Oxidation of wüstite rich iron oxide nanoparticles via post-synthesis annealing

Article abstract of DOI:10.1016/j.jmmm.2021.168405

When forming magnetic nanoparticles, the decomposition of organo-metallic precursors causes a reduction of Fe(III) to Fe(II) which leads to the formation of an antiferromagnetic rock salt phase of FeO. The antiferromagnetic phase reduces the nanoparticle magnetization, so a new method of oxidation was developed that can convert FeO rich particles to Fe₃O₄ particles. Iron oxide nanoparticles with different sizes were synthesized to validate the oxidation method. We demonstrate that iron oxide nanoparticles can be oxidized by post synthesis annealing without addition of oxidizing agents. The oxidized particles were measured with XRD, VSM and AC calorimetry to show the effective oxidation by comparing to the as prepared sample. The resulting 20 nm oxidized particles have a saturation magnetization of 72 Am²/kg at 300 K and a specific absorption rate of 181 W/g under a 212 kHz, 33 mT AC field.

Full text of DOI:10.1016/j.jmmm.2021.168405

Journal of Magnetism and Magnetic Materials 539 (2021) 168405
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Research articles
Oxidation of wüstite rich iron oxide nanoparticles via
post-synthesis annealing
,
*
Zichun Yana, Sara FitzGeraldb, Thomas M. Crawfordb, O. Thompson Mefforda
a Department of Materials Science & Engineering, Clemson University, Clemson, SC 29634, USA
b Department of Physics and Astronomy, SmartState Center for Experimental Nanoscale Physics, University of South Carolina, Columbia, SC 29208, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
When forming magnetic nanoparticles, the decomposition of organo-metallic precursors causes a reduction of Fe
(III) to Fe(II) which leads to the formation of an antiferromagnetic rock salt phase of FeO. The antiferromagnetic
phase reduces the nanoparticle magnetization, so a new method of oxidation was developed that can convert FeO
rich particles to Fe3O4 particles. Iron oxide nanoparticles with different sizes were synthesized to validate the
oxidation method. We demonstrate that iron oxide nanoparticles can be oxidized by post synthesis annealing
without addition of oxidizing agents. The oxidized particles were measured with XRD, VSM and AC calorimetry
to show the effective oxidation by comparing to the as prepared sample. The resulting 20 nm oxidized particles
have a saturation magnetization of 72 Am2/kg at 300 K and a specific absorption rate of 181 W/g under a 212
kHz, 33 mT AC field.
Magnetic nanoparticle
Nanoparticle synthesis
Materials characterization
Oxidation
Wüstite
Magnetite
1. Introduction
temperatures increases the possibility of oxidizing the organic ligands
and requires an additional process to remove the excess.
In recent years, developments in thermal decomposition synthesis
have improved control over the size, morphology, and composition of
iron oxide-based magnetic nanoparticles [1–4]. Usually the reaction
requires organo-metallic complexes as precursors. As the complexes
decompose at high temperature, the organic ligands also decompose and
generate reducing agents such as carbon or carbon monoxide. These
cause reduction of metal (typically iron, from trivalent to divalent)
during the high-temperature reaction and result in the formation of
unwanted wüstite structure, [5–9] which leads to a loss of magnetization
this affects applications such as magnetic resonance imaging [10],
magnetic hyperthermia, [11,12], and magnetic particle imaging
[13,14]. To solve this problem, different approaches were developed to
oxidize the divalent iron to trivalent iron inside the particles either
during particle formation or after particles are synthesized. These
include adding oxygen during synthesis [5,15], adding oxygen to
oxidize the particles after synthesis [16], aging the particles [9,17], as
well as adding solid oxidizing agents [13,18]. Although these methods
succeeded in oxidizing the particles from wüstite to magnetite or
maghemite to some extent, they are not perfect. For example, adding
oxygen to a high-temperature organic liquid increases the risk of igni-
tion of reagents. Adding oxidizing agents to the reaction at high
Based on the phase diagram of iron oxide (Fig. 1), wüstite (FeO), is
not a thermodynamically favorable phase at a relatively low tempera-
ture [19]. Thus the existence of wüstite in the nanoparticles was caused
by quenching. As shown in Fig. 1, to transform FeO (wüstite) to Fe3O4
(magnetite), one can either decrease the temperature at a certain oxygen
level or increase the oxygen partial pressure at a specific temperature. In
practice, this can be done easily when dealing with ceramics by sintering
the iron oxide solids in a controlled stoichiometry and atmosphere.
