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Physica E 27 (2005) 341–350

www.elsevier.com/locate/physe

Electrochemical formation of platinum nanoparticles by anovel rotating cathode method

Min Zhoua, Shenhao Chena,b,�, Haipeng Rena, Ling Wua, Shiyong Zhaoa

aDepartment of Chemistry, Shandong University, Jinan 250100, PR ChinabState Key Laboratory for Corrosion and Protection, Shenyang 110016, PR China

Received 7 December 2004; accepted 20 December 2004

Abstract

Platinum nanoparticles in aqueous solution were synthesized for the first time by a novel electrochemical reduction of

ionic platinum in the presence of Poly(N-vinylpyrrolidone) (PVP) which is used as a protecting agent. The utilization of

a rotating cathode and the selection of a suitable protecting agent played an important role in these methods. PVP not

only protects metallic particles from agglomeration but also promotes metal nucleation, which tends to produce small

metal particles. Using a rotating cathode effectively solves the technological difficulty of rapidly transferring the

(electrochemically synthesized) metallic nanoparticles from the cathode vicinity to the bulk solution, avoiding the

occurrence of flocculates in the vicinity of the cathode, and ensuring the monodispersity of the particles. The effects of

polymer stabilizer concentration, chloroplatinic acid concentration and reaction time on the particle size were studied.

The platinum nanoparticles synthesized by the electrochemical method were characterized by TEM, UV–vis

spectroscopy and XRD.

r 2005 Elsevier B.V. All rights reserved.

PACS: 61.46.+w; 82.45.Aa

Keywords: Rotating cathode; Electrochemical method; Platinum nanoparticles; Polymer stabilizer; UV–vis absorption spectrum

1. Introduction

Nanoparticles have drawn considerable interestin various fields of science and engineering because

e front matter r 2005 Elsevier B.V. All rights reserve

yse.2004.12.012

ng author. Tel.: +86531 8364959;

65167.

ss: shchen@sdu.edu.cn (S. Chen).

of their unique physical and chemical propertiesleading to potential applications in electronics,for optical and magnetic devices [1–6]. Platinumhas attracted much attention because it is anexcellent catalyst for many purposes; for instance,the nanoparticles supported on porous aluminaare used to eliminate NO generated in a combus-tion process [7]. The most popular method of

d.

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M. Zhou et al. / Physica E 27 (2005) 341–350342

preparation is the reduction of PtCl2�6 by citrate asdescribed by Turkevich [8]. The formation ofvarious shapes and sizes of polymer-protectedplatinum nanoparticles by H2 gas reduction of Ptsalts was reported by Ahmadi [9]. Recently,Fumitaka synthesized stable platinum nanoparti-cles by laser ablation in water [10]. Pt nanoparti-cles can be synthesized by their interaction withsuitable agents such as surfactants, ligands, poly-mers, etc., and as a result of electrostatic repulsionor steric hindrance, further aggregation is pre-vented.Metal nanoparticles have been obtained by

thermal [11], photochemical [12], radiolytic [13],sonochemical [14] or electrochemical [15] methodsby using various reagents. It has been confirmedthat electrochemical methods have some advan-tages over traditional methods in synthesis ofmetal nanoparticles [16,17]. Reetz [15] developed asacrificial anode method to prepare size-selectivemetal particles in an organic phase. In thismethod, tetraalkylammonium salts served as thesupporting electrolyte and stabilizer for the metalnanoclusters. The salient features of this methodinclude ease of operation, high yield, and theabsence of undesired side products: these featuresare especially good for the wide application of thismethod to the electrochemical synthesis of metallicnanostructured materials. For example, Yu [18]and Mohamed [19] synthesized gold nanorods inaqueous solution via this electrochemical methodby introducing a shape-inducing cosurfactant;Rodrigues-Sanchez [17] prepared silver nanoparti-cles, ranging in size from 2 to 7 nm, in acetonitrilein an analogous manner. Small metal particlesindicate the high purity of the particles and thepossibility of a precise particle size control can beachieved by adjusting current density or appliedpotential. Recently, the formation of polymer-stabilized Ag and Au nanoparticles was alsoreported by Ma et al. [20,21] with the electro-chemical method. In this paper, we first report anelectrochemical procedure for preparing polymer-stabilized Pt nanoparticles using a rotating cath-ode. In this process, the Pt nanoparticles wereprepared by the electrochemical reduction of aplatinum precursor salt, and the nanoparticleswere formed in aqueous solutions during polymer

stabilization. We present for the UV–vis spectro-scopy, transmission electron microscopy and X-ray diffraction evidence for the electrochemicalformation of well-dispersed Pt colloids stabilizedby Poly(Vinyl-pyrrolidone) (PVP). The influenceof preparation parameters on the particle size andsize distribution is also discussed.

