Gradient Cationic Vacancies Enabling Inner-To-Outer Tandem Homojunctions: Strong Local Internal Electric Field and Reformed Basic Sites Boosting CO Photoreduction.

The slow charge dynamics and large activation energy of CO severely hinder the efficiency of CO photoreduction. Defect engineering is a well-established strategy, while the function of common zero-dimensional defects is always restricted to promoting surface adsorption. In this work, a gradient layer of tungsten vacancies with a thickness of 3-4 nm is created across Bi WO nanosheets. This gradient layer enables the formation of an inner-to-outer tandem homojunction with an internal electric field, which provides a strong driving force for the migration of photoelectrons from the bulk to the surface. Meanwhile, W vacancies change the coordination environment around O and W atoms, leading to an alteration in the basic sites and the mode of CO adsorption from weak/strong adsorption to moderate adsorption, which ultimately decreases the formation barrier of the key intermediate *COOH and facilitates the conversion thermodynamics for CO . Without any cocatalyst and sacrificial reagent, W-vacant Bi WO shows an outstanding photocatalytic CO reduction performance with a CO production rate of 30.62 µmol g  h , being one of the best catalysts in similar reaction systems. This study reveals that gradient vacancies as a new type of defect will show huge potential in regulating charge dynamics and catalytic reaction thermodynamics.