However, in the case of nanoparticles, the sintering method does not
apply since it will destroy the organic ligands and cause particle merg-
ing. Theoretically, the FeO phase has a thermodynamic potential to relax
to Fe3O4 and finally α-Fe2O3 (hematite) according to the phase diagram,
therefore, if given enough energy to overcome the kinetic energy bar-
rier, the phase transition process should be spontaneous. It has been
previously proposed [20], that the transformation between wüstite and
magnetite requires a re-coordination of iron cations which is likely to
happen as both wüstite and magnetite exhibit cubic close packing. While
the transformation between wüstite and hematite requires a shift of the
oxygen planes from a cubic close packed arrangement to a hexagonal
close packed arrangement, this process is predominantly facilitated by
the anionic diffusion of oxygen. Thus, if the particles are annealed at an
* Corresponding author.
E-mail address: mefford@clemson.edu (O.T. Mefford).
https://doi.org/10.1016/j.jmmm.2021.168405
Received 31 May 2021; Received in revised form 9 August 2021; Accepted 9 August 2021
Available online 14 August 2021
0304-8853/© 2021 Elsevier B.V. All rights reserved.

Z. Yan et al.
Journal of Magnetism and Magnetic Materials 539 (2021) 168405
particles were annealed at 250 ◦C, 265 ◦C, and 280 ◦C for 1 h and 1.5 h,
respectively, to study the effect of parameters during annealing. The 20
nm particles were annealed at 250 ◦C for 2 h to validate the annealing
procedure, and the annealed particles were used for further magnetic
characterizations. The resulting products were purified and stored with
the same procedures mentioned above.
2.2. Materials characterization
Electron microscopy, X-ray diffraction, vibrating sample magne-
tometry, and inductively coupled plasma-optical emission spectroscopy
methods were described in Chapter 2.
3. Results and discussion
3.1. Particle synthesis and post-synthesis annealing
Iron oxide nanoparticles were synthesized using a relatively mature
method that uses iron (III) as the thermal decomposition precursor. This
method was introduced by Park et al. [2,8] and was widely used for
preparing iron oxide nanoparticles with narrow size distributions. As
shown in Fig. 2, spherical iron oxide particles were prepared. The size
distributions for each batch were relatively narrow and the mean sizes
were close to the targeted value. Higher concentration of precursors,
higher reaction temperature, and longer reaction time was used to
produce particles with larger sizes; however, such changes may also
result in wider size distribution. The synthesized particles have mean
diameters of 15.4 ± 1.3 nm (targeted 15 nm), 18.6 ± 1.2 nm (targeted
20 nm), and 38 ± 7 nm (targeted 30 nm) respectively. It is important to
mention that in Fig. 2 a) and b), some particles show what appears to be
a “core/shell” like structure. This structure likely occurs because the
particles are slightly oxidized on the surface before an annealing treat-
ment. The oxidized regions are more prominent on particles with
smaller sizes, like the 15 nm and 20 nm particles, while larger particles
(i.e., 30 nm) do not show significant core/shell structures due to a
relatively lower specific surface area [21]. This hypothesis was further
confirmed by XRD analysis. Before annealing, larger particles have a
higher fraction of wüstite than smaller particles. A more detailed dis-
cussion of this phase behavior can be found in the following paragraphs.
The size distribution of particles after annealing remained unchanged
and the core/shell images disappeared.
Fig. 1. Phase diagram of Fe-O compounds at 400–1500 ◦C and certain oxygen
partial pressures. Figure is reprinted from ref.19 with permission. Copyright
2019 Elsevier.
ideal temperature for a sufficient time, the wüstite-rich particles will
automatically turn into magnetite with minimum addition of oxygen.
