Scalable and Low Cost Synthesis of Highly Conducting Polypyrrole

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Scalable and Low Cost Synthesis of Highly Conducting Polypyrrole Nanofibers Using Oil-Water Interfacial Polymerization Under Constant Stirring Jayanta Hazarika, and Ashok Kumar J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b03179 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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The Journal of Physical Chemistry

Scalable and Low Cost Synthesis of Highly Conducting Polypyrrole Nanofibers Using Oil-Water Interfacial Polymerization Under Constant Stirring J. Hazarika and A. Kumar* Materials Research Laboratory, Department of Physics, Tezpur University, Tezpur 784028, Assam, India *Email: [email protected]; Phone: +91 3712 275553, Fax: +91 3712 267006 Abstract In the present work, we report a low cost and scalable oil-water interfacial polymerization method to synthesize one dimensional (1-D) highly conducting polypyrrole (PPy) nanofibers doped with p-Toluenesulfonic (p-TSA) and hydrochloric (HCl) acids. Polymerization of pyrrole (monomer) has been carried out at the interface formed between the immiscible oil and aqueous water droplets under constant magnetic stirring at room temperature. Formation of smaller diameter (16-20 nm) PPy nanofibers has been confirmed from the high resolution transmission electron microscopy (HRTEM) studies and average diameter of p-TSA doped PPy nanofibers is found to be smaller than that of HCl doped nanofibers. The polymer chain ordering or crystallinity of both p-TSA and HCl doped PPy nanofibers have been studied with X-ray diffraction (XRD). Studies of Fourier transform infrared (FTIR) spectra suggest the presence of all the characteristic vibration bands in doped PPy nanofibers. The doping of PPy nanofibers has been confirmed from the formation of polaron and bipolaron bands in their UV-visible spectra. The optical band gap energy ( E g ) , and Urbach energy ( EU ) for PPy nanofibers doped with p-TSA and HCl doped PPy nanofibers determined from their UV-vis absorption spectra. The red shifting of polaron absorption band in p-TSA doped PPy nanofibers confirms higher conjugation length of the

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polymer nanofibers chains than that in the HCl doped PPy nanofibers. Thermo-gravimetric (TGA) and derivative plots of TGA studies predict that PPy nanofibers doped with p-TSA are thermally and structurally more stable as compared to HCl doped PPy nanofibers. Currentvoltage (I-V) characteristics exhibit non-linear behavior with voltage in both p-TSA and HCl doped PPy nanofibers.

1. Introduction Conducting polymers (CPs), also known as conjugated polymers or synthetic metals, are a class of polymers with highly π-conjugated sp2 hybridized polymeric main chains due to which they exhibit electrical properties comparable to the inorganic materials such as semiconductors and metals1. Having excellent properties of high electrical conductivity, good thermal stability and electrochemical activity, flexibility, light weight, ease of synthesis, unique optical properties and biocompatibility, CPs can be used as organic metallic conductors as the substitute for traditional inorganic metals2. Up to now, a variety of CPs such as polyaniline (PAni), polypyrrole (PPy), polythiophene (PTh), and poly(3,4ethylenedioxythiophene) have been extensively studied for the need of basic importance and technological aspects over other conducting polymers. Despite their various excellent properties, CPs has limited use due to their poor reaction control and processability with conventional morphologies. The nanostructured conducting polymers, on the other hand, have demonstrated, increase in processability, higher surface area, and improved consistency and stability in aqueous dispersions, which allow them for expanded commercial development as the promising materials3. Over the past decade, there has been increasing demand to synthesize nanostructured CPs with unique properties over their bulk counterparts. Due to improve physico-chemical properties, nanostructured CPs has received special attention as promising candidate in many areas of nanoscience and nanotechnology.


