Use of ionic liquids (ILs) for the IL-anion size-dependent formation of Cr, Mo and W nanoparticles from metal carbonyl M(CO)6 precursors

Article abstract of DOI:10.1039/b718055a

Stable chromium, molybdenum and tungsten nanoparticles are obtained reproducibly by thermal or photolytic decomposition under argon from mononuclear metal carbonyl precursors M(CO)₆ (M = Cr, Mo, W) suspended in the ionic liquids BMim⁺BF₄^-, BMim ⁺OTf^- and BtMA⁺Tf₂N^- (BMim⁺ = n-butyl-methyl-imidazolium, BtMA⁺ = n-butyl-trimethyl-ammonium, Tf₂N = N(O₂SCF ₃)₂, OTf = O₃SCF₃) with a very small and uniform size of 1 to 1.5 nm in BMim⁺BF₄^- which increases with the molecular volume of the ionic liquid anion to ～100 nm in BtMA⁺Tf₂N^- [characterization by transmission electron microscopy (TEM), dynamic light scattering and transmission electron diffraction (TED) analysis]. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718055a

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Use of ionic liquids (ILs) for the IL-anion size-dependent formation of
Cr, Mo and W nanoparticles from metal carbonyl M(CO)6 precursorsw
Engelbert Redel,a Ralf Thomannb and Christoph Janiak*a
Received (in Cambridge, UK) 22nd November 2007, Accepted 3rd March 2008
First published as an Advance Article on the web 14th March 2008
DOI: 10.1039/b718055a
Stable chromium, molybdenum and tungsten nanoparticles are
obtained reproducibly by thermal or photolytic decomposition
under argon from mononuclear metal carbonyl precursors
M(CO)6 (M = Cr, Mo, W) suspended in the ionic liquids
BMim+BF4À, BMim+OTfÀ and BtMA+Tf2NÀ (BMim+
=
n-butyl-methyl-imidazolium, BtMA+ = n-butyl-trimethyl-am-
monium, Tf2N = N(O2SCF3)2, OTf = O3SCF3) with a very
À
small and uniform size of 1 to 1.5 nm in BMim+BF4 which
increases with the molecular volume of the ionic liquid anion to
B100 nm in BtMA+Tf2NÀ [characterization by transmission
electron microscopy (TEM), dynamic light scattering and trans-
mission electron diffraction (TED) analysis].
Scheme 1 Formation of Cr, Mo and W nanoparticles by thermal or
photolytic decomposition of metal carbonyls under argon in ionic
liquids.
Syntheses and applications of transition-metal nanoparticles
are of contemporary interest in several areas of science.1 In
particular, W and Mo nanoparticles can be used for olefin
metathesis reactions.2 The synthesis of defined and stable
metal nanoparticles (MNPs) is of high importance.3
the range of about 1 to 1.5 nm with a narrow, albeit not
monodisperse, size dÀistribution could be reproducibly synthe-
sized in BMim+BF4 (Fig. 1 and Fig. 2, Table 1). Nanopar-
ticles of this small size are novel. Nanoparticles obtained in
BtMA+Tf2NÀ are larger, ranging from about 70 to 150 nm
(Table 1). Such large nanoparticles can easily be separated, e.g.
by simple centrifugation (10 min at 2000 rpm under argon)
from the IL. TED (transmission electron diffraction) studies
show that the larger Cr, Mo and W NPs produced under
argon are crystalline (Fig. 3, ESIw Fig. S14, S18, S23), with the
diffraction patterns corresponding to the metal lattices, there-
by proving their metallic character and the absence of
significant oxidation.
