Zero-valent iron nanoparticles preparation

Article abstract of DOI:10.1016/j.materresbull.2012.02.026

Zero-valent iron nanoparticles were synthesized by hydrogenating [Fe[N(Si(CH₃)₃)₂]₂] at room temperature and a pressure of 3 atm. To monitor the reaction, a stainless steel pressure reactor lined with PTFE and mechanically stirred was designed. This design allowed the extraction of samples at different times, minimizing the perturbation in the system. In this way, the shape and the diameter of the nanoparticles produced during the reaction were also monitored. The results showed the production of zero-valent iron nanoparticles that were approximately 5 nm in diameter arranged in agglomerates. The agglomerates grew to 900 nm when the reaction time increased up to 12 h; however, the diameter of the individual nanoparticles remained almost the same. During the reaction, some byproducts constituted by amino species acted as surfactants; therefore, no other surfactants were necessary.

Full text of DOI:10.1016/j.materresbull.2012.02.026

Materials Research Bulletin 47 (2012) 1478–1485
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Zero-valent iron nanoparticles preparation
a
b
a,
S. Oropeza a, M. Corea a, , C. Gomez-Yanez , J.J. Cruz-Rivera , M.E. Navarro-Clemente
*
*
´
´ ˜
a Instituto Polite´cnico Nacional, ESIQIE, UPALM, Edificio Z-6, Primer Piso, C.P. 07738, Col. San Pedro Zacatenco, Me´xico D.F., Mexico
b Universidad Auto´noma de San Luis Potosı´, Instituto de Metalurgia, Sierra Leona 550, San Luis Potosı´, C.P. 78210, Mexico
A R T I C L E I N F O
A B S T R A C T
Article history:
Zero-valent iron nanoparticles were synthesized by hydrogenating [Fe[N(Si(CH3)3)2]2] at room
temperature and a pressure of 3 atm. To monitor the reaction, a stainless steel pressure reactor lined
with PTFE and mechanically stirred was designed. This design allowed the extraction of samples at
different times, minimizing the perturbation in the system. In this way, the shape and the diameter of the
nanoparticles produced during the reaction were also monitored. The results showed the production of
zero-valent iron nanoparticles that were approximately 5 nm in diameter arranged in agglomerates. The
agglomerates grew to 900 nm when the reaction time increased up to 12 h; however, the diameter of the
individual nanoparticles remained almost the same. During the reaction, some byproducts constituted by
amino species acted as surfactants; therefore, no other surfactants were necessary.
Received 19 June 2011
Received in revised form 19 December 2011
Accepted 19 February 2012
Available online 28 February 2012
Keywords:
A. Organometallic compounds
A. Nanostructures
B. Chemical synthesis
C. Electron microscopy
ß 2012 Elsevier Ltd. All rights reserved.
1. Introduction
solution, but these techniques are often unable to yield simulta-
neous control over nanoparticle structure, surface chemistry,
Nanoparticles can be made up from most of elements of the
periodic table, and they can be classified based on their
components as organics [1,2], inorganics [3,4], metallic [5,6],
semiconductors [7], ionics [8] and moleculars [9].
monodispersity, crystal structure and assembly [23–25].
Among ‘‘bottom-up’’ methods, there are a great variety of
chemical methods for the synthesis of metallic nanoparticles in
which it is possible to control the shape, size distribution and
properties of nanoparticles with kinetics parameters such as the
temperature and, the concentration of stabilizing agents. These
methods include the thermolysis [26], hydrolysis [27] or reduction
[28] of precursors of organometallic and coordination compounds
[29–32]. Recently, several colloidal chemical synthetic procedures
have been developed to produce monodisperse nanoparticles;
these methods have a series of stages that take place in the liquid
phase [33] in the presence of a surfactant. First, metallic atoms can
be obtained by reducing precursors using chemical redactors in
solution [34,35]. In a second stage, the atoms begin to nucleate, and
these nuclei begin to grow slowly until they form nanoparticles
[36–38]. The composition and the size of the formed particles
depend on parameters such as the reaction time, the temperature
and the surfactant molecule length [39].
Recently, several colloidal chemical synthetic procedures have
been developed to produce zero-valent iron nanoparticles. One of
the most common procedures involves the decomposition of iron
pentacarbonyl, Fe(CO)5, to generate metallic iron atoms [40–43].
Other important organometallic precursors include the amido
complexes. These are easily accessible precursors used obtain
zero-valent iron nanoparticles; they decompose under hydrogen
pressure, and the resulting residue can serve as a surfactant that
surround the nanoparticles. An example of this type complex is bis
[bis(trimethylsylil)amido] iron (II), Fe[N(SiMe3)2]2. The decompo-
sition of this complex has been reported in solution at 150 8C under
Metallic nanoparticles are of particular interest the because of
their quantum size effects and their large surface areas.
Nanoparticles with magnetic properties, superparamagnetism
and quantum tunneling of magnetization, have received consider-
able attention [10–12] because they have a wide variety of
biomedical applications [13], such as their uses as contrast
enhancement agents for magnetic resonance imaging (MRI)
[14,15], bioprobes [16] and cell sorters [17]. Most of these types
of nanoparticles can enter the cell, and even the nucleus, and
interact with DNA and cellular proteins [18]. They have the
potential to improve therapies such as cancer drug delivery
[19,20]. Monodisperse nanoparticles of zero-valent iron are ideal
for these kinds of applications because they have a high saturation
magnetization at room temperature and a high reactivity [21].
Metallic nanoparticles can be obtained by ‘‘top-down’’ methods
that include the mechanical grinding of bulk metals and
subsequent stabilization of the resulting nanosize metal particles;
however, it is difficult to obtain narrow particle size distributions
in this way [22]. The ‘‘bottom-up’’ methods form nanoparticles by
starting from either atomic or molecular precursors in gas or
* Corresponding authors. Tel.: +52 5557296000x55396; fax: +52 5557296000.
E-mail addresses: mcoreat@yahoo.com.mx, mcorea@ipn.mx (M. Corea),
mnavarroc@ipn.mx (M.E. Navarro-Clemente).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2012.02.026

