Polycrystalline iron oxide nanoparticles prepared by C-dot-mediated aggregation and reduction for supercapacitor application

Abstract

Pseudocapacitive transition-metal oxides are widely used as excellent electrode materials in high-performance supercapacitors due to their high capacitance, low cost and environmentally friendliness. Here, α-Fe₂O₃/Fe₃O₄ heterostructure nanoparticles as a three-dimensional, multicomponent, multiphase oxide were successfully prepared by a simple solvothermal method and C-dot-mediated aggregation and reduction. The working electrode based on heterostructure nanoparticles could be simultaneously used as an anodic and cathodic electrode, resulting in a specific capacitance of 150 F g⁻¹ and 40.1 F g⁻¹ at a current density of 1 A g⁻¹ and 0.5 A g⁻¹, respectively. The synergistic effect of heterostructure iron oxide allowed electron transport and ion diffusion between electrode and electrolyte and then electrochemical performance was improved. Moreover, when α-Fe₂O₃/Fe₃O₄ heterostructure nanoparticles were used as a cathodic electrode, the behavior of electrochemical double layer capacitors was observed. The results proved that heterostructure nanoparticle-based electrodes are promising candidates for supercapacitors.

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1. Introduction

With the increased demand for portable and clean energy, electrochemical capacitors (i.e., supercapacitors) are the most promising candidates for next-generation power devices because of their desirable properties, including fast charging and discharging, excellent cycling stability, small size and low mass.^1–7 Based on the charge storage mechanism, supercapacitors are classified into electrical double layer capacitors and pseudocapacitors. Because the charge is stored at the electrode/electrolyte interface through reversible ion adsorption, the application of high-surface-area porous carbon electrodes for electrical double layer capacitors is limited by unsatisfying energy density, low specific capacitance and rate capability.^8,9 Currently, many works are focused on improving the conductive and specific surface area of carbonaceous materials, however, the specific capacitance severely decreases under high current.^10–14 On the other hand, novel electrode materials (i.e., MnO₂, Co₃O₄, CuO, MoO₂, NiO, TiO₂, Fe₃O₄, ZnO, VO_x, etc.) are designed to possess high specific surface area, specific capacitance, electronic conductivity and power, and energy densities.^15–26 These transition metal oxide materials exhibit pseudocapacitor behavior, storing the charge using the redox-based faradaic reactions, which can provide higher energy densities but usually suffer from shorter cyclic lifetimes.²⁷ These problems can only be partially resolved by using hybrid materials. Various Fe₃O₄- and α-Fe₂O₃-based nanohybrid structures, including Fe₃O₄/reduced graphene oxide,²⁸ Fe₃O₄/activated carbon,²⁹ Fe₃O₄/CNFs,³⁰ Fe₃O₄@C,³¹ Fe₃O₄/MWNT,³² V₂O₅-doped α-Fe₂O₃,³³ graphene/Fe₂O₃/polyaniline nanocomposites³⁴ and α-Fe₂O₃/carbon nanocomposite,³⁵ have been used and demonstrated as electrode active materials with enhanced electrochemical performance. However, few work reports on composite capacitance materials combining two transition metal oxides, which show enhanced electrochemical performance compared with single component.⁵

Iron oxides (e.g., α-Fe₂O₃ and Fe₃O₄), were discovered to show superior performance when used as active electrode materials for the pseudocapacitors.^36,37 The utilization in energy storage devices should benefit from their unique properties of low cost, good stability, nontoxic, environmentally friendly, and high redox activity. Although iron oxides exhibit excellent pseudocapacitive behaviors with long-term cyclic stability, the measured values of specific capacitance of Fe₃O₄ and α-Fe₂O₃ are still far below their theoretical value, because they generally suffer from serious agglomeration and poor electrical conductivity.^30,35 Meanwhile, hybrid metal oxides (e.g., α-Fe₂O₃/Fe₃O₄) with variable oxidation states remarkably facilitate the efficient redox charge transfer. Carbon dots (C-dots) with abundant surface functional groups have gradually become a rising star in the material family. Although it has been used as a precursor for the synthesis of hybrid nanomaterials in a handful of papers, but most of them are focus on precious metal.^38,39 Herein, heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles is synthesized by C-dots-mediated aggregation and reduction as well as solvothermal reaction. Therefore, it is possible for heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles as anodic and cathodic electrode to achieve enhanced electrochemical performance by combining functions of individual components and intruding synergistic effects into the pseudocapacitor electrode system.

