Magnetic properties of Cr doped Fe3O4 porous nanoparticles prepared through a co-precipitation method using surfactant

Magnetic Cr3+xFe3+2 − xFe2+O4 (0 ≦̸ x ≦̸ 0.1) porous nanoparticles were prepared by the aqueous co-precipitation method. The resulting magnetic nanoparticles were characterized by using an x-ray diffraction (XRD), field enhanced scanning electron microscope (FESEM), transmission electron microscope (TEM), and vibrating sampling magnetometer (VSM). The nitrogen gas adsorption/desorption isotherm showed a microporous structure of the obtained magnetic materials. A rod and round shape of Fe3O4 was observed as using polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) surfactant, respectively. The Fe3O4 nanoparticles exhibited superparamagnetic properties with easy separation and re-dispersion in solution by using an external magnet. More remarkably, the saturation magnetization (Ms) was enhanced up to 1.2 times for doping Cr3+ into the Fe3O4 lattice. The effect of surfactants and Cr3+ doping concentrations on size and the magnetic properties of Fe3O4 nanoparticles are studied.


Introduction
Spinel ferrite nanoparticles with superparamagnetic behavior have attracted much attention in nanoscience and nanotechnology because of their potential applications for magnetic resonance imaging, drug targeting, catalysis and highdensity magnetic recording devices [1][2][3][4][5][6]. Each type of application requires magnetic nanoparticles with specific physicochemical and magnetic properties that can be engineered during the synthesis process [7,8]. The magnetic properties could be improved by doping transition metals such as Co, Mn, Nd into Fe 3 O 4 lattice due to their enhanced crystal anisotropy [9,10]. There are many methods to prepare Fe 3 O 4 nanoparticles, including co-precipitation, hydrothermal, micro-emulsion, sol-gel, and using porous templates [7,[11][12][13][14]. The co-precipitation route shows outstanding advantages such as producing well water-dispersible nanoparticles in high yields, being cost-effective, less time-consuming, easily scalable for industrial applications, and environmentally friendly without using hazardous solvents [13,14]. However, the control of the particle size, morphology, and magnetic properties through this route has met with very limited success.
In this work, superparamagnetic spinel ferrite Fe 3 O 4 nanoparticles were prepared by co-precipitation method at temperature 70°C. The morphology of magnetic materials changed from nano-round to nano-rod when using cetyltrimethylammonium bromide (CTAB) and polyvinylpyrrolidone (PVP) surfactant, respectively. The surfactants act as micelle templates in forming different shapes with porous structures. Magnetic nanoparticles with porous structure will be promising materials for advanced catalysts because of their easy recyclability, good dispersion and high shape selectivity for both reactants and products [15,16]. In order to improve the magnetic properties, transition metal ion Cr 3+ was introduced into Fe 3 2+ :Cr 3+ ∼2 − x:1:x) (0 ⩽ x ⩽ 0.1) was dissolved in 100 ml of distilled and deoxygenated water containing 2.5 gram surfactant. Then, 50 ml of a 5.0 M NaOH solution was dropped gradually into the mixture solution under vigorous stirring and heating at 60°C for 2 h. The resulting nanoparticles were washed with distilled water and ethanol several times and dried at 60°C for 5 h in an oven. During washing, magnetic powder was easily separated from solution by magnet, as shown in figure 1.

Characterization
The resulting powders were characterized by XRD analysis using an x-ray diffractometer Bruker D8 Advence and Rigaku DMAX-2200PC with Cu-kα radiation. Fourier transform infrared (FTIR) spectra were recorded by using a Bruker Equinox 55 FTIR spectrometer. Transmission electron microscope (TEM, JEOL JEM 1400) and field enhanced scanning electron microscopy (FESEM JSM-6700F, JEOL, Japan) were used to observe size and morphology of magnetic porous nanoparticles. Nitrogen adsorption-desorption isotherms were measured using a NOVA 1000e system. The catalyst samples were outgassed for 3 h at 150°C before the measurements. The average pore-size and pore volume were determined by Barrett-Joyner-Halenda (BJH) method, and surface area was calculated by Brunauer-Emmett-Teller (BET) model.

