Enhanced electrochemical behavior and hydrophobicity of crystalline polyaniline@graphene nanocomposite synthesized at elevated temperature

Composites Part B 87 (2016) 281e290

Contents lists available at ScienceDirect

Composites Part B journal homepage: www.elsevier.com/locate/compositesb

Enhanced electrochemical behavior and hydrophobicity of crystalline polyaniline@graphene nanocomposite synthesized at elevated temperature Nazish Parveen a, Neelima Mahato a, Mohd Omaish Ansari b, Moo Hwan Cho a, * a b

School of Chemical Engineering, Yeungnam University, Gyeongsan-si, Gyeongbuk 712-749, South Korea Center of Nanotechnology, King Abdulaziz University, Jeddah-21589, Saudi Arabia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 December 2014 Received in revised form 30 March 2015 Accepted 31 October 2015 Available online 11 November 2015

This paper reports a simple route for the synthesis of graphene (GN) using an electrochemical method as well as its composite with polyaniline (Pani). The resulting nanocomposite contained high crystalline regions due to the in-situ oxidative polymerization performed at elevated temperatures. Thus prepared GN, Pani and Pani@GN nanocomposite were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). Morphological studies showed that Pani formed a smooth coating over the surface of GN. A shift in the FTIR, Raman and XPS spectra of the Pani@GN nanocomposite also supports the strong interactions between Pani and GN, confirming the successful synthesis of the Pani@GN nanocomposite. XRD and selected area electron diffraction revealed the highly crystalline nature of Pani in the Pani@GN nanocomposite, highlighting the efficacy of the preparation method. The Pani@GN nanocomposite showed higher DC electrical conductivity and capacitance than Pani. The enhanced performance of the Pani@GN nanocomposite was attributed to the large surface area provided by GN, facilitating the rapid transport of electrolyte ions into the electrode during the redox process. The Pani@GN nanocomposite also exhibited better hydrophobicity due to the incorporation of GN. © 2015 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures B. Electrical properties D. Surface analysis D. Polyaniline@Graphene

1. Introduction The 1977 chemical communication report on conducting polyacetylene provided new insights into the bright and exciting world of conducting polymers [1]. Later, many other polymers, such as polyaniline (Pani), polythiophene, polypyrrole, etc., were also found to be conducting. Among all these polymers, Pani has attracted considerable attention because of its ease of synthesis, low cost, good environmental stability, interesting electroactivity, unusual doping/dedoping chemistry, and long shelf life [2e4]. On the other hand, Pani normally suffers from poor thermal stability and low capacitance. To improve the performance further or extend the functions of the devices, Pani normally needs to be nanostructured, i.e. synthesis of either nanometer-sized polymer particles or nanocomposites with conducting materials to improve the

* Corresponding author. E-mail address: [email protected] (M.H. Cho). http://dx.doi.org/10.1016/j.compositesb.2015.10.029 1359-8368/© 2015 Elsevier Ltd. All rights reserved.

conductive path structure. Small sized nanomaterials can provide high electroactive regions and a short diffusion path [5,6], which are essential for the effective access of an electrolyte to the electrode in both double-layer and redox mechanisms. The high conductivity of electroactive materials is another important parameter to meet the rapid electron transmission for the high rate redox reactions of Pani. Therefore, to achieve high capacitance, a nanocomposite of Pani with carbon based materials, particularly graphene (GN), can be effective owing to its high conductivity [7,8]. GN is comprised of sp2 hybridized aromatic carbon atoms arranged in a honeycomb network, and has unique electrical and thermal properties [9,10] as well as considerable mechanical strength [11]. For applications in supercapacitor systems, GN can provide good stability during the charge/discharge process. On the other hand, the capacitance achieved is generally low due to the unavoidable aggregation of nanosheets. Pani has been used as a supercapacitor material because of its high conductivity, high flexibility, low cost, and multi-redox state [12]. In addition, it has particularly high theoretical pseudocapacitance, but has limited

282

N. Parveen et al. / Composites Part B 87 (2016) 281e290

using a 4-in-line probe electrical conductivity measuring instrument in a PID controlled oven (Scientific Equipment's, Roorkee, India), and were calculated using the following equation:

applications as supercapacitors because of its poor stability during the charge/discharge process. Several studies have reported the incorporation of GN into Pani matrices to produce nanocomposites for supercapacitors electrodes. Nevertheless, the specific capacitance obtained is generally low, i.e. approximately 111 F/g for GN [13] and 233 F/g for Pani@GN composites [14]. The capacitance of the Pani@GN composites is dominated mainly by the pseudocapacitance from Pani. The electrical double-layer capacitors (EDLC) for GN are not high, possibly due to the agglomerated large numbers of layer-like structures of the GN sheets [15]. A few layered GN and its composite with Pani are expected to overcome the drawbacks of GN and Pani due to the synergism between the constituents, which might lead to good supercapacitor performance and better charge acceptance behavior. This paper reports the synthesis of few layered GN nanosheets and highly crystalline Pani@GN nanocomposites via a simple insitu oxidative polymerization technique. Aniline was polymerized by the drop wise addition of an oxidant, at 60
C, which is considerably different from the conventional method of polymerization. The combination of GN and Pani resulted in a Pani@GN nanocomposite with enhanced capacitance (613 F/g) and super hydrophobic behavior compared to Pani because GN not only serves as a conducting support material, but also provides a large surface area. The electrochemical performance, morphology and chemical structure of the nanocomposite were also investigated.

