Silver doping in lanthanum manganite materials: structural and electrical properties

Article abstract of DOI:10.1007/s10973-020-10095-1

Ag-doped LaMnO₃ nanoparticles have been synthesized through sol–gel process using citric acid as a chelating agent. Silver effect on physicochemical properties was studied. The physicochemical properties and electrical performance of the obtained materials were also investigated, using thermogravimetric analysis (TGA) and X-ray diffraction (XRD)?techniques. Thermogravimetric analysis shows that for the studied materials a decomposition stage between 3.7% and 4.1% can be observed, and mass increases from the LMN-Ag1 to the LMN-Ag3 sample. Also, in all those three situations an endothermic process with a maximum around 600?°C is observed. The diffractograms of the perovskite materials showed single-phase crystalline with high purity and rhombohedral structure with R-3c space group. The electrical measurements show that the conductivity spectrum is formed by both the static component σ_DC corresponding to the low frequencies and the component σ_AC corresponding to high frequencies. The values of σ_DC of samples are significantly influenced by the silver ions concentration.

Full text of DOI:10.1007/s10973-020-10095-1

Journal of Thermal Analysis and Calorimetry
https://doi.org/10.1007/s10973-020-10095-1
Silver doping in lanthanum manganite materials: structural
and electrical properties
1
2
2
3
3
1
Paula Sfrloaga · Gabriela Vlase · Titus Vlase · Iosiꢀ Malaescu · Catalin Nicolae Marin · Paulina Vlazan
Received: 29 January 2020 / Accepted: 16 July 2020
©
Akadémiai Kiadó, Budapest, Hungary 2020
Abstract
Ag-doped LaMnO nanoparticles have been synthesized through sol–gel process using citric acid as a chelating agent. Silver
3
efect on physicochemical properties was studied. The physicochemical properties and electrical perꢀormance oꢀ the obtained
materials were also investigated, using thermogravimetric analysis (TGA) and X-ray difraction (XRD) techniques. Ther-
mogravimetric analysis shows that ꢀor the studied materials a decomposition stage between 3.7% and 4.1% can be observed,
and mass increases ꢀrom the LMN-Ag1 to the LMN-Ag3 sample. Also, in all those three situations an endothermic process
with a maximum around 600 °C is observed. The difractograms oꢀ the perovskite materials showed single-phase crystalline
with high purity and rhombohedral structure with R-3c space group. The electrical measurements show that the conductivity
spectrum is ꢀormed by both the static component σ corresponding to the low ꢀrequencies and the component σ cor-
DC
AC
responding to high ꢀrequencies. The values oꢀ σ oꢀ samples are signiꢁcantly inꢂuenced by the silver ions concentration.
DC
Keywords Thermal analysis · Lanthanum manganite · Ag-doped · Electrical properties
Introduction
Doping is a process oꢀ purposely adding heteroatoms into
a target lattice, which modulates the stability and improves
the properties oꢀ semiconductors, while it does not change
the host crystal basic characteristics and structure [4].
From the specialized studies, it can be as certain that Ag
can be incorporated into the grid or it can substitute the lan-
The perovskites, in general, have ꢀormula ABO , where
3
cation A is larger than the smaller cation B. This structure
has B cation in sixꢀold coordination; respectively, cation A
is 12-ꢀold coordinated by oxygen anions. Also, the thermal
stability oꢀ perovskites is deꢁned by the cations at A and
B position [1]. Metal oxides with perovskite structure rep-
resent an important class oꢀ materials due to their electri-
cal, mechanical, optical, magnetic and catalytic outstanding
properties [2]. They have good thermal stability and band
gap around 3–4 eV and are interesting choice ꢀor gas sensing
materials [3].
3
+
+
thanum ions. The ionic radii oꢀ La and Ag in 12-coordi-
nation are 1.36 and 1.22 Å, respectively [5]. The doped man-
ganese oxides have attracted considerable attention due to
their electronic, magnetic and structural properties required
ꢀor diferent applications. Silver, among various metallic
species, is a good candidate ꢀor various reasons: it is appro-
priate to be used as electrode material because it is stable in
alkaline solutions and the overall electrical conductivity oꢀ
composite is improved by the addition oꢀ Ag [6]. In the last
decades, new methods oꢀ synthesis have been introduced in
order to obtain materials with a high speciꢁc surꢀace area,
which exceed the typical low values obtained by traditional
ceramic methods [7–9]. Multiple methods have been utilized
to synthesize perovskites; among them are solid phase, car-
bonate co-precipitation and sol–gel. Compared to alternative
synthesis methods, the sol–gel technique has advantages,
such as homogeneously obtained materials, oꢀ controlled
shape and size, as well as reduced reaction time [10].
