- Email: [email protected]
Morphology dependent ammonia sensing with 5-sulfosalicylic acid doped nanostructured polyaniline synthesized by several routes
Sensors and Actuators B 181 (2013) 544–550
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Morphology dependent ammonia sensing with 5-sulfosalicylic acid doped nanostructured polyaniline synthesized by several routes Krishanu Chatterjee a , Palash Dhara b , Saibal Ganguly c , Kajari Kargupta b , Dipali Banerjee a,∗ a b c
Department of Physics, Bengal Engineering & Science University, Shibpur, Howrah 711 103, West Bengal, India Department of Chemical Engineering, Jadavpur University, Kolkata 700 032, India Chemical Engineering Department, Universiti Teknologi Petronas, Malaysia
a r t i c l e
i n f o
Article history: Received 24 May 2012 Received in revised form 24 January 2013 Accepted 11 February 2013 Available online 17 February 2013 Keywords: Polyaniline 5-Sulfosalicylic acid Nanostructure Ammonia sensing Response percentage Surface to volume ratio
a b s t r a c t Polyaniline (PANI) doped with 5-sulfosalicylic acid (SSA) has been synthesized employing several techniques, viz. polymerization, Langmuir Blodgett (LB), spin-coating and electrodeposition. The influence of process variation on the structure, electrical conductivity and ammonia vapour sensing performance (response percentage and response time) has been investigated. The synthesized samples have been structurally characterized by transmission electron microscopy (TEM), UV–vis, and FTIR spectra. TEM analysis reveals formation of nanorods and nanospheres of PANI. Nanorods are formed for the samples synthesized by electrodeposition and LB technique whereas for the samples synthesized by polymerization and spin-coating, nanospheres are formed. Among these four different synthesized samples, the one deposited by LB technique exhibits highest conductivity due to the ordered molecular architecture. An exploration of variation of conductivity of these samples after exposure to ammonia reveals that the polymerized sample having highest surface to volume ratio exhibits fastest response (least response time), while the spin coated sample show sluggish response. The importance lies in the mapping between response percentages of different samples with the surface to volume ratio of the various nanostructures. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The last several decades have witnessed the progress in the synthesis of electrically conducting or conjugated polymers that are extremely interesting new class of materials. Since the discovery in early seventies, these polymers have made a significant impact and provide a vast field for a number of growing new technologies such as energy storage devices [1,2], electromagnetic interface shielding (EMI) [3,4], micro-electronics devices [5], displays [6], sensors and actuators [7], etc. The importance of environmental protection is well understood which led to focus in the development of suitable gas sensitive conducting polymer based material [8]. Most of the conducting polymers exhibit highly reversible redox behaviour with a distinguishable chemical memory and thus considered to be the potential candidates for the fabrication of sensors. The advantages of conducting polymers over inorganic components used are their diversity, intrinsic conductivity, fast response, easy synthesis, stability in air and particularly sensitivity at room temperature [9,10].
∗ Corresponding author. Tel.: +91 9830299253; fax: +91 3326684561. E-mail addresses: [email protected], banerjee [email protected] (D. Banerjee). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.02.042
Though polypyrrole has been one of the first polymers used as gas sensors, yet a low sensitivity, high response and an incomplete de-absorption of gas molecules is observed in sensor response [9]. Thus research workers switched over to polyaniline (PANI). PANI gained its importance as a potential candidate to be used in gas sensors due to its mechanical flexibility, high environmental stability, ease of processing, simple and reversible doping/dedoping chemistry and modifiable electrical conductivity [11–13]. It is observed that PANI has been used as a sensing material for different vapours like HCl, hydrogen, ammonia as well as humidity [14–18]. Studies show that PANI has three main oxidation states, which can be observed from the colour changes ranging from transparent leucoemaraldine and yellow/green emaraldine to blue/black pernigraniline. The doping/dedoping chemistry is attributed to the presence of NH-N group in the polymer backbone whose protonation/deprotonation bring a change in electronic conductivity as well as in the colour. For this aspect, out of the different methods used for measuring sensitivity of gases, electrical detection is mostly used one, based on the change in resistance of the sensor on exposure to gases. Recently nano conducting polymers based sensors are thought to create a major impact on the environmental monitoring. Based on the investigation of sensing properties of conducting polymer, PANI specially in its nanoform has been proposed as one
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
of the promising material to be used as sensors [19–27]. Micro and nanostructural polyanilines show a great promise as good sensing materials because of their high surface to volume ratio favouring the diffusion of gas molecules [28,29]. Microstructure of PANI–camphosulfonic acid (CSA) thin films [19] deposited by spincoating technique show the importance of hexagonal structure for a fast response in NH3 . Special attention has been developed in nanostructures of PANI which have been extensively used as gas sensors in the detection of different gases such as NO2 , NH3 , N2 H4 , CO, and H2 [20–26]. Virji et al. [20,22,23] have observed the performance of nanostructures of PANI towards ammonia, HCl, hydrazine and hydrogen sulphide. Synthesized nanorods of PANI–CSA and nanoparticles of PANI–paratoluene sulfonic acid (PTSA) [27] by LB technique has been studied for NH3 sensing, where PANI–CSA exhibits higher response than PANI–PTSA. Different group of workers have investigated gas sensing property of PANI. But reports of the formation of nanostructure of several morphology employing various procedures and their effect on the gas sensing property are scanty. In the present work an attempt has been made to correlate the surface to volume ratio of the nanostructure with the response percentage of the material for ammonia vapour sensing. PANI doped with SSA has been synthesized by polymerization, Langmuir Blodgett technique, spin coating and electro deposition. The polymerized material has been used to prepare samples by other techniques. The structural and electrical characterizations of the samples are undertaken. Nanostructures of several sizes and shapes with diverse values of electrical conductivities that are obtained have been used to investigate the response behaviour of the samples towards ammonia vapour. A correlation between surface to volume ratio and response percentage of various nanostructures has been drawn.
545
made in pellet form (sample 1) by applying a pressure of 9 tones using cold press which was used for the detection of ammonia. 2.3. Langmuir Blodgett deposition of PANI Solution of PANI was prepared by dissolving a small amount of PANI in methanol. The solution was kept under soft sonication (using piezo-U-Sonic, Concepts International) for about 4 h. The ultrasonicated solution is filtered. The concentration of the final solution was found to be 0.42 mg/ml. 10 ml of solution was spread over a LB trough (Model LB 2007, Apex Instrument Co. Ltd., India) at air/water interface and a time of 1 h was allowed for solvent evaporation. Water was used as sub-phase with a low pH of 2. It was proposed that low pH gives the desired level of doping [31] and enhanced monolayer stability [32]. The transfer pressure was maintained at 19 mN/m. For recording the surface pressure against area/molecule isotherm whatman filter paper was used. The dipper lifting and the barrier compression speeds are kept at 1 mm/min and 10 mm/min, respectively. ITO coated glass, washed with water and methanol, was used as substrate. Z type monolayer of PANI was obtained (sample 2) on ITO coated glass with a transfer ratio (the decrease in monolayer area to the area of coated substrate) close to 1 during upstroke. Ten such monolayers were obtained to develop multilayer of PANI. 2.4. Spin coating deposition of PANI
2. Experimental
20 ml solution of PANI emeraldine salt was prepared by dissolving the synthesized PANI in dimethyl formamide in the ratio 3 mg/ml. The solution was sonicated for 4 h and filtered. The filtered solution was poured a little on ITO coated glass and rotated using Apex spin coating unit model no. SCU 2007, Apex Instruments Co. with a speed of 1800 rpm for 8 s (sample 3). The sample prepared was dried in vacuum at 60 ◦ C for 1 h.
2.1. Materials used
2.5. Electro deposition of PANI
Aniline (99%), ammonium peroxydisulfate (APS), 5sulfosalicylic acid (SSA), dimethyl formamide (DMF), ethanol and ammonia (25%) were obtained from Merck specialties and filter paper was purchased from Whatman. Water was purchased from hydro lab. All the chemicals were used as received.
60 ml solution of PANI emeraldine salt was prepared by dissolving the synthesized sample in dimethyl formamide in the ratio 2 mg of synthesized PANI in 1 ml of DMF. The solution is sonicated for 4 h. This solution was transferred to a standard electrochemical cell of volume 80 ml for deposition of PANI on cathode. Graphite was used as anode and indium tin oxide (ITO) coated glass, washed with water as well as methanol, was used as cathode. A voltage of 20 volts was applied across the electrodes. The deposited PANI on ITO (sample 4) was oven dried in vacuum for 2 h at 60 ◦ C.
