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 PII: DOI: Reference: S0927-7757(14)00444-0 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.05.001 COLSUA 19201 To appear in: Colloids and Surfaces A: Physicochem Eng Aspects Received date: Accepted date: 24-2-2014 1-5-2014 Please cite this article as: N.V Long, Y Yang, C Minh Thi, T Nann, M Nogami, Ultra-High Stability and Durability of Iron Oxide Micro- and Nano-Structures with Discovery of New Three-Dimensional Structural Formation of Grain and 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 proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain 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 Masayuki Nogami a,e f and a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai ip t Institute of Ceramics, Chinese Academy of Science,1295, Dingxi Road, Shanghai 200050, China cr 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 us 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 an e Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Ian Wark Research Institute, ARC Special Research Centre, University of South Australia M f * Corresponding author Emails: nguyenviet_long@yahoo.com; nguyenvietlong01@gmail.com d State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, 1295, Dingxi Road, Shanghai 200050, China, te Tel: 86-21-52414321; Fax: 86-21-52414219; Mobile: +81(0)90-9930-9504; +84(0)946293304 Ac ce p 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 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 αFe2O3 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 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 Page of 28 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 ip t 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 cr the new technologies with particle heat treatment is very crucial to make novel micro- and nano- utilized in catalysis, energy and environment for future us structures of ultra-high durability and stability with the grains and the boundaries that can be an KEYWORDS: Magnetic materials; Surfaces; Crystal growth; Crystallization; Heat treatment; M Microstructure d Introduction te 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 Ac ce p 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 Page of 28 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], ip t 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 cr applications, which are of interest to scientists, such as pure α-Fe2O3, pure Fe3O4 So far, there have us 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 an 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 M and boundaries [25-30] At present, there is no any research and evidences of microparticle te and the boundaries d sintering or nanoparticle heat treatment of α-Fe2O3 regarding particles containing the nanograins In our present research, modified polyol methods with NaBH4 have been used for synthesizing large Ac ce p 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 Under these experimental conditions, large polyhedral shape and morphology were achieved in the final product containing α-Fe2O3 microparticles dried at low temperature in richoxygen 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 Page of 28 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 ip t Experimental cr 2.1 Chemical In our typical synthetic processes, chemicals can be used from chemical companies, such as Aldrich, us Sigma-Aldrich or Wako Our synthetic processes enable large-scale production of large, sharp, and an 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), M 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 d used as good protective agents for very high stabilization of large Fe oxide microparticles Here, we te 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 Ac ce p 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-Fe2O3 oxide microparticles In this process, mL of EG, 1.5 mL of 0.0625 M FeCl3, 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 Page of 28 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 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 ip t 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 cr grains and the boundaries in the large α-Fe2O3 microparticles, particle or nanoparticle heat us treatment of large α-Fe2O3 microparticles was applied at very high temperature with