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Withdrawn: Chemical co-precipitation synthesis of spinel manganese ferrite nanoparticles (MnFe2O4): Morphological characterizations and magnetic properties
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Chemical co-precipitation synthesis of spinel manganese ferrite nanoparticles (MnFe2O4): Morphological characterizations and magnetic properties Ramin Ghatreh-Samani a, Amir Mostafaei b,n a b
Department of Materials Engineering, Sahand University of Technology, Tabriz 51335-1996, Iran Young Researchers and Elites Club, Tehran North Branch, Islamic Azad University, Tehran, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2014 Received in revised form 1 November 2014 Accepted 3 November 2014
Manganese ferrite nanoparticles (MnFe2O4) were synthesized by the chemical co-precipitation method and as-synthesized sample was annealed at various temperatures up to 560 °C to attain single powders with single phase. To assess crystalline phases and functional groups in the magnetic nanoparticles, samples were analyzed by X-ray diffraction (XRD) and Fourier transforms infrared spectroscopy (FT-IR), respectively. Morphology and particle size were characterized by electron microscopy methods including transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The obtained nano-powders have spherical structure with the particle size in the ranges of 12–21 nm. The specific surface area of MnFe2O4 nanoparticles was determined by using BET method which was achieved about 122 m2/g and magnetic properties were studied by using vibrating sample magnetometer (VSM). Obtained results revealed that proper magnetic feature could be achieved in the sample annealed at the temperature of 560 °C. The room temperature M–H hysteresis loops indicated that MnFe2O4 nanoparticles showed ferromagnetic behavior with specific saturation magnetization (Ms) values in the ranges of 18–43 emu/g. The roles of particle size and cation distribution of the manganese ferrite nanoparticles on the magnetic property were discussed. & 2014 Elsevier B.V. All rights reserved.
Keywords: Manganese ferrite nanoparticles Chemical co-precipitation synthesis Nanostructures Magnetic properties X-ray diffraction pattern Electron microscopy
1. Introduction In recent years, researchers have widely studied nano-size materials because of their interesting physical and chemical properties compared to their counterpart bulk materials. Spinel manganese ferrite compound with the formulation of MnFe2O4 is one of the most important magnetic oxides which can be used in wide range of applications such as information storage, drug delivery, water treatment, electronic devices, gas sensing, catalysis, biosensing and MRI technology with tunable magnetic properties [1]. Manganese ferrites compounds belong to a group of soft ferrite materials characterized by high magnetic permeability and low losses. As it has been known, the crystallographic, electrical and magnetic properties of ferrites compounds strongly depend on chemical composition, cation distribution, annealing temperature, substitution of cations, shape and size of the prepared powders [2–4]. n
Corresponding author. Fax: þ 98 21 88638514. E-mail address: [email protected] (A. Mostafaei).
Recently, different synthesis methods have been used to attain spinel ferrite nanoparticles with unique electrical and magnetic properties. Some of the more ordinary applied methods are coprecipitation, microemulsion, solid state reaction, chemical autocombustion route, sol–gel, conventional ceramic process and thermal decomposition [1–18]. In addition, different precipitation agents have been utilized to produce manganese ferrite nanostructured with a controlled shape and size such as surfactant and ammonia in the reverse micelles and micro-emulsion methods, organic matrices in the sol–gel method, high temperature and pressure in the hydrothermal method, and metal hydroxide in the co-precipitation method. Although these methods lead to the production of particles of the required size and shape, they are impractical for large-scale applications since they need complicated and expensive instruments, high reaction temperatures, long reaction times, and toxic reagents. By the addition of impurity in a low weight percent, effective modification in magnetic properties of a material can be attained. In the case of spinel ferrites, presence of metal cations of different valance states results in various tetrahedral (A) and octahedral (B) sites distributions [4]. Generally, spinel ferrite structure
http://dx.doi.org/10.1016/j.jmmm.2014.11.005 0304-8853/& 2014 Elsevier B.V. All rights reserved.
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consists of 32 oxygen atoms in cubic close packing with 8 tetrahedral (A) and 16 octahedral (B) occupied sites. Mn2 þ and Fe3 þ ions can occupy either tetrahedral (A) or octahedral (B) interstitial sites. Furthermore, Magnetization of spinel ferrites originates from difference in the magnetic moments of the cations distributed at tetrahedral A and octahedral B sites and this depends on the superexchange interactions through oxygen [1,6,7]. In the present work, as-synthesized nano-powders were initially synthesized by chemical co-precipitation method which offered the advantages of simplicity, low cost, low temperature processing and better homogeneity for the fabrication of multicomponent materials and consequently formation of the homogenized particles of ferrites. Then obtained powders were annealed at different temperature to reach single phase and crystalline structured manganese ferrite nanoparticles. Agglomeration is a drawback in the preparation of magnetic nanoparticles. Thus, one practical method can be calcination of the as-synthesized powders in the higher temperature. The synthesized nanostructured materials were characterized by XRD, FTIR, FESEM, TEM, BET surface area measurements and VSM. Finally, the effects of particle size and cation distribution on the magnetic property were discussed.
2. Experimental 2.1. Chemicals and materials The simplified diagram of the process is presented in Fig. 1. Ferric chloride hexahydrate (FeCl3 6H2O) and manganese chloride tetrahydrate (MnCl2 4H2O) was purchased from Merck
(Germany). NH4OH solution (24%, Merck, code 105422) was used as a reactant agent and NaNO3 (Merck) was also utilized to keep the ionic strength constant in the solution. Analytical grade reagents were used in all experiments without further purification. Deionized water was used in all experiments. 2.2. Synthesis of manganese ferrite nanoparticles The following procedure was used to synthesize manganese ferrite nanoparticles by using chemical co-precipitation method. Initially, 1.73 g of MnCl2 and 3.35 g of FeCl3 were continuously dissolved in deionized water and mixed with a molar ratio of 1:2 under vigorous mechanical stirring. Then after, the mixture was heated up to 85 °C in a water bath and was protected by Ar gas. At this stage, an agent was used to synthesize ferrite nanoparticles including sodium hydroxide chemical compound. 2 M NaOH solution was added drop wisely into the mixture till solution pH attained suitable condition for co-precipitation of the cation from the solution. In this study, the preferred solution pH was 11. When a brown-dark precipitate became visible in the solution, the addition of NaOH was stopped and the solution was stirred for 2 h at the temperature of 85 °C to ensure complete crystallization and growth of the nanoparticles. Then the stirrer was turned off and the magnetic precipitates were isolated by a small magnet from the solution. To obtain pure and product with neutral pH, synthesized materials were washed 5 times with deionized water. The precipitate was then dried by freeze-drier to avoid any agglomeration in the obtained nanoparticles and enhanced their adsorption properties. Chemical reactions could be described as the following equation:
2FeCl3 + MnCl2 +8NaOH=MnFe2 O4 +8NaCl+4H2 O
(1)
Finally, the magnetic nanoparticles were annealed at different temperature including 150, 300, 450, and 560 °C to consider optimum annealing temperature to obtain a crystalline structure with proper magnetic properties. 2.3. Characterization of MnFe2O4 nanoparticles The following methods were utilized to evaluate obtained powders:
Fig. 1. Conceptual flow-diagram of the synthesis process and characterization of obtained manganese ferrite nano-powders.
