Synthesis and characterization of graphene via electrochemical exfoliation technique and study its electrochemical properties

In this study, we present a one-pot approach for the large-scale synthesis of multilayer to few-layer graphene nanosheets in an aqueous medium that is cost-effective, environmentally friendly, high-yielding, and simple. This electrochemical exfoliation method produces low defect and high yield graphene nanosheet products and is more efficient than chemical exfoliation methods. Two highly oriented pyrolytic graphite plates (HOPG) are utilized as the anode and cathode in this method, which also incorporates two electrode geometry configurations. XRD, HRTEM, AFM, XPS, and Raman spectroscopy are used to describe graphene nano-sheets. Raman characterization supports graphene’s inherent qualities. By using cyclic voltammetry, the electrochemical performance of the produced nano-sheets is evaluated.


Introduction
In the last four to five years, graphene, the substance that underlies all graphitic forms has emerged as one of the most fascinating areas of study [1][2][3][4][5][6].This two-dimensional substance is a brand-new type of nanocarbon made up of layers of six-member rings of carbon atoms.In contrast to carbon nanotubes and fullerenes, it differs noticeably and possesses unusual features that have intrigued scientists.Generally, the fractional quantum hall effect at ambient temperature [7][8][9], the ambipolar electric field effect together with the ballistic conduction of charge carriers [10], and high elasticity [11] are essential characteristics of graphene.Despite expectations that graphene would be perfectly flat, heat fluctuations generate ripples [1].Although a single sheet of graphene is the ideal structure, samples with two or more layers are also being studied with interest.Graphene can be classified into three main types: single-layer, few-layer (four layers or less), and multi-layer (ten layers or more).Several methods have been used to create graphene with various numbers of layers [4,5,12].In addition, there is no established method for producing graphene with a specific number of layers.A rapidly developing area of study, graphene is applicable to a wide range of technological applications, including transparent conducting films, catalytic supports, lithium-ion batteries, solar cells, electrochemical capacitors, and more [13].With the isolation of graphene nanosheets using the scotch tape method, researchers have been working to create ways for producing graphene nanosheet products on a large scale [13].The chemical oxidation of graphene powders (Hummer's process), chemical vapour deposition, liquid-phase exfoliation of graphite, sublimation of Si from SiC, and mechanical cleavage from graphite paper are examples of well-studied graphene nanosheet production methods, according to research [13].The creation of an effective technique for producing high-quality graphene nanosheets by exfoliating graphite powders, however, remains a significant hurdle.For instance, the Hummer's approach and its modified version are often used to chemically exfoliate graphite powder, but the presence of oxide functionalities on the GO sheets considerably lowers their electrical conductivity due to an inevitable disruption of long range conjugation [14,15].About CVD, its high energy consumption because it needs a high temperature, high material consumption since it needs a metal catalyst, and a difficult transfer process to the required substrate are its limitations.The yield and controllability of the mechanical cleavage process are too low to be commercially feasible, despite the ability to manufacture high-quality graphene [13].Under the aforementioned parameters, the electrochemical exfoliation approach has been used to begin the process of producing high-yield and high-quality graphene nanosheets.Due to its quick synthesis, ease of use, and environmentally benign approach, the electrochemical exfoliation process has gradually gained popularity [13].Different types of electrolytes, such as LiCl in propylene carbonate (PC) [14], LiClO4 in PC [16], LiCl in dimethyl sulfoxide [17], (NH4)2SO4 aqueous solution [15], and H2SO4 aqueous solution [13], can be used to electrochemically exfoliate graphite into graphene with one or more stacks of graphene-like sheets.It has been demonstrated that the electrochemical exfoliation approach using electrolytes in the presence of SO4 2-ions has enhanced exfoliation efficiency (using 20 min).This is because SO4 2-ions, which are produced during the electrolysis of water, tend to intercalate faulty sites at the margins or grain boundaries of graphite.The release of SO2 gas and anion depolarization are made possible by SO4 2-ions, increasing the distance between graphite layers.In this study, we present a two-step, effective electrochemical approach for producing high-quality graphene, as well as the basic reaction kinetics of this method's electrochemical exfoliation in the presence of sulphate ions.

Experimental
During the electrochemical exfoliation process, two HOPG with a size of 3×0.5×0.2 cm 3 were utilized as an electrode and a source of graphene nanosheets.In order to make the electrolyte solution, 1.1g of FeSO4 were dissolved in 40ml of distilled water.The two electrodes were dipped into the electrolyte solution at a distance of 2cm after stirring the mixture for 10 minutes.The scientific HM5041 dual output power supply's negative and positive sources were linked to the graphite rods.A 15V power supply was used to begin the exfoliation process on positive rods, which lasted for 6 hours.The anode graphite rod corroded once the potential was applied, and black precipitate gradually developed at the reaction cell's bottom.The experimental setup used in this work is displayed in figure 1(a-e).To get rid of any ferrous ions that might have remained in the samples, they were filtered and repeatedly washed with distilled water and ethanol.The samples were then dried for 10 hours at 60° C.

