Controlled synthesis and characterization of iron oxide nanostructures with potential applications Nguyen Viet Long,*abcdefYong Yang,*aMasayoshi Yuasa,eCao Minh Thi,fYanqin Cao,a Thomas
Trang 1Controlled synthesis and characterization of iron oxide nanostructures with potential applications
Nguyen Viet Long,*abcdefYong Yang,*aMasayoshi Yuasa,eCao Minh Thi,fYanqin Cao,a Thomas Nanngand Masayuki Nogamiah
found that the highly homogeneous iron oxide microparticles' novel structure is the best pure crystal phase
new method of heat treatment or atomic surface deformation allowed for the discovery of a new large
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,1–4such as
their previously undiscovered ferromagnetic,
antiferromag-netic, and ferrimagnetic magnetism So far, the crystal
nanostructures of various Fe oxides and magnetic Fe oxide nanoparticles have the same specic crystal nanostructures as magnetite (Fe3O4) and maghemite nanoparticles (g-Fe2O3) and hematite (a-Fe2O3).5 –10Recently, scientists and researchers have discovered important practical applications of a-Fe2O3 nano-particles 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.11–15,46In addition, a reduced graphene oxide platelet/Fe2O3nanoparticle composite can be used in the anode for Li-ion batteries with high-performance as well as high durability and stability.16In 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 modied 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 Fe3O4nanoparticles and maghemite g-Fe2O3nanoparticles under size and morphology control, have been used in drug delivery vehicles.17–20At 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
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: nguyenviet_long@yahoo.com; Fax:
+86-21-52414219; 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,
861-8580, 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-Fe 2 O 3 microparticles produced with a modi ed polyol method with
NaBH 4 at 200 –300 C for 30 min SEM images of the very large a-Fe 2 O 3
microparticles with surface deformations and the grains under the same
conditions and microparticle heat treatment See DOI: 10.1039/c3ra45925j
Cite this: RSC Adv., 2014, 4, 6383
Received 18th October 2013
Accepted 14th November 2013
DOI: 10.1039/c3ra45925j
www.rsc.org/advances
PAPER
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Trang 2durability and stability Magnetite Fe3O4 nanoparticles can be
used for magnetic hyperthermia, high contrast agents and MRI
technology, and targeted drug delivery vehicles.17–24Due to their
high biocompatibility and relatively low toxicity in animals and
humans, maghemite g-Fe2O3nanoparticles 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
charac-teristics, 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-Fe2O3structure in the range of 1–
5 mm Herein, we have successfully used a modied polyol
method with the addition of an extra amount of NaBH4 in
ethylene glycol (EG)
2.1 Chemical
For the chemical synthesis processes to make the pure a-Fe2O3
oxide nanoparticles, we used chemicals from Aldrich,
Sigma-Aldrich and Wako These include poly(vinylpyrrolidone) (PVP)
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 purication Deionized and distilled water
with high purity prepared by a Milli-Pore purication system
available in our laboratory was used for the washing and
cleaning of containers during experimental synthesis processes
2.2 Synthesis ofa-Fe2O3oxide microparticles
Briey, 3 mL of EG, 1.5 mL of 0.0625 M FeCl3, 3 mL of 0.375 M
PVP, and 0.028 g NaBH4were used for making Sample 1 in a
typical process of the controlled synthesis of the large
poly-hedral a-Fe2O3oxide microparticles The details and steps of
the known process procedures were previously presented.19,25In
general, FeCl3was completely reduced with the extra amount of
NaBH4in EG at 200–230C for 30 min As a result, black
solu-tions containing polyhedral a-Fe2O3oxide microparticles with
large sizes, shapes and morphologies were obtained as thenal
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 500C for
1 h for SEM measurement and analysis Sample 3 was heated at
900C for 1 h for SEM analysis All of the experimental
condi-tions for making Samples 1–3 corresponding to the XRD results
are presented in Table 1 and section 3 (Results and discussion)
