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Sensors and Actuators B 132 (2008) 116–124
Humidity sensitive poly(2,5-dimethoxyaniline)/ WO3 composites Dewyani Patil a , You-Kyong Seo b , Young Kyu Hwang b , Jong-San Chang b , Pradip Patil b,∗ b
a Department of Physics, North Maharashtra University, Jalgaon 425 001, Maharashtra, India Research Center for Nanocatalysts, Korea research Institute of Chemical Technology, Post Box 107, Yuseong, Daejeon 305-343, South Korea
Received 23 October 2007; received in revised form 7 January 2008; accepted 9 January 2008 Available online 20 January 2008
Abstract Humidity sensitive poly(2,5-dimethoxyaniline)/WO3 (PDMA/WO3 ) composites with different weight percents of WO3 have been prepared by a simple mechanical mixing method. These composites were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Humidity sensing characteristics, including the resistance versus relative humidity (RH), humidity hysteresis, response time, repeatability and long-term stability of these composites have been investigated. The PDMA/WO3 composites show better sensing properties than pure PDMA, such as higher sensitivity, quicker response and small hysteresis. The resistance of the composite with 30 wt% of WO3 changes linearly over the humidity range (23–84% RH). Furthermore, it exhibits a maximum percentage response factor (∼651 at 87% RH), quick response (humidification, 27 s and desiccation, 136 s), narrow hysteresis (∼5%) and an excellent repeatability of the response. © 2008 Elsevier B.V. All rights reserved. Keywords: Conducting polymers; Poly(2,5-dimethoxyaniline); Poly(2,5-dimethoxyaniline)/WO3 composites; Humidity sensor
1. Introduction In recent years, conducting polymers have been dominating the research areas of advanced materials due to their applications in many technological areas such as rechargeable batteries, sensors, electromagnetic interference (EMI) shielding, electrochromic display devices, smart windows, molecular devices, energy storage systems, membrane for gas separation, etc. [1,2]. Conducting polymers have shown very promising results for sensor applications in comparison to other classical sensor materials employed in the fabrication of sensors [3,4]. Conducting polymers such as polyaniline (PANI) [5] and polypyrrole (PPY) [6] show a change in their electrical properties when exposed to a humid atmosphere. This property provides a possibility of using these polymers as a humidity sensing material. Jain et al. [7] investigated the behavior of the humidity sensors based on the PANI doped with different weak acid dopants.
∗
Corresponding author. E-mail address:
[email protected] (P. Patil).
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.01.021
They have synthesized PANI by a chemical polymerization method using maleic acid (Mac), camphosulphonic acid (CSA) and diphenyl phosphate (DPPH) as weak acid dopants. It was observed that over the major range of relative humidity (between 30 and 80%) the behavior of these sensors is almost linear and the highest sensitivity is obtained with PANI doped with CSA. More recently, Geng et al. [6] investigated the effect of polymerization time on the humidity sensing properties of PPY. They observed that the humidity sensing properties of PPY can be improved greatly by extending the polymerization time. The incorporation of substituents in the polymer skeleton is a common technique to synthesize polymers having improved properties. Poly(o-anisidine) (POA) and poly(Nmethylaniline) (PNMA), which are derivatives of PANI, were also reported to be humidity sensitive by Kulkarni et al. [8,9]. The di-substituted derivatives of conducting polymers are also potential candidates in many technological applications. The applicability of poly(2,3-dimethylaniline) (P23DMLA) and poly(2,5-dimethylaniline) (P25DMLA), which are disubstituted derivatives of PANI, as a sensor material for humidity was investigated by Kulkarni and Athawale [10]. They observed
