Enhanced Cyclability for Sulfur Cathode Achieved by a Water-Soluble

Jul 1, 2011 - The zeta potentials were measured with a Zeta-Meter4.0+. .... Figure 2. CV curves of the sulfur cathodes at a scanning rate of 0.1 mV sâ...
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Enhanced Cyclability for Sulfur Cathode Achieved by a Water-Soluble Binder Min He,†,‡ Li-Xia Yuan,*,† Wu-Xing Zhang,† Xian-Luo Hu,† and Yun-Hui Huang*,† †

School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mold Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China ‡ School of Science, Wuhan University of Science and Technology, Wuhan, Hubei 430065, China ABSTRACT: The electrochemical properties of sulfur cathodes based on commercially available sulfur powder (S) and water-soluble binder have been investigated. The mixture of styrene butadiene rubber (SBR) and sodium carboxyl methyl cellulose (CMC) is used as the binder. Compared with conventional poly(vinylidene fluoride) (PVDF) binder, the SBR CMC binder significantly improves cycling performance of the sulfur cathode. A high specific capacity of 580 mA h g 1 after 60 cycles has been achieved. Studies on the electrode slurries show that the SBR CMC mixture is not only a high adhesion agent but also a strong dispersion medium, which favors the uniform distribution between insulating sulfur and conductive carbon black (CB) and ensures a good electrical contact, leading to a high sulfur utilization. Furthermore, our experiments show that the improvement in cyclability is ascribed to structural stability of the sulfur cathode promoted by the SBR CMC binder during charge/discharge cycles due to the combined effects of homogeneous distribution of the S and CB particles in the composite cathode, the low electrolyte uptake, and the suppressed agglomeration of Li2S.

1. INTRODUCTION Lithium sulfur rechargeable batteries have attracted growing attention in recent years due to their high energy density that can meet the requirement for large-scale power systems. As a cathode material, elemental sulfur (S) shows almost the highest theoretical specific capacity of 1675 mA h g 1 and the highest theoretical specific energy density of 2500 W h kg 1 among all the known cathode materials for lithium rechargeable batteries, assuming the complete reaction of lithium with sulfur to Li2S. In addition to the high capacity, elemental sulfur also benefits from advantages of natural abundance, low cost, and environmental friendliness, which are important for the next generation of lithium batteries. However, rechargeable Li S batteries suffer from low sulfur utilization and poor rechargeability. The problems can be ascribed to the electrically insulating nature of S and the discharge products (Li2S2, Li2S) and to the loss of active material in the form of soluble reaction intermediates of polysulfides (Li2Sx, 2 < x e 8).1,2 Many efforts have been made to solve these problems, such as preparing various sulfur composites via incorporating sulfur into conducting materials to improve the conductivity,3 8 optimizing the cathode,9,10 or electrolyte composition11 13 to alleviate the dissoluble loss of sulfur in liquid electrolytes. Among them, the composites with sulfur embedded within conductive mesoporous carbon framework have been proven to be promising.5,6,8 By contacting with these mesoporous conductive species, the conductivity of the sulfur electrode can be greatly improved, and the r 2011 American Chemical Society

dissoluble loss of sulfur in liquid electrolyte can also be restrained due to the sorption properties of the carbon, leading to a remarkably improved reversible capacity even as high as 1320 mA h g 1 that corresponds to a 78% utilization of sulfur.8 Although many works have been devoted to optimizing the active materials, there are only a few reports on the effect of the electrode structure for the sulfur cathodes. For the insulating nature of S and its reduction products (Li2S2, Li2S), the redox reactions of sulfur cathode can only occur at the surface of the conductive species. The facile transport of the polysulfides to carbon matrix is very important to achieve high sulfur utilization. Furthermore, the electrochemical reduction of sulfur cathode is a complicated process composed of a series of electron transfer reactions, accompanied with repeated dissolution/deposition of the sulfur species. Therefore, the structure of the sulfur electrode will be inevitably changed during discharge/charge cycling.14 18 It was found that the sulfur composite electrodes expanded when discharging and shrank when charging again, and the change in the thickness of the electrode was measured to be about 22%.17 Cheon et al.19 indicated that the capacity fading in a high-sulfurloading cathode was mainly due to the structural collapse by physical crack propagation of the electrode and subsequent formation of the electrochemically irreversible Li2S layer at the Received: May 10, 2011 Revised: June 30, 2011 Published: July 01, 2011 15703

