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Structural evolution from mesoporous a-Fe2O3 to Fe3O4@C and g-Fe2O3

nanospheres and their lithium storage performances†

Shuming Yuan,a Zhen Zhou*a and Guang Li*b

Received 29th November 2010, Accepted 19th April 2011

DOI: 10.1039/c0ce00902d

Mesoporous a-Fe2O3 nanospheres were synthesized through a solvothermal route, with polyethylene

glycol as soft templates. The mesoporous nanospheres were then coated with carbon through

hydrothermal treatment of glucose. After being calcined at 500 �C under Ar atmosphere for 4 h,

mesoporous Fe3O4@C nanospheres were obtained. When further heated at 260 �C in air for 2 h,

Fe3O4@C was transformed into g-Fe2O3 mesoporous nanospheres. Significant phase changes occurred

without destroying the structure of mesoporous nanospheres. All the three mesoporous nanospheres

showed superior Li storage performances at the initial charge and discharge cycle; however, the cyclic

stabilities of the three samples were quite different; Fe3O4@C exhibited a stable discharge capacity of

�570 mA h g�1 after 30 cycles, while the discharge capacities of a-Fe2O3 and g-Fe2O3 faded rapidly to

�100 mA h g�1 after 30 cycles.

1. Introduction

Mesoporous materials with tunable pore structures and high

surface areas have many applications in catalysis, electron

transfer, energy conversion and storage, and magnetism.1–7

Especially, transitionmetal oxides withmesoporous structure can

confine d-electrons to the thin walls between pores and endow

such materials with unusual magnetic, electrochemical and

optical properties, and meanwhile the high internal pore surface

area can lead to new and unique catalytic performances.8–11 For

example, mesoporous Nb2O5$nH2O exhibits much higher cata-

lytic activity for the hydrolysis of cellobiose than super-

microporous and bulk Nb2O5$nH2O.12 Mesoporous MnO2/

carbon aerogel composites have been employed as supercapacitor

electrodes with a high specific capacitance and good cyclic

ability.13 Mesoporous Co3O4 nanobelts with a main pore size of

15 nm showed a higher capacitance and more stable reversibility

than other forms as lithium ion battery (LiB) anodes.14Therefore,

mesoporous transitionmetal oxides have receivedmore andmore

attention.

Mesoporous iron oxides (a-Fe2O3, g-Fe2O3 and Fe3O4) have

also attracted much interest due to the magnetic, catalytic and

sensoric behaviors. For example, mesoporous a-Fe2O3 hollow

microspheres presented high remnantmagnetization and coercive

aInstitute of New Energy Material Chemistry, Key Laboratory ofAdvanced Energy Materials Chemistry (Ministry of Education), NankaiUniversity, Tianjin, 300071, China. E-mail: zhouzhen@nankai.edu.cnbSchool of Physics andMaterials Science, Anhui University, Hefei, 230039,China. E-mail: liguang64@163.com

† Electronic supplementary information (ESI) available: Nitrogenadsorption/desorption isotherm and XRD patterns. See DOI:10.1039/c0ce00902d

This journal is ª The Royal Society of Chemistry 2011

force.15 Mesoporous a-Fe2O3 with various morphologies such as

mesoporous nanoparticles,16 nanorods,17,18 films,19 hollow

microspheres,15 and walls20–23 have been successfully prepared

through chemical and physical routes.

So far, ironoxides (FexOy) have been extensively investigated as

potential LIB anode materials for the high theoretical capacity

(900–1000 mA h g�1), low cost, good stability, and environmental

friendliness.24–27 However, fundamental changes occur to the

structure during the lithium inclusion and exclusion, resulting in

the destruction of the solid electrolyte interphase (SEI) film on the

electrode surface. The repeated formation/decomposition of the

SEI film consumes the Li supplied by the cathodes and leads to

poor cyclic performances. Meanwhile, the generated Fe nano-

grains with large specific surface have high catalytic activity in

some irreversible reactions.38–31 Mesoporous structure is an

effective way to improve the electrochemical performances of

oxide anode materials, which can increase the interfaces between

active materials and the electrolyte. Moreover, the mesopores in

materials can supply the space to buffer the volume changes.32

Unfortunately the increased interfaces may raise the risk of side

reactions. Currently carbon coating is an effective technique to

overcome the side reactions between active materials and the

electrolyte.33–37 Carbon-coated FexOy nanorods, nanospindles,

and nanowires have been synthesized and presented better elec-

trochemical performances.38–43

Preparation of g-Fe2O3 mesoporous nanospheres is difficult

due to the fact that nano-size g-Fe2O3 nanoparticles tend to

aggregate into large particles, which makes the g–a phase tran-

sition easier.44 Herein, we report the synthesis of mesoporous

g-Fe2O3 nanospheres through a conversion process from meso-

porous a-Fe2O3. In a typical procedure, we successfully synthe-

sizedmesoporous a-Fe2O3, Fe3O4@C, andg-Fe2O3 nanospheres,

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and all the productsmaintained their original porousmorphology

