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This article can be cited before page numbers have been issued, to do this please use: P. DEB and K. Bhattacharya, Dalton Trans., 2015, DOI: 10.1039/C5DT00296F.

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View Article Online Hybrid nanostructured C-dot decorated Fe3O4 electrode materials for DOI: 10.1039/C5DT00296F

superior electrochemical energy storage performances

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Department of Physics, Tezpur University (Central University), Napaam, Tezpur – 784028, India *Corresponding Author: [email protected]

Abstract: Research on energy storage devices has created a niche owing to the ever increasing demand of alternative energy production as well as its efficient utilisation. Here, a novel composite of Fe3O4 nanospheres and carbon quantum dots (C-dots) have been synthesized by a two step chemical route. The hybrids of C-dot with metal oxides can contribute to the charge storage capacity due to the combined effect of Faradaic pseudocapacitance from the Fe3O4 and the excellent electrical properties of the C-dots, the promising new member of the carbon family. The structural as well as morphological properties of the obtained Fe3O4-C hybrid nanocomposites were extensively studied. Detailed electrochemical study manifests the high performance of magnetically responsive Fe3O4-C hybrid nanocomposites as an efficient supercapacitor electrode material. The remarkable improvement in the electrochemical performances of Fe3O4-C hybrid nanocomposites is attributed to the Faradaic pseudocapacitance of Fe3O4 coupled with the high electrical conductivity of the C-dot which aided in fast transport and ionic motion during the charge-discharge cycles. The cyclic voltammetry as well as galvanostatic charge discharge studies of Fe3O4-C hybrid nanocomposites show that the nanosystem delivers a maximum specific capacitance of ̴ 208 F g-1. The results demonstrate that the novel Fe3O4-C hybrid nanocomposite holds great potential as high performance electrode materials for supercapacitors. Keywords: Supercapacitors, hybrid nanocomposite, energy storage, metal oxides, carbon dot 1. Introduction: Rapid depletion of the fossil resources has called for the development of smart technologies that can enable greater efficiency of energy consumption and appropriate utilization of renewable energy sources. In the recent years, supercapacitors have gained wide interest in the scientific community owing to their higher power density and higher energy density 1   

Dalton Transactions Accepted Manuscript

K. Bhattacharya and P. Deb*

Dalton Transactions

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Article Online compared to conventional batteries and capacitors. This new age power source has View find DOI: 10.1039/C5DT00296F

excellent applications in electric vehicles, burst power generation, memory back-up devices and other related devices which require high-power pulses [1-5]. Based on the energy storage mechanism, supercapacitors are widely classified into: (i) electrical double-layer

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at the electrode-electrolyte interfaces, and (ii) redox capacitors, also known as pseudocapacitors, where the capacitance is due to the Faradaic reactions occurring at the interface. Carbonaceous materials with high surface area are generally used for electrical double layer supercapacitors while transition metal oxides and conducting polymers find their use in pseudocapacitors. The Faradaic pseudocapacitors offer much higher specific capacitance and energy density in comparison to the ELDCs, because of their reversible multielectron redox Faradaic reactions [6-8]. Transitional metal oxides like RuO2, Co3O4, NiO, Fe2O3, MnO2, etc have been widely used and accepted as supercapacitor materials, owing to their large capacitance and fast redox kinetics [9-14]. Recently, Fe3O4 based supercapacitor electrodes have attracted significant attention because of their easy redox reaction, low cost and less environmental impact [1517].However, poor conductivity of Fe3O4 poses as a major hindrance for its application as supercapacitor electrodes. Recent studies have showed improved conductivity of iron oxide based supercapacitor electrodes in porous flower-like α-Fe2O3 nanostructures which exhibited a specific capacitance of 127 F g-1 at a current density of 1 A g-1[18]. Also, several attempts have been made to couple iron oxide with carbonaceous materials to develop hybrid nanostructure based electrode materials because carbon coating layers can significantly enhance the electronic conductivity of electrode materials, further improving their electrochemical performances. A recent report depicted modified Fe3O4 nanosheets with carbonaceous materials which delivered a specific capacitance of 135 F g-1at a current density of 0.42 A g-1 [19]. C-dots have been recognized as a potent next generation carbon material owing to their unique optical and electrical properties [20]. Generally, C-dots are known as the zerodimensional oxygenous carbon nanoparticles having size below 10 nm displaying the graphitic sp2 π-bonds. In view of the pronounced quantum confinement effect, C-dots have showed novel properties in comparison to conventional chalcogenide semiconductor nanocrystals and also the bulk carbon counterparts, such as high aqueous solubility, high resistance to photo bleaching, strong and tunable photoluminescence, good electrical 2   

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supercapacitors (EDLCs) where the capacitance is achieved by the accumulation of charges

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conductivity, chemical stability and benignity.

