Highly dispersed spherical phosphors obtained by encapsulating silica hollow spheres with SnO2: Eu3+ layers for enhancing red light emission

Orange light (593 nm, ⁵ D ₀→ ⁷ F ₁) and red light (613 nm, ⁵ D ₀→ ⁷ F ₂) are the two most important emission lines belonging to Eu ³⁺ ions in high and low symmetries, respectively, making them important indicators for judging the nanoenvironment around Eu ³⁺ ions. Therefore, obtaining significant red light emission in Eu ³⁺ doped SnO ₂ nanocrystals requires anchoring Eu ³⁺ ions onto lattice sites with low symmetries, which can be achieved through maximizing surface doping. To achieve this goal, we coated very thin Eu ³⁺ doped SnO ₂ nanocrystal (10–20 nm) coatings on extremely dispersed SiO ₂ hollow spheres (500 nm in diameter) to investigate doping dependent luminescent properties. A significant increase in red emission was observed in such a system, accompanied by a significant attenuation of orange light, and the intensity of red emission increases with increasing Eu ³⁺ concentration. When the concentration of Eu ³⁺ exceeds 3 at%, the intensity of red light emission sharply decreases due to the quenching effect. For the optimal doped hollow sphere sample (3 at%), the asymmetry ratio (red/orange light integration intensity ratio) under indirect excitation is 5.58, which further rises to 11.3 under direct excitation, while the asymmetry ratio of SnO ₂ nanopowder with the same doping concentration is less than 0.2 under both excitation modes, indicating that the current structural design plays a significant role in the modulation of Eu ³⁺ ion luminescence. Our research not only demonstrates an effective method to enhance the red light emission of Eu ³⁺ doped SnO ₂ nanocrystals, but also provides a method for manipulating the polychromatic luminescence of such systems.