Article
Conversion Study on the Formation of Mechanochemically
Synthesized BaTiO3
Gábor Kozma 1, * , Kata Lipták 1 , Cora Deák 1 , Andrea Rónavári 1 , Ákos Kukovecz 1 and Zoltán Kónya 1,2
1
2
*
Citation: Kozma, G.; Lipták, K.;
Deák, C.; Rónavári, A.; Kukovecz, Á.;
Kónya, Z. Conversion Study on the
Formation of Mechanochemically
Synthesized BaTiO3 . Chemistry 2022,
4, 592–602. https://doi.org/10.3390/
Department of Applied and Environmental Chemistry, University of Szeged, 6720 Szeged, Hungary;
liptak.katalina@gmail.com (K.L.); cora@chem.u-szeged.hu (C.D.); ronavari@chem.u-szeged.hu (A.R.);
kakos@chem.u-szeged.hu (Á.K.); konya@chem.u-szeged.hu (Z.K.)
MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, 6720 Szeged, Hungary
Correspondence: kozmag@chem.u-szeged.hu
Abstract: Mechanochemistry is a method that can cover the energy demand of reaction pathways
between solid materials. This requires enough energy to maintain the reactions between the starting
materials. This is called “high-energy milling”. In our case, a planetary ball mill provided the required
energy. Using the Burgio-equation, the required energy is determinable; the energy released during a
single impact of a milling ball (Eb ), as well as during the whole milling process (Ecum ). The aim of this
work was the one-step production of BaTiO3 from BaO and TiO2 starting materials. Whereas during
mechanochemical reactions it is possible to produce nanoparticles of up to 10 nm, the essence of this
study is to develop the preparation of BaTiO3 with a perovskite structure even without subsequent
heat treatment, since sintering at high temperatures is associated with a rapid increase in the size
of the particles. By describing the synthesis parameters and their energy values (Eb and Ecum ), it is
possible to transpose experimental conditions, so that in the case of other types of planetary ball mills
or grinding vessel made of other materials, the results can be used. In this study, the mechanical
treatment was carried out with a Fritsch Pulverisette-6 planetary ball mill and the transformation of
the starting materials was investigated by X-ray diffractometric, Raman and Energy-dispersive X-ray
spectroscopic, and transmission electron microscopic measurements.
Keywords: mechanochemistry; perovskite; BaTiO3 ; ball-milling; nanoparticles
chemistry4020042
Academic Editors: Marcela
Achimovičová, Matej Baláž and
1. Introduction
Abhishek Lokhande
Ceramics are produced and used in huge quantities all over the world due to the wide
variety of probable applications. In total, there are only a few minerals with applications
and technological uses that dominate the industrial application (e.g., quartz, calcium
silicates, alumina, and titanium dioxide). These materials are characterized by both the
crystal structure and the composition and are important due to their specific properties
and applications. The question arose whether there is a structure that is multifunctional
and crystallographically suitable for the development of useful properties. Considering
the three-component crystal structures, there are only a dozen ceramics that are widely
used. Among them, the A2 BX4 spinel and ABX3 perovskite excel and perovskite is the only
structure, the chemical modification of which results in an extremely wide range of phases
with completely different properties [1]. Due to its unique electrical properties, the family
of chemical compounds with perovskite-structure permits a wide range of electrotechnical
applications: semiconductor dielectrics, superionic conductors, combined with ionic and
electron conductivity for high-temperature superconductors [2].
The possibility of fine grinding, mechanical activations and chemical reactions that can
be carried out in planetary ball mills has long been known. However, several factors affect
the success of each milling, since the energy generated during milling must be in balance
with the properties of the desired product, therefore predetermined, optimal parameters
Received: 29 April 2022
Accepted: 13 June 2022
Published: 15 June 2022
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Chemistry 2022, 4, 592–602. https://doi.org/10.3390/chemistry4020042
https://www.mdpi.com/journal/chemistry
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Chemistry 2022, 4
are very important. It is necessary to consider the material quality of the grinding vessel
and balls; the rotational speed; the time of treatment; the applied atmosphere and temperature [3], the number of balls and the filling ratio of the balls and reactants; and the physical
and chemical properties of the reactants [4]. These parameters are also very interdependent
and play an important role in the development of optimal milling energy, thus achieving
the best product yield in the shortest reaction time [5]. Obviously, higher milling energy
can be achieved by increasing the rotational speed or using a grinding vessel of higher
hardness. In our case, grinding vessels made of three different materials (silicon nitride,
stainless steel, tungsten carbide) were used, although there are studies in which BaTiO3
was produced in zirconia grinding vessels [6,7]. The intensity of grinding increases the
particle size of crystalline materials, or when powders are ground, compounds of different
compositions can be formed with a temperature change. However, it should be noted that
too much energy may cause the onset of secondary reactions, such as product degradation
or transformation.
