Use of long chain amine as a reducing agent for the synthesis of high quality monodisperse iron(0) nanoparticles

Article abstract of DOI:10.1039/c1jm12127h

This article reports the synthesis of iron(0) nanoparticles at moderate temperature - from 120 °C to 150 °C - using the reduction of the organometallic iron(ii) precursor {Fe[N(SiMe₃)₂] ₂}₂ by hexadecylamine (HDA) in the absence of dihydrogen (H₂). The nanoparticles are monodisperse in size and self-assemble into 2D super-lattices suitable for transport measurements. The nanoparticles are stabilized in mesitylene by a mixture of HDA and hexadecylammonium chloride (HDA·HCl). The resulting truncated single-crystalline nanocubes have a narrow size distribution and a high magnetization close to the bulk value. The products are characterized by transmission electronic microscopy (TEM and HRTEM), SQUID measurements, Moessbauer and Infra-Red spectroscopies. Fe(ii) reduction is accompanied by oxidation of amines into imines which was detected as a by-product. This reduction occurs at 120 °C and above. The temperature, in conjunction with the reaction time, allows for a fine control of the nano-objects final size. The latter can also be tuned with the HDA·HCl concentration. Finally, this one-pot synthesis produces high-quality magnetic nanoparticles with mean sizes in the range 6 to 10 nm depending on the conditions.

Full text of DOI:10.1039/c1jm12127h

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PAPER
Use of long chain amine as a reducing agent for the synthesis of high quality
monodisperse iron(0) nanoparticles
a
a
Anca Meffre, Sebastien Lachaize, Christophe Gatel,b Marc Respauda and Bruno Chaudreta
*
ꢀ
Received 13th May 2011, Accepted 28th June 2011
DOI: 10.1039/c1jm12127h
This article reports the synthesis of iron(0) nanoparticles at moderate temperature—from 120 ꢀC to 150
ꢀC—using the reduction of the organometallic iron(II) precursor {Fe[N(SiMe3)2]2}2 by hexadecylamine
(HDA) in the absence of dihydrogen (H2). The nanoparticles are monodisperse in size and self-assemble
into 2D super-lattices suitable for transport measurements. The nanoparticles are stabilized in
mesitylene by a mixture of HDA and hexadecylammonium chloride (HDA$HCl). The resulting
truncated single-crystalline nanocubes have a narrow size distribution and a high magnetization close
to the bulk value. The products are characterized by transmission electronic microscopy (TEM and
€
HRTEM), SQUID measurements, Mossbauer and Infra-Red spectroscopies. Fe(II) reduction is
accompanied by oxidation of amines into imines which was detected as a by-product. This reduction
occurs at 120 ꢀC and above. The temperature, in conjunction with the reaction time, allows for a fine
control of the nano-objects final size. The latter can also be tuned with the HDA$HCl concentration.
Finally, this one-pot synthesis produces high-quality magnetic nanoparticles with mean sizes in the
range 6 to 10 nm depending on the conditions.
Due to their potential interest in many application fields, iron(0)
nanoparticles (NPs) well-dispersed in a common solvent and
with mean sizes that cover the range from 1 to 20 nm are highly
desirable. Indeed, metallic iron has a high magnetization at room
temperature, a superparamagnetic behaviour for sizes ranging
from 1 to 12 nm and a ferromagnetic behaviour above those.
These properties, from a physical point of view, make it a mate-
rial of choice for biomedical applications such as magnetic fluid
hyperthermia (MFH) and magnetic resonance imaging (MRI).1
Furthermore, their relatively high reactivity and low cost make
iron(0) NPs ideal candidates for catalysis in a non-oxidizing
environment (e.g. formation or cleavage of stable bonds).1 Iron
NPs can also be interesting building blocks for applications in
electronic and spin-dependent tunnelling devices too. In the field
of spintronics, only two groups have measured a magneto-
resistive response in assemblies of ferromagnetic metallic NPs.2
The main difficulty is related to the unclean surface state, where
the oxidation leads to an insulating surface with a high degree of
spin disordering. Well controlled elaboration routes under
preserved atmosphere are thus necessary, the stabilizing ligands
should preserve the surface spin polarisation. Moreover, the
control of the particle growth to get real monodisperse size
distributions is a key parameter to get long-range well ordered
two dimensional and three dimensional assemblies. However,
partly due to its high sensitivity to oxidation and despite
a number of efforts recently reported in the literature, synthesis
of such full metallic Fe nano-objects remains a challenge.
