ARTICLE pubs.acs.org/JPCC
Carbon Nanocapsules as Nanoreactors for Controllable Synthesis of Encapsulated Iron and Iron Oxides: Magnetic Properties and Reversible Lithium Storage Ping Wu, Ning Du, Hui Zhang, Jingxue Yu, and Deren Yang* State Key Lab of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: This paper reports the synthesis of carbon-encapsulated Fe3O4, γ-Fe2O3, and Fe nanostructures using FeOOH as a precursor and carbon nanocapsules (CNCs) as nanoreactors via controllable thermal transformation processes. The magnetic property investigation reveals that the as-synthesized Fe3O4-CNCs, γ-Fe2O3-CNCs, and Fe-C nanostructures all exhibit ferromagnetic behavior with quite high saturation magnetizations. Moreover, the Fe3O4-CNCs with a controlled carbon coating are prepared and their comparative lithium storage properties are investigated. It is found that optimized Fe3O4-CNCs exhibit high capacity and good cycling performance.
1. INTRODUCTION Nanoscopic reactions that exhibit a unique mechanism and high activity have been widely explored in the past decades.1,2 Various organic and inorganic nanostructures, such as protein cages,3,4 mesoporous silica,5 alumina,6 and carbon,7-10 which contain inner hollow cavities, have been proposed as nanoreactors due to the confinement effect. Among them, tubular carbon nanostuctures, for example, carbon nanotubes (CNTs) and CNCs, are of particular interest due to the unique inner environment. In addition, carbon can act as a reducing agent and participate in the nanoscopic reactions, making the desired reactions easier to take place.7,8 For example, FexB,7 SiC,8 Mg3N2,9 and Co3O410 nanostructures have been successfully synthesized using CNTs as nanoreactors. Iron (Fe) and iron oxides (Fe3O4 and γ-Fe2O3) have been intensively studied due to their potential applications in catalysts,11 ferrofluids,12 Li-ion batteries,13 and various biomedical fields.14,15 However, one of the major obstacles is the structural variation and morphological change of nanoiron or nanoiron oxides due to oxidation by oxygen or erosion by acid. Up to now, much effort has been focused on the development of iron or iron oxides-based core-shell nanostructures with a protecting layer, such as polyaniline,16 polystyrene,17 silica,18,19 and Au,20,21 to solve the above-mentioned problems somehow. Compared to the polymer, silica, or Au layer, the carbon layer has many advantages, such as higher chemical and thermal stability, better conductivity, and higher biocompatibility. Therefore, carbon-encapsulated Fe, Fe3O4, and γ-Fe2O3 nanosturctures are more attractive. For example, Fe-CNT nanocomposites exhibit a higher FischerTropsch synthesis activity and excellent microwave-adsorption characterisics.22,23 Fe3O4-C nanocomposites show high reversible capacity and enhanced cycling performance as superior anode materials for lithium-ion batteries.24-28 r 2011 American Chemical Society
Up to now, extensive investigations have been carried out for the fabrication of carbon-encapsulated iron and iron oxide nanostructures.29-32 However, the synthetic processes often face the disadvantages of high temperatures, pressure, and harsh conditions, which restrict their practical applications.29-32 It is desirable to obtain these nanostructures through more facile and economic processes. Moreover, despite the successful synthesis of Fe-C29,30 or Fe3O4-C31,32 nanostructures, to the best of our knowledge, there has been no report about the controllable synthesis of carbon-encapsulated Fe, Fe3O4, and γ-Fe2O3 nanostructures from an individual precursor. Herein, we reported the synthesis of carbon-encapsulated Fe3O4, γ-Fe2O3, and Fe nanostructures (Fe3O4-CNCs, γFe2O3-CNCs, and Fe-C nanostructures) using FeOOH as a precursor and CNCs as nanoreactors via controllable thermal transformation processes. The magnetic properties of Fe3O4CNCs, γ-Fe2O3-CNCs, and Fe-C nanostructures were also evaluated. Moreover, the Fe3O4-CNCs with a controlled carbon coating were prepared, and their comparative lithium storage properties were investigated.
