Controlled synthesis and characterization of iron oxide micro-particles ...

Controlled synthesis and characterization of iron oxide micro-particles for Fe-air battery electrode material Nguyen Viet Long, Yong Yang, Cao Minh Thi, Bui Thi Hang, Yanqin Cao & Masayuki Nogami Colloid and Polymer Science Kolloid-Zeitschrift und Zeitschrift für Polymere ISSN 0303-402X Volume 293 Number 1 Colloid Polym Sci (2015) 293:49-63 DOI 10.1007/s00396-014-3363-0

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Author's personal copy Colloid Polym Sci (2015) 293:49–63 DOI 10.1007/s00396-014-3363-0

ORIGINAL CONTRIBUTION

Controlled synthesis and characterization of iron oxide micro-particles for Fe-air battery electrode material Nguyen Viet Long & Yong Yang & Cao Minh Thi & Bui Thi Hang & Yanqin Cao & Masayuki Nogami

Received: 11 June 2014 / Revised: 20 July 2014 / Accepted: 30 July 2014 / Published online: 12 September 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract In this research, novel homogeneous iron (Fe) oxide particles with the pure α-Fe2O3 structure are successfully synthesized with controlling and shaping via a modified polyol method with NaBH4 as an efficient reducing agent according to drying and heat treatment processes. In the critical synthetic and experimental conditions, large α-Fe2O3 particles exhibited homogeneously large sizes in the certain ranges of 1-5 μm and 1-10 μm, which are regarded as a discovery of controlled and shaped synthesis. The electrochemical measurements indicated that oxide powders containing as-prepared pure αFe2O3 microparticles were successfully used in the electrodes. Accordingly, the cyclic voltammetry (CV) and galvanostatic cycling measurements indicated their potential applications for next-generation Fe-air battery technology in comparison with commercial oxide products of Fe2O3 particles. Finally, we suggest that the sharp polyhedral shape and morphology of the engineered micro-particles, such as metal, alloy, and oxide

micro-particles, are of importance because they have very high stability and durability with respect to their applied properties. Keywords Colloidal oxide . Surfaces . Crystal structure . Fe2O3 oxides . Battery

Introduction At present, Fe (iron) is one of the most important metals for its use in various modern technologies. For example, Fe-based inorganic particles can be used for the electrodes in batteries and energy storage and conversion [1, 2]. In the controlled synthesis of inorganic particles, the roles of both the size and the shape are crucial to potential applications in catalysis and biomedicine [3], such as surface-enhanced Raman scattering (SER) as well as various undiscovered potential applications

N. V. Long (*) : Y. Yang : Y. Cao State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, 1295, Dingxi Road, Shanghai 200050, China e-mail: [email protected]

N. V. Long Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka 861-8580, Japan

N. V. Long e-mail: [email protected]

N. V. Long : C. M. Thi Ho Chi Minh City University of Technology, 144/24 Dien Bien Phu, Ward 25Binh Thach Ho Chi Minh City, Vietnam

N. V. Long e-mail: [email protected] Y. Yang e-mail: [email protected] N. V. Long Posts and Telecommunications Institute of Technology, km 10 Nguyen Trai, Hanoi, Vietnam N. V. Long Laboratory for Nanotechnology, Ho Chi Minh Vietnam National University, Linh TrungThu Duc Ho Chi Minh, Vietnam

B. T. Hang International Training Institute for Materials Science, Hanoi University of Science and Technology, No. 1 Dai Co Viet Str., Hanoi, Vietnam e-mail: [email protected] M. Nogami Toyota Physical and Chemical Research Institute, Toyota Motor Corporation, 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan e-mail: [email protected]