The two well-known synthesis routes to iron(0) NPs are the
thermal decomposition and the chemical reduction of an iron
molecular complex. In the thermal decomposition method, the
most popular precursor is Fe(CO)5.3 For example, upon sonol-
ysis of Fe(CO)5 in the presence of polyvinylpyrrolidone or oleic
acid, amorphous iron nanoparticles are produced in good yields.
Similarly, Hyeon et al. have described the synthesis of elongated
bcc iron NPs, but with a magnetization either depleted or not
reported. Farell and coworkers reported a high temperature
decomposition of Fe(CO)5 in the presence of oleic acid and/or
oleylamine leading to core/shell Fe/Fe3O4 NPs of controlled size
between 7.0 and 11.2 nm and a magnetization of up to 200 A m2
kgꢁ1 in spite of the oxide shell. More recently, Yang reported
a large scale synthesis of iron NPs (from 5.8 to 9.3 nm) by
decomposition of Fe(CO)5 at 180 ꢀC, with a magnetization of
150 A m2 kgꢁ1 for the final product. In summary the thermal
decomposition route can quite easily provide Fe or Fe/Fe3O4
NPs with a rather good size control between 5 and 11 nm. Their
structure is either amorphous or bcc single-crystalline and their
magnetization can be as high as 200 A m2 kgꢁ1. However, as far
as we know, none of the procedures using Fe(CO)5 entirely fulfil
the needs defined above in terms of size range coverage and of
aLPCNO, Universiteꢀ de Toulouse, INSA/UPS/CNRS, F-31077 Toulouse
Cedex 04, France. E-mail: sebastien.lachaize@insa-toulouse.fr; Fax: +33
561559697; Tel: +33 561559663
bCEMES—CNRS, 29 avenue Jeanne Marvig, F-31055 Toulouse Cedex 04,
France. E-mail: christophe.gatel@cemes.fr; Fax: +33 562257999; Tel: +33
562257893
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magnetization. Concerning the chemical reduction, our group
has been investigating the synthesis of pure metallic iron NPs
from bis(amido)iron(II) complexes for over 8 years.4 The complex
Fe[N(SiMe3)2]2(THF) (Me ¼ CH3, THF ¼ tetrahydrofuran) and
then the dimer {Fe[N(SiMe3)2]2}2 were chosen as iron sources
since they are reduced by H2 under mild conditions. The first iron
(0) nanocube superlattices reported by Dumestre and coworkers
in 2004 have clearly demonstrated the potential of this strategy to
form single-crystalline and monodispersed NPs with the
magnetization of bulk iron(0). Preliminary results were obtained
with oleic acid/HDA or HDA$HCl/HDA as surfactant mixture.
This article was the starting point of our study aiming at
controlling the size and shape of these objects on a larger range.
By using a mixture of palmitic acid/HDA, we were then able to
produce either ultra small Fe(0) clusters of ca. 1.5 nm, poly-
crystalline spherical NPs with a mean diameter between 5 and
10 nm, or single-crystalline nanocubes over 13 nm which were
again self-organised into micrometric superlattices. In all cases,
the magnetization of the final iron NPs was higher than 210 A m2
kgꢁ1 at 2 K (i.e. very close to bulk iron). Despite our efforts, we
were however still facing a size limit of 11 nm for well-dispersed
NPs (generally the spherical ones). While looking for a solution
by exploring the nature of the reducing agent, we found a dihy-
drogen free synthesis which would keep the high magnetization
associated with metallic iron and would allow tuning the NPs size
within a quite large range.
