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Self-Assembled and One-Step Synthesis of Interconnected 3D Network of Fe3O4/Reduced Graphene Oxide Nanosheets Hybrid for High-Performance Supercapacitor Electrode Rajesh Kumar,*,† Rajesh K. Singh,*,‡ Alfredo R. Vaz,† Raluca Savu,† and Stanislav A. Moshkalev† †

Centre for Semiconductor Components and Nanotechnology (CCS Nano), University of Campinas (UNICAMP), 13083-870 Campinas, Sao Paulo, Brazil ‡ School of Physical & Material Sciences, Central University of Himachal Pradesh (CUHP), Kangra, Dharamshala, Himachal Pradesh 176215, India S Supporting Information *

ABSTRACT: In the present work, we have synthesized threedimensional (3D) reduced graphene oxide nanosheets (rGO NSs) containing iron oxide nanoparticles (Fe3O4 NPs) hybrids (3D Fe3O4/rGO) by one-pot microwave approach. Structural and morphological studies reveal that the as-synthesized Fe3O4/rGO hybrids were composed of faceted Fe3O4 NPs induced into the interconnected network of rGO NSs. The morphologies and structures of the 3D hybrids have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectrometer (XPS). The electrochemical studies were analyzed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy, which demonstrate superior electrochemical performance as supercapacitors electrode application. The specific capacitances of 3D hybrid materials was 455 F g−1 at the scan rate of 8 mV s−1, which is superior to that of bare Fe3O4 NPs. Additionally, the 3D hybrid shows good cycling stability with a retention ratio of 91.4 after starting from ∼190 cycles up to 9600 cycles. These attractive results suggest that this 3D Fe3O4/rGO hybrid shows better performance as an electrode material for high-performance supercapacitors. KEYWORDS: 3D Fe3O4/rGO hybrids, interconnected network, faceted Fe3O4 nanoparticles, reduced graphene oxide, microwave, supercapacitor

1. INTRODUCTION With rapid global economical development leading to reduction of fossil fuels and rising environmental pollution, there is a critical call for competent, sustainable, and clean sources of energy along with latest technologies related with energy conversion and storage.1,2 In several application areas, the electrochemical energy conversion and storage are through batteries, fuel cells, and electrochemical capacitors (ECs).3,4 The ECs, also called supercapacitors (SCs) have attracted considerable attention since SCs, are used in electronic devices for energy storage because of their higher capacitance, fast charging/discharging rate, long cycle-life, high power density, and low equivalent series resistance.5,6 The SCs can be mainly divided into two categories according to their energy storage mechanisms: (i) high surface area and conductive carbon-based different dimensional materials that store high charge at the interface between electrolyte and electrode, known as electrical double-layer capacitance (EDLC)7,8 and (ii) redox active materials such as conducting polymers and transition metal oxides, that undergo fast and reversible Faradaic redox reactions © 2017 American Chemical Society

during electrochemical measurement such as charge/discharge process,9,10 termed as pseudocapacitance. The EDLC behavior, leads to high power density at the cost of low energy density and limiting rate capability.11 However, the pseudocapacitance behavior shows high energy density (with low power density) lacking cyclic stability.12 A variety of different materials including, carbon based materials (activated carbon, carbon aerogels, meso-porous carbon, carbon nanotubes and carbidederived carbons etc.), metal oxides nanoparticles (NPs) (NiO, RuO2, MnO2, ZnO, SnO2, Co3O4, etc.) and conducting polymers (polyaniline, polypyrrole and polythiophene etc.) have widely been studied as electrode materials for highperformance SCs.11,13 The hybrid materials, composed of porous carbon based materials and metal oxides have been extensively studied toward developing ultra high energy storage system that exhibits the advantages of both EDLC and Received: November 16, 2016 Accepted: February 22, 2017 Published: February 22, 2017 8880

DOI: 10.1021/acsami.6b14704 ACS Appl. Mater. Interfaces 2017, 9, 8880−8890

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ACS Applied Materials & Interfaces

