Ion exchange-promoted surface reduction strategy: For improving the water-resistance of K2SiF6:Mn4+ red phosphors

Mn4+-doped fluoride red emission phosphors with excellent luminescence properties have become an important part of white light-emitting diodes (WLEDs), but the poor moisture resistance of the phosphors hinders their practical application. Aiming to solve this problem, this paper proposes an ion exchange-promoted surface reduction (IESR) strategy in which K2SiF6:Mn4+ (KSF: Mn4+) phosphor is treated with NaNO2 in a mixed solution of NH4F/HCl, and a thick and clean Mn4+-free shell layer is constructed on its surface to improve water resistance. The thickness of the Mn4+-free shell layer of the optimal sample is determined to be about 600 nm by focused ion beam (FIB) and scanning electron microscope (SEM) characterization techniques. The internal quantum yield (QYi) of the treated phosphor (IESR-KSF:0.05Mn4+) is 88.86%, and it maintains a high QYi (75.69%) under the extreme hydrolysis environment of high temperature and high humidity, which is better than the results treated by the reverse cation-exchange strategy (22.81%) and the surface passivation strategy (7.9%). Moreover, IESR-KSF:0.05Mn4+ has a strong negative thermal quenching (NTQ) effect, which could be attributed to the electron-phonon interaction mechanism. The LED device encapsulated with the phosphor is aged at 85 °C and 85% relative humidity, and the results demonstrate that the IESR strategy can effectively improve the luminescence stability of fluoride phosphors.