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Electrochemical synthesis of poly(aniline-co-fluoroaniline) films and their application as humidity sensing material
Thin Solid Films 517 (2009) 3350–3356
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electrochemical synthesis of poly(aniline-co-fluoroaniline) films and their application as humidity sensing material Amit L. Sharma ⁎ Materials Research Division (MRD), Central Scientific Instruments Organisation (CSIO), Sector-30C, Chandigarh-160030, India
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Article history: Received 12 June 2007 Received in revised form 15 November 2008 Accepted 17 November 2008 Available online 27 November 2008 Keywords: Copolymer Polyaniline Poly(2-fluoroaniline) SEM Cyclic voltammetry Humidity sensing
a b s t r a c t In the present manuscript, humidity sensing properties of a copolymer, poly(aniline-co-fluoroaniline) have been reported. The copolymer was prepared on indium-tin-oxide coated glass plates as well as platinum surface in the form of films using electrochemical technique (versus standard calomel electrode) in acidic medium. Synthesis of copolymer films was supported by Fourier transform infra-red, ultraviolet–visible, scanning electron microscope and cyclic voltammetry techniques. Molecular weight and electrical conductivity of these films were measured at different temperature. Polyaniline and poly(2-fluoroaniline) films were also synthesized using the same technique to compare the data with copolymer film. On exposure to humid atmosphere, the response behaviour of copolymer film exhibited a change in resistance with respect to relative humidity (RH). This copolymer film was found to be most sensitive in the 30–65% RH range and shows a linear behaviour with in this range. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, conducting polymers have been the subject of great interest to chemists and physicists because these materials have several advantages over metallic conductors [1–3]. Their conductivity sometimes similar to that of metals is tunable by varying the level of doping agents [4]. The preparation of a variety of conducting polymers has been intensively investigated because of their potential applications such as rechargeable batteries [5,6], electrochromic devices [7,8], microelectronics [9,10] and sensors [11–13]. Due to the poor processability of conducting polymers in their oxidized form, it is difficult to process these materials in the form of films, which is the main requirement to use these materials for device fabrication. Among the various polymer synthesis techniques, electrochemical technique is a well established method to synthesis organic conducting polymers [14–17]. In comparison to chemical route, this technique provides a clean, one step production of polymeric material directly onto the desired substrate. It provides the direct formation of conducting polymers with better control of polymer film thickness and is easy to handle for characterization purpose [18]. A wide choice of counter ions from different electrolytes can be used for the facile variation of polymer films properties. Redox behavior, diffusion coefficient estimation, electrochemical and degradation studies can be done using this technique. Redox couples can be
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characterized from the peaks obtained on the cyclic voltammogram (CV) that is formed by varying the scan rate. The desired material properties can be achieved by organizing the conducting polymers in specific arrangements, which can be useful for the development of molecule based devices. Several studies have been reported on conducting copolymers on their advantages over homopolymers for conductivity, stability and solubility [19–23]. These reports suggest that the copolymerization provides a convenient synthetic method to prepare conducting polymers with desired properties. The properties of copolymers can be modified by varying either the ratio of various constituents or the manner by which these are chemically attached. Conducting polymer based sensors are highly sensitive towards volatile organic compounds gases at room temperature [24]. Extensive studies on the gas sensing properties of these materials with various oxidizing and reducing gases have also been reported [25]. These materials also show variation in conductivity upon interaction with humidity [26–28]. Synthesis and characterization of poly(aniline-co-fluoroaniline) [poly(An-FAn)] using chemical route has already been reported [23] in earlier work. In the present work, poly(An-FAn) has been deposited on indium-tin-oxide (ITO) coated glass plates and platinum surface using cyclic voltammetry technique. These films were characterized using Fourier transform infera red (FT-IR) and ultraviolet visible (UV–visible) spectroscopy. CV of the copolymeric films has been recorded in 1 M HCl at various scan rates. Molecular weight and electrical conductivity of these films were measured at different temperatures. The results obtained for poly(An-FAn) were compared with the electrochemically
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synthesized polyaniline (PANI) and poly(2-fluoroaniline) (PFANI) films. Copolymer films were found to be sensitive with humidity and exhibits quick response in terms of variation in electrical resistance with nearly linear and repeatable resistance.
and UV–visible spectroscopy and molecular weight measurements. Whereas, surface morphology, CV and electrical conductivity measurements were carried out on copolymers films deposited on ITO glass plate.
