Graphene Composite Nanosheets with Enhanced

Oct 27, 2016 - It reveals that amorphous Fe2O3 nanoparticles with an average diameter of 5 nm ... buffer the volume change of host materials during so...
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Amorphous Fe2O3/Graphene Composite Nanosheets with Enhanced Electrochemical Performance for Sodium-Ion Battery Dan Li, Jisheng Zhou,* Xiaohong Chen, and Huaihe Song* State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: With the increasing use of sodium-ion batteries (SIBs), developing cost-effective anode materials, such as metal oxide, for Na-ion storage is one of the most attractive topics. Due to the obviously larger ion radius of Na than that of Li, most metal oxide electrode materials fail to exhibit the same high performance for SIBs like that of Li-ion batteries. Herein, iron oxide was employed to demonstrate a concept that rationally designing an amorphous structure should be useful to enhance Na-ion storage performance of a metal oxide. Amorphous Fe2O3/graphene composite nanosheets (Fe2O3@GNS) were successfully synthesized by a facile approach as anodes for SIBs. It reveals that amorphous Fe2O3 nanoparticles with an average diameter of 5 nm were uniformly anchored on the surface of graphene nanosheets by the strong C−O−Fe oxygen-bridge bond. Compared to well-crystalline Fe2O3, amorphous Fe2O3@GNS exhibited superior sodium storage properties such as high electrochemical activity, high initial Coulombic efficiency of 81.2%, and good rate performance. At a current density of 100 mA/g, amorphous Fe2O3@GNS composites show a specific capacity of 440 mAh/g, which is obviously higher than the specific capacity of 284 mAh/g of crystalline Fe2O3. Even at a high current density of 2 A/g, amorphous Fe2O3@GNS composites still exhibit a specific capacity as high as 219 mAh/g. The excellent electrochemical performance should be attributed to the amorphous structures of Fe2O3 as well as strongly interfacial interaction between Fe2O3 and GNS, which not only accommodate more electrochemical active sites and provide the more transmission channels for sodium ions but also benefit electron transfer as well as effectively buffer the volume change of host materials during sodiation and desodiation. This concept for designing amorphous iron oxide anodes for SIBs is also expected to facilitate preparation of various amorphous nanostructure of other metal oxides and improve their Na-ion storage performance. KEYWORDS: amorphous, iron oxide, graphene, anode, sodium-ion batteries

1. INTRODUCTION Cost-effective, high-efficiency, and secure electric energy storage devices are very crucial for fulfilling the rapid development of electric vehicles, portable electronics, and renewable energy storage. Up to now, lithium-ion batteries (LIBs)1,2 with high energy density, safety, and eco-friendliness have attained the fastest commercial development in the field of energy storage.3,4 However, due to lower abundance and uneven distributions of lithium in the earth, at present, the cost of LIBs makes it unaffordable for the development of the electric vehicles and renewable energy storage.5,6 Therefore, it is necessary to develop new and cheap batteries based on abundant elements. Sodium is the sixth richest element on earth.7 In addition, sodium-ion batteries (SIBs) possess electrochemical working principles similar to lithium-ion batteries.8 Therefore, it is expected that SIBs may be more commercially competitive compared to LIBs9,10 because of additional energy storage. It is significantly crucial for developing cost-effective SIBs to look for inexpensive anode materials with high electrochemical performance.11,12 Iron oxide material (Fe2O3),13,14 as one of the cheapest materials, has attracted great attention in © 2016 American Chemical Society

electrochemical energy storage in LIBs because of abundant sources, environment friendliness, safety, and better electrochemical activity. Recently, sodium-storage performance of iron oxide has also been investigated. However, similar to that in LIBs, iron oxide in SIBs also suffers from low electronic conductivity and volume changes.13,15−17 To solve these problems, one of the classical strategies is to composite with a carbon matrix.18 With the increasing use of graphene, graphene-based iron oxide as anode for SIBs is drawing more attention.19−24 For example, in a recent report,23 the ultrafine Fe3O4 nanoparticles anchored onto the surfaces of reduced graphene oxide were used as anode for SIBs and exhibit a specific capacity of 204 mAh/g. Another report24 indicated that the Fe2O3-reduced graphene oxide composite showed a specific capacity of about 250 mAh/g and good cycling performance. However, the investigations on the iron oxide anode materials for SIBs are limited until now. Additionally, the specific capacity for most graphene-based iron oxide composites needs to be Received: July 29, 2016 Accepted: October 27, 2016 Published: October 27, 2016 30899