adjustment at an 900-910 °C 2.3 Characterization M 2.3.1 X-ray diffraction d To carry out XRD analysis and measurement, we have used the as-prepared products of the black te 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 Ac ce p 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 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) Page of 28 2.3.2 Surface analysis: SEM and TEM In order to study the size and shape of α-Fe2O3 microparticle product, we have used field emission scanning electron microscope (SEM) (JEOL-JSM-634OF) operated at 5, 10, and 15 kV (5-15 kV), ip t 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 cr characterize the α-Fe2O3 oxide microparticles with very large sizes of 1-10 μm in TEM measurements, copper grids containing the α-Fe2O3 microparticles were maintained under vacuum us 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- an 2100F or JEM-2010) The TEM images were obtained using a transmission electron microscope M (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 d the digital image data of α-Fe2O3 microparticles with very large size and shape Because the α- te Fe2O3 microparticles have very large particle size (1-10 μm), it is very hard to obtain TEM images Ac ce p 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 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 Page of 28 Results and Discussion 3.1 Structure of α-Fe2O3 oxide microparticles Figure shows SEM images of large polyhedral microparticles with α-Fe2O3 structure by polyol ip t 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- cr 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 us 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 an small nanosystem from micro- to-nanosize range by simple chemical and physical methods Thus, M 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 d interesting results, large polyhedral α-Fe2O3 microparticles can be considered as a nanosystem te containing the monodispersed α-Fe2O3 microparticles They exhibited as a homogeneous characterization of size, shape, morphology, surface and internal structure We suggested that the Ac ce p nucleation, growth, and formation mechanisms and processes occurred so fast, which leads to a nanosystem containing large polyhedral α-Fe2O3 microparticles 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 Page of 28 are confirmed in our SEM observations of α-Fe2O3 oxide microparticles in Figure 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 ip t 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 cr observed in experimental evidences of synthesis of microparticles Figure shows the models of us sharp, large, and polyhedral shapes of α-Fe2O3 oxide microparticles in our SEM images in Figure The best models are used for the crystallization of perfect large crystals with large crystal faces to an 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- M 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 D36d  R c [31] It is known that d hematite (α-Fe2O3) have many specific properties, such as parasitic ferromagnetism, te antiferromagnets (α-Fe2O3) [31] and other works [45-47] More recently, most scholars have Ac ce p 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 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-Fe2O3 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 wellknown α-FeC equilibrium diagram in metallography of steel from 500 to 910 °C [26,29-30] In Page of 28 Figures 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 ip t 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 cr metallurgy technology [25-30] We suggest that this is a new phenomenon of plastic and elastic us particle deformation of large microparticles due to sintering or heat treatment The various interfaces between the α-Fe2O3 grains were clearly shown in Figures and The grain boundaries an 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 M 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 d XRD measurements (Figure 5) The average size of the α-Fe2O3 grain is estimated in a size range te of 100-300 nm to the small α-Fe2O3 grains, and in a size range of 400-1000 nm to the large α-Fe2O3 Ac ce p 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 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 Page of 28 mechanical, electrical and catalytic properties