Microwave assisted fabrication of a nanostructured reduced graphene oxide (rGO)/Fe 2 O 3 composite as a promising next generation energy storage material †

Herein, we report a facile two-step process involving homogenous precipitation followed by microwave assisted reduction to fabricate a rGO – Fe 2 O 3 composite. The applicability of the composite as an electrode material for supercapacitors has been evaluated by a cyclic voltammetry (CV) and galvanostatic charging – discharging (GCD) study. The composite displays excellent supercapacitor performance compared to bare rGO and generates a high speci ﬁ c capacitance of 577.5 F g (cid:1) 1 , at a current density of 2 A g (cid:1) 1 . A high rate performance is also observed by retaining a speci ﬁ c capacitance of 437.5 F g (cid:1) 1 , at a high current density of 10 A g (cid:1) 1 . Finally, the electrodes have been analyzed through electrochemical impedance spectroscopy (EIS) to probe the charge transfer characteristics and the results have been found to be consistent with other electrochemical measurements. The remarkable electrochemical performance of the rGO – Fe 2 O 3 composite can be attributed to the positive synergistic e ﬀ ects between rGO platelets and Fe 2 O 3 nanoparticles.


Introduction
Recent times have witnessed an increasing energy demand as a result of global warming, depletion of natural energy resources, the development of a global economy and increasing pollution.4][15] Currently, EDLC capacitors consist of carbon materials such as graphene, carbon nanotubes or activated carbon in which the capacitance arises due to the charge accumulated at the electrolyte/electrode interface. 16Although, they exhibit good cyclic stability, but suffer from poor capacitance.On the other hand, conducting polymers or transition metal oxides are generally used as pseudocapacitor materials which undergoes reversible faradic redox reactions. 15Though, pseudocapacitors display high capacitance but the response time is considerably longer than EDLC capacitors.Therefore, to overcome these issues, a perfect combination of metal oxides and carbon materials is required, which can enhance the energy storage efficiency synergistically.
9][20][21][22] However, Fe 2 O 3 alone as nanoparticles suffer from severe issues like low conductivity, high agglomeration and structural degradation during the charge and discharge cycling process which causes rapid loss of capacity caused by volume changes. 214][25][26][27][28][29][30][31] It has been investigated that rGO connes the Fe 2 O 3 nanoparticles and allows in situ transformation between Fe 2 O 3 nanoparticles and Fe atoms, which can maintain the structural integrity. 13,19The rGO-Fe 2 O 3 composites perfectly exhibit the synergistic effects of EDLC and faradaic pseudocapacitance, which signicantly improves the electrochemical performance.For example, Lee and co-workers fabricated Fe 2 O 3 nanotubes anchored on rGO with a specic capacitance seven times higher than that of bare Fe 2 O 3 . 17Gao and co-workers developed porous Fe 2 O 3 on graphene and achieved a specic capacitance of 343.7 F g À1 at a current density of 3 A g À1 . 19Similarly, Wang and co-workers reported hybrid of Fe 2 O 3 mesocrystals/graphene which displayed higher specic capacitance (226 F g À1 ) than rGO (116 F g À1 ) when employed as supercapacitor electrode material. 32][18][19] Herein, we report a facile synthesis of rGO-Fe 2 O 3 composite involving homogenous precipitation of FeCl 3 in graphene oxide (GO)/urea suspension and subsequently hydrazine assisted reduction of GO under microwave irradiation.In general, the hydrothermal method has been used for synthesizing rGO-Fe 2 O 3 composites, but to the best of our knowledge, this is the rst example of microwave-driven synthesis of rGO-Fe 2 O 3 composite employed as a supercapacitor electrode material.Moreover, for the rst time, a detailed impedimetric evaluation has been correlated with other electrochemical techniques.The results show that the Fe 2 O 3 nanoparticles are uniformly distributed on the rGO matrix, which exhibits excellent capacitive and charge discharge behavior, making it suitable for charge storage devices.

