Sensors and Actuators B 77 (2001) 657±663
Acrylic acid doped polyaniline as an ammonia sensor V.V. Chabukswar, Sushama Pethkar, Anjali A. Athawale* Department of Chemistry, University of Pune, Pune 411 007, India Received 6 November 2000; received in revised form 12 March 2001; accepted 22 March 2001
Abstract Chemically synthesised acrylic acid doped polyaniline (PANI:AA) has been utilised as an ammonia vapour sensor in a broad range of concentrations, viz. 1±600 ppm. The response, in terms of decrease in dc electric resistance on exposure to ammonia was observed. The change in resistance, DR, is found to increase linearly with NH3 concentration upto 58 ppm and saturates thereafter. The decrease in resistance has been explained on the basis of removal of proton from the free acrylic acid (AA) dopant by the ammonia molecules thereby rendering free conduction sites in the polymer matrix. These results are well supported by FTIR spectral analysis and the X-ray diffraction studies. The FTIR spectra show a remarkable increase in benzenoid and quinoid vibrations. Also, simultaneous appearance of ±COO and ammonium ion vibrations is indicative of the interaction of ammonia molecules with acrylic acid. The degree of crystallinity was found to increase substantially upto 58 ppm concentration. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Acrylic acid doped polyaniline; Ammonia vapour sensor; Resistance; Crystallinity
1. Introduction
2. Experimental
Conductive polymers have made a signi®cant impact upon a number of different technologies, since the discovery that, the semiconducting polyacetylene could be converted into a highly conducting form by proper chemical doping [1,2]. This class of polymers have a wide range of applications, such as electromagnetic interference shields, displays, capacitors, junction devices, antistatic coatings, etc. [3±6]. Several researchers have utilised conducting polymers as chemical sensors for air-borne volatile organic compounds especially, alcohols, ethers, halocarbons, ammonia, NO2 and CO2 [7±16]. The applicability of these materials as sensors is based on their selectivity over a wide range of analyte molecules and low level of gas concentration. Although, conducting polymer sensors have been developed for ammonia they are found to exhibit insuf®cient gas sensitivity as well as reversibility [10,11]. In the present work, we report the results of the acrylic acid doped polyaniline as a new material for sensing ammonia. It is found to exhibit high sensitivity over a broad range of concentrations from 1 ppm upto ca. 600 ppm. The response is highly reversible even upto 20 cycles.
Reagent grade aniline (Merck) was distilled prior to use. All other chemicals used were of analytical reagent grade. The polymerisation was carried out in aqueous acid solution. The acrylic acid doped polymer was synthesised by in situ oxidative polymerisation of aniline monomer after addition of 0.03 M of acrylic acid to the reaction mixture maintained at 58C. The total amount of 1 M HCl was used as a protonic acid. Ammonium peroxodisulphate solution prepared in double distilled water was added dropwise to the reaction mixture over a period of 1 h under constant stirring. After complete addition of the oxidant the reaction was kept under stirring for 24 h. The precipitated polymer salt was recovered from the reaction vessel by ®ltration and washing followed by drying at 60±708C for 36 h. The synthesised polymer was made in pellet form (diameter 12 mm, thickness 3 mm) by applying a pressure of 7 tonnes using Pye±Unicam system and used as sensor [17]. The sensing performance of the polymer was tested by subjecting the polymer pellets to ammonia vapours in a closed glass container at room temperature. Water was used as a diluent for preparing the various analyte concentrations. Sensing measurements were carried out for different ammonia concentrations from 1 ppm upto ca. 600 ppm for three different set of samples. The samples were characterised by measuring the dc electrical resistance FTIR spectra and X-ray diffractogrammes (XRD). The FTIR spectra of the samples
* Corresponding author. E-mail address:
[email protected] (A.A. Athawale).
