Cobalt nanocubes in ionic liquids: Synthesis and properties

Article abstract of DOI:10.1002/anie.200804200

(Chemical Equation Presented) Cubes and spheres: The simple thermal decomposition of [Co₂(CO)₈] dispersed in 1-alkyl-3-methyl imidazolium ionic liquids can preferentially generate either ligand-free cobalt nanocubes or spherical nano

Full text of DOI:10.1002/anie.200804200

Angewandte
Chemie
DOI: 10.1002/anie.200804200
Nanostructures
Cobalt Nanocubes in Ionic Liquids: Synthesis and Properties**
Morgana Scariot, Dagoberto O. Silva, Jackson D. Scholten, Giovanna Machado,
SØrgio R. Teixeira, Miguel A. Novak, Günter Ebeling, and Jairton Dupont*
The magnetic, optical, and catalytic properties of soluble
metal nanoparticles (MNPs) depend primarily on their size,
anion, and reaction conditions, ligand-free cobalt MNPs with
either cubic or spherical shapes can be prepared. Moreover,
we present the magnetic and catalytic properties of these
naked ligand-free MNPs in 1-alkyl-3-methylimidazolium ILs.
The in situ decomposition of [Co (CO) ] dispersed in 1-n-
[
1–6]
shape, and type, and on the nature of the stabilizer.
The
generation of MNPs of controlled size and shape has been
achieved by using a variety of methods that are mainly based
2
8
[7,8]
on the use of ligands.
Indeed, the vast majority of stable
decyl-3-methylimidazolium N-bis(trifluoromethanesulfonyl)-
and soluble MNPs formed from transition metals have a
ligand and/or oxide environment that has a substantial effect
imidate ([DMI][NTf ]) at 1508C over 1 h afforded a black
2
0
solution containing [Co ] NPs with a cubic shape, together
n
[
7,9]
on the metal-surface properties of the nanoparticles.
The
with MNPs with an irregular shape (Figure 1 and Figure S1 in
synthesis of stable, soluble, naked, and ligand-free MNPs of
controlled size and shape still remains a challenge. Although
these MNPs can be produced in organic solvents (e.g.,
alcohols or tetrahydrofuran) by the simple decomposition of
[
10]
organometallic precursors, the properties of these nano-
particles cannot be investigated in solution because of their
[11]
poor stability and the volatility of the solvents.
Since
imidazolium ionic liquids (ILs) possess preorganized struc-
tures that can adapt or are adaptable to many species—as
they provide hydrophobic or hydrophilic regions with high
directionality—they are emerging as alternative liquid tem-
plates for the generation of a plethora of size- and shape-
[12–18]
controlled nanostructures.
In particular, the size of
“
soluble” MNPs is apparently directly related to IL self-
0
[
19]
Figure 1. a) Selected TEM micrograph of the [Co ]
n
particles prepared
organization,
and can thus, in principle, be tuned by
by decomposition of the [Co (CO) ] dispersed in [DMI][NTf ] at 1508C
for 1 h and b) a histogram showing the size distribution.
2
8
2
modulating the length of the N-alkyl imidazolium side
[20]
[21]
[22,23]
chains,
anion coordination ability.
imidazolium ILs that possess very low vapor pressure
and high thermal stability has opened the way for the
investigation of processes in solution using physical methods,
for example, transmission electron microscopy (TEM) and
X-ray photoelectron spectroscopy,
conditions such as high vacuum. We report herein that, with
the proper combination of N-alkyl imidazolium side chain,
reaction temperature,
anion volume,
or
[
24,25]
Moreover, the advent of
[
26,27]
the Supporting Information). These cobalt particles show a
bimodal size distribution with a mean diameter of (79 Æ
17) nm for the larger particles (cubic shape) and (11 Æ 3) nm
for the smaller particles (mainly spherical in shape; Figure 1).
However, nanoparticles with an exclusively cubic shape
((53 Æ 22) nm) were obtained by decomposition of
[Co (CO) ] in 1-n-decyl-3-methylimidazolium trifluoro-tris-
[
28]
[29]
which require special
2
8
(
1
pentafluoroethane) phosphate ([DMI][FAP]) after 5 min at
508C (Figure 2). Interestingly, MNPs with an irregular shape
were obtained from reactions in 1-n-butyl-3-methylimidazo-
[*] M. Scariot, D. O. Silva, J. D. Scholten, Prof. S. R. Teixeira,
Prof. G. Ebeling, Prof. J. Dupont
À
À
À
Institute of Chemistry and Institute of Physics
UFRGS, Avenida Bento Gonçalves 9500
lium (BMI) ILs associated with NTf2 , FAP , and BF ions
4
under similar reaction conditions. Therefore, the formation of
cubic-shaped cobalt MNPs is related to the IL self-organ-
ization that is, they are formed preferentially in the ILs that
91501-970, Porto Alegre, RS (Brazil)
Fax: (+55)5133087304
E-mail: dupont@iq.ufrgs.br
+
have more preorganized structures (with DMI rather than
Prof. G. Machado
Departamento de Engenharia Química
Universidade de Caxias do Sul (Brazil)
+
À
À
[20,30]
BMI and FAP rather than BF4 ions).
The cobalt
MNPs were characterized by transmission electron micro-
scopy (TEM), scanning electron microscopy (SEM) coupled
with an electron dispersive spectroscopy (EDS) detector, and
X-ray diffraction (XRD).
Prof. M. A. Novak
Institute of Physics, UFRJ, Cidade Universitµria
21941-972, Rio de Janeiro (Brazil)
[
**] We thank PETROBRAS for financial support.
The formation of the MNPs in ILs was followed by in situ
TEM analysis of aliquots removed from the reaction mixture
at different times (2, 5, 15, 40, 60, and 300 min). In the case of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804200.
Angew. Chem. Int. Ed. 2008, 47, 9075 –9078
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9075