2.2. PANI synthesis Chemical oxidative polymerization process has been employed for preparing nanostructured PANI doped with SSA. PANI is synthesized using template-free oxidative polymerization of aniline in aqueous solution of 5-sulfosalicylic acid, using ammonium peroxydisulfate as an oxidant [30]. 2.0 ml of aniline was dissolved in 190 ml of aqueous solution containing 1.4 g of SSA. The solution was stirred with magnetic stirrer and heated to boiling and cooled to room temperature. An aqueous solution of APS used as oxidant was prepared by dissolving 5.02 g of APS in 100 ml of water. To the aqueous solution of the aniline monomer, the APS solution was mixed drop wise to start the oxidation, and the reaction mixture was stirred for 6 h. Throughout the reaction time the temperature of the reaction mixture was kept between 0 and 5 ◦ C. A dark green precipitate was formed indicating PANI emaraldine salt (doped with SSA) which was recovered from the reaction vessel by filtration. The precipitate was washed with water for several times to remove any of the oxidant present till the filtered water becomes colourless. It was rinsed with 5 × 10−3 M SSA to compensate the loss of dopant and again washed with water. Finally the prepared sample was dried in vacuum oven at 60 ◦ C for 24 h. A part of the synthesized sample was
2.6. Characterization The Fourier transform infrared (FTIR) spectrum of the polymerized sample was recorded on a Shimadzu Spectrophotometer within the wave number range of 400–4000 cm−1 . The sample was prepared in the form of a pellet by mixing the material with potassium bromide (KBr). A UV–vis spectrum of the polymerized sample was recorded on a Perkin Elmer spectrophotometer in a wavelength range 300–1000 nm. To figure out the structural morphology of the nanoparticles of PANI deposited by different methods, transmission electron microscopic study was executed. The conductivity at room temperature was measured using an electrometer (Keithley model no. 6517 A) and microvoltmeter (Keithley 177 Microvolt DMM). 2.7. Gas-sensitivity characterization For sensing properties a simple set up was used. The sample was fixed to a holder and placed in the mouth of a bottle containing 25% ammonia solution and covered up as shown in Fig. 1 [27,28].
546
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
Fig. 2. FTIR spectra of SSA doped PANI before and after exposure to aqueous ammonia.
Fig. 1. Experimental setup for performance study of different nanostructures of SSA doped PANI exposed to aqueous ammonia.
The ammonia vapour (96.3% ammonia vapour and 3.7% water vapour in equilibrium with the ammonia solution) comes in contact with the sample as they are released from the solution. Moisture absorber, fused CaCl2 , is placed inside the hood while performing the experiments to take care of the influence of the humidity. The dc electrical resistances of the samples exposed to the vapour were determined using a four probe configuration. The electrical contacts on the sample surfaces were made by copper wires and silver paste. The resistances of the samples were measured as a function of time with electrometer and micro voltmeter. Thus from the dimension the conductivity was found out as a function of time.
bending and/or out-of-plane bending of SSA ring, 804 cm−1 and 883 cm−1 due to C H vibration of SSA ring and ∼1029 cm−1 due to symmetric stretching of SO3 group confirm that PANI–SSA samples are doped with 5-sulfosalicylate anions [35–37]. There is a shoulder at ∼1077 cm−1 in the spectra of sample (unexposed to ammonia) which corresponds to SO3 symmetric stretching of HSO4 − anions [36]. Further the characteristic band observed at 1672–1678 cm−1 due to C O stretching of COOH is a signature of the presence of SSA as dopant in PANI. When exposed to ammonia there is a decrease in peak intensity at 667 cm−1 corresponding to in-plane bending and/or out-of-plane bending of SSA ring. Also there is a shift in the peak towards left at 595 cm−1 (out-of-plane bending of SSA ring), at 1029 cm−1 (symmetric stretching of SO3 group) and at 1077 (SO3 symmetric stretching of HSO4 − anions) cm−1 . The shift towards higher wave number [38] along with the decrease in the intensity
3. Results and discussions 3.1. Spectral studies 3.1.1. FTIR analysis Fig. 2 shows the FTIR spectrum in the range 550–3500 cm−1 of the SSA doped polymerized sample before and after exposure to observe any structural modification introduced by ammonia. The characteristic peaks of PANI emaraldine salt are observed at 823 cm−1 associated with the aromatic C H bending out of the plane for 1,4 di-substituted benzene ring [33], 1151 cm−1 due to B NH+ Q stretching, 1247 cm−1 owing to C N+ stretching, 1303 cm−1 due to C N stretching of secondary aromatic amine, at around ∼1484 cm−1 for benzenoid (B) ring stretching, 1566 and 1585 cm−1 for C C quiononoid (Q) ring stretching in emaraldine salt and base respectively [26]. After exposure to ammonia vapour there is a decrease in the peak intensities assigned to quinonoid and benzenoid ring stretching at 1566 cm−1 and 1484 cm−1 respectively. This observation confirms the process of dedoping [34] when exposed to ammonia. The band observed at 2900–3430 cm−1 is attributed to the N H stretching vibration of secondary amine in the backbone of PANI [35]. The distinctive bands observed at 595 cm−1 attributed to outof-plane bending of SSA ring, 667 cm−1 corresponding to in-plane
Fig. 3. UV–vis spectra of SSA doped PANI before and after exposure to aqueous ammonia.
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
547
Table 1 Morphologies and different parameters of SSA doped PANI synthesized by various methods. Process of synthesis
Sample name
Structure
Conductivity (S cm−1 )
Response (%)
Response time (s)
Thickness (mm)
Surface to volume ratio (nm−1 )
Polymerization Langmuir Blodgett
Sample 1 Sample 2
1.13 200.65
99.62 59.44
50 225
4.78 4.3 × 10−3
0.600 0.177
Spin coating Electro deposition
Sample 3 Sample 4
Nanosphere of 10 nm diameter Short nanorods average diameter and length of 25 nm and 115 nm Nanosphere of 50 nm diameter Long nanorods average diameter and length of 50 nm and 350 nm
183.77 34.83
14.92 11.08
325 175
7.88 × 10−2 3.72 × 10−2
0.120 0.086
of the peak [34] suggest dedoping which lowers the value of electrical conductivity when exposed to ammonia vapour as has been observed in the present case. 3.1.2. UV–vis analysis Fig. 3 shows the UV–vis spectra of the samples unexposed and exposed to ammonia vapour. For the sample before exposure to ammonia two bands appear at around 634 nm and 917 nm. The shoulder at ∼465 nm and peak at 917 nm are due to the localized polarons [39] which are the signature of doped conducting PANI. The bands centred at 634 nm correspond to – transition of benzonoid and quinonoid rings respectively [40–42]. After exposure to ammonia vapour the peaks are found at 628 nm and 916 nm. The
shoulder at ∼465 nm is missing. Comparing the peak positions in the two cases (for unexposed and exposed samples) it is observed there is reduction in the intensity of the peak at 916 nm (characteristics of the doped PANI) [43] for exposed sample. Further there is increase in the intensity of the peak at 628 nm (characteristics of the undoped PANI) for the exposed sample. Both these observations confirm the process of deprotonation. The absence of shoulder in the ammonia exposed sample also points to the process of dedoping. Measurement of electrical conductivity () shows that there is decrease in the value of for the exposed samples. All these observations namely deprotonation, dedoping and lower value of electrical conductivity for exposed samples conformity with each other.
Fig. 4. TEM images of nanostructure of SSA doped PANI synthesized by (a) polymerization, (b) Langmuir Blodgett (LB) technique, (c) spincoating and (d) electrodeposition.
548
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
3.2. Structural characterization of PANI Fig. 4 shows the transmission electron microscopic (TEM) images of PANI which reveals that the morphology of the PANI (emeraldine salt) nanostructure depends on the synthesis procedures, viz. polymerization, Langmuir Blodgett technique, spin-coating and electrodeposition. The particle size and the morphology of these samples are presented in Table 1. PANI doped with SSA was synthesized by polymerization (sample 1). The prepared sample was then used to deposit films by other three techniques. Nanospheres of PANI are obtained for sample 1 as shown in Fig. 4a prepared by polymerization and sample 3 as shown in Fig. 4c deposited by spin-coating. The spheres of different dimensions with an average of 10 nm in diameter for sample 1 (polymerization) and an average of 50 nm diameter for sample 3 (spin-coating) are produced as observed from the TEM images. Nanorods are seen for sample 2 (Langmuir Blodgett technique) and sample 4 (electrodeposition) as shown in Fig. 4b and d respectively. Sample 4 are of larger dimension, with an average diameter and length of 50 nm and 350 nm respectively, than those of sample 2 with an average diameter and length of 25 nm and 115 nm respectively. Thus it is revealed that different procedures yield various types of nanostructures of PANI. In LB technique by Z type molecular film deposition, an ampiphilic molecular layer is transferred on the substrate [44]. The nanorods are supposed to be formed due to the cross linking between the molecules of PANI emaraldine salt [27]. The lateral attractive force owing to cross linking in a cross-linked structure supports the formation of connected nanorods. The rods like structures are separated from each other as a result of the electrostatic repulsion between cationic sites. In electrodeposition, a similar type of mechanism due to electrostatic force may be responsible for the formation of nanorods. The sample prepared by polymerization yielded nanosphere like structure. The polymerized sample (10 nm) dispersed by sonication in the solvent (DMF) was then used to deposit the film by spin coating. While spinning the solvent is evaporated resulting in agglomeration of the nanosphere that grow in size (50 nm) and become dispersed. 3.3. Room temperature conductivities The room temperature electrical conductivities of different samples (viz. Samples 1, 2, 3 and 4) were measured as presented in Table 1. Maximum conductivity was obtained for sample 2 (200.65 S cm−1 ) deposited by LB technique. This is probably due to molecular architecture achieved by this technique as a consequence of deposition of ordered hydrophilic layers of the material. The minimum conductivity is obtained for sample 1 (1.1305 S cm−1 ) prepared by polymerization. The sample prepared by polymerization was in powder form. Pelletalization of the powder for measuring conductivity introduced random orientations of the nanocrystals, leading to diminish the electrical conductivity.
