Less is better: Insight into the concentration dependency of the photoreduction of Eu3+ in Ba2SiO4

Photoinduced electron transfer processes in luminescent materials drive numerous applications like medical imaging, ionizing radiation dosimetry and safety signage. However, the concentration dependency of the photoreduction of the dopants during this process is still poorly understood. Here, optical spectroscopy and electron paramagnetic resonance (EPR) experiments are systematically performed to reveal the doping concentration-dependent photoreduction behavior in Ba2-xEuxSiO4 where a fraction of the Eu3+ dopants can be reduced to Eu2+ upon irradiation with X-rays. EPR measurements show that the conversion ratio of Eu3+→Eu2+ decreases with increasing concentration at the same irradiation dose, resulting in the concentration dependency of the photoreduction behavior in Ba2-xEuxSiO4. Optical spectroscopy measurements show that although the incorporated Eu3+ ions occupy at least three crystallographic sites, Eu3+ can only be photoreduced on one of these sites. Meanwhile, the relative occupancy of this photoreducible site decreases as the doping concentration increases. The combination of these two factors might be responsible for the decreased conversion ratio of Eu3+→Eu2+ as the doping concentration increases. These findings provide insights into the photoreduction of dopants in luminescent materials and can facilitate the exploration of this functional behavior for advanced devices.