Communications
0
Figure 3. XRD analysis of the [Co ] particles prepared at different
n
times during the decomposition reaction of [Co (CO) ] in the IL
2
8
[DMI][NTf2].
0
Figure 2. Selected TEM micrographs of the [Co ] particles prepared by
n
noteworthy the e-cobalt phase was always observed, inde-
pendent of reaction time (Figure 3). However, after a reaction
time of 2 min, Bragg reflections at 2q = 12.378, 12.758, and
decomposition of the [Co (CO) ] precursor dispersed in [DMI][FAP] at
2
8
1
508C after a) 5 min ((53Æ22) nm), b) 15 min, and c) 300 min
(
(5.5Æ1.1) nm).
1
4.288 were observed, which correspond to the intermediate
[33]
[
Co (CO) ]; this observation indicated that the reduction
4 12
[34]
the IL [DMI][NTf ], both large cubic and small spherical
of the cobalt precursor was not complete after this time.
2
particles were observed in all samples, except for the aliquot
taken after 2 min, in which only irregular spherical particles
The material isolated from the IL was also analyzed by SEM/
EDS, but in these cases oxygen was detected together with
cobalt since the samples were exposed to air for analysis
(Figure S3 in the Supporting Information).
were detected together with the organometallic cobalt
0
precursor. TEM pictures of [Co ] NPs show that the mean
n
diameter of the cubes decreases with longer reaction times
Figure 4 shows the thermal magnetization curve of the
(
from about 88 nm at 15 min to 61 nm after 5 h), and the
cobalt sample prepared in [DMI][NTf ] after a reaction time
2
percentage of spherically shaped particles increases from
about 25% at the beginning of the decomposition to about
of 1 h. The zero field cooling (ZFC) curve shows two features,
a clear maximum around 7 K, associated with the blocking
temperature of the small spherical particles with a mean
diameter of about 3 nm, and a broad shoulder around 150 K,
associated with the blocking of larger particles with a
diameter 10 nm (indicated by arrows in Figure 4).
9
0% after 5 h (see Figure S2 in the Supporting Information).
These results indicate that the large cubic particles are
gradually transformed into small spherical particles through-
[
31]
out the reaction. Indeed, the cubic-shaped NPs obtained in
DMI][FAP] are completely transformed into irregular NPs
[
The increase in the ZFC curve above 200 K corresponds
to a reorientation of the magnetic easy axis, which results
(
(5.5 Æ 1.1) nm) after 5 h at 1508C (Figure 2).
In addition, the reaction was also followed by XRD
from the melting of [DMI][NTf ]; this occurs in an analogous
2
0
analysis of the [Co ] NPs embedded in the IL [DMI][NTf ]
n
2
(
Figure 3). Only peaks corresponding to metallic cobalt were
observed together to that of the IL (broad peaks below 2q =
08), and no cobalt oxide peaks were detected in the sample,
3
indicating that the IL forms a protective layer around the
metal surface and prevents its oxidation since the sample was
air exposed for analysis. However, the pattern found does not
correspond to either of the two most common structures of
cobalt (hexagonal close packed and face-centered cubic). This
phase was indexed as cubic with space group P4 32, which is
1
defined as e-cobalt. The Bragg reflections corresponding to
crystalline cobalt MNPs were observed at 2q = 44.578, 47.128,
and 49.558, which correspond to the indexed planes of e-
cobalt(0) crystals: (2 2 1), (3 1 0), and (3 1 1), respectively, with
unit cell parameters of a = (6.091 Æ 0.001) Š. The lattice
parameters and the electron density obtained are quite
0
Figure 4. Thermal magnetization curve at 52 Oe of [Co ] NPs prepared
n
[32]
similar to the values obtained by Bawendi et al.
It is
in [DMI][NTf ] at 1508C for 1 h.
2
9
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Angewandte
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[
20]
manner to that reported for [BMI][BF ]. Assuming that the
In summary, we have demonstrated that the simple
thermal decomposition of [Co (CO) ] in ILs at 1508C
4
spherical particles are uni-axial and that only the e-phase is
present in the sample (according to X-ray measurements), the
