Lei Chen,Zhuming Cheng,Guifang Zheng,Gang Yao,Liangrui He,Lei Wang,Jianzhe Liu,Haiwu Zheng,Shizhong Wei,Haiyong Ni
The green-emitting phosphor SrSi 2 O 2 N 2 : Eu 2+ , along with the cyan-emitting phosphor BaSi 2 O 2 N 2 : Eu 2+ , is a key component for phosphors used to package natural sunlight full-spectrum LED devices. Thereby, the synthesis of high-efficiency and thermal-stability luminous SrSi 2 O 2 N 2 : Eu 2+ with an economic approach is very important for the development of LED industry. In this work, the syntheses of SrSi 2 O 2 N 2 : Eu 2+ phosphors were optimized from four aspects: strontium sources, reaction atmospheres, reaction vessels, and reaction fluxes. Meanwhile, the relationship among photoluminescence performances, crystal structures, and morphologies were examined. The results show that the synthesis of SrSi 2 O 2 N 2 : Eu 2+ from SrO and Sr(NO 3 ) 2 sources are beneficial to obtain N-rich oxynitide structure, which is helpful to improve luminescence. By employing SrCO 3 and Sr(NO 3 ) 2 as strontium sources, however, the SrSi 2 O 2 N 2 : Eu 2+ is O-rich and the by-product Sr 2 Si 5 N 8 :Eu is introduced. The optimal conditions for the synthesis of SrSi 2 O 2 N 2 : Eu 2+ is by utilizing SrO as a strontium source, in N 2 atmosphere, adopting Mo crucible, and adding NH 4 Cl as flux. The quantum yield of the optimal SrSi 2 O 2 N 2 : Eu 2+ phosphor under the excitation of 380 nm reaches up to 100%.
Nowadays, considerable efforts have been extensively devoted to explore a general strategy for improving the color uniformity and thermal stability of phosphors, which is vital for its applications in health and comfort lighting. In this study, the SrSi2O2N2:Eu2+/g-C3N4 composites were successfully prepared via a facile and effective solid-state method to improve their photoluminescence properties and thermal stability. The coupling microstructure and chemical composition of the composites were demonstrated by high-resolution transmission electron microscopy (HRTEM) and EDS line-scanning analyses. Notably, the dual emissions at ∼460 nm (blue) and ∼520 nm (green) were observed for the SrSi2O2N2:Eu2+/g-C3N4 composite under near-ultraviolet (NUV) excitation, attributed to the g-C3N4 and 5d–4f transition of Eu2+ ions, respectively. The coupling structure will be beneficial to the color uniformity of the blue/green emitting light. Further, SrSi2O2N2:Eu2+/g-C3N4 composites exhibited a similar photoluminescence intensity compared with the SrSi2O2N2:Eu2+ phosphor even after thermal treatment at 500 °C for 2 h due to the protection of g-C3N4. The decreased decay time (1798.3 ns) of green emission for SSON/CN compared with SSON phosphor (1835.5 ns) indicated that the coupling structure suppressed the non-radiative transition and improved photoluminescence properties and thermal stability. This work provides a facile strategy to construct SrSi2O2N2:Eu2+/g-C3N4 composites with coupling structure for improved color uniformity and thermal stability.
A phase pure SrSi2O2N2:Eu2+ green phosphor was synthesized by a solid state reaction through careful control of the Sr : Si ratio in the starting powder consisting of SrCO3, Si3N4 and Eu2O3. The thermal degradation of the phosphor was investigated by baking it at high temperatures for 2 h. The surface states of the samples before and after baking were analyzed by SEM, HRTEM, XPS, TGA/DTA, and high temperature in situ X-ray diffraction. The results showed that the thermal degradation became intense when the temperature was higher than 500 °C, and the degradation was caused by the formation of SrSiO3 on the particle surface and the oxidation of Eu2+ to Eu3+. It is suggested that the thermal stability can be enhanced by achieving high crystallinity as well as high phase purity of the phosphor.
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