Gradient Cationic Vacancies Enabling Inner‐To‐Outer Tandem Homojunctions: Strong Local Internal Electric Field and Reformed Basic Sites Boosting CO2 Photoreduction

The slow charge dynamics and large activation energy of CO₂ severely hinder the efficiency of CO₂ 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 Bi₂ WO₆ 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 CO₂ 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 CO₂ . Without any cocatalyst and sacrificial reagent, W-vacant Bi₂ WO₆ shows an outstanding photocatalytic CO₂ reduction performance with a CO production rate of 30.62 µmol g^-1 h^-1 , 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.