Magnesium Ferrite (MgFe2O4) Nanostructures Fabricated by Electrospinning
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Magnesium ferrite (MgFe2O4) nanostructures were successfully fabricated by electrospinning method. X-ray diffraction, FT-IR, scanning electron microscopy, and transmission electron microscopy revealed that calcination of the as-spun MgFe2O4/poly(vinyl pyrrolidone) (PVP) composite nanofibers at 500–800 °C in air for 2 h resulted in well-developed spinel MgFe2O4nanostuctures. The crystal structure and morphology of the nanofibers were influenced by the calcination temperature. Crystallite size of the nanoparticles contained in nanofibers increased from 15 ± 4 to 24 ± 3 nm when calcination temperature was increased from 500 to 800 °C. Room temperature magnetization results showed a ferromagnetic behavior of the calcined MgFe2O4/PVP composite nanofibers, having their specific saturation magnetization (Ms) values of 17.0, 20.7, 25.7, and 31.1 emu/g at 10 Oe for the samples calcined at 500, 600, 700, and 800 °C, respectively. It is found that the increase in the tendency ofMsis consistent with the enhancement of crystallinity, and the values ofMsfor the MgFe2O4samples were observed to increase with increasing crystallite size.
KeywordsMagnesium ferrite Nanofibers Electrospinning Electron microscopy X-ray diffraction Magnetic properties Nanofabrication
Spinel ferrites with the general formula AFe2O4 (A = Mn, Co, Ni, Mg, or Zn) are very important magnetic materials because of their interesting magnetic and electrical properties with chemical and thermal stabilities . Magnesium ferrite (MgFe2O4) is one of the most important ferrites. It has a cubic structure of normal spinel-type and is a soft magnetic n-type semiconducting material, which finds a number of applications in heterogeneous catalysis, adsorption, sensors, and in magnetic technologies . Recently, nanostructures of magnetic materials have received more and more attention due to their novel material properties that are significantly different from those of their bulk counterparts [3, 4, 5, 6, 7]. The ordered magnetic materials such as nanorods and nanowires have currently attracted a great interest due to their enhanced magnetic property [8, 9]. So far, reported nanostructures MgFe2O4 are mostly in the form of nanoparticle [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22], whereas other nanostructured forms of MgFe2O4 have not been reported. Large surface-to-volume ratio is an attractive characteristic that can be achieved from nanofiberization of magnetic materials. With such feature, their technological application should be expressed into many areas including nanocomposites, nanocatalysts, nanosensors, nano-electronics, and photonics.
A number of methods have been developed to fabricate materials with nanofibrous structures, including an electrospinning which is a simple and convenient method for preparing polymer fibers and ceramic fibers with both solid and hollow interiors that are exceptionally long in length, uniform in diameter ranging from tens of nanometers to several micrometers, and diversified in compositions [23, 24]. In an electrospinning process , an electrical potential is applied between a droplet of a polymer solution held at the end of the nozzle of the spinneret and a ground target. When the applied electric field overcomes the surface tension of the droplet, a charged jet of polymer solution is ejected. The route of the charged jet is controlled by the electric field. The jet exhibits bending instabilities caused by repulsive forces between the charges carried with the jet. The jet extends through spiralling loops. As the loops increase in diameter the jet grows longer and thinner until it solidifies or is collected on the target.
To date, electrospun ferrite nanofibers of NiFe2O4, CoFe2O4, MnFe2O4, and CuFe2O4 have been reported. To the best of our knowledge, electrospinning of MgFe2O4 has not yet been reported. Thus, the present work investigated the fabrication of MgFe2O4 nanofibers by electrospinning using a solution that contained poly(vinyl pyrrolidone) (PVP) and cheap Mg and Fe nitrates as metal sources. The samples of as-spun and calcined MgFe2O4/PVP composite were characterized by thermogravimetric-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), FT-IR, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The magnetic properties of calcined MgFe2O4/PVP composite samples were investigated using a vibrating sample magnetometer (VSM) at room temperature. The effects of calcination temperature on morphology, structure, and magnetic properties of the fabricated samples were also studied.
