Polypyrrole microspheroidals decorated with Ag nanostructure: Synthesis and their characterization

Applied Surface Science 280 (2013) 950–956

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Polypyrrole microspheroidals decorated with Ag nanostructure: Synthesis and their characterization Yasir Ali a , Kashma Sharma b , Vijay Kumar a,∗ , R.G. Sonkawade c , A.S. Dhaliwal a a b c

Department of Physics, Sant Longowal Institute of Engineering and Technology, Longowal District Sangrur, 148106 Punjab, India Department of Chemistry, Shoolini University of Biotechnology and Management Sciences, Solan 173212, India School of Physical Sciences, BBA University (A Central University), Lucknow 226025, India

a r t i c l e

i n f o

Article history: Received 2 April 2013 Received in revised form 16 May 2013 Accepted 24 May 2013 Available online 31 May 2013 Keywords: Electrochemical synthesis Cyclic voltammetery Nanostructure Composite

a b s t r a c t In this work, polypyrrole (PPy) microspheroidals were synthesized by chronopotentiometry on conducting indium tin oxide (ITO) substrate. These prepared PPy microspheroidals were subjected to surface modification by Ag nanostructures using cyclic voltammetery (CV). The decoration of Ag nanostructures on PPy surface was characterized by UV–visible spectroscopy (UV–vis), X-ray diffraction (XRD), Fourier transform infrared (FTIR), energy dispersion X-ray spectroscopy (EDX) and scanning electron microscopy (SEM). SEM images show nanostar like structures with slight aggregation of Ag nanostructures on the PPy surface. The average particle size of PPy was found to increase from 7.3 to 30 nm after decoration with Ag nanostructure. Further, the XRD patterns shows a decrease in interchain separation and inter planar distance after the impregnation of Ag nanostructures in the PPy film when compared with pure PPy film. FTIR and Raman spectra displayed main vibrational bands; including the characteristic peaks. Raman spectra also show the signature of polaron and bipolaron states. Electrical conductivity of the Ag nanostructure decorated PPy microspheroidals has been measured by the two probe method. A possible mechanism for the formation of Ag-PPy nanostructure has been proposed. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Recently, conducting polymers (CPs) have attracted a lot of attention due to their unique mechanical [1], optical [2] and electronic properties [3–5]. They find prospective applications in many fields like sensors [6], actuators [7], catalysis [8], field effect transistors [9], light emitting diodes [10], supercapacitors [11], biofuel cells [12] etc. Among the CPs, polypyrrole (PPy) has been intensively studied due to its ease in synthesis, environmental stability, and property of entrapping the counter ions during synthesis [13–15]. In addition, there have been several reports on synthesis and characterization of noble metal nanoparticles (NPs) due to their unique electronic, optical and catalytic properties [16–18]. The incorporation of noble metal nanoparticles in different polymer matrix has been reported by various authors [19–22]. Recently, metal-conducting polymer nanocomposites have been the subject of great interest for their ability to enhance the electrical, optical and mechanical properties by synergistic effects during the interaction of the two components. For example, Wang et al. [23] deposited different kind of silver nanostructure on the surface of PPy films by silver mirror reaction and studied the application

∗ Corresponding author. Tel.: +91 1672 253186; fax: +91 1672 280057. E-mail address: vijays [email protected] (V. Kumar). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.116

of SERS for active substrate. Li et al. [24] reported the synthesis of Ag-PPy composite films via enhanced redox reaction of metal ions. Further they reported that the composite is highly conductive and has high superhydrophilicity. Chen et al. [25] fabricated Ag/PPy/polyacrylonitrile composite fibrous materials using the electro-spinning technique. Zhao et al. [26] reported on the synthesis of Ag-PPy composite colloids using a simple wet chemical method. Yao et al. [27] addressed the preparation of raspberry-like PPy composites onto silica substrates via oxidation polymerization with applications in catalysis. Wu et al. [28] studied the improvement in colloidal gold super-particles by PPy modification using oxidative polymerization of pyrrole monomer. Wang and Shi [29] demonstrated the fabrication of uniformly sized (Ag/PPy) core–shell nanoparticles by a one-step hydrothermal reaction of PPy and silver nitrate in the presence of polyvinyl pyrrolidone (PVP) as protection agent. In spite of a significant amount of work on the synthesis and characterization of Ag-PPy nanocomposites, there have been no reports on surface modification of PPy film by Ag nanostructures using two step electrochemical synthesis and their characterizations. In our previous work, two step electrochemical synthesis of Au nano particles decorated polyaniline nanofiber was reported [30]. The electrochemical method for synthesizing conducting polymers films is very versatile and provides a facile way to tailor in a controlled way the different morphologies and properties by simply

