Synthesis of hollow spheres with mesoporous silica nanoparticles shell

Synthesis of hollow spheres with mesoporous silica nanoparticles shell

Materials Chemistry and Physics 111 (2008) 5–8 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsev...

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Materials Chemistry and Physics 111 (2008) 5–8

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Materials science communication

Synthesis of hollow spheres with mesoporous silica nanoparticles shell Mu Yang a,∗ , Ge Wang a , Zhenzhong Yang b a b

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China

a r t i c l e

i n f o

Article history: Received 9 November 2007 Received in revised form 4 March 2008 Accepted 20 March 2008 Keywords: Hollow spheres Mesoporous Polystyrene Dual templates

a b s t r a c t Hollow spheres with mesoporous silica nanoparticles shell were synthesized with the use of cetyltrimethylammonium bromide (CTAB) and polystyrene (PS) hollow spheres as dual templates. The key to this study is that the uneven surface of the template provides nucleation sites for mesoporous nanoparticles, resulting in the formation of hollow spheres with mesoporous silica nanoparticles shell. The final products with hierarchical mesopores can be obtained through a simple one-step approach. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Mesoporous silica has attracted great research interest since the first report of MCM-41 materials in 1992 [1,2]. Various morphologies of mesoporous silica, such as, polyhedral particles [3,4], fibers [5], membranes [6,7] and spheres [8–11], have been synthesized. Among these hierarchical structures, mesoporous hollow spheres are of particular importance because of their potential applications in encapsulation, controlled drug release, catalysis and separation. Generally, mesoporous hollow spheres are synthesized by two approaches: interfacial synthesis and template technique [12–20]. Interfacial synthesis approach obtains hollow spheres of mesoporous silica by condensing silica/surfactant assembly in oil/water emulsion system and also spray-drying system, but strict condition is required to synthesize intact hollow spheres. The sizes of hollow spheres are dispersed widely at the range of micrometer [12–15]. On the other hand, template technique, also known as core–shell approach, is an effective method to obtain mesoporous hollow ¨ spheres. Unger used the Stober approach and the Kaiser approach to synthesize the submicrometer-size solid core\mesoporous shell silica spheres with alkyltrialkoxysilane as porogen to generate the porosity [16]. Qiu derived the progress to sulfate-modified polystyrene spheres template and synthesized hollow mesoporous silica spheres under highly alkaline condition [17]. Rankin prepared hollow silica spheres with ordered mesoporous shells using

∗ Corresponding author. Tel.: +86 10 82376882; fax: +86 10 62320883. E-mail address: [email protected] (M. Yang). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.03.014

cetyltrimethylammonium bromide (CTAB) and PS spheres dual templates in concentrated aqueous ammonia media [18]. However, all these approaches generally obtained hollow spheres with smooth mesoporous silica shell layer. Sadasivan reported the synthesis of hollow microspheres with MCM-41 nanoparticles shell by alternatively deposit preformed MCM-41 particles and polyelectrolytes onto polystyrene microspheres [19,20], but it was a time-consuming procedure and multilayer of nanoparticles was needed to obtain stable hollow spheres. Here we are proposing a simple, novel approach to synthesize hollow spheres with mesoporous silica nanoparticles shell. The hollow silica spheres were synthesized using polystyrene hollow spheres and CTAB as dual templates under condition similar to those of some mesoporous solid silica spheres prepared without latex template [10]. 2. Experimental 2.1. Material Aqueous polystyrene hollow spheres with diameter of 400 nm (solid content 37.5 wt%) were purchased from Rohm & Hass Company. The hollow spheres were washed with water and freeze-dried before use. Aqueous ammonia (25 wt% NH3 ), ethanol, tetraethoxysilane (TEOS) and CTAB were obtained from Beijing Chemical Reagent Company. Tetraethoxysilane was distilled in vacuum immediately before use. All other chemicals were used as purchased. 2.2. Synthesis Hollow mesopore silica spheres were synthesized by a precipitated process, using CTAB and hollow spheres as dual templates at 30 ◦ C. In a typical synthesis procedure, 5.0 g ethanol (108.70 mmol), 1.4 g aqueous ammonia (20.59 mmol) and 0.21 g CTAB (0.58 mmol) were added into 3.85 mL water (213.89 mmol) under stirring. After 15 min of stirring, 0.20 g of freeze-dried hollow spheres was added and