2. Experimental section

2.1. Materials and chemicals

5-Sulfosalicylic acid dihydrate, diethylene glycol (DEG), sodium acetate anhydrous, ethanol and lithium hydroxide were analytical grade and purchased from Shantou west long chemical Co., Ltd in China. Iron(III) chloride anhydrous (98%) was purchased from Alfa Aesar. All chemicals and solvents were used as received. All aqueous solutions were prepared using ultrapure water (18 MU) from a Milli-Q system (Millipore).

2.2. Synthesis of carbon dots (C-dots)

The synthetic procedure of C-dots in detail was described as follows. 5-Sulfosalicylic acid dihydrate (1.0 g) and diethylene glycol (DEG, 20 mL) were added into three-necked flash. The reaction mixture was heated to 200 °C with a constant heating rate of 5 °C min⁻¹, maintained for 5 min, and the resulted solution could be directly used for the synthesis of α-Fe₂O₃/Fe₃O₄ heterostructured nanoparticles. For obtaining pure and high quality of C-dots, the reaction product was centrifuged at 9000 rpm for 20 min after cooling to room temperature. The yellow supernatant solution was then subjected to dialysis to completely remove the non-reactive 5-sulfosalicylic acid. Resultant C-dots were stored in the dark for future characterization.

2.3. Synthesis of heterostructure α-Fe₂O₃ or Fe₃O₄ nanoparticles

The precursors for the synthesis of α-Fe₂O₃ or Fe₃O₄ nanoparticles were ferric chloride (1.0 g), anhydrous sodium acetate (3.0 g) and ultrapure water (10 mL) or ferric chloride (1.0 g), hydroxy propyl cellulose (0.1 g) and anhydrous sodium acetate (3.0 g), which were dissolved in diethylene glycol solution (20 mL) to form a homogeneous solution. The mixture was dispersed by ultrasound for 10 min, transferred into a Teflon-lined stainless-steel autoclave, heated at 200 °C for 10 h, and cooled to room temperature. The products were washed several times with ethanol and water and dried at 80 °C.

2.4. Synthesis of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles

Ferric chloride (1.0 g) and anhydrous sodium acetate (3.0 g) were dissolved in C-dots dispersive diethylene glycol solution (20 mL, from C-dots prepared procedure) to form a homogeneous solution. The mixture was dispersed by ultrasound for 10 min, transferred into a Teflon-lined stainless-steel autoclave, heated at 200 °C for 10 h, and cooled to room temperature. The products were washed several times with ethanol and water and dried at 80 °C.

