Thermal stability of the solvothermal-synthesized MnFe 2O4 [610242]

Thermal stability of the solvothermal-synthesized MnFe 2O4
nanopowder
Marcela Stoia1,2•Cornelia Pa ˘curariu1•Eliza-Cornelia Muntean1
Received: 23 September 2015 / Accepted: 10 January 2016
/C211Akade ´miai Kiado ´, Budapest, Hungary 2016
Abstract Manganese ferrite nanopowder was prepared
by a new solvothermal method, using 1,2 propanediol as
solvent and KOH as precipitant. The as-synthesized pow-
der, by solvothermal treatment in autoclave at 195 /C176C, for
12 h, consisted of fine manganese ferrite nanoparticles.
The further thermal treatment of the initial manganese
ferrite powder to higher temperature resulted in manganese
ferrite decomposition due to Mn(II) oxidation to Mn(III),
as observed by X-ray diffraction. FT-IR spectroscopy has
evidenced that the oxidation takes place even at 400 /C176C.
The oxidation of Mn(II) to Mn(III) was studied by TG/DSC
simultaneous thermal analysis. It was shown that Mn(II)
oxidation takes place in a very small extent up to 400 /C176C.
The main oxidation step occurs around 600 /C176C, when a
clear mass gain is registered on TG curve, associated with a
sharp exothermic effect on DSC curve. The exothermic
effect is smaller in case of the powder annealed at 400 /C176C,
confirming the superficial oxidation of Mn(II) up to
400/C176C. In order to avoid Mn(II) oxidation, the powder
obtained at 400 /C176C was further annealed at 800 /C176C in argon
atmosphere, without degassing, when manganese ferrite
MnFe 2O4was obtained as major crystalline phase (69 %).
All manganese ferrite powders showed a superparamag-
netic behavior, with maximum magnetization of51 emu g-1in case of the as-synthesized powder, charac-
teristic of magnetic ferrite nanopowders.
Keywords Manganese ferrite /C1Nanopowder /C1
Solvothermal /C11,2-Propanediol /C1Thermal stability
Introduction
Nanoscale magnetic spinel ferrites have drawn major
attention in the last decade, because of their technological
applications in magnetic recording, ferrofluids and cata-
lysts. It is well known that magnetic ferrites’ nanoparticles
have different characteristics in comparison with the bulk
material, due to their reduced size and the effects of
magnetic interactions between particles [ 1–3].
Manganese ferrite is a spinel magnetic ferrite, exten-
sively used in various technological applications including
magnetic devices, gas sensor, adsorbent material for hot
gas [ 4] and anode materials for lithium-ion batteries [ 5].
The properties of manganese ferrite are strongly dependent
on its composition, on nanoparticles morphology and size,
which are closely connected with the processing conditions
[4]. Several techniques have been reported for the synthesis
of MnFe 2O4nanopowders, such as solid-phase reactions
[6,7], mechanical ball-milling [ 8], thermal decomposition
of precursors [ 9,10], hydrothermal method [ 11],
solvothermal method [ 5], coprecipitation method [ 12],
combustion method [ 13] and microemulsion method [ 14].
It was reported that at elevated temperatures, MnFe 2O4
is unstable in air and Mn2?ions on the surface oxidize to
form Mn3?ions resulting in the dissociation of the formed
MnFe 2O4[15] into the individual oxides (Mn 2O3and
Fe2O3). Thus, it was concluded that any preparation
&Marcela Stoia
[anonimizat]
1Faculty of Industrial Chemistry and Environmental
Engineering, Politehnica University Timis ¸oara, 6 Pı ˆrvan Blv,
300223 Timisoara, Romania
2Research Institute for Renewable Energy, Politehnica
University Timis ¸oara, P-ta Victoriei No. 2,
300006 Timisoara, Romania
123J Therm Anal Calorim
DOI 10.1007/s10973-016-5249-5

method involving a calcination step is not suitable for the
preparation of manganese ferrite nanoparticles [ 13].
