Synthesis and magnetism of hierarchical iron oxide particles

Materials and Design 86 (2015) 797–808

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Synthesis and magnetism of hierarchical iron oxide particles Nguyen Viet Long a,b,c,⁎, Yong Yang a, Toshiharu Teranishi d, Cao Minh Thi c, Yanqin Cao a, Masayuki Nogami e a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, 1295, Dingxi Road, Shanghai 200050, China Posts and Telecommunications Institute of Technology, km 10 Nguyen Trai, Hanoi, Viet Nam c Ho Chi Minh City University of Technology, 144/24 Dien Bien Phu, Ward-25, Binh Thach, Ho Chi Minh City, Viet Nam d Faculty of Information, Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan e Toyota Physical and Chemical Research Institute, 41-1 Yokomichi, Nagakute 480-1192, Japan b

a r t i c l e

i n f o

Article history: Received 20 February 2015 Received in revised form 13 July 2015 Accepted 28 July 2015 Available online 3 August 2015 Keywords: Magnetic materials α-Fe2O3 CoFe2O4 Crystal structure Heat treatment Energy and environment

a b s t r a c t The hierarchical polyhedral α-Fe2O3 oxide particles, and hierarchical spherical CoFe2O4 oxide particles in the microsized ranges have been successfully prepared via modified polyol methods with NaBH4 and heat treatment process. An exact comparison was presented in the soft magnetic properties and structures of new hierarchical α-Fe2O3 and CoFe2O4 oxide particles. To study Fe-based oxide microparticles, X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometer have been used to characterize their microstructures and magnetic properties according to the preparation processes. The hierarchical α-Fe2O3 and CoFe2O4 oxide microparticles with oxide grain and grain boundary textures were perfectly formed under heat treatment at high temperature without any chemical self-assembly methods. For this exciting research, the various kinds of homogeneous hierarchical Fe-based oxide particles can be successfully produced in the large amount for commercialization. It is discovered that MS of hierarchical CoFe2O4 particles is much larger than that of hierarchical α-Fe2O3 particles about 93 times. The coordination numbers of Co and Fe cations at tetrahedral and octahedral sites lead to determine its magnetic behavior. Finally, hierarchical Fe-based oxide particles are proposed for promising and potential candidates for energy and environment related technologies associated with safety and sustainability. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Recently, magnetic Fe-based metal, alloy, and oxide particles have been widely investigated because of their implications for life, medicine, catalysis, clean energy and environment, typically such as α-Fe2O3 and Fe3O4 particles with both various micro and nanostructures [1–7]. Today, there are various synthesis methods of Fe oxide particles, such as co-precipitation, thermal decomposition, microemulsion, hydrothermal synthesis, sonochemical synthesis etc. [1–7]. Here, Fe oxides existed in various compounds, e.g. Fe3O4 (magnetite), α-Fe2O3 (hematite), γ-Fe2O3, FeO, ε-Fe2O3, β-Fe2O3 with various micro and nanostructures [1–5]. The known synthesis methods of various α- and γ-Fe2O3 nanoparticles were introduced for catalytic applications with controlled crystal-phase and shape with the micro to nano size ranges

Abbreviations: M, magnetization (emu g−1); MR, remanence or remanent magnetization (emu g−1); MS, magnetic saturation (emu g−1); H, magnetic field strength (Oe); Hc, coercivity (Oe); TC, Curie temperature. ⁎ Corresponding author at: (1) Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan; (2) Shanghai Institute of Ceramics, Chinese Academy of Science, 1295, Dingxi Road, Shanghai 200050, China. E-mail addresses: [email protected], [email protected] (N.V. Long).

http://dx.doi.org/10.1016/j.matdes.2015.07.157 0264-1275/© 2015 Elsevier Ltd. All rights reserved.

[6]. In addition, magnetic α-Fe2O3 materials have various great technologies and applications for gas sensors, batteries, energy conversion and storage [8–12]. On doping Co element into Fe oxides, and occupation of Co ions into octahedral or tetrahedral sites, CoFeO4 ferrite materials, i.e. Fe-based AB2O4 ferrite materials or modified ferrite materials with doping compositions and methods are formed with the inverse spinel structures [13,14], which have exhibited a large positive anisotropy constant. Its anisotropy energy places its magnetic properties between soft and hard ferrite for new practical applications [15,19]. Therefore, the 3d inner electron configuration of the ferrite ions was necessarily known. Additionally, they can be used for magnetoelastic and magnetoelectric transducers [15,19]. Besides, the more complicated structure of FeCobased alloy as (Fe70Co30)100 − xCux was successfully prepared by highenergy ball milling [16]. Today, Fe-based ferrite materials are best suited for designing acoustooptic devices, tunable optical oscillators, and microwave components for protection, radar receivers and transmitters against microwave spikes. In industrial development, Fe-, Co-, and Nibased materials have a diversity of practical applications for transformers [17–19]. The magnetism of Fe, Ni, and Co ions is due to unpaired 3d electrons while the magnetism of rare earth elements is due to unpaired 4f electrons [17–19]. In particular, the combinations of alloyand oxide-based ferrite materials have been considerably used in electronic components, and permanent magnets with critical magnetic

