Recently, we demonstrated the fabrication of anodic alumina nanofibers via anodizing in a new electrolyte, pyrophosphoric acid (H4P2O7). Interestingly, the aluminum surface covered by the anodic alumina nanofibers exhibited a superhydrophilic behavior with a contact angle measuring less than 10°. In the present investigation, we described the fabrication of superhydrophilic and superhydrophobic aluminum alloys via pyrophosphoric acid anodizing and subsequent SAM modification.
Two kinds of aluminum specimens (5N aluminum and 3004/1N30/8021 aluminum alloys) were ultrasonically degreased and chemically polished. The specimens were anodized in a concentrated pyrophosphoric acid solution at a constant cell voltage of 25-75 V for 60 min. The anodized specimens were immersed in a 0.5 mM fluorinated phosphoric acid SAM (3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid: FOPA)/ethanol solution at 293-323 K for 48 h. anodized specimens were examined by field emission scanning electron microscopy (FE-SEM). The water contact angles were measured by an optical contact angle meter (2 µL in water volume for hydrophilic measurements and 4 µL for hydrophobic measurements).
Anodizing of aluminum and its alloys in a pyrophosphoric acid solution caused the formation of numerous anodic alumina nanofibers on the aluminum surface. During anodizing of aluminum alloys, many intermetallic compounds were exposed to the surface with anodizing time increase, and the nanofiber-tangled intermetallic particles were formed on the surface. The contact angle decreased with the anodizing time, and the nanofiber-covered aluminum alloys exhibited a superhydrophilic behavior measuring less than 3° in the contact angle (Figure 1, anodizing voltage: Ua = 75 V). As the surface of the nanofiber-covered aluminum alloys was modified with fluorinated phosphoric acid SAMs (solution temperature: Tim = 293-313 K), the contact angle drastically increased with the immersion time and temperature. These aluminum alloys exhibited a superhydrophobic behavior measuring more than 150° (Figure 2).
Figure 1. Changes in the water contact angle, θ, with the anodizing time, ta, on the 5N aluminum and 3004 aluminum alloy surfaces after anodizing in a pyrophosphoric acid solution (293 K) at 75 V.
Figure 2. Changes in the water contact angle, θ, with the immersion time, tim, on the nanofiber-covered aluminum alloys (3004). The specimens were anodized at 75 V for 10 min, and then were immersed in a 0.5 mM FOPA-SAM/ethanol solution.