Metal nanoparticles can be stabilized by the ionic charge,
high polarity, high dielectric constant and supramolecular
network4 of ionic liquids (ILs). According to the DLVO
(Derjaugin–Landau–Verwey–Overbeek) theory5 ILs provide
an electrostatic protection in the form of a ‘‘protective shell’’
for MNPs, so that no extra stabilizing molecules are needed.6
Most known metal nanoparticle syntheses in ILs are carried
out through the reduction of metal salts by hydrogen gas,7–9
photochemical10 or electroreduction/electrodeposition.11 Re-
cently, Fe2O3 nanoparticles were synthesized by thermal de-
composition of Fe(CO)5 + stabilizers in BMim+Tf2NÀ.12
Here we report the preparation of Cr, Mo and W MNPs by
thermal or photochemical decomposition under argon of the
mononuclear metal carbonyls M(CO)6 in ILs (Scheme 1).w13
Green Cr, brown Mo or gray-blue W nanoparticle disper-
sions (ESIw Fig. S25–S27) are obtained through decomposi-
tion from their metal carbonyl in ionic liquids. The dispersions
are found to be stable for six months and longer under argon
atmosphere. Also, extremely small MNPs of Cr, Mo and W in
Furthermore, we synthesized the M-oxide nanoparticles
(Fig. 4) for comparison to the Cr, Mo and W NPs. For the
M-oxides the M(CO)6/IL mixture was subjected to the same
decomposition conditions, albeit under air.w The M-oxide NPs
have different colors from the MNP suspensions (ESIw Fig.
a Institut fur Anorganische und Analytische Chemie, Universitat
¨
¨
Freiburg, Albertstr. 21, D-79104 Freiburg, Germany. E-mail:
janiak@uni-freiburg.de; Fax: +49 761 2036147; Tel: +49 761
2036127
b FMF (Freiburger Material Forschungszentrum), Universitat
¨
Freiburg, Stefan-Meier-Str. 21, D-79104 Freiburg, Germany. E-mail:
ralf.thomann@fmf.uni-freiburg.de; Tel: +49 761 2035379
w Electronic supplementary information (ESI) available: Materials
and instrumentation, synthesis, statistical graphs for dynamic light
scattering, additional TEM and TED pictures, photographs of
dispersions. See DOI: 10.1039/b718055a
Fig. 1 TEM of W NPs from W(CO)6 in BMim+BF4 by thermal
À
decomposition (entry 1 in Table 1). See footnote e in Table 1.
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À
Fig. 2 TEM of Mo NPs from Mo(CO)6 in BMim+BF4 by photo-
Fig. 4 TEM of Cr-oxide NPs from Cr(CO)6 in BtMA+Tf2NÀ by
lytic decomposition (entry 7 in Table 1). See footnote e in Table 1.
thermal decomposition (entry 12 in Table 1).
Fig. 3 TED patterns of a single W NP (left) and an ensemble of W
NPs (right) from W(CO)6 in BtMA+Tf2NÀ by thermal decomposition
under Ar (entry 4 in Table 1). The black bar is the beam stopper. The
d-values match with the d-spacing of W metal.14
Fig. 5 Correlation between the molecular volume of the ionic liquid
anion (VIL) and the observed W nanoparticle size with standard
deviations as error bars (from TEM and dynamic light scattering,
ESIw Fig. S1–S11).
S23–25), have a broader size distribution (Table 1) and show
no crystallinity (ESIw Fig. S15 and S24).
A comparison of Mo NPs produced under thermal und
photolytic decomposition shows only slight differences in
median size and size distribution (Table 1). Nanoparticles
produced by photolysis give somewhat larger particles because
of a faster decomposition process in the ionic liquid.