2. EXPERIMENTAL SECTION 2.1. Synthesis of FeOOH-CNCs. All the chemicals were analytical grade without further purification. FeOOH nanocapsules were prepared by a hydrothermal route described elsewhere.33 FeOOH-CNCs were synthesized by a simple glucose hydrothermal process.34-36 Briefly, 0.4 g of FeOOH nanocapsules were dissolved in 40 mL of 0.25 M aqueous glucose Received: November 30, 2010 Revised: January 8, 2011 Published: February 11, 2011 3612
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Figure 1. FESEM image (a) of pure FeOOH nanocapsules and morphological and structural characterizations of FeOOH-CNCs (12 h, 0.25 M glucose): (b) FESEM image, (c) TEM image, and (d) XRD patterns of pure FeOOH nanocapsules (curve a) and FeOOH-CNCs (curve b).
solution (concentrations in the range of 0.1-0.5 M were investigated). After sonication for 30 min, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed, and maintained at 180 °C for 12 h. After the reaction was finished, the resulting solid products were centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final products, and finally dried at 60 °C in air. 2.2. Synthesis of Fe3O4-CNCs, γ-Fe2O3-CNCs, and Fe-C Nanostructures. Fe3O4-CNCs and γ-Fe2O3-CNCs were prepared by thermal transformation of the FeOOH-CNCs at 400 and 500 °C for 3 h under an Ar atmosphere, respectively. When the annealing temperature was 600 °C, hollow CNCs and Fe-C nanostructures were obtained. 2.3. Characterization. The obtained samples were characterized by X-ray powder diffraction (XRD) using a Rigaku D/maxga X-ray diffractometer with graphite monochromatized Cu KR radiation (γ = 1.54178 Å). The morphology and structure of the samples were examined by field emission scanning electron microscopy (FESEM, Hitachi S-4800), transmission electron microscopy (TEM, PHILIPS CM200UT), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010). The Fourier transform infrared (FT-IR) spectra were carried out on an Avatar 360 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on an AXIS-Ultra instrument from Kratos Analytical, using monochromatic Al KR radiation (225 W, 15 mA, 15 kV) and low-energy electron flooding for charge compensation. Magnetization measurements were carried out using a physical property measurement system (PPMS-9, Quantum Design). Thermogravimetric analysis (TGA) was tested on an SDT Q600 V8.2 Bulid 100. 2.4. Electrochemical Measurements of Fe3O4-CNCs. Electrochemical measurements were carried out using two-electrode cells with lithium metal as the counter and reference electrodes. The working electrodes were composed of the active material
(Fe3O4-CNCs), conductive materials (acetylene black, AB), and binder (polyvinyldifluoride, PVDF) in a weight ratio of Fe3O4CNCs/AB/PVDF = 70:15:15, and pasted on Cu foil. The electrolyte solution was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) with the volume ratio of EC/PC/DEC = 3:1:1. The cell assembly was performed in a glovebox filled with pure argon (99.999%) in the presence of an oxygen scavenger and a sodium drying agent. The electrode capacity was measured by a galvanostatic discharge-charge method at a current density of 100 mA 3 g-1 at 20 °C. Charge-discharge cycles were tested with a current density of 100 mA 3 g-1 in the potential range of 0.01-3 V.
3. RESULTS AND DISCUSSION FeOOH-CNCs were synthesized by a simple glucose hydrothermal process. Figure 1 shows the morphological and stuctural characterizations of FeOOH-CNCs (12 h, 0.25 M glucose). As can be seen, the capsule-like shape is retained after coating of a carbon-rich layer (Figure 1b) compared with the naked FeOOH nanocapsules (Figure 1a). The TEM image (Figure 1c) indicates that the uniform coating layer is smooth and continuous and the thickness of the layer is about 5-10 nm. The hydroxyl groups on the surface of FeOOH nanocapsules facilitate the redox reaction, hence leading to the uniform carbonaceous layer.34 Figure 1d shows the XRD patterns of pure FeOOH nanocapsules (curve a) and FeOOH-CNCs (curve b). It can be seen from curve a that all of the diffraction peaks can be indexed to tetragonal structured FeOOH, consistent with the values in the standard card (JCPDS: 75-1594). After surface coating, the diffraction peaks become much weaker due to the existence of an amorphous carbonaceous layer. Hydrothermal carbonization of glucose has been widely employed as a facile route for coating a carbonaceous layer on 3613
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Figure 2. TEM images of FeOOH-CNCs prepared with different hydrothermal conditions: (a) 2 h, 0.25 M glucose; (b) 4 h, 0.25 M glucose; (c) 12 h, 0.1 M glucose; and (d) 4 h, 0.5 M glucose.