DOI: 10.1021/acsami.6b09444 ACS Appl. Mater. Interfaces 2016, 8, 30899−30907

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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Synthesis of Fe2O3 and GNS Composites

structure,37 it is expected that amorphous iron oxide will accommodate more electrochemical active species, contact more tightly with electron donors and electrolyte, buffer more efficiently the volume expansion, and provide more transmission channels for sodium ions. In this work, amorphous Fe2O3 nanoparticles uniformly anchored on the surfaces of graphene nanosheets (referred to as Fe2O3@GNS) by the strong C−O−Fe oxygen-bridge bonds were successfully synthesized through a facile approach as anodes for sodium-ion batteries (Scheme 1). For comparison, as-prepared amorphous Fe2O3@GNS was converted to wellcrystalline composite by the additional annealing at 500 °C, which was denoted as Fe2O3@GNS-500. The amorphous Fe2O3@GNS composites exhibited excellent sodium storage properties over well-crystalline Fe2O3@GNS-500 with high specific capacity (440 mAh/g) and good rate performance, and even at the current density of 2 A/g, the reversible capacity still remained at 219 mAh/g. Amorphous Fe2O3@GNS also exhibits obviously superior Na-ion storage performance compared to crystalline ones reported in the previous works.19−24 This concept for designing amorphous iron oxide anodes for SIBs is also expected to be used for preparation of various amorphous nanostructure of other metal oxide materials and to improve their Na-ion storage performance.

improved for practical applications. In addition, many elaborate designs have enhanced largely the specific capacity of graphenebased iron oxide composite,19−24 but the key parameters for electrochemical performances are still unsatisfactory, including reduced Coulombic efficiency and unsatisfactory rate performance for sodium-ion storage. For example, Zhou et al.19 reported recently that the Fe2O3 nanocrystals anchored onto GNS, prepared by nanocasting, exhibited a higher specific capacity of 410 mAh/g used as anode for SIBs, but the initial Coulombic efficiency is slightly low, and rate performance still needs to be improved. In order to improve Na-ion storage performance of graphene-based iron oxide anode materials, on one hand, it should enhance interfacial interaction between graphene and iron oxide. In previous reports,25,26 it has been found that the performances of graphene/metal oxide composites with strongly interfacial interaction are superior to those of corresponding composites prepared by physical mixing when used as anode materials for LIBs. Weak interfacial connection is detrimental to smooth electronic transport between iron oxide and graphene as well as effectively buffering of volume change.25,26 Recently, Wang et al.27 also suggested that P/ CNTs hybrids with strongly P−O−C bonds had a better cyclic stability than that with weak physical interaction in SIBs. A similar strategy should be also expected to enhance the Na-ion storage performance of graphene-based iron oxide anode materials. However, most of the previous efforts only focus on the synthesis strategies and Na-ion storage tests of graphene-based iron oxide.23,24 To the best of our knowledge, there has been little focus on the design of graphene-based iron oxide with strongly interfacial interaction to improve the sodium-ion storage performance. On the other hand, considering the larger ion radius of sodium than that of lithium,28 it is necessary to design iron oxide nanostructures with more suitable accommodations for Na-ion diffusion and storage. Larger ion radius makes the sodium ion difficult to reversibly insert into and extract from host materials, and it also prompts host materials with high Liion storage activity to become low active and even inactive for Na-ion.29,30 Iron oxide nanocrystals own a rock-salt structure without any available empty site for ion storage,31 and therefore, iron oxide as anode materials for SIBs will suffer from more volume change in charge/discharge processes than in LIBs.32−34 A recent report35,36 suggested that disorder and even amorphous carbon materials can provide more suitable accommodation space for sodium ions than those in highcrystalline graphite. Thereby we envisioned that the composite based on amorphous Fe2O3 nanoparticles might overcome the ionic radius disadvantages of sodium to improve sodium-ion storage performance. Due to the loosely packed amorphous

