DSpace at VNU: Controlled synthesis and characterization of iron oxide nanostructures with potential applications for gas sensors and the environment

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Controlled synthesis and characterization of iron
oxide nanostructures with potential applications
for gas sensors and the environment†
Nguyen Viet Long,*abcdef Yong Yang,*a Masayoshi Yuasa,e Cao Minh Thi,f Yanqin Cao,a
Thomas Nanng and Masayuki Nogamiah
In the present research, large iron oxide microparticles with large sizes in the range of 1–5 mm have been
facilely synthesized by a modified polyol method with NaBH4 as a versatile strong reducing agent. We
found that the highly homogeneous iron oxide microparticles' novel structure is the best pure crystal phase
of a-Fe2O3 in terms of polyhedral morphology and shape in existence. There are no diffraction peaks of

Received 18th October 2013
Accepted 14th November 2013

other crystal phases from impurities in a-Fe2O3 microparticle products in the crystal growth. Interestingly, a
new method of heat treatment or atomic surface deformation allowed for the discovery of a new large
a-Fe2O3 structure with controlled specific a-Fe2O3 oxide grains in the crystal structure. The severe surface

DOI: 10.1039/c3ra45925j

deformation of sharp, polyhedral, large a-Fe2O3 microparticles under a sintering treatment was found to

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give un-sharp, polyhedral large a-Fe2O3 microparticles with specific grains and boundaries.

1. Introduction
Typically, magnetic nanoparticles (MNs) such as nickel (Ni) and
cobalt (Co) are used in catalysis, biology and medicine. Iron (Fe)
based nanoparticles, and magnetic Fe based nanostructures
offer increasingly excellent performance in the aforementioned
applications due to their great magnetic properties,1–4 such as
their previously undiscovered ferromagnetic, antiferromagnetic, and ferrimagnetic magnetism. So far, the crystal
a

State Key Laboratory of High Performance Ceramics and Superne Microstructure,
Shanghai Institute of Ceramics, Chinese Academy of Science, 1295, Dingxi Road,
Shanghai 200050, China. E-mail: ; Fax: +86-2152414219; Tel: +86-21-52414321

b

Posts and Telecommunications Institute of Technology, km 10 Nguyen Trai, Hanoi,
Vietnam. Tel: +84 (0)946293304

c

Laboratory for Nanotechnology, Ho Chi Minh Vietnam National University,
LinhTrung, Thu Duc, Ho Chi Minh, Vietnam
d

Department of Molecular and Material Sciences, Interdisciplinary Graduate School of

Engineering Sciences, Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka, 8618580, Japan

e

Department of Materials Science, Faculty of Engineering Sciences, Kyushu University,
Kasuga-koen 6-1, Kasuga-shi, Fukuoka, 816-8580, Japan

f

Ho Chi Minh City University of Technology, 144/24 Dien Bien Phu, Ward 25,
BinhThach, Ho Chi Minh City, Vietnam

g
Ian Wark Research Institute, ARC Special Research Centre, University of South
Australia, Australia
h

Department of Materials Science and Engineering, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan. Tel: +81 (0)90-9930-9504

† Electronic supplementary information (ESI) available: SEM images of the very
large a-Fe2O3 microparticles produced with a modied polyol method with
NaBH4 at 200–300 C for 30 min. SEM images of the very large a-Fe2O3
microparticles with surface deformations and the grains under the same
conditions and microparticle heat treatment. See DOI: 10.1039/c3ra45925j

