Optical properties of multicolor, hierarchical nanocomposite films based on anodized aluminum oxide

With three distinct layers, multicolor, hierarchical nanocomposite films based on anodized aluminum oxide (AAO) were fabricated on the 6063 aluminum alloy surface by anodizing and alternating current electrodeposition. The impact of the electrolysis time on the nanostructures and optical properties of the as-grown film was investigated. It was found that with the extension of electrolysis time, the sample color could be varied from gray, yellow, red, blue, to green, realizing almost full-spectrum colors of the AAO-based films in the visible light range. The nanostructure characterizations and the UV–Vis reflectance spectroscopy of the AAO-based films showed that the thickness of the bottom one (i.e., layer 3) of the three layers was dictated by the electrolysis time over the range of 55–145 s, corresponding to the thickness from 135 to 264 nm. Additionally, the optical pathway of the AAO-based film with hierarchical composite nanostructures was analyzed. An increase in the layer 3 thickness would cause a shift in the peak wavelength in the corresponding reflectance spectra, conforming to the Bragg-Snell formula. The colors of AAO films were speculated to originate from constructive interference between the light reflected from the layer 1-layer 2 interface and the light reflected from the layer 3-aluminum alloy substrate interface.