A Reverse Strategy to Restore the Moisture-deteriorated Luminescence Properties and Improve the Humidity Resistance of Mn⁴⁺ -doped Fluoride Phosphors

Fluoride phosphors as red components for warm white LEDs have attracted a tremendous amount of research attention. But these phosphors are extremely sensitive to moisture, which seriously limits their practical industrial applications. To tackle this problem, unlike all the straightforward preventive strategies, a reverse strategy "Good comes from bad" was successfully developed to treat the degraded K₂ SiF₆ : Mn⁴⁺ (D-KSFM) phosphor in the present study, which not only completely restores the luminescence properties, but also significantly enhances the moisture resistance at the same time. After treatment with an oxalic acid solution as restoration modifier, the emission intensity of the D-KSFM phosphor can be restored to 103.7% of the original K₂ SiF₆ : Mn⁴⁺ red phosphor (O-KSFM), and the moisture resistance is remarkably improved. The restored K₂ SiF₆ : Mn⁴⁺ (R-KSFM) maintains approximately 62.3% of its initial relative emission intensity after immersing in deionized water for 300 min, while the reference commercial K₂ SiF₆ : Mn⁴⁺ with a protective coating (C-KSFM) is only 33.2%. As a proof of general applicability, this strategy was also conducted to K₂ TiF₆ : Mn⁴⁺ phosphor, which is less moisture-stable than K₂ SiF₆ : Mn⁴⁺ . The luminescence intensity of the degraded K₂ TiF₆ : Mn⁴⁺ (D-KTFM) phosphor can be restored to 162.6% of original level of the K₂ TiF₆ : Mn⁴⁺ synthesized through a cation exchange approach without any treatment (O-KTFM). The emission intensity of the restored K₂ TiF₆ : Mn⁴⁺ (R-KTFM) phosphor retains 62.8% of its initial emission intensity after soaking in deionized water for 300 min. Finally, the R-KSFM phosphors were packaged into white light-emitting diodes with blue InGaN chips and Y₃ Al₅ O₁₂ : Ce³⁺ yellow phosphors. The WLEDs display excellent color rendition with higher color rendering index, lower color temperature (WLED-II: R_a =83.6, R₉ =57.3, 3743 K, η_l =199.68 lm/W; WLED-III: R_a =90.4, R₉ =94.2, 2892 K, η_l =183.3 1 m/W). The above results show that the reverse strategy can be applied in those phosphor materials with poor moisture resistance to restore luminescence properties and improve moisture resistance without excessively care about the deterioration during the production, storage and transportation.