Fe2O3-Graphene Rice-on-Sheet Nanocomposite for High and Fast

ARTICLE pubs.acs.org/JPCC

Fe2O3-Graphene Rice-on-Sheet Nanocomposite for High and Fast Lithium Ion Storage Yuqin Zou, Jin Kan, and Yong Wang* Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shangda Road 99, Shanghai, P. R. China 200444

bS Supporting Information ABSTRACT: Graphene nanosheet (GNS)-supported Fe2O3 nanorice and nanoparticles were synthesized by a microwaveassisted hydrothermal technique. The Fe2O3-GNS rice-onsheet composite showed a better suitability as an anode material for Li-ion batteries. It exhibited a very large reversible capacity of 1184 mAh/g at a small current of 100 mA/g as well as good cycling performances. A good rate capability was also observed for Fe2O3-GNS rice-on-sheet composite, which exhibited large capacities of 825 and 633 mAh/g at large currents of 1000 and 5000 mA/g, respectively. The outstanding electrochemical behavior was ascribed to the increased electrical conductivity and mechanical stability of Fe2O3 nanorice by GNS support and prevented aggregation of few-layer GNS by the Fe2O3 decoration during the repetitive cycling with lithium ions.

1. INTRODUCTION As an n-type semiconductor, Fe2O3 has been extensively studied for various applications such as Li-ion batteries, gas sensors, catalysts, magnetic materials, and water treatment.1
20 Its abundant raw materials, low-cost, environmentally friendliness, and high corrosion resistance are preferable features for large-scale production and applications. In particular, Fe2O3 has been suggested as a promising anode candidate for rechargeable lithium ion batteries basically owing to its higher theoretical specific capacity (1005 mAh/g) than commercial graphite (theoretical capacity: 372 mAh/g).3
12 The storage mechanism is based on a well-known reversible Li-ion reactions with transitional metal oxides.3
5 However, its Li-ion storage properties, especially highrate performance are still limited because Fe2O3 exhibits poor electronic conductivity and there is also volume expansion/shrinkage during the Li-ion reactions with Fe2O3.3
12 The Fe2O3 electrodes are inclined to lose the electrical contact and structural integrity during repetitive cycling. This phenomenon is more prominent at high discharge and charge current rates.9
12 As conductive, flexible, and active anode materials, carbonaceous materials have been demonstrated as effective “matrix” materials to increase the electrical conductivity of high-capacity transitional metal oxides and release the mechanical stress induced by lithium insertion and extraction with transitional metal oxides.13
20 Since their discovery in 2004 as a new form carbonaceous material, graphene and its derivative composites have drawn a great deal of research interest for various applications owing to excellent electrical conductivity, mechanical stability, large surface areas, and so on.21
32 These intriguing merits suggest that graphene nanosheets (GNSs) are also good “matrix” materials to r 2011 American Chemical Society

be composited with high-capacity metal oxides.21,22,29
32 A number of reports have been reported recently for graphenebased Fe2O333,34 and Fe3O4 composites.35
39 The cycling performances of these composites were substantially increased compared with bare iron oxides anodes. Notably, these efforts have been concerned only with graphene or reduced grapheneoxide-supported iron oxide nanoparticles.33
39 Considering that extensive research efforts have demonstrated that intriguing properties and applications may be generated by tailoring the shape and size of functional nanomaterials,6
12 we believe that it is worthwhile to explore graphene-Fe2O3 composites with a carefully crafted control of Fe2O3 morphology and size. In this Article, Fe2O3 nanorice and nanoparticles were synthesized on GNSs by a microwave-assisted hydrothermal technique. Fe2O3-GNS rice-on-sheet nanocomposite showed a high reversible capacity of 1184 mAh/g at 100 mA/g as well as significantly enhanced cycling performance and high rate capability compared with bare Fe2O3 and Fe2O3
GNS particle-on-sheet composites.

2. EXPERIMENTAL METHODS 2.1. Preparation of Fe2O3-GNS Rice-on-Sheet and Particleon-Sheet Composites. Graphene oxide (GO) was synthesized

from natural graphite powder by a modified Hummers method. GNSs were obtained by a thermal reduction of GO. The preparation details have been described in a previous publication.31 Received: July 19, 2011 Revised: September 8, 2011 Published: September 14, 2011 20747

