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Langmuir 2004, 20, 554-555
Preparation of Monodisperse SiO2 Nanoparticles by Membrane Emulsification Using Ideally Ordered Anodic Porous Alumina Takashi Yanagishita,† Yasuyuki Tomabechi,† Kazuyuki Nishio,†,‡ and Hideki Masuda*,†,‡ Department of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji, Tokyo 192-0397, Japan, and Kanagawa Academy of Science and Technology, 5-4-21 Nishihashimoto, Sagamihara, Kanagawa 229-1131, Japan Received July 30, 2003. In Final Form: October 28, 2003 Monodisperse SiO2 particles of nanometer dimensions were fabricated by membrane emulsification using ideally ordered anodic porous alumina. For the preparation of monodisperse emulsion droplets, the dispersed phase was pressed through a porous alumina membrane into the continuous phase. After solidification treatment of the emulsion droplets, prepared spherical SiO2 nanoparticles with uniform sizes were obtained. From scanning electron microscope observation of the obtained particles, it was confirmed that the size distribution of SiO2 nanoparticles is relatively narrow.
Introduction The preparation of monodisperse nanoparticles with uniform shapes and sizes has been of growing interest due to the utilization for various types of functional application fields, such as electronic, optical, and chemical devices. Although there have been a large number of techniques for preparing monodisperse particles, the uniformity in shape and size has not been satisfied in most techniques. Membrane emulsification, in which emulsions are formed by passing pressurized liquid through a porous membrane, is one promising process for preparing monodisperse emulsion droplets.1-4 In some cases, the obtained droplets can be solidified to yield monodisperse solid particles after post-treatment. There have been several works on the preparation of monodisperse particles based on membrane emulsification. In these works, a porous glass membrane or a lithographically formed Si microchannel plate has been used as a membrane for emulsification.1-3 However, the lower size limit of the obtained emulsion has been in the range from several tens of microns to microns. In the present work, we describe the preparation of monodisperse particles of nanometer dimensions using anodic porous alumina as a membrane for emulsification. In the membrane emulsification, the droplet size and its uniformity are strongly dependent on the geometrical structure of the membrane used for emulsification. For the preparation of monodisperse emulsions with reduced sizes, membranes with uniform pores of reduced sizes are essential. Anodic porous alumina, which is formed by anodization of Al in acidic solution, is a typical porous material. Under appropriate anodizing conditions, this material has a highly ordered hole array structure composed of straight cylindrical holes.5 Recently, we have shown that an ideally * To whom correspondence should be addressed. E-mail:
[email protected]. † Tokyo Metropolitan University. ‡ Kanagawa Academy of Science and Technology. (1) Nakashima, T.; Shimizu, M.; Kukizaki, M. Key Eng. Mater. 1991, 61, 513. (2) Omi, S.; Katami, K.; Yamamoto, A.; Iso, M. J. Appl. Polym. Sci. 1994, 51, 1. (3) Kawakatu, T.; Kikuchi, Y.; Nakajima, M. J. Am. Oil Chem. Soc. 1997, 74, 317. (4) Schro¨der, V.; Schubert, H. Colloids Surf. 1999, 152, 103.
ordered anodic porous alumina could be obtained by a pretexturing process of Al.6,7 The intervals and diameters of the anodic porous alumina prepared by this process can be precisely controlled on nanometer dimensions by adjusting the preparation conditions. In the present report, we describe the preparation of monodisperse SiO2 particles of nanometer dimensions by membrane emulsification using an ideally ordered anodic porous alumina membrane. To our knowledge, the preparation of small particles of nanometer dimensions using membrane emulsification has not been reported so far. Experimental Section Figure 1 shows the experimental procedure for the fabrication of SiO2 nanoparticles. The preparation of an ideally ordered anodic porous alumina membrane was performed by a process reported previously.6,7 In such a process, the nanoindentation of Al is conducted using a Ni mold with a hexagonally arranged array of convexities. The shallow concavities prepared by nanoindentation of Al act as initiation sites of the holes, and an ideally ordered channel configuration with high aspect ratios is produced by subsequent anodization. In the present study, the imprinting of Al was carried out using a mold with a 500 nm periodicity. Pretextured Al was anodized under constant voltage conditions of 200 V at 0 °C for 90 min in 0.1 M phosphoric acid. After the anodization, Al was removed in a saturated HgCl2 solution. The bottom part of the oxide film was subsequently removed using an Ar ion milling apparatus (JEOL JIT-100) at a beam voltage of 6 kV for 45 min. The anodic porous alumina was kept in a chamber filled with [(CH3)3Si]2NH vapor to make its surface hydrophobic. After the hydrophobic treatment, the obtained porous alumina membrane was fixed to a filter holder using epoxy resin. This filter holder was subsequently attached to a microsyringe. Using the ideally ordered anodic porous alumina as a membrane for emulsification, 1 M aqueous Na2SiO3 solution was pushed out into a mixture of hexane and cyclohexane (1:4 by volume) containing 2 wt % surfactant (Tween 85) in order to prepare monodisperse emulsion droplets. The water-in-oil (w/o) emulsion obtained by membrane emulsification was added to 4 wt % aqueous (NH4)2CO3 solution. The suspension was agitated using a magnetic stirrer at 40 °C for 8 h to synthesize SiO2 particles. The obtained SiO2 nanoparticles were trapped on a filter membrane with a pore diameter of 20 nm (Anodisc, (5) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (6) Masuda, H.; Yamada, H.; Satoh, M.; Asoh, H.; Nakao, M.; Tamamura, T. Appl. Phys. Lett. 1997, 71, 2770. (7) Asoh, H.; Nishio, K.; Nakao, M.; Tamamura, T.; Masuda, H. J. Electrochem. Soc. 2001, 148, B152.