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Fig. 1. Representative electron micrographs of the obtained hollow spheres with mesoporous silica nanoparticles shell: (a) SEM image; (b and c) TEM images; (d) microtone TEM image.

then 0.39 g of tetraethoxysilane (1.88 mmol) was added dropwise into the above solution. The solution was continuingly stirred for 4 h. The as-synthesized sample was centrifuged, washed with ethanol and water, and then dried under vacuum. The organic templates were removed after being calcinated at 450 ◦ C with a heating rate of 1 ◦ C min−1 in air and the hollow spheres with mesoporous silica nanoparticles shell were obtained. 2.3. Characterization Transmission electron microscope (TEM) was performed using a JEOL 100CX microscope operating at 100 kV. Highly diluted suspension of the particles in ethanol was dispersed onto carbon-coated copper grids for observation. Some samples were embedded in epoxy resins and sectioned to 30–50 nm thick slices using Leica ultracut UCT ultramicrotome at room temperature. Scanning electron microscope (SEM) was performed with a HITACHI S-4300 instrument equipped with a field-emission electron gun operating at 15 kV. Samples were deposited from ethanol dispersion onto silicon wafers and vacuum sputtered with ca. 3 nm platinum. Power X-ray diffraction pattern of the sample was recorded with a Rigaku D/max-2500 diffractomer using Cu K␣ radiation over a 2 range from 1◦ to 10◦ . Thermogravimetric analysis was carried out on a PerkinElmer TGA 7 instrument. Nitrogen adsorption–desorption isotherms at 77 K were performed on a Micromeritics ASAP 2020 M Porosity Analyzer. Prior to each adsorption experiment, the calcined samples were outgassed at 573 K for at least 2 h under vacuum.

indicating the spheres are hollow structures. Most particles are single spheres although few agglomeration is visible. Transmiss electron microscopy images show the hollow spherical morphologies (Fig. 1b and c). The size of the cavity is around 400 nm, which is consistent with the average size of the template sphere. Along with spheres a very few solid mesoporous silica nanoparticles are also observed, whose sizes are below 200 nm. Microtone TEM image shows (Fig. 1d) that the shell of the hollow spheres consist of disordered mesopores, and the wormhole structure of the pore is

3. Results and discussion A hierarchically organized structure could be obtained through the combination surfactant self-assembly with hollow spheres template. Fig. 1a shows a representative scanning electron microscopic image of the synthesized hollow silica spheres. It can be observed clearly that the spheres are composited of 60–120 nm nanoparticles. The spheres are nearly uniform and the size of the spheres is about 600 nm. Some broken spheres are also observed,

Fig. 2. Powder X-ray diffraction pattern of the hollow spheres with mesoporous silica nanoparticles shell.

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Fig. 3. Nitrogen adsorption–desorption isotherm of the hollow spheres with mesoporous silica nanoparticles shell. Inset showing the corresponding pore size distribution curve.

Fig. 4. The thermogravimetric analysis (TGA) plots of the as-synthesized composite hollow spheres.

