DSpace at VNU: Ultra-high stability and durability of iron oxide micro- and nano-structures with discovery of new three-dimensional structural formation of grain and boundary

The sharp polyhedral and large Fe oxide microparticles exhibited a uniform characterization of size, shape and morphology with the pure α-Fe2O3 structure.. A new structure was found in

Accepted Manuscript

Title: Ultra-High Stability and Durability of Iron Oxide

Micro- and Nano-Structures with Discovery of New

Three-Dimensional Structural Formation of Grain and

Boundary

Author: Nguyen Viet Long Yong Yang Cao Minh Thi Thomas

Nann Masayuki Nogami

Boundary, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014),

http://dx.doi.org/10.1016/j.colsurfa.2014.05.001

This is a PDF file of an unedited manuscript that has been accepted for publication

As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain

Accepted Manuscript

Ultra-High Stability and Durability of Iron Oxide Micro- and Nano-Structures with

Discovery of New Three-Dimensional Structural Formation of Grain and Boundary

Nguyen Viet Long a,b,c,d,*, Yong Yang a, Cao Minh Thi d, Thomas Nann f and

Masayuki Nogami a,e

a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Science,1295, Dingxi Road, Shanghai 200050,

China

b Posts and Telecommunications Institute of Technology, km 10 Nguyen Trai, Hanoi, Vietnam

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

Duc, Ho Chi Minh, Vietnam

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

Chi Minh City, Vietnam

e Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho,

Showa-ku, Nagoya 466-8555, Japan

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

*Corresponding author Emails: nguyenviet_long@yahoo.com; nguyenvietlong01@gmail.com

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Science, 1295, Dingxi Road, Shanghai 200050, China,

Tel: 86-21-52414321; Fax: 86-21-52414219; Mobile: +81(0)90-9930-9504; +84(0)946293304

ABSTRACT In this research, we have applied a facile polyol method with the addition of NaBH4

to synthesize polyhedral α-Fe2O3 oxide microparticles with the large size of 1-10 μm at 200-230 °C

for about 25-30 min The sharp polyhedral shapes and morphologies have been formed in the good

assistance of NaBH4 as versatile strong reducing agent The sharp polyhedral and large Fe oxide

microparticles exhibited a uniform characterization of size, shape and morphology with the pure

α-Fe2O3 structure To study durability and stability of the microparticles under temperature, we have

rationally carried out isothermalheat treatment of the prepared α-Fe2O3 oxide microparticles at high

annealing temperature about 500 °C and 900-910 °C for 1 h A new structure was found in the pure

α-Fe2O3 particles with micro- and nano-structure co-existed with the very good formation of a

three-dimensional (3D) structure with the oxide grains and the boundaries The interesting

phenomena of the deformation of surface, size, shape, and structure are discovered in α-Fe2O3 oxide

Accepted Manuscript

microparticles by isothermalheat treatment Apart from surface deformation, the new micro- and

nano-structure of large α-Fe2O3 microparticles cannot be destroyed but they can be retained in their

specific characteristics of size, shape, and morphology because of both plastic and elastic

deformation co-existed The interesting surface deformation by heat treatment is observed in

comparison with the case without heat treatment The typical magnetic properties of α-Fe2O3

microparticles were investigated in the appearance of grain and boundary Finally, our proposal of

the new technologies with particle heat treatment is very crucial to make novel micro- and

nano-structures of ultra-high durability and stability with the grains and the boundaries that can be

utilized in catalysis, energy and environment for future

KEYWORDS: Magnetic materials; Surfaces; Crystal growth; Crystallization; Heat treatment;

Microstructure

1 Introduction

So far, platinum (Pt) and iron (Fe) based nanoparticles (NPs) with specific nanostructures have been

intensively investigated in many works because of their importance in catalysis, high-performance

batteries [1-3], gas-sensing sensors [4-6], biological and bio-medical applications, energy and

environment technologies [7-11], and thermoelectrics [11] Traditionally, the pure Pt nanoparticles

were used as efficient catalyst for energy conversion and fuel cell (FC) technology, Pt-Fe2O3 for

next FCs technologies [9,10], and Pt- and Fe-based nanoparticles for next-generation biosensors

