Photocatalytic overall water splitting is one of the ultimate goals in solar conversion and the activation and cleavage of H2O molecule is the initial and often rate-determining step in this reaction. Through DFT calculations, we designed Pt cluster decorated Ni(OH)(2) nanoparticles (Pt/Ni(OH)(2)) as robust cocatalyst capable of activating H2O and dissociating HO-H bonds. Then the unique structure was fabricated through in-situ photo oxidation of Ni2P to Ni(OH)(2) on semiconductor like C3N4 and subsequent selective photo-deposition of Pt on Ni (OH)(2) surface. Pt/Ni(OH)(2)-C3N4 is much more active in photocatalytic HER, OER and overall water splitting compared with Pt, Ni(OH)(2) and spatially separated co-loaded Ni(OH)(2)-Pt on C3N4. And Pt/Ni(OH)(2)-C3N4 achieves an AQE of L8% at 420 nm in overall water splitting by using only 0.3 wt% Pt, superior to most solid-state photocatalytic systems to date. Moreover, Pt/Ni(OH)(2) composite enables TiO2 to split pure water in good stoichiometry. This work emphasizes the importance of H2O activation and may pave the way for enabling single semiconductors to efficiently split pure water.