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 7 8 0 - 8
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were recorded on a Perkin-Elmer 1600 FTIR spectrophotometer. X-ray diffraction patterns were obtained on a Philips PW 172Q X-ray diffractometer using Cu Ka radiation. Both FTIR and XRD measurements were performed by exposing the pellet to ammonia vapours in the sample port. 3. Results and discussion Fig. 1 shows the response of the acrylic acid doped polyaniline (PANI:AA) exposed to different concentrations of ammonia from 1 to 600 ppm. The resistance of the polymers is found to decrease when exposed to ammonia vapours. In comparison with polyaniline sensors reported earlier [10±12] the present responses exhibit an inverse nature which is rather odd. The change in resistance (DR) is found to be linearly proportional to the concentration of ammonia upto 58 ppm (Table 1). However, at higher concentrations (>58 ppm) the observed DR seems to be practically independent of ammonia concentration which limits the sensitivity (DR/R). The response time, recovery time (calculated when the DR is equal to 90% of the steady resistance) and the sensitivities in terms of DR/R for different ammonia concentrations are presented in Table 1. The response time is found to decrease with NH3 concentration, while the recovery time increases. An unusual response in magnitude and sign observed in the present case suggests a signi®cant role of the incorporated acrylic acid (AA) in the polymeric chain. In conventional PANI doped with HA type acid (where A: Cl , Br , ClO4 , etc.) the sensing mechanism is governed by the protonation/deprotonation phenomena. The reversibility in such PANI has been explained on the basis of the formation of energetically more favourable ammonium NH4 ion at N±H adsorption centres leading to a rise in the resistance on exposure to ammonia [10±12]. The same ammonium ion decomposes back into ammonia leaving out the proton on the adsorption site in the air environment. As the responses observed in the present case are in contrast with that of the conventional trend, the sensing behaviour is likely to involve a different process, especially interaction of ammonia with AA. Although, the exact mechanism of sensing is not very clear at this stage, the probable phenomena for reduction in Table 1 Results of the measurements of dc electrical resistance of polyaniline sensor subjected to different ammonia concentrations Concentration of NH3 (ppm)
DR (kO)
Sensitivity, DR/R
Response time (min)
Recovery time (min)
1 2 11 34 58 460 600 (ca.)
79 99 227 37975 38957 33909 7970
0.71 0.82 0.87 0.99 0.99 0.99 0.98
2.5 2.5 2.0 1.5 1.0 1.0 1.0
4.5 4 4 4 4 6 6
Fig. 1. Response of acrylic acid doped polyaniline exposed to different ammonia concentrations: (a) 1; (b) 2; (c) 11; (d) 34; (e) 58; (f) 460; (g) 600 ppm.
the resistance on exposure to ammonia can be interpreted by considering the interaction of NH3 with the dopant AA. In the ammonia vapour environment NH3 can attack two adsorption sites, viz. N±H sites originated from small HCl content present during polymerisation reaction and AA molecules trapped in the polymeric chains. At the latter site, ammonia molecule may take up a hydrogen from AA forming NH4 species which creates ±COO anion in the polymeric chain simultaneously leaving one site free for
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conduction. This explains the reduction in resistance imparted by self doping in the presence of AA on exposure to ammonia. The above interpretation is well supported by FTIR spectra. Fig. 2 a shows the FTIR spectrum of the bare PANI:AA sample. The major absorption bands having usual signi®cance are quoted in Table 2. The broad bands ca. 3100±3700 and 1145 cm 1 represent partial doping and protonation of
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amine and imine nitrogen atoms [18,19]. The presence of peak ca. 2920 cm 1 can be attributed to the C±H stretching vibrations of AA. Apart from this, the polymer shows C±N aromatic stretching vibrations at ca. 1299 cm 1. The absorption peaks at ca. 1571 and 1486 cm 1 are assigned to quinoid (Q) and benzenoid (B) structures of emraldine form of polymer, respectively. On the other hand, Fig. 2b±f depict the FTIR spectra of the PANI:AA samples exposed to
Fig. 2. The FTIR absorption spectra of acrylic acid doped polyaniline before and after exposure to different ammonia concentrations: (a) acrylic acid doped polyaniline; (b) 1; (c) 2; (d) 11; (e) 34; (f) 58; (g) 460; (h) 600 ppm.