The hydro-/solvothermal method represents one of the
most common liquid-phase methods used to prepare high-purity materials and to control the morphology of materials
[16]. Solvothermal method is a suitable synthesis method
for manganese ferrite as it leads directly to the designedoxides system at low temperature. This method seems to be
effective in avoiding the oxidation, hydrolysis and
volatilization of the reactants, and also favorable for
products crystallization due to the particular reaction con-
dition inside the sealed autoclave [ 17]. The main advantage
of the solvothermal method is the possibility to obtain
oxides with tailored properties (shape and size of the
nanoparticles) by tuning the reaction conditions (tempera-ture, filling degree of the autoclave, concentration,
solvent).
The current study introduces an original, simple
solvothermal method to synthesize magnetic MnFe
2O4
nanopowders, using Mn(II) and Fe(III) chlorides as pre-cursors, 1,2-propanediol as solvent and potassiumhydroxide as precipitant. The thermal stability of the
obtained manganese ferrite nanopowder in air and inert
atmosphere was performed by means of TG/DSC simul-taneous analysis. The effect of thermal treatment on their
structural and magnetic properties was investigated.
Experimental
The starting reagents were: iron chloride hexahydrate extrapure FeCl
3/C16H2O (Ph Eur, from Scharlau), manganese
chloride tetrahydrate, MnCl 2/C14H2O (for synthesis, purity
[99 %, from Merck), 1,2 propanediol C 3H8O2(12PG, for
synthesis, purity [99 %, from Merck), potassium hydrox-
ide (C99.5 %, from Sigma-Aldrich) and ethanol C 2H6O
(analytical reagent grade, from Merck). All reagents were
used as received, without further purification.
Synthesis
In a typical synthesis, the necessary amounts of FeCl 3and
MnCl 2were dissolved in a 30 mL of 1,2-propanediol. The
necessary amount of precipitant (KOH), considering 40 %
excess, was dissolved also in 30 cm31,2-propanediol. The
obtained clear solutions have been mixed, by gradually
adding the KOH solution into metal chlorides solution,
when black slurry was obtained. This slurry was left undermagnetic stirring for half an hour, than it was transferred
into a Teflon-lined stainless steel autoclave of 75 cm
3
capacity (leading to a 80 % degree filling of the autoclave).
The autoclave was placed into an oven and kept at 195 /C176C
for 12 h, and then allowed to cool to room temperature.The precipitate was separated from the liquid phase by
filtration. The product was washed with distilled water and
ethanol several times to remove the residual organics and
Cl-ions (detected by reaction with Ag?ions) that
remained in the precipitate. The washed precipitate was left
to dry at room temperature for 1 day and then was milled,
and the obtained powder was further characterized as-synthesized (sample MnFeSv-195) and annealed at differ-
ent temperatures: 400 /C176C (MnFeSv-400), 500 /C176C
(MnFeSv-500), 700 /C176C (MnFeSv-700) and 1000 /C176C
(MnFeSv-1000).
Characterization methods
Thermal behavior of the as-synthesized powders and the
ones annealed at 400 and 500 /C176C was performed on a
Netzsch STA 449C instrument, in air and in N
2atmo-
sphere, respectively, at a flow rate of 20 mL min-1. The
TG/DSC curves were recorded in the range of 25–1000 /C176C
with a heating rate of 10 K min-1, using alumina crucibles.
The phase composition of the samples was determined
by XRD, using a Rigaku Ultima IV diffractometer (Cu Ka
radiation). FTIR spectra were carried out using a Shimadzu
Prestige-21 spectrometer in the range 400–4000 cm-1,
using KBr pellets and resolution of 4 cm-1. The mor-
phology of the nanopowders was investigated by scanning
electron microscopy (SEM), using a FEI Quanta FEG 250microscope. The behavior in external magnetic field of the
obtained nanoparticles was studied under AC (50 Hz)
applied magnetic fields of amplitudes up to 160 kA m
-1,
by means of a conventional induction hysteresisgraph [ 18],
calibrated to a high-purity (99.98 %) Ni reference; the
output signals of the device were recorded in numericformat (ASCII files) to a PC, by means of a data acquisition
board.