Chemicals
All chemicals were purchased from Merck.Throughout the synthesis, deionized water (DI water, 18.2 MU cm) was used.Digital ultrasonic cleaner LMUC SERIES (Labman Scientic Instruments) was used throughout the experiment.Microwave reactor (Anton Paar, Monowave 300) was employed for microwave irradiation.

Characterization
Powder X-ray diffraction (PXRD) spectra were recorded on a Bruker D2 phaser X-ray diffractometer equipped with Cu-Ka radiations (1.54 Å).Surface morphology was determined using Supra55 Zeiss Field-Emission Scanning Electron Microscope equipped with EDX facility.Transmission electron microscopy (TEM) results were obtained through TECNAI-120 kV system.Thermogravimetric analysis (TGA) was conducted on METTLER TOLEDO (TGA/DSC 1) by using soware STAR e system.

Electrochemical measurements
All electrochemical experiments were conducted at room temperature on computer controlled Autolab PGSTAT 204N using NOVA soware (version 1.10).The standard three electrode conguration was used where Ag/AgCl electrode was used as a reference electrode and platinum wire was incorporated as a counter electrode.Electrochemical testing was performed on glassy carbon electrodes (GCE of 3 mm diameter).Cyclic voltammetry (CV) study were performed at room temperature at different scan rates in 1 M Na 2 SO 4 .Galvanostatic charging discharging test was performed by chronopotentiometry technique at different current densities.The equations to calculate supercapacitor parameters are given in ESI.†

Synthesis of rGO
The rGO was synthesized by employing modied Hummer's method.In a typical synthesis, 4 g graphite powder and 2 g NaNO 3 were mixed together and slowly added into concentrated 170 mL H 2 SO 4 on an ice bath.Thereaer, 12 g KMnO 4 was drop wise added into the mixture under vigorous stirring keeping its temperature less than 20 C. Aer some times, the ice bath was removed and mixture was continuously stirred for next 18 h.As a result, the mixture became viscous and brownish colored appeared.Subsequently, the mixture was diluted with 220 mL DI water by maintaining the temperature below 50 C.Further, the mixture was treated with 10 mL of 30% H 2 O 2 , which induced bubble formation and the mixture color turned into bright yellow indicating the reaction completion.The mixture was further stirred for next 4 h and then it was ltered and carefully washed with 10% HCl, DI water and ethanol to remove any other impurity ions.The as-formed product i.e. graphite oxide was vacuum dried and collected. 11,39Later on, 300 mg graphite oxide was rst dispersed in DI water and then sonicated for 1 h to transform it into graphene oxide (GO).The reduction of graphene oxide was carried out by dissolving 3 g NaBH 4 in 18 mL DI water, further gradually added into the GO dispersion and then reuxed at 100 C for 24 h under continuous stirring.Finally, the reuxed product was ltered and thoroughly washed with DI water and ethanol.The washed powder was vacuum dried and referred to as rGO. 8,11,39nthesis of rGO-Fe 2 O 3 composite The rGO-Fe 2 O 3 composite was prepared by homogeneous precipitation followed by reduction with hydrazine hydrate under microwave irradiation.In a typical synthesis, FeCl 3 (5 mmol; 405 mg) and urea (150 mmol; 4.5 g) were separately dissolved in DI water (25 mL).Further, both solutions were slowly and sequentially added to graphite oxide suspension (25 mL, 2 mg mL À1 ) under continuous stirring and the as formed mixture was sonicated for 1 h.Aer sonication, the brown colored mixture was heated at 90 C for 2 h and then cooled down to room temperature.Further, hydrazine hydrate (0.5 mL) was introduced to the mixture under constant stirring.Aer 2 h of stirring, the mixture was irradiated for 2 min in a microwave reactor at 150 C with a power of 300 W, which transformed the mixture color from brown to black and it was ltered and washed properly with DI water to remove any unreacted part and rGO-Fe 2 O 3 composite was obtained by drying at 60 C for 24 h under vacuum. 39

Fabrication of modied glassy carbon electrodes
The rGO-Fe 2 O 3 composite was further employed as a modier for glassy carbon electrode (GCE) under optimized conditions.In this process, rst 20 mg of rGO-Fe 2 O 3 composite was dispersed in 8 mL ethanol using ultra-sonication to form a stable suspension and then 10 mL of this suspension was drop cast onto the working area of GCE by micropipette and then dried under infrared lamp irradiation.Aer drying, the modi-ed GCE (rGO-Fe 2 O 3 /GCE) was rinsed over several times with DI water and dried before employing it for electrochemical testing.Similarly other electrode of rGO (rGO/GCE) was also prepared.

