Long-Lived Photoexcited Carrier Dynamics of d–d Excitations in Spinel Ordered Co₃O₄

The spectrum and dynamics of excited carriers in a spinel-ordered transition metal oxide, Co 3 O 4, were investigated by both selective photoexcitation of all major optical transitions and selectively filling electronic states through an applied voltage. Co 3 O 4 contains strong absorptions at all relevant optical excitations common to transition-metal oxides, inclusive of ligand-to-metal charge transfer, metal-to-metal charge transfer, and intravalence d – d transitions. We find that carriers initially excited across the charge-transfer excitations quickly (∼3 ps) convert to d – d excitations due to strong electron–phonon coupling. Subsequent recombination from weakly coupled, localized excited d states to the ground state occurs at a much longer, nanosecond time scale. These results suggest that d – d excitations represent a special type of long-lived recombination center intrinsic to a transition-metal oxide. Such carrier dynamics may apply to a wider range of transition metal oxides actively being integrated in photocatalytic and photovoltaic devices.