where I, V, W, and S are the current (A), voltage (V), thickness of the pellet (cm), and probe spacing (cm), respectively, and s is the DC electrical conductivity in S/cm [16]. For the electrical conductivity studies, a pellet of Pani and the Pani@GN nanocomposite was prepared using a hydraulic pressure instrument at a 50 kN pressure for 10 min. All electrochemical measurements were performed using a potentiostat Versa STAT 3, Princeton Research, USA. The working electrode was prepared by casting a Nafion-impregnated sample into a carbon paper electrode with area of 1 cm2. In a typical process, 5 mg of each sample (GN, Pani and Pani@GN nanocomposite) was dispersed in 1 mL of an ethanol solution containing 5 mL of a Nafion solution in an ultrasonic bath for 20 min and then coated on a carbon paper electrode. A three electrode cell system was used to evaluate the electrochemical performance by CV and EIS in 1 M H2SO4 as the electrolyte. GN, Pani and the Pani@GN nanocomposite were coated separately on carbon paper and used as the working electrodes. Platinum gauze and Ag/AgCl (3.0 M KCl) were used as the counter and reference electrodes, respectively.

2. Experimental

2.3. Electrochemical synthesis of graphene

2.1. Materials

The Pani@GN nanocomposite was synthesized using GN, which was prepared via electrochemical route and its subsequent polymerization with aniline by in situ oxidative polymerzition technique. The electrochemical cell consisted of a graphite sheet and platinum gauze as the anode and cathode, respectively. The complete assembly was dipped in an aqueous electrolyte solution containing 100 mL of 1% H2SO4 and 3 g of Na2HPO4. The electrodes were connected to the respective terminals of the electrochemical workstation at a fixed voltage of ±10 V. Soon after the start of the reaction, the solution gradually turned gray and finally to an intense black. The graphite anode dissociated constantly into flaky GN and exfoliated graphite particles throughout the course of the reaction, which settled at the bottom of the bottle. After 1.5 h, the electrolyte solution containing exfoliated graphite and GN was centrifuged at 8000 rpm for 10 min to isolate GN, which was then washed with water and an excess of ethanol followed by drying in an oven at 80
C.

Aniline was acquired from Sigma Aldrich. Potassium persulphate (PPs), hydrochloric acid (HCl), sulfuric acid (H2SO4), ammonia solution (35%), sodium phosphate dibasic (Na2HPO4), methyl alcohol, and ethanol were purchased from Duksan Pure Chemicals Co. Ltd., South Korea. The graphite sheets for the synthesis of GN were supplied by KOMAX, South Korea. The water used in these experiments was de-ionized water obtained from a PURE ROUP 30 water purification system. 2.2. Methods The microstructures of GN, Pani and Pani@GN were examined by scanning electron microscopy (SEM, HITACHI-S4800), and field emission transmission microscopy (FE-TEM, Tecnai G2 F20, FEI, USA) at an accelerating voltage of 200 kV. The mean diameter of the particles was calculated using image J software. Phase analysis was performed by X-ray diffraction (XRD, PANalytical, X'pert PRO-MPD, Netherland) using Cu Ka radiation (l ¼ 0.15405 nm). Raman spectroscopy was recorded on Lab Ram HR 800 UV Raman microscope (Horiba Jobin-Yvon, France, l ¼ 514 nm). The Raman spectra were acquired at the Korea Basic Science Institute, South Korea. The functional groups and their interactions were examined by Fourier transform infrared (FTIR, Excalibur series FTS 3000 Bio-Rad spectrometer) spectroscopy. The UVevisible diffuse absorbance/reflectance spectra were measured using an ultravioletevisibleenear infrared spectrophotometer (UVeVISeNIR, Cary 5000, VARIAN, USA). X-ray photoelectron spectroscopy (XPS, K-ALPHA) was performed using a monochromatized Al Ka X-ray source (hn ¼ 1486.6 eV) with a 400 mm spot size. Thermogravimetric analysis (TGA, Perkin Elmer, Pyris Diamond) was performed by heating the samples from 20 to 900
C at 10
C min1 at a N2 flow rate of 200 mL min1. Electrochemical studies, such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), were carried out using a potentiostat (VersaSTAT 3, Princeton Research, USA). The DC electrical conductivity (s) was measured

s ¼ ½ln 2ð2S=WÞ=½2pS ðV=IÞ

(1)

2.4. Synthesis of the Pani@GN nanocomposite The Pani@GN nanocomposite was synthesized via the simple insitu oxidative polymerization of aniline in the presence of GN using PPs as an oxidizing agent. In a typical process, 0.1 g of GN was added to 500 mL of 1 M HCl followed by ultrasonic agitation for proper dispersion of the GN nanosheets, which was followed by the addition of 5 mL of the aniline monomer. The entire system was stirred vigorously on a hot plate at a constant temperature of 60
C. A solution of the oxidant (0.5 M PP in 500 mL 1 M HCl) was added drop wise to the above dispersion of GN and aniline to initiate the polymerization of aniline on the GN nanosheets. The reaction mixture was then stirred constantly for 12 h, after which the solution was filtered. The Pani@GN slurry after filtration was washed with an excess of water and methanol to remove the residual PPs and Pani oligomers. The nanocomposite dedoped by 1 M ammonia solution and then washed sequentially with water and methanol. The prepared emeraldine base of the Pani@GN nanocomposite was doped with 100 mL of a 1 M HCl solution for 12 h, filtered and

N. Parveen et al. / Composites Part B 87 (2016) 281e290

283

washed with water and methanol. Pure Pani was prepared using a similar method in the absence of a GN nanosheet. The prepared conducting Pani and Pani@GN nanocomposite were dried in an air oven at 80
C and later stored in a desiccator for further experiments.

