INVESTIGATION OF POLYOL PROCESS FOR THE SYNTHESIS OF HIGHLY PURE BIFEO3 OVOID-LIKE SHAPE NANOSTRUCTURED POWDERS - MDPI
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nanomaterials
Article
Investigation of Polyol Process for the Synthesis
of Highly Pure BiFeO3 Ovoid-Like Shape
Nanostructured Powders
Manel Missaoui 1,2 , Sandrine Coste 1, * , Maud Barré 1 , Anthony Rousseau 1 , Yaovi Gagou 3 ,
Mohamed Ellouze 2 and Nirina Randrianantoandro 1
1 Institut des Molécules et Matériaux du Mans (IMMM)–UMR CNRS 6283, Le Mans Université,
Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France; manel.missaoui.11@gmail.com (M.M.);
Maud.barre@univ-lemans.fr (M.B.); anthony_rousseau@univ-lemans.fr (A.R.);
Nirina.Randrianantoandro@univ-lemans.fr (N.R.)
2 Laboratoire des Matériaux Multifonctionnels et Applications (LMMA)—LR16ES18, Faculté des Sciences de
Sfax (FSS), Route de Soukra km 3.5, B.P.1171, 3000 Sfax, Tunisie; mohamed.ellouze@fss.rnu.tn
3 Laboratoire de Physique de la matière condensée (LPMC), Université de Picardie Jules Verne, Chemin du
Thil, CS 52501 80025 Amiens Cedex 1, France; yaovi.gagou@u-picardie.fr
* Correspondence: sandrine.coste@univ-lemans.fr; Tel.: +33-243833353
Received: 25 October 2019; Accepted: 13 December 2019; Published: 20 December 2019
Abstract: Exclusive and unprecedented interest was accorded in this paper to the synthesis of BiFeO3
nanopowders by the polyol process. The synthesis protocol was explored and adjusted to control the
purity and the grain size of the final product. The optimum parameters were carefully established
and an average crystallite size of about 40 nm was obtained. XRD and Mössbauer measurements
proved the high purity of the synthesized nanostructurated powders and confirmed the persistence
of the rhombohedral R3c symmetry. The first studies on the magnetic properties show a noticeable
widening of the hysteresis loop despite the remaining cycloidal magnetic structure, promoting the
enhancement of the ferromagnetic order and consequently the magnetoelectric coupling compared to
micrometric size powders.
Keywords: bismuth ferrite; polyol process; nanoparticles; multiferroic; cycloid
1. Introduction
Multiferroic materials present a combination of electrical, magnetic and/or elastic features.
They were classified in two types: type (I), in which the ferroelectricity coexists with the magnetic order
spontaneously, and type (II), where the ferroelectricity is induced by the magnetic order (e.g., TbMnO3).
The prototype material bismuth ferrite (BiFeO3 ), denoted as BFO, belongs to the type (I). Actually,
the ferroelectric order in BFO was described by the lone pair model [1] of bismuth (Bi, 6s2 ) which
causes, by the hybridization with oxygen orbitals, a displacement of bismuth from the center of the
perovskite. Furthermore, early studies have proven the existence of G-type antiferromagnetic order
and weak ferromagnetic order, and that was explained by the Dzyaloshinskii-Moriya (DM) spin canting
interaction [2]. However, in order to allow the unusual coexistence of both ferroelectric and magnetic
orders, a structural distortion of the crystal cell and of the rotating FeO6 octahedra is required.
From practical point of view, BFO is the unique simple perovskite that exhibits spontaneous
magnetoelectric coupling at room temperature, endowed with a high structural, magnetic and
ferroelectric stability in a large range of temperatures. Indeed, the ferroic transitions occur at TN (bulk)
∼643 K and TC (bulk) ∼1103 K, depending on the quality of the sample [3]. However, two major
obstacles are inhibiting the understanding of intrinsic BFO properties and the use of such a rare
Nanomaterials 2020, 10, 26; doi:10.3390/nano10010026 www.mdpi.com/journal/nanomaterials
Nanomaterials 2020, 10, 26 2 of 16
multiferroic and multifunctional material. First of all, the synthesis of the pure BFO phase is a hard
task to accomplish, due to the thermodynamic competition between the BFO phase and the Fe-rich and
Bi-rich phases (known as millite and sillenite, respectively) [4,5]. Second of all, beneath the magnetic
transition temperature, BFO presents a complex incommensurate magnetic structure, repeating each
62 nm, causing the compensation of most of the resulting magnetic moments which weakens the
ferromagnetism and lowers the magnetoelectric coupling. Therefore, the efficiency of BFO, particularly
for ambient applications, can be limited. By stabilizing the BFO phase and suppressing the cycloidal
structure, this material can be used in a variety of technological devices, such as spintronic devices,
Read-Write units for magnetic devices [6], gas sensors [7], capacitors, Magnetic Resonance Imaging
(MRI) and in the biomedical field [8], thanks to its magnetoelectric coupling [9] and nonlinear optical
properties, and also in photovoltaic and photocatalysis applications [10–12].
For these reasons, many chemical routes were developed to synthesize the pure BFO phase and
also to control the grain size and morphology, and it was found that in most cases, BFO nanoparticles
exhibit much better chemical and physical properties compared to micrometric powders [8,13,14].
Among the numerous soft chemical routes which were published, no study has been reported
up to date about the synthesis of BFO by the polyol process, despite the popularity of this technique
basically because of its simplicity, versatility and the possibility to obtain reproducible [15] high quality
materials with low-cost starting products and simple equipment [16,17].