660
Polyaniline (acrylic acid doped)
Ammonia concentration, 1 ppm
Ammonia concentration, 2 ppm
Ammonia concentration, 11 ppm
Ammonia concentration, 34 ppm
Ammonia concentration, 58 ppm
Ammonia concentration, 460 ppm
Ammonia concentration, 600 ppm
Peak assignment
649 809 1145 ± 1299 1486 1571 1730 2920 3100±3700
618 800 1120 1232 1291 1400 1547 1737 2916 3116
617 800 1118 1233 1292 1401 1575 1732 2924 3124
616 806 1119 1232 1292 1401 1560 1737 2918 3110
617 800 1119 1232 1292 1401 1576 1744 2925 3122
616 801 1119 1233 1292 1401 1569 1737 2916 3132
616 807 1119 1226 1293 1401 1585 1738 2923 3127
616 809 1118 1223 1292 1401 1587 1744 2915 3123
Out of plane ±C±H bending vibration Paradisubstituted benzene ring B±NH±B stretching vibration (±COO ) stretching band Aromatic (C±N) stretching band Benzenoid ring stretching Quinoid ring and (±COO ) stretching band Carbonyl (±C=O) stretching band ±C±H stretching band ±N±H stretching band
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Table 2 FTIR spectra of acrylic acid doped polyaniline before and after exposure to various concentrations of ammonia
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variable concentrations of ammonia. A comparison of these spectra with the spectrum of the bare sample reveal a signi®cant increase in the intensity of Q and B bands. This increase can be attributed to ±COO band suggesting self doping, resulting from the formation of additional site available for conduction due to the interaction of ammonia with AA as mentioned earlier. However, the band corresponding to the formation of ±COO , i.e. carboxylate anion stretching seems to overlap with the band ca. 1575 cm 1 representing the quinoid ring stretching vibrations. Hence, it can be stated that quinoid stretching vibrations grow during self-doping due to AA on exposure to ammonia. The band corresponding to benzenoid structure ca. 1486 cm 1 (Fig. 2b±f) is found to shift towards lower energy (ca. 1400 cm 1) with increased intensity and sharpness in ammonia environment. Both benzenoid and quinoid vibrations have been reported to appear stronger on higher doping levels of HCl [18]. Similarly, all the spectra of the exposed samples exhibit a shoulder ca. 1487 cm 1 corresponding to the ammonium ion and increase in B±NH±B intensity (ca. 1120 cm 1) with ammonia concentration. The shift in the peak corresponding to the dopant anion ca. 1145 cm 1 (Fig. 2a) towards lower wave number, i.e. ca. 1120 cm 1 with increased sharpness also implies higher doping level on exposure to ammonia [8,20]. The simultaneous appearance of these bands is indicative of interaction of AA with NH3 resulting in NH4 ion formation. Additionally, the broad band ca. 3100±3700 cm 1 (Fig. 2a) appear more intense at ca. 3100 cm 1 on exposure to NH3. The responses to ammonia at various concentration levels show a remarkable change in R0 (starting resistance of the pellet) values after 1st cycle apparently affecting DR above 11 ppm. This could be due to the fact that during 1st cycle NH3 molecules also react with N±H sites created by Cl dopant undergoing deprotonation and probably neutralising the dopant anion leading to a considerable increase in R0 after 1st cycle. The extent of this deprotonation also seems to be a function of ammonia concentration which can be expected. Hence, as seen from the responses, beyond 58 ppm (Fig. 1) DR as well as change in R0 is found to be nearly constant irrespective of ammonia concentration. Further, as seen from Table 1 the shorter response time observed with increasing concentration of ammonia upto 58 ppm can be attributed to the higher number of NH3 molecules interacting with AA resulting in the increase in the conduction sites over the polymer chain in smaller time. Contrary to this, the recovery time increases drastically, since the time required for the ammonia molecules to desorb at higher concentration would be greater than when the concentration is small. The decrease in resistance of PANI:AA on exposure to ammonia has also been re¯ected in the XRD presented in Fig. 3. The XRD of bare PANI:AA is found to exhibit partial crystallinity (Fig. 3a) as observed from the presence of several peaks at 2y values of 18.0, 19.5, 24.0, 26.5, 30.8, 36.0 and 41.08. Whereas, the degree of crystallinity is found
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Fig. 3. X-ray diffraction patterns of acrylic acid doped polyaniline before and after exposure to different ammonia concentrations: (a) acrylic acid doped polyaniline; (b) 1; (c) 2; (d) 11; (e) 34; (f) 58; (g) 460; (h) 600 ppm.
