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

Un-doped (Mg1.5Al2O4.5) and 0.88% Cd2+ doped MgxAl2O3+x nano-powders were synthesized via citrate sol-gel method. The Mgx concentration was varied in the range of 0.25≤x≤4.5. The X-ray diffraction (XRD) data revealed that Mg0.25Al2O3.25:0.88% Cd2+ consists of poorly crystalline and amorphous phases. For the MgxAl2O3+x:0.88% Cd2+ (0.5≤x≤4.5) series, the XRD revealed that the nano-powders consist of face centered cubic MgAl2O4 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 14nm, 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% Cd2+ doped nano-powders generally emits a violet emission at 407nm, which is attributed to the bandgap defects within the host material. The emission peaks at 495 and 656nm 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 MgAl2O4 to MgO. The maximum PL intensity was observed from the Mg1.5Al2O4.5:0.88% Cd2+ sample. Increasing the Mgx content up-to 1.5 lead to luminescence intensity enhancement, while a further increase of Mgx 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 Mgx content.