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One-dimensional (1-D) CPs nanostructures (such as nanofibers and nanotubes) are promising materials for fabricating polymeric nanodevices such as chemical and bio-sensors, field-effect transistors, electrochromic display devices, supercapacitors etc. and they exhibit clear advantages over their bulk counterparts in many types of applications4-6. The synthesis of 1-D CPs nanofibers can be carried out via chemical and electrochemical methods by polymerizing the monomer with the help of soft or hard templates. The hard templates include zeolite channels, track-etched, polycarbonate, and anodized alumina7-9. Soft templates, such as surfactants and micelles have been successfully reported as the directing agent to growth one dimensional nanostructures of CPs10,11. However, the use of hard templates to guide the nanostructures of CPs undergoes major difficulty of processability and post synthesis process, which limits their applicability in various nanodevices; although uniform ordered nanostructures can be obtained by this method. The soft template method, on the other hand is a cost effective method as the surfactants which act as micelles for polymerization to occur, also act as dopant anions. A relatively new approach, called the interfacial polymerization is carried out at the bulk interface between the immiscible organic and aqueous medium to synthesize 1-D nanofibers of conducting polymers. After polymerization, the resulting nanofibers diffuse away from the interface to the aqueous medium due to their hydrophilic nature. Nanofibers of CPs with diameter of a few nanometers and length to several micrometers are remarkable for device applications due to their high surface area, small pore size and very high porosity and a low basis weight. In previous reports, many researchers have successfully synthesized 1-D CPs nanofibers using both chemical oxidative and electrochemical methods. J. Huang et al.12 reported the facile synthesis of PAni nanofibers with uniform diameters between 30-50 nm and lengths varying from 500 nm to several micrometers for chemical sensing applications of HCl and NH3. X. Yang et al.13 synthesized large scale production of PPy nanofibers via a reactive template



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method for measuring NH3 gas sensitivity. X. Li et al.14 reported the template-sacrificed synthesis of polypyrrole nanofibers for lithium battery application. In another report, S. Virji et al.15 synthesized PAni nanofibers using interfacial polymerization approach for gas sensing application. H. D. Tran et al.16 reported the template free method for production of PPy nanofibers with an average diameter of 20 nm. A. Wu et al.17 et al. reported the chemical synthesis of PPy nanofibers with average diameter in the range of 20-40 nm in presence of Cetyltrimethylammonium bromide (CTAB) as micelles. I. S. Chronakis et al.18 reported the electrochemical synthesis of conductive PPy nanofibers with average diameter in the range of 70-300 nm. J. Zang et al.19 reported the template free electrochemical synthesis of superhydrophilic PPy nanofibers with diameter range of 50-220 nm. Although the interfacial polymerization method is proven to be a versatile synthesis method for fabricating 1-D nanofibers of CPs, it also suffers from major disadvantage of using equal amount of organic phase to the aqueous phase making the synthesis method costly and highly toxic. Keeping this in mind, one has to search for a new approach for synthesizing 1-D nanofibers of CPs, which is a low cost, reliable, environmentally friendly synthesis method capable of making high quality, pure, uniform and template-free PPy nanofibers with smaller diameters. In a previous study, S. Xing et al.20 have reported the low cost synthesis of PAni nanofibers with diameters in the range of 50-80 nm using the novel interfacial polymerization under magnetic stirring at room temperature. The synthesized PAni nanofibers were greatly affected by the used xylene amount (0.2-5 mL) and the electrical conductivity varied from 6.02 S/cm to 13.07 S/cm. Their results showed that the use of more amounts of xylene and longer reaction time reduced the doping level of the resulting PAni and produced aggregates, though water-dispersity of the products increased under such conditions. On the other hand, a slower stirring rate was favorable for obtaining higher


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doping level, but the morphology of the resulting nanofibers was not as good as that obtained from the higher stirring rate. In this work, we report an oil-water interfacial polymerization method to synthesize PPy nanofibers using very small amount of organic solvent xylene. In this method, pyrrole (monomer) was dissolved in small amount (1mL) of xylene and added directly to the aqueous medium containing dopant p-Toluenesulfonic acid (p-TSA) and hydrochloric acid (HCl), respectively in presence of ammonium persulphate (APS) as oxidant to induce polymerization at the interfaces of oil-water droplets under constant magnetic stirring at room temperature. The morphology, structural, optical and thermal properties of the resulting PPy nanofibers doped with p-TSA and HCl have been investigated using HRTEM, XRD, FTIR, UV-vis and TGA characterization techniques. The electrical properties of PPy nanofibers have been studied by their current-voltage (I-V) characteristics.

