Carbon 43 (2005) 2536–2543

www.elsevier.com/locate/carbon

Simple synthesis of mesoporous carbon with magneticnanoparticles embedded in carbon rods

Jinwoo Lee a, Sunmi Jin a, Yosun Hwang b, Je-Geun Park b,Hyun Min Park c, Taeghwan Hyeon a,*

a National Creative Research Initiative Center for Oxide Nanocrystalline Materials, School of Chemical and Biological Engineering,

Seoul National University, Seoul 151-744, Republic of Koreab Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea

c New Material Evaluation Center, Korea Research Institute of Standards and Science, Taejon 305-600, Republic of Korea

Received 17 December 2004; accepted 7 May 2005

Available online 24 June 2005

Abstract

Magnetically separable ordered mesoporous carbon containing magnetic nanoparticles embedded in the carbon walls was syn-

thesized using a simple synthetic procedure. The resulting magnetically separable mesoporous carbon was denoted as M-OMC

(magnetically separable ordered mesoporous carbon) poly(pyrrole) with residual Fe2+ ions in the mesoporous channel was con-

verted to carbon material containing superparamagnetic nanoparticles. The size of the magnetic nanoparticles obtained was

restricted by the channel size of the SBA-15 silica template, which resulted in the generation of superparamagnetic nanoparticles

embedded in the carbon rods. The blocking temperature of M-OMC is 110 K. Pore size and textural property of M-OMC is similar

to that of hexagonally ordered mesoporous carbon fabricated using SBA-15 silica as a template. The saturation magnetization of M-

OMC is ca. 30.0 emu/g at 300 K, high enough for magnetic separation.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Porous carbon; Carbonization; BET surface area; Magnetic properties, Particle size

1. Introduction

Many mesoporous materials have been synthesized

for a variety of applications involving large molecules,

which cannot be accomplished using conventional

microporous zeolitic materials [1,2]. Ordered mesopor-

ous carbons with various structures have been synthe-sized using appropriate mesoporous silica templates,

and have been successfully developed as electrode

materials for supercapacitors and fuel cells, adsorbents

for large molecules, and as catalyst supports [3–11].

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

doi:10.1016/j.carbon.2005.05.005

* Corresponding author. Tel./fax: +82 2 886 8457.

E-mail address: thyeon@plaza.snu.ac.kr (T. Hyeon).

Moreover, the synthesis of nanostructured magnetic

materials has been intensively pursued because of their

broad applications including magnetic storage media,

ferrofluids, as magnetic resonance imaging (MRI) con-

trast materials, and as magnetic carriers for drug target-

ing [12–17]. When nanostructured magnetic materials

are incorporated into mesoporous materials withoutsignificant pore blockage, the resulting materials can

be used as host materials that can be separated by

magnet, which is a more convenient possibility for large

scale applications than separation by filtration or cen-

trifugation. Nonetheless, only few reports have been

issued on the synthesis of mesoporous materials contain-

ing magnetic nanoparticles [18,19]. Generally, to incor-

porate magnetic nanoparticles in mesoporous silica,

J. Lee et al. / Carbon 43 (2005) 2536–2543 2537

post-synthetic impregnation methods have been em-

ployed, which require multiple complicated synthetic

steps. Furthermore, it is sometimes not easy to avoid

the formation of the non-magnetic a-Fe2O3 phase [20].

The post-synthetic incorporation of magnetic nanoparti-

cles in porous hosts also could block the main pores ofmesoporous materials. Recently, to overcome these

drawbacks, Wiesner and co-workers reported on the

synthesis of mesoporous aluminosilicate containing

superparamagnetic c-Fe2O3 particles embedded in its

walls [21,22]. This novel magnetic mesoporous silica

was obtained by mixing iron ethoxide and aluminosili-

cate source during the self-assembly of amphiphilic

block copolymers. The authors anticipated that thesematerials could be used for the separation of magneti-

cally labeled biological molecules. Schuth and co-work-

ers fabricated magnetically separable mesoporous silica

by adsorbing cobalt nanoparticles on the surface of mes-

oporous silica particles, followed by carbonization of

poly(furfuryl alcohol) to cap the magnetic nanoparticles

[23]. In these materials, the main channels are not

blocked by magnetic nanoparticles [21–23]. Xu and co-workers reported on the fabrication of magnetic meso-

porous composite by coating mesoporous silica on the

micrometer sized magnetite [24]. But the surface area

is too low (below �60 m2/g) for the accommodation of

large molecules [24].