various nanostructures (such as nobel metals,35 silica,37 Te,38 and SnO239-41) by different groups. However, the formation of the carbonaceous layer is quite complex, and the optimized hydrothermal conditions for various nanostructures are different.41 Figure 2a,b shows TEM images of FeOOH-CNCs prepared with different hydrothermal times while keeping other conditions identical (0.25 M glucose). It can be seen that the thickness of the carbonaceous layer is about 1-2 nm with a 2 h hydrothermal reaction (Figure 2a). When the reaction time was prolonged to 4 and 12 h, the thickness of carbonaceous layer increased to 2-3 and 5-10 nm, respectively (Figures 2b and 1c). In addition, the effect of glucose concentrations (0.1 and 0.5 M) was also investigated. Only a very thin carbonaceous layer is obtained with a glucose concentration of 0.1 M even if the hydrothermal time is increased to 12 h, as shown in Figure 2c. When the glucose concentration is increased to 0.5 M, FeOOH nanocapsules are partially corroded due to the increased acid environment even if at a short hydrothermal time (4 h, Figure 2d). Therefore, the thickness of the carbonaceous layer can be easily manipulated by controlling the hydrothermal time and the concentrations of glucose. Fe3O4-CNCs, γ-Fe2O3-CNCs, and Fe-C nanostructures can be synthesized by using FeOOH as a precursor and CNCs as nanoreactors via controllable thermal transformation processes. Several advantages can be expected from this synthetic approach. First, CNCs are introduced by surface coating on FeOOH, which can avoid the difficulty of filling precursors into nanoreactors, such as reported before. Second, FeOOH is inexpensive, nontoxic, and easy to be synthesized. Third, Fe3O4-CNCs, γ-Fe2O3CNCs, and Fe-C nanostructures can all be obtained through simple thermal transformation processes of FeOOH-CNCs. Fe3O4-CNCs (black product) was prepared by thermal transformation of FeOOH-CNCs at 400 °C for 3 h under an Ar atmosphere. During the thermal treatment process, the carbonaceous layer of FeOOH-CNCs was carbonized and the inner
FeOOH was reduced to Fe3O4 by the outer carbon layer. In addition, the outer carbon layer can preserve the capsule-like morphology of FeOOH-CNCs, which can be verified from the TEM image of the product (Figure 3a). Figure 3c (curve a) shows the XRD pattern of the black product. The observed crystalline phases can be assigned to cubic Fe3O4 (magnetite, JCPDS: 19-0629), whereas the carbon shell is not well-crystallized. Furthermore, when the annealing temperature was increased to 500 °C, the product color changed from black to brown (γ-Fe2O3-CNCs) while the capsule-like morphology was retained (Figure 3b). In this temperature, carbon is not able to reduce Fe(III) in the precursor to metallic Fe,42,43 and γ-Fe2O3 seems to be more stable than other types of iron oxides.44 Therefore, the precursor FeOOH can fully transform into γFe2O3 and exist stably in this type in our experiments. The XRD pattern (Figure 3c, curve b) of the brown product agrees well with cubic γ-Fe2O3 (maghemite, JCPDS: 39-1346), which is very close to that of Fe3O4 (Figure 3c, curve a). The three peaks between 50° and 70° from the brown product shift slightly to higher angles (Figure 3c, inset), and there are three additional peaks in the lower-angle region resulting from the (110), (210), and (211) planes of simple cubic of γ-Fe2O3.45 IR spectroscopy and XPS analysis were employed to further characterize the products. It can be seen from Figure 3d (curve a, FeOOH-CNCs) that the peaks at 1700 and 1625 cm-1 can be attributed to CdO and CdC vibrations, respectively, indicating aromatization of glucose during hydrothermal treatment.35,37 After thermal transformation processes, the carbonaceous layer from FeOOH-CNCs has been fully carbonized to a carbon layer, leading to the disappearance of CdO vibrations peaks, which can be confirmed from curve b (Fe3O4-CNCs) and curve c (γFe2O3-CNCs). Moreover, Fe3O4 and γ-Fe2O3, which have the same cubic inverse spinel structure, can be described as (Fe3þ)A(Fe3þFe2þ)BO42- and (Fe3þ)A(Fe3þ5/3Δ1/3)BO42-, respectively, where Δ stands for a vacancy and labels A and B 3614
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Figure 3. Morphological, structural, and compositional characterizations of Fe3O4-CNCs and γ-Fe2O3-CNCs: (a) TEM image of Fe3O4-CNCs; (b) TEM image of γ-Fe2O3-CNCs; (c) XRD patterns of Fe3O4-CNCs (curve a) and γ-Fe2O3-CNCs (curve b); (d) infrared spectra of FeOOH-CNCs (curve a), Fe3O4-CNCs (curve b), and γ-Fe2O3-CNCs (curve c); (e) enlarged infrared spectrum of Fe3O4-CNCs; and (f) enlarged infrared spectrum of γ-Fe2O3-CNCs.
