Growth Behavior of Anodic Alumina Nanofibers Fabricated By Pyrophosphoric Acid Anodizing and Their Hydrophilicity
Authors: Nakajima Daiki , Kikuchi Tatsuya , Natsui Shungo , Suzuki Ryosuke O. 
Published: February 2020

Introduction
Anodizing of aluminum is a simple electrochemical technique that has been widely investigated in the fields of surface science and engineering for corrosion protection, electronic devices, and optical materials. Anodic oxide films fabricated via anodizing can be typically classified into the following two groups: barrier type oxide films formed in neutral solutions and porous type oxide films (porous alumina) formed in acidic and alkaline solutions. Here, we have demonstrated the fabrication of novel anodic alumina nanofibers via a simple anodizing technique in a new electrolyte, pyrophosphoric acid (H4P2O7).
Experimental
High-purity aluminum specimens (99.99 wt%, 110–400 µm thick) were ultrasonically degreased in ethanol for 10 min. After drying, the specimens were electropolished in a 13.6 M CH3COOH/2.56 M HClO4 mixture (T = 280 K) at a constant voltage of 28 V for 1 min. The electropolished specimens were immersed in a concentrated pyrophosphoric acid solution (74.0-78.0 wt%, T = 293 K), and then were anodized at a constant voltage of 10 to 95 V using a PC-connected direct-current power supply. A platinum plate was used as the cathode for pyrophosphoric acid anodizing. The surface nanomorphology of the anodized specimens was examined by field emission scanning electron microscopy (FE-SEM) and scanning transmission electron microscopy (STEM). The water contact angles on the aluminum surface anodized via each anodizing condition were measured by an optical contact angle meter. For the wettability measurements, the volume of distilled water placed on the surface of the specimens was adjusted to a relatively large amount of 4 µL to evaluate the superhydrophilic behavior in the initial stage of the measurements.
Results and discussion
Figure 1 shows the SEM images of the aluminum specimen anodized in pyrophosphoric acid at 50 V and 293 K for 12 min. The thin barrier anodic oxide, which was several tens of nanometers in thickness, was observed on the aluminum substrate by SEM observation of the fracture cross-section (Fig. 1a). This thin barrier oxide was immediately formed on the aluminum substrate by applying a constant voltage for 30 s. Due to additional anodizing for 4 min, a honeycomb oxide structure with narrow hexagonal walls was formed on the aluminum surface (Fig. 1b). Although the nanomorphology of the oxide is very similar to the structure of anodic porous alumina, the nanopores in the honeycomb layer are considerably larger than those obtained by typical anodizing in other acidic solutions. Therefore, vigorous chemical dissolution of the anodic oxide occurs during anodizing. When the anodizing time increased to 12 min, fibrous nanostructures were formed on the honeycomb layer, and several nanofibers became tangled and bundled together due to bending under their own weight (Fig. 1c). The bottom honeycomb alumina contains phosphorus that is due to the incorporation of anions originating from the electrolyte, which is similar to typical porous alumina. However, phosphorus is not found in the alumina nanofibers. The anodic oxides without anions remain during long-term pyrophosphoric acid anodizing because of their slow dissolution rate, and numerous anodic alumina nanofibers cover the aluminum surface during anodizing.
The water contact angle on the nanofiber-covered aluminum surface decreased with time after a 4-µL droplet was placed on the surface, and a superhydrophilic behavior with a contact angle measuring 2.2° was observed within 2 s; this contact angle is considerably lower than those observed for electropolished and porous alumina-covered aluminum surfaces.

Figure 1