a. Crystalline structure of MnFe2O4 nanoparticles was investigated by X-ray powder diffraction (XRD-D8 ADVANCEBRUCKER) with Cu Kα radiation operating at 40 kV and 44 mA with counting time about 0.05 s per step. The grain size of the nanoparticles was determined using Sherrer's equation and a Gaussian fitting of three different diffraction peaks. b. As-synthesized powders and annealed samples were characterized by Fourier transform infrared spectroscopy (FTIR). IR spectra were recorded with Bruker Tensor 27 spectrometer over the wave number range of 4000–400 cm 1. The spectra of magnetic nanomaterials were taken as KBr disks. c. The particle size distribution and morphology of nanoparticles were examined by field emission scanning electron microscopy (HITACHI S4160-15 kV) and transmission electron microscopy (Ziess 100 kV). d. The surface area and pore volume of MnFe2O4 nanoparticles were measured by performing N2 adsorption and desorption at 77 K using surface area and porosity analyzer (ChemBET 3000 analyzer Quantochrom model) and the surface area was calculated by Brunauer–Emmett–Teller (BET) equation as well. e. Magnetic behavior of nanoparticles was measured by vibrating sample magnetometer (VSM).
Please cite this article as: R. Ghatreh-Samani, A. Mostafaei, Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/ 10.1016/j.jmmm.2014.11.005i
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3
the following equation [7,18]:
3. Results and discussions
λ 2
[h2
+
k2
+ sin θ
l2]1/2
3.1. XRD studies of synthesized powders
a=
The main Bragg diffraction peaks obtained from each phase are shown in Fig. 2. The miller indices (h k l) help us to identify the obtained powders. The h k l indices of the samples are (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (6 2 0), and (5 3 3). In order to achieve MnFe2O4 compound, it was necessary to anneal the magnetic powders up to the calcination temperature in order to complete the crystallization, as it carried out at four different temperatures (150, 300, 450, and 560 °C) under Ar atmosphere for about 4 h. The XRD pattern obtained from the magnetic powder annealed at 560 °C shows better crystalline structure of manganese ferrite phase with low content of magnetite phase. Although it has been pointed out that the annealing at higher temperature such as 1100 °C may lead to formation of single phase crystalline manganese ferrite, however further increase in the annealing temperature from 560 °C coalesce small particles and consequently it may result in preventing one to achieve particles in the nanometer size [5,6]. From the (3 1 1) peak the crystalline size of the synthesized materials were calculated. The average crystallite size of the synthesized materials was estimated using Debye– Scherrer's equation:
As the results show in Fig. 3b and Table 1, the lattice constant was slightly affected by the calcination temperature. Because the ionic radius of Fe3 þ is smaller than that of Mn3 þ , the replacement of iron by manganese ions leads to an increase in the lattice parameter and hence the size of the unit cell. Mn3 þ ions predominately occupy the octahedral sites, which is consistent with their preference for large octahedral site energy. Finally, the XRD density (ρ) for all the prepared samples has been calculated based on the following equation [2]:
D = kλ /β cos θ
(2)
Here, D is average crystallite size, β is full width at half maxima (FWHM) (rad), λ is wavelength of X-rays (λ ¼ 1.5406 Å), θ is diffraction angle, and k is a constant value which depends on several factors including the Miller index, index of reflection plane and the shape of the crystal, and is usually assigned a value of 0.9 [2]. The calculated crystal sizes of the manganese ferrite particles (D) vary in the range of 12–21 nm (Fig. 3a). In addition, the lattice parameters have been calculated by using d-spacing values and the respective plane parameters from
ρx =
(3)
8M Na3
(4)
where M is the molecular weight, N is Avogadro's constant (6.022 1023), and a is the lattice constant. Summary of the obtained results are given in Table 1 and also presented in Fig. 3c. As can be seen, density of the powders decreased within increasing calcination temperature. The appearance of the plane (1 1 3) can be attributed to the formation of the α-Fe2O3 phase at 560 °C (as shown with green color in Fig. 2), which is evidence of the transfer of Fe3 þ ions from B site to A site in the mixed spinel structure of MnFe2O4 nanoparticles [9]. Additionally, the lattice constant increases with increasing manganese concentration, which can be explained based on the relative ionic radius. The ionic radius of (0.82 Å) of Mn2 þ ions is bigger than ionic radius (0.64 Å) of Fe3 þ ions. Thus, a significant fraction of Mn2 þ ions occupies the octahedral sites and forces Fe3 þ to the tetrahedral sites against their chemical preferences. Moreover, the intensities of the (2 2 0) and (4 0 0) or (4 4 0) planes are more sensitive to the cations on tetrahedral and octahedral sites, respectively [20,21]. As mentioned earlier, Mn2 þ ions have a strong preference to occupy B sites. The intensities of the above three planes can be easily seen in XRD pattern shown in Fig. 2. It can be observed that the intensities of the (4 0 0) or (4 4 0) planes increase with increasing the annealing temperature, which
Fig. 2. X-ray diffraction patterns of MnFe2O4 nanoparticles (a) as-synthesized, and annealed for 4 h at temperatures of (b) 150 °C, (c) 300 °C, (d) 450 °C, and (e) 540 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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R. Ghatreh-Samani, A. Mostafaei / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 1 Data of obtained characteristic parameters including average crystalline size using Scherrer's formula, lattice parameter, average grain size by TEM, densities calculated from XRD results of the MnFe2O4 samples calcined at 150, 300, 450, and 560 °C for 2 h. Samples
Crystal size (nm)
Lattice parameter (Å)
Density, ρx (g/cm3) Average grain size by TEM (nm)
1: Before annealing 2: Annealed 150 °C 3: Annealed 300 °C 4: Annealed 450 °C 5: Annealed 560 °C
11
8.421
5.130
13
at
12
8.426
5.121
14
at
14
8.433
5.109
16
at
17
8.441
5.096
20
at
21
8.457
5.065
26
Fig. 4. FTIR spectra of MnFe2O4 magnetic nanoparticles.
