α-Fe2O3: Hydrothermal Synthesis, Magnetic and Electrochemical

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J. Phys. Chem. C 2010, 114, 10671–10676

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r-Fe2O3: Hydrothermal Synthesis, Magnetic and Electrochemical Properties Jianmin Ma, Jiabiao Lian, Xiaochuan Duan, Xiaodi Liu, and Wenjun Zheng* Department of Materials Chemistry, College of Chemistry, Nankai UniVersity, Tianjin, People’s Republic of China ReceiVed: August 15, 2009; ReVised Manuscript ReceiVed: May 7, 2010

The size- and shape-controlled fabrication of R-Fe2O3 has been successfully realized via a faicle templatefree hydrothermal route, only simply changing reaction time and solvent used. The formation mechanisms of various nanostructures are proposed and the controlling factors on the morphology of the final product are also discussed. Furthermore, magnetic hysteresis measurements demonstrate that the as-obtained R-Fe2O3 nanostructures show structure-dependent magnetic properties. And the as-obtained R-Fe2O3 nanopolyhedra exhibits ultrahigh reversible capacity, and excellent capacity retention over 20 cycles. It is expected that the adjustable magnetic properties and high discharge capacity of the as-prepared samples make them useful with potential applications in magnetic nanodevices and high-energy batteries. 1. Introduction Synthesis of inorganic nanocrystals with controlled size and structure has attracted considerable attention due to their socalled size- and shape-dependent physical properties.1 The ability to manipulate the size, shape, composition, crystal structure, and surface properties of the nanocrystals is essential for uncovering their intrinsic properties unaffected by sample heterogeneity.2 Thus, controlled growth of nanocrystals is critical for understanding and exploiting their unique properties.3 Hematite (R-Fe2O3) is one of the most studied nanocrystals due to its wide range of applications, such as in catalysts,4 pigments,5 magnetic materials,6 gas sensors,7 and lithium ion batteries.8 Inspired by its excellent characteristics, much effort has been made to fabricate the nanostructured R-Fe2O3 with different sizes and shapes because of its strong size- and shape-dependent properties. Over the past decades, R-Fe2O3 with various nanostructures, such as zero-dimensional nanostructures (such as nanoparticles,9 nanocrystals,10 and nanocubes11), one-dimensional nanostructures (nanorods,12 nanowires,13 nanobelts,14 and nanotubes15), two-dimensional flake-like nanostructures,16 and three-dimensional hierarchical nanostructures17 have been fabricated by various synthetic techniques, including sol-gel process, gas-solid growth route, two-step reaction process, hydrothermal approach, high ergergy ball milling, chemical precipitation, microwave heat method, thermal oxidation at high temperature, etc. Hydrothermal synthesis is an attractive technique among these methods with several advantages: (i) effective control of size, morphology, and degree of agglormeration of the particles, (ii) incorporation of fewer impurities in the hydrolyzed product, (iii) relatively low reaction temperature, (iv) a cost-effective route, and (v) an environmentally benign route. Recently, Tang et al.18 have successfully fabricated hollow hematite spindles and microspheres using FeCl3, oxalic acid, and bases as reactants at first, and then calcined the precursors in air at 400 °C for 2 h. Yang et al.19 have reported the synthesis of hollow sea urchin-like R-Fe2O3 nanostructures and R-Fe2O3 nanocubes by an anions-assisted hydrothermal process with subsequent cal* To whom correspondence should be addressed. Phone: +86-2223507951. Fax: +86-22-23502458. E-mail: [email protected].

