Rapid Synthesis of Well-Dispersed Aqueous-Phase Magnetite
Nanoparticles by Atmospheric Pressure Non-thermal Microplasma
Ruixue Wang1, Shasha Zuo1, Weidong Zhu2 , Jue Zhang1,3,*, Jing Fang1,3
1
Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P.R.China.
Department of Applied Science and Technology, Saint Peter’s University, Jersey City, New Jersey
07306, USA
3
College of Engineering, Peking University, Beijing 100871, P.R.China.
* Authors to whom correspondence should be addressed. Electronic addresses: [email protected]
2
We present a facile method of producing magnetite nanoparticles (Fe3O4 NPs) in liquid by an
atmospheric-pressure non-thermal argon microplasma. These Fe3O4 NPs are well-dispersed in liquid
with an average size of 12.45±2.43nm. They show good aqueous-phase stability and excellent
superparamagnetic properties with a saturation magnetization of 51.23 emu/g and a small coercivity
(<35 Oe). Our findings demonstrate the feasibility of plasma induced magnetite nanoparticles synthesis
in liquid.
Key words: Magnetite Nanoparticles, Microplasma, Electrochemical cell
Many different plasma processes have been used for
gas-phase nanoparticles synthesis, including AC, DC, RF or
Microwave systems. 1 , 2Although the production rate is high,
it is generally difficult to synthesize well-dispersed
nanoparticles. 3 , 4 , 5 In addition, those plasma processes are
usually associated with low pressure (<1 Torr) and high
temperature, making conventional plasma tools costly. 6 In
recent years, plasma-liquid interactions at atmospheric
pressure have drawn much attention as a novel nanoparticle
synthesis method. 7,8 By reducing the critical dimensions of
the devices down to micrometer range enables plasma to be
sustained at atmospheric pressure and room temperature. 9
The high density of electrons (with energies in excess of 10
eV) 10 and various reactive oxygen species (ROS) present in
these plasmas allow the reduction/oxidation of metal
cations/metal in aqueous phase. Plasma can therefore be
used as a “contactless” electrode to avoid the difficulty of
production isolation. Recently, C. Richmonds et al 11 showed
that microplasmas can electro-chemically reduce noble
metal cations and generate noble metal (e.g., silver and gold)
nanoparticles at ambient conditions. However, synthesis of
magnetic metal oxide nanoparticles in liquid phase by
microplasmas has not yet been reported.
Magnetic metal oxide nanoparticles have many
applications across biomedicine and environmental
engineering because they combine the large surface area of
particles with nanosize dimension and the strong response of
the material under an applied magnetic field. 12,13 Magnetite
nanoparticles (Fe3O4 NPs) are frequently used due to their
high saturation magnetization value. 14 They have been
previously synthesized by a variety of methods including
sol-gel techniques, 15 microwave hydro-thermal synthesis, 16
micro-emulsion methods 17 and chemical coprecipitation
from a solution of ferrous/ferric mixed salts. 18 Those
strategies may be able to prepare magnetite NPs with
controllable particle sizes and shapes. However, they are
usually time consuming, expensive, and the produced
magnetite NPs are poorly soluble in water. To date, direct
synthesis of well-dispersed aqueous NPs has met with very
limited success. In this study, we report the generation of
magnetite NPs directly in liquid by an atmospheric pressure
non-thermal microplasma.
The experimental setup of the Fe3O4 NPs synthesis
system is shown schematically in Fig.1. The reaction was
performed in a U-shaped electrochemical cell, which
consisted of a microplasma cathode and a platinum (Pt) foil
anode. The precursors contained 2 mM ferric chloride and 1
mM ferrous chloride mixture with an initial pH of 5.0
(adjusted by sodium hydroxide). A copper capillary tube (7
cm long with pore size of 0.355 mm in diameter, KS
Engineering, US) was positioned 1 mm above the surface of
the solution and was pressurized with argon (Ar) gas
(99.99%) at a constant flow rate of 50 SCCM (standard
cubic centimeter per minute). The electrochemical cell was
driven by a direct current negative-polarity high-voltage
power supply (Matsuada AU5R120) through a 5.1 kΩ
ballast resistor.
Fig.1. A schematic diagram of the electrochemical cell with
an atmospheric-pressure non-thermal microplasma cathode
and a Pt foil anode.
When the cathode end of the electrochemical cell is open
to the ambient, the Ar flow in the copper capillary tube pulls
in surrounding air, which results in the production of αFe2O3 nanoparticles in the solution (over-oxidation), a
material that is non-magnetic at room temperature.
Therefore, Ar gas was used to fill the head space of the
reaction apparatus through port B1 (Fig.1). B2 served as the
cathode exhaust port. Upon the application of a DC high
voltage, a jet-like microplasma formed in the space between
the end of the copper capillary tube and the surface of the
solution. During the NP synthesis, the microplasma
discharge was sustained at a voltage around 400 V with an
operating current of 13 mA. The solution under the
microplasma turned black within seconds, indicating the
effective formation of Fe3O4 NPs. After a 10 minute reaction,
the synthesized Fe3O4 NPs were isolated from the solution
by a 0.6 T magnet (isolation only took seconds) and dried
overnight at room temperature. A total of 3.7 mg Fe3O4 NPs
were collected.
