Multiwalled Carbon

Article pubs.acs.org/IECR

Silver Nanoparticles Decorated Polyaniline/Multiwalled Carbon Nanotubes Nanocomposite for High-Performance Supercapacitor Electrode Saptarshi Dhibar and Chapal Kumar Das* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, India ABSTRACT: In the modern era, it is still a challenge to develop an easy, inexpensive, and scalable technique to fabricate an energy storage system. Here, a low-cost and simple process was used to fabricate a silver−polyaniline/multiwalled carbon nanotubes ((Ag-PANI)/MWCNTs) nanocomposite for high-performance supercapacitor electrodes. The possible interactions between Ag and PANI were characterized by Fourier transform infrared and UV−visible spectroscopies. Morphological study confirmed the formation of Ag nanoparticles in the PANI surface, and the MWCNTs were uniformly coated by PANI with the presence of Ag nanoparticles. The nanocomposite showed better electrical conductivity of 4.24 S/cm at room temperature and also attained nonlinear current−voltage characteristics. The highest specific capacitance of 528 F/g has been obtained for the nanocomposite at 5 mV/s scan rate. The nanocomposite also showed better energy as well as power density. Ag-PANI/CNT based supercapacitors with outstanding energy and power density make them a potentially promising candidate for future energy storage systems. and mechanical flexibility.7−9 Carbon based materials, including activated carbon, carbon nanotubes (CNTs), and graphene, have been used in electrochemical double layer supercapacitors due to their outstanding chemical and physical properties.10−13 The higher conductivity and better charge transfer channels of CNTs make them a most promising material for energy saving applications. In recent times CNTs have been regarded as highpower electrode materials due to their improved electrical conductivity as well as high accessible surface area. It was also observed that the introduction of multiwalled carbon nanotubes (MWCNTs) into the polymeric matrices enhances the specific surface area and thereby improves the electrical conductivity and mechanical properties.14,15 Polymer metal based nanocomposites studies have generated enormous research interests due to their exciting properties. Out of these metals, silver (Ag) is an important metal for producing nanoparticles due to its high conductivity and also high thermal stability. Silver nanoparticles have potential applications in conductive inks, catalysis, adhesives for many electronic components, and photonics and also in the photography industry.16,17 Introduction of Ag into the various polymer matrices enhances the thermal, optical, mechanical, and conducting properties resulting in a new class of important materials suitable for their applications in memory devices and sensors.18,19 In recent time many efforts have been focused on the synthesis of Ag-PANI and Ag-PANI/CNT composites, using various synthetic procedures and being analyzed by different characterization techniques. Oliveira et al. reported silver nanoparticles/polyaniline composites synthesized by two-

1. INTRODUCTION In response to the fossil fuel energy reduction and global warming problems, energy storage becomes an enormous challenge to the major world powers and scientific society.1 With the rapid-growing market for handy electronic devices and the development of hybrid electric vehicles, there has been a rising and urgent demand for energy storage devices that are high power density and energy density. Though Li ion batteries have excellent energy performance, their lower power performance restricts them from many power demanding applications. Supercapacitors, also known as electrochemical double layer capacitors (EDLCs) or ultracapacitors, have consumed much more attention in recent years due to their long life cyclic, pulse power supply, high dynamics of charge propagation, and easy operational mechanism.2 They have comparatively large energy density and high power capability compared to conventional capacitors, which allow supercapacitors to be applied in a varity of energy storage systems. A latest application is the use of supercapacitors in emergency doors on Airbus A380 airplanes2 and they are also used in memory back-up systems, industrial power supplies, consumer electronics, and energy management.3 Based on the different energy storage mechanisms, supercapacitors can be divided into two categories: (i) EDLCs which store energy by the adsorption of both cations and anions and (ii) pseudocapacitors that store energy through fast surface redox reaction. Conducting polymers have received a lot of research interest due to their high conductivity and have become trendy basic materials for advanced applications such as energy storage and conversion devices, electrochromic displays, batteries, membranes, sensors, and anticorrosive coatings, etc.4−6 Among all of the conducting polymers polyaniline (PANI) has been attracting much more attention due to its superior properties such as easy synthesis, being quite cheap, high electrical conductivity, chemical stability, good environmental stability, © 2014 American Chemical Society

Received: Revised: Accepted: Published: 3495

July 8, 2013 February 10, 2014 February 13, 2014 February 13, 2014 dx.doi.org/10.1021/ie402161e | Ind. Eng. Chem. Res. 2014, 53, 3495−3508

Industrial & Engineering Chemistry Research

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about their potential application in conducting inks, nobel metal recovery, printed electronics, antimicrobial materials, catalysts, and sensors.38 So we have seen that there are numerous works done by various research groups on Ag-PANI and Ag-PANI/CNT nanocomposites, although the synthesis of silver nanoparticles by using dodecylbenzenesulfonic acid (DBSA) and silver nitrate (AgNO3) in the presence of PANI and MWCNTs is not yet done. To the best of our knowledge, there are no scientific reports also available on the electrochemical properties of Ag-PANI/CNT nanocomposites. So, our main motivation of this work to synthesized silver nanoparticles decorated PANI−MWCNTs nanocomposites for high-performance supercapacitor electrode materials. In this work, pure PANI, Ag-PANI, and A-PANI/CNT nanocomposites were synthesized by in situ polymerization methods using ammonium persulfate (APS) as an oxidizing agent in the presence of DBSA and AgNO3 and investigated as electrode materials for supercapacitors. Though AgNO3 can be used as an oxidant for the polymerization of aniline, the rate of polymerization is very slow.39 However, in this present study AgNO3 is used as a precursor material25 for the synthesis of Ag nanoparticles, which also acted as an oxidant for the polymerization process. On the other hand APS is used as oxidant for the polymerization of aniline as the polymerization becomes faster in the presence of APS. The electrochemical properties of all of the composites were investigated by cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy (EIS) analysis with a three-electrode system. Further, the thermal, morphological and electrical properties as well as chemical interactions of all of the composites were examined by various characterization techniques.

