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 (8S7/2) 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.