cination. These synthesis processses seem to be inconvenient because calcinating precursors at elevated temperature is necessary. Zhu and his co-workers20 have realized the shapecontrollable synthesis of R-Fe2O3 by changing the concentration of PO43- anions under hydrothermal conditions. Unfornately, they were not completely removed and remained on the final products although the additives are very effective in aspect of morphology control. Therefore, it is meaningful to develop a simple, one-step, and free-template hydrothermal route to synthesize nanostructured R-Fe2O3 with various sizes and shapes. In this study, we conduct the size- and shape-controlled synthesis of R-Fe2O3 via a free-template, environmentally benign route at 180 °C with different reaction time and solvent. The as-prepared samples are nanoparticles, nanopolyhedra, and nanoparticles-aggregated microcubes, respectively. Furthermore, the magnetic properties of the as-obtained R-Fe2O3 samples strongly depend on their size and shape. And, electrochemical experiments show different electrochemical behaviors toward lithium storage for the three R-Fe2O3 samples. It is found that the particle size and shape also has a remarkable effect on the lithium insertion/extraction behavior. This study provides a new facile controlled synthesis of R-Fe2O3 with various sizes and shapes, as well as documents size- and shape-dependent properties for R-Fe2O3. 2. Experimental Section 2.1. Synthesis of r-Fe2O3 with Various Sizes and Shapes. In a typical synthesis, 0.27 g of FeCl3 · 6H2O was first dissolved into 15 mL of ammonia-water (commercial ammonia-water (w, 25%) and distilled water with a volume ratio of 1:2) under mild stirring. The brown precipitate floccules immediately formed, then were transferred into a 20 mL Teflon-lined stainless steel autoclave, which was sealed and then heated to 180 °C. After autoclaving at 180 °C for 8 h, the resulting red product was centrifuged, rinsed with distilled water, and finally dried at 60 °C under vacuum. The obtained red solid products were collected for the following experiments and characterizations. For the synthesis of R-Fe2O3 nanoparticles and nanoparticlesaggregated R-Fe2O3 microcubes, the synthetic procedure was the same as the procedure described above except for different

10.1021/jp102243g  2010 American Chemical Society Published on Web 06/01/2010

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Figure 1. XRD patterns of as-prepared samples: (a) nanoparticles, (b) nanopolyhedra, and (c) nanoparticles-aggregated microcubes. The standard diffraction lines of bulk rhombohedral Fe2O3 (JCPDS card 87-1165) are given at the bottom of the lower frame.

reaction solvent and reaction time. For the synthesis of R-Fe2O3 nanoparticles, the solvent used was distilled water, and the reaction time was 8 h. While the reaction time was prolonged to 24 h, nanoparticles-aggregated R-Fe2O3 microcubes could be obtained. 2.2. Characterization. The samples of as-prepared R-Fe2O3 nanostructures were characterized by X-ray powder diffraction (XRD) with a Rigaku D/max Diffraction System, using a Cu KR source (λ ) 0.15406 nm). Scanning electron microscopy (SEM) images were taken with a JEOLJSM-6700F field emission scanning electron microscope (15 kV). High-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL-2010 TEM at an acceleration voltage of 200 kV. 2.3. Magnetic and Electrochemical Measurements. The magnetic properties of R-Fe2O3 were measured at 300 K in the applied magnetic field sweeping from -10 to 10 kOe with a vibrating sample magnetometer. The electrochemical Li intercalation performance was investigated in Li test cells for the R-Fe2O3 samples with various morphologies. The R-Fe2O3 was mixed with acetylene black and polytetrafluoroethylene (PVdF) with a weight ratio of 75: 15:10 in ethanol to ensure homogeneity. After ethanol was evaporated, the mixture was rolled into a sheet and cut into circular strips of 8 mm diameter. The strips were then dried at 120 °C for 10 h in air. Lithium metal was used as the counter and reference electrodes. The electrolyte was composed of a 1 M LiPF6 solution in ethylene carbonate (EC)/dimethyl carbonate (DMC) with a weight ratio of 1:1. The above three parts were assembled into test cells in an argon-filled dry glovebox, and then the cells were measured at a current density of 50 mA/g within a voltage range of 0.01-3.0 V with a Land CT2001 battery tester. 3. Results and Discussion 3.1. Characterization. The purity and crystallinity of asprepared samples were examined by XRD technique (Figure 1). Figure 1, patterns a-c, show the XRD patterns of the asprepared R-Fe2O3 nanoparticles, nanopolyhedra, and nanoparticles-aggregated microcubes, respectively. It is evident that all of the expected peaks can be indexed to the rhombohedral structure of R-Fe2O3 [space group: R3cj(167)] with structural parameters of a ) b ) 5.035 Å, c ) 13.75 Å, R ) β ) 90°, and γ ) 120°, which are in good agreement with the reference data (i.e., JCPDS Card 87-1165). The narrow sharp peaks suggest that the R-Fe2O3 samples are highly crystalline. No other peaks are observed, indicating high purity of the as-prepared samples.

