Sol-gel synthesis and characterization of MAl2O4 (M = Zn or Mg) spinels doped, co-doped and triply doped nano-phosphors
Motloung, Setumo Victor
MetadataShow full item record
The luminescent and structural properties of the MAl2O4 (M = Zn, Mg) (which are thereafter referred as hosts) phosphors prepared by sol-gel methods at a relatively low temperature (~80 °C) are discussed. Zinc, magnesium and aluminium nitrates and citric acid were used as the starting materials for the hosts preparations. The prepared gels were dried in an oven and subsequently annealed in air either at 800 0C for an hour. In order to study the effects of the different dopants into the hosts matrix, the dopants concentration were varied. The host material was either singly doped or co-doped or triply doped. Furthermore, in order to study the effects of the catalyst content on the prepared powders, the optimum concentrations for the singly doped phosphors were prepared and the catalyst content was varied during synthesis. Generally, the surface morphologies, surface topographies, crystal structure, photoluminescence (PL), Ultraviolet-visible (UV-Vis) properties were influenced by the dopant concentration and catalyst content. The incorporation of the foreign atoms seems to populate the hosts with more defects. For the ZnAl2O4: x% Pb2+ samples, the Thermo gravimetric analysis (TGA) showed that the minimum annealing temperature required to obtain single phase ZnAl2O4 is above 400 °C. Undoped and Pb2+-doped ZnAl2O4 nanoparticles exhibit the violet emission at slightly different positions, which suggests the possibilities that the luminescence centre can either be due to the defects level in the host or Pb2+ ions. The emission peaks at 390 and 399 nm are ascribed to the typical ultra-violet (UV) transitions 3P0,1 → 1S0 in Pb2+ ions. On the study of ZnAl2O4: x% Cr3+ (0 ≤ x ≤0.3), Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) analysis confirmed the presence of all expected ions in the powder material. The results showed that Cr3+ can occupy multiple-sites in the host matrix. It was interesting to see, once again, that the PL results showed that the host and the Cr3+-doped exhibit violet emission slightly at different peak positions, which suggests that the luminescence can originate from the host or Cr3+ ion. Emission from the host is attributed to the band-gap defects in the host material, while the emission from the Cr3+ is attributed to the 4T1 → 4A2 transition. At the higher mol% there is an emission at 692 nm, which is attributed to the 2E → 4A2 transition in Cr3+. On the case-study of the co-doped ZnAl2O4: 0.1% Ce3+, x% Eu3+ (0 ≤ x ≤ 2mol%), the results showed that the nanopowders microstructure consists of non-uniform sizes and the loss in lattice fringes as the Eu3+ mol% increase suggest the increase in strain or disorder. The incorporation of the co-activator (Eu3+) at the higher mol% resulted in the radiative energy transfer from Ce3+→ Eu3+. The International Commission on Illumination (CIE) color coordinates show the shift from the blue to orange visible region as the Eu3+ concentration is increased. From the triply doped MgAl2O4: 0.1% Ce3+, 0.1% Eu2+, x% Tb3+ (0 ≤ x ≤ 2%) study, the PL results revealed the existence of the energy transfer from Eu2+ → Tb3+ → Ce3+. CIE colour chromaticity showed that the colour can be tuned from bluish → greenish by changing the Tb3+ mol% and the excitation wavelength. In both studies of the effects of the catalyst content in ZnAl2O4:1.5% Pb2+ and ZnAl2O4:0.01% Cr3+, the results showed that the increase in the catalyst content lead to the morphological evolution and transformation from small particles to rods-like-needles. In addition, at the higher catalyst content, the extra peak associated with the ZnO impurities are observed. The emission intensity was influenced by the catalyst content. The catalysts content does not affect the emission colour in the case of ZnAl2O4:1.25% Pb2+. However, in the case of ZnAl2O4:0.01% Cr3+, the results revealed the possibilities of tuning the emission colour by varying the catalyst content.