2.5. Characterization and electrochemical measurement

The morphology of composites during preparations process and final products were characterized by scanning electron microscopy (SEM, Hitachi S-4800) with an accelerating voltage of 10 kV. The crystal structure of composites was tested by X-ray diffractions analysis master (XRD, D/MAX in Japan-TTRIII). Transmissions electron microscopy (TEM) was obtained with Tecnai G2 20 S-TWIN at an accelerating voltage of 200 kV. The TEM samples were prepared by doping solution onto a copper grid and dried in air. A homogeneous slurry was prepared by the mixing of active material (α-Fe₂O₃/Fe₃O₄ heterostructured nanoparticles, 80 wt%), conductivity agent (acetylene black, 10 wt%) and binder (polytetrafluoroethylene, PTFE, 10 wt%) in ethanol. This slurry was coated and pressed onto the surface of the cleansed nickel foam with an area of 1 cm² followed by drying at 50 °C for 24 h as working electrode. A three electrodes method were used, consisting of a nickel foam as working electrode, Pt wire and Ag/AgCl (saturated KCl) electrodes as counter and reference electrodes. LiOH (1.0 mol L⁻¹) was used as the electrolyte. For anode electrode, a potential window with a range of 0 to 0.6 V and a different scan rates of 10, 50, 80 and 100 mV s⁻¹ were employed in the CV measurements and 0–0.5 V with current densities of 3, 5, 8 or 10 A g⁻¹ was used in the galvanostatic (GV) charge–discharge experiments. For cathode electrode, a potential window with a range of −0.8 to 0 V was employed in the CV measurements at different scan rates of 100, 500, 1000 and 2000 mV s⁻¹ and the GV charge–discharge experiments were carried at −0.8 to 0 V with various current densities of 0.5, 1, 2, 3, 4 and 5 A g⁻¹. In addition, before each electrochemical test, the electrode was put into the electrolyte for four hours. Cyclic voltammetry and galvanostatic charge/discharge studies were used to characterize the electrochemical behavior of supercapacitor devices using a CHI660D electrochemical working station (Shanghai, China) at room temperature.

3. Results and discussion

3.1. Material characterizations

The powder of 5-sulfosalicylic acid as precursor with solvent DEG was used for the first time to synthesize carbon dots (C-dots). TEM images in Fig. S1† clearly revealed that a uniform dispersion of the C-dots in water with the average diameter of 3 nm. X-ray photoelectron spectroscopy (XPS) was used to further investigate the chemical nature of the atoms in the C-dots. The XPS survey spectrum in Fig. S2† showed the S, C, and O at a binding energy of 159.2 eV, 284.8 eV and 532.8 eV, respectively. It indicated that hydroxy, sulfonic acid and carboxyl group on the surface of C-dots. The construction of α-Fe₂O₃/Fe₃O₄ heterostructured nanoparticles was shown in Fig. S3.† Under ultrasonication, FeCl₃ powder and NaAc were dissolved in C-dots/DEG suspension, iron ions were coordinated and aggregated onto C-dots with their surface sulfonic acid group and carboxyl group and then hydrolyzed, resulted in the formation of C-dots/Fe(OH)₃. After hydrothermal process, most of Fe(OH)₃ were converted to α-Fe₂O₃ particles and their crystallinities were improved, part of Fe(III) was reduced to Fe(II) due to the reductive ability of C-dots,^38,39 and then Fe(II)Fe(III)(OH)_x were coexisted, which resulted in the formation of Fe₃O₄ phase. The α-Fe₂O₃/Fe₃O₄ heterostructured nanoparticles was made up of many small α-Fe₂O₃ and Fe₃O₄ nanoparticles (Fig. S4†). The forming mechanism of Fe₂O₃/Fe₃O₄ nanoparticles mediated by C-dots was confirmed by the spectrum data of Fig. S5.† Without using C-dots in the hydrothermal synthesis, single α-Fe₂O₃ phase particles with micron sizes and ruleless morphology were obtained and shown in Fig. S6.† The C-dots were important to control the morphology, structure, and function of iron oxide particles.

As the electrode for capacitance, the conductivity of metal oxides was typically poor,⁴⁰ acetylene black and PTFE were used as conductive agent and adhesive, respectively. Heterostructured α-Fe₂O₃/Fe₃O₄ nanoparticles were smeared directly and equably onto nickel foam substrate and used as working electrode. A three electrode system (Fig. 1c) was used for electrochemical capacitance measurements of these working electrodes in a 1 M lithium hydroxide aqueous electrolyte, because of its stable curve, clear oxidation and reduction peaks, and high resolution peak current and peak potential.

Fig. 1 Working electrode fabrication and capacitance measurement. (a) Schematic illustration of the preparation process of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles (a), direct smear materials on nickel foam (b), and a three-electrode electrochemical cell constructed from WE, RE, CE and electrolyte (c).

