Chem.Mater.,2013,25,3357–Stretchable and Self-Healing Graphene Oxide–Polymer Composite Hydrogels

Stretchable and Self-Healing Graphene Oxide?Polymer Composite Hydrogels:A Dual-Network Design

Huai-Ping Cong,*,?,?Ping Wang,?and Shu-Hong Yu*,?

?Division of Nanomaterials and Chemistry,Hefei National Laboratory for Physical Sciences at Microscale,Department of Chemistry, University of Science and Technology of China,Hefei,Anhui230026,People’s Republic of China

?Anhui Key Lab of Controllable Chemical Reaction&Material Chemical Engineering,School of Chemical Engineering,Hefei University of Technology,Hefei230009,People’s Republic of China

*Supporting Information

interconnected,attributed to the interactions of polar groups of

and oxygen-containing groups of GO nanosheets.

enhanced mechanical property,self-healing

I.INTRODUCTION

For a long time,improving the mechanical properties of hydrogels has been an important topic in the?eld of gel science due to the limitations imposed by poor mechanical behavior on its application scope.Double-network polymer hydrogels and nanocomposite hydrogels are two typical and widely studied models for preparation of hydrogels with enhanced mechanical properties.Herein,double-network polymer hydrogel refers to a step-wise synthesized hydrogel composed of a?rst,highly cross-linked and second,loosely cross-linked polymer net-work.1?3For nanocomposite hydrogel,nano?llers such as clays,4,5carbon nanotubes,6ferritin particles,7and graphene peroxides8are involved in the polymerization of hydrogel as large cross-linkers.Recently,we have utilized a new double-network model involving an ion-induced network and a hydrogen-bonding network to enhance the mechanical proper-ties of a graphene oxide-polyacrylamide hydrogel.9

Self-healing,a fascinating characteristic of the living organism,refers to the ability to automatically and sponta-neously repair itself in response to damage and regenerate its original function.White et al.in200110reported an early self-healing system:upon the occurrence of damage,the micro-capsulated healing monomer would release and polymerize with the assistance of suspended catalysts,resulting in recombining of the ruptures.Afterward,strategies to devise the self-healing materials were based on either release of the repair agents upon damage10or awakening the latent molecular functionality in the polymer structure to heal the cracks via molecular di?usion and entanglement upon external light,11?14 thermal,15,16electric,17and magnetic stimuli,18etc.Cui and del Campo19and Phadke et al.20used hydrogen-bonding interactions widely in the design and construction of smart materials.Therefore,it is signi?cant to develop strategies to design and construct powerful hydrogels with both outstanding mechanical behavior and self-healing property.Nowadays, graphene oxide(GO)is widely used as a reliable and promising component material and has attracted great interests in the ?elds of electrocatalysis,21photocatalysis,22?24etc. Herein,we expand the intertwined double-network mecha-nism to fabricate a novel kind of graphene oxide(GO)/poly(N-acryloyl-6-aminocaproic acid)(PAACA)hydrogel endowed with enhanced mechanical properties and self-healing capa-bility.Without use of organic cross-linkers,GO nanosheets and calcium ions as double cross-linkers trigger the formation of robust and?exible double networks in the composite hydrogel. On the one hand,a3D hydrogen-bonding network is formed between the polar groups of PAACA side chains with both oxygen-containing groups of GO nanosheets and the other

Received:June13,2013

Revised:July21,2013

Published:July23,2013

polar groups of PAACA side chains.On the other hand,a Ca2+-induced coordination network results from the interactions of Ca2+with the PAACA side chains and oxygen-containing groups of GO nanosheets.Owing to this?exible cross-linked network,the GO/PAACA composite hydrogel exhibits higher mechanical strength of65.8kPa at a stretch of1190%than that of PAACA hydrogel,with a fracture stress of8.9kPa at an elongation of350%.Importantly,the composite hydrogel shows rapid self-healing to pH stimulus,relying on the hydrogen bonds between PAACA side chains mediated by the optimal balance between hydrophilic and hydrophobic moieties,which did not deteriorate while its mechanical properties were improved.