denote tetrahedral (Td) and octahedral (Oh) sites, respectively. γ-Fe2O3 differs from Fe3O4 by the presence of vacancies within Oh sites. Therefore, IR analysis, which shows high sensitivity to vacancy ordering, can easily differentiate Fe3O4 from γ-Fe2O3. The IR spectra of Fe3O4-CNCs and γ-Fe2O3-CNCs in 850-450 cm-1 are given in Figure 3e,f, respectively. Only one peak at about 570 cm-1 can be observed from Figure 3e, which is characteristic of stoichiometric Fe3O4.46 Multipeaks can be observed from Figure 3f, which is characteristic of partially ordered γ-Fe2O3.46,47 Figure 4 shows the XPS spectra of Fe3O4-CNCs (curve a) and γ-Fe2O3-CNCs (curve b). As observed from Figure 4a, iron, oxygen, and carbon elements are expected from the composition of iron oxides and CNCs, respectively. The XPS of the regions of Fe 2p is shown in Figure 4b. Only two peaks (710.6 and 724.6 eV) can be observed from Fe3O4-CNCs (curve a), which can be assigned to Fe 2p3/2 and Fe 2p1/2 for Fe3O4.48,49 In contrast, the Fe 2p3/2 and Fe 2p1/
in γ-Fe2O3-CNCs (curve b) are accompanied by satellite structures on their high-binding-energy side. The satellite peak at about 719 eV is a characteristic peak of the ferric cation, suggesting that the product is γ-Fe2O3 with high purity.48-50 Therefore, the XRD patterns, IR spectra, and XPS spectra of the products give enough evidence for the successful selective synthesis of Fe3O4-CNCs and γ-Fe2O3-CNCs. Hollow carbon shells can be obtained by a chemical method31 or by thermal treatments at high temperatures34,39 to remove the inner cores from carbon-encapsulated materials. Herein, hollow CNCs and Fe-C nanostructures were obtained when FeOOHCNCs was annealed at 600 °C. As can be seen from Figure 5a, there are some bigger congeries besides CNCs, indicating that the morphology of FeOOH-CNCs has been destroyed. Figure 5b shows the XRD pattern of the products. A mixed phase of cubic Fe together with graphite appears besides a little Fe3C and iron oxides as byproducts. Normally, reduction of
2 peaks
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Figure 4. XPS spectra of the Fe3O4-CNCs (curve a) and γ-Fe2O3-CNCs (curve b): (a) survey spectrum and (b) multiplex spectrum.
Figure 5. Morphological, structural, and compositional characterizations of Fe-C nanostructures and hollow CNCs: (a) FESEM image, (b) XRD pattern, (c) TEM image of hollow carbon nanocapsules, (d) TEM image, (e) EDX spectrum, and (f) magnified TEM image of Figure 4d. 3616
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Figure 6. Room-temperature magnetization curves of Fe3O4-CNCs (curve a) and γ-Fe2O3-CNCs (curve b).
Figure 7. Room-temperature magnetization curves of Fe-C nanostructures.
FeOOH to Fe and crystallization of CNCs need high temperatures (>1100 °C). Rencently, Chen et al. investigated a lowtemperature (600 °C) synthesis of CNT-Fe capsules from a CNTs-confined R-Fe2O3 system.42 Cao et al. pointed out that reduction of Fe3O4 to Fe in a CNTs-confined Fe3O4 system happened at a temperature between 532 and 570 °C.43 The reduction of reaction temperatures benefits from the CNTsconfined environment with unique electronic properties.42,43 In our experiments, the reduction of FeOOH to Fe takes place at a relatively low temperature (600 °C) due to the high reaction activity in the FeOOH-CNCs system, which further demonstates the superiority of CNCs as nanoreactors. Unlike Fe3O4 and γFe2O3, Fe tends to form congeries instead of preserving the morphology of FeOOH-CNCs, remaining hollow CNCs, which can be confirmed by TEM images (Figure 5c,d). Moreover, the magnified TEM image (Figure 5f) indicates that Fe exsists in the form of carbon-coated nanostructures. It can be seen from the EDX spectrum (Figure 5e) that there are only strong peaks for Fe and C elements, which further confirms the formation of iron instead of iron oxides. The magnetic properties of the Fe3O4-CNCs, γ-Fe2O3CNCs, and Fe-C nanostructures were investigated at room temperature (Figures 6 and 7). As can be seen, Fe3O4-CNCs (Figure 6, curve a) exhibits a typical ferromagnetic behavior, with a saturation magnetization, Ms = 48.9 emu/g; remnant magnetization, Mr = 15.6 emu/g; and coercive field, Hc = 179.4 Oe. The Ms of Fe3O4-CNCs is lower than that of bulk Fe3O4 (92 emu/ g)51 but much higher than those of previously reported Fe3O4-C
nanowires (5.11 emu/g)31 and coaxial nanofibers (27.5 emu/ g).32 The reduction in the value of Ms could be due to the existence of nonmagnetic CNCs31,32 and the high shape anisotropy of Fe3O4-CNCs.52 In addition, the Hc value is much higher than that of bulk Fe3O4 (
