Synthesis process dependent white LPL in Zn2GeO4 ceramic and the long afterglow mechanism

Abstract

The white long persistent luminescence (LPL) material Zn2GeO4 ceramic was prepared by high temperature solid-state method. It is the first time to report the afterglow can last for more than half an hour, and the color does not change with time delay. Such a long afterglow time is enough for photocatalysis research. The structure, luminescence and afterglow properties were analyzed by X-ray diffraction, fluorescence spectra and afterglow decay curves. It is the first time to find the dosage ratio of Zinc Oxide (ZnO) to Germanium Oxide (GeO2) and the annealing temperature affect the crystal phase and optical properties of the products. These two aspects were studied in detail. It was found that GeO2 played the role of fluxing agent in the reaction system because of its low melting point. The amount of GeO2 must be more than the stoichiometric amount because it has a low boiling point and can volatilize at high temperature. Too high sintering temperature and much excessive GeO2 led to crystallization of Zn2GeO4 ceramic during cooling process. The experimental results showed that the initial afterglow spectrum ranged widely from 400 nm to 700 nm after the sample was excited by 254 nm UV light and this broad band could be fitted to three or four Gaussian curves. The Gausses fitted curves peak at 450 nm, 510 nm, 590 nm and 680 nm. It is concluded that the white light is a combination of these light peaking at different wavelength and each peak corresponds to different defects inside the material. The internal defects are decreased a lot because of ceramic crystallization. As a result, the luminescence intensity and afterglow intensity decrease with the decrease of defects, but the afterglow time is not affected.