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Synthesis of unit-cell-thick α-Fe2O3 nanosheets and their transformation to γ-Fe2O3 nanosheets with enhanced LIB performances
Accepted Manuscript Synthesis of unit-cell-thick α-Fe2O3 nanosheets and their transformation to γFe2O3 nanosheets with enhanced LIB performances Ying Jin, Liyun Dang, Hao Zhang, Chuang Song, Qingyi Lu, Feng Gao PII: DOI: Reference:
S1385-8947(17)30916-6 http://dx.doi.org/10.1016/j.cej.2017.05.155 CEJ 17054
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
28 January 2017 25 May 2017 25 May 2017
Please cite this article as: Y. Jin, L. Dang, H. Zhang, C. Song, Q. Lu, F. Gao, Synthesis of unit-cell-thick α-Fe2O3 nanosheets and their transformation to γ-Fe2O3 nanosheets with enhanced LIB performances, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.05.155
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Synthesis of unit-cell-thick α-Fe2O3 nanosheets and their transformation to γ-Fe2O3 nanosheets with enhanced LIB performances Ying Jin,1,3 Liyun Dang,1 Hao Zhang,1 Chuang Song,2 Qingyi Lu,1* Feng Gao2* 1
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering;
Collaborative Innovation Center of Advanced Microstructures, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China, E-mail: [email protected] 2
, Department of Materials Science and Engineering, Collaborative Innovation Center of Advanced
Microstructures, Nanjing University, Nanjing 210093, P. R. China, E-mail: [email protected] 3
College of Biological and Chemical Engineering, Anhui Polytechnic University, Wuhu 241000,
Anhui P. R. China
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Abstract The synthesis of ultrathin magnetic nanosheets is significant for the development of next-generation nanodevices but remains to be a big challenge. Herein, we present facile approaches for large-scaled production of cell-unit-thick single-crystalline iron oxide nanostructures. The ultrathin hematite (α-Fe2O3) nanosheets with unit-cell thickness of 1.3 nm are firstly synthesized using a metal-ion-intervened hydrothermal method. Then the α-Fe2O3 nanosheets are converted to magnetic maghemite (γ-Fe2O3) nanosheets with unit-cell thickness of 0.8 nm by a simple calcination process. The Li-ion battery studies reveal that unit-cell-thick γ-Fe2O3 nanosheets exhibit great enhanced electrochemical performances with high specific capacitance approaching to its theoretical value and excellent cycling stability. Keywords: Hematite; Maghemite; Nanosheets; Unit cell thickness; Anode materials
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Introduction Dimensionality is one of the crucial parameters in nanoscience; nanomaterials with same chemical composition but with different dimensionality would display significantly different physical and chemical properties.1-3 Compared to zero-dimensional (0D, e.g. cage molecules) and one-dimensional (1D, e.g. nanotubes) materials, two-dimensional materials haven’t drawn much attention until the discover of graphene. Over the last decade, researches on graphene, a 2D single layer of carbon atoms with honeycomb lattice structure, highlighted a new nanomaterial with outstanding properties and led to a great number of interests from both academia and industry.4,5 More importantly, this kind of material with atomic-layer thickness and 2D morphology revolutionizes the traditional ideology of nano-science and technology and opens up a door to novel 2D nanosystems.6-8 As graphene-based materials arouse great research interest, tremendous efforts have been made to synthesize other inorganic 2D materials with graphene-like intrinsic layered structure. Graphene and its inorganic analogues with strong covalent bonds in layers and weak van der waals interaction between layers can be simply gained from their bulk counterparts by exfoliation or peeling off process.9-11 Representive ultrathin nanosheets of transition metal dichalcogenide such as MoS2,12,13 MoSe2,14 WS2,15,16 WSe2,17,18 TiS2,19 TiSe2,20 VS221,22 and VSe223 have lately been reported. However, though these new 2D crystals with atomic thickness can be fabricated by CVD growth, mechanical cleavage or liquid exfoliation strategy,24-27 their species are mainly limited to layered materials with weak van der waals forces between the layers. Driven by the promising properties and potential applications of graphene-like layer materials, researchers began to expand the vein of 2D graphene-like crystals and explored other 2D materials with nonlayered crystal structures, which would not only enrich the family of 2D crystals with atomic thickness, but also give birth to new properties and applications. However, in contrast to graphene and its inorganic analogues with weak van der walls force between the layers, nonlayered phases possess relatively strong bonds between different layers. Due to the lack of intrinsic driving force for 2D anisotropic growth, nonlayered nanostructures are difficult to be exfoliated into ultrathin nanosheets using simple exfoliation processes and controlled assembly of small building blocks is considered as
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the exclusive way for the preparation of nonlayered 2D crystals.28-30 So far, just several kinds of these structures including Co9Se8, PbS and CeO2 have been reported.29-31 Developing new methods for new types of 2D nonlayered materials is extremely urgent but still a major challenge. Iron oxides have recently received increased attention as promising anode materials for rechargeable lithium-ion batteries (LIBs) because of their low cost, chemical-stable, and high theoretical capacity. However, Fe2O3 anode materials always display poor cycling stability, due to large volume changes (>200%) and particle pulverization during lithium insertion and extraction. 2D nanostructures offer ideal frameworks for rapid lithium storage due to their large exposed surfaces, short lithium diffusion paths and good resistance to the agglomeration during cycling. Now, there are many 2D α-Fe2O3 nanostructured LIB anodes reported in literatures32-36. However, much of the earlier works has been focused on the enhancement of storage capacity and the cyclic stability of α-Fe2O3 (hematite) and Fe3O4 (magnetite), while only very few articles on lithium storage properties of γ-Fe2O3 (maghemite). Although α-Fe2O3 is thermodynamically more stable (less ∆Go) than γ-Fe2O3 in bulk, γ-Fe2O3 at nanosize less than 16 nm was reported to be more stable than α-Fe2O3 due to the differences in the excess surface contributions, which predicts that γ-Fe2O3 with grain size smaller than 16 nm may exhibit better storage and cyclic performances.37,38 Herein, we developed a metal-ion-intervened technique for the growth of hematite graphene-like nanosheets with the unit cell thickness of 1.3 nm. The α-Fe2O3 nanosheets can be converted to maghemite (γ-Fe2O3) by a subsequent phase transition process while preserving the graphene-like 2D morphology due to crystal lattice matching. Furthermore, the transformation from α-Fe2O3 to γ-Fe2O3 with unit-cell thickness leads to the enhanced LIB performances and the obtained γ-Fe2O3 nanosheets exhibit excellent electrochemical performances with high specific capacitance approaching to its theoretical value and outstanding cycling stability.
