Synthesis of various magnetic nanostructures and the microwave characterizations 7

Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
113

Chapter 7
Synthesis and microwave absorption of Fe
3
O
4
particles with
various structures by chemical reduction route
7.1 Introduction
Chapter 6 presented that the resonance frequency and the permeability of
as-synthesized Zn-ferrite were higher than those of Fe
3
O
4
. Hence the enhanced
saturation magnetization could extend the Snoek’s limitation. Another optional
method to extend the Snoek’s limitation is to induce the shape anisotropic field into
Fe
3
O
4
particles. The shape effect on the Snoek’s law could be described as following:



 
















(1.11 in Chapter 1)
The value on the right side would be extended due to 

 

. In this case, the
permeability of Fe
3
O
4
may be high at relative high frequency range (GHz range). In
this chapter, we succeeded in obtaining uniform Fe
3
O
4
particles with different shapes
and studied the effect of various shapes on the resonance frequency. The synthesis

process reported here is so called chemical reduction method, which is used to convert
-Fe
2
O
3
to Fe
3
O
4
with the morphology being preserved.
For several decades, shape control over iron oxide nanocrystals is one of the most
interesting topics because their physical and chemical properties can be manipulated
through variations on their morphology and size. Unique electron-transport behavior
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
114

was shown by Fe
3
O
4
nanowires.[1] Very high specific capacity (~ 749 mA·h·g
-1
at
C/5 and ~ 600 mA·h·g
-1
at C/2) was exhibited by carbon coated Fe
3
O
4

nanospindles
when used as an anode material for Li-Ion batteries.[2] Room temperature
magnetoresistance as high as ~ 1.2% was observed in MgO/Fe
3
O
4
core-shell
nanowires.[3] Relative high luminescence and very strong magnetic resonance T2*
effect was displayed by quantum dot capped Fe
3
O
4
nanorings.[4] Extra high coercive
fields of 76.5 ( 1.5) mT was detected in Fe
3
O
4
tube arrays.[5] With these intriguing
properties reported, the fabrication of various shapes of iron oxides, especially Fe
3
O
4

particles, attracts more and more attentions. So far, many synthesis methods have
been developed for synthesis of Fe
3
O
4
particles, such as sol-gel in reverse micelles,[6]
hydrothermal,[7] co-precipitation[8] and thermal decomposition.[9,10] However,

these methods tend to form Fe
3
O
4
nanoparticles with isotropic shapes. Recently,
Fe
3
O
4
with hollow structures (rings, tubes ad capsules, etc.) and 1-dimensional
structures (wires, rods and spindles, etc.) were successfully synthesized by template
method[11] - dehydration or reduction of premade isotropic -FeOOH or -Fe
2
O
3

particles.[5,12] Compared with dehydration, reduction of premade -Fe
2
O
3
seems to
be a simple and effective method. Usually, the reduction of -Fe
2
O
3
involves an
annealing process in reductive atmosphere at elevated temperatures (typically in the
range of 300-500 ℃). This annealing treatment of nanostructured materials may
result in undesirable aggregation and sintering. In this work, we have developed a
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles

with various structures by chemical reduction route
115

chemical reduction method, which allows the reduction process to be carried out in
organic solvent. The developed method aims to obtain pure Fe
3
O
4
phase after the
reduction with preserving the morphology of premade -Fe
2
O
3
template. In this study,
two kinds of reducing agents, i.e. oleic acid and H
2
-involved gas (5%H
2
-95%Ar gas
mixture), are used, and their effects on the reduction process are investigated. The
developed reduction method is suitable for massive production, which makes it
possible to investigate the microwave absorption performance of as-reduced Fe
3
O
4

particles with various structures.
Since the shape of formed Fe
3
O

4
nanoparticles is most related with that of premade
-Fe
2
O
3
template, it is important for us to prepare -Fe
2
O
3
with various shapes prior
to the reduction process. Unlike Fe
3
O
4
, the existing methods are more effective to
form high-quality -Fe
2
O
3
particles with varied shapes. -Fe
2
O
3
nanobelts, nanowires
and nanoflakes could be produced by thermal oxidation of iron in oxygen
atmosphere[13,14] or by calcination of -FeOOH in air.[15] -Fe
2
O
3

nanotubes and
nanorods could be formed by template method[16,17] as well as hydrothermal
method.[18] -Fe
2
O
3
nanorings, nanodiscs and capsules could also be prepared by
hydrothermal method.[19-21] Based on the references, the hydrothermal route is
found to be versatile and able to synthesize -Fe
2
O
3
nanoparticles of different shapes,
such as rings, tubes, capsules, rods as well as discs. Hence, on the purpose to enrich
the library of the shapes of Fe
3
O
4
nanostructures, the hydrothermal method was
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
116

employed in the current study. As an extension work of our previous report,[22]
ammonium phosphate (NH
4
H
2
PO
4

