Large scale production of nanoporous graphene sheets and their application in lithium ion battery

Large scale production of nanoporous graphene sheets and their application in lithium ion battery

Accepted Manuscript Large Scale Production of Nanoporous Graphene Sheets and Their Application in Lithium Ion Battery Jianan Zhang, Binghao Guo, Yongq...

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Accepted Manuscript Large Scale Production of Nanoporous Graphene Sheets and Their Application in Lithium Ion Battery Jianan Zhang, Binghao Guo, Yongqiang Yang, Wenzhuo Shen, Yanmei Wang, Xuejiao Zhou, Haixia Wu, Shouwu Guo PII: DOI: Reference:

S0008-6223(14)01200-7 http://dx.doi.org/10.1016/j.carbon.2014.12.039 CARBON 9572

To appear in:

Carbon

Received Date: Accepted Date:

27 August 2014 11 December 2014

Please cite this article as: Zhang, J., Guo, B., Yang, Y., Shen, W., Wang, Y., Zhou, X., Wu, H., Guo, S., Large Scale Production of Nanoporous Graphene Sheets and Their Application in Lithium Ion Battery, Carbon (2014), doi: http://dx.doi.org/10.1016/j.carbon.2014.12.039

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Large Scale Production of Nanoporous Graphene Sheets and Their Application in Lithium Ion Battery

Jianan Zhanga, Binghao Guo b, Yongqiang Yanga, Wenzhuo Shena, Yanmei Wanga, Xuejiao Zhoua, Haixia Wua*, and Shouwu Guo a* a

Department of Electronic Engineering, School of Electronic Information and

Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China b

High School Affiliated to Shanghai Jiao Tong University, Shanghai, 200439,P. R.

China Tel/Fax: 0086-21-34206915, E-mail: [email protected], [email protected]

Abstract Nanoporous graphene sheets were generated through a simple thermal annealing procedure using composites of ferrocene nanoparticles and graphene oxide sheets as precursors in a large scale. The morphology, composition, and formation mechanism of the as-obtained nanoporous graphene sheets were studied complementarily with

scanning

electron

microscopy,

transmission

electron

microscopy, X-ray powder diffraction, and other spectroscopy techniques. We found that the density of nanopores on the graphene sheet was determined by the surface distribution of oxygen-containing groups on the original graphene oxide sheets. The coin cells using nanoporous graphene sheets as anode materials showed higher 1

specific lithium ion storage capacity, better discharge/charge rate capability and higher cycling stability when compared to the coin cells with graphene or chemically reduced graphene sheets as anodes.

2

1. Introduction Owing to its ultra-large specific surface area, high carrier mobility and extraordinary lithium ion/atom storage capacity, graphene sheet has been considered as one of the most promising anode materials for lithium ion battery (LIB) [1-7]. However, there are two problems remained to be overcome using bare graphene sheets as anode materials[2]. First, graphene sheets aggregate severely due to the π-π stacking interaction, which affects lithium ion insertion/extraction and limits the permeation of electrolyte. As a result, the specific capacity, rate capability, and cycling stability of the anode made up of bare graphene sheets are distressed[8, 9]. Second, when using pristine graphene sheet as anode material, the cross-plane diffusion of electron and lithium ion is extremely low, which severely influences the rate capability under high discharge/charge current density[10-14]. Therefore, derivatives of graphene sheets and graphene based composites have been rationally designed and prepared as anodes of LIBs. For instance, by doping heteroatoms or introducing functional groups, a series of functionalized graphene sheets were fabricated[15-21], and these modifications can not only suppress the inter-plane aggregation of graphene sheets, but also improve their electrochemical properties to some extent[15-21]. However, the cost for effective fabrication of functionalized graphene sheets in bulk scale is still very high. Meanwhile, various composites of graphene with different nanostructured materials have been used as anodes in LIBs as well. The aggregation of these graphene sheets was inhibited[15], and the lithium ion capacity of graphene sheets was enhanced accordingly[7, 16, 17].In some cases, the 3

