Effects of anion incorporation on the local structure of porous-type amorphous alumina prepared by anodization: NMR study of Al coordination numbers

• Published in: Results in physics Vol. 44; p. 106177
• Main Authors: Hashimoto, Hideki, Fujita, Yuki, Yazawa, Koji, Asoh, Hidetaka
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2023; Elsevier
• Subjects: Amorphous alumina; Anion incorporation; Anodic porous alumina; Coordination number; Solid-state NMR; Solid-state NMR; Coordination number; Anodic porous alumina; Amorphous alumina; Anion incorporation
• ISSN: 2211-3797; 2211-3797
• DOI: 10.1016/j.rinp.2022.106177

•Factors affecting local structure of porous-type anodic amorphous alumina was clarified.•Local structure of high-purity amorphous alumina does not depend on electrolytic factors.•Average coordination number of anion-incorporated alumina is affected by its composition.•Considering the depth of anion incorporation helps accurate interpretation of experimental data. Although anodization is an essential technology that has supported the aluminum industry for nearly a century, the atomistic structure of the formed alumina films has not completely understood. Here we used NMR spectroscopy to clarify the factors affecting local structure of anodic porous alumina films prepared under various electrolytic conditions. For anion-free amorphous alumina films prepared using chromic acid, the local structure does not depend on electrolytic factors. For films prepared using phosphoric acid, the average coordination number was lower than that of anion-free alumina. On the other hand, films prepared using oxalic and sulfuric acids showed the average coordination numbers higher than that of anion-free alumina. We reveal that aluminum coordination number of anodic porous alumina are affected by anion species and anion content. In addition, the depth of anion incorporation into cell walls, or compositional uniformity, is quite important for accurate interpretation of NMR spectra.