2. Experimental details
3. Results and discussions
The grade and source of chemical used for the synthesis of polymer films are listed as below: The monomers, aniline (AR, Qualigens) and 2-fluoroaniline (Fluka) were purified by vacuum distillation before use. Methanol (AR), Nitric acid (AR), Hydrochloric acid (35%, AR), tetrahydrofuran (THF) and N-methyl 2-pyrrolidone (NMP) were procured from Merck were used as received. Calcium chloride was procured from Merck. IR grade potassium bromide (KBr) was procured from Fluka. Aqueous solutions were prepared in double distilled deionized water (N18 MΩ) using PURE Lab ELGA water purified system was used throughout the experiments. The synthesis of poly(An-FAn), PANI and PFANI films and their CV studies were carried out using a Potentiostat/Galvanostat (Model 1285 Potentiostat, Solartron). The polymerization of the monomers (aniline, 2-fluoroaniline and aniline-fluoroaniline) was carried out using an electrochemical cell with three-electrode configuration. ITO coated glass plates having dimension of 1.5 × 0.5 cm2 was used as working electrodes. Whereas, smooth platinum foils of same dimension (1.5× 0.5 cm2) were used as working electrode as well as counter electrode. Saturated calomel electrode (SCE) was used as reference electrode. Before deposition of the films, ITO glass plates were cleaned by ultrasonic method with methanol and distilled water. Platinum foils were cleaned properly using nitric acid and distilled water. The thickness of copolymer films was measured using a TALYSTEP instrument. Structural information of the copolymer and other polymer samples was obtained using FT-IR (Model 510 P, Nicolet) spectrophotometer. UV–visible spectra were recorded on a dual beam spectrophotometer (Model 160 A, Shimadzu) in the wavelength range of 200–1100 nm. Molecular weight of the polymer samples were measured using gel permeation chromatography (GPC) technique (Waters) after calibrating the system with polystyrene samples of different molecular weights as standard. Scanning electron microscopy (SEM) studies were carried out using LEO 440 at 10 KV. The electrical conductivity measurements were done out using fourpoints-probe technique. A constant current source (Keithley Model 224) was used to supply a steady current through the two outermost contact points. The voltage drop across inner two contact points were measured by a digital multimeter (Fluke Model 189). During the preparation of poly(An-FAn), PANI and PFANI films, a potential range of −0.2 to 1.0 V versus SCE was varied during CV experiments. During the copolymerization, the cell was filled with the mixture of aniline and 2-fluoroaniline in 10 ml 1 M HCl having a molar ratio of 1:1. The monomers, aniline and 2-fluoroaniline were used separately in 10 ml of 1 M HCl for the deposition of PANI and PFANI films, respectively. The area of the polymeric films deposited was kept 1.0 cm2. The thickness of the films obtained was about 68 µm. Prior to the polymerization, oxygen was removed from the solution by continuously purging it with dry nitrogen gas for about 30 min. After preparation, these polymeric films were washed several time with distilled water and then dried at 70 °C in a vacuum oven and then characterization were done. To prove that the poly(An-FAn) films, which were prepared by adopting electrochemical technique, were copolymeric films and not a mixture of homopolymers, composite of electrochemically synthesized PANI and PFANI films was prepared in 1:1 ratio. Since the films of poly(An-FAn), PANI and PFANI were not found as free-standing. So, the films were scratched separately from the platinum substrate and were powdered. Then the powders of PANI and PFANI were mixed manually in the 1:1 ratio. PANI and PFANI powders were analyzed using FT-IR
The current–voltage curve during the growth of polymeric films of poly(An-FAn), PANI and PFANI is shown in Fig. 1(a–c). The redox current was found to be increased gradually with cycles, indicating the continuous growth of polymer film on electrode with each cycle in case of poly(An-FAn). Growth rate of copolymer is found to be lower in comparison to PANI but found to be higher in comparison of growth rate of PFANI. It indicates that the growth of copolymer was influenced by attachment of fluoro group. During the polymerization, the value of charge passed was found to be increased with time in all cases, which indicates the inclusion and expulsion of the anions (counter ions) into and from polymer films. Tentative mechanism for the polymerization of aniline, 2-fluoroaniline and co-polymerization of aniline-fluoroaniline is shown in Scheme I.
Fig. 1. Current–voltage curves for the growth of (a) Poly(An-FAn), (b) PANI and (c) PFANI.
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Fig. 2. FT-IR spectra of powders of (a) Poly(An-FAn), (b) PANI, (c) PFANI and (d) 1:1 composite of the PANI and PFANI. Scheme I. Tentative mechanism for the polymerization of aniline, 2-fluoroaniline and co-polymerization of aniline-fluoroaniline.
observed in the IR spectra of PANI, PFANI and 1:1 composite of the PANI and PFANI. It can be the characteristic of the copolymer.
3.1. Molecular weight Molecular weight of poly(An-FAn) in its undoped state was carried out in THF at room temperature (22 °C ± 1 °C) using GPC analysis and found to be about 2.9 × 104. Whereas, it was found about 5.6 × 104, 3.6 × 103 and 3.0 × 104 for PANI, PFANI and 1:1 composite of the PANI and PFANI, respectively under the same conditions (Table 1). The lower value for molecular weight of poly(An-FAn) as compared to PANI reveals the less conjugation length of the copolymer due to the incorporation of fluoro moieties into the polymer backbone. 3.2. FT-IR spectra The FT-IR spectroscopic studies of all polymeric samples were conducted on pressed pellets of polymer samples mixed with KBr (1:100). Fig. 2(a–d) represents the IR spectra of poly(An-FAn), PANI, PFANI and 1:1 composite of the PANI and PFANI. The arising bands were compared in Table 2. It is clear from the table that poly(An-FAn) film exhibits two additional peaks at 1381 and 1128 cm− 1 in the IR spectra, which may arise due to the introduction of C–H and C–N stretching bands during the copolymerization. These peaks were not
Table 1 Molecular weight of poly(An-FAn), PANI, PFANI and 1:1 composite of PANI and PFANI measured at different temperatures Sample Poly(An-FAn) PANI PFANI 1:1 composite of PANI and PFANI
Molecular weight 22 °C ± 10
15 °C ± 10
5 °C±10
2.9 × 104 5.6 × 104 3.6 × 103 4.1 × 104
3.1 × 104 5.8 × 104 3.7 × 103 4.5 × 104