*
Paula Sꢁrloaga
paulasꢁrloaga@gmail.com
1
National Institute ꢀor Research and Development
in Electrochemistry and Condensed Matter, Timisoara, P.
Andronescu No.1, 300224 Timisoara, Romania
2
3
Research Center ꢀor Thermal Analysis in Environmental
Problems, West University oꢀ Timisoara, Str. Pestalozzi
No. 16, 300115 Timisoara, Romania
West University oꢀ Timisoara, Vasile Parvan 4,
3
00223 Timisoara, Romania
Vol.:(0
1
123456789)
3

P. Sꢀrloaga et al.
1
00
5
These materials have signiꢁcant physicochemical proper-
ties, giving them special importance in the use oꢀ advanced
technologies, such as solid oxide ꢀuel cells [11], catalysts
4
95
90
85
80
75
70
TG
[
12], chemical sensors [13], photocatalysis [14] and biosen-
LMN.Ag1
sors [15].
3
2
1
0
–
LMN.Ag2
LMN.Ag3
The synthesis oꢀ the Ag-doped LaMnO nanomaterials
3
via sol–gel process, using citric acid as a chelating agent,
is presented in this paper. Also, the Ag doping efect on
the structural and electric properties as well as the thermal
stability oꢀ LaMnO system was investigated.
3
Materials and method
1
All the chemicals were oꢀ analytical grade and were used
without ꢀurther puriꢁcation. The ꢀollowing materials and
reagents were used ꢀor the gel synthesis: La(NO ) ·6H O,
NormHF
– 2
700 750
3
0
100
200
300
400
500
600
Temperature/°C
3
3
2
Mn(NO ) , NaOH and AgNO as the dopant source. The
3
2
3
materials were obtained by the sol–gel method ꢀollowing the
same steps as in the paper [16]. In addition, in order to carry
out a comparative study on the physicochemical properties
oꢀ the obtained materials, three diferent dopant concentra-
tions were used: Ag 0.05%, Ag 0.10% and Ag 0.15%, being
Fig. 1 Thermoanalytical curves ꢀor all samples in air atmosphere with
1
1
0 °C min− in the thermal range oꢀ 25–750 °C
LMN-Ag1 sample in the air atmosphere showed, on the
TG curve, three decomposition processes. The ꢁrst process
takes place in the range oꢀ 45–270 °C with a loss oꢀ 6.28%
oꢀ the sample mass. The decomposition is accompanied by
an exothermic process with a maximum at 115.89 °C and
noted as ꢀollows: Ag 0.05%-doped LaMnO : LMN-Ag1; Ag
3
0
.10%-doped LaMnO : LMN-Ag2; and Ag 0.15%-doped
3
LaMnO : LMN-Ag3.
3
To determine the thermal stability, a PerkinElmer
Thermogravimetric/Diferential Thermal Analyser TG/
DTA Diamond in dynamic air and nitrogen atmosphere
was used. The crystal phase oꢀ the as-synthesized products
was established by X-ray difraction using a PANalytical
−
1
a ∆H = 174.26 J g . The second decomposition process
occurs in the range oꢀ 270–370 °C accompanied by an exo-
−
1
thermic efect with ∆H=90.76 J g and a loss oꢀ 3.8% oꢀ
the sample mass.
difractometer, with Cu-K radiations (λ = 0.15406 nm)
The last stage oꢀ decomposition is composed oꢀ several
overlapping processes with a continuous loss oꢀ mass. On
the HF curve, there are several peaks, the most visible being
the one with the maximum at 603.65 °C.
α
in a 2θ range ꢀrom 20° to 80°, and the crystallite aver-
age size was determined ꢀollowing the Scherrer equation:
D = Kλ/βcosθ, where D represents the average crystallite
size, K the Scherrer constant, λ the wavelength oꢀ light
used ꢀor the difraction, β the line broadening at halꢀ the
maximum intensity (FWHM), aꢀter subtracting the instru-
mental line broadening in radians, and θ the Bragg angle.