[7,8] For Fe-based metal and oxide nanoparticles, the scientists have mainly focused on their

controlled synthesis, processing, structure, property, and performance in optimization of their

practical applications [11-17] However, the Fe-based nanoparticles have a diversity of sizes, shapes

and morphologies [11-13] Therefore, the scientists have tried to shaping the Fe-based nanoparticles

in sharp shape and morphology in respective to their certain size ranges according to their applied

properties, and with pure oxides [18-24] For warning detection of toxic gases (SO2, NOX, CO) in

environment, it is known that Fe-based metal and oxide nanoparticles can be used in gas sensors In

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energy technology, Pt-Fe bimetallic nanoparticles were used for energy conversion and FCs [9,10]

However, Fe oxides have a diversity of various structures, such as FeO (Wϋstite), Fe3O4

(Magnetite), α-Fe2O3 (Hematite), β-Fe2O3, γ-Fe2O3 (Maghemite), and ε-Fe2O3 nanoparticles [11-13],

which can be potentially used for energy conversion and storage in high-performance battery [1-3],

imaging agent and MRI technology, as well as biomedicine [7-8] At present, the controlled

synthesis of one pure phase of Fe oxide particles or nanoparticles is very crucial to their practical

applications, which are of interest to scientists, such as pure α-Fe2O3, pure Fe3O4 So far, there have

rarely been recent works of sintering or heat treatment of Fe- and Pt-based nanomaterials and

nanoparticles These can lead the improvements of their applied properties Interestingly, the

important phenomena of grain and boundary formation were discovered in various steels and

nanomaterials but no research was found on hard evidences of large microparticles with the grains

and boundaries [25-30] At present, there is no any research and evidences of microparticle

sintering or nanoparticle heat treatment of α-Fe2O3 regarding particles containing the nanograins

and the boundaries

In our present research, modified polyol methods with NaBH4 have been used for synthesizing large

polyhedral Fe oxide particles or microparticles of 1-10 μm with very high yield in pure α-Fe2O3

structural phase In this process, the critical experimental conditions are selected in controlled

synthesis of a homogeneous system of large polyhedral α-Fe2O3 microparticles at 200-230 °C for

30-35 min Under these experimental conditions, large polyhedral shape and morphology were

achieved in the final product containing α-Fe2O3 microparticles dried at low temperature in

rich-oxygen medium In particular, we discovered that heat treatment of α-Fe2O3 microparticles at very

high temperature around 500 °C and 900-910 °C is very crucial to obtain new α-Fe2O3 structures

with the fine grains and the boundaries on their large surfaces Ordinarily, we suggest that the

appearance of Fe oxide microparticles with grain and boundary was due to plastic and elastic

deformation as the same as to plastic and elastic nanoparticle deformation Therefore, a new method

of nanoparticle or particle heat treatment was to highlight various important aspects of phase

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transformations of crystal structures of large α-Fe2O3 particles or microparticles for ultra-high

durability and stability Finally, heat treatment has led to densification, grain growth and interesting

microstructure of large polyhedral α-Fe2O3 nanoparticles

2 Experimental

2.1 Chemical

In our typical synthetic processes, chemicals can be used from chemical companies, such as Aldrich,

Sigma-Aldrich or Wako Our synthetic processes enable large-scale production of large, sharp, and

polyhedral Fe oxide microparticles with pure α-Fe2O3 structure after drying and isothermalheat

treatment The necessary industrial chemicals imported from US are poly(vinylpyrrolidone) (PVP),

sodium borohydride (NaBH4), and iron (III) chloride hexanhydrate (FeCl3.6H2O), ethylene glycol

(EG) PVP polymer is a chemical kind with high formula weight (Fw) of 55,000 (Aldrich) It is

used as good protective agents for very high stabilization of large Fe oxide microparticles Here, we

can use Fe precursor of typical chemical kinds (FeCl3: Aldrich, No 451649, Mw: 162.20)

