2778 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 10, OCTOBER 2014
Optical Diagnostics of Iron Oxide Nanoparticle
Synthesis in Microwave Oxygen Plasma
Miroslav Šnírer, Peter Zelina, and Vít Kudrle
Abstract—Iron oxide nanoparticle synthesis in microwave
plasma is studied by optical diagnostics. The arrangement of the
plasma reactor results in well-defined reactant mixing, which
makes different parts of the reaction zone clearly observable.
Besides the reaction zone, digital imaging also reveals the presence
of heated nanoparticles. The optical emission spectroscopy
is used to determine important plasma parameters.
Index Terms—Nanotechnology, optical spectroscopy, plasma
CVD, plasma materials processing applications.
SYNTHESIS of nanomaterials with advanced properties
is an important part of nanotechnology and a fast
developing field with many promising applications. Magnetic
iron and iron-oxide-based nanopowders show great potential
in medicine (MRI contrast agent, targeted drugs), chemical
catalysis, environmental science (decontamination of toxic
wastes), in biochemistry (in vivo and in vitro reactions controlled
by magnetic fields) and many other areas of science and
technology.
Traditional wet techniques of synthesis of iron-based
nanopowders are well established and are able to produce
this material at industrial quantities. However, they are rather
complicated and use potentially harmful substances during
the many steps. In contrast, plasma enhanced chemical vapor
deposition (PECVD) is a rather straight–forward process producing
nanopowders, which consequently do not need to be
dissolved, precipitated, dried, or purified. This makes PECVD
an attractive and advantageous method, however, currently
only suitable for low–quantity purposes.
The experiment uses a low–pressure microwave plasma
reactor to produce iron oxide nanoparticles from Fe(CO)5 precursor
vapors. Use of a surface–wave driven discharge permits
to spatially separate the excitation and the reaction zones.
The waveguide plasma applicator—surfaguide [1], is used to
couple the microwave power into the plasma. The discharge
is sustained in oxygen atmosphere by 2.45 GHz microwaves
using a magnetron generator with adjustable output power in
the range of 150–1500 W. The plasma is contained in fused
Manuscript received November 1, 2013; revised March 4, 2014; accepted
April 24, 2014. Date of publication May 20, 2014; date of current version
October 21, 2014. This work was supported by the Project Research and
Development Center for Low-Cost Plasma and Nanotechnology Surface
Modifications under Grant CZ.1.05/2.1.00/03.0086.
The authors are with the Department of Physical Electronics, Masaryk
University, Brno 611 37, Czech Republic (e-mail: snirer@mail.muni.cz;
zelina@mail.muni.cz; kudrle@sci.muni.cz).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPS.2014.2321995
silica tube (1 m long, 45 mm diameter). The synthesis is
operated in the flow regime, flow of oxygen being set by a
mass flow controller. Vacuum is sustained by an oil rotary
vane pump. Typical operating pressure during the synthesis is
∼1–2 kPa depending on the oxygen flow.
Saturated vapor pressure of iron pentacarbonyl at room
temperature is higher than the typical operating pressure of
the apparatus, therefore, vapors flow from an evaporator into
the reaction zone freely. Their flow is only limited by the
vacuum conductance of the tubing, the surface area of the
evaporator, and a throttling valve. Vapors are injected into
the plasma flow by a pyrex tube (8 mm outer diameter,
5.2 mm inner diameter) reaching as high as 22 cm below
the surfaguide. After entering the plasma, vapors are dissociated
mainly into iron and carbonyl. The iron atoms oxidize
and agglomerate immediately due to the presence of oxygen
plasma, forming iron oxide nanoparticles, which are stable
under air atmosphere [2]. The nanopowder is collected on finemesh
filter at the output of the plasma reactor. Typical size of
nanoparticles is in 10–50 nm range [2].
High–resolution digital images are taken using Canon EOS
50D camera with Canon EF-S 17–85 mm f/4–5.6 IS USM lens.
Optical emission spectroscopy is carried out using Jobin Yvon
TRIAX 550 spectrometer with CCD detector and an optic fiber
placed perpendicularly to the discharge vertical axis. Spectral
sensitivity of the instrument was calibrated using a tungsten
halogen lamp.
Counter–flowing iron pentacarbonyl does not turbulently
mix with oxygen. Instead it forms a glowing mushroomlike
shape [Fig. 1(a)]. We can distinguish two parts of
the mushroom—a cap and a stem. The mushroom becomes
smaller as the operating pressure reaches the vapor saturation
pressure of Fe(CO)5.
The long cylindrical stem forms as a result of
oxidizing precursor vapors. Undissociated iron pentacarbonyl
flows through the center, oxidizing mainly at its edges. It has
yellow–orange color. The length and shape of the stem
depend on the experimental conditions. Its length increases
with decreasing oxygen flow while the edges become more
diffused due to lower total pressure [Fig. 1(b)]. The length of
the stem also depends on the Fe(CO)5 flow rate.
The cap formed on the top of the stem is predominantly
orange and much brighter that the stem. This suggests excessive
heating in this region due to the exothermic iron oxidation.
The baseline of the spectrum (i.e. without the spectral lines
of Fe and O) was fitted with the blackbody spectrum giving
the temperature of 1850 K. Dark current was not subtracted
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ŠNÍRER et al.: OPTICAL DIAGNOSTICS OF IRON OXIDE NANOPARTICLE SYNTHESIS 2779
Fig. 1. Oxygen plasma with Fe(CO)5 admixture during nanoparticle synthesis. (a) Mushroom-like effect formed by counter flowing iron pentacarbonyl has
two components—a cap and a stem. (b) Its shape as a function of oxygen flow (700 W power, F/5.6, 1/5s, ISO 800, f = 47 mm). (c) Emission spectrum of
the cap corrected for spectral sensitivity, fitted with the black body radiation function (power 500 W, O2 flow 200 sccm).
from the detector signal and hence, the region below 675 nm
differs from the Planck spectrum.
Optical diagnostics is able to visualize phenomena occurring
during the synthesis of nanoparticles and provide valuable
information, hence, improving our understanding of the
processes involved. It can also serve as a basis for optimization
and future enhancements, e.g. in situ functionalization.
REFERENCES
[1] M. Moisan and Z. Zakrzewski, “Plasma sources based on the propagation
of electromagnetic surface waves,” J. Phys. D, Appl. Phys., vol. 24,
no. 7, p. 1025, 1991.
[2] P. Zelina et al., “Versatile low–pressure plasma–enhanced
process for synthesis of iron and iron–based magnetic
nanopowders,” World J. Eng., vol. 9, no. 2, pp. 161–166,
2012.