Initially, the polyol process was used to produce metals [18] and alloys [19] and hence noble
metals (Au, Pt . . . ) due to its reducing capacity, and then it was enlarged for the synthesis of oxides [20]
and perovskites [21] displaying improved physical properties, and most of the time totally new
emerging electrical and optical features as a consequence of the changed shape, color, and particle
size and structure [22] and the enlarged specific area [23,24]. It is interesting to recognize the large
range of the possible sizes, going from the mesoscale down to 4 nm [25], which can be useful if
one needs to investigate the overturning of physico-chemical properties of some phases based on
the particles size. Actually, the polyol process gained its high reliability from its great efficiency to
inhibit the agglomeration of particles, to provide a remarkable uniformity of grain shape and narrow
size distribution, and to be able to mix reactants at the molecular level [26]. Those outstanding
characteristics actually refer to the role of polyols which act as solvents and reducing agents and
stabilize the nucleation and growth of particles by limiting their agglomeration, due to their chelating
effect. They also help to lower the synthesis temperature [27], since a major part of the nucleation
and interdiffusion has already happened inside polyol medium during the reflux; in some cases, final
phases can be obtained without further heat treatment [26]. Also, the colloidal suspension of the
mixed elements in the dielectric polyol medium enables the growth of specific grain morphologies
(nanopeanuts [28], nanowires [29], nanocubes [30], flower-like shapes [31]). However, despite the
overall easiness of manipulating polyol synthesis, the external conditions must be vigorously controlled
to ensure the reproducibility of the obtained phase and the maintaining of the morphology of the
nanoparticles. In our experiment, we were confronted with a major difficulty related to high room
temperature and humidity in the hot season that affected the quality of the starting nitrate salts we
used and resulted in an impure compound.
In association to our research purpose, many studies reported a surprising high magnetization of
iron [32] and ferrite alloys, e.g., FeCo [33–36] and NiFe [37], once synthesized by the polyol process,
with a remarkable control of their size and form. With this aim, we explored the polyol process for the
synthesis of BFO nanoparticles. The optimization of the heat treatment of the precursor obtained by
the polyol process and the effect of the nanostructuration of the powder on the magnetic properties,
in comparison with micrometric size powders, are the subject of another article [38]. In this work,
we focus on the effect of different synthesis parameters on the structure and microstructure of the
powders and on their impact on the magnetic response.
Nanomaterials 2020, 10, 26 3 of 16
2. Materials and Methods
2.1. Synthesis of the BiFeO3 Nanoparticles
Bismuth nitrate (Bi(NO3 )3 .5H2 O) and iron nitrate (Fe(NO3 )3 .9H2 O) (Alfa Aesar, Thermo Fisher,
Karlsruhe, Germany, 98%) were dissolved in diethylene glycol (DEG, Alfa Aesar, Thermo Fisher,
Karlsruhe, Germany, 99%), with a 1:1 molar ratio, and heated up to 200 ◦ C under reflux for 2, 3 or 6 h.
During heating, the orange transparent solution started to darken on reaching 150 ◦ C and a precipitate
started to appear. The dissolution of the nitrates was done in two different ways: by direct magnetic
stirring in DEG while heating, or by sonication until the total dissolution of the salts before the reflux
heating step. The obtained brown precipitates were washed with acetone and centrifuged three times
to eliminate DEG, as much as possible. Then they were collected in a beaker with additional acetone,
sonicated for 15 min to disperse the precipitate (“for quicker dryness”) and then dried at 120 ◦ C.
The resulting precursors were ground in agate mortar and disposed in a platinum crucible
to be heated, under air, at 500 ◦ C for 2 h with a heating rate of 5 ◦ C/min and then quenched at
room temperature in order to remove the organic groups from the precursor and then to allow the
crystallization of the BFO compound. The organic materials were mainly of the hydroxyl, carbonyl,
alkyl or nitro groups. It has to be pointed out that, when the precursor is placed in the crucible before
heating, it must be well spread in a way to form only a thin layer. Otherwise, impurities form during
the thermal treatment [38].
2.2. Characterization Techniques
The purity of the powders was checked by powder X-ray diffraction (XRD). The crystallite size was
also determined according to the Scherrer equation for the Bragg reflection (0 1 2)hex (the contribution
of the apparatus on the FWHM being deduced). The patterns were recorded at room temperature on a
PANalytical θ-θ Bragg Brentano X’pert MPD PRO diffractometer (Malvern, Panalytical B.V., Almelo,
Netherlands) equipped with a Cobalt source (λKαCo = 1789 Å).
The morphology and grain size of nanopowders were investigated with a JEOL JEM 2100 HR
transmission electron microscope (JEOL Ltd., Tokyo, Japan) (TEM).
The magnetic behavior of BFO powders was explored by Mössbauer spectrometry, employing a
57 Co source, in transmission configuration with a constant velocity of 12 mm·s−1 .
Magnetic results were obtained using Quantum Design PPMS DynaCool (Quantum Design, North
America, San Diego, CA, USA) which offers versatile vibrating sample magnetometer (VSM), AC
and DC susceptibility measurements. This equipment offers a continuous low or high temperature
control sweep mode and a precise magnetic field, integrating a cryopump compatible with all the other
measurement options from 0 up to 9 T magnetic field and a temperature range from 2 to 1000 K.
3. Results and Discussion
3.1. Effect of Synthesis Parameters on the Microstructure
3.1.1. Effect of the Solvent
Diethylene glycol shows more stability and can be considered as a better candidate for reflux at
different temperatures below its boiling temperature of 246 ◦ C. Thus, it was proven to be useful for the
synthesis of nanosized perovskite materials [21,24]. However, several reports have shown that the
use of ethylene glycol for the synthesis of various types of compounds [39,40] has an impact on the
microstructure [41].
Nanomaterials 2020, 10, x FOR PEER REVIEW 4o
temperature of EG is 189 °C, the heating under reflux of the solution was realized at 190 °C. In bo
Nanomaterials 2020, 10, 26 4 of 16
polyols, the dissolution of the iron nitrate was easy, the solution, initially transparent, becomi
orange. Indeed, Insimple stirring ifwas
order to determine the useenough
of ethylenefor its(EG)
glycol complete dissolution.
instead of diethylene Thishas
glycol (DEG) may an be due to t
dissolving role
impactofon polyols
the structureon and transition
microstructure metals [16].a synthesis
of the powders, However, thewith
was made bismuth nitrate cannot
EG (Alfa Aesar,
Thermo Fisher, Karlsruhe, Germany, 99%) as solvent instead of DEG. As the boiling temperature of EG
dissolved at room temperature with magnetic stirring. Actually, bismuth grains remain visible un
is 189 ◦ C, the heating under reflux of the solution was realized at 190 ◦ C. In both polyols, the dissolution
the solution starts to become
of the iron nitrate wasobscure when the
easy, the solution, temperature
initially reachesorange.
transparent, becoming 150 °CIndeed,
during the heating und
simple
reflux. After centrifuging
stirring was enough and drying
for its completethe precipitate,
dissolution. This may the
be precursors wererole
due to the dissolving heat treated
of polyols on at 500 °C fo
transition metals [16]. However, the bismuth nitrate cannot be dissolved at room temperature with
h under air. As evidenced by XRD (Figure 1), the use of EG leads to the development of seconda
magnetic stirring. Actually, bismuth grains remain visible until the solution starts to become obscure
phases, whilewhen
thetheDEG was suitable
temperature reaches 150 for◦ Cthe formation
during of pure
the heating under BFO
reflux. (rhombohedral
After R3c symmetry)
centrifuging and drying
the precipitate, the precursors were heat treated at 500 ◦ C for 2 h under air. As evidenced by XRD
the polyol process.