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[4]. Among magnetic particles [5], Fe-based particles are considered ideally suitable for use in nano-medicine because of their very high biocompatibility. Additionally, Fe-based particles have great biological and environmental applications. In much of the research on biomedical applications, Fe metal and Fe oxidebased particles have been used in drug delivery, high contrast agents, targeted imaging and therapy, therapeutic hyperthermia, and magnetic resonance imaging (MRI) [6–14]. So far, various Fe-based nano-structures have been intensively investigated and developed for a better life [15]. In a facile method, the controlled synthesis of Fe oxide particles was carried out under solvothermal conditions using a mixture of K3[Fe(CN)6], N2H4 solution, and sodium carboxymethyl cellulose at 160 °C for 6 h for the formations of two kinds of Fe oxide crystals in the hexagonal crystal system. The system of Fe-based particles includes tetrakaidecahedra and oblique parallelepipeds with high-index facets with a size range from 200 to 400 nm [16], and other size ranges as common concepts of examples of particles and related phenomena in the range from 1 nm to 10 μm [17–27]. So far, the facile synthesis of Fe-based nanostructures has been presented in many important works concerning energy and environment [18–23]. The more facile synthesis methods to the products of Fe-based particles with controlled sizes and shapes have been confirmed [24–26]. In particular, MnFe2O4 particles/C catalysts were found as electrocatalysts for oxygen reduction reaction (ORR) in 0.1-M KOH solution. It has a similar ORR to commercial Pt/C catalyst in 0.1 M KOH. It will be a new category of catalyst with a potential use in replacing expensive precious metals for ORR in alkaline media [28]. To date, the Fe oxide particles with various specific structures include α-Fe2O3, γ-Fe2O3, and Fe3O4, or the mixture of the above specific structures [29–38]. Indeed, there are various controlled synthesis methods of Fe-based oxide nano-structures for practical applications [8–14, 39–45]. So far, a great deal of the increasing efforts has been carried out to synthesize oxide nano-structures via various chemical synthetic methods, such as polyol methods [46, 47]. In recent years, there has been much progress in the synthesis of nano-sized metals, alloys, and oxides for energy applications [48–51], as well as the known various oxide-based nano-structures [52–81]. In the characterization of particle size, there are three size ranges of Fe-based particles including the nano-sized range less than 100 nm (1–100 nm), less than 1,000 nm (100–1,000 nm), and less than 10,000 nm (1,000–10,000 nm or 1–10 μm). In nature, a large metal particle has many good advantages, such as higher strength, higher hardness, higher durability, and stability in its structure than a small metal particle. Thus, the very large Febased oxide particles are urgently needed for the above applications. Until now, no physical and chemical synthesis technology (polyol methods) has been well developed for making the large and polyhedral Fe-based particles in the range of 1,000 to 10,000 nm and for studying and distinguishing between the

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two most important size ranges in both the micro-size range and the nano-size range with respect to their properties and applications. The chemically shape-controlled synthesis of the small, homogeneous, polyhedral, or spherical Fe metal and oxide particles in the range of 10 nm is facile and feasible [5]. However, the chemically shape-controlled synthesis of the large, relatively homogeneous, and polyhedral Fe metal and oxide particles is still extremely difficult, which is of fascination to scientists and researchers. Therefore, synthesis technology for producing a homogeneous nano-system with large particles (the idea of shaping particle systems in all the micro-size range) is of necessity in comparison with a homogeneous Fe nano-system with very small particles (the idea of shaping particle systems in all the nano-size range). Moreover, the comparison of applied properties between the various nano- and micro-size ranges is of importance in new discoveries of their practical applications in catalysis, energy, and biomedicine [1, 2, 8–14]. In addition, oxide-based particles are of very attraction to the scientists because their preparation was carried out by the facile polyol methods with cheap raw materials and precursors at low cost, but they exhibit a wide range of applications with excellent advantages. In our research, a modified polyol method with NaBH4 and ethylene glycol (EG) is adopted to control the large and polyhedral α-Fe2O3 particle powder with very good characterization of size, shape, and morphology. In addition, the oxide powders of α-Fe2O3 polyhedral micro-particles with large size ranges of 1–5 μm were successfully used in the electrodes for testing in batteries. For those applications, the results have led to the use of the large and polyhedral α-Fe2O3 particles in batteries for energy storage and conversion. In our present results, we have presented a new phenomenon of the particle size enlargement of Fe-based particles with sharp and flat surfaces, sharp shapes, and crystal morphologies by modified polyol method. To make use of modified chemical polyol methods, this research also gives a new general method or idea of shaping metal, alloy, and oxide-based particle products under controlled size and shape, ranging in size from microto nano-particles that can be fully exploited in materials for batteries.

Experimental section Synthesis Chemical In our chemical synthesis of α-Fe2O3 oxide particles, we have used industrial chemicals from Aldrich or Sigma-Aldrich, Ishida, or Wako. They are poly(vinylpyrrolidone) (PVP) (FW=55,000) as efficient protective polymer agent (Aldrich),

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FeCl3·4H2O, and other Fe precursors (Aldrich, No. 451649) (Aldrich, No. 236489), typical chemical kinds according to the specifications of research of the American Chemical Society (ACS). In particular, sodium borohydride (NaBH4) was empirically used as a strong and efficient reducing agent for controlled synthesis of α-Fe2O3 oxide particles, ethylene glycol (EG, Aldrich) as both solvent and weak reducing agent from Aldrich, ethanol, acetone, and hexane (Aldrich or Japanese companies). Here, chemicals used were of analytical standard grade and were used without any further purification. The containers hold various kinds of ionized and distilled water with high purity prepared by Millipore purification system available in our laboratory. They were used for washing and cleaning throughout experimental synthesis processes.

In all the processes, our results of particle products indicated that large α-Fe2O3 oxide particles or micro-particles have the particle size of 1–5 μm for 30 min (sample 1), the particle sizes of 1–10 μm for 35 min (sample 2), 1–5 μm for 30 min (sample 3), and 1–5 μm for 30 min (samples like sample 1). The large particles exhibited sharp and polyhedral shape and morphology with large interesting crystal surfaces. The synthesis processes for samples 1–3 were very stable and successful. In our experiments, we have recognized that our particle products can be kept in glass bottles for long time. In particular, the above large particle products with the homogeneous characterization of surface, shape, morphology, and size are of appeal, interest, and importance to the scientist’s present research.