Y. Ali et al. / Applied Surface Science 280 (2013) 950–956

varying the process parameters such as pH, monomer and dopant concentration, applied current density and the time of deposition. Furthermore, the electrochemical synthesis allows control over the thickness of the deposited films, which is not possible by other methods. Therefore, with this genesis a report is communicated with two different procedures of electrochemical technique for the decoration of Ag nanostructures on the surface of the PPy film. On the other hand Ag being the most conducting among noble metals. Its exceptional electronic and optical properties with high conductivity and stability make it a resourceful material for the fabrication of composite with polymers [26,31]. The selectivity and sensitivity in sensors are improved by metal-polymer composites than bulk polymers [32]. The first step comprised a chronopotentiometeric/galvanostatic deposition of PPy microspheroidals on an ITO substrate with optimized process parameters (viz. current density, monomer and supporting electrolyte concentration and deposition time). The second step was followed by surface modification of the synthesized PPy microspheroidals by silver nanostructures for fabrication of Ag-PPy composite using cyclic voltammetery. This study shows the strong interaction of Ag nanostructures with the PPy matrix. The nanocomposites were characterized to determine their structure and morphology using X-ray diffraction (XRD), microRaman and scanning electron microscopy (SEM). Compositional analysis was done using EDX. Optical and electrical characterization was carried out using UV–vis and I–V measurement respectively. It is worth noting that the system selected for the present study is of practical importance since; this system has the potential for improving the functional properties in chemical and bio-chemical sensing. 2. Experimental 2.1. Materials Pyrrole monomer (Merck, 99.5% purity), pTS (Merck, >99% purity) and AgNO3 (Sigma Aldrich) were used in the present investigation. All these reagents were analytical grade reagents (A.R.) and used without further purifications. All the experiments were carried out with double-distilled water. 2.2. Electrochemical synthesis Electrochemical synthesis was carried out using a CHI 660D electrochemical workstation under computer control. The standard three electrode setup was employed in one a compartment electrochemical cell. A rectangular conducting ITO sheet of size 20 × 10 × 0.7 mm3 was used as a working electrode whereas a platinum sheet of size 20 × 40 × 0.25 mm3 was used as a counter electrode. The reference electrode was an Ag/AgCl electrode. The first step involves the aqueous electropolymerization of pyrrole containing 80 ml aqueous solution onto an ITO substrate using a chronopotentiometery technique. The electrolyte solution was composed of 0.15 M pyrrole monomer and 0.60 M pTS. The surface modification of synthesized PPy microspheroidals was conducted in an aqueous solution of 80 ml containing 0.001 M AgNO3 using cyclic voltammetery (CV) between potential windows of −0.4 to 0.7 V at a scan rate of 40 mV s−1 . The surface of PPy film was uniformly decorated by Ag nanostructures by controlling the scan rate and applied potential during the synthesis process. The thickness of the synthesized composite film was 15 ± 1 ␮m. 2.3. Characterizations SEM images and EDX spectra were obtained using a JEOL JSM-6490LV microscope at 25 kV after covered with a thin layer

951

Fig. 1. Chronopotentiogram recorded during synthesis of PPy-pTS film with optimized process parameters.