) was chose as additives to facilitate the hydrolysis
of FeCl
3
to produce Fe
3+
ions and control the growth of -Fe
2
O
3
nanoparticles.
Besides the -Fe
2
O
3
rings and tubes reported by the previous works, single-crystalline
-Fe
2
O
3
rods with tunable sizes were also developed by adjusting the ratio of
[Fe
3+
]/[H
2
PO
4
-
]. To the best of my knowledge, it is the first report on the synthesis of
-Fe
2

O
3
rings, tubes and rods by one single route. It is worthy to mention that the
sizes of developed rods can be well controlled by adjusting the concentration of
starting materials. The formation mechanism on the variety of as-prepared -Fe
2
O
3

particles by hydrothermal method is also described in this chapter.
7.2 Experimental results
7.2.1 Synthesis of -Fe
2
O
3
with various shapes by hydrothermal treatment
7.2.1.1 Mechanism on the formation of -Fe
2
O
3
nanoparticles with different
morphology
In this work, large-scale -Fe
2
O
3
nanoparticles with various shapes and sizes can be
prepared via a facile hydrothermal treatment. The formation of -Fe
2
O

3
nanocrystals
starts from the -Fe
2
O
3
monomers generated by the hydrolysis of Fe
3+
ions at 220 ℃
in the presence of phosphate ions. The formed monomers possess very high surface
energy and tend to aggregate rapidly. The aggregation process is affected greatly by
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
117

the concentration of Fe
3+
ions as well as the phosphate ions, resulting in -Fe
2
O
3
of
different shapes. Similar mechanism has been already revealed by some previous
reports regarding to the hollow nanocrystals, such as nanorings and
nanotubes.[19,22,23] For the formation of nanorings, -Fe
2
O
3
monomers aggregate to
form a disk first, followed by a subsequent ‘etching’ of the -Fe

2
O
3
nanodisks by
phosphate ions at the central part, leading to ring-structure particles formed. In our
experimental results, when the concentrations of Fe
3+
and H
2
PO
4
-
ions are 5 mM and
0.72 mM, 150 nm disks would be formed if the heating period at 220 ℃ is around 10
h; while 154 nm rings would be obtained if the heating period at 220 ℃ is prolonged
to 48 h, as shown in Fig. 7.1. The SEM image in Fig. 7.1b shows a mixture of disks
and freshly formed rings, which is a kind of intermediate product before the final

formation of nanorings. The morphology evolution of formed particles from disk to
ring is consistent with the previous reports.[19] However, the mechanism (why the
aggregation of -Fe
2
O
3
monomers forms disk at the middle stage) is still not well
understood. Although we know that molar ratio of Fe
3+
to H
2
PO

4
-
ions is crucial to
this problem, no specific answer could be given so far. Nevertheless, researchers have
Fig. 7.1 SEM images of (a) -Fe
2
O
3
disks (10 h at 220 ℃
); (b) mixed product of
disk and rings (20 h at 220 ℃) and (c) -Fe
2
O
3
rings (48h at 220 ℃
). The scale
bars for all the images stand for 200 nm.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
118

tried to explain the dissolution of the hematite particles by the following reactions:
Fe
2
O
3
+ 6 H
+
 2 Fe
3+

+ 3 H
2
O (7.1)
Fe
3+
+ xH
2
PO
4
-
 [Fe(H
2
PO
4
)
x
]
3-x
(7.2)
According to Eq. (7.2), the formation of [Fe(H
2
PO
4
)
x
]
3-x
will consume Fe
3+
ions and

lower the concentration of Fe
3+
in the aqueous solution. Then the decomposition of
Fe
2
O
3
to Fe
3+
ions is forced by the lack of Fe
3+
ions to reach a thermodynamic
equilibrium state, as indicated by Eq. (7.1). The central part of as formed disk is rich
of H
2
PO
4
-
because of the aggregation of -Fe
2
O
3
monomers starts from there, leading
to a final ring-structure after the dissolution process.
The above mechanism is also valid for the formation of -Fe
2
O
3
tubes. With using
different concentrations of FeCl