Results and discussion
The rGO-Fe 2 O 3 composite was in situ prepared by a facile method including homogeneous precipitation followed by hydrazine hydrate assisted reduction under microwave irradiation (Scheme 1).In most of earlier reports, hydrothermal method was used for the composite fabrication which is a timeconsuming process and reactions take place at harsh conditions such as high temperature and pressure, whereas in the present work a simple microwave reaction for 2 min was used.Further, the composite was characterized by various techniques and then used as a modier for the GCE to form modied electrode rGO-Fe 2 O 3 /GCE to study its electrochemical activity.The results were compared with separately prepared electrode of rGO (rGO/ GCE).

Characterization of rGO and rGO-Fe 2 O 3 composite
The crystallinity and phase purity analysis of rGO and rGO-Fe 2 O 3 composite were examined by powder X-ray diffractometer (XRD) in the range of 10-80 .Fig. 1a represents the diffraction pattern of bare rGO, where the peaks located at 2q ¼ $25 and $43 correspond to (002) and (100) planes conrm the formation of rGO. 27The XRD spectrum of graphene oxide (GO) is shown in Fig. S1, † exhibiting its characteristic peak at $10 corresponding to 001 plane, which is shied to $25 in the XRD spectrum of rGO, clearly indicates that GO has been successfully reduced into rGO. 9In the XRD spectrum of rGO-Fe 2 O 3 composite, the diffraction peaks obtained at 2q ¼ 24.07 , 33  104), ( 110), ( 113), (024), ( 116), ( 018), ( 214), (300), ( 119) and (220), respectively with the lattice parameters a ¼ b ¼ 5.0356 Å, c ¼ 13.7489 Å, a ¼ b ¼ 90 and g ¼ 120 .The indexed peaks represent as trigonal phase of hematite (JCPDS no.00-033-0664), which are consistent with previous reports (Fig. 1b). 13,32The average crystallite size is calculated to be 15.1 nm using Debye-Scherrer's equation.Additionally, no peak emerges at around 2q ¼ 25 in the XRD spectrum of composite, which suggests that the rGO platelets have been homogenously dispersed on the surface of the Fe 2 O 3 nanoparticles.Furthermore, no diffraction peak corresponding to any impurity ions is detected, conrming the high phase purity of material.To further conrm the phase purity of rGO-Fe 2 O 3 composite, EDX analysis was carried out.The EDX result shows the phase purity of rGO-Fe 2 O 3 composite with desired elemental composition as C, O, Fe with no obvious impurities (Fig. S2 †), which is consistent with the XRD result.
Fig. 2a and b shows the FESEM images of rGO displaying wavy and corrugated appearance forming platelets.1][42] TEM image (Fig. 2c) further reveals the presence of thin rGO platelets and conrms the wavy wrinkled structures.Fig. 2d-f  as an interconnected 3D network structure, which witnesses healthy alliance between the Fe 2 O 3 nanoparticles and rGO nanoplatelets leading to high stability.The average size of these Fe 2 O 3 nanoparticles is determined to be $46 nm.These Fe 2 O 3 nanoparticles are seen to be uniformly anchored on the rGO platelets and some nanoparticles are observed to be properly bound to the surface of the rGO platelets (Fig. 2e).Moreover, a considerable portion of the Fe 2 O 3 nanoparticles is encapsulated within the rGO platelets, which possibly prevents the aggregation of nanoparticles (Fig. 2f).These engagements of Fe 2 O 3 nanoparticles within the rGO platelets enhances their interfacial contact and prevents the agglomeration of Fe 2 O 3 nanoparticles, which in turn results in high electrochemical performance and better structural integrity for supercapacitor  electrode material. 22TEM images further conrm the proper distribution of Fe 2 O 3 nanoparticles on rGO matrix and disclose the size range between 30-80 nm with an average particle size of 51 AE 25 nm (Fig. 2g-i).
Thermal stability of rGO and rGO-Fe 2 O 3 composite was compared by thermogravimetric analysis (TGA) at a scan rate of 5 C min À1 under N 2 atmosphere up to 900 C (Fig. 3).As can be seen, rGO initially loses $18% of its weight up to 100 C due to physically absorbed water molecules (Fig. 3a).However, for the rGO-Fe 2 O 3 composite, $7% weight loss is observed up to 220 C, which can be attributed to the loss of physically absorbed water molecules and removal of remaining oxygen groups present on the surface of GO (Fig. 3b).Further, signicant weight loss ($19%) is observed in rGO-Fe 2 O 3 composite between 220-530 C, which can be associated with the gasication of carbonaceous material due to the oxidation of rGO.Above 530 C, no signicant weight loss occurs and almost constant weight is observed up to 900 C, whereas more than 90% of the rGO content completely decompose up to 900 C. Based on the comparison of TGA prole of both rGO and rGO-Fe 2 O 3 composite, the weight percentage of Fe 2 O 3 nanoparticles in the composite is estimated to be 79.32%.From these results, it can be concluded that an alliance between Fe 2 O 3 nanoparticles and rGO platelets leads to positive synergism with a high temperature stability to the as prepared rGO-Fe 2 O 3 composite, as compared to bare rGO. 43

