Controlled Synthesis and Magnetic Properties of

Controlled Synthesis and Magnetic Properties of Uniform Hierarchical Polyhedral α-Fe2O3 Particles Nguyen Viet Long, Yong Yang, Cao Minh Thi, Le Hong Phuc & Masayuki Nogami Journal of Electronic Materials ISSN 0361-5235 Journal of Elec Materi DOI 10.1007/s11664-017-5360-9

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DOI: 10.1007/s11664-017-5360-9 Ó 2017 The Minerals, Metals & Materials Society

Controlled Synthesis and Magnetic Properties of Uniform Hierarchical Polyhedral a-Fe2O3 Particles NGUYEN VIET LONG,1,2,6 YONG YANG,1,2 CAO MINH THI,4 LE HONG PHUC,5 and MASAYUKI NOGAMI1,2,3 1.—Ceramics and Biomaterials Research Group, Ton Duc Thang University, 19 Nguyen Huu Tho Str., Tan Phong Ward, District 7, Ho Chi Minh City, Vietnam. 2.—Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam. 3.—Toyota Physical and Chemical Research Institute, Nagakute, Aichi, Japan. 4.—Ho Chi Minh City University of Technology, 475A Dien Bien Phu Street, Binh Thanh District, Ho Chi Minh City, Vietnam. 5.—Department for Novel and Nanostructure Materials, Ho Chi Minh City Institute of Physics, Academy of Science and Technology, 01 Mac Dinh Chi St., Dist 01, Ho Chi Minh City, Vietnam. 6.—e-mail: [email protected]

The controlled synthesis of uniform hierarchical polyhedral iron (Fe) micro-/nanoscale oxide particles with the a-Fe2O3 structure is presented. The hierarchical polyhedral iron oxide particles were synthesized by modified polyol methods with sodium borohydride as a powerful and efficient reducing agent. A critical heat treatment process used during the synthesis allowed for the interesting formation of a-Fe2O3 hematite with a micro-/nanoscale structure. The structure and weak ferromagnetism of the a-Fe2O3 particles were investigated by x-ray diffraction with whole pattern fitting and Rietveld refinement, scanning electron microscopy, and by vibrating sample magnetometry. The as-prepared a-Fe2O3 particles and the three dimensional models presented have promising practical applications for energy storage and conversion in batteries, capacitors, and fuel cells, and related spintronic devices and technologies. Key words: a-Fe2O3, hematite, grains and grain boundaries, hierarchical particles

M MR MS H Hc

Abbreviations Magnetization (emu g 1) Remanence or remanent magnetization (emu g 1) Magnetic saturation (emu g 1) Magnetic field strength (Oe) Coercivity (Oe)

INTRODUCTION Thus far, iron oxides have shown very stable oxidation states because of oxygen coordination in their crystal structure along with ferromagnetic properties. Fe2O3 powders are commonly prepared

(Received October 9, 2016; accepted February 2, 2017)

with a, c, b, and/or e phases.1–3 Among them, octahedral a-Fe2O3 has proven to be a very stable and durable crystal structure in air and various gas environments. Recently, a-Fe2O3 oxide powders with micro-/nanoscale structures have shown very promising potential in a wide range of applications such as gas sensing, medicine (nanomedicine), spintronics and spintronic devices, and energy storage and conversion processes because of inexpensive cost, low toxicity, and very good optical, electrical, and magnetic properties.1–7 For example, a-Fe2O3micro-/nanoscale materials have been employed as a negative electrode for high-performance supercapacitors. It is known that a-Fe2O3 micro-/nanoscale materials can be synthesized by various methods and processes such as hydrothermal and solvothermal methods, electrodeposition, green synthesis methods, sol–gel chemistry, and precipitation methods, etc.8–15 The formation of large a-Fe2O3 particles by a modified polyol method