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that the P23DMLA exhibits almost linear response over the humidity range (6.4–97.3%) with higher sensitivity than pure PANI. However, considerable work is still needed to understand the basic issues related to the polymerization of substituted anilines and to explore the possibility of utilizing them as an alternative to PANI for humidity sensors. Recently, conducting polymer/inorganic composites have been considered as a new class of materials due to their improved properties compared with those of pure conducting polymers and inorganic materials [11]. Several attempts have been made towards the synthesis of conducting polymer/inorganic composites by using chemical and electrochemical polymerization routes for gas [12,13] and humidity sensor applications [14–18]. Parvatikar et al. synthesized composites of PANI with metal oxides such as WO3 [14], Co3 O4 [15] and CeO2 [16] by using chemical polymerization and investigated the humidity sensing properties. They have shown that these composites in pellet form exhibit good humidity sensing properties. However, the humidity sensing properties of pure PANI have not investigated and compared with those of composites. Also, other aspects of the humidity sensing such as hysteresis, response and recovery time and long term stability have not been studied by these authors. Suri et al. [17] prepared PPY/Fe2 O3 nanocomposites by simultaneous gelation and polymerization process and studied the gas and humidity sensing properties. More recently, Su and Huang [18] synthesized TiO2 /polypyrrole composite films on alumina substrates using an in situ photopolymerization route and investigated the humidity sensing properties by impedance spectroscopy. These composite films showed higher sensitivity, better linearity, smaller hysteresis, faster response (40 s) and recovery time (20 s), smaller temperature influence between 15 and 35 ◦ C and better long term stability than those without TiO2 nanoparticles. In the present work, we have prepared the organic–inorganic composite containing PDMA as the organic part and WO3 as the inorganic part by a simple mechanical mixing method and examined the possibility of utilizing it as a sensor material for humidity sensing application. To the best of our knowledge, there are no reports in the literature dealing with the synthesis and study of humidity sensing properties of PDMA/WO3 composites. 2. Experimental 2.1. Materials All chemicals were of analytical grade. The monomer 2,5dimethoxyaniline was procured from Fluka and it was doubly distilled prior to being used for the synthesis. Tungsten trioxide (WO3 ), ammonium persulphate ((NH4 )2 S2 O8 ) and hydrochloric acid (HCl) were purchased from Sigma and used as-received. Bi-distilled water was used to prepare all the solutions. 2.2. Synthesis of PDMA The PDMA was synthesized by a standard chemical polymerization method. An aqueous monomer solution containing 1 M
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HCl and 0.1 M 2,5-dimethoxyaniline was prepared and it was cooled down in an ice bath to 0–5 ◦ C under constant stirring. After 2 h, an aqueous solution of oxidant, 0.1 M ammonium persulphate was slowly added to the monomer solution under constant stirring. This polymerization process was carried out up to 20 h. Subsequently, the solution was filtered and washed with double distilled water repeatedly to remove the impurities and finally washed with a 1 M HCl solution to obtain dark green precipitate. The precipitate was dried under dynamic vacuum for constant weight. The PDMA powder was stored in a desiccator at room temperature for later use. 2.3. Preparation of PDMA/WO3 composites Different PDMA/WO3 composites were prepared by a simple mechanical mixing method using 10, 30 and 50 wt% of WO3 . The PDMA powder was ground together with WO3 powder which had been previously dried at 800 ◦ C for a few hours. The prepared composites were abbreviated by taking into consideration the weight percentage of WO3 used for the preparation of composites. For example, the PDMA/WO3 composite prepared with 10 wt% of WO3 was abbreviated as PDMAWO10. 2.4. Characterization of PDMA/WO3 composites X-ray diffraction (XRD) measurements were performed with a Rigaku diffractometer (Miniflex Model, Rigaku, Japan) having Cu K␣ (λ = 0.1542 nm) radiation over the 2θ range from 1.5◦ to 80◦ at a scan rate of 2◦ /min. Fourier transform infrared (FTIR) transmission spectra were recorded in the powder form (compressed KBr pellets) with a Nicolet FTIR (IMPACT 420 DSP) in the spectral range 4000–400 cm−1 . The morphological aspects of PDMA and its composites were examined by scanning electron microscopy (SEM) with a Philips, XL, 30S FEG 440 microscope. 