under the calcination conditions. Also, we investigated the

performances of electrochemical lithium storage in mesoporous

a-Fe2O3, g-Fe2O3 and Fe3O4@C nanospheres.

2. Experimental

2.1. Synthesis

All chemicals were analytical grade and used as received without

further purification. Mesoporous a-Fe2O3 was synthesized

through a solvothermal process. In a typical case, FeCl3$6H2O

(0.3 g) with 0.4 g polyethylene glycol (PEG-6000) and 0.4 g

hexamethylenetetramine was added into a 50 ml of Teflon-lined

autoclave and the autoclave was filled with 20 ml of distilled H2O

and 20 ml of n-butanol. The autoclave was heated at 150 �C for

24 h and allowed to cool to room temperature. The obtained

composite (sample A) was collected and washed with deionized

water and absolute ethanol, and then 0.35 g composite was

transferred to a clear solution containing 3.5 g glucose dissolved

in 40 ml deionized water. The mixture was placed in a 50 ml

Teflon-sealed autoclave, ultrasonically dispersed for 30 min, and

maintained at 180 �C in a drying oven for 6 h. The product was

centrifuged and washed with deionized water and absolute

ethanol at least five times and dried in oven at 100 �C for 12 h,

and then further heated at 500 �C for 4 h in an argon atmosphere

(sample B). Finally, the product was also sintered at 260 �C for

2 h in air (sample C).

2.2. Characterization

X-ray diffraction (XRD) patterns were recorded under a Japan

Rigaku Rotaflex diffractometer equipped with a rotating anode

and using Cu-Ka radiation over the range 10� # 2q # 80�.Scanning electron microscopy (SEM) was performed on a JEOL

JSM-6700F field-emission scanning electron microscope. Trans-

mission electron microscopy (TEM) was performed on a JEOL

JEM-2100F field-emission electron microscope. Nitrogen

adsorption/desorption measurements were conducted at 77.35 K

on a Micromeritics Tristar 3000 analyzer. The Brunauer–

Emmett–Teller (BET) surface areawas estimatedwith adsorption

data. The specific surface areas (SBET) of the samples were

calculated following the multi-point BET procedure. The pore

size distributions were determined by using the BJH (Barett–

Joyner–Halenda) method.

Fig. 1 Typical SEM (a), TEM (b) and HRTEM (c) images of a-Fe2O3.

2.3. Electrochemical tests

Electrochemical performances of the samples were evaluated in

Li test cells. The working electrodes comprised active material,

acetylene black and polytetrafluoroethylene (PTFE) with the

weight ratio of 15 : 4 : 1. Lithium metal was used as the counter

and reference electrode. The electrolyte was 1 M LiPF6 dissolved

in a 1 : 1 : 1 mixture of ethylene carbonate (EC), ethylene methyl

carbonate (EMC), and dimethyl carbonate (DMC). The cells

were assembled in a glove box filled with high-purity argon.

Discharge/charge measurements of the cells were performed at

different current densities between the potentials of 0.01–3.00 V

(vs Li+/Li) under a LAND-CT2001A battery tester. The specific

4710 | CrystEngComm, 2011, 13, 4709–4713

capacity was calculated according to the corresponding active

material (a-Fe2O3, Fe3O4@C and g-Fe2O3) in each electrode.

3. Results and discussion

3.1. Characterization of three iron oxide samples

Mesoporous a-Fe2O3 samples were synthesized as precursors.

Low magnification SEM images show that the as-obtained a-

Fe2O3 sample is composed of uniform spheres (Fig. 1a). The high

resolution TEM images (Fig. 1b) reveal that larger light contrast

areas randomly distributed throughout the crystal indicate the

presence of mesopores with nearly the same size, and also show

the crystallinity of the microporous framework (regular light

dots), as well as randomly distributed mesoporosity (Fig. 1b).