In the literature, many examples can be found in the production of barium titanate
from barium-oxide [8] or barium-peroxide [9] and titanium-dioxide precursors. In many
of these cases, mechanical activation is used [10]. It is important to note that this does not
mean the transformation of the starting materials during grinding, but is only used for the
thorough mixing of precursors, while the perovskite-structured barium titanate is formed
during the subsequent heat treatment [8–10]. High-energy milling may be suitable for
creating the conditions for the temperature required for subsequent calcination. Research
of this kind began as early as the 1960s when Bowden and Yoffe introduced the “Hot-spot”
theory [11]. Later, Weicher and Schiner experimentally demonstrated that the area carrying
extra energy is about 1 mm2 with a temperature of 1000–1500 K, which keeps this state
for about 10−4 –10−3 s [12]. By providing this extra energy, it is also possible to develop
BaTiO3 mechanochemically. It should be noted that, in addition to the mechanochemical
process, BaTiO3 perovskite can also be produced in several other ways, including a wetchemical [13], hydrothermal [14], or microwave-assisted hydrothermal reaction [15]. In
contrast, the mechanochemical process has the advantage that it does not require an
expensive solvent, it can be produced in one step, and, as we present later, there is no need
for subsequent heat treatment [16]. The advantage of this is that, it avoids the increase in
the size of the particles at high temperatures and avoids the undesirable transformation of
the product, which in many cases can happen even during mechanical treatment [17].
In this study, we followed the formation of perovskite-structured barium titanate
from BaO and TiO2 precursors with X-ray diffractometric (XRD) and Raman spectroscopic
measurements. To do this, we used three grinding drums of different hardness and
methodically changed the number of grinding balls, the rotational speed, and the time of
grinding. Using the Energy-dispersive X-ray spectroscopic (EDS) technique, we measured
the barium-titanium ratio of the powder mixture. Since BaO is water-soluble (~3.6 g/100 mL
at 20 ◦ C) as opposed to BaTiO3 and TiO2 , which are practically insoluble in water, after
washing with distilled water the ratio of the two metals must change.
2. Materials and Methods
2.1. The Ball-Milling Experiments
The grinding was carried out in a Fritsch Pulverisette-6 planetary ball mill. Such mills
are suitable for high-energy milling, which allows them to provide the energy needed to
form BaTiO3 . In each case, grinding balls with a diameter of 10 mm were used in the three
grinding vessels (80 mL) of the same material. To produce BaTiO3 2.00 g BaO (99.99%) and
1.04 g TiO2 (≥99%) were measured in all cases. The energy released during the millings has
been predetermined. This was made possible by the energy model introduced by Burgio
et al. [18]. The applicability of the model to our system has already been proven by our
research team experimentally [19]. The Equation (1) can be used to determine two energy
values: the Eb (1), which represents the total energy available during an impact event of a
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Chemistry 2022, 4
milling ball, and Ecum (2), which means the energy transferred to 1 g of the powder during
the whole milling:
! "
#
πd3b
ωv 2 dv − db 2
ωv 2 dv − db 2
ωv dv − db
ωv
1
2
ωp (
)
) − 2rp (
)(
)−
(1 − 2
Eb = ϕb K ρb
2
6
ωd
2
ωd
ωd
2
ωd
2
(1)
where K is the geometric constant of the mill, ϕb is the obstruction factor, ρb is the density
of the milling balls, db is the diameter of the balls, dv is the diameter of the grinding vessel,
ωp and ωv is the rotational speed of the disc and the vessel and rp is the distance between
the rotational axes of the disc and the vessel [19].
