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Current–voltage characteristics of electrochemically synthesized multi-layer graphene with polyaniline
Accepted Manuscript Current-Voltage characteristics of electrochemically synthesized multi-layer graphene with polyaniline Seema Awasthi, Praveen S. Gopinathan, A. Rajanikanth, C. Bansal PII:
S2468-2179(17)30227-7
DOI:
10.1016/j.jsamd.2018.01.003
Reference:
JSAMD 140
To appear in:
Journal of Science: Advanced Materials and Devices
Received Date: 12 December 2017 Revised Date:
20 January 2018
Accepted Date: 23 January 2018
Please cite this article as: S. Awasthi, P.S. Gopinathan, A. Rajanikanth, C. Bansal, Current-Voltage characteristics of electrochemically synthesized multi-layer graphene with polyaniline, Journal of Science: Advanced Materials and Devices (2018), doi: 10.1016/j.jsamd.2018.01.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Current-Voltage characteristics of electrochemically synthesized multi-layer graphene with polyaniline Seema Awasthi*, Praveen S. Gopinathan, A. Rajanikanth and C. Bansal School of Physics, University of Hyderabad, Hyderabad-500046, India
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ABSTRACT Graphene nanosheets (GNS) synthesized through commercially available pencil lead were used to prepare composites with polyaniline on ITO coated PET substrate. I-V
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characteristics of GNS-PANI composite device shows a decrease in band gap of PANI from 2.8 eV to 6.9 meV at 20 wt% loading of GNS in PANI. SEM and Raman spectroscopy show good
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dispersion of GNS and interfacial interaction with GNS-PANI composite by significant shift in the G peak at 20 wt% GNS-PANI, further confirming the formation of composites at 20 wt%. Keywords: graphene, polyaniline, I-V characteristics, composite, band gap
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Corresponding author email: [email protected]
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1. Introduction Graphene possesses extremely high carrier mobility (100 000 cm2/V.s), strength, flexibility, high current carrying capacity (109 A/cm2), and high thermal conductivity. These
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properties of graphene can be used for the applications in the area of energy storage, supercapacitors, transparent electrodes, sensors and in composite materials [2-6]. The unusual electronic properties of graphene make it a promising candidate for future electronic
13]. polymers have been studied with graphene as fillers.
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applications. Polyaniline, polypyrrole, polythiophene, polystyrene, polymethyl methacrylate [9-
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The electrochemical exfoliation of graphite in presence of suitable electrolytic medium has opened a new single step one pot method for the preparation of graphene [17-20]. Su et al prepared high quality and large area graphene sheets by electrochemical exfoliation of natural graphite flakes in sulfuric acid as electrolyte medium [21]. Guo and coworkers has synthesized
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high quality GNS in large quantity through electrochemical reduction of exfoliated graphite oxide precursor at cathodic potentials [22]. Singh et al have produced high quality GNS from pencil graphite by electrochemical exfoliation in ionic liquid medium following non-aqueous
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route [23].
In the present investigation, GNS have been prepared by aqueous anodic exfoliation of pencil
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graphite in acidic medium at different dc voltages (5V to 12V). The GNS prepared at optimum voltage were mixed with polyaniline (PANI) polymer and drop casted on indium tin oxide (ITO)/Polyethylene terephthalate (PET) substrate to study the device I-V characteristics and band gap measurments. The room temperature to low temperature I-V characteristics of the as prepared polymer-GNS devices were measured. Commercially available pencil graphite was used as working electrode.
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2. Experimental 2.1 Materials
received. 2.2 Synthesis of graphene and graphene polyanline nanocomposite
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All chemicals (Polyaniline, DMF, H2SO4) were purchased from Sigma Aldrich and used as
Electrochemical exfoliation of pencil graphite has been performed in 0.2M H2SO4 as electrolyte
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and pencil graphite graphite(0.7mm×50mm) as working electrode, Ag/AgCl as reference electrode and platinum wire as counter electrode on a Gamry Instruments model Reference
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600TMPotentiostat/Galvanostat/ZRA. A voltage of 1V was applied for 5 min by a potentiostat. Electrochemical reaction started very slowly at 1.8V. This condition was held for 5 min to make the pencil graphite totally occupied with sulfate ions [21]. The exfoliation was performed at different voltages from 5V to 12V. In every experiment, the reaction was held till the whole
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pencil graphite consumes in the process. After complete exfoliation, the black precipitate from the vessel was filtered, washed with de-ionized water and the exfoliated sheets have been collected. The as prepared GNS was used at different (3, 6, 15, 20) wt% to prepare composites
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with polyaniline. PANI was dissolved in di-methylformamide (DMF) solution and ultrasonicated GNS was added to the polymer solution followed by magnetic stirring for 60mins.