2. Experimental details 2.1. Materials and Methods Pyrrole (99.9 %, monomer) and ammonium persulphate (APS, oxidant) were procured from Sigma Aldrich. The dopants such as p-Toluenesulfonic acid (p-TSA) and hydrochloric acid (HCl) were purchased from Merck. Pyrrole was purified by distillation in vacuum before use and other chemicals were used without purification. The synthesis of PPy nanofibers using interfacial polymerization at the interfaces of the oil-water droplets were carried out as follows: Initially 0.25 M pyrrole (monomer) was dissolved in small amount of xylene, 1 mL (=0.008 M). In the second step, 0.5 M of each of the dopants viz., p-TSA and HCl was added in 100 mL of aqueous solution containing 0.25 M of APS (oxidant) under a constant stirring at room temperature. Subsequently the organic solution was added dropwise into the dopant-oxidant mixed solution and the reaction was



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allowed to occur for 12 h under constant stirring at room temperature. The immiscible xylene results droplet size of equal volume into the aqueous solution under the constant stirring, which provides relatively large interfacial area for polymerization. After complete polymerization, the precipitates were collected and washed with ethanol, acetone followed by double distilled water and then dried under vacuum at room temperature for 72 h. During polymerization, the pyrrole to APS molar ratio was kept as 1:1 in a total volume of 100 ml. In this method, the polymerization is characterized by the formation of walls via rapid polymerization of monomers at the surface of the water (aqueous) droplets or particles of dispersed core material as depicted in Fig. 1. As the monomer solution is dispersed in an aqueous phase, polymerization take place instantly at the surfaces of the water droplets in presence of the oxidant.

2.2. Characterization The morphology of the PPy powders was characterized with high resolution transmission electron microscope (HRTEM) Jeol, JEM-2100 model. The X-ray diffraction (XRD) patterns were recorded using a Rigaku Miniflex X-ray diffractometer in 2θ range of at a scan rate of 2°/min. The Fourier transmission infrared (FTIR) spectra were recorded using a Nicolet Impact 410 spectrometer. The UV-visible absorption spectra were recorded using a Lambda spectrometer in the wavelength of 300 nm to 900 nm. The thermal stability of PPy nanofibers was measured using a Perkin Elmer Thermo-gravimetric analyser (TGA) model STA-6000 under constant nitrogen atmosphere. The current-voltage (I-V) measurements and electrical conductivity on the pressed pellets of PPy nanofibers have been carried out using a Keithley source meter, model 2400 at room temperature. The room temperature electrical conductivity of the pressed pellets of PPy nanofibers has been measured with a four probe resistivity measurement set-up using the following equation21:


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Resistivity (ρ in ohm-cm) = πt (V/I) ln2 = 4.53 × t × (resistance)

(1)

Conductivity (σ in S/cm) = 1/ρ

(2)

3. Results and discussion 3.1. HRTEM studies The HRTEM micrographs of p-TSA and HCl doped PPy powders at different magnifications are shown in Fig. 2 (a-c). Both the micrographs show the formation of uniform, high yields nanofibrilliar morphology of PPy. The diameter of the resulting PPy nanofibers ranges from 16-20 nm with lengths of few microns. Uniform nanofibres with diameter 20 nm results for p-TSA doped PPy nanofibers, while the diameter of the HCl doped PPy nanofibers is 16 nm. Also, the p-TSA doped nanofibers are more twisted than that of the inorganic HCl doped PPy nanofibers. The selected area electron diffraction (SAED) pattern of the p-TSA doped PPy nanofibers is shown in Fig. 2 (d). The diffused ring pattern is attributed to the absence of long range order of PPy nanofibers chains doped with p-TSA.

3.2. Formation mechanism of PPy nanofibers Upon addition of pyrrole monomer solution drop wise to the aqueous (oxidant + dopant) solution under constant stirring at room temperature, thermodynamically stable spontaneous nano-droplets (monomer) form without any surfactant in the immiscible aqueous solution. The presence of small amount of xylene with monomer droplets provides higher interfacial area for polymerization. The polymerization occurs at the interfaces of the oilwater droplets in presence of the homogenously mixed APS oxidant molecules in the bulk solution. After a short introduction period, as the polymerization reaction begins, the transparent color of the bulk organic-aqueous bulk solution become darker and finally stops changing indicating the completion of the reaction. This oil-water interfacial polymerization



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does not rely on specific templates or structure directing molecules. Due to the homogenous mixture of the initiator molecules with the monomer molecules in bulk solution, each molecule of initiator (APS oxidant) is capable of producing nucleation centres for polymerization and thus results PPy nanofibers. This is the main reason for large scalable synthesis of PPy nanofibers using this approach. The amount of xylene present in the aqueous dopant-oxidant mixed solution greatly affects the morphology of the resulting PPy nanofibers. Due to the insolubility of xylene in aqueous water, under constant magnetic stirring it disperses in the reaction solution as liquid droplets. The interfacial area and the reaction sites for the polymerization of pyrrole monomer are greatly increased in presence of small amount of xylene and the efficiency of formation of PPy nanofibers significantly improves. However, the addition of large amount of xylene produces bigger size droplets in aqueous solution under constant magnetic stirring, thereby providing larger reaction space for polymerization of pyrrole monomer that can result in the longer chain PPy nanofibers with larger diameters nanofibers20.