3.2 × 104 5.9 × 104 3.9 × 103 4.6 × 104
3.3. UV–visible spectra Optical absorption measurements of poly(An-FAn), PANI, PFANI and 1:1 composite of the PANI and PFANI in their respective undoped states have been conducted at room temperature in UV–visible region and shown in Fig. 3(a–d). The spectra were obtained after dissolving the polymer samples in NMP. Two absorption peaks at 322 and 620 nm were found in the case of poly(An-FAn) (Fig. 3a), which attributed to π-π⁎ and n-π⁎ transition, respectively [29–30]. The UV– visible spectra of PANI shows absorption peaks at 340 and 623 nm (Fig. 3b). The UV–visible spectra of PFANI (Fig. 3c) show the absorption peaks at 278 and 559 nm, whereas the absorption spectra of 1:1 composite of the PANI and PFANI shows the peaks at 305 and 618 nm (Fig. 3d). Comparing the absorption spectra of poly(An-FAn) with PANI and PFANI, a notable blue shift was observed. This may be due to the presence of electron withdrawing group in the copolymer backbone. The band gap calculations for all polymer samples were made using their respective optical absorption spectra. The plot of (αhν)−2 versus photon energy (hν) shows (not seen in manuscript) the value of energy band gap for poly(An-FAn) as 2.39 eV, which found to be
Table 2 IR vibration bands of electrochemically synthesized poly(An-FAn) films Sample
Vibration bands (cm− 1)
PANI Poly(An-FAn) PFANI 1:1 composite of PANI and PFANI
1593 1599 1574 1595
1495 1504 1506 1508
– 1381 – –
1305 1302 1317 1315
1165 1174 1186 1169
– 1128 – –
– – 1103 1103
1010 1012 1025 1026
829 825 833 830
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3.5. Cyclic voltammetry Fig. 5(a–c) shows the CV of electrochemically synthesized poly(AnFAn), PANI and PFANI films obtained in 1 M HCl at a scan rate of 10 mV/s. In the case of poly(An-FAn) (Fig. 5a), three anodic peaks at 0.26, 0.64 and 0.89 V were found. The first peak (0.26 V) is associated with the oxidation of monomer to redical cation and related to the conjugation length of the polymer. The second peak (0.89 V) arises due to the oxidation of radical cation to radical dication [32]. The CV of PANI (Fig. 5(b)) shows three peaks at 0.18, 0.48 and 0.76 V versus SCE, whereas CV of PFANI (Fig. 5(c)) shows two anodic peaks at 0.41 and 0.61 V versus SCE. On comparison, it was found that the first oxidation peak obtained for poly(An-FAn) (0.26 V) is shifted towards the higher potential as compared to PANI (0.18 V) and towards the lower potential as compared to PFANI (0.41 V). These results suggest that the conjugation length of the copolymer lies in between PANI and PFANI. The shift in oxidation peaks to higher potential may be due to the decreased conjugation length [33] that results in the lower value of electrical conductivity [34,35].
Fig. 3. UV–visible spectra of powders of (a) Poly(An-FAn), (b) PANI, (c) PFANI and (d) 1:1 composite of the PANI and PFANI .
higher than that of PANI (2.23 eV) and lower than that of PFANI (2.79 eV) [31]. 3.4. SEM studies SEM micrograph of poly(An-FAn) film shows the fibrillar morphology (Fig. 4). The non-uniformity in thickness of the fibrils can be clearly seen which may be due to the incorporation of fluoro group in to the PANI backbone. Such type of structure can be used in the fabrication of sensors especially biosensor because this type of structure provides an increased space for the immobilization of enzyme.
Fig. 4. SEM Micrograph of electrochemically synthesized poly(An-FAn) film.
Fig. 5. CV curves of (a) poly(An-FAn), (b) PANI and (c) PFANI.
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The CV of poly(An-FAn) has been recorded at various scan rates from 10 to 100 mV/s (Fig. 6), which shows a proportional variation of peak current as a function of varying scans rates. Figure shows that oxidation peaks shifted towards the higher potential with scan rate, whereas the reduction peaks shifted towards the lower potential. The presence of sharp and higher anodic peaks reveals the conducting state of the polymer film. On the other hand, the cathodic peaks are smaller and broader in comparison to anodic peaks, indicating the conversion of the polymer into an electrically insulating reduced state. The peak current versus (scan rate)1/2 shows a straight line (not shown in manuscript), which indicate that the peak current (anodic/cathodic) depends on the square root of scan rates [33]. This behavior is in accordance with the diffusion controlled redox processes. The CV of poly(An-FAn) was also carried out in different potential ranges (Fig. 7(a–c)) in 1 M HCl at a scan rate of 20 mV/s. In comparison to lower potential range (i.e. −0.2 to 0.8 V vs SCE), a new peak, arises at around 0.64 V in the case of higher potential range (i.e. from −0.2 to 1.0 V and −0.2 to 1.2 V versus SCE), which become sharper with increase in potential. It is noticed that the first oxidation peak in the case of lower potential range was shifted towards the higher side when potential range exceeds from −0.2 to 0.8 V vs SCE. This shift indicates that the peak is related to the degradation of polymer as well as the concentration of the medium [36]. In the case of PANI, this peak attributed to the presence of phenazine rings, which has been associated with the formation of cross-linked polymer chain by direct coupling of anilinium radical cation and oxidized polymer cation [36]. As the potential exceed from 0.8 V onwards, the peak current shifts towards the positive side. This peak has also been observed when the copolymer was cycled in other medium such as H2SO4, HClO4 (not seen in text) and found to be more prominent with increasing the concentration of the medium. The similar peak has also been observed in PANI and other substituted polyanilines like poly(o-toludine), poly (o-anisidine) and poly(o-ethylaniline) [37]. Lower scan rate accelerates degradation as well as loss in electroactivity. On the basis of these results it can be stated that the conjugation length decreases at high potential range and it can be correlated as one of the factor for the degradation of polymer. 3.6. Electrical conductivity The values of electrical conductivity of poly(An-FAn), PANI and PFANI films were measured at room temperature and found to be 1.32 × 10− 4, 1.12 and 2.14 × 10− 5 S/cm, respectively. The lower value of conductivity of poly(An-FAn) in comparison to PANI is due to the presence of fluoro (electron withdrawing in nature) group in the polymer chain, which suppresses the oxidation rate [38]. It may also
Fig. 7. CV curves of poly(An-FAn) at (a) 0.8, (b) 1.0 and (c) 1.2 V versus SCE.