View Article Online Therefore, the environment friendly C-dots DOI: 10.1039/C5DT00296F

are potential for applications in bioimaging, pollutant sensors, biosensors, drug delivery, solar cells, catalysis etc [21-23]. The brilliant electronic properties of C-dots as electron donors and acceptors endow them with wide potential for their use as electrodes in energy storage

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carbon family, 2D graphene nanosheets and 1D carbon nanotubes are well exploited for energy storage purpose. But very little is known about the electrochemical properties of the confined 0D C-dots, the promising new member of the carbon family. In the present work, we report a novel Fe3O4-C hybrid nanocomposite as a potential electrode material for supercapacitors. The hybrid nanocomposite has been developed by a two step process where the pristine Fe3O4 nanospheres were developed by a cost effective hydrothermal method. The C-dots were developed using a green solvothermal method. Finally, the C-dots were engrafted on the Fe3O4 nanospheres via an impregnation method and the hybrid structure is referred as Fe3O4-C hybrid nanocomposite. The electrochemical capacitative behaviour of the C-dot decorated Fe3O4 hybrid nanocomposite and the pristine Fe3O4 nanospheres were measured by cyclic voltammetry, galvanostatic charge–discharge method and electrochemical impedance spectroscopy. 2. Experimental Methods 2.1 Materials and Methods: Anhydrous ferrous chloride (FeCl3), sodium acetate (NaOAc), ethylene glycol, copper acetate (Cu(acac)), ascorbic acid and ethanol were purchased from Merck Specialities Pvt. Ltd. Polyvinylpyrrolidone (PVP) was purchased from Sigma-Aldrich. The reagents used in the synthesis procedure were purely analytical grade and used without further purification. 2.2 Synthesis of pristine Fe3O4 nanospheres Fe3O4 nanospheres were prepared by a modified hydrothermal method described elsewhere [24]. In a typical preparation, 1 g of sodium acetate was dissolved in 30 ml of ethylene glycol under vigorous magnetic stirring to obtain a clear solution. After that 1.5 g FeCl3 1 g PVP and 2 g NaOAc were added to the above solution to form a homogenous mixture. The final solution was stirred magnetically for 30 min and the mixture was transferred to a Teflon- lined stainless steel autoclave. The autoclave was subjected to a high temperature of 200oC for 8 hours. After that the autoclave was allowed to cool down naturally and the resulting black solution was 3   

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devices like batteries and supercapacitors. It is well known that the popular members of the

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View Article Online centrifuged. The black precipitate obtained was washed thoroughly with ethanolDOI: and distilled 10.1039/C5DT00296F

