Anodization of FeAl intermetallic alloys for bandgap tunable nanoporous mixed aluminum-iron oxide

Abstract

Using a two-step self-organizing anodization of FeAl intermetallic alloy in sulfuric acid, a mixed nanoporous anodic aluminum-iron oxide composite with a voltage-controlled morphology and bandgap was obtained. The chemical composition of nanoporous oxide composites formed with Al, Fe and O elements was determined by X-ray photoelectron spectroscopy. It was demonstrated that bandgap of the resulting anodic oxide composites can be tuned from 3.65eV (samples prepared at 5V) to 2.06eV (samples prepared at 17.5V), which was attributed to the increase in the composition ratio of the oxyhydroxide MOOH (where M=Al and Fe). Thus, water is more involved in the formation of oxide MOOH. After annealing at 600°C, X-ray diffraction confirmed formation of a spinel phase of FeAl2O4. FE-SEM observations of the formed oxide demonstrated that ultra-small nanopores with a diameter of 12.8±3.0nm were formed at 5V. The pore diameter and interpore distance were found to be linearly dependent on the voltage; however, slopes of the fitted curves were much larger than that of nanoporous anodic oxide formed on aluminum. Large current densities recorded during anodization allowed for formation of nanoporous anodic oxide with a growth rate of up to 743.0±17.9μm/h (20V).