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Chemistry Letters Vol.33, No.5 (2004)
Fabrication of Highly Ordered Anodic Porous Alumina
Using Self-organized Polystyrene Particle Array
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Hideki Masuda, Yoshitaka Matsui, Masato Yotsuya, Futoshi Matsumoto, and Kazuyuki Nishio
Department of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji, Tokyo 192-0397
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Kanagawa Academy of Science and Technology, 5-4-30, Nishihasimoto, Sagamihara, Kanagawa 229-1131
(Received February 9, 2004; CL-040147)
Highly ordered anodic porous alumina was fabricated by
anodization of vapor-deposited Al that had been deposited on
a self-organized periodic array of polystyrene particles with a
submicron-scale diameter on a glass plate. The ordered bumpy
structure of the particle array surface was replicated on the de-
posited Al surface. Anodization was initiated at the concaves
of the Al surface. Employment of the self-organized particle ar-
ray to produce highly ordered anodic porous alumina with a sub-
micron-scale channel structure was shown to be feasible.
Recently, anodic porous alumina with pores on the submi-
cron or nanometer scale has attracted a great deal of attention
as a key material in the fabrication of various submicro- and
nanodevices in various fields such as electronics, optics, magnet-
ism, biomedical science, and electrochemistry, because of its or-
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dered arrangement and high aspect ratio of channels. We have
reported that a straight channel array structure with regular chan-
nel diameter of submicron order and an ideal hexagonal arrange-
ment could be obtained by adding anodization of Al to the pre-
Figure 1. Schematic diagram of fabrication of highly ordered po-
rous alumina using self-organized 2-D polystyrene particle array.
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texturing process of Al. In the pretexturing process, a master,
which has a hexagonal array of convexes, is placed on an Al
sheet and pressed to generate the array of concaves on the sur-
face of the Al sheet. The array of concaves initiates the develop-
ment of pores and guides the growth of channels. However, the
master used in the pretexturing process was a SiC mold prepared
using expensive electron-beam lithography and ion-etching ap-
paratuses. In this paper, we propose a novel preparation process
of anodic porous alumina with an highly ordered submicron-
channel array structure, by means of a polystyrene particle array
(
1) deposition of Al on the polystyrene particles array, (2) cleavage
of the glass plate from the deposited Al, (3) removal of polystyrene
particles by immersing the Al substrate into toluene, (4) anodiza-
tion for formation of porous alumina. (a) glass plate, (b) self-organ-
ized polystyrene particle array, (c) Al, (d) porous alumina.
array using a vapor deposition apparatus (ALVAC EX-201) with
Al (99.99% purity, Wako) serving as the vapor deposition
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source, at a deposition rate of 20 nmꢀs for 2.5 min. The depos-
ited Al was mechanically cleaved from the glass plate, and then
immersed in toluene to remove polystyrene particles from the Al
surface. After removal of the particles, anodization was conduct-
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formed naturally by self-organization of the particles. The
structure is considered suitable for use in the formation of texture
onto an Al surface for the development of highly ordered pores,
because of the periodic bumpy structure of the particle array sur-
face.
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ed at a constant voltage of 80 V at 17 C for 3 min using a direct-
current source. Pores in the porous alumina were widened by
etching the sidewall of the pores in 5 wt % phosphoric acid solu-
Figure 1 schematically illustrates the procedure for the fab-
rication of highly ordered anodic porous alumina using self-or-
ganized 2-D polystyrene particle arrays. A glass plate was im-
mersed in ethanol and sonicated in an ultrasonic bath to clean
the surface. A silicon rubber O-ring of 0.35-cm diameter was
placed on the glass substrate. Polystyrene latex (20 mL,
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tion at 30 C for 80 min. The structure of the obtained porous
alumina was observed using a scanning electron microscope
(
SEM: JEOL JSM 6100, Hitachi S-5000).
Figure 3 shows a surface SEM image of Al fabricated by de-
positing Al onto the polystyrene two-dimensional array. This
SEM image demonstrates that highly ordered concaves can be
replicated from the polystyrene particle array onto the surface
of the deposited Al. The concaves fabricated on the deposited
Al have a pore diameter of 200 nm and pore period (center-to-
center distance between adjacent pores) of 200 nm.
Figure 4 shows surface and cross-sectional SEM images of
the porous alumina after anodization in 0.5 to 2.8 wt % oxalic
acid solutions and pore widening in a 5 wt % phosphoric acid so-
lution for 80 min. The images reveal an almost ideally arranged
0.2 wt%, diameter: 0.212 mm, Nissin EM) dispersed in water
was dropped into the region surrounded by the silicon rubber
O-ring on the glass substrate, and the droplet was uniformly
spread over the inside of the silicone rubber O-ring. The sub-
strate was dried by slowly evaporating the water solvent in an
airtight box. The polystyrene particles were self-organized
two-dimensionally inside the ring on the glass plate (Figure 2).
An Al layer of 3-mm thickness was deposited on the 2-D particle
Copyright ꢀ 2004 The Chemical Society of Japan
Masuda, Hideki