(FeCl3·6H2O: Aldrich, No 236489, Mw: 270.30 g/mol) for synthesis of large α-Fe2O3

microparticles after heat treatment according to the specifications of the American Chemical

Society (ACS) In our process, NaBH4 (CAS: 16940-66-2, Mw: 37.83 g/mol) was used as an

efficient and strong reducing agent for the assistance of controlled synthesis of large α-Fe2O3

microparticles Primarily, EG (Aldrich or Sigma-Aldrich) can be used as both solvent and weak

reducing agent for synthesis, ethanol, acetone, and hexane (Aldrich or Japanese companies, Wako)

Here, all chemicals used were of analytical standard grade and were used without any further

purification

2.2 Synthesis of large polyhedral α-Fe 2 O 3 oxide microparticles

In this process, 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 controlled synthesis of large, sharp and polyhedral α-Fe2O3 oxide microparticles The

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experimental details were previously presented [10] In general, FeCl3 was completely reduced with

an efficient assistance of NaBH4 in EG at 200-230 °C for 25-30 min Finally, we have obtained the

dark-brown solutions (Samples) containing polyhedral α-Fe2O3 oxide microparticles with large

sizes, sharp shapes and morphologies in the range of 1-10 µm Similarly, we have used the same

processes to synthesize other samples for XRD, SEM, and TEM analyses and measurements, and

other supplement measurements To investigate stability, durability, and interesting formation of the

grains and the boundaries in the large α-Fe2O3 microparticles, particle or nanoparticle heat

treatment of large α-Fe2O3microparticles was applied at very high temperature with adjustment at

900-910 °C

2.3 Characterization

2.3.1 X-ray diffraction

To carry out XRD analysis and measurement, we have used the as-prepared products of the black

solution containing the PVP protected α-Fe2O3 oxide microparticles First, PVP were removed by

centrifugation process by a centrifuge, Xiang Yi, H-205R (China) or Kubota (Japan) Then, the

prepared α-Fe2O3 oxide nanoparticles were washed many times with the use of hexane and ethanol

in order to remove a remaining amount of PVP on their surfaces and other contamination impurities

The pure product of ethanol and α-Fe2O3 oxide microparticles after many washing and cleaning

processes was dried in order to receive the Fe-based nano-powder After drying or heat treatment at

high temperature, they were set on the glass substrate for XRD analysis and measurement To study

stability and durability as well as the interesting formation of the grain and boundaries, high heat

treatment of our Samples was carried out in the air flow (20 mL/min) or mixture of H2/air (10

ml/min for H2, and 10 ml/min for air) at 500 °C and 900 ° C for 1 h in the ovens The X-ray

diffraction patterns were recorded by a X-ray diffractometer (Rigaku D/max 2550V) at 40kV/200

mA and using Cu Kα radiation (1.54056 Å)

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2.3.2 Surface analysis: SEM and TEM

In order to study the size and shape of α-Fe2O3microparticle product, we have used field emission

scanning electron microscope (SEM) (JEOL-JSM-634OF) operated at 5, 10, and 15 kV (5-15 kV),

and probe current around 12 μA (1-12 μA) The SEM images of large polyhedral Fe microparticles

were focused by suitably fine focus level and adjustment in a suitable condition of probe current To

characterize the α-Fe2O3 oxide microparticles with very large sizes of 1-10 μm in TEM

measurements, copper grids containing the α-Fe2O3microparticles were maintained under vacuum

by using a vacuum cabinet Prior to the TEM measurements, the copper grids were maintained

overnight under high vacuum conditions in the transmission electron microscope (JEOL

JEM-2100F or JEM-2010) The TEM images were obtained using a transmission electron microscope

(JEOL JEM-2100F and JEM-2010) operated at 200 kV Finally, DigitalMicrograph software (Gatan,