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-Fe2O3oxide 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/airow (20 mL min1) or a mixture of 10 mL min1 for O2, and 10 mL min1for air at 500C and 900C 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
Ka radiation (1.54056 ˚A) Finally, only the crystal phase of 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 suitablene 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 micropar-ticles were maintained under vacuum by using a vacuum cabinet
Fig 1 and 2 show the SEM images of the as-prepared large a-Fe2O3oxide microparticles with polyhedral morphologies and shapes of a certain size of about 1–5 mm.19,20It should be noted that the as-prepared nanoparticles were observed to have poly-hedral 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 conrmed from our heat treatment process of the as-prepared Fe oxide microparticles at 900C (Fig 3(b) and (c), S1 and S2 (ESI†)) but
no clear and signicant structural changes at the surfaces of the large a-Fe2O3microparticles at 500C (Fig 3(a))
Very interestingly, most large polyhedral a-Fe2O3 oxide microparticles contained smaller a-Fe2O3nanoparticles in their very large nano-textures All the large a-Fe2O3crystal surfaces were clearly deformed in the nanoparticle heat treatment at
900C C1 in Fig 4(c) shows the new micro-nano structure of one as-prepared large polyhedral Fe based microparticle aer heat treatment at about 900C Although the particle size of this large microparticle was not signicantly changed, all the large crystal surfaces were signicantly changed into the new
Trang 3micro-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 1 grains or 35 1 small or intermediate
nano-particles 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-Fe2O3microparticle of about 3 mm
The arrangements in order show that there are six nanoparticles
or grains (6 1) in one row, and six nanoparticles or grains (6
1) in one column Therefore, there are 35 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 Fe2O3grains clearly exhibited curvature on their surfaces
Moreover, each a-Fe2O3grain is classied as a single a-Fe2O3
crystal by the XRD method However, the boundaries between
the grains are also clearly distinguished
The large oxide grains have two categories in normal and abnormal grain-growth regimes.26–32Certain grains in our new micro-nano structures have both small and large sizes with respect to the abnormal grain-growth regimes They show un-sharp curvature boundaries (our results) or un-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-Fe2O3crystal Surprisingly, the new 3D structures can be considered to be excellent evidence of 3D grain growth without particle collapse or cracking This is of
stock solutions
3 mL of 0.375 M PVP
solutions
3 mL of 0.375 M PVP
solutions
(Sample 1).
polyol method (Sample 1) (b) SEM image of an orthorhombic crystal, and (c) its model.
Trang 4high 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 scientic interest at present.32At
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 –35Here, we have
appropriately selected two temperature points for our method
of nanoparticle heat treatment to produce nanoparticles with an a-Fe2O3structure the same as the well-known a-FeC equilibrium diagram from 500 to 910C.33,34This is an interestingnding for making a-Fe2O3microparticles 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 nano-particles are the same as their equilibrium diagrams in metal-lurgy Aer annealing, there is the appearance of small and large a-Fe2O3grains in the large crystal surfaces because of renuclea-tion and recrystallizarenuclea-tion processes The severe deformarenuclea-tion of the large,at, and smooth crystal surfaces into concave, convex, rough, and distorted crystal surfaces of a-Fe2O3microparticles is very crucial to achieve new nano-textures (Fig 3(b) and (c), S1 and S2 (ESI†), and Fig 4 with models) These Fe2O3structures may be very stiff and permanent aer nanoparticle heat treatment in the 500–900C 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 signicant changes in the particle size distribution Thus, an appropriate annealing process can control the grain sizes and boundaries among the interfaces of the grains
boundaries in one two dimensional (2D) large crystal surface of the
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.