Fig. 3. Variation of (a) crystal size, (b) lattice parameter, and (c) density (ρx) with annealing temperature.
infers that the Mn2 þ has preferentially occupied the B site, i.e. the octahedral site on the (4 0 0) or (4 4 0) planes. However, the intensity of (2 2 0) plane increases slightly by the annealing process at higher temperature indicating the preferential occupation of A sites by Fe3 þ ions [20,21]. It is worthy to mention that the determination of cation distribution of the ferrites by X-ray diffraction can be difficult task since the X-ray scattering contrast is small in spinel ferrites' compounds. 3.2. Fourier transform infrared spectroscopy studies The FTIR spectra recorded for as-synthesized powders and annealed samples are shown in Fig. 4. The IR spectra indicated that
two main metal–oxygen bands due to vibrations of the Fe–O and Mn–O at the tetrahedral site (ν1) appear in the frequency range 500–650 cm 1 and also their vibrations at the octahedral site (ν2) in the frequency range 400–475 cm 1 [1,4,9]. Also, the adsorption broad band at the range of 3200–3450 cm 1 represents a stretching mode of –OH groups and H2O molecules. What is more, as can be seen in Fig. 4, the band around 1530–1660 cm 1 corresponds to the bending mode of H2O molecules and it disappears in the samples which have been annealed at 450 and 560 °C. In the case of as-synthesized powders, weak adsorption bands appeared at 1000–1250 cm 1 and 2320–2710 cm 1 which can be attributed to the formation of symmetrical and asymmetrical stretching vibration of the O–H mode, C–O mode, C ¼H stretching-mode and CH2 groups as organic sources in the magnetic nanoparticles. However, these organic functional groups did not exist in the sample which was annealed at 560 °C indicated
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5
Fig. 5. SEM images of MnFe2O4 nanoparticles (a) as-synthesized materials annealed samples at various annealing temperature of (b) 150 °C, (c) 300 °C, (d) 450 °C, and (e) 560 °C.
that unwanted ions and pollutant compounds were omitted at the consequence of annealing at high temperature [1,4,9]. The obtained results confirmed XRD patterns in which annealing temperature had a crucial role in the fabrication of pure manganese ferrite nanoparticles.
3.3. Electron microscopy investigations Scanning electron images of the as-synthesized materials and annealed samples were shown in Fig. 5. Agglomeration can be easily seen in the as-synthesized materials and also the samples which were annealed at temperatures of 150 and 300 °C. With increasing annealing temperature, grain growth took place and reduction in agglomeration achieved. Morphology and particle
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Fig. 6. TEM image of MnFe2O4 nanoparticles prepared at various annealing temperatures of (a) 150 °C, (b) 300 °C, (c) 450 °C, and (d) 560 °C.
size of MnFe2O4 powders were examined by transmission electron microscopic micrographs. Fig. 6 shows TEM images taken from annealed powders after different annealing temperatures. It is clear that the manganese ferrite nanopowders which were prepared at higher annealing temperature were uniformly distributed and have spherical shaped morphology. Generally, the prepared nanoparticles have particle size in the range of 14–26 nm. The larger particle size of the nanopowders annealed at 560 °C can be attributed to grain growth. Summary of the particle size was given in Table 1. 3.4. BET analysis To ensure the formation of nano-scale size MnFe2O4 particles, BET measurements were performed and results were summarized in Table 2. Specific surface areas of the manganese ferrite nanopowders were measured by BET method. Moreover, average particle size of the products was calculated by following equation [19]:
d = 6000/Sρ
(5)
where d is the average particle size of the products (nm), S is specific surface (m2/g) and ρ is density of the particles (g/cm3). In this equation, density of the manganese ferrite nanoparticles is
Table 2 Data of obtained characteristic parameters including surface area measurements and crystal size calculated by BET of the MnFe2O4 samples calcined at 150, 300, 450, and 560 °C for 2 h. Samples
1: Before annealing 2: Annealed at 150 °C 3: Annealed at 300 °C 4: Annealed at 450 °C 5: Annealed at 560 °C
BET surface area (g/m2) Crystal size calculated by BET (nm) 81.7 88.4
14.36 13.25
93.8
12.52
107.8
10.96
122.1
9.70
calculated based on the Eq. (4) which are given in Table 1. As evident, manganese ferrite nanoparticles which were annealed at the temperature of 560 °C have a high specific surface area about 122.1 m2/g. Additionally, the average particle size of MnFe2O4 nanoparticles were calculated and obtained results confirmed electron microscopy images and X-ray diffraction patterns results (Table 1). As the BET values indicated, surface area of the annealed samples increased with increasing annealing temperature. It seemed somehow in conflict with the fact that grain growth
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7
Fig. 7. Magnetization curve of the (a) as-synthesized materials and prepared MnFe2O4 magnetic nanoparticles at different annealing temperatures of (b) 150 °C, (c) 300 °C, (d) 450 °C, and (e) 560 °C.
