Reaction Mechanism of Hydrogen Generation and Nitrogen Fixation at Carbon Nitride/Double Perovskite Heterojunctions

Photocatalytically active heterojunctions based on metal halide perovskites (MHPs) are drawing significant interest for their chameleon ability to foster several redox reactions. The lack of mechanistic insights into their performance, however, limits the ability of engineering novel and optimized materials. Herein, we report on a composite system including a double perovskite, Cs2AgBiCl6/g-C3N4, used in parallel for solar-driven hydrogen generation and nitrogen reduction. The composite efficiently promotes the two reactions, but its activity strongly depends on the perovskite/carbon nitride relative amounts. Through advanced spectroscopic investigation and density function theory modelling we studied the H2 and NH3 production reaction mechanisms, finding perovskite halide vacancies as the primary reactive sites for hydrogen generation, withstanding a positive contribution of low loaded g-C3N4, in reducing carrier recombination. For nitrogen reduction, instead, the active sites are g-C3N4 nitrogen vacancies, and the heterojunction best performs at low perovskites loadings, as the composites maximizes light absorption and reduced carrier losses. We believe these insights are important add-ons towards universal exploitation of MHPs in contemporary photocatalysis.