II.EXPERIMENTAL SECTION

Preparations of GO/PAACA and PAACA Hydrogels.In a typical synthesis,the N-acryloyl-6-aminocaproic acid(AACA) monomer and GO dispersion were?rst prepared according to the literature25and our previous work,26respectively.Then1M AACA monomer was dissolved into NaOH solution.Afterward,di?erent amounts of GO and Ca(NO3)2were added into the solution at weight ratios of GO to AACA(W GO/W AACA)of0.1?3.0%and weight ratios of Ca2+to the sum of GO and AACA[W Ca2+/(W GO+W AACA)]of1.9?19%.Polymerization was initiated after adding ammonium persulfate at0.5mol%of the AACA amount,and the mixture was kept in the heating oven at60°C for3h.For the synthesis of PAACA hydrogel, we adopted a similar procedure to that of the composite hydrogel but without the addition of GO.

Swelling and Deswelling Performances.For measurement of the pH-response swelling ratio,hydrogel cubes of1×1×1cm3were immersed into excess water at di?erent pH values.After reaching equilibrium for2days at room temperature,the cubes were taken out and weighed.For investigating the swelling kinetics of the hydrogel, the weights of the cubic hydrogel were measured at di?erent time intervals.The swelling ratio(SR)was calculated as SR=(W t?W d)/ W d.Herein,W t and W d represent the weights of swollen hydrogel at time t and dry hydrogel,respectively.

The deswelling kinetics of the hydrogel was performed in acid solution at pH1.5after swelling to equilibrium in neutral solution.The hydrogel was weighed at di?erent time intervals.Water retention (WR)was calculated as WR=(W t?W d)/W s.In the equation,W t, W d,and W s described the weights of deswollen hydrogel at time t,dry hydrogel,and swollen hydrogel at equilibrium,respectively.

Drug Release Experiment.Doxorubicin was used as the drug release model in our experiment.Doxorubicin(0.5mg/mL)was embedded during preparation of the hydrogel.Then the loaded hydrogel was immersed into acid solution of pH1.5and shaken at200 rpm.At di?erent hour intervals,1mL of the immersion solution was taken out and replaced with1mL of fresh solution.The amount of released drug was calculated through UV?vis spectrophotometric measurement at480nm.

Materials Characterization.Scanning electron microscope (SEM)images were carried out on a Zeiss Supra40scanning electron microscope at an acceleration voltage of5kV.Fourier transform infrared(FT-IR)spectra were taken with a Bruker Vector-22FTIR spectrometer from4000to400cm?1at room temperature.X-ray di?raction(XRD)patterns were recorded on a PW1710instrument with Cu Kαradiationλ=0.15406nm.Brunauer?Emmett?Teller (BET)measurements were made on a Micromeritics ASAP-2000 nitrogen adsorption apparatus.Tensile stress?strain tests were performed on the striplike samples by using Instron5565A at a speed of1mm/s.UV?vis spectra were recorded on a Shimadzu UV-240spectrophotometer scanning from400to800nm at room temperature.III.RESULTS AND DISCUSSION

Preparation of GO/PAACA Hydrogels.In a typical synthesis,GO dispersion and Ca(NO3)2were added into the NaOH solution of AACA precursor.Subsequently,the GO/ PAACA composite hydrogel was fabricated by initiating the polymerization of monomer attached to the GO nanosheets with the help of ammonium persulfate as an initiator.GO nanosheets had the advantages of being easily functionalized and high dispersibility in aqueous medium due to the abundant carboxyl groups at the GO edges and epoxy and hydroxy groups on the planes.For one thing,GO nanosheets formed a loose3D network originating from the coordination interactions between oxygen-containing groups of GO and calcium ions.27For another,GO nanosheets and Ca2+were regarded as double cross-linkers to form another intercon-nected?exible polymer network in the polymerization process. The polar functional groups of the PAACA side chains interacted with both the oxygen-containing groups of GO nanosheets in the form of hydrogen bonds and Ca2+via coordination interactions.