3. Result and discussions 3.1. Proposed mechanism for the synthesis of the Pani@GN nanocomposite The Pani@GN nanocomposite was synthesized by the simple insitu oxidative polymerization of aniline in the presence of GN nanosheets using an electrochemical method. The nanocomposite, consisting mainly of well-arranged fibers, was obtained. As the polymerization of aniline is an exothermic process, the high temperature polymerization process decreases the rate of polymerization. This leads to a better interaction between aniline and GN before the polymerization process. In acidic media, aniline first changed to the anilinium cation (C6H5NHþ 3 ) and the presence of oxygen functional groups in GN might interact with these anilinium cations via an electrostatic interaction, hydrogen bonding and p-p stacking interaction between them. As a result, the anilinium cation grows uniformly over the surface of GN and the subsequent addition of PPs produces a highly crystalline Pani@GN nanocomposite. Fig. 1 shows a schematic diagram of the polymerization mechanism and the interaction of GN with Pani.

3.2. XRD analysis Fig. 2 shows a XRD pattern of GN, Pani and Pani@GN nanocomposites. In the case of GN, a single large diffraction peak at 26.53
2q with a typical inter-spacing of 3.35 Å was observed, which is very close to the XRD peak of the (002) crystalline plane of the graphite sheet that is indicative of the presence of GN [17,18], corresponding to the d-spacing between the GN layers. The other peaks at 45.29
, 44.58
and 54.47
2q were assigned to the (100) (101) and (004) planes, respectively, which also confirmed the successful synthesis of GN [19].

Fig. 2. XRD pattern of Pani, GN and Pani@GN nanocomposite.

In the case of Pani, the characteristic peaks were observed at 19.26
, 25.61
and 35.7q
2q, which is consistent with the report of Yan et al. [20]. In the case of Pani@GN, peaks corresponding to both Pani and GN are present. On the other hand, Pani@GN showed an additional peak at 15.15
2q, which was suppressed considerably in Pani, as well as a much sharper peak at 19.26
2q with a typical inter-spacing of 3.41 Å, corresponding to the (110) plane, indicating that Pani@GN is more crystalline than Pani [21]. This suggests that both Pani and Pani@GN showed improved crystalline features (as evident to the sharp XRD peaks) compared to other reports [14], which might be due to the slow polymerization of aniline at elevated temperatures. Compared to Pani, the higher crystallinity of Pani@GN was attributed to the adsorption of aniline on the GN template and its subsequent polymerization. The aniline monomers are first adsorbed on the GN template and later undergo slow polymerization at elevated temperatures, which lead to a highly ordered alignment and arrangements of Pani, resulting in an increase in the intensity of the peaks in the Pani@GN nanocomposite.

3.3. FTIR analysis Fig. 3 shows the FTIR spectra of Pani and the Pani@GN nanocomposite in the region, 500e4000 cm1. In the case of Pani, the bands at 1582 and 1496 cm1 were assigned to the C]C and C]N stretching vibrations, respectively, showing the deformation of quinoid and benzenoid rings [22]. The other band at 1296 cm1 was assigned to electron delocalization in the CeN stretching vibrations of the quinoid rings, which is a measure of the degree of electron

3600 Fig. 1. Schematic diagram of the synthesis of Pani@GN nanocomposite and possible p-p interaction between Pani and GN.

3000 2400 1800 1200 -1 Wavenumber (cm )

821

1582 1496 1296 1156 1113

Pani 3228

Transmittance

(a.u.)

Pani@GN

600

Fig. 3. FTIR spectra of Pani and Pani@GN nanocomposite.

284

N. Parveen et al. / Composites Part B 87 (2016) 281e290

delocalization as an “electronic-like bond”. This bond is a characteristic of conducting Pani, indicating that the Pani was in the doped state [23], whereas the absorption band at 1156 cm1 was attributed to the vibration mode of the eNHþ]. The other bands at 1113 and 821 cm1 were assigned to the CeH plane bending and CeH out of plane deformation bending of the benzene ring, respectively. These absorption bands are characteristic of Pani, confirming its successful synthesis. In the case of the Pani@GN nanocomposite, no new bands were observed but all the bands were red shifted due to p-p interactions between the GN sheets and Pani backbone [24,25].

3.4. UVevis diffuse absorption spectroscopy Fig. 4 shows the UVevis diffuse spectra of Pani and the Pani@GN nanocomposite. Pani revealed absorption peaks at ~280 and ~550 nm. The band at ~280 nm was assigned to the p-p transition in the benzenoid structure, whereas the absorption in the visible range at ~550 nm was attributed to exciton formation in the quinoid rings. In the case of the Pani@GN nanocomposite, the peaks were red shifted, indicating the coordination of GN with the imine nitrogen atom of Pani via the p-conjugated system of both constituents [23,26].

