Enhancement of afterglow in ZnS:Cu,Co water-soluble nanoparticles by aging

In addition to illuminations and lighting, water-soluble afterglow nanoparticles may also find applications in disease treatment and medical imaging. Afterglow efficiency and longevity are the key parameters for practical applications. It was observed that the afterglow efficiency of 3-mercaptopropionic acid stabilized water-soluble ZnS:Cu,Co afterglow nanoparticles can be improved by using much less S^2- precursor (Na₂S) in the synthesis. In addition, the afterglow intensity increased more than 7 times after the sample was aged for two weeks at room temperature. To figure out the mechanism for the enhancement, the nanoparticle structures and sizes were measured by X-ray diffraction and high-resolution transmission electron microscopy. The crystal structure and the particle sizes are almost the same for the samples before and after aging. The optical absorption spectra show not much difference for the two samples. However, the X-ray photoelectron spectroscopy measurement indicates that oxidation occurred on the surfaces of ZnS:Cu,Co nanoparticles due to the aging. The oxidation may lead to more S vacancies and electron traps that might be responsible for the afterglow enhancement.