2. Experimental section

2.1. Materials

PVP K30 (weight-average molecular weight(Mw), 30,000) was supplied from BASF Co.Germany. Chloroplatinic acid (H2PtCl6) andpotassium nitrate (KNO3) were purchased fromChinese Shanghai Regent Co. All chemicals wereof analytical grade and used without furtherpurification. All solutions were prepared withtriply distilled water.

2.2. Preparation of the rotating copper cathode

electrolysis system

A 2.0mm diameter copper rod (Aldrich, 99.9%)was employed to prepare the rotating copperelectrode. The copper specimen was embedded ina Teflon mold, leaving only its cross-sectionexposed to the aqueous solution. The rotationspeed of the electrode was controlled mechani-cally. A 0:5 cm� 2:0 cm platinum sheet was usedhere as anode, the two electrodes being 5 cm apart.

2.3. Preparation of Pt nanoparticles

The synthesis procedure of platinum nanopar-ticles is similar to that reported by Ma et al. [20].The process was performed in a simple two-electrode cell by using an EG&G M173 potentio-stat/galvanostat. Typically, 5mL of 6�10�3 mol=dm3 of H2PtCl6 solution, 30mL of5 g=dm3 PVP solution and 1mL of 0:1mol=dm3

KNO3 solution were added while stirring. All thesolutions were purged by nitrogen. The Electro-lysis was carried out in a potentiostatic manner atappropriate temperature (� 80 �C). The rotatingspeed of the working electrode was 1000 rpm.

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2.4. Characterization with UV– vis

spectrophotometer, TEM and XRD

The solutions before and right after reductionwere measured at room temperature by a Hitachi4100 UV–vis spectrophotometer using a 1 cmpath-length quartz cell. Transmission electronmicrographs were taken with a Hitachi H-800transmission electron microscope (TEM) operatedat 100 kV accelerating voltage. Samples wereprepared by adding ethanol to a fraction of theplatinum colloid synthesized, and a droplet of itwas dropped on a carbon-coated copper grid. Theplatinum nanoparticles were identified by X-raydiffraction (XRD Rigaku D/Max 2200PC diffract-ometer with Cu–K radiation and graphite mono-chromator) with a scanning rate of 8�min�1 in the2 range from 20� to 70�:

Scheme 1. Polymer molecule (n denotes the polymerization-

number).

3. Results and discussion

3.1. Formation of the Pt nanoparticles on a rotating

cathode

In the present work, we demonstrated for thefirst time that spherical platinum nanoparticlesmay be electrochemically synthesized by the directelectroreduction of PtCl2�6 ions in PVP-containingaqueous electrolytes. There are two competitivecathode surface processes in the electroreductionof platinum ions: one is the formation of platinumparticles and the other is the deposition ofplatinum film on the cathode surface [19]. Gen-erally, platinum ions are inclined to form platinumon the cathode surface because the second processdominates over the first one in most cases. So, themost important requirement for synthesis ofplatinum nanoparticles is the reduced formationof a platinum film on the cathode surface. Once acathode surface is completely covered by anelectrodeposited platinum film, the particle forma-tion process will not proceed any more. We usedtwo measures to prepare platinum nanoparticles:(1) PVP (Scheme 1) was added to the electrolyte inorder to enhance the particle formation rate and toreduce the platinum film deposition rate; and (2) acopper electrode was employed as the cathode in

order to reduce the tendency of platinum ions todeposit there, because the great difference in radiusand lattice parameters for platinum and copperfavor particle formation.Samples of PVP-stabilized platinum nanoparti-

