Preparation and characterization of spindle-like Fe3O4 mesoporous nanoparticles
© Zhang et al; licensee Springer. 2011
Received: 18 May 2010
Accepted: 17 January 2011
Published: 17 January 2011
Magnetic spindle-like Fe3O4 mesoporous nanoparticles with a length of 200 nm and diameter of 60 nm were successfully synthesized by reducing the spindle-like α-Fe2O3 NPs which were prepared by forced hydrolysis method. The obtained samples were characterized by transmission electron microscopy, powder X-ray diffraction, attenuated total reflection fourier transform infrared spectroscopy, field emission scanning electron microscopy, vibrating sample magnetometer, and nitrogen adsorption-desorption analysis techniques. The results show that α-Fe2O3 phase transformed into Fe3O4 phase after annealing in hydrogen atmosphere at 350°C. The as-prepared spindle-like Fe3O4 mesoporous NPs possess high Brunauer-Emmett-Teller (BET) surface area up to ca. 7.9 m2 g-1. In addition, the Fe3O4 NPs present higher saturation magnetization (85.2 emu g-1) and excellent magnetic response behaviors, which have great potential applications in magnetic separation technology.
In the past few decades, porous materials have been used in many fields, such as filters, catalysts, cells, supports, optical materials, and so on [1–3]. In general, porous materials can be classified into three types depending on their pore diameters, namely, microporous (<2 nm), meso- or transitional porous (2-50 nm), and macroporous (>50 nm) materials, respectively . Currently, the mesoporous materials have attracted growing research interests and have great impact in the applications of catalysis, separation, adsorption and sensing due to their special structural features such as special surface area and interior void [2, 5–8]. On the other hand, iron oxide nanomaterials have been extensively studied by material researchers in recent years, due to their novel physicochemical properties and advantages (high saturation magnetization, easy synthesis, low cost, etc.) and wide applications in many fields (magnetic recording, pigment, magnetic separation, and magnetic resonance imaging, MRI) [9–16].
However, it is crucial to realize the magnetic iron oxide materials with mesoporous structure which can further adjust the physical and chemical properties of iron oxides for expanding application. According to the previous studies, the porous iron oxide nanomaterials have remarkable magnetic properties, special structures and greatly potential applications in targetable or recyclable carriers, catalyst and biotechnology [17, 18]. For example, Yu et al.  fabricated novel cage-like Fe2O3 hollow spheres on a large scale by hydrothermal method. In the report carbonaceous polysaccharide spheres were used as templates, and the prepared Fe2O3 hollow spheres exhibit excellent photocatalytic activity for the degradation of rhodamine B aqueous solution under visible-light illumination. Wu et al.  successfully developed porous iron oxide-based nanorods used as nanocapsules for drug delivery, and this porous magnetic nanomaterial exhibited excellent biocompatibility and controllability for drug release.
It is well known that the intrinsic properties of an iron oxide nanomaterial are mainly determined by its size, shape, and structure. A key problem of synthetically controlling the shape and structure of iron oxide nanomaterials has been intensively concerned by many researchers. In previous studies, there have been various porous iron oxide nanomaterials, such as porous α-Fe2O3 nanorods, Fe3O4 nanocages, and so on [9, 21–25]. However, to our best knowledge, there are few reports for fabricating the mesoporous structure of monodisperse spindle-like Fe3O4 NPs. Thus, we employ forced hydrolysis method to prepare spindle-like α-Fe2O3 NPs first. Then as-prepared α-Fe2O3 NPs were reduced by hydrogen gas at different temperatures. The structure, morphology, and magnetic properties of samples were investigated by multiple analytical technologies. The results reveal that spindle-like Fe3O4 mesoporous NPs could be obtained after annealing at 350°C.
Ferric chloride hexahydrate (FeCl3·6H2O) was purchased from Tianjin Kermel Chemical Reagent CO., Ltd. (Tianjin, China), ethanol (C2H5OH, 95% (v/v)) and sodium dihydrogen phosphate dihydrate (NaH2PO4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and all regents used were analytically pure (AR) and as received without further purification. The used water was double distilled water.
Synthesis of α-Fe2O3 and Fe3O4 NPs
Forced hydrolysis method is normally used for the synthesis of α-Fe2O3 NPs . In the typical procedure, NaH2PO4·2H2O (0.0070 g) was dissolved into 100 ml of water. After completely dissolving, the solution was transferred to a flask (100 ml) and heated to 95°C. Then 1.8 ml of FeCl3 solution (1.48 mol l-1) was added dropwise into the flask, and the mixture was aged at 100°C for 14 h. After the resulting mixture was cooled down to room temperature naturally, the product was centrifuged and washed with double distilled water and ethanol. The as-obtained α-Fe2O3 NPs was labeled as S1. The dried α-Fe2O3 powder was annealed at 250, 300, 350, 400, and 450°C in hydrogen atmosphere for 5 h. These annealed powders were labeled as S2, S3, S4, S5, S6, respectively. All the samples were dispersed into ethanol solution.
XRD patterns of the samples were obtained by using an X'Pert PRO X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm) at a rate of 0.002° 2θ s-1, which was operated at 40 kV and 40 mA. TEM images and selected area electron diffraction (SAED) patterns were performed by a JEOL JEM-2010 (HT) transmission electron microscope operated at 200 kV, the samples were dissolved in ethanol and dropped directly onto the carbon-covered copper grids. SEM analysis of the samples was carried out with a FEI SIRION FESEM operated at an acceleration voltage of 25 kV. The BET surface area of the sample was measured by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The samples were degassed before the measurement. Magnetic hysteresis loops of samples were performed in Quantum Design PPMS (Physical Property Measurement System) equipped with a vibrating sample magnetometer (VSM) at room temperature with the external field up to 15 kOe. ATR-FTIR spectra were performed on a Thermo Fisher Nicolet iS10 FT-IR.
Results and discussion
In the hydrolysis process, the features that affect the products of the experiment generally include additive, reaction temperature, aging time, PH value. On the basis of previous reports, the addition anions have great effect on the shape of α-Fe2O3 NPs. The used PO4 3- anions will adsorb onto the crystal planes parallel to the c-axis of α-Fe2O3, which causes the growing of the α-Fe2O3 NPs along the c-axis direction and promotes the formation of spindle-like α-Fe2O3 NPs [22, 29, 30]. More detailed formation mechanisms in this study are currently under way.
In conclusion, spindle-like α-Fe2O3 NPs were fabricated by forced hydrolysis of FeCl3 in the presence of PO4 3- anions. The as-prepared α-Fe2O3 NPs were then reduced in hydrogen at 350°C and transformed into spindle-like Fe3O4 NPs with mesoporous structure. The as-obtained mesoporous Fe3O4 NPs possess a high BET surface area of 7.876 m2 g-1. In addition, the obtained Fe3O4 NPs possessed a high saturation magnetization of 85.18 emu g-1 and a coercivity of 86.01 Oe. Owing to its excellent magnetic separation property and special mesoporous structure, the as-obtained Fe3O4 NPs may have a great potential application in the future.
attenuated total reflection fourier transform infrared spectroscopy
field emission scanning electron microscopy
magnetic resonance imaging
selected area electron diffraction
transmission electron microscopy
vibrating sample magnetometer
The author thanks the National Basic Research Program of China (973 Program, No. 2009CB939704), National Mega Project on Major Drug Development (2009ZX09301-014-1), the National Nature Science Foundation of China (No. 10905043, 11005082), Young Chenguang Project of Wuhan City (No. 200850731371, 201050231055), and the Fundamental Research Funds for the Central Universities for financial support.
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