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properties. The preparation methods and processes of the pure and stable structures of α-Fe2O3 and CoFeO4 particles with the high crystallization levels are of challenging interest to scientists. In this research, hierarchical magnetic α-Fe2O3 and CoFe2O4 microparticles with the controlled size ranges were prepared by modified polyol method with NaBH4 and heat treatment at 900 °C for 1 h. The hierarchical α-Fe2O3 and CoFe2O4 microparticles contained various grain and grain boundary forms. Here, Co2+ and Fe3+ ions occupied either tetrahedral or octahedral sites in CoFe2O4 spinel ferrite structure. In this context, it is suggested that Co2+ ions preferred to occupy the tetrahedral sites for the best inverse spinel structure. The comparisons of microstructure and magnetism of the two kinds of α-Fe2O3 and CoFe2O4 particles were presented in detail. For α-Fe2O3, the satellite peaks for Fe2p1/2 and Fe2p3/2 are reduced after etching process but the main peaks of Fe2p core levels are preserved in the hierarchical structure of Fe2O3. For CoFe2O4, the satellite peaks for Fe2p1/2 and Fe2p3/2 are not only preserved after etching process but also the main peaks of Co2p core levels are preserved in the hierarchical structure of CoFe2O4 as well. In this case, all the satellite and main peaks were preserved in XPS spectra of CoFe2O4 before and after etching process. This led to prove that the very high ability of the occupation of Co ions into the best inverse structure occurred at the ideal tetrahedral sites. 2. Experimental procedure In the controlled synthesis of α-Fe2O3 and CoFe2O4 particles, starting precursors were prepared as described in previous detailed works for αFe2O3 oxide particles about 20–30 min [8–12], and with drying and heat treatment (Scheme 1). Briefly, a lot of attention and time were paid to develop our preparation processes. In a typical process, 10 mL of EG, 3 mL of 0.0625 M FeCl3 (FeCl3 · 6H2O precursor), 1.5 mL of 0.0625 M CoCl2 (CoCl2 · 6H2O) precursor, 10 mL of 0.375 M PVP, and 0.048 g NaBH4 were used for controlled synthesis. The stock solutions of Fe and Co precursors were pumped into the center of reaction flask (250 mL) under stirring according to an exactly fixed 2:1 ratio of the stock solutions of Fe3+/Co2+ precursors in volume for the precisely controlled synthesis. The reaction periods for synthesis of PVP–CoFe particles via the reduction of precursors by NaBH4 were done for 45 min. Then, the PVP-particles were achieved in the resulting black solutions. They were kept at room temperature for some days to obtain the black products at the bottom. The clean black products were obtained by removing PVP on the surfaces of as-prepared particles according to centrifugation processes, washing and cleaning procedures [20]. They were maintained in the containers in the forms of powders. Then, the dried particle powders were re-dispersed into ethanol and dried at 60 °C. Overall, to obtain black-brown oxide products of CoFe2O4 particles, these black powders were isothermally heated at 900 °C for 1 h with ceramic containers in air (Scheme 1). Similarly, we prepared various samples for X-ray diffraction (XRD), scanning electron microscopy (SEM) investigation and analysis. The most typical characterizations of α-Fe2O3 and CoFe2O4 particles were investigated by XRD, SEM, and VSM methods. The X-ray diffraction patterns of α-Fe2O3 and CoFe2O4 particles were recorded in a 2θ range of 5–95(°) by X-ray diffractometer (Rigaku-D/max 2550 V, 40 kV/40 mA, CuKα radiation at 1.54056 Å). Finally, the features of size, shape, and morphology were investigated by field emission (FE)SEM (Magellan-400, FEI, Eindhoven, Netherlands) operated at 15 kV with a combination of SEM and energy dispersive spectroscopy (EDS) methods, and with electron backscatter diffraction (EBSD). We have used field emission scanning electron microscope (SEM) (JEOL-JSM-634OF) operated at 5, 10, and 15 kV (5–15 kV), and probe current around 12 μA (1–12 μA). Here, VSM method was applied for determination of magnetic properties of magnetic α-Fe2O3 and CoFe2O4 particles mentioned. We have utilized a vibrating sample magnetometer (VSM), Model EV11, which is used for analyzing magnetic characteristics of α-Fe2O3 and CoFe2O4 materials evaluated at room temperature (RT), about 293 K in a wide range of applied field from

−20 kOe to 20 kOe. Here, EV11-VSM can reach fields up to 31 kOe at a sample space of 5 mm and 27 kOe with the temperature chamber, with Signal noise to be 0.1 μemu, and 0.5 μemu, respectively. The surface chemical bonding was characterized by X-ray photoelectron spectroscopy (XPS) (Escalab 250, Thermo Scientific, Britain). In the XPS analysis, we obtained the information from the initial surfaces and the etched surfaces at 2 kV, 1 μA, 1.0 mm × 1.0 mm for 10 s before testing in the removal of most of the surface impurities. All the peaks have been adjusted with taking C285 as the reference. For the XPS analysis, the samples with Fe2O3 and CoFe2O4 oxide microparticles were pre-etched. Thus, the composition of the elements in the prepared oxide microparticles was determined. 3. Results and discussion 3.1. Crystal structure of iron-based oxide particles Fig. 1a shows our results of XRD pattern of the as-prepared Fe2O3 particles of 10 μm (1–10 μm) for hematite, i.e. α-Fe2O3 according to PDF87-1166 (XRD powder data). In our observation and evaluation, Fig. 1a shows the most typical peaks are characterized by a set of miller indices of (012), (104), (110), (113), (024), (116), (122) or (018), (214), (300), (208), (1010), and (220), and more (hkl), respectively. The corresponding values of 2θ (°) are roughly estimated at about 21.6, 24.3, 33.2, 35.6, 40.8, 49.5, 54.1, 57.7, 62.5, 64.2, 69.8, 72.0, and 75.4, respectively in respective to a certain range of 20–95°. The XRD data indicated the best crystallographic α-Fe2O3 structure, crystallite size or grain size. That was α-Fe2O3-hematite system, space group R3C (167). The hexagonal crystal lattice of α-Fe2O3 was a = 5.0353 Å, b = 5.0353 Å, c = 13.7495 Å, c/a = 2.730, (α = β = 90°, γ = 120°) according to PDF87-1166, respectively. It is noted that the phase identification was confirmed in α-Fe2O3 with strong 15 lines by our observation for rhombohedral hematite, and 19 lines by pattern indexing with MDI Jade software (Table 1), and 28 lines by standard pattern (PDF87-1166). In the reflections from lattice constants, the values of d-I parameters were shown in Table 1. In phase identification, the strongest line was revealed to be from the reflections of (104) planes. The polyhedral α-Fe2O3 microparticles were heated under high temperature at 900 °C. It is certain that hematite exhibited ultra-high stability and durability in a wide range from room temperature to 900 °C. An exciting new hybrid miro-nano structure was primarily found in terms of the most interesting shape, size, and morphology of α-Fe2O3 microparticles that contained various grains and boundaries. Each grain was a pure single α-Fe2O3 crystal. Additionally, α-Fe2O3 grains have various particle sizes in both micro- and nano-size ranges. Among the Fe oxides, the structure of α-Fe2O3 has been considered to be the most stable and durable structure. Finally, we suggested that the new hybrid miro-nano particles made of from hundreds to thousands of the grains would be very hot topics in the development of functional metal oxide particles in next research. Fig. 1b shows the most important diffraction peaks of CoFe2O4 particles at (111), (220), (311), (222), (400), (422), (511), (440), (533), (731), and possible (hkl) indices, respectively. The corresponding values of 2θ (°) were estimated at 18.304, 30.122, 35.489, 37.111, 43.144, 53.505, 57.036, 62.622, 74.107, 89.808, and more 2θ, respectively. The crystal structure of CoFe2O4 particles was confirmed by our XRD investigation. After pattern indexing, we obtained CoFe2O4 with cubic spinel structure (Fd3m-277: a = b = c = 8.3954 Å), all the parameters were listed in Table 1, which are in agreement with the strongest (311) line of PDF-22-1086 and it has the corresponding values of 2θ(°) at 18.288, 30.084, 35.437, 37.057, 43.058, 53.445, 56.973, 62.585, 74.009, 89.669, and more 2θ, respectively. Therefore, the parameters are good agreement with the standard pattern for the most typical CoFe2O4 ferrite materials. Typically, the main diffraction peaks were found in the cubic spinel crystal structure of CoFe2O4 in its crystal growth by using Software of Materials Data JADE and MDI Material data for XRD pattern processing. In the reflections from lattice

