Ionic additives and weak magnetic fields in the thermal decomposition of octacarbonyldicobalt - Tools to control the morphology of cobalt nanoparticles

Article abstract of DOI:10.1002/ejic.201100947

In the presence of ionic compounds, the thermal decomposition of octacarbonyldicobalt, Co₂(CO)₈, in an inert solvent leads exclusively to Iμ-Co nanocubes. The ionic species can be added directly or generated in situ by a chemical reaction between an additive and the precursor or between two additives. The additional presence of an inhomogeneous magnetic field leads to disc-shaped crystalline nanoparticles, which assemble to chains. Copyright

Full text of DOI:10.1002/ejic.201100947

SHORT COMMUNICATION
DOI: 10.1002/ejic.201100947
Ionic Additives and Weak Magnetic Fields in the Thermal Decomposition of
Octacarbonyldicobalt – Tools To Control the Morphology of Cobalt
Nanoparticles
[
a]
[b]
[b]
[c]
[c]
Axel Dreyer, Michael Peter, Jochen Mattay, Katrin Eckstädt, Andreas Hütten, and
Peter Jutzi*[
a]
Keywords: Cobalt / Nanoparticles / Synthetic methods / Surfactants / Magnetic properties
In the presence of ionic compounds, the thermal decomposi-
tion of octacarbonyldicobalt, Co (CO) , in an inert solvent
leads exclusively to ε-Co nanocubes. The ionic species can
between an additive and the precursor or between two addi-
tives. The additional presence of an inhomogeneous mag-
netic field leads to disc-shaped crystalline nanoparticles,
which assemble to chains.
2
8
be added directly or generated in situ by a chemical reaction
Introduction
decomposition of Co (CO) in the presence of ionic addi-
2 8
tives leads exclusively to Co nanocubes with average edge
lengths between 35 and 95 nm, depending on the reaction
conditions. For shape control, it is not important whether
the ionic additive is present already from the beginning or
formed only during the thermal decomposition.
Only very recently, we have investigated in detail the ther-
mal decomposition of decamethylstannocene, (Me C ) Sn,
5
5 2
in organic solvents, which resulted in the formation of dif-
ferent morphologies of nanostructured β-tin particles. Sur-
prisingly, the presence of ionic additives during the thermol-
ysis led exclusively to the formation of cubes with controlla-
The influence of an external magnetic field during the
[
26–29]
synthesis of cobalt nanoparticles
zation of cobalt nanoparticles under the influence of an ex-
ternal magnetic field
and the self-organi-
[
1]
ble size. In the context of our ongoing interest in the in-
vestigation of the magnetic properties of cobalt nanopar-
ticles prepared from the thermal decomposition of octacar-
[
15,14]
are documented in the literature.
One-dimensional assemblies have been observed in all cases,
and the resulting physical effects have been studied. To the
best of our knowledge, the influence of an external mag-
netic field applied during the thermal decomposition of
[
2–4]
bonyldicobalt, Co (CO) , in organic solvents,
we now
2
8
investigated the influence of ionic additives on the mor-
phology of cobalt nanoparticles. The effect of ionic com-
pounds as additives on the decomposition of Co (CO) has
2
8
[30]
Co (CO) has not been studied so far. Here we describe
2
8
not been described in the literature so far, though this syn-
the effect of an additional weak external field, which leads
to a change in the shape of the cobalt nanoparticles from
cubes to discs, which are self-organized to chains.
[
5–7]
thetic strategy represents a classical procedure
for the
preparation of Co nanoparticles,[
8–16]
together with the de-
[17]
composition of other organometallic precursors and with
the reduction of cobalt salts.[
18–22]
Very recent studies have
shown that changing the shape of cobalt nanoparticles from
spherical to cubic can fundamentally change their magnetic
Results and Discussion
[
15,23–25]
behavior.
Here we describe our observation that the
An overview of the performed experiments with a sche-
matic representation of the particle shapes obtained to-
gether with a collection of TEM images is presented in Fig-
ure 1. The respective XRD patterns are collected in Fig-
ure 2. The magnetic properties of the Co nanoparticles
[
a] Bielefeld University, Department of Chemistry, Institute of
Inorganic Chemistry,
Universitätsstrasse 25, 33615 Bielefeld, Germany
Fax: +49-521-106 6026
E-mail: peter.jutzi@uni-bielefeld.de
[
[
b] Bielefeld University, Department of Chemistry, Institute of correspond to those already described in the litera-
Organic Chemistry,
[11,15,20]
[31]
ture
and will not be discussed.
Universitätsstrasse 25, 33615 Bielefeld, Germany
c] Bielefeld University, Department of Physics, Institute of Thin
Films & Physics of Nanostructures,
The influence of the following additives has been investi-
gated: (1) cetyltrimethylammonium bromide (CTAB),
Universitätsstrasse 25, 33615 Bielefeld, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100947.
(2) benzylpyridine (BPy), and (3) mixtures of oleic acid
(OS) and oleylamine (OA).
198
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 198–202