Fig. 5. Variation of conductivity with exposure time to aqueous ammonia for the four samples – (a) polymerization and electrodeposition, (b) Langmuir Blodgett technique and spincoating.
to the deprotonation of PANI emeraldine salt and formation of emeraldine base with time. The sensing mechanism of nanostructures of PANI in the presence of ammonia vapor can be represented schematically as shown in Fig. 7. The mechanism involves a combined process of diffusion and chemisorptions of ammonia vapour into the PANI emaraldine salt through deprotonation of PANI salt.
3.4. Dynamic response of nanostructures of PANI in the presence of ammonia vapor The variation of electrical conductivity with time for the four samples when exposed to ammonia is as shown in Fig. 5. The conductivity of the sample was found to decrease when exposed to ammonia vapor released from a 25% ammonia solution (96.3% ammonia vapour and 3.7% water vapour in equilibrium with the ammonia solution) kept in the bottle inside the experimental chamber. Variation in the conductivity was observed as a function of time. The response value is defined as ( o − f )/ o , where o is the initial conductivity and f is the final conductivity. Fig. 6 shows the variation of response with time. It is seen that the response increases with time and attains saturation. This is due
Fig. 6. Percentage response with time of SSA doped PANI synthesized by different methods.
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
549
Fig. 7. Sensing mechanism of aqueous ammonia.
If A− is the anionic group of the dopant, the conversion of PANI-A− to PANI emeraldine base results in the sensing mechanism when exposed to ammonia vapour. The lone pair electron of ammonia enters into a coordination bonding with the dopant proton resulting in deprotonation of nitrogen atoms of PANI. As a result of deprotonation of PANI nitrogen atoms the charge carriers namely polaron disappears and hence the electrical conductivity decreases [27]. In the reverse cycle, in presence of air or inert gases, ammonia desorbs and diffuses back from PANI base and it is reversibly converted to PANI salt. Response percentage and response time (time corresponding to maximum response) of the samples are presented in Table 1. It is seen that response is maximum for sample 1 (polymerized – 50 s). This value is comparable for PANI doped with CSA (52 s) [27], low for PANI doped with acrylic acid (60 s) [16], PTSA (60 s) [27], and much lower for PANI doped perflourooctanoic acid (2 min) [27] and composite (∼5 min) [45]. Looking at several morphologies, surface to volume ratio of all the four samples were calculated and plotted against the response percentage as shown in Fig. 8. The same trend has been observed as seen in Fig. 6 (response vs time). Due to increase in the surface to volume ratio of the nanostructure the diffusional resistance of the sensing material towards ammonia vapor decreases and the overall reaction rate becomes faster which enhances the magnitude of percentage response [(( o − f )/ o ) × 100] of the sample. In the present case surface to volume ratio is the maximum for sample 1 (polymerized)
Fig. 8. Variation of percentage response with surface to volume ratio of various nanostructures of SSA doped PANI.
and minimum for sample 4 (electrodeposition) which validates the experimental observation. Further in case of chemisorptions overall response time depends on the rate of reaction as well as time scale of diffusion of the species through the material. In bulk material diffusional resistance increases with thickness of the material and hence the time of response also increases. On the contrary in this work for the case of nanostructured material surface to volume ratio plays a crucial role in determining the response time rather than the thickness of the material. Thicknesses of the samples differ depending on method of synthesis (Table 1). Comparison of the response times with the thickness of films and with the surface to volume ratio (Table 1) reveals that the pellet with maximum thickness and the highest surface to volume ratio shows the least response time. 4. Conclusion PANI doped with SSA are synthesized by varying the process parameter, viz. polymerization, Langmuir Blodgett technique, spin coating and electrodeposition. For structural characterization, FTIR, UV–vis and TEM have been performed. It may be concluded that: i. Polymerization and spin coating technique yielded nanospheres of diameters 10 nm and 50 nm respectively whereas nanorods were formed by LB technique and electrodeposition. On an average the electrodeposited rods are 350 nm in length and 50 nm in diameter and nanorods deposited by LB technique are 115 nm in length and 25 nm in diameter. Thus different synthesis processes produce various morphologies of nanostructured PANI. ii. The FTIR and UV–vis studies indicate the process of deprotonation/dedoping when the samples were exposed to ammonia. This is further confirmed from the results of electrical conductivity which decreases with time when exposed to ammonia. The value of electrical conductivity of the sample 2 is found to be maximum (200.65 S cm−1 ) which may be due to the ordered molecular architecture. iii. From the variation of response with time it is noticed that the response increases for all the samples and attains a saturation value. The surface to volume ratio calculated from the dimensions come out to be maximum for sample 1 and minimum for sample 4. A correlation is mapped between the surface to volume ratio and response percentage which indicates that an increase in surface to volume ratio enhances the diffusion of ammonia thereby increasing the response percentage. From the literature review it is seen that the value of response time obtained is satisfactory as compared to the other reports. The significance of this work lies in sensing ammonia vapor with
550
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
nanostructured PANI due to their lower diffusional resistance and higher surface to volume ratio. Also contrary to the bulk material effect of surface to volume ratio of the nanostructured material masks the effect of thickness to determine the time of response of the sensing material when exposed to ammonia vapour. Acknowledgements The authors acknowledge the Department of Science and Technology, India, for financial support to carry out the research project. One of the authors, Krishanu Chatterjee, acknowledges Council of Scientific and Industrial Research, India for financial assistance. References [1] B.J. Feldman, P. Burgmayer, R.W. Marray, The potential dependence of electrical conductivity and chemical charge storage of polypyrrole films on electrodes, Journal of the American Chemical Society 107 (1985) 872. [2] N. Li, Y. Lee, L.H. Ong, A polyaniline and nafion composite film as a rechargeable battery, Journal of Applied Electrochemistry 22 (1992) 512–516. [3] S.K. Dhawan, D.C. Trivedi, EMI shielding with organic conducting polymers, Journal of Electromagnetic Compatibility 4 (1991) 1–4. [4] F. Colaneri, L.W. Shacklett, EMI shielding measurement of conducting polymer blends, IEEE Transactions on Instrumentation and