Ecum =
Eb × f × t
mp
(2)
where f is the frequency of impacts, t is the milling time and mp is the mass of the measured
sample. Thus, it is possible to produce our product in similar energy conditions using
different parameters. This answers our question as to whether the perovskite structure
can be formed in all three grinding vessels. In the experiments, we changed the rotational
speed from 300 to 500 rpm, the number of grinding balls from 10 to 25. In all cases, the
grinding took 3 h. For XRD measurements, samples were taken hourly, while for Raman
and EDS measurements 3-h samples were used.
2.2. Characterization
The powder X-ray diffraction patterns were obtained using a Rigaku Miniflex II
XRD (Rigaku Corporation, Tokyo, Japan) instrument operating with Cu Kα radiation
(λ = 1.5406 Å). The 2Θ Bragg angles were scanned over a range of 5–90◦ at a rate of
1.0◦ min−1 . Transmission Electron Microscope (TEM) analysis was performed with a FEI
Tecnai G2 20 X-TWIN instrument with a point resolution of 0.26 nm. Samples were placed
on holey carbon-coated copper grids of 300 mesh. Raman characterization was performed
at the excitation wavelength of 532 nm, and a nominal laser power of 12.5 mW (Senterra
Bruker Optik GmbH, Ettlingen, Germany). The spectral resolution was set to ca. 3–5 cm−1 ,
and the interferometer resolution was 1.5 cm−1 . The elemental composition of the prepared
samples was characterized by energy-dispersive X-ray spectroscopy (Hitachi Co., Tokyo,
Japan) operating at 20 kV, equipped with a Röntec energy dispersive spectrometer with a
12 mm working distance. Transmission Electron Microscope (TEM) analysis was performed
by an FEI-Tecnai G2/20/X-TWIN (FEI Company, Hillsboro, OR, USA) instrument with a
point resolution of 0.26 nm. Samples were placed on holey carbon-coated copper grids of
300 mesh.
3. Results
9 different settings were determined based on the energy model. These values are
shown in the table below.
It can be seen that higher density grinding vessels and rotational speed increase the
value of the Eb , although there are smaller overlaps, such as SiN500/25 and stainless steel
SN300/25 settings.
In the case of Ecum values, this increase is not so gradual, since the number of grinding
balls used increases the frequency of impacts. Therefore, a higher Ecum may be obtained
for the lower density Si3 N4 vessel, where 25 grinding balls have been inserted than for the
FeNiCr vessel with a density of 2.3 times higher, where this value is lower due to slower
rotation and fewer grinding balls.
Raman spectroscopic measurements were performed on all 9 samples at excitation
wavelengths of 532 nm (Figure 1). On the spectra, the characteristic peaks of TiO2 and
BaTiO3 are clearly observed. In the case of TiO2 , characteristic peaks of the anatase structure
were observed [20]. TiO2 Raman-active lattice vibrations assigned as follows: (A1 ) 516 cm−1
+ (B1 ) 636 cm−1 + (B1 ) 395 cm−1 + (E) 635 cm−1 + (E) 144 cm−1 + (E) 198 cm−1 the weak
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Chemistry 2022, 4
band at 796 cm−1 was assigned as the first overtone of the B1 mode [21]. The Raman
− −1
spectrum of BaTiO3 crystals assigned as follows: (A1 ) 186 cm
+ (A1 ) 265 −cm−1 + (B1 )
−
1
−
1
−
−
303 cm and an asymmetric broadband (A1 /E) 520 cm and a broad weak peek at (A1 /E)
720− cm−1 [22]. Of the weak bands of BaO, only the most intense peak signal was detected at
(A1 ) 193 cm−−1 [23,24]. Due to the overlap between the starting materials and the vibrations
of the product, the progress of conversion cannot be clearly stated only based on the most
intense peak of BaTiO3 [25]. Therefore, it is possible to deduce from the decrease in the
intensity of the individual vibrations of TiO2 the re-evaluation of the precursors [26].
Figure 1. Raman spectra of all samples treated at different ball-impact energy (Eb ).
