Investigation of chemical bonding and supercapacitivity properties of Fe3O4-rGO nanocomposites for supercapacitor applications

Journal Pre-proof Investigation of chemical bonding and supercapacitivity properties of Fe3O4-rGO nanocomposites for supercapacitor applications

N. Aruna Devi, Sumitra Nongthombam, Sayantan Sinha, Rabina Bhujel, Sadhna Rai, W. Ishwarchand Singh, Prajnamita Dasgupta, Bibhu P. Swain PII:

S0925-9635(19)30932-X

DOI:

https://doi.org/10.1016/j.diamond.2020.107756

Reference:

DIAMAT 107756

To appear in:

Diamond & Related Materials

Received date:

30 November 2019

Revised date:

13 February 2020

Accepted date:

13 February 2020

Please cite this article as: N.A. Devi, S. Nongthombam, S. Sinha, et al., Investigation of chemical bonding and supercapacitivity properties of Fe3O4-rGO nanocomposites for supercapacitor applications, Diamond & Related Materials (2018), https://doi.org/ 10.1016/j.diamond.2020.107756

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Journal Pre-proof Investigation of Chemical Bonding and Supercapacitivity Properties of Fe3O4rGO nanocomposites for supercapacitor applications 1 N. Aruna Devi, 1Sumitra Nongthombam, 1Sayantan Sinha, 2Rabina Bhujel, 2Sadhna Rai, 1W. Ishwarchand Singh, 3Prajnamita Dasgupta, 1Bibhu P. Swain 1 Department of Physics, National Institute of Technology Manipur, Langol, Imphal West-795004 2 Centre for Materials Science and Nanotechnology, Sikkim Manipal University, Rangpo-737136 3 University Science Instrumentation Centre, North Bengal University, West Bengal734013 Corresponding email: [email protected], [email protected] Abstract

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Iron oxide decorated reduced graphene oxide (Fe3O4/rGO)nanocomposites were

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synthesized using a one-step chemical reduction method.The XRD results reveal the

-p

diffraction planes at 2θ= 36.53° and 43.04° corresponding to the planes (311) and

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(400) respectivelyfor Fe nanoparticles and the broadened peak at 26.52° was observed corresponds to plane (002) for rGO which confirmed the formation of Fe3O4in rGO

, 1559.36 cm-1, 2358.27cm-1, 2987.45 cm-1 and 3360 cm-1attributes for the Fe-O, C-

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1

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sheets. FTIR results shows the chemical bonding at around 580.96 cm-1, 1191.61 cm-

O, C=C,C=O, C-H2and O-H bonds respectively whereas Raman shift for Fe3O4 was

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found in the range of 100-800 cm-1. The ID/IG ratios varied from 2.5 to 1.55 as Fe(NO3)3 concentration increases from 5mM to 25mM. The estimated bandgap for rGO and rGO/Fe3O4are 2.60 eV,and from 2.52 to 2.34 eVrespectivelyasthe Fe(NO3)3concentration increases from 5mM to 25mM. The atomic percentage of Fe(2p), C(1s),andO(1s)was varied as 0.75-7.11at.%, 86.09-69.58 at.%and 13.16-23.31 at.% as the Fe(NO3)3 concentration increases from 5mM to 25mM.The maximum specific capacitance was achieved at 416 F/g for 25mM of Fe3O4/rGO nanocompositewith cyclic stability of 88.57% at a current density of 5Ag-1over 1000 cycles.Hence, Fe3O4/rGO nanocomposite can be considered as a good candidate for the supercapacitor electrode applications.

Journal Pre-proof Keywords: Fe3O4/rGO nanocomposites; Functional groups; cyclic voltammetry;UVVis Introduction Nowadays, energy storage has become the biggest challengein order to improve the shortcomings that occur like longer charging time, higher internal resistance, lower energy density, etc. Electrical double-layer capacitors (EDLCs) and pseudocapacitors are the two mechanisms for the energy storage of supercapacitors

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where the energy is stored between the electrolyte and electrode through the charge

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accumulation occurstowards the interface and the carbon-based materials like

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graphene, carbon nanotubes, and activated carbon were used as an electrode in

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EDLCswhich exhibits a high specific area thatleads to achieving a high capacitance.On the other hand, pseudocapacitor accumulates energy by reversible

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Faradaic redox reaction of a conductive polymer or metal oxide exhibiting high

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capacitance and power densities which makes it interesting to study. But there are drawbacks of exhibiting relatively low electrical conductivity and poor stability. To

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increase their conductivity, other conducting materials such as graphene and its derivatives were blended with it and used for various supercapacitors applications.In hybrid supercapacitors, there is the used of both the mechanisms of the faradaic and non-faradaic charge storage mechanism. It overcomes the shortcomings by owing high power densities and high energy at once. The study of graphene-based metal oxide nanocomposites had a great interest in research for supercapacitor applications, in which graphene improved the electrical and chemical stability along with metal oxide exhibiting fast faradaic reaction [1].The composite of metal-oxide decorated on graphene are considered as a potential solution in the various technological fields such as energy storage devices [2], solar cells [3], nanoelectronics [4] and transparent