The Scherrer constant (K) ꢀrom the above ꢀormula stands
In the nitrogen atmosphere, the thermal degradation pro-
cesses are similar to the decomposition in the air atmosphere
up to 530 °C (Fig. 2). Above this temperature, the thermal
behavior is diferent, in the case oꢀ the LMN-Ag1 sample on
the HF curve; the endothermic process is no longer present
in the air atmosphere. Above 700 °C a weak endothermic
process begins to show but does not end at 750 °C. The
same change in thermal behavior is also visible on the DTG
curve with the illustration oꢀ a decomposition process that
starts above 640 °C.
ꢀ
or the shape oꢀ the particle and in general has the value oꢀ
.9. Electrical measurements were perꢀormed using a RLC
meter, Agilent E4980A type, over the ꢀrequency range oꢀ
0 Hz to 2 MHz, at room temperature [17].
0
2
In the case oꢀ the LMN-Ag2 sample in the air atmos-
phere, the presence oꢀ the same decomposition stages
is observed as in the case oꢀ LMN-Ag1 sample. The ꢁrst
decomposition stage occurs in the range oꢀ 33–268 °C with
a loss oꢀ 7.6% oꢀ the sample mass being accompanied by an
exothermic process with the maximum at 112.26 °C and a
Results and discussion
Thermal analysis
The thermal decomposition process and phase transꢀor-
−
1
mations ꢀor Ag-doped LaMnO are shown in Fig. 1. The
∆H=155.94 J g . The second process with a maximum on
3
−
1
thermogravimetric analysis perꢀormed in the case oꢀ the
the HF curve at 331.14 °C and a ∆H= 82.75 J g occurs
1
3

Silver doping in lanthanum manganite materials: structural and electrical properties
(
a) 2.0
(b) 1.5
101
3.0
2.5
2.0
1.5
100
6
5
Ag 0.10% (air)
95
Ag 0.05% (air)
1.0
TG
9
9
8
8
5
0
5
0
TG
Ag 0.10% (nitrogen)
1
1
0
0
.5
.0
.5
.0
Ag 0.05% (nitrogen)
90
0.5
8
8
5
0
4
0.0
75
3
– 0.5
1.0
70
75
–
–
1.0
1.5
2.0
DTG
2
65
60
55
0.5
7
6
6
5
0
5
0
5
–
–
–
–
0.5
1.0
1.5
2.0
0
–
.0
DTG
1
–
0.5
1.0
5
4
0
5
0
– 2.5
– 3.0
–
NormHF
NormHF
700
50
– 1
40
– 1.3
750
3
5
100
200
300
400
500
600
700 750
30
100
200
4.0
300
400
500
600
Temperature/°C
Temperature/°C
(
c)
1
1
0
0
.5
.0
.5
.0
100
Ag 0.15% (air)
3.5
9
5
0
5
0
5
0
5
0
5
Ag 0.15% (nitrogen)
TG
3.0
9
8
8
7
7
6
6
5
2
2
.5
.0
1.5
–
–
–
–
–
–
0.5
1.0
1.5
2.0
2.5
3.0
1.0
DTG
0.5
0
–
–
–
–
.0
0.5
1.0
1.5
2.0
Norm.HF
100
50
– 2.282
30
200
300
400
500
600
700
750
Temperature/°C
Fig. 2 Comparative thermoanalytical curves ꢀor LMN-Ag1 (a), LMN-Ag2 (b), LMN-Ag3 (c) samples in air and nitrogen atmosphere with
−
1
1
0 °C min in the thermal range oꢀ 25–750 °C
in the range oꢀ 270–370 °C with a loss oꢀ 3.96 ꢀrom the
sample mass. The last stage oꢀ decomposition takes place
through complex overlapping processes, accompanied by a
continuous loss oꢀ mass. In the nitrogen atmosphere in the
case oꢀ the LMN-Ag2 sample, as in the previous sample,
the lack oꢀ the endothermic process (see the HF curve) with
the maximum at 590 °C can be observed and also the pres-
ence oꢀ a decomposition process that begins above 680 °C
accompanied by a weak endothermic process which does not
end in the studied temperature range.
stages oꢀ decomposition are accompanied by complex pro-
cesses that cannot be separated under these conditions. The
total loss in the analyzed temperature range is 18.5% oꢀ the
initial mass oꢀ the sample. The thermal analysis perꢀormed
ꢀor the LMN-Ag3 sample in the nitrogen atmosphere showed
the same behavior as ꢀor the previous samples.
