Two-Step Oxidation Synthesis of Sulfur with a Red Aggregation-Induced Emission

Sulfur is not normally considered a light-emitting material, even though there have been reports of a dim luminescence of this compound in the blue-to-green spectral region. Now, it is shown how to make red-emissive sulfur by a two-step oxidation approach using elemental sulfur and Na₂S as starting materials, with a high photoluminescence quantum yield of 7.2 %. Polysulfide is formed first and is partially transformed into Na₂S₂O₃ in the first step, and then turns back to elemental S in the second step. The elevated temperature and relatively oxygen-deficient environment during the second step transforms Na₂S₂O₃ into Na₂SO₃ incorporated with oxygen vacancies, thus resulting in the formation of a solid-state powder consisting of elemental S embedded in Na₂SO₃. It shows aggregation-induced emission properties, attributed to the influence of oxygen vacancies on the emission dynamics of sulfur by providing additional lower energy states that facilitate the radiative relaxation of excitons.