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Scheme 1. Synthesis of α-Fe2O3 and CoFe2O4.

constants, the values of d-I were shown to be 4.8470 Å/10.0%, 2.9680 Å/ 30.0%, 2.5310 Å/100.0%, 2.4240 Å/8.0%, 2.0990 Å/20.0%, 1.9260 Å/1.0%, 1.7130 Å/10.0%, 1.6150 Å/30.0%, 1.4830 Å/40.0%, 1.2798 Å/9.0%, 1.2114

Å/2.0%, 1.1214 Å/4.0%, and 1.0925 Å/2.0% in Table 2. The strongest outstanding line was revealed to be from the reflections of (311) planes. Therefore, the crystallization of crystal structure of CoFe2O4 was

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N.V. Long et al. / Materials and Design 86 (2015) 797–808 Table 2 Pattern indexing of CoFe2O4 (Cobalt iron ferrite). Sample

2-Theta

d(Å)

I% (h k l)

CoFe2O4 (Cobalt iron ferrite) a = 8.3954 Å, b = 8.3954 Å, c = 8.3954 Å, c/a = 1, α = β = γ = 90°

18.304 30.122 35.489 37.111 43.144 47.191 53.505 57.036 62.622 74.107 79.044 86.847 89.808

4.8470 2.9680 2.5310 2.4240 2.0990 1.9260 1.7130 1.6150 1.4830 1.2798 1.2114 1.1214 1.0925

10.0 (1 1 1) 30.0 (2 2 0) 100.0 (3 1 1) 8.0 (2 2 2) 20.0 (4 0 0) 1.0 (3 3 1) 10.0 (4 2 2) 30.0 (5 1 1) 40.0 (4 4 0) 9.0 (5 3 3) 2.0 (4 4 4) 4.0 (6 4 2) 2.0 (7 3 1)

3.2. Size and shape of iron-based oxide particles

Fig. 1. XRD patterns of (a) α-Fe2O3, and (b) CoFe2O4.

relatively high in our preparation process. To calculate crystallite size of α-Fe2O3 and CoFe2O4 particles, we have used Debye–Sherrer's equation with D = 0.89λ/(βcosθ) (D: crystallite size, λ: wavelength of X-ray radiation, β: Full width at half maximum (FWHM) with the use of the most intense peak in XRD patterns, θ: Bragg angle). For α-Fe2O3 and CoFe2O4 oxide particles, their values of D are 13.76 Å for the strongest (104) line, and 16.86 Å for the strongest (311) line, respectively.

Table 1 Pattern indexing of α-Fe2O3 (Hematite). Sample

2-Theta

d(Å)

I% (h k l)

α-Fe2O3 (Hematite) a = 5.0353 Å, b = 5.0353 Å, c = 13.7495 Å, c/a = 2.730, α = β = 90°, γ = 120°

24.145 33.162 35.626 39.322 40.873 43.594 49.446 54.076 56.339 57.611 62.424 63.983 69.775 71.972 75.442 77.740 82.973 84.920 88.569

3.6824 2.6995 2.5176 2.2916 2.2066 2.0783 1.8412 1.6947 1.6365 1.5990 1.4862 1.4536 1.3498 1.3113 1.2588 1.2275 1.1630 1.1409 1.1033

31.4 (0 1 2) 100 (1 0 4) 71.3 (1 1 0) 1.9 (0 0 6) 19.2 (1 1 3) 1.8 (2 0 2) 34.1 (0 2 4) 41.1 (1 1 6) 0.5 (2 1 1) 8.6 (0 1 8) 26.4 (2 1 4) 25.6 (3 0 0) 2.5 (2 0 8) 8.8 (1 0 10) 5.5 (2 2 0) 1.9 (0 3 6) 4.3 (0 2 10) 6.4 (1 3 4) 5.9 (2 2 6)

Fig. 2 illustrated the most typical SEM images of hierarchical α-Fe2O3 (10 μm) and CoFe2O4 (5 μm) oxide particles with grain and grain boundary forms. We have selected the most typical two particles for calculation based on scale bar 10 μm. Their sizes are approximately to be 1.73 and 10 μm as shown in Fig. 2a. Similarly, we have also calculated the most typical two particles with the particle sizes to be 3.72 and 1.73 μm based on scale bar 5 μm as shown in Fig. 2c. It is suggested that they show the characterizations of size, shape and morphology with deformation [8–10]. We suggest that the two samples of α-Fe2O3 and CoFe2O4 particles proved grain and grain boundary inside various porous structures after high heat treatment at 900 °C for 1 h in air. Here, two typical products of Fe2O3 and CoFe2O4 oxide particles were regarded as hierarchical micro/nanostructured oxide materials. Fig. 3a shows the most typical polyhedral models of hierarchical α-Fe2O3 particles with grain and grain boundary in respect with our primary experimental results of SEM observation and investigation. We supposed four systems of α-Fe2O3 particles. The polyhedral α-Fe2O3 particles with a uniform size range of 1 μm are proposed. Here, α-Fe2O3 particles can have the uniform larger size range of 5 μm proposed (Fig. 3a Model B1 to B8). The models of α-Fe2O3 particles can have many different sizes in a limit range of 10 μm (Fig. 3a model C1 to C9), and models of α-Fe2O3 particles with a uniform size range of 10 μm. They consist of hundreds to thousands various small and large grains and grain boundaries. In general, their crystal structure is illustrated in Fig. 3b with chemical formula α-Fe2O3, Cod-ID 9000139, Space group: R-3c:H, Cell parameters: a = 5.038, b = 5.038, c = 13.772, and α = 90°, β = 90°, and γ = 120°, respectively. The typical crystal structure data of α-Fe2O3 was defined in the crystallography open database (COD), which was similar to typical structure of corundum (Al2O3), R3c(167)[hR10] in respect with three-dimensional crystal structure (Fig. 3b). Similarly, Fig. 3c shows the spherical models of hierarchical CoFe2O4 particles with grain and grain boundary forms in our obtained SEM results (Fig. 2c) associated with three-dimensional crystal structure (Fig. 3d). Like hierarchical α-Fe2O3 particles, we supposed four systems of CoFe2O4 particles. The first category is uniform particle system but a small size range of 1 μm (Fig. 3c: A1–A15). The second category is uniform but a large size range of 5 μm (Fig. 3c: B1–B8). The different system is not uniform with many different sizes (Fig. 3c: Model C1 to C10). Finally, uniform system has the limit range of 10 μm (Fig. 3(c): Model D1 to D4). Fig. 3d shows general chemical structure and formula of CoFe2O4 particles with the sites of Co atoms, the sites of Fe atoms, the sites of O atoms in respective to Cod-ID 5910063, Space group: Fd3 m:2, cell parameters: a = 8.35, b = 8.35, c = 8.35, α = 90°, β = 90°, and γ = 90°, respectively. In the experimental results, Fe oxide particles have various shapes in the same case as those of Fig. 3a from model C1 to model C10 for hierarchical polyhedral α-Fe2O3 particles with grain and grain boundary, and those of Fig. 3c from model C1 to