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The Journal of Physical Chemistry C cracked surface of carbon particles. Therefore, for the highperformance sulfur cathode, it is crucial to control the morphology of the cathode to attain stable structure and uniform carbon matrix. The role of polymer binders becomes particularly important for the morphology. In general, the cathode is obtained by depositing the electrochemically active material and conductive additives on a current collector assisted by a polymer binder. For fabrication of the electrode, the primary role played by the binder is to link different types of small particles together and to ensure the active material to adhere to the current collector. The most common binder used for sulfur cathode is poly(vinylidene fluoride) (PVDF) due to its high electrochemical stability and good connection between the electrode materials and current collectors.8,13,20 However, PVDF is always dissolved into some organic solvents with high boiling point, such as N-methy-l-2-pyrrolidone (NMP). To remove the NMP, a temperature higher than 80 °C is needed. Such a high temperature may lead to the loss of active sulfur by sublimation. On the other hand, NMP is a volatile and combustible solvent, which causes problems of safety and severe pollution.21,22 Moreover, PVDF is readily swollen, gelled, or dissolved by nonaqueous liquid electrolytes to form a viscous fluid or gel polymer electrolyte, which results in desquamation of electrode particles and hence capacity fading and cycle life shortening.23 25 Recent works have confirmed that an optimized binder shows a great positive impact on the electrochemical performance of sulfur cathodes. Jung et al. proved that a mixed polymer binder system of poly(vinylpyrrolidone) (PVP) and poly(ethyleneimine) (PEI) helped to maintain the initial morphology of the sulfur electrodes.26 Sun et al. reported that a gelatin binder was effective to improve the capacity of sulfur cathode.27 31 In these two systems, in addition to the conventional function to connect the electrode species together, the binder also acts as an effective dispersion agent to suppress the agglomeration of sulfur and carbon, helping to improve the electrochemical performance of the sulfur cathode. In the present work, a water-soluble binder composed of styrene butadiene rubber (SBR) and sodium carboxyl methyl cellulose (CMC) was applied to prepare the sulfur cathode. Compared with PVDF, the elastomeric SBR possesses higher flexibility, stronger binding force, and higher heat resistance.32 CMC, which has two functional groups, carboxylate anion and hydroxyl, is well-known as an effective dispersion and thickener agent for aqueous suspension. Recent research has shown that the SBR CMC binder can effectively stabilize the cyclability of Si-based anodes since the extended conformation of CMC in solution can facilitate the formation of a network among the conductive additive and Si particles during fabrication of the composite electrode.33 36 Similar to the Si-based anodes, the sulfur cathode also suffers from the poor conductivity and the mechanical stress induced by volume change that is caused by redox reaction of sulfur during the charge/discharge cycles. Therefore, we expect that the SBR CMC binder is useful for the sulfur cathode. In addition, the SBR CMC binder is soluble in environmentally friendly solvents such as water and ethyl acetate, which is very important for the future green batteries. Here, we focus on comparison between the water-soluble SBR CMC mixture binder and the traditional PVDF binder working on the sulfur cathode. The structure and electrochemical performance of the sulfur cathode during the charge/discharge process have been carefully investigated.