dx.doi.org/10.1021/jp206876t | J. Phys. Chem. C 2011, 115, 20747–20753

The Journal of Physical Chemistry C We dispersed 0.025 g graphene in 40 mL of deionized water by ultrasonication to form a suspension. This suspension was mixed with 20 mL of 0.016 M FeCl3 and 0.32 mM NH4H2PO4 aqueous solution. The molar ratio of Fe3+ to (H2PO4)
was 50:1, and the theoretical weight ratio of Fe2O3 to GNS in the composite was 1:1. The mixture suspension was sealed in a specialized glass tube under microwave irradiation in a single-mode microwave reactor (Nova, EU Microwave Chemistry). The microwave-induced hydrolysis was performed at 200 °C (pressure: ∼20 bar) with continuous magnetic stirring for 20 min. The Fe2O3-GNS riceon-sheet composite was collected by centrifuging and washing with deionized water, followed by drying in an electrical oven at 80 °C. The Fe2O3-GNS particle-on-sheet composite was prepared by a similar microwave irradiation procedure except for the absence of NH4H2PO4. Fe2O3 nanorice and nanoparticles were prepared accordingly by a similar method except for the absence of GNS or both GNS and NH4H2PO4. 2.2. Materials Characterizations. The obtained products were characterized by X-ray diffraction (XRD, Rigaku D/max2550 V, Cu Kα radiation), field-emission scanning electron microscopy (FE-SEM, JSM-6700F) with an energy dispersive X-ray spectrometer (EDS), and transmission electron microscopy/selected area electron diffraction/energy dispersive X-ray spectrometer (TEM/SAED/EDS, JEOL JEM-200CX and JEM2010F) in the Instrumental Analysis and Research Center, Shanghai University. Raman spectroscopy was recorded on Renishaw in plus laser Raman spectrometer (excitation wavelength: 785 nm, excitation power: 3 mW, spot size: ∼1.2 μm). Fourier transform infrared (FT-IR) spectra were collected by a BIO-RAD FTS 135 FT-IR spectrophotometer using the KBr pellet method. The electrical conductivity was measured by a four-electrode method using a conductivity detection meter (Shanghai Fortune Instrument, FZ-2010). The carbon and sulfur elements were measured by a high-frequency infrared carbon
sulfur analyzer (Keguo Instrument, HCS-500P). 2.3. Electrochemical Measurements. The working electrodes were composed of 80 wt % of active material, 10 wt % of the conductivity agent (acetylene black), and 10 wt % of the binder (poly(vinylidene difluoride), PVDF, Aldrich). Lithium foil (China Energy Lithium) was used as counter and reference electrode. The electrolyte was 1 M LiPF6 in a 50:50 w/w mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Electrochemical measurements were performed on a LAND-CT2001C test system. The Swagelok-type cells were discharged (lithium insertion) and charged (lithium extraction) at a constant current (100 mA/g, 0.1 C, 1 C = 1000 mA/g) in the fixed voltage range 5 mV to 3.0 V. Higher hourly rates (1, 2, or 5 C) were also used, and the first cycle discharging was kept at 0.1 C. Cyclic voltammetry (CV) was performed on a CHI660D electrochemical workstation at a scan rate of 0.1 mV/s.

3. RESULTS AND DISCUSSION The crystallographic structures of Fe2O3 nanoparticles, Fe2O3 nanorice, Fe2O3-GNS particle-on-sheet, and rice-on-sheet nanocomposites were analyzed by X-ray powder diffraction (XRD) in Figure 1a. A few characteristic peaks such as (012), (104), (110), and (300) planes were observed at 2θ = 24.1, 33.2, 35.6, and 64.0°, respectively, for all of these products. These peaks could be readily indexed to the standard hematite (PDF 33-0664). The characteristic (002) peak of carbon was shadowed by the (012)

ARTICLE

Figure 1. (a) Powder X-ray diffraction (XRD) patterns of Fe2O3 nanoparticles, Fe2O3 nanorice, Fe2O3-GNS particle-on-sheet, and Fe2O3-GNS rice-on-sheet composites. (b) Raman spectrum. (c) FTIR spectra.

peak of Fe2O3 crystals at the similar degree. Figure 1b shows the Raman spectra of GNS, Fe2O3-GNS particle-on-sheet, and Fe2O3-GNS rice-on-sheet nanocomposites. Two characteristic peaks of the D and G bands from GNS were observed at ∼1330 and 1580 cm
1. The intensity ratios of D and G band (ID:IG) were 1.21, 1.45, and 1.47, respectively, for GNS, Fe2O3-GNS particle-on-sheet, and Fe2O3-GNS rice-on-sheet nanocomposites. The more disordered carbon structure in the GNS- Fe2O3 composites compared with bare GNS may be due to the possible insertion of Fe2O3 into GNS layers. This agrees well with the previous observation on NiO-GNS composite.31 A large number of defects or cavities may be generated on the surface of GNS, which could be applied to store an extra number of lithium 20748

dx.doi.org/10.1021/jp206876t |J. Phys. Chem. C 2011, 115, 20747–20753

The Journal of Physical Chemistry C

ARTICLE

Figure 2. SEM images of (a) Fe2O3 nanoparticles, (b) Fe2O3-GNS particle-on-sheet composite, (c) Fe2O3 nanorice, and (d,e) Fe2O3-GNS rice-onsheet composite. (f) EDS spectrum of Fe2O3-GNS composites. Scale bar = 100 nm.