Results and discussion
The as-synthesized powder (195 /C176C) was annealed in air
for 2 h at 400 /C176C, 500, 700 and 1000 /C176C in order to study
the evolution of the systems’ crystalline phases. All pow-ders have been characterized by X-ray diffraction and FT-
IR spectroscopy.
Figure 1shows the XRD patterns of the as-synthesized
powder (195 /C176C) and of the powders annealed at different
temperatures, while the data derived from XRD analysis
are given in Table 1.
The XRD patterns of the as-synthesized powder
(MnFeSv-195) and of the powder annealed at 400 /C176C
(MnFeSv-400) evidence the presence of manganese ferrite(MnFe
2O4) as the only crystalline phase. Manganese ferrite
completely decomposes after annealing at 500 /C176C into anM. Stoia et al.
123

iron-rich manganese iron oxide (Mn 0.176Fe1.824O3[19]).
The rest of the manganese is probably present in the system
as an amorphous phase, which crystallizes at higher tem-perature (700 /C176C) as Mn
0.74Fe1.26O3. The later phase
becomes the major crystalline phase in case of the powder
annealed at 1000 /C176C.One can notice from the data shown in Table 1that the
crystallite size of manganese ferrite decreases after the
annealing at 400 /C176C, due to the decrease in its lattice
parameter (Table 1). The annealing at 500 /C176C determines a
significant increase in the crystallite size from 9.3 to
55.9 nm. This sudden increase might suggest a possible
10 20 30 40 50 60 70 800200040006000800010,00012,00014,00016,000
1000 °C
700 °C
500 °C
400 °C
195 °CMnFe2O4
Mn0.176Fe1.824O3
Mn0.74Fe1.26O301-071-4919
04-011-9587
04-007-2773
2 Theta/°Intensity/countsFig. 1 XRD patterns of the as-
synthesized powder (195 /C176C)
and of the powders annealed at
different temperatures
Table 1 Data derived from XRD analysis
Sample Temp/ /C176C Phase (DB card number) Lattice parameters/A ˚ dXRD/A˚ %
abc
MnFeSv-195 195 Jacobsite MnFe 2O4
(01-071-4919)8.445 8.445 8.445 140 100
MnFeSv-400 400 Jacobsite MnFe 2O4
(01-071-4919)8.397 8.397 8.397 93 100
MnFeSv-500 500 Manganese Iron Oxide
Mn 0.176Fe1.824O3
(04-011-9587)5.037 5.037 13.74 559 100
MnFeSv-700 700 Manganese Iron Oxide
Mn 0.176Fe1.824O3
(04-011-9587)5.046 5.046 13.76 366 71
Bixbyite
Mn 0.74Fe1.26O3
(04-007-2773)9.453 9.453 9.453 318 29
MnFeSv-1000 1000 Manganese Iron Oxide
Mn 0.176 Fe1.824O3
(04-011-9587)5.044 5.044 13.78 932 14
Bixbyite
Mn 0.74Fe1.26O3
(04-007-2773)9.427 9.427 9.427 271 86Thermal stability of the solvothermal-synthesized MnFe 2O4nanopowder
123

sintering of the particles at this low temperature, confirmed
by the SEM images of the powders (Fig. 2). The SEM
image of the powder annealed at 500 /C176C shows the
beginning of a strange sintering phenomenon, revealingbeside the nanoparticles of *20 nm, the presence of very
big particles ( *300 nm), which may explain the results of
XRD data refinement. The sintering process is moreadvanced in case of the powders annealed at 700 /C176C
(Fig. 2d) and 1000 /C176C (Fig. 2e).
