Spin states investigation of delafossite oxides by means of X-ray absorption and photoemission spectroscopy

Highlights: • We investigated the local atomic structure of delafossite oxides including AgCoO2 and CuCoO2. • Shortening of Co-O bond length indicated that Co 3d orbitals were in low spin states. • Non-splitting of Co 3s core level spectra also agreed with the low spin states. • Fully-filled t2g orbital and its metallic surface states is likely associated with high catalytic activity. -- Abstract: The d-electron occupancy of transition metal (TM) oxides is one of the most dominant factors in the catalytic activity in the oxygen evolution reaction (OER). To investigate the spin states and the local atomic structure around the TM atoms in delafossite oxides, CuCrO₂, CuMnO₂, AgFeO₂, CuFeO₂, AgCoO₂, and CuCoO₂, were investigated using X-ray absorption spectroscopy. Based on Shannon radii, the extracted TM-O bond lengths showed that TM = Co delafossite oxides are in the low spin (LS) state, while the other delafossite oxides are in the high spin state. Furthermore, non-splitting of Co 3s core level spectra were observed in photoemission spectroscopy, also indicating LS states. In the LS states, Fully-filled t_2g orbital and its metallic surface states distributing normal to (001) surface is likely associated with high OER activity in TM = O delafossite oxides.