O emulsion method

Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 208–212

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Synthesis and characterization of hollow silica microspheres functionalized with magnetic particles using W/O emulsion method Chul Oh, Yong-Geun Lee, Chan-Uk Jon, Seong-Geun Oh ∗ Department of Chemical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul, Republic of Korea

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Article history: Received 13 March 2008 Received in revised form 8 December 2008 Accepted 11 December 2008 Available online 24 December 2008 Keywords: Magnetic particles Silica Hollow Micropore Emulsion

a b s t r a c t Hollow silica particles functionalized with magnetic particles (HSFMs) were prepared without templates using the W/O emulsion system containing magnetic nanoparticles synthesized by the redox reaction of Fe salts. Since the removal process like etching step is not required, our system is a simple and easy method to obtain the functionalized shell and composite hollow materials. Here, the magnetite nanoparticles not only give the hollow silica particles, the magnetic properties but also influence the formation process of the hollow silica microspheres. It is verified by X-ray diffraction (XRD), energy-dispersive Xray (EDS), and vibrating sample magnetometer (VSM) measurements that the structure and magnetic property of the magnetic particles were not changed in quality before and after the synthesis of hollow silica particles. From the Brunauer, Emmett, Teller (BET - Nitrogen adsorption–desorption measurements) analysis, hollow silica microspheres functionalized by magnetic nanoparticles (HSFMs) reveal the typical Type I isotherm in the IUPAC classification, which shows a long saturation plateau and involves micropore filling. This method can be extended to the synthesis of hollow silica particles functionalized by other metal oxide materials as well as iron oxide. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Ceramic hollow particles have been extensively studied in recent years due to their potential applicability to industrial and biological fields [1]. These hollow particles often exhibit properties that are substantially different from those of general particles and bulk materials (for example, low density, large specific surface area, stability, and surface permeability), thus making them attractive from both scientific and technological viewpoints. Nanostructures with hollow interiors are commonly prepared by various methods, including spray drying techniques, templating procedures, and so on [2]. In a typical templating method, hollow particles are synthesized by coating the surface of colloidal particles with thin layers of the desired materials or its precursors, followed by selective removal of the colloidal templates through wet chemical etching or calcinations [3]. This process has the advantage of preparing monodisperse hollow particles, while, unfortunately, it has complex and difficult stages for selectively removing the template. Moreover, more complex procedures are required to synthesize hollow particles with composite shells functionalized by other valuable materials. For example, to prepare a hollow particle involving Fe3 O4 particles, some steps are needed. First, CTAB-modified magnetic nanoparticles were synthesized and they were added in O/W

∗ Corresponding author. Tel.: +82 2 2220 0485; fax: +82 2 2294 4568. E-mail address: [email protected] (S.-G. Oh). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.12.010

emulsion again. And then TEOS reacted with as-prepared magnetic particles [4]. However, for the preparation of hollow particles with a simpler and easier method that exclude the template-removing process, in this study, we used W/O emulsion as a reaction medium. Magnetic materials are one of the most conspicuous materials and have been extensively used in the industrial fields such as high-density magnetic data storage arrays, magneto-optical switches, and sensors [5]. Recently, magnetic particles are applied to biology and medicine, for example, magnetic resonance imaging (MRI) contrast agent, separation of oligonucleotids, cells and biocomponents, and protein and enzyme immobilization [6]. Ulman reported the successful immobilization method of proteins on pure maghemite nanoparticles for biological uses [7]. In addition, nanometer-sized materials have attracted in the material science community because of their special properties [8]. Generally, as a particle size is smaller, surface area increases. Using this unique property, various metal catalysts having a nanometer-size were made. The relatively large surface area and highly active surface sites of nanoparticles enable them a higher adsorption capacity compared with the large size of particles. However, pure magnetic particles may not be useful in practical applications because large aggregates are easily found. To overcome such limitations, many literatures have reported on the surface-modification of magnetic particles and coating by organic and inorganic materials [9]. Of these materials, the magnetite-containing spherical silica nanoparticles are noticed by many research groups because of high stability and biocompatibility [10].

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Fig. 1. Schematic representation of synthesis process for hollow silica particles functionalized by magnetic particles (HSFMs): (a) formation of Fe3 O4 nanoparticles inside a water droplet of W/O emulsion by redox reaction, (b) condensation between as-prepared magnetic particles and hydrolysed TMOS molecules in W/O interface and (c) formation of HSFMs and adsorption of a few residual silica-coated magnetic particles coated with silica on the inner surface during the drying process.