2. EXPERIMENTAL SECTION

pseudocapacitance, including high power density, high energy density, and long-term cycle stability.11,14 Thus, carbon-based materials with metal oxide can remove the problems of low energy and power density along with rate limiting capability and lack of stability during cycling due to the occurrence of two simultaneous different charge/discharge storage mechanisms in SCs.15,16 In the present time, new carbon materials known as “graphene”, with a one-atom-thick two-dimensional (2D) planar sheet of sp2-bonded carbon atoms, are considered as a suitable candidate due to its extraordinary high surface area (2630 m2 g−1), excellent electronic conductivity, high chemical and thermal stability, large range potential windows, and high surface chemistry.17−19 One specific branch of graphene research deals with graphene oxide, which consists of various oxygen-containing functional groups on their basal planes (epoxy, hydroxyl) and edges (carbonyl, carbonyl).18,20 Because of oxygen functionalities attached on surfaces and edges, it shows hydrophilic nature and can be easily dispersed in water and organic solvents to form stable colloidal suspensions for the attachment of metal oxide NPs.21 Moreover, these different types of oxygen-containing groups can be further utilized to graft polymers and metal oxides to form graphene-based hybrids materials.22,23 Graphene-based Fe3O4 hybrids materials as electrodes have attracted intensive scientific research interest due to their positive synergistic effects of pseudocapacitance and EDLC between graphene and metal oxides.24,25 Several other metal oxides, such as NiO, Co3O4, Cu2O, RuO2, MnO2, V2O5 SnO2, ZnO, TiO2, etc. have been used to synthesize graphene-based metal oxide hybrids.10,26−28 Lately, hybrids materials composed of 2D Fe3O4/rGO have been reported to be good electrode materials providing high electrochemical capacitance and show the excellent materials for high capacitance, abundant sources and minimal environmental impact.11,29,30 The electrochemical performances of SCs are strongly influenced by morphology, microstructure, and interaction of the graphene and Fe3O4 NPs. These structural parameters for hybrids materials can be controlled and optimized during the synthesis processes. The graphene-Fe3O4 composites has been synthesized using various routes such as electrochemical transformation,30 solvothermal,31 hydrothermal,32 hydrolysis process,33 solution precipitation and hydrogen reducing,7 electrophoretic deposition,34 etc. These reported synthesis routes show it is rather difficult and complex to control the final structure. So it is required that we look at a simpler, cost-effective, and fast synthesis route for graphene- based Fe3O4 SCs hybrids for enhanced electrochemical performance. Synthesis of hybrids with different morphologies has always fascinated researchers because of their versatile range of scientific and technological applications. Herein, we report a facile, simple, and fast synthesis approach to fabricate 3D Fe 3 O 4 /rGO hybrids by microwave for SCs electrode application. The 3D interconnected network of rGO NSs contains the faceted Fe3O4 NPs and the vertical rGO NSs provides high surface area. The performance of 3D Fe3O4/rGO hybrids was investigated using cyclic voltammetry (CV), galvanostatic charge/discharge, cycling performance, and electrochemical impedance spectroscopy (EIS). The Cs value of 3D Fe3O4/rGO hybrid is higher compared to that of pure 3D rGO NSs as well as Fe3O4 NPs.