3
and NH
4
H
2
PO
4
, as listed in Table 2.3, the
aggregation of -Fe
2
O
3
monomers forms spindle-like crystals first, as revealed by our
previous study,[22] and followed by a dissolution along the central axis. As reported,
for -Fe
2
O
3
tubes fabricated by this hydrothermal route, the central axis is always
along the c axis of trigonal Fe
2
O
3
, corresponding to [0 0 1] crystal orientation. The
preferential growth along [0 0 1] direction may be dominated by the selective
adsorption of phosphate ions on different crystal facets of Fe
2
O
3
other than (0 0 1)

facets. As investigated by Jia et al.,[23] the adsorption capacity and affinity of (0 0 1)
plane to phosphate ions are much lower than that of other planes, such as (1 1 0), (0 1
2) and (1 0 4) planes, originating from the absence of singly coordinated hydroxyl
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
119

groups on (0 0 1) plane. In other words, the attachment of phosphate ions on the facet
will hinder the growth in the direction normal to the facet. And the density of attached
phosphate ions, depending on the molar ratio of phosphate ions to iron precursor, is
very important to the morphology of formed -Fe
2
O
3
particles. The function of
phosphate ions in the employed hydrothermal route is similar with that of oleic acid
and oleylamine used in the thermal decomposition method for synthesis of shaped
metal oxides.[24,25]
7.2.1.2 Shape controllable synthesis of -Fe
2
O
3
nanoparticles
As a key factor that influences the morphology of as-synthesized -Fe
2
O
3

nanocrystals, the molar ratio of iron precursor to phosphate ions, i.e. [Fe
3+

]/[H
2
PO
4
-
],
was adjusted in this work. Besides the repeated work on the synthesis of -Fe
2
O
3
rings and tubes by following the previous works, we further developed -Fe
2
O
3
balls
and rods, as shown in Fig. 7.2. The concentrations of used materials are listed in Table
2.3. From Table 2.3, we further observed that 74 nm -Fe
2
O
3
rings and 70 nm
-Fe
2
O
3
tubes were produced with using the same ratio of [Fe
3+
]/[H
2
PO

4
-
] but
different concentrations of iron precursors. Due to their similar outer diameters, tubes
with longer sizes could be seen as elongated rings. This observation may reveal that (a)
the morphology of -Fe
2
O
3
nanoparticles is mainly controlled by the ratio of
[Fe
3+
]/[H2PO
4
-
]; while (b) the size of -Fe
2
O
3
nanoparticles is mainly dependent on
the concentration of iron precursor when the ratio of [Fe
3+
]/[H
2
PO
4
-
] is fixed to keep
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route

120

the morphology. This finding is further supported by the experimental work on the
synthesis of -Fe
2
O
3
rods with different sizes.

7.2.1.3 Size controllable synthesis of -Fe
2
O3 rods
For the synthesis of -Fe
2
O
3
rods,

the ratio of [Fe
3+
]/[H
2
PO
4
-
] was adjusted to be
20:0.36. The concentration of iron precursor was tuned by adding different amount of
distilled water, as demonstrated by Table 2.3. The results indicated that the size of
as-synthesized -Fe
2

O
3
rods could be successfully controlled by only adjusting the
concentration of precursor. A trend that higher concentrations of the iron precursor
lead to larger sizes of -Fe
2
O
3
rods was observed. Referring to the size control of
as-obtained -Fe
2
O
3
rods, a maximum length of 120 nm was reached. However, the
uniformity and quality is poor as shown by the SEM images in Fig. 7.3a. Some
fragments of -Fe
2
O
3
rods, as pointed out by the arrowheads, could be found. The
Fig. 7.2 SEM images of -Fe
2
O
3
with different shapes: (a) 117 nm 
-Fe
2
O
3
balls;

(b) 74 nm -Fe
2
O
3
rings; (c) 70 nm -Fe
2
O
3
tubes and (d) 98 nm 
-Fe
2
O
3
rod. The
scale bars for all the images stand for 200 nm.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
121

TEM images in Fig. 7.3b indicate that some hollow capsules and broken ones are
involved in the sample of 120 nm -Fe
2
O
3
rods. Hence, there exists a maximal size
limitation for as-prepared -Fe
2
O
3
rods by the hydrothermal method with employing

