Citrate sol-gel synthesis of BaAl₂O₄:x% Cu²⁺ (0 ≤ x ≤ 1) nano-phosphors: Structural, morphological and photoluminescence properties

This paper reports on the effects of varying the Cu²⁺ doping concentration on the structure, morphology and optical properties of the BaAl₂O₄ phosphor. BaAl₂O₄:x% Cu²⁺ (0 ≤ x ≤ 1) series were successfully synthesized via citrate sol-gel method. The X-ray diffraction (XRD) results revealed that the prepared phosphor sample consist of a single phase hexagonal structure. The presence of Ba, Al, O and Cu were confirmed by the energy dispersive X-ray spectroscopy (EDS). Scanning electron microscope (SEM) revealed that the morphology of the prepared samples highly depends on the Cu²⁺ concentration. Transmission electron microscopy (TEM) results revealed the hollow tubular and nano-nature of the crystallite sizes. When the un-doped sample was excited at 283 nm, the photoluminescence (PL) results revealed six emission peaks located at 420, 435, 457, 521, 612 and 722 nm, which were attributed to the intrinsic intra band gap defects within the BaAl₂O₄ (host). The emission at 425 nm observed for the BaAl₂O₄:Cu²⁺ samples was attributed to the 3d⁸4s¹ → 3d⁹ transition in Cu²⁺ ion. The optimum doping Cu²⁺ concentration was found to be 0.075% Cu²⁺. Critical energy transfer distance (R_c) of Cu²⁺ ions was found to be 12.01 Å, which suggested that the multipole-multipole interaction was the main reason for the luminescence quenching. The International Commission on Illumination (CIE) colour showed that the bluish emission colour of the prepared samples depends on the excitation wavelength and Cu²⁺ concentration. Thermo stimulated luminescence (TSL) of the host material showed the presence of both the swallow and deep traps respectively located at 75 and 240 °C.