water and later dried in a vacuum oven at 60oC. 2.3 Synthesis of C-dots

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used as the carbon source and Cu(Ac)2.H2O was used as a catalyst. The ascorbic acid solution was prepared by dissolving 3.522 g of ascorbic acid in 380mL water to which 20mL of Cu(acac) solution was added with rigorous stirring. The mixture was then heated in an oil bath under constant stirring for 5 hours. With the advancement of the reaction, the colour of the mixture changes from an orange suspension to a yellowish golden hue, indicating the formation of the carbon quantum dots. After cooling down, the solution was centrifuged at 7000 rpm to remove the unreacted catalyst and the supernatant containing C-dots was stored. 2.4 Fe3O4 nanospheres decorated with C-dots 0.3 g of pristine Fe3O4 nanospheres were dispersed in 15mL of the as prepared C-dots solution and then subjected to ultrasonication for 30 min. The mixture was then dried in vacuum at 80o C for 12 hours. The end product was washed with ethanol and distilled water for a few times and finally dried at 80o C for 12 hours to obtain the hybrid nanocomposites. 3. Characterization techniques Powder X-Ray diffraction analyses was performed with the aid of a Rigaku Minilflex unit using CuKα radiation (λ=0.15418 nm) over the 2θ range of 10-70o. The IR spectra were recorded in a Nicolet (Impact 410, Madison, WI) spectrophotometer on pressed KBr pellets. Scanning electron microscope (SEM) images and energy dispersive X-ray spectroscopy (EDX) were obtained on a JEOL JSM Model no. 6390 LV. High resolution transmission electron microscope (TEM) images were recorded on JEOL JEM-2100 microscope operated at 200kV. The room temperature and low temperature magnetization was studied in 9T Quantum Design PPMS. Cyclic Voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy were carried out with the help of Biologic-SP 150 workstation. The working electrodes were prepared by mixing 80 wt % of Fe3O4 (or Fe3O4-C hybrid nanocomposite) ,15 wt% acetylene black and 5 wt% polytetrafluoro-ethylene (PTFE) to form a slurry. The slurry was pressed into a stainless steel grid and later dried in oven at 80oC for 12 hours. The 4   

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Fluorescing C-dots were synthesized via a solvothermal route [25] where ascorbic acid was

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View Article Online electrochemical measurements were carried out in a three-electrode system with 1M 2SO3 as DOI: Na 10.1039/C5DT00296F

the aqueous electrolyte. The capacitative behaviour of C-dot decorated Fe3O4 was compared to pristine Fe3O4 nanospheres using cyclic voltammetry test and galvanostatic charge–discharge

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4. Results and Discussions 4.1 Formation mechanism and morphological study The formation of the porous Fe3O4 nanospheres followed a complex mechanism which involved nucleation of nanoseeds, growth of nanocrystals and then self assembly of the primary nanocrystals to form the large porous structure. In this method, ethylene glycol served as the solvent as well as the reducing agent. The iron precursor FeCl3 when dissolved in ethylene glycol was partially reduced to Fe2+ and Fe3+ ions. Sodium acetate (NaOAc) was used as the precipitating agent which was responsible for the nucleation of the Fe3O4 nanoseeds. In the second stage, the primary nanostructures were then assembled into microspheres with porous structure. The role of PVP was to provide capping over the Fe3O4 nanospheres which prevented the oxidation of the Fe3O4 nanocrystals. In case of the formation of the C-dots, the high concentration of ascorbic acid led to the nucleation of C-dots. The resulting nuclei then grow uniformly and isotropically by diffusion of solutes towards the particle surface. Ascorbic acid is very reactive and acidic in nature and hence the reaction can take place at low temperature as well. But it usually takes a prolonged time for the formation of C-dots at lowered temperature. To overcome this, Cu(Ac)2.H2O was added to the reaction medium to generate Cu2+ ions which in turn shortens the reaction time by accelerating the oxidization of the ascorbic acid . Ultimately, Cu or Cu2O which is generated as a by-product is conveniently removed by centrifugation at 15000 rpm. The C-dots were engrafted into the porous Fe3O4 nanospheres to form Fe3O4-C hybrid nanocomposites with the aid of ultrasonication [26]. With the agitation created by the ultrasonic waves, the C-dots present in the solution came into contact with the open porous structure of the Fe3O4 nanospheres and got settled down in the porous network. In the process, it is believed that the C-dots have knocked out some of the Fe3O4 nanocrystals on the surface thus diminishing the size of the nanospheres. X-Ray diffraction (XRD) measurements were carried out to realize the phase and structure of the pristine Fe3O4 nanospheres, the C-dots and the C-dot decorated Fe3O4 hybrid 5   

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tests.

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View Article Online nanocomposites. The diffraction peaks in pristine Fe3O4 were corroborating to DOI: (220), (311), 10.1039/C5DT00296F

(400), (422), (511) and (440) planes. From Figure 1, it is evident that the observed peaks in pristine Fe3O4 are indexed to the magnetite phase of iron oxide (JCPDS PDF no. 89-0951). The corresponding diffraction pattern of the C-dots shows a broad diffraction peak centred at