(∼10 nm) of sputtered gold. SEM micrographs are taken at different resolution. UV–vis spectra of PPy and Ag-PPy nanocomposite in reflectance mode were recorded on a JASCO V-530 spectrophotometer in the wavelength range 200–740 nm. The micro-RAMAN investigation was carried out using a Renishaw InVia Raman microscope. The Ar ion laser excitation at 514 nm and at very low power (<1 mW, 20× objective) was used to avoid any heating effect. XRD spectra of Ag-PPy composite film was recorded on a Phillips X˚ for wide range ray diffractometer with Cu-K␣ radiation (1.54 A) of Bragg’s angle 2 (20◦ <  < 50◦ ) at the scanning rate of 1◦ per min. The operating voltage and current for the X-ray gun were 40 kV and 40 mA, respectively. FTIR spectral study was carried out using a Testscan Shimadzu FTIR-800 series. The analysis conditions were: wavenumber range of 500–2500 cm−1 , 4 cm−1 resolution, 40 scans, and room temperature (25 ◦ C). I–V characteristics measurements were made using a Keithley 2400-C source meter. 3. Results and discussion Galvanostatic deposition of PPy film was carried out with optimized process parameters (viz. monomer concentration, supporting electrolyte, current density and deposition time). These parameters have a strong influence on the nature of the polymerization process. It is well reported in literature that the concentration of dopants usually determines the morphology, conductivity, rate of the polymer growth during electrochemical polymerization, and has an influence on the degradation process [33,34]. Thus, by optimizing the various process parameters one can synthesize desired nanostructure. The optimized values were 0.15 M, 0.6 M, 0.8 mA/cm2 and 800 s, respectively. Synthesized films exhibited microspheroidals morphology and good adhesivity. Chronopotentiogram recorded during synthesis of the PPy film with optimized process parameters is shown in Fig. 1. In our previous group it was reported that Chronopotentiograms revealed a low polymerization potential that ensures higher conductivity and uniform surface morphology [30,35–37]. The similar study was conducted by Li et al. [38] for electrochemical deposition of PPy onto the different substrates. They have concluded that the galvanostatically deposited PPy film has the highest redox peaks with highest electrochemical reactivity. The obtained chronopotentiogram has low polymerization potential which is in agreement with the Gade et al. [39] reported results. During the galvanostatic process of the electrochemical synthesis the polymerization initiates with the generation of radical

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-e-

C4H5NH+

C4H5NH +

C4H5NH

(1) +

+

(C4H5NH )2

+ C4H5NH

(2)

The process proceeds till the polymerization process terminates. During cyclic voltammetery the reaction mechanism is carried out as given below.

Ag+ + NO3-

AgNO3 (aq)

2NO3- (oxidisedatanode) +

Ag

-

+ e +

(3)

2NO2 + O2 + 2eAg

-

2C4H5NH + 2e + Ag

(4) (5)

2C4H5N

Ag(co-ordinationbonding) (6)

Scheme 1. The proposed mechanism for the reaction of pyrrole with AgNO3 . Fig. 2. The cyclic voltammogram of Ag-PPy composite between potential windows of −0.4 V to 0.7 V in a solution of 0.001 M AgNO3 with scan rate of 40 mV s−1 for 4-sweep segments.

cations at the anode, the reaction of two radicals causes chain propagation, pairing the spins and elimination of protons for production of neutral dimmer [40]. The potential increases to oxidize the monomer presumably the dimmer and higher oligomers are also oxidized to the corresponding radical cations [41]. The chronopotentiogram (Fig. 1) exhibits low polymerization potential indicating the full thickness and conducting nature of PPy film. The polymerization potential at the peak clearly indicates the nucleation process and subsequent polymerization of the PPy on the ITO substrate by the redox process. The decreasing slope and plateau formation indicates the polymerization growth rate and increase in thickness of the film with deposition time. In the present manuscript, the PPy microspheroidals deposited on the ITO substrate act as the working electrode in an 80 ml aqueous solution of AgNO3 (0.001 M) in for 4-sweep segments of CV to modify the surface of the PPy microspheroidals under 40 mV s−1 scan rate in a potential window of −0.4 to 0.7 V (Fig. 2). During the electrochemical process, the aqueous solution of AgNO3 with the oxidative PPy film as working electrode, the dissociation of AgNO3 takes place into NO3 − as anion radical and Ag+ as cation radical. This redox process is further preceded with oxidation of NO3 − at the anode as shown in Eq. (4) (see Scheme 1). The reduction process of Ag+ takes place at the cathode i.e. the PPy working electrode to produce silver metal particle. During this CV redox process, the Ppy+ is dehydrogenated to reduce Ppy. The reduced Ag metal particles are bound to the PPy film at the working electrode by ␲-electron donation (co-ordinate bonding) and these Ag metal particles are uniformly coupled on the PPy surface to form a metal polymer composite. During chronopotentiometery the mechanism of formation of the PPy film proceeds with the formation of radicals to form as polaronic state and the combination of radicals leads to combination of two radicals with zero spin as bipolaronic state involving the creation of dimmers and higher oligomers leads to the polymerization process. The proposed mechanism for the reaction of pyrrole with AgNO3 is shown in Scheme 1. From the above study, a mechanism has been proposed for the electrochemical synthesis of Ag-PPy composite film. The formation mechanism of the Ag-PPy composite is depicted in Scheme 2.

