Nano-Size Effects on Decay Dynamics of Photo-Excited Polarons in CeO₂

The study of polaron dynamics in complex materials has garnered significant attention owing to its implications for various technological applications, including catalysis, solid-state devices, and energy storage. This paper investigates the photo-excited electron and hole polaron dynamics in cerium dioxide (CeO₂) using time-resolved X-ray absorption spectroscopy, with an emphasis on the nano-size effect. Additionally, density functional theory and multiplet calculations have been utilized to reveal the photo-excited polaron dynamics in CeO₂ single crystal (SC) and nanocrystal (NC). The electron polaron is observed to decay into a deep trap site with a short duration of ≈5 ps, while electrons in the traps stay for more than 1400 ps. The most significant observation is the behavior of holes in NC, which tends to stay longer (≈150 ps) compared to SC (<10 ps) suggesting hole existence more at the surface than at bulk. The fast dissociation of the electron polarons the prolonged lifetime of the electrons above the Fermi level and the enhanced hole lifetime at the surface are proposed to be among the various factors that influence the high reactivity of CeO₂.