Gradient Cationic Vacancies Enabling Inner-to-outer Tandem Homojunction: Strong local Internal Electric Field and Reformed basic sites Boosting CO 2 Photoreduction.

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 vacancy defect is always restricted to promoting surface adsorption. Here, we report that gradient tungsten vacancies layer with a thickness of 3∼4 nm is created across the Bi 2 WO 6 nanosheets by a post-etching approach. It enables formation of an inner-to-outer tandem homojunction with a strong internal electric field directing from outer to inner within the gradient vacancies layer, which provides a strong driving force for the migration of the photoelectron from the bulk to the surface of catalyst. Meanwhile, the introduction of gradient W vacancies changes the coordination environment around O and W atoms, leading to an alteration in the basic sites and the mode of CO 2 adsorption on the catalyst surface from weak/strong adsorption (O sites) to moderate adsorption (O and W sites), which ultimately decreases the formation barrier of key intermediate *COOH and facilitates the conversion thermodynamics for CO 2 . Without any cocatalysts and sacrificial reagents, 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 and a selectivity of 99%, being one of the best catalysts in similar reaction system. This study discloses that gradient vacancies as a new type of defect will show huge potential in regulating charge dynamics and catalytic reaction thermodynamics. This article is protected by copyright. All rights reserved.