Synthesis of zero-valent iron nanoparticles via laser ablation in a formate ionic liquid under atmospheric conditions

Article abstract of DOI:10.1039/C8CC03350A

Transition metal nanoparticles (NPs) are promising materials for use as catalysts in many processes, although they are easily oxidized under ambient conditions. In this communication, a novel synthetic method is proposed for producing zero-valent iron (Fe) NPs by laser ablation under atmospheric conditions using the reducing properties of a formate-based ionic liquid solvent. The valence state of Fe was confirmed using X-ray absorption near edge structure (XANES) spectroscopy. The Fe NPs adopt a face centered cubic structure after synthesis, which gradually transforms to a body centered cubic structure after one month. The method can be extended to the synthesis of other transition metal NPs that are easily oxidized.

Full text of DOI:10.1039/C8CC03350A

ChemComm
View Article Online
View Journal
COMMUNICATION
Synthesis of zero-valent iron nanoparticles via
laser ablation in a formate ionic liquid under
atmospheric conditions†
Cite this:DOI: 10.1039/c8cc03350a
Received 25th April 2018,
Accepted 20th June 2018
a
b
c
c
Shinya Okazoe, Yoshiro Yasaka, Masaki Kudo, Hiroshi Maeno,
cd
a
Yasukazu Murakami and Yoshifumi Kimura
*
DOI: 10.1039/c8cc03350a
rsc.li/chemcomm
5
Transition metal nanoparticles (NPs) are promising materials for use
Fe NPs are used in the dehalogenation of pollutants and the
6
as catalysts in many processes, although they are easily oxidized hydrogenation of harmful chemicals. Their magnetic properties
under ambient conditions. In this communication, a novel synthetic also are of interest to physical chemists. Improving the oxidation
method is proposed for producing zero-valent iron (Fe) NPs by laser resistance of Fe NPs is important, because these materials are
ablation under atmospheric conditions using the reducing properties easily oxidized upon exposure to atmospheric oxygen or water.
of a formate-based ionic liquid solvent. The valence state of Fe Fe NPs typically are synthesized and stored under nitrogen or
was confirmed using X-ray absorption near edge structure (XANES) argon to prevent oxidation. For example, Fe NPs have been
3
+
spectroscopy. The Fe NPs adopt a face centered cubic structure after synthesized by reducing Fe with sodium borohydride in
7
synthesis, which gradually transforms to a body centered cubic solutions that have been deoxygenated by an inert gas.
structure after one month. The method can be extended to the Special treatments also are required to prevent oxidation after
synthesis of other transition metal NPs that are easily oxidized.
preparation. As an example, X-ray diffraction and X-ray absorp-
tion spectroscopy samples must be prepared under anaerobic
7
Among the transition metal nanoparticles (NPs) employed in many conditions.
1
fields, those composed of noble metals such as platinum,
Fe NPs have been coated with silicon dioxide to overcome
2
3
8
rhodium, and palladium are widely used as catalysts. The their susceptibility to oxidation. Although this procedure
materials are stable under atmospheric conditions and have increases resistance to oxidation, the reactive surface is blocked
high catalytic activity. However, the expense of noble metals is a by the protecting layer. In this communication, we produce
significant drawback in commercial applications. Base metals zero-valent Fe NPs by laser ablation of metallic Fe in an ionic
are much more abundant and are possible substitutes for noble liquid (IL) that provides a reducing environment. Ionic liquids
metals. Although base metals are widely used in bulk materials, (ILs) are low-melting-point salts composed of organic cations
their use in NPs is not popular. Base metal NPs are easily oxidized and anions. ILs have unique properties including high ionic
9
at the nanoscale and form an oxidized shell under atmospheric conductivity and negligible vapor pressure. The properties of
4
conditions, which reduces activity. If this oxidation can be ILs can be modulated by changing their cationic and anionic
prevented, the applicability of base metal NPs can be expanded. constituents. We have selected the combination of a phosphonium
In this report, we describe the preparation of zero-valent iron (Fe) cation and a formate anion, each of which plays an independent
NPs via a method that prevents their nanoscale oxidation.
role. The phosphonium cation acts as a stabilizer to prevent
nanoparticle aggregation, and the formate anion serves as a
reducing reagent. ILs have previously been used to protect metal
a
Department of Applied Chemistry, Graduate School of Science and Engineering,
10
NPs during syntheses by means of chemical reduction, metal
Doshisha University, Kyotanabe, Kyoto 610-0321, Japan.
