Light absorption by metal oxides creates electron-hole pairs that drive chemical reactions, but carriers must localize or trap on the surface of a photoactive solid to drive interfacial redox reactions. In many insulators, electrons or holes self-trap due to electron-phonon coupling to form localized carriers known as polarons. Polarons move sluggishly by thermal hopping, but relatively little is known about the time scales behind their formation and about their transient mobility under the nonequilibrium conditions of photoexcitation.

We are studying the dynamics of photogenerated polarons in cerium oxide (CeO2) nanoparticles (nanoceria). The ‘rare earth’ element cerium is more abundant than cobalt, lead, and tin. Cerium oxide has a similar absorption onset as TiO2 and is also an attractive material for photocatalysis. Cerium oxide is highly redox mutable and a large fraction of cerium ions at the surface of nanoceria in aqueous solution can be switched from the +4 to the +3 oxidation state. We are investigating how carrier separation, recombination, and trapping are influenced by the presence of Ce(III) defects. This redox mutability allows ceria to act as a mimic of superoxide dismutase, inhibiting cellular damage by reactive oxygen species in vivo, but the underlying mechanisms are uncertain. We are studying the tension between the localization of electrons and holes, which is necessary for facilitating redox reactions at the surface, and delocalization, which can increase the charge mobility.