Microwave synthesis and inherent stabilization of metal nanoparticles in 1-methyl-3-(3-carboxyethyl)-imidazolium tetrafluoroborate

Article abstract of DOI:10.1039/c1dt10795j

The synthesis of Co-NPs and Mn-NPs by microwave-induced decomposition of the metal carbonyls Co₂(CO)₈ and Mn₂(CO) ₁₀, respectively, yields smaller and better separated particles in the functionalized IL 1-methyl

Full text of DOI:10.1039/c1dt10795j

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Microwave synthesis and inherent stabilization of metal nanoparticles in
1-methyl-3-(3-carboxyethyl)-imidazolium tetrafluoroborate
Dorothea Marquardt,a Zailai Xie,b Andreas Taubert,*b,c Ralf Thomannd and Christoph Janiak*a
Received 29th April 2011, Accepted 7th June 2011
DOI: 10.1039/c1dt10795j
The synthesis of Co–NPs and Mn–NPs by microwave-
induced decomposition of the metal carbonyls Co2(CO)8
and Mn2(CO)10, respectively, yields smaller and better sep-
arated particles in the functionalized IL 1-methyl-3-(3-carbo-
xyethyl)-imidazolium tetrafluoroborate [EmimCO2H][BF4]
(1.6 0.3 nm and 4.3 1.0 nm, respectively) than in the non-
functionalized IL 1-n-butyl-3-methylimidazolium tetrafluo-
roborate [Bmim][BF4]. The particles are stable in the absence
of capping ligands (surfactants) for more than six months
although some variation in particle size could be observed by
TEM.
ILs11,12 and ionic liquid crystals (ILCs)13,14 has attracted tremen-
dous interest in the recent past.
A scientifically interesting and technologically promising ap-
proach is the preparation and stabilization of M–NPs from
metal carbonyls in ILs.3,15 The stabilization of M–NPs in non-
functionalized ILs such as 1-butyl-3-methylimidazolium tetraflu-
oroborate [Bmim][BF4] is primarily based on the IL-anions, which
form the immediate layer around the nanoparticle because the
molecular volume of the IL anion influences sizes and shapes
of the resulting metal nanoparticles.3,10,16,17 Functional amino-,18
carboxylic acid-,19 hydroxyl-,20 nitrile-21 or thiol-,22 groups on the
imidazolium cations exert an additional stabilization on M–NPs
because of specific interactions of the functional group with the
particle surface. The donor atom(s) of the functional group can
attach to the metal nanoparticle much like an extra stabilizing
capping ligand.19 Then, the stabilization of metal-nanoparticles in
functionalized-imidazolium-based ILs occurs through the cation
with its functional group (Fig. 1).11,23 For both non-functionalized
and functionalized ILs equally charged layers around the M–NPs
lead to their separation through electrostatic repulsion and, thus,
prevent their aggregation or Ostwald ripening.24
Metal carbonyls Mx(CO)y1 or other M(0) complexes2 are interest-
ing precursors for the synthesis of metal nanoparticles (M–NPs)
because the metal atoms are already in their final zero-valent
oxidation state. Thus, no reducing agent is necessary and the
carbonyl (CO) side product is largely given off to the gas phase and
thereby removed from the M–NP dispersion. Contamination from
by- or decomposition products which are otherwise generated
during the M–NP synthesis are therefore significantly reduced
and very clean particles can thus be obtained.3
The small size of nanoparticles results in a large fraction of
surface atoms.4 Unprotected small particles interact via van der
Waals and (if the particles are magnetic) magnetic interactions
which will lead to agglomeration or aggregation from the cohesive
surface energy.5 As a result of their colloidal instability, many
nanoparticles need to be stabilized via additional (capping)
agents such as surfactants or polymers, which provide a steric,
electrostatic or electrosteric particle stabilization.4,6
Ionic liquids (ILs) can stabilize metal nanoparticles through
their high ionic charge, their polarity and high dielectric
constants.7,8 ILs can therefore function both as stabilizer and
solvent for the preparation of small (< 5 nm) and (generally)
kinetically stabilized M–NPs.9,10 For these reasons, the chemistry
of inorganic compounds and inorganic materials synthesis in
Fig. 1 Three possible stabilization modes of metal-nanoparticles by
IL-imidazolium cations with functional groups (FG).