Author's personal copy Long, Yang, Thi, Phuc, and Nogami

with the use of controlling agents such as KOH, NH4OH, NaI, NaCl, NaBH4, and NaOH with a heat treatment at high temperature has been successfully presented in references.8–15 In addition, through low-temperature sol–gel synthesis, the formation of an Fe oxide crystal structure and a change between a-Fe2O3, c-Fe2O3, and Fe3O4 were identified in Refs. 16, 21, and 22. However, at present, the research results of uniform, large aFe2O3 particles with grain and grain boundaries are relatively rare, since special annealing processes with critical conditions of time and temperature are needed. In general, Fe oxide products are formed by the combination of FeO, a-Fe2O3, b-Fe2O3, and FexOy phases. The crystal phase of the pure Fe particles can be changed into that of FexOy particles via heat treatment. Researchers have shown that the preparation procedure of the pure phase has proven to be very challenging due to the mixture of minor crystal phases in the main phase. Therefore, heat treatment processes on a-Fe2O3 particles or hematite powders have been the focus of scientists and researchers. To gain the properties for their practical application, Fe oxide micro-/nanoscale powders can be annealed in air or in various gases like N2, and H2, in order to make doped Fe oxide and alloy structures, with the possibility to create properties such as high strength and durability. Though heat treatment, hierarchical 3D a-Fe2O3 oxide particles have some unique and technologically promising properties. In this work, our present results of the hierarchical three-dimensional (3D) aFe2O3 with grain and boundary structures will contribute significant opportunities for improving Pt/oxide catalyst materials for fuel cells (FCs), and batteries, and possibly enhancing the performance of gas sensors. Similarly, a-, b-, c-Fe2O3-, Fe3O4-, AB2O4-, and ABO3-based technologies, and especially AB2O4-based ferrites technology, expand research on magnet materials and applications.12–15 Thus, fine magnetic particle powders, and grains and grain boundary structures need to be intensively studied in order to understand magnetic domains and magnetic walls. In this trend, our present studies are involved in the controlled synthesis and characterization of various metal and oxide particles with micro-/nanoscale structures associated with the related practical applications. Accordingly, metal and magnetic Fe-based oxide particles with micro-/nanoscale structures with grains and grain boundary textures have large potential applications for our health, life, engineering, academic research, science and technology, magnetic recording, speakers, electronic devices, motors, power generation, equipment, gas sensors, etc., and for dealing with the problems and challenges of the energy crisis and pollution of the environment.1,2 In this work, we have successfully developed a chemical polyol process to synthesize Fe-based particles by heat treatment. Based on our results,

the models of grains and grain boundaries are provided. In addition, the role of heat treatment in the preparation of hierarchical polyhedral micro-/nanoscale a-Fe2O3 particle powder products is investigated with attention paid to the various aspects of particle deformation, such as the deformation of size, shape, morphology, surface, internal structure, composition, and other crystal parameters during sintering and final densification. The durability, stability, and strength of the uniform polyhedral particles in the size range of 10 lm after high-temperature heat treatment are all significantly enhanced. These important improvements and modification processes are the keys for discovering applied properties of hierarchical polyhedral a-Fe2O3 particles. EXPERIMENTAL Synthesis of Hierarchical a-Fe2O3 Microparticles Figure 1 shows the controlled synthesis of hierarchical polyhedral a-Fe2O3 particles. The starting precursors were prepared as described in previous detailed works for a-Fe2O3 oxide particles. In general, much attention and time was paid to developing increasingly stringent preparation processes. In

Fig. 1. Synthesis of hierarchical polyhedral a-Fe2O3 microparticles. (a) Chemicals, (b) polyol methods with alcohols or polyols, (c) modified polyol methods with ethylene glycol, NaBH4 and NaOH, (d) wet particle powder products, (e) cleaning, drying, and heat treatment and (f) hierarchical particle products.

Author's personal copy Controlled Synthesis and Magnetic Properties of Uniform Hierarchical Polyhedral a-Fe2O3 Particles

a typical process, 10 mL of EG, 3 mL of 0.0625 M FeCl3 (FeCl3.6H2O precursor), 10 mL of 0.375 M Polyvinylpyrrolidone (PVP), and 0.048 g NaBH4 were used for synthesis. The stock solutions of Fe precursors were pumped into the center of a reaction flask (250 mL) while stirring for the precisely