2. EXPERIMENTAL SECTION 2.1. Materials. Iron nitrate (Fe(NO3)3, 98.5%), sodium borohydride (NaBH4, 98%), and absolute alcohol (99.7%) were provided by the Tianjin Fuchen Chemical Reagents Factory, China. The graphene nanosheets (GNSs) were fabricated by the modified staudenmaier’s method in our previous work.38 All of the chemicals were used as received without further purification. 2.2. Synthesis of Amorphous Fe2O3@GNS. The amorphous Fe2O3@GNS composite was synthesized by reaction between Fe(NO3)3 and NaBH4 under the condition of ice bath, followed by oxidation in the air at 250 °C for 3 h (Scheme 1). In the experiment, 3.3 mmol of graphene (0.040 g) and 0.8 mmol of Fe(NO3)3·9H2O (0.336 g) were sonicated in 20 mL of ethanol with an atom ratio of 4:1 for 30 min with a sonicator at 80 W to form a homogeneous solution. After evaporating the alcohol at 90 °C under the strong stirring, the mixture of Fe(NO3)3 and GNS was added to the solution of NaBH4 (1 M) to react for 4 h in an ice bath under the strong stirring. Then, amorphous iron oxide/GNS composites can be obtained by centrifugal separation and subsequent dying at 50 °C for 1 h under vacuum. Finally, in order to obtain pure Fe2O3, amorphous iron oxide/GNS composites were oxidized to further prepare the amorphous Fe2O3@ GNS composites in the air at 250 °C for 3 h in the oven. For comparison, the amorphous Fe2O3@GNS composites were converted to crystalline Fe2O3/GNS composites, denoted as Fe2O3@ GNS-500, by annealing at 500 °C in the Ar. 2.3. Structural and Physical Characterization. The morphology and structure of the obtained composites were characterized by 30900

DOI: 10.1021/acsami.6b09444 ACS Appl. Mater. Interfaces 2016, 8, 30899−30907

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ACS Applied Materials & Interfaces scanning electron microscopy (SEM, ZEISS SUPRATM 55 field emission microscope). The high-angle annular dark-field scanning transmission electron microscopy (HAADF-TEM) images and corresponding high-resolution transmission electron microscopy (HRTEM) were measured by a TECNAI G2 F30. Elemental dispersive spectroscopy (EDS) mapping were carried out on the electron microscope (TECNAI G2 F30) to determine the distribution of C, O, and Fe in the composites. X-ray diffraction (XRD) was performed by a Rigaku D/max-2500B2+/PCX system using Cu Kα radiation (λ = 1.5406 Å) over the range of 5−90° (2θ) at room temperature. Thermogravimetric analysis (TGA) was measured by a STA449C produced by NETZSCH Company in Germany with a heating rate of 10 °C/min from room temperature to 1000 °C. X-ray photoelectron energy spectra (XPS) were recorded using monochromatic Al Kα (1486.6 eV) X-ray source with 30 eV pass energy in 0.5 eV step over an area of 650 μm × 650 μm to the sample. 2.4. Electrochemical Characterization. The electrochemical properties of the samples were characterized with CR2025 type cell. Pure Na foil was used as the counter electrode, a glass fiber as separator, and one molar NaSO3CF3 solution in diglyme was used as electrolyte. The working electrode was prepared by mixing the active materials, carbon black, and polyvinylidene fluoride (PVDF) with a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP) to form a slurry, which was then coated onto a nickel foil, dried at 80 °C for 4 h, and further dried at 120 °C for 12 h. The mass of active materials in every electrode can be controlled in the range of 1.5−2.0 mg. The cells were discharged and charged in the voltage window from 0.01 to 2.50 V using a Land battery tester (CT2001A, China) at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectral (EIS) measurements with a frequency range from 100 kHz to 0.01 Hz were tested on an electrochemical workstation (ZAHNER ZENNIUM).