Inc.) was used in the SEM, TEM, and HRTEM studies to acquire, visualize, analyze, and process

the digital image data of α-Fe2O3 microparticles with very large size and shape Because the

α-Fe2O3microparticles have very large particle size (1-10 μm), it is very hard to obtain TEM images

with high-resolution TEM measurements

2.3.3 Magnetic characteristics

We have used Vibrating Sample Magnetometer (VSM), Model EV11 at Institute of Physics (IOP),

Vietnam Academy of Science and Technology (VAST), Hochiminh City, Vietnam, for analysizing

magnetic characteristics of α-Fe2O3 microparticles The EV11-VSM can reach fields up to 31 kOe

at a sample space of 5 mm and 27 kOe with the temperature chamber, with Signal noise to be 0.1

μemu, and 0.5 μemu, respectively Here, all our samples are powder samples of α-Fe2O3

microparticles All the samples measured by EV11-VSM system were evaluated at room

temperature

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3 Results and Discussion

3.1 Structure of α-Fe 2 O 3 oxide microparticles

Figure 1 shows SEM images of large polyhedral microparticles with α-Fe2O3 structure by polyol

method with an efficient assistance of NaBH4 as a very strong reducing agent in respective to heat

treatment Here, the large α-Fe2O3 oxide nanoparticles synthesized are confirmed in the pure

α-Fe2O3 structure with the good, fine, and large crystal surface formation of α-Fe2O3 (PDF-89-0597)

in the typical XRD patterns by Jade software after drying at low temperature, and heat treatment at

high temperatures in Figures 3, 4, and 5(a) At present, a pure crystal nanosystem is of interest to

scientists who want to understand extensively the interesting formations of both very large and very

small nanosystem from micro- to-nanosize range by simple chemical and physical methods Thus,

large polyhedral Fe oxide particles can be achieved by adjusting the pH, by changing the

temperature or by adjusting the magnetic mixing rate reaction time in the reaction mixture In our

interesting results, large polyhedral α-Fe2O3 microparticles can be considered as a nanosystem

containing the monodispersed α-Fe2O3 microparticles They exhibited as a homogeneous

characterization of size, shape, morphology, surface and internal structure We suggested that the

nucleation, growth, and formation mechanisms and processes occurred so fast, which leads to a

nanosystem containing large polyhedral α-Fe2O3microparticles They have good characterization of

uniform shape and morphology of sharpness, flatness and smoothness with uniform particle size of

10 µm (1-10 µm) in critical experimental condition for controlled synthesis at 200-230 °C We

suggest that large crystal growth mechanism and process of α-Fe2O3 are very fast during

crystallization from a homogeneous EG solution The fast growth of such large polyhedral crystals

needs to be clarified in comparison with that of very tiny crystal by a polyol method Typically,

each α-Fe2O3 oxide microparticle shows its characteristics of the sharp angles (α,β,γ) and right

edges (a,b,c) in Figure 2A-A1 The edges and the angles can be similar or different from

experimental measurements and evidences in SEM images Here, an orthorhombic α-Fe2O3 oxide

particle has different edges (a≠b≠c) but the same angles (α=β=γ) The six very large crystal surfaces

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are confirmed in our SEM observations of α-Fe2O3 oxide microparticles in Figure 1 The large

polyhedral α-Fe2O3 oxide crystals including tetrahedra, cube, octahedra have the highest symmetric

characteristics However, their characteristic forms by chemical method gave different crystal

species with lower symmetric characteristics It is the crystal habit formation of α-Fe2O3 oxide

microparticles or α-Fe2O3 oxide crystal habit We know that there are seven crystal systems

belonging to solids Consequently, there are seven crystal shapes and morphologies, which can be

observed in experimental evidences of synthesis of microparticles Figure 2 shows the models of

sharp, large, and polyhedral shapes of α-Fe2O3 oxide microparticles in our SEM images in Figure 1

The best models are used for the crystallization of perfect large crystals with large crystal faces to

describe the common crystal systems approximately, such as orthorhombic, nonoclinic,

rhombohedral systems and others In Figure 1, the sharp polyhedral shapes and morphologies of