3.3. XRD studies Fig. 3 shows XRD diffraction patterns for the p-TSA and HCl doped PPy nanofibers at room temperature. The broad diffraction peak centred around 2θ = 22° in p-TSA doped PPy nanofibers reveals the amorphous structure and short range arrangement of the PPy chains and this peak arises due to the diffraction from the parallel periodicity of the PPy chains22. However in case of HCl doped PPy nanofibers, the broad diffraction peak shifts to

2θ = 25° which signifies that HCl doped PPy nanofibers are denser than that of the p-TSA doped nanofibers. The p-TSA doped PPy nanofibers show 23 % crystallinity as compared to the HCl doped nanofibers that exhibit 18.60 % crystallinity. The larger electrostatic interaction between the SO42- dopant anions with the polymer chains due to delocalized


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charges on the dopant anions results in ordered alignment of the PPy chains in p-TSA doped PPy nanofibers, whereas the charges are highly localized on the chloride anions in HCl doped PPy nanofibers. The higher crystallinity in p-TSA doped PPy nanofibers can lead to higher electrical conductivity due to less scattering of the charge carriers23. The average crystallite size (L) calculated using the Scherrer formula24 for p-TSA doped PPy nanofibers is 6.81 Å while that for HCl doped nanofibers is 7.85 Å.

3.4. FTIR studies The FTIR spectra of both p-TSA and HCl doped PPy nanofibers are shown in Fig. 4. The strong vibration band positioned at 3400 cm-1 corresponds to the N-H stretching vibration25. It is observed that this particular band exhibits a low frequency shift for the pTSA doped PPy nanofibers, indicating presence of a larger number of hydrogen bonded N-H groups. The band at 1567 cm-1 is assigned to C=C or C-C stretching vibrations, whereas the band at 1474 cm-1 is attributed to the C-N stretching vibration in pyrrole rings26. The vibration band at about 1046 cm-1 is corresponded to the C-H deformation vibration, whereas the bands at 1210 cm-1 and 927 cm-1 are ascribed to the stretching vibrations in the doped PPy nanofibers26. Moreover, the vibration band at around 1700 cm-1 is ascribed to the presence of carbonyl group formed by the nucleophilic attack of water molecules during the preparation27. The band at 793 cm-1 is assigned to the C-H out-of-plane ring deformation of the pyrrole ring. A significant shifting of the vibration bands positioned at 927 cm-1 and 793 cm-1 (C-H out-of-plane ring deformation) to higher frequency of 1035 cm-1 and 805 cm-1 are observed for PPy nanofibers doped with p-TSA doped. Furthermore, the intensity of the vibration band positioned at 1210 cm-1 is higher in p-TSA doped PPy nanofibers than that for HCl doped PPy nanofibers. These findings suggest that the dopant has significant effect on the vibration modes of PPy nanofibers. This may be attributed to the better morphology with



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more ordered polymer chains in p-TSA doped PPy nanofibers which also corroborates the XRD results.

3.5. UV-visible studies The UV-vis absorption spectra of p-TSA and HCl doped PPy nanofibers are shown in Fig. 5. As observed from Fig. 5, the first absorption band in HCl doped PPy nanofibers positioned at 330 nm is due to π-π* interband transition i.e. transition from the HOMO (Highest occupied molecular orbital) to LUMO (Lowest unoccupied molecular orbital) of the neutral state of PPy, the second absorption band at about 470 nm represents a transition from the HOMO to the anti-bonding polaron state because of polaron absorption and a broad tail absorption band centred around 700 nm or above higher wavelength is assigned to a transition from the HOMO to the bipolaron band28. Thus UV-vis studies reveal the existence of both polarons and bipolarons as charge carriers in doped state of PPy. However, the band position corresponding to π-π* transition does not change in p-TSA doped PPy nanofibers, but the polaron band undergoes a red shifting to 510 nm which may be due to the higher conjugation length of charge carriers in p-TSA doped PPy nanofibers. Also, intensity of the polaron absorption band is more than that of the bipolaron band in both HCl and p-TSA doped PPy nanofibers suggesting that the polarons are dominant charge carriers in PPy nanofibers. The optical absorption is one of the most powerful tools to investigate the optical band gap energy of optically active materials. The optical properties of a material are governed by the interaction between the material and electric field of the electromagnetic wave. In compounds, a typical absorption band edge can be ascribed to any of the three processes: (i) residual below-gap absorption; (ii) Urbach tails and (iii) interband absorption29.