Experimental Synthesis of ultrathin iron oxide nanosheets: In a typical procedure for the synthesis of α-Fe2O3 nanosheets, 1 mmol of iron nitrate and 0.25 mmol of aluminum sulfate was dissolved in 10 mL of
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deionized water under magnetic stirring to form a transparent solution. While stirring, 3 ml of triethylamine was dropwisely added into the transparent solution and stirring for 20 more minutes. Then the mixed solution was transferred and sealed in a 50 mL Teflon-lined autoclave, which was maintained at 160 °C for 24 h. The red precipitates at the bottom of autoclave were collected by centrifugation, washed and then dried at 60 °C. For the transformation from α-Fe2O3 nanosheets to γ-Fe2O3 nanosheets, the obtained α-Fe2O3 nanosheets were heated to 600 °C with a heating rate of 5 °C/min under N2 atmosphere and kept at 600 °C for 5 h. Characterizations: Powder X-ray diffraction (XRD) patterns were collected by using a Bruker D8 ADVANCE diffractometer with CuKα radiation (λ=1.5418 Ǻ). The structures and morphologies of the samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL-2010). Electrochemical experiments were performed with coin cell (CR2032) using Li foil as counter electrode. Atomic force microscopy (AFM) studies were performed using a Veeco DI Nano-scope MultiMode V system. Electrochemical measurements: For the electrode preparation, 60 wt% of the as-prepared samples, 30 wt% acetylene black, and 10 wt% poly(vinylidene difluoride) (PVDF) were dissolved in N-methyl-2-pyrrolidinone (NMP). The obtained slurry was coated onto Cu foil disks to form working electrodes, which were then dried in vacuum at 80 °C for 12 h to remove the solvent. After that, the copper foil with the active material was cut into round disks with the mass of the active material in each cell of about 0.80–0.90 mg. Electrochemical measurements were carried out on CR2032 (3V) coin-type cells with lithium metal as the counter/reference electrode, a Celgard 2400 membrane as the separator, and electrolyte solution obtained by dissolving 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC–DMC, 1:1, v/v). The coin cells were assembled in an argon-filled glovebox (oxygen and water concentration below 5 ppm). The electrochemical measurements were performed on LAND CT-2001A instrument (Wuhan, China).
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2θ/degree Figure 1 XRD patterns of the obtained nanosheets: (a) α-Fe2O3 and (b) γ-Fe2O3. The ultrathin α-Fe2O3 nanosheets were prepared by treating Fe(NO3)3 solution with addition of triethylamine and aluminum sulfate under hydrothermal conditions. Figure 1a shows XRD pattern of the as-prepared sample controlled with the addition of Al3+. All the diffraction peaks can be indexed to the hexagonal phase α-Fe2O3 (JCPDS card No 33-0664) and no characteristic peaks of impurities can be detected, indicating the high purity of the sample and no formations of any alumina compounds. SEM investigations (Figure 2a and b) show that the sample is composed of 2D nanosheets. A TEM image (Figure 2c) clearly demonstrates flexible and ultrathin sheet-like structure of the obtained product. Similar to graphene, the edges of the nanosheets roll up due to the high surface tension. High-resolution TEM image (HRTEM) shows clear lattice fringes with an interfringe spacing of 0.25 nm (Figure 2d), matching well with the lattice distance of (110) planes of hexagonal α-Fe2O3. The inset SAED pattern further reveals microstructure information, clearly demonstrating that the nanosheets are a hexagonal single crystal with [001] orientation. In addition, it can be clearly observed that the nanosheets are ultrathin, which is further confirmed by atomic force microscope (AFM) investigations. The tapping-mode AFM image and corresponding height profile were carried out and shown in Figure 2e. The thickness of the α-Fe2O3 nanosheets was evaluated to be in the range from 1.36 nm to 1.38 nm, which agrees well with the thickness (1.37 nm) of a single-layered α-Fe2O3 slab
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Figure 2 (a, b) SEM images; (c) TEM image; (d) HRTEM image (inset: SAED pattern); (e) AFM image and the corresponding height profile; (f) cell unit of the obtained ultrathin α-Fe2O3 nanosheets. (Figure 2f) along the [001] direction, providing direct and solid evidence for the formation of single-layers. In this synthesis method, the addition of Al3+ is very critical for obtaining α-Fe2O3 nanosheets. In a control experiment, when no Al3+ was added under the same reaction conditions, only α-Fe2O3 particles with irregular morphology can be collected (Supporting information, Figure S1a, b). With the addition of aluminum sulfate, under high alkalinity with trimethylamine ferrite nanoparticles might form at the beginning of the hydrothermal process. As reaction goes on, the existence of trimethylamine would make Al dissolve from ferrite into the solution due to the amphoteric property of Al3+ and result in the formation of α-Fe2O3 nanoparticles. It is known that aluminium ion can adsorb on the (001) surface of α-Fe2O3.39 Thus, in the process of reaction, Al3+ ions would play the role of structure and surface director and the growth of (001) is restrained with the presence of Al3+, and the α-Fe2O3 nanoparticles grow along [100] direction, which leads to the formation of unit-cell thick α-Fe2O3 nanosheets. In order to prove our assumption that the Al3+ ions are the structure director
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rather than SO42-, AlCl3 was also used to replace Al2(SO4)3 as the additive. As shown in Figure S1c and d, the obtained product is also composed of ultrathin α-Fe2O3 nanosheets, confirming that the addition of Al3+ is the main reason to direct the growth of α-Fe2O3 nanocrystals. These results demonstrated that with the assistance of triethylamine and Al3+, ultrathin α-Fe2O3 nanosheets with cell unit height can be obtained under the mild conditions.
Figure 3 (a, b) SEM images; (c) TEM image; (d) HRTEM image; (e) AFM image and the corresponding height profile; (f) cell unit of the obtained ultrathin γ-Fe2O3 nanosheets. Iron oxides are a kind of chemically simple binary compound that has diversified phase diagrams with many thermodynamically stable crystal structures and stoichiometries including α-Fe2O3, γ-Fe2O3 and Fe3O4, which makes them attractive to investigate nanocrystal shape and phase polymorphism. After the synthesis of the α-Fe2O3 nanosheets, we investigated the transformation of ultrathin α-Fe2O3 nanosheets to other iron oxides. Heat treatment provides a simple route to realize the phase transformation. In this study, we calcined the α-Fe2O3 nanosheets at 600 °C in nitrogen atmosphere for 5 h. The XRD pattern of the product is shown in Figure 1b. Through the comparison to
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Figure 1a and JCPDS cards (No 39-1346), it can be found that the α-Fe2O3 nanosheets are transformed to maghemite γ-Fe2O3 in N2 atmosphere. Figure 3 displays SEM, TEM and AFM characterizations of the obtained maghemite γ-Fe2O3. SEM and TEM investigations (Figure 3a~c) show that the samples are composed of 2D nanosheets like α-Fe2O3. It is clear that the sheet-like 2D morphology has been perfectly retained after the heat treatments and there are no apparent broken or collapsed structures in the final samples, suggesting good structural stability of this 2D structure. HRTEM images (Figure 3d) shows clear lattice fringes with interfringe spacing of 0.296 nm, matching well with the lattice distance of (220) planes of γ-Fe2O3. The SAED pattern in the inset Figure 3d further suggests the single-crystal nature of γ-Fe2O3 nanosheets. The thickness of the obtained γ-Fe2O3 nanosheets was determined by AFM characterizations (Figure 3e). It was found that after transformation, the nanosheets became thinner. The statistical height profiles of nanosheets of γ-Fe2O3 show a smooth 2D sheet with thickness in the range of 0.82−0.84 nm (Figure 3e), which is very close to a unit cell thickness of the γ-Fe2O3 lattice (0.8351 nm, Figure 3f). α-Fe2O3 nanosheets have the crystal structure of hexagonal phase while γ-Fe2O3 nanosheets are with cubic phase. In the process of calcination, the transformation from hexagonal to cubic structure makes iron atoms and oxygen atoms in original unit cell move and rearrange, which induce the thickness change of the products and result in the formation of γ-Fe2O3 nanosheets with thickness of about 0.83 nm. So, using the simple calcination procedures, we could easily obtain single-layered magnetic iron oxides from the single-layered α-Fe2O3 nanosheets. The magnetism studies (Figure S2) reveals that these single-layered thick γ-Fe2O3 nanosheets are magnetic, which might bring great opportunities for the development of next-generation magnetic nanodevices. As an important member of the transition-metal oxide family, iron oxides have long been regarded as promising anode materials for lithium ion battery because of high theoretical capacity (1054 mAh/g), nontoxicity, high corrosion resistance and low processing cost.40,41 The application of iron oxides as anode materials in lithium ion batteries are usually focused on α-Fe2O342, and researches on γ-Fe2O3 as anode material are relatively little explored.32 The achievement of the novel unit-cell thick iron oxide graphene-like nanostructures in this study brings us great possibilities to