2.5. Humidity sensing study The mechanically mixed PDMA and WO3 powder was pressed into pellet form of diameter ∼1 cm and thickness ∼0.1 cm for the humidity sensing study. The electrical contact leads were fixed 0.7 cm apart with the help of silver paste on the surface of the pellet. The electrical resistance of the pellet was measured as a function of relative humidity by using a simple two probe configuration with a sensitive digital multimeter (Scientific, SW 5015, India) controlled by a personal computer. Continuous variation in humidity was achieved in a simple experimental set-up fabricated in our laboratory in order to investigate the humidity sensing properties. A two temperature method was used to measure the relative humidity. The experimental set-up mainly consisted of a closed flask (1000 ml) with two necks for inserting thermometers and the sensing element (i.e. the pellet of the composite). The flask was partially filled with water and kept in a polystyrene container. The sensing element along with a thermometer was mounted on the sensor holder. The sensor holder was kept inside the flask at a height of 6 cm from the surface of the water. The external polystyrene
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container was filled with ice up to the level of water which was present in the flask. The variations in temperature of the water, Tw (inside the flask) and the sensing element, Ts were recorded continuously. The relative humidity (RH) in percentage inside the flask was calculated by using the relation [19]. % RH =
Pw (Tw ) Pw (Ts )
where Pw (Tw ) and Pw (Ts ) denote saturated water vapour pressures at the temperatures of the water and the sensing element, respectively. 3. Results and discussion 3.1. Preparation of PDMA/WO3 composites Fig. 1 depicts the XRD patterns of PDMA, WO3 and PDMAWO30. The XRD pattern of PDMA [Fig. 1(a)] exhibits the diffraction peaks at 2θ values of 6.90◦ and 25.24◦ , which are assigned to the scattering from PDMA chains at interplanar spacing. The XRD pattern of the WO3 powder [Fig. 1(b)] indicates the diffraction peaks at 2θ values of 23.18◦ , 23.66◦ , 24.34◦ , 26.64◦ , 28.34◦ , 33.32◦ , 34.18◦ , 49.90◦ and 56.0◦ , which
Fig. 1. XRD patterns of: (a) PDMA, (b) WO3 and (c) PDMAWO30.
are attributed to the triclinic phase of WO3 . The XRD pattern of the PDMAWO30 composite in Fig. 1(c) is almost similar to that observed for WO3 . The diffraction peaks corresponding to PDMA cannot be distinguished in the XRD pattern due to the overlapping of peaks with those of WO3 .
Fig. 2. FTIR spectra of: (a) PDMA, (b) WO3 and (c) PDMAWO30.
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The FTIR spectra of PDMA, WO3 and PDMAWO30 are shown in Fig. 2. The FTIR spectrum of PDMA [Fig. 2(a)] shows main characteristic bands of PDMA, which are assigned as follows [20]. A broad and weak band at ∼3432 cm−1 is due to the N–H stretching mode, the C N and C C stretching modes for the quinoid (Q) and benzoid (B) rings occur at 1590 and 1500 cm−1 , bands at ∼1207 and 1008 cm−1 are assigned to the presence of o-methoxy groups in PDMA, a band at ∼1112 cm−1 is attributed to the plane bending vibration of C–H, which is formed during protonation, and a band at ∼803 cm−1 indicates the ortho-substituted benzene ring. The FTIR spectrum of WO3 [Fig. 2(b)] indicates the presence of a strong doublet around ∼775 cm−1 due to WO3 [21]. The FTIR spectrum of PDMAWO30 is shown in Fig. 2(c). It exhibits the main bands corresponding to pure PDMA and a characteristic doublet around ∼775 cm−1 due to WO3 . SEM images of PDMA and PDMAWO30 are shown in Fig. 3. Pristine PDMA shows an aggregated structure, while the PDMAWO30 composite exhibits an aggregated morphology with the presence of large globules. 3.2. Humidity sensing properties 3.2.1. Relative humidity-resistance characteristics Variations in resistance of PDMA and PDMAWO10 as a function of RH are shown in Fig. 4. It is observed that the resistance decreases with an increase in RH for PDMA as well
Fig. 3. SEM images of: (a) PDMA and (b) PDMAWO30.