specific lithium ion capacity of graphene based composites could be larger than the theoretical capacity due to their unique three dimensional (3D) structures. We previously reported a novel category of composites of chemically reduced graphene oxide (GO) sheets with carbon nanoshperes assuming uniform 3D network as anode materials, and the 3D composites showed the highest reversible specific capacity of 925 mAh g-1 and 604 mAh g-1 at charge-discharge current densities of 5 A g-1 ( C-rate of 13.5 C, based on a theoretical graphite capacity of 372 mAh/g of graphite) and 10 A g-1, respectively[18]. The carbon nanoshperes in these 3D composites were fully cladded and bridged with graphene sheets forming cavities and pores. However, introducing extra components to graphene based composites may decline the final volume energy density or even the gravimetric energy density of lithium ion batteries[12, 19, 20]. More recently, porous graphene (also called holey graphene) sheets were applied as electrode materials in LIBs and supercapacitors[20-23]. It shows that the porous feature can increase its specific surface area and provide more lithium ion binding sites, thus increase the energy density of the electrode. Additionally, the pores on graphene sheets may serve as channels for the cross-plane ion and electron transportation, which can enhance the rate capability in theory. The high density nanopores might also reduce the π-π stacking interaction between graphene sheets, which should be beneficial to the cycling stability of the electrode. Hence, long term ultrasonication of GO under harsh acidic condition[20], oxidization of GO by H2O2 with gold or AgO nanoparticle as catalyst[23, 24], and photo-degradation of GO 4

sheets on the top of ZnO nanorods arrays[25] have been employed for the fabrication of porous graphene sheets. However, to cost-effectively fabricate nanoporous graphene sheets in bulk scale remains challenging. Herein we reported a simple chemical approach to nanoporous graphene sheets in a large scale using the composites of ferrocene and graphene oxide as precursors. The application of the as-prepared porous graphene sheets as anode materials for lithium ion battery was investigated. The anode prepared with the as-obtained porous graphene sheets showed larger specific capacity, higher rate capability, and better cycling stability than pristine graphene sheets. 2. Experimental Fabrication of nanoporous graphene sheets: GO sheets used in this work were prepared from graphite powder through a modified Hummers method that we described previously[26, 27]. As the precursor for nanoporous graphene sheets, the composite of ferrocene and GO (ferrocene/GO) was prepared through a solvothermal procedure. In a typical experiment, the ethanol dispersion of GO (1 mg/mL) was prepared under ultrasonication. Subsequently, different amounts of ferrocene powder were added into GO ethanol dispersion to get the GO to ferrocene ratios (in weight) of 20:1, 1:1, 1:20, and 1:50. The as-prepared dispersions were transferred into Teflon-lined autoclaves (50 mL), and solvothermally treated at 180°C for 4 h. The solid products, ferrocene/GO composites, were separated through filtration, washed thoroughly using ethanol to remove the residue ferrocene, and finally dried in air. The as-obtained ferrocene/GO composites were put into a quartz tube and annealed at 5

900°C for 4 h in a flow of Ar containing 5% of H2 at a flow rate of 100 mL/min, meanwhile the ferrocene was decomposed into iron/Austenite nanosphere. Finally, the iron/Austenite nanosphere was removed with 10% of HCl and then porous graphene sheets were separated by filtration and annealed in nitrogen at 900°C for 30 min. Characterization: Scanning electron microscopy (SEM) images were acquired on an Ultra 55 field-emission scanning electron microscope (FESEM, Zeiss, Germany).

Transmission electron microscopy (TEM) and

High resolution

transmission electron microscopy (HRTEM) images were obtained using a JEM-2010 transmission electron microscope (JEOL Ltd., Japan). The TEM samples were prepared by putting a droplet of water suspension of the composite on a copper grid, followed by the removal of the solution by tilting the grid on a piece of filter paper. X-ray powder diffraction (XRD) patterns were obtained on a D/max-2600 PC diffractometer (Rigaku, Japan) using Cu/Kα radiation (λ = 1.55406 Å). The FT-IR spectra were recorded on a VERTEX70 FT-IR spectrometer (Bruker, Germany). The specimens for FT-IR measurement were prepared by grinding the dried powder of the composites with KBr together and then compressed into thin pellets. Electrochemical property measurement: The electrochemical performance of nanoporous graphene sheets was carried out on 2025 type coin cells. In the coin cells, the working electrodes were composed of nanoporous graphene sheets and poly(vinyl difluoride) (PVDF) with a weight ratio of 4:1 using the copper foil as current collector. The thin lithium plate was used as the counter electrode. The electrolyte was 1 M of LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, in volume). The 6