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Fig. 2. (a)–(b) SEM images of the as-prepared Fe2O3 oxide particles. (c)–(e) SEM images of the as-prepared CoFe2O4 oxide particles with grain and grain boundary. (d) is a snapshot of (c) marked. Scale bars: (a) 10 μm. (b) 5 μm. (c)–(d) 5 μm. (e) 2 μm.

model C10 for hierarchical spherical CoFe2O4 particles with grain and grain boundary. Thus, we indicated that the hierarchical oxide particles are classified with the relative uniform shapes and morphology. 3.3. Magnetic properties of iron-based oxide particles In this research, the M–H hysteresis loops of were typically characterized for their magnetic properties of as-prepared α-Fe2O3 and CoFe2O4 particles. In our investigation of related magnetism of α-Fe2O3 and CoFe2O4 particles, the most important measurement parameters include MR, MS, HC, HS, and S etc. [8], which were calculated,

and analyzed in magnetic hysteresis loops. Fig. 4 shows typical hysteresis loops of α-Fe2O3 (Fig. 4a), and CoFe2O4 particles (Fig. 4b) taken to MS at the specific S and S' points in the comparison of their magnetism, which are S and S' points in M–H curves. The two hysteresis loops show MR which M measured at H = 0, MS at maximum M measured in forward and reverse saturations, coercive field strength at which M/H changes sign, squareness parameter (MR/MS) etc., respectively. For the two samples of α-Fe2O3 and CoFe2O4 particles, hysteresis loops indicated the magnetic parameters of upward part, downward part, and average value, respectively. The typical magnetic parameters of hysteresis loops of α-Fe2O3 and CoFe2O4 materials by the VSM

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Fig. 3. (a) Important new models of grain and grain boundary belonging to polyhedral α-Fe2O3 particles with various size scales. Each particle contains hundreds to thousands grains inside its structure. (b) Hematite structure. (c) Important models of grain and grain boundary belonging to spherical CoFe2O4 particles with various size scales. Each particle contains hundreds to thousands grains inside its inner structure. (d) Cobalt iron spinel ferrite structure. (b)–(d) Structures hematite and cobalt iron ferrite from VESTA Software, Copyright 2006–2014, K. Momma, F. Izumi [25a,b,c].

methods were listed in Table 3. We find that MS of magnetic CoFe2O4 particles is much larger than that of magnetic α-Fe2O3 particles about 93 times. Additionally, we have found α-Fe2O3 oxide particles have low MS exhibiting a trend changing from antiferromagnetic or weak ferromagnetic to superparamagnetic property. In contrast, ferrimagnetic CoFe2O4 particles have high MS exhibiting a trend changing from ferrimagnetic to super paramagnetic property in comparison with superparamagnetic alloy and oxide nanoparticles with the very small sizes less than 10 nm and 100 nm investigated. Both antiferromagnetism of α-Fe2O3 and ferrimagnetism of as-prepared CoFe2O4 are explained by molecular field theory (MFT) between the specific two sublattices in their spin structures through exchange interactions between Co and Fe ions at tetrahedral and octahedral sites according to Curie temperature (TC) under external magnetic field [1,4,19]. The exchange interactions between ions in the inverse spinel structure of CoFe2O4 included the exchange interaction between A, i.e. Co and B, i.e. Fe. Among CoFe, CoCo, and FeFe interactions, CoFe interaction was commonly the strongest kind [1], as well as relative distribution of magnetic Fe ions on tetrahedral and octahedral sites with the contents of Co

ions into tetrahedral and octahedral sites of CoFe2O4 spinel structure. However, CoFe2O4 particles have spontaneous M [1b]. According to MFT and TC [1b], the magnetizations of AB2O4 and sublattices are M, MA(Co), and MB(Fe) showing the absolute values. In that case, MFT between the two sublattices in the AB2O4 structure has led that the total magnetization of CoFe2O4 is M = MA + MB, i.e. MA = MA(Co) and MB = MB(Fe). For the case of α-Fe2O3 oxide, MA(Co) is equal to 0. In order to explain the results in Fig. 4, it is known that CoFe2O4 has relative spontaneous magnetization because MA(Co) is different from MB(Fe) at H = 0. When H(H ≠ 0) is applied, and enough strong, we can obtain total magnetization M = |MA(Co)| − |MB(Fe)|. When H increases, all the directions of MA(Co), MB(Fe), and H are parallel so that M is equal to M = MA(Co) + MB(Fe) [1]. In the intermediate range, M was mainly depended external magnetic field. As shown in Fig. 4a and b, MS has shown the highest magnetic saturation of the two sublattices in CoFe2O4. In the case of the only crystal lattice, i.e. α-Fe2O3, MS of αFe2O3 was much smaller than MS of CoFe2O4 with the strong CoFe interaction. In our research, α-Fe2O3 microparticles with grain and grain boundary also exhibited very weak ferrimagnetism. Through a first-

N.V. Long et al. / Materials and Design 86 (2015) 797–808

Fig. 4. Magnetism of (a) α-Fe2O3 oxide particles, and (b) CoFe2O4 oxide particles.