Thermal Decomposition of Octacarbonyldicobalt
2 8
Figure 1. Schematic overview of the morphologies of Co nanoparticles obtained by the thermal decomposition of Co (CO) under the
influence of ionic additives with and without an external magnetic field and under the influence of non-ionic additives. TEM images
show the respective morphologies. The chosen additives are: (a) cetyltrimethylammonium bromide (CTAB; the cubic structure of some
particles is exemplarily marked), (b) 4-benzylpyridine (BPy), (c and d) an OS/OA mixture in the absence (c) and in the presence of a
magnetic field (d), (e) OA, and (f) OS.[ Scale bar: 50 nm. Additional TEM images with higher magnification are presented in section B
of the Supporting Information.
8]
as a result of oxidation at the particle surface during the
measurement. The crystallite size was determined as
[
32]
34.9 nm.
Benzylpyridine (BPy)
The synthesis of Co nanoparticles in the presence of BPy
(0.12 mmol) led to the formation of nanocubes of ε-Co
with an average edge length of 57.6Ϯ21.5 nm (TEM image
in Figure 1b and XRD pattern in Figure 2b). The crystallite
size of 26.3 nm indicates a polycrystalline texture.
During the usual work-up, a deep green solution was ob-
tained after centrifugation. The conductivity of this solu-
–1
tion was measured to be 0.5 μScm . Evaporation of the
solvent led to an air-sensitive, viscous, red-brown residue.
Its IR spectrum shows absorptions at 1895, 1863 and
–
1
–
1
unit.
701 cm , which can be assigned to the Co(CO)
4
[
34–36]
Figure 2. XRD patterns[33] of cobalt nanoparticles with various
Hieber has reported that Co (CO) dispropor-
2 8
morphologies synthesized by the thermal decomposition of tionates with pyridine (Py) already at room temperature to
2
+
– [37]
Co
2
(CO)
8
in the presence of (a) cetyltrimethylammonium bromide give the red-brown salt [CoPy ] [Co(CO) ] .
We con-
clude that, during the synthetic procedure (see Experimen-
6
4 2
(
(
CTAB), (b) 4-benzylpyridine (BPy), (c and d) an OS/OA mixture
) in the absence (c) and in the presence of a magnetic field (d), (e)
OA, and (f) OS.
2+
tal Section) the soluble ionic compound [Co(BPy) ] -
Co(CO)4] 2 is formed in addition to the Co nanocubes (for
6
–
[
further details, see section C of the Supporting Infor-
mation).
Cetyltrimethylammonium Bromide (CTAB)
The synthesis of Co nanoparticles in the presence of
CTAB (0.16 mm) led to the formation of small nanocubes
of ε-Co with an edge length of 4.1Ϯ1.9 nm (TEM image
in Figure 1a and XRD pattern in Figure 2a). The XRD
Mixtures of Oleic Acid (OS) and Oleylamine (OA)
The synthesis of Co nanoparticles in the presence of a
pattern indicates small contributions of fcc cobalt(II) oxide mixture of OS (0.12 mmol) and OA (0.12 mmol) led to the
Eur. J. Inorg. Chem. 2012, 198–202
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www.eurjic.org
199

