Combustion synthesis of Eu-, Tb- and Tm- doped Ln2O2S (Ln=Y, La, Gd) phosphors

Powder phosphors of (Y1−xREx)2O2S, (La1−xREx)2O2S, and (Gd1−xREx)2O2S where RE=Eu3+, Tb3+, or Tm3+ were prepared by combustion reactions from mixed metal nitrate reactants and dithiooxamide (CSNH2)2 with ignition temperatures of 300–350°C. The X-ray diffraction patterns of as-synthesized powders revealed that the Ln2O2S (Ln=Y, La, and Gd) phase crystallized directly from the combustion reaction with fuel to oxidizer ratios of ∼2.0. Scanning electron microscope images of as-synthesized powders showed a foamy, porous agglomeration and a continuous three-dimensional network. The agglomerates ranged in size between 10 and 30 μm, while the primary particles ranged in size between 100 and 200 nm. The phosphors prepared by combustion reaction were cathodoluminescent and photoluminescent immediately after combustion without additional heating. They exhibited the characteristic emission spectrum of Eu3+, Tb3+, or Tm3+. The cathodoluminescent efficiency of as-synthesized, non-optimized Gd2O2S:Tb3+ (1.0 mol%) phosphors was lower than that of commercial phosphors at all electron beam energies (30–15% at 1–4 kV). Increased water during combustion was shown to favor the formation of oxides or sulfates over oxysulfides. Grinding was shown to reduce and heat treatment was shown to increase the PL intensities.