Monodisperse nanoparticles of metal oxides prepared by membrane emulsification using ordered anodic porous alumina

Abstract

Uniform-sized nanoparticles of metal oxides were fabricated by membrane emulsification using ordered anodic porous alumina as a membrane and subsequent solidification. The size of the nanoparticles could be controlled by adjusting the pore size of the anodic porous alumina membrane. This process allows the high throughput preparation of monodisperse nanoparticles of various types of metal oxide with controlled size due to the capability of continuous formation of monodisperse emulsion droplets through the porous membrane.

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Introduction

There has been growing interest in the preparation of monodisperse particles of metal oxides of nanometer scale owing to their potential applications in various fields, such as electronic, optoelectronic and biodevices.^1–4 Various techniques, such as precipitation and chemical vapor deposition (CVD) synthesis, have been proposed for the preparation of nanoparticles of metal oxides.^5–10 However, processes that can generate monodisperse nanoparticles with controlled size have not yet been established. To optimize the performance of devices fabricated from nanoparticles, control of the size and uniformity of the particles is essential. Among the various processes for the preparation of nanoparticles, membrane emulsification is a promising method for producing an emulsion containing uniform-size droplets.^11,12 In this process, monodisperse emulsion droplets are formed by injecting a liquid with a dispersed phase into another liquid (continuous phase) through the pores in a membrane. The size of the droplets can be controlled by changing the diameter of the pores of the membrane used for emulsification. In addition to the formation of liquid droplets, uniform-size solid particles can also be obtained through the solidification of the obtained liquid droplets.¹³ There have been several reports on the preparation of emulsions containing monodisperse droplets by membrane emulsification. The most commonly used porous membrane for emulsification is porous glass, which is formed by the selective etching of a split-phase binary glass. However, the size of the droplets obtained using the porous glass was limited to a range from submicron size to several tens of microns. This is because of the difficulty in obtaining a membrane with uniform-sized pores on the nanometer scales in porous glass. In addition, the uniformity of the obtained droplets was low owing to the lack of uniformity of the pores in the glass membrane.

To obtain an emulsion containing monodisperse droplets on the nanometer scale, we have developed a membrane emulsification method using anodic porous alumina.^14,15 Anodic porous alumina, which is formed by the anodization of Al in an acidic electrolyte, is a typical self-ordered porous material and is promising for membrane emulsification.¹⁶ In our previous work, we showed that SiO₂ nanoparticles can be obtained by the solidification of droplets in an emulsion, which were formed by membrane emulsification using anodic porous alumina.¹⁴ In this process, SiO₂ particles were prepared through the solidification of droplets containing Na₂SiO₃. The uniformity of the obtained SiO₂ particles was low owing to the difficulty in maintaining uniformity in the size of droplets during the synthesis. In the present report, we describe a new method for the preparation of monodisperse nanoparticles of various metal oxides based on membrane emulsification using anodic porous alumina. Here, monodisperse nanoparticles were prepared by the drying of emulsion droplets, which contain small primary particles, in a continuous phase. The most characteristic feature of this process is that the droplets containing fine primary particles are solidified through in situ drying in a continuous phase composed of kerosene. The water in the droplets migrates to the continuous phase in situ. This process allows the generalized preparation of monodisperse nanoparticles of various types of metal oxide with controlled size. The in situ drying process of the liquid droplets in the continuous phase significantly contributes to the strain-free solidification of the droplets and generates uniform-size nanoparticles.

Experimental section

Fig. 1 shows a schematic of the preparation process of highly ordered anodic porous alumina through-hole membrane used for the emulsification. The highly ordered anodic porous alumina used for membrane emulsification was prepared using a previously described procedure.¹⁷ In this process, a pretextured pattern formed by imprinting on Al generates an ideally arranged array of pores in porous alumina after anodization. In the present work, the pretexturing of aluminum was carried out by nanoimprinting using Ni mold with an ordered array of convexes on the surface. After imprinting, anodization was conducted at a constant applied voltage in oxalic acid or phosphoric acid solution. Selective removal of the Al layer was performed by dissolving Al in saturated iodine methanol solution. To obtain a through-hole membrane, the bottom part of the anodic porous alumina was removed using an Ar ion milling apparatus. The pore size was adjusted by etching treatment using 10 wt% phosphoric acid solution at 30 °C.

