 200 8C, originated from the evaporation of NMP solvent molecules. The third step corresponded to the loss of acid dopants and the decomposition of polyaniline backbone; the decomposition temperatures found were 400 8C for PANI/HCl and 500 8C for PANI/CH3COOH. Finally, we determined the molecular weight of the polyaniline emeraldine base synthesized by GPC. Two samples from two separate syntheses but of identical procedure were used. The average Mw and Mn values found were 89,000 and 21,000, respectively and the corresponding polydispersity was 4.16. 3.2. Electrical conductivity Subsequent studies on speci®c conductivity of all polyaniline ®lms were carried out with the ®lms which had been previously dried in a vacuum oven at 40±50 8C for a period of 3 days to reduce water contents to be as low as 2±3 wt.%. In all speci®c electrical conductivity measurements, temperature was kept between 25 and 27 8C and the relative open air humidity was between 65 and 70%. Fig. 1a and b show speci®c conductivity of PANI/HCl and PANI/CH3COOH ®lms of various doping molar ratios under the exposures to air, water and pure ethanol. In air, the speci®c conductivity of PANI/HCl ®lms rises and then declines with the doping molar ratio, Na/NEB. On the other hand, the speci®c conductivity of the PANI/CH3COOH ®lms
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Fig. 1. (a) The specific conductivity of PANI/HCl films vs. doping molar ratio, Na/NEB, under the exposures to air, water, and pure ethanol. The temperature was between 25 and 27 8C and the open air relative humidity was between 65 and 70%. (b) The specific conductivity of PANI/ CH3COOH films vs. doping molar ratio, Na/NEB, under the exposures to air, water, and pure ethanol. The temperature was between 25 and 27 8C and the open air relative humidity was between 65 and 70%.
gradually increases toward a constant value of about 0.01 S/cm when Na/NEB is large. The rise and decline in speci®c conductivity of PANI/HCl ®lms re¯ect the similar behavior observed in crystallinity. At low doping molar ratios, the X-ray pattern consists only of a broad band at 2y 258, characteristic of an amorphous region [32]. At the doping molar ratio of 98, the X-ray pattern contains several distinct peaks, suggesting some crystalline domains present. The characteristic peak at 2y Ê , represents the average distance 11.08, d-spacing 8.04 A between the polymer chains. The larger peak at 2y 14.68, Ê , corresponds to the distance between N d-spacing 5.94 A and the Cl counterions. These two X-ray peaks identi®ed are
consistent with those in a previously published result [32] Ê , respecwhere they found d-spacings of 9.57 and 5.94 A tively. Other characteristic peaks of 2y 21.4, 26.0, 27.5, 28.8 30.5, 32.7, and 39.18 which appeared are similar to those found previously [32]. At the doping molar ratios of 980 and 9800, the X-ray patterns show only broad bands at 2y 258. For the PANI/CH3COOH ®lms, we found no evidence for crystalline domains from their X-ray diffraction patterns. Only a broad band at 2y 258 appeared, characteristic of an X-ray diffraction pattern of an amorphous region [32]. The gradual increase in speci®c conductivity of PANI/ CH3COOH ®lms can be mainly attributed to higher doping levels or larger numbers of charge carriers available. We note that HCl is a relatively stronger acid whose pKa value is 6.10, whereas CH3COOH is a weaker acid with its pKa value of 4.76 [33]. HCl with a lower pKa value has a higher ef®ciency to donate H; polaron and bipolaron states can be formed at very low acid/emeraldine base molar ratios and induce crystallinity observed. When PANI/HCl and PANI/CH3COOH ®lms were exposed to water and pure ethanol at temperature between 25 and 27 8C, their speci®c conductivity increased. The increments in speci®c conductivity upon exposure to water were larger than those found when exposed to ethanol for both PANI/HCl and PANI/CH3COOH ®lms, and at all doping molar ratios investigated. This is equivalent to a negative change in ®lm resistance observed under an exposure to ethanol vapor [18]. The mechanism for the increase in speci®c conductivity under the water exposure can be given in terms of the interchain H-transfer [4] where water molecules act as carriers to transfer H from one chain to another. The interchain H-transfer results in (a) the protonation on some quinoid segments in which electrons can delocalize more effectively along the chain, and (b) an increase in the interchain charge mobility, and hence the increase in the speci®c electrical conductivity observed. The same interchain Htransfer mechanism can be applied to the ®lms exposed to ethanol. However, the smaller increase in speci®c conductivity observed under the exposure to ethanol can be partially attributed to the larger size of ethanol molecules which have a lower molecular diffusivity and therefore a lower interchain electron mobility. The difference in electrical conductivity response may also re¯ect the differences in solvatochromic parameters between water and ethanol: ethanol has larger polarizability and dispersion interactions, whereas dipolar interactions are nearly the same [17]. A higher H-bond basicity of water is the main factor here. Water molecule has a higher ability to cause the H transfer and hence induce a higher ionic conductivity. A recent model of a variable size metallic island, based on the increased size of the metallic islands in the presence of water, has also been proposed [20,21]. The interaction between water and ethanol molecules with PANI/HCl was investigated by means of FT-IR spectroscopy. Fig. 2 shows three FT-IR spectra of the polyaniline
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Fig. 2. The FT-IR spectra of PANI/HCl films (Na =NEB 98): (a) without exposures to water and ethanol; (b) after exposure to water; (c) after exposure to pure ethanol.