3.5. SEM analysis SEM was used to examine the morphology of GN, Pani and Pani@GN nanocomposite. The exfoliated GN showed a flaky wrinkled and folded sheet-like morphology, which is in accordance with many other reports indicating the successful synthesis of GN [17,19,27]. The SEM image of Pani (Fig. 5b) revealed a fibrous morphology along with a large number of interconnected tubules. The fibrous nature was attributed to the soft template-assisted growth of Pani, as explained earlier [28]. As the reaction was performed at elevated temperatures, the polymerization of aniline occurred slowly due to the exothermic nature of the reaction. This leads to a much ordered alignment and arrangements of the Pani chains, resulting in improved crystalline behavior. The Pani@GN nanocomposite also showed similar morphological features. GN was coated with Pani due to the adsorption of the aniline monomers on GN with subsequent polymerization, as evident from Fig. 5c. The mean diameter of Pani and Pani@GN was 80 and 90 nm, respectively. The increase in the diameter of Pani@GN also indicates the polymerization of aniline on the GN template.

3.6. TEM analysis TEM was performed to examine the inner nanostructures of GN, Pani and the Pani@GN nanocomposite. The TEM image of GN revealed a sheet like structure with many wrinkles and scrolls [29], which is analogous to the microstructure of the individual GN sheet (Fig. 6a). In addition, the selected area electron diffraction (SAED) pattern (inset of Fig. 6a) revealed a ring like pattern, indicating the polycrystalline nature of the as prepared GN. Pani (Fig. 6b) shows tubular morphology but in the case of Pani@GN (Fig. 6c), the morphology was relatively tubular but some flakey structures could also be seen. This was attributed to the adsorption of aniline on the GN template and its subsequent polymerization. This also resulted in an increase in the diameter of Pani@GN nanocomposite compared to pure Pani (Fig. 6c). The mean diameter of the Pani fibers was measured to be 85 nm, suggesting that they are nanofibers. The SAED diffraction pattern of Pani@GN (inset of Fig. 6c) is in accordance with the XRD observations showing its polycrystalline nature. HR-TEM analysis clearly revealed lattice fringes in the Pani@GN nanocomposite confirming its highly crystalline nature, but small areas with little or no GN were observed, as evident from Fig. 7. 3.7. Raman analysis Raman spectroscopy is one of the most successful tools for examining the electronic and structural properties of materials [30,31]. The surface composition of GN, Pani and the Pani@GN nanocomposite was characterized by Raman spectroscopy, as shown in Fig. 8. In the case of GN, the bands at 1350 and 1590 cm1 were assigned to the well-documented D and G bands, respectively [17,20,35]. For Pani, the bands at 1614 and 1569 cm1 were assigned to the CeC stretching vibration of the benzenoid ring and the C]C stretching vibration in the quinoid units respectively, whereas the other band in close proximity at 1528 cm1 was assigned to the NeH deformation vibration associated with the semiquinoid structure [32e35]. The band at 1348 cm1 corresponds to the CeN vibration of the delocalized polaronic structure (protonated band) [36]. The bands at 1236, 1185 and 814 cm1 were assigned to deformation of the quinonoid segments, benzenoid units (owing to the CeH bending vibration of the semiquinoid ring) and benzene ring deformation, respectively [37]. In the case of the Pani@GN nanocomposite, a reduced peak intensity with a slight red shift of all the bands was observed. This might be due to the surface coating of GN by Pani, which leads to specific p-p interactions at different sites; hence, a decrease in peak intensity [38]. This result also corresponds to FTIR and XRD analysis. 3.8. Thermogravimetric analysis

Fig. 4. UVevis diffuse absorbance spectra of Pani and the Pani@GN nanocomposite.

TGA was performed to examine the thermal stability of Pani and the Pani@GN nanocomposite (Fig. 9). In the case of Pani, the weight loss up until ~120
C was assigned to the loss of physically adsorbed water and other volatile impurities. The second weight loss between 150 and 280
C was attributed to the loss of dopant HCl molecules, leading to deprotonation of the Pani backbone [39]. The major weight loss of ~31% from ~450 to 600
C was assigned to the exothermic thermal decomposition of Pani with different degrees of polymerization [25]. Compared to the TGA of the Pani, Pani@GN showed much smoother weight loss, and there was no sharper weight loss trend between any temperature regions. The relatively higher stability of Pani@GN compared to Pani was attributed to the interaction of GN with Pani, which hinders the decomposition of the Pani chains, thereby contributing to the stability of the Pani@GN nanocomposite [40]. The residual weight after the

N. Parveen et al. / Composites Part B 87 (2016) 281e290

Fig. 5. SEM images of (a) GN, (b) Pani, and (c) the Pani@GN nanocomposite.

Fig. 6. TEM images of (a) GN, (b) Pani, and (c) Pani@GN nanocomposite. The inset shows the SAED pattern of the respective materials.

285

286

N. Parveen et al. / Composites Part B 87 (2016) 281e290

Fig. 9. TGA curves of Pani and Pani@GN nanocomposite.

Fig. 7. HR-TEM image of the Pani@GN nanocomposite.

degradation of both Pani and Pani@GN might be due to the degradation products, such as aniline, ammonia, p-phenylenediamine, N-phenyl-1, 4-benzenediamine, carbazole, N-phenylaniline, pyridine-based heterocycles, acetylene, methane, etc. [41].