In this study, we report a new synthetic route of hollow silica microspheres functionalized by Fe3 O4 magnetite nanoparticles using W/O reverse emulsion system. Since template materials such as polymer beads are not employed, the removal process of templates is not required. Here, the magnetite nanoparticles not only gave the hollow silica particles the magnetic properties but also influenced the formation process of the hollow silica microspheres. To stabilize the W/O emulsion system and obtain the spherical shape of silica particles, hydroxypropyl cellulose (HPC) as well as Span 80, the oil-soluble surfactant, were employed in the oilcontinuous phase. 2. Experimental 2.1. Materials Tetramethyl orthosilicate (TMOS, 98%), hydroxypropyl cellulose (HPC, average Mw ca. 370,000), span80, iron(II) chloride tetrahydrate, and iron(III) chloride hexahydrate were purchased from Sigma–Aldrich. The 1-octanol and ammonium hydroxide (NH4 OH) were purchased from Junsei Chemical Company. Ethanol (95%) as a washing reagent was obtained from Teamin Chemical Company (Korea). All Chemicals were used as received without further purification. Water used in this study was deionized and double distilled. (Millipore, France); it had electrical resistivity 18.2 M.

phase containing Fe2+ and Fe3+ was prepared. To synthesize Fe3 O4 by improved chemical co-precipitation method, 0.4 g of FeCl2 ·4H2 O and 1.04 g of FeCl3 ·6H2 O were dissolved in 5 ml of deionized water, such that Fe2+ /Fe3+ = 0.5. Finally, the water phase containing Fe ions was added to the external oil phase. The weight ratio of water phase to oil phase in the emulsion was kept at 1:9. This ratio is very important to synthesize magnetic particles. To disperse the water phase into the oil phase as a form of droplet, agitation was performed using the magnetic stirrer at 40 ◦ C for 1 h. Synthesis of pure magnetic particles and HSFMs: to synthesize magnetic particles, 5 ml of 1.5 M NH4 OH solution was injected into the W/O emulsion medium. The growth of Fe3 O4 nanocrystalline was allowed to proceed for 1 h inside the water droplets of W/O emulsion. The water droplets as a micro-reactor maintained the spherical structure for synthesis of magnetic particles. The orange color of W/O emulsion was changed into black, which appears in the synthesis of magnetite. Then, TMOS, the silica source, was added into W/O emulsion containing magnetic particles. TMOS is soluble in the oil-continuous phase of the W/O emulsion but it becomes water-soluble after the hydrolysis by the catalyst. The molar ratios of H2 O to TMOS (Rw) were 10 and 20. After 2 h for sol–gel reaction of TMOS, the samples were centrifuged at 3000 rpm for 15 min to obtain HSFMs particles. They were washed with ethanol three times. The products were dried in a vacuum oven at 40 ◦ C for 1 day and were finally calcined in air at 600 ◦ C for 4 h using electric muffle furnace (JEIL SCIENCETIFIC IND. CO. LTD., Korea).

2.2. Synthesis procedures 3. Results and discussion Preparation of W/O emulsion: first, an external oil phase was prepared by dissolving hydroxypropyl cellulose (HPC, Mw = 370,000) in n-octanol (45 g) at 80 ◦ C for 6 h. After it was cooled and kept up to 40 ◦ C, 1.5 g of Span 80, a low HLB surfactant, was added into the oil phase. HPC polymer and Span 80 increased the stability of the W/O emulsion structure. Secondly, the inner aqueous

The scheme for the hollow silica microspheres functionalized by magnetic nanoparticles was illustrated in Fig. 1. In W/O emulsion system, water droplets containing the iron ions Fe2+ and Fe3+ serve as the micro-reactors. Although magnetic nanoparticles are formed by the redox reaction, water droplets in the W/O emulsion

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Fig. 2. SEM (a)–(d) and TEM (e)–(f) images of hollow silica particles functionalized with magnetic particles (a) HSFMs synthesized at Rw = 10, (b) HSFMs synthesized at Rw = 20, (c) silica-coated magnetic particles deposited in the inner surface of silica, (d) TEM image of magnetic particles and (e)–(f) TEM images of HSFMs synthesized at Rw = 20.

system keep the micro-sized spherical shapes. It was reported that magnetic particles prepared in the aqueous phase have a number of hydroxy ( OH) groups at the surface [11]. After the magnetic particles are completely formed in the water droplets, TMOS molecules are added into the oil-continuous phase because they are initially hydrophobic. When they are contacted with the interface of water droplets containing ammonium hydroxide, the hydrolysis of TMOS molecules takes place. In general sol–gel reaction, when basic catalysts are employed, successive hydrolysis of TMOS occurs and then fully hydrolysed TMOS molecules, Si(OH)4 , are condensed. As a consequence, highly cross-linked primary sol particles are obtained, which eventually link to form secondary spherical particles with large pores between the interconnected primary particles [12]. However, in the case of our study, the magnetic nanopar-

ticles with a number of hydroxy groups at the surface already exist in water droplets. They prevent cross-linking condensation between hydrolysed TMOS molecules and promote the condensation between as-prepared magnetic particles and hydrolysed TMOS at W/O interface. Since this peculiar condensation takes place in the W/O interface of water droplets, the shell of silica-coated magnetic particles formed at the surface of water droplets prevents the contact between TMOS molecules in the oil-continuous phase and water containing ammonium hydroxide. As a result, hydrolysis and condensation reaction of TMOS molecules are terminated by the shell and hollow silica microspheres functionalized by magnetic particles are obtained. The structure and morphology of samples were first investigated by scanning electron microscopy (SEM) and transmission