®lms under the exposures to air, water and ethanol respectively. The ®lms exposed to water and ethanol were allowed to dry at room temperature before sample preparations and measurements were taken. All three FT-IR spectra were nearly the same except that two peaks are now more visible in the water-exposed ®lm. The more pronounced peaks are at 1493 and 1317 cm 1, representing the stretching mode of Nbenzenoid±N segment, and the stretching modes of quinoid± benzenoid±quinoid segment/quinoid±benzenoid±benzenoid segment/benzenoid±benzenoid±quinoid segment [34], respectively. These two absorption bands indicate that some quinoid segments may have been protonated and more benzenoid segments were present. The absorption peak at 1153 cm 1, which represents the broken symmetry mode of quinoid structure, also increased because of a greater degree of protonation. For the PANI/HCl ®lm exposed to ethanol, no new peak occurred in the FT-IR spectrum shown. Similarly, the FT-IR spectrum of PANI/CH3COOH ®lm exposed to ethanol showed the absorption bands identical to those of a ®lm without an exposure. This suggests that ethanol molecules do not permanently change with the polyaniline emeraldine salt. 3.3. Effect of ethanol concentration Fig. 3a and 3b show speci®c conductivity versus ethanol concentration of PANI/HCl and PANI/CH3COOH ®lms at 25 8C. The doping molar ratios, Na/NEB, were 98 and 5900 corresponding to their maximum speci®c electrical conductivity attainable in air, respectively. There are three response regimes depending upon ethanol concentration. At low ethanol concentrations, speci®c electrical conductivity was independent of ethanol concentration, indicating that no measurable physical or chemical adsorption taking place. The minimum detectable concentration can be de®ned as the concentration required for a polyaniline ®lm to become sensitive to the presence of ethanol. They were 1.7 and 2.8 M for the PANI/HCl and PANI/CH3COOH ®lms, respec-
Fig. 3. (a) The specific conductivity of PANI/HCl films (Na =NEB 98) vs. ethanol concentration. The temperature was between 25 and 27 8C, and the open air relative humidity was between 65 and 70%. (b) The specific conductivity of PANI/CH3COOH films (Na =NEB 5900) vs. ethanol concentration. The temperature was between 25 and 27 8C, and the open air relative humidity was between 65 and 70%.
tively. At intermediate ethanol concentrations, speci®c electrical conductivity was inversely proportional to ethanol concentration with sensitivity equal to 1:91 10 3 and 1:1 10 4 S/(cm M) for the PANI/HCl and PANI/ CH3COOH ®lms, respectively. The last regime corresponds to the concentration range in which speci®c conductivity was independent of ethanol concentration; polyaniline ®lms were saturated with ethanol molecules. The saturated concentrations were 8.3 M and 10.2 M for the PANI/HCl and PANI/CH3COOH ®lms, respectively. The large difference in concentration dependence observed for the PANI/HCl ®lm, by an order of magnitude, can be attributed to the Cl anions present and the greater degree of crystallinity induced.
