Cerium oxide (CeO2) is a wide bandgap semiconductor that has a number of similarities with better-studied TiO2. Both metal oxides have similar bandgaps and UV irradiation of nanocrystalline CeO2 also generates OH radicals that can be used to mineralize organic pollutants in aqueous solution. One important difference with respect to TiO2 is the ease with which the oxidation state of cerium can be switched between +4 and +3 in CeO2. The fluorite structure adopted by cerium oxide is highly tolerant of this redox change and this property facilitates the formation of catalytically active oxygen vacancies. The effects of Ce3+ defects on cerium oxide photoprocesses are poorly understood. For this reason, femtosecond and steady-state spectroscopic measurements were carried out on CeO2 nanoparticles in aqueous solution to study carrier dynamics in CeO2 and the photochemical reaction pathways that lead to OH radical generation. In the course of this work, it was discovered that CeO2 nanoparticles several nm in diameter can be prepared spontaneously by simply dissolving cerium(IV) ammonium nitrate in water in mM concentrations. The UV-vis absorption spectrum changes on the seconds and minutes time scale as the initial mononuclear complexes age into relatively disperse CeO2 nanoparticles. Growth occurs significantly more slowly in D2O compared to H2O, revealing that proton transfer plays a role in the rate-limiting dehydration that is thought to accompany nanoparticle growth. Femtosecond pump-probe measurements on these and larger nanoparticles are providing insight into electron-hole recombination and trapping in this very attractive photocatalytic material.

Carrier dynamics in cerium oxide nanoparticles observed by femtosecond transient absorption spectroscopy

251st National Meeting (2016)

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