NMR Spectroscopic Analysis of the Local Structure of Porous-Type Amorphous Alumina Prepared by Anodization

Abstract

Alumina is classified as an intermediate oxide that cannot form a glass and is industrially used as a multifunctional material. Intensive studies on the atomistic structure of amorphous alumina have been conducted because a fundamental understanding of its structure can be meaningful to the design of new materials and devices. Here we focused on anodic alumina as a model material for clarifying the atomistic structure of amorphous alumina and prepared anion-free and anion-incorporated poroustype amorphous alumina by anodization using chromic acid electrolyte and typical electrolytes (e.g., sulfuric acid, oxalic acid, and phosphoric acid), respectively. The local structure around aluminum atoms in the anodic alumina was investigated by nuclear magnetic resonance spectroscopy. We found that the structure of anodic amorphous alumina comprises AlO 4 , AlO 5 , and AlO 6 units with predominant fractions of AlO 5 . We also observed that the fraction of each unit was 37.7, 54.3, and 8.0%, with an average coordination number (N Al−O ) of 4.70 in the anion-free sample. The results of 1 H− 27 Al cross-polarization with magic angle spinning measurements showed that the prevalence of AlO 6 units decreased with the elimination of physisorbed water by heat treatment. We also suggest that the average N Al−O is affected by the depth, content, and/or species of incorporated anions and that the average N Al−O increases with increasing anion content in the case of samples prepared using sulfuric acid electrolyte. The results are quite meaningful because they provide precise information about the local structure of anodic amorphous alumina; previous reports on this subject have been inconsistent and controversial. We expect that the present results will accelerate structural analysis research on the atomistic structures of amorphous alumina.

Coordination Geometry‐Dependent Multi‐Band Emission and Atypically Deep‐Trap‐Dominated NIR Persistent Luminescence from Chromium‐Doped Aluminates

“…As show in Figure b, the dominant peaks centering at 8 and 17 ppm correspond to six‐coordinated Al sites ( VI Al 3+ ) . The peaks at 65 ppm are related to a tetrahedral Al coordination ( IV Al 3+ ), whereas the other peaks at ‐75, and 109 ppm can be identified as spinning sidebands . The various configurations of Al ions demonstrated in the NMR results offer a powerful structural anticipation to activate the multi‐band emission.…”

The Sealing Step in Aluminum Anodizing: A Focus on Sustainable Strategies for Enhancing Both Energy Efficiency and Corrosion Resistance

“…Hashimoto et al [82] recently investigated the local structure around aluminum atoms in anodic alumina using nuclear magnetic resonance spectroscopy, critiqued earlier studies on the subject matter, and reported that aluminum cations coordinated with oxygen anions in three types of co-ordination; AlO 4 (tetra-coordination), AlO 5 (penta-coordination), and AlO 6 (octahedral-coordination), and that penta-coordination was predominant while the proportion of octahedrally coordinated (AlO 6 ) units decreased with removal of physisorbed water. They [82] also evaluated the range of ratios of each mode of coordination to be 30%-40% for AlO 4 , 50%-60% for AlO 5 , and 4%-15% for AlO 6 . The roles that surface coordination plays in precipitation and dissolution of mineral phases had been discussed elsewhere [83][84][85][86].…”

“…However, while some workers [77,78] have reported that Al cation in anodized layer are coordinated to oxygen anions in two ways (tetrahedral (AlO4) and octahedral (AlO6) coordination), other researchers [79][80][81][82] have reported 3 types of co-ordination of Al cations with oxygen anions (tetrahedral (AlO4), pentahedral (AlO4), and octahedral (AlO6) coordination). Hashimoto et al [82] recently investigated the local structure around aluminum atoms in anodic alumina using nuclear magnetic resonance spectroscopy, critiqued earlier studies on the subject matter, and reported that aluminum cations coordinated with oxygen anions in three types of coordination; AlO4 (tetra-coordination), AlO5 (penta-coordination), and AlO6 (octahedralcoordination), and that penta-coordination was predominant while the proportion of octahedrally coordinated (AlO6) units decreased with removal of physisorbed water. They [82] also evaluated the range of ratios of each mode of coordination to be 30%-40% for AlO4, 50%-60% for AlO5, and 4%- The chemical and phase composition of the anodized layer on aluminum has been a subject of much interest [64][65][66][67][68][69][70][71][72].…”

“…Hashimoto et al [82] recently investigated the local structure around aluminum atoms in anodic alumina using nuclear magnetic resonance spectroscopy, critiqued earlier studies on the subject matter, and reported that aluminum cations coordinated with oxygen anions in three types of coordination; AlO4 (tetra-coordination), AlO5 (penta-coordination), and AlO6 (octahedralcoordination), and that penta-coordination was predominant while the proportion of octahedrally coordinated (AlO6) units decreased with removal of physisorbed water. They [82] also evaluated the range of ratios of each mode of coordination to be 30%-40% for AlO4, 50%-60% for AlO5, and 4%- The chemical and phase composition of the anodized layer on aluminum has been a subject of much interest [64][65][66][67][68][69][70][71][72]. From analysis of these numerous reports on the subject, there appears to be a consensus that the porous anodized layer on aluminum prior to sealing is predominantly comprised of amorphous alumina [64,69,73,74], some boehmite (AlOOH) [66,75,76], and aluminum oxide compounds whose actual compositions are determined by the bath electrolyte anion chemistry.…”

“…The structure of amorphous alumina which constitute much of the anodic layer has been studied extensively using a variety of techniques suitable for probing structural details of non-crystalline and poorly crystalline materials. However, while some workers [77,78] have reported that Al cation in anodized layer are coordinated to oxygen anions in two ways (tetrahedral (AlO 4 ) and octahedral (AlO 6 ) coordination), other researchers [79][80][81][82] have reported 3 types of co-ordination of Al cations with oxygen anions (tetrahedral (AlO 4 ), pentahedral (AlO 4 ), and octahedral (AlO 6 ) coordination). Hashimoto et al [82] recently investigated the local structure around aluminum atoms in anodic alumina using nuclear magnetic resonance spectroscopy, critiqued earlier studies on the subject matter, and reported that aluminum cations coordinated with oxygen anions in three types of co-ordination; AlO 4 (tetra-coordination), AlO 5 (penta-coordination), and AlO 6 (octahedral-coordination), and that penta-coordination was predominant while the proportion of octahedrally coordinated (AlO 6 ) units decreased with removal of physisorbed water.…”