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4. Conclusion We have studied the effect of dopant type and concentration on speci®c conductivity of polyaniline ®lms under atmospheric conditions and under exposures to water and ethanol. PANI/HCl ®lms fabricated contained some crystalline domains causing a greater speci®c conductivity than that of PANI/CH3COOH ®lms for the same doping molar ratio; HCl is a relatively stronger acid with a greater ef®ciency to protonate the emeraldine base. The speci®c electrical conductivity of the PANI/HCl and PANI/ CH3COOH ®lms increases upon the exposures to ethanol and water, with greater increments occurring with water at all doping molar ratios studied. Interchain H-transfer is suggested to be a common mechanism that induces ionic electron mobility, whereas an additional protonation occurs along the polymer chain upon the exposure to water. Ethanol molecules do not permanently interact with the PANI/HCl studied, only ionic conduction occurs. The speci®c conductivity of the PANI/HCl and PANI/CH3COOH ®lms studied is inversely proportional to ethanol concentration. The sensitivity to ethanol concentration can be varied by at least an order of magnitude by changing the type of the acid dopant used; a stronger acid induces higher crystallinity and gives a polyaniline ®lm which has a greater ethanol concentration sensitivity. Acknowledgements L.R. and A.S. would like to acknowledge the ®nancial support from the Rajadapisek Fund of Chulalongkorn University, the RGJ grant from TRF, no. PHD/4/2541, and the BGJ award from TRF, no. BGJ/5/2543. The authors are grateful of the laboratory assistance provided from P. Saengsawang, J. Amornlertratanatada, and C.P.O. Poon Arjpru. References [1] W.R. Slalneck, I. Lundstrom, B. Ranby, Conjugated Polymers and Related Materials: The Interconnection of Chemical and Electronic Structure, Oxford Science, Oxford, 1993.
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Electrical conductivity response of polyaniline ®lms to ethanol±water mixtures L. Tarachiwina, P. Kiattibutra, L. Ruangchuaya, A. Sirivata,*, J. Schwankb a
The Petroleum and Petrochemical College, Chulalongkorn University, Phya Thai Road Soi Chulalongkorn, Bangkok 10330, Thailand b Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA Received 8 May 2001; received in revised form 23 January 2002; accepted 6 May 2002
Abstract Polyaniline emeraldine base was chemically synthesized and converted to polyaniline emeraldine salts through the protonation doping process using HCl and CH3COOH as the acid dopants. The doped polyanilines were characterized by ultraviolet±visible (UV±VIS) spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, TGA, GPC. The four-point probe technique was used to evaluate the effect of dopant type and doping molar ratio on the speci®c conductivity. Weak acid doping, by CH3COOH, produces ®lms with the speci®c conductivity which depends solely on the degree of protonation, or the number of charge carriers. On the other hand, stronger acid doping by HCl can induce crystalline domains, a greater electron mobility and hence a greater speci®c conductivity value. The speci®c conductivity of the HCl-doped and CH3COOH-doped polyaniline ®lms responds with positive increments upon exposed to water and ethanol. The interchain H-transfer is suggested to be a common mechanism which increases electron mobility upon exposure to water and ethanol, whereas additional protonation occurs only with the exposure to water. No evidence for ethanol molecules to interact chemically with the doped polyaniline ®lms was found. The ®lm electrical conductivity sensitivity is inversely proportional to ethanol concentration, with a higher sensitivity to concentration found in the ®lm doped with the acid with a lower pKa value. # 2002 Published by Elsevier Science B.V. Keywords: Polyaniline emeraldine salts; Protonic acid doping; Speci®c electrical conductivity; Exposures to water and ethanol
1. Introduction Polyaniline constitutes an important class of conductive polymers known and it can be synthesized either by chemical or electrochemical oxidative polymerizations [1,2]. In the emeraldine base form, or the half-oxidized form, the number of amine nitrogen atoms is equal to the number of imine nitrogen atoms; therefore, emeraldine base is also called poly(phenylene amine±imine) [1]. The structure of emeraldine base consists of two amine nitrogen atoms followed by two imine nitrogen atoms. The imine nitrogen indeed contributes a single p electron, with its lone pair lying within the molecular plane. Emeraldine base can be doped through a doping process called the protonation doping. Reaction of polyaniline emeraldine base in aqueous HCl solution results in a complete or partial protonation of the imine nitrogens to give a protonated emeraldine hydrochloride salt [3]. The protonated polyaniline is actually polysemiquinone radical cation [1,4]; one resonance form consists of two separated * Corresponding author. Tel.: 66-2-218-4131; fax: 66-2-611-7221. E-mail address: [email protected] (A. Sirivat).