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nanostructures of various Fe oxides and magnetic Fe oxide
nanoparticles have the same specic crystal nanostructures as

magnetite (Fe3O4) and maghemite nanoparticles (g-Fe2O3) and
hematite (a-Fe2O3).5–10 Recently, scientists and researchers have
discovered important practical applications of a-Fe2O3 nanoparticles and nano-structures in lithium ion batteries, energy
storage and materials for various gas sensors. In recent work,
the addition of metal nanoparticles has led to better sensitivity
and better selectivity in various oxide sensor devices.11–15,46 In
addition, a reduced graphene oxide platelet/Fe2O3 nanoparticle
composite can be used in the anode for Li-ion batteries with
high-performance as well as high durability and stability.16 In
most cases, the characteristics such as size, shape, morphology,
and particle composition of the Fe based nanoparticles need to
be controlled with the addition of various metals: Ni, Co, Zn,
Cu, etc. For example, the special nanostructures of modied
MFe2O4 ferrite nanoparticles (M ¼ Co, Ni, Zn), or magnetic
multimetal oxide nanoparticles, can be potentially used in
magnetic resonance imaging (MRI) technology. During recent
years, super-paramagnetic iron oxide nanoparticles (SPIONs),
such as superparamagnetic magnetite Fe3O4 nanoparticles and
maghemite g-Fe2O3 nanoparticles under size and morphology
control, have been used in drug delivery vehicles.17–20 At present,
magnetic iron metal and iron oxide nanoparticles also have
important applications in experimental catalysts, high contrast
agents for magnetic resonance imaging (MRI), and therapeutic
agents for the treatments of dangerous tumors and cancers.17–24
Beside the interesting magnetic properties of iron alloy and iron
oxide based nanostructures and nanomaterials, some of the
most important characteristics are that iron alloy and iron oxide
based nanostructures and nanomaterials have ultra-high

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durability and stability. Magnetite Fe3O4 nanoparticles can be
used for magnetic hyperthermia, high contrast agents and MRI
technology, and targeted drug delivery vehicles.17–24 Due to their
high biocompatibility and relatively low toxicity in animals and
humans, maghemite g-Fe2O3 nanoparticles can also offer great
applications in biomedicine. So far, various synthesis and
preparation methods of MNs have been utilized to control the
size characteristics, the surface shape and morphology characteristics, and the internal characteristics (large and small crystal
structures, large and small crystal surfaces, as well as low and
high porosity etc.).21–24
In this research, we present a novel synthesis process to
control the size, shape and morphology of large polyhedral Fe
oxide microparticles with a-Fe2O3 structure in the range of 1–
5 mm. Herein, we have successfully used a modied polyol
method with the addition of an extra amount of NaBH4 in
ethylene glycol (EG).

2.

Experimental section

2.1. Chemical

For the chemical synthesis processes to make the pure a-Fe2O3
oxide nanoparticles, we used chemicals from Aldrich, SigmaAldrich and Wako. These include poly(vinylpyrrolidone) (PVP)
(FW ¼ 55 000) as a good protective agent (Aldrich) and
FeCl3$4H2O (Aldrich, no. 451649 and 236489). In particular,
sodium borohydride (NaBH4) was used as a strong reducing
agent for the synthesis of a-Fe2O3 microparticles, ethylene
glycol (EG) from Aldrich was used as both a solvent and a weak
reducing agent, and ethanol, acetone, and hexane were
procured from Aldrich or Japanese companies. Here, all
chemicals were of standard analytical grade and were used
without any further purication. Deionized and distilled water
with high purity prepared by a Milli-Pore purication system
available in our laboratory was used for the washing and
cleaning of containers during experimental synthesis processes.
2.2. Synthesis of a-Fe2O3 oxide microparticles
Briey, 3 mL of EG, 1.5 mL of 0.0625 M FeCl3, 3 mL of 0.375 M
PVP, and 0.028 g NaBH4 were used for making Sample 1 in a
typical process of the controlled synthesis of the large polyhedral a-Fe2O3 oxide microparticles. The details and steps of
the known process procedures were previously presented.19,25 In
general, FeCl3 was completely reduced with the extra amount of
NaBH4 in EG at 200–230  C for 30 min. As a result, black solutions containing polyhedral a-Fe2O3 oxide microparticles with
large sizes, shapes and morphologies were obtained as the nal
product. They have a particle size of 1–5 mm with a polyhedral
shape and morphology. Similarly to Sample 1, we used the same
processes for Sample 2 and Sample 3 for XRD and SEM
measurements. Sample 1 was also used for XRD and SEM
measurement and analysis. Sample 2 was heated at 500  C for
1 h for SEM measurement and analysis. Sample 3 was heated at
900  C for 1 h for SEM analysis. All of the experimental conditions for making Samples 1–3 corresponding to the XRD results
are presented in Table 1 and section 3 (Results and discussion).