In this study, Mg(NO3)2 · 6H2O (99% purity, Kanto Chemicals, Japan), Fe(NO3)3 · 9H2O (99.99% purity, Kanto Chemicals, Japan) and PVP (Mn = 1,300,000, Aldrich),N,N-Dimetylformamide (DMF) (99.8% purity, Fluka, Switzerland), acetic acid (100% purity, BDH, England), and ethanol (100% purity, BDH, England) were used as the starting chemicals. In the preparation of the solution for electrospinning, we used a solution that contained PVP mixed with Mg(NO3)2 · 6H2O and Fe(NO3)3 · 9H2O. A PVP/ethanol solution was prepared using a ratio of 1.0 g PVP to 9 mL ethanol. A metal nitrates/DMF solution was prepared by dissolving 0.01 mol Mg(NO3)26H2O and 0.02 mol Fe(NO3)3 · 9H2O in 10 mL of DMF and stirred for 5 h. Subsequently, the metal nitrates/DMF solution (4 mL) was added slowly to the PVP/ethanol solution (50 mL) under vigorous stir at 27 °C for 5 h to obtain a well-dissolved solution. This final solution was used for electrospinning.
The as-spun MgFe2O4/PVP composite nanofibers were subjected to TG-DTA using Pyris Diamond TG/DTA (PerkinElmer Instrument, USA). This was done to determine the temperatures of possible decomposition and crystallization (or phase changes) of the as-spun nanofibers. The analyses were performed with a heating rate of 5 °C/min in static air up to 1000 °C. The composite nanofibers were calcined at 500, 600, 700, and 800 °C for 2 h in air in box furnace (Lenton Furnaces, UK), using heating and cooling rates of 5 °C/min. The final products obtained were brown MgFe2O4samples. The as-spun and calcined composite nanofibers were characterized by means of XRD using CuK a radiation withλ = 0.15418 nm (PW3040 mpd control, The Netherlands), FT-IR spectroscopy (Spectrum One FT-IR Spectrometer, PerkinElmer Instruments, USA), SEM (Hitachi FE-SEM S–4700, Japan), and TEM (Philips Tecnai 12 G2 TEM, at 120 kV, The Netherlands). The average diameters of the as-spun and calcined composite nanofibers were determined from about 300 measurements. The magnetic properties of the calcined samples were examined at room temperature (20 °C) using a VSM (Lake Shore VSM 7403, USA).
Results and Discussion
Average crystal sizes from XRD, spinel lattice parametera calculated from XRD spectra, the specific magnetization (Ms), remnant magnetization (Mr), the ratio of the ratio of remnant magnetization to bulk saturation magnetization (Mr/Ms), and coercive forces (Hc) of the MgFe2O4/PVP composite samples calcined in air at 500, 600, 700, and 800 °C for 2 h
Average crystallite size from XRD (nm)
Spinel lattice parametersa(nm)
Msat 10 kOe (emu/g)
Calcined at 500 °C
15 ± 4
0.8372 ± 0.0007
Calcined at 600 °C
17 ± 1
0.8362 ± 0.0012
Calcined at 700 °C
23 ± 2
0.8353 ± 0.0011
Calcined at 800 °C
24 ± 3
0.8346 ± 0.0030
The specific magnetization curves of the calcined MgFe2O4/PVP composite nanofibers obtained from room temperature VSM measurement are shown in Fig. 8. These curves are typical for a soft magnetic material and indicate hysteresis ferromagnetism in the field range of ±500 Oe, while outside this range the specific magnetization increases with increasing field and tends to saturate in the field range investigated (±10 kOe). The specific saturation magnetization (Ms) values of 17.0, 20.7, 25.7, and 31.1 emu/g at 10 kOe were observed for the MgFe2O4/PVP composite nanofibers calcined at 500, 600, 700, and 800 °C, respectively. It is found that the increase in the tendency of Ms is consistent with the enhancement of crystallinity, and the values of Ms for the MgFe2O4 samples were observed to increase with increasing crystallite size. This type of behavior is entirely consistent with a model of crystal growth in such a way that the difference in the magnetic parameters is associated with the change in crystallite size . Noted that the saturation value of 31.1 emu/g obtained in the sample calcined at 800 °C (crystallite size of 24 ± 3 nm) is close to the values of 33.4 emu/g for bulk MgFe2O4 and 30.6 emu/g for sol–gel/combustion synthesized MgFe2O4 (crystallite size of ~78 nm) , while it is higher than the values of ~14.09 emu/g for coprecipitation-synthesized MgFe2O4 nanoparticles (diameters of ~34.4 nm)  and 15.3 emu/g for sol–gel-derived MgFe2O4 nanoparticles (diameters of ~42 nm) .
