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A surface with a wettability gradient can be used as a surface where water droplets transport in one direction. In this report, a surface with a wettability gradient was formed by Al anodization and subsequent gradient etching. Since the surface wettability of anodic porous alumina formed by the Al anodization was dependent on hole size, anodic porous alumina with gradient hole size was prepared to form a surface with a wettability gradient. The surface obtained by this method showed continuously changing wettability, and it was possible to control the range and amount of change in the contact angle of the water droplets by adjusting the hole size. In addition, water droplets were observed to transport spontaneously from an area with a high contact angle to that with a low contact angle on the surface of anodic porous alumina with gradient hole size. The obtained surface with a wettability gradient can be used in various applications, such as microfluidic devices, water collection, and oil-water separation.

Polymer nanofibers prepared via continuous spinning using anodic porous alumina are reported.

Polysiloxane antireflective structures composed of tapered nanopillar arrays were prepared by nanoimprinting using anodic porous alumina molds with tapered pores. Because polysiloxane is a heat-resistant material, the resulting tapered nanopillar array structures were maintained even after heat treatment at 200ºC. In addition, no significant changes in antireflection properties were observed before and after heat treatment at 200ºC. These results indicate that the polysiloxane nanopillar arrays obtained in this study can be applied as heat-resistance antireflection structures.

An anodic porous alumina mold with tapered pores, which can be used to form moth-eye structures, was formed by repetitions of anodization and etching. It was shown that the controllability of the pore shape of the anodic porous alumina mold improved with the number of repetitions of anodization and etching. However, it was found that even when the total anodization time or anodic charge (electrical current × time) was kept constant, the thickness of anodic films was not constant because the total etching time varied. This is because the etching of the anodic porous alumina mold not only increases the pore size but also reduces the thickness of the barrier layer so that pore growth proceeds after the barrier layer is re-formed during re-anodization. Therefore, it was found that if anodization is performed with the additional anodic charge required to re-form the barrier layer, anodic porous alumina mold with tapered pores and uniform film thickness can be produced even if the etching time is varied. Nanoimprinting using the resulting anodic porous alumina mold was shown to form a moth-eye structure with a reflectance of less than 0.1% over the entire visible light range

Anodic porous oxides with ordered nanohole array structures were prepared by the formation of concave arrays on the surface of Cu, Zn, and Ni substrates and the subsequent anodization of the prepatterned substrates. The concave arrays on the surface of the substrate were formed by Ar ion milling using an alumina mask. Although the anodization of Cu, Zn, and Ni substrates without prepatterning generates spongelike porous structures, ordered arrays of cylindrical nanoholes were obtained by the anodization of prepatterned substrates. The interpore distance of the obtained nanohole arrays was controlled by changing the period of the concave arrays. Crystallized ordered nanohole arrays of Cu

Ordered alumina through-hole membrane with an interhole distance of 1 μm.

Renewable superhydrophobic surfaces based on laminated polymer films with nanopillar array structures were prepared. Polymer nanopillar arrays exhibiting superhydrophobic properties were prepared by nanoimprinting using anodic porous alumina as a mold. The hydrophobic properties of the obtained polymer nanopillar arrays could be controlled and optimized by changing the geometrical structures of anodic porous alumina molds used for nanoimprinting. The polymer films were laminated using a photocurable monomer. The ordered polymer nanopillar array structures could be maintained even after delamination of the films. Renewed polymer nanopillar arrays exposed by peeling off the upper films exhibited a water contact angle higher than 150°. Using this process, superhydrophobic surfaces could be obtained repeatedly by delamination of a film even when superhydrophobicity deteriorated with the collapse of surface patterns. The obtained renewable superhydrophobic surfaces can be used for various applications requiring high durability.

Ordered anodic porous alumina with controlled-size holes on the order of a single-nanometer scale was obtained by the atomic layer deposition (ALD) of Al2O3 or TiO2. The thin metal oxide layers of uniform thickness were formed successfully on the inner wall of the hole of the ordered anodic porous alumina with high aspect ratios by ALD. The hole diameter of the ordered anodic porous alumina could be controlled precisely by adjusting the number of cycles of ALD. The obtained anodic porous alumina with an ordered hole arrangement of reduced holes will be applied to various application fields requiring uniform-sized holes on the order of a single-nanometer scale, such as the starting material for preparing various types of quantum effect devices.