The FT-IR spectra of the powders obtained at different
temperatures (Fig. 3) support the changes in the cations
placement in the tetrahedral or octahedral sites by thechanges of the bands located in the wavenumber range
400–800 cm
-1, characteristic of M–O bonds vibrations. It
is known that the band located around 700 cm-1corre-
sponds to the intrinsic vibrations of tetrahedral site and the
band at *400 cm-1is attributed to the vibrations of
octahedral site [ 20]. The difference in bond position isexpected because of the difference in the metal–oxygen
bond lengths for the octahedral and tetrahedral sites [ 21].
According to the literature [ 22], the vibrational frequencies
depend on the cation mass, cation–oxygen bonding force,distance and unit cell parameter. It was also observed that
the frequency of the absorption bands gradually increases
with an increased volume of unit cell [ 22]. Thus, FT-IR
spectroscopy is a good technique for evidencing the
changes in cation distribution, within the different sites of
spinel structure.
MnFe
2O4is a partially inverted spinel structure where
the iron and manganese cations are distributed between thetetrahedral (A) and octahedral (B) sites available in a close
packing of oxygen anions. The structural formula for a
generic spinel compound, MFe
2O4, can be written as:
[M1-iFei]A[MiFe2-i]BO4(A-tetrahedral sites and B-octa-
hedral sites) and i is the inversion parameter. For a normal
spinel, i=0 and for an inverted spinel i=1.
Fig. 2 SEM images of the aas-synthesized powder (195 /C176C) and powders annealed at different temperatures: b400/C176C;c500/C176C;d700/C176C and
e1000 /C176CM. Stoia et al.
123

Stoichiometric manganese ferrite is reported to be a par-
tially inverted spinel structure with manganese atoms pre-dominantly in the tetrahedral sites (low degree of
inversion). However, the cation distributions over tetrahe-
dral and octahedral sites in manganese ferrites were foundto be different depending on the synthesis methods (in
particular with different annealing temperatures and cool-
ing rates) [ 23].
The FT-IR spectrum of the as-obtained powder (195 /C176C)
shows a strong absorption band located at 563 cm
-1which
can be assigned to the vibrations of the Mn2?and Fe3?
cations located in tetrahedral sites. A very weak band
located at 424 cm-1can be observed, corresponding to the
vibrations of M–O bonds, of cations located in octahedralsites. The weakness and even the absence of the lower
frequency band was explained in the literature by the dis-
ordered structure of nanoparticles obtained at low tem-peratures [ 24] and by the fact that the tetrahedral site
dimension is less compared to the octahedral site dimen-
sion, and the intensity of absorption band has an inverserelationship with the bond length [ 25].
The annealing at 400 /C176C of the initial powder causes
changes in the FT-IR spectrum. Thus, a supplementaryband appears at 640 cm
-1, suggesting some changes
regarding the cations placed in the tetrahedral sites. This
may be due to a partial oxidation of Mn2?ions to Mn3?;i t
was reported in the literature that part of Mn in purejacobsite is present as Mn3?, but in a small amount, too
low to cause a significant change of the XRD peak posi-
tions [ 23]. The FT-IR spectrum of the powder annealed at
500/C176C evidences some significant changes in the range
400–700 cm-1: the band located at 640 cm-1disappears,
and two bands located at 551 and 488 cm-1appear. These
bands are only present in the FT-IR spectrum of the powderannealed at 700 /C176C. According to the XRD patterns of the
powders annealed at 500 and 700 /C176C (Fig. 1), Mn
2?ions
are completely oxidized at Mn3?. The only phase, and the
predominant phase, respectively, in these cases is
Mn 0.176Fe1.824O3hexagonal phase (very close to a-Fe 2O3),
with only octahedrally coordinated cations. The two bands
can be assigned to the stretching and bending vibration
mode of Fe–O bonds, respectively [ 26].
The FT-IR spectrum of the powder annealed at 1000 /C176C
exhibits three bands located at 488, 576 and 661 cm-1, due to
the fact that at this temperature all ferrite was transformedinto Mn
0.74Fe1.26O3(with bixbyite structure—Table 1).