2.1. Synthesis of Graphite Oxide. Graphite oxide (GO) was synthesized by chemical oxidation of natural graphite powder, using the modified Staudenmaier’s method.35 Graphite powder (5 g) was continuously stirred with a combination of sulfuric acid (H2SO4) (90 mL) and nitric acid (HNO3) (45 mL) solution at room temperature. The solution container was placed into an ice−water bath to make sure constant temperature and subsequently, potassium chlorate (KClO3) (55 g) was gradually poured into the solution to reduce the risk of explosion due to exothermic reaction. This solution was kept for 5 days under continuous magnetic stirring at room temperature for superior oxidation of the graphite powder.28 The as obtained GO product was washed with DI water and further 10% hydrochloric (HCl) solution was added to eliminate sulfate and other ion impurities. It was then again washed with DI water several times until a pH of 7 was reached.28 Afterward, the GO powder was dried at 70 °C under vacuum and used in the next step for the synthesis of 3D Fe3O4/rGO hybrids. 2.2. Synthesis of 3D Fe3O4/rGO Hybrids. The graphite oxide suspension (1 mg/mL) was obtained by mixing 50 mg of graphite oxide powder in 50 mL of C2H5OH assisted with continuous magnetic stirring (5 min) and sonication. To prepare the Fe3O4/rGO hybrids, dropwise solution of FeCl3·6H2O (0.25 g in 10 mL water) was added into the as-prepared graphite oxide suspension under continuous stirring. After 10 min, diluted ammonia (NH3·H2O) (0.2 M) solution (10 mL) was dropwise added into the mixture for 10 min.36 Finally, the solution was dried in open atmosphere for complete evaporation of C2H5OH and dried power was treated with microwave (ConsulCMW30AB) heating for the formation of 3D Fe3O4/rGO hybrids. 2.3. Preparation of Electrode and Electrochemical Testing. The as-synthesized hybrid materials were used as the working electrode, platinum19 wire as the counter electrode and Ag/AgCl as the reference electrode. All the measurements were carried out in a 2.0 M KOH electrolyte solution at room temperature.37 The glassy carbon electrode (GCEs) (5 mm diameter) was polished with a 1.0 and 0.05 mm alumina slurry and then sonicated in ethanol for cleaning and elimination of contamination. After that, 1.0 mg of the synthesized hybrid material was dispersed in 1.0 mL of dimethylformamide (DMF) with the aid of ultrasonic agitation to give a 1.0 mg/mL black suspension. Then, 10 μL of black suspension was dropped on the GCE electrode with the help of a microsyringe and the solvent was evaporated in air.28 Thus, a uniform film of electrode material coating is formed on the surface of the GCE. Finally, 5 mL of Nafion (5 wt %) was cast and used as a net to stably hold the materials on the electrode surface. The solvent was allowed to evaporate before use and this final electrode was used for the electrochemical analysis. The cyclic voltammetry (CV) and charge−discharge measurements were carried out using a three-electrode system employing a VersaSTAT 3 (Princeton Applied research) between −1.0 and 0.4 V by varying scan rates and current densities. The electrochemical impedance spectroscopic (EIS) measurement was performed in the frequency range from 0.01 Hz to 100 kHz with ac-voltage amplitude of 10 mV. All the measurements were carried out in 2 M KOH electrolyte solution at room temperature. 2.4. Characterization. The crystal phases of the as-prepared samples were determined using X-ray diffractometer (XRD D/MAX2500/PC; Rigaku Co., Tokyo, Japan) over the 2θ range 10 to 75°. The surface morphology and elemental data were investigated using scanning electron microscope (SEM - Dual Beam FIB/FEG model Nova 200) equipped with energy-dispersive X-ray spectroscope (EDS). Transmission electron microscopic (TEM) images were obtained by using a TECNAI 20 with a operating voltage of 200 kV. Raman measurements were carried out using a spectrometer with a 473 nm laser (NT-MDT NTEGRA Spectra). The X-ray photoelectron spectroscopy (XPS) spectrum was recorded on a MultiLab 2000 photoelectron spectrometer (Thermo-VG Scientific, USA) with Al Kα (1486.6 eV) as the X-ray source. All XPS spectra were corrected using the C 1s line at 284.6 eV. To determine the weight ratio of synthesized samples Thermogravimetric analysis (TGA) was carried out employ8881


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ACS Applied Materials & Interfaces ing PerkinElmer (TA Instruments) in air from room temperature to 900 °C at a heating rate of 10 °C/min.

3. RESULTS AND DISCUSSION 3.1. Morphological and Structural Characterization. Figure 1 shows the SEM images of the 3D rGO NSs sample. It

Figure 2. SEM images of 3D Fe3O4/rGO hybrids at different magnifications: (a) 5 μm, (b) 4 μm, (c) 2 μm, and (d) 500 nm.

because of this, its shows high values of electrochemical capacitances.38,39 To further explore the inner morphology, we took TEM images of 3D Fe3O4/rGO hybrids as shown in Figure 3. It is worth noting that the multifaceted Fe3O4 NPs with an average size of 30−150 nm are strongly attached to the rGO NSs even after ultrasonic treatment, implying the strong interaction between the Fe3O4 NPs and rGO NSs.40 The Fe3O4 NPs are dispersed on the surface of rGO NSs and less aggregation was found in the synthesized 3D Fe3O4/rGO hybrids (Figure 3a, b). Figure 3d, reveals that faceted Fe3O4 NPs are spread on rGO NSs. Figure 3d, e shows a high-resolution TEM (HRTEM) image of individual crystalline faceted Fe3O4 NPs on rGO having different orientations. Also, Figure 3d shows the clear lattice fringes of rGO NSs which gives the support to Fe3O4 NPs. The rGO NSs have the interplanar distance of 0.34 nm corresponding to the (002) crystalline plane of graphene. The Fe3O4 NPs contains multiple crystalline domains with different orientations as observed in the HRTEM image. Inset of Figure 3e reveals a 0.26 nm interspacing corresponding to the (311) plane of crystalline Fe3O4 NPs. The selected area electron diffraction (SAED) pattern suggested that the Fe3O4 NPs are high crystalline. The Figure 3f shows a selected area electron diffraction (SAED) pattern, where the labeled diffraction dots and rings shows the (220), (311), (400), and (440) planes and confirming the formation of Fe3O4 phase. These results are well-matched with the XRD data, indicating the crystalline characteristic of the Fe3O4 NPs. Figure 4a shows the EDS of the 3D Fe3O4/rGO hybrids which contains the characteristic peaks of C, O and Fe. The content (at %) of C, O, and Fe were 93.2, 3.08, and 3.71, respectively. The crystal formation of the as-synthesized Fe3O4/ rGO hybrids was examined by XRD analysis (Figure 4b). The broad diffraction peaks observed at 2θ = 25.9 is corresponds to (002) hkl for rGO NSs and indicates that there are few layers in NSs. The other diffraction peaks at 2θ = 18.1, 30.4, 35.3, 37.2, 43.1, 53.4, 57.2, and 62.8° can be assigned to (111), (220), (311), (222), (400), (422), (511), and (440) reflections respectively for Fe3O4 (JCPDS No. 72−2303).41 As for 3D Fe3O4/rGO hybrids, all the characteristic peaks of Fe3O4 NPs