Self-ordered anodic porous alumina with a long-range ordered hole arrangement having an inter-hole spacing of over 1.5 μm was prepared by anodization in a citric acid electrolyte containing a small amount of phosphoric acid. The inter-hole spacing of the ordered anodic porous alumina could be controlled within a range of 700 nm to 1.8 μm by adjusting the anodization conditions. The use of an electrolyte with a mixture of water and ethylene glycols, especially tetraethylene glycol, was effective for preparing ordered anodic porous alumina with large inter-hole spacings. The obtained anodic porous alumina is suitable for various applications that require highly ordered hole array structures with large inter-hole spacings.

Polymer infiltration into metal surface structures during injection molding results in a direct and tight metal–polymer joint. Sizes of metal surface structures were maintained within the 50–500 nm range by anodization, and the infiltration depth into several sizes of the structures under various injection speed conditions were investigated in this study. Replicated structures were evaluated using atomic force microscopy after dissolving the metal parts of the joint specimens. The experimental results revealed that the structure size caused a transition of the infiltration dependence on the injection speed. In the case of polybutylene terephthalate–aluminum alloy joining, the infiltration depth increased with the low injection speed the pores equal to or smaller than 300 nm, while the high injection speed led to deep infiltration for the large pores. In structures of the order of tens of nanometers, the viscoelasticity of flowing polymer and molecular orientation could affect the polymer deformation and infiltration much more than the solidification process. The findings in this study provide insights into the anchoring mechanism of injection molded direct joining and could improve the micro/nano replication in the injection molding process.

Ordered alumina through-hole membranes were obtained by a combination of the anodization of Al, formation of a TiO

Ideally ordered TiO2 through-hole membranes were obtained by a combination of Ti substrate pre-patterning and two-layer anodization. The Ti substrate was pre-patterned by Ar ion milling using ideally ordered porous alumina as an etching mask. Each concave pit formed by dry etching acted as an initiation site for hole development during anodization, and ideally ordered anodic porous TiO2 was produced by anodization using an electrolyte containing fluoride ions. Two-layered anodic porous TiO2 samples with different solubilities were formed by heat treatment for the crystallization of the first anodized oxide layer and a subsequent second anodization. The selective dissolution of the lower part of the two-layered porous TiO2 produced an ideally ordered TiO2 through-hole membrane. Using the present process, the repeated preparation of ideally ordered TiO2 membranes can be conducted because the ideally ordered concave array, which corresponds to the hole arrangement at the bottom of the detached TiO2 membrane, is maintained even after detaching the membrane. The obtained samples can be applied to various functional devices, such as photonic crystals and filtration membranes with photocatalytic properties.

A surface with a simple nanopillar array and hierarchical micro-/nano structures consisting of microconvexes and a surface with a nanopillar array were fabricated by nanoimprinting using two types of anodic porous alumina mold. On both surfaces, the height of polymer nanopillars could be controlled by adjusting the anodization time for the preparation of the anodic porous alumina mold. Regarding water-repellent properties, a higher contact angle was observed on the surface with a hierarchical structure than on the surface with a simple nanopillar array. In the measurement of the water droplet sliding angle, the surface with a hierarchical structure showed higher performance characteristics than the surface with a simple nanopillar array. This is because the surface with a hierarchical structure can more easily trap air between the substrate surface and the water droplet than the surface with a simple nanopillar array. The hierarchical micro-/nano structures could be prepared repeatedly by nanoimprinting using a single mold. The method developed here for preparing a superhydrophobic surface is used for various applications, such as those requiring superhydrophobic surfaces with sliding angles.

Electrodeposition and electrodissolution of Li metal in an anodic porous alumina that has an ideal hole array structure were demonstrated. Electrodeposition and electrodissolution of Li metal in the hole array structures could be repeatedly carried out without any formation of dendritic Li. Although the hole diameter of the anodic porous alumina was reduced from the micrometer to nanometer scale, deposition and dissolution of Li metal could be stably conducted. The present process is expected to be applicable to the fabrication of a safe rechargeable Li-metal battery.

