Stability improvement of moisture sensitive CaS:Eu2+ micro-particles by coating with sol–gel alumina

Here we report on the usage of alumina coating for the protection of moisture sensitive CaS:Eu ²⁺ micro-particles. Alumina sol was prepared using a water-free sol–gel technique and single crystal CaS:Eu ²⁺ luminescent micro-particles were synthesized via solvothermal way. After deposition of the particles on a substrate, they were coated with an alumina layer and heat treated at 500°C for 30min. In addition to in situ measurements of accelerated ageing of the luminescent particles, photoluminescence (PL) spectra of coated and uncoated particles were compared. Coated particles showed a broad band PL emission with a maximum at 650nm identical to that of uncoated particles. Coating with an alumina layer drastically increased the resistivity of the luminescent material against moisture.