Thermal Hardening and Defects in Anodic Aluminum Oxide Obtained in Oxalic Acid: Implications for the Template Synthesis of Low-Dimensional Nanostructures

This study addresses the investigation of the nature of defects generated during the anodization of aluminum using oxalic acid electrolyte and their influence on the structure and properties of anodic aluminum oxide films (AAO). AAO films, which are obtained by anodization in oxalic acid, and their powdered samples are subjected to thermal annealing at temperatures up to 1050 °C and are subsequently investigated by 27Al solid-state nuclear magnetic resonance (NMR), electron spin resonance (ESR), powder X-ray diffraction, scanning electron microscopy (SEM), and nanoindentation. By NMR and continuous wave ESR, it is found that the anodization obtained in oxalic acid not only produces amorphous porous alumina, but also gives rise to bulk H defects and unpaired electrons originating from the decomposition of the oxalate ions. Paramagnetic defects are healed by annealing at temperatures of ∼800 °C in air, possibly by an oxidative conversion to CO2 with the oxygen in air, while the removal of the H defects requires temperatures of at least 1050 °C. The observed hardening of the films and color changes can be explained by changes in structure on an atomic scale and changes in composition: amorphous AAO is converted to a poorly crystalline intermediate phase containing η-Al2O3 at ∼800 °C and subsequently to crystalline α-Al2O3 (corundum) at ∼1050 °C. Moreover, thermal hardening of an AAO passivation layer on top of metallic aluminum with the flame of a H2/O2 burner was demonstrated.