Applied Surface Science 258 (2011) 1317 1321

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Applied Surface Science

j our nal ho me p age: www.elsev ier .com

Facile s semtheir el

Mei Kong hanSchool of Chem l Chem

a r t i c l

Article history:Received 18 MReceived in reAccepted 30 AAvailable onlin

Keywords:CuOHollow nanospSynthesisKirkendall effeLithium ion ba

rage theraract

miche baollow

and y nanuO n

of ntance

contalithium ion insertion and extraction.

2011 Elsevier B.V. All rights reserved.

1. Introdu

Hollow to their higpotential apdevices anddeveloped fors includinand ultrasobased methof hollow ndation and from the didiffusion co

CuO, an gap (Eg = 1.optical switsidering its been prepahollow micrnanoribbon

CorresponE-mail add

0169-4332/$ doi:10.1016/j.ction

nanostructures have been paid much attention dueh specic surface area, low density and widespreadplications in drug delivery, chemical sensors, photonic

lightweight ller [1,2]. A variety of methods have beenor the synthesis of nanostructures with hollow interi-g template methods [3,4], hydrothermal treatment [5]nic treatment [6]. In particularly, the Kirkendall effect-ods have attracted great interests for the preparationanomaterials in recent years, such as oxidation, sul-phosphorization of metal nanoparticles, which resultsfferent diffusion rates between two components in auple [7,8].important p-type semiconductor with a narrow band2 eV), has potential applications in sensors, catalysts,ches, lithium ion batteries and solar cells [9,10]. Con-wide applications, various CuO hollow structures havered. Liu and Zeng [11] synthesized dandelion-like CuOospheres with diameters of 48 m, assembled by CuOs, using Cu(NO3)23H2O, NH3H2O, NaOH and NaNO3

ding author. Tel.: +86 551 2901450; fax: +86 551 2901450.ress: wxzhang@hfut.edu.cn (W. Zhang).

as reactants by solvothermal method at 100 C for 24 h. Liu andXue [12] reported the synthesis of porous CuO hollow architectureswith diameters of 11.7 m by thermal oxidation of CuS and Cu2Ssolid precursors at 700 C for 4 h, respectively. The precursors weresynthesized by solvothermal method using Cu(NO3)23H2O andthiourea as reactants with the assistance of PVP at 120 C for 20 h.Bourret and Lennox [13] synthesized porous CuO hollow micro-spheres assembled by nanoribbons with diameter of 2.55 m bythermolysis of porous Cu(OH)2 hollow microspheres at 600 C for15 h. The Cu(OH)2 precursor was fabricated by template method atroom temperature for 35 min, using CuCl22H2O, n-butylamine asreactants and water-in-oil emulsion as soft template in H2O/CH2Cl2system.

Among all those potential applications, CuO as anode materi-als for lithium ion batteries have attracted much interests becauseof their high theoretical capacity (670 mAh g1), high safety andlow cost. It has been reported that the morphology and size ofCuO could inuence its electrochemical performances [14,15]. Forinstance, Zheng et al. [16] synthesized CuO nanoparticles with anaverage size of 10 nm via thermal decomposition of CuC2O4 pre-cursor at 400 C for 4 h, which could deliver an initial dischargecapacity of 810 mAh g1 at a current density of 0.1 mA cm2. Thesecond and third discharge capacities were about 350 mAh g1

and 120 mAh g1, respectively. The CuO nanoparticles had a poorcycle performance. Li et al. reported [17] the synthesis of CuO nan-otubes by heating Cu nanowires at 600 C for 3 h, which showed

see front matter 2011 Elsevier B.V. All rights reserved.apsusc.2011.08.127ynthesis of CuO hollow nanospheres asectrochemical performance