cells were assembled in an argon-filled glove box. The electrochemical property measurements were conducted at various current densities in a voltage range of 0.01-3.00 V. Electrochemical impedance spectroscopy (EIS) were acquired on an electrochemical working station (CHI 660C) (Shanghai, China) by applying a perturbation voltage of 5 mV in a frequency range of 0.01 Hz to 100 KHz. The cyclic voltammetry (CV) was obtained over the voltage range from 0.01 to 3.00 V at a scanning rate of 0.1 mV/s, 0.5 mV/s and 1mV/s using an electrochemical working station (CHI 660C) (Shanghai, China). 3. Results and discussion As aforementioned, GO sheets can be converted into porous graphene sheets through deep oxidizations by using noble metals or metal oxides as catalysts [23, 24], which limits the bulk scale preparation of porous graphene sheets. In this work, the porous graphene sheets were generated from the composites of ferrocene/GO which certainly costs less. Figure 1 shows the ferrocene/GO composites prepared with different initial ratios (in weight) of ferrocene to GO. When the ratio of ferrocene to GO is 1:20, there are almost no clusters generated on the GO surface, as shown in Figure 1a. Once the ratios were raised to 1:1, 20:1 and 50:1, as illustrated in Figure 1b, c and d, the amount of clusters on GO surface was increased considerably. It is worthwhile noting that the average size of the clusters is about 80 nm in diameter estimated from the SEM images irrespective of the surface density change of ferrocene clusters. The results were confirmed independently with TEM images (Figure S1). To ascertain the composition of clusters formed on the GO surface, 7

XRD patterns of the composites were acquired and compared with bare ferrocene crystalline powder, as shown in Figure 1e. It could be found that the major diffraction peaks of the composite, at 15.26°, 17.50°, 18.38° and 19°, fit well with the ferrocene crystals[28]. This implies that the clusters formed on GO surface are exactly the ferrocene. Besides the ferrocene to GO ratio, the effects of solvothermal temperature and reaction time on the formation of ferrocene clusters on GO surface were also studied. However, as shown in Figure S2, the sizes and surface densities of ferrocene clusters on GO surface almost keep the same when the solvothermal temperatures vary from 110°C to 220°C, and the reaction times range from 1 to 4 h. There are two factors affecting the surface density of the ferrocene clusters: first one is the initial ratio of ferrocene to GO, the other important one is the oxygen-containing groups on GO since they serve as nucleation sites for ferrocene clusters. To prove this assumption, the GO sheets were pre-reduced and then solvothermally treated with ferrocene (ferrocene to GO ratio of 50:1). As shown in Figure S3, the surface density of the ferrocene clusters was decreased dramatically, because the majority of oxygen-containing groups on GO were removed by the pre-reduction and there were not enough binding/anchoring sites for ferrocene clusters. The ferrocene powder can be decomposed into Fe and gaseous organic species at 900-1000°C in hydrogen atmosphere[29]. The as-obtained ferrocene/GO composites were thermally annealed at 900°C in a flow of Argon containing 5% of H2 for 4 hours accordingly. As depicted in Figure 2, the ferrocene clusters were converted 8