Despite the fact that in this case all cations are octahedrally
coordinated, the multiplication of the bands may be causedby the difference in Mn–O and Fe–O bonds due to the Jahn–
Teller distortion of the [MnO
6] octahedra [ 27].
The oxidation of Mn(II) to Mn(III) in air was evi-
denced by TG/DSC thermal analysis. Figure 4presents
the thermal curves recorded in air for the as-obtained
manganese ferrite (Fig. 4a) and for the powders annealed
at 400 /C176C (Fig. 4b) and 500 /C176C (Fig. 4c). In order to
confirm the oxidizing role of oxygen, the thermal curves
of the as-obtained powder were also recorded in nitrogenatmosphere (Fig. 4a).
The thermoanalytical curves of the as-obtained powder
(195/C176C) recorded in air (Fig. 4a) exhibit several exother-
mic processes with mass gain (oxidation processes). Thus
up to 400 /C176C, there are two very weak oxidation processes:
one around 250 /C176C (more visible on DSC curve) and
another around 400 /C176C (less visible on DSC curve). These
processes can be caused by a superficial Mn(II) oxidation
to Mn(III), with no changes in the present phases, asrevealed by XRD. This hypothesis confirms the findings of
our FT-IR study. The most important exothermic process is
the one recorded around 600 /C176C, with clear mass gain of
0.43 % and a sharp exothermic effect; in this range, a
crystallization process might also occur (as resulted from
the XRD study). The theoretic mass gain for the oxidationof Mn(II) from manganese ferrite to Mn(III) is of 0.829 %.
The smaller mass gain recorded in air at *600/C176C confirms
the partial oxidation of Mn(II) up to 400 /C176C, even if this
process does not modify the phases in the XRD pattern
(Fig. 1). It is also possible that in the annealing conditions
(thicker powder layer in the crucible compared with theone used in TG analysis) the oxidation process might occur
more in the superficial layer and less in the depth of the
1000 900 800 700 600 500 4001000 °C700 °C500 °C400 °C195 °C
661
576488549480551488640 576563424
Wavenumber/cm–1Transmittance/a.u.
Fig. 3 FT-IR spectra of the aas-synthesized powder (195 /C176C) and
powders annealed at different temperaturesThermal stability of the solvothermal-synthesized MnFe 2O4nanopowder
123

sample. When the thermoanalytical curves were recorded
in nitrogen (Fig. 4a), the thermal behavior was similar to
the one in air, but the processes were significantly weaker;the oxidation processes in this case are sustained by the air
remained in the pores of the sample. As a consequence, the
mass gain in nitrogen atmosphere was lower (0.18 %) ascompared to the mass gain in air (0.43 %). In case of the
sample annealed at 400 /C176C (Fig. 4b), the mass gain regis-
tered in air at 600 /C176C is higher than the one registered in
the sample obtained at 195 /C176C (0.55 % instead of 0.43 %),
probably due to the lower mass loss registered up to
150/C176C. The TG curve of the powder annealed at 500 /C176C
exhibits no mass changes, and no visible effects on DSCcurve were recorded.
In order to avoid Mn(II) oxidation, the powder annealed
at 400 /C176C was further annealed for 2 h in argon atmosphere
at 800 /C176C, than characterized by thermal analysis in air
(Fig. 5). In this case, two oxidation processes with mass
gain can be observed, one up to 400 /C176C and the second
around 600 /C176C, accompanied by a weak exothermic effect.
The first mass gain may be due to the superficial oxidationof Mn(II), while the second mass gain is due to the final
complete oxidation of Mn(II) in the entire mass of the
sample.