peak at 456 nm, which is the Polaron absorption peak of the conducting PPy [42,43]. It is reported that the UV–vis spectrum of Ag nanoparticles gives an absorption band at around 420 nm due to surface plasmon of silver [44]. Analogous absorption spectra with a peak at 477 nm appeared in the absorption spectrum of Ag decorated PPy film, which corresponds to the plasmon resonance of Ag nanostructures. The shift in absorption band after decoration with Ag nanostructure clearly shows the strong interaction between Ag and PPy. This shift in the absorption peak indicated a decrease in the energy band gap which gives rise to the increase in the conductivity of Ag-PPy film as compare to pure PPy. The spectral shift indicated oxidative and morphological changes in the polymer matrix, which is also confirmed by Raman and SEM analysis. Not only this, we observed noticeable broadening of the absorption edge after decoration with Ag nanostructures, which reflects the aggregation of Ag nanostructures and hence average particle size increase, which is in line with the XRD calculation. From the absorption spectra, the band gap of PPy and Ag-PPy nanostructure was calculated by the linear part of the Tauc’s plots [43] by the extrapolation of the plot of photon absorption (˛h)2 versus (h) on the h axes. The value of the band gap energy for PPy was 2.3 eV and 2.15 eV for the Ag-PPy film. The decrement in the band gap energy is attributed to the strong interaction of Ag nanostructures with the PPy film.

3.1. UV–vis spectroscopic studies UV–vis spectra of PPy and Ag-PPy nanocomposites are shown in Fig. 3. The absorption spectra of PPy showed a characteristic

Fig. 3. UV–vis spectra of PPy and Ag decorated PPy film synthesized by electrochemical method.

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Scheme 2. Schematic diagram for the electrochemical synthesis of Ag-PPy composite.

3.2. X-ray diffraction studies The XRD was carried out to spot the Ag particles in the Ag-PPy film. X-ray diffraction pattern of PPy and Ag-PPy composite film are shown in Fig. 4. Fig. 4(a) represents XRD pattern of PPy microspheroidals, which has a peak at 26.1◦ , a characteristic peak of semi crystalline PPy [43]. Fig. 4(b) presents XRD spectrum of Ag-PPy nanocomposite revealing the crystalline nature of the composite. The peak at 2 = 38.1◦ and 44.2◦ representing Braggs reflection from (1 1 1) and (2 0 0) planes of Ag and were in good agreement with the reported data, reflecting the presence of Ag in the Ag-PPy composite [24,45,46]. Also, the peak in PPy microspheroidals at 2 = 26.1◦ slightly shifted to higher angle (2 = 27.3◦ ) after decoration of Ag particles. The intensity of peak at 2 = 21.5◦ decreases after decoration with Ag nanoparticles. These analyses indicated that the structure of the Ag nanostructures decorated on the PPy surface was altered during the electrochemical synthesis. Furthermore, the average particle size (L), inter-chain separation (R) and inter-planer distance (d) were calculated [47,48]. The structural parameters viz. L, R and d are calculated with respect to the prominent peak of the PPy and Ag-PPy composite

