Ultra-thin polymer-encapsulation of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor for enhanced hydrolytic resistance

Abstract

Eu²⁺ and Dy³⁺ doped strontium aluminates (SrAl₂O₄:Eu²⁺, Dy³⁺) are well known green light emitting materials with a long-lasting afterglow property; however, SrAl₂O₄:Eu²⁺, Dy³⁺ is readily hydrolized under ambient moisture conditions, which impedes industrial applications. In this work, SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor was encapsulated by layer-by-layer (LbL) and coacervation methods for the formation of ultra-thin polymeric capsulation layer. Morphology of the phosphor and polymer-encapsulated phosphors was characterized by scanning electron microscopy (SEM). The amount and thickness of the encapsulated polymer layers were analyzed by thermogravimetric analysis (TGA) and atomic force microscopy (AFM), and the polymer content for the phosphor encapsulated by LbL was estimated to be 0.08 wt% with a shell thickness of 11 nm. After pristine phosphor and encapsulated phosphor samples were immersed in water for 24 h, the photoluminescence (PL) intensity was measured to compare the water-resistant performance. The polymer-encapsulated phosphors by LbL and coacervation methods were maintained for 85.7 and 93.5% of pristine phosphors, respectively, while PL intensity of the pristine phosphor was observed as 18.1% after immersion in water for 24 h. The change in pH was monitored under the same conditions. The pH for the dispersion of polymer-encapsulated phosphors by LbL and coacervation methods was measured as 10.27 and 9.63, respectively, while pH for the dispersion of the pristine phosphor was observed as 11.99. This research may provide a wider option for the encapsulation methods of particulate materials for applications where encapsulation with very thin layer of polymer is needed, because the encapsulation process inevitably lowers both the content of active ingredients and consequently their performance as well.