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Recent years have seen enormous research interests on the direct water splitting using solar light radiation [1-4].Here charged-slab first principles calculations integrated with periodic continuum solvation model is utilized to analyze the initiating steps of water splitting on two most common TiO2 surfaces,namely,rutile(110)and anatase(101)at the solid-water interface.The rutile(110)surface is more active for water splitting with the calculated barrier of O-H bond breaking being 0.2 eV lower compared to that on anatase(101).The first proton removal of water(H2O+hole+→OH+H+)is both phase and surface structure-sensitive due to the heat-driven first-step of O-H bond breaking [5-7].Unexpectedly,the photo-generated hole does not promote the O-H bond breaking,and the charge transfer occurs after the H2O dissociation when the surface O nearby the dissociated OH anion trapping the hole.The solvation plays important catalytic role to stabilize and remove protons from the reaction site,which inhibits effectively the charge-recombination of the dissociated OH anion with the proton.The theory presented here shows that the chemical properties of surface play significant role in photocatalytic process and a strategy based on simple structural parameters is proposed towards the design of new photocatalyst.