Magnetically separable mesoporous carbon materials

are important for the application to catalyst support,

separation technology and adsorption of biomolecules.It is generally known that carbon is difficult to separate

from solution. Non-magnetic a-Fe2O3 was incorporated

into CMK-3 mesoporous carbon by post-impregnation

after synthesizing the ordered mesoporous carbon

CMK-3 [25]. Schuth group fabricated cobalt nanoparti-

cles immobilized on an ordered mesoporous carbon,

CMK-3, and successfully applied mesoporous carbon

to magnetically separable adsorbent and hydrogenationcatalyst supports [26]. But for the synthesis of Co-OMC,

magnetic nanoparticles should be first synthesized using

toxic organometallic precursor, Co2(CO)8 and then

deposited on the surface of SBA-15/carbon composite.

To prevent leaching and oxidation of deposited cobalt

nanoparticles, furfuryl alcohol deposition, polymeriza-

tion and carbonization were performed. The synthetic

procedure for Co-OMC is rather complex. This longand complicated multi-step synthetic procedure ham-

pers broad application of the magnetically separable

mesoporous carbon, despite of its many important char-

acteristics. A short and simple synthetic procedure

should be developed for the practical applications of

such novel carbon materials. Herein, we report on the

simple synthesis of ordered magnetic mesoporous car-

bon with magnetic iron oxide nanoparticles embeddedin the carbon walls, which we denote as M-OMC (mag-

netic ordered-mesoporous-carbon).

2. Experimental

2.1. Materials and methods

Pluronic P123 (EO20PO70EO20, Mav = 5800), was

purchased from BASF. N2 adsorption and desorptionisotherms were measured at 77 K using a Micromeritics

ASAP 2000 Gas Adsorption Analyzer after the meso-

porous materials were degassed at 423 K at 10 lTorrfor 5 h. The pore size distribution was calculated from

the analysis of the adsorption branch of the nitrogen iso-

therm using the BJH (Barrett–Joyner–Halenda) method.

Transmission electron micrographs (TEM) were

obtained on a JEOL JEM-2010 electron microscope.X-ray diffraction patterns were obtained with a Rigaku

D/Max-3C diffractometer equipped with a rotating an-

ode and a Cu Ka radiation source (k = 0.154056 nm).

Synchrotron SAXS measurements were performed on

the 4C2 Beamline at the Pohang Light Source (Korea).

The primary beam was monochromatized with a cou-

pled Si(111) single crystal at a wavelength of

0.1608 nm (the photon energy of X-ray is 7.78 keV, aresolution k/k ffi 0.0001), and then it was focused on a

detector plane by means of a bent cylindrical mirror.

A 2-D CCD camera (Roper Scientific Inc., PI-SCX-

2048) was used to collect the scattered X-rays. We used

the SEBS block copolymer (32.5 nm in d spacing) as a

periodic calibrant, in order to calibrate the image from

the 2-D CCD camera [27].

2.2. Synthesis of M-OMC

The typical synthetic procedure used to produce M-

OMCs is as follows. The template, SBA-15 silica is pre-

pared by following a reported procedure [28]. Briefly, 4 g

of P123 was dissolved in solution composed of 130 ml of

deionized water and 20 ml of hydrochloric acid

(37 wt%), and the temperature of the solution was raisedto 313 K. 9.2 ml of TEOS was added to the solution and

stirred vigorously and the solution was remained at

313 K for 20 h, followed by aging at 373 K for 24 h.

The resulting white precipitate was filtered, dried at

room temperature and finally calcined at 823 K to re-

move P123. Pyrrole monomer based on the pore volume

of SBA-15, was incorporated into SBA-15 by vapor

phase infiltration. The resulting pyrrole/SBA-15 nano-composite was dispersed in H2O containing 2.3 molar

equivalents of FeCl3 relative to the amount of pyrrole,

and was stirred for 3 h to polymerize the pyrrole inside

the mesoporous SBA-15 silica template. After recover-

ing the poly(pyrrole)/SBA-15 nanocomposite by filtra-

tion, it was carbonized in a N2 atmosphere at 973 K

for 3 h at a heating rate 1.5 K/min. The silica template

was removed by boiling the silica/carbon composite in1 M NaOH solution dissolved in 50:50 mixture of water

and ethanol for more than 1 h twice.