Journal Pre-proof electrodes for displays [5]. It also helps in enhancing efficiency, lifetime and selectivity of the core functioning materials in the electrocatalysis and electrosensing setups [6-9]. Recent investigation have been conducted to synthesize supercapacitor electrodes by combining pseudocapacitive materials such as copper oxide (CuO) [10],manganese dioxide (MnO2) [11], nickel oxide (NiO) [12], titanium dioxide(TiO2)[13], cobalt oxide (Co3O4) [14, 15] etc.withgraphene to increase energy density. Among the several metal oxides, iron oxide (Fe3O4) is considered as one of

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the most suitable and promising material as it is cost-effective and is not harmful in

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the environment. Fe3O4 possesses high pseudo-charge but it exhibits low capacitance

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value and poor electrical conductivity which limits the diffusion of ion.To improve its

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capacitive and electrical properties, the addition of materials which exhibits good conductive propertylike activated carbon and carbon nanotubes (CNT), etc. were

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reported earlier [16].Recent investigation revealed that Fe3O4/rGOnanocomposites

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can be considered as a promising candidate in multiple applications including hyperthermia treatment of cancer cells [17], drug delivery [18], chemical sensors [19],

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and electromagnetic field attenuation devices [20]. Further, Fe3O4/rGO and Fe2O3/rGOnanocomposite has found its use for energy storage devices like electrochemical capacitors and lithium-ion batteries [21-23].Kim et al. reported microwave-assisted Fe3O4/rGO nanocompositeand achievedmaximum capacitance of 972F/gfor 0.4 M Fe3O4 at current density of 1Ag-1, which is higher than that of the pristine rGO found to be 251 F/gand Fe3O4 as 183 F/g respectively [24]. Bhujel et al. obtained 50 F/gas a maximum capacitance for Fe2O3/rGO nanocomposite. Furthermore, the charge transfer resistance (RCT) value was observed to be varied from 91.1 Ωto 21.64 Ωas the concentration of Fe(NO3)3increases from 0.1g to 0.5g[25]. Huang et al. synthesized Fe3O4/rGO composite for Li-ion capacitor

Journal Pre-proof application and obtained a high specificcapacity of 1065 mAh g-1at1Ag1

showinggoodrate capability and cyclicstability [26]. Furthermore,Cheng et

al.investigated the effect of Fe3O4/rGO composite for advanced lithium-sulfur batteries(LSB) and found that LSB with a modified separator achieved excellent cycling stability and high discharge capacities [27].Though above researcher investigate Fe3O4/rGO nanocomposite for supercapacitor applications but the role of iron oxide by using different concentrations of Fe(NO3)3solutionusing a simple one-

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step chemical reduction method has not been studied properly. Furthermore, the

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chemically functionalized graphene with iron oxideincreases the electrochemical

can

take

place.

Therefore,

it

is

essential

to

study

the

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adsorption

-p

surface area through the enhancement of the number of sites where the electrolyte

Fe3O4/rGOnanocomposite to improve and enhance the electrochemical properties.

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Therefore in the present work Fe3O4/rGO nanocompositeswere synthesized by

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a simple cost-effective chemical reduction methodwith varying the content of Fe(NO3)3 from 5mM to 25mM. The studies of structural, morphological, elemental

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composition, optical and vibrational properties were done for Fe3O4/rGO nanocomposites. Moreover, the investigations of electrochemical properties of Fe3O4/rGO nanocomposites were also carried out. 2. Experimental Procedure 2.1 Synthesis of GO The modified Hummer’s method was used for the synthesis of Graphene oxide (GO) [25]. In this synthesis process, graphite powder of 5g was added into a mixture of 110ml of concentrated H2SO4 (98%) and 2.5g of sodium nitrate. At room temperature, the above mixture was stirred for 30mins. Brown slurry was formed after the mixing is done properly. The reaction beaker was then kept in a cold water bath to bring

Journal Pre-proof down the temperature of the reactants to below 20°C. After the temperature goes well below 20°C, 15g of potassium permanganate powder was added pinch by pinch under the vigorous stirring condition with maintaining the temperature below 20°C to avoid explosion and overheating. After that, raised the temperature viz. 35°C and stirring is done for 2h. During this period the mixture becomes highly dense and dark brownish orange color, which indicates the oxidation of graphite. Then by slowly adding 300ml of DI waterit was diluted. Then the temperature was raised to 98°C and it was stirred

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for 1h. Then 15ml of hydrogen peroxide (30%) was added to the solution to end the

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reaction. To remove metal ions from the resulting substances filtration and washing

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was doneusingHCl (10%) solution followed by DI water. The washed and filtered GO

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are then kept to dry at room temperature for 12h. 2.2Synthesis of Fe3O4/rGO nanocomposites

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The preparation of Fe3O4/rGO nanocompositewasdone using a simple chemical

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reduction method. For the preparation ofFe3O4/rGO nanocomposite 0.2g GO was dispersed in 100ml deionized water to make its suspension and stirring was done for