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2.3. Material characterization
2.3.1. X-ray diffraction method. In the XRD method for
crystal analysis, we used the as-prepared products of the black
solution containing the PVP-aFe2O3 oxide microparticles
(Sample 1). The PVP-aFe2O3 oxide nanoparticles were washed
many times in order to obtain clean a-Fe2O3 oxide nanoparticles
by our standard procedures with the use of a centrifuge. The
black solution of Fe based microparticles was dried in order to
leave an Fe based nano-powder on the glass substrate for XRD
analysis. The high heat treatment of our samples was carried
out in a gas/air ow (20 mL minÀ1) or a mixture of 10 mL minÀ1
for O2, and 10 mL minÀ1 for air at 500  C and 900  C for 1 h in
ovens. The X-ray diffraction patterns were recorded by an X-ray
diffractometer (Rigaku D/Max 2550V) at 40 kV/200 mA using Cu
˚ Finally, only the crystal phase of
Ka radiation (1.54056 A).
a-Fe2O3 was found in the pure as-prepared a-Fe2O3
microparticles.
2.3.2. Scanning electron microscopy. In order to study the
size and shape of the as-prepared a-Fe2O3 microparticles
(Samples 1–3), we used a eld emission scanning electron
microscope (SEM) (JEOL-JSM-634OF) operated at 5, 10, and
15 kV (5–15 kV), with a probe current around 12 mA. The SEM
images of the as-prepared Fe microparticles were focused by
using a suitable ne focus level adjustment. To characterize the

a-Fe2O3 oxide microparticles with very large sizes of 1–5 mm,
copper or copper brass grids containing the a-Fe2O3 microparticles were maintained under vacuum by using a vacuum
cabinet.

3.

Results and discussion

Fig. 1 and 2 show the SEM images of the as-prepared large
a-Fe2O3 oxide microparticles with polyhedral morphologies and
shapes of a certain size of about 1–5 mm.19,20 It should be noted
that the as-prepared nanoparticles were observed to have polyhedral morphologies, such as cubes, octahedra, and tetrahedra
etc. This is possibly because sodium borohydride (NaBH4) is a
very strong reducing agent, leading to the fast crystal growth of
large polyhedral Fe2O3 microparticles. Here, the polyhedral
a-Fe2O3 microparticles have three large crystal surfaces with
crystal planes of (100), (011), and (111). They have large
homogeneous sizes and sharp, smooth, polyhedral surfaces. In
particular, our discovery of a new nano-structure is conrmed
from our heat treatment process of the as-prepared Fe oxide
microparticles at 900  C (Fig. 3(b) and (c), S1 and S2 (ESI†)) but
no clear and signicant structural changes at the surfaces of the
large a-Fe2O3 microparticles at 500  C (Fig. 3(a)).
Very interestingly, most large polyhedral a-Fe2O3 oxide
microparticles contained smaller a-Fe2O3 nanoparticles in their
very large nano-textures. All the large a-Fe2O3 crystal surfaces
were clearly deformed in the nanoparticle heat treatment at
900  C. C1 in Fig. 4(c) shows the new micro-nano structure of
one as-prepared large polyhedral Fe based microparticle aer
heat treatment at about 900  C. Although the particle size of this

large microparticle was not signicantly changed, all the
large crystal surfaces were signicantly changed into the new

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Table 1

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Experimental conditions for the preparation of the samples, and their a-Fe2O3 crystal structure

