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