Synthesis and properties of magnetite/polypyrrole core-shell nanocomposites and polypyrrole hollow spheres

Article abstract of DOI:10.1016/j.electacta.2010.11.044

In this work a new route for preparation of core-shell nanoparticles composed of an iron oxide core and a polypyrrole (PPy) shell is explored. During the preparation procedure the initially formed iron(0) core is converted to magnetite. It is demonstrated, that the magnetite cores can completely be dissolved by reaction with acid. Furthermore the dissolution of iron oxide cores by electrolysis also is possible. The resulting PPy hollow spheres as well as the core-shell nanocomposites are electrochemically active.

Full text of DOI:10.1016/j.electacta.2010.11.044

Electrochimica Acta 56 (2011) 3616–3619
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and properties of magnetite/polypyrrole core–shell nanocomposites
and polypyrrole hollow spheres
K.-M. Mangold ,1, J. Schuster, C. Weidlich
∗
DECHEMA e.V., Karl-Winnacker-Institut, Theodor-Heuss-Allee 25, 60486 Frankfurt am Main, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2010
Received in revised form
In this work a new route for preparation of core–shell nanoparticles composed of an iron oxide core and
a polypyrrole (PPy) shell is explored. During the preparation procedure the initially formed iron(0) core
is converted to magnetite. It is demonstrated, that the magnetite cores can completely be dissolved by
reaction with acid. Furthermore the dissolution of iron oxide cores by electrolysis also is possible. The
resulting PPy hollow spheres as well as the core–shell nanocomposites are electrochemically active.
1
1 November 2010
Accepted 13 November 2010
Available online 21 November 2010
©
2010 Elsevier Ltd. All rights reserved.
Keywords:
Core–shell particles
Nanocomposites
Hollow spheres
Polypyrrole
Iron oxide
1
. Introduction
microwave-absorbing material. Due to the magnetic properties and
the possibility of drug release by the PPy shell a further interest-
ing application for this type of material is a combined chemo and
hyperthermia cancer therapy [7].
In this work a new synthetic route for core–shell systems is pre-
sented. The core–shell system is based on iron(0) particles which
were covered by polypyrrole. This approach enables the combina-
tion of a highly reactive iron(0) core with a conducting polymer
shell. Furthermore hollow spheres of PPy are formed by dissolu-
tion of the iron or rather iron oxide cores by treatment with acid or
by electrochemical methods.
Core–shell nanocomposites of inorganic and organic material
are of special interest due to the combination of several very dif-
ferent properties. A core–shell system composed of a metal oxide
core which is covered by a conducting polymer shell is an example
of this type of material. The inorganic compound, in this case iron
oxide, provides magnetic properties and the organic compound,
the conducting polymer, provides tuneable electronic conductivity.
Examples are the core–shell systems magnetite@polypyrrole [1]
and maghemite@polypyrrole [2]. The diameters of the core–shell
particles range between 10 nm [3] and 100 nm [7]. The powder con-
ductivity of this type of core–shell particles can accomplish 11 S/cm
2. Experimental
[
[
1]. The core–shell particles with iron oxide cores are ferromagnetic
4].
2
.1. Preparation of Fe O @PPy nanocomposites and PPy hollow
3
4
This type of core–shell system mostly is synthesized in a two-
nanocapsules
step procedure. In the first step the iron oxide core, e.g. magnetite
or maghemite, is formed. In the next step this core is covered by a
conducting polymer, e.g. polypyrrole (PPy) [5]. Hollow spheres of a
conducting polymer can be produced by dissolving the iron oxide
core with acid [6].
Iron(II)
sulfate
FeSO ·7H O
(Fluka),
iron(III)
sul-
4
2
fate Fe (SO ) ·5H O (Fluka) sodium borohydride NaBH4
2
4
3
2
(
Sigma–Aldrich), sodium hydroxide NaOH (Merck), sulfuric
acid (Merck) and cetylpyridinium bromide monohydrate CPB
Sigma–Aldrich) were of analytical grade and were used as
Possible applications of this type of nanocomposites are
electromagnetic shielding, electro-chemical display devices and
(
received. Pyrrole (Aldrich) was distilled before use. The preparation
and purification were made in deionised water (18 Mꢀcm).
In the first step iron nano-particles are synthesized by the reduc-
tion of FeSO in alkaline solution. This preparation method is based
on the reports of Wang and Zhang [8], Zhang [9] and Shin et al.
[10]. Wang et al. described the formation of iron nano-particles
∗ Corresponding author. Tel.: +49 69 75 64 327; fax: +49 69 75 64 388.
4
E-mail address: mangold@dechema.de (K.-M. Mangold).
1
ISE member.
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.11.044