S. Oropeza et al. / Materials Research Bulletin 47 (2012) 1478–1485
1479
Fig. 2. Progress of the synthesis reaction of zero-valent iron nanoparticles as a
function of time.
Fig. 1. Scheme of the stainless steel reactor used during the reaction.
a hydrogen atmosphere at 3 bars of pressure over 48 h in the
presence of hexadecylamine and long chain acids. The results
showed zero-valent nanoparticles with a cubic shape and an
average diameter of 7 nm [44]. Moreover, the synthesis of Rh and
Fe nanoparticles has also been reported from the hydrogenation of
Rh(C3H3)3 and [Fe[N(Si(CH3)3)2]2], which produced core-shell
nanoparticles with an Rh core and a Fe shell. The average particles
size was 2.1 nm [45].
pressure of 3 atm. A stainless steel pressure reactor lined with PTFE
was adapted for the synthesis. Several samples were taken at
different times during the reaction to study the progress of the
reaction and the shape and the diameter of the nanoparticles,
during the entire process. The progress of the reaction was
determined by gravimetric methods. The results of the dynamic
light scattering of pure nanoparticles established that the average
diameter of the particles increased with the reaction time because
of the formation of aggregates of nanoparticles. The transmission
electron microscopy analysis confirmed a spherical shape and a
In this work, zero-valent iron nanoparticles were synthesized
by hydrogenating [Fe[N(Si(CH3)3)2]2] at room temperature and a
Fig. 3. Images of samples taken at (a) 0.5 and 1.5 h, (b) 3 h, (c) 8 h and (d) 12 h of reaction.