To obtain the structural information and phase identification, the X-ray diffraction (XRD) measurements were conducted. A series of peaks appeared the XRD pattern of α-Fe₂O₃/Fe₃O₄ heterostructured nanoparticles (Fig. 2) could be ascribed to crystal planes of α-Fe₂O₃ (PDF card no. 33-0664, including (012), (024), (104), (110), (113), (116), (119), (214) and (300)) and Fe₃O₄ (PDF card no. 19-0629, including (012), (220), (222), (311), (400), (422), (440) and (511)), respectively. The phase ratio of α-Fe₂O₃ : Fe₃O₄ in heterostructure nanoparticles was 57 : 43, which was calculated base on XRD results. In addition, compared with the pure Fe₃O₄ and α-Fe₂O₃, the diffraction peaks were sharp and intense, the results indicated that the nanocomposites were high purity and good crystallinity with a mixture of two phases, i.e., Fe₃O₄ and α-Fe₂O₃.

Fig. 2 XRD patterns of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles (red line), α-Fe₂O₃ (black line), and Fe₃O₄ (blue line).

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (see Fig. 3) showed the representative morphology of the heterostructure sample. Fig. 3a showed a low-magnification SEM image of the products, the nanoparticles with a average diameter of 79.7 nm were isolated, uniform and spherical, according to a statistics of 100 particles. Inset high-magnification SEM revealed that the surface of α-Fe₂O₃/Fe₃O₄ nanoparticles was rough and rugged due to C-dots-mediated disorder aggregation. As was shown in Fig. 3b, X-ray elemental mapping indicated that heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles mainly contained Fe and O elements, the C element was few. The EDS in Fig. S7† showed the same result. TGA measures the weight loss with respect to temperature under controlled environment. The C-dots were stable at 110 °C and loss their weight at about 450 °C.⁴¹ The amount of C dots in the heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles was trace (about 2.1%, seen in Fig. S8†). The amount of materials used in electrode was at mg level, so the weight of electrodeactive material was not considered by us. Fig. 3c and d showed the low magnification TEM image of the heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles, which were uniform with multicomponent stack structure. The diameter of nanoparticle was about 70–90 nm, which was in agreement with the results of statistical analysis for size and distribution based on SEM. The HRTEM image further confirmed the structural information on the nanoparticles. As shown in Fig. 3e, the lattice fringes in the typical HRTEM image were separated by 2.52 Å and 2.96 Å, which agreed well with the (110) and (220) lattice spacing of α-Fe₂O₃ and Fe₃O₄, respectively. In addition, the BET surface area of as-prepared α-Fe₂O₃/Fe₃O₄ nanoparticles was 59.13 m² g⁻¹ (Fig. S9†).

Fig. 3 Morphology characterization of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles. (a) SEM image, (b) SEM image with an X-ray elemental mapping recorded and corresponding elemental mappings of different elements. (c and d) TEM images, (e) high-resolution TEM image.