(Figure 1), the use of EG leads to the development of secondary phases, while the DEG was suitable for
the formation of pure BFO (rhombohedral R3c symmetry) by the polyol process.
Figure 1. XRD patterns showing the effect of the nature of the polyol (ethylene glycol (EG) or diethylene
glycol (DEG)) on the purity of bismuth ferrite (BFO) powders (precursors heat treated at 500 ◦ C for 2 h).
Figure 1. XRD patterns showing the effect of the nature of the polyol (ethylene
(Dotted lines correspond to the BFO phase).
glycol (EG) or
diethylene glycol (DEG)) on the purity of bismuth ferrite (BFO) powders (precursors heat treated at
3.1.2. Effect of the Reflux Temperature and Duration
500 °C for 2 h). (Dotted lines correspond to the BFO phase).
In order to determine if the reflux conditions have an influence on the crystallite size and the
purity of the powder, the temperature and duration of the heating of the solution under reflux were
3.1.2. Effect ofvaried
the (CReflux Temperature
= 0.06 mol·L and Duration
−1 ). On one hand, in addition to the reflux temperature of 200 ◦ C that was
initially used, two other reflux temperatures were tested: 190 ◦ C and 230 ◦ C (reflux duration of 3 h).
In orderOn tothe
determine
other hand, theif the
initialreflux
solutionconditions
containing bismuthhaveandaniron
influence on theincrystallite
nitrates dissolved DEG was size and t
heated under reflux for durations of 2, 3 or 6 h, the reflux temperature being fixed at 200 ◦ C.
purity of the powder, the temperature and duration of the heating of the solution under reflux we
In the case of various heating temperatures of the solution under reflux, the purity of the final
varied (C = 0.06
powdersmol·L −1). On one hand, in addition to the reflux◦ temperature of 200 °C that w
was largely affected (Figure 2). For the synthesis performed at 230 C, in order to be closer to
initially used, two other reflux temperatures
the DEG boiling temperature, were tested:
the obtained precursor 190
was already °C andand
crystallized 230this°C (reflux duration of 3
crystallization
during reflux would not allow the formation of pure BFO powder, as it was always observed when a
On the otherproblem
hand,occurred
the initial solution containing bismuth and iron nitrates dissolved in DEG w
during the synthesis of other precursors which were already crystallized. Heating
heated underunderreflux for
reflux durations
at 190 ◦ C led, after ofthe2,heat
3 or 6 h, the
treatment reflux
of the temperature
precursor being
at 500 ◦ C for 2 h, fixed atof 200 °C.
to the presence
In the case of various heating temperatures of the solution under reflux, the purity of the fin
powders was largely affected (Figure 2). For the synthesis performed at 230 °C, in order to be clo
to the DEG boiling temperature, the obtained precursor was already crystallized and t
crystallization during reflux would not allow the formation of pure BFO powder, as it was alwa
Nanomaterials 2020, 10, 26 5 of 16
about 10 wt% of Bi2 O3 (or Bi-rich phase, that present quite the same XRD pattern) in addition to the
BFO phase.
It can be also noted that any unexpected stop of heating, or any bad regulation of temperature
Nanomaterials
during the2020, 10, x under
heating FOR PEER step, exceeding 200 ◦ C ± 5 ◦ C, led to the perturbation of the nucleation 5 of 16
REVIEW
reflux
and/or growing process, as the solution rapidly cooled down, and then led to an impure final powder
after heat treatment at 500 ◦ C under air.
Figure 2. XRD patterns for the determination of the suitable heating temperature under reflux with DEG
Figure 2. XRD
(C = 0.06 mol·Lpatterns fortothe
−1 ) in order determination
obtain of theafter
pure final powder suitable
a heatheating
treatmenttemperature
at 500 ◦ C (Theunder reflux with
crystallized
DEG (C = 0.06 mol·L −1) in◦ order to obtain pure final ◦ powder after a heat treatment at 500 °C (The
precipitate obtained at 230 C was not heat treated at 500 C) (Dotted lines correspond to the BFO phase).
crystallized precipitate obtained at 230 °C was not heat treated at 500 °C) (Dotted lines correspond to
theConcerning
BFO phase).the effect of the duration of the heating under reflux, the powders obtained after
heat treatment of the precursor at 500 ◦ C, under air, were pure according to XRD patterns, whatever
the duration (2, 3 or 6 h) (Figure 3b). However, Mössbauer spectra revealed the presence of 7% of
Concerning the effect of the duration of the heating under reflux, the powders obtained after
paramagnetic impurity (here Millite) when the solution was refluxed for only 2 h, as shown in Figure 3a.
heat treatment of the precursor at 500 °C, under air, were pure according to XRD patterns, whatever
For reflux durations of 3 or 6 h, no impurity was observed by Mössbauer spectrometry, which is more
the duration (2, 3 or 6 h) (Figure 3b). However, Mössbauer spectra revealed the presence of 7% of
sensitive than XRD for compounds containing iron. Moreover, STEM (Scanning Transmission Electron
paramagnetic
Microscopy) impurity
image and(here X maps Millite) when thefor
were performed solution wassynthesized
the sample refluxed for withonly 2 h,ofas3 h
a reflux shown
at in
Figure 3a.and
200 ◦ C Forheat
reflux durations
treated at 500 ◦ Coffor
3 or
2 h6under
h, noairimpurity was
(Figure 4). observed
They evidence bythat
Mössbauer spectrometry,
the particles were
which is more sensitive
homogeneous in composition. than ItXRD for be
can also compounds
noted that the containing
grains shape iron. Moreover,
slightly tendedSTEM
towards (Scanning
an
Transmission
ovoid shape Electron
for the longerMicroscopy)
refluxes (6image and X maps
h), as observed by TEM were performed
(Figure 9a–d). for the sample synthesized
with a It
reflux of 3 that
indicates h ata200 °Ctime
reflux and ofheat treated
at least 3 h isatprobably
500 °C for 2 h under
necessary to letair
the(Figure 4). They
precipitation evidence
of iron
and
that thebismuth
particlesproceed.
were Eventually,
homogeneous this longer time also allows
in composition. It can a partial
also be dissolution
noted that of the
theobtained
grains shape
precipitate and again precipitation of the dissolved species. Consequently,
slightly tended towards an ovoid shape for the longer refluxes (6 h), as observed by TEM (Figure the final precipitate presents
a higher homogeneity in the distribution of the different metals and leads to pure BFO after the
9a–d).
thermal treatment.