Sample

Chemicals and precursor solution

Experimental

Heat treatment

Crystal structure by XRD

Sample 1

 3 mL of EG
 1.5 mL of 0.0625 M FeCl3
 3 mL of 0.375 M PVP
 0.028 g NaBH4

 3 mL of EG
 1.5 mL of 0.0625 M FeCl3
 3 mL of 0.375 M PVP
 0.028 g NaBH4

 200–230  C for 30 min

 Drying in air or mixture of
oxygen–air

a-Fe2O3 (PDF-89-0597)

 Heated at 500  C in air or
mixture of oxygen–air

a-Fe2O3 (PDF-89-0597)

 Heated at 900  C in air or
mixture of oxygen–air

a-Fe2O3 (PDF-89-0597)

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Sample 2

Sample 3

 3 mL of EG
 1.5 mL of 0.0625 M FeCl3

 3 mL of 0.375 M PVP
 0.028 g NaBH4

 Pumping method of
stock solutions
 200–230  C for 30 min
 Pumping method of stock
solutions
 200–230  C for 30 min
 Pumping method of stock
solutions

SEM image of the uniform Fe oxide nanoparticles and large (a)Fe2O3 microparticles synthesized by a modified polyol method
(Sample 1).
Fig. 1

micro-nano surface structures. Fig. 4(d) and (e) present two
models for possibility of very slow crack propagation along the
grain boundaries for intergranular fractures (red and blue
lines). 35 Æ 1 grains or 35 Æ 1 small or intermediate nanoparticles were observed in the large crystal surfaces or the
crystal planes. The various sizes of the nanoparticles were about
100–300 nm for the small a-Fe2O3 nanoparticles, and about
300–500 nm for the intermediate a-Fe2O3 nanoparticles,
compared to the very large a-Fe2O3 microparticle of about 3 mm.
The arrangements in order show that there are six nanoparticles
or grains (6 Æ 1) in one row, and six nanoparticles or grains (6 Æ
1) in one column. Therefore, there are 35 Æ 2 nanoparticles as a
raw estimation. Thus, we estimate there are about 266 to 280
small nanoparticles (or grains) in the large microparticle. The
oxide grains are strongly connected and linked in one large

particle as a three dimensional (3D) microparticle. In addition,
all the Fe2O3 grains clearly exhibited curvature on their surfaces.
Moreover, each a-Fe2O3 grain is classied as a single a-Fe2O3
crystal by the XRD method. However, the boundaries between
the grains are also clearly distinguished.

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Fig. 2 (a) SEM image of the large uniform and polyhedral Fe oxide
nanoparticles and (a)-Fe2O3 microparticles synthesized by a modified
polyol method (Sample 1). (b) SEM image of an orthorhombic crystal,
and (c) its model.

The large oxide grains have two categories in normal and
abnormal grain-growth regimes.26–32 Certain grains in our new
micro-nano structures have both small and large sizes with
respect to the abnormal grain-growth regimes. They show unsharp curvature boundaries (our results) or sharp boundaries
(our proposed model). Thus, there are important mechanisms
and processes of recovery, re-crystallization, and development
of grain growth in every large a-Fe2O3 crystal. Surprisingly, the
new 3D structures can be considered to be excellent evidence of
3D grain growth without particle collapse or cracking. This is of

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Fig. 3 (a) SEM images of a-Fe2O3 nanoparticles with a sharp polyhedral shape and morphology in a sensor device calcined at 500  C
with no changes in the a-Fe2O3 nanostructures (Sample 2). (b) and (c)
SEM images of a-Fe2O3 nanoparticles with a sharp polyhedral shape
and morphology in a sensor device calcined at 900  C (Sample 3).

