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Journal of Technological and Space Plasmas, Vol. 1, Issue 1 (2020)
Temperature influence on the diethylamine sensing
abilities of CuO nanoparticles deposited by atmospheric
pressure plasma
G. Filipič1and J. Gruenwald2
1Jozef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
2Gruenwald Laboratories, Taxberg 50, 5660 Taxenbach, Austria
In this work, we present a copper oxide nanostructured analysed as a gas sensor, but the focus of the paper
is on the temperature dependence of the sensor sensing properties. As a case study, temperature dependent
diethylamine sensing is presented. The CuO nanoflakes were deposited and evenly distributed on intercalated
electrodes by an atmospheric pressure plasma source. The sensor was electrically connected to ohmmetre
and inserted in an oven chamber where it was isolated from the atmosphere and heated to the desired tem-
peratures. The intrinsic resistance of the sensor was measured in dependence of the temperature and the
temperature change rate. Then the possibility to detect diethylamine was investigated. Furthermore, the
amine influence on the sensor resistance in the correlation to the amine effect on the sensor temperature
was shown. Finally, the temperature dependence of the amine detection was explored. It was demonstrated
that the plasma deposited nanostructures could be a way towards developing reliable sensing of the amine at
low temperatures of 100 ◦C and below.
DOI: 10.31281/jtsp.v1i1.10
gregor.filipic@guest.arnes.si
I. Introduction
Metal–oxide semiconductors are a popular choice for
chemical sensors due to the ease of manufacturing, struc-
ture, and morphology manipulations they offer. In the
case of gas sensing, common materials are SnO2[1, 2, 3],
ZnO [4, 5, 6], In2O3[7, 8, 9] and CuO [10, 11, 12]. How-
ever, they come with the downside of not being energy ef-
ficient due to the need of heating them to a working tem-
perature that can be quite high, even above 300 ◦C [13].
Despite many attempts to lower the needed temperature
and the availability of some sensors operating at room
temperature [14, 15], this remains a pertinent issue. This
paper aims at obtaining a better understanding of the
temperature dependence of the resistive type of gas sen-
sor. As a case study, we have chosen a copper-oxide
nanomaterial to detect diethylamine. The ability to detect
amines even in low concentrations is of high importance
due to health hazards, which are connected to these or-
ganic compounds. It was found, for example, that diethy-
lamine (DEA), which is used in the industrial production of
rubber, pharmaceuticals or pesticides, is responsible for
excessive bone growth (hyperostosis) in the nasal cavities
(turbinates) of lab mice [16] as well as for irritation of the
nose and eyes [17]. These negative effects on the health
were shown for DEA concentrations down to some ppm.
The prepared sensor was first exposed to heating to gain
an insight into its resistance dependence to temperature
and checked for the repeatability of the process. Then we
analysed the response – change of its resistance – to the
DEA injection and the desorption process. The tempera-
ture was simultaneously measured to detect any anoma-
lies in the sensing due to possible temperature perturba-
tion. Finally, we have repeated the detection experiments
of DEA at different temperatures of the sensor.
II. Experimental Setup
The gas sensing surface was homogeneously covered
with CuO nanoflakes that were deposited on intercalated
gold electrode substrates with a non-thermal plasma jet
at atmospheric pressure, which is described in detail else-
where [18, 19]. The depositions with the atmospheric
pressure plasma source were conducted with a mixture
of highly pure 5.0 gases; 1000 sccm of Ar and 7000 sccm of
N2gas flow. The input power was 150 W fixed at 30 mA in-
put current. The copper was introduced into the plasma
via a sacrificial electrode made of Cu with 99.99 purity.
The plasma source was fastened in a vertical position, and
samples were exposed for deposition 6 mm underneath
the nozzle. The morphology of the deposited material is
seen in Fig. 1. The CuO grows in the form of thin flakes
(d < 10 nm). The hight of the accumulated flake layer is
roughly 1 µm.
As it is known from a previous study regarding the chem-
ical compositions of these depositions, the nanoparticles
consist of CuO on top of a Cu2O layer [19]. The analysis
of the chemical variety of copper oxide in the aforemen-
tioned paper was done by depth profile measurements
with XPS. The nanoflakes are homogeneously distributed
over the copper and gold substrates used in our experi-
ments (Fig. 2). The large area that is achieved with such a
kind of nanoparticles enables excellent gas sensing prop-
erties in general.