3.2. Electrochemical performance

The performance of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles on nickel foam substrates as working electrodes was evaluated. For proving the advantages of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles as the electrode materials for supercapacitors, the individual component such as Fe₂O₃ or Fe₃O₄, or α-Fe₂O₃ + Fe₃O₄ (i.e., mixing two pure phase refer to the phase ratio of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles), has been prepared by same method and different surface modification agent, used for the preparation of the electrodes, and then their performances were compared under the same testing conditions. The nanoparticles of α-Fe₂O₃, Fe₃O₄, α-Fe₂O₃ + Fe₃O₄ and heterostructure α-Fe₂O₃/Fe₃O₄ were used as anode electrode materials and their CV curves were obtained at the scan rate of 100 mV s⁻¹ in 1 M KOH solution and shown in Fig. 4a. The CV curve of pure α-Fe₂O₃ and Fe₃O₄ phase was much narrower than that of heterostructure α-Fe₂O₃/Fe₃O₄, which indicated smaller specific capacitances. It was worthy noted that the performance of α-Fe₂O₃ + Fe₃O₄ was not satisfactory, although its phase ratio was as same as heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles. Fig. 4b displayed the CV curves of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles anode electrode recorded at different scan rates. A pair of pronounced redox peaks was clearly visible within a voltage window ranging from 0 to 0.6 V, which indicated that the pseudocapacitance mainly came from the faradic redox reaction of Fe(II) and Fe(III). Increasing scan rate response with a slight shift of the redox peaks, indicated that the materials was in favor of fast and reversible redox reactions. Even at a high scan rate of 100 mV s⁻¹, the shape retention of CV curve was achieved. The discharge behavior of the electrode was examined by a galvanostatic charge/discharge test in the potential range from 0 to 0.5 V at different current densities. As shown in Fig. 4c, the discharge branch involved two sections, a sudden drop followed by a slow decay of the voltage, bound up with the internal resistance and the capacitive feature of the electrode.^33,42 The specific capacitance as a function of current density was showed in Fig. 4d. The specific capacitance was calculated from the corresponding galvanostatic discharge curves according to the following equation:
where C_S was the specific capacitance in F g⁻¹, I was the current density, m was the mass of active materials in the electrode, and V was the voltage difference. A specific capacitance of 150 F g⁻¹ was achieved for heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles at a current density of 1 A g⁻¹. The attenuation of capacitance was vividly found at a high current density.³³ Even so, a specific capacitance values of 88 F g⁻¹ was achieved with the charge/discharge rate up to 10 A g⁻¹. The long-term cycling life was another critical factor to estimate the electrochemical performance of electrode materials for their practical applications. The cycling stability of α-Fe₂O₃, Fe₃O₄, α-Fe₂O₃ + Fe₃O₄ and heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles as anode electroactive materials at the current density of 1 A g⁻¹ was shown in Fig. 4e. The specific capacitance of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles (150 F g⁻¹) was more higher than that of pure α-Fe₂O₃ (50 F g⁻¹) and Fe₃O₄ (94 F g⁻¹) phase or even α-Fe₂O₃ + Fe₃O₄ (49 F g⁻¹), respectively (Fig. 4f). After 1000 charge–discharge cycles, the capacitance retention of α-Fe₂O₃, Fe₃O₄, and heterostructure α-Fe₂O₃/Fe₃O₄ was 84%, 87.8%, and 97.3%, respectively, which indicated the superiority of heterostructure α-Fe₂O₃/Fe₃O₄ with long-term electrochemical stability.

Fig. 4 Electrochemical properties of nanoparticles as anode electrode materials in 1 M LiOH solution. (a) CV curves at the scan rate of 100 mV s⁻¹, (b) CV curves of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles at different scan rates, (c) galvanostatic charge/discharge curves of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles at different current densities, (d) rate performance of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles based on the weight, (e) cycle stability at the current density of 1 A g⁻¹, and (f) initial specific capacitance and capacitance retention after 1000 charge–discharge cycles.

When using nanomaterials as cathode electrode, the CV curve of α-Fe₂O₃, Fe₃O₄ and α-Fe₂O₃ + Fe₃O₄ particles were much narrower than that of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles at the same scan rate of 100 mV s⁻¹, which indicated smaller specific capacitances. As can be seen from the Fig. 5b and c, the CV testing showed rectangular curves from 0 to −0.8 V even at a high scan rate of 1000 mV s⁻¹. Meanwhile, a nearly symmetric triangular shape with small voltage drops at the initial point of the discharge curve was observed from its galvanostatic charge/discharge curve. Both results suggested an excellent electrical-double-layer capacitive behavior in heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles based cathode electrode. Compared with carbon-based cathode electrode materials, higher specific capacitance was achieved for heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles. A maximum specific capacitance of 40.1 F g⁻¹ was obtained at a current density of 0.5 A g⁻¹, even the current density was increased up to 5 A g⁻¹, its specific capacitance could still reach 12.5 F g⁻¹ (Fig. 5d). Fig. 5e displayed the capacitance of α-Fe₂O₃, Fe₃O₄, α-Fe₂O₃ + Fe₃O₄ and heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles vs. cycle number at a current density of 1 A g⁻¹. Fig. 5f displayed the initial specific capacitance and capacitance retention after 1000 charge–discharge cycles. The specific capacitance of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles (21.9 F g⁻¹) was about 10 times higher than that of α-Fe₂O₃ (1.3 F g⁻¹) and Fe₃O₄ (2.2 F g⁻¹) phase or even α-Fe₂O₃ + Fe₃O₄ (1.3 F g⁻¹), respectively. Moreover, after 1000 charge/discharge cycles at a current density of 1 A g⁻¹, this cathode electrode maintained its excellent cycling stability, 80.7% of its initial capacitance was retained (Fig. 5f). The stable and reversible characteristics of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles as cathode electrode materials were confirmed.