Herein, we propose a facile and efficient way to convert the as-
prepared wüstite-rich particles into pure magnetite particles by accel-
erating the oxidation with annealing. The success of this conversion was
confirmed via x-ray powder diffraction (XRD) and zero-field cooling/
field cooling (ZFC/FC) magnetometry.
2. Experimental
2.1. Particle synthesis and post-synthesis annealing
Thermal decomposition of oleate precursors was conducted in a “one
pot” reaction that was adopted from a previous study with changes [2].
Iron (III) oleate precursors were made by reacting ferric chloride with
sodium oleate in a refluxing solvent mixture of hexane, ethanol, and
water prior to the particle synthesis. The iron oleate was obtained by
evaporating hexane from the upper layer and precipitated by washing
with acetone. The final waxy solid was dried in a vacuum oven at 80℃
overnight before use. To synthesize spherical wüstite-rich nanoparticles
with different diameters different quantities of reagents, reaction time,
and reaction temperatures were used. Briefly, 15 nm iron oxide particles
were made by refluxing 4 mmol iron (III) oleate, 4.2 mL oleic acid, and
12 mL n-octadecene at 325 ◦C for 1.5 h under nitrogen. 20 nm iron oxide
nanoparticles were made by refluxing 9 mmol iron (III) oleate, 5 g
docosane, 9 mL oleic acid, and 5 mL n-octadecene at 365 ◦C for 2 h under
nitrogen. 30 nm iron oxide nanoparticles were made by refluxing 7.47
mmol iron (III) oleate, 3.73 mmol iron (II) oleate, 11.2 mL oleic acid, 5 g
docosane, and 10 mL n-octadecene at 365℃ for 2 h under nitrogen. To
ensure the formation of wüstite, all reactions underwent a degassing
The 15 nm particles were used for the study of oxidation via the
addition of oxidizing agents. Before oxidation the as-prepared 15 nm
iron oxide nanoparticles showed a combination of spinel and rock salt
phase as illustrated in Fig. 3. According to the Rietveld refinement, the
weight fraction of the spinel phase is approximately 83% and the weight
fraction of the rock salt phase is around 17%. Such particles were treated
by the annealing methods with or without the addition of oxidizing
agents. As described in previous studies, TMAO or oxygen were typically
used as an oxidizing agent for the post-synthesis oxidation of iron oxide
nanoparticles. Herein, we hypothesized that the addition of oxidizing
agents was not the critical factor that converted wüstite to magnetite; if
given enough thermal energy and annealing time, particles could be
converted to pure spinel without adding any oxidizing agents. A set of
control experiments were conducted with the as-prepared 15 nm iron
oxide nanoparticles by controlling the parameters during annealing. As
shown in Fig. 3, the four groups with post-synthesis treatment had very
similar XRD results, and they all exhibit a pure spinel structure. This
indicates that the driving force of converting rock salt phase to spinel
phase is not the addition of oxidizing agents since even the sample with
no addition of TMAO or air (orange line, Wüstite-anneal-N2) showed a
100% spinel structure. Most likely, the anneal process was done without
a degassing procedure and the oxygen that dissolved in the organic
solvent (n-octyl ether) is enough for the oxidation of wüstite to
magnetite [22,23]. It is important to check the reproducibility of the
annealing process. To further validate the hypothesis, wüstite particles
◦
step where the reagents were heated to 100 C under vacuum for 1 h
before reaction.
To purify the products, the final product was added to a washing
solution containing hexanes (5 mL), ethanol (30 mL), and acetone (30
mL); this mixture was vortexed and centrifuged at 10000 rpm for 5 min.
The precipitation was collected by decanting the supernatant and repeat
the washing process with the same washing solution until the superna-
tant became clear. The final precipitation was then redispersed and
stored in hexanes.