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S. Oropeza et al. / Materials Research Bulletin 47 (2012) 1478–1485
[Fe[N(Si(CH3)3)2]2] was obtained by means of the following
reaction [47]:
Et2
O
FeBr2ðTHFÞ2 þ 2LiN½SiðCH3Þ3ꢀ2ꢁ!Fe½NðSiðCH3ÞsÞ2ꢀ2 þ 2LiBr
þ 2THF
Fe[N(Si(CH3)3)2]2 was extracted and purified in pentane. In this
reaction, approximately 1.4 g (0.27 mmol) of compound was
obtained. The efficiency of the reaction was approximately 55%.
The Fe[N(Si(CH3)3)2]2 obtained was dissolved in 200 mL of
pentane. The solution was loaded into a stainless steel pressure
reactor. The reactor was pressurized to 3 atm of H2 at room
temperature, and the reaction time was 12 h. The iron nanopar-
ticles were produced by means of the following reaction:
pentene
Fe½NðSiMe3Þ2ꢀ þ H2 ꢁ! Fe0 þ 2HNðSiMe3Þ2
2
An austenitic 230 mL stainless steel reactor (Parr Instruments
Company model 452HC2-T316) was used to synthesize the iron
nanoparticles. The reactor was lined with PTFE, and it was
equipped with a mechanical stirrer connected to a Pitched Blade
Impeller, which was also lined with PTFE. For the feeding of the
reactants and the sample collection, the reactor was adapted
according to the scheme in Fig. 1.
Fig. 4. Z-average diameter as
a function of reaction time for unpurified
nanoparticles.
diameter less than 5 nm for the zero-valent iron nanoparticles
during all reaction times.
With this system, 9 samples of 9 mL were taken at different
reaction times to observe the growth of the nanoparticles. In all
cases, a black precipitate was produced. The particles formed were
transferred to a Schlenk flask and dried with a vacuum. The
samples were put in a vacuum chamber where they were stored in
glass ampoules and sealed at the temperature of liquid nitrogen to
prevent oxidation before characterization.
2. Experimental
2.1. Reagents
The hexamethyldisilazane (HN[Si(CH3)3]2) and the iron bromine
(II) (FeBr2) (from Aldrich) were reagent grade, and they were used as
received. The n-butyl lithium (C4H9Li) was placed in 2.5 M hexane
(from Aldrich). The THF (>99.0%), diethyl ether (99.9% anhydrous)
and pentane (98%) (from Aldrich) were distilled, dried in the
presence of CaH2, degassed and stored in an anaerobicchamber [46].
The precursor of the iron particles, bis[bis(trimethylsilyl)amido]ir-
on(II) [Fe[N(Si(CH3)3)2]2], was used as received [47].
2.3. Gravimetry
The progress of the zero-valent iron nanoparticles synthesis
reaction was calculated by gravimetric techniques. From each
sample, 3.5 mL of reaction solution was extracted. The extraction
was made with a syringe that had a filter on the needle so that the
iron nanoparticles were not extracted. Five samples (0.2, 0.4, 0.6,
0.8 and 1 mL) from each resulting solution were placed in a 1 mL
vials with rubber caps and aluminum seals saturated with
nitrogen. The vials had been weighed previously. The precursor
was then separated from the reaction solution by evaporating the
pentane. The precursor was quantified to determine the conver-
sion as a function of the reaction time.
2.2. Synthesis of nanoparticles
The reactions for the synthesis of nanoparticles were carried
out as
a batch process. Bis[bis(trimethylsilyl)amido]iron(II)
Fig. 5. (a) Transmission electron micrograph and (b) EDX spectra for the sample taken at 1.25 h.

S. Oropeza et al. / Materials Research Bulletin 47 (2012) 1478–1485
1481
Fig. 6. Particle size distribution of the sample of purified iron nanoparticles taken at 12 h measured at intervals of (a) 3 min, (b) 6 min, (c) 9 min and (d) 12 min.
2.4. Dynamic light scattering (DLS)
2.5. Electron diffraction (ED), transmission electronic microscopy
(TEM), X-ray energy dispersive (EDX) spectroscopy
The hydrodynamic diameter of the iron nanoparticles was
determined using dynamic light scattering (DLS). Samples of
unpurified and purified nanoparticles were measured at 25 8C,
using a Malvern Zetasizer Nano ZS instrument. To prevent the
oxidation of nanoparticles, a glass cell with a Shclenk valve type
was used, which was designed and made for this propose. The
nanoparticles were purified with pentane washes. The purified
particles were re-dispersed in pentane with Auto Science model
AS5150B ultrasound equipment. The particle size distribution was
calculated using the software provided by Malvern in the light
scattering equipment.
The size and morphology of zero-valent iron particles were
determined using a JEOL JEM 1230 microscope at 100 kV, which
was coupled to an EDX model EDS7215, SPEC:143 eV with an
INCA model ATW2 window. The sample was purified with
pentane washes and placed onto carbon-coated copper grids
inside a vacuum chamber. The samples were added to glass
ampoules saturated with nitrogen to prevent oxidation. The
presence of Fe0 and the qualitative analysis characteristic of the
nanoparticles were determined by means of X-ray energy
dispersive spectroscopy at 100 kV and in the energy range
Fig. 7. Particle size distribution by DLS for samples taken at (a) 2 h, (b) 5 h and (c) 8 h of reaction in the synthesis of zero-valent iron nanoparticles.