high importance, it is considered to be the “ideal” grain growth
in materials optimization,29–31 as well as the recrystallization
and annealing phenomena of the nanoparticles being of both
technological importance and scientic interest at present.32 At
present, 3D metal and oxide structures have been simulated
under different grain growth regimes, but experimental
evidence has not been shown in a micro- or nano-system. Thus,
our results are an interesting discovery regarding the structure
and synthesis design of new micro-nano sized structures. Here,
each large a-Fe2O3 microparticle became an oxide nanograin
system. Clear and complete models of micro-nano surfaces and
boundaries of nano-structures were previously proposed for
heat treatments at high temperature.26–28,30 The important
properties of nano-materials and nano-structures involved in
the grain and boundary structure were predicted by modeling
and simulation.29
At present, the creation of homogeneous and ne oxide
grains in very large oxide microparticles is a big challenge for
scientists. From our proposed models in Fig. S2(e) and (f) (ESI†)
and Fig. 4, the oxide grains have created large crystal surfaces
with various different degrees of concave or convex curvature
and roughness. In fact, the oxide grains can be split in two
categories, the coarse-grain forms and the ne-grain forms.

These are very crucial to predict the properties of engineered
nanostructures in both theory and practice.26–35 Here, we have
appropriately selected two temperature points for our method

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Plastic and surface deformation: (a) the various grains and
boundaries in one two dimensional (2D) large crystal surface of the
grains with round boundaries of large a-Fe2O3 nanoparticles, as in SEM
image C1; (b) the configuration of the boundaries in the 2D system
observed in SEM image C3; (c)–(e) proposed models for two cases: (c)
no heat treatment; (d) and (e) with heat treatment. (d) Large 3D particle
made up of grains with un-sharp curvature boundaries (our results); (e)
or with sharp boundaries (our proposal) as estimated in C3.
Fig. 4

of nanoparticle heat treatment to produce nanoparticles with an
a-Fe2O3 structure the same as the well-known a-FeC equilibrium
diagram from 500 to 910  C.33,34 This is an interesting nding for
making a-Fe2O3 microparticles with deformed surface states. For
a given method of nanoparticle heat treatment, it is possibly true
that the various pure structures of metal, alloy, and oxide nanoparticles are the same as their equilibrium diagrams in metallurgy. Aer annealing, there is the appearance of small and large
a-Fe2O3 grains in the large crystal surfaces because of renucleation and recrystallization processes. The severe deformation of
the large, at, and smooth crystal surfaces into concave, convex,
rough, and distorted crystal surfaces of a-Fe2O3 microparticles is
very crucial to achieve new nano-textures (Fig. 3(b) and (c), S1 and
S2 (ESI†), and Fig. 4 with models). These Fe2O3 structures may be
very stiff and permanent aer nanoparticle heat treatment in the

500–900  C range investigated. We suggest that there was plastic
deformation in the a-Fe2O3 crystal surfaces of every Fe2O3
microparticle, but also elastic deformation in the a-Fe2O3
microparticles with no signicant changes in the particle size
distribution. Thus, an appropriate annealing process can control
the grain sizes and boundaries among the interfaces of the
grains.

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The phenomenon of grain growth has been explained even
for metals, alloys and oxides.26–33 The plastic and surface
deformation mechanisms and stress–strain behaviors have
generally led to various methods to strengthen and regulate the
mechanical characteristics of steel (FeC) or ferrite, which are
important in metallurgy.29,33 These allow new methods of
nanoparticle heat treatment for engineered nanoparticles. So
far, most annealing and heat treatments have generally led to
new structures of metals, alloys, glasses, ceramics and oxides
with interesting new (and some already known) discoveries
such as higher strength, better toughness, higher durability and
stability through small grains, creep resistance and reduction
through large grains.32 These are due to the special characteristics of both small grain and large grain systems. Plastic and

