Long-Lived Photoexcited Carrier Dynamics of d–d Excitations in Spinel Ordered Co3O4
- Matthias M. Waegele, Hoang Q. Doan, and Tanja Cuk*
- J. Phys. Chem. C, 2014, 118 (7), pp 3426–3432 DOI: 10.1021/jp4113443. Download
The spectrum and dynamics of excited carriers in a spinel-ordered transition metal oxide, Co3O4, were investigated by both selective photoexcitation of all major optical transitions and selectively filling electronic states through an applied voltage. Co3O4 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.