The image reveals that lattice fringes extend over the entire

particle, indicating that the entire particle is monocrystalline

structure (Fig. 1c). Fig. 2 shows the XRD pattern of sample A. It

can be seen that the XRD pattern is consistent with rhombohe-

dral a-Fe2O3 (a ¼ b ¼ 5.038 �A, c ¼ 13.772 �A, JCPDS Card No.

33-0664). No characteristic peaks were observed for the impu-

rities such as g-Fe2O3 and Fe3O4.

BET gas sorptometry measurements were conducted to

examine the porous nature of sample A. Fig. 3 shows the N2

adsorption/desorption isotherm and the pore-size distribution

(inset) of the products. The isotherms are identified as type IV,

which is characteristic of mesoporous materials. The pore-size

distribution obtained from the isotherm indicates a number of

pores smaller than 10 nm in the sample. These pores presumably

arise from the spaces among the nanoparticles. The sharp

distribution of the mesopores around 2 nm suggests that the

nanospheres have high monodispersity, and a few mesopores of

around 35 nm appear in the sample. The BET specific surface

area of the sample was calculated from N2 isotherms at �196.6

8 �C, to be about 18.87 m2 g�1. The single-point total volume of

pores at P/P0 ¼ 0.975 was 0.50 cm�3 g�1. The BET surface area

and large total pore volume strongly support the fact that the

nanospheres have mesoporous structures.

Fig. 4a shows the XRD patterns of sample B. It can be seen

that all XRD diffraction peaks are in conformity with cubic

Fe3O4 (a ¼ b ¼ c ¼ 8.396 �A, Fd3m (227), JCPDS: 19-0629). No

characteristic peaks were observed for the impurities. The higher

background from 10� to 30� (Fig. 4a) is due to the presence of

amorphous carbon, which is confirmed in Fig. 6. All peaks can be

assigned to the diffraction from the (220), (311), (400), (511), and

(440) planes of the cubic Fe3O4, respectively. This indicates that

crystalline Fe3O4 formed from the reduction of a-Fe2O3 by

coated carbon in a calcination process at 500 �C in Ar. Fig. 4b

shows that sample C is very close to Fe3O4, but there are three

new peaks in lower-angle region, which agree well with the

This journal is ª The Royal Society of Chemistry 2011

Fig. 2 XRD patterns of a-Fe2O3.

Fig. 6 HRTEM image of Fe3O4@C.

Fig. 5 Typical SEM and HRTEM images of Fe3O4@C (a, c, and d) and

g-Fe2O3 (b, e, and f).

Fig. 3 Nitrogen adsorption/desorption isotherm and Barrett–Joyner–

Halenda (BJH) pore size distribution plot (inset) of mesoporous a-Fe2O3

nanospheres (Sample A).

Fig. 4 XRD patterns of Fe3O4@C (a) and g-Fe2O3 (b).

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standard XRD pattern of g-Fe2O3 (maghemite, JCPDS: 39-

1346), and result from (110), (210) and (211) faces of g-Fe2O3.45

The result demonstrates that Fe3O4 was transformed into

g-Fe2O3.

This journal is ª The Royal Society of Chemistry 2011

SEM observations (Fig. 5a and 5b) show that the as-obtained

Fe3O4@C and g-Fe2O3 samples are still sphere-like architectures

with uniform size and shape similar to a-Fe2O3 precursors. TEM

images (Fig. 5c and 5e) reveal that the Fe3O4 and g-Fe2O3

samples still keep their mesoporous structures. Compared with

mesoporous a-Fe2O3 spheres, Fe3O4@C and g-Fe2O3 samples

only show increased mesopore sizes, which are in conformity

with the BET analysis (See Fig. S1 in ESI†). HRTEM images

(Fig. 5d and 5f) show the lattice fringes with a d-spacing of

0.252 nm corresponding to the spacing of the (311) planes of

Fe3O4, and a d-spacing of 0.296 nm corresponding to the spacing

of the (220) planes of g-Fe2O3.

TEMandN2 adsorption/desorption have both ensured that the

conversion of a-Fe2O3 to Fe3O4 involves a change from a hexag-

onal close-packed oxide ion array (a-Fe2O3) to a cubic close-

packed array (Fe3O4). However, significant structural changes

occurred without the destruction of the mesoporous structure.

