Atomically　precise　metal　nanoclusters　(NCs)　have　been　deemed　as　an　emerging　class　of　metal　nanomaterials　owing　to　fascinating　size-dependent　physicochemical　properties,　discrete　energy　band　structure,　and　quantum　confinement　effect,　which　are　distinct　from　conventional　metal　nanoparticles　(NPs).　Nevertheless,　metal　NCs　suffer　from　photoinduced　self-oxidative　aggregation　accompanied　by　in-situ　transformation　to　metal　NPs,　markedly　reducing　the　photosensitization　of　metal　NCs.　Herein,　maneuvering　the　generic　instability　of　metal　NCs,　we　perform　the　charge　transport　impetus　comparison　between　atomically　precise　metal　NCs　and　plasmonic　metal　NPs　counterpart　obtained　from　in-situ　self-transformation　of　metal　NCs　in　photoelectrochemical　(PEC)　water　splitting　reaction.　For　conceptual　demonstration,　we　proposed　two　quintessential　heterostructures,　which　include　TNTAs-Au25　heterostructure　fabricated　by　electrostatically　depositing　glutathione　(GSH)-protected　Au25(GSH)18　NCs　on　the　TiO2　nanotube　arrays　(TNTAs)　substrate,　and　TNTAs-Au　heterostructure　constructed　by　triggering　self-transformation　of　Au25(GSH)18　NCs　to　plasmonic　Au　NPs　in　TNTAs-Au25　via　calcination.　The　results　indicate　that　photoelectrons　produced　over　Au25　NCs　are　superior　to　hot　electrons　of　plasmonic　Au　NPs　in　stimulating　the　interracial　charge　transport　toward　solar　water　oxidation.　This　is　mainly　ascribed　to　the　significantly　accelerated　carrier　transport　kinetics,　prolonged　carrier　lifespan,　and　substantial　photosensitization　effect　of　Au25　NCs　compared　with　plasmonic　Au　NPs,　resulting　in　the　considerably　enhanced　PEC　water　splitting　performance　of　TNTAs-Au25　relative　to　plasmonic　TNTAs-Au　counterpart　under　visible　light　irradiation.　Our　work　would　provide　important　implications　for　rationally　designing　atomically　precise　metal　NCs-based　photosystems　toward　solar　energy　conversion.　©　2023