Carbon-coated core shell structured copper and nickel nanoparticles synthesized in an ionic liquid

Article abstract of DOI:10.1021/jp063842e

A simple synthetic route to prepare carbon-coated copper or nickel nanoparticles is developed in an ionic liquid under microwave heating. The obtained products are characterized by XRD, UV-spectroscopy, and Raman spectroscopy. The morphologies are studied with the help of TEM, HRSEM, and HRTEM. A bulk transport property for carbon coated nickel is reported in this letter.

Full text of DOI:10.1021/jp063842e

17711
2006, 110, 17711-17714
Published on Web 08/22/2006
Carbon-Coated Core Shell Structured Copper and Nickel Nanoparticles Synthesized in an
Ionic Liquid
David S. Jacob,† Isaschar Genish,‡ Lior Klein,‡ and Aharon Gedanken*,†
Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center for
AdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, 52900, Ramat-Gan, Israel, and
Department of Physics, Bar-Ilan UniVersity, Ramat-Gan, Israel 52900
ReceiVed: June 20, 2006; In Final Form: August 8, 2006
A simple synthetic route to prepare carbon-coated copper or nickel nanoparticles is developed in an ionic
liquid under microwave heating. The obtained products are characterized by XRD, UV-spectroscopy, and
Raman spectroscopy. The morphologies are studied with the help of TEM, HRSEM, and HRTEM. A bulk
transport property for carbon coated nickel is reported in this letter.
Introduction
(cetyltrimethylammonium bromide) as a surfactant.20 The
precursors used for CoPt alloy preparation were the metal
acetates, whereas the metal acetates on thermal decomposition
yield the respective metal.21 Recently, it has been reported that
a large-scale synthesis of single-crystal gold nanosheets in an
IL is accomplished without any additional template agent.22 Ni
nanoparticles were synthesized in IL via the thermolysis of
nickel formate, which undergoes a self-redox reaction.23
High-speed synthesis using microwave (MW) radiation has
attracted a considerable amount of attention in recent years.24
The efficient in-core volumetric heating with the help of MW
radiation has helped to reduce chemical reaction times from
hours to minutes, and it is also known to reduce side reactions,
increase yields, and improve reproducibility.25 The observed rate
enhancement is a purely thermal/kinetic effect, i.e., a conse-
quence of the high reaction temperatures (superheated solvents
above their boiling temperature) that can rapidly be attained by
irradiating polar materials in a MW field. The ionic nature and
thermal stability of the RTIL make them very good susceptors
of MW radiation. This gives RTILs an advantage over other
solvents in the synthesis of inorganic nanomaterials.15-23,26
This combination effect of an IL and MW radiation has a
unique advantage for our research on the fabrication of metal
nanoparticles, as well as for the synthesis of metallic nanopar-
ticles with a coating of a carbon shell.
Nanoparticles have properties intermediate between those of
bulk and single particles.1 Carbon-coated nanomaterials, espe-
cially metals, are of great interest due to their stable nature
toward oxidation and degradation, and they show potential
applications.2-8 Whereas bare metal nanoparticles can be easily
oxidized in an oxidizing medium, it is well known that an outer
layer or a carbon shell coated on a nanomaterial provides
excellent protection against air oxidation.3 There are various
known methods for obtaining carbon coating on metal nano-
particles, the most popular being the arc-discharge technique.9
In general, the various methods for obtaining carbon-coated
metal nanomaterials are very chaotic, and closed systems with
very harsh conditions, such as high pressure, are required to
attain the desired products.10,11
In this paper we describe a simple reaction, which combines
the use of an ionic liquid (IL) and microwave irradiation to
obtain metal nanoparticles coated with carbon in an IL solvent.
Room temperature ionic liquids (RTILs) are known as “green”
solvents due to their unusual extreme physical properties. In
the literature they are well-known solvents, mostly used as
solvents for organic synthesis, separation techniques, and
electrochemistry.12-14 Recently, RTILs have been gaining
increasingly more importance in inorganic chemistry. There are
reports of the synthesis of inorganic nanometals in IL. For
example, Mu et al. report on a general ethanol reduction method
for the preparation of PVP-stabilized noble metal nanoparticles
in RTIL.15 Nanoparticles of metals such as platinum, palladium,
and rhodium, were synthesized in RTIL.15,16 Tellurium nano-
wires have been prepared in an IL where NaBH4 was used as
a reducing agent and PVP acts as a stabilizer.17 Ir and Rh
nanoparticles have been synthesized in ILs by a chemical
reduction.18 Anisotropic gold nanoparticles have been synthe-
sized by photochemical reduction in an ionogel template.19 CoPt
alloy nanorods have been synthesized in an IL using CTAB
Experimental Section
1-Butyl-3-methylimidazolium tetrafluoroborate (BMIBF4),
(10 g, 44.2mM, for each reaction) IL solvent and the metal
precursors copper (3.38mM) or nickel nitrate (3.43mM) salts
(1 g) were heated for 10 min in a domestic microwave oven
(2.45 GHz, Kenwood Microwave 900 W). After completion of
the reaction time the products were washed with acetone and
ethanol for several times to remove the IL and other organic
impurities and centrifuged for 10 min at 9000 rpm. The washed
products were dried under vacuum and used for further
investigations. The quantitative yields of the products obtained
after drying are in the range of 80-95%.
* Corresponding author.
† Department of Chemistry and Kanbar Laboratory for Nanomaterials.
‡ Department of Physics.
10.1021/jp063842e CCC: $33.50 © 2006 American Chemical Society