Measurement 2 (1992) 291–297. [5] E.W. Paul, A.J. Riccio, M.S. Wrighton, Resistance of polyaniline films as a function of electrochemical potential and the fabrication of polyanilinebased microelectronic devices, Journal of Physical Chemistry 89 (1985) 1441–1447. [6] F. Huang, H.L. Wang, M. Fieldstone, A.G. MacDiarmid, B.R. Hsieh, A.J. Epstein, Application of thin film of conjugated polymers in electrochemical and conventional light emitting devices and related photovoltaic devices, Synthetic Metals 85 (1997) 1283–1284. [7] J. Ago, J.M. Samisen, H.L. Wang, Chemical vapor driven polyaniline sensor/actuators, Synthetic Metals 135 (2003) 809–810. [8] N.E. Abort, M.C. Petty, A.P. Monk man, Polyaniline thin films for gas sensing, Sensors and Actuators B: Chemical 28 (1995) 173–179. [9] D.N. Debar not, F.P. Pillared, Polyaniline as a new sensitive layer for gas sensors, Analytica Chimica Acta 475 (2003) 1–15. [10] J. Janet, M. Mosaics, Conducting polymers in electronic chemical sensors, Nature Materials 2 (2003) 19–24. [11] Y. Coo, P. Smith, A.J. Hedger, Counter ion induced processability of conducting polyaniline, Synthetic Metals 57 (1993) 3514–3519. [12] J.C. Chang, A.G. MacDiarmid, Polyaniline–protonic acid doping of the emaraldine from to the metallic regime, Synthetic Metals 13 (1986) 193–205. [13] W.S. Huang, B.D. Humphrey, A.G. MacDiarmid, Polyaniline, a novel conducting polymer-morphology and chemistry of its oxidation and reduction in aqueouselectrolytes, Journal of the Chemical Society, Faraday Transactions 82 (1986) 2385 (Part 8). [14] S. Koul, S.K. Dhawan, S. Chandra, R. Chandra, Conducting PANI as a sensor material for the detection of HCl vapours, Indian Journal of Chemistry 36A (1997) 901–904. [15] C. Conn, S. Sestak, A.T. Baker, J. Unsworth, A polyaniline-based selective hydrogen sensor, Electroanalysis 10 (1998) 1137. [16] V.V. Chabuskar, S. Pethkar, A.A. Athawale, Acrylic acid doped polyaniline as an ammonia sensor, Sensors and Actuators B 77 (2001) 657–663. [17] A.L. Kukla, Y.M. Shirshov, S.A. Piletsky, Ammonia sensors based on sensitive polyaniline films, Sensors and Actuators B 37 (1996) 135–140. [18] G.E. Collins, L.J. Buckley, Conductive polymer-coated fabrics for chemical sensing, Synthetic Metals 78 (1996) 93–101. [19] D. Verma, V. Dutta, Role of novel microstructure of polyaniline-CSA thin film in ammonia sensing at room temperature, Sensors and Actuators B 134 (2008) 373–376. [20] S. Virji, J.X. Huang, R.B. Kaner, B.H. Weiller, Polyaniline nanofibers gas sensors: examination of response mechanism, Nano Letters 4 (2004) 491–496. [21] M. Joubert, M. Bouhadid, D. Begue, P. Iratcabal, N. Redon, J. Desbrieres, S. Reynaud, Conducting polyaniline composite: from syntheses in waterborne systems to chemical sensor devices, Polymer 51 (2010) 1716. [22] S. Virji, R.B. Kaner, B.H. Weiller, Hydrazine detection of polyaniline using fluorinated alcohol derivatives, Chemistry of Materials 17 (2005) 1256–1260. [23] S. Virji, J.D. Fowler, C.O. Baker, J.X. Huang, R.B. Kaner, B.H. Weiller, Polyaniline nanofibers composites with metal salts: chemical sensors for hydrogen sulphide, Small 1 (2005) 624–627. [24] V. Dixit, S.C.K. Misra, B.S. Sharma, Carbon monoxide sensitivity of vacuum deposited polyaniline semiconducting thin films, Sensors and Actuators B 104 (2005) 90–93. [25] R. Arsat, X.F. Yu, Y.X. Li, W. Wlodarski, K.K. Zadeh, Hydrogen gas sensor based on highly ordered polyaniline nanofibers, Sensors and Actuators B 137 (2009) 529–532.
[26] X.B. Yan, Z.J. Han, Y. Yang, B.K. Tay, NO2 gas sensing with polyaniline nanofibers synthesized by a facile aqueous/organic interfacial polymerization, Sensors and Actuators B 123 (2007) 107–113. [27] S. Manigandan, A. Jain, S. Majumder, S. Ganguly, K. Kargupta, Formation of nannorods and nanoparticles of polyaniline using Langmuir Blodgett technique: performance study for ammonia sensor, Sensors and Actuators B 133 (2008) 187–194. [28] S. Manigandan, S. Majumdar, A. Suresh, S. Ganguly, K. Kargupta, D. Banerjee, Electric field induced dewetting and pattern formation in thin conducting polymer film, Sensors and Actuators B 144 (2010) 170–175. [29] J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, Nanostructured polyaniline sensors, Chemistry: A European Journal 10 (2004) 1314. ´ c-Marjanovi ´ G. Ciri ´ ´ B. Marjanovic, ´ P. Holler, M. Trchová, J. Stejskal, c, [30] A. Janoˇsevic, Synthesis and characterization of conducting polyaniline 5-sulfosalicylate nanotubes, Nanotechnology 19 (2008) 1356906. [31] P. Granholm, J. Paloheimo, H. Stubb, Conducting Langmuir–Blodgett films of polyaniline: fabrication and charge transport properties, Synthetic Metals 84 (1997) 783–784. [32] V.I. Troitsky, T.S. Verzina, M.P. Fontana, Langmuir–Blodgett assemblies with patterned conductive polyaniline layers, Materials Science and Engineering C 22 (2002) 239–244. [33] I. Sapurina, A.Y. Osadchev, B.Z. Volchek, M. Trchová, A. Riede, J. Stejskal, In-situ polymerized polyaniline films: 5. Brush-like chain ordering, Synthetic Metals 129 (2002) 29–37. [34] S. Jain, A.B. Samui, M. Patri, V.R. Hande, S.V. Bhoraskar, FEP/polyaniline based multilayered chlorine sensor, Sensors and Actuators B 106 (2005) 609–613. [35] G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables and Charts of Work, Wiley, Chichester, NY, 2001, pp. 94–99, 107, 221. [36] D.C. Trivedi, S.K. Dhawan, Investigations on the effect of 5-sulfosalicylic acid on the properties of polyaniline, Synthetic Metals 58 (1993) 309–324. [37] H.T. Varghese, C.Y. Panicker, D. Philip, I.R. Raman, SERS spectra of 5sulphosalicylic acid dihydrate, Journal of Raman Spectroscopy 38 (2007) 309–315. [38] A. Choudhury, Polyaniline/silver nanocomposites: dielectric properties and ethanol vapour sensitivity, Sensors and Actuators B 138 (2009) 318–325. [39] F. Yakuphanoglu, B.F. S¸enkal, Electronic and thermoelectric properties of polyaniline organic semiconductor and electrical characterization of Al/PANI MIS diode, Journal of Physical Chemistry C 111 (2007) 1840–1846. [40] X. Zhang, J. Zhang, Z. Liu, Tubular composite of doped polyaniline with multiwalled carbon nanotubes, Applied Physics A 80 (2005) 1813–1817. [41] E.E. Havinga, M.M. Bouman, E.W. Meijer, A. Pomp, M.M.J. Simenon, Large induced optical activity in the conduction band of polyaniline doped with (1S)-(+)-10-camphorsulfonic acid, Synthetic Metals 66 (1994) 93–97. [42] W. Feng, F. Zhou, X. Wang, X. Bai, K. Yoshino, Fabrication of composite films by controlling molecular doping processes between polyaniline and soluble multiwalled nanotubes and their optical characteristics, Japanese Journal of Applied Physics 42 (2003) 5726–5730. [43] F. Yakuphanoglu, B.F. S¸enkal, A. Sarac¸, Electrical conductivity, thermoelectric power, and optical properties of organo-soluble polyaniline organic semiconductor, Journal of Electronic Materials 37 (2008) 930–934. [44] L.H. Huo, L.X. Coo, D.M. Wang, H.N. Cui, G.F. Zeng, S.Q. Xi, Preparation and characterization of doped polyaniline films, Thin Solid Films 350 (1999) 5–9. [45] S. Koul, R. Chandra, S.K. Dhawan, Conducting polyaniline composite: a reusable sensor material for ammonia vapour, Sensors and Actuators B 75 (2001) 151–159.
