Regulation of Co(II) coordination in nano-hydroxyapatite for effective photocatalytic CO₂-to-ethanol conversion in...

Artificial photosynthesis of C₂₊ hydrocarbon fuels still faces a fundamental challenge in the rational design of active sites on photocatalysts. Herein, high-spin tetrahedral Co(II) sites (Co²⁺_Td) are implanted through the controllable substitution of Ca_II ions in nano-hydroxyapatite (HAP). Co-inclusion not only effectively regulates the electronic structure to promote photogenerated charge carrier migration but also modulates the surface properties of CoCaAP nanosticks to enhance CO₂ capture and activation. Ex situ XRD and XPS results indicate that CoCaAP transforms into more stable hydrated CoCa phosphate, and partial Co²⁺ sites are oxidized to Co^δ+ with a higher oxidation state in photocatalytic CO₂ reduction, thus effectively promoting H₂O oxidation and proton transfer. As a result, an exceptional performance for selective CO₂-to-ethanol conversion is achieved by CoCaAP, with an ethanol production rate of 779 µmol g^-1 h^-1 in the absence of any co-catalysts or sacrificial agents under xenon-light irradiation. In situ FT-IR spectra indicate CO₂ reduction to ethanol via the CO₂ → *HCO₃ → *COCOH → *OCH₂CH₃ pathway. This work provides an accessible strategy for engineering single-atom active sites in HAP to advance efficient solar-to-fuel systems.