Nickel Single-Atom Modified g-C₃N₄/TiO₂ Heterojunctions for Sacrificial Reagent-Free CO₂ Photoreduction

A heterojunction photocatalyst composed of single-atom, atomically dispersed Ni sites on g-C₃N₄/TiO₂ was developed for sacrificial-agent-free CO₂-to-CO reduction under simulated solar irradiation. The optimized catalyst, containing 0.78 wt% Ni and 46 wt% g-C₃N₄, delivered 70% selectivity to CO over H₂ and exhibited 16-fold enhancement in activity compared with bare g-C₃N₄/TiO₂ and Ni-single-atom catalysts supported on either g-C₃N₄ or TiO₂ alone. High-resolution transmission electron microscopy (HRTEM) revealed intimate interfacial coupling within the g-C₃N₄/TiO₂ heterojunction, while atomically dispersed Ni species were predominantly anchored on g-C₃N₄ nanosheets coating the TiO₂ surface. X-ray photoelectron spectroscopy revealed pronounced interfacial electronic redistribution following heterojunction formation and Ni incorporation, indicating strong electronic communication between the semiconductor components. Electrochemical impedance spectroscopy (EIS) and steady-state photoluminescence measurements showed significantly suppressed charge recombination, whereas transient absorption spectroscopy revealed that isolated Ni sites act as efficient electron traps, extracting photogenerated electrons and directing them toward catalytic reduction centers. Combined with the comparative photocatalytic performance of individual components, these findings identify the Ni-N_x moieties on g-C₃N₄ as active sites for CO₂-to-CO conversion and support the S-scheme charge-transfer pathway, in which TiO₂ preferentially consumes holes while highly reducing electrons accumulate on the g-C₃N₄-supported Ni single-atom sites to drive selective CO₂ reduction in water.