K.-M. Mangold et al. / Electrochimica Acta 56 (2011) 3616–3619
3617
by reduction of iron(II) salts with NaBH . Zhang reported that an
4
excess of borohydride is needed to accelerate the synthesis reac-
tion and ensure uniform growth of iron particles. NaOH is added
because of the higher stability of iron in alkaline solution. Shin et al.
described the use of cetylpyridinium salt as cationic surfactant in
the preparation of gold nano-particles covered with polypyrrole.
3
0 ml of an aqueous solution of 0.08 mol NaBH and 0.0625 mol
4
NaOH were added drop-wise to 400 ml of a stirred aqueous solution
of 0.016 mol FeSO . Because of the air-sensitivity of iron particles
4
which easily form iron oxides the solution were kept in an inert gas
atmosphere (N ) at room temperature. A black magnetic precipi-
2
tate was formed. The precipitate was washed several times with
deionised water and dried in desiccator.
In the second step the iron particles are covered by CPB. 0.2 g
particles are dispersed in 90 ml deionised water and separated from
the precipitate of aggregated particles. Then 0.5 m mol CPB was
added. The suspension was shaken for 30 min and stored for 3 h.
In the third step pyrrole was concentrated in the CPB shell at
the surface of the iron particles. 7 m mol pyrrole was added to the
suspension. The dispersion was shaken for 30 min and stored for
7
h.
In the fourth step the chemical polymerization of pyrrole was
performed. 2 m mol of the oxidant Fe (SO ) was added to the sus-
2
4 3
Scheme 1. Synthesis of Fe3O4@PPy core–shell particles and PPy hollow spheres.
pension. The suspension was shaken for 2 h and stored over night.
The suspension slowly turned black.
The dissolution of the iron oxide core was achieved by treat-
ment of the core–shell particles in 0.25 M sulfuric acid at room
temperature for one day.
measurements reported by Nurmi et al., who described a broad
◦
reflex at 2ꢁ = 44.7 for iron(0) particles smaller than 1.5 nm [11].
CPB is added ((b) in Scheme 1) in order to minimize aggregation
of the particles and also as a layer which concentrates pyrrole which
is added in the next step ((c) in Scheme 1).
2.2. Characterization of the particles
The oxidation of pyrrole to form PPy is accomplished by the
addition of iron(III) salt as an oxidant ((d) in Scheme 1). The gener-
The crystal structure of the nanoparticles was measured by X-
ray diffraction analysis (XRD) of samples in powder form (Siemens
Kristalloflex; Cu K␣).
Iron ions in solution were detected by atomic absorption spec-
troscopy AAS (Perkin-Elmer 1100B).
TEM images are made with a Philips EM420 transmission elec-
tron microscope.
◦
◦
ated core–shell particles showed in XRD peaks at 2ꢁ = 30.1 , 35.4 ,
4
◦ ◦ ◦ ◦
3.1 , 53.5 , 57.1 , 62.7 which correspond to the Bragg reflections
of magnetite Fe O4 [3] 220, 311, 400, 422, 511 and 440, respec-
3
tively. Magnetite was also identified by Mössbauer spectroscopy
results not shown here). It is not clear which reaction step leads
(
to the conversion of iron(0) to crystalline magnetite. There are two
possibilities: an oxidation due to contamination with oxygen dur-
ing the addition of CPB and the following washing procedure or a
parallel reaction to the oxidation of pyrrole favored by hydroxide
groups at the surface of the iron particles in combination with the
addition of Fe (SO ) . Further studies to explain this conversion are
Electrochemical measurements were made by cyclic voltamme-
try (CV) with a three-electrode electrochemical cell (potentiostat:
EG&G 263A). The working electrode was either a carbon fleece (SGL
3
Carbon; volume 1 cm ; connected with a platinum wire) soaked
2
4 3
with particles or a carbon paste electrode (BAS; MF-2010; cylin-
in progress.
Fig. 1A shows TEM images of core–shell Fe O @PPy particles
3
drical volume of 0.03 cm ). The carbon fleece was consecutively
3
4
washed withethanoland water in anultrasonic bath. Then the dried
fleece was soaked by a suspension of particles. The carbon paste
electrode was prepared by mixing a particle suspension with an
oil based carbon paste (BAS). The counter electrode was a platinum
foil. Reference electrode was a Ag/AgCl/saturated KCl electrode. The
and Fig. 1B of PPy hollow spheres. The black dots are iron oxide
cores and the grey spheres are polypyrrole shells. The diameters of
the spherical core–shell particles are between 10 and 30 nm. The
diameters of the spherical hollow spheres are less than 50 nm with
broad size distribution due to aggregation.
The dissolving of the iron oxide core ((e) in Scheme 1) is achieved
by sulfuric acid. AAS analysis showed an increasing concentration
of iron ions after treatment of the core–shell particles with sulfu-
ric acid. The comparison of TEM images (Fig. 1) of core–shell and
hollow spheres shows that all iron oxide cores are dissolved. In
contrast to the core–shell particles the hollow spheres exhibit no
magnetic properties and XRD measurements of the hollow spheres
showed no peaks. These results indicate a complete dissolving of
the iron oxide cores.
−1
measurements were made in a 0.25 mol l Na SO solution in an
2
4
inert gas atmosphere (N ) at room temperature.
2
3
. Results and discussion
3.1. Preparation of nanocomposites and hollow spheres
The preparation steps of the core–shell nanocomposites and of
the PPy hollow spheres are shown in Scheme 1. The preparation
start with die reduction of iron(II) salt to get iron(0) nanoparti-
cles ((a) in Scheme 1). In order to prevent immediate dissolution of
the iron(0) particles the reaction is accomplished in alkaline solu-
tion. This favors the formation of a protective iron hydroxide and/or
oxide layer at the surface of the iron particles. The iron nanopar-
ticles show no peaks in XRD measurements. Also no peaks of iron
oxides are observed in XRD. This result is in contrast to the XRD
3.2. Electrochemical characterization of nanocomposites and
hollow spheres
Electrochemical switchable Fe O @PPy core–shell particles and
3
4
PPy hollow spheres are of interest for technical applications, e.g.
switchable ion exchangers or catalyst support. The electrochemi-