Biographies Krishanu Chatterjee received his M.Sc. degree from University of Calcutta. He is pursuing Ph.D. in the department of Physics, Bengal Engineering and Science University, Shibpur. His interests are in synthesis and characterization of inorganic and organic (conducting polymer) nanostructure materials and its composites for various device applications. Palash Dhara received his B.E. from West Bengal University of Technology. He is pursuing M.Tech at Jadavpur University, Kolkata. His research interests are in development of nanostructure materials for sensor and thermoelectric applications. Dr. Saibal Ganguly received his Ph.D. from Indian Institute of Technology, Kanpur, in 1993. He is currently working as Professor in the Chemical Engineering Department, Universiti Teknologi Petronas, Malaysia. His field of interest includes process engineering, real time control and automation, energy engineering, fuel cell, property up-gradation and modification of industrial polymer and coal, modelling and simulation. Kajari Kargupta received her Ph.D. from Indian Institute of Technology, Kanpur, in 2001. She is presently working as a Professor in Chemical Engineering Department, Jadavpur University, Kolkata. Her area of research includes nano-science, conducting polymers, gel polymer, controlled drug delivery, polymer composite, modelling and simulation. Dipali Banerjee received her Ph.D. form Jadavpur University, Kolkata in the year 1990. She is now working as a Professor of Physics, Bengal Engineering and Science University, Shibpur. Her area of research includes nanostructured materials, conducting polymer, composite materials, thermoelectric materials and applications.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Morphology dependent ammonia sensing with 5-sulfosalicylic acid doped nanostructured polyaniline synthesized by several routes Krishanu Chatterjee a , Palash Dhara b , Saibal Ganguly c , Kajari Kargupta b , Dipali Banerjee a,∗ a b c
Department of Physics, Bengal Engineering & Science University, Shibpur, Howrah 711 103, West Bengal, India Department of Chemical Engineering, Jadavpur University, Kolkata 700 032, India Chemical Engineering Department, Universiti Teknologi Petronas, Malaysia
a r t i c l e
i n f o
Article history: Received 24 May 2012 Received in revised form 24 January 2013 Accepted 11 February 2013 Available online 17 February 2013 Keywords: Polyaniline 5-Sulfosalicylic acid Nanostructure Ammonia sensing Response percentage Surface to volume ratio
a b s t r a c t Polyaniline (PANI) doped with 5-sulfosalicylic acid (SSA) has been synthesized employing several techniques, viz. polymerization, Langmuir Blodgett (LB), spin-coating and electrodeposition. The influence of process variation on the structure, electrical conductivity and ammonia vapour sensing performance (response percentage and response time) has been investigated. The synthesized samples have been structurally characterized by transmission electron microscopy (TEM), UV–vis, and FTIR spectra. TEM analysis reveals formation of nanorods and nanospheres of PANI. Nanorods are formed for the samples synthesized by electrodeposition and LB technique whereas for the samples synthesized by polymerization and spin-coating, nanospheres are formed. Among these four different synthesized samples, the one deposited by LB technique exhibits highest conductivity due to the ordered molecular architecture. An exploration of variation of conductivity of these samples after exposure to ammonia reveals that the polymerized sample having highest surface to volume ratio exhibits fastest response (least response time), while the spin coated sample show sluggish response. The importance lies in the mapping between response percentages of different samples with the surface to volume ratio of the various nanostructures. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The last several decades have witnessed the progress in the synthesis of electrically conducting or conjugated polymers that are extremely interesting new class of materials. Since the discovery in early seventies, these polymers have made a significant impact and provide a vast field for a number of growing new technologies such as energy storage devices [1,2], electromagnetic interface shielding (EMI) [3,4], micro-electronics devices [5], displays [6], sensors and actuators [7], etc. The importance of environmental protection is well understood which led to focus in the development of suitable gas sensitive conducting polymer based material [8]. Most of the conducting polymers exhibit highly reversible redox behaviour with a distinguishable chemical memory and thus considered to be the potential candidates for the fabrication of sensors. The advantages of conducting polymers over inorganic components used are their diversity, intrinsic conductivity, fast response, easy synthesis, stability in air and particularly sensitivity at room temperature [9,10].
∗ Corresponding author. Tel.: +91 9830299253; fax: +91 3326684561. E-mail addresses: [email protected], banerjee [email protected] (D. Banerjee). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.02.042
Though polypyrrole has been one of the first polymers used as gas sensors, yet a low sensitivity, high response and an incomplete de-absorption of gas molecules is observed in sensor response [9]. Thus research workers switched over to polyaniline (PANI). PANI gained its importance as a potential candidate to be used in gas sensors due to its mechanical flexibility, high environmental stability, ease of processing, simple and reversible doping/dedoping chemistry and modifiable electrical conductivity [11–13]. It is observed that PANI has been used as a sensing material for different vapours like HCl, hydrogen, ammonia as well as humidity [14–18]. Studies show that PANI has three main oxidation states, which can be observed from the colour changes ranging from transparent leucoemaraldine and yellow/green emaraldine to blue/black pernigraniline. The doping/dedoping chemistry is attributed to the presence of NH-N group in the polymer backbone whose protonation/deprotonation bring a change in electronic conductivity as well as in the colour. For this aspect, out of the different methods used for measuring sensitivity of gases, electrical detection is mostly used one, based on the change in resistance of the sensor on exposure to gases. Recently nano conducting polymers based sensors are thought to create a major impact on the environmental monitoring. Based on the investigation of sensing properties of conducting polymer, PANI specially in its nanoform has been proposed as one
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
of the promising material to be used as sensors [19–27]. Micro and nanostructural polyanilines show a great promise as good sensing materials because of their high surface to volume ratio favouring the diffusion of gas molecules [28,29]. Microstructure of PANI–camphosulfonic acid (CSA) thin films [19] deposited by spincoating technique show the importance of hexagonal structure for a fast response in NH3 . Special attention has been developed in nanostructures of PANI which have been extensively used as gas sensors in the detection of different gases such as NO2 , NH3 , N2 H4 , CO, and H2 [20–26]. Virji et al. [20,22,23] have observed the performance of nanostructures of PANI towards ammonia, HCl, hydrazine and hydrogen sulphide. Synthesized nanorods of PANI–CSA and nanoparticles of PANI–paratoluene sulfonic acid (PTSA) [27] by LB technique has been studied for NH3 sensing, where PANI–CSA exhibits higher response than PANI–PTSA. Different group of workers have investigated gas sensing property of PANI. But reports of the formation of nanostructure of several morphology employing various procedures and their effect on the gas sensing property are scanty. In the present work an attempt has been made to correlate the surface to volume ratio of the nanostructure with the response percentage of the material for ammonia vapour sensing. PANI doped with SSA has been synthesized by polymerization, Langmuir Blodgett technique, spin coating and electro deposition. The polymerized material has been used to prepare samples by other techniques. The structural and electrical characterizations of the samples are undertaken. Nanostructures of several sizes and shapes with diverse values of electrical conductivities that are obtained have been used to investigate the response behaviour of the samples towards ammonia vapour. A correlation between surface to volume ratio and response percentage of various nanostructures has been drawn.
545
made in pellet form (sample 1) by applying a pressure of 9 tones using cold press which was used for the detection of ammonia. 2.3. Langmuir Blodgett deposition of PANI Solution of PANI was prepared by dissolving a small amount of PANI in methanol. The solution was kept under soft sonication (using piezo-U-Sonic, Concepts International) for about 4 h. The ultrasonicated solution is filtered. The concentration of the final solution was found to be 0.42 mg/ml. 10 ml of solution was spread over a LB trough (Model LB 2007, Apex Instrument Co. Ltd., India) at air/water interface and a time of 1 h was allowed for solvent evaporation. Water was used as sub-phase with a low pH of 2. It was proposed that low pH gives the desired level of doping [31] and enhanced monolayer stability [32]. The transfer pressure was maintained at 19 mN/m. For recording the surface pressure against area/molecule isotherm whatman filter paper was used. The dipper lifting and the barrier compression speeds are kept at 1 mm/min and 10 mm/min, respectively. ITO coated glass, washed with water and methanol, was used as substrate. Z type monolayer of PANI was obtained (sample 2) on ITO coated glass with a transfer ratio (the decrease in monolayer area to the area of coated substrate) close to 1 during upstroke. Ten such monolayers were obtained to develop multilayer of PANI. 2.4. Spin coating deposition of PANI
2. Experimental
20 ml solution of PANI emeraldine salt was prepared by dissolving the synthesized PANI in dimethyl formamide in the ratio 3 mg/ml. The solution was sonicated for 4 h and filtered. The filtered solution was poured a little on ITO coated glass and rotated using Apex spin coating unit model no. SCU 2007, Apex Instruments Co. with a speed of 1800 rpm for 8 s (sample 3). The sample prepared was dried in vacuum at 60 ◦ C for 1 h.
2.1. Materials used
2.5. Electro deposition of PANI
Aniline (99%), ammonium peroxydisulfate (APS), 5sulfosalicylic acid (SSA), dimethyl formamide (DMF), ethanol and ammonia (25%) were obtained from Merck specialties and filter paper was purchased from Whatman. Water was purchased from hydro lab. All the chemicals were used as received.
60 ml solution of PANI emeraldine salt was prepared by dissolving the synthesized sample in dimethyl formamide in the ratio 2 mg of synthesized PANI in 1 ml of DMF. The solution is sonicated for 4 h. This solution was transferred to a standard electrochemical cell of volume 80 ml for deposition of PANI on cathode. Graphite was used as anode and indium tin oxide (ITO) coated glass, washed with water as well as methanol, was used as cathode. A voltage of 20 volts was applied across the electrodes. The deposited PANI on ITO (sample 4) was oven dried in vacuum for 2 h at 60 ◦ C.