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2. EXPERIMENTAL METHODS All the cathodes were composed of 60 wt % active material (S powder, 99.5%, analytically grade, Shanghai, China), 30 wt % conductive additive (carbon black, CB, Shanghai, China), and 10 wt % binder. The elastomeric binder is a mixture of SBR (Shanghai, China) and NaCMC (analytical grade, DS = 0.9 1.0, Shanghai, China) in an empirically optimized weight ratio of 1:1. SBR was used as purchased in the form of an aqueous emulsion solution, and NaCMC was added to act as the thixotropic agent. For brevity, the mixture is hereafter referred to SBR CMC binder, and the sulfur electrode bound by this mixture is referred to as SBR CMC cathode. The slurry for the SBR CMC cathode was prepared by thoroughly mixing the components with deionized (DI) water by ball milling for 5 h. The PVDF (HSV900, ARKEMA, France) electrode in which PVDF was used as the binder was prepared by a similar route with the solid components dispersed in N-methylpyrrolidone (NMP). The cathodes were attained by casting the slurry onto Al foil and then being dried in a vacuum at 70 °C for 24 h. The dispersion morphologies for CB and/or S particles in the slurry systems with different binders were observed by highmagnification confocal microscopy (BX51M, Olympus). The zeta potentials were measured with a Zeta-Meter4.0+. For measurement, a small amount of CB and/or S suspension was dispersed in SBR CMC/DI water or PVDF/NMP solution, then diluted in the same solvent (DI water or NMP) by ultrasonic dispersion for 10 min, and finally equilibrated for 24 h before measurements. CR2032 coin-type cells were assembled in an Ar-filled glovebox with H2O and O2 content lower than 10 ppm. Sulfur electrode was used as cathode, microporous polypropylene membrane as separator, lithium sheet as anode, and 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (V/V = 1/1) containing 1 mol L 1 lithium bis(trifluoromethane sulfonyl)imide (LiN(SO2CF3)2, LiTFSI) as electrolyte. Cyclic voltammetry (CV) measurement was performed on a PARSTAT 2273 electrochemical workstation at a scanning rate of 0.1 mV s 1. The discharge/charge tests were carried out in the range of 1.0 3.0 V (vs Li+/Li) at a current density of 100 mA g 1 based on the mass of sulfur with cell testing machine (LAND Electronic Co. CT2001A, China). Electrochemical impedance spectroscopy (EIS) was measured with the fresh cells and the cells after 50 cycles at open circuit voltage. The frequency range applied was from 100 kHz to 100 mHz at potentiostatic signal amplitudes of 5 mV. All experiments were conducted at 25 °C. To measure the liquid weight absorbed by the film, several pieces of the sulfur cathodes prepared above were first accurately weighed and then immersed into the electrolyte solution of DME and DOL (V/V = 1/1) for up to 24 h at 25 °C. At regular intervals, the samples were taken out, pressed lightly between two sheets of clean blotting to remove the surface liquid, and then weighed. The swelling ratio was calculated by the weight ratio of the amount of solvent absorbed to the dry coat of the tested cathode sheets. In addition to the sulfur cathode, we also prepared SBR CMC and PVDF polymer films via the same method to get a direct comparison between the binders. The morphologies of the sulfur cathode before and after charge discharge test were observed with a field-emission scanning electron microscopy (FESEM; Sirion 200, Philips-FEI Co., Holland). Before FESEM measurement, the remaining soluble polysulfide in the cathode was completely washed with DME in 15704

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Figure 1. Dispersion morphology of (a1) S/CB/SBR CMC, (a2) CB/SBR CMC, (a3) S/SBR CMC, (b1) S/CB/PVDF, (b2) CB/PVDF, and (b3) S/PVDF.

an Ar-filled glovebox. Therefore, only solid compounds were observed in the FESEM images.

3. RESULTS AND DISCUSSION 3.1. Dispersion of S and/or CB Particles in the Electrode Slurries. In the electrode slurries, a three-dimensional network is

formed due to bridging of the particles by polymer chains since segments of polymer chain adsorb on different particles and the adsorbed chains form entanglements. After solvent evaporation, the dried composite electrode retains the initial morphology in the wet state and the particles are tightened altogether by the polymer chains. For the slurry preparation, the underlying goal is to get a homogeneous dispersion of all the components in the solvent. Parts a1 and b1 of Figure 1 show the dispersion morphologies of S and CB particles connected with SBR CMC in water and with PVDF in NMP, respectively. Both the slurries consist of aggregates or clusters of a few aggregates. For the SBR CMC suspensions, the mixture particles (S and CB) with a mean size of several micrometers were well dispersed, whereas the PVDF systems show much larger agglomerate size of hundreds of micrometers. Parts a2 and b2 of Figure 1 give the size distribution of CB particles in different binder systems. For the sulfur cathode, a good dispersion of CB particles can help to form an efficient conductive network, which has a special significance due to the high insulating nature of S and Li2S. Pure CB powder cannot be dispersed in water because it is hydrophobic. The high interfacial free energy between CB and water prevents the liquid from wetting the CB powder. Adding CMC allows CB to be dispersed into water efficiently. CMC adsorbs onto the CB surface. The presence of the carboxylate groups within the adsorbed layer of CMC gives rise to an effective surface charge on CB and therefore stabilizes the CB dispersion through an electrostatic double-layer repulsion mechanism.35 The quality of the CB dispersion with PVDF/NMP is much poorer than that with SBR CMC/DI water, as shown in Figure 1a2 and b2, indicating that the binding effect of SBR CMC binder is more efficient than that of PVDF. The distribution of the sulfur particles is similar to that of the CB particles, as shown in Figure 1a3 and b3.