Here we present a simple synthesis method for cobalt and
manganese nanoparticles (Co–NPs and Mn–NPs) from their
metal-carbonyl precursors Co2(CO)8 and Mn2(CO)10, respectively,
in the carboxylic acid functionalized imidazolium ionic liquid
(IL) 1-methyl-3-(3-carboxyethyl)-imidazolium tetrafluoroborate
[EmimCO2H][BF4], under microwave irradiation (MWI) (Fig.
2).† The results show that the additional carboxylic acid on the IL
(Fig. 3) does in both cases (Mn and Co) lead to a more uniform
product and a better stabilization of the nanoparticles. Co– and
Mn–NPs can be used as catalysts for Fischer–Tropsch reactions
(Co–NPs also prepared from Co2(CO)8)25 and in the reduction of
polycyclic aromatic hydrocarbons and heterocyclic hydrocarbons
(Mn and Co).26
aInstitut fu¨r Anorganische Chemie und Strukturchemie, Universita¨t
Du¨sseldorf, Universita¨tsstr. 1, D-40225, Du¨sseldorf. E-mail: janiak@
uni-duesseldorf.de; Tel: +49 (0)211 81 12286
bInstitute of Chemistry, University of Potsdam, D-14476, Golm, Germany.
E-mail: ataubert@uni-potsdam.de; Tel: +49 (0)331 977 5773
cMax Planck Institute of Colloids and Interfaces, D-14476, Golm, Germany
dFreiburger Material Forschungszentrum (FMF), Universita¨t Freiburg,
Stefan-Meier-Str., 21-31, 79104, Freiburg, Germany
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Fig. 2 Formation of Co and Mn nanoparticles by microwave irradia-
tion-induced decomposition of metal carbonyls Mx(CO)y under argon in
ILs.
Fig. 3 Chemical structure of the ILs relevant in this study.
ILs are attractive media for microwave reactions because of
their high interaction cross-section resulting from their high ionic
charge, high polarity, and high dielectric constant.1,27 Fig. 4 shows
representative transmission electron microscopy (TEM) images of
the nanoparticles obtained from the decomposition of Co2(CO)8 in
[EmimCO2H][BF4] and [Bmim][BF4]. Small Co–NPs with narrow
size distribution are reproducibly obtained from Co2(CO)8 by
microwave irradiation in the functionalized IL [EmimCO2H][BF4]
(Fig. 4). All samples grown in [EmimCO2H][BF4] contain small
and monodisperse particles with a diameter of 1.6 0.3 nm. Six
months after synthesis the particle size has increased to 3.0
0.6 nm; the size distribution is still narrow. In contrast, Co–NPs
synthesized from Co2(CO)8 under the same conditions, but in
the non-functionalized IL [Bmim][BF4] exhibit larger, relatively
highly aggregated nanoparticles of 5.1 0.9 nm in diameter with
somewhat irregular shapes.15
Fig. 4 (a) TEM-images of Co–NPs in [EmimCO2H][BF4] six weeks after
synthesis; (b) SAED-image of Co–NPs (the black bar in the SAED is
the beam stopper) indicating amorphous material (no well-defined sharp
reflections). (c) TEM of Co–NPs in [EmimCO2H][BF4] 6 months after
synthesis; (d) SAED of Co–NPs in [EmimCO2H][BF4] after 6 months,
still indicating an amorphous material; (e) Co–NPs from Co2(CO)8 in
[Bmim][BF4].
The sample identity and crystallinity can be evaluated from
selected area electron diffraction (SAED). SAED patterns do
not show reflections indicative of a crystalline material. We
therefore conclude that the particles obtained from the synthesis
are amorphous Co–NPs stabilized by the IL.