Figure 1. SEM images of 3D rGO NSs at different magnifications. (a) 100 μm, (b) 5 μm, (c) 1 μm, and (d) 500 nm.

can be clearly seen that rGO NSs are interconnected to each others to form a network structure with hollow interior. It can be seen that the 3D rGO NSs possess cotton-shaped fluffy structure, and GNSs surfaces are open and interconnected to each other possessing network like morphology. The rGO NSs have a micrometer-sized open structure, which makes them more porous. Vertically, rGO NSs possess high surface area and large reactive edges, which is good for the attachment of NPs. Such “open” large amounts of reactive edges are favorable for the holding of Fe3O4 NPs. The surface microstructures of 3D Fe3O4/rGO hybrids are presented in the SEM image in Figure 2. The graphene interconnected networks contains Fe3O4 NPs as shown in Figure 2a−d, where the Fe3O4 NPs appear to be well-dispersed into the framework of rGO NSs. The Fe3O4/rGO hybrids are composed of many faceted Fe3O4 NPs decorated into interconnected rGO network surfaces. In addition each Fe3O4 NPs were also caged by the thin continuous rGO NSs and its flexibility are believed to be the reason for the interconnecting graphene web. High magnification SEM image in Figure 2d, shows that faceted Fe3O4 NPs with a size distribution of about 50−200 nm are encapsulated within the spaces between the rGO NSs. The open edges of rGO NSs hold the Fe3O4 NPs with the help of functional group for the final formation of Fe3O4/rGO hybrids, which can efficiently prevent them from agglomerating and restacking and hence enhancing the present active surface for better electrochemical reactions. The highresolution SEM image in Figure 2d demonstrates that there are several faceted Fe3O4 NPs tightly adjusted into the rGO interconnected network. Filling of 3D rGO NSs interconnect network by faceted Fe3O4 NPs, facilitates the electron transport between Fe3O4 and rGO NSs. The existence of such network formation by Fe3O4 NPs and rGO NSs may be advantageous for fast diffusion with easy accessibility of the redox phase; 8882


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Figure 3. (a−c) TEM, (d, e) HRTEM, and (f) SAED pattern images of 3D Fe3O4/rGO hybrids.

Figure 4. (a) EDS spectrum and (b) XRD patterns of 3D Fe3O4/rGO hybrids. (c) Raman spectra of 3D rGO NSs and 3D Fe3O4/rGO hybrids.

band) and ∼1580 cm−1 (G band), respectively. The intensity ratio of the D-band to the G-band (ID/IG) is a measure of the disordered sp3 and ordered sp2 carbon domains.43 The ID/IG ratios were 0.281 and 0.537 for rGO NSs network and Fe3O4/ rGO hybrids, respectively. It was observed that the ID/IG ratio increases in the case of Fe3O4/rGO hybrids. Hence, it confirms the creation of defects onto the rGO NSs network by Fe3O4 NPs for the formation of Fe3O4/rGO hybrids. The changes in the relative intensity of G and 2D band are due to variation in the number of graphene layers. The increase in intensity of 2D band shows that the numbers of layers are decreased after the

and rGO NSs are observed, demonstrating the coexistence of Fe3O4 and rGO in the hybrid networks. Raman spectroscopy is a well-established and powerful technique for structural characterization of carbon-based materials. The Raman spectra provide the information about the structural changes in 3D Fe3O4/rGO hybrids as shown in Figure 4c. The G band is related to the in-plane vibration of sp2 carbon atoms in a 2D hexagonal lattice, and the D band is assigned to the vibrations of sp3 carbon atoms of disordered graphite.42 Raman spectra of rGO NSs network and Fe3O4/ rGO hybrids displayed two prominent peaks at ∼1355 cm−1 (D 8883