In a typical synthesis of iron NPs, HDA$HCl (415.5 mg, 1.5
mmol) and HDA (483 mg, 2 mmol) were added to a green
solution of {Fe[N(SiMe3)2]2}2 (376.5 mg, 0.5 mmol) in mesity-
lene (20 mL) and the mixture was heated at 150 ꢀC for two days
under vigorous magnetic stirring. The solvent was then removed
under vacuum and the collected black powder was washed with
toluene. It yielded a material that contained 85% of iron; the
isolated yield was 76% (50 mg). The iron content was obtained by
ICP-MS analysis. Each synthesis reported in this article was
reproduced at least 3 times with a deviation of the final NPs mean
size always lower than the size distribution width.†
Characterization of the NPs
The size, the morphology and the structure of as-synthesized
samples were measured by TEM and high-resolution trans-
mission microscopy (HRTEM). Conventional bright-field
images were performed using JEOL microscopes (Model 1400F
or Model 1011F) working at 100 kV and HRTEM was con-
ducted on a FEI Tecnai-F20 microscope working at 200 kV. The
samples were prepared by solution drop deposition onto
a carbon-coated copper grid in the glove box. However, the
observed nanoparticles were partially oxidized due to the air
exposure during the transfer into the microscope chamber. Size
histograms were obtained by an automatic counting process over
more than 1000 particles,6 and size distributions were fitted by
the Gaussian law. The results are expressed in terms of the
calculated mean size and the standard deviation (s).
In this paper we report on a high yield method to synthesize
colloidal solutions of single-crystalline iron(0) NPs which are
very uniform in size and shape and self-assemble into a 2D super-
lattice. Here the marked difference from our previous work is the
absence of any dihydrogen to ensure the iron(II) precursor
reduction. The {Fe[N(SiMe3)2]2}2 reduction is indeed possible at
relatively low temperature and under argon thanks to the
replacement of the previously used carboxylic acid (oleic acid or
palmitic acid) by the ammonium chloride HDA$HCl. In addi-
tion to its surfactant role, we provide evidence that the amine
also plays the role of reducing agent.
The magnetization and the coercive field of as-synthesized iron
NPs were measured on a Quantum Design Model MPMS 5.5
SQUID magnetometer. The absolute magnetization was
deduced from the iron(0) content. The iron state and its envi-
€
ronment were analyzed by Mossbauer spectroscopy (WISSEL,
57Co source). These studies were carried out on powder samples
that were prepared and sealed under an argon atmosphere to
preserve the metallic character of iron. Extreme care was taken to
avoid oxidation during the transfer to apparatus.
FTIR spectra were recorded using a Perking–Elmer FTIR
spectrometer Paragon 100. A few mg of the powder were
deposited on the diamond-anvil cell without any particular
precaution.
Experimental methods
Synthesis
All the syntheses are performed under an argon atmosphere by
using Fischer–Porter bottles techniques, a glove box and argon/
vacuum lines. Mesitylene (99%, VWR Prolabo), toluene (99%,
VWR Prolabo), tetrahydrofuran (THF, 99% VWR Prolabo) and
pentane (99%, VWR Prolabo) were dried according to general
procedures. The solvents were distilled and degassed prior use.
Hexadecylamine (HDA, 99%) and hydrochloric acid in dieth-
ylether (HCl, 1 mol Lꢁ1) were purchased from Sigma Aldrich and
used without any additional purification; the bis(amido)iron(II)
dimer {Fe[N(SiMe3)2]2}2 was purchased from Nanomeps. The
hexamethylammonium chloride (HDA$HCl) was prepared
according to a published procedure.5 Addition of the HCl in
diethylether (15 mL, 1.5 mmol of HCl) to a pentane solution (50
mL) of HDA (241.5 mg, 1 mmol) led to the immediate precipi-
tation of a white solid that was filtered off, washed three times
with THF (15 mL) and dried under vacuum overnight. The yield
was 82% (230 mg).