Synthesis of α-Fe2O3 oxide particles

Material characterization

In a typical process of the controlled synthesis of large αFe2O3 oxide particles, 3 mL of EG, 1.5 mL of 0.0625 M FeCl3 from one typical FeCl3·4H2O precursor, 3 mL of 0.375 M PVP, and 0.028 g NaBH4 were used for making sample 1. Previously, the method of pumping the stock solution of precursors according to their volume (μL) was used for synthesis [48]. Similarly, we have used the same processes as sample 1 for the many same samples for X-ray diffraction (XRD) (powder samples), energy-dispersive X-ray spectroscopy (EDS), SEM analysis and investigation, and cyclic voltammetry (CV) measurement (powder samples). In experimental conditions, it turns out that the details and steps of the successful process procedures were previously presented in many successful cases of polyhedral Pt nano-particles with the use of pumping the solutions of precursors carefully [48]. The modified method of fast pumping the stock solutions of precursors was very carefully used under magnetic stirring with high speed at 6,000 rpm. By this way, synthesis time was significantly reduced. In general, FeCl3 was completely reduced with NaBH4 with the extra content associated with EG solvent at high temperature in the range of about 200–230 °C and at about 30 min (sample 1). Another sample was prepared in the same process as sample 1 but synthesis time for 35 min (sample 2). Sample 3 was prepared in the same process as to sample 1 for the SEM measurements. More samples like sample 1 were also prepared in the same process for electrochemical measurements and SEM analysis. Sample 1 was used for UV–Vis measurements, analysis, and structural investigation by XRD as well as for use in the electrochemical measurements. We observed that the yellow color of the precursor solution containing all precursors was changed into the black color of the product solution containing the synthesized particles. As a result, the black solutions containing large and polyhedral α-Fe2O3 oxide particles with large sizes, shape, and morphologies were obtained as the final product.

UV–Vis–NIR spectroscopy and X-ray diffraction UV–Vis–NIR spectroscopy To study the reduction of FeCl3 precursor with NaBH4 in EG, we used 30 μL of the stock solution of 0.0625 M FeCl3 and 30 μL of the solution product containing PVP-protected Fe-based particles with 3 mL ethanol in the analysis of UV–Vis spectroscopy (Ubest 570 UV– Vis–near-infrared (NIR) spectrometer) in a range of wavelength of 200–1,100 nm for the investigation of the fast reduction of FeCl3 with NaBH4 as qualitative analysis of the formation of the particles. X-ray diffraction: determination of α-Fe2O3 structure In the quantitative XRD method for crystal determination and analysis, we have used the as-prepared products of the black solution containing the PVP-protected α-Fe2O3 oxide microparticles (sample 1). Then, PVP-protected Fe-based microparticles were washed many times by hexane, ethanol, and mixture of hexane/ethanol (1/3 in volume) in order to obtain pure clean α-Fe2O3 oxide micro-particles with complete removal of PVP by our standard procedures with the use of centrifuge at 10,000 rpm for 10 min (Kabuta 3740 model). To obtain samples containing oxide particle powder, the black solution of clean and pure α-Fe2O3 oxide micro-particles was dried in order to receive the dry α-Fe2O3 particle powder on the glass substrate for XRD analysis. For crystal analysis, the X-ray diffraction pattern was recorded by a diffractometer (X’Pert Phillips) at 45 kV/45 mA and using Cu Kα radiation (0.54056 nm). As a result, only crystal phase of α-Fe2O3 was found in the pure as-prepared Fe-based micro-particles. Electron microscopy: SEM and TEM In order to study the size and shape of the large as-prepared αFe2O3 oxide micro-particles (sample 1 and other samples), we

