Dual-Band Emissive CaF₂: Eu²⁺, Eu³⁺ Nanoparticles and Their Composite Films: Synthesis and Properties

To achieve an efficient light conversion agent with blue-violet and red-orange light emission, CaF₂: Eu²⁺, Eu³⁺ nanoparticles were prepared in the presence of citric acid by a hydrothermal method at a relatively low temperature. CaF₂: Eu²⁺, Eu³⁺ nanoparticles were incorporated into polylactic acid (PLA) to prepare transparent composite films. Citric acid was not only bonded to the surface of the nanoparticles but also showed the reduction characteristic under closed high-pressure reaction condition, so that Eu³⁺ was partially reduced to Eu²⁺. The investigation of luminescence properties showed that CaF₂: Eu²⁺, Eu³⁺ nanoparticles can emit strong blue-violet and red-orange fluorescence with a maximum peak at 460 and 615 nm, which can be ascribed to the 4f⁶5d¹ → 4f⁷ (⁸S_7/2) transition of Eu²⁺ ions and ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. Under excitation at wavelengths of 370 and 267 nm, the composite films mainly emit blue-violet and red-orange fluorescence, respectively. As the nanoparticle concentration in composite films increased, the blue-violet fluorescence intensity gradually increased, whereas the red-orange fluorescence showed an initial increase followed by a subsequent decrease. These findings confirm the coexistence of Eu²⁺ and Eu³⁺ in the CaF₂ matrix, even under an air condition.