Improved luminescence properties and thermal stability of SrSi₂O₂N₂:Eu²⁺ phosphor with single phase via the formation of Eu³⁺ on surface structure

The green-emitting SrSi₂O₂N₂:Eu²⁺ phosphors with single phase have been successfully synthesized by a simple two-step method. During the first-step process, Sr₂SiO₄:Eu³⁺ precursor was formed and emitted red light under UV light excitation. The excessive amount of Si₃N₄ in reactants promoted the formation of pure SrSi₂O₂N₂ phase when molar ratio of Si₃N₄/Sr₂SiO₄ was located in the range of 1.1–1.5. The luminescence intensities of SrSi₂O₂N₂:Eu²⁺ phosphors were enhanced by Si₃N₄/Sr₂SiO₄ molar ratio of 1.1. Phase analysis, luminescence properties and thermal stability of the samples were intensively investigated by X-ray diffraction, SEM, XPS and TGA/DSC. The trivalent europium ions were identified by XPS results on the surface of as-synthesized SrSi₂O₂N₂:Eu²⁺ particles with approximate theoretical atomic ratio of Eu/Sr, which prevent the further oxidation of Eu²⁺ to Eu³⁺ in the bulk structure and evidently improved the thermal stability of the phosphors. For the thermal oxidation mechanism, it was suggested that the oxidation of SrSi₂O₂N₂ to form SrSiO₃ layer firstly occurred on the surface after a calcination process in air.