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have used field emission scanning electron microscope (FE-SEM) (JEOL-JSM-634OF) operated at 5, 10, and 15 kV (5–15 kV) and probe current in the range of around 12 μA. The SEM images of the as-prepared Fe-based micro-particles were exactly focused by suitably fine focus level and adjustment. To characterize the Fe-based oxide micro-particles with very large size ranges of 5 and 10 μm by SEM, copper grids containing the αFe2O3 oxide micro-particles were maintained under vacuum by using a vacuum cabinet (sample 1 or 2). In SEM method, energy-dispersive X-ray spectroscopy (EDS) acquisition and element analysis can be processed by Voyager software and Voyager environment for Spectral Display with Snapshot V3.5.1 program. Image data was set up to the downloaded EDS spectra from Spectral Voyager unit through EFFTP program. In TEM method, the copper grids were maintained over night under high vacuum conditions in transmission electron microscope (JEOL JEM-2100F or JEM-2010) prior to the TEM measurements. The TEM images were obtained using a transmission electron microscope (JEOL JEM-2100F and JEM2010) operated at 200 kV. Finally, Digital Micrograph versatile software (Gatan, Inc.) was used in TEM and HRTEM studies to acquire, visualize, analyze, and process the digital image data of α-Fe2O3 oxide microparticles with large size and shape. However, because the particle size of as-prepared α-Fe2O3 oxide microparticles is very large in a size range of 10 μm, it is hard to control and adjust high resolution TEM. Therefore, SEM method can be efficiently utilized in surface analysis in all samples. Electrochemical measurements To determine the electrochemical behavior of as-prepared α-Fe 2 O 3 , we prepared an electrode sheet by mixing 45 wt% AB, 45 wt% α-Fe2 O3 powder, and 10 wt% polytetrafluoroethylene (PTFE; Daikin Co.) followed by rolling. Each electrode was made into a 1-cm-dia. pellet. Cyclic voltammetry (CV) and charge/discharge measurements were carried out in three-electrode glass cells with α-Fe2O3/AB composite electrode as the working electrode, Pt mesh as the counter-electrode, and Hg/HgO as the reference electrode. The electrolyte was 8 mol dm−3 KOH aqueous solution. CV measurements were taken at a scan rate of 0.5 mV s−1 and within a range of −1.3 to −0.1 V. After the 15th redox cycle, the Fe/C composite electrodes were removed, washed with deionized water, dried, and observed by SEM and X-ray energy-dispersive spectroscopy (EDS). For the galvanostatic cycling performance measurements, in the charging course, we carried out a coulostatic process with a cutoff capacity of 1,007 mAh g−1 Fe2O3. The charge current density was

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5.0 mAcm-2. In the discharge course, a constant current density of 2.0mAcm−2 was carried out on the electrode with a cutoff potential of −0.1 V. In all electrochemical measurements, we used fresh electrodes without precycling.

Results Formation of the PVP-protected α-Fe2O3 oxide micro-particles Figure 1a shows the UV–Vis absorption spectrum of aliquot 1 (30 μL of Fe precursor in ethanol), showing three main peaks at 201.79, 232.36, and 318.78 nm. Here, UV–Vis spectrum clearly shows the very stable formation of Fe3+ complexes in ethanol. In our opinion, these typical peaks indicate the ligand-to-metal charge transfer absorption due to the complex formation of the Fe ions in the solution of EG, PVP polymer, and ethanol in the testing aliquot part with yellow color. However, the UV– Vis absorption spectrum of aliquot 2 (30 μL of PVPprotected α-Fe2O3 oxide micro-particles in ethanol for sample 1) also shows the only one peak at 211 nm. This indicated the experimental evidence of the final formation of PVP-protected α-Fe2O3 oxide micro-particles in the solution with the testing aliquot part with brown black color (Inset photo of Fig. 1a). Interestingly, the strong peak intensities of at 232.36 and 318.78 nm disappeared in the UV–Vis absorption spectrum because of the complete reduction of FeCl3 with NaBH4 in EG. In this way, this is a rough analysis and measure of the reduction of Fe precursors, leading to the formation of the various Febased nano-structures. Therefore, the qualitative results indicated a possibility of the final formation of polyhedral α-Fe2O3 oxide micro-particles of 1–5 μm (Fig. 2) in agreement with our confirmation of the XRD measurement of the pure α-Fe2O3 crystal phase and structure (Fig. 1b) in respect with EDS analysis in Fig. 3. It is clear that Fig. 1b shows the XRD pattern of a sample of the pure α-Fe2O3 oxide micro-particles with the good, fine, and large crystal formation of α-Fe2O3 (JCPDS 33–0664). In addition, the special color of our sample 1 (inset photo in Fig. 1a) is brown black in the same color that coincides with alpha (α)-Fe2O3 color product or Hematite, and other Fe oxide products [80, 85–89]. To meet the urgent huge needs of applications for Fe-based micro-particles in chemical controlled synthesis, likewise the Fe-based products prepared possess various colors identified such as goethite, lepidocrocite, akaganéite, hematite, magnetite, maghemite, ferrihydrite, feroxyhyte, schwertmannite, and haematite [80]. It is very important

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Fig. 1 a UV–Vis spectra of aliquot 1: 30 μL of FeCl2·4H2O precursor in EG (0.0625 M) and ethanol; aliquot 2: 30 μL of PVPprotected Fe-based microparticles and ethanol (sample 1). The black solution product with the as-prepared Fe-based microparticles. b XRD pattern of a sample of the Fe2O3 oxide microparticles (sample 1)

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that we should controllably synthesize Fe oxide microparticles with a pure crystal phase because Fe oxides have various structures such as various the pure ε, γ, β, and α crystal structures or hybrid crystal structures. In general, the common formation of Fe metal nano-particles occurs as follows [82]. 4Fe3þ þ 3BH4− þ 9H2 O ¼ 4Fe0 þ 3H2 BO3− þ 12Hþ þ 6H2 There is a large volume of H2 gas that was generated during the synthetic process according to the interesting the above reaction equation. In the full explanation of nucleation,