A correlation exists between the molecular volume of the
anion in the ionic liquid and the synthesized metal nanopar-
ticles, shown here for, but not limited to, tungsten. The size of
the Cr, Mo and W nanoparticles increases with the molecular
volume of the IL anion (Table 1, Fig. 5). MNPs (core) are
Table 1 MNP and M-oxide NP (M = Cr, Mo and W) size and size distribution in different ionic liquids
VIL-anion/nm3 18
(standard deviation s)
Metal
carbonyl
TEM NP median diameter/nm,
(standard deviation s)b
Dynamic light scattering NP median
diameter/nm, (standard deviation s)c
Ionic liquida
À
1
2
3
4
5
6
7
8
BMim+BF4
0.073 (Æ0.009)
0.131 (Æ0.015)
0.232 (Æ0.015)
0.232 (Æ0.015)
0.232 (Æ0.015)
0.073 (Æ0.009)
0.073 (Æ0.009)
0.232 (Æ0.015)
0.232 (Æ0.015)
0.073 (Æ0.009)
0.232 (Æ0.015)
0.232 (Æ0.015)
W(CO)6
W(CO)6
W(CO)6
W(CO)6
W(CO)6/air
Mo(CO)6d
Mo(CO)6
Mo(CO)6
Mo(CO)6/air
Cr(CO)6
r1.5 (Æ0.3)e
3.1 (Æ0.5)
11.7 (Æ2.3)
45 (Æ11)
BMim+OTfÀ
BMim+Tf2NÀ
BtMA+Tf2NÀ
BtMA+Tf2NÀ
5.7 (Æ2.1)
33 (Æ11)
67 (Æ32)
97 (Æ33)
91 (Æ83)
163 (Æ47)
2.5 (Æ0.6)
3.8 (Æ1.1)
258 (Æ89)
— (layers)
3.0 (Æ0.6)
51 (Æ12)
BMim+BF4À
r1.5 (Æ0.3)e
B1.0–2.0 (Æ0.6)e
150 (Æ30)
À
BMim+BF4
BtMA+Tf2NÀ
9
BtMA+Tf2NÀ
— (layers, ESI S19)
r1.5 (Æ0.3)e
—
À
10
11
12
BMim+BF4
BtMA+Tf2NÀ
BtMA+Tf2NÀ
Cr(CO)6
Cr(CO)6/air
33 (Æ10)
62 (Æ16)
a
b
c
See Scheme 1. From TEM measurements, statistical evaluation of the total sample pictures. Hydrodynamic radius, median diameter form the
d
e
first 3 measurements at 633 nm in n-butyl-imidazole (see ESI Fig. S1–S11). Photolytic decomposition for 15 min at 200 to 450 nm. The TEM
pictures with particles of average median diameters of less than 1.5 nm show electron dense cloudy structures. Due to scattering caused by the
surrounding ionic liquid resolution of the TEM is limited and particles below 1.5 nm are hardly resolved.
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considered stabilized in the ILs by the formation of ‘‘protec-
tive’’ anionic and cationic layers (shells) around them in a
‘‘core–shell system’’.6,15 We suggest that the thickness of the
stabilizing shells around an MNP depends on the IL molecular
ion volumes. According to DLVO theory5 the first inner shell
must be anionic, then the IL anion will have the greatest
influence on the size and electrostatic stabilization of the Cr,
Mo and W nanoparticle.
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The anion molecular volume determines the region of the
nanoparticle size. IL parameters like density, viscosity, con-
ductivity and surface tension also correlate with the volume of
the anion in the ionic liquid and could influence nanoparticle
nucleation and growth,16 although the supramolecular imida-
zolium-anion clusters of the IL should be taken into account.4
The IL cation can be used as a fine-tuning tool in nanosynth-
esis.9 ‘‘Pure’’ imidazolium based ILs should be considered as
three-dimensional networks of anions and cations, linked by
weak interactions (such as hydrogen bonds, van der Waals and
Coulomb forces). ILs should be regarded as supramolecular
polymeric structures with a high degree of self-organisation
and weak interactions. When mixed with other molecules or
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We describe here a simple and reproducibly method for the
synthesis and size tailoring of Cr, Mo and W metal nanopar-
ticles with median diameters between B1–100 nm and narrow
size distribution in ionic liquids. The synthesis uses easily
commercially available M(CO)6 metal carbonyls and ILs and
can readily be expanded to the broad range of other metal
carbonyl complexes. Metal carbonyls are attractive starting
materials for nanosyntheses, available in high purity or easily
purifiable, e.g. by sublimation.w19
12 Y. Wang, S. Maksimuk, R. Shen and H. Yang, Green Chem., 2007,
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13 The IL BMim+BF4À is stable up to 423 1C. The ILs were dried at
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14 STOE WinXPow version 1.10, data base, STOE & Cie GmbH,
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We thank Dr T. Schubert from IoLiTec for donation of ILs.
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