are given in Table 1. The average particle size of the characteristics peak of PPy in pure PPy and Ag-PPy composite has increased after decoration with silver particles. The increase in crystallite size is due to the aggregation of Ag particles in the surface of PPy film. The aggregation of Ag particles can be seen from the SEM micrographs. Interestingly, the interchain separation decreased after incorporation of Ag particles in the PPy surface. This clearly shows the decoration of silver particles in the PPy surface due the formation of ␲–␲ co-ordination bond. Tyler et al. [49] demonstrated that interchain separation has strong influence on the spectral properties of polymers for sensing applications. They further reported that interchain separation of polymeric materials changes the photoluminescence properties of conjugated polymers. Therefore, it can be worth to mention that the inter-chain separation of metal polymer composite plays an important role as chemo-sensors. This interchain separation swallows the composite structure and offers quick diffusion of gases to enhance sensitivity and low detection limit in case of chemical-sensor. However, the interplanar distance decreased after decoration with Ag nanostructures. The decrement in interplanar distance reflected better mechanical properties of the composite. In other words, these results show the strong interaction of Ag nanostructures with PPy. These results are in good agreement with the Raman analysis. 3.3. Raman studies of Ag-PPy composite Raman spectroscopy was performed to characterize the structure of the synthesized composite. Fig. 5(a) shows Raman spectrum of PPy microspheroidals at room temperature. The characteristics Raman bands of the PPy are given in Table 2. It is well documented in the literature that, the peak pointed in the range from 1560 to 1630 cm−1 shows the C C backbone stretching of PPy [46,50]. The peaks in this range can be used to calculate the conjugating length of polymer chains, which in terms confirms the conducting nature of PPy [51]. The band 959 cm−1 corresponds to ring deformation Table 1 XRD parameters of PPy and Ag-PPy composite film.

Fig. 4. XRD pattern of (a) PPy and (b) Ag-PPy composite.

Sample

2 (◦ )

FWHM b (◦ )

R (Å)

D (Å)

C. S., L (nm)

PPy Ag-PPy Ag-PPy (1 1 1)

26.1 27.3 38.1

1.1 0.7 0.28

4 4 2.95

3.22 3.26 2.36

7.3 11.6 30

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Fig. 5. Raman spectrum of (a) PPy and (b) Ag-PPy composite.

Fig. 6. FTIR spectra of (a) pure PPy and (b) Ag-PPy composite.

with associated cation [52]. The bands at 1122 and 1167 cm−1 are assigned to C H plane deformation of oxidized PPy [53]. It is interesting to compare the spectra of PPy with Ag nanostructures decorated PPy film. The Raman shifts and relative intensity are observed in the Ag decorated PPy film (Fig. 5(b)). This shifts in Raman bands and intensity shows the interfacial strapping amalgamation of Ag nanostructures with the PPy matrix. Almost all the Raman bands show increase in Raman intensity after incorporation of Ag nanostructures on the surface of PPy film. The characteristic peaks of PPy microspheroidals at 1219 and 1488 cm−1 appear at 1221 and 1493 cm−1 in the composite film. The shifts in the Raman bands clearly showed the deposition of Ag nanostructures on the surface of PPy. The small Raman band at 1423 cm−1 assigned to the formation of bipolarons [54]. The peak represented at 751–777 cm−1 is due to CH out of plane vibration. The small band 527 cm−1 gets disappear after decoration with Ag nanostructures. In the spectrum, the small band centred at around 954 cm−1 is assigned to the ring deformation associated with the dication [55].

vibration [57]. Moreover, the spectra shows shift in the peaks position toward higher wavenumber region after decoration with Ag nano structure. This may be due to the strong interaction between PPy and silver particles. Thus, the IR results confirm the formation of Ag-PPy composite film [51,56,57].

3.4. FTIR measurements Fig. 6 shows the FTIR spectra of PPy (a) and Ag-PPy nanocomposite (b). The FTIR spectrum of pure PPy shows characteristics peaks at 1564 cm−1 and 1486 cm−1 , due to the C–C stretching of pyrrole ring and the conjugated C–N stretching mode, respectively. However, the corresponding peaks in Ag-PPy composites appeared at 1571 cm−1 and 1492 cm−1 . The typical bipolaron bands at 926 and 1214 cm−1 showed that PPy is in its doped state [51,56]. The peaks at 1280 and 1049 cm−1 are linked to the in-plane vibrations of C H and the peak at 795 cm−1 is ascribed to C H wagging Table 2 Raman bands of PPy on ITO substrate and assignments. Peaks (cm−1 ) in this work