occurred when the sample was calcined at higher temperature. However, our investigations indicated that samples which were annealed at lower temperature tended to agglomerate and consequently, it might lead to reduction in an active surface area as well. 3.5. Magnetic properties of MnFe2O4 nanoparticles Various parameters affect magnetic properties of the ferrite compounds including chemical composition, cation distribution and average. It has been known that the occupancy of cations in A and B sites are able to exhibit ferromagnetic, antiferromagnetic and paramagnetic behaviors [2,18]. Moreover, particle size and chemical composition of the magnetic materials have a significant role in the determination of the magnetic properties of the nanopowders and possible applications [1]. The magnetic properties of the as-synthesized material and manganese ferrite nanoparticles annealed at various temperatures including 150 °C, 300 °C, 450 °C, and 560 °C were investigated at the ambient temperature (25 °C) under an applied field using the VSM technique. Fig. 7 indicated hysteresis loops of the MnFe2O4 powders in asprepared and annealed conditions. As can be seen in Fig. 7, a little hysteresis occurs in the magnetization curves of nanoparticles. The magnetic remanence of MnFe2O4 nanoparticles was measured before annealing and obtained 1.304 emu/g while it increased to 3.14 emu/g after annealing at the temperature of 560 °C (Fig. 8a). The coercivity of MnFe2O4 nanoparticles was 18.01 Oe before annealing and when the as-synthesized powders were annealed, the value of the coercivity increased (Fig. 8b). Obtained results revealed the fact that increasing in the value of coercivity was the highest in the case which was calcined at the temperature of 560 °C. The change in magnetization after annealing at higher temperature can be attributed to the change of nanoparticle's phase and corresponding magneto-crystalline parameters of manganese ferrite. In addition, as the TEM images indicated, particle size increased within increasing annealing temperature and consequently, it resulted in the increasing of saturation magnetization values [6]. However, the negligible remanence and coercivity obtained for the MnFe2O4 particles indicated that the particles almost exhibited superparamagnetic behavior. The saturation magnetization obtained was 23.229 emu/g for the as-prepared sample and after annealing at different annealing temperatures, it increased and reached to 48.07 emu/g in the case that powders were annealed at the temperature of 560 °C (Fig. 8c).
Fig. 8. Variation of (a) magnetic remanence, (c) Magnetization (Ms) with annealing temperature.
(b)
coercivity
(Hc),
and
Table 3 summarized magnetic properties of the all synthesized manganese ferrite nanopowders. In the other words, saturation magnetization usually declines with decrease in particle size due to spin canting occurring in the disordered surface layer, and thus, saturation magnetization of the samples which were annealed at 150, 300, 450, and 560 °C increases with increase in manganese ferrite particle size. Generally, bulk manganese ferrite has two magnetic sub-lattice including A and B sites in AB2O4 structure which were separated by oxygen atoms. The magnetic interaction between A and B sites is super-exchange interaction that is mediated by oxygen onions. Also, the magnetic moments have ferromagnetic alignment between A and B sublattices [6–9].
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Table 3 Data of obtained magnetic characteristic parameters of the MnFe2O4 samples calcined at 150, 300, 450, and 560 °C for 2 h. Samples
Magnetic remanence (emu/g)
Coercivity, Hc (Oe)
Magnetization, Ms (emu/g)
1: Before annealing 2: Annealed 150 °C 3: Annealed 300 °C 4: Annealed 450 °C 5: Annealed 560 °C
1.304
18.01
23.21
at
1.618
19.89
25.47
at
2.014
21.45
31.09
at
2.591
25.76
42.53
at
3.140
29.13
48.07
were presented in Fig. 9. It is clear that annealing at higher temperature resulted in improved magnetic properties which could be attained for the sample annealed at 560 °C. Particle size has a close relationship with the magnetization values and after annealing at the temperature of 560 °C, the particle size will be increased as demonstrated by Amighian et al. [5]. In high temperatures with changes in particle size, the magnetic properties such as saturation magnetization, remanence and coercivity were increased [1,5]. It has a face-centered cubic structure with a large unit cell containing eight formula units. There are two kinds of lattices for cation occupancy including A and B sites which have tetrahedral and octahedral coordinations, respectively. Commonly, the M2 þ and Fe3 þ cations distribute at both sites. Generally, it is known that the interactions between A and B sub-lattices in the spinel lattice system (AB2O4) consist of three kinds of interactions including inter-sub-lattice (A–B) super-exchange interactions and intra-sub-lattice (A–A) and (B–B) exchange interactions. Inter-sublattice super-exchange interactions of the cations on the (A–B) are much stronger than the (A–A) and (B–B) intra-sub-lattice exchange interactions. Also, it is worthwhile to mention that by increasing the annealing temperature of the MnFe2O4 nanoparticles, Fe3 þ ions transferred from B site to A site, so the accumulation of Fe3 þ ions increased in A site; however, the FeA3 þ –FeB3 þ superexchange interactions increased (FeA3 þ –FeB3 þ interactions were twice as strong as the MnA2 þ –FeB3 þ interactions), and this can lead to an increase in saturation magnetization in MnFe2O4 nanoparticles [9,20].
4. Conclusions Manganese ferrite nanoparticles were successfully synthesized by the chemical co-precipitation method. X-ray diffraction patter studies indicated that after annealing at the temperature of 560 °C, most of the crystal structure of nanopowders contains single MnFe2O4 phase. FTIR spectroscopy methods confirmed the presence of Mn–O and Fe–O stretching vibrations in the synthesized manganese ferrite nanoparticles. Also, the influences of particle size and cation distribution on the magnetic properties of the manganese ferrite nanoparticles were discussed. Electron microscopic investigations confirmed that manganese ferrite powders have uniform spherical morphology and a mean particle size of around 12–21 nm. Also, surface area measurements indicated that obtained powders have nano-scaled dimensions as well. The magnetic properties were verified using VSM. It was found that the magnetic properties of manganese ferrite nanoparticles increased with increasing the annealing temperature. Magnetization measurement indicated that nanoparticles had a little corecivity before and after annealing.
Acknowledgments
Fig. 9. Variations in (a) magnetic susceptibility against magnetization and (b) magnetic susceptibility against applied field.
