Nanoporous α-Alumina Membranes with Pore Diameters Tunable over Wide Range of 30-350 nm
Authors: Asoh Hidetaka , Masuda Tatsuya , Ono Sachiko 
Published: February 2020

Porous anodic oxide film, which is formed by anodizing of aluminum, is a typical self-ordered nanoporous material. Over the past several decades, studies have been carried out on the protection or design of aluminum surfaces for commercial applications. Recently, anodic alumina films have attracted considerable attention as a key material due to their potential technological applications in area such as filters, catalyst supports, biological applications, electronic, magnetic and optical devices. However, commercially available anodic porous alumina membranes have insufficient pore arrangement and low chemical resistance because the porous alumina films obtained by typical anodizing processes are amorphous. To expand the application of anodic porous alumina with ordered pore arrays, it is necessary to control the cell dimensions (e.g., pore interval, pore diameter, and pore depth) and to enhance the chemical resistance of alumina membranes in extreme environments such as high temperature, high vapor pressure, and high-concentration acid/base solutions. Several fundamental studies have been reported on the fabrication of crystalline anodic porous alumina aimed at improving the chemical properties of α-alumina membranes using a combination of anodizing and heat treatment processes. Since the 2000s, the crystallization of free-standing alumina membranes to the α-phase has also been attempted; however, curling and cracking as a result of thermal deformation caused by the change in density and decomposition and desorption of electrolyte anions from the outer anion-incorporated layer in the cell wall were unavoidable during the transition to crystalline alumina in addition to the desorption of bound water at lower temperatures.
In this study, we investigated the optimum conditions for the fabrication of α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm by the anodizing of aluminum and subsequent heat treatment. The morphological and structural change of the porous alumina membrane during heat treatment was also evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), and thermogravimetry-mass spectrometry (TG-MS). Because the pore diameter could be adjusted by changing the anodizing conditions including the electrolyte species and formation voltage, anodizing of high-purity aluminum sheets was conducted in sulfuric acid, oxalic acid and phosphoric acid at each specific voltage. Here, we applied two-step anodizing process to improve the regularity of the pore arrangement of the anodic film. As a result, the deformation of alumina membrane such as warp and cracking during heat treatment for crystallization was effectively suppressed [1, 2]. By optimizing the conditions for anodizing, subsequent detachment, and heat treatment, nanoporous and single phase α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm were successfully fabricated. Even in the case of anodic porous film with short pore interval of approximately 60 nm, which was formed at 25 V in sulfuric acid, α-alumina membrane that maintained straight channels was obtained. The α-alumina membranes exhibited high chemical resistance in various concentrated acidic and alkaline solutions as well as when exposed to high temperature steam under pressure [2]. These results open a new route to technological and scientific applications.
[1] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Electrochemistry, 82, 448-455 (2014)
[2] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Materials, 8, 1350-1368 (2015)