Various metal nanoparticles produced by accelerated electron beam irradiation of room-temperature ionic liquid

Article abstract of DOI:10.1039/c2cc16183d

Various metal nanoparticles including base metal were produced by a brief accelerated electron beam irradiation of 1-alkyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide room-temperature ionic liquid without a stabilizing agent, which is usually employed so as to prevent aggregation.

Full text of DOI:10.1039/c2cc16183d

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Various metal nanoparticles produced by accelerated electron beam
irradiation of room-temperature ionic liquidw
Tetsuya Tsuda,*ab Taiki Sakamoto,b Yoshitomo Nishimura,b Satoshi Seino,a
Akihito Imanishicd and Susumu Kuwabata*bd
Received 5th October 2011, Accepted 12th December 2011
DOI: 10.1039/c2cc16183d
Various metal nanoparticles including base metal were produced
by a brief accelerated electron beam irradiation of 1-alkyl-3-
methylimidazolium bis((trifluoromethyl)sulfonyl)amide room-
temperature ionic liquid without a stabilizing agent, which is
usually employed so as to prevent aggregation.
reduction,5 plasma reduction,6 electron beam irradiation,7
magnetron sputtering.8 From a practical application standpoint
of metal nanoparticle preparation in RTILs, we are keeping an eye
on g-ray and accelerated electron beam irradiation methods that
can be conducted at an existing industrial plant for sterilizing
medical kits in order to establish a mass production method for the
metal nanoparticles in RTILs without any stabilizing and reducing
agents. We have already succeeded in using this approach for
preparation of Au nanoparticles.9 In this article, we report
efforts toward preparation of various metal nanoparticles
including base metal, which cannot be thermodynamically
obtained in aqueous solution, by accelerated electron beam irradia-
tion of RTIL, 1-alkyl-3-methylimidazolium bis((trifluoromethyl)-
sulfonyl)amide ([R1MeIm][Tf2N]), containing metal salts. The
aim of this study is to prepare various metal nanoparticles not
covered with a stabilizing agent by using the accelerated
electron beam irradiation method, leading to the mass production
of the metal nanoparticles.
After discovery of water- or air-stable room-temperature
ionic liquids (RTILs), e.g., 1-ethyl-3-methylimidazolium tetra-
fluoroborate ([EtMeIm][BF4]) and 1-butyl-3-methylimidazolium
bis((trifluoromethyl)sulfonyl)amide ([BuMeIm][Tf2N]), they have
been investigated in various fields as one of the next-generation
reaction media and liquid materials because most RTILs show
attractive unique physical and chemical properties such as negligible
vapor pressure, wide electrochemical window, wide handling
temperature, and flame resistance.1 Due to the interesting
physicochemical properties, a great number of future technologies
have been proposed by using RTILs. They have brought about
new trends and directions in science and technology.2
It is well-known that ionizing radiation, e.g., g-ray and
accelerated electron beam, yields strong reducing agents (solvated
Metal nanoparticles, which have notable features including
large surface area, surface plasmon resonance, fluorescent
emission etc., are expected to be key materials for further
development of science and technology.3 The number of
articles related to metal nanoparticles continues to increase.
The reaction medium for preparations using a wet process is
usually aqueous solution. RTILs are just beginning to be
employed in the preparation of metal nanoparticles as reaction
media.4 It is interesting to note that RTILs work as an
excellent medium for nanoparticle formation and behave as
a stabilizing agent for the produced nanoparticles regardless of
the types of the preparation methods such as chemical
electrons, esolv , hydrogen atoms, Hꢀ, etc.) in many solvents.10
À
Such generated reactive species can reduce many types of
metal ions to lower valences and metal atoms. Finally metal
nanoparticles are produced by the reactive species. We reported
in a previous paper that similar reactions occur in RTILs by
g-ray and accelerated electron beam irradiations,9 although
most RTILs are known to have better radiochemical stability
than aqueous solution.11 As shown in Fig. S1 (ESIw), FAB-
MS experiments for RTILs before and after accelerated
electron beam irradiation revealed that reactive species for
reducing metal ions can be generated in [BuMeIm][Tf2N] by the
electron beam irradiation but not in tributylmethylammonium
bis((trifluoromethyl)sulfonyl)amide ([Bu3MeN][Tf2N]) which is
very stable to the ionizing radiation.11 Based on these facts, in
this study, we employed [R1MeIm][Tf2N] and accelerated electron
beam, which can give a higher irradiation dose in a short period
of time, as reaction medium and ionizing radiation, respectively,
so as to produce a wide variety of metal nanoparticles including
base metal.
a Frontier Research Base for Global Young Researchers, Graduate
School of Engineering, Osaka University, 2-1 Yamada-oka, Suita,
Osaka 565-0871, Japan. E-mail: ttsuda@chem.eng.osaka-u.ac.jp
b Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
E-mail: kuwabata@chem.eng.osaka-u.ac.jp
c Department of Chemistry, Graduate School of Engineering Science,
Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531,
Japan
d Core Research for Evolutional Science and Technology (CREST),
Japan Science and Technology Agency (JST), Kawaguchi, Saitama
332-0012, Japan
Fig. 1 shows the UV-vis spectra of the metal nanoparticles in
Group 11, i.e., Au, Ag, and Cu, prepared in [BuMeIm][Tf2N]
with 5 mmol LÀ1 metal salts by means of accelerated electron
beam irradiation at 6 and 20 kGy. Obvious absorption, that is,
w Electronic supplementary information (ESI) available: Experimental
details, TEM images, UV-vis spectra, etc. See DOI: 10.1039/
c2cc16183d
c
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Chem. Commun., 2012, 48, 1925–1927 1925