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investigate their performances in Li-ion battery. Figure 4a shows the galvanostatic discharge-charge voltage performances of the two ultrathin iron oxides. The first discharge capacity of α-Fe2O3 nanosheets is calculated to be 1191.9 mAh·g-1. There is a vivid plateau at 0.85 V (vs. Li/Li+) in the discharge process, demonstrating that the reduction of Fe2O3 to form Fe0 and Li2O (Fe2O3 + 6Li++ 6e↔ 2Fe +3Li2O).40 The γ-Fe2O3 nanosheets display similar discharge-charge plateaus and deliver discharge capacities of 1377.5 mAh·g-1. It’s clear that γ-Fe2O3 nanosheets can deliver a much higher capacity than α-Fe2O3 nanosheets. Figure 4b shows the typical cycle voltammetric (CV) curves of γ-Fe2O3 nanosheets at a scan rate of 0.1 mV·s-1 in the voltage of 0.01~3.0 V. At the 1st cycle, two main redox peaks at ~0.79 V and ~1.8 V observed are corresponding to lithium intercalating in and out of Fe2O3.41 The subsequent curves with a cathodic and anodic peaks pair at around 0.81 V and 1.8–1.9 V exhibits little change, indicating good reversibility and structure stability of γ-Fe2O3 nanosheets as anode material. For comparison, the CV curves of α-Fe2O3 nanosheets at a scan rate of
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0.1 mV·s-1 are shown in Figure S3 and from the CV curves it can be found that the peak intensity reduces in the magnitude of α-Fe2O3 nanosheets greater than that of γ-Fe2O3 nanosheets in the cycles, indicating the worse reversibility of α-Fe2O3 nanosheets in the early cycles. Figure 4c displays the 1st, 2nd, 10th, 40th, 80th, and 120th discharge and charge curves of γ-Fe2O3 nanosheets in the voltage range of 0.05-3.0 V at the current of 100 mA·g-1. The second discharge and charge capacities declined and the capacity fading of electrode might be attributed to the electrolyte decomposition and formation of solid electrolyte interface (SEI) layer. After 40 cycles, the discharge and charge capacities began to rise up, which means the cell was activated after cycles. More importantly, the capacities continue to rise and at 100th cycle the discharge capacity of γ-Fe2O3 nanosheets reaches 1000 mAh· g-1, which almost equal to its theoretical capacity, and maintains stable from then on. The cycling performances of the γ-Fe2O3 nanosheets can be clearly seen in Figure 4d. The discharge and charge capacity is 1047.1 mAh· g-1 and 1021.1 mAh· g-1 at 120th cycle. The cycling performances of γ-Fe2O3 nanosheets at larger current densities of 400, 600 and 1000 mA·g-1 were also studied and the results are shown in Figure S4. As shown in Figure S4a, it can be found that the discharge capacities of the electrode at different current densities decline rapidly at first, but after about 50 cycles, the discharge capacities increase slowly with cycle number, which is consistent with the variation tendency of current density of 100 mA·g-1. This phenomenon might be attributed to the growth and dissolution of polymeric gel-like film made of decomposition products of the LiPF6/EC-DMC electrolyte on the active materials, which would release extra discharge capacity as cycling goes on.42,43 Figure S4b presents the rate capability of the γ-Fe2O3 electrode at different current densities ranging from 100 to 1000 mA· g-1. With the current density increasing from 100 to 200, 500, and 1000 mA g-1, the discharge specific capacity decreases and when the current density return to 100 mA g-1, the discharge specific capacity is recovered to a high level, implying a very stable cycling performance. Furthermore, at the different current densities, the specific capacities of γ-Fe2O3 nanosheets are all higher than those of α-Fe2O3 nanosheets. The remarkable lithium storage properties are probably related to the unique nanostructure of these γ-Fe2O3 nanosheets. Specifically, 2D graphene-like structures of γ-Fe2O3 would provide large effective surface areas that facilitate penetration of the electrolyte and transport of
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lithium ions in the electrode.44 The BET specific surface area of α-Fe2O3 and γ-Fe2O3 nanosheets are calculated to be 35.70 m2·g-1 and 30.27 m2· g-1, respectively. Meanwhile, the γ-Fe2O3 nanosheets with high crystallinity and structural stability would also help in retaining the pristine nanostructure upon cycling. TEM and XRD analyses of the Fe2O3 nanosheets after initial cycle are shown in Figure S5 and S6. Although the amount of the active material in the electrode is very small, besides a broad peak centered at 27° arising from impurities and substrate, the diffraction peaks can be still clearly observed that are indexed to cubic γ-Fe2O3, indicating the phase conversation of the nanosheets after long time cycling. From the TEM images, although the nanosheets are not smooth due to the lithium intercalating in and out of Fe2O3 nanosheets and there are also some impurities because reagents addition in electrode preparation, both samples basically keep the original two-dimensional morphology after lithiation, confirming the structural stability. As comparison, after 120 cycles the reversible capacity for α-Fe2O3 nanosheets was only 419.7 mAh· g-1, 65 % loss of its original value. It has been reported that with size less than 16 nm, γ-Fe2O3 is more stable than α-Fe2O3 owing to thermodynamic difference, which results in γ-Fe2O3 materials actually exhibits better storage and cyclic performance compared to α-Fe2O3 nanosheets.38,39 Conclusions In summary, graphene-like hematite α-Fe2O3 nanosheets are synthesized via a facile metal-ion-intervened method. The α-Fe2O3 nanosheets are single crystalline with a unit-cell thickness of 1.3 nm. Through simple calcination procedure, the α-Fe2O3 nanosheets are converted to magnetic maghemite (γ-Fe2O3) nanosheets. They are also singly-layered with thickness of about 0.85 nm. The Li-ion battery studies reveal that the ultrathin γ-Fe2O3 nanosheets exhibit great electrochemical performances with high capacity, excellent cycling stability and after 100 cycles, the capacities can reach its theoretical value. We believe that these ultrathin iron oxide nanosheets could be promising function materials with great application potentials in various fields. Acknowledgement: This work is supported by the National Basic Research Program of China (Grant No. 2013CB922102), the National Natural Science Foundation of China (Grant No. 21471076 and
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51172106) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Competing Financial Interests: The authors declare no competing financial interests.
Supporting Information: SEM images, TEM images, CV curves and magnetism curves.
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References 1.
M.A. El-Sayed, Acc. Chem. Res. 37 (2004) 326.
2.
H.H. Han, S.J. Lee, J.W. Cheon, Chem. Soc. Rev. 42 (2013) 2581.
3.
S. Ithurria, M.D. Tessier, B. Mahler, R.P.S.M. Lobo, B. Dubertret, A.L. Efros, Nat. Mater. 10 (2011) 936.
4.
A.K. Geim, Science 324 (2009) 1530.
5.
A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183.
6.
F. Bonaccorso, Z. Sun, T. Hasan, A. C. Ferrari, Nat. Photonics 4 (2010) 611.
7.
K.P. Loh, Q.L. Bao, G. Eda, M. Chhowalla, Nat. Chem. 2 (2010) 1015.
8.
K.P. Wang, J. Wang, J.T. Fan, M. Lotya, A. O’Neill, D. Fox, et. al. ACS Nano 7 (2013) 9260.