In addition, the very large surfaces of magnetic Fe oxides can enable to have good surfactant adsorption, e.g useful small molecules, biomolecules, surfactants, polymers and dendrimers, drugs against cancer and tumors onto their large solid surfaces [48] Therefore, they will be employed in potential applications of biology and medicine ip t Importantly, a nanosystem of uniform, pure, large colloidal oxide microparticles (Fe2O3, Fe3O4, and others) with large particle size of 10 µm can be used as the very good supports for noble metal cr nanoparticles (Pt, Pd, Rh, Ru and others) with very small particle size of 10 nm [9] Noble metal us nanoparticles can be firmly fixed or supported freely on large faces of large oxide nanoparticles Similarly, core-shell bimetal nanoparticles, such as Pt-Pd, Pt-Cu, Pt-Ni, and others can be supported an on the large oxide nanosystem [9,49] Upon heat treat, large 3D α-Fe2O3 oxide nanoparticles by heat treatment will have some unique and technologically promising properties Ultimately, the M metal, bimetal/oxide nanoparticles by heat treatment can be concerned primarily with grain and boundary structures the testing of gas sensing, catalysis, biology and medicine [50-55], especially in d gas sensors with the highest sensitivity and stability in very urgent needs at present Our present te results of α-Fe2O3 with grain and boundary structures will contribute significant opportunity of Ac ce p improving the performance of next-generation gas sensors 3.4 Crystallization and re-crystallization of α-Fe2O3 structure Based on Figures 1, and 4, we obtain important conclusions as follows The large α-Fe2O3 microparticles synthesized in solution of EG and PVP have the high crystallization They exhibited very large crystal faces about tens µm2, which are sharp, flat, and near perfect They can be (100), (111), and (110), and more other large faces (Figure 1) During synthesis and magnetic mixing, the collisions of colloidal particles in the solution of EG and PVP easily occurred, which are very crucial to find an efficient synthetic process with very high yield of polyhedral shape and morphology Typically, there are most of the particles after the elastic collisions like “Brownian motion” but the size and the shape are retained and considerably unchanged In contrast, there are 14 Page 14 of 28 microparticles after the elastic or inelastic collisions leading to the microparticle breaking or particle deformation in Figure (F5, G3, and G4) They can combine into a new group of the larger particles However, the important roles of PVP polymer for protection of large α-Fe2O3 oxide particles are also very crucial to be against their self-aggregation To crystallization and ip t recrystallization of large polyhedral α-Fe2O3 microparticles, Figure shows the typical XRD pattern of three samples One Sample was dried at low temperature in air medium (or air/H2) Other cr Samples heated at 500 °C and 900 °C in air (or air/N2) XRD results showed that all the prepared us microparticles have α-Fe2O3 structure The pure α-Fe2O3 oxide microparticles heated at 500 °C and 900 °C have the exact rhombohedral crystal structure The most typical peaks were characterized by an (012), (104), (110), (113), (024), (116), (122) or (018), (214), (300), (208), (1010), and (220), and more (hkl), respectively The corresponding values of 2θ (degrees) are estimated at about 24.3, 33.2, M 35.6, 40.8, 49.5, 54.1, 57.7, 62.5, 64.2, 69.8, 72.0, and 75.4 °(degree), and more 2θ, respectively in d the range of 5-95 ° te The insets (A1, B1, and C1) show SEM images of the large microparticles corresponding to Sample (Dried Sample), Sample (500 °C), and Sample (900 °C), respectively Figure also indicated Ac ce p the sharp narrow diffraction peaks as the evidences of pure high crystallization of the pure α-Fe2O3 crystal structure in the final microparticle products The α-Fe2O3 structures with the crystallographic space group R=3C[167] have lattice constants (a,b,c) equal to 5.039 Å, 5.039 Å, and 13.770 Å, respectively, and with a ratio of c/a=2.733 (PDF-89-0597) by using Software of Materials Data JADE for XRD pattern processing and MDI material data Therefore, the prepared large microparticles by XRD method indicated the sharp narrow diffraction peaks as evidence of pure high crystallization of