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growth, and formation, Fe particles are formed in the full reduction of FeCl3 with the extra amount of NaBH4 in EG solvent. Firstly, Fe nuclei were formed. Then, Fe clusters and nano-clusters were formed by the common mechanisms and processes of collision, self-attachment, self-aggregation, and self-assembly in solution. Next, the very small Fe nanoparticles were formed in the form of pure Fe. According to time (about 30 min), the final brown black solution containing the large and polyhedral Fe oxide micro-particles was taken out of the flask. The pH level was measured about 7–8 of black solution product containing the micro-particles. After that, the pure surfaces of the Fe nano-particles can possibly oxidized according to time in the formation of FeO; then, the formation of the thin Fe oxide shells attributes to the oxidation

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phenomena in solvent such as EG. Thus, the α-Fe2O3 oxide structures can be formed by the gradual and slow or fast oxidation in EG, ethanol, and other solvents in their containers, especially in drying process and heat treatment. In most general features, the structural transformations investigated among Fe (Iron), FexOy, such as FeO (Wüstite), Fe3O4 (Magnetite), γ-Fe2O3 (Maghemite), and α-Fe2O3 (Hematite) can quickly occur through heat treatment [82]. Figure 1b shows a typical XRD pattern of the prepared polyhedral and large α-Fe2O3 oxide micro-particles. Here, the pure α-Fe2O3 oxide micro-particles have the main rhombohedral structure. The most typical peaks were characterized by (012), (104), (110), (113), (024), (116), (018), (214), and (300), respectively, corresponding to the values of 2θ (degree) of about 24.31, Fig. 2 a, b SEM images of the uniform Fe oxide micro-particles or alpha (α)-Fe2O3 synthesized by modified polyol methods in the range of 5 μm (sample 1). a, b Scale bars 100 μm, a, b insets 1 μm

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33.20, 35.59, 40.84, 49.50, 54.05, 57.69, 62.48, and 64.24°, respectively, in the range of 20–80° in the XRD diagram. The crystallographic space group R=3C[167] has lattice constants (a, b, c) equal to 5.036, 5.036, and 13.749 Å, respectively, with a ratio of c/a=2.730 (PDF-33-0664) by using software of Materials Data JADE for XRD pattern processing and MDI material data. Therefore, the XRD results are also in good agreement with the EDS of SEM data. As a conclusion, we have clearly proved that a product of the pure α-Fe2O3 microparticles could be controllably synthesized in both the uniform large size and polyhedral shape. However, in many special cases, we suggested that XRD method is not sufficient in order to resolve the differences in the fine crystal structures between Fe 3 O 4 phase and γ-Fe 2 O 3 . Although the XRD peaks

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corresponding to α-Fe2O3 were detected in this work. It should be noted that other fine crystal phases could not be completely revealed within the resolution of the XRD techniques. Structure of large α-Fe2O3 oxide micro-particles Figure 2 shows the SEM images of large monodisperse αFe2O3 oxide particles with a very high yield of 100 % for sharp polyhedral morphologies and shapes and with the large size in the certain size range of about 1–5 μm. The polyol process was carried out with the use of NaBH4 in EG solvent for 30 min with a special emphasis on the interesting size enlargement phenomenon. Because of importance in shape and morphology, it should be noted that the large, sharp polyhedral as-prepared Fe-based particles were observed in the large, good, sharp, and flat morphologies and surfaces, typically such as cubes, octahedra, and tetrahedra. It is commonly known that main rhombohedral-like particles have the very large, flat, sharp, and smooth crystal surfaces of (012), (104), and (101), respectively [18, 80, 83]. Here, the polyhedral αFe2O3 particles usually have three main crystal surfaces such as (100), (011), and (111). As discussed, the inset image in Fig. 2a shows a large α-Fe2O3 crystal. It seems to be cubic-like crystal (a≈b≈c), but it really seems to be a large orthorhombic α-Fe2O3 crystal with the different distances of all the sharp and straight edges (a≠b≠c), but all the same angles nearly equal to 90° among their sharp and straight edges or triclinic α-Fe2O3 crystal (a≠b≠c) (all the same angles α≠β≠γ≠90) etc. In particular, the perfect and sharp orthorhombic shape with the sharp different edges (a≠b≠c) and all the same angles between them nearly equal to 90° (inset image in Fig. 2b). The large crystal surfaces of α-Fe2O3 crystals are sharp, flat, smooth, and very large. The very fast growths have lead to the final formation of large crystal particles in the same polyol method. Here, we suggested that the very fast growth rates of iron nuclei, clusters, nano-clusters, and very small nano-particles during their very fast collision, attachment, collision, aggregation or agglomeration, and self-assembly, as well as other unknown mechanisms, are the main causes of the final formation of the large polyhedral α-Fe2O3 oxide micro-particles in the certain range of 5 and 10 μm. They have large homogeneous size as well as large homogeneous polyhedral shape and morphology. It is proved that the various stable, flat, sharp, and smooth crystal surfaces of one Fe oxide particle showed a very large crystal (Fig. 2a, b). Therefore, the interesting formation of such large crystal surfaces of Febased particles in the range of 1,000–10,000 nm from the solution needs to be intensively investigated in the certain experimental conditions with that of the small crystal

