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A self-assembly template approach to form hollowhexapod-like, flower-like and tube-like carbon materials

Liqiang Xu a, Jianwei Liu b, Jin Du a, Yiya Peng a, Yitai Qian a,*

a Hefei National Laboratory for Physical Sciences at Microscale and Department of Chemistry,

University of Science and Technology of China, PR Chinab Department of Materials Science & Engineering and Department of Chemistry, University of Science and Technology of China, PR China

Received 9 November 2004; accepted 28 December 2004

Available online 23 February 2005

Keywords: Carbon composites; Pyrolysis; Magnetic properties

Hollow carbon materials have a wide range of struc-

tures, textures, properties, and resulting extensive appli-cations owing to their low density, high surface area and

surface permeability [1–3], therefore, since the discovery

of carbon nanotubes [4], considerable efforts have been

made to produce new kinds of them such as carbon

cubes [5], cones [6], spheres [7] and hollow carbon cala-

bashes [8]. To date, shape and dimension control have

raised significant concern and have made great progress

in the fabrication of novel hollow carbon materials asmentioned above, however, they still have been difficult

to achieve and turns out to be a great challenge for the

future. In this report, hexapod-like and flower-like

Fe3O4@C microcrystals with ferromagnetic properties

and the corresponding hollow carbon materials were

prepared via a self-assembly template approach by using

ferrocene and absolute ethanol as reactants in an auto-

clave and employing a quartz tube as inner react wallat 600 �C. In addition, bundles of carbon nanotubes

were produced with a relative high yield when the reac-

tion temperature was increased to 800 �C. This report isa further progress of our previous work [9].

In a typical procedure, absolute ethanol (18 ml) and

ferrocene (1.0 g) were loaded into a quartz tube of20 ml capacity. After the quartz tube was put into a

65 ml stainless-steel autoclave, the autoclave was sealed

tightly and put into an electronic furnace. Then the fur-

nace was heated from room temperature to 600 �C with

an increasing speed of 10 �C /min and maintained at

600 �C for 16 h. After it was allowed to cool down to

room temperature naturally, the dark precipitate in the

quartz tube was collected (labeled as Sample 1). Andpart of them was heated in HCl acidic solution (1 M)

at 80 �C for 24 h and was filtered and washed with dis-

tilled water and absolute ethanol for several times. Fi-

nally it was dried in a vacuum at 60 �C for 4 h and

was collected for characterization. The only difference

for synthesis of bundles of carbon nanotubes (labeled

as Sample 2) was that the reaction temperature was

raised from room temperature to 800 �C. X-ray powderdiffraction (XRD) patterns of the products were re-

corded on a Philips X’pert diffractometer with CuKa1radiation (k = 1.5418 A). The morphology, composition

and structure of the products were examined with field

emission scanning electron microscopy (FSEM, JEOL

JSM-6300F), FSEM-energy dispersive X-ray analyses,

transmission electron microscopy (TEM, HITACHI

800) and high-resolution TEM (HRTEM, JEOL 2010).Magnetic hysteresis loops of the Fe3O4@C materials

* Corresponding author. Tel.: +86 551 3602942; fax: +86 551

3607402.

E-mail addresses: xulq@mail.ustc.edu.cn (L. Xu), ytqian@ustc.

edu.cn (Y. Qian).

0008-6223/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2004.12.030

1560 Letters to the Editor / Carbon 43 (2005) 1557–1583

were recorded on a vibrating sample magnetometer

(BHV-55, Riken, Japan).

Fig. 1(a) shows XRD pattern of Sample 1 before acid

treatment. The sharp diffraction peaks with relative high

peak intensity in this figure were indexed as crystalline

face-centered cubic (fcc) Fe3O4 (JCPDS card no. 19–

629), while the broad peaks with low peak intensity ob-

served from magnified Fig. 1(a) and (b) could be bothindexed as amorphous graphite indicating that crystal-

line Fe3O4 was removed after the acid treatment process.

Fig. 1(c) and (d) show XRD patterns of Sample 2 before

and after acid treatment, respectively, which reveals that

only carbon materials were left after the HCl acid treat-

ment process.