9.
Q.H. Wang, K. Kalantar-Zadeh, A. Kis, J.N. Coleman, M.S. Strano, Nat. Nanotechnol. 7 (2012) 699.
10. S.Z. Butler, S.M. Hollen, L. Cao, Y. Cui, J.A. Gupta, H.R. Gutiérrez, et. al. ACS Nano 7 (2013) 2898. 11. V. Nicolosi, M. Chhowalla, M.G. Kanatzidis, M.S. Strano, J.N. Coleman, Science 340 (2013) 1420. 12. Z. Zeng, Z. Yin, X. Huang, H. Li, Q. He, G. Lu, et. al. Angew. Chem. Int. Ed. 50 (2011) 11093. 13. S.S. Chou, B. Kaehr, J. Kim, B.M. Foley, M. De, P.E. Hopkins, et. al. Angew. Chem. Int. Ed. 52 (2013) 4160. 14. S. Tongay, J. Zhou, C. Ataca, K. Lo, T.S. Matthews, J. Li, et. al. Nano Lett. 12 (2012) 5576. 15. J.W. Seo, Y.W. Jun, S.W. Park, H. Nah, T. Moon, B. Park, et. al. Angew. Chem. Int. Ed. 46 (2007) 8828. 16. K.G. Zhou, N.N. Mao, H.X. Wang, Y. Peng, H.L. Zhang, Angew. Chem. Int. Ed. 50 (2011) 10839. 17. H. Fang, S. Chuang, T.C. Chang, K. Takei, T. Takahashi, A. Javey, Nano Lett. 12 (2012) 3788. 18. Z. Zeng, T. Sun, J. Zhu, X. Huang, Z. Yin, G. Lu, et. al. Angew. Chem. Int. Ed. 51 (2012) 9052. 19. C. Lin, X. Zhu, J. Feng, C. Wu, S. Hu, J. Peng, et. al. J. Am. Chem. Soc. 135 (2013) 5144.
- 14 -
20. P. Goli, J. Khan, D. Wickramaratne, R.K. Lake, A.A. Balandin, Nano Lett. 12 (2012) 5941. 21. J. Feng, X. Sun, C. Wu, L. Peng, C. Lin, S. Hu, et. al. J. Am. Chem. Soc. 133 (2011) 17832. 22. J. Feng, L. Peng, C. Wu, X. Sun, S. Hu, C. Lin, et. al. Adv. Mater. 24 (2012) 1969. 23. K. Xu, P.Z. Chen, X.L. Li, C.Z. Wu, Y.Q. Guo, J.Y. Zhao, et. al. Angew. Chem. Int. Ed. 52 (2013) 10477. 24. K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, et. al. Proc. Natl. Acad. Sci. U. S. A. 102 (2005) 10451. 25. Y.H. Lee, X.Q. Zhang, W. Zhang, M.T. Chang, C.T. Lin, K.D. Chang, et. al. Adv. Mater. 24 (2012) 2320. 26. K.K. Liu, W. Zhang, Y.H. Lee, Y.C. Lin, M.T. Chang, C.Y. Su, et. al. Nano Lett. 12 (2012) 1538. 27. J.N. Coleman, M. Lotya, A. O’Neill, S.D. Bergin, P.J. King, U. Khan, et. al. Science 331 (2011) 568. 28. X.D. Zhang, Y. Xie, Chem. Soc. Rev. 42 (2013) 8187 29. X. Zhang, J. Zhang, J. Zhao, B. Pan, M. Kong, J. Chen, et. al. J. Am. Chem. Soc. 134 (2012) 11908. 30. C. Schliehe, B.H. Juarez, M. Pelletier, S. Jander, D. Greshnykh, M. Nagel, et. al. Science 329 (2010) 550. 31. T. Yu, B. Lim, Y. Xia, Angew. Chem. Int. Ed. 49 (2010) 4484. 32. J.H. Wang, M.X. Gao, H.G. Pan, Y.F. Liu, Z. Zhang, J.X. Li, et. al. J. Mater. Chem. A, 3(2015) 14178. 33. Y.K. Wang, L.C. Yang, R.Z. Hu, W. Sun, J.W. Liu, Y.K. Wang, et. al. J. Power Sources, 288 (2015) 314. 34. Y.K. Wang, L.C. Yang, R.Z. Hu, L.Z. Ouyang, M. Zhu, Electrochim Acta, 125 (2014) 421. 35. L.Z. Ouyang, Z.J. Cao, H. Wang, R.Z. Hu, M. Zhu, J. Alloys Compd. 691 (2017) 422. 36. J.J. Zhang, T. Huang, Z.L. Liu, A.H. Yu, Electrochem Commun. 29 (2013) 17. 37. B.M. Oscar, M. Lena, N. Alexandra, V.V. Sabino, Chem. Mater. 20(2008) 591.
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38. H. Srirama, S. Kuppan, B. Palani, Electrochem Solid-State Lett. 13(2010) A132. 39. R.M. Liu, Y.W. Jiang, Q.Y. Lu, W. Du, F. Gao, Cryst. Eng. Comm. 15 (2013) 443. 40. N. Kang, J.H. Park, J. Choi, J. Jin, J. Chun, I.G. Jung, et. al. Angew. Chem. Int. Ed. 51 (2012) 6626. 41. B. Wang, J.S. Chen, H.B. Wu, Z.Y. Wang, X W. Lou, J. Am. Chem. Soc. 133 (2011) 17146. 42. F.M. Courtel, H. Duncan, Y. Abu-Lebdeh, I.J. Davidson, J. Mater. Chem. 21 (2011) 10206. 43. S.H. Choi, Y.C. Kang, ChemSusChem 6 (2013) 2111. 44. L. Zhang, H.B. Wu, S. Madhavi, H.H. Hng, X.W. Lou, J. Am. Chem. Soc. 134 (2012) 17388.
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Figure captions Figure 1 XRD patterns of the obtained nanosheets: (a) α-Fe2O3 and (b) γ-Fe2O3. Figure 2 (a, b) SEM images; (c) TEM image; (d) HRTEM image (inset: SAED pattern); (e) AFM image and the corresponding height profile; (f) cell unit of the obtained ultrathin α-Fe2O3 nanosheets. Figure 3 (a, b) SEM images; (c) TEM image; (d) HRTEM image; (e) AFM image and the corresponding height profile; (f) cell unit of the obtained ultrathin γ-Fe2O3 nanosheets. Figure 4 (a) First discharge-charge curves of the two ultrathin iron oxide nanosheets; b) Cyclic voltammograms (CVs) of ultrathin γ-Fe2O3 nanosheets; c) discharge-charge curves of ultrathin γ-Fe2O3 nanosheets at the 1st, 2nd, 10th, 40th, 80th, 120th cycle at the current density of 100 mA·g-1; d) cycling performances of the two ultrathin iron oxide nanosheets.
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Highlights Unit-cell thick α-Fe2O3 nanosheets with thickness of 1.3 nm were firstly synthesized. The nanosheets were synthesized through a metal-ion-intervened hydrothermal method. The α-Fe2O3 nanosheets were converted to magnetic γ-Fe2O3 nanosheets by a calcination process. The obtained magnetic γ-Fe2O3 nanosheets were with unit-cell thickness of 0.8 nm The unit-cell-thick γ-Fe2O3 nanosheets exhibit great enhanced electrochemical performances.