E-mail: yokimura@mail.doshisha.ac.jp
1
1
12
vapor deposition,
thermal decomposition,
and laser
1
3
b
ablation. One of the present authors has reported the synthesis
of gold (Au) NPs via laser ablation in imidazolium-based ILs.
Department of Molecular Chemistry and Biochemistry, Faculty of Science and
1
3a
Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan
c
The Ultramicroscopy Research Center, Kyushu University, Fukuoka 819-0395,
According to this report, Au NP stability is greater in ILs
with longer alkyl chains. Therefore, we have selected the
Japan
d
Department of Applied Quantum Physics and Nuclear Engineering,
+
tetraoctylphosphonium ([P8888] ) cation as the protecting agent.
Kyushu University, Fukuoka 819-0395, Japan
The imidazolium cation is not suitable in this instance,
because it is unstable in the presence of the formate anion.
†
Electronic supplementary information (ESI) available: Experimental details and
supplemental figures. See DOI: 10.1039/c8cc03350a
This journal is ©The Royal Society of Chemistry 2018
Chem. Commun.

View Article Online
Communication
ChemComm
Formate was chosen as the IL anion based on its reducing
properties and the expectation that it will reduce oxidized
Fe NPs. In a previous paper, we demonstrated the reducing
capability of formate ions in the synthesis of Au and silver NPs
14
from their corresponding metal ions in formate-containing ILs.
Laser ablation is used to synthesize Fe NPs in this work.
Irradiation of the target with a laser pulse locally melts and
vaporizes the metal. Some metal atoms in the solution emit
electrons and become ionized. Hot metal atoms or cations are
cooled by the surrounding medium and form metal NPs. The
emitted electrons recombine with cations. Thus, the valence
Fig. 2 A TEM image of Fe NPs (left) and the size distribution of NPs (right).
The average diameter of Fe NPs was obtained from TEM image, and it was
state of the metal does not change upon laser ablation. Zero- 7.1 Æ 3.5 nm.
valent Fe NPs are easily oxidized, if oxygen or water is present in
the solution. In an [P8888]HCOO IL, it is expected that oxidized
EDS, together with a high-angle annular dark-field STEM
HAADF-STEM) image. As shown in the figure, the distribution
Fe NPs will be reduced by the formate ion or that oxygen and
water will be deactivated by the reaction with formate.
(
of Fe element overlaps the shape of the STEM image, and the
NPs are confirmed to be composed of Fe element. P and O
elements derived from the formate IL are also confirmed by the
mapping, which cover the Fe NPs. The distribution of oxygen is
somewhat broader than that of phosphorus, probably because
this image was taken on the grid covered by a holey amorphous
carbon film and because of the effect of the adsorbed oxygen on
the grid. The results of EDS mapping anticipate that the NPs do
not suffer from oxidization.
Fe NPs were synthesized by ablation with a Nd:YAG laser
under ambient conditions using two ILs: tetraoctylphosphonium
formate ([P8888]HCOO) and tetraoctylphosphonium bis(trifluoro-
methylsulfonyl)imide ([P8888]TFSI). The ILs were evacuated under
1
Pa at 60 1C for 16 h. A schematic illustration of the experimental
set-up is shown in Fig. 1. The Fe target was ablated in a vessel
filled with the IL. All samples were stored in the atmosphere after
preparation. The water content of the Fe colloidal solution was
1
estimated using the H NMR signal of water (Fig. S1, ESI†). The
To clarify the valence, structure, and stability of the synthe-
sized Fe NPs, X-ray absorption near edge structure (XANES)
spectra were measured at different times after preparation.
Fig. 3 shows the XANES spectra of Fe NPs in ILs one day and
one month after preparation and the spectra of the reference
water content was 3 mol% (0.1 wt%). Transmission electron
microscopy (TEM) images and scanning transmission electron
microscopy (STEM) images of Fe NPs were obtained using
JEM-3200FSK and JEM ARM200F, respectively. The X-ray
absorption near edge structure was measured at BL-12C of
the Photon Factory in the fluorescence mode. Details are
described in the ESI.†
A TEM image of Fe NPs produced via laser ablation in
[P8888]HCOO and its size distribution histogram are shown
in Fig. 2. It was difficult to collect clear lattice images that
reveal the atomic arrangement of the Fe NPs due to residual
substances (Fe foil and a-Fe
signal at 7112 eV, which appears as a shoulder on the first peak
at 7131 eV. The a-Fe powder shows a pre-edge peak at
115 eV and a maximum at 7134 eV. As shown in Fig. 2, the
XANES spectrum of Fe NPs in [P8888]HCOO one day after
preparation differs greatly from those of Fe foil and a-Fe O .