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Fig. 4. Variation in resistance with change in relative humidity (%) for: (a) PDMA and (b) PDMAWO10.
as for the composite. The decrease in resistance of PDMA and PDMAWO10 composite with an increase in RH can be attributed to the adsorption of water. This implies that PDMA and its composite with WO3 are humidity sensitive. The resistance of PDMA [Fig. 4(a)] decreases from 3.69 to 1.47 M with increasing the humidity from 22 to 44% RH and thereafter, no significant change in resistance is observed with a further increase in RH. It is observed that the resistance of PDMAWO10 [Fig. 4(b)] decreases systematically with an increase in RH. As can be seen in Fig. 4(b), the resistance of PDMAWO10 changes from 4.90 to 2.73 M when RH increases from 22 to 100%. It is found that the resistance of PDMAWO10 decreases exponentially (y = 2.5908 + 4.9465 × e−x/29.4673 , R2 = 0.9987, where x, y and R2 represent the % RH, resistance and correlation coefficient, respectively) with an increase in RH. The broken curve shows the exponential fit to the experimental data, illustrating clearly good quality of the fit. This observation reveals that WO3 in the composite plays an important role in sensing water molecules. The variation in resistance of PDMAWO30 as a function of RH is shown in Fig. 5(b). It is observed that when the wt% of WO3 in the composite is increased to 30%, the resistance of the composite varies linearly with RH and the linear fit (y = −0.1959x + 18.2881, S.D. = 0.1820, where S.D. represents standard deviation) to the experimental data indicates good quality of the fit. The resistance of PDMAWO30 varies from 13.89 to 2.2 M with an increase in RH from 23 to 84%. The resistance of PDMAWO50 in Fig. 6 decreases almost linearly (y = 13.9905 − 0.03357x, S.D. = 0.03185) with increasing the humidity from 27 to 100% RH. The resistance of the PDMAWO50 exhibits a change from 13.10 to 10.66 M with an increase in RH from 27 to 100%. 3.2.2. Response factor (Rf ) The response factor for detecting the humidity was calculated to reveal the characteristics of the PDMA/WO3 composite
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Fig. 5. Variation in resistance with change in relative humidity (%) for (a) PDMA and (b) PDMAWO30.
Fig. 7. Variation of percentage response factor Rf with change in relative humidity (%) for PDMAWO30.
towards moisture. The percentage response factor (Rf ) for detecting the humidity is defined as Rf = Rd /Rh × 100, where Rd and Rh are the values of the resistance recorded at 27% RH and at a particular RH, respectively. The greater the value of Rf , the higher the sensitivity of the material towards moisture. The Rf values calculated at 87% RH for PDMAWO10, PDMAWO30 and PDMAWO50 are found to be 160, 651 and 118%, respectively. The variation of the response factor with RH for the PDMAWO30 composite is shown in Fig. 7. It is observed that the response factor increases with an increase in RH. The PDMAWO30 composite showing the maximum percentage response factor was chosen for further study to evaluate the hysteresis, response time and stability.
Fig. 6. Variation in resistance with change in relative humidity (%) for PDMAWO50.
3.2.3. Hysteresis The hysteresis between the humidification and desiccation process was also measured in the range between 25 and 95% RH as shown in Fig. 8. It is seen that PDMA, Fig. 8(a) exhibits
Fig. 8. Humidity hysteresis for: (a) PDMA and (b) PDMAWO30. The symbols () and (䊉) denote humidification and desiccation processes, respectively.
