Effects of Mg2+ concentration on the structure and optical properties of MgxAl2O3+x:0.88% Cd2+ (0.25 ≤ x ≤ 4.5) nano-powders synthesized via citrate sol-gel

Abstract

Abstract Un-doped (Mg 1.5 Al 2 O 4.5 ) and 0.88% Cd 2+ doped Mg x Al 2 O 3+x nano-powders were synthesized via citrate sol-gel method. The Mg x concentration was varied in the range of 0.25 ≤ x ≤ 4.5. The X-ray diffraction (XRD) data revealed that Mg 0.25 Al 2 O 3.25 :0.88% Cd 2+ consists of poorly crystalline and amorphous phases. For the Mg x Al 2 O 3+x :0.88% Cd 2+ (0.5 ≤ x ≤ 4.5) series, the XRD revealed that the nano-powders consist of face centered cubic MgAl 2 O 4 spinel crystal structure for x ≤ 2 and face centered cubic MgO crystal structure for x ≥ 2.5. The estimated crystal size ranged from 4 to 14 nm, which the estimated crystal size of the un-doped and the doped was confirmed by the transmission electron microscopy (TEM). The photoluminescence (PL) results showed that the un-doped and the 0.88% Cd 2+ doped nano-powders generally emits a violet emission at 407 nm, which is attributed to the bandgap defects within the host material. The emission peaks at 495 and 656 nm are attributed to the F center and the abundance of oxygen vacancies within lattice due to a higher calcination temperature of 800 °C. The PL and XRD results showed the phase mutation from MgAl 2 O 4 to MgO. The maximum PL intensity was observed from the Mg 1.5 Al 2 O 4.5 :0.88% Cd 2+ sample. Increasing the Mg x content up-to 1.5 lead to luminescence intensity enhancement, while a further increase of Mg x content leads to concentration quenching. The International Commission on Illumination (CIE) color chromaticity showed that the color shifts from blue to greenish (blue-green vertex region) when ranging Mg x content.