2 3
O ). The Fe foil shows a K-edge
2 3
O
7
2
3
Therefore, the Fe NP structure is different from those of
the references. However, the near edge structure (7112 eV)
of Fe NPs in [P8888]HCOO is quite similar to that of Fe foil,
[
P8888]HCOO, which contributes a significant background to
the TEM observations. The NPs are roughly spherical and are
composed of Fe which was confirmed by energy dispersive
X-ray spectroscopy (EDS). Analyzing the histogram of the size
distribution, the average diameter of Fe NPs is found to be
7
.1 Æ 3.5 nm, here variance is the standard deviation (Fig. 2).
This dimension is slightly smaller than the average diameter
of NPs prepared in solvents such as water. In Fig. S3 (ESI†),
1
5
we show the element mapping of Fe, O and P conducted using
Fig. 3 XANES spectra of reference materials and Fe NPs produced via
laser ablation in ILs. The inset figure is an enlargement of the Fe and Fe NP
in [P8888]HCOO spectra over a smaller energy range.
Fig. 1 Schematic illustration of experimental setup. The target Fe foil is
rotated during laser ablation.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2018

View Article Online
ChemComm
Communication
Table 1 Comparison of peak energies from the XANES spectra of Fe conducted as shown in Fig. S4 (ESI†). Because [P8888]HCOO has
nanoparticles prepared via laser ablation and of fcc and bcc Fe in the
sufficient viscosity, the IL remains on the surface during ablation.
After ablation, the IL-containing Fe NPs were collected, and
bulk state
Peak top energy/eV
XANES spectra were obtained at the Fe K-edge. The XANES
spectrum of this product, which is shown in Fig. S5 (ESI†),
contains a pre-edge peak at 7115 eV and an oscillatory structure
a
Peak number
bcc Fe
fcc Fe
One day after
One month after
First
Second
Third
7134
7142
7153
7128
7136
7160
7130
7135
7158
7129b
2 3
after the pre-edge which is similar to that of a-Fe O .
7156
—
Therefore, Fe NPs produced from thin-coated [P8888]HCOO
are oxidized. We think that the depth of the IL solution is an
important factor preventing oxidation. In normal ablation, the
depth of the interface in contact with air is typically 2 cm.
However, the estimated thickness of the coated IL layer is
a
b
The peak energies of fcc Fe are taken from ref. 16 (C. Marini). This
peak corresponds to the second and third peaks of bcc Fe.
which suggests that Fe NPs are not oxidized in [P8888]HCOO, 0.5 mm. During expansion of the Fe droplet produced by the
although the solution was exposed to air. On the other hand, laser pulse, particles may come into contact with air and be
the X-ray absorption fine structure (XAFS) after the edge clearly oxidized. This process is inhibited in normal ablation. This
À
differs from that of Fe foil. The Fe NPs in [P8888]HCOO show result indicates that HCOO lacks the ability to reduce Fe
2
O
3
À
peaks at 7130, 7135, and 7158 eV, whereas the Fe foil shows once it is produced and that residual HCOO is present after
peaks at 7134, 7142, and 7153 eV (Table 1).
ablation.
A cyclic voltammogram of [P8888]HCOO in acetonitrile
We noticed that the spectrum of Fe NPs in [P8888]HCOO one
À1
day after preparation is very similar to that of fcc Fe. According (1 mol kg ) is shown in Fig. S6 (ESI†). The anodic peak at
to ref. 16, fcc Fe produces peaks at 7128, 7136, and 7160 eV À1.5 V corresponds to the oxidation of the formate ion based
(Table 1). Based on this similarity, the freshly prepared NP in on a comparison of the [P8888]HCOO voltammogram with that
1
7
[P8888]HCOO is not oxidized and has an fcc structure unlike the of formic acid in aqueous solution. The reaction is described
typical bcc structure of Fe under ambient conditions. Because by the following equation:
Fe NPs produced via laser ablation are formed at high tem-
À
+
À
HCOO - CO + H + 2e
(1)
2
perature and rapidly cooled by the surrounding medium, the
particles are quenched in an fcc structure, which is stable
at high temperature and pressure. The XANES spectrum of Fe
NPs in [P8888]HCOO one month after preparation clearly differs
from that one day after preparation, indicating a change in the
structure from fcc. Compared with the spectrum one day after
preparation, the first peak (7130 eV) is higher and the second
peak is absent. The position of the third peak shifts to a lower
energy (7156 eV). However, because the intensity of the edge
peak at the shoulder is similar to that of the Fe foil, the Fe
atoms of Fe NPs in [P8888]HCOO one month after preparation
are considered to be unoxidized. The change in the XANES
spectrum after the edge is attributed to a partial change in the
structure from fcc to bcc. The spectrum is similar to that
observed during the bcc to fcc transition of Fe (Fig. 5 in
ref. 16). The results reflect the structural instability of Fe NPs
and their transformation into a stable structure over time.