The carbon layers on the outer surface of the hematite meso-

porous nanospheres were obtained by pyrolysis of glucose under

hydrothermal conditions.46,47 With the heating procedure, the

inner hematite mesoporous nanospheres were reduced to

magnetite Fe3O4@C by the outer carbon layers. Fig. 6 shows the

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HRTEM image of Fe3O4 nanospheres coated with a uniform

carbon shells with the thickness of 3.9–5.7 nm.

The heating temperature is themain parameter in the formation

of Fe3O4@C nanospheres and the maintenance of their meso-

porous structures. When the products were heated at 400 �C, theinner hematitemesoporous nanospheres were partially reduced to

magnetite Fe3O4 by the outer carbon layers (See Fig. S2 in ESI†).

When the products were heated at temperatures higher than

600 �C, the inner hematitemesoporous nanospheres were reduced

to magnetite Fe3O4, but their mesoporous structures were

destroyed.

3.2 Electrochemical tests

The electrochemical performances of the as-preparedmesoporous

nanospheres were tested in Li test cells. Fig. 7 shows the charge–

discharge curves for the initial 30 cycles ofa-Fe2O3,Fe3O4@Cand

g-Fe2O3 between 0.01 and 3.00 V at 0.1 C. All three samples

display highdischarge specific capacities of 1100–1350mAhg�1 at

the first cycle, while the capacities decrease to 700–1000 mA h g�1

in the second discharge. Such results are attributed to the irre-

versible reactions including the formation of the SEI film and

further lithium consumption via interfacial reactions due to the

charge separation at themetal/Li2O phase boundary. As shown in

Fig. 7d, the discharge capacities of a-Fe2O3 and g-Fe2O3 fade

rapidly to �100 mA h g�1 after 30 cycles, but Fe3O4@C exhibits

a more stable capacity of �570 mA h g�1 after 30 charge and

discharge cycles.

Fast capacity degradation occurred to a-Fe2O3 and g-Fe2O3

without carbon coating. The lithium storage mechanism in iron

oxides can be interpreted as, FexOy + 2yLi+ + 2ye� 4 xFe +

yLi2O. Large volume changes happen to the structure after the

lithium inclusion, and the irreversible reactions between active

materials and electrolyte result in the poor capacity retention over

extended cycles.Meanwhile, the SEI film on the iron oxide anodes

may be destroyed due to the large volume change, and the

repeated formation/decomposition of the SEI film consumes Li

and active materials, leading to the poor cyclic performances.28–31

The carbon coating improves the lithium storage performances

of Fe3O4 by providing high electronic conductivity, good lithium

Fig. 7 Typical charge–discharge curves of a-Fe2O3 (a), Fe3O4@C (b)

and g-Fe2O3 (c) mesoporous nanospheres and the cyclic performances of

the samples (d).

4712 | CrystEngComm, 2011, 13, 4709–4713

permeability, and flexible accommodation of volume

change.33,34,43 The presence of amorphous carbon layers on the

surface of Fe3O4 nanospheres reduces the risk of side reactions

and the SEI film on the carbon shell can be maintained without

decomposition during each charge/discharge process to consume

the storedLi capacity.48,49Low-crystallization amorphous carbon

withmany defects can allow Li ions to pass through carbon layers

to react with inner Fe3O4 nanorods.50 In addition, the mesopores

in the Fe3O4 nanospheres may also play an important role in

relieving the impact of volume change of active materials by

providing the space for the volume expansion and contraction,

and releasing the stress on the carbon shells. The carbon shells

would suffer severe volumetric changes and might be destroyed

without the buffering effects of mesopores during high-rate long

charge/discharge cycles.

4. Conclusion

Mesoporous FexOy nanospheres were successfully synthesized by

a structural evolution from mesoporous a-Fe2O3 nanospheres.

After coating with carbon, the hematite mesoporous nanospheres

were converted into Fe3O4@C with retention of the mesoporous

structures, and then cubicFe3O4@Cwas changed intog-Fe2O3 by

calcination in air, without destroying the structure ofmesoporous

nanospheres. All the three samples showed superior Li storage

performances at initial charge/discharge cycles; however, only

Fe3O4@C exhibited a stable cyclic performance through tens of

charge/discharge cycles. The discharge capacities of a-Fe2O3 and

g-Fe2O3 degraded rapidly. Therefore, carbon coating is critical

for oxide anode materials with good cyclic stability for practical

application to Li ion batteries.