The zeta potentials of S and/or CB particles with different binders are listed in Table 1. The potential values of S, CB, and their mixture in SBR CMC/DI water are 30.65, 40.31, and 37.81 mV, respectively. High value means strong electrostatic repulsive force between the particles, which can efficiently stabilize the particle dispersion in the suspension. As for the PVDF/NMP slurry, the potential values of CB and S/CB mixture are respectively 6.964 and 7.315 mV, much lower than those in the SBR CMC/DI water system. The CB and/or S suspensions in PVDF/NMP tend to gather together into aggregates due to low charge. The results of the zeta potential also confirm that the SBR CMC binder does favor a uniform distribution of the CB among the S particles and prevents the CB particles from forming large-scale aggregates in composite electrode, which is likely to enhance the sulfur utilization of the electrode. 3.2. Electrochemical Performance of the Sulfur Cathodes. Figure 2 shows CV curves of the sulfur electrodes with the two different binders at a scan rate of 0.1 mV s 1. Both curves show two distinct reduction peaks and one oxidation peak, which are in accordance with the typical CV characteristics of the sulfur cathode.7,28,37,38 The two reduction peaks can be attributed to the multiple reaction steps: S8f Li2Sn (n < 4), Li2Sn (n < 4) f Li2S or Li2S2, and the oxidation peak should be assigned to the oxidation of the low-order lithium polysulfides to high-order lithium polysulfides. For the SBR CMC cathode, three peaks appear at 2.0, 2.3, and 2.5 V, whereas for the PVDF cathode, the peaks are located at 1.9, 2.2, and 2.9 V. Furthermore, the reduction and oxidation peaks of the SBR CMC cathode appear much sharper than those of the PVDF cathode. Obviously the voltage difference ΔE between oxidation and reduction peaks for the PVDF electrode is much larger than that for the SBR CMC electrode. Considering that the ΔE is determined by the potential polarization of the active material during the charge and discharge process, the lower ΔE and the sharper peak both demonstrate that the redox reactions in the SBR CMC cathode behave more likely as a Nernst system. Specific discharge and charge capacities are plotted against cycle number in Figure 3. For each cell, the capacity gradually increases in the initial 3 5 cycles, which may be explained by the 15705

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Table 1. Zeta Potential of Different Slurries slurry

S/S Ca

ζ potential (mV) a

30.65

CB/S C

S/CB/S C

40.31

37.81

S/PVDF

CB/PVDF 6.964

S/CB/PVDF 7.315

S C stands for SBR CMC.

Figure 2. CV curves of the sulfur cathodes at a scanning rate of 0.1 mV s 1.

Figure 3. Discharge charge curves of sulfur cathodes with different binders at 100 mA g 1.

fact that the electrochemical redox reaction of elemental sulfur is an activation step due to gradual phase change from solid state to dissolved polysulfide state. Here we show the fifth cycle instead of the first cycle to represent the initial reversible capacity. As shown in Figure 3, all the discharge curves show two plateaus corresponding to the multistep of reduction reaction of sulfur during the discharge process as mentioned above. Theoretical potentials of the two plateaus are 2.33 and 2.18 V vs Li/Li+. However, the discharge voltage usually drops in practical cell system, which should be contributed to IR drop generated by internal resistance of the cell. Here, the internal resistance consists of ionic resistance of the electrolyte, electronic resistance of electrode-current collector, and interfacial resistance between electrode and electrolyte, etc. The SBR CMC cathode exhibits less voltage drop than the PVDF cathode, which should benefit

