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

Abstract The slow charge dynamics and large activation energy of CO 2 severely hinder the efficiency of CO 2 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 2 WO 6 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 2 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 2 . Without any cocatalyst and sacrificial reagent, W‐vacant Bi 2 WO 6 shows an outstanding photocatalytic CO 2 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.