Ordered anodic porous WO3 was prepared by the anodization of a W substrate with a depression pattern under a constant voltage of 18 V in concentrated phosphoric acid at 120 °C. However, the behavior of hole formation induced in depressions was found to be strongly affected by the depth and diameter of the depressions. Multiple holes were formed in a shallow depression during the initial stage of anodization, whereas only one hole was formed in a deep depression. In addition, when the depression diameter was small, the growth of fine holes around the induced holes was observed. Since the fine holes formed on the surface of anodic porous WO3 prevented the induced holes from growing in the depth direction, the formation of large-diameter depressions allowed the induced holes to grow deeper. The obtained ordered anodic porous WO3 can be used for various applications such as in photocatalysis and electrochromic devices.

A nanohole array structure with a regular array of cylindrical holes of a high aspect ratio with a period of 63 nm can be obtained by anodizing 304 stainless steel with a concave pattern. It was shown that matching the concave period to the anodization voltage is important to achieving the fabrication of nanohole arrays with a high aspect ratio. This is because the interhole distance of anodic porous oxide obtained by the anodization of 304 stainless steel in ethylene glycol containing 0.1 M ammonium fluoride depends on the anodization voltage. By optimizing the fabrication conditions, it was possible to fabricate highly ordered nanohole arrays with aspect ratios exceeding 100. The nanohole arrays obtained in this study are promising as key materials for fabricating various functional devices.

In this study, highly ordered anodic porous metal oxides of transition metals (W, Fe, Co, and Nb) were prepared by anodization combined with a pretexturing process. The formation of an array of dimples on the metal surface was achieved by dry etching using ordered anodic porous alumina as a mask. Subsequent anodization of the pretextured metals generated metal oxides (WO3, Fe2O3, Co3O4, and Nb2O4) with an ordered hole array structure because each dimple acted as a starting point for hole development. The hole periodicity of the resulting ordered anodic porous metal oxide could be controlled by changing the periodicity of the dimples formed on the substrate. Ordered anodic porous films obtained by this process could be used in various functional devices that require ordered hole array structures.

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Anodic porous alumina templates with controlled microscale geometrical structures were prepared by a process combining mask formation and subsequent selective etching of the alumina layer. In this process, the anisotropic etching of anodic porous alumina allows the preparation of anodic porous alumina with microhole array patterns having high aspect ratios. The electrodeposition of Ni using the obtained alumina templates generated an array of Ni micropillars with high aspect ratios. The height of Ni micropillars could also be controlled by adjusting the thickness of the anodic porous alumina. The obtained Ni micropillar array with a high aspect ratio was applied as a mold for imprinting. The ordered microstructures of TiO2 with high aspect ratios were prepared by imprinting using the Ni mold.

Polymer nanofibers with controlled diameters were formed by continuous spinning using ordered anodic porous alumina as a spinneret. The diameter of the polymer nanofibers could be controlled by cha...

Al substrates with smooth curved surfaces were fabricated by peeling off Al foil sputtered on the surface of concaved substrates. Anodic porous alumina with tapered pores on the curved surface was obtained by repeated anodization and etching of the obtained Al substrate. The moth-eye structure was formed on the curved surface by nanoimprinting using the anodic porous alumina as a mold. From the reflectance measurement, it was shown that the reflection of the incident light was suppressed on the curved surface where the moth-eye structure was formed. This process can be used to fabricate moth-eye structures on the surface of substrates of various shapes by changing the substrate used for sputtering and is expected to expand the range of applications for moth-eye structures.