Neutron diffraction has revealed a mixed spinel structure of manganese ferrite compound and Fe3 þ and Mn2 þ ions coexist in the two sublattices [6,9,18]. When the annealing temperature increases from 450 to 560 °C, the occupation ratio of Fe3 þ ions at the octahedral sites decrease as well. To further investigate magnetic properties of the MnFe2O4 nanoparticles, variations of magnetic susceptibility against magnetization and applied field were studied and obtained results
The authors would like to acknowledge Iranian Nanotechnology Initiative Council (INIC) for the financial support.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Chemical co-precipitation synthesis of spinel manganese ferrite nanoparticles (MnFe2O4): Morphological characterizations and magnetic properties Ramin Ghatreh-Samani a, Amir Mostafaei b,n a b
Department of Materials Engineering, Sahand University of Technology, Tabriz 51335-1996, Iran Young Researchers and Elites Club, Tehran North Branch, Islamic Azad University, Tehran, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2014 Received in revised form 1 November 2014 Accepted 3 November 2014
Manganese ferrite nanoparticles (MnFe2O4) were synthesized by the chemical co-precipitation method and as-synthesized sample was annealed at various temperatures up to 560 °C to attain single powders with single phase. To assess crystalline phases and functional groups in the magnetic nanoparticles, samples were analyzed by X-ray diffraction (XRD) and Fourier transforms infrared spectroscopy (FT-IR), respectively. Morphology and particle size were characterized by electron microscopy methods including transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The obtained nano-powders have spherical structure with the particle size in the ranges of 12–21 nm. The specific surface area of MnFe2O4 nanoparticles was determined by using BET method which was achieved about 122 m2/g and magnetic properties were studied by using vibrating sample magnetometer (VSM). Obtained results revealed that proper magnetic feature could be achieved in the sample annealed at the temperature of 560 °C. The room temperature M–H hysteresis loops indicated that MnFe2O4 nanoparticles showed ferromagnetic behavior with specific saturation magnetization (Ms) values in the ranges of 18–43 emu/g. The roles of particle size and cation distribution of the manganese ferrite nanoparticles on the magnetic property were discussed. & 2014 Elsevier B.V. All rights reserved.
Keywords: Manganese ferrite nanoparticles Chemical co-precipitation synthesis Nanostructures Magnetic properties X-ray diffraction pattern Electron microscopy
1. Introduction In recent years, researchers have widely studied nano-size materials because of their interesting physical and chemical properties compared to their counterpart bulk materials. Spinel manganese ferrite compound with the formulation of MnFe2O4 is one of the most important magnetic oxides which can be used in wide range of applications such as information storage, drug delivery, water treatment, electronic devices, gas sensing, catalysis, biosensing and MRI technology with tunable magnetic properties [1]. Manganese ferrites compounds belong to a group of soft ferrite materials characterized by high magnetic permeability and low losses. As it has been known, the crystallographic, electrical and magnetic properties of ferrites compounds strongly depend on chemical composition, cation distribution, annealing temperature, substitution of cations, shape and size of the prepared powders [2–4]. n
Corresponding author. Fax: þ 98 21 88638514. E-mail address: [email protected] (A. Mostafaei).
Recently, different synthesis methods have been used to attain spinel ferrite nanoparticles with unique electrical and magnetic properties. Some of the more ordinary applied methods are coprecipitation, microemulsion, solid state reaction, chemical autocombustion route, sol–gel, conventional ceramic process and thermal decomposition [1–18]. In addition, different precipitation agents have been utilized to produce manganese ferrite nanostructured with a controlled shape and size such as surfactant and ammonia in the reverse micelles and micro-emulsion methods, organic matrices in the sol–gel method, high temperature and pressure in the hydrothermal method, and metal hydroxide in the co-precipitation method. Although these methods lead to the production of particles of the required size and shape, they are impractical for large-scale applications since they need complicated and expensive instruments, high reaction temperatures, long reaction times, and toxic reagents. By the addition of impurity in a low weight percent, effective modification in magnetic properties of a material can be attained. In the case of spinel ferrites, presence of metal cations of different valance states results in various tetrahedral (A) and octahedral (B) sites distributions [4]. Generally, spinel ferrite structure
http://dx.doi.org/10.1016/j.jmmm.2014.11.005 0304-8853/& 2014 Elsevier B.V. All rights reserved.
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consists of 32 oxygen atoms in cubic close packing with 8 tetrahedral (A) and 16 octahedral (B) occupied sites. Mn2 þ and Fe3 þ ions can occupy either tetrahedral (A) or octahedral (B) interstitial sites. Furthermore, Magnetization of spinel ferrites originates from difference in the magnetic moments of the cations distributed at tetrahedral A and octahedral B sites and this depends on the superexchange interactions through oxygen [1,6,7]. In the present work, as-synthesized nano-powders were initially synthesized by chemical co-precipitation method which offered the advantages of simplicity, low cost, low temperature processing and better homogeneity for the fabrication of multicomponent materials and consequently formation of the homogenized particles of ferrites. Then obtained powders were annealed at different temperature to reach single phase and crystalline structured manganese ferrite nanoparticles. Agglomeration is a drawback in the preparation of magnetic nanoparticles. Thus, one practical method can be calcination of the as-synthesized powders in the higher temperature. The synthesized nanostructured materials were characterized by XRD, FTIR, FESEM, TEM, BET surface area measurements and VSM. Finally, the effects of particle size and cation distribution on the magnetic property were discussed.
2. Experimental 2.1. Chemicals and materials The simplified diagram of the process is presented in Fig. 1. Ferric chloride hexahydrate (FeCl3 6H2O) and manganese chloride tetrahydrate (MnCl2 4H2O) was purchased from Merck
(Germany). NH4OH solution (24%, Merck, code 105422) was used as a reactant agent and NaNO3 (Merck) was also utilized to keep the ionic strength constant in the solution. Analytical grade reagents were used in all experiments without further purification. Deionized water was used in all experiments. 2.2. Synthesis of manganese ferrite nanoparticles The following procedure was used to synthesize manganese ferrite nanoparticles by using chemical co-precipitation method. Initially, 1.73 g of MnCl2 and 3.35 g of FeCl3 were continuously dissolved in deionized water and mixed with a molar ratio of 1:2 under vigorous mechanical stirring. Then after, the mixture was heated up to 85 °C in a water bath and was protected by Ar gas. At this stage, an agent was used to synthesize ferrite nanoparticles including sodium hydroxide chemical compound. 2 M NaOH solution was added drop wisely into the mixture till solution pH attained suitable condition for co-precipitation of the cation from the solution. In this study, the preferred solution pH was 11. When a brown-dark precipitate became visible in the solution, the addition of NaOH was stopped and the solution was stirred for 2 h at the temperature of 85 °C to ensure complete crystallization and growth of the nanoparticles. Then the stirrer was turned off and the magnetic precipitates were isolated by a small magnet from the solution. To obtain pure and product with neutral pH, synthesized materials were washed 5 times with deionized water. The precipitate was then dried by freeze-drier to avoid any agglomeration in the obtained nanoparticles and enhanced their adsorption properties. Chemical reactions could be described as the following equation:
2FeCl3 + MnCl2 +8NaOH=MnFe2 O4 +8NaCl+4H2 O
(1)
Finally, the magnetic nanoparticles were annealed at different temperature including 150, 300, 450, and 560 °C to consider optimum annealing temperature to obtain a crystalline structure with proper magnetic properties. 2.3. Characterization of MnFe2O4 nanoparticles The following methods were utilized to evaluate obtained powders:
Fig. 1. Conceptual flow-diagram of the synthesis process and characterization of obtained manganese ferrite nano-powders.