Trang 5The 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.32These are due to the special
character-istics 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.26,33However,
we also suggest that the curvature boundaries between the
grains were possibly due to both plastic and elastic
deforma-tion, 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
deforma-tion 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
micropar-ticle, it is not difficult to observe possible cracking and
propa-gation cracking in the prepared microparticles The good
recovery characteristics of the large shape and morphology of
a-Fe2O3microparticles was shown aer strong surface
deforma-tion and plastic deformadeforma-tion in the formed a-Fe2O3 grains
Thus, we can expect that the boundaries of thene grains in
one such large microparticle can be controlled by the
nano-particle sintering process This also illustrates the well ordered
arrangement of the small and intermediate a-Fe2O3 oxide
nanoparticles with specic ne grain boundaries, and also
within the oxide grains on the surfaces annealed at 900C for
1 h that were evidenced by one as-prepared large a-Fe2O3oxide
microparticle (or large a-Fe2O3oxide 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
poly-hedral a-Fe2O3oxide 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
as-prepared large a-Fe2O3oxide microparticles of about 3 mm with
an orthorhombic shape and morphology The crystal a-Fe2O3
oxide grains and grain boundaries ofnite sizes can be clearly
distinguished The small crystal grains are normally located at
the corners of the intermediate a-Fe2O3oxide grains In the
as-prepared large a-Fe2O3oxide microparticles, their shapes and
morphologies haveat and smooth crystal surfaces Aer 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-Fe2O3oxide 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 thiseld
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 specic oxide grains Our latest results regarding the pure a-Fe2O3oxide 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-Fe2O3oxide crystals was caused during heat treatment at 900 C The
a-Fe2O3oxide 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-Fe2O3oxide microparticles in the range between
500C and 900C 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-Fe2O3nanostructures have very large sizes, we did not characterize them by TEM measurements in our subse-quent 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, NaBH4can usually be used in excess amounts for the full reduction of metal precursors.17 –24Here, we suggest that the fast formation of very small Fe metal nano-particles during the synthetic process occurred at 200–230C.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 oxi-dised As crucial evidence for this, structural transformations among FeO (Wustite), 3-Fe2O3, Fe3O4, g-Fe2O3, and a-Fe2O3can be carried out through suitable heat treatments.9,36However, 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 as-prepared 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
Trang 6Thenal formation of the pure a-Fe2O3structure in EG was
done with the long-term stabilization of various PVP polymers by
a facile method with the use of NaBH4as 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
mate-rials for high performance lithium-ion batteries.39,40In addition,
we have evaluated the crystal structure of the as-prepared
samples of large Fe2O3microparticles Fig 5 shows the typical
XRD patterns of one dried sample, and two samples aer
calci-nation at 500 and 900C Here, the pure a-Fe2O3oxide
micro-particles 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
crystal-lization of the pure a-Fe2O3 crystal structure produced by the
modied polyol method with NaBH4 The a-Fe2O3crystal
struc-ture (hematite system) belonging to the crystallographic space
group R3C[167] has lattice constants (a,b,c) equal to 5.039 nm,
5.039 nm, and 13.770 nm, respectively, with a ratio of c/a¼ 2.733 (ICDD/JCPDS PDF-89-0597) using the JADE soware (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/a¼ 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-Fe2O3microparticles without any mixture of other phases, such as FeO (Wustite), a-FeOOH, 3-Fe2O3, Fe3O4, g-Fe2O3etc As shown in Fig 5, the calcination of samples at 500 and 900C also resulted in the formation of a-Fe2O3 (PDF-89-0597) crystal phase structure Importantly, high crystallization of the pure rhombohedral hematite a-Fe2O3was obtained between 500C and 900C As shown in Fig 2(a)–(c) and S1 and S2 (ESI†), the degree of densication of the prepared oxide microparticles at 500C is a little smaller than that of the prepared oxide microparticles at
900C 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, andat surfaces, as well as sharp right-angled edges and corners In the a-Fe2O3 microparticles aer heat treatment at 500C (Sample 2) and 900C (Sample 3), the structures were considerably changed The microparticles were changed in their deformation at 500C but show no signicant changes in their size and shape in Fig 3(a) They gradually became polycrystals under high heat treatment At an annealing temperature of about 900C, the microparticles were changed due to their very signicant 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-Fe2O3microparticles and a-Fe2O3nanoparticles, 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 signicance for ecosystems and environmental appli-cations,41a-Fe2O3tetradecahedra 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,42and other a-Fe2O3 structures also have practical applications in gas sensing.42–45Interestingly, the important roles of the metal or alloy or oxide grains were known in the signicant reduction of lattice thermal conductivity for an enhanced ZT applied in new thermal nano-structured materials.30The topic of nanoparticle heat treatment will be an important subject for scientists, and specialists In addition, various methods of nanoparticle heat
nanoparticles prepared at various temperatures: (a) dried sample
Trang 7treatment need to be signicantly considered in the
develop-ment of new nanomaterials with grain and boundary
structures.45
In this research, we have successfully prepared large a-Fe2O3
microparticles by a modied 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 aer nanoparticle heat treatment at
900C This new structure of a-Fe2O3microparticles 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 signicant efforts of Mr Michael
Igna-towich (PhD student), California Institute of Technology for
checking and editing the manuscript
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