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d of the (002) peak is found to be 0.42 nm, which is greater than the interlayer spacing of graphite i.e. 0.34 nm. This increase in d value also suggests the amorphous nature of the C-dot system which is attributed to the introduction of more oxygen containing groups. The XRD pattern of the Fe3O4-C hybrid nanocomposite system is in good agreement with the characteristic peaks of the magnetite phase. However, no diffraction peaks of carbon can be indexed because of the amorphous nature of the carbon or may be due to the low carbon concentration [28]. A single line analysis method employing a pseudo-Voigt profile shape function was used to determine the crystallite size and lattice strain [29]. The most intense peak has been deconvoluted and fitted accordingly. Here, D = λ/βfCcos θ and e = βfG/4 tanθ , where, D is the crystallite size, e the lattice strain, λ the wavelength of x-ray, βfC and βfG the integral breadths of Cauchy and Gaussian fit of the structurally broadened profile and θ is the Bragg angle. For the pristine Fe3O4 nanospheres, the crystallite size was calculated to be around 6.9 nm with a lattice strain of 1.51x10-2. Insert Figure 1

Figure 1 XRD patterns of Fe3O4-C hybrid nanocomposites, C-dots and pristine Fe3O4 nanospheres  

The interesting morphological features of the as prepared pristine Fe3O4 nanospheres, the Cdots and the Fe3O4-C hybrid nanocomposites were studied using SEM and TEM. As shown in figure 2 (a), the pristine Fe3O4 nanospheres are spherical and well separated from each other. It is evident from the figure 2 (b) that with the introduction of the C-dots the spherical morphology of the as synthesized nanospheres has been altered. Energy-dispersive X-ray (EDX) spectrum of pristine Fe3O4 nanospheres (Fig. 2c) indicates the presence of Fe and O while in case of Fe3O4-C hybrid nanocomposite system (Fig. 2 d), the presence of Fe, O and C are evident. Insert Figure 2 6   

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around 23.8oC demonstrating the amorphous nature of the system [27]. The interplanar spacing

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View Article Online Figure 2 SEM images of (a) Fe3O4 (b) Fe3O4-C hybrid nanocomposite and EDX DOI: spectra of (c) 10.1039/C5DT00296F

Fe3O4 (d) Fe3O4-C hybrid nanocomposite Representative TEM images show the morphology of the pristine Fe3O4 nanospheres as well as

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3b) that the morphology of the pristine Fe3O4 has undergone significant change with the incorporation of the C-dots. It is clearly evident from the TEM micrographs that after the formation of the hybrid nanocomposite, the smooth surfaced iron oxide nanospheres have transformed into flower like porous spherical structure. The reduced size of the composite system caters to more surface area which will be beneficial for the charge storage behaviour. It is worthwhile to mention that such porous structures furnish extra ion storage spaces along with fast ion transport pathways, which results in high supercapacitative performance. The enhanced surface area due to the porous structure provides an abundance of electroactive sites for Faradaic reactions which in turn increases the specific capacitance of the composite. The porous nature of the composites helps in transport and diffusion of electrolyte ions during fast charge-discharge process and also accommodates the volume changes while the chargedischarge process, resulting in high cycling ability of the nanocomposites. Insert Figure 3

Figure 3 TEM images of (a), (c) pristine Fe3O4 and (b), (d) Fe3O4-C hybrid nanocomposites (inset displays HRTEM image showing presence of Fe3O4 and carbon quantum dots) 4.2 Magnetic Property Insert Figure 4

Figure 4 Field dependence magnetization curve of (a) Fe3O4 and Fe3O4-C hybrid nanocomposites and temperature dependence magnetization curve of (b) Fe3O4 and Fe3O4-C hybrid nanocomposites The magnetic properties of the pristine Fe3O4 as well as the Fe3O4-C hybrid nanocomposites have been investigated by studying the field dependence magnetization at room temperature and the temperature dependence at constant external magnetic field. Figure 4 (a,b) shows the M-H curves of the pristine Fe3O4 as well as the Fe3O4-C hybrid nanocomposite system measured at 300 K. At 300 K, a very narrow hysteresis is obtained in both the systems 7   

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the Fe3O4-C dot hybrid nanocomposite system. It is evident from the TEM micrograph (Fig.

