Synthesis, magnetic and optical properties of core/shell Co1-x Zn x Fe2O4/SiO2 nanoparticles
© Girgis et al; licensee Springer. 2011
Received: 22 April 2011
Accepted: 20 July 2011
Published: 20 July 2011
The optical properties of multi-functionalized cobalt ferrite (CoFe2O4), cobalt zinc ferrite (Co0.5Zn0.5Fe2O4), and zinc ferrite (ZnFe2O4) nanoparticles have been enhanced by coating them with silica shell using a modified Stöber method. The ferrites nanoparticles were prepared by a modified citrate gel technique. These core/shell ferrites nanoparticles have been fired at temperatures: 400°C, 600°C and 800°C, respectively, for 2 h. The composition, phase, and morphology of the prepared core/shell ferrites nanoparticles were determined by X-ray diffraction and transmission electron microscopy, respectively. The diffuse reflectance and magnetic properties of the core/shell ferrites nanoparticles at room temperature were investigated using UV/VIS double-beam spectrophotometer and vibrating sample magnetometer, respectively. It was found that, by increasing the firing temperature from 400°C to 800°C, the average crystallite size of the core/shell ferrites nanoparticles increases. The cobalt ferrite nanoparticles fired at temperature 800°C; show the highest saturation magnetization while the zinc ferrite nanoparticles coated with silica shell shows the highest diffuse reflectance. On the other hand, core/shell zinc ferrite/silica nanoparticles fired at 400°C show a ferromagnetic behavior and high diffuse reflectance when compared with all the uncoated or coated ferrites nanoparticles. These characteristics of core/shell zinc ferrite/silica nanostructures make them promising candidates for magneto-optical nanodevice applications.
Keywordsnanostructures oxides cobalt ferrite cobalt zinc ferrite zinc ferrite magnetic properties diffuse reflectance.
Synthesis of magnetic nanoparticles have been intensively pursued due to their unique functional properties and their wide variety of potential applications in high density magnetic recording [1–4], ferrofluids technology , biomedical drug delivery [6, 7], and magnetic resonance imaging [8, 9], data storage, biosensors , biocompatible magnetic nanoparticles for cancer treatment [11–14], and magneto-optical devices [15–17] among others.
In recent years, Spinel ferrite nanoparticles have been widely studied because of their excellent and convenient magnetic and electrical properties [18, 19]. Among spinel ferrites, CoFe2O4 is of interest due to its high intrinsic coercivity (5,400 Oe) and moderate saturation magnetization (about 80 emu/g) as well as remarkable chemical stability and mechanical hardness, which makes it a good candidate for recording media [20, 21]. Also, studies indicate that the magnetic properties of CoFe2O4 depend strongly on its morphology and are greatly affected by the size of the particles [22, 23]. In addition, the magnetic properties of spinel structure CoFe2O4 can be altered by cation substitution. According to recent research, Zn2+ substituting for Co2+ in CoFe2O4 nanoparticles (Co1-x Zn x Fe2O4) exhibited improvement in properties such as excellent chemical stability, high corrosion resistivity, magneto-crystalline anisotropy, magneto-striction, and magneto-optical properties. Cobalt zinc ferrites nanoparticles have been prepared by different methods, such as co-precipitation, usual ceramic technique, microwave-hydrothermal method, and the solvothermal method [24–30].
In the present decade, core/shell structured nanoparticles have received much attention, due to their enhanced combination of optical, electronic, and magnetic properties compared to those of single-component nanomaterials . Thus, coating magnetic nanoparticles with silica is becoming a promising and important approach in the development of magnetic nanoparticles for both fundamental studies as well as technological applications. Silica formed on the surface of magnetic nanoparticles could screen the magnetic dipolar attraction between magnetic nanoparticles, which improves the dispersion of magnetic nanoparticles in liquid media and protects them from leaching in an acidic environment. In addition, the core/shell structure enhances the thermal and chemical stability of the magnetic nanoparticles due to the silica shell which provides a chemically inert surface for magnetic nanoparticles in biological systems. Therefore, silica-coated magnetic nanoparticles can be easily allowed to conjugate its surface with various functional groups [32, 33]. Also, the silica shell can enhance the optical properties of the nanoparticles . The optical properties of the nanostructures have been investigated earlier using many techniques, among them is the diffuse reflectance spectroscopy .
The main objective of this study is to investigate the effect of Zn2+ partially substituting for Co2+ in CoFe2O4 nanoparticles (Co1-x Zn x Fe2O4; x = 0, 0.5, and 1) and shelling with silica on the magnetic and optical properties of the ferrite nanoparticles for a variety of magneto-optical nanodevice applications. From a synthesis point of view exploring the effect of firing temperatures (400°C, 600°C and 800°C) is of interest to investigate.
The chemicals used for preparation of the samples were ferric nitrate (Fe(NO3)3·9H2O, Mw = 404.00 g/mol, Alpha Chemika™, Mumbai, India), cobalt (II) nitrate (Co(NO3)2·6H2O, Mw = 291.04 g/mol, WinLab, UK), and zinc nitrate (Zn(NO3)2·6H2O, Mw = 297.47 g/mol, WinLab, Laboratory chemicals reagent fine chemicals), citric acid monohydrate gritty, puriss, (C6H8O7·H2O, Mw = 210.14 g/mol, Riedel-Dehaën, Sigma-Aldrich, Labor Chemika Lien, GmbH, St. Louis, MO, USA), ammonia solution (30%), and tetraethyl orthosilicate (TEOS, C8H20O4Si, Mw = 208.33 g/mol, Merck Schuchardt OHG, Hohenbrunn, Germany).