The fabrication of anodic porous alumina with anisotropic structures, i.e., an anisotropic hole arrangement and anisotropic elliptical hole shape, was demonstrated. Through the optimization of the pattern used for the pretexturing and the anodizing conditions, anodic porous alumina with holes having an elliptical cross section could be satisfactory obtained. The diameter of the elliptical nanoholes could be controlled by an additional wet-etching process. The obtained anodic porous alumina with elliptical nanoholes is expected to be applied to various functional optical devices, such as localized plasmonic devices, requiring control of the anisotropy of the incident light. Keywords: Anodization, Anodic porous alumina, Elliptical nanohole, Pretexturing

Ideally ordered anodic porous alumina with controlled interpore distances was formed by fabricating a resist mask using a flexible mold and subsequent anodization. Prior to forming the resist pattern on the surface of an Al substrate, Al was pre-anodized at 10 V to prepare the fine porous structure, which acts as a resist adhesive layer. After the formation of the resist mask using a flexible mold, an arranged array of cavities with Al exposed at the bottom was formed by the selective dissolution of the oxide layer at resist openings. The subsequent anodization of the sample with the cavity array generated ideally ordered anodic porous alumina because alumina holes were formed at the bottom of cavities during anodization. This process allows the preparation of ideally ordered anodic porous alumina even on a curved Al surface owing to the flexibility of the mold. In addition, this process can also be applied to the preparation of an ideally ordered anodic porous alumina with a large sample area because the Al substrate can be patterned without high pressure. The obtained sample can be used for various applications requiring an ideally ordered hole array structure.

Anodic porous alumina membranes with site controlled through-holes were prepared by the formation of a masking layer on the surface of anodic porous alumina and subsequent selective second anodization in concentrated sulfuric acid to form a readily soluble layer. After the anodization, the residual Al substrate was removed, and the highly soluble alumina layer formed in concentrated sulfuric acid was dissolved selectively by wet etching. An advantageous point of this process is the controllability of the pattern of through-holes, and the preparation of large samples with selective through-holes is possible. The pattern of through-holes was controlled by changing the mask pattern formed on the surface of anodic porous alumina. The alumina membranes obtained by this process are expected to be used for various applications that require porous alumina membranes with site controlled through-holes.

Polymer micro/nano hierarchical structures were successfully formed by photo-nanoimprinting using anodic porous alumina molds. Anodic porous alumina molds with hierarchical structures were prepared by the anodization of an Al substrate with a micro-concave array. The obtained surfaces with hierarchical structures exhibited superhydrophobicity. The hydrophobic properties of the obtained samples were dependent on the surface structures and could be optimized by changing the micro- and nanopatterns in the hierarchical structure.

Nanoporous alumina spheres with an ordered hole arrangement were prepared through a two-step anodization of small Al particles. The hole periodicity in the ordered anodic porous alumina could be controlled by adjusting the anodizing conditions. Nanoporous hollow spheres were also obtained by removal of residual Al in an etchant. Additionally, nanoporous spheres loaded with Au nanoparticles on their surfaces were obtained through electrochemical or chemical deposition of Au nanoparticles. The obtained Au/alumina composite hollow spheres were used as a substrate for surface-enhanced Raman scattering measurements.

By anodically etching a (100)-oriented Al foil with an etching mask in hydrochloric acid solution, an ideally ordered array of Al holes was obtained. The Al hole array structure showed wavelength selective thermal radiation property. Wavelength band of the thermal radiation could be tuned by controlling the aperture size of the hole. The aperture size of the Al hole was controlled by optimizing geometrical structures of the etching mask and anode etching conditions.

Geometrically controlled Li nanohole arrays with high aspect features were fabricated by a two-step replication process using ordered anodic porous alumina as a template. The use of an appropriate electrolyte for Li electrodeposition and organic solvent for the removal of the polymer (PMMA) template allowed the preparation of Li nanohole arrays with precisely replicated nanostructures. The application of a template with the structures for the electrodeposition of Li contributed to the suppression of the formation of Li dendrites, contributing to the precise replication.

We studied the formation of ideally ordered nanoporous zirconia by anodizing Zr having a texturing pattern. A texturing pattern was formed on a Zr surface by peeling off a sputtered zirconium film from a metal (Ni) mold having an ordered array of convexes. In contrast to texturing by nanoimprinting, the present texturing process never requires the pressing of a mold onto the Zr surface. During the anodization of Zr that has a texturing pattern on its surface, which is an ideally ordered array of concaves, each concave acted as a starting point of the formation of nanoholes. By the present process, porous anodic zirconia having an ideally ordered array of nanoholes was obtained. The interval between nanoholes in the porous anodic zirconia agreed with the texturing pattern.