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G. Filipič et al. J. of Technol. and Space Plasmas, Vol. 1, Issue 1 (2020)
Figure 1: Secondary electrons (SE) SEM image of the
nanostructured CuO semiconductor surface.
Figure 2: Simple schematic of the sensor chip; A) the
intercalated gold electrode on a polymer substrate with
a representation of nanomaterial deposition; B) the size
of the electrodes and the space between them in mm;
C) the approximate circular size of the deposited
material in mm.
The gas sensing experiments were conducted at a low
pressure in a vacuum system to control the sensor
environment and for the safety of the researchers. The
experimental setup was the same throughout all of the
sensing experiments: sensor was electrically connected
to a 2450 Keithley source-metre and inserted into a
centre of a 1 m long quartz tube with an 80 mm ODE in a
vacuum furnace with 45 cm heating zone (OTF-1200X-II,
MTI Corp.). A thermocouple was inserted in the tube -
from the opposite side, 2 cm from the gas sensor, also in
the axis of the tube - to monitor the temperature in the
vicinity of the sensor. The voltage on the thermocouple
was recorded by the same source-metre, and it was
translated to temperature, T, in ◦C via:
T=−
V
41 ·10−6+ 21 (1)
The chamber was evacuated and kept at low pressure
by a rotary pump. The pump was able to reduce
the pressure in the chamber down to 2.5 Pa. Dur-
ing the amine sensing experiments, the amine was in-
jected into the chamber from the opposite side by utilis-
ing a Bronkhorst’s vaporising system (CEM W-102A). The
vaporising system comprises two inlets, one for liquid
(amine) that is introduced under pressure through a liq-
uid mass-flow-metre, and another for carrying gas (ar-
gon), which is injected through a mass-flow controller.
The gas and the liquid are mixed in the vaporiser, and the
vapour is introduced to the vacuum chamber.
The first sets of experiments, results shown in sections
III.a) and III.b), were performed to test the reaction
of the sensor resistance to temperature change. The
sensor was in a vacuum while the furnace was set to
heat up from room temperature to 100 ◦C, stay there for
10 min, and then naturally cool down. The experiment
was repeated three times. The following part of the
experiment - results presented in section III.c) - was
performed to determine the sensor response to a DEA
injection into the system. In parallel, the influence of
amine on the temperature of the sensor was monitored
by thermocouple. The sensor was put in the chamber,
which was again evacuated to 2.5 Pa, heated to 100 ◦C,
and left there for the sensor to thermalise. Then, DEA
was injected into the system with a rate of 250 sccm
carried by 300 sccm of Ar. The changes in the resistance
of the sensor and temperature of the thermocouple
were measured against the time. They were continued
to be monitored for a couple of minutes after the flow of
amine was switched off. In the last experiment - III.d) -
DEA response at different temperatures was evaluated.
Thus, the same experiment as in III.c) was repeated with
several cycles of the DEA injection/evacuation at different
set temperatures: 100, 115, 130, and 145 ◦C.
III. Experimental Results
III.a) Resistance change due to temperature
change
The experimental results regarding the connection
between temperature and response of the CuO sensors
are presented and discussed. First, the measurements
of the resistance of the sensor during heating and
cooling are shown in Fig. 3. There have been three
Figure 3: Resistance of the sensor measured during
heating and cooling. The measurements were done
three times.
cycles of heating and cooling done in the vacuum. The
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G. Filipič et al. J. of Technol. and Space Plasmas, Vol. 1, Issue 1 (2020)
resistance curve during the heating in the first cycle,
M1, is noticeably different from the other two measure-
ments. The assumption is that this is due to the slow
desorption of various substances that were adsorbed on
the surface during the sensor shelf-time, which affects
the conductivity of the surface. The measurements
M2 and M3 were done after finishing with M1 without
exposing the sensor to the atmosphere. Their curves are
almost identical. Another thing one can notice is that
during the heating the resistance is proportional to the
inverse Boltzmann factor, which is to be expected since
resistance is inverse proportional to the density number
of the charge carriers of a semiconductor:
n0= 2 m∗kT
2π¯h23/2
exp −
(Ec−Ef)
kT (2)
The resistance dependence on the temperature during
cooling is quite linear as it can also be noticed from the
graph in Fig. 3.