cles were characterized by TEM. Representativemicrographs and the size distribution histogramswere presented for Pt-PVP in Fig. 1. It can be seenin Fig. 1a that the platinum particles demonstrateddominatingly regular spherical particles and fairlyeven dispersion. PVP seemed to be a goodstabilizer for platinum nanoparticles since particlesize and particle size distribution were small andnarrow as shown in Fig. 1. And the particles’average sizes were 6–12 nm. The particle sizedistribution is shown as a histogram in Fig. 1b,from which it was estimated that the meandiameter of the particles was 9 nm, and thestandard deviation (SD) for the Gaussian fit was2.4. Compared with Pt particles (Fig. 5) which wasnot protected by PVP, the average particlediameters were much smaller and there were noconglomeration in the solution. The temperatureof the reaction was investigated. We selected for20; 40; 60 and 80 �C in the reaction. We noted thatPt nanoparticles could be obtained only when thetemperature was 80 �C: When the temperatureswere 20; 40; and 60 �C; we did not observe changein the solution and UV–vis spectra. Therefore, wehave chosen 80 �C as the appropriate temperaturefor the reaction. The above nanoparticles stored inan air-sealed bottle at ambient conditions werefound to be very stable, since no apparent changewas observed by UV–vis even after about 6months.The formation process of PVP-protected Pt

nanoparticles synthesized in H2PtCl6–KNO3–PVPaqueous solution is presented as a representative

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Fig. 1. (a) TEM image and (b) particle size distribution of

platinum nanoparticles electrochemically synthesized in an

aqueous solution of 6� 10�3 mol=dm3 H2PtCl6 þ

0:1mol=dm3 KNO3 þ 50 g=dm3 PVP, where PVP was the

stabilizer for the platinum clusters. Electrolysis time: 15min;

rotative velocity: 1000 rpm.

M. Zhou et al. / Physica E 27 (2005) 341–350344

example in Fig. 3 at a voltage of 5V withelectrolysis time at 15min. At the beginning ofthe reaction, the solution was pale yellow andshowed a peak at 262 nm in its UV–vis spectrum,

which is attributed to ligand-to-metal charge-transfer transition of the PtCl2�6 ions [22]. It canbe observed in Fig. 3 that the intensity of this bandgradually decreased after 5min of electrolysis,while the full-width at half-maximum (FWHM)and peak position changed only slightly. At thesame time, the color of the solution changed frompale yellow to brown. When the time of electro-lysis was 15min, the absorption peak at 262 nmdisappeared and the color of the solution becamedark brown. Such a tendency in UV–vis spectrawas the same as that reported by Duff et al. andsuggests the formation of Pt nanoparticles [23,24].The decrease in the intensity of the adsorptionband with electrolysis time was due to the increasein concentration of platinum nanoparticles. Theconcentration of platinum particles became higherand higher as the electrolysis proceeded. Thechanges in color of the solution also contributedto the formation of more and more platinumnanoparticles. But in the present experimentalcondition, longer electrolysis time did not affectthe size of the particles because all the platinumions were utilized for atom formation and theatoms ultimately formed the colloidal platinumparticles, in good agreement with the UV–visresults.The crystal structures of Pt nanoparticles which

were examined by XRD were shown in Fig. 4. Thetypical diffraction peaks which belong to FCCplatinum can be clearly observed for the particles.As expected, the XRD peaks of the nanocrystal-lites were considerably broadened compared tothose of the bulk Pt due to the finite size of thesecrystallites. These diffraction features appearing at2y ¼ ca: 39:9; 46:5; 68:4; were in accord with thecrystal data of platinum and correspond to the{1 1 1}, {2 0 0}, and {2 2 0} planes of the cubicphase of Pt, respectively [25]. The average crystal-line sizes of the platinum nanoparticles weredetermined from the FWHM of the X-raydiffraction lines according to the Debye–Scherrerequation, and were calculated to be 8.86 nm (PVP/Pt 500:1). These values agreed with the TEMobservations.It should be emphasized that the platinum

clusters formed must be rapidly transferred fromthe cathode vicinity to the bulk solution in order to