α-Fe2O3 oxide microparticles describe the best models of magnetic lattices of the orthorhombic system

It can be assigned to the crystallographic space group to be 6

D R c [31] It is known that hematite (α-Fe2O3) have many specific properties, such as parasitic ferromagnetism,

antiferromagnets (α-Fe2O3) [31] and other works [45-47] More recently, most scholars have

suggested that controlled synthesis of metal and oxide particles with polyhedral surface, shape, and

morphology as well as very sharp corners in the limit ranges of particle sizes are of importance in

both science and practice, especially in electro-catalysis [1,2,9,10] To control particle

characteristics, such as size, shape, surface, internal structure, and composition, we have used the

addition of an amount of NaBH4 for the critical fast reduction of FeCl3 in EG at high temperature

(200-230 °C) for 30 min Clearly, the prepared α-Fe2O3 microparticles were well stabilized under

the good protection of PVP for a very long time

3.2 Structure of α-Fe 2 O 3 oxide particle with grain and boundary: Micro-nano structure

In this research, we have used a method of nanoparticle or particle heat treatment for making them

with α-Fe2O3 structure at 900 °C for 1h The annealing temperature was selected from the

well-known α-FeC equilibrium diagram in metallography of steel from 500 to 910 °C [26,29-30] In

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Figures 3 and 4, the α-Fe2O3 grains and the boundaries between them were clearly observed in

microstructures of the heated α-Fe2O3 microparticles directly attributable to plastic and elastic

deformation at micro- and nano-scale ranges [25-30] Here, the microparticles in Figure 4-C1-C4

and D1-D4 are the same images with the best operation and measurement conditions of SEM The

interesting plastic and surface deformation mechanisms and stress-strain behaviors were very

crucial to strengthen and regulate the mechanical characteristics of steel (FeC) or ferrite in

metallurgy technology [25-30] We suggest that this is a new phenomenon of plastic and elastic

particle deformation of large microparticles due to sintering or heat treatment The various

interfaces between the α-Fe2O3 grains were clearly shown in Figures 3 and 4 The grain boundaries

in the α-Fe2O3 microparticles exhibited the separations between the grains of the same α-Fe2O3

crystal phase Therefore, the α-Fe2O3 oxide microparticles are considered as new 3D α-Fe2O3 oxide

nanostructures with the small and large α-Fe2O3 oxide grains Thus, they also exhibited the same

crystal structures as α-Fe2O3 oxide bulk This is exactly determined by the typical evidences of

XRD measurements (Figure 5) The average size of the α-Fe2O3 grain is estimated in a size range

of 100-300 nm to the small α-Fe2O3 grains, and in a size range of 400-1000 nm to the large α-Fe2O3

grains Large α-Fe2O3 oxide microparticles consisted of both the coarse grain and the fine grain

The low-angle and high-angle boundaries (θ < 20 °, θ > 45 °) can be seen in the α-Fe2O3 oxide

microparticle (Figure 4E (E1,E2)) The model in Figure 4 also shows possibilities of crack

propagation along grain boundaries for intergranular fracture (Blue lines) Thus, there are the

various crack-propagation directions The grain boundaries clearly show curvature's radius

Through appropriate heat treatment with the addition of metal or oxide composition, we can expect

to control the homogeneous size of the grains, such as for a significant improvement of stability and

durability of α-Fe2O3 nanostructures as well as their alignment and order according the various

crystal directions Therefore, the grain growth was confirmed to be due to heating and annealing to

the prepared Fe oxide microparticles Based on calculation of a total number of the grains on each

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surface of large α-Fe2O3 nanoparticles in the three dimensions, we can expect to control heat

treatment or sintering in order to obtain a particle with a number of their desirable grains and sizes