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2.3 Preparation of GNS-PANI thin film devices Indium tin oxide (ITO) coated Polyethylene terephthalate (PET) as the flexible substrate was used for the device fabrication. The source-drain was deposited on this substrate through vacuum coating of silver (Ag) wire. A mask of gold wire (dia. 0.127mm) was used to form a channel between the source-drain. The as prepared GNS-PANI solution has been drop casted onto Ag source-drain deposited PET/ITO substrate and left to dry overnight.
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2.4 Characterization techniques The as synthesized GNS have been characterized by X-ray diffraction technique (INEL X-ray Diffractometer (XRG-3000 with CoKα 1.79Å) for structural and by scanning electron
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microscopy (FESEM, Model Ultra55 from Carl Zeiss Germany) for microstructural morphology. Raman spectroscopy (LabRam HR-800-Horiba JobinYvon) at 514nm excitation wavelength of 0.9mW power was used for spectroscopic characterization.
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2.5 IV characteristics measurements
The room temperature current-voltage (I-V)characteristics have been measured with the
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semiconductor device analyzer (Agilent B1500A) and low temperature I-V measurements were performed in cryogenic probe station (Model CRX-4K from lakeshore Cryotronics). 3. Results and discussions 3.1 Synthesis of GNS
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Fig. 1(a) shows the schematic diagram of electrochemical exfoliation of pencil graphite as working electrode in 0.2M H2SO4 electrolyte with Ag/AgCl as reference, and Pt wire as counter electrode. The energy dispersive X-ray spectrum of pencil graphitewas displayed in fig. 1 (b).
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The pencil graphite was found to contain mainly carbon (C). Si was found as impurity. No other elements are present in the pencil graphite confirming that GNS prepared from this pencil
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graphite are free from impurities. Fig. 1(c) shows the schematic diagram of ITO coated PET GNS-PANI flexible device with polymer-GNS as channel and source-drain of Ag. The channel width in this case is 0.1 mm. 3.2 Structural and morphological analysis Fig. 2(a) shows the X-ray diffraction pattern of pencil graphite and as synthesized GNS at 9V to 12V. The XRD of pencil graphite clearly shows a sharp and intense peak at 30.50. The
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characteristic graphitic (002) peak which was coming in pencil graphite has significantly reduced and broadened in the exfoliated samples at different voltages.The broadening of (002) peak clearly shows the reduction in number of graphene layers. Lorentzian curve fitting of (002) peaks
2 (b). A minimum of ~5 layers was obtained at a voltage of 10V.
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[24] was done for calculating the number of layers in the as synthesized samples as shown in fig.
Fig. 3 (a-f) shows SEM images of pencil graphite and as synthesized GNS from 8V to 12V,
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respectively. Fig. 3 (a) shows SEM image of pencil graphite displaying continuous sheet like structure. After applying a dc voltage the sheets began to exfoliate. The exfoliated sheets at 8V
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and 9V are shown in fig. 3 (b)& (c). Thick sheet like morphology are clearly visible. As we can see in the Fig. 3 (c) thick graphite flakes are produced at 9V, while in Fig. 3(d) these graphite flakes are being exfoliated at 10V. The trend of exfoliation increases as we increased the voltage to 9V and it can clearly be seen from Fig. 3 (d) the transparent, sheet like and crumpled structure
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of GNS has been formed. The SEM image shows a typical worm-like shape with thin layered and wrinkled graphene structure.
In Fig. 4 (e&f) SEM image is composed of fluffy aggregates of crumpled sheets obtained at 11V
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and 12V respectively.The morphology of GNS-PANIcomposite on ITO/PET substrate with various GNS loading was investigated through FESEM, shown in Fig. 4. Fig. 4 (a) shows the
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SEM images of GNS-PANI composite at 6wt% GNS in PANI. Due to the lower concentration of GNS within the channel region very few GNS sheets are forming the conducting network. In Fig. 4 (b) at 15wt% GNS in PANI, GNS have started forming conducting network throughout the channel region, giving rise to good conductivity of the composite device. The GNS sheets appearing separated in Fig. 4 (a) were started connecting and forming a good conducting
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network as evident from Fig 4 (b) within the channel region. At 20wt% loading of GNS in Fig. 4 (c), agglomerated GNS sheets was observed. 3.3 Raman Spectroscopy of GNS and GNS-PANI composite devices
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Fig. 5 (a) shows the Raman spectra of pencil graphite and as synthesized GNS at 9V to 12V.While 2D-peak changesits shape, width and position with increasing number of layers, G peak position shows a down-shift with number of layers [25]. The Raman spectrum of pencil
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graphite displays a prominent G peak at 1581 cm-1 and a D peak at 1355 cm-1and a 2D peak at 2722 cm-1. At 10V, the position of G peak, which is down-shifted from the pencil graphite, and
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also position of D (1352cm-1) and 2D (2692cm-1) peak shows good agreement with the multilayer graphene Raman spectrum. It is well-known that the intensity ratio ID/IG will quantify the amount of defects. As it is clear from the ID/IG, the defect level has been increased because of the exfoliation of pencil graphite from 9V to 12V. The ID/IG is inversely proportional to in-plane size
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of the graphene sheets (La), according to the formula La = (2.4×10-10) ×λ4 (IG/ID)[27]. Since pencil graphite is made by compressing polycrystalline graphite particles as it is evident from SEM micrographs. The in plane size of these particles according to the above relation was found
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to be 24 nm, the size of as synthesized GNS was found to be 16.84 nm, 16.47 nm, and 15.84 nm, 20.24 nm at 9V, 10V, 11V, and 12V respectively. Table-1 summarizes the characteristics Raman
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D-, G- and 2D-peak and their intensity ratio is given for pencil graphite and as synthesized GNS from electrochemical exfoliation of pencil graphite. 3.4 Raman Spectroscopy of GNS-PANI composite devices Raman spectroscopy can also provide the information related to interfacial interaction
mechanism at the filler and polymer interface [28]. In the present investigation, the interaction between the PANI polymer matrix and GNS is correlated through Raman spectroscopy and
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conductivity values of the as prepared devices. The polymer surroundings in the graphene sheets induce the local strain, change in interatomic distance, which leads to the changes in the C-C bond vibrational frequency of G peak. The 2D band is also very sensitive to these deformations.