FeCl
3
-NH
4
H
2
PO
4
system. When the concentration of iron precursor decreased, we can
obtain shorter but high-quality rods, as shown in Fig. 7.4a&b. The size and size
distribution of as-synthesized -Fe
2
O
3
particles was obtained by counting 80 to 100
particles in the SEM images. The statistical results were also listed in Table 2.3. The
average value of outer diameter is adopted to name the sample. Take 98-rod as an
example, the outer diameter is 98 nm with a deviation less than 8 nm.
As far as I know, it is the first time that -Fe
2
O
3
rods with tunable sizes via
hydrothermal method are reported. To learn more about the structure of as-prepared
-Fe
2
O
3
rods, HRTEM images and SAED patterns are acquired. The results reveal a
perfect single crystal structure for all -Fe

2
O
3
rods with different sizes. The lattice
spacing in Fig. 7.4c is measured at about 0.253 nm, which is close to the standard d
Fig. 7.3 (a) SEM images of 120 nm 
-Fe
2
O
3
rods and (b) TEM images of capsules
and broken ones involved in as-prepared 120 nm -Fe
2
O
3
rods.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
122

spacing of {1 1 0} at 0.252 nm for the hematite. The SAED patterns and HRTEM
analyses reveal that the nanorods grow along [0 0 1] (c axis), as labeled in the image.
The oriented growth of -Fe
2
O
3
rods along [0 0 1] direction further illustrates the
weak adsorption affinity of (0 0 1) phosphate ions onto (0 0 1) plane of trigonal
hematite. Different from the previously reported hollow structure, 98 nm-rods, 61
nm-rod as well as 55 nm-rods are solid. This may be due to the high ratio of

[Fe
3+
]/[H
2
PO
4
-
] and the low concentration of phosphate ions in the aqueous solution,
resulting in insufficient phosphate ions for the dissolution process, as displayed by Eq.
(7.2).

Fig. 7.4 SEM images of (a) 61 nm Fe
3
O
4
rods and (b) 55 nm Fe
3
O
4
rods; (c) the
HRTEM image for as-synthesized 
-Fe
2
O
3
rod and the corresponding SAED
pattern (inset).
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
123

7.2.2 Chemical reduction of -Fe
2
O
3
to Fe
3
O
4
nanoparticles
7.2.2.1 Effect of reducing agent (oleic acid) on the reduction process
For the reduction process, trioctylamine (TOA) with a boiling point above 365℃ was
chosen as the solvent, and oleic acid was used as reducing agent. To investigate the
effect of oleic acid on the phase transformation from -Fe
2
O
3
to Fe
3
O
4
in the current
work, a set of experiments was applied to 74 nm -Fe
2
O
3
rings firstly. The
experimental conditions were listed in Table 7.1. With using 100 mg Fe
2
O

3
rings as
starting material, the amount of oleic acid was adjusted from 0.5 g to 2 g, then to 3.5 g,
corresponding to as-obtained sample B, C and D. To be noted that the reduction
process for these three samples was under pure Ar gas flow, indicating that no other
reducing agent but only oleic acid was used. After phase identification by XRD,
Table 7.1 Reduction conditions for phase transformation from -Fe
2
O
3
to Fe
3
O
4

nanorings.
Sample
No.
Oleic acid
(g)
Gas
Flow rate
(sccm)
Phase
Ms
(emu/g)
B
0.5
Ar
80

α-Fe
2
O
3
+ Fe
3
O
4

50.9
C
2
Ar
80
Fe
3
O
4

59.7
D
3.5
Ar
80
Fe
3
O
4

67.3

E
3.5
5% H
2
+ 95% Ar
80
Fe
3
O
4

69.1
F

5% H
2
+ 95% Ar
80
α-Fe
2
O
3
+ Fe
3
O
4

12.9
G

5% H
2
+ 95% Ar
120
α-Fe
2
O
3
+ Fe
3
O
4

48.2
Note: For each batch of experiments, 100 mg of 74 nm

-Fe
2
O
3
rings dispersed
in 35 mL TOA was used as starting materials (so called sample A).
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
124

sample B was examined to be a mixture of hematite and magnetite, as shown in Fig.
7.5. Most of as-reduced particles in sample B were broken into pieces. The saturation
magnetization of sample B (only 50.9 emu/g) was relatively low due to the existence
of hematite phase, as recorded by the magnetic hysteresis loops in Fig. 7.6. With

