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
Angew. Chem. Int. Ed. 2008, 47, 9075 –9078
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9075