Sol-gel synthesis and characterization of MAl2O4 (M = Zn or Mg) spinels doped, co-doped and triply doped nano-phosphors

[摘要] The luminescent and structural properties of the MAl2O4 (M = Zn, Mg) (which are thereafterreferred 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 thestarting materials for the hosts preparations. The prepared gels were dried in an oven andsubsequently annealed in air either at 800 0C for an hour. In order to study the effects of thedifferent dopants into the hosts matrix, the dopants concentration were varied. The hostmaterial was either singly doped or co-doped or triply doped. Furthermore, in order to studythe effects of the catalyst content on the prepared powders, the optimum concentrations forthe singly doped phosphors were prepared and the catalyst content was varied duringsynthesis.Generally, the surface morphologies, surface topographies, crystal structure,photoluminescence (PL), Ultraviolet-visible (UV-Vis) properties were influenced by thedopant concentration and catalyst content. The incorporation of the foreign atoms seems topopulate the hosts with more defects. For the ZnAl2O4: x% Pb2+ samples, the Thermogravimetric analysis (TGA) showed that the minimum annealing temperature required toobtain single phase ZnAl2O4 is above 400 °C. Undoped and Pb2+-doped ZnAl2O4nanoparticles exhibit the violet emission at slightly different positions, which suggests thepossibilities that the luminescence centre can either be due to the defects level in the host orPb2+ 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 �?.3), Time-of-Flight Secondary Ion MassSpectroscopy (TOF-SIMS) analysis confirmed the presence of all expected ions in thepowder material. The results showed that Cr3+ can occupy multiple-sites in the host matrix. Itwas interesting to see, once again, that the PL results showed that the host and the Cr3+-dopedexhibit violet emission slightly at different peak positions, which suggests that theluminescence can originate from the host or Cr3+ ion. Emission from the host is attributed tothe band-gap defects in the host material, while the emission from the Cr3+ is attributed to the4T1 �?4A2 transition. At the higher mol% there is an emission at 692 nm, which is attributedto the 2E �?4A2 transition in Cr3+.On the case-study of the co-doped ZnAl2O4: 0.1% Ce3+, x% Eu3+ (0 �?x �?2mol%), the resultsshowed that the nanopowders microstructure consists of non-uniform sizes and the loss inlattice fringes as the Eu3+ mol% increase suggest the increase in strain or disorder. Theincorporation of the co-activator (Eu3+) at the higher mol% resulted in the radiative energytransfer from Ce3+�?Eu3+. The International Commission on Illumination (CIE) colorcoordinates show the shift from the blue to orange visible region as the Eu3+ concentration isincreased.From the triply doped MgAl2O4: 0.1% Ce3+, 0.1% Eu2+, x% Tb3+ (0 �?x �?2%) study, the PLresults revealed the existence of the energy transfer from Eu2+ �?Tb3+ �?Ce3+. CIE colourchromaticity showed that the colour can be tuned from bluish �?greenish by changing theTb3+ 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 morphologicalevolution and transformation from small particles to rods-like-needles. In addition, at thehigher catalyst content, the extra peak associated with the ZnO impurities are observed. Theemission intensity was influenced by the catalyst content. The catalysts content does notaffect the emission colour in the case of ZnAl2O4:1.25% Pb2+. However, in the case ofZnAl2O4:0.01% Cr3+, the results revealed the possibilities of tuning the emission colour byvarying the catalyst content.

[发布机构] University of the Free State