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M. Zhou et al. / Physica E 27 (2005) 341–350 345

obtain the nearly monodispersed nanoparticles.Otherwise, when the concentration of platinumnanoparticles in the vicinity of the cathode is toohigh, the increase of interaction between alkylchains of PVP will lead to the appearance offlocculates. The static cathode and rotating cath-ode were used to accelerate the transfer ofplatinum clusters to the bulk solution. Fig. 2shows the TEM image of platinum nanoparticlesynthesis by static cathode. It can be seen that theparticles spherically and multiply dispersed, andaggregation of particles occurred. Clearly, staticreduction cannot transfer the platinum clustersformed from the cathodic vicinity to the bulk. Itsrole is just to increase the collision frequencybetween the platinum clusters formed and thecathode. Formation of large particles can beexplained on the basis of the theory of suspendedelectrode. Excited by the ultrasonic waves, thesuspended platinum particles move in the solution

Fig. 2. TEM image of platinum particles synthesized using a

static cathode and without stirring. The other experimental

conditions are the same as used in Fig. 1.

near the cathode, come into collision with thecathode, accept its potential, and then travel backto the solution. These charged particles behave aspart of the cathode and can make platinum ionselectrodeposit on them and therefore grow intolarge particles. When mechanical rotation wasused instead of ultrasonication, the electrolyticsolution showed a beautiful variation in visiblecolor, from light yellow to deep yellow and finallyto dark brown, depending on electrolysis time, andno black precipitate was formed until the end ofelectrolysis reaction. The color change is asso-ciated with the increase of the concentration ofplatinum nanoparticles in the electrolyte, and theTEM image of the particles is shown in Fig. 1. It isobvious that the nanoparticles in Fig. 1a aresmaller in size and much better in size distributionthan those shown in Fig. 2. Compared to the staticreduction, the mechanical rotation can signifi-cantly accelerate the transfer of platinum clustersproduced from the cathodic vicinity to the bulksolution and make the platinum clusters distributerelatively uniformly in the solution. Thus, mostplatinum clusters far from the cathode growuniformly in the bulk under protection of PVP,which is favorable for formation of the well-dispersed nanoparticles. Despite all this, mechan-ical rotation cannot completely eliminate theformation of suspended electrode. A small amountof platinum clusters near the cathode still hasenough chances to grow into large particlesaccording to the mechanism of suspended elec-trode formation, as stated earlier (Figs. 3–5).

3.2. Influence of Pt nanoparticles by concentrations

of PVP and chloroplatinic ion

The effect of PVP concentration on the plati-num particle size was studied using UV–visspectroscopy. Fig. 6 presents the absorptionband of the PVP-Pt nanoparticles synthesizedat various molar ratios of PVP/Pt in thesolution. It can be seen that the obvious absorp-tion band for PtCl2�6 showed up when theN-vinylpyrrolidone monomer=PtCl2�6 molar ratiois relatively low, as described by curve A, curve Band curve C; and the color of the solutions was notchanged. In addition, the electrolytes after 15min

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Fig. 4. The XRD patterns of platinum nanoparticles electro-

chemically synthesized in the same experimental conditions as

used in Fig. 1.

Fig. 3. The UV–vis absorption spectra of a solution of 6�

10�3 mol=dm3 H2PtCl6 þ 0:1mol=dm3 KNO3 þ 50 g=dm3 PVP

with electrolysis time.

Fig. 5. TEM image of platinum particles electrochemically

synthesized in the absence of PVP. The other experimental

conditions are the same as used in Fig. 1.

Fig. 6. UV–vis spectra for platinum nanoparticles synthesized

by electrochemical reduction in electrolytes with different PVP

monomer=PtCl2�6 molar ratios under mechanical stirring

(electrolysis time is 15min). PVP=PtCl2�6 molar ratios are

indicated in the plot. The concentrations of H2PtCl6 and KNO3

are 6� 10�3 and 0:1mol=dm3; respectively.

