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graphene nanofibers for transparent electrode applications
Synthetic Metals 191 (2014) 113–119
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrical measurement of PVA/graphene nanofibers for transparent electrode applications K. Jothi Ramalingam, N.R. Dhineshbabu, S.R. Srither, B. Saravanakumar, R. Yuvakkumar, V. Rajendran ∗ Centre for Nanoscience and Technology, K. S. Rangasamy College of Technology, Tiruchengode 637 215, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 20 December 2013 Received in revised form 17 February 2014 Accepted 4 March 2014 Keywords: Graphene Electrospinning PVA/graphene hybrid nanofibers Raman spectra Electrical conductivity
a b s t r a c t In this study, using alkaline solution, we synthesized graphene-based nanosheets via general chemical reduction of exfoliated graphene oxide by Hummers method. The prepared sheets were characterized using Raman spectra, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Polyvinyl alcohol (PVA) was synthesized using sol–gel method. The PVA/graphene hybrid nanofibers were produced by electrospinning technique with different concentrations (10%, 30%, 50%, 70%, and 90%). The characteristics of the prepared hybrid nanofibers were confirmed through SEM and TEM analyses. The results showed an enhanced electrical conductivity of 10.7 × 10−6 S cm−1 for higher concentration of PVA/graphene hybrid nanofiber compared with 3.7 × 10−12 S cm−1 for pure PVA. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is an easy and versatile method to produce nanofibers that have a potential use as a barrier/membrane and act as a polymeric material [1]. The one-dimensional (1D) nanostructures have many engineering applications, such as protective textile, tissue engineering, drug release, and air/water filtration [2–5]. To enhance the optical, thermal, and electrical properties of nanocomposite fibers, the produced unbreakable polymeric nanofibers are incorporated with nanofillers such as 1D (needlelike structure) and 2D (sheet-like structure) materials. The 2D fillers are based on the carbon materials such as graphite nanoplatelets, graphene oxide (GO), carbon nanotubes, and graphene [6–8]. Among these, graphene is the one of the essential materials that can enhance the physicochemical properties of potential applications [9]. A single layer of graphene does not interact with the polymeric solution because of the absence of free bonds. Until today, interfacial interactions between layered graphene and polymer matrix are reported [10,11]. Hence, to overcome this, several attempts have been made to develop a graphene that can interact with the polymer by the addition of polar solvents such as NaOH, KOH, NaBH4 , and hydrazine [12,13]. Recently,
∗ Corresponding author. Tel.: +91 4288 274741–4/4288 274860; fax: +91 4288 274880/4288 274860. E-mail address: [email protected] (V. Rajendran). http://dx.doi.org/10.1016/j.synthmet.2014.03.004 0379-6779/© 2014 Elsevier B.V. All rights reserved.
functionalized graphene–polymer nanocomposite is prepared with some polymers such as polystyrene, polyvinyl acetate, polyvinyl alcohol (PVA), and poly (methyl methacrylate) [14,15]. On the basis of the reported studies, PVA, a polyhydroxy polymer, is one of the biodegradable polymers known for their excellent physical and mechanical properties and chemical resistance for broad applications [16,17]. In this investigation, we synthesized graphene from natural graphite by Hummers method, which is an intermediate, chemically effective, and low-cost method used to obtain graphene from its oxides [18]. PVA nanofibers are produced using electrospinning technique with graphene nanofillers at five concentrations (i.e., 10%, 30%, 50%, 70%, and 90%). The prepared hybrid nanofibers were tested for electrical conductivity for transparent electrode because of the higher concentration of graphene present in the material. 2. Materials and methods 2.1. Materials Graphite powders (99.5% purity), sulfuric acid (H2 SO4 ; 98% purity), sodium nitrate (NaNO3 ; 99% purity), potassium permanganate (KMnO4 ; 98.5% purity), hydrogen peroxide (H2 O2 ; ≥30% purity), hydrochloric acid (HCl; ≥30% purity), PVA (MW: 20,000; 78% purity), all from Merck, and deionized water were used without any further purification for the experiment. The horizontal electrospinning setup established using syringe pump (Cole-Parmer,