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surfaces in the certain range of 10 (1–10 nm), 100 (1– 100 nm), and 1,000 nm (100–1,000 nm), respectively, in the same synthetic polyol method. In our conclusion, the very small Fe particles were formed by nucleation, collision, aggregation, or agglomeration, self-assembly phenomena. They are involved in Fe nuclei during the strong reduction process of Fe salt with NaBH4 in EG solvent. They bind together into the large agglomerates, and then, the large colloidal particles formed by random collision via Brownian motion, self-attachment, renucleation, regrowth, and reassembly as well as other bonding mechanisms among them at 200–230 °C and 30 min were applied. Thus, the random mechanisms of Brownian motions and collisions of the very tiny nano-particles in EG with good stabilization of PVP polymer concerned here are very crucial to the final formation of the very big αFe 2 O 3 oxide micro-particles. Here, Brownian motion mechanism of the as-prepared particles in the solution is considered as an important mechanism, leading to fast collision processes such as elastic and inelastic collisions under experimental strong mixing conditions allowing the formation of very large crystals. Generally, when their structural changes occurred, the large agglomerates were formed, but there are no any discoveries concerning a new discovery of the size enlargement method of the metal, alloy, and oxide particles by novel modified polyol methods with certain experimental conditions, such as temperature, pressure, concentration, time, pH, mixing speed, stabilization of polymer and dendrimer agents, Fe precursors, and structure-controlling agents [48, 84]. Most importantly, we suggest the homogeneous nano-system of the large and polyhedral Fe oxide particles exhibiting a new phenomenon of size enlargement by the same modified polyol method with the use of NaBH4 in EG solvent. In the essential requirements of the shaped and sized

Fig. 3 The existence of the elements in our sample was Fe(La), Fe(Ka), Fe(Kb1), O (Ka), and C (Ka) by EDS method (sample 1)

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range, this is an important remark for the controlled synthesis of metal, alloy, and oxide particles with desirable shapes and forms but very large sizes and shapes through a same polyol process. In addition, it is important to note that the prepared α-Fe2O3 particles have sharp and flat crystal surfaces, which are attractive to scientists because of their specific properties, such as electrical and magnetic properties, and catalysis. In EDS analysis of the large as-prepared α-Fe2O3 particles (Fig. 3), the Fe (La), Fe (Ka), Fe (Kb1), and O (Ka) elements existed

Fig. 4 SEM images of a nanosystem of the uniform Fe oxide micro-particles or alpha (α)Fe2O3 synthesized by modified polyol methods in our breakthroughs of synthesis processes (sample 2). The Fe oxide micro-particles with the certain size range of 10 μm and sharp polyhedral-like shape and morphology. a, b Scale bars 100 μm

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were observed with the strong peaks at 0.71, 6.41, 7.06, and 0.52 keV. There are two peaks with the existence of Fe and O elements at near the same positions at 0.52 and 0.71 keV. Because the as-prepared α-Fe 2 O 3 nanostructures had very large sizes, we did not characterize them by the TEM measurements in detail. Figures 4 and 5 show SEM images of sample 2 with the same process, but the particle size is larger than that of sample 1 because of the fact that experimental steps are slightly a little different from experimental operations. Additionally, most of

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Fig. 5 SEM images of a nanosystem of the uniform Fe2O3 oxide micro-particles or alpha (α)-Fe2O3 synthesized by modified polyol methods in our breakthroughs of synthesis processes (sample 2). The Fe oxide micro-particles with the certain size range of 1–10 μm and sharp polyhedral-like shape and morphology. a, b Scale bars 10 μm

the large polyhedral as-prepared Fe-based particles in the size range of 10 μm for synthesis time for 35 min (sample 2) have various sharp surfaces, sharp edges, and sharp corners as well as fine, smooth, and flat crystal surfaces. Figure 6 also shows SEM images of sample 3 with the same process as to sample 1 with synthesis time for 30 min. The SEM and TEM images of large α-Fe2O3 micro-particles show the particle size of 1–5 μm. TEM image of orthorhombic particle has the size of about 2 μm

with orthorhombic shape with large surfaces. The main shapes are the most typical sharp rhombohedral system. However, there are some typical sharp cube and tetrahedra in our products. Our product shows the large as-prepared Fe-based particles with nearly 100 % of the polyhedral crystal shapes and morphologies and with a very high yield in our synthesis. This proved that the degrees of the crystallinities are very high in a homogeneous nanosystem of the as-prepared large α-Fe2O3 particles in our

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controlled chemical synthesis. It is known that the Fe oxides have various forms. Their existent pure structures include goethite, lepidocrocite, akaganéite, ferrihydrite, hematite, magnetite, maghemite, etc. [80, 82]. The only appearance of the pure crystal phase of α-Fe2O3 oxide particles prepared by modified method with the use of NaBH4 in EG has been very meaningful in both the theory and practice. In addition, the obtained product provides the new scientific results and highlights of the large polyhedral α-Fe2O3 oxide particles and the large new α-Fe2O3 nano-organizations.