super-plastic deformations were characterized in FeC, Fe
oxides, and Fe alloys during steel heat treatment.26,33 However,
we also suggest that the curvature boundaries between the
grains were possibly due to both plastic and elastic deformation, permanent plastic deformation of the external surface and
internal structure, and elastic deformation where the particle
size was retained in the elastic recovery of particle shape.
Furthermore, a-Fe2O3 microparticles with sharp and straight
edges (Fig. 1, 2, and 3a) were observed in both plastic and elastic
deformation processes, but higher localized plastic deformation on all surfaces was also observed (Fig. 3(b) and (c) and
models (a), (b), (d), and (e) in Fig. 4). Our interesting evidence
regarding plastic and elastic atomic surface deformation and
grain growth is of importance in simulation and modeling at
present.
Although the grain boundaries of the a-Fe2O3 grains are
clearly distinguishable on the surface of one large microparticle, it is not difficult to observe possible cracking and propagation cracking in the prepared microparticles. The good
recovery characteristics of the large shape and morphology of aFe2O3 microparticles was shown aer strong surface deformation and plastic deformation in the formed a-Fe2O3 grains.
Thus, we can expect that the boundaries of the ne grains in
one such large microparticle can be controlled by the nanoparticle sintering process. This also illustrates the well ordered
arrangement of the small and intermediate a-Fe2O3 oxide
nanoparticles with specic ne grain boundaries, and also
within the oxide grains on the surfaces annealed at 900  C for
1 h that were evidenced by one as-prepared large a-Fe2O3 oxide
microparticle (or large a-Fe2O3 oxide crystal). The boundaries of
a-Fe2O3 oxide grains and a-Fe2O3 oxide domains were also
observed on the large crystal surfaces and we predict their
existence inside the internal structure of the pure large polyhedral a-Fe2O3 oxide microparticles. There are two small holes
on the crystal surface, which are observed because there is a
certain degree of the porosity in the large microparticle.
Fig. S2(b) (ESI†) shows the micro-nano structure of the asprepared large a-Fe2O3 oxide microparticles of about 3 mm with
an orthorhombic shape and morphology. The crystal a-Fe2O3

oxide grains and grain boundaries of nite sizes can be clearly
distinguished. The small crystal grains are normally located at
the corners of the intermediate a-Fe2O3 oxide grains. In the asprepared large a-Fe2O3 oxide microparticles, their shapes and

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morphologies have at and smooth crystal surfaces. Aer an
appropriate heat treatment, the Fe based grains (or Fe based
nanocrystals) appeared on the surfaces. Each large crystal
surface will become more coarse because of the concave and
convex local regions of a-Fe2O3 oxide grains that are caused. So
far, our samples are considered to be the best examples of very
large a-Fe2O3 microparticles with oxide grains, oxide grain
domains, and sharp and un-sharp boundaries for future studies
in this eld.
In addition, Fig. S2(c) and (d) (ESI†) show interesting a-Fe2O3
oxide grains in the forms of micro and nano oxide crystals.
Crystallization and re-crystallization transformations were
observed in the deformation of sharp, at, and smooth large
oxide crystals into un-sharp, distorted, rough, convex and
concave large oxide crystals with specic oxide grains. Our latest
results regarding the pure a-Fe2O3 oxide microstructures and
nanostructures with respect to the structural phase transitions
and mechanisms are the most important examples at present.
Thus, the high roughness of the small and large a-Fe2O3 oxide
crystals was caused during heat treatment at 900  C. The aFe2O3 oxide grains also appeared at all of the six large crystal
surfaces of the as-prepared large polyhedral a-Fe2O3 oxide
microparticles. The most important thing is that there are no