Electrochemical properties of rGO and rGO-Fe 2 O 3 composite
The electrochemical properties of in situ generated rGO-Fe 2 O 3 composite were rstly investigated by cyclic voltammetry (CV) measurements in 1 M Na 2 SO 4 solution in the potential window (À0.8 to 0 V) at different scan rates.The comparison of CV proles of rGO and rGO-Fe 2 O 3 composite at a scan rate of 100 and 500 mV s À1 is shown in Fig. 4a and b.As expected, rGO-Fe 2 O 3 /GCE demonstrates excellent charge propagation and enhanced integrated area compared to rGO/GCE, due to synergistic contribution of rGO nanoplatelets and Fe 2 O 3 nanoparticles.Moreover, no obvious redox peaks were observed exhibiting nearly rectangular shaped prole conrms good supercapacitive properties.The overall charge storage capacity and charge propagation time of rGO-Fe 2 O 3 composite have been considerably improved by the sum of pseudocapacitance of Fe 2 O 3 and the EDLC of rGO.The presence of wrinkles on wavy rGO prevents restacking among the platelets and enhances the ion transportation on electrode surface by shortening the ion diffusion path, [40][41][42] resulting in enhanced electrochemical output.The CVs of both electrodes at different scan rates (10-500 mV s À1 ) are shown in Fig. 4c and  d and it is observed that the area under the CV curves increases with the increasing scan rate in both cases.As expected, CV of rGO/GCE exhibits good charge propagation at the electrode surface following EDLC mechanism.On the other hand, CVs of rGO-Fe 2 O 3 /GCE show excellent charge propagation and exhibit quasi-rectangular prole at low scan rates, but interestingly at higher scan rates, it tries to achieve nearly rectangular shape due to faster diffusion of electrolyte ions resulting in enhanced charge storage capacity. 8,23This observation concludes that at higher scan rates EDLC mechanism predominates.It is also observed that even at a highest scan rate of 500 mV s À1 , the rGO-Fe 2 O 3 /GCE still maintains a good rectangular shape without any distortion suggesting high rate ability, small equivalent resistance and supreme electrochemical reversibility. 8o discuss more about the potential of in situ fabricated rGO-Fe 2 O 3 composite as supercapacitor electrode material, galvanostatic charge-discharge (GCD) measurements are performed with 1 M Na 2 SO 4 solution between potential windows (0 to À0.8 V) at different current densities.The comparison of charge-discharge proles of rGO/GCE and rGO-Fe 2 O 3 /GCE at a current density of 2 A g À1 is shown in Fig. 5a.During the charge-discharge process, the charging curve of rGO-Fe 2 O 3 / GCE is nearly as symmetrical to its corresponding discharging counterpart as the rGO/GCE electrode, which is representative of excellent capacitive behavior.Additionally, no considerable potential drop occurs, suggesting the lack of any signicant internal resistance in rGO-Fe 2 O 3 /GCE.Moreover, the chargingdischarging time of rGO-Fe 2 O 3 /GCE has remarkably enhanced due to positive synergism between Fe 2 O 3 nanoparticles and rGO platelets than the rGO/GCE, this results in higher charge storage efficiency due to involvement of more electrolyte ions.The GCD proles of rGO/GCE and rGO-Fe 2 O 3 /GCE at different current densities display charge-discharge lines with almost constant slopes in the whole range of the potential as shown in Fig. 5b and c, further signifying the nearly ideal capacitive type behavior.19][20][21]32 The rate performance of rGO/GCE and rGO-Fe 2 O 3 /GCE is also analyzed by plotting specic capacitance against current density.The specic capacitance kept on decreasing with increasing current density due to incomplete  utilization of active ionic species at high current densities (Fig. 5d).However, rGO-Fe 2 O 3 /GCE exhibits high rate ability by retaining 75.75% (437.5 F g À1 ) of its initial capacitance even at a high current density of 10 A g À1 , suggesting rGO-Fe 2 O 3 composite a promising electrode material for charge storage devices.The electrochemical stability of rGO/GCE and rGO-Fe 2 O 3 /GCE is also examined by repeating 1000 charge discharge cycles at a current density of 2 A g À1 (Fig. 5e).The capacitance retention of rGO-Fe 2 O 3 /GCE is nearly as high (85.71%)as rGO, suggesting the high long-term cyclic stability of the rGO-Fe 2 O 3 / GCE.The maximum energy and power densities for rGO-Fe 2 O 3 / GCE are determined to be 51.33 W h kg À1 and 796.55 W kg À1 respectively at a current density of 2 A g À1 .A Ragone plot of rGO-Fe 2 O 3 /GCE is presented in Fig. 5f, which demonstrates its remarkable rate performance and shows that energy density decreases and power density increases on increasing current density from 2-10 A g À1 .The separate plots of energy and power density versus current density rGO-Fe 2 O 3 /GCE are presented in Fig. S3.† A comparative statement of presently fabricated supercapacitor electrode material (rGO-Fe 2 O 3 ) with previously reported similar electrode materials is shown in Table 1, which clearly shows that microwave assisted fabrication of rGO-Fe 2 O 3 composite has an edge over existing composites as a supercapacitor electrode material.