phase water/toluene interfacial reactions and found the conductivity of 2.3 S/cm and diode-like I−V characteristics for the composites.20 Patil et al. synthesized a Ag/PANI electrode by chemical polymerization techniques and obtained the highest specific capacitance of 512 F/g at 5 mV/s scan rate, energy density of 50.01 Wh/kg at 1 mA/cm2, and direct current (DC) electrical resistivity of 5.19 × 102 Ω at 0.9 wt % doping of Ag.21 Gao et al. reported silver@polyaniline nanofibers through a simple mix-and-wait method and get core−shell nanofibers from morphological analysis.22 The highest specific capacitance of 850 F/g at 10 mV/s sweep rate was achieved for polyaniline−Ag nanocable arrays that were reported by Xie et al.23 Xia et al. reported polyaniline−silver complex synthesized by in situ techniques and obtained a maximum conductivity of 535.22 S/cm.24 Tamboli et al. synthesized the Ag-PANI nanocomposite by in situ chemical polymerization methods and observed excellent antibacterial activity for Ag-PANI nanocomposite.25 Correa et al. describe a one-pot synthesis of Ag-PANI nanocomposite and obtained the electrical conductivity of 100 S/cm at room temperature.26 Bober et al. reported polyaniline−silver composites synthesized by oxidation of aniline with silver nitrate in formic acid solutions and found the electrical conductivity of 43 S/cm.27 Li et al. synthesized Ag-PANI nanocomposite via in situ ultraviolet photoredox mechanism and obtained the electrical conductivity of 96 S/cm and higher specific capacitance for the Ag− polyaniline nanocomposite.28 Jia et al. reported Ag-PANI nanocomposites synthesized by one-step approach without using any reducing agent and achieved their antimicrobial effectiveness on Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), and yeast bacteria.29 Karim et al. prepared Ag-PANI nanocomposites by in situ γ-radiation-induced chemical polymerization methods and obtained core−shell morphology and also better thermogravimertic stability for the Ag-PANI nanocomposite.30 Stejskal et al. synthesized Ag-PANI composite and found electrical conductivity of 22.8 S/cm.31 Kang et al. reported Ag-PANI nanocomposites prepared by γ-rayirradiation method and saw hexangular-type morphology for the Ag nanoparticles in the Ag-PANI nanocomposite when the Ag nanoparticles were protected by aniline molecule.32 Narang et al. synthesized Ag-PANI/CNT composites by using electrochemical techniques and investigated the biosensor application and obtained rapidity (4 s), detection limit (0.3 μM), sensitivity (0.477 μA/μM), and storage stability (3 months) for the Ag-PANI/CNT nancoomposite.33 Reddy et al. prepared Ag-PANI/CNT nanocomposite synthesized by in situ chemical oxidative polymerization techniques and achieved the room-temperature electrical conductivity of 5.04 S/cm for AgPANI/CNT composite.34 Nguyen et al. reported Ag-PANI/ CNT nanocomposites by in situ polymerization and found the highest electrical conductivity of 15.4 S/cm for the Ag-PANI/ CNT nanocomposite and also expected to have potential applications as cathode materials for lithium batteries and supercapacitor due to high electrochemical activity and better cyclic stability.35 Grinou et al. synthesized Ag-PANI/CNT hybrid nanocomposites by emulsion polymerization methods and obtained better electrical conductivity of 1.685 S/cm for the Ag-PANI/CNT nanocomposite.36 Ciric-Marjanovic reported a review article based on PANI composites with different metals, metalloids, and nonmetals, PANI composites with CNTs and graphene, and also PANI/metal/nonmetals ternary composites.37 Stejskal also reported a review article based on conducting polymer−silver composites and discussed

2. EXPERIMENTAL SECTION 2.1. Materials Used. The monomer, aniline, used in this study was supplied by Merck, Darmstadt, Germany. Ammonium persulfate [(NH4)2S2O8] used in this study was also supplied by Merck. Dodecylbenzenesulfonic acid (DBSA) and silver nitrate (AgNO3) were obtained from Sigma-Aldrich, Bangalore, India. MWCNTs (MWCNT-100) were purchased from Iljin Nanotechnology, Seoul, South Korea. These MWCNTs had a length of 20 μm, a diameter of 10−20 nm, and aspect ratios of 1000. All of the solutions were made using double distilled water during the synthesis. Dimethylformamide (DMF) was purchased from Merck. 2.2. Modification of the MWCNTs. Initially, 1 g of raw MWCNTs was mixed with mixed acid solution containing 3 mol/L concentrated H2SO4 and 1 mol/L concentrated HNO3 (H2SO4:HNO3 = 3:1) solutions. At 1:100 weight ratios, the MWCNTs were mixed with the concentrated acid solutions. After adding the MWCNTs to the mixed acid, the whole solution was stirred for 24 h at 60 °C. After that, the whole solution was washed with double distilled water until the pH of the solution becomes neutral. Then the resulting solution was centrifuged for 15 min at 3000 rpm to separate out the product. Lastly, it was transferred into a Petri dish and dried at 100 °C for 24 h to get carboxylic acid functionalized MWCNTs (AMWCNTs). 2.3. Synthesis of PANI. Pure PANI was synthesized by chemical oxidative polymerization techniques where APS was used as oxidant.40 Briefly, 1 mL of aniline was dissolved in 100 mL of double distilled water in a 200 mL beaker. After that, this solution was stirred for 15 min. In another beaker, 2 g of APS 3496