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View Article Online indicating the soft ferromagnetic nature. The saturation magnetization value ofDOI:the pristine 10.1039/C5DT00296F

Fe3O4 nanospheres is 77 emu/g while in case of the hybrid nanocomposite it is lowered to 47.8 emu/g. The reduced MS in Fe3O4-C hybrid nanocomposites is possibly due to the counteraction between the different magnetic ordering of the iron oxide nanosystem and the non magnetic C-

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less magnetic moment per unit mass than that of pristine Fe3O4, which results in a reduced saturation magnetization. The temperature dependent magnetization of the pristine Fe3O4 as well as the Fe3O4-C hybrid nanocomposites was studied in a zero field cooled (ZFC) and field cooled (FC) conditions. In ZFC measurements, the samples are initially cooled in a zero magnetic field from room temperature to 5 K and then the magnetization was measured from 5 K to room temperature in presence of a small field. In contrast, FC measurements were recorded after cooling the sample in presence of an applied field of 500 Oe. As seen from Fig 4(c) the ZFC magnetization rises sharply upon heating at low temperatures. At higher temperatures there is a linear increase but it is found that the ZFC magnetization is lower than the FC magnetization in the entire temperature range under study. It is seen that the ZFC curve exhibits a broad maximum around 25 K and it never collapses into the FC curve. It is known that after zero field cooling the randomly aligned spins in the system are freezed. It is in the heating process that with the increase in temperature the smallest particles in the system unblocks themselves and align themselves in the direction of the applied field. Likewise in this case the broad ZFC curve at 25 K corresponds to the unblocking of a fraction of particles of both the systems with average energy barrier. However, the observed pattern of the ZFC-FC curve for both the system indicates that at room temperature a large fraction of particles are still in the blocked state which suggests that both the nanosystems have a broad distribution of the anisotropy energy barriers for the reorientation of the magnetic moments. 4.3 Electrochemical Property study Cyclic Voltammetry studies and galvanostatic charge discharge studies were carried out in a three electrode system in the presence of 1M Na2SO3 electrolyte. Figure 5 (a,b) shows the CV curves of Fe3O4-C hybrid nanocomposite in comparison to pristine Fe3O4 at different scan rates in a potential range from 0 V to 1 V. The CV curve for both the systems exhibits a typical pseudocapacitative behaviour, displaying a pair of obvious redox peaks of the voltammetry characteristics. The presence of the redox peaks indicates that the capacitative response comes 8   

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dots [30]. The presence of the disordered amorphous C-dots on the Fe3O4 nanospheres leads to

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View Article Online from the Faradaic redox reactions related to the M-O/M-O-OH [31]. In case DOI: of 10.1039/C5DT00296F Fe3O 4 the

pseudocapacitative behaviour originates from the surface redox reaction of sulphur in the form of sulphite anions, while the redox reactions between Fe2+ and Fe3+ are followed by intercalation of sulphite ions which counterpoises the extra charge with the iron oxide layers FeO + SO - ↔ FeSO4 + 2e

(1)

2FeIIO + SO - ↔ (FeIIIO) + SO - (FeIIIO)+ + 2e

(2)

The internal area and the peak intensity of the CV curve for Fe3O4-C hybrid nanocomposites is evidently more enhanced than the pristine Fe3O4 nanospheres which implies that the specific capacitance of Fe3O4-C hybrid nanocomposites is higher than that of pristine Fe3O4. With the increase in scan rate the shape of the CV curves show a minimal change which demonstrates that the Fe3O4-C hybrid nanocomposites is a good electron conductor with small equivalent series resistance. However, the anodic and cathodic peaks have shifted with increasing scan rate due to internal resistance of the electrode [33]. The specific capacitance can be calculated from the area under the CV curve using the following formula:









(3)



where m is the mass of the active material in the electrodes (g), v is the potential scan rate (mV s-1), Va is the anodic potential (V), Vc is the cathodic potential (V), I(V) is the response current density (A) and V is the potential (V). The specific capacitances of Fe3O4-C hybrid nanocomposites are found to be 203.4, 146.2, 123.1, 106.8, 96.5, 86.4 F g-1 at scan rates of 5, 10, 20, 50, 100 and 200 mV s-1 respectively. However for pristine Fe3O4 nanospheres the calculated specific capacitance is much lower i.e. 153.7, 92.2, 64.5, 43.9, 41.7 and 35.4 F g-1 at scan rates of 5, 10, 20, 50, 100 and 200 mV s-1. It is seen that the specific capacitance value drops with increasing scan rates. Two different mechanisms explain the process of charge storage

in

oxide

based

materials.