The valence state of Fe NPs changes upon changing the
The broad peak near À0.9 V is attributed to the oxidation of the
material on the platinum electrode surface following formate
oxidation. The peak also is detected on the negative going
scan and is presumed to arise from the oxidation of residual
formate. The redox potential of [P8888]HCOO in acetonitrile
determined from the more negative voltammetric peak is
À0.7 V versus the standard hydrogen electrode (SHE) following
+
correction relative to the Ag/Ag potential. A potential of À0.7 V
is sufficient to reduce oxidized Fe, because Fe ions are reduced
at 0.44 V versus SHE. However, oxidized NPs are not reduced by
the formate ion in the experimental system. There are two
plausible reasons for this. One is that the oxidation potential of
the neat IL is not as negative as that of the IL diluted in
X Y
acetonitrile. Another is that the Fe O redox potential in NPs
2+
is not the same as that of bulk Fe . At present, we do not have
an explanation for the observed behavior.
The success in forming zero-valent Fe NPs likely depends on
the efficient reaction of the formate ion with oxygen dissolved
in the IL, which prevents the oxidation of Fe. The proposed
reaction scheme is shown in eqn (2).
À
À
anion from HCOO to TFSI . The XANES spectrum of Fe
NPs produced in [P8888]TFSI differs from those produced in
[P8888]HCOO. A small pre-edge peak arising from oxidized Fe is
found on the XANES shoulder peak, which points to the
oxidation of Fe NPs prepared with [P8888]TFSI. The vibrational
À
3À
2HCOO + O
2
- 2HCO
(2)
structure after the pre-edge is very similar to that of a-Fe
2 3
O .
Therefore, most Fe atoms in Fe NPs produced in [P8888]TFSI are
In summary, we have demonstrated a new synthetic method
oxidized during preparation or storage, which is consistent for preparing zero-valent Fe NPs under atmospheric conditions.
À
with the ability of HCOO to protect Fe NPs from oxidation.
The procedure for producing Fe NPs that are resistant to
Ablation conditions also are important in controlling the oxidation involves simple laser pulse irradiation of a Fe
valence of Fe NPs. We attempted the synthesis of more con- foil target immersed in [P8888]HCOO. The method can be
centrated Fe NPs by reducing the volume of [P8888]HCOO. extended to the preparation of other easily oxidizable transition
Here, the IL was coated on the Fe foil, and laser ablation was metal NPs.
This journal is ©The Royal Society of Chemistry 2018
Chem. Commun.

View Article Online
Communication
ChemComm
This work is supported by a fund from the AMADA FOUNDATION
7 Y. P. Sun, X. Q. Li, W. X. Zhang and H. P. Wang, Colloids Interface
Sci. A: Eng. Aspects, 2007, 308, 60.
(
AF-2015210) and partially by a fund from the MEXT-Supported
Program for the Strategic Research Foundation at Private Universities
015–2019 (S1511025). TEM observations were performed as part
of a program conducted by the Advanced Characterization Nano-
technology Platform sponsored by the MEXT, Japan. XANES
measurements were supported by the Photon Factory Program
Advisory Committee (Proposal No. 2017G564). S. O. and Y. K.
thank Prof. Sakae Takenaka (Doshisha University) and Prof.
Hiroshi Kitagawa (Kyoto University) for the helpful discussion.
8
S. Yamamoto, G. Ruwan, Y. Tamada, K. Kohara, Y. Kusano, T. Sasano,
K. Ohno, Y. Tsuji, H. Kageyama, T. Ono and M. Takano, Chem. Mater.,
2011, 23, 1564.
2
9
(a) H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan and
M. Watanabe, J. Phys. Chem. B, 2004, 108, 16593; (b) H. Tokuda,
K. Hayamizu, K. Ishii, M. A. B. H. Susan and M. Watanabe, J. Phys.
Chem. B, 2005, 109, 6103.