Figure 2. SEM images (a, b) of as-prepared Fe2O3@GNS composites. TEM image (c) elemental distributions of Fe2O3@GNS at HAADF model: (i) HAADF-TEM image of Fe2O3@GNS composite, (ii) enlarged STEM image in red box of image (i), and corresponding elemental mapping of C (iii), O (iv), and Fe (v).

the GNSs. The average diameter of particles in Fe2O3@GNS is ca. 5 nm. HRTEM reveals (Figure 3b) an amorphous structure of ultrasmall iron oxide nanoparticles due to no clear crystal lattice fringes, which is consistent with the XRD pattern (Figure 1). Furthermore, the selected area diffraction (SAED) pattern (Figure 3c) confirms the presence of amorphous-like phases, in which only hexagonal diffractional dots belong to graphene,40 and there is no clear diffraction rings or dots ascribed to iron oxide. Because it is difficult for XRD and TEM to determine the chemical compositions of iron oxide in the composites, XPS measurements were carried out to reveal the elemental chemical state of the composites. XPS survey spectra (Figure 4a) reveal that Fe2O3 @GNS is composed of C, O, and Fe elements, while the GNS contains C and O. In Figure 4b, the Fe 2p3/2 and Fe 2p1/2 core levels are present at 711.3 and 725.0 eV, respectively, which are consistent with those of Fe2O3 reported in the previous reports.41,42 The result confirms that iron oxide in the Fe2O3@GNS is Fe2O3. Moreover, the XPS spectra indicate that the strong interfacial interaction exists between iron oxide and GNSs in the Fe2O3 @ GNS. In Figure 4c, the C 1s spectra of pure graphene and two composites are mainly composed of three bonds: nonoxygenated C (CC/C−C) in aromatic rings (284.9 eV) and the C in C−O (286.5 eV) and O−CO (289.4 eV).43,44 There is no peak at 283.3 eV, illustrating the absence of C−Fe bond in two composites.45 In Figure 4d, the O 1s spectra confirm iron oxide nanoparticles are linked with GNSs by C− O−Fe oxygen-bridged bonds. The O 1s spectrum of GNS is composed of two peaks at 531.8 and 533.7 eV, which are attributed to CO and epoxy C−O groups, respectively.43,44 The O 1s spectrum of Fe2O3@GNS is composed of three peaks at 533.7, 532.1, and 530.5 eV, respectively. Compared with that of pure graphene, the peak at 533.7 eV should correspond to epoxy C−O groups in graphene, and the peak at 530.5 eV should be from the Fe2O3.46 The dominant peak at 532.1 eV should be attributed to C−O−Fe bonds according to previous reports.25,47−49 This chemical bonding between carbon matrix and metal oxide could improve the electrochemical properties.25−27 On the one hand, formation of a chemical bonding

3. RESULTS AND DISCUSSION The morphology and structure of the as-prepared Fe2O3@GNS composites were characterized by XRD, SEM and TEM, which reveal in further the ultrathin size and amorphous structures of iron oxide nanoparticles. The pattern of the Fe2O3@GNS (Figure 1) shows broad reflection peaks and no sharp peak

Figure 1. XRD patterns of Fe2O3@GNS and Fe2O3@GNS-500.

appears, which indicates the existence of amorphous phase.35,39 The low-magnification image in Figure 2a shows that the Fe2O3@GNS composites possess a curled morphology with a thin wrinkled structure, which is not much different from that of original wrinkled GNSs. Due to their ultrasmall size, the iron oxide nanoparticles could not be observed obviously in the SEM (Figure 2a), and still cannot be distinguished after further amplification (Figure 2b). However, HAADF-STEM and TEM images (Figure 2c and 3a) confirm that ultrasmall nanoparticles are anchored on the surface of GNSs. Elemental mapping images (Figure 2c(i−v)) clearly demonstrate that C, Fe, and O elemental distributions are overlapped highly, indicating further that ultrasmall iron oxide nanoparticles are uniformly grown on 30901

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Figure 3. (a) TEM, (b) HRTEM images, and (c) SAED patterns of as-prepared amorphous Fe2O3@GNS and (d) TEM, (e) HRTEM images, and (f) SAED patterns of high-crystalline Fe2O3@GNS-500.