10.1021/la030314a CCC: $27.50 © 2004 American Chemical Society Published on Web 01/07/2004
Letters
Langmuir, Vol. 20, No. 3, 2004 555
Figure 3. SEM image of obtained SiO2 nanoparticles on a filter membrane.
Figure 4. Distribution histogram for the diameter of SiO2 nanoparticles prepared by membrane emulsification using ideally ordered anodic porous alumina. Figure 1. Schematic drawing of the preparation of monodisperse SiO2 nanoparticles by membrane emulsification: (a) pressing of the dispersed phase through the pore of a membrane into the continuous phase, (b) preparation of monodisperse emulsion droplets, and (c) synthesis of SiO2 nanoparticles.
Figure 2. SEM micrograph of the surface view of ideally ordered anodic porous alumina after removal of the bottom layer of alumina by ion milling treatment. Anodization was conducted in 0.1 M phosphoric acid at 0 °C at 200 V for 90 min. Whatman Ltd.) and then washed with distilled water and dried at room temperature for scanning electron microscope (SEM) observation. The sizes of the prepared silica nanoparticles were determined by SEM observation.
Results and Discussion Figure 2 shows a SEM image of the anodic porous alumina used as a membrane for emulsification. From the SEM image, it was observed that the holes were arranged ideally over the sample and that the interval was in good agreement with that of the pretextured pattern. The hole diameter and thickness of the porous membrane were 125 nm and 15 µm, respectively. The relative standard deviation of the hole diameters was 1.9%. Each pore in the porous alumina membrane has a circular cross section, and the distribution of hole diameters was very narrow.
Figure 3 shows a typical SEM image of the SiO2 nanoparticles obtained by membrane emulsification using anodic porous alumina. The SiO2 nanoparticles were observed by SEM after being trapped on a filter membrane of 20 nm diameter. From Figure 3, it was confirmed that the obtained SiO2 particles had uniform size and shape. Figure 4 shows the typical size distribution of the SiO2 nanoparticles determined by SEM observation. The average diameter of the SiO2 particles was 70 nm, and the relative standard deviation was 26%. The mean diameter of the SiO2 nanoparticles was somewhat smaller than that of the pores of anodic porous alumina. This difference in size is due to the volume shrinkage of SiO2 particles during the synthesis of SiO2 particles from emulsion droplets of aqueous Na2SiO3 solution. The precise ratio of volume shrinkage upon solidification is not clear because it is difficult to obtain the droplet size in the emulsion before solidification at the present stage. In the case of using anodic porous alumina without any hydrophobic treatment, the size uniformity of the obtained SiO2 nanoparticles was lower than that of the SiO2 nanoparticles prepared using the hydrophobic membrane. This is because the hydrophilic surface is easily wetted with aqueous Na2SiO3 solution and is covered with the dispersed phase. Consequently, emulsion droplets with low uniformity of sizes resulted. Conclusions SiO2 nanoparticles were prepared by membrane emulsification using an ideally ordered anodic porous alumina. The average size of the obtained SiO2 nanoparticles was 70 nm, and the relative standard deviation was 26%. Thus, an ideally ordered anodic porous alumina is useful as a membrane for emulsification. This method will be applicable to the preparation of monodisperse nanoparticles of several types of material. LA030314A