enhancing the amount of oleic acid to 2 g, an improvement could be observed based
on the XRD and SEM results. No hematite phase could be found for sample C and
fewer broken particles were shown compared with sample B, and the magnetization
was enhanced as well. These results allow us to speculate that oleic acid acts not only
as reducing agent but also as capping agent. The function of oleic acid as reducing
agent is trying to break down the particles to finish the reduction from Fe
3+
to Fe
2+
;
Fig. 7.5 SEM images of (a) 74 nm 
-Fe
2
O
3
rings, i.e. sample A; and as reduced
samples: (b) sample B; (c) sample C; (d) sample D. The scale bars on these images
stand for 200 nm. (e) The photo image of two samples, the one labeled with letter
‘A’ is for sample A dispersed in TOA, the other one with ‘T’ is for transparent
solution obtained after reduction process when the ratio of oleic acid to -Fe
2
O
3
rings is adjusted to be 29:1. The color of sample B, C and D seems the same, as
shown by the inset photo in figure (c). (f) The XRD patterns for different samples.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
125

the other function as capping agent is responsible for protecting the particles from

being broken during the redox reaction. The multifunction of oleic acid has been
investigated in other chemical reactions, such as the synthesis of Fe
3
O
4
nanoparticles
via thermal decomposition method.[26,27] Compared with these reference works, the
refluxing temperature in the current study is much higher (350 ℃ versus 280 ℃ or
290 ℃). As revealed by Dieste et al.,[28] the higher the temperature, the less C=O
double bonds of oleic acid could be detected due to the decomposition of carboxyl
group. When the temperature reaches 430 ℃ or above, no C=O bonds could be
detected because of the complete decomposition of carboxyl group. This also means
stronger reducibility of oleic acid but weaker stability as capping agent at higher
temperatures, because more CO, H
2
and C will be produced as a result of the
decomposition of carboxyl group.[29] Our first concern is the phase conversion from
-Fe
2
O
3
to Fe
3
O
4
. Hence, we set the refluxing temperature of the reduction process at
a relative high value, i.e. 350 ℃. At this temperature, the oleic acid still performs
multifunction but with a relative strong reducibility. Temperature higher than 350 ℃
will damage the magnetic stirrer severely.
Fig. 7.6 The M-H loops of as-reduced

samples B, C and D.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
126

As such, when we increased the amount of oleic acid to 3.5 g (sample D), the
morphology of as-reduced particles was further improved, as shown by the SEM
image Fig. 7.5d. This indicates that the increase of oleic acid results in a stronger
stability. But, a further increase of oleic acid (5 g - corresponding to a molar ratio of
29:1) is not helpful to the reduction because the TOA solvent involving -Fe
2
O
3
particles would become transparent and show a red color due to the presence of Fe
3+

ions after high temperature reaction, as displayed by the photo labeled with letter ‘T’
in Fig. 7.5e. This indicated that all the Fe
3+
ions were released from particles instead
of being reduced to Fe
2+
ions due to excessive oleic acid. No Fe
3
O
4
particles could be
obtained. The reason is unknown so far. What we know is that the molar ratio of oleic
acid to -Fe
2

O
3
particles should be well controlled to make sure the complete
reduction from hematite to magnetite as well as the unchanged morphology of
particles. For 74 nm -Fe
2
O
3
rings, a molar ratio around 20:1 (oleic acid to -Fe
2
O
3
particles) was found to be effective to accomplish the reduction process. Nevertheless,
there were still some defects on the particles surface, which could be seen directly
from the SEM image in Fig. 7.5d.
7.2.2.2 Effect of 5%H
2
/95%Ar protection gas on the reduction process
In order to improve the quality of Fe
3
O
4
nanoparticles, a gas mixture including 5% H
2

and 95% Ar instead of pure Ar was adopted as protection atmosphere to facilitate the
reduction process. The gas flow rate was controlled by a gas mass flow meter. Sample
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
127

E, F and G were obtained under the protection of 5%H
2
/95%Ar gas, as listed in Table
7.1. By comparing the SEM images of sample E (Fig. 7.7a) and D (Fig. 7.5d), we
could find that the usage of 5%H
2
/95%Ar gas made a great improvement on the
surface morphology of as-reduced particles.

