Effect of annealing period on the structure, morphology, and optical properties of CaAl₂O₄:0.1% Sm³⁺ prepared by citrate sol-gel method

We report on the variation of annealing period (AP) in the range 1–8 h on samarium doped calcium aluminate (CaAl₂O₄:0.1%Sm³⁺) powders prepared by sol-gel method. The effect of AP was investigated on the structure, morphology and optical properties of the powders. The X-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS) findings showed that Sm³⁺ ions were successfully doped into CaAl₂O₄. XRD confirmed that AP does not have any effect on the phase of the prepared nanopowders, hence all the samples have a monoclinic structure. Scanning electron microscope (SEM) showed that as AP increases the particles became more agglomerated and smoother. The energy band gap (E_g) was affected by the increase in AP. Ultraviolet-visible (UV–vis) diffuse reflection spectroscopy demonstrated that E_g can be tuned between 5.56 and 5.71 eV. The photoluminescence (PL) emission spectrum upon excitation of 403 nm displayed ten emission peaks located at around (559, 564 and 576), (601 and 608), (643, 648, 653 and 662) and 708 nm. These emission peaks can be ascribed to the ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, ⁴G_5/2→⁶H_9/2 and ⁴G_5/2→⁶H_11/2 transitions of Sm³⁺ ion, respectively. It further showed that the optimum luminescence intensity occurred for AP = 7 h. The Commission Internationale de l'Eclairage (CIE) analysis revealed that AP has tuned the emission colour from greenish to orange-red. The results indicate that the prepared nanopowders can potentially be used in the orange/red emitting phosphors.