Ultrafast Electron Injection and Recombination Dynamics of Coumarin 343-Sensitized Cerium Oxide Nanoparticles

Abstract

Dye-sensitized electron injection into cerium oxide (CeO2) nanoparticles is studied using femtosecond transient absorption spectroscopy. Following the 430 nm photoexcitation of coumarin 343 molecules adsorbed on CeO2 nanoparticles, mid-infrared (mid-IR) transient absorption signals appear within 500 fs. Mid-IR signals are assigned to electrons injected into broadly distributed trap or defect states found roughly midway between the Ce 5d conduction band edge and the bottom of the empty band of Ce 4f states. These states are proposed to have Ce 5d character. In contrast, mid-IR signals are not observed when electrons in bare CeO2 nanoparticles are promoted from the valence band to the band built from Ce 4f orbitals by UV excitation. Transient absorption signals from the oxidized dye rise within 300 fs and decay somewhat more slowly than the mid-IR signals, suggesting that electron relaxation occurs within CeO2 in competition with back-electron transfer to the oxidized dye. These results reveal elementary events important for understanding photocatalysis using CeO2 nanoparticles.