M. Zhou et al. / Physica E 27 (2005) 341–350346

electrolysis seemed to be grayish and precipitatewas formed, which implies formation of largeplatinum particles due to insufficient protection ofPVP. As the molar ratio of PVP/Pt was increased,the maximum of plasmon peak diminished gradu-ally. However, when the ratio increased to 50:1,the characteristic plasmon band for PtCl2�6 at262 nm disappeared. These phenomena proved theformation of platinum nanoparticles as the

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foregoing statement. Moreover, upon increasingthe ratio from 50:1 to 500:1, the band displayed aslight difference. These features were related tothe small size and uniform size distribution forthe platinum nanoparticles. The appropriatemolar ratio between the N-vinylpyrrolidonemonomer and chloroplatinic ions ranged from50:1 to 500:1 based on our results. The higherN-vinylpyrrolidone monomer/PtCl26- molar ratio isdisadvantageous to the electrochemical synthesisof platinum nanoparticles, since that will makethe electrolyte more viscous and therefore slowthe transfer rate of chloroplatinic ions toward thecathode.Accordingly, the amount of PVP added to the

solution is expected to affect the growth processfor the Pt nanoparticles. Therefore, the change inthe size of the Pt nanoparticles was investigated by

Fig. 7. TEM image of platinum nanoparticles electrochemically synth

0:1� 10�3 mol=dm3 KNO3 at various molar ratios of PVP/Pt: (a) 1:1

varying the amount of PVP in the solution. Fig. 7presented the TEM images of the PVP-Pt nano-particles synthesized at various molar ratios ofPVP/Pt in the solution. It can be seen that themolar ratios had a great influence on the particlesize and degree of aggregation. In particular, anobvious aggregation phenomenon between parti-cles took place when the molar ratios were 1:1 (7a)and 5:1 (7b). Fig. 7c showed that the size anddistribution of particles was more narrow and eventhan Figs. 7a and b, and the condition in Fig. 7dwas more improved than in Fig. 6c. Sphericalplatinum particles with a small size and a narrowsize distribution were obtained when the molarratios was controlled at relatively high molarratios (100:1 and 500:1), as is shown in Figs. 7eand f. The decrease in particle size could be owingto the fact that the presence of a larger amount of

esized in an aqueous solution of 6� 10�3 mol=dm3 H2PtCl6 þ

; (b) 5:1; (c) 10:1; (d) 50:1; (e) 100:1; (f) 500:1.

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the stabilizer would lead to a higher dispersion ofthe metal ions on the polymer matrix and hencethe metal atoms formed; the unchanging particlesize may be because of the fact that a smalleramount of stabilizing polymer is already sufficientfor the stabilization. By decreasing the PVP/Ptratio, the growth of the particles was thought toproceed by the lack of the number of protectinggroups of PVP. Therefore, the particle size couldbe significantly controlled by the amount of PVPðPVP=Pt450: 1Þ at the solution, the size distribu-tion remaining quite narrow.We also investigate the influence of the con-

centration of the chloroplatinic ion. In Fig. 8, theconcentration of PtCl2�6 was increased graduallywhile keeping the PVP/Pt molar ratios constant at500:1. It can be seen that the concentration ofPtCl2�6 had a great influence in the electrochemicalsynthesis of platinum nanoparticles too. From thechange of the absorbance band as shown in Fig. 8,the reduction and particle formation process endedat about 15min after the reaction. The disappear-ance of 260 nm absorbance for Fig. 8 was foundwhen the concentration of PtCl2�6 was o1�10�3 mol=dm3 (Fig. 8), indicating that platinumnanoparticles were synthesized. However, when

Fig. 8. UV–vis spectra for platinum nanoparticles synthesized

by electrochemical reduction in electrolytes with different

concentration of the chloroplatinic ion under mechanical

stirring (electrolysis time is 15min). The concentration of the

chloroplatinic ion is indicated in the plot. The concentrations of

PVP and KNO3 are 50 g=dm3 and 0:1� 10�3mol=dm3;respectively.

the concentration was o1� 10�4 mol=dm3; nochange of solution was observed by the UV–visspectra because the very low concentration ofPtCl2�6 allowed enough platinum ions to formclusters.