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USA), high-voltage power supply, rotating drum, and variable speed controller (Annai Electronics, India) was used to synthesize hybrid nanofibers in our laboratory [19]. 2.2. Preparation and characterization of graphene Graphene oxide was prepared from graphite powder and graphene was obtained from reduction of GO using Hummers method. To prepare graphene, we placed 2 g graphite powder into 35 ml concentrated H2 SO4 with continuous stirring in a dry ice bath. Then, 1 g of NaNO3 and 3 g KMnO4 was gradually added into the above solution with continuous vigorous stirring for 6 h. Further, temperature was gradually increased from 20 to 30 ◦ C for 2 h while the dry ice bath was removed. The solution was diluted by addition of 90 ml deionized water at 80 ◦ C. The color of the suspension was dark brown. The reaction was maintained for 40 min to increase oxidation of the GO product. Then, bright-yellow suspension was observed by addition of 140 ml distilled water followed by 30 ml H2 O2 . The obtained solution was centrifuged at 10,000 rpm (Remi, India) and washed with HCL solution. Then, the centrifuged solution was rinsed three times with distilled water until the pH of the solution became neutral. The collected wet sample was dried overnight at 80 ◦ C, and finally black powder was observed. The required graphene nanopowder was prepared by reduction of GO. The obtained GO powder (20 mg) was added into 200 ml deionized water under continuous stirring for 15 min. The suspension was further treated with ultrasonication (25 kHz) for 30 min using an ultrasonicator (Sonic, USA). The dispersion of GO sheets was observed in the suspension. The aqueous NaOH solution (1 wt%) was added drop-wise to the GO suspension in order to adjust the pH up to 10. Finally, yellow-brown GO suspension became black under sonication. The obtained suspension was centrifuged with deionized water and dried at 60 ◦ C for 12 h. The Raman spectra of the graphene sample were measured using a Raman spectroscopy (HORIBA Jobin Yvon, France) with a confocal microscopy equipped with a solid-state crystal laser ( = 532 nm) as the excitation source. The obtained spectrum was adjusted by image acquisition software (LabSpec 5.1; HORIBA Jobin Yvon) along with Raman spectroscopy. The infrared transmittance spectra were analyzed at room temperature using wave number range from 4000 to 400 cm−1 by a computerized Fourier transform infrared spectrophotometer (Spectrum 100; PerkinElmer, USA). The sample was prepared with KBr pellet method with a weight ratio of 1:100. A small quantity of the powder sample was placed on a double-sided carbon-coated tape attached to the sample-holder while analyzing by a scanning electron microscope equipped with an energy-dispersive spectroscopy (SEM-EDS; JSM-6390LV; JEOL, Japan). The individual morphology of the powder sample was obtained by transmission electron microscopy (TEM). The prepared powder sample was dispersed in ethanol in an ultrasonic bath and then placed drop-wise on the carbon-coated copper grid. Then, the sample was dried for 5 min and again placed on the sample-holder for TEM analysis. 2.3. Production and characterization of nanofibers The PVA powder (10 g) was dissolved in 90 ml deionized water under continuous stirring at 50 ◦ C for 60 min. The prepared graphene powder was added into the aqueous solution of PVA in the following composition (w/w between graphene and PVA solution concentration): 0%, 10%, 30%, 50%, 70%, and 90% to obtain composite solutions. The aqueous solution and the composite solutions were used to produce nanofiber employing electrospinning technique. The aqueous solution was filled in a capillary tube with steel needle (inner
Fig. 1. Characterization of graphene sheets. (a) Raman spectra and (b) FTIR spectra.
diameter 0.3 mm) placed on the syringe pump. The tip-to-collector distance was approximately 19 cm and feed rate of the solution was 0.09 ml h−1 . The positive terminal of applied voltage (19 kV) was connected to the steel needle and the negative terminal was joined to the rotating drum (30 rpm) and then grounded. During the spinning process, the nanofiber web was collected for 10 min on an aluminum foil of ∼15 m thickness. The same procedure was used to prepare composite PVA/graphene nanofibers. The collected nanofibers were used for comprehensive characterization. Hereafter, PVA and PVA/graphene hybrid nanofibers are termed as PF and PGF. The microstructure of the sample was studied using SEM-EDS (JSM-6390LV: JEOL). A small (1 × 1 cm) section of the nanofiber mat was placed on the double-sided carbon-coated tape for gold sputtering before the SEM analysis. The nanofibers were applied on the carbon-coated copper grid for TEM analysis along with a selected area energy diffraction pattern (CM200; Philips, USA). Electrochemical impedance spectroscopy (EIS) measurements were carried out using a frequency response analyzer-equipped Autolab PGSTAT128N bipotentiostat under potentiostatic mode at 10 mV with frequency range 0.1–100 kHz. A home-made twoprobe conductivity cell (Perspex) designed with two copper plates
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Fig. 2. Micrographical analysis of graphene sheets. (a) Surface morphology and (b) primary morphology.