Fig. 5 Electrochemical properties of nanoparticles as cathode electrode materials in 1 M LiOH solution. (a) CV curves at the scan rate of 100 mV s⁻¹, (b) CV curves of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles at different scan rates, (c) galvanostatic charge/discharge curves of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles at different current densities, (d) rate performance of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles based on the weight, (e) cycle stability at the current density of 1 A g⁻¹, and (f) initial specific capacitance and capacitance retention after 1000 charge–discharge cycles.

The advantages of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles as both anodic and cathodic electroactive materials were outstanding when compared with Fe₂O₃, Fe₃O₄ and α-Fe₂O₃ + Fe₃O₄. Based on the synergistic effects of Fe₃O₄ and α-Fe₂O₃, this electrode exhibited a high specific capacitance and good reversibility. The role of heterostructure in enhancing the capacitance performance was further investigated by electrochemical impedance. According to analysis of Nyquist plots (Fig. 6), the charge-transfer resistance of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles electrode for supercapacitor was lower than that of α-Fe₂O₃, Fe₃O₄ and α-Fe₂O₃ + Fe₃O₄ particles, which was mainly attributed to the C-dots-mediated aggregation and high specific surface area of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles. The charge transfer and ion insertion/extraction during electrochemical reactions were thus facilitated.

Fig. 6 Nyquist plots of particles electrode in 1 M LiOH electrolyte.

The superior performance of heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles electrode could be attributed to the following reasons: (i) during the charge/discharge process, enough pore channels in three-dimensional heterostructure were beneficial for easy electrolyte immersion and diffusion and then enabled an effective redox reaction, (ii) as heterostructure formed between Fe₃O₄ and α-Fe₂O₃ nanoparticles, both electric contact and electron transport could be improved by the presence of Fe₃O₄, which enhanced the rate performance and cycle stability, (iii) the agglomeration of Fe₃O₄ nanoparticles could be prevented by α-Fe₂O₃, which could increase their effective availability and accessible capacitance. From the above, the synergistic effects of Fe₃O₄ and α-Fe₂O₃ could be used for the understanding of electrochemical performance enhancement. As supercapacitor electrode material, heterostructure pseudocapacitive iron oxides with the characteristics of low cost, good stability and environmentally friendly could offer additional advantages than bare Fe₃O₄, α-Fe₂O₃ and even their hybrid materials with those two pure phase particles.

4. Conclusion

In summary, for the first time, a three-dimensional, heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles, has been simultaneously used as anodic and cathodic electroactive materials. Based on the synergistic effects of Fe₃O₄ and α-Fe₂O₃, this electrode exhibited a high specific capacitance and a good reversibility. Using a simple solvothermal method and C-dots-mediated aggregation and reduction, the as-prepared heterostructure α-Fe₂O₃/Fe₃O₄ nanoparticles showed promising potential as an advanced electrode active material for supercapacitors.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 20775067, 20977074, 21175115 and 21475055), the Science & Technology Committee of Fujian Province, China (No. 2012Y0065), and the Program for New Century Excellent Talents in University (NCET-110904).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05968f

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