The

first

mechanism

is

based

on

the

intercalation/deintercalation of protons or alkaline metal cations, which induces the full utilization of the electrode material. When the scan rate is high, protons or alkaline metal cations can access only the outer surface layer of the electrode and as a result the interior pores of the Fe3O4 material remains unutilized. This might be responsible for the lower specific capacitance at higher scan rates. Other mechanism is based on the adsorption process of anions on the surface of the electrode. At higher scan rate the adsorption mechanism becomes predominant which leads to the decrease in specific capacitance [19, 34]. 9   

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[32]. The reactions can be expressed as:

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Insert Figure 5

DOI: 10.1039/C5DT00296F

Figure 5 Cyclic voltammetry curves of (a) Fe3O4 and (b) Fe3O4-C hybrid nanocomposites Galvanostatic charge-discharge measurements of Fe3O4-C hybrid nanocomposites and

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discharge curves displays a sudden potential drop followed by a slow potential decay which is the inherent characteristic of pseudocapacitative materials. The specific capacitance from the galvanostatic charge-discharge measurements can be evaluated from the following equation: Cs =



(4)



where Cs is the specific capacitance (F g-1), I is the charge-discharge current (A), t is the charge-discharge time (s), m is the mass of the active material in the electrode (g) and V is the charge-discharge potential range (V). The variation of the specific capacitance with discharge current is shown in Fig. 6 (b). It is apparent from the figure that the capacitance values decreases with the increasing current density. The specific capacitance of pristine Fe3O4 and Fe3O4-C hybrid nanocomposites at a current density of 1 A g-1 is found to be 157.64 and 208.58 F g-1 respectively. The low specific capacitance in case of pristine Fe3O4 can be accounted for the large internal resistance in Fe3O4. Hence, it is confirmed that the enhancement of the specific capacitance of the porous Fe3O4-C hybrid nanocomposites is due to the presence of the electrically conductive C-dots. The high electrical conductivity of the Cdots [35] in the Fe3O4-C hybrid nanocomposites decreased the internal charge transfer resistance of pristine Fe3O4. Also it is known that carbon provides electrochemical capacitance which leads to a synergistic effect due to C-dots and Fe3O4 leading to an improved specific capacitance. Insert Figure 6

Figure 6 (a) Galvanostatic charge discharge of Fe3O4-C hybrid nanocomposites (b) Specific capacitance change of Fe3O4-C hybrid nanocomposites as a function of current density (c) cyclic stability of Fe3O4-C hybrid nanocomposites at a current density of 1 A g-1 and (d) Nyquist impedance plot For practical applications, long cycle life of supercapacitors is a major issue. To investigate the cyclic ability galvanostatic charge-discharge was performed for 200 cycles at 1 A g-1. As observed in Figure 6c, the specific capacitance retention for Fe3O4-C hybrid nanocomposites is 10   

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pristine Fe3O4 performed at different current densities are shown in Figure 6a.The charge-

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View Article Online around 86% but in case of Fe3O4 nanospheres it drops to around 71.2 %. The drop specific DOI: of 10.1039/C5DT00296F

capacitance may be attributed to various reasons such as the consumption of the electrolyte due to the irreversible reaction between the electrolyte and the electrode [36]; chemical degradation and transformation of magnetite in aqueous electrolyte to other forms of iron oxide such as

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C-dots not only improved the electronic and ionic conductivity of the Fe3O4-C hybrid nanocomposites