the pure α-Fe2O3 crystal structure by modified polyol method with NaBH4 and heat treatments at various temperatures In Figure 5, we suggested that Samples of α-Fe2O3 oxide, e.g., Dried Sample, Heated Sample at 500 °C exhibited much lower porosity than Samples of α-Fe2O3 oxide heated at 900 °C in respective to 15 Page 15 of 28 their typical XRD data Certainly, the highest improved crystallization of Samples heated at 900 °C for for h was proved in Figure 5(c) with the very sharp and narrow diffraction peaks in the grain and boundary structure ip t 3.5 Magnetic properties of α-Fe2O3 microparticles Figure with the snapshot shows the exciting magnetic properties of α-Fe2O3 microparticles by a cr magnetometer (EV11-VSM) for Sample 1, Sample and Sample All the samples were measured us in an applied filed range from −16 to +16 kOe Here, remanent magnetization shows M at H=0 in the lood In each hysteresis loop, MR parameter was characterized by the upward and downward an part of hysteresis loop in respective to its average value Similarly, MS, MHmax, HC, and BHmax in respective to their average values are typical parameters of one hysteresis loop In physical meaning, M MS indicates that saturation magnetization shows the maximum of M to be measured There is a value of MHmax when the magnetization was measured at the maximum of magnetic field Here, d coercive field shows magnetic field at which M/H changes sign An important magnetic parameter te is stored energy inside the magnetic material showing its performance It is characterized by Ac ce p maximum energy product (BHmax) In addition, µmax shows a maximum permeability measured in all the samples All the magnetic characteristics of Samples 1, and were listed in Table To Sample 1, the magnetic properties of α-Fe2O3 microparticles include MR, MS, MHmax, HC, BHmax, and µmax, respectively In hysteresis parameters of α-Fe2O3 microparticles, MR, MS, MHmax, HC, BHmax, and µmax have the typical values to be 0.142 emu g-1, 0.543 emu g-1, 0.543 emu g-1, 696.48 Oe, 0.075 MGsOe, and 0.000371 in the whole hysteresis loop MR and MS increased with decreasing HC and BHmax while µmax also increased in respective to the Sample (Drying), Sample (heating at 500 C) and Sample (heating at 900 C) We found that the ratio of MR/MS is small and less than In addition, Sample showed the maximum value of HC = 696 Oe but the minimum values of MR and µmax because of a number of magnetic domains less than that of Sample In our obtained results, we have found a clear trend of the changes from ferromagnetic to 16 Page 16 of 28 superparamagnetic in respective to α-Fe2O3 oxide dried at low temperature and α-Fe2O3 oxide heated at over 900 °C to all the samples that were investigated in comparison with other works [58,59] These were explained by the evidences of appearance of grain and boundary structures in α-Fe2O3 oxide microparticles in the heated sample with magnetic multi-domains more than those of ip t the dried samples Thus, all the as-prepared α-Fe2O3 oxide microparticles can be used as potential cr candidate for magnetic applications us Conclusions In this research, we have applied modified polyol methods with NaBH4 as well as drying and heat an treatment for synthesizing large polyhedral α-Fe2O3 microparticles within the 1-10 μm range The sharp, large polyhedral shapes of α-Fe2O3 microparticles were discovered in critical experimental M conditions at 200-230 C for 30-35 They can be synthesized in a large amount through strong reduction of FeCl3 by NaBH4 and EG in a facile process This synthetic method is very competitive d to produce the α-Fe2O3 nano or microparticle product In addition, the proposed methods of nano te and microparticle heat treatment can produces the interesting new large 3D α-Fe2O3 particles with Ac ce p grain and boundary structure in ultra-high durability and stability, which are of importance to scholars in the areas of nano and microparticle 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Mater 61 (2013) 974-1000 57 S Vahdatifara, A.A Khodadadi, Y Mortazavi, Sens Actuators, B, 191 (2014) 421-430 58 Q A Pankhurst, N T K Thanh, S K Jones and J Dobson, J Phys D: Appl Phys 42 (2009) 224001 59 X Zhang, Y Chen, H Liu, Y Wei and W Wei, CrystEngComm 15 (2013) 6184-6190 20 Page 20 of 28 Highlights  Large polyhedral α-Fe2O3 particles