The dynamics of charge carriers formed by UV excitation of few nm CeO2 nanoparticles (nanoceria) were studied by femtosecond transient absorption spectroscopy. Transient absorption bands of photogenerated electrons and holes are spectrally well separated greatly aiding the elucidation of their localization and trapping dynamics. Excitation above the optical band gap forms an electron small polaron (ESP) in the bulk of the nanoparticle and a localized hole at the surface in hundreds of fs. Ultrafast charge separation occurs because holes have much greater mobility than electrons in crystalline CeO2. From the mean first passage time for ESPs to diffuse to the particle surface, an activation barrier of 0.15 eV was determined for thermal hopping. While self-trapped excitons are not formed in the bulk of the nanoparticle, they form easily at the defect-rich surface when exciting below the optical band gap. The resulting surface polaron exciton decays nonradiatively with a half-life of 5 ps. This work offers the insight that the effectiveness of nanoceria, and possibly other metal oxides such as TiO2, as a photocatalyst arises from the self-trapping of just one of the carriers in the nanoparticle interior. It also shows that strategies that extend absorption to longer wavelengths by creating surface defects spoil the asymmetry and will likely not be productive for improving photocatalyst performance.
Pham et al. (Tue,) studied this question.