Results and discussion
We first present the temperature influence on the produced
nanoparticles as well as their full characterization. Then we
describe the evidence for the iron(II) reduction assisted by the
primary amine and the kinetic study of a typical reaction by
€
Mossbauer spectroscopy. The influence of the acid concentration
on the mean size of the NPs is finally reported.
Effect of the temperature
In this first series of experiments we report the effect of the
temperature on the reaction of 0.5 mmol of {Fe[N(SiMe3)2]2}2 in
mesitylene with 1.5 equivalent per iron atom of HDA$HCl and 2
† The reproducibility was especially evidenced on one synthesis
performed more than 20 times (HDA$HCl/HDA ¼ 1.5 : 2, 135 ꢀC, 4
days). The mean sizes always were within one reference distribution.
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ꢀ
equiv. of HDA. After 4 days at 150 C, it produced Fe(0) NPs
with an average diameter of 10.4 nm and a narrow size distri-
bution (s ¼ 0.9 nm). Fig. 1a shows a typical transmission elec-
tron micrograph of the centrifuged material. These NPs
spontaneously form a highly ordered arrangement on the TEM
grid, either cubic or hexagonal, as a result of their narrow size
distribution. The well-defined inter-particles spacing of about
2 nm is correlated to the iron surface stabilizing the organic shell
composed of HDA$HCl and HDA.
More precise analyses indicate that the NPs adopt an octa-
hedral shape analogous to Wulff polyhedra of bcc structure with
facets making 135ꢀ angles between them (see Fig. 1b and the
corresponding inset showing High-Resolution TEM).7
The largest sides are elongated along the h100i directions and the
smallest ones along the h110i directions. The bcc structure
corresponds to the one of bulk iron.
Fig. 2 Characterizations of the NPs obtained after 3 days at 150 ꢀC by
SQUID which ascertain their metallic state and bulk magnetization.
From a magnetic point of view, these particles display a high
magnetization (212 (ꢂ10%) A m2 kgꢁ1 at 2 K and 202 (ꢂ10%) A
m2 kgꢁ1 at 300 K) and a ferromagnetic behaviour up to room
temperature as a result of magnetic dipolar interactions (Hc ¼
14.5 mT at 2 K and Hc ¼ 3.4 mT at 300 K, see Fig. 2). No
exchange phenomenon has been evidenced on the magnetization
curve measured after a field cooling process at 2 K. All these
features agree with the absence of any ferrimagnetic or spin glass
layer related to an oxide shell.
only 2 days at 150 ꢀC, the average diameter of nanoparticles
being of 8.7 nm (s ¼ 0.5 nm) (see Fig. 4d). Keeping the solution
at 150 ꢀC for 2 extra-days actually only results in the ripening of
these NPs that grow up to 10.4 nm with a related size de-focal-
€
ization (s ¼ 0.9 nm). The Mossbauer study also confirms that
complete Fe reduction into metallic Fe NPs can be reached at
lower temperature when increasing the reaction time. Thus, in
a reaction performed at 135 ꢀC, the Fe(0) contribution is
90 (ꢂ5)% after 4 days; at 120 ꢀC, it is 80 (ꢂ5)% after 7 days (see
Fig. 3).
Since the absence of a reducing atmosphere could lead to an
incomplete reduction of iron, the pure metallic nature of the Fe
€
NPs was also confirmed by Mossbauer spectroscopy studies. As
However, the reduction kinetics is drastically slowed down at
105 ꢀC since we observed very few NPs on the TEM grid and no
shown in Fig. 3, the spectra recorded at 5 K on the raw powder
are composed only of a single sextuplet the parameters of which
being similar to bulk iron. The reduction is actually total after
€
Fe(0) contribution was detected by Mossbauer spectroscopy,
even after 7 days (see Fig. 3 and 4a). An average diameter of
10.4 nm (s ¼ 0.8 nm) was obtained for the particles produced at
€
Fig. 3 Mossbauer spectra of the iron nanoparticles prepared after (a) 7
Fig. 1 (a) TEM and (b) HRTEM images of the iron nanoparticles
prepared at 150 ꢀC after 4 days.
days at 105 ꢀC; (b) 7 days at 120 ꢀC; (c) 4 days at 135 ꢀC and (d) 2 days at
150 ꢀC.