pressed against each other using spring action was used to measure the through-plane conductivity. All the composites were placed between the two Cu disk electrodes having 4 mm diameter, and the conductivity measurements were carried out at 25 ± 1 ◦ C. The electrical properties of prepared samples and their interface with electrodes are analyzed by impedance spectroscopy, which is relatively a new and powerful method of characterizing electrical properties and investigating the dynamics of bound or mobile charge in the bulk or interfacial regions of any kind of solid including ionic or mixed electronic–ionic materials and even insulators [20]. 3. Results and discussion The obtained Raman spectrum (HORIBA; = 633 nm) in Fig. 1a clearly reveals a prominent peak, G band, at 1580 cm−1 , which shows the single-crystalline graphene with in-plane vibrational mode [21]. In addition, the D band observed at 1334 cm−1 confirms the increase in size of the in-plane sp2 domains upon reduction of the exfoliated GO [22]. The position of D and G bands indicates that formation of sp2 -bonded linear chains with structural and topological disorders could be in the form of distortion in the bond length, bond angle, and delocalization of -states around the
sp2 chains [20–25]. The 2D band observed at 2680 cm−1 shows an increase in the number layers of graphene sheets [26]. Fig. 1b shows functional group of the prepared graphene nanosheets. The peak observed at 3440 cm−1 corresponds to the strong vibrations of O H group [10], whereas the peak observed at 1390 cm−1 corresponds to the symmetric stretching vibrations of C O bond. The vibration of water molecules or nonoxidized graphitic fields was observed at 1640 cm−1 [26]. Further, the peak observed at 1046 cm−1 corresponds to C O bond stretching [26,27]. The electron microscopy images of the prepared graphene sheets are shown in Fig. 2, which confirm the layer-like morphology of the graphene powder. The size of the obtained graphene layer is 200 nm. The SEM images of graphene sheets along with the EDS pattern are shown in Fig. 2a. The percentages of C, O, S, and Mn obtained from the EDS pattern are 90.83, 7.72, 0.39, and 1.06 wt%, respectively. The transparent layer of internal microstructure of graphene sheet is studied through TEM images (Fig. 2b). The size of the layer is approximately 100 nm with non-circular morphology. The XRD pattern of pure PVA (PF) and graphene sheet loaded PVA nanofibers (PGF) are shown in Fig. 3a–f. Fig. 3a shows the semicrystalline peak at 2 = 20.2◦ of pure PVA nanofiber. Fig. 3b–d reveals the lower concentration of graphene loaded PVA nanofibers are examined the crystalline peak at 2 = 26.7◦ . On the other hand,
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Fig. 3. XRD pattern of nanofibers with and without graphene at different concentrations.
Fig. 3e and f shows the peaks are observed at 2 = 26.7◦ and 43.2◦ corresponds to the characteristic peaks (0 0 2) and (1 0 0) plane reflection of graphene sheets. The crystalline peaks of graphene are related to the standard powder diffraction card file No. 010646 [28]. From the observed XRD peaks (Fig. 3b–f) confirm the nano-graphene sheets are inside the PVA nanofiber matrix. The homogeneous PVA and PVA/graphene nanofibers are shown in Fig. 4. In electrospinning, polymeric composite solution is ejected under high DC voltage to form an ultrafine fiber of 200–400 nm.