EIS analysis
The charge transfer kinetics and capacitive components of prepared materials were further probed by electrochemical impedance spectroscopy (EIS).In EIS, the data is generally expressed by Nyquist plots. 44In the present work, the EIS study was performed by applying an AC signal in the frequency range of 1 Hz to 60 kHz under open circuit potential conditions.6][47][48][49] The measurements of impedance provide information on real and imaginary parts of complex impedance in an electrochemical system.Fig. 6 shows the Nyquist plot of rGO/GCE as well as rGO-Fe 2 O 3 /GCE.
As the semicircles obtained via EIS appear depressed having a center below the real axis, a constant phase element (CPE) is introduced instead of capacitance.The impedance is represented by the below equation: If a simple equivalent circuit (a series array of parallel RC elements) is applied wherein each RC combination represents contributions due to different components (such as double layer, charge transfer, etc.), then the total complex impedance is represented as the sum of these components as follows: wherein, the two components represent the bulk (depletion layer) and grain boundary (interfacial/Helmholtz layer) properties of the material, aer excluding the properties from the electrode itself (in this case, GCE). 50,51The impedance of the CPE is described by the following equation: where u is the angular frequency and n (0 # n # 1) is a distributing factor, as the material shows distributive  impedance. 50,51When n ¼ 1, the circuit describes an ideal capacitor, and when n ¼ 0, the circuit describes an ideal resistor.The true capacitance can be resolved from the CPE value by using the following equation: where R, A and n are resistance and CPE parameter values determined from the applied model, respectively. 51The inset of Fig. 6 shows the equivalent circuit used for tting the EIS data, using the true capacitance derived from above mentioned equation from the CPE values.The values of n are in the range of 0.4 to 0.8, which indicates phase inhomogeneity in the electrode, which is expected given that the electrode consists of at least two phases (including GCE material).However, it should be noted that the average value of n is higher for rGO-Fe 2 O 3 /GCE, indicating that the material is more homogenous compared to the purely rGO/ GCE.It should also be noted that the value of R gb (which is the interfacial resistance or grain-boundary resistance) decreases notably for the rGO-Fe 2 O 3 /GCE, which is an indicator of good crystallinity of the composite electrode in comparison with purely rGO/GCE.This is consistent with increased currents obtained with rGO-Fe 2 O 3 /GCE in CV scan, and indicates greater electronic mobility and conductivity for the rGO-Fe 2 O 3 /GCE.It also agrees with the results of the XRD described previously.To the best of our knowledge, there are no similar studies on the materials described here in the context of a supercapacitor application.