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Fig. 5 (b) & (c) show the corresponding Raman spectra of GNS-PANI devices at 15 and 20wt% of GNS in PANI. The change in intensity and shifting of G peak has been recorded with respect to the increased wt% of GNS in GNS-PANI composites devices. In fig. 5 (b) the G peak at 1589
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cm-1, which was shifted from 1580 cm-1 of GNS prepared at 10V, quantifies the interfacial interaction within the composite. The interfacial interaction between the filler and polymer
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matrix is a crucial requirement for preparation of compositeand for sufficient property enhancement. In Fig. 5 (c) the G peak present at 1585 cm-1 shows relatively low interfacial interaction between PANI matrix and GNS due to aggregation of GNS, this was also evident from SEM images. Table 2 summarizes the D-, G-, and 2D-peak positions and their ratios for
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GNS-PANI devices at 15 and 20 wt% loading of GNS in PANI.
3.5 Low temperature IV characteristics of GNS-PANI composite devices The room temperature current-voltage characteristics of as prepared GNS-PANI devices have
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been measured. Fig. 6 shows the room temperature I-V characteristics of GNS-PANI at 3 and 6 wt% GNS loading in PANI. At 3 wt% concentration of GNS, the resistance of 1.034 GΩ was
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obtained. As the loading of GNS was increased from 3 wt% to 6 wt%, the resistance has been decreased to the value of 4.106 MΩ, i.e. a ~3 orders of magnitude increased in conductance were observed.Fig. 7 (a-c) shows I-V characteristics of GNS-PANI at 6, 15, 20 wt% loading of GNS in PANI from 5K to 300K, respectively. A voltage of -2.5V to +2.5Vwas applied to the source drain ofGNS-PANI devices and I-V characteristics was measured from 300K to 5K with a step of 10K. In Fig. 7 (a) it was noticed that while going from 300K to 5K current is decreasedfrom
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~1µA to 200 nA, and consecutively a considerable increase in resistance.This decrease in resistance with decreasing temperature can be attributed to the freezing of charge carrier or flow of electrons which are responsible for the conductance of the device.
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From Fig. 7 (b) as the GNS concentration in PANI was increased to 15 wt% the curve is showing true ohmic behavior at room temperature with a current of the order of ~10µA. At 300K resistance was 258 kΩ, means conductance has been increased to ~6 orders in comparison to
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3wt% GNS-PANI. At 250K and 200K the current reduces to almost half of its value at 300K. From 150K to 5K current flowing in the device is negligible corresponding to the voltage
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applied. The increase in the resistance while cooling to cryogenic temperature is due to the fact that polymer i.e. PANI and GNS are losing contacts due to the contraction of PANI, because of this contraction the GNS network is disturbed or loss of contact between the graphene layers. When wt% of GNS was increased to 20wt% in PANI in Fig. 7 (c), a current of ~6µA was
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flowing through the channel at 300K. The resistance was found to be 708.4 kΩ, which is slightly less than the 15wt% GNS-PANI. The behavior of these curves also slightly shifted from ohmic to non-ohmic. This may be the fact that upon increasing the concentration to 20 wt%, GNS
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aggregation may have started in channel regime thereby reducing the current and increasing the resistance with a small value. Fig. 7 (d) shows the resistance versus temperature curve of GNS-
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PANI at15wt% loading of GNS in PANI, it is showing a change in resistance with respect to temperature showing semiconducting nature of the composite device. The measured room temperature resistances for GNS-PANI devices are summarized in table 3. 4. Conclusions
In conclusion, graphene nanosheets were synthesized by electrochemical exfoliation of pencil graphite by acidic aqueous route.The optimum voltage was found to be 10V at which minimum
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of ~5 layers of GNS was prepared. I-V characteristics of as prepared devices have been measured at room temperature as well as low temperature i.e. at 5K. The highest current or lowest resistance was observed at 15 wt% GNS-PANI composite i.e. 10µA at 300K. The
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interaction between GNS and PANI were studied with Raman spectroscopy, the remarkable shift in G peak of composite with respect to the G peak of GNS confirms that there is a sign of interaction with GNS and PANI. The SEM and Raman characteristics of the composites show
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good agreement with the IV characteristics of the as prepared composites devices. Acknowledgements
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Authors are acknowledged to DST, DRDO, DAE and UGC for funding. One of the authors Seema Awasthi is thankful to Dr DS Kothari PDF fellowship scheme of UGC for funding (award letter no-F.4-2/2006 (BSR)/13-769/2012 (BSR)). REFERENCES
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Applications. Wiley-VCH, 206-207.