soluble species HFeO2 [49, 50, 62, 67, 75–79] of which concentration is strongly dependent on temperature [60, 72, 73], i.e.,

Electrochemical characterization

In fact, the second oxidation step of iron (Fe) electrode involves in Eq. (6) of Chakkaravarthy et al. [49]

The cyclic voltammograms of the α-Fe2O3/AB electrodes are shown in Fig. 7. Two oxidation peaks were observed around −0.85 V (a1) and −0.65 V (a2) while two reduction peaks occurred around −0.9 V (c1) and −1.1 V (c2), respectively. Along with the appearance of four peaks, a1, a2 and c1, c2, a small anodic peak a0 was observed around −1.0 V on the forward scan and hydrogen evolution observed around −1.15 V (c3). In the previous investigation [60], it is indicated that the clear surface of iron was never exposed to the electrolyte and over a partially oxidized surface adsorption of hydroxyl ion takes place. The dissolution of the oxide or underlying metal by the ion transport through the oxide can also take place. The overall electrochemical behavior involved in the passivation and dissolution of iron in alkaline solution was proposed earlier [49–51, 60–79] containing two main steps, the first of which is Eq. (1) of Chakkaravarthy et al. [49]

FeðOHÞ2 þ OH− ⇄FeOOH þ H2 O þ e

Fe þ 2OH− ⇄FeðOHÞ2 þ 2e

ð1Þ

E0 ¼ −0:975 V vs: Hg=HgO

According to the scholars’ results [60, 62, 72, 73], Eq. (1) involves in the following steps in conjunction with the adsorption of OH− ion: Fe þ 2OH− ⇄½FeðOHÞad þ e

ð2Þ

½FeðOHÞad þ OH− ⇄FeðOHÞ2 þ e

ð3Þ

Remarkably, most scholars agreed that the clear formation of Fe(OH)2 proceeds through the formation of intermediate

½FeðOHÞad þ 2OH− ⇄HFeO−2 þ H2 O þ e

ð4Þ

and HFeO−2 þ H2 O⇄FeðOHÞ2 þ OH−

E0 ¼ −0:658 V vs: Hg=HgO

ð5Þ

ð6Þ

and/or Eq. (7) of Kalaignan et al. and Micka and Zabransky [60, 61] 3

FeðOHÞ2 þ OH− ⇄Fe3 O4 :4H2 O þ 2e E0 ¼ −0:758 V vs: Hg=HgO

ð7Þ

Theoretically, the first and second anodic peaks (a1 and a2) can be attributed to oxidation of Fe to Fe(II) (Eq. 1) and Fe(II)/ Fe(III) (Eq. 6 or 7) while cathodic peaks (c1 and c2) correspond to the reduction of Fe(III)/Fe(II) and Fe(II)/Fe, respectively. Thus, a1 and c2 correspond to Fe/Fe(II) redox couple while a2 and c1 correspond to Fe(II)/Fe(III) redox couple. Related to oxidation peak (a0), Cerny et al. [60] summarized the results of numerous literatures and interpreted that the first step (a0) in the oxidation of iron appears probably more than oxidation of hydrogen, whereas some authors did not observe this peak on their curves. This step should be formulated as Fe: [Fe(OH)]ads (Eq. 2). At the first scan, the Fe2O3 was conversed to Fe(II) (c1) at low potential around −1.05 V, and the reduction peak of Fe(II)/Fe (c2) was not observed. The redox current of Fe(II)/ Fe(III) couple was much higher than that Fe/Fe(II) couple. With further cycling, the redox current of the Fe(II)/Fe(III) couple was decreased while redox current of the Fe/Fe(II) couple increased. This could be ascribed to the insulating nature of the Fe(OH)2 active material forming during cycling. In comparison with CV result of the commercial nanopowder α-Fe2O3 (Aldrich) with an average diameter of ca. 2–3 nm (Fig. 8), it can be observed that both the CV results are similar. However, in the case of commercial nano-powder αFe2O3, the reduction peak of iron deposition occurred at a low potential (around −1.15 V) together with hydrogen evolution.