The particles were weighed after being transferred into tared glass
vials where the hexane was evaporated and were then dispersed into n-
octyl ether with a concentration of 20 mg/mL. To study the effect of the
◦
oxidant, the 15 nm particles were annealed at 280 C for 1 h with or
without trimethylamine N-oxide (TMAO) (0.05 mmol/mL). 30 nm
2

Z. Yan et al.
Journal of Magnetism and Magnetic Materials 539 (2021) 168405
Fig. 2. TEM images of as-prepared iron oxide nanoparticles. a) 15.4 ± 1.3 nm (targeted 15 nm); b) 18.6 ± 1.2 nm (targeted 20 nm); and c) 38 ± 7 nm (targeted 30
nm). Scale bars are 20 nm.
with larger sizes were made. Larger particles have better XRD signal-to-
noise ratio and sharper peaks which will make the quantitative analysis
more persuasive. Moreover, the annealing process needs to be optimized
before establishing it as a standard operating procedure. Therefore, the
as-prepared 30 nm iron oxide nanoparticles were annealed at three
different temperatures for 1 h and 1.5 h. The 30 nm particles were
selected because they have the lowest specific surface area among the
three batches, and intuitively the 30 nm batch should have the purest
phase as prepared. Moreover, for a post-reaction oxidation process, the
larger the particles are, the longer time it will take to complete the
process [24]. Therefore if the procedure works for large particles it
should apply for smaller ones. The products after annealing were
analyzed by XRD and Rietveld refinement to know the efficiency of
annealing with different parameters. The results are shown in Table 1
and the XRD results of the sample annealed at 265℃ are shown in Fig. 4.
The diffraction patterns of samples annealed at different temperatures
Table 1
The weight percentage of spinel and rock salt phases in 30 nm particles before
and after annealing.
As-prep
250 ◦
C
265 ◦
C
280 ◦
C
Fig. 3. XRD results of 15 nm particles before and after annealing with addition
of oxidizing agents.
1 h
1.5 h
1 h
1.5 h
1 h
1.5 h
Spinel
16.0%
84.0%
75.6%
24.4%
97.4%
2.6%
75.3%
24.7%
97.6%
2.4%
71.8%
28.2%
97.0%
3.0%
Rock salt
3

Z. Yan et al.
Journal of Magnetism and Magnetic Materials 539 (2021) 168405
Fig. 5. Magnetization curves of 20 nm particles before and after oxidation. The
inset shows the curves at the region close to zero field. Both as-prepared and
oxidized samples showed a superparamagnetic behavior at 300 K. The satura-
tion magnetization of as prepared sample is 30 Am2/kg (300 K), and the
saturation magnetization of oxidized sample is 72 Am2/kg (300 K). The coer-
civity of as prepared sample is 145 mT (116000 A/m) (5 K) and coercivity of
oxidized sample is 43 mT (34300 A/m) (5 K).
Fig. 4. XRD result of 30 nm iron oxide particles annealed at 265 ◦C.
are very similar if the annealing times are the same. It seems that the
annealing time plays a more significant role in the phase transition,
which makes sense as the kinetics of this thermo dynamically driven
processes are more dominated by time if the thermal energy surpasses a
minimum threshold. For example, the sharp wüstite peaks from the as-
prepared 30 nm sample after 1 h of annealing, the rock salt phase was
partially converted to spinel phase, and if we kept annealing the sample
for another half hour, the particles became nearly pure spinel. As the
annealing time increases, the peaks with low-intensities started to
appear in the diffractogram, such as the peaks at 18◦ and 75◦. The 30 nm
particles provided clearer XRD results which illustrated the phase
transition process during annealing. It is worth emphasizing that the 30
nm samples were annealed under constant nitrogen flow without the
addition of oxidizing agents. This phenomenon validates the former
hypothesis, meaning the wüstite-rich particles can be successfully
oxidized to spinel iron oxide particles by annealing. Considering a study
on the oxidation of unsaturated aliphatic acid [25], the oleic acid could
be oxidized by the oxygen in the air at high temperature and cause inter-
particle crosslinking, so the annealing was done under nitrogen. As
discussed in the study by Kemp et al. [5] the unsaturated solvent n-
octadecene can be another cause for the reduction of iron. Thus, for the
annealing process, n-octyl ether was selected as solvent. It was also
mentioned in their study that the iron (II content increased when the
temperature increased. Therefore, further annealing was done under
nitrogen at 250 ◦C for 2 h in n-octyl ether. 20 nm as-prepared iron oxide
nanoparticles were treated using this procedure, and the XRD results and
the Rietveld refinement are shown in Fig. 5. The as-prepared sample
consisted of both wüstite phase (68 wt%) and magnetite phase (32 wt%),
while after annealing, the 20 nm sample was successfully oxidized to
100 wt% magnetite. The crystallite sizes of the as-prepared and oxidized
20 nm sample were calculated using the highest peaks and the results are
shown in Table 2.
