Surface Passivation toward Highly Stable Mn⁴⁺‐Activated Red‐Emitting Fluoride Phosphors and Enhanced Photostability for White LEDs

Abstract

Mn4+‐activated red‐emitting fluoride phosphors are indispensable candidates for white light emitting diodes (WLEDs) with enhanced color rendition, however, their intrinsic hydrolysis characteristics seriously restricted the durability applications. Here, a facile and general postsynthetic hydrogen peroxide (H2O2) surface passivation strategy is proposed to treat the K2XF6:Mn4+ (KXF, X = Ti, Si, Ge) phosphors, in which a Mn4+‐rare surface protective layer appears and further covers the corresponding particles. The environment moisture can be effectively isolated by the Mn4+‐rare surface passivation layer with low solubility, which will be sacrificed to protect the phosphor particles even if under extreme hydrolysis conditions. The relative external quantum efficiency of the optimized phosphors still maintains over 96% after the passivation treatment, and the relative luminous intensity still remain 97% even when soaked in water after 12 h. The correlated color temperature of high‐power WLEDs fabricated by the passivated phosphors has no remarkable change during aging process (100 days) in the high temperature (85 °C) and high humidity atmosphere (85%). It is expected that such a surface‐redox strategy can be expanded to other doped materials systems, and also opening a new perspective for the development of luminescence materials with enhanced surface stability and device duration.

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

“…The integrated emission intensities of O‐KSFM, D‐KSFM and R‐KSFM samples are demonstrated in Figure b. The reason why the emission intensity of D‐KSFM is lower than O‐KSFM is the brown coating heavily absorbs light and the effective Mn 4+ concentration is reduced because of the generation of Mn oxides and hydroxides . The luminescence of D‐KSFM sample is restored because the dark‐brown coating was removed by oxalic acid solution .…”

“…As we know, K 2 SiF 6 : Mn 4 + and K 2 TiF 6 : Mn 4 + samples can reach hydrolytic equilibrium after immersing in water for 15 minutes, and the corresponding pH value is between 3-4. [30] Therefore, the absorption bands from 1200 to 1850 cm À 1 can be ascribed to the vibration of C=O, C-OH, C=OOH and symmetric and asymmetric stretching vibration of C=OO from residual oxalic acid. [40] The broad peak located at about 3437 cm À 1 belongs to the symmetric stretching vibrations of OÀ H group in -COOH group from residual oxalic acid and inevitably few H 2 O molecule.…”

“…The reason why the emission intensity of D-KSFM is lower than O-KSFM is the brown coating heavily absorbs light and the effective Mn 4 + concentration is reduced because of the generation of Mn oxides and hydroxides. [30] The luminescence of D-KSFM sample is restored because the dark-brown coating was removed by oxalic acid solution. [37] The emission intensity of R-KSFM (103.68%) is somewhat higher than that of O-KSFM due to the reduction of surface defects and tiny particles of the phosphor, which is consistent with the results of SEM analysis (Figure 4).…”

“…Peng et al [29] found that a homogeneous K 2 SiF 6 shell on K 2 SiF 6 : Mn 4 + surface can be constructed by ethanol-induced deposition. Xia et al [30] also adopted H 2 O 2 surface passivation strategy to treat the K 2 XF 6 : Mn 4 + (X = Ti, Si, Ge) based on previous studies. For inorganic-organic hybrid, Liu et al [31] constructed a double-shell coating composed of OA and SiO 2 to protect the K 2 TiF 6 : Mn 4 + phosphor.…”

“…found that a homogeneous K 2 SiF 6 shell on K 2 SiF 6 : Mn 4+ surface can be constructed by ethanol‐induced deposition. Xia et al . also adopted H 2 O 2 surface passivation strategy to treat the K 2 XF 6 : Mn 4+ (X=Ti, Si, Ge) based on previous studies.…”