3
618
K.-M. Mangold et al. / Electrochimica Acta 56 (2011) 3616–3619
1
1
1
4
2
0
8
6
4
2
0
2
4
6
8
d)
c)
b)
a)
-
-
-
-
-
-
10
12
-1000 -800 -600 -400 -200
0
200 400 600 800 1000 1200
E vs (Ag/AgCl)/V
Fig. 2. Cyclic voltammogram of Fe3O4@PPy core–shell particles fixed in a carbon
fleece electrode at different scan rates: (a) 10 mV/s, (b) 25 mV/s, (c) 50 mV/s, (d)
−
1
1
00 mV/s (measured in 0.25 mol l Na2SO4 solution).
0
0
.06
.03
0
a)
b)
-
-
-
0.03
0.06
0.09
-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
E vs (Ag/AgCl)/V
Fig. 3. (a) Cyclic voltammogram of PPy hollow spheres embedded in carbon paste
electrode and (b) background current of the carbon paste (scan rate 50 mV/s; mea-
−
1
sured in 0.25 mol l Na2SO4 solution).
currents in multi sweep CV. This opens a new route to synthesize
PPy hollow spheres by electrolysis.
Fig. 3 exhibits a CV of PPy hollow spheres in a carbon paste elec-
trode. The hollow spheres show a capacitive like current curve. The
current values are about 100 times higher compared to the back-
ground current. In the core–shell system the current caused by PPy
is very low compared to the current caused by the redox reactions
described by Eqs. (1) and (2). Therefore currents caused by redox
reactions of PPy are not observable in Fig. 2.
The electrochemical experiments demonstrated that the
core–shell and also the PPy hollow spheres are electrochemical
active.
Fig. 1. TEM images of Fe3O4@PPy core–shell particles (A) and PPy hollow spheres
B).
(
cal properties of these particles are measured by CV. In Fig. 2, the CV
of Fe O @PPy core–shell particles adsorbed on a carbon fleece elec-
3
4
trode for different scan rates are shown. White et al. [12] assigned
the anodic peak of magnetite at 0.69 V vs. Ag/AgCl/KClsat to the oxi-
dation of Fe(II) to Fe(III) leading to a conversion of magnetite to
2+
maghemite (Fe O ) and to dissolved Fe ions (Eq. (1)).
2
3
[Fe Fe2 ]O → 4[Fe ]O + Fe + 2e−
II
III
III
2+
4. Conclusions
3
(1)
4
2
3
The cathodic peak at 0.18 V vs. Ag/AgCl/KClsat is due to the reduction
of Fe(III) followed by a dissolution of magnetite (Eq. (2)).
In this work a new route for preparation of core–shell nanopar-
ticles composed of an iron(0) core and a PPy shell is explored.
However, during the preparation procedure the initial formed
iron(0) is converted to magnetite. In case this oxidation could be
avoided, highly reactive iron cores enclosed by a PPy shell can be
realized. Corresponding studies are in progress.
Fe Fe ]O + 8H + 2e− → 3Fe2+ + 4H O
II
III
+
[
(2)
2
4
2
The dissolution of magnetite cores as a result of polarization is
proofed by the loss of magnetic properties and decreasing peak

K.-M. Mangold et al. / Electrochimica Acta 56 (2011) 3616–3619
3619
The iron oxide cores can be completely dissolved by treatment
[6] X. Lu, H. Mao, W. Zhang, Polym. Compos. 30 (2009) 847.
[
7] S.C. Wuang, K.G. Neoh, E.-T. Kang, D.W. Pack, D.E. Leckband, Biomaterials 29
with acid which is demonstrated by different analytical methods.
Electrochemical measurements demonstrated that core–shell
particles and also the PPy hollow spheres are electrochemically
active. Furthermore it is demonstrated, that the electrolytic dis-
solution of magnetite cores to produce PPy hollow spheres is
possible.
(
2008) 2270.
[
8] C. Wang, W. Zhang, Environ. Sci. Technol. 31 (1997) 2154.
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