SEM images in Figure1a,b show the interior microstructure of the GO/PAAC composite hydrogel,prepared with weight

ratio of GO to AACA of2.0%and weight ratio of Ca2+to the sum of GO and AACA of9.5%.The well-de?ned and interconnected network of composite hydrogel was composed of?exible porous structure with sizes ranging from sub-micrometer to several micrometers.It was interesting to note that the GO nanosheets were completely covered with polymers due to the hydrogen-bond interactions between oxygen-containing groups of GO and polar functional groups of the PAACA side chains(Figure1c);no bare nanosheets were exposed in the gel structures.Oppositely,as shown from SEM images(Supporting Information,Figure S1a),the PAACA hydrogel prepared under identical conditions as the composite hydrogel but without the addition of GO,exhibited a thick block with sparse pores dispersed.The XRD pattern of GO/ PAACA hydrogel displayed a similar amorphous di?raction peak at the2θposition of20.9°as that of PAACA

hydrogel Figure1.(a,b)SEM images with di?erent magni?cations of GO/ PAACA hydrogel.(c)Chemical structure of AACA monomer.(d)FT-IR spectra of GO powder,PAACA hydrogel,and GO/PAACA hydrogel.For preparation of GO/PAACA composite hydrogel,the weight ratio of GO to AACA was2.0%,and the weight ratio of Ca2+to the sum of GO and AACA was9.5%.

(Figure 2).There was no characteristic GO peak at 9.2°with the interlayer spacing of 9.5?detected in the XRD pattern of composite hydrogel,suggesting that GO nanosheets were uniformly dispersed in the polymer gels without aggrega-tion.16,28Brunauer ?Emmett ?Teller (BET)surface area of GO/PAACA hydrogel of 10.5m 2/g was much higher than that of PAACA hydrogel (0.32m 2/g)due to the uniform GO incorporated,although GO nanosheets were covered with a thick layer of polymer (Supporting Information,Figure S2).Moreover,the corresponding Barrett ?Joyner ?Halenda (BJH)adsorption cumulative pore volumes of GO/PAACA and PAACA hydrogel were 0.10and 0.005cm 3/g,respectively,in accordance with their microstructures as shown from SEM images.Possible interactions between GO nanosheets and PAACA chains were further clari ?ed by analyzing the FT-IR spectra of GO powder,PAACA hydrogel,and GO/PAACA hydrogel (Figure 1d).The obvious peaks at 1730,1620,1407,1228,and 1055cm ?1in the FT-IR spectrum of GO powder were attributed to C=O carbonyl stretching,aromatic C=C stretching,O ?H deformation vibration,and asymmetric and symmetric C ?O stretching in C ?O ?C group,respectively.29In the FT-IR spectrum of PAACA hydrogel,the bands at 3347,2937,and 2880cm ?1in the high-frequency region were typical of N ?H stretching and asymmetric and symmetric vibration of C ?H stretching,respectively.The band at 1714cm ?1was assigned to C=O stretching vibration of the carboxylic acid group of the PAACA side chains.The characteristic vibrational bands of amide I and amide II modes presented at 1650and 1548cm ?1,respectively.20Another strong band at 1388cm ?1

was related to O ?H deformation of the carboxylic acid group.The spectral results revealed the intermolecular hydrogen bonds of carboxyl groups with both the amide and carboxyl groups of the opposing polymer molecules in the PAACA hydrogel.This kind of noncovalent interaction endowed the PAACA hydrogel with rapid self-healing functionality.20In the FT-IR spectrum of GO/PAACA hydrogel,most of the bands belonging to GO powder were signi ?cantly decreased (1407,1228,and 1055cm ?1)or even disappeared (1730and 1620cm ?1).Furthermore,the amide I and amide II bands of the PAACA hydrogel were weakened and the marked typical O ?H deformation at 1388cm ?1also disappeared.The weakness and disappearance of the featured bands suggested intertwined networks in the GO/PAACA hydrogel through hydrogen bonding of oxygen-containing groups of GO sheets with polar groups of PAACA side chains and the coordination interactions of GO with calcium ions.30?32

Based on the above analyses,the schematic illustration in Scheme 1a shows two types of hydrogen-bonding con ?g-

urations in PAACA cross-linked network between the polar groups of the polymer side chains as reported in the literature:face-on and interleaved con ?gurations.20Furthermore,the intertwined double networks of hydrogen bonds and coordination interactions were involved in the formation process of the composite hydrogel,as visually shown in the illustration of Scheme 1b.On the one hand,the powerful hydrogen-bonding networks originated not only from the polar groups of PAACA side chains interacting with oxygen-containing groups of GO but also from the interactions of PAACA side chains.On the other hand,the Ca 2+-induced 3D network was due to coordination interactions of Ca 2+with both oxygen-containing groups of GO and polar groups of PAACA side chains.27,31