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Figure 5. (a) XPS survey of 3D Fe3O4/rGO hybrids. Deconvoluted XPS spectra of (b) C 1s, (c) O 1s, and (d) Fe 2p.

rGO hybrids. Two main peaks at the positions of 711.5 and 724.9 eV can be assigned to Fe 2p3/2 and Fe 2p1/2, respectively, which corroborates to the reported values for Fe3O4 NPs.49,50 For the two peaks at the L2 edge (719−725 eV), the left peak is relatively stronger than the right one, agrees well with the case reported in the literature for Fe3O4 NPs.51 The absence of the satellite peak, which is the fingerprints of the electronic structures of iron oxides like α- Fe2O3 and γ- Fe2O3, demonstrates that the Fe2O3 phase does not exist in the hybrids. For thermal stability and compositional analysis of as synthesized 3D rGO NSs and 3D Fe3O4/rGO hybrids were carried using TGA measurements in air atmosphere as shown in Figure 6. In the case of 3D rGO NSs, small weight loss (∼3 wt %) appears below ∼325 °C temperature, which may be due to the vaporization of moisture and decomposition of some functional groups attached on the surfaces of rGO NSs. Above 325 °C, the TGA curve sharply decreases up to ∼650 °C envisaging high weight loss (∼94.5 wt %) ascribes to the oxidation and decomposition of carbon frame of rGO to CO2.52,53 The TGA curve of 3D Fe3O4/rGO hybrids shows the weight loss in two steps (room temperature to 325 and 325 °C to 650 °C) and after these weight losses (below 650 °C), its

formation of Fe3O4/rGO hybrids. For a much layered graphene structure, the 2D band is broadened and I2D/IG ratio decreases.44 The calculated I2D/IG ratio of rGO NSs network (0.377) was lower as compared to Fe3O4/rGO hybrids (0.443), which further confirmed that the continued exposure to microwave irradiation for the formation of Fe3O4/rGO hybrids also reduces the number of graphene layers. The weak peaks toward lower wave no. shows the Raman scattering peaks for Fe3O4 NPs at 301, 542, and 668 cm−1 corresponding to the Eg, T2g, and A1g vibration modes, respectively.45 The surface composition of the hybrids and the valence state of Fe were characterized by X-ray photoelectron spectroscopy (XPS). Figure 5a shows the survey spectrum of 3D Fe3O4/rGO hybrids, which contains only C, O, and Fe elements. The deconvoluted XPS spectra of C 1s in Figure 5b displays considerable degree of oxidation corresponding to carbon atoms in different functional groups: C−C/CC (284.6 eV) and C−O (286.6 eV), groups.46 The O 1s XPS spectrum (Figure 5c) deconvolute into three peaks located at 530.0, 531.5, and 533.1 eV, which are attributed to oxygen in the lattice (Fe−O),47 oxygen atoms in the surface hydroxyl groups (H−O), and oxygen in the lattice (C−O), respectively.48 Figure 5d shows the Fe 2p core-level XPS spectra of the Fe3O4/ 8884