into nanospheres with the average size of 50 nm in diameter after annealing, which is smaller than the original ferrocene cluster (~80 nm). Moreover, the TEM images (Figure 3a and b) showed that the nanospheres assume a core-shell structural motif. The electron density of the core is darker than the shell. To define the composition of the as-generated core-shell nanospheres, their XRD data were collected and shown in Figure 4a. The diffraction peaks corresponding to ferrocene nanoparticles disappeared. Instead, new diffraction peaks at 43.6º, 50.8º and 74.9º corresponding to the diffractions of (111), (200), (220) of Austenite (JCPDS, file 23-0298), and the diffractions of (110), (200) of Fe at 44.5º and 65.0º (JCPDS, file 01-1262) were detected. These results reveal that the as-generated nanospheres should assume Fe/Austenite core-shell motif. To further confirm the core-shell structure, the HRTEM images were acquired. As depicted in Figure 3c and 3d, the core showed the Fe (110) lattice fringes with a d-spacing of ~0.21 nm, and the shell exhibited the Austenite (200) lattice fringes with a d-spacing of ~0.16 nm, demonstrating the Fe/Austenite core-shell structural motif clearly. More interestingly, there were nanopores generated near the Fe/Austenite core-shell nanopheres on the GO surface, as shown in Figure 2b, c, and d. The average size of nanopores is 40 nm in diameter, and their distribution is closely related to the number of nanospheres. It demonstrates that when ferrocene nanoclusters are decomposed into Fe nanoparticles and gaseous organic species at a relatively high temperature (~900°C) [29], the as-generated Fe nanoparticles snatch carbon atoms from the adjacent areas of GO sheets forming Austenite structure, and generated nanopores on the GO sheets. Additionally, GO sheets were reduced to 9

graphene sheets during thermal annealing[27, 30-32]. To prove this, the FTIR spectrum of the composite after thermal annealing was acquired and compared with GO sheets. As shown in Figure 4b, the vibration band of -OH at 3380 cm-1, the stretching vibration bands of C=O, and C-O of epoxy and alkoxy at 1728 cm-1, 1218 and 1060 cm-1, respectively, were decreased or disappeared after thermal annealing treatment, undoubtedly illustrating that GO sheets in the composites were thermally reduced to graphene. It is well known that Fe can absorb carbon atoms at high temperature, which is the key factor to the formation of nanopores. What’s more sophisticated is the choice of temperature. When the temperature is higher than 1000°C, the as-generated Fe nanoparticles melt and coagulated randomly, and this process impairs the original distribution of Fe nanospheres on graphene sheets, which then certainly change the uniform distribution and morphology of nanopores on the graphene sheets[33]. When the temperature is lower than 900°C, the annealing process cannot get nanopores on the graphene sheets, as shown in Figure S4. The Fe/Austenite nanospheres in the annealed composites were removed by washing with diluted HCl (10%), and porous graphene sheets were obtained as shown in Figure 5. There are more pores generated at the edge of GO sheets as shown in Figure 5d, possibly because there are more oxygen-containing groups at the periphery area of original GO sheets[34-37], where more ferrocene clusters could be deposited leading to large amounts of Fe/Austenite and nanopores at this area accordingly. The as-generated nanoporous graphene sheets were further characterized with TEM, as shown in Figure S5. It could be found that the sheet-like motif of graphene was 10

preserved well and the average pore size was in agreement with the SEM results. On the basis of these results, the formation mechanism of nanoporous graphene sheets was summarized and shown in Figure 6. Briefly, the ferrocene clusters were grown solvothermally on GO with the oxygen-containing groups as nucleation sites. Subsequently, the ferrocene clusters were decomposed into iron and gaseous organic species, and GO sheets were thermally reduced to graphene at high temperature. Meanwhile, the as-generated Fe nanoparticles grabbed the carbon atoms from graphene forming Austenite shell on it and generated nanopores on graphene. Finally, the Fe/Austenite nanosphere was removed with diluted HCl generating pure nanoporous graphene sheets. To explore the application of nanoporous graphene sheets in lithium ion battery, the coin cells using porous graphene sheets as anode materials were assembled. As depicted in Figure 7a, at a current density of 0.2 A g-1 (0.53 C), the first cycle discharge and charge capacities of coin cells can reach 1769.7 and 980.1 mA h g-1, respectively. However, the estimated Coulombic efficiency (CE) is only of ~ 56%. The low Columbic efficiency showed during first charge/discharge cycle can be assigned to the reaction of residual oxygen-containing groups with Li ion, the high specific surface area of graphene, and the subsequent SEI formation. Nevertheless, starting from the second discharge/charge cycle, the CE was increased rapidly to 90.4%, and the specific discharge and charge capacities were remained at 1036.2 and 936.3 mA h g-1, respectively. Pronouncedly, after 100 cycles of discharge/charge, the specific capacity of the coin cell still retained at 800 mA h g-1, and the CE was up to 11