3.3. Formation mechanism of Pt nanoparticles

The key to electrochemical synthesis of platinumnanoparticles in the aqueous phase is to avoid theformation of platinum plating and to forceplatinum particles reduced to leave the cathodesurface as rapidly as possible. In this sense, PVPmay fully meet the requirements for electrochemi-cal synthesis of platinum nanoparticles. Theprotective polymer, PVP, apparently stabilizedthe Pt nanoparticles by preventing them fromaggregating. We employed UV–vis, FT-IR mea-surements to clarify the protecting mechanism ofPVP. As mentioned above, the mixed solution ofPVP and H2PtCl6 before electrolysis shows a peakat 262 nm in its UV–vis spectrum due to theligand-to-metal charge-transfer transition of thePtCl2�6 ions. On the other hand, H2PtCl6 solutionwithout PVP shows a peak at 258 nm (Fig. 9). Thisdifference means that the ligand field splitting of Pt5d orbital slightly expands due to the coordinationof N and/or O atoms of PVP to Pt4þ; whichprovide stronger ligand field than Cl�: In FT-IRspectra (Fig. 10), a new peak assigned to afrequency of a carbonyl stretching band appearedat a lower frequency (1652:5 cm�1) for the mixtureof PVP and H2PtCl6 before electrolysis (PVP-H2PtCl6) in addition to that of pure PVP(1670:1 cm�1), suggesting some interaction be-tween the carbonyl groups and Pt4þ ions. Theband for the PVP-Pt nanoparticles (1664:3 cm�1)was higher than that of PVP-H2PtCl6; suggestingthat fewer carbonyl groups interacted with thesurface Pt atoms of the Pt nanoparticles. Teranishi[22] reported that the N and O atoms of PVP makecoordinate bonds with Pt4þ ions and with parts ofthe surface Pt atoms of PVP-Pt. Consequently, theformation process of the PVP-Pt nanoparticlesmay be summarized as follows (Scheme 2). ThePVP and PtCl2�6 ions form complexes at firstthrough N and O atoms of PVP, which isaccompanied by the elimination of some Cl� ions

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Scheme 2. Schematic of diagram showing formation of electrochemically produced polymer-stabilized platinum nanoparticles.

Fig. 9. UV–vis spectra of H2PtCl6 and PVP-H2PtCl6ðPVP=H2PtCl6 ¼ 500: 1Þ:

Fig. 10. FTIR spectra for the PVP, PVP-Pt and PVP-H2PtCl6ifn wavenumber between 1800 and 1500 cm�1:

M. Zhou et al. / Physica E 27 (2005) 341–350 349

from PtCl2�6 : The Pt4þ ions are reduced to Pt0

atoms, and the Pt nuclei form between 10 and20min. In the next 10min, the reduction of Pt4þ

and the growth of the Pt nuclei proceed at the

same time to form the Pt nanoparticles with thecomplete elimination of Cl�: Although the forma-tion mechanism and the shape of the Pt nuclei areuncertain at present, the Pt nuclei may grow byreducing the Pt4þ ions mainly on the mostcatalytically active {1 1 1} surfaces of Pt nuclei toform truncated octahedral nanoparticles, aspointed out by Petroski et al. [26]. Finally, the Ptnanoparticles are stabilized by the coordination ofparts of the surface Pt atoms to the N and O atomsof PVP. Part of the main chain of PVP is expectedto be adsorbed on the surface Pt atoms byhydrophobic interaction.

4. Conclusions

Nanosize dispersions of platinum with a narrowparticle size distribution could be synthesized byelectrochemical reduction of H2PtCl6 in thepresence of PVP in the aqueous solution. Theaddition of PVP was necessary to prevent inter-particle conglomeration. The amount of PVPadded was found to be critical because an excessinhibits completely the reduction of the metalcomplex, thus preventing the formation of metallicplatinum. Control of the average particle size anddegree of conglomeration of these dispersions waspossible by using different quantities of metal-starting materials and PVP, and by modifying theexperimental equipment of particle preparation.Colloidal dispersions of Pt stayed stable afterseveral months. The most distinguished features ofthe method include simple operation, high yield,well-dispersity of particles, quite high stability ofmetal colloids prepared, and easy control ofparticle size, and there were especially good forfuture application in synthesis of metal nanopar-ticles on a large scale (Schemes 1 and 2).

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Acknowledgements

The authors thank the Chinese National ScienceFund (20373038) for the support of this research.

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