principles study, the electronic and magnetic properties of the various structures and formula of (Co1− xFex)Tet(CoxFe2− x)OctO4 have been intensively studied for obtaining the inverse spinel structures [21]. When x = 1, CoFe2O4 energetically favors inverse spinel and both Fe and Co always prefer the high spin configurations [1,4]. Therefore, Co ions can be occupied at Tet sites or Oct sites, which do not indicate the importance of two sites to Co ions. These are in agreement with our results in both structure and ferrimagnetic properties. The two snapshots (Fig. 4) show the magnetic parameters MR, − MR, HC, and − HC of ferrimagnetic CoFe2O4 particles much larger than those of ferromagnetic α-Fe2O3 oxide particles due to magnetic exchange interactions and interphase coupling between two corresponding sublattices (Table 3) despite their similar grain and grain boundary forms and different sizes [1,4,19]. The above magnetic characterizations were due to the specific structure of α-Fe2O3 and CoFe2O4 oxide particles with grain

Table 3 Typical magnetic parameters of hysteresis loops by VSM method for α-Fe2O3 and CoFe2O4 materials. Symbols: MR: Remanent magnetization, MS: Saturation magnetization, HC: Coercive field, S: Squareness (MR/MS). Samples

Magnetic parameters

Unit

Upward

Downward

Average

α-Fe2O3

MR MS HC S (MR/MS) MR MS HC S (MR/MS)

emu g−1 emu g−1 Oe Constant emu g−1 emu g−1 Oe Constant

−0.163 0.831 95.82 0.20 −26.682 77.124 415.53 0.240

0.195 −0.829 −94.65 0.24 26.747 −77.167 −411.75 0.244

0.179 0.830 95.24 0.22 26.714 77.146 413.64 0.242

CoFe2O4

803

and boundary forms in the structural models in Fig. 3a. These structures are illustrated from model C1 to model C10 for polyhedral-like α-Fe2O3 particles, and Fig. 3c from model C1 to model C10 for spherical-like CoFe2O4 particles, which are regarded as various magnetic multidomains and walls as well as different arrangement of magnetic moments in their structures. Here, α-Fe2O3 oxide particles were considered as antiferromagnetic material because of neighboring atomic moments and exchange coupling effects [1,4], and CoFe2O4 oxide particles as aniferromagnetic or ferrimagnetic material. In structural model approach, each particle contain the large grains, the small grains, i.e. possible smaller than 1 μm, and much smaller than this range if this structure is carefully investigated, and nano or microscale grains and grain boundaries, which are the most interesting structural characteristics and challenges to scientists [8]. In comparison with cobalt ferrites with inverse spinel structure [22–27], and with many composition of CoMnxFe2 − xO4 [22], Cu0.5Co0.5Fe2O4 and Co0.2Ni0.3Zn0.5Fe2O4 [23], CoxMn1 − xFe2O4 nanoferrites [24], and BaTiO3–CoFe2O4 [26] etc., the prepared products in our present research shows good soft magnetic properties, which indicates that generalized ferrimagnetism is relatively similar to other above works in both micro and nanosized ranges. Among with magnetic oxides, α-Fe2O3, γ-Fe2O3, Fe3O4, and CoFe2O4, the structure of CoFe2O4 has the advantages for practical magnetic applications because it shows the higher and better stability and durability than other remaining oxides. Fe3O4 can be expressed as FeO · Fe2O3 or FeFe2O4 and The Fe ions exist in + 2 and + 3 valence states in a ratio of 1:2. In nature, Fe3+ ions preferred to occupy octahedral and tetrahedral sites but Fe2 + preferred to occupy octahedral sites. In addition, γ-Fe2O3 can be expressed as 3[(Fe3+)O·(Fe3+5/3V1/3)O3] with V to be vacancy in its structure, i.e. cation-deficient AB2O4 spinel structure. This formula may be true to the case of α-Fe2O3. Therefore, α-Fe2O3 oxide is parasite ferromagnetic materials but it shows a very week ferrimagnetism in comparison with that of CoFe2O4 oxide in our results. The ferrimagnetism is determined by the relative amount of Fe3+ ions at the 8a sites (4 oxygen ions) or tetrahedral sites, and at the 16d sites (16 oxygen ions) or octahedral sites. It is known that α-Fe2O3 oxide has hexagonal crystal lattice a = b ≠ c, and lattice constant c is much larger than a and b with a ratio of c/a or c/b = 2.73. When Co ions were introduced into α-Fe2O3 oxide by the same synthetic process, this led to obtain a = b = c in the crystal structure of CoFe2O4. It means that the crystal direction of c axis of α-Fe2O3 oxide was significantly decreased while their directions of a and b axes were increased in order to achieve the limitation of crystal lattice when the parameters are equal a = b = c for cubic spinel structures, which led to obtain the high ferrimagnetism of asprepared CoFe2O4 oxide in its cubic crystal lattice with lattice constant a = b = c, and the ratio of c/a or c/b = 1. The hexagonal crystal lattice of α-Fe2O3 was changed into the cubic crystal lattice of CoFe2O4 depending on the ability and amount of Co ion integrated into α-Fe 2O 3 . To get the best inverse spinel structures MeFe2O4 (Me = Co, Ni, Cu …), we need to use the amount of Co ion so that MeFe2O4 has specific cubic crystal lattices, and lattice constant c varying from c N a = b into c = a = b (Supplementary data). In fact, α-Fe2O3 is a special case of CoFe2O4 when the content of Co goes to be equal to zero in the same synthetic process. Therefore, various MFe2O4 oxides with normal and inverse structures (M metal = Zn, Mg, Ni, Cu etc) can be facilely produced in the above same processes. It is clear that our results show advantages of CoFe2O4 ferrite particles with the good inverse spinel structure. We suggest that there is a trend of combination between hard-magnetic and soft-magnetic compounds to utilize two soft-hard phases of Fe–Co-based alloy and oxide with NdFe12Nx, NdxFeyBz, and other compounds for the new magnet materials [28,29]. In all the parts, they are typical nanocrystalline soft magnetic oxide materials for related energy and environment applications as well as waste water treatment [3,6,30], magnetic recording materials, microwave ferrite materials for circulators, phase shifters, and filters [19,30].

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Fig. 5. Chemical elements are found by SEM-EDS methods.