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The optical band gap energy ( E g ) of PPy nanofibers can be calculated using Tauc’s relation30,

αhν = A(hν − E g ) n

(3)

where A is a constant and the value of exponent n= 1 2 , 2 , 3 2 and 3 corresponds to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. The allowed direct band gap of PPy nanofibers can be determined by extrapolating the straight line portion of (αhν ) 2 vs. ( hν ) graphs to the (hν ) axis as shown in Fig. 6. As calculated from Fig. 6, value of E g corresponding to π-π* transition is 2.97 eV, which is same for both HCl and p-TSA doped PPy nanofibers. However, value of E g corresponding to polaron band in HCl doped PPy nanofibers are calculated as 2.37 eV, whereas its value is 2.24 eV for the p-TSA doped PPy nanofibers. The smaller optical band gap energy in p-TSA doped PPy nanofibers may be attributed to the formation of more polaron bands within their band gap. The absorption spectra in Fig. 5 show an extending tail for lower energies below the band edge. This corresponds to the transition from the localized states in the valence band tail, which is formed due to the extrinsic origins arising from the defects or impurities, to extended states in the conduction band. It is also assumed that the absorption coefficient near the band edge shows an exponential dependence on photon energy and this dependence is described by the Urbach formula31:

 hυ    EU 

α = α 0 exp

(4)

where α 0 is a constant and EU is Urbach energy interpreted as width of the exponential absorption edge. In both crystalline and amorphous materials, it is not known the origin of the exponential dependence of absorption coefficient on energy. This dependence may arise from



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the random fluctuations of the internal fields associated with the structural disorder in many amorphous materials32. The dependence of optical absorption coefficient with photon energy may be ascribed to the electronic transitions between the localized states. The logarithm of absorption coefficient α (ν ) as a function of photon energy (hν ) is plotted in Fig. 7 for PPy nanofibers doped with p-TSA and HCl. The values of Urbach energy EU is calculated by taking the reciprocal of the slopes of the linear portion in the region of lower photon energy of these curves, and are calculated as 6.25 eV and 5.56 eV for p-TSA and HCl doped PPy nanofibers, respectively. The density of these states falls exponentially with energy which is consistent with the Tauc’s theory. Eq. (3) can be written as

 β hυ    kT 

α = α 0 exp

(5)

where β is called the steepness parameter, which characterizes the broadening of the absorption edge due to electron-phonon interaction or exciton-phonon interaction33. If width of the edge, EU is related to Eq. (4), the parameter β is found as β = kT EU . The values of

β are calculated using this relationship taking T=300 K and are found to be 4.13 meV and 4.64 meV for the p-TSA and HCl doped PPy nanofibers, respectively. Values of optical band gap energy ( E g ) , Urbach energy ( EU ) and steepness parameter ( β ) for both the p-TSA and HCl doped PPy nanofibers are recorded in Table 1.

3.6. Thermogravimetric analysis The TGA thermographs for p-TSA and HCl doped PPy nanofibers are shown in Fig. 8 (a). As observed from Fig. 8(a), HCl doped PPy nanofibers undergoes three degradation stages, the first weight loss stage occurs from room temperature to 150 ºC which is due to the loss of moisture and volatile impurities present in the sample. The second stage of weight loss occurs in the temperature range of 150-427 ºC which is due to the elimination of the dopant 12 ACS Paragon Plus Environment