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Table 1 Summary of metal nanoparticles prepared by using the
accelerated electron beam irradiation of [BuMeIm][Tf2N]
6 kGy
20 kGy
irradiationa irradiationb
Concentration/ dc/
SDd/ dc/
nm nm
SDd/
nm
Metal Added salts
mmol LÀ1
nm
Au
Ag
Cu
Ni
Pd
Pt
Mg
Fe
Zn
Al
NaAuCl4Á2H2O
5
5
5
5
5
5.44 0.96 8.21 1.18
15.3 5.36 20.4 6.64
20.3 6.48 26.3 8.81
18.1 3.53 19.2 11.4
Ag[Tf2N]
Cu[Tf2N]2
C10H14NiO4
K2PdCl4
o1.0
—
—
o1.0
—
—
K2PtCl4
MgCl2
Fe[Tf2N]2
Zn[Tf2N]2
[EtMeIm][Al2Cl7]
SnCl2
5
o1.0
o1.0
0.5
10
10
4
1
5
40.7 6.53 65.6 13.6
7.73 1.51 7.05 1.29
4.79 0.75 10.8 2.31
61.0 16.4 194
124
Sn
43.2 5.86 52.8 7.63
3.33 0.47 4.13 0.87
FePte Fe[Tf2N]2
K2PtCl4
5
a
b
The irradiation time was ca. 2 s. The irradiation time was ca. 7 s.
c
d
e
Mean diameter. Standard deviation. The alloy phase could not
be identified in this study.
Pd and Pt nanoparticles produced at 6 kGy irradiation. Mean
particle sizes of the produced Pd and Pt nanoparticles were 3.51 nm
(SD: 0.53 nm) and 2.13 nm (SD: 0.34 nm), respectively.
Fig. 1 UV-vis spectra of [BuMeIm][Tf2N] with
(a) NaAuCl4Á2H2O, (b) Ag[Tf2N], and (c) Cu[Tf2N]2 after accelerated
electron beam irradiation at ( ) 6 or ( ) 20 kGy. Arrows show
5
mmol LÀ1
It is very difficult to produce base-metal nanoparticles by
using conventional methods such as vapor-phase synthesis,14
laser ablation method,15 mechanical milling/alloying process,16
and chemical reduction reaction in organic solvents,17 because
base-metal nanoparticles have a highly-reactive nature that brings
about unfavorable reactions with moisture, oxygen, and solvent.
Our approach can be completed in an airtight container without
any contamination and most RTILs show high physicochemical
stability to base metal. If we can produce base-metal nano-
particles by this approach, it will greatly contribute to develop-
ment of nanomaterials science. Herein, base-metal nanoparticles
were prepared in [BuMeIm][Tf2N] RTIL. After the accelerated
electron beam irradiation of the RTILs containing base-metal
salts, we verified existence of base-metal nanoparticles in the
RTIL with TEM observation. Typical TEM images of the
nanoparticles yielded at 6 kGy and 20 kGy irradiation are
depicted in Fig. 2 and Fig. S7 (ESIw), respectively. The particle
size distributions were estimated from TEM observation of
the resulting nanoparticles (Fig. S8 and S9, ESIw), and the
characterization was conducted by the EDX analysis of the
nanoparticles (Fig. S10, ESIw). The mean diameters of the nano-
particles are given in Table 1. Similar to the noble metal nano-
particles, their particle size showed a tendency to become large at
an irradiation dose of 20 kGy except that Fe nanoparticles
maintained a roughly constant particle size.
peaks of surface plasmon resonance absorption.
surface plasmon resonance absorption appeared. The absorption
peak wavelength moved to higher wavelength at higher irradiation
dose. In general, plasmon absorption is very sensitive to the
nanoparticle size and moves to higher wavelength if the particle
size becomes large.12 In order to visually estimate the particle size
and determine the composition, the resulting nanoparticles were
observed with transmission electron microscope (TEM) and
characterized by energy dispersive X-ray analysis (EDX).
Fig. S2, S3, and S4 (ESIw) show typical TEM images, particle
size distributions, and EDX spectra of the obtained nanoparticles,
respectively. The mean diameters of the nanoparticles determined
with the TEM observation are given in Table 1. Each nano-
particle had a spherical shape and became larger at the 20 kGy
irradiation. Then the metal nanoparticles obtained at 20 kGy
showed the plasmon absorption at higher wavelength as
exhibited in Fig. 1. The increase in the particle size should
have been caused by further metal deposition onto metal
nanoparticles from unreacted metallic ionic species. Similarly,
we also investigated preparation of metal nanoparticles in
Group 10 (Ni, Pd, Pt) by the same strategy. After the irradiation,
we could obtain Ni nanoparticles without difficulty (Fig. S5,
ESIw), but measurable Pd and Pt nanoparticles were not observed
in the [BuMeIm][Tf2N] while we could confirm that very small
crystals less than 1 nm exist in the RTIL. In [R1MeIm][Tf2N]-
based RTILs, the imidazolium cations absorbed onto metal
nanoparticles cause gauche defects that increase with a decrease
in alkyl chain length;13 that is, crystal growth of nanoparticles
is enhanced in the RTILs having shorter alkyl chain like ethyl
groups. Next, in order to make the Pt and Pd crystals larger,
1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide
([EtMeIm][Tf2N]) was exploited as a reaction medium so that
we can verify them visually. Fig. S6 (ESIw) shows typical TEM
images, particle size distributions, and EDX spectra of the
It is noteworthy that all the metal nanoparticles prepared in
this investigation were very stable without a stabilizing additive for
more than three months. It is highly likely that RTIL itself works
as a stabilizing agent for the nanoparticles.4 In some cases RTIL
gives functionality to the nanoparticles.18 The features will be an
advantage to using RTILs as reaction media for nanoparticle
preparation. We examined a catalytic activity for electrochemical
oxygen reduction reaction (ORR) of the Pt nanoparticles prepared
in this investigation. Pt nanoparticle-modified glassy carbon
electrode (Pt-GCE) was fabricated using the preparation method
c
1926 Chem. Commun., 2012, 48, 1925–1927
This journal is The Royal Society of Chemistry 2012

