A facile surface passivation strategy for Na2SiF6:Mn4+,Li+ phosphors to achieve high moisture resistance and luminescent thermal stability

Abstract

Recently, Mn4+-doped fluoride red phosphors become widely used as red emitting components of WLEDs (white light-emitting diodes), but such phosphors are easy to hydrolyze in humidity environment and it limits their application. Herein, LNSF:Mn4+-CA (LNSF: Mn4+ = Na2SiF6: 0.066Mn4+,0.06 Li+, CA = passivation with citric acid solution) with high water resistance, luminescent thermal stability and luminescent intensity was synthesized though the H2O2-free hydrothermal method and surface passivation process. The water resistance, luminescent thermal stability and luminescent intensity are obviously enhanced by surface passivation. The high thermal stability is caused by strong negative thermal quenching (NTQ). The mechanism of the strong NTQ is attributed to the increase of electron traps after surface passivation. Furthermore, mechanism for improvement of water resistance is suggested as that a homogeneous Mn4+-rare surface protective layer is in situ formed on surface of the sample after surface passivation. The layer isolates external moisture and then makes the sample resistive to interaction with water. Improvement of water resistance is demonstrated as that after immersing in water for 6 h (0.25 day), luminescent intensity of the un-passivation sample drops to 42.73% of the initial one, while that of the passivation sample remains to have 92.33% of its initial one. The prototype WLEDs with good luminescent performances (CCT = 3916 K and Ra = 89.6) was assembled by using the optimal coated sample. The results suggest that the optimal passivated sample is potential red emitting phosphor for the application in blue-based warm WLEDs.