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Fig. 1 (a) Schematic diagram of electrochemical exfoliation of 0.7mm pencil lead as working
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electrode in 0.2M H2SO4 electrolyte with Ag/AgCl as reference, and spiral Pt wire as counter electrode, (b) Energy dispersive X ray spectra of pencil which was used as working electrode in
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electrochemical exfoliation, (c) Schematic diagram of GNS-polymer device with Ag source drain
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(b)
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(a)
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Fig. 2 (a) XRD of pencil and as synthesized GNS at 9V to 12V, (b) Lorentzian curve fitting of XRD of as synthesized GNS
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(b) 8V
(c) 9V
(d) 10V
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(a) Pencil
(f) 12V
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(e) 11V
Fig. 3 SEM images of as synthesized GNS at (a) pencil, (b) 8V, (c) 9V, (d) 10V, (e) 11V, (f) 12V
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(a)
(c)
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(b)
Fig. 4 SEM images of (a) 6wt% GNS-PANI, (b) 15wt% GNS-PANI, (c) 20wt% GNS-PANI devices
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(a)
(c)
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(b)
Fig. 5 (a) Raman Spectra of (a) pencil and GNSs, GNS-PANI device at (b) 15wt%, (c) 20wt%
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PANI + 6 wt% GNS
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PANI + 3 wt% GNS
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Fig. 6 Room temperature I-V characteristics of GNS-PANI devices
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(a) PANI-GNS 6wt%
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(b) PANI-GNS 15wt%
300K
300K
5K
(d)
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(c) PANI-GNS 20wt%
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5K
300K
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5K
Fig. 7 I-V characteristics of GNS-PANI device at (a) 6wt% GNS-PANI (b) 15wt% GNS-PANI, (c) 20wt% GNS-PANI, (d) Resistance Vs Temperature curve of GNS-PANI
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2.
9V
3.
10V
4.
11V
5.
12V
1360 cm-1 1350 cm-1 1352 cm-1 1359 cm-1 1355 cm-1
1581cm-1 1570 cm1
1580 cm1
1578 cm1
1569 cm1
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Width (La= 2.4×1010 4 λ IG/ID) 24 nm
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Pencil
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1.
2D peak 2722 cm-1 2698 cm-1 2692 cm-1 2701 cm-1 2673 cm-1
G peak
ID/IG I2D/IG 0.7
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S.No. Material D-peak
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Table-1 the characteristics D-, G- and 2D-peak of Raman spectra of pencil and as synthesized GNS from electrochemical exfoliation of pencil
0.4
1.00
0.29
1.02
0.26
1.06
0.22
0.83
0.34
16.84 nm 16.47 nm 15.84 nm 20.24 nm
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1.
10V
2.
G peak 1580 cm-1 1589 cm-1 1585 cm-1
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3.
15wt% GNSPANI 20wt% GNSPANI
Dpeak 1352 cm-1 1352 cm-1 1352 cm-1
2D peak 2692 cm-1 2692 cm-1 2699 cm-1
ID/IG I2D/IG
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Materials
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S.No.
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Table-2 the characteristics D-, G- and 2D-peak of Raman spectra of GNS-PANI devices at 15and 20wt% loading
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1.02
0.26
1.13
0.13
1.06
0.58
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GNS-PANI (3 wt%)
2.
GNS-PANI (6 wt%)
3.
GNS-PANI (15 wt%)
4.
GNS-PANI (20 wt%)
Resistance 1.034 GΩ
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Sample details
4.106 MΩ 258 kΩ
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S. No.
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Table-3 The room temperature resistance of GNS-PANI devices
10
708.4 kΩ
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Highlights 1. Electrochemical exfoliation of commercially available pencil was performed to produce multi- layer graphene nanosheets (GNS) at different voltages
GNS in PANI.
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2. The as produced GNS was used to prepare polyaniline composite at different wt% of
3. GNS-PANI devices were prepared by drop casting method onto Ag source drain deposited ITO coated PET flexible substrate.
well as low temperature i.e from 5K to 300K.
