Oleic acid as the capping agent in the synthesis of noble metal nanoparticles in imidazolium-based ionic liquids

Article abstract of DOI:10.1039/b604269d

This paper reports the synthesis and automatic separation of solvent dispersible silver and platinum nanoparticles in 1-butyl-3-methylimidazolium bis(triflylmethyl-sulfonyl) imide ([BMIM][Tf₂N]) ionic liquid (IL) using oleic acid as the major c

Full text of DOI:10.1039/b604269d

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Oleic acid as the capping agent in the synthesis of noble metal
nanoparticles in imidazolium-based ionic liquids{
Yong Wang and Hong Yang*
Received (in Berkeley, CA, USA) 22nd March 2006, Accepted 25th April 2006
First published as an Advance Article on the web 15th May 2006
DOI: 10.1039/b604269d
1
7
This paper reports the synthesis and automatic separation
of solvent dispersible silver and platinum nanoparticles in
(TFT). In this paper, we used silver trifluoroacetate as precursor
17–20
which can be readily reduced to form Ag nanoparticles.
Although this precursor had a low solubility in [BMIM][Tf N] at
room temperature, the mixture of silver trifluoroacetate, oleic acid,
and [BMIM][Tf N] formed a homogenous and light yellowish
1-butyl-3-methylimidazolium bis(triflylmethyl-sulfonyl) imide
2
(
[BMIM][Tf N]) ionic liquid (IL) using oleic acid as the major
2
capping agent.
2
solution at y150 uC.
Recent results suggest that ILs can be the preferable solvents in the
synthesis and modification of nanostructured materials because of
1
their unique properties, such as extended hydrogen bonding. They
The reaction was conducted at either 160 or 200 uC. Under these
conditions, the product could settle out from [BMIM][Tf N] ILs
2
after about 20 min. Upon the completion of the reaction, the color
have been used in making mesoporous metal oxides, polyaniline
of the mixture turned into transparent yellow. The ionic liquid
could be easily removed from the reaction vessel directly by a
pipette, leaving behind the black solid stuck to the wall of the flask.
This product could then be dispersed readily in hexane.{ Fig. 1
nanoparticles, morphologically interesting nanoflakes and
2–8
nanosheets, and in controlling the composition of PtCo alloys.
Imidazolium-based ionic liquids have also been used in the surface
9
modification of carbon nanotubes. While ionic liquids have been
2
shows a photograph of two vials that contained the [BMIM][Tf N]
used in the synthesis of nanostructured materials, identification of
appropriate surface capping agents in these new solvents remains
an important issue. Several currently used capping agents tend to
IL mixture after the reaction and the hexane suspension of
nanoparticles. The suspension contained highly concentrated
nanoparticles, which could be further cleaned and separated using
ethanol as anti-solvent.
8,10
cause irreversible aggregation of nanoparticles.
This phenom-
2
enon has been successfully applied in making mesoporous titania.
Amphiphilic molecules have been used for the synthesis of uniform
nanoparticles in non-hydrolytic systems, as they not only govern
the size and shape but also the stability of resulting particles. Long
alkane chain carboxylic acids and amines have been proven very
effective for synthesis conducted in various conventional non-
Fig. 2 shows a TEM image of the nanoparticles made at silver
triflouroacetate/oleic acid molar ratio of 1 : 6. These particles had
an average diameter of 4.5 ¡ 0.4 nm. Unlike the nanoparticles
made in ILs previously reported, an extended ordered hexagonal
arrange was observed, suggesting monodispersity in size and
excellent solvent dispersity of these nanoparticles. The average size
of the nanoparticles increased to 5.1 ¡ 0.7 nm if the precursor
concentration was double while all other conditions were kept the
same. The resultant particles however had a standard deviation of
about 14% in size, which was broader than that for 4.5 nm
particles, Fig. S1.{ The silver triflouroacetate/oleic acid molar ratio
played an important role in determining the particle size and size
distribution. The yield of silver nanoparticles was also affected by
11,12
hydrolytic solvents.
This work shows some unique aspects of
N] IL is used as
using oleic acid as capping agent when [BMIM][Tf
2
solvent. It can not only stabilize the metal nanoparticles formed
but also lead to a settling process which the resultant nanoparticles
automatically separate out from the IL reaction mixtures. Unlike
most of the previously reported colloidal nanostructures made in
ILs, these metal nanoparticles can be readily dispersed in
conventional organic solvents and the synthetic approach can
potentially facilitate the development of continuous production of
high quality nanoparticles.
Silver is chosen in part because of its size-dependent surface
plasmon and other unique properties, which have found increas-
13–15
ingly broad applications.
Oleic acid-capped Ag nanoparticles
also possess an interesting surface mediated phase transfer
1
6
property. Most recently, oleic acid-capped Ag nanoparticles
have been found to be an excellent candidate as ink for printable
Ohmic contacts for high-mobility organic thin-film transistors
Department of Chemical Engineering, University of Rochester, Gavett
Hall 206, Rochester, NY 14627, USA.
E-mail: hongyang@che.rochester.edu; Fax: +1 585 273 1348;
Tel: +1 585 275 2110
Fig. 1 Photograph of the [BMIM][Tf
2
N] IL mixture after the reaction
(left vial) and the dispersion of nanoparticles in hexane (right vial). The
{
Electronic supplementary information (ESI) available: Experimental
reaction was conducted using silver trifluoroacetate and oleic acid at
details and TEM images of silver nanoparticles made in ILs under various
conditions. See DOI: 10.1039/b604269d
200 uC. The reaction time was 40 min.
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Chem. Commun., 2006, 2545–2547 | 2545