Relative Pressure (P/P0)

0.0 0.2 0.4 0.6 0.8 1.0

Vol

ume

adso

rbed

(cc

/g)

100

200

300

400

500

600

Pore Diameter (nm)

0 5 10 15 20

dV/d

log

D

0

1

2

3

4

5

6

7

a

b

Fig. 2. (a) N2 isotherm of SBA-15 silica which was used as a template

for M-OMC. (b) Corresponding pore size distribution obtained from

adsorption isotherm calculated by BJH (Barret–Joyner–Halenda)

method.

2538 J. Lee et al. / Carbon 43 (2005) 2536–2543

3. Result and discussion

The synthetic procedure for M-OMC is presented in

Fig. 1. The BET surface area and the single point total

pore volume of the SBA-15 silica template are 663 m2/

g and 0.80 cm3/g, respectively. N2 adsorption–desorp-tion isotherm of SBA-15 template yields H1-type hyster-

esis that is typical of mesoporous materials with 1D

cylindrical channels (Fig. 2(a)). Pore size is centered at

7.5 nm. During the oxidative catalytic polymerization

of pyrrole in the pores of the SBA-15 silica template,

Fe3+ ions were converted to Fe2+ ions [29]. During high

temperature carbonization at 973 K, Fe2+ ions, which

are present inside the mesopores of the silica templatealong with poly(pyrrole), were converted to magnetic

a-Fe and Fe3C nanoparticles. The formation of a-Feand Fe3C was characterized by XRD pattern (Fig. 3).

Gedanken and co-workers reported formation of mag-

netic a-Fe and Fe3C nanocomposite through sonication

of Fe(CO)5 in the presence of diphenylmethane (DPhM)

solution that acted as a carbon source [30].

Moreover, the sizes of most of magnetic nanoparti-cles are restricted to the channel size of SBA-15, which

results in the formation of superparamagnetic nanopar-

ticles in the carbon rods. Silica removal by NaOH etch-

ing then generates M-OMC. During this etching

process, part of a-Fe and Fe3C nanoparticles embedded

in the carbon walls are oxidized to magnetite. This syn-

thetic procedure is very simple because the catalyst Fe2+

ions that are generated after the oxidative polymeriza-tion are converted to magnetic nanoparticles during

the carbonization step. Jang and Yoon also reported

on the formation of magnetic carbon nanotubes via

the carbonization of poly(pyrrole) nanotubes synthe-

sized by Fe3+ catalyst [31].

Fig. 1. Schematic representation for the synthesis of M-OMC.

Fig. 3. XRD pattern of SBA-15/magnetic carbon composite before

NaOH etching.

Fig. 4. (a) Transmission electron microscopy (TEM) image of M-

OMC. (b) High resolution transmission electron microscopy

(HRTEM) image of M-OMC showing highly crystalline nature of

magnetic nanoparticles.

J. Lee et al. / Carbon 43 (2005) 2536–2543 2539

To make magnetic mesoporous carbon with good

quality, the following points are critical. Firstly, after

polymerization of pyrrole/SBA-15 silica in the FeCl3solution, the resulting polymerized poly(pyrrole)/SBA-

15 composite was separated by suction filtration. To

prevent leaching of Fe2+ ions present in poly(pyrrole) in-side the mesoproes of SBA-15, very small amount of

water should be used for washing Fe2+ ions only on

the external surface of SBA-15. If we used too much

water for washing the filtrated materials, the result-

ing carbonized sample cannot be separated by applied

magnetic field. Secondly, the heating rate is critical

to get large amount of small sized superparamag-

netic nanoparticles. If the heating rate is over 3 K/min,large sized (>50 nm) ferromagnetic nanoparticles were

formed outside the mesoporous carbon particles. In that

case, we also observed carbon nanotubes catalytically

grown from the magnetic metal particles. Thirdly, poly-

merization should be performed for sufficiently long

time to get ordered mesoporous carbon. If poly(pyr-

role)/SBA-15 composite, obtained by polymerizing for

shorter than 2 h, was carbonized, poor quality meso-porous carbon material with low surface area was

produced.

The obtained M-OMC was characterized by X-ray

diffraction (XRD), small-angle X-ray scattering

(SAXS), transmission electron microscopy (TEM), and

by using a superconducting quantum interference device

(SQUID). TEM images (Fig. 4) of M-OMC showed

one-dimensionally ordered structure, similar to CMK-3 carbon. As was expected, magnetic nanoparticles

generated by the conversion of Fe2+ ions were found

embedded in the carbon rods over the entire M-OMC

particles (Fig. 4(a)). The size of most of the nano-

particles was similar to the diameter of the carbon

rods, which was similar to the channel size of the

SBA-15 silica template. High-resolution TEM (Fig.