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4. The electrical properties of these devices have been measured at room temperature as
5. The interfacial interaction was studied by Raman spectroscopy and it is in good
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agreement with resistance values of the devices.
S2468-2179(17)30227-7
DOI:
10.1016/j.jsamd.2018.01.003
Reference:
JSAMD 140
To appear in:
Journal of Science: Advanced Materials and Devices
Received Date: 12 December 2017 Revised Date:
20 January 2018
Accepted Date: 23 January 2018
Please cite this article as: S. Awasthi, P.S. Gopinathan, A. Rajanikanth, C. Bansal, Current-Voltage characteristics of electrochemically synthesized multi-layer graphene with polyaniline, Journal of Science: Advanced Materials and Devices (2018), doi: 10.1016/j.jsamd.2018.01.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Current-Voltage characteristics of electrochemically synthesized multi-layer graphene with polyaniline Seema Awasthi*, Praveen S. Gopinathan, A. Rajanikanth and C. Bansal School of Physics, University of Hyderabad, Hyderabad-500046, India
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ABSTRACT Graphene nanosheets (GNS) synthesized through commercially available pencil lead were used to prepare composites with polyaniline on ITO coated PET substrate. I-V
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characteristics of GNS-PANI composite device shows a decrease in band gap of PANI from 2.8 eV to 6.9 meV at 20 wt% loading of GNS in PANI. SEM and Raman spectroscopy show good
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dispersion of GNS and interfacial interaction with GNS-PANI composite by significant shift in the G peak at 20 wt% GNS-PANI, further confirming the formation of composites at 20 wt%. Keywords: graphene, polyaniline, I-V characteristics, composite, band gap
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Corresponding author email: [email protected]
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1. Introduction Graphene possesses extremely high carrier mobility (100 000 cm2/V.s), strength, flexibility, high current carrying capacity (109 A/cm2), and high thermal conductivity. These
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properties of graphene can be used for the applications in the area of energy storage, supercapacitors, transparent electrodes, sensors and in composite materials [2-6]. The unusual electronic properties of graphene make it a promising candidate for future electronic
13]. polymers have been studied with graphene as fillers.
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applications. Polyaniline, polypyrrole, polythiophene, polystyrene, polymethyl methacrylate [9-
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The electrochemical exfoliation of graphite in presence of suitable electrolytic medium has opened a new single step one pot method for the preparation of graphene [17-20]. Su et al prepared high quality and large area graphene sheets by electrochemical exfoliation of natural graphite flakes in sulfuric acid as electrolyte medium [21]. Guo and coworkers has synthesized
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high quality GNS in large quantity through electrochemical reduction of exfoliated graphite oxide precursor at cathodic potentials [22]. Singh et al have produced high quality GNS from pencil graphite by electrochemical exfoliation in ionic liquid medium following non-aqueous
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route [23].
In the present investigation, GNS have been prepared by aqueous anodic exfoliation of pencil
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graphite in acidic medium at different dc voltages (5V to 12V). The GNS prepared at optimum voltage were mixed with polyaniline (PANI) polymer and drop casted on indium tin oxide (ITO)/Polyethylene terephthalate (PET) substrate to study the device I-V characteristics and band gap measurments. The room temperature to low temperature I-V characteristics of the as prepared polymer-GNS devices were measured. Commercially available pencil graphite was used as working electrode.
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2. Experimental 2.1 Materials
received. 2.2 Synthesis of graphene and graphene polyanline nanocomposite
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All chemicals (Polyaniline, DMF, H2SO4) were purchased from Sigma Aldrich and used as
Electrochemical exfoliation of pencil graphite has been performed in 0.2M H2SO4 as electrolyte
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and pencil graphite graphite(0.7mm×50mm) as working electrode, Ag/AgCl as reference electrode and platinum wire as counter electrode on a Gamry Instruments model Reference
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600TMPotentiostat/Galvanostat/ZRA. A voltage of 1V was applied for 5 min by a potentiostat. Electrochemical reaction started very slowly at 1.8V. This condition was held for 5 min to make the pencil graphite totally occupied with sulfate ions [21]. The exfoliation was performed at different voltages from 5V to 12V. In every experiment, the reaction was held till the whole
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pencil graphite consumes in the process. After complete exfoliation, the black precipitate from the vessel was filtered, washed with de-ionized water and the exfoliated sheets have been collected. The as prepared GNS was used at different (3, 6, 15, 20) wt% to prepare composites
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with polyaniline. PANI was dissolved in di-methylformamide (DMF) solution and ultrasonicated GNS was added to the polymer solution followed by magnetic stirring for 60mins.
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2.3 Preparation of GNS-PANI thin film devices Indium tin oxide (ITO) coated Polyethylene terephthalate (PET) as the flexible substrate was used for the device fabrication. The source-drain was deposited on this substrate through vacuum coating of silver (Ag) wire. A mask of gold wire (dia. 0.127mm) was used to form a channel between the source-drain. The as prepared GNS-PANI solution has been drop casted onto Ag source-drain deposited PET/ITO substrate and left to dry overnight.
