Photocatalytic overall water splitting by using layered polymeric materials has been considered a promising way for clean fuel H-2 production, however the catalytic performance is still unsatisfactory due to the sluggish charge separation as well as serious backward reaction of water splitting. We herein demonstrate the clever design and construction of alkali-metal-oxides membrane coated ultrasmall Pt sub-nanoparticle that is deposited on alkali-metal-atoms intercalated carbon nitride for efficient overall water splitting. We verify that the intercalated alkali-metal-atoms (K, Na, Li) would bridge the adjacent layers of carbon nitride for efficient charge interlayer transportation, boosting charge carrier separation; while the formation of mixed alkali-metal-oxides shell on the ultrasmall Pt sub-nanopartile would function as a membrane that is H-2 permeable but O-2 impermeable. Such a selective permeable membrane is found to block H-2/O-2 recombination to form H2O. Benefited from these advantages, the as-prepared photocatalyst exhibits excellent photocatalytic H-2 evolution performance (vacuum condition: AQY = 37%, air condition: AQY = 4.9%) as well as overall water splitting activity in a stoichiometric ratio of 2:1 with suppressed H-2/O-2 recombination. The innovative design demonstrated here provides new ideas for designing advanced photocatalysts with high charge separation for efficient overall water splitting with no H-2/O-2 recombination. (C) 2019 Elsevier Inc. All rights reserved.