be attributed to the shorter chain length of the copolymer during the polymerization as confirmed by CV data. The electrical conductivity of the copolymer synthesized at different temperatures (22, 15 and 5 °C) using different pH of electrolyte was also measured and presented in Table 3. Copolymer, synthesized at low temperature shows high conductivity value in comparison to copolymer synthesized at room temperature. It may be due to the high conjugation length of copolymer film during the synthesis at low temperature. GPC analysis of these films also confirms this fact as the molecular weight was found to be increased in comparison of the molecular weight of copolymer synthesized at room temperature (Table 1). 4. Response of poly(An-FAn) film with humidity Sensing characteristics of the copolymer was carried out by exposing the copolymer films to humidity in a test chamber as shown in Fig. 8. A container of 1000 ml capacity was used as chamber, which was kept in a thermocole container of 2000 ml to avoid the temperature loss. The chamber was partially filled (100 ml) with water. A mixture of ice and salt was filled between thermocole and test chamber up to the height of water. The polymer film was kept in the chamber above 5 cm from water level. For recording the temperature, two thermometers (0–60 °C range) were used, one for measuring the temperature of water bath and another for measuring the temperature of the sensing film. The change in resistance of copolymer film with humidity was measured by using two-pointsprobe technique. Before experiments, the resistance (Ri) of the copolymer film was measured by controlling the humidity by using calcium chloride at room temperature. After that water vapours were introduced inside the chamber in a controlled manner to increase the humidity level. Relative humidity (% RH) inside the chamber was noted down using a standard pre-calibrated humidity meter. It was noted that the temperature of the sample was higher than the temperature of water during the experiment with a range of 2–5 °C. The temperature of
Table 3 Electrical conductivity of the poly(An-FAn) film at different temperature and pH values
Fig. 6. CV curves of poly(An-FAn) films at different scan rates for poly(An-FAn).
S. No.
pH
Temperature (°C) ± 10
Conductivity (×10−4 S/cm)
1. 2. 3. 4. 5. 6.
1.02 2.03 3.00 4.02 1.05 1.05
22 22 22 22 15 10
1.32 1.25 1.17 1.08 1.55 1.81
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Fig. 10. Time dependence of resistance of electrochemically synthesized poly(An-FAn) film on exposure and removal with 50% RH. Fig. 8. Schematic view of experimental arrangement for humidity sensing measurements.
water inside the chamber was adjusted with the addition of ice-salt mixture so that the temperature of the chamber varies from 22 to 3 °C and accordingly % RH was also changed. The response measurements on poly(An-FAn) film were done on 10 copolymer films at different % RH ranging from 10–95%. The response of the sensing films was found to be almost linear and repeatable for all ten films. The change in film resistance as a function of % RH is shown in Fig. 9(a). The resistance of the HCl doped film decreases with increased humidity but in the range of 30–65% RH, the decrease in resistance is very prominent. This behaviour reveals that the film is very sensitive in this humidity range. Response time measurements of poly(An-FAn) films were carried out at a 50% of % RH with and without exposure of the film to humidity and is shown in Fig. 10. Almost repeatable results were obtained with each on and off cycle. It was observed that up to three cycles, the resistance of the sensor did not attain the base value and attains higher value than the original value but after that it become almost constant for next cycles. Variation of % sensitivity with response and recovery time is shown in Fig. 9(b). The sensitivity of the sensor is defined as S = [(Rf ~ Ri)/Ri], where Ri and Rf are the initial (in the absence of humidity) and final (when expose to humidity) resistance values. The sensing film was found to be most sensitive in the % RH range of 30 to 65%, which confirms the results of Fig. 9(a).
The change in resistance of copolymer films when exposed in humid conditions can be explained on the basis of proton transfer mechanism between the copolymer and the adsorbed water on the copolymer film. From the literature [39–41] it has been observed that the mobile dopant ions present in the polymer chains are responsible for this change in resistance. The mobility of these ions remains unaffected at low humidity and increases with increase in humidity. Literature shows that at a very low humidity or nearly dry conditions (b10% RH), the polymer chains tend to curl up into a compact coil. As the polymer absorbs water molecules, polymer chain swells. As a result, these compact coils turn in to the straight chains and try to align with each other. Such type of configuration provides the path for the free movement of dopant ions and the charge transfer across the polymer chains become easier thereby decreasing the resistance or increasing the conductivity of the polymer. On the basis of oxidation–reduction phenomenon, this decrease in resistance in presence of water molecules can be explained. When poly (An-FAn) films are exposed in humid conditions, copolymer absorbs the water molecules and the exchange of proton from polymer takes place. The role of water molecules is crucial in this mechanism. The water molecules gets dissociates in to H+ and OH− ions at the imine centre. The presence of these ions acts as an acidic reagent and increases the effective doping of the copolymer. The protons incorporate into the polymer chain and π-conjugation of aromatic rings, which promotes the easier electron transfer. In the absence of humidity (absence of water molecules) the reverse process takes place due to desorption of water molecules. This process acts as the de-doping of the polymer and hence responsible for the increase in resistance of copolymer. The copolymer samples were reused many times and the repeatable results were found for about 17 times. Beyond this limit the film starts to suffer from bad quality. 5. Conclusion
Fig. 9. Variation in (a) film resistance and (b) % sensitivity with % RH for poly(An-FAn) film.