Mechanical Properties of GO/PAACA Hydrogels.The photograph in Figure S1b (Supporting Information)shows the stretchability of striplike GO/PAACA composite hydrogel ?xed at two clamps withstanding the tensile stress at an elongation

several times its original length.The tensile stress ?strain curves in Figure 3a quanti ?ed the mechanical properties of hydrogels with and without GO nanosheets in the cross-linked networks,respectively.The composite hydrogel was fractured up to an elongation of 1190%its original length at a tensile stress of 65.8kPa.For pure PAACA hydrogel,its maximum elongation and the corresponding tensile stress were lowered to 350%and 8.9kPa,respectively,much weaker than those of the composite hydrogel.The stress ?strain curve during a loading ?

unloading

Figure 2.XRD patterns of GO powder,PAACA hydrogel,and GO/PAACA hydrogel.Preparation conditions for GO/PAACA hydrogel were as described for Figure 1.Scheme 1.(a)Schematic Illustration of Hydrogen-Bonding Interactions of the PAACA Side Chains at Low pH a and (b)Illustration of Assembly Mechanism of GO/PAACA

Hydrogel

a

Green

curves represent the backbones of polymer chains.Face-on (red)and interleaved (blue)hydrogen-bonding con ?gurations were

proposed.Figure 3.(a)Tensile stress ?strain curves of PAACA and GO/PAACA hydrogels.

(b)Tensile stress ?strain

curves of PAACA and GO/

PAACA hydrogels during a loading ?unloading cycle at a stretch of 300%.Preparation conditions for GO/PAACA hydrogel were as

described for Figure 1.

cycle (Figure 3b)revealed the capability of energy dissipation,another index to assess the mechanical properties.33At a stretch of 300%,the stress ?strain curve of the composite hydrogel showed an obvious hysteresis loop,indicating its strong energy dissipation ability to support the outstanding mechanical properties.Furthermore,the elaborate stress ?strain curves were performed on the GO/PAACA hydrogel with di ?erent setting stretches from 200%to 900%,displaying the ?ne hysteresis loops and small permanent deformations during loading ?unloading cycles (Supporting Information,Figure S3).However,two nearly overlapping stress ?strain curves during the loading ?unloading process were drawn for PAACA hydrogel (Figure 3b).In the preparation process,formation of the composite hydrogel was signi ?cantly in ?uenced by the GO content.It was investigated that a weight ratio of GO to AACA ≥1.0%was desired to form the integrated GO/PAACA composite hydrogel as shown from photographs with low weight ratios of GO and AACA from 0.1%to 1.0%(Supporting Information,Figure S4).This was due to the competition of hydrogen bonds in the cross-linked network,which were either through the PAACA chains or between the polar groups of PAACA side chains and oxygen-containing groups of GO sheets.The stress ?strain curves of the GO/PAACA hydrogels with di ?erent weight ratios of GO and AACA precursor (≥1.0%)were measured to further illuminate the e ?ect of GO content on their mechanical behaviors (Figure 4a).With low GO

content of 1.0%,the fracture stress of the composite hydrogel was evidently improved 2.2times to 19.9kPa at a stretch of 661%,compared with that of the PAACA hydrogel.At a weight ratio of 2.0%,the corresponding tensile stress was signi ?cantly strengthened to 65.8kPa at the critical elongation of 1190%.However,upon further improving the weight ratio to 3.0%,the tensile stress was a bit increased to 74.1kPa but with obviously decreased stretchability to 790%.Meanwhile,we found that Ca 2+content was the other important factor to in ?uence the mechanical properties of the GO/PAACA composite hydrogel.The weight ratio of Ca 2+to the sum of GO and AACA precursor [W Ca 2+/(W GO +W AACA )]was de ?ned as the Ca 2+content due to coordination of Ca 2+with both oxygen-containing groups of GO sheets and polar groups of PAACA side chains.Stress ?strain curves were performed on a series of composite hydrogels with identical weight ratio of GO to AACA monomer of 2.0%and di ?erent Ca 2+contents (Figure 4b).With increasing Ca 2+content from 1.9%to 5.7%to 9.5%,the fracture stress was improved from 30.6to 40.4to 65.8kPa,and the corresponding critical stretch values were 816%,927%,and 1190%,respectively.However,further increasing the Ca 2+content to 19%led to decreased tensile stress of 47.7kPa at a comparative critical stretch of 1100%,due to the balanced interactions of Ca 2+with both GO and PAACA.These results revealed that upon moderately increasing the Ca 2+content,the mechanical behavior was enhanced in the form of improved fracture stress at the critical stretch.Swelling and Deswelling Behaviors of GO/PAACA Hydrogels.The swelling/deswelling behavior was systemati-cally investigated to further reveal the structural features of the GO/PAACA hydrogel.It was well-known that the swelling/deswelling behavior of the hydrogel depended on the cross-linked density of the gel network.28,34The prepared composite hydrogels presented the obvious pH-dependent swelling behaviors,with weaker swelling capacity than PAACA hydrogel due to its more robust network (Figure 5a).At lower pH than the p K a of 6-aminocaproic acid of 4.4,the carboxyl groups of the PAACA side chains as the protonated form were allowed to interact with other polar groups via hydrogen-bonding interactions,resulting in the weak swelling capacity.However,at high pH,the hydrogen bonds were signi ?cantly weakened due to the strong electrostatic repulsion of the deprotonated carboxyl groups.Therefore,upon improving the pH value to 5,higher than the p K a of the carboxyl group of 4.3,a sharply increased swelling ratio was shown in the curves for both PAACA and GO/PAACA hydrogels.The maximum capacity of the hydrogel to absorb water corresponded to neutral and basic solution,due to the large ionic degree and the high osmotic pressure.35