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Fig. 3 PXRD patterns (left panel) and UV-vis spectra (right panel) of
A) 5.1 and (B) 4.5 nm silver nanoparticles, respectively.
(
Fig. 2 TEM images of silver nanoparticles made at silver trifluoroace-
12–13%. The overall yields of the nanoparticles were substantially
tate/oleic acid molar ratio of 1 : 6.
lower for those reactions conducted at 160 uC. Slow nucleation
rate and the inefficiency in size focusing of nanoparticles at the
this molar ratio. This value increased from about 54% to 70%,
when the precursor/surfactant ratio changed from 1 : 3 to 1 : 6,
Table 1. We noted that some insoluble yellowish solid could be
found at the bottom of the hexane suspension. TEM and powder
X-ray diffraction (PXRD) analyses indicated that this solid could
be the products of the intermediates formed from oleic acid and
the silver precursor. It appears that oleic acid was essential in this
reaction. It not only stabilized the resulting metal nanoparticles,
but also facilitated the settling of nanoparticles from the IL
reaction mixture and ensured an excellent dispersity of nanopar-
ticles in conventional organic solvents, such as hexane. This auto-
separation phenomenon was most likely due to the changing
interactions between ionic liquid and capped nanoparticles.
Fig. 3 shows the PXRD patterns and UV-vis absorption spectra
of 4.5 and 5.1 nm particles obtained at 200 uC. These two samples
had the similar X-ray diffraction patterns, which could be assigned
to metal silver in Fm3m space group, [ICDD PDF: 04-0783]. They
had the maximum absorbance centered at y416 nm, which could
be attributed to the surface plasmon resonance of the silver
nanoparticles. Their full width at half maximum (FWHM) was
y84 nm. The UV-vis absorption appeared to be not sensitive
enough to detect the difference in size and size distribution between
these two types of samples.
11
relatively low temperature could be the main reasons. Table 1
summarizes the size and yield of the nanoparticles obtained under
the above conditions. The temperature threshold for the formation
of relatively uniform Ag nanoparticles in [BMIM][Tf N] was
2
around 160 uC, while the acceptable surfactant/precursor molar
ratio was around three or above.
The size and size distribution of silver particle formed in the
2
[BMIM][Tf N] reaction mixtures were also affected by the reaction
time. The diameters of the Ag particles formed were 4.8 ¡ 1.1 nm
for reaction time of 2 h at 160 uC, Fig. S3.{ This value decreased
slightly to 4.4 ¡ 1.0 nm at 200 uC. The long reaction time resulted
in the broadening of particle size distribution.
The IL mixtures after reaction were studied using both UV-vis
spectrometer and TGA based on the 5.1 nm silver nanoparticles at
200 uC. No absorption peak was observed between 400 and 800 nm
for the mixture after the synthesis. This observation indicated that
the Ag nanoparticles settled out completely from the IL mixtures.
The change in the interaction between this ionic liquid and the
nanoparticles during the growth could be important for the settling
1
6
process. The silver precursors could form complexes with oleic
acid through the carboxyl acid groups and gradually grew into
2
0
clusters. As these clusters were covered by the alkyl chains
pointing toward surrounding IL environment, they exhibited a
dramatically different solubility (or dispersity) from the molecular
precursors and were eventually driven out from the ionic liquid
upon reaching the critical sizes. Based on the TGA data, the onset
temperature of the decomposition of this IL mixture was 438 uC,
The particles formed at 160 uC became less uniform in size in
comparison with those formed at 200 uC, Fig. S2.{ The average
diameters were 4.1 ¡ 0.5 nm for particles made at the precursor/
surfactant molar ratio of 1 : 6, and 6.1 ¡ 0.8 nm for those with
this ratio of 1 : 3. These values represented a standard deviation of
2
Fig. 4, which was the same as that for pure [BMIM][Tf N]. This
result suggests that the IL kept its thermal stability after the
reaction.
Table 1 Comparison of average diameter and yield of the silver
nanoparticles made at different temperatures and precursor/surfactant
molar ratios.
A control experiment with no addition of oleic acid was
conducted at 200 uC and a concentration of silver precursor of
a
Yield (%)
T/uC
Molar ratio
Average diameter/nm
0.06 M. In this case, the settling of particles did not occur. The
1
2
3
4
a
200
200
160
160
1 : 6
1 : 3
1 : 6
1 : 3
4.5 ¡ 0.4
5.1 ¡ 0.7
4.1 ¡ 0.5
6.1 ¡ 0.8
69.9
53.9
21.2
17.7
silver species remained in the IL reaction mixtures and could not
be extracted out from the mixture of [BMIM][Tf N] IL using
2