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2.4 Characterization techniques The as synthesized GNS have been characterized by X-ray diffraction technique (INEL X-ray Diffractometer (XRG-3000 with CoKα 1.79Å) for structural and by scanning electron
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microscopy (FESEM, Model Ultra55 from Carl Zeiss Germany) for microstructural morphology. Raman spectroscopy (LabRam HR-800-Horiba JobinYvon) at 514nm excitation wavelength of 0.9mW power was used for spectroscopic characterization.
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2.5 IV characteristics measurements
The room temperature current-voltage (I-V)characteristics have been measured with the
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semiconductor device analyzer (Agilent B1500A) and low temperature I-V measurements were performed in cryogenic probe station (Model CRX-4K from lakeshore Cryotronics). 3. Results and discussions 3.1 Synthesis of GNS
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Fig. 1(a) shows the schematic diagram of electrochemical exfoliation of pencil graphite as working electrode in 0.2M H2SO4 electrolyte with Ag/AgCl as reference, and Pt wire as counter electrode. The energy dispersive X-ray spectrum of pencil graphitewas displayed in fig. 1 (b).
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The pencil graphite was found to contain mainly carbon (C). Si was found as impurity. No other elements are present in the pencil graphite confirming that GNS prepared from this pencil
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graphite are free from impurities. Fig. 1(c) shows the schematic diagram of ITO coated PET GNS-PANI flexible device with polymer-GNS as channel and source-drain of Ag. The channel width in this case is 0.1 mm. 3.2 Structural and morphological analysis Fig. 2(a) shows the X-ray diffraction pattern of pencil graphite and as synthesized GNS at 9V to 12V. The XRD of pencil graphite clearly shows a sharp and intense peak at 30.50. The
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characteristic graphitic (002) peak which was coming in pencil graphite has significantly reduced and broadened in the exfoliated samples at different voltages.The broadening of (002) peak clearly shows the reduction in number of graphene layers. Lorentzian curve fitting of (002) peaks
2 (b). A minimum of ~5 layers was obtained at a voltage of 10V.
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[24] was done for calculating the number of layers in the as synthesized samples as shown in fig.
Fig. 3 (a-f) shows SEM images of pencil graphite and as synthesized GNS from 8V to 12V,
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respectively. Fig. 3 (a) shows SEM image of pencil graphite displaying continuous sheet like structure. After applying a dc voltage the sheets began to exfoliate. The exfoliated sheets at 8V
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and 9V are shown in fig. 3 (b)& (c). Thick sheet like morphology are clearly visible. As we can see in the Fig. 3 (c) thick graphite flakes are produced at 9V, while in Fig. 3(d) these graphite flakes are being exfoliated at 10V. The trend of exfoliation increases as we increased the voltage to 9V and it can clearly be seen from Fig. 3 (d) the transparent, sheet like and crumpled structure
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of GNS has been formed. The SEM image shows a typical worm-like shape with thin layered and wrinkled graphene structure.
In Fig. 4 (e&f) SEM image is composed of fluffy aggregates of crumpled sheets obtained at 11V
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and 12V respectively.The morphology of GNS-PANIcomposite on ITO/PET substrate with various GNS loading was investigated through FESEM, shown in Fig. 4. Fig. 4 (a) shows the
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SEM images of GNS-PANI composite at 6wt% GNS in PANI. Due to the lower concentration of GNS within the channel region very few GNS sheets are forming the conducting network. In Fig. 4 (b) at 15wt% GNS in PANI, GNS have started forming conducting network throughout the channel region, giving rise to good conductivity of the composite device. The GNS sheets appearing separated in Fig. 4 (a) were started connecting and forming a good conducting
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network as evident from Fig 4 (b) within the channel region. At 20wt% loading of GNS in Fig. 4 (c), agglomerated GNS sheets was observed. 3.3 Raman Spectroscopy of GNS and GNS-PANI composite devices
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Fig. 5 (a) shows the Raman spectra of pencil graphite and as synthesized GNS at 9V to 12V.While 2D-peak changesits shape, width and position with increasing number of layers, G peak position shows a down-shift with number of layers [25]. The Raman spectrum of pencil
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graphite displays a prominent G peak at 1581 cm-1 and a D peak at 1355 cm-1and a 2D peak at 2722 cm-1. At 10V, the position of G peak, which is down-shifted from the pencil graphite, and
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also position of D (1352cm-1) and 2D (2692cm-1) peak shows good agreement with the multilayer graphene Raman spectrum. It is well-known that the intensity ratio ID/IG will quantify the amount of defects. As it is clear from the ID/IG, the defect level has been increased because of the exfoliation of pencil graphite from 9V to 12V. The ID/IG is inversely proportional to in-plane size
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of the graphene sheets (La), according to the formula La = (2.4×10-10) ×λ4 (IG/ID)[27]. Since pencil graphite is made by compressing polycrystalline graphite particles as it is evident from SEM micrographs. The in plane size of these particles according to the above relation was found
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to be 24 nm, the size of as synthesized GNS was found to be 16.84 nm, 16.47 nm, and 15.84 nm, 20.24 nm at 9V, 10V, 11V, and 12V respectively. Table-1 summarizes the characteristics Raman
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D-, G- and 2D-peak and their intensity ratio is given for pencil graphite and as synthesized GNS from electrochemical exfoliation of pencil graphite. 3.4 Raman Spectroscopy of GNS-PANI composite devices Raman spectroscopy can also provide the information related to interfacial interaction
mechanism at the filler and polymer interface [28]. In the present investigation, the interaction between the PANI polymer matrix and GNS is correlated through Raman spectroscopy and
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conductivity values of the as prepared devices. The polymer surroundings in the graphene sheets induce the local strain, change in interatomic distance, which leads to the changes in the C-C bond vibrational frequency of G peak. The 2D band is also very sensitive to these deformations.