dx.doi.org/10.1021/ie402161e | Ind. Eng. Chem. Res. 2014, 53, 3495−3508


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was dissolved in 50 mL of double distilled water. Then, the APS solution was added drop by drop to the aniline solution. After the addition of ASP solution, the total solution became deep greenish in color. The entire solution was then stirred at constant speed for 5 h at room temperature. After the polymerization process, the whole solution was filtered, washed several times with double distilled water and ethanol, and dried at 60 °C for 12 h, and pure PANI was obtained. 2.4. Synthesis of Ag-PANI Composite. The Ag-PANI composite was synthesized by in situ chemical polymerization methods. In a round-bottom flask, added 0.01 M AgNO3 solution made with double distilled water and stirred for 30 min using a magnetic stirrer at room temperature. Then 0.1 M DBSA solution was added, and stirring continued for another 15 min. The precooled solution of aniline monomer (0.01 M) was then added to the above solution with stirring for 8 h at room temperature. After that, the oxidizing agent, APS (0.01 M), was added drop by drop to initiate the polymerization of aniline. The monomer and oxidizing agent ratio was kept as 1:2. The whole solution was again stirred for another 8 h at room temperature, in order to ensure complete polymerization of aniline. The greenish black precipitate was then filtered and washed several times with double distilled water and ethanol, to remove the unreacted monomer, excess acid, and oxidant. Finally the product was dried in an oven at 80 °C for 24 h.25 2.5. Synthesis of Ag-PANI/CNT Nanocomposite. For the synthesis of nanocomposites, a typical in situ chemical polymer technique was used. In 150 mL of double distilled water, 1.24 g of cetyltrimethylammonium bromide (CTAB) and 60 mg of A-MWCNTs were added and sonicated for 45 min at room temperature. In another round-bottom flask, 100 mL of 0.01 M AgNO3 solution was taken and stirred for 30 min at room temperature. Then, 0.01 M DBSA solution was added to the above solution and stirred for another 15 min. After that, the well-dispersed suspension of the A-MWCNTs solution was added to the AgNO3 solution and stirred for 15 min. The precooled 0.01 M aniline solution was added dropwise and stirred for 8 h at room temperature. Afterward, a 0.01 M solution of APS was added drop by drop to start the polymerization of aniline. The monomer and oxidizing agent ratio was the same as before. The reaction was further continued with stirring for another 8 h at room temperature. Then, the whole solution was kept at 1−5 °C for 12 h for complete polymerization. After that, the greenish black precipitate was filtered and washed several times with double distilled water and ethanol, to remove the impurities. Lastly, the product was vacuum-dried at 80 °C for 24 h to extract the nanocomposite. The schematic representation of the synthesis of the nanocomposite is illustrated in Figure 1. The compositions of the composites are given in Table 1. Mainly CTAB was used in our study to disperse the carbon nanotubes. Here CTAB acted as surfactant. When the CTAB was added to the solution, it adsorbed on the nanotubes surface, and further ultrasonication helped the CTAB to debundle the carbon nanotubes by static or electrostatic repulsion.41 Also, addition of CTAB created micelles around the carbon nanotubes surface. Due to the formation of micelles, they repelled each other by electrostatic repulsion and converted from their network or coillike structure to a nonnetwork structure. Homogeneous dispersion is also required for uniform coating on carbon nanotubes surfaces. Here CTAB serves the purpose appropriately.

Figure 1. Schematic representation of the nanocomposite synthesis process.

Table 1. Composition of the Composites sample codes

aniline (wt %)

AgNO3 (wt %)

A-MWCNTs (wt %)

PANI Ag-PANI Ag-PANI/CNT

100 92 86

0 8 8

0 0 6

3. CHARACTERIZATION 3.1. Fourier Transform Infrared Analysis. To characterize the bonding properties of the nanocomposite, Fourier tranform infrared (FTIR) analysis was carried out. The FTIR spectra of pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite were recorded using a NEXUS 870 FT-IR (Thermo Nicolet) instrument in the range from 400 to 4000 cm−1. The samples were prepared in the pellet form using spectroscopic grade potassium bromide (KBr) powder. The KBr and the composites were mixed at a weight ratio of 10:1. 3.2. UV−Visible Spectroscopy. The UV−visible spectra of pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite were recorded using a Perkin-Elmer, Lambda 750 spectrophotometer. The UV−visible spectra were taken within the wavelength range of 200−800 nm. UV−visible spectroscopy analysis was performed by dissolving all of the electrode materials in DMF solvent. 3.3. X-ray Diffraction Study. X-ray diffraction analysis of the powder samples was performed by a PW X-ray diffractometer with Cu Kα target (λ = 0.15404 nm) at 2 mm slits at a scanning rate of 2 deg/min with operating voltage and current at 40 kV and 20 mA, respectively. The polymeric materials were scanned in the 2θ range from 10 to 80°. 3.4. Field Emission Scanning Electron Microscopy Analysis. To examine the surface morphology of the pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite, a field emission scanning electron microscope (Carl Zeiss-SUPRA 40 field emission scanning electron microscope) was used. The powder samples were dispersed in acetone and put dropwise in aluminum foil. This foil was then placed in to carbon tap and sputter coat with gold. The field emission scanning electron microscopy (FESEM) micrographs were taken at an operating voltage of 40 kV. To check the elemental composition of the polymeric materials, energy-dispersive X-ray spectroscopy (EDX) analysis was carried out. 3.5. Transmission Electron Microscopy Analysis. To examine the formation of Ag nanoparticles in the PANI surface 3497



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Figure 2. FTIR spectra of (a) pristine MWCNTs and A-MWCNTs and (b) PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite.