Figure 4. Cycle performance of sulfur cathodes with different binders at 100 mA g 1.

from the highly dispersed CB and hence the low internal resistance. Moreover, the SBR CMC cathode shows discharge capacity of 870 mA h g 1 and coulomb efficiency of 76%, both of which are higher than those of the PVDF cathode (810 mA h g 1 and 56%). The discharge platform keeps almost unchanged over cycling for the SBR CMC cathode, whereas the platform decreases significantly over cycling for the PVDF cathode. It should be noted that the capacity of the SBR CMC cathode remains 580 mA h g 1 after 60 cycles, much higher than that of the PVDF cathode (370 mA h g 1), showing a great improvement in cyclability, as displayed in Figure 4. In addition, sulfur utilization and cycle performance of the present cathode are comparable with those of the reported sulfur cathodes with PVP PEI and gelatin as binders.26,27 Figure 5 shows typical Nyquist plots of the sulfur cathodes before the first discharge and after 50 cycles of discharge/charge. For the original cathodes (see Figure 5a1), their Nyquist curves are composed of a depressed semicircle at high frequencies and a short inclined line in low-frequency regions. The semicircle corresponds to the internal resistance of the cathode including bulk impedance and interfacial impedance; the line is due to the Li-ion diffusion within the cathode. For the cathodes after 50 cycles, the Nyquist plots exhibit two depressed semicircles followed by a sloping line. According to our previous work,39 the semicircle at high frequencies reflects the charge transfer process at interface, and the semicircle in the middle frequency range relates to the formation of Li2S (or Li2S2) film. The equivalent circuits are shown in the insets of Figure 5a and b. In the equivalent circuits, Re represents the impedance contributed by the resistance of the electrolyte, and Rct is the charge transfer resistance at the interface of the conductive agent. The CPE1 is used instead of double-layer capacitance (Cdl), CPE2 describes the space charge capacitance of the Li2S (or Li2S2) film, and Rg is the resistance in the Li2S (or Li2S2) film. WO is the Warburg impedance due to the diffusion of the polysulfides within the cathode. From parts a and b of Figure 5, we can see that the 15706

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Figure 5. Nyquist plots for the sulfur cathodes in the frequency range of 100 mHz 100 kHz: (a) before discharge, (b) after 50 cycles. Inset: the equivalent circuits.

Figure 6. Swelling ratio vs swelling time in DME/DOL (1:1 by vol) for different sheets.

SBR CMC cathode exhibits not only lower internal resistance, but also lower charge transfer impedance compared with the PVDF cathode. It can be concluded that the SBR CMC cathode can provide a more effective electronically conductive network and a more stable interface structure than the traditional PVDF cathode.