2.2. PANI synthesis Chemical oxidative polymerization process has been employed for preparing nanostructured PANI doped with SSA. PANI is synthesized using template-free oxidative polymerization of aniline in aqueous solution of 5-sulfosalicylic acid, using ammonium peroxydisulfate as an oxidant [30]. 2.0 ml of aniline was dissolved in 190 ml of aqueous solution containing 1.4 g of SSA. The solution was stirred with magnetic stirrer and heated to boiling and cooled to room temperature. An aqueous solution of APS used as oxidant was prepared by dissolving 5.02 g of APS in 100 ml of water. To the aqueous solution of the aniline monomer, the APS solution was mixed drop wise to start the oxidation, and the reaction mixture was stirred for 6 h. Throughout the reaction time the temperature of the reaction mixture was kept between 0 and 5 ◦ C. A dark green precipitate was formed indicating PANI emaraldine salt (doped with SSA) which was recovered from the reaction vessel by filtration. The precipitate was washed with water for several times to remove any of the oxidant present till the filtered water becomes colourless. It was rinsed with 5 × 10−3 M SSA to compensate the loss of dopant and again washed with water. Finally the prepared sample was dried in vacuum oven at 60 ◦ C for 24 h. A part of the synthesized sample was
2.6. Characterization The Fourier transform infrared (FTIR) spectrum of the polymerized sample was recorded on a Shimadzu Spectrophotometer within the wave number range of 400–4000 cm−1 . The sample was prepared in the form of a pellet by mixing the material with potassium bromide (KBr). A UV–vis spectrum of the polymerized sample was recorded on a Perkin Elmer spectrophotometer in a wavelength range 300–1000 nm. To figure out the structural morphology of the nanoparticles of PANI deposited by different methods, transmission electron microscopic study was executed. The conductivity at room temperature was measured using an electrometer (Keithley model no. 6517 A) and microvoltmeter (Keithley 177 Microvolt DMM). 2.7. Gas-sensitivity characterization For sensing properties a simple set up was used. The sample was fixed to a holder and placed in the mouth of a bottle containing 25% ammonia solution and covered up as shown in Fig. 1 [27,28].
546
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
Fig. 2. FTIR spectra of SSA doped PANI before and after exposure to aqueous ammonia.
Fig. 1. Experimental setup for performance study of different nanostructures of SSA doped PANI exposed to aqueous ammonia.
The ammonia vapour (96.3% ammonia vapour and 3.7% water vapour in equilibrium with the ammonia solution) comes in contact with the sample as they are released from the solution. Moisture absorber, fused CaCl2 , is placed inside the hood while performing the experiments to take care of the influence of the humidity. The dc electrical resistances of the samples exposed to the vapour were determined using a four probe configuration. The electrical contacts on the sample surfaces were made by copper wires and silver paste. The resistances of the samples were measured as a function of time with electrometer and micro voltmeter. Thus from the dimension the conductivity was found out as a function of time.
bending and/or out-of-plane bending of SSA ring, 804 cm−1 and 883 cm−1 due to C H vibration of SSA ring and ∼1029 cm−1 due to symmetric stretching of SO3 group confirm that PANI–SSA samples are doped with 5-sulfosalicylate anions [35–37]. There is a shoulder at ∼1077 cm−1 in the spectra of sample (unexposed to ammonia) which corresponds to SO3 symmetric stretching of HSO4 − anions [36]. Further the characteristic band observed at 1672–1678 cm−1 due to C O stretching of COOH is a signature of the presence of SSA as dopant in PANI. When exposed to ammonia there is a decrease in peak intensity at 667 cm−1 corresponding to in-plane bending and/or out-of-plane bending of SSA ring. Also there is a shift in the peak towards left at 595 cm−1 (out-of-plane bending of SSA ring), at 1029 cm−1 (symmetric stretching of SO3 group) and at 1077 (SO3 symmetric stretching of HSO4 − anions) cm−1 . The shift towards higher wave number [38] along with the decrease in the intensity
3. Results and discussions 3.1. Spectral studies 3.1.1. FTIR analysis Fig. 2 shows the FTIR spectrum in the range 550–3500 cm−1 of the SSA doped polymerized sample before and after exposure to observe any structural modification introduced by ammonia. The characteristic peaks of PANI emaraldine salt are observed at 823 cm−1 associated with the aromatic C H bending out of the plane for 1,4 di-substituted benzene ring [33], 1151 cm−1 due to B NH+ Q stretching, 1247 cm−1 owing to C N+ stretching, 1303 cm−1 due to C N stretching of secondary aromatic amine, at around ∼1484 cm−1 for benzenoid (B) ring stretching, 1566 and 1585 cm−1 for C C quiononoid (Q) ring stretching in emaraldine salt and base respectively [26]. After exposure to ammonia vapour there is a decrease in the peak intensities assigned to quinonoid and benzenoid ring stretching at 1566 cm−1 and 1484 cm−1 respectively. This observation confirms the process of dedoping [34] when exposed to ammonia. The band observed at 2900–3430 cm−1 is attributed to the N H stretching vibration of secondary amine in the backbone of PANI [35]. The distinctive bands observed at 595 cm−1 attributed to outof-plane bending of SSA ring, 667 cm−1 corresponding to in-plane
Fig. 3. UV–vis spectra of SSA doped PANI before and after exposure to aqueous ammonia.
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
547
Table 1 Morphologies and different parameters of SSA doped PANI synthesized by various methods. Process of synthesis
Sample name
Structure
Conductivity (S cm−1 )
Response (%)
Response time (s)
Thickness (mm)
Surface to volume ratio (nm−1 )
Polymerization Langmuir Blodgett
Sample 1 Sample 2
1.13 200.65
99.62 59.44
50 225
4.78 4.3 × 10−3
0.600 0.177
Spin coating Electro deposition
Sample 3 Sample 4
Nanosphere of 10 nm diameter Short nanorods average diameter and length of 25 nm and 115 nm Nanosphere of 50 nm diameter Long nanorods average diameter and length of 50 nm and 350 nm
183.77 34.83
14.92 11.08
325 175
7.88 × 10−2 3.72 × 10−2
0.120 0.086
of the peak [34] suggest dedoping which lowers the value of electrical conductivity when exposed to ammonia vapour as has been observed in the present case. 3.1.2. UV–vis analysis Fig. 3 shows the UV–vis spectra of the samples unexposed and exposed to ammonia vapour. For the sample before exposure to ammonia two bands appear at around 634 nm and 917 nm. The shoulder at ∼465 nm and peak at 917 nm are due to the localized polarons [39] which are the signature of doped conducting PANI. The bands centred at 634 nm correspond to – transition of benzonoid and quinonoid rings respectively [40–42]. After exposure to ammonia vapour the peaks are found at 628 nm and 916 nm. The
shoulder at ∼465 nm is missing. Comparing the peak positions in the two cases (for unexposed and exposed samples) it is observed there is reduction in the intensity of the peak at 916 nm (characteristics of the doped PANI) [43] for exposed sample. Further there is increase in the intensity of the peak at 628 nm (characteristics of the undoped PANI) for the exposed sample. Both these observations confirm the process of deprotonation. The absence of shoulder in the ammonia exposed sample also points to the process of dedoping. Measurement of electrical conductivity () shows that there is decrease in the value of for the exposed samples. All these observations namely deprotonation, dedoping and lower value of electrical conductivity for exposed samples conformity with each other.
Fig. 4. TEM images of nanostructure of SSA doped PANI synthesized by (a) polymerization, (b) Langmuir Blodgett (LB) technique, (c) spincoating and (d) electrodeposition.
548
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
3.2. Structural characterization of PANI Fig. 4 shows the transmission electron microscopic (TEM) images of PANI which reveals that the morphology of the PANI (emeraldine salt) nanostructure depends on the synthesis procedures, viz. polymerization, Langmuir Blodgett technique, spin-coating and electrodeposition. The particle size and the morphology of these samples are presented in Table 1. PANI doped with SSA was synthesized by polymerization (sample 1). The prepared sample was then used to deposit films by other three techniques. Nanospheres of PANI are obtained for sample 1 as shown in Fig. 4a prepared by polymerization and sample 3 as shown in Fig. 4c deposited by spin-coating. The spheres of different dimensions with an average of 10 nm in diameter for sample 1 (polymerization) and an average of 50 nm diameter for sample 3 (spin-coating) are produced as observed from the TEM images. Nanorods are seen for sample 2 (Langmuir Blodgett technique) and sample 4 (electrodeposition) as shown in Fig. 4b and d respectively. Sample 4 are of larger dimension, with an average diameter and length of 50 nm and 350 nm respectively, than those of sample 2 with an average diameter and length of 25 nm and 115 nm respectively. Thus it is revealed that different procedures yield various types of nanostructures of PANI. In LB technique by Z type molecular film deposition, an ampiphilic molecular layer is transferred on the substrate [44]. The nanorods are supposed to be formed due to the cross linking between the molecules of PANI emaraldine salt [27]. The lateral attractive force owing to cross linking in a cross-linked structure supports the formation of connected nanorods. The rods like structures are separated from each other as a result of the electrostatic repulsion between cationic sites. In electrodeposition, a similar type of mechanism due to electrostatic force may be responsible for the formation of nanorods. The sample prepared by polymerization yielded nanosphere like structure. The polymerized sample (10 nm) dispersed by sonication in the solvent (DMF) was then used to deposit the film by spin coating. While spinning the solvent is evaporated resulting in agglomeration of the nanosphere that grow in size (50 nm) and become dispersed. 3.3. Room temperature conductivities The room temperature electrical conductivities of different samples (viz. Samples 1, 2, 3 and 4) were measured as presented in Table 1. Maximum conductivity was obtained for sample 2 (200.65 S cm−1 ) deposited by LB technique. This is probably due to molecular architecture achieved by this technique as a consequence of deposition of ordered hydrophilic layers of the material. The minimum conductivity is obtained for sample 1 (1.1305 S cm−1 ) prepared by polymerization. The sample prepared by polymerization was in powder form. Pelletalization of the powder for measuring conductivity introduced random orientations of the nanocrystals, leading to diminish the electrical conductivity.