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participate a key role in controlling the structural morphology of Fe3O4 NPs by performing as nucleation active sites through interaction with metal ions. In our case, maximum Fe3O4 NPs are inside the open structure and it is expected that these Fe3O4 NPs are connected with the surface attached functional groups. Varying the microwave treatment time can effectively adjust the size of metal oxide NPs in hybrids materials. The mechanism for the formation of 3D Fe3O4/rGO hybrids is schematically illustrated in Figure 7. During the microwave irradiation, graphite oxide was converted into fully exfoliated 3D rGO NSs having some oxygen containing functional group attached of the surfaces. The FeCl3 then decomposes and provides Fe ions. The rGO NSs are negatively charged due to considerable oxygen-containing functional groups such as hydroxyl, epoxy on the surfaces and carboxyl and carbonyl on the edges of rGO NSs. The negatively rGO NSs surfaces can electro statically attract and strongly interacts with Fe3+ to form Fe3+-rGO NSs. The Fe3+ ions are adsorbed onto rGO NSs surface to form Fe3+-rGO NSs mixtures. The Fe3+ interact with the functional groups of rGO NSs and form the Fe(OH)3/rGO at high temperature, due to microwave heating.33 Afterward, for longer time irradiation, the Fe(OH)3 convert in to Fe3O4 NPs and the final resultant product obtained are 3D Fe3O4/rGO hybrids. For higher microwave treatment, Fe3O4 gets sufficient time to grow and form faceted structure inside the interconnected network of 3D rGO NSs. Also, the functional groups on the rGO NSs surface effectively hindered the diffusion, growth, and agglomeration of Fe3O4 NPs in the hybrids.55 3.2. Electrochemical Performance of 3D Fe3O4/rGO Hybrids. The electrochemical performance of the 3D Fe3O4/ rGO hybrids electrodes material was investigated by cyclic voltammetry. The specific capacitance (Cs) (F g−1) of the

Figure 6. TGA curves of 3D rGO NSs and 3D Fe3O4/rGO hybrids.

shows a slightly constant weight stability above 650 °C. Room temperature to 325 °C temperature, the weight loss (18.4 wt %) is due to the removal of moisture and functional groups present in synthesized 3D Fe3O4/rGO hybrids. From 325 to 650 °C temperature, the sharp weight loss (24.5 wt %) is due to the oxidation of rGO NSs into CO2. There are several reports for the phase transition of Fe3O4 into Fe2O3 and FeO.54 The TGA curves of 3D Fe3O4/rGO hybrids, shows that Fe3O4 NPs loading on rGO NSs is ∼57% by weight without considering any phase transition. The detailed studies on the basis of morphological and structural analysis clearly show that the hybrids are strongly influenced by the shape of Fe3O4 NPs and structure of rGO NSs. It is well recognized that rGO NSs have some randomly dispersed functional groups on the edges and surfaces, which

Figure 7. Schematic formation mechanism of 3D Fe3O4/rGO hybrids. 8885


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Figure 8. (a) CV measurement of Fe3O4 NPs, 3D rGO NSs and 3D Fe3O4/rGO hybrids at 8 mV/s scan rate, (b) CV measurement of 3D Fe3O4/ rGO hybrids at different scan rates, (c) Cs of 2D Fe3O4/rGO nanostructure and 3D rGO NSs hybrids at different scan rates, and (d) charge/ discharge measurement of 3D Fe3O4/rGO hybrids at different current densities.

mainly due to EDLC. However, for the complete EDLC behavior, the CV curve should be an ideal rectangular shape. Also, the CV curve of rGO NSs is nearly of rectangular shape, indicating EDLC character, whereas that of Fe3O4 NPs shows pseudocapacitance behavior because of the redox peaks and its correspondence to the reversible reactions in Fe(II) ↔ Fe(III).12,56 The electrochemical performance of Fe3O4/rGO hybrids was studied by CV curves as shown in Figure 8b at various scan rates of 8, 12, 18, and 27 mV s−1. The CV curves are rectangular in shape indicating charge conduction at the surfaces of the Fe3O4/rGO hybrids electrode. At higher scan rate (above 27 mV s−1), the CV curves does not much distort in shape but small change at the end on curve was found due to the increase of resistance at the higher scan rate (Figure S2). The rectangular and symmetric CV curves indicate the combination of both, a major contribution of EDLC from rGO NSs and minor effect due to Faradaic pseudocapacitive behavior originated from Fe3O4 NPs. The pseudocapacitance in the curve due to Fe3O4 NPs are remarkably depressed may be due to the low content of Fe3O4 NPs in the 3D Fe3O4/rGO hybrids.7 Also, it can be seen that the shapes of different CV

active material can be calculated from the CV curves according to the following equation Cs =

∫ I dV υmΔV

(1)

where I is the oxidation/reduction current (A), V is the voltage (V), ΔV represents the voltage window (V), υ is the potential scan rate (mV s−1), and m is the mass of the electroactive materials in the electrodes (g). Figure 8a shows the CV curves of rGO NSs, Fe3O4 NPs, and Fe3O4/rGO hybrids at the scan rate of 8 mV s−1. The CV curves of pure Fe3O4 NPs and pure rGO NSs at the same scan rate having low area than that of the Fe3O4/rGO hybrids. It indicates smaller value of Cs since Cs is directly related with the average areas of CV curves as shown in eq 1. The area shown in Figure 8a also indicates that Fe3O4/rGO hybrids have the largest Cs compared with rGO NSs and pure Fe3O4 NPs. On the basis of the above discussion, Fe3O4/rGO hybrids have the best capacitive performance. The shape of the CV curves (for rGO NSs and 3D Fe3O4/rGO hybrids) reveals that the curves are nearly rectangular and the capacitance characteristic is 8886