Figure 1 shows the Raman spectra of all samples between 370–700 1/cm. In this period
there are three intensive peaks specific to TiO2 and one peak typical of BaTiO3 . The peaks
of 516 and 520 1/cm largely overlap with each other [26]. Nevertheless, by increasing the
Eb , the change in intensity at the peaks of TiO2 395 and 636 1/cm can be traced, which
gradually disappears towards with the increasing energies. In the case of samples treated at
the highest Eb (FN500/10; TC400/10; TC500/10), these peaks are barely detectable, while
the peak typical of BaTiO3 in this range dominates at 456 1/cm.
The rate of transformation of starting materials is well suited to the rate of the growing
Eb . The only exceptions were the FN400/15 and FN400/25 samples, where even though
in the latter case the Eb was 3.11% less, the rate of transformation was slightly higher.
This phenomenon can be explained by an increase in the number of grinding balls and,
with it, by a higher value of Ecum , which was 61.5% higher in case the of the sample
FN400/25 (Table 1). This is supported by previous experience: the reaction rate is basically
determined by the input energy, but the frequency of impacts also counts, especially if the
energy transferred may be accumulating to some extent [19].
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Chemistry 2022, 4
Table 1. Values of Ecum and Eb for each grinding vessel. ωp : rotational speed of the grinding vessel.
Nb : number of grinding balls.
Grinding Vessel
Si3 N4
FeNiCr
TC
ωp (1/s)
Nb (pcs)
ωb (J/hit)
E
Ecum (J/g)
Short Name
300
25
0.0197
5907
SN300/25
ω
500
25
0.0548
27,350
SN500/25
300
25
0.0455
13,613
FN300/25
400
25
0.0809
32,269
FN400/25
400
15
0.0835
19,989
FN400/15
500
10
0.132
26,311
FN500/10
300
25
0.0845
25,282
TC300/25
400
10
0.157
25,018
TC400/10
500
10
0.245
48,865
TC500/10
In addition to Raman spectroscopic measurements, the formation of BaTiO3 has been
measured by XRD in this case of different grinding vessels (Figure 2). The typical reflections
of BaTiO3 between 2 theta 20–80◦ are listed below: 2 theta 22.04◦ (100); 31.44◦ (110); 38.76◦
(111); 45.08◦ (200); 50.72◦ (210); 56.02◦ (211); 65.68◦ (220); 70.20◦ (300); 74.66◦ (310); 78.98◦
(311) [27]. BaTiO3 produced at a sintering temperature above 1000 ◦ C, is typically tetragonal,
which is changed in hexagonal structure at 1400 ◦ C [28]. The evolution of the starting
materials and the reflections characteristic of BaTiO3 can be clearly tracked by increasing
the milling energy [26]. With mechanochemical treatment, the energy required for the
formation of the tetragonal structure was provided during the 3-h milling process, which
was detected first in the SN500/25 sample. The Eb limit for the transformation of the
precursors is therefore above 50 mJ/hit. Below this value, the perovskite structure does not
form despite further 6 h of grinding. This is additional information compared to Raman
results, as those measurements did not clearly show the beginning of the conversion due to
the overlap of the peaks of the precursors and the product.
Figure 2. XRD diffractograms of all samples treated at different ball-impact energy (Eb ). •: BaTiO3 ,
: TiO2 , ×: BaO reflections.
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Chemistry 2022, 4
●
□
As in the previous results, the exchange of FN400/25 and FN400/15 samples (by the
value of Eb ) based on intensities was observed in XRD measurements. This supports the
hypothesis that, in addition to the appropriate Eb , sufficient treatment time (Ecum ) should
be ensured.
For better visualization of the XRD results, diffractograms were used to track the
θ
formation of BaTiO3 product at the intensity of most typical peaks at 2θ 31.4◦ , i.e., based on
the fact that the intensity of this peak shows the increasing appearance of the product in
the grinding vessel (Figure 3). As a result, a so-called “milling-map” was created [29]. A
good correlation between the performed Eb and the reflection intensity is observed. The
threshold for ball-impact energy required to produce the BaTiO3 is well-drawn, which is
around 0.05 J/hit. In mechanochemistry, this is a typical limit, i.e., below Eb = 0.05 J/hit
low-energy, while above it is high-energy milling [29,30]. The latter arises from the limiting
factor of typical single-axis mills that, when using such a mill at too high a rotational speed,
the balls already move together with the walls of the grinding vessel above a certain value.