Fig. 5 shows representative TEM data of the nanopar-
ticles obtained from the decomposition of Mn2(CO)10 in
[EmimCO2H][BF4] and [Bmim][BF4]. The Mn–NPs are larger
than the Co–NPs and have a diameter of 4.3 1.0 nm. The
nucleation rate (possibly controlled by the decomposition rate of
the carbonyl) may be different for the two precursors. Thus, if the
two carbonyls decompose at different rates, the supersaturation
will not be the same and consequently, the particle number will
not be identical. If we have a fast decomposition, more and smaller
particles may form, if the decomposition is slower, we may generate
larger particles. Still, the small particle sizes and the narrow size
distribution for Mn–NPs are remarkable because under the same
conditions in [Bmim][BF4] the particle diameter is 28.6 11.5
nm. Again, SAED shows that the Mn-particles obtained in the
microwave process are amorphous and remain so over time.
Interestingly, TEM images of Mn–NPs in [EmimCO2H][BF4]
stored under nitrogen atmosphere show a decrease of the NP sizes
to 2.4 0.5 nm with prolonged storage of the NP dispersion.
Such a decrease in size is counter-intuitive to the expected slow
aggregation or growth of kinetically stabilized small (< 5 nm)
metal nanoparticles. With the good coordinating ability of the
carboxylate group some chemical etching of the M–NPs by the
IL may be possible. Binnemans et al. use the metal-complex-
forming abilities of ILs with carboxyl-functionalized cations
for the dissolution of metal oxides and metal hydroxides in
carboxylated ILs. The carboxylic acid moiety provides this class
of ILs with a good solubilizing ability for basic metal oxides and
metal hydroxides leading to molecular metal-carboxylate species
in such functionalized ILs.28,29 The main difference between the
current work and the Binnemans study is that in the earlier case,
the oxidation state of the metals is positive and the reaction leading
to the complexes is an acid–base reaction. In the current case
we suspect that a slow redox reaction between reactive surface
metal atoms and carboxylic acid yields some molecular metal-
carboxylate complexes and H2, effectively leading to a reduced
particle size after a few months. A reason for the selectivity of this
process (towards Mn– and not Co–NPs) could be the less-noble
character of the former (standard reduction potentials Mn2+/Mn =
-1.18 V, Co2+/Co = -0.28 V, both in acidic aqueous solution).30
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Fig. 5 (a) TEM-image of Mn–NPs in [EmimCO2H][BF4] six weeks after
synthesis; (b) SAED-image of Mn–NP (the black bar in the SAED is
the beam stopper) indicating amorphous material (no well-defined sharp
reflections). (c) TEM of Mn–NPs in [EmimCO2H][BF4] six months after
synthesis; (d) SAED of Mn–NPs in [EmimCO2H][BF4] after 6 months,
still indicating an amorphous material; (e) Mn–NPs from Mn2(CO)10 in
[Bmim][BF4].
Fig. 6 ATR-IR spectra of (a) Co2(CO)8/IL (gray curve) and the resulting
Co–NP (0.5 wt.%)/IL-dispersion after microwave treatment and of (b)
Mn2(CO)10/IL (gray curve) and Mn–NP (0.5 wt.%)/IL after microwave
treatment (20 W, 6 min).
As the energy input in the nanoparticle synthesis is rather
low (and hence some metal carbonyl may survive the reaction),
complete decomposition of the metal carbonyl was ascertained by
attenuated total reflection infrared (ATR-IR) spectroscopy.‡ Fig.
6 compares ATR-IR spectra of the freshly prepared carbonyl/IL
dispersion and of the reaction mixture after microwave treatment.
The spectra of the initial metal carbonyl/IL dispersion show
the typical band structure of metal carbonyls (2002 cm-1 and
1810 cm-1 for terminal and bridging CO ligands, respectively,
in Co2(CO)8 and 2011 cm-1 and 1943 cm-1 for the terminal CO
ligands in Mn2(CO)10). These bands disappeared after microwave
treatment. ATR-IR spectroscopy thus indicates that the metal
carbonyl precursors are completely decomposed after irradiation
for 6 min with 20 W.
