Facile preparation of Eu³⁺-activated Ca₇(VO₄)₄O nanoparticles: A blue light-triggered red-emitting platform for indoor solid-state lighting

Using the sol-gel technique, Ca₇(VO₄)₄O nanoparticles with various doping concentrations of Eu³⁺ ions were achieved. With an excitation wavelength of 462 nm, the as-synthesized compounds can emit visible red light, which exhibited a high color purity of 82.8%, arising from the 4f-4f transitions of Eu³⁺ ions. The emission intensity of the Eu³⁺ ions in the Ca₇(VO₄)₄O host lattices was sensitive to the dopant content with an optimum value of 10 mol%. The critical distance for the Eu³⁺ ions in the Ca₇(VO₄)₄O host lattices was 32.64 Å, and dipole-dipole interactions resulted in the concentration quenching effect. By employing the Judd-Ofelt theory, the optical transition intensity parameters of the Eu³⁺ ions were calculated and the values of Ω₂ and Ω₄ were determined to be 2.81 × 10^-20 cm^-2 and 0.54 × 10^-20 cm^-2, respectively. The thermal stability of the resultant nanoparticles was verified by the temperature-dependent emission spectra, and the activation energy was determined to be around 0.47 eV. Additionally, the developed light-emitting diode device, which contained a blue chip, commercial Y₃Al₅O₁₂:Ce³⁺ yellow-emitting phosphor and the prepared nanoparticles could emit visible white light with satisfactory colorific behavior; it had a proper color rendering index (76.5) and a low correlated color temperature (5435 K).