It indicates that a reflux time of at least 3 h is probably necessary to let the precipitation of iron
and bismuth proceed. Eventually, this longer time also allows a partial dissolution of the obtained
precipitate and again precipitation of the dissolved species. Consequently, the final precipitate
presents a higher homogeneity in the distribution of the different metals and leads to pure BFO after
the thermal treatment.
Nanomaterials 2020, 10, 26 6 of 16
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 16
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 16
(a) (b)
(a) (b)
Figure
Figure 3. 3.
(a)(a) Mössbauerspectra,
Mössbauer spectra, registered
registered at
at room
room temperature,
temperature,showing
showing thethe
presence
presence of aof7%
a 7%
Figure 3. (a) Mössbauer spectra, registered at room temperature, showing the presence of a 7%
paramagnetic impurity (Millitephase)
phase)forforthe
theshorter
shorter reflux
reflux duration
duration22hh(C (C==0.060.06mol·L ) while in in
−1 −1
paramagnetic impurity (Millite mol·L ) while
paramagnetic impurity (Millite phase) for the shorter reflux duration 2 h (C = 0.06 mol·L−1) while in
the
the the (b)
(b) (b)
XRD XRD pattern
pattern of
of of
thethe same samples,only
same only a BFO phase
phase is detected for 2,2,3 3oror
6 h of reflux.
XRD pattern the samesamples,
samples, only aa BFO
BFO phase isisdetected
detectedforfor2, 3 or 6 h6 of
h of reflux.
reflux.
Figure 4. STEM image (a) and X maps of the precursor prepared with C = 0.06 mol·L−1 and a reflux of
Figure 4. STEM image (a) and X maps of the precursor prepared with C = 0.06 mol·L−1 and a reflux of
Figure STEM
4. 200
3 h at °C image (a)heat
and then andtreated
X mapsatof500
the°Cprecursor prepared with
for 2 h, evidencing C = 0.06 mol·L
the homogeneity −1 and a reflux
in composition of of
3 h at 200
◦ °C and then heat treated at 500 ◦ °C for 2 h, evidencing the homogeneity in composition of
the200
3 h at particles.
C and(pink:
then bismuth (b), yellow:
heat treated at 500 iron
C for(c), green:
2 h, oxygen the
evidencing (d)).homogeneity in composition of the
the particles. (pink: bismuth (b), yellow: iron (c), green: oxygen (d)).
particles. (pink: bismuth (b), yellow: iron (c), green: oxygen (d)).
Nanomaterials 2020,10,
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x FOR PEER REVIEW 77ofof16
16
3.1.3. Effect of the Dissolution Method
3.1.3. Effect of the Dissolution Method
As explained previously, iron nitrate can be instantly dissolved in DEG, contrary to bismuth
As explained previously, iron nitrate can be instantly dissolved in DEG, contrary to bismuth
nitrate, which is insoluble in polyols at room temperature, under stirring. Thus, in order to ensure
nitrate, which is insoluble in polyols at room temperature, under stirring. Thus, in order to ensure
the equal dispersion of both reagents in the solution before starting the nucleation process under
the equal dispersion of both reagents in the solution before starting the nucleation process under
reflux-heating, the suspension was sonicated in a beaker for about 20 min until the complete
reflux-heating, the suspension was sonicated in a beaker for about 20 min until the complete dissolution
dissolution of bismuth nitrate. Then, the homogenous orange transparent solution was poured into
of bismuth nitrate. Then, the homogenous orange transparent solution was poured into the reactor
the reactor and the rest of the liquid was collected with 2 mL of ethanol.
and the rest of the liquid was collected with 2 mL of ethanol.
The purity of both powders issued from the dissolution of the reagents by sonication or only by
The purity of both powders issued from the dissolution of the reagents by sonication or only by
magnetic stirring (without any prior sonication) was determined by XRD and Mössbauer
magnetic stirring (without any prior sonication) was determined−1by XRD and Mössbauer measurements
measurements for the concentration [Bi + Fe] of 0.06 mol·L (Figure 5). Whatever the dissolution
for the concentration [Bi + Fe] of 0.06 mol·L−1 (Figure 5). Whatever the dissolution method employed,
method employed, after a heat treatment◦ of precursors at 500 °C under air, the resulting powder is
after a heat treatment of precursors at 500 C under air, the resulting powder is pure (only BFO phase)
pure (only BFO phase) according to the XRD characterizations. This result is confirmed by the
according to the XRD characterizations. This result is confirmed by the analysis of the Mössbauer
analysis of the Mössbauer spectra of the different samples, as there is no variation of the hyperfine
spectra of the different samples, as there is no variation of the hyperfine parameter values of both
parameter values of both samples, as shown in Figure 5b. Moreover, the sonication does not
samples, as shown in Figure 5b. Moreover, the sonication does not drastically influence the crystallite
drastically influence the crystallite size as it is of around 42 nm, with or without sonication.
size as it is of around 42 nm, with or without sonication.
(a)
(b)
Figure 5. (a) XRD patterns and (b) Room temperature Mössbauer spectra of the powders obtained
Figure 5. (a) XRD patterns and (b) Room temperature Mössbauer spectra of the powders obtained
after heat treatment at 500 ◦ C for 2 h of the precursors synthesized with or without sonication of the
after heat treatment at 500 °C for 2 h of the precursors synthesized with or without sonication of the
nitrates in polyol solution before heating under reflux at 200 ◦ C for 2 h (C = 0.06 mol·L−1 ).
nitrates in polyol solution before heating under reflux at 200 °C for 2 h (C = 0.06 mol·L−1).