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Fig. 4 TEM images of the iron nanoparticles prepared after (a) 7 days at
105 ꢀC; (b) 7 days at 120 ꢀC; (c) 4 days at 135 ꢀC and (d) 2 days at 150 ꢀC.
135 ꢀC (4 days) (see Fig. 4c), while NPs of 10.7 nm (s ¼ 0.5 nm)
(see Fig. 4b) are formed at 120 ꢀC after 7 days. Since the reaction
time was adjusted to reach comparable reaction advancement
states, working at 135 ꢀC or 120 ꢀC does not affect much the sizes
and morphologies of the NPs. In both cases, we obtained single-
crystalline NPs as ascertained by HRTEM and a magnetization
Fig. 5 FTIR spectra of HDA and HDA$HCl; then the reaction row
mixture obtained after 7 days at 105 ꢀC, 6 h at 150 ꢀC, and 4 days at
150 ꢀC.
of up to 200 A m2 kgꢁ1
.
€
reduction of iron(II) species to form iron(0). This Mossbauer
spectrum showed more contributions corresponding to at least
three intermediate species. The ratio of these three contributions
evolves with time toward the formation of the sole iron(0) after
2 days.
Reduction mechanism
It is noteworthy that this method does not require the use of any
additional reducing agents such as dihydrogen for the formation
of iron(0) NPs. In this reaction, we propose that the long-chain
amine HDA can serve as both reducing agent and surfactant, the
former reaction leading to the formation of an imine. Such long
chain amines are generally used for the synthesis of noble metal
NPs, which present a very low reduction potential.8 Carenco and
coworkers have however recently extended this chemical reduc-
tion to the case of Ni(II) precursor, using oleylamine to form Ni
(0) NPs.9 M. Chen and co-workers have used FTIR spectroscopy
to investigate the growth mechanism of silver NPs capped by
oleylamine.9 In the present work, we combine FTIR and
ꢀ
TEM pictures (Fig. 7) obtained at 150 C after 6 hours reac-
tion time display some 6.5 nm nanoparticles, but also very small
spherical particles around them (<2 nm). After 1 day, some very
small NPs were still present, but the sample was predominantly
composed of larger and monodispersed spherical nanoparticles
with a mean diameter of about 7.7 nm (see Fig. 7). After 2 days,
we finally observed octahedral nanocrystals with a narrow size
distribution centred at 8.3 nm (s ¼ 0.6 nm) (see Fig. 4d).
Interestingly, the FTIR spectra recorded on the sample
prepared at 105 ꢀC for 7 days do not display the imine absorption
band. This is consistent with the fact that no metallic ferro-
€
Mossbauer spectra with TEM images recorded for different
€
magnetic iron contribution is detected on the Mossbauer spec-
reaction conditions, to probe the oxidative transformation of the
primary amine into imine and the reduction of the iron precursor
into iron(0) nanoparticles. At 150 ꢀC, the oxidation of the
primary amine into imine was evidenced within the first 6 h of the
reaction together with the reduction of the iron precursor into
trum and agrees with a very slow reduction kinetic at that
temperature (see Fig. 3 and 4a). In summary, these data are
consistent with the fact that HDA is the reducing agent which is
€
iron(0). Fig. 5 and 6 show the FTIR and the Mossbauer spectra
of representative samples obtained for reactions carried out at
150 ꢀC for different reaction times, and at 105 ꢀC during 7 days.