The use of a carboxylic acid-functionalized IL has already
been described for the synthesis of Pt(0) and Au(0) nanoparticles
from H2PtCl6 and HAuCl4 by reduction with NaBH4 in aqueous
solution. Particle sizes of 2.5 0.5 nm (Pt) and 3.5 1.0 nm
(Au) were obtained.19 The IL used in this previous study, 1-
carboxymethyl-3-methylimidazolium chloride, is quite similar to
the one used in the current work, but it was significantly more
hydrophilic and contained a coordinating anion. The effect of the
carboxylic acid functionalization could not clearly be discerned
because the chloride anions also have a pronounced stabilization
effect towards nanoparticles, in particular towards Au–NPs10 and
no comparison to a non-functionalized IL was made.
The main finding of the current work is that carboxylated
ILs can also be efficient reaction media and stabilizers if used
as pure phases and not in aqueous solution. In particular the
acidic group appears to lead to a well-controlled nucleation and
growth process, which is illustrated by the narrow size distribution
of the resulting particles. Moreover, the carboxylic acid moiety
stabilizes the particles over extended periods of time, which (1)
is an indication of a rather strong interaction of the IL with the
nanoparticles and (2) is useful for practical applications, such as
functional fluids. An intriguing feature is the size decrease of the
Mn–NPs with time, but, as pointed out above, this could be due
to a surface redox process between the NP and the acid group. A
more detailed investigation of this process is, however, necessary
to conclude on the details of the process.
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Acknowledgements
Financial support by the University of Potsdam, through DFG
grant Ja466/17-1, the Fonds der Chemischen Industrie, and the
MPI of Colloids and Interfaces (Colloid Chemistry Department),
is gratefully acknowledged. Z.L.X. acknowledges a Chinese
Science Council Doctoral Fellowship.
Notes and references
† Experimental: Materials. Co2(CO)8 and Mn2(CO)10 were obtained
from Acros Organics, [Bmim][BF4] from IoLiTec. 1-methylimidazole, 3-
chloropropionic acid, and NaBF4 were used as received from Acros.
[EmimCO2H][BF4] Synthesis. Under an inert nitrogen atmosphere,
equimolar quantities of 1-methylimidazole and 3-chloropropionic acid
were dissolved in methanol at room temperature. The mixtures were heated
to 75 ◦C with stirring and allowed to react for 20 h. The resulting reaction
mixture was treated with NaBF4 (1.2 equiv) and allowed to stir for 18 h at
room temperature. After separating the NaCl precipitant, the filtrate was
concentrated by rotary evaporation. The yellow and viscous product was
washed with diethyl ether and dried under vacuum to afford the neat IL.
1H NMR (D2O, ppm relative to TMS): 8.61 (s, 1H), 7.42 (d, 1H), 7.50
(d, 1H), 4.22 (t, 2H), 3.83 (s, 3H), 2.10 (t, 2H).31 IR (ATR): 3568, 3160,
3115, 2959, 1728, 1572, 1376, 1282, 1169, 1015, 846, 752 cm-1. Elemental
analysis: calc. for C7H11BF4N2O2: C 34.74%, H 4.58%, N 11.58%. Found:
C 34.39%, H 4.60%, N 11.64%. Total yield: 68%.
Nanoparticle synthesis. Microwave vials with a teflon septum were used.
They were charged with ILs and metal-carbonyls by placing the vial in
a Schlenck tube or by handling in a glovebox. The general procedure
for preparing an 0.5 wt% M–NP dispersion in [EmimCO2H][BF4] was to
add the required mass of metal carbonyl [Mx(CO)y, M = Mn 26.80 mg;
Co 21.80 mg) under nitrogen to the dried and degassed IL (0.5 ml, 0.75
g, density 1.51 g mL-1). A 0.5 wt% dispersion of M–NP in [Bmim][BF4]
was prepared by adding the required mass of metal carbonyl [Mx(CO)y,
M = Mn 21.30 mg; Co 17.40 mg) under nitrogen to the dried and
degassed IL (0.5 ml, 0.6 g, density 1.20 g mL-1). The metal carbonyls
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under nitrogen. The magnetic stir bars were removed from the dispersion
and the reaction mixture was subsequently heated to 250 ◦C (20 W, 6 min)
by microwave irradiation (MWI, CEM Discover). Dark brown to black
metal nanoparticle dispersions were obtained. They were degassed under
reduced pressure for 30 min to remove CO.
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