In the case oꢀ thermal decomposition oꢀ LMN samples
doped with increasing concentration oꢀ Ag, it is ꢀound that
the ꢁrst decomposition process noticed in the air atmosphere
is completed at slightly lower temperatures with increasing
dopant concentration. The thermal decomposition oꢀ the
samples up to 500 °C shows two stages resulting in a mixture
oꢀ La O and Mn oxides at diferent oxidation states oꢀ Mn
The thermogravimetric analysis carried out ꢀor the LMN-
Ag3 sample in the air atmosphere reveals the same decom-
position stages as in the previous cases. The ꢁrst decomposi-
tion stage is in the range oꢀ 45–259 °C with a loss oꢀ 5.64%
oꢀ the sample mass and a maximum at 107.95 °C with a
2
3
depending on the amount oꢀ oxygen present. In the atmos-
phere oꢀ air at the decomposition oꢀ LaMnO , La O and
3
2
3
−
1
∆
H=128.95 J g . The second process has a maximum on
MnO are ꢀormed and in inert nitrogen environment La O
2
2
3
−
1
the HF curve at 268 °C and a ∆H = 90.08 J g . The last
and Mn O are ꢀormed.
2
3
1
3

P. Sꢀrloaga et al.
In the air atmosphere, the decomposition above 550 °C
begins with the transition oꢀ MnO to Mn O , a process that
The structural data ꢀor all samples were analyzed by the
Rietveld reꢁnement method with FullProꢀ soꢀtware [19],
and the ꢀollowing spatial groups reported in the literature
ꢀor the perovskite phase were tested: Pnma, R-3c, Pm-3m
and C12/c. Rietveld reꢁnement with space group: R-3 c and
Pm-3m, gave the best results, but no clear distinction can
be made between these two. According to Markelova et al.
[20], the Ag-doped perovskite samples were indexed within
the R-3c space group. From the x-ray difraction analysis,
it can be noticed that the structural parameters (Table 1)
show a slight increase oꢀ the c lattice parameters with the
increase in dopant concentration. On the other hand, unit
cell volumes ꢀor Ag concentration x=0.05 and 0.1%, respec-
tively, increase with the increase in dopant concentration and
2
2
3
is not ꢀound in the case oꢀ the nitrogen atmosphere when
Mn O is compose [18]. Thus, we can argue the diferent
2
3
behavior oꢀ doped samples in the atmosphere oꢀ synthetic
air, respectively nitrogen. Even the decomposition oꢀ Mn O
2
3
is shiꢀted to higher temperatures in the presence oꢀ the inert
nitrogen atmosphere. The amount oꢀ dopant does not greatly
inꢂuence the decomposition oꢀ samples above 550 °C.
XRD analysis
The XRD patterns oꢀ Ag-doped LaMnO nanoparticles syn-
3
thesized through sol–gel technique and ꢀollowed by calcina-
tions at 600 °C ꢀor 6 h are shown in Fig. 3. The structural
characterization ꢀor the synthesized samples was achieved
by powder X-ray difraction (XRD) using a PANalytical
X′Pert Pro MPD-type difractometer with Cu-Kα radiation
decrease ꢀor x=0.15% sample; this may be due to the limited
+
solubility oꢀ Ag ions in LaMnO , which generate a change
3
in stoichiometry oꢀ the compound [21].
(
λCu=1.54060 Å). The difraction angle (2Theta) ranged
Electrical measurements
between 15° and 80° and a 2θ-step oꢀ 0.2°/min.
The XRD difraction peaks are well deꢁned, and it reveals
that the samples are well crystallized indexed JCPDS:01086-
Figure 4 shows the dependence on ꢀrequency (f), over the
range 20 Hz–2 MHz, oꢀ the real (Z′) and imaginary (Z″)
*
1
22. The intense and sharp peaks were observed and cor-
components oꢀ the complex impedance (Z ), at room
respond to the well-known perovskite phase without any
secondary phases.
temperature.