From the above mentioned studies, it has been concluded that poly (An-FAn) can be synthesized by adopting electrochemical technique in the form of films. IR studies were conducted on polymeric samples. CV studies reveal that the copolymer having less conjugation length in comparison to PANI but found to be higher than that of PFANI. Cyclic voltammogram of copolymer films were recorded at various potential ranges versus SCE. The interpretation of third peak was also discussed. It can be concluded that the degradation of the polymer depends upon the potential applied and the concentration of the medium. The less electrical conductivity of poly(An-FAn) in comparison to PANI confirm the presence of fluoro moieties in to the polymer chain. On the basis of these results the formation of copolymer has also been confirmed. The influence of water molecules absorbed by the copolymer on its resistance has also been studied. The change in resistance of the
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copolymer film when exposed to humid conditions was observed. The films were found to be most sensitive in the % RH range from 30–65%. On the basis of response measured, it can be stated that this copolymer can be used as a humidity sensing material. Acknowledgements Author is grateful to Dr. Pawan Kapur, Director, CSIO for his constant encouragement. Author is also grateful to Dr. ML Singla, Head, MRD for valuable suggestions. References [1] [2] [3] [4]
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electrochemical synthesis of poly(aniline-co-fluoroaniline) films and their application as humidity sensing material Amit L. Sharma ⁎ Materials Research Division (MRD), Central Scientific Instruments Organisation (CSIO), Sector-30C, Chandigarh-160030, India
a r t i c l e
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Article history: Received 12 June 2007 Received in revised form 15 November 2008 Accepted 17 November 2008 Available online 27 November 2008 Keywords: Copolymer Polyaniline Poly(2-fluoroaniline) SEM Cyclic voltammetry Humidity sensing
a b s t r a c t In the present manuscript, humidity sensing properties of a copolymer, poly(aniline-co-fluoroaniline) have been reported. The copolymer was prepared on indium-tin-oxide coated glass plates as well as platinum surface in the form of films using electrochemical technique (versus standard calomel electrode) in acidic medium. Synthesis of copolymer films was supported by Fourier transform infra-red, ultraviolet–visible, scanning electron microscope and cyclic voltammetry techniques. Molecular weight and electrical conductivity of these films were measured at different temperature. Polyaniline and poly(2-fluoroaniline) films were also synthesized using the same technique to compare the data with copolymer film. On exposure to humid atmosphere, the response behaviour of copolymer film exhibited a change in resistance with respect to relative humidity (RH). This copolymer film was found to be most sensitive in the 30–65% RH range and shows a linear behaviour with in this range. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, conducting polymers have been the subject of great interest to chemists and physicists because these materials have several advantages over metallic conductors [1–3]. Their conductivity sometimes similar to that of metals is tunable by varying the level of doping agents [4]. The preparation of a variety of conducting polymers has been intensively investigated because of their potential applications such as rechargeable batteries [5,6], electrochromic devices [7,8], microelectronics [9,10] and sensors [11–13]. Due to the poor processability of conducting polymers in their oxidized form, it is difficult to process these materials in the form of films, which is the main requirement to use these materials for device fabrication. Among the various polymer synthesis techniques, electrochemical technique is a well established method to synthesis organic conducting polymers [14–17]. In comparison to chemical route, this technique provides a clean, one step production of polymeric material directly onto the desired substrate. It provides the direct formation of conducting polymers with better control of polymer film thickness and is easy to handle for characterization purpose [18]. A wide choice of counter ions from different electrolytes can be used for the facile variation of polymer films properties. Redox behavior, diffusion coefficient estimation, electrochemical and degradation studies can be done using this technique. Redox couples can be
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characterized from the peaks obtained on the cyclic voltammogram (CV) that is formed by varying the scan rate. The desired material properties can be achieved by organizing the conducting polymers in specific arrangements, which can be useful for the development of molecule based devices. Several studies have been reported on conducting copolymers on their advantages over homopolymers for conductivity, stability and solubility [19–23]. These reports suggest that the copolymerization provides a convenient synthetic method to prepare conducting polymers with desired properties. The properties of copolymers can be modified by varying either the ratio of various constituents or the manner by which these are chemically attached. Conducting polymer based sensors are highly sensitive towards volatile organic compounds gases at room temperature [24]. Extensive studies on the gas sensing properties of these materials with various oxidizing and reducing gases have also been reported [25]. These materials also show variation in conductivity upon interaction with humidity [26–28]. Synthesis and characterization of poly(aniline-co-fluoroaniline) [poly(An-FAn)] using chemical route has already been reported [23] in earlier work. In the present work, poly(An-FAn) has been deposited on indium-tin-oxide (ITO) coated glass plates and platinum surface using cyclic voltammetry technique. These films were characterized using Fourier transform infera red (FT-IR) and ultraviolet visible (UV–visible) spectroscopy. CV of the copolymeric films has been recorded in 1 M HCl at various scan rates. Molecular weight and electrical conductivity of these films were measured at different temperatures. The results obtained for poly(An-FAn) were compared with the electrochemically
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synthesized polyaniline (PANI) and poly(2-fluoroaniline) (PFANI) films. Copolymer films were found to be sensitive with humidity and exhibits quick response in terms of variation in electrical resistance with nearly linear and repeatable resistance.
and UV–visible spectroscopy and molecular weight measurements. Whereas, surface morphology, CV and electrical conductivity measurements were carried out on copolymers films deposited on ITO glass plate.