3.4. Role of particle heat treatment to the appearance of grain and grain boundary structure In the case of the controlled synthesis of α-Fe2O3 particles, 10 mL of EG, and 3 mL of 0.0625 M FeCl3 were used for synthesis that led faster reduction of FeCl2 by NaBH4 that led polyhedral morphology of the pure Fe particles that were heated to form α-Fe2O3 particles. In the case of the controlled synthesis of CoFe2O4, 10 mL of EG, 3 mL of 0.0625 M FeCl3, 1.5 mL of 0.0625 M CoCl2 precursor were used for synthesis that led the possible slower reductions of FeCl3 and CoCl2 by NaBH4 that led spherical morphology and shape of CoFe particles that were heated to form CoFe2O4 particles. We confirmed that α-Fe2O3 and CoFe2O4 oxide particles with grain and boundary structures were just formed by heat treatment at high temperature. Based on the obtained results, we suggest that particle heat treatment is very important and

indispensable in order to harden the microstructure of α-Fe2O3 and CoFe2O4 oxide nanomaterials in final crystal growth and formation after heating at 900 °C. The prepared oxide microparticles exhibited the heavy particle deformation of size, shape, surface, and inner structure associated with the plastic, inelastic and elastic deformation at micro- and nanoscale ranges with crack propagation, and without the particle collapse or destroying the structures of oxide microparticles but the sizes and shapes possibly retain well with high durability and stability [8]. In the very wide annealing temperature range less than 900 °C, it ensured that the pure crystal structure was found to be αFe2O3. The arrangements and distributions of grain size on surface, and inside α-Fe2O3 and Cofe2O4 oxide particles are important [8]. The small and large grain sizes led the higher coercivity. In the critical process, it has been accepted that hierarchical α-Fe2O3 and CoFe2O4 microparticles with important evidences of various grain and boundary forms

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were formed at high temperature. The particle deformation was observed in surfaces, sizes, shapes, and internal structures. Therefore, magnetism of hierarchical α-Fe2O3 and CoFe2O4 microparticles were significantly enhanced in order to achieve an increase in their higher quality, better durability and stability for practical applications. In one research, scientists confirmed that the important physical and mechanical parameters of the real 3D objects were intensively studied in a nondestructive way through recent tomographic reconstruction modalities and algorithms [31]. It included crystal orientation, random or order grain orientation, lattice distortion and misorientation, elastic, and

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plastic components of the strain tensor, etc. In next studies, develop annealing procedures are proposed and tested to make cubic inverted spinel structures, i.e. our formula A1 − xB2 − yO4, such as CoFe2O4 with x = 0 and y = 0 for the formation of the best inverse spinel structure, and the best inverse spinel level in its crystalline structure. Thus, the proposed particle heat treatment methods of the most intensively used oxide products can be used for research directions in academic and industrial applications [32,33]. Therefore, in most of magnetic nanostructures with various alloys and oxides, the issues of the formation of grain and grain boundary are

Fig. 6. XPS spectra of the α-Fe2O3 microparticles before (A1–A3, C1), and after etching (B1–B3, D1). The satellite peaks (1) and (3) are reduced after etching process to α-Fe2O3 microparticles.

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very important to understand magnetism of soft and hard magnetic materials with or without rare earth [32,33]. The smaller particle size of magnetic particles can lead to superparamagnetism, which

can be best understood by Mössbauer technique [34,35]. Similarly, the site occupancy of Co was exactly interpreted using Mössbauer spectroscopy.

Fig. 7. XPS spectra of the CoFe2O4 microparticles before (A1–A4, C1, C2) and after etching (B1–B4, D1, D2).

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3.5. Effect of Co integration inside the inverse spinel structure of CoFe2O4 Fig. 5a–f shows SEM image of a CoFe2O4 oxide particle. It shows new microstructure of small and large grains and boundaries. Based on chemical analysis of CoFe2O4 oxide particle about 2.5 μm in size were carried out with EDS and the high solution SEM images of the CoFe2O4 particle. The particle indicated the existence of the elements of CoFe2O4 oxide particles consisting of C Kα1,2, Co Lα1-2, O Kα1, Fe Kα1, respectively. According to Table 4 and Supplementary data, the wt% ratio of Co:Fe for K series was 1.893, respectively, and especially the atomic% ratio of Co:Fe K series was 1.997 reaching approximately to be 2 as the hard evidence in this research. This shows this experimental process approaching very close to the theoretical formula of CoFe2O4 or CoO · Fe2O3. The surface properties of both α-Fe2O3 and CoFe2O4 prepared were characterized using XPS to determine the existence of the elements and their valence. These critical issues were proved in XPS results in Fig. 6 (α-Fe2O3) and Fig. 7 (CoFe2O4). Here, green lines indicate the background line of all the XPS measurements. Fig. 6A(A1–A4) shows XPS spectra of α-Fe2O3 microparticles with the initial surfaces. Fig. 5B(B1–B4): XPS spectra of α-Fe2O3 microparticles with etched surfaces. Fig. 6C1 and D1 shows the comparison of the initial surfaces and etched surfaces of the α-Fe2O3 microparticles by XPS, corresponding to the Fe oxidation states inside the α-Fe2O3 oxide. The two satellite peaks on the surfaces of the α-Fe2O3 microparticles were reduced by etching for 10 s. Fig. 7A(A1–A4) shows XPS spectra of the CoFe2O4 microparticles with the initial surfaces. Fig. 7B(B1–B4) shows XPS spectra of the CoFe2O4 microparticles with etched surfaces. Fig. 7C(C1, C2), 7(D1, D2) indicate the relative comparison of the initial surfaces and etched surfaces of the CoFe2O4 microparticles by XPS, mainly corresponding to the Fe and Co oxidation states of the CoFe2O4 oxide surfaces. In Figs. 6 and 7, the C1s peaks and regions show the C285 reference peak and the described adventitious hydrocarbons inside the prepared sample. The most common O1s peaks and regions were visible due to preparation processes α-Fe2O3 and CoFe2O4 oxides and surrounding environment. The XPS spectra of the Fe2p and Co2p core levels of the prepared CoFe2O4 oxide microparticles, i.e. CoO · Fe2O3 are shown with the typical two peaks at around 711 and 724 eV, as the important surface peaks of α-Fe2O3 for the presence of Fe3 + inside CoFe2O4 oxide as well as the two common satellite peaks at 719 and 734 eV for Fe oxidation states inside the prepared CoFe2O4. These two above satellite peaks on the surfaces of the CoFe2O4 microparticles were reduced by etching for 10 s. The Co2p spectrum exhibited the two main peaks observed at around 781 and 796 eV, and with the two satellite peaks observed at around 803 and 787 eV, respectively. The Co2p1/2 and Co2p3/2 spectra proved the Co2+ valence states in the inverse spin structure of CoFe2O4. The two main peaks and the two satellite peaks led to the hard evidences of the presence of Co2 + highly integrated into the prepared CoFe2O4 oxide microparticles for the best inverse spinel structures. Therefore, Co ions occupied tetrahedral sites, and Fe ions occupied octahedral sites to form the ideal inverse spin structure in the formula CoO · Fe2O3. Thus, the relative cation distribution at tetrahedral and octahedral sites to Co2+ and Fe3+ show inverse level and oxidation states. This proved that CoFe2O4 have much higher magnetization than αFe2O3. It led that the useful formula CoO · Fe2O3 is best proved to being the useful presentation for CoFe2O4 ferrite oxides and other ferrite oxides MeO · Fe2O3. Normally, the integration of Co into Fe oxide ensured that Co ions need to occupy tetrahedral sites, and Fe ions need to occupy octahedral sites for the best inverse level. Therefore, the structural results, and surface analysis of CoFe2O4 ferrite oxide by XPS method revealed that they are in very good agreement with other XRD and SEM measurements. 4. Conclusion In this study, the hierarchical α-Fe2O3 and CoFe2O4 microparticles with the specific grain and boundary textures were made by an efficient