different from the usually observed hexagonally arranged mesopores in MCM-41. The small angle X-ray diffraction (XRD) pattern of the calcined product is shown in Fig. 2. The scattering pattern exhibits one intense peak at 2 value 2.44◦ of representing d spacing of 3.168 nm along with a broad shoulder, which indicating a mesoporous silica structure lacks long-range ordering in the arrangement of mesopores. This is consistent with the wormlike pores structure observed in the TEM image (Fig. 1d). The N2 adsorption–desorption isotherm (Fig. 3) shows that the sample possess a type IV isotherm, according to the IUPAC nomenclature. An initial rise at low partial pressure attributes to monolayer nitrogen adsorption, and a steep adsorption at relative pressure (P/P0 ) around 0.2–0.3 is ascribed to capillary condensation of nitrogen in the framework-confined mesopores. A slight adsorption occurs near P/P0 = 0.95–1.0, which is the impact of textural mesoporosity created by the packing of the nanoparticles in the hollow sphere shell. The sample possesses a high Brunauer–Emmet–Teller (BET) surface area of about 1064 m2 g−1 and a specific pore volume of 0.88 cm3 g−1 , which are consistent with those reported in the literature for solid spheres samples [10]. The corresponding pore size distribution obtained from Barret–Joyner–Halenda (BJH) method shows a narrow pore distribution centered at 2.3 nm (the inset of Fig. 3). Fig. 4 shows thermogravimetric analysis (TGA) curve of the assynthesized hybrid hollow spheres using a nitrogen atmosphere and a heating rate of 5 ◦ C min−1 . The first step of weight loss (19.9%) centred at 220 ◦ C is due to the decomposition of CTAB in nitrogen (the melting and decomposition temperatures for CTAB are around 230 ◦ C). This weight loss is only half of the CTAB amount

which added in the composition of hybrid particles (39.6%), which possible be owing to the release of surfactant after formation of the as-synthesized mesophase [21]. The second step of weight loss (50.6%) centred at 440 ◦ C corresponds to the decomposition of polymer sphere template, and the weight of the residual silica is 29.5%. These data are in accordance with that of polymer spheres added (47.1%) and the accounted silica content (28.2%) after decreased the amount of CTAB which did not enter into the as-synthesized spheres, indicating that the mesoporous silica has high yield. In order to further confirm the effect of hollow spheres template, another experiment was carried out. All reaction conditions were the same as above except that the hollow spheres template was replaced by solid polystyrene spheres (400 nm in diameter). The mesoporous silica particles were difficult to deposit on the surfaces of polystyrene spheres. Single solid mesoporous silica particles of 100–400 nm in diameter, instead of hollow mesoporous silica spheres, were obtained. A model for the formation of hollow spheres with mesoporous silica nanoparticles is suggested (Scheme 1). The hollow spheres template has a polystyrene shell with transverse hydrophilic channels which is composed of polymer containing of carbonyl groups [22]. The negatively charged polymer in the surface of hollow sphere template could absorbed positively charged surfactant micelle via electrostatic interaction, which provide nucleation sites for the self-assembly of surfactant micelles and silicate species on the surface of PS hollow spheres. The increase of mesoporous silica particles leads to the formation of mesoporous silica nanoparticles shell. The obtained hollow silica spheres have disorder mesopores of nanoparticles and the interparticle pores generated by ran-

Scheme 1. Schematic drawing of the suggesting formation process of hollow spheres with mesoporous silica nanoparticles shell.

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domly packing of nanoparticles, which increase the diffusibility of the guest molecules into and out of the hollow spheres. In addition, the materials to be released could be previously loaded into the hollow spheres template before the synthesis of mesoporous silica hollow spheres. This will provide a novel template for mesoporous hollow sphere synthesis and the obtained mesoporous silica hollow spheres are expected to find application as reactors, containers. 4. Conclusions A new approach has been demonstrated to synthesize hollow spheres with mesoporous silica nanoparticles shell. The polymer hollow spheres with uneven surface were first used to synthesize hollow mesoporous silica spheres. The hollow spheres with mesoporous silica nanoparticles shell were prepared from the selfassembly of the surfactant CTAB and silica species on the surface of hollow PS spheres in aqueous ammonia medium by simple one-step method. The obtained hollow spheres have hierarchical mesopores. Acknowledgements We thank the National High Technology Research and Development Program of China (863 program: 2006AA03Z459) and the Program for New Century Excellent Talents in University (NCET-050103) for support.

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