3.1.4. Effect of the Concentration
3.1.4. Effect of the Concentration
In order to reduce the BFO particle size, we varied the molar concentration of cations [Bi + Fe] in
In orderastofollows:
the solutions reduce the
0.1,BFO
0.06,particle size,
0.04, 0.02 we−1
mol·L varied theare
, which molar concentration
considered of cations
to be very low for[Bi + Fe] in
chemical
the methods.
soft solutions The
as follows: 0.1, 0.06,
precursors 0.04,after
obtained 0.02 the
mol·L −1, which are considered to be very low for chemical
heating of the solution under reflux for 3 h, were heat
treated at 500 ◦ C
soft methods. Theforprecursors obtained
2 h. The final powdersafterwere
the heating
pure forof thethe solution under
concentrations reflux
0.06 for 3mol·L
and 0.04 −1
h, were
heat treated
(Figure at 500
6), while °C forconcentration
a higher 2 h. The finalofpowders −1
0.1 mol Lwereled pure
to afor the concentrations
crystallized precursor,0.06 and 0.04
the powder
mol·L−1 (Figure
obtained after the6), while
heat a higher
treatment at 500 ◦ C for 2 h under
concentration of 0.1air
mol L−1 led
always to a crystallized
containing, with such precursor, the
crystallized
precursors, impurities in addition to the BFO phase. For the lower concentration C = 0.02 mol·L ,
powder obtained after the heat treatment at 500 °C for 2 h under air always containing, with −1
such
crystallized
the precursors,
final powders impurities
contained in addition
impurities (8 wt% toof the BFO
Bi2 O phase. For the
3 ). Moreover, the formation
lower concentration C = 0.02
of the precipitate
mol·L−1, the final powders contained impurities (8 wt% of Bi2O3). Moreover, the formation of the
precipitate during the heating of the solution under reflux was reduced and in the case of the
concentration 0.02 mol·L−1, the quantity of precipitate that was collected was very low.
Nanomaterials 2020, 10, 26 8 of 16
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during the heating of the solution under reflux was reduced and in the case of the concentration
These−1results
0.02 mol·L , the quantity
evidence ofthat
precipitate that was collected
the concentration was
range that veryus
allow low.
to obtain pure BFO powders is
Thesethe
reduced, results evidence
crystallite size that
beingtheofconcentration range
about 43 nm for that
both C =allow us C
0.04 or to=obtain pure−1BFO
0.06 mol·L . powders is
reduced, the crystallite size being of about 43 nm for both C = 0.04 or C = 0.06 mol·L−1 .
Figure 6. XRD patterns of the precursors prepared with concentrations of 0.02, 0.04, 0.06 and 0.1 mol·L−1
Figure 6. XRD patterns of the precursors prepared with concentrations of 0.02, 0.04, 0.06 and 0.1
and heat treated at 500 ◦ C for 2 h, illustrating the effect of the initial concentration of polyol solution on
mol·L −1 and heat treated at 500 °C for 2 h, illustrating the effect of the initial concentration of polyol
the purity of the final powders.
solution on the purity of the final powders.
3.1.5. Effect of the Addition of Hydroxide or Hydronium Ions
3.1.5. Effect of the Addition of Hydroxide or Hydronium Ions
In order to control the morphology of the powders, the influence of the addition of hydroxide or
In order
hydronium to control
ions the morphology
was determined. Indeed, ofthe
theaddition
powders,ofthe − ions can,
OHinfluence of for
theinstance,
addition influence
of hydroxide the
or hydroniumofions
morphology was determined.
particles Indeed, the
[42]. More generally, theaddition
presence OHhydronium
of of –
ions can, for instance, influence
or hydroxide groups can the
morphology
influence of particles
particle growth [42]. More generally, the presence of hydronium or hydroxide groups can
and stability.
influence
In thisparticle growth and
way, different stability.
acids (nitric acid, citric acid or acetic acid) and/or urea (that leads to the
In thisofway,
formation different
hydroxide acidswhen
groups (nitric acid, citric acid
it decomposes under or heating
acetic acid) and/or
of the ureawere
solution) (thatadded
leads to the
the
formation solution
optimized of hydroxide
(bismuthgroups
and when it decomposes
iron nitrates dissolvedunder
in DEG with Cof=the
heating 0.06solution)−1
mol·L ).wereThisadded
solutionto
the optimized
was then heatedsolution (bismuth
under reflux at 200and◦ Ciron
for 3nitrates
h and thedissolved
precursorin DEG withtreated
was heat C = 0.06 mol·L
at 500 ◦ C−1for
). This
2h
solution
under air.was then heated under reflux at 200 °C for 3 h and the precursor was heat treated at 500 °C
for 2Pure
h under
BFOair.
powders were obtained, as revealed by the XRD patterns (Figure 7), when only urea or
Pure and
both urea BFOacetic
powders
acid were added
obtained, as revealed
to the mixture of byDEG
the XRD patterns
and nitrate (Figure
salts. 7), when
This could onlytourea
be due the
or both
fact thaturea and acetic
the addition ofacid
aceticwere
acidadded to thethe
enhanced mixture of DEG
dissolution ofand nitrate
bismuth salts. by
nitrate This could bestirring
magnetic due to
the factthe
before that the addition
heating of acetic
of the solution acid reflux.
under enhanced the dissolution
However, of bismuth(Figure
TEM observations nitrate9g,h)
by magnetic
showed
stirring
more before the heating
agglomerated grains andof the solution under
an undefined reflux. the
morphology, However, TEM
crystallite sizeobservations
determined from(Figurethe 9g,h)
XRD
showed being
patterns more approximately
agglomerated the grains
same and an without
than undefined morphology,
these additions (Tablethe crystallite size the
1). Moreover, determined
addition
from
of the XRD nitric
concentrated patterns
acidbeing
led toapproximately
the formation of the same than without
a bismuth-rich phase, asthese
well additions (Table
as citric acid 1).
or only
acetic acid. The obtained phases are summarized in Table 1 and the crystallite sizes are indicated. as
Moreover, the addition of concentrated nitric acid led to the formation of a bismuth-rich phase,
well as citric acid or only acetic acid. The obtained phases are summarized in Table 1 and the
crystallite sizes are indicated.