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Fig. 3. XRD data of (a) pure magnetic particles and (b) HSFMs calcined at 600 ◦ C for 4 h.

electron microscopy (TEM). Fig. 2a–c is the typical SEM images of HSFMs. From the images, it is verified that micro-sized welldispersed spherical silica particles are synthesized. Also, rugged inner surface of HSFMs illustrated in Fig. 2c shows that a few residual silica-coated magnetic particles inside were deposited in previously synthesized silica–magnetite composite shell during a sol–gel process. As shown in Fig. 2a, when the molar ratio of water to TMOS (Rw) is 10, HSFMs have a spherical shape with micrometersize around 3–5 ␮m. Similarly, Fig. 2b also shows that HSFMs in the case of Rw = 20 have a spherical morphology. Although, amount of Rw = 20 is very small, it can be possible to prepare a hollow structured spherical HSFMs particles stably in W/O emulsion. Further evidence for the hollow structure can also be found from the TEM images shown in Fig. 2d–f. Fig. 2d shows TEM picture of magnetic particles. Magnetic particle have a spherical shape and the particle size is about 5–10 nm. Fig. 2e–f is seen that there is a strong contrast difference in all of the spheres with dark edge and bright center containing small dark spots, confirming their hollow structure. The dark parts in the bright center are the magnetite nanoparticles that are aggregated (Fig. 2e) or regularly dispersed (Fig. 2f) in silica shells. Fig. 3 shows the X-ray diffraction pattern of pure magnetic particles and HSFMs. It is apparent from JCPDS database that magnetite and maghemite have a spinel structure. Although their lines are close and it is difficult to distinguish them from one another by XRD pattern, it is thought that magnetic particles of HSFM is mostly composed of magnetite nanoparticles because (1) the diffraction peak of (2 2 2) is relatively cleared, (2) the strength of (4 4 0) peak is stronger than (2 2 0) peak, and (3) the obtained particles are the black precipitate. In addition, two peaks appeared in 2 = 18 and 26 shows the presence of the small amount of maghemite particles. In the case of HSFMs, the spinel structure of the magnetic nanoparticles dispersed in silica is hardly changed though they are calcined at 600 ◦ C for 4 h. The broad peak appeared in the range from 16◦ to 28◦ indicates the existence of amorphous silica in the coating layer [13]. The magnetic properties of HSFM and pure magnetic particles were measured by VSM. Fig. 4 shows their magnetization curves. The saturation of HSFMs, which was revealed to be equal to 0.39 emu/g, is comparable to the pure magnetic particles of 1.65 emu/g. The difference of magnetization takes place because the amount of magnetic particles in the HSFM is reduced about a quarter of the sample of pure magnetic particles as measured by EDX (not shown). The magnetization of ferromagnetic materials is very sensitive, depending on the size and the structure. If a sample consists of small particles, its magnetization decrease with the particle size when compared to the large particles at a certain applied field. It is also known that below some critical size magnetic par-

Fig. 4. VSM measurements of (a) pure magnetic particles and (b) HSFMs synthesized at Rw = 20.

ticles become single domain because of the interplay between the energy of dipole fields and domain wall creation [14]. The curve displays the typical feature of nanosized single domain particles assembled in hollow shell structure. The BET surface area and pore parameters of HSFMs were measured by nitrogen adsorption–desorption isotherm measurement on an ASAP 2000 analyzer. Fig. 5 shows the nitrogen adsorption– desorption isotherm of HSFM synthesized at Rw = 20. In contrast

Fig. 5. Nitrogen adsorption and desorption isotherms of HSFMs synthesized at Rw = 20.

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with our previous results [15], it reveals the typical Type I isotherm in the IUPAC classification, which shows a long saturation plateau and involves micropore filling. The analysis data shows that it had a large surface area of 573.86 m2 /g and an average pore volume of 0.298 cm3 /g. 4. Conclusions Hollow silica particles functionalized by magnetic particles are prepared without templates using the W/O emulsion system, including magnetic nanoparticles synthesized by the redox reaction of Fe salts. Since the removal process like etching step is not required, our system is a simple and easy method to obtain the functionalized shell and composite hollow materials. It is verified by XRD, EDS and VSM measurements that the structure and magnetic property of the magnetic particles are not changed in quality before and after the synthesis of hollow silica particles. HSFMs or HSFMs modified with nucleophilic functional groups containing amine, sulfur, and phosphorous can be employed in various fields such as composite materials and biotechnology. Moreover, this method can be extended to the synthesis of hollow silica particles functionalized by other metal oxide materials as well as iron oxide.

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