a. Crystalline structure of MnFe2O4 nanoparticles was investigated by X-ray powder diffraction (XRD-D8 ADVANCEBRUCKER) with Cu Kα radiation operating at 40 kV and 44 mA with counting time about 0.05 s per step. The grain size of the nanoparticles was determined using Sherrer's equation and a Gaussian fitting of three different diffraction peaks. b. As-synthesized powders and annealed samples were characterized by Fourier transform infrared spectroscopy (FTIR). IR spectra were recorded with Bruker Tensor 27 spectrometer over the wave number range of 4000–400 cm 1. The spectra of magnetic nanomaterials were taken as KBr disks. c. The particle size distribution and morphology of nanoparticles were examined by field emission scanning electron microscopy (HITACHI S4160-15 kV) and transmission electron microscopy (Ziess 100 kV). d. The surface area and pore volume of MnFe2O4 nanoparticles were measured by performing N2 adsorption and desorption at 77 K using surface area and porosity analyzer (ChemBET 3000 analyzer Quantochrom model) and the surface area was calculated by Brunauer–Emmett–Teller (BET) equation as well. e. Magnetic behavior of nanoparticles was measured by vibrating sample magnetometer (VSM).
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3
the following equation [7,18]:
3. Results and discussions
λ 2
[h2
+
k2
+ sin θ
l2]1/2
3.1. XRD studies of synthesized powders
a=
The main Bragg diffraction peaks obtained from each phase are shown in Fig. 2. The miller indices (h k l) help us to identify the obtained powders. The h k l indices of the samples are (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (6 2 0), and (5 3 3). In order to achieve MnFe2O4 compound, it was necessary to anneal the magnetic powders up to the calcination temperature in order to complete the crystallization, as it carried out at four different temperatures (150, 300, 450, and 560 °C) under Ar atmosphere for about 4 h. The XRD pattern obtained from the magnetic powder annealed at 560 °C shows better crystalline structure of manganese ferrite phase with low content of magnetite phase. Although it has been pointed out that the annealing at higher temperature such as 1100 °C may lead to formation of single phase crystalline manganese ferrite, however further increase in the annealing temperature from 560 °C coalesce small particles and consequently it may result in preventing one to achieve particles in the nanometer size [5,6]. From the (3 1 1) peak the crystalline size of the synthesized materials were calculated. The average crystallite size of the synthesized materials was estimated using Debye– Scherrer's equation:
As the results show in Fig. 3b and Table 1, the lattice constant was slightly affected by the calcination temperature. Because the ionic radius of Fe3 þ is smaller than that of Mn3 þ , the replacement of iron by manganese ions leads to an increase in the lattice parameter and hence the size of the unit cell. Mn3 þ ions predominately occupy the octahedral sites, which is consistent with their preference for large octahedral site energy. Finally, the XRD density (ρ) for all the prepared samples has been calculated based on the following equation [2]:
D = kλ /β cos θ
(2)
Here, D is average crystallite size, β is full width at half maxima (FWHM) (rad), λ is wavelength of X-rays (λ ¼ 1.5406 Å), θ is diffraction angle, and k is a constant value which depends on several factors including the Miller index, index of reflection plane and the shape of the crystal, and is usually assigned a value of 0.9 [2]. The calculated crystal sizes of the manganese ferrite particles (D) vary in the range of 12–21 nm (Fig. 3a). In addition, the lattice parameters have been calculated by using d-spacing values and the respective plane parameters from
ρx =
(3)
8M Na3
(4)
where M is the molecular weight, N is Avogadro's constant (6.022 1023), and a is the lattice constant. Summary of the obtained results are given in Table 1 and also presented in Fig. 3c. As can be seen, density of the powders decreased within increasing calcination temperature. The appearance of the plane (1 1 3) can be attributed to the formation of the α-Fe2O3 phase at 560 °C (as shown with green color in Fig. 2), which is evidence of the transfer of Fe3 þ ions from B site to A site in the mixed spinel structure of MnFe2O4 nanoparticles [9]. Additionally, the lattice constant increases with increasing manganese concentration, which can be explained based on the relative ionic radius. The ionic radius of (0.82 Å) of Mn2 þ ions is bigger than ionic radius (0.64 Å) of Fe3 þ ions. Thus, a significant fraction of Mn2 þ ions occupies the octahedral sites and forces Fe3 þ to the tetrahedral sites against their chemical preferences. Moreover, the intensities of the (2 2 0) and (4 0 0) or (4 4 0) planes are more sensitive to the cations on tetrahedral and octahedral sites, respectively [20,21]. As mentioned earlier, Mn2 þ ions have a strong preference to occupy B sites. The intensities of the above three planes can be easily seen in XRD pattern shown in Fig. 2. It can be observed that the intensities of the (4 0 0) or (4 4 0) planes increase with increasing the annealing temperature, which
Fig. 2. X-ray diffraction patterns of MnFe2O4 nanoparticles (a) as-synthesized, and annealed for 4 h at temperatures of (b) 150 °C, (c) 300 °C, (d) 450 °C, and (e) 540 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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R. Ghatreh-Samani, A. Mostafaei / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 1 Data of obtained characteristic parameters including average crystalline size using Scherrer's formula, lattice parameter, average grain size by TEM, densities calculated from XRD results of the MnFe2O4 samples calcined at 150, 300, 450, and 560 °C for 2 h. Samples
Crystal size (nm)
Lattice parameter (Å)
Density, ρx (g/cm3) Average grain size by TEM (nm)
1: Before annealing 2: Annealed 150 °C 3: Annealed 300 °C 4: Annealed 450 °C 5: Annealed 560 °C
11
8.421
5.130
13
at
12
8.426
5.121
14
at
14
8.433
5.109
16
at
17
8.441
5.096
20
at
21
8.457
5.065
26
Fig. 4. FTIR spectra of MnFe2O4 magnetic nanoparticles.
