Shiny laminated alumina films prepared by ac oxidation

Abstract

Laminated alumina films, resembling shell nacre structures, were fabricated via the oxidation of aluminum in sulfuric acid using alternating current (ac). Columnar structures were observed on the surface of the laminated structures when the raising speeds of the ac voltage were smaller than 0.5 V/s; these structures decreased and disappeared as the raising speeds increased. The reflectance spectra of the films showed a peak when the raising speed was higher than 0.5 V/s, resulting in the observed structural color without the presence of dopants such as dyes. The reflectance peak of the film exhibited the highest intensity when the film was prepared at a raising speed of 1 V/s for 1500 s. The peaks in the reflectance spectra of the films tend to shift toward longer wavelengths with the increase in the applied voltage, and the change in color depends on the peak positions. The thickness of each layer increased with an increase in the ac voltage. These results suggest that the structural color affects the thickness of each layer. In particular, the films prepared at an applied voltage of 60 V showed the highest intensities in the reflectance spectra, with a pearlescent appearance.