where ρ is the resistivity (ohm·cm), V is the measured voltage, I is the source current, and tp is the thickness of the pellets. 3.9. Current−Voltage (I−V) Relationship. The current− voltage (I−V) characteristics of pure PANI, Ag-PANI, and AgPANI/CNT nanocomposite were recorded by use of a Keithley 2400 source meter at room temperature at 0.1 V/s scan rate. 3.10. Thermogravimertic Analysis. The thermal stability of pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite were studied by thermogravimertic analyzer (TGA 209F, NETZSCH, Seib, Germany). The TGA analysis was performed with a heating rate of 10 °C/min under nitrogen atmosphere.

and also the coating of PANI on A-MWCNTs surfaces, analysis was done by transmission electron spectroscopy (HR-TEM, JEOL 2100). A small amount of sample was put in the acetone and sonicated for 20 min. From this well-dispersed solution, one drop of solution was placed on the copper grid for TEM analysis. 3.6. Atomic Force Microscopy Analysis. Atomic force microscopy (AFM) analysis was studied by an Agilent 5500 AFM (N9410S) system mounted on a vibration controlled box to check the surface morphology of the Ag-PANI and AgPANI/CNT nanocomposite. The AFM analysis was carried out by silicon nitride tip in tapping mode. 3.7. Electrochemical Characterization. Electrochemical studies such as CV, GCD, and EIS were carried on a GAMRY Reference 3000 instrument (750 mA and 1.6 V) with a threeelectrode system, where platinum and saturated calomel electrodes (SCE) were used as counter and reference electrodes, respectively. CV measurement was performed in a 1 M KCl solution at various scan rates from 5 to 200 mV/s. The specific capacitances of all of the electrode materials were calculated by the following equation.42−44 Csp = (I+ − I −)/vm

4. RESULTS AND DISCUSSION 4.1. FTIR Analysis. Figure 2a shows the FTIR spectra of raw and acid functionalized MWCNTs. The A-MWCNTs show typical peaks at 1212, 1400, 1548, 1632, and 1723 cm−1. The characteristic peaks at 1212 and 1723 cm−1 represent the C−O stretching and CO stretching of carboxylic acids, respectively, in the A-MWCNTs. The typical peaks at 1400 and 1548 cm−1 correspond to O−H bending and CC stretching, respectively. Further, the characteristic peak at 1632 cm−1 represents the H-bonded carbonyl group (CO) conjugates with CC in the graphene wall.45 A broad absorption band at 3415 cm−1 further confirms the presence of hydroxyl groups (-OH) on the A-MWCNTs surface, which might arise from either ambient atmosphere moisture or oxidation during acidification of pristine MWCNTs.46 Figure 2b represents the FTIR spectra of pure PANI, AgPANI, and Ag-PANI/CNT nanocomposite. Pure PANI showed absorption bands at 1594 cm−1 (CC stretching deformation of quinonoid), 1482 cm−1 (benzenoid ring), 1310 cm−1 (C−N stretching vibration), 1130 cm−1 (NQN, Q denotes quinonoid), and 812 cm−1 (C−H out of plane bending vibration), respectively.47 Similar bands with some additional bands are observed for Ag-PANI and Ag-PANI/CNT nanocomposite. The broad band at 3455 cm−1 and the characteristic peak at 3258 cm−1 were due to the N−H stretching vibration. The absorption peaks at 2946 and 2876 cm−1 correspond to C−H stretching vibration. The characteristic peak at 1594 cm−1 shifted to 1606 and 1608 cm−1, the peak at 1482 cm−1 shifted to 1523 and 1519 cm−1, and the peak at 1310 cm−1 shifted to 1320 and 1323 cm−1, respectively, for Ag-PANI and Ag-PANI/ CNT nanocomposite. This peak shifting phenomenon indicates the presence of interaction between PANI and Ag.21 Also, the

(1)

where I+ and I− are maximum currents in positive and negative voltage scan, respectively, v is the scan rate, and m is the mass of the composite materials. A 1 M aliquot of aqueous KCl solution was used for impedance measurements. The samples were prepared by pressing the composite materials at 8 MPa pressure. No polymer binder was used for electrochemical characterization. 3.8. Electrical Conductivity Measurements. The electrical conductivity of the pure PANI, Ag-PANI, and Ag-PANI/ CNT nanocomposite were measured by conventional fourelectrode methods (Lakeshore resistivity and Hall measurement setup) with compressed pellets. The pellets are prepared by pressing the samples at 8 MPa pressure. All of the pellets were about 0.05 cm thick. The electrical conductivity (σ) was calculated by using the following equation: ρ = πt p/ln 2(V /I ) = 4.53t p × resistance σ /(S/cm) = 1/ρ 3498



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Figure 3. UV−visible spectra of (a) A-MWCNTs, (b) PANI, (c) Ag-PANI, and (d) Ag-PANI/CNT nanocomposite.

characteristic peak at 1130 cm−1 shifted to 1156 cm−1 for AgPANI/CNT nanocomposite due to strong electrostatic interaction between the Ag and the PANI functionalized AMWCNTs, signifying that there was an effective increase in the conductivity of the polymer chains.34 It is observed that the peak intensity of all of the electrode materials is more or less the same, though Ag-PANI and Ag-PANI/CNT nanocomposite showed a small decrease in peak intensity as compared to PANI. This small decrease in peak intensity is due to the presence of Ag nanoparticles and also the presence of AMWCNTs in the nanocomposite. For both Ag-PANI and AgPANI/CNT nanocomposite a peak at 1040 cm−1 is observed which corresponds to the stretching vibration of the -SO3H group, which arises from the incorporation of DBSA. Additionally, the band at 650 cm−1 is observed for Ag-PANI/ CNT nanocomposite, corresponding to the C−S stretching of the benzenoid ring of DBSA.25 4.2. UV−Visible Spectroscopy Analysis. The UV−visible spectrum was used to examine the electronic properties of the polymer and the nanocomposite. Figure 3 shows the UV− visible spectra of A-MWCNTs, PANI, Ag-PANI and Ag-PANI/ CNT nanocomposite, respectively. As shown in Figure 3a, no absorption peaks were observed for A-MWCNTs. Pure PANI exhibited two absorption bands at 365 and 600 nm, shown in Figure 3b. The band at 365 nm is attributed to a π−π′ transitions in the benzonoid units of the PANI chains, while the absorption band at 600 nm is attributed to the exciton-like transition in quinonoid units. After magnifying the UV−vis spectrum of PANI (500−800 nm), it is clearly seen that the increase in the absorption from 650 nm to higher up to 800 nm (indicated as a red arrow in Figure 3b). This is the characteristic of PANI in its most conductive form. The magnified portion (500−800 nm) is put into the inset portion