In Figure 3, we have calculated a number of the nano-grains about 55  2 grains (or small and large

nanoparticles) on one surface, e.g S1 in Figure 3D-(D1), and Figure 3D-(D5) with a number of the

nano-grains about 56  2 grains on one surface S1, and Figure 3F-(F3) with a number of the grains

about 60  2 grains on one surface S1 Similarly, we can see a large cube about 2.5 µm in size with

a number of the grains to be about 27 grains (Surface S1) and 30 grains (Surface S2) in Figure

3G-(G5) In reality, all the grains on the surfaces of α-Fe2O3 lead to form a concave and convex plane

or a roughing plane or a roughing surface It is also suggested that the large oxide grains have two

typical categories in normal and abnormal grain-growth regimes [25] Scientists proved that

sintering and heat treatment processes are very important to improve nanomaterials's properties,

such as higher strength, better toughness, higher durability and stability through the small grains,

creep resistance and reduction through the large grains, due to the special characteristics of both

fine small grain and large grain systems In addition, there were plastic and super-plastic

deformations during steel heat treatment that is very crucial to create steels with super-strength,

super-durability and super-stability [25-30] In this context, our results can lead to produce a

nanosystem of Fe oxide microparticles with super-strength, super-durability and super-stability In

Figure 4B-(B1), we can calculate a number of the nano-grains about 129  2 grains on one surface

S1, another surface S2 to be about 812 grains A total number of grains was estimated as

(81+129)2  8 equal to 420  8 grains on all four surfaces of that large microparticle Here, we did

not calculate a number of the grains of the two remaining surfaces Then, we can consider the large

particle as grain layers consisting of 9 layers containing the grains or the nine same surfaces with

129  2 grains, a total number of grains in the volume of that microparticle was 9129  92 equal

to 1161 18 grains as a rough estimation For case of Figure 4B-(B3), a total number of the grains

on surfaces was evaluated as 75  2 grains (Surface S1), 81  2 grains (Surface S2), and 78  2

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grains (Surface S3) Here, surfaces S1, S2, and S3 were used for calculate all the grains on the

surface of the large microparticle equal to 2(75+81+78)  12 grains or 486  12 grains The total

number of the grains belonging to the large microparticle can be calculated to be (75 2) 10, equal

to be 750  20 grains in the case we use S1 surface and 10 layers of surface S1 of the grains In

Figure 4C-(C1), we can calculate a number of the grains on one surface to be 71  2 grains In

Figure 4D-(D3), we can estimate a number of the grains on one surface to be 75  2 grains Our

results of large α-Fe2O3 microparticles containing the grains and the boundaries can be considered

as the densification models of “ideal” grain growth in materials optimization [11,39] The grains in

metal oxide nanostructures were considered as the important factors to enhance gas sensing

property and better performance in large grain films of gas sensor devices [4], and thermoelectric

nanostructured bulk materials [11,32,44] In general, the scholars have used computer simulation of

3-D grain growth or 3-D nanostructures using phase-field model method that is the best agreement

with our experimental results obtained [11,36] Thus, we have successfully produced large

three-dimensional (3D) microparticles with the fine grains and the boundaries This is one of the largest

challenges in nanotechnology for 3D nano-textures In fact, various scholars proved that there are

the crystal transformations of α-FeOOH (goethite) into α-Fe2O3 by oxidation, then into magnetite

Fe3O4 by reduction, and finally into γ-Fe2O3 due to oxidation through drying and sintering

processes or heat treatment [16,22,23] In Figures 3 and 4, the α-Fe2O3 oxide microparticles show

long-term stability and high durability without the structure collapses or cracking and destroying the

α-Fe2O3 oxide microparticles from room temperature to 900 °C In addition, the shrinkage in

α-Fe2O3 oxide microparticles in the dimensions was relatively low in the final stage heating This

proves ultra-high stability and durability of α-Fe2O3 oxide microparticles synthesized by modified

polyol methods and heat treatment Consequently, the structural variations because of heat

treatment at 900 °C led to the specific surface deformations from smoothness, flatness, and

sharpness surfaces into various smaller convex or concave curvature surfaces of the specific

α-Fe2O3 oxide grains while the size and the shape were retained According to researchers, the