collapses of the nano- and micro-structures of the large
as-prepared a-Fe2O3 oxide microparticles in the range between
500  C and 900  C. Therefore, the important issues of achieving
high stability and durability are possibly dealt with by using
higher heat treatments for microsystems and nanosystems.
Because the a-Fe2O3 nanostructures have very large sizes, we did
not characterize them by TEM measurements in our subsequent further investigation. As a facile method for the controlled
synthesis of Fe oxide based microparticles, we suggest that NaBH4
can be successfully used for the very strong reduction of Fe
precursors in various common solvents, such as EG, alcohols and
water. A moderate addition of control agents such as NaOH,
NH4OH, NaI, HCl etc. can be carried out during the controlled
synthesis. By this method, NaBH4 can usually be used in excess
amounts for the full reduction of metal precursors.17–24 Here, we
suggest that the fast formation of very small Fe metal nanoparticles during the synthetic process occurred at 200–230  C.19
We also suggest that, rst, Fe nanoparticles were formed by the
full reduction of Fe precursors such as FeCl3$xH2O (or
FeCl2$xH2O) with the addition of NaBH4. Then, the surfaces of the
Fe nanoparticles are oxidized in the initial formation of the Fe
oxide shells. According to the synthesis time, the formation of
metal oxide shell can be understood to be a gradual and slow
oxidation. In general, this can lead to the complete internal
structure of the prepared microparticles being completely oxidised. As crucial evidence for this, structural transformations
among FeO (Wustite), 3-Fe2O3, Fe3O4, g-Fe2O3, and a-Fe2O3 can be
carried out through suitable heat treatments.9,36 However, there
have been many considerable difficulties encountered when
scientists have tried to make large nano-textures from smaller
nanoparticles by self-assembly methods. It is clear that our asprepared products, the Fe based nanoparticles with the controlled
homogeneous features of size, shape, and morphology, can be
used in order to meet the very high demands of sensor materials.

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The nal formation of the pure a-Fe2O3 structure in EG was
done with the long-term stabilization of various PVP polymers by
a facile method with the use of NaBH4 as a strong reducing agent
for the Fe precursors. The successfully controlled synthesis of
magnetite nanoparticles has been shown in some recent works.
These works also tried to focus on the facile synthesis of the
crystal phases of MNs, such as Fe3O4, g-Fe2O3, and a-Fe2O3
nanoparticles,37 and on producing a-Fe2O3 nanoparticles for
promising applications in lithium ion batteries,38 as well as
nanostructured Fe oxide materials for much more durable and
stable advanced energy conversion and storage devices, such as
nano-sized transition-metal oxides as negative-electrode materials for high performance lithium-ion batteries.39,40 In addition,
we have evaluated the crystal structure of the as-prepared
samples of large Fe2O3 microparticles. Fig. 5 shows the typical
XRD patterns of one dried sample, and two samples aer calcination at 500 and 900  C. Here, the pure a-Fe2O3 oxide microparticles display a rhombohedral crystal structure. The prepared
large microparticles analysed by the XRD method had sharp and
narrow diffraction peaks, which is evidence of the high crystallization of the pure a-Fe2O3 crystal structure produced by the
modied polyol method with NaBH4. The a-Fe2O3 crystal structure (hematite system) belonging to the crystallographic space
C[167] has lattice constants (a,b,c) equal to 5.039 nm,
group R3

Fig. 5 XRD powder diffraction patterns of the samples of the Fe based
nanoparticles prepared at various temperatures: (a) dried sample
(Sample 1), (b) 500  C (Sample 2), and (b) 900  C (Sample 3). The
stability and durability of the a-Fe2O3 nanostructure during nanoparticle heat treatment at high temperatures were identified.

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5.039 nm, and 13.770 nm, respectively, with a ratio of c/a ¼ 2.733
(ICDD/JCPDS PDF-89-0597) using the JADE soware (Materials
Data) for XRD pattern processing and MDI materials data.
Table 2 lists the powder pattern indexing of Samples 1–3. For
Sample 1 (c/a ¼ 2.730815) and Sample 2 (c/a ¼ 2.733914), the c/a
ratios are the same as that of a standard sample (ICDD/JCPDS
PDF-89-0597). For Sample 3, the c/a ratio of our sample is equal to
2.726895 at 900  C, which is a little smaller than that of the
standard sample.
The narrow and sharp peaks from the XRD show the very
high crystallization of large polyhedral a-Fe2O3 microparticles
without any mixture of other phases, such as FeO (Wustite),
a-FeOOH, 3-Fe2O3, Fe3O4, g-Fe2O3 etc. As shown in Fig. 5, the
calcination of samples at 500 and 900  C also resulted in the
formation of a-Fe2O3 (PDF-89-0597) crystal phase structure.
Importantly, high crystallization of the pure rhombohedral
hematite a-Fe2O3 was obtained between 500  C and 900  C. As
shown in Fig. 2(a)–(c) and S1 and S2 (ESI†), the degree of
densication of the prepared oxide microparticles at 500  C is a
little smaller than that of the prepared oxide microparticles at