3.10. DC electrical conductivity of Pani and Pani@GN nanocomposite

3.9. XPS analysis XPS was performed to determine the chemical and different electronic states of the as-synthesized GN, Pani and Pani@GN nanocomposite as well as to study the different possible

Intensity (a.u.)

Pani@GN

GN

Pani

1200

1300

interactions between them. Fig. 10 shows the C 1s and N 1s core level spectra of GN, Pani and the Pani@GN nanocomposite. The C 1s spectrum of GN (Fig. 10a) can be divided into three Gaussian peaks at 284.6, 285.4 and 286.38 eV, which are assigned to the C]C, CeOH and C]O bands, respectively [18]. In the case of Pani (Fig. 10b) and the Pani@GN nanocomposite (Fig. 10c), the detected carbon element was revealed from the peaks at binding energies of 284.18, 284.98, 285.69, 286.99, and 288.08 eV as well as at 284.23, 284.78, 285.39, and 287.78 eV, which correspond to the CeC, C]C, CeOH, p-p* interaction and C]O bands in the respective materials [42]. Therefore, a small shift in the binding energy occurred in the case of a nanocomposite [43]. The C 1s spectrum also clearly showed that the binding energy (CeOH and C]O) of Pani@GN is higher than Pani. The N 1s core level of Pani (Fig. 10d) consists of two peaks with binding energies at 399.41 eV (eNHe), and 401.02 eV (eNþe). For the Pani@GN nanocomposite (Fig. 10e), the eNþe peak (400.72 eV) had a higher binding energy than pure Pani, which was attributed to the protonated amine units [44]. The decreased C]C binding energy intensity in the Pani@GN nanocomposite and the shift in the eNþe suggests a strong p-p interaction between GN and Pani chains, which may facilitate the electron transfer process.

1400

1500

1600

1700

1800

The DC electrical conductivity of Pani and the Pani@GN nanocomposite was measured using the 4-in-line probe method. The DC electrical conductivity of the HCl doped Pani and Pani@GN nanocomposite was 2.1 and 2.4 S/cm, respectively, as evident from Fig. 11. The increase in the DC electrical conductivity after the incorporation of GN in Pani was attributed to the additional effects of both Pani and GN because both are conducting. This shows that there is a p-p interaction between the GN sheets and the Pani backbone, which leads to a higher conjugated system (Fig. 1). This suggests that GN had a remarkable impact on the microstructure of the resulting Pani@GN nanocomposite, which might also lead to an increase in the mobility of the charge carriers in the Pani@GN nanocomposite; hence, an increase in DC electrical conductivity. In addition, the highly ordered structure of the Pani@GN nanocomposite, as evident from XRD, might also be a favorable factor for the higher conductivity. 3.11. Cyclic voltammetry

-1

Raman shift (cm ) Fig. 8. Raman spectra of GN, Pani and Pani@GN nanocomposite.

Fig. 12a shows the cyclic voltammograms of the Pani@GN nanocomposite along with those of Pani and GN. The two pairs of

N. Parveen et al. / Composites Part B 87 (2016) 281e290

C-OH

Intensity (a.u.)

Intensity (a.u.)

(a)

C-C

C-OH

(b)

C=C

ππ C=O

C=O

280

285

290 280

Binding energy (eV)

399

402

405

ππ C=C

408

Binding energy (eV)

C=O

285

290

295

N 1s

-N-H-

Intensity (a.u.)

Intensity (a.u.)

-N

(c)

Binding energy (eV)

N 1s

(d)

C-OH

295 280

285 290 Binding energy (eV)

-N-H-

396

C 1s

C 1s

C-C

Intensity (a.u.)

C 1s C=C

287

(e) +

-N

396

399

402

Binding energy (eV)

405

408

Fig. 10. XPS spectra of the C 1s region of (a) GN, (b) Pani, (c) Pani@GN nanocomposite and N 1s region of (d) Pani and (e) Pani@GN nanocomposite.

DC electrical conductivity (S/cm)

redox waves shown in CV of Pani and Pani@GN nanocomposite were attributed to the redox transition of Pani (i.e. from the leucoemeraldineeemeraldine transition and emeraldineepernigraniline transition) [45]. Both the CV of Pani and Pani@GN nanocomposite had a larger rectangular area than GN, suggesting that both have a large double layer capacitance compared to GN. On the other hand, for Pani and the Pani@GN nanocomposite, with the introduction of GN into Pani, the Pani@GN nanocomposite showed a larger capacitance than Pani. This might be due to the large surface area or increase in porosity provided by GN onto which Pani is adhered, facilitating the rapid transport of electrolyte ions into the electrode during the redox process. Another reason is the high electrical conductivity of GN, which offers a highly conducting path for the movement of charge carriers [46]. The specific capacitance of the Pani@GN nanocomposite was measured to be 613 F/g at a scan rate of 10 mV/s, which is much higher than either GN (115 F/g) or Pani (528 F/g), highlighting the efficacy of the Pani@GN

2.5 2.0 1.5 1.0 0.5 0.0

Pani

Pani@GN

Fig. 11. DC electrical conductivity of Pani and the Pani@GN nanocomposite.