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about 1h. 0.06g of Fe(NO3)2.9H2Owas dissolved into DI water (30ml)to make 5mM Fe(NO3)3solution and is added to GO suspension under the constant stirring condition and stir it for another 30mins to make dispersion nicely. Then, 4ml hydrazine hydrate was put into the suspension and its stirring is done for about 2h at temperature 80℃. The obtained substance was then filtrated and washed by using DI water several times followed by alcohol. The same process is done for the preparation of 10mm, 15mM, 20mM, and 25mM Fe3O4/rGO nanocomposites. 2.3 Characterization Fe3O4/rGO nanocomposites The morphology of Fe3O4/rGO nanocomposites was done by JEOL JSM-IT 100 Scanning Electron Microscope. The chemical bonding of Fe3O4/rGO nanocomposites

Journal Pre-proof was characterized by an FTIR spectrometer using PerkinElmer Spectrum Two with a step size of 1cm-1. To analyze the Raman spectra, Raman spectrometer was carried out using WITec Alpha 300RS having excitation wavelength 532nm with a step size 1cm-1. The structural characterization of the Fe3O4/rGO nanocomposites was done by Bruker D8 Advance X-ray diffractometer using CuKα radiation of wavelength 1.54056 Å witha step size 0.02°. The XPS study was carried out by using VG ESCALABMK II with anMg(Kα) X-ray (1253.6eV) source.The synthesized nanocomposites

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were dissolved in ethanol and dispersed using the ultrasonicator method for about an

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hour. Then the solutions were placed in a cuvette and its absorbance is recorded on a

-p

UV using Shimadzu UV-1800 spectrophotometer within the wavelength ranging from

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200 nm to 1000 nm under normal pressure and ambienttemperature. Cyclic voltammetry measurement was used to study the electrochemical properties of nanocomposites

using

CH

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Fe3O4/rGO

Instruments,

Inc.

(Electrochemical

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AnalyzerCHI608E). As a reference electrode, Ag/AgCl was used, for the counter and the working electrode, Pt wire and glassy carbon electrode were used. For grinding

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the synthesized Fe3O4/rGO nanocomposite, the mortar pestle was used and after that, slurry paste is formed by adding a little amount of ethanol. The preparation of electrode was done by dripping the distribution into the electrolytic solution containing a 1M H2SO4 solution and the glassy carbon electrode. 3. Results and discussion 3.1 Morphology of Fe3O4/rGO nanocomposites Fig. 1 shows SEM images of Fe3O4/rGO nanocomposite with different Fe(NO3)3 concentrations varying from 5mM to 25mM. A wrinkled and crinkled layered microstructure of rGO with the uniformly structural pattern for Fe3O4 in the composites was observed inFig. 1 [24].Further, the smallspherical Fe3O4

Journal Pre-proof nanoparticlesare well dispersed and have good contact with rGO sheets [2].The particle

size

wasin

the

range

between

100

nm

to

250nm

nanoparticles.Moreover, in higher magnified SEM images of

for

Fe3O4

Fe3O4/rGO

nanocomposites, it was observed that with the increase in Fe(NO3)3 concentrations, the deposition amount of Fe3O4 nanoparticles in the rGO sheets increases.Hence, the SEM images confirmed the attachment of Fe3O4on therGO sheets. 3.2 X-ray diffraction (XRD)

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The XRD pattern for the prepared Fe3O4/rGO nanocomposites with different

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concentrations of Fe(NO3)3 were shown in Fig. 2. The diffraction peaks at 2θ= 36.53°

-p

and 43.04° corresponding to the planes (311) and (400) respectivelywere observed for

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Fe nanoparticles [24, 28]. The broadened peak at 26.52° was observed thatcorresponds to a plane (002) for rGO [29].XRD confirmed the formation of Fe3O4

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in rGOagreeing to the JCPDS Card No. 19-0629.The interplanar spacing of 5mM,

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10mM, 15mM, 20mM and 25 mM of Fe3O4/rGO nanocomposite are calculated by the Bragg’s law which is expressed as

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2dsinθ=nλ………………….. (1) where is λ=1.54Å, the wavelength of CuKα radiation, ‘θ’represents the Bragg angle for the diffraction peak.The d-spacing obtained are almost same for all the Fe3O4/rGO nanocomposites which is0.34 nmthat corresponds to (002) peak [29].crystallite size of Fe3O4 is calculated by using the Debye’s Scherrerformula which is given as D= kλ/(βCosθ)…………………(2) where k is the dimensionless shape factor with value0.9, λ=1.54Å, the wavelength of CuKα radiation, ‘θ’represents the Bragg angle for the diffraction peak and β is the FWHM of the corresponding diffraction peak.The crystallite size varied from 9.41nm to 21.69nm with increasingthe Fe(NO3)3 concentrationsfrom 5mM to 25mM [29, 30].

Journal Pre-proof 3.3 Fourier-transform infrared spectroscopy (FTIR) Fig. 3 shows the FTIR spectra of the Fe3O4/rGO nanocomposites with different concentrations of Fe(NO3)3 varying from 5mM to 25mM. The spectrum for the composite Fe3O4 with rGO shows vibrational signatures at 580.96cm-1, 3360 cm-1, 2987.45 cm-1,2358.27 cm-1, 1559.36 cm-1, and 1191.61 cm-1 which corresponds to FeO stretching, O-H, C-H2, C=O, C=C,and epoxy or alkoxy C-O stretching bonds respectively confirming the formation of Fe3O4with rGO sheets [25, 30-31] .