Meanwhile,as shown from the swelling kinetics in Figure 5b,the PAACA hydrogel presented a faster rate of absorbing water than the GO/PAACA hydrogel.Due to the pH sensitivity of the hydrogel,the deswelling kinetics was measured in acid solution at pH 1.5after swelling to equilibrium in pH 7solution.Water retention of the GO/PAACA hydrogel decreased as a function of time at a lower rate than for the PAACA hydrogel (Figure 5c).Herein,the GO nanosheet was regarded as a kind of cross-linker and formed more cross-link points with higher cross-linked density,resulting in reduction

of

Figure 4.(a)Tensile stress ?strain curves of GO/PAACA

hydrogels

with identical Ca 2+

content of 9.5%and di ?erent weight ratios of GO to AACA.(b)Tensile stress ?strain curves of GO/PAACA hydrogels with identical weight ratio of GO to AACA of 2.0%and di ?erent

weight ratios of Ca 2+content [W Ca 2+/(W GO +W AACA

)].Figure 5.(a)Equilibrium swelling ratios of PAACA and GO/PAACA hydrogels with di ?erent pH values.(b)Swelling kinetics of PAACA and

GO/PAACA hydrogels at pH 7.(c)Deswelling kinetics of PAACA and GO/PAACA hydrogels at pH 1.5.(d)Curves of released doxorubicin

from PAACA and GO/PAACA

hydrogels

in acid solution (pH 1.5)as a function of time.Preparation conditions for GO/

PAACA hydrogel were as described for Figure 1.

swelling and deswelling capacities.The di ?erences in swelling ?deswelling behaviors con ?rmed the more robust network of the GO/PAACA hydrogel than that of the PAACA hydrogel.In view of the interesting swelling and deswelling responses to pH stimulus of the hydrogel,doxorubicin was embedded into the hydrogel as a model for drug release.Afterward,the embedded hydrogel was incubated in the simulated gastric acid solution of pH 1.5for assessing drug delivery e ?ciency.From the drug-release curves as a function of time (Figure 5d),after 48h,only 68%of the drug was released from GO/PAACA hydrogel.However,in the same period,86%of the drug was released from PAACA hydrogel at a faster rate.The drug release experiment indicated that the composite hydrogel had a superior ability to the pure PAACA hydrogel in drug delivery.It was mainly attributed to the robust interconnected cross-linking network and the permeability barrier of the 2D GO sheets,limiting the di ?usion of the loaded drug molecules.36Self-Healing of GO/PAACA Hydrogels.The novelty of the GO/PAACA composite hydrogel was revealed not only in the enhanced toughness and stretchability but also in the remaining self-healing response to pH stimuli.The photographs in Figure 6a clearly show the pH response of the GO/PAACA hydrogel at di ?erent time intervals ranging from seconds to minutes in a reversible way.The hydrogel was opaque as soon as it was immersed in the HCl solution at pH <3.When the immersion time was prolonged,the opaque degree of the hydrogel was obvious.Up to 10min,the surface of the hydrogel was wrinkled with a deeper opaque color.It was interesting to note that when the hydrogel was reintroduced to neutral or basic solution at pH ≥7,the above process was

