Active anion manipulation for emergence of active functions in the nanoporous crystal 12CaO·7Al2O3 : a case study of abundant element strategy

• Published in: Journal of materials science Vol. 42; no. 6; pp. 1872 - 1883
• Main Authors: HOSONO, Hideo, HAYASHI, Katsuro, HIRANO, Masahiro
• Format: Conference Proceeding Journal Article
• Language: English
• Published: Heidelberg Springer 01.03.2007; Springer Nature B.V
• Subjects: Anions; Cages; Calcium aluminate; Chemistry; Cold cathodes; Colloidal state and disperse state; Conductors; Continuous radiation; Conversion; Crystal lattices; Electric fields; Electrical resistivity; Electron beams; Electron irradiation; Electron paramagnetic resonance; Exact sciences and technology; First principles; General and physical chemistry; Heat treatment; High temperature; Materials science; Organic chemistry; Oxidation; Oxygen; Porous materials; Production methods; Rare gases; Synthesis gas; Thin films; Work functions; Oxygen; Calcium oxide; Inorganic free radical; Crystals; Nanostructure; Review; Alumina; Porous material; Nanoporosity
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-006-1316-9

This article reviews our approach to render 12CaO · 7Al2O3 (C12A7) electronically active using a new concept of ‘active anion manipulation’, where nanostructures embedded within the C12A7 crystal lattice are intentionally utilized to generate chemically unstable (‘water-free active’) anions. Anionic active oxygen radicals, O− and O2−, are formed efficiently in C12A7 cages under high oxygen activity conditions. The configuration and dynamics of O2− in cages are revealed by a combination of continuous-wave and pulsed electron paramagnetic resonance (EPR). It is demonstrated that metal-loaded C12A7 is a promising oxidation catalyst for syngas (CO + H2) formation from methane. Furthermore, the O− ion, the strongest oxidant among active oxygen species, can be extracted from the cage into an external vacuum by applying an electric field with thermal assistance, generating a high-density O− beam in the order of μA cm−2. In contrast, heat treatment of C12A7 in a hydrogen atmosphere forms H− ions in the cages. The resultant C12A7:H− exhibits a persistent insulator-conductor conversion upon ultraviolet-light or electron-beam irradiation. The irradiation-induced conversion mechanism is examined by first-principle theoretical calculations. Furthermore, the presence of a severely reducing environment causes the complete substitution of electrons for anions in the cages. The resulting C12A7:e−, which exhibits excellent stability and an electrical conductivity greater than 100 S cm−1, is regarded as an ‘electride’, an ionic compound in which electrons serve as anions. The C12A7 electride exhibits a high potential for applications involving cold cathode and thermal field electron emissions due to its small work function. Electride fabrication methods suitable for large-scale production via melt processing are described. It is also demonstrated that proton or inert gas ion implantations into C12A7 thin films at elevated temperatures are effective for both H− and electron doping.