Results and discussion
The x-ray spectra of Fe 3 O 4 samples prepared using different sufactants is shown in figure 2. All diffraction peaks can be indexed to (220), (311), (400), (511) and (440) [12], which match well with the database of cubic spinel magnetite in ICSD (Fe 3 O 4 , ICSD No. 29129). On the other hand, the broad refection peaks with low intensity also indicate magnetic particles of very small crystalline size. The average crystalline diameters (D) of magnetic Fe 3 O 4 particles without using sufactant, with using CTAB and PVP are 20, 24, and 32 nm, respectively. The crystalline diameter was estimated from the Debye-Scherrer equation: D = kλ/βcos θ. Here, λ is the x-ray wavelength, θ the angle of Bragg diffraction and β the full-width at half-maximum (FWHM) in radians in the 2θ scale [11]. Figure 3 presents the diffraction peaks of Cr 3+ x Fe 3+ 2 − x Fe 2+ O 4 samples corresponding to that of the standard pattern of cubic spinel magnetite ICSD No. 29129. However, it is noteworthy that the diffraction peaks shift toward smaller angles with the incorporation of Cr 3+ ions into the lattices, which implies that the lattice constants increase with the Cr 3+ incorporation. The major peak at 2θ = 35.76°for  Figure 5 shows TEM images of the Fe 3 O 4 nanoparticles. Without using surfactant in the preparation process, near round-like Fe 3 O 4 crystals in aggregation with an average size of 10 nm are presented ( figure 5(a)). A slight increase in particle size and decrease in aggregation of Fe 3 O 4 nanoparticles prepared using CTAB is seen in figure 5(b). However, when PVP is used, Fe 3 O 4 nanoparticles appear as rods in shape with an average diameter of 30 nm and length of 100 nm. The addition of surfactant (PVP or CTAB) is a vital factor in the morphology. The surfactants form various micelles that are used as the backbones to make various crystalline shapes. FESEM images of Cr doped Fe 3 O 4 magnetic nanoparticles are also observed in figure 6, the results reveal that the magnetic nanoparticles are round shaped. The porous parameters of Cr doped Fe 3 O 4 nanoparticles are evaluated from N 2 adsorption-desorption isotherms with the average pore volume (V P ) equal to 0.039 cm 3 g −1 , the average pore size (D p ) equal to 1.44 nm, and the surface area (S BET ) equal to 72.39 m 2 g −1 .
As observed in figure 7, the plots of magnetization versus applied magnetic field (hysteresis curves) of Fe 3 O 4 prepared with various surfactants at room temperature, there is almost immeasurable coercivity (H c ), and no remaining magnetization when the external magnetic field is removed. This indicates that the Fe 3 O 4 samples reveal a superparamagnetic behavior [17]. The slight decrease in saturation magnetization (M s ) of PVP used Fe 3 O 4 nano-rods are ascribed to the increase in crystal size. The enhancement of M s value due to doping Cr 3+ ions at low concentration (x = 0.01-0.05) into the Fe 3 O 4 lattice can be observed in

Conclusion
Superparamagnetic porous nanoparticles were prepared by the co-precipitation method using surfactants. The Fe 3 O 4 nanoparticles possessed inverse spinel structure with round and rod shape when using CTAB and PVP, respectively. The Fe 3 O 4 nano-rods had an average diameter of 30 nm and length of 100 nm. The round-particle size of the magnetic Fe 3 O 4 was approximately 15 nm. The saturation magnetization of Fe 3 O 4 nanoparticles was 48.8 emu g −1 . Enhancement of saturation magnetization up to 54.7-57.8 emu g −1 was observed on doping 1.0-5.0% Cr 3+ into the Fe 3 O 4 lattice. The magnetic nanoparticles were readily isolated from solution by external magnet and re-dispersed in solution once magnetic field was removed. The results indicated the obtained magnetic nanoparticles as potential materials for magnetic and catalytic applications.