Facile synthesis of a cationic-doped [Ca₂₄Al₂₈O₆₄]⁴⁺(4e⁻) composite<i>via</i>a rapid citrate sol–gel method

Abstract

One of the greatest challenges in the enhancement of the electrical properties of conductive mayenite [Ca₂₄Al₂₈O₆₄]⁴⁺(4e^-) (hereinafter C12A7:e^-) is the design of a more suitable/simple synthesis strategy that can be employed to obtain the required properties such as excellent stable electrical conductivity, a high electron concentration, outstanding mobility, and an exceptionally large surface area. Therefore, to synthesize C12A7:e^- in the metallic state, we proposed a facile, direct synthesis strategy based on an optimized sol-gel combustion method under a nitrogen gas environment using the low-cost precursors Ca(NO₃)₂·4H₂O and Al(NO₃)₃·9H₂O. Using this developed strategy, we successfully synthesized moderately conductive nanoscale C12A7:e^- powder, but with unexpected carbon components (reduced graphene oxide (rGO) and/or graphene oxide (GO)). The synthesized C12A7:e^- composite at room temperature has an electrical conductivity of about 21 S cm^-1, a high electron concentration of approximately 1.5 × 10²¹ cm^-3, and a maximum specific surface area of 265 m² g^-1. Probably, the synthesized rGO was coated on nanocage C12A7:e^- particles. In general, the C12A7:e^- electride is sensitive to the environment (especially to oxygen and moisture) and protected by an rGO coating on C12A7:e^- particles, which also enhances the mobility and keeps the conductivity of C12A7:e^- electride stable over a long period. Doped mayenite electride exhibits a conductivity that is strongly dependent on the substitution level. The conductivity of gallium-doped mayenite electride increases with the doping level and has a maximum value of 270 S cm^-1, which for the first time has been reported for the stable C12A7:e^- electride. In the case of Si-substituted calcium aluminate, the conductivity has a maximum value of 222 S cm^-1 at room temperature.