4(b)) revealed that the magnetic nanoparticles embed-ded in the carbon walls are highly crystalline. Mag-

netic nanoparticle embedded in carbon walls were

characterized using high resolution transmission elec-

tron microscopy (HRTEM) and a fast Fourier trans-

formation (FFT) pattern (Fig. 4(b) inset). The

interplanar distance is estimated to be 2.03 A, which is

in good agreement with the {110} planes of the bcc

a-Fe. This result indicates that the magnetic nanoparti-cles deeply embedded in carbon walls are resistant to

oxidation.

During the conversion to magnetic nanoparticles, the

size of magnetic nanoparticles seems to be restricted by

the channel size of the SBA-15 silica. The size of parti-

cles was expected to be similar to the channel size of

the SBA-15. However, the diameter of the final carbon

rods will be smaller than the channel size of SBA-15, be-cause the poly(pyrrole) rods formed in the channels of

SBA-15 would shrink during carbonization. Conse-

quently, the size of the magnetic nanoparticles is slightly

larger than the diameter of carbon nanorods.

The energy-dispersive X-ray spectroscopy (EDS)

analysis of M-OMC (69.0 wt% C, 4.5 wt% O, 0.85 wt%

Si, and 25.5 wt% Fe) confirmed that the silica template

was successfully removed by the NaOH etching process.

As shown in Fig. 5, the black M-OMC powder was eas-ily attracted by a magnet, demonstrating that M-OMC

can be used as a magnetically separable adsorbent or

catalyst support.

N2 sorption isotherms and corresponding pore size

distributions of M-OMC are presented in Fig. 6(a).

The N2 isotherm is similar to that of CMK-3 carbon

fabricated using SBA-15 silica as a template [7]. Pore

size distribution showed that the pores were rela-tively uniform and centered at 2.9 nm. The BET sur-

face area and the single point total pore volume

were 643 m2/g and 0.60 cm3/g, respectively. The X-ray

diffraction pattern of M-OMC revealed that magnetite

(Fe3O4) co-existed with bcc iron (a-Fe) (Fig. 7(a)). Con-sidering that the molar ratio of iron and oxygen ob-

tained by EDS measurement was 7.0:4.4, a-Fe and

Fe3C is partially oxidized into Fe3O4. We expect thatonly the exposed surface of generated nanoparti-

cles is converted to Fe3O4 during the NaOH etching

Relative Pressure (P/P0)0.0 0.2 0.4 0.6 0.8 1.0

Por

e V

olum

e (c

c/g)

0

100

200

300

400

500

Pore Diameter (nm)

0 2 4 6 8 10 12 14 16 18 20

dV/d

logD

0.0

0.2

0.4

0.6

0.8

1.0

a

b

Fig. 6. (a) N2 adsorption–desorption isotherms of M-OMC. (b) Pore

size distribution of M-OMC obtained from adsorption isotherms

calculated by BJH (Barret–Joyner–Halenda) method.

q [nm-1]

0.3 0.6 0.9 1.2 1.5 1.8

Inte

nsity

0

100

200

300

400

(100)

2

10 20 30 40 50 60 70 80 90

Inte

nsity

0

200

400

600

800

1000

Fe3O4

θ

•: α-Fe•

♦

♦ ♦♦♦

♦

a

b

Fig. 7. (a) XRD pattern of M-OMC. (b) Small-angle X-ray scattering

(SAXS) pattern of M-OMC.

Fig. 5. Image showing M-OMC can be separated by applied magnetic

field.

2540 J. Lee et al. / Carbon 43 (2005) 2536–2543

process. The preservation of the SBA-15 silica template

ordered structure was shown by the (100) peak of the

hexagonal structure of M-OMC in the SAXS pattern

(Fig. 7(b)).The magnetic properties of M-OMC were investi-

gated by measuring the temperature dependence of the

magnetization with zero-field-cooling (ZFC) and field-

cooling (FC) procedures in an applied magnetic field

of 100 Oe between 2 and 350 K using a commercial

superconducting quantum interference device (SQUID)

magnetometer (Quantum Design, MPMS5XL). The

plots of temperature versus magnetization for M-OMCwith zero-field-cooling (ZFC) and field-cooling (FC)

are presented in Fig. 8(a). The blocking temperature of

M-OMC is 110 K, which indicates that superparamag-

netic nanoparticles are formed in the carbon rods. The

formation of superparamagnetic nanoparticles rather

than ferromagnetic bulk materials is extremely impor-

tant for practical applications, because the magnetic

support should retain no residual magnetism after themagnetic field is removed [32]. The magnetization curve

also exhibited superparamagnetic behavior. At 2 K,

which is far below the blocking temperature, large hys-

teresis was observed, which is typical of superparamag-

netic nanoparticles. Remanent magnetization value

(MR) is 6.21 emu/g at 2 K and at 300 K a small amount

of hysteresis was observed, which seems to be the result

0 100 200 300 4000

1

2

3

4

TB=110 K

H [Oe]