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S1385-8947(17)30916-6 http://dx.doi.org/10.1016/j.cej.2017.05.155 CEJ 17054
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
28 January 2017 25 May 2017 25 May 2017
Please cite this article as: Y. Jin, L. Dang, H. Zhang, C. Song, Q. Lu, F. Gao, Synthesis of unit-cell-thick α-Fe2O3 nanosheets and their transformation to γ-Fe2O3 nanosheets with enhanced LIB performances, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.05.155
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of unit-cell-thick α-Fe2O3 nanosheets and their transformation to γ-Fe2O3 nanosheets with enhanced LIB performances Ying Jin,1,3 Liyun Dang,1 Hao Zhang,1 Chuang Song,2 Qingyi Lu,1* Feng Gao2* 1
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering;
Collaborative Innovation Center of Advanced Microstructures, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China, E-mail: [email protected] 2
, Department of Materials Science and Engineering, Collaborative Innovation Center of Advanced
Microstructures, Nanjing University, Nanjing 210093, P. R. China, E-mail: [email protected] 3
College of Biological and Chemical Engineering, Anhui Polytechnic University, Wuhu 241000,
Anhui P. R. China
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Abstract The synthesis of ultrathin magnetic nanosheets is significant for the development of next-generation nanodevices but remains to be a big challenge. Herein, we present facile approaches for large-scaled production of cell-unit-thick single-crystalline iron oxide nanostructures. The ultrathin hematite (α-Fe2O3) nanosheets with unit-cell thickness of 1.3 nm are firstly synthesized using a metal-ion-intervened hydrothermal method. Then the α-Fe2O3 nanosheets are converted to magnetic maghemite (γ-Fe2O3) nanosheets with unit-cell thickness of 0.8 nm by a simple calcination process. The Li-ion battery studies reveal that unit-cell-thick γ-Fe2O3 nanosheets exhibit great enhanced electrochemical performances with high specific capacitance approaching to its theoretical value and excellent cycling stability. Keywords: Hematite; Maghemite; Nanosheets; Unit cell thickness; Anode materials
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Introduction Dimensionality is one of the crucial parameters in nanoscience; nanomaterials with same chemical composition but with different dimensionality would display significantly different physical and chemical properties.1-3 Compared to zero-dimensional (0D, e.g. cage molecules) and one-dimensional (1D, e.g. nanotubes) materials, two-dimensional materials haven’t drawn much attention until the discover of graphene. Over the last decade, researches on graphene, a 2D single layer of carbon atoms with honeycomb lattice structure, highlighted a new nanomaterial with outstanding properties and led to a great number of interests from both academia and industry.4,5 More importantly, this kind of material with atomic-layer thickness and 2D morphology revolutionizes the traditional ideology of nano-science and technology and opens up a door to novel 2D nanosystems.6-8 As graphene-based materials arouse great research interest, tremendous efforts have been made to synthesize other inorganic 2D materials with graphene-like intrinsic layered structure. Graphene and its inorganic analogues with strong covalent bonds in layers and weak van der waals interaction between layers can be simply gained from their bulk counterparts by exfoliation or peeling off process.9-11 Representive ultrathin nanosheets of transition metal dichalcogenide such as MoS2,12,13 MoSe2,14 WS2,15,16 WSe2,17,18 TiS2,19 TiSe2,20 VS221,22 and VSe223 have lately been reported. However, though these new 2D crystals with atomic thickness can be fabricated by CVD growth, mechanical cleavage or liquid exfoliation strategy,24-27 their species are mainly limited to layered materials with weak van der waals forces between the layers. Driven by the promising properties and potential applications of graphene-like layer materials, researchers began to expand the vein of 2D graphene-like crystals and explored other 2D materials with nonlayered crystal structures, which would not only enrich the family of 2D crystals with atomic thickness, but also give birth to new properties and applications. However, in contrast to graphene and its inorganic analogues with weak van der walls force between the layers, nonlayered phases possess relatively strong bonds between different layers. Due to the lack of intrinsic driving force for 2D anisotropic growth, nonlayered nanostructures are difficult to be exfoliated into ultrathin nanosheets using simple exfoliation processes and controlled assembly of small building blocks is considered as
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the exclusive way for the preparation of nonlayered 2D crystals.28-30 So far, just several kinds of these structures including Co9Se8, PbS and CeO2 have been reported.29-31 Developing new methods for new types of 2D nonlayered materials is extremely urgent but still a major challenge. Iron oxides have recently received increased attention as promising anode materials for rechargeable lithium-ion batteries (LIBs) because of their low cost, chemical-stable, and high theoretical capacity. However, Fe2O3 anode materials always display poor cycling stability, due to large volume changes (>200%) and particle pulverization during lithium insertion and extraction. 2D nanostructures offer ideal frameworks for rapid lithium storage due to their large exposed surfaces, short lithium diffusion paths and good resistance to the agglomeration during cycling. Now, there are many 2D α-Fe2O3 nanostructured LIB anodes reported in literatures32-36. However, much of the earlier works has been focused on the enhancement of storage capacity and the cyclic stability of α-Fe2O3 (hematite) and Fe3O4 (magnetite), while only very few articles on lithium storage properties of γ-Fe2O3 (maghemite). Although α-Fe2O3 is thermodynamically more stable (less ∆Go) than γ-Fe2O3 in bulk, γ-Fe2O3 at nanosize less than 16 nm was reported to be more stable than α-Fe2O3 due to the differences in the excess surface contributions, which predicts that γ-Fe2O3 with grain size smaller than 16 nm may exhibit better storage and cyclic performances.37,38 Herein, we developed a metal-ion-intervened technique for the growth of hematite graphene-like nanosheets with the unit cell thickness of 1.3 nm. The α-Fe2O3 nanosheets can be converted to maghemite (γ-Fe2O3) by a subsequent phase transition process while preserving the graphene-like 2D morphology due to crystal lattice matching. Furthermore, the transformation from α-Fe2O3 to γ-Fe2O3 with unit-cell thickness leads to the enhanced LIB performances and the obtained γ-Fe2O3 nanosheets exhibit excellent electrochemical performances with high specific capacitance approaching to its theoretical value and outstanding cycling stability.
Experimental Synthesis of ultrathin iron oxide nanosheets: In a typical procedure for the synthesis of α-Fe2O3 nanosheets, 1 mmol of iron nitrate and 0.25 mmol of aluminum sulfate was dissolved in 10 mL of
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deionized water under magnetic stirring to form a transparent solution. While stirring, 3 ml of triethylamine was dropwisely added into the transparent solution and stirring for 20 more minutes. Then the mixed solution was transferred and sealed in a 50 mL Teflon-lined autoclave, which was maintained at 160 °C for 24 h. The red precipitates at the bottom of autoclave were collected by centrifugation, washed and then dried at 60 °C. For the transformation from α-Fe2O3 nanosheets to γ-Fe2O3 nanosheets, the obtained α-Fe2O3 nanosheets were heated to 600 °C with a heating rate of 5 °C/min under N2 atmosphere and kept at 600 °C for 5 h. Characterizations: Powder X-ray diffraction (XRD) patterns were collected by using a Bruker D8 ADVANCE diffractometer with CuKα radiation (λ=1.5418 Ǻ). The structures and morphologies of the samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL-2010). Electrochemical experiments were performed with coin cell (CR2032) using Li foil as counter electrode. Atomic force microscopy (AFM) studies were performed using a Veeco DI Nano-scope MultiMode V system. Electrochemical measurements: For the electrode preparation, 60 wt% of the as-prepared samples, 30 wt% acetylene black, and 10 wt% poly(vinylidene difluoride) (PVDF) were dissolved in N-methyl-2-pyrrolidinone (NMP). The obtained slurry was coated onto Cu foil disks to form working electrodes, which were then dried in vacuum at 80 °C for 12 h to remove the solvent. After that, the copper foil with the active material was cut into round disks with the mass of the active material in each cell of about 0.80–0.90 mg. Electrochemical measurements were carried out on CR2032 (3V) coin-type cells with lithium metal as the counter/reference electrode, a Celgard 2400 membrane as the separator, and electrolyte solution obtained by dissolving 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC–DMC, 1:1, v/v). The coin cells were assembled in an argon-filled glovebox (oxygen and water concentration below 5 ppm). The electrochemical measurements were performed on LAND CT-2001A instrument (Wuhan, China).