, Weixin Zhang , Zeheng Yang, Shaoying Weng, Zical Engineering, Anhui Key Laboratory of Controllable Chemical Reaction and Materia

e i n f o

ay 2011vised form 29 August 2011ugust 2011e 29 September 2011

heres

cttteries

a b s t r a c t

CuO hollow nanospheres with an avesuccessfully synthesized via a simpleprecursor. The products have been chand eld emission scanning electronresults from the Kirkendall effect on telectrochemical performance of CuO hbeen evaluated by cyclic voltammetryCuO hollow nanospheres assembled bcycle performance than the reported Cdemonstrated to take the advantagesshorten the lithium ion transport disprovide suitable electrode/electrolyte/ loc ate /apsusc

bled by nanoparticles and

gxian Chenical Engineering, Hefei University of Technology, Hefei 230009, China

diameter of 400 nm and shell thickness of 40 nm have beenmal oxidation strategy with Cu2O solid nanospheres as theerized by X-ray diffraction, transmission electron microscopyroscopy. The formation of CuO hollow nanospheres mainlysis of temperature-dependent experiments. Furthermore, the

nanospheres as anode materials for lithium ion batteries hasgalvanostatic discharge-charge experiments. The as-preparedoparticles exhibit higher initial discharge capacity and betteranoparticles. The hierarchical hollow nanospheres have beenanoparticles and hollow architectures, which could not only

and increase the kinetics of conversion reactions, but alsoct area and accommodate the volume change associated with

1318 M. Kong et al. / Applied Surface Science 258 (2011) 1317 1321

an initial discharge capacity of 910 mAh g1 and the 10th dis-charge capacity of about 100 mAh g1 at a current density of50 mA g1.

In this paper, we report a facile route to synthesize CuO hol-low nanospoxidation oCu2O solid scale througout any surCuO hollowfor lithium show highecompared wets from tnanoparticl

2. Experim

2.1. Synthe

All the rand used wnanosphere[18]. The wperature unCuSO45H2Otilled waterof NaOH sosolution in quickly pouyellow precdeionized wat 60 C.

The CuOtion of Cu25 C min1. product wa

2.2. Charac

The as-pdiffraction a Cu K rad80 mA. Fieldsurement wmicroscopesion electrodiffraction (mission eleof 200 kV. Fon a Perkindard KBr pe

2.3. Electro

The electype cells (Cwere prepaacetylene bratio of 80slurry was u70 C for 4 hwith lithiumlene membmixture of e

1

1 1

1|

Rel

ativ

e In

tens

ity

1 1

11 1

1

RD pnt at 4

voluonduina) 13.

on aent

of 0

ults

com by X

of tre. A

No. t obt

in w CuOed inical

2. Af nanhe hsurfay of here

andters s shre. F, it cah an

TEM investigation was used to provide further insight intoO hollow structure. Fig. 3a shows the TEM image of therecursor, indicating that the Cu2O nanospheres have a solidre with a diameter of 400 nm. The TEM image of CuO is

in Fig. 3b. The contrast in brightness between the darknd bright centers conrms the hollow structure of CuO with

hickness of about 40 nm, which is consistent with FESEM. The corresponding SAED patterns in Fig. S1a and b (Sup-

g information) display the characteristic diffraction rings ofu2O and monoclinic CuO, respectively, indicating the poly-line nature of the products.study the formation mechanism of CuO hollow structure,s were collected after heating the Cu2O precursor in air at

and 300 C for 2 h, respectively. After 2 h of thermal treat-n air at 250 C, a core-shell structure with a little void starts toheres with uniform diameters in large scale by thermalf Cu2O solid nanospheres at 400 C for 2 h. The uniformnanospheres as the precursor were prepared in largeh a solution-phase method at room temperature with-

factant or organic solvent. The formation mechanism of nanospheres has been proposed. As anode materialsion batteries, the as-prepared CuO hollow nanospheresr initial discharge capacity and better cycle performanceith the reported CuO nanoparticles, which mainly ben-he hierarchical hollow nanostructures assembled byes.

ental details

sis

eagents used in the experiments were analytical gradeithout further purication. The synthesis of Cu2O solids as the precursor was referred to our previous reporthole reaction was conducted in a beaker at room tem-der constant magnetic stirring. Typically, 0.375 g of

and 2 g of glucose were dissolved in 100 mL of dis-. Then, 25 mL of NH3H2O solution (0.04 M) and 25 mLlution (0.20 M) were added dropwise into the aboveturn. 10 min later, 50 mL of ascorbic acid (0.03 M) wasred into the mixture. After stirring for another 1 h, theipitates were collected by centrifugation, washed withater and ethanol for several times and then dried in air

hollow nanospheres were prepared by thermal oxida-O precursor in air at 400 C for 2 h at a heating rate ofAfter cooling to room temperature naturally, the blacks obtained.