Fig. 5. Variation of conductivity with exposure time to aqueous ammonia for the four samples – (a) polymerization and electrodeposition, (b) Langmuir Blodgett technique and spincoating.
to the deprotonation of PANI emeraldine salt and formation of emeraldine base with time. The sensing mechanism of nanostructures of PANI in the presence of ammonia vapor can be represented schematically as shown in Fig. 7. The mechanism involves a combined process of diffusion and chemisorptions of ammonia vapour into the PANI emaraldine salt through deprotonation of PANI salt.
3.4. Dynamic response of nanostructures of PANI in the presence of ammonia vapor The variation of electrical conductivity with time for the four samples when exposed to ammonia is as shown in Fig. 5. The conductivity of the sample was found to decrease when exposed to ammonia vapor released from a 25% ammonia solution (96.3% ammonia vapour and 3.7% water vapour in equilibrium with the ammonia solution) kept in the bottle inside the experimental chamber. Variation in the conductivity was observed as a function of time. The response value is defined as ( o − f )/ o , where o is the initial conductivity and f is the final conductivity. Fig. 6 shows the variation of response with time. It is seen that the response increases with time and attains saturation. This is due
Fig. 6. Percentage response with time of SSA doped PANI synthesized by different methods.
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
549
Fig. 7. Sensing mechanism of aqueous ammonia.
If A− is the anionic group of the dopant, the conversion of PANI-A− to PANI emeraldine base results in the sensing mechanism when exposed to ammonia vapour. The lone pair electron of ammonia enters into a coordination bonding with the dopant proton resulting in deprotonation of nitrogen atoms of PANI. As a result of deprotonation of PANI nitrogen atoms the charge carriers namely polaron disappears and hence the electrical conductivity decreases [27]. In the reverse cycle, in presence of air or inert gases, ammonia desorbs and diffuses back from PANI base and it is reversibly converted to PANI salt. Response percentage and response time (time corresponding to maximum response) of the samples are presented in Table 1. It is seen that response is maximum for sample 1 (polymerized – 50 s). This value is comparable for PANI doped with CSA (52 s) [27], low for PANI doped with acrylic acid (60 s) [16], PTSA (60 s) [27], and much lower for PANI doped perflourooctanoic acid (2 min) [27] and composite (∼5 min) [45]. Looking at several morphologies, surface to volume ratio of all the four samples were calculated and plotted against the response percentage as shown in Fig. 8. The same trend has been observed as seen in Fig. 6 (response vs time). Due to increase in the surface to volume ratio of the nanostructure the diffusional resistance of the sensing material towards ammonia vapor decreases and the overall reaction rate becomes faster which enhances the magnitude of percentage response [(( o − f )/ o ) × 100] of the sample. In the present case surface to volume ratio is the maximum for sample 1 (polymerized)
Fig. 8. Variation of percentage response with surface to volume ratio of various nanostructures of SSA doped PANI.
and minimum for sample 4 (electrodeposition) which validates the experimental observation. Further in case of chemisorptions overall response time depends on the rate of reaction as well as time scale of diffusion of the species through the material. In bulk material diffusional resistance increases with thickness of the material and hence the time of response also increases. On the contrary in this work for the case of nanostructured material surface to volume ratio plays a crucial role in determining the response time rather than the thickness of the material. Thicknesses of the samples differ depending on method of synthesis (Table 1). Comparison of the response times with the thickness of films and with the surface to volume ratio (Table 1) reveals that the pellet with maximum thickness and the highest surface to volume ratio shows the least response time. 4. Conclusion PANI doped with SSA are synthesized by varying the process parameter, viz. polymerization, Langmuir Blodgett technique, spin coating and electrodeposition. For structural characterization, FTIR, UV–vis and TEM have been performed. It may be concluded that: i. Polymerization and spin coating technique yielded nanospheres of diameters 10 nm and 50 nm respectively whereas nanorods were formed by LB technique and electrodeposition. On an average the electrodeposited rods are 350 nm in length and 50 nm in diameter and nanorods deposited by LB technique are 115 nm in length and 25 nm in diameter. Thus different synthesis processes produce various morphologies of nanostructured PANI. ii. The FTIR and UV–vis studies indicate the process of deprotonation/dedoping when the samples were exposed to ammonia. This is further confirmed from the results of electrical conductivity which decreases with time when exposed to ammonia. The value of electrical conductivity of the sample 2 is found to be maximum (200.65 S cm−1 ) which may be due to the ordered molecular architecture. iii. From the variation of response with time it is noticed that the response increases for all the samples and attains a saturation value. The surface to volume ratio calculated from the dimensions come out to be maximum for sample 1 and minimum for sample 4. A correlation is mapped between the surface to volume ratio and response percentage which indicates that an increase in surface to volume ratio enhances the diffusion of ammonia thereby increasing the response percentage. From the literature review it is seen that the value of response time obtained is satisfactory as compared to the other reports. The significance of this work lies in sensing ammonia vapor with
550
K. Chatterjee et al. / Sensors and Actuators B 181 (2013) 544–550
nanostructured PANI due to their lower diffusional resistance and higher surface to volume ratio. Also contrary to the bulk material effect of surface to volume ratio of the nanostructured material masks the effect of thickness to determine the time of response of the sensing material when exposed to ammonia vapour. Acknowledgements The authors acknowledge the Department of Science and Technology, India, for financial support to carry out the research project. One of the authors, Krishanu Chatterjee, acknowledges Council of Scientific and Industrial Research, India for financial assistance. References [1] B.J. Feldman, P. Burgmayer, R.W. Marray, The potential dependence of electrical conductivity and chemical charge storage of polypyrrole films on electrodes, Journal of the American Chemical Society 107 (1985) 872. [2] N. Li, Y. Lee, L.H. Ong, A polyaniline and nafion composite film as a rechargeable battery, Journal of Applied Electrochemistry 22 (1992) 512–516. [3] S.K. Dhawan, D.C. Trivedi, EMI shielding with organic conducting polymers, Journal of Electromagnetic Compatibility 4 (1991) 1–4. [4] F. Colaneri, L.W. Shacklett, EMI shielding measurement of conducting polymer blends, IEEE Transactions on Instrumentation and Measurement 2 (1992) 291–297. [5] E.W. Paul, A.J. Riccio, M.S. Wrighton, Resistance of polyaniline films as a function of electrochemical potential and the fabrication of polyanilinebased microelectronic devices, Journal of Physical Chemistry 89 (1985) 1441–1447. [6] F. Huang, H.L. Wang, M. Fieldstone, A.G. MacDiarmid, B.R. Hsieh, A.J. Epstein, Application of thin film of conjugated polymers in electrochemical and conventional light emitting devices and related photovoltaic devices, Synthetic Metals 85 (1997) 1283–1284. [7] J. Ago, J.M. Samisen, H.L. Wang, Chemical vapor driven polyaniline sensor/actuators, Synthetic Metals 135 (2003) 809–810. [8] N.E. Abort, M.C. Petty, A.P. Monk man, Polyaniline thin films for gas sensing, Sensors and Actuators B: Chemical 28 (1995) 173–179. [9] D.N. Debar not, F.P. Pillared, Polyaniline as a new