Fig. 1 Schematic of the preparation of ordered porous alumina through-hole membrane.

Fig. 2 shows a schematic of the preparation process of monodisperse nanoparticles by membrane emulsification using anodic porous alumina. The obtained alumina through-hole membrane was treated with octadecyltrichlorosilane to give the surface hydrophobicity. The alumina porous membrane was then attached to the holder used for membrane emulsification using epoxy resin. As the dispersed phase, commercially available sol solutions containing primary nanoparticles of metal oxides, including 10 wt% TiO₂/SiO₂/ZnO₂ (Catalysts and Chemicals Ind. Co. Ltd., Japan), SiO₂, NbO, SeO, and TiO₂ (Taki Chemical Co., Ltd., Japan), were used. The size of the primary particles was ca. 10 nm. The continuous phase was kerosene containing 3 wt% span 80 as a detergent, which ensures the formation of droplets in the continuous phase. The dispersed phase was injected into the continuous phase through the porous alumina membrane. The pressure was controlled using a gas (N₂) operation system. The obtained emulsion was then heated at 70 °C for 1 h to induce solidification through the drying of the droplets. After the heat treatment, nanoparticles were trapped on a filter for observation by SEM.

Fig. 2 Schematic of the preparation of monodisperse nanoparticles by membrane emulsification using anodic porous alumina: (a) membrane emulsification; (b) monodisperse emulsion droplets of sol solution; (c) preparation of nanoparticles by heat treatment.

Results and discussion

Fig. 3 shows SEM images of porous alumina through-hole membrane. From the surface image shown in Fig. 3(a), it was observed that uniform sized holes were arranged hexagonally. Cross-sectional image shown in Fig. 3(b) revealed that each hole was straight and perpendicular to the surface.

Fig. 3 SEM images of ordered porous alumina thorough-hole membrane: (a) surface and (b) cross-sectional images.

Fig. 4(a) shows an optical microscopic image of droplets formed on the surface of a porous alumina membrane by membrane emulsification. The pore size in the alumina membrane used for the membrane emulsification was 130 nm. The small dark dots arranged at equal intervals over the entire image correspond to the pores of the membrane with a 500 nm interval. Observation of the droplets by optical microscopy at a high resolution revealed that uniform-size droplets were formed at the sites of pores. They then became detached from the membrane and propagated to the continuous phase once they reached a certain size. Fig. 4(b) shows an SEM image of monodisperse nanoparticles of metal oxides (TiO₂/SiO₂/ZnO₂) prepared by the solidification of droplets. In the experiment, part of the fed dispersed phase was emulsified. Concerning the emulsified droplets, all of them were changed to the solid particles. From the SEM image in Fig. 4(b), it was observed that spherical particles with a uniform shape and size were formed. This was because the droplets were solidified by strain-free drying in the continuous phase. The surface of the obtained particles was smooth owing to the small size of the primary particles relative to the obtained particles. The obtained particles were mechanically stable and could be maintained spherical shapes even after sonication treatment for a few minutes. Fig. 4(c) shows the size distribution of the solidified nanospheres. The size distribution of droplets in the emulsion is also shown for comparison. The sizes of the droplets and nanoparticles were determined by optical microscope and SEM observations, respectively. To obtain the distributions, more than 1000 particles were counted. From Fig. 4(c), it was confirmed that both size distributions were relatively narrow. This means that the size uniformity of emulsion droplets was maintained even after solidification. The average diameters of the emulsion droplets and nanospheres were 1.26 μm and 475 nm, respectively. The average size of the nanospheres was 38% of that of the droplets before the heat treatment. This difference in size is caused by the volume shrinkage of solidified particles during drying of emulsion droplets of aqueous sol solution. The value of volume shrinkage is in good agreement with the change in the volume calculated from the concentration of the primary particles in the sol solution used for the dispersed phase.