Nanomaterials2020,
Nanomaterials 2020,10,
10,26
x FOR PEER REVIEW 99of
of16
16
Figure 7. XRD patterns of the powders obtained by heat treatment at 500 ◦ C of the precursor synthesized
with the7.addition
Figure of acids of
XRD patterns and/or urea in order
the powders to determine
obtained by heat the pH effect
treatment at on
500the
°Cpurity
of theofprecursor
the final
powders. (N.A.:
synthesized withnitric acid, C.A.:
the addition of citric
acids acid, A.A.:
and/or ureaacetic = urea). the pH effect on the purity
acid,toUdetermine
in order
of the final powders. (N.A.: nitric acid, C.A.: citric acid, A.A.: acetic acid, U = urea).
Table 1. Study of the effect of acids and urea on BFO stability and crystallite sizes (R = [acid]/[Bi + Fe]
and U = [urea]/[Bi + Fe]).
Table 1. Study of the effect of acids and urea on BFO stability and crystallite sizes (R = [acid]/ [Bi + Fe]
and U = [urea]/ [Bi + Fe]). Quantities Phases Observed
Acid/Base Crystallites Sizes (nm)
in 30 mL of DEG by XRD
Quantities Phases Observed
Acid/Base BiFeO Crystallites Sizes (nm)
Nitric acid = 3.5of DEG
in 30R mL by XRD3 31
Bi2 O3 (25 wt%) *
BiFeO
BiFeO33
Nitric
Citric acidacid RR==33.5 31 40
BiBi
2O2O3 (25 wt%)
3 (14 wt%)**
BiFeO33
BiFeO
GlacialCitric
Aceticacid
acid RR = =1 3 40 30
Bi2 O3 (9 wt%) *
Bi2O3 (14 wt%) *
Urea (powder) U=2 BiFeO3 43
Urea + Acetic acid U = 2 + R = 1 BiFeO
BiFeO 3
Glacial Acetic acid R=1 3 30 43
Bi2O3 (9 wt%) *
* notice the limitation of XRD measurements to identify Bi rich phases or Bi2 O3 since they represent the same
Urea (powder)
cubic parameters. U=2 BiFeO3 43
Urea + Acetic acid U=2+R=1 BiFeO3 43
3.1.6. *Effect
noticeof the
the Addition
limitation of of
XRDWater or of a Surfactant
measurements to identify Bi rich phases or Bi2O3 since they represent
the same cubic parameters.
In order to control the microstructure of the powders, the addition of water or of a surfactant,
polyvinylpyrrolidone (PVP), was also tested.
3.1.6. Effect of the Addition of Water or of a Surfactant
Deionized water (h = [water]/[Bi + Fe] = 10) was added to the reference solution with a
In order of
concentration to 0.06
control the−1microstructure
mol·L . By following theof the powders,
same process the
thataddition of water
we established or of a surfactant,
previously (heating
polyvinylpyrrolidone
of (PVP),
the solution under reflux forwas also
about 3 htested.
at 200 C and heat treatment of the precursor at 500 ◦ C for 2 h
◦
underDeionized water (h
air), we obtained = [water]/[Bi
a BFO phase with+ aFe] = 10)size
particle wasof added to the
48 nm but withreference solution
the presence withofa
of 6 wt%
concentration of 0.06 mol·L −1. By following the same process that we established previously (heating
Bi2 O3 (Figure 8). Moreover, TEM observation evidenced that water enhanced grain agglomeration but
of the solution
without under reflux
any difference for about 3ofhthe
in morphology at 200 °C and
grains heat9e).
(Figure treatment of the precursor at 500 °C for 2
h under air), we obtained a BFO phase with a particle size of 48 nm but with the presence of 6 wt% of
Bi2O3 (Figure 8). Moreover, TEM observation evidenced that water enhanced grain agglomeration
but without any difference in morphology of the grains (Figure 9e).
Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 16
Nanomaterials 2020, 10, 26 10 of 16
Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 16
For the surfactant, PVP, the first experiment done with 1.44 g in 15 mL of DEG did not lead to
the formation of a precipitate
For the surfactant, PVP, the of Bi and
first Fe, and the
experiment finalwith
done solution
1.44 gremained
in 15 mLorange
of DEGtransparent
did not lead with
to
agglomerated PVP that do not completely dissolved. After the optimization
the formation of a precipitate of Bi and Fe, and the final solution remained orange transparent withof the required quantity
to ensure the PVP
agglomerated occurrence
that doofnot thecompletely
precipitation, we determined
dissolved. that the valueofofthe
After the optimization C required
= 4.10−4·mol·L −1 of
quantity
PVP
to leadsthe
ensure thetooccurrence
a higher purity
occurrence theof
of the
of the powder.we
precipitation,
precipitation, However,
we 9 wt%
determined
determined ofthe
that
that thevalue
the Bi2Oof
value 3 impurity −4
4.10was
ofCC= =4.10 detected
−4·mol·L
·mol·L −1
−1
of
(Figure
PVP leads8). to
Moreover,
higher the
a higher crystallite
purity
purity of the
of sizes obtained
the powder.
powder. However,with99PVP
wt%polyol
wt% thesolution
of the
of Bi22O
Bi or without,
O33 impurity are quite
was detected
similar 8).
(Figure (39Moreover,
nm with PVP
Moreover, instead ofsizes
the crystallite about 43 nm).
obtained In addition,
with PVP polyol TEM images
solution or do not evidence
without, are quitea
lowering
similar (39 of
(39nm the agglomeration
nmwith
withPVPPVP instead
instead of the powders,
of about
of about as it
43 nm).
43 nm). could be expected
In addition,
In addition, TEM images (Figure
TEM images 9h).
do not evidence
do not evidence a loweringa
lowering
of of the agglomeration
the agglomeration of the powders,
of the powders, as it couldasbeitexpected
could be (Figure
expected (Figure 9h).
9h).
Figure 8. 8.
Figure XRDXRD patterns
patterns of powders
of powders prepared with prepared
additionwith addition
of water of orwater
(red line) (red line) or
polyvinylpyrrolidone
polyvinylpyrrolidone
(PVP) (black line) to (PVP)
the (black
initial line)
solution to
that the
was initial
heatedsolution
under that was
reflux at 200 ◦ C for
heated under
3 h reflux at 200
(precursor °C
heat
Figure 8. XRD patterns of powders prepared with addition of water (red line) or
for 3
treatedh at 500 ◦ C for
(precursor heat
2 treated
h). at 500 °C for 2 h).
polyvinylpyrrolidone (PVP) (black line) to the initial solution that was heated under reflux at 200 °C
for 3 h (precursor heat treated at 500 °C for 2 h).