controlled synthesis. The precursor FeCl2 is used for replacing FeCl3 or both FeCl3 and FeCl2 for synthesis. The reaction periods for the synthesis of PVP-Fe particles via the fast reduction of precursors by NaBH4 was 45 min. Then, the PVP-particles were formed in a resulting black solution. The product was kept at room temperature for a few days to obtain the black products at the bottom. The clean black products were obtained by removing PVP on the surfaces of the as-prepared particles according to centrifugation processes, washing, cleaning procedures, and heat treatment processes. They were kept in a container in the form of powder. Then, the dried particle powders were re-dispersed into ethanol and dried at 60°C. To obtain the blackbrown oxide products of a-Fe2O3 particles, the black powders were isothermally heated at 900°C for 1 h in ceramic containers in air, and other gas elements, such as N2 and H2 gases. Characterization A series of various a-Fe2O3 particle powder samples were prepared for x-ray diffraction (XRD), scanning electron microscope (SEM), and a vibrating sample magnetometer (VSM) investigation and analysis. The crystal structure was found from the x-ray diffraction patterns of the a-Fe2O3 particles recorded in a 2h range of 5°–95° by a Rigaku-D/max 2550 V (40 kV/200 mA, CuKa radiation at ˚ ). The features of size, shape, and mor1.54056 A phology were investigated by field emission (FE)SEM (JEOL-JSM-634OF) operated at 5, 10, and 15 kV (5–15 kV) with a probe current around 12 lA (1–12 lA). The VSM (Model EV11), which was used for analyzing the magnetic characteristics of a-Fe2O3 micro-/nanoscale materials, was operated at room temperature (293 K) in a wide range of applied fields from 20 kOe to 20 kOe with noise levels between 0.1 lemu and 0.5 lemu. RESULTS AND DISCUSSION

Fig. 2. (a) XRD patterns of the as-synthesized polyhedral a-Fe2O3 particles and (b) crystal cell of a-Fe2O3 (VESTA Software).

Figure 2 shows the result of XRD of the asprepared polyhedral a-Fe2O3 particles with particle

Table I. 9/11 Peaks to index to a-Fe2O3 2h (°)

hkl

2h (c)

d (c)

d (o)

I (%)

24.062 33.061 35.540 40.763 49.380 53.997 57.500 62.341 63.900 71.902 75.376

012 104 110 113 024 116 018 214 300 1010 220

24.111 33.105 35.575 40.796 49.381 53.978 57.499 62.330 63.892 71.817 75.319

3.6881 2.7037 2.5215 2.2100 1.8440 1.6973 1.6015 1.4885 1.4558 1.3134 1.2607

3.6954 2.7072 2.5239 2.2118 1.8441 1.6968 1.6015 1.4882 1.4556 1.3120 1.2599

31.3 100.0 74.5 26.2 48.3 60.5 10.9 39.8 42.5 13.6 11.6


Fig. 3. Whole pattern fitting and Rietveld refinement of XRD of hierarchical polyhedral a-Fe2O3 particles.

size of 10 llm (for hematite, a-Fe2O3 according to ICDD/JCPDS-PDF#33-0664). Figure 2 shows the most typical peaks are characterized by the set of Miller indices of (012), (104), (110), (113), (024), (116), (122) or (018), (214), (300), (208), (1010), and (220). The corresponding values of 2h (°) are estimated in Table I. The XRD data indicate that the aFe2O3 oxide phase was formed with high crystallization. The standard pattern is PDF#33-0664 for ˚, a-Fe2O3 with the 45 lines, CuKa1, k = 1.5406 A 2h = 24.138–147.961 (°). This is hexagonal crystal system, space group R-3C(167) with 8 lines to be strong. The strongest intensity is the (104) line. The lattice parameters are a = 5.042, b = 5.042, and ˚ , and a = 90°, b = 90°, and c = 120° with c = 13.773 A the 11 lines confirmed. The strongest line is the (104) line that was characterized in the crystal phase of a-Fe2O3 crystal structure derived from Database code 0017806 (Fig. 2b).17 It is noted that the phase identification was confirmed in a-Fe2O3 by pattern indexing with MDI Jade software (Table I) and by standard pattern PDF#33-0664. In the reflections from the lattice constants, the values of the d–I parameters are shown in Table I. Here, the values of 2h and d are the observed values of 2h (o) and d (o) as well as the calculated values of 2h (c) and d (c), respectively. In the phase identification, the strongest line was revealed to be from the reflections of the (104) planes. Figure 3 shows the whole pattern fitting and Rietveld refinement of the XRD pattern of the hierarchical polyhedral a-Fe2O3 particles. Several R-indices were also determined at iteration times,18 i.e. Round 4, R = 61.64%. Here, the profile shape function for all the phases is pseudo-Voigt with a fixed background. There was only the crystal phase ˚ , c = 13.73709 A ˚ , c/ for a-Fe2O3, a = b = 5.03503 A a = 2.72830, a = b = 90°, c = 120°. The value c/a of the hierarchical a-Fe2O3 particles is smaller than

Fig. 4. (a) SEM of hierarchical polyhedral a-Fe2O3 particles with grains and grain boundaries. (b) Enlargement of the marked local area of (a).