Results and Discussion
Typical XRD patterns of electrochemically exfoliated natural graphite powder and graphene nanosheets are shown in figure 2. Both graphene and graphite exhibit a reflection peak at 2θ = 26.4°with (002) however, as a result of the breakdown of interplanar carbon inside the graphitic structure, the intensity of exfoliated graphene is significantly lower than that of natural graphite.In addition to (002) plane (004) is also seen at 2θ=54.4°, in both cases with a low intensity value.However, the absence of graphene oxide characteristic peak around 16°confirms that the exfoliated samples majorly contained reduced graphene layers.According to the Debye-Scherer equation [18], the average crystallite width (D) is the perpendicular dimension within which the graphite ordering is maintained.
where, λ is the x-ray wavelength, β is the full width at half maximum (FWHM) expressed in radians, θ is the half diffraction angle of peak corresponding to inter-layer spacing (2θ⁓26°) for graphene and graphite), and k is a parameter related to crystal structure.In a similar manner, in plane periodicity peak (2θ⁓54°) [18] can be used to express in plane crystallite size (L) as follows: The average crystallite size has been determined to be substantially smaller in EG (D = 27 nm; L = 61 nm) than in graphite (D = 59 nm; L = 120 nm).As a result of the expulsion of graphene layers from domains during reduction, the graphitic domain may contract, and extra grain boundaries or lateral deflects may arise.

Figure 2. XRD spectra of exfoliate graphene and natural graphite
Absorption or reflectance spectroscopy in the UV-Visible range is referred to as UV-Visible spectroscopy.The absorption bands of a molecule's functions are mapped onto the UV-visible spectrum.Figure 3 displays the UV-Visible absorption spectrum of EG.The electrochemically exfoliated samples showed the absorbance peak at the wavelength (max) of 267 nm, which is associated with the π* transition of the aromatic C-C bonds in graphene [19].The exfoliated products were proven to be devoid of graphite oxide by the absence of the absorbance peak about 230 nm, which is associated with the π* transition of the aromatic C-C bond of graphene oxide [19].

Figure 3. UV-Visible spectra of exfoliated graphene
Figure 4 shows a HRTEM image of exfoliated graphene.The HRTEM findings offer more evidence that the graphene nanosheets are made up of isolated graphene layers or several layers.The interaction of H2O with expanded graphite, which was electrochemically peeled by the production of O2 and development of SO2 gas, is the main source of the synthesis of graphene layers [13].As hydroxyl (OH) and oxygen (O .) radicals are produced when water is oxidized, these active radicals can cause expanded graphite to corrode at edges, grain boundaries, or defect sites, which can lead to the dissolution of carbon crystals [20].The following steps are included in the reaction kinetics for electrochemical exfoliation: I ionic intercalation (ii) anionic insertion and polarisation (iii) electrolysis of water and SO2 evolution (iv) bubble expansion which is formulated as follows: H2O→H + +OH -+ e -→2H + +O -+e - (3) SO4 2-+4H + +4e -→SO2↑+3H2O (4) H2O→2H + +2e -+1/2 O2↑ (5) The graphene sheet's circle-marked area was where the SAED pattern was captured, and the resulting SAED pattern is displayed in figure 4(b).The SAED pattern exhibits hexagonal diffraction spots that come from sp 2 bonded carbon frameworks, indicating that the majority of the produced material is composed of few-layered graphene with few flaws.These flaws primarily result from the electrochemical process' introduction of oxygen functions.Strong 6-fold symmetric diffraction in the designated area is evident from the intensity of the spots with Bravais-Miller(hkil) indices-(1-210) plane and later from the (0101) plane, demonstrating the strong crystallinity of graphene [21].Moreover, it was discovered that the inner and outer spots' relative intensities were 1, confirming multi-layered graphene [21].The AFM 3D image of exfoliated graphene is shown in figure 4(c).Spin coating of graphene dissolved in NMP on a glass substrate was used to prepare the AFM sample.To assess the quality of EG as it is made, X-ray photo electron spectroscopy (XPS) is used.In figure 5 the C1s spectrum of EG is displayed.The sample's C1s spectra reveal three different types of carbon bonds, with peaks at 287.4 eV, 285 eV, and 285.9 eV C=C, C-O, and C-O-C, respectively.The sample has a significant concentration of sp 2 carbon, as evidenced by the strong peak at 284.7 eV [22].The sample's C/O atomic ratio, which is approximately 10.13, demonstrates the high calibre of the graphene sheets in their as-prepared state.In comparison to those reported by Badrayyana Subramanya et.al [22], the reported C/O value is much greater.According to the intensity ratios I2D/IG of ˃2, 1-2, and ˂1, respectively, graphene is one layer, a few layers, and multiple layers.Graphene nanosheets with many layers can be verified by an I2D/IG intensity ratio of 0.437.We also show how to employ EG as conductive ink, which is a crucial component of printable electronics of the future.The preparation of conductive graphene ink involved scattering EG flakes in NMP.Using a paintbrush and produced graphene ink, A-4 size paper was quickly converted into an electrically conductive sheet.Paper's high degree of porosity gives the EG ink a strong capillary force that enhances solvent absorption and results in conformal coatings of EG ink on paper.We investigated the potential of EG coated paper for flexible supercapacitors to show the multifunctionality of the EG paper.To achieve this, an overnight solidification of a PVA/H2SO4 gel was drop cast over the top surface of EG coated paper.After that, a supercapacitor was constructed utilizing two EG paper electrodes without the use of an additional current connection.By using cyclic voltammetry and an electrochemical window of 0.4V, the device's electrochemical properties were examined.The CV curves of EG electrodes at a scan rate of 10-90 mVs -1 are shown in figure 7. Calculating a specific gel electrolyte's specific capacitance requires knowledge of the CV curve's shape.At various scan rates, the EG paperbased supercapacitor that had been binder-and additive-free manufactured displayed normal double capacitive performance.With an increase in scan rate, it was discovered that the specific capacitance decreased.The range of the specific capacitance values is from 126 to 52.5µFcm -2 .A faster redox reaction occurs at a higher scan rate, which reduces electrolyte ion diffusion into the active material and, as a result, lowers capacitance.