A. Dreyer, M. Peter, J. Mattay, K. Eckstädt, A. Hütten, P. Jutzi
SHORT COMMUNICATION
formation of polycrystalline nanocubes of ε-Co with an benzene showed a conductivity of 1.2Ϯ0.3 μScm–1 at
average edge-length of 68.6Ϯ14.0 nm (TEM image in Fig- 30 °C, which increased linearly with increasing temperature
ure 1c and XRD pattern in Figure 2c). The average crystal- up to 2.3Ϯ0.3 μScm–1 at 100 °C (see section C of the Sup-
lite size was determined to be 22.2 nm (Figure 2c). Other porting Information). As expected, solutions of the pure
OS/OA ratios also led to the formation of nanocubes. Inter- components did not show any conductivity.
estingly, the size of the nanocubes strongly depends on the
OS/OA ratio: Figure 3 shows that the average edge-length
decreases with an increase of the OA component.
Weak Magnetic Field during Synthesis
The influence of a weak magnetic field on the shape of
the Co particles was investigated exemplarily in a system
containing an OS/OA mixture as additive. The synthesis
was performed in the presence of a heterogeneous magnetic
field of 0.03 T.
A
permanent magnet (Nd Fe B,
2 12
30ϫ5ϫ10 mm) was positioned horizontally underneath
the reaction flask. Nanodiscs of ε-Co with an average dia-
meter of 9.1Ϯ2.3 nm and with an average thickness of
4
.5Ϯ1.8 nm were formed. The discs are organized in form
[
10,11]
of chains by magnetic dipole interactions.
Tilted chain
segments confirm the existing disc shape (Figure 1d). The
disc size is influenced only to a small extent by the OS/
OA ratio; the size increases with the increase of the OA
component (Figure 3). XRD investigations prove the pres-
ence of monocrystalline Co with a crystallite size of 5.8 nm
Figure 3. Edge length of Co nanocubes (squares) and diameter of
Co nanodiscs (dots) depending on the mixing ratio of the additives
(Figure 2d). A structure assignment (ε- or hcp-Co) remains
uncertain because of broad Bragg reflections.
(OS/OA). Nanocubes (in the absence of a magnetic field; squares)
and nanodiscs (in the presence of a magnetic field; dots) were ob-
tained.
Conclusions
The synthesis of Co nanoparticles in the presence of only
a single additive (long-chain amine or carboxylic acid) has
Our experiments clearly show that the presence of ionic
already been described in the literature in detail: The use of species and of magnetic fields during the thermal decompo-
hexadecylamine (HDA) as additive leads to polycrystalline sition of Co (CO) allows to control the morphology of the
2
8
hcp-Co spheres and that of OS to monocrystalline ε-Co resulting Co nanoparticles. Ionic additives lead exclusively
[
11]
spheres. For comparison, we have performed similar ex- to the formation of nanocubes. The ionic species are added
periments with OA (0.32 mmol) and with OS (0.52 mmol) in the form of a stable salt (CTAB) or are formed by a
as single additives. As expected, the formation of spherical chemical reaction between the additive and the precursor
nanoparticles was observed. In the presence of OA, small molecule [BPy/Co (CO) ] or between additives (OS/OA). In
2
8
crystallites of fcc-Co are formed, which aggregate to larger the OS/OA additive system, the additional presence of a
particles (27.7Ϯ13.1 nm, Figures 1e and 2e). In the pres- weak inhomogeneous magnetic field leads to the formation
ence of OS, polycrystalline Co particles are observed of nanodiscs, which assemble to chains.
(4.3Ϯ1.1 nm, Figures 1f and 2f). The small particles and
Only very recently, Scariot et al. have observed that
the resulting broad Bragg reflections do not allow a struc- nanocubes are formed in the thermal decomposition of
[16]
ture assignment.
Co (CO) performed in an ionic liquid as solvent.
Our
2
8
In mixtures of organic carboxylic acids and amines, the experiments demonstrate that an inert organic solvent and
formation of the corresponding salts is expected (RCOOH the presence of ionic additives is sufficient to initiate a com-
+
–
+
RNH i RNH + RCOO ). The presence of ionic spe- parable selectivity in particle formation. Puntes et al. have
2 3
cies has been confirmed by IR spectroscopic and conductiv- reported that the thermal decomposition of Co (CO) in
2
8
ity measurements. In IR spectroscopic studies of the system the presence of the additive mixture OS/HDA leads to hcp-
[
11]
OS/OA, vibrations assignable to the COOH unit appear in Co nanodiscs.
We assume that a magnetic stirring bar
–
1
[38]
pure OS at 1703 and 933 cm , whereas vibrations assign- has been used in these experiments.
able to the COO unit in the corresponding salt formed in observations, the weak inhomogeneous magnetic field of a
an OS/OA mixture appear at 1557 and 1396 cm (for fur- stirring bar is sufficient to induce a change in shape from
ther details see section D of the Supporting Information). nanocubes to nanodiscs.
According to our
–
–
1
Conductivity measurements in the OS/OA system were not
In future work, the mechanisms that are responsible for
conclusive because of the restricted ion mobility. Alterna- the influence of electric charges and heterogeneous mag-
tively, we have measured the conductivity of the system con- netic fields on the formation of anisotropic nanoparticles
sisting of hexanoic acid (HS) and hexylamine (HA). A mix- have to be investigated in detail. With regard to the influ-
ture of 3 mmol HS and 3 mmol HA in 12 mL 1.2-dichloro- ence of electric charges, we suppose that a mechanism sim-
200
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Thermal Decomposition of Octacarbonyldicobalt
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Acknowledgments
We wish to thank the University of Bielefeld and the Faculties of
Chemistry and Physics for support with experiments and analyses,
and the Deutsche Forschungsgemeinschaft (DFG) for financial
support within the framework of the FOR 945 project 3. We are
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α
α
[
15]
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A. Dreyer, M. Peter, J. Mattay, K. Eckstädt, A. Hütten, P. Jutzi
SHORT COMMUNICATION
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202
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Eur. J. Inorg. Chem. 2012, 198–202