of the UV−vis spectrum of pure PANI in Figure 3b. For AgPANI two absorption bands are observed at 377 and 502 nm, Figure 3c. The red shift of the absorption band from 365 to 377 nm may be due to the interaction of Ag to the PANI chains. The absorption band at 502 nm indicates the surface plasmon resonance of Ag nanoparticles.48 The Ag-PANI/CNT nanocomposite exhibited two absorption bands at 347 and 602 nm shown in Figure 3d. Here the characteristic band at 602 nm is observed due to the deprotonation of PANI. The blue shift in the band indicates the interaction between quinonoid and benzenoid rings and the A-MWCNTs. Here the plasmon band is absent. The band may be overlapped by stronger absorption of PANI and cause the shift of the absorption maximum of the emeraldine base to lower wavelengths.48 4.3. XRD Analysis. The XRD patterns of pure PANI, AgPANI, and Ag-PANI/CNT nanocomposite are shown in Figure 4. For the Ag-PANI/CNT nanocomposite, the sharp crystalline peaks appearing at 2θ values of 38.10°, 44.28°, 64.44°, and 77.38° corresponded to the face-centered cubic (fcc) phase of silver (111), (200), (220), and (311), respectively.49 The same crystalline peaks were observed for the Ag-PANI composite. The existence of sharp peaks in both Ag-PANI and Ag-PANI/ CNT nanocomposite clearly indicates the presence of Ag nanoparticles in the nanocomposite with their crystalline nature. For pure PANI two broad peaks at 19.9° and 25.38° were observed, representing the periodicity parallel and perpendicular to the polymer chain of PANI.50 It was observed that all of the XRD patterns show a broad peak at 2θ values of ∼17−30°. This is mainly because of the amorphous behavior of PANI. 4.4. Surface Morphology Study. To investigate the surface morphology of the Ag-PANI/CNT nanocomposite along with pure PANI and Ag-PANI composite, FESEM 3499



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analysis was carried out. Figure 5 shows the FESEM images of pure PANI, Ag-PANI, and the Ag-PANI/CNT nanocomposite. Parts a and b of Figure 5 represent the FESEM images of AgPANI composites at lower and higher magnification. It is observed that there is a formation of nonagglomerated uniformly packed Ag nanoparticles in the PANI surfaces. The average diameter of the Ag nanoparticles were about 50−95 nm. The same types of Ag nanoparticles were also obtained by Mark et al.51 The high surface area to volume ratio of the Ag nanoparticles provides a high charge/discharge rate and also specific capacitance. The Ag has several positions for doping and tends to bind with nitrogen sites of PANI leading to interchain linkage between many adjacent PANI chains by coordination.52 The FESEM images of the Ag-PANI/CNT nanocomposite at both lower and higher magnification are shown in Figure 5c,d. It is observed that the A-MWCNTs surfaces are uniformly coated by PANI and between the coated A-MWCNTs there is the presence of Ag nanoparticles. These Ag nanoparticles are uniformly distributed into the PANI coated A-MWCNTs surfaces. The average diameter of the coated A-MWCNTs in the nanocomposite were about 20−30

Figure 4. XRD patterns of PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite.

Figure 5. FESEM micrographs of Ag-PANI (a) at lower magnification and (b) at higher magnification, Ag-PANI/CNT nanocomposite (c) at lower magnification and (d) at higher magnification, and (e) PANI. (f) EDX spectra of Ag-PANI/CNT nanocomposite. 3500



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particles (indicated by yellow circles) are attached in the coated A-MWCNTs surfaces. Here the average diameter of the AMWCNTs was about 15−20 nm and the PANI coated AMWCNTs have the diameter of 40−60 nm. After the addition of the aniline monomer into the A-MWCNTs, there is a π−π* interaction between A-MWCNTs and the aniline monomer as well as the hydrogen bonding interaction among the carboxyl groups of the A-MWCNTs and the amino groups of the aniline monomers, which is the main reason behind the uniform coating on the A-MWCNTs surfaces (indicated by red arrows). The selected area electron diffraction (SAED) patterns of the Ag-PANI and Ag-PANI/CNT nanocomposite are shown in Figure 7. The ringlike diffraction pattern specifies the Ag

nm. Well-resolved circular grains are obtained for pure PANI shown in Figure 5e. The presence of Ag nanoparticles in the Ag-PANI/CNT nanocomposite was analyzed by using EDX spectroscopy. Figure 5f shows the EDX spectra of Ag-PANI/ CNT nanocomposite. The EDX spectrum strongly revealed the presence of the Ag nanoparticles in the nanocomposite, which is also confirmed by XRD analysis. 4.5. TEM Analysis. The morphological characteristics of the A-MWCNTs, pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite are analyzed by TEM, and the images are shown in Figure 6. Figure 6a represents the TEM image of A-

Figure 7. SAED patterns of (a) Ag-PANI and (b) Ag-PANI/CNT nanocomposite.