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2θ/degree Figure 1 XRD patterns of the obtained nanosheets: (a) α-Fe2O3 and (b) γ-Fe2O3. The ultrathin α-Fe2O3 nanosheets were prepared by treating Fe(NO3)3 solution with addition of triethylamine and aluminum sulfate under hydrothermal conditions. Figure 1a shows XRD pattern of the as-prepared sample controlled with the addition of Al3+. All the diffraction peaks can be indexed to the hexagonal phase α-Fe2O3 (JCPDS card No 33-0664) and no characteristic peaks of impurities can be detected, indicating the high purity of the sample and no formations of any alumina compounds. SEM investigations (Figure 2a and b) show that the sample is composed of 2D nanosheets. A TEM image (Figure 2c) clearly demonstrates flexible and ultrathin sheet-like structure of the obtained product. Similar to graphene, the edges of the nanosheets roll up due to the high surface tension. High-resolution TEM image (HRTEM) shows clear lattice fringes with an interfringe spacing of 0.25 nm (Figure 2d), matching well with the lattice distance of (110) planes of hexagonal α-Fe2O3. The inset SAED pattern further reveals microstructure information, clearly demonstrating that the nanosheets are a hexagonal single crystal with [001] orientation. In addition, it can be clearly observed that the nanosheets are ultrathin, which is further confirmed by atomic force microscope (AFM) investigations. The tapping-mode AFM image and corresponding height profile were carried out and shown in Figure 2e. The thickness of the α-Fe2O3 nanosheets was evaluated to be in the range from 1.36 nm to 1.38 nm, which agrees well with the thickness (1.37 nm) of a single-layered α-Fe2O3 slab
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Figure 2 (a, b) SEM images; (c) TEM image; (d) HRTEM image (inset: SAED pattern); (e) AFM image and the corresponding height profile; (f) cell unit of the obtained ultrathin α-Fe2O3 nanosheets. (Figure 2f) along the [001] direction, providing direct and solid evidence for the formation of single-layers. In this synthesis method, the addition of Al3+ is very critical for obtaining α-Fe2O3 nanosheets. In a control experiment, when no Al3+ was added under the same reaction conditions, only α-Fe2O3 particles with irregular morphology can be collected (Supporting information, Figure S1a, b). With the addition of aluminum sulfate, under high alkalinity with trimethylamine ferrite nanoparticles might form at the beginning of the hydrothermal process. As reaction goes on, the existence of trimethylamine would make Al dissolve from ferrite into the solution due to the amphoteric property of Al3+ and result in the formation of α-Fe2O3 nanoparticles. It is known that aluminium ion can adsorb on the (001) surface of α-Fe2O3.39 Thus, in the process of reaction, Al3+ ions would play the role of structure and surface director and the growth of (001) is restrained with the presence of Al3+, and the α-Fe2O3 nanoparticles grow along [100] direction, which leads to the formation of unit-cell thick α-Fe2O3 nanosheets. In order to prove our assumption that the Al3+ ions are the structure director
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rather than SO42-, AlCl3 was also used to replace Al2(SO4)3 as the additive. As shown in Figure S1c and d, the obtained product is also composed of ultrathin α-Fe2O3 nanosheets, confirming that the addition of Al3+ is the main reason to direct the growth of α-Fe2O3 nanocrystals. These results demonstrated that with the assistance of triethylamine and Al3+, ultrathin α-Fe2O3 nanosheets with cell unit height can be obtained under the mild conditions.
Figure 3 (a, b) SEM images; (c) TEM image; (d) HRTEM image; (e) AFM image and the corresponding height profile; (f) cell unit of the obtained ultrathin γ-Fe2O3 nanosheets. Iron oxides are a kind of chemically simple binary compound that has diversified phase diagrams with many thermodynamically stable crystal structures and stoichiometries including α-Fe2O3, γ-Fe2O3 and Fe3O4, which makes them attractive to investigate nanocrystal shape and phase polymorphism. After the synthesis of the α-Fe2O3 nanosheets, we investigated the transformation of ultrathin α-Fe2O3 nanosheets to other iron oxides. Heat treatment provides a simple route to realize the phase transformation. In this study, we calcined the α-Fe2O3 nanosheets at 600 °C in nitrogen atmosphere for 5 h. The XRD pattern of the product is shown in Figure 1b. Through the comparison to
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Figure 1a and JCPDS cards (No 39-1346), it can be found that the α-Fe2O3 nanosheets are transformed to maghemite γ-Fe2O3 in N2 atmosphere. Figure 3 displays SEM, TEM and AFM characterizations of the obtained maghemite γ-Fe2O3. SEM and TEM investigations (Figure 3a~c) show that the samples are composed of 2D nanosheets like α-Fe2O3. It is clear that the sheet-like 2D morphology has been perfectly retained after the heat treatments and there are no apparent broken or collapsed structures in the final samples, suggesting good structural stability of this 2D structure. HRTEM images (Figure 3d) shows clear lattice fringes with interfringe spacing of 0.296 nm, matching well with the lattice distance of (220) planes of γ-Fe2O3. The SAED pattern in the inset Figure 3d further suggests the single-crystal nature of γ-Fe2O3 nanosheets. The thickness of the obtained γ-Fe2O3 nanosheets was determined by AFM characterizations (Figure 3e). It was found that after transformation, the nanosheets became thinner. The statistical height profiles of nanosheets of γ-Fe2O3 show a smooth 2D sheet with thickness in the range of 0.82−0.84 nm (Figure 3e), which is very close to a unit cell thickness of the γ-Fe2O3 lattice (0.8351 nm, Figure 3f). α-Fe2O3 nanosheets have the crystal structure of hexagonal phase while γ-Fe2O3 nanosheets are with cubic phase. In the process of calcination, the transformation from hexagonal to cubic structure makes iron atoms and oxygen atoms in original unit cell move and rearrange, which induce the thickness change of the products and result in the formation of γ-Fe2O3 nanosheets with thickness of about 0.83 nm. So, using the simple calcination procedures, we could easily obtain single-layered magnetic iron oxides from the single-layered α-Fe2O3 nanosheets. The magnetism studies (Figure S2) reveals that these single-layered thick γ-Fe2O3 nanosheets are magnetic, which might bring great opportunities for the development of next-generation magnetic nanodevices. As an important member of the transition-metal oxide family, iron oxides have long been regarded as promising anode materials for lithium ion battery because of high theoretical capacity (1054 mAh/g), nontoxicity, high corrosion resistance and low processing cost.40,41 The application of iron oxides as anode materials in lithium ion batteries are usually focused on α-Fe2O342, and researches on γ-Fe2O3 as anode material are relatively little explored.32 The achievement of the novel unit-cell thick iron oxide graphene-like nanostructures in this study brings us great possibilities to
