Effect of Tb³⁺ concentration on the structure and optical properties of triply doped ZnAl₂O₄:1% Ce³⁺,1% Eu³⁺,x% Tb³⁺ nano-phosphors synthesized via citrate sol-gel method

Ce³⁺, Eu³⁺ and Tb³⁺ singly and triply doped ZnAl₂O₄ nano-powders were successfully synthesized via the citrate sol-gel method. X-ray diffraction (XRD) analysis showed that the powder samples consisted of the cubic crystalline phase. Energy dispersive X-ray spectroscopy (EDS) results confirmed the presence of all expected elements and the EDS map showed that the foreign ions were distributed homogeneously on the surface. Scanning electron microscopy (SEM) results revealed that the prepared powder morphology was influenced by doping. The transmission electron microscopy (TEM) results confirmed that the prepared powders were in the nano-scale region. The ultraviolet–visible (UV–vis) spectra showed that different dopants and varying the Tb³⁺ concentration influenced the effective band gap (E_g) of the host. The photoluminescence (PL) results showed that the emission at 395 nm was due to the defects emission within the host material. Ce³⁺ doped samples showed a broad emission peak centered at 425 nm which is attributed to the 5d → 4f transition of Ce³⁺ ions. The sample doped with Eu³⁺ showed two emission peaks at 592 and 616 nm ascribed to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺ ions, respectively. Tb³⁺ singly-doped sample showed four peaks located at 489, 545, 587 and 616 nm, which were respectively ascribed to ⁵D₄ → ⁷F₆, ⁷F₅, ⁷F₄ and ⁷F₃ transitions of Tb³⁺ ions. The Commission Internationale de l'Elcairage (CIE), or International Commission on Illumination, co-ordinates showed that the emission colour could be tuned by varying the Tb³⁺ concentration.