nanocomposite for electrochemical purposes. The Pani@GN nanocomposite exhibited a maximum specific capacitance of 375 F/g at a scan rate of 100 mV/s, which is much higher than previously reported value (261 F/g), highlighting the efficiency of the method for preparing a few layered GN and subsequently its composite with Pani [47]. The specific capacitance decreased with increasing scan rate from 10 to 100 mV1 (Fig. 12c). This was attributed to the swelling and shrinking of the fibers during the redox reactions. At a higher scan rate, the diffusion of the electrolyte ions was limited by the structural properties and only the outer active surface could be used for charge storage, thereby decreasing the electrochemical properties. 3.12. Electrochemical impedance spectroscopy EIS was used to examine the redox reaction resistance and equivalent series resistance. The impedance measurements were performed on Pani and the Pani@GN nanocomposite in a 1 M H2SO4 aqueous electrolyte solution by EIS and the data was recorded over the frequency range of 1e104 Hz. The EIS data was analyzed by Nyquist plots, which showed the frequency response of the electrode/electrolyte system and are a plot of the imaginary component (Z00 ) as a function of the real component (Z0 ). Each data point is at a different frequency with the lower portion of the curve corresponding to the higher frequency (Fig. 13). The curve of the Pani@GN nanocomposite in the low frequency region was more vertical than that of Pani, as evident from Fig. 13, indicating that the Pani@GN nanocomposite exhibits better charge acceptance behavior. The semicircular loop of Pani at a higher frequency can be assigned to the charge transfer resistance of the electrode. After the incorporation of GN, the Pani@GN nanocomposite showed a much lower charge transfer resistance, which might be due to the highly conducting nature of GN. GN acts as a charge collector and reduces

288

N. Parveen et al. / Composites Part B 87 (2016) 281e290

0.10

0.020

(a)

0.08 Pani@GN

0.015

0.06 Pani GN Pani@GN

0.005

0.04

Current (A)

0.010

Current (A)

100 mV

0.000

0.02 0.00

10 mV

-0.02

(b)

-0.04

-0.005

-0.06 -0.010

-0.08 -0.10

-0.015 0.0

0.1

0.2 0.3 0.4 0.5 0.6 0.7 Potential (V vs Ag/AgCl)

0.8

0.0

0.1

0.2 0.3 0.4 0.5 0.6 0.7 Potential (V vs Ag/AgCl)

0.8

Fig. 12. (a) Cyclic voltammogram of GN, Pani and the Pani@GN nanocomposite at scan rate of 10 mV s1, (b) Cyclic voltammogram of the Pani@GN nanocomposite at different scan rate (10, 20, 30, 50, 70 and 100 mV s1), and (c) Specific capacitance of GN, Pani and Pani@GN nanocomposite at different scan rates from 10 to 100 mV1.

the interfacial charge transfer resistance of the Pani@GN nanocomposite. Owing to the redox reaction resistance and relatively low conductivity of the Pani electrode, the semicircular loop of Pani was larger than that of the Pani@GN nanocomposite.

3.13. Contact angle measurements Contact angle measurements were performed to examine the hydrophobicity of the Pani and the Pani@GN nanocomposite. In general, a larger contact angle is indicative of greater hydrophobicity because it resists water molecules [48]. Fig. 14 shows the changes in the contact angles of Pani and Pani@GN. The various assemblies exhibited varying affinities toward water, indicating a

Pani Pani@GN

25

-Z''/ohm

20

difference in hydrophobicity. The water contact angles of Pani (Fig. 14a) and Pani@GN (Fig. 14b) were 49 ± 3
and 71 ± 1
, respectively. The Pani@GN membrane showed the highest water contact angle of 71 ± 1
, indicating that the surfaces of the Pani@GN nanocomposite are relatively more hydrophobic in nature. A lower contact angle indicates a more hydrophilic surface, which results in an increase in water permeability of the membrane. The water droplet placed on the Pani surface with a contact angle 49 ± 3
indicated that Pani is an intrinsically hydrophilic material. An array of such relatively nanofibers are not sufficiently hydrophobic on their own. On the contrary, water droplets placed on the surface penetrate into the fibers immediately. This specific property has been attributed to the porous fibrous nature of Pani, which is expected to possess a high surface energy that causes the water to sweep into the voids of the fibers [49]. In general, the GN nanosheets cause an increase in the hydrophobicity of the surfaces [50]. Therefore, in the case of the Pani@GN nanocomposite, the incorporation of GN nanosheets led to an increase in hydrophobicity compared to Pani.

15

4. Conclusions

10

In this study, Pani was coated on the surface of GN via simple insitu polymerization. The synergistic effect of GN and Pani facilitated the charge transfer reactions. CV showed that the Pani@GN nanocomposite had a higher specific capacitance (613 F/g) than Pani (528 F/g). The enhanced capacitance of the Pani@GN nanocomposite electrode was attributed to the combined effects of GN and Pani. The incorporation of GN into Pani not only increases the conductivity, but also increases the number of active sites, which improves the charge transfer and ion transportation. The static

5 0 0

5

10

15

20

25

30

35

Z'/ohm Fig. 13. Nyquist plot of Pani and Pani@GN nanocomposite.

