Surface and Defect Engineering Coupling of Halide Double Perovskite Cs₂NaBiCl₆ for Efficient CO₂ Photoreduction

Non-toxic halide double perovskite materials have many advantages of lead halide perovskite. Whereas, they usually exhibit poor stability and very low intrinsic photocatalytic CO₂ reduction activity due to the insufficient separation of photogenerated charges and the lack of active sites. In this work, stable chlorine-deficient 3D hierarchical Cs₂NaBiCl₆ porous microspheres assembled by highly crystalline nanoflakes were prepared by a simple grinding method. An unprecedented CO yield of 30.22 µmol g⁻¹ h⁻¹ was achieved in the gas-solid photocatalytic reduction of CO₂ without sacrificial agents, which is the highest value among lead-free halide perovskite photocatalysts. Experimental results and density-functional theory calculations show that the chlorine vacancy plays the triple role of suppressing photogenerated electron-holes recombination, enhancing CO₂ adsorption, and significantly reducing the free energy barrier for the key intermediate COOH* generation. In comparison with the pristine Cs₂NaBiCl₆, coupling of surface and defect engineering of the hierarchical sample brings 12.34 times enhancement of CO₂ photoreduction activity. This work proposes a simple method to synthesize a chlorine-vacancy rich 3D hierarchical lead-free halide perovskite and offers a new design idea to substantially enhance the photocatalytic activity, opening a door for the prospective contribution of these materials to carbon neutralization.