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Fig. 5 (b) & (c) show the corresponding Raman spectra of GNS-PANI devices at 15 and 20wt% of GNS in PANI. The change in intensity and shifting of G peak has been recorded with respect to the increased wt% of GNS in GNS-PANI composites devices. In fig. 5 (b) the G peak at 1589
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cm-1, which was shifted from 1580 cm-1 of GNS prepared at 10V, quantifies the interfacial interaction within the composite. The interfacial interaction between the filler and polymer
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matrix is a crucial requirement for preparation of compositeand for sufficient property enhancement. In Fig. 5 (c) the G peak present at 1585 cm-1 shows relatively low interfacial interaction between PANI matrix and GNS due to aggregation of GNS, this was also evident from SEM images. Table 2 summarizes the D-, G-, and 2D-peak positions and their ratios for
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GNS-PANI devices at 15 and 20 wt% loading of GNS in PANI.
3.5 Low temperature IV characteristics of GNS-PANI composite devices The room temperature current-voltage characteristics of as prepared GNS-PANI devices have
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been measured. Fig. 6 shows the room temperature I-V characteristics of GNS-PANI at 3 and 6 wt% GNS loading in PANI. At 3 wt% concentration of GNS, the resistance of 1.034 GΩ was
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obtained. As the loading of GNS was increased from 3 wt% to 6 wt%, the resistance has been decreased to the value of 4.106 MΩ, i.e. a ~3 orders of magnitude increased in conductance were observed.Fig. 7 (a-c) shows I-V characteristics of GNS-PANI at 6, 15, 20 wt% loading of GNS in PANI from 5K to 300K, respectively. A voltage of -2.5V to +2.5Vwas applied to the source drain ofGNS-PANI devices and I-V characteristics was measured from 300K to 5K with a step of 10K. In Fig. 7 (a) it was noticed that while going from 300K to 5K current is decreasedfrom
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~1µA to 200 nA, and consecutively a considerable increase in resistance.This decrease in resistance with decreasing temperature can be attributed to the freezing of charge carrier or flow of electrons which are responsible for the conductance of the device.
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From Fig. 7 (b) as the GNS concentration in PANI was increased to 15 wt% the curve is showing true ohmic behavior at room temperature with a current of the order of ~10µA. At 300K resistance was 258 kΩ, means conductance has been increased to ~6 orders in comparison to
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3wt% GNS-PANI. At 250K and 200K the current reduces to almost half of its value at 300K. From 150K to 5K current flowing in the device is negligible corresponding to the voltage
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applied. The increase in the resistance while cooling to cryogenic temperature is due to the fact that polymer i.e. PANI and GNS are losing contacts due to the contraction of PANI, because of this contraction the GNS network is disturbed or loss of contact between the graphene layers. When wt% of GNS was increased to 20wt% in PANI in Fig. 7 (c), a current of ~6µA was
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flowing through the channel at 300K. The resistance was found to be 708.4 kΩ, which is slightly less than the 15wt% GNS-PANI. The behavior of these curves also slightly shifted from ohmic to non-ohmic. This may be the fact that upon increasing the concentration to 20 wt%, GNS
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aggregation may have started in channel regime thereby reducing the current and increasing the resistance with a small value. Fig. 7 (d) shows the resistance versus temperature curve of GNS-
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PANI at15wt% loading of GNS in PANI, it is showing a change in resistance with respect to temperature showing semiconducting nature of the composite device. The measured room temperature resistances for GNS-PANI devices are summarized in table 3. 4. Conclusions
In conclusion, graphene nanosheets were synthesized by electrochemical exfoliation of pencil graphite by acidic aqueous route.The optimum voltage was found to be 10V at which minimum
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of ~5 layers of GNS was prepared. I-V characteristics of as prepared devices have been measured at room temperature as well as low temperature i.e. at 5K. The highest current or lowest resistance was observed at 15 wt% GNS-PANI composite i.e. 10µA at 300K. The
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interaction between GNS and PANI were studied with Raman spectroscopy, the remarkable shift in G peak of composite with respect to the G peak of GNS confirms that there is a sign of interaction with GNS and PANI. The SEM and Raman characteristics of the composites show
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good agreement with the IV characteristics of the as prepared composites devices. Acknowledgements
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Authors are acknowledged to DST, DRDO, DAE and UGC for funding. One of the authors Seema Awasthi is thankful to Dr DS Kothari PDF fellowship scheme of UGC for funding (award letter no-F.4-2/2006 (BSR)/13-769/2012 (BSR)). REFERENCES
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25. S.C.Tjong (2012) Polymer composites with carbonaceous nanofillers: Properties and
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Applications. Wiley-VCH, 206-207.
