XXXIV ICPIG & ICRP-10, July 14-19, 2019, Sapporo, Hokkaido, Japan Topic 12 
Influence of admixtures on atmospheric pressure microwave plasma torch
used for graphene nanosheets synthesis 
V. Kudrle​, M. Snirer, J. Toman, O. Jasek, J. Jurmanova 
Department of physical electronics, Masaryk University, Brno, Czech Republic 
The influence of molecular admixtures on processes in microwave plasma torch operating in
atmospheric pressure argon was investigated. The goal was to to optimise working gas
composition during graphene nanosheets plasma deposition from ethanol as precursor. The
admixtures affected the process of filamentation, plasma temperature and properties of the
deposited material.
 
1. Introduction 
Atmospheric pressure microwave plasma
torch is versatile device which can operate under
wide range of experimental conditions. It was
successfully used for various applications, such
as advanced material synthesis, VOC
decomposition, plasma surface treatment or as a
source for analytic spectrochemistry. 
Its operation at atmospheric pressure induces
plasma channel contraction and filamentation.
This spatial inhomogeneity, while advantageous
from the point of view of power density, can
negatively affect the intended application. It is
known that presence of various admixtures can
strongly influence the filament formation and
behaviour. 
In this contribution such influence is
investigated with the goal of optimizing the
plasma deposition of graphene nanosheets. 
 
2. Experimental 
The plasma torch was excited by 2.45 GHz
magnetron (Muegge) with power variable in
10-300 W range. Microwave waveguide line was
quite standard, i.e. magnetron head, circulator,
reflectometer and matching. It ended with a
waveguide-to-rigid coaxial transition, able to
operate at substantially higher power levels than
flexible coaxial cable. 
Its central conductor (6 mm diameter) was
hollow which enabled passing of multiple gas
lines inside. It ended in exchangeable carbon or
stainless steel nozzle with dual flow
configuration. The central (axial) channel (0.8
mm diameter) was used for argon
(300-1000 sccm). While the intended admixtures
can be introduced directly to the central channel
it was advantageous to use a secondary channel
(annulus with outer radius 8.4 mm and inner
radius 7.7 mm), as this configuration increases a
stability of operation. 
For graphene synthesis, ethanol admixture
was used. It was vaporised in bubbler by the
carrying argon gas (10 - 700 sccm). The flow of
the carrying gas controlled the amount of ethanol
in 0-10 sccm range. Other admixtures (O​2​, H​2​,
N​2​) were also used, either alone or in
combination with the ethanol. 
Plasma reactor consisted of fused silica tube
(80 mm diameter, 200 mm long) terminated by
duralumin flange. Deposited material was Gas
collected from the reactor walls and/or the flange.
Samples were analysed using Raman
spectroscopy and scanning electron microscopy
(SEM). 
Optical emission spectroscopy was carried out
using Jobin Yvon Triax 550 spectrometer.
Rotational OH temperature was calculated by
MassiveOES software [1]. High resolution digital
imaging used consumer digital camera Nikon
D5200 with Nikkor 200 mm lens. 
 
3. Results and discussion 
Microwave plasma torch as many other
atmospheric pressure discharges exhibits a
tendency to strong radial contraction of the
plasma, forming a relatively thin and dense
plasma channel. For detailed discussion of
underlying phenomena, see e.g. [2].
When a small amount of hydrogen is
introduced to the outer channel, the filamentary
character of the plasma is maintained (see Fig.1)
while its appearance changes. Molecular
admixture, due to its low lying excitation levels
XXXIV ICPIG & ICRP-10, July 14-19, 2019, Sapporo, Hokkaido, Japan Topic 12 
effectively removes the energy from electrons
and so the plasma channel length decreases.
Reddish colour is easily attributed to H​α
emissions.
One may observe also changes in radial
emission profile, as the discharge becomes more
diffusive near the tip.
 
 
Fig. 1. Influence of hydrogen admixture on
appearance of the plasma filament​. The filament is
reflected by the silica wall, forming false second
filament to the right.
The presence of molecular admixture strongly
affects the energy transfer from electrons to a
movement of atoms/molecules. In pure Ar, the
electronic excitations do not couple well with the
translations, making the plasma rather cold. With
admixtures, the plasma becomes much hotter.
From Fig.2, where OH rotational temperatures
are plotted, it is seen, that the effect is not same
for all admixtures. Effect of hydrogen is
strongest, while nitrogen and oxygen behave
nearly same. The curve for ethanol does not
exhibit a saturation in the observed range.
Gas admixtures influence a balance between
C/H/O. It was reported by ​[3] that ethanol and
dimethyl ether (DME) have the same C/H/O ratio
and can be used as a precursor for graphene
synthesis. In our investigation, if several sccm of
O​2 were added to 2 sccm of ethanol only gas
products were formed during the synthesis. In the
case of H​2 admixture a yellow polymer-like
amorphous layer was formed. This result is in
contradiction with results achieved by Tsyganov
et al.​[4]​, where addition of 1 sccm of H​2 into
surface wave discharge led to increased graphene
nanosheets quality.
When the ethanol flow rate was increased to
10 sccm and flow rates of oxygen and hydrogen
were varied from 10 to 50 sccm, the amount of
defects in the graphene nanosheets structure was
increasing with amount of admixture gas as
shown by SEM and Raman spectroscopy.
 
Fig. 2. Evolution of plasma temperature (from OH
rotation) with increasing admixture. 
 
4. Conclusion 
We investigated influence of reactive gas
admixture on the stability and filament
formation in the dual-channel microwave
plasma torch discharge. The addition of
hydrogen containing gases, H​2 and ethanol,
led to substantial increase of discharge
temperature and in case of higher flow rates
of ethanol to the formation of defective
graphene nanosheets.
 
Acknowledgement 
This work was supported by the Czech
Science Foundation (GA ČR) under project
18-08520S and in part by the project LO1411
(NPU I) funded by Ministry of Education, Youth
and Sports of Czech Republic. 
 
References 
1. J. Voráč et al., Plasma Sourc. Sci.
Tech. 26(2) (2017) 025010.
2. Yu. B. Golubovskii et al., Plasma
Sourc. Sci. Tech 20 (5) (2011)
053002
3. A. Dato, M. Frenklach, New Journal
of Physics​ ​12(12) (2010) 125013​.
4. D. Tsyganov et al., Plasma Sourc.
Sci. Tech. 25 (1) (2016) 015013.