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process via modified polyol method with NaBH4 and heat treatment at 900 °C for 1 h in order to obtain adequate material solidification under heating. Their structures with high crystallization levels were confirmed by XRD method. As a result, the α-Fe2O3 microparticles show antiferromagnetism or very weak ferrimagnetism, and CoFe2O4 microparticles with ferrimagnetism at room temperature. Here, hierarchical α-Fe2O3 and CoFe2O4 microparticles with specific magnetism in grain and grain boundary structures have been studied under heat treatment at high temperature. Here, we have emphasized that we have selected the correct 2:1 ratio of Fe and Co precursor ratio in volume to obtain the best inverse structure of CoFe2O4 produced. With a very small extra volume of Co or Fe precursor and other errors in weight during balancing the precursors, the minor phases of various Co or Fe oxides can be certainly formed in the whole preparation process from synthesis, drying, and heat treatment. They are difficult to be resolved by XRD analysis and evaluation. The highest inverse level and behavior of ferrite materials have significant engineering importance. In next studies, heat treatment processes and operations or annealing procedures will be developed for producing Fe-based oxides to investigate their effects on material structures and properties for the products of better α-Fe2O3 and CoFe2O4 oxide powders with the small modifications [8], and with common modifiers such as Ni, Mn, Cu, Ba, rare earth and other elements. Here, we originally newly propose the modified and improved chemical methods with heat treatment for synthetic processes of Febased oxides with or without rare earth via the liquid-phase chemical reactions, and heat treatment processes in air or mixture of H2/O2 or N2/O2 gases etc. Then, the prepared Fe-based oxides are changed into Fe-based alloys with grain and grain boundary structures via solidphase chemical reactions and stages with the typical efficient strong reducing agents, like Ca compounds in heat treatment processes with various Ar, H2, N2, Ar/H2 gases etc, and via the interface and/or internal chemical reactions for making new hard magnetic materials with rare earth, such as NdFe, SmFe, SmNdFe, NdFeB, SmNdFeB, and their magnetic superalloys etc, which are different from physical and/or chemical metallurgy technologies, and other conventional methods and approaches, i.e. high-energy ball-milling mechanical methods [8,33]. These new ideas and approach methods will open new, efficient, facile, and inexpensive ways of making soft and hard magnetic nanomaterials with grain and grain boundary in both nano and microscale ranges by chemical methods and approaches by the leading researcher, S. Hirosawa at National Institute of Materials Science (NIMS), Japan [33]. The above products of the nano and microparticle powders can be introduced for useful practical applications for our life. Acknowledgment We are grateful to precious supports from Shanghai Institute of Ceramics, Chinese Academy of Science, Dingxi Road 1295, Shanghai 200050, China. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.matdes.2015.07.157. References [1] (a) S. Chikazumi, Physics of Ferromagnetism (International Series of Monographs on Physics), 2nd ed. Oxford University Press Inc, United States, New York, 2009; (b) B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, 2nd ed. Wiley, 2009. [2] A. Goldman, Modern Ferrite Technology, 2nd ed. Springer: Science-Business Media, Inc, 2006. [3] R.M. Cornell, U. Schwertmann, The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses, Verlag GmbH&Co. KGaA, John Wiley&Sons Inc, Weinheim, 2003. [4] H. Kronmüller, S. Parkin, Handbook of Magnetism and Advanced Magnetic Materials, Verlag GmbH&Co. KGaA, John Wiley&Sons Inc, Weinheim, 2007.

