(Invited) Preparation of Ordered through-Hole Membranes with Controlled Geometrical Structures By Two-Layer Anodization Processes

Abstract

Ordered through-hole membranes, which have a unique geometrical structure composed of hexagonally arranged array of straight holes with uniform diameters, have attracted growing interest owing to its various applications such as filtration membranes, sensors, and templates. Anodization process of metals is an effective method for the preparation of ordered hole array structures of metal oxides. One advantageous point of this process is controllability of the geometrical structures of the obtained porous oxides based on the anodization conditions [1]. In addition, the naturally occurring self-ordered ordered nanoporous structures can be obtained under the appropriate anodization conditions. In our previous work, we reported a high-throughput process for the preparation of ordered alumina through-hole membranes by two-layer anodization using concentrated sulfuric acid [2-4]. In this process, the second highly soluble alumina layer is formed under the first porous alumina layer by the anodization of samples in concentrated sulfuric acid. The porous alumina through-hole membrane can be obtained by selective dissolution of the second alumina layer formed in concentrated sulfuric acid. This process allows repeated preparation of ordered through-hole membranes from a single metal substrate by repetition of the process. Based on this process, ordered TiO2 through-hole membranes were also obtained using an electrolyte containing fluoride ions[5]. The hole period of the obtained TiO2 membranes could be controlled by changing the anodizing voltage. In the present report, preparation of various types of metal oxide through-hole membranes will be presented. For example, the preparation of ideally ordered TiO2 through-hole membranes by the combination process of two-layer anodization and pretexturing of Ti was described. H. Masuda and K. Fukuda, Science, 268, 1466 (1995).T. Yanagishita and H. Masuda, Electrochim. Acta, 184, 80 (2015).T. Yanagishita, A. Kato, T. Kondo, and H. Masuda, Jpn. J. Appl. Phys., 56, 035202 (2017).T. Yanagihsita, A. Kato, T. Kondo, and H. Masuda, Jpn. J. Appl. Phys., 59, 038002 (2020).T. Yanagishita, H. Inada, T. Kondo, N. T. Nguyen, P. Schmuki, and H. Masuda, J. Electrochem. Soc., 165, E763 (2018).