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plant for sterilizing medical kits with future mass production
of metal nanoparticles prepared in RTIL in mind. The
industrial plant can irradiate 200 vials  100 mL with the
accelerated electron beam at a time and the process can be
completed within 10 s.
Part of this research was supported by Grant-in-Aid for
Scientific Research on Innovative Areas (Area No. 2206),
Grant No. 23107518, from Japanese Ministry of Education,
Culture, Sports, Science and Technology (MEXT) and General
Sekiyu R&D Encouragement Assistance Foundation.
Notes and references
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Fig. 2 TEM images of (a) Mg, (b) Fe, (c) Zn, (d) Al, (e) Sn, and (f)
FePt nanoparticles prepared by accelerated electron beam irradiation
at 6 kGy in [BuMeIm][Tf2N]. The concentration of the metal salts
added to the RTIL is shown in Table 1.
6 M. Brettholle, O. Hofft, L. Klarhofer, S. Mathes, W. Maus-
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given in the ESI.w We found that the Pt nanoparticles become
large upon heating the accelerated electron beam-irradiated
5 mmol LÀ1 K2PtCl4 RTIL solutions at 573 K for 30 min
(Fig. S11 and S12, ESIw). Variation in the Pt nanoparticle size
after the process is summarized in Table S1 (ESIw). Catalytic
activity for electrochemical ORR of the Pt-GCE was estimated
by cyclic voltammograms recorded in O2-saturated 0.5 mol LÀ1
H2SO4 aqueous solution. As shown in Fig. S13 (ESIw), there is
no wave for ORR in the voltammogram recorded at a bare
GC electrode, but if the Pt-GCE was employed, current increment
for electrochemical ORR initiated at ca. +0.93 V vs. NHE and a
distinct redox couple for H+/H2 reaction appeared at ca. 0 V due
to the Pt nanoparticles modified on the GCE. The Pt-GCE
prepared in this work exhibited a favorable electrocatalytic
activity to ORR as an electrode catalyst for PEM fuel cell.
In conclusion, a wide variety of metal nanoparticles were
obtained by the accelerated electron beam irradiation of the
[R1MeIm][Tf2N] RTIL without a stabilizing agent. Even base-metal
nanoparticles were produced without any difficulties, although
the nanoparticles may not be sufficiently characterized because
of their instability in air. In addition to this, Pt nanoparticles
prepared in this investigation showed a favorable catalytic
activity for electrochemical ORR. The most important thing is
that this study was carried out at an existing common industrial
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1925–1927 1927