oxidized to form an amorphous iron oxide sheath with ca. 1−5 nm. Generally speaking, the oxidation reaction can occur very fast, which takes in the range of ca. 0.2 fs to tens of seconds.52 Here, the reduction between Fe(NO3)3 and NaBH4 was carried out under the condition of an ice bath, so ultrasmall Fe0 nanoparticles can be formed. Ultrasmall Fe0 nanoparticles can be oxidized fully into amorphous iron oxide (Supporting Information Figure S1). Finally, amorphous Fe2O3@GNS was obtained by fully oxidizing amorphous iron oxide into trivalent iron oxide in the air. In the Fe2O3@GNS composites, amorphous Fe2O3 can be bonded with functional groups and/or defects on the GNSs to form C−O−Fe bonds according to previous reports.26,53 Compared with that of amorphous Fe2O3@GNS, crystallinity of Fe2O3@GNS-500, obtained by annealing amorphous Fe2O3@GNS at 500 °C, is improved largely. The XRD pattern of Fe2O3@GNS-500 after heat-treatment shows the sharp peaks, which matches well with Fe2O3 (JCPDS: 39-1346) and illustrates the high crystallinity and high phase purity. The content of Fe2O3 in Fe2O3@GNS calculated by TGA carried out in an O2 atmosphere is 61 wt % (Supporting Information Figure S2). The crystallinity of Fe2O3@GNS-500 is also improved obviously according to HRTEM and SAED measurements. Contrary to the HRTEM image in Figure 3b, the HRTEM image of Fe2O3@GNS-500 in Figure 3e shows clear crystal lattice fringes. The interlayer spacing is 2.622 Å, belonging to (311) plane of Fe2O3. SAED pattern (Figure 3f) suggests obvious diffraction rings composed of bright spots, which also confirms the presence of nanocrystalline phases. The average diameter of Fe2O3 nanoparticles in the Fe2O3@GNS500 is slightly increased to 10 nm (Figure 3d). Therefore, the specific surface area of the Fe2O3@GNS-500 is also decreased to 159.8 m2/g from 241.5 m2/g of Fe2O3@GNS (Supporting Information Figure S3). The Fe 2p spectrum (Figure 4b) confirms that the composites after annealing are still Fe2O3,41 which is consistent with XRD pattern. However, after annealing, the intensity of C−O−Fe bond in Fe2O3@GNS500 (Figure 4d) is much weaker than that in Fe2O3@GNS, indicating that the interfacial interaction in Fe2O3 @GNS is stronger than that in Fe2O3@GNS-500. The stronger chemical bond combination might be beneficial to the fast electron transfer between metal oxide and graphene with the Na+ reversible insertion/extraction, so as to affect the electrochemical performance of composites according to previous reports.26,49

Figure 4. (a) XPS survey spectra of pristine GNS, Fe2O3@GNS and Fe2O3@GNS-500, and their Fe 2p (b), C 1s (c), and O 1s (d) spectra.

interface combination could accelerate the electrons’ transformation between the carbon matrix and metal oxide. On the other hand, the chemical bonding connection could effectively avoid the metal oxide particles drop from the carbon matrix during sodiation and desodiation processes. It is very interesting to reveal the formation of amorphous Fe2O3@GNS with oxygen-bridge bonds. The ferric solution can be reduced to zerovalent iron (Fe0) by NaBH4 solution.41 According to previous reports,42,50,51 Fe0 nanoparticles can be 30902

DOI: 10.1021/acsami.6b09444 ACS Appl. Mater. Interfaces 2016, 8, 30899−30907

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Fe2O3@GNS have obvious redox peaks. In the first cathodic scan of Fe2O3@GNS, three weak peaks were recorded at the first sodiation process. The peak presented at 1.10 V was ascribed to the sodium ion’s first insertion into the amorphous iron oxide, and the complete reduction of Fe (III) to Fe (0) is recorded at 0.73 V. The third peak at 0.20 V is related to the decomposition of the electrolyte and SEI film formation.54 The second and third cycles are overlap, which suggests the excellent reversibility performance. Beyond the first cycle, the primary reduction peak at 0.73 V becomes sharper, which reflects the reversibility of conversion reaction and low-phase transformation resistance. In the anodic sweeps, three peaks recorded at 0.75, 1.25, and 1.70 V are assigned to the oxidation of Fe (0) to Fe (III). The conversion reaction of Fe2O3@GNS occurs in a potential range of ca. 0.97 V. The whole process can be described by follow reaction:

