Abstract: S-scheme heterostructure photocatalysts can achieve highly efficient solar energy utilization. Here, single-atom Ni species were deposited onto TiO₂/g-C₃N₄ (TCN) composite photocatalyst with an S-scheme heterojunction for highly efficient photocatalytic water splitting to produce hydrogen. Under solar irradiation, it realized the hydrogen production activity of 134 µmol g^-1 h^-1, about 5 times higher than the TCN without atomic Ni. In-situ Kelvin probe force microscopy characterization and the density functional calculation certify that by forming the S-scheme heterojunction, the photo-excited electrons from the TiO₂ combine with the photogenerated holes at the coupled g-C₃N₄ driven by a built-in electric field. More importantly, the single-atom Ni species stabilized the photogenerated electrons from the g-C₃N₄ could effectively enhance the charge separation between the holes on the valence band of TiO₂ and electrons at the conduction band of g-C₃N₄. Meanwhile, the Ni atoms act as the surface catalytic centers for the water reduction reaction, which greatly improves the reactivity of the photocatalyst. The present work provides a new approach for developing noble metal-free heterojunctions for high-efficiency photocatalysis.

Key words: S-scheme, Single-atom catalysis, TiO₂/g-C₃N₄, Hydrogen production, Built-in electric field