Acknowledgements

This work is supported by the 973 Program (2009CB220100) in

China.

References

1 D. M. Antonelli and J. Y. Ying, Angew. Chem., Int. Ed. Engl., 1995,34, 2014.

2 (a) F. Schuth, Chem. Mater., 2001, 13, 3184; (b) P. Yang, T. Deng,D. Zhao, P. Feng, D. Pine, B. F. Chmelka, G. M. Whitesides andG. D. Stucky, Science, 1998, 282, 2244; (c) X. He and D. Antonelli,Angew. Chem., Int. Ed., 2002, 41, 214; (d) P. Behrens, Angew.Chem., Int. Ed. Engl., 1996, 35, 515; (e) Q. S. Huo, D. I. Margolese,U. Ciesla, D. G. Demuth, P. Y. Feng, T. E. Gier, P. Sieger,A. Firouzi, B. F. Chmelka, F. Schuth and G. D. Stucky, Chem.Mater., 1994, 6, 1176.

3 C. S. Guo, M. Ge, L. Liu, G. D. Gao, Y. C. Feng and Y. Q. Wang,Environ. Sci. Technol., 2010, 44, 419.

4 T. Brezesinski, J. Wang, S. H. Tolbert and B. Dunn, Nat. Mater.,2010, 9, 146.

5 H. Takahashi, B. Li, T. Sasaki, C. Miyazaki, T. Kajino andS. Inagaki, Chem. Mater., 2000, 12, 3301.

6 M. Vallet-Regi, A. Ramila, R. P. del Real and J. Perez-Pariente,Chem. Mater., 2001, 13, 308.

7 P. Horcajada, A. Ramila, J. Perez-Pariente and M. Vallet-Regi,Microporous Mesoporous Mater., 2004, 68, 105.

8 P. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B. F. Chmelka,G. M. Whitesides and G. D. Stucky, Science, 1998, 282, 2244.

9 X. He and D. Antonelli, Angew. Chem., Int. Ed., 2002, 41, 214.10 P. Behrens, Angew. Chem., Int. Ed. Engl., 1996, 35, 515.

This journal is ª The Royal Society of Chemistry 2011

Publ

ishe

d on

24

May

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1. D

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Uni

vers

ity o

f St

elle

nbos

ch o

n 09

/10/

2014

04:

40:5

0.

View Article Online

11 Q. S. Huo, D. I. Margolese, U. Ciesla, D. G. Demuth, P. Y. Feng,T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schuth andG. D. Stucky, Chem. Mater., 1994, 6, 1176.

12 K. Nakajima, T. Fukui, H. Kato, M. Kitano, J. N. Kondo,S. Hayashi and M. Hara, Chem. Mater., 2010, 22, 3332.

13 G. R. Li, Z. P. Feng, Y. N. Ou, D. C. Wu, R. W. Fu and Y. X. Tong,Langmuir, 2010, 26, 2209.

14 L. Tian, H. L. Zou, J. X. Fu, X. F. Yang, Y. Wang, H. L. Guo,X. H. Fu, C. L. Liang, M. M. Wu, P. K. Shen and Q. M. Gao,Adv. Funct. Mater., 2010, 20, 617.

15 J. B. Lian, X. C. Duan, J. M. Ma, P. Peng, T. G. Kim andW. J. Zheng, ACS Nano, 2009, 3, 3749.

16 L. Andrei, T. Michel, M. T. Georgi, E. W. Lowell and A. David,J. Phys. Chem. B, 2004, 108, 5211.

17 X. L. Gou, G. X. Wang, X. Y. Kong, D. Wexler, J. Horvat, J. Yangand J. Park, Chem.–Eur. J., 2008, 14, 5996.

18 Z. A. Zang, H. B. Yao, Y. X. Zhou, W. T. Yao and S. H. Yu, Chem.Mater., 2008, 20, 4749.

19 T. Brezesinski,M. Groenewolt,M. Antonietti and B. Smarsly,Angew.Chem., Int. Ed., 2006, 45, 781.

20 F. Jiao and P. G. Bruce, Angew. Chem., Int. Ed., 2004, 43, 5958.21 A. S. Malik, M. J. Duncan and P. G. Bruce, J. Mater. Chem., 2003,