The FTIR reference spectra of HDA and HDA$HCl are shown
for comparison. Interestingly, all FTIR spectra collected on the
samples prepared at 150 ꢀC contained an absorption band at
1670 cmꢁ1 that can be associated to the stretching mode of an
imine group. However, we have not detected any stretching band
corresponding to a cyanide group as reported by Chen and co-
workers. This suggests that the oxidation process of HDA at
150 ꢀC occurs rapidly and is limited to the formation of imines in
our conditions.
€
As shown by Mossbauer spectroscopy (see Fig. 6), the
€
Fig. 6 Mossbauer spectra of the reaction row mixture obtained after 7
ꢀ
oxidation of HDA after 6 h of reaction at 150 C has led to the
days at 105 ꢀC; 6 h at 150 ꢀC; 24 h at 150 ꢀC and 48 h at 150 ꢀC.
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Fig. 7 TEM images of the reaction row mixture obtained after (a) 2 h at
150 ꢀC; (b) 6 h at 150 ꢀC; and (c) 24 h at 150 ꢀC.
Fig. 8 TEM images of iron nanoparticles obtained at 150 ꢀC with a ratio
HDA$HCl/HDA of (a) 0.5 : 2 for 7 days; (b) 1 : 2 for 2 days; and (c)
1.5 : 2 for 2 days.
dehydrogenated into an imine while the iron(II) species are
reduced. The HDA concentration was tuned within the range 2
to 6 equiv. while keeping the HDA$HCl at 1.5 equiv., but no
significant change on the final size and shape was observed after 4
Conclusions
ꢀ
In conclusion, this paper reports a facile and efficient synthesis of
single crystalline bcc iron(0) NPs displaying a very narrow size
distribution and a saturation magnetization similar to bulk iron.
The reaction simply involves the reduction of the dimer {Fe[N
(SiMe3)2]2}2 by a primary amine under an inert atmosphere to
form iron(0) NPs in 90% yield together with an imine. This
reducing agent does not affect the NPs surface and preserve their
magnetization close to the bulk one. At 150 ꢀC, an increase of the
hexadecylammonium chloride concentration leads to an increase
of the mean size of the NPs within the range 6.9 nm to 9.5 nm.
Reaction parameters like time and temperature can also be used
to control the size of the nanoparticles. Finally, this simple
method provides promising high quality magnetic nanoparticles
for use in magnetoresistive devices.
days at 150 C. This suggests that the rate-limiting step of the
reduction does not involve free HDA but should rather concern
one or two of the intermediate iron(II) species observed in the
€
Mossbauer spectra. A dedicated kinetic study will be performed
to quantify the role of the reducing agent and to determine the
nature of these key intermediates. Whether or not the chloride
anions play a role by themselves in this process was not evidenced
but they are probably involved in the composition of the inter-
mediate species as explained below.
Effect of the acid concentration
The HDA$HCl concentration clearly influenced the mean size of
the final product. Keeping the HDA concentration at 2 equiva-
lents with respect to the molecular iron, we varied the acid
concentration from 0.5 to 1.5 equiv. (temperature and reaction
time were 150 ꢀC and 7 days respectively). At the lowest acid
concentration, we obtained spherical nanoparticles of 6.9 nm
(s ¼ 0.7 nm) (see Fig. 8a). Increasing the acid concentration led
to NPs of 8.3 nm (1.0 equiv, s ¼ 0.8 nm) (see Fig. 8b) and
10.4 nm (1.5 equiv, s ¼ 0.9 nm) (see Fig. 8c).
Acknowledgements
ꢀ
We thank the InNaBioSante foundation and the Conseil
ꢀ
Regional de Midi-Pyrenees for financial support, and A. Mari
ꢀ ꢀ
€
for SQUID measurements, J.-F. Meunier for Mossbauer exper-
ꢁ
iments and V. Colliere for HRTEM.
The reaction of the iron(II) precursor with HDA$HCl to yield
more stable aminochloroiron(II) species favoured the growth at
the expense of the nucleation, hence yielding larger NPs. Such
a behaviour has previously been observed when using long chain
carboxylic acid and dihydrogen as a reducing agent.4
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