101
100
99
98
97
96
95
94
100 200 300 400 500 600 700 800 900 1000100 200 300 400 500 600 700 800 900 1000
100 200 300 400 500 600 700 800 900 1000–4–3–2–10
–4–3–2–10
ExoExoExo
–4
–5–3–2–10
DSC/mW mg–1
Temperature/°C
Temperature/°CTemperature/°CMass/% Mass/% Mass/%103
102
101
100
99
98
97
96
95
94
103104
102
101
100
99
98
97
96
95615
627
TG
DSCTG
DSCDSC-airTG-N2DSC-N2
TG-air
(a)
(b)
(c)
DSC/mW mg–1DSC/mW mg–1
Fig. 4 TG/DSC curves of the aas-synthesized powder (195 /C176C) and
powders annealed at different temperatures: b400/C176C and c500/C176C
100 200 300 400 500 600 700 800 900 10009698100102104
–5–4–3–2–10
617
TG
DSCExoMass/%
Temperature/°C
DSC/mW mg–1
Fig. 5 TG/DSC curves of the powder annealed at 800 /C176C in argon
10 20 30 40 50 60 70 800200400600800100012001400
MnFe2O4
Mn0.176Fe1.824O301-071-4919
04-011-9587
2 Theta/°Intensity/counts
Fig. 6 XRD pattern of MnFe 2O4powder obtained at 400 /C176C and
annealed at 800 /C176C in argonM. Stoia et al.
123

The weakness of the oxidation process was due to the
partial oxidation of the manganese ferrite, probably
because of the residual oxygen from manganese ferrite
powders’ pores (due to the fact that the powder was notpreviously degassed), as resulted from the XRD pattern
(Fig. 6). The major crystalline phase in this case is man-
ganese ferrite (69 %), with 31 % Mn
0.176Fe1.824O4as sec-
ondary phase (caused by the presence of residual oxygen in
the pores of the initial powder). Thus, to obtain pure
manganese ferrite powder well crystallized, the powderobtained in air at 400 /C176C must be degassed and annealed at
higher temperatures in inert atmosphere.
The behavior of the powders obtained in air at different
temperatures was studied by measurement in magneticfield of maximum 5 kOe. The obtained magnetization
cycles are shown in Fig. 7. In case of the powders annealed
at 700 and 1000 /C176C, there was no magnetization. As results
from Fig. 7, the powder obtained at 195 /C176C has the highest
magnetization at 5 kOe (51 emu g
-1), while the powder
annealed at 400 /C176C, despite its higher crystallinity, has a
slightly smaller magnetization (46 emu g-1), due to the
superficial oxidation of Mn(II) to Mn(III) as discussedabove. After annealing at 500 /C176C, the magnetization
severely decreases to almost 0 (0.31 emu g
-1) due to
manganese ferrite decomposition by oxidation. All pow-ders exhibited superparamagnetic behavior ( H
c=0 kOe)
due to the very small size of manganese ferrite crystallites.
Conclusions
Manganese ferrite was successfully synthesized as finenanopowder, by a new solvothermal method, using 1,2
propanediol as solvent. The evolution of the oxidic systemwith the annealing temperature in air was studied. It was
established that manganese ferrite was stable up to 400 /C176C; at
higher temperatures, it decomposes, leading to Mn(III) andFe(III) mixed oxides. The oxidation of Mn(II) to Mn(III) was
evidenced on both TG and DSC curves of the powders
obtained at 195 /C176C (as synthesized) and annealed at 400 /C176C.
The annealing at 500 /C176C for 2 h in air ensures the complete
Mn(II) oxidation, as the characteristic exothermic effect was
no longer present at 600 /C176C on DSC curve and no mass gain
was registered on TG curve. In order to avoid manganese
ferrite oxidation, the annealing at temperatures higher than
400/C176C must be performed in inert atmosphere, but the
degassing of the powders is first necessary. The magnetic
behavior of the manganese ferrite nanopowders was super-
paramagnetic, with a maximum magnetization of51 emu g
-1for the as-synthesized powder (195 /C176C).
Acknowledgements This work was supported by a grant of the
Romanian National Authority for Scientific Research and Innovation,
CNCS—UEFISCDI, project number PN-II-RU-TE-2014-4-0514.
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–60–40–200204060
–60–40–200204060
0.4
0.2
0
–0.2
–0.4–6 –4 –2 0 2 4 6
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