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[5] J. Lee, Y. Huh, Y. Jun, J. Seo, J. Jang, H. Song, Artificially engineered magnetic nanoparticles for ultra-sensitive molecular imaging, Nat. Med. 13 (2007) 95–99. [6] X. Mou, X. Wei, Y. Li, W. Shen, Tuning crystal-phase and shape of Fe2O3 nanoparticles for catalytic applications, CrystEngComm 14 (2012) 5107–5120. [7] R.C. Pullar, Prog. Mater. Sci. 57 (2012) 1191–1334. [8] (a) N.V. Long, Y. Yang, T. Teranishi, C.M. Thi, Y. Cao, M. Nogami, Related magnetic properties of CoFe2O4 cobalt ferrite particles synthesised by the polyol method with NaBH4 and heat treatment: new micro and nanoscale structures, RSC Adv. 5 (2015) 56560–56569; (b) N.V. Long, Y. Yang, C.M. Thi, Y. Cao, M. Nogami, Ultra-high stability and durability of α-Fe2O3 oxide micro- and nano-structures with discovery of new 3D structural formation of grain and boundary, Colloids Surf. A 456 (2014) 184–194. [9] N.V. Long, Y. Yang, M. Yuasa, C.M. Thi, Y. Cao, T. Nann, et al., Gas-sensing properties of p-type α-Fe2O3 polyhedral particles synthesized via a modified polyol method, RSC Adv. 4 (2014) 8250–8255. [10] N.V. Long, Y. Yang, M. Yuasa, C.M. Thi, Y. Cao, T. Nann, et al., Controlled synthesis and characterization of iron oxide nanostructures with potential applications for gas sensors and the environment, RSC Adv. 4 (2014) 6383–6390. [11] N.V. Long, Y. Yang, B.T. Hang, Y. Cao, C.M. Thi, M. Nogami, Controlled synthesis and characterization of iron oxide micro-particles for Fe-air battery electrode material, Colloid Polym. Sci. 293 (2015) 49–63. [12] N.V. Long, Y. Yang, M. Yuasa, C.M. Thi, N.V. Minh, M. Nogami, The development of mixture, alloy, and core-shell nano-catalysts with the support nano-materials for energy conversion in low temperature fuel cells, Nano Energy 2 (2013) 636–676. [13] P.C. Fannin, C.N. Marin, I. Malaescu, N. Stefu, P. Vlazan, S. Novaconi, et al., Microwave absorbent properties of nanosized cobalt ferrite powders prepared by coprecipitation and subjected to different thermal treatments, Mater. Des. 32 (2011) 1600–1604. [14] K. Tahmasebi, A. Barzegar, J. Ding, T.S. Herng, A. Huang, S. Shannigrahi, Magnetoelectric effect in Pb(Zr0.95Ti0.05)O3 and CoFe2O4 heteroepitaxial thin film composite, Mater. Des. 32 (2011) 2370–2373. [15] A.R. Jha, Rare Earth Materials Properties and Applications, CRC Press, New York, 2014 (Taylor & Francis Group 6000 Broken Sound Parkway, Suite 300, Boca Raton, FL33487-2742). [16] A. Sharifati, S. Sharafi, Structural and magnetic properties of nanostructured (Fe70Co30)100 − xCux alloy prepared by high energy ball milling, Mater. Des. 41 (2012) 8–15. [17] Y. Liu, D.J. Sellmyer, D. Shindo, Handbook of Advanced Magnetic Materials, Springer Science: Business Media Inc, 2006. [18] C. Yuan, H.B. Wu, Y. Xie, X.W. Lou, Mixed transition-metal oxides: design, synthesis, and energy-related applications, Angew. Chem. Int. Ed. 53 (2014) 1488–1504. [19] (1) V.G. Harris, Microwave Magnetic Materials, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Imprint, University of Amsterdam, Elsevier B.V., Northholland 2012, pp. 1–63; (2) M.A. Willard, M. Daniil, Nanocrystalline Soft Magnetic Alloys Two Decades of Progress, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Imprint, University of Amsterdam, Elsevier B.V., North-holland 2013, pp. 173–342. [20] N.V. Long, N.D. Chien, T. Hayakawa, H. Hirata, G. Lakshminarayana, M. Nogami, The synthesis and characterization of platinum nanoparticles: a method of controlling the size and morphology, Nanotechnology 21 (2010) 035605.

[21] Y.H. Hou, Y.J. Zhao, Z.W. Liu, H.Y. Yu, X.C. Zhong, W.Q. Qiu, et al., Structural, electronic and magnetic properties of partially inverse spinel CoFe2O4: a firstprinciples study, J. Phys. D. Appl. Phys. 43 (2010) 445003. [22] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, et al., Multiferroic BaTiO3–CoFe2O4 Nanostructures, Science 303 (2004) 661–663. [23] M.K. Surendra, R. Dutta, M.S. Rao, Realization of highest specific absorption rate near superparamagnetic limit of CoFe2O4 colloids for magnetic hyperthermia applications, Mater. Res. Express 1 (2014) 026107. [24] Y. Hirayama, Y. Takahashi, H. Satoshi, K. Hono, NdFe12Nx hard-magnetic compound with high magnetization and anisotropy field, Scr. Mater. 95 (2015) 70–72. [25] V.H. Ky, N.H. Dan, N.C. Kien, L.T. Minh, V.M. Quang, L.V. Hong, et al., Influence of quenching rate on magnetic properties of Nd25Fe30Co30Al10B5 alloy, J. Magn. Magn. Mater. 272–276 (2004) 1404–1405. [26] S.P. Gubin, Magnetic Nanoparticles, Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim, 2009. [27] A. Goyal, S. Bansal, V. Kumar, J. Singh, S. Singhal, Mn substituted cobalt ferrites (CoMnxFe2 − xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)): As magnetically separable heterogeneous nanocatalyst for the reduction of nitrophenols, Appl. Surf. Sci. 324 (2015) 877–889. [28] (a) Q. Lin, G. Yuan, Y. He, W. Wang, J. Dong, Y. Yu, The influence of La-substituted Cu0.5Co0.5Fe2O4 nanoparticles on its structural and magnetic properties, Mater. Des. 78 (2015) 80–84; (b) H. Huili, B. Grindi, A. Kouki, G. Viau, L.B. Tahar, Effect of sintering conditions on the structural, electrical, and magnetic properties of nanosized Co0.2Ni0.3Zn0.5Fe2O4, Ceram. Int. 41 (2015) 6212–6225. [29] M.P. Reddy, X.B. Zhou, A. Yann, D. Shiyu, Q. Huang, Low temperature hydrothermal synthesis, structural investigation and functional properties of CoxMn1 − xFe2O4 (0x1.0) nanoferrites, Superlattice. Microst. 81 (2015) 233–242. [30] (a) K. Momma, F. Izumi, VESTA-3 for three-dimensional visualization of crystal, volumetric and morphology data, J. Appl. Crystallogr. 44 (2011) 1272–1276; (b) http://www.crystallography.net/; (c) L. Pauling, S.B. Hendricks, Crystal structures of hematite and corundum, J. Am. Chem. Soc. 47 (1925) 781–790; (d) R.W.G. Wyckoff, Structure of Crystals, 2nd ed. The Chemical Catalog Company, Inc, New York, 1931. [31] N. Baimpas, M. Xie, X. Song, F. Hofmann, B. Abbey, J. Marrow, M. Mostafavi, J. Mi, A.M. Korsunsky, Rich tomography techniques for the analysis of microstructure and deformation, Int. J. Comput. Methods 11 (2014) 1343006–1343018. [32] M. Sugimoto, The Past, Present, and Future of Ferrites, J. Am. Ceram. Soc. 82 (1999) 269–280. [33] (a) S. Hirosawa, Current status of research and development toward permanent magnets free from critical elements, J. Magn. Soc. Jpn. 39 (2015) 85–95; (b) S. Hirosawa, Permanent Magnets Beyond Nd-Dy-Fe-B, JOM 67 (2015) 1304–1305. [34] A. Sinha, S. Nayar, G.V.S. Murthy, P.A. Joy, V. Rao, P. Ramachandrarao, Biomimetic synthesis of superparamagnetic iron oxide particles in proteins, J. Mater. Res. 18 (2003) 1309–1313. [35] B.P. Rao, P.S.V.S. Rao, G.V.S. Murthy, K.H. Rao, Mössbauer study of the system Ni0.65Zn0.35Fe2 − xScxO4, J. Magn. Magn. Mater. 268 (2004) 315–320.