ions.21,22 The FTIR spectra of GO, Fe2O3-GNS particle-onsheet, and Fe2O3-GNS rice-on-sheet nanocomposites are shown in Figure 1c. A few stretches of 1700 and 1234 cm
1 could be attributed to the presence of abundant OdC and O
C functional groups on the GO surface. After a thermal reduction, these oxygen-containing functional groups disappeared in two Fe2O3GNS composites, indicating that GOs have been reduced to GNSs. A new small stretch around 1600 cm
1 was observed in two Fe2O3-GNS composites, which has been ascribed to the skeletal vibration of GNSs.28 FESEM images in Figure 2a
e show the surface morphologies of Fe2O3 nanoparticles, Fe2O3 nanorice, Fe2O3-GNS particle-on-sheet, and Fe2O3-GNS rice-on-sheet nanocomposites. Pristine Fe2O3 nanoparticles with nearly nanocube-like morphology were around 50
80 nm in size (Figure 2a). A large number of uniform Fe2O3 nanorice are shown in Figure 2c. Their lengths were around 150
200 nm. In the presence of GNS

materials, Fe2O3-GNS particle-on-sheet (Figure 2b) and Fe2O3GNS rice-on-sheet (Figure 2d
e) nanocomposites were obtained accordingly under similar experimental condition for preparing Fe2O3 nanoparticles and nanorice. The presence of phosphate anions has been demonstrated to be a key factor to affect the Fe2O3 product morphologies.2,15 The corresponding EDS spectroscopy measured in SEM is shown in Figure 2f. A few elements such as C, O, and Fe were present in the composite. The Si element was observed because Si substrate was used to disperse SEM sample, thus removing the carbon effect from the common substrate (carbon paste). The compositions of two Fe2O3-GNS nanocomposites were measured by a high-frequency infrared carbon analyzer. The carbon contents were determined to be 45.3 and 52.6% for Fe2O3-GNS particle-on-sheet and Fe2O3GNS rice-on-sheet, respectively. These values were slightly different with the theoretical value (50 wt % carbon) in two Fe2O3-GNS composites. 20749

dx.doi.org/10.1021/jp206876t |J. Phys. Chem. C 2011, 115, 20747–20753

The Journal of Physical Chemistry C

ARTICLE

Figure 3. TEM images of (a) Fe2O3 nanoparticles (scale bar = 100 nm), (b) Fe2O3-GNS particle-on-sheet composite (scale bar = 100 nm), (c) Fe2O3 nanorice (scale bar = 100 nm), and (d) Fe2O3-GNS rice-on-sheet composite (scale bar = 100 nm). (e) HRTEM image of a Fe2O3 nanorice (scale bar = 5 nm). The inset is the SAED pattern showing the single crystalline structure and [100] growth direction. (f) EDS spectrum of Fe2O3-GNS rice-on-sheet composite.

Figure 4. Schematic illustration of the growth process of Fe2O3-GNS particle-on-sheet and rice-on-sheet composites. 20750

dx.doi.org/10.1021/jp206876t |J. Phys. Chem. C 2011, 115, 20747–20753

The Journal of Physical Chemistry C

Figure 5. (a) Cyclic voltammograms of Fe2O3 nanorice and Fe2O3GNS rice-on-sheet composite at a scan rate of 0.1 mV/s. (b) First-cycle discharge (lithium insertion) and charge (lithium extraction) curves of the products. (c) Cycling performances of the products at 0.1C.

Microstructures and sizes of various products were further characterized by TEM technique. TEM images in Figure 3a
e show the Fe2O3-based nanomaterials with different morphologies. A number of Fe2O3 nanocube-like nanoparticles were observed in Figure 3a,b. There was a detectable agglomeration of Fe2O3 nanoparticles despite the presence of GNS support. Figure 3c,d shows bare Fe2O3 nanorice and Fe2O3-GNS rice-onsheet nanostructures. Fe2O3 nanorice were ∼200 nm in length, and their diameters were decreased from ∼40 nm in the middle section to ∼3
5 nm in the tips. The HRTEM image in Figure 3e shows the lattice structures of Fe2O3-GNS rice-on-sheet. Two

ARTICLE

interlayer distances of 0.25 and 0.43 nm were observed, which could be ascribed to the (
120) and (100) planes of Fe2O3 crystals (PDF 33-0664). The corresponding SAED in the inset figure shows the single crystal Fe2O3 structure. All diffraction data were determined to be taken from the [001] zone axis. The preferential growth direction of Fe2O3 nanorice was along the [100] direction. The EDS pattern in Figure 3f confirmed the presence of Fe2O3 materials in the Fe2O3-GNS composite. A schematic sketch of the growth process of GNS-supported Fe2O3 nanoparticles and Fe2O3 nanorice is shown in the Figure 4. Microwave irradiation promoted the hydrolysis of FeCl3 and formed Fe2O3 nanocrystals in the sealed microwave reactor. Phosphate anions played an important role in the defined ricelike morphology. In the absence of phosphate anions, Fe2O3 nanoparticle products were produced as the final products. The reaction temperature and time effect on the microstructures of Fe2O3 nanorice were also explored in Figures S1 and S2 in the Supporting Information. When the temperature was below 200 °C and microwave-irradiation time was