sensitive layer for gas sensors, Analytica Chimica Acta 475 (2003) 1–15. [10] J. Janet, M. Mosaics, Conducting polymers in electronic chemical sensors, Nature Materials 2 (2003) 19–24. [11] Y. Coo, P. Smith, A.J. Hedger, Counter ion induced processability of conducting polyaniline, Synthetic Metals 57 (1993) 3514–3519. [12] J.C. Chang, A.G. MacDiarmid, Polyaniline–protonic acid doping of the emaraldine from to the metallic regime, Synthetic Metals 13 (1986) 193–205. [13] W.S. Huang, B.D. Humphrey, A.G. MacDiarmid, Polyaniline, a novel conducting polymer-morphology and chemistry of its oxidation and reduction in aqueouselectrolytes, Journal of the Chemical Society, Faraday Transactions 82 (1986) 2385 (Part 8). [14] S. Koul, S.K. Dhawan, S. Chandra, R. Chandra, Conducting PANI as a sensor material for the detection of HCl vapours, Indian Journal of Chemistry 36A (1997) 901–904. [15] C. Conn, S. Sestak, A.T. Baker, J. Unsworth, A polyaniline-based selective hydrogen sensor, Electroanalysis 10 (1998) 1137. [16] V.V. Chabuskar, S. Pethkar, A.A. Athawale, Acrylic acid doped polyaniline as an ammonia sensor, Sensors and Actuators B 77 (2001) 657–663. [17] A.L. Kukla, Y.M. Shirshov, S.A. Piletsky, Ammonia sensors based on sensitive polyaniline films, Sensors and Actuators B 37 (1996) 135–140. [18] G.E. Collins, L.J. Buckley, Conductive polymer-coated fabrics for chemical sensing, Synthetic Metals 78 (1996) 93–101. [19] D. Verma, V. Dutta, Role of novel microstructure of polyaniline-CSA thin film in ammonia sensing at room temperature, Sensors and Actuators B 134 (2008) 373–376. [20] S. Virji, J.X. Huang, R.B. Kaner, B.H. Weiller, Polyaniline nanofibers gas sensors: examination of response mechanism, Nano Letters 4 (2004) 491–496. [21] M. Joubert, M. Bouhadid, D. Begue, P. Iratcabal, N. Redon, J. Desbrieres, S. Reynaud, Conducting polyaniline composite: from syntheses in waterborne systems to chemical sensor devices, Polymer 51 (2010) 1716. [22] S. Virji, R.B. Kaner, B.H. Weiller, Hydrazine detection of polyaniline using fluorinated alcohol derivatives, Chemistry of Materials 17 (2005) 1256–1260. [23] S. Virji, J.D. Fowler, C.O. Baker, J.X. Huang, R.B. Kaner, B.H. Weiller, Polyaniline nanofibers composites with metal salts: chemical sensors for hydrogen sulphide, Small 1 (2005) 624–627. [24] V. Dixit, S.C.K. Misra, B.S. Sharma, Carbon monoxide sensitivity of vacuum deposited polyaniline semiconducting thin films, Sensors and Actuators B 104 (2005) 90–93. [25] R. Arsat, X.F. Yu, Y.X. Li, W. Wlodarski, K.K. Zadeh, Hydrogen gas sensor based on highly ordered polyaniline nanofibers, Sensors and Actuators B 137 (2009) 529–532.
[26] X.B. Yan, Z.J. Han, Y. Yang, B.K. Tay, NO2 gas sensing with polyaniline nanofibers synthesized by a facile aqueous/organic interfacial polymerization, Sensors and Actuators B 123 (2007) 107–113. [27] S. Manigandan, A. Jain, S. Majumder, S. Ganguly, K. Kargupta, Formation of nannorods and nanoparticles of polyaniline using Langmuir Blodgett technique: performance study for ammonia sensor, Sensors and Actuators B 133 (2008) 187–194. [28] S. Manigandan, S. Majumdar, A. Suresh, S. Ganguly, K. Kargupta, D. Banerjee, Electric field induced dewetting and pattern formation in thin conducting polymer film, Sensors and Actuators B 144 (2010) 170–175. [29] J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, Nanostructured polyaniline sensors, Chemistry: A European Journal 10 (2004) 1314. ´ c-Marjanovi ´ G. Ciri ´ ´ B. Marjanovic, ´ P. Holler, M. Trchová, J. Stejskal, c, [30] A. Janoˇsevic, Synthesis and characterization of conducting polyaniline 5-sulfosalicylate nanotubes, Nanotechnology 19 (2008) 1356906. [31] P. Granholm, J. Paloheimo, H. Stubb, Conducting Langmuir–Blodgett films of polyaniline: fabrication and charge transport properties, Synthetic Metals 84 (1997) 783–784. [32] V.I. Troitsky, T.S. Verzina, M.P. Fontana, Langmuir–Blodgett assemblies with patterned conductive polyaniline layers, Materials Science and Engineering C 22 (2002) 239–244. [33] I. Sapurina, A.Y. Osadchev, B.Z. Volchek, M. Trchová, A. Riede, J. Stejskal, In-situ polymerized polyaniline films: 5. Brush-like chain ordering, Synthetic Metals 129 (2002) 29–37. [34] S. Jain, A.B. Samui, M. Patri, V.R. Hande, S.V. Bhoraskar, FEP/polyaniline based multilayered chlorine sensor, Sensors and Actuators B 106 (2005) 609–613. [35] G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables and Charts of Work, Wiley, Chichester, NY, 2001, pp. 94–99, 107, 221. [36] D.C. Trivedi, S.K. Dhawan, Investigations on the effect of 5-sulfosalicylic acid on the properties of polyaniline, Synthetic Metals 58 (1993) 309–324. [37] H.T. Varghese, C.Y. Panicker, D. Philip, I.R. Raman, SERS spectra of 5sulphosalicylic acid dihydrate, Journal of Raman Spectroscopy 38 (2007) 309–315. [38] A. Choudhury, Polyaniline/silver nanocomposites: dielectric properties and ethanol vapour sensitivity, Sensors and Actuators B 138 (2009) 318–325. [39] F. Yakuphanoglu, B.F. S¸enkal, Electronic and thermoelectric properties of polyaniline organic semiconductor and electrical characterization of Al/PANI MIS diode, Journal of Physical Chemistry C 111 (2007) 1840–1846. [40] X. Zhang, J. Zhang, Z. Liu, Tubular composite of doped polyaniline with multiwalled carbon nanotubes, Applied Physics A 80 (2005) 1813–1817. [41] E.E. Havinga, M.M. Bouman, E.W. Meijer, A. Pomp, M.M.J. Simenon, Large induced optical activity in the conduction band of polyaniline doped with (1S)-(+)-10-camphorsulfonic acid, Synthetic Metals 66 (1994) 93–97. [42] W. Feng, F. Zhou, X. Wang, X. Bai, K. Yoshino, Fabrication of composite films by controlling molecular doping processes between polyaniline and soluble multiwalled nanotubes and their optical characteristics, Japanese Journal of Applied Physics 42 (2003) 5726–5730. [43] F. Yakuphanoglu, B.F. S¸enkal, A. Sarac¸, Electrical conductivity, thermoelectric power, and optical properties of organo-soluble polyaniline organic semiconductor, Journal of Electronic Materials 37 (2008) 930–934. [44] L.H. Huo, L.X. Coo, D.M. Wang, H.N. Cui, G.F. Zeng, S.Q. Xi, Preparation and characterization of doped polyaniline films, Thin Solid Films 350 (1999) 5–9. [45] S. Koul, R. Chandra, S.K. Dhawan, Conducting polyaniline composite: a reusable sensor material for ammonia vapour, Sensors and Actuators B 75 (2001) 151–159.
Biographies Krishanu Chatterjee received his M.Sc. degree from University of Calcutta. He is pursuing Ph.D. in the department of Physics, Bengal Engineering and Science University, Shibpur. His interests are in synthesis and characterization of inorganic and organic (conducting polymer) nanostructure materials and its composites for various device applications. Palash Dhara received his B.E. from West Bengal University of Technology. He is pursuing M.Tech at Jadavpur University, Kolkata. His research interests are in development of nanostructure materials for sensor and thermoelectric applications. Dr. Saibal Ganguly received his Ph.D. from Indian Institute of Technology, Kanpur, in 1993. He is currently working as Professor in the Chemical Engineering Department, Universiti Teknologi Petronas, Malaysia. His field of interest includes process engineering, real time control and automation, energy engineering, fuel cell, property up-gradation and modification of industrial polymer and coal, modelling and simulation. Kajari Kargupta received her Ph.D. from Indian Institute of Technology, Kanpur, in 2001. She is presently working as a Professor in Chemical Engineering Department, Jadavpur University, Kolkata. Her area of research includes nano-science, conducting polymers, gel polymer, controlled drug delivery, polymer composite, modelling and simulation. Dipali Banerjee received her Ph.D. form Jadavpur University, Kolkata in the year 1990. She is now working as a Professor of Physics, Bengal Engineering and Science University, Shibpur. Her area of research includes nanostructured materials, conducting polymer, composite materials, thermoelectric materials and applications.