Author's personal copy Colloid Polym Sci (2015) 293:49–63

59

Fig. 6 a SEM images of a nanosystem of the uniform α-Fe2O3 micro-particles synthesized by modified polyol methods in our breakthroughs of synthesis processes (sample 3). The αFe2O3 oxide micro-particles with the certain size range of 1–5 μm and sharp polyhedral-like shape and morphology. b Orthorhombic model with large surfaces of (101). c, d TEM image of orthorhombic particle. Scale bar a 10 μm, c, d 1 μm

In addition, the redox current of both Fe/Fe(II) and Fe(II)/ Fe(III) couples decreased with repeated cycling. These results revealed that the nano-Fe2O3/AB composite electrodes had larger internal resistances than our micro-Fe2O3/AB composite electrodes. The cycle performance of α-Fe2O3/AB composite electrode is shown in Fig. 9 (named as micro-Fe2O3/AB composite electrode). For comparison, the cycle performance of nano-Fe2O3/AB composite electrode (using commercial nano-powder Fe2O3 (Aldrich)) is also shown (Fig. 9). The

discharge capacities of micro-Fe2O3/AB composite electrode increased in the initial cycles, then gradually decreased and achieved the stable capacities after few tens of cycles. This is due to the passivation of the electrode caused by Fe(OH)2 formed during the cycling. However, the tendency of decrease in discharge capacity for the micro-Fe2O3/AB composite electrode agrees with the CV results. In comparison with the cycle performance result of nano-Fe2O3/AB composite electrode (Aldrich) in Fig. 9, it can be identified that both the electrode

10

a1 a0

-5

c3

Current / mA

Current / mA

5

0

40

1st 2nd 3rd 4th 5th

a2

c2

-1.4

-1.2

-1.0

-0.6

-0.4

-0.2

0.0

Potential / V vs. Hg/HgO Fig. 7 Voltammogams of α-Fe2O3/AB composite electrode in KOH aqueous solution (arrows present the tendency of the current during the cycling)

1st 2nd 3rd 4th 5th

c2 c1

-40 -60

-100 -1.4

-0.8

a2

0 -20

-80

c1

-10

a0 a1

20

c3 -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Potential / V vs. Hg/HgO Fig. 8 Voltammogams of nano-α-Fe2O3/AB composite electrode (nanopowder α-Fe2O3: Aldrich) in KOH aqueous solution (arrows present the tendency of the current during the cycling)

Author's personal copy Colloid Polym Sci (2015) 293:49–63

Discharge capacity / mAhg

-1

60 400 Micro Fe2O3 Nano Fe2O3 (Aldrich)

300

200

100

0 0

10

20

30

40

Cycle number Fig. 9 Cycle performance of α-Fe2O3/AB composite electrode in 8 M KOH solution

Fig. 10 SEM images and distribution of iron and carbon of α-Fe2O3/AB electrodes a before and b after the 15th redox cycle

materials α-Fe2O3 shows the same tendency: The capacity increased in the initial cycles and then gradually decreased. Although the discharge capacity of micro-Fe2O3/AB electrode was lower than that of nano-Fe2O3/AB electrode, after long cycles, micro-Fe2O3/AB electrode provided stable capacity higher than that of nano-Fe2O3/AB electrode. The main reason is because nano-Fe2O3/AB composite electrode had larger internal resistances than micro-Fe2O3/AB composite electrode. In our previous works [87, 88], we revealed that some Fe species are distributed on carbon surface throughout the electrochemical dissolution/deposition process of Fe, and consequently, the active surface area may increase. In order to confirm the mode of distribution of iron on carbon surface, EDS measurements, together with SEM observations, were carried out, and the results are shown in Fig. 10. Before

Author's personal copy Colloid Polym Sci (2015) 293:49–63

cycling (Fig. 10a), most iron species were only observed as particles that were tens of micrometers in size. After the 15th cycle (Fig. 10b), the iron was dispersed only a small amount on carbon surface via charge–discharge cycles. The particle size was still large in micrometer scale. Thus, the passivation caused by the formation of Fe(OH)2 during the charge/ discharge could be resulted in the decrease of capacity on further cycling. With further investigation of electrode preparation, our α-Fe2O3 microparticles with uniform cubic structure are expected to be potential candidate for use in Fe-air battery anode, and promisingly potential support for Pt-based catalysts in fuel cell technology as well as nanomedicine [90–92].

Conclusions In our research, the homogeneous large polyhedral α-Fe2O3 oxide micro-particles were synthesized by modified polyol method with the use of NaBH4 in EG homogeneous solvent, and the CV and galvanostatic cycling measurements were performed. The interesting results show the α-Fe2O3 nanostructures of 5 and 10 μm. They were possibly used in the αFe2O3 electrodes, which exhibited the good advantages of the CV characterization and cycle performance when Fe oxide micro- and nano-materials become increasingly important to applications of energy conversion and storage for Fe-air battery technology. Acknowledgments This work is supported by NAFOSTED Grant No. 103.02-2014.45, 2014, Vietnam. This is also supported by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant No. 103.02-2014.20, 2014. We greatly thank and appreciate the financial supports and projects sponsored by Chinese Academy of Sciences through Visiting Fellowship for Researchers from Developing Countries (Grant No. 2013FFGB0007) and China Postdoctoral Science Foundation (No. 2014M551462) from Shanghai Institute of Ceramics (SIC), Chinese Academy of Sciences (CAS), Dingxi Road 1295, Shanghai 200050, China, and other Universities for our research on Novel Magnetic Nanoparticles for Catalysis, Biology and Medicine (Nanomedicine). The work was also supported by National Natural Science Foundation of China (Grant No. 51471182).

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