10 (a) M. Migowski, D. Zanchet, G. Machado, M. A. Gelesky, S. R.
Teixeira and J. Dupont, Phys. Chem. Chem. Phys., 2010, 12, 6826;
(
b) K. L. Luska and A. Moores, Green Chem., 2012, 14, 1736;
(c) J. Dupont and J. D. Scholten, Chem. Soc. Rev., 2010, 39, 1780.
1 (a) T. Torimoto, K. Okazaki, T. Kiyama, K. Hirahara, N. Tanaka and
S. Kuwabata, Appl. Phys. Lett., 2006, 89, 243117; (b) E. Vanecht,
K. Binnemans, S. Patskovsky, M. Meunier, J. W. Seo, L. Stappers and
J. Fransaer, Phys. Chem. Chem. Phys., 2012, 14, 5662.
1
Conflicts of interest
12 (a) S. Wegner and C. Janiak, Top. Curr. Chem., 2017, 375, 65;
There are no conflicts to declare.
(
b) C. Janiak, Z. Naturforsch., B: J. Chem. Sci., 2013, 68, 1059.
1
3 (a) Y. Kimura, H. Takata, M. Terazima, T. Ogawa and S. Isoda, Chem.
Lett., 2007, 36, 1130; (b) H. Wender, M. L. Andreazza,
R. R. B. Correia, S. R. Teixeira and J. Dupont, Nanoscale, 2011,
3, 1240; (c) M. A. Gelesky, A. P. Umpierre, G. Machado, R. R. B.
Correia, W. C. Magno, J. Morais, G. Ebeling and J. Dupont, J. Am.
Chem. Soc., 2005, 127, 4588; (d) H. P. S. Castro, V. S. Souza,
J. D. Scholten, J. H. Dias, J. A. Fernandes, F. S. Rodembusch,
R. D. Reis, J. Dupont, S. R. Teixeira and R. R. B. Correia, Chem. –
Eur. J., 2016, 22, 138.
Notes and references
1
(a) Z. Peng and H. Yang, Nano Today, 2009, 4, 143; (b) S. Guo,
S. Zhang and S. Sun, Angew. Chem., Int. Ed., 2013, 52, 8526.
(a) S. K. Singh, X.-B. Zhang and Q. Xu, J. Am. Chem. Soc., 2009,
2
1
4
31, 9894; (b) N. Yan, Y. Yuan and P. J. Dyson, Chem. Commun., 2011,
7, 2529; (c) Q. Yao, Z.-H. Lu, Y. Jia, X. Chen and X. Liu, Int.
J. Hydrogen Energy, 2015, 40, 2207.
3
4
(a) X. Chen, G. Wu, J. Chen, X. Chen, Z. Xie and X. Wang, J. Am. 14 S. Okazoe, Y. Yasaka, M. Ueno and Y. Kimura, Chem. Lett., 2017,
Chem. Soc., 2011, 133, 3693; (b) H. Wu, H. Li, Y. Zhai and Xi. Xu and
Y. Jin, Adv. Mater., 2012, 24, 1594.
46, 1344.
15 V. Amendola, M. Meneghetti, G. Granozzi, S. Agnoli, S. Polizzi,
P. Riello, A. Boscaini, C. Anselmi, G. Fracasso, M. Colombatti,
C. Innocenti, D. Gatteschid and C. Sangregoriode, J. Mater. Chem.,
2011, 21, 3803.
(a) X. Song, S. Sun, W. Zhang and Z. Yin, J. Colloid Interface Sci.,
2
004, 273, 463; (b) N. Lasemi, U. Pacher, C. Rentenberger, O. B.
Miguel and W. Kautec, Chem. Phys. Chem., 2017, 18, 1118.
5
6
B. C. Reinsch, B. Forsberg, R. L. Penn, C. S. Kim and G. V. Lowry, 16 C. Marini, F. Occelli, O. Mathon, R. Torchio, V. Recoules,
Environ. Sci. Technol., 2010, 44, 3455. S. Pascarelli and P. Loubeyre, J. Appl. Phys., 2014, 115, 093513.
(a) J. Xu, S. A. Baig, X. Lv and X. Xu, J. Hazard. Mater., 2013, 244–245, 628; 17 J. John, H. Wang, E. D. Rus and H. D. Abru n˜ a, J. Phys. Chem. C, 2012,
b) C. B. Wang and W. X. Zhang, Environ. Sci. Technol., 1997, 31, 2154. 116, 5810.
(
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2018