In a planetary ball mill, the complex rotation determined by the two axes, up to a very high
rotational speed, does not prevent the grinding balls detach and impact into the walls of
the grinding vessel [31]. Low-intensity reflections will only be replaced in samples ground
with the highest impact energy with signals higher than 400 cps, which clearly show the
presence of tetragonal BaTiO3 .
Figure 3. XRD measurement-based (BaTiO3 peak intensity at
θ 2θ 31.4◦ ,) milling-map of BaTiO3
samples in different grinding vessels.
Based on the Raman and XRD results, the minimum Eb required for the formation of
the perovskite structure can be determined (~0.05 J/hit). However, to be able to draw on the
relationship between the Eb and Ecum and the amount of BaTiO3 converted, a quantification
was also necessary, thus, the energy-dispersive X-ray spectrum of all 9 samples has been
measured (Figure 4). Since the water solubility of BaO is ~36 g/L at 20 ◦ C, assuming that the
weighed 2.0 g is not converted to BaTiO3 at all, it can dissolve in a minimum of ~57 mL of
water. In comparison, to prove that all Ba present in the form of BaO has been removed, all
samples have been washed with 2 L of deionized water. High-intensity characteristic peaks
corresponding to Ba (La —4.47; Bb1 —4.83; Bb2 —5.17 keV) and Ti (Ka —4.50; Kb —4,91 keV)
elements were noticed in the EDS patterns of the nanoparticles [32]. The percentage of Ba
and Ti relative to each other was plotted from the resulting values.
Chemistry 2022, 4
598
Figure 4. The atomic percentage (at% ) of Ba ■() and Ti●(•) based on EDS measurements, and the
calculated conversions ▲
(N) of the precursors. Dashed lines in the figure are guides for the eye.
Figure 4 shows the changes in the percentage distribution of the two elements examined and the calculated conversions. As the reaction progresses, the barium content
of the BaO precursor is gradually integrated into the water-insoluble BaTiO3 , so that its
amount will increase continuously in the sample that remains after distilled water wash.
The absolute amount of titanium should not change, as it is still in water-insoluble form as a
precursor and after being incorporated into BaTiO3 , but its relative amount will decrease in
relation to barium. If all starting materials were to be converted to BaTiO3 , the Ba-Ti ratios
would have to be 50–50%. In the sample milled with the highest Eb and Ecum (TC500/10),
the conversion value was 94%. Further conversion trends can be seeded from the path of
the pasted curve, as it gradually saturates. The 94% value could be increased by extending
the milling time, but the 100% cannot be achieved due to the trapping of the precursors
(on the wall of the grinding vessel, in particular at the junction of the lid and the vessel
wall) [33].
Figure 5 summarizes conversion data calculated from EDS measurements, Eb and Ecum
values as a function of each sample. The Eb values of the samples (FN400/25, FN400/15,
TC300/25) within the area framed by the dotted line are almost the same, the differences
are below 5%. In Ecum , however, there are significant differences due to the number of
grinding balls and the rotational speed. This discrepancy can be tracked in the conversion
of precursors, which follows the Ecum values. It can be concluded that the Eb necessary for
the formation of BaTiO3 perovskite is available, but most of it is still lost during grinding (in
the form of heat or friction). For this reason, the corrective effect of Ecum is necessary, which
counteracts this relatively low Eb . For this reason, the conversion of starting materials
follows the course of Ecum .
In contrast, in the case of samples (TC300/25, FN500/10, TC400/10) framed by a
dashed line (Figure 5), the values of Ecum are close to each other, with the largest difference
being less than 5%. The Eb values are as follows: 0.0845; 0.132; 0.157 J/hit. The difference
relative to the maximum value (TC400/10) is 46.2% for TC300/25 and 16.0% for FN500/10
samples. But the difference in the conversion of the starting materials is 28.6% for the
TC300/25 and only 4.8% for the FN500/10 sample. From this, it can be concluded that,
under the circumstances, further increases in the Eb will no longer bring such a significant
increase in the transformation of the precursors. This is confirmed by the fact that, in the
case of the TC500/10 compared to the TC400/10 sample, an increase in Eb by 56.1% and
95.3% in Ecum only causes an increase in conversion by 12%. Based on these results, the two
values can be determined from the point of view of both Eb and the Ecum , between which
the perovskite structured BaTiO3 is formed, and the speed of production can be influenced
without compromising the quality of the product.