but

also

prevented

the

loss

of

cyclicity

during

repetitive

incorporation/extraction processes due to the flexible C-dots, which would feasibly buffer the mechanical stress [38,39]. The Nyquist plots of both the Fe3O4 nanospheres and Fe3O4-C hybrid nanocomposites (Figure 6c) were recorded at −0.3 V with amplitude of 5 mV in a frequency range of 1000 kHz to 0.01Hz. The plots show a semi circle in the high frequency range and inclined line in the high frequency region. The broad semicircle in case of Fe3O4 nanospheres electrodes mark a high ion diffusion resistance however in case of the Fe3O4-C hybrid nanocomposites electrodes showed a lower resistance. It can be inferred that the embedded C-dots might have reduced the charge transfer resistance of the pristine Fe3O4 leading to improved electronic and ionic conduction of the Fe3O4-C hybrid nanocomposites electrodes through charge transfer interactions. 5. Conclusion: In summary, a novel flower shaped Fe3O4-C hybrid nanocomposite were developed using a two step chemical route for their use in next generation supercapacitor applications. Porous Fe3O4 nanospheres were developed by employing a simple template-free hydrothermal method and the electrically conductive C-dots were synthesized using a simple solvothermal method. The electrochemical investigations of the hybrid nanocomposites revealed that the Fe3O4-C hybrid nanocomposites hold excellent capacitative performance and stable cyclability due to the presence of the electrically conducting C-dots present over the porous nanospheres. The porous structure of the hybrid nanocomposites further enhanced charge transfer due to larger interfaces which in turn provided larger surface area in contact with the electrolyte, so more active materials can effectively contribute to the capacitance. Hence, it can be inferred that the flower shaped Fe3O4-C hybrid nanocomposites hold great prospect for its application in next generation supercapacitors.

11   

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maghemite and hematite in the presence of oxygen [37]. Hence, it is found that the conductive

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DOI: 10.1039/C5DT00296F

Acknowledgement The authors acknowledge financial support from Department of Science and Technology, Govt. of India (INSPIRE) and Department of Biotechnology (DBT), Govt. of India vide grant

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extending the Transmission electron microscopy support. KB would also like to acknowledge Bikash Chandra Nath for helping with the electrochemical measurements. References 1. Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li, L. Zhang, Int. J. Hydrogen. Energ. 34 (2009) 4889. 2. R. Kotz, M. Carlen, Electrochim. Acta 45 (2000) 2483. 3. A. Burke, J. Power Sources 91 (2000) 37. 4. J. P. Liu, J. Jiang, C. W. Cheng, H. X. Li, J. X. Zhang, H. Gongand H. J. Fan, Adv. Mater., 23 (2011) 2076. 5. A. Sumboja, X. Wang, J. Yan and P. S. Lee, Electrochim. Acta 65 (2012) 190. 6. J. Jiang, J. Liu, R. Ding, J. Zhu, Y. Li, A. Hu, X. Li and X. Huang, ACS Appl. Mater. Interfaces 3 (2010) 99. 7. Y. F. Ke, Y. S. Tsai and Y. S. Huang, J. Mater. Chem.15 (2005) 2122. 8.J. Chang, J. Sun, C. H. Xu, H. Xu and L. Gao, Nanoscale 4(2012) 6786 9 D. C. Wang, W. B. Ni, H. Pang, Q. Y. Lu, Z. J. Huang and J. W. Zhao, Electrochim. Acta 55(2010) 6830 10. S. Li and C.A. Wang, J Colloid Interf Sci 438 (2015) 61. 11. J. Xu, L. Gao, J. Y. Cao, W. C. Wang and Z. D. Chen, Electrochim. Acta 56 (2010)732. 12. H. Pang, J. Deng, J. Du, S. Li, J. Li, Y. Ma, J. Jhang and J. Chen, Dalton Trans., 41(2012) 10175. 13. Y.T. Chen, P. He, Y. Liang, X.F. Yi, J.T. Sun and Q.Y. Jiang, ECS Transactions 28(2010) 107 14.C. Yuan, B. Gao, L. Su and X. Zhang, J Colloid Interf Sci 322 (2008) 545 15.W. Shi, J. Zhu, D.H. Sim, Y.Y. Tay, Z. Lu, X. Zhang, Y. Sharma, M. Srinivasan, H. Zhang, H.H. Hng and Q. Yan , J. Mater. Chem., 21 (2011) 3422

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no. BT/357/NE/TBP/2012 dated 08/04/2013. The authors would like to thank SAIF, NEHU for

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18. S. Shivakumara, T. R. Penki, and N. Munichandraiah, ECS Electrochemistry Letters 2 (2013) A60

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Hybrid nanostructured C-dot decorated Fe3O4 electrode materials for superior electrochemical energy storage performance.

Research on energy storage devices has created a niche owing to the ever increasing demand for alternative energy production and its efficient utilisa...
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