The morphology of the fiber is changed with the homogeneous solution that creates the occurrence of without beads in fiber. The surface morphology of the homogenous PVA and graphene-loaded PF clearly shows long fiber mats due to the relevant high electric field. The formation of graphene in PF is increased with the increase in the concentration of graphene. The graphene nanopowder is well arranged inside the PF. The average diameter of PGF is calculated from SEM and is shown in Table 1. The average diameter of nanofibers with and without graphene increases on increasing
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Fig. 4. Surface morphology of nanofibers with and without graphene at different concentrations. Table 1 The average diameter of PGF with different concentrations. Samples
PF 0.1 PGF 0.3 PGF 0.5 PGF 0.7 PGF 0.9 PGF a
Weight % of PVA
10 wt% 10 wt% 10 wt% 10 wt% 10 wt% 10 wt%
Error value of the fiber diameter: ±15 nm.
Parameter Voltage (kV)
Distance (cm)
Feed rate (ml h−1 )
Diameter (nm)a
18 18 18 18 18 18
19 19 19 19 19 19
0.5 0.5 0.5 0.5 0.5 0.5
150 200 214 235 254 262
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the concentration of graphene from 10% to 90%. An average of 8–10 nanofibers were taken from SEM images for the measurement of the diameter of the fiber. The average value of the diameter of the fiber is obtained. The percentage of error in the determination of diameter is ±20 nm. The error bar for the measurement of diameter is included in Fig. 5. The internal microstructures of the spun PF and PGF are shown in Fig. 6. Fig. 6 clearly shows that the filler amount of graphene sheets is present in the PF matrix. The observed results clearly depict that the prepared graphene is arranged in the middle of the PF. In addition, the TEM observation clearly indicates that a good affinity between PVA and graphene resulted in the homogeneous fibers without beads. The diameter of the PF is approximately
Fig. 5. The diameter of nanofiber with various concentration of graphene.
Fig. 6. TEM image of nanofibers with and without graphene at different compositions.
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Table 2 Obtained resistance and calculated conductivity with different composition. Graphite loading in PVA (g) 0 0.1 0.3 0.5 0.7 0.9
Film thickness (m)
Resistance ()
Conductivity (×10−6 S cm−1 )
90 60 70 50 80 80
1.93 × 10 0.07614 0.05744 0.07902 0.05969 0.05993
3.7 5.23 6.93 7.70 10.6 10.7
5
was characterized and its electrical conductivity was measured by the electrical impedance spectroscopy. The ratio of graphene was increased in PF, which creates dramatic changes in electrical measurements. Hence, the electrical conductivity of loaded graphene (70% and 90%) in PF shows a similar relation. In conclusion, 70% and above ratio of loaded graphene is used for nanoelectronics, transparent electrode, and supercapacitor applications. Acknowledgment We acknowledge the financial support (ERIPR/ER/0905103/ M/01/1279) provided by Defence Research and Development Organisation (DRDO), New Delhi, for carrying out the research. References
Fig. 7. EIS spectra of PVA nanofibers with and without graphene.
150 ± 15 nm. Moreover, the increase in graphene concentration in PVA also increased the diameter of the nanofiber (Table 1). In this study, EIS has been used to study the resistive behavior of materials with their interface with electrodes. The measured impedance can be applied to examine and qualitatively determine the electronic/ionic conduction in electrode and electrolytes; interfacial charging at either the surface films or the double-layer, charge transfer processes; and the mass transfer effects [29]. From, Fig. 7 shows the comparison of Nyquist plots of different concentration of graphene over polyvinyl alcohol (PGF), and the plots are recorded at 10 mV perturbation potential with the frequency range between 100 kHz and 0.1 Hz. From Fig. 5, we can observe that the linearity behavior for all the samples depicts the conducting behavior of the PGFs. The true ohmic resistance of the composite is measured by taking the real impedance at the point where the imaginary impedance is equal to zero [30]. In addition, the linear trend in its conductivity increases with an increase in the amount of graphite in PGFs. The maximum conductivity is found to be ∼10.6 S cm−1 for 0.7 g PGF and no appreciable change is observed with increase in the graphite loading. It clearly shows that 0.7 g PGF is compatible composition with improved conductivity than other composition and pure PVA. The obtained resistance and calculated conductivity for all composition are compared in Table 2 and clearly show that the graphene nanofibers prepared by this method possess promising electrical conductivity, which can be used for transparent electrode applications and could be a promising material for the optoelectronic devices. 4. Conclusion In this study, the solubilized graphene sheets were successfully synthesized using Hummers method and the required PGFs were produced by electrospinning technique with different concentrations of loaded graphene. The microstructure of the nanofibers
