Highly Stable K₂SiF₆:Mn⁴⁺@K₂SiF₆ Composite Phosphor with Narrow Red Emission for White LEDs

Poor water resistance and nongreen synthesis remain great challenges for commercial narrow red-emitting phosphor A₂MF₆:Mn⁴⁺ (A = alkali metal ion; M = Si, Ge, Ti) for solid-state lighting and display. We develop here a simple and green growth route to synthesize homogeneous red-emitting composite phosphor K₂SiF₆:Mn⁴⁺@K₂SiF₆ (KSFM@KSF) with excellent water resistance and high efficiency without the usage of toxic and volatile hydrogen fluoride solution. After immersing into water for 6 h, the as-obtained water-resistant products maintain 76% of the original emission intensity, whereas the emission intensity of non-water-resistant ones steeply drops down to 11%. A remarkable result is that after having kept at 85% humidity and at 85 °C for 504 h (21 days), the emission intensity of the as-obtained water-resistant products is at 80-90%, from its initial value, which is 2-3 times higher than 30-40% for the non-water-resistant products. The surface deactivation-enabled growth mechanism for these phosphors was proposed and investigated in detail. We found that nontoxic H₃PO₄/H₂O₂ aqueous solution promotes the releasing and decomposition of the surface [MnF₆]^2- ions and the transformation of the KSFM surface to KSF, which finally contributes to the homogeneous KSFM@KSF composite structure. This composite structure strategy was also successfully used to treat KSFM phosphor prepared by other methods. We believe that the results obtained in the present paper will open the pathway for the large-scale environmentally friendly synthesis of the excellent antimoisture narrow red-emitting A₂MF₆:Mn⁴⁺ phosphor to be used for white light-emitting diode applications.