Well-dispersed (Y0.95−xGdxEu0.05)(B(OH)4)CO3 colloidal spheres as a novel precursor for orthoborate red phosphor and the effects of Gd3+ doping on structure and luminescence

Abstract

One-micron-sized uniform red-phosphor spheres of an (Y,Gd)BO₃ ternary system have been converted from their colloidal moydite precursor spheres facilely synthesized using modified homogenous precipitation. The moydite spheres, possessing the composition (Y_0.95−xGd_xEu_0.05)(B(OH)₄)CO₃ (0 ≤ x ≤ 0.50), are orthorhombic in structure. Gd³⁺ is more effective than Y³⁺ in raising nucleation density, leading to a decreased average size of the precursor particles with a higher addition of Gd³⁺. The lattice parameters, a and b, increase linearly with increasing Gd incorporation, while the c value shrinks with a higher Gd content. The moydite spheres completely decomposed into hexagonal orthoborate upon heating to 800 °C, while retaining the spherical shape and excellent dispersion of the original particles. However, they lost their spherical shape and underwent significant aggregation at a higher temperature. The lattice constant of (Y_0.95−xGd_xEu_0.05)BO₃ calcined at 800 °C follows Vegard's law and increases linearly with an increase in the value of x from 0 to 0.30. Upon excitation with 207 nm wavelength light, (Y_0.95−xGd_xEu_0.05)BO₃ spheres show typical Eu³⁺ emission, with the strongest emission at 610 and 627 nm (⁵D₀ → ⁷F₂ transition of Eu³⁺) and almost the same CIE chromaticity coordinates of (∼0.632, ∼0.373). The incorporation of Gd³⁺ resulted in enhanced luminescence intensity and shorter lifetimes (∼5.77–6.56 ms). The optimized doping concentration of Gd³⁺ is 15 at%. The uniform phosphor spheres obtained in this work are expected to have wide applications for the contemporary interest in high-resolution display technologies.