The coercive forces (Hc) were obtained to be 35.8, 37.6, 71.2, and 98.9 Oe for the MgFe2O4/PVP composite nanofibers calcined at 500, 600, 700, and 800 °C, respectively. These values are comparable to the values of 48.86–75.99 Oe for coprecipitation-synthesized MgFe2O4 nanoparticles (diameters of ~27.2–112 nm) , but are lower than the value of 165 Oe for sol–gel/combustion-synthesized MgFe2O4 (crystallite size of ~78 nm) . It is seen from our results that the Hc values of the calcined MgFe2O4/PVP composite nanofibers increased with crystallite size. It is known that the variation of Hc with particle size can be explained on the basis of domain structure, critical diameter, and the anisotropy of the crystal [39, 40, 41, 42]. Rashad  reported that Hc increased from 48.86 for 27.2-nm MgFe2O4 nanoparticles to 75.99 for 34.4-nm MgFe2O4 nanoparticles and then decreased to 68.11 Oe for 112-nm MgFe2O4 nanoparticles. In this case, the particle size of the 112-nm MgFe2O4 nanoparticles is possibly larger than that of the critical size and thus results in the decrease in Hc, while the particle sizes of our electrospun MgFe2O4 samples have not reached their critical size and therefore Hc was increased with increase in crystal size. The values of specific magnetization at 10 kOe, remnant magnetization (Mr), the ratio of remnant magnetization to bulk saturation magnetization (Mr/Ms), and coercive forces (Hc) are also tabulated in Table 1.
Nanostructures of MgFe2O4have been successfully fabricated using an electrospinning technique. Polycrystalline MgFe2O4nanostructures (crystallite size of ~15–24 nm) as confirmed by SEAD analysis, XRD and FT-IR were formed after calcination of the as-spun MgFe2O4/PVP composite nanofibres in air at above 500 °C for 2 h. The calcined samples consisted of the structure of packed particles or crystallites of <50 nm, as revealed by SEM and TEM. The crystal structure and morphology of the calcined samples were influenced by the calcination temperature. All of the electrospun MgFe2O4samples are ferromagnetic, having the specific magnetizations of 17.0, 20.7, 25.7, and 31.1 emu/g at 10 kOe for the samples calcined at 500, 600, 700, and 800 °C, respectively. We believe that the electrospun MgFe2O4nanostructures could have potential in some new applications as ferromagnetic nanostructures for nanocomposites, separation, anodic material in lithium ion batteries, catalysts, and as electronic material for nanodevices and storage devices.
The authors would like to thank the Department of Chemistry, Khon Kaen University for providing TG-DTA, FT-IR, and VSM facilities, the Science Lab Center, Naresuan University for providing TEM facilities, the Department of Physics, Faculty of Science, Ubon Ratchathani University for providing XRD facilities, and the Thai Microelectronics Center (TMEC) for FE-SEM facilities. This study is supported by The National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence.
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