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3.4 Raman spectroscopy

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To get the information of rGO in the Fe3O4/rGO nanocomposites, the Raman analysis

-p

was performedfor 5mM, 15mM and 25mM Fe3O4/rGO nanocompositewhich are

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shown in Fig. 4. A typical Fe3O4peak at 665.64 cm-1, 669.58 cm-1and 679.29cm-1were observed in the Raman spectra for the 5mM, 15mM and25mM Fe3O4/rGO

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nanocomposites respectively, which is attributed to the A1g vibration [26,32]. Two

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sharp peaks appeared at1600.89 cm-1 and 1335.87 cm-1that corresponds to G-band and D-band respectively for 5mM Fe3O4/rGO nanocomposite [33]. In the Raman analysis

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of15mM Fe3O4/rGO nanocomposite, D and G bands were observed at 1336.90 cmand 1621.70 cm-1 respectively.Furthermore, the Raman spectrum of 25mM

Fe3O4/rGO nanocomposite exhibits two characteristic peaks at 1343.42 cm-1 (D band) and 1596.54 cm-1(G band). The D band indicates the defects and disorder in the rGO which contains oxygen and carbon functional groups on its surface and the presence of G band assigns the in-plane vibration of sp2hybridized carbon atoms of rGO sheet.Two additional peaks at around 2700 cm-1and 2930 cm-1 were also seen attributed to 2D and D+G bands respectively [33, 34].The second harmonic of the D band, 2D gives idea about the structure ofgraphene layer, while the appearance of D+G band might be due to the resonance of D and G longitudinal optical(LO) phonon

Journal Pre-proof modes.Moreover, Fe3O4/rGO nanocomposites show several weak peaks in the range of 100-800 cm-1. The peaks were located at about 257.19, 320.52, 403.33, 568.64 and 670 cm-1, which designates the presence of Fe3O4[34, 35]. The peak intensity ratio of D and G band (ID/IG) is interconnected with the ratio of the disordered sp3 and ordered sp2 carbon domains [36].The calculated ID/IGvalues of 5mM, 15mM, and 25mM Fe3O4/rGO nanocomposites were 2.50, 2.55 and 1.55 respectively [25,35].The ID/IG value for 25mM Fe3O4/rGO nanocomposite is lower than that of the other Fe3O4/rGO

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nanocomposites, which may be attributed that the increased of the amount of

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Fe3O4fills the holes which are left after the removal of the oxygen species leading to

-p

reduce the defect density [33].

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3.5 X-ray photoelectron spectroscopy (XPS)

Fig. 5(a) shows the binding energy of core orbital spectrum of Fe3O4/rGO

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nanocomposites of different concentration of 5mM and 25mM. The binding energy

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ofFe(2p), O(1s) and C(1s)orbitals were located at 710.34eV, 531.29 eVand 285.73 eVrespectively [25, 33]. The compositional of the Fe3O4/rGO nanocomposites is

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estimated by the following formula ( ⁄ )

⁄ ………………………….(3) ∑( ⁄ )

where ‘Ai’represents the area under curve of the XPS and Si value is 1.964, 0.733, and 0.314for Fe2p3,O(1s) and C(1s) respectively, which is the relative sensitivity factor.The atomic percentage was varied as 86.09-69.58at.%,13.16-23.31at.% and 0.75-7.11 at.% for C, O, and Fe as the Fe(NO3)3concentration increases from 5mM to 25mM.

Journal Pre-proof Toanalyze the various carbon's functioning groups in the prepared material, the deconvolution ofthe core orbital of C(1s) was done which is presented in Fig. 5(b). The binding energies at 284.61, 285.15, 286.13, 289.09 and 290.76 eV were obtained which corresponds to C=C, C-C, C-O, C=O, and O=C-O for 5mM Fe3O4/rGO nanocomposites [25, 28, 37]. An additional peak was observed with an increase in Fe3O4/rGO nanocomposite of 25mM concentration at around 283.90 eV which can correspond to C-Fe and slightly change in binding energy was also observed at

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284.71, 285.11, 286, 288.61 and 289.98 eV which corresponds to C=C, C-C, C-O,

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C=O, and O=C-O [25].The FWHM of C=C, C-C, C-O, C=O, and O=C-O bonds in

-p

the core orbital ofC(1s) of 5mM Fe3O4/rGOnanocompositewere 0.94, 1.43, 2.60, 1.67

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and 2.44 eV respectively. For 25mM Fe3O4/rGO nanocomposite, the FWHM were

O=C-O bonds respectively.