0379-6779/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 1 1 2 - 1
polarons resulting from an ``internal'' redox reaction. The polarons are then separated into polaron lattice to minimize the eletrostatic repulsion between positive charges [1]. The overall structure is expected to have extensive spins and charge delocalization, resulting in a half-®lled polaron conduction band and an increase in the electrical conductivity by several orders of magnitude without any changes in the number of electrons on the polyaniline backbone [3,4]. In recent applications of polyaniline, in its conductive form of emeraldine salt, it has been used as a sensor material in solution sensor applications. Polyaniline was used to entrap enzyme glucose oxidase [5±11], used as a pH transducer [12], used as a chlorine biosensor [13], used as a deposited ferrocene-modi®ed polyaniline ®lm to detect hydrogen peroxide [14], used in fabricating a glucose amperometric sensor [15], used in fabricating a urea biosensor [16], and used as a sensor to detect aliphatic alcohol vapors [17,18]. Recently, interactions with water [19±21] and methanol [22] have been speci®cally investigated. In the present work, we are interested in utilizing conductive polymers, in particular doped polyaniline, in detecting ethanol concentration in ethanol±water mixtures.
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Ethanol molecule is relatively small compared to other aliphatic alcohols; it is used widely in foods and beverages and possibly as a substitute for regular gasoline. The essential requirements for an effective sensor material are: (a) sensitivity which includes the change in sensor speci®c conductivity for a given concentration, the lowest detectable concentration, and the highest saturated concentration; (b) selectivity for a particular chemical species; and (c) temporal response. We shall report our study on the effect of dopant type and doping molar ratio on the sensitivity and selectivity of the polyaniline electrical conductivity upon exposed to ethanol±water mixtures. The effect on temporal response shall be reported elsewhere. 2. Experimental 2.1. Materials Aniline monomer (Merck) was puri®ed prior to use. Ammonium peroxydisulphate ((NH4)2S2O8) (Merck) was used as an oxidant. HCl (BDH Laboratory) and CH3COOH (Lab-Scan) were used as received as acid dopants. Nmethyl-2-pyrrolidone (Lab-Scan) was used as the solvent in preparing polyaniline solutions for the ultraviolet±visible (UV±VIS) spectra and for ®lm casting. N-Methyl-2-pyrrolidone (Lab-Scan), HPLC grade, was used as the mobile phase in gel permeation chromatography. 2.2. Synthesis of polyaniline emeraldine base Polyaniline emeraldine base was synthesized according to the method described earlier [23]. Aniline monomer was puri®ed by distillation under reduced pressure at 50±60 8C. A 0.05 mol of puri®ed aniline monomers was dissolved in 300 ml of 1 M HCl aqueous solution and cooled at 0±5 8C in an ice bath. A 0.01 mol of (NH4)2S2O8 was dissolved in 100 ml of 1 M HCl aqueous solution, then it was slowly added into the previous aniline solution and the mixture was vigorously stirred for 2 h at 0±5 8C. The polyaniline hydrochloride, PAN/Cl , was ®ltrated and repeatedly washed with 750 ml of 1 M HCl aqueous solution, 750 ml of deionized water, and 750 ml of 0.1 M NH4OH until the ®ltrate was colorless. Polyaniline hydrochloride was treated in a 500 ml of 0.1 M NH4OH solution, by stirring vigorously for a period of 5 h and ®nally converted to emeraldine base. The product, the polyaniline emeraldine base, was ®ltrated and washed with 750 ml of 1 M NH4OH solution and deionized water until the ®ltrate was neutral. Polyaniline emeraldine base powder was dried in a vacuum oven at 40±50 8C for a period of 3 days and then stored in a dessicator at room temperature for later use. 2.3. Preparation of doped polyaniline emeraldine salt films Four grams of polyaniline emeraldine base was dissolved in 1000 ml of N-methyl-2-pyrrolidone and the solution was