95.2%. This reflects the good cycling stability of the anode prepared with nanoporous graphene sheets. As illustrated in Figure 7c, when the current densities of charge/discharge increased to 0.5, 1, 2, 5, and 10 A g-1 sequentially, the as-measured specific capacities were 717.1, 626.1, 518.4, 429.9 and 346.5 mAhg-1, respectively, exhibiting a good rate capability. The sequential cyclic voltammetry on the coin cells was also acquired and the results were shown in Figure S6. The lithium ion diffusion coefficient calculated using the Randles-Servick equation is of ~1.0×10-9cm2/s. For comparison, the coin cells with bare hydrothermally reduced GO sheets as anodes were also assembled and their discharge/charge voltage profiles were acquired. As shown in Figure 7b, the first cycle discharge and charge capacities were 808.6 and 489 mA h g-1, respectively, with the current density of 0.2 A g-1 (0.53C). When the charge/discharge current densities increased to 0.5, 1, 2, 5, and 10 A g-1, the as-measured specific capacity of the coin cells fell down to 348.1, 255.8, 214.5, 161.3, and 133.1 mA h g-1, which were much lower than the coin cells made up of nanoporous graphene sheets as anodes (Figure 7c). It has been reported that the porous structures on graphene sheets can increase the specific capacity through a defect-induced lithium metal plating mechanism[38]. To confirm this, two coin-cells, one was lithiated and the other one was delithiated, were unlocked and the XRD patterns of the anode materials were acquired (Figure S7). Clearly, the diffraction peaks of Li and Li3C8[39] were observed from the lithiated anode using the nanoporous graphene sheets, but not from the delithiated one. The results indicate that the defect-induced lithium plating on the nanoporous graphene sheets do occur in the 12

work. To investigate the charge-transfer ability of porous graphene sheets, the electrochemical impedance spectroscopy (EIS) was acquired. As illustrated in Figure 7d, the diameters of semicircles in both high and medium frequency areas become smaller evidently after 10 cycles in comparison with the first two cycles revealing the decrease of the contact and charge-transfer impedances, which are similar to the most carbonaceous anode reported so far[18, 40, 41]. Notably, the impedance plots after 10 and 100 cycles are almost identical reflecting a good cycling stability of the anode, which is in agreement with the discharge/charge measurements. For comparison, the EIS spectrum of the chemically reduced graphene oxide sheets was also recorded (Figure S8). The corresponding equivalent circuits were proposed, and their charge-transfer resistances were estimated and shown in Table S1. Notably, charge-transfer resistance of nanoporous graphene sheets seems much lower than that of the chemically reduced graphene sheets. This may partially explain the enhanced electrochemical performance of the coin cells with nanoporous graphene sheets as anode. In general, the graphene sheet assumes larger theoretical specific capacity than graphite, but the rate capability and cycling stability are relatively poor due to the irreversible aggregation of graphene sheets during the discharge/charge process[3, 10]. As illustrated above, the nanoporous graphene sheets show high specific capacity for lithium ion storage as well as greater rate capability and cycling stability. It suggests that nanopores suppress the π-π stacking interactions among the graphene sheets. To get insight on the status of nanoporous graphene sheets after the discharge/charge 13

cycling, the coin cells were unlocked after 500 discharge/charge cycles, and the cycled anode materials were studied. As shown in Figure 8a, the SEM images of anode materials illustrated that the sheet-like motif of the nanoporous graphene was preserved very well after the discharge/charge cycling. Conversely, the chemically reduced GO sheets, as shown in Figure 8b, aggregated severely. These illustrated unambiguously that the nanopores play a key role in suppressing the irreversible aggregation of the graphene sheets. What’s more, the nanopores of graphene provided channels for the lithium ion and the electrolyte cross-plane transportation. Therefore, the nanoporous graphene sheet should be one of the optimal anode materials for lithium ion batteries[42-44]. 4. Conclusion A facile and scalable approach to prepare nanoporous graphene sheets is developed. We demonstrate that under a solvothermal condition ferrocene nanoclusters can be grown onto the GO sheets with oxygen-containing groups as nucleation sites, forming ferrocene/GO composites. The ferrocene nanoclusters are then decomposed into Fe nanoparticles and the GO sheets are reduced to graphene at high temperature in a reduction atmosphere. Meanwhile, Fe nanoparticles snatch the carbon atoms from graphene sheets forming Fe/Austenite core-shell nanoparticles, and generate nanopores on graphene sheets. Nanoporous graphene sheets are finally obtained by removing the Fe/Austenite nanoparticles. We also emphasize that the anode prepared with nanoporous graphene sheets exhibit higher specific lithium ion storage capacity, better discharge/charge rate capability and higher cycling stability 14