volume of this type of particle can be calculated using known
2
8
0
preferentially affords ligand-free [Co ] nanocubes or spher-
n
ical MNPs, depending on the type of IL used and the reaction
time. The mean diameter of the nanocubes was estimated to
be (53 Æ 22) nm and (79 Æ 17) nm by TEM analysis for the
[35]
values of the anisotropy constant.
The calculations give
mean diameters of 3.3 and 9.7 nm, which correspond to the
signals at temperatures of 7 and 150 K, respectively. These
results corroborate the TEM analysis, in which spherical
cobalt particles with diameters in the range 3-25 nm can be
observed. Moreover, for the cubic particles ( ꢀ 79 nm by TEM
analysis) it is difficult and perhaps impossible to estimate
their volume from the magnetic measurements because the
blocking temperature is above 300 K, and in addition they can
samples prepared in [DMI][FAP] and [DMI][NTf ], respec-
2
tively. The shape of the “soluble” cobalt particles is appa-
rently directly related to self-organization in the IL and thus
can, in principle, be tuned by modulating the length of the N-
alkyl imidazolium side chains, the anion type (volume or
coordination ability), and the reaction conditions. Moreover,
XRD measurements indicate the presence of e-cobalt (a
[
36,37]
0
be considered neither as monodomains nor as uni-axial.
distorted cubic structure) for the [Co ] NPs prepared in
n
The magnetization curves saturate for applied fields
around 1.0 T, showing that the sample is formed from
ferromagnetic particles with a weak anisotropy embedded
in the IL. At 200 K, small coercivity and remanence values of
[DMI][NTf ] as well as the absence of cobalt oxide, which
2
suggests that the IL produces a protective layer around the
MNPs and prevents their oxidation. An adsorption experi-
ment shows the typical carbonyl bands of CO adsorbed on an
activated cobalt surface. In addition, these e-cobalt MNPs
demonstrate selectivity for the formation of diesel-like
products in the FTS.
À5
2
0
.042 T and 3.44 ” 10 Am , respectively, are observed in the
hysteresis curve. At 5 K, as expected for such a system, both
the hysteresis and remanence increase to twice the values
obtained at 200 K (see Figure S4 in the Supporting Informa-
tion).
As a typical experiment to better evaluate the properties Experimental Section
of the metallic surface, CO adsorption (20 atm at 258C for
0 h) was performed with the isolated cobalt MNPs (prepared
A solution of [Co
added in small portions under a stream of argon to a mechanically
2 8
(CO) ] (0.2 mmol in ca. 25 mL of hexane) was
2
[
46]
stirred system consisting of 1 mL of the IL (preheated to 1508C).
After the addition of the precursor, the flow of argon was stopped and
stirring (250 rpm) was maintained for the desired reaction time. The
reaction mixture was cooled to room temperature and the black
solution thus obtained was analyzed by TEM and XRD, and its
magnetic properties were determined.
in [DMI][NTf ]) in a DRIFT cell (DRIFT= diffuse reflec-
2
tance infrared Fourier transform; Figure S5 in the Supporting
Information). Typical bands for CO adsorbed on the activated
À1
cobalt surface were observed around 2210–2050 cm (termi-
À1
nal CO stretching), at 1732 cm (CO adsorbed in a bridged
[
38]
À1
À1
mode),
group of carbonate), while the band at 750 cm is typical of
1367 cm , and 1213 cm (assigned to the CO
[
39]
À1
Received: August 25, 2008
Published online: October 23, 2008
[39]
a bridged CO carbonate.
0
The isolated [Co ] NPs prepared in [DMI][NTf ] were
n
2
Keywords: cobalt · Fischer–Tropsch synthesis · ionic liquids ·
nanocubes · nanoparticles
tested as catalysts in the Fischer–Tropsch synthesis (FTS). The
.
reaction of syngas (20 atm, H /CO 2:1) over the isolated
2
MNPs at 2108C for 20 h afforded mainly hydrocarbons (8–26
carbon atoms) in the liquid phase (see Figure S6in the
Supporting Information). The chain-growth probability was
estimated by using a modified Anderson–Shulz–Flory (ASF)
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(see Figure S7 in the Supporting Information).
À5
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