22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Formation of metal and metal-oxide nanoparticles by plasma-induced liquid
chemistry
N. Tarasenko, V. Burakov, N. Tarasenka and V. Kiris
B.I.Stepanov Institute of Physics National Academy of Sciences of Belarus, Minsk, Belarus
Abstract: Plasma-induced liquid chemistry was used to synthesize metallic and oxide
nanoparticles (NPs). The phase composition, morphology and optical properties of the
synthesized NPs were characterized by X-ray diffraction (XRD), transmission electron
microscopy (TEM) and ultraviolet–visible spectroscopy. The main mechanisms and
reaction paths that contribute to the NPs formation process are discussed.
Keywords: atmospheric pressure microplasma, metal–oxides, nanofabrication
1. Introduction
Recently, non-thermal plasma in and with liquids has
attracted considerable interest for a wide range of
applications. In particular, microplasma of discharge with
a liquid electrode has potential use in analytical
techniques and nanomaterials synthesis. Microplasma–
liquid interactions initiate new synthetic and
functionalization approach that is different from both
standard liquid electrochemistry as well as from the
submerged discharge method. The successful application
of such plasmas in field of nanofabrication relies on
plasma non-equilibrium nature, which can offer large
amounts of radical species, molecular fragments,
metastables, and highly energetic electrons. New
chemical pathways may be possible by microplasmaproduced species that are not achievable with traditional
plasma processes [1, 2].
In the present study, we have used non-equilibrium
atmospheric pressure microplasma which directly
interacts with liquids to synthesize colloidal metallic and
oxide NPs.
power supply. The discharge current was kept constant in
the range of 1 - 4 mA during the synthesis process. The
formed NPs were characterized by TEM, SEM and X-ray
diffraction (XRD) in order to determine NPs composition,
its crystalline structure, lattice parameters and grain size.
Samples for XRD measurements were prepared by drying
of the colloidal solutions.
2. Experimental
The principle of synthetic process is clear from the
Fig.1, where the laboratory model of the experimental
reactor is presented. Our experiments are focused on the
anodic dissolution of metallic (Ag, Zn, Fe, Ni) electrodes
followed by reduction of aqueous metal cations with
charged, energetic species directed from a microplasma.
Metal electrodes are placed inside a distilled water or
aqueous solution consisting of 1 mM HNO 3 (or HCl) with
10 mM glucose or without glucose. The acid is necessary
to increase the solution conductivity; the glucose is a
stabilizer that prevents uncontrolled particle growth and
agglomeration. A stainless steel capillary tube served as
the cathode (500 µm inside diameter, 5 cm length) was
located 3 cm away from the metal electrode with a
distance up to 3 mm between the tube end and the liquid
surface. Argon gas flow (approximately 15 sccm) was
directed through the capillary tube. The discharge was
ignited by applying of a high voltage of 3.6 kV using a dc
Fig. 1. Atmospheric pressure microplasma setup used for
the synthesis of NPs
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3. Results and discussion
In a standard electrochemical cell, redox reactions
occur by flowing current between solid electrodes and the
electrolyte. Igniting a discharge above the solution results
in current flow through the electrolyte and reactions are
observed at the anode and cathode. At the anode,
oxidation reactions lead to dissolution of the solid metal
into metal cations which are then reduced at the cathode
by energetic species supplied by the microplasma. The
observations suggest that gas-phase electrons can react
with a solution at the plasma–liquid boundary to initiate
redox reactions. The reduction of metal cations provides
an approach for nanoparticle synthesis from either bulk
metals or metal salt solutions.
In a typical experiment, igniting the microplasma in
contact with a solution caused a colour change as a result
1
Fig.2 Typical TEM image of Ag nanoparticles prepared
by the discharge with liquid electrode
Using optical emission spectroscopy plasma
composition, electron density and temperature have been
evaluated depending on the discharge parameters. Optical
emission has also been used to analyze the plasma species
and identify the elementary processes responsible for the
formation of specific nanostructures.
Emission spectra of the gas–liquid interfacial discharge
plasma generated in argon flow in the process of Ag
nanoparticles synthesis have been recorded for different
argon flow rates (Fig.3). The main components of the
spectra were found to be Ag lines and N 2 bands which
disappeared with the argon flow rate increase. No argon
lines were detected evidently due to their high excitation
2
3998 N2
4059 N2
3576,9 N2
3710 N2
3804 N2
2
Ag I 3280,6 3371 N
2
4
Ag I 3382
6
3136 N2
3159 N2
potentials.
I, a.u.
of particle growth. The particles were observed to
nucleate at the plasma-liquid interface and diffuse into
the solution volume.
Liquid chemistry initiated by microplasma at the
plasma–liquid interface is responsible for the nucleation
and corresponding synthesis of NPs in liquid. It should be
noted that direct electrochemistry of the used aqueous
solution with two electrodes both immersed in solution
under the same electrical conditions that were used for the
plasma-based NPs did not result in the formation of NPs.
This confirms that plasma-induced reactions are essential
for reducing the metal ions and are fundamentally
different from reactions promoted with a standard
electrochemical cell arrangement.
The growth kinetics is closely coupled to process
parameters including current density, acid and stabilizer
concentration. We applied this method for the preparation
of pure Ag and composite ZnO/Ag nanostructures. The
recent experiments showed a possibility of application
this technique for preparation of magnetic iron oxide NPs
and to develop the similar procedures for the preparation
of magnetic Zn and Co ferrites NPs based on this plasma
source as well.
Fig.2 shows the typical TEM photograph of the
prepared AgNPs. TEM analysis of the as-grown NPs
confirms the synthesis of NPs with sizes in the range of
about 10-15 nm.
0
3000
3500
4000
4500
λ, A
Fig.3 Emission spectra of the gas–liquid interfacial
discharge plasma
Plasma temperature was evaluated using relative
intensity values of silver lines Ag I λ=3280.7 and 3382.9
nm. According to the evaluation data the plasma
temperature decreases from 4000 to 2000 К with the
increase of the argon flow rate.
4. Conclusion
Nanostructured metal and metal–oxides produced by
the developed
technique can find many different
applications that include photonics, optoelectronics,
biomedicine, etc. In comparison to conventional wet
chemical methods, plasma-assisted synthesis of colloidal
metal NPs is simple, rapid, and clean since the particles
are grown without any chemical reducing agents. This is
particularly attractive because NPs are produced at
ambient conditions (atmospheric pressure and room
temperature), making the approach safe and
biocompatible. In addition the approach is low cost,
scalable and should allow a wide range of nanoparticle
materials to be synthesized. NPs have been grown with
and without stabilizer molecules which allows the
particles to be selectively functionalized. For example, the
NPs can be modified with the amino-, hydroxyl-, and
thiol- groups on NPs’ surface with preliminary
silanization
procedure.
Organic
compounds
functionalized iron oxide NPs provide not only the basic
magnetism characteristics of magnetic NPs, but also
possess good biocompatibility and biodegradability of the
materials. The superparamagnetic behavior was found for
the obtained samples containing the particle with the
mean size of 10-15 nm.
This research has been supported by the Belarusian
Foundation for Fundamental Researches under Grant No.
F 12MS-006.
5. References
[1] D. Mariotti, R M. Sankaran. J. Phys. D: Appl. Phys.
43, 323001 (2010)
[2] Chen Q., Kaneko T., and Hatakeyama R. Appl. Phys.
Express 5, 086201-3 (2012)
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