2. Experimental details
3. Results and discussions
The grade and source of chemical used for the synthesis of polymer films are listed as below: The monomers, aniline (AR, Qualigens) and 2-fluoroaniline (Fluka) were purified by vacuum distillation before use. Methanol (AR), Nitric acid (AR), Hydrochloric acid (35%, AR), tetrahydrofuran (THF) and N-methyl 2-pyrrolidone (NMP) were procured from Merck were used as received. Calcium chloride was procured from Merck. IR grade potassium bromide (KBr) was procured from Fluka. Aqueous solutions were prepared in double distilled deionized water (N18 MΩ) using PURE Lab ELGA water purified system was used throughout the experiments. The synthesis of poly(An-FAn), PANI and PFANI films and their CV studies were carried out using a Potentiostat/Galvanostat (Model 1285 Potentiostat, Solartron). The polymerization of the monomers (aniline, 2-fluoroaniline and aniline-fluoroaniline) was carried out using an electrochemical cell with three-electrode configuration. ITO coated glass plates having dimension of 1.5 × 0.5 cm2 was used as working electrodes. Whereas, smooth platinum foils of same dimension (1.5× 0.5 cm2) were used as working electrode as well as counter electrode. Saturated calomel electrode (SCE) was used as reference electrode. Before deposition of the films, ITO glass plates were cleaned by ultrasonic method with methanol and distilled water. Platinum foils were cleaned properly using nitric acid and distilled water. The thickness of copolymer films was measured using a TALYSTEP instrument. Structural information of the copolymer and other polymer samples was obtained using FT-IR (Model 510 P, Nicolet) spectrophotometer. UV–visible spectra were recorded on a dual beam spectrophotometer (Model 160 A, Shimadzu) in the wavelength range of 200–1100 nm. Molecular weight of the polymer samples were measured using gel permeation chromatography (GPC) technique (Waters) after calibrating the system with polystyrene samples of different molecular weights as standard. Scanning electron microscopy (SEM) studies were carried out using LEO 440 at 10 KV. The electrical conductivity measurements were done out using fourpoints-probe technique. A constant current source (Keithley Model 224) was used to supply a steady current through the two outermost contact points. The voltage drop across inner two contact points were measured by a digital multimeter (Fluke Model 189). During the preparation of poly(An-FAn), PANI and PFANI films, a potential range of −0.2 to 1.0 V versus SCE was varied during CV experiments. During the copolymerization, the cell was filled with the mixture of aniline and 2-fluoroaniline in 10 ml 1 M HCl having a molar ratio of 1:1. The monomers, aniline and 2-fluoroaniline were used separately in 10 ml of 1 M HCl for the deposition of PANI and PFANI films, respectively. The area of the polymeric films deposited was kept 1.0 cm2. The thickness of the films obtained was about 68 µm. Prior to the polymerization, oxygen was removed from the solution by continuously purging it with dry nitrogen gas for about 30 min. After preparation, these polymeric films were washed several time with distilled water and then dried at 70 °C in a vacuum oven and then characterization were done. To prove that the poly(An-FAn) films, which were prepared by adopting electrochemical technique, were copolymeric films and not a mixture of homopolymers, composite of electrochemically synthesized PANI and PFANI films was prepared in 1:1 ratio. Since the films of poly(An-FAn), PANI and PFANI were not found as free-standing. So, the films were scratched separately from the platinum substrate and were powdered. Then the powders of PANI and PFANI were mixed manually in the 1:1 ratio. PANI and PFANI powders were analyzed using FT-IR
The current–voltage curve during the growth of polymeric films of poly(An-FAn), PANI and PFANI is shown in Fig. 1(a–c). The redox current was found to be increased gradually with cycles, indicating the continuous growth of polymer film on electrode with each cycle in case of poly(An-FAn). Growth rate of copolymer is found to be lower in comparison to PANI but found to be higher in comparison of growth rate of PFANI. It indicates that the growth of copolymer was influenced by attachment of fluoro group. During the polymerization, the value of charge passed was found to be increased with time in all cases, which indicates the inclusion and expulsion of the anions (counter ions) into and from polymer films. Tentative mechanism for the polymerization of aniline, 2-fluoroaniline and co-polymerization of aniline-fluoroaniline is shown in Scheme I.
Fig. 1. Current–voltage curves for the growth of (a) Poly(An-FAn), (b) PANI and (c) PFANI.
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Fig. 2. FT-IR spectra of powders of (a) Poly(An-FAn), (b) PANI, (c) PFANI and (d) 1:1 composite of the PANI and PFANI. Scheme I. Tentative mechanism for the polymerization of aniline, 2-fluoroaniline and co-polymerization of aniline-fluoroaniline.
observed in the IR spectra of PANI, PFANI and 1:1 composite of the PANI and PFANI. It can be the characteristic of the copolymer.
3.1. Molecular weight Molecular weight of poly(An-FAn) in its undoped state was carried out in THF at room temperature (22 °C ± 1 °C) using GPC analysis and found to be about 2.9 × 104. Whereas, it was found about 5.6 × 104, 3.6 × 103 and 3.0 × 104 for PANI, PFANI and 1:1 composite of the PANI and PFANI, respectively under the same conditions (Table 1). The lower value for molecular weight of poly(An-FAn) as compared to PANI reveals the less conjugation length of the copolymer due to the incorporation of fluoro moieties into the polymer backbone. 3.2. FT-IR spectra The FT-IR spectroscopic studies of all polymeric samples were conducted on pressed pellets of polymer samples mixed with KBr (1:100). Fig. 2(a–d) represents the IR spectra of poly(An-FAn), PANI, PFANI and 1:1 composite of the PANI and PFANI. The arising bands were compared in Table 2. It is clear from the table that poly(An-FAn) film exhibits two additional peaks at 1381 and 1128 cm− 1 in the IR spectra, which may arise due to the introduction of C–H and C–N stretching bands during the copolymerization. These peaks were not
Table 1 Molecular weight of poly(An-FAn), PANI, PFANI and 1:1 composite of PANI and PFANI measured at different temperatures Sample Poly(An-FAn) PANI PFANI 1:1 composite of PANI and PFANI
Molecular weight 22 °C ± 10
15 °C ± 10
5 °C±10
2.9 × 104 5.6 × 104 3.6 × 103 4.1 × 104
3.1 × 104 5.8 × 104 3.7 × 103 4.5 × 104
