Reduced reflectivity and golden color of porous anodic aluminum oxide nanostructures filled with maghemite nanoparticles

The reflectivity and consequent color of anodic aluminum oxide (AAO) containing maghemite (γ-Fe2O3) nanoparticles (NPs) were investigated. The hexagonally ordered pores of AAO were completely filled with maghemite NPs with ∼20 nm in diameter by the dip-coating process. The AAOs containing maghemite NPs have a reduced reflectivity of light at the wavelength below ∼600 nm, resulting in golden color. The reduced reflectivity and golden color of AAOs with NPs was explained by the absorption of light from blue to violet wavelength range by maghemite NPs. These results indicate the potential color-engineering of AAO by means of wavelength-selective absorption by incorporated NPs.