III.b) Varying heating-cooling rates
We have conducted another experiment with different
rates of heating and cooling. We started at around
1.1 ◦C/min heating, then increased the heating to
4.2 ◦C/min until reaching the maximum temperature of
about 100 ◦C. The cooling has started with 3.3 ◦C/min
decrease and finished with a linear estimation of around
1◦C/min cooling. The results are depicted in Fig. 4; first,
the temperature and resistance change with time are pre-
sented in 4A and then combined into a resistance change
against temperature plot in 4B. It is observed that dif-
ferent rates of temperature change yield different re-
sponses.
At small temperature changes, it seems the change of
the resistance with the temperature could be linearly fit-
ted, but with different slopes for the different rates of the
temperature change. Furthermore, even in the Maxwell-
Boltzmann distribution fit, the parameters would have
significantly changed between the different rates of the
temperature changes. Further research is needed to
fully understand these dependencies, while it is clear that
the sensor’s resistance can be easily detected already at
50 ◦C.
III.c) Sensor case study
For a better response, the choice to heat the sensor to
100 ◦C was made. First, the sensor was inserted into the
chamber close to the thermocouple, as in the previous ex-
periments. The chamber was evacuated to the base pres-
sure (2.5 Pa) and heated to the pre-set temperature. After
the sensor got thermalised, Ar was leaked into the sys-
tem with 300 sccm of flow, while still pumping the cham-
ber. After the thermalisation, the DEA was injected into
the system with 250 sccm - the pressure rose to about
70 Pa. After 10 min, the DEA flow was terminated, and the
desorption started. The resistance of the sensor and the
temperature measured with the thermocouple are pre-
sented in Fig. 5.
Figure 4: Influence of different rates of temperature
change on the resistance of the sensor: A) measurement
of resistance and temperature against time;
B) presentation of the resistance against temperature.
Figure 5: Graphs of the sensor resistance and the
thermocouple temperature before (black), during (red),
and after the DEA injection of the system (blue).
It can be seen that after the DEA injection, the tempera-
ture of the thermocouple is dropping. This is ascribed to
the cooling by the evaporating DEA. The temperature is
falling until the equilibrium between the DEA adsorption
and desorption is reached, and thermodynamic equilib-
rium to furnace heating is established. In this process,
the temperature changes for about 1 ◦C. During this time
the resistance increases for around 9 MΩ. The resistance
is increasing fast after the DEA hits the surface of the sen-
sor and continues rising even after the temperature is in
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G. Filipič et al. J. of Technol. and Space Plasmas, Vol. 1, Issue 1 (2020)
equilibrium. In the beginning stage, one cannot decou-
ple the temperature effect from the sensing response in
the initial phase. However, the cooling for about 1 ◦C at
the fastest rate in Fig. 4 caused only about 5 MΩresis-
tance increase. In this case, the fastest rate contributed
less than 0.5 ◦C, which would contribute only 2.5 MΩto
the resistance increase according to Fig. 4. In the later
phase, while the temperature is constant, it is clear that
the resistance increases due to DEA adsorption on the
surface. The time during the slow drop of the tempera-
ture then contributed to the 6.5 MΩresistance increase,
which clearly can not be all attributed to the tempera-
ture change. Furthermore, the resistance curve shape
follows the path towards a saturated value, which is ex-
pected. After the DEA source is closed, a fast evapora-
tion phase follows. The resistance temporary increases
in a jump before it starts to decrease steadily. The jump
can be explained by rapid desorption, which cools the
surface as seen from the temperature curve. As it is evi-
dent from Fig. 4, fast changes in temperature yield more
significant changes in resistance; since the temperature
dropped in an almost discrete jump, the resistance of the
sensor is increased. When the temperature starts slowly
rising, the resistance change is governed mostly by the
amount of DEA adsorbed on the surface and its desorp-
tion. In the last part of the experiment, the temperature is
equalised again. Since the sensor resistance did not reach
the saturation level in the 10 min in the DEA atmosphere,
the sensing can be done again even before the complete
amine desorption.