3.2 × 104 5.9 × 104 3.9 × 103 4.6 × 104
3.3. UV–visible spectra Optical absorption measurements of poly(An-FAn), PANI, PFANI and 1:1 composite of the PANI and PFANI in their respective undoped states have been conducted at room temperature in UV–visible region and shown in Fig. 3(a–d). The spectra were obtained after dissolving the polymer samples in NMP. Two absorption peaks at 322 and 620 nm were found in the case of poly(An-FAn) (Fig. 3a), which attributed to π-π⁎ and n-π⁎ transition, respectively [29–30]. The UV– visible spectra of PANI shows absorption peaks at 340 and 623 nm (Fig. 3b). The UV–visible spectra of PFANI (Fig. 3c) show the absorption peaks at 278 and 559 nm, whereas the absorption spectra of 1:1 composite of the PANI and PFANI shows the peaks at 305 and 618 nm (Fig. 3d). Comparing the absorption spectra of poly(An-FAn) with PANI and PFANI, a notable blue shift was observed. This may be due to the presence of electron withdrawing group in the copolymer backbone. The band gap calculations for all polymer samples were made using their respective optical absorption spectra. The plot of (αhν)−2 versus photon energy (hν) shows (not seen in manuscript) the value of energy band gap for poly(An-FAn) as 2.39 eV, which found to be
Table 2 IR vibration bands of electrochemically synthesized poly(An-FAn) films Sample
Vibration bands (cm− 1)
PANI Poly(An-FAn) PFANI 1:1 composite of PANI and PFANI
1593 1599 1574 1595
1495 1504 1506 1508
– 1381 – –
1305 1302 1317 1315
1165 1174 1186 1169
– 1128 – –
– – 1103 1103
1010 1012 1025 1026
829 825 833 830
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3.5. Cyclic voltammetry Fig. 5(a–c) shows the CV of electrochemically synthesized poly(AnFAn), PANI and PFANI films obtained in 1 M HCl at a scan rate of 10 mV/s. In the case of poly(An-FAn) (Fig. 5a), three anodic peaks at 0.26, 0.64 and 0.89 V were found. The first peak (0.26 V) is associated with the oxidation of monomer to redical cation and related to the conjugation length of the polymer. The second peak (0.89 V) arises due to the oxidation of radical cation to radical dication [32]. The CV of PANI (Fig. 5(b)) shows three peaks at 0.18, 0.48 and 0.76 V versus SCE, whereas CV of PFANI (Fig. 5(c)) shows two anodic peaks at 0.41 and 0.61 V versus SCE. On comparison, it was found that the first oxidation peak obtained for poly(An-FAn) (0.26 V) is shifted towards the higher potential as compared to PANI (0.18 V) and towards the lower potential as compared to PFANI (0.41 V). These results suggest that the conjugation length of the copolymer lies in between PANI and PFANI. The shift in oxidation peaks to higher potential may be due to the decreased conjugation length [33] that results in the lower value of electrical conductivity [34,35].
Fig. 3. UV–visible spectra of powders of (a) Poly(An-FAn), (b) PANI, (c) PFANI and (d) 1:1 composite of the PANI and PFANI .
higher than that of PANI (2.23 eV) and lower than that of PFANI (2.79 eV) [31]. 3.4. SEM studies SEM micrograph of poly(An-FAn) film shows the fibrillar morphology (Fig. 4). The non-uniformity in thickness of the fibrils can be clearly seen which may be due to the incorporation of fluoro group in to the PANI backbone. Such type of structure can be used in the fabrication of sensors especially biosensor because this type of structure provides an increased space for the immobilization of enzyme.
Fig. 4. SEM Micrograph of electrochemically synthesized poly(An-FAn) film.
Fig. 5. CV curves of (a) poly(An-FAn), (b) PANI and (c) PFANI.
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The CV of poly(An-FAn) has been recorded at various scan rates from 10 to 100 mV/s (Fig. 6), which shows a proportional variation of peak current as a function of varying scans rates. Figure shows that oxidation peaks shifted towards the higher potential with scan rate, whereas the reduction peaks shifted towards the lower potential. The presence of sharp and higher anodic peaks reveals the conducting state of the polymer film. On the other hand, the cathodic peaks are smaller and broader in comparison to anodic peaks, indicating the conversion of the polymer into an electrically insulating reduced state. The peak current versus (scan rate)1/2 shows a straight line (not shown in manuscript), which indicate that the peak current (anodic/cathodic) depends on the square root of scan rates [33]. This behavior is in accordance with the diffusion controlled redox processes. The CV of poly(An-FAn) was also carried out in different potential ranges (Fig. 7(a–c)) in 1 M HCl at a scan rate of 20 mV/s. In comparison to lower potential range (i.e. −0.2 to 0.8 V vs SCE), a new peak, arises at around 0.64 V in the case of higher potential range (i.e. from −0.2 to 1.0 V and −0.2 to 1.2 V versus SCE), which become sharper with increase in potential. It is noticed that the first oxidation peak in the case of lower potential range was shifted towards the higher side when potential range exceeds from −0.2 to 0.8 V vs SCE. This shift indicates that the peak is related to the degradation of polymer as well as the concentration of the medium [36]. In the case of PANI, this peak attributed to the presence of phenazine rings, which has been associated with the formation of cross-linked polymer chain by direct coupling of anilinium radical cation and oxidized polymer cation [36]. As the potential exceed from 0.8 V onwards, the peak current shifts towards the positive side. This peak has also been observed when the copolymer was cycled in other medium such as H2SO4, HClO4 (not seen in text) and found to be more prominent with increasing the concentration of the medium. The similar peak has also been observed in PANI and other substituted polyanilines like poly(o-toludine), poly (o-anisidine) and poly(o-ethylaniline) [37]. Lower scan rate accelerates degradation as well as loss in electroactivity. On the basis of these results it can be stated that the conjugation length decreases at high potential range and it can be correlated as one of the factor for the degradation of polymer. 3.6. Electrical conductivity The values of electrical conductivity of poly(An-FAn), PANI and PFANI films were measured at room temperature and found to be 1.32 × 10− 4, 1.12 and 2.14 × 10− 5 S/cm, respectively. The lower value of conductivity of poly(An-FAn) in comparison to PANI is due to the presence of fluoro (electron withdrawing in nature) group in the polymer chain, which suppresses the oxidation rate [38]. It may also
Fig. 7. CV curves of poly(An-FAn) at (a) 0.8, (b) 1.0 and (c) 1.2 V versus SCE.
