Mechanism and active site of photocatalytic water splitting on titania in aqueous surroundings

Abstract

Photocatalytic water splitting is regarded as an important route for generating renewable energy. Here, charged-slab first principles calculations integrated with a periodic continuum solvation model is utilized to analyze the initiating steps of water splitting on the two most common TiO₂ surfaces, namely, rutile (110) and anatase (101), at the solid–water interface. It is found that the first proton removal of water (H₂O + hole⁺ → OH + H⁺) is sensitive to the crystalline phase and surface. 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 higher activity of rutile is not due to the redox level of the hole (the position of the valence band maximum), but caused by the more favorable local bonding geometry of the surface. Unexpectedly, the photogenerated hole does not promote O–H bond breaking, and the charge transfer occurs after the H₂O dissociation when the surface O nearby the dissociated OH anion traps the hole. The solvation plays an important catalytic role to stabilize and remove protons from the reaction site, which effectively inhibits the charge-recombination of the dissociated OH anion with the proton. The theory presented here shows that the chemical properties of the surface play a significant role in the photocatalytic process, and a strategy based on simple structural parameters is proposed towards the design of new photocatalysts.