Such as samples F and G, were prepared with using only H
2
as the reducing agent in
the experiments. Without the presence of oleic acid, the as-reduced particles of
samples F and G possess perfect ring-structure as well as smooth surface, as shown by
the SEM images in Fig. 7.7. When the gas flow rate of H
2
gas was 80 sccm (sample
F), the reduction effect from H
2
gas was so weak that a very strong hematite peak (1 0
4) was observed in the XRD pattern (Fig. 7.7d). The brown color shown by the
as-reduced precipitate also demonstrated the large proportion hematite phase in
Sample F, as seen from the photo in Fig. 7.7b (inset). With the gas flow rate
Fig. 7.7 SEM images of as-reduced samples: (a) sample E; (b) sample F; (c) sample
G. The scale bars on these images stand for 200 nm. Photo images of sample B and
sample C dispersed in TOA solution are also displayed by insets. (d) The XRD
patterns for different samples.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route

128

increasing to 120 sccm (sample G), the precipitates became black and the hematite
peak in XRD pattern got less strong, indicating more magnetite phase transformed
after reduction process compared with sample F. The magnetic properties could also
illustrate the effect of H
2
on the transformation from hematite to magnetite phase. The


values read from the hysteresis loops (Fig. 7.8) were 12.9 emu/g and 48.2 emu/g,
corresponding to
sample F and G. The increment of magnetization could prove that Sample G had less
hematite phase, resulting from the higher gas flow of 5%H
2
/95%Ar gas employed for
reduction process. Even the gas flow rate was adjusted to 120 sccm, the phase
transformation was still uncompleted. This phenomenon further indicates that oleic
acid is necessary for the reduction process in the current method, while the H
2
gas
facilitates the phase transformation from hematite to magnetite and benefits to the
maintenance of morphology.
7.2.2.3 Characterizations on the as-reduced Fe
3
O
4
nanoparticles
Based on above analysis, we have learnt that both the ratio of oleic acid to -Fe
2

O
3
Fig. 7.8 The M-H loops of as-reduced
samples E, F and G.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
129

particles and the gas flow of H
2
are key factors for a successful reduction work. A
usage of 3.5 g oleic acid combined with a gas flow around 80 sccm of H
2
is sufficient
to accomplish the reduction of 100 mg of 74 nm -Fe
2
O
3
rings. This optimized
condition was further applied to other kinds of as-synthesized -Fe
2
O
3
particles, such
as 164 nm rings, 70 nm tubes as well as the rods with three different sizes. The SEM
images of as-obtained Fe
3
O
4
particles were shown in Fig. 7.9 and Fig. 7.10.

Comparing with unreduced -Fe
2
O
3
particles, the perfect ring-, tube- and
rod-structure were maintained after reduction process. Before the reduction process,
the phase of as-prepared -Fe
2
O
3
particles was confirmed as the rhombohedral
-Fe
2
O
3
(JCPDS 88-2359) and no impurities were observed, as shown by the X-ray
diffraction patterns in Fig. 7.11a. After reduction, the XRD result showed pure Fe
3
O
4
phase, which match well with the cubic structure magnetite (JCPDS 82-1533). The
Fig. 7.9 SEM images of as-reduced samples: (a) 154 nm rings; (b) 70 nm tubes; (c)
98 nm rods. The scale bars on these images stand for 500 nm. All the scale bars
stand for 500 nm. (d) The M-H loops for as reduced samples.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
130

apparent difference between the XRD patterns of the as-synthesized -Fe
2

O
3
particles
and the as-reduced Fe
3
O
4
particles clearly shows the structure conversion from
corundum to spinel. To further confirm that the magnetite phase rather than
maghemite phase was obtained after the reduction process, XPS spectrum was
recorded and shown in Fig. 7.11b. Both of the two curves show two main peaks at the
binding energies of 711 eV and 724 eV, corresponding to Fe2p1/2 and Fe2p3/2 peaks.
However, a feature on the Fe2p spectra line shapes for -Fe
2
O
3
is the small satellite
peak,[30] which is used to differentiate Fe
2
O
3
and Fe
3
O
4
. Obviously, in our Fe
3
O
4

samples, no satellite peak was detected. This further proves that the phase conversion
from hematite to magnetic was complete.

Fig. 7.10c shows HRTEM images and SAED patterns of as-reduced Fe
3
O
4
rods. A
Fig. 7.10 SEM images of as reduced sample: (a) 61 nm Fe
3
O
4
rods; (b) 55 nm Fe
3
O
4
rods. The scale bars on these images stand for 200 nm. (c) The HRTEM image for
as-synthesized Fe
3
O
4
rod and the corresponding SAED pattern (inset). (d) The
M-H loops of as-reduced Fe
3
O
4
rods with different sizes.

Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route

131

perfect single crystal structure was clearly observed and the measured d spacing from
HRTEM image was around 0.47 nm, which is close to the standard d spacing of {111}
at 0.48 nm for the magnetite. The <111> crystallographic direction is along the
longitudinal direction of the magnetite rods. The results show a preserved single
crystal structure after phase transformation from -Fe
2
O
3
to Fe
3
O
4
. Meanwhile, the
crystal orientations change from [0 0 1]
hematite
 [1 1 1]
magnetite
and [1 1 0]
hematite
 [3
1 1]
magnetite
, as indexed on SAED patterns.

The particles are successfully reduced from -Fe
2
O
3

to Fe
3
O
4
phase, which is further
proved by the magnetic hysteresis loops in Fig. 7.9d. All of as-reduced Fe
3
O
4
particles
show ferromagnetic property. The saturation magnetization (

) value of 154
nm-rings is 89.3 emu/g, comparable with bulk Fe
3
O
4
(85-90 emu/g).[31,32] For Fe
3
O
4
rods, the magnetization seems to be size/volume dependent. With their outer
diameters decreasing from 98 nm to 55 nm, the 

values reduced from 89 emu/g to
71 emu/g (Fig. 7.10d). This trend allows us to synthesize different size of rod
structure Fe
3
O
4

particles according to the required magnetic property.
Fig. 7.11 (a) The X-ray patterns and (b) XPS spectra of -Fe
2
O
3
and Fe
3
O
4
samples.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
132

7.2.3 Microwave characterizations on as-reduced Fe
3
O
4
particles
As mentioned above, Fe
3
O
4
nanoparticles with various structures, including rings,
tubes and rods are synthesized by employing chemical reduction method. The
microwave absorption performance of these magnetic nanostructures is further
investigated. The Fe
3
O
4

particles (74 nm rings, 160 nm rings, 70 nm tubes and 98 nm
rods) are dispersed into paraffin wax at a volume concentration of 20% for the
electromagnetic parameters (

 

 

; 

 

 

)measurements. Based on
the measured parameters, the reflection loss curves are further obtained through
calculation work using Eq. (2.5) and Eq. (2.6). The acquired electromagnetic spectra
(the left side of each figure) and reflection loss curves are showed in Fig. 7.12 to Fig.
7.15 for different samples. When compared with as-synthesized Fe
3
O
4
nanocrystals
with octahedron-structures, the enhancement in the resonance frequency has been
brought by as-reduced Fe
3
O
4
particles with different shapes. Some characterized
parameters are summarized in Table 7.2.

As we can see from Table 7.2, the resonance frequency is enhanced from 1.57 GHz
for octahedral Fe
3
O
4
to 3.53 GHz for 74 nm rings and to 4.01 GHz for 154 nm rings.
The smaller size of 74 nm rings results in a relative lower saturation magnetization


than 154 nm rings. As we know from the Snoek’s law, for the materials with
similar structure, the saturation magnetization 

is decisive to the product of
frequency multiplies permeability. The higher 

of 154 nm rings may account for
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
133


Fig. 7.15 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the
calculated frequency dependent reflection loss plots for 98 nm-Fe
3
O
4
rods.
Fig. 7.13 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the
calculated frequency dependent reflection loss plots for 154 nm-Fe
3

O
4
rings.
Fig. 7.14 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the
calculated frequency dependent reflection loss plots for 70 nm-Fe
3
O
4
tubes.
Fig. 7.12 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the
calculated frequency dependent reflection loss plots for 74 nm-Fe
3
O
4
rings.
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
134

its larger permeability at a higher resonance frequency relative to that of 74 nm rings,
resulting in a more effective microwave absorption. The reflection loss of 154 nm
rings can reach -28 dB at the relative small thickness  = 4.2 mm. There is no
optimal thickness observed in the reflection loss curves of 74 nm rings, which is
similar to that of Fe
3
O
4
nanocrystals (Fig. 5.15b in Chapter 5). Their resonance
frequency shifts to lower band with lower the reflection loss when the thickness
increases.