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anions from the PPy chains. The third and final weight loss stage takes place in the temperature range of 427-690 ºC, which is due to the complete degradation or decomposition of the polymer main chains and it shows a total of 88.13 % weight loss up to the temperature at around 680 ºC and beyond this temperature no more weight loss is observed. The p-TSA doped PPy nanofibers, on the other hand loss their weight via four degradation stages over the measured temperature regions. The first stage of weight loss occurs from room temperature to 130 ºC, which is same as that for HCl doped PPy nanofibers, while the second weight loss stage occurs from 160-290 ºC which may be the elimination of SO42- dopant anions (supplied from p-TSA dopant) from the polymer chains. The third weight loss begins at 290 ºC and continues to 385 ºC, which may be the decomposition of low molecular weight oligomers of PPy. The fourth and final weight loss occurs at and above 510 ºC with a total weight loss of 70 % up to the 785 ºC, which is ascribed to the degradation or decomposition of the main PPy chains. As observed from Fig. 7(a), the degradation temperatures at different percentage (%) weight losses for both the samples have been calculated and are recorded in Table 2. The degradation temperature at every percentage weight loss is higher for p-TSA doped PPy nanofibers than that for HCl doped PPy nanofibers. These studies reveal that pTSA doped PPy nanofibers are thermally more stable than that of HCl doped nanofibers, which can be correlated with XRD results where the polymer chain ordering is higher for pTSA doped PPy nanofibers. The derivative plots of TGA thermographs for both p-TSA and HCl doped PPy nanofibers are shown in Fig. 8(b). The onset decomposition temperature (Tonset), where the decomposition of the polymer chains begins is 427 °C for PPy nanofibers doped with HCl, whereas Tonset shifts to higher temperature 510 °C for p-TSA doped PPy nanofibers. Moreover, the decomposition rate of the main polymer chains corresponding to their maximum decomposition temperature (Tm) is smaller in PPy nanofibers doped with p-TSA



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doped than that for the HCl doped PPy nanofibers. The values of onset decomposition temperature (Tonset), maximum decomposition temperature (Tm) and decomposition rate for pTSA and HCl doped PPy nanofibers are recorded in Table 3. These results further confirm the higher thermal stability of p-TSA doped PPy nanofibers.

3.7. Current-Voltage (I-V) characteristics The room temperature (300 K) current-voltage characteristics (I-V) of PPy nanofibers doped with p-TSA and HCl are shown in Fig. 9. As observed from the figure, the I-V characteristics of both p-TSA and HCl doped PPy nanofibers show nonlinear behavior which is symmetric with respect to the both polarity in the applied voltages of -10 V to + 10 V. As depicted in Fig. 8, at a particular applied voltage, the value of current is higher for p-TSA doped PPy nanofibers than that for HCl doped nanofibers, which may result from the availability of increased charge carries as well as from the enhanced crystallinity of the polymer chains in PPy nanofibers doped with p-TSA due to strong electrostatic interaction between the dopant ions and polymer backbone chains. The insight into the conduction in PPy nanofibers can be obtained by plotting the current-voltage (I-V) data on a log-log scale as shown in Fig. 10. The I-V characteristics show two distinct linear regions over the applied voltage region (-10 V to + 10 V), with a gradual transition between the two regions. The two distinct linear regions on the log-log plot can be fitted to a power law equation with different exponents. The power-law can be expressed as follows,

I = KV m

(6)

where K is a constant and m is the exponent, which can be determined from the slope of the fitted curve. At low voltages (0 6 V), the charge transport mechanism in both the p-TSA and HCl doped PPy nanofibers is dominated by the space-charge-limited conduction. However, the critical voltage, Vc at which the spacecharge-limited conduction dominates, is smaller for p-TSA doped PPy nanofibers. The enhanced thermal stability of p-TSA doped PPy nanofibers may be ascribed to the higher crystallinity of the PPy nanofibers as a result of stronger electrostatic interaction of the larger p-TSA anions with that of the polymer chains. The scalable and low cost synthesis of PPy nanofibers reported in this work may be explored such as supercapacitors, sensors, molecular wires and composite materials.

Acknowledgement The research is funded by the Inter University Accelerator Centre (IUAC), New Delhi, India through the grant IUAC/XIII.7/UFR-48310/1985 dated July 16, 2010. The


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authors acknowledge the Sophisticated Analytical Instrument Facility (SAIF), North-Eastern Hill University (NEHU), Shillong for the help during HRTEM imaging.