As can be observed ꢀrom Fig. 4, the real component Z′
remains constant over a wide range oꢀ ꢀrequencies (up to
a) Ag 0.05%
b) Ag 0.1%
c) Ag 0.15%
160
(1) Ag 0.05%
(2) Ag 0.10%
(3) Ag 0.15%
1
1
1
40
20
00
1
Z ′
c)
2
3
fmax,1
8
6
0
0
fmax,2
x
fmax,3
b)
40
Z ″
1
2
2
0
0
3
x
a)
0
.02
0.1
1
10
100
1000
JCPDS:01-086-1229
f/kHz
20
30
40
50
60
70
80
2
θ/°
Fig. 4 Frequency dependence oꢀ the real (Zʹ) and imaginary (Z″)
components oꢀ the complex impedance oꢀ LaMnO samples with diꢀ-
3
Fig. 3 X-ray difractograms ꢀor perovskite materials
ꢀerent Ag concentrations
Table 1 Crystallographic data
or Ag-doped LaMnO
Unit cell volume/Å3
Sampler
Lattice parameters
Average diameter
oꢀ crystallites/nm
ꢀ
3
a/Å
b/Å
c/Å
LMN-Ag1
LMN-Ag2
LMN-Ag3
5.47290
5.48170
5.47656
5.47290
5.48170
5.47656
13.35383
13.34735
13.33640
346.395
347.342
346.405
33.5
29.8
29.8
1
3

Silver doping in lanthanum manganite materials: structural and electrical properties
about 10 kHz) and then decreases rapidly with the increasing
oꢀ ꢀrequency, ꢀor all samples. At the same time, ꢀor a constant
Table 2 The electrical parameters oꢀ samples obtained by electrical
measurements
Sample
σDC/S m−1
Rg/kΩ
C /pF
ꢀrequency f, the Z′ component decreases by increasing the x
fmax/kHz
τ/μs
g
concentration oꢀ Ag ions. The imaginary component Z″ shows
3
LMN-Ag1
LMN-Ag2
LMN-Ag3
139.16
190.88
212.44
1.14
0.83
0.75
3.25·10−
4.54·10−
5.00·10−
134
96
88
8.54
8.68
8.52
a maximum ꢀor each sample at an f ꢀrequency that moves
max
3
to higher values when the x concentration increases (Table 2),
3
indicating the existence oꢀ an electrical relaxation process in
the samples. Based on the f
values and using the Debye
max
Eq. (1), we computed the relaxation time τ corresponding to
√
each sample [22], the values being listed in Table 2.
where�Z � = Z�2 + Z��2 is the complex impedance modu-
∗
lus oꢀ the sample, d is the length and A is the transversal
sectional area oꢀ the sample. The ꢀrequency dependence oꢀ
the electrical conductivity oꢀ the samples is shown in Fig. 6.
The conductivity spectrum, σ(f), ꢀrom Fig. 6 consists oꢀ
two regions, illustrating two components oꢀ σ: (1) a compo-
nent that does not depend on ꢀrequency but only on tempera-
ture and corresponds to region oꢀ low ꢀrequencies (compo-
2
ꢀf
ꢁ = 1
max
(1)
The Nyquist diagram, Z″(Z′), ꢀor the investigated samples,
at room temperature is shown in Fig. 5.
From Fig. 5 it can be seen that each Nyquist diagram oꢀ
the samples consists oꢀ a semicircular arc. As a result, one
can assert that the microstructure oꢀ the sample (consisting
oꢀ grains oꢀ LaMnO doped and separated by air gaps) is cor-
nent σ ) and (2) a component that increases rapidly with
3
DC
related with the electrical process that occurs in each sample.
Given the microstructure oꢀ each sample, the equivalent elec-
trical model is ꢀormed oꢀ an R -C parallel circuit (R is the
ꢀrequency and temperature (component σ ), corresponding
AC
to the region oꢀ high ꢀrequencies, according to the Jonscher’s
universal power law [26]:
g
g
g
resistance oꢀ grains and C their electrical capacitance) [23].
g
n(T)
ꢀ
= ꢀDC + ꢀAC = ꢀDC + A(T)ꢁ
(4)
The diameter oꢀ semicircle represents the resistance R [24,
g
2
5] and the capacitance C can be computed ꢀrom relation
g
In Eq. (4), A(T) and n(T) are parameters which depend
(
2). The parameters thus obtained ꢀor each sample are shown
only on temperature and the exponent n is smaller than unity
in Table 2.
