CuFeO2 and Fe2O3 are closely related earth-abundant photocatalysts with band gaps ideally suited for solar light harvesting. However, photoelectrochemical properties differ significantly between these two materials: Fe2O3 is a water oxidation catalyst, which suffers from poor conversion, while CuFeO2 is an efficient photocathode with activity for both H2 evolution and CO2 reduction. Using ultrafast extreme ultraviolet (XUV) and optical spectroscopy combined with density functional theory (DFT), we present a detailed study of the electron and hole dynamics in CuFeO2 and Fe2O3. Using XUV spectroscopy it is possible to track electrons and holes independently in the Fe 3d, Cu 3d, and O 2p states comprising the band structure of these semiconducting metal oxides. Despite similar electron dynamics, only CuFeO2 shows cathodic photocurrent. Spectroscopic results show that this may be the result of ultrafast (500 fs) hole thermalization from O 2p to hybridized Cu 3d states in CuFeO2 leading to spatial charge separation within the layered CuFeO2 delafossite lattice. These results are supported by DFT calculations confirming strong covalency of Cu 3d and O 2p states at the valence band edge. The ability to elucidate site-specific charge carrier dynamics in real time provides important criteria for the rational design of catalysts for efficient solar energy harvesting based on their underlying photophysics.

Ultrafast electron dynamics in CuFeO2 solar photocathodes measured by XUV reflection-absorption spectroscopy

256th National Meeting (2018)

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