Table 2
Crystallite size and composition analysis results of 20 nm sample from the XRD
data.
Magnetite (311)
Wüstite (200)
Crystallite size
(nm)
Percentage
(wt.%)
Crystallite size
(nm)
Percentage
(wt.%)
As-prep
3.6
32
11.5
\
68
\
Oxidized
14.8
100
wüstite. Both samples exhibited superparamagnetic behavior at 300 K
showing no coercivity. However, at 5 K, the as-prepared sample had a
coercivity around 145 mT (116000 A/m) larger than the coercivity of
the oxidized sample, 43 mT (34300 A/m). This phenomenon was also
seen in previous studies [26–28], which can be explained by exchange
coupling between the antiferromagnetic core and the ferrimagnetic
shell. The ZFC/FC measurement was conducted in a 10 mT (7977 A/m)
field and the magnetization vs temperature curves are plotted in Fig. 6.
As seen in Fig. 6, the curves of the two samples look very different. The
FC curve of the as-prepared sample showed a drop in magnetization at
around 210 K when cooling down from 325 K, while the annealed
sample’s magnetization kept increasing till a plateau. This phenomenon
is commonly seen in the antiferromagnetic materials. When the tem-
´
perature is above its Neel temperature the material behaves para-
magnetically, and all the magnetic moments are magnetized to align
´
with the field. [16,27] However, below its Neel temperature the mate-
rial returns to its antiferromagnetic structure resulting in a loss of total
magnetization. This result coincides with the conclusion from the
structural characterizations: the as-prepared sample has a combination
of wüstite and magnetite. The ZFC curves of the two samples are plotted
3.2. The effect of post-synthesis annealing on the magnetic properties
´
in Fig. 6 (right). To get the Neel temperature and the blocking temper-
As wüstite is antiferromagnetic, the magnetic properties of the par-
ticles before and after oxidation should be very different. To show the
effectiveness of oxidation and the change in magnetic properties after
treatment. 20 nm iron oxide particles were measured with VSM at 5 K
and 300 K under a maximum field of 2T (1495400 A/m). The magne-
tization curves were plotted in Fig. 5. The oxidized sample showed
higher saturation magnetization and susceptibility than the as-prepared
sample since magnetite contributes more magnetic moments than
ature, two regimes were used [29,30]. If the vertex of the ZFC curve was
used to define the blocking temperature, the blocking temperatures are
235 K and 283 K for the as-prepared and oxidized samples, respectively.
If the vertex of the derivative curve was used to define the blocking
temperature, then the blocking temperatures are 209 K and 139 K,
respectively, as seen in Table 3. With the same rationale, we can esti-
´
mate the Neel temperature of the as-prepared sample by measuring the
4

Z. Yan et al.
Journal of Magnetism and Magnetic Materials 539 (2021) 168405
Fig. 6. Left: ZFC/FC curve of 20 nm iron oxide nanoparticles. Right: ZFC magnetization vs temperature curve and derivative of magnetization vs temperature curve.
points) to be 181 W/g using [33–36]
Table 3
ρ
C⋅ΔT
´
Summary of Neel temperature (TN) and Blocking temperature (TB) obtained
SAR =
from ZFC/FC curves.