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3.3. Electrolyte Solvent Uptake. To evaluate the binder, we need to consider the swelling in electrolyte solvents. Generally, if a binder is seriously swollen in electrolyte, the adhesion of the electrode materials to other particles and to the current collector deteriorates, which will lead to internal contact loss and, as a consequence, to an increase in the contact resistance. As to the sulfur cathode, as mentioned above, since the electrode reaction involves multiple reduction/oxidation processes coupled with repeated dissolution/deposition of the sulfur species, the structure of the electrode will be unavoidably changed while cycling. The structural reorganization is likely to lead to the isolation of carbon particles as well as formation of a lot of interstices inside the electrode. Therefore, serious uptake of the electrolyte will undoubtedly result in the deterioration of the electrode structure. As shown in Figure 6, the swelling ratios of the SBR CMC mixture binder and its sulfur cathode are respectively about 40% and 82%, much lower than those of PVDF and its sulfur cathode (∼130% and 175%). High solvent uptake of the PVDF binder in the electrolyte results in the disruption of electrode structure, giving rise to a large potential difference between charge and discharge plateaus (see Figure 3). Low swelling ratio for the SBR CMC cathode helps to stabilize the electrode structure and the low internal resistance, thereby improving cycle performance. 3.4. Morphologies of the Cathodes. We use SEM analysis for further insight into the excellent performance of the SBR CMC cathode. Figure 7 compares the SEM images of the SBR CMC cathode and the PVDF cathode. Before SEM measurement, the remaining soluble polysulfides in the cathode have been completely washed off with DME. Therefore, only solid compounds are observed in the SEM images. As shown in Figure 7a1 and b1, both the as-prepared SBR CMC and PVDF cathodes are typical porous ones containing well-distributed S and CB. However, at the end of the 50th discharge, the surfaces of the two cathodes are covered with solid film (see Figure 7a2 and b2). According to our previous work,39 this solid film is identified as the final discharge product, Li2S. As shown in Figure 7a2, the Li2S film on the SBR CMC cathode surface still shows a porous morphology, which can offer an easy mass transport for the dissolved polysulfide to the carbon matrix, keeping a high sulfur utilization. Compared with the SBR CMC cathode, the PVDF cathode surface covered by Li2S (or Li2S2) is much denser and more congregative, which might be responsible for the low sulfur utilization by cutting out the progress of discharge reaction prematurely. Parts a3 and b3 of Figure 7 show the surface morphologies of the two cathodes at the end of the 50th charge (full charging). We know that when recharging after the first discharge, Li2S is only recovered to long-chain polysulfide not to elemental sulfur even at 100% depth of charge.15,39 Long-chain polysulfide can be dissolved into the electrolyte. For an ideal sulfur cathode with 100% sulfur utilization, it is requested that only carbon matrix can be observed at full charging. But for a practical sulfur cathode, the content of Li2S at charge state always increases with increasing cycle number.16,17 This indicates that the irreversible Li2S phase is continuously accumulated and then saturated in the carbon matrix with increasing cycle. By comparing Figure 7a3 and b3, we can see that the residual irreversible Li2S on the PVDF cathode surface is much more severe than that on the SBR CMC cathode, which is responsible for its poor cyclic durability. Since the SBR CMC and PVDF cathodes show similar porous morphology at the initial state, and the only difference between these two cathode systems is the binder, we 15707

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Figure 7. SEM images of the sulfur cathodes: (a1) the as prepared SBR CMC cathode, (a2) the SBR CMC cathode at the end of the 50th discharge, (a3) the SBR CMC cathode at the end of the 50th charge, (b1) the as prepared PVDF cathode, (b2) the PVDF cathode at the end of the 50th discharge, and (b3) the PVDF cathode at the end of the 50th charge.

can conclude that the SBR CMC binder can help to stabilize the structure of the sulfur cathode during discharge charge cycles by suppressing the agglomeration of Li2S.

4. CONCLUSIONS We have successfully used water-soluble SBR CMC instead of traditional PVDF as a binder for the sulfur cathode. The SBR CMC cathode exhibits a reversible capacity of 580 mA h g 1 after 60 cycles, showing a remarkably improved cyclability as compared with the traditional PVDF cathode. Zeta potential data confirm that the SBR CMC binder favors a uniform distribution of the CB among the S particles. EIS measurement shows a low internal resistance and a low charge transfer impedance for the SBR CMC cathode, which indicates that the SBR CMC binder can provide an effective electronically conductive network and a stable interface structure for the cathode. Furthermore, SEM images confirm the SBR CMC binder can suppress the agglomeration of Li2S and hence stabilize the structure of the sulfur cathode during discharge charge cycles. The improvement of cycle property of the SBR CMC cathode is suggested to arise from the above combined effects. Our results clearly show that the SBR CMC mixture is a high efficient binder for the sulfur cathode to improve the electrochemical performance with a high sulfur utilization. ’ AUTHOR INFORMATION Corresponding Author

*Tel./fax: +86 27 87558241 (Y.-H.H.). E-mail: huangyh@mail. hust.edu.cn (Y.-H.H.), [email protected] (L.-X.Y.).

’ ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (Grant Nos. 50825203 and 20803042), the 863 program from the MOST (Grant Nos. 2009AA03Z225 and 2011AA11290), the PCSIRT (Program for Changjiang Scholars and Innovative Research Team in University), and the Fundamental Research Funds for the Central Universities (HUST,

2010QN048). In addition, the authors thank the Analytical and Testing Center of Huazhong University of Science and Technology for SEM measurement.

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