Due to the amorphous structure and stronger interfacial interaction, the Fe2O3@GNS exhibits much better sodium-ion storage performance than crystalline Fe2O3@GNS-500. Figure 5a shows that the cyclic voltammetry (CV) curves of the

6Na + + 6e− + Fe2O3 ↔ 3Na 2O + 2Fe

For comparison, the CV curves of the nanocrystalline Fe2O3@GNS-500 are shown in Figure 5b, and inferior performance was clearly distinguished. On the one hand, during the reversible cycle, the location of cathodic peak of Fe2O3@GNS-500 electrode is consistent with that in the CV curves of Fe2O3@GNS electrode in Figure 5a, whereas the highest anodic peak of Fe2O3@GNS-500 recorded at 1.59 V is slightly lower than that of Fe2O3@GNS (1.70 V). Therefore, it is obvious to see that the conversion reaction of crystalline Fe2O3 occurred in a narrower potential range of ca. 0.86 V, whereas the phase transformation of amorphous Fe2O3 (Figure

Figure 5. CV curves at a scan rate of 0.1 mV/s for Fe2O3@GNS (a) and Fe2O3@GNS-500 (b) between 0.01 and 3 V vs Na+/Na.

Figure 6. Initial three discharge−charge curves for Fe2O3@GNS (a) and Fe2O3@GNS-500 (b) at 100 mA/g. (c) Cycling performance at 100 mA/g and Coulombic efficiency of Fe2O3@GNS and Fe2O3@GNS-500. (d) Rate capability of Fe2O3@GNS and Fe2O3@GNS-500. (e) Cycling performance for Fe2O3@GNS and pure grahene nanosheets cycled at a current of 2 A/g, and Coulombic efficiency of the Fe2O3@GNS sample. (f) Electrochemical impedance plots and Randles equivalent circuit of Fe2O3@GNS and Fe2O3@GNS-500 after three cycles at 100 mA/g. (g) Schematic illustration of transport paths of Na+ ions into the amorphous Fe2O3 and crystalline Fe2O3, and the conversion reaction of sodium-ion batteries. 30903

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electrode and irreversible Na-ion storage in host materials.9 The Nyquist plots reveal (Figure 6f) that the diameter of the semicircle of Fe2O3@GNS is much smaller compared to Fe2O3@GNS-500. By simulation, the SEI film resistance Rf (3.18Ω) of Fe2O3@GNS is much lower than that (5.11Ω) of Fe2O3@GNS-500, indicating that less electrolyte decompose to form SEI films on the surface of Fe2O3@GNS (Table S1), which means less irreversible capacity from the SEI films. In addition, amorphous materials have smaller volume change than crystalline ones,56,57 as a result with more effective releasing of volume strain and less irreversible Na-ion storage in the host materials. Therefore, the amorphous Fe2O3@GNS has obviously higher initial Coulombic efficiency, which is very important for practical application used as electrode materials for SIBs. The amorphous Fe2O3@GNS composite also exhibits the superior rate performance compared to the crystalline Fe2O3@ GNS-500 (Figure 6d). The stable discharge−charge capacity of 350, 295, 239, and 194 mAh/g can be delivered at various currents of 200, 500, 1000, and 2000 mA/g, respectively. When the current is returned to 100 mA/g, the capacity can also be close to the original value, which indicated the good tolerance at high current. However, the capacities of Fe2O3@GNS-500 were 265, 212, 156, and 126 mAh/g, respectively, at currents of 200, 500, 1000, and 2000 mA/g. It can be seen that with increasing of current density for 200 to 2000 mA/g, the capacity ratio between Fe2O3@GNS and Fe2O3@GNS-500 increases from ca. 1.3 to 1.7. In addition, rate performance of amorphous Fe2O3@GNS is also better than that of ultrathin crystalline iron oxide in previous reports.19 For example, Zhou et al. prepared Fe2O3 nanocrystals/GNSs composite anode materials for SIBs, in which the particle size of Fe2O3 nanocrystals (ca. 2 nm) is obviously smaller than that of amorphous Fe2O3@GNS (ca. 5 nm). The stable specific capacities of Fe2O3 nanocrystals/GNSs composite in their work are 400, 350, 260, 190, and 110 mAh/g at various current densities of 100, 200, 500, 1000, and 2000 mA/g, respectively. It can be seen that the specific capacity of amorphous Fe2O3@ GNS in our work is comparable to that of iron oxide nanocrystals at the lower current densities, but the capacities of amorphous oxide become obviously higher and higher than those of Fe2O3 nanocrystals/GNSs composite with the increase of current density. It can be calculated that the capacity of amorphous iron oxide is ca. 1.3 and 1.8 times higher than that of Fe2O3 nanocrystals/GNSs composite at 1000 and 2000 mA/ g, respectively. Obviously higher rate performance of Fe2O3@ GNS should be attributed to amorphous structure and the stronger interaction between amorphous iron oxide and graphene in [email protected],26 Stronger interfacial interaction results in charge transfer resistance Rct (4.36Ω) of Fe2O3@GNS that is much lower than the value (16.04Ω) for Fe2O3@GNS500, as determined by EIS spectra (Figure 6f and Table S1), indicating the formation of higher electronic transfer network in Fe2O3@GNS. In addition, the amorphous structure has more channels for Na-ion diffusion at high rate. To reveal the cycling stability of Fe2O3@GNS at a high rate, the Fe2O3@GNS electrode was charged/discharged up to 500 cycles at 2000 mA/g (ca. 5 C). The capacity remains at 110 mAh/g after 500 cycles (Figure 6e). It can be calculated that the capacity fading is only ca. 0.15 mAh/g per cycle at ca. 5 C. The pure graphene nanosheets only showed a much lower specific capacity of 44 mAh/g. It can be calculated that the actual deliverable specific capacity of amorphous iron oxide in