hexane. This control study further indicated the dual functions of
oleic acid.
The yield was estimated based on silver. After the reaction, the
particles were extracted using a designed amount of hexane. About
00 mL of the extracted product was transferred into an alumina pan
Besides the single-molecule precursor approach, the reduction of
a metal salt by a reducing agent was also examined. The latter
route can have broad ramifications because of the easy access to
various metal salts. Platinum was chosen for this demonstration, in
part because of its excellent catalytic property in the hydrogenation
1
in a thermogravimetric analyzer (TGA). The stable value at ambient
room temperature was the weight of particles with surfactants. The
residue weight obtained after heating the specimen to 800 uC under
N
2
flow was used to calculate the amount of silver nanoparticles in
the sample, and subsequently the yield of silver nanoparticles.
21
and other reactions.
2
546 | Chem. Commun., 2006, 2545–2547
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Fig. 5 (A) TEM image and (B) SAED of platinum nanoparticles.
show the potentials of using ionic liquids in the development of a
recyclable method for making metal nanoparticles.
We thank U.S. National Science Foundation (CAREER
Award, DMR-0449849 and SGER Grant, CTS-0417722), and
Environmental Protection Agency (EPA-STAR R831722) for
support.
Fig. 4 TGA traces of (A) pure [BMIM][Tf
mixture after the syntheses (B) with and (C) without the presence of oleic
acid. T : temperature onset for the thermal decomposition of various IL
mixtures.
2 2
N], the [BMIM][Tf N] IL
d
2
Platinum acetylacetonate (Pt(acac) ) was used as the Pt
precursor and 1,2-hexandecandiol was the reduction agent.
Besides oleic acid, oleylamine was used as the coordinate capping
agent, as it has been shown in previous studies that long chain
Notes and references
1
(a) M. Antonietti, D. B. Kuang, B. Smarsly and Z. Yong, Angew.
Chem., Int. Ed., 2004, 43, 4988; (b) J. G. Huddleston, A. E. Visser,
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Chem., 2001, 3, 156; (c) G. A. Baker, S. N. Baker, S. Pandey and
F. V. Bright, Analyst, 2005, 130, 800.
12
amine can help to stabilize Pt colloidal nanoparticles. The
reaction was conducted in [BMIM][Tf N] IL at 230 uC following a
similar procedure. The color of the reaction mixture turned bright
yellow after Pt(acac) and 1,2-hexandecandiol dissolved at about
5 uC. The color of this mixture subsequently turned brown at
2
2 Y. Zhou and M. Antonietti, J. Am. Chem. Soc., 2003, 125, 14960.
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2
7
4
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H. Itoh, K. Naka and Y. Chujo, J. Am. Chem. Soc., 2004, 126, 3026.
A. Taubert, Angew. Chem., Int. Ed., 2004, 43, 5380.
about 200 uC and became black when the reaction temperature
reached 230 uC.
Fig. 5A shows a representative TEM image of the Pt
nanoparticles obtained. The nanoparticles could also settle out
from the IL during the reaction. The particles were found having
an average diameter of 4.5 ¡ 0.8 nm with a standard size
deviation of less than 20%. The selected area electron diffraction
6 Y. J. Zhu, W. W. Wang, R. J. Qi and X. L. Hu, Angew. Chem., Int. Ed.,
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1
(
SAED) pattern indicated these particles were face centered cubic
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11 Y. Yin and A. P. Alivisatos, Nature, 2005, 437, 664.
Pt metal (Fm3m), Fig. 5B. The combination of oleic acid and
oleylamine was necessary in this synthesis. The reaction without
oleic acid resulted in large faceted and irregularly shaped crystals.
When only oleic acid was used, aggregates consisting of small
particles with diameter of y5 nm were obtained. These aggregates
could not be dispersed readily as individual nanoparticles using
hexane or other common solvents.
1
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In short, noble metal nanoparticles with relatively narrow size
distributions can be obtained using an imidazolium-based ionic
liquid as solvent. The combination of oleic acid with
20 P. J. Cowdery-Corvan and D. R. Whitcomb, in Handbook of Imaging
Materials, ed. A. S. Diamond and D. S. Weiss, Dekker Inc., New York,
[BMIM][Tf N] ionic liquid leads to an automatic separation of
2
2002.
colloidal metal nanoparticles from the IL mixtures through a
2
1 A. Wieckowski, E. R. Savinova and C. G. Vayenas, Catalysis and
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settling process. No obvious degradation in IL has been observed
based on the onset of the decomposition temperature. Our results
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 2545–2547 | 2547