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

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 highdensity O– beam in the order of lA 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.