CoFe2O4, ZnFe2O4, and Co0.5Zn0.5Fe2O4 nanoparticles have been prepared using modified citrate gel method [36, 37]. Co(NO3)2·6H2O solution (0.25 M), Zn(NO3)2·6H2O solution (0.25 M), and Fe(NO3)3·9H2O solution (0.25 M) were prepared by dissolving the metal nitrates in distilled water. The prepared solutions were mixed in molar ratio of Me2+/Fe3+ = 0.5 (Me2+ = Co2+, Zn2+, and 0.5 Co2+ + 0.5 Zn2+ for CoFe2O4, ZnFe2O4, and Co0.5Zn0.5Fe2O4, respectively) under constant stirring to get homogeneous solution with the heating rate of 5°C/min up to 80°C for 1 h. This mixture solution was added to the citric acid solution (0.25 M) maintaining the molar ratio between metal nitrates solution and citric acid solution as 1:1 and stirred for 2 h. Ammonia was added to reach pH equal to 7.5. Increasing the temperature during the stirring process leads to form a viscous gel. The gel was dried and fired at temperatures of 400°C, 600°C, and 800°C for 2 h to form CoFe2O4 (CF), ZnFe2O4 (ZF), and Co0.5Zn0.5Fe2O4 (CZF) nanoparticles.
Silica-coated magnetic nanoparticles were prepared using the modified Stöber method. The nanoparticles (fired at 400°C) were first treated by citric acid solution (0.01 M) under constant stirring for 1 h. The presence of citrate increases the organosilane affinity of the particle surface. These particles were separated and washed with distilled water several times. After that, the particles were redispersed in a mixture of absolute ethanol (80 ml) and distilled water (20 ml) the ammonia was added to the solution as a catalyst. Subsequently, 6 ml of TEOS was injected to the above solution, drop by drop at constant stirring for 24 h at room temperature to ensure the hydrolysis, after that, the condensation of TEOS on the surface of nanoparticles was achieved. Finally, the core/shell CoFe2O4/SiO2, Co0.5Zn0.5Fe2O4/SiO2, and ZnFe2O4/SiO2 particles were separated using external magnet, and washed with ethanol and water several times. The samples have been dried at 40°C for 24 h and fired at temperatures 400°C, 600°C, and 800°C, respectively, for 2 h.
The morphology of uncoated and coated nanoparticles was studied using transmission electron microscopy, TEM (JEOL 1230, JEOL, Tokyo, Japan). The phase composition and average crystallite size of the core/shell ferrite nanoparticles were investigated using X-ray diffractometer (Model Bruker D8 Advance (Bruker AXS, Madison, WI, USA), Cu-Kα1 (λ = 1.54058 Å) radiation with secondary monochromator at a scanning speed of 1°/min). In addition, vibrating samples magnetometer (model is Princeton FM-1, Princeton Applied Research, Oak Ridge, TN, USA) and UV/VIS double-beam spectrophotometer (model is no. Lambda 35, Perkin Elmer, Waltham, MA, USA) were used to measure the magnetic properties and diffuse reflectance of the prepared ferrite nanoparticles, respectively.
Results and discussion
On the other hand, the hysteresis loop is much wider for the cobalt ferrite samples coated with silica shell (CFS) and fired at 400°C compared with cobalt ferrite samples fired at 800°C. This confirms that by increasing the firing temperature, the crystallite size increases leading to decrease of the switching field. Also, it was found that, for the cobalt ferrite nanoparticles coated with silica (CFS), the magnetic moment increases with increasing the firing temperature from 400°C to 800°C. As mentioned earlier from the XRD analysis, with increasing the firing temperature, the amorphous silica starts to disappear and the diffraction peaks of spinel cobalt ferrite phase only are found at higher temperatures due to the formation of robust core/shell structure (Figure 1a). This leads to creation of a very thin layer of cobalt ferrite silicate at the surface of these cobalt ferrite nanoparticles which decrease the effect of the amorphous silica shell and hence increase the magnetic moment at higher firing temperature.
Figure 5b shows the hysteresis loops of core/shell zinc ferrite nanoparticles coated with silica shell (ZFS) fired at 400°C and 800°C. It is clear that at 400°C, the zinc ferrite/silica nanoparticles show a ferromagnetic behavior compared with the sample fired at 800°C which shows a paramagnetic behavior.
Summary of the magnetization saturation and switching field (HC) values at room temperature (27°C)
Core/shell Co1-x Zn x Fe2O4/SiO2 (x = 0, 0.5, and 1) nanoparticles were prepared using modified citrate gel technique and coated with silica shell. The samples have been fired at 400°C, 600°C, and 800°C, respectively. It is concluded that cobalt ferrite nanoparticles fired at 800°C showed the highest magnetic properties, while zinc ferrite nanoparticles coated with silica and fired at 800°C shows the best enhanced optical properties. X-ray diffraction patterns show the presence of spinel ferrite crystalline phase as the main phase in all prepared core/shell ferrite nanoparticles. In addition, the average crystallite size increases on increasing the firing temperature from 400°C up to 800°C. Zinc ferrite nanoparticles coated with silica shell and fired at 400°C show a ferromagnetic behavior and high diffuse reflectance compared with all uncoated and coated nanoparticles due to the presence of zinc ions and the silica shell which play an important role on the optical properties enhancement. The firing temperatures as well as the crystallite size parameters have great effect on the magnetic and the optical properties of core/shell ferrite nanoparticles. Core/shell ferrite nanoparticles coated with silica are found to enhance the optical properties of the magnetic nanoparticles. Core/shell zinc ferrite nanoparticles coated with silica shell and fired at 400°C show promising results for photo-magnetic nanodevice applications and for magneto-optical recording industry.
We would like to thank the Swedish Research Foundation SIDA for supporting the present work under grant # 348-2007-6992.
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