Ma et al. The size and shape of the as-prepared samples were examined with SEM, TEM, and HR-TEM techniques, respectively. When ferric chloride hydrolysized in the ammonia-water, R-Fe2O3 nanopolyhedra (Figure 2a,b) could be obtained. The product consisted of different nanopolyhedra with dominant octahdra nanocrystals. Three typical octahedra are shown in the inset in Figure 2b. These polyhedra have a size of about 90-110 nm. Figure 2c shows a typical HR-TEM image of one surface of polyhedron nanocrystal. As shown in the inset in Figure 2c, the lattice interplanar spacing is measured to be 0.27 nm, corresponding to the (104) plane of rhombohedral Fe2O3. The clear lattice fringes in the HRTEM image indicate that the nanopolyhedra were well crystalline. Irregular R-Fe2O3 nanoparticles were obtained under hydrothermal condition at 180 °C for 8 h (Figure 2d). The particles were mainly composed of irregular nanoparticles of about 80 nm in diameter (inset in Figure 2d), which finally congregated into microcubes (Figure 2e). The HRSEM image (Figure 2f) indicates that the size of the cubes is about 1 µm in edge and the surface of microcubes, which are composed of nanoparticles (inset in Figure 2f). 3.2. Influences of Reaction Parameters. Final morphology of nanomaterials is determined not only by thermodynamics but also by kinetics.21 In this study, it was found that reaction time and solvent used play crucial roles in determining morphologies of final products. The formation of nanoparticles-aggregated R-Fe2O3 microcubes can be well explained by thermodynamics. This is due to the fact that the surfaces of the fresh nanoparticles were not passivated by the stabilizing reagent and thus were highly activated.22 Consequently, the R-Fe2O3 nanoparticles were not stable due to their high surface energy and were inclined to aggregate to eliminate the interfaces and reduce the total energy of the system. The reduction in surface energy is the driving force for their aggregation. This process is thermodynamically favorable because the surface energy and the total energy of the system are substantially reduced when the interfaces are eliminated.23 Moreover, acid solution environment may be another reason for the aggregation of nanoparticles. When the R-Fe2O3 nanoparticles (Figure 2d) were formed, the pH of reaction media reached 0.5-1. Strong hydrogen interaction between R-Fe2O3 nanoparticles could lead to fresh nanoparticles to aggregate into the well-defined microcubes. In contrast, the nanopolyhedra did not aggregate in alkaine media (pH, 10-10.5). As a result, the nanoparticles spontaneously aggregated into the well-defined R-Fe2O3 microcubes in acidic media. In the experiments, the formation of the R-Fe2O3 nanopolyhedra was found to strongly depend on the effects of the ammonia-water. When FeCl3 · 6H2O was dissolved into ammonia-water, the brown floccules immediately formed. The morphology of the brown floccules before being subjected to the hydrothermal process was shown to be aggregates of nanoparticles, which indicates that the subsequent hydrothermal process involves a process of recrystallization (shown in Figure S1, Supporting Information). This similar phenomenon in Cd(OH)2 nanowires has been reported by Tang et al.24 Furthermore, it was found that R-Fe2O3 nanopolyhedra (shown in Figure S2, Supporting Information) could be produced in different concentrations of ammonia-water (commercial ammonia-water and distilled water with varying volume ratio from 1:14 to 7:8). We did not conduct experiments with further enhancing the concentration of ammonia-water, because there is potential danger due to high vapor pressure. Therefore, the presence of ammonia-water is one of the important factors influencing the recrystallization process and the growth of R-Fe2O3 nanopolyhedra, although the concentration of ammonia-water has little

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Figure 2. (a) SEM, (b) TEM, and (c) HRTEM images of R-Fe2O3 nanopolyhedra; (d) SEM image of R-Fe2O3 nanoparticles; (e and f) SEM images of nanoparticles-aggregated R-Fe2O3 microcubes.