5a) during conversion reaction occurred in a broader potential range, indicating higher stress/strain for crystalline Fe2O3 compared with amorphous Fe2O3.55 On the other hand, the redox current peaks of crystalline Fe2O3 are much weaker than those of amorphous Fe2O3, which is due to the high phasetransformation resistance. Therefore, the conversion reaction of amorphous Fe2O3@GNS is more reversible than that of crystalline Fe2O3@GNS-500. It is well-known that conversion-type metal oxide usually suffers from the repeated decomposition and reconstruction during the cycling. In order to rule out the effect of cycling on the crystal structure of iron oxide, morphology and structure of electrode materials after cycling were investigated (Figure S5 and S6). Both cycled electrodes retained their original structure, indicating good stability (Figure S5). Additionally, the HRTEM image and SAED pattern show that Fe2O3 in the Fe2O3@GNS500 electrode still keep the crystalline structures after a complete discharge/charge cycle (Figure S6). Therefore, higher electrochemical activity of Fe2O3@GNS should be attributed to the amorphous structure of Fe2O3. In addition, it is worthwhile to point out that the CV curves of nanocrystalline Fe2O3 as anodes for SIBs in previous reports,19−22,24 just like the curves of Fe2O3@GNS-500 in Figure 5b, have no obvious redox peak. Therefore, it is clear that amorphous iron oxide exhibits higher reversible electrochemical activity than crystalline iron oxide. The galvanostatic discharge−charge measurement indicates that Fe2O3@GNS electrode has higher specific capacity. Figure 6a shows that the initial discharge specific capacity and reversible capacity of Fe2O3@GNS are 542 and 440 mAh/g at 100 mA/g, respectively. The reversible capacity of graphene is ca. 250 mAh/g (Supporting Information Figure S4), and therefore, on the basis of the mass content of iron oxide, it can be calculated that the actual deliverable specific capacity of amorphous iron oxide in Fe2O3@GNS will be 561 mAh/g (related calculation can be seen in the Supporting Information). However, the nanocrystalline Fe2O3@GNS-500 only showed a reversible capacity of 284 mAh/g (Figure 6b), which is much lower than that of Fe2O3@GNS. Thus, crystalline iron oxide in Fe2O3@GNS-500 only delivers 253 mAh/g of specific capacity, which is ca. 45% that of amorphous iron oxide. Na-storage capacity of amorphous Fe2O3@GNS is also higher than most of nanocrystalline iron oxide and/or iron oxide/nanocarbon composites in previous reports.20−24 For example, Zhang et al.23 synthesized the ultrafine Fe3O4 nanoparticles anchored onto the surfaces of reduced graphene oxide, and the composites exhibit a specific capacity of 204 mAh/g when used as anodes for SIBs. Liu et al.24 reported Fe2O3-reduced graphene oxide composites and found that composite electrode showed a specific capacity of about 250 mAh/g. In fact, those results in previous reports are lower than or only comparable to the reversible capacity of nanocrystalline [email protected],24 Therefore, it indicates that amorphous iron oxide has more Nastorage active sites than crystalline iron oxide. Additionally, it is remarkable that the initial Coulombic efficiency of Fe2O3@GNS can reach up to as high as 81.2%, while the initial Coulombic efficiency of Fe2O3@GNS-500 is only 64.8%. Recent reports19−24 showed that the initial Coulombic efficiency of nanocrystalline iron oxide and/or iron oxide/nanocarbon composites are also very low, which are in the range of ca. 40−60%. Higher initial Coulombic efficiency should be attributed to lower initial irreversible capacity, which should be mainly resulted from the irreversible formation of a SEI (solid electrolyte interphase) layer on the surface of 30904

