Single-site nickel-grafted anatase TiO2 for hydrogen production: Toward understanding the nature of visible-light photocatalysis

•Atomically-isolated Ni species result in the visible-light photocatalytic properties of Ni/TiO2.•Ti-O-Ni linkage units function as the so-called “p–n heterojunction”.•The visible-light absorption originates from metal-to-metal charge transfer of the TiONi linkage unites.•Electrons produced by MMCT transfer to oxygen vacancies of TiO2 to participant H2 releasing reaction.•Single-site Ni atoms work as electron-trapping and hydrogen-evolving sites under UV light. [Display omitted] This work provides an engineering guide to constructing active sites on TiO2 with the surface organometallic chemistry of nickelocene for unveiling the molecular nature of so-called “p–n heterojunction”-induced visible-light photocatalysis. H2 evolution was used as a model reaction to evaluate the photocatalytic properties of Ni/TiO2 materials containing different Ni-oxo species, which are prepared by three methods. Ni nuclearity-dependent H2 evolution was shown by comparison of H2 production rates over such Ni/TiO2 photocatalysts. Detailed characterizations clearly revealed the triple role of atomically isolated Ni species as light-harvesting, electron-trapping, and hydrogen-evolving sites in the photocatalytic reaction. The TiONi molecular junctions formed at the NiO–TiO2 interface perform the function of p–n heterojunctions and create visible light absorption and photocatalysis. It was well established that visible light photocatalysis follows a physical mechanism of metal-to-metal charge transfer in TiONi linkages severing as visible-light chromophores. Hydrogen gas is reduced and evolved at oxygen vacancies of the TiO2 surface.