effect on the morphology of the final product. Herein, two roles of ammonia-water should be mentioned. On one hand, the ammonia-water not only serves as the alkali source but also provides a buffering solution as the reaction processes, which produces the reaction under a relative stable pH (10-10.5). On the other hand, ammonia can significantly decrease the viscosity of the solution, which in turn increases the mobility of the components in the system. Faster ionic motion usually ensures a reversible pathway between the fluid phase and solid phase and allows ions to adopt correct positions in developing crystal lattices.25 Thus, addition of ammonia-water provides a favorable environment for the growth of nanopolyhedra in our system. In addition, it should be noted that the morphology of nanopolyhedra did not further change with reaction time increasing. To clearly demonstrate the formation of various R-Fe2O3 nanostructures, a schematic illustration of the growth process of R-Fe2O3 nanostructures is given in Scheme 1. 3.3. Magnetic Properties for r-Fe2O3 Nano-/Microstructures. The magnetic properties of a material are sensitive to its size and shape due to the dominating role of anisotropy in magnetism.26 The shape of magnetic materials is unambiguously one of the crucial factors controlling their magnetic behavior.27 Recently, some work has focused on controlling the size and shape of nanostructured R-Fe2O3 to understand their size- and

shape-dependent magnetic properties.6 To investigate the effects of particle size and shape on the magnetic properties, we carried out preliminary magnetic hysteresis measurements for the three R-Fe2O3 with various sizes and shapes. Figure 3 shows the room temperature hysteresis loops of the as-synthesized R-Fe2O3 with various nanostructures (nanoparticles, nanopolyhedra, and nanoparticles-aggregated microcubes). No saturation of the magnetization as a function of the field is observed up to the maximum applied magnetic field for all three samples. The magnetizations at the maximum applied magnetic field of 1 T (Mmax) are 0.4307, 0.4389, and 0.3672 emu/g for nanoparticles, nanopolyhedra, and nanoparticles-aggregated microcubes, respectively. In addition, the three R-Fe2O3 samples have significantly different magnetic behaviors, although they all exhibit a slight hysteresis feature. Compared with those of nanoparticles and nanopolyhedra, nanoparticles-aggregated microcubes have wide-open M-H loops with a higher coercivity (Hc, 3756 Oe) (Figure 3c). Reported by Fu et al.28 and Zheng et al.,12e the assembly of the small and oriented subparticles into the microcubes results in the change of the single domain to the multidomain, leading to the higher coercivity. On the other hand, compared with nanoparticles (Hc, 666.3 Oe), the R-Fe2O3 nanopolyhedra exhibits a smaller coercivity (Hc, 345 Oe) and weak hysteresis loop, which may be the result of its

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SCHEME 1: Schematic Illustration of the Formation Process of r-Fe2O3 Nanostructures with Various Sizes and Shapes

larger size.29 In addition, the shape anisotropy of the R-Fe2O3 nanopolyhedra may be another reason for the smaller coercivity and weak hysteresis loop. Further work should be done to research the difference of the magnetic properties and influencing factors in these R-Fe2O3 samples. 3.4. Electrochemical Performance toward Lithium Storage. It is well-known that the lithium intercalation performance is related to the intrinsic crystal structure, where the lithium ions can intercalate into interlayer, tunnels, and holes in the crystal structure.30 Thackeray et al.31 first explored the structural changes of R-Fe2O3 during lithium insertion and predicted the potential application for lithium ion batteries. Recently, Xie et al.7a have well explained that hematite (R-Fe2O3) could be applied to insert/extract lithium ions from the viewpoint of crystal structure. Up to now, some work has also been done to investigate the influence of R-Fe2O3 with various sizes and shapes on electrochemical performance.8 To better understand the particle size and shape effects on the electrochemical properties of the three R-Fe2O3 nano-/microstructures, we carried out a preliminary investigation of their electrochemical performance with respect to Li insertion/extraction in this work. Figure 4 shows the first cycle discharge-charge voltage profiles for the three R-Fe2O3 samples with different sizes and shapes on the first cycle with a cutoff voltage of 0.01 V at a current density of 50 mA/g. It can be seen that there are two obvious flat discharge plateaus (1.05 and 0.85 V vs Li+/Li) for the lithium reaction with R-Fe2O3 nanopolyhedra and nanoparticles, which are similar to those of the R-Fe2O3 nanoparticles and nanotubes.32 However, there is only a typical flat discharge