(
0<n<1) [27].
ꢀ
= 1∕R Cg
g
Based on Fig. 6, we have determined the experimental
max
(2)
values oꢀ DC conductivity (σ ) oꢀ samples and in Table 2
DC
Based on the complex impedance measurements, the elec-
are listed these values obtained. The ꢀrequency dependence
trical conductivity σ was computed using the equation:
oꢀ the DC conductivity (σ ) oꢀ samples is presented in
DC
Fig. 7.
Z�
d
ꢀ
=
The conductivity value oꢀ the investigated samples is
typical ꢀor semiconductors. In granular semiconductors, the
(3)
2
∗
A
ꢀ
Z ꢀ
(
(
(
1) Ag 0.05%
2) Ag 0.10%
3) Ag 0.15%
1
40
20
00
0
0
0
0
0
0
0
.30
.25
.20
.15
.10
.05
.00
(
(
(
1) Ag 0.05%
2) Ag 0.10%
3) Ag 0.15%
1
1
x
ωmax = 1/R Cg
g
σAC
8
6
4
2
0
0
0
0
0
σ
x
σ DC
3
1
0.02
0.1
1
10
100
1000
0
20
40
60
80
100
120
140
f/kHz
Z′/kΩ
Fig. 6 Frequency dependence oꢀ the conductivity σ oꢀ LaMnO sam-
3
Fig. 5 The Nyquist plots ꢀor investigated samples
ples with diferent Ag concentrations
1
3

P. Sꢀrloaga et al.
6
5
5
4
4
3
3
.0
.5
.0
.5
.0
.5
.0
perꢀormed in the nitrogen atmosphere shows us, in the
case oꢀ all the samples, a diferent decomposition than the
decomposition realized in the air atmosphere. The complex
(
(
(
1) Ag 0.05%
2) Ag 0.10%
3) Ag 0.15%
x
3
*
impedance measurements, Z (f)=Z′(f)−iZ″(f), over the ꢀre-
2
quency range oꢀ 20 Hz–2 MHz, at room temperature, reveal
that the imaginary component Z″ oꢀ the samples, presents
a maximum corresponding to an electrical relaxation pro-
cess. From these measurements, the relaxation times (τ) oꢀ
the samples were calculated and the results show that by
increasing the Ag concentration, τ decreases. The results
show that the conductivity spectrum is ꢀormed by both the
σ
σDC
1
0
.02
0.1
1
10 20
f/KHz
static component σ corresponding to the low ꢀrequencies
DC
and the component σ corresponding to high ꢀrequencies.
AC
Fig. 7 Frequency and concentration dependence oꢀ Ag ions oꢀ static
conductivity oꢀ investigated samples
The values oꢀ σ oꢀ samples are signiꢁcantly inꢂuenced by
DC
σ
DC
the dopant concentration.
Acknowledgements This paper was ꢀinancially supported by the
National Nucleu Project PN 19220201, Ctr. 40N/2019. The authors
thank to Maria Poienar (INCEMCT) ꢀor their help during the materi-
als’ characterization.
electrical conduction process is determined by the mecha-
nism oꢀ hopping oꢀ the charge carriers between the localized
states, in the vicinity oꢀ the Fermi level [28].
The change in the concentration oꢀ Ag ions changes
the density oꢀ the localized states near the Fermi level,
between which the hopping oꢀ the electric charge carriers
is perꢀormed; consequently, the electrical conductivity oꢀ
the investigated samples changes, increasing ꢀrom about
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−
3
−1
3
.3·10 S m (measured at the ꢀrequency oꢀ 20 Hz, ꢀor a
concentration oꢀ Ag in order oꢀ 0.05%) at the approximate
−
3
−1
value oꢀ 5·10 S m (measured at the ꢀrequency oꢀ 20 Hz,
or a concentration oꢀ Ag in order oꢀ 0.15%).
As can be observed in Fig. 7, the DC conductivity (σ
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