be attributed to the shorter chain length of the copolymer during the polymerization as confirmed by CV data. The electrical conductivity of the copolymer synthesized at different temperatures (22, 15 and 5 °C) using different pH of electrolyte was also measured and presented in Table 3. Copolymer, synthesized at low temperature shows high conductivity value in comparison to copolymer synthesized at room temperature. It may be due to the high conjugation length of copolymer film during the synthesis at low temperature. GPC analysis of these films also confirms this fact as the molecular weight was found to be increased in comparison of the molecular weight of copolymer synthesized at room temperature (Table 1). 4. Response of poly(An-FAn) film with humidity Sensing characteristics of the copolymer was carried out by exposing the copolymer films to humidity in a test chamber as shown in Fig. 8. A container of 1000 ml capacity was used as chamber, which was kept in a thermocole container of 2000 ml to avoid the temperature loss. The chamber was partially filled (100 ml) with water. A mixture of ice and salt was filled between thermocole and test chamber up to the height of water. The polymer film was kept in the chamber above 5 cm from water level. For recording the temperature, two thermometers (0–60 °C range) were used, one for measuring the temperature of water bath and another for measuring the temperature of the sensing film. The change in resistance of copolymer film with humidity was measured by using two-pointsprobe technique. Before experiments, the resistance (Ri) of the copolymer film was measured by controlling the humidity by using calcium chloride at room temperature. After that water vapours were introduced inside the chamber in a controlled manner to increase the humidity level. Relative humidity (% RH) inside the chamber was noted down using a standard pre-calibrated humidity meter. It was noted that the temperature of the sample was higher than the temperature of water during the experiment with a range of 2–5 °C. The temperature of
Table 3 Electrical conductivity of the poly(An-FAn) film at different temperature and pH values
Fig. 6. CV curves of poly(An-FAn) films at different scan rates for poly(An-FAn).
S. No.
pH
Temperature (°C) ± 10
Conductivity (×10−4 S/cm)
1. 2. 3. 4. 5. 6.
1.02 2.03 3.00 4.02 1.05 1.05
22 22 22 22 15 10
1.32 1.25 1.17 1.08 1.55 1.81
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Fig. 10. Time dependence of resistance of electrochemically synthesized poly(An-FAn) film on exposure and removal with 50% RH. Fig. 8. Schematic view of experimental arrangement for humidity sensing measurements.
water inside the chamber was adjusted with the addition of ice-salt mixture so that the temperature of the chamber varies from 22 to 3 °C and accordingly % RH was also changed. The response measurements on poly(An-FAn) film were done on 10 copolymer films at different % RH ranging from 10–95%. The response of the sensing films was found to be almost linear and repeatable for all ten films. The change in film resistance as a function of % RH is shown in Fig. 9(a). The resistance of the HCl doped film decreases with increased humidity but in the range of 30–65% RH, the decrease in resistance is very prominent. This behaviour reveals that the film is very sensitive in this humidity range. Response time measurements of poly(An-FAn) films were carried out at a 50% of % RH with and without exposure of the film to humidity and is shown in Fig. 10. Almost repeatable results were obtained with each on and off cycle. It was observed that up to three cycles, the resistance of the sensor did not attain the base value and attains higher value than the original value but after that it become almost constant for next cycles. Variation of % sensitivity with response and recovery time is shown in Fig. 9(b). The sensitivity of the sensor is defined as S = [(Rf ~ Ri)/Ri], where Ri and Rf are the initial (in the absence of humidity) and final (when expose to humidity) resistance values. The sensing film was found to be most sensitive in the % RH range of 30 to 65%, which confirms the results of Fig. 9(a).
The change in resistance of copolymer films when exposed in humid conditions can be explained on the basis of proton transfer mechanism between the copolymer and the adsorbed water on the copolymer film. From the literature [39–41] it has been observed that the mobile dopant ions present in the polymer chains are responsible for this change in resistance. The mobility of these ions remains unaffected at low humidity and increases with increase in humidity. Literature shows that at a very low humidity or nearly dry conditions (b10% RH), the polymer chains tend to curl up into a compact coil. As the polymer absorbs water molecules, polymer chain swells. As a result, these compact coils turn in to the straight chains and try to align with each other. Such type of configuration provides the path for the free movement of dopant ions and the charge transfer across the polymer chains become easier thereby decreasing the resistance or increasing the conductivity of the polymer. On the basis of oxidation–reduction phenomenon, this decrease in resistance in presence of water molecules can be explained. When poly (An-FAn) films are exposed in humid conditions, copolymer absorbs the water molecules and the exchange of proton from polymer takes place. The role of water molecules is crucial in this mechanism. The water molecules gets dissociates in to H+ and OH− ions at the imine centre. The presence of these ions acts as an acidic reagent and increases the effective doping of the copolymer. The protons incorporate into the polymer chain and π-conjugation of aromatic rings, which promotes the easier electron transfer. In the absence of humidity (absence of water molecules) the reverse process takes place due to desorption of water molecules. This process acts as the de-doping of the polymer and hence responsible for the increase in resistance of copolymer. The copolymer samples were reused many times and the repeatable results were found for about 17 times. Beyond this limit the film starts to suffer from bad quality. 5. Conclusion
Fig. 9. Variation in (a) film resistance and (b) % sensitivity with % RH for poly(An-FAn) film.
From the above mentioned studies, it has been concluded that poly (An-FAn) can be synthesized by adopting electrochemical technique in the form of films. IR studies were conducted on polymeric samples. CV studies reveal that the copolymer having less conjugation length in comparison to PANI but found to be higher than that of PFANI. Cyclic voltammogram of copolymer films were recorded at various potential ranges versus SCE. The interpretation of third peak was also discussed. It can be concluded that the degradation of the polymer depends upon the potential applied and the concentration of the medium. The less electrical conductivity of poly(An-FAn) in comparison to PANI confirm the presence of fluoro moieties in to the polymer chain. On the basis of these results the formation of copolymer has also been confirmed. The influence of water molecules absorbed by the copolymer on its resistance has also been studied. The change in resistance of the
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copolymer film when exposed to humid conditions was observed. The films were found to be most sensitive in the % RH range from 30–65%. On the basis of response measured, it can be stated that this copolymer can be used as a humidity sensing material. Acknowledgements Author is grateful to Dr. Pawan Kapur, Director, CSIO for his constant encouragement. Author is also grateful to Dr. ML Singla, Head, MRD for valuable suggestions. References [1] [2] [3] [4]
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