Effect of annealing temperature on structural and optical properties of ZnAl2O4:1.5% Pb2+ nanocrystals synthesized via sol-gel reaction

Lead-doped zinc aluminate (ZnAl2O4:1.5% Pb2+) spinel nanocrystals powders were prepared by the citric assisted sol-gel process. The effects of annealing temperature (AT) at a fixed dopant concentration on the structure, morphology and optical properties of the powders were investigated. Thermogravimetric analysis (TGA) showed that the minimum annealing temperature required to obtain single phase ZnAl2O4 must be above 400 °C. Fourier transform infrared spectroscopy (FTIR) results suggest that heat treating destroys some of the bonds. From the XRD results, it was observed that the required structure started to grow at the AT of 500 °C. With an increase in AT > 500 °C the structure transformed to single phase of cubic ZnAl2O4 spinel. The crystallinity increases upon further rise of AT to 800 °C. On heating at 600-800 °C, the powders had the average crystallite sizes of around 18-22 nm. Energy dispersive x-ray spectroscopy (EDS) analysis confirmed the presence of the expected elements (Zn, Al, O and Pb). The morphology of the prepared phosphor were influenced by the AT. Transmission electron microscopy (TEM) results revealed that the prepared powders are in the nano-scale range. The band gap of the sample annealed at 800 °C were estimated to be 3.2 eV. The emission position and intensity of the phosphor was dependent on the AT. Emissions arising from both the host and Pb2+ ion were observed. Emission wavelength of the prepared phosphors can be tuned by varying the AT. The results suggest that ZnAl2O4:1.5% Pb2+ is a promising candidate for blue-light emitting phosphor.