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.　H-2　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　H-2　evolution　was　shown　by　comparison　of　H-2　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　Ti-O-Ni　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　Ti-O-Ni　linkages　severing　as　visible-light　chromophores.　Hydrogen　gas　is　reduced　and　evolved　at　oxygen　vacancies　of　the　TiO2　surface.　(C)　2014　Elsevier　Inc.　All　rights　reserved.