are prepared by polyol method ip t with NaBH4 cr  Very high crystallization of large α-Fe2O3 particles is identified  Large polyhedral α-Fe2O3 particles with grain and boundary are us found at 900 C an  Methods of particle heat treatment are proposed for α-Fe2O3 with grain and boundary M  There are surface and structure deformations of α-Fe2O3 particles Ac ce p te d due to heat treatment 21 Page 21 of 28 Figure(s) A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 C1 C2 C3 C4 D1 D2 D3 E1 E2 E3 F1 F2 B C5 G1 G H1 E4 an E5 F4 F5 M D5 F3 Ac ce pt e E F D4 d D us cr C ip t A G2 H2 Broken face G3 H3 G4 G5 H4 H5 Broken face H Figure The very large crystal surfaces, shapes and morphologies of large polyhedral α-Fe2O3 particles according to their crystallization from solution by polyol method with the addition of NaBH (Dried Sample) Scale bars: A-H: µm Page 22 of 28 Figure(s) A2 A1 A a α β B1 A3 A4 A5 c γ b B4 B3 B2 B5 C1 C3 C2 C4 us D3 D4 D5 an D2 E2 E1 E3 E4 E5 F1 F G1 G H1 Ac ce pt e d E D1 M D C5 cr C ip t B F2 G2 H2 F4 F3 G3 F5 G4 H3 G5 H4 H5 H Figure The models of sharp polyhedral shapes and morphologies of α-Fe2O3 oxide particles are experimentally investigated in our research The large polyhedral α-Fe2O3 oxide crystal models are proposed in the illustrations with the experimental results of SEM images in Figure (A-H) Page 23 of 28 Figure(s) A1 A2 A3 A4 A5 B B1 B2 B3 B4 B5 C C1 C2 C3 C4 D D1 D2 D3 ip t A us cr C5 D4 E1 E2 F1 F2 G G1 H1 H d F3 Ac ce pt e F E3 M E an S1 D5 S1 E4 E5 F4 F5 S1 G2 G3 G4 G5 H2 H3 H4 H5 Figure (A-H) The evolution of α-Fe2O3 microstructure from their re-nucleation and re-crystallization through particle heat treatment in a mixture of H2/air at high temperature (around 900 °C) for h Scale bars: D3: 100 nm; A, B, C (1-5), D(1,2,4,5), E, F, G, H (1-5): µm Page 24 of 28 Figure(s) A A1 A2 A3 A4 A5 B B1 B2 B3 B4 B5 S3 C1 C2 C3 D2 D3 S1 C4 D4 Small grain E M S1 E1 D5 an D1 us S1 D C5 cr C S2 S1 ip t S2 E2  Ac ce pt e d Large grain  α-Fe O Un-sharp curvature boundary and surface of the α-Fe2O3 grains Figure (A-D) The evolution of α-Fe2O3 microstructure in critical heat treatment at high temperature (around 900 °C) for h (E): E1 Model of new 3D nanostructure of large α-Fe2O3 nanoparticle with the small grains and the large grains for Figure 4C(C1) E2 Model of new grains on surface of large α-Fe2O3 nanoparticle for Figure 4E(D5) Scale bars: B5, C3-5, D5: 100 nm; A (1-5), B(1-4), C(1-4), D(1,2): µm Page 25 of 28 Figure(s)  C1  α-Fe2O3           an  us  d  Ac ce pt e α-Fe2O3  M  (a)  10 20  30  B1 α-Fe2O3        cr (b)  ip t (c) 40        A1     50  60        70 80  90 2/degree Figure XRD patters of the samples of the as-prepared α-Fe2O3 particles (a) Dried Sample, (b) Sample annealed at 500 C for h, and (c) Sample annealed at 900 C for h Insets: (a) SEM images of large α-Fe2O3 nanoparticles with drying; (b) SEM images of large α-Fe2O3 particles with nanoparticle heat treatment; (c) SEM images of large α-Fe2O3 particles: The very stability and durability of the α-Fe2O3 nanostructure with the grains and boundaries were confirmed during drying or particle heat treatment Scale bars: (a) μm; (b) 100 μm; (c) 10 μm The insets (A1, B1, and C1) show SEM images of the large particles corresponding to Sample (Dried Sample), Sample (500 C), and Sample (900 C) Page 26 of 28 Figure(s) (a) Sample Sample Sample ip t M(emu/g) -1 cr -2 -3 5000 10000 15000 (b) 1.0 Sample 0.8 HC 0.2 Ac ce pt e 0.0 Sample d 0.4 M MR 0.6 M(emu/g) an H(Oe) us -15000 -10000 -5000 -0.2 Sample -0.4 -0.6 -0.8 -1.0 -600 -400 -200 200 400 600 H(Oe) Figure Hysteresis loops M-H of magnetic α-Fe2O3 microparticles at room temperature Page 27 of 28 Graphical abstract e f g i j k d cr c l Ac ce pt e d M h us b an a ip t The SEM images of new large polyhedral three dimensional (3D) α-Fe2O3 particles with grain and boundary configuration Page 28 of 28 .. .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... deformation 12 Page 12 of 28 including plastic and elastic deformation from nano- to micro-scale The heat treatment caused the structure deformation or particle deformation in the formation of. .. particles with grain and boundary are us found at 900 C an  Methods of particle heat treatment are proposed for α-Fe2O3 with grain and boundary M  There are surface and structure deformations of