that of a standard sample. The XRD data of the pure a-Fe2O3 did not show the existence of the minor phases of various Fe2O3 structures. Figure 4 illustrates typical SEM images of aFe2O3 (10 lm) oxide particles after high heat treatment at 900°C h in air for 1 h, with visible grains and grain boundaries. The most typical two particles were selected for calculation. It is suggested that they show deformations of size, shape and morphology.8–10 The two samples of a-Fe2O3 particles show grains and grain boundaries inside various porous structures. Here, the products of the aFe2O3 oxide particles were regarded as hierarchical micro-/nanostructured oxide materials. Figure 5 also shows the most typical polyhedral models of hierarchical a-Fe2O3 particles with grains and grain boundaries in our experimental results of SEM observation and investigation. The main roles of chemicals, pH, synthetic processes, reaction time and temperature during synthesis, annealing and heating processes with time at high temperature,


Fig. 5. (a)–(c) High-resolution SEM of hierarchical polyhedral a-Fe2O3 particles with grains and grain boundaries. (d) Snapshot of the marked area of (c) for grains and grain boundary growth.

etc., have been presented in previous works with good results of the pure hierarchical a-Fe2O3 particles with grains and grain boundaries.8,9,11–13 Figures 6 and 7 show the models of hierarchical polyhedral a-Fe2O3 particles with grains and grain boundaries. There is one a-Fe2O3 oxide particle (models A1–A10) with its assumed particle size about 1 lm or 1000 nm (model A1), and a-Fe2O3 oxide particle (model A10) about 10 lm for low temperature ranges in in situ experimental observations (Fig. 6), and for high temperature in in situ theoretical observation via modeling and simulation (Fig. 7). The assumed oxide particle models A1–A10 in respect with the sizes of hierarchical magnetic oxide systems to be 1 lm (1000 nm), 2 lm, 3 lm, 4 lm, 5 lm, 6 lm, 7 lm, 8 lm, 9 lm, and 10 lm (10,000 nm), respectively, are relative uniform in the sizes, shapes, and morphologies. It is possible that the number of grains could number from hundreds to thousands. The results suggest that large and small grains tend to be highly concave and convex in becoming larger or smaller in their forms. The particle sizes of the grains were identified at both nano- and microscale. However, the largest particles in the micron range can be shown in the experimental SEM in Figs. 4 and 5 at low temperature in SEM measurement conditions in vacuum. The abnormal grain growth shown in Fig. 5 shows a large grain size of about 1 lm (or 1000 nm) and

much smaller grains of about 0.1 lm or 100 nm. The rough surfaces with the grains indicate heavy particle deformation, such as the formation of voids or blanks on the surfaces or inside the prepared particles after heat treatment in Fig. 5d. The color of the grains and grain boundaries of the oxide particles in Fig. 7 shows the possibilities and positions of the development of grain growth in the case of high-temperature heat treatment. The formation and disappearance of the grains at the fixed high temperatures are simulated and modeled by phase field models.19 Further, the evidence of the formation of grains and grain boundaries in the hierarchical 3D a-Fe2O3 micro-/nanoscale oxide particles is presented. This formation can be possible in various materials, such as metals, alloys, oxides, and ceramics micro-/nanoscale materials with grains and grain boundaries. Here, the main focus should be on the important roles of the atomic arrangement in the nanoscale range of hierarchical metals, alloys, ceramics and oxide particles at nanoscale (1–30 nm) and related properties for practical applications as well as the important roles of the grains and grain boundary arrangement (less than 1 lm, and 1–10 lm) at microscale and related properties with the heat treatment methods and processes. The measurement parameters including magnetization (M), magnetic field strength (H), remanent


Fig. 6. Proposed models of hierarchical polyhedral a-Fe2O3 particles with grains and grain boundaries according to our SEM evidence. Scale bars for the proposed particle sizes: A1 1 lm, A2 2 lm, A3 3 lm, A4 4 lm, A5 5 lm, A6 lm, A7 lm, A8 lm, A9 lm, and A10 10 lm, respectively (Designed by N.V. Long).

magnetization (MR), magnetic saturation (MS), coercivity (HC), M at maximum magnetic field strength (Hmax), and squareness (S = MR/MS), etc., were calculated and analyzed from the magnetic hysteresis loops. Figure 8 shows the hysteresis loops of aFe2O3 taken to magnetization saturation. The grains of 3D hierarchical magnetic particles can be considered as magnetic domains which were magnetized to their MS. The adjacent domains are separated by domain boundaries or ‘‘magnetic walls’’, across which the direction of magnetization changes under the external field. With the increasing field, the spin alignment and rotation processes occur.