17712 J. Phys. Chem. B, Vol. 110, No. 36, 2006
Letters
Figures 3c-f, respectively. The HRTEM images show that the
copper and the nickel nanoparticles are coated with an amor-
phous carbon layer on their surface and are shown by an arrow.
The Fourier transform image also confirms the carbon layer,
which shows a d spacing of 3.3 Å. The observed spacing of
the fringes detected in the HRTEM images is well matched with
the d value (2.08 Å, Cu) and (2.03 Å, Ni) of the respective
metals. The UV-vis spectrum of the sample is recorded on a
Perkin-Elmer UV-visible spectrophotometer. The surface plas-
mon band around 570 nm is characteristic of copper nanopar-
ticles (10 nm).27,28 The shifts in plasmon band depend on the
size and shape of the nanoparticles. Copper nanoparticles with
30-50 nm diameter show plasmon band at 579 nm, whereas
nanorods with 50-70 nm size show a plasmon band at 586
nm.29 The individual isolated nanoparticles and aggregated
nanoparticles also affect the absorption spectra.27 As we can
see in Figure 4a the UV-vis plasmon band for carbon coated
copper nanoparticles shows a broad band that is red shifted to
higher wavelengths (608 nm). This is due to an increase in the
particle size to 100 nm. The red shift is expected in comparison
with refs 27 and 29. The broadening of the plasmon band results
also from the particle size. Raman spectrum is recorded on a
Jobin Yvon Horiba Raman system. The 524 nm line of an Ar+
laser was used as the excitation source. The Raman spectra are
shown (Figure 4b) to confirm the presence of a carbon coating
on the copper and nickel nanoparticles.11 The Raman spectra
show the characteristic bands at 1354 cm-1 for disordered
D-band carbon and at 1589 cm-1 graphitic G-band carbon.
Figure 1. XRD patterns of (a) nickel nanoparticles (b) copper
nanoparticles.
Results and Discussion
The products were characterized by XRD measurements on
a Bruker AXSD advanced powder X-ray diffractometer using
Cu KR radiation at 40 kV. The XRD patterns of the desired
products are given in Figures 1a and 1b. The crystalline XRD
patterns of the products (Figure 1a) fit well with that of nickel
(hexagonal, PDF-45-1027), and the patterns in Figure 1b match
that of copper nanoparticles (face-centered cubic, PDF-1-85-
1326).
The structural morphology was studied with HRSEM (high
resolution scanning electron microscope) using a JEOL JSM-
7000F field emission scanning electron microscope with an
accelerating voltage of 15 kV. TEM (transmission electron
microscopy) measurements were conducted on a JEOL-TEM
100SX microscope and the HRTEM (high resolution transmis-
sion electron microscopy) on a JEOL-2010, using an accelerat-
ing voltage of 200 kV. Samples for HRTEM measurements were
prepared by ultrasonically dispersing the products into absolute
ethanol, then placing a drop of this suspension onto a copper
grid coated with an amorphous carbon film and then drying in
air. The HRSEM images of copper and nickel nanoparticles are
shown in Figures 2a and 2b, respectively. The HRSEM images
illustrate that both the copper and nickel nanoparticles are in
the size range of 100 nm and show aggregation. The regular
TEM images are shown in Figures 3a and 3b, where we see
that the metal nanoparticles are embedded in a matrix of car-
bon. The carbon layer is indicated by an arrow. The HRTEM
images for copper and nickel nanoparticles are depicted in
The transport properties of the carbon-coated nickel nano-
particles were studied using the Van der Pauw method30 on a
pellet sample (Θ6 mm, h1.5 mm). The resistivity, measured
between 12 and 300 K, exhibits strong temperature dependence
and it diverges as the temperature is decreased (see Supporting
Information, Figure S7).
The results of XRD and the HRTEM indicate that the metal
ions undergo a reduction to the metallic state. The presence of
the amorphous carbon observed clearly in the HRTEM images
suggests that the reduction of the metal ions is, perhaps, assisted
by the amorphous carbon. The remaining question is how is
the amorphous carbon formed? We attribute this formation to
the decomposition of the IL, which absorbs the MW radiation
Figure 2. HRSEM images of (a) copper nanoparticles (b) nickel nanoparticles.