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0.47, 0.73, 1.13, 2.26, 2.34 and 5.36 eV for the C-Fe, C=C, C-C, C-O, C=O, and

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Fig. 5(c) shows the deconvolution of the core orbital of O(1s) orbital to analyze the different functional groups of oxygenexist in the Fe3O4/rGO

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nanocomposites. O(1s) XPS spectra of 5mM Fe3O4/rGO nanocomposite shows six peaks due to O-Fe, O=C, O-C, O-C-O, O-O-C, and O=O which appears at binding energies 530.26eV, 531.80eV, 532.90eV, 533.8eV, 533.89eV and 536.01 eV respectively [25, 28]. With increasing the amount of Fe(NO3)3concentration as 25mM in Fe3O4/rGO nanocomposite, the binding energy is seen at 530.41eV, 531.31eV, 531.98eV, 533.06eV, 534.34eV and 535.99 eV which signify the O-Fe, O=C, O-C, OC-O, O-O-C, and O=O respectively. In the core orbital of O(1s), the FWHM of the OFe bond increased from 1.08 to 1.25 eV in the Fe3O4/rGO nanocomposite as the Fe(NO3)3 concentration increased from 5mM to 25mM. Moreover, FWHM of O=C, O-C,O-C-O, O-O-C and O=O bond for 5mM and 25mM Fe3O4/rGO nanocomposite

Journal Pre-proof were found (2.6eV, 1.22eV), (2.7eV, 1.41eV), (0.55eV, 2.01eV), (1.88eV, 2.28eV) and (2.95eV, 5.17eV) respectively. In Fig. 5(d), the deconvolution of Fe(2p) orbital is presented. In XPS spectra of Fe(2p)in Fe3O4/rGO nanocomposite, two peaks are locatedat 710.34 eV and 724.59 eV of the binding energy which attributes Fe-Fe. The first and second peaks signify Fe2p3/2 and Fe2p1/2 of Fe3O4 respectively [28]. Further, the obtained Fe2p3/2and Fe2p1/2 peak were again deconvoluted into eight peaks which are assigned

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as Fe-Fe(709.94eV), Fe2-C(710.94eV), Fe-C(712.29eV), Fe-O(719.01eV), Fe-

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Fe(725.05eV), Fe2-C(727.93eV), Fe-C(730eV) and Fe-O(732.2eV) respectively [25].

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Further for the 25mM Fe3O4/rGO nanocomposite, XPS spectrum of Fe2p3/2 and

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Fe2p1/2 shows the binding energies at Fe-Fe(710.52eV), Fe2-C(711.43eV), FeC(712.9eV), Fe-O(719.34eV), Fe-Fe(725.59eV), Fe2-C(726.74eV), Fe-C(729.01eV)

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and Fe-O(732.2eV). Further, the FWHM of the eight deconvoluted peaks in Fe2p3/2

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peak and Fe2p1/2 peak for 5mM Fe3O4/rGO nanocomposite were found to be 0.228eV, 1.77eV, 4.39eV, 11.21eV, 2.92eV, 2.26eV, 0.89eV and 6.26 eV for the Fe-Fe, Fe2-C,

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Fe-C, Fe-O, Fe-Fe, Fe2-C, Fe-C, and Fe-O bonds respectively. And for the 25mM Fe3O4/rGO nanocomposite, the FWHM is obtained as 1.16eV, 2.27eV, 4.37eV, 9eV, 2.48eV, 2.77eV, 2.22eV and 6.93 eV concerning theFe-Fe, Fe2-C, Fe-C, Fe-O, Fe-Fe, Fe2-C, Fe-C, and Fe-O bonds. Thus, Fig. 5(a-d) confirms the formation of Fe3O4 with rGO in Fe3O4/rGO nanocomposites. 3.6 UV-Visible Fig. 6(a) shows the absorption of rGO and Fe3O4/rGO nanocomposites within the wavelength ranging from 200 nm to 1000 nm. The UV-visible spectra of rGO and Fe3O4/rGO nanocomposites of different concentrations of Fe(NO3)3 were obtained as shown in Fig. 6(a). In the case of rGO, absorption peaks at 245 nm and 279 nm were

Journal Pre-proof observed becauseof the presence ofπ→π* transition of C-C bonds[30]. While in the case of Fe3O4/rGO nanocomposite, graphene oxideabsorption spectra and the restoration of C=C bonds in the graphene sheets might be responsible for the appearance of absorption peak at around 245 nm [38].The characteristics peaks at around 330 nm, 404 nm were observed which gives the idea about the interaction of rGO with Fe3O4 nanoparticles thatappears due to the presence of Fe-O-C functioning during the formation of Fe3O4 in rGOsheets [30]. Moreover, a redshift in the spectrum

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of Fe3O4/rGO nanocomposites with some distinct peaks at 328, 330, 347, 382, 404,

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412, 504, 633, 699, 797, 882, 925, and 987 nm were observed [38].The shift and

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enhanced absorption of Fe3O4/rGO nanocomposites into the longer wavelength region

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are attributed to the chemical linkage formed between the rGO and Fe3O4 nanoparticles after incorporation of graphene into Fe3O4 nanoparticles [38].Hence the

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observed peaks confirmed the attachment of iron nanoparticlesinto rGO sheets.

