Preparation of red phosphor K2SiF6:Mn4+ with high water-resistant and negative thermal quenching effect induced by citric acid-passivation

Mn4+-doped fluoride red phosphors are an outstanding choice for warm white light LEDs (WLEDs) and wide color gamut liquid crystal displays due to their broad absorption spectrum in the blue region, narrow emission spectrum in the red region, and high fluorescence quantum efficiency at room temperature. However, these phosphors face limited heat and water resistance challenges, which may impede the practical application of WLEDs. To overcome these issues, we synthesized K2SiF6:Mn4+ red phosphors with enhanced luminous efficiency and heat resistance using a hydrothermal method at H2O2-free condition. Furthermore, we employed citric acid (CA) to create a surface shell layer on the phosphor through room temperature surface passivation, significantly enhancing the phosphor's water resistance without compromising its luminescent properties. After immersing the two phosphors in deionized water for 360 min, the fluorescence emission intensity of KSFM (K2SiF6:0.06Mn4+) and P-KSFM (CA(0.66 wt)-K2SiF6:0.06Mn4+) decreased from 100% to 44.98% and 95.21%, respectively. Warm WLEDs encapsulated with the P-KSFM red phosphor exhibited a low correlated color temperature (CCT) of 3992 K, a high color rendering index of 87.3, and a luminous efficacy of 103.5 lm/W. These results demonstrate the significant potential of P-KSFM as a highly efficient red phosphor with excellent water resistance.