Fig. 3. Variation of (a) crystal size, (b) lattice parameter, and (c) density (ρx) with annealing temperature.
infers that the Mn2 þ has preferentially occupied the B site, i.e. the octahedral site on the (4 0 0) or (4 4 0) planes. However, the intensity of (2 2 0) plane increases slightly by the annealing process at higher temperature indicating the preferential occupation of A sites by Fe3 þ ions [20,21]. It is worthy to mention that the determination of cation distribution of the ferrites by X-ray diffraction can be difficult task since the X-ray scattering contrast is small in spinel ferrites' compounds. 3.2. Fourier transform infrared spectroscopy studies The FTIR spectra recorded for as-synthesized powders and annealed samples are shown in Fig. 4. The IR spectra indicated that
two main metal–oxygen bands due to vibrations of the Fe–O and Mn–O at the tetrahedral site (ν1) appear in the frequency range 500–650 cm 1 and also their vibrations at the octahedral site (ν2) in the frequency range 400–475 cm 1 [1,4,9]. Also, the adsorption broad band at the range of 3200–3450 cm 1 represents a stretching mode of –OH groups and H2O molecules. What is more, as can be seen in Fig. 4, the band around 1530–1660 cm 1 corresponds to the bending mode of H2O molecules and it disappears in the samples which have been annealed at 450 and 560 °C. In the case of as-synthesized powders, weak adsorption bands appeared at 1000–1250 cm 1 and 2320–2710 cm 1 which can be attributed to the formation of symmetrical and asymmetrical stretching vibration of the O–H mode, C–O mode, C ¼H stretching-mode and CH2 groups as organic sources in the magnetic nanoparticles. However, these organic functional groups did not exist in the sample which was annealed at 560 °C indicated
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5
Fig. 5. SEM images of MnFe2O4 nanoparticles (a) as-synthesized materials annealed samples at various annealing temperature of (b) 150 °C, (c) 300 °C, (d) 450 °C, and (e) 560 °C.
that unwanted ions and pollutant compounds were omitted at the consequence of annealing at high temperature [1,4,9]. The obtained results confirmed XRD patterns in which annealing temperature had a crucial role in the fabrication of pure manganese ferrite nanoparticles.
3.3. Electron microscopy investigations Scanning electron images of the as-synthesized materials and annealed samples were shown in Fig. 5. Agglomeration can be easily seen in the as-synthesized materials and also the samples which were annealed at temperatures of 150 and 300 °C. With increasing annealing temperature, grain growth took place and reduction in agglomeration achieved. Morphology and particle
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Fig. 6. TEM image of MnFe2O4 nanoparticles prepared at various annealing temperatures of (a) 150 °C, (b) 300 °C, (c) 450 °C, and (d) 560 °C.
size of MnFe2O4 powders were examined by transmission electron microscopic micrographs. Fig. 6 shows TEM images taken from annealed powders after different annealing temperatures. It is clear that the manganese ferrite nanopowders which were prepared at higher annealing temperature were uniformly distributed and have spherical shaped morphology. Generally, the prepared nanoparticles have particle size in the range of 14–26 nm. The larger particle size of the nanopowders annealed at 560 °C can be attributed to grain growth. Summary of the particle size was given in Table 1. 3.4. BET analysis To ensure the formation of nano-scale size MnFe2O4 particles, BET measurements were performed and results were summarized in Table 2. Specific surface areas of the manganese ferrite nanopowders were measured by BET method. Moreover, average particle size of the products was calculated by following equation [19]:
d = 6000/Sρ
(5)
where d is the average particle size of the products (nm), S is specific surface (m2/g) and ρ is density of the particles (g/cm3). In this equation, density of the manganese ferrite nanoparticles is
Table 2 Data of obtained characteristic parameters including surface area measurements and crystal size calculated by BET of the MnFe2O4 samples calcined at 150, 300, 450, and 560 °C for 2 h. Samples
1: Before annealing 2: Annealed at 150 °C 3: Annealed at 300 °C 4: Annealed at 450 °C 5: Annealed at 560 °C
BET surface area (g/m2) Crystal size calculated by BET (nm) 81.7 88.4
14.36 13.25
93.8
12.52
107.8
10.96
122.1
9.70
calculated based on the Eq. (4) which are given in Table 1. As evident, manganese ferrite nanoparticles which were annealed at the temperature of 560 °C have a high specific surface area about 122.1 m2/g. Additionally, the average particle size of MnFe2O4 nanoparticles were calculated and obtained results confirmed electron microscopy images and X-ray diffraction patterns results (Table 1). As the BET values indicated, surface area of the annealed samples increased with increasing annealing temperature. It seemed somehow in conflict with the fact that grain growth
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Fig. 7. Magnetization curve of the (a) as-synthesized materials and prepared MnFe2O4 magnetic nanoparticles at different annealing temperatures of (b) 150 °C, (c) 300 °C, (d) 450 °C, and (e) 560 °C.