III.d) Sensor response at different tempera-
tures
As the pre-tests were successful, the testing of the sensor
for the detection of DEA at different temperatures was in-
teresting. The known experiment for sensing the DEA was
repeated at 100, 115, 130, and 145 ◦C; at each tempera-
ture two cycles were run, each with 10 min amine flow
and 20 min desorption. The results are shown in Fig. 6 as
the change in resistance from the initial value before the
DEA injection.
The sensor response function is calculated as a relative
change of resistance presented in Eq.3:
S=R−R0
R0
(3)
The measurements show that the resistance increases for
between 4 and 6 MΩin the first 10 s and between 6 and
8 in the first 20 s. As we saw in the previous chapter, less
than 3 MΩcan be caused purely by the cooling effect.
Thus, the sensor response after the first 10 seconds can
be already used for the estimation of the vapour concen-
tration based on the measuring of the transient response
parameters [20]. Also at much longer times, longer than
5 mins, the temperature effect on the response can also
be seen from the Fig. 7 - higher the temperature of the
sensor, the higher the response is. Even more so, the
response dependence on temperature is not linear. Re-
turning to the number density of charge carriers at a spe-
cific temperature and the Eq. 2, one can again expect
Figure 6: Graph of change in resistance of the sensor at
four different temperatures during two cycles of amine
(DEA) injections and desorption. The DEA injection
periods are noted by the colouring of the graphs under
the curves.
an exponential dependence of the response to the same
concentration of the analyte at different temperatures.
This was confirmed by this experiment and is depicted
in Fig. 7 where the maximum responses after 10 min of
sensor exposure to DEA are plotted against temperatures
together with an exponential fit to guide the eye. The sat-
uration level of the resistance was not observed - the sen-
sor response and recovery times are well above 5 min.
However, one needs to keep in mind that we were op-
erating the sensor at very low temperatures and that if
the semiconductor sensors temperature is ramped up to
250 ◦C and above the response and recovery times be-
come much shorter [21].
Figure 7: The sensor response to amine injection at
different temperatures and the exponential fit with
temperature in the denominator of the exponent. The
error bars represent the difference in the response
between the two cycles at each temperature. The flow of
DEA was 250 and Ar 300 sccm, respectively.
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G. Filipič et al. J. of Technol. and Space Plasmas, Vol. 1, Issue 1 (2020)
IV. Conclusions
We have deposited copper oxide nanoflakes with a large
surface area on intercalated electrodes and shown that
it is possible to utilise it to sense diethylamine. The sen-
sor response research was done at low pressure to de-
termine intrinsic sensor properties. It was presented that
the resistance of the sensor changes with the tempera-
ture obeying exponential temperature dependence. Fur-
thermore, the results are reproducible if the temperature
is changed in the same manner in all cases. However,
if the rate of temperature change is varied, also the re-
sistance dependence on temperature changes. This phe-
nomenon needs further examination in order to evalu-
ate the sensor operation in a possible temperature fluc-
tuating environment. Furthermore, this work illuminates
the sensor resistance response during the initial contact
of the vapour of the detection interests with the surface
of the sensor. In this phase, the temperature drops very
fast, which can have an influence on the resistance, as
seen in the DEA detection experiment. The same be-
haviour needs to be tested in the live-world conditions at
atmospheric pressure, and higher temperatures to esti-
mate if these results need to be taken into account when
trying to use sensors transient times for the analyte con-
centration detection. In addition, there is a curious sud-
den resistance spike at the time of DEA switch off, which
was denoted to rapid DEA desorption from the surface
in the low–pressure atmosphere and consequent tempo-
rary cooling of the sensor. By demonstrating the ability of
CuO nanoflakes to respond to DEA even at low tempera-
tures, it was also possible to indirectly prove that atmo-
spheric pressure plasma sources can be successfully used
as a tool for gas sensor fabrication. Since the plasma de-
position occurs at ambient pressure directly on conduct-
ing substrates, it is a very efficient fabrication technology.
In the future, the sensor interaction with different atmo-
spheres (e.g. water vapour, oxygen) will be studied to find
how these gases influence the sensing possibilities and
temperature dependencies.
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