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The direct bandgap of Fe3O4/rGOnanocomposite of different concentrations of Fe(NO3)3 has been evaluated using the Tauc formula. The relation of the absorption

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coefficient α and bandgap Egof indirect &direct bandgapisgiven by the formula αℎ υ=B(ℎ υ-Eg)n ………………….......(4)

where B is a proportionality constant,Eg is the bandgap (eV) of the substance,hυrepresents the incident photon energy (eV), and n is an exponent which is equal to 1/2, and 2 based on characteristics of the electronic transition. For the direct bandgap calculation, n=1/2is used and n=2 is used for the indirect bandgap calculation.Tauc plots shown in Fig.6 (b) shows the relation between (αhυ)2 Vs. hυ for direct bandgap. The bandgap for rGO is found to be 2.60 eV and for the Fe3O4/rGOnanocomposites, its bandgap varied from 2.52eV to 2.34 eV as the

Journal Pre-proof Fe(NO3)3 concentration increases from 5mM to 25mM [29]. It was observed that at the higher concentration of Fe(NO3)3, the bandgap decreases. 3.7 (a) Electrochemical measurement The Cyclic Voltammetry (CV) curves for different concentrations of Fe3O4/rGO nanocomposites were plotted in Fig. 7(a). It was observed that all the CV curves of Fe3O4/rGO nanocomposites exhibits nearly rectangular shapes and minimal changes were seen in its shape even at the higher scan rates which indicates good

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supercapacitive behaviour. The specific capacitance of 5mM, 10mM, 15mM, 20mM,

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and 25mM Fe3O4/rGO nanocomposites at different scan rate varying from 5mV/sec to

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50mV/sec in the potential window range of -0.4 V to 0.6 V were calculated by

is the specific capacitance, I is the current(in A) of the CV curve, mis the

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Where

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))……………….(5)

mass of active electrode materials which is 0.1mg, v is the scan rate,Vb and

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Vaindicates high and low potential limit of the CV measurements.The maximum specific capacitance was obtained at 5mV/sec scan rate for all the Fe3O4/rGO

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nanocomposites of 5mM, 10mM, 15mM, 20mM and 25mM Fe(NO3)3 concentrations achieving 398F/g, 400 F/g, 412 F/g, 413.6 F/gand 416 F/grespectively. The high specific capacitance values of the Fe3O4/rGO nanocomposites obtained are due to the combined effect of rGO possessing EDLC behaviour and Fe3O4 possessing pseudocapacitive behaviour in Fe3O4/rGO nanocomposites owing good electron transport and electrical contact along with exhibiting fast faradaic reaction due to Fe3O4 at once. In addition, with the increase of the Fe(NO3)3concentrationsit was observed that specific capacitance increases, which is attributed to the enhancedpseudocapacitanceowing to the increased amount of Fe3O4 nanoparticles. It can be observed from Fig. 7(a)that the capacitance of the electrodes decreases when

Journal Pre-proof the scan rate is increased. It can be explained based on the fact thatnear the electrodes, the electrolyte ions were transported limitedly [39].Yan J. et al. explained that there is a limitation of effective sites nearthe electrodes surface area at high scan rates, while most of the inner active surface area of the porous electrode material takes part in the electrochemical reactions at low scan rates which lead to high performance [40]. (b) Galvanostatic charging-discharging The measurements of Galvanostatic charging-discharging (GCD) were also carried

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out for different concentrations of Fe3O4/rGO nanocomposites and the GCD curves

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are shown in Fig. 7(b). The specific capacitance of Fe3O4/rGO nanocomposites were

………………………. (6)

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Csc=I t/m

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calculated from the curves of charge-discharge by theformula

Where‘m’ is the mass of the substance on the electrode which is 0.1mg,

is the

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potential window, ∆t is the discharge time, and ‘I’ indicates the discharge current. A

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triangle-shaped charge-discharge curve was observed for all the Fe3O4/rGO nanocomposites as shown in Fig. 7(b) at different current densities varying from

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0.5Ag-1 to 5Ag-1 in the potential window range -0.4 V to 0.6 Vwhich shows a good capacity behavior inFe3O4/rGO nanocomposites. The specific capacitance for Fe3O4/rGOnanocompositeswere found to be 238.44 F/g, 171.17F/g, 244.87F/g, 135.68 F/g and 286.91 F/g for 5mM, 10mM, 15mM, 20mM and 25mM Fe(NO3)3 concentrations respectively.Further, the cyclic stability is an important parameter to study for practical supercapacitor application. Fig. 7(c) shows the capacitance retention curvefor25mM Fe3O4/rGO nanocomposite at a current density of 5Ag1

operated over 1000 cycles of GCD curve which exhibits capacitive retention of

88.57% to that of the initial capacitance. Thus, it shows goodand stable capacitance retention.

Journal Pre-proof Fig. 7(d) shows theNyquist plots for Fe3O4/rGO nanocomposites for different concentrations of Fe(NO3)3. The semicircle curveswhich are seen in the Nyquist plots of Fe3O4/rGO nanocomposites signify the charge transfer resistance (RCT). Thesemicircle’s diameterobserved in the Nyquist plot gives thecharge transfer resistance(RCT) scale [41]. Fig. 7(d)showsimpedance having less semi-circular curves which lead to obtained smaller charge-transfer resistance that is favorable for supercapacitor

materials

which

enhances

the

electrochemicalperformance

of

inFe3O4/rGO nanocomposites.The smallest value of RCT obtained is 3.33 Ω for 25mM

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Fe3O4/rGO nanocomposite as compared to other composites. The capacitive

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behaviorof the material increases as the value of RCT decreases due to the interaction