Peaks (cm−1 ) shown in the literature

Peaks assignments

959

969

1122, 1167

1000–1150

1318 1488 1561, 1590, 1615

1314 1485 1560, 1570, 1584, 1610

Ring deformation associated with radical cation [52] C-H plane deformation of oxidized PPy [53] Ring stretching [50] Skeletal band [52] C C backbone stretching [46,50]

3.5. SEM results SEM is carried out to provide confirmation of the PPy microspheroidals and decoration of Ag nanostructure on the surface of PPy microspheroidals. Fig. 7(a)–(d) shows the SEM images of synthesized Ag-PPy composite film at different magnifications. From these SEM images, it is clear that PPy film shows spheroidal and globular like structure with uniformly decorated with Ag nano structures. The image exhibits homogeneous nucleation and uniform growth during the polymerization process indicates the defined optimization of process parameters for fabrication of desired PPy film using chronopotentiometery and subsequent deposition of Ag nanostructures on the surface of polymer film using CV. The Ag nanostructures look likes nano stars. Further, the Ag nanostructures show aggregation, which is also confirmed by XRD and UV–vis analysis. This reflects strong interaction between polymer and metal particles that enhances mechanical property. The diameter of these spheroids is in micro-meter range. Interestingly it is reported that the size of the polymer films strongly dependent upon the oxidation ability of monomer during the electrochemical process. Therefore, higher oxidation potential generated larger diameter and rough surface as shown in the Chronopotentiogram (Fig. 1) [26]. At the higher oxidation level, the polarons are combined to produce spineless bipolarons, and wide bipolaron bands are present in the gaps in the highly conducting regime [40]. The obtained results clearly explore the possibility to use Ag nanostructures decorated PPy composite film in gas sensing and enzyme immobilization applications. The presence of Ag-particles on the surface of PPy was ascertained by EDAX results (not shown). The Ag-peak strongly indicates that Ag-particles are decorated on the PPy surface. The EDX measurements were performed in a complete area of square millimeter range to ensure the uniform distribution of Ag-nanostructures. 3.6. Current–voltage characteristics The mechanism of charge transport in the conducting polymers is reported via different charge carrier (viz. polarons, bipolarons

Y. Ali et al. / Applied Surface Science 280 (2013) 950–956

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Fig. 7. SEM micrographs of Ag-PPy composite films on ITO substrate at different magnifications (a) 1000×, (b) 2000×, (c) 3000×, and (d) 7000×.

etc.) over large number of monomer units in the polymer chain [58]. In order to avoid the percolation of silver, thick PPy films were deposited of the mother solution. Electrical connection was made by highly conducting silver paste and metal wires. It is interesting to demonstrate the effect of Ag nanostructure on the conductivity of the PPy film. The surface conductivities of the PPy and Ag decorated PPy nanocomposite film was measured by using two probes setup at room temperature. The typical I–V characteristics of the films are shown in Fig. 8. It has been found that the

I–V characteristics are linear in nature like Ohms law. On the other hand, we can notice that the conductivity of Ag decorated PPy film is more than pure PPy film. The lower conductivity of the PPy film may be due to the lower protonation of the PPy chain. The increase in conductivity in the present investigations may be due to the generation of polarons or bipolarons owing to the creation of defects during chain rearrangement during electrochemical decoration of Ag nanostructures on the surface of PPy film. In other words, the enhancement in conductivity of Ag decorated PPy film reflects the dispersion of Ag nanostructures on the surface of PPy film. Various authors reported enhancement in conductivity of metal conducting polymer nanocomposites [56,24].

4. Conclusion

Fig. 8. I–V characteristics of PPy and Ag-PPy film.

In summary, a simple two step electrochemical method has been employed for the synthesis of Ag-PPy composite films. The decoration of PPy with Ag nanostructures results in the enhancement of conductivity which is in agreement with the UV–vis study. The XRD pattern was used to calculate structural parameters viz. crystallite size, interchain separation and inter planar distance of PPy microspheroidals and Ag decorated PPy film. The XRD results show increase in crystallite size after impregnation of Ag nano structure in the PPy film. SEM investigations show nano star like morphology of Ag nano structure on PPy microspheroidals. The characterization revealed that Ag-PPy matrix enhances certain electrical, optical and mechanical properties which can be explored in chemical and biochemical sensing and the work in this direction is in progress. This method can be used to fabricate other metal polymer composites for various application purposes.

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