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Fig. 1 (a) Schematic diagram of electrochemical exfoliation of 0.7mm pencil lead as working
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electrode in 0.2M H2SO4 electrolyte with Ag/AgCl as reference, and spiral Pt wire as counter electrode, (b) Energy dispersive X ray spectra of pencil which was used as working electrode in
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electrochemical exfoliation, (c) Schematic diagram of GNS-polymer device with Ag source drain
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(b)
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(a)
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Fig. 2 (a) XRD of pencil and as synthesized GNS at 9V to 12V, (b) Lorentzian curve fitting of XRD of as synthesized GNS
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(b) 8V
(c) 9V
(d) 10V
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(a) Pencil
(f) 12V
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(e) 11V
Fig. 3 SEM images of as synthesized GNS at (a) pencil, (b) 8V, (c) 9V, (d) 10V, (e) 11V, (f) 12V
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(a)
(c)
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Fig. 4 SEM images of (a) 6wt% GNS-PANI, (b) 15wt% GNS-PANI, (c) 20wt% GNS-PANI devices
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(a)
(c)
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Fig. 5 (a) Raman Spectra of (a) pencil and GNSs, GNS-PANI device at (b) 15wt%, (c) 20wt%
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PANI + 6 wt% GNS
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PANI + 3 wt% GNS
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Fig. 6 Room temperature I-V characteristics of GNS-PANI devices
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(a) PANI-GNS 6wt%
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(b) PANI-GNS 15wt%
300K
300K
5K
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(c) PANI-GNS 20wt%
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5K
300K
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5K
Fig. 7 I-V characteristics of GNS-PANI device at (a) 6wt% GNS-PANI (b) 15wt% GNS-PANI, (c) 20wt% GNS-PANI, (d) Resistance Vs Temperature curve of GNS-PANI
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2.
9V
3.
10V
4.
11V
5.
12V
1360 cm-1 1350 cm-1 1352 cm-1 1359 cm-1 1355 cm-1
1581cm-1 1570 cm1
1580 cm1
1578 cm1
1569 cm1
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Width (La= 2.4×1010 4 λ IG/ID) 24 nm
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Pencil
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1.
2D peak 2722 cm-1 2698 cm-1 2692 cm-1 2701 cm-1 2673 cm-1
G peak
ID/IG I2D/IG 0.7
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S.No. Material D-peak
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Table-1 the characteristics D-, G- and 2D-peak of Raman spectra of pencil and as synthesized GNS from electrochemical exfoliation of pencil
0.4
1.00
0.29
1.02
0.26
1.06
0.22
0.83
0.34
16.84 nm 16.47 nm 15.84 nm 20.24 nm
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1.
10V
2.
G peak 1580 cm-1 1589 cm-1 1585 cm-1
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15wt% GNSPANI 20wt% GNSPANI
Dpeak 1352 cm-1 1352 cm-1 1352 cm-1
2D peak 2692 cm-1 2692 cm-1 2699 cm-1
ID/IG I2D/IG
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Materials
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S.No.
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Table-2 the characteristics D-, G- and 2D-peak of Raman spectra of GNS-PANI devices at 15and 20wt% loading
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1.02
0.26
1.13
0.13
1.06
0.58
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GNS-PANI (3 wt%)
2.
GNS-PANI (6 wt%)
3.
GNS-PANI (15 wt%)
4.
GNS-PANI (20 wt%)
Resistance 1.034 GΩ
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Sample details
4.106 MΩ 258 kΩ
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S. No.
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Table-3 The room temperature resistance of GNS-PANI devices
10
708.4 kΩ
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Highlights 1. Electrochemical exfoliation of commercially available pencil was performed to produce multi- layer graphene nanosheets (GNS) at different voltages
GNS in PANI.
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2. The as produced GNS was used to prepare polyaniline composite at different wt% of
3. GNS-PANI devices were prepared by drop casting method onto Ag source drain deposited ITO coated PET flexible substrate.
well as low temperature i.e from 5K to 300K.
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4. The electrical properties of these devices have been measured at room temperature as
5. The interfacial interaction was studied by Raman spectroscopy and it is in good
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agreement with resistance values of the devices.