Chemistry 2022, 4
599
Figure 5. The conversion of the precursors (#), the ball-impact (Eb , ∆) energy and the cumulative
○
∆
energy (Ecum , ) in case of all grinding sets. The areas with the dotted and dashed lines show the
□
samples compared in the text.
Figure 6 shows the TEM images of end-product in the case of SN300/25, FN300/25,
and TC300/25 samples. By increasing the density of the grinding vessels, the morphology
of the particles becomes sharper. While only a mixture of starting materials can be seen in
the Si3 N4 grinding vessel (SN300/25), as confirmed by the XRD results, individual particles
can be distinguished in the samples made in the FeNi (FN300/25) and TC (TC300/25)
grinding vessel. By the TEM images, size distribution histograms were made in the case
of FN300/25 and TC300/25 samples. Due to the sufficient Eb , particle growth is inhibited
during the formation of the BaTiO3 structure. It follows that, the size of the particles falls
within the nano range. The conspicuous difference between the material produced in
the two grinding vessels is that the increase in particle size was measurable in the TC
vessel, which provides more impact energy (Eb ). This phenomenon may have been caused
by excessive Ecum , which thus led to the sintering of particles. This process is orders of
magnitude greater in the case of subsequent heat treatment, while in this case, it allows the
final size distribution of the product to be regulated. Overall, the average diameter of the
BaTiO3 perovskite particles is 13.2 nm (FN300/25) and 19.2 nm (TC300/25).
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Chemistry 2022, 4
Figure 6. TEM images of BaTiO3 perovskites synthesized in the Si3N4 (SN300/25), FeNi (FN300/25)
and TC (TC300/25) grinding vessels.
4. Conclusions
The formation of BaTiO3 from BaO and TiO2 was studied in grinding vessels made
of different materials, at different rotational speeds and a different number of grinding
balls. Based on XRD and Raman measurements, the lowest ball-impact energy (Eb ) with
which perovskite-structured BaTiO3 can be formed under the studied experimental conditions has been determined (Eb = 50 mJ/hit). Above this value, within a wide range, the
transformation of starting materials is almost continuously increasing. It has been shown
that to produce the BaTiO3 perovskite in sufficient quantities, the synchronization of Eb
and cumulative energy (Ecum ) is essential. Based on the results obtained, the highest Eb
value can be determined, which can be used to speed up the formation of the product
(Eb = 160 mJ/hit). Above this value, the additional energy no longer contributes to increasing the rate of reaction. This excess energy often causes the product transformation,
crystallization, or growth of the individual particles through the process of sintering. In
this case, by accurately defining the grinding conditions, the parameters necessary for
the transformation have been successfully determined without further transformation of
the product. Choosing the necessary Eb and Ecum allows for the formation of the BaTiO3
structure, and in addition, increasing these energies, also allows to control of the final
size. In addition to the controllability of mechanochemical perovskite-synthesis, the results
support the goodness of the model for calculating Eb and Ecum . This allows the results
of the experiments to be quantified, thereby converting the grinding parameters between
grinding vessels of different materials or different types of planetary ball mills.
Author Contributions: G.K.: Conceptualization, Methodology, Formal analysis, Visualization, Validation, Writing—original draft. K.L.: measurements, laboratory work. C.D.: measurements, formal
analysis. A.R.: Writing—review & editing. Á.K.: Supervision, Funding acquisition. Z.K.: Supervision,
Funding acquisition. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the János Bolyai Research Fellowship of the Hungarian
Academy of Sciences (BO/00835/19/7 for G.K. and BO/00384/21/7 for A.R.), and by the professional
support of the New National Excellence Program of the Ministry of Innovation and Technology
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Chemistry 2022, 4
ÚNKP-21-5-SZTE-547 for G.K. and ÚNKP-21-5-SZTE-876 for A.R. Project No. TKP2021-NVA-19
has been implemented with the support provided by the Ministry of Innovation and Technology
of Hungary from the National Research, Development and Innovation Fund, financed under the
TKP2021-NVA funding scheme.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this paper.
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