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improved properties of metals and oxides after heat treatment at high temperature ranges can be

obtained in the appearance of small and large grain [29-31,34,35] In general, we suggest that the

characteristic comparisons of the complete crystallization of metals and oxides microparticles after

their synthesis from solution and the complete re-crystallization of the same metals and oxides

microparticles after heat treatment are very important to potential applications in catalysis,

biochemistry, and energy and storage materials, such as high-performance batteries Above all, the

roles of the grains in thermoelectric nanostructures are also very necessary to improve Seebeck

coefficient for high-performance of thermoelectric materials [11,32,44]

3.3 Effect of heat treatment to formation of micro-nano α-Fe 2 O 3 structure

The based materials are often heat-treated to improve their properties During heat treatment,

Fe-based microparticles were completely oxidized in air or air/H2 medium in Figure 5 for the best

crystal phase of α-Fe2O3 structure It is true that heat treatments that are used to strengthen and

harden Fe oxides [26-30] Heat treatment to large Fe oxide particles at 900 °C for 1 h in our

research makes the important changes in their microstructure in respective to recovery,

recrystallization, and grain growth The new surfaces of large α-Fe2O3 oxide microparticles are

formed with grain and boundary Thus, it is important to understand a process of heat treatment in

terms of appropriate annealing time and temperature The ability to arrange grains on large surfaces

of large oxide microparticles through heat treatment provides opportunities to develop the applied

properties It is known that the order in grain arrangement on the surfaces and insides of large oxide

nanoparticles can be distinguished in the order in atomic arrangement on the surfaces and insides of

very small metal nanoparticles These changes of the microstructure of Fe oxide microparticles with

grain and boundary can lead the new research directions for researchers in the same area It will

open a new method of nano and microparticle heat treatment for making the new surfaces with

grain and boundary of the large particles in order or in disorder In the formation of micro-nano

α-Fe2O3 structure, the heat treatment is the main cause to nanoparticle or particle deformation

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including plastic and elastic deformation from nano- to micro-scale The heat treatment caused the

structure deformation or particle deformation in the formation of the grains and the boundaries of

prime α-Fe2O3 oxides Indeed, the heat treatment can also lead to atomic surface and structure

deformation, plastic and elastic deformation The scientists proved that the cracks are one of the

main causes that lead to nucleate in the grain boundary at various locations in steel [33] Therefore,

our method of high heat treatment of α-Fe2O3 microparticles can lead to create ultrafine-grained

structures [34,35] Here it may be noted that the most important characterizations of α-Fe2O3

microparticles are the size, the shape, the morphology retained during heat treatment around 900 °C

The grain growth in our research can be explained in terms of pure single-phase α-Fe2O3 system

The grains in metal oxide nanostructures were considered as the important factors to improve gas

sensing property in gas sensor devices To obtain smaller grain size requires the elaborate controls

of the other characteristics of large polyhedral oxide particles Likewise, the significant effects of

the structural compositions of metals, alloys, and oxides by heat treatment can be understood in

detail for the discoveries of durability and stability of nano and microparticles and nanostructures

via heat treatment In modeling and simulation, scientists have tried to optimize properties of

nanomaterials through the grain and boundary structures for practical applications in energy and

environment Their scientific challenges are how to produce the evidences 3D particles from

experimental [36-44] Therefore, our modified polyol methods with NaBH4 to prepare large

polyhedral α-Fe2O3 microparticles have many advantages of producing a large amount of

microparticle products in comparison with other methods for preparation of large α-Fe2O3

nanoparticles [45-47] We suggested that new phenomena of nano or microparticle deformation,

surface deformation, and co-existence in both plastic and elastic deformation are discovered in

interesting evidences of SEM images of α-Fe2O3 because of isothermalheat treatment at 900 C

The large 3D nano or microparticles can contribute to deal with the challenges in the topics of

simulation, modeling and materials design [36-44] In future, large 3D Fe oxide nano and

microparticles can be designed with hundreds to thousands grains and boundaries with good