Figure 9. Cont.Nanomaterials 2020, 10, 26 11 of 16
Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 16
Figure
Figure 9.9. TEM
TEM images
images showing
showing the the precursors
precursors heat
heat treated
treated at
at 500
500 °C◦ C for
for 22 hh and
and synthesized
synthesized with
with (a)
(a)
reflux for 6 h, (b,c) reflux for 3 h (d) reflux for 2 h, (e) addition of water, (f) addition of urea,
reflux for 6 h, (b,c) reflux for 3 h (d) reflux for 2 h, (e) addition of water, (f) addition of urea, (g) addition (g)
addition of acetic
of acetic acid acidand
and urea and(h)
urea and (h)
addition addition
of PVP, (C =of PVP,
0.06 (C−1=).0.06
mol·L Themol·L
−1). The last four samples
last four samples were refluxed
were refluxed
◦
at 200 C for 3 h. at 200 °C for 3 h.
3.1.7. Conclusion on the Effect of the Different Parameters
The effects of the different parameters on the purity of the powders and on the microstructure
(Figure 9) were determined. The best microstructures, with a low grain size and a higher purity,Nanomaterials 2020, 10, 26 12 of 16
3.1.7. Conclusion on the Effect of the Different Parameters
Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 16
The effects of the different parameters on the purity of the powders and on the microstructure
(Figure 9) werefor
were obtained determined.
the simplest Thesynthesis,
best microstructures,
based on awith a low
nitrate grain size
mixture and aand
in DEG higher purity,
heating were
under
obtained
reflux forsolution
of the the simplest
of atsynthesis, based on a shape
least 3 h. Ovoid-like nitrategrains
mixturecaninbe
DEG and
well heating
defined andunder reflux seem
the grains of the
solution of at least 3 h. Ovoid-like shape grains can be well defined and the
to be less agglomerated (Figure 9a–c) compared to the other preparations, according to all the grains seem to be less
agglomerated (Figure 9a–c) compared to the other preparations, according to all
images observed for each sample. The specific ovoid shape was not achieved for short reflux (2 h in the images observed
for each
Figure 9d),sample.
and theThe specific
grains tended ovoid shape was not
to agglomerate moreachieved for short refluxagents
when supplementary (2 h inwere
Figure 9d),(see
added and
the grains tended to agglomerate more when supplementary agents were added
in Figure 9e–h referring respectively to the addition of water, urea, acetic acid and urea and finally(see in Figure 9e–h
referring
PVP). respectively
Therefore, to the addition
we selected the best of water,
protocolurea, acetic
that acidhomogenous,
allows and urea and well-formed
finally PVP). grains
Therefore,
of
we selected the best protocol that allows homogenous, well-formed grains of
narrowly distributed sizes that can be obtained in a reasonable amount of time, which corresponds narrowly distributed
sizes
to 3 hthat can be obtained
of heating in a reasonable
of the solution amount
of nitrates salts of
in time,
DEG which
under corresponds
reflux at 200to°C,3 hand
of heating
then a of the
heat
solution ofofnitrates salts inatDEG ◦
treatment the precursor 500 under
°C for reflux at 200
2 h under air. C, and then a heat treatment of the precursor at
500 ◦ C for 2 h under air.
3.2. Magnetic Properties
3.2. Magnetic Properties
In this section, we present the physical properties of synthesized BFO nanostructured particles
In this section, we present the physical properties of synthesized BFO nanostructured particles
obtained from the best protocol, with C = 0.06 mol·L−1−1, 3 h of reflux of nitrate salts in DEG at 200 °C,
obtained from the best protocol, with C = 0.06 mol·L , 3 h of reflux of nitrate salts in DEG at 200 ◦ C,
the precursor being heat treated at 500 °C for 2 h under air.
the precursor being heat treated at 500 ◦ C for 2 h under air.
The microstructural and structural studies reported above showed that the final product
The microstructural and structural studies reported above showed that the final product consists
consists of a pure BFO phase. XRD and Mössbauer experiences have not detected the presence of
of a pure BFO phase. XRD and Mössbauer experiences have not detected the presence of secondary
secondary phases. Figure 10 shows that the Mössbauer spectra, recorded at room and liquid
phases. Figure 10 shows that the Mössbauer spectra, recorded at room and liquid nitrogen (77 K)
nitrogen (77 K) temperatures, consist of a pure BFO phase, recognizable by asymmetric lines.
temperatures, consist of a pure BFO phase, recognizable by asymmetric lines.
Figure 10. Mössbauer spectra recorded at room temperature (300 K) and at nitrogen liquid temperature
Figure 10. Mössbauer spectra recorded at room temperature (300 K) and at nitrogen liquid
(77 K) of obtained BFO nanoparticles. Symbol plot and red full lines represent experimental data and
temperature (77 K) of obtained BFO nanoparticles. Symbol plot and red full lines represent
refined curves based on cycloidal model respectively.
experimental data and refined curves based on cycloidal model respectively.
Mössbauer data were analyzed using MOSFIT, a Lorentzian line fitting program [43] allowing
for the use of a discrete distribution of hyperfine parameters. Based on the cycloidal model described
above, we obtained an Isomer Shift of δ = 0.41 (0.5) mm·s−1, a quadripolar splitting of 2ε = 0.46 (0.51)
mm·s−1, and a hyperfine field of Bhf = 49.3 (54.5) T at room temperature (nitrogen liquid). TheseNanomaterials 2020, 10, 26 13 of 16
Mössbauer data were analyzed using MOSFIT, a Lorentzian line fitting program [43] allowing for
Nanomaterials
the use of a 2020, 10, xdistribution
discrete FOR PEER REVIEW
of hyperfine parameters. Based on the cycloidal model described 13 of 16
above,
we obtained an Isomer Shift of δ = 0.41 (0.5) mm·s , a quadripolar splitting of 2ε = 0.46 (0.51) mm·s−1 ,
−1
values are typicalfield
of the high-spin Fe3+ and clearly show that synthesized BFO particles have a Fe3+
and a hyperfine of B hf = 49.3 (54.5) T at room temperature (nitrogen liquid). These values are
valence
typical ofstate only. From
the high-spin Fethese
3+ andMössbauer
clearly show data
thatitsynthesized
is worth noting that, even
BFO particles though
have a Fe3+the crystallite
valence state
size (of around 40 nm) is lower than 62 nm, the obtained BFO nanoparticles conserve
only. From these Mössbauer data it is worth noting that, even though the crystallite size (of around the cycloidal
spin
40 nm)structure.
is lower than 62 nm, the obtained BFO nanoparticles conserve the cycloidal spin structure.