nanoparticles are crystalline. The diffraction rings are indexed on the basis of the fcc structure of Ag. The rings arise because of the reflections from (111), (200), (220), and (311) lattice planes of fcc Ag. 4.6. AFM Analysis. Parts a and c of Figure 8 represent the two-dimensional (2D) and three-dimensional (3D) AFM images of Ag-PANI/CNT nanocomposite, respectively. It is observed that the A-MWCNTs are uniformly coated by PANI and there is the presence of globular Ag nanoparticles. These Ag nanoparticles are uniformly distributed in the PANI coated A-MWCNTs surface. Parts b and d of Figure 8 signify the 2D and 3D images of Ag-PANI/CNT nanocomposite at different positions. Here only PANI coated A-MWCNTs are observed. 4.7. Electrochemical Characterization. The electrochemical characterization of PANI, Ag-PANI, and the AgPANI/CNT nanocomposite were carried out by CV, GCD, and EIS. 4.7.1. Cyclic Voltammetry. In order to check the oxidation and reduction potentials and the effect of Ag nanoparticles on the electrochemical performance, CV analysis was carried out. The electrode materials were analyzed within −0.5 to +1 V and within −0.8 to +0.8 V regions at various scan rates of 5, 10, 20, 50, 100, and 200 mV/s in 1 M KCl solution. Figure 9 represents the CV curves of pure PANI, Ag-PANI, and AgPANI/CNT nanocomposite at different scan rates. The specific capacitance of all of the electrode materials was evaluated by using eq 1. In the CV curve, the negative current region denotes the cathodic reduction and the positive current region indicates the anodic oxidation, respectively. The nonrectangular shape of the CV curves indicates the redox behavior due to the occurrence of functional groups and/or a wide pore size distribution. From the CV curve of PANI (Figure 9a) it is observed that the presence of two pairs of redox peaks at 0.36, 0.65 V and 0.16,

Figure 6. TEM micrographs of (a) A-MWCNTs, (b) PANI, (c, d) AgPANI at different positions, and (e, f) Ag-PANI/CNT nanocomposites at different places.

MWCNTs, which shows that the average diameter of the AMWCNTs was 15−20 nm. Figure 6b shows the TEM images of pure PANI. Parts c and d of Figure 6 present the TEM images of Ag-PANI composites at different positions. It is observed from the images the formation of Ag nanoparticles in the PANI surfaces. The Ag nanoparticles are uniformly distributed with a spherical shape on the PANI surface having an average diameter of 5−30 nm, shown in Figure 6c. The agglomerated Ag nanoparticles are also observed to have a diameter of 50−95 nm, shown in Figure 6d. Parts e and f of Figure 6 show the TEM images of Ag-PANI/CNT nanocomposites at different locations. It is observed that the PANI is uniformly coated over A-MWCNTs surfaces. The Ag nano3501



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Figure 8. AFM images of Ag-PANI/CNT nanocomposite at different positions: (a, b) two-dimensional (2D); (c, d) three-dimensional (3D).

0.51 V, respectively. This is due to the redox transition of PANI between the semiconducting state (leucoemeraldine form) and the conducting state (polaronic emeraldine form), which resulted in the redox capacitance. For Ag-PANI composite (Figure 9b) one additional anodic peak was observed at 0.13 V. This is mainly because of the oxidation of Ag which again proves the presence of Ag in the Ag-PANI composite. The same type of anodic peaks is observed at 0.07 V for Ag-PANI/ CNT nanocomposite also, shown in Figure 9c with some peak shifting. This anodic peak shifting phenomenon of the nanocomposite is due to electrode resistance. It is observed that the CV curve for Ag-PANI/CNT nanocomposite showed local minima at 0.02 V and local maxima at 0.07 V. These peaks are the oxidation peak of silver in the nanocomposite.21 These oxidation peaks further prove the existence of silver nanoparticles in the Ag-PANI/CNT nanocomposite. Among all of the electrode materials, Ag-PANI/CNT nanocomposite showed the highest specific capacitance value of 528 F/g at the 5 mV/s scan rate, whereas pure PANI and Ag-PANI exhibited specific capacitance of 376 and 425 F/g at the 5 mV/s scan rate, respectively. The better specific capacitance of AgPANI compared to pure PANI may be due the presence of Ag nanoparticles which enhances the capacitance value. The increase in the specific capacitance of Ag-PANI/CNT nanocomposite can be attributed to the following possible reasons: (a) the presence of A-MWCNTs having high surface area in the Ag-PANI/CNT nanocomposite enhances the specific capacitance; (b) the presence of Ag nanoparticles in the nanocomposite may be responsible for the enhancement of the specific capacitance; (c) there is good interaction between AMWCNTs and Ag-PANI composite; and (d) the uniform coating of PANI on the A-MWCNTs surfaces and in the coated A-MWCNTs surfaces in the presence of Ag nanoparticles gives a superior structure for easy electrolyte accessibility, thereby enhancing the specific capacitance. It is observed for all of the

electrode materials that with an increase in the scan rate the specific capacitance values decrease, shown in Figure 10. It is accepted that, at low scan rate due to the presence of inner active sites, complete redox transition can occur and a higher specific capacitance value can be produced, whereas, at the higher scan rate, the capacitance value decreases mainly due to the diffusion effect of proton within the electrode.53 The electrochemical properties of a material also depend on the energy density and power density. The major drawback of the supercapacitors is their lower energy density. In our study, all of the electrode materials showed superior energy density as well as power density values. The energy densities (E) of the electrode materials were evaluated by using the following equation:

E=

1 (CV 2) 2

(2)

where C and V denote the specific capacitance (F/g) and operating voltage, respectively. Power densities (P) of the electrode materials were calculated by using the following equation:

P = E/t

(3)

where t represents the time in seconds for the complete cycle. The Ag-PANI/CNT nanocomposite attained the maximum energy density value of 187.73 Wh/kg at 5 mV/s scan rate. The maximum power density of 4185 W/kg was obtained for AgPANI/CNT nanocomposite at 200 mV/s scan rate. Figure 11 showed the plot of energy density vs power density (Ragone plot), which revealed an increase in the energy density of the electrode materials with a decrease in the power density. 4.7.2. Galvanostatic Charge Discharge. The galvanostatic charge/discharge is a steady method to estimate the electrochemical performance of electrode materials under controlled current conditions. The galvanostatic charge/discharge plot of pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite at 3502



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Figure 9. Cyclic voltammograms at different scan rates of (a) PANI, (b) Ag-PANI, and (c) Ag-PANI/CNT nanocomposite.