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(2) Baker, C. O.; Huang, X.; Nelson, W.; Kaner R. B. Polyaniline Nanofibers: Broadening Applications for Conducting Polymers. Chem. Soc. Rev. 2017, 46, 1510-1525. (3) Ghosh, S.; Maiyalaganb, T.; Basu R. N. Nanostructured Conducting Polymers for Energy Applications: Towards a Sustainable Platform. Nanoscale, 2016, 8, 6921-6947. (4) Tran, H. D.; Li, D.; Kaner, R. B. One-Dimensional Conducting Polymer Nanostructures: Bulk Synthesis and Applications. Adv. Mater. 2009, 21, 1487-99. (5) MacDiarmid, A. G. Synthetic Metals: A Novel Role for Organic Polymers. Angew. Chem. Int. Ed. 2001, 40, 2581-2890. (6) Li, C.; Bai, H.; Shi, G. Q. Conducting Polymer Nanomaterials: Electrosynthesis and Applications. Chem. Soc. Rev. 2009, 38, 2397-3409. (7) Wu, C. G.; Bein, T. Conducting Polyaniline Filaments in a Mesoporous Channel Host. Science, 1994, 264, 1757-1759. (8) Martin, C. R. Membrane-Based Synthesis of Nanomaterials. Chem. Mater. 1996, 8, 1739-1746. (9) Wang, C. W.; Wang, Z.; Li, M. K.; Li, H. L. Well-Aligned Polyaniline Nano-Fibril Array Membrane and its Field Emission Property. Chem. Phy. Lett. 2001, 341, 431-434. (10) Michaelson, J. C.; McEvoy, A. J. Interfacial Polymerization of Aniline. Chem. Commun.

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(11) Pan, L.; Qiu, H.; Dou, C.; Li, Y.; Pu, Lin.; Xu, J.; Shi, Yi. Conducting Polymer Nanostructures: Template Synthesis and Applications in Energy Storage. Int. J. Mol. Sci. 2010, 11, 2636-2657. (12) Huang, J. Polyaniline Nanofibers: Facile Synthesis and Chemical Sensors. J. Am. Chem. Soc. 2003, 125, 314-315. (13) Yang, X. Polypyrrole Nanofibers Synthesized via Reactive Template Approach and their NH3 Gas Sensitivity. Synth. Met. 2010, 160, 1365-1367. (14) Li, X. Template-Sacrificed Synthesis of Polypyrrole Nanofibers for Lithium Battery. J. Mater. Sci. 2016, 51, 9526-9533. (15) Virji, S.; Huang, J.; Kaner, R. B.; Weiller, B. H. Polyaniline Nanofiber Gas Sensors: Examination of Response Mechanisms. Nano Lett. 2004, 4, 491-496. (16) Tran, H. D.; Shin, K.; Hong, W. G.; D'Arcy, J. M.; Kojima, R. W.; Weiller, B. H.; Kaner R. B. A Template-Free Route to Polypyrrole Nanofibers. Macromol. Rapid Commun. 2007, 28, 2289-2293. (17) Wu, A.; Kolla, H.; Manohar, S. K. Chemical Synthesis of Highly Conducting Polypyrrole Nanofiber Film. Macromolecules 2005, 38, 7873-7875. (18) Chronakis, I. S.; Grapenson, S.; Jakob, A. Conductive Polypyrrole Nanofibers via Electrospinning: Electrical and Morphological Properties. Polymer, 2006, 47, 15971603. (19) Zang, J.; Li, C. M.; Bao, S. J.; Cui, X.; Bao, Q.; Sun, C. Q. Template-Free Electrochemical Synthesis of Superhydrophilic Polypyrrole Nanofiber Network. Macromolecules, 2008, 41, 7053-7057. (20) Xing, S.; Zheng, H.; Zhao, G. Preparation of Polyaniline Nanofibers via a Novel Interfacial Polymerization Method. Synth. Met. 2008, 158, 59-63.