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Figure 9. (a) Long-term cycling stability of the 3D Fe3O4/rGO hybrids electrode at a constant current density of 3.8 A g−1 over 9500 cycles (inset: time vs potential in charge/discharge of the 3D Fe3O4/rGO hybrids) and (b) Nyquist plot of Fe3O4 NPs and 3D Fe3O4/rGO hybrids (inset: magnified view of Nyquist plot at high-frequency range).

curves at various scan rates (8 to 27 mV s−1), does not shows much significant changes except increase in area, implying good electron conduction within the electrodes. The Cs were calculated from the CV curve (using eq 1) and shown in Figure 8c. The 3D Fe3O4/rGO hybrids electrode exhibits Cs of 455, 363, 322, and 317 F g−1 at scan rates of 8, 12, 18, and 27 mV s−1, respectively. The 2D Fe3O4/rGO nanostructure (Figure S1: SEM images of 2D Fe3O4/rGO nanostructure) shows a decrease in Cs, obtained at same scan rates. The comparatively lower values of Cs for 2D Fe 3O 4-rGO nanostructure may be due to the absence of 3D rGO NSs interconnected network and faceted Fe3O4 NPs which provides the confined spaces for Fe3O4 NPs and hence enhancing the electrochemical properties. The Cs of 3D Fe3O4/rGO hybrids normally decreases with increasing scan and assumed to be related to the diffusion of protons into the electrode material, because of the intercalation/deintercalation of protons or alkaline metal cations.57 At lower scan rates, protons or alkaline metal cations easily intercalate inside the 3D Fe3O4/rGO hybrids electrode. However, at higher scan rates, they are able to fix to only the external surface sheet of the Fe3O4/rGO hybrids electrode and show a decrease in the Cs.57,58 The galvanostatic charge/discharge response of Fe3O4/rGO electrode materials were studied at various current densities, as shown in Figure 8d. The Cs (F g−1) was calculated from the corresponding galvanostatic discharge curves using the following equation

Cs =

I Δt ΔVm

contribution from EDLC.58 The slight asymmetry arises from the pseudocapacitive behavior of oxygen containing functional group attached with rGO NSs and Fe3O4 NPs. The calculated energy density for 3D Fe3O4/rGO hybrids were found to be 80.9, 87.1, 102.4, and 124.0 Wh/kg and corresponding power densities were 2.74, 2.44, 2.27, and 2.06 kW/kg at current densities of 4.4, 4, 3.8, and 3.6 A/g, respectively. The cyclic stability of Fe3O4/rGO hybrids electrodes materials were further examined by continuous charge/ discharge repeating up to 9500 cycle at a constant current density of 3.8 A g−1 within the voltage range of −0.4 to 1.0 V. The capacitance retention as a function of cycle number is shown in Figure 9a. A characteristic triangular charge/discharge curve was observed as revealed in the inset of Figure 9a. There is a nominal decrease of 8.6% in capacitance over the first ∼190 cycles and afterward remains almost stable for more than 9500 cycles. The initial degradation in capacitance can be recognized to the consumption of electrolyte arising from the irreversible reaction between the electrolyte and electrodes.58 The performance of the Fe3O4/rGO hybrids retains up to ∼91% of its initial capacity over 9500 cycles, exhibiting long-term stability for SCs application. The achieved Cs values, long-term stability and retention properties of as synthesized 3D Fe3O4/ rGO hybrids shows better performance with reported articles (Table S1) on Fe3O4/rGO composite/nanostructure-based supercapacitors.7,32−34,58−65 On the basis of the above results, it can be said that the synergy linking the Fe3O4 NPs and 3D rGO NSs is accountable for the enhanced electrochemical performance of 3D Fe3O4/ rGO hybrids. This may result from the low resistance which is further investigated in detail employing electrochemical impedance spectroscopy (EIS). Figure 9b shows Nyquist plots of the Fe3O4 NPs and Fe3O4/rGO hybrid electrodes. Normally, the resistance in an electrochemical cell has an electronic and ionic involvement. The ionic part is ascribed to the separator resistance due to the diffusion of ions through the pores of the electrodes. The electronic resistance includes the bulk resistivity of the electrode material including contact resistance between current collector and the electrode materials.66,67 At high applied frequencies, the intercept of