m⋅Δt
´
Neel temperature TN (K)
Blocking temperature TB (K)
where C is the heat capacity of the measured mixture (close to the heat
Max point
Derivative
Max point
Derivative
capacity of toluene, 1.71 J/g∙K),
ρ is the density of the solvent used
(0.867 g/mL for toluene), m is the concentration of magnetic nano-
particles in the measured suspension (4 mg/mL), and ΔT/Δt is the slope
of the fitted line (0.4893 K/s in Fig. 7). The wüstite-rich particles
(without post-synthesis annealing) showed no heating (SAR = 0) under
such a condition (212 kHz, 26 kA/m). Previous reports showed a SAR
(130 W/g) for iron oxide nanoparticles with a similar size (17 ± 0.1 nm)
under an AMF (205 kHz, 20 mT) [37]. A study on the commercialized
iron oxide nanoparticles (25 nm) showed SAR values close to our sample
(Sigma Aldrich 900026: 145.6 ± 6.3 W/g and Sigma Aldrich 900027:
176.2 ± 27.1 W/g) under an AMF (223 kHz/, 41 kA/m) [35]. Since SAR
is related to the size of the nanoparticles and the field parameters, it is
better to compare SAR with the same field parameters to compare the
heating efficiency of the particles. Comparing to the previous study
using the same field parameters, the annealed 20 nm particles showed
better heating efficiency, and the SAR is increased more than four times
(20 nm, SAR = 40 W/g) [38]. Thus, post-synthesis annealing signifi-
cantly improved the heating efficiency.
As-prep
210
\
194
\
235
283
209
139
Oxidized
FC curve. The results are shown in Table 3, and are comparable with the
´
´
reported Neel temperature of bulk wüstite (198 K) [31]. The Neel
temperature refers to the critical temperature when the thermal energy
becomes high enough to disorder the anti-parallel magnetic moments
that are held together by exchange interactions [32]. Therefore, only
´
particles with an antiferromagnetic structure show a Neel temperature,
´
and there is no Neel temperature listed in the table for the oxidized
sample. The 20 nm particles were dissolved in toluene with a magnetic
core concentration of 4 mg/mL and then measured under a 212 kHz, 26
kA/m (33 mT) AC field. The temperature profile is plotted in Fig. 7. The
as-prepared sample did not show a temperature increase, while the
oxidized sample heated from 37 ◦C to 63 ◦C within 2 min. The specific
absorption rate (SAR) was calculated (linear fitting of the first 6 data
4. Conclusion and outlook
Iron oxide nanoparticles with different diameters that are rich in the
wüstite phase can be oxidized by post-synthesis annealing without the
addition of oxidizing agents. The XRD results showed that the oxidized
particles have a pure magnetite structure. The saturation magnetization
of the particles increased after annealing. The magnetic heating ability
significantly increased after annealing compared to the untreated par-
ticles. With the success in the oxidation of iron oxide particles, more
studies need to be done to see if the same strategy applies for substituted
ferrites, such as MnFe2O4, CoFe2O4, and NiFe2O4 to ensure pure spinel
phase nanoparticles.
CRediT authorship contribution statement
Zichun Yan: Project administration, Conceptualization, Methodol-
ogy, Investigation, Data curation, Writing – original draft, Writing –
review & editing. Sara FitzGerald: Data curation. Thomas M. Craw-
ford: Supervision, Funding acquisition. O. Thompson Mefford: Su-
pervision, Project administration, Funding acquisition, Writing – review
& editing.
Fig. 7. Heating profile of 20 nm iron oxide particles under a 212 kHz, 33 mT
AC field.
5

Z. Yan et al.
Journal of Magnetism and Magnetic Materials 539 (2021) 168405
synthesis of iron oxide nanoparticles with diminished magnetic dead layer by
Declaration of Competing Interest
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The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
magnetic properties of nanoparticles arising from defect structures: Topotaxial
oxidation of Fe1- xO|Fe 3-δO4 core|shell nanocubes to single-phase particles, ACS
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Foundation award #OIA-1655740 Grants for Exploratory Academic
Research (GEAR). The imaging was funded by Clemson Core Incentiv-
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