The hysteresis loop of the a-Fe2O3 particles indicated the magnetic parameters of the upward part, downward part, and average value, respectively. The typical magnetic parameters of hysteresis of the a-Fe2O3 materials by the VSM method are listed in Table II. We have found a-Fe2O3 oxide particles have a very low magnetization saturation (MS), about 3 emu/g, exhibiting the very weak ferromagnetism. It is known that a-Fe2O3 has antiferromagnetism.20 Previous works have shown the roomtemperature hysteresis measurement of a-Fe2O3 with a size less than 1000 nm has a high coercivity but small MS, exhibiting weak ferromagnetic behavior or parasitic ferromagnetism20,21 but most a-


Fig. 7. Proposed models of hierarchical polyhedral a-Fe2O3 particles with grains and grain boundaries according to theoretical and experimental evidence. Scale bars for the proposed particle sizes: A1 1 lm, A2 2 lm, A3 3 lm, A4 4 lm, A5 5 lm, A6 lm, A7 lm, A8 lm, A9 lm, and A10 10 lm, respectively (Designed by N.V. Long).

Fe2O3 typically exhibit weak ferromagnetism at room temperature, with a MS value smaller than 1 emu/g.22 In contrast, the polyhedral a-Fe2O3 particles essentially have low magnetization saturation (MS) exhibiting weak ferromagnetism; a trend changing from the ferrimagnetism to the superparamagnetism in comparison with the superparamagnetic alloys and oxide nanoparticles with very small sizes of less than 10 and 100 nm investigated in the past. The very weak ferromagnetism or the weak ferrimagnetism of hierarchical a-Fe2O3 particles has been identified. Based on the obtained results, it is suggested that particle heat treatment is very important to harden the microstructure of the aFe2O3 oxide micro-/nanoscale materials in final crystal growth and formation after heating at

900°C, with heavy particle deformation of the size, shape, surface, and inner structure. Therefore, the trend of weak ferrimagnetism was identified in order to achieve better quality, high durability and stability of the crystal structure of the hierarchical Fe-, Co-, and Ni-based oxide particles.21 CONCLUSION In this study, hierarchical a-Fe2O3 micro-/nanoscale particles with grains and grain boundaries were made by an efficient process via a modified polyol method with NaBH4 and an innovative and sophisticated heat treatment at 900°C for 1 h in order to obtain adequate material solidification under heating. Here, a-Fe2O3 structures with high


ment (NAFOSTED) under Code 103.02-2016.92. This research is supported and funded by Ceramics and Biomaterials Research Group, Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam. N.V. Long is thankful to Prof. Thomas Nann for his open and positive discussion to this paper. N.V. Long is thankful to Prof. Satoshi Hirosawa at Elements Strategy Initiative Center for Magnetic Materials, RCMSM, National Institute for Materials Science (NIMS) for his research direction. He is very grateful for the precious supports of machines and measurements through Fellowship and Postdoctoral programs for Researchers from Developing Countries from Shanghai Institute of Ceramics, Chinese Academy of Science from China, and Nagoya Institute of Technology, Kyushu University, and Kyoto University from Japan.

REFERENCES

Fig. 8. (a) Magnetism of hierarchical polyhedral a-Fe2O3 particles. (b) Determination of remanence and coercivity.

Table II. Hysteresis parameters Hysteresis loop

Up

HC (Oe) MS (emu/g) M at Hmax (emu/g) MR: Remanence (emu/g) HS (Oe)

103.014 2.668 2.668 0.549 13,733.85

Down 103.014 2.696 2.696 0.549 9998.00

Average 103.014 2.682 2.682 0.549 11,865.93

crystallization levels were confirmed by XRD and whole pattern fitting with the Rietveld refinement. The a-Fe2O3 microparticles show weak ferromagnetism or the trend to become weakly ferrimagnetic at room temperature. The goal of this research is to study, understand, and carry out the controlled synthesis of hierarchical micro-/nanoscale Fe-based oxide particles with large sizes and with grains and grain boundaries in order to contribute to the fields of energy conversion and storage, fuel cells, batteries, capacitors, gas sensors, promising magnetic applications and devices. ACKNOWLEDGEMENTS This research is funded by the Vietnam National Foundation for Science and Technology Develop-

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