Ultrafast Charge-Carrier Dynamics and Photocatalytic Mechanisms in Lanthanum-Doped Cerium Oxide Nanoparticles

Abstract Cerium oxide nanoparticles (nanoceria) doped with trivalent lanthanum (La) ions and synthesized by a coprecipitation process at ambient temperature were studied by steady-state and femtosecond transient absorption spectroscopy to understand the photocatalytic mechanisms of this wide band gap photocatalyst in alkaline aqueous solution. UV irradiation reduces a significant fraction of cerium ions near the surface to Ce3+ centers that persist for hours in air. La doping increases the percentage of cerium ions that can be photoreduced, while also increasing the rate of photocatalytic degradation of an added dye molecule. Adding methanol eliminates dye photodegradation, showing that photogenerated hydroxyl radicals are the active species. Unexpectedly, the transient spectral signatures and dynamics of photogenerated electrons and holes are nearly unaffected by La doping. Fewer electron small polarons are formed in the La-doped nanoceria, but the dynamics of electron–hole recombination are unchanged during the 0–3 ns window probed. Furthermore, electrons and holes recombine according to a power law with an exponent that is inversely proportional to the Urbach energy determined from the steady-state absorption spectrum. These results are explained by a model in which a crystalline core of cerium oxide that lacks oxygen vacancies is surrounded by a defective shell that likely consists of an amorphous hydrous oxide phase containing Ce and La ions. The recombination of carriers trapped in this shell is unchanged on subnanosecond time scales, and La doping is proposed to enhance photocatalytic performance by enabling more holes to escape into the solvent via slow surface reactions.