Unraveling the spin-lattice-band coupling driven photocatalytic selectivity in CuFeO₂ single crystals

In this study, the crystal and electronic structures of CuFeO₂ were identified using a (scanning) transmission electron microscope equipped with a novel CEOS energy-filtering and imaging device (CEFID), (S)TEM-EELS, which unambiguously confirmed the rhombohedral 3 R structure with valences of Fe and Cu are + 3 and + 1, respectively. Valence EELS (VEELS) combined with density function theory (DFT) was further used to investigate its photocatalytic characteristics. By integrating experimental evidence with theoretical analysis, we provide a physical explanation for why 3R-CuFeO₂ acts exclusively as a single-function photocatalyst active for hydrogen evolution reactions (HER). Upon photoexcitation, electrons occupied in the σ*_Cu–O antibonding states are promoted into the conduction band, yielding a negative Gibbs free energy change (ΔG < 0), indicating that the HER process occurs spontaneously under solar irradiation at ambient conditions. In contrast, the oxygen evolution reaction (OER) would require the oxidation process from Fe^{3 +} to Fe⁴⁺, which entails breaking the exceptionally stable half-filled 3d⁵ configuration. Under the same conditions, this oxidation process corresponds to a positive Gibbs free energy change (ΔG > 0), rendering it thermodynamically forbidden for OER. These findings elucidate the intrinsic origin of the single-function behavior of 3R-CuFeO₂ and provide new insights for the rational design of photocatalytic materials.