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investigate their performances in Li-ion battery. Figure 4a shows the galvanostatic discharge-charge voltage performances of the two ultrathin iron oxides. The first discharge capacity of α-Fe2O3 nanosheets is calculated to be 1191.9 mAh·g-1. There is a vivid plateau at 0.85 V (vs. Li/Li+) in the discharge process, demonstrating that the reduction of Fe2O3 to form Fe0 and Li2O (Fe2O3 + 6Li++ 6e↔ 2Fe +3Li2O).40 The γ-Fe2O3 nanosheets display similar discharge-charge plateaus and deliver discharge capacities of 1377.5 mAh·g-1. It’s clear that γ-Fe2O3 nanosheets can deliver a much higher capacity than α-Fe2O3 nanosheets. Figure 4b shows the typical cycle voltammetric (CV) curves of γ-Fe2O3 nanosheets at a scan rate of 0.1 mV·s-1 in the voltage of 0.01~3.0 V. At the 1st cycle, two main redox peaks at ~0.79 V and ~1.8 V observed are corresponding to lithium intercalating in and out of Fe2O3.41 The subsequent curves with a cathodic and anodic peaks pair at around 0.81 V and 1.8–1.9 V exhibits little change, indicating good reversibility and structure stability of γ-Fe2O3 nanosheets as anode material. For comparison, the CV curves of α-Fe2O3 nanosheets at a scan rate of
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0.1 mV·s-1 are shown in Figure S3 and from the CV curves it can be found that the peak intensity reduces in the magnitude of α-Fe2O3 nanosheets greater than that of γ-Fe2O3 nanosheets in the cycles, indicating the worse reversibility of α-Fe2O3 nanosheets in the early cycles. Figure 4c displays the 1st, 2nd, 10th, 40th, 80th, and 120th discharge and charge curves of γ-Fe2O3 nanosheets in the voltage range of 0.05-3.0 V at the current of 100 mA·g-1. The second discharge and charge capacities declined and the capacity fading of electrode might be attributed to the electrolyte decomposition and formation of solid electrolyte interface (SEI) layer. After 40 cycles, the discharge and charge capacities began to rise up, which means the cell was activated after cycles. More importantly, the capacities continue to rise and at 100th cycle the discharge capacity of γ-Fe2O3 nanosheets reaches 1000 mAh· g-1, which almost equal to its theoretical capacity, and maintains stable from then on. The cycling performances of the γ-Fe2O3 nanosheets can be clearly seen in Figure 4d. The discharge and charge capacity is 1047.1 mAh· g-1 and 1021.1 mAh· g-1 at 120th cycle. The cycling performances of γ-Fe2O3 nanosheets at larger current densities of 400, 600 and 1000 mA·g-1 were also studied and the results are shown in Figure S4. As shown in Figure S4a, it can be found that the discharge capacities of the electrode at different current densities decline rapidly at first, but after about 50 cycles, the discharge capacities increase slowly with cycle number, which is consistent with the variation tendency of current density of 100 mA·g-1. This phenomenon might be attributed to the growth and dissolution of polymeric gel-like film made of decomposition products of the LiPF6/EC-DMC electrolyte on the active materials, which would release extra discharge capacity as cycling goes on.42,43 Figure S4b presents the rate capability of the γ-Fe2O3 electrode at different current densities ranging from 100 to 1000 mA· g-1. With the current density increasing from 100 to 200, 500, and 1000 mA g-1, the discharge specific capacity decreases and when the current density return to 100 mA g-1, the discharge specific capacity is recovered to a high level, implying a very stable cycling performance. Furthermore, at the different current densities, the specific capacities of γ-Fe2O3 nanosheets are all higher than those of α-Fe2O3 nanosheets. The remarkable lithium storage properties are probably related to the unique nanostructure of these γ-Fe2O3 nanosheets. Specifically, 2D graphene-like structures of γ-Fe2O3 would provide large effective surface areas that facilitate penetration of the electrolyte and transport of
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lithium ions in the electrode.44 The BET specific surface area of α-Fe2O3 and γ-Fe2O3 nanosheets are calculated to be 35.70 m2·g-1 and 30.27 m2· g-1, respectively. Meanwhile, the γ-Fe2O3 nanosheets with high crystallinity and structural stability would also help in retaining the pristine nanostructure upon cycling. TEM and XRD analyses of the Fe2O3 nanosheets after initial cycle are shown in Figure S5 and S6. Although the amount of the active material in the electrode is very small, besides a broad peak centered at 27° arising from impurities and substrate, the diffraction peaks can be still clearly observed that are indexed to cubic γ-Fe2O3, indicating the phase conversation of the nanosheets after long time cycling. From the TEM images, although the nanosheets are not smooth due to the lithium intercalating in and out of Fe2O3 nanosheets and there are also some impurities because reagents addition in electrode preparation, both samples basically keep the original two-dimensional morphology after lithiation, confirming the structural stability. As comparison, after 120 cycles the reversible capacity for α-Fe2O3 nanosheets was only 419.7 mAh· g-1, 65 % loss of its original value. It has been reported that with size less than 16 nm, γ-Fe2O3 is more stable than α-Fe2O3 owing to thermodynamic difference, which results in γ-Fe2O3 materials actually exhibits better storage and cyclic performance compared to α-Fe2O3 nanosheets.38,39 Conclusions In summary, graphene-like hematite α-Fe2O3 nanosheets are synthesized via a facile metal-ion-intervened method. The α-Fe2O3 nanosheets are single crystalline with a unit-cell thickness of 1.3 nm. Through simple calcination procedure, the α-Fe2O3 nanosheets are converted to magnetic maghemite (γ-Fe2O3) nanosheets. They are also singly-layered with thickness of about 0.85 nm. The Li-ion battery studies reveal that the ultrathin γ-Fe2O3 nanosheets exhibit great electrochemical performances with high capacity, excellent cycling stability and after 100 cycles, the capacities can reach its theoretical value. We believe that these ultrathin iron oxide nanosheets could be promising function materials with great application potentials in various fields. Acknowledgement: This work is supported by the National Basic Research Program of China (Grant No. 2013CB922102), the National Natural Science Foundation of China (Grant No. 21471076 and
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51172106) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Competing Financial Interests: The authors declare no competing financial interests.
Supporting Information: SEM images, TEM images, CV curves and magnetism curves.
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References 1.
M.A. El-Sayed, Acc. Chem. Res. 37 (2004) 326.
2.
H.H. Han, S.J. Lee, J.W. Cheon, Chem. Soc. Rev. 42 (2013) 2581.
3.
S. Ithurria, M.D. Tessier, B. Mahler, R.P.S.M. Lobo, B. Dubertret, A.L. Efros, Nat. Mater. 10 (2011) 936.
4.
A.K. Geim, Science 324 (2009) 1530.
5.
A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183.
6.
F. Bonaccorso, Z. Sun, T. Hasan, A. C. Ferrari, Nat. Photonics 4 (2010) 611.
7.
K.P. Loh, Q.L. Bao, G. Eda, M. Chhowalla, Nat. Chem. 2 (2010) 1015.
8.
K.P. Wang, J. Wang, J.T. Fan, M. Lotya, A. O’Neill, D. Fox, et. al. ACS Nano 7 (2013) 9260.
9.
Q.H. Wang, K. Kalantar-Zadeh, A. Kis, J.N. Coleman, M.S. Strano, Nat. Nanotechnol. 7 (2012) 699.