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of Fe3O4 and rGO[42].The capacitance of the interfacial reaction was also calculated from the Nyquist plot by using the formula

, τ is the time constant, ωmax is the frequency at which the maximum

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Where

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……………………. (7)

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imaginary impedance, RCT is the charge transfer resistance and Cdl gives the doublelayer capacitance. The electrolyte solution provides a resistance (Re) which is also obtained from the Nyquist plot itself.The time constant remains almost the same for all the Fe3O4 and rGO nanocomposites which are found to be 1.59 x 10-1s. For the interfacial reaction, there was a variation of specific capacitance from 2.9 x 10-2 to 4.7x 10-2 Fcm-2 as the Fe(NO3)3concentration increases from 5mM to 25mM. The Bode plot of Fe3O4/rGO nanocompositesfor different concentrations ofFe(NO3)3varying from 5mM to 25mMin the range of frequency 1 to 106 Hzwas depicted in Fig. 7(e).At the low frequency, the maximum impedance is obtained within the frequency region 1 to 106 Hz. The value of total resistance (Re+RCT) that is provided by the electrode and the H2SO4 electrolyte was obtained at the low-

Journal Pre-proof frequency region viz. below 10Hz with the corresponding phase angle of ~φ = 0o. The value of impedance remains constant at the high-frequency region i.e., within the range 102 to 106 Hz which gives the electrolytic resistance (Re). The approximate values for Re for 5mM, 10mM, 15mM, 20mM and 25mM Fe3O4/rGO nanocomposites were 6.4, 7.5, 5.7, 5.6 and 5.6Ω respectively. Moreover, the values of RCT were 80.7, 70.1, 72.2, 61.7 and 268Ω approximately for 5mM, 10mM, 15mM, 20mM and 25mM Fe3O4/rGO nanocomposites respectively.Further, the estimated φmax values were

of

14.7°, 9°, 17.3°, 17.6° and 16.2° obtained for different Fe(NO3)3concentrations in the

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nanocomposite of Fe3O4/rGO respectively.

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4. Conclusion

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The formation of Fe3O4 in rGO sheets was successfully done by using a chemical reduction method. The crystallite size varied from 9.41 nm to 21.69 nm as the

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Fe(NO3)3 concentration increases from 5mM to 25mM.Chemical bondingdue to Fe-O,

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C-O, C=C, C=O, C-H2, and O-H bonds was obtained from FTIR analysis. The Raman spectrum of Fe3O4/rGO nanocomposites displays the two prominent peaks at around

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1600.89 cm-1for(G-band) and 1335.87cm-1for (D-band)respectively. The ID/IG ratios decrease from2.50 to 1.55 as the Fe(NO3)3 concentration increaseswhich may be attributed that the increased of the amount of Fe3O4fills up the holes which are left after the removal of oxygen species that leads to reduce the defect density.The bandgap of rGO/Fe3O4 nanocomposite varied from 2.52eV to 2.34 eV as the Fe(NO3)3 concentration increases from 5mM to 25mM. It was observed that in the higher Fe(NO3)3concentration, the bandgap decreases.From XPS, the binding energy ofFe (2p), O(1s) and C(1s)orbitals appeared at 710.34, 531.29 and 285.73 eV and the atomic percentage of Fe(2p), O(1s)and C(1s) varied as0.75-7.11at.%, 13.16-23.31 at.% and86.09-69.58 at.%respectively with increasing Fe(NO3)3 concentration from

Journal Pre-proof 5mM to 25mM.A maximum specific capacitance of 416 F/g was achieved for 25mMFe3O4/rGOnanocomposite. A triangle-shaped charge-discharge curvewas observed for all the rGO/Fe3O4 nanocomposites from the GCD plots at the current densities which are varied from 0.5Ag-1 to 5Ag-1 which shows a good capacitive behaviour. Furthermore, 25mM Fe3O4/rGO nanocomposite shows a good cycling stability of 88.57% at 5Ag-1 over 1000 cycles.Thus, Fe3O4/rGO nanocomposites can

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be considered as a good candidate for the supercapacitor electrode applications.

Journal Pre-proof Acknowledgments The authorswould like to express thanks to the Department of Chemistry, NIT Manipur for the XRD and FTIR measurements. The authors wouldlike to acknowledgeDr. Laushambam Herojit Singh. Asst. Prof., Physics, NIT Manipur for the UV measurement.Finally, the authorswould like tothank the Department of Physics, NIT Manipur for giving the internet service which helps in accessing many online journals.

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public, commercial, or not-for-profit sectors.

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Funding: This research did not receive any specific grant from funding agencies in the

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Journal Pre-proof Schemes captions Scheme. 1Schematic illustration for the preparation of graphene oxide (GO). Scheme. 2 Schematicillustration for the synthesis of rGOwith Fe3O4nanocomposites. Figure captions Figure 1SEM images of (a) 5mM, (b) 10mM, (c) 15mM, (d) 20mM and (e) 25mM Fe3O4/rGO nanocomposites. Figure 2 XRD peaks of Fe3O4/rGO nanocomposites with different concentrations of

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Figure 3 FTIR spectra of Fe3O4/rGO nanocomposites with different concentrations of

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Figure 4 Raman spectra of (a) 5mM Fe3O4/rGO nanocomposite, (b) 15mM Fe3O4/rGO nanocomposite and (c) 25mM Fe3O4/rGO nanocomposite.