T/TB

-10000-8000-6000-4000-2000 0 2000 4000 6000 800010000

M [e

mu/

g]

Hc/

Hco

M [e

mu/

g]

-60

-40

1.0

0.8

0.6

0.4

0.2

0.0

-20

0

20

40

60

300 K2 K

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

10

c

b

a

Fig. 8. (a) The temperature dependence of ZFC and FC magnetization curve for M-OMC. (b) Field dependence magnetic property of M-OMC. (c)

The normalized coercive field (Hc/Hc0) as a function of normalized temperature (T/TB) for each sample. The line is a theoretical curve for a single

domain of fine particles.

J. Lee et al. / Carbon 43 (2005) 2536–2543 2541

of large-sized ferromagnetic particles formed on the

outer surfaces of M-OMC grains. The saturation mag-

netization of M-OMC is ca. 30.0 emu/g at 300 K, high

enough for magnetic separation. However, the amount

of residual magnetization was very small (MR = 0.76emu/g) compared with saturation magnetization value,

thus demonstrating that small amount of these large-

sized ferromagnetic particles are present in M-OMC.

This saturation magnetization value is higher than those

of any other magnetically separable mesoporous host

materials except low surface area magnetic mesoporous

composite [24].

Another interesting point is the temperature depen-dence of the coercive field of M-OMC, as shown in

Fig. 8(c). The dotted line is a theoretical curve for single

domain nanoparticles: Hc/Hc0= 1�(T/TB)

1/2, where Hc

is the measured coercive field, Hc0is the estimated coer-

cive field at T = 0 K, and TB is the measured blocking

temperature [33]. Although the number of data points

is rather small, we suggest that the temperature depen-

dence of the measured coercive field of M-OMC can

be reasonably explained by the simple theoretical curve

representing single domain particles.Long-term magnetic stability is critical for the indus-

trial application of M-OMC carbon. To check the sta-

bility of magnetic mesoporous carbon, magnetic

characterization was conducted after exposing in air

for 5 months (Fig. 9(a)). The saturation magnetization

value at 2 K was 40.90 emu/g at initial characteriza-

tion. After five months, the value was decreased to

26.78 emu/g. Even after 5 months, the saturationmagnetization value is higher than that of magnetic hex-

agonally ordered mesoporous carbon developed by

Schuth group [23]. But blocking temperature of mag-

netic nanoparticles did not change after 5 months

(Fig. 9(b)).

H (T)-0.5 0.0 0.5

M(e

mu/

g)

-60

-40

-20

0

20

40

60Initial CharacterizationCharacterization after 5 months

0 100 200 300 4000

1

2

3

41st5 month

T[K]

M[e

mu/

g]

TB=110K

a

b

Fig. 9. (a) Field dependence magnetic property of M-OMC showing

the magnetic stability. (measured at 2 K). (b) The temperature

dependence of ZFC and FC magnetization curve for M-OMC showing

the change of blocking temperature after 5 months.

2542 J. Lee et al. / Carbon 43 (2005) 2536–2543

4. Conclusion

Magnetically separable ordered mesoporous carbon

(M-OMC) containing magnetic nanoparticles embedded

in the carbon walls was synthesized using a simple syn-thetic procedure. Poly(pyrrole) with residual Fe2+ ions

was converted to carbon material containing superpara-

magnetic nanoparticles. The sizes of the magnetic nano-

particles obtained were restricted by the channel size of

the SBA-15 silica template, which resulted in the gener-

ation of superparamagnetic nanoparticles embedded in

the carbon rods. This magnetically separable M-OMC

may find large-scale applications as catalyst supportsor as adsorbents. The straightforward approach de-

scribed can be extended to the synthesis of magnetically

separable ordered mesoporous carbons with containing

various pore structures. The M-OMC has potential

application to catalyst support, adsorbent, and electrode

materials for bioelectrocatalysis [34].

Acknowledgement

TH would like to thank the financial support by

the Korean Ministry of Science and Technology

through the National Creative Research Initiative

Program.

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