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a relatively wide hysteresis loop, which indicates that the regeneration process is slower. In the case of PDMAWO30, Fig. 8(b), the differences in resistance between the humidification and desiccation process in the range between 25 and 95% RH are within 5%. Moreover, a pathway of the desorption process is located at the lower position of the loop, which indicates that the rate of desiccation of the adsorbed water is slower than that of the humidification. This result reveals that the presence of WO3 significantly improves the humidity sensing characteristics of PDMA. 3.2.4. Response and recovery behavior The response rate to the variation of RH was examined via measurement of the response time required to attain a steady resistance value when the samples were exposed to the atmosphere with a certain RH. The time taken by a sensor to achieve 90% of the total resistance change is defined as the response time [22]. The resistance of PDMA and PDMAWO30 was monitored at two extreme humidity atmospheres (25 and 100% RH). The variation in resistance of PDMA with time when RH was changed between these two extremes is shown in Fig. 9(a). The PDMA exhibits the response time of ∼204 and 306 s at the humidification and desiccation steps, respectively. In order to examine the repeatability, the response of the PDMA was measured by exposing repeatedly to 25% RH and then to 100% RH atmosphere for 180 s each. The corresponding response and recovery curves are shown in Fig. 9(b). The asymmetric variation in resistance of PDMA is observed when RH was changed between 25 and 100%. This suggests that the humidity sensing process is partially reversible, which substantiates the hysteresis observed between the humidification and desiccation for PDMA. The variation in resistance of the PDMAWO30 with time when RH was changed between 25 and 100% is shown in Fig. 10(a). The PDMAWO30 exhibits the response time of ∼27 and 136 s at the humidification and desiccation steps, respectively. The response and recovery curves for the PDMAWO30 composite are shown in Fig. 10(b) where RH was changed between 25 and 100%. It is observed that the resistance value of PDMAWO30 reverts always to the original one when RH is restored to the former state, which indicates that the humidity sensing process is extremely reversible. Thus, the PDMAWO30 composite in our experiment exhibits good stability as well as an excellent repeatability of the response. This suggests that the PDMA/WO3 composites can be used as a reusable sensor material for humidity sensing. 3.2.5. Stability The stability of the PDMAWO30 composite was also examined at three testing points of 40, 60 and 90% RH. The variation of resistance at 40, 60 and 90% RH with days is shown in Fig. 11. It is observed that the resistance of the PDMAWO30 composite shows a negligibly small variation at the three testing points for 30 days. Thus, the PDMAWO30 composite appears to be very stable over 30 days.
Fig. 9. (a) Variation in resistance of PDMA with time and (b) response/recovery curves of PDMA at 25 and 100% RH.
3.3. Sensing mechanism The decrease in resistance of PDMA and PDMA/WO3 composites with an increase in relative humidity can be attributed to the adsorption of water. The interaction of PDMA with water occurs via the proton exchange mechanism Fig. 12(a), which is in accordance with that reported by Jain et al. [7] for PANI. The PDMA pellet is comparatively less compact than those of PDMA/WO3 composites. Consequently, when the PDMA is exposed to a humid atmosphere water molecules are easily adsorbed onto the PDMA particles and it quickly becomes saturated with water content. The resistance of the PDMA decreases due to the saturated adsorbed water, and therefore it exhibits lower humidity sensing property when RH changes from 20 to 100%. The proposed humidity sensing mechanism for PDMA/WO3 composites is depicted in Fig. 12(b). In the case of PDMA/WO3 composites, there are two kinds of sites for the adsorption of water molecules, namely PDMA and WO3 . Therefore, the
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response of the composites to RH is due to the interaction of water molecules with both PDMA and WO3 . As stated previously, the interaction of water with PDMA is through the proton exchange mechanism. Alternatively, the interaction of water molecules with the WO3 surface takes place via the electrondonating adsorption [24]. The adsorption of water molecules on the WO3 surface takes place via a dissociative chemisorption process which results in the formation of hydroxyl groups at the surface [17]. The overall dissociative chemisorption process can be described in a two-step process as given below. (a) The water molecules adsorbed on the WO3 grain surface reacts with the lattice W as H2 O + Oo + W ↔ 2OH–W + Vo + 2e− where Oo represents the lattice oxygen at the oxygen site and Vo is the vacancy created at the oxygen site according to the reaction. Oo ↔ O2− + Vo (b) The doubly ionized oxygen, displaced from the lattice, reacts with the H+ coming from the dissociation of water molecules to form a hydroxyl group as given below. H+ + O2− ↔ OH−
Fig. 10. (a) Variation in resistance of PDMAWO30 with time and (b) response/recovery curves of PDMAWO30 at 25 and 100% RH.
Fig. 11. Long term stability of the PDMAWO30 composite measured at 40, 60 and 90% RH.