900  C. Thus, the as-prepared microparticles are single crystals
or monocrystallites because they show a sharp polyhedral shape
and morphologies that are continuous and unbroken to the
edges and corners of the microparticles in Sample 1, without
the existence of any grains or boundaries. All the microparticles
have sharp, smooth, and at surfaces, as well as sharp rightangled edges and corners. In the a-Fe2O3 microparticles aer
heat treatment at 500  C (Sample 2) and 900  C (Sample 3), the
structures were considerably changed. The microparticles were
changed in their deformation at 500  C but show no signicant
changes in their size and shape in Fig. 3(a). They gradually
became polycrystals under high heat treatment. At an annealing
temperature of about 900  C, the microparticles were changed
due to their very signicant deformations, with the appearance
of grains and boundaries between them. The grains have
particle sizes in both the microsize range and the nanosize
range. Additionally, the polycrystalline a-Fe2O3 microparticles
or a-Fe2O3 polycrystallites have various crystallites of varying
size and orientation. Each microparticle became a polycrystal or
a polycrystallite. Here, each microparticle has much smaller
a-Fe2O3 microparticles and a-Fe2O3 nanoparticles, or so-called
grains. However, each grain can be considered a single crystal or
single crystallite because of their continuous shape and
morphology.
At present, the a-Fe2O3 oxide based nanostructures have
special signicance for ecosystems and environmental applications,41 a-Fe2O3 tetradecahedra can be used in gas sensing by
a facile hydrothermal method with the use of K4Fe(CN)6$3H2O,
sodium carboxymethyl cellulose solution, PVP, and N2H4$3H2O
solution at room temperature at 200  C for 6 h,42 and other
a-Fe2O3 structures also have practical applications in gas
sensing.42–45 Interestingly, the important roles of the metal or

alloy or oxide grains were known in the signicant reduction of
lattice thermal conductivity for an enhanced ZT applied in new
thermal nano-structured materials.30 The topic of nanoparticle
heat treatment will be an important subject for scientists, and
specialists. In addition, various methods of nanoparticle heat

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Table 2

RSC Advances
The indexing of powder diffraction patterns of a-Fe2O3 oxide

Sample

a (nm)

b (nm)

c (nm)

c/a

Space group

ICDD/JCPDS-PDF

Sample 1
Sample 2
Sample 3

5.056
5.051
5.053

5.056
5.051
5.053

13.807
13.809
13.779

2.730815
2.733914
2.726895

C(161)
R3
C(161)
R3
C(161)
R3

ICDD/JCPDS PDF-89-0597
ICDD/JCPDS PDF-89-0597
ICDD/JCPDS PDF-89-0597

treatment need to be signicantly considered in the development of new nanomaterials with grain and boundary
structures.45

4. Conclusion
In this research, we have successfully prepared large a-Fe2O3
microparticles by a modied polyol method with NaBH4. A new
a-Fe2O3 microparticle with a-Fe2O3 grains was discovered by
chance following external surface deformation, and did not
show structural collapse aer nanoparticle heat treatment at
900  C. This new structure of a-Fe2O3 microparticles containing
micro-grains, nano-grains and boundaries would potentially
exhibit good properties in future gas sensors.

Acknowledgements
In this research, we are very grateful to the precious support
from the Structural Ceramics Engineering Center, Shanghai
Institute of Ceramics, Chinese Academy of Science, Dingxi Road
1295, Shanghai 200050, China. This study was also supported in
part by a fund from the National Natural Science Foundation of
China (NSFC, contract nos. 51071167 and 51102266). Lastly, we
would like to thank the signicant efforts of Mr Michael Ignatowich (PhD student), California Institute of Technology for
checking and editing the manuscript.

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