40

N. Parveen et al. / Composites Part B 87 (2016) 281e290

289

Fig. 14. Contact angle measurements of (a) Pani and (b) Pani@GN.

contact angle of water on the Pani@GN nanocomposite surface was measured to be 71 ± 1
, highlighting its hydrophobic nature. The Pani@GN nanocomposites with high capacitance are promising electrode materials, which may have a large number of applications in a range of electronic devices. Acknowledgments This study was supported by 2013 Yeungnam University Research Grant. References [1] Shirakawa H, Louis EJ, MacDiarmid AG, Ghiang CK, Heeger AJ. Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x. Chem Commun 1977:577e80. [2] Li D, Huang J, Kaner BR. Polyaniline nanofibers: a unique polymer nanostructure for versatile applications. Acc Chem Res 2009;42:135e45. [3] Bai H, Shi G. Gas sensors based on conducting polymers. Sensors 2007;7: 267e307. [4] Ryu SK, Kim KM, Park NG, Park YJ, Chang SH. Symmetric redox supercapacitor with conducting polyaniline electrode. J Power Sources 2002;103:305e9. [5] Wang YG, Li HQ, Xia YY. Ordered whisker like polyaniline grown on the surface of mesoporous carbon and its electrochemical capacitance performance. Adv Mater 2006;1(8):2619e23. [6] Fan LZ, Hu YS, Maier J, Adelhelm P, Smarsly B, Antonietti M. High electroactivity of polyaniline in supercapacitors by using a hierarchically porous carbon monolith as a support. Adv Funct Mater 2007;17:3083e7. [7] Stankovich S, Dikin DA, Dommett1 GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442:282e6. [8] Ping WW, Yuan PC. Preparation and characterization of poly (methyl methacrylate)-intercalated graphite oxide/poly(methyl methacrylate) nanocomposite. Polym Eng Sci 2004;44:2335e9. [9] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666e9. [10] Booth TJ, Blake P, Nair RR, Jiang D, Hill EW, Bangert U, et al. Macroscopic graphene membranes and their extraordinary stiffness. Nano Lett 2008;8: 2442e6. [11] Lee C, Wei X, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008;321:385e8. [12] Da Silva JEP, de Torresi SIC, Temperini MLA. Redox behavior of crosslinked polyaniline films. J Braz Chem Soc 2000;11:91e4. [13] Yu AP, Roes I, Davies A, Chen Z. Ultrathin, transparent and flexible graphene films for supercapacitor application. Appl Phys Lett 2010;96:253105. [14] Wang DW, Li F, Zhao J, Ren W, Chen ZG, Tan J, et al. Fabrication of graphene/ polyaniline composite paper via in situ anodic electropolymerization for high performance flexible electrode. ACS Nano 2009;3:1745e52. [15] Zhang K, Zhang LL, Zhao XS, Wu J. Graphene/Polyaniline nanofiber composites as supercapacitor electrodes. Chem Mater 2010;22:1392e401. [16] Ansari MO, Mohammad F. Thermal stability and electrical properties of dodecyl-benzene-sulfonic-acid doped nanocomposites of polyaniline and multi-walled carbon nanotubes. Compos Part B 2012;43:3541e8. [17] Singh VV, Gupta G, Batra A, Nigam AK, Boopathi M, Gutch PK, et al. Greener electrochemical synthesis of high quality graphene nanosheets directly from pencil and its SPR sensing application. Adv Funct Mater 2012;22:2352e62. [18] Qiu F, Hao Y, Li X, Wang B, Wang M. Functionalized graphene sheets filled isotactic polypropylene nanocomposites. Compos Part B 2015;71:175e83.

[19] Parvez K, Wu ZS, Li R, Liu X, Graf R, Feng X, et al. Exfoliation of graphite into graphene in aqueous solutions of inorganic salts. J Am Chem Soc 2014;136: 6083e91. [20] Yan J, Wei T, Shao B, Fan Z, Qian W, Zhang M, et al. Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance. Carbon 2010;48:487e93. [21] Du FP, Wang JJ, Tang CY, Tsui CP, Xie XL, Yung KF. Enhanced electrochemical capacitance of polyaniline/graphene hybrid nanosheets with graphene as templates. Compos Part B 2013;53:376e81. [22] Shao W, Jamal R, Xu F, Ubul A, Abdiryim T. The effect of a small amount of water on the structure and electrochemical properties of solid-state synthesized polyaniline. Materials 2012;5:1811e25. [23] Kim J, Park SJ, Kim S. Capacitance behaviors of polyaniline/graphene nanosheet composites prepared by aniline chemical polymerization. Carbon Lett 2013;14:51e4. [24] Wang L, Ye Y, Lu X, Wen Z, Li Z, Hou H, et al. Hierarchical nanocomposites of polyaniline nanowire arrays on reduced graphene oxide sheets for supercapacitors. Sci Rep 2013;3:3568. [25] Goswami S, Maiti UN, Maiti S, Nandy S, Mitra MK, Chattopadhyay KK. Preparation of grapheneepolyaniline composites by simple chemical procedure and its improved field emission properties. Carbon 2011;49:2245e52. [26] Lin YC, Hsu FH, Wu TM. Enhanced conductivity and thermal stability of conductive polyaniline/graphene composite synthesized by in situ chemical oxidation polymerization with sodium dodecyl sulfate. Synth Met 2013;184: 29e34. [27] Moriche R, Prolongo SG, S anchez M, Suarez AJ, Sayagues MJ, Urena A. Morphological changes on graphene nanoplatelets induced during dispersion into an epoxy resin by different methods. Compos Part B 2015;72:199e205. [28] Joshi A, Bajaj A, Singh R, Anand A, Alegaonkar PS, Datar S. Processing of graphene nanoribbon based hybrid composite for electromagnetic shielding. Compos Part B 2015;69:472e7. [29] Zhou M, Tang J, Cheng Q, Xu G, Cui P, Qin LC. Few-layer graphene obtained by electrochemical exfoliation of graphite cathode. Chem Phys Lett 2013;572: 61e5. [30] Ansari MO, Khan MM, Ansari SA, Lee J, Cho MH. Enhanced thermo electric behaviour and visible light activity of Ag@TiO2/polyaniline nanocomposite synthesized by biogenic-chemical route. RSC Adv 2014;4:23713e9. [31] Parveen N, Ansari MO, Cho MH. Simple route for gram synthesis of less defective few layered graphene and its electrochemical performance. RSC Adv 2015;5(4). 4920e4927. [32] Boyer MI, Quillard S, Louarn G, Froyer G, Lefrant S. Vibrational study of the FeCl3-doped dimer of polyaniline; a good model compound of emeraldine salt. J Phys Chem B 2000;104:8952e61. [33] Niaura G, Mazeikiene R, Malinauskas A. Structural changes in conducting form of polyaniline upon ring sulfonation as deduced by near infrared resonance Raman spectroscopy. Synth Met 2004;145:105e12. [34] Mazeikiene R, Niaura G, Malinauskas A. In situ Raman spectroelectrochemical study of electrocatalytic processes at polyaniline modified electrodes: redox vs. metal-like catalysis. Electrochem Commun 2005;7:1021e6. [35] Nascimento GMD, Temperini MLA. Studies on the resonance Raman spectra of polyaniline obtained with near-IR excitation. J Raman Spectrosc 2008;39: 772e8. [36] Mashat LA, Shin K, Zadeh KK, Plessis JD, Han SH, Kojima RW, et al. Graphene/ polyaniline nanocomposite for hydrogen sensing. J Phys Chem C 2010;114: 16168e73. [37] da Silva JEP, Torresi SIC, deFariaa DLA, Temperin MLA. Raman characterization of polyaniline induced conformational changes. Synth Met 1999;101:834e5. [38] Aymen M, Sami S, Ahmed S, Fethi G, Abdellatif BM. Correlation between raman spectroscopy and electrical conductivity of graphite/polyaniline composites reacted with hydrogen peroxide. J Phys D Appl Phys 2013;46:335103. [39] Kocherginsky NM, Wang Z. Redox reactions of polyaniline films doped with d, l-camphor sulfonic acid. React Funct Polym 2006;66:1384e93.

290

N. Parveen et al. / Composites Part B 87 (2016) 281e290

[40] Golczak S, Kanciurzewska A, Fahlman M, Langer K, Langer JJ. A comparative XPS surface study of polyaniline thin films. Solid state Ionics 2008;179: 2234e9. [41] Liu Y, Deng R, Wang Z, Liu H. Carboxyl-functionalized graphene oxidepolyaniline composite as a promising supercapacitor material. J Mater Chem 2012;22:13619e24. [42] Bandgar DK, Khuspe GD, Pawar RC, Lee CS, Patil VB. Facile and novel route for preparation of nanostructured polyaniline (PANi) thin films. Appl Nanosci 2014;4:27e36. [43] Ansari MO, Yadav SK, Cho JW, Mohammad F. Thermal stability in terms of DC electrical conductivity retention and the efficacy of mixing technique in the preparation of nanocomposites of graphene/polyaniline over the carbon nanotubes/polyaniline. Compos Part B 2013;47:155e61. [44] Ansari MO, Mohammad F. Thermal stability of HCl-doped-polyaniline and TiO2 nanoparticles-based nanocomposites. J App Polym Sci 2011;124: 4433e42. [45] Yoon SB, Yoon EH, Kim KB. Electrochemical properties of leucoemeraldine, emeraldine, and pernigraniline forms of polyaniline/multi-wall carbon

[46]

[47]

[48]

[49]

[50]

nanotube nanocomposites for supercapacitor applications. J Power Sources 2011;196:10791e7. Li Y, Peng H, Li G, Chen K. Synthesis and electrochemical performance of sandwich-like polyaniline/graphene composite nanosheets. Eur Polym J 2012;48:1406e12. Li X, Song H, Zhang Y, Wang H, Du K, Li H, et al. Enhanced capacitance of graphene nanosheets coating with polyaniline for supercapacitors. Int J Electrochem Sci 2012;7:5163e71. Abreu SB, Skinner W. Determination of contact angles, silane coverage, and hydrophobicity heterogeneity of methylated quartz surfaces using ToF-SIMS. Langmuir 2012;28:7360e7. Dhawale DS, Salunkhe RR, Jamadade VS, Dubal DP, Pawar SM, Lokhande CD. Hydrophilic polyaniline nanofibrous architecture using electrosynthesis method for supercapacitor application. Curr Appl Phys 2010;10:904e9. Li Z, Wang Y, Kozbial A, Shenoy G, Zhou F, McGinley R, et al. Effect of airborne contaminants on the wettability of supported graphene and graphite. Nat Mater 2013;12:925e31.