10. S.Z. Butler, S.M. Hollen, L. Cao, Y. Cui, J.A. Gupta, H.R. Gutiérrez, et. al. ACS Nano 7 (2013) 2898. 11. V. Nicolosi, M. Chhowalla, M.G. Kanatzidis, M.S. Strano, J.N. Coleman, Science 340 (2013) 1420. 12. Z. Zeng, Z. Yin, X. Huang, H. Li, Q. He, G. Lu, et. al. Angew. Chem. Int. Ed. 50 (2011) 11093. 13. S.S. Chou, B. Kaehr, J. Kim, B.M. Foley, M. De, P.E. Hopkins, et. al. Angew. Chem. Int. Ed. 52 (2013) 4160. 14. S. Tongay, J. Zhou, C. Ataca, K. Lo, T.S. Matthews, J. Li, et. al. Nano Lett. 12 (2012) 5576. 15. J.W. Seo, Y.W. Jun, S.W. Park, H. Nah, T. Moon, B. Park, et. al. Angew. Chem. Int. Ed. 46 (2007) 8828. 16. K.G. Zhou, N.N. Mao, H.X. Wang, Y. Peng, H.L. Zhang, Angew. Chem. Int. Ed. 50 (2011) 10839. 17. H. Fang, S. Chuang, T.C. Chang, K. Takei, T. Takahashi, A. Javey, Nano Lett. 12 (2012) 3788. 18. Z. Zeng, T. Sun, J. Zhu, X. Huang, Z. Yin, G. Lu, et. al. Angew. Chem. Int. Ed. 51 (2012) 9052. 19. C. Lin, X. Zhu, J. Feng, C. Wu, S. Hu, J. Peng, et. al. J. Am. Chem. Soc. 135 (2013) 5144.
- 14 -
20. P. Goli, J. Khan, D. Wickramaratne, R.K. Lake, A.A. Balandin, Nano Lett. 12 (2012) 5941. 21. J. Feng, X. Sun, C. Wu, L. Peng, C. Lin, S. Hu, et. al. J. Am. Chem. Soc. 133 (2011) 17832. 22. J. Feng, L. Peng, C. Wu, X. Sun, S. Hu, C. Lin, et. al. Adv. Mater. 24 (2012) 1969. 23. K. Xu, P.Z. Chen, X.L. Li, C.Z. Wu, Y.Q. Guo, J.Y. Zhao, et. al. Angew. Chem. Int. Ed. 52 (2013) 10477. 24. K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, et. al. Proc. Natl. Acad. Sci. U. S. A. 102 (2005) 10451. 25. Y.H. Lee, X.Q. Zhang, W. Zhang, M.T. Chang, C.T. Lin, K.D. Chang, et. al. Adv. Mater. 24 (2012) 2320. 26. K.K. Liu, W. Zhang, Y.H. Lee, Y.C. Lin, M.T. Chang, C.Y. Su, et. al. Nano Lett. 12 (2012) 1538. 27. J.N. Coleman, M. Lotya, A. O’Neill, S.D. Bergin, P.J. King, U. Khan, et. al. Science 331 (2011) 568. 28. X.D. Zhang, Y. Xie, Chem. Soc. Rev. 42 (2013) 8187 29. X. Zhang, J. Zhang, J. Zhao, B. Pan, M. Kong, J. Chen, et. al. J. Am. Chem. Soc. 134 (2012) 11908. 30. C. Schliehe, B.H. Juarez, M. Pelletier, S. Jander, D. Greshnykh, M. Nagel, et. al. Science 329 (2010) 550. 31. T. Yu, B. Lim, Y. Xia, Angew. Chem. Int. Ed. 49 (2010) 4484. 32. J.H. Wang, M.X. Gao, H.G. Pan, Y.F. Liu, Z. Zhang, J.X. Li, et. al. J. Mater. Chem. A, 3(2015) 14178. 33. Y.K. Wang, L.C. Yang, R.Z. Hu, W. Sun, J.W. Liu, Y.K. Wang, et. al. J. Power Sources, 288 (2015) 314. 34. Y.K. Wang, L.C. Yang, R.Z. Hu, L.Z. Ouyang, M. Zhu, Electrochim Acta, 125 (2014) 421. 35. L.Z. Ouyang, Z.J. Cao, H. Wang, R.Z. Hu, M. Zhu, J. Alloys Compd. 691 (2017) 422. 36. J.J. Zhang, T. Huang, Z.L. Liu, A.H. Yu, Electrochem Commun. 29 (2013) 17. 37. B.M. Oscar, M. Lena, N. Alexandra, V.V. Sabino, Chem. Mater. 20(2008) 591.
- 15 -
38. H. Srirama, S. Kuppan, B. Palani, Electrochem Solid-State Lett. 13(2010) A132. 39. R.M. Liu, Y.W. Jiang, Q.Y. Lu, W. Du, F. Gao, Cryst. Eng. Comm. 15 (2013) 443. 40. N. Kang, J.H. Park, J. Choi, J. Jin, J. Chun, I.G. Jung, et. al. Angew. Chem. Int. Ed. 51 (2012) 6626. 41. B. Wang, J.S. Chen, H.B. Wu, Z.Y. Wang, X W. Lou, J. Am. Chem. Soc. 133 (2011) 17146. 42. F.M. Courtel, H. Duncan, Y. Abu-Lebdeh, I.J. Davidson, J. Mater. Chem. 21 (2011) 10206. 43. S.H. Choi, Y.C. Kang, ChemSusChem 6 (2013) 2111. 44. L. Zhang, H.B. Wu, S. Madhavi, H.H. Hng, X.W. Lou, J. Am. Chem. Soc. 134 (2012) 17388.
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Figure captions Figure 1 XRD patterns of the obtained nanosheets: (a) α-Fe2O3 and (b) γ-Fe2O3. Figure 2 (a, b) SEM images; (c) TEM image; (d) HRTEM image (inset: SAED pattern); (e) AFM image and the corresponding height profile; (f) cell unit of the obtained ultrathin α-Fe2O3 nanosheets. Figure 3 (a, b) SEM images; (c) TEM image; (d) HRTEM image; (e) AFM image and the corresponding height profile; (f) cell unit of the obtained ultrathin γ-Fe2O3 nanosheets. Figure 4 (a) First discharge-charge curves of the two ultrathin iron oxide nanosheets; b) Cyclic voltammograms (CVs) of ultrathin γ-Fe2O3 nanosheets; c) discharge-charge curves of ultrathin γ-Fe2O3 nanosheets at the 1st, 2nd, 10th, 40th, 80th, 120th cycle at the current density of 100 mA·g-1; d) cycling performances of the two ultrathin iron oxide nanosheets.
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20
30
40
50
2θ/degree
Figure 1
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440 214 300
116
024
113
a
511
400
311
220
104 110
012
Intensity/a.u.
b
60
γ-Fe2O3
α-Fe2O3
70
80
Figure 2
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Figure 3
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b
3 γ-Fe2O3 α-Fe2O3
2
0.3 0.2
Current/mA
+
Potential/V vs Li / Li
a
1
0.1 0.0 -0.1
3rd cycle 1st cycle
-0.2 0 -0.3 0
400
800
1200
0
1
-1
2
3
+
Capacity/mAh g
Potential/V vs. Li /Li
d 1600
c
-1
Specific capacity/mAh g
+
Potential/V vs Li / Li
3
2
1
1200
γ-Fe2O3 α-Fe2O3
800
400
0 th
40
th
80
th
10
nd
2
th
st
120
1
0
0
400
800
1200
0
-1
20
40
60
Cycle number
Capacity/mAh g
Figure 4
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80
100
120
Highlights Unit-cell thick α-Fe2O3 nanosheets with thickness of 1.3 nm were firstly synthesized. The nanosheets were synthesized through a metal-ion-intervened hydrothermal method. The α-Fe2O3 nanosheets were converted to magnetic γ-Fe2O3 nanosheets by a calcination process. The obtained magnetic γ-Fe2O3 nanosheets were with unit-cell thickness of 0.8 nm The unit-cell-thick γ-Fe2O3 nanosheets exhibit great enhanced electrochemical performances.
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