A transmission electron microscope (TEM, FEI Tecnai
F-20, USA) was used to evaluate the size, shape and
morphology of the resultant NPs. A drop of the Fe3O4 NPs
solution was applied to a 300 mesh copper grid (coated with
a continuous carbon support film) and dried at room
temperature overnight. The sample was then analyzed in the
TEM at an accelerating voltage of 200 kV. As shown in Fig.
2, the NPs are spherical in shape and have an average size of
12.45±2.43 nm (based on 200 NPs). The inset in Fig.2
shows the size distribution histogram of the NPs.
An X-Ray Diffraction (XRD) system (Rigaku D/Max2400, Japan) was used to elucidate the crystal structure of
the obtained NPs. As shown in Fig.3, the XRD pattern
indicates that these NPs preserve the typical features of the
magnetite spinel phase, with a series of characteristic peaks
at 2 θ = 18.299° , 30.100° , 35.454° , 43.088° ,
53.455° , 56.983° , 62.574° , 74.026° and
89.685° , which correspond to the [111], [220], [311],
[400], [422], [511], [440], [533] and [731] Bragg reflections,
respectively. This is in agreement with the standard
magnetite (Fe3O4) XRD JCPDS file (PDF No.65-3107) by
comparison with other iron oxide compounds [Fe(OH)3,
hematite, maghemite, and goethite]. 19,20,21,22
Measurements of the magnetic properties of these NPs
were carried out in a vibrating sample magnetometer (VSM,
LDJ9400, LDJ Electronics, US) at room temperature.
Magnetic hysteresis curve is shown in Fig.4. The saturation
magnetization of the synthesized NPs is 51.23 emu/g under
an applied magnetic field of 1 T, and the coercivity is small
(<35Oe) at room temperature. This result suggests that the
Fe3O4 NPs synthesized directly in liquid by the atmospheric
pressure non-thermal microplasma have excellent
superparamagnetic property with a low saturation field and a
high susceptibility, making them a promising rapidresponding MRI contrast agents.
Fig.4. Magnetic hysterisis curve of the Fe3O4 NPs at room
temperature
Fig.2. TEM image of the Fe3O4 NPs; the average size of the
NPs (based on 200 NPs) is calculated to be 12.45±2.43nm.
The inset figure shows the size distribution histogram of the
NPs.
The actual chemical reaction in the microplasmasolution system is rather complex. We believe the following
pathways were involved in the NPs production process:
(1) Ar + e → Ar *
(Excitation of argon atoms)
(2) Ar * + H 2O → Ar + H + OH 
(Production of hydroxyl radicals)
(3) OH  + OH  → 2 H 2O2
(Radicals interaction)
(4) Fe 2+ + H 2O2 → Fe3+ + OH  + OH −
(Fenton Reaction)
(5) Fe3+ + e → Fe 2+
(Reduction)
(6) Fe 2+ + 2 Fe3+ + 8OH − → Fe3O4 + 4 H 2O
Fig.3. XRD pattern of the synthesized Fe3O4 NPs
(Formation of Fe3O4 nanoparticles)
The excessive electrons supplied by the microplasma
cathode should also in principle be able to reduce the metal
cations to zero-valent irons (ZVI), which may in turn be
partially oxidized to Fe3O4. Additional experiments will be
conducted to further investigate this aspect.
The pH value of the ferrous and ferrite chlorite solution
is essential for the generation of NPs in liquid. The original
value of the pH was around 2-3 due to the hydrolysis of
ferric/ferrite ions. Under such a condition, the synthesized
Fe3O4 NPs quickly re-dissolved in the acidic solution.
Stabilization of the Fe3O4 NPs in the solution was achieved
only when the pH value was adjusted to around 5 by sodium
hydroxide. This pH value is also favored by the Fenton
reaction. It is also worth noting that potassium iodide starch
test paper placed near the anode exhaust port B3 (Fig.1)
became blue over time, indicating the formation of chlorine
gas at the grounded platinum electrode. The pH value of the
solution decreased accordingly near the Pt electrode, but
hardly, if at all affected the solution near the microplasma
cathode.
In summary, an atmospheric pressure non-thermal
microplasma cathode is used to replace the solid cathode in
a conventional electro-chemical cell for magnetite
nanoparticle systhesis. Well-dispersed Fe3O4 nanoparticles
with an average size of 12.45±2.43nm were produced in the
solution under the right pH and argon gas buffer. These
naked Fe3O4 NPs can be further decorated with many
different functional groups by adding corresponding
surfactants into the electrolyte. Although the Fe3O4 NP
production rate is only ~0.37 mg/min, parallel operation of
these microplasmas in the same cell or in multiple cells can
significantly improve the yield. The simplicity of the
apparatus and room temperature operation make this
plasma-liquid interaction based approach a promising new
method for the production of metal oxide NPs in liquid for
various nano-technological applications.
This research was sponsored by the Bioelectrics Inc.
(U.S.A.). The authors want to thank Dr. Juergen Kolb of the
Institute for Plasma Science and Technology, INP
Greifswald for his suggestions to this work.
Acknowledgment is also made to Jixiang Liu of the Physics
department at Peking University for his help with the NP
magnetic property measurements.
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