Figure 10. Plot of specific capacitance vs scan rate. Figure 11. Ragone plot of PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite.

constant current of 0.5 A/g is shown in Figure 12a. The constant current charge/discharge plot of PANI and Ag-PANI shows a somewhat similar nature with three stages of voltage drop, from 0.8 to 0.65 V, from 0.65 to 0.2 V, and from 0.2 to −0.2 V. Both PANI and Ag-PANI exhibit a quick discharge within the first potential region, but relative delay in the second potential region, indicating better specific capacitance. The AgPANI/CNT exhibits three stages of voltage drop, having larger discharge duration in the second stage. The rapid discharge in

the 0.8−0.70 V potential region is associated with the EDL capacitance, while the second voltage drop in the 0.7−0.08 V region with larger discharge duration indicates the combination of both EDL capacitance and pseudocapacitance. Figure 12b depicts the galvanostatic charge/discharge plot of Ag-PANI/ CNT nanocomposite at different current densities of 0.5, 1, 1.5, 2, and 3 A/g. It is observed that with increasing current density 3503



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Figure 12. (a) Comparative galvanostatic charge/discharge plots of PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite at a constant charge/ discharge current density of 0.5 A/g and (b) GCD plot of Ag-PANI/CNT nanocomposite at different constant current densities of 0.5, 1, 1.5, 2, and 3 A/g.

frequency behavior of supercapacitor electrode. It gives information about the charge transport behavior of the electrode material at the electrode/electrolyte interface. A Nyquist plot is the most extensively used plot for the EIS analysis. It is the plot of real component (Z′) of the impedance with the imaginary component (Z″). The Nyquist plots are mainly interpreted by fitting the experimental data with the equivalent electrical circuits. The fitted Nyquist plots of pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite are shown in Figure 14. The area at higher frequency represents the electrolyte properties, and the region in the middle frequency is related to the electrode/electrolyte interface processes. The intercepts of the curves with the real axis is the solution resistance (Rs), and the depressed semicircle at the higher frequency region represents the charge transfer resistance (Rct) of the electrode materials. The appropriate equivalent circuit for pure PANI, Ag-PANI, and Ag-PANI/ CNT nanocomposite is shown in Figure 15, and the fitting data are given in Table 2. Ideal capacitors are not possible in the real-world system. These imperfect capacitors are signified by constant phase element (CPE). CPE may be arises from (1) dynamic disorder associated with diffusion, (2) the nature of the electrode, (3) porosity, and (4) inhomogeneity at the electrode/electrolyte interface. From the fitted Nyquist plot the solution resistances of 1.54, 1.43, and 1.38 Ω have been determined for pure PANI, AgPANI, and Ag-PANI/CNT nanocomposite, respectively. The lowest Rs value of the Ag-PANI/CNT nanocomposite indicates the better conductivity and enhances the specific capacitance. The lower Rs value may be due to the presence of the Ag nanoparticles in the Ag-PANI/CNT nanocomposite. It is observed that the Ag-PANI/CNT nanocomposite showed lower Rct indicating an easy charge transfer resistance. The lower Rct value may be due to the presence of Ag nanoparticles, which enhanced the charge transfer within the whole matrix. The Warburg resistance (Wd) is also lower for the Ag-PANI/ CNT nanocomposite, revealing a short diffusive path of the electrolyte ions within the nanocomposite. For any supercapacitor electrode the most important parameter is the frequency factor (n), which calculates the ideality of a composite toward supercapacitive behavior. n = 1 indicates

the discharge time of Ag-PANI/CNT nanocomposite decreases. The discharge time of the Ag-PANI/CNT nanocomposite extends as long as 2000 s, which shows a higher specific capacitance. The variation of specific capacitance with th enumber of cycles indicates the cyclic stability of the composite which is an important parameter in determining the application potential of the supercapacitor electrode for practical purposes. The cyclic stability of all of the electrode materials was done in 1 M KCl solution at 1 A/g current density, shown in Figure 13. The Ag-

Figure 13. Plot of the variation of specific capacitance as a function of the cycle number of PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite at 1 A/g constant current density.

PANI/CNT nanocomposite maintained 94% specific capacitance of their initial capacitance over 1000 cycles, indicating the long-term electrochemical stability of the nanocomposite, whereas PANI and Ag-PANI composite showed specific capacitance retention of 84% and 91% after 1000 cycles. The higher cyclic stability of the Ag-PANI/CNT nanocomposite can be attributed to the presence of Ag nanoparticles over PANI coated A-MWCNTs surfaces. 4.7.3. Electrochemical Impedance Spectroscopy. EIS is a very useful technique for the determination of electrochemical 3504



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Figure 14. Nyquist plots of (a) PANI, (b) Ag-PANI, and (c) Ag-PANI/CNT nanocomposite after fitting with an equivalent electrical circuit.

