Fabrication of Alumina Membrane Filters with Framework

Membrane filters are becoming increasingly important because of their wide range of applications. Precise control of the pore size is essential for precise separation using membrane filters. In addition, a high solution permeability is required for efficient filtration. We previously reported that a large alumina membrane filter with a high density of pores and uniform diameter can be obtained by detaching anodic porous alumina from Al substrates by two-layer anodization using concentrated sulfuric acid [1]. Using this method, the pore size and thickness of the resulting alumina membrane can be controlled by adjusting the preparation conditions, making it possible to produce alumina membrane filters suitable for various applications. However, the resulting alumina membrane filters have a nanohole array structure with cylindrical pores, which makes it difficult to achieve efficient filtration because the solution permeability deteriorates as the thickness of the membrane increases. On the other hand, reducing the membrane thickness to improve the solution permeability results in the deterioration of the mechanical durability. Therefore, alumina membrane filters have a problem in that they cannot achieve both solution permeability and mechanical durability. In this report, we describe the preparation of membrane filters with a structure consisting of a thin filter layer formed at the openings of a thick framework that serves as the support layer. The structure of the membrane filter proposed in this study allows the mechanical strength of the membrane to be reinforced by a thick framework and the filter layer to be thin. This allows the membrane filter to achieve a high permeability and mechanical strength, which is a problem with conventional ordered alumina through-pore membranes. Anodic porous alumina, consisting of a thick framework and a thin filter layer, was prepared by selective etching of anodic porous alumina with a resist mask and re-anodization [2]. Finally, an alumina membrane with a framework was fabricated by detaching porous alumina from the Al substrate via two-layer anodization using concentrated sulfuric acid. The framework structure can be controlled by changing the resist pattern formed on the anodic porous alumina. The thicknesses of the filter and framework layers can also be controlled by adjusting the anodization time. The results of the water permeation measurements using the obtained alumina membrane filter confirmed that it exhibited high permeation performance. The membrane filter with the framework obtained in this study is expected to efficiently separate and filter various substances, such as microplastics, viruses, and bacteria. References [1] T. Yanagisihta and H. Masuda, Electrochim. Acta, 184, 80 (2015). [2] Y. Boushi and T. Yanagishita, Langmuir, 40, 5278 (2024).