Transient optical spectroscopy of Co3O4 finds that excited electrons thermalize within a few picoseconds to the mid-gap, near infrared states associated with crystal field d-d transitions and polarons in correlated, 3d transition metal oxides. The fast, ps thermalization from above band-gap excitations at 2 eV to low energy transitions at ~0.8 eV and possibly below, combined with the slow, ~0.6 ns recovery due to hole capture, indicates that these states are efficient electron traps. The dependence of the transient probe spectrum on excitation density, pump wavelength, and across different morphologies of Co3O4 supports the primary conclusion. Aspects of the transient probe spectrum are attributed to transient absorptions, transient bleaches, and electric field effects arising from a carrier density located at the mid-gap infrared states. The results are complemented by transient x-ray absorption spectroscopy elucidating the more detailed elemental and electronic structure of these electron trap states. How hole capture by such bulk electron trap states could mediate charge flow during the water oxidation reaction on Co3O4and materials with similar mid-gap states is discussed. Future experiments will investigate the extent to which this type of carrier relaxation is generic to transition metal oxides with prominent mid-gap infrared states.

Mid-gap, infrared crystal field transitions as efficient electron trap states in Co<sub>3</sub>O<sub>4</sub> with implications for the water oxidation reaction

246th National Meeting (2013)

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Mid-gap, infrared crystal field transitions as efficient electron trap states in Co3O4 with implications for the water oxidation reaction

Mid-gap, infrared crystal field transitions as efficient electron trap states in Co₃O₄ with implications for the water oxidation reaction