Table 3. Electrical Conductivity Measurementsa

a

Figure 15. Equivalent circuit used for fitting the Nyquist plots.

sample

d,cm

resistance (ohms)

ρ, ohm·cm

σ, S/cm


0.05 0.05 0.05

10.26 1.75 1.04

2.32 0.39 0.23

0.43 2.52 4.24

d = thickness; ρ = resistivity; σ = conductivity..

nanocomposite (4.24 S/cm). Again there is a good interaction between Ag nanoparticles and PANI coated A-MWCNTs which results in greater conductivity. 4.9. Current−Voltage Relationship. The current−voltage relationships of pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite were measured at room temperature within the voltage range of −10 to 10 V and shown in Figure 16. It is observed for all of the electrode materials that the current increases with an increase in the applied voltage in a nonlinear manner. This signifies the nonohmic nature of the electrode materials. This nonlinearity of the I−V curves specify the semiconducting behavior of the electrode materials. Thus these materials can be used in electronic devices. 4.10. Thermogravimertic Analysis. The effect of Ag nanoparticles on thermal stability of the electrode materials was analyzed by TGA analysis. Figure 17 shows the TGA thermogram of pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite. For all of the electrode materials, the initial weight loss is observed only because of the presence of some

the ideal supercapacitive behavior which is not possible, n = 0.5−1 indicates the moderate supercapacitive behavior, and n = 0.5 designates as low supercapacitor behavior. In this study, all of the electrode materials show the moderate supercapacitive behavior. The highest n value of 0.74 is obtained for the AgPANI/CNT nanocomposite. 4.8. Electrical Conductivity Study. The electrical properties of the pure PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite were studied by four-probe measurements at room temperature, and their values are summarized in Table 3. The pure PANI shows electrical conductivity of 0.43 S/cm, whereas Ag-PANI shows electrical conductivity of 2.52 S/cm. This increase in conductivity of the Ag-PANI composite is due to the incorporation of the Ag nanoparticles in the composite. The introduction of high surface area and large aspect ratio AMWCNTs and good electrical conductor Ag nanoparticles enhanced the electrical conductivity of the Ag-PANI/CNT

Table 2. Fitting Data of Equivalent Circuit Elements Obtained by Simulation of Impedance Spectra sample

Rs (Ω)

Rct (Ω)

W(S−s0.5) × 10−2

CPE(S−sn) × 10−3

n

PANI Ag-PANI Ag-PANI CNT

1.54 1.43 1.38

51.53 43.92 38.5

5.1 2.67 1.77

0.46 0.42 0.50

0.65 0.70 0.74

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Figure 16. I−V plots of PANI, Ag-PANI, and Ag-PANI/CNT nanocomposite.

becomes preserved.55,56 The TGA values at various temperature regions are shown in Table 4. Table 4. Thermal Stability Data for PANI, Ag-PANI, and AgPANI/CNT Nanocomposite Extracted from TGA Thermogram weight loss at given temp (%) sample

100 °C

200 °C

350 °C


10.18 2.90 1.20

16.05 3.60 2.26

33.71 34.49 32.47

5. CONCLUSION We have successfully prepared a high-performance electrode material based on Ag nanoparticles decorated on PANI coated A-MWCNTs by a simple and inexpensive in situ oxidative polymerization technique. The formation of Ag nanoparticles in the PANI coated A-MWCNTs surface was confirmed by morphological analyses: FESEM, TEM, and AFM. The presence of Ag nanoparticles enhances the electrochemical properties of the Ag-PANI/CNT nanocomposite. The nanocomposite achieved the highest specific capacitance of 528 F/g at 5 mV/s scan rate. The nanocomposite also attained a better energy density of 187.73 Wh/kg and power density of 4185 W/ kg at 5 mV/s and 200 mV/s scan rates respectively. The maximum electrical conductivity of 4.24 S/cm was observed for Ag-PANI/CNT nanocomposite due to the occurrence of highly conductive Ag nanoparticles and A-MWCNTs. The nonlinearity of the I−V curves indicates the semiconducting behavior of the electrode materials. The nanocomposite also showed better thermal stability up to 350 °C. The typical synthetic procedure and superior properties revealed that the Ag-PANI/CNT nanocomposite can be used as a promising electrode material for supercapacitor.

Figure 17. TGA thermograms of (a) PANI, (b) Ag-PANI, and (c) AgPANI/CNT nanocomposite.

volatile impurities or the removal of water molecules. At 100 °C, PANI shows 10.18% weight loss whereas Ag-PANI and AgPANI/CNT show 2.90% and 1.20% weight loss, respectively. The same trend is also observed at 200 °C. The higher thermal stability is observed for Ag-PANI/CNT nanocomposite up to 350 °C. This increase in the thermal stability is due to the presence of the Ag nanoparticles in the uniformly PANI coated A-MWCNTs surfaces. Here both Ag nanoparticles and AMWCNTs take part to increase the thermal stability. Ag-PANI achieved better thermal stability compared to pure PANI; this is only because of the uniform distribution of Ag nanoparticles in the PANI surfaces. But after 350 °C the nanocomposite reduced their thermal stability. It is observed from the TGA curve that the thermal stability of the Ag-PANI and Ag-PANI/ CNT nanocomposite deteriorates suddenly from their thermal stability after 350 °C temperature. The possible explanation is that after 350 °C the bonded interaction between PANI and Ag is not stable after that temperature. As a result sudden decreases in thermal stabilities of Ag-PANI and Ag-PANI/CNT nanocomposite were observed. The same types of TGA curves were also observed by Giri et al. for Pd doped PANI composite.54 So, the as prepared electrode materials can be used in different types of high-temperature (up to 350 °C) device applications. It is observed that all of the electrode materials left around a 40− 46 wt % residue after being heated to 800 °C. When the electrode materials were heated in nitrogen atmosphere, PANI converted to nitrogen-containing carbon, i.e. the chemical nature of the materials changed, and only the morphology

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91-3222-283978. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Defence Research & Development Organization (DRDO), India, for financial support. We gratefully acknowledge Souvik Ghosh, Department of Chemical Engineering, 3506



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Case Western Reserve University for AFM analysis. We are also thankful to IIT Kharagpur for instrumental facilities.

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