As a result of this reaction, the electrons are accumulated at the WO3 surface and consequently, the resistance of WO3 decreases with an increase in RH. Thus, the harmony between the electron-donating adsorption of water molecules on the WO3 surface and interaction of the water molecules with PDMA via the proton exchange mechanism results in the observed humidity sensing behavior of the PDMA/WO3 composites. In a low RH range, only a few water molecules were adsorbed due to the compact nature of the composites and as a result the coverage of water on the surface was not continuous. The resistance values of the composite in the lower humidity range are observed to be higher than that of PDMA and increase with an increase in the amount of WO3 in the composite. Due to the discontinuous water layer, the transfer of H+ and H3 O+ is so difficult that PDMA exhibits higher resistance at lower RH. At the same time, the interaction of water with WO3 is also very weak due to the adsorption of a few water molecules. This results in the higher resistance of the composites than that of pure PDMA at low RH. Geng et al. [6] showed that as RH increases, the average particle size of PPY decreases, which results in formation of large voids among the particles. Therefore, it can be said that when RH increases to a middle region, one or several serial water layers are formed which accelerates the transfer of H+ or H3 O+ . The quick transfer of ions on the water layer results in a rapid decrease of the resistance of PDMA. Furthermore, due to the adsorption of a large amount of water molecules on the WO3 surface at middle and higher RH, more electrons will be accumulated at the WO3 surface. These combined reactions hence result in a rapid decrease of the resistance of the composites at middle and higher RH.
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Fig. 12. (a) Proton exchange mechanism in PDMA and (b) proposed humidity sensing mechanism for PDMA/WO3 composites.
4. Conclusions
References
Humidity sensitive PDMA/WO3 composites were prepared by a simple mechanical mixing method and their humidity sensing characteristics were investigated. The humidity sensing characteristics of the PDMA/WO3 composites were observed to be better than those of pure PDMA. The composite with 30 wt% of WO3 exhibited the maximum percentage response factor (Rf ) ∼651 at 87% RH, narrow hysteresis (∼5%), excellent repeatability and long term stability (for more than 30 days). This investigation demonstrates that a simple mechanical mixing of WO3 with PDMA significantly improves the humidity sensing properties of PDMA.
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Acknowledgements The financial support from University Grants Commission (UGC), New Delhi, India under SAP–DRS programme is gratefully acknowledged. Dewyani Patil is thankful to North Maharashtra University, Jalgaon, India for awarding the research fellowship through Chief Minister’s fund, while Pradip Patil gratefully acknowledges KOFST for awarding Brain Pool fellowship at KRICT, South Korea.
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Biographies Dewyani Patil is a research scholar in Department of Physics, North Maharashtra University, Jalgaon, India, doing research work in the field of conducting polymers. She has obtained master’s degree (MSc) in electronics from North Maharashtra University, Jalgaon, India in 2004. She is currently pursuing a PhD in physics. Her research interests include conducting polymers and conducting polymer composites for chemical and biological sensors. You-Kyong Seo received his BS and MS in 2004 and 2006, respectively, in chemistry from Inje University. He is currently pursuing a PhD in chemistry at Korea Research Institute of Chemical Technology (KRICT), South Korea. His research work is focused on development of nanoporous materials, sensors and mesoporous materials. Young Kyu Hwang received his BE, MS and PhD degrees in 1995, 1997 and 2003, respectively in chemistry from Sungkyunkwan University. He has been a senior scientist at Korea Research Institute of Chemical Technology since 2003. His current research work is focused on development of new types of nanoporous materials, thin films and inorganic functional materials as well as gas sensors. Jong-San Chang received BS from Seoul National University in 1986, and MS and PhD from Korea Advanced Institute of Science and Technology (KAIST) in 1988 and 1996, respectively. He has been working for Korea Research Institute of Chemical Technology (KRICT) since 1988, and he is currently the director of the Research Center for Nanocatalysts. In 1999 he spent a sabbatical period at the Materials Research Laboratory, University of California, Santa Barbara, USA. He has been engaged in the fields of energy-related catalysis and green chemistry. His present interests cover microwave synthesis, characterization and applications of nanoporous materials and inorganic–organic hybrid materials. Pradip Patil is presently a visiting scientist in Korea Research Institute of Chemical technology (KRICT), Daejon, South Korea, and is working as professor in physics at Department of Physics, North Maharashtra University, Jalgaon, India. He received his master’s degree (MSc) in physics in 1983 and PhD degree in 1988 from the University of Pune, Pune, India. He joined North Maharashtra University, Jalgaon, India as the Head, Department of Physics since its inception. His main interests include development of conducting polymers and conducting polymer nanocomposites for their applications as corrosive protective coatings, chemical and biological sensors.