(Invited) Fabrication of Anodic Porous Alumina via Anodizing Aluminum in Novel Electrolyte Solutions
Authors: Kikuchi Tatsuya , Iwai Mana , Nakajima Daiki 
Published: December 2024

Anodizing aluminum in several appropriate acidic electrolyte solutions, such as sulfuric, dicarboxylic, phosphoric, and chromic acids, causes the formation of anodic porous alumina. The porous alumina film possesses vertical nanoscale pores in its structure, and is typically used for corrosion protection and nanostructure fabrication. Particularly, since the self-ordering behavior of nanoscale pores in the porous alumina film under the appropriate anodizing conditions was reported by Masuda et al. in 1995, the ordered porous alumina film is widely used for the fabrication of various nanomaterials, such as plasmonic devices, antireflection structures, filters, memory devices, and sensors.
The nanostructure and property of the porous alumina film strongly depends on the electrolyte used during anodizing. The discovery of additional electrolytes for anodizing aluminum would cause the novel nanostructures and properties of anodic oxide, thus expand their applicability. In this lecture, we describe various electrolytes for the formation of novel anodic oxide films on the aluminum surface. In addition, many interesting nanostructures and properties exhibited by these anodic oxide films will be presented.
The interpore distance of porous alumina, D
int, increases with the applied voltage, U, during anodizing. We studied anodizing aluminum in various acidic electrolyte solutions for the formation of porous alumina, and the self-ordering behaviors with new applied voltages and interpore distance regions are found via anodizing in selenic acid at U = 42-48 V for D
int = 95-112 nm, phosphonic acid at 150-180 V for 370-440 nm, and etidronic acid at 210-270 V for 530-670 nm. The interpore distance of these and typical porous alumina films was proportional to the applied voltage with a proportionality constant of k = 2.5 nmV-1. Recently, we reported a novel anodizing method in several alkaline electrolyte solutions. For example, anodizing in sodium tetraborate (Na2B4O7) solution causes the self-ordering behavior at 90-190 V for 260-590 nm. Interestingly, the interpore distance obtained in this alkaline solution also had a linear relationship with the voltage, but the proportionality constant obviously increased by 1.2 times, with k = 3.0 nmV-1 under the new regime. A wide interpore distance range for the self-ordering of porous alumina can be obtained by anodizing in these novel electrolyte solutions.
Pyrophosphoric acid is a unique electrolyte that produces anodic alumina nanofibers and provides higher controllability of their nanomorphologies. As the aluminum specimen is anodized in pyrophosphoric acid, the anodic alumina without anions at the apices of the honeycomb structure remains during anodizing due to their slow dissolution rate, thus numerous alumina nanofibers measuring less than 10 nm in diameter are formed on the extremely thin, porous alumina film. As a result, sub-10 nm alumina nanofibers completely cover the aluminum surface. The nanofiber-covered aluminum surface exhibits superhydrophilic behaviors with the rapid spreading of a water droplet. In addition, superhydrophobic surfaces can be fabricated by modification with hydrophobic self-assembled monolayers (SAMs) on the surface of alumina nanofibers. Moreover, opposite water slipping properties such as slippery and sticky states can be obtained by controlling the nanomorphology of alumina nanofibers.
As described above, the nanostructure of the anodic oxide changes significantly depending on the type of electrolyte used, thus we can obtain interesting nanomorphologies and properties by anodizing in various electrolyte solutions. However, even now there are many unknown behaviors during anodizing. Therefore, anodizing science and technology will continue to evolve in the future.

Figure 1