Surface modification of K₂TiF₆:Mn⁴⁺ phosphor with SrF₂ coating to enhance water resistance

Abstract

K 2 TiF 6 :Mn 4+ is an attractive narrow-band red-emitting phosphor for warm white light-emitting diodes (LEDs). Nevertheless, the hexafluoride phosphor is liable to deliquesce in moist environments, which leads to a sharp deterioration performance of luminescence. Surface modification of K 2 TiF 6 :Mn 4+ phosphor with SrF 2 coating has been introduced, with the aid of KHF 2 transition layer to moderate the lattice mismatch. The reaction mechanism is discussed in detail, as so as the influence of SrF 2 coating on the luminescence intensity. The SrF 2 coating is able to prevent the hydrolysis of internal [MnF 6 ] 2− group; thereby, the luminescence intensity retains over 90% of initial value after being immersed in distilled water for 2 h. The LED devices fabricated with commercial Y 3 Al 5 O 12 :Ce 3+ and as-modified K 2 TiF 6 :Mn 4+ phosphors exhibit bright white light with tunable chromaticity coordinate, correlated color temperature, and color rendering index. It enlightens a convenient method to enhance the moisture resistance of Mn 4+ doped fluoride phosphors for commercial application in the field of white LEDs.

Recent Research Progress of Mn4+-Doped A2MF6 (A = Li, Na, K, Cs, or Rb; M = Si, Ti, Ge, or Sn) Red Phosphors Based on a Core–Shell Structure

“…Dong et al [ 41 ] added K 2 TiF 6 : Mn 4+ to a KF and Ca(NO 3 ) 2 solution, and Yu et al [ 63 ] added K 2 SiF 6 : Mn 4+ to a HF and Ca(NO 3 ) 2 solution via mixing and stirring, resulting in the obtaining of KTF@CaF 2 and KSF: Mn 4+ @CaF 2 , respectively. Fang et al [ 43 ] used K 2 TiF 6 : Mn 4+ , KHF 2 , and Sr(NO 3 ) 2 as raw materials to obtain K 2 TiF 6 : Mn 4+ @SrF 2 . KHF 2 played a bridging role in the synthesis of the shell structure, which could alleviate lattice mismatch.…”

“…Dong et al [ 41 ] and Yu et al [ 42 ] modified the surface of K 2 TiF 6 : Mn 4+ and K 2 SiF 6 : Mn 4+ with CaF 2 , which strengthened the humidity resistance and luminescence properties. Fang et al [ 43 ] constructed a water-resistant SrF 2 coating on the surface of K 2 TiF 6 : Mn 4+ . SrF 2 was uniformly covered on K 2 TiF 6 : Mn 4+ to eliminate lattice defects and improve the emission efficiency.…”

Insoluble oxalates modified K2XF6:Mn4+ (X = Ti, Ge, Si) red-emitting phosphors exhibiting excellent moisture resistance and luminescence for warm white light-emitting diodes

“…The thermal stability improvement may be ascribed to three possible aspects: (i) The surface thermal resistances and specific surface areas increase after modification. 18 (ii) The oxalate coatings absorb heat to lose the crystal water or decompose, and then the actual temperature of inner phosphors is lower than the apparent one, which improves the thermal quenching of luminescence. (iii) The decomposition of oxalate coatings leads to thinner shells and thus less light scattering at the interfaces.…”

“…17 In our previous work, K 2 TiF 6 : Mn 4+ was modified with SrF 2 coatings with the aid of KHF 2 transition layer to moderate the lattice mismatch. 18 These protective layers significantly improve the moisture resistance of fluoride phosphors. Despite all this, it is worth developing a novel waterproof strategy for Mn 4+ -doped fluoride phosphors.…”

Towards improved waterproofness of Mn4+-activated fluoride phosphors

“…After soaking in water for 6 h, the luminescence intensity of K 2 SiF 6 :0.055Mn 4+ decreased to 41.68% of the initial one, while that of K 2 SiF 6 :0.055Mn 4+ @CaF 2 (20 wt%) were retained 88.24% of the initial one. Similarly, Fang et al 16 coated KSF:Mn 4+ with SrF 2 by adding a Sr(NO 3 ) 2 solution to a K 2 TiF 6 :Mn 4+ /KHF 2 solution. The luminescence intensities of KTF:Mn 4+ @ x SrF 2 ( x = 0, 0.2, 0.4, 0.6, and 0.8) phosphors were still retained over 90% of initial values after soaking in water for 2 h.…”