Anodizing is one of the most important surface finishing techniques for aluminum and its alloys in various industrial applications. Recently, we reported a novel anodic aluminum oxide formed via anodizing in a new electrolyte, pyrophosphoric acid solution (H₄P₂O₇). Pyrophosphoric acid anodizing leads to the formation of numerous alumina nanofibers measuring less than 10 nm in diameter on the aluminum substrate. In the present investigation, we report the fabrication of a superhydrophobic aluminum surface modified by anodic alumina nanofibers and self-assembled monolayers (SAMs).

High-purity aluminum specimens (4N, 400 µm thick) were ultrasonically degreased and electropolished. The specimens were immersed in a concentrated pyrophosphoric acid solution (74.0 wt%, T = 273–293 K), and then were anodized at a constant voltage of 10 to 75 V to form anodic alumina nanofibers. After anodizing, the specimens were immersed in 5 mM n-alkylphosphonic acid / ethanol solutions (T_i= 293–313 K) for up to 48 h to form SAMs on the anodic alumina nanofibers. Seven types of n-alkylphosphonic acid with different chain lengths were used for SAM-modifications. The surface nanomorphology of the anodized specimens was examined by field emission scanning electron microscopy (FE-SEM). The water contact angles on the specimens were measured by an optical contact angle meter. Distilled water droplets (2–3 µL volume) were placed on the specimens, and the contact angles of the droplets were measured by a charge-coupled device (CCD) camera.

Figure 1 shows the appearances of water droplets on the a) electropolished aluminum surface and b) nanofiber-covered aluminum surface after TDPA-SAM modifications (T_i = 313 K, t_i= 24 h). The contact angle greatly increased from approximately 110° to 160° by pyrophosphoric acid anodizing. Because the pyramidal alumina bundles were formed on the aluminum specimen by pyrophosphoric acid anodizing (Figure 2), the water droplet was supported by the top of the pyramidal alumina bundles with TDPA-SAMs, and a large amount of air was trapped in the spaces between the alumina bundles.

As the aluminum specimens were anodized at the low temperature of 273 K, the contact angle increased with anodizing time, and the aluminum surface exhibited a superhydrophobic behavior after anodizing for 4 h. However, the contact angle gradually decreased after that time, and the superhydrophobic angles above 150° no longer appeared after the excess anodizing process. High-aspect-ratio anodic alumina nanofibers then bent under their own weight, and the aluminum surface was completely covered by the long alumina nanofibers. The contact angle decreases by long-term anodizing at low temperature because of the loss of the air trapping space caused by the excess growth of the alumina nanofibers.