The resonance peaks of 70 nm Fe
3
O
4
tubes and 98 nm Fe
3
O
4
rods shift to even higher
frequency, i.e. 4.46 GHz and 4.82 GHz, respectively. We may speculate that the shape
anisotropy contributes to the resonance frequency somehow. For 70 nm Fe
3
O
4
tubes,
the optimal thickness is at 4.2 mm corresponding to a reflection loss of -20 dB. There
Table 7.2 Characterized parameters for various magnetic structures summarized
from the measured electromagnetic spectra and the calculated reflection loss curves.
Samples



(emu/g)



(GHz)



Optimal thickness
 (mm)
RL
(dB)
114nm octahedron_Fe
3
O
4

90
1.57
0.8
6.0
-17
104nm octahedron_ZnFe
2
O
4

104
3.45
1.4
4.2
-38
74nm ring_Fe
3
O
4

69

3.53
0.35
6.0
-12
154nm ring_Fe
3
O
4

81
4.01
0.53
4.2
-28
70nm tube_Fe
3
O
4

73
4.46
0.35
4.2
-20
98nm rod_Fe
3
O
4

85

4.82
0.34
3.6
-15
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
135

is also no optimal thickness observed in the reflection loss for 98 nm Fe
3
O
4
rods.
Unlike the trend shown by Fe
3
O
4
nanooctahedron and 74 nm rings, the resonance
peak of 98 nm rods shifts to higher frequency band along with lower reflection loss
when the thickness decreases. This means that the resonance peak could shift to
higher frequency band along with lower reflection loss values if the thickness could
be further decreased.
Although the mechanism on the structure effect to electromagnetic performance is
still unclear, we could find that the magnetic structures do impact the microwave
performance. Based on our work, the 98 nm Fe
3
O
4
rods display the highest resonance
frequency at 4.82 GHz; while Zn-ferrite octahedra display the lowest reflection loss of

-38 dB. Hence we could conclude that the anisotropic structure is promising to
enhance the resonance frequency to higher band and the high saturation magnetization
is necessary for an effective microwave absorber.
7.3 Summary
In this chapter, we synthesized -Fe
2
O
3
nanoparticles of different shapes and sizes via
hydrothermal method. The dependence of the shape and size of as-synthesized
-Fe
2
O
3
on the reactant concentration was studied. -Fe
2
O
3
rods with controllable
sizes were developed. The length of as-produced rods was found to increase with the
reactant concentration. In a subsequent step, Fe
3
O
4
nanoparticles of various shapes
were obtained after a phase conversion from as-prepared -Fe
2
O
3
by a chemical

Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
136

reduction process. The phase transformation from hematite to magnetite mostly relies
on the reducing agent, such as oleic acid and 5%H
2
-95%Ar gas. As investigated in
this work, oleic acid is effective to reduce the Fe
3+
to Fe
2+
even at the inner part of
particles. But this usually leads to broken particles after the reduction process. Instead,
H
2
gas is conducive to maintain the morphology of as-reduced particles but its
reduction effect is not strong enough to realize the complete reduction of -Fe
2
O
3
particles. Based on the investigation, an optimal molar ratio of 20:1 (oleic acid to
-Fe
2
O
3
particles) was chosen in this work, as well as 5%H
2
plus 95% Argon gas was
used to facilitate the reduction process. The results indicate that pure hematite phase

could be obtained after reduction process, and the morphology of unreduced hematite
particles, including rings, tubes as well as rods, is well kept. In other words, the
chemical reduction method makes it possible to obtain Fe
3
O
4
particles with various
structures as long as the template of -Fe
2
O
3
particles with well-shaped surface are
provided. Furthermore, this method is favorable to large-scale synthesis of Fe
3
O
4

particles, which is required in many applications. Through the comparison of the
microwave performance of as-prepared Fe
3
O
4
with various structures (114 nm
octahedra, 74 nm rings, 154 nm rings, 79 nm tubes and 98 nm rods), we have found
that the anisotropic structures could contribute to the enhancement of resonance
frequency of materials. The 98 nm Fe
3
O
4
rods show a highest resonance peak at 4.82

GHz. The balance between the resonance frequency and the permeability of a
Chapter 7 Synthesis and microwave absorption of Fe3O4 particles
with various structures by chemical reduction route
137

magnetic material is important to its microwave absorption property. As such, the 154
nm Fe
3
O
4
rings show relative low reflection loss of -28 dB corresponding to an
optimal thickness of 4.2 mm. This point could be further proved by the 104 nm
Zn-ferrite particles, which show the lowest reflection loss of -38 dB.
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