DOI: 10.1021/acsami.6b09444 ACS Appl. Mater. Interfaces 2016, 8, 30899−30907

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ACS Applied Materials & Interfaces Notes

Fe2O3@GNS is still ca. 152 mAh/g at the current density of 2 A/g after 500 cycles, confirming the effectiveness of interfacial interaction made by oxygen bridge bonds. Compared with some results reported,20−22,24,26 the amorphous Fe2O3@GNS composite still had a high initial Coulombic efficiency and showed superior electrochemical properties over crystallinephase materials. In addition, some reports20,22 showed excellent rate performance, but the preparation methods are complicated, so it highlights the advantages of our materials. On the basis of the analyses above, the enhanced sodium-ion storage properties of Fe2O3@GNS should be attributed to the following factors, as illustrated in Figure 6g. First, compared to crystalline structures, the amorphous Fe2O3 could provide more suitable accommodation space for large sodium ions to reversibly insert into and extract from host materials. Second, the amorphous structure will provide more transmission channels for sodium ions so as to significantly reduce the Naions diffusion paths. Third, the strong interfacial interaction caused by oxygen bridge bonds can be helpful to electron transfer as well as to buffer effectively the volume change during sodiation and desodiation processes. All of these contributed to the improvement of electrochemical performance.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51572015 and 51272019), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001).

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4. CONCLUSIONS In summary, amorphous Fe2O3 nanoparticles uniformly anchored on the surface of graphene nanosheets by the strong C−O−Fe oxygen-bridge bond was successfully synthesized by a facile approach. When used as anode materials for SIBs, amorphous Fe2O3@GNS composites exhibit a high reversible specific capacity of 440 mAh/g at the current density of 100 mA/g. At the higher current density of 2 A/g, the reversible capacity still remained at 219 mAh/g. The electrochemical performance of amorphous Fe2O3@GNS composites are obviously better than that of crystalline Fe2O3@GNS-500 composites, which should be attributed to amorphous structures as well as the strong interfacial interaction. The excellent performance of Fe2O3@GNS makes it possible to be a candidate for cost-effective anode materials for sodium-ion batteries. The idea for designing an amorphous structure is also expected to be extended to synthesis other high-performance metal oxide electrode materials for Na-ion storage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09444. XRD pattern and SEM image of the Fe2O3@GNS before oxidation; TG curves of the Fe2O3@GNS and Fe2O3@ GNS-500 samples; nitrogen adsorption−desorption isotherms of Fe 2O3@GNS and Fe2O 3@GNS-500; electrochemical performance of original GNS and GNS annealed at 500 °C; SEM images of cycled electrodes of Fe2O3@GNS and Fe2O3@GNS-500; TEM, HRTEM images, and SAED patterns of high-crystalline Fe2O3@ GNS-500 after the first charge/discharge cycle at 100 mA/g; dynamical parameters of Randles equivalent circuit for Fe2O3@GNS and Fe2O3@GNS-500 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 30905

DOI: 10.1021/acsami.6b09444 ACS Appl. Mater. Interfaces 2016, 8, 30899−30907

Research Article

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