Letters
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17713
complete reduction of Cu2+ ions to copper occurs within a 10
min MW irradiation of the IL. The percentage of carbon formed
in a 5 min MW reaction of copper salt is less (11% mole) than
that of 10 min MW reaction (16% mole).
We also carried out MW irradiation in the same IL of the
same precursor with the addition of water. There was no other
byproduct as the time duration remains the same for all the
reactions. It was found that the formation of metals is unaffected
by the addition of water. To be sure that the reduction of metal
ions is due to the presence of carbon, which is the decomposi-
tion product of IL, and not due to water or to a MW heating
effect, we have carried out MW reaction in the presence of
distilled water (10 mL) and metals salts (1 g) without IL. The
XRD results show no evidence of metals or metal oxides (see
Supporting Information). We have also examined the possibility
of reducing the solid metal salts under MW. No metallic
products are detected. It can be stated and confirmed with the
results obtained (HRTEM) that during the reaction a decom-
position temperature is reached for the IL, which decomposes
to obtain carbon, which absorbs microwave energy32 and acts
as a reducing agent33 forming a shell around the nanoparticles.
These synthesized carbon-coated metal nanoparticles are stable
in air atmosphere but are oxidized in acidic medium. The acid
treated product may be of great importance in attaining carbon-
cage types of materials.34
Conclusion
In conclusion, we are able to synthesize copper and nickel
nanoparticles coated with carbon. It was also found that in our
reactions carried out in an IL under microwave heating, the
decomposition of this liquid takes place to form carbon. This
carbon acts as a reducing agent for the copper and nickel ions
to get pure metal nanoparticles and also forms a layer structure
over the metal nanoparticles.
Figure 3. TEM images of (a) copper nanoparticles (b) nickel
nanoparticles, HRTEM images of (c) and (d) carbon coated copper,
and (e) and (f) carbon coated nickel nanoparticles. The inset shows
the electron diffraction scattering and the Fourier transform image of
the copper and nickel, respectively. In all the TEM and HRTEM images
the carbon coating is indicated with the arrow in the pictures.
Supporting Information Available: Raman spectrum,
XRD patterns, HRTEM images, and resistance measure-
ments. This material is available free of charge via the Internet
at http://pubs.acs.org.
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