reversible,but a longer time was needed.The reversible process of the hydrogel fully indicated reconstruction of the hydrogen bonds in the interconnected cross-linking networks in response to pH stimuli.The photographs of Figure 6b demonstrate the self-healing process of the GO/PAACA hydrogel.With the punched hydrogel immersed into acid solution (pH <3)completely for several seconds,the separated parts were recombined together,capable of withstanding the repeated stretches.SEM images indicated the well rehealed network at the interface of the two recombined parts (Supporting Information,Figure S5).After release of the stress,the united hydrogel recovered its original shape and size.Importantly,it was a reversible process:when the healed hydrogel was reintroduced into the high-pH solution,it was separated again.The photographs in Figure 6c give another example to show the fast and facile self-healing process of the composite hydrogel.From these demonstrations,it was suggested that the composite hydrogel exhibited the fast self-healing capability and reversibility of healing ?separation ?rehealing process to the pH stimulus,which was attributed to abundant hydrogen bonding in PAACA cross-linking network between the polar groups of the polymer side chains.Furthermore,the ?exible side chains were long enough to allow the polar groups across the interface to reach each other,but too-long chains would bring greater steric hindrance and increase the hydrophobic interaction.The optimal balance of hydrophilic and hydrophobic interactions of the PAACA structures resulted in the excellent self-healing of the GO/PAACA hydrogel.Meaningfully,the fast self-healing ability of the prepared composite hydrogel did not deteriorate and kept well at the time of improving its mechanical properties through introduction of GO nanosheets and calcium ions as double cross-linkers.

IV.CONCLUSION

In summary,a kind of novel GO/PAACA composite hydrogel has been fabricated under the recently proposed double-network mechanism triggered by GO and calcium ions,9

where

Figure 6.(a)Photographs showing the reversible response process of GO/PAACA hydrogel to pH stimulus at di ?erent time intervals.(b)Photographs showing the self-healing and recycling properties of GO/PAACA hydrogel.(c)Photographs of the facile self-healing process of the GO composite hydrogel with two manual ruptures.The hydrogel was healed just by dripping two drops of acid solution into the ruptures and pushing the fresh surface into contact.

the one-step polymerization strategy without introducing any organic cross-linkers is applied,and the cross-linked hydrogen-bonding network is attributed to the polar groups of the PAACA side chains interacting both with other PAACA side chains and with oxygen-containing groups of GO nanosheets. Meanwhile,calcium ions function with both the polar groups of the PAACA side chains and oxygen-containing groups of GO nanosheets via coordination interactions.This kind of new GO/PAACA hydrogel exhibits enhanced mechanical properties and retains fast self-healing capability,which make it a promising candidate for drug delivery,external coating,and

biological sca?olding.

■ASSOCIATED CONTENT

*Supporting Information

Five?gures showing SEM images,N2adsorption and desorption isotherms,tensile stress?strain curves of GO/ PAACA composite hydrogel during loading?unloading cycles, and photographs of GO/PAACA hydrogels(PDF).This material is available free of charge via the Internet at http://

https://www.wendangku.net/doc/3a10017465.html,.

■AUTHOR INFORMATION

Corresponding Author

*E-mail hpcong@https://www.wendangku.net/doc/3a10017465.html,(H.-P.C.)or shyu@https://www.wendangku.net/doc/3a10017465.html, (S.-H.Y.).

Author Contributions

The manuscript was written through contributions of all authors.

Notes

The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS

S.-H.Y.acknowledges funding support from the National Basic Research Program of China(Grant2010CB934700),the National Natural Science Foundation of China(Grants 91022032,91227103,21061160492,and J1030412),the Chinese Academy of Sciences(Grant KJZD-EW-M01-1),and the International Science&Technology Cooperation Program of China(2010DFA41170).H.-P.C.thanks the National Natural Science Foundation of China(Grant21001099),the Fundamental Research Funds for the Central Universities, Hefei Institutes of Physics Science,CAS(Grant 2012FXCX008),China Postdoctoral Science Foundation (Grant20110490086),and the Foundation for the Author of

Excellent Doctoral Dissertation of CAS.

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