to be enhanced as observed from the increase in the peak intensities for all 2y values as seen in the XRD patterns of the samples after exposure to ammonia (Fig. 3b±h). In particular, development of intense peaks at 2y ca. 20, 26 and 30.58 can be attributed to the sub chain alignment induced by NH3 vapours as these peaks represent highly crystalline phase [21±23]. The presence of additional peaks at 2y values of 13.0, 15.0, 17.5, 20.5, 33.4, 36.0, 38.0 and 39.08 also contributes to new crystalline domains developed in presence of NH3. This supports the above discussion related to the interaction of NH3 vapours with the trapped AA in the polymeric chains. The formation of ±COO on
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Fig. 5 shows the long term response of PANI:AA for different NH3 concentrations measured upto 120 days. The responses do not exhibit signi®cant change in DR/R values indicative of long term stability of the sensing material. 4. Conclusion In this paper, we have presented an ammonia sensor using acrylic acid doped polyaniline. In contrast to the conventional PANI sensors PANI:AA shows a decrease in resistance on exposure to ammonia. The sensor is found to be chemically stable and highly sensitive to even low concentrations of ammonia (1 ppm.) at room temperature. The sensor exhibits stable responses upto 120 days suggesting long term stability of the sensing material.
Fig. 4. Variation of DR/R of acrylic acid doped polyaniline exposed to different ammonia concentrations.
exposure to ammonia would rearrange and reorient the polymeric chains resulting in improved crystallinity as observed in Fig. 3. Fig. 4 depicts the variation of DR/R with ammonia concentration, indicating linear increase in DR/R with NH3 concentration upto 58 ppm. No signi®cant change in DR/R is observed for concentrations beyond 58 ppm, which may be due to the limiting number of AA molecules available for interaction with NH3 molecules. The error estimates in sensitivity (measured for three different samples) is found to be ca. 1% which suggest good tolerance as well as reproducibility of the sensor.
Fig. 5. Long term stability of acrylic acid doped polyaniline exposed to various ammonia concentrations.
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Biographies Anjali A. Athawale Lecturer in the Department of Chemistry, University of Pune, Pune, India since 1991. She received her M.Sc. Degree in Physical Chemistry in 1982 and the Ph.D. in the area of Radiation Chemistry in 1988 from the University of Pune, Pune. She has worked as a Post Doctoral Fellow in ``Institute of Applied Radiation Chemistry'' Lodz, Poland on Pulse Radiolysis of Glassy Matrices. Over the last ten years she has been actively involved in synthesis of conducting polyaniline and polypyrrole by electrochemical and chemical method. Her current research is based on the development of the chemical sensors using conducting polymers as well as their application as electrocatalyst. She has published 35 papers in International Journals. Vasant V. Chabukswar Lecturer in Organic Chemistry at the Nowrosjee Wadia College, Pune. He received his M.Sc. Degree in Organic Chemistry in 1990 from University of Pune, India. In 1999 he joined as a Teachers' Fellow under the UGC IX plan scheme. His research interest includes the synthesis of organically conducting polymers and their charcterisation by using various analytical techniques. His current interests involved in the development of conducting polymer for organic vapour sensor.