occurred when the sample was calcined at higher temperature. However, our investigations indicated that samples which were annealed at lower temperature tended to agglomerate and consequently, it might lead to reduction in an active surface area as well. 3.5. Magnetic properties of MnFe2O4 nanoparticles Various parameters affect magnetic properties of the ferrite compounds including chemical composition, cation distribution and average. It has been known that the occupancy of cations in A and B sites are able to exhibit ferromagnetic, antiferromagnetic and paramagnetic behaviors [2,18]. Moreover, particle size and chemical composition of the magnetic materials have a significant role in the determination of the magnetic properties of the nanopowders and possible applications [1]. The magnetic properties of the as-synthesized material and manganese ferrite nanoparticles annealed at various temperatures including 150 °C, 300 °C, 450 °C, and 560 °C were investigated at the ambient temperature (25 °C) under an applied field using the VSM technique. Fig. 7 indicated hysteresis loops of the MnFe2O4 powders in asprepared and annealed conditions. As can be seen in Fig. 7, a little hysteresis occurs in the magnetization curves of nanoparticles. The magnetic remanence of MnFe2O4 nanoparticles was measured before annealing and obtained 1.304 emu/g while it increased to 3.14 emu/g after annealing at the temperature of 560 °C (Fig. 8a). The coercivity of MnFe2O4 nanoparticles was 18.01 Oe before annealing and when the as-synthesized powders were annealed, the value of the coercivity increased (Fig. 8b). Obtained results revealed the fact that increasing in the value of coercivity was the highest in the case which was calcined at the temperature of 560 °C. The change in magnetization after annealing at higher temperature can be attributed to the change of nanoparticle's phase and corresponding magneto-crystalline parameters of manganese ferrite. In addition, as the TEM images indicated, particle size increased within increasing annealing temperature and consequently, it resulted in the increasing of saturation magnetization values [6]. However, the negligible remanence and coercivity obtained for the MnFe2O4 particles indicated that the particles almost exhibited superparamagnetic behavior. The saturation magnetization obtained was 23.229 emu/g for the as-prepared sample and after annealing at different annealing temperatures, it increased and reached to 48.07 emu/g in the case that powders were annealed at the temperature of 560 °C (Fig. 8c).
Fig. 8. Variation of (a) magnetic remanence, (c) Magnetization (Ms) with annealing temperature.
(b)
coercivity
(Hc),
and
Table 3 summarized magnetic properties of the all synthesized manganese ferrite nanopowders. In the other words, saturation magnetization usually declines with decrease in particle size due to spin canting occurring in the disordered surface layer, and thus, saturation magnetization of the samples which were annealed at 150, 300, 450, and 560 °C increases with increase in manganese ferrite particle size. Generally, bulk manganese ferrite has two magnetic sub-lattice including A and B sites in AB2O4 structure which were separated by oxygen atoms. The magnetic interaction between A and B sites is super-exchange interaction that is mediated by oxygen onions. Also, the magnetic moments have ferromagnetic alignment between A and B sublattices [6–9].
Please cite this article as: R. Ghatreh-Samani, A. Mostafaei, Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/ 10.1016/j.jmmm.2014.11.005i
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Table 3 Data of obtained magnetic characteristic parameters of the MnFe2O4 samples calcined at 150, 300, 450, and 560 °C for 2 h. Samples
Magnetic remanence (emu/g)
Coercivity, Hc (Oe)
Magnetization, Ms (emu/g)
1: Before annealing 2: Annealed 150 °C 3: Annealed 300 °C 4: Annealed 450 °C 5: Annealed 560 °C
1.304
18.01
23.21
at
1.618
19.89
25.47
at
2.014
21.45
31.09
at
2.591
25.76
42.53
at
3.140
29.13
48.07
were presented in Fig. 9. It is clear that annealing at higher temperature resulted in improved magnetic properties which could be attained for the sample annealed at 560 °C. Particle size has a close relationship with the magnetization values and after annealing at the temperature of 560 °C, the particle size will be increased as demonstrated by Amighian et al. [5]. In high temperatures with changes in particle size, the magnetic properties such as saturation magnetization, remanence and coercivity were increased [1,5]. It has a face-centered cubic structure with a large unit cell containing eight formula units. There are two kinds of lattices for cation occupancy including A and B sites which have tetrahedral and octahedral coordinations, respectively. Commonly, the M2 þ and Fe3 þ cations distribute at both sites. Generally, it is known that the interactions between A and B sub-lattices in the spinel lattice system (AB2O4) consist of three kinds of interactions including inter-sub-lattice (A–B) super-exchange interactions and intra-sub-lattice (A–A) and (B–B) exchange interactions. Inter-sublattice super-exchange interactions of the cations on the (A–B) are much stronger than the (A–A) and (B–B) intra-sub-lattice exchange interactions. Also, it is worthwhile to mention that by increasing the annealing temperature of the MnFe2O4 nanoparticles, Fe3 þ ions transferred from B site to A site, so the accumulation of Fe3 þ ions increased in A site; however, the FeA3 þ –FeB3 þ superexchange interactions increased (FeA3 þ –FeB3 þ interactions were twice as strong as the MnA2 þ –FeB3 þ interactions), and this can lead to an increase in saturation magnetization in MnFe2O4 nanoparticles [9,20].
4. Conclusions Manganese ferrite nanoparticles were successfully synthesized by the chemical co-precipitation method. X-ray diffraction patter studies indicated that after annealing at the temperature of 560 °C, most of the crystal structure of nanopowders contains single MnFe2O4 phase. FTIR spectroscopy methods confirmed the presence of Mn–O and Fe–O stretching vibrations in the synthesized manganese ferrite nanoparticles. Also, the influences of particle size and cation distribution on the magnetic properties of the manganese ferrite nanoparticles were discussed. Electron microscopic investigations confirmed that manganese ferrite powders have uniform spherical morphology and a mean particle size of around 12–21 nm. Also, surface area measurements indicated that obtained powders have nano-scaled dimensions as well. The magnetic properties were verified using VSM. It was found that the magnetic properties of manganese ferrite nanoparticles increased with increasing the annealing temperature. Magnetization measurement indicated that nanoparticles had a little corecivity before and after annealing.
Acknowledgments
Fig. 9. Variations in (a) magnetic susceptibility against magnetization and (b) magnetic susceptibility against applied field.
Neutron diffraction has revealed a mixed spinel structure of manganese ferrite compound and Fe3 þ and Mn2 þ ions coexist in the two sublattices [6,9,18]. When the annealing temperature increases from 450 to 560 °C, the occupation ratio of Fe3 þ ions at the octahedral sites decrease as well. To further investigate magnetic properties of the MnFe2O4 nanoparticles, variations of magnetic susceptibility against magnetization and applied field were studied and obtained results
The authors would like to acknowledge Iranian Nanotechnology Initiative Council (INIC) for the financial support.
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