13, 2123.22 F. Jiao, A. Harrison, J. C. Jumas, A. V. Chadwick, W. Kockelmann

and P. G. Bruce, J. Am. Chem. Soc., 2006, 128, 5468.23 F. Jiao, J. C. Jumas, M. Womes, A. V. Chadwick, A. Harrison and

P. G. Bruce, J. Am. Chem. Soc., 2006, 128, 12905.24 P. G. Bruce, B. Scrosati and J. M. Tarascon, Angew. Chem., Int. Ed.,

2008, 47, 2930.25 P. Poizot, S. Lauruelle, S. Grugeon, L. Dupont and J. M. Tarascon,

Nature, 2000, 407, 496.26 J. Chen, L. N. Xu, W. Y. Li and X. L. Gou, Adv. Mater., 2005, 17,

582.27 W. Y. Li, L. N. Xu and J. Chen, Adv. Funct. Mater., 2005, 15, 851.28 M. Armand and J. M. Tarascon, Nature, 2008, 451, 652.29 A. Debart, L. Dupont, P. Poizot, J. B. Leriche and J. M. Tarascon,

J. Electrochem. Soc., 2001, 148, A1266.30 P. Balaya, H. Li, L. Kienle and J. Maier,Adv. Funct. Mater., 2003, 13,

621.

This journal is ª The Royal Society of Chemistry 2011

31 O. Delmer, P. Balaya, L. Kienle and J. Maier, Adv. Mater., 2008, 20,501.

32 H. S. Zhou, D. L. Li, M. Hibino and I. Honma, Angew. Chem., Int.Ed., 2005, 44, 797.

33 X. W. Lou, C. M. Li and L. A. Archer, Adv. Mater., 2009, 21,2536.

34 Y. S. Hu, R. D. Cakan, M. M. Titirici, J. O. Muller, R. Schloge,M. Antonietti and J. Maier, Angew. Chem., Int. Ed., 2008, 47,1645.

35 M. M. Ren, Z. Zhou, X. P. Gao, W. X. Peng and J. P. Wei, J. Phys.Chem. C, 2008, 112, 5689.

36 Y. Zhang, C. S. Sun and Z. Zhou, Electrochem. Commun., 2009, 11,1183.

37 L. W. Su, Z. Zhou and M. M. Ren, Chem. Commun., 2010, 46, 2590.38 T.Muraliganth, A. V.Murugan and A.Manthiram,Chem. Commun.,

2009, 7360.39 H. Liu, G. X. Wang, J. Z. Wang and D. Wexler, Electrochem.

Commun., 2008, 10, 1879.40 W. M. Zhang, X. L. Wu, J. S. Hu, Y. G. Guo and L. J. Wan, Adv.

Funct. Mater., 2008, 18, 3941.41 Y. Z. Piao, H. S. Kim, Y. E. Sung and T. Hyeon, Chem. Commun.,

2010, 46, 118.42 C. M. Ban, Z. C. Wu, D. T. Gillaspie, L. Chen, Y. F. Yan,

J. L. Blackburn and A. C. Dillon, Adv. Mater., 2010, 22, E145.43 S. M. Yuan, J. X. Li, L. T. Yang, L. W. Su, L. Liu and Z. Zhou, ACS

Appl. Mater. Interfaces, 2011, 3, 705.44 P. Ayyub, M. Multani, M. Barma, V. R. Plkar and

R. Vijayaraghavan, J. Phys. C: Solid State Phys., 1988, 21, 2229.45 H. Deng, X. L. Li, Q. Peng, X.Wang, J. P. Chen and Y. D. Li,Angew.

Chem., Int. Ed., 2005, 44, 2782.46 X. M. Sun and Y. D. Li, Angew. Chem., Int. Ed., 2004, 43, 597.47 W. M. Zhang, J. S. Hu, Y. G. Guo, S. F. Zheng, L. S. Zhong,

W. G. Song and L. J. Wan, Adv. Mater., 2008, 20, 1160.48 J. Hu, H. Li, X. Huang and L. Chen, Solid State Ionics, 2006, 177,

2791.49 S. A. Needham, G. X. Wang, K. Konstantinov, Y. Tournayre, Z. Lao

and H. K. Liu, Electrochem. Solid-State Lett., 2006, 9, A315.50 L. Cheng, X. L. Li, H. J. Liu, H. M. Xiong, P. W. Zhang and

Y. Y. Xia, J. Electrochem. Soc., 2007, 154, A692.

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