Figure 1 .
Figure 1.(a) Experimental Setup (b) Colour of the solution at 0min (c) colour of the solution after 30 min (d) Solution after 6hr (e) After sonication EG appears at the top surface Figure4shows a HRTEM image of exfoliated graphene.The HRTEM findings offer more evidence that the graphene nanosheets are made up of isolated graphene layers or several layers.The interaction of H2O with expanded graphite, which was electrochemically peeled by the production of O2 and development of SO2 gas, is the main source of the synthesis of graphene layers[13].As hydroxyl (OH) and oxygen (O .) radicals are produced when water is oxidized, these active radicals can cause expanded graphite to corrode at edges, grain boundaries, or defect sites, which can lead to the dissolution of carbon crystals[20].The following steps are included in the reaction kinetics for electrochemical exfoliation: I ionic intercalation (ii) anionic insertion and polarisation (iii) electrolysis of water and SO2 evolution (iv) bubble expansion which is formulated as follows:H2O→H + +OH -+ e -→2H + +O -+e - (3) SO4 2-+4H + +4e -→SO2↑+3H2O (4) H2O→2H + +2e -+1/2 O2↑(5) The graphene sheet's circle-marked area was where the SAED pattern was captured, and the resulting SAED pattern is displayed in figure4(b).The SAED pattern exhibits hexagonal diffraction spots that come from sp 2 bonded carbon frameworks, indicating that the majority of the produced material is composed of few-layered graphene with few flaws.These flaws primarily result from the electrochemical process' introduction of oxygen functions.Strong 6-fold symmetric diffraction in the designated area is evident from the intensity of the spots with Bravais-Miller(hkil) indices-(1-210) plane and later from the (0101) plane, demonstrating the strong crystallinity of graphene[21].Moreover, it was discovered that the inner and outer spots' relative intensities were 1, confirming multi-layered graphene[21].The AFM 3D image of exfoliated graphene is shown in figure4(c).Spin coating of graphene dissolved in NMP on a glass substrate was used to prepare the AFM sample.The fluffy morphology caused by heat fluctuation is shown in the AFM image.

Figure 4 .
Figure 4. (a) HRTEM image of EG at 20KX magnification (b) SAED pattern of EG and (c) AFM 3D image of EG The EG's Raman spectra are shown in figure6.Three prominent peaks were visible in the Raman spectra, which correspond to the D-band at 1343 cm -1 , the G-band at 1590 cm -1 , and the 2D band at 2706 cm -1 .The existence of sp3 defects is indicated by the D-band, the G-band by the in-plane phonon mode vibration of sp 2 carbon atoms, and the 2D band by the two-phonon lattice vibration.The ratio of the integrated intensity of the 2D band (I2D) to the G-band (IG) reveals the quantity of graphene layers.