M(H)
M(H) loop
loopmeasurements
measurementsatatdifferent
differenttemperatures,
temperatures, ranging from
ranging 4 K4 to
from K 300 K, were
to 300 carried
K, were out
carried
to check the macroscopic behavior of the magnetization of the BFO nanostructured
out to check the macroscopic behavior of the magnetization of the BFO nanostructured particles. particles. Figure
11 shows
Figure clearlyclearly
11 shows that the
thatM(H) curvecurve
the M(H) for the
for BFO
the BFOnanoparticles (≈ (≈40
nanoparticles 40 nm)nm)made
madewith
withthe
the polyol
polyol
process exhibits a non-saturated ferromagnetic behavior compared to a micrometric
process exhibits a non-saturated ferromagnetic behavior compared to a micrometric size BFO powder size BFO
powder (≈ 200 nm) with a G-type antiferromagnetic
(≈200 nm) with a G-type antiferromagnetic order. order.
Figure 11. Hysteresis loops of BFO nanostructured particles synthesized by the polyol process and of
Figure 11. Hysteresis
BFO micrometric sizeloops of BFO
particles nanostructured
obtained particles
by hydrothermal synthesized
process, by the
recorded polyol
at room process and of
temperature.
BFO micrometric size particles obtained by hydrothermal process, recorded at room temperature.
The appearance of the hysteresis loop indicates that when the crystallite size of BFO becomes
lowerThe appearance
than the period of of
theitshysteresis
cycloidalloop spinindicates
structure, that
thewhen the crystallite
compensation size of BFO
of magnetic becomes
moments in
lower than the period of its cycloidal spin structure, the compensation
the G-type antiferromagnetic order is no longer achieved and a ferromagnetic-type order appears. of magnetic moments in the
G-type antiferromagnetic
This phenomenon order is no longer
is the consequence of theachieved
break of and a ferromagnetic-type
symmetry order appears.
of the incommensurable This
cycloidal
phenomenon
spin structureisdue the consequence
to the finite of sizetheofbreak of symmetry
particles. In otherofwords,
the incommensurable cycloidal
if the polyol method spin
allows
structure
a size reduction below the critical size value of 62 nm, the nanoparticles obtained preservesize
due to the finite size of particles. In other words, if the polyol method allows a the
reduction
non-collinearbelow
magneticthe order,
criticalas size
observedvaluefrom of Mössbauer
62 nm, the nanoparticles
spectrometry, obtained
but have a largerpreserve
spontaneous the
non-collinear
magnetization.magnetic
The measured order, as observed
values of coercivefrom Mössbauer
field and remanence spectrometry,
are in order ofbutµ0 HC
have≈ 0.6
a larger
T and
spontaneous magnetization.
2 −1
Mr ≈ 0.07 Am kg , respectively. The measured values of coercive field and remanence are in order of
µ0HC ≈ 0.6 T and Mr ≈ 0.07 Am kg , respectively.
2 −1
4. Conclusions
4. Conclusions
Pure ovoid-like shaped BFO nanostructured particles were successfully synthesized for the first
timePure
by theovoid-like shapedusing
polyol process BFO nanostructured
DEG as a solvent. particles were successfully
The synthesis parameters synthesized for the first
were controlled and
time by the
allowed us topolyol
obtainprocess
an average using DEG size
particle as aofsolvent.
∼40 nm,The synthesis
which is underparameters wereofcontrolled
the periodicity the modulatedand
allowed
cycloidalus to obtain an
arrangement average
of iron spins. particle
Surprisingly, ∼40asymmetric
size of the nm, whichMössbauer
is under the periodicity
spectrum of the
revealed the
modulated
persistence cycloidal arrangement
of the cycloidal magnetic ofstructure
iron spins. Surprisingly,
at this the asymmetric
scale. However, Mössbauer
a spectacular spectrum
room temperature
revealed
hysteresisthe
looppersistence of thewhich
was registered, cycloidal magnetic
explains structure atofthis
the enhancement the scale. However,
ferromagnetic a spectacular
order compared
room temperature hysteresis
to the antiferromagnetic (AFM) bulkloop material.
was registered, which ofexplains
This exaltation the enhancement
the macroscopic magnetization of can
the
ferromagnetic order compared to the antiferromagnetic (AFM) bulk material. This exaltation of the
macroscopic magnetization can be explained by a symmetry breaking, due to the finite size of the
BFO nanostructured particles which induces a non-compensation of the total magnetization of Fe3+
sublattices in the cycloidal spin structure.Nanomaterials 2020, 10, 26 14 of 16
be explained by a symmetry breaking, due to the finite size of the BFO nanostructured particles
which induces a non-compensation of the total magnetization of Fe3+ sublattices in the cycloidal
spin structure.
To summarize, the physical characterization presented in this paper showed a transformation of the
overall magnetic order and an exalted spontaneous magnetization when the particle size was reduced
below 62 nm. This can improve the magnetoelectric coupling and makes these BFO nanostructured
powders low-cost potential candidates for spintronic technology applications in various domains,
such as a magnetic and electric field sensors, MRA memories and electromagnetic devices.
Author Contributions: Formal analysis: M.M., M.B. and N.R.; Funding acquisition: N.R.; Investigation: M.M.,
N.R., A.R. and Y.G.; Methodology: S.C., M.B. and N.R.; Resources: S.C.; Supervision: S.C., M.B., N.R. and M.E.;
Validation: S.C., N.R. and M.B.; Visualization: M.M. and N.R.; Writing—original draft: M.M.; Writing—review &
editing: S.C., N.R. and M.B. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Ministère de l’Enseignement Supérieur et de la Recherche Scientifique
of the Tunisian Republic, the Doctoral School Matière, Molécules et Matériaux (3M) (ED596), the Eiffel scholarship
program of excellence (Campus France) and the Agence Erasmus+ (MIC funding) for their financial support.
Acknowledgments: We would like to acknowledge the Electronic microscopy platform of the IMMM and to the
XRD platform and the Mössbauer spectrometry technical facility.
Conflicts of Interest: The authors declare no conflict of interest.
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