(2)

where I is the charge/discharge current (A), Δt is the discharge time (s), ΔV is the potential window during discharge process, and m is the mass of the Fe3O4/rGO hybrids.34 The Cs of Fe3O4/rGO hybrids, obtained discharge curve according to eq 2, are 455, 376, 320, and 297 F g−1 at 3.6, 3.8, 4.0, and 4.4 A g−1, respectively. The values of Cs decreases slowly with increasing the current density due to slow rate of redox reactions under high current density.7 The charge/discharge plots at different current densities show triangular nature and have nearly symmetric profile, suggesting the dominant 8887



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real part (Z) represents the ionic resistance of electrolyte.33 It reveals that the ionic resistance of electrolyte is almost 0.8 Ω and is almost same for both impedance spectra. Also, Nyquist plots of Fe3O4 NPs and Fe3O4/rGO hybrids electrodes exhibit a semicircular part and an inclined line. It is identified that a large semicircle for the electrode suggests high interfacial charge-transfer resistance, probably resulting from the poor electrical conductivity of active materials.68,69 The semicircle at high frequency regime could be ascribed to the charge transfer resistance (Rct) and the inclined line at low frequencies correspond to the diffusion of ions in the electrodes materials. Among both electrodes, the 3D Fe3O4/rGO hybrids electrode illustrates low Rct value of 4 Ω and less inclined line, signifying improved capacitive performance of the hybrids electrode than that of the Fe3O4 electrode (11 Ω). This may be due to the reversible redox reactions associated with Fe3O4 nanomaterials along with the better electrical conductivity of 3D rGO NSs. The excellent and high capacitive performance for Fe3O4/ rGO hybrids electrode may be due to the constructive synergistic effects caused by the attachment of Fe3O4 NPs and rGO NSs.26,30 In comparison with common spherical Fe3O4 NPs, the faceted Fe3O4 NPs are better candidate to produce high performance graphene-based electrode material, which might be due to the distinctive face to face contact between rGO NSs and faceted Fe3O4 NPs.59,70 In this study, first, rGO NSs interconnected network give support to Fe3O4 NPs with well-defined faceted shape to anchor inside the open spaces and suppressing the restacking of rGO NSs. Second, the Fe3O4 NPs remains separated by rGO NSs and are free from agglomerations. Third, the open space inside the rGO NSs for Fe3O4 NPs provides east path for electrolyte diffusion during the electrochemical measurements. These three properties are suitable for the electronic and ionic transport. Such an interconnected rGO NSs structure with Fe3O4 NPs provide an elastic buffer to the volume expansion or contraction of Fe3O4 NPs and confine them during the redox reaction process and exhibits better cycling performance with high rate capability. Also, the larger surface area of interconnected network of rGO NSs gives a highly conducive path for electron transfer during the charge/discharge processes.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14704. SEM image of 2D Fe3O4/rGO nanostructure, CV curve at higher scan rates comparison for different rGO/Fe3O4 composite/nanostructures reported for electrochemical supercapacitor application (PDF)

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AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected] (R.K). *E-mail: [email protected] (R.K.S). ORCID

Rajesh Kumar: 0000-0001-7065-3259 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.K., A.R.V., and S.A.M. acknowledge CNPq and FAPESP (Brazil) for financial support.

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REFERENCES

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4. CONCLUSIONS We have hereby synthesized 3D hybrid materials constituting Fe3O4 NPs in an rGO NSs interconnected network using a simple, facile, and effective in situ microwave method. The presence of Fe3O4 NPs between the 3D network rGO NSs effectively keeps the neighboring graphene NSs separated. The 3D interconnected network contains faceted Fe3O4 NPs into rGO NSs. The specific capacitance of the 3D hybrid materials is highly dependent upon the surface morphology of rGO NSs and Fe3O4 NPs and their interconnection. The 3D Fe3O4/rGO hybrids material shows good capacitive behavior in terms of excellent specific capacitance, good rate capability and excellent cycling performance (91.4% after 190 cycles). Considering the low price of the electrode materials, 3D Fe3O4/rGO hybrids are a potential material for energy storage in SCs application. Furthermore, the process stated in this article may offer a fast, straightforward, and efficient route for the synthesis of other carbon/metal-oxide hybrids. 8888


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