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Figure 5 (a)XPS survey scan of chemically preparedFe3O4/rGO nanocomposites with

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orbital for (i) 5mM and (ii) 25mM Fe3O4/rGO nanocomposites. Figure 6 (a) UV-visible spectra of Fe3O4/rGO nanocomposites for different concentrations of Fe(NO3)3 and (b)direct bandgap of Fe3O4/rGO nanocomposites for different concentrations of Fe(NO3)3. Figure 7 (a)Cyclic-voltammetry curves ofFe3O4/rGO nanocomposites with different concentrations of Fe(NO3)3, (b)Galvanostatic charging-discharging curves of Fe3O4/rGO nanocomposites with different concentrations of Fe(NO3)3, (c) Capacitance retention of Fe3O4/rGO nanocomposite at a current density of 5Ag-1, (d) Nyquist plotsand (e)Bode plot of different concentrations of Fe(NO3)3varying from 5mM to 25mM in Fe3O4/rGO nanocomposite.

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Scheme 1

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Scheme 2

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(v)25mM Fe3O4/rGO

(002)

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(iv)20mM Fe3O4/rGO

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(iii)15mM Fe3O4/rGO

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2000

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(v)25mM Fe3O4/rGO

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(c)25mM Fe3O4/rGO

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699 nm 777 nm 279 nm

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Absorbance

Absorbance

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882 nm 412 nm

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987 nm 925 nm

(vi) 25mM Fe3O4/rGO 633 nm 797 nm 245 nm

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Journal Pre-proof Figure 6(b)

25mM Fe3O4/rGO

(b)

Eg=2.34 eV 20mM Fe3O4/rGO

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(µhu)2 (a.u)

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Eg=3.08eV

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rGO

Eg=2.60 eV

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5mV/s 10mV/s 20mV/s 30mV/s 40mV/s 50mV/s

5mV/s 10mV/s 20mV/s 30mV/s 40mV/s 50mV/s

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Journal Pre-proof Figure 7(b)

0 0.78

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(b)

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0.39 0.00 -0.39 0.66 0.33

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Potential (V)

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Current density 0.5 Ag-1 1 Ag-1 2 Ag-1 3 Ag-1 4 Ag-1 5 Ag-1

Current density 0.5 Ag-1 1 Ag-1 2 Ag-1 3 Ag-1 4 Ag-1 5 Ag-1

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10mM Fe3O4/rGO

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Current density 0.5 Ag-1 1 Ag-1 2 Ag-1 3 Ag-1 4 Ag-1 5 Ag-1

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5mM Fe3O4/rGO

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Current density 0.5 Ag-1 1 Ag-1 2 Ag-1 3 Ag-1 4 Ag-1 5 Ag-1

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(C)

80

88.57%

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Figure 7(d)

5mM Fe3O4/rGO 10mM Fe3O4/rGO 15mM Fe3O4/rGO 20mM Fe3O4/rGO 25mM Fe3O4/rGO

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Img Impedance (Z")

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Figure 7(e)

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Phase (f)

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5mM Fe3O4/rGO 10mM Fe3O4/rGO 15mM Fe3O4/rGO 20mM Fe3O4/rGO 25mM Fe3O4/rGO

Journal Pre-proof AUTHOR’S STATEMENT Miss. N. Aruna Devi has synthesized Fe3O4/rGO nanocomposites, analyzed the data and written the manuscript. Miss. Sumitra Nongthombam helped during the preparation of graphene oxide Mr. Sayantan Sinha helped in the analysis of the obtained data Miss. Rabina Bhujel helped in the electrochemical measurement Miss. Sadhna Rai has measured the cyclic stability of Fe3O4/rGO nanocomposites Mr. W. Ishwarchand Singh helped in XRD measurement Miss. Prajnamita Dasgupta has performed SEM measurement Dr. Bibhu P. Swain* is supervisor of Miss N. Aruna Devi, Miss Sumitra Nongthombam, Mr. Sayantan Sinha, Miss. Rabina Bhujel, Miss. Sadhna Rai, Mr. W. Ishwarchand Singh and over all coordination and data analysis.

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* Dr. Bibhu Prasad Swain is the corresponding author of the manuscript entitled “Investigation of Chemical Bonding and Supercapativity Properties of Fe3O4rGO nanocomposites for supercapacitor applications”

Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

All authors agreed with above statement and we do not have any conflict of interest.

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Dr. Bibhu Prasad Swain

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Research Highlights  Fe3O4/rGO was successfully synthesized by using a one-step chemical reduction method.  Optical, chemical network and electrochemical properties of Fe3O4/rGO were investigated.  The crystallite size varied from 9.41 nm to 21.69 nm with increasing Fe(NO3)3 concentrations.  FTIR and Raman spectra confirmed the rGO and Fe nanoparticles in Fe3O4/rGO nanocomposites.

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 The maximum specific capacitance was achieved at 416 F/g with capacitance

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retention of 88.57% for Fe3O4/rGO nanocomposite.

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