terization

repared samples were characterized by X-ray powder(XRD) in a Rigaku D/max-B X-ray diffractometer withiation source ( = 0.154178 nm) operated at 40 kV and-emission scanning electron microscopy (FESEM) mea-as carried out with a FEI Sirion-200 scanning electron

operated at an acceleration voltage of 5 kV. Transmis-n microscopy (TEM) images and selected area electronSAED) patterns were taken with a Hitachi H-800 trans-ctron microscope performed at an accelerating voltageourier Transform Infrared (FTIR) spectra were recordedElmer Spectrum 100 FTIR Spectrometer using the stan-llet technique in the range of 4004000 cm1.

chemical measurements

trochemical measurements were carried out using coin-R2032) at room temperature. The working electrodes

red by mixing the as-prepared CuO hollow nanospheres,lack and polyvinylidene uoride (PVDF) at a weight:10:10 in N-methylpyrrolidone (NMP). The resultingniformly spread onto a Cu foil and dried in vacuum at. The cells were assembled in an argon-lled glove box

disk as counter electrode, microporous polypropy-rane (Celgard-2400) as separator and 1 M LiPF6 in athylene carbonate (EC) and dimethyl carbonate (DMC)

Fig. 1. Xtreatme

(1:1 inwere cCo., Chof 0.00formedInstrumrange

3. Res

Theminedpatternperatu(JCPDSproducfor 2 h,oclinicdetect

Typin Fig.sists oscale. Trough pholognanosp400 nmdiamespherestructuFig. 2dis roug

Thethe CuCu2O pstructushownedges ashell timagesportincubic Ccrystal

To sample250 Cment i0 20 30 40 50 60 70

(b) 1 1 0

2 2

0

2 0

0

1 1

1

1 1

0

2 / degre e

|||

2 2

03

1 1

1 1

32

0 2

0 2

0

2 0

2

(a)

atterns of (a) the precursor and (b) the nal product after thermal00 C for 2 h in air.

me) as electrolyte. Galvanostatic chargedischarge testscted on a BTS battery test system (Shenzhen Newareat a current density of 67 mA g1 in a potential range0 V. Cyclic voltammetry (CV) measurements were per-

CHI604C electrochemical analyzer (Shanghai Chenhua Co., China) at a scan rate of 0.1 mV s1 in the potential3.0 V.

and discussion

position and phase purity of the products were deter--ray powder diffraction (XRD). Fig. 1a shows the XRDhe precursor precipitated in the solution at room tem-ll the diffraction peaks can be indexed to cubic Cu2O05-0667). Fig. 1b displays the XRD pattern of the nalained by thermal oxidation of Cu2O precursor at 400 Chich all the diffraction peaks can be indexed to the mon-

(JCPDS No. 48-1548). No impurities could be obviously the XRD patterns.FESEM images of the as-prepared products are showns can be seen in Fig. 2a, the Cu2O precursor con-ospheres with an average diameter of 400 nm in largeigh-magnication FESEM image in Fig. 2b displays thece of these nanospheres. Fig. 2c and d shows the mor-CuO prepared through thermal oxidation of the Cu2Os. The average diameter and shell thickness are about

40 nm, respectively, without any obvious changes ofcompared with Cu2O precursor. Many broken hollowown in Fig. 2c reveal the existence of hollow nano-rom the high-magnication FESEM image displayed inn be seen that the shell of the CuO hollow nanospheresd some small nanoparticles can be clearly observed.

M. Kong et al. / Applied Surface Science 258 (2011) 1317 1321 1319

(c, d)

form, indicainformationbecome larg

Similar pcollected that 400 C forlution aftershown in Fiheating at 4layer and thing time atand the core15 min of hlow sphere

Fig. 3. TEM im

pleg at 4the b

struginnihin Chereh thFig. 2. FESEM images of (a, b) Cu2O nanospheres and

ting condensation of abundant vacancies (Supporting, Fig. S2a). When heated at 300 C for 2 h, the voidser (Supporting information, Fig. S2b).henomena were also observed when the samples wererough thermal oxidation of the Cu2O solid nanospheres

different time. The TEM images of the morphology evo- heating the samples at 400 C for different time wereg. S3 (Supporting information). It can be seen that after

are comheatin

On hollowthe beand a tnanospthroug00 C for 5 min, a sphere with voids between the surfacee inside core can be observed (Fig. S3a). When the heat-

400 C is prolonged to 8 min, the voids become larger-shell structure becomes more obvious (Fig. S3b). Aftereating at 400 C, the core almost disappears and a hol-gradually forms (Fig. S3c). Finally the solid nanospheres

ages of (a) Cu2O nanospheres and (b) CuO hollow nanospheres.

