Carbon Nitrides with Grafted Dual-Functional Ligands as Electron Acceptors and Active Sites for Ultra-stable Photocatalytic H2O2 Production

Photocatalytic production of H2O2 from earth-abundant water and oxygen using low-cost metal-free carbon nitrides (CNs) through oxygen reduction is a prospective route toward a greener future. However, the H2O2 productivity is restricted by rapid electron–hole separation and the low oxygen reduction activity of CNs. Herein, we rationally designed a series of CNs with covalently bonded dual-functional ligands acting as electron acceptors and active sites to achieve high photocatalytic H2O2 production and superior stability. The best-performing carbon nitride displays a H2O2 production rate of 7.3 mmol/g h with an apparent quantum efficiency of 20.2% at 420 nm using formic acid as the electron donor. Moreover, the modified CNs show excellent stable H2O2 generation over 110 h without significant decline. Mechanistic studies reveal that H2O2 was produced through a 2e– oxygen reduction reaction route. Photoluminescence, photo-electrochemical, and Kelvin probe force microscopy results together with theoretical calculations have revealed that the excellent photocatalytic performance originates from the dual-functional ligand. It not only acts as an electron acceptor to promote photogenerated charge carrier separation by withdrawing electrons but also works as an active site to accelerate oxygen reduction by lowering the oxygen adsorption and activation energy. Moreover, this facial strategy of grafting ligands provides a universal approach to synthesize photocatalysts with enhanced reactivity under mild conditions by choosing the proper ligands for a specific reaction.