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Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrical measurement of PVA/graphene nanofibers for transparent electrode applications K. Jothi Ramalingam, N.R. Dhineshbabu, S.R. Srither, B. Saravanakumar, R. Yuvakkumar, V. Rajendran ∗ Centre for Nanoscience and Technology, K. S. Rangasamy College of Technology, Tiruchengode 637 215, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 20 December 2013 Received in revised form 17 February 2014 Accepted 4 March 2014 Keywords: Graphene Electrospinning PVA/graphene hybrid nanofibers Raman spectra Electrical conductivity
a b s t r a c t In this study, using alkaline solution, we synthesized graphene-based nanosheets via general chemical reduction of exfoliated graphene oxide by Hummers method. The prepared sheets were characterized using Raman spectra, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Polyvinyl alcohol (PVA) was synthesized using sol–gel method. The PVA/graphene hybrid nanofibers were produced by electrospinning technique with different concentrations (10%, 30%, 50%, 70%, and 90%). The characteristics of the prepared hybrid nanofibers were confirmed through SEM and TEM analyses. The results showed an enhanced electrical conductivity of 10.7 × 10−6 S cm−1 for higher concentration of PVA/graphene hybrid nanofiber compared with 3.7 × 10−12 S cm−1 for pure PVA. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is an easy and versatile method to produce nanofibers that have a potential use as a barrier/membrane and act as a polymeric material [1]. The one-dimensional (1D) nanostructures have many engineering applications, such as protective textile, tissue engineering, drug release, and air/water filtration [2–5]. To enhance the optical, thermal, and electrical properties of nanocomposite fibers, the produced unbreakable polymeric nanofibers are incorporated with nanofillers such as 1D (needlelike structure) and 2D (sheet-like structure) materials. The 2D fillers are based on the carbon materials such as graphite nanoplatelets, graphene oxide (GO), carbon nanotubes, and graphene [6–8]. Among these, graphene is the one of the essential materials that can enhance the physicochemical properties of potential applications [9]. A single layer of graphene does not interact with the polymeric solution because of the absence of free bonds. Until today, interfacial interactions between layered graphene and polymer matrix are reported [10,11]. Hence, to overcome this, several attempts have been made to develop a graphene that can interact with the polymer by the addition of polar solvents such as NaOH, KOH, NaBH4 , and hydrazine [12,13]. Recently,
∗ Corresponding author. Tel.: +91 4288 274741–4/4288 274860; fax: +91 4288 274880/4288 274860. E-mail address: [email protected] (V. Rajendran). http://dx.doi.org/10.1016/j.synthmet.2014.03.004 0379-6779/© 2014 Elsevier B.V. All rights reserved.
functionalized graphene–polymer nanocomposite is prepared with some polymers such as polystyrene, polyvinyl acetate, polyvinyl alcohol (PVA), and poly (methyl methacrylate) [14,15]. On the basis of the reported studies, PVA, a polyhydroxy polymer, is one of the biodegradable polymers known for their excellent physical and mechanical properties and chemical resistance for broad applications [16,17]. In this investigation, we synthesized graphene from natural graphite by Hummers method, which is an intermediate, chemically effective, and low-cost method used to obtain graphene from its oxides [18]. PVA nanofibers are produced using electrospinning technique with graphene nanofillers at five concentrations (i.e., 10%, 30%, 50%, 70%, and 90%). The prepared hybrid nanofibers were tested for electrical conductivity for transparent electrode because of the higher concentration of graphene present in the material. 2. Materials and methods 2.1. Materials Graphite powders (99.5% purity), sulfuric acid (H2 SO4 ; 98% purity), sodium nitrate (NaNO3 ; 99% purity), potassium permanganate (KMnO4 ; 98.5% purity), hydrogen peroxide (H2 O2 ; ≥30% purity), hydrochloric acid (HCl; ≥30% purity), PVA (MW: 20,000; 78% purity), all from Merck, and deionized water were used without any further purification for the experiment. The horizontal electrospinning setup established using syringe pump (Cole-Parmer,