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S. Oropeza et al. / Materials Research Bulletin 47 (2012) 1478–1485
emitted by the Fe is 6.403 eV for the K-alpha emission and
0.776 eV for the L-alpha emission.
3. Results and discussion
The preparation process for the zero-valent iron nanoparticles
was modified from the previously reported methods [44,48,49].
The first modification was that the reaction was carried out in a
PTFE-lined stainless steel reactor and that had a mechanical stirrer.
This system allowed to take samples at different reaction times;
samples were taken at 0.5, 1.25, 2, 3, 4, 5, 6, 8, 10 and 12 h to follow
the conversion, growth and shape change of the nanoparticles. A
second modification of the commonly reported process entailed
pressurizing the reaction system to 3 atm with hydrogen at room
temperature. Finally, during the reaction, some amino ligands
were produced as byproducts, and these amino ligands stabilized
the system [45] hence, it was not necessary to use a surfactant,
which influences the formation of small nanoparticles.
The percentage of the conversion of nanoparticles, Np, in each
sample was calculated using Eq. (1):
Fig. 8. Z-average size of purified nanoparticles as a function of reaction time.
mp
Np ¼ 1 ꢁ
ꢂ 100
(1)
Fig. 2. The conversion of nanoparticles increased with the reaction
time and reached 100% conversion at 12 h.
ms
Images of the samples taken at different reaction times are
shown in Fig. 3. After 30 min it was possible to observe that a
portion of the coordination compound had reacted with hydrogen,
and the color of the solution changed from green to black,
indicating that the decomposition of the Fe precursor and the
formation of zero-valent iron nanoparticles had occurred.
where Np is the conversion of nanoparticles, mp, is the precursor
mass and ms, is the solution mass without nanoparticles.
The conversion of zero-valent iron nanoparticles was obtained
by taking into account that the formation of nanoparticles had a
1:1 stoichiometry. In this way, the results of the conversion of zero-
valent iron nanoparticles as a function of time are presented in
Fig. 9. Transmission electron micrographs of zero-valent iron nanoparticles for (a) 2 h, (b) 5 h and (c) 12 h of reaction time.

S. Oropeza et al. / Materials Research Bulletin 47 (2012) 1478–1485
1483
From the results of the measurements of dynamic light
scattering (DLS) of the unpurified and purified samples of
nanoparticles taken at different reaction times, different particle
diameter moments (number average diameter, Dn; weight average
diameter, DW; and Z-average diameter, DZ) were calculated using
Eqs. (2)–(4), and the polydispersity index (PDI) was determined
using Eq. (5) [50]:
These results were confirmed by means of transmission
electron microscopy. Fig. 5 shows a micrograph and the EDX
spectra of the sample taken 1.25 h after the reaction was initiated.
A
micelle aggregate swollen with Fe[N(Si(CH3)3)2]2 can be
observed, and the spectra confirm the presence of Si. This figure
demonstrates that, at this reaction time, the conversion to
nanoparticles was less frequent.
The samples of iron nanoparticles were purified and re-
dispersed in pentane. The re-dispersed particles were measured
by dynamic light scattering at intervals of 3, 6, 9 and 12 min. The
results of the particle size distribution at these intervals for the
sample taken at 12 h are shown in Fig. 6.
The particle size distributions obtained by DLS for purified
samples of zero-valent iron nanoparticles at 2, 5 and 12 h of
reaction are shown in Fig. 7 as examples of the samples taken
during all reaction times for which the average size was 239, 324
and 850 nm, respectively. All the nanoparticles samples presented
high polydispersity because the polydispersity value was larger
than 1.
The results of the Z-average size as a function of the reaction
time for the purified zero-valent iron nanoparticles are shown in
Fig. 8. The volume taken from each sample was always the same. As
the reaction time increases, the average size also increases up to
values of approximately 900 nm; however, these large entities
consisted of aggregates because the sample was purified. These
results suggest that, as the reaction progressed, more and more
particles were produced, and causing the aggregate to become
enlarged. This hypothesis was confirmed with transmission
electron microscopy images, which are shown in Fig. 9.
P
niDi
P
niDi4
niDi3
Dn
¼
(2)
(3)
ni
P
DW
¼
P
P
P
niDi5
niDi4
DZ
¼
(4)
(5)
Dn
PDI ¼
DW
where ni is the number of nanoparticles with diameter Di.
The results of the Z-average diameters as a function of the
reaction time the unpurified nanoparticles are shown in Fig. 4. The
iron nanoparticles produced had an average diameter smaller than
10 nm, except for the samples taken at 1.25 h and 12 h. This
discrepancy can be explained by the observation that the
byproduct produced during the reaction forms micelles that are
swollen with the precursor.
Fig. 10. Transmission electronic micrographs of zero-valent iron nanoparticles after (a) 2 h, (b) 5 h and (c) 12 h of reaction time.

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S. Oropeza et al. / Materials Research Bulletin 47 (2012) 1478–1485
Fig. 11. The EDX spectrum and the ED pattern for zero-valent iron nanoparticles.
The typical images obtained by transmission electron micros-
Acknowledgement
copy of pure nanoparticles collected after 2, 5 and 12 h of reaction
are presented in Fig. 8 to illustrate the formation of zero-valent
iron nanoparticles.
We would like to thank to Consejo Nacional de Ciencia y
´
Tecnologıa (CONACyT) for the student grant during two years.
The nanoparticles began to aggregate as the reaction pro-
gressed. The images show that the aggregates are formed by
nanoparticles with spherical shapes.
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lower material exchange rate, resulting in the formation of a
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shapes [52].
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