Fig. 4 (a) Optical microscope image of monodisperse emulsion droplets prepared by membrane emulsification. (b) SEM images of TiO₂/SiO₂/ZnO₂ monodisperse nanoparticles after solidification. (c) Size distributions of solidified nanoparticles and droplets prepared by membrane emulsification.

When the size uniformity of the small particles is high, the formation of an ordered two dimensional (2D) array of the particles in a triangular lattice can be expected. Fig. 5 shows a typical example of an ordered array of metal oxide particles (TiO₂/SiO₂/ZnO₂). The ordered 2D structure of the particles was obtained based on the “moving accumulation technique”, in which the ordered array of particles formed through the flow of the liquid in a single direction. From Fig. 5, the formation of an ordered array of small particles with a triangular lattice can be confirmed, also illustrating the size uniformity of the particles obtained by the present process.

Fig. 5 SEM images of ordered two-dimensional array of particles: (a) low-magnification and (b) high-magnification images.

In the membrane emulsification technique, the size of the emulsion droplets can be controlled by changing the size of the holes in the porous membrane. Fig. 6 shows SEM images of the nanoparticles consisted of the mixtures of TiO₂/SiO₂/ZnO₂ primary particles. The nanoparticles with controlled sizes were prepared by membrane emulsification using anodic porous alumina with different pore sizes. For the preparation of nanoparticles with different sizes, anodic porous alumina membrane with pore diameters of 75, 60, and 50 nm were used. From the SEM images in Fig. 6(a)–(c), it was confirmed that monodisperse nanospheres were also formed by membrane emulsification using anodic porous alumina with reduced pore sizes. The size of the nanospheres decreased with decreasing hole diameter in the porous membrane. Fig. 6(d) shows the size distributions of the obtained nanoparticles shown in Fig. 6(a)–(c), in which the average diameters of the nanoparticles were 146, 88, and 56 nm with relative standard deviations of 7.0, 7.5, and 10.3%, respectively. These results confirmed that the size of the nanospheres can be controlled by adjusting the hole size of the anodic porous alumina and that monodisperse nanospheres with a size of less than 100 nm can be formed by the present process.

Fig. 6 (a)–(c) SEM images of monodisperse nanoparticles prepared by membrane emulsification using anodic porous alumina with different pore diameters: (a) 75, (b) 60, and (c) 50 nm. (d) Size distributions of the monodisperse nanoparticles shown in (a)–(c).

This process can be applied to form nanoparticles of various types of metal oxide by using a sol solution containing primary particles of the metal oxide. Fig. 7 shows SEM images of SiO₂, NbO, SeO, and TiO₂ nanoparticles. The pore size in the alumina membrane was 130 nm. From the SEM images in Fig. 7, it was confirmed that uniform-size spherical particles could be prepared by membrane emulsification in each case. This means that the present process is useful as a general process for preparing monodisperse nanoparticles of various types of material. A small variation in the particle size was observed in nanoparticles shown in Fig. 7. The variation is considered to originate from differences in the concentration of primary particles in each sol solution used as a dispersed phase. However, the uniformity of the obtained nanoparticles was high.

Fig. 7 SEM images of nanoparticles prepared using various sol solutions: (a) SiO₂, (b) NbO, (c) SeO, and (d) TiO₂.

Conclusion

Monodisperse nanoparticles of various types of metal oxide were prepared by membrane emulsification using anodic porous alumina and subsequent solidification. The size of the nanoparticles could be controlled precisely by changing the size of the pores in the anodic porous alumina used for emulsification. The present process is simple and can be used as a general-purpose high-throughput process for forming monodisperse nanoparticles of metal oxides owing to its unique solidification process involving the drying of droplets in the liquid. The monodisperse nanoparticles prepared by this process will be useful for the preparation of various types of functional device requiring uniform-size nanoparticles of metal oxides.

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