Conclusions
In summary, we have described a simple two-step microwave assisted in situ strategy to fabricate rGO-Fe 2 O 3 composite.The positive synergism between rGO and Fe 2 O 3 nanoparticles leads to the formation of mechanically robust, electrically conductive and thermally stable composite.Further, the composite was applied as a working electrode material to demonstrate its potential candidature to be a promising electrode material for high performance hybrid supercapacitors.The rGO-Fe 2 O 3 composite based electrode displays a very high specic capacitance of 577.5 F g À1 at a current density of 2 A g À1 .It also demonstrates high rate performance by retaining 75.75% (437.5 F g À1 ) of its initial capacitance even at a very high current density of 10 A g À1 .In addition, these results are supported by the EIS analysis, which suggests high phase homogeneity, high capacitance and improved charge-transfer characteristics.Based on the results, it can be concluded that the microwavesupported synthesis of rGO-Fe 2 O 3 composites could provide effective enhancements in desirable properties for high performance supercapacitor electrode materials.
Scheme 1 Schematic of fabrication of rGO-Fe 2 O 3 composite.

Fig. 2
Fig. 2 (a) and (b) FESEM images of rGO at different magnifications, (c) TEM image of rGO, (d-f) FESEM images of rGO-Fe 2 O 3 composite at different magnifications, and (g-i) TEM image of rGO-Fe 2 O 3 composite at different magnifications.

Fig. 4
Fig. 4 (a) and (b) Comparison of CV profiles of rGO/GCE and rGO-Fe 2 O 3 /GCE at a scan rate of 100 and 500 mV s À1 , respectively, (c) and (d) CV profiles of rGO and rGO-Fe 2 O 3 /GCE at different scan rates (10-500 mV s À1 ) in 1 M Na 2 SO 4 .

Fig. 6
Fig. 6 Nyquist plots for (a) rGO-Fe 2 O 3 /GCE and (b) rGO/GCE vs. Ag/ AgCl over the frequency range of 1 Hz to 60 kHz in 1 M Na 2 SO 4 .Here Z 0 ¼ real impedance and Z 00 ¼ imaginary impedance, inset shows the equivalent circuit.

Table 1
Comparison of performance of present supercapacitor with other reported supercapacitor based of rGO-Fe 2 O 3 composite Table2presents the parameters determined from the tting, as well as the true capacitance values obtained from the CPE by means of above equations.It can be clearly seen that the values of R s are nearly constant, which indicate the resistance values due to the experimental setup, wires, etc.The contributions of capacitance from the bulk (or charge transfer-impedance, C bulk ), and the grain boundary (or Helmholtz layer, C gb ) are clearly higher for the rGO-Fe 2 O 3 / GCE than for the rGO/GCE.The total capacitance is a combination of the bulk as well as grain-boundary capacitance, and in this respect, the rGO-Fe 2 O 3 /GCE provides a total capacitance much higher than the solely rGO/GCE, due to synergistic effects of the two nanomaterials being combined in a composite.This is further strengthened by the specic capacitance tests described earlier.