Sample 1 includes hexapod-like and flower-like

Fe3O4@C materials (with a yield of �60%) and carbonmicrospheres as can be seen from Fig. 2(a) and (b). The

former product with six pods and each pod has a length

in the range of 4–5 lm and points to the direction nearly

along (001); while the flower-like carbon have two mis-

matched layers. Each layer has three petals with similar

angles between two adjacent ones of the same layer. Its

ED pattern (Fig. 2(c)) recorded from one edge of a ran-

domly selected petal contains sharp spots and diffusedrings. The former could be indexed as (31�1), (311)

and (620) reflections from fcc Fe3O4 with a zone axis

of [001] and the latter could be indexed as graphite

002 and 10 reflections. Fig. 2(d)–(i) show typical FSEM

and TEM images of Sample 1 after acid treatment. It is

observed that almost all of the hexapod-like and flower-

like products are broken and hollow. The composition

analyses result of sample 1 before acid treatment show

that the average atomic ratio of elements C, Fe and Ois 55.0:19.5:25.5, and the calculated atomic ratio of O/

Fe (�1.31) is close to the theoretical calculation result

of Fe3O4.

Magnetization measurements were carried out on

Sample 1 (before acid treatment) to examine the mag-

netic nature of the Fe3O4@C microcrystals. The typical

hysteresis loop of the product reveals a ferromagnetic

behavior with saturation magnetization (Ms), remnantmagnetization (Mr), and coercivity (Hc) values of ca.

56.10 emu/g, 14.77 emu/g, and 226.70 Oe, respectively,

and the values of the Ms are higher than that of the

Fe3O4 encapsulated in carbon microtubes [9].

Based on the statistical analyses result of TEM and

SEM observation, Sample 2 is composed of carbon

nanotubes (CNTs) (�75%), solid carbon microspheres

(�20%), and carbon nanobelts, while hexapod-likemicrocrystals as arrowed in Fig. 3(a) are occasionally

encountered. It is interesting to note that most of the

CNTs shown in Fig. 3(a)–(c) are straight and in bundles,

and generally have both open ends. These CNTs have

diameters in the range of 25–200 nm and lengths up to

tens of micrometers, and typically with a wall thickness

in the range of 10–15 nm. Fig. 3(d) shows typical ED

pattern of a randomly selected carbon nanotube, thearcs and rings in the pattern can be indexed as 002

and 10 reflections of hexagonal graphite. The structure

of the carbon nanotube walls is shown to be multi-

walled and crystallined as illustrated by its HRTEM

image (Fig. 3(e)). It is worth noting that some iron

containing materials are still remained in the cavity of

a carbon nanotube (before acid treatment) as can be

seen from Fig. 3(f).In this experiment, it is thought that the quartz-

tube plays a substrate role and favors the formation of

hexapod-like and flower-like Fe3O4@C microcrystals:

10 20 30 40 50 60 70 80

200400600800

(d)

2θ /degrees

1000200030004000

(c)

0

200

400 (b)

Inte

nsity

(a.u

.)

1000200030004000500060007000

(a)

Fig. 1. XRD patterns of Sample 1 (a,b) and Sample 2 (c,d) before and

after HCl acid treatments, respectively.

Fig. 2. Typical FSEM, TEM images and ED pattern of Sample 1 with different treatments: (a)–(c) Before acid treatment; (d)–(i) after acid treatment.

Letters to the Editor / Carbon 43 (2005) 1557–1583 1561

without it, only little amount of them could be observed.

The relative low temperature and pressure leads to the

growth of Fe3O4@C microcrystals with multiple pods

or petals. The overall reaction involved in this experi-

ment may be tentatively written as follows:

3FeC10H10 þ 5C2H5OH ! 40Cþ Fe3O4 þH2Oþ 29H2

The as-formed Fe3O4 single crystals (or iron containing

materials produced at high temperature) might act as a

self-assembly template and catalytic roles in the final

formation of the product as evidenced by the above

analyses; The flow of hydrogen gas leads to temperatureand concentration gradient, which derives carbon atom

(decomposed from ferrocene and ethanol) diffusion rap-

idly over and through the Fe3O4 microcrystals to form

the final products under the present temperature and

pressure, however, the exact formation mechanism still

needs further research.

In summary, a self-assembly template method was

developed to prepare hexapod-like and flower-like car-bon microcrystals with or without Fe3O4 single crystals

encapsulated in their hollow cores. In addition, bundles

of carbon nanotubes with multi-shells could be produced

in relative high yield when the reaction temperature was

increased to 800 �C. These hollow carbon structures may

find a range of potential applications as catalysis, coat-

ings, protecting sensitive agents, and would offer oppor-

tunities to explore their novel properties.

Acknowledgement

The authors gratefully acknowledge the National

Natural Science Foundation of China, the 973 Project

of China and the Postdoctoral Fund of China.

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Fig. 3. Typical FSEM, TEM and HRTEM images of Sample 2 obtained with different treatments: (a)–(e) after acid treatment; (f) before acid

treatment.

1562 Letters to the Editor / Carbon 43 (2005) 1557–1583