Liquid-ammonia synthesis of microporous Mg3N2 showing intense red-light emission

Abstract

Magnesium nitride (Mg₃N₂) is prepared via lyotropic phases with liquid ammonia (lq-NH₃). To avoid oxide contamination, the synthesis was performed in the absence of any oxygen source (e.g., solvents, starting materials, and surfactants). Specifically, the lyotropic phase was established by lq-NH₃ at −50 °C as the polar phase, heptane as the non-polar dispersant phase, dimethyldioctylammonium iodide (DDAI) as the surfactant, hexylamine as the cosurfactant, and di-n-butylmagnesium as the starting material. Ammonolysis was completed by slow heating to 500–600 °C in a nitrogen atmosphere (N₂). Structure and purity – especially the absence of oxygen – were studied in detail with different electron microscopic and electron spectroscopic techniques. The as-obtained yellow Mg₃N₂ (heated at 500 °C) exhibits high porosity (specific surface area: 176 m² g⁻¹; total pore volume: 0.41 cm³ g⁻¹; and micropore volume: 0.13 cm³ g⁻¹) and good selectivity regarding CO₂ adsorption (CO₂ uptake: 108 mg g⁻¹; N₂ uptake: 9 mg g⁻¹; both at 90 bars). After heating to 600 °C, the as-obtained yellow Mg₃N₂ (band gap: 2.89 eV) shows intense red emission (600–900 nm) with a quantum yield of 29 ± 3%. Liquid-phase synthesis, oxygen-free Mg₃N₂ and intense red emission are reported for the first time, including nanostructured as well as bulk Mg₃N₂.