Controlled Fabrication of Iron Oxide Mesoporous Silica Core

© 2013 American Chemicale core and a mesoporous silica shell is still a challenging topic. Herein, we report a simple one-pot route for synthesizing iron oxide@mesoporous silica core−shell structures. First, a novel and environmentally benign green route was applied to synthesize the iron oxide with pseudocubic morphology via the solvothermal method. The monodisperse iron oxide was obtained by treating a solution of Fe(NO 3 ) 3 · 9H 2 O, CH3COOH, and CH3CH2OH in the presence of polyvinylpyrrolidone (PVP) at 200 °C. Then, the as-prepared solution provided an ideal system for the continuous coating of mesoporous silica shells via a modified Stöber process, which consists of the alkaline hydrolysis of tetraethyl orthosilicate (TEOS) in an ethanol/water mixture in the presence of iron oxide nanoparticles. In addition, the applications and the formation mechanism of such core−shell nanostructures are also discussed.
EXPERIMENTAL SECTION Materials. All of the chemicals were of analytical grade and used as received without any further purification. Ferric nitrate
Received: May 20, 2013 Revised: August 26, 2013 Published: September 5, 2013
INTRODUCTION The controlled fabrication of core−shell nanocomposites has attracted much attention because of the combined functionality of cores and shells and great application potential in various fields.1−3 Iron oxide@silica core−shell nanostructures possess controllable magnetism, low cytotoxicity, and a chemically modifiable surface. They have shown great potential in applications such as drug delivery, magnetic resonance imaging (MRI), and catalysis.4−7 Many efforts have been made to fabricate iron oxide@silica colloidal materials.8−13 However, few of these studies organically couple iron oxide synthesis routes with a silica sol−gel process because of the different sensitive processes for either the iron oxide synthesis or the silica sol−gel process.2 In these studies, the formation of the iron oxide core and the silica shell was achieved in different processes. Iron oxide was first prepared and separated from the mother liquid in order to remove impurities. Second, the asobtained iron oxide was redispersed in silica sol−gel after surface modification for the deposition of silica shell. Such complicated procedures usually led to poor dispersion and irregular agglomeration of the obtained nanostructures and made scaling-up more difficult, which would limit the application of the final colloidal materials. Therefore, the facile and scalable synthesis of core−shell nanocomposites with a dimensionally and morphologically well-defined iron oxide
Article /JPCC
Controlled Fabrication of Iron Oxide/Mesoporous Silica Core−Shell Nanostructures
Mingwei Zhang,†,‡ Kegong Fang,† Minggui Lin,† Bo Hou,† Liangshu Zhong,§ Yan Zhu,§ Wei Wei,*,§ and Yuhan Sun*,†,§
21529
/10.1021/jp4049583 | J. Phys. Chem. C 2013, 117, 21529−21538
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The Journal of Physical Chemistry C (Fe(NO3)3·9H2O), acetic acid, ethanol, tetraethyl orthosilicate (TEOS) and cetyltrimethylammonium bromide (CTAB) were purchased from Tianjin Chemical Reagent Co., Ltd. (Tianjin, China). Polyvinylpyrrolidone (PVP, Mw = 1 300 000) was obtained from Sigma-Aldrich. Preparation of Monodisperse Iron Oxide Nanoparticles. The iron oxide nanocubes were synthesized by a novel solvothermal approach. In brief, 1.01 g of Fe(NO3)3· 9H2O, 1.01 g of PVP, and 2 mL of acetic acid were dissolved in 100 mL of ethanol under magnetic stirring at room temperature. Then the formed solution was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 200 °C for 24 h. Afterward, the autoclave was allowed to cool to room temperature, and the iron oxide nanocubes were obtained. Preparation of Iron Oxide Nanoparticles Embedded in Mesoporous Silica Spheres (Fe2O3@SiO2 Core−Shell Composites). The as-prepared iron oxides in ethanol (100 mL) were dissolved in distilled water (100 mL), and an additional 100 mL of ethanol was added. Then 9 mL aqueous ammonia (25 wt %) was added to this solution to adjust the pH to 9. Afterward, cetyltrimethylammonium bromide (CTAB, 0.2 g) was dissolved in the above solution. Finally, tetraethoxysilane (TEOS, 0.625 mL) was added slowly. After the solution was stirred for 24 h at room temperature, the nanoparticles were separated by filtering, washed thoroughly with water and ethanol, and then dried in an oven at 100 °C. The calcination of as-prepared core−shell composite nanoparticles was carried out for 5 h at 550 °C in the presence of static air. Then iron oxides coated with mesoporous silica (20 nm shell thickness) composites with perfect core−shell structure were at last obtained. Additionally, two samples were prepared with an identical procedure but with increased amounts of TEOS (1.25 mL, 2.5 mL) and CTAB (0.4g, 0.8g) to change the thickness of the SiO2 shell (35 nm, 60 nm). To investigate the effects of various synthesis conditions in the sol−gel process, a series of experiments were conducted by changing the volume ratio of alcohol to water (VE/W), the feeding amount of aqueous ammonia, and CTAB. Preparation of Hollow Silica Particles. Hollow SiO2 spheres were prepared by etching Fe2O3@SiO2 nanoparticles with 18% HCl. In a typical etching process, one gram of core− shell powder was added to 20 mL of an 18% HCl solution, and the suspension was treated by ultrasonication for 2 h. During this process, the mother solution turned a yellowish color, indicating that ferric ions were leaching out. Finally, the prepared hollow-type silica particles were collected by centrifugation and washed with deionized water and ethanol to remove FeCl3. Preparation of Magnetic Core/Mesoporous Silica Shell (MCMS) Particles. Magnetic core/mesoporous silica shell (MCMS) particles were obtained by H2 reduction of Fe2O3@SiO2 nanoparticles. The reduction was carried out by the thermal treatment of the Fe2O3@SiO2 particles in mixed H2 and Ar (5:95) gases at different temperature (350, 450, or 650 °C) for 4 h to obtain MCMS particles with different compositions and magnetic properties. The synthesis process of Fe2O3@SiO2 and its derivative nanoparticles is illustrated in Scheme 1. Characterization. X-ray powder diffraction (XRD) measurements were performed on a D8 Advance Bruker AXS diffractometer, using a Cu Kα radiation (λ = 1.5406 Å) at 40 kV, 40 mA, employing a scanning rate of 0.02° s−1 in the 2θ