plateau (0.85 V vs Li+/Li) for nanoparticles-aggregated R-Fe2O3 microcubes (Figure 4). In the enlarged profile below 60 mAh/g (inset of Figure 4), there is one short plateau I′ (1.5-1.6 V vs Li+/Li) for the three samples. As reported about plateau I′ in the literature,32a a small amount of lithium can be inserted into the crystal structure of R-Fe2O3 before the structural transformation of the close-packed anionic array from hexagonal to cubic stacking occurs. Furthermore, the inserted amount of lithium is markedly influenced by the particle size. In the present study, only a small degree of Li insertion (0.04 mol for plateau I′) into nanoparticles-aggregated microcubes (ca. 1 µm in size) was observed. In contrast, in the case of both R-Fe2O3 nanoparticles (ca. 80 nm in size) and nanopolyhedra (ca. 90-110 nm in size) a substantially higher amount of 0.12 mol of Li (plateau I′) can be inserted. For plateau I, the inserted amount of lithium is also strongly affected by particle size, which is similar to that of plateau I′. The inserted amount of lithium for nanoparticlesaggregated R-Fe2O3 microcubes was not detected. As shown in Figure 4, there is a long plateau II at approximately 0.85 V, which corresponds to the reversible reaction between cubic Li2Fe2O3 and Fe. Although the length of plateau II for the nanoparticles-aggregated microcubes is larger than that of plateau II for nanoparticles and nanopolyhydra, the three plateau capacities (I′ + I + II) observed in the first discharge curves for all three R-Fe2O3 nanostructures are close to a theoretical capacity of 1007 mAh/g, which are associated with the following conversion reaction:Fe2O3 + 6Li+ 6e f 3Li2O + 2Fe. This result is similar to the other reports.8e,32a The following sloping

Figure 3. Magnetic hysteresis loops of as-prepared samples measured at room temperature: (a) nanoparticles, (b) nanopolyhedra, and (c) nanoparticles-aggregated R-Fe2O3 microcubes.

Figure 4. First charge-discharge curves of R-Fe2O3 with different morphologies: (a) nanoparticles, (b) nanopolyhedra, and (c) nanoparticles-aggregated microcubes at a current density of 50 mA/g.

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Figure 5. Cycling performance of R-Fe2O3 with different morphologies: (a) nanoparticles, (b) nanopolyhedra, and (c) nanoparticlesaggregated microcubes.

region III is the biggest different part in the discharge curves for the three R-Fe2O3 nanostructures. This process might be attributed to the formation of a surface polymeric layer, which in general contributes to additional capacity. For nanopolyhedra, the initial discharge capacity is 1660 mAh/g, corresponding to 9.9 Li per R-Fe2O3, respectively. For nanoparticles and nanoparticles-aggregated microcubes, the first discharge capacity is 1438 and 1378 mAh/g, corresponding to 8.57 and 8.2 Li per R-Fe2O3, respectively. As can be seen, the difference of the lithium storage ability for the three R-Fe2O3 nano-/microstructures is mainly attributed to the different capacity in the region (0.01-0.8 V), which is closely related to the size, shape, and surface area of the samples.33 Figure 5 shows the cycle performance of the R-Fe2O3 samples with different size and shape at a current density of 50 mA/g. After 20 cycles, the reversible capacity for nanopolyhedra is still as high as 668 mAh/g while that for nanoparticles and microcubes is only 405 and 556 mAh/g, respectively. All of them are much higher than the theoretical specific capacity of currently used graphite (LiC6, 372 mAh/g). On the basis of all the above results, it is evident that the R-Fe2O3 nanopolyhedra are superior among the three samples with the highest lithium storage capacity, lowest irreversible loss, and excellent cycling performance. In view of their excellent lithium storage properties and simplicity in synthesis, these R-Fe2O3 nanopolyhedra will be of interest for next-generation lithium ion batteries. 4. Conclusion In conclusion, we have developed a facile template-free synthetic route to the controlled fabrication of various R-Fe2O3 nanostructures, such as nanoparticles, nanopolyhedra, and nanoparticles-aggregated microcubes, by simply controlling the synthesis parameters such as reaction time and solvent. Magnetic hysteresis measurements demonstrate that the as-obtained R-Fe2O3 nanostructures show structure-dependent magnetic properties. In addition, the as-obtained R-Fe2O3 nanopolyhedra exhibits ultrahigh reversible capacity, and excellent capacity retention over 20 cycles. We expect that there are good prospects for using R-Fe2O3 nanopolyhedra as anode materials in lithium ion cells. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 20571044 and 20971070). We also thank Dr. Liao Chang from the University of Southampton for many useful discussions about magnetics of R-Fe2O3 nanostructures. Supporting Information Available: Figure showing the morphology evoluation of R-Fe2O3 nanopolyhedra samples with different times and images of R-Fe2O3 samples synthesized in

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