Because of outward thsequently, eappears.

Interestiby Lis grodimethylfotransformein air at 500The possiblin our workglucose, is To further spectra of investigatemation). Fignanosphereresponds todue to adsoand 1610 cmbending, reregion corr1460 cm1

ated with tconrms thFTIR spectrthere still eCuO hollow nanospheres.

tely transformed into hollow nanospheres after 2 h of00 C (Fig. 3d).asis of above observations, the formation process of CuOcture is much related with the Kirkendall effect [7,8]. Atng of the thermal oxidation, Cu2O reacts with O2 in airuO layer would be formed on the surface of Cu2O solids. Subsequently, the Cu+ in the core diffuse outwarde CuO shell, meanwhile the O2 in air diffuse inward.

the different diffusion rates, Cu+ diffuse much fasteran O2 inward and the hollow interior forms [19]. Con-xcess vacancies occur in the core and hollow structure

ngly, the result here is quite different from that reportedup [20], who used a solution-phase method in N,N-rmamide at 8595 C to get Cu2O solid nanospheres andd them into CuO solid nanospheres by thermal oxidation

C for 2 h, without any obvious change of morphology.e reason may be that the Cu2O nanosphere precursor, synthesized at room temperature in the presence ofdifferent from the one reported in the literature [20].characterize the as-prepared Cu2O nanospheres, FTIRthe as-prepared Cu2O nanospheres and glucose wered, respectively, as shown in Fig. S4 (Supporting infor-. S4a shows the FTIR spectra of the as-prepared Cu2Os. The characteristic absorption band at 629 cm1 cor-

the CuO vibration of the Cu2O nanocrystals. The bandsrptions of water molecules are observed at 3300 cm11, which are attributed to the OH stretching and OH

spectively. Besides, the bands in the 28003000 cm1

espond to the CH stretching. The bands located atand 1270 cm1 are assigned to the vibrations associ-he CH2 group, and typically the band near 1050 cm1

e existence of carbohydrate [21]. Compared with thea of glucose in Fig. S4b, the result clearly suggests thatxist some glucose molecules in the Cu2O spheres even

1320 M. Kong et al. / Applied Surface Science 258 (2011) 1317 1321

0.0 0.5 1.0 1.5 2.0 2.5 3.0-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

0.83 V

1.01 V

2.36 V

0.73 V 3rd 2nd

1st

Cur

rent

/ m

A

Potential / V vs. Li+/Li

1.15 V

Fig. 4. Cyclic voltammograms of the CuO/Li cell in the rst, second and third cyclesat a scan rate of 0.1 mV s1.

after washing the samples with water and ethanol several times[22,23]. Thus, the aggregates of Cu2O nanoparticles with the mod-ication of the glucose molecules may play an important role inthe facile formation of CuO hollow nanostructures [24,25]. Mean-while, durinmolecules igases, whicthe formati

The eleclow nanosplithium iongrams of thcycle, two are attributtrolyte inteat 2.36 V is tice of CuO 0.73 V and indicating tcycle, the pdecreases s

Fig. 5 diat a currencess, there and 0.80.0CV measur

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Pote

ntia

l / V

vs.