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(21) Bose, S.; Kuila, T.; Uddin, M. E.; Kim, N. H.; Lau, A. K. T.; Lee, J. H. In-Situ Synthesis and Characterization of Electrically Conductive Polypyrrole/Graphene Nanocomposites. Polymer, 2010, 51, 5921-5928. (22) Wynne, K. J.; Street, G. B. Poly (pyrrol-2-ylium tosylate), Electrochemical Synthesis and Physical and Mechanical Properties. Macromolecules, 1985, 18, 2361-2368. (23) Jeong, C.K.; Jung, J. H.; Kim, B. H.; Lee, S. Y., Lee, D. E.; Jang, S. H.; Ryu, K.S.; Joo, J. Electrical, Magnetic, and Structural Properties of Lithium Salt Doped Polyaniline. Synth. Met. 2001, 117, 99-103. (24) Cai, J.; Yu, Q.; Zhang, X.; Lin, J.; Jiang, L. Control of Thermal Cross-Linking Reactions and the Degree of Crystallinity of Syndiotactic 1, 2-Polybutadiene. J. Polym. Sci. Part B Polym. Phys. 2005, 43, 2885-2897. (25) Chalmers, J. M.; Hannah, R. W.; Mayo, D. W. Spectra-Structure Correlation: Polymer Spectra. Handbook of Vibrational Spectroscopy, John Wiley and Sons, New York, 2002, 1783-1812. (26) Jeon, S. S.; Park, J. K.; Yoon, C. S.; Im S. S. Organic Single-Crystal Surface-Induced Polymerization of Conducting Polypyrroles. Langmuir, 2009, 25, 11420-11424. (27) Maia, G.; Ticianelli, E. A.; Nart, F. C. FTIR Investigation of the Polypyrrole Oxidation in Na2SO4 and NaNO3 Aqueoussolutions. Zeitschrift für Physikalische Chemie International journal of research in physical chemistry and chemical physics. 1994, 186, 245-257. (28) Zang, J.; Li, C. M.; Bao, S. J.; Cui, X.; Bao, Q.; Sun, C. Q. Template-Free Electrochemical Synthesis of Superhydrophilic Polypyrrole Nanofiber Network. Macromolecules, 2008, 41, 7053-7057. (29) Mott, N.F.; Davis, E.A. Electronic Processes in Non-crystalline Materials. Clarendon Press Oxford, 1979, 1-583.



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Figure Captions Fig. 1: Pictorial diagram for synthesis of PPy nanofibers using oil-water interfacial polymerization method.

Fig. 2: High magnification HRTEM micrographs of (a) p-TSA and (b) HCl doped with PPy nanofibers, (c) low magnification HRTEM micrographs of p-TSA doped with PPy nanofibers and (d) SAED pattern of p-TSA doped PPy nanofibers.

Fig. 3: XRD diffraction patterns for (a) p-TSA and (b) HCl doped PPy nanofibers. Fig. 4: FTIR spectra for (a) p-TSA and (b) HCl doped PPy nanofibers. Fig. 5: Plots of UV-vis absorption spectra for (a) p-TSA and (b) HCl doped PPy nanofibers at room temperature.

Fig. 6: Plots of (αhν ) 2 vs. (hν ) for (a) p-TSA and (b) HCl doped PPy nanofibers.


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Fig. 7: Plots of logarithm of absorption coefficient α (ν ) vs. photon energy (hν ) for (a) pTSA and (b) HCl doped PPy nanofibers at room temperature.

Fig. 8: Plots of (a) TGA thermographs and (b) derivative plots of TGA thermographs for pTSA and HCl doped PPy nanofibers.

Fig. 9: Room temperature (300 K) current-voltage characteristics (I-V) of PPy nanofibers doped with (a) p-TSA and (b) HCl.

Fig. 10: Plots of forward current-voltage (I-V) data on a log-log scale for (a) p-TSA and (b) HCl doped PPy nanofibers at room temperature (300 K).

Table Captions Table 1: Values of optical band gap energy ( E g ) , Urbach energy ( EU ) and steepness parameter ( β ) for doped p-TSA and HCl doped PPy nanofibers.

Table 2: Degradation temperatures at different percentage (%) weight losses for both p-TSA and HCl doped PPy nanofibers.

Table 3: Values of onset decomposition temperature (Tonset), maximum decomposition temperature (Tm) and decomposition rate for p-TSA and HCl doped PPy nanofibers.



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List of Figures

Fig. 1

Fig. 2


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Fig. 3

Fig. 4

Fig. 5



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Fig. 6

Fig. 7

Fig. 8


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Fig. 9

Fig. 10

List of Tables Table 1

PPy nanofibers

Optical band gap energy,

Urbach energy,

Steepness parameter,

(eV)

(eV)

(meV)

HCl doped

2.37

5.56

4.13

p-TSA doped

2.24

6.25

4.64



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Table 2 Degradation temperature (°C) at different percentage (%) weight losses PPy 5

10

20

30

40

50

60

65

%

%

%

%

%

%

%

%

HCl doped

94

240

326

397

451

503

544

567

p-TSA doped

114

265

364

471

572

646

710

750

nanofibers

Table 3 PPy

Onset

decomposition

Maximum decomposition

Decomposition rate

nanofibers

temperature, Tonset (°C)

temperature, Tm (°C)

HCl doped

427

588

0.258

p-TSA doped

510

625

0.047

TOC Graphic


(dW/dT)m, (%/°C)

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TOC Graphic 84x47mm (96 x 96 DPI)

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Scalable and Low Cost Synthesis of Highly Conducting Polypyrrole

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