stirred for 5 h at room temperature. Eight milliliters of the solution was then mixed with an acid dopant of a known concentration, and the mixture was stirred for 15±20 h at room temperature. Two acids were used as the dopants: HCl and CH3COOH with the corresponding pKa values of 6.10 and 4.76, respectively. A 0.5 ml of doped polyaniline solution was cast on a glass slide and the solvent was allowed to evaporate in a vacuum oven for a period between 96 and 100 h at 40±50 8C. The casting process was repeated several times and the typical ®lm thickness obtained was between 12 and 20 mm. 2.4. Characterizations of undoped and doped polyanilines The ultraviolet±visible (UV±VIS) spectra of polyaniline were obtained from a UV±VIS spectrometer (Perkin-Elmer, model Lambda 10) in the wavelength range of 300±900 nm. The light source was a deuterium lamp. N-methyl-2-pyrrolidone was used as the solvent to prepare the sample polyaniline solutions at the concentration of 0.1 g/l. The Fourier transform-infrared (FT-IR) spectra of polyaniline were obtained from a FT-IR spectrometer (Bruker, model FRA 106/S) in the absorbance mode with 20 scans at the resolution of 4 cm 1 covering the wavenumber between 400 and 4000 cm 1. A deuterated triglycine sulfate detector was used. Spectroscopy grade KBr (Carlo Erba) was used as the binding material. Polyaniline samples were mixed with dried KBr, ground and pelletized. A thermal gravimetric analyzer (Dupont, model TGA 2950) was used to study thermal stability and to determine the decomposition temperature of polyanilines synthesized at a heating rate of 20 8C/min. A gel permeation chromatography (Waters, 150C-Plus) was used to determine the molecular weight of polyaniline emeraldine base synthesized. We used an ultrastyragel column at 85 8C with NMP (Lab-Scan, HPLC grade) as the mobile phase, and known molecular weight polystyrenes were used as the standards for the calibration curve. Elution time was detected by a refractometer. Polyaniline emeraldine base powder was ®rst dissolved in NMP at the concentration of 0.6 wt.%, and the solution was ®ltered through a 0.2 mm PTFE ®lter. An X-ray diffractometer (Rigaku, model D/MAX-2000) was used to obtain the diffraction patterns and the degree of crystallinity. The measurements were carried out in the continuous mode with a scan speed of 58/min covering the angle 2y between 5 and 508. Cu Ka1 was used as the X-ray source. Custom made four-point probe and associated electrical units [24] were used to measure speci®c conductivity of polyaniline ®lms under atmospheric conditions or under the exposures to water, ethanol or their mixtures. The four-point probe built was of a rectangular array geometry. A current of 0.05 mA was applied between probes numbered 1 and 2 and the resultant voltage was measured across the probes numbered 3 and 4; the direction between the probes numbered 1
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and 2 is parallel to that of the probe numbered 3 and 4. The speci®c conductivity was calculated from s 1=KVt, where K is the geometric correction factor or the ratio of the probe width (distance between probes numbered 1 and 3) divided by the probe length (distance between probes numbered 1 and 2), V the measured voltage drop over the probe length, and t the polyaniline ®lm thickness. In preliminary tests, the current used was varied in order to verify that the voltage drop was in the Ohmic regime. The geometric correction factor K was calibrated and obtained from SiO2 sheets of known resistance values. In the study of the effect of exposures to water, ethanol and their mixtures, 2.0 ml of prepared solution was pipetted and dropped onto a polyaniline ®lm without allowing any contact with the four probes. The ®lm and solution temperatures were maintained at 25 8C before measurements were taken. 3. Results and discussion 3.1. Characterizations of polyaniline emeraldine base and salts We veri®ed that the undoped and doped polymers synthesized were polyaniline emearaldine base and salts by investigating their UV±VIS spectra. At a low doping molar ratio (Na/NEB) for HCl-doped polyaniline, two absorption bands were observed: the bands at 325 and 625 nm. These two absorption peaks corresponded to the p±p transition of the benzenoid ring and the exciton absorption of the quinoid ring, respectively [25]. These two absorption peaks were also found in an undoped polyaniline emeraldine base synthesized. At a higher doping molar ratio, two new absorption peaks were identi®ed. The peak at 865 nm represented the polaron species, replacing the absorbance peak of 625 nm which disappeared completely. The shoulder peak at 420 nm represented the bipolaron species which were also present, but in a smaller proportion relative to the polaron species. For the other protonic