Li+

/Li

Fig. 5. The di67 mA g1 in aperformance o

which is higher than the theoretical capacity of CuO (670 mAh g1).The extra discharge capacity is mainly attributed to the formationof a solid electrolyte interface (SEI) lm during the rst dis-charge/charge process [27,30]. The capacity fades to 425 mAh g1

in the secocapacity grCuO hollowinitial dischprevious reonly deliverent density120 mAh g

chical holloand hollowresult fromion transporeactions. Thierarchicaelectrode/echange asso

4. Conclus

s where

of C. Thrisonhereollow

fromachies. Thignis, ther lith4 mAanopructuain e

suitnopa

the d in

wled

authl Scg the thermal oxidation process, the remnant glucosen the Cu2O spheres were decomposed into CO2 and H2Oh would emit out of the spheres and were benecial toon of the CuO hollow nanostructures [26].trochemical performances of the as-prepared CuO hol-heres have been evaluated as anode materials for

batteries. Fig. 4 shows the rst three cyclic voltammo-e CuO/Li cell at a scan rate of 0.1 mV s1. In the rstcathodic peaks located at 1.01 and 0.83 V (vs. Li+/Li)ed to the electrode reaction and growth of solid elec-rface, respectively [27,28]. Meanwhile, the anodic peakcorresponding to the Li extraction from the crystal lat-[29]. In the following cycle, two cathodic peaks shift to1.15 V along with the decrease of each peak intensity,hat the reversible capacity greatly loses. In the thirdeak potentials remain similar and the peak intensitylightly.splays the dischargecharge curves of the CuO/Li cellt density of 67 mA g1. During the rst discharge pro-are two obvious plateaus at the potential of 1.221.0 V1 V (vs. Li+/Li), which is consistent with the results ofement. The rst discharge capacity is 1134 mAh g1,

1000

1200

mA

h g

1

2nd3rd

Thinanospdationfor 2 hcompananospCuO hresultseasily procesgreat sBesiderials foof 113CuO nnanostthe stroffer athe nareduceions an

Ackno

TheNatura0 1 2 3 4 5 6 7 8 9 100

200

400

600

800

Spec

ific

capa

city

/

Cycle nu mber

0 20 0 40 0 60 0 800 10 00 120 0

3rd

2nd

1st

Specific capacity / mAh g1

1st

schargecharge curves of the CuO/Li cell at a current density of potential range of 0.0013.0 V. The inset is the corresponding cyclicf the cell.

20976033 aCentral Uniment of An

Appendix A

Supplemthe online v

References

[1] S.W. Cao,[2] Y.F. Zhu, [3] G. Jia, H.P[4] X.X. Li, Y.[5] M.W. Xu,[6] J.J. Zhu, Snd discharge cycle and after 10 cycles the dischargeadually decreased to 190 mAh g1 (inset in Fig. 5). The

nanospheres assembled by nanoparticles show higherarge capacity and better cycle performance than theport of CuO nanoparticles with size of 10 nm, which canr an initial discharge capacity of 810 mAh g1 at a cur-

of 0.1 mA cm2 and the discharge capacity decreased to1 after 3 cycles [16]. The results indicate that the hierar-w nanospheres can take the advantages of nanoparticles

architectures. The high initial discharge capacity may the nanoparticles, which could shorten the lithiumrt distance and increase the kinetics of conversionhe good cycle performance is much related with thel hollow nanostructures, which could provide suitablelectrolyte contact area and accommodate the volumeciated with lithium ion insertion and extraction.

ions

ork demonstrates the synthesis of CuO hollows with diameters of about 400 nm by thermal oxi-u2O solid nanospheres as the precursor in air at 400 Ce morphology of CuO does not change obviously in

with Cu2O precursor and the as-prepared CuO hollows are assembled by nanoparticles. The formation of

nanospheres from Cu2O solid nanospheres mainly the Kirkendall effect. The whole process can be

ved without any surfactant, organic solvent or etchingis facile, green and low-cost synthesis route may be ofcance to prepare other metal oxide hollow structures.

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gements

ors are grateful to the nancial supports of the Nationalience Foundation of China (NSFC Grants 20871038,nd 21176054), the Fundamental Research Funds for theversities (2010HGZY0012) and the Education Depart-hui Provincial Government (TD200702).

. Supplementary data

entary data associated with this article can be found, inersion, at doi:10.1016/j.apsusc.2011.08.127.

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Facile synthesis of CuO hollow nanospheres assembled by nanoparticles and their electrochemical performance1 Introduction2 Experimental details2.1 Synthesis2.2 Characterization2.3 Electrochemical measurements

3 Results and discussion4 ConclusionsAcknowledgementsAppendix A Supplementary dataAppendix A Supplementary data