than anodes made up of graphene sheets. We believe that nanopores play a key role in suppressing aggregation of graphene sheets during the discharge/charge cycling and also serve as short-cuts for cross-plane diffusion of the lithium ions and electrolytes. We therefore envisage that the as-prepared nanoporous graphene sheets should be an optimal anode material for lithium ion battery.

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Figure 1 a-d) SEM images of the composites of ferrocene nanoclusters and GO (ferrocene/GO) prepared with different initial ratios of ferrocene to GO (in weight) of 1: 20, 1:1, 20:1, and 50:1, respectively. e) X-ray powder diffraction patterns of the ferrocene/GO composites, and bare ferrocene before and after the solvothermal treatment (180°C, 4 h).

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Figure 2 a-d) SEM images of ferrocene/GO composites with initial ferrocene to GO ratios (in weight) of (a) 1:20, (b) 1:1, (c) 20:1, and (d) 50:1 after being annealed in argon containing 5% of H2 at 900°C for 4 h.

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Figure 3 a, b) TEM images of ferrocene/GO composites with initial ferrocene to GO ratios (in weight) of 20:1 after being annealed in argon containing 5% of H2 at 900°C for 4 h, revealing the core/shell structural motif of the as-generated nanoparticles and the formation of the nanopores. c, d) HRTEM images of Fe/Austenite nanoparticles showing the lattice structure of Fe and Austenite, and the interface.

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Figure 4 a) X-ray powder diffraction patterns of the ferrocene/GO composites after being annealed at 300 °C, 500 °C, 700 °C, 900 °C for 4 h. b) FTIR spectra of the as-generated nanoporous graphene and thermally reduced GO sheets

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Figure 5 SEM images of nanoporous graphene sheets prepared with the ferrocene/GO composites with initial ferrocene to GO ratios (in weight) of a) 1:20, b) 1:1, c) 20:1, and d) 50:1.

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Figure 6 Schematic representation of the formation mechanism of nanoporous graphene.

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Figure 7 a) The discharge/charge voltage profiles for nanoporous graphene (generated from the ferrocene/GO composite with initial ferrocene to GO ratio of 50:1) acquired with a current density of 0.2A/g (C-rate of 0.53 C) during 1st, 2nd, 5th, 10th, and 100th discharge/charge cycle. b) The discharge/charge voltage profiles for thermally reduced GO sheets obtained with a current density of 0.2A/g (C-rate of 0.53 C) during 1st, 2nd, 5th, 10th, and 100th discharge/charge cycles. c) The discharge/charge performance of nanoporous graphene (50:1) and thermally reduced GO sheets measured at different current densities revealing higher specific capacity and good rate capability of nanoporous graphene. d) AC impedance spectra of the electrode using nanoporous graphene sheet after 1, 2, 5, 10 and 100 charge/discharge cycles acquired at an amplitude of 5 mV versus the open circuit potential with a frequency range of 0.01 Hz-100 kHz.

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Figure 8 a) SEM image of nanoporous graphene (50:1) after 500 discharge/charge cycles at a current density of 0.1 A/g (C-rate of 0.26C) b) SEM image of thermally reduced GO after 500 discharge/charge cycles at a current density of 0.1A/g (C-rate of 0.26C).

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ACKNOWLEDGEMENT We thank National “973 Program” of China (No. 2014CB260411), the National Science foundation of China (No. 90923041, 11374205), the Science and Technology Commission of Shanghai Municipality (No. 12nm0503500), and National “863” Program of China (No. 2012AA022603) for financial support of this work.

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