acid used, CH3COOH, the absorption bands in the UV±VIS spectra at 420 and 865 nm appeared in all of the doped polyanilines, provided doping molar ratio was suf®ciently large. Our UV±VIS spectra results are consistent with those reported previously [4]. A FT-IR spectrum of the polyaniline emeraldine base was measured. There were ®ve important absorption bands present. The band at 1590 cm 1 represented the CN stretching of the quinoid ring. The band at 1495 cm 1 indicated the stretching of the benzenoid ring. The C±N stretching of the benzenoid ring appeared as the 1299 cm 1 band. The band at 1163 cm 1 re¯ected the in-plane C±H bending motion of the quinoid ring and the band at 827 cm 1 was identi®ed with the out-of-plane bending of C±H bond in the aromatic ring. The peaks identi®ed are consistent with previously published data [26,27]. FT-IR spectra of the PANI/HCl ®lms of various molar ratios were measured. There were ®ve new bands present:
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1735, 1154, 1054, 1014, and 854 cm 1. The band at 1735 cm 1 represented the CO stretching of NMP molecules in the polyaniline ®lms [23]. The band at 1154 cm 1, appearing at high values of the molar ratio, indicated the broken symmetry mode of the quinoid ring [28]. The bands of 1054 and 1014 cm 1, not present in the FT-IR spectra at the low value of Na/NEB but visible at high value of Na/NEB, represented the substitutions of excess chloride ions onto the meta-position and ortho-position, respectively [29]. The band at 854 cm 1, representing the out of plane bending of the aromatic ring [27], resulted from the shift of the band at 827 cm 1 of the emeraldine base due to the change from the quinoid structure to the benzenoid structure upon doping. FT-IR spectra of the PANI/CH3COOH ®lms were also measured and the peaks were identical to those of the undoped PANI ®lms because CH3COOH is a weak acid and has less ef®ciency to protonate H on the polymer chain even at relatively high doping molar ratios used. Thermal gravimetric analysis thermogram of polyaniline emeraldine base was recorded in N2 atmosphere showing a two-step weight loss behavior. The ®rst step, with 1±2 wt.% loss occurring at temperature around 70 8C, can be attributed to the loss of water molecules [30]. The second step occurred at 510 8C where the skeletal of emeraldine base decomposed [31]. For both PANI/HCl and PANI/CH3COOH polyaniline ®lms, a three-step weight loss behavior was observed. The ®rst step, occurring between 50 and 100 8C, was due to loss of water. The second step, at approximately 200 8C, originated from the evaporation of NMP solvent molecules. The third step corresponded to the loss of acid dopants and the decomposition of polyaniline backbone; the decomposition temperatures found were 400 8C for PANI/HCl and 500 8C for PANI/CH3COOH. Finally, we determined the molecular weight of the polyaniline emeraldine base synthesized by GPC. Two samples from two separate syntheses but of identical procedure were used. The average Mw and Mn values found were 89,000 and 21,000, respectively and the corresponding polydispersity was 4.16. 3.2. Electrical conductivity Subsequent studies on speci®c conductivity of all polyaniline ®lms were carried out with the ®lms which had been previously dried in a vacuum oven at 40±50 8C for a period of 3 days to reduce water contents to be as low as 2±3 wt.%. In all speci®c electrical conductivity measurements, temperature was kept between 25 and 27 8C and the relative open air humidity was between 65 and 70%. Fig. 1a and b show speci®c conductivity of PANI/HCl and PANI/CH3COOH ®lms of various doping molar ratios under the exposures to air, water and pure ethanol. In air, the speci®c conductivity of PANI/HCl ®lms rises and then declines with the doping molar ratio, Na/NEB. On the other hand, the speci®c conductivity of the PANI/CH3COOH ®lms
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Fig. 1. (a) The specific conductivity of PANI/HCl films vs. doping molar ratio, Na/NEB, under the exposures to air, water, and pure ethanol. The temperature was between 25 and 27 8C and the open air relative humidity was between 65 and 70%. (b) The specific conductivity of PANI/ CH3COOH films vs. doping molar ratio, Na/NEB, under the exposures to air, water, and pure ethanol. The temperature was between 25 and 27 8C and the open air relative humidity was between 65 and 70%.
gradually increases toward a constant value of about 0.01 S/cm when Na/NEB is large. The rise and decline in speci®c conductivity of PANI/HCl ®lms re¯ect the similar behavior observed in crystallinity. At low doping molar ratios, the X-ray pattern consists only of a broad band at 2y 258, characteristic of an amorphous region [32]. At the doping molar ratio of 98, the X-ray pattern contains several distinct peaks, suggesting some crystalline domains present. The characteristic peak at 2y Ê , represents the average distance 11.08, d-spacing 8.04 A between the polymer chains. The larger peak at 2y 14.68, Ê , corresponds to the distance between N d-spacing 5.94 A and the Cl counterions. These two X-ray peaks identi®ed are
consistent with those in a previously published result [32] Ê , respecwhere they found d-spacings of 9.57 and 5.94 A tively. Other characteristic peaks of 2y 21.4, 26.0, 27.5, 28.8 30.5, 32.7, and 39.18 which appeared are similar to those found previously [32]. At the doping molar ratios of 980 and 9800, the X-ray patterns show only broad bands at 2y 258. For the PANI/CH3COOH ®lms, we found no evidence for crystalline domains from their X-ray diffraction patterns. Only a broad band at 2y 258 appeared, characteristic of an X-ray diffraction pattern of an amorphous region [32]. The gradual increase in speci®c conductivity of PANI/ CH3COOH ®lms can be mainly attributed to higher doping levels or larger numbers of charge carriers available. We note that HCl is a relatively stronger acid whose pKa value is 6.10, whereas CH3COOH is a weaker acid with its pKa value of 4.76 [33]. HCl with a lower pKa value has a higher ef®ciency to donate H; polaron and bipolaron states can be formed at very low acid/emeraldine base molar ratios and induce crystallinity observed. When PANI/HCl and PANI/CH3COOH ®lms were exposed to water and pure ethanol at temperature between 25 and 27 8C, their speci®c conductivity increased. The increments in speci®c conductivity upon exposure to water were larger than those found when exposed to ethanol for both PANI/HCl and PANI/CH3COOH ®lms, and at all doping molar ratios investigated. This is equivalent to a negative change in ®lm resistance observed under an exposure to ethanol vapor [18]. The mechanism for the increase in speci®c conductivity under the water exposure can be given in terms of the interchain H-transfer [4] where water molecules act as carriers to transfer H from one chain to another. The interchain H-transfer results in (a) the protonation on some quinoid segments in which electrons can delocalize more effectively along the chain, and (b) an increase in the interchain charge mobility, and hence the increase in the speci®c electrical conductivity observed. The same interchain Htransfer mechanism can be applied to the ®lms exposed to ethanol. However, the smaller increase in speci®c conductivity observed under the exposure to ethanol can be partially attributed to the larger size of ethanol molecules which have a lower molecular diffusivity and therefore a lower interchain electron mobility. The difference in electrical conductivity response may also re¯ect the differences in solvatochromic parameters between water and ethanol: ethanol has larger polarizability and dispersion interactions, whereas dipolar interactions are nearly the same [17]. A higher H-bond basicity of water is the main factor here. Water molecule has a higher ability to cause the H transfer and hence induce a higher ionic conductivity. A recent model of a variable size metallic island, based on the increased size of the metallic islands in the presence of water, has also been proposed [20,21]. The interaction between water and ethanol molecules with PANI/HCl was investigated by means of FT-IR spectroscopy. Fig. 2 shows three FT-IR spectra of the polyaniline
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Fig. 2. The FT-IR spectra of PANI/HCl films (Na =NEB 98): (a) without exposures to water and ethanol; (b) after exposure to water; (c) after exposure to pure ethanol.
®lms under the exposures to air, water and ethanol respectively. The ®lms exposed to water and ethanol were allowed to dry at room temperature before sample preparations and measurements were taken. All three FT-IR spectra were nearly the same except that two peaks are now more visible in the water-exposed ®lm. The more pronounced peaks are at 1493 and 1317 cm 1, representing the stretching mode of Nbenzenoid±N segment, and the stretching modes of quinoid± benzenoid±quinoid segment/quinoid±benzenoid±benzenoid segment/benzenoid±benzenoid±quinoid segment [34], respectively. These two absorption bands indicate that some quinoid segments may have been protonated and more benzenoid segments were present. The absorption peak at 1153 cm 1, which represents the broken symmetry mode of quinoid structure, also increased because of a greater degree of protonation. For the PANI/HCl ®lm exposed to ethanol, no new peak occurred in the FT-IR spectrum shown. Similarly, the FT-IR spectrum of PANI/CH3COOH ®lm exposed to ethanol showed the absorption bands identical to those of a ®lm without an exposure. This suggests that ethanol molecules do not permanently change with the polyaniline emeraldine salt. 3.3. Effect of ethanol concentration Fig. 3a and 3b show speci®c conductivity versus ethanol concentration of PANI/HCl and PANI/CH3COOH ®lms at 25 8C. The doping molar ratios, Na/NEB, were 98 and 5900 corresponding to their maximum speci®c electrical conductivity attainable in air, respectively. There are three response regimes depending upon ethanol concentration. At low ethanol concentrations, speci®c electrical conductivity was independent of ethanol concentration, indicating that no measurable physical or chemical adsorption taking place. The minimum detectable concentration can be de®ned as the concentration required for a polyaniline ®lm to become sensitive to the presence of ethanol. They were 1.7 and 2.8 M for the PANI/HCl and PANI/CH3COOH ®lms, respec-
Fig. 3. (a) The specific conductivity of PANI/HCl films (Na =NEB 98) vs. ethanol concentration. The temperature was between 25 and 27 8C, and the open air relative humidity was between 65 and 70%. (b) The specific conductivity of PANI/CH3COOH films (Na =NEB 5900) vs. ethanol concentration. The temperature was between 25 and 27 8C, and the open air relative humidity was between 65 and 70%.
tively. At intermediate ethanol concentrations, speci®c electrical conductivity was inversely proportional to ethanol concentration with sensitivity equal to 1:91 10 3 and 1:1 10 4 S/(cm M) for the PANI/HCl and PANI/ CH3COOH ®lms, respectively. The last regime corresponds to the concentration range in which speci®c conductivity was independent of ethanol concentration; polyaniline ®lms were saturated with ethanol molecules. The saturated concentrations were 8.3 M and 10.2 M for the PANI/HCl and PANI/CH3COOH ®lms, respectively. The large difference in concentration dependence observed for the PANI/HCl ®lm, by an order of magnitude, can be attributed to the Cl anions present and the greater degree of crystallinity induced.
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4. Conclusion We have studied the effect of dopant type and concentration on speci®c conductivity of polyaniline ®lms under atmospheric conditions and under exposures to water and ethanol. PANI/HCl ®lms fabricated contained some crystalline domains causing a greater speci®c conductivity than that of PANI/CH3COOH ®lms for the same doping molar ratio; HCl is a relatively stronger acid with a greater ef®ciency to protonate the emeraldine base. The speci®c electrical conductivity of the PANI/HCl and PANI/ CH3COOH ®lms increases upon the exposures to ethanol and water, with greater increments occurring with water at all doping molar ratios studied. Interchain H-transfer is suggested to be a common mechanism that induces ionic electron mobility, whereas an additional protonation occurs along the polymer chain upon the exposure to water. Ethanol molecules do not permanently interact with the PANI/HCl studied, only ionic conduction occurs. The speci®c conductivity of the PANI/HCl and PANI/CH3COOH ®lms studied is inversely proportional to ethanol concentration. The sensitivity to ethanol concentration can be varied by at least an order of magnitude by changing the type of the acid dopant used; a stronger acid induces higher crystallinity and gives a polyaniline ®lm which has a greater ethanol concentration sensitivity. Acknowledgements L.R. and A.S. would like to acknowledge the ®nancial support from the Rajadapisek Fund of Chulalongkorn University, the RGJ grant from TRF, no. PHD/4/2541, and the BGJ award from TRF, no. BGJ/5/2543. The authors are grateful of the laboratory assistance provided from P. Saengsawang, J. Amornlertratanatada, and C.P.O. Poon Arjpru. References [1] W.R. Slalneck, I. Lundstrom, B. Ranby, Conjugated Polymers and Related Materials: The Interconnection of Chemical and Electronic Structure, Oxford Science, Oxford, 1993.
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