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USA), high-voltage power supply, rotating drum, and variable speed controller (Annai Electronics, India) was used to synthesize hybrid nanofibers in our laboratory [19]. 2.2. Preparation and characterization of graphene Graphene oxide was prepared from graphite powder and graphene was obtained from reduction of GO using Hummers method. To prepare graphene, we placed 2 g graphite powder into 35 ml concentrated H2 SO4 with continuous stirring in a dry ice bath. Then, 1 g of NaNO3 and 3 g KMnO4 was gradually added into the above solution with continuous vigorous stirring for 6 h. Further, temperature was gradually increased from 20 to 30 ◦ C for 2 h while the dry ice bath was removed. The solution was diluted by addition of 90 ml deionized water at 80 ◦ C. The color of the suspension was dark brown. The reaction was maintained for 40 min to increase oxidation of the GO product. Then, bright-yellow suspension was observed by addition of 140 ml distilled water followed by 30 ml H2 O2 . The obtained solution was centrifuged at 10,000 rpm (Remi, India) and washed with HCL solution. Then, the centrifuged solution was rinsed three times with distilled water until the pH of the solution became neutral. The collected wet sample was dried overnight at 80 ◦ C, and finally black powder was observed. The required graphene nanopowder was prepared by reduction of GO. The obtained GO powder (20 mg) was added into 200 ml deionized water under continuous stirring for 15 min. The suspension was further treated with ultrasonication (25 kHz) for 30 min using an ultrasonicator (Sonic, USA). The dispersion of GO sheets was observed in the suspension. The aqueous NaOH solution (1 wt%) was added drop-wise to the GO suspension in order to adjust the pH up to 10. Finally, yellow-brown GO suspension became black under sonication. The obtained suspension was centrifuged with deionized water and dried at 60 ◦ C for 12 h. The Raman spectra of the graphene sample were measured using a Raman spectroscopy (HORIBA Jobin Yvon, France) with a confocal microscopy equipped with a solid-state crystal laser ( = 532 nm) as the excitation source. The obtained spectrum was adjusted by image acquisition software (LabSpec 5.1; HORIBA Jobin Yvon) along with Raman spectroscopy. The infrared transmittance spectra were analyzed at room temperature using wave number range from 4000 to 400 cm−1 by a computerized Fourier transform infrared spectrophotometer (Spectrum 100; PerkinElmer, USA). The sample was prepared with KBr pellet method with a weight ratio of 1:100. A small quantity of the powder sample was placed on a double-sided carbon-coated tape attached to the sample-holder while analyzing by a scanning electron microscope equipped with an energy-dispersive spectroscopy (SEM-EDS; JSM-6390LV; JEOL, Japan). The individual morphology of the powder sample was obtained by transmission electron microscopy (TEM). The prepared powder sample was dispersed in ethanol in an ultrasonic bath and then placed drop-wise on the carbon-coated copper grid. Then, the sample was dried for 5 min and again placed on the sample-holder for TEM analysis. 2.3. Production and characterization of nanofibers The PVA powder (10 g) was dissolved in 90 ml deionized water under continuous stirring at 50 ◦ C for 60 min. The prepared graphene powder was added into the aqueous solution of PVA in the following composition (w/w between graphene and PVA solution concentration): 0%, 10%, 30%, 50%, 70%, and 90% to obtain composite solutions. The aqueous solution and the composite solutions were used to produce nanofiber employing electrospinning technique. The aqueous solution was filled in a capillary tube with steel needle (inner
Fig. 1. Characterization of graphene sheets. (a) Raman spectra and (b) FTIR spectra.
diameter 0.3 mm) placed on the syringe pump. The tip-to-collector distance was approximately 19 cm and feed rate of the solution was 0.09 ml h−1 . The positive terminal of applied voltage (19 kV) was connected to the steel needle and the negative terminal was joined to the rotating drum (30 rpm) and then grounded. During the spinning process, the nanofiber web was collected for 10 min on an aluminum foil of ∼15 m thickness. The same procedure was used to prepare composite PVA/graphene nanofibers. The collected nanofibers were used for comprehensive characterization. Hereafter, PVA and PVA/graphene hybrid nanofibers are termed as PF and PGF. The microstructure of the sample was studied using SEM-EDS (JSM-6390LV: JEOL). A small (1 × 1 cm) section of the nanofiber mat was placed on the double-sided carbon-coated tape for gold sputtering before the SEM analysis. The nanofibers were applied on the carbon-coated copper grid for TEM analysis along with a selected area energy diffraction pattern (CM200; Philips, USA). Electrochemical impedance spectroscopy (EIS) measurements were carried out using a frequency response analyzer-equipped Autolab PGSTAT128N bipotentiostat under potentiostatic mode at 10 mV with frequency range 0.1–100 kHz. A home-made twoprobe conductivity cell (Perspex) designed with two copper plates
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Fig. 2. Micrographical analysis of graphene sheets. (a) Surface morphology and (b) primary morphology.
pressed against each other using spring action was used to measure the through-plane conductivity. All the composites were placed between the two Cu disk electrodes having 4 mm diameter, and the conductivity measurements were carried out at 25 ± 1 ◦ C. The electrical properties of prepared samples and their interface with electrodes are analyzed by impedance spectroscopy, which is relatively a new and powerful method of characterizing electrical properties and investigating the dynamics of bound or mobile charge in the bulk or interfacial regions of any kind of solid including ionic or mixed electronic–ionic materials and even insulators [20]. 3. Results and discussion The obtained Raman spectrum (HORIBA; = 633 nm) in Fig. 1a clearly reveals a prominent peak, G band, at 1580 cm−1 , which shows the single-crystalline graphene with in-plane vibrational mode [21]. In addition, the D band observed at 1334 cm−1 confirms the increase in size of the in-plane sp2 domains upon reduction of the exfoliated GO [22]. The position of D and G bands indicates that formation of sp2 -bonded linear chains with structural and topological disorders could be in the form of distortion in the bond length, bond angle, and delocalization of -states around the
sp2 chains [20–25]. The 2D band observed at 2680 cm−1 shows an increase in the number layers of graphene sheets [26]. Fig. 1b shows functional group of the prepared graphene nanosheets. The peak observed at 3440 cm−1 corresponds to the strong vibrations of O H group [10], whereas the peak observed at 1390 cm−1 corresponds to the symmetric stretching vibrations of C O bond. The vibration of water molecules or nonoxidized graphitic fields was observed at 1640 cm−1 [26]. Further, the peak observed at 1046 cm−1 corresponds to C O bond stretching [26,27]. The electron microscopy images of the prepared graphene sheets are shown in Fig. 2, which confirm the layer-like morphology of the graphene powder. The size of the obtained graphene layer is 200 nm. The SEM images of graphene sheets along with the EDS pattern are shown in Fig. 2a. The percentages of C, O, S, and Mn obtained from the EDS pattern are 90.83, 7.72, 0.39, and 1.06 wt%, respectively. The transparent layer of internal microstructure of graphene sheet is studied through TEM images (Fig. 2b). The size of the layer is approximately 100 nm with non-circular morphology. The XRD pattern of pure PVA (PF) and graphene sheet loaded PVA nanofibers (PGF) are shown in Fig. 3a–f. Fig. 3a shows the semicrystalline peak at 2 = 20.2◦ of pure PVA nanofiber. Fig. 3b–d reveals the lower concentration of graphene loaded PVA nanofibers are examined the crystalline peak at 2 = 26.7◦ . On the other hand,
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Fig. 3. XRD pattern of nanofibers with and without graphene at different concentrations.
Fig. 3e and f shows the peaks are observed at 2 = 26.7◦ and 43.2◦ corresponds to the characteristic peaks (0 0 2) and (1 0 0) plane reflection of graphene sheets. The crystalline peaks of graphene are related to the standard powder diffraction card file No. 010646 [28]. From the observed XRD peaks (Fig. 3b–f) confirm the nano-graphene sheets are inside the PVA nanofiber matrix. The homogeneous PVA and PVA/graphene nanofibers are shown in Fig. 4. In electrospinning, polymeric composite solution is ejected under high DC voltage to form an ultrafine fiber of 200–400 nm.
The morphology of the fiber is changed with the homogeneous solution that creates the occurrence of without beads in fiber. The surface morphology of the homogenous PVA and graphene-loaded PF clearly shows long fiber mats due to the relevant high electric field. The formation of graphene in PF is increased with the increase in the concentration of graphene. The graphene nanopowder is well arranged inside the PF. The average diameter of PGF is calculated from SEM and is shown in Table 1. The average diameter of nanofibers with and without graphene increases on increasing
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Fig. 4. Surface morphology of nanofibers with and without graphene at different concentrations. Table 1 The average diameter of PGF with different concentrations. Samples
PF 0.1 PGF 0.3 PGF 0.5 PGF 0.7 PGF 0.9 PGF a
Weight % of PVA
10 wt% 10 wt% 10 wt% 10 wt% 10 wt% 10 wt%
Error value of the fiber diameter: ±15 nm.
Parameter Voltage (kV)
Distance (cm)
Feed rate (ml h−1 )
Diameter (nm)a
18 18 18 18 18 18
19 19 19 19 19 19
0.5 0.5 0.5 0.5 0.5 0.5
150 200 214 235 254 262
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the concentration of graphene from 10% to 90%. An average of 8–10 nanofibers were taken from SEM images for the measurement of the diameter of the fiber. The average value of the diameter of the fiber is obtained. The percentage of error in the determination of diameter is ±20 nm. The error bar for the measurement of diameter is included in Fig. 5. The internal microstructures of the spun PF and PGF are shown in Fig. 6. Fig. 6 clearly shows that the filler amount of graphene sheets is present in the PF matrix. The observed results clearly depict that the prepared graphene is arranged in the middle of the PF. In addition, the TEM observation clearly indicates that a good affinity between PVA and graphene resulted in the homogeneous fibers without beads. The diameter of the PF is approximately
Fig. 5. The diameter of nanofiber with various concentration of graphene.
Fig. 6. TEM image of nanofibers with and without graphene at different compositions.
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Table 2 Obtained resistance and calculated conductivity with different composition. Graphite loading in PVA (g) 0 0.1 0.3 0.5 0.7 0.9
Film thickness (m)
Resistance ()
Conductivity (×10−6 S cm−1 )
90 60 70 50 80 80
1.93 × 10 0.07614 0.05744 0.07902 0.05969 0.05993
3.7 5.23 6.93 7.70 10.6 10.7
5
was characterized and its electrical conductivity was measured by the electrical impedance spectroscopy. The ratio of graphene was increased in PF, which creates dramatic changes in electrical measurements. Hence, the electrical conductivity of loaded graphene (70% and 90%) in PF shows a similar relation. In conclusion, 70% and above ratio of loaded graphene is used for nanoelectronics, transparent electrode, and supercapacitor applications. Acknowledgment We acknowledge the financial support (ERIPR/ER/0905103/ M/01/1279) provided by Defence Research and Development Organisation (DRDO), New Delhi, for carrying out the research. References
Fig. 7. EIS spectra of PVA nanofibers with and without graphene.
150 ± 15 nm. Moreover, the increase in graphene concentration in PVA also increased the diameter of the nanofiber (Table 1). In this study, EIS has been used to study the resistive behavior of materials with their interface with electrodes. The measured impedance can be applied to examine and qualitatively determine the electronic/ionic conduction in electrode and electrolytes; interfacial charging at either the surface films or the double-layer, charge transfer processes; and the mass transfer effects [29]. From, Fig. 7 shows the comparison of Nyquist plots of different concentration of graphene over polyvinyl alcohol (PGF), and the plots are recorded at 10 mV perturbation potential with the frequency range between 100 kHz and 0.1 Hz. From Fig. 5, we can observe that the linearity behavior for all the samples depicts the conducting behavior of the PGFs. The true ohmic resistance of the composite is measured by taking the real impedance at the point where the imaginary impedance is equal to zero [30]. In addition, the linear trend in its conductivity increases with an increase in the amount of graphite in PGFs. The maximum conductivity is found to be ∼10.6 S cm−1 for 0.7 g PGF and no appreciable change is observed with increase in the graphite loading. It clearly shows that 0.7 g PGF is compatible composition with improved conductivity than other composition and pure PVA. The obtained resistance and calculated conductivity for all composition are compared in Table 2 and clearly show that the graphene nanofibers prepared by this method possess promising electrical conductivity, which can be